# Patent

## Title
Sodium channel protein type III α-subunit splice variant

## Abstract

The present invention is directed to a splice variant of a human sodium channel alpha subunit and methods and compositions for making and using the same.

## Background
This application is a divisional of U.S. patent application Ser. No. 12/416,777 (issued as U.S. Pat. No. 7,915,385), filed Apr. 1, 2009, which is a divisional of U.S. patent application Ser. No. 11/357,518 (issued as U.S. Pat. No. 7,531,523), filed Feb. 17, 2006, which claims the benefit of priority of U.S. Provisional Application No. 60/654,019, filed Feb. 17, 2005, which is incorporated herein by reference in its entirety.


BACKGROUND
1. Field of the Invention
The present invention is generally directed to voltage-gated sodium channel NaV 1.3 splice variants. The invention further describes methods and compositions for the stable expression of such splice variants and methods of use of such compositions for identifying compounds that modulate the activity of sodium channels.
2. Background of the Related Art
The electrical activity of neuronal and muscle cells are governed by the activity of sodium channels on the plasma membrane of such cells. Rapid entry of sodium ions into the cell through such a channel causes depolarization of the membrane and generation of an action potential. Entry of sodium ions through sodium channels in response to a voltage change on the plasma membrane in excitable cells plays a functional role in the excitation of neurons in the central nervous system and the peripheral nervous system.
Sodium channels are voltage-gated transmembrane proteins that form ion channels within the membrane and have been the target of significant pharmocologic study, due to their potential role in a variety of pathological conditions. These sodium channels are responsible for the cellular uptake of sodium during the transmission of an electrical signal in cell membranes. The channels are members of a multigene family of proteins and are typically composed of a number of subunits. Typically, the pore of the channel is formed by the α-subunit and there are four accessory β-subunits, termed β1, β2, β3 and β3.
The β-subunits are involved in the modulation of the activity of sodium channel but the α-subunit is all that is required for the channel to form a functional ion pore. Co-expression of the β-subunits with the α-subunit has been shown to produce a more positive membrane potential. Further not all of the β-subunits are required, for example, it has been shown that the β3-subunits alone is sufficient to cause an increase in sodium current (Qu et al., Mol. Cell. Neurosci., 18(5):570-80, 2001).
The amino acid sequence of the sodium channel has been evolutionarily conserved. The channel is comprised of a signal polypeptide containing four internal repeats (domains I-IV). Each domain folds into six transmembrane α-helices or segments, of which five are hydrophobic and one is a highly-charged domain containing lysine and arginine residues (S4 segment). The highly-charged S4 segment is involved in the voltage gating properties of the sodium channel. The positively-charged side chains of the amino acids of the S4 segment are thought to be paired with the negatively-charged side chains of the other five segments such that upon membrane depolarization there is a shift in the position of one of the helices relative to the other resulting in an opening of the channel.
There are numerous variants of sodium channel α-subunit. These variants may be classified according to their sensitivity to tetrodotoxin (TTX). Those subunits that are inhibited by nanomolar quantities of TTX are classified as tetrodotoxin-sensitive channels, whereas those that require at least micromolar quantities of TTX for inhibition are referred to as tetrodotoxin-insensitive (1-5 micromolar). Those channels that require greater that 100 micromolar quantities of the TTX are termed tetrodotoxin-resistant. TTX is a toxin that blocks the conduction of nerve impulses along the axons and leads to paralysis. It binds to sodium channels and blocks the flow of sodium ions. It is believed that the positively-charged group of the toxin interacts with a negatively-charged carboxylate at the mouth of the channel on the extracellular side of the membrane thereby blocking the conductance of the pathway.
It has been noted that following nerve injury there is hyperexcitability (or an increased rate of spontaneous impulse firing in neurons) in peripheral sensory ganglia. It has been suggested that this hyperexcitability in neurons is due to altered sodium channel expression in some chronic pain syndromes (Tanaka et al., Neuroreport 1998; 9 (6): 967-72). Increased numbers of sodium channels leading to inappropriate, repetitive firing of the neurons have been reported in the tips of injured axons in various peripheral nervous tissues such as the DRG, which relay signals from the peripheral receptors to the central nervous system. Indeed, it has been noted that there is an increase in expression of an α1 NaV 1.3 subunit in axotomized DRG neurons together with elevated levels of α1 NaV1.1 and α1 NaV1.2 mRNAs (Waxman et al, Brain Res Mol Brain Res 1994; 22 (1-4): 275-89).
The peripheral input that drives pain perception is thought to depend upon the presence of functional voltage-gated sodium channels in peripheral nerves. It has been noted that there is a positive correlation between increased sodium channel expression in peripheral nerves. Some studies have also shown increased expression in association with neuropathic pain. In particular, it has been recognized that acute, inflammatory, and neuropathic pain can all be attenuated or abolished by local treatment with sodium channel blockers such as lidocaine. Remarkably, two voltage-gated sodium channel genes (Nav1.8 and Nav1.9) are expressed selectively in damage-sensing peripheral neurons, while a third channel (Nav1.7) is found predominantly in sensory and sympathetic neurons. An embryonic channel (Nav1.3) is also upregulated in damaged peripheral nerves and associated with increased electrical excitability in neuropathic pain states. Using antisense and knock-out studies, it has been shown that these sodium channels play a specialized role in pain pathways, and pharmacological studies (Wood et al., J Neurobiol., 61(1):55-71, 2004).
Most patients with traumatic spinal cord injury (SCI) report moderate to severe chronic pain that is refractory, or only partially responsive, to standard clinical interventions (Balazy, Clin J Pain 8: 102-110, 1992; Turner et al., Arch Phys Med Rehabil 82: 501-509, 2001). Experimental contusion SCI in rodents can produce long-lasting central neuropathic pain (Hulsebosch et al., J Neurotrauma 17: 1205-1217, 2000; Lindsey et al., Neurorehabil Neural Repair 14: 287-300, 2000; Hains et al., Neuroscience 116: 1097-1110, 2001; Mills et al., J Neurotrauma 18: 743-756, 2001). In spinally injured animals, alterations in electrophysiologic properties of dorsal horn neurons (Hao et al., Pain 45: 175-185, 1991; Yezierski and Park, Neurosci Lett 157: 115-119, 1993; Drew et al., Brain Res 893: 59-69, 2001; Hains et al., Neuroscience 116: 1097-1110, 2003a; Hains et al., Brain Res 970: 238-241, 2003b) are thought to contribute to changes in somatosensory responsiveness.
The TTX-sensitive Nav1.3-sodium channel is expressed at relatively high levels in embryonic dorsal root ganglion (DRG) neurons but is barely detectable in adult DRG neurons and its expression is decreased in the adult spinal cord and CNS throughout development. However, the expression of Nav1.3 mRNA and protein is markedly upregulated in DRG neurons of adult rats after axotomy of peripheral projections, after chronic constriction injury, and after tight spinal nerve ligation. This produces a rapidly repriming TTX-S current that permits neuronal firing at higher than normal frequencies. It has been shown that an increase in expression of Nav1.3, similar to the changes in DRG neurons after peripheral axotomy, takes place in lumbar dorsal horn sensory neurons after SCI. It has further been shown that knock-down or reduction of expression of Nav1.3 mRNA and protein results in a reduction in hyperexcitability of dorsal horn sensory neurons and pain-related behaviors in animals (Hains et al., J. Neurosci., 23(26):0881-8892, 2003).
There are various sodium channels that remain to be characterized. Identification of such channels will facilitate further studies and identification and characterization of further isotype-specific antagonists of sodium channel blockers. Such sodium channel blockers or antagonists will be useful in the management of pain. Preferably, such analgesic agents are such that treatment of pain is facilitated without having deleterious side effects due to cardiac, central nervous system or neuromuscular complications.
SUMMARY OF THE INVENTION
The present invention is directed to a splice variant of a human sodium channel alpha subunit and methods and compositions for making and using the same. More specifically, one embodiment of the invention is directed to an isolated recombinant nucleic acid encoding a human sodium channel NaV1.3 polypeptide wherein the polypeptide is encoded by the nucleic acid sequence presented in SEQ ID NO: 1.
Another embodiment of the invention describes an isolated recombinant nucleic acid encoding a human sodium channel NaV1.3 recombinant protein having the amino acid sequence of SEQ ID NO: 2. Also contemplated herein is an isolated recombinant nucleic acid comprising the sequence presented in SEQ ID NO: 1, the mature protein coding portion thereof, or a complement thereof. One preferred embodiment of the invention contemplates an isolated recombinant nucleic acid encoding a polypeptide of SEQ ID NO: 2. The nucleic acids described herein may be genomic DNA, cDNA, or RNA.
Conservative variants of the sequences of the present invention are particularly contemplated, for example, the invention is directed to an isolated nucleic acid comprising a nucleotide sequence encoding a polypeptide that is a conservative variant of the amino acid sequence set forth in SEQ ID NO:2, wherein the variant encodes a sodium channel α-subunit with the proviso that residue 208 of SEQ ID NO:2 is an aspartic acid residue and an insert of 33 amino acids is found after residue 623, as defined in SEQ ID NO:2.
Expression constructs that comprise an isolated nucleic acid encoding a protein having an amino acid sequence of SEQ ID NO:2 or the mature protein portion thereof wherein the mature protein region comprises an aspartic acid residue at the residue that corresponds to amino acid residue 208 in SEQ ID NO:2, an insert of 33 amino acids is found after residue 623, as defined in SEQ ID NO:2 and a promoter operably linked to the polynucleotide also form part of the invention. In specific embodiments, the expression construct is such that the nucleic acid comprises a mature protein coding sequence as set forth in SEQ ID NO:1. The expression construct is an expression construct selected from the group consisting of an adenoassociated viral construct, an adenoviral construct, a herpes viral expression construct, a vaccinia viral expression construct, a retroviral expression construct, a lentiviral expression construct and a naked DNA expression construct.
Also part of the invention are recombinant host cell stably transformed or transfected with a nucleic acid or an expression construct of the invention in a manner that allows the expression in the host cell of a protein of SEQ ID NO:2. Preferably, the nucleic acid transforming the host cell comprises a mature protein encoding sequence as set forth in SEQ ID NO:1, wherein the mature protein encoded by the sequence is a sodium channel NaV1.3 polypeptide that has an aspartic acid residue at the amino acid that corresponds to amino acid residue 208 of SEQ ID NO:2 and an insert of 33 amino acids is found after residue 623, as defined in SEQ ID NO:2. Recombinant host cells stably transformed or transfected with an expression construct of the invention in a manner allowing the expression in the host cell of a protein product of the expression construct also are contemplated.
The host cells may be mammalian, a bacterial, yeast cells, or insect cells. It may be advantageous that the recombinant host cells produced by the invention further express one or more β-subunits of a sodium channel selected from the group consisting of β1, β2, β3 and β4. In specific embodiments, the host cell is a HEK293 cell line.
The invention further provides an isolated and purified protein comprising an amino acid sequence selected from the group consisting of an amino acid sequence set forth in SEQ ID NO:2 and the mature protein portion of SEQ ID NO:2, wherein the mature protein portion comprises an aspartic acid residue at the amino acid residue that corresponds to amino acid residue 208 of SEQ ID NO:2 and an insert of 33 amino acids is found after residue 623, as defined in SEQ ID NO:2. In particular embodiments, the isolated and purified protein comprises an amino acid sequence that is 99% identical to the complete sequence set forth in SEQ ID NO:2. In other embodiments, the isolated and purified protein comprises an amino acid sequence that is 95% identical to the complete sequence of SEQ ID NO:2 and contains a 33 amino acid insert of SEQ ID NO:3.
The invention also comprises a diagnostic kit for detecting a nucleic acid that encodes a sodium channel α-subunit polypeptide, the polypeptide being encoded by the sequence presented in SEQ ID NO: 1, comprising an isolated nucleic acid probe complementary to the complete sequence of SEQ ID NO: 1, and means for containing the nucleic acid.
Methods of identifying a modulator of a human sodium channel α-subunit expression or activity are contemplated wherein the modulator is identified by a method comprising the steps of contacting a cell that expresses a nucleic acid of SEQ ID NO:1 with the candidate modulator substance; monitoring the expression or ion channel activity of a protein of SEQ ID NO:2; and comparing the expression or ion channel activity of a protein of SEQ ID NO:2 in the presence and absence of the candidate substance; wherein an alteration in the expression or ion channel activity of a protein of SEQ ID NO:2 indicates that the substance is a modulator of human sodium channel α-subunit expression or activity. The modulator of human sodium channel α-subunit expression or activity may be a small molecule ion channel blocker or inhibitor, an oligonucleotide, an antisense oligonucleotide, a DNA oligonucleotide, an RNA oligonucleotide, an RNA oligonucleotide having at least a portion of the RNA oligonucleotide capable of hybridizing with RNA to form an oligonueleotide-RNA duplex, or a chimeric oligonucleotide.
The invention also provides methods of identifying a test compound that binds to a sodium channel comprising providing a cell that expresses a sodium channel having a sequence of SEQ ID NO:2; contacting the host cell with the test compound and determining the binding of the test compound to the sodium channel; and comparing the binding of the test compound to the host cell determined in step (b) to the binding of the test compound with a cell that does not express a sodium channel.
Also provided is an assay for identifying a test compound that modulates the activity of a sodium channel comprising providing a host cell that expresses a functional sodium channel comprising at least one polypeptide comprising the amino acid sequence of SEQ ID NO: 2, contacting the host cell with a test compound under conditions that would activate sodium channel activity of the functional sodium channel in the absence of the test compound; and determining whether the host cell contacted with the test compound exhibits a modulation in activity of the functional sodium channel. In particular embodiments, the host cell has been genetically engineered to express or overexpress the functional sodium channel. In other embodiments, the host cell has been genetically engineered by the introduction into the cell of a nucleic acid molecule having a nucleotide sequence encoding a polypeptide comprising the amino acid sequence set forth in SEQ ID NO: 2. Preferably, the host cell has been genetically engineered to upregulate the expression of a nucleic acid encoding a polypeptide comprising the amino acid sequence of SEQ ID NO: 2. In particular embodiments, the upregulated nucleic acid is endogenous to the host cell. Preferably, the modulation of the functional sodium channel activity is an inhibition of that activity.
A method of producing a purified human sodium channel α-subunit protein also is provided, the method comprising preparing an expression construct comprising a nucleic acid of SEQ ID NO:1 operably linked to a promoter; transforming or transfecting a host cell with the expression construct in a manner effective to allow the expression of a protein having an amino acid sequence of SEQ ID NO:2, or the mature protein portion thereof in the host cell; culturing the transformed or transfected cell under conditions to allow the production of the protein by the transformed or transfected host cell; and isolating the human sodium channel α-subunit protein from the host cell.
Other embodiments contemplate treatment of a disorder by administering to a subject in need thereof a pharmaceutical composition that comprises a compound identified according to the methods described herein and a pharmaceutically acceptable carrier, excipient or diluent.
Other features and advantages of the invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, because various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.



BRIEF DESCRIPTION OF THE DRAWINGS
The following drawings form part of the present specification and are included to further illustrate aspects of the present invention. The invention may be better understood by reference to the drawings in combination with the detailed description of the specific embodiments presented herein.
FIGS. 1A through 1F shows the sequence alignment of the sequence of the present invention (SEQ ID NO:2) with sequences (SEQ ID NO:4; SEQ ID NO:5; SEQ ID NO:6; SEQ ID NO:7; SEQ ID NO:8) identified from Genbank.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
Human NaV1.3 is a voltage-gated sodium channel α-subunit whose expression has been shown to be upregulated in neurons that have been subjected to various injuries such as e.g., after axotomy of peripheral projections, after chronic constriction injury, and after tight spinal nerve ligation. Reduction or knockout Nav1.3 alleviates the pain associated with such injuries. Thus, therapies designed to knock-out or block the action of Nav1.3 are important in alleviating neuropathy. The present application is directed to a novel splice variant of the α1 subunit of human Nav 1.3.
The splice variant of the human NaV1.3 channel α subunit of the present invention was identified from human spinal cord RNA using RT-PCR to amplify the message in three overlapping fragments as described in the Examples herein below. The three fragments were then ligated together to create the full-length sequence. The cDNA clone isolated is a novel splice variant that differs from previously reported Nav1.3 cDNAs. This Nav1.3 cDNA was used to express human Nav1.3 in cultured cell lines. The Nav 1.3-expressing line was used in high-throughput screening to identify antagonists of Nav 1.3 and has been used to characterize the activity of such agents against the channel. Methods and compositions for making and using the splice variant of the present invention are described in further detail below.
Polypeptide and Fragments Thereof.
According to the present invention, there has been identified a gene that encodes a novel splice variant of human Nav1.3. It is contemplated that this gene and the protein encoded by the same may be used in studies of sodium channels and in the identification of modulators thereof. In this regard, it is noted that sodium channel α-subunits are well known to those of skill in the art and have been described e.g., in U.S. Pat. No. 6,479,259; U.S. Pat. No. 6,335,172; U.S. Pat. No. 6,184,349; U.S. Pat. No. 6,060,271; U.S. Pat. No. 6,030,810; U.S. Pat. No. 5,892,018; U.S. Pat. No. 5,776,859; U.S. Pat. No. 5,693,756; U.S. Pat. No. 5,437,982; U.S. Pat. No. 5,380,836. Each of the foregoing patents are incorporated herein by reference as providing specific teaching of how to make and use sodium channel proteins and nucleic acids that encode the same. The methods taught therein for using such compositions to identify therapeutic agents, e.g., sodium channel blockers or even β-subunits that are modulators of sodium channel α-subunits may readily be adapted using the protein and nucleic acid compositions of the present invention. As discussed above, Nav1.3 sodium channels are involved in mediating pain associated with neuronal injury. As such, it is contemplated that it will be will be desirable to inhibit, decrease, ablate, reduce or otherwise diminish the expression of the Nav1.3 gene or the activity of the protein product of the gene expression described herein. It is contemplated that inhibition of activity of the encoded protein or the expression of this gene will have a beneficial effect in treating pain. Inhibition of the gene expression may even be helpful in regenerative studies or overcoming the deleterious effects of spinal cord injury.
In the therapeutic aspects, guidance may be gained from the functional and therapeutic aspects of sodium channels described in e.g., in U.S. Pat. No. 6,479,259; U.S. Pat. No. 6,335,172; U.S. Pat. No. 6,184,349; U.S. Pat. No. 6,060,271; U.S. Pat. No. 6,030,810; U.S. Pat. No. 5,892,018; U.S. Pat. No. 5,776,859; U.S. Pat. No. 5,693,756; U.S. Pat. No. 5,437,982; U.S. Pat. No. 5,380,836 been recognized as being involved in pain and have been used as targets for therapy. Sodium channel blockers or modulators have been described e.g., in U.S. Pat. No. 6,756,400; U.S. Pat. No. 6,646,012; U.S. Pat. No. 6,613,345; U.S. Pat. No. 6,607,741; U.S. Pat. No. 6,559,154; U.S. Pat. No. 6,479,498 (each incorporated by reference) and such patents provide guidance as to methods and compositions for identification of additional such therapeutic agents once new targets such as the Nav1.3 splice variant of the present invention, are identified. While treatment of pain and the like will involve inhibition or blocking of Nav1.3 activity, it is contemplated that in certain embodiments, it may be desirable to increase the expression of Nav1.3. For example, in specific embodiments, it would be desirable to increase, augment or otherwise supplement endogenous Nav1.3 expression and/or activity in commercial or experimental endeavors where it would be desirable to produce animal models or cells that have an increased Nav1.3 expression and are phenotypically models for neuropathic pain.
The Nav1.3 splice variant encoding gene has been cloned by the present inventors and is taught herein to have a nucleic acid sequence as shown in SEQ ID NO:1. The coding region of this gene encodes a protein of SEQ ID NO:2. It is noted that the encoded protein has an aspartic acid residue at an amino acid that corresponds to residue number 208 of SEQ ID NO:2. A sequence alignment of the sequence of SEQ ID NO:2 with other sodium channel alpha subunit proteins is shown in FIG. 1. As can be seen in FIG. 1, there is an aspartic acid residue at amino acid residue 208. Further, SEQ ID NO:2 comprises an insert of NVSQASM SSRMVPGLPANGKMHSTVDCNGVVSL that is not present in other sodium channel alpha subunits examined in FIG. 1. This 33 amino acid insert that starts at residue 624 of the NAV1.3 splice variant of the present invention. This region is the linker between domain 1 and domain 2 of the sodium channel alpha subunit. Protein kinase A phosphorylation sites of the sodium channel are located in this region and are close to this splice site. The sodium channel activity of the protein of SEQ ID NO:2 or a variant thereof that contains aspartic acid at an amino acid residue that corresponds to amino acid 208 of the Nav1.3 splice variant of the present invention may be readily tested using techniques well known to those of skill in the art.
In addition to the entire Nav1.3 protein molecule of SEQ ID NO:2, de compositions of the present invention also may employ fragments of the polypeptide of SEQ ID NO:2 that retain the ability/activity to form a sodium channel and retain an aspartic acid at a residue that corresponds to amino acid residue 208 of SEQ ID NO:2. Fragments, including the N-terminus or C terminus of the molecule may be generated by genetic engineering of translation start or stop sites within the coding region (discussed below). Alternatively, treatment of the Nav1.3 splice variant protein molecule with proteolytic enzymes, (proteases), can produce a variety of N-terminal, C-terminal and internal fragments. Examples of fragments may include contiguous residues of the Nav1.3 splice variant protein sequence of SEQ ID NO:2 of 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 30, 35, 40, 45, 50, 55, 60, 65, 75, 80, 85, 90, 95, 100, or more amino acids in length. Such fragments preferably retain one or more of the biological activities of Nav1.3 protein and/or retain an immunological (antigenic) property of Nav1.3 protein. These fragments may be purified according to known methods, such as precipitation (e.g., ammonium sulfate), HPLC, ion exchange chromatography, affinity chromatography (including immunoaffinity chromatography) or various size separations (sedimentation, gel electrophoresis, gel filtration).
When the present application refers to the function of Nav1.3 splice variant protein or “wild-type” activity, it is meant that the molecule in question has the ability to form a sodium channel in a plasma membrane fraction. The Nav1.3 protein of the present invention has a sequence of SEQ ID NO:2. An assessment of the particular molecules that possess such activities may be achieved using standard assays familiar to those of skill in the art. For example, the immunological studies will readily reveal whether the Nav1.3 splice variant binds to antibodies directed against Nav1.3 or other sodium channel alpha subunits. Such antibodies are known to those of skill in the art and may be readily generated using routine methods.
In certain embodiments, Nav1.3 protein analogs and variants may be prepared and will be useful in a variety of applications. Amino acid sequence variants of the polypeptide can be substitutional, insertional or deletion variants. Deletion variants lack one or more residues of the native protein which are not essential for function or immunogenic activity. A common type of deletion variant is one lacking secretory signal sequences or signal sequences directing a protein to bind to a particular part of a cell. Insertional mutants typically involve the addition of material at a non-terminal point in the polypeptide. This may include the insertion of an immunoreactive epitope or simply a single residue. Terminal additions, also called fusion proteins, are discussed below.
Substitutional variants typically exchange one amino acid of the wild type for another at one or more sites within the protein, and may be designed to modulate one or more properties of the polypeptide, such as stability against proteolytic cleavage, without the loss of other functions or properties. Substitutions of this kind preferably are conservative, that is, one amino acid is replaced with one of similar shape and charge. Conservative substitutions are well known in the art and include, for example, the changes of: alanine to serine; arginine to lysine; asparagine to glutamine or histidine; aspartate to glutamate; cysteine to serine; glutamine to asparagine; glutamate to aspartate; glycine to proline; histidine to asparagine or glutamine; isoleucine to leucine or valine; leucine to valine or isoleucine; lysine to arginine; methionine to leucine or isoleucine; phenylalanine to tyrosine, leucine or methionine; serine to threonine; threonine to serine; tryptophan to tyrosine; tyrosine to tryptophan or phenylalanine; and valine to isoleucine or leucine.
In order to construct such mutants, one of skill in the art may employ well known standard technologies. Specifically contemplated are N-terminal deletions, C-terminal deletions, internal deletions, as well as random and point mutagenesis.
N-terminal and C-terminal deletions are forms of deletion mutagenesis that take advantage for example, of the presence of a suitable single restriction site near the end of the C- or N-terminal region. The DNA is cleaved at the site and the cut ends are degraded by nucleases such as BAL31, exonuclease III, DNase I, and S1 nuclease. Rejoining the two ends produces a series of DNAs with deletions of varying size around the restriction site. Alternatively, deletions can be generated using polymerase chain reaction (PCR) amplification of cDNAs using primers that exclude regions of the coding sequence corresponding to the desired polypeptide deletion. Proteins expressed from such mutants can be assayed for appropriate activity as voltage-gated sodium channels, as described throughout the specification. Similar techniques may be employed for internal deletion mutants by using two suitably placed restriction sites, thereby allowing a precisely defined deletion to be made, and the ends to be religated as above. as above. Similarly, PCR can be used to amplify the sequences flanking the internal deletion and then ligation used as described above to generate the DNA clone containing the desired deletion.
Also contemplated are partial digestion mutants. In such instances, one of skill in the art would employ a “frequent cutter”, which cuts the DNA in numerous places depending on the length of reaction time. Thus, by varying the reaction conditions it will be possible to generate a series of mutants of varying size, which may then be screened for activity.
A random insertional mutation may also be performed by cutting the DNA sequence with a DNase I, for example, and inserting a stretch of nucleotides that encode, 3, 6, 9, 12 etc., amino acids and religating the end. Once such a mutation is made the mutants can be screened for various activities presented by the wild-type protein.
Point mutagenesis also may be employed to identify with particularity which amino acid residues are important in particular activities associated with Nav1.3 splice variant protein. Thus, one of skill in the art will be able to generate single base changes in the DNA strand to result in an altered codon and a missense mutation.
The amino acids of a particular protein can be altered to create an equivalent, or even an improved, second-generation molecule. Such alterations contemplate substitution of a given amino acid of the protein without appreciable loss of interactive binding capacity with structures such as, for example, antigen-binding regions of antibodies or binding sites on substrate molecules or receptors. Since it is the interactive capacity and nature of a protein that defines that protein's biological functional activity, certain amino acid substitutions can be made in a protein sequence, and its underlying DNA coding sequence, and nevertheless obtain a protein with like properties. Thus, various changes can be made in the DNA sequences of genes without appreciable loss of their biological utility or activity, as discussed below. Codon tables that show the codons that encode particular amino acids are well known to those of skill in the art. In making changes to the sequence of SEQ ID NO:2, the hydropathic index of amino acids may be considered, which contributes to the secondary structure of the resultant protein, which in turn defines the interaction of the protein with other molecules, for example, enzymes, substrates, receptors, DNA, antibodies, antigens, and the like (Kyte & Doolittle, J. Mol. Biol., 157(1):105-132, 1982, incorporated herein by reference). Generally, amino acids may be substituted by other amino acids that have a similar hydropathic index or score and still result in a protein with similar biological activity, i.e., still obtain a biological functionally equivalent protein. Hydrophilicity is another parameter that may be used to determine amino acid substitution (see e.g., U.S. Pat. No. 4,554,101).
Exemplary amino acid substitutions that may be used in this context of the invention include but are not limited to exchanging arginine and lysine; glutamate and aspartate; serine and threonine; glutamine and asparagine; and valine, leucine and isoleucine. Other such substitutions that take into account the need for retention of some or all of the biological activity whilst altering the secondary structure of the protein will be well known to those of skill in the art.
Another type of variant that is specifically contemplated for the preparation of polypeptides according to the invention is the use of peptide mimetics. Mimetics are peptide-containing molecules that mimic elements of protein secondary structure. See, for example, Johnson et al., “Peptide Turn Mimetics” in BIOTECHNOLOGY AND PHARMACY, Pezzuto et al., Eds., Chapman and Hall, New York (1993). The underlying rationale behind the use of peptide mimetics is that the peptide backbone of proteins exists chiefly to orient amino acid side chains in such a way as to facilitate molecular interactions, such as those of antibody and antigen. A peptide mimetic is expected to permit molecular interactions similar to the natural molecule. These principles may be used, in conjunction with the principles outlined above, to engineer second generation molecules having many of the natural properties of Nav1.3 protein, but with altered and even improved characteristics.
Other mutants that are contemplated are those in which entire domains of the Nav1.3 protein are switched with those of another related protein. For example, other sodium channels exist and chimeric sodium channels may be produced where domains from e.g., a Nav1.8 protein are switched with domains from the Nav1.3. Domain switching is well-known to those of skill in the art and is particularly useful in generating mutants having domains from related species.
Domain switching involves the generation of chimeric molecules using different but related polypeptides. For example, by comparing the sequence of Nav1.3 protein with that of similar sequences from another source and with mutants and allelic variants of these polypeptides, one can make predictions as to the functionally significant regions of these molecules. Thus, it is contemplated then to switch related domains of these molecules in an effort to determine the criticality of these regions to Nav1.3 protein function. These molecules may have additional value in that these “chimeras” can be distinguished from natural molecules, while possibly providing the same or even enhanced function.
In addition to the mutations described above, the present invention further contemplates the generation of a specialized kind of insertional variant known as a fusion protein. This molecule generally has all or a substantial portion of the native molecule, linked at the N- or C-terminus, to all or a portion of a second polypeptide. For example, fusions typically employ leader sequences from other species to permit the recombinant expression of a protein in a heterologous host. Another useful fusion includes the addition of an immunologically active domain, such as an antibody epitope, to facilitate purification of the fusion protein. Inclusion of a cleavage site at or near the fusion junction will facilitate removal of the extraneous polypeptide after purification. Other useful fusions include linking of functional domains, such as active sites from enzymes, glycosylation domains, cellular targeting signals or transmembrane regions.
There are various commercially available fusion protein expression systems that may be used in the present invention. Particularly useful systems include but are not limited to the glutathione S-transferase (GST) system (Pharmacia, Piscataway, N.J.), the maltose binding protein system (NEB, Beverley, Mass.), the FLAG system (IBI, New Haven, Conn.), and the 6×His system (Qiagen, Chatsworth, Calif.). These systems are capable of producing recombinant polypeptides bearing only a small number of additional amino acids, which are unlikely to affect the antigenic ability of the recombinant polypeptide. For example, both the FLAG system and the 6×His system add only short sequences, both of which are known to be poorly antigenic and which do not adversely affect folding of the polypeptide to its native conformation. Another N terminal fusion that is contemplated to be useful is the fusion of a Met Lys dipeptide at the N terminal region of the protein or peptides. Such a fusion may produce beneficial increases in protein expression or activity.
A particularly useful fusion construct may be one in which a Nav1.3 splice variant of the present invention is fused to a hapten to enhance immunogenicity of a Nav1.3 protein fusion construct. Such fusion constructs to increase immunogenicity are well known to those of skill in the art, for example, a fusion of Nav1.3 protein with a helper antigen such as hsp70 or peptide sequences such as from Diptheria toxin chain or a cytokine such as IL-2 will be useful in eliciting an immune response. In other embodiments, fusion construct can be made which will enhance the targeting of the Nav1.3 protein-related compositions to a specific site or cell.
Other useful fusions include Nav1.3 protein fused to specific peptide or polypeptide domains that serve to increase cell surface expression of membrane proteins. Examples of such domains include one known to increase the cell surface expression of potassium channels by facilitating exit from the endoplasmic reticulum (see Zerangue et al Neuron 22, 537).
Other fusion constructs including a heterologous polypeptide with desired properties also are contemplated. Other fusion systems produce polypeptide hybrids where it is desirable to excise the fusion partner from the desired polypeptide. In one embodiment, the fusion partner is linked to the recombinant Nav1.3 protein polypeptide by a peptide sequence containing a specific recognition sequence for a protease. Examples of suitable sequences are those recognized by the Tobacco Etch Virus protease (Life Technologies, Gaithersburg, Md.) or Factor Xa (New England Biolabs, Beverley, Mass.).
It will be desirable to purify Nav1.3 protein or variants thereof. Protein purification techniques are well known to those of skill in the art. These techniques involve, at one level, the crude fractionation of the cellular milieu to polypeptide and non-polypeptide fractions. Having separated the polypeptide from other proteins, the polypeptide of interest may be further purified using chromatographic and electrophoretic techniques to achieve partial or complete purification (or purification to homogeneity). Analytical methods particularly suited to the preparation of a pure peptide are ion-exchange chromatography, exclusion chromatography; polyacrylamide gel electrophoresis; isoelectric focusing; affinity columns specific for protein fusion moieties; affinity columns containing Nav1.3-specific antibodies. A particularly efficient method of purifying peptides is fast protein liquid chromatography or even HPLC.
Certain aspects of the present invention concern the purification, and in particular embodiments, the substantial purification, of an encoded protein or peptide. The term “purified protein or peptide” as used herein, is intended to refer to a composition, isolatable from other components, wherein the protein or peptide is purified to any degree relative to its naturally-obtainable state. A purified protein or peptide therefore also refers to a protein or peptide, free from the environment in which it may naturally occur. As is common with purification of transmembrane proteins such as ion channels, it is possible that the purified protein may need to be incorporated in synthetic liposomes to retain biological activity.
Incorporation of proteins into liposomes is well known to those of skill in the art. The most frequently used strategy for reconstitution and crystallization of transmembrane proteins in lipid bilayers is comicellization of the proteins and lipids, both solubilized with detergent, which is removed after mixing the separate solutions (e.g., Jap et al., Ultramicroscopy. 46:45-84, 1992; Kühlbrandt, Q. Rev. Biophys. 25:1-49, 1992; Rigaud et al., Struct. Biol. 118:226-235, 1995; Mosser, Micron. 32:517-540, 2001). Additional chemical agents are usually added to the solution, including e.g., detergents such as octyl thioglucoside (Scheuring et al., EMBO J. 20:3029-3035, 2001), octyl glucoside, octyl glucopyranoside (Montoya et al., J. Mol. Biol. 250:1-10, 1995), organic solvents such as pentane, hexane, or other chemicals such as glucose (Walz et al., J. Mol. Biol. 282:833-845, 1998) or glycerol (Ikeda-Yamasaki et al., FEBS Lett. 425:505-508, 1998). The concentration of the detergent molecules in these solutions is then gradually reduced, either by dialysis or by the addition of Bio-Beads (Rigaud et al., J. Struct. Biol. 118:226-235, 1997). As the concentration of the detergent decreases from these lipid-detergent and lipid-protein-detergent micellar solutions, lipid bilayers are progressively formed in which the transmembrane proteins are incorporated. Usually, the morphology of the resulting 2D crystals depends on several poorly defined factors, and depending on the circumstances, various structures can be obtained such as planar sheets, proteo-liposomes, multilayered stacked sheets, thin three-dimensional crystals, and tubes (Lacapere et al., Biophys. J. 75:1319-1329, 1998; Mosser, Micron. 32:517-540, 2001). Specifically contemplated compositions of the present invention include the Nav1.3 splice variants of the present invention incorporated into a liposomal preparation using techniques such as those outlined above. The splice variant protein compositions for incorporation into the liposomes may be prepared and purified using any standard protein preparation techniques.
Generally, “purified” will refer to a protein or peptide composition that has been subjected to fractionation to remove various other components, and which composition substantially retains its expressed biological activity. Where the term “substantially purified” is used, this designation will refer to a composition in which the protein or peptide forms the major component of the composition, such as constituting about 50%, about 60%, about 70%, about 80%, about 90%, about 95% or more of the proteins in the composition.
Various methods for quantifying the degree of purification of the protein or peptide will be known to those of skill in the art in light of the present disclosure. These include, for example, determining the specific activity of an active fraction, or assessing the amount of polypeptides within a fraction by SDS/PAGE analysis. A preferred method for assessing the purity of a fraction is to calculate the specific activity of the fraction, to compare it to the specific activity of the initial extract, and to thus calculate the degree of purity, herein assessed by a “-fold purification number.” The actual units used to represent the amount of activity will, of course, be dependent upon the particular assay technique chosen to follow the purification and whether or not the expressed protein or peptide exhibits a detectable activity.
Various techniques suitable for use in protein purification will be well known to those of skill in the art. These include, for example, precipitation with ammonium sulphate, PEG, antibodies and the like or by heat denaturation, followed by centrifugation; chromatography steps such as ion exchange, gel filtration, reverse phase, hydroxylapatite and affinity chromatography; isoelectric focusing; gel electrophoresis; and combinations of such and other techniques. As is generally known in the art, it is believed that the order of conducting the various purification steps may be changed, or that certain steps may be omitted, and still result in a suitable method for the preparation of a substantially purified protein or peptide.
There is no general requirement that the protein or peptide always be provided in their most purified state. Indeed, it is contemplated that less substantially purified products will have utility in certain embodiments. Partial purification may be accomplished by using fewer purification steps in combination, or by utilizing different forms of the same general purification scheme. For example, it is appreciated that a cation-exchange column chromatography performed utilizing an HPLC apparatus will generally result in a greater “-fold” purification than the same technique utilizing a low pressure chromatography system. Methods exhibiting a lower degree of relative purification may have advantages in total recovery of protein product, or in maintaining the activity of an expressed protein.
It is known that the migration of a polypeptide can vary, sometimes significantly, with different conditions of SDS/PAGE. It will therefore be appreciated that wider differing electrophoresis conditions, the apparent molecular weights of purified or partially purified expression products may vary.
In addition to the full length Nav1.3 protein of SEQ ID NO:2 described herein, smaller Nav1.3 protein-related peptides derived from the sequence of SEQ ID NO:2 and containing at least the aspartic acid residue at the relative position number 208 of SEQ ID NO:2 (i.e., in SEQ ID NO:2 the aspartic acid residue in question is at position 208, in other proteins that residue may be at another position along the amino acid sequence but it is in a position that corresponds to or is derived from position 208 of SEQ DI NO:2) may be useful in various embodiments of the present invention. Such peptides or indeed even the full length protein, of the invention can also be synthesized in solution or on a solid support in accordance with conventional techniques. Various automatic synthesizers are commercially available and can be used in accordance with known protocols. See, for example, Stewart and Young, Solid Phase Peptide Synthesis, 2d. ed., Pierce Chemical Co., (1984); Tam et al., J. Am. Chem. Soc., 105:6442, (1983); Merrifield, Science, 232: 341-347, (1986); and Barany and Merrifield, The Peptides, Gross and Meienhofer, eds, Academic Press, New York, 1-284, (1979), each incorporated herein by reference. The Nav1.3 active protein or portions of the protein, which correspond to the selected regions described herein, can be readily synthesized and then screened in screening assays designed to identify reactive peptides.
Alternatively, recombinant DNA technology may be employed wherein a nucleotide sequence which encodes a peptide of the invention is inserted into an expression vector, transformed or transfected into an appropriate host cell and cultivated under conditions suitable for expression as described herein below.
U.S. Pat. No. 4,554,101 (incorporated herein by reference) also teaches the identification and preparation of epitopes from primary amino acid sequences on the basis of hydrophilicity. Thus, one of skill in the art would be able to identify epitopes from within any amino acid sequence encoded by any of the DNA sequences disclosed herein.
The protein of SEQ ID NO:2 or proteins and peptides derived thereform, may be useful as antigens for the immunization of animals relating to the production of antibodies. It is envisioned that such protein, or portions thereof, may be coupled, bonded, bound, conjugated or chemically-linked to one or more agents via linkers, polylinkers or derivatized amino acids. This may be performed such that a bispecific or multivalent composition or vaccine is produced. It is further envisioned that the methods used in the preparation of these compositions will be familiar to those of skill in the art and should be suitable for administration to animals, i.e., pharmaceutically acceptable. Preferred agents are the carriers are keyhole limpet hemocyannin (KLH) or bovine serum albumin (BSA).
NAV1.3 Splice Variant Encoding Nucleic Acids
The present invention also provides, in another embodiment, an isolated nucleic acid encoding the Nav1.3 splice variant protein of the invention. Preferred embodiments of the present invention are directed to nucleic acid constructs comprising a sequence of SEQ ID NO:1. Preferably, the sequence is operably linked to a heterologous promoter. The present invention is not limited in scope to the particular gene(s) identified herein, however, seeing as one of ordinary skill in the art could, using the nucleic acids corresponding to the Nav1.3 gene, readily identify related homologs in various other species (e.g., rat, rabbit, monkey, gibbon, chimp, ape, baboon, cow, pig, horse, sheep, cat and other species).
In addition, it should be clear that the present invention is not limited to the specific nucleic acids disclosed herein. As discussed below, a “Nav1.3 gene” may contain a variety of different nucleic acid bases and yet still produce a corresponding polypeptide that is functionally indistinguishable, and in some cases structurally, from the human gene disclosed herein. In preferred embodiments, the nucleic acids encode SEQ ID NO:2. In other embodiments the nucleic acids encode a function sodium channel alpha subunit based on the amino acid sequence of SEQ ID NO:2 which at least comprises an aspartic acid residue at the amino acid that corresponds to the amino acid at position 208 of SEQ ID NO:2, and has an insertion of a linker between Domain 1 and Domain 2. The term “Nav1.3 gene” may be used to refer to any nucleic acid that encodes such a protein, peptide or polypeptide and, as such, is intended to encompass both genomic DNA, mRNA and cDNA.
Similarly, any reference to a nucleic acid should be read as encompassing a host cell containing that nucleic acid and, in some cases, capable of expressing the product of that nucleic acid. Such cells expressing nucleic acids of the present invention are contemplated to be particularly useful in the context of screening for agents that induce, repress, inhibit, augment, interfere with, block, abrogate, stimulate or enhance the function of Nav1.3 gene or protein product. Such compounds identified in these screening assay embodiments also will be useful as sodium channel modulators of other sodium channels (e.g., Nav1.8, Nav1.6 and the like)
Nucleic acids according to the present invention (which include genomic DNA, cDNA, mRNA, as well as recombinant and synthetic sequences and partially synthetic sequences) may encode an entire Nav1.3 protein of SEQ ID NO:2, or polypeptide, or allelic variant, a domain of the protein that expresses an activity of the wild-type sodium channel and has an aspartic acid residue at a position that corresponds to residue 208 of SEQ ID NO:2, or any other fragment or variant of the Nav1.3 protein sequences set forth herein, as long as those variant comprise a linker between Domain 1 and Domain 2 as described herein.
The nucleic acid may be derived from genomic DNA, i.e., cloned directly from the genome of a particular organism. In preferred embodiments, however, the nucleic acid would comprise complementary DNA (cDNA). Also contemplated is a cDNA plus a natural intron or an intron derived from another gene; such engineered molecules are sometime referred to as “mini-genes.” At a minimum, these and other nucleic acids of the present invention may be used as molecular weight standards in, for example, gel electrophoresis.
The term “cDNA” is intended to refer to DNA prepared using messenger RNA (mRNA) as template. The advantage of using a cDNA, as opposed to genomic DNA or DNA polymerized from a genomic, non- or partially-processed RNA template, is that the cDNA primarily contains coding sequences of the corresponding protein. There may be times when the full or partial genomic sequence is preferred, such as where the non-coding regions are required for optimal expression or where non-coding regions such as introns are to be targeted in an antisense strategy.
It also is contemplated that due to the redundancy of the genetic code, a given Nav1.3 gene from a given species may be represented by degenerate variants that have slightly different nucleic acid sequences but, nonetheless, encode the same protein.
As used in this application, the term “a nucleic acid encoding a Nav1.3 protein” refers to a nucleic acid molecule that has been isolated from total cellular nucleic acid. In preferred embodiments, the invention concerns a nucleic acid sequence essentially as net forth in SEQ ID NO:1. The term “functionally equivalent codon” is used herein to refer to codons that encode the same amino acid, such as the six codons for arginine or serine (Table 1, below), and also refers to codons that encode biologically equivalent amino acids, as discussed in the following pages.










TABLE 1





Amino Acids


Codons







Alanine
Ala
A
GCA GCC GCG GCU


 


Cysteine
Cys
C
UGC UGU


 


Aspartic acid
Asp
D
GAC GAU


 


Glutamic acid
Glu
E
GAA GAG


 


Phenylalanine
Phe
F
UUC UUU


 


Glycine
Gly
G
GGA GGC GGG GGU


 


Histidine
His
H
CAC CAU


 


Isoleucine
Ile
I
AUA AUC AUU


 


Lysine
Lys
K
AAA AAG


 


Leucine
Leu
L
UUA UUG CUA CUC CUG CUU


 


Methionine
Met
M
AUG


 


Asparagine
Asn
N
AAC AAU


 


Proline
Pro
P
CCA CCC CCG CCU


 


Glutamine
Gln
Q
CAA CAG


 


Arginine
Arg
R
AGA AGG CGA CGC CGG CGU


 


Serine
Ser
S
AGC AGU UCA UCC UCG UCU


 


Threonine
Thr
T
ACA ACC ACG ACU


 


Valine
Val
V
GUA GUC GUG GUU


 


Tryptophan
Trp
W
UGG


 


Tyrosine
Tyr
Y
UAC UAU









Nucleotide sequences that have at least about 95% of nucleotides that are identical to the nucleotides of the entire sequence of SEQ ID NO:1 are preferred. Sequences that are essentially the same as those set forth in SEQ ID NO:1 may also be functionally defined as sequences that are capable of hybridizing to a nucleic acid segment containing the complement of SEQ ID NO:1 under standard conditions.
The DNA segments of the present invention include those encoding biologically functional equivalent Nav1.3 proteins and peptides as described above. Such sequences may arise as a consequence of codon redundancy and amino acid functional equivalency that are known to occur naturally within nucleic acid sequences and the proteins thus encoded. Alternatively, functionally equivalent proteins or peptides may be created via the application of recombinant DNA technology, in which changes in the protein structure may be engineered, based on considerations of the properties of the amino acids being exchanged. Changes designed by man may be introduced through any means described herein or known to those of skill in the art.
The present invention also encompasses DNA segments that are complementary, or essentially complementary, to the sequence set forth in SEQ ID NO:1. Nucleic acid sequences that are “complementary” are those that are capable of base-pairing according to the standard Watson-Crick complementary rules. As used herein, the term “complementary sequences” means nucleic acid sequences that are substantially complementary, as may be assessed by the same nucleotide comparison set forth above, or as defined as being capable of hybridizing to the nucleic acid segment of SEQ ID NO:1 under highly stringent conditions. Such sequences may encode the entire Nav1.3 protein of SEQ ID. NO:2 or functional or non-functional fragments thereof.
Alternatively, the hybridizing segments may be shorter oligonucleotides. Sequences of about 17 bases long should occur only once in the human genome and, therefore, suffice to specify a unique target sequence. Antisense nucleic acids directed against the sequence of SEQ ID NO:1 are particularly useful.
Suitable hybridization conditions will be well known to those of skill in the art. In certain applications, it is appreciated that lower stringency conditions may be required. Under these conditions, hybridization may occur even though the sequences of probe and target strand are not perfectly complementary, but are mismatched at one or more positions. Conditions may be rendered less stringent by increasing salt concentration and decreasing temperature. For example, a medium stringency condition could be provided by about 0.1 to 0.25 M NaCl at temperatures of about 37° C. to about 55° C., while a low stringency condition could be provided by about 0.15 M to about 0.9 M salt, at temperatures ranging from about 20° C. to about 55° C. Thus, hybridization conditions can be readily manipulated, and thus will generally be a method of choice depending on the desired results.
In other embodiments, hybridization may be achieved under conditions of, for example, 50 mM Tris-HCl (pH 8.3), 75 mM KCl, 3 mM MgCl2, 10 mM dithiothreitol, at temperatures between approximately 20° C. to about 37° C. Other hybridization conditions utilized could include approximately 10 mM Tris-HCl (pH 8.3), 50 mM KCl, 1.5 mM MgCl2, at temperatures ranging from approximately 40° C. to about 72° C. Formamide and SDS also may be used to alter the hybridization conditions.
Another way of exploiting probes and primers of the present invention is in site-directed, or site-specific mutagenesis. Site-specific mutagenesis is a technique useful in the preparation of individual peptides, or biologically functional equivalent proteins or peptides, through specific mutagenesis of the underlying DNA. The technique further provides a ready ability to prepare and test sequence variants, incorporating one or more of the foregoing considerations, by introducing one or more nucleotide sequence changes into the DNA. Site-specific mutagenesis allows the production of mutants through the use of specific oligonucleotide sequences which encode the DNA sequence of the desired mutation, as well as a sufficient number of adjacent nucleotides, to provide a primer sequence of sufficient size and sequence complexity to form a stable duplex on both sides of the deletion junction being traversed. Typically, a primer of about 17 to 25 nucleotides in length is preferred, with about 5 to 10 residues on both sides of the junction of the sequence being altered.
The technique typically employs a bacteriophage vector that exists in both a single stranded and double stranded form. Typical vectors useful in site-directed mutagenesis include vectors such as the M13 phage. These phage vectors are commercially available and their use is generally well known to those skilled in the art. Double stranded plasmids also are routinely employed in site directed mutagenesis, which eliminates the step of transferring the gene of interest from a phage to a plasmid.
In general, site-directed mutagenesis is performed by first obtaining a single-stranded vector, or melting of two strands of a double stranded vector which includes within its sequence a DNA sequence encoding the desired protein. An oligonucleotide primer bearing the desired mutated sequence is synthetically prepared. This primer is then annealed with the single-stranded DNA preparation, taking into account the degree of mismatch when selecting hybridization conditions, and subjected to DNA polymerizing enzymes such as E. coli polymerase I Klenow fragment, in order to complete the synthesis of the mutation-bearing strand. Thus, a heteroduplex is formed wherein one strand encodes the original non-mutated sequence and the second strand bears the desired mutation. This heteroduplex vector is then used to transform appropriate cells, such as E. coli cells, and clones are selected that include recombinant vectors bearing the mutated sequence arrangement. Site-directed mutagenesis can also be accomplished using PCR to introduce the desired alteration in the coding sequence. In this case, one of the amplification primers contains the alteration(s) of choice and results in a DNA fragment containing the desired mutation(s) that can then be incorporated into the full-length construct.
Of course site-directed mutagenesis is not the only method of generating potentially useful mutant protein species and as such is not meant to be limiting. The present invention also contemplates other methods of achieving mutagenesis such as for example, treating the recombinant vectors carrying the gene of interest mutagenic agents, such as hydroxylamine, to obtain sequence variants.
It will be useful to inhibit the expression of Nav1.3 to decrease the activity of the encoded protein and effect and ameliorative outcome on pain. One may advantageously disrupt the activity or expression of a protein using a variety of methods known to those of skill in the art. For example, nucleic acid-based methods of disrupting or block Nav1.3 expression are contemplated. Polynucleotide products which are useful in this endeavor include antisense polynucleotides, ribozymes, RNAi, and triple helix polynucleotides that modulate the expression of Nav1.3.
Antisense polynucleotides and ribozymes are well known to those of skill in the art. Crooke and B. Lebleu, eds. Antisense Research and Applications (1993) CRC Press; and Antisense RNA and DNA (1988) D. A. Melton, Ed. Cold Spring Harbor Laboratory Cold Spring Harbor, N.Y. Anti-sense RNA and DNA molecules act to directly block the translation of mRNA by binding to targeted mRNA and preventing protein translation. An example of an antisense polynucleotide is an oligodeoxyribonucleotide derived from the translation initiation site, e.g., between −10 and +10 regions of the relevant nucleotide sequence.
Antisense methodology takes advantage of the fact that nucleic acids tend to pair with “complementary” sequences. By complementary, it is meant that polynucleotides are those which are capable of base-pairing according to the standard Watson-Crick complementarity rules. That is, the larger purines will base pair with the smaller pyrimidines to form combinations of guanine paired with cytosine (G:C) and adenine paired with either thymine (A:T) in the case of DNA, or adenine paired with uracil (A:U) in the case of RNA. Inclusion of less common bases such as inosine, 5-methylcytosine, 6-methyladenine, hypoxanthine and others in hybridizing sequences does not interfere with pairing.
Targeting double-stranded (ds) DNA with polynucleotides leads to triple-helix formation; targeting RNA will lead to double-helix formation. Antisense polynucleotides, when introduced into a target cell, specifically bind to their target polynucleotide and interfere with transcription, RNA processing, transport, translation and/or stability. Antisense RNA constructs, or DNA encoding such antisense RNA's, may be employed to inhibit gene transcription or translation or both within a host cell, either in vitro or in vivo, such as within a host animal, including a human subject.
Antisense constructs may be designed to bind to the promoter and other control regions, exons, introns or even exon-intron boundaries of a gene. It is contemplated that the most effective antisense constructs will include regions complementary to intron/exon splice junctions. Thus, it is proposed that a preferred embodiment includes an antisense construct with complementarily to regions within 50-200 bases of an intron-exon splice junction. It has been observed that some exon sequences can be included in the construct without seriously affecting the target selectivity thereof. The amount of exonic material included will vary depending on the particular exon and intron sequences used. One can readily test whether too much exon DNA is included simply by testing the constructs in vitro to determine whether normal cellular function is affected or whether the expression of related genes having complementary sequences is affected.
As stated above, “complementary” or “antisense” means polynucleotide sequences that are substantially complementary over their entire length and have very few base mismatches. For example, sequences of fifteen bases in length may be termed complementary when they have complementary nucleotides at thirteen or fourteen positions. Naturally, sequences which are completely complementary will be sequences which are entirely complementary throughout their entire length and have no base mismatches. Other sequences with lower degrees of homology also are contemplated. For example, an antisense construct which has limited regions of high homology, but also contains a non-homologous region (e.g., ribozymes) could be designed. These molecules, though having less than 50% homology, would bind to target sequences under appropriate conditions.
It may be advantageous to combine portions of genomic DNA with cDNA or synthetic sequences to generate specific constructs. For example, where an intron is desired in the ultimate construct, a genomic clone will need to be used. The cDNA or a synthesized polynucleotide may provide more convenient restriction sites for the remaining portion of the construct and, therefore, would be used for the rest of the sequence.
As indicated above, the DNA and protein sequences for the specific splice variant of the present invention are provided in SEQ ID NO:1 and SEQ ID NO:2, respectively. Related protein and/or nucleic acid sequences from other sources may be identified using probes directed at the sequences of SEQ ID NO:1. Such additional sequences may be useful in certain aspects of the present invention. Although antisense sequences may be full length genomic or cDNA copies, they also may be shorter fragments or oligonucleotides e.g., polynucleotides of 100 or less bases. Although shorter oligomers (8-20) are easier to make and more easily permeable in vivo, other factors also are involved in determining the specificity of base pairing. For example, the binding affinity and sequence specificity of an oligonucleotide to its complementary target increases with increasing length. It is contemplated that oligonucleotides of 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, 40, 45, 50, or more base pairs will be used.
Ribozymes are enzymatic RNA molecules capable of catalyzing the specific cleavage of RNA. The mechanism of ribozyme action involves sequence specific interaction of the ribozyme molecule to complementary target RNA, followed by an endonucleolytic cleavage. Within the scope of the invention are engineered hammerhead or other motif ribozyme molecules that specifically and efficiently catalyze endonucleolytic cleavage of RNA sequences encoding protein complex components.
Specific ribozyme cleavage sites within any potential RNA target are initially identified by scanning the target molecule for ribozyme cleavage sites which include the following sequences, GUA, GUU and GUC. Once identified, short RNA sequences of between 15 and 20 ribonucleotides corresponding to the region of the target gene containing the cleavage site may be evaluated for predicted structural features, such as secondary structure, that may render the oligonucleotide sequence unsuitable. The suitability of candidate targets may also be evaluated by testing their accessibility to hybridization with complementary oligonucleotides, using ribonuclease protection assays. See, Draper PCT WO 93/23569; and U.S. Pat. No. 5,093,246.
Nucleic acid molecules used in triple helix formation for the inhibition of transcription are generally single stranded and composed of deoxyribonucleotides. The base composition must be designed to promote triple helix formation via Hoogsteen base pairing rules, which generally require sizeable stretches of either purines or pyrimidines to be present on one strand of a duplex. Nucleotide sequences may be pyrimidine-based, which will result in TAT and CGC+ triplets across the three associated strands of the resulting triple helix. The pyrimidine-rich molecules provide base complementarity to a purine-rich region of a single strand of the duplex in a parallel orientation to that strand. In addition, nucleic acid molecules may be chosen that are purine-rich, for example, containing a stretch of G residues. These molecules will form a triple helix with a DNA duplex that is rich in GC pairs, in which the majority of the purine residues are located on a single strand of the targeted duplex, resulting in GGC triplets across the three strands in the triplex.
Alternatively, the potential sequences that can be targeted for triple helix formation may be increased by creating a so called “switchback” nucleic acid molecule. Switchback molecules are synthesized in an alternating 5′-3′, 3′-5′ manner, such that they base pair with first one strand of a duplex and then the other, eliminating the necessity for a sizeable stretch of either purines or pyrimidines to be present on one strand of a duplex.
Another technique that is of note for reducing or disrupting the expression of a gene is RNA interference (RNAi), also known as small interfering RNA (siRNA). The term “RNA interference” was first used by researchers studying C. elegans and describes a technique by which post-transcriptional gene silencing (PTGS) is induced by the direct introduction of double stranded RNA (dsRNA: a mixture of both sense and antisense strands). Injection of dsRNA into C. elegans resulted in much more efficient silencing than injection of either the sense or the antisense strands alone (Fire et al., Nature 391:806-811, 1998). Just a few molecules of dsRNA per cell is sufficient to completely silence the expression of the homologous gene. Furthermore, injection of dsRNA caused gene silencing in the first generation offspring of the C. elegans indicating that the gene silencing is inheritable (Fire et al., Nature 391:806-811, 1998). Current models of PTGS indicate that short stretches of interfering dsRNAs (21-23 nucleotides; siRNA also known as “guide RNAs”) mediate PTGS. siRNAs are apparently produced by cleavage of dsRNA introduced directly or via a transgene or virus. These siRNAs may be amplified by an RNA-dependent RNA polymerase (RdRP) and are incorporated into the RNA-induced silencing complex (RISC), guiding the complex to the homologous endogenous mRNA, where the complex cleaves the transcript. Thus, siRNAs are nucleotides of a short length (typically 18-25 bases, preferably 19-23 bases in length) which incorporate into an RNA-induced silencing complex in order to guide the complex to homologous endogenous mRNA for cleavage and degradation of the transcript.
While most of the initial studies were performed in C. elegans, RNAi is gaining increasing recognition as a technique that may be used in mammalian cell. It is contemplated that RNAi, or gene silencing, will be particularly useful in the disruption of tissue-specific gene expression. By placing a gene fragment encoding the desired dsRNA behind an inducible or tissue-specific promoter, it should be possible to inactivate genes at a particular location within an organism or during a particular stage of development.
Variations on RNA interference (RNAi) technology is revolutionizing many approaches to experimental biology, complementing traditional genetic technologies, mimicking the effects of mutations in both cell cultures and in living animals. (McManus & Sharp, Nat. Rev. Genet. 3, 737-747 (2002)). RNAi has been used to elicit gene-specific silencing in cultured mammalian cells using 21-nucleotide siRNA duplexes (Elbashir et al., Nature, 411:494-498, 2001; Fire et al., Nature 391, 199-213 (1998), Harmon, G. J., Nature 418, 244-251 (2002))). In the same cultured cell systems, transfection of longer stretches of dsRNA yielded considerable nonspecific silencing. Thus, RNAi has been demonstrated to be a feasible technique for use in mammalian cells and could be used for assessing gene function in cultured cells and mammalian systems, as well as for development of gene-specific therapeutics. In particularly preferred embodiments, the siRNA molecule is between 20 and 25 oligonucleotides in length an is derived from the sequence of SEQ ID NO:1. Particularly preferred siRNA molecules are 21-23 bases in length.
Anti-sense RNA and DNA molecules, ribozymes, RNAi and triple helix molecules can be prepared by any method known in the art for the synthesis of DNA and RNA molecules. These include techniques for chemically synthesizing oligodeoxyribonucleotides well known in the art including, but not limited to, solid phase phosphoramidite chemical synthesis. Alternatively, RNA molecules may be generated by in vitro and in vivo transcription of DNA sequences encoding the antisense RNA molecule. Such DNA sequences may be incorporated into a wide variety of vectors which incorporate suitable RNA polymerase promoters such as the T7 or SP6 polymerase promoters. Alternatively, antisense cDNA constructs that synthesize antisense RNA constitutively or inducibly, depending on the promoter used, can be introduced stably or transiently into cells.
Commercial providers such as Ambion Inc. (Austin, Tex.), Darmacon Inc. (Lafayette, Colo.), InvivoGen (San Diego, Calif.), and Molecula Research Laboratories, LLC (Herndon, Va.) generate custom siRNA molecules. In addition, commercial kits are available to produce custom siRNA molecules, such as SILENCERTM siRNA Construction Kit (Ambion Inc., Austin, Tex.) or psiRNA System (InvivoGen, San Diego, Calif.). These siRNA molecules may be introduced into cells through transient transfection or by introduction of expression vectors that continually express the siRNA in transient or stably transfected mammalian cells. Transfection may be accomplished by well known methods including methods such as infection, calcium chloride, electroporation, microinjection, lipofection or the DEAE-dextran method or other known techniques. These techniques are well known to those of skill in the art.
Recombinant Protein Production.
Given the above disclosure of SEQ ID NO:1, it is possible to produce a protein of SEQ ID NO:2 by recombinant techniques. A variety of expression vector/host systems may be utilized to contain and express such a protein coding sequence. These include but are not limited to microorganisms such as bacteria transformed with recombinant bacteriophage, plasmid or cosmid DNA expression vectors; yeast transformed with yeast expression vectors; insect cell systems infected with virus expression vectors (e.g., baculovirus); plant cell systems transfected with virus expression vectors (e.g., cauliflower mosaic virus, CaMV; tobacco mosaic virus, TMV) or transformed with bacterial expression vectors (e.g., Ti or pBR322 plasmid); or animal cell systems. Mammalian cells that are useful in recombinant protein production include but are not limited to VERO cells, HeLa cells, Chinese hamster ovary (CHO) cell lines, COS cells (such as COS-7), W138, BHK, HepG2, 3T3, RIN, MDCK, A549, PC12, K562 and 293 cells. Exemplary protocols for the recombinant expression of a protein of amino acid sequence of SEQ ID NO:2 in bacteria, yeast and other invertebrates are described herein below.
The DNA sequence encoding the mature form of the protein is amplified by PCR and cloned into an appropriate vector for example, pGEX 3X (Pharmacia, Piscataway, N.J.). The pGEX vector is designed to produce a fusion protein comprising glutathione S transferase (GST), encoded by the vector, and a protein encoded by a DNA fragment inserted into the vectors cloning site. The primers for the PCR may be generated to include for example, an appropriate cleavage site.
Treatment of the recombinant fusion protein with thrombin or factor Xa (Pharmacia, Piscataway, N.J.) is expected to cleave the fusion protein, releasing the proapoptotic factor from the GST portion. The pGEX 3X/Nav1.3 protein construct is transformed into E. coli XL 1 Blue cells (Stratagene, La Jolla Calif.), and individual transformants were isolated and grown. Plasmid DNA from individual transformants is purified and partially sequenced using an automated sequencer to confirm the presence of the desired protein-encoding gene insert in the proper orientation.
Knowledge of SEQ ID NO:1 gene sequences allows for modification of cells to permit or increase expression of endogenous Nav1.3 splice variant of the present invention. The cells can be modified (heterologous promoter is inserted in such a manner that it is operably linked to, e.g., by homologous recombination) to provide increased protein expression by replacing, in whole or in part the naturally occurring promoter with all or part of a heterologous promoter so that the cells express such a protein at higher levels. The heterologous promoter is inserted in such a manner that it is operably linked to gene sequence of SEQ ID NO:1. (e.g., PCT International Publication No. WO96/12650; PCT International Publication No. WO 92/20808 and PCT International Publication No. WO 91/09955). It is contemplated that, in addition to the heterologous promoter DNA, amplifiable marker DNA (e.g., ada, dhfr and the multifunctional CAD gene which encodes carbamyl phosphate synthase, aspartate transcarbamylase and dihydroorotase) and/or intron DNA may be inserted along with the heterologous promoter DNA. If linked to the gene sequence, amplification of the marker DNA by standard selection methods results in co-amplification of the Nav1.3 splice variant with the marker sequence in the cells.
While certain embodiments of the present invention contemplate producing the Nav1.3 splice variant protein using synthetic peptide synthesizers and subsequent FPLC analysis and appropriate refolding of the cysteine double bonds, it is contemplated that recombinant protein production also may be used. For example, induction of the fusion protein containing the protein of interest fused to GST is achieved by growing the transformed XL 1 Blue culture at 37° C. in LB medium (supplemented with carbenicillin) to an optical density at wavelength 600 nm of 0.4, followed by further incubation for 4 hours in the presence of 0.5 mM Isopropyl β-D Thiogalactopyranoside (Sigma Chemical Co., St. Louis Mo.).
The fusion protein, expected to be produced as an insoluble inclusion body in the bacteria, may be purified using standard techniques in which cells are harvested, washed, lysed and the protein extracted and purified e.g., using the GST Purification Module (Pharmacia Biotech). The GST may be cleaved using thrombin digestion.
The recombinant protein also may be prepared using a yeast system e.g., the Pichia Expression System (Invitrogen, San Diego, Calif.), following the manufacturer's instructions. Alternatively, the cDNA encoding the given Nav1.3 splice variant protein may be cloned into the baculovirus expression vector pVL1393 (PharMingen, San Diego, Calif.). This vector is then used according to the manufacturer's directions (PharMingen) to infect Spodoptera frugiperda cells in sF9 protein free media and to produce recombinant protein. The protein is purified and concentrated from the media using a heparin Sepharose column (Pharmacia, Piscataway, N.J.) and sequential molecular sizing columns (Amicon, Beverly, Mass.), and resuspended in PBS. SDS PAGE analysis shows a single band and confirms the size of the protein, and Edman sequencing on a Porton 2090 Peptide Sequencer confirms its N terminal sequence. In still other alternatives, the an insect system expression system may be used.
Mammalian host systems for the expression of the recombinant protein also are well known to those of skill in the art and are most preferred. Host cell strains may be chosen for a particular ability to process the expressed protein or produce certain post translation modifications that will be useful in providing protein activity. Such modifications of the polypeptide include, but are not limited to, acetylation, carboxylation, glycosylation, phosphorylation, lipidation and acylation. Post-translational processing which cleaves a “prepro” form of the protein may also be important for correct insertion, folding and/or function. Different host cells such as CHO, HeLa, MDCK, 293, WI38, and the like have specific cellular machinery and characteristic mechanisms for such post-translational activities and may be chosen to ensure the correct modification and processing of the introduced, foreign protein.
It is preferable that the transformed cells are used for long-term, high-yield protein production and as such stable expression is desirable. Once such cells are transformed with vectors that contain selectable markers along with the desired expression cassette, the cells may be allowed to grow for 1-2 days in an enriched media before they are switched to selective media. The selectable marker is designed to confer resistance to selection and its presence allows growth and recovery of cells which successfully express the introduced sequences. Resistant clumps of stably transformed cells can be proliferated using tissue culture techniques appropriate to the cell.
A number of selection systems may be used to recover the cells that have been transformed for recombinant protein production. Such selection systems include, but are not limited to, HSV thymidine kinase, hypoxanthine-guanine phosphoribosyltransferase and adenine phosphoritiosyltransferase genes, in tk-, hgprt- or aprt-cells, respectively. Also, anti-metabolite resistance can be used as the basis of selection for dhfr, that confers resistance to methotrexate; gpt, that confers resistance to mycophenolic acid; neo, that confers resistance to the aminoglycoside G418; als which confers resistance to chlorsulfuron; and hygro, that confers resistance to hygromycin. Additional selectable genes that may be useful include trpB, which allows cells to utilize indole in place of tryptophan, or hisD, which allows cells to utilize histinol in place of histidine. Markers that give a visual indication for identification of transformants include anthocyanins, glucuronidase and its substrate, GUS, and luciferase and its substrate, luciferin.
Vectors for Cloning, Gene Transfer and Expression
As discussed in the previous section, expression vectors are employed to express the protein of interest, which can then be purified and, for example, be used to vaccinate animals to generate antisera or monoclonal antibody with which further studies may be conducted.
Expression requires that appropriate signals be provided in the vectors, and which include various regulatory elements, such as enhancers/promoters from both viral and mammalian sources that drive expression of the genes of interest in host cells. Elements designed to optimize messenger RNA stability and translatability in host cells also are defined. The conditions for the use of a number of dominant drug selection markers for establishing permanent, stable cell clones expressing the products also are provided, as is an element that links expression of the drug selection markers to expression of the polypeptide.
Throughout this application, the term “expression construct” or “expression vector” is meant to include any type of genetic construct containing a nucleic acid coding for gene products in which part or all of the nucleic acid encoding sequence is capable of being transcribed. The transcript may be translated into a protein, but it need not be. In certain embodiments, expression includes both transcription of a gene and translation of mRNA into a gene product.
The nucleic acid encoding a gene product is under transcriptional control of a promoter. A “promoter” refers to a DNA sequence recognized by the synthetic machinery of the cell, or introduced synthetic machinery, required to initiate the specific transcription of a gene. The phrase “under transcriptional control” means that the promoter is in the correct location and orientation in relation to the nucleic acid to control RNA polymerase initiation and expression of the gene.
The particular promoter employed to control the expression of a nucleic acid sequence of interest is not believed to be important, so long as it is capable of directing the expression of the nucleic acid in the targeted cell. Thus, where a human cell is targeted, it is preferable to position the nucleic acid coding region adjacent to and under the control of a promoter that is capable of being expressed in a human cell. Generally speaking, such a promoter might include either a human or viral promoter. Exemplary promoters include the human cytomegalovirus (CMV) immediate early gene promoter, the SV40 early promoter, the Rous sarcoma virus long terminal repeat, β-actin, rat insulin promoter, the phosphoglycerol kinase promoter and glyceraldehyde-3-phosphate dehydrogenase promoter, all of which are promoters well known and readily available to those of skill in the art, can be used to obtain high-level expression of the coding sequence of interest. The use of other viral or mammalian cellular or bacterial phage promoters which are well-known in the art to achieve expression of a coding sequence of interest is contemplated as well, provided that the levels of expression are sufficient for a given purpose. By employing a promoter with well known properties, the level and pattern of expression of the protein of interest following transfection or transformation can be optimized. Inducible promoters e.g., inducible ecdysone system (Invitrogen, Carlsbad, Calif.), or the Tet-OffTM or Tet-OnTM system which are designed to allow regulated expression of a gene of interest in mammalian cells also may be used.
Modified versions of the CMV promoter that are less potent have also been used when reduced levels of expression of the transgene are desired. When expression of a transgene in hematopoietic cells is desired, retroviral promoters such as the LTRs from MLV or MMTV are often used. Other viral promoters that may be used depending on the desired effect include SV40, RSV LTR, HIV-1 and HIV-2 LTR, adenovirus promoters such as from the E1A, E2A, or MLP region, AAV LTR, cauliflower mosaic virus, HSV-TK, and avian sarcoma virus.
Tissue specific promoters may be used to effect transcription in specific tissues or cells so as to reduce potential toxicity or undesirable effects to non-targeted tissues.
It is envisioned that any of the above promoters alone or in combination with another may be useful according to the present invention depending on the action desired. In addition, this list of promoters should not be construed to be exhaustive or limiting, and those of skill in the art will know of other promoters that may be used in conjunction with the promoters and methods disclosed herein.
Another regulatory element contemplated for use in the present invention is an enhancer. These are genetic elements that increase transcription from a promoter located at a distant position on the same molecule of DNA. Enhancers are organized much like promoters. That is, they are composed of many individual elements, each of which binds to one or more transcriptional proteins. The basic distinction between enhancers and promoters is operational. An enhancer region as a whole must be able to stimulate transcription at a distance; this need not be true of a promoter region or its component elements. On the other hand, a promoter must have one or more elements that direct initiation of RNA synthesis at a particular site and in a particular orientation, whereas enhancers lack these specificities. Promoters and enhancers are often overlapping and contiguous, often seeming to have a very similar modular organization. Enhancers useful in the present invention are well known to those of skill in the art and will depend on the particular expression system being employed (Scharf D et al Results Probl Cell Differ 20: 125-62, 1994; Bittner et al Methods in Enzymol 153: 516-544, 1987).
Where a cDNA insert is employed, one will typically desire to include a polyadenylation signal to effect proper polyadenylation of the gene transcript. The nature of the polyadenylation signal is not believed to be crucial to the successful practice of the invention, and any such sequence may be employed such as human or bovine growth hormone and SV40 polyadenylation signals. Also contemplated as an element of the expression cassette is a terminator. These elements can serve to enhance message levels and to minimize read through from the cassette into other sequences.
In certain embodiments of the invention, the use of internal ribosome entry site (IRES) elements is contemplated to create multigene, or polycistronic, messages. In specific embodiments herein it is contemplated that host cells are created which comprise both a Nav1.3 alpha subunit and one or more beta subunits. IRES elements can be linked to heterologous open reading frames for such endeavors. IRES elements are able to bypass the ribosome scanning model of 5′ methylated Cap dependent translation and begin translation at internal sites (Pelletier and Sonenberg, Nature, 334:320-325, 1988). IRES elements from two members of the picornavirus family (poliovirus and encephalomyocarditis) have been described (Pelletier and Sonenberg, 1988 supra), as well an IRES from a mammalian message (Macejak and Sarnow, Nature, 353:90-94, 1991). Multiple open reading frames can be transcribed together, each separated by an IRES, creating polycistronic messages. By virtue of the IRES element, each open reading frame is accessible to ribosomes for efficient translation. Multiple genes can be efficiently expressed using a single promoter/enhancer to transcribe a single message.
There are a number of ways in which expression constructs may be introduced into cells. In certain embodiments of the invention, the expression construct comprises a virus or engineered construct derived from a viral genome. In other embodiments, non-viral delivery such as lipid- or chemical-mediated transfection is contemplated. The ability of certain viruses to enter cells via receptor-mediated endocytosis, to integrate into host cell genome and express viral genes stably and efficiently have made them attractive candidates for the transfer of foreign genes into mammalian cells (Ridgeway, In: Rodriguez R L, Denhardt D T, ed. Vectors: A survey of molecular cloning vectors and their uses. Stoneham Butterworth, 467 492, 1988; Nicolas and Rubenstein, In: Vectors: A survey of molecular cloning vectors and their uses, Rodriguez & Denhardt (eds.), Stoneham: Butterworth, 493 513, 1988; Baichwal and Sugden, In: Gene Transfer, Kucherlapati R, ed., New York, Plenum Press, 117 148, 1986; Temin, In: gene Transfer, Kucherlapati (ed.), New York: Plenum Press, 149 188, 1986). The first viruses used as gene vectors were DNA viruses including the papovaviruses (simian virus 40, bovine papilloma virus, and polyoma) (Ridgeway, 1988 supra; Baichwal and Sugden, 1986 supra) and adenoviruses (Ridgeway, 1988 supra; Baichwal and Sugden, 1986 supra). These have a relatively low capacity for foreign DNA sequences and have a restricted host spectrum. Furthermore, their oncogenic potential and cytopathic effects in permissive cells raise safety concerns. They can accommodate only up to 8 kb of foreign genetic material but can be readily introduced in a variety of cell lines and laboratory animals (Nicolas and Rubenstein, 1988 supra; Temin, 1986 supra).
It is now widely recognized that DNA may be introduced into a cell using a variety of viral vectors. In such embodiments, expression constructs comprising viral vectors containing the genes of interest may be adenoviral (see for example, U.S. Pat. No. 5,824,544; U.S. Pat. No. 5,707,618; U.S. Pat. No. 5,693,509; U.S. Pat. No. 5,670,488; U.S. Pat. No. 5,585,362; each incorporated herein by reference), retroviral (see for example, U.S. Pat. No. 5,888,502; U.S. Pat. No. 5,830,725; U.S. Pat. No. 5,770,414; U.S. Pat. No. 5,686,278; U.S. Pat. No. 4,861,719 each incorporated herein by reference), adeno-associated viral (see for example, U.S. Pat. No. 5,474,935; U.S. Pat. No. 5,139,941; U.S. Pat. No. 5,622,856; U.S. Pat. No. 5,658,776; U.S. Pat. No. 5,773,289; U.S. Pat. No. 5,789,390; U.S. Pat. No. 5,834,441; U.S. Pat. No. 5,863,541; U.S. Pat. No. 5,851,521; U.S. Pat. No. 5,252,479 each incorporated herein by reference), an adenoviral-adenoassociated viral hybrid (see for example, U.S. Pat. No. 5,856,152 incorporated herein by reference) or a vaccinia viral or a herpesviral (see for example, U.S. Pat. No. 5,879,934; U.S. Pat. No. 5,849,571; U.S. Pat. No. 5,830,727; U.S. Pat. No. 5,661,033; U.S. Pat. No. 5,328,688 each incorporated herein by reference) vector. In preferred embodiments, retroviral vectors are used to introduce the expression construct into HEK293 cells.
Screening for Modulators of Sodium Channel Protein
The present invention also contemplates the use of human Nav1.3 splice variant of the present invention and active fragments thereof in the screening of compounds that modulate (increase or decrease activity) of sodium channels. Such modulators and particularly sodium channel blockers will be useful as therapeutic agents. Assays for the identification of these agents may make use of splice variants of the invention in a variety of different formats and may depend on the kind of “activity” for which the screen is being conducted.
a. Assay Formats.
The present invention provides methods of screening for modulators of human Nav1.3 sodium channel activity in vitro and in vivo in the presence and absence of the candidate substance and comparing such results. It is contemplated that this screening technique will prove useful in the general identification of compounds of therapeutic value against e.g., pain, inflammation, and other diseases or disorders associated with sodium channel activity. In preferred embodiments, it will be desirable to identify inhibitors of sodium channel activity. However, in other embodiments, stimulators of such activity also may be desirable.
In the screening embodiments, the present invention is directed to a method for determining the ability of a candidate substance to alter the sodium channel activity of cells that either naturally express Nav1.3 splice variant protein or have been engineered to express such a protein as described herein. Alternatively, the present application teaches the use of models for determining the in vivo effects of such compounds. The cells or animals also may then be contacted with additional sodium channel blockers in combination with a putative modulator of sodium channel function in order to determine whether the effect of such sodium channel blockers is increased or decreased as a result of the presence of the candidate substance.
An alteration in sodium channel activity, expression or processing in the presence of the candidate substance will indicate that the candidate substance is a modulator of the activity.
While the above method generally describes a sodium channel splice variant protein activity, it should be understood that candidate substance may be an agent that alters the expression of sodium channel protein, thereby increasing or decreasing the amount of Nav1.3 protein present as opposed to the per unit activity of the protein.
To identify a candidate substance as being capable of inhibiting protein activity, one would measure or determine the protein activity in the absence of the added candidate substance. One would then add the candidate inhibitory substance to the cell and determine the activity of protein in the presence of the candidate inhibitory substance. A candidate substance which is inhibitory would decrease the sodium channel activity. Exemplary such assays are described below.
b. Candidate Substances.
As used herein the term “candidate substance” refers to any molecule that is capable of modulating sodium channel Nav1.3 splice variant protein activity or expression. The candidate substance may be a protein or fragment thereof, a small molecule inhibitor, or even a nucleic acid molecule. It may prove to be the case that the most useful pharmacological compounds for identification through application of the screening assay will be compounds that are structurally related to other known modulators of sodium channels. The active compounds may include fragments or parts of naturally-occurring compounds or may be only found as active combinations of known compounds which are otherwise inactive. However, prior to testing of such compounds in humans or animal models, it will be necessary to test a variety of candidates to determine which have potential.
Accordingly, the active compounds may include fragments or parts of naturally-occurring compounds or may be found as active combinations of known compounds which are otherwise inactive. Accordingly, the present invention provides screening assays to identify agents which inhibit or otherwise treat a disorder or associated with sodium channel activity. It is proposed that compounds isolated from natural sources, such as animals, bacteria, fungi, plant sources, including leaves and bark, and marine samples may be assayed as candidates for the presence of potentially useful pharmaceutical agents.
It will be understood that the pharmaceutical agents to be screened could also be derived or synthesized from chemical compositions or man-made compounds. Thus, it is understood that the candidate substance identified by the present invention may be polypeptide, polynucleotide, small molecule inhibitors or any other inorganic or organic chemical compounds that may be designed through rational drug design starting from known agents that are used in the intervention of pain, inflammation or other diseases/disorders associated with sodium channel activity.
The candidate screening assays are simple to set up and perform. Thus, in assaying for a candidate substance, after obtaining a cell expressing functional Nav1.3 splice variant protein of the invention, one will admix a candidate substance with the cell, under conditions which would allow measurable sodium channel activity to occur. Exemplary sodium channel assays are provided below. In this fashion, one can measure the ability of the candidate substance to stimulate the activity of the cell in the absence of the candidate substance. Likewise, in assays for inhibitors after obtaining a cell expressing functional Nav1.3 splice variant protein, the candidate substance is admixed with the cell. In this fashion the ability of the candidate inhibitory substance to reduce, abolish, or otherwise diminish a biological effect mediated by Nav1.3 splice variant protein from said cell may be detected.
“Effective amounts” in certain circumstances are those amounts effective to reproducibly alter sodium channel associated activity of the cell or animal in comparison to the normal levels of such an event. Compounds that achieve significant appropriate changes in such activity will be used.
Significant changes in a given sodium channel activity or in vivo function (discussed below) of at least about 30%-40%, and most preferably, by changes of at least about 50%, with higher values of course being possible. The active compounds of the present invention also may be used for the generation of antibodies which may then be used in analytical and preparatory techniques for detecting and quantifying further such inhibitors.
Proteins are often used in high throughput screening (HTS) assays known in the art, including melanophore assays to investigate receptor ligand interactions, yeast based assay systems and mammalian cell expression systems. For a review see Jayawickreme and Kost, Curr. Opin. Biotechnol. 8: 629 634 (1997). Automated and miniaturized HTS assays are also contemplated as described for example in Houston and Banks Curr. Opin. Biotechnol. 8: 734 740 (1997)
There are a number of different libraries used for the identification of small molecule modulators including chemical libraries, natural product libraries and combinatorial libraries comprised or random or designed peptides, oligonucleotides or organic molecules. Chemical libraries consist of structural analogs of known compounds or compounds that are identified as hits or leads via natural product screening or from screening against a potential therapeutic target. Natural product libraries are collections of products from microorganisms, animals, plants, insects or marine organisms which are used to create mixtures of screening by, e.g., fermentation and extractions of broths from soil, plant or marine organisms. Natural product libraries include polypeptides, non-ribosomal peptides and non-naturally occurring variants thereof. For a review see Science 282:63 68 (1998). Combinatorial libraries are composed of large numbers of peptides oligonucleotides or organic compounds as a mixture. They are relatively simple to prepare by traditional automated synthesis methods, PCR cloning or other synthetic methods. Of particular interest will be libraries that include peptide, protein, peptidomimetic, multiparallel synthetic collection, recombinatorial and polypeptide libraries. A review of combinatorial libraries and libraries created therefrom, see Myers Curr. Opin. Biotechnol. 8: 701 707 (1997). A candidate modulator identified by the use of various libraries described may then be optimized to modulate activity of Nav1.3 splice variant protein through, for example, rational drug design.
It will, of course, be understood that all the screening methods of the present invention are useful in themselves notwithstanding the fact that effective candidates may not be found. The invention provides methods for screening for such candidates, not solely methods of finding them.
c. In Vitro Assays.
Those of skill in the art are aware of numerous variations of in vitro methods for measuring sodium channel activity. Cells that express the given sodium channel being tested e.g., a neuronal cell line (from any eukaroyotic, preferably mammalian source) that has been transformed or transfected with a nucleic acid that encodes a protein of SEQ ID NO:2 or alternatively, a primary mammalian cell culture e.g., neurons that naturally express a protein of SEQ ID NO:2 are obtained. Primary cells from e.g., a rat source can be prepared as taught in Gallo et al., 1990 (J. Neurochem: 54, 1619-25 or Example 155 of U.S. Pat. No. 6,756,400). The cells are plated in an appropriate support e.g., in 96-well poly-D-lysine-coated black wall-clear bottom culture plates at a suitable concentration (e.g., 1-2×105 cells/well of a 96-well plate). The cells are maintained at 37° C. in an atmosphere containing 5% CO2.
To measure sodium channel activity, veratridine-evoked increases in intracellular Ca2+ ([Ca2+]i) in fluo-4/AM loaded cerebellar granule neurons may be monitored, in real-time, using a Fluorescent Imaging Plate Reader (FLIPRTM, Molecular Devices, Sunnyvale, Calif.). The cells are incubated with 4 mM fluo-4/AM in HBSS buffer containing 2.5 mM probenecid and 0.04% pluronic acid for 45 min at 37° C. The cells are then washed three times with HBSS containing 2.5 mM probenecid (FLIPRTM buffer). The plates are transferred to the FLIPRTM and the cells incubated for 5 min in FLIPRTM buffer, in the absence (control) or presence of the test compound, prior to addition of veratridine (40 μM). Cell fluorescence (λEx=488 nm; λEm=510 nm) is monitored both before and after the addition of veratridine. Peak fluorescence intensity, after veratridine addition, is determined using the FLIPRTM software. Curve fitting and parameter estimation (pIC50) were performed using GraphPad. Stock solutions (10 mM) of compounds were made in 100% DMSO.
As an alternative to FLIPRTM, the VIPRTM (Aurora Biosciences Corporation) assay may be used. In such an alternative assay, HEK293 cells expressing hNav1.3 channels are cultured on 96- or 384-well plates (Costar tissue culture treated 96-well flat bottom plates, Corning). To prevent detachment of cells during plate washing, these plates are pre-coated with 0.5% Growth Factor Reduced matrigel matrix in DMEM for 1 hour at room temperature before use for cell culture. About 40,000 cells are seeded to each well and incubated at 38° C. for 24 hours before assay. Assay is performed at room temperature. The cell plates are first washed three times with bath solution using automatic plate washer (ELx405, Biotek), leaving a residual volume of 50 μL/well. Subsequently, cells are incubated with mixed dye solution for 30 min in the dark at room temperature. The mixed dye solution is prepared with External solution and consists of 10 μM CC2-DMPE (chlorocoumarin-2-dimyristoyl phosphatidylethanolamine), 2.4 μM DISBAC6(3) (bis-(1,3-dihexyl-thiobarbituric acid) trimethine oxonol), 0.5% β-cyclodextrin, 20 μg/ml pluronic F-127 and ESS Acid Yellow 17 (ESS AY-17). Thereafter, the cells are washed three times again with bath solution and then incubated with bath solution containing 0.5 mM ESS AY-17 and test compounds at desired concentrations for 10 min before assay.
A VIPR is equipped with instrumentation capable of electrical stimulation of cells expressing NaV1.3 (see U.S. Pat. No. 6,686,193). This allows manipulation of the membrane potential and modulates the NaV1.3 conductance. Sodium channels have brief ( ̃1-3 ms) open times, so a train of electric field pulses is used to cycle the channel through open and closed conformations repeatedly. Membrane potential changes caused by the sodium influx through the channels is converted to optical signals using the Aurora FRET voltage sensitive dyes, described above. Cells stained with CC2-DMPE and DiSBAC6(3) are excited at 405 nm. The instrument is able to continually monitor the fluorescent output at two wavelengths for FRET measurement. Fluorescence responses are obtained at two wavelengths, 460 nm for CC2-DMPE and 580 nm for DiSBAC6(3).
The IC50 values of the tested compounds may be determined using an assay such as the one set forth above or any other conventional assay that measures sodium channel activity. Compounds that are effective in such in vitro assays may be tested in subsequent in vivo assays as described below.
Such assays are highly amenable to automation and high throughput. High throughput screening of compounds is described in WO 84/03564.
Of particular interest in this format will be the screening of a variety of different Nav1.3 splice variant protein mutants. These mutants, including deletion, truncation, insertion and substitution mutants, will help identify which domains are involved with the functional channel forming activity of the Nav1.3 splice variant of the invention. Once this region(s) or amino acids particularly important to the channel forming properties of the Nav1.3 splice variant protein that has the sequence of SEQ ID NO:2 has been determined, it will be possible to identify which of these mutants have altered structure but retain some or all of the biological functions of the sodium channel. As noted above, SEQ ID NO:2 comprises a 33-amino acid (NVSQASM SSRMVPGLPANGKMHSTVDCNGVVSL; SEQ ID NO:3) that starts at residue 624 of the NAV1.3 splice variant of the present invention. This region is part of the linker between domain 1 and domain 2 of the sodium channel alpha 1 subunit. PICA phosphorylation sites of the sodium channel are located in this region and are close to this splice site. It is particularly contemplated that amino acids in this linker region, and particularly, each of the residues in SEQ ID NO:3 will be mutated to assess the effect of such mutation on the sodium channel activity. For example, each of the amino acids in this domain may be separately switched to an alanine residue and an “alanine scan” performed to determine which of the residues is important in determining activity. In additional embodiments, it is contemplated that each of the amino acids in this domain may be separately switched to another amino acid that is a conservative substitution of the native residue depending on the hyrophobicity, hydrophilicity or other characteristics of the amino acid at a given residue. To this effect, the SIFT (Sorting Intolerant From Tolerant) program is an exemplary program that allows the skilled artisan to predict whether an amino acid substitution affects protein function and can distinguish between functionally neutral and deleterious amino acid changes in mutagenesis studies (Ng and Henikoff, Nucleic Acid. Res. 31(13): 3812-3814, 2003)
Purified Nav1.3 splice variant protein or its binding agent can be coated directly onto plates for use in the aforementioned drug screening techniques. However, non-neutralizing antibodies to the polypeptide can be used to immobilize the polypeptide to a solid phase. Also, fusion proteins containing a reactive region (preferably a terminal region) may be used to link the Nav1.3 protein active region to a solid phase.
Other forms of in vitro assays include those in which functional readouts are taken. For example cells in which a Nav1.3 protein polypeptide is expressed can be treated with a candidate substance. In such assays, the substance would be formulated appropriately, given its biochemical nature, and contacted with the cell. Depending on the assay, culture may be required. The cell may then be examined by virtue of a number of different physiologic assays, as discussed above. Alternatively, molecular analysis may be performed in which the cells characteristics are examined. This may involve assays such as those for protein expression, enzyme function, substrate utilization, mRNA expression (including differential display of whole cell or polyA RNA) and others. Yet another assay format that can be contemplated is the use of a binding assay with a suitably labeled ligand that binds to the expressed protein. An example of such an assay would be the displacement by a small molecule of a radiolabeled or fluorescently labeled ligand from the expressed Nav1.3 protein. Such an assay can be used to identify potential small molecule modulators of the channel especially if the site where the labeled ligand binds is known to affect channel activity or regulation.
d. In Vivo Assays.
The present invention also encompasses the use of various animal models. In exemplary embodiments, the in vivo assays are set up to identify agents that modulate the sodium channel and are effective as analgesic or anti-inflammatory agents. The ability of an agent or a combination of agents to treat pain can be determined using known pharmacological models (for example see Kim, S. H. and Chung, J. M., Pain, 1992, 50, 355-363), or using models that are similar to known models. For example, to test baseline pain responses, tests such as mechanical withdrawal frequencies by application of different forces of calibrated von Frey monofilaments (mN: 0.24, 1.47, 4.33, 8.01, 23.69, 40.31) (Stoelting, Wood Dale, Ill.) to the plantar hind paw surface, or thermal withdrawal latencies after the application of radiant heat to the plantar hind paw surface may be used (Mansikka et al., Exp Neurol 162: 343-349, 2000; Tao et al., Neuroscience 98: 201-206, 2000). Mechanical withdrawal frequencies are assessed by applying calibrated von Frey monofilaments 0.24 and 4.33 mN to the plantar hind paw surface (Fairbanks et al., Proc Natl Acad Sci USA 97: 10584-10589 2000; Mansikka et al., Exp Neurol 162: 343-349, 2000).
Male Sprague-Dawley rats are pre-screened to determine their baseline 50% withdrawal threshold using a set of von Frey filaments. The 50% withdrawal threshold for mechanical stimulation to the hind paw is determined by the up-down method described by Dixon W. J., Ann. Rev. Pharmacol. Toxicol., 1980, 20, 441-462. Briefly, 8 von Frey filaments with approximately equal logarithmic incremental (0.22) bending forces are chosen (von Frey numbers: 3.65, 3.87, 4.10, 4.31, 4.52, 4.74, 4.92, and 5.16; equivalent to: 0.45, 0.74, 1.26, 2.04, 3.31, 5.50, 8.32, and 14.45 g). A von Frey filament is applied perpendicularly to the plantar surface with sufficient force to bend it slightly and held for 3-5 seconds. An abrupt withdrawal of the foot during stimulation or immediately after the removal of stimulus is considered a positive response.
Whenever there is a positive or negative response, the next weaker or stronger filament is applied, respectively. The test is continued until six stimuli after the first change in response has been obtained. The pattern of positive and negative responses may then be converted into a 50% threshold value using various formulae known to those of skill in the art. One such formula is: 50% threshold=10(X+kd)/104, where X=the value of the final von Frey filament used (in log units), k=the tabular value for the pattern of positive/negative responses [obtained from Dixon, Annu Rev Pharmacol Toxicol 20:441-462], and d=the mean difference between stimuli in log units (0.22). In the cases where continuous positive or negative responses are observed all the way out to the end of the stimulus spectrum, values of 0.3 g or 15.0 g are assigned, respectively. For ED50 calculations, a linear regression is determined for responses one either side of the 50% reversal and then an approximation is determined based upon the value which intersects the 50% point.
Other in vivo methods of testing pain include hotplate analgesia meter determinations. Such hotplate methods evaluate the reaction time of mice (or rats) dropped onto a heated surface and confronted with a heat stimulus applied to their plantar surface. When an analgesic agent is administered to the animals, their reaction time is markedly increased. Such methods may be assessed using e.g., the SDI Hotplate Analgesia Meter (San Diego Instruments, San Diego, Calif., USA).
Treatment of animals with test compounds will involve the administration of the compound, in an appropriate form, to the animal. Administration will be by any route that can be utilized for clinical or non-clinical purposes, including but not limited to oral, nasal, buccal, rectal, vaginal or topical. Alternatively, administration may be by intratracheal instillation, bronchial instillation, intradermal, subcutaneous, intramuscular, intraperitoneal or intravenous injection. Specifically contemplated are systemic intravenous injection, regional administration via blood, cerebrospinal fluid (CSF) or lymph supply and intratumoral injection.
Determining the effectiveness of a compound in vivo may involve a variety of different criteria. Such criteria include, but are not limited to, survival, reduction of tumor burden or mass, inhibition or prevention of inflammatory response, increased activity level, improvement in immune effector function and improved food intake.
Therapeutic Methods and Pharmaceutical Compositions
The present invention deals with the treatment of diseases that result from the increased activity (or expression) of sodium channel proteins. Compositions that inhibit the expression or overexpression of Nav1.3 protein or block its sodium channel activity will be useful in treating or preventing a disease or condition associated with sodium channel activity.
The phrase “disease or condition associated with sodium channel activity” includes all disease states and/or conditions that are acknowledged now, or that are found in the future, to be associated with the activity of sodium channels. Such disease states include, but are not limited to, pathophysiological disorders, including hypertension, cardiac arrhythmogenesis, angina, insulin-dependent diabetes, non-insulin dependent diabetes mellitus, diabetic neuropathy, seizures, tachycardia, ischemic heart disease, cardiac failure, myocardial infarction, transplant rejection, autoimmune disease, sickle cell anemia, muscular dystrophy, gastrointestinal disease, mental disorder, sleep disorder, anxiety disorder, eating disorder, neurosis, alcoholism, inflammation, multiple sclerosis, cerebrovascular ischemia, CNS diseases, epilepsy, stroke, Parkinson's disease, asthma, incontinence, urinary dysfunction, micturition disorder, irritable bowel syndrome, restenosis, subarachnoid hemorrhage, Alzheimers disease, drug dependence/addiction, schizophrenia, Huntington's chorea, tension-type headache, trigeminal neuralgia, cluster headache, migraine (acute and prophylaxis), inflammatory pain, neuropathic pain and depression.
Nucleic acid sequences, antisense molecules, PNAs, purified protein, antibodies, antagonists or inhibitors directed against Nav1.3 can all be used as pharmaceutical compositions. Delivery of these molecules for therapeutic purposes is further described below. The most appropriate therapy depends on the patient, the specific diagnosis, and the physician who is treating and monitoring the patient's condition.
Where clinical applications are contemplated, it will be necessary to prepare the small molecules, analgesic compounds, viral expression vectors, antibodies, peptides, nucleic acids and other compositions identified by the present invention as pharmaceutical compositions, i.e., in a form appropriate for in vivo applications. Generally, this will entail preparing compositions that are essentially free of pyrogens, as well as other impurities that could be harmful to humans or animals.
One will generally desire to employ appropriate salts and buffers to render delivery vectors stable and allow for uptake by target cells. Buffers also will be employed when recombinant cells are introduced into a patient. Aqueous compositions of the present invention comprise an effective amount of the vector to cells, dissolved or dispersed in a pharmaceutically acceptable carrier or aqueous medium. Such compositions also are referred to as inocula. The phrase “pharmaceutically or pharmacologically acceptable” refer to molecular entities and compositions that do not produce adverse, allergic, or other untoward reactions when administered to an animal or a human. As used herein, “pharmaceutically acceptable carrier” includes any and all solvents, dispersion media, coatings, antibacterial and antifungal agents, isotonic and absorption delaying agents and the like. The use of such media and agents for pharmaceutically active substances is well known in the art. Except insofar as any conventional media or agent is incompatible with the compositions of the present invention, its use in therapeutic compositions is contemplated. Supplementary active ingredients also can be incorporated into the compositions.
The active compositions of the present invention include classic pharmaceutical preparations. Administration of these compositions according to the present invention will be via any common route so long as the target tissue is available via that route. The pharmaceutical compositions may be introduced into the subject by any conventional method, e.g., by intravenous, intradermal, intramusclar, intramammary, intraperitoneal, intrathecal, intraocular, retrobulbar, intrapulmonary (e.g., term release); by oral, sublingual, nasal, anal, vaginal, or transdermal delivery, or by surgical implantation at a particular site, e.g., embedded under the splenic capsule, brain, or in the cornea. The treatment may consist of a single dose or a plurality of doses over a period of time. A “subject” or “individual” as used herein, is a vertebrate, preferably a mammal, more preferably a human. Mammals include research, farm and sport animals, and pets.
The active compounds may be prepared for administration as solutions of free base or pharmacologically acceptable salts in water suitably mixed with a surfactant, such as hydroxypropylcellulose. Dispersions also can be prepared in glycerol, liquid polyethylene glycols, and mixtures thereof and in oils. Under ordinary conditions of storage and use, these preparations contain a preservative to prevent the growth of microorganisms.
The pharmaceutical forms suitable for injectable use include sterile aqueous solutions or dispersions and sterile powders for the extemporaneous preparation of sterile injectable solutions or dispersions. In all cases the form must be sterile and must be fluid to the extent that easy syringability exists. It must be stable under the conditions of manufacture and storage and must be preserved against the contaminating action of microorganisms, such as bacteria and fungi. The carrier can be a solvent or dispersion medium containing, for example, water, ethanol, polyol (for example, glycerol, propylene glycol, and liquid polyethylene glycol, and the like), suitable mixtures thereof, and vegetable oils. The proper fluidity can be maintained, for example, by the use of a coating, such as lecithin, by the maintenance of the required particle size in the case of dispersion and by the use of surfactants. The prevention of the action of microorganisms cart be brought about by various antibacterial and antifungal agents, for example, parabens, chlorobutanol, phenol, sorbic acid, thimerosal, and the like. In many cases, it will be preferable to include isotonic agents, for example, sugars or sodium chloride. Prolonged absorption of the injectable compositions can be brought about by the use in the compositions of agents delaying absorption, for example, aluminum monostearate and gelatin.
Sterile injectable solutions are prepared by incorporating the active compounds in the required amount in the appropriate solvent with various of the other ingredients enumerated above, as required, followed by filtered sterilization. Generally, dispersions are prepared by incorporating the various sterilized active ingredients into a sterile vehicle which contains the basic dispersion medium and the required other ingredients from those enumerated above. In the case of sterile powders for the preparation of sterile injectable solutions, the preferred methods of preparation are vacuum-drying and freeze-drying techniques which yield a powder of the active ingredient plus any additional desired ingredient from a previously sterile-filtered solution thereof.
A typical composition for intramuscular or intrathecal administration will consist of a suspension or solution of active ingredient in an oil, for example arachis oil or sesame oil. A typical composition for intravenous or intrathecal administration will consist of a sterile isotonic aqueous solution containing, for example active ingredient and dextrose or sodium chloride, or a mixture of dextrose and sodium chloride. Other examples are lactated Ringer's injection, lactated Ringer's plus dextrose injection, Normosol-M and dextrose, Isolyte E, acylated Ringer's injection, and the like. Optionally, a co-solvent, for example, polyethylene glycol; a chelating agent, for example, ethylenediamine tetracetic acid; a solubilizing agent, for example, a cyclodextrin; and an anti-oxidant, for example, sodium metabisulphite, may be included in the formulation. Alternatively, the solution can be freeze dried and then reconstituted with a suitable solvent just prior to administration.
As used herein, “pharmaceutically acceptable carrier” includes any and all solvents, dispersion media, coatings, antibacterial and antifungal agents, isotonic and absorption delaying agents and the like. The use of such media and agents for pharmaceutical active substances is well known in the art. Except insofar as any conventional media or agent is incompatible with the active ingredient, its use in the therapeutic compositions is contemplated. Supplementary active ingredients also can be incorporated into the compositions.
For oral administration the polypeptides of the present invention may be incorporated with excipients and used in the form of non-ingestible mouthwashes and dentifrices. A mouthwash may be prepared incorporating the active ingredient in the required amount in an appropriate solvent, such as a sodium borate solution (Dobell's Solution). Alternatively, the active ingredient may be incorporated into an antiseptic wash containing sodium borate, glycerin and potassium bicarbonate. The active ingredient may also be dispersed in dentifrices, including: gels, pastes, powders and slurries. The active ingredient may be added in a therapeutically effective amount to a paste dentifrice that may include water, binders, abrasives, flavoring agents, foaming agents, and humectants.
The compositions of the present invention may be formulated in a neutral or salt form. Pharmaceutically-acceptable salts include the acid addition salts (formed with the free amino groups of the protein) and which are formed with inorganic acids such as, for example, hydrochloric or phosphoric acids, or such organic acids as acetic, oxalic, tartaric, mandelic, and the like. Salts formed with the free carboxyl groups also can be derived from inorganic bases such as, for example, sodium, potassium, ammonium, calcium, or ferric hydroxides, and such organic bases as isopropylamine, trimethylamine, histidine, procaine and the like.
The compositions of the present invention may be formulated in a neutral or salt form. Pharmaceutically-acceptable salts include the acid addition salts (formed with the free amino groups of the protein) and which are formed with inorganic acids such as, for example, hydrochloric or phosphoric acids, or such organic acids as acetic, oxalic, tartaric, mandelic, and the like. Salts formed with the free carboxyl groups also can be derived from inorganic bases such as, for example, sodium, potassium, ammonium, calcium, or ferric hydroxides, and such organic bases as isopropylamine, trimethylamine, histidine, procaine and the like.
Upon formulation, solutions will be administered in a manner compatible with the dosage formulation and in such amount as is therapeutically effective. The formulations are easily administered in a variety of dosage forms such as injectable solutions, drug release capsules and the like. For parenteral administration in an aqueous solution, for example, the solution should be suitably buffered if necessary and the liquid diluent first rendered isotonic with sufficient saline or glucose. These particular aqueous solutions are especially suitable for intravenous, intramuscular, subcutaneous and intraperitoneal administration.
In the clinical setting a “therapeutically effective amount” is an amount sufficient to effect beneficial or desired clinical results. An effective amount can be administered in one or more doses. The term “therapeutically effective amount” refers to an amount sufficient to effect treatment when administered to a patient in need of treatment. The team “treatment” as used herein refers to the treatment of a disease or medical condition in a patient, such as a mammal (particularly a human) which includes: preventing the disease or medical condition from occurring, i.e., prophylactic treatment of a patient; ameliorating the disease or medical condition, i.e., eliminating or causing regression of the disease or medical condition in a patient; suppressing the disease or medical condition, i.e., slowing or arresting the development of the disease or medical condition in a patient; or alleviating the symptoms of the disease or medical condition in a patient. Thus, in terms of treatment, a “therapeutically effective amount” of the given therapeutic agent is an amount sufficient to palliate, ameliorate, stabilize, reverse, slow or delay the progression of a disease or condition associated with sodium channel activity or otherwise reduce the pathological consequences of such a disease or condition. The effective amount is generally determined by the physician on a case-by-case basis and is within the skill of one in the art. Several factors are typically taken into account when determining, an appropriate dosage. These factors include age, sex and weight of the patient, the condition being treated, the severity of the condition and the form of the antibody being administered.
The therapeutic compositions can also comprise one or more additional agents effective in the treatment of a disease or disorder associated with sodium channel activity. Other compositions which inhibit the expression, activity or function of Nav1.3 protein (e.g., antagonists) also are contemplated for use in such treatment methods. Thus, combination therapy for the treatment of a disease or disorder associated with sodium channel activity is specifically contemplated. In the context of the present invention, it is contemplated that Nav1.3 inhibition therapy could be used similarly in conjunction with other analgesic agents or sodium channel blocker that are used in the treatment of such disorders.
To achieve the appropriate therapeutic outcome using the methods and compositions of the present invention, one would generally administer a first therapeutic agent that is a Nav1.3, inhibitor or blocker as discussed herein and at least one other therapeutic agent (second therapeutic agent). These compositions would be provided in a combined amount effective to produce the desired therapeutic outcome. This process may involve contacting the cells with the expression construct and the agent(s) or factor(s) at the same time. This may be achieved by contacting the cell with a single composition or pharmacological formulation that includes both agents, or by contacting the cell with two distinct compositions or formulations, at the same time, wherein one composition includes the expression construct and the other includes the second therapeutic agent.
Alternatively, the first therapeutic agent may precede or follow the other agent treatment by intervals ranging from minutes to weeks. In embodiments where the second therapeutic agent and expression construct are applied separately to the cell, one would generally ensure that a significant period of time did not expire between the time of each delivery, such that the agent and expression construct would still be able to exert an advantageously combined effect on the cell. In such instances, it is contemplated that one would contact the cell with both modalities within about 12-24 hours of each other and, more preferably, within about 6-12 hours of each other, with a delay time of only about 12 hours being most preferred. In some situations, it may be desirable to extend the time period for treatment significantly, however, where several days (2, 3, 4, 5, 6 or 7) to several weeks (1, 2, 3, 4, 5, 6, 7 or 8) lapse between the respective administrations.
Local delivery of the first therapeutic agent (i.e., the inhibitor, stimulator or other agent that decreases or increases the amount or activity of Nav1.3 in the individual) to patients may be a very efficient method for delivering a therapeutically effective gene to counteract a clinical disease. Similarly, the second therapeutic agent may be directed to a particular, affected region of the subject's body. Alternatively, systemic delivery of the first and/or second therapeutic agent may be appropriate in certain circumstances.
The active compound(s) is effective over a wide dosage range and is generally administered in a therapeutically effective amount. It, will be understood, however, that the amount of the compound actually administered will be determined by a physician, in the light of the relevant circumstances, including the condition to be treated, the chosen route of administration, the actual compound administered and its relative activity, the age, weight, and response of the individual patient, the severity of the patient's symptoms, and the like.
According to the invention, a compound can be administered in a single daily dose or in multiple doses per day. The treatment regimen may require administration over extended periods of time, for example, for several days or for from one to six weeks.
“Unit dose” is defined as a discrete amount of a therapeutic composition dispersed in a suitable carrier. Parenteral administration may be carried out with an initial bolus followed by continuous infusion to maintain therapeutic circulating levels of drug product. Those of ordinary skill in the art will readily optimize effective dosages and administration regimens as determined by good medical practice and the clinical condition of the individual patient. Suitable doses of sodium channel blockers are in the general range of from 0.01-100 mg/kg/day, preferably 0.1-50 mg/kg/day. For an average 70 kg human, this would amount to 0.7 mg to 7 g per day, or preferably 7 mg to 3.5 g per day. In general, an effective amount of a compound of this invention is a dose between about 0.5 and about 100 mg/kg. A preferred dose is from about 1 to about 60 mg/kg of active compound. A typical daily dose for an adult human is from about 50 mg to about 5 g.
The frequency of dosing will depend on the pharmacokinetic parameters of the agents and the routes of administration. The optimal pharmaceutical formulation will be determined by one of skill in the art depending on the route of administration and the desired dosage. See for example Remington's Pharmaceutical Sciences, 18th Ed. (1990, Mack Publ. Co. Easton Pa. 18042) pp 1435 1712, incorporated herein by reference. Such formulations may influence the physical state, stability, rate of in vivo release and rate of in vivo clearance of the administered agents. Depending on the route of administration, a suitable dose may be calculated according to body weight, body surface areas or organ size. Further refinement of the calculations necessary to determine the appropriate treatment dose is routinely made by those of ordinary skill in the art without undue experimentation, especially in light of the dosage information and assays disclosed herein as well as the pharmacokinetic data observed in animals or human clinical trials.
Appropriate dosages may be ascertained through the use of established assays for determining blood levels in conjunction with relevant dose response data. The final dosage regimen will be determined by the attending physician, considering factors which modify the action of drugs, e.g., the drug's specific activity, severity of the damage and the responsiveness of the patient, the age, condition, body weight, sex and diet of the patient, the severity of any infection, time of administration and other clinical factors. As studies are conducted, further information will emerge regarding appropriate dosage levels and duration of treatment for specific diseases and conditions.
In a preferred embodiment, the present invention is directed at treatment of human disorder, disease or condition associated with sodium channel activity, or may be alleviated by administering a sodium channel blocker or inhibitor. A variety of different routes of administration are contemplated. For example, a classic and typical therapy will involve direct, injection of a discrete area.
Further Uses of Compositions of the Invention
It is contemplated that in certain diseases or disorders, the Nav1.3 splice variant is overexpressed. In certain embodiments therefore, methods of diagnosing a disorder in which Nav1.3 splice variant is overexpressed or aberrantly expressed are contemplated. Such diagnostic methods of the present invention are achieved through the detection of the Nav1.3 protein or a fragment thereof that is expressed in abundance as compared to normal expression. Such a protein may be detected using antibodies specific for the protein in any of a number of formats commonly used by those of skill in the art for such detection.
In another aspect, the present invention contemplates that an antibody that is immunoreactive with any sodium channel alpha subunit may be immunoreactive with the protein molecule of the present invention. Indeed, antibodies may be generated using the protein of SEQ ID NO:2, which specifically include antibodies that detect the 32-amino acid linker between Domain 1 and Domain 2. Such antibodies include, but are not limited to, polyclonal, monoclonal, chimeric, single chain, Fab fragments and fragments produced by a Fab expression library, bifunctional/bispecific antibodies, humanized antibodies. CDR grafted antibodies, human antibodies and antibodies which include portions of CDR sequences specific for sodium channel protein of the present invention. Means for preparing and characterizing antibodies are well known in the art (see, e.g., Harlow and Lane, ANTIBODIES: A LABORATORY MANUAL, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., 1988). Antibodies specifically immunoreactive to the splice variant that comprises a sequence of SEQ ID NO:2 are particularly preferred.
It is proposed that antibodies specific for sodium channels will be useful in standard immunochemical procedures, such as ELISA and Western blot methods and in immunohistochemical procedures such as tissue staining to determine the distribution of sodium channels.
EXAMPLE(S)
The following examples are included to demonstrate preferred embodiments of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples which follow represent techniques discovered by the inventor to function well in the practice of the invention, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention.
The present example teaches the characterization and isolation of a novel Nav1.3 cDNA clone isolated from human spinal cord RNA using RT-PCR to amplify the message in three overlapping fragments (experimental details below). The three fragments were then ligated together to create the full-length sequence. The cDNA clone isolated is a novel splice variant that differs from previously reported Nav1.3 cDNAs (see FIG. 1).
The Nav1.3 cDNA of the present invention was used to express human Nav1.3 in cultured cell lines. The Nav1.3-expressing line was used in high-throughput screening to identify antagonists of Nav1.3 and has been used to measure activity against the sodium channel.
The cell line expressing NaV1.3 was created as follows. Human Embryonic Kidney (HEK) cells were chosen as the most suitable host cell line. The cells were transduced by retrovirus. VSV-G pseudotyped retroviral vectors were generated through a three-plasmids co-transfection. After a stable pool of cells were generated from retroviral transduction, the cells were FACS (Fluorescence Activated Cell Sorter) sorted on light scatter, selecting for individual viable cells on 96 well plates. Daughter plates were made from these plates 2 weeks after sorting and were assayed on the EVIPR by the assay described below.
HEK293 cells expressing hNav1.3 channels are cultured on 96- or 384-well plates (Costar tissue culture treated 96-well flat bottom plates, Corning). To prevent detachment of cells during plate washing, these plates are pre-coated with 0.5% Growth Factor Reduced matrigel matrix in DMEM for 1 hour at room temperature before use for cell culture. About 40,000 cells are seeded to each well and incubated at 38° C. for 24 hours before assay. Assay is performed at room temperature. The cell plates are first washed three times with bath solution using automatic plate washer (ELx405, Biotek), leaving a residual volume of 50 μL/well. Subsequently, cells are incubated with mixed dye solution for 30 min in the dark at room temperature. The mixed dye solution is prepared with External solution and consists of 10 μM CC2-DMPE (chlorocoumarin-2-dimyristoyl phosphatidylethanolamine), 2.4 μM DISBAC6(3) (bis-(1,3-dihexyl-thiobarbituric acid) trimethine oxonol), 0.5% β-cyclodextrin, 20 μg/ml pluronic F-127 and ESS Acid Yellow 17 (ESS AY-17). Thereafter, the cells are washed three times again with bath solution and then incubated with bath solution containing 0.5 mM ESS AY-17 and test compounds at desired concentrations for 10 min before assay.
A VIPR is equipped with instrumentation capable of electrical stimulation of cells expressing NaV1.3 (EVIPR U.S. Pat. No. 6,686,193). This allows manipulation of the membrane potential and modulates the NaV1.3 conductance. Sodium channels have brief ( ̃1-3 ms) open times, so a train of electric field pulses is used to cycle the channel through open and closed conformations repeatedly. Membrane potential changes caused by the sodium influx through the channels is converted to optical signals using the Aurora FRET voltage sensitive dyes, described above. Cells stained with CC2-DMPE and DiSBAC6(3) are excited at 405 nm. The instrument is able to continually monitor the fluorescent output at two wavelengths for FRET measurement. Fluorescence responses are obtained at two wavelengths, 460 nm for CC2-DMPE and 580 nm for DiSBAC6(3).
A. Sequence Comparison of Vertex Nav1.3 Clone Against Other Human Nav1.3 sequences
FIG. 1 shows a pilup in which the sequence of the present invention (SEQ ID NO:2) was compared with other human clones that were identified by a blast search at NCI. The amino acid at residue 208 is aspartic acid in the sequence identified in the present invention, however in some other sequences analyzed this residue was found to be a serine residue. Amino acid residue 208 is in the S3-S4 linker in Domain 1 of the sodium channel. It is a very short linker and changes here can have a significant effect on sodium channel activation because S4 residues must pivot in response to changes in transmembrane voltage. Variation at this site arises by alternative splicing and insertion of one of two exons that differ only at residue 208; S is found in neonatal brain NaV1.3, while D is found in adult brain. In addition, it was seen that there is an amino acid insert that starts at residue 624 of SEQ ID NO:2. This region is the linker between domain 1 and domain 2. Protein kinase A phosphorylation sites are in this region and are close to this splice site.
B. Cloning of Nav1.3
The nucleotide sequence for a human Nav1.3 cDNA (Genbank accession #AJ251507) was used to establish oligonucleotide primer sequences and RT-PCR strategies for the cloning of a full-length hNav1.3 cDNA. First strand cDNA synthesis (Clontech, Advantage RT-for-PCR, #639505) was carried out using 100 ng of whole brain RNA (Clontech, #636530) with three Nav1.3 specific antisense oligos (X1-26α, X2-22α and X3-29α, 1 μl of 20 μM for each oligonucleotide) in a total of reaction volume of 20 μl at 42° C. for 45 minutes.
An hNav1.3 cDNA was subsequently subcloned in three fragments (A, B and C) overlapping at unique internal restriction sites (Nsi I and Bgl II) using the first-strand cDNA as template and nested PCR. The individual PCR reactions were carried out initially using an outer set of primers (X1-20s+X1-26α, X2-22s+X2-22α, X3-22s+X3-29α) and the reaction products subsequently used as template with an inner set of oligonucleotide primers (X1-32s+X1-25α, X2-20s+X2-21α, X3-21s+X3-) for further PCR amplification and isolation of the three Nav1.3 overlapping fragments. Restriction sites were inserted into RT-PCR oligonucleotides at the 5′ (Xho I) and 3′ (Not I) ends. PCR reaction conditions for both the outer and inner set of oligonucleotides was carried out for 15 cycles of 1′ at 95° C., 1′ at 55° C. and 6′ at 72° C. and then another 15 cycles of 1′ at 95° C., 1′ at 58° C. and 6′ at 72° C. Reaction samples were run on 1% agarose gels, the expected molecular weight band sizes were isolated (frag A=1.2 kb, frag B=1.37 kb and frag C=2.45 kb), subsequently blunt-end ligated into the TOPO II vector (Invitrogen) and individual miniprep DNA samples of the fragments were sequenced.
The three fragments were then subcloned in sequence into the vector pLBCX vector, which is derived from the pLNCX vector (Clontech) but replaces the gene for neomycin resistance with one providing resistance blasticidin. The completed expression vector pLBChNav1.3, containing a novel human Nav1.3 splice product, was utilized to generate retroviral supernatants for infection of HEK293 cells. The infected HEK293 cells were selected in 5 ug/ml blasticidin for two weeks and then single cells were FACSorted for isolation of distinct clonal cell lines, which were tested for Nav channel expression using voltage-sensitive optical dyes and electrical stimulation on eVIPR. Clonal lines with the expected depolarizing responses were further subjected to patch clamp recording techniques and pharmacological characterization on eVIPR. Clonal lines were chosen and an eVIPR assay protocol optimized as described above.










Table of Oligonucleotide Primers:


















X1-20s
CTA CAC GTA ATT AAA TGT GC




(SEQ ID NO: 9)



 



X1-26α
AAT GGA TCC ATA ACA ATT AAA TTC 




AC




(SEQ ID NO: 10)



 



X1-32s
NNN NNC TCG AGA GGA TGA AAA GAT  




GGC ACA GGC




(SEQ ID NO: 11)



 



X1-25α
GTT TTA CTT TTA ACC ATG CAT CAC




(SEQ ID NO: 12)



 



X2-22s
ATT TGC CAA TGT GTT CTT GAT C




(SEQ ID NO: 13)



 



X2-22α
TTG TGC TCA ACA ATA CTG TAG C




(SEQ ID NO: 14)



 



X2-20s
CTG GGA CTG CTG TGA TGC AT




(SEQ ID NO: 15)



 



X2-21α
GAA GAT TCC ACC AGA TCT TCC




(SEQ ID NO: 16)



 



X3-22s
AGA AGA CCT GTC AAG TAA GTA C




(SEQ ID NO: 17)



 



X3-29α
CA CAA AGA TAA TTC TTT GTT TCT  




TTT TAC




(SEQ ID NO: 18)



 



X3-21s
GAA GAA GGC AAA GGG AAG ATC




(SEQ ID NO: 19)



 



X3-
NNN NNG CGG CCG CTT TTT ACT TTT  




GAT TTT CTC TGA CC




(SEQ ID NO: 20)










All of the compositions and/or methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention. More specifically, it will be apparent that certain agents which are both chemically and physiologically related may be substituted for the agents described herein while the same or similar results would be achieved. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims.
The references cited herein throughout, to the extent that they provide exemplary procedural or other details supplementary to those set forth herein, are all specifically incorporated herein by reference.

## Claims

1. An in vitro method of identifying a modulator of a human sodium channel α-subunit expression or activity, the method comprising the steps of:
i) contacting a cell that expresses a nucleic acid comprising a nucleotide sequence encoding a protein having the amino acid sequence of SEQ ID NO: 2 with a candidate modulator substance;
ii) monitoring the expression or ion channel activity of the protein having the amino acid sequence of SEQ ID NO: 2; and
iii) comparing the expression or ion channel activity of the protein having the amino acid sequence of SEQ ID NO: 2 with expression or ion channel activity of the protein having the amino acid sequence of SEQ ID NO: 2 in a cell not exposed to said candidate modulator substance;
wherein an alteration in the expression or ion channel activity of the protein having the amino acid sequence of SEQ ID NO: 2 in a cell exposed to the candidate modulator substance compared to a cell not exposed to the candidate modulator substance indicates that the candidate modulator substance is a modulator of human sodium channel α-subunit expression or activity.

2. The method of claim 1 wherein the candidate modulator substance comprises a small molecule ion channel blocker or inhibitor, an oligonucleotide, an antisense oligonucleotide, a DNA oligonucleotide, an RNA oligonucleotide, or a chimeric oligonucleotide.

3. The method of claim 2, wherein the candidate modulator substance comprises a nucleotide sequence capable of hybridizing with RNA to form an RNA oligonucleotide-RNA duplex.

4. An in vitro method for identifying a compound that modulates the activity of a sodium channel comprising:
i) contacting a cell that expresses a sodium channel comprising a polypeptide comprising the amino acid sequence of SEQ ID NO: 2 with a test compound under conditions that activate sodium channel activity of said sodium channel in the absence of the test compound; and
ii) determining whether the cell contacted with the test compound exhibits a modulation in activity of the sodium channel compared to a cell not contacted with the test compound.

5. The method of claim 4, wherein the cell has been genetically engineered to express or overexpress the polypeptide.

6. The method of claim 4, wherein the cell has been genetically engineered by introducing into the cell a nucleic acid having a nucleotide sequence encoding a polypeptide comprising the amino acid sequence set forth in SEQ ID NO: 2.

7. The method of claim 4, wherein the cell has been genetically engineered to upregulate the expression of a nucleic acid encoding a polypeptide comprising the amino acid sequence of SEQ ID NO: 2.

8. The method of claim 7, wherein the upregulated nucleic acid is endogenous to the cell.

9. The method of claim 4, wherein the method comprises determining whether the cell contacted with the test compound exhibits a decrease in sodium channel activity.