IMMORTALIZED BRAIN ENDOTHELIAL CELLS

Abstract: A brain microvessel endothelial cell having a Middle-T antigen gene from papovirus is disclosed. The brain microvessel endothelial cell exhibits normal cell phenotype, maintains the phenotype in culture and forms monolayers substantially impermeable to low molecular weight molecules. The cells are useful for in vitro studies involving the blood brain barrier. Also disclosed are methods of using and processes of making these cells.

Description: Immortalized Brain Endothelial Cells Related Applications This international application claims priority to U . S . Application Ser . No . 09/200 , 063 , filed November 25 , 1998 Technical Field of Invention The invention relates to immortalized cellular clones of brain microvessel endothelial cells produced by transfection with a plasmid containing the middle-T antigen gene from polyoma virus. The cells described herein display and maintain an endothelial cell phenotype and spontaneously form confluent monolayers substantially impermeable to small molecules. The cells are therefore useful for in vi tro models studying the blood brain barrier. Background of the Invention The blood brain barrier (BBB) is composed of brain microvessel endothelial cells (referred to hereafter as BMECs) and acts as a regulatory interface for permeability of drugs and solutes between the blood and central nervous system (CNS) . Isolation and culture of BMECs have led to the development of in vi tro model systems for studying the BBB (Bowman et. al, 1983, Audus and Borchardt, 1986) . In vi tro BBB model systems have been successfully derived from bovine, canine, human, murine, porcine, and rat brains and were found to have similar permeability properties due to similarity of the physiological characteristics of the BBB among all mammals (Cserr et al, 1984, Audus et al, 1990). In these models, BMECs retain the characteristics of brain endothelial cells in vivo including morphology, specific BBB enzyme markers, and tight intercellular junctions which can be useful for studying a variety of CNS drug delivery issues ranging from passive diffusion, carrier mediated transport, and metabolism to specific factors affecting the BBB permeability. However, passaging of BMECs results in loss of specific endothelial and BBB markers as well as tight intercellular junctions (Brightman and Neuwelt, 1989). Presently, only primary cultures of BMECs have been developed to address the BBB permeability issues in vi tro . Isolated and cultured primary brain cells developed previously have exhibited different properties primarily due to considerable variety in the starting material. For example, with respect to transcellular transport, rigorous comparison of data between different laboratories has been very difficult (Pardridge et al . , 1990, Masereeuw et al., 1994) . Passaging primary cells can affect the differentiation of cells and lead to the selection of the most rapidly proliferating clones. Furthermore, the expression of some marker enzymes such as gamma-glutmayl transpeptidase as well as tight junctional complexity has been shown to decrease with time in culture and passage number (Meresse et . al . , 1989) . Papoviruses such as SV-40 and polyoma virus are known to produce tumors in rodents. The polyoma virus DNA sequence has been determined and the λ early region' which includes the small, middle and large T antigens has been indicated to be responsible for tumor formation. See Soeda et al., 1980, incorporated herein by reference. Previous methods using the SV 40 large T antigen gene disclose immortalization of bovine brain capillary endothelial cells in which the cells retain the differentiated phenotype and appear morphologically normal (Durieu-Trautmann et al., 1991) . Fetal rat stems cells transfected with the middle T antigen gene produced two cell lines of endothelial cells that were not oncogenic, however, these cell lines do not spontaneously form a barrier to small hydrophilic molecules ( Juillerat-Jeanneret et al . , 1992). Thus, it is apparent that the presently available clones of immortalized BMEC cultures suffer from individual drawbacks in terms of phenotype expression and homogeneic maintenance of that expression. This leads to difficulties with respect to accuracy and reproducibility in studies utilizing BMECs. Summary of the Invention All of the above factors provide a strong incentive for the development of immortalized BMECs capable of maintaining a differentiated phenotype in culture and therefore, measurements such as permeability can be made without the need for repeated isolations of BMECs. The BMECs of the present invention provide a convenient, ready-to-use bank of cells as an efficient alternative to the use primary BMECs. It is therefore an object of the invention to provide immortalized BMECs possessing a nucleic acid sequence encoding the middle-T antigen gene from a papovirus, such cells are capable of forming monolayers substantially impermeable to low molecular weight molecules. In preferred embodiments, there are provided BMECs having the middle-T antigen gene from the polyoma papovirus. It is a further object of the present invention to provide a method of using the BMECs described herein for measuring permeability via passive diffusion or active transport, carrier mediated transport, efflux transport system such as P-glycoprotein and multi-drug resistance associated proteins (MRP) , and metabolism of substances crossing the blood brain barrier such as pharmaceuticals. It is yet a further object of the present invention to provide a process of making the immortalized BMECs described herein by providing primary culture BMECs, subsequently transfecting them with a vector containing the middle-T antigen gene from a papovirus, incubating the vectors and cells and subsequently isolating the immortalized BMECs possessing the middle-T antigen nucleic acid sequence. These and other objectives will be readily appreciated by those of ordinary skill in the art based upon the following detailed disclosure of this invention. Brief Description of the Drawings Pig. 1 : shows results of the PCR amplification performed various primers from the polyoma middle-T antigen coding region . Pig. 2 : shows morphology BMEC by light microscopy (200x) . Fig. 3 : shows morphology of TBMEC, pll by light microscopy (200x) . Detailed Description of the Invention The subject cell line referred to herein as TBMEC, Pll has the A.T.C.C. designation number CRL-12414 having been deposited on October 7, 1997. The present invention relates to immortalized BMECs, preferably mammalian BMECs, more preferably bovine BMECs, processes of making and methods of using these cells. For the first time, the present invention provides BMECs with the capability of preserving a fully differentiated phenotype and thus, the endothelial nature of these cells. This is accomplished by transfecting the BMECs with a vector such as a plasmid, containing the entire coding region of middle-T antigen gene from papoviruses such as polyoma or SV40. In preferred embodiments, there are provided a plasmid which carries the nucleic acid sequence encoding Middle T antigen from polyoma virus, such plasmid referred to herein as polyoma middle T (pyMT) . Clones of the transfected cells are characterized in that they express and retain their differentiated phenotype. The BMECs according to the present invention grow as monolayers and show substantial impermeability to small substances imparted by their ability to form confluent monolayers. The aforementioned substances include low molecular weight hydrophilic and lipophilic pharmaceuticals and other chemicals. Examples of these low molecular weight molecules include: acetylsalicylic acid, acyclovir, aminopyrine, caffeine, chlorpromazine, cimetidine, clonidine, corticosterone, desipramine, diazepam, dopamine, estradiol, hydrocortisone, indomethacin, mannitol, methyl scopolamine, nicotine, phenytoin, pirenzepine, progesterone, propranolol, salicylic acid, scopolamine, sucrose, sulfasalazine, terbutaline, testosterone, uracil, dexamethasone, urea, warfarin, bremazocine, meloxicam, nevirapine, ranitidine, didanosine, zalcitabine, stavudine and Saquinavir. The term "differentiated phenotype" as used in the present invention shall be understood to mean BMECs forming continuous monolayers of oblong contact inhibited cells. The term "substantially impermeable" as used in the present invention shall be understood to mean having low permeability coefficients similar to that of sucrose, a non- metabolizable paracellular marker. Immortalized BMECs produced by transfection with a plasmid containing the middle-T antigen gene from polyoma virus displayed a non-transformed endothelial cell phenotype, i.e., they stained for phenotypical genetic markers such as Factor VIII and LDL receptor, and retained tight junction formation and permeability characteristics. Cell morphology of one of the BMEC clones according to the present invention, TBMEC, Pll and BMEC cultures was examined with a microscope using phase contrast optics. TBMEC, Pll cells form continuous non-overlapping tightly packed monolayers of contact inhibited oblong cells with similar morphology to the primary BMECs when grown on collagen-coated and fibronectin treated petri dishes. With respect to studying maintenance of differentiated endothelial phenotype, the TBMEC, Pll was grown as monolayers in Transwell ® plates and the permeability of sucrose, a non- metabolizable paracellular marker, was measured in primary BMECS and the TBMEC, Pll and was shown to be comparable. The permeability coefficient values for acyclovir and leucine were also shown to be comparable at similar concentrations. See Example 4. It is therefore also an object of the present invention to provide a method of measuring blood brain barrier permeability of a substance by providing BMECs according to the invention in one or more confluent monolayers, contacting the substance with said confluent monolayer ( s ) ; and measuring the amount of said substance permeated across the BMEC monolayer (s) . The method can measure passive diffusion or active transport and in addition can be varied, as appreciated by those skilled in the art, to perform characteristic measurements such as carrier mediated transport wherein permeability values as described in Example 3 are measured as a function of concentration of the solute or in the presence of ATP inhibitors such as sodium azide. Efflux transport can be measured wherein measuring permeability values as described in Example 3 is performed in the presence of inhibitors of the efflux pumps such as, but not limited to, cyclosporin-A, dexamethasone, digoxin, FK506, methadone, phenytoin, prazosin, quinidine, rhodamine 123, tamoxifen, verapamil, and yohimibine as well as all those listed by Seelig 1997, incorporated herein by reference. The methods of the present invention can also be used to measure blood brain barrier metabolism of a substance by measuring permeability values as described in Example 3 and profiling the metabolic degradation of compounds of interest as a function of time using quantitative analytical techniques such as high pressure liquid chromatography and mass spectrometry . Immortalized BMECs provided by the present invention can be prepared by the following general method: Primary cultures of BMECs can be prepared according to known methods. One example is bovine brain endothelium prepared from fresh calf brains as described previously (Audus and Borchardt, 1986). Briefly, three to four freshly excised bovine brains were purchased from a local slaughter house and transported to the laboratory in ice cold MEM. The capillaries and meninges were removed from each brain and the gray matter was scraped off and minced. The gray matter suspension was treated with protease and collagenase/dispase to separate the endothelial cells from the fat, myelin, and other cells. Microvessel endothelial cells were collected from a Percoll gradient, washed with MEM, and pelleted. They were then resuspended in a culture medium made of approximate equal parts MEM and F-12 Ham nutrient mix containing about 20% horse serum and about 5% DMSO and frozen under liquid nitrogen for 0 - 12 months, preferably not more then six months . The BMECs above are then transfected as follows: The BMECs are transfected with a plasmid containing the middle-T antigen gene from polyoma virus . The plasmid pyMT (see example 5) carries the entire coding region of the Middle T antigen from polyoma virus {Soeda et al . , 1980). Here, both the pSV2 neo plasmid, which carries the neo gene (Southern and Berg) for resistance to the aminoglycoside G418, and the pyMT plasmid were cotransfected into a primary stock of BMECs at about 50% confluency. About 20μg of each vector was used in a calcium phosphate transfection procedure. Plasmids were allowed to incubate with the cells for about 30 minutes at room temperature and then incubated in complete culture medium for about 2.5 hours at about 37 °C. The complete culture medium was made up of equal parts MEM and F-12 Ham nutrient mix containing amphotericin B (2.5 μg/mL) , ECGS (15 μg/mL) , HEPES (10 mM) , horse serum (10%), penicillin (100 Units/ L) , polymoxin B (50 μg/mL), and streptomycin (100 μg/mL) . After about 3 hours the cells were subjected to a DMSO (10%) osmotic shock for about 10 minutes. The cells are then washed, preferably about 7 times, with complete culture medium and returned to the incubator. Cells were split 1:1 into complete culture medium supplemented with G418 (300μg/ml, activity) after they reached confluency (about 48 hours) . Selective medium was changed about every 2 to 3 days. Foci appeared within 10 days. The foci were trypsinized, pooled and cultured through several passages. All clones had normal phenotypes but more regular and faster dividing time than the original primary BMECs (Figure 2B) . One of these clones, named TBMEC, Pll was further characterized and used in permeability experiments. In order that this invention be more fully understood, the following examples are set forth. These examples are for the purpose of illustrating preferred embodiments of this invention, and are not to be construed as limiting the scope of the invention in any way. Example 1 Detection of pyMT gene in TBMEC, Pll The presence of PyMT gene in TBMEC, Pll cells was determined by polymerase chain reaction (PCR). Genomic DNA was extracted from non-transfected (WT) and transfected (TX) BMEC and used at 10 ng/ml for PCR amplification. PyMT plasmid DNA was used as a positive control. The primers used for PCR amplification were as follows and were based on the published sequence (Soeda et.al, 1980) . 230 (S) - 5'CCCAGACAACTATGGGGGGAT 3' 281 (S) - 5'CAGTCACTGCTACTGCACCCA 3' 971 (S) - 5'GCAACCCGACCTATTCTGTTA 3' 660 (AS) - 5'CGGGTTGGTGTTCCAAACCAT 3' 1030 (AS)- 5'GTTGGAGAACTCGGGTTGGGG 3' 1150 (AS) - 5'CCAGCTGGTCTTGGTCGCTTT 3' PCR reactions were set up using 45 μl PCR Supermix (Gibco BRL) , 2.5 μl each of the sense (S) and antisense (AS) primers (20uM stock) and lOul of template DNA (lOng/ml) . Thirty cycles of PCR amplification was performed by melting DNA at 95°C for 1 min, annealing at 50°C for 1 min and extending at 72°C for lmin for each cycle using GeneAmp PCR System 9600 (Perkin Elmer) . After 30 cycles, the final extension was performed for 7 mins at 72°C and the products were resolved using 1.0% agarose gels Detection of transfected pyMT gene encoding Middle T Antigen : Since TBMEC, Pll cells were co-transfected with a target DNA (pyMT) and a plasmid containing a selectable marker (pSV2neo) , and then selected for G418 antibiotic resistance, it was preferable to detect pyMT gene in G418- resistant clones. Results of the PCR amplification performed using various primers from the polyoma middle T antigen coding region are shown in Figure 1. Set A represents amplification using primers 230 (S) and 1030 (AS) generating a PCR product of 701 bp, set B using primers 230 (S) and 1150 (AS) for a product of 821 bp, set C using primers 281 (S) and 660 (AS) for a product of 380 bp and set D represents amplification using primers 971 (S) and 1150 (AS) giving rise to a PCR product of 180 bp . PCR amplification was detected only in the reactions where PyMT plasmid DNA and genomic DNA from transfected cells were used as templates. Template DNA from non-transfected BMEC cells did not show PCR amplification. These results confirm the presence of PyMT gene in TBMEC, Pll cells selected for G418 resistance. Example 2 Histochemical Techniques Low density lipoprotein (LDL) uptake and Factor VIII antigen and gamma-glutamyl transpeptidase expression, specific markers found only in endothelial cells, were examined by immunohistochemical techniques. Cells were grown to near confluence on non-autofluorescent 25 mm plastic dishes. Uptake of acetylated low density lipoprotein labelled with 1, 1 ' =dioctadecyl-3, 3, 3 ' , 3 ' - tetramethylinocabocyanine perchlorate (Dil-Ac-LDL) at lOμg/ml in PBS was performed for 4h at 37°C. The cells were examined by fluorescence microscopy. Negative controls used 50% rabbit serum in HBSS as a substitute for the primary antibody. BMECs grown in culture were also shown to stain histochemically positive for alkaline phosphatase, an enzyme marker for brain endothelial cells. Example 3 Permeability Experiments BMEC and TBMEC, Pll cells were seeded at densities of 75,000 and 60,000 cells/mL respectively, onto polycarbonate filters in Transwell®, 6-well plates (Costar Transwell ® 12mm diameter, 0.4μm pore size, Corning Costar Corp., Cambridge, MA) and were cultured in complete culture medium. The plates were maintained in an incubator at 37 °C and 5% C0 2 and were used for permeability experiments within 10 to 12 days postseeding for BMEC cells and 4 to 6 days postseeding for TBMEC, Pll cells- Drug solutions were prepared in HBSS. All drug solutions also contained 0. ImM of sucrose (~0.15μCi/mL of radiolabeled) as an internal standard paracellular marker. The permeability of sucrose for each well was determined along with that of the compound of interest to ascertain the confluence of the monolayers. Transport rates for both the monolayers and the filters were then determined by taking samples at discrete time intervals and plotting the cumulative amount permeated as a function of time. Samples were analyzed by HPLC (Hewlett Packard, HP 1090) or counted on a liquid scintillation counter (Packard, Tri-Carb, A2700) . Monolayers were deemed to be confluent and acceptable for permeability measurements when sucrose leakage was less than 15% per hour. Permeability coefficients were then determined according to equation 1 : P = J/AC 0 (1) where J was the rate of appearance of the drug in the receiver chamber, C 0 was the initial concentration of the drug in the donor chamber, and A was the surface area of the filter. The in vi tro BBB permeability was reported as the permeability coefficient of drugs across the monolayers, P m , and was determined according to equation 2 : where P e was the effective permeability coefficient across the monolayer and the filter and P f was the permeability coefficient across the filter only. Permeability measurements were done in at least triplicates for each compound. Example 4 Permeability Measurements Transmembrane permeability of TBMEC, Pll monolayers was quantitated by sucrose, a non-metabolizable, membrane impermeant marker. The permeability coefficients of 0.1M sucrose solutions in HBSS for TBMEC, Pll monolayers compared to BMEC monolayers are presented in Table 1. Permeability of Select Compounds: The permeability coefficients of acyclovir and leucine at 0.1M in TBMEC, Pll and BMEC monolayers are shown in Table II . It can be seen that the permeability coefficients for these compounds in both transfected and primary cells are similar. These results demonstrate that the BMECs according to the present invention form monolayers substantially impermeable to low molecular weight molecules. Example 5 Construction of plasmid pyMT Construction of a modified polyoma virus early region encoding only the middle-T protein was made by the methods as described by Treisman, Novak, Favaloro and Kamen, Nature, Vol 292, 13 August 1981, incorporated herein by reference. In brief, a recombinant plasmid containing the full length viral (polyoma) wild type viral DNA was cut with the endonuclease Ava 1 to remove an intervening sequence that contained the middle-T antigen coding region. This fragment was ligated into a recombinant ampicillin- resistant vector, at sites previously cut with Msp 1 plus Ava 1 and dephosphorylated with calf alkaline phosphatase. The ligation product was transfected into E . coli strain HB101, and ampicillin-resistant colonies were selected. Small preparations of DNA was prepared from the resulting colonies and digested with Aval, Avail, Hinfl and Mspl and run on agarose-polyacrylamide gels to determine that the colonies contained the middle-T coding region fragment. A colony containing the appropriate digestion pattern by electrophoresis was termed pPyMTl and propagated to produce milligram quantities of DNA by cesium chloride gradient purification, for transfection of mammalian cells. Studies from the reference cited above had determined that this recombinant plasmid containing the middle-T antigen could transform Fisher rat cells (F2408 cell line) . Cells transfected with the pPyMTl plasmid alone formed foci, grew poorly on plastic and grew poorly at low density. REFERENCES Audus, K.L., Bartel, R.L., Hidalgo, I.J., and Borchardt, R.T., The use of cultured epithelial and endothelial cells for drug transport and metabolism studies, Pharτa . Res . 7, 435-451 (1990) Masereeuw, R.; Jaehde, U . ; Langemeijer, M.W.E.; de Boer, A.G.; Breimer, D.D. In vitro and in vivo transport of zidovudine (AZT) across the blood-brain barrier and the effect of transport inhibitors. Pharm . Res . 1994, 11, 324- 330. Meresse, S., Dehouck, M-P., Delorme, P., Bensaϊd, M. , Tauber, J-P., Delbart, C, Fruchart, J-C, and Ceccelli, R., Bovine brain endothelial cells express tight junctions and monoamine oxidase activity in long-term culture, J. Neuorchem . 53, 1363-1371 (1989) Pardridge, W.M., Triguero, D., Yang, J., and Cancilla, P. A., Comparison of in vi tro and in vivo models of drug transcytosis through the blood-brain barrier, J. Pharma col . Exp . Thera . 253, 884-891 (1990) Soeda, E., Arrand, J.R., Smolar, N., Walsh, J.E., and Griffin, B.E., Na ture 283, 445-453 (1980) Cserr, H.F. and Bindgaard, M., Blood-brain interfaces in vertebrates: a comparative approach, Am . J. Physiol . 246, R277-R288 (1984) Juillerat-Jeanneret , L., Aguzzi, A, Wiestler, O.D., Darekar, P., ands Jnazer, R.C., Dexamethasone selectively regulates the activity of enzymatic markers of cerebral endothelial cells lines, In Vi tro Cell Dev. Biol . 28A, 537-543 (1992) Soeda E., Arrand, J.R., Smolar, N., Walsh, J.E., and Griffin, B.E., Coding potential and regulatory signals of the polyoma virus genome, Na ture , 283, 445-453 (1980) Durieu-Tautmann, O. , Foignant-Chaverot , N. , Perdomo, J. , Gounon, P., Strosberg, A.D., and Conrad, P.O., Immortalization of brain capillary endothelial cells with maintenance of structural characteristics of the blood-brain barrier endothelium, In vi tro Cell . Dev. Bio . 27A, 771-778 (1991) Seelig, A. , A general pattern for substrate recognition by P- glycoprotein, Eur. J. Bioch . 251, 252-261 (1998) Treisman, R. , Novak, U. , Favaloro, J. & Kamen, R. Transformation of rat cells by an altered polyoma virus genome expressing only the middle-T protein. 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Claims: What is Claimed is : 1. A brain microvessel endothelial cell comprising a nucleic acid sequence encoding Middle-T antigen gene from a papoviruse, wherein said cell is capable of forming monolayers substantially impermeable to low molecular weight molecules . 2. The brain microvessel endothelial cell according Claim 1 wherein the papovirus is polyoma. 3. The brain microvessel endothelial cell according Claim 1 wherein the low molecular weight molecule is a hydrophilic and/or lipophilic pharmaceutical. 4. The brain microvessel endothelial cell according Claim 2 wherein the low molecular weight molecule is a hydrophilic and/or lipophilic pharmaceutical. 5. A method of measuring blood brain barrier permeability of a substance, wherein the permeability measured is passive diffusion or active transport, said method comprising: providing BMECs according to claim 1 in one or more confluent monolayers; contacting said substance with said confluent monolayer (s) ; and measuring the amount of said substance permeated across the BMEC monolayer ( s ) . 6. The method of measuring blood brain barrier permeability of a substance according to claim 5 wherein said substance is a low molecular weight hydrophilic or lipophilic pharmaceutical . 7. A method of measuring blood brain barrier carrier mediated transport of a substance, said method comprising : providing BMECs according to claim 1 in one or more confluent monolayers; contacting said substance with said confluent monolayer (s) in the presence of one or more ATP inhibitors; and measuring the amount of said substance permeated across the BMEC monolayer ( s ) . 8. A method of measuring blood brain barrier efflux transport of a substance, said method comprising: providing BMECs according to claim 1 in one or more confluent monolayers; contacting said substance with said confluent monolayer (s) in the presence of one or more inhibitors of the efflux pumps; and measuring the amount of said substance permeated across the BMEC monolayer ( s ) . 9. A method of measuring blood brain barrier metabolism of a substance, said method comprising: providing BMECs according to claim 1 in one or more confluent monolayers; Incubating said substance with said confluent monolayer (s) ; and measuring the amount metabolites of said substance permeated across the BMEC monolayer (s) . 10. A process of making immortalized brain microvessel endothelial cells comprising nucleic acid sequence encoding Middle-T antigen from a papovirus , said method comprising: providing a primary culture brain microvessel endothelial cells; transfecting the brain microvessel endothelial cells with a vector containing the nucleic acid sequence encoding middle-T antigen from the papovirus; incubating the vectors and cells; and isolating the immortalized brain microvessel endothelial cells having the nucleic acid sequence encoding Middle-T antigen from papovirus. 11. The process according to Claim 10 wherein the papovirus is polyoma virus. 12. The process according to Claim 11 wherein the vector is a plasmid. 13. The process according to Claim 12 wherein the brain microvessel endothelial cells are co-transfected with a plasmid containing a selectable marker. 14. The brain microvessel endothelial cell according Claim 1 wherein the cell is TBMEC, Pll having the A.T.C.C. designation number CRL-12414.

Inventors: BORMANN BARBARA J, YAZDANIAN MEHRAN, BORMANN, BARBARA, J., YAZDANIAN, MEHRAN
Applicants: BOEHRINGER INGELHEIM PHARMA, BOEHRINGER INGELHEIM PHARMACEUTICALS, INC.
Application year: 1999
Main classification: C12N5
IPC codes: C07K14, C12N5, C12N15, C12Q1, C12R1