Patent Publication Number: US-8987208-B2

Title: Botulinum neurotoxin a receptor and the use thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/014,971, filed Jan. 27, 2011, which is a continuation of U.S. patent application Ser. No. 11/546,880, filed Oct. 12, 2006 and issued as U.S. Pat. No. 7,985,554 on Jul. 26, 2011, which claims the benefit of U.S. Provisional Application No. 60/726,879, filed on Oct. 14, 2005, all of which are herein incorporated by reference in their entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under AI057153 and AI057744 awarded by the National Institutes of Health. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Botulinum neurotoxin A (BoNT/A) is one of seven botulinum neurotoxins (designated BoNT/A-G) produced by the anaerobic bacteria strain  Clostridium botulinum  (Schiavo G et al.,  Physiol. Rev.  80:717-766, 2000). BoNTs block neurotransmitter release by cleaving members of the membrane fusion machinery composed of SNAP-25, vamp-2/synaptobrevin (Syb), and syntaxin (Jahn R and Niemann H,  Ann. NY Acad. Sci.  733:245-255, 1994; Schiavo G. et al., supra, 2000). Cleavage of these proteins in motor nerve terminals blocks acetylcholine release at the neuromuscular junction (NMJ) which causes paralysis and may lead to death due to respiratory failure (Schiavo G. et al., supra, 2000; Simpson L L,  Ann. Rev. Pharmacol. Toxicol.  44:167-193, 2004). Due to extreme potency and lethality as well as ease of use and transport, BoNTs are considered one of the six most dangerous potential bioterrorism threats (designated by Center for Disease Control of United States) (Arnon S. et al.,  JAMA  285:1059-1070, 2001). According to the American Medical Society, as little as one gram of crystalline toxin is sufficient to kill one million people. 
     Currently, the standard test for BoNTs is the mouse bioassay available at the Centers for Disease Control and Prevention (CDC) and select laboratories across the country. The test involves treating mice with clinical samples suspected of carrying one of the BoNTs. The mice are immunized against the various BoNTs, and only those mice immunized against the specific BoNT present in the sample will survive. Although the test is sensitive in that it can detect as little as 0.03 ng of a BoNT, it is expensive and takes days to complete. On the treatment side, equine antitoxin containing antibodies against a BoNT is the therapy of choice and its effectiveness depends on timely treatment. This treatment, however, has all the disadvantages of a horse serum product such as the risks of anaphylaxis and serum sickness. Many times, treatment begins before botulism is confirmed as the diagnostic test takes days which is too long to wait for effective treatment. Therefore, there is a need in the art for alternative detection and treatment strategies. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is based on the identification of synaptic vessel glycoprotein SV2 as the BoNT/A receptor and the further identification of various BoNT/A-binding fragments of SV2. The disclosure here provides new tools for diagnosing and treating botulism. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The file of this patent contains at least one drawing executed in color. Copies of this patent with color drawings will be provided by the Patent and Trademark Office upon request and payment of the necessary fee. 
         FIG. 1  shows that stimulating synaptic vesicle exocytosis increases BoNT/A binding to diaphragm motor nerve terminals and cultured hippocampal neurons. Panel a: Mouse hemi-diaphragm preparations were exposed to BoNT/A (25 nM) in either resting conditions (control buffer) or stimulated conditions (High K +  buffer: 45 mM KCl). The tissue was fixed and permeabilized. Neuromuscular junctions (NMJs) were labeled with Alexa-488 conjugated α-BTX. BoNT/A was detected with a polyclonal BoNT/A antibody and a Cy3-conjugated secondary antibody. The overlay of regions indicated by rectangles was enlarged to show that BoNT/A staining mirrors α-BTX staining at individual NMJs. Right panel: BoNT/A binding to NMJs was quantified. NMJ regions (region of interest, ROI) were defined by α-BTX signals. The intensity of BoNT/A staining was normalized to α-BTX signals and the ratios were plotted on the Y-axis. Stimulation with high K +  buffer resulted in an approximate 6-fold increase in BoNT/A intensity. Error bars represent SEM (n=27−28 images). Panel b: Cultured rat hippocampal neurons were exposed to BoNT/A (10 nM) and an antibody against the luminal domain of synaptotagmin I (Syt I N  Ab; CI604.4, 1:40) for 1 min in three different buffer conditions: (1) Normal buffer (PBS), (2) High K +  (56 mM KCl), and (3) High K + /No Ca 2+  (without Ca 2+ ). Stimulating neurons with high K +  increased Syt I N  Ab immunofluorescence signals. This increase was not seen without extracellular Ca 2+ . BoNT/A signals largely co-localize with Syt I N  Ab signals. The third image column on the right shows images obtained from differential interference contrast microscopy (DIC). Panel c: Rat hippocampal neurons pretreated with BoNT/B (30 nM, 24 h) were exposed to BoNT/A (10 nM) for 10 min in High K +  buffer. Cells were fixed and immunostained for Syb II and BoNT/A. BoNT/A binding is abolished by BoNT/B treatment, which cleaves Syb II in neurons. Neurons that were not treated with BoNT/B served as controls. The third image column on the right shows images obtained from DIC microscopy. 
         FIG. 2  shows that BoNT/A binds directly to SV2 luminal domains.  FIG. 2A : Monoclonal antibodies against the synaptic vesicle proteins synaptophysin (Syp, CI7.2) and SV2 (pan-SV2), were used to co-immunoprecipitate BoNT/A (100 nM) from rat brain detergent extract. Control samples without antibodies (No Ab) were carried out in parallel. Immunoprecipitated toxin and vesicle proteins were detected by western blot. BoNT/A co-immunoprecipitated with SV2, but not Syp.  FIG. 2B : Co-immunoprecipitations of BoNT/A from mouse brain detergent extract were carried out with pan-SV2 antibodies, with or without exogenous gangliosides (mixture of bovine brain gangliosides, 0.6 mg/ml), at the indicated BoNT/A concentrations. Adding gangliosides increased the amount of BoNT/A co-immunoprecipitated by SV2 antibodies.  FIG. 2C : The 4 th  luminal domains of all three SV2 isoforms (see  FIG. 2F  for SV2 topology) were purified as GST-tagged proteins and immobilized on glutathione-sepharose beads. Pull down assay were carried out using 8 μg immobilized proteins and 100 nM toxins (BoNT/A, B or E). Bound materials were analyzed by western blot with anti-BoNT/A,B,E antibodies. BoNT/A binds directly to all SV2 isoforms with the luminal domain of SV2C showing the highest apparent affinity.  FIG. 2F : Schematic view of putative SV2 topology. Each circle represents a residue. Filled circles indicate conserved residues in all SV2A, B and C isoforms, gray circles are residues conserved in two of SV2 isoforms, and open circles represent non-conserved residues. SV2 contains 12 transmembrane domains with its N- and C-terminus facing the cytoplasm. The 4 th  luminal domain (L4) lies inside vesicles and contains three putative N-glycosylation sites, indicated. The critical region in SV2C for BoNT/A binding is indicated by arrows (see  FIG. 2E  for details).  FIG. 2D : A series of truncation mutants within SV2C-L4 were generated as GST fusion proteins and tested for BoNT/A binding. Binding assays were performed as described in  FIG. 2C  and analyzed by western blot. The critical region for BoNT/A binding was mapped to a short fragment (residues 529-566 in SV2C), which alone maintains the ability to bind BoNT/A. Immobilized GST fusion proteins were shown by Ponceau S staining to ensure that equal amount of immobilized protein were used in the assay.  FIG. 2E  The protein sequence of this region is aligned with regions of SV2A and B (all are rat sequences), with putative N-glycosylation sites indicated by asterisks. 
         FIG. 3  shows the block of BoNT/A binding and entry into hippocampal neurons and motor nerve terminals by an SV2C luminal fragment. Panel a, left panel: hippocampal neurons were exposed to BoNT/A (10 nM) and Syt I N  Ab in High K +  buffers for 10 min, with the presence of either control protein (soluble GST, 10 μM) or SV2C-L4 (soluble GST tagged SV2C-L4 fragment, 10 μM). Cells were washed and fixed. Binding and uptake of Syt I N  Ab and BoNT/A were analyzed through subsequent immunostaining SV2C-L4 did not affect Syt I N  Ab uptake into neurons, but reduced BoNT/A binding to the same neurons. Panel a, right panel: The experiment was carried out as described above except using BoNT/B instead of BoNT/A. SV2C-L4 did not affect BoNT/B binding to neurons. Panel b: Hippocampal neurons were incubated with BoNT/A (10 nM), in the presence of either GST proteins or SV2C-L4, for 10 min in High K +  buffers. Cells were washed three times and further incubated for 6 hrs in culture medium. Cells were then fixed and permeabilized. Cleavage of SNAP-25 was detected using a monoclonal antibody (anti-SNAP-25-C) that only recognizes cleaved SNAP-25 (but not intact full-length SNAP-25). SV2C-L4 prevented the cleavage of native SNAP-25 by BoNT/A. The third image column on the right shows images obtained from DIC microscopy. Panel c: Mouse hemi-diaphragm preparations were exposed to BoNT/A (10 nM) or BoNT/B (10 nM) in the presence of either GST protein or SV2C-L4 for 30 min in High K +  buffer. Tissues were washed, fixed and permeabilized. NMJs were labeled with α-BTX. BoNT/A and B were detected with their polyclonal antibodies, respectively. SV2C-L4 specifically reduced binding of BoNT/A to NMJs, while it has no effect on BoNT/B binding. Panel d: Binding of BoNT/A and B to NMJs, based on images collected in panel c, were quantified as described in  FIG. 1   a . SV2C-L4 significantly reduced BoNT/A binding (65% reduction compared to control, P&lt;0.0001, t-test, n=76-90 images), but did not affect BoNT/B binding (P&gt;0.05, t-test, n=49-55 images). Error bars represent SEM. 
         FIG. 4  shows that BoNT/A binding is abolished in SV2A and B knockout hippocampal neurons. Panel a: Hippocampal neurons from SV2B knockout (SV2B(−/−)) mice and wild-type (WT) littermate controls were cultured. Neurons were exposed to BoNT/A (15 nM) and BoNT/B (7.5 nM) in High K +  for 10 min. They were washed three times to reduce surface bound toxins, fixed and permeabilized. Immunofluorescence signals for BoNT/A and B were detected and quantified, and plotted as normalized intensity ratios (% WT signals). SV2B(−/−) neurons displayed significantly reduced BoNT/A uptake (28% reduction compared to WT, P&lt;0.0001, t-test, n=18 images). BoNT/B uptake level remained the same for SV2B(−/−) and WT neurons (P&gt;0.05, t-test, n=22 images). Error bars represent SD. Panel b: Hippocampal neurons from littermates with the following genotypes: SV2A(+/+)SV2B(−/−), SV2A(+/+)SV2B(−/−), and double knockout SV2A(−/−)SV2B(−/−) were cultured. Cultures were exposed to BoNT/A (10 nM) and BoNT/B (7.5 nM) simultaneously for 10 min. Cells were washed three times, fixed and permeabilized. Triple immunostaining was performed (BoNT/B: rabbit anti-BoNT/B; BoNT/A: human anti-BoNT/A; SV2: mouse pan-SV2). Binding of BoNT/B to neurons was not altered between different genotypes. SV2B knockout and SV2A heterozygotes (SV2A(+/−)SV2B(−/−)) showed reduced BoNT/A binding. SV2A/B double knockouts showed no binding of BoNT/A. Panel c: Images collected in panel b were thresholded to only include neurons. The average intensity (background subtracted) was plotted as normalized data (% of SV2A(+/+)). SV2A(+/−)SV2B(−/−) neurons displayed a 53% reduction compared to SV2A(+/+)SV2B(−/−), and SV2A/B double knockouts showed no binding of BoNT/A. The binding of BoNT/B remained the same for all genotypes (P&gt;0.05, t-test, n=11 images). 
         FIG. 5  shows that introducing SV2A, B or C in SV2A/B double knockout neurons rescues BoNT/A binding. Rat SV2A, B and C were subcloned into a lentiviral vector under control of a neuronal specific synapsin promoter, and were used to transfect hippocampal neurons from SV2A/B double knockout mice. Forty-eight hrs after transfection, neurons were exposed to BoNT/A (10 nM) for 10 min. Cells were washed three times, fixed, and permeabilized. Immunostaining for SV2 and BoNT/A were performed as described in  FIG. 4   b . Transfected neurons were identified by GFP expression, which is under control of a separated synapsin promoter in the vector. Expression of exogenous un-tagged SV2 isoforms were confirmed by SV2 staining, and BoNT/A selectively bound to transfected cells. The overlay images of regions indicated by white rectangles are enlarged to show the high degree of colocalization between SV2 expression and BoNT/A signals. 
         FIG. 6  shows that SV2 knockout mice have reduced BoNT/A binding at diaphragm motor nerve terminals and are more resistant to BoNT/A. Panel a: Mouse diaphragms were prepared from wild-type (WT) and SV2A(+/−)SV2B(−/−) mice. Half of the diaphragm was exposed to BoNT/A (25 nM) in stimulated conditions for 1 hr. The tissue was fixed and immunostained with α-BTX and BoNT/A antibody as described in  FIG. 1   a . The other half of the diaphragm was exposed to BoNT/B (25 nM), and immunostained in parallel. Representative images are shown. Panel b: Images collected in panel a were quantified as described in  FIG. 1   a . SV2A(+/−)SV2B(−/−) mice showed significantly less BoNT/A binding compared to WT (72% reduction, P&lt;0.001, t-test, n=47 images), while BoNT/B binding is the same (P&gt;0.05, t-test, n=20 images). Panel c: The susceptibility of SV2B(−/−) mice and their WT littermates to BoNT/A was determined by a rapid time-to-death assay. The same amount of BoNT/A was injected into each mouse, and their survival time (time-to-death) recorded. The average effective toxicity (LD 50 /ml) were estimated from time-to-death data using a standard curve. SV2B(−/−) mice live significantly longer than WT mice (43 min versus 33 min, P&lt;0.05, paired t-test). The effective toxicity of injected BoNT/A in WT mice is about 2.5-fold greater than SV2B knockout mice. 
         FIG. 7  shows that a peptide containing the BoNT/B binding site specifically inhibits BoNT/B, but not BoNT/A binding to hippocampal neurons, and SV2C-L4 does not affect BoNT/E binding. Panel a: Peptide P21 is derived from the synaptotagmin II luminal domain (Dong M et al.,  J. Cell. Biol.  162:1293-1303, 2003). P21S is a scrambled version of P21 that serves as a control (Dong M et al., supra, 2003). Cultured hippocampal neurons were exposed to BoNT/B (10 nM) and Syt I N  Ab in High K +  buffers for 10 min, in the presence of P21 (30 μM) or P21S. Cells were washed and fixed. Binding and uptake of Syt I N  Ab and BoNT/B were analyzed through subsequent immunostaining as described in  FIG. 3   a . P21 inhibited BoNT/B binding to neurons, while uptake of Syt I N  Ab is not affected. Panel b: Experiments were carried out as described in panel a with BoNT/A instead of BoNT/B. P21 peptide did not affect BoNT/A binding to hippocampal neurons. Panel c: Hippocampal neurons were exposed to BoNT/E (10 nM) and Syt I N  Ab in High K +  buffers for 10 min, in the presence of GST (10 μM) or SV2C-L4 (10 μM). Binding of BoNT/E was detected with a polyclonal anti-BoNT/E antibody. SV2C-L4 did not affect BoNT/E binding to neurons. 
         FIG. 8  shows that motor nerve terminals at mouse diaphragm express SV2A, B and C. Mouse hemi-diaphragms were excised, immediately fixed in 4% paraformaldehyde for 30 min, permeabilized, and blocked. Expression of SV2A, B or C was detected by their specific polyclonal antibodies (1:1000). NMJs were labeled with α-BTX. All SV2 isoforms were observed at NMJs, presumably at presynaptic nerve terminals. 
         FIG. 9  shows an alignment of partial sequences of rat SV2C (SEQ ID NO:6), human SV2A (SEQ ID NO:14), human SV2B (SEQ ID NO:16), human SV2C (SEQ ID NO:18), mouse SV2A (SEQ ID NO:8), mouse SV2B (SEQ ID NO:10), and mouse SV2C (SEQ ID NO:12). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is based on the identification of synaptic vessel glycoprotein SV2 as the BoNT/A receptor as well as the identification of various BoNT/A-binding fragments of SV2. The disclosure here provides new prevention and treatment strategies for BoNT/A toxicity and the botulism disease. The disclosure here also provides new tools for identifying agents that can reduce SV2-BoNT/A binding, BoNT/A cellular entry, and BoNT/A toxicity. 
     In some species (e.g., human, rat, and mouse), three SV2 isoforms, namely SV2A, SV2B, and SV2C, have been identified. Using rat SV2 as an example, the inventors found that all three isoforms are capable of binding to and serve as the receptor for BoNT/A. In other species such as bovine and electric ray ( Discopyge ommatta ), only one isoform has been identified so far. The bovine SV2 cDNA is closer to SV2A than SV2B and SV2C, and the electric ray SV2 cDNA is closer to SV2C than SV2A and SV2B. It is known in the art that the function and amino acid sequences of SV2A, SV2B, and SV2C are conserved across animal species (mammalian species in particular). At protein level, there is at least 62% identity among known SV2 proteins (human, mouse, rat, bovine, and electric ray) and at least 57% identity among the luminal domains of known SV2 proteins. For known SV2A and bovine SV proteins, the amino acid sequence identity is over 98% for the whole protein and 100% for the luminal domain. For known SV2B proteins, the amino acid sequence identity is over 94% for the whole protein and over 96% for the luminal domain. For known SV2C and electric ray SV proteins, the amino acid sequence identity is over 79% (over 96% for mammalian species) for the whole protein and over 76% (over 97% for mammalian species) for the luminal domain. The amino acid sequence identity among rat SV2A, B, and C luminal domains is 76% and the amino acid sequence identity among mouse SV2A, B, and C luminal domains is 75%. Although the disclosure here is based on the discovery made with rat SV2A, SV2B, and SV2C, it applies to all animal species including all mammalian species. For example, while certain rat SV2C fragments have been shown to be capable of binding to BoNT/A, corresponding fragments from rat SV2A, rat SV2B as well as corresponding fragments from other SV2 homologs are expected to be capable of binding to BoNT/A. Corresponding domains and fragments among all SV2 proteins can be identified using any alignment program familiar to a skilled artisan. For example, the GCG software from Accelrys (San Diego, Calif.) can be used for this purpose (e.g., the MegaAlign program with default parameters). 
     An SV2 protein typically contains 12 transmembrane domains, 7 cytoplasmic domains, and one large luminal domain (luminal domain 4, L4) (Janz R and Sudhof T C,  Neuroscience  94:1279-1290, 1999). In the case of rat SV2A, SV2B, and SV2C, the luminal domain spans from amino acid 468 to amino acid 595, amino acid 411 to amino acid 536, and amino acid 454 to amino acid 580, respectively. The inventors have determined that BoNT/A binds to an SV2 protein at its luminal domain. In particular, the inventors have demonstrated that rat SV2C luminal domain fragments amino acids 529-562 and amino acids 454-546 and various other fragments containing the above fragments are capable of binding to BoNT/A. Fragment amino acids 529-566 binds almost as efficiently as the luminal domain itself. Fragments shorter than that spanning amino acids 529-562 or 454-546 may also be able to bind to BoNT/A and a skilled artisan can readily identify these fragments by routine truncation experiments. 
     Furthermore, a peptide that is at least 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95% identical to any of the BoNT/A-binding fragments of an SV2 protein discussed above and any such binding fragments with one or more conservative substitutions are expected to be able to bind to BoNT/A. It is well known in the art that the amino acids within the same conservative group can typically substitute for one another without substantially affecting the function of a protein. For the purpose of the present invention, such conservative groups are set forth in Table 1 based on shared properties. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Conservative substitution. 
               
            
           
           
               
               
               
            
               
                   
                 Original Residue 
                 Conservative Substitution 
               
               
                   
                   
               
               
                   
                 Ala (A) 
                 Val, Leu, Ile 
               
               
                   
                 Arg (R) 
                 Lys, Gln, Asn 
               
               
                   
                 Asn (N) 
                 Gln, His, Lys, Arg 
               
               
                   
                 Asp (D) 
                 Glu 
               
               
                   
                 Cys (C) 
                 Ser 
               
               
                   
                 Gln (Q) 
                 Asn 
               
               
                   
                 Glu (E) 
                 Asp 
               
               
                   
                 His (H) 
                 Asn, Gln, Lys, Arg 
               
               
                   
                 Ile (I) 
                 Leu, Val, Met, Ala, Phe 
               
               
                   
                 Leu (L) 
                 Ile, Val, Met, Ala, Phe 
               
               
                   
                 Lys (K) 
                 Arg, Gln, Asn 
               
               
                   
                 Met (M) 
                 Leu, Phe, Ile 
               
               
                   
                 Phe (F) 
                 Leu, Val, Ile, Ala 
               
               
                   
                 Pro (P) 
                 Gly 
               
               
                   
                 Ser (S) 
                 Thr 
               
               
                   
                 Thr (T) 
                 Ser 
               
               
                   
                 Trp (W) 
                 Tyr, Phe 
               
               
                   
                 Tyr (Y) 
                 Trp, Phe, Thr, Ser 
               
               
                   
                 Val (V) 
                 Ile, Leu, Met, Phe, Ala 
               
               
                   
                   
               
            
           
         
       
     
     The cDNA and amino acid sequences for rat SV2A (cDNA sequence is set forth in SEQ ID NO:1 and amino acid sequence is set forth in SEQ ID NO:2), SV2B (cDNA sequence is set forth in SEQ ID NO:3 and amino acid sequence is set forth in SEQ ID NO:4), and SV2C (cDNA sequence is set forth in SEQ ID NO:5 and amino acid sequence is set forth in SEQ ID NO:6) can be found at GenBank Accession Nos. NM — 057210, L10362, and NM — 031593, respectively. The cDNA and amino acid sequences for mouse SV2A (cDNA sequence is set forth in SEQ ID NO:7 and amino acid sequence is set forth in SEQ ID NO:8), SV2B (cDNA sequence is set forth in SEQ ID NO:9 and amino acid sequence is set forth in SEQ ID NO:10), and SV2C (cDNA sequence is set forth in SEQ ID NO:11 and amino acid sequence is set forth in SEQ ID NO:12) can be found at GenBank Accession Nos. NM — 022030, NM — 153579, and XM — 991257, respectively. The cDNA and amino acid sequences for human SV2A (cDNA sequence is set forth in SEQ ID NO:13 and amino acid sequence is set forth in SEQ ID NO:14), SV2B (cDNA sequence is set forth in SEQ ID NO:15 and amino acid sequence is set forth in SEQ ID NO:16), and SV2C (cDNA sequence is set forth in SEQ ID NO:17 and amino acid sequence is set forth in SEQ ID NO:18) can be found at GenBank Accession Nos. NM — 014849, BC030011, and BC100827, respectively. The cDNA and amino acid sequences for bovine SV2 (cDNA sequence is set forth in SEQ ID NO:19 and amino acid sequence is set forth in SEQ ID NO:20) can be found at GenBank Accession No. NM — 173962. The cDNA and amino acid sequences for electric ray ( Discopyge ommatta ) SV2 (cDNA sequence is set forth in SEQ ID NO:21 and amino acid sequence is set forth in SEQ ID NO:22) can be found at GenBank Accession No. L23403. 
     Polypeptides, Nucleic Acids, Vectors, and Host Cells 
     The term “isolated polypeptide” or “isolated nucleic acid” used herein means a polypeptide or nucleic acid isolated from its natural environment or prepared using synthetic methods such as those known to one of ordinary skill in the art. Complete purification is not required in either case. The polypeptides and nucleic acids of the invention can be isolated and purified from normally associated material in conventional ways such that in the purified preparation the polypeptide or nucleic acid is the predominant species in the preparation. At the very least, the degree of purification is such that the extraneous material in the preparation does not interfere with use of the polypeptide or nucleic acid of the invention in the manner disclosed herein. The polypeptide or nucleic acid is preferably at least about 85% pure, more preferably at least about 95% pure and most preferably at least about 99% pure. 
     Further, an isolated nucleic acid has a structure that is not identical to that of any naturally occurring nucleic acid or to that of any fragment of a naturally occurring genomic nucleic acid spanning more than one gene. An isolated nucleic acid also includes, without limitation, (a) a nucleic acid having a sequence of a naturally occurring genomic or extrachromosomal nucleic acid molecule but which is not flanked by the coding sequences that flank the sequence in its natural position; (b) a nucleic acid incorporated into a vector or into a prokaryote or eukaryote genome such that the resulting molecule is not identical to any naturally occurring vector or genomic DNA; (c) a separate molecule such as a cDNA, a genomic fragment, a fragment produced by polymerase chain reaction (PCR), or a restriction fragment; and (d) a recombinant nucleotide sequence that is part of a hybrid gene, i.e., a gene encoding a fusion protein. Specifically excluded from this definition are nucleic acids present in mixtures of clones, e.g., as these occur in a DNA library such as a cDNA or genomic DNA library. An isolated nucleic acid can be modified or unmodified DNA or RNA, whether fully or partially single-stranded or double-stranded or even triple-stranded. A nucleic acid can be chemically or enzymatically modified and can include so-called non-standard bases such as inosine. 
     As used in this application, “percent identity” between amino acid or nucleotide sequences is synonymous with “percent homology,” which can be determined using the algorithm of Karlin and Altschul ( Proc. Natl. Acad. Sci. USA  87, 2264-2268, 1990), modified by Karlin and Altschul ( Proc. Natl. Acad. Sci. USA  90, 5873-5877, 1993), or other methods familiar to a skilled artisan. The noted algorithm is incorporated into the NBLAST and XBLAST programs of Altschul et al. ( J. Mol. Biol.  215, 403-410, 1990). BLAST nucleotide searches can be performed with the NBLAST program, score=100, wordlength=12, to obtain nucleotide sequences homologous to a polynucleotide of interest. BLAST protein searches can be performed with the XBLAST program, score=50, wordlength=3, to obtain amino acid sequences homologous to a reference polypeptide. To obtain gapped alignments for comparison purposes, Gapped BLAST can be utilized as described in Altschul et al. ( Nucleic Acids Res.  25, 3389-3402, 1997). When utilizing BLAST and Gapped BLAST programs, the default parameters of the respective programs (e.g., XBLAST and NBLAST) can be used. An example of another program for aligning two amino acid sequences (MegaAlign, GCG) is provided earlier in the specification. 
     In one aspect, the present invention relates to an isolated polypeptide containing an amino acid sequence that is at least 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95% identical to that of a BoNT/A-binding fragment of an SV2 protein over the entire length of the binding fragment or an amino acid sequence of a BoNT/A-binding fragment of an SV2 protein with one or more conservative substitutions. Preferably, the above isolated polypeptide is capable of binding to BoNT/A. Specifically excluded from the polypeptide of the present invention is one that contains a full length SV2 protein. In one embodiment, an isolated polypeptide that consists of an SV2 luminal domain or that contains an SV2 luminal domain wherein the domain is flanked at one or both ends by a non-native flanking amino acid sequence is also excluded from the present invention. Examples of BoNT/A binding fragments of SV2 proteins include but are not limited to (i) amino acids 529-562 of rat SV2C, (ii) amino acids 486 to 519 of rat SV2B, (iii) amino acids 543 to 576 of rat SV2A, (iv) a fragment of a homolog of the rat SV2C, SV2B, or SV2A wherein the fragment corresponds to amino acids 529-562 of rat SV2C, amino acids 486 to 519 of rat SV2B, or amino acids 543 to 576 of rat SV2A, respectively (see  FIG. 9  for examples), (v) amino acids 454-546 of rat SV2C, (vi) amino acids 411 to 503 of rat SV2B, (vii) amino acids 468 to 560 of rat SV2A, and (viii) a fragment of a homolog of the rat SV2C, SV2B, or SV2A wherein the fragment corresponds to amino acids 454-546 of rat SV2C, amino acids 411 to 503 of rat SV2B, or amino acids 468 to 560 of rat SV2A, respectively (see  FIG. 9  for examples). 
     Preferred BoNT/A binding fragments of SV2 proteins include but are not limited to (i) amino acids 529-566 of rat SV2C, (ii) amino acids 486 to 523 of rat SV2B, (iii) amino acids 543 to 580 of rat SV2A, (iv) a fragment of a homolog of the rat SV2C, SV2B, or SV2A wherein the fragment corresponds to amino acids 529-566 of rat SV2C, amino acids 486 to 523 of rat SV2B, or amino acids 543 to 580 of rat SV2A, respectively. Other preferred BoNT/A binding fragments include the luminal domains of SV2 proteins. 
     In one embodiment, the polypeptide of the present invention is about the size of an SV2 luminal domain or shorter. For example, the polypeptide of the present invention can be shorter than 129, 128, 127, or 126 amino acids. In another embodiment, the polypeptide of the present invention is shorter than 125, 120, 110, 100, 90, 80, 70, 60, 50, or 40 amino acids. 
     In another embodiment, the polypeptide of the present invention is soluble in an aqueous solvent (e.g., water with or without other additives). By soluble in an aqueous solvent, we mean that the polypeptide exhibits a solubility of at least 10 μg/ml, preferably at least 50 μg/ml or 100 μg/ml, more preferably at least 500 μg/ml, and most preferably at least 1,000 Ξg/ml in an aqueous solvent. Whether a polypeptide is soluble in an aqueous solution can be readily determined by a skilled artisan based on its amino acid sequence or through routine experimentation. Examples of soluble polypeptides of the present invention include those that contain all or part of the luminal domain of an SV2 protein but lack at least part of and preferably the entire adjacent transmembrane domain(s). Soluble polypeptides are typically more suitable than insoluble polypeptides for intravenous administration. 
     The isolated polypeptide of the invention can include one or more amino acids at either or both N-terminal and C-terminal ends of a BoNT/A-binding sequence of an SV2 protein, where the additional amino acid(s) do not materially affect the BoNT/A binding function. Any additional amino acids can, but need not, have advantageous use in purifying, detecting, or stabilizing the polypeptide. 
     In order to improve the stability and/or binding properties of a polypeptide, the molecule can be modified by the incorporation of non-natural amino acids and/or non-natural chemical linkages between the amino acids. Such molecules are called peptidomimics (H. U. Saragovi et al.,  Bio/Technology  10:773-778, 1992; S. Chen et al.,  Proc. Nat&#39;l. Acad. Sci. USA  89:5872-5876, 1992). The production of such compounds is restricted to chemical synthesis. It is understood that a polypeptide of the present invention can be modified into peptidomimics without abolishing its function. This can be readily achieved by a skilled artisan. 
     In another aspect, the present invention relates to an isolated nucleic acid containing a coding polynucleotide or its complement wherein the coding polynucleotide has an uninterrupted coding sequence that encodes a polypeptide of the invention as set forth above. A nucleic acid containing a polynucleotide that can hybridize to the coding polynucleotide or its complement, under either stringent or moderately stringent hybridization conditions, is useful for detecting the coding polypeptide and thus is within the scope of the present invention. Stringent hybridization conditions are defined as hybridizing at 68° C. in 5×SSC/5×Denhardt&#39;s solution/1.0% SDS, and washing in 0.2×SSC/0.1% SDS+/−100 μg/ml denatured salmon sperm DNA at room temperature, and moderately stringent hybridization conditions are defined as washing in the same buffer at 42° C. Additional guidance regarding such conditions is readily available in the art, for example, by Sambrook et al., 1989 , Molecular Cloning, A Laboratory Manual , Cold Spring Harbor Press, N.Y.; and Ausubel et al. (eds.), 1995 , Current Protocols in Molecular Biology , (John Wiley &amp; Sons, N.Y.) at Unit 2.10. A nucleic acid containing a polynucleotide that is at least 80%, 85%, 90%, or 95% identical to the coding polynucleotide or its complement over the entire length of the coding polynucleotide can also be used as a probe for detecting the coding polynucleotide and is thus within the scope of the present invention. Specifically excluded from the present invention is a nucleic acid that contains a nucleotide sequence encoding a full length SV2 protein. In one embodiment, a nucleic acid that consists of a polynucleotide that encodes an SV2 luminal domain and a nucleic acid that comprises a polynucleotide that encodes a polypeptide having an SV2 luminal domain wherein the domain is flanked at one or both ends by a non-native amino acid sequence are excluded. 
     In a related aspect, any nucleic acid of the present invention described above can be provided in a vector in a manner known to those skilled in the art. The vector can be a cloning vector or an expression vector. In an expression vector, the polypeptide-encoding polynucleotide is under the transcriptional control of one or more non-native expression control sequences which can include a promoter not natively found adjacent to the polynucleotide such that the encoded polypeptide can be produced when the vector is provided in a compatible host cell or in a cell-free transcription and translation system. Such cell-based and cell-free systems are well known to a skilled artisan. Cells comprising a vector containing a nucleic acid of the invention are themselves within the scope of the present invention. Also within the scope of the present invention is a host cell having the nucleic acid of the present invention integrated into its genome at a non-native site. 
     Ligand-Polypeptide Complexes 
     In another aspect, the present invention relates to a complex of a ligand and a polypeptide, wherein the polypeptide comprises a member to which the ligand binds, the member being selected from (i) amino acids 529-562 of rat SV2C, (ii) amino acids 486 to 519 of rat SV2B, (iii) amino acids 543 to 576 of rat SV2A, (iv) a fragment of a homolog of the rat SV2C, SV2B, or SV2A wherein the fragment corresponds to amino acids 529-562 of rat SV2C, amino acids 486 to 519 of rat SV2B, or amino acids 543 to 576 of rat SV2A, respectively, (v) amino acids 454-546 of rat SV2C, (vi) amino acids 411 to 503 of rat SV2B, (vii) amino acids 468 to 560 of rat SV2A, (viii) a fragment of a homolog of the rat SV2C, SV2B, or SV2A wherein the fragment corresponds to amino acids 454-546 of rat SV2C, amino acids 411 to 503 of rat SV2B, or amino acids 468 to 560 of rat SV2A, respectively, (ix) an amino acid sequence that is at least 70% identical to any of the amino acid sequences in (i) to (viii) and is capable of binding to BoNT/A, and (x) an amino acid sequence from (i) to (viii) with conservative substitutions and is capable of binding to BoNT/A, with the proviso that where the polypeptide is a full length SV2 protein, the ligand is not a botulinum toxin. The complexes disclosed herein include both those formed in vitro and in vivo. 
     In one embodiment, the polypeptide in the complex is a full length SV2 protein. 
     In another embodiment, the polypeptide in the complex is one of the BoNT/A-binding polypeptides of the present invention provided in the section of “polypeptides, polynucleotides, vectors, and host cells.” 
     In a preferred embodiment, the polypeptide in the complex comprises a member selected from (i) amino acids 529-566 of rat SV2C, (ii) amino acids 486 to 523 of rat SV2B, (iii) amino acids 543 to 580 of rat SV2A, (iv) a fragment of a homolog of the rat SV2C, SV2B, or SV2A wherein the fragment corresponds to amino acids 529-566 of rat SV2C, amino acids 486 to 523 of rat SV2B, or amino acids 543 to 580 of rat SV2A, respectively, (v) amino acids 454-546 of rat SV2C, (vi) amino acids 411 to 503 of rat SV2B, (vii) amino acids 468 to 560 of rat SV2A, and (viii) a fragment of a homolog of the rat SV2C, SV2B, or SV2A wherein the fragment corresponds to amino acids 454-546 of rat SV2C, amino acids 411 to 503 of rat SV2B, or amino acids 468 to 560 of rat SV2A, respectively. 
     The polypeptide in the complex may be a synthetic or recombinant peptide and it may contain an affinity tag and/or a ganglioside binding site. 
     In one embodiment, the ligand in the complex is an antibody against the polypeptide or a BoNT/A fragment that binds to the polypeptide. Such an antibody and BoNT/A fragment can reduces the binding between the polypeptide and BoNT/A. 
     Methods for Reducing BoNT/A Neuro-Toxicity 
     In another aspect, the present invention relates to a method for reducing BoNT/A cellular toxicity in target cells such as neurons. As a result, botulism disease can be prevented or treated. In one embodiment, the method is used to reduce BoNT/A toxicity in a human or non-human animal by administering to the human or non-human animal an agent that can reduce BoNT/A toxicity. 
     The term “reducing BoNT/A cellular toxicity” encompasses any level of reduction in BoNT/A toxicity. The BoNT/A toxicity can be reduced by reducing the level of an SV2 protein in target cells, by inhibiting BoNT/A-related cellular functions of an SV2 protein in target cells, or by reducing the binding between BoNT/A and an SV2 protein located on the cellular surface of target cells. The binding between BoNT/A and an SV2 protein can be reduced by either blocking the binding directly or by reducing the amount of SV2 proteins available for binding. 
     There are many methods by which cellular protein levels such as the level of an SV2 protein can be reduced. The present invention is not limited to a particular method in this regard. As an example, the cellular level of an SV2 protein can be reduced by using the antisense technology. For instance, a 20-25mer antisense oligonucleotide directed against the 5′ end of an SV2 mRNA can be generated. Phosphorothioate derivatives can be employed on the last three base pairs on the 3′ and 5′ ends of the antisense oligonucleotide to enhance its half-life and stability. A carrier such as a cationic liposome can be employed to deliver the antisense oligonucleotide. In this regard, the oligonucleotide is mixed with the cationic liposome prepared by mixing 1-alpha dioleylphatidylcelthanolamine with dimethldioctadecylammonium bromide in a ratio of 5:2 in 1 ml of chloroform. The solvent is evaporated and the lipids resuspended by sonication in 10 ml of saline. Another way to use an antisense oligonucleotide is to engineer it into a vector so that the vector can produce an antisense cRNA that blocks the translation of an SV2 mRNA. Similarly, RNAi techniques, which are now being applied to mammalian systems, are also suited for inhibiting the expression of an SV2 protein. (See Zamore,  Nat. Struct. Biol.  8:746-750, 2001, incorporated herein by reference as if set forth in its entirety). 
     Dominant Negative SV2 
     In another aspect, the present invention relates to identifying a dominant negative SV2 that can negate the effects of BoNT/A on cells that express the corresponding wild-type SV2. A dominant negative SV2 can be identified by introducing a mutation into a wild-type SV2 gene, expressing the mutated SV2 and the wild-type SV2 in the same host cell and determining the effect of the mutated SV2 on parameters that relate to BoNT/A toxicity, which include but are not limited to susceptibility of the host cell to BoNT/A, integration of newly formed SV2 into the host cell membrane, binding of wild-type SV2 to BoNT/A, and uptake of BoNT/A into the cells. The wild-type SV2 expressed in the host cell can be the endogenous SV2 or an SV2 introduced into the host cell. Any dominant negative SV2 identified is within the scope of the present invention. The identified dominant negative SV2 can be used to negate the effect of BoNT/A. 
     Blocking the Binding Between BoNT/A and SV2 
     The identification of SV2 as the BoNT/A receptor as well as the BoNT/A-binding sequences on SV2 enable those skilled in the art to block the binding between BoNT/A and its receptor through many strategies available in the art. One strategy involves the use of monoclonal and polyclonal antibodies specific for the BoNT/A-binding sequences on SV2. It is well within the capability of a skilled artisan to generate such monoclonal and polyclonal antibodies. The antibodies so generated are within the scope of the present invention. 
     Another strategy involves the use of a BoNT/A-binding polypeptide, preferably a soluble BoNT/A-binding polypeptide, to compete with the receptor for BoNT/A binding. For example, the BoNT/A-binding polypeptide of the present invention described above in the section of “polypeptides, polynucleotides, vectors, and host cells” can be employed for this purpose. Other polypeptides that can be employed include those that comprise a full length SV2 protein, those that consist of an SV2 luminal domain, and those that comprise an SV2 luminal domain wherein the domain is flanked at one or both ends by a non-native flanking amino acid sequence. 
     To block the binding between BoNT/A and its receptor in an animal (human or non-human), a BoNT/A-binding polypeptide from both the same and a different species can be used. The polypeptide can be introduced into the animal by administering the polypeptide directly or by administering a vector that can express the polypeptide in the animal. 
     Those skilled in the art understand that mutations such as substitutions, insertions and deletions can be introduced into a BoNT/A-binding sequence of an SV2 protein without abolishing their BoNT/A binding activity. Some mutations may even enhance the binding activity. A polypeptide containing such modifications can be used in the method of the present invention. Such polypeptides can be identified by using the screening methods described below. 
     In addition, as gangliosides may promote formation of stable BoNT/A-SV2 complexes, the binding between BoNT/A and an SV2 protein may be reduced through reducing the binding between the gangliosides and the SV2 protein or through reducing the amount of gangliosides available for binding to the SV2 protein. In a related aspect, when a BoNT/A-binding polypeptide is used for reducing BoNT/A toxicity by forming a complex with BoNT/A, gangliosides may be included to facilitate the formation of the complex. 
     Identifying Agents that can Block Binding Between BoNT/A and SV2 
     Agents that can block binding between BoNT/A and SV2 can be screened by employing BoNT/A and a polypeptide that contains a BoNT/A-binding sequence of an SV2 protein under the conditions suitable for BoNT/A to bind the polypeptide. Gangliosides are optionally included in the reaction mixture. The binding between BoNT/A and the polypeptide can be measured in the presence of a test agent and compared to that of a control that is not exposed to the test agent. A lower than control binding in the test group indicates that the agent can block binding between BoNT/A and the SV2 protein. Other BoNT/A-binding polypeptides that can be employed in the method include those of the present invention as described above in the section of “polypeptides, polynucleotides, vectors, and host cells.” 
     There are many systems with which a skilled artisan is familiar for assaying the binding between BoNT/A and a BoNT/A-binding polypeptide. Any of these systems can be used in the screening method. Detailed experimental conditions can be readily determined by a skilled artisan. For example, the binding between BoNT/A and the polypeptide described above can be measured in vitro (cell free system). A cell culture system in which an SV2 protein is expressed and translocated onto the cellular membrane can also be used. For the cell culture system, in addition to the binding between BoNT/A and the SV2 protein, the cellular entry of BoNT/A and a number of other parameters can also be used as an indicator of binding between BoNT/A and SV2. 
     Any method known to one of ordinary skill in the art for measuring protein-protein interaction can be used to measure the binding between BoNT/A and a BoNT/A-binding polypeptide. Coimmunoprecipitation and affinity column isolation are two commonly used methods. 
     Surface plasmon resonance (SPR) is another commonly used method. SPR uses changes in refractive index to quantify binding and dissociation of macromolecules to ligands covalently linked onto a thin gold chip within a micro flow cell. This technique has been used to study protein-protein interactions in many systems, including the interactions of PA63 with EF and LF (Elliott, J. L. et al.,  Biochemistry  39:6706-6713, 2000). It provides high sensitivity and accuracy and the ability to observe binding and release in real time. Besides the equilibrium dissociation constant (Kd), on- and off-rate constants (ka and kd) may also be obtained. Typically, a protein to be studied is covalently tethered to a carboxymethyl dextran matrix bonded to the gold chip. Binding of a proteinaceous ligand to the immobilized protein results in a change in refractive index of the dextran/protein layer, and this is quantified by SPR. A BIAcore 2000 instrument (Pharmacia Biotech) can be used for these measurements. 
     For the cell culture system, the binding of BoNT/A to a BoNT/A-binding polypeptide can be assayed by staining the cells, the examples of which are described in the example section below. 
     Identifying Agents that can Bind to a BoNT/A-Binding Sequence of SV2 
     Agents that can bind to a BoNT/A-binding sequence of an SV2 protein can be used to block the binding between BoNT/A and the SV2 protein. Such agents can be identified by providing a polypeptide that contains a BoNT/A-binding sequence of an SV2 protein to a test agent, and determining whether the agent binds to the BoNT/A-binding sequence. Other BoNT/A-binding polypeptides that can be employed in the method include those of the present invention as described above in the section of “polypeptides, polynucleotides, vectors, and host cells.” Any agent identified by the method can be further tested for the ability to block BoNT/A entry into cells or to neutralize BoNT/A toxicity. A skilled artisan is familiar with the suitable systems that can be used for the further testing. Examples of such systems are provided in the example section below. 
     The skilled artisan is familiar with many systems in the art for assaying the binding between a polypeptide and an agent. Any of these systems can be used in the method of the present invention. Detailed experimental conditions can be readily determined by a skilled artisan. For example, a polypeptide that contains a BoNT/A-binding sequence of an SV2 protein can be provided on a suitable substrate and exposed to a test agent. The binding of the agent to the polypeptide can be detected either by the loss of ability of the polypeptide to bind to an antibody or by the labeling of the polypeptide if the agent is radioactively, fluorescently, or otherwise labeled. In another example, a polypeptide that contains a BoNT/A-binding sequence of an SV2 protein can be expressed in a host cell, and the cell is then exposed to a test agent. Next, the polypeptide can be isolated, e.g., by immunoprecipitation or electrophoresis, and the binding between the polypeptide and the agent can be determined. As mentioned above, one way to determine the binding between the polypeptide and the agent is to label the agent so that the polypeptide that binds to the agent becomes labeled upon binding. If the test agent is a polypeptide, examples of specific techniques for assaying protein/protein binding as described above can also be used. It should be noted that when a BoNT/A-binding sequence of an SV2 protein used in the screening assay have flanking sequences, it may be necessary to confirm that an agent binds to the BoNT/A-binding sequence rather than the flanking sequences, which can be readily accomplished by a skilled artisan. 
     Agents that can be Screened 
     The agents screened in the above screening methods can be, for example, a high molecular weight molecule such as a polypeptide (including, e.g., a polypeptide containing a modified BoNT/A-binding sequence of an SV2 protein, or a monoclonal or polyclonal antibody against a BoNT/A-binding sequence of an SV2 protein), a polysaccharide, a lipid, a nucleic acid, a low molecular weight organic or inorganic molecule, or the like. 
     Batteries of agents for screening are commercially available in the form of various chemical libraries including peptide libraries. Examples of such libraries include those from ASINEX (i.e. the Combined Wisdom Library of 24,000 manually synthesized organic molecules) and CHEMBRIDGE CORPORATION (i.e. the DIVERSet™ library of 50,000 manually synthesized chemical compounds; the SCREEN-Set™ library of 24,000 manually synthesized chemical compounds; the CNS-Set™ library of 11,000 compounds; the Cherry-Pick™ library of up to 300,000 compounds) and linear library, multimeric library and cyclic library (Tecnogen (Italy)). Once an agent with desired activity is identified, a library of derivatives of that agent can be screened for better molecules. Phage display is also a suitable approach for finding novel inhibitors of the interaction between BoNT/A and SV2. 
     Methods of detecting BoNT/A or  Clostridium botulinum    
     In another aspect, the present invention relates to a method of detecting BoNT/A or  Clostridium botulinum . The method involves exposing a sample suspected of containing BoNT/A to an agent that contains a polypeptide having a BoNT/A-binding sequence of an SV2 protein, and detecting binding of the polypeptide to BoNT/A. Other BoNT/A-binding polypeptides that can be employed in the method include those of the present invention as described above in the section of “polypeptides, polynucleotides, vectors, and host cells.” 
     Methods for Identifying Polypeptides that can Bind to BoNT/A 
     In another aspect, the present invention relates to a method for identifying polypeptides that can bind to BoNT/A. The method involves providing a polypeptide that comprises a BoNT/A-binding sequence of an SV2 protein, modifying the polypeptide at the BoNT/A-binding sequence, and determining whether the modified polypeptide can bind to BoNT/A. 
     Kits 
     Any product of the invention described herein can be combined with one or more other reagent, buffer or the like in the form of a kit (e.g., a diagnosis, prevention, or treatment kit) in accord with the understanding of a skilled artisan. 
     The invention will be more fully understood upon consideration of the following non-limiting example. 
     EXAMPLE 
     In this example, we demonstrate that BoNT/A binds to all three SV2 isoforms (SV2A, SV2B, and SV2C). Particular binding fragments such as amino acids 529-562, 529-566, and 454-546 of the rat SV2C were also identified. Recombinant SV2 fragments inhibit BoNT/A binding to hippocampal neurons and motor nerve terminals. Significantly, BoNT/A binding to hippocampal neurons was abolished in SV2A/B knockout mice and this binding can be restored by transfecting neurons with SV2. Consistently, BoNT/A binding was reduced at diaphragm motor nerve terminals in SV2 knockout mice, and SV2B knockout mice displayed reduced sensitivity to BoNT/A. These data establish SV2 as the protein receptor for BoNT/A, which mediates toxin entry through synaptic vesicle recycling. 
     Materials and Methods 
     Materials, Antibodies and SV2 Knockout Mouse Lines: 
     Alexa 488-conjugated αt-BTX was purchased from Molecular Probes, Inc. (OR). A mAb that recognizes SNAP-25 after it has been cleaved by BoNT/A (anti-SNAP-25-C) was purchased from Research &amp; Diagnostic Antibodies, Inc. (CA). mAbs directed against SV2 (pan-SV2), Syp (Cl 7.2), Syb II (Cl 69.1) and Syt I (Syt I N  Ab, Cl 604.4) were generously provided by R. Jahn (Max-Planck-Institute for Biophysical Chemistry, Gottingen, Germany). A human antibody directed against BoNT/A (RAZ-1) was generously provided by J. Marks (University of California—San Francisco, Calif.). Cy2, Cy3, Cy5, Alexa 546 and Alexa 647 conjugated secondary antibodies were purchased from Jackson Laboratories (ME) and Molecular Probes, Inc. Rabbit polyclonal anti-BoNT/A, B and E antibodies and anti-SV2A, B and C antibodies were described in Dong M et al.,  J. Cell. Biol.  162:1293-1303, 2003; and Janz R and Sudhof T C,  Neuroscience  94:1279-1290, 1999, both are herein incorporated by reference in their entirety). BoNT/A, B and E were purified as described in Malizio C G, Methods and Protocols, O. Holst, ed. (Humana Press), pp. 27-39, 2000, which is incorporated by reference in its entirety. A mixture of bovine brain gangliosides was purchased from Matreya LLC (PA). The SV2 knockout mouse lines used in this study were described in Janz R et al.,  Neuron  24:1003-1016, 1999, which is incorporated by reference in its entirety. Mice were genotyped by PCR as described in Janz R et al., supra, 1999. 
     cDNA, Constructs and Transfection: 
     Rat SV2A, B and C cDNAs were described in Bajjalieh S M et al.  Science  257: 1271-3, 1992; Feany M B et al.  Cell  70: 861-7, 1992; Bajjalieh S M et al.  Proc Natl Acad Sci USA  90: 2150-4, 1993; and Janz R &amp; Sudhof T C  Neuroscience  94: 1279-90, 1999, all of which are herein incorporated by reference in their entirety. Various SV2 luminal domain fragments were generated by PCR, subcloned into pGEX-2T and purified as GST fusion proteins (Lewis J L et al.  J Biol Chem  276: 15458-65, 2001). GST and GST tagged SV2C-L4 proteins were also purified using magnetic GST beads according to the manufacturers protocol (Promega, Wis.), eluted with 40 mM Glutathione (Sigma), and subsequently dialyzed to produce high concentrations of soluble protein. 
     To transfect hippocampal neurons with SV2 isoforms, full length SV2A, B and C were subcloned into the Lox-Syn-Syn lentivirus vector (provided by P. Scheiffele, Columbia University, NY). This vector is a modified version of pFUGW (Lois C et al.,  Science  295:868-872, 2002) and contains separate neuronal-specific (synapsin) promoters. One promoter controls the expression of SV2 isoforms inserted between BamHI and NotI sites and the other promoter controls expression of EGFP to detect transfected cells. Transfections were performed on neurons 7-10 DIV using Lipofectamine 2000 (Invitrogen) as described in Dean C et al.,  Nat. Neurosci.  6:708-716, 2003 (incorporated by reference in its entirety) and analyzed 48 hrs later. Note: The BamHI site inside the SV2C sequence has been mutated (GGATCC to GGATAC, preserving the amino acid sequence) to simplify subcloning. 
     Neuronal Cell Cultures, BoNT Uptake, Immunocytochemistry: 
     Cultures of hippocampal neurons were prepared from E18-19 rats, and SV2 knockout mouse neuron cultures were prepared from P1 mice. Neurons were plated on poly-D-lysine coated glass coverslips (12 mm) at a density of 50,000/cm 2  and cultured in Neurobasal medium supplemented with B-27 (2%) and Glutamax (2 mM). Experiments were carried out on neurons 10-14 days old. 
     To assay for BoNT/A uptake under different conditions ( FIG. 1   b ), hippocampal neurons were incubated in one of the following assay buffers (200 μl) containing BoNT/A (10 nM) and Syt I N  Ab (604.4, 1:40) for 1 min. These buffers are: control buffer (PBS: 140 mM NaCl, 3 mM KCl, 1.5 mM KH 2 PO 4 , 8 mM Na 2 HPO 4 , 1 mM CaCl 2 , 0.5 mM MgCl 2 ), high K +  (same as control buffer but adjusted to 56 mM KCl and 87 mM NaCl), and high K + /No Ca 2+  buffer (same as high K +  buffer but lacking CaCl 2 ). Neurons were then washed in PBS (3×500 μl) and fixed in 4% paraformaldehyde for 15 min. After permeabilization with 0.3% Triton X-100, neurons were blocked with 10% goat serum and stained with a polyclonal anti-BoNT/A antibody (1:200) for 1 hr at room temperature. The secondary antibodies were Cy2-conjugated goat-anti-mouse and Cy3-conjugated goat-anti-rabbit. Immunofluorescence images were acquired using a confocal microscope (Olympus FV1000, 60× water-immersion objective). Identical gain and laser settings were used for images that were directly compared in the figures. For all experiments using hippocampal neurons after  FIG. 1   b , neurons were incubated in high K +  buffer for 10 min in order to increase the amount of toxin entry. 
     For triple staining of BoNT/A, BoNT/B and SV2 ( FIG. 4   b ), BoNT/B was detected with a rabbit polyclonal antibody (1:200) and a Cy2-conjugated secondary antibody; BoNT/A was detected with a human antibody (RAZ-1, 1:300) and Alexa-546 conjugated secondary antibody; and SV2 expression was detected with a mouse monoclonal antibody (pan-SV2 Ab, 1:400), and Alexa-647 conjugated secondary antibody. 
     To detect SNAP-25 that has been cleaved by BoNT/A ( FIG. 3   b ), neurons were washed three times after exposure to BoNT/A, and further incubated in culture media for 6 hrs. Cells were then fixed, permeabilized, and stained with the anti-SNAP-25-C monoclonal antibody (1:50) and the rabbit anti-BoNT/A antibody. 
     ImageJ software (NIH) was used to quantify fluorescence intensities shown in  FIG. 4 . Briefly, a fixed threshold was first chosen for each channel (BoNT/B and BoNT/A) to exclude background signals from regions lacking neurons, and the average intensity of the fluorescence signals from decorated neurons was measured. Neurons that were not exposed to BoNT/A and B were fixed and stained with the same antibodies in parallel. The average intensity of these images was subtracted from samples treated with the toxins. Two-tailed t tests were used to determine statistical significance. 
     Co-Immunoprecipitation and Pull-Down Assays: 
     Rat or mice brain detergent extracts were made as described in Lewis J L et al.,  J. Biol. Chem.  276:15458-15465, 2001, which is incorporated by reference in its entirety. BoNT/A was premixed with brain extracts (400 μl, 3-6 mg/ml) for 1 hr at 4° C. before adding antibodies (5 μl), and then further incubated for 1 hr. Protein G Fast Flow beads (40 μl, Amersham Biosciences) were added and incubated for 1 hr. Beads were washed three times in TBS (20 mM Tris, 150 mM NaCl, pH 7.4) plus 0.5% Triton X-100. Bound material was subjected to SDS-PAGE and western blot analysis. 
     Recombinant GST fusion proteins were purified and immobilized on glutathione-Sepharose beads. Pull down assays were carried out as described in Dong M et al. (supra, 2003), using 8 μg immobilized proteins and 100 nM toxins in 100 μl TBS plus 0.5% Triton X-100. Bound material was subjected to SDS-PAGE and western blot analysis. 
     Mouse Hemi-Diaphragm Experiments: 
     Mouse hemi-diaphragms were kept in either control buffer ( FIG. 1   a , mammalian Ringer, in mM: NaCl 138.8, KCl 4, NaHCO 3  12, KH 2 PO 4  1, MgCl 2  2, CaCl 2  2, and glucose 11), or high K +  buffer (same as control buffer but adjusted to 98 mM NaCl and 45 mM KCl), warmed to 37° C. and gased with 95% O 2 /5% CO 2 . Hemi-diaphragms were incubated with the indicated BoNTs (25 nM) for 1 hr at room temperature (note: 10 nM BoNT/A was used and incubated for only 30 min in  FIG. 3   c ). After incubation, diaphragms were washed and fixed with 4% paraformaldehyde for 30 min at room temperature, permeabilized and blocked in 5% goat serum plus 0.5% Triton X-100. Diaphragms were incubated with Alexa-488-conjugated α-BTX (1:250) and rabbit anti-BoNT/A or B antibodies (1:1000) at 4° C., overnight. A Cy3-conjugated anti-rabbit secondary antibody was used at a dilution of 1:800. For staining SV2A, B or C in the NMJ ( FIG. 8 ), the hemi-diaphragms were excised and immediately fixed. A 1:1000 dilution of the specific rabbit anti-SV2A, B or C antibodies were used. All images were collected using a confocal microscope (Olympus FV1000, 60× water-immersion objective). 
     To quantify fluorescent signals, the α-BTX channel was pseudo-colored green and BoNT/A (or BoNT/B) channel was pseudo-colored red. Merged green and red images were imported into MetaMorph software (Improvision). The regions of interests (ROI) marking NMJs were determined by thresholding the α-BTX green channel. The same threshold values were used throughout the diaphragm experiments. The average intensity of green and red channels within ROIs were measured and the ratio between them used to determine the level of toxin binding. Two-tailed t tests were used to determine statistical significance between pairwise sets of data. 
     Rapid BoNT Toxicity Assay in Mice: 
     BoNT/A effective toxicity in mice was estimated using the intravenous method described in Boroff D A and Fleck U,  J. Bacteriol.  92:1580-1581, 1966 (incorporated by reference in its entirety); Dong M et al., supra, 2003; and Malizio C G, supra, 2000. Briefly, BoNT/A (type A1) isolated from Hall strain was diluted to 10 μg/ml in 30 mM sodium phosphate buffer (pH 6.3 plus 0.2% gelatin). Each mouse was injected intravenously (lateral tail vein) with 0.1 ml of the diluted toxin and their time-to-death was recorded. The time-to-death values were converted to intraperitoneal LD 50 /ml using a standard curve described in Malizio C G, supra, 2000. SV2B knockout mice used in these experiments had been crossed for 6 generation against C57B16/J mice. 
     Results 
     The BoNT/A Receptor Resides on Synaptic Vesicles: 
     The physiological target for BoNT/A is peripheral motor nerve terminals (Dolly J O et al.,  Nature  307:457-460, 1984). Stimulation of neuronal activity (i.e. neurotransmitter release) can accelerate the rate of paralysis caused by BoNT/A (Hughes R W,  J. Physiol . (Lond.) 160:221-233, 1962). However, it is not known whether synaptic vesicle exocytosis directly increases BoNT/A binding and entry into neurons. To address this question, we visualized BoNT/A binding to motor nerve terminals in mouse diaphragm preparations. The neuromuscular junctions (NMJs) in this tissue were labeled with α-bungarotoxin (α-BTX), which binds to postsynaptic acetylcholine receptors (Astrow S H et al.,  J. Neurosci.  12:1602-1615, 1992). As shown in  FIG. 1   a , high K +  solution (45 mM KCl), which triggers synaptic vesicle exocytosis, increased BoNT/A binding to NMJs by approximately 6-fold compared to control conditions, indicating that synaptic vesicle exocytosis exposes BoNT/A receptors. 
     To further analyze whether the BoNT/A receptor is on synaptic vesicles, we used cultured rat hippocampal neurons as a model system. Synaptic vesicle recycling was monitored through the uptake of an antibody that recognizes the luminal domain of synaptotagmin I (Syt I N  Ab) (Dong M et al.,  J. Cell. Biol.  162:1293-1303, 2003), an abundant synaptic vesicle membrane protein. As shown in  FIG. 1   b , uptake of Syt I N  Ab was greatly increased by a short stimulation with high K +  (56 mM KCl, 1 min), and inhibited by depletion of extracellular Ca 2+ . Interestingly, uptake of BoNT/A to the same neurons mimicked Syt I N  Ab behavior and largely co-localize with Syt I N  Ab signals ( FIG. 1   b ). To further confirm this finding, we pretreated these neurons with BoNT/B, which specifically blocks synaptic vesicle exocytosis by cleaving Syb II, a synaptic vesicle membrane protein required for exocytosis (Schiavo G et al.,  Nature  359:832-835, 1992). As shown in  FIG. 1   c , BoNT/B treatment abolished uptake of BoNT/A under stimulated conditions, indicating that the BoNT/A receptor resides on vesicles containing Syb II in neurons. Together, these evidences suggest that the receptor for BoNT/A is on synaptic vesicles. 
     BoNT/A Binds to the Luminal Domain of SV2: 
     Synaptic vesicles are well-studied organelles, and most, if not all, integral synaptic vesicle proteins have been identified (Fernandez-Chacon R and Sudhof T C  Ann. Rev. Physiol.  61:753-776, 1999). We screened known synaptic vesicle membrane proteins for BoNT/A interactions by using their specific antibodies to co-immunoprecipitate BoNT/A from rat brain detergent extracts. As shown in  FIG. 2   a , an SV2 monoclonal antibody (pan-SV2) was able to immunoprecipitate BoNT/A. This interaction is specific since an antibody against synaptophysin (Syp), another abundant vesicle protein, failed to pull down significant amounts of BoNT/A ( FIG. 2   a ). 
     We next assessed whether BoNT/A-SV2 interactions are affected by gangliosides. We increased ganglioside concentration in brain detergent extracts by adding exogenous gangliosides. Immunoprecipitations were performed at various BoNT/A concentrations. As indicated in  FIG. 2   b , adding exogenous gangliosides increased the level of co-immunoprecipitation of BoNT/A. This effect was not significant at high BoNT/A concentration (100 nM), but became more apparent at low toxin concentration (e.g., 20 nM), indicating gangliosides may promote formation of stable BoNT/A-SV2 complexes at low toxin concentration. 
     SV2 is a conserved membrane protein on synaptic vesicles and endocrine secretory vesicles in vertebrates (Buckley K and Kelly R B,  J. Cell. Biol.  100:1284-1294, 1985; Lowe A W et al.,  J. Cell. Biol.  106:51-59, 1988). Three highly homologous isoforms have been identified, denoted as SV2A, B and C (Bajjalieh S M et al.,  Proc. Natl. Acad. Sci. USA  90:2150-2154, 1993; Bajjalieh S M et al.,  Science  257:1271-1273, 1992; Feany M B et al.,  Cell  70:861-867, 1992; Janz R and Sudhof T C,  Neuroscience  94:1279-1290, 1999). SV2A and B are widely expressed throughout the brain, while the expression of SV2C is more restricted to evolutionarily older brain regions (Bajjalieh S M et al.,  J. Neurosci.  14:5223-5235, 1994; Janz R and Sudhof T C, supra, 1999). The antibody used in  FIG. 2   a  recognizes all three isoforms (Lowe A W et al., supra, 1988). As depicted in  FIG. 2   d , SV2 isoforms share a similar topology, containing 12 putative transmembrane domains and only one relatively large luminal domain (luminal domain 4, L4) (Janz R and Sudhof T C, supra, 1999). Because the luminal domain of SV2 is the only region exposed to the outside of cells after vesicle exocytosis, we first examined whether BoNT/A binding is mediated by SV2 luminal domains. The major luminal domain (L4) of SV2A, B and C were purified as GST fusion proteins, immobilized on beads and tested for their ability to pull down BoNT/A, B and E from solution. As shown in  FIG. 2   c , we observed direct binding of BoNT/A, but not BoNT/B or E, to all SV2 isoforms. SV2C showed the most robust binding, and SV2B pulled down the least amount of BoNT/A. 
     To determine the critical BoNT/A binding region, we made a series of truncation mutants within the SV2C-L4 region. As shown in  FIG. 2   e , a short fragment (amino acids 529-566) was able to pull down similar levels of BoNT/A to the full L4 region. Sequence alignment showed that this region, indicated in  FIG. 2   d  by arrows, is relatively conserved among SV2 isoforms and includes two putative N-glycosylation sites. It was further observed that a shorter fragment, amino acids 529-562, was also able to pull down BoNT/A, although not as effectively as the 529-566 fragment. In addition, various fragments containing amino acids 454-546 were also able to pull down BoNT/A. 
     SV2C Luminal Fragments Inhibit BoNT/A Binding to Neurons: 
     Among the three SV2 isoforms, hippocampal neurons were found to express SV2A and B, but not SV2C (Bajjalieh S M et al., supra, 1994; Janz R and Sudhof T C, supra, 1999). To determine whether SV2 mediates BoNT/A binding in these neurons, we used soluble recombinant SV2C-L4 fragments, which showed the highest apparent affinity for BoNT/A, to inhibit BoNT/A binding to neurons by competing with endogenous SV2A/B. Neurons were exposed to BoNT/A and Syt I N  Ab for 10 min in the presence of an excess amount of either control protein (GST) or GST-tagged SV2C-L4 fragment. As shown in  FIG. 3   a , SV2C-L4 reduced BoNT/A binding to neurons compared to GST. Neurons in both conditions showed similar level of Syt I N  Ab uptake, indicating that the reduction in BoNT/A binding is not due to nonspecific changes in synaptic vesicle recycling. 
     To further demonstrate the specificity of this inhibition, we tested in parallel another BoNT, BoNT/B, which has been shown to use the synaptic vesicle protein synaptotagmin I/II to enter cells (Dong M et al., supra, 2003; Nishiki T et al.,  J. Biol. Chem.  269:10498-10503, 1994; Nishiki T et al.,  Biochim. Biophys. Acta  1158:333-338, 1993). BoNT/B is an ideal control since it has similar structure and size to BoNT/A and uses the same entry pathway. As shown in  FIG. 3   a  (right panel), SV2C-L4 did not affect BoNT/B binding to neurons. Furthermore, binding of BoNT/B can be inhibited by adding a peptide (P21) derived from its receptor, synaptotagmin II (Syt II) (Dong M et al., supra, 2003), and addition of this peptide did not affect BoNT/A binding ( FIGS. 7   a  and  b ). Interestingly, SV2C-L4 also did not affect the binding of BoNT/E, another major BoNT, suggesting BoNT/E does not use SV2 to enter neurons ( FIG. 7   c ). 
     We further assessed whether the reduction in BoNT/A binding by addition of SV2C-L4 correlates with the protection of endogenous SNAP-25. Taking advantage of a specialized antibody, anti-SNAP-25-C, which only recognizes SNAP-25 fragments cleaved by BoNT/A ( FIG. 3   b ), we monitored the level of cleaved fragments by immunostaining Neurons were first treated with BoNT/A for 10 min in the presence of either GST or SV2C-L4. These neurons were washed and further incubated for 6 hrs, fixed and immunostained for cleaved SNAP-25 fragments. As shown in  FIG. 3   b , reduced BoNT/A binding by addition of SV2C-L4 resulted in decreased levels of SNAP-25 cleavage, indicating that SV2C-L4 prevented the functional entry of BoNT/A into neurons. 
     We extended this study to peripheral motor nerve terminals, the physiological target of BoNT/A in vivo. Using SV2 isoform specific antibodies, we found that motor nerve terminals at NMJs in the diaphragm express all three SV2 isoforms ( FIG. 8 ). Pre-incubation of BoNT/A with a high concentration of SV2C-L4 fragments (30 μM) significantly reduced BoNT/A binding to NMJs (65% reduction compared to control, P&lt;0.0001, t-test) ( FIGS. 3   c  and  d ). This decrease is specific to BoNT/A since SV2C-L4 did not affect BoNT/B binding ( FIGS. 3   c  and  d ). These data suggest that binding of BoNT/A to motor nerve terminals is mediated by direct interactions with SV2 luminal domains. 
     BoNT/A Binding is Abolished in SV2A/B Knockout Neurons: 
     To determine definitively whether SV2 is the receptor for BoNT/A, we turned to available SV2A and B single knockout and SV2A/B double knockout mice (Janz R et al.,  Ann. NY Acad. Sci.  733:345-255, 1999). Mice lacking SV2A (SV2A single knockout and SV2A/B double knockout) display severe seizures and die within 2-3 weeks of birth, while SV2B single knockout mice are normal. These phenotypes may be because SV2A has wider distribution than SV2B and these two isoforms are functionally redundant (Janz R et al., supra, 1999). Cultured hippocampal neurons from SV2A/B knockout mice develop normal synaptic structures and are capable of releasing neurotransmitter (Janz R et al., supra, 1999). Because these neurons only express SV2A and B, neurons from SV2A/B double knockout mice become an ideal loss-of-function model to study the role of SV2 for BoNT/A binding. 
     We first tested the function of SV2B by comparing BoNT/A binding to neurons from SV2B knockout (SV2B(−/−)) mice and wild-type littermate controls (WT). Neurons were exposed to BoNT/A and B simultaneously for 10 min in High K +  buffer, washed and fixed. Binding of BoNT/A and B to neurons was quantified by measuring immunofluorescence intensity (see Methods for details). Normalized average intensities (% WT) are shown in  FIG. 4   a . SV2B knockout neurons showed significantly reduced BoNT/A binding (28% reduction compared to WT, P&lt;0.0001, t-test), while BoNT/B binding remained the same. These data suggest that lack of SV2B does not affect the toxin entry pathway—synaptic vesicle recycling—in general, but rather specifically reduced BoNT/A binding surface binding sites. 
     Because SV2B(−/−) neurons still express SV2A, we asked whether remaining binding of BoNT/A is mediated by SV2A. By breeding SV2A(+/−)SV2B(−/−) mice, we generated littermates that have no SV2B but wild-type levels of SV2A (SV2A(+/+)SV2B(−/−)), no SV2B and half the levels of SV2A (SV2A(+/−)SV2B(−/−)), and SV2A/B double knockouts (SV2A(−/−)SV2B(−/−)) ( FIG. 4   b ). Neurons cultured from these littermates were exposed to BoNT/A and B, washed and fixed. Triple immunostaining of BoNT/A, BoNT/B and SV2 were performed and representative images from each genotype are shown in  FIG. 4   b . Quantification of immunofluorescence intensity showed that BoNT/A binding to SV2A(+/−) SV2B(−/−) neurons is only 47% of SV2A(+/+)SV2B(−/−) neurons ( FIG. 4   c ). Interestingly, the majority of BoNT/A binding in SV2A(+/−)SV2B(−/−) neurons colocalized with SV2A expression ( FIG. 4   b  middle frames). Strikingly, there is virtually no binding of BoNT/A to SV2A/B double knockout neurons ( FIGS. 4   b  and  c ). These changes in binding are specific to BoNT/A, since BoNT/B binding to each genotype remained the same ( FIGS. 4   b  and  c ). This indicates that SVA/B knockouts specifically abolished BoNT/A recognition sites instead of causing other entry pathway defects. Together with the fact that SV2C luminal fragments were able to inhibit BoNT/A binding, we have established that BoNT/A binding to hippocampal neurons is mediated by direct interactions with SV2A and B. 
     Expression of SV2 Restores BoNT/A Binding to SV2A/B Knockout Neurons: 
     Using hippocampal neurons from SV2A/B double knockout mice, we carried out rescue studies to determine whether expression of SV2A, B or C can restore BoNT/A binding. Rat SV2A, B or C were transfected into these neurons, with a lentiviral vector that can express SV2 and GFP simultaneously under separated neuronal specific promoters (synapsin promoter, details described in Methods). Forty eight hrs post-transfection, neurons were exposed to BoNT/A for 10 min, washed, and binding of BoNT/A assessed by immunostaining Transfected neurons were identified by GFP fluorescence signals and expression of SV2 in these neurons were confirmed by immunostaining. As shown in  FIG. 5 , BoNT/A binding was only observed on neurons transfected with SV2A, B or C, while other neurons in the same field showed no binding. Enlarged overlay images between SV2 and BoNT/A signals showed a high degree of colocalization at synapses ( FIG. 5 , overlay). The high level of SV2 expression in the cell soma is likely due to over-expression of exogenous proteins since it is not found in immunostaining of endogenous SV2 in wild-type neurons. Overexpression of another synaptic vesicle protein, synaptotagmin I, using the same viral vector did not resulted in detectable BoNT/A fluorescence signals, confirming the specificity of the rescue effect. SV2A, B or C all rescued BoNT/A binding, indicating all three isoforms can mediate BoNT/A binding to neurons once expressed. 
     BoNT/A Binding to Motor Terminals is Reduced in SV2A/B Knockout Mice: 
     To determine whether SV2 function as a receptor for BoNT/A at its physiological target, we examined BoNT/A binding to diaphragm nerve terminals from SV2 knockout mice. These nerves normally express all three SV2 isoforms ( FIG. 8 ) and we expect they all contribute to BoNT/A recognition of motor terminals. Because SV2A knockout and SV2A/B double knockout mice do not survive to adulthood and SV2C knockout mice are not available, we compared diaphragm preparations from available adult knockout mice that have the least amount of SV2 expression (SV2A(+/−) SV2B(−/−)), to wild-type (WT). As shown in  FIGS. 6   a  and  b , BoNT/A binding to NMJs from SV2A(+/−)SV2B(−/−) mice is significantly reduced compared to WT (72% reduction, P&lt;0.001, t-test), while the levels of BoNT/B binding are the same (P&gt;0.05, t-test), suggesting that SV2A and B are important for BoNT/A binding to motor nerve terminals. The remaining level of BoNT/A binding in SV2A(+/−)SV2B(−/−) NMJs is likely mediated by SV2C, which is not altered in these mice, and remaining reduced level of SV2A. 
     SV2B Knockout Mice have Reduced Sensitivity to BoNT/A: 
     To further establish the physiological meaning of these findings, we extended our studied to the whole animal. Among available SV2 knockout mice lines, SV2A knockout and SV2A/B double knockout mice both die within weeks after birth, suggesting SV2A is essential for maintaining normal synaptic transmission. In contrast, SV2B single knockout mice (SV2B(−/−)) have no apparent difference to wild-type (WT) mice. To minimize the potential defect in vivo on synaptic transmission, we chose to compare BoNT/A sensitivity of SV2B knockout mice and WT littermates. Sensitivity to BoNT/A was assessed with an established rapid assay, in which large amount of toxins are injected intravenously and the survival time (Time-to-death) after the injection were recorded (Boroff D A and Fleck U,  J. Bacteria  92:1580-1581, 1966; Dong M et al., supra, 2003; Malizio C G, Methods and Protocols O. Holst, ed. (Humana Press), pp. 27-39, 2000). This survival time can be converted to apparent toxicity from a previously established standard curve (Malizio CG, supra, 2000). 
     Identical amounts of BoNT/A (10 4 -10 6  LD 50 /ml) were injected into SV2B(−/−) and WT mice and their survival time is shown in  FIG. 6   c . SV2B knockout mice survived significantly longer than wild-type littermates (43.7  min versus 32.6  min). The effective toxin concentration estimated from the survival time of WT mice is about 2.5-fold more than SV2B(−/−) mice (8.4 ×10 5  LD 50 /ml versus 3.4 ×10 5  LD 50 /ml,  FIG. 6   c ). The difference in effective toxicity reflects the shift of LD 50  value in SV2B knockout mice, i.e., these mice require approximately 2.5 fold more BoNT/A for a lethal dose than their WT littermates. The remaining toxicity in SV2B(−/−) mice is likely mediated by SV2A and C in their motor nerve terminals. These results provided functional evidence that SV2 is the physiological receptor for BoNT/A in vivo. 
     The present invention is not intended to be limited to the foregoing example, but rather to encompass all such variations and modifications as come within the scope of the appended claims.