Thèse (M.Sc.)--Université Laval, 2003. / Bibliogr.: f. 78-84. Publié aussi en version électronique.

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Influenza is an important human pathogen that causes fatal disease in 250,000-500,000 people worldwide each year. Because of high levels of variation between influenza strains, vaccines are not always effective and must be administered annually. Influenza virus, which replicates primarily in the lung epithelium, encodes only 10 proteins and relies heavily on host products to replicate. Determining which cellular factors are important for influenza replication represents an important area of virology and cellular biology research, and could elucidate proteins or pathways to target for antiviral therapies. We developed a high throughput screening method in primary human bronchial epithelium (HBECs) to identify novel regulators of influenza replication. We first used this method to functionally examine 1745 genes that were identified as potential influenza regulators due to transcriptional regulation by virus or viral products, direct interaction with viral proteins via yeast two-hybrid, or through computational analysis. This screen confirmed some known regulators of influenza replication while identifying novel viral interactors as influenza regulators (e.g. USHBP1, ZMAT4). We also found that the WNT, p53, and ER stress pathways, among others, affect viral replication and interferon production. The life cycle of influenza involves extensive intracellular trafficking of viral components. We again used RNAi to systematically examine the roles of vesicle, RNA, and protein trafficking genes in the production of infectious influenza A virus in primary lung cells. Among the factors that significantly impact viral infection, we identify a set of five genes with strong antiviral effects that are mutated in patients with Hermansky-Pudlak syndrome (HPS). Depletion of HPS genes leads to elevated viral RNA at an early stage of influenza infection prior to transcription. In contrast, depletion of these genes does not alter the innate immune response to virus or interferon. Using an HPS-1 patient cell line, we find an increase in viral fusion to endosomal compartments but no change in viral binding to the cell surface or entry into the early endosome. Our studies uncover a potential role for many trafficking factors in the influenza life cycle, and point to an HPS1-dependent process that inhibits viral entry prior to viral membrane fusion.

京都大学 / 0048 / 新制・課程博士 / 博士(医学) / 甲第22003号 / 医博第4517号 / 新制||医||1038(附属図書館) / 京都大学大学院医学研究科医学専攻 / (主査)教授 長船 健二, 教授 川口 義弥, 教授 柳田 素子 / 学位規則第4条第1項該当 / Doctor of Medical Science / Kyoto University / DFAM

Cytotoxic T lymphocytes (CTL) kill virally infected and tumourigenic cells via the regulated secretion of specialised secretory lysosomes. These secretory lysosomes contain cytolytic effector molecules, such as perforin and granzymes, which are able to induce apoptosis in target cells. Secretion occurs at the contact point between the CTL and its target, in a highly structured region termed the immunological synapse (IS). Upon formation of the IS, CTL undergo polarisation of their microtubule cytoskeleton and movement of the microtubule organising centre (MTOC) to the IS. Secretory lysosomes are then able to polarise along microtubules, fuse with the plasma membrane and deliver their effector molecules to the IS. The Adaptor Protein 3 complex (AP-3) sorts transmembrane proteins to lysosomes and deficiency in AP-3 results in missorting of proteins from the lysosomal to plasma membrane. CTL from AP-3 deficient patients, who suffer from Hermansky-Pudlak Syndrome Type 2 (HPS2), show reduced killing of target cells. This thesis describes two new patients with HPS2, both with homozygous mutations in the AP3B1 gene, which codes for the β3A subunit of the AP-3 complex. CTL from the new HPS2 patients show reduced cytotoxicity, which is shown here to be due to impaired secretory lysosome polarisation towards the IS. This impairment is common to HPS2 CTL, but varies between patients. In order to determine differences between HPS2 and wild type CTL, the localisation of a range of lysosomal, cytolytic, transmembrane, inhibitory and activation marker proteins is examined. This shows that in HPS2 CTL, LAMP1, CD63 and CD9 are potential AP-3 cargos. In addition, a possible effect on the key lytic effector perforin is identified. Preliminary experiments to allow proteomic comparison of HPS2 and wild type CTL are also presented. Further investigation of these results will help to shed light on the mechanisms involved in secretory lysosome polarisation in CTL.

Melanosome biogenesis is a highly regulated endosomal maturation process wherein structural fibers harbouring immature melanosomes acquires its biosynthetic proteins through the secretory pathway and finally matures into a functional organelle. These processes were shown to be dependent on several cytosolic protein complexes such as AP (adaptor protein)-1, AP-3, BLOC (biogenesis of lysosome-related organelles complex)-1, -2 and -3; in addition to kinesin motor KIF13A and Rab GTPases 7, 32 or 38. Mutations in the subunits of these complexes or Rab38 result into defective melanosome maturation leading to occulocutaneous albinism, a clinical phenotype commonly observed in Hermansky-Pudlak syndrome (HPS). Moreover, molecular function of these complexes in regulating the biogenesis of melanosome is partially known. The delivery of cargo to maturing melanosomal membranes requires fusion machinery that includes Rab GTPases, tethering factors and SNARE (soluble N-ethylmaleimide sensitive factor attachment protein receptor) proteins. However, the SNAREs involved in the transport of cargo to melanosomes is poorly understood. In this study entitled as “understanding the SNARE dynamics during melanosome biogenesis” we focus on functional role of endosomal Qa-SNARE protein, Syntaxin 13 (formally called STX12, herein referred to as STX13) in the organelle biogenesis and its transport in and out of melanosome. Moreover, these studies show that STX13-mediated cargo transport require a melanosomal membrane localized R-SNARE VAMP7 and these SNAREs are interdependent on each other in regulating their steady state distribution. In addition, this study illustrated the possible mechanism of SNARE recycling which occurs indirectly through AP-3 complex. Thus, these studies underscore the STX13‟s role in cargo transport to maturating melanosomes and its trafficking routes to and from the melanosomes. Chapter-I describes the literature review on melanosome biogenesis; Chapter-II lists the experimental procedures used in this study and Chapter-III to V focuses on results and discussion, segregated into three sections. Chapter-III: Screening and identification of endosomal SNAREs involved in the trafficking of melanosomal proteins. Our preliminary RNAi screen for SNAREs involved in melanosome biogenesis revealed STX13 as one of the Qa-SNARE affecting pigmentation and cargo transport. STX13, a recycling endosomal SNARE has been reported to interact with pallidin, a subunit of BLOC-1; however the functional role of this interaction in pigment formation is unknown. In addition, previous studies from our lab have shown that STX13 colocalize with endosomal Rab11 and partially with EEA1- or Rab5-positive organelles in melanocytes. Together, these observations insinuated us to characterize the functional role of STX13 in melanosome biogenesis. Upon STX13 inactivation, wild type mouse melanocytes showed hypopigmentation due to mistargeting of cargo such as TYRP1 and TYR to lysosomes. Knockdown of STX13 dramatically decrease the population of immature and mature melanosomes. Moreover, STX13 associate with the melanosome cargo on endosomal tubular structures. In addition, deletion of regulatory domain in STX13 increases the cargo transport to melanosomes due to its increased SNARE activity. This is possibly due to loss in intracellular regulation of SNARE occur through multiple factors such as SM (Sec1p/Munc18) proteins. Together this data suggests that STX13 mediates cargo transport to melanosomes from recycling endosomes. Chapter-IV: Functional characterization of the SNAREs involved in melanosomal maturation. Several in vitro studies have shown that a set of four SNAREs such as Qa, Qb, Qc (or Qbc) and R control the membrane fusion event duing the cargo transport. Additionally, this process is further regulated by SM proteins in in vivo. Electron microscopic studies in melanocytes have shown that melanosomal proteins were delivered to the melanosomal membrane through recycling endosomal tubular domains. Moreover, our RNAi screen show that STX13 possibly acts as Qa-SNARE in mediating the fusion events between melanosomal membranes and the endosomal tubular or vesicular intermediates. However, the role of other SNAREs for this membrane transport is unknown. It has been shown that the expression of VAMP family SNAREs such as VAMP3, VAMP7 and VAMP8 increased with melanogenesis upon differentiation of melanoma cells. VAMPs belong to the class of R-SNAREs, in which VAMP7 is known to interact with VARP (abbreviation) and AP-3 (mediates the trafficking of TYR) separately, and these molecules are known to regulate the cargo transport to melanosomes. However, the precise role of VAMP7 in pigment granule maturation is unknown. Therefore, we set out to characterize the functional role of VAMP7 in melanosome biogenesis. VAMP7 has been shown to localizes to multiple sub-cellular compartments and regulate the several transport steps in other cell types. Our study found that GFP-epitope tagged either human or rat VAMP7 localize to melanosomes at steady state in wild type mouse melanocytes. Knockdown of VAMP7 causes hypopigmentation of melanocytes and misroutes the cargo to lysosomes. Further, the inactivation of VAMP7 in melanocytes phenocopies the STX13 depletion, suggesting both the SNAREs are required for the melanosome biogenesis. In addition, knockdown of STX13 target the VAMP7 to lysosomes; while inactivation of VAMP7 affect the localization of STX13 to recycling tubular structures. Subsequently, the dominant active mutants of STX13 were not able to rescue the pigmentation or cargo transport defects in VAMP7 knockdown melanocytes. Together, the data suggests that STX13 functions from recycling endosomes and VAMP7 on melanosome membrane for the transport of cargo to melanosomes Chapter-V: Understanding the mechanism of STX13 recycling during melanosome biogenesis. At steady state, SNAREs are localized to the membranes of specific organelles where they mediate or regulate the membrane fusion. During this process, three or two Q-SNAREs on one membrane (in a trans-SNARE complex, possibly formed by Qa, Qb, Qc or Qbc) interact with a R-SNARE on another member to form a SNAREpin complex. Post-fusion, SNAREs are disassembled by SNAP and NSF proteins and then recycled back to the original compartment for next round of fusion. Here, we address the mechanism of post-fusion recycling of STX13 from melanosomes to endosomes. Previous studies have shown that STX13 mislocalize to melanosomes in AP-3-deficient melanocytes, suggesting a role for AP-3 in recycling the SNARE from melanosomes. Bioinformatic analysis of the N-terminal region of STX13 revealed the presence of two canonical adaptor binding motifs 3YGP6L and KETNE80L81L, resembling the tyrosine-based (YXXø) and dileucine-based motif [DE]XXXL[LI], recognized by several adaptor proteins. Point mutagenesis of these motifs in STX13 had no effect on their steady state distribution indicating that STX13 possibly uses non-canonical residues for its recycling. Further, deletion of the N-terminal region (either 1-129 or 14-129 aa) in STX13 redistributes the SNARE to melanosomes. Moreover, the activity and the trafficking of recycling defective STX13 mutants are dependent on another HPS complex, BLOC-2 and the SNARE, VAMP7. Absence of 1-129 region in STX13 or mutations in the subunits of AP-3 perturbs the steady state localization of STX13 suggesting an indirect role for AP-3 in recycling of STX13 to endosome via non canonical motifs present in its 1-129 aa region.

Recycling embosoms (REs) are transient intermediates of endosomal network, constantly generated from early/sorting endosomes (EEs/SEs). Conventionally, these organelles function in recycling of many growth/nutrient/signalling receptors from SEs to the cell surface and maintain the cellular homeostasis in all cell types. Recent studies have shown that REs slightly diverted their function in specialized cells such as melanocytes for the delivery of melanogenic cargo to a set of lysosome-related organelles (LROs) called melanosomes. However, it is unknown how melanocytes modulate the trafficking routes of REs towards the biogenesis of melanosomes. Any alterations in this process result in occulocutaneous albinism, commonly observed in autosomal recessive disorder, Hermansky-Pudlak Syndrome (HPS). HPS is caused by mutations in nine genes in human and fifteen genes in mouse and the protein products of these genes were grouped in multiple endosomal protein complexes; BLOC (Biogenesis of Lysosome-related Organelles Complex)-1, -2, -3, AP (Adaptor Protein)-3 and HOPS (homotypic fusion and protein sorting). Studies from our laboratory and others have shown that REs deliver the melanin-synthesizing enzymes to melanosome in BLOC-1 and BLOC-2 dependent manner. On the other side, studies in fibroblasts have shown that the adaptor AP-1 and microtubule-dependent motor, KIF13A also regulates the formation of REs. In these studies, it was proposed that AP-1 binds to the cargo tails and interacts with motor KIF13A to generate the RE tubules, where BLOC-1 initiates the biogenesis. Nevertheless, the mechanism behind the biogenesis of REs and how these molecules synergistically control these processes is largely unknown. Additionally, the role of BLOC-2 in REs biogenesis never been implicated. Here we have attempted to study the mechanism of RE biogenesis and their role in pigment granule formation using HeLa and mouse melanocytes as model systems. In general, Rab GTPases (Rabs) regulate the several process of membrane trafficking including cargo sorting, membrane domain organization, tethering and fusion. We hypothesized that the biogenesis of RE is also regulated by one of the endosome localized Rab GTPases. Our RNAi screening against Rabs involved in regulating the RE length/number showed Rab22A as a potential candidate. Thus, we aim to study the role of Rab22A in RE biogenesis and its regulation in melanocyte pigmentation. The current study entitled as “Understanding the role of Rab22A in recycling endosome biogenesis and melanocyte pigmentation” is divided into five chapters. Chapter-I outlines the review of literature on cell biology of intracellular organelles such as endocytic network and melanosomes. Chapter-II details the experimental procedures used in the study. Chapter-III to Chapter-V describes the results and discussion. Chapter-III: Identification of endosomal Rab GTPases required for the dynamics of recycling endosomes Endosomal Rabs are known to regulate various functions such as vesicle biogenesis, transport, tethering and fusion, but their role in generation of tubulo-vesicular carriers of endocytic system, REs is unknown. It has been shown that REs possibly derived from EEs/SEs and characterized by the association/localization of multiple proteins such as transferrin receptor (TfR), SNARE STX13, Rab11 and motor KIF13A. In this study, we have used YFP-KIF13A as a marker to label the REs. YFP-KIF13A in HeLa cells localized to long tubular structures throughout the cell and also to the clusters of peripheral endosomes. To identify the endosomal Rabs that regulate the RE dynamics (both length and number), we have transfected the HeLa cells with shRNA against endosomal Rabs such as Rab4A, Rab5A, Rab5B, Rab5C, Rab7A, Rab9A, Rab11A, Rab14A and Rab22A. Post transfection and shRNA selection, cells were transfected with YFP-KIF13A, analyzed and quantified the RE dynamics using ImageJ. Here, we have measured two parameters for the identification of Rab/s that potentially regulates the REs biogenesis: first, average number of tubules per cell and second, average length of tubules per cell. These studies identified Rab22A as a potential candidate, depletion of this Rab affects both number and average length of KIF13A-positive tubules. As described above, REs deliver several melanocyte specific cargoes to melanosomes in melanocytes. However, the function of Rab22A in controlling these transport steps to melanosome/its biogenesis or pigmentation has not been addressed. Thus, we have studied the mechanism of Rab22A in RE biogenesis and its role in pigmentation in the following sections. Chapter-IV: Characterization of Rab22A function in regulating the recycling endosomes Initially, we tested whether Rab22A localizes to the REs. Our co-expression studies show that Rab22A localizes to KIF13A- or STX13-positive RE compartments in HeLa or melanocytes, respectively. In general, Rab GTPases mediate their function through cycling between GTP (membrane bound) and GDP (cytosol) bound state. These states can be achieved by point mutation of active site residues in the protein. We have generated Rab22A constitutive active mutant (Rab22AQ64L, defective in GTP hydrolysis) and dominant negative mutant (Rab22AS19N, defective in GTP binding) to understand the role of Rab22A in regulating REs. Interestingly, overexpression of Rab22AQ64L mutant in HeLa cells increases the average number of KIF13A-positive REs relative to the wild-type Rab22A (Rab22AWT). As predicted, overexpression of Rab22AS19N mutant reduces the number as well as length of RE tubules relative to the control HeLa cells. Consistent to these studies, Rab22A-knockdown did not affect the endogenous KIF13A protein levels or its recruitment to endosomes, however recycling of TfR (measured through Tf-Alexa 594) was significantly affected in these cells. These studies suggest that Rab22A possibly regulates the formation or function of REs. Likewise, overexpression of Rab22AQ64L and Rab22AS19N mutants in melanocytes resulted in reduction of total melanin content in the cells. To confirm these results, we have performed immunofluorescence microscopy (IFM) analysis, which showed Rab22AQ64L localized to the enlarged vacuolar structures, positive for melanosomal cargo TYRP1 (tyrosinase-related protein 1), whereas Rab22AS19N localized to the cytosol. Further, Rab22A depletion in melanocytes causes the hypopigmentation in the cells concurrently reduces the stability of TYRP1 but not other melanocyte specific proteins, indicating a role for Rab22A in regulating TYRP1 transport to melanosomes. Altogether, our studies suggests that Rab22A regulates the TfR recycling in HeLa cells and TYRP1 transport in melanocytes by controlling the RE dynamics. Chapter-V: Molecular mechanism of recycling endosome biogenesis: a role for Rab22A Rabs perform their function by recruiting specific effector/s to the membrane upon Rab activation. It is unknown, how Rab22A regulates REs through its effectors. We hypothesize that Rab22A may regulate the recruitment and function of BLOC-1 and BLOC-2 complexes during RE formation. To validate these hypothesis, we carried out the knockdown of individual BLOC-1 and -2 subunits (destabilize the entire complex) separately in HeLa and studied the dynamics of RE through YFP-KIF13A expression. As expected, the length and number of KIF13A-postive tubules were significantly reduced in both BLOC-1- and BLOC-2-deficient HeLa cells and was phenocopying the Rab22A knockdown cells. Moreover, subcellular fractionation in HeLa, co-fractionated Rab22A with BLOC-1 (Muted) or BLOC-2 (HPS6) subunits along with KIF13A. Additionally, endogenous subunit levels of BLOC-1 and BLOC-2 were moderately reduced in Rab22A knockdown HeLa cells. Consistent to these results, recycling kinetics of Transferrin (Tf) was altered in Rab22A depleted cells as similar to BLOC-1- or BLOC-2-deficient cells as reported earlier. Likewise, Rab22A knockdown in melanocytes affected STX13-positive tubules and also the stability of endogenous BLOC-1 subunit, Pallidin, suggesting that Rab22A possibly works with BLOC-1 and BLOC-2 independent of cell types. To understand the regulation among these molecules, we overexpressed Rab22A in BLOC-1-deficient cells and analyzed the cells for BLOC-1-deficient rescue phenotypes such as pigmentation and cargo localization. However, Rab22A could not compensate the BLOC-1 function, suggesting that Rab22A possibly functions upstream of BLOC-1. Our subcellular and membrane associated fractionation studies of homogenates depleted with Rab22A, BLOC-1 and BLOC-2 showed that subunit levels of BLOC-1 and BLOC-2 in the membrane pool were significantly reduced upon Rab22A depletion compared to control cells. However, membrane association of Rab22A in BLOC-1 deficient cells was not affected. Further, our biochemical interaction studies showed that Rab22A interacts physically with BLOC-1 and BLOC-2 subunits as well as with KIF13A. Thus, these studies indicate that Rab22A possibly recruits and interacts with BLOC-1 and BLOC-2 for the generation of REs. We have summarized the study by proposing a model wherein Rab22A localizes to the limiting membrane of endosomes that are positive for KIF13A and then recruits and associates with BLOC-1 and BLOC-2 complexes which subsequently pulled by KIF13A for the generation of RE tubules.

Trombocitopenia e diminuição da função plaquetária são achados comuns em pacientes picados por serpentes do gênero Bothrops. Sabe-se que o veneno de Bothrops jararaca (VBj) apresenta compostos com características pró e anti-agregantes plaquetárias, porém existem poucos estudos sobre a influência do veneno total assim como das principais famílias de proteínas que o compõem sobre as funções plaquetárias. A utilização de modelos experimentais é essencial para entender as desordens plaquetárias em humanos que culminam em sangramentos diversos. Portanto, o objetivo deste estudo foi (i) comparar as respostas ex vivo de plaquetas lavadas de humanos e de camundongos frente ao VBj, assim como entender a participação de serinaproteinases (SVSP) e metaloproteinases (SVMP) presentes no veneno sobre a agregação dessas plaquetas; e (ii) delimitar dentro do complexo proteico do veneno os compostos que induzem a trombocitopenia em camundongos após 3 horas do início do envenenamento botrópico. As plaquetas lavadas de humanos e camundongos BALB/c, C57BL/6 e do mutante natural pérola (Ap3b1-/-) apresentaram respostas de agregação máxima ao VBj na concentração de 24,4 ?g/mL, porém esta concentração provocou uma agregação menos intensa em plaquetas humanas quando comparada àquela observada nas linhagens BALB/c e C57BL/6. Mesmo em plaquetas de camundongos pérolas, deficientes em corpos densos plaquetários, o VBj se mostrou um potente agonista, promovendo a agregação plaquetária sem a necessidade da secreção do conteúdo granular. A ação agonista do veneno promoveu a secreção de ATP presente nos corpos densos plaquetários de humanos e das linhagens BALB/c e C57BL/6 de forma tão intensa quanto à provocada pela trombina, assim como a secreção de PF4 presente nos grânulos ? de plaquetas humanas e de camundongos BALB/c. Já em relação à secreção lisossomal, observou-se que as plaquetas humanas secretam níveis mais baixos de ?-hexosaminidase quando estimuladas com VBj do que pela trombina, enquanto que em plaquetas de camundongos BALB/c a secreção lisossomal ao VBj foi superior à constatada com a trombina. Os baixos níveis de lactato desidrogenase (LDH) no sobrenadante das plaquetas estudadas mostraram ausência de atividade direta citotóxica pelo VBj. Para verificar se as principais classes de famílias de enzimas do VBj, SVMP e SVSP, estavam envolvidas na ativação plaquetária ex vivo, elas foram inibidas com Na2EDTA (13 mM) e AEBSF (8 mM), respectivamente. Observou-se que em plaquetas humanas as serinaproteases são importantes para a agregação ex vivo, enquanto a agregação das plaquetas de camundongos BALB/c foi independente dessa classe de toxinas. Os resultados dos ensaios in vivo demonstraram que as proteínas do VBj com peso molecular inferior a 50 kDa (UF < 50 kDa) são importantes para o estabelecimento da trombocitopenia em camundongos BALB/c que receberam essa fração por via subcutânea e que esse quadro é independente de manifestações hemorrágicas e do desenvolvimento de coagulopatia durante o envenenamento botrópico. A caracterização dos compostos presentes no UF < 50 kDa foi realizada por espectrometria de massas e foi observada a presença predominante de metaloproteinases (37%) e proteínas similares às lectinas do tipo-C (33%), enquanto serinaproteases (17%), fosfolipases A2 (10%) e outros compostos (3%) somaram 30% da fração UF < 50 kDa. Em conclusão, o veneno é um potente agonista plaquetário que promove agregação, aglutinação e secreção de plaquetas humanas e de camundongos, independente da secreção de corpos densos plaquetários, e a fração do veneno responsável pela trombocitopenia em camundongos BALB/c tem peso molecular menor que 50 kDa / Thrombocytopenia and platelet dysfunction are common findings in patients bitten by Bothrops jararaca snakes. Pro- and anti-aggregating toxins have been isolated from Bothrops jararaca venom (BjV), but only few studies have been carried out about the effects of crude BjV and its main families of enzymes on platelet function ex vivo, as well as to understand their relevance to the pathophysiological events that occur during B. jararaca envenomation. Animal models have been used to understand platelet disorders in humans that culminate in bleeding manifestations. Thus, the aim of this study was to investigate (i) the effects of BjV, and snake venom serine proteinases (SVSP) and snake venom metalloproteinases (SVMP) contained therein, on aggregation and secretion in suspensions of washed human and mouse platelets, and (ii) to determine the BjV fractions, obtained by ultrafiltration, that induce thrombocytopenia in BALB/c mice after 3 h of administration of Bothrops envenomation. Washed platelets from humans and BALB/c, C57BL/6 and pearl (Ap3b1-/-) mice showed maximal aggregation responses to BjV at the concentration of 24.4 ?g/mL. However, this concentration aggregated less intensely platelets from humans than BALB/c or C57BL/6 mice. Even in pearl mouse platelets, which are deficient in dense bodies, BjV proved to be a potent agonist, promoting platelet aggregation without the requirement of granule content secretion. Nonetheless, BjV induced secretion of ATP, present in dense bodies, and PF4, present in ? granules, in the same extent as thrombin, from platelets of humans and mice. In regard to lysosomal secretion, it was observed that human platelets secreted low ?-hexosaminidase levels when stimulated by BjV than thrombin, whereas in BALB/c platelets higher secretion was induced by BjV than by thrombin. Release of lactate dehydrogenase (LDH) was similar between BjV and thrombin, evidencing the absence of cytotoxic activity by BjV on platelets. Inhibition of SVMP and SVSP, using Na2EDTA (13 mM) and AEBSF (8 mM), respectively, demonstrated that SVSP are important for ex vivo aggregation only in human platelets, whilst BALB/c platelet aggregation was independent of both of them. In in vivo studies, only BjV toxins with molecular weight less than 50 kDa (UF50) caused thrombocytopenia when administered s.c. to BALB/c mice, and it was independent of hemorrhagic manifestations and consumptive coagulopathy. Characterization by mass spectrometry of these toxins present in UF50 showed the predominant presence of SVMP (37%) and type-C lectin proteins (33%) were observed, while SVSP (17%), phospholipases A2 (10%) and other proteins (3%) accounted for the remaining toxins in UF50. In conclusion, BjV is a potent platelet activating agent that promotes aggregation, agglutination and secretion of human and mouse platelets, independent of secretion of dense platelet bodies, and the fraction of venom responsible for thrombocytopenia in BALB / c mice has a molecular weight less than 50 kDa