In addition to their role in hemostasis, platelets appear to contribute to vascular inflammatory diseases. Platelets achieve this through the secretion of various prothrombotic and pro-inflammatory molecules. Platelet secretion is mediated by integral membrane proteins called Soluble NSF Attachment protein REceptors (SNAREs). SNAREs come from both granule/vesicle membranes (v-SNAREs) and target membranes (t-SNAREs) to form a trans-bilayer complex that promotes membrane fusion and subsequent granule cargo release. The work described in this dissertation dissects various, yet related aspects of platelet secretion in both physiological relevant and pathological circumstances. Atherosclerosis is a leading cause of death in the westernized countries and a major contributor to heart attacks and strokes. Given the potential involvement of platelets in atherosclerosis and previous work from our laboratory showing that VAMP-8 is the primary v-SNARE for platelet secretion, one part of this dissertation focuses on the role of VAMP-8- mediated secretion in atherosclerosis. The data showed that the deletion of VAMP-8 in the ApoE-/- null model of chronic atherosclerosis attenuated plaque development compared to the wild type littermates. Aged (50 week) VAMP-8-/-/ApoE-/- mice showed a reduction in lesion size compared to ApoE-/- controls, as measured by Oil Red-O staining of the plaques in the aortic sinus and by en face analysis of plaques in the aortic arch. These data show that the loss of VAMP-8 attenuates the development of atherosclerotic plaques and suggest that platelet secretion contributes to atherosclerosis. Considering the vital role of platelet secretion in both physiological and pathological conditions, it is essential to understand how it is regulated. SNARE proteins are controlled by a range of regulatory molecules that affect where, when, and with whom they form trans-bilayer complexes for membrane fusion. One family of such regulators is the Munc18 family: platelets contain three (Munc18a-c). The second part of this dissertation focuses on the role of Munc18b/STXBP2. Mutations in the Munc18b/STXBP2 gene underlie Familial Hemophagocytic Lymphohistocytosis type 5 (FHL5), which is a life- threatening disease caused by dysregulation of the immune system. Platelets from two biallelic FHL5 patients had almost undetectable levels of Munc18b/STXBP2 levels; the levels of Munc18a increased slightly and Munc18c levels were unaffected. Syntaxin 11 levels were affected but the levels of other secretory machinery proteins were normal. Platelet secretion from dense and alpha granule in two biallelic patients and the one heterozygous patient was decreased. The release of serotonin from dense granules, and platelet factor 4 (PF4) from alpha granules was profoundly affected in the biallelic patients and partially affected in the heterozygote heterozygous patient. Lysosome release was affected only from the platelets of the biallelic patients. These data indicate that Munc18b plays a key role in platelet secretion. Ras is the prototypical member of a family of low molecular weight, GTP-binding proteins. It affects various cellular functions by cycling between an active, guanine triphosphate (GTP) and an inactive guanine diphosphate (GDP) -bound state. Little is known about the role of Ras activation in platelets. The third part of this dissertation focuses on what could be learned about Ras’ role by analyzing platelets from patients with Noonan Syndrome. Specific mutations in the genes encoding elements of Ras signaling pathways are associated with Noonan Syndrome. Platelets from Noonan Syndrome patients with a mutation in kRas (F156V) were analyzed and shown to have a defect in aggregation in response to low levels of agonist. These data suggest that Ras may play a functionally relevant role in platelet activation. In summary, the experiments presented in investigations of this dissertation support a role for platelet secretion in several pathological conditions and suggest that platelet secretion assays may serve as useful as diagnostic tools for some genetic diseases. In addition, these studies elucidate the importance of understanding the regulation of platelet exocytosis, to drive the development of new antithrombotic therapeutics.

Platelet exocytosis is essential for hemostasis and for many of its sequelae. Platelets release numerous bioactive molecules stored in their granules enabling them to exert a wide range of effects on the vascular microenvironment. Are these granule cargo released thematically in a context-specific pattern or via a stochastic, kinetically-controlled process? My work describes platelet exocytosis using a systematic examination of platelet secretion kinetics. Platelets were stimulated for increasing times with different agonists (i.e. thrombin, PAR1-agonist, PAR4-agonist, and convulxin) and micro-ELISA arrays were used to quantify the release of 28 distinct α-granule cargo molecules. Agonist potency directly correlated with the speed and extent of release. PAR4-agonist induced slower release of fewer molecules while thrombin rapidly induced the greatest release. Cargo with opposing actions (e.g. pro- and anti-angiogenic) had similar release profiles, suggesting limited thematic response to specific agonists. From the release time-course data, rate constants were calculated and used to probe for underlying patterns. Probability density function and operator variance analyses were consistent with three classes of release events, differing in their rates. The distribution of cargo into these three classes was heterogeneous suggesting that platelet secretion is a stochastic process potentially controlled by several factors such as cargo solubility, granule shape, and/or granule-plasma membrane fusion routes. Sphingosine 1 phosphate (S1P) is a bioactive lipid that is stored in platelets. S1P is essential for embryonic development, vascular integrity, and inflammation. Platelets are an abundant source of S1P due to the absence of the enzymes that degrade it. Platelets release S1P upon stimulation. My work attempts to determine how this bioactive lipid is released from platelets. Washed platelets were stimulated with agonists for defined periods of time and the supernatant and pellet fractions were separated by centrifugation. Lipids were separated by liquid phase extraction and S1P was quantified with a triple quadrapole mass spectrometer. A carrier molecule (BSA) is required to detect release of S1P. Further, there is a dose-dependent increase in total S1P with increasing BSA. S1P release shows characteristics similar to other platelet granule cargo e.g. platelet factor IV (PF4). Platelets from Unc13-d Jinx mice and VAMP8-/- mice, which are secretion-deficient (dense granule, alpha granule and lysosome), were utilized to understand the process of S1P release. S1P release was more affected in Unc13-d Jinx mice mirroring their dense granule secretion defect. Fluorescence microscopy and sub-cellular fractionation were used to examine localization of S1P in platelets. S1P was observed to be enriched in a granule population. These studies indicate the existence of two pools of S1P, a readily extractable agranular pool, sensitive to BSA, and a granular pool that requires the secretion machinery for release. The secretion machinery of platelets in addition to being involved in the release of normal granule cargo is thus proved to be involved in the release of bioactive lipid molecules like S1P.

While some rare and severe forms of platelet function disorders are now well characterized, many common types of platelet function disorders are not yet characterized. My hypothesis was that uncharacterized platelet function disorders that impair platelet function in aggregation and/or dense granule ATP release assays are associated with increased bleeding risk. The main goal of the thesis was to study the phenotype and bleeding risks for uncharacterized platelet function disorders, through analysis of the results from clinical laboratory tests of platelet function and for a detailed analysis of their reported bleeding symptoms. First, I assessed if lumi-aggregometry provides useful diagnostic information on platelet function and can be used to help decide if an individual has a bleeding disorder. Two cohorts of individuals were studied that had dense granule ATP release assessed in response to multiple agonists as part of a work-up for a bleeding disorder. Cohort I was comprised of individuals tested between January 2007 and June 2013 and cohort II was comprised of subjects tested at least twice by this assay prior to September 2015. Among subjects tested more than once for dense granule release defects as part of the work up for a bleeding disorder (cohort I; n=133; cohort II; n=17), normal findings with all tested agonists were often confirmed by the second test (cohort I: 83%; cohort II: 100%), but impaired release with multiple agonists was not often confirmed (cohort I: 34%; cohort II: 54%) and even if it was present, the finding was not predictive of a bleeding disorder. Consequentially, it was recommended that lumi-aggregometry should not be used to diagnose platelet function disorders. Next, I studied the bleeding risks associated with uncharacterized platelet function disorders, by evaluating subjects who had abnormal findings by validated assays, namely subjects who had defective aggregation responses to two or more agonists and/or dense granule deficiency. Bleeding history was evaluated using the International Society for Thrombosis and Haemostasis bleeding assessment tool (ISTH BAT) and the likelihood for bleeding symptoms/ problems, was estimated using odds ratios (OR) collected using the clinical history assessment tool - platelet (CHAT-P) for all affected subjects, a subgroup family with a mutation RUNX1, unaffected family v members and general population controls. Individuals with platelet function disorders (n=29) and the affected members of the family with the RUNX1 mutation (n=6) had elevated ISTH BAT scores (median: 9; range:0-18 and median: 8.5, range 4-15, respectively) and an increased risk of abnormal bruising (OR 15-65 and 11-67), nosebleeds (OR 23-40 and 19-121), menorrhagia (OR 6.5-29) and excessive bleeding after trauma or dental/surgical procedures (OR 9.5-44 and 15-77 ) and wound healing problems (OR 13 and 38) compared to general population control (n=60) and unaffected (n=12) family members. Overall, the platelet function disorders in the study present with a significantly increased risk of mild, rather than severe bleeding problems. These findings are important for individuals and healthcare providers to promote evidence-based care of common uncharacterized inherited platelet function disorders for individuals with RUNX1 mutations, dense granule deficiency and/or impaired aggregation responses. / Thesis / Master of Science (MSc) / Platelets are small blood cells that help stop bleeding. People who have platelets that do not work properly are more likely to bleed. Determining who has platelet problems can be challenging as there are limitations to diagnostic tests for these conditions. Additionally, the risks for bleeding in individuals with platelet problems are unknown. We looked at individuals with bleeding problems and found that a recommended test to assess platelet dense granule release, called lumi-aggregometry, wasn’t able to reliably identify persons with bleeding problems. Based on this, we recommend that lumi-aggregometry should not be used to diagnose platelet function disorders. We also found that individuals with uncharacterized platelet function disorders have increased risks for wound healing problems and experiencing bruising, nosebleeds, menorrhagia, and excessive bleeding after dental or surgical procedures. These risks are common among other mild bleeding disorders and will be important to differentiate bleeding risk from other platelet disorders.

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