Summary Platelets are formed elements of blood. Upon activation, they externalize phosphatidylserine, thus accelerating membrane‐dependent reactions of blood coagulation. Activated platelets form two subpopulations, only one of which expresses phosphatidylserine. This study aimed to identify signalling pathways responsible for this segregation. Gel‐filtered platelets, intact or loaded with calcium‐sensitive dyes, were activated and labelled with annexin V and antibodies, followed by flow cytometric analysis. Calcium Green and Fura Red dyes were compared, and only the latter was able to detect calcium level differences in the platelet subpopulations. Phosphatidylserine‐positive platelets produced by thrombin had stably high intracellular calcium level; addition of convulxin increased and stabilized calcium level in the phosphatidylserine‐negative subpopulation. PAR1 agonist SFLLRN also induced calcium rise and subpopulation formation, but the resulting platelets were not coated with alpha‐granule proteins. Adenylatecyclase activator forskolin inhibited phosphatidylserine‐positive platelets formation several‐fold, while its inhibitor SQ22536 had no effect. This suggests that adenylatecyclase inactivation is necessary, but not rate‐limiting, for subpopulation segregation. Inhibition of mitogen‐activated protein kinase kinase (U0126) and glycoprotein IIb‐IIIa (Monafram®) was without effect, whereas inhibitors of phosphatidylinositol 3‐kinase (wortmannin) and Src tyrosine kinase (PP2) decreased the procoagulant subpopulation threefold. These data identify the principal signalling pathways controlling platelet heterogeneity.

Binding of coagulation factors X (fX) and Xa (fXa) to activated platelets is required for the formation of membrane-dependent enzymatic complexes of intrinsic tenase and prothrombinase. We carried out an in-depth characterization of fX/fXa binding to phospholipids and gel-filtered, thrombin-activated platelets. Flow cytometry, surface plasmon resonance, and computational modeling were used to investigate interactions of fX/fXa with the membranes. Confocal microscopy was employed to study fXa binding to platelet thrombi formed in flowing whole blood under arterial conditions. Binding of fX/fXa to either vesicles or procoagulant platelets did not follow a traditional one-step reversible binding model. Their dissociation was a two-step process resulting in a plateau that was up to 10-fold greater than the saturation value observed in the association experiments. Computational modeling and experimental evidence suggested that this was caused by a combination of two-step association (mainly for fX) and multimerization on the membrane (mainly for fXa). Importantly, fX formed multimers with fXa, thereby improving its retention. The same binding/dissociation hysteresis was observed for annexin V known to form trimers on the membranes. Experiments with platelets from gray syndrome patients showed that alpha-granular factor Va provided an additional high-affinity binding site for fXa that did not affect the hysteresis. Confocal microscopy observation of fXa binding to platelet thrombi in a flow chamber and its wash-out confirmed that this phenomenon persisted under physiologically relevant conditions. This suggests its possible role of "locking" coagulation factors on the membrane and preventing their inhibition in plasma and removal from thrombi by flow.

С целью разработки технологии восстановительной терапии костных тканей получен секретом мезенхимных стволовых клеток (МСК) человека после остеогенной дифференцировки МСК линии FetMSC in vitro. Для получения стандартного образца секретома использовали бессывороточную кондиционированную среду (БКС), в которой культивировали большое число клеток FetMSC (до 700 млн). Все этапы клеточного процессинга проводили в автоматизированной системе CompacT SelectT (Sartorius, Великобритания). БКС концентрировали путем ультрафильтрации, затем подвергали диализу, высушивали в вакуумном ротационном испарителе. Остеоиндуктивные свойства концентрата БКС (КБКС) тестировали на клетках линии FetMSC при его добавлении в ростовую среду в разных концентрациях. Не было выявлено изменений морфологии клеток в процессе культивирования в присутствии КБКС. Анализ экспрессии транскрипционных факторов Runx2 и YAP1 (маркеров остеогенной дифференцировки клеток), с помощью иммунофлуоресцентного окрашивания и полимеразной цепной реакции в реальном времени (RT-PCR), показал повышение уровня их экспрессии в клетках в присутствии КБКС. Результаты позволяют сделать заключение о возможности использования КБКС, полученного от остеогенных МСК, для индукции дифференцировки других МСК. Полученные результаты могут быть использованы при разработке биомедицинских клеточных продуктов для восстановления костной ткани на основе секретома МСК.