The Role of Platelets in Angiogenesis

Platelet activation near point-like source of agonist: Experimental insights and computational model

Disorders of hemostasis resulting in bleeding or thrombosis are leading cause of mortality in the world. Blood platelets are main players in hemostasis, providing the primary response to the vessel wall injury. In this case, they rapidly switch to the activated state in reaction to the exposed chemical substances such as ADP, collagen and thrombin. Molecular mechanisms of platelet activation are known, and detailed computational models are available. However, they are too complicated for large-scale problems (e.g. simulation of the thrombus growth) where less detailed models are required, which still should take into account the variation of agonist concentration and heterogeneity of platelets. In this paper, we present a simple model of the platelet population response to a spatially inhomogeneous stimulus. First, computational nodes modeling platelets are placed randomly in space. Each platelet is assigned the specific threshold for agonist, which determines whether it becomes activated at a given time. The distribution of the threshold value in a population is assumed to be log-normal. The model was validated against experimental data in a specially designed system, where the photorelease of ADP was caused by localized laser stimulus. In this system, a concentration of ADP obeys 2-dimensional Gaussian distribution which broadens due to the diffusion. The response of platelets to the point-like source of ADP is successfully described by the presented model. Our results advance the understanding of platelet function during hemostatic response. The simulation approach can be incorporated into larger computational models of thrombus formation.

De novo hematopoiesis from the fetal lung

Hemogenic endothelial cells (HECs) are specialized cells that undergo endothelial-to-hematopoietic transition (EHT) to give rise to the earliest precursors of hematopoietic progenitors that will eventually sustain hematopoiesis throughout the lifetime of an organism. Although HECs are thought to be primarily limited to the aorta-gonad-mesonephros (AGM) during early development, EHT has been described in various other hematopoietic organs and embryonic vessels. Though not defined as a hematopoietic organ, the lung houses many resident hematopoietic cells, aids in platelet biogenesis, and is a reservoir for hematopoietic stem and progenitor cells (HSPCs). However, lung HECs have never been described. Here, we demonstrate that the fetal lung is a potential source of HECs that have the functional capacity to undergo EHT to produce de novo HSPCs and their resultant progeny. Explant cultures of murine and human fetal lungs display adherent endothelial cells transitioning into floating hematopoietic cells, accompanied by the gradual loss of an endothelial signature. Flow cytometric and functional assessment of fetal-lung explants showed the production of multipotent HSPCs that expressed the EHT and pre-HSPC markers EPCR, CD41, CD43, and CD44. scRNA-seq and small molecule modulation demonstrated that fetal lung HECs rely on canonical signaling pathways to undergo EHT, including TGFβ/BMP, Notch, and YAP. Collectively, these data support the possibility that post-AGM development, functional HECs are present in the fetal lung, establishing this location as a potential extramedullary site of de novo hematopoiesis.

Comparison of Mean Platelet Counts in Preterm Infants with and without Retinopathy of Prematurity

Platelets are a primary source of pro- and anti-angiogenic cytokines. However, the evidence of their role in retinopathy of prematurity (ROP) is controversial. This retrospective study aimed to compare mean weekly platelet counts between infants with and without ROP over the first 6 weeks of life. A total of 93 infants matched by gestational age and birth weight were recruited (31 with ROP, 62 without ROP). Weekly mean platelet counts and other related risk factors were documented. The repeated measure analysis of variance (ANOVA) and the repeated measure analysis of covariance (ANCOVA) were used to compare mean platelet counts over time between the two groups, with and without adjusting for confounders. We found significant differences in the weekly mean platelet counts of infants with and without ROP over the first 6 weeks of life (p = 0.002). These differences disappeared after adjusting for covariates (p = 0.489). Lower mean platelet counts in ROP infants are not directly related to ROP, but rather to the presence of other risk factors for ROP, such as culture-proven sepsis, blood transfusion and bronchopulmonary dysplasia.