The human heart is believed to grow by enlargement but not proliferation of cardiomyocytes (heart muscle cells) during postnatal development. However, recent studies have shown that cardiomyocyte proliferation is a mechanism of cardiac growth and regeneration in animals. Combined with evidence for cardiomyocyte turnover in adult humans, this suggests that cardiomyocyte proliferation may play an unrecognized role during the period of developmental heart growth between birth and adolescence. We tested this hypothesis by examining the cellular growth mechanisms of the left ventricle on a set of healthy hearts from humans aged 0-59 y (n = 36). The percentages of cardiomyocytes in mitosis and cytokinesis were highest in infants, decreasing to low levels by 20 y. Although cardiomyocyte mitosis was detectable throughout life, cardiomyocyte cytokinesis was not evident after 20 y. Between the first year and 20 y of life, the number of cardiomyocytes in the left ventricle increased 3.4-fold, which was consistent with our predictions based on measured cardiomyocyte cell cycle activity. Our findings show that cardiomyocyte proliferation contributes to developmental heart growth in young humans. This suggests that children and adolescents may be able to regenerate myocardium, that abnormal cardiomyocyte proliferation may be involved in myocardial diseases that affect this population, and that these diseases might be treatable through stimulation of cardiomyocyte proliferation.heart failure | pediatrics H eart failure, a leading public health problem worldwide (1), is linked to the loss of cardiomyocytes (2-4). The only currently available, definitive therapy-heart transplantation-is limited by donor availability. New approaches, such as cell transplantation, have shown encouraging results in clinical trials (5, 6). However, a third, complementary strategy has emerged, based on stimulating endogenous regenerative mechanisms. One approach for developing such regeneration strategies is to examine the cellular mechanisms of myocardial growth, since mechanisms of regeneration should be similar to the mechanisms of development.Although stem and progenitor cells are important for morphogenesis of the myocardium, developmental growth in a number of nonhuman species is largely driven by cardiomyocyte proliferation (7-9). In biological models that, unlike adult humans, regenerate myocardium, cardiomyocyte proliferation is important for regeneration as well as postnatal heart growth (10, 11). For example, in mice, developmental cardiomyocyte proliferation continues for up to day 7 after birth, which coincides with the loss of regenerative capacity (11,12). The close temporal relationship between cardiomyocyte proliferation and heart regeneration in animals raises the question of whether and to what age and extent cardiomyocyte proliferation plays a role in humans. The answer may help us understand the endogenous regenerative potential of the human heart and possibly indicate strategies for stimulating cardiomyocyte proliferation to reg...

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The existence of a hematopoietic stem cell niche as a spatially confined regulatory entity relies on the notion that hematopoietic stem and progenitor cells (HSPCs) are strategically positioned in unique bone marrow (BM) microenvironments with defined anatomical and functional features. Here, we employ a powerful imaging cytometry platform to perform a comprehensive quantitative analysis of HSPC distribution in BM cavities of femoral bones. We find that HSPCs preferentially localize in endosteal zones, where the majority closely interacts with sinusoidal and non-sinusoidal BM microvessels, which form a distinctive circulatory system. In situ tissue analysis reveals that HSPCs exhibit a hypoxic profile, defined by strong retention of pimonidazole and expression of HIF-1α, regardless of localization throughout the BM, adjacency to vascular structures or cell cycle status. These studies argue that the characteristic hypoxic state of HSPCs is not solely the result of a minimally oxygenated niche but may be partially regulated by cell-specific mechanisms.

The full neutrophil heterogeneity and differentiation landscape remains incompletely characterized. Here we profiled >25,000 differentiating and mature mouse neutrophils using single-cell RNA sequencing to provide a comprehensive transcriptional landscape of neutrophil maturation, function, and fate decision in their steady state and during bacterial infection. Eight neutrophil populations were defined by distinct molecular signatures. The three mature peripheral blood neutrophil subsets arise from distinct maturing bone marrow neutrophil subsets. Driven by both known and uncharacterized transcription factors, neutrophils gradually acquire microbicidal capability as they traverse the transcriptional landscape, representing an evolved mechanism for fine-tuned regulation of an effective but balanced neutrophil response. Bacterial infection reprograms the genetic architecture of neutrophil populations, alters dynamic transition between each subpopulation, and primes neutrophils for augmented functionality without affecting overall heterogeneity. In summary, these data establish a reference model and general framework for studying neutrophil-related disease mechanisms, biomarkers, and therapeutic targets at single-cell resolution.

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The related adhesion focal tyrosine kinase (RAFTK), also known as Pyk2, undergoes autophosphorylation upon its stimulation. This leads to cascades of intracellular signaling that result in the regulation of various cellular activities. However, the molecular mechanism of RAFTK autophosphorylation is not yet known. Using various RAFTK constructs fused with two different tags, we found that the autophosphorylation of RAFTK was mediated by a trans-acting mechanism, not a cis-acting mechanism. In addition, overexpression of kinase-mutated RAFTK inhibited wild type RAFTK autophosphorylation in a dose-dependent manner by a trans-acting interaction. Trans-acting autophosphorylation was also observed between endogenous and exogenous RAFTK upon potassium depolarization of neuroendocrine PC12 cells. Using immunoprecipitation and affinity chromatography, we detected RAFTK self-association that was not affected by deletion of a single region or domain of RAFTK. Furthermore, RAFTK autophosphorylation occurred only at site Tyr 402 in a Src kinase activity-independent manner. However, Src significantly enhanced RAFTK-mediated paxillin phosphorylation, suggesting a key role for Src in RAFTK activation and phosphorylation of downstream substrates. Our results indicate that the activation of RAFTK occurs in several steps. First, upon stimulus, RAFTK trans-autophosphorylates Tyr 402 . Second, phosphorylated Tyr 402 recruits and activates Src kinase that in turn phosphorylates RAFTK and enhances its kinase activity. Lastly, the enhanced RAFTK activity induces the activation of downstream signaling molecules. Taken together, these studies provide insights into the molecular mechanism of RAFTK autophosphorylation and the specific role of Src in the regulation of RAFTK activation.

LRRC8A recognizes a ligand expressed on thymic epithelial cells and its expression on thymocytes is required for their development and function.

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