Substratum stiffness controls the subcellular localization of Rac1b, a highly activated splice variant of the small GTPase Rac1. On stiff substrata, Rac1b localizes to the plasma membrane, forming a complex with NADPH oxidase and generating ROS, thus inducing the expression of the transcription factor Snail and downstream signaling to EMT.

Cardiomyocyte cell division and replication in mammals proceed through embryonic development and abruptly decline soon after birth. The process governing cardiomyocyte cell cycle arrest is poorly understood. Here we carry out whole exome sequencing in an infant with evidence of persistent postnatal cardiomyocyte replication to determine the genetic risk factors. We identify compound heterozygous ALMS1 mutations in the proband, and confirm their presence in her affected sibling, one copy inherited from each heterozygous parent. Next, we recognise homozygous or compound heterozygous truncating mutations in ALMS1 in four other children with high levels of postnatal cardiomyocyte proliferation. Alms1 mRNA knockdown increases multiple markers of proliferation in cardiomyocytes, the percentage of cardiomyocytes in G2/M phases, and the number of cardiomyocytes by 10% in cultured cells. Homozygous Alms1-mutant mice have increased cardiomyocyte proliferation at two weeks postnatal compared to wild-type littermates. We conclude that deficiency of Alström protein impairs postnatal cardiomyocyte cell cycle arrest.

One of the leading causes of death worldwide is heart failure. Despite advances in the treatment and prevention of heart failure, the number of affected patients continues to increase. We have recently developed 3D-bioprinted biomaterial-free cardiac tissue that has the potential to improve cardiac function. This study aims to evaluate the in vivo regenerative potential of these 3D-bioprinted cardiac patches. The cardiac patches were generated using 3D-bioprinting technology in conjunction with cellular spheroids created from a coculture of human-induced pluripotent stem cell-derived cardiomyocytes, fibroblasts, and endothelial cells. Once printed and cultured, the cardiac patches were implanted into a rat myocardial infarction model (n = 6). A control group (n = 6) without the implantation of cardiac tissue patches was used for comparison. The potential for regeneration was measured 4 weeks after the surgery with histology and echocardiography. 4 weeks after surgery, the survival rates were 100% and 83% in the experimental and the control group, respectively. In the cardiac patch group, the average vessel counts within the infarcted area were higher than those within the control group. The scar area in the cardiac patch group was significantly smaller than that in the control group. (Figure S1) Echocardiography showed a trend of improvement of cardiac function for the experimental group, and this trend correlated with increased patch production of extracellular vesicles. 3D-bioprinted cardiac patches have the potential to improve the regeneration of cardiac tissue and promote angiogenesis in the infarcted tissues and reduce the scar tissue formation.

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The use left ventricular assist devices (LVAD) as a bridge-to-transplant (BTT) has become a common modality to treat end-stage heart failure. We sought to examine the impact of BTT on long-term survival and quality of life after heart transplant. The population was all adult patients undergoing isolated heart transplantation in the United States between 2007 and 2017. Inclusion criteria covered BTT patients with a LVAD (only Heartmate II [HMII] or HeartWare Ventricular Assist System [HVAD]) and compared these with patients undergoing de novo heart transplantation. Our primary end-point was survival at 1, 2, and 5 years. Secondary end-points were functional status, return to work, and rates of hospital readmission and graft rejection. Unconditional and conditional survival was estimated with the Kaplan-Meier method. The independent influence of BTT on risk-adjusted mortality was determined using Cox proportional hazards models. In this period, 5,584 patients were bridged with an LVAD and 12,295 underwent de novo transplantation. Unconditional survival was 2% higher in de novo patients at 1, 2, and 5 years. After risk adjustment, BTT was associated with increased mortality at each time point. Unadjusted 5 year survival, conditional on 90 day survival, was similar between groups (82.6% vs. 83.4%; p = 0.15). Functional status, return to work, and unadjusted rates of hospital readmission and graft rejection were similar at 1, 2, 5 years. Bridge-to-transplant with LVADs provides excellent survival and similar quality of life to that of patients undergoing de novo heart transplantation. Bridge-to-transplant patients experience a slightly higher mortality rate within 90 days of transplantation.

The regenerative capacity of the mammary gland following post-lactational involution depends on the presence of multipotent stem or progenitor cells. Mammary progenitor cells exist as a quiescent and self-renewing population capable of differentiating into luminal epithelial and myoepithelial cells and generating ductal and alveolar structures. The fate choices of these cells are regulated by several soluble signals as well as their surrounding extracellular matrix. Whereas matrix stiffness has been implicated in organ-specific differentiation of embryonic and mesenchymal stem cells, the effects of substratum compliance on the more limited fate switches typical of tissue-specific progenitor cells is unknown. Here we examined how the mechanical properties of the microenvironment affect the differentiation of mammary progenitor cells. Immortalized human mammary progenitor cells were cultured on synthetic hydrogels of varying stiffness and their self-renewal and fate decisions were quantified. We found that cells cultured on soft substrata differentiated preferentially into luminal epithelial cells, whereas those cultured on stiff substrata differentiated preferentially into myoepithelial cells. Furthermore, pharmacological manipulations of cytoskeletal tension in conjunction with analysis of gene expression revealed that mechanical properties of the microenvironment signal through the small GTPase RhoA and cytoskeletal contractility to modulate the differentiation of mammary progenitor cells. These data suggest that subtle variations in the mechanical compliance of a tissue can direct the fate decisions of its resident progenitor cells.

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