ß-Catenin drives distinct transcriptional networks in proliferative and nonproliferative cardiomyocytes.

The inability of the adult mammalian heart to regenerate represents a fundamental barrier in heart failure management. By contrast, the neonatal heart retains a transient regenerative capacity, but the underlying mechanisms for the developmental loss of cardiac regenerative capacity in mammals are not fully understood. Wnt/ß-catenin signalling has been proposed as a key cardioregenerative pathway driving cardiomyocyte proliferation. Here, we show that Wnt/ß-catenin signalling potentiates neonatal mouse cardiomyocyte proliferation in vivo and immature human pluripotent stem cell-derived cardiomyocyte (hPSC-CM) proliferation in vitro By contrast, Wnt/ß-catenin signalling in adult mice is cardioprotective but fails to induce cardiomyocyte proliferation. Transcriptional profiling and chromatin immunoprecipitation sequencing of neonatal mouse and hPSC-CMs revealed a core Wnt/ß-catenin-dependent transcriptional network governing cardiomyocyte proliferation. By contrast, ß-catenin failed to re-engage this neonatal proliferative gene network in the adult heart despite partial transcriptional re-activation of a neonatal glycolytic gene programme. These findings suggest that ß-catenin might be repurposed from regenerative to protective functions in the adult heart in a developmental process dependent on the metabolic status of cardiomyocytes.

Sex-Specific Control of Human Heart Maturation by the Progesterone Receptor.

BACKGROUND: Despite in-depth knowledge of the molecular mechanisms controlling embryonic heart development, little is known about the signals governing postnatal maturation of the human heart. METHODS: Single-nucleus RNA sequencing of 54 140 nuclei from 9 human donors was used to profile transcriptional changes in diverse cardiac cell types during maturation from fetal stages to adulthood. Bulk RNA sequencing and the Assay for Transposase-Accessible Chromatin using sequencing were used to further validate transcriptional changes and to profile alterations in the chromatin accessibility landscape in purified cardiomyocyte nuclei from 21 human donors. Functional validation studies of sex steroids implicated in cardiac maturation were performed in human pluripotent stem cell-derived cardiac organoids and mice. RESULTS: Our data identify the progesterone receptor as a key mediator of sex-dependent transcriptional programs during cardiomyocyte maturation. Functional validation studies in human cardiac organoids and mice demonstrate that the progesterone receptor drives sex-specific metabolic programs and maturation of cardiac contractile properties. CONCLUSIONS: These data provide a blueprint for understanding human heart maturation in both sexes and reveal an important role for the progesterone receptor in human heart development.

Stage-dependent cardiac regeneration in Xenopus is regulated by thyroid hormone availability.

Despite therapeutic advances, heart failure is the major cause of morbidity and mortality worldwide, but why cardiac regenerative capacity is lost in adult humans remains an enigma. Cardiac regenerative capacity widely varies across vertebrates. Zebrafish and newt hearts regenerate throughout life. In mice, this ability is lost in the first postnatal week, a period physiologically similar to thyroid hormone (TH)-regulated metamorphosis in anuran amphibians. We thus assessed heart regeneration in Xenopus laevis before, during, and after TH-dependent metamorphosis. We found that tadpoles display efficient cardiac regeneration, but this capacity is abrogated during the metamorphic larval-to-adult switch. Therefore, we examined the consequence of TH excess and deprivation on the efficiently regenerating tadpole heart. We found that either acute TH treatment or blocking TH production before resection significantly but differentially altered gene expression and kinetics of extracellular matrix components deposition, and negatively impacted myocardial wall closure, both resulting in an impeded regenerative process. However, neither treatment significantly influenced DNA synthesis or mitosis in cardiac tissue after amputation. Overall, our data highlight an unexplored role of TH availability in modulating the cardiac regenerative outcome, and present X. laevis as an alternative model to decipher the developmental switches underlying stage-dependent constraint on cardiac regeneration.

Vegfc/d-dependent regulation of the lymphatic vasculature during cardiac regeneration is influenced by injury context.

The lymphatic vasculature mediates essential physiological functions including fluid homeostasis, lipid and hormone transport, and immune cell trafficking. Recent studies have suggested that promoting lymphangiogenesis enhances cardiac repair following injury, but it is unknown whether lymphangiogenesis is required for cardiac regeneration. Here, we describe the anatomical distribution, regulation, and function of the cardiac lymphatic network in a highly regenerative zebrafish model system using transgenic reporter lines and loss-of-function approaches. We show that zebrafish lacking functional vegfc and vegfd signaling are devoid of a cardiac lymphatic network and display cardiac hypertrophy in the absence of injury, suggesting a role for these vessels in cardiac tissue homeostasis. Using two different cardiac injury models, we report a robust lymphangiogenic response following cryoinjury, but not following apical resection injury. Although the majority of mutants lacking functional vegfc and vegfd signaling were able to mount a full regenerative response even in the complete absence of a cardiac lymphatic vasculature, cardiac regeneration was severely impaired in a subset of mutants, which was associated with heightened pro-inflammatory cytokine signaling. These findings reveal a context-dependent requirement for the lymphatic vasculature during cardiac growth and regeneration.

Development of a human skeletal micro muscle platform with pacing capabilities.

Three dimensional engineered culture systems are powerful tools to rapidly expand our knowledge of human biology and identify novel therapeutic targets for disease. Bioengineered skeletal muscle has been recently shown to recapitulate many features of native muscle biology. However, current skeletal muscle bioengineering approaches require large numbers of cells, reagents and labour, limiting their potential for high-throughput studies. Herein, we use a miniaturized 96-well micro-muscle platform to facilitate semi-automated tissue formation, culture and analysis of human skeletal micro muscles (hµMs). Utilising an iterative screening approach we define a serum-free differentiation protocol that drives rapid, directed differentiation of human myoblast to skeletal myofibres. The resulting hµMs comprised organised bundles of striated and functional myofibres, which respond appropriately to electrical stimulation. Additionally, we developed an optogenetic approach to chronically stimulate hµM to recapitulate known features of exercise training including myofibre hypertrophy and increased expression of metabolic proteins. Taken together, our miniaturized approach provides a new platform to enable high-throughput studies of human skeletal muscle biology and exercise physiology.

Evolution, comparative biology and ontogeny of vertebrate heart regeneration.

There are 64,000 living species of vertebrates on our planet and all of them have a heart. Comparative analyses devoted to understanding the regenerative potential of the myocardium have been performed in a dozen vertebrate species with the aim of developing regenerative therapies for human heart disease. Based on this relatively small selection of animal models, important insights into the evolutionary conservation of regenerative mechanisms have been gained. In this review, we survey cardiac regeneration studies in diverse species to provide an evolutionary context for the lack of regenerative capacity in the adult mammalian heart. Our analyses highlight the importance of cardiac adaptations that have occurred over hundreds of millions of years during the transition from aquatic to terrestrial life, as well as during the transition from the womb to an oxygen-rich environment at birth. We also discuss the evolution and ontogeny of cardiac morphological, physiological and metabolic adaptations in the context of heart regeneration. Taken together, our findings suggest that cardiac regenerative potential correlates with a low-metabolic state, the inability to regulate body temperature, low heart pressure, hypoxia, immature cardiomyocyte structure and an immature immune system. A more complete understanding of the evolutionary context and developmental mechanisms governing cardiac regenerative capacity would provide stronger scientific foundations for the translation of cardiac regeneration therapies into the clinic.