MBNL1 Regulates Programmed Postnatal Switching Between Regenerative and Differentiated Cardiac States

BACKGROUND

Discovering determinants of cardiomyocyte maturity is critical for deeply understanding the maintenance of differentiated states and potentially reawakening endogenous regenerative programs in adult mammalian hearts as a therapeutic strategy. Forced dedifferentiation paired with oncogene expression is sufficient to drive cardiac regeneration, but elucidation of endogenous developmental regulators of the switch between regenerative and mature cardiomyocyte cell states is necessary for optimal design of regenerative approaches for heart disease. MBNL1 (muscleblind-like 1) regulates fibroblast, thymocyte, and erythroid differentiation and proliferation. Hence, we examined whether MBNL1 promotes and maintains mature cardiomyocyte states while antagonizing cardiomyocyte proliferation.

METHODS

MBNL1 gain- and loss-of-function mouse models were studied at several developmental time points and in surgical models of heart regeneration. Multi-omics approaches were combined with biochemical, histological, and in vitro assays to determine the mechanisms through which MBNL1 exerts its effects.

RESULTS

MBNL1 is coexpressed with a maturation-association genetic program in the heart and is regulated by the MEIS1/calcineurin signaling axis. Targeted MBNL1 overexpression early in development prematurely transitioned cardiomyocytes to hypertrophic growth, hypoplasia, and dysfunction, whereas loss of MBNL1 function increased cardiomyocyte cell cycle entry and proliferation through altered cell cycle inhibitor transcript stability. Moreover, MBNL1-dependent stabilization of estrogen-related receptor signaling was essential for maintaining cardiomyocyte maturity in adult myocytes. In accordance with these data, modulating MBNL1 dose tuned the temporal window of neonatal cardiac regeneration, where increased MBNL1 expression arrested myocyte proliferation and regeneration and MBNL1 deletion promoted regenerative states with prolonged myocyte proliferation. However, MBNL1 deficiency was insufficient to promote regeneration in the adult heart because of cell cycle checkpoint activation.

CONCLUSIONS

Here, MBNL1 was identified as an essential regulator of cardiomyocyte differentiated states, their developmental switch from hyperplastic to hypertrophic growth, and their regenerative potential through controlling an entire maturation program by stabilizing adult myocyte mRNAs during postnatal development and throughout adulthood. Targeting loss of cardiomyocyte maturity and downregulation of cell cycle inhibitors through MBNL1 deletion was not sufficient to promote adult regeneration.

MBNL1 regulates programmed postnatal switching between regenerative and differentiated cardiac states

Discovering determinants of cardiomyocyte maturity and the maintenance of differentiated states is critical to both understanding development and potentially reawakening endogenous regenerative programs in adult mammalian hearts as a therapeutic strategy. Here, the RNA binding protein Muscleblind-like 1 (MBNL1) was identified as a critical regulator of cardiomyocyte differentiated states and their regenerative potential through transcriptome-wide control of RNA stability. Targeted MBNL1 overexpression early in development prematurely transitioned cardiomyocytes to hypertrophic growth, hypoplasia, and dysfunction, whereas loss of MBNL1 function increased cardiomyocyte cell cycle entry and proliferation through altered cell cycle inhibitor transcript stability. Moreover, MBNL1-dependent stabilization of the estrogen-related receptor signaling axis was essential for maintaining cardiomyocyte maturity. In accordance with these data, modulating MBNL1 dose tuned the temporal window of cardiac regeneration, where enhanced MBNL1 activity arrested myocyte proliferation, and MBNL1 deletion promoted regenerative states with prolonged myocyte proliferation. Collectively these data suggest MBNL1 acts as a transcriptome-wide switch between regenerative and mature myocyte states postnatally and throughout adulthood.

Correcting dilated cardiomyopathy with fibroblast-targeted p38 deficiency

Inherited mutations in contractile and structural genes, which decrease cardiomyocyte tension generation, are principal drivers of dilated cardiomyopathy (DCM)- the leading cause of heart failure ^1,2 . Progress towards developing precision therapeutics for and defining the underlying determinants of DCM has been cardiomyocyte centric with negligible attention directed towards fibroblasts despite their role in regulating the best predictor of DCM severity, cardiac fibrosis ^3,4 . Given that failure to reverse fibrosis is a major limitation of both standard of care and first in class precision therapeutics for DCM, this study examined whether cardiac fibroblast-mediated regulation of the heart's material properties is essential for the DCM phenotype. Here we report in a mouse model of inherited DCM that prior to the onset of fibrosis and dilated myocardial remodeling both the myocardium and extracellular matrix (ECM) stiffen from switches in titin isoform expression, enhanced collagen fiber alignment, and expansion of the cardiac fibroblast population, which we blocked by genetically suppressing p38α in cardiac fibroblasts. This fibroblast-targeted intervention unexpectedly improved the primary cardiomyocyte defect in contractile function and reversed ECM and dilated myocardial remodeling. Together these findings challenge the long-standing paradigm that ECM remodeling is a secondary complication to inherited defects in cardiomyocyte contractile function and instead demonstrate cardiac fibroblasts are essential contributors to the DCM phenotype, thus suggesting DCM-specific therapeutics will require fibroblast-specific strategies.

MBNL1 drives dynamic transitions between fibroblasts and myofibroblasts in cardiac wound healing

Dynamic fibroblast to myofibroblast state transitions underlie the heart's fibrotic response. Because transcriptome maturation by muscleblind-like 1 (MBNL1) promotes differentiated cell states, this study investigated whether tactical control of MBNL1 activity could alter myofibroblast activity and fibrotic outcomes. In healthy mice, cardiac fibroblast-specific overexpression of MBNL1 transitioned the fibroblast transcriptome to that of a myofibroblast and after injury promoted myocyte remodeling and scar maturation. Both fibroblast- and myofibroblast-specific loss of MBNL1 limited scar production and stabilization, which was ascribed to negligible myofibroblast activity. The combination of MBNL1 deletion and injury caused quiescent fibroblasts to expand and adopt features of cardiac mesenchymal stem cells, whereas transgenic MBNL1 expression blocked fibroblast proliferation and drove the population into a mature myofibroblast state. These data suggest MBNL1 is a post-transcriptional switch, controlling fibroblast state plasticity during cardiac wound healing.

Direct coculture of human pluripotent stem cell-derived cardiac progenitor cells with epicardial cells induces cardiomyocyte proliferation and reduces sarcomere organization

Epicardial cells (EpiCs) are necessary for myocardium formation, yet little is known about crosstalk between EpiCs and cardiomyocytes (CMs) during development and the potential impact of EpiCs on CM maturation. To investigate the effects of EpiCs on CM commitment and maturation, we differentiated human pluripotent stem cells (hPSCs) to cardiac progenitor cells (CPCs) and EpiCs, and cocultured EpiCs and CPCs for two weeks. When EpiCs were allowed to form epicardial-derived cells, we observed increased expression of cTnI in developing CMs. In the presence of the TGFβ inhibitor A83-01, EpiCs remained in the epicardial state and induced CM proliferation, increased MLC2v expression, and led to less organized sarcomeres. These effects were not observed if CPCs were treated with EpiC-conditioned medium or if CPCs were indirectly cocultured with EpiCs. Finally, single cell RNA sequencing identified that EpiC-CPC coculture had bi-directional effects on transcriptional programs in EpiCs and CMs, and biased EpiC lineages from a SFRP2-enriched population to a DLK1- or C3-enriched population. This work suggests important crosstalk between EpiCs and CMs during differentiation which can be used to influence cell fate and improve the ability to generate cardiac cells and tissues for in vitro models and development of cardiac cellular therapies.

Controlling cardiac fibrosis through fibroblast state space modulation

The transdifferentiation of cardiac fibroblasts into myofibroblasts after cardiac injury has traditionally been defined by a unidirectional continuum from quiescent fibroblasts, through activated fibroblasts, and finally to fibrotic-matrix producing myofibroblasts. However, recent lineage tracing and single cell RNA sequencing experiments have demonstrated that fibroblast transdifferentiation is much more complex. Growing evidence suggests that fibroblasts are more heterogenous than previously thought, and many new cell states have recently been identified. This review reexamines conventional fibroblast transdifferentiation paradigms with a dynamic state space lens, which could enable a more complex understanding of how fibroblast state dynamics alters fibrotic remodeling of the heart. This review will define cellular state space, how it relates to fibroblast state transitions, and how the canonical and non-canonical fibrotic signaling pathways modulate fibroblast cell state and cardiac fibrosis. Finally, this review explores the therapeutic potential of fibroblast state space modulation by p38 inhibition, yes-associated protein (YAP) inhibition, and fibroblast reprogramming.

Coculture of Endothelial Cells with Human Pluripotent Stem Cell-Derived Cardiac Progenitors Reveals a Differentiation Stage-Specific Enhancement of Cardiomyocyte Maturation

Cardiomyocytes (CMs) generated from human pluripotent stem cells (hPSCs) are immature in their structure and function, limiting their potential in disease modeling, drug screening, and cardiac cellular therapies. Prior studies have demonstrated that coculture of hPSC-derived CMs with other cardiac cell types, including endothelial cells (ECs), can accelerate CM maturation. To address whether the CM differentiation stage at which ECs are introduced affects CM maturation, the authors coculture hPSC-derived ECs with hPSC-derived cardiac progenitor cells (CPCs) and CMs and analyze the molecular and functional attributes of maturation. ECs have a more significant effect on acceleration of maturation when cocultured with CPCs than with CMs. EC coculture with CPCs increases CM size, expression of sarcomere, and ion channel genes and proteins, the presence of intracellular membranous extensions, and chronotropic response compared to monoculture. Maturation is accelerated with an increasing EC:CPC ratio. This study demonstrates that EC incorporation at the CPC stage of CM differentiation expedites CM maturation, leading to cells that may be better suited for in vitro and in vivo applications of hPSC-derived CMs.