Glucocorticoid stress hormones are powerful modulators of brain function and can affect mood and cognitive processes. The hippocampus is a prominent glucocorticoid target and expresses both the glucocorticoid receptor (GR: Nr3c1) and the mineralocorticoid receptor (MR: Nr3c2). These nuclear steroid receptors act as ligand‐dependent transcription factors. Transcriptional effects of glucocorticoids have often been deduced from bulk mRNA measurements or spatially informed individual gene expression. However, only sparse data exists allowing insights on glucocorticoid‐driven gene transcription at the cell type level. Here, we used publicly available single‐cell RNA sequencing data to assess the cell‐type specificity of GR and MR signaling in the adult mouse hippocampus. The data confirmed that Nr3c1 and Nr3c2 expression differs across neuronal and non‐neuronal cell populations. We analyzed co‐expression with sex hormones receptors, transcriptional coregulators, and receptors for neurotransmitters and neuropeptides. Our results provide insights in the cellular basis of previous bulk mRNA results and allow the formulation of more defined hypotheses on the effects of glucocorticoids on hippocampal function.
Adult mammalian hearts do not regenerate following ischemic injury, causing permanent damage to the myocardium, often leading to heart failure. In contrast, neonatal mouse hearts can fully regenerate after injury, however this ability is lost few days after birth. Loss of regenerative capacity coincides with profound changes in the epigenetic landscape. Yet, the mechanisms controlling cardiomyocyte proliferation remain poorly understood. To identify epigenetic mechanisms that underlie cardiomyocyte regeneration in response to ischemic injury, we subjected mice to sham or ischemia-reperfusion injury (IR) and performed RNA-Seq at multiple timepoints after injury. Multiple SWI/SNF chromatin remodeling complex subunits were upregulated after IR, including the AT-rich interactive domain-containing protein 1A ( Arid1a ), which has previously been implicated in tissue regeneration. Here, we show that Arid1a is abundantly expressed in cardiomyocytes during development, and is reactivated in a subset of adult cardiomyocytes after IR. Moreover, ARID1A is highly expressed in cardiomyocytes in human failing hearts, suggesting an important function in injury response. Cardiomyocyte-specific Arid1a ablation around birth ( Arid1a cKO ) in mice induced cardiomyocyte hyperplasia, and severe cardiac enlargement at 2 weeks of age. Arid1a cKO hearts displayed increased expression of key cell cycle genes, and HIPPO target genes, suggesting Arid1a is required for cell cycle withdrawal in neonatal cardiomyocytes. When Arid1a was inducibly removed from adult cardiomyocytes ( Arid1a icKO ), hearts had normal gross morphology without visible signs of cardiac pathology. Next, we performed IR injury on Arid1a icKO mice and observed increased cell cycle activity in mutant border zone cardiomyocytes. To further explore the mechanisms by which Arid1a functions in cardiomyocytes, we are currently performing ChIP-Seq and protein interaction studies. These data suggest that Arid1a regulates cardiomyocyte proliferation and function. Upregulation of Arid1a in cardiomyocytes after injury may suppress proliferation and regeneration. Suppression of Arid1a after ischemic injury may prove to be a novel target for therapeutics to enhance cardiac regeneration.
The inability of adult human cardiomyocytes to proliferate is an obstacle to efficient cardiac regeneration after injury. Understanding the mechanisms that drive postnatal cardiomyocytes to switch to a non-regenerative state is therefore of great significance. Here we show that Arid1a, a subunit of the switching defective/sucrose non-fermenting (SWI/SNF) chromatin remodeling complex, suppresses postnatal cardiomyocyte proliferation while enhancing maturation. Genome-wide transcriptome and epigenome analyses revealed that Arid1a is required for the activation of a cardiomyocyte maturation gene program by promoting DNA access to transcription factors that drive cardiomyocyte maturation. Furthermore, we show that ARID1A directly binds and inhibits the proliferation-promoting transcriptional coactivators YAP and TAZ, indicating ARID1A sequesters YAP/TAZ from their DNA-binding partner TEAD. In ischemic heart disease, Arid1a expression is enhanced in cardiomyocytes of the border zone region. Inactivation of Arid1a after ischemic injury enhanced proliferation of border zone cardiomyocytes. Our study illuminates the pivotal role of Arid1a in cardiomyocyte maturation, and uncovers Arid1a as a crucial suppressor of cardiomyocyte proliferation.
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