Tissue-specific regulation of Igf2r/Airn imprinting during gastrulation

Background Appropriate epigenetic regulation of gene expression during lineage allocation and tissue differentiation is required for normal development. One example is genomic imprinting, which is defined as parent-of-origin mono-allelic gene expression. Imprinting is established largely due to epigenetic differences arriving in the zygote from sperm and egg haploid genomes. In the mouse, there are approximately 150 known imprinted genes, many of which occur in imprinted gene clusters that are regulated together. One imprinted cluster includes the maternally expressed Igf2r, Slc22a2, and Slc22a3 genes and the paternally expressed long non-coding RNA (lncRNA) Airn. Although it is known that Igf2r and Airn are reciprocally imprinted, the timing of imprinted expression and accompanying epigenetic changes have not been well characterized in vivo. Results Here we show lineage- and temporal-specific regulation of DNA methylation and histone modifications at the Igf2r/Airn locus correlating with differential establishment of imprinted expression during gastrulation. Our results show that Igf2r is expressed from both alleles in the E6.5 epiblast. After gastrulation commences, the locus becomes imprinted in the embryonic lineage with the lncRNA Airn expressed from the paternal allele and Igf2r restricted to maternal allele expression. We document differentially enriched allele-specific histone modifications in extraembryonic and embryonic tissues. We also document for the first time allele-specific spreading of DNA methylation during gastrulation concurrent with establishment of imprinted expression of Igf2r. Importantly, we show that imprinted expression does not change in the extraembryonic lineage even though maternal DMR2 methylation spreading does occur, suggesting distinct mechanisms at play in embryonic and extraembryonic lineages. Conclusions These results indicate that similar to preimplantation, gastrulation represents a window of dynamic lineage-specific epigenetic regulation in vivo

SWI/SNF chromatin-remodeling complexes in cardiovascular development and disease

Our understanding of congenital heart defects has been recently advanced by whole exome sequencing projects, which have identified de novo mutations in many genes encoding epigenetic regulators. Notably, multiple subunits of SWI/SNF chromatin-remodeling complexes have been identified as strong candidates underlying these defects because they physically and functionally interact with cardiogenic transcription factors critical to cardiac development, such as TBX5, GATA-4, and NKX2-5. While these studies indicate a critical role of SWI/SNF complexes in cardiac development and congenital heart disease, many exciting new discoveries have identified their critical role in the adult heart in both physiological and pathological conditions involving multiple cell types in the heart, including cardiomyocytes, vascular endothelial cells, pericytes, and neural crest cells. This review summarizes the role of SWI/SNF chromatinremodeling complexes in cardiac development, congenital heart disease, cardiac hypertrophy, and vascular endothelial cell survival. Although the clinical relevance of SWI/SNF mutations has traditionally been focused primarily on their role in tumor suppression, these recent studies illustrate their critical role in the heart whereby they regulate cell proliferation, differentiation, and apoptosis of cardiac derived cell lines

The role of ubiquitin ligases in cardiac disease

Rigorous surveillance of protein quality control is essential for the maintenance of normal cardiac function, while the dysregulation of protein turnover is present in a diverse array of common cardiac diseases. Central to the protein quality control found in all cells is the ubiquitin proteasome system (UPS). The UPS plays a critical role in protein trafficking, cellular signaling, and most prominently, protein degradation. As ubiquitin ligases (E3s) control the specificity of the UPS, their description in the cardiomyocyte has highlighted how ubiquitin ligases are critical to the turnover and function of the sarcomere complex, responsible for the heart’s required continuous contraction. In this review, we provide an overview of the UPS, highlighting a comprehensive overview of the cardiac ubiquitin ligases identified to date. We then focus on recent studies of new cardiac ubiquitin ligases outlining their novel roles in protein turnover, cellular signaling, and the regulation of mitochondrial dynamics and receptor turnover in the pathophysiology of cardiac hypertrophy, cardiac atrophy, myocardial infarction, and heart failure

BRG1 and BRM SWI/SNF ATPases redundantly maintain cardiomyocyte homeostasis by regulating cardiomyocyte mitophagy and mitochondrial dynamics in vivo

There has been an increasing recognition that mitochondrial perturbations play a central role in human heart failure. Discovery of mitochondrial networks, whose function is to maintain the regulation of mitochondrial biogenesis, autophagy (‘mitophagy’) and mitochondrial fusion/fission, are new potential therapeutic targets. Yet our understanding of how the molecular underpinning of these processes is just emerging. We recently identified a role of the SWI/SNF ATP-dependent chromatin remodeling complexes in the metabolic homeostasis of the adult cardiomyocyte using cardiomyocyte-specific and inducible deletion of the SWI/SNF ATPases BRG1 and BRM in adult mice (Brg1/Brm double mutant mice). To build upon these observations in early alterated metabolism, the present study looks at the subsequent alterations in mitochondrial quality control mechanisms in the impaired adult cardiomyocyte. We identified that Brg1/Brm double-mutant mice exhibited an increased mitochondrial biogenesis, increases in ‘mitophagy’, and alterations in mitochondrial fission and fusion that led to small, fragmented mitochondria. Mechanistically, increases in the autophagy and mitophagy-regulated proteins Beclin1 and Bnip3 were identified, paralleling changes seen in human heart failure. Cardiac mitochondrial dynamics were perturbed including decreased mitochondria size, reduced number, and altered expression of genes regulating fusion (Mfn1, Opa1) and fission (Drp1). We also identified cardiac protein amyloid accumulation (aggregated fibrils) during disease progression along with an increase in pre-amyloid oligomers and an upregulated unfolded protein response including increased GRP78, CHOP, and IRE-1 signaling. Together, these findings described a role for BRG1 and BRM in mitochondrial quality control, by regulating mitochondrial number, mitophagy, and mitochondrial dynamics not previously recognized in the adult cardiomyocyte. As epigenetic mechanisms are critical to the pathogenesis of heart failure, these novel pathways identified indicate that SWI/SNF chromatin remodeling functions are closely linked to mitochondrial quality control mechanisms

Tonic Activation of Bax Primes Neural Progenitors for Rapid Apoptosis through a Mechanism Preserved in Medulloblastoma

Commitment to survival or apoptosis within expanding progenitor populations poses distinct risks and benefits to the organism. We investigated whether specialized mechanisms regulate apoptosis in mouse neural progenitors and in the progenitor-derived brain tumor medulloblastoma. Here, we identified constitutive activation of proapoptotic Bax, maintained in check by Bcl-xL, as a mechanism for rapid cell death, common to postnatal neural progenitors and medulloblastoma. We found that tonic activation of Bax in cerebellar progenitors, along with sensitivity to DNA damage, was linked to differentiation state. In cerebellar progenitors, active Bax localized to mitochondria, where it was bound to Bcl-xL. Disruption of Bax:Bcl-xL binding by BH3-mimetic ABT 737 caused rapid apoptosis of cerebellar progenitors and primary murine medulloblastoma cells. Conditional deletion of Mcl-1, in contrast, did not cause death of cerebellar progenitors. Our findings identify a mechanism for the sensitivity of brain progenitors to typical anticancer therapies and reveal that this mechanism persists in medulloblastoma, a malignant brain tumor markedly sensitive to radiation and chemotherapy

SWI/SNF chromatin-remodeling complexes in cardiovascular development and disease

Our understanding of congenital heart defects has been recently advanced by whole exome sequencing projects, which have identified de novo mutations in many genes encoding epigenetic regulators. Notably, multiple subunits of SWI/SNF chromatin-remodeling complexes have been identified as strong candidates underlying these defects because they physically and functionally interact with cardiogenic transcription factors critical to cardiac development, such as TBX5, GATA-4, and NKX2-5. While these studies indicate a critical role of SWI/SNF complexes in cardiac development and congenital heart disease, many exciting new discoveries have identified their critical role in the adult heart in both physiological and pathological conditions involving multiple cell types in the heart, including cardiomyocytes, vascular endothelial cells, pericytes, and neural crest cells. This review summarizes the role of SWI/SNF chromatinremodeling complexes in cardiac development, congenital heart disease, cardiac hypertrophy, and vascular endothelial cell survival. Although the clinical relevance of SWI/SNF mutations has traditionally been focused primarily on their role in tumor suppression, these recent studies illustrate their critical role in the heart whereby they regulate cell proliferation, differentiation, and apoptosis of cardiac derived cell lines

BRG1 and BRM SWI/SNF ATPases redundantly maintain cardiomyocyte homeostasis by regulating cardiomyocyte mitophagy and mitochondrial dynamics in vivo

There has been an increasing recognition that mitochondrial perturbations play a central role in human heart failure. Discovery of mitochondrial networks, whose function is to maintain the regulation of mitochondrial biogenesis, autophagy (‘mitophagy’) and mitochondrial fusion/fission, are new potential therapeutic targets. Yet our understanding of how the molecular underpinning of these processes is just emerging. We recently identified a role of the SWI/SNF ATP-dependent chromatin remodeling complexes in the metabolic homeostasis of the adult cardiomyocyte using cardiomyocyte-specific and inducible deletion of the SWI/SNF ATPases BRG1 and BRM in adult mice (Brg1/Brm double mutant mice). To build upon these observations in early alterated metabolism, the present study looks at the subsequent alterations in mitochondrial quality control mechanisms in the impaired adult cardiomyocyte. We identified that Brg1/Brm double-mutant mice exhibited an increased mitochondrial biogenesis, increases in ‘mitophagy’, and alterations in mitochondrial fission and fusion that led to small, fragmented mitochondria. Mechanistically, increases in the autophagy and mitophagy-regulated proteins Beclin1 and Bnip3 were identified, paralleling changes seen in human heart failure. Cardiac mitochondrial dynamics were perturbed including decreased mitochondria size, reduced number, and altered expression of genes regulating fusion (Mfn1, Opa1) and fission (Drp1). We also identified cardiac protein amyloid accumulation (aggregated fibrils) during disease progression along with an increase in pre-amyloid oligomers and an upregulated unfolded protein response including increased GRP78, CHOP, and IRE-1 signaling. Together, these findings described a role for BRG1 and BRM in mitochondrial quality control, by regulating mitochondrial number, mitophagy, and mitochondrial dynamics not previously recognized in the adult cardiomyocyte. As epigenetic mechanisms are critical to the pathogenesis of heart failure, these novel pathways identified indicate that SWI/SNF chromatin remodeling functions are closely linked to mitochondrial quality control mechanisms

The role of ubiquitin ligases in cardiac disease

Rigorous surveillance of protein quality control is essential for the maintenance of normal cardiac function, while the dysregulation of protein turnover is present in a diverse array of common cardiac diseases. Central to the protein quality control found in all cells is the ubiquitin proteasome system (UPS). The UPS plays a critical role in protein trafficking, cellular signaling, and most prominently, protein degradation. As ubiquitin ligases (E3s) control the specificity of the UPS, their description in the cardiomyocyte has highlighted how ubiquitin ligases are critical to the turnover and function of the sarcomere complex, responsible for the heart’s required continuous contraction. In this review, we provide an overview of the UPS, highlighting a comprehensive overview of the cardiac ubiquitin ligases identified to date. We then focus on recent studies of new cardiac ubiquitin ligases outlining their novel roles in protein turnover, cellular signaling, and the regulation of mitochondrial dynamics and receptor turnover in the pathophysiology of cardiac hypertrophy, cardiac atrophy, myocardial infarction, and heart failure