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

Abstract

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.

BRG1 and BRM function antagonistically with c-MYC in adult cardiomyocytes to regulate conduction and contractility

“…We previously documented conditional loss of Brg1 in cardiomyocytes within 7 days of tamoxifen treatment in this model by PCR and IHC (Supplemental Fig. 1A–C) [11,12]. These mice (herein referred to as Brg1/Brm double mutants), which are null for BRG1 and BRM in cardiomyocytes, die at 6–22 days (mean of 11.6 ± 1.5 days) relative to the first day of tamoxifen treatment (Supplemental Fig.…”

“…These mice (herein referred to as Brg1/Brm double mutants), which are null for BRG1 and BRM in cardiomyocytes, die at 6–22 days (mean of 11.6 ± 1.5 days) relative to the first day of tamoxifen treatment (Supplemental Fig. 1D) [12]. We have demonstrated that Brg1 conditional mutants on a wild-type background are viable, as are Brm −/− mice, indicating that the two catalytic subunits are functionally redundant in adult cardiomyocytes (Supplemental Fig.…”

“…We have demonstrated that Brg1 conditional mutants on a wild-type background are viable, as are Brm −/− mice, indicating that the two catalytic subunits are functionally redundant in adult cardiomyocytes (Supplemental Fig. 1D) [12]. …”

“…IHC was performed with a BRG1 antibody (Millipore #07-478) as described previously [12], and western blots were performed with c-MYC (Abcam ab#32072) and GAPDH (Sigma G8795) antibodies following standard procedures.…”

“…To test the hypothesis that BRG1 and BRM functionally compensate in the adult cardiomyocyte, we generated Brg1 fl/fl mice carrying an inducible, cardiomyocyte-specific αMHC-Cre-ERT2 transgene that were also Brm −/− . Indeed, these Brg1/Brm double mutants died within 3 weeks following the loss of Brg1 , and they exhibited metabolic perturbations and mitochondrial defects in the heart [11,12]. However, their cause of death is unclear, and it is not known whether SWI/SNF is required for cardiac conduction or if this model is relevant to the study of heart failure.…”