Bid homeostatically regulates mitochondrial cristae structure and necrotic cell death to protect against cardiac disease and bone marrow failure

Abstract

The Bcl-2 family protein Bid was originally characterized as a cytochrome c releasing factor during intrinsic apoptotic death. We and others noted that Bid is localized to the mitochondria in the absence of cell death. Bid-/- myeloid progenitors paradoxically display decreased viability in vitro. We took a multidisciplinary approach to determine how mitochondrial Bid impacts survival. Firstly, Bid-/- myeloid progenitor cells and cardiomyocytes revealed a significant decrease in mitochondrial cristae number, perturbation of structure, and consequently decreased respiratory function. Bid-/- cardiomyocytes have decreased ATP production, and mice are prone to cardiac dysfunction under acute stress. Two genetic approaches uncovered the impact of BID on human disease. PrediXcan analysis, which evaluates gene expression based on germline variation, uncovered decreased BID associated with myocardial infarction (MI) in multiple independent cohorts, including BioVU. Whole exome sequencing (WES) of BioVU patients identified rare coding variation within BID’s membrane binding domain associated with MI. The helix 6 single nucleotide polymorphism (SNP), M148T, identified by WES, results in loss of function when this point mutation is combined with a rescue mutant of Bid, and disrupts an interaction with the Bcl-2 protein Mcl-1, previously implicated in cristae maintenance. Importantly, regulation of cell homeostasis by Bid also extends beyond the mitochondria. Loss of Bid in the absence of Bax and Bak (VavCre +Bax F/FBak -/-Bid -/- (TKO) mice) results in bone marrow failure driven by Rip1-kinase necrosis. This bone marrow failure results in disrupted hematopoiesis and erythropoiesis that phenotypically mimics the human disorder Myelodysplastic Syndrome (MDS). Genetically altering Ripk1 function modulates inflammation and erythropoiesis in TKO mice as well as mice with loss of the epigenetic regulator TET2 (VavCre+Tet2F/F). In sum, these findings are important for understanding the role of the Bcl-2 family in mitochondrial and hematopoietic homeostasis, and highlight our complimentary approach combining cell biology with human genetics.