Abstract 17075: Preferential Targeting of Bnip3 Isoforms to Mitochondria or Endoplasmic Reticulum During Metabolic Stress Promotes Autophagy or Cell Survival of Post-Natal Ventricular Myocytes

Abstract

Autophagy is an evolutionary conserved process that allows cells to recycle or discard macromolecular constituents or damaged organelles during times of metabolic crisis. Alternative mRNA splicing is a versatile mechanism by which cells generate proteins with different or even antagonistic properties. Herein, we show that inclusion or skipping of exon3 of Bnip3 mRNA generates proteins with distinct and opposing actions on autophagy and cell survival. Metabolic stress imposed by hypoxia or nutrient deprivation resulted in the synthesis of two Bnip3 mRNA isoforms in post-natal ventricular myocytes in vitro and in vivo. Notably, one Bnip3 mRNA comprised of exons 1 through 6 encoded a protein of 26kDa, while a second mRNA generated by the fusion of exon2 and exon4 encoded a truncated Bnip3 protein of 8.2kDa. Sequence analysis revealed the truncated isoform encodes a conserved C-terminus domain that exclusively targets Bnip3 to the endoplasmic reticulum (ER) and not mitochondrion. While the 26kDa Bnip3 induced mitochondrial perturbations and autophagy, the spliced isoform suppressed Bnip3 induced mitochondrial defects and autophagy. Interestingly switching from glucose to fatty metabolism by supplementing nutrient deprived ventricular myocytes with carnitine but not pyruvate preferentially accumulated the spliced isoform of Bnip3 and the association of Bcl-2/Beclin-1 complexes at the ER, while suppressing autophagy. Furthermore, C-terminal domain mutations of the spliced variant defective for ER targeting sensitized cardiac myocytes to mitochondrial perturbations and apoptosis. To our knowledge our data provide the first direct evidence for a novel survival mechanism whereby the metabolic status of the cell programs autophagy or apoptosis by preferentially targeting Bnip3 isoforms to mitochondria or ER during metabolic stress.