BNIP3L/Nix is required for mitochondrial elimination through mitophagy and the subsequent elimination of endoplasmic reticulum during the lens fiber cell differentiation program.

Abstract

Purpose: Degradation of mitochondria and other organelles is a key feature of cellular differentiation and remodeling. Mitochondria elimination is a key step in the program of lens cell differentiation during formation of the organelle free zone (OFZ) making the lens an outstanding model for revealing the role of mitophagy in normal cellular functions. Building on our recent discovery that mitophagy is responsible for the elimination of mitochondria during lens cell differentiation, we investigated the role of the mitophagy regulatory protein BNIP3L/Nix in this process and the potential effects on loss of other lens organelles

Methods: High-throughput RNA sequencing and bioinformatic analysis identified that the mitochondrial depolarizing protein BNIP3L/Nix exhibits a spatial expression pattern that parallels lens mitochondrial elimination during lens cell differentiation. The requirement for BNIP3L/Nix in differentiation-state dependent removal of mitochondria and other organelles during lens cell differentiation was determined through immunolocalization and biochemical analysis in chick lens growing in organ culture following overexpression of either wild-type or recombinant mutant forms of BNIP3L/Nix, or silencing of the BNIP3L/Nix gene, as well as in the lenses of BNIP3L/Nix knockout mice.

Results: Silencing of BNIP3L/Nix in cultured lenses or deletion of the BNIP3L/Nix gene in mouse lenses in vivo prevented mitochondria from being eliminated during lens cell differentiation. In contrast, over-expression of BNIP3L/Nix resulted in accelerated mitochondrial elimination in lens fiber cells. Elimination of endoplasmic reticulum but not nuclei was dependent on the action of BNIP3L/Nix.<br />

Conclusions: BNIP3L/Nix is required for elimination of mitochondria during lens fiber cell differentiation. Interestingly, BNIP3L/Nix function also is required for eliminating endoplasmic reticulum but not nuclei for formation of the organelle free zone. This establishes a novel requirement for BNIP3L/Nix in the lens cell differentiation program that has implications for understanding the roles for BNIP3L/Nix in mitochondrial regulation for homeostasis, development, differentiation and disease in more complex tissues.