The Mitochondrial Inner Membrane Protein Mitofilin Controls Cristae Morphology

TLDR

Mitochondria are complex organelles with a highly dynamic distribution and internal organization. The study demonstrates that mitofilin, a mitochondrial protein of previously unknown function, controls cristae morphology and is essential for normal mitochondrial function. Mitofilin localizes to the narrow space between the inner boundary and outer membranes, forming homotypic interactions and assembling into a large multimeric complex. Mitofilin knockdown in HeLa cells caused reduced proliferation, increased apoptosis, disorganized cristae lacking tubular structures and junctions, an elevated inner‑to‑outer membrane ratio, heightened reactive oxygen species and membrane potential, and a mismatch between increased metabolic flux and unchanged oxidative phosphorylation.

Abstract

Mitochondria are complex organelles with a highly dynamic distribution and internal organization. Here, we demonstrate that mitofilin, a previously identified mitochondrial protein of unknown function, controls mitochondrial cristae morphology. Mitofilin is enriched in the narrow space between the inner boundary and the outer membranes, where it forms a homotypic interaction and assembles into a large multimeric protein complex. Down-regulation of mitofilin in HeLa cells by using specific small interfering RNA lead to decreased cellular proliferation and increased apoptosis, suggesting abnormal mitochondrial function. Although gross mitochondrial fission and fusion seemed normal, ultrastructural studies revealed disorganized mitochondrial inner membrane. Inner membranes failed to form tubular or vesicular cristae and showed as closely packed stacks of membrane sheets that fused intermittently, resulting in a complex maze of membranous network. Electron microscopic tomography estimated a substantial increase in inner:outer membrane ratio, whereas no cristae junctions were detected. In addition, mitochondria subsequently exhibited increased reactive oxygen species production and membrane potential. Although metabolic flux increased due to mitofilin deficiency, mitochondrial oxidative phosphorylation was not increased accordingly. We propose that mitofilin is a critical organizer of the mitochondrial cristae morphology and thus indispensable for normal mitochondrial function.