MFF-dependent mitochondrial fission regulates presynaptic release and axon branching by limiting axonal mitochondria size

Tommy L Lewis,Franck Polleux,Reuben Shaw,Seok-Kyu Kwon,Annie Lee

doi:10.1038/s41467-018-07416-2

Abstract

Neurons display extreme degrees of polarization, including compartment-specific organelle morphology. In cortical, long-range projecting, pyramidal neurons (PNs), dendritic mitochondria are long and tubular whereas axonal mitochondria display uniformly short length. Here we explored the functional significance of maintaining small mitochondria for axonal development in vitro and in vivo. We report that the Drp1 ‘receptor’ Mitochondrial fission factor (MFF) is required for determining the size of mitochondria entering the axon and then for maintenance of their size along the distal portions of the axon without affecting their trafficking properties, presynaptic capture, membrane potential or ability to generate ATP. Strikingly, this increase in presynaptic mitochondrial size upon MFF downregulation augments their capacity for Ca2+ ([Ca2+]m) uptake during neurotransmission, leading to reduced presynaptic [Ca2+]c accumulation, decreased presynaptic release and terminal axon branching. Our results uncover a novel mechanism controlling neurotransmitter release and axon branching through fission-dependent regulation of presynaptic mitochondrial size.

Highlights

Neurons display extreme degrees of polarization, including compartment-specific organelle morphology
In order to visualize mitochondrial morphology and function in developing and adult cortical neurons in vitro and in vivo, we implemented a strategy for sparse labeling via ex utero or in utero electroporation (EUE and IUE, respectively)[33]
Our results demonstrate that loss of Mitochondrial fission factor (MFF) activity increased mitochondria length by reducing fission for both mitochondria entering the axon as well as along the axonal shaft

Summary

Introduction

Neurons display extreme degrees of polarization, including compartment-specific organelle morphology. We report that the Drp1 ‘receptor’ Mitochondrial fission factor (MFF) is required for determining the size of mitochondria entering the axon and for maintenance of their size along the distal portions of the axon without affecting their trafficking properties, presynaptic capture, membrane potential or ability to generate ATP. This increase in presynaptic mitochondrial size upon MFF downregulation augments their capacity for Ca2+ ([Ca2+]m) uptake during neurotransmission, leading to reduced presynaptic [Ca2+]c accumulation, decreased presynaptic release and terminal axon branching. Mitochondrial fusion is required for proper development as each of the Mfn[1], Mfn[2], and Opa[1] knockouts are lethal[15,31,32], their role in cortical neuron development is less well studied

Methods

Results

Conclusion