Mechanisms Regulating Movement and Distribution of Neuronal Mitochondria

Abstract

Mitochondria are highly mobile double-membraned organelles that serve as intracellular hubs for metabolic reactions like ATP generation, nutrient sensing and calcium signaling. The movement of mitochondria is coordinated by multiple signaling mechanisms to position mitochondria according to local metabolic requirements within a cell. Neurons are particularly reliant on a mitochondrial distribution that is responsive to local metabolism, as the rates of nutrient uptake and metabolic demands are transient and heterogeneous along their arbors. Dysregulated mitochondrial motility results in an abnormal distribution and is a hallmark of neurodegenerative and neurodevelopmental disorders. Knowing the different ways by which mitochondrial motility is regulated is fundamental to understanding the pathologies that arise from its misregulation and developing potential therapies. Mitochondria move along intracellular tracks called microtubules and their movement is powered by microtubule-based molecular motors. The motors bind mitochondria through a complex of two adaptor proteins, Miro and Milton. During the course of my thesis, I have identified a new mechanism, in which the protein FHL2 (four and a half LIM domains protein 2) anchors the mitochondrial motor-adaptor complex to the actin cytoskeleton and overrides the otherwise active molecular motors. Using rat hippocampal neurons, I have shown that the actin-dependent mitochondrial anchoring is activated by glucose sensing and adapts mitochondrial motility to glucose supply. I have found this mechanism of modulating mitochondrial motility to be ubiquitous across multiple cell types and species, including humans. I have also developed algorithms to autonomously analyze mitochondrial movement and distribution. In conjunction with robotic imaging, my algorithms have been used to study mitochondrial movement in several thousand neuronal cultures, each treated with individual bio-active compounds. This work has led to the discovery of two proteins as novel regulators of mitochondrial motility. In chapter 1 of my thesis, I provide a general introduction and relevant background. In chapter 2, I present my major discovery, the mechanism of anchoring mitochondria to filamentous actin. In chapter 3, I discuss my algorithms and highlight their utility to study mitochondrial transport especially in combination with high-throughput imaging. Lastly, I discuss some outstanding questions and future directions in Chapter 4.