2023 Theses Doctoral

The role of ER-mitochondria contact sites in the regulation of glucose metabolism: a tale of two mitochondria and its relevance to amyotrophic lateral sclerosis

Tamucci, Kirstin Arianna

The mechanisms by which mitochondria convert nutrients into cellular energy have been described in intricate detail, and yet, the regulation and compartmentalization of such metabolic pathways are poorly understood. As a result, the underlying causes of mitochondrial dysfunction and bioenergetic deficiency in diseases such as amyotrophic lateral sclerosis (ALS) remain elusive. To address this longstanding gap in the field, we first sought to understand how the metabolism of glucose and glucose-derived pyruvate are regulated in the cell.

Previous research has suggested that this metabolic regulation is mediated by specialized lipid raft domains of the endoplasmic reticulum (ER) in close contact with mitochondria, referred to as mitochondria-associated ER membranes (MAM). Using density gradient ultracentrifugation and immunoblotting techniques, we found that MAM domains play a role in the compartmentalization of glycolysis by recruiting and promoting the interaction of specific glycolytic enzymes. We then performed a series of bioenergetic, proteomic, and lipidomic analyses to determine how the establishment of ER-mitochondria contact sites at MAM affects the biology of mitochondria attached at these domains.

We observed a novel distinction between mitochondria in contact with ER-MAM domains (MER) and those that are free from the ER (FM), with MER displaying a higher capacity for pyruvate-driven respiration and FM being specialized for fatty acid-driven energy production. Finally, using cell and mouse models of ALS with mutations in superoxide dismutase 1 (SOD1), we found that the glycolytic deficiency in ALS is a direct consequence of the progressive disruption of MAM structure and function, which thereby hinders the use of glucose-derived pyruvate as a mitochondrial fuel.

This triggers a shift in mitochondrial substrate from pyruvate to fatty acids that, when sustained over time, contributes to the death of motor neurons and the progression of this fatal disease. Overall, this work aims to advance our understanding of metabolic compartmentalization, mitochondrial substrate specificity, and the relevance of both to ALS etiology.