VAPB mediated Lipid Dysregulation and its role in the pathogenesis of Amyotrophic Lateral Sclerosis

Abstract

Lipids are the integral component of membranes and serve key biological functions. The endoplasmic reticulum (ER) acts as a hub for lipid synthesis and transport. VAMP Associated Protein B (VAPB/VAP) is an ER-based, cytoplasmic-facing, transmembrane protein implicated in multiple critical functions of the cell. It helps maintain intracellular membrane:membrane contact sites (MCS) of the ER with other organelle membranes. VAP interacts with lipid transfer proteins (LTPs) via their FFAT motif, facilitating lipid transfer to different organelles. Mutations in VAP can cause Amyotrophic Lateral Sclerosis (ALS) in humans, with ALS8/VAP(P56S) being the predominant mutation. It leads to progressive neurodegenerative disease marked by motor neuron loss leading to gradual paralysis and death within 2-5 years post-diagnosis. An orthologous mutation VAP(P58S) is used in Drosophila to model ALS8 pathogenesis. VAP(P58S) flies develops normally; however, adult animals exhibit a shortened lifespan and age-dependent progressive motor dysfunction. In the adult VAP(P58S) brain, we observe mutant protein inclusions, ER stress, and increased inflammation. Expression of VAP(WT) can rescue all disease phenotypes. However, its impact on lipid metabolism is poorly understood. We investigated lipid dysbiosis in the VAP(P58S) brain using a combination of lipid mass spectrometry and genetic analysis. We observed age-dependent accumulation of cholesterol esters and sphingolipids, accompanied by a concomitant decrease in cholesterol, which could be rescued by VAP(WT) expression. Lipid Droplet (LD) homeostasis is also perturbed in ALS8 brains, with our investigations reveal a glial role in LD regulation. Ceramide Transfer (CERT), an interactor of VAP, is a cytosolic protein that transfers ceramide from ER to Golgi. The Golgi uses the transported ceramide to form other sphingolipids. Modulating CERT in neurons and glia of VAP(P58S) flies revealed differential effects on motor function. Further, my exploration uncovered a novel role of CERT in glial LD regulation, where VAP interaction with CERT being important. Our study highlights the critical roles of VAP-enriched membrane contact sites in lipid transfer and LD biogenesis, which, in turn, affect cellular physiology. Lipid imbalance, as observed in the ALS8/VAP(P58S) mutant, points towards a vital role of lipid metabolism in ALS pathogenesis.