Investigating Cell-Autonomous Mechanisms of Microglia Dysfunction in PFN1-ALS

Abstract

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by loss of motor neurons. Cumulative evidence shows that microglia contribute to disease progression, but the underlying mechanisms are unclear. Several ALS-related genes are highly expressed in microglia compared to neurons, including profilin-1 (PFN1). This raises the possibility that ALS-linked PFN1 mutations could induce microglia cell-autonomous dysfunction. Here, I sought to interrogate this possibility by differentiating human pluripotent stem cells (iPSCs) into microglia-like cells (iMGs). My work uncovered that ALS-PFN1 iMGs accumulate undegraded phagocytosed cargo in endo-lysosomal compartments which is recapitulated in vivo. ALS-PFN1 iMGs also exhibit dysregulation in the expression and cellular localization of crucial components of the endo-lysosomal pathway, impairments in the autophagy flux, and accumulation of lipid droplets. Intriguingly, rapamycin treatment ameliorates the accumulation of phagocytosed material in ALS-PFN1 iMGs and rescues the defects in the autophagy pathway, suggesting that an impaired autophagy flux contributes to ALS-PFN1-linked defects in microglial phagocytosis. In vitro experimentation uncovered that PFN1 interacts with phosphatidylinositol-3phosphate, a signaling molecule essential for autophagy and phagocytosis, and that this interaction is altered when PFN1 is mutated in ALS. Collectively, these findings implicate that ALS-PFN1 causes microglia dysfunction by hindering the autophagy flux, perturbing the endo-lysosomal pathway, and, in turn, causing delays in the degradation process during phagocytosis and inducing lipid dysmetabolism. These alterations may be partially driven by ALS-PFN1 distorted interactions with phosphoinositides. My work provides insight into PFN1 biology and opens new perspectives regarding microglia cell-autonomous defects in ALS that may contribute to neurodegeneration.