Investigation of a Misfolded, Destabilized Profilin-1 Species as a Toxic Molecule in ALS Pathogenesis

Dominant mutations in profilin-1 (PFN1) are associated with amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease characterized by motor neuron loss, paralysis, and death from respiratory failure. Our lab recently demonstrated that PFN1 mutant proteins are destabilized—they unfold at milder conditions during thermal and chemical denaturation. Furthermore, we and others have shown that mutant PFN1 is more prone to misfold and aggregate. This misfolding alters PFN1’s protein-protein interactions, as demonstrated by an affinity purification-mass spectrometry screen. While ALS-associated mutants do not show loss of interaction, several have altered interactions with several formin family proteins, a group of proteins that interacts with profilins to regulate actin polymerization. These perturbations in profilin-formin interaction result in changes in actin metabolism, as shown by stress fiber formation in a HeLa model and neurite outgrowth in an iPSC-derived neuron model. Additionally, one mutant shows increased actin filament survival time in a microfluidic experiment, indicative of tighter binding in the actin-profilin-formin complex at the growing end of a filament. Misfolding and aggregation also puts additional stress on the cell’s proteostasis pathways. A cell culture model shows that misfolded Pfn1 is processed primarily by the proteasome, with modest contributions from autophagy. Together, this evidence provides additional support for two theories of Pfn1 ALS pathogenesis: disruptions in cytoskeletal function and proteostatic stress