Background: FUS is an RNA/DNA binding proteins that mislocalized to the cytoplasm and causes a severe form of Amyotrophic Lateral Sclerosis (ALS). Recently, we identified a novel isoform of FUS, termed extracellular FUS (EC-FUS). The actin cytoskeleton is a key determinant of neuronal function. Rapid assembly and disassembly of actin filaments, involving the polymerisation from monomeric G-actin to polymeric F-actin, regulates vital responses in neurons. This is mediated by actin binding proteins, including the phosphorylation of cofilin, which promotes the formation of F-actin. However, the interplay between actin dynamics, FUS and EC-FUS pathology has not been characterized previously.

Methods and Materials:  This study used three homozygous and three non-transgenic brain mice. G-actin and F-actin in vivo assay and western blotting for anti pCofilin/Cofilin, pLIMK/LIMK and profilin-1 used to measure actin dysregulation. We also used two days zebrafish lysates in three groups: EGFP-FUS (n=3), EGFP-WT (n=3) and non-transgenic controls (n=3). HA-stable HEK cells expressing EC-FUS and their conditioned media, used to monitor the actin dynamics in this cell line.

Key Findings: More F-actin (polymerized actin) relative to G-actin (monomeric actin) were present in FUS transgenic mice and zebrafish brains in comparison to non-transgenic controls. Also increased phosphorylation of Cofilin, pLIMK1/2 and profilin-1 were also detected, providing mechanistic insights into these observations. FUS mislocalisation resulted after pharmacological actin polymerization, linking dysregulation of actin dynamics to FUS pathology in ALS. These findings reveal dysregulation of actin dynamics as one of the novel mechanisms in the pathophysiology of FUS and EC-FUS in ALS.