ALS-associated mutations in FUS disrupt the axonal distribution and function of SMN

Abstract

Mutations in the RNA binding protein fused in sarcoma/translated in liposarcoma (FUS/TLS) cause amyotrophic lateral sclerosis (ALS). Although ALS-linked mutations in FUS often lead to a cytosolic mislocalization of the protein, the pathogenic mechanisms underlying these mutations remain poorly understood. To gain insight into these mechanisms, we examined the biochemical, cell biological and functional properties of mutant FUS in neurons. Expression of different FUS mutants (R521C, R521H, P525L) in neurons caused axonal defects. A protein interaction screen performed to explain these phenotypes identified numerous FUS interactors including the spinal muscular atrophy (SMA) causing protein survival motor neuron (SMN). Biochemical experiments showed that FUS and SMN interact directly and endogenously, and that this interaction can be regulated by FUS mutations. Immunostaining revealed co-localization of mutant FUS aggregates and SMN in primary neurons. This redistribution of SMN to cytosolic FUS accumulations led to a decrease in axonal SMN. Finally, cell biological experiments showed that overexpression of SMN rescued the axonal defects induced by mutant FUS, suggesting that FUS mutations cause axonal defects through SMN. This study shows that neuronal aggregates formed by mutant FUS protein may aberrantly sequester SMN and concomitantly cause a reduction of SMN levels in the axon, leading to axonal defects. These data provide a functional link between ALS-linked FUS mutations, SMN and neuronal connectivity and support the idea that different motor neuron disorders such as SMA and ALS may be caused, in part, by defects in shared molecular pathways.

Unraveling the Pathways to Neuronal Homeostasis and Disease: Mechanistic Insights into the Role of RNA-Binding Proteins and Associated Factors

“…In addition, FUS mutants were shown to redistribute SMN towards cytosolic FUS aggregates, eventually decreasing available levels of axonal SMN. Overexpression of SMN rescued the axonal defects induced by mutant FUS, suggesting that FUS mutations cause axonal defects, in part, through SMN sequestration [395]. Furthermore, FUS mutants, but not wild-type FUS, assembled into perinuclear stress granules in response to oxidative stress or heat-shock indicating that they may influence SG dynamics [396].…”

Sequestration of PRMT1 and Nd1-L mRNA into ALS-linked FUS mutant R521C-positive aggregates contributes to neurite degeneration upon oxidative stress

“…FUS is involved in many cellular processes such as transcriptional regulation, RNA splicing, DNA repair, genomic integrity, and responses to DNA damage5. In polarized neurons, FUS has been shown to regulate spine formation and maintenance of spine stability, transport of mRNA (such as actin-regulating mRNA) in dendrites, axonal transport of survival motor neuron (SMN) protein, regulation of stability of GluA1 (a subunit of AMPA subtype of ligand-gated glutamate receptors) mRNA, and activity-dependent synaptic homeostasis, all suggesting an important role in synaptic formation and function6789.…”

Pyramidal signs in a Caucasian patient with spinal muscular atrophy: a case report

“…Synapses and distal axonal compartments are frequently involved in the early stages of ALS [6]. Mutations in FUS induce a redistribution of SMN from the axon to cytosolic FUS accumulations, which disrupt the function of SMN, thus leading to axonal defects [3]. Moreover, a study of 600 sporadic ALS patients found that an abnormal number of SMN1 copies (one or three rather than two) occurred more frequently in cases than controls (OR = 2.8, 95% CI: 1.8-4.4) [8].…”