The myokine FGF21 associates with enhanced survival in ALS and mitigates stress-induced cytotoxicity

Amyotrophic lateral sclerosis (ALS) is an age-related and fatal neurodegenerative disease characterized by progressive muscle weakness. There is marked heterogeneity in clinical presentation, progression, and pathophysiology with only modest treatments to slow disease progression. Molecular markers that provide insight into this heterogeneity are crucial for clinical management and identification of new therapeutic targets. In a prior muscle miRNA sequencing investigation, we identified altered FGF pathways in ALS muscle, leading us to investigate FGF21. We analyzed human ALS muscle biopsy samples and found a large increase in FGF21 expression with localization to atrophic myofibers and surrounding endomysium. A concomitant increase in FGF21 was detected in ALS spinal cords which correlated with muscle levels. FGF21 was increased in the SOD1^G93A mouse beginning in presymptomatic stages. In parallel, there was dysregulation of the co-receptor, β-Klotho, with higher levels detected in ALS muscle biopsies and lower levels in post-mortem muscle compared to controls. Plasma FGF21 levels were increased in ALS patients and high levels correlated with slower disease progression, prolonged survival, and increased body mass index. <i>In cellulo</i>, FGF21 was induced in differentiating muscle cells and ectopic treatment with FGF21 enhanced muscle differentiation. Ectopic FGF21 mitigated oxidative stress-induced loss of viability in iPSC-derived ALS motor neurons and muscle cells expressing SOD1^G93A. In summary, FGF21 is a novel biomarker in ALS which exerts trophic effects in the neuromuscular system.