Amyotrophic lateral sclerosis (ALS) is a neuromuscular disease characterised by progressive loss of motor function culminating in death 3 to 4 years after clinical onset, typically due to respiratory failure (van Es et al., 2017). A complex interplay between genetic, environment and lifestyle factors is believed to cause the disease in the majority of patients (Vasta et al., 2022). In addition to rare variants that cause familial monogenic forms of the disease, genetic variants that are commonly found in the population have also been associated with disease risk. The latest landmark cross-ancestry genome-wide association study (GWAS) identified several risk loci including UNC13A and SCFD1 (van Rheenen et al., 2021). The relationship of these risk genes with disease mechanism has remained unresolved. However, most recently, two compelling studies have demonstrated that for UNC13A, mis-splicing of its mRNA transcript in ALS patients eventually results in lower protein levels with dire consequences for synaptic maintenance (Brown et al., 2022; Ma et al., 2022). Although SCFD1 was also shown to be a top-most significant expression Quantitative Trait Locus (eQTL) for ALS and differential expression was detected in patient-derived motor cortex tissue compared to controls (Iacoangeli et al., 2021), it is presently unknown whether loss of SCFD1 expression contributes to the pathogenesis of ALS.

While an in vivo analysis of SCFD1 function in the neuromuscular system is lacking, the molecular and cellular functions of SCFD1 have been well studied. A member of the Sec1/Munc18-like (SM) protein family, SCFD1 regulates Endoplasmic Reticulum (ER) to Golgi anterograde transport. It does so by ensuring the correct assembly in addition to opposing the disassembly of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, itself required for the fusion of ER-derived vesicles with Golgi membranes (Lobingier et al., 2014; Peng and Gallwitz, 2002). A role for SCFD1 in intra-Golgi and Golgi-to-ER retrograde transport has also been proposed due to a direct interaction with the conserved oligomeric Golgi (COG) tethering complex (Laufman et al., 2009). Impairment of these functions by depletion of SCFD1 in mammalian cells induces an ER stress response that leads to autophagy (Renna et al., 2011). To date, however, studies on animal models with complete or conditional SCFD1 function have been limited, which would be critical for assessing whether this gene and its product play a crucial role in maintaining the motor system in vivo.

Here we report on the use of RNAi-mediated gene silencing in the Drosophila model system to investigate the in vivo consequences of SCFD1 depletion. Drosophila has a highly conserved SCFD1 orthologue known as Slh. Adult flies with a moderate deficiency of Slh in either muscle or neurons exhibited motoric deficits. A stronger knockdown of Slh in either tissue induced paralysis in flies at an earlier developmental stage as demonstrated by a severe reduction in the larval contraction rate and high puparial axial ratios. This phenotype was accompanied by defects in the neuromuscular junction (NMJ) between the innervating motor neurons and muscle. Transcriptome analysis revealed several alterations that can explain the motor dysfunction resulting from Slh depletion. Importantly, a general downregulation of protein folding pathways can explain why loss of SCFD1 function is a meaningful contributor to ALS.