Post-transcriptional Inhibition of Hsc70-4/HSPA8 Expression Leads to Synaptic Vesicle Cycling Defects in Multiple Models of ALS

Abstract

Summary Amyotrophic Lateral Sclerosis (ALS) is a synaptopathy accompanied by the presence of cytoplasmic aggregates containing TDP-43, an RNA binding protein linked to ~97% of ALS cases. Using a Drosophila model of ALS, we show that TDP-43 OE in motor neurons results in decreased expression of the Hsc70-4 chaperone at the neuromuscular junction (NMJ). Mechanistically, mutant TDP-43 sequesters hsc70-4 mRNA and impairs its translation. Expression of Hsc70-4’s ortholog, HSPA8 is also reduced in primary motor neurons and NMJs of mice expressing mutant TDP-43. Electrophysiology, imaging, and genetic interaction experiments reveal TDP-43 dependent defects in synaptic vesicle endocytosis. These deficits can be partially restored by OE of Hsc70-4, cysteine-string protein (Csp) or dynamin. This suggests that TDP-43 toxicity results in part from impaired activity of the synaptic CSP/Hsc70 chaperone complex impacting dynamin function. Finally, Hsc70-4/HSPA8 expression is also post-transcriptionally reduced in fly and human iPSC C9orf72 models, suggesting a common disease pathomechanism.

Lost in Translation: Evidence for Protein Synthesis Deficits in ALS/FTD and Related Neurodegenerative Diseases

“…Recently, an interesting mechanistic connection has been identified between ribostasis and proteostasis [15]. Using the same Drosophila model of ALS [35, 36] the authors identified hsc70-4 mRNA as a candidate target of mutant but not wild-type TDP-43 [15].…”

“…A similar post-transcriptional reduction was observed in C9 ALS fly and patient-derived motor neurons, although it remains to be determined whether this is caused by translation inhibition as was the case with TDP-43 models. Notably, restoration of Hsc70-4 through genetic overexpression mitigated ALS phenotypes in a variant-dependent manner suggesting that although both wild-type and mutant TDP-43 contribute to ALS pathogenesis, they do so through distinct mechanisms [15]. …”

Dynamic duo – FMRP and TDP-43: Regulating common targets, causing different diseases

“…Indeed, under normal conditions, TDP-43 is required for preventing the downregulation of the microtubule stabilizing protein Futsch and maintain synaptic integrity (Godena et al, 2011; Romano et al, 2016). In disease, TDP-43 appears to cause toxicity by sequestering mRNAs and reducing their availability to the translation machinery as shown for futsch and hsc70-4 mRNAs (Coyne et al, 2014; Coyne et al, 2017a). Hsc70-4 is a chaperone required for synaptic vesicle cycling and its downregulation in TDP-43 proteinopathy provides a mechanistic explanation for synaptic dysfunction in ALS (Coyne et al, 2017a).…”

“…In disease, TDP-43 appears to cause toxicity by sequestering mRNAs and reducing their availability to the translation machinery as shown for futsch and hsc70-4 mRNAs (Coyne et al, 2014; Coyne et al, 2017a). Hsc70-4 is a chaperone required for synaptic vesicle cycling and its downregulation in TDP-43 proteinopathy provides a mechanistic explanation for synaptic dysfunction in ALS (Coyne et al, 2017a). In neuroblastoma cells, TDP-43 interacts directly with ribosomes via RACK1, which impacts translation globally and may have implications in disease (Russo et al, 2017).…”

“…In recent years, an avalanche of molecular and phenotypic studies showed that FMRP and TDP-43, two RNA binding proteins with seemingly different roles in the nervous system work together closely and regulate common mRNA targets (Coyne et al, 2014; Coyne et al, 2015; Coyne et al, 2017a; Fallini et al, 2012; Majumder et al, 2016; Wang et al, 2008; Yu et al, 2012). Given their involvement in diseases affecting synaptic function during early development (FMRP, Fragile X Mental Retardation Protein) versus aging neurons (TDP-43, TAR-DNA Binding Protein), these intriguing findings provide an opportunity to uncover common and unique aspects of RNA based mechanisms during lifespan.…”