Interaction between alpha-COP and SMN ameliorates disease phenotype in a mouse model of spinal muscular atrophy

Abstract

We report that expression of the α-COP protein rescues disease phenotype in a severe mouse model of Spinal Muscular Atrophy (SMA).. Lentiviral particles expressing α-COP were injected directly into the testes of genetically pure mouse strain of interest resulting in infection of the spermatagonial stem cells. α-COP was stably expressed in brain, skeletal muscle, and spinal cord without altering SMN protein levels. SMA mice transgenic for α-COP live significantly longer than their non-transgenic littermates, and showed increased body mass and normal muscle morphology at postnatal day 15. We previously reported that binding between SMN and α-COP is required for restoration of neurite outgrowth in cells lacking SMN, and we report similar finding here. Lentiviral-mediated transgenic expression of SMN where the dilysine domain in exon 2b was mutated was not able to rescue the SMA phenotype despite robust expression of the mutant SMN protein in brain, muscle and spinal cord. These results demonstrate that α-COP is a validated modifier of SMA disease phenotype in a mammalian, vertebrate model and is a potential target for development of future SMN-independent therapeutic interventions.

Local Translation in Axons: When Membraneless RNP Granules Meet Membrane-Bound Organelles

“…In the absence of SMN, loss of axonal RNP granules and significant decrease in axonal mRNA levels are observed (Rage et al, 2013; Fallini et al, 2014, 2016; Saal et al, 2014), indicating that defective RNP assembly and subsequent axonal transport defects may at least partially lead to SMA. Interestingly, specifically altering the COPI/SMN interaction impairs the developmental function of SMN in axons (Custer et al, 2013, 2019; Li et al, 2015), suggesting that loosing the tight interactions between axonal RNP granules and membrane-bound organelles might also play a role in disease progression. Further supporting the importance of such interactions, mutations in the late endosome protein Rab7 known to be causally linked to the Charcot-Marie-Tooth disease type 2B (CMT2B), disrupted axonal mRNA translation when expressed in cultured retinal neurons (Cioni et al, 2019).…”

What Genetics Has Told Us and How It Can Inform Future Experiments for Spinal Muscular Atrophy, a Perspective

“…Mutations in alpha-COP were also shown to remove the SMN interaction [182]. It was shown that overexpression of alpha-COP was capable of rescuing growth of axons in cell culture and zebrafish, but only increased survival of SMA mice from 11 to 18 days with no motor functions assessed [182][183][184]. This interaction is interesting, as our experiments have shown that expression of an Smn lacking exon 2B, which contains the lysine residues responsible for alpha-COP interaction, completely rescues survival and snRNP assembly in Smn −/D7 iMEFs [77].…”

“…As in the example for SCA1, where a mutant protein removed the ability to bind CIC resulted in removal of the toxic action of the glutamine expansion in ATXN1, we can ask if expression of an Smn which lacks the ability to bind alpha-COP modifies the motor outcomes and survival in SMA mice. It should be noted that an experiment regarding this question has been published, however expression from the mutated SMN transgene is quite low and this mutant has not been assayed for the ability to perform snRNP assembly [184]. Furthermore, if the Smn lacking exon 2B is crossed onto an Smn null, it can be determined whether the resulting mouse presents with an overt phenotype.…”

A survey of transcripts generated by spinal muscular atrophy genes

“…The spectrum of SMA is broad ranging from embryonic lethality to a nearly normal life expectancy [4,6]. The expression of SMN and SMA modifying factors determine the severity of the disease and the timing of its manifestation [56][57][58][59][60][61][62][63][64][65]. Based on a recent study that analyzed insurance claims, patients frequently reported initial pathology associated with peripheral tissues, including issues with male fertility, prior to diagnosis with mild SMA [66].…”