The ASO Nusinersen ameliorates motor function and prevents Cajal body disassembly and abnormal poly(A) RNA distribution in motor neurons from a Spinal Muscular Atrophy mouse model

Abstract

ABSTRACT : Spinal muscular atrophy (SMA) is a devastating autosomal recessive neuromuscular disease characterized by degeneration of spinal cord alpha motor neurons (αMNs). It is caused by the homozygous deletion or mutation of the survival motor neuron 1 (SMN1) gene, resulting in reduced expression of SMN protein, which leads to αMN degeneration and muscle atrophy. A second gene, SMN2, contains a mutation that promotes exclusion of exon 7 and causes the SMN2 transcripts to generate a truncated, nonfunctional, form of SMN: the SMN∆7 protein. A recent treatment with antisense oligonucleotides (ASOs) has been developed for SMA – Nusinersen (Spinraza, BIOGEN), which works by promoting the inclusion of exon 7 in SMN2 transcripts. In this work, we used the αMNs of the SMN∆7 mouse model of SMA to study the effects of Nusinersen in the behavior of Cajal bodies (CBs) and the cellular distribution of polyadenylated mRNAs. CBs are nuclear structures involved in RNA metabolism that can be disrupted in response to cellular stress in SMA αMNs. Moreover, an abnormal accumulation of polyadenylated mRNAs in nuclear granules (PARGs) have been previously shown in αMNs from the SMN∆7 mouse model of SMA. Notably, in our study we demonstrate that the treatment with Nusinersen, administered via intracerebroventricular injection, rescues growth curve and improves motor behavior in these mice. Importantly, Nusinersen also recovers the number of canonical CBs and reduces the abnormal accumulation of polyadenylated RNAs in nuclear granules or PARGs. Furthermore, the treatment with this ASO normalizes the expression of the pre-mRNAs encoding chondrolectin and choline acetyltransferase, two key factors for αMNs homeostasis. In light of these results, we propose that the mechanism of action of Nusinersen is partially mediated by the rescue of both CB biogenesis and the normal processing and distribution of polyadenylated mRNAs. It also becomes apparent that the rescue of SMN levels in the spinal cord has a beneficial effect on αMNs and skeletal myofibers, but the rescue of SMN expression in muscle is needed for the complete recovery of motor function.