Survival Motor Neuron Expression Research Articles

SMN Deficiency Induces an Early Non-Atrophic Myopathy with Alterations in the Contractile and Excitatory Coupling Machinery of Skeletal Myofibers in the SMN∆7 Mouse Model of Spinal Muscular Atrophy.

Spinal muscular atrophy (SMA) is caused by a deficiency of the ubiquitously expressed survival motor neuron (SMN) protein. The main pathological hallmark of SMA is the degeneration of lower motor neurons (MNs) with subsequent denervation and atrophy of skeletal muscle. However, increasing evidence indicates that low SMN levels not only are detrimental to the central nervous system (CNS) but also directly affect other peripheral tissues and organs, including skeletal muscle. To better understand the potential primary impact of SMN deficiency in muscle, we explored the cellular, ultrastructural, and molecular basis of SMA myopathy in the SMNΔ7 mouse model of severe SMA at an early postnatal period (P0-7) prior to muscle denervation and MN loss (preneurodegenerative [PND] stage). This period contrasts with the neurodegenerative (ND) stage (P8-14), in which MN loss and muscle atrophy occur. At the PND stage, we found that SMN∆7 mice displayed early signs of motor dysfunction with overt myofiber alterations in the absence of atrophy. We provide essential new ultrastructural data on focal and segmental lesions in the myofibrillar contractile apparatus. These lesions were observed in association with specific myonuclear domains and included abnormal accumulations of actin-thin myofilaments, sarcomere disruption, and the formation of minisarcomeres. The sarcoplasmic reticulum and triads also exhibited ultrastructural alterations, suggesting decoupling during the excitation-contraction process. Finally, changes in intermyofibrillar mitochondrial organization and dynamics, indicative of mitochondrial biogenesis overactivation, were also found. Overall, our results demonstrated that SMN deficiency induces early and MN loss-independent alterations in myofibers that essentially contribute to SMA myopathy. This strongly supports the growing body of evidence indicating the existence of intrinsic alterations in the skeletal muscle in SMA and further reinforces the relevance of this peripheral tissue as a key therapeutic target for the disease.

SMN depletion impairs skeletal muscle formation and maturation in a mouse model of SMA.

Spinal muscular atrophy (SMA) is characterized by low levels of the ubiquitously expressed Survival Motor Neuron (SMN) protein, leading to progressive muscle weakness and atrophy. Skeletal muscle satellite cells play a crucial role in muscle fiber maintenance, repair, and remodelling. While the effects of SMN depletion in muscle are well documented, its precise role in satellite cell function remains largely unclear. Using the Smn2B/- mouse model, we investigated SMN-depleted satellite cell biology through single fiber culture studies. Myofibers from Smn2B/- mice were smaller in size, shorter in length, had reduced myonuclear domain size, and reduced sub-synaptic myonuclear clusters-all suggesting impaired muscle function and integrity. These changes were accompanied by a reduction in the number of myonuclei in myofibers from Smn2B/- mice across all disease stages examined. Although the number of satellite cells in myofibers was significantly reduced, those remaining retained their capacity for myogenic activation and proliferation. These findings support the idea that a dysregulated myogenic process could be occurring as early in muscle stem cells during muscle formation and maturation in SMA. Targeting those pathways could offer additional options for combinatorial therapies for SMA.

Systemic LPS Administration Stimulates the Activation of Non-Neuronal Cells in an Experimental Model of Spinal Muscular Atrophy.

Spinal muscular atrophy (SMA) is a neurodegenerative disease caused by deficiency of the survival motor neuron (SMN) protein. Although SMA is a genetic disease, environmental factors contribute to disease progression. Common pathogen components such as lipopolysaccharides (LPS) are considered significant contributors to inflammation and have been associated with muscle atrophy, which is considered a hallmark of SMA. In this study, we used the SMNΔ7 experimental mouse model of SMA to scrutinize the effect of systemic LPS administration, a strong pro-inflammatory stimulus, on disease outcome. Systemic LPS administration promoted a reduction in SMN expression levels in CNS, peripheral lymphoid organs, and skeletal muscles. Moreover, peripheral tissues were more vulnerable to LPS-induced damage compared to CNS tissues. Furthermore, systemic LPS administration resulted in a profound increase in microglia and astrocytes with reactive phenotypes in the CNS of SMNΔ7 mice. In conclusion, we hereby show for the first time that systemic LPS administration, although it may not precipitate alterations in terms of deficits of motor functions in a mouse model of SMA, it may, however, lead to a reduction in the SMN protein expression levels in the skeletal muscles and the CNS, thus promoting synapse damage and glial cells' reactive phenotype.

Autophagy in spinal muscular atrophy: from pathogenic mechanisms to therapeutic approaches.

Spinal muscular atrophy (SMA) is a devastating neuromuscular disorder caused by the depletion of the ubiquitously expressed survival motor neuron (SMN) protein. While the genetic cause of SMA has been well documented, the exact mechanism(s) by which SMN depletion results in disease progression remain elusive. A wide body of evidence has highlighted the involvement and dysregulation of autophagy in SMA. Autophagy is a highly conserved lysosomal degradation process which is necessary for cellular homeostasis; defects in the autophagic machinery have been linked with a wide range of neurodegenerative disorders, including amyotrophic lateral sclerosis, Alzheimer's disease and Parkinson's disease. The pathway is particularly known to prevent neurodegeneration and has been suggested to act as a neuroprotective factor, thus presenting an attractive target for novel therapies for SMA patients. In this review, (a) we provide for the first time a comprehensive summary of the perturbations in the autophagic networks that characterize SMA development, (b) highlight the autophagic regulators which may play a key role in SMA pathogenesis and (c) propose decreased autophagic flux as the causative agent underlying the autophagic dysregulation observed in these patients.

Synergistic Effect of an Antisense Oligonucleotide and Small Molecule on Splicing Correction of the Spinal Muscular Atrophy Gene.

Spinal muscular atrophy (SMA) is treated by increasing the level of Survival Motor Neuron (SMN) protein through correction of SMN2 exon 7 skipping or exogenous expression of SMN through gene therapy. Currently available therapies have multiple shortcomings, including poor body-wide distribution, invasive delivery, and potential negative consequences due to high doses needed for clinical efficacy. Here we test the effects of a combination treatment of a splice-correcting antisense oligonucleotide (ASO) Anti-N1 with the small compounds risdiplam and branaplam. We show that a low-dose treatment of Anti-N1 with either compound produces a synergistic effect on the inclusion of SMN2 exon 7 in SMA patient fibroblasts. Using RNA-Seq, we characterize the transcriptomes of cells treated with each compound as well as in combination. Although high doses of each individual treatment trigger widespread perturbations of the transcriptome, combination treatment of Anti-N1 with risdiplam and branaplam results in minimal disruption of gene expression. For individual genes targeted by the 3 compounds, we observe little to no additive effects of combination treatment. Overall, we conclude that the combination treatment of a splice-correcting ASO with small compounds represents a promising strategy for achieving a high level of SMN expression while minimizing the risk of off-target effects.

Brain magnetic resonance imaging of patients with spinal muscular atrophy type 2 and 3

Background and objectiveProximal spinal muscular atrophy (SMA) is caused by deficiency of the ubiquitously expressed survival motor neuron protein. Although primarily a hereditary lower motor neuron disease, it is probably also characterized by abnormalities in other organs. Brain abnormalities and cognitive impairment have been reported in severe SMA. We aimed to systematically investigate brain structure in SMA using MRI. MethodsWe acquired high-resolution T1-weighted images of treatment-naive patients with SMA, age- and sex-matched healthy and disease controls with other neuromuscular diseases, on a 3 T MRI scanner. We performed vertex-wise whole brain analysis and region of interest analysis of cortical thickness (CT), and volumetric analysis of the thalamus and compared findings in patients and controls using multiple linear regression models and Wald test. We correlated structural abnormalities with motor function as assessed by the Hammersmith Functional Motor Scale Expanded (HFMSE) and SMA Functional Rating Scale (SMA-FRS). ResultsWe included 30 patients, 12–70 years old, with SMA type 2 and 3, 30 age- and sex-matched healthy controls and 17 disease controls (with distal SMA, hereditary motor and sensory neuropathy, multifocal motor neuropathy, progressive muscular atrophy and segmental SMA). We found a reduced CT in patients with SMA compared to healthy controls at the precentral, postcentral and medial orbitofrontal gyri and at the temporal pole (mean differences −0.059(p = 0.04); −0.055(p = 0.04), −0.06(p = 0.04); −0.17 mm(p = 0.001)). Differences at the precentral gyrus and temporal pole were most pronounced in SMA type 2 (mean differences −0.07(p = 0.045); −0.26 mm(p < 0.001)) and were also present compared to disease controls (mean differences −0.08(p = 0.048); −0.19 mm(p = 0.003)). There was a positive correlation between CT at the temporal pole with motor function. Compared to healthy controls, we found a reduced volume of the whole thalamus (mean difference −325 mm3(p = 0.03)) and of the anterior, ventral and intralaminar thalamic nuclei (mean differences −9.9(p = 0.02); −157(p = 0.01); −24.2 mm3(p = 0.02) in patients with SMA and a positive correlation between these volumes and motor function. ConclusionMRI shows structural changes in motor and non-motor regions of the cortex and the thalamus of patients with SMA type 2 and 3, indicating that SMA pathology is not confined to motor neurons.

Optimization of base editors for the functional correction of SMN2 as a treatment for spinal muscular atrophy.

Spinal muscular atrophy (SMA) is caused by mutations in SMN1. SMN2 is a paralogous gene with a C•G-to-T•A transition in exon 7, which causes this exon to be skipped in most SMN2 transcripts, and results in low levels of the protein survival motor neuron (SMN). Here we show, in fibroblasts derived from patients with SMA and in a mouse model of SMA that, irrespective of the mutations in SMN1, adenosine base editors can be optimized to target the SMN2 exon-7 mutation or nearby regulatory elements to restore the normal expression of SMN. After optimizing and testing more than 100 guide RNAs and base editors, and leveraging Cas9 variants with high editing fidelity that are tolerant of different protospacer-adjacent motifs, we achieved the reversion of the exon-7 mutation via an A•T-to-G•C edit in up to 99% of fibroblasts, with concomitant increases in the levels of the SMN2 exon-7 transcript and of SMN. Targeting the SMN2 exon-7 mutation via base editing or other CRISPR-based methods may provide long-lasting outcomes to patients with SMA.

SMN haploinsufficiency promotes ischemia/ reperfusion-induced AKI-to-CKD transition by endoplasmic reticulum stress activation.

Acute kidney injury (AKI) and chronic kidney disease (CKD) are interconnected syndromes that represent a global public health challenge. Here, we identified a specific role of survival of motor neuron (SMN) in ischemia/reperfusion (I/R)-induced kidney injury and progression of CKD. SMN was an essential protein in all cell type and was reported to play important roles in multiple fundamental cellular homeostatic pathways. However, the function of SMN in experimental models of I/R-induced kidney fibrosis has not extensively studied. Genetic ablation of SMN or small interfering RNA-base knockdown of SMN expression aggravated the tubular injury and interstitial fibrosis. Administration of scAAV9-CB-SMN or epithelial cell overexpression of SMN reduced I/R-induced kidney dysfunction and attenuated AKI-to-CKD transition, indicating that SMN is vital for the preservation and recovery of tubular phenotype. Our data showed that the endoplasmic reticulum stress (ERS) induced by I/R was persistent and became progressively more severe in the kidney without SMN. On the contrary, overexpression of SMN prevented against I/R-induced ERS and tubular cell damage. In summary, our data collectively substantiate a critical role of SMN in regulating the ERS activation and phenotype of AKI-to-CKD transition that may contribute to renal pathology during injury and repair.

NADPH oxidase 4 inhibition is a complementary therapeutic strategy for spinal muscular atrophy.

Spinal muscular atrophy (SMA) is a fatal neurodegenerative disorder, characterized by motor neuron (MN) degeneration and severe muscular atrophy and caused by Survival of Motor Neuron (SMN) depletion. Therapies aimed at increasing SMN in patients have proven their efficiency in alleviating SMA symptoms but not for all patients. Thus, combinational therapies are warranted. Here, we investigated the involvement of NADPH oxidase 4 (NOX4) in SMA-induced spinal MN death and if the modulation of Nox4 activity could be beneficial for SMA patients. We analysed in the spinal cord of severe type SMA-like mice before and at the disease onset, the level of oxidative stress and Nox4 expression. Then, we tested the effect of Nox4 inhibition by GKT137831/Setanaxib, a drug presently in clinical development, by intrathecal injection on MN survival and motor behaviour. Finally, we tested if GKT137831/Setanaxib could act synergistically with FDA-validated SMN-upregulating treatment (nusinersen). We show that NOX4 is overexpressed in SMA and its inhibition by GKT137831/Setanaxib protected spinal MN from SMA-induced degeneration. These improvements were associated with a significant increase in lifespan and motor behaviour of the mice. At the molecular level, GKT137831 activated the pro-survival AKT/CREB signaling pathway, leading to an increase in SMN expression in SMA MNs. Most importantly, we found that the per os administration of GKT137831 acted synergistically with a FDA-validated SMN-upregulating treatment. The pharmacological inhibition of NOX4 by GKT137831/Setanaxib is neuroprotector and could represent a complementary therapeutic strategy to fight against SMA.

SMN Is Physiologically Downregulated at Wild-Type Motor Nerve Terminals but Aggregates Together with Neurofilaments in SMA Mouse Models.

Survival motor neuron (SMN) is an essential and ubiquitously expressed protein that participates in several aspects of RNA metabolism. SMN deficiency causes a devastating motor neuron disease called spinal muscular atrophy (SMA). SMN forms the core of a protein complex localized at the cytoplasm and nuclear gems and that catalyzes spliceosomal snRNP particle synthesis. In cultured motor neurons, SMN is also present in dendrites and axons, and forms part of the ribonucleoprotein transport granules implicated in mRNA trafficking and local translation. Nevertheless, the distribution, regulation, and role of SMN at the axons and presynaptic motor terminals in vivo are still unclear. By using conventional confocal microscopy and STED super-resolution nanoscopy, we found that SMN appears in the form of granules distributed along motor axons at nerve terminals. Our fluorescence in situ hybridization and electron microscopy studies also confirmed the presence of β-actin mRNA, ribosomes, and polysomes in the presynaptic motor terminal, key elements of the protein synthesis machinery involved in local translation in this compartment. SMN granules co-localize with the microtubule-associated protein 1B (MAP1B) and neurofilaments, suggesting that the cytoskeleton participates in transporting and positioning the granules. We also found that, while SMN granules are physiologically downregulated at the presynaptic element during the period of postnatal maturation in wild-type (non-transgenic) mice, they accumulate in areas of neurofilament aggregation in SMA mice, suggesting that the high expression of SMN at the NMJ, together with the cytoskeletal defects, contribute to impairing the bi-directional traffic of proteins and organelles between the axon and the presynaptic terminal.

P.52 Determination of DLC1 isoform 1 (DLC1-i1) as a gene therapy for the treatment of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a motor neuron (MN) disease caused by loss of the ubiquitously expressed survival motor neuron (SMN) spliceosome protein, resulting in selective degeneration of spinal motor neurons (MNs) but the mechanism underlying the specific loss of MNs remains unknown. A previous report showed that Deleted in Liver Cancer 1 (DLC1) is the most down-regulated gene in MNs derived from a SMA patient but its roles in MN development and SMA pathogenesis remain to be elucidated. Here, we found a specific increase in the expression level of DLC1 isoform 1 (DLC1-i1) in human induced pluripotent stem cells (hiPSCs)-derived MNs compared to other isoforms. Knockdown of DLC1-i1 in MN progenitors resulted in reduced MN formation, axonal outgrowth, synapse number and cell survival, whereas overexpression of DLC1-i1 promoted MN differentiation with extensive neurite projections. Consistent with previous findings, we detected downregulated expression of DLC1-i1 in MNs derived from SMA patients' urine-derived iPSCs compared with healthy controls, likely due to intron retention of DLC1-i1 pre-mRNA. In addition, SMA patients-derived neuromuscular organoids (NMOs) showed reduced MNs and skeletal muscles generation, resulting in defective formation of neuromuscular junctions. Overexpression of DLC1-i1 partly restored both neural and muscle fibre formation in SMA NMOs. Similarly, DLC1-i1 expression was markedly reduced in the lumber spinal cord of SMA mice compared with that of wild-type, implying conservation of dysregulated DLC1-i1 expression in both the human SMA MNs and the spinal neurons of SMA mice. Injection of AAV9-SMN into the brain of SMA mice not only restored their weight and locomotor function but also DLC1-i1 expression level in the lumber spinal cord. Importantly, SMA mice treated with AAV9-DLC1-i1 together with AAV9-SMN resulted in synergistic improvement of their motor activity and lifespan compared with single treatment. Altogether, these findings demonstrate that MN-specific DLC1-i1 functions downstream of SMN to regulate MN maturation, survival, and neuromuscular junctions formation, revealing a new therapeutic target for the treatment of SMA.

Survival Motor Neuron Enhances Pluripotent Gene Expression and Facilitates Cell Reprogramming.

Survival motor neuron (SMN) plays important roles in snRNP assembly and mRNA splicing. Deficiency of SMN causes spinal muscular atrophy (SMA), a leading genetic disease causing childhood mortality. Previous studies have shown that SMN regulates stem cell self-renewal and pluripotency in Drosophila and mouse and is abundantly expressed in mouse embryonic stem cells. However, whether SMN is required for establishment of pluripotency is unclear. In this study, we show that SMN is gradually upregulated in preimplantation mouse embryos and cultured cells undergoing cell reprogramming. Ectopic expression of SMN increased cell reprogramming efficiency, whereas knockdown of SMN impeded induced pluripotent stem cell (iPSC) colony formation. iPSCs could be derived from SMA model mice, but impairment in differentiation capacity may be present. The ectopic overexpression of SMN in iPSCs can upregulate the expression levels of some pluripotent genes and restore the neuronal differentiation capacity of SMA-iPSCs. Taken together, our findings not only demonstrate the functional relevance of SMN in establishment of cell pluripotency but also propose its potential application in facilitating iPSC derivation.

Antisense Oligonucleotide Induction of the hnRNPA1b Isoform Affects Pre-mRNA Splicing of SMN2 in SMA Type I Fibroblasts.

Spinal muscular atrophy (SMA) is a severe, debilitating neuromuscular condition characterised by loss of motor neurons and progressive muscle wasting. SMA is caused by a loss of expression of SMN1 that encodes the survival motor neuron (SMN) protein necessary for the survival of motor neurons. Restoration of SMN expression through increased inclusion of SMN2 exon 7 is known to ameliorate symptoms in SMA patients. As a consequence, regulation of pre-mRNA splicing of SMN2 could provide a potential molecular therapy for SMA. In this study, we explored if splice switching antisense oligonucleotides could redirect the splicing repressor hnRNPA1 to the hnRNPA1b isoform and restore SMN expression in fibroblasts from a type I SMA patient. Antisense oligonucleotides (AOs) were designed to promote exon 7b retention in the mature mRNA and induce the hnRNPA1b isoform. RT-PCR and western blot analysis were used to assess and monitor the efficiency of different AO combinations. A combination of AOs targeting multiple silencing motifs in hnRNPA1 pre-mRNA led to robust hnRNPA1b induction, which, in turn, significantly increased expression of full-length SMN (FL-SMN) protein. A combination of PMOs targeting the same motifs also strongly induced hnRNPA1b isoform, but surprisingly SMN2 exon 5 skipping was detected, and the PMO cocktail did not lead to a significant increase in expression of FL-SMN protein. We further performed RNA sequencing to assess the genome-wide effects of hnRNPA1b induction. Some 3244 genes were differentially expressed between the hnRNPA1b-induced and untreated SMA fibroblasts, which are functionally enriched in cell cycle and chromosome segregation processes. RT-PCR analysis demonstrated that expression of the master regulator of these enrichment pathways, MYBL2 and FOXM1B, were reduced in response to PMO treatment. These findings suggested that induction of hnRNPA1b can promote SMN protein expression, but not at sufficient levels to be clinically relevant.

Activating ATF6 in spinal muscular atrophy promotes SMN expression and motor neuron survival through the IRE1α-XBP1 pathway.

Spinal muscular atrophy (SMA) is a neuromuscular disease caused by survival of motor neuron (SMN) deficiency that induces motor neuron (MN) degeneration and severe muscular atrophy. Gene therapies that increase SMN have proven their efficacy but not for all patients. Here, we explored the unfolded protein response (UPR) status in SMA pathology and explored whether UPR modulation could be beneficial for SMA patients. We analysed the expression and activation of key UPR proteins by RT-qPCR and by western blots in SMA patient iPSC-derived MNs and one SMA cell line in which SMN expression was re-established (rescue). We complemented this approach by using myoblast and fibroblast SMA patient cells and SMA mouse models of varying severities. Finally, we tested in vitro and in vivo the effect of IRE1α/XBP1 pathway restoration on SMN expression and subsequent neuroprotection. We report that the IRE1α/XBP1 branch of the unfolded protein response is disrupted in SMA, with a depletion of XBP1s irrespective of IRE1α activation pattern. The overexpression of XBP1s in SMA fibroblasts proved to transcriptionally enhance SMN expression. Importantly, rebalancing XBP1s expression in severe SMA-like mice, induced SMN expression and spinal MN protection. We have identified XBP1s depletion as a contributing factor in SMA pathogenesis, and the modulation of this transcription factor proves to be a plausible therapeutic avenue in the context of pharmacological interventions for patients.

Pharmacological and clinical profile of Onasemnogene Aveparvovec, the first gene therapy for spinal muscular atrophy (SMA)

Onasemnogene abeparvovec (Zolgensma®; formerly AVXS-101) is a one-time gene therapy designed to address the genetic root cause of spinal muscular atrophy (SMA) by replacing the function of the missing or nonworking SMN1 gene via an adeno-associated AAV9 viral vector. On March 19, 2020, the Japanese Ministry of Health, Labor and Welfare approved onasemnogene abeparvovec for the treatment of SMA patients <2 years of age, including presymptomatic patients with a genetic diagnosis. Patients must be negative for elevated anti-AAV9 antibodies. Onasemnogene abeparvovec is administered through a single intravenous infusion, delivering a new working copy of the SMN gene into a patient's cells. Intravenous administration of onasemnogene abeparvovec to SMA model mice resulted in sustained expression of survival motor neuron (SMN) protein, weight gain, improvement of motor function, and prolongation of survival. Its clinical efficacy and safety have been demonstrated through the Phase I START and Phase III STR1VE-US, STR1VE-EU, and SPR1NT trials, and their long-term extension studies. SMA and presymptomatic patients treated with onasemnogene abeparvovec have achieved rates of survival not observed in the natural history of SMA. Treatment has led to rapid motor function improvement, often within one month of dosing, and developmental milestone achievement, including the ability to sit without support. The most commonly observed adverse effects after treatment were elevated liver enzymes, which often resolved with a course of prednisolone, and vomiting. This review discusses the rationale underlying gene replacement therapy for SMA, and describes the basic science, clinical trial experience, and use of onasemnogene abeparvovec.

NOVA1 promotes SMN2 exon 7 splicing by binding the UCAC motif and increases SMN protein expression.

Spinal muscular atrophy (SMA) is a rare hereditary neuromuscular disease with a high lethality rate in infants. Variants in the homologous genes survival of motor neuron (SMN)1 and SMN2 have been reported to be SMA pathogenic factors. Previous studies showed that a high inclusion rate of SMN2 exon 7 increased SMN expression, which in turn reduced the severity of SMA. The inclusion rate of SMN2 exon 7 was higher in neural tissues than in non-neural tissues. Neuro-oncological ventral antigen (NOVA) is a splicing factor that is specifically and highly expressed in neurons. It plays a key role in nervous system development and in the induction of nervous system diseases. However, it remains unclear whether this splicing factor affects SMA. In this study, we analyzed the inclusion of SMN2 exon 7 in different tissues in a mouse model of SMA (genotype smn-/-SMN22tg/0) and littermate controls (genotype smn+/-SMN22tg/0). We found that inclusion level of SMN2 exon 7 was high in the brain and spinal cord tissue, and that NOVA1 was also highly expressed in nervous system tissues. In addition, SMN2 exon 7 and NOVA1 were expressed synchronously in the central nervous system. We further investigated the effects of NOVA1 on disease and found that the number of neurons in the anterior horn of spinal cord decreased in the mouse model of SMA during postnatal days 1-7, and that NOVA1 expression levels in motor neurons decreased simultaneously as spinal muscular atrophy developed. We also found that in vitro expression of NOVA1 increased the inclusion of SMN2 exon 7 and expression of the SMN2 protein in the U87MG cell line, whereas the opposite was observed when NOVA1 was knocked down. Finally, point mutation and RNA pull-down showed that the UCAC motif in SMN2 exon 7 plays a critical role in NOVA1 binding and promoting the inclusion of exon 7. Moreover, CA was more essential for the inclusion of exon 7 than the order of Y residues in the motif. Collectively, these findings indicate that NOVA1 interacts with the UCAC motif in exon 7 of SMN2, thereby enhancing inclusion of exon 7 in SMN2, which in turn increases expression of the SMN protein.

Fluorescence Correlation Spectroscopy Reveals Survival Motor Neuron Oligomerization but No Active Transport in Motor Axons of a Zebrafish Model for Spinal Muscular Atrophy.

Spinal Muscular Atrophy (SMA) is a progressive neurodegenerative disease affecting lower motor neurons that is caused by a deficiency in ubiquitously expressed Survival Motor Neuron (SMN) protein. Two mutually exclusive hypotheses have been discussed to explain increased motor neuron vulnerability in SMA. Reduced SMN levels have been proposed to lead to defective snRNP assembly and aberrant splicing of transcripts that are essential for motor neuron maintenance. An alternative hypothesis proposes a motor neuron-specific function for SMN in axonal transport of mRNAs and/or RNPs. To address these possibilities, we used a novel in vivo approach with fluorescence correlation spectroscopy (FCS) in transgenic zebrafish embryos to assess the subcellular dynamics of Smn in motor neuron cell bodies and axons. Using fluorescently tagged Smn we show that it exists as two freely diffusing components, a monomeric, and a complex-bound, likely oligomeric, component. This oligomer hypothesis was supported by the disappearance of the complex-bound form for a truncated Smn variant that is deficient in oligomerization and a change in its dynamics under endogenous Smn deficient conditions. Surprisingly, our FCS measurements did not provide any evidence for an active transport of Smn in axons. Instead, our in vivo observations are consistent with previous findings that SMN acts as a chaperone for the assembly of snRNP and mRNP complexes.

Survival motor neuron deficiency slows myoblast fusion through reduced myomaker and myomixer expression.

BackgroundSpinal muscular atrophy is an inherited neurodegenerative disease caused by insufficient levels of the survival motor neuron (SMN) protein. Recently approved treatments aimed at increasing SMN protein levels have dramatically improved patient survival and have altered the disease landscape. While restoring SMN levels slows motor neuron loss, many patients continue to have smaller muscles and do not achieve normal motor milestones. While timing of treatment is important, it remains unclear why SMN restoration is insufficient to fully restore muscle size and function. We and others have shown that SMN‐deficient muscle precursor cells fail to efficiently fuse into myotubes. However, the role of SMN in myoblast fusion is not known.MethodsIn this study, we show that SMN‐deficient myoblasts readily fuse with wild‐type myoblasts, demonstrating fusion competency. Conditioned media from wild type differentiating myoblasts do not rescue the fusion deficit of SMN‐deficient cells, suggesting that compromised fusion may primarily be a result of altered membrane dynamics at the cell surface. Transcriptome profiling of skeletal muscle from SMN‐deficient mice revealed altered expression of cell surface fusion molecules. Finally, using cell and mouse models, we investigate if myoblast fusion can be rescued in SMN‐deficient myoblast and improve the muscle pathology in SMA mice.ResultsWe found reduced expression of the muscle fusion proteins myomaker (P = 0.0060) and myomixer (P = 0.0051) in the muscle of SMA mice. Suppressing SMN expression in C2C12 myoblast cells reduces expression of myomaker (35% reduction; P < 0.0001) and myomixer, also known as myomerger and minion, (30% reduction; P < 0.0001) and restoring SMN levels only partially restores myomaker and myomixer expression. Ectopic expression of myomixer improves myofibre number (55% increase; P = 0.0006) and motor function (35% decrease in righting time; P = 0.0089) in SMA model mice and enhances motor function (82% decrease in righting time; P < 0.0001) and extends survival (28% increase; P < 0.01) when administered in combination with an antisense oligonucleotide that increases SMN protein levels.ConclusionsHere, we identified reduced expression of muscle fusion proteins as a key factor in the fusion deficits of SMN‐deficient myoblasts. This discovery provides a novel target to improve SMA muscle pathology and motor function, which in combination with SMN increasing therapy could enhance clinical outcomes for SMA patients.

A high-throughput genome-wide RNAi screen identifies modifiers of survival motor neuron protein.

SUMMARYSpinal muscular atrophy (SMA) is a debilitating neurological disorder marked by degeneration of spinal motor neurons and muscle atrophy. SMA results from mutations in survival motor neuron 1 (SMN1), leading to deficiency of survival motor neuron (SMN) protein. Current therapies increase SMN protein and improve patient survival but have variable improvements in motor function, making it necessary to identify complementary strategies to further improve disease outcomes. Here, we perform a genome-wide RNAi screen using a luciferase-based activity reporter and identify genes involved in regulating SMN gene expression, RNA processing, and protein stability. We show that reduced expression of Transcription Export complex components increases SMN levels through the regulation of nuclear/cytoplasmic RNA transport. We also show that the E3 ligase, Neurl2, works cooperatively with Mib1 to ubiquitinate and promote SMN degradation. Together, our screen uncovers pathways through which SMN expression is regulated, potentially revealing additional strategies to treat SMA.

Gain of toxic function by long-term AAV9-mediated SMN overexpression in the sensorimotor circuit.

The neurodegenerative disease spinal muscular atrophy (SMA) is caused by deficiency in the survival motor neuron (SMN) protein. Currently approved SMA treatments aim to restore SMN, but the potential for SMN expression beyond physiological levels is a unique feature of AAV9-SMN gene therapy. Here, we show that long-term AAV9-mediated SMN overexpression in mouse models induces dose-dependent, late-onset motor dysfunction associated with loss of proprioceptive synapses and neurodegeneration. Mechanistically, aggregation of overexpressed SMN in the cytoplasm of motor circuit neurons sequesters components of small nuclear ribonucleoproteins, leading to splicing dysregulation and widespread transcriptome abnormalities with prominent signatures of neuroinflammation and innate immune response. Thus, long-term SMN overexpression interferes with RNA regulation and triggers SMA-like pathogenic events through toxic gain of function mechanisms. These unanticipated, SMN-dependent and neuron-specific liabilities warrant caution on the long-term safety of treating SMA patients with AAV9-SMN and the risks of uncontrolled protein expression by gene therapy.