SMN1 functions as a novel inhibitor for TRAF6-mediated NF-κB signaling

Genetic Variability in Oxidative Stress, Inflammatory, and Neurodevelopmental Pathways: Impact on the Susceptibility and Course of Spinal Muscular Atrophy

The spinal muscular atrophy (SMA) phenotype strongly correlates with the SMN2 gene copy number. However, the severity and progression of the disease vary widely even among affected individuals with identical copy numbers. This study aimed to investigate the impact of genetic variability in oxidative stress, inflammatory, and neurodevelopmental pathways on SMA susceptibility and clinical progression. Genotyping for 31 genetic variants across 20 genes was conducted in 54 SMA patients and 163 healthy controls. Our results revealed associations between specific polymorphisms and SMA susceptibility, disease type, age at symptom onset, and motor and respiratory function. Notably, the TNF rs1800629 and BDNF rs6265 polymorphisms demonstrated a protective effect against SMA susceptibility, whereas the IL6 rs1800795 was associated with an increased risk. The polymorphisms CARD8 rs2043211 and BDNF rs6265 were associated with SMA type, while SOD2 rs4880, CAT rs1001179, and MIR146A rs2910164 were associated with age at onset of symptoms after adjustment for clinical parameters. In addition, GPX1 rs1050450 and HMOX1 rs2071747 were associated with motor function scores and lung function scores, while MIR146A rs2910164, NOTCH rs367398 SNPs, and GSTM1 deletion were associated with motor and upper limb function scores, and BDNF rs6265 was associated with lung function scores after adjustment. These findings emphasize the potential of genetic variability in oxidative stress, inflammatory processes, and neurodevelopmental pathways to elucidate the complex course of SMA. Further exploration of these pathways offers a promising avenue for developing personalized therapeutic strategies for SMA patients.

Anti-Obesity Activity of Sanghuangporus vaninii by Inhibiting Inflammation in Mice Fed a High-Fat Diet

Obesity is an unhealthy condition associated with various diseases characterized by excess fat accumulation. However, in China, the prevalence of obesity is 14.1%, and it remains challenging to achieve weight loss or resolve this issue through clinical interventions. Sanghuangpours vaninii (SPV) is a nutritional fungus with multiple pharmacological activities and serves as an ideal dietary intervention for combating obesity. In this study, a long-term high-fat diet (HFD) was administered to induce obesity in mice. Different doses of SPV and the positive drug simvastatin (SV) were administered to mice to explore their potential anti-obesity effects. SPV regulated weight, serum lipids, and adipocyte size while inhibiting inflammation and hepatic steatosis. Compared with the vehicle-treated HFD-fed mice, the lowest decreases in total cholesterol (TC), triglycerides (TG), and low-density lipoprotein cholesterol (LDL-C) were 9.72%, 9.29%, and 12.29%, respectively, and the lowest increase in high-density lipoprotein cholesterol (HDL-C) was 5.88% after treatment with different doses of SPV. With SPV treatment, the analysis of gut microbiota and serum lipids revealed a significant association between lipids and inflammation-related factors, specifically sphingomyelin. Moreover, Western blotting results showed that SPV regulated the toll-like receptor (TLR4)/nuclear factor kappa B (NF-κB) signaling pathway in HFD-diet mice, which is related to inflammation and lipid metabolism. This research presents empirical proof of the impact of SPV therapy on obesity conditions.

What could be the function of the spinal muscular atrophy-causing protein SMN in macrophages?

Spinal Muscular Atrophy (SMA), a neurodegenerative disorder, extends its impact beyond the nervous system. The central protein implicated in SMA, Survival Motor Neuron (SMN) protein, is ubiquitously expressed and functions in fundamental processes such as alternative splicing, translation, cytoskeletal dynamics and signaling. These processes are relevant for all cellular systems, including cells of the immune system such as macrophages. Macrophages are capable of modulating their splicing, cytoskeleton and expression profile in order to fulfil their role in tissue homeostasis and defense. However, less is known about impairment or dysfunction of macrophages lacking SMN and the subsequent impact on the immune system of SMA patients. We aimed to review the potential overlaps between SMN functions and macrophage mechanisms highlighting the need for future research, as well as the current state of research addressing the role of macrophages in SMA.

Cell-mediated cytotoxicity within CSF and brain parenchyma in spinal muscular atrophy unaltered by nusinersen treatment

5q-associated spinal muscular atrophy (SMA) is a motoneuron disease caused by mutations in the survival motor neuron 1 (SMN1) gene. Adaptive immunity may contribute to SMA as described in other motoneuron diseases, yet mechanisms remain elusive. Nusinersen, an antisense treatment, enhances SMN2 expression, benefiting SMA patients. Here we have longitudinally investigated SMA and nusinersen effects on local immune responses in the cerebrospinal fluid (CSF) - a surrogate of central nervous system parenchyma. Single-cell transcriptomics (SMA: N = 9 versus Control: N = 9) reveal NK cell and CD8+ T cell expansions in untreated SMA CSF, exhibiting activation and degranulation markers. Spatial transcriptomics coupled with multiplex immunohistochemistry elucidate cytotoxicity near chromatolytic motoneurons (N = 4). Post-nusinersen treatment, CSF shows unaltered protein/transcriptional profiles. These findings underscore cytotoxicity's role in SMA pathogenesis and propose it as a therapeutic target. Our study illuminates cell-mediated cytotoxicity as shared features across motoneuron diseases, suggesting broader implications.

Dysregulation of innate immune signaling in animal models of spinal muscular atrophy

BACKGROUND

Spinal muscular atrophy (SMA) is a devastating neuromuscular disease caused by hypomorphic loss of function in the survival motor neuron (SMN) protein. SMA presents across a broad spectrum of disease severity. Unfortunately, genetic models of intermediate SMA have been difficult to generate in vertebrates and are thus unable to address key aspects of disease etiology. To address these issues, we developed a Drosophila model system that recapitulates the full range of SMA severity, allowing studies of pre-onset biology as well as late-stage disease processes.

RESULTS

Here, we carried out transcriptomic and proteomic profiling of mild and intermediate Drosophila models of SMA to elucidate molecules and pathways that contribute to the disease. Using this approach, we elaborated a role for the SMN complex in the regulation of innate immune signaling. We find that mutation or tissue-specific depletion of SMN induces hyperactivation of the immune deficiency (IMD) and Toll pathways, leading to overexpression of antimicrobial peptides (AMPs) and ectopic formation of melanotic masses in the absence of an external challenge. Furthermore, the knockdown of downstream targets of these signaling pathways reduced melanotic mass formation caused by SMN loss. Importantly, we identify SMN as a negative regulator of a ubiquitylation complex that includes Traf6, Bendless, and Diap2 and plays a pivotal role in several signaling networks.

CONCLUSIONS

In alignment with recent research on other neurodegenerative diseases, these findings suggest that hyperactivation of innate immunity contributes to SMA pathology. This work not only provides compelling evidence that hyperactive innate immune signaling is a primary effect of SMN depletion, but it also suggests that the SMN complex plays a regulatory role in this process in vivo. In summary, immune dysfunction in SMA is a consequence of reduced SMN levels and is driven by cellular and molecular mechanisms that are conserved between insects and mammals.

Dysregulation of innate immune signaling in animal models of Spinal Muscular Atrophy

Background

Spinal Muscular Atrophy (SMA) is a devastating neuromuscular disease caused by hypomorphic loss of function in the Survival Motor Neuron (SMN) protein. SMA presents across broad spectrum of disease severity. Unfortunately, vertebrate models of intermediate SMA have been difficult to generate and are thus unable to address key aspects of disease etiology. To address these issues, we developed a Drosophila model system that recapitulates the full range of SMA severity, allowing studies of pre-onset biology as well as late-stage disease processes.

Results

Here, we carried out transcriptomic and proteomic profiling of mild and intermediate Drosophila models of SMA to elucidate molecules and pathways that contribute to the disease. Using this approach, we elaborated a role for the SMN complex in the regulation of innate immune signaling. We find that mutation or tissue-specific depletion of SMN induces hyperactivation of the Immune Deficiency (IMD) and Toll pathways, leading to overexpression of antimicrobial peptides (AMPs) and ectopic formation of melanotic masses in the absence of an external challenge. Furthermore, knockdown of downstream targets of these signaling pathways reduced melanotic mass formation caused by SMN loss. Importantly, we identify SMN as a negative regulator of an ubiquitylation complex that includes Traf6, Bendless and Diap2, and plays a pivotal role in several signaling networks.

Conclusions

In alignment with recent research on other neurodegenerative diseases, these findings suggest that hyperactivation of innate immunity contributes to SMA pathology. This work not only provides compelling evidence that hyperactive innate immune signaling is a primary effect of SMN depletion, but it also suggests that the SMN complex plays a regulatory role in this process in vivo. In summary, immune dysfunction in SMA is a consequence of reduced SMN levels and is driven by cellular and molecular mechanisms that are conserved between insects and mammals.

Dysregulation of Tweak and Fn14 in skeletal muscle of spinal muscular atrophy mice

BACKGROUND

Spinal muscular atrophy (SMA) is a childhood neuromuscular disorder caused by depletion of the survival motor neuron (SMN) protein. SMA is characterized by the selective death of spinal cord motor neurons, leading to progressive muscle wasting. Loss of skeletal muscle in SMA is a combination of denervation-induced muscle atrophy and intrinsic muscle pathologies. Elucidation of the pathways involved is essential to identify the key molecules that contribute to and sustain muscle pathology. The tumor necrosis factor-like weak inducer of apoptosis (TWEAK)/TNF receptor superfamily member fibroblast growth factor-inducible 14 (Fn14) pathway has been shown to play a critical role in the regulation of denervation-induced muscle atrophy as well as muscle proliferation, differentiation, and metabolism in adults. However, it is not clear whether this pathway would be important in highly dynamic and developing muscle.

METHODS

We thus investigated the potential role of the TWEAK/Fn14 pathway in SMA muscle pathology, using the severe Taiwanese Smn^-/-; SMN2 and the less severe Smn^2B/- SMA mice, which undergo a progressive neuromuscular decline in the first three post-natal weeks. We also used experimental models of denervation and muscle injury in pre-weaned wild-type (WT) animals and siRNA-mediated knockdown in C2C12 muscle cells to conduct additional mechanistic investigations.

RESULTS

Here, we report significantly dysregulated expression of Tweak, Fn14, and previously proposed downstream effectors during disease progression in skeletal muscle of the two SMA mouse models. In addition, siRNA-mediated Smn knockdown in C2C12 myoblasts suggests a genetic interaction between Smn and the TWEAK/Fn14 pathway. Further analyses of SMA, Tweak^-/-, and Fn14^-/- mice revealed dysregulated myopathy, myogenesis, and glucose metabolism pathways as a common skeletal muscle feature, providing further evidence in support of a relationship between the TWEAK/Fn14 pathway and Smn. Finally, administration of the TWEAK/Fn14 agonist Fc-TWEAK improved disease phenotypes in the two SMA mouse models.

CONCLUSIONS

Our study provides mechanistic insights into potential molecular players that contribute to muscle pathology in SMA and into likely differential responses of the TWEAK/Fn14 pathway in developing muscle.

How Inflammation Pathways Contribute to Cell Death in Neuro-Muscular Disorders

Neuro-muscular disorders include a variety of diseases induced by genetic mutations resulting in muscle weakness and waste, swallowing and breathing difficulties. However, muscle alterations and nerve depletions involve specific molecular and cellular mechanisms which lead to the loss of motor-nerve or skeletal-muscle function, often due to an excessive cell death. Morphological and molecular studies demonstrated that a high number of these disorders seem characterized by an upregulated apoptosis which significantly contributes to the pathology. Cell death involvement is the consequence of some cellular processes that occur during diseases, including mitochondrial dysfunction, protein aggregation, free radical generation, excitotoxicity and inflammation. The latter represents an important mediator of disease progression, which, in the central nervous system, is known as neuroinflammation, characterized by reactive microglia and astroglia, as well the infiltration of peripheral monocytes and lymphocytes. Some of the mechanisms underlying inflammation have been linked to reactive oxygen species accumulation, which trigger mitochondrial genomic and respiratory chain instability, autophagy impairment and finally neuron or muscle cell death. This review discusses the main inflammatory pathways contributing to cell death in neuro-muscular disorders by highlighting the main mechanisms, the knowledge of which appears essential in developing therapeutic strategies to prevent the consequent neuron loss and muscle wasting.

Bioinformatic analysis of SMN1-ACE/ACE2 interactions hinted at a potential protective effect of spinal muscular atrophy against COVID-19-induced lung injury

Patients with spinal muscular atrophy (SMA) are susceptible to the respiratory infections and might be at a heightened risk of poor clinical outcomes upon contracting coronavirus disease 2019 (COVID-19). In the face of the COVID-19 pandemic, the potential associations of SMA with the susceptibility to and prognostication of COVID-19 need to be clarified. We documented an SMA case who contracted COVID-19 but only developed mild-to-moderate clinical and radiological manifestations of pneumonia, which were relieved by a combined antiviral and supportive treatment. We then reviewed a cohort of patients with SMA who had been living in the Hubei province since November 2019, among which the only 1 out of 56 was diagnosed with COVID-19 (1.79%, 1/56). Bioinformatic analysis was carried out to delineate the potential genetic crosstalk between SMN1 (mutation of which leads to SMA) and COVID-19/lung injury-associated pathways. Protein-protein interaction analysis by STRING suggested that loss-of-function of SMN1 might modulate COVID-19 pathogenesis through CFTR, CXCL8, TNF and ACE. Expression quantitative trait loci analysis also revealed a link between SMN1 and ACE2, despite low-confidence protein-protein interactions as suggested by STRING. This bioinformatic analysis could give hint on why SMA might not necessarily lead to poor outcomes in patients with COVID-19.

Survival motor neuron protein regulates oxidative stress and inflammatory response in microglia of the spinal cord in spinal muscular atrophy

The deficiency of survival motor neuron (SMN) protein can result in the onset of spinal muscular atrophy (SMA), an autosomal recessive disorder characterized by a progressive loss of motor neurons and skeletal muscle atrophy. The mechanism underlying SMA pathology remains unclear. Here, we demonstrate that SMN protein regulates oxidative stress and inflammatory response in microglia. Antisense oligonucleotide, which increases SMN protein expression (SMN-ASO), attenuated SMA model mice phenotypes and suppressed the activation of microglia in the spinal cord. The expression of oxidative stress marker in microglia was decreased by SMN-ASO injection in SMA model mice. Increased reactive oxygen species production and subsequent antioxidative stress reaction was observed in SMN protein-depleted RAW264.7. Furthermore, nuclear factor kappa B (NFκB) and c-Jun amino terminal kinase (JNK) signaling, which mainly mediate the inflammatory response, are activated in SMN protein-depleted RAW264.7. Tumor necrosis factor-α (TNF-α) production is also increased in SMN protein-depleted RAW264.7. These findings suggest that SMN protein regulates oxidative stress and inflammatory response in microglia, supporting current claims that microglia can be an effective target for SMA therapy.

Mislocalization of SMN from the I-band and M-band in human skeletal myofibers in spinal muscular atrophy associates with primary structural alterations of the sarcomere

Spinal muscular atrophy (SMA) is caused by a deletion or mutation of the survival motor neuron 1 (SMN1) gene. Reduced SMN levels lead to motor neuron degeneration and muscular atrophy. SMN protein localizes to the cytoplasm and Cajal bodies. Moreover, in myofibrils from Drosophila and mice, SMN is a sarcomeric protein localized to the Z-disc. Although SMN participates in multiple functions, including the biogenesis of spliceosomal small nuclear ribonucleoproteins, its role in the sarcomere is unclear. Here, we analyzed the sarcomeric organization of SMN in human control and type I SMA skeletal myofibers. In control sarcomeres, we demonstrate that human SMN is localized to the titin-positive M-band and actin-positive I-band, and to SMN-positive granules that flanked the Z-discs. Co-immunoprecipitation assays revealed that SMN interacts with the sarcomeric protein actin, α-actinin, titin, and profilin2. In the type I SMA muscle, SMN levels were reduced, and atrophic (denervated) and hypertrophic (nondenervated) myofibers coexisted. The hypertrophied myofibers, which are potential primary targets of SMN deficiency, exhibited sites of focal or segmental alterations of the actin cytoskeleton, where the SMN immunostaining pattern was altered. Moreover, SMN was relocalized to the Z-disc in overcontracted minisarcomeres from hypertrophic myofibers. We propose that SMN could have an integrating role in the molecular components of the sarcomere. Consequently, low SMN levels might impact the normal sarcomeric architecture, resulting in the disruption of myofibrils found in SMA muscle. This primary effect might be independent of the neurogenic myopathy produced by denervation and contribute to pathophysiology of the SMA myopathy.

Glial cells involvement in spinal muscular atrophy: Could SMA be a neuroinflammatory disease?

Spinal muscular atrophy (SMA) is a severe, inherited disease characterized by the progressive degeneration and death of motor neurons of the anterior horns of the spinal cord, which results in muscular atrophy and weakness of variable severity. Its early-onset form is invariably fatal in early childhood, while milder forms lead to permanent disability, physical deformities and respiratory complications. Recently, two novel revolutionary therapies, antisense oligonucleotides and gene therapy, have been approved, and might prove successful in making long-term survival of these patients likely. In this perspective, a deep understanding of the pathogenic mechanisms and of their impact on the interactions between motor neurons and other cell types within the central nervous system (CNS) is crucial. Studies using SMA animal and cellular models have taught us that the survival and functionality of motor neurons is highly dependent on a whole range of other cell types, namely glial cells, which are responsible for a variety of different functions, such as neuronal trophic support, synaptic remodeling, and immune surveillance. Thus, it emerges that SMA is likely a non-cell autonomous, multifactorial disease in which the interaction of different cell types and disease mechanisms leads to motor neurons failure and loss. This review will introduce the different glial cell types in the CNS and provide an overview of the role of glial cells in motor neuron degeneration in SMA. Furthermore, we will discuss the relevance of these findings so far and the potential impact on the success of available therapies and on the development of novel ones.

Efficacy and Mechanism of Polymerized Anthocyanin from Grape-Skin Extract on High-Fat-Diet-Induced Nonalcoholic Fatty Liver Disease

We investigated the therapeutic potential of polymerized anthocyanin (PA) on a nonalcoholic fatty liver disease (NAFLD) model in mice. C57BL/6 mice were fed a high-fat diet (HFD) for 8 weeks to establish the NAFLD mouse model and randomly divided into four groups: control diet (con), NAFLD mice treated with saline (NAFLD), NAFLD mice treated with PA (PA), and NAFLD mice treated with orlistat (Orlistat) for four weeks. Mice were euthanized at the end of the four weeks. Total cholesterol (TC) and triglyceride (TG) levels were estimated, and pathological changes in the liver, white adipose tissue, and signaling pathways related to lipid metabolism were evaluated. Results revealed that the body, liver, and white fat weight of the NAFLD group was significantly increased compared to that of the con group, while that of the PA group showed significant reduction. NAFLD led to an increase in blood lipids in mice (except for HDL). Conversely, PA effectively reduced TC and LDL-C. Compared to the control group, the degree of steatosis in the mice of PA group was decreased. Moreover, PA also regulated the NAFLD signaling pathway. In agreement with improved lipid deposition, PA supplementation inhibited the activation of inflammatory pathways, depressing oxidative stress through increased antioxidant levels, and increasing β-oxidation to inhibit mitochondrial dysfunction. Taken together, our results demonstrate that PA can improve the liver function of NAFLD mice, regulating blood lipids, reducing liver-fat accumulation, and regulating lipid metabolism.

Survival Motor Neuron (SMN) Protein Insufficiency Exacerbates Renal Ischemia/Reperfusion Injury

The survival of motor neuron (SMN) protein is ubiquitously involved in spliceosome assembly and ribonucleoprotein biogenesis. SMN protein is expressed in kidney and can affect cell death processes. However, the role of SMN in acute kidney injury (AKI) is largely unknown. In the current study, we found that the expression of SMN in the kidney was significantly reduced in both clinical ischemic AKI and a mouse model of renal ischemia-reperfusion injury (IRI). We then used SMN heterozygous knockout (SMN+/-) mice and found that the declines in renal function, tubular injury, and tubular cell apoptosis after experimental IRI were significantly more severe in SMN+/- mice than those in their wild-type littermates. Concomitantly, the canonical transcription factor nuclear factor-κb (NFκb) signaling was enhanced in ischemic SMN+/- mice. In vitro, cobalt dichloride (CoCl₂) treatment reduced SMN expression in proximal tubular epithelial cells. In addition, CoCl₂-induced apoptosis and activation of NFκb signaling pathway were enhanced by transient transfection of a small-interfering RNA (siRNA) against SMN while attenuated by transient transfection of a full-length SMN plasmid. Taken together, this study for the first time supported the protective role of SMN in ischemic AKI.

SMN deficiency causes pain hypersensitivity in a mild SMA mouse model through enhancing excitability of nociceptive dorsal root ganglion neurons

Spinal muscular atrophy (SMA) is a devastating motor neuron degeneration disease caused by a deficiency of the SMN protein. Majority of patients also suffer from chronic pain. However, the pathogenesis of pain in the context of SMA has never been explored. In this study, using various pain tests, we found that a mild SMA mouse model presents with multiple forms of pain hypersensitivity. Patch-clamp recording showed that nociceptive neurons in SMA mouse dorsal root ganglia (DRGs) are hyperexcitable and their sodium current densities are markedly increased. Using quantitative RT-PCR, western blotting and immunofluorescence, we observed enhanced expression of two main voltage-gated sodium channels Na_v1.7 and Na_v1.8 in SMA mouse DRGs, which is at least in part due to increase in both expression and phosphorylation of NF-κB p50/p65 heterodimer. Moreover, we revealed that plasma norepinephrine levels are elevated in SMA mice, which contributes to mechanical hypersensitivity via the β2-adrenergic receptor. Finally, we uncovered that β2-adrenergic signaling positively modulates expression as well as phosphorylation of p50 and p65 in SMA mouse DRGs. Therefore, our data demonstrate that SMA mice, similar to humans, also develop pain hypersensitivity, and highlight a peripheral signaling cascade that elicits the mechanical sensitization in the mouse model, suggesting potential targets for therapeutic intervention.

Composition of the Survival Motor Neuron (SMN) Complex in Drosophila melanogaster

Spinal Muscular Atrophy (SMA) is caused by homozygous mutations in the human survival motor neuron 1 (SMN1) gene. SMN protein has a well-characterized role in the biogenesis of small nuclear ribonucleoproteins (snRNPs), core components of the spliceosome. SMN is part of an oligomeric complex with core binding partners, collectively called Gemins. Biochemical and cell biological studies demonstrate that certain Gemins are required for proper snRNP assembly and transport. However, the precise functions of most Gemins are unknown. To gain a deeper understanding of the SMN complex in the context of metazoan evolution, we investigated its composition in Drosophila melanogaster Using transgenic flies that exclusively express Flag-tagged SMN from its native promoter, we previously found that Gemin2, Gemin3, Gemin5, and all nine classical Sm proteins, including Lsm10 and Lsm11, co-purify with SMN. Here, we show that CG2941 is also highly enriched in the pulldown. Reciprocal co-immunoprecipitation reveals that epitope-tagged CG2941 interacts with endogenous SMN in Schneider2 cells. Bioinformatic comparisons show that CG2941 shares sequence and structural similarity with metazoan Gemin4. Additional analysis shows that three other genes (CG14164, CG31950 and CG2371) are not orthologous to Gemins 6-7-8, respectively, as previously suggested. In D.melanogaster, CG2941 is located within an evolutionarily recent genomic triplication with two other nearly identical paralogous genes (CG32783 and CG32786). RNAi-mediated knockdown of CG2941 and its two close paralogs reveals that Gemin4 is essential for organismal viability.

The role of survival motor neuron protein (SMN) in protein homeostasis

Ever since loss of survival motor neuron (SMN) protein was identified as the direct cause of the childhood inherited neurodegenerative disorder spinal muscular atrophy, significant efforts have been made to reveal the molecular functions of this ubiquitously expressed protein. Resulting research demonstrated that SMN plays important roles in multiple fundamental cellular homeostatic pathways, including a well-characterised role in the assembly of the spliceosome and biogenesis of ribonucleoproteins. More recent studies have shown that SMN is also involved in other housekeeping processes, including mRNA trafficking and local translation, cytoskeletal dynamics, endocytosis and autophagy. Moreover, SMN has been shown to influence mitochondria and bioenergetic pathways as well as regulate function of the ubiquitin-proteasome system. In this review, we summarise these diverse functions of SMN, confirming its key role in maintenance of the homeostatic environment of the cell.

Self-oligomerization regulates stability of survival motor neuron protein isoforms by sequestering an SCFSlmb degron

Spinal muscular atrophy (SMA) is caused by homozygous mutations in human SMN1 Expression of a duplicate gene (SMN2) primarily results in skipping of exon 7 and production of an unstable protein isoform, SMNΔ7. Although SMN2 exon skipping is the principal contributor to SMA severity, mechanisms governing stability of survival motor neuron (SMN) isoforms are poorly understood. We used a Drosophila model system and label-free proteomics to identify the SCFSlmb ubiquitin E3 ligase complex as a novel SMN binding partner. SCFSlmb interacts with a phosphor degron embedded within the human and fruitfly SMN YG-box oligomerization domains. Substitution of a conserved serine (S270A) interferes with SCFSlmb binding and stabilizes SMNΔ7. SMA-causing missense mutations that block multimerization of full-length SMN are also stabilized in the degron mutant background. Overexpression of SMNΔ7S270A, but not wild-type (WT) SMNΔ7, provides a protective effect in SMA model mice and human motor neuron cell culture systems. Our findings support a model wherein the degron is exposed when SMN is monomeric and sequestered when SMN forms higher-order multimers.

Pathogenic commonalities between spinal muscular atrophy and amyotrophic lateral sclerosis: Converging roads to therapeutic development

Spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS) are the two most common motoneuron disorders, which share typical pathological hallmarks while remaining genetically distinct. Indeed, SMA is caused by deletions or mutations in the survival motor neuron 1 (SMN1) gene whilst ALS, albeit being mostly sporadic, can also be caused by mutations within genes, including superoxide dismutase 1 (SOD1), Fused in Sarcoma (FUS), TAR DNA-binding protein 43 (TDP-43) and chromosome 9 open reading frame 72 (C9ORF72). However, it has come to light that these two diseases may be more interlinked than previously thought. Indeed, it has recently been found that FUS directly interacts with an Smn-containing complex, mutant SOD1 perturbs Smn localization, Smn depletion aggravates disease progression of ALS mice, overexpression of SMN in ALS mice significantly improves their phenotype and lifespan, and duplications of SMN1 have been linked to sporadic ALS. Beyond genetic interactions, accumulating evidence further suggests that both diseases share common pathological identities such as intrinsic muscle defects, neuroinflammation, immune organ dysfunction, metabolic perturbations, defects in neuron excitability and selective motoneuron vulnerability. Identifying common molecular effectors that mediate shared pathologies in SMA and ALS would allow for the development of therapeutic strategies and targeted gene therapies that could potentially alleviate symptoms and be equally beneficial in both disorders. In the present review, we will examine our current knowledge of pathogenic commonalities between SMA and ALS, and discuss how furthering this understanding can lead to the establishment of novel therapeutic approaches with wide-reaching impact on multiple motoneuron diseases.