SMN deficiency negatively impacts red pulp macrophages and spleen development in mouse models of spinal muscular atrophy

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.

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.

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.

Innovating spinal muscular atrophy models in the therapeutic era

Spinal muscular atrophy (SMA) is a severe, monogenetic, neuromuscular disease. A thorough understanding of its genetic cause and the availability of robust models has led to the development and approval of three gene-targeting therapies. This is a unique and exciting development for the field of neuromuscular diseases, many of which remain untreatable. The development of therapies for SMA not only opens the door to future therapeutic possibilities for other genetic neuromuscular diseases, but also informs us about the limitations of such treatments. For example, treatment response varies widely and, for many patients, significant disability remains. Currently available SMA models best recapitulate the severe types of SMA, and these models are genetically and phenotypically more homogeneous than patients. Furthermore, treating patients is leading to a shift in phenotypes with increased variability in SMA clinical presentation. Therefore, there is a need to generate model systems that better reflect these developments. Here, we will first discuss current animal models of SMA and their limitations. Next, we will discuss the characteristics required to future-proof models to assist the field in the development of additional, novel therapies for SMA.

Changes in Ventilatory Support Requirements of Spinal Muscular Atrophy (SMA) Patients Post Gene-Based Therapies

Spinal muscular atrophy (SMA) is a genetic neuromuscular disease resulting in global muscular weakness and, frequently, in respiratory failure and premature death. Gene-based therapies like Nusinersen are now available for patients with SMA. The aim of this review was to assess in "real world" studies, whether novel treatments would have a positive impact on the mechanical ventilatory support requirements of SMA patients, already initiated on ventilatory support prior to treatment administration. A literature search was performed in Pubmed using multiple combinations of MESH terms and the snowball procedure. A total of 14 publications were discussed in this review. Considering all patients included in the published studies who were on ventilatory support and were treated with Nusinersen, 13/172 (7.5%) had reduced needs for ventilatory support, 1/172 (0.6%) did not need ventilation post-treatment, and 122/172 (70.9%) were maintained on the same ventilator settings. Moreover, 2/41 (4.9%) children who were offered gene therapy had no need for further ventilatory support and 12/41 (29.2%) had reduced requirements. In conclusion, available evidence suggests that among children with SMA, who are on mechanical respiratory support either noninvasively or via tracheostomy at the time of gene-based treatment, only a few will be weaned off the ventilator or have reduced ventilator needs per 24 h. Children will usually require the same level of support as before treatment.

Restoring SMN Expression: An Overview of the Therapeutic Developments for the Treatment of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disorder and one of the most common genetic causes of infant death. It is characterized by progressive weakness of the muscles, loss of ambulation, and death from respiratory complications. SMA is caused by the homozygous deletion or mutations in the survival of the motor neuron 1 (SMN1) gene. Humans, however, have a nearly identical copy of SMN1 known as the SMN2 gene. The severity of the disease correlates inversely with the number of SMN2 copies present. SMN2 cannot completely compensate for the loss of SMN1 in SMA patients because it can produce only a fraction of functional SMN protein. SMN protein is ubiquitously expressed in the body and has a variety of roles ranging from assembling the spliceosomal machinery, autophagy, RNA metabolism, signal transduction, cellular homeostasis, DNA repair, and recombination. Motor neurons in the anterior horn of the spinal cord are extremely susceptible to the loss of SMN protein, with the reason still being unclear. Due to the ability of the SMN2 gene to produce small amounts of functional SMN, two FDA-approved treatment strategies, including an antisense oligonucleotide (AON) nusinersen and small-molecule risdiplam, target SMN2 to produce more functional SMN. On the other hand, Onasemnogene abeparvovec (brand name Zolgensma) is an FDA-approved adeno-associated vector 9-mediated gene replacement therapy that can deliver a copy of the human SMN1. In this review, we summarize the SMA etiology, the role of SMN, and discuss the challenges of the therapies that are approved for SMA treatment.

Cerebrospinal Fluid and Clinical Profiles in Adult Type 2-3 Spinal Muscular Atrophy Patients Treated with Nusinersen: An 18-Month Single-Centre Experience

BACKGROUND AND OBJECTIVES

Nusinersen was approved as the first disease-modifying therapy in spinal muscular atrophy (SMA). Our aim was to analyse therapy-related changes in cerebrospinal fluid (CSF) and serum parameters of adult type 2-3 SMA and to correlate biochemical data with motor functional status.

METHODS

Nine adult SMA type 2-3 patients and ten control subjects without neurodegenerative diseases were included in our single-centre study. Cross-sectional analysis of CSF routine parameters, CSF neurofilament light chain, CSF Tau, CSF phospho-Tau and serum creatinine was performed between SMA patients at baseline (T0) and control subjects. The above-mentioned fluid parameters were longitudinally analysed in the SMA cohort after loading dose (T1) and after four maintenance doses (T2, T3, T4, T5). Hammersmith Functional Motor Scale Expanded (HFMSE), Revised Upper Limb Module (RULM) and the 6-minute walking test (6MWT) were used to evaluate motor outcomes.

RESULTS

Improvements in HFMSE, RULM and 6MWT were observed only after the loading dose of nusinersen. No significant differences in routine CSF parameters and CSF markers of neurodegeneration were found between SMA patients and control subjects. Serum creatinine levels were significantly lower in SMA patients than in control subjects. CSF/serum albumin ratio (Qalb) significantly increased from T0 to each time point, without any further increase after the maintenance doses. Persistent systemic oligoclonal bands (OCBs) were found in five patients from baseline. Three more patients developed persistent systemic OCBs from T1; one patient showed intrathecal OCBSs from baseline to T5. Markers of neurodegeneration did not change during the follow-up and did not correlate with motor scores at baseline and at each timepoint. Serum creatinine levels significantly correlated with HFMSE and RULM at each time point.

CONCLUSIONS

The increase of the Qalb values and the development of systemic OCBs in some SMA patients could be due to repeated lumbar puncture and to the immunogenic effect of nusinersen. On the other hand, the presence of OCBs in serum and/or CSF at baseline should be further investigated. Furthermore, biomarkers of neurodegeneration did not play a prognostic role in our cohort of adult SMA patients.

Spinal muscular atrophy: Broad disease spectrum and sex-specific phenotypes

Spinal muscular atrophy (SMA) is one of the major genetic disorders associated with infant mortality. More than 90% of cases of SMA result from deletions of or mutations in the Survival Motor Neuron 1 (SMN1) gene. SMN2, a nearly identical copy of SMN1, does not compensate for the loss of SMN1 due to predominant skipping of exon 7. The spectrum of SMA is broad, ranging from prenatal death to infant mortality to survival into adulthood. All tissues, including brain, spinal cord, bone, skeletal muscle, heart, lung, liver, pancreas, gastrointestinal tract, kidney, spleen, ovary and testis, are directly and/or indirectly affected in SMA. Accumulating evidence on impaired mitochondrial biogenesis and defects in X chromosome-linked modifying factors, coupled with the sexual dimorphic nature of many tissues, point to sex-specific vulnerabilities in SMA. Here we review the role of sex in the pathogenesis of SMA.

Short-duration splice promoting compound enables a tunable mouse model of spinal muscular atrophy

We describe drug treatment paradigms that allow investigation of cellular and molecular pathogenesis at different stages of spinal muscular atrophy in a mouse model.

Spinal muscular atrophy (SMA) is a motor neuron disease and the leading genetic cause of infant mortality. SMA results from insufficient survival motor neuron (SMN) protein due to alternative splicing. Antisense oligonucleotides, gene therapy and splicing modifiers recently received FDA approval. Although severe SMA transgenic mouse models have been beneficial for testing therapeutic efficacy, models mimicking milder cases that manifest post-infancy have proven challenging to develop. We established a titratable model of mild and moderate SMA using the splicing compound NVS-SM2. Administration for 30 d prevented development of the SMA phenotype in severe SMA mice, which typically show rapid weakness and succumb by postnatal day 11. Furthermore, administration at day eight resulted in phenotypic recovery. Remarkably, acute dosing limited to the first 3 d of life significantly enhanced survival in two severe SMA mice models, easing the burden on neonates and demonstrating the compound as suitable for evaluation of follow-on therapies without potential drug–drug interactions. This pharmacologically tunable SMA model represents a useful tool to investigate cellular and molecular pathogenesis at different stages of disease.

Comparison of the efficacy of MOE and PMO modifications of systemic antisense oligonucleotides in a severe SMA mouse model

Spinal muscular atrophy (SMA) is a motor neuron disease. Nusinersen, a splice-switching antisense oligonucleotide (ASO), was the first approved drug to treat SMA. Based on prior preclinical studies, both 2′-O-methoxyethyl (MOE) with a phosphorothioate backbone and morpholino with a phosphorodiamidate backbone—with the same or extended target sequence as nusinersen—displayed efficient rescue of SMA mouse models. Here, we compared the therapeutic efficacy of these two modification chemistries in rescue of a severe mouse model using ASO10-29—a 2-nt longer version of nusinersen—via subcutaneous injection. Although both chemistries efficiently corrected SMN2 splicing in various tissues, restored motor function and improved the integrity of neuromuscular junctions, MOE-modified ASO10-29 (MOE10-29) was more efficacious than morpholino-modified ASO10-29 (PMO10-29) at the same molar dose, as seen by longer survival, greater body-weight gain and better preservation of motor neurons. Time-course analysis revealed that MOE10-29 had more persistent effects than PMO10-29. On the other hand, PMO10-29 appears to more readily cross an immature blood-brain barrier following systemic administration, showing more robust initial effects on SMN2 exon 7 inclusion, but less persistence in the central nervous system. We conclude that both modifications can be effective as splice-switching ASOs in the context of SMA and potentially other diseases, and discuss the advantages and disadvantages of each.

SMN-deficiency disrupts SERCA2 expression and intracellular Ca2+ signaling in cardiomyocytes from SMA mice and patient-derived iPSCs

Spinal muscular atrophy (SMA) is a neurodegenerative disease characterized by loss of alpha motor neurons and skeletal muscle atrophy. The disease is caused by mutations of the SMN1 gene that result in reduced functional expression of survival motor neuron (SMN) protein. SMN is ubiquitously expressed, and there have been reports of cardiovascular dysfunction in the most severe SMA patients and animal models of the disease. In this study, we directly assessed the function of cardiomyocytes isolated from a severe SMA model mouse and cardiomyocytes generated from patient-derived IPSCs. Consistent with impaired cardiovascular function at the very early disease stages in mice, heart failure markers such as brain natriuretic peptide were significantly elevated. Functionally, cardiomyocyte relaxation kinetics were markedly slowed and the T₅₀ for Ca²⁺ sequestration increased to 146 ± 4 ms in SMN-deficient cardiomyocytes from 126 ± 4 ms in wild type cells. Reducing SMN levels in cardiomyocytes from control patient IPSCs slowed calcium reuptake similar to SMA patent-derived cardiac cells. Importantly, restoring SMN increased calcium reuptake rate. Taken together, these results indicate that SMN deficiency impairs cardiomyocyte function at least partially through intracellular Ca²⁺ cycling dysregulation.

Motor transmission defects with sex differences in a new mouse model of mild spinal muscular atrophy

Background

Mouse models of mild spinal muscular atrophy (SMA) have been extremely challenging to generate. This paucity of model systems has limited our understanding of pathophysiological events in milder forms of the disease and of the effect of SMN depletion during aging.

Methods

A mild mouse model of SMA, termed Smn^2B/−;SMN2^+/−, was generated by crossing Smn^−/−;SMN2 and Smn^2B/2B mice. This new model was characterized using behavioral testing, histology, western blot, muscle-nerve electrophysiology as well as ultrasonography to study classical SMA features and extra-neuronal involvement.

Findings

Smn^2B/−;SMN2^+/− mice have normal survival, mild but sustained motor weakness, denervation and neuronal/neuromuscular junction (NMJ) transmission defects, and neurogenic muscle atrophy that are more prominent in male mice. Increased centrally located nuclei, intrinsic contractile and relaxation muscle defects were also identified in both female and male mice, with some male predominance. There was an absence of extra-neuronal pathology.

Interpretation

The Smn^2B/−;SMN2^+/− mouse provides a model of mild SMA, displaying some hallmark features including reduced weight, sustained motor weakness, electrophysiological transmission deficit, NMJ defects, and muscle atrophy. Early and prominent increase central nucleation and intrinsic electrophysiological deficits demonstrate the potential role played by muscle in SMA disease. The use of this model will allow for the understanding of the most susceptible pathogenic molecular changes in motor neurons and muscles, investigation of the effects of SMN depletion in aging, sex differences and most importantly will provide guidance for the currently aging SMA patients treated with the recently approved genetic therapies.

Funding

: This work was supported by Cure SMA/Families of SMA Canada (grant numbers KOT-1819 and KOT-2021); Muscular Dystrophy Association (USA) (grant number 575466); and Canadian Institutes of Health Research (CIHR) (grant number PJT-156379).

Light modulation ameliorates expression of circadian genes and disease progression in spinal muscular atrophy mice

Physiology and behaviour are critically dependent on circadian regulation via a core set of clock genes, dysregulation of which leads to metabolic and sleep disturbances. Metabolic and sleep perturbations occur in spinal muscular atrophy (SMA), a neuromuscular disorder caused by loss of the survival motor neuron (SMN) protein and characterized by motor neuron loss and muscle atrophy. We therefore investigated the expression of circadian rhythm genes in various metabolic tissues and spinal cord of the Taiwanese Smn-/-;SMN2 SMA animal model. We demonstrate a dysregulated expression of the core clock genes (clock, ARNTL/Bmal1, Cry1/2, Per1/2) and clock output genes (Nr1d1 and Dbp) in SMA tissues during disease progression. We also uncover an age- and tissue-dependent diurnal expression of the Smn gene. Importantly, we observe molecular and phenotypic corrections in SMA mice following direct light modulation. Our study identifies a key relationship between an SMA pathology and peripheral core clock gene dysregulation, highlights the influence of SMN on peripheral circadian regulation and metabolism and has significant implications for the development of peripheral therapeutic approaches and clinical care management of SMA patients.

Developmental and degenerative cardiac defects in the Taiwanese mouse model of severe spinal muscular atrophy

Spinal muscular atrophy (SMA), an autosomal recessive disease caused by a decrease in levels of the survival motor neuron (SMN) protein, is the most common genetic cause of infant mortality. Although neuromuscular pathology is the most severe feature of SMA, other organs and tissues, including the heart, are also known to be affected in both patients and animal models. Here, we provide new insights into changes occurring in the heart, predominantly at pre- and early symptomatic ages, in the Taiwanese mouse model of severe SMA. Thinning of the interventricular septum and dilation of the ventricles occurred at pre- and early symptomatic ages. However, the left ventricular wall was significantly thinner in SMA mice from birth, occurring prior to any overt neuromuscular symptoms. Alterations in collagen IV protein from birth indicated changes to the basement membrane and contributed to the abnormal arrangement of cardiomyocytes in SMA hearts. This raises the possibility that developmental defects, occurring prenatally, may contribute to cardiac pathology in SMA. In addition, cardiomyocytes in SMA hearts exhibited oxidative stress at pre-symptomatic ages and increased apoptosis during early symptomatic stages of disease. Heart microvasculature was similarly decreased at an early symptomatic age, likely contributing to the oxidative stress and apoptosis phenotypes observed. Finally, an increased incidence of blood retention in SMA hearts post-fixation suggests the likelihood of functional defects, resulting in blood pooling. These pathologies mirror dilated cardiomyopathy, with clear consequences for heart function that would likely contribute to potential heart failure. Our findings add significant additional experimental evidence in support of the requirement to develop systemic therapies for SMA capable of treating non-neuromuscular pathologies.

New insights into SMA pathogenesis: immune dysfunction and neuroinflammation

Spinal muscular atrophy (SMA) is a neuromuscular disorder characterized by motor neuron degeneration, although defects in multiple cell types and tissues have also been implicated. Three independent laboratories recently identified immune organ defects in SMA. We therefore propose a novel pathogenic mechanism contributory to SMA, resulting in higher susceptibility to infection and exacerbated disease progression caused by neuroinflammation. Overall, compromised immune function could significantly affect survival and quality of life of SMA patients. We highlight the recent findings in immune organ defects, their potential consequences on patients, our understanding of neuroinflammation in SMA, and new research hypotheses in SMA pathogenesis.