Advances in modeling and treating spinal muscular atrophy

Mitigating aberrant Cdk5 activation alleviates mitochondrial defects and motor neuron disease symptoms in spinal muscular atrophy

Spinal muscular atrophy (SMA), the top genetic cause of infant mortality, is characterized by motor neuron degeneration. Mechanisms underlying SMA pathogenesis remain largely unknown. Here, we report that the activity of cyclin-dependent kinase 5 (Cdk5) and the conversion of its activating subunit p35 to the more potent activator p25 are significantly up-regulated in mouse models and human induced pluripotent stem cell (iPSC) models of SMA. The increase of Cdk5 activity occurs before the onset of SMA phenotypes, suggesting that it may be an initiator of the disease. Importantly, aberrant Cdk5 activation causes mitochondrial defects and motor neuron degeneration, as the genetic knockout of p35 in an SMA mouse model rescues mitochondrial transport and fragmentation defects, and alleviates SMA phenotypes including motor neuron hyperexcitability, loss of excitatory synapses, neuromuscular junction denervation, and motor neuron degeneration. Inhibition of the Cdk5 signaling pathway reduces the degeneration of motor neurons derived from SMA mice and human SMA iPSCs. Altogether, our studies reveal a critical role for the aberrant activation of Cdk5 in SMA pathogenesis and suggest a potential target for therapeutic intervention.

Motor defects in a Drosophila model for spinal muscular atrophy result from SMN depletion during early neurogenesis

Spinal muscular atrophy (SMA) is the most common autosomal recessive neurodegenerative disease, and is characterised by spinal motor neuron loss, impaired motor function and, often, premature death. Mutations and deletions in the widely expressed survival motor neuron 1 (SMN1) gene cause SMA; however, the mechanisms underlying the selectivity of motor neuron degeneration are not well understood. Although SMA is degenerative in nature, SMN function during embryonic and early postnatal development appears to be essential for motor neuron survival in animal models and humans. Notwithstanding, how developmental defects contribute to the subversion of postnatal and adult motor function remains elusive. Here, in a Drosophila SMA model, we show that neurodevelopmental defects precede gross locomotor dysfunction in larvae. Furthermore, to specifically address the relevance of SMN during neurogenesis and in neurogenic cell types, we show that SMN knockdown using neuroblast-specific and pan-neuronal drivers, but not differentiated neuron or glial cell drivers, impairs adult motor function. Using targeted knockdown, we further restricted SMN manipulation in neuroblasts to a defined time window. Our aim was to express specifically in the neuronal progenitor cell types that have not formed synapses, and thus a time that precedes neuromuscular junction formation and maturation. By restoring SMN levels in these distinct neuronal population, we partially rescue the larval locomotor defects of Smn mutants. Finally, combinatorial SMN knockdown in immature and mature neurons synergistically enhances the locomotor and survival phenotypes. Our in-vivo study is the first to directly rescue the motor defects of an SMA model by expressing Smn in an identifiable population of Drosophila neuroblasts and developing neurons, highlighting that neuronal sensitivity to SMN loss may arise before synapse establishment and nerve cell maturation.

Spinal muscular atrophy (SMA) is the most common genetic cause of infant mortality and leads to the degeneration of the nerves that control muscle function. Loss-of-function mutations in the widely expressed survival motor neuron 1 (SMN1) gene cause SMA, but how low levels of SMN protein cause the neuronal dysfunction is not known. Although SMA is a disease of nerve degeneration, SMN function during nerve cell development may be important, particularly in severe forms of SMA. Nevertheless, how the defects during development and throughout early life contribute to the disease is not well understood. We have previously demonstrated that SMN protein becomes enriched in neuroblasts, which are the cells that divide to produce neurons. In the present study, motor defects observed in our fly model for SMA could be rescued by restoring SMN in neuroblasts alone. In addition, we show that knocking down SMN in healthy flies within the same cell type causes impaired motor function. The present study shows that the manipulation of SMN in a developmentally important cell type can cause motor defects, indicating that a period of abnormal neurodevelopment may contribute to SMA.

Cell-penetrating peptide-conjugated Morpholino rescues SMA in a symptomatic preclinical model

Spinal muscular atrophy (SMA) is a motor neuron disease and the leading genetic cause of infant mortality. Recently approved SMA therapies have transformed a deadly disease into a survivable one, but these compounds show a wide spectrum of clinical response and effective rescue only in the early stages of the disease. Therefore, safe, symptomatic-suitable, non-invasive treatments with high clinical impact across different phenotypes are urgently needed. We conjugated antisense oligonucleotides with Morpholino (MO) chemistry, which increase SMN protein levels, to cell-penetrating peptides (CPPs) for better cellular distribution. Systemically administered MOs linked to r6 and (RXRRBR)₂XB peptides crossed the blood-brain barrier and increased SMN protein levels remarkably, causing striking improvement of survival, neuromuscular function, and neuropathology, even in symptomatic SMA animals. Our study demonstrates that MO-CPP conjugates can significantly expand the therapeutic window through minimally invasive systemic administration, opening the path for clinical applications of this strategy.

Current diagnosis and management of rare pediatric diseases in China

This review categorizes and summarizes the rare pediatric diseases that have been included in the First List of Rare Diseases that was jointly published by the National Health Commission and four other government departments in China in 2018. In total, 58 diseases that develop during childhood are included. These diseases involve nine organ systems, including the musculoskeletal, respiratory, immune, endocrine and metabolic, nervous, cardiovascular, hematological, urinary, and integumentary systems. Affected children often have multiorgan involvement with various presentations. Severe diseases can cause acute symptoms starting in the neonatal period that lead to increased morbidity and mortality without prompt management. Early diagnosis and treatment can significantly change the course of a disease and improve its prognosis. This work systemically reviews the status of rare pediatric diseases with a relatively high incidence in the First List of Rare Diseases.

Sumoylation regulates the assembly and activity of the SMN complex

SMN is a ubiquitously expressed protein and is essential for life. SMN deficiency causes the neurodegenerative disease spinal muscular atrophy (SMA), the leading genetic cause of infant mortality. SMN interacts with itself and other proteins to form a complex that functions in the assembly of ribonucleoproteins. SMN is modified by SUMO (Small Ubiquitin-like Modifier), but whether sumoylation is required for the functions of SMN that are relevant to SMA pathogenesis is not known. Here, we show that inactivation of a SUMO-interacting motif (SIM) alters SMN sub-cellular distribution, the integrity of its complex, and its function in small nuclear ribonucleoproteins biogenesis. Expression of a SIM-inactivated mutant of SMN in a mouse model of SMA slightly extends survival rate with limited and transient correction of motor deficits. Remarkably, although SIM-inactivated SMN attenuates motor neuron loss and improves neuromuscular junction synapses, it fails to prevent the loss of sensory-motor synapses. These findings suggest that sumoylation is important for proper assembly and function of the SMN complex and that loss of this post-translational modification impairs the ability of SMN to correct selective deficits in the sensory-motor circuit of SMA mice.

2020 Update to Spinal Muscular Atrophy Management in Saudi Arabia

Novel therapeutic strategies have shown some promise in treating spinal muscular atrophy (SMA). However, the outcomes and acceptance of these new strategies are yet to be explored. We aimed to investigate physicians' opinions and perceptions toward management strategies of SMA across Saudi Arabia. This is a cross-sectional survey using a self-administered, structured questionnaire sent to physicians who care for SMA patients during the Saudi Pediatric Neurology Society annual conference. A total of 72 clinicians of different neurological subspecialties were included. 48.6% prescribed nusinersen to their patients, with 39% of them having patients started on nusinersen. Though, 8.3% prescribed onasemnogene abeparvovec for 1-3 patients, while none of their patients started on the treatment. 64.3% stated that the only treatment available for SMA in their settings is supportive care. Around 69.4% described having a moderate to high knowledge on SMA gene therapy, and 79.2% would recommend it. 48.6% confirmed they would prescribe gene therapy at the age of 6 months, and 78.3% would prescribe it for type-I SMA. Pediatric neurologists are receptive to novel and innovative therapies for SMA in Saudi Arabia. However, the high treatment acquisition cost, strict regulations, logistical issues, and budget constraints delay their adoption and implementation.

Stasimon Contributes to the Loss of Sensory Synapses and Motor Neuron Death in a Mouse Model of Spinal Muscular Atrophy

Reduced expression of the survival motor neuron (SMN) protein causes the neurodegenerative disease spinal muscular atrophy (SMA). Here, we show that adeno-associated virus serotype 9 (AAV9)-mediated delivery of Stasimon—a gene encoding an endoplasmic reticulum (ER)-resident transmembrane protein regulated by SMN—improves motor function in a mouse model of SMA through multiple mechanisms. In proprioceptive neurons, Stasimon overexpression prevents the loss of afferent synapses on motor neurons and enhances sensory-motor neurotransmission. In motor neurons, Stasimon suppresses neurodegeneration by reducing phosphorylation of the tumor suppressor p53. Moreover, Stasimon deficiency converges on SMA-related mechanisms of p53 upregulation to induce phosphorylation of p53 through activation of p38 mitogen-activated protein kinase (MAPK), and pharmacological inhibition of this kinase prevents motor neuron death in SMA mice. These findings identify Stasimon dysfunction induced by SMN deficiency as an upstream driver of distinct cellular cascades that lead to synaptic loss and motor neuron degeneration, revealing a dual contribution of Stasimon to motor circuit pathology in SMA.

SMN deficiency causes motor circuit dysfunction in SMA. Simon et al. show that Stasimon—an ER-resident protein regulated by SMN—contributes to sensory synaptic loss and motor neuron death in SMA mice through distinct mechanisms. In motor neurons, Stasimon dysfunction induces p38 MAPK-mediated phosphorylation of p53 whose inhibition prevents neurodegeneration.

Minor snRNA gene delivery improves the loss of proprioceptive synapses on SMA motor neurons

Spinal muscular atrophy (SMA) is an inherited neuromuscular disorder caused by reduced expression of the survival motor neuron (SMN) protein. SMN has key functions in multiple RNA pathways, including the biogenesis of small nuclear ribonucleoproteins that are essential components of both major (U2-dependent) and minor (U12-dependent) spliceosomes. Here we investigated the specific contribution of U12 splicing dysfunction to SMA pathology through selective restoration of this RNA pathway in mouse models of varying phenotypic severity. We show that virus-mediated delivery of minor snRNA genes specifically improves select U12 splicing defects induced by SMN deficiency in cultured mammalian cells, as well as in the spinal cord and dorsal root ganglia of SMA mice without increasing SMN expression. This approach resulted in a moderate amelioration of several parameters of the disease phenotype in SMA mice, including survival, weight gain, and motor function. Importantly, minor snRNA gene delivery improved aberrant splicing of the U12 intron-containing gene Stasimon and rescued the severe loss of proprioceptive sensory synapses on SMA motor neurons, which are early signatures of motor circuit dysfunction in mouse models. Taken together, these findings establish the direct contribution of U12 splicing dysfunction to synaptic deafferentation and motor circuit pathology in SMA.

A subset of SMN complex members have a specific role in tissue regeneration via ERBB pathway-mediated proliferation

Spinal muscular atrophy (SMA) is the most common genetic disease in children. SMA is generally caused by mutations in the gene SMN1. The survival of motor neurons (SMN) complex consists of SMN1, Gemins (2-8), and Strap/Unrip. We previously demonstrated smn1 and gemin5 inhibited tissue regeneration in zebrafish. Here we investigated each individual SMN complex member and identified gemin3 as another regeneration-essential gene. These three genes are likely pan-regenerative, since they affect the regeneration of hair cells, liver, and caudal fin. RNA-Seq analysis reveals that smn1, gemin3, and gemin5 are linked to a common set of genetic pathways, including the tp53 and ErbB pathways. Additional studies indicated all three genes facilitate regeneration by inhibiting the ErbB pathway, thereby allowing cell proliferation in the injured neuromasts. This study provides a new understanding of the SMN complex and a potential etiology for SMA and potentially other rare unidentified genetic diseases with similar symptoms.

R-Roscovitine Improves Motoneuron Function in Mouse Models for Spinal Muscular Atrophy

Neurotransmission defects and motoneuron degeneration are hallmarks of spinal muscular atrophy, a monogenetic disease caused by the deficiency of the SMN protein. In the present study, we show that systemic application of R-Roscovitine, a Ca_v2.1/Ca_v2.2 channel modifier and a cyclin-dependent kinase 5 (Cdk-5) inhibitor, significantly improved survival of SMA mice. In addition, R-Roscovitine increased Ca_v2.1 channel density and sizes of the motor endplates. In vitro, R-Roscovitine restored axon lengths and growth cone sizes of Smn-deficient motoneurons corresponding to enhanced spontaneous Ca²⁺ influx and elevated Ca_v2.2 channel cluster formations independent of its capability to inhibit Cdk-5. Acute application of R-Roscovitine at the neuromuscular junction significantly increased evoked neurotransmitter release, increased the frequency of spontaneous miniature potentials, and lowered the activation threshold of silent terminals. These data indicate that R-Roscovitine improves Ca²⁺ signaling and Ca²⁺ homeostasis in Smn-deficient motoneurons, which is generally crucial for motoneuron differentiation, maturation, and function.

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R-Roscovitine prolongs survival of SMA mice

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R-Roscovitine increases Ca²⁺ influx and growth cone size of SMA motoneurons

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R-Roscovitine beneficially affects neurotransmission in SMA motor nerve terminals

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R-Roscovitine wakes up dormant synapses of SMA motoneurons

Converging Mechanisms of p53 Activation Drive Motor Neuron Degeneration in Spinal Muscular Atrophy

The hallmark of spinal muscular atrophy (SMA), an inherited disease caused by ubiquitous deficiency in the SMN protein, is the selective degeneration of subsets of spinal motor neurons. Here, we show that cell-autonomous activation of p53 occurs in vulnerable but not resistant motor neurons of SMA mice at pre-symptomatic stages. Moreover, pharmacological or genetic inhibition of p53 prevents motor neuron death, demonstrating that induction of p53 signaling drives neurodegeneration. At late disease stages, however, nuclear accumulation of p53 extends to resistant motor neurons and spinal interneurons but is not associated with cell death. Importantly, we identify phosphorylation of serine 18 as a specific post-translational modification of p53 that exclusively marks vulnerable SMA motor neurons and provide evidence that amino-terminal phosphorylation of p53 is required for the neurodegenerative process. Our findings indicate that distinct events induced by SMN deficiency converge on p53 to trigger selective death of vulnerable SMA motor neurons.

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.

Dysregulation of Mdm2 and Mdm4 alternative splicing underlies motor neuron death in spinal muscular atrophy

Van Alstyne et al. show that loss of SMN-dependent regulation of Mdm2 and Mdm4 alternative splicing underlies p53-mediated death of motor neurons in SMA, establishing a causal link between snRNP dysfunction and neurodegeneration.

Ubiquitous deficiency in the survival motor neuron (SMN) protein causes death of motor neurons—a hallmark of the neurodegenerative disease spinal muscular atrophy (SMA)—through poorly understood mechanisms. Here, we show that the function of SMN in the assembly of spliceosomal small nuclear ribonucleoproteins (snRNPs) regulates alternative splicing of Mdm2 and Mdm4, two nonredundant repressors of p53. Decreased inclusion of critical Mdm2 and Mdm4 exons is most prominent in SMA motor neurons and correlates with both snRNP reduction and p53 activation in vivo. Importantly, increased skipping of Mdm2 and Mdm4 exons regulated by SMN is necessary and sufficient to synergistically elicit robust p53 activation in wild-type mice. Conversely, restoration of full-length Mdm2 and Mdm4 suppresses p53 induction and motor neuron degeneration in SMA mice. These findings reveal that loss of SMN-dependent regulation of Mdm2 and Mdm4 alternative splicing underlies p53-mediated death of motor neurons in SMA, establishing a causal link between snRNP dysfunction and neurodegeneration.

Glial Activation and Central Synapse Loss, but Not Motoneuron Degeneration, Are Prevented by the Sigma-1 Receptor Agonist PRE-084 in the Smn2B/- Mouse Model of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is characterized by the loss of α-motoneurons (MNs) with concomitant muscle denervation. MN excitability and vulnerability to disease are particularly regulated by cholinergic synaptic afferents (C-boutons), in which Sigma-1 receptor (Sig1R) is concentrated. Alterations in Sig1R have been associated with MN degeneration. Here, we investigated whether a chronic treatment with the Sig1R agonist PRE-084 was able to exert beneficial effects on SMA. We used a model of intermediate SMA, the Smn2B/- mouse, in which we performed a detailed characterization of the histopathological changes that occur throughout the disease. We report that Smn2B/- mice exhibited qualitative differences in major alterations found in mouse models of severe SMA: Smn2B/- animals showed more prominent MN degeneration, early motor axon alterations, marked changes in sensory neurons, and later MN deafferentation that correlated with conspicuous reactive gliosis and altered neuroinflammatory M1/M2 microglial balance. PRE-084 attenuated reactive gliosis, mitigated M1/M2 imbalance, and prevented MN deafferentation in Smn2B/- mice. These effects were also observed in a severe SMA model, the SMNΔ7 mouse. However, the prevention of gliosis and MN deafferentation promoted by PRE-084 were not accompanied by any improvements in clinical outcome or other major pathological changes found in SMA mice.

Evolving views of human genetic variation and its relationship to neurologic and psychiatric disease

Recent advances in exome and genome sequencing in populations are beginning to define the genetic architecture of neurologic and psychiatric disease. At the same time these findings are changing our perspective of genetic variant contributions to disease, implicating both rare and common genetic variation in common diseases. Most of what we know about genetic contributions to disease so far comes from analysis of mutations in protein-coding genes. Since most genetic variation lies in nonprotein-coding regions of the genome whose presumed function is entirely regulatory, understanding gene regulation in a cell type and developmental state-specific manner will be important to connect human genetic variation to disease mechanisms.

ERK and ROCK functionally interact in a signaling network that is compensationally upregulated in Spinal Muscular Atrophy

Spinal Muscular Atrophy (SMA) is a motoneuron disease caused by low levels of functional survival of motoneuron protein (SMN). Molecular disease mechanisms downstream of functional SMN loss are still largely unknown. Previous studies suggested an involvement of Rho kinase (ROCK) as well as the extracellular signal-regulated kinases (ERK) pathways in the pathomechanism. Both pathways are bi-directionally linked and inhibit each other. Thus, we hypothesize that both pathways regulate SMA pathophysiology in vivo in a combined manner rather than acting separately. Here, we applied the repurposed drugs, selumetinib, an ERK inhibitor, and the ROCK inhibitor fasudil to severe SMA mice. Thereby, separately applied inhibitors as well as a combination enabled us to explore the impact of the ROCK-ERK signaling network on SMA pathophysiology. ROCK inhibition specifically ameliorated the phenotype of selumetinib-treated SMA mice demonstrating an efficient ROCK to ERK crosstalk relevant for the SMA pathophysiology. However, ERK inhibition alone aggravated the condition of SMA mice and reduced the number of motoneurons indicating a compensatory hyper-activation of ERK in motoneurons. Taken together, we identified a regulatory network acting downstream of SMN depletion and upstream of the SMA pathophysiology thus being a future treatment target in combination with SMN dependent strategies.

Splicing arrays reveal novel RBM10 targets, including SMN2 pre-mRNA

Background

RBM10 is an RNA binding protein involved in message stabilization and alternative splicing regulation. The objective of the research described herein was to identify novel targets of RBM10-regulated splicing. To accomplish this, we downregulated RBM10 in human cell lines, using small interfering RNAs, then monitored alternative splicing, using a reverse transcription-PCR screening platform.

Results

RBM10 knockdown (KD) provoked alterations in splicing events in 10–20% of the pre-mRNAs, most of which had not been previously identified as RBM10 targets. Hierarchical clustering of the genes affected by RBM10 KD revealed good conservation of alternative exon inclusion or exclusion across cell lines. Pathway annotation showed RAS signaling to be most affected by RBM10 KD. Of particular interest was the finding that splicing of SMN pre-mRNA, encoding the survival of motor neuron (SMN) protein, was influenced by RBM10 KD. Inhibition of RBM10 resulted in preferential expression of the full-length, exon 7 retaining, SMN transcript in four cancer cell lines and one normal skin fibroblast cell line. SMN protein is expressed from two genes, SMN1 and SMN2, but the SMN1 gene is homozygously disrupted in people with spinal muscular atrophy; as a consequence, all of the SMN that is expressed in people with this disease is from the SMN2 gene. Expression analyses using primary fibroblasts from control, carrier and spinal muscle atrophy donors demonstrated that RBM10 KD resulted in preferential expression of the full-length, exon 7 retaining, SMN2 transcript. At the protein level, upregulation of the full-length SMN2 was also observed. Re-expression of RBM10, in a stable RBM10 KD cancer cell line, correlated with a reversion of the KD effect, demonstrating specificity.

Conclusion

Our work has not only expanded the number of pre-mRNA targets for RBM10, but identified RBM10 as a novel regulator of SMN2 alternative inclusion.

Electronic supplementary material

The online version of this article (doi:10.1186/s12867-017-0096-x) contains supplementary material, which is available to authorized users.

Diverse role of survival motor neuron protein

The multifunctional Survival Motor Neuron (SMN) protein is required for the survival of all organisms of the animal kingdom. SMN impacts various aspects of RNA metabolism through the formation and/or interaction with ribonucleoprotein (RNP) complexes. SMN regulates biogenesis of small nuclear RNPs, small nucleolar RNPs, small Cajal body-associated RNPs, signal recognition particles and telomerase. SMN also plays an important role in DNA repair, transcription, pre-mRNA splicing, histone mRNA processing, translation, selenoprotein synthesis, macromolecular trafficking, stress granule formation, cell signaling and cytoskeleton maintenance. The tissue-specific requirement of SMN is dictated by the variety and the abundance of its interacting partners. Reduced expression of SMN causes spinal muscular atrophy (SMA), a leading genetic cause of infant mortality. SMA displays a broad spectrum ranging from embryonic lethality to an adult onset. Aberrant expression and/or localization of SMN has also been associated with male infertility, inclusion body myositis, amyotrophic lateral sclerosis and osteoarthritis. This review provides a summary of various SMN functions with implications to a better understanding of SMA and other pathological conditions.