SMN protein promotes membrane compartmentalization of ribosomal protein S6 transcript in human fibroblasts.

Alterations of RNA homeostasis can lead to severe pathological conditions. The Survival of Motor Neuron (SMN) protein, which is reduced in Spinal Muscular Atrophy, impacts critical aspects of the RNA life cycle, such as splicing, trafficking, and translation. Increasing evidence points to a potential role of SMN in ribosome biogenesis. Our previous study revealed that SMN promotes membrane-bound ribosomal proteins (RPs), sustaining activity-dependent local translation. Here, we suggest that plasma membrane domains could be a docking site not only for RPs but also for their encoding transcripts. We have shown that SMN knockdown perturbs subcellular localization as well as translation efficiency of RPS6 mRNA. We have also shown that plasma membrane-enriched fractions from human fibroblasts retain RPS6 transcripts in an SMN-dependent manner. Furthermore, we revealed that SMN traffics with RPS6 mRNA promoting its association with caveolin-1, a key component of membrane dynamics. Overall, these findings further support the SMN-mediated crosstalk between plasma membrane dynamics and translation machinery. Importantly, our study points to a potential role of SMN in the ribosome assembly pathway by selective RPs synthesis/localization in both space and time.

SMN Protein Contributes to Skeletal Muscle Cell Maturation Via Caspase-3 and Akt Activation.

BACKGROUND/AIM: In spinal muscular atrophy (SMA), systemic deficiency of survival motor neurons (SMN) caused by loss or mutation of SMN1 leads to SMA symptoms. SMA was, for a long time, considered as a selective motor-neuron disease. However, accumulated evidence suggests that skeletal muscle cells are affected by low levels of SMN protein. The purpose of this study was to elucidate the function of SMN protein in skeletal cell differentiation and maturation. MATERIALS AND METHODS: In SMNΔ7 mice, which exhibit a systemic reduction of SMN protein, muscle atrophy was evaluated. To direct the effect of SMN against muscle cells, SMN functions were examined by knockdown of SMN in mouse myoblasts cell line C2C12 using siRNA. RESULTS: SMNΔ7 mice showed muscle atrophy accompanied by decreased both expression of a myogenesis marker and a proliferating marker. In SMN-knockdown myoblasts, early expression of myosin heavy chain and reduced multinuclear myotube formation were found. Decreased caspase-3 activity and reduced phosphorylation of Akt were observed at an early stage of differentiation in SMN-knockdown myoblasts. CONCLUSION: A critical role of SMN protein in muscle cell differentiation via caspase-3 and Akt activation was shown.

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.

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.

Hyperexcitability precedes motoneuron loss in the Smn2B/- mouse model of spinal muscular atrophy.

Spinal motoneuron dysfunction and loss are pathological hallmarks of the neuromuscular disease spinal muscular atrophy (SMA). Changes in motoneuron physiological function precede cell death, but how these alterations vary with disease severity and motoneuron maturational state is unknown. To address this question, we assessed the electrophysiology and morphology of spinal motoneurons of presymptomatic Smn2B/- mice older than 1 wk of age and tracked the timing of motor unit loss in this model using motor unit number estimation (MUNE). In contrast to other commonly used SMA mouse models, Smn2B/- mice exhibit more typical postnatal development until postnatal day (P)11 or 12 and have longer survival (~3 wk of age). We demonstrate that Smn2B/- motoneuron hyperexcitability, marked by hyperpolarization of the threshold voltage for action potential firing, was present at P9-10 and preceded the loss of motor units. Using MUNE studies, we determined that motor unit loss in this mouse model occurred 2 wk after birth. Smn2B/- motoneurons were also larger in size, which may reflect compensatory changes taking place during postnatal development. This work suggests that motoneuron hyperexcitability, marked by a reduced threshold for action potential firing, is a pathological change preceding motoneuron loss that is common to multiple models of severe SMA with different motoneuron maturational states. Our results indicate voltage-gated sodium channel activity may be altered in the disease process.NEW & NOTEWORTHY Changes in spinal motoneuron physiologic function precede cell death in spinal muscular atrophy (SMA), but how they vary with maturational state and disease severity remains unknown. This study characterized motoneuron and neuromuscular electrophysiology from the Smn2B/- model of SMA. Motoneurons were hyperexcitable at postnatal day (P)9-10, and specific electrophysiological changes in Smn2B/- motoneurons preceded functional motor unit loss at P14, as determined by motor unit number estimation studies.

Combined deficiency of Senataxin and DNA-PKcs causes DNA damage accumulation and neurodegeneration in spinal muscular atrophy.

Chronic low levels of survival motor neuron (SMN) protein cause spinal muscular atrophy (SMA). SMN is ubiquitously expressed, but the mechanisms underlying predominant neuron degeneration in SMA are poorly understood. We report that chronic low levels of SMN cause Senataxin (SETX)-deficiency, which results in increased RNA-DNA hybrids (R-loops) and DNA double-strand breaks (DSBs), and deficiency of DNA-activated protein kinase-catalytic subunit (DNA-PKcs), which impairs DSB repair. Consequently, DNA damage accumulates in patient cells, SMA mice neurons and patient spinal cord tissues. In dividing cells, DSBs are repaired by homologous recombination (HR) and non-homologous end joining (NHEJ) pathways, but neurons predominantly use NHEJ, which relies on DNA-PKcs activity. In SMA dividing cells, HR repairs DSBs and supports cellular proliferation. In SMA neurons, DNA-PKcs-deficiency causes defects in NHEJ-mediated repair leading to DNA damage accumulation and neurodegeneration. Restoration of SMN levels rescues SETX and DNA-PKcs deficiencies and DSB accumulation in SMA neurons and patient cells. Moreover, SETX overexpression in SMA neurons reduces R-loops and DNA damage, and rescues neurodegeneration. Our findings identify combined deficiency of SETX and DNA-PKcs stemming downstream of SMN as an underlying cause of DSBs accumulation, genomic instability and neurodegeneration in SMA and suggest SETX as a potential therapeutic target for SMA.

Pilot study of population-based newborn screening for spinal muscular atrophy in New York state.

PurposeTo determine feasibility and utility of newborn screening for spinal muscular atrophy (SMA) in New York State.MethodsWe validated a multiplex TaqMan real-time quantitative polymerase chain reaction assay using dried blood spots for SMA. From January 2016 to January 2017, we offered, consented, and screened 3,826 newborns at three hospitals in New York City and tested newborns for the deletion in exon 7 of SMN1.ResultsNinety-three percent of parents opted in for SMA screening. Overall the SMA carrier frequency was 1.5%. We identified one newborn with a homozygous SMN1 deletion and two copies of SMN2, which strongly suggests the severe type 1 SMA phenotype. The infant was enrolled in the NURTURE clinical trial and was first treated with Spinraza at age 15 days. She is now age 12 months, meeting all developmental milestones, and free of any respiratory issues.ConclusionOur pilot study demonstrates the feasibility of population-based screening, the acceptance by families, and the benefit of newborn screening for SMA. We suggest that SMA be considered for addition to the national recommended uniform screening panel.

Neuronal activity regulates DROSHA via autophagy in spinal muscular atrophy.

Dysregulated miRNA expression and mutation of genes involved in miRNA biogenesis have been reported in motor neuron diseases including spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS). Therefore, identifying molecular mechanisms governing miRNA expression is important to understand these diseases. Here, we report that expression of DROSHA, which is a critical enzyme in the microprocessor complex and essential for miRNA biogenesis, is reduced in motor neurons from an SMA mouse model. We show that DROSHA is degraded by neuronal activity induced autophagy machinery, which is also dysregulated in SMA. Blocking neuronal activity or the autophagy-lysosome pathway restores DROSHA levels in SMA motor neurons. Moreover, reducing DROSHA levels enhances axonal growth. As impaired axonal growth is a well described phenotype of SMA motor neurons, these data suggest that DROSHA reduction by autophagy may mitigate the phenotype of SMA. In summary, these findings suggest that autophagy regulates RNA metabolism and neuronal growth via the DROSHA/miRNA pathway and this pathway is dysregulated in SMA.

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

Survival motor neuron (SMN) is a 38-kDa protein, whose deficiency in humans develops spinal muscular atrophy (SMA), an autosomal recessive neurodegenerative disease with muscular atrophy due to motor neuron death in the spinal cord. We now report that SMN prevents the activation of TRAF6 and IκB kinase (IKK) and thereby negatively regulates the NF-κB signaling processes. SMN physically interacted with TRAF6 and with each component of the IKK complex, IKK-α, IKK-ß, and IKK-γ in BV2 microglia cells. Moreover, SMN1 inhibited the E3 ubiquitin ligase activity of TRAF6 as well as the kinase activity of IKK. Furthermore, depletion of endogenous SMN by RNA interference enhanced the IL-1ß-induced activation of IKK and production of inflammatory mediators such as TNF-α and nitric oxide in BV2 cells. Consistently, the potentiation of IL-1ß-induced IKK activity was also found in SMA patient fibroblasts, compared with that of normal ones. Our results thus suggest that SMN functions as a natural inhibitor for IL-1ß-induced NF-κB signaling by targeting TRAF6 and the IKK complex.

Compensatory axon sprouting for very slow axonal die-back in a transgenic model of spinal muscular atrophy type III.

KEY POINTS: Smn+/- transgenic mouse is a model of the mildest form of spinal muscular atrophy. Although there is a loss of spinal motoneurons in 11-month-old animals, muscular force is maintained. This maintained muscular force is mediated by reinnervation of the denervated fibres by surviving motoneurons. The spinal motoneurons in these animals do not show an increased susceptibility to death after nerve injury and they retain their regenerative capacity. We conclude that the hypothesized immaturity of the neuromuscular system in this model cannot explain the loss of motoneurons by systematic die-back. ABSTRACT: Spinal muscular atrophy (SMA) is a common autosomal recessive disorder in humans and is the leading genetic cause of infantile death. Patients lack the SMN1 gene with the severity of the disease depending on the number of copies of the highly homologous SMN2 gene. Although motoneuron death in the Smn+/- transgenic mouse model of the mildest form of SMA, SMA type III, has been reported, we have used retrograde tracing of sciatic and femoral motoneurons in the hindlimb with recording of muscle and motor unit isometric forces to count the number of motoneurons with intact neuromuscular connections. Thereby, we investigated whether incomplete maturation of the neuromuscular system induced by survival motoneuron protein (SMN) defects is responsible for die-back of axons relative to survival of motoneurons. First, a reduction of ∼30% of backlabelled motoneurons began relatively late, at 11 months of age, with a significant loss of 19% at 7 months. Motor axon die-back was affirmed by motor unit number estimation. Loss of functional motor units was fully compensated by axonal sprouting to retain normal contractile force in four hindlimb muscles (three fast-twitch and one slow-twitch) innervated by branches of the sciatic nerve. Second, our evaluation of whether axotomy of motoneurons in the adult Smn+/- transgenic mouse increases their susceptibility to cell death demonstrated that all the motoneurons survived and they sustained their capacity to regenerate their nerve fibres. It is concluded the systematic die-back of motoneurons that innervate both fast- and slow-twitch muscle fibres is not related to immaturity of the neuromuscular system in SMA.

SMN is required for the maintenance of embryonic stem cells and neuronal differentiation in mice.

Survival motor neuron (SMN) is the determining factor in spinal muscular atrophy, the most common genetic cause of childhood mortality. We have previously found that SMN regulates stem cell division, proliferation and differentiation in Drosophila. However, it is unknown whether a similar effect exists in vertebrates. Here, we show that SMN is enriched in highly proliferative embryonic stem cells (ESCs) in mice and reduction of SMN impairs the pluripotency of ESCs. Moreover, we find that SMN reduction activates ERK signaling and affects neuronal differentiation in vitro. Teratomas with reduced SMN grow more slowly and show weaker signals of neuronal differentiation than those with a normal level of SMN. Finally, we show that over-expression of SMN is protective for ESCs from retinoic acid-induced differentiation. Taken together, our results suggest that SMN plays a role in the maintenance of pluripotent ESCs and neuronal differentiation in mice.

Overexpression of survival motor neuron improves neuromuscular function and motor neuron survival in mutant SOD1 mice.

Spinal muscular atrophy results from diminished levels of survival motor neuron (SMN) protein in spinal motor neurons. Low levels of SMN also occur in models of amyotrophic lateral sclerosis (ALS) caused by mutant superoxide dismutase 1 (SOD1) and genetic reduction of SMN levels exacerbates the phenotype of transgenic SOD1(G93A) mice. Here, we demonstrate that SMN protein is significantly reduced in the spinal cords of patients with sporadic ALS. To test the potential of SMN as a modifier of ALS, we overexpressed SMN in 2 different strains of SOD1(G93A) mice. Neuronal overexpression of SMN significantly preserved locomotor function, rescued motor neurons, and attenuated astrogliosis in spinal cords of SOD1(G93A) mice. Despite this, survival was not prolonged, most likely resulting from SMN mislocalization and depletion of gems in motor neurons of symptomatic mice. Our results reveal that SMN upregulation slows locomotor deficit onset and motor neuron loss in this mouse model of ALS. However, disruption of SMN nuclear complexes by high levels of mutant SOD1, even in the presence of SMN overexpression, might limit its survival promoting effects in this specific mouse model. Studies in emerging mouse models of ALS are therefore warranted to further explore the potential of SMN as a modifier of ALS.

Spinal muscular atrophy: a motor neuron disorder or a multi-organ disease?

Spinal muscular atrophy (SMA) is an autosomal recessive disorder that is the leading genetic cause of infantile death. SMA is characterized by loss of motor neurons in the ventral horn of the spinal cord, leading to weakness and muscle atrophy. SMA occurs as a result of homozygous deletion or mutations in Survival Motor Neuron-1 (SMN1). Loss of SMN1 leads to a dramatic reduction in SMN protein, which is essential for motor neuron survival. SMA disease severity ranges from extremely severe to a relatively mild adult onset form of proximal muscle atrophy. Severe SMA patients typically die mostly within months or a few years as a consequence of respiratory insufficiency and bulbar paralysis. SMA is widely known as a motor neuron disease; however, there are numerous clinical reports indicating the involvement of additional peripheral organs contributing to the complete picture of the disease in severe cases. In this review, we have compiled clinical and experimental reports that demonstrate the association between the loss of SMN and peripheral organ deficiency and malfunction. Whether defective peripheral organs are a consequence of neuronal damage/muscle atrophy or a direct result of SMN loss will be discussed.

Chronic treatment with lithium does not improve neuromuscular phenotype in a mouse model of severe spinal muscular atrophy.

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by defective levels of the survival motor neuron (SMN) protein. SMA causes spinal motoneuron (MN) loss, and progressive muscle weakness and paralysis. Currently, there is no effective therapy to cure this disease. Although different strategies focused on increasing the expression of functional SMN protein have been assayed, numerous SMN-independent therapeutic approaches have been demonstrated to have potential effectiveness in improving the SMA phenotype in mouse models and clinical trials. Recent works have shown that compounds which inhibit GSK-3ß activity are effective in promoting MN survival and ameliorating lifespan in models of MN diseases including SMA. Taking into account the reported neuroprotective actions of lithium (Li) through the inhibition of GSK-3ß in different studies, we tested here its potential efficiency as a therapeutic agent in a mouse model of severe SMA (SMNΔ7 mice). We show that the chronic treatment with Li initiated before the appearance of disease symptoms, although inhibited GSK-3ß, did not improve the median survival, motor behavior, and spinal MN loss linked to SMA. Li administration did not either ameliorate the microglial and astroglial reaction in the spinal cord or the depletion of glutamatergic synapses on MNs observed in SMNΔ7 animals. Moreover, Li treatment did not mitigate muscle atrophy or calcitonin gene-related peptide (CGRP) downregulation in the neuromuscular junctions linked to the disease. However, a significant reduction in apoptotic cell death found in the skeletal muscle of SMA mice was observed after Li treatment.

Spinal muscular atrophy and the antiapoptotic role of survival of motor neuron (SMN) protein.

Spinal muscular atrophy (SMA) is a devastating and often fatal neurodegenerative disease that affects spinal motor neurons and leads to progressive muscle wasting and paralysis. The survival of motor neuron (SMN) gene is mutated or deleted in most forms of SMA, which results in a critical reduction in SMN protein. Motor neurons appear particularly vulnerable to reduced SMN protein levels. Therefore, understanding the functional role of SMN in protecting motor neurons from degeneration is an essential prerequisite for the design of effective therapies for SMA. To this end, there is increasing evidence indicating a key regulatory antiapoptotic role for the SMN protein that is important in motor neuron survival. The aim of this review is to highlight key findings that support an antiapoptotic role for SMN in modulating cell survival and raise possibilities for new therapeutic approaches.

A novel function for the survival motoneuron protein as a translational regulator.

SMN1, the causative gene for spinal muscular atrophy (SMA), plays a housekeeping role in the biogenesis of small nuclear RNA ribonucleoproteins. SMN is also present in granular foci along axonal projections of motoneurons, which are the predominant cell type affected in the pathology. These so-called RNA granules mediate the transport of specific mRNAs along neurites and regulate mRNA localization, stability, as well as local translation. Recent work has provided evidence suggesting that SMN may participate in the assembly of RNA granules, but beyond that, the precise nature of its role within these structures remains unclear. Here, we demonstrate that SMN associates with polyribosomes and can repress translation in an in vitro translation system. We further identify the arginine methyltransferase CARM1 as an mRNA that is regulated at the translational level by SMN and find that CARM1 is abnormally up-regulated in spinal cord tissue from SMA mice and in severe type I SMA patient cells. We have previously characterized a novel regulatory pathway in motoneurons involving the SMN-interacting RNA-binding protein HuD and CARM1. Thus, our results suggest the existence of a potential negative feedback loop in this pathway. Importantly, an SMA-causing mutation in the Tudor domain of SMN completely abolished translational repression, a strong indication for the functional significance of this novel SMN activity in the pathology.

Human axonal survival of motor neuron (a-SMN) protein stimulates axon growth, cell motility, C-C motif ligand 2 (CCL2), and insulin-like growth factor-1 (IGF1) production.

Spinal muscular atrophy is a fatal genetic disease of motoneurons due to loss of full-length survival of motor neuron protein, the main product of the disease gene SMN1. Axonal SMN (a-SMN) is an alternatively spliced isoform of SMN1, generated by retention of intron 3. To study a-SMN function, we generated cellular clones for the expression of the protein in mouse motoneuron-like NSC34 cells. The model was instrumental in providing evidence that a-SMN decreases cell growth and plays an important role in the processes of axon growth and cellular motility. In our conditions, low levels of a-SMN expression were sufficient to trigger the observed biological effects, which were not modified by further increasing the amounts of the expressed protein. Differential transcriptome analysis led to the identification of novel a-SMN-regulated factors, i.e. the transcripts coding for the two chemokines, C-C motif ligands 2 and 7 (CCL2 and CCL7), as well as the neuronal and myotrophic factor, insulin-like growth factor-1 (IGF1). a-SMN-dependent induction of CCL2 and IGF1 mRNAs resulted in increased intracellular levels and secretion of the respective protein products. Induction of CCL2 contributes to the a-SMN effects, mediating part of the action on axon growth and random cell motility, as indicated by chemokine knockdown and re-addition studies. Our results shed new light on a-SMN function and the underlying molecular mechanisms. The data provide a rational framework to understand the role of a-SMN deficiency in the etiopathogenesis of spinal muscular atrophy.

Spliceosomal small nuclear ribonucleoprotein biogenesis defects and motor neuron selectivity in spinal muscular atrophy.

The SMN protein is essential and participates in the assembly of macromolecular complexes of RNA and protein in all cells. The best-characterized function of SMN is as an assembler of spliceosomal small nuclear ribonucleoproteins (snRNPs). SMN performs this function as part of a complex with several other proteins called Gemins. snRNPs are assembled in the cytoplasm in a stepwise manner and then are imported to the nucleus where they participate globally in the splicing of pre-mRNA. Mutations in the SMN1 gene result in the motor neuron disease, spinal muscular atrophy (SMA). Most of these mutations result in a reduction in the expression levels of the SMN protein, which, in turn, results in a reduction in snRNP assembly capacity. This review highlights current studies that have investigated the mechanism of SMN-dependent snRNP assembly, as well as the downstream effects on pre-mRNA splicing that result from a decrease in SMN. This article is part of a Special Issue entitled "RNA-Binding Proteins".

The spinal muscular atrophy disease protein SMN is linked to the Golgi network.

Proximal spinal muscular atrophy (SMA) is a neurodegenerative disorder caused by deficiency of the ubiquitous Survival of Motor Neuron (SMN) protein. SMN has been shown to be transported in granules along the axon and moved through cytoskeletal elements. However, the role and nature of SMN granules are still not well characterized. Here, using immunocytochemical methods and time-lapse studies we show that SMN granules colocalize with the Golgi apparatus in motor neuron-like NSC34 cells. Electron microscopy clearly revealed that SMN granules are transported into the Golgi stack and aggregate in the trans-Golgi apparatus. SMN granules are characterized as either coated or un-coated and behave like regulated secretory granules. Treatment of cells with monensin to disrupt Golgi-mediated granule secretion decreased SMN expression in neurites and caused growth cone defects similar to those seen in SMN knockdown cells. Knockdown of Cop-α, the protein that coats vesicles transporting proteins between the Golgi compartments, caused SMN granule accumulation in the Golgi apparatus. In addition to the well-studied role of SMN in small nuclear ribonucleoprotein (SnRNP) assembly, this work links SMN granules with the Golgi network and thus sheds light on Golgi-mediated SMN granule transport.