Improvement of neuromuscular synaptic phenotypes without enhanced survival and motor function in severe spinal muscular atrophy mice selectively rescued in motor neurons

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.

CSF IL-8 Associated with Response to Gene Therapy in a Case Series of Spinal Muscular Atrophy

Gene therapies have greatly changed the outlook in spinal muscular atrophy (SMA), and this disorder provides a rare opportunity to study longitudinal biomarker changes correlated with reduced disease burden and improved clinical outcomes. Recent work suggests clinical response to correlate with declining cerebrospinal fluid (CSF) levels of the neurodegenerative marker neurofilament light chain (NfL) in children receiving serial anti-sense oligonucleotide therapy. However, change in CSF NfL levels is no longer a practical biomarker as more children undergo single-dose gene replacement therapy. Here we leverage serial CSF samples (median of 4 per child) collected in 13 children with SMA undergoing anti-sense oligonucleotide therapy to characterize the longitudinal profiles of NfL as well as inflammatory and neuronal proteins. In contrast to neurodegeneration in adults, we found NfL levels to first decrease following initiation of treatment but then increase upon further treatment and improved motor functions. We then examined additional CSF inflammatory and neuronal markers for linear association with motor function during SMA treatment. We identified longitudinal IL-8 levels to inversely correlate with motor functions determined by clinical examination (F(1, 47) = 12.903, p = 0.001) or electromyography in the abductor pollicis brevis muscle (p = 0.064). In keeping with this, lower baseline IL-8 levels were associated with better longitudinal outcomes, even though this difference diminished over 2 years in the younger group. We thus propose CSF IL-8 as a biomarker for baseline function and short-term treatment response in SMA, and a candidate biomarker for future treatment trials in other neurodegenerative disorders.

Glial fibrillary acidic protein in cerebrospinal fluid of patients with spinal muscular atrophy

Objective

Activated astroglia is involved in the pathophysiology of neurodegenerative diseases and has also been described in animal models of spinal muscular atrophy (SMA). Given the urgent need of biomarkers for treatment monitoring of new RNA‐modifying and gene replacement therapies in SMA, we examined glial fibrillary acidic protein concentrations in cerebrospinal fluid (cGFAP) as a marker of astrogliosis in SMA.

Methods

58 adult patients and 21 children with genetically confirmed 5q‐associated SMA from four German motor neuron disease specialist care centers and 30 age‐ and sex‐matched controls were prospectively included in this study. cGFAP was measured and correlated to motor performance and disease severity. Additionally, we compared cGFAP with neurofilament light chain concentrations in cerebrospinal fluid (cNfL).

Results

cGFAP concentrations did not differ from controls but showed higher levels in more severely affected patients after adjustment for patients' age. Normalized cNfL values were associated with disease severity. Within 14 months of nusinersen treatment, cGFAP concentrations did not change, while cNfL decreased significantly.

Interpretation

cGFAP is not an outstanding biomarker in SMA, but might support the hypothesis that glial activation is involved in SMA pathology. Unlike previously suggested, cNfL may be a promising biomarker also in adult patients with SMA, which should be subject to further investigations.

In Search of a Cure: The Development of Therapeutics to Alter the Progression of Spinal Muscular Atrophy

Until the recent development of disease-modifying therapeutics, spinal muscular atrophy (SMA) was considered a devastating neuromuscular disease with a poor prognosis for most affected individuals. Symptoms generally present during early childhood and manifest as muscle weakness and progressive paralysis, severely compromising the affected individual’s quality of life, independence, and lifespan. SMA is most commonly caused by the inheritance of homozygously deleted SMN1 alleles with retention of one or more copies of a paralog gene, SMN2, which inversely correlates with disease severity. The recent advent and use of genetically targeted therapies have transformed SMA into a prototype for monogenic disease treatment in the era of genetic medicine. Many SMA-affected individuals receiving these therapies achieve traditionally unobtainable motor milestones and survival rates as medicines drastically alter the natural progression of this disease. This review discusses historical SMA progression and underlying disease mechanisms, highlights advances made in therapeutic research, clinical trials, and FDA-approved medicines, and discusses possible second-generation and complementary medicines as well as optimal temporal intervention windows in order to optimize motor function and improve quality of life for all SMA-affected individuals.

Motor unit changes in children with symptomatic spinal muscular atrophy treated with nusinersen

OBJECTIVES

To elucidate the motor unit response to intrathecal nusinersen in children with symptomatic spinal muscular atrophy (SMA) using a novel motor unit number estimation technique.

METHODS

MScanFit MUNE studies were sequentially undertaken from the abductor pollicis brevis muscle after stimulation of the median nerve in a prospective cohort of symptomatic children with SMA, undergoing intrathecal treatment with nusinersen at a single neuromuscular centre from June 2017 to August 2019. Electrophysiological measures included compound muscle action potential (CMAP), motor unit number estimation (MUNE), motor unit number contributing to 50%-100% of CMAP (N50) and measures of collateral reinnervation including largest single motor unit potential (LSMUP) and amplitude of the smallest unit contributing to N50 (A50).

RESULTS

Twenty children (median age 99 months, range 4-193) were followed for a median of 13.8 (4-33.5) months. Therapeutic intervention was an independent and significant contributor to an increase in CMAP (p = 0.005), MUNE (p = 0.001) and N50 (p = 0.04). The magnitude of this electrophysiological response was increased in children with shorter disease durations (p<0.05). Electrophysiological changes delineated children who were functionally stable from those who attained clinically significant gains in motor function.

INTERPRETATION

Nusinersen therapy facilitated functional innervation in SMA through recovery of smaller motor units. Delineation of biomechanisms of therapeutic response may be the first step in identifying potential novel targets for disease modification in this and other motor neuropathies. MScanFit MUNE techniques may have a broader role in establishing biomarkers of therapeutic response in similar adult-onset diseases.

Temporal evolution of the microbiome, immune system and epigenome with disease progression in ALS mice

Amyotrophic lateral sclerosis (ALS) is a terminal neurodegenerative disease. Genetic predisposition, epigenetic changes, aging and accumulated life-long environmental exposures are known ALS risk factors. The complex and dynamic interplay between these pathological influences plays a role in disease onset and progression. Recently, the gut microbiome has also been implicated in ALS development. In addition, immune cell populations are differentially expanded and activated in ALS compared to healthy individuals. However, the temporal evolution of both the intestinal flora and the immune system relative to symptom onset in ALS is presently not fully understood. To better elucidate the timeline of the various potential pathological factors, we performed a longitudinal study to simultaneously assess the gut microbiome, immunophenotype and changes in ileum and brain epigenetic marks relative to motor behavior and muscle atrophy in the mutant superoxide dismutase 1 (SOD1G93A) familial ALS mouse model. We identified alterations in the gut microbial environment early in the life of SOD1G93A animals followed by motor dysfunction and muscle atrophy, and immune cell expansion and activation, particularly in the spinal cord. Global brain cytosine hydroxymethylation was also altered in SOD1G93A animals at disease end-stage compared to control mice. Correlation analysis confirmed interrelationships with the microbiome and immune system. This study serves as a starting point to more deeply comprehend the influence of gut microorganisms and the immune system on ALS onset and progression. Greater insight may help pinpoint novel biomarkers and therapeutic interventions to improve diagnosis and treatment for ALS patients.This article has an associated First Person interview with the joint first authors of the paper.

Astrocytes in Motor Neuron Diseases

Motor neuron disorders are highly debilitating and mostly fatal conditions for which only limited therapeutic options are available. To overcome this limitation and develop more effective therapeutic strategies, it is critical to discover the pathogenic mechanisms that trigger and sustain motor neuron degeneration with the greatest accuracy and detail. In the case of Amyotrophic Lateral Sclerosis (ALS), several genes have been associated with familial forms of the disease, whilst the vast majority of cases develop sporadically and no defined cause can be held responsible. On the contrary, the huge majority of Spinal Muscular Atrophy (SMA) occurrences are caused by loss-of-function mutations in a single gene, SMN1. Although the typical hallmark of both diseases is the loss of motor neurons, there is increasing awareness that pathological lesions are also present in the neighbouring glia, whose dysfunction clearly contributes to generating a toxic environment in the central nervous system. Here, ALS and SMA are sequentially presented, each disease section having a brief introduction, followed by a focussed discussion on the role of the astrocytes in the disease pathogenesis. Such a dissertation is substantiated by the findings that built awareness on the glial involvement and how the glial-neuronal interplay is perturbed, along with the appraisal of this new cellular site for possible therapeutic intervention.

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 Smn^2B/- 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, Smn^2B/- mice exhibit more typical postnatal development until postnatal day (P)11 or 12 and have longer survival (~3 wk of age). We demonstrate that Smn^2B/- 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. Smn^2B/- 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 Smn^2B/- model of SMA. Motoneurons were hyperexcitable at postnatal day (P)9-10, and specific electrophysiological changes in Smn^2B/- motoneurons preceded functional motor unit loss at P14, as determined by motor unit number estimation studies.

Systemic nature of spinal muscular atrophy revealed by studying insurance claims

Objective

We investigated the presence of non-neuromuscular phenotypes in patients affected by Spinal Muscular Atrophy (SMA), a disorder caused by a mutation in the Survival of Motor Neuron (SMN) gene, and whether these phenotypes may be clinically detectable prior to clinical signs of neuromuscular degeneration and therefore independent of muscle weakness.

Methods

We utilized a de-identified database of insurance claims to explore the health of 1,038 SMA patients compared to controls. Two analyses were performed: (1) claims from the entire insurance coverage window; and (2) for SMA patients, claims prior to diagnosis of any neuromuscular disease or evidence of major neuromuscular degeneration to increase the chance that phenotypes could be attributed directly to reduced SMN levels. Logistic regression was used to determine whether phenotypes were diagnosed at significantly different rates between SMA patients and controls and to obtain covariate-adjusted odds ratios.

Results

Results from the entire coverage window revealed a broad spectrum of phenotypes that are differentially diagnosed in SMA subjects compared to controls. Moreover, data from SMA patients prior to their first clinical signs of neuromuscular degeneration revealed numerous non-neuromuscular phenotypes including defects within the cardiovascular, gastrointestinal, metabolic, reproductive, and skeletal systems. Furthermore, our data provide evidence of a potential ordering of disease progression beginning with these non-neuromuscular phenotypes.

Conclusions

Our data point to a direct relationship between early, detectable non-neuromuscular symptoms and SMN deficiency. Our findings are particularly important for evaluating the efficacy of SMN-increasing therapies for SMA, comparing the effectiveness of local versus systemically delivered therapeutics, and determining the optimal therapeutic treatment window prior to irreversible neuromuscular damage.

Notch Signaling Mediates Astrocyte Abnormality in Spinal Muscular Atrophy Model Systems

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder characterized by the degeneration of spinal motor neurons and muscle atrophy. The disease is mainly caused by low level of the survival motor neuron (SMN) protein, which is coded by two genes, namely SMN1 and SMN2, but leads to selective spinal motor neuron degeneration when SMN1 gene is deleted or mutated. Previous reports have shown that SMN-protein-deficient astrocytes are abnormally abundant in the spinal cords of SMA model mice. However, the mechanism of the SMN- deficient astrocyte abnormality remains unclear. The purpose of this study is to identify the cellular signaling pathways associated with the SMN-deficient astrocyte abnormality and propose a candidate therapy tool that modulates signaling. In the present study, we found that the astrocyte density was increased around the central canal of the spinal cord in a mouse SMA model and we identified the dysregulation of Notch signaling which is a known mechanism that regulates astrocyte differentiation and proliferation, in the spinal cord in both early and late stages of SMA pathogenesis. Moreover, pharmacological inhibition of Notch signaling improved the motor functional deficits in SMA model mice. These findings indicate that dysregulated Notch signaling may be an underlying cause of SMA pathology.

Enhancing GABAergic Transmission Improves Locomotion in a Caenorhabditis elegans Model of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a neuromuscular disease characterized by degeneration of spinal motor neurons resulting in variable degrees of muscular wasting and weakness. It is caused by a loss-of-function mutation in the survival motor neuron (SMN1) gene. Caenorhabditis elegans mutants lacking SMN recapitulate several aspects of the disease including impaired movement and shorted life span. We examined whether genes previously implicated in life span extension conferred benefits to C. elegans lacking SMN. We find that reducing daf-2/insulin receptor signaling activity promotes survival and improves locomotor behavior in this C. elegans model of SMA. The locomotor dysfunction in C. elegans lacking SMN correlated with structural and functional abnormalities in GABAergic neuromuscular junctions (NMJs). Moreover, we demonstrated that reduction in daf-2 signaling reversed these abnormalities. Remarkably, enhancing GABAergic neurotransmission alone was able to correct the locomotor dysfunction. Our work indicated that an imbalance of excitatory/inhibitory activity within motor circuits and underlies motor system dysfunction in this SMA model. Interventions aimed at restoring the balance of excitatory/inhibitory activity in motor circuits could be of benefit to individuals with SMA.

Spinal muscular atrophy

Autosomal-recessive proximal spinal muscular atrophy (Werdnig-Hoffmann, Kugelberg-Welander) is caused by mutation of the SMN1 gene, and the clinical severity correlates with the number of copies of a nearly identical gene, SMN2. The SMN protein plays a critical role in spliceosome assembly and may have other cellular functions, such as mRNA transport. Cell culture and animal models have helped to define the disease mechanism and to identify targets for therapeutic intervention. The main focus for developing treatment has been to increase SMN levels, and accomplishing this with small molecules, oligonucleotides, and gene replacement has been quite. An oligonucleotide, nusinersen, was recently approved for treatment in patients, and confirmatory studies of other agents are now under way.

Increasing Agrin Function Antagonizes Muscle Atrophy and Motor Impairment in Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a pediatric genetic disease, characterized by motor neuron (MN) death, leading to progressive muscle weakness, respiratory failure, and, in the most severe cases, to death. Abnormalities at the neuromuscular junction (NMJ) have been reported in SMA, including neurofilament (NF) accumulation at presynaptic terminals, immature and smaller than normal endplates, reduced transmitter release, and, finally, muscle denervation. Here we have studied the role of agrin in SMAΔ7 mice, the experimental model of SMAII. We observed a 50% reduction in agrin expression levels in quadriceps of P10 SMA mice compared to age-matched WT controls. To counteract such condition, we treated SMA mice from birth onwards with therapeutic agrin biological NT-1654, an active splice variant of agrin retaining synaptogenic properties, which is also resistant to proteolytic cleavage by neurotrypsin. Mice were analyzed for behavior, muscle and NMJ histology, and survival. Motor behavior was significantly improved and survival was extended by treatment of SMA mice with NT-1654. At P10, H/E-stained sections of the quadriceps, a proximal muscle early involved in SMA, showed that NT-1654 treatment strongly prevented the size decrease of muscle fibers. Studies of NMJ morphology on whole-mount diaphragm preparations revealed that NT-1654-treated SMA mice had more mature NMJs and reduced NF accumulation, compared to vehicle-treated SMA mice. We conclude that increasing agrin function in SMA has beneficial outcomes on muscle fibers and NMJs as the agrin biological NT-1654 restores the crosstalk between muscle and MNs, delaying muscular atrophy, improving motor performance and extending survival.

Decreased Motor Neuron Support by SMA Astrocytes due to Diminished MCP1 Secretion

Spinal muscular atrophy (SMA) is an autosomal-recessive disorder characterized by severe, often fatal muscle weakness due to loss of motor neurons. SMA patients have deletions and other mutations of the survival of motor neuron 1 (SMN1) gene, resulting in decreased SMN protein. Astrocytes are the primary support cells of the CNS and are responsible for glutamate clearance, metabolic support, response to injury, and regulation of signal transmission. Astrocytes have been implicated in SMA as in in other neurodegenerative disorders. Astrocyte-specific rescue of SMN protein levels has been shown to mitigate disease manifestations in mice. However, the mechanism by which SMN deficiency in astrocytes may contribute to SMA is unclear and what aspect of astrocyte activity is lacking is unknown. Therefore, it is worthwhile to identify defects in SMN-deficient astrocytes that compromise normal function. We show here that SMA astrocyte cultures derived from mouse spinal cord of both sexes are deficient in supporting both WT and SMN-deficient motor neurons derived from male, female, and mixed-sex sources and that this deficiency may be mitigated with secreted factors. In particular, SMN-deficient astrocytes have decreased levels of monocyte chemoactive protein 1 (MCP1) secretion compared with controls and MCP1 restoration stimulates outgrowth of neurites from cultured motor neurons. Correction of MCP1 deficiency may thus be a new therapeutic approach to SMA.SIGNIFICANCE STATEMENT Spinal muscular atrophy (SMA) is caused by the loss of motor neurons, but astrocyte dysfunction also contributes to the disease in mouse models. Monocyte chemoactive protein 1 (MCP1) has been shown to be neuroprotective and is released by astrocytes. Here, we report that MCP1 levels are decreased in SMA mice and that replacement of deficient MCP1 increases differentiation and neurite length of WT and SMN-deficient motor-neuron-like cells in cell culture. This study reveals a novel aspect of astrocyte dysfunction in SMA and indicates a possible approach for improving motor neuron growth and survival in this disease.

The Antisense Transcript SMN-AS1 Regulates SMN Expression and Is a Novel Therapeutic Target for Spinal Muscular Atrophy

The neuromuscular disorder spinal muscular atrophy (SMA), the most common inherited killer of infants, is caused by insufficient expression of survival motor neuron (SMN) protein. SMA therapeutics development efforts have focused on identifying strategies to increase SMN expression. We identified a long non-coding RNA (lncRNA) that arises from the antisense strand of SMN, SMN-AS1, which is enriched in neurons and transcriptionally represses SMN expression by recruiting the epigenetic Polycomb repressive complex-2. Targeted degradation of SMN-AS1 with antisense oligonucleotides (ASOs) increases SMN expression in patient-derived cells, cultured neurons, and the mouse central nervous system. SMN-AS1 ASOs delivered together with SMN2 splice-switching oligonucleotides additively increase SMN expression and improve survival of severe SMA mice. This study is the first proof of concept that targeting a lncRNA to transcriptionally activate SMN2 can be combined with SMN2 splicing modification to ameliorate SMA and demonstrates the promise of combinatorial ASOs for the treatment of neurogenetic disorders.

Defective Acetylcholine Receptor Subunit Switch Precedes Atrophy of Slow-Twitch Skeletal Muscle Fibers Lacking ERK1/2 Kinases in Soleus Muscle

To test the role of extracellular-signal regulated kinases 1 and 2 (ERK1/2) in slow-twitch, type 1 skeletal muscle fibers, we studied the soleus muscle in mice genetically deficient for myofiber ERK1/2. Young adult mutant soleus was drastically wasted, with highly atrophied type 1 fibers, denervation at most synaptic sites, induction of “fetal” acetylcholine receptor gamma subunit (AChRγ), reduction of “adult” AChRε, and impaired mitochondrial biogenesis and function. In weanlings, fiber morphology and mitochondrial markers were mostly normal, yet AChRγ upregulation and AChRε downregulation were observed. Synaptic sites with fetal AChRs in weanling muscle were ~3% in control and ~40% in mutants, with most of the latter on type 1 fibers. These results suggest that: (1) ERK1/2 are critical for slow-twitch fiber growth; (2) a defective γ/ε-AChR subunit switch, preferentially at synapses on slow fibers, precedes wasting of mutant soleus; (3) denervation is likely to drive this wasting, and (4) the neuromuscular synapse is a primary subcellular target for muscle ERK1/2 function in vivo.

Defects in Motoneuron-Astrocyte Interactions in Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a motoneuron disease caused by loss or mutation in Survival of Motor Neuron 1 (SMN1) gene. Recent studies have shown that selective restoration of SMN protein in astrocytes partially alleviates pathology in an SMA mouse model, suggesting important roles for astrocytes in SMA. Addressing these underlying mechanisms may provide new therapeutic avenues to fight SMA. Using primary cultures of pure motoneurons or astrocytes from SMNΔ7 (SMA) and wild-type (WT) mice, as well as their mixed and matched cocultures, we characterized the contributions of motoneurons, astrocytes, and their interactions to synapse loss in SMA. In pure motoneuron cultures, SMA motoneurons exhibited normal survival but intrinsic defects in synapse formation and synaptic transmission. In pure astrocyte cultures, SMA astrocytes exhibited defects in calcium homeostasis. In motoneuron-astrocyte contact cocultures, synapse formation and synaptic transmission were significantly reduced when either motoneurons, astrocytes or both were from SMA mice compared with those in WT motoneurons cocultured with WT astrocytes. The reduced synaptic activity is unlikely due to changes in motoneuron excitability. This disruption in synapse formation and synaptic transmission by SMN deficiency was not detected in motoneuron-astrocyte noncontact cocultures. Additionally, we observed a downregulation of Ephrin B2 in SMA astrocytes. These findings suggest that there are both cell autonomous and non-cell-autonomous defects in SMA motoneurons and astrocytes. Defects in contact interactions between SMA motoneurons and astrocytes impair synaptogenesis seen in SMA pathology, possibly due to the disruption of the Ephrin B2 pathway.

Transcriptional enhancement of Smn levels in motoneurons is crucial for proper axon morphology in zebrafish

An unresolved mystery in the field of spinal muscular atrophy (SMA) is why a reduction of the ubiquitously expressed Smn protein causes defects mostly in motoneurons. We addressed the possibility that this restricted vulnerability stems from elevated Smn expression in motoneurons. To explore this, we established an ex vivo zebrafish culture system of GFP-marked motoneurons to quantitatively measure Smn protein and smn mRNA levels as well as promoter activity in motoneurons versus other cell types. Importantly, we uncovered that Smn levels are elevated in motoneurons by means of transcriptional activation. In addition, we identified the ETS family transcription factor Etv5b to be responsible for increased smn transcription in motoneurons. Moreover, we established that the additional supply of Smn protein in motoneurons is necessary for proper axonogenesis in a cell-autonomous manner. These findings demonstrate the reliance of motoneurons on more Smn, thereby adding a novel piece of evidence for their increased vulnerability under SMA conditions.

Restoration of SMN in Schwann cells reverses myelination defects and improves neuromuscular function in spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neuromuscular disease caused by low levels of SMN protein, primarily affecting lower motor neurons. Recent evidence from SMA and related conditions suggests that glial cells can influence disease severity. Here, we investigated the role of glial cells in the peripheral nervous system by creating SMA mice selectively overexpressing SMN in myelinating Schwann cells (Smn^−/−;SMN2^tg/0;SMN1^SC). Restoration of SMN protein levels restricted solely to Schwann cells reversed myelination defects, significantly improved neuromuscular function and ameliorated neuromuscular junction pathology in SMA mice. However, restoration of SMN in Schwann cells had no impact on motor neuron soma loss from the spinal cord or ongoing systemic and peripheral pathology. This study provides evidence for a defined, intrinsic contribution of glial cells to SMA disease pathogenesis and suggests that therapies designed to include Schwann cells in their target tissues are likely to be required in order to rescue myelination defects and associated disease symptoms.

Selective Neuromuscular Denervation in Taiwanese Severe SMA Mouse Can Be Reversed by Morpholino Antisense Oligonucleotides

Spinal muscular atrophy (SMA) is an autosomal recessive motor neuron disease caused by deficiency of the survival of motor neuron (SMN) protein, which leads to synaptic defects and spinal motor neuron death. Neuromuscular junction (NMJ) abnormalities have been found to be involved in SMA pathogenesis in the SMNΔ7 SMA mouse model. However, whether similar NMJ pathological findings present in another commonly used mouse model, the Taiwanese SMA mouse, has not been fully investigated. To examine the NMJs of the Taiwanese severe SMA mouse model (Smn-/-; SMN2tg/0), which is characterized by severe phenotype and death before postnatal day (P) 9, we investigated 25 axial and appendicular muscles from P1 to P9. We labelled the muscles with anti-neurofilament and anti-synaptophysin antibodies for nerve terminals and α-bungarotoxin for acetylcholine receptors (AChRs). We found that severe NMJ denervation (<50% fully innervated endplates) selectively occurred in the flexor digitorum brevis 2 and 3 (FDB-2/3) muscles from P5, and an increased percentage of fully denervated endplates correlated with SMA progression. Furthermore, synaptophysin signals were absent at the endplate compared to control littermate mice, suggesting that vesicle transport might only be affected at the end stage. Subsequently, we treated the Taiwanese severe SMA mice with morpholino (MO) antisense oligonucleotides (80 μg/g) via subcutaneous injection at P0. We found that MO significantly reversed the NMJ denervation in FDB-2/3 muscles and extended the survival of Taiwanese severe SMA mice. We conclude that early NMJ denervation in the FDB-2/3 muscles of Taiwanese severe SMA mice can be reversed by MO treatment. The FDB-2/3 muscles of Taiwanese severe SMA mice provide a very sensitive platform for assessing the effectiveness of drug treatments in SMA preclinical studies.

Neuromuscular Junctions as Key Contributors and Therapeutic Targets in Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a recessive autosomal neuromuscular disease, representing the most common fatal pediatric pathology. Even though, classically and in a simplistic way, it is categorized as a motor neuron (MN) disease, there is an increasing general consensus that its pathogenesis is more complex than expected. In particular, neuromuscular junctions (NMJs) are affected by dramatic alterations, including immaturity, denervation and neurofilament accumulation, associated to impaired synaptic functions: these abnormalities may in turn have a detrimental effect on MN survival. Here, we provide a description of NMJ development/maintenance/maturation in physiological conditions and in SMA, focusing on pivotal molecules and on the time-course of pathological events. Moreover, since NMJs could represent an important target to be exploited for counteracting the pathology progression, we also describe several therapeutic strategies that, directly or indirectly, aim at NMJs.

Vascular Defects and Spinal Cord Hypoxia in Spinal Muscular Atrophy

OBJECTIVE

Spinal muscular atrophy (SMA) is a major inherited cause of infant death worldwide. It results from mutations in a single, ubiquitously expressed gene (SMN1), with loss of lower motor neurons being the primary pathological signature. Systemic defects have also been reported in SMA patients and animal models. We investigated whether defects associated with the vasculature contribute to motor neuron pathology in SMA.

METHODS

Development and integrity of the capillary bed was examined in skeletal muscle and spinal cord of SMA mice, and muscle biopsies from SMA patients and controls, using quantitative morphometric approaches on immunohistochemically labeled tissue. Pimonidazole hydrochloride-based assays were used to identify functional hypoxia.

RESULTS

The capillary bed in muscle and spinal cord was normal in presymptomatic SMA mice (postnatal day 1), but failed to match subsequent postnatal development in control littermates. At mid- and late-symptomatic time points, the extent of the vascular architecture observed in two distinct mouse models of SMA was ∼50% of that observed in control animals. Skeletal muscle biopsies from human patients confirmed the presence of developmentally similar, significant vascular depletion in severe SMA. Hypovascularity in SMA mouse spinal cord was accompanied by significant functional hypoxia and defects in the blood-spinal cord barrier.

INTERPRETATION

Our results indicate that vascular defects are a major feature of severe forms of SMA, present in both mouse models and patients, resulting in functional hypoxia of motor neurons. Thus, abnormal vascular development and resulting hypoxia may contribute to the pathogenesis of SMA.

Chronic Treatment with the AMPK Agonist AICAR Prevents Skeletal Muscle Pathology but Fails to Improve Clinical Outcome in a Mouse Model of Severe Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a genetic neuromuscular disorder characterized by spinal and brainstem motor neuron (MN) loss and skeletal muscle paralysis. Currently, there is no effective treatment other than supportive care to ameliorate the quality of life of patients with SMA. Some studies have reported that physical exercise, by improving muscle strength and motor function, is potentially beneficial in SMA. The adenosine monophosphate-activated protein kinase agonist 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) has been reported to be an exercise mimetic agent that is able to regulate muscle metabolism and increase endurance both at rest and during exercise. Chronic AICAR administration has been shown to ameliorate the dystrophic muscle phenotype and motor behavior in the mdx mouse, a model of Duchenne muscular dystrophy. Here, we investigated whether chronic AICAR treatment was able to elicit beneficial effects on motor abilities and neuromuscular histopathology in a mouse model of severe SMA (the SMNΔ7 mouse). We report that AICAR improved skeletal muscle atrophy and structural changes found in neuromuscular junctions of SMNΔ7 animals. However, although AICAR prevented the loss of glutamatergic excitatory synapses on MNs, this compound was not able to mitigate MN loss or the microglial and astroglial reaction occurring in the spinal cord of diseased mice. Moreover, no improvement in survival or motor performance was seen in SMNΔ7 animals treated with AICAR. The beneficial effects of AICAR in SMA found in our study are SMN-independent, as no changes in the expression of this protein were seen in the spinal cord and skeletal muscle of diseased animals treated with this compound.

SMN deficiency does not induce oxidative stress in SMA iPSC-derived astrocytes or motor neurons

Spinal muscular atrophy (SMA) is a genetic disorder characterized by loss of motor neurons in the spinal cord leading to muscle atrophy and death. Although motor neurons (MNs) are the most obviously affected cells in SMA, recent evidence suggest dysfunction in multiple cell types. Astrocytes are a crucial component of the motor circuit and are intimately involved with MN health and maintenance. We have previously shown that SMA astrocytes are altered both morphologically and functionally early in disease progression, though it is unclear what causes astrocytes to become reactive. Oxidative stress is a common feature among neurodegenerative diseases. Oxidative stress can both induce apoptosis in neurons and can cause astrocytes to become reactive, which are features observed in the SMA induced pluripotent stem cell (iPSC) cultures. Therefore, we asked if oxidative stress contributes to SMA astrocyte pathology. We examined mitochondrial bioenergetics, transcript and protein levels of oxidative and anti-oxidant factors, and reactive oxygen species (ROS) production and found little evidence of oxidative stress. We did observe a significant increase in endogenous catalase expression in SMA iPSCs. While catalase knockdown in SMA iPSCs increased ROS production above basal levels, levels of ROS remained lower than in controls, further arguing against robust oxidative stress in this system. Viral delivery of survival motor neuron (SMN) reversed astrocyte activation and restored catalase levels to normal, without changing mitochondrial respiration or expression of oxidative stress markers. Taken together, these data indicate that SMN deficiency induces astrocyte reactivity, but does not do so through an oxidative stress-mediated process.

SMN expression is required in motor neurons to rescue electrophysiological deficits in the SMNΔ7 mouse model of SMA

Proximal spinal muscular atrophy (SMA) is the most frequent cause of hereditary infant mortality. SMA is an autosomal recessive neuromuscular disorder that results from the loss of the Survival Motor Neuron 1 (SMN1) gene and retention of the SMN2 gene. The SMN2 gene produces an insufficient amount of full-length SMN protein that results in loss of motor neurons in the spinal cord and subsequent muscle paralysis. Previously we have shown that overexpression of human SMN in neurons in the SMA mouse ameliorates the SMA phenotype while overexpression of human SMN in skeletal muscle had no effect. Using Cre recombinase, here we show that either deletion or replacement of Smn in motor neurons (ChAT-Cre) significantly alters the functional output of the motor unit as measured with compound muscle action potential and motor unit number estimation. However ChAT-Cre alone did not alter the survival of SMA mice by replacement and did not appreciably affect survival when used to deplete SMN. However replacement of Smn in both neurons and glia in addition to the motor neuron (Nestin-Cre and ChAT-Cre) resulted in the greatest improvement in survival of the mouse and in some instances complete rescue was achieved. These findings demonstrate that high expression of SMN in the motor neuron is both necessary and sufficient for proper function of the motor unit. Furthermore, in the mouse high expression of SMN in neurons and glia, in addition to motor neurons, has a major impact on survival.

Low levels of Survival Motor Neuron protein are sufficient for normal muscle function in the SMNΔ7 mouse model of SMA

Spinal Muscular Atrophy (SMA) is an autosomal recessive disorder characterized by loss of lower motor neurons. SMA is caused by deletion or mutation of the Survival Motor Neuron 1 (SMN1) gene and retention of the SMN2 gene. The loss of SMN1 results in reduced levels of the SMN protein. SMN levels appear to be particularly important in motor neurons; however SMN levels above that produced by two copies of SMN2 have been suggested to be important in muscle. Studying the spatial requirement of SMN is important in both understanding how SMN deficiency causes SMA and in the development of effective therapies. Using Myf5-Cre, a muscle-specific Cre driver, and the Cre-loxP recombination system, we deleted mouse Smn in the muscle of mice with SMN2 and SMNΔ7 transgenes in the background, thus providing low level of SMN in the muscle. As a reciprocal experiment, we restored normal levels of SMN in the muscle with low SMN levels in all other tissues. We observed that decreasing SMN in the muscle has no phenotypic effect. This was corroborated by muscle physiology studies with twitch force, tetanic and eccentric contraction all being normal. In addition, electrocardiogram and muscle fiber size distribution were also normal. Replacement of Smn in muscle did not rescue SMA mice. Thus the muscle does not appear to require high levels of SMN above what is produced by two copies of SMN2 (and SMNΔ7).

Motor deficit in a tauopathy model is induced by disturbances of axonal transport leading to dying-back degeneration and denervation of neuromuscular junctions

Several neurodegenerative diseases are characterized by both cognitive and motor deficits associated with accumulation of tau aggregates in brain, brainstem, and spinal cord. The Tg30 murine tauopathy model expresses a human tau protein bearing two frontotemporal dementia with Parkinsonism linked to chromosome 17 pathogenic mutations and develops a severe motor deficit and tau aggregates in brain and spinal cord. To investigate the origin of this motor deficit, we analyzed the age-dependent innervation status of the neuromuscular junctions and mutant tau expression in Tg30 mice. The human transgenic tau was detected from postnatal day 7 onward in motoneurons, axons in the sciatic nerve, and axon terminals of the neuromuscular junctions. The development and maturation of neuromuscular junctions were not disrupted in Tg30 mice, but their maintenance was disturbed in adult Tg30 mice, resulting in a progressive and severe muscle denervation. This muscle denervation was associated with early electrophysiological signs of muscle spontaneous activities and histological signs of muscle degeneration. Early loss of synaptic vesicles in axon terminals preceding motor deficits, accumulation of Gallyas-positive aggregates, and cathepsin-positive vesicular clusters in axons in the sciatic nerve suggest that this denervation results from disturbances of axonal transport. This physiopathological mechanism might be responsible for motor signs observed in some human tauopathies, and for synaptic dysfunction resulting from alterations at the presynaptic level in these diseases.

Astrocytes influence the severity of spinal muscular atrophy

Systemically low levels of survival motor neuron-1 (SMN1) protein cause spinal muscular atrophy (SMA). α-Motor neurons of the spinal cord are considered particularly vulnerable in this genetic disorder and their dysfunction and loss cause progressive muscle weakness, paralysis and eventually premature death of afflicted individuals. Historically, SMA was therefore considered a motor neuron-autonomous disease. However, depletion of SMN in motor neurons of normal mice elicited only a very mild phenotype. Conversely, restoration of SMN to motor neurons in an SMA mouse model had only modest effects on the SMA phenotype and survival. Collectively, these results suggested that additional cell types contribute to the pathogenesis of SMA, and understanding the non-autonomous requirements is crucial for developing effective therapies. Astrocytes are critical for regulating synapse formation and function as well as metabolic support for neurons. We hypothesized that astrocyte functions are disrupted in SMA, exacerbating disease progression. Using viral-based restoration of SMN specifically to astrocytes, survival in severe and intermediate SMA mice was observed. In addition, neuromuscular circuitry was improved. Astrogliosis was prominent in end-stage SMA mice and in post-mortem patient spinal cords. Increased expression of proinflammatory cytokines was partially normalized in treated mice, suggesting that astrocytes contribute to the pathogenesis of SMA.

Amyotrophic lateral sclerosis: mechanisms and therapeutics in the epigenomic era

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease of the motor neurons, which results in weakness and atrophy of voluntary skeletal muscles. Treatments do not modify the disease trajectory effectively, and only modestly improve survival. A complex interaction between genes, environmental exposure and impaired molecular pathways contributes to pathology in patients with ALS. Epigenetic mechanisms control the hereditary and reversible regulation of gene expression without altering the basic genetic code. Aberrant epigenetic patterns-including abnormal microRNA (miRNA) biogenesis and function, DNA modifications, histone remodeling, and RNA editing-are acquired throughout life and are influenced by environmental factors. Thus, understanding the molecular processes that lead to epigenetic dysregulation in patients with ALS might facilitate the discovery of novel therapeutic targets and biomarkers that could reduce diagnostic delay. These achievements could prove crucial for successful disease modification in patients with ALS. We review the latest findings regarding the role of miRNA modifications and other epigenetic mechanisms in ALS, and discuss their potential as therapeutic targets.

Non-aggregating tau phosphorylation by cyclin-dependent kinase 5 contributes to motor neuron degeneration in spinal muscular atrophy

Mechanisms underlying motor neuron degeneration in spinal muscular atrophy (SMA), the leading inherited cause of infant mortality, remain largely unknown. Many studies have established the importance of hyperphosphorylation of the microtubule-associated protein tau in various neurodegenerative disorders, including Alzheimer's and Parkinson's diseases. However, tau phosphorylation in SMA pathogenesis has yet to be investigated. Here we show that tau phosphorylation on serine 202 (S202) and threonine 205 (T205) is increased significantly in SMA motor neurons using two SMA mouse models and human SMA patient spinal cord samples. Interestingly, phosphorylated tau does not form aggregates in motor neurons or neuromuscular junctions (NMJs), even at late stages of SMA disease, distinguishing it from other tauopathies. Hyperphosphorylation of tau on S202 and T205 is mediated by cyclin-dependent kinase 5 (Cdk5) in SMA disease condition, because tau phosphorylation at these sites is significantly reduced in Cdk5 knock-out mice; genetic knock-out of Cdk5 activating subunit p35 in an SMA mouse model also leads to reduced tau phosphorylation on S202 and T205 in the SMA;p35(-/-) compound mutant mice. In addition, expression of the phosphorylation-deficient tauS202A,T205A mutant alleviates motor neuron defects in a zebrafish SMA model in vivo and mouse motor neuron degeneration in culture, whereas expression of phosphorylation-mimetic tauS202E,T205E promotes motor neuron defects. More importantly, genetic knock-out of tau in SMA mice rescues synapse stripping on motor neurons, NMJ denervation, and motor neuron degeneration in vivo. Altogether, our findings suggest a novel mechanism for SMA pathogenesis in which hyperphosphorylation of non-aggregating tau by Cdk5 contributes to motor neuron degeneration.

SMN deficiency disrupts gastrointestinal and enteric nervous system function in mice

The 2007 Consensus Statement for Standard of Care in Spinal Muscular Atrophy (SMA) notes that patients suffer from gastroesophageal reflux, constipation and delayed gastric emptying. We used two mouse models of SMA to determine whether functional GI complications are a direct consequence of or are secondary to survival motor neuron (Smn) deficiency. Our results show that despite normal activity levels and food and water intake, Smn deficiency caused constipation, delayed gastric emptying, slow intestinal transit and reduced colonic motility without gross anatomical or histopathological abnormalities. These changes indicate alterations to the intrinsic neural control of gut functions mediated by the enteric nervous system (ENS). Indeed, Smn deficiency led to disrupted ENS signaling to the smooth muscle of the colon but did not cause enteric neuron loss. High-frequency electrical field stimulation (EFS) of distal colon segments produced up to a 10-fold greater contractile response in Smn deficient tissues. EFS responses were not corrected by the addition of a neuronal nitric oxide synthase inhibitor indicating that the increased contractility was due to hyperexcitability and not disinhibition of the circuitry. The GI symptoms observed in mice are similar to those reported in SMA patients. Together these data suggest that ENS cells are susceptible to Smn deficiency and may underlie the patient GI symptoms.

Muscle-derived extracellular signal-regulated kinases 1 and 2 are required for the maintenance of adult myofibers and their neuromuscular junctions

The Ras-extracellular signal-regulated kinase 1 and 2 (ERK1/2) pathway appears to be important for the development, maintenance, aging, and pathology of mammalian skeletal muscle. Yet no gene targeting of Erk1/2 in muscle fibers in vivo has been reported to date. We combined a germ line Erk1 mutation with Cre-loxP Erk2 inactivation in skeletal muscle to produce, for the first time, mice lacking ERK1/2 selectively in skeletal myofibers. Animals lacking muscle ERK1/2 displayed stunted postnatal growth, muscle weakness, and a shorter life span. Their muscles examined in this study, sternomastoid and tibialis anterior, displayed fragmented neuromuscular synapses and a mixture of modest fiber atrophy and loss but failed to show major changes in fiber type composition or absence of cell surface dystrophin. Whereas the lack of only ERK1 had no effects on the phenotypes studied, the lack of myofiber ERK2 explained synaptic fragmentation in the sternomastoid but not the tibialis anterior and a decrease in the expression of the acetylcholine receptor (AChR) epsilon subunit gene mRNA in both muscles. A reduction in AChR protein was documented in line with the above mRNA results. Evidence of partial denervation was found in the sternomastoid but not the tibialis anterior. Thus, myofiber ERK1/2 are differentially required for the maintenance of myofibers and neuromuscular synapses in adult mice.

SMA-causing missense mutations in survival motor neuron (Smn) display a wide range of phenotypes when modeled in Drosophila

Mutations in the human survival motor neuron 1 (SMN) gene are the primary cause of spinal muscular atrophy (SMA), a devastating neuromuscular disorder. SMN protein has a well-characterized role in the biogenesis of small nuclear ribonucleoproteins (snRNPs), core components of the spliceosome. Additional tissue-specific and global functions have been ascribed to SMN; however, their relevance to SMA pathology is poorly understood and controversial. Using Drosophila as a model system, we created an allelic series of twelve Smn missense mutations, originally identified in human SMA patients. We show that animals expressing these SMA-causing mutations display a broad range of phenotypic severities, similar to the human disease. Furthermore, specific interactions with other proteins known to be important for SMN's role in RNP assembly are conserved. Intragenic complementation analyses revealed that the three most severe mutations, all of which map to the YG box self-oligomerization domain of SMN, display a stronger phenotype than the null allele and behave in a dominant fashion. In support of this finding, the severe YG box mutants are defective in self-interaction assays, yet maintain their ability to heterodimerize with wild-type SMN. When expressed at high levels, wild-type SMN is able to suppress the activity of the mutant protein. These results suggest that certain SMN mutants can sequester the wild-type protein into inactive complexes. Molecular modeling of the SMN YG box dimer provides a structural basis for this dominant phenotype. These data demonstrate that important structural and functional features of the SMN YG box are conserved between vertebrates and invertebrates, emphasizing the importance of self-interaction to the proper functioning of SMN.

Spinal Muscular Atrophy (SMA) is a prevalent childhood neuromuscular disease, which in its most common form causes death by the age of two. One in fifty Americans is a carrier for SMA, making this genetic disease a serious health concern. SMA is caused by loss of function mutations in the survival motor neuron 1 (SMN1) gene. SMN is an essential protein and has a well-characterized function in the assembly of small nuclear ribonucleoproteins (snRNPs), which are core components of the spliceosome. To elucidate the phenotypic consequences of disrupting specific SMN protein interactions, we have generated a series of SMA-causing point mutations, modeled in Drosophila melanogaster. Using this system, we have shown that key aspects of SMN structure and function are conserved between humans and flies. Intragenic complementation analyses reveal the potential for dominant negative interactions between wild-type and mutant SMN subunits, highlighting the essential nature of the YG box in formation of higher-order SMN multimers. These results provide a basis for future studies investigating therapy targeted at restoration of functional SMN oligomers.

Electrophysiological Biomarkers in Spinal Muscular Atrophy: Preclinical Proof of Concept

Objective

Preclinical therapies that restore survival motor neuron (SMN) protein levels can dramatically extend survival in spinal muscular atrophy (SMA) mouse models. Biomarkers are needed to effectively translate these promising therapies to clinical trials. Our objective was to investigate electrophysiological biomarkers of compound muscle action potential (CMAP), motor unit number estimation (MUNE) and electromyography (EMG) using an SMA mouse model.

Methods

Sciatic CMAP, MUNE, and EMG were obtained in SMNΔ7 mice at ages 3–13 days and at 21 days in mice with SMN selectively reduced in motor neurons (ChAT^Cre). To investigate these measures as biomarkers of treatment response, measurements were obtained in SMNΔ7 mice treated with antisense oligonucleotide (ASO) or gene therapy.

Results

CMAP was significantly reduced in SMNΔ7 mice at days 6–13 (P < 0.01), and MUNE was reduced at days 7–13 (P < 0.01). Fibrillations were present on EMG in SMNΔ7 mice but not controls (P = 0.02). Similar findings were seen at 21 days in ChAT^Cre mice. MUNE in ASO-treated SMNΔ7 mice were similar to controls at day 12 and 30. CMAP reduction persisted in ASO-treated SMNΔ7 mice at day 12 but was corrected at day 30. Similarly, CMAP and MUNE responses were corrected with gene therapy to restore SMN.

Interpretation

These studies confirm features of preserved neuromuscular function in the early postnatal period and subsequent motor unit loss in SMNΔ7 mice. SMN restoring therapies result in preserved MUNE and gradual repair of CMAP responses. This provides preclinical evidence for the utilization of CMAP and MUNE as biomarkers in future SMA clinical trials.