Abnormal motoneuron migration, differentiation, and axon outgrowth in spinal muscular atrophy

The spinal muscular atrophy gene product regulates actin dynamics

Spinal Muscular Atrophy (SMA) is a neuromuscular disease caused by low levels of the Survival of Motoneuron (SMN) protein. SMN interacts with and regulates the actin-binding protein profilin2a, thereby influencing actin dynamics. Dysfunctional actin dynamics caused by SMN loss disrupts neurite outgrowth, axonal pathfinding, and formation of functional synapses in neurons. Whether the SMN protein directly interacts with and regulates filamentous (F-) and monomeric globular (G-) actin is still elusive. In a quantitative single cell approach, we show that SMN loss leads to dysregulated F-/G-actin fractions. Furthermore, quantitative assessment of cell morphology suggests an F-actin organizational defect. Interestingly, this is mediated by an interaction of SMN with G- and F-actin. In co-immunoprecipitation, in-vitro pulldown and co-localization assays, we elucidated that this interaction is independent of the SMN-profilin2a interaction. Therefore, we suggest two populations being relevant for functional actin dynamics in healthy neurons: SMN-profilin2a-actin and SMN-actin. Additionally, those two populations may influence each other and therefore regulate binding of SMN to actin. In SMA, we showed a dysregulated co-localization pattern of SMN-actin which could only partially rescued by SMN restoration. However, dysregulation of F-/G-actin fractions was reduced by SMN restoration. Taken together, our results suggest a novel molecular function of SMN in binding to actin independent from SMN-profilin2a interaction.

Isogenic patient-derived organoids reveal early neurodevelopmental defects in spinal muscular atrophy initiation

Whether neurodevelopmental defects underlie postnatal neuronal death in neurodegeneration is an intriguing hypothesis only recently explored. Here, we focus on spinal muscular atrophy (SMA), a neuromuscular disorder caused by reduced survival of motor neuron (SMN) protein levels leading to spinal motor neuron (MN) loss and muscle wasting. Using the first isogenic patient-derived induced pluripotent stem cell (iPSC) model and a spinal cord organoid (SCO) system, we show that SMA SCOs exhibit abnormal morphological development, reduced expression of early neural progenitor markers, and accelerated expression of MN progenitor and MN markers. Longitudinal single-cell RNA sequencing reveals marked defects in neural stem cell specification and fewer MNs, favoring mesodermal progenitors and muscle cells, a bias also seen in early SMA mouse embryos. Surprisingly, SMN2-to-SMN1 conversion does not fully reverse these developmental abnormalities. These suggest that early neurodevelopmental defects may underlie later MN degeneration, indicating that postnatal SMN-increasing interventions might not completely amend SMA pathology in all patients.

Total tau in cerebrospinal fluid detects treatment responders among spinal muscular atrophy types 1-3 patients treated with nusinersen

AIMS

Considering the substantial variability in treatment response across patients with spinal muscular atrophy (SMA), reliable markers for monitoring response to therapy and predicting treatment responders need to be identified. The study aimed to determine if measured concentrations of disease biomarkers (total tau protein, neurofilament light chain, and S100B protein) correlate with the duration of nusinersen treatment and with scores obtained using functional scales for the assessment of motor abilities.

METHODS

A total of 30 subjects with SMA treated with nusinersen between 2017 and 2021 at the Department of Pediatrics, University Hospital Centre Zagreb, Croatia, were included in this study. Cerebrospinal fluid (CSF) samples were collected by lumbar puncture prior to intrathecal application of nusinersen. Protein concentrations in CSF samples were determined by enzyme-linked immunosorbent assay in 26 subjects. The motor functions were assessed using functional motor scales.

RESULTS

The main finding was significantly decreased total tau correlating with the number of nusinersen doses and motor improvement in the first 18-24 months of treatment (in all SMA patients and SMA type 1 patients). Neurofilament light chain and S100B were not significantly changed after administration of nusinersen.

CONCLUSIONS

The measurement of total tau concentration in CSF is a reliable index for monitoring the biomarker and clinical response to nusinersen therapy in patients with SMA.

Multi-omics profiling of CSF from spinal muscular atrophy type 3 patients after nusinersen treatment: a 2-year follow-up multicenter retrospective study

Spinal muscular atrophy (SMA) is a neurodegenerative disorder caused by mutations in the SMN1 gene resulting in reduced levels of the SMN protein. Nusinersen, the first antisense oligonucleotide (ASO) approved for SMA treatment, binds to the SMN2 gene, paralogue to SMN1, and mediates the translation of a functional SMN protein. Here, we used longitudinal high-resolution mass spectrometry (MS) to assess both global proteome and metabolome in cerebrospinal fluid (CSF) from ten SMA type 3 patients, with the aim of identifying novel readouts of pharmacodynamic/response to treatment and predictive markers of treatment response. Patients had a median age of 33.5 [29.5; 38.25] years, and 80% of them were ambulant at time of the enrolment, with a median HFMSE score of 37.5 [25.75; 50.75]. Untargeted CSF proteome and metabolome were measured using high-resolution MS (nLC-HRMS) on CSF samples obtained before treatment (T0) and after 2 years of follow-up (T22). A total of 26 proteins were found to be differentially expressed between T0 and T22 upon VSN normalization and LIMMA differential analysis, accounting for paired replica. Notably, key markers of the insulin-growth factor signaling pathway were upregulated after treatment together with selective modulation of key transcription regulators. Using CombiROC multimarker signature analysis, we suggest that detecting a reduction of SEMA6A and an increase of COL1A2 and GRIA4 might reflect therapeutic efficacy of nusinersen. Longitudinal metabolome profiling, analyzed with paired t-Test, showed a significant shift for some aminoacid utilization induced by treatment, whereas other metabolites were largely unchanged. Together, these data suggest perturbation upon nusinersen treatment still sustained after 22 months of follow-up and confirm the utility of CSF multi-omic profiling as pharmacodynamic biomarker for SMA type 3. Nonetheless, validation studies are needed to confirm this evidence in a larger sample size and to further dissect combined markers of response to treatment.

Molecular Biomarkers for the Diagnosis, Prognosis, and Pharmacodynamics of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a progressive degenerative illness that affects 1 in every 6 to 11,000 live births. This autosomal recessive disorder is caused by homozygous deletion or mutation of the SMN1 gene (survival motor neuron). As a backup, the SMN1 gene has the SMN2 gene, which produces only 10% of the functional SMN protein. Nusinersen and risdiplam, the first FDA-approved medications, act as SMN2 pre-mRNA splicing modifiers and enhance the quantity of SMN protein produced by this gene. The emergence of new therapies for SMA has increased the demand for good prognostic and pharmacodynamic (response) biomarkers in SMA. This article discusses current molecular diagnostic, prognostic, and pharmacodynamic biomarkers that could be assessed in SMA patients' body fluids. Although various proteomic, genetic, and epigenetic biomarkers have been explored in SMA patients, more research is needed to uncover new prognostic and pharmacodynamic biomarkers (or a combination of biomarkers).

Mitochondrial Dysfunction in Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a devastating neuromuscular disorder caused by recessive mutations in the SMN1 gene, globally affecting ~8-14 newborns per 100,000. The severity of the disease depends on the residual levels of functional survival of motor neuron protein, SMN. SMN is a ubiquitously expressed RNA binding protein involved in a plethora of cellular processes. In this review, we discuss the effects of SMN loss on mitochondrial functions in the neuronal and muscular systems that are the most affected in patients with spinal muscular atrophy. Our aim is to highlight how mitochondrial defects may contribute to disease progression and how restoring mitochondrial functionality may be a promising approach to develop new therapies. We also collected from previous studies a list of transcripts encoding mitochondrial proteins affected in various SMA models. Moreover, we speculate that in adulthood, when motor neurons require only very low SMN levels, the natural deterioration of mitochondria associated with aging may be a crucial triggering factor for adult spinal muscular atrophy, and this requires particular attention for therapeutic strategies.

The Proteome Signatures of Fibroblasts from Patients with Severe, Intermediate and Mild Spinal Muscular Atrophy Show Limited Overlap

Most research to characterise the molecular consequences of spinal muscular atrophy (SMA) has focused on SMA I. Here, proteomic profiling of skin fibroblasts from severe (SMA I), intermediate (SMA II), and mild (SMA III) patients, alongside age-matched controls, was conducted using SWATH mass spectrometry analysis. Differentially expressed proteomic profiles showed limited overlap across each SMA type, and variability was greatest within SMA II fibroblasts, which was not explained by SMN2 copy number. Despite limited proteomic overlap, enriched canonical pathways common to two of three SMA severities with at least one differentially expressed protein from the third included mTOR signalling, regulation of eIF2 and eIF4 signalling, and protein ubiquitination. Network expression clustering analysis identified protein profiles that may discriminate or correlate with SMA severity. From these clusters, the differential expression of PYGB (SMA I), RAB3B (SMA II), and IMP1 and STAT1 (SMA III) was verified by Western blot. All SMA fibroblasts were transfected with an SMN-enhanced construct, but only RAB3B expression in SMA II fibroblasts demonstrated an SMN-dependent response. The diverse proteomic profiles and pathways identified here pave the way for studies to determine their utility as biomarkers for patient stratification or monitoring treatment efficacy and for the identification of severity-specific treatments.

Suppression of the necroptotic cell death pathways improves survival in Smn2B/− mice

Spinal muscular atrophy (SMA) is a monogenic neuromuscular disease caused by low levels of the Survival Motor Neuron (SMN) protein. Motor neuron degeneration is the central hallmark of the disease. However, the SMN protein is ubiquitously expressed and depletion of the protein in peripheral tissues results in intrinsic disease manifestations, including muscle defects, independent of neurodegeneration. The approved SMN-restoring therapies have led to remarkable clinical improvements in SMA patients. Yet, the presence of a significant number of non-responders stresses the need for complementary therapeutic strategies targeting processes which do not rely solely on restoring SMN. Dysregulated cell death pathways are candidates for SMN-independent pathomechanisms in SMA. Receptor-interacting protein kinase 1 (RIPK1) and RIPK3 have been widely recognized as critical therapeutic targets of necroptosis, an important form of programmed cell death. In addition, Caspase-1 plays a fundamental role in inflammation and cell death. In this study, we evaluate the role of necroptosis, particularly RIPK3 and Caspase-1, in the Smn^2B/− mouse model of SMA. We have generated a triple mutant (TKO), the Smn^2B/−; Ripk3^−/−; Casp1^−/− mouse. TKO mice displayed a robust increase in survival and improved motor function compared to Smn^2B/− mice. While there was no protection against motor neuron loss or neuromuscular junction pathology, larger muscle fibers were observed in TKO mice compared to Smn^2B/− mice. Our study shows that necroptosis modulates survival, motor behavior and muscle fiber size independent of SMN levels and independent of neurodegeneration. Thus, small-molecule inhibitors of necroptosis as a combinatorial approach together with SMN-restoring drugs could be a future strategy for the treatment of SMA.

Identification of specific gene methylation patterns during motor neuron differentiation from spinal muscular atrophy patient-derived iPSC

Spinal muscular atrophy is a progressive motor neuron disorder caused by deletions or point mutations in the SMN1 gene. It is not known why motor neurons are particularly sensitive to a decrease in SMN protein levels and what factors besides SMN2 underlie the high clinical heterogeneity of the disease. Here we studied the methylation patterns of genes on sequential stages of motor neuron differentiation from induced pluripotent stem cells derived from the patients with SMA type I and II. The genes involved in the regulation of pluripotency, neural differentiation as well as those associated with spinal muscular atrophy development were included. The results show that the PAX6, HB9, CHAT, ARHGAP22, and SMN2 genes are differently methylated in cells derived from SMA patients compared to the cells of healthy individuals. This study clarifies the specificities of the disease pathogenesis and extends the knowledge of pathways involved in the SMA progression.

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

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

The Need for SMN-Independent Treatments of Spinal Muscular Atrophy (SMA) to Complement SMN-Enhancing Drugs

Spinal Muscular Atrophy (SMA) is monogenic motoneuron disease caused by low levels of the Survival of Motoneuron protein (SMN). Recently, two different drugs were approved for the treatment of the disease. The antisense oligonucleotide Nusinersen/Spinraza® and the gene replacement therapy Onasemnogene Abeparvovec/Zolgensma® both enhance SMN levels. These treatments result in impressive benefits for the patients. However, there is a significant number of non-responders and an intervention delay has a strong negative impact on the efficacy. Obviously, later stages of motoneuron degeneration cannot be reversed by SMN-restoration. Therefore, complementary, SMN-independent strategies are needed which are able to address such SMN-irreversible degenerative processes. Those are defined as pathological alterations which are not reversed by SMN-restoration for a given dose and intervention delay. It is crucial to tailor SMN-independent approaches to the novel clinical situation with SMN-restoring treatments. On the molecular level, such SMN-irreversible changes become manifest in altered signaling modules as described by molecular systems biology. Based on our current knowledge about altered signaling, we introduce a network approach for an informed decision for the most potent SMN-independent treatment targets. Finally, we present recommendations for the identification of novel treatments which can be combined with SMN-restoring drugs.

Lumbosacral ventral spinal nerve root atrophy identified on MRI in a case of spinal muscular atrophy type II

Spinal muscular atrophies are rare genetic disorders most often caused by homozygous deletion mutations in SMN1 that lead to progressive neurodegeneration of anterior horn cells. Ventral spinal root atrophy is a consistent pathological finding in post-mortem examinations of patients who suffered from various subtypes of spinal muscular atrophy; however, corresponding radiographic findings have not been previously reported. We present a patient with hypotonia and weakness who was found to have ventral spinal root atrophy in the lumbosacral region on MRI and was subsequently diagnosed with spinal muscular atrophy. More systematic analyses of imaging studies in spinal muscular atrophy will help determine whether such findings have the potential to serve as reliable diagnostic markers for clinical evaluations or as outcome measure for clinical trials.

Gender-Specific Amelioration of SMA Phenotype upon Disruption of a Deep Intronic Structure by an Oligonucleotide

Spinal muscular atrophy (SMA), the leading genetic disease of children, is caused by low levels of survival motor neuron (SMN) protein. Here, we employ A15/283, an antisense oligonucleotide targeting a deep intronic sequence/structure, to examine the impact of restoration of SMN in a mild SMA mouse model. We show gender-specific amelioration of tail necrosis upon subcutaneous administrations of A15/283 into SMA mice at postnatal days 1 and 3. We also demonstrate that a modest increase in SMN due to early administrations of A15/283 dramatically improves testicular development and spermatogenesis. Our results reveal near total correction of expression of several genes in adult testis upon temporary increase in SMN during early postnatal development. This is the first demonstration of in vivo efficacy of an antisense oligonucleotide targeting a deep intronic sequence/structure. This is also the first report of gender-specific amelioration of SMA pathology upon a modest peripheral increase of SMN.

Spinal Muscular Atrophy Patient iPSC-Derived Motor Neurons Have Reduced Expression of Proteins Important in Neuronal Development

Spinal muscular atrophy (SMA) is an inherited neuromuscular disease primarily characterized by degeneration of spinal motor neurons, and caused by reduced levels of the SMN protein. Previous studies to understand the proteomic consequences of reduced SMN have mostly utilized patient fibroblasts and animal models. We have derived human motor neurons from type I SMA and healthy controls by creating their induced pluripotent stem cells (iPSCs). Quantitative mass spectrometry of these cells revealed increased expression of 63 proteins in control motor neurons compared to respective fibroblasts, whereas 30 proteins were increased in SMA motor neurons vs. their fibroblasts. Notably, UBA1 was significantly decreased in SMA motor neurons, supporting evidence for ubiquitin pathway defects. Subcellular distribution of UBA1 was predominantly cytoplasmic in SMA motor neurons in contrast to nuclear in control motor neurons; suggestive of neurodevelopmental abnormalities. Many of the proteins that were decreased in SMA motor neurons, including beta III-tubulin and UCHL1, were associated with neurodevelopment and differentiation. These neuron-specific consequences of SMN depletion were not evident in fibroblasts, highlighting the importance of iPSC technology. The proteomic profiles identified here provide a useful resource to explore the molecular consequences of reduced SMN in motor neurons, and for the identification of novel biomarker and therapeutic targets for SMA.

Spinal muscular atrophy: journeying from bench to bedside

Spinal muscular atrophy (SMA) is a frequently fatal neuromuscular disorder and the most common inherited cause of infant mortality. SMA results from reduced levels of the survival of motor neuron (SMN) protein. Although the disease was first described more than a century ago, a precise understanding of its genetics was not obtained until the SMA genes were cloned in 1995. This was followed in rapid succession by experiments that assigned a role to the SMN protein in the proper splicing of genes, novel animal models of the disease, and the eventual use of the models in the pre clinical development of rational therapies for SMA. These successes have led the scientific and clinical communities to the cusp of what are expected to be the first truly promising treatments for the human disorder. Yet, important questions remain, not the least of which is how SMN paucity triggers a predominantly neuromuscular phenotype. Here we review how our understanding of the disease has evolved since the SMA genes were identified. We begin with a brief description of the genetics of SMA and the proposed roles of the SMN protein. We follow with an examination of how the genetics of the disease was exploited to develop genetically faithful animal models, and highlight the insights gained from their analysis. We end with a discussion of ongoing debates, future challenges, and the most promising treatments to have emerged from our current knowledge of the disease.

Congenital brain anomalies and chromosomal aberrations from the Zagreb Collection of human brains

The Zagreb Collection of developing and adult human brains consists of approximately 1,300 brains of fetuses, children and adults that were collected following routine autopsies in the period from 1974 to 2014. The collection comprises brains of different normal developmental stages that may serve for investigation of normal human brain development. Previous studies on this material have led to several important contributions on human cortical development, such as the discovery of the transient fetal subplate zone. The Zagreb Collection, however, also contains approximately 100 brains with different anomalies including chromosomal aberrations such as Down syndrome. We have analyzed all the available material from the Zagreb Collection and identified 44 brains of fetuses and children with Down syndrome, 10 with Patau syndrome, 6 with Edwards syndrome as well as 7 holoprosencephalic, 7 hydrocephalic and 4 microcephalic brains. The largest part of the Collection is available for further research using modern genetic, immunocytochemical and imaging methods, especially magnetic resonance imaging. Furthermore, the histological slides from the Zagreb Collection are currently being digitally scanned and made available as virtual slides to general scientific audience. The Zagreb Collection represents unique and versatile resource for the future study of normal and abnormal human brain development.

Neurodegeneration in spinal muscular atrophy: from disease phenotype and animal models to therapeutic strategies and beyond

Of the numerous inherited diseases known to afflict the pediatric population, spinal muscular atrophy (SMA) is among the most common. It has an incidence of approximately one in 10,000 newborns and a carrier frequency of one in 50. Despite its relatively high incidence, SMA remains somewhat obscure among the many neurodegenerative diseases that affect humans. Nevertheless, the last two decades have witnessed remarkable progress in our understanding of the pathology, underlying biology and especially the molecular genetics of SMA. This has led to a genuine expectation within the scientific community that a robust treatment will be available to patients before the end of the decade. The progress made in our understanding of SMA and, therefore, towards a viable therapy for affected individuals is in large measure a consequence of the simple yet fascinating genetics of the disease. Nevertheless, important questions remain. Addressing these questions promises not only to accelerate the march towards a cure for SMA, but also to uncover novel therapies for related neurodegenerative disorders. This review discusses our current understanding of SMA, considers the challenges ahead, describes existing treatment options and highlights state-of-the-art research being conducted as a means to a better, safer and more effective treatment for the disease.

Selective vulnerability of spinal and cortical motor neuron subpopulations in delta7 SMA mice

Loss of the survival motor neuron gene (SMN1) is responsible for spinal muscular atrophy (SMA), the most common inherited cause of infant mortality. Even though the SMA phenotype is traditionally considered as related to spinal motor neuron loss, it remains debated whether the specific targeting of motor neurons could represent the best therapeutic option for the disease. We here investigated, using stereological quantification methods, the spinal cord and cerebral motor cortex of ∆7 SMA mice during development, to verify extent and selectivity of motor neuron loss. We found progressive post-natal loss of spinal motor neurons, already at pre-symptomatic stages, and a higher vulnerability of motor neurons innervating proximal and axial muscles. Larger motor neurons decreased in the course of disease, either for selective loss or specific developmental impairment. We also found a selective reduction of layer V pyramidal neurons associated with layer V gliosis in the cerebral motor cortex. Our data indicate that in the ∆7 SMA model SMN loss is critical for the spinal cord, particularly for specific motor neuron pools. Neuronal loss, however, is not selective for lower motor neurons. These data further suggest that SMA pathogenesis is likely more complex than previously anticipated. The better knowledge of SMA models might be instrumental in shaping better therapeutic options for affected patients.

Chondrolectin affects cell survival and neuronal outgrowth in in vitro and in vivo models of spinal muscular atrophy

Spinal muscular atrophy (SMA) is characterized by the selective loss of spinal motor neurons owing to reduced levels of survival motor neuron (Smn) protein. In addition to its well-established role in assembling constituents of the spliceosome, diverse cellular functions have been proposed for Smn, but the reason why low levels of this widely expressed protein result in selective motor neuron pathology is still debated. In longitudinal studies of exon-level changes in SMA mouse model tissues, designed to determine the contribution of splicing dysfunction to the disease, we have previously shown that a generalized defect in splicing is unlikely to play a causative role in SMA. Nevertheless, we identified a small subset of genes that were alternatively spliced in the spinal cord compared with control mice before symptom onset, indicating a possible mechanistic role in disease. Here, we have performed functional studies of one of these genes, chondrolectin (Chodl), known to be highly expressed in motor neurons and important for correct motor axon outgrowth in zebrafish. Using in vitro and in vivo models of SMA, we demonstrate altered expression of Chodl in SMA mouse spinal motor neurons, show that Chodl has distinct effects on cell survival and neurite outgrowth and that increasing the expression of chodl can rescue motor neuron outgrowth defects in Smn-depleted zebrafish. Our findings thus link the dysregulation of Chodl to the pathophysiology of motor neuron degeneration in SMA.

Notch signaling pathway is activated in motoneurons of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neurodegenerative disease produced by low levels of Survival Motor Neuron (SMN) protein that affects alpha motoneurons in the spinal cord. Notch signaling is a cell-cell communication system well known as a master regulator of neural development, but also with important roles in the adult central nervous system. Aberrant Notch function is associated with several developmental neurological disorders; however, the potential implication of the Notch pathway in SMA pathogenesis has not been studied yet. We report here that SMN deficiency, induced in the astroglioma cell line U87MG after lentiviral transduction with a shSMN construct, was associated with an increase in the expression of the main components of Notch signaling pathway, namely its ligands, Jagged1 and Delta1, the Notch receptor and its active intracellular form (NICD). In the SMNΔ7 mouse model of SMA we also found increased astrocyte processes positive for Jagged1 and Delta1 in intimate contact with lumbar spinal cord motoneurons. In these motoneurons an increased Notch signaling was found, as denoted by increased NICD levels and reduced expression of the proneural gene neurogenin 3, whose transcription is negatively regulated by Notch. Together, these findings may be relevant to understand some pathologic attributes of SMA motoneurons.

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.

Abnormal fatty acid metabolism in spinal muscular atrophy may predispose to perioperative risks

A 15 year old boy with SMA type II underwent spinal fusion and suffered a mitochondrial Reye-like catabolic crisis 4 days postop with hypoketotic hypoglycemia, lactic acidaemia, hyperammonemia and liver failure, with 90% coagulative necrosis and diffuse macro- and microvesicular steatosis, requiring orthotopic liver transplantation. This crisis responded in part to mitochondrial therapy and anabolic rescue. He made a dramatic sustained neurological recovery, though his post-transplant liver biopsies revealed micro- and macrosteatosis. We hypothesize that a combination of surgical stress-catecholamine induced lipolysis, prolonged general anaesthesia with propofol and sevoflurane, and perioperative fasting on a background of decreased β-oxidation were potential risk factors for the mitochondrial decompensation.

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.

Spinal muscular atrophy: the role of SMN in axonal mRNA regulation

Spinal muscular atrophy (SMA) is a neurodegenerative disease caused by homozygous mutations or deletions in the survival of motor neuron (SMN1) gene, encoding the ubiquitously expressed SMN protein. SMN associates with different proteins (Gemins 2-8, Unrip) to form a multimeric complex involved in the assembly of small nuclear ribonucleoprotein complexes (snRNPs). Since this activity is essential for the survival of all cell types, it still remains unclear why motor neurons are selectively vulnerable to low levels of SMN protein. Aside from its housekeeping role in the assembly of snRNPs, additional functions of SMN have been proposed. The well-documented localization of SMN in axonal transport granules and its interaction with numerous mRNA-binding proteins not involved in splicing regulation suggest a role in axonal RNA metabolism. This review will focus on the neuropathological and experimental evidence supporting a role for SMN in regulating the assembly, localization, or stability of axonal messenger ribonucleoprotein complexes (mRNPs). Furthermore, how defects in this non-canonical SMN function may contribute to the motor neuron pathology observed in SMA will be discussed. This article is part of a Special Issue entitled RNA-Binding Proteins.

Motor neuron pathology and behavioral alterations at late stages in a SMA mouse model

Spinal muscular atrophy (SMA) is a neurogenetic autosomal recessive disorder characterized by degeneration of lower motor neurons. The validation of appropriate animal models is key in fostering SMA research. Recent studies set up an animal model showing long survival and slow disease progression. This model is knocked out for mouse SMN (Smn(-/-)) gene and carries a human mutation of the SMN1 gene (SMN1A2G), along with human SMN2 gene. In the present study we used this knock out double transgenic mouse model (SMN2(+/+); Smn(-/-); SMN1A2G(+/-)) to characterize the spinal cord pathology along with motor deficit at prolonged survival times. In particular, motor neuron loss was established stereologically (44.77%) after motor deficit reached a steady state. At this stage, spared motor neurons showed significant cell body enlargement. Moreover, similar to what was described in patients affected by SMA we found neuronal heterotopy (almost 4% of total motor neurons) in the anterior white matter. The delayed disease progression was likely to maintain fair motor activity despite a dramatic loss of large motor neurons. This provides a wonderful tool to probe novel drugs finely tuning the survival of motor neurons. In fact, small therapeutic effects protracted over considerable time intervals (even more than a year) are expected to be magnified.

SMN deficiency attenuates migration of U87MG astroglioma cells through the activation of RhoA

Spinal muscular atrophy (SMA) is a neurodegenerative disease that affects alpha motoneurons in the spinal cord caused by homozygous deletion or specific mutations in the survival motoneuron-1 (SMN1) gene. Cell migration is critical at many stages of nervous system development; to investigate the role of SMN in cell migration, U87MG astroglioma cells were transduced with shSMN lentivectors and about 60% reduction in SMN expression was achieved. In a monolayer wound-healing assay, U87MG SMN-depleted cells exhibit reduced cell migration. In these cells, RhoA was activated and phosphorylated levels of myosin regulatory light chain (MLC), a substrate of the Rho kinase (ROCK), were found increased. The decrease in cell motility was related to activation of RhoA/Rho kinase (ROCK) signaling pathway as treatment with the ROCK inhibitor Y-27632 abrogated both the motility defects and MLC phosphorylation in SMN-depleted cells. As cell migration is regulated by continuous remodeling of the actin cytoskeleton, the actin distribution was studied in SMN-depleted cells. A shift from filamentous to monomeric (globular) actin, involving the disappearance of stress fibers, was observed. In addition, profilin I, an actin-sequestering protein showed an increased expression in SMN-depleted cells. SMN is known to physically interact with profilin, reducing its actin-sequestering activity. The present results suggest that in SMN-depleted cells, the increase in profilin I expression and the reduction in SMN inhibitory action on profilin could lead to reduced filamentous actin polymerization, thus decreasing cell motility. We propose that the alterations reported here in migratory activity in SMN-depleted cells, related to abnormal activation of RhoA/ROCK pathway and increased profilin I expression could have a role in developing nervous system by impairing normal neuron and glial cell migration and thus contributing to disease pathogenesis in SMA.

Modeling spinal muscular atrophy in Drosophila links Smn to FGF signaling

FGF signaling in neurons is regulated by Survival Motor Neuron, a component of a complex that regulates snRNP biogenesis and FGF receptor expression.

Spinal muscular atrophy (SMA), a devastating neurodegenerative disorder characterized by motor neuron loss and muscle atrophy, has been linked to mutations in the Survival Motor Neuron (SMN) gene. Based on an SMA model we developed in Drosophila, which displays features that are analogous to the human pathology and vertebrate SMA models, we functionally linked the fibroblast growth factor (FGF) signaling pathway to the Drosophila homologue of SMN, Smn. Here, we characterize this relationship and demonstrate that Smn activity regulates the expression of FGF signaling components and thus FGF signaling. Furthermore, we show that alterations in FGF signaling activity are able to modify the neuromuscular junction defects caused by loss of Smn function and that muscle-specific activation of FGF is sufficient to rescue Smn-associated abnormalities.

The Zagreb Collection of human brains: a unique, versatile, but underexploited resource for the neuroscience community

The Zagreb Collection of developing and adult human brains was founded in 1974 by Ivica Kostović and consists of 1,278 developing and adult human brains, including 610 fetal, 317 children, and 359 adult brains. It is one of the largest collections of developing human brains. The collection serves as a key resource for many focused research projects and has led to several seminal contributions on mammalian cortical development, such as the discovery of the transient fetal subplate zone and of early bilaminar synaptogenesis in the embryonic and fetal human cerebral cortex, and the first description of growing afferent pathways in the human fetal telencephalon. The Zagreb Collection also serves as a core resource for ever-growing networks of international collaboration and represents the starting point for many young investigators who now pursue independent research careers at leading international institutions. The Zagreb Collection, however, remains underexploited owing to a lack of adequate funding in Croatia. Funding could establish an online catalog of the collection and modern virtual microscopy scanning methods to make the collection internationally more accessible.

Smn deficiency causes neuritogenesis and neurogenesis defects in the retinal neurons of a mouse model of spinal muscular atrophy

The eye is an excellent model for the study of neuronal development and pathogenesis of central nervous system disorders because of its relative ease of accessibility and the well-characterized cellular makeup. We have used this model to study spinal muscular atrophy (SMA), an autosomal recessive neuromuscular disease caused by deletions or mutations in the survival of motor neuron 1 gene (SMN1). We have investigated the expression pattern of mouse Smn mRNA and protein in the neural retina and the optic nerve of wild type mice. Smn protein is present in retinal ganglion cells and amacrine cells within the neural retina as well as in glial cells in the optic nerve. Histopathological analysis in phenotype stage SMA mice revealed that Smn deficiency is associated with a reduction in ganglion cell axon and glial cell number in the optic nerve, as well as compromised cellular processes and altered organization of neurofilaments in the neural retina. Whole mount preparation and retinal neuron primary culture provided further evidence of abnormal synaptogenesis and neurofilament accumulation in the neurites of Smn-deficient retinal neurons. A subset of amacrine cells is absent, in a cell-autonomous fashion, in the retina of SMA mice. Finally, the retinas of SMA mice have altered electroretinograms. Altogether, our study has demonstrated defects in axodendritic outgrowth and cellular composition in Smn-depleted retinal neurons, indicating a role for Smn in neuritogenesis and neurogenesis, and providing us with an insight into pathogenesis of SMA.

Survival of motor neuron protein over-expression prevents calpain-mediated cleavage and activation of procaspase-3 in differentiated human SH-SY5Y cells

Spinal muscular atrophy (SMA), a neurodegenerative disorder primarily affecting motor neurons, is the most common genetic cause of infant death. This incurable disease is caused by the absence of a functional SMN1 gene and a reduction in full length survival of motor neuron (SMN) protein. In this study, a neuroprotective function of SMN was investigated in differentiated human SH-SY5Y cells using an adenoviral vector to over-express SMN protein. The pro-survival capacity of SMN was assessed in an Akt/PI3-kinase inhibition (LY294002) model, as well as an oxidative stress (hydrogen peroxide) and excitotoxic (glutamate) model. SMN over-expression in SH-SY5Y cells protected against Akt/phosphatidylinositol 3-kinase (PI3-kinase) inhibition, but not oxidative stress, nor against excitotoxicity in rat cortical neurons. Western analysis of cell homogenates from SH-SY5Y cultures over-expressing SMN harvested pre- and post-Akt/PI3-kinase inhibition indicated that SMN protein inhibited caspase-3 activation via blockade of calpain-mediated procaspase-3 cleavage. This study has revealed a novel anti-apoptotic function for the SMN protein in differentiated SH-SY5Y cells. Finally, the cell death model described herein will allow the assessment of future therapeutic agents or strategies aimed at increasing SMN protein levels.

SMN deficiency disrupts brain development in a mouse model of severe spinal muscular atrophy

Reduced expression of the survival motor neuron (SMN) gene causes the childhood motor neuron disease spinal muscular atrophy (SMA). Low levels of ubiquitously expressed SMN protein result in the degeneration of lower motor neurons, but it remains unclear whether other regions of the nervous system are also affected. Here we show that reduced levels of SMN lead to impaired perinatal brain development in a mouse model of severe SMA. Regionally selective changes in brain morphology were apparent in areas normally associated with higher SMN levels in the healthy postnatal brain, including the hippocampus, and were associated with decreased cell density, reduced cell proliferation and impaired hippocampal neurogenesis. A comparative proteomics analysis of the hippocampus from SMA and wild-type littermate mice revealed widespread modifications in expression levels of proteins regulating cellular proliferation, migration and development when SMN levels were reduced. This study reveals novel roles for SMN protein in brain development and maintenance and provides the first insights into cellular and molecular pathways disrupted in the brain in a severe form of SMA.

Drosophila melanogaster as a model organism of brain diseases

Drosophila melanogaster has been utilized to model human brain diseases. In most of these invertebrate transgenic models, some aspects of human disease are reproduced. Although investigation of rodent models has been of significant impact, invertebrate models offer a wide variety of experimental tools that can potentially address some of the outstanding questions underlying neurological disease. This review considers what has been gleaned from invertebrate models of neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, metabolic diseases such as Leigh disease, Niemann-Pick disease and ceroid lipofuscinoses, tumor syndromes such as neurofibromatosis and tuberous sclerosis, epilepsy as well as CNS injury. It is to be expected that genetic tools in Drosophila will reveal new pathways and interactions, which hopefully will result in molecular based therapy approaches.

Spinal muscular atrophy and a model for survival of motor neuron protein function in axonal ribonucleoprotein complexes

Spinal muscular atrophy (SMA) is a neurodegenerative disease that results from loss of function of the SMN1 gene, encoding the ubiquitously expressed survival of motor neuron (SMN) protein, a protein best known for its housekeeping role in the SMN-Gemin multiprotein complex involved in spliceosomal small nuclear ribonucleoprotein (snRNP) assembly. However, numerous studies reveal that SMN has many interaction partners, including mRNA binding proteins and actin regulators, suggesting its diverse role as a molecular chaperone involved in mRNA metabolism. This review focuses on studies suggesting an important role of SMN in regulating the assembly, localization, or stability of axonal messenger ribonucleoprotein (mRNP) complexes. Various animal models for SMA are discussed, and phenotypes described that indicate a predominant function for SMN in neuronal development and synapse formation. These models have begun to be used to test different therapeutic strategies that have the potential to restore SMN function. Further work to elucidate SMN mechanisms within motor neurons and other cell types involved in neuromuscular circuitry hold promise for the potential treatment of SMA.

Pathogenesis of proximal autosomal recessive spinal muscular atrophy

Although it is known that deletions or mutations of the SMN1 gene on chromosome 5 cause decreased levels of the SMN protein in subjects with proximal autosomal recessive spinal muscular atrophy (SMA), the exact sequence of pathological events leading to selective motoneuron cell death is not fully understood yet. In this review, new findings regarding the dual cellular role of the SMN protein (translocation of beta-actin to axonal growth cones and snRNP biogenesis/pre-mRNA splicing) were integrated with recent data obtained by detailed neuropathological examination of SMA and control subjects. A presumptive series of 10 pathogenetic events for SMA is proposed as follows: (1) deletions or mutations of the SMN1 gene, (2) increased SMN mRNA decay and reduction in full-length functional SMN protein, (3) impaired motoneuron axono- and dendrogenesis, (4) failure of motoneurons to form synapses with corticospinal fibers from upper motoneurons, (5) abnormal motoneuron migration towards ventral spinal roots, (6) inappropriate persistence of motoneuron apoptosis due to impaired differentiation and motoneuron displacement, (7) substantial numbers of motoneurons continuing to migrate abnormally ("heterotopic motoneurons") and entering into the ventral roots, (8) attracted glial cells following these heterotopic motoneurons, which form the glial bundles of ventral roots, (9) impaired axonal transport of actin, causing remaining motoneurons to become chromatolytic, and (10) eventual death of all apoptotic, heterotopic and chromatolytic neurons, with apoptosis being more rapid and predominating in the earlier stages, with death of heterotopic and chromatolytic neurons occurring more slowly by necrosis during the later stages of SMA. According to this model, the motoneuron axonopathy is more important for pathogenesis than the ubiquitous nuclear splicing deficit. It is also supposed that individually variable levels of SMN protein, together with influences of other phenotype modifier genes and their products, cause the clinical SMA spectrum through differential degree of motoneuron functional loss.