Survival motor neuron (SMN) protein in rat is expressed as different molecular forms and is developmentally regulated

Biomarkers in 5q-associated spinal muscular atrophy-a narrative review

5q-associated spinal muscular atrophy (SMA) is a rare genetic disease caused by mutations in the SMN1 gene, resulting in a loss of functional SMN protein and consecutive degeneration of motor neurons in the ventral horn. The disease is clinically characterized by proximal paralysis and secondary skeletal muscle atrophy. New disease-modifying drugs driving SMN gene expression have been developed in the past decade and have revolutionized SMA treatment. The rise of treatment options led to a concomitant need of biomarkers for therapeutic guidance and an improved disease monitoring. Intensive efforts have been undertaken to develop suitable markers, and numerous candidate biomarkers for diagnostic, prognostic, and predictive values have been identified. The most promising markers include appliance-based measures such as electrophysiological and imaging-based indices as well as molecular markers including SMN-related proteins and markers of neurodegeneration and skeletal muscle integrity. However, none of the proposed biomarkers have been validated for the clinical routine yet. In this narrative review, we discuss the most promising candidate biomarkers for SMA and expand the discussion by addressing the largely unfolded potential of muscle integrity markers, especially in the context of upcoming muscle-targeting therapies. While the discussed candidate biomarkers hold potential as either diagnostic (e.g., SMN-related biomarkers), prognostic (e.g., markers of neurodegeneration, imaging-based markers), predictive (e.g., electrophysiological markers) or response markers (e.g., muscle integrity markers), no single measure seems to be suitable to cover all biomarker categories. Hence, a combination of different biomarkers and clinical assessments appears to be the most expedient solution at the time.

A Deletion of the Nuclear Localization Signal Domain in the Fus Protein Induces Stable Post-stress Cytoplasmic Inclusions in SH-SY5Y Cells

Mutations in Fused-in-Sarcoma (FUS) gene involving the nuclear localization signal (NLS) domain lead to juvenile-onset Amyotrophic Lateral Sclerosis (ALS). The mutant protein mislocalizes to the cytoplasm, incorporating it into Stress Granules (SG). Whether SGs are the first step to the formation of stable FUS-containing aggregates is still unclear. In this work, we used acute and chronic stress paradigms to study the SG dynamics in a human SH-SY5Y neuroblastoma cell line carrying a deletion of the NLS domain of the FUS protein (homozygous: ΔNLS^–/–; heterozygous: ΔNLS^+/–). Wild-type (WT) cells served as controls. We evaluated the subcellular localization of the mutant protein through immunoblot and immunofluorescence, in basal conditions and after acute stress and chronic stress with sodium arsenite (NaAsO₂). Cells were monitored for up to 24 h after rescue. FUS was expressed in both nucleus and cytoplasm in the ΔNLS^+/– cells, whereas it was primarily cytoplasmic in the ΔNLS^–/–. Acute NaAsO₂ exposure induced SGs: at rescue,>90% of ΔNLS cells showed abundant FUS-containing if compared to less than 5% of the WT cells. The proportion of FUS-positive SGs remained 15–20% at 24 h in mutant cells. Cycloheximide did not abolish the long-lasting SGs in mutant cells. Chronic exposure to NaAsO₂ did not induce significant SGs formation. A wealth of research has demonstrated that ALS-associated FUS mutations at the C-terminus facilitate the incorporation of the mutant protein into SGs. We have shown here that mutant FUS-containing SGs tend to fail to dissolve after stress, facilitating a liquid-to-solid phase transition. The FUS-containing inclusions seen in the dying motor neurons might therefore directly derive from SGs. This might represent an attractive target for future innovative therapies.

Pathogenesis and therapeutic targets in spinal muscular atrophy (SMA)

Autosomal-recessive spinal muscular atrophy (SMA) is characterized by the loss of specific motor neurons of the spinal cord and skeletal muscle atrophy. SMA is caused by mutations or deletions of the survival motor neuron 1 (SMN1) gene, and disease severity correlates with the expression levels of the nearly identical copy gene, SMN2. Both genes ubiquitously express SMN protein, but SMN2 generates only low levels of protein that do not fully compensate for the loss-of-function of SMN1. SMN protein forms a multiprotein complex essential for the cellular assembly of ribonucleoprotein particles involved in diverse aspects of RNA metabolism. Other studies using animal models revealed a spatio-temporal requirement of SMN that is high during the development of the neuromuscular system and later, in the general maintenance of cellular and tissues homeostasis. These observations define a period for maximum therapeutic efficiency of SMN restoration, and suggest that cells outside the central nervous system may also participate in the pathogenesis of SMA. Finally, recent innovative therapies have been shown to mitigate SMN deficiency and have been approved to treat SMA patients. We briefly review major findings from the past twenty-five years of SMA research. © 2020 French Society of Pediatrics. Published by Elsevier Masson SAS. All rights reserved.

Update on Biomarkers in Spinal Muscular Atrophy

The availability of disease modifying therapies for spinal muscular atrophy (SMA) has created an urgent need to identify clinically meaningful biomarkers. Biomarkers present a means to measure and evaluate neurological disease across time. Changes in biomarkers provide insight into disease progression and may reveal biologic, physiologic, or pharmacologic phenomena occurring prior to clinical detection. Efforts to identify biomarkers for SMA, a genetic motor neuron disease characterized by motor neuron degeneration and weakness, have culminated in a number of putative molecular and physiologic markers that evaluate biological media (eg, blood and cerebrospinal fluid [CSF]) or nervous system function. Such biomarkers include SMN2 copy number, SMN mRNA and protein levels, neurofilament proteins (NFs), plasma protein analytes, creatine kinase (CK) and creatinine (Crn), and various electrophysiology and imaging measures. SMN2 copy number inversely correlates with disease severity and is the best predictor of clinical outcome in untreated individuals. SMN mRNA and protein are commonly measured in the blood or CSF of patients receiving SMA therapies, particularly those aimed at increasing SMN protein expression, and provide insight into current disease state. NFs have proven to be robust prognostic, disease progression, and pharmacodynamic markers for SMA infants undergoing treatment, but less so for adolescents and adults. Select plasma proteins are altered in SMA individuals and may track response to therapy. CK and Crn from blood correlate with motor function and disease severity status and are useful for predicting which individuals will respond to therapy. Electrophysiology measures comprise the most reliable means for monitoring motor function throughout disease course and are sensitive enough to detect neuromuscular changes before overt clinical manifestation, making them robust predictive and pharmacodynamic biomarkers. Finally, magnetic resonance imaging and muscle ultrasonography are non-invasive techniques for studying muscle structure and physiology and are useful diagnostic tools, but cannot reliably track disease progression. Importantly, biomarkers can provide information about the underlying mechanisms of disease as well as reveal subclinical disease progression, allowing for more appropriate timing and dosing of therapy for individuals with SMA. Recent therapeutic advancements in SMA have shown promising results, though there is still a great need to identify and understand the impact of biomarkers in modulating disease onset and progression.

25 years of the SMN genes: the Copernican revolution of spinal muscular atrophy

The new era of advanced therapies has influenced and changed the views and perspectives of a neuromuscular disease such as spinal muscular atrophy (SMA). Being an autosomal recessive motor neuron disorder, characterized by different degrees of muscle weakness, after 25 years of the discovery of the determinant and modifier genes (SMN1 and SMN2, respectively) three SMN-dependent specific therapies are already approved by FDA (two by EMA), so that worldwide patients are currently under clinical investigation and treatment. This success was the combined effort mainly of patients and families, physician and researchers, advocacy groups and several Institutions together with the support of pharmaceutical companies. Progression trajectories, phenotypes, follow-up and care of the patients are continously evolving. Clinical investigations are currently demonstrating that early diagnosis and intervention are essential for better and more effective response to treatment, consistently improving prognosis. This scenario has created the need for awareness, early diagnosis and even implementation of of newborn screening programs. New views and perspectives of patient and family expectations, genetic counselling and multidisciplinary care: a truly Copernican revolution in neuromuscular and genetic diseases.

Spinal Muscular Atrophy Modeling and Treatment Advances by Induced Pluripotent Stem Cells Studies

Spinal Muscular Atrophy (SMA) is a neurodegenerative disease characterized by specific and predominantly lower motor neuron (MN) loss. SMA is the main reason for infant death, while about one in 40 children born is a healthy carrier. SMA is caused by decreased levels of production of a ubiquitously expressed gene: the survival motor neuron (SMN). All SMA patients present mutations of the telomeric SMN1 gene, but many copies of a centromeric, partially functional paralog gene, SMN2, can somewhat compensate for the SMN1 deficiency, scaling inversely with phenotypic harshness. Because the study of neural tissue in and from patients presents too many challenges and is very often not feasible; the use of animal models, such as the mouse, had a pivotal impact in our understanding of SMA pathology but could not portray totally satisfactorily the elaborate regulatory mechanisms that are present in higher animals, particularly in humans. And while recent therapeutic achievements have been substantial, especially for very young infants, some issues should be considered for the treatment of older patients. An alternative way to study SMA, and other neurological pathologies, is the use of induced pluripotent stem cells (iPSCs) derived from patients. In this work, we will present a wide analysis of the uses of iPSCs in SMA pathology, starting from basic science to their possible roles as therapeutic tools.

Age-dependent SMN expression in disease-relevant tissue and implications for SMA treatment

BACKGROUNDSpinal muscular atrophy (SMA) is caused by deficient expression of survival motor neuron (SMN) protein. New SMN-enhancing therapeutics are associated with variable clinical benefits. Limited knowledge of baseline and drug-induced SMN levels in disease-relevant tissues hinders efforts to optimize these treatments.METHODSSMN mRNA and protein levels were quantified in human tissues isolated during expedited autopsies.RESULTSSMN protein expression varied broadly among prenatal control spinal cord samples, but was restricted at relatively low levels in controls and SMA patients after 3 months of life. A 2.3-fold perinatal decrease in median SMN protein levels was not paralleled by comparable changes in SMN mRNA. In tissues isolated from nusinersen-treated SMA patients, antisense oligonucleotide (ASO) concentration and full-length (exon 7 including) SMN2 (SMN2-FL) mRNA level increases were highest in lumbar and thoracic spinal cord. An increased number of cells showed SMN immunolabeling in spinal cord of treated patients, but was not associated with an increase in whole-tissue SMN protein levels.CONCLUSIONSA normally occurring perinatal decrease in whole-tissue SMN protein levels supports efforts to initiate SMN-inducing therapies as soon after birth as possible. Limited ASO distribution to rostral spinal and brain regions in some patients likely limits clinical response of motor units in these regions for those patients. These results have important implications for optimizing treatment of SMA patients and warrant further investigations to enhance bioavailability of intrathecally administered ASOs.FUNDINGSMA Foundation, SMART, NIH (R01-NS096770, R01-NS062869), Ionis Pharmaceuticals, and PTC Therapeutics. Biogen provided support for absolute real-time RT-PCR.

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

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

Unraveling the Pathways to Neuronal Homeostasis and Disease: Mechanistic Insights into the Role of RNA-Binding Proteins and Associated Factors

The timing, dosage and location of gene expression are fundamental determinants of brain architectural complexity. In neurons, this is, primarily, achieved by specific sets of trans-acting RNA-binding proteins (RBPs) and their associated factors that bind to specific cis elements throughout the RNA sequence to regulate splicing, polyadenylation, stability, transport and localized translation at both axons and dendrites. Not surprisingly, misregulation of RBP expression or disruption of its function due to mutations or sequestration into nuclear or cytoplasmic inclusions have been linked to the pathogenesis of several neuropsychiatric and neurodegenerative disorders such as fragile-X syndrome, autism spectrum disorders, spinal muscular atrophy, amyotrophic lateral sclerosis and frontotemporal dementia. This review discusses the roles of Pumilio, Staufen, IGF2BP, FMRP, Sam68, CPEB, NOVA, ELAVL, SMN, TDP43, FUS, TAF15, and TIA1/TIAR in RNA metabolism by analyzing their specific molecular and cellular function, the neurological symptoms associated with their perturbation, and their axodendritic transport/localization along with their target mRNAs as part of larger macromolecular complexes termed ribonucleoprotein (RNP) granules.

Smn-Deficiency Increases the Intrinsic Excitability of Motoneurons

During development, motoneurons experience significant changes in their size and in the number and strength of connections that they receive, which requires adaptive changes in their passive and active electrical properties. Even after reaching maturity, motoneurons continue to adjust their intrinsic excitability and synaptic activity for proper functioning of the sensorimotor circuit in accordance with physiological demands. Likewise, if some elements of the circuit become dysfunctional, the system tries to compensate for the alterations to maintain appropriate function. In Spinal Muscular Atrophy (SMA), a severe motor disease, spinal motoneurons receive less excitation from glutamatergic sensory fibers and interneurons and are electrically hyperexcitable. Currently, the origin and relationship among these alterations are not completely established. In this study, we investigated whether Survival of Motor Neuron (SMN), the ubiquitous protein defective in SMA, regulates the excitability of motoneurons before and after the establishment of the synaptic contacts. To this end, we performed patch-clamp recordings in embryonic spinal motoneurons forming complex synaptic networks in primary cultures, and in differentiated NSC-34 motoneuron-like cells in the absence of synaptic contacts. Our results show that in both conditions, Smn-deficient cells displayed lower action potential threshold, greater action potential amplitudes, and larger density of voltage-dependent sodium currents than cells with normal Smn-levels. These results indicate that Smn participates in the regulation of the cell-autonomous excitability of motoneurons at an early stage of development. This finding may contribute to a better understanding of motoneuron excitability in SMA during the development of the disease.

Decay in survival motor neuron and plastin 3 levels during differentiation of iPSC-derived human motor neurons

Spinal muscular atrophy (SMA) is a neuromuscular disease caused by mutations in Survival Motor Neuron 1 (SMN1), leading to degeneration of alpha motor neurons (MNs) but also affecting other cell types. Induced pluripotent stem cell (iPSC)-derived human MN models from severe SMA patients have shown relevant phenotypes. We have produced and fully characterized iPSCs from members of a discordant consanguineous family with chronic SMA. We differentiated the iPSC clones into ISL-1+/ChAT+ MNs and performed a comparative study during the differentiation process, observing significant differences in neurite length and number between family members. Analyses of samples from wild-type, severe SMA type I and the type IIIa/IV family showed a progressive decay in SMN protein levels during iPSC-MN differentiation, recapitulating previous observations in developmental studies. PLS3 underwent parallel reductions at both the transcriptional and translational levels. The underlying, progressive developmental decay in SMN and PLS3 levels may lead to the increased vulnerability of MNs in SMA disease. Measurements of SMN and PLS3 transcript and protein levels in iPSC-derived MNs show limited value as SMA biomarkers.

Impaired Muscle Mitochondrial Biogenesis and Myogenesis in Spinal Muscular Atrophy

IMPORTANCE

The important depletion of mitochondrial DNA (mtDNA) and the general depression of mitochondrial respiratory chain complex levels (including complex II) have been confirmed, implying an increasing paucity of mitochondria in the muscle from patients with types I, II, and III spinal muscular atrophy (SMA-I, -II, and -III, respectively).

OBJECTIVE

To investigate mitochondrial dysfunction in a large series of muscle biopsy samples from patients with SMA.

DESIGN, SETTING, AND PARTICIPANTS

We studied quadriceps muscle samples from 24 patients with genetically documented SMA and paraspinal muscle samples from 3 patients with SMA-II undergoing surgery for scoliosis correction. Postmortem muscle samples were obtained from 1 additional patient. Age-matched controls consisted of muscle biopsy specimens from healthy children aged 1 to 3 years who had undergone analysis for suspected myopathy. Analyses were performed at the Neuromuscular Unit, Istituto di Ricovero e Cura a Carattere Scientifico Foundation Ca' Granda Ospedale Maggiore Policlinico-Milano, from April 2011 through January 2015.

EXPOSURES

We used histochemical, biochemical, and molecular techniques to examine the muscle samples.

MAIN OUTCOMES AND MEASURES

Respiratory chain activity and mitochondrial content.

RESULTS

Results of histochemical analysis revealed that cytochrome-c oxidase (COX) deficiency was more evident in muscle samples from patients with SMA-I and SMA-II. Residual activities for complexes I, II, and IV in muscles from patients with SMA-I were 41%, 27%, and 30%, respectively, compared with control samples (P < .005). Muscle mtDNA content and cytrate synthase activity were also reduced in all 3 SMA types (P < .05). We linked these alterations to downregulation of peroxisome proliferator-activated receptor coactivator 1α, the transcriptional activators nuclear respiratory factor 1 and nuclear respiratory factor 2, mitochondrial transcription factor A, and their downstream targets, implying depression of the entire mitochondrial biogenesis. Results of Western blot analysis confirmed the reduced levels of the respiratory chain subunits that included mitochondrially encoded COX1 (47.5%; P = .004), COX2 (32.4%; P < .001), COX4 (26.6%; P < .001), and succinate dehydrogenase complex subunit A (65.8%; P = .03) as well as the structural outer membrane mitochondrial porin (33.1%; P < .001). Conversely, the levels of expression of 3 myogenic regulatory factors-muscle-specific myogenic factor 5, myoblast determination 1, and myogenin-were higher in muscles from patients with SMA compared with muscles from age-matched controls (P < .05).

CONCLUSIONS AND RELEVANCE

Our results strongly support the conclusion that an altered regulation of myogenesis and a downregulated mitochondrial biogenesis contribute to pathologic change in the muscle of patients with SMA. Therapeutic strategies should aim at counteracting these changes.

More than a bystander: the contributions of intrinsic skeletal muscle defects in motor neuron diseases

Spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS), and spinal-bulbar muscular atrophy (SBMA) are devastating diseases characterized by the degeneration of motor neurons. Although the molecular causes underlying these diseases differ, recent findings have highlighted the contribution of intrinsic skeletal muscle defects in motor neuron diseases. The use of cell culture and animal models has led to the important finding that muscle defects occur prior to and independently of motor neuron degeneration in motor neuron diseases. In SMA for instance, the muscle specific requirements of the SMA disease-causing gene have been demonstrated by a series of genetic rescue experiments in SMA models. Conditional ALS mouse models expressing a muscle specific mutant SOD1 gene develop atrophy and muscle degeneration in the absence of motor neuron pathology. Treating SBMA mice by over-expressing IGF-1 in a skeletal muscle-specific manner attenuates disease severity and improves motor neuron pathology. In the present review, we provide an in depth description of muscle intrinsic defects, and discuss how they impact muscle function in these diseases. Furthermore, we discuss muscle-specific therapeutic strategies used to treat animal models of SMA, ALS, and SBMA. The study of intrinsic skeletal muscle defects is crucial for the understanding of the pathophysiology of these diseases and will open new therapeutic options for the treatment of motor neuron diseases.

Solving the puzzle of spinal muscular atrophy: what are the missing pieces?

Spinal muscular atrophy (SMA) is an autosomal recessive, lower motor neuron disease. Clinical heterogeneity is pervasive: three infantile (type I-III) and one adult-onset (type IV) forms are recognized. Type I SMA is the most common genetic cause of death in infancy and accounts for about 50% of all patients with SMA. Most forms of SMA are caused by mutations of the survival motor neuron (SMN1) gene. A second gene that is 99% identical to SMN1 (SMN2) is located in the same region. The only functionally relevant difference between the two genes identified to date is a C → T transition in exon 7 of SMN2, which determines an alternative spliced isoform that predominantly excludes exon 7. Thus, SMN2 genes do not produce sufficient full length SMN protein to prevent the onset of the disease. Since the identification of the causative mutation, biomedical research of SMA has progressed by leaps and bounds: from clues on the function of SMN protein, to the development of different models of the disease, to the identification of potential treatments, some of which are currently in human trials. The aim of this review is to elucidate the current state of knowledge, emphasizing how close we are to the solution of the puzzle that is SMA, and, more importantly, to highlight the missing pieces of this puzzle. Filling in these gaps in our knowledge will likely accelerate the development and delivery of efficient treatments for SMA patients and be a prerequisite towards achieving our final goal, the cure of SMA.

Applicability of histone deacetylase inhibition for the treatment of spinal muscular atrophy

Spinal muscular atrophy (SMA), a neurodegenerative disease with potentially devastating and even deadly effects on affected individuals, was first described in the late nineteenth century. Although the survival of motor neuron (SMN) gene was identified nearly 2 decades ago to be causative of the disease, neither an effective treatment nor a cure are currently available. Yet efforts are on-going to test a multitude of treatment strategies with the potential to alleviate disease symptoms in human and clinical trials. Among the most studied compounds for the treatment of SMA are histone deacetylase inhibitors. Several of these epigenetic modifiers have been shown to increase expression of the crucial SMN gene in vitro and in vivo, an effect linked to increased histone acetylation and remodeling of the chromatin landscape surrounding the SMN gene promoter. Here, we review the history and current state of use of histone deacetylase inhibitors in SMA, as well as the success of clinical trials investigating the clinical applicability of these epigenetic modifiers in SMA treatment.

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

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

Aberrant splicing in neurological diseases

Splicing of precursor messenger RNA (pre-mRNA) removes the intervening sequences (introns) and joins the expressed regions (exons) in the nucleus, before an intron-containing eukaryotic mRNA transcript can be exported and translated into proteins in the cytoplasm. While some sequences are always included or excluded (constitutive splicing), others can be selectively used (alternative splicing) in this process. Particularly by alternative splicing, up to tens of thousands of variant transcripts can be produced from a single gene, which contributes greatly to the proteomic diversity for such complex cellular functions as 'wiring' neurons in the nervous system. Disruption of this process leads to aberrant splicing, which accounts for the defects of up to 50% of mutations that cause certain human genetic diseases. In this review, we describe the different mechanisms of aberrant splicing that cause or have been associated with neurological diseases.

The neuroprotective drug riluzole acts via small conductance Ca2+-activated K+ channels to ameliorate defects in spinal muscular atrophy models

Spinal muscular atrophy (SMA), a recessive neuromuscular disorder, is caused by diminished function of the Survival Motor Neuron (SMN) protein. To define the cellular processes pertinent to SMA, parallel genetic screens were undertaken in Drosophila and Caenorhabditis elegans SMA models to identify modifiers of the SMN loss of function phenotypes. One class of such genetic modifiers was the small conductance, Ca(2+)-activated K(+) (SK) channels. SK channels allow efflux of potassium ions when intracellular calcium increases and can be activated by the neuroprotective drug riluzole. The latter is the only drug with proven, albeit modest, efficacy in the treatment of amyotrophic lateral sclerosis. It is unclear if riluzole can extend life span or ameliorate symptoms in SMA patients as previous studies were limited and of insufficient power to draw any conclusions. The critical biochemical target of riluzole in motor neuron disease is not known, but the pharmacological targets of riluzole include SK channels. We examine here the impact of riluzole in two different SMA models. In vertebrate neurons, riluzole treatment restored axon outgrowth caused by diminished SMN. Additionally, riluzole ameliorated the neuromuscular defects in a C. elegans SMA model and SK channel function was required for this beneficial effect. We propose that riluzole improves motor neuron function by acting on SK channels and suggest that SK channels may be important therapeutic targets for SMA patients.

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.

Co-regulation of survival of motor neuron and Bcl-xL expression: implications for neuroprotection in spinal muscular atrophy

Spinal muscular atrophy (SMA), a fatal genetic motor disorder of infants, is caused by diminished full-length survival of motor neuron (SMN) protein levels. Normally involved in small nuclear ribonucleoprotein (snRNP) assembly and pre-mRNA splicing, recent studies suggest that SMN plays a critical role in regulating apoptosis. Interestingly, the anti-apoptotic Bcl-x isoform, Bcl-xL, is reduced in SMA. In a related finding, Sam68, an RNA-binding protein, was found to modulate splicing of SMN and Bcl-xL transcripts, promoting SMNΔ7 and pro-apoptotic Bcl-xS transcripts. Here we demonstrate that Bcl-xL expression increases SMN protein by ∼2-fold in SH-SY5Y cells. Conversely, SMN expression increases Bcl-xL protein levels by ∼6-fold in SH-SY5Y cells, and ∼2.5-fold in the brains of transgenic mice over-expressing SMN (PrP-SMN). Moreover, Sam68 protein levels were markedly reduced following SMN and Bcl-xL expression in SH-SY5Y cells, suggesting a feedback mechanism co-regulating levels of both proteins. We also found that exogenous SMN expression increased full-length SMN transcripts, possibly by promoting exon 7 inclusion. Finally, co-expression of SMN and Bcl-xL produced an additive anti-apoptotic effect following PI3-kinase inhibition in SH-SY5Y cells. Our findings implicate Bcl-xL as another potential target in SMA therapeutics, and indicate that therapeutic increases in SMN may arise from modest increases in total SMN.

Partial restoration of cardio-vascular defects in a rescued severe model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a leading genetic cause of infantile death. Loss of a gene called Survival Motor Neuron 1 (SMN1) and, as a result, reduced levels of the Survival Motor Neuron (SMN) protein leads to SMA development. SMA is characterized by the loss of functional motor neurons in the spinal cord. However, accumulating evidence suggests the contribution of other organs to the composite SMA phenotype and disease progression. A growing number of congenital heart defects have been identified in severe SMA patients. Consistent with the clinical cases, we have recently identified developmental and functional heart defects in two SMA mouse models, occurring at embryonic stage in a severe SMA model and shortly after birth in a less severe model (SMN∆7). Our goal was to examine the late stage cardiac abnormalities in untreated SMN∆7 mice and to determine whether gene replacement therapy restores cardiac structure/function in rescued SMN∆7 model. To reveal the extent of the cardiac structural/functional repair in the rescued mice, we analyzed the heart of untreated and treated SMN∆7 model using self-complementary Adeno-associated virus (serotype 9) expressing the full-length SMN cDNA. We examined the characteristics of the heart failure such as remodeling, fibrosis, oxidative stress, and vascular integrity in both groups. Our results clearly indicate that fibrosis, oxidative stress activation, vascular remodeling, and a significant decrease in the number of capillaries exist in the SMA heart. The cardiac structural defects were improved drastically in the rescued animals, however, the level of impairment was still significant compared to the age-matched wildtype littermates. Furthermore, functional analysis by in vivo cardiac magnetic resonance imaging (MRI) revealed that the heart of the treated SMA mice still exhibits functional defects. In conclusion, cardiac abnormalities are only partially rescued in post-birth treated SMA animals and these abnormalities may contribute to the premature death of vector-treated SMA animals with seemingly rescued motor function but an average life span of less than 70 days as reported in several studies.

New insights into the pathogenesis of spinal muscular atrophy

To clarify the pathomechanism of spinal muscular atrophy (SMA) with mutations in the gene for survival motor neuron (SMN) protein, postmortem neuropathological analyses were performed on spinal cords obtained at autopsy from 2 fetuses with SMA, 5 infants and a low teenager with SMA type 1, and a higher teenager with SMA type 2; the diagnosis of all of them was confirmed clinically and genetically. Histopathologically, it was noted that lower motor neurons (LMNs) in the SMA cases showed immature profiles characterized by fine Nissl bodies restricted to the periphery of small round somata with a few cell processes in the fetal period, and showed small-sized profiles in the postnatal period. LMNs began to reduce in size and number in the fetal period, ballooned neurons (BNs) appeared postnatally, and the remaining LMNs including BNs diminished with age. BNs were filled with phosphorylated neurofilament protein, and morphologically similar to but smaller than typical chromatolytic neurons as axonal reaction. The population of survived LMNs was relatively preserved in an SMA type 2 case, who lived to 17-year-old, as compared to SMA type 1 cases. Immunohistochemical analysis demonstrated expression of Bcl-2, Bax, activated caspase-3 and SMN in the LMNs prominent in the fetal cases. There was no significant difference in staining for these substances between the control and SMA cases. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assay revealed no significant signal in the control and SMA cases. Given that downregulation of SMN leads to a failure in neurite outgrowth and neuromuscular contact of LMNs, the present results suggest the involvement of a fetal developmental maturation error as well as a postnatal retrograde dying-back degeneration of LMNs in SMN-mutated SMA.

Reduced levels of survival motor neuron protein leads to aberrant motoneuron growth in a Xenopus model of muscular atrophy

Spinal muscular atrophy (SMA) is a neurodegenerative disease characterized by motor neuron loss and skeletal muscle atrophy. The loss of function of the smn1 gene, the main supplier of survival motor neuron protein (SMN) protein in human, leads to reduced levels of SMN and eventually to SMA. Here, we ask if the amphibian Xenopus tropicalis can be a good model system to study SMA. Inhibition of the production of SMN using antisense morpholinos leads to caudal muscular atrophy in tadpoles. Of note, early developmental patterning of muscles and motor neurons is unaffected in this system as well as acetylcholine receptors clustering. Muscular atrophy seems to rather result from aberrant pathfinding and growth arrest and/or shortening of motor axons. This event occurs in the absence of neuronal cell bodies apoptosis, a process comparable to that of amyotrophic lateral sclerosis. Xenopus tropicalis is revealed as a complementary animal model for the study of SMA.

Induction of full-length survival motor neuron by polyphenol botanical compounds

The loss of survival motor neuron-1 (SMN1) is responsible for the development of the neurodegenerative disorder spinal muscular atrophy (SMA). A nearly identical copy of SMN1 is present on the same chromosomal region called SMN2. While SMN2 encodes a normal SMN protein, the majority of SMN2-derived transcripts are alternatively spliced, resulting in a truncated protein that lacks the 16 amino acids encoded by SMN exon 7. Numerous studies have shown that the SMN2-derived protein product, called SMNDelta7, is unstable and dysfunctional. Therefore, identifying molecules that stimulate full-length SMN expression from the SMN2 gene could lead to the development of effective therapies for a broad range of SMA patient populations. Polyphenol compounds have been shown to provide benefit in varied genetic disease contexts. For example, epigallocatechin galate (EGCG) was found to correct aberrant alternative mRNA splicing in familiar dysautonomia (FD). A series of polyphenols were screened and a subset was shown to increase full-length SMN expression from SMN2. Curcumin, EGCG, and resveratrol increased exon 7 inclusion of SMN2 transcripts in transient reporter assays. In SMA patient fibroblasts, these compounds stimulated the production of full-length SMN RNA and protein as well as the formation of SMN-containing nuclear gems. Collectively, these compounds elevated total SMN concentrations in SMA patient fibroblasts, potentially through the modulation of SMN2 exon 7 alternative splicing.

Molecular mechanisms of spinal muscular atrophy

Significant strides have been made during the past decade in the understanding of the molecular mechanisms that lead to the autosomal recessive motor neuron disease spinal muscular atrophy. Genetic studies revealed that spinal muscular atrophy is caused by mutation of the telomeric copy of the survival motor neuron gene ( SMN1), with all patients retaining at least one copy of the centromeric form of the gene, SMN2. SMN2 produces reduced amounts of full-length SMN messenger ribonucleic acid because of alterative splicing of SMN2 -derived transcripts, a process that is governed by specific cisand trans-acting factors. The resulting insufficient expression level of full-length SMN protein likely causes the disease manifestations of spinal muscular atrophy; however, the mechanism for the selective vulnerability of the motor unit to deficiency of this ubiquitously expressed protein remains unknown. It also remains unclear specifically when and where in the motor unit SMN is required. Despite the remaining questions, progress has been made in developing therapeutic strategies targeted to specific points along the pathogenetic pathway of spinal muscular atrophy. Histone deacetylase inhibitors will be discussed as an example.

Expression and Developmental Regulation of the Cystine/Glutamate Exchanger (x c − ) in the Rat

The cystine/glutamate exchanger (antiporter x (c) (-) ) is a membrane transporter involved in the uptake of cystine, the rate-limiting amino acid in the synthesis of glutathione. Recent studies suggest that the antiporter plays a role in the slow oxidative excitotoxity and in the pathological effects of beta-N-oxalylamino-L: -alanine, the molecule responsible for neurolathyrism, a neurotoxic upper motor neuron disease. The mouse cystine/glutamate exchanger has been cloned and showed to be composed of two distinct proteins, one of which being a novel protein, named xCT, of 502 amino acids and 12 putative trans-membrane domains. We have generated and purified a polyclonal antibody to mouse xCT and studied its expression in rat brain and in different cultured cells (astrocytes, fibroblasts and neurons) using Western blot and immunocytochemical techniques. Expression of xCT was also studied in rat brain and muscle at different developmental stages. Parallel experiments were carried out with antibodies to the heavy chain of 4F2 surface antigen, the non-specific subunit of the antiporter x (c) (-) . xCT antibody detected in all cell and tissue extracts a specific band of about 40 kDa. Subcellular fractionation demonstrated that xCT is concentrated mainly in the microsomal-mitochondrial fraction, in accord with its structure as transmembrane protein. Immunocytochemical analysis showed a strong staining in all cells examined, included neurons. Furthermore, both xCT and the heavy chain of 4F2 surface antigen increased in the brain during development, reaching the highest expression in adulthood. The study of the expression and developmental profile of xCT represents a first step towards a better characterization of its biochemical properties and function, which in turn may help to understand the relative contribution of the x (c) (-) antiporter in the pathogenesis of certain neurodegenerative diseases.

SMN, the product of the spinal muscular atrophy-determining gene, is expressed widely but selectively in the developing human forebrain

The expression pattern of the survival motor neuron (SMN) protein has been investigated immunohistochemically in the human fetal forebrain from 14 to 38 weeks of gestation. Mutations in the SMN gene cause spinal muscular atrophy (SMA), an autosomal recessive disease characterized by degeneration of lower motor neurons in the spinal cord leading to progressive muscle wasting. SMN is a multifunctional protein and has been implicated in diverse cytoplasmic and nuclear processes. The monoclonal murine SMN antibody used in this study recognized a major band at approximately 34 kDa. In spinal cord anterior horn motor neurons at 13 weeks of gestation, the soma, proximal neurites, and nucleus were immunostained. In the nucleus, SMN immunolabeling was observed at the nuclear membrane, at the nucleolus, and at dot-like structures in the nucleoplasm likely to be coiled bodies and gems. In the fetal forebrain, SMN was immunodetected as early as 14 weeks of gestation. From 14 to 24 weeks of gestation, intense immunostaining was observed in the basal nucleus of Meynert, a major source of cholinergic afferents to the cortex. Less intensely labeled cells at lower packing density were also observed in the thalamus, reticular and perireticular nucleus, globus pallidus, hippocampus, amygdala, and enthorinal cortex. Immunolabeled cells were still detectable at 38 gestational weeks, the latest time point investigated. These findings provide an anatomical basis for future investigations of SMN functions during brain development and for the neuropathological characterization of severe SMA cases.

Motoneuron injury and repair: New perspectives on gonadal steroids as neurotherapeutics

In this review, we will summarize recent work from our laboratory on the role of gonadal steroids as neuroprotective agents in motoneuron viability following cell stress. Three motoneuron models will be discussed: developing axotomized hamster facial motoneurons (FMNs); adult axotomized mouse FMNs; and immortalized, cultured mouse spinal motoneurons subjected to heat shock. New work on two relevant motoneuron proteins, the survival of motor neuron protein, and neuritin or candidate plasticity-related gene 15, indicates differential steroid regulation of these two proteins after axotomy. The concept of gonadal steroids as cellular stress correction factors and the implications of this for acute neurological injury situations will be presented as well.

Distinct domains of the spinal muscular atrophy protein SMN are required for targeting to Cajal bodies in mammalian cells

Mutations of the survival motor neuron gene SMN1 cause the inherited disease spinal muscular atrophy (SMA). The ubiquitous SMN protein facilitates the biogenesis of spliceosomal small nuclear ribonucleoproteins (snRNPs). The protein is detected in the cytoplasm, nucleoplasm and enriched with snRNPs in nuclear Cajal bodies. It is structurally divided into at least an amino-terminal region rich in basic amino acid residues, a central Tudor domain, a self-association tyrosine-glycine-box and an exon7-encoded C-terminus. To examine the domains required for the intranuclear localization of SMN, we have used fluorescently tagged protein mutants transiently overexpressed in mammalian cells. The basic amino acid residues direct nucleolar localization of SMN mutants. The Tudor domain promotes localization of proteins in the nucleus and it cooperates with the basic amino acid residues and the tyrosine-glycine-box for protein localization in Cajal bodies. Moreover, the most frequent disease-linked mutant SMNΔex7 reduces accumulation of snRNPs in Cajal bodies, suggesting that the C-terminus of SMN participates in targeting to Cajal bodies. A reduced number of Cajal bodies in patient fibroblasts associates with the absence of snRNPs in Cajal bodies, revealing that intranuclear snRNA organization is modified in disease. These results indicate that direct and indirect mechanisms regulate localization of SMN in Cajal bodies.

Is spinal muscular atrophy the result of defects in motor neuron processes?

The hereditary neurodegenerative disease spinal muscular atrophy (SMA) with childhood onset is one of the most common genetic causes of infant mortality. The disease is characterized by selective loss of spinal cord motor neurons leading to muscle atrophy and is the result of mutations in the survival motor neuron (SMN) gene. The SMN protein has been implicated in diverse nuclear processes including splicing, ribosome formation and gene transcription. Even though the genetic basis of SMA is well understood, it is not clear how defects in these ubiquitous processes result in motor neuron degeneration leaving other tissues unaffected. Recent evidence from animal and cell culture models of SMA points to roles for SMN in neurite outgrowth and axonal transport. Disruption of these functions might be particularly detrimental to motor neurons given their high metabolic demands and precise connectivity requirements, thus providing a possible explanation for the specificity of motor neuron susceptibility in SMA. Understanding the molecular mechanisms of SMN activity in neuronal processes may generate new targets for future therapeutic strategies.

Spinal muscular atrophy: a delayed development hypothesis

Spinal muscular atrophy is an inherited neuromuscular disorder. The gene responsible for the disease has been identified and named the SMN gene. This review is prompted by recent advances in understanding cellular function of the SMN gene and its gene product and by the increasing evidence that maturation of all parts of the neuromuscular system is delayed in spinal muscular atrophy patients. We suggest that the timing of developmental changes in motoneurons and muscles is critical for their survival. Delayed maturation of either motoneuron or muscle can cause these cells to die so the molecules that are involved in controlling their rate of maturation are crucial for normal development. We suggest that SMN gene/protein is one such molecule, because the neuromuscular system develops more slowly in spinal muscular atrophy patients, where SMN protein is absent, and in animals models, where SMN protein is reduced.

The role of histone acetylation in SMN gene expression

Increasing survival motor neuron 2 (SMN2) gene expression may be an effective strategy for the treatment of spinal muscular atrophy (SMA). Histone deacetylase (HDAC) inhibitors have been shown to increase SMN transcript and protein levels, but the specific role of histone acetylation in regulating SMN gene expression has not been explored. Using chromatin immunopreciptation, we investigated the levels of acetylated H3 and H4 histones and HDACs associated with different regions of the human and mouse SMN genes in both cultured cells and tissues. We show that the SMN gene has a reproducible pattern of histone acetylation that is largely conserved among different tissues and species. A limited region of the promoter surrounding the transcriptional start site has relatively high levels of histone acetylation, whereas regions further upstream or downstream have lower levels. After HDAC inhibitor treatment, acetylated histone levels increased, particularly at upstream regions, correlating with a 2-fold increase in promoter activity. During development in mouse tissues, histone acetylation levels decreased and associated HDAC2 levels increased at the region closest to the transcriptional start site, correlating with a 40-60% decrease in SMN transcript and protein levels. These data indicate that histone acetylation modulates SMN gene expression and that pharmacological manipulation of this epigenetic determinant is feasible. HDAC2, in particular, may be a future therapeutic target for SMA.

Post-translational modifications in the survival motor neuron protein

Spinal muscular atrophy (SMA) is an autosomal recessive disease characterized by a progressive loss of the spinal motoneurons. The SMA-determining gene has been termed survival motor neuron (SMN) and is deleted or mutated in over 98% of patients. The encoded gene product is a protein expressed as different isoforms. In particular, we showed that the rat SMN cDNA produces two isoforms with M(r) of 32 and 35kDa, both localized in nuclear coiled bodies, but the 32kDa form is also cytoplasmic, whereas the 35kDa form is also microsomal. To determine the molecular relationship between these two isoforms and potential post-translational modifications, we performed transfection experiments with a double-tagged rat SMN. Immunoblot and immunostaining studies demonstrated that the 32kDa SMN isoform derives from the full length 35kDa, through a proteolytic cleavage at the C-terminal. Furthermore, the 35kDa SMN isoform is physiologically phosphorylated in vivo. This may modulate its interaction with molecular partners, either proteins or nucleic acids.

The b1 isoform of protocadherin-gamma (Pcdhγ) interacts with the microtubule-destabilizing protein SCG10

Due to their structural characteristics and their diversity, the 22 members of the protocadherin-gamma (Pcdhgamma) family have been suggested to contribute to the establishment of specific connections in the nervous system. Here, we focus on a single isoform, Pcdhgamma-b1. Its expression is found in different brain regions and in developing spinal cord it is restricted to scattered cells, whereas all cells are labeled using an antibody that recognizes all Pcdhgamma isoforms. As a first step to understanding the signaling mechanisms downstream of Pcdhgamma, we identify the microtubule-destabilizing protein SCG10 as a cytoplasmic interactor for Pcdhgamma-b1 and other isoforms of the Pcdhgamma-b subfamily, and show that SCG10 and Pcdhgamma-b1 are found together in certain neuronal growth cones.

Altered mRNA expression of genes related to nerve cell activity in the fracture callus of older rats: A randomized, controlled, microarray study

Background

The time required for radiographic union following femoral fracture increases with age in both humans and rats for unknown reasons. Since abnormalities in fracture innervation will slow skeletal healing, we explored whether abnormal mRNA expression of genes related to nerve cell activity in the older rats was associated with the slowing of skeletal repair.

Methods

Simple, transverse, mid-shaft, femoral fractures with intramedullary rod fixation were induced in anaesthetized female Sprague-Dawley rats at 6, 26, and 52 weeks of age. At 0, 0.4, 1, 2, 4, and 6 weeks after fracture, a bony segment, one-third the length of the femur, centered on the fracture site, including the external callus, cortical bone, and marrow elements, was harvested. cRNA was prepared and hybridized to 54 Affymetrix U34A microarrays (3/age/time point).

Results

The mRNA levels of 62 genes related to neural function were affected by fracture. Of the total, 38 genes were altered by fracture to a similar extent at the three ages. In contrast, eight neural genes showed prolonged down-regulation in the older rats compared to the more rapid return to pre-fracture levels in younger rats. Seven genes were up-regulated by fracture more in the younger rats than in the older rats, while nine genes were up-regulated more in the older rats than in the younger.

Conclusions

mRNA of 24 nerve-related genes responded differently to fracture in older rats compared to young rats. This differential expression may reflect altered cell function at the fracture site that may be causally related to the slowing of fracture healing with age or may be an effect of the delayed healing.

Active transport of the survival motor neuron protein and the role of exon-7 in cytoplasmic localization

Spinal muscular atrophy (SMA) is a neurodegenerative disease caused by deletion and/or mutation of the survival motor neuron protein Gene (SMN1) that results in the expression of a truncated protein lacking the C terminal exon-7. Whereas SMN has been shown to be an important component of diverse ribonucleoprotein (RNP) complexes, its function in neurons is unknown. We hypothesize that the active transport of SMN may be important for neurite outgrowth and that disruption of exon-7 could impair its normal intracellular trafficking. SMN was localized in granules that were associated with cytoskeletal filament systems and distributed throughout neurites and growth cones. Live cell imaging of enhanced green fluorescent protein (EGFP)-SMN granules revealed rapid, bidirectional and cytoskeletal-dependent movements. Exon-7 was necessary for localization of SMN into the cytoplasm but was not sufficient for granule formation and transport. A cytoplasmic targeting signal within exon-7 was identified that could completely redistribute the nuclear protein D-box binding factor 1 into the cytoplasm. Neurons transfected with SMN lacking exon-7 had significantly shorter neurites, a defect that could be rescued by redirecting the exon-7 deletion mutant into neurites by a targeting sequence from growth-associated protein-43. These findings provide the first demonstration of cytoskeletal-based active transport of SMN in neuronal processes and the function of exon-7 in cytoplasmic localization. Such observations provide motivation to investigate possible transport defects or inefficiency of SMN associated RNPs in motor neuron axons in SMA.

The SMN genes are subject to transcriptional regulation during cellular differentiation

Proximal spinal muscular atrophy (SMA) is an autosomal recessive disease characterized by degeneration of alpha-motor neurons and muscular atrophy. The causal survival motor neuron (SMN) gene maps to a complex region of chromosome 5q13 harbouring an inverted duplication. Thus, there are two SMN genes, SMN1 and SMN2, but SMN1-deficiency alone causes SMA. In this study we demonstrate, for the first time, down-regulation of SMN promoter activity during cellular differentiation. Specifically, the minimal SMN promoter is four times more active in undifferentiated embryonal carcinoma P19 cells compared to cells treated with retinoic acid (RA) to initiate neuronal differentiation. This effect is mediated by sequences contained within the minimal core promoter that we have confined to the 257 nucleotides upstream of exon 1. We have identified seven regions that are highly conserved between the mouse and human SMN core promoters and this region contains the consensus sequence for a number of transcription factors. Most notably, AhR, HNF-3 and N-Oct3 have already been shown to respond to RA treatment of EC cells, while E47, HNF-3, MAZ, N-Oct3 and Pit-1a have been implicated in embryonic, muscle or neural development. In addition, we have mapped two strong transcription initiation sites upstream of SMN exon 1. The novel -79 site identified in this study is preferentially utilized during human foetal development. Furthermore, analysis of RNA from SMA patients with deletions of the entire SMN1 gene or chimpanzees that lack SMN2 suggests that the level of transcription initiation at these sites may be different for the SMN1 and SMN2 genes. Taken together, this work provides the first demonstration of transcriptional regulation of these genes during cellular differentiation and development. Deciphering the underlying mechanisms responsible for regulating SMN transcription may provide important clues towards enhancing SMN2 gene expression, one target for the treatment of SMA.

Death of motoneurons induced by trophic deprivation or by excitotoxicity is not prevented by overexpression of SMN

The telomeric copy of the survival motor neuron gene (SMN1) is deleted or mutated in all spinal muscular atrophy (SMA) patients and these patients present mainly a loss in spinal motoneurons. Although studies performed in HeLa cells suggest that SMN may be involved in the biogenesis and possibly in recycling of spliceosomal small nuclear ribonucleoproteins (snRNPs), no link has been established between this function and the consequence of the absence of SMN in the specific loss of motoneurons. We attempted to answer the question of whether SMN plays a direct role in motoneuron survival by transducing cultured motoneurons with lentiviral vectors coding either for an antisense Smn mRNA or for full-length or truncated forms of SMN. We studied their effect on survival under different anti- or proapoptotic culture conditions. Our results show that increased levels of SMN are unable to protect motoneurons from death induced by trophic deprivation or by excitotoxicity. These results suggest that SMN is not a survival factor per se for motoneurons. In addition, overexpression of a truncated form of SMN shown to induce a modified subcellular localization and to exert a dominant-negative effect on snRNP biogenesis and RNA splicing in HeLa cells was ineffective in modifying both localization and survival in motoneurons.

Expression and subcellular localization of two isoforms of the survival motor neuron protein in different cell types

The survival motor neuron (SMN) gene is deleted or mutated in over 98% of spinal muscular atrophy patients who show specific motoneuron loss. By performing transfection experiments with rat smn cDNA, we show that two isoforms of SMN with Mr of 32 kDa and 35 kDa are produced by the same cDNA. In cultured motoneurons, both forms colocalize in coiled bodies and not in GEMS bodies as shown for HeLa cells. Subcellular fractionation of cells acutely dissociated from rat embryonic ventral spinal cord shows that the two SMN isoforms have a different subcellular localization, namely, that the 32 kDa isoform is enriched in the cytosol, whereas the 35 kDa isoform is segregating in the microsomal fraction. We show that the 35 kDa isoform of SMN is part of an insoluble complex but is absent from the cytoplasmic membranes and from the mitochondria. Immunostaining studies show that neither SMN isoform colocalizes with Bcl-2, the mitochondrial antiapoptotic protein suggested to bind to SMN in HeLa cells. Our results show that the isoforms of SMN protein have different subcellular localization and may therefore play independent biological roles. Moreover, the absence of colocalization of SMN with Bcl-2 in motoneurons suggests that some of the interactors of SMN may vary depending on the cell type, and this underscores the importance of identifying motoneuron-specific SMN interactors.

A mouse model for spinal muscular atrophy

The survival motor neuron gene is present in humans in a telomeric copy, SMN1, and several centromeric copies, SMN2. Homozygous mutation of SMN1 is associated with proximal spinal muscular atrophy (SMA), a severe motor neuron disease characterized by early childhood onset of progressive muscle weakness. To understand the functional role of SMN1 in SMA, we produced mouse lines deficient for mouse Smn and transgenic mouse lines that expressed human SMN2. Smn-/- mice died during the peri-implantation stage. In contrast, transgenic mice harbouring SMN2 in the Smn-/- background showed pathological changes in the spinal cord and skeletal muscles similar to those of SMA patients. The severity of the pathological changes in these mice correlated with the amount of SMN protein that contained the region encoded by exon 7. Our results demonstrate that SMN2 can partially compensate for lack of SMN1. The variable phenotypes of Smn-/-SMN2 mice reflect those seen in SMA patients, providing a mouse model for this disease.

Programmed cell death of embryonic motoneurons triggered through the Fas death receptor

About 50% of spinal motoneurons undergo programmed cell death (PCD) after target contact, but little is known about how this process is initiated. Embryonic motoneurons coexpress the death receptor Fas and its ligand FasL at the stage at which PCD is about to begin. In the absence of trophic factors, many motoneurons die in culture within 2 d. Most (75%) of these were saved by Fas-Fc receptor body, which blocks interactions between Fas and FasL, or by the caspase-8 inhibitor tetrapeptide IETD. Therefore, activation of Fas by endogenous FasL underlies cell death induced by trophic deprivation. In the presence of neurotrophic factors, exogenous Fas activators such as soluble FasL or anti-Fas antibodies triggered PCD of 40-50% of purified motoneurons over the following 3-5 d; this treatment led to activation of caspase-3, and was blocked by IETD. Sensitivity to Fas activation is regulated: motoneurons cultured for 3 d with neurotrophic factors became completely resistant. Levels of Fas expressed by motoneurons varied little, but FasL was upregulated in the absence of neurotrophic factors. Motoneurons resistant to Fas activation expressed high levels of FLICE-inhibitory protein (FLIP), an endogenous inhibitor of caspase-8 activation. Our results suggest that Fas can act as a driving force for motoneuron PCD, and raise the possibility that active triggering of PCD may contribute to motoneuron loss during normal development and/or in pathological situations.

Spinal muscular atrophy: molecular pathophysiology

Spinal muscular atrophy is an autosomal recessive disease characterized by motor neurone loss, muscle atrophy and weakness. Deletion or mutation of the SMN1 gene reduces intracellular survival motor neurone protein levels causes spinal muscular atrophy, most likely by interfering with spliceosome assembly. A range of clinical severity and corresponding survival motor neurone levels is seen because of the presence of copies of the transcriptionally inefficient SMN2 gene and possibly other modifying genes. The delineation of SMN1 as the gene that causes spinal muscular atrophy and the identification of genes that modify spinal muscular atrophy raise the prospect of gene therapy or in-vivo gene activation treatment for this frequently fatal disorder.

Subcellular distribution of survival motor neuron (SMN) protein: possible involvement in nucleocytoplasmic and dendritic transport

Spinal muscular atrophy (SMA) is among the most common recessive autosomal diseases and is characterized by the loss of spinal motor neurons. A gene termed 'Survival of Motor Neurons' (SMN) has been identified as the SMA-determining gene. Recent work indicates the involvement of the SMN protein and its associated protein SIP1 in spliceosomal snRNP biogenesis. However, the function of SMN remains unknown. Here, we have studied the subcellular localization of SMN in the rat spinal cord and more generally in the central nervous system (CNS), by light fluorescence and electron microscopy. SMN immunoreactivity (IR) was found in the different regions of the spinal cord but also in various regions of the CNS such as the brainstem, cerebellum, thalamus, cortex and hippocampus. In most neurons, we observed a speckled labelling of the cytoplasm and a discontinuous staining of the nuclear envelope. For some neurons (e.g. brainstem nuclei, dentate gyrus, cortex: layer V) and, in particular in motoneurons, SMN-IR was also present as prominent nuclear dot-like-structures. In these nuclear dots, SMN colocalized with SIP1 and with fibrillarin, a marker of coiled bodies. Ultrastructural studies in the anterior horn of the spinal cord confirmed the presence of SMN in the coiled bodies and also revealed the protein at the external side of nuclear pores complexes, in association with polyribosomes, and in dendrites, associated with microtubules. These localizations suggest that, in addition to its involvement in the spliceosome biogenesis, the SMN protein could also play a part in nucleocytoplasmic and dendritic transport.