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

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.

Revisiting the role of mitochondria in spinal muscular atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive motor neuron disease of variable clinical severity that is caused by mutations in the survival motor neuron 1 (SMN1) gene. Despite its name, SMN is a ubiquitous protein that functions within and outside the nervous system and has multiple cellular roles in transcription, translation, and proteostatic mechanisms. Encouragingly, several SMN-directed therapies have recently reached the clinic, albeit this has highlighted the increasing need to develop combinatorial therapies for SMA to achieve full clinical efficacy. As a subcellular site of dysfunction in SMA, mitochondria represents a relevant target for a combinatorial therapy. Accordingly, we will discuss our current understanding of mitochondrial dysfunction in SMA, highlighting mitochondrial-based pathways that offer further mechanistic insights into the involvement of mitochondria in SMA. This may ultimately facilitate translational development of targeted mitochondrial therapies for SMA. Due to clinical and mechanistic overlaps, such strategies may also benefit other motor neuron diseases and related neurodegenerative disorders.

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.

Survival motor neuron protein protects H9c2 cardiomyocytes from hypoxia-induced cell injury by reducing apoptosis

BACKGROUND

Hypoxia induces cell injury in cardiomyocytes and leads to the development of cardiovascular diseases. The survival motor neuron protein (SMN) is a crucial ubiquitous protein whose functional deficiency causes motor neuron loss seen in spinal muscular atrophy. SMN has shown protective effects on the cardiovascular system and the aim of the present study was to investigate the cardioprotective effects of SMN on hypoxia-induced cell injury.

METHODS

Cobalt chloride (CoCl₂ ) was used to induce chemical hypoxia in H9c2 cardiomyocytes. Cell proliferation was determined by the MTT assay and the mRNA levels of SMN were evaluated by real-time polymerase chain reaction. The protein expression levels of SMN, hypoxia-inducible transcription factor 1α (HIF-1α), and apoptosis-related proteins, such as cytochrome c (Cyt c), B cell lymphoma-2 (Bcl-2), Bcl-2 associated X protein (Bax), and cleaved caspase-3 were evaluated by western blot analysis. Cell apoptosis was analysed using annexin V/propidium iodide (PI) staining.

RESULTS

Treatment with CoCl₂ significantly reduced H9c2 cell viability; the level of HIF-1α, which is a hypoxia-related indicator increased whereas the expression of SMN protein decreased. Hypoxia also induced cardiomyocyte apoptosis, indicated by reduced Bcl-2 expression and elevated cleaved caspase-3, Bax, and cytochrome c levels. Interestingly, SMN, which is a neuron protection factor, ameliorated CoCl₂ -induced cell damage by reducing cardiomyocyte apoptosis through upregulation of Bcl-2 and inhibition of cytochrome c, cleaved caspase-3, and Bax expression.

CONCLUSION

Survival motor neuron prevents hypoxia-induced cell apoptosis through inhibition of the mitochondrial apoptotic pathway, and thereby exerts a protective effect on H9c2 cardiomyocytes.

Ubiquitously Expressed Proteins and Restricted Phenotypes: Exploring Cell-Specific Sensitivities to Impaired tRNA Charging

Aminoacyl-tRNA synthetases (ARS) are ubiquitously expressed, essential enzymes that charge tRNA with cognate amino acids. Variants in genes encoding ARS enzymes lead to myriad human inherited diseases. First, missense alleles cause dominant peripheral neuropathy. Second, missense, nonsense, and frameshift alleles cause recessive multisystem disorders that differentially affect tissues depending on which ARS is mutated. A preponderance of evidence has shown that both phenotypic classes are associated with loss-of-function alleles, suggesting that tRNA charging plays a central role in disease pathogenesis. However, it is currently unclear how perturbation in the function of these ubiquitously expressed enzymes leads to tissue-specific or tissue-predominant phenotypes. Here, we review our current understanding of ARS-associated disease phenotypes and discuss potential explanations for the observed tissue specificity.

The role of survival motor neuron protein (SMN) in protein homeostasis

Ever since loss of survival motor neuron (SMN) protein was identified as the direct cause of the childhood inherited neurodegenerative disorder spinal muscular atrophy, significant efforts have been made to reveal the molecular functions of this ubiquitously expressed protein. Resulting research demonstrated that SMN plays important roles in multiple fundamental cellular homeostatic pathways, including a well-characterised role in the assembly of the spliceosome and biogenesis of ribonucleoproteins. More recent studies have shown that SMN is also involved in other housekeeping processes, including mRNA trafficking and local translation, cytoskeletal dynamics, endocytosis and autophagy. Moreover, SMN has been shown to influence mitochondria and bioenergetic pathways as well as regulate function of the ubiquitin-proteasome system. In this review, we summarise these diverse functions of SMN, confirming its key role in maintenance of the homeostatic environment of the cell.

ALS-Related Mutant FUS Protein Is Mislocalized to Cytoplasm and Is Recruited into Stress Granules of Fibroblasts from Asymptomatic FUS P525L Mutation Carriers

BACKGROUND

Amyotrophic lateral sclerosis (ALS) shows a strong genetic basis, with SOD1, FUS, TARDBP, and C9ORF72 being the genes most frequently involved. This has allowed identification of asymptomatic mutation carriers, which may be of help in understanding the molecular changes preceding disease onset.

OBJECTIVES

We studied the cellular expression of FUS protein and the effect of heat-shock- and dithiothreitol-induced stress in fibroblasts from FUS P525L mutation carriers, healthy controls, and patients with sporadic ALS.

METHODS

Western blots and immunocytochemistry were performed to study the subcellular localization of FUS protein. Control and stressed cells were double stained with FUS and the stress marker TIA-R.

RESULTS

Fibroblasts from healthy controls and sporadic ALS cases showed a prominent nuclear FUS expression. In the 2 FUS P525L mutation carriers, instead, most cells showed a protein localization in both nucleus and cytoplasm, or exclusively in the cytoplasm. Stress prompted the formation of cytoplasmic granules in all subjects and in sporadic ALS FUS mislocalization to the cytoplasm. Cytoplasmic FUS was recruited into stress granules, which showed a time-dependent decrease in all subjects. However, in the FUS P525L fibroblasts, the granules persisted longer, and they were more numerous than those detected in the cells from controls and sporadic ALS patients.

CONCLUSIONS

We show that in fibroblasts of FUS P525L mutation carriers, FUS mislocalized to the cytoplasm where it redistributed into stress granules with likely a dose effect, i.e. a higher number of cells with granules, which persist longer, than in controls and ALS cases. These data represent an early molecular change occurring before ALS onset, suggesting a transient preaggregative state.

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.

Systemic gene delivery in large species for targeting spinal cord, brain, and peripheral tissues for pediatric disorders

Adeno-associated virus type 9 (AAV9) is a powerful tool for delivering genes throughout the central nervous system (CNS) following intravenous injection. Preclinical results in pediatric models of spinal muscular atrophy (SMA) and lysosomal storage disorders provide a compelling case for advancing AAV9 to the clinic. An important translational step is to demonstrate efficient CNS targeting in large animals at various ages. In the present study, we tested systemically injected AAV9 in cynomolgus macaques, administered at birth through 3 years of age for targeting CNS and peripheral tissues. We show that AAV9 was efficient at crossing the blood–brain barrier (BBB) at all time points investigated. Transgene expression was detected primarily in glial cells throughout the brain, dorsal root ganglia neurons and motor neurons within the spinal cord, providing confidence for translation to SMA patients. Systemic injection also efficiently targeted skeletal muscle and peripheral organs. To specifically target the CNS, we explored AAV9 delivery to cerebrospinal fluid (CSF). CSF injection efficiently targeted motor neurons, and restricted gene expression to the CNS, providing an alternate delivery route and potentially lower manufacturing requirements for older, larger patients. Our findings support the use of AAV9 for gene transfer to the CNS for disorders in pediatric populations.

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.

Mitochondrial dysfunction in a neural cell model of spinal muscular atrophy

Mutations of the survival motor neuron (SMN) gene in spinal muscular atrophy (SMA) lead to anterior horn cell death. The cause is unknown, but motor neurons depend substantially on mitochondrial oxidative phosphorylation (OxPhos) for normal function. Therefore, mitochondrial parameters were analyzed in an SMA cell culture model using small interfering RNA (siRNA) transfection that decreased Smn expression in NSC-34 cells to disease levels. Smn siRNA knock-down resulted in 35% and 66% reduced Smn protein levels 48 and 72 hr posttransfection, respectively. ATP levels were reduced by 14% and 26% at 48 and 72 hr posttransfection, respectively, suggesting decreased ATP production or increased energy demand in neural cells. Smn knock-down resulted in increased mitochondrial membrane potential and increased free radical production. Changes in activity of cytochrome c oxidase (CcO), a key OxPhos component, were observed at 72 hr with a 26% increase in oxygen consumption. This suggests a compensatory activation of the aerobic pathway, resulting in increased mitochondrial membrane potentials, a condition known to lead to the observed increase in free radical production. Further testing suggested that changes in ATP at 24 hr precede observable indices of cell injury at 48 hr. We propose that energy paucity and increased mitochondrial free radical production lead to accumulated cell damage and eventual cell death in Smn-depleted neural cells. Mitochondrial dysfunction may therefore be important in SMA pathology and may represent a new therapeutic target.

Fugu rubripes and human survival motor neuron genes: structural and functional similarities in comparative genome studies

The compactness of the Fugu rubripes (Fugu) genome has supported its use in comparative genome analysis. Nevertheless, as Fugu is distinct evolution-wise from humans, it is essential to determine the similarity between a Fugu gene and its human counterpart to confirm its potential for comparative genome analysis. We cloned and analyzed the Fugu survival motor neuron gene (fsmn) for similarities with human SMN gene (huSMN). The Fugu genome has a single fsmn that is 13.4 times smaller than huSMN. fsmn and huSMN are highly similar in their genome organization and tissue expression patterns. The functional domains of the Fugu smn and human SMN molecules are also highly conserved. In human MCF-7 cells, expression of fsmn protein resulted in the formation of "gems" in the cytoplasm and nucleus, similar to observations reported for huSMN protein. In these cells, fsmn RNA was also processed correctly and produced alternatively spliced transcripts like huSMN2. These findings indicate close structural and functional similarities between fsmn and huSMN, suggesting that regulation of the two genes may also be similar and supporting the use of fsmn in comparative genome studies for the identification of functional regulatory elements of huSMN.

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.

Spinal muscular atrophy: present state

Spinal muscular atrophy (SMA) is a hereditary neurodegenerative disease caused by homozygous deletions or mutations in the SMN1 gene on Chr.5q13. SMA spans from severe Werdnig-Hoffmann disease (SMA 1) to relatively benign Kugelberg-Welander disease (SMA 3). Onset before birth possibly aggravates the clinical course, because immature motoneurons do not show compensatory sprouting and collateral reinnervation, and motor units in SMA 1, in contrast to those in SMA 3, are not enlarged. Genetic evidence indicates that SMN2, a gene 99% identical to SMN1, can attenuate SMA severity: in patients, more SMN2 copies and higher SMN protein levels are correlated with milder SMA. There is evidence that SMN plays a role in motoneuron RNA metabolism, but it has also been linked to apoptosis. Several mouse models with motoneuron disease have been successfully treated with neurotrophic factors. None of these models is, however, homologous to SMA. Recently, genetic mouse models of SMA have been created by introducing human SMN2 transgenes into Smn knockout mice or by targeting the Smn gene knockout to neurons. These mice not only provide important insights into the pathogenesis of SMA but are also crucial for testing new therapeutic strategies. These include SMN gene transfer, molecules capable to up-regulate SMN expression and trophic or antiapoptotic factors.

Fine-mapping and candidate gene analysis of bovine spinal muscular atrophy

Bovine spinal muscular atrophy (SMA), an autosomal recessive neurodegenerative disease, has been mapped at moderate resolution to the distal part of Chromosome 24. In this article we confirm this location and fine-map the SMA locus to an interval of approximately 0.8 cM at the very distal end of BTA24. Despite remarkable similarity to human SMA, the causative gene SMN can be excluded in bovine SMA. However, the interval where the disease now has been mapped contains BCL2, like SMN an antiapoptotic factor, and shown to bind to SMN. Moreover, knockout mice lacking the BCL2 gene show rapid motor neuron degeneration with early postnatal onset, as observed in bovine SMA. A comparative cattle/human map of the distal end of BTA24, based on the emerging bovine genome sequencing data, shows conserved synteny to HSA18 with hints of a segmental duplication and pericentric inversion just after the last available bovine marker DIK4971. This synteny lets us conclude that SMA is in immediate vicinity of the telomere. Candidate gene analysis of BCL2, however, excludes most of this gene, except its promoter region, and draws attention to the neighboring gene VPS4B, part of the endosomal protein-sorting machinery ESCRT-III which is involved in several neurodegenerative diseases.

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.

Degradation of survival motor neuron (SMN) protein is mediated via the ubiquitin/proteasome pathway

Homozygous deletion or mutation in the survival motor neuron (SMN)1 gene causes proximal spinal muscular atrophy (SMA), whereas SMN2 acts as a modifying gene that can influence the severity of SMA. It has been suggested that restoration of the SMN protein level in neuronal cells may prevent cell loss and may be helpful for treatment of SMA. Recent studies indicate that the ubiquitin/proteasome pathway is a major system for proteolysis of intracellular proteins. In this study, we investigate whether SMN protein is degraded via the ubiquitin/proteasome pathway. Primary fibroblasts were established from the skin biopsies of SMA patients and the effect of a proteasome inhibitor MG132 and lysosome inhibitor NH(4)Cl on SMN protein level was examined. We found that MG132, but not NH(4)Cl, significantly increased the amount and nuclear accumulation of SMN protein in SMA patient's fibroblasts. Immunoprecipitation/western blot analysis indicated that SMN protein was ubiquitinated in cells. In vitro protein ubiquitination assay also demonstrated that SMN protein could be conjugated with ubiquitin. Taken together, we have provided clear evidences that degradation of SMN protein is mediated via the ubiquitin/proteasome pathway and suggest that proteasome inhibitors may up-regulate SMN protein level and may be useful for the treatment of SMA.

Mapping of the bovine spinal muscular atrophy locus to Chromosome 24

A hereditary form of spinal muscular atrophy (SMA) caused by an autosomal recessive gene has been reported for American Brown-Swiss cattle and in advanced backcrosses between American Brown-Swiss and many European brown cattle breeds. Bovine SMA (bovSMA) bears remarkable resemblance to the human SMA (SMA1). Affected homozygous calves also show progressive symmetric weakness and neurogenic atrophy of proximal muscles. The condition is characterized by severe muscle atrophy, quadriparesis, and sternal recumbency as result of neurogenic atrophy. We report on the localization of the gene causing bovSMA within a genomic interval between the microsatellite marker URB031 and the telomeric end of bovine Chromosome (Chr) 24 (BTA24). Linkage analysis of a complex pedigree of German Braunvieh cattle revealed a recombination fraction of 0.06 and a three-point lod score of 11.82. The results of linkage and haplotyping analysis enable a marker-assisted selection against bovSMA based on four microsatellite markers most telomeric on BTA24 to a moderate accuracy of 89-94%. So far, this region is not orthologous to any human chromosome segments responsible for twelve distinct disease phenotypes of autosomal neuropathies. Our results indicate the apoptosis-inhibiting protein BCL2 as the most promising positional candidate gene causing bovSMA. Our findings offer an attractive animal model for a better understanding of human forms of SMA and for a probable anti-apoptotic synergy of SMN-BCL2 aggregates in mammals.

Characterization of an anti-apoptotic glycoprotein encoded by Kaposi's sarcoma-associated herpesvirus which resembles a spliced variant of human survivin

We have investigated the expression and function of a novel protein encoded by open reading frame (ORF) K7 of Kaposi's sarcoma-associated herpesvirus (KSHV). Computational analyses revealed that K7 is structurally related to survivin-DeltaEx3, a splice variant of human survivin that protects cells from apoptosis by an undefined mechanism. Both K7 and survivin-DeltaEx3 contain a mitochondrial-targeting sequence, an N-terminal region of a BIR (baculovirus IAP repeat) domain and a putative BH2 (Bcl-2 homology)-like domain. These suggested that K7 is a new viral anti-apoptotic protein and survivin-DeltaEx3 is its likely cellular homologue. We show that K7 is a glycoprotein, which can inhibit apoptosis and anchor to intracellular membranes where Bcl-2 resides. K7 does not associate with Bax, but does bind to Bcl-2 via its putative BH2 domain. In addition, K7 binds to active caspase-3 via its BIR domain and thus inhibits the activity of caspase-3. The BH2 domain of K7 is crucial for the inhibition of caspase-3 activity and is therefore essential for its anti-apoptotic function. Furthermore, K7 bridges Bcl-2 and activated caspase-3 into a protein complex. K7 therefore appears to be an adaptor protein and part of an anti-apoptotic complex that presents effector caspases to Bcl-2, enabling Bcl-2 to inhibit caspase activity. These data also suggest that survivin-DeltaEx3 might function by a similar mechanism to that of K7. We denote K7 as vIAP (viral inhibitor-of-apoptosis protein).

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.