Characterization of a nuclear 20S complex containing the survival of motor neurons (SMN) protein and a specific subset of spliceosomal Sm proteins.
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Spinal muscular atrophy (SMA) is a neurodegenerative disease of motor neurons caused by reduced levels of functional survival of motor neurons (SMN) protein. Cytoplasmic SMN directly interacts with spliceosomal Sm proteins and facilitates their assembly onto U snRNAs. Nuclear SMN, in contrast, mediates recycling of pre-mRNA splicing factors. In this study, we have addressed the function of SMN in the nucleus. We show that a monoclonal antibody directed against SMN inhibits pre-mRNA splicing. Interestingly, the mode of inhibition suggests a novel role for SMN in splicing that occurs prior to, or in addition to, its role in recycling. Using biochemical fractionation and anti-SMN immunoaffinity chromatography, we identified two distinct nuclear SMN complexes termed NSC1 and NSC2. The biochemical properties and protein composition of NSC1 were determined in detail. NSC1 migrates in sucrose gradients as a U snRNA-free 20S complex containing at least 10 proteins. In addition to SMN, these include the SMN-interacting protein 1 (SIP-1), the putative helicase dp103/Gemin3, the novel dp103/Gemin3-interacting protein GIP1/Gemin4 and three additional proteins with apparent masses of 43, 33 and 18 kDa, respectively. Most surprisingly, NSC1 also contains a specific subset of spliceosomal Sm proteins. This shows that the SMN-Sm protein interaction is not restricted to the cytoplasm. Our data imply that nuclear SMN affects splicing by modulating the Sm protein composition of U snRNPs. (+info)
CREB-binding protein sequestration by expanded polyglutamine.
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Spinal and bulbar muscular atrophy (SBMA) is one of eight inherited neurodegenerative diseases known to be caused by CAG repeat expansion. The expansion results in an expanded polyglutamine tract, which likely confers a novel, toxic function to the affected protein. Cell culture and transgenic mouse studies have implicated the nucleus as a site for pathogenesis, suggesting that a critical nuclear factor or process is disrupted by the polyglutamine expansion. In this report we present evidence that CREB-binding protein (CBP), a transcriptional co-activator that orchestrates nuclear response to a variety of cell signaling cascades, is incorporated into nuclear inclusions formed by polyglutamine-containing proteins in cultured cells, transgenic mice and tissue from patients with SBMA. We also show CBP incorporation into nuclear inclusions formed in a cell culture model of another polyglutamine disease, spinocerebellar ataxia type 3. We present evidence that soluble levels of CBP are reduced in cells expressing expanded polyglutamine despite increased levels of CBP mRNA. Finally, we demonstrate that over-expression of CBP rescues cells from polyglutamine-mediated toxicity in neuronal cell culture. These data support a CBP-sequestration model of polyglutamine expansion disease. (+info)
Animal models of spinal muscular atrophy.
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Proximal spinal muscular atrophy (SMA) is the second most common autosomal recessive inherited disorder in humans. It is the most common genetic cause of infant mortality. As yet, there is no cure for this neuromuscular disorder which affects the lower motor neurons and proximal muscles of the limbs and trunk. In the last decade, significant advances have been made in understanding this disease, from linkage analysis to isolating the defective gene and identifying its protein product. This review summarizes the most recent advance in SMA research: the development of animal models of the disease, in particular mouse models of SMA. The SMA mice that we describe here present with symptoms similar to those seen in SMA patients. They promise to further the understanding of the molecular basis of this disease and demonstrate the feasibility of using the intact SMN2 gene, found in all SMA patients, as a means of treating this disorder. (+info)
Survival motor neuron protein modulates neuron-specific apoptosis.
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Spinal muscular atrophy (SMA) is attributed to mutations in the SMN1 gene, leading to loss of spinal cord motor neurons. The neurotropic Sindbis virus vector system was used to investigate a role for the survival motor neuron (SMN) protein in regulating neuronal apoptosis. Here we show that SMN protects primary neurons and differentiated neuron-like stem cells, but not cultured cell lines from virus-induced apoptotic death. SMN also protects neurons in vivo and increases survival of virus-infected mice. SMN mutants (SMNDelta7 and SMN-Y272C) found in patients with SMA not only lack antiapoptotic activity but also are potently proapoptotic, causing increased neuronal apoptosis and animal mortality. Full-length SMN is proteolytically processed in brains undergoing apoptosis or after ischemic injury. Mutation of an Asp-252 of SMN abolished cleavage of SMN and increased the antiapoptotic function of full-length SMN in neurons. Taken together, deletions or mutations of the C terminus of SMN that result from proteolysis, splicing (SMNDelta7), or germ-line mutations (e.g., Y272C), produce a proapoptotic form of SMN that may contribute to neuronal death in SMA and perhaps other neurodegenerative disorders. (+info)
The exon 2b region of the spinal muscular atrophy protein, SMN, is involved in self-association and SIP1 binding.
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Spinal muscular atrophy (SMA) is caused by mutations in the SMN (survival of motor neurons) gene and there is a correlation between disease severity and levels of functional SMN protein. Studies of structure-function relationships in SMN protein may lead to a better understanding of SMA pathogenesis. Self-association of the spinal muscular atrophy protein, SMN, is important for its function in RNA splicing. Biomolecular interaction analysis core analysis now shows that SMN self-association occurs via SMN regions encoded by exons 2b and 6, that exon 2b encodes a binding site for SMN-interacting protein-1 and that interaction occurs between exon 2- and 4-encoded regions within the SMN monomer. The presence of two separate self-association sites suggests a novel mechanism by which linear oligomers or closed rings might be formed from SMN monomers. (+info)
Motor neuron diseases in the university hospital of Fortaleza (Northeastern Brazil): a clinico-demographic analysis of 87 cases.
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In this retrospective (1980-1998) study, we have analyzed clinico-demographically, from the records of the University Hospital of Fortaleza (Brazil), a group of 87 patients showing signs and symptoms of motor neuron diseases (MNDs). Their diagnosis was determined clinically and laboratorially. The WFN criteria were used for amyotrophic lateral sclerosis (ALS) diagnosis. The clinico-demographic analysis of the 87 cases of MNDs showed that 4 were diagnosed as spinal muscular atrophy (SMA), 5 cases as ALS subsets: 2 as progressive bulbar paralysis (PBP), 2 as progressive muscular atrophy (PMA) and 1 as monomelic amyotrophy (MA), and 78 cases of ALS. The latter comprised 51 males and 27 females, with a mean age of 42.02 years. They were sub-divided into 4 groups according to age: from 15 to 29 years (n= 17), 30 to 39 years (n= 18), 40 to 69 years (n= 39) and 70 to 78 years (n= 4). From the 78 ALS patients, 76 were of the classic sporadic form whilst only 2 were of the familial form. The analysis of the 87 patients with MNDs from the University Hospital of Fortaleza showed a predominance of ALS patients, with a high number of cases of juvenile and early onset adult sporadic ALS. (+info)
A cell system with targeted disruption of the SMN gene: functional conservation of the SMN protein and dependence of Gemin2 on SMN.
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The motor neuron degenerative disease spinal muscular atrophy is caused by reduced expression of the survival motor neuron (SMN) protein. Here we report a genetic system developed in the chicken pre-B cell line DT40, in which the endogenous SMN gene is disrupted by homologous recombination, and SMN protein is expressed from a chicken SMN cDNA under control of a tetracycline (tet)-repressible promoter. Addition of tet results in depletion of SMN protein and consequent cell death, which directly demonstrates that SMN is required for cell viability. The tet-induced lethality can be rescued by expression of human SMN, indicating that the function of SMN is highly conserved between the two species. Cells expressing low levels of SMN display slow growth proportional to the amount of SMN they contain. Interestingly, the level of the SMN-interacting protein Gemin2 decreases significantly following depletion of SMN, supporting the conclusion that SMN and Gemin2 form a stable complex in vivo. This system provides a powerful setting for studying the function of SMN in vivo and for screening for potential therapeutics for spinal muscular atrophy. (+info)
Interferons and IRF-1 induce expression of the survival motor neuron (SMN) genes.
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BACKGROUND: Spinal muscular atrophy (SMA) is a common recessive disorder, characterized by degeneration of motor neurons of the spinal cord. Deletions, conversions, or mutations of the survival motor neuron gene (SMN) are responsible for SMA. A highly homologous centromeric copy of the SMN gene (SMNc) remains intact in SMA patients. However, there is an inverse correlation between the amount of the SMNc gene product and the clinical severity of the disease. An understanding of SMN and SMNc gene regulation is, therefore, an important step towards therapy for SMA. RESULTS: We identified a candidate Interferon-Stimulated Response Element (ISRE), overlapping with an Interferon Regulatory Factors binding motif (IRF-E) in the promoter region of SMN and SMNc genes. Both ISRE and IRF-E motifs are involved in mediating transcriptional induction of interferon-stimulated gene expression. We, therefore, investigated whether SMN and SMNc genes were regulated by interferons (IFN). Here we show that both IFN-beta and IFN-gamma rapidly induced SMN and SMNc mRNA and protein expression in various cell lines. The transcription factor IRF-1 bound to the candidate ISRE/IRF-E sequence of SMN and SMNc genes in vitro and overexpression of IRF-1 induced expression of both genes in transfection assays. IRF-1 is, therefore, at least in part responsible for the induction of SMN and SMNc by IFNs. In primary culture of fibroblasts from SMA patients, IFN-beta and IFN-gamma induced SMNc gene expression and restored protein defect. (+info)