Characterization of a nuclear 20S complex containing the survival of motor neurons (SMN) protein and a specific subset of spliceosomal Sm proteins

Viruses and Cajal Bodies: A Critical Cellular Target in Virus Infection?

Nuclear bodies (NBs) are dynamic structures present in eukaryotic cell nuclei. They are not bounded by membranes and are often considered biomolecular condensates, defined structurally and functionally by the localisation of core components. Nuclear architecture can be reorganised during normal cellular processes such as the cell cycle as well as in response to cellular stress. Many plant and animal viruses target their proteins to NBs, in some cases triggering their structural disruption and redistribution. Although not all such interactions have been well characterised, subversion of NBs and their functions may form a key part of the life cycle of eukaryotic viruses that require the nucleus for their replication. This review will focus on Cajal bodies (CBs) and the viruses that target them. Since CBs are dynamic structures, other NBs (principally nucleoli and promyelocytic leukaemia, PML and bodies), whose components interact with CBs, will also be considered. As well as providing important insights into key virus-host cell interactions, studies on Cajal and associated NBs may identify novel cellular targets for development of antiviral compounds.

The SMN complex drives structural changes in human snRNAs to enable snRNP assembly

Spliceosomal snRNPs are multicomponent particles that undergo a complex maturation pathway. Human Sm-class snRNAs are generated as 3'-end extended precursors, which are exported to the cytoplasm and assembled together with Sm proteins into core RNPs by the SMN complex. Here, we provide evidence that these pre-snRNA substrates contain compact, evolutionarily conserved secondary structures that overlap with the Sm binding site. These structural motifs in pre-snRNAs are predicted to interfere with Sm core assembly. We model structural rearrangements that lead to an open pre-snRNA conformation compatible with Sm protein interaction. The predicted rearrangement pathway is conserved in Metazoa and requires an external factor that initiates snRNA remodeling. We show that the essential helicase Gemin3, which is a component of the SMN complex, is crucial for snRNA structural rearrangements during snRNP maturation. The SMN complex thus facilitates ATP-driven structural changes in snRNAs that expose the Sm site and enable Sm protein binding.

A novel CARM1-HuR axis involved in muscle differentiation and plasticity misregulated in spinal muscular atrophy

Spinal muscular atrophy (SMA) is characterized by the loss of alpha motor neurons in the spinal cord and a progressive muscle weakness and atrophy. SMA is caused by loss-of-function mutations and/or deletions in the survival of motor neuron (SMN) gene. The role of SMN in motor neurons has been extensively studied, but its function and the consequences of its loss in muscle has also emerged as a key aspect of SMA pathology. In this study, we explore the molecular mechanisms involved in muscle defects in SMA. First, we show in C2C12 myoblasts, that arginine methylation by CARM1 controls myogenic differentiation. More specifically, the methylation of HuR on K217 regulates HuR levels and subcellular localization during myogenic differentiation, and the formation of myotubes. Furthermore, we demonstrate that SMN and HuR interact in C2C12 myoblasts. Interestingly, the SMA-causing E134K point mutation within the SMN Tudor domain, and CARM1 depletion, modulate the SMN-HuR interaction. In addition, using the Smn2B/- mouse model, we report that CARM1 levels are markedly increased in SMA muscles and that HuR fails to properly respond to muscle denervation, thereby affecting the regulation of its mRNA targets. Altogether, our results show a novel CARM1-HuR axis in the regulation of muscle differentiation and plasticity as well as in the aberrant regulation of this axis caused by the absence of SMN in SMA muscle. With the recent developments of therapeutics targeting motor neurons, this study further indicates the need for more global therapeutic approaches for SMA.

TOR signaling regulates liquid phase separation of the SMN complex governing snRNP biogenesis

The activity of the SMN complex in promoting the assembly of pre-mRNA processing UsnRNPs correlates with condensation of the complex in nuclear Cajal bodies. While mechanistic details of its activity have been elucidated, the molecular basis for condensation remains unclear. High SMN complex phosphorylation suggests extensive regulation. Here, we report on systematic siRNA-based screening for modulators of the capacity of SMN to condense in Cajal bodies and identify mTOR and ribosomal protein S6 kinase β-1 as key regulators. Proteomic analysis reveals TOR-dependent phosphorylations in SMN complex subunits. Using stably expressed or optogenetically controlled phospho mutants, we demonstrate that serine 49 and 63 phosphorylation of human SMN controls the capacity of the complex to condense in Cajal bodies via liquid-liquid phase separation. Our findings link SMN complex condensation and UsnRNP biogenesis to cellular energy levels and suggest modulation of TOR signaling as a rational concept for therapy of the SMN-linked neuromuscular disorder spinal muscular atrophy.

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.

miR-206 Reduces the Severity of Motor Neuron Degeneration in the Facial Nuclei of the Brainstem in a Mouse Model of SMA

Spinal muscular atrophy (SMA) is a severe neuromuscular disease affecting infants caused by alterations of the survival motor neuron gene, which results in progressive degeneration of motor neurons (MNs). Although an effective treatment for SMA patients has been recently developed, the molecular pathway involved in selective MN degeneration has not been yet elucidated. In particular, miR-206 has been demonstrated to play a relevant role in the regeneration of neuromuscular junction in several MN diseases, and particularly it is upregulated in the quadriceps, tibialis anterior, spinal cord, and serum of SMA mice. In the present paper, we demonstrated that miR-206 was transiently upregulated also in the brainstem of the mouse model of SMA, SMAΔ7, in the early phase of the disease paralleling MN degeneration and was down-regulated in the late symptomatic phase. To prevent this downregulation, we intracerebroventricularly injected miR-206 in SMA pups, demonstrating that miR-206 reduced the severity of SMA pathology, slowing down disease progression, increasing survival rate, and improving behavioral performance of mice. Interestingly, exogenous miRNA-206-induced upregulation caused a reduction of the predicted target sodium calcium exchanger isoform 2, NCX2, one of the main regulators of intracellular [Ca²⁺] and [Na⁺]. Therefore, we hypothesized that miR-206 might exert part of its neuroprotective effect modulating NCX2 expression in SMA disease.

Composition of the Survival Motor Neuron (SMN) Complex in Drosophila melanogaster

Spinal Muscular Atrophy (SMA) is caused by homozygous mutations in the human survival motor neuron 1 (SMN1) gene. SMN protein has a well-characterized role in the biogenesis of small nuclear ribonucleoproteins (snRNPs), core components of the spliceosome. SMN is part of an oligomeric complex with core binding partners, collectively called Gemins. Biochemical and cell biological studies demonstrate that certain Gemins are required for proper snRNP assembly and transport. However, the precise functions of most Gemins are unknown. To gain a deeper understanding of the SMN complex in the context of metazoan evolution, we investigated its composition in Drosophila melanogaster Using transgenic flies that exclusively express Flag-tagged SMN from its native promoter, we previously found that Gemin2, Gemin3, Gemin5, and all nine classical Sm proteins, including Lsm10 and Lsm11, co-purify with SMN. Here, we show that CG2941 is also highly enriched in the pulldown. Reciprocal co-immunoprecipitation reveals that epitope-tagged CG2941 interacts with endogenous SMN in Schneider2 cells. Bioinformatic comparisons show that CG2941 shares sequence and structural similarity with metazoan Gemin4. Additional analysis shows that three other genes (CG14164, CG31950 and CG2371) are not orthologous to Gemins 6-7-8, respectively, as previously suggested. In D.melanogaster, CG2941 is located within an evolutionarily recent genomic triplication with two other nearly identical paralogous genes (CG32783 and CG32786). RNAi-mediated knockdown of CG2941 and its two close paralogs reveals that Gemin4 is essential for organismal viability.

Spinal muscular atrophy

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

Nanos genes and their role in development and beyond

The hallmark of Nanos proteins is their typical (CCHC)2 zinc finger motif (zf-nanos). Animals have one to four nanos genes. For example, the fruit fly and demosponge have only one nanos gene, zebrafish and humans have three, and Fugu rubripes has four. Nanos genes are mainly known for their evolutionarily preserved role in germ cell survival and pluripotency. Nanos proteins have been reported to bind the C-terminal RNA-binding domain of Pumilio to form a post-transcriptional repressor complex. Several observations point to a link between the miRNA-mediated repression complex and the Nanos/Pumilio complex. Repression of the E2F3 oncogene product is, indeed, mediated by cooperation between the Nanos/Pumilio complex and miRNAs. Another important interaction partner of Nanos is the CCR4-NOT deadenylase complex. Besides the tissue-specific contribution of Nanos proteins to normal development, their ectopic expression has been observed in several cancer cell lines and various human cancers. An inverse correlation between the expression levels of human Nanos1 and Nanos3 and E-cadherin was observed in several cancer cell lines. Loss of E-cadherin, an important cell-cell adhesion protein, contributes to tumor invasion and metastasis. Overexpression of Nanos3 induces epithelial-mesenchymal transition in lung cancer cell lines partly by repressing E-cadherin. Other than some most interesting data from Nanos knockout mice, little is known about mammalian Nanos proteins, and further research is needed. In this review, we summarize the main roles of Nanos proteins and discuss the emerging concept of Nanos proteins as oncofetal antigens.

SMN regulation in SMA and in response to stress: new paradigms and therapeutic possibilities

Low levels of the survival of motor neuron (SMN) protein cause the neurodegenerative disease spinal muscular atrophy (SMA). SMA is a pediatric disease characterized by spinal motor neuron degeneration. SMA exhibits several levels of severity ranging from early antenatal fatality to only mild muscular weakness, and disease prognosis is related directly to the amount of functional SMN protein that a patient is able to express. Current therapies are being developed to increase the production of functional SMN protein; however, understanding the effect that natural stresses have on the production and function of SMN is of critical importance to ensuring that these therapies will have the greatest possible effect for patients. Research has shown that SMN, both on the mRNA and protein level, is highly affected by cellular stress. In this review we will summarize the research that highlights the roles of SMN in the disease process and the response of SMN to various environmental stresses.

Antibodies against the mono-methylated arginine-glycine repeat (MMA-RG) of the Epstein-Barr virus nuclear antigen 2 (EBNA2) identify potential cellular proteins targeted in viral transformation

The Epstein-Barr virus is a human herpes virus with oncogenic potential. The virus-encoded nuclear antigen 2 (EBNA2) is a key mediator of viral tumorigenesis. EBNA2 features an arginine-glycine (RG) repeat at amino acids (aa)339-354 that is essential for the transformation of lymphocytes and contains symmetrically (SDMA) and asymmetrically (ADMA) di-methylated arginine residues. The SDMA-modified EBNA2 binds the survival motor neuron protein (SMN), thus mimicking SMD3, a cellular SDMA-containing protein that interacts with SMN. Accordingly, a monoclonal antibody (mAb) specific for the SDMA-modified RG repeat of EBNA2 also binds to SMD3. With the novel mAb 19D4 we now show that EBNA2 contains mono-methylated arginine (MMA) residues within the RG repeat. Using 19D4, we immune-precipitated and analysed by mass spectrometry cellular proteins in EBV-transformed B-cells that feature MMA motifs that are similar to the one in EBNA2. Among the cellular proteins identified, we confirmed by immunoprecipitation and/or Western blot analyses Aly/REF, Coilin, DDX5, FXR1, HNRNPK, LSM4, MRE11, NRIP, nucleolin, PRPF8, RBM26, SMD1 (SNRDP1) and THRAP3 proteins that are either known to contain MMA residues or feature RG repeat sequences that probably serve as methylation substrates. The identified proteins are involved in splicing, tumorigenesis, transcriptional activation, DNA stability and RNA processing or export. Furthermore, we found that several proteins involved in energy metabolism are associated with MMA-modified proteins. Interestingly, the viral EBNA1 protein that features methylated RG repeat motifs also reacted with the antibodies. Our results indicate that the region between aa 34-52 of EBNA1 contains ADMA or SDMA residues, while the region between aa 328-377 mainly contains MMA residues.

Current and emerging treatment options for spinal muscular atrophy

Spinal muscular atrophy is one of the most common inherited neuromuscular conditions; our understanding of the genetic pathology and translational research coming from this insight has made significant progress over the past decade. This short review provides the background of the disease along with the bench to bedside progress of some promising treatment options to develop better understanding of the present state of the disease.

Expression of Muscle-Specific MiRNA 206 in the Progression of Disease in a Murine SMA Model

Spinal muscular atrophy (SMA) is a severe neuromuscular disease, the most common in infancy, and the third one among young people under 18 years. The major pathological landmark of SMA is a selective degeneration of lower motor neurons, resulting in progressive skeletal muscle denervation, atrophy, and paralysis. Recently, it has been shown that specific or general changes in the activity of ribonucleoprotein containing micro RNAs (miRNAs) play a role in the development of SMA. Additionally miRNA-206 has been shown to be required for efficient regeneration of neuromuscular synapses after acute nerve injury in an ALS mouse model. Therefore, we correlated the morphology and the architecture of the neuromuscular junctions (NMJs) of quadriceps, a muscle affected in the early stage of the disease, with the expression levels of miRNA-206 in a mouse model of intermediate SMA (SMAII), one of the most frequently used experimental model. Our results showed a decrease in the percentage of type II fibers, an increase in atrophic muscle fibers and a remarkable accumulation of neurofilament (NF) in the pre-synaptic terminal of the NMJs in the quadriceps of SMAII mice. Furthermore, molecular investigation showed a direct link between miRNA-206-HDAC4-FGFBP1, and in particular, a strong up-regulation of this pathway in the late phase of the disease. We propose that miRNA-206 is activated as survival endogenous mechanism, although not sufficient to rescue the integrity of motor neurons. We speculate that early modulation of miRNA-206 expression might delay SMA neurodegenerative pathway and that miRNA-206 could be an innovative, still relatively unexplored, therapeutic target for SMA.

Newborn screening for spinal muscular atrophy: Anticipating an imminent need

Spinal muscular atrophy (SMA) is the most common genetic cause of infant mortality. Children with type I SMA typically die by the age of 2 years. Recent progress in gene modification and other innovative therapies suggest that improved outcomes may soon be forthcoming. In animal models, therapeutic intervention initiated before the loss of motor neurons alters SMA phenotype and increases lifespan. Presently, supportive care including respiratory, nutritional, physiatry, and orthopedic management can ameliorate clinical symptoms and improve survival rates if SMA is diagnosed early in life. Newborn screening could help optimize these potential benefits. A recent report demonstrated that SMA detection can be multiplexed at minimal additional cost with the assay for severe combined immunodeficiency, already implemented by many newborn screening programs. The public health community should remain alert to the rapidly changing developments in early detection and treatment of SMA.

GEMIN4 functions as a coregulator of the mineralocorticoid receptor

The mineralocorticoid receptor (MR) is a member of the nuclear receptor superfamily. Pathological activation of the MR causes cardiac fibrosis and heart failure, but clinical use of MR antagonists is limited by the renal side effect of hyperkalemia. Coregulator proteins are known to be critical for nuclear receptor-mediated gene expression. Identification of coregulators, which mediate MR activity in a tissue-specific manner, may allow for the development of novel tissue-selective MR modulators that confer cardiac protection without adverse renal effects. Our earlier studies identified a consensus motif among MR-interacting peptides, MPxLxxLL. Gem (nuclear organelle)-associated protein 4 (GEMIN4) is one of the proteins that contain this motif. Transient transfection experiments in HEK293 and H9c2 cells demonstrated that GEMIN4 repressed agonist-induced MR transactivation in a cell-specific manner. Furthermore, overexpression of GEMIN4 significantly decreased, while knockdown of GEMIN4 increased, the mRNA expression of specific endogenous MR target genes. A physical interaction between GEMIN4 and MR is suggested by their nuclear co-localization upon agonist treatment. These findings indicate that GEMIN4 functions as a novel coregulator of the MR.

The catalytically inactive tyrosine phosphatase HD-PTP/PTPN23 is a novel regulator of SMN complex localization

This first systematic and comprehensive screen of human phosphatases for a regulatory role in the survival motor neuron (SMN) complex identifies the catalytically inactive, non–receptor-type tyrosine phosphatase PTPN23/HD-PTP as a novel SMN complex regulator. PTPN23 maintains a highly phosphorylated state of SMN, which is important for its function in snRNP assembly.

The survival motor neuron (SMN) complex fulfils essential functions in the assembly of snRNPs, which are key components in the splicing of pre-mRNAs. Little is known about the regulation of SMN complex activity by posttranslational modification despite its complicated phosphorylation pattern. Several phosphatases had been implicated in the regulation of SMN, including the nuclear phosphatases PPM1G and PP1γ. Here we systematically screened all human phosphatase gene products for a regulatory role in the SMN complex. We used the accumulation of SMN in Cajal bodies of intact proliferating cells, which actively assemble snRNPs, as a readout for unperturbed SMN complex function. Knockdown of 29 protein phosphatases interfered with SMN accumulation in Cajal bodies, suggesting impaired SMN complex function, among those the catalytically inactive, non–receptor-type tyrosine phosphatase PTPN23/HD-PTP. Knockdown of PTPN23 also led to changes in the phosphorylation pattern of SMN without affecting the assembly of the SMN complex. We further show interaction between SMN and PTPN23 and document that PTPN23, like SMN, shuttles between nucleus and cytoplasm. Our data provide the first comprehensive screen for SMN complex regulators and establish a novel regulatory function of PTPN23 in maintaining a highly phosphorylated state of SMN, which is important for its proper function in snRNP assembly.

Truncating and missense mutations in IGHMBP2 cause Charcot-Marie Tooth disease type 2

Using a combination of exome sequencing and linkage analysis, we investigated an English family with two affected siblings in their 40s with recessive Charcot-Marie Tooth disease type 2 (CMT2). Compound heterozygous mutations in the immunoglobulin-helicase-μ-binding protein 2 (IGHMBP2) gene were identified. Further sequencing revealed a total of 11 CMT2 families with recessively inherited IGHMBP2 gene mutations. IGHMBP2 mutations usually lead to spinal muscular atrophy with respiratory distress type 1 (SMARD1), where most infants die before 1 year of age. The individuals with CMT2 described here, have slowly progressive weakness, wasting and sensory loss, with an axonal neuropathy typical of CMT2, but no significant respiratory compromise. Segregating IGHMBP2 mutations in CMT2 were mainly loss-of-function nonsense in the 5' region of the gene in combination with a truncating frameshift, missense, or homozygous frameshift mutations in the last exon. Mutations in CMT2 were predicted to be less aggressive as compared to those in SMARD1, and fibroblast and lymphoblast studies indicate that the IGHMBP2 protein levels are significantly higher in CMT2 than SMARD1, but lower than controls, suggesting that the clinical phenotype differences are related to the IGHMBP2 protein levels.

DEAD-box helicase DP103 defines metastatic potential of human breast cancers

Despite advancement in breast cancer treatment, 30% of patients with early breast cancers experience relapse with distant metastasis. It is a challenge to identify patients at risk for relapse; therefore, the identification of markers and therapeutic targets for metastatic breast cancers is imperative. Here, we identified DP103 as a biomarker and metastasis-driving oncogene in human breast cancers and determined that DP103 elevates matrix metallopeptidase 9 (MMP9) levels, which are associated with metastasis and invasion through activation of NF-κB. In turn, NF-κB signaling positively activated DP103 expression. Furthermore, DP103 enhanced TGF-β-activated kinase-1 (TAK1) phosphorylation of NF-κB-activating IκB kinase 2 (IKK2), leading to increased NF-κB activity. Reduction of DP103 expression in invasive breast cancer cells reduced phosphorylation of IKK2, abrogated NF-κB-mediated MMP9 expression, and impeded metastasis in a murine xenograft model. In breast cancer patient tissues, elevated levels of DP103 correlated with enhanced MMP9, reduced overall survival, and reduced survival after relapse. Together, these data indicate that a positive DP103/NF-κB feedback loop promotes constitutive NF-κB activation in invasive breast cancers and activation of this pathway is linked to cancer progression and the acquisition of chemotherapy resistance. Furthermore, our results suggest that DP103 has potential as a therapeutic target for breast cancer treatment.

The splicing factor U1-70K interacts with the SMN complex and is required for nuclear gem integrity

The nuclear SMN complex localizes to specific structures called nuclear gems. The loss of gems is a cellular marker for several neurodegenerative diseases. Here, we identify that the U1-snRNP-specific protein U1-70K localizes to nuclear gems, and we show that U1-70K is necessary for gem integrity. Furthermore, we show that the interaction between U1-70K and the SMN complex is RNA independent, and we map the SMN complex binding site to the unstructured N-terminal tail of U1-70K. Consistent with these results, the expression of the U1-70K N-terminal tail rescues gem formation. These findings show that U1-70K is an SMN-complex-associating protein, and they suggest a new function for U1-70K in the formation of nuclear gems.

Phosphoregulation of the human SMN complex

The survival motor neuron (SMN) complex is a macromolecular machine comprising 9 core proteins: SMN, Gemins2-8 and unrip in vertebrates. It performs tasks in RNA metabolism including the cytoplasmic assembly of spliceosomal small nuclear ribonucleoprotein particles (snRNPs). The SMN complex also localizes to the nucleus, where it accumulates in Cajal Bodies (CB) and may function in transcription and/or pre-mRNA splicing. The SMN complex is subject to extensive phosphorylation. Detailed understanding of SMN complex regulation necessitates a comprehensive analysis of these post-translational modifications. Here, we report on the first comprehensive phosphoproteome analysis of the intact human SMN complex, which identify 48 serine/threonine phosphosites in the complex. We find that 7 out of 9 SMN components of the intact complex are phosphoproteins and confidently place 29 phosphorylation sites, 12 of them in SMN itself. By the generation of multi non-phosphorylatable or phosphomimetic variants of SMN, respectively, we address to which extent phosphorylation regulates SMN complex function and localization. Both phosphomimetic and non-phosphorylatable variants assemble into intact SMN complexes and can compensate the loss of endogenous SMN in snRNP assembly at least to some extent. However, they partially or completely fail to target to nuclear Cajal bodies. Moreover, using a mutant of SMN, which cannot be phosphorylated on previously reported tyrosine residues, we provide first evidence that this PTM regulates SMN localization and nuclear accumulation. Our data suggest complex regulatory cues mediated by phosphorylation of serine/threonine and tyrosine residues, which control the subcellular localization of the SMN complex and its accumulation in nuclear CB.

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.

Genome-wide identification of mRNAs associated with the protein SMN whose depletion decreases their axonal localization

Spinal muscular atrophy is a neuromuscular disease resulting from mutations in the SMN1 gene, which encodes the survival motor neuron (SMN) protein. SMN is part of a large complex that is essential for the biogenesis of spliceosomal small nuclear RNPs. SMN also colocalizes with mRNAs in granules that are actively transported in neuronal processes, supporting the hypothesis that SMN is involved in axonal trafficking of mRNPs. Here, we have performed a genome-wide analysis of RNAs present in complexes containing the SMN protein and identified more than 200 mRNAs associated with SMN in differentiated NSC-34 motor neuron-like cells. Remarkably, ~30% are described to localize in axons of different neuron types. In situ hybridization and immuno-fluorescence experiments performed on several candidates indicate that these mRNAs colocalize with the SMN protein in neurites and axons of differentiated NSC-34 cells. Moreover, they localize in cell processes in an SMN-dependent manner. Thus, low SMN levels might result in localization deficiencies of mRNAs required for axonogenesis.

The functional interactome landscape of the human histone deacetylase family

This study presents the first global protein interaction network for all 11 human HDACs in T cells and an integrative mass spectrometry approach for profiling relative interaction stability within isolated protein complexes.

T-cell lines stably expressing each of the human HDACs (1 - 11), C-terminally tagged with both EGFP and FLAG, were generated using retroviral transduction.

Affinity purification coupled to mass spectrometry-based proteomics (AP-MS) was used to build the first global protein interaction network for all eleven human HDACs in T cells.

An optimized label free AP-MS and computational workflow was developed for profiling relative interaction stability among isolated protein complexes.

HDAC11 is a member of the “survival of motor neuron” protein complex with a functional role in mRNA splicing.

Histone deacetylases (HDACs) are a diverse family of essential transcriptional regulatory enzymes, that function through the spatial and temporal recruitment of protein complexes. As the composition and regulation of HDAC complexes are only partially characterized, we built the first global protein interaction network for all 11 human HDACs in T cells. Integrating fluorescence microscopy, immunoaffinity purifications, quantitative mass spectrometry, and bioinformatics, we identified over 200 unreported interactions for both well-characterized and lesser-studied HDACs, a subset of which were validated by orthogonal approaches. We establish HDAC11 as a member of the survival of motor neuron complex and pinpoint a functional role in mRNA splicing. We designed a complementary label-free and metabolic-labeling mass spectrometry-based proteomics strategy for profiling interaction stability among different HDAC classes, revealing that HDAC1 interactions within chromatin-remodeling complexes are largely stable, while transcription factors preferentially exist in rapid equilibrium. Overall, this study represents a valuable resource for investigating HDAC functions in health and disease, encompassing emerging themes of HDAC regulation in cell cycle and RNA processing and a deeper functional understanding of HDAC complex stability.

Therapeutic strategies for the treatment of spinal muscular atrophy

Spinal muscular atrophy (SMA) is an inherited neurodegenerative disease that results in progressive dysfunction of motor neurons of the anterior horn of the spinal cord. SMA is caused by the loss of full-length protein expression from the survival of motor neuron 1 (SMN1) gene. The disease has a unique genetic profile as it is autosomal recessive for the loss of SMN1, but a nearly identical homolog, SMN2, acts as a disease modifier whose expression is inversely correlated to clinical severity. Targeted therapeutic approaches primarily focus on increasing the levels of full-length SMN protein, through either gene replacement or regulation of SMN2 expression. There is currently no US FDA approved treatment for SMA. This is an exciting time as multiple efforts from academic and industrial laboratories are reaching the preclinical and clinical testing stages.

Antisense genes to induce exon inclusion

Many inherited diseases are associated with changed splicing patterns, and alternative splicing influences several biological processes as well as the replication of certain viral pathogens. For this reason, there is a broad interest in modulating individual splicing events for therapeutic purposes. Based on the small nuclear RNA (snRNA) U7, we have developed expression vectors for short antisense RNAs that accumulate in the cell nucleus where splicing occurs and that can very specifically modulate the splicing of individual exons. More specifically, in the context of the fatal neuromuscular disorder Spinal Muscular Atrophy (SMA), we have shown that U7 snRNA constructs can restore the inclusion of exon 7 in the SMN2 gene and thereby alleviate or even fully cure disease symptoms in a severe mouse model for SMA. Here we describe more generally procedures to produce U7 constructs to induce exon inclusion and to test their efficiency in cell culture experiments at the level of RNA as well as protein. The analytical methods comprise reverse transcription (RT-)PCR to detect the splicing changes, quantitative real-time RT-PCR to measure U7 snRNA expression levels and western blot and immunofluorescence methods to detect a restoration of protein expression. Additionally, we indicate how U7 cassettes can be introduced into gene transfer vectors for in vivo experiments in animal models or to transduce cell systems that are not readily amenable to DNA transfection.

Functional mammalian spliceosomal complex E contains SMN complex proteins in addition to U1 and U2 snRNPs

Spliceosomes remove introns from primary gene transcripts. They assemble de novo on each intron through a series of steps that involve the incorporation of five snRNP particles and multiple non-snRNP proteins. In mammals, all the intermediate complexes have been characterized on one transcript (MINX), with the exception of the very first, complex E. We have purified this complex by two independent procedures using antibodies to either U1-A or PRPF40A proteins, which are known to associate at an early stage of assembly. We demonstrate that the purified complexes are functional in splicing using commitment assays. These complexes contain components expected to be in the E complex and a number of previously unrecognized factors, including survival of motor neurons (SMN) and proteins of the SMN-associated complex. Depletion of the SMN complex proteins from nuclear extracts inhibits formation of the E complex and causes non-productive complexes to accumulate. This suggests that the SMN complex stabilizes the association of U1 and U2 snRNPs with pre-mRNA. In addition, the antibody to PRPF40A precipitated U2 snRNPs from nuclear extracts, indicating that PRPF40A associates with U2 snRNPs.

NANOS1 and PUMILIO2 bind microRNA biogenesis factor GEMIN3, within chromatoid body in human germ cells

Nanos and pumilio bind each other to regulate translation of specific mRNAs in germ cells of model organisms, such as D. melanogaster or C. elegans. Recently described human homologues NANOS1 and PUMILIO2 form a complex similar to their ancestors. This study was aimed to identify the proteins interacting with NANOS1-PUMILIO2 complex in the human spermatogenic cells. Here, using the yeast two-hybrid system we found that NANOS1 and PUMILIO2 proteins interact with RNA DEAD-box helicase GEMIN3, a microRNA biogenesis factor. Moreover, GEMIN3 coimmunoprecipitates with NANOS1 and PUMILIO2 in transfected mammalian cells. By double immunofluorescence staining, we observed that complexes built of NANOS1, PUMILIO2 and GEMIN3 are located within cytoplasmic region of germ cells. These proteins condense to form a compact aggregate in the round spermatids of the human and mouse germ cells. This aggregate was reminiscent of the chromatoid body (CB), a perinuclear structure present in the mammalian male germ line. This structure is considered evolutionary remnant of germ plasm, a hallmark structure of germ cells in lower metazoan. Using a CB marker VASA protein, we demonstrated that CBs are present in the human round spermatids, as they are in the mouse. Moreover, NANOS1, PUMILIO2 and GEMIN3 colocalize with VASA protein. We demonstrated for the first time that a mammalian Nanos-Pumilio complex functions within CB, a center of RNA storing and processing, involving microRNAs. NANOS1-PUMILIO2 complex, together with GEMIN3 and small noncoding RNAs, possibly regulate mRNA translation within CB of the human germ cells.

Prolactin increases SMN expression and survival in a mouse model of severe spinal muscular atrophy via the STAT5 pathway

Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disease that is characterized by the loss of motor neurons, resulting in progressive muscle atrophy. It is caused by the loss of functional survival motor neuron (SMN) protein due to mutations or deletion in the SMN1 gene. A potential treatment strategy for SMA is to upregulate levels of SMN protein. Several agents that activate STAT5 in human and mouse cell lines enhance SMN expression from the SMN2 gene and can compensate, at least in part, for the loss of production of a functional protein from SMN1. Here, we have shown that prolactin (PRL) increases SMN levels via activation of the STAT5 pathway. PRL increased SMN mRNA and protein levels in cultured human and mouse neuronal cells. Administration of STAT5-specific siRNA blocked the effects of PRL, indicating that the PRL-induced transcriptional upregulation of the SMN-encoding gene was mediated by activation of STAT5. Furthermore, systemic administration of PRL to WT mice induced SMN expression in the brain and spinal cord. Critically, PRL treatment increased SMN levels, improved motor function, and enhanced survival in a mouse model of severe SMA. Our results confirm earlier work suggesting STAT5 pathway activators as potential therapeutic compounds for the treatment of SMA and identify PRL as one such promising agent.

SMN deficiency reduces cellular ability to form stress granules, sensitizing cells to stress

Spinal Muscular Atrophy (SMA) is a neurodegenerative disease that is caused by deletion of the SMN (Survival of Motor Neuron) gene. The SMN protein is essential for cell survival and co-localized with TIA-1/R and G3BP, two characteristic markers of stress granules (SGs). To further study the SMN function in stress granules and in response to stress, we generated stable cell lines with SMN knockdown. Our data indicate that suppression of SMN drastically reduces cellular ability to form stress granules in response to stress treatment. In addition, we show that SMN deficiency sensitizes cells to sodium arsenite and H(2)O(2), two well-known stress inducers, leading to cell death at a much lower concentration of inducers in SMN knockdown cells than in control cells. Interestingly, the cell death is correlated with formation of stress granules, suggesting that involvement of SMN in formation of stress granules may play an important role in cell survival. Furthermore, rescue of SGs formation by overexpression of G3BP can reverse the defective formation of stress granules and results in partial abrogation of cell death against SMN deficiency. We deduce that modulation of stress response may be useful for potential SMN treatment.

Identification of novel interacting protein partners of SMN using tandem affinity purification

Mutations in the survival motor neuron (SMN) gene cause spinal muscular atrophy (SMA), a neuromuscular disease associated with muscle weakness that progresses to paralysis, respiratory distress, and ultimately death. Both neurons and muscle are severely affected in this disease. Tandem affinity purification (TAP) has emerged as a useful tool for studying protein complexes in vitro. We have used this purification system to discover novel SMN interacting partners in C2C12 muscle and PC12 neuronal cells. To do so, we fused a TAP-tag, consisting of a HIS hexamer and FLAG epitope separated by the tobacco etch virus (TEV) protease cleavage site, to either the N- or C-terminal region of SMN. Interestingly, the profile of SMN interacting proteins varies depending on the cell type and stage. We have identified a number of novel SMN interacting proteins in both C2C12 and PC12 cells, and from among these we have validated Annexin II and myosin regulatory light chain (MRLC). The discovery of these proteins will lead to a better understanding of the mechanisms underlying the pathophysiology of SMA.

Spinal muscular atrophy: why do low levels of survival motor neuron protein make motor neurons sick?

Many neurogenetic disorders are caused by the mutation of ubiquitously expressed genes. One such disorder, spinal muscular atrophy, is caused by loss or mutation of the survival motor neuron1 gene (SMN1), leading to reduced SMN protein levels and a selective dysfunction of motor neurons. SMN, together with partner proteins, functions in the assembly of small nuclear ribonucleoproteins (snRNPs), which are important for pre-mRNA splicing. It has also been suggested that SMN might function in the assembly of other ribonucleoprotein complexes. Two hypotheses have been proposed to explain the molecular dysfunction that gives rise to spinal muscular atrophy (SMA) and its specificity to a particular group of neurons. The first hypothesis states that the loss of SMN's well-known function in snRNP assembly causes an alteration in the splicing of a specific gene (or genes). The second hypothesis proposes that SMN is crucial for the transport of mRNA in neurons and that disruption of this function results in SMA.

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

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

Gemin3 is an essential gene required for larval motor function and pupation in Drosophila

The assembly of metazoan Sm-class small nuclear ribonucleoproteins (snRNPs) is an elaborate, step-wise process that takes place in multiple subcellular compartments. The initial steps, including formation of the core RNP, are mediated by the survival motor neuron (SMN) protein complex. Loss-of-function mutations in human SMN1 result in a neuromuscular disease called spinal muscular atrophy. The SMN complex is comprised of SMN and a number of tightly associated proteins, collectively called Gemins. In this report, we identify and characterize the fruitfly ortholog of the DEAD box protein, Gemin3. Drosophila Gemin3 (dGem3) colocalizes and interacts with dSMN in vitro and in vivo. RNA interference for dGem3 codepletes dSMN and inhibits efficient Sm core assembly in vitro. Transposon insertion mutations in Gemin3 are larval lethals and also codeplete dSMN. Transgenic overexpression of dGem3 rescues lethality, but overexpression of dSMN does not, indicating that loss of dSMN is not the primary cause of death. Gemin3 mutant larvae exhibit motor defects similar to previously characterized Smn alleles. Remarkably, appreciable numbers of Gemin3 mutants (along with one previously undescribed Smn allele) survive as larvae for several weeks without pupating. Our results demonstrate the conservation of Gemin3 protein function in metazoan snRNP assembly and reveal that loss of either Smn or Gemin3 can contribute to neuromuscular dysfunction.

TDRD3, a novel Tudor domain-containing protein, localizes to cytoplasmic stress granules

Our previous work has demonstrated that the Tudor domain of the ‘survival of motor neuron’ protein and the Tudor domain-containing protein 3 (TDRD3) are highly similar and that they both have the ability to interact with arginine-methylated polypeptides. TDRD3 has been identified among genes whose overexpression has a strong predictive value for poor prognosis of estrogen receptor-negative breast cancers, although its precise function remains unknown. TDRD3 is a modular protein, and in addition to its Tudor domain, it harbors a putative nucleic acid recognition motif and a ubiquitin-associated domain. We report here that TDRD3 localizes predominantly to the cytoplasm, where it co-sediments with the fragile X mental retardation protein on actively translating polyribosomes. We also demonstrate that TDRD3 accumulates into stress granules (SGs) in response to various cellular stresses. Strikingly, the Tudor domain of TDRD3 was found to be both required and sufficient for its recruitment to SGs, and the methyl-binding surface in the Tudor domain is important for this process. Pull down experiments identified five novel TDRD3 interacting partners, most of which are potentially methylated RNA-binding proteins. Our findings revealed that two of these proteins, SERPINE1 mRNA-binding protein 1 and DEAD/H box-3 (a gene often deleted in Sertoli-cell-only syndrome), are also novel constituents of cytoplasmic SGs. Taken together, we report the first characterization of TDRD3 and its functional interaction with at least two proteins implicated in human genetic diseases and present evidence supporting a role for arginine methylation in the regulation of SG dynamics.

Evolution of an RNP assembly system: a minimal SMN complex facilitates formation of UsnRNPs in Drosophila melanogaster

In vertebrates, assembly of spliceosomal uridine-rich small nuclear ribonucleoproteins (UsnRNPs) is mediated by the SMN complex, a macromolecular entity composed of the proteins SMN and Gemins 2-8. Here we have studied the evolution of this machinery using complete genome assemblies of multiple model organisms. The SMN complex has gained complexity in evolution by a blockwise addition of Gemins onto an ancestral core complex composed of SMN and Gemin2. In contrast to this overall evolutionary trend to more complexity in metazoans, orthologs of most Gemins are missing in dipterans. In accordance with these bioinformatic data a previously undescribed biochemical purification strategy elucidated that the dipteran Drosophila melanogaster contains an SMN complex of remarkable simplicity. Surprisingly, this minimal complex not only mediates the assembly reaction in a manner very similar to its vertebrate counterpart, but also prevents misassembly onto nontarget RNAs. Our data suggest that only a minority of Gemins are required for the assembly reaction per se, whereas others may serve additional functions in the context of UsnRNP biogenesis. The evolution of the SMN complex is an interesting example of how the simplification of a biochemical process contributes to genome compaction.

In vitro and in cellulo evidences for association of the survival of motor neuron complex with the fragile X mental retardation protein

Spinal muscular atrophy (SMA) is caused by reduced levels of the survival of motor neuron (SMN) protein. Although the SMN complex is essential for assembly of spliceosomal U small nuclear RNPs, it is still not understood why reduced levels of the SMN protein specifically cause motor neuron degeneration. SMN was recently proposed to have specific functions in mRNA transport and translation regulation in neuronal processes. The defective protein in Fragile X mental retardation syndrome (FMRP) also plays a role in transport of mRNPs and in their translation. Therefore, we examined possible relationships of SMN with FMRP. We observed granules containing both transiently expressed red fluorescent protein(RFP)-tagged SMN and green fluorescent protein(GFP)-tagged FMRP in cell bodies and processes of rat primary neurons of hypothalamus in culture. By immunoprecipitation experiments, we detected an association of FMRP with the SMN complex in human neuroblastoma SH-SY5Y cells and in murine motor neuron MN-1 cells. Then, by in vitro experiments, we demonstrated that the SMN protein is essential for this association. We showed that the COOH-terminal region of FMRP, as well as the conserved YG box and the region encoded by exon 7 of SMN, are required for the interaction. Our findings suggest a link between the SMN complex and FMRP in neuronal cells.

QNQKE targeting motif for the SMN-Gemin multiprotein complexin neurons

Spinal muscular atrophy (SMA) is a heritable neurodegenerative disease affecting motor neurons that is caused by the impaired expression of the full-length form of the survival of motor neuron protein (SMN), which may have a specialized function in neurons related to mRNA localization. We have previously shown that a population SMN complexes contain Gemin ribonucleoproteins and traffic in the form of granules to neuronal processes and growth cones of cultured neurons. A QNQKE sequence within exon 7 has been shown to be necessary for both cytoplasmic localization of SMN and axonal function. Here we show that the QNQKE sequence can influence the nucleocytoplasmic distribution of the SMN-Gemin complex and its localization into neuronal processes. QNQKE exerted a stronger effect on SMN localization in primary neurons compared with COS-7 cells. By using double-label fluorescence in situ hybridization and immunofluorescence, SMN granules within neuronal processes colocalized with poly-(A) mRNA and PABP. These findings provide further evidence in support of a neuronal function for SMN and motivation to investigate for impaired assembly and/or localization of mRNP complexes as an underlying cause of SMA.

Identification of a novel 82 kDa proMMP-9 species associated with the surface of leukaemic cells: (auto-)catalytic activation and resistance to inhibition by TIMP-1

MMP-9 (matrix metalloproteinase 9) plays a critical role in tumour progression. Although the biochemical properties of the secreted form of proMMP-9 are well characterized, little is known about the function and activity of cell surface-associated proMMP-9. We purified a novel 82 kDa species of proMMP-9 from the plasma membrane of THP-1 leukaemic cells, which has substantial differences from the secreted 94 kDa proMMP-9. The 82 kDa form was not detected in the medium even upon stimulation with a phorbol ester. It is truncated by nine amino acid residues at its N-terminus, lacks O-linked oligosaccharides present in the 94 kDa proMMP-9, but retains N-linked carbohydrates. Incubation of 94 kDa proMMP-9 with MMP-3 generated the well-known 82 kDa active form, but the 82 kDa proMMP-9 was converted into an active species of 35 kDa, which was also produced by autocatalytic processing in the absence of activating enzymes. The activated 35 kDa MMP-9 efficiently degraded gelatins, native collagen type IV and fibronectin. The enzyme was less sensitive to TIMP-1 (tissue inhibitor of metalloproteinase 1) inhibition with IC50 values of 82 nM compared with 1 nM for the 82 kDa active MMP-9. The synthetic MMP inhibitor GM6001 blocked the activity of both enzymes, with similar IC50 values below 1 nM. The 82 kDa proMMP-9 is also produced in HL-60 and NB4 leukaemic cell lines as well as ex vivo leukaemic blast cells. It is, however, absent from neutrophils and mononuclear cells isolated from peripheral blood of healthy individuals. Thus, the 82 kDa proMMP-9 expressed on the surface of malignant cells may escape inhibition by natural TIMP-1, thereby facilitating cellular invasion in vivo.

Molecular functions of the SMN complex

The SMN complex is essential for the biogenesis of spliceosomal small nuclear ribonucleoproteins and likely functions in the assembly, metabolism, and transport of a diverse number of other ribonucleoproteins. Specifically, the SMN complex assembles 7 Sm proteins into a core structure around a highly conserved sequence of ribonucleic acid (RNA) found in small nuclear RNAs. The complex recognizes specific sequences and structural features of small nuclear RNAs and Sm proteins and assembles small nuclear ribonucleoproteins in a stepwise fashion. In addition to the SMN protein, the SMN complex contains 7 additional proteins known as Gemin2-8, each likely to play a role in ribonucleoprotein biogenesis. This review focuses on the current understanding of the mechanism of the role of the SMN complex in small nuclear ribonucleoprotein assembly and considers the relationship of this function to spinal muscular atrophy.

Absence of gemin5 from SMN complexes in nuclear Cajal bodies

Background

Spinal muscular atrophy is caused by reduced levels of the survival of motor neurons (SMN) protein. SMN is found in large complexes with Sm proteins and at least eight other proteins, including seven "gemins". These complexes are involved in the assembly of snRNPs in the cytoplasm and their transport into the nucleus, but the precise roles of the individual protein components are largely unknown.

Results

We have investigated the subcellular distribution of gemins using novel antibodies against gemins 3–7, and existing mAbs against SMN, gemin2, unrip, fibrillarin and profilin II. Most gemins were equally distributed between nuclear and cytoplasmic fractions of HeLa cells, but gemin5 and unrip were more abundant in the cytoplasm. In a cytoplasmic extract obtained by mild disruption of HeLa cells, nearly all the SMN and gemins 2–4 were in large complexes, but most of the gemin5 sedimented separately with a lower S value. Most of the unrip sedimented with gemins 6 and 7 near the top of the sucrose density gradients, separate from both SMN and gemin5. Anti-SMN mAbs pulled down gemin5 from cytoplasmic extracts, but not from nuclear extracts, and gemin5 did not co-sediment with large SMN complexes in nuclear extracts. These data suggest that gemin5 is easily detached from SMN-gemin complexes in the nucleus. By immuno-histochemistry, gemin5 was rarely detectable in nuclear gems/Cajal bodies, although it was accessible to antibody and easily detectable when present. This suggests that gemin5 is normally absent from SMN complexes in these nuclear storage sites.

Conclusion

We conclude that SMN complexes usually exist without gemin5 in nuclear gems/Cajal bodies. Gemin5 is believed to be involved in capturing snRNA into SMN complexes in the cytoplasm for transport into the nucleus. We hypothesize that gemin5, though present in the nucleus, is no longer needed for SMN complex function during the time these complexes are stored in gems/Cajal bodies.

Spinal muscular atrophy: SMN2 pre-mRNA splicing corrected by a U7 snRNA derivative carrying a splicing enhancer sequence

Spinal muscular atrophy (SMA) is a lethal hereditary disease caused by homozygous deletion/inactivation of the survival of motoneuron 1 (SMN1) gene. The nearby SMN2 gene, despite its identical coding capacity, is only an incomplete substitute, because a single nucleotide difference impairs the inclusion of its seventh exon in the messenger RNA (mRNA). This splicing defect can be corrected (transiently) by specially designed oligonucleotides. Here we have developed a more permanent correction strategy based on bifunctional U7 small nuclear RNAs (snRNAs). These carry both an antisense sequence that allows specific binding to exon 7 and a splicing enhancer sequence that will improve the recognition of the targeted exon. When expression cassettes for these RNAs are stably introduced into cells, the U7 snRNAs become incorporated into small nuclear ribonucleoprotein (snRNP) particles that will induce a durable splicing correction. We have optimized this strategy to the point that virtually all SMN2 pre-mRNA becomes correctly spliced. In fibroblasts from an SMA patient, this approach induces a prolonged restoration of SMN protein and ensures its correct localization to discrete nuclear foci (gems).

The spinal muscular atrophy gene product regulates neurite outgrowth: importance of the C terminus

Spinal muscular atrophy is a neurodegenerative disease accompanied by a loss of motoneurons. Either mutations or deletions in the survival of motoneuron (SMN) gene are responsible for this defect. SMN is an assembly protein for RNA-protein complexes in the nucleus and is also found in axons of neurons. However, it is unclear which dysfunctions of SMN are important for disease progression. In this study we analyzed the contributions of different SMN regions for localization and neuronal differentiation associated with outgrowth of neurites. Suppression of endogenous SMN protein levels significantly decreased the growth of neurites. Down-regulation of the interacting protein gemin2 had the opposite effect. Surprisingly, selective overexpression of the SMN C-terminal domain promoted neurite outgrowth similar to full-length protein and could rescue the SMN knock-down effects. The knock-down led to a significant change in the G-/F-actin ratio, indicating a role for SMN in actin dynamics. Therefore, our data suggest a functional role for SMN in microfilament metabolism in axons of motoneurons.

A Comprehensive Interaction Map of the Human Survival of Motor Neuron (SMN) Complex

Assembly of the Sm-class of U-rich small nuclear ribonucleoprotein particles (U snRNPs) is a process facilitated by the macromolecular survival of motor neuron (SMN) complex. This entity promotes the binding of a set of factors, termed LSm/Sm proteins, onto snRNA to form the core structure of these particles. Nine factors, including the SMN protein, the product of the spinal muscular atrophy (SMA) disease gene, Gemins 2-8 and unrip have been identified as the major components of the SMN complex. So far, however, only little is known about the architecture of this complex and the contribution of individual components to its function. Here, we present a comprehensive interaction map of all core components of the SMN complex based upon in vivo and in vitro methods. Our studies reveal a modular composition of the SMN complex with the three proteins SMN, Gemin8, and Gemin7 in its center. Onto this central building block the other components are bound via multiple interactions. Furthermore, by employing a novel assay, we were able to reconstitute the SMN complex from individual components and confirm the interaction map. Interestingly, SMN protein carrying an SMA-causing mutation was severely impaired in formation of the SMN complex. Finally, we show that the peripheral component Gemin5 contributes an essential activity to the SMN complex, most likely the transfer of Sm proteins onto the U snRNA. Collectively, the data presented here provide a basis for the detailed mechanistic and structural analysis of the assembly machinery of U snRNPs.

Survival motor neuron function in motor axons is independent of functions required for small nuclear ribonucleoprotein biogenesis

Spinal muscular atrophy (SMA) is a motor neuron degenerative disease caused by low levels of the survival motor neuron (SMN) protein and is linked to mutations or loss of SMN1 and retention of SMN2. How low levels of SMN cause SMA is unclear. SMN functions in small nuclear ribonucleoprotein (snRNP) biogenesis, but recent studies indicate that SMN may also function in axons. We showed previously that decreasing Smn levels in zebrafish using morpholinos (MO) results in motor axon defects. To determine how Smn functions in motor axon outgrowth, we coinjected smn MO with various human SMN RNAs and assayed the effect on motor axons. Wild-type SMN rescues motor axon defects caused by Smn reduction in zebrafish. Consistent with these defects playing a role in SMA, SMN lacking exon 7, the predominant form from the SMN2 gene, and human SMA mutations do not rescue defective motor axons. Moreover, the severity of the motor axon defects correlates with decreased longevity. We also show that a conserved region in SMN exon 7, QNQKE, is critical for motor axon outgrowth. To address the function of SMN important for motor axon outgrowth, we determined the ability of different SMN forms to oligomerization and bind Sm protein, functions required for snRNP biogenesis. We identified mutations that failed to rescue motor axon defects but retained snRNP function. Thus, we have dissociated the snRNP function of SMN from its function in motor axons. These data indicate that SMN has a novel function in motor axons that is relevant to SMA and is independent of snRNP biosynthesis.

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.