Spinal Muscular Atrophy (SMA) is an autosomal, recessive form of childhood motor neuron disease and the most common genetic cause of infant mortality in the western world. SMA displays the characteristic hallmarks of a motor neuron disease, including loss of motor neurons in the spinal cord and atrophy of skeletal muscles. However, mounting evidence suggests that multiple tissues and body systems, beyond the neuromuscular system, are affected in SMA. Previous studies have highlighted alterations in the vascular system in both SMA patients and in a variety of mouse models of the disease, reporting alterations in vessel structure and perfusion abnormalities in peripheral tissues. In this project a detailed morphological investigation of the capillary beds of skeletal muscle and the spinal cord, two of the key pathological tissues in SMA was undertaken. This work was conducted in the Smn-/-;SMN2, Smn-/-;SMN2tg/+ and Smn-/-;SMN2;Δ7 mouse models of SMA. Significant alterations in the form and extent of the skeletal muscle and spinal cord capillary bed in SMA mice were identified, the most striking of which being a reduction in capillary density in SMA tissue when compared to control littermate tissue. In skeletal muscle, this reduction in capillary density was found to be a postnatal phenomenon, which occurred independently of denervation, in a variety of phenotypically distinct muscles and in all three SMA mouse models investigated. In the spinal cord, the capillary defect was seen to develop in a similar postnatal pattern to that observed in skeletal muscle. Importantly, a reduction in capillary density was observed in the ventral horn of the spinal cord, which houses motor neuron cell bodies, a known pathological target in SMA. These motor neurons were seen to be surrounded by fewer capillaries than their control counterparts. Using an injectable marker of hypoxia, it was determined that the cells of the ventral horn of SMA spinal cords are hypoxic. This suggests that the capillary defect identified has a functional impact on the tissues it is observed in. Having established the presence of capillary defect in SMA tissue, the effect of potential SMA therapeutics on the capillary defect was then investigated. The effect of HDAC inhibitors, which have been successfully shown to increase the levels of the disease causing Smn protein, was investigated. Treatment with the HDAC inhibitor SAHA was found to ameliorate the capillary defect, significantly improving capillary density in SMA skeletal muscle. This implies that the capillary defect is related to Smn levels in tissue and is amenable to therapeutics which increase Smn levels. Having characterised the capillary defect in SMA tissues in detail, a selection of tools were then used to investigate the underlying mechanisms resulting in the defect. First, using primary cell cultures, the growth and morphology of the key cellular component of capillaries, the endothelial cell, was examined. While displaying reduced levels of the Smn protein, endothelial cells isolated from SMA tissues showed no difference in growth rate, morphology or endothelial cell marker expression when compared to endothelial cells isolated from control tissue. This suggests that the defects seen in SMA capillary beds are not the result of defects in the structure and growth of endothelial cells. Second, retinas from SMA mice were found to exhibit similar capillary defects to those observed in SMA skeletal muscle and spinal cord. Given the entirely postnatal development of the retinal capillary network, the retina was identified as a useful experimental preparation for the further investigation of the mechanisms underlying the capillary defect in SMA. In summary, this work highlights the incidence and importance of capillary defects in mouse models of spinal muscular atrophy.

Spinal Muscular Atrophy (SMA) is a neuronal degenerative disease caused by the mutation or loss of the Survival Motor Neuron (SMN) gene. The cause for the specific motor neuron susceptibility in SMA has not been identified. The high axonal transport/localization demand on motor neurons may be one potentially disrupted function, more specific to these cells. We therefore used a large-scale immunoprecipitation (IP) experiment, to identify potential interactors of SMN involved in neuronal transport and localization of mRNA targets. We identified KH-type splicing regulatory protein (KSRP), a multifunctional RNA-binding protein that has been implicated in transcriptional regulation, neuro-specific alternative splicing, and mRNA decay. KSRP is closely related to chick zipcode-binding protein 2 and rat MARTA1, proteins involved in neuronal transport/localization of beta-actin and microtubule-associated protein 2 mRNAs, respectively. We demonstrated that KSRP is arginine methylated, a novel SMN interactor (specifically with the SMN Tudor domain; and not with SMA causing mutants). We also found this protein to be misregulated in the absence of SMN, resulting in increased mRNA stability of KSRP mRNA target, p21cip/waf1. A role for SMN as an axonal chaperone of methylated RBPs could thus be key in SMA pathophysiology.

Spinal Muscular Atrophy (SMA) is a neuronal degenerative disease caused by the mutation or loss of the Survival Motor Neuron (SMN) gene. The cause for the specific motor neuron susceptibility in SMA has not been identified. The high axonal transport/localization demand on motor neurons may be one potentially disrupted function, more specific to these cells. We therefore used a large-scale immunoprecipitation (IP) experiment, to identify potential interactors of SMN involved in neuronal transport and localization of mRNA targets. We identified KH-type splicing regulatory protein (KSRP), a multifunctional RNA-binding protein that has been implicated in transcriptional regulation, neuro-specific alternative splicing, and mRNA decay. KSRP is closely related to chick zipcode-binding protein 2 and rat MARTA1, proteins involved in neuronal transport/localization of beta-actin and microtubule-associated protein 2 mRNAs, respectively. We demonstrated that KSRP is arginine methylated, a novel SMN interactor (specifically with the SMN Tudor domain; and not with SMA causing mutants). We also found this protein to be misregulated in the absence of SMN, resulting in increased mRNA stability of KSRP mRNA target, p21cip/waf1. A role for SMN as an axonal chaperone of methylated RBPs could thus be key in SMA pathophysiology.

Spinal Muscular Atrophy (SMA) is a neuronal degenerative disease caused by the mutation or loss of the Survival Motor Neuron (SMN) gene. The cause for the specific motor neuron susceptibility in SMA has not been identified. The high axonal transport/localization demand on motor neurons may be one potentially disrupted function, more specific to these cells. We therefore used a large-scale immunoprecipitation (IP) experiment, to identify potential interactors of SMN involved in neuronal transport and localization of mRNA targets. We identified KH-type splicing regulatory protein (KSRP), a multifunctional RNA-binding protein that has been implicated in transcriptional regulation, neuro-specific alternative splicing, and mRNA decay. KSRP is closely related to chick zipcode-binding protein 2 and rat MARTA1, proteins involved in neuronal transport/localization of beta-actin and microtubule-associated protein 2 mRNAs, respectively. We demonstrated that KSRP is arginine methylated, a novel SMN interactor (specifically with the SMN Tudor domain; and not with SMA causing mutants). We also found this protein to be misregulated in the absence of SMN, resulting in increased mRNA stability of KSRP mRNA target, p21cip/waf1. A role for SMN as an axonal chaperone of methylated RBPs could thus be key in SMA pathophysiology.

Spinal Muscular Atrophy (SMA) is a neurodegenerative, inherited disease caused by an insufficient amount of functional Survival of Motor Neurone protein (SMN), though the exact mechanism underlying this is not fully understood. The primary function of SMN is assembling a ring of Sm proteins around small nuclear RNA (snRNA) in an early, cytoplasmic stage of small nuclear ribonucleoprotein (snRNP) biogenesis, a process essential in eukaryotes. SMN, together with several mRNA binding proteins, has been linked to neural transport of mRNA towards areas of growth in Motor neurons for local translation of transcripts. Previous research in our group has found that this may involve Coatomer protein-containing vesicles transported by Dynein and requiring the Sm family protein, SmB, for maintenance. Little is known, however, about what other proteins are also present and required for correct transport and localisation of these vesicles. To further investigate this, we have produced plasmids expressing each Sm protein tagged to fluorescent proteins to help track their behaviour, in some cases for the first time, and developed a detergent-free fractionation protocol to enrich for SMN containing vesicles, providing tools that can be used to further probe behaviour and interactions in the future. Using these approaches, SmN, a neural specific Sm protein, was identified to also be present in SMN-containing vesicles similarly to SmB. Analysis of the interactomes of different Sm proteins identified a novel interactor of SMN, Neurochondrin (NCDN), that appears to be required for the correct localisation of SMN in neural cells. NCDN was found to not associate with snRNPs, indicating an snRNP-independent interaction with SMN. NCDN and SMN both independently associated and co-enriched with Rab5, indicating a potential endocytic and cell polarity role for the interaction. This interaction has the potential to be key in SMA pathology and may have therapeutic potential.

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder characterized by widespread loss of lower motor neurons from the spinal cord. Lower motor neuron degeneration leads to a progressive decline in motor development, manifesting as muscle atrophy and weakness. It is now well characterised that ubiquitin homeostasis is altered in SMA and that reduction of the ubiquitin-like modifier-activating enzyme 1 (UBA1) is central to this disruption. UBA1 is responsible for activating ubiquitin as the first step in the ubiquitin conjugation process, marking unwanted proteins for degradation by the proteasome. While it is known that therapies targeting UBA1 rescue neuromuscular phenotypes in SMA models, the mechanism by which UBA1 mediates neurodegeneration is unclear. In fact, very little is known about the function of UBA1 beyond its canonical role in the ubiquitin proteasome system. To better understand the role of UBA1 in motor neuron degeneration, a robust set of antibodies for both in vivo and in vitro work to study UBA1 have been identified. This enabled a novel characterisation of UBA1 distribution throughout disease progression in SMA spinal motor neurons to be performed, revealing that UBA1 reduction is an important pre-symptomatic molecular feature of SMA. To identify downstream targets of UBA1 critical for UBA1-mediated degeneration in SMA, label-free proteomics was performed on HEK293 cells after overexpression or knockdown of UBA1. The proteomics data was analysed across multiple platforms, including Biolayout, IPA and DAVID to identify UBA1-dependent pathways and demonstrated that modulation of UBA1 levels lead to disruption of key cellular pathways including translation elongation, nuclear transport, and tRNA synthetases. Validation of target proteins from these UBA1-dependent pathways identified that the tRNA synthetease GARS behaves in a UBA1-dependent manner across a range of model systems in vitro and in vivo. It was then identified that GARS expression is significantly dysregulated across a range of neuronal tissues in a mouse model of SMA. Interestingly, mutations in GARS cause Charcot-Marie-Tooth disease type 2D (CMT2D), an axonal neuropathy, in which a disruption to sensory neuron fate in dorsal root ganglia has recently been identified. In a mouse model of SMA we identified a phenotype consistent with that in the CMT2D mouse model and showed that disruption to sensory neuron fate is reversible and dependent on changes in UBA1 and GARS expression in SMA. In conclusion, modulation of UBA1 levels leads to disruption of key cellular pathways, with dysregulation of tRNA synthetases a prominent feature that is likely to play a role in the pathogenesis of SMA.

Les techniques de criblage actuelles (in vitro et in silico) sont dépendantes des efforts menés en biologie médicinale pour identifier des cibles biologiques pertinentes ; cibles difficiles à définir pour les maladies génétiques dites "perte de fonction". De plus, les composés issus de ces cribles s'avèrent souvent inefficaces et/ou toxiques une fois confrontés à la complexité physiologique d'un organisme entier. Pour contourner ce problème, nous proposons d'utiliser le nématode C. elegans, notamment pour des maladies répondant aux critères suivants : i) physiopathologie complexe et/ou mal comprise excluant le développement à court terme de médicaments sur une base rationnelle, ii) peu d'espoir de thérapie génique/cellulaire à court terme, iii) conservation chez C. elegans du gène relié à la maladie humaine et induisant un phénotype exploitable une fois inactivé. Nous démontrons ici que ce petit nématode permet de tester, à moindre coût, un grand nombre de composés chimiques tout en conservant la complexité physiologique d'un animal entier. De plus, la souplesse génétique de cet animal permet d'apporter rapidement des informations sur le mode d'action des composés identifiés. Ainsi, en plus du but initial visant à identifier des molécules bioactives à intérêt thérapeutique, cette approche peut permettre de dégager de nouvelles cibles moléculaires utiles pour l'industrie chimique, et cruciales pour la recherche de traitements contre les maladies perte de fonction. Finalement, nous présentons comment mettre en place une telle stratégie, notamment pour la myopathie de Duchenne, l'amyotrophie spinale et le syndrome de Schwartz-Jampel. Enfin, nous présentons les résultats obtenus lors des différentes campagnes de criblage, les validations des molécules les plus prometteuses et les travaux effectués pour tenter de comprendre leur mode d'action chez le nématode.

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Spinal muscular atrophy (SMA) and Charcot-Marie-Tooth disease type 2D (CMT2D)/distal SMA type V (dSMAV) are two incurable neuromuscular disorders that predominantly manifest during childhood and adolescence. Both conditions are caused by mutations in widely and constitutively expressed genes that encode proteins with essential housekeeping functions, yet display specific lower motor neuron pathology. SMA results from recessive inactivating mutations in the survival motor neuron 1 (SMN1) gene, while CMT2D/dSMAV manifests due to dominant point mutations in the glycyl-tRNA synthetase (GlyRS) gene, GARS. Using a number of different model systems, ranging from Caenorhabditis elegans to the mouse, this thesis aimed to identify potential novel therapeutic compounds for SMA, and to increase our understanding of the mechanisms underlying both diseases. I characterised a novel C. elegans allele, which possesses a point mutation in the worm SMN1 orthologue, smn-1, and showed its potential for large-scale screening by highlighting 4-aminopyridine in a screen for compounds able to improve the mutant motility defect. Previously, the gene encoding three isoforms of chondrolectin (Chodl) was shown to be alternatively spliced in the spinal cord of SMA mice before disease onset. I performed functional analyses of the three isoforms in neuronal cells with experimentally reduced Smn levels, and determined that the dysregulation of Chodl likely reflects a combination of compensatory mechanism and contributor to pathology, rather than mis-splicing. Finally, working with two Gars mutant mice and a new Drosophila model, I have implicated semaphorin-plexin pathways and axonal guidance in the GlyRS toxic gain-of-function disease mechanism of CMT2D/dSMAV.

Les techniques de criblage actuelles (in vitro et in silico) sont dépendantes des efforts menés en biologie médicinale pour identifier des cibles biologiques pertinentes ; cibles difficiles à définir pour les maladies génétiques dites "perte de fonction". De plus, les composés issus de ces cribles s'avèrent souvent inefficaces et/ou toxiques une fois confrontés à la complexité physiologique d'un organisme entier. Pour contourner ce problème, nous proposons d'utiliser le nématode C. elegans, notamment pour des maladies répondant aux critères suivants : i) physiopathologie complexe et/ou mal comprise excluant le développement à court terme de médicaments sur une base rationnelle, ii) peu d’espoir de thérapie génique/cellulaire à court terme, iii) conservation chez C. elegans du gène relié à la maladie humaine et induisant un phénotype exploitable une fois inactivé. Nous démontrons ici que ce petit nématode permet de tester, à moindre coût, un grand nombre de composés chimiques tout en conservant la complexité physiologique d'un animal entier. De plus, la souplesse génétique de cet animal permet d'apporter rapidement des informations sur le mode d'action des composés identifiés. Ainsi, en plus du but initial visant à identifier des molécules bioactives à intérêt thérapeutique, cette approche peut permettre de dégager de nouvelles cibles moléculaires utiles pour l'industrie chimique, et cruciales pour la recherche de traitements contre les maladies perte de fonction. Finalement, nous présentons comment mettre en place une telle stratégie, notamment pour la myopathie de Duchenne, l'amyotrophie spinale et le syndrome de Schwartz-Jampel. Enfin, nous présentons les résultats obtenus lors des différentes campagnes de criblage, les validations des molécules les plus prometteuses et les travaux effectués pour tenter de comprendre leur mode d'action chez le nématode. / Current high-throughput screening methods for drug discovery rely on the existence of targets. Moreover, most of the hits generated during screenings turn out to be invalid after further testing in animal models. To by-pass these limitations, efforts are now being made to screen chemical libraries on whole animals. One of the most commonly used animal model in biology is the murine model Mus musculus. However, its cost limits its use in large-scale therapeutic screening. In contrast, the nematode Caenorhabditis elegans is gaining momentum as screening chemical tool. This tiny worm combines genetic amenability, low cost, and culture conditions that are compatible with large-scale screens. Its main advantage is to allow high-throughput screening in a whole-animal context. Moreover, its use is not dependent on the prior identification of a target and permits the selection of compounds with an improved safety profile. Here, we introduce this approach with the Duchenne Muscular Dystrophy, the Spinal Muscular Dystrophy and the Schwartz-Jampel syndrome. We present the methodology used with each model to screen up to 7,000 compounds and the results of these screening campaigns. We further present the validation of our best hits and try to understand their mechanism of action.