Aneuploidy has been observed in various physiological tissues, however the frequency of this error remained highly debatable. This discovery challenges the current view that suggested extra-centrosomes as only contributor to spindle multipolarity and provides a rational to understand chromosome instability typical of polyploid cells.The aim of the second part of my PhD project was to generate a novel tool to quantitively probe chromosome loss in vivo in Drosophila tissues. Indeed, laser ablation to disrupt and increase in microtubule stability and length to bypass the DNA-barrier could rescue bipolar spindle formation. Further I combined DNA and spindle perturbations with computer modelling and found that in polyploid cells, the presence of excessive DNA acts as a physical barrier blocking spindle pole coalescence and bipolarity. Then I identified contributors to the multipolar status using in vivo approaches in Drosophila neural stem cells and in vitro culture of cancer cells. I found that the presence of extra DNA and extra centrosomes generated invariably multipolar spindles. For example, variations to genome content are associated with chromosome instability and cancer development, however their exact contribution to cancer genome remains poorly understood.In the first part of my PhD project, I investigated the consequences of polyploidy during cell division. Importantly, whole genome duplications and aneuploidy have also been associated to pathological conditions. Polyploidy is the doubling of the whole chromosome set and aneuploidy concerns the gain or loss of whole chromosomes. However, numerical deviations to the diploid set are observed in healthy tissues. This is essential for the maintenance of genome stability, tissue and organism homeostasis. Establishment of proper bipolar mitotic spindle containing two centrosomes, one at each pole contributes to accurate chromosome segregation. Most animal cells are diploid, containing two copies of each chromosome. Our in-vivo study is the first to directly rescue the motor defects of an SMA model by expressing Smn in an identifiable population of Drosophila neuroblasts and developing neurons, highlighting that neuronal sensitivity to SMN loss may arise before synapse establishment and nerve cell maturation. Finally, combinatorial SMN knockdown in immature and mature neurons synergistically enhances the locomotor and survival phenotypes. By restoring SMN levels in these distinct neuronal population, we partially rescue the larval locomotor defects of Smn mutants. Our aim was to express specifically in the neuronal progenitor cell types that have not formed synapses, and thus a time that precedes neuromuscular junction formation and maturation. Using targeted knockdown, we further restricted SMN manipulation in neuroblasts to a defined time window.
Furthermore, to specifically address the relevance of SMN during neurogenesis and in neurogenic cell types, we show that SMN knockdown using neuroblast-specific and pan-neuronal drivers, but not differentiated neuron or glial cell drivers, impairs adult motor function. Here, in a Drosophila SMA model, we show that neurodevelopmental defects precede gross locomotor dysfunction in larvae. Notwithstanding, how developmental defects contribute to the subversion of postnatal and adult motor function remains elusive. Although SMA is degenerative in nature, SMN function during embryonic and early postnatal development appears to be essential for motor neuron survival in animal models and humans. Mutations and deletions in the widely expressed survival motor neuron 1 ( SMN1 ) gene cause SMA however, the mechanisms underlying the selectivity of motor neuron degeneration are not well understood.
Spinal muscular atrophy (SMA) is the most common autosomal recessive neurodegenerative disease, and is characterised by spinal motor neuron loss, impaired motor function and, often, premature death.
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