Dégénérescence locale et réparation anormale de la jonction neuromusculaire dans un modèle de la sclérose latérale amyotrophique

Amyotrophic lateral sclerosis (ALS) is an aggressive neurodegenerative disease characterized by the loss of upper and lower motor neurons, and of their connections to muscle cells, the neuromuscular junctions (NMJs). In the last century, a plethora of novel genetic causes of ALS were identified and were associated with numerous mechanisms of cellular toxicity. Notably, it is now well appreciated that motor neurons do not degenerate by themselves and that glial cells contribute to numerous aspects of ALS pathogenesis. Nevertheless, no consensus has been reached on the origin of the selective degeneration of these neurons.  Multiple recent studies identified local alterations at the level of NMJs, including notably a dysfunction of the glial cells at this synapse, the perisynaptic Schwann cells (PSCs). These alterations suggest that an early destabilization of NMJs could contribute to ALS. Nevertheless, the sequence of events leading to NMJ loss in ALS is ill-defined. Notably, whether the loss of NMJs is caused by the selective degeneration of a subpopulation of motor neurons or by the local degeneration of individual axonal branches remains a controversial question. This distinction is of great importance as the mechanisms underlying these two events potentially differ greatly. Furthermore, the extent and efficiency of compensatory events remain uncertain, even though ample elements suggesting that some motor neurons attempt to compensate for the loss of NMJs.  Hence, the goal of this thesis was first to characterize the degenerative and compensatory events taking place within the axonal arborization of individual motor neurons during disease progression in a classical model of ALS, the SOD1G37R mice. Through the repeated in vivo imaging of individual axonal arborization, we show that some axonal branches progressively retract from their NMJs while others are maintained for several weeks before global axonal degeneration. This asynchronous dismantlement of axon terminals is accompanied by the formation new axonal branches, reestablishing synaptic connections with NMJs which were not initially innervated by this motor neuron. Interestingly, we show that these compensatory events were much more frequent in females, but were associated with a slightly faster disease progression, suggesting that excessive compensation could be deleterious. Moreover, we show that, despite the presence of some types of compensatory events (axonal sprouting), other types of compensatory events seemed absent or inefficient (terminal sprouting), suggesting that mechanisms regulating NMJ repair are altered in ALS. Hence, we next wanted to investigate one of the mechanisms which could contribute to NMJ instability and repair abnormalities, namely the dysfunction of PSCs. Specifically, we evaluated if the alteration of PSC properties affected their capacity to promote NMJ repair and contributed to the deficits observed in vivo. We identified that PSCs failed to adapt their phenotype upon the dismantling of NMJs. Notably, we show that PSCs did not upregulate their expression of Galectin-3, a factor associated with glial phagocytosis of axonal debris. Importantly, this incapacity of PSCs to promote reinnervation reflected the selective vulnerability of different types of NMJs in ALS, suggesting that PSCs could be one of the factors contributing to this vulnerability gradient.  Altogether, these studies show that the inital loss of NMJs in SOD1 mice is caused by the local dismantlement of individual axonal branches. This phenomenon is accompanied by a maladapted response of PSCs which potentially contributes to NMJ repair deficits in ALS. These evidence strongly suggest that local mechanisms, at the level of axon terminals and their neighboring cells, lead to NMJ loss in ALS. Therapies aimed at stabilizing NMJs could thus benefit ALS patients by improving their motor function.

Keyphrases: SOD1, Amyotrophic lateral sclerosis, motor neurons, neuromuscular junctions, perisynaptic Schwann cells