Le rôle des cellules gliales de Müller dans la mort des cellules ganglionnaires de la rétine par des mécanismes cellulaires non-autonomes

Glial cells are essential for the functioning of the nervous system. In the retina, the Müller glial cells ensure the homeostasis of this tissue as well as the protection of neurons including the retinal ganglion cells (RGCs).  The main hypothesis of this thesis is that Müller cells play a predominant role in neuronal survival both at the levels of neurotrophin/proneurotrophin signaling following injury and excitotoxic mechanisms. Unlike the brain-derived neurotrophic factor (BDNF), the nerve growth factor (NGF) is unable to induce RGCs survival following optic nerve transection. The first objective of the thesis was therefore to describe the expression of the two receptors of NGF, p75NTR and TrkA, in the adult retina and to address their functional role by using peptidomimetic agonistic or antagonistic ligands specific for each receptor. Our results showed that TrkA is overexpressed by RGCs following axotomy, whereas p75NTR is specifically expressed by Müller cells. While NGF by itself does not promote RGC survival, selective activation of TrkA receptors using peptidomimetic ligands is markedly neuroprotective. Surprisingly, selective blockers of p75NTR, or the absence of p75NTR, protect RGCs from axotomy-induced death. Moreover, combination of NGF or TrkA agonists with p75NTR antagonists functions synergistically to enhance RGC survival. These results reveal a new mechanism by which p75NTR expression by Müller glia may profoundly influence neuronal survival. Next, we wanted to address the effect of proneurotrophins in the adult retina. We showed that injection of proNGF induces the death of RGCs in rats and mice by a p75NTR-dependent signaling mechanism. Expression of p75NTR in the adult retina being confined to Müller glial cells, we tested the hypothesis that proNGF activates a non-cell autonomous signaling pathway to induce RGC death. Consistent with this notion, we showed that proNGF induced a robust expression of tumor necrosis factor α (TNFα) in Müller cells, and that genetic or biochemical ablation of TNFα blocked proNGF-induced death of retinal neurons. Mice rendered null for p75NTR, its co-receptor sortilin, or the adaptor protein NRAGE were defective in proNGF-induced glial TNFα production and did not undergo proNGF-induced retinal ganglion cell death. We concluded that proNGF activates a non-cell autonomous signaling pathway that causes TNFα-dependent death of retinal neurons in vivo.  The central hypothesis of excitotoxicity is that excessive stimulation of neuronal N-Methyl-D-Aspartate (NMDA)-sensitive glutamate receptors is harmful to neurons and contributes to a variety of neurological disorders. Glial cells have been proposed to participate in excitotoxic neuronal loss, but their precise role is poorly defined.  In this in vivo study, we showed that NMDA induces a strong NF-κB activation in Müller glia, but not in retinal neurons. Intriguingly, NMDA-induced death of retinal neurons was effectively blocked by inhibitors of NF-κB activity. We demonstrated that TNFα protein produced in Müller glial cells via an NMDA-induced NF-κB dependent pathway plays a crucial role in the excitotoxic loss of retinal neurons. This cell loss occurs mainly through a TNFα-dependent increase in Ca2+-permeable AMPA receptors on susceptible neurons. Thus, our data reveal a novel non-cell-autonomous mechanism by which glial cells can profoundly exacerbate neuronal death following excitotoxic injury.

Keyphrases: Nerve growth factor, pro-NGF, p75NTR, Nuclear Factor K B