NGF and RAGE-p75NTR signaling in models of amyotrophic lateral sclerosis

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Abstract Neurotrophins are essential for differentiation and survival of neurons during development and modulate neuronal plasticity in the adult nervous system. Nerve growth factor (NGF), the best-characterized member of the family, exerts its actions by signaling mainly through two transmembrane receptors, the tyrosine kinase receptor TrkA and the p75 neurotrophin receptor (p75NTR). Besides promoting survival, NGF can also induce cell death by signaling through p75NTR. p75NTR is re-expressed at high levels by stressed neurons in pathological conditions, including amyotrophic lateral sclerosis (ALS). ALS is the most common adult onset motor neuron disease, characterized by the progressive degeneration of upper and lower motor neurons, leading to paralysis and death one to five years after symptoms onset. We have shown that in an ALS-mouse model, the re-expression of p75NTR by motor neurons coincides with an increase production of NGF by reactive astrocytes. However, the ability of mature NGF to induce cell death has been challenged because high concentrations of the neurotrophin are required to induce cell death in vitro, at least an order of magnitude higher than the required to promote survival. More than ten years ago, pro-neurotrophins were identified as the pro-apoptotic ligands of p75NTR, able to induce cell death at subnanomolar concentrations. Since then, pro- neurotrophins are considered the physiologically relevant pro-apoptotic ligands of p75NTR, and the field has focused on the regulation of pro-neurotrophin processing as the switch regulating the pro-survival/pro-death signaling of neurotrophins. Our proposal challenges this paradigm and adds a new layer of complexity. It shows that the pro-survival/pro-death activity of NGF could also be regulated by post-translational modification of the mature neurotrophin. Our preliminary data indicate that nitration and glycation confer mature NGF the exceptional ability to induce cell death at physiologically relevant concentrations (10,000-fold less compared to that required by native NGF). In addition, it identified RAGE, the receptor for advance glycation end products, as a new partner of p75NTR in the induction of cell death by post-translational modified-mature NGF. The primary goal of this proposal is to identify the mechanism by which post-translational modifications confer upon NGF the ability to signal through p75NTR and RAGE to promote cell death, and to establish its pathophysiological relevance in models of ALS. We will focus on the following specific aims: Aim 1-To identify the signaling pathways activated by post-translational modified-NGF. Aim 2-To elucidate the role of post- translational modified-NGF signaling in ALS cell culture models. Aim 3-To evaluate the effect of inhibiting RAGE-p75NTR signaling on the progression of the disease in ALS mouse models. This proposal will contribute to the current understanding of neurotrophin-mediated death signaling and it will provide in vitro evidence for the involvement of RAGE-p75NTR signaling in ALS pathology. Finally, it will provide in vivo proof for the role of RAGE signaling in astrocytes versus motor neurons and its value as a therapeutic target in ALS.