Resumen de Role of ps/γ-secretase-mediated signaling during neuronal development and degeneration
Presenilin-1 (PS1), the catalytic component of γ-secretase that regulates the processing of multiple transmembrane proteins is mutated in the majority of cases of familial Alzheimer’s disease (FAD). Recent evidence indicates that FAD-linked PS1 mutations reduce the γ-secretase cleavage of several transmembrane proteins, suggesting a loss-of-function mechanism. Indeed, PS1 inactivation during embryogenesis leads to morphological defects, whereas genetic inactivation of both PS in the adult brain causes age-dependent memory impairments and neurodegeneration. Moreover, the participation of PS in the proteolysis of signaling molecules involved in the development of nervous system, including ErbB4, suggest that these signaling pathways could contribute to neurodegeneration.
In this doctoral thesis we have studied the role of PS1/γ-secretase-dependent cleavage of EphA3 and Nrg1/ErbB4 signaling in neuronal development and neurodegeneration. Our results show that PS1/γ-secretase is required for axon growth in the developing brain. PS1/γ-secretase mediates axon elongation through the cleavage of EphA3 at Tyr560 resulting in the generation of an ICD fragment. EphA3 ICD regulates negatively RhoA, and interacts with and increases phosphorylation (S1943) of non-muscle myosin IIA (NMIIA) leading to filament disassembly and axon growth. In contrast to the classical ephrin/EphA3 signaling, PS/γ-secretase-dependent EphA3 signaling is independent of ligand. This result suggests for the first time opposite roles of EphA3 on inhibiting (ligand-dependent) and enhancing (PS/γ-secretase-dependent processing) axon growth in neurons. Second, we show that PS/γ-secretase regulates Nrg1 type III expression, mediates the processing of Nrg1 type III and ErbB4 and regulates negatively synaptogenesis through Nrg1. Taken together, our results show that PS1/γ-secretase regulates axon growth and synaptogenesis by regulating ligand-independent EphA3 signaling and Nrg1/ErbB4 processing/signaling, respectively. Our investigation paves the way for exploring new relationships between neurodevelopment and neurodegeneration, providing insights of the existence of a crosstalk among the signaling pathways involved in these processes.
