MAG, Nogo-A and NgR in Hippocampal Development and Regeneration

Abstract

The adult central nervous system (CNS) has a very little capability to regrow its connections after lesion. The lack of axonal regeneration is a multifactorial problem, which is mainly due to the presence of inhibitory molecules in the CNS. Two main families of inhibitors exist, that of the so-called myelin-associated inhibitors, and that of the chondoritin-sulfate proteoglycans (CSPG). The question this thesis aims to address is the <i>role of myelin-associated inhibitors in the regeneration of cortical connections</i>. To do so, the model we have used is the entorhino-hippocampal connection and the main questions addressed were i) characterization of the temporal expression of MAG, Nogo-A and NgR, ii) analysis of their regulation after lesion, and iii) study of the effect of their blockade on axonal regeneration.<br/>Our results demonstrate that developmental expression of the myelin-associated inhibitors MAG and Nogo-A temporally coincides with the loss of regeneration capability of the entorhino-hippocampal connection also known as perforant pathway (PP). Moreover, expression of NgR (the common receptor for MAG and Nogo-A) also matches regeneration loss. After lesioning of the PP, MAG is overexpressed by mature oligodendrocytes which experiment a process of glial reactivity. Similarly, Nogo-A is re-expressed by reactive astrocytes in the deafferented hippocampus. The regulation of both inhibitors following injury supports a direct implication in the preventing axonal regeneration. To confirm this implication, we performed the blockade of MAG and the inhibitory domains of Nogo-A in an in vitro model (entorhino-hippocampal organotypic cultures). Both the blockade of MAG with Neuraminidase and the blockade of Nogo-A with NEP1-40 induced regeneration of the PP. Moreover, Blockade of CSPG with ChABC proved to be efficient in the same model. However, combination of NEP1-40 and ChABC, in order to block simultaneously Nogo-A-induced and CSPG-induced inhibition, did not display additive effects, suggesting a convergence in the inhibition mechanisms employed by both inhibitors. Last, we addressed the possibility of delaying treatment delivery as a therapeutic approach. While we confirmed that PP keeps its capability to regenerate during at least five days after lesion, we also determined that ChABC-based treatments are greatly compromised and lose efficiency when treatment is delayed.<br/>In conclusion, we have characterized the developmental expression pattern of the inhibitors MAG and Nogo-A and their common receptor NgR. The expression of the three proteins is strongly regulated following lesion and the blockade of MAG and Nogo-A is proved to be successful to promote axonal regeneration. We also determined the convenience of combined treatments and the most successful delivery schedule. Our data provide an insight for the development of new strategies addressed to induce axonal regeneration of injured cortical connections.