Endocrine Abstracts

Central nervous system neurones are generally incapable of regenerating their axons after injury due to the limited availability of neurotrophins, the development of a glial scar, and the presence of multiple axon growth inhibitors. We therefore designed short interfering RNA (siRNA) sequences to knock down components of the inhibitory signalling cascade and tested their ability to disinhibit the growth of FGF2-stimulated dorsal root ganglia neurone (DRGN) neurites in the presence of inhibitory CNS myelin. siRNA sequences to p75^NTR, NgR and Rho-A mRNA caused 70%, 100% and 100% knock down of the relevant protein, respectively, while changes in neither protein levels nor cellular immunoreactivity in DRGN were detected using the relevant scrambled siRNA control sequences, when analysed by Western blotting and immunocytochemistry. Importantly, siRNA-mediated knock down of p75^NTR, NgR and Rho-A in FGF2-stimulated DRGN promoted 5-, 3.5- and 6.5-fold increased neurite outgrowth in the presence of CNS myelin. Neurites grew longer after siRNA-mediated knock down of Rho-A than after knock down of p75^NTR and NgR. One reason for this could be that knock down of p75NTR and NgR only disinhibits neurite growth in the presence of myelin-derived axonal growth inhibitors such as MAG, Nogo and OMgp. Since our crude myelin preparation probably contains other inhibitory ligands, knock down of Rho-A will disinhibit growth on almost all known inhibitory ligands that signal growth cone collapse through the Rho-A cascade. Hence, Rho-A knock down might be the most effective therapeutic disinhibition strategy to promote CNS axon regeneration in vivo.