The ability of adult CNS neurons to successfully regenerate their axons upon injuries such as
stroke and spinal cord injury is very limited. Over the last two decades researches have focused
on the challenging question of how to overcome this regenerative failure. The identification of
the myelin associated neurite outgrowth inhibitor Nogo-A shed light on the inhibitory nature,
compared to the PNS, of CNS myelin. Subsequent Nogo-A neutralization experiments led to
successful CNS regeneration in animal models of spinal cord injury. The first part of Chapter 1
summarizes important findings regarding Nogo-A function as a neurite outgrowth inhibitor and
gives insight into emerging roles of Nogo-A during development and disease.
Nogo-A contains two inhibitory regions for neurite outgrowth inhibition: the Nogo66 region,
also common to Nogo-B and Nogo-C, and the Nogo-A specific region, NogoΔ20. Whereas the
signaling mechanisms of the Nogo66 region are well characterized, comparatively little is known
about the signaling complex of NogoΔ20. Since blocking of the Nogo66 receptor complex does
not completely abolish myelin inhibition of neurite outgrowth, it is of great importance to
elucidate the Nogo66 receptor independent mechanism of Nogo-A signaling.
Over the last few years it has become clear that endocytosis plays an active role during cellular
communication. A number of surface ligand-receptor complexes are known to internalize into
cells and signal from endosomes. The second part of Chapter 1 introduces endocytosis as a
possible mechanism for NogoΔ20 signaling and reviews the known endocytic pathways.
In Chapter 2 we find that the soluble, active NogoΔ20 fragment is internalized into neuronal
cells. The internalization into early endosomes does not follow a conventional clathrin-and
dynamin dependent route, it rather depends on the Pincher protein and the small GTPase Rac.
This Pincher-mediated macroendocytosis of NogoΔ20 results in the formation of NogoΔ20
signalosomes that direct RhoA activation and growth cone collapse. In compartmentalized
chamber cultures, NogoΔ20 is endocytosed into neurites and retrogradely transported to the cell
bodies of DRG neurons, triggering RhoA activation en route and decreasing pCREB levels in
cell bodies. The decrease of pCREB points towards an antagonistic regulation of the growth
machinery by Nogo-A and neurotrophins. Taken together, the findings of the present thesis
suggest that Pincher-dependent macroendocytosis may lead to the formation of a Nogo-A
signaling endosome that is retrogradely transported from the axons to the cell body thus
suppressing the neuronal growth program.