Rho Signaling and Axon Regeneration - International Review of Neurobiology - page 124

to either cell bodies or axon tips can promote neurite outgrowth, and delayed
treatment of crushed optic nerve with cell-permeable C3 promotes regen-
eration of RGCs across the lesion scar ( Bertrand et al., 2007; Bertrand,
Winton, Rodriguez-Hernandez, Campenot, & McKerracher, 2005 ). The
Benowitz lab has shown that transfection of RGCs with C3 promoted
the regeneration of RGC axons in the optic nerve ( Fischer, Petkova,
Thanos, & Benowitz, 2004 ). Studies with ROCK inhibitors also show
axon regeneration in the optic nerve. Treatment with Y-27632 promotes
RGC regeneration, and cotreatment with growth factor further enhances
the effect ( Lingor et al., 2007, 2008 ).
Many independent groups have documented axon regeneration in
injured spinal cord, but the results have been more variable. In the first
studies, permeable C3 was applied to the site of overhemisection injury
in adult mice ( Dergham et al., 2002 ). Anterograde labeling techniques were
used to show that Rho inactivation promotes sprouting rostral to the lesion,
with some long-distance regeneration through the lesion site and past the
lesion. In addition, in situ hybridization was used to show an increase in
the level of growth-associated protein 43 (GAP43) in the motor cortex of
treated animals. In an independent study by Fournier et al. (2003) , histolog-
ical observations suggested that treatment with C3 reduced the formation of
the glial scar after SCI, but axon regeneration was not observed. This failure
to replicate the results is likely because C3 lacking a transport sequence
was used and the delivery methods were different. More recently, we
have compared the ability of C3 and cell-permeable C3 (Cethrin) to
penetrate the spinal cord, and find that C3 has poor penetration compared
to Cethrin, which can diffuse readily throughout the spinal cord
( Lord-Fontaine et al., 2008 ). Thus, for studies of neurotrauma and SCI, ad-
equate penetration of reagents is critical, and this challenge is even greater
when translated to the larger spinal cord of patients.
ROCK inhibitors also have been used to show the importance of
inactivation of the Rho signaling pathway to promote regeneration of axons
in the spinal cord. Y-27362 promotes sprouting and regeneration in the
injured spinal cord, including cortical spinal tract axons ( Chan et al.,
2005; Dergham et al., 2002; Fournier et al., 2003 ). Experiments with
dominant negative ROCK show potential of rubrospinal axons to
regenerate ( Wu et al., 2009 ). In a dorsal rhizotomy model, inhibiting
ROCK with Y-27632 induced growth of serotonergic and tyrosine
hydroxylase-positive axons and also rescued the development of pain
sensitivity after SCI ( Ramer, Borisoff, & Ramer, 2004 ). Together, the
diverse experimental approaches by multiple laboratories provide strong