Mechanisms governing neuronal degeneration and axonal regeneration in the mature retinofugal system

Abstract

The ability of mammalian CNS neurons to regrow their lesioned axons declines during late embryonic and postnatal development. Consequently, adult retinal ganglion cells of mammals respond to injuries with rapid anterograde and protracted retrograde (Wallerian) degeneration. To monitor the cascade of events initiated by neuronal injuries, and to explore whether the regressive events of this cascade can be blocked or reversed, axotomy-induced ganglion cell responses were investigated in adult rats. The aim of the experiments was to block degradation of axotomized ganglion cells with enzymes which inhibit proteolytic activities within the retina (protease inhibitors). To achieve this goal, a new fluorescence technique was employed to assess both the chronotopological pattern of degradation and the efficacy of the protease inhibitors and anti-inflammatory treatment in preventing cell death. Injection of protease inhibitors alone or combined with dexamethasone into the vitreous body of animals whose optic nerves were transected, protected ganglion cells from degradation and prevented endocytosis-dependent tracing of microglia. Two major functions of rescued ganglion cells proved their viability: (1) the numbers of ganglion cell axons extended from retinal stripes that were explanted 1 week after axotomy and cultured in vitro, were significantly higher when the retinal pieces originated from retinae pretreated with protease inhibitors and dexamethasone at the time of optic nerve transection than from untreated retinae; (2) the numbers of ganglion cells which regenerated axons into transplanted peripheral nerve pieces were more than doubled when the eyes were injected with protease inhibitors and dexamethasone during axotomy. The results show that blocking of the retinal proteases, which are presumably localized in microglial cells, and simultaneous treatment of the intraretinal inflammation, are key steps in understanding the intraretinal responses to axotomy and for beneficially manipulating the numbers of surviving neurons. In addition to the supporting influence of neurotrophic factors and to nonpermissive features of oligodendroglia, the microglia co-regulate whether neurons can regenerate their axons.