Wodarczyk, Linda

Mammals, unlike lower vertebrates, cannot normally regenerate injured central nervous system neurons. Although rat retinal ganglion cells (RGCs), following optic nerve crush, will undergo an initial period of sprouting, axon outgrowth is limited and subsequently aborted. This study examined how extensive the changes in fast transported proteins (FTPs) were during the early response to RGC damage and whether these changes were comparable to those known to occur in lower vertebrate RGCs. Changes in mRNA for several known proteins were also analyzed. It was found that, within 2 days, axotomized rat RGCs initiated a program of cell growth, involving the differential synthesis and transport of a broad range of FTPs. This response is very similar to that of lower vertebrates and indicates that rat RGCs are capable of initiating the metabolic responses necessary for regeneration to begin. This response, however, was not sustained beyond 5 days axotomy.

The survival of rat retinal ganglion cells (RGCs) after axotomy has been shown to be enhanced by Brain Derived Neurotrophic Factor (BDNF). It was, therefore, of interest to determine whether previously observed changes in the differential regulation of fast axonally transported proteins (FTPs) occur in rat RGCs during the early response to axotomy or whether such changes are obviated by the action of BDNF at the cell body level. It was of further interest to determine whether these regeneration-associated changes are sustained during the period of BDNF-enhanced cell survival. It was found that, within 2 days of injury and BDNF injection, rat RGCs initiate a growth-like cellular response that includes the differential synthesis and transport of the same profile of FTPs found to be induced in axotomized animals following injection of a saline control solution. Thus, supplementation of rat RGCs with BDNF does not obviate the changes required to reinstate active cellular regrowth. It is, therefore, unlikely that the loss of a trophic factor, such as BDNF, is the signal for axotomy-induced changes. Although a single injection of BDNF at the time of injury prolongs cell survival to at least 5 days, it is not sufficient to sustain the elevation in FTPs. This result indicates that the regulatory mechanisms that promote cell growth are distinct and separate from those that promote cell survival. This study extended beyond the above findings to affirm that apoptosis of axotomized rat RGCs is mediated by the activation of the cysteine protease, caspase-3. Such activation was demonstrated within 12 hours of axotomy and appeared to become increasingly prevalent in a central to peripheral gradient, as might be anticipated by the loss of glial derived neurotrophic support. Such activation was completely prevented by intraocular injection of BDNF, indicating that BDNF acts upstream of caspase-3 to prevent the proteolytic cascade that leads to apoptosis.