NEUROPROTECTIVE PROPERTIES OF CILIARY NEUROTROPHIC FACTOR ON THE SURVIVAL OF AXOTOMIZED RETINAL GANGLION CELLS IN VIVO

Abstract

<p>Over 90% of retinal ganglion cells (RGCs) die after intraorbital optic nerve transection close to the eye. Several factor contribute to RGC death including: loss of neurotrophic support, overproduction of free radicals, glutamate-mediated excitotoxicty, activation of pro-apoptotic caspases (3 and 9), and reactive glia. In the present study, the model of optic nerve transection was used to study the neuroprotective mechanisms and effects of ciliary neurotrophic factor on the survival of axotomized retinal ganglion cells. It was demonstrated that axotomized RGCs die by apoptosis since overexpression of Neuronal Inhibitory Apoptosis Protein (NAIP), a potent and selective inhibitor of caspase 3, protected axotomized RGCs that would have otherwise died if untreated. Protection of axotomized RGCs was further enhanced by overexpression of Ciliary Neurotrophic Factor (CNTF) using adenoviral and lentiviral vectors. It was demonstrated that intraocular administration of adenoviral vectors selectively transduced retinal Muller cells, and injection of this vector into the optic nerve stump at the time of optic nerve transection selectively transduced a small percentage of RGCs. Regardless of the route of administration, adenoviral-mediated overexpression of CNTF protected axtomized RGCs. In comparison to adenoviral-mediated delivery, the delivery of CNTF using lentiviral vectors protected greater numbers of axotomized RGCs and for an extended period of time not typically seen using the optic nerve transection model. It was assumed that viral-mediated transfer of CNTF rescued axotmized RGCs by directly activating the high affinity CNTF receptor alpha (CNTFRα) expressed on RGCs. However, CNTF can also protect axotomized RGCs indirectly, by activating the low affinity leukemia inhibitory receptor beta (LIFAβ) expressed on retinal astrocytes and Muller cells. It was demonstrated that viral-mediated overexpression of CNTF resulted in phenotypic changes in retinal glial cells (astrocytes and Muller cells) that may have increased their neuroprotective function. Overexpression of CNTF increased retinal levels of the gap junction protein, connexin 43 (Cx43), the intermediate filament glial fibrillary acidic protein (GFAP), the astrocyte-specific glutamate/aspartate trasporter-1, GLAST-1, and the astrocyte specific enzyme, glutamine synthetase (GS). Taken together, these results suggest that CNTF is capable of protecting axotomized RGCs by directly binding to injured neurons or by modulating surrounding glia. Together, these results suggest that pharmacological interventions that maintain or upregulate CNTF expression may confer a clinically beneficial neuroprotection.</p>