Purpose: Few proteins are known to be selectively expressed in retinal ganglion cells (RGCs), the neurons directly affected by optic nerve stroke and glaucoma. In addition, subsets of RGCs are reported to project to various CNS areas via the retinohypothalamic pathway in rodents and primates. Many of these areas exhibit immunoreactivity for the brain-expressed X-linked (Bex) proteins Bex1 and Bex2. This prompted us to evaluate expression of these proteins in retina.
Methods: We utilized rats and transgenic mice, coupled with a new rodent model of isolated optic nerve stroke (rodent anterior ischemic optic neuropathy, rAION). An anti-Bex1 antibody was reacted to retinal tissue extracts. To evaluate short term effects of rAION on RGC Bex expression, a double transgenic mouse strain was employed expressing cyan fluorescent protein (CFP) under control of the Thy-1 protein promotor, and beta-galactosidase (lacZ) under control of the immediate early stress c-fos gene promotor. Positive identification of rat RGCs was performed by retrograde fluorogold labeling via stereotactic CNS injection. Retinas were analyzed using both diaminobenzidine (DAB)-linked immunochemistry and confocal microscopy.
Results: Bex immunoreactivity is present at high levels in the retina. Bex1 and Bex2 are selectively expressed in RGCs and differentially expressed in a subset of large RGC neurons. Bex signal levels are lower in small RGC neurons, which preferentially express high levels of the transcription factor Brn3b. Post-stroke, Bex accumulates in the RGC cytoplasm, consistent with the optic nerve edema produced by clinical AION.
Conclusions: Bex immunoreactivity can be used to evaluate, ex vivo, the distribution of RGC cell bodies and their axons in the retina and rAION effects on RGC axonal loss. Thus, Bex can be utilized to evaluate both long- and short-term effects of optic nerve stroke and may play a significant role in regulating RGC functions in both the axonal and cell body components of RGC neurons.