A Role for Neurotrophic Factors in Sensory Cell Protection

Neurotrophic factors are proteins that promote survival of specific neuronal populations during development and promote neuronal health and continuity in the adult. During development, they may be required for differentiation, maturation, survival, axon sprouting, synapse stabilization, or synaptic efficacy, or a combination of these. Here we focus on a role for neurotrophic factors in increasing survival of sensory cells, in particular, photoreceptors. Photoreceptors degenerate in much the same way as hair cells, undergoing an apoptotic degeneration. Later steps in the cell death cascade are likely to be more similar among different cell types than are early steps. However, investigations into the earliest steps can provide important insights. If we look at the primary causes of sensory cell degeneration with clinical relevance, we find both acute (noise-induced damage and aminoglycosides) and chronic (genetic) sources in hair cells. In photoreceptors, most clinically relevant degeneration is induced by chronic genetic insults. Blocking apoptosis downstream in the cascade is unlikely to ameliorate chronic insults. Action at upstream steps, such as trophic factors, holds greater promise. The rd mouse is a model of retinitis pigmentosa that has a known defect in a protein in the phototransduction cascade, cyclic guanosine monophosphate (cGMP)-phosphodiesterase (PDE). In the rd mouse, the photoreceptor cells, which sit in the outer nuclear layer (ONL), are reduced by approximately 90% by 21 days after birth. We have investigated the effects of trophic factors in an organ culture model of the rd retina. In this model, retinas are isolated at postnatal day 2 and grown in Dulbecco's Modified Essential Media (Gibeo, Rockville, MD) with 10% fetal calf serum for 27 days in vitro. The retinal architecture is well-maintained, with the ONL maintained at five to six rows of nuclei in the wild-type organ culture and reduced to a monolayer in the rd culture. We tested a variety of trophic factors for effects on photoreceptors in this model and found no effect with ciliary neurotrophic factor (CNTF), brain-derived neurotrophic factor (BDNF), fibroblast growth factor-2, glial-derived neurotrophic factor, neurturin, or persephin. However, the addition of CNTF and BDNF in combination resulted in a dramatic increase in photoreceptor survival comparable to that seen in the wild-type controls. The underlying mechanism could involve action upstream from the genetic defect, such as modification of expression of phototransduction-associated proteins, or downstream from the defect, such as alternate regulation of cGMP. To investigate this mechanism, we have looked at expression of photoreceptor-specific proteins. Crx is a transcription factor that regulates expression of several photoreceptor specific genes, including the photopigment rhodopsin and PDE. Immunohistochemical staining demonstrated that Crx is unaffected by treatment with either CNTF or BDNF. In contrast, rhodopsin-like immunoreactivity is fully suppressed by CNTF alone or with BDNF but not by BDNF alone, in spite of the fact that CNTF alone has no effect on photoreceptor survival. Future studies will investigate whether BDNF acts on expression of other photoreceptor specific genes and whether either or both factors act downstream to alter cGMP levels.