Ciliary neurotrophic factor (CNTF) acts as a potent neuroprotective agent in multiple retinal degeneration animal models. Recently, CNTF has been evaluated in clinical trials for the inherited degenerative disease retinitis pigmentosa (RP) and for dry agerelated macular degeneration (AMD). Despite its potential as a broad-spectrum therapeutic treatment for blinding diseases, the target cells of exogenous CNTF and its mechanism of action remain poorly understood. We have shown previously that constitutive expression of CNTF prevents photoreceptor death but alters the retinal transcriptome and suppresses visual function. Here, we use a lentivirus to deliver the same secreted human CNTF used in clinical trials to a mouse model of RP. We found that low levels of CNTF halt photoreceptor death, improve photoreceptor morphology, and correct opsin mislocalization. However, we did not detect corresponding improvement of retinal function as measured by the electroretinogram. Disruption of the cytokine receptor gp130 gene in Müller glia reduces CNTF-dependent photoreceptor survival and prevents phosphorylation of STAT3 and ERK in Müller glia and the rest of the retina. Targeted deletion of gp130 in rods also demolishes neuroprotection by CNTF and prevents further activation of Müller glia. Moreover, CNTF elevates the expression of LIF and endothelin 2, thus positively promoting Müller and photoreceptor interactions. We propose that exogenous CNTF initially targets Müller glia, and subsequently induces cytokines acting through gp130 in photoreceptors to promote neuronal survival. These results elucidate a cellular mechanism for exogenous CNTF-triggered neuroprotection and provide insight into the complex cellular responses induced by CNTF in diseased retinas.C iliary neurotrophic factor (CNTF) belongs to a small subfamily of cytokines and has long been recognized as a potent neuroprotective molecule in the vertebrate retina (1). Enhancement of photoreceptor survival by CNTF has been demonstrated in multiple animal models of retinal degeneration, ranging from zebrafish to canine (2). Interestingly, CNTF is effective in rescuing retinal degeneration due to various causes, including mutations in genes expressed by photoreceptors or the retinal pigment epithelium (RPE), as well as those induced by strong light, neurotoxins, or antibodies. In addition, CNTF prolongs the survival of retinal ganglion cells (3-5) and promotes axonal growth in optic nerve crush or transection models (6-8). Furthermore, CNTF may affect the physiology and survival of RPE cells (9, 10), which are critical in normal vision and retinal degenerative diseases. Based on its significant and broad neuroprotective effects in damaged retinas, a secreted form of human CNTF delivered from an encapsulated cell device has been tested in clinical trials and approved by the Food and Drug Administration (FDA) to treat retinitis pigmentosa (RP) and geographic atrophy (GA), a subset of age-related macular degeneration (AMD) (11-14). However, despite its clinical signifi...

has no effect on rod photoreceptor differentiation in vitro. Furthermore, disrupting the function of epidermal growth factor (EGF) receptors, which modulate rod development in vivo, indicates that the EGF family of ligands does not mediate the inhibitory effect of cytokine on rod differentiation. These results demonstrate that cytokine signal transduction is dynamic and heterogeneous in the developing retina, and that endogenous ligand-induced STAT activation in retinal progenitor and/or photoreceptor precursor cells plays an important role in regulating photoreceptor development.

Despite the rescue from cell death, continuous exposure to CNTF changed photoreceptor cell profiles, especially resulting in the loss of cone immunoreactivity. In addition, the Müller glia and bipolar cells became disorganized, and the number of cells expressing Müller and bipolar cell markers increased. Constitutive CNTF production resulted in sustained activation of cytokine signal transduction and altered the expression of a large number of genes. Therefore, stringent regulation of CNTF may be necessary for its therapeutic application in preventing retinal degeneration.

Ciliary neurotrophic factor (CNTF) exhibits multiple biological effects during vertebrate retinal development, including regulating the differentiation of photoreceptor cells and promoting the survival and axonal growth of ganglion cells. We report here that in addition to affecting the differentiation of retinal neurons, CNTF also promotes Müller glia genesis in the postnatal mouse retina. In both retinal monolayer and explant cultures, CNTF increases the number of progenitor cells adopting the Müller cell fate. Exogenous CNTF induces phosphorylation of signal transducers and activators of transcription (STAT)3 and extracellular signal-regulated kinase (ERK) among neonatal progenitor cells and newborn Müller cells. In addition, increased levels of endogenous STAT3 and ERK phosphorylation have been observed at around postnatal day 5, coinciding with the peak of Müller glia genesis. Perturbation of STAT and ERK signaling using protein kinase inhibitors and a dominant-negative STAT3 mutant demonstrates that both CNTF-induced STAT and ERK activation are involved in promoting Müller cell production. Moreover, absorbing epidermal growth factor (EGF) signals with a neutralizing antibody did not affect CNTF-induced Müller glial genesis, indicating that the effect of CNTF is not mediated by the known Müller-enhancing activity of EGF. Together, these results support a novel function of CNTF-like cytokines in retinal gliogenesis.