Retinal ganglion cells (RGCs) comprise a heterogenous group of projection neurons that transmit visual information from the retina to the brain. Progressive degeneration of these cells, as it occurs in inflammatory, ischemic, traumatic or glaucomatous optic neuropathies, results in visual deterioration and is among the leading causes of irreversible blindness. Treatment options for these diseases are limited. Neuroprotective approaches aim to slow down and eventually halt the loss of ganglion cells in these disorders. In this review, we have summarized preclinical studies that have evaluated the efficacy of cell-based neuroprotective treatment strategies to rescue retinal ganglion cells from cell death. Intraocular transplantations of diverse genetically nonmodified cell types or cells engineered to overexpress neurotrophic factors have been demonstrated to result in significant attenuation of ganglion cell loss in animal models of different optic neuropathies. Cell-based combinatorial neuroprotective approaches represent a potential strategy to further increase the survival rates of retinal ganglion cells. However, data about the long-term impact of the different cell-based treatment strategies on retinal ganglion cell survival and detailed analyses of potential adverse effects of a sustained intraocular delivery of neurotrophic factors on retina structure and function are limited, making it difficult to assess their therapeutic potential.

We recently showed, in a mouse optic nerve crush model, that a sustained cell-based intravitreal administration of ciliary neurotrophic factor (CNTF) and glial cell line-derived neurotrophic factor (GDNF) synergistically slowed the lesion-induced degeneration of retinal ganglion cells (RGCs), resulting in the presence of approximately 35% viable RGCs eight months after the lesion. However, the combinatorial neuroprotective treatment was initiated shortly after the lesion. To mimic a more clinically relevant situation, we co-administered both factors either three or five days after an intraorbital nerve crush when approximately 35% or 57% of the RGCs were degenerated, respectively. Analyses of the retinas at different time points after the lesion consistently revealed the presence of significantly more surviving RGCs in retinas co-treated with CNTF and GDNF than in retinas treated with either factor alone. For example, when the neurotrophic factors were administered five days after the nerve crush and the animals were analyzed two months after the lesion, retinas co-treated with CNTF and GDNF contained approximately 40% of the RGCs present at the start of treatment. In comparison, monotherapy with either CNTF or GDNF protected only about 15% or 10% of the RGCs present at baseline, respectively. The number of regenerating axons in the distal nerve stumps was similar in CNTF- and CNTF/GDNF-treated animals, despite the significantly higher number of rescued RGCs in the latter group. These findings have potential implications for studies aimed at developing neuroprotective treatments for optic neuropathies such as glaucoma.

Progranulin (PGRN) is a secreted protein composed of 7.5 granulin domains. The protein is implicated in various functions, including cell survival, inflammation, lysosomal homeostasis, tumorigenesis, and aging. Haploinsufficiency and complete loss of PGRN function cause the neurodegenerative disorders frontotemporal lobar degeneration and neuronal ceroid lipofuscinosis type 11, respectively. In the nervous system, administration of exogenous PGRN has been shown to promote the survival of various nerve cell types under different pathological conditions and to stimulate neurite outgrowth in vitro and axonal regeneration in vivo. In the retina, PGRN dysfunction results in photoreceptor and retinal ganglion cell (RGC) loss, whereas PGRN administration promotes photoreceptor cell survival. In the present study, we analyzed whether a sustained intravitreal administration of PGRN promotes the survival of axotomized RGCs and the regrowth of the lesioned axons. To this end, we generated a PGRN-overexpressing clonal neural stem cell line and injected the cells into the vitreous cavity of a mouse optic nerve crush model. The progression of the lesion-induced degeneration of RGCs was studied at different time points after the nerve crush. The regeneration of the injured RGC axons into the distal optic nerve stump was analyzed one month after nerve lesioning. We found that the intravitreally administered PGRN slowed the degeneration of the injured RGCs for up to four months, the latest post-lesion interval analyzed. Furthermore, PGRN stimulated the regeneration of some RGC axons over long distances into the distal optic nerve stumps. Taken together, our results identify PGRN as a novel neurotrophic factor for retinal ganglion cells.