The Neurogenic Capacity of Müller Glia and Synaptic Remodeling in the Mammalian Retina: Implications for Regenerative Therapies
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Stem cell based therapies offer hope for the treatment of retinal disease. In the mammalian retina, Müller glia possess some stem cell-like characteristics. However, their capacity for neurogenesis remains limited. To identify factors influencing the neurogenic potential of Müller glia in vitro, a conditionally immortalized mouse Müller cell line (ImM10) was generated and characterized. In addition to Müller glial genes, ImM10 cells express some genes characteristic of retinal progenitor cells. Following treatment with specific growth factors and media supplements, a subset of Müller glia acquired a more neuronal morphology and expressed neuronal genes. To determine the role of environment on Müller glia derived neurogenesis, Müller glia were cultured in three dimensional (3D) peptide hydrogels. Cells encapsulated within 3D hydrogels expressed different neuronal genes, compared to cells cultured on 2D flat plastic substrates. The roles of RE-1 silencing transcription factor (REST) and Notch signaling in Müller glia derived neurogenesis were also investigated. REST represses neuronal genes in nonneuronal tissues, whereas Notch can promote gliogenesis during retinal development. Inhibition of Rest expression by RNA interference combined with pharmacological inhibition of Notch signaling with DAPT led to the upregulation of some additional neuronal genes in ImM10 Müller glia. Together, these results demonstrated a modest neurogenic capacity for Müller glia. A potential barrier for transplantation of these or any stem-like cells for therapeutic purposes is retinal remodeling of surviving cells following retinal disease or injury. Neuronal and synaptic remodeling was analyzed in the rd10 mouse model of retinitis pigmentosa. Following photoreceptor degeneration, second order neurons (horizontal, and bipolar cells) showed dendrite retraction, cellular migration, and a loss of post-synaptic elements that increased with disease progression. Surprisingly, the inner retina remained relatively intact even at advanced stages of degeneration. Furthermore, survival of small numbers of cone photoreceptors preserved second-order neuronal circuitry locally. This study suggests that the inner retina may remain receptive to interventions even at late stages of disease, and that early cellular transplantation may help prevent or slow retinal remodeling.