The molecular mechanisms underlying the generation of the various types of cells in the vertebrate retina are largely unknown. We investigated the possibility that genes belonging to the basic helix-loop-helix (bHLH) family of transcriptional factors participate in cell-type specification during retinal neurogenesis. Chick neuroD was isolated from an embryonic cDNA library and its deduced amino acid sequence showed 75% identity with mouse neuroD. In situ hybridization showed that neuroD was expressed in cells located at the outer portion of the developing retinal neuroepithelium, the location where prospective photoreceptors reside. Misexpression of neuroD in retinal neuroepithelium through replication-competent, transformation-deficient retroviruses produced a retina with three, instead of two, layers of photoreceptor cells; the number of cells that express visinin, a marker for cone photoreceptors, increased over 50% compared to control embryos misexpressing the green fluorescent protein. No significant changes were observed in the number of other retinal neurons, including those that express RA4 (ganglion cells), pax6 (ganglion cells and amacrine cells), and chx10 (bipolar cells). Retroviral-driven misexpression of neuroD in monolayer cultures of retinal pigment epithelium yielded de novo production of photoreceptor cells with no other types of retinal neurons detected. We propose that neuroD is important for photoreceptor cell production in the vertebrate retina.
neurogenin2 (ngn2) encodes a basic helix-loop-helix transcription factor and plays an important role in neurogenesis from migratory neural crest cells. Its role in retinal development is poorly understood. We observed that in the developing chick retina, ngn2 was expressed in a subpopulation of proliferating progenitor cells. Ectopic expression of ngn2 in nonneural, retinal pigment epithelial cell culture triggered de novo generation of cells that expressed neural-specific markers and exhibited neuronal morphologies. Further molecular and morphological analyses showed that the main products of the induced neurogenesis were cells resembling young photoreceptor cells and cells resembling retinal ganglion cells. The generation of multiple cell types suggests that ngn2 induces various retinal pathways. Thus, unlike in the peripheral nervous system where ngn2 specifies one type of sensory neuron, ngn2 in the retina is likely involved in a common step leading to different cellular pathways. Our finding that ngn2 can instruct nonneural retinal pigment epithelial cells to differentiate toward retinal neurons demonstrates one possible way to induce de novo retinal neurogenesis.
SignificanceCoffee consumption is linked with reduced risk of Parkinson’s disease (PD), and caffeine is generally believed to be the protective agent. However, several lines of evidence suggest the presence of additional compound(s) in coffee that can be protective as well. Here we show that eicosanoyl-5-hydroxytryptamide, which we purified from coffee as an agent that leads to enhanced enzymatic activity of the specific phosphatase PP2A that dephosphorylates the pathogenic protein α-synuclein, works in synergy with caffeine in protecting against mouse models of PD and Dementia with Lewy bodies. The mechanism of this synergy is also through enhancing PP2A, which is dysregulated in the brains of individuals with these α-synucleinopathies.
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