Sphingosine-1-Phosphate Signaling through Müller Glia Regulates Neuroprotection, Accumulation of Immune Cells, and Neuronal Regeneration in the Rodent Retina

The purpose of this study was to investigate how Sphingosine-1-phosphate (S1P) signaling regulates glial phenotype, neuroprotection, and reprogramming of Müller glia (MG) into neurogenic MG-derived progenitor cells (MGPCs) in the adult male and female mouse retina. We found that S1P-related genes were dynamically regulated following retinal damage. <i>S1pr1</i> (S1P receptor 1) and <i>Sphk1</i> (sphingosine kinase 1) are expressed at low levels by resting MG and are rapidly upregulated following acute damage. Overexpression of the neurogenic bHLH transcription factor Ascl1 in MG downregulates <i>S1pr1</i>, and inhibition of Sphk1 and S1pr1/3 enhances Ascl1-driven differentiation of bipolar-like cells. Treatments that activate S1pr1 or increase retinal levels of S1P initiate proinflammatory NFκB signaling in MG, whereas treatments that inhibit S1pr1 or decreased levels of S1P suppress NFκB signaling in MG. Conditional knock-out (cKO) of <i>S1pr1</i> in MG, but not <i>Sphk1</i>, enhances the accumulation of immune cells in damaged retinas. cKO of <i>S1pr1</i> promotes the survival of ganglion cells, whereas cKO of <i>Sphk1</i> promotes the survival amacrine cells in damaged retinas. Consistent with these findings, pharmacological treatments that inhibit S1P receptors or inhibit Sphk1 have protective effects upon inner retinal neurons. We conclude that the S1P signaling pathway is activated in MG after damage and this pathway restricts the accumulation of immune cells, impairs neuron survival, and suppresses the reprogramming of MG into neurogenic progenitors in the adult mouse retina.