A microRNA, mir133b, suppresses melanopsin expression mediated by failure dopaminergic amacrine cells in RCS rats

The photopigment melanopsin and melanopsin-containing RGCs (mRGCs or ipRGCs) represent a brand-new and exciting direction in the field of visual field. Although the melanopsin is much less sensitive to light and has far less spatial resolution, mRGCs have the unique ability to project to brain areas by the retinohypothalamic tract (RHT) and communicate directly with the brain. Unfortunately, melanopsin presents lower expression levels in many acute and chronic retinal diseases. The molecular mechanisms underlying melanopsin expression are not yet really understood. MicroRNAs play important roles in the control of development. Most importantly, the link of microRNA biology to a diverse set of cellular processes, ranging from proliferation, apoptosis and malignant transformation to neuronal development and fate specification is emerging. We employed Royal College of Surgeon (RCS) rats as animal model to investigate the underlying molecular mechanism regulating melanopsin expression using a panel of miRNA by quantitative real-time reverse transcription polymerase chain reaction. We identified a microRNA, mir133b, that is specifically expressed in retinal dopaminergic amacrine cells as well as markedly increased expression at early stage during retinal degeneration in RCS rats. The overexpression of mir133b downregulates the important transcription factor Pitx3 expression in dopaminergic amacrine cells in RCS rats retinas and makes amacrine cells stratification deficit in IPL. Furthermore, deficient dopaminergic amacrine cells presented decreased TH expression and dopamine production, which lead to a failure to direct mRGCs dendrite to stratify and enter INL and lead to the reduced correct connections between amacrine cells and mRGCs. Our study suggested that overexpression of mir133b and downregulated Pitx3 suppress maturation and function of dopaminergic amacrine cells, and overexpression of mir133b decreased TH and D2 receptor expression as well as dopamine production, which finally resulted in reduced melanopsin expression.