The Hippo Pathway Blocks Mammalian Retinal Müller Glial Cell Reprogramming

Müller glial cells (MGs) of the zebrafish retina exhibit the remarkable ability to reprogram to proliferative, retinal progenitor-like cells that can regenerate lost photoreceptors and restore vision. Unfortunately, mammalian MGs are not regenerative and this is at least in part due to the inability of MGs to undergo sustained cell cycle re-entry in response to retinal damage. Here, we identify the Hippo pathway as the core regulatory mechanism that normally prevents mammalian MG reprogramming to a proliferative, progenitor-like state. Specifically, in adult MGs within the damaged retina, Hippo signaling represses the activity of the transcription cofactor YAP, which would otherwise promote sustained MG cell cycle re-entry. MG-specific deletion of Hippo pathway components Lats1 and Lats2 resulted in Cyclin D1 upregulation and spontaneous MG proliferation. These data suggest that sustained YAP activity in reactive MGs induces MG proliferation and reprogramming, but this response is normally repressed by the Hippo pathway. Generation of MGs that express a variant of YAP (YAP5SA) that is non-responsive to Hippo signaling, resulted in cellular reprogramming whereby MG identity was lost and the cells acquired a highly proliferative, progenitor-like state. Together, our results reveal that MGs may have latent regenerative capacity that can be “reawakened” by blocking Hippo signaling. Adult Glast-CreERT2+/tg; Yap5SA+/tg; R26R-tdTomato+/tg mice were induced with tamoxifen followed by NMDA injection. After 2 days, tdTomato+ cells were isolated by fluorescence-activated cell sorting (FACS) followed by scRNA-seq using the 10X Genomics platform.