The Rax homeoprotein in Müller glial cells is required for homeostasis maintenance of the postnatal mouse retina

Muller glial cells, which are the most predominant glial subtype in the retina, play multiple important roles, including the maintenance of structural integrity, homeostasis, and physiological functions of the retina. We have previously found that the Rax homeoprotein is expressed in postnatal and mature Muller glial cells in the mouse retina. However, the function of Rax in postnatal and mature Muller glial cells remains to be elucidated. In the current study, we first investigated Rax function in retinal development using retroviral lineage analysis and found that Rax controls the specification of late -born retinal cell types, including Muller glial cells in the postnatal retina. We next generated Rax tamoxifen-induced conditional KO (Rax iCKO) mice, where Rax can be depleted in mTFP-labeled Muller glial cells upon tamoxifen treatment, by crossing Raxflox/floxmice with Rlbp1-CreERT2 mice, which we have produced. Immunohistochemical analysis showed a characteristic of reactive gliosis and enhanced gliosis of Muller glial cells in Rax iCKO retinas under normal and stress conditions, respectively. We performed RNA-seq analysis on mTFP-positive cells purified from the Rax iCKO retina and found significantly reduced expression of suppressor of cytokine signaling-3 (Socs3). Reporter gene assays showed that Rax directly transactivates the Socs3 promoter. We observed decreased expression of Socs3 in Muller glial cells of Rax iCKO retinas by immunostaining. Taken together, the present results suggest that Rax suppresses inflammation in Muller glial cells by transactivating Socs3. This study sheds light on the transcriptional regulatory mechanisms underlying retinal Muller glial cell homeostasis.