SOX2 Maintains the Quiescence of Müller Glial Cells

Abstract

Purpose: : Although the primary function of Müller glia is to provide trophic and structural support to the retina, they also preserve progenitor cell morphology and gene expression. In addition, Müller glia can re-enter the cell cycle in response to retinal injury in order to replace neurons in the damaged tissue. Because of the shared qualities of Müller glia and retinal progenitors, we propose to determine how a gene critical for progenitor cell competence, SOX2, is important for Müller glial progenitor cell characteristics. SOX2 is a HMG-box transcription factor that is associated with the maintenance of progenitor cell populations. As neural retinal progenitor cells differentiate into mature neurons, SOX2 is downregulated. However, SOX2 is maintained in mature Müller glia, suggesting that it is involved in preserving their proliferative potential.

Methods: : In order to study the function of SOX2 in Müller glia, we used mice carrying a conditional Sox2 allele (Sox2^c) to ablate its function in a spaciotemporal-specific manner. These mice also contain one of the tamoxifen-inducible alleles, CAG-CreER^TM or Glast-CreER^T2, in order to ablate Sox2 ubiquitously or in a Müller glia-specific manner, respectively. Isolated retinal tissue from various ages are exposed to 4-OH-tamoxifen and cultured for 5 days using an in vitro retinal explant system. After fixation, retinas are cryosectioned and immunostained with an array of antibodies.

Results: : The conditional postnatal ablation of Sox2 in the early postnatal retina results in the disruption of structural integrity of the retinal cell layers. In contrast to the mitotically quiescent Müller glia in wild type retinas, a subset of SOX2-deficient Müller glia re-enter the cell cycle. These cellsupregulate the expression of proliferation genes, and their nuclei travel to the apical retinal surface to divide. Time lapse imaging reveals the nuclear migration and mitotic separation into two daughter cells in the SOX2-deficient Müller glia.

Conclusions: : These data suggest that SOX2 maintains Müller glial cell structure and mitotic quiescence, possibly via the regulation of cell cycle components. In addition, SOX2 maintains the progenitor cell characteristics of Müller glia by preventing cell cycle progression.

Keywords: Muller cells • proliferation • retinal development