DNA Damage and p53 Restrict Proliferation of Müller Cells in the Mouse Retina in Response to the Influence of N-Methyl-N-Nitrosourea

Abstract

Systemic administration of N-methyl-N-nitrosourea in rats resulted in the death of retinal photoreceptors followed by differentiation of retinal Muller glial cells into photoreceptor-like cells [27]. However, mammalian Muller glial cells exhibit an extremely limited proliferative capacity, which correlates with the expression of histone γH2AX and p21 protein. These proteins are known to be components of the cellular response to DNA damage [26]. The restriction of proliferation of human Muller glial cells prevents retinal replacement therapy by cell transplantation. On the other hand, the mechanism that limits the proliferation of Muller glia in the mammalian retina remains to be elucidated. We examined the Muller glial proliferative response and the DNA damage response in Muller glia in the postreplicative stage, as well as the expression of the p53 protein in response to the influence of retinotoxic N-methyl-N-nitrosourea. It was shown that N‑methyl-N-nitrosourea induced retinal degeneration in mice via apoptosis of photoreceptors, whereas the other retinal layers retained intact morphology. Nevertheless, the formation of DNA breaks and alkali-labile sites was observed in all retinal cells 5 h after the N-methyl-N-nitrosourea injection; these formations completely disappeared 15 h after N-methyl-N-nitrosourea injection. By 72 h, a significant increase in the number of DNA breaks in Muller glial cells was observed. The absence of bromodeoxyuridine incorporation into the retinal cells later testifies to the absence of proliferation of Muller glial cells and DNA repair synthesis. At the same time, an increased expression of the p53 protein, a universal marker of DNA damage, was observed in the retina. Thus, our findings support the concept of the “DNA damage response” with respect to Muller glial cells, according to which the DNA damage in Muller glial cell is related to the restricted proliferation of these cells in mice. Postreplicative repair is considered as a probable mechanism of the formation of DNA breaks in postreplicative Muller glial cells.