Age- and cell cycle-related expression patterns of transcription factors and cell cycle regulators in Müller glia

Agarose disk electroporation method for ex vivo retinal tissue cultured at the air-liquid interface reveals electrical stimulus-induced cell cycle reentry in retinal cells

It is advantageous to culture the ex vivo murine retina along with many other tissue types at the air-liquid interface. However, gene delivery to these cultures can be challenging. Electroporation is a fast and robust method of gene delivery, but typically requires submergence in a liquid buffer to allow electric current flow. We have developed a submergence-free electroporation technique using an agarose disk that allows for efficient gene delivery to the ex vivo murine retina. This method advances our ability to use ex vivo retinal tissue for genetic studies and can easily be adapted for any tissue cultured at an air-liquid interface. We found an increased ability to transfected Muller glia at 14 days ex vivo and an increase in BrdU incorporation in Muller glia following electrical stimulation. Use of this method has revealed valuable insights on the state of ex vivo retinal tissues and the effects of electrical stimulation on retinal cells.

Cell cycle-dependent activation of proneural transcription factor expression and reactive gliosis in rat Müller glia

Retinal Müller glia have a capacity to regenerate neurons in lower vertebrates like zebrafish, but such ability is extremely limited in mammals. In zebrafish, Müller glia proliferate after injury, which promotes their neurogenic reprogramming while inhibiting reactive gliosis. In mammals, however, how the cell cycle affects the fate of Müller glia after injury remains unclear. Here, we focused on the expression of proneural transcription factors, Ngn2 and Ascl1, and a gliosis marker glial fibrillary acidic protein (GFAP) in rat Müller glia after N-methyl-N-nitrosourea (MNU)-induced photoreceptor injury and analyzed the role of Müller glia proliferation in the regulation of their expression using retinal explant cultures. Thymidine-induced G1/S arrest of Müller glia proliferation significantly hampered the expression of Ascl1, Ngn2, and GFAP, and release from the arrest induced their upregulation. The migration of Müller glia nuclei into the outer nuclear layer was also shown to be cell cycle-dependent. These data suggest that, unlike the situation in zebrafish, cell cycle progression of Müller glia in mammals promotes both neurogenic reprogramming and reactive gliosis, which may be one of the mechanisms underlying the limited regenerative capacity of the mammalian retina.

Nestin Regulates Müller Glia Proliferation After Retinal Injury

Purpose

The proliferative and neurogenic potential of retinal Müller glia after injury varies widely across species. To identify the endogenous mechanisms regulating the proliferative response of mammalian Müller glia, we comparatively analyzed the expression and function of nestin, an intermediate filament protein established as a neural stem cell marker, in the mouse and rat retinas after injury.

Methods

Nestin expression in the retinas of C57BL/6 mice and Wistar rats after methyl methanesulfonate (MMS)-induced photoreceptor injury was examined by immunofluorescence and Western blotting. Adeno-associated virus (AAV)-delivered control and nestin short hairpin RNA (shRNA) were intravitreally injected to rats and Müller glia proliferation after MMS-induced injury was analyzed by BrdU incorporation and immunofluorescence. Photoreceptor removal and microglia/macrophage infiltration were also analyzed by immunofluorescence.

Results

Rat Müller glia re-entered the cell cycle and robustly upregulated nestin after injury whereas Müller glia proliferation and nestin upregulation were not observed in mice. In vivo knockdown of nestin in the rat retinas inhibited Müller glia proliferation while transiently stimulating microglia/macrophage infiltration and phagocytic removal of dead photoreceptors.

Conclusions

Our findings suggest a critical role for nestin in the regulation of Müller glia proliferation after retinal injury and highlight the importance of cross species analysis to identify the molecular mechanisms regulating the injury responses of the mammalian retina.