Modulation of the Progenitor Cell and Homeostatic Capacities of Müller Glia Cells in Retina Focus on α2-Adrenergic and Endothelin Receptor Signaling Systems
Abstract: Müller cells are major glial cells in the retina and have a broad range of functions that are vital for the retinal neurons. During retinal injury gliotic response either leads to Müller cell dedifferentiation and formation of a retinal progenitor or to maintenance of mature Müller cell functions. The overall aim of this thesis was to investigate the intra- and extracellular signaling of Müller cells, to understand how Müller cells communicate during an injury and how their properties can be regulated after injury. Focus has been on the α2-adrenergic receptor (α2-ADR) and endothelin receptor (EDNR)-induced modulation of Müller cell-properties after injury.The results show that α2-ADR stimulation by brimonidine (BMD) triggers Src-kinase mediated ligand-dependent and ligand-independent transactivation of epidermal growth factor receptor (EGFR) in both chicken and human Müller cells. The effects of this transactivation in injured retina attenuate injury-induced activation and dedifferentiation of Müller cells by attenuating injury-induced ERK signaling. The attenuation was concomitant with a synergistic up-regulation of negative ERK- and RTK-feedback regulators during injury. The data suggest that adrenergic stress-signals modulate glial responses during retinal injury and that α2-ADR pharmacology can be used to modulate glial injury-response. We studied the effects of this attenuation of Müller cell dedifferentiation on injured retina from the perspective of neuroprotection. We analyzed retinal ganglion cell (RGC) survival after α2-ADR stimulation of excitotoxically injured chicken retina and our results show that α2-ADR stimulation protects RGCs against the excitotoxic injury. We propose that α2-ADR-induced protection of RGCs in injured retina is due to enhancing the attenuation of the glial injury response and to sustaining mature glial functions. Moreover, we studied endothelin-induced intracellular signaling in Müller cells and our results show that stimulation of EDNRB transactivates EGFR in Müller cells in a similar way as seen after α2-ADR stimulation. These results outline a mechanism of how injury-induced endothelins may modulate the gliotic responses of Müller cells.The results obtained in this thesis are pivotal and provide new insights into glial functions, thereby uncovering possibilities to target Müller cells by designing neuroprotective treatments of retinal degenerative diseases or acute retinal injury.
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