Müller glia (MG) are the predominant type of support cell in the retina that are common to all vertebrate classes. MG perform many important glial functions including structural support, synaptic support, osmotic homeostasis, and metabolic support. However, MG also have the extraordinary capability to de-differentiate, proliferate, acquire progenitor phenotype, and generate new neurons (Bernardos et al., 2007; Fausett et al., 2008; Fischer and Reh, 2001; Karl et al., 2008; Ooto et al., 2004). Reprogramming of MG involves activation, downregulation of glial genes, upregulation of progenitor-associated factors, proliferation, followed by neuronal differentiation (Hoang et al., 2020). The capacity of MG to regenerate retinal neurons varies greatly between vertebrate classes. The MG in fish retinas have an extraordinary ability to effectively become proliferating progenitor cells that produce functional neurons to restore vision to damaged retinas (reviewed by (Lahne et al., 2020; Wan and Goldman, 2016)). In the mouse retina, MG respond to damage by becoming reactive and rapidly activate networks of genes to restore quiescence (Hoang et al., 2020). The regenerative capacity of MG in chick retinas lies somewhere in between that of fish and mammals. MG in the chick retina form numerous proliferating MGPCs with limited neurogenic potential in response to acute neuronal damage or in the absence of damage when treated with insulin and FGF2 (Fischer et al., 2002; Fischer and Reh, 2001).

In the mouse model, recent studies indicated that forced-expression of the transcription factor Ascl1, in combination with NMDA-induced damage and HDAC-inhibitor, stimulates reprogramming of MG into functional, light-responsive neurons (Jorstad et al., 2017; Pollak et al., 2013; Ueki et al., 2015). The numbers of neurons produced by Ascl1-over expressing MG can be enhanced by inhibiting Jak/Stat (Jorstad et al., 2020), ablating microglia from the retina (Todd et al., 2020), inhibiting NFkB, inhibiting ID (Inhibitor of DNA binding) factors, inhibiting TGFβ/Smad3 (Palazzo et al., 2022) and combined forced expression of Ascl1 and Atoh1 (Todd et al., 2021). Cell signaling cascades are broadly and rapidly activated in MG during the process of reprogramming. Pathways that promote the formation of MGPCs include MAPK (A.J. Fischer et al., 2009a, Fischer et al., 2009b), Jak/Stat (Todd et al., 2016; Wan et al., 2014a), Wnt/β-catenin (Gallina et al., 2016; Ramachandran et al., 2011; Wan et al., 2014b), Hedgehog (Stenkamp et al., 2008; Todd and Fischer, 2015), Notch (Ghai et al., 2010; Hayes et al., 2007), retinoic acid (Todd et al., 2018), BMP/Smad1/5/8 (Todd et al., 2017), cannabinoid (Campbell et al., 2021a), midkine (Campbell et al., 2021b) and mTor (Zelinka et al., 2016). Pathways that suppress the formation of MGPCs include glucocorticoid (Gallina et al., 2014), TGFβ/Smad2/3 (Lee et al., 2020; Todd et al., 2017) and NFkB (Palazzo et al., 2022, Palazzo et al., 2020). The majority of these cell signaling pathways involve kinase-dependent cascades, and it is assumed these pathways are negatively regulated by phosphatases. The purpose of this study was to understand how phosphatases regulate the responses of MG to damage and become proliferating progenitor-like cells. Accordingly, we investigated patterns of expression of different protein phosphatases in MG following neuronal damage or treatment with insulin and FGF2, and how inhibition of different families of phosphatases influences glial de-differentiation and the proliferation of MGPCs in vivo.