Linking YAP to Müller glia quiescence exit in the degenerative retina

Abstract

Contrasting with fish or amphibian, retinal regeneration from Müller glial cells is largely limited in mammals. In our quest towards the identification of molecular cues that may boost their stemness potential, we investigated the involvement of the Hippo pathway effector YAP, which we previously found to be upregulated in Müller cells following retinal injury. We report that conditional Yap deletion in Müller cells prevents the upregulation of cell cycle genes that normally accompanies reactive gliosis upon photoreceptor cell death. This occurs as a consequence of defective EGFR signaling. Consistent with a function of YAP in triggering Müller glia cell cycle re-entry, we further show that in Xenopus, a species endowed with efficient regenerative capacity, YAP is required for their injurydependent proliferative response. Finally, and noteworthy, we reveal that YAP overactivation in mouse Müller cells is sufficient to induce their reprogramming into highly proliferative cells. Overall, we unravel a pivotal role for YAP in tuning Müller cell response to injury and highlight a novel YAP-EGFR axis by which Müller cells exit their quiescence state, a critical step towards regeneration. Key words: Müller cells, reactive gliosis, retinal regeneration, Hippo/YAP pathway, EGFRpathway Neurodegenerative retinal diseases, such as retinitis pigmentosa or age-related macular degeneration, ultimately lead to vision loss, as a consequence of photoreceptor cell death. Driving retinal self-repair from endogenous neural stem cells in patients represents an attractive therapeutic strategy. Among cellular sources of interest are Müller cells, the major glial cell type in the retina. These cells are essential for maintaining retinal homeostasis 1 but also proved their potential for endogenous regeneration. In certain species, such as zebrafish or Xenopus, they behave as genuine stem cells, endowed with the ability to reprogram into a progenitor-like state upon retinal damage, proliferate and regenerate lost photoreceptors 2-4 . In mammals, however, their proliferative response to injury is extremely limited. Following acute retinal damage, mouse Müller glial cells rapidly re-enter G1-phase of the cell cycle, as inferred by increased cyclin gene expression, but they rarely divide 5 . Suggesting that they nonetheless retain remnants of repair capacities, their proliferation and neurogenic potential can be stimulated, for instance by supplying exogenous growth factors such as Heparin-binding EGF-like growth factor (HB-EGF), or by overexpressing the proneural gene Ascl1a 3,6-8 . Our understanding of the genetic/signaling network sustaining Müller cell stemness potential is however far from being complete. Identifying novel molecular cues is thus of utmost importance to foresee putative candidates that could be targeted for regenerative medicine. We here investigated whether the Hippo pathway effector YAP might influence Müller cell reactivation and how it would intersect with other critical signaling pathways. The Hippo pathway is a kina...

Cross-species transcriptomic and epigenomic analysis reveals key regulators of injury response and neuronal regeneration in vertebrate retinas

“…4J). In mice, we observed that a subset of gliotic MG resembled the proliferative branch in the zebrafish trajectory, which is consistent with observations that mouse MG transiently upregulate cell cycle regulators such as Ccnd1 and Cdk4 following injury (5,19). In zebrafish, MG in the resting, gliosis and proliferative branches were close to chick MG from corresponding branches ( Fig.…”

“…S21A). YAP was recently shown to be required for MG to exit quiescence in both Xenopus and mice (5,19). We found that morpholino-mediated knockdown of Yap1 protein in the light-damaged zebrafish retina significantly reduced the number of proliferating MG relative to the Standard Control morphant ( Fig.…”

“…Studies in zebrafish identified a number of genes that are rapidly induced in MG following injury and regulate neurogenic competence. Multiple extrinsic signaling pathways, including Wnt, BMP, FGF, TNFɑ, and cytokines, were found to regulate MGPC formation (5)(6)(7)(8)(9)(10)(11)(12)(13)(14). These induce neurogenic factor genes, such as ascl1a (15) and the RNA binding protein lin28a (16), which are necessary for MG reprogramming.…”

“…Interestingly, forced expression of Ascl1 in mouse MG, in combination with neuronal injury and histone deacetylase inhibitor treatment, can induce direct conversion of MG to inner retinal cells that show neuronal morphology and respond to light (17,18). Conversely, inhibition of the Hippo pathway stimulates MG proliferation in mice following injury, but does not induce neurogenic competence (5,19). This suggests that key molecular components of MG reprogramming remain to be identified.…”

Yaphaploinsufficiency leads to Müller cell dysfunction and late-onset cone dystrophy

“…Briefly, Yap flox/+ mice (Reginensi et al, 2013) were crossed with PKG Cre mice (PGK-Cre transgene is maternally expressed and serves as a tool for early and uniform activation of the Cre site-specific recombinase (Lallemand et al, 1998)) to generate the Yap +/mice, that are viable and fertile. Yap flox/flox ;Rax-CreER T2 mice were obtained as previously described (Hamon et al, 2019) by mating Yap flox/flox mice (Reginensi et al, 2013) with heterozygous Rax-CreER T2 knock-in mice (Pak et al, 2014). The Cre activity was induced through a single intraperitoneal injection of 4-hydroxy-t-moxifen (4-OHT; 1 mg/kg) at P10.…”