Microglia, the principal resident immune cell of the CNS, exert significant influence on neurons during development and in pathological situations. However, if and how microglia contribute to normal neuronal function in the mature uninjured CNS is not well understood. We used the model of the adult mouse retina, a part of the CNS amenable to structural and functional analysis, to investigate the constitutive role of microglia by depleting microglia from the retina in a sustained manner using genetic methods. We discovered that microglia are not acutely required for the maintenance of adult retinal architecture, the survival of retinal neurons, or the laminar organization of their dendritic and axonal compartments. However, sustained microglial depletion results in the degeneration of photoreceptor synapses in the outer plexiform layer, leading to a progressive functional deterioration in retinal light responses. Our results demonstrate that microglia are constitutively required for the maintenance of synaptic structure in the adult retina and for synaptic transmission underlying normal visual function. Our findings on constitutive microglial function are relevant in understanding microglial contributions to pathology and in the consideration of therapeutic interventions that reduce or perturb constitutive microglial function.SIGNIFICANCE STATEMENTMicroglia, the principal resident immune cell population in the CNS, has been implicated in diseases in the brain and retina. However, how they contribute to the everyday function of the CNS is unclear. Using the model of the adult mouse retina, we examined the constitutive role of microglia by depleting microglia from the retina. We found that in the absence of microglia, retinal neurons did not undergo overt cell death or become structurally disorganized in their processes. However, connections between neurons called synapses begin to break down, leading to a decreased ability of the retina to transmit light responses. Our results indicate that retinal microglia contribute constitutively to the maintenance of synapses underlying healthy vision.

Retinal dystrophies are a major cause of blindness for which there are currently no curative treatments. Transplantation of stem cell‐derived neuronal progenitors to replace lost cells has been widely investigated as a therapeutic option. Another promising strategy would be to trigger self‐repair mechanisms in patients, through the recruitment of endogenous cells with stemness properties. Accumulating evidence in the past 15 year0s has revealed that several retinal cell types possess neurogenic potential, thus opening new avenues for regenerative medicine. Among them, Müller glial cells have been shown to be able to undergo a reprogramming process to re‐acquire a stem/progenitor state, allowing them to proliferate and generate new neurons for repair following retinal damages. Although Müller cell–dependent spontaneous regeneration is remarkable in some species such as the fish, it is extremely limited and ineffective in mammals. Understanding the cellular events and molecular mechanisms underlying Müller cell activities in species endowed with regenerative capacities could provide knowledge to unlock the restricted potential of their mammalian counterparts. In this context, the present review provides an overview of Müller cell responses to injury across vertebrate model systems and summarizes recent advances in this rapidly evolving field. Developmental Dynamics 245:727–738, 2016. © 2015 The Authors. Developmental Dynamics published by Wiley Periodicals, Inc.

Chemokine signaling via CX3CL1-CX3CR1 helps regulate homeostasis in the population of microglia in the adult mouse retina.

Contrasting with fish or amphibian, retinal regeneration from Müller glia is largely limited in mammals. In our quest toward the identification of molecular cues that may boost their stemness potential, we investigated the involvement of the Hippo pathway effector YAP (Yes-associated protein), which is upregulated in Müller cells following retinal injury. Conditional Yap deletion in mouse Müller cells prevents cell-cycle gene upregulation that normally accompanies reactive gliosis upon photoreceptor cell death. We further show that, in Xenopus, a species endowed with efficient regenerative capacity, YAP is required for their injury-dependent proliferative response. In the mouse retina, where Müller cells do not spontaneously proliferate, YAP overactivation is sufficient to induce their reprogramming into highly proliferative cells. Overall, we unravel a pivotal role for YAP in tuning Müller cell proliferative response to injury and highlight a YAP-EGFR (epidermal growth factor receptor) axis by which Müller cells exit their quiescence state, a critical step toward regeneration.

Leber congenital amaurosis (LCA) encompasses a set of early-onset blinding diseases that are characterized by vision loss, involuntary eye movement, and nonrecordable electroretinogram (ERG). At least 19 genes are associated with LCA, which is typically recessive; however, mutations in homeodomain transcription factor CRX lead to an autosomal dominant form of LCA. The mechanism of CRX-associated LCA is not understood. Here, we identified a spontaneous mouse mutant with a frameshift mutation in Crx (Crx^Rip). We determined that Crx^Rip is a dominant mutation that results in congenital blindness with nonrecordable response by ERG and arrested photoreceptor differentiation with no associated degeneration. Expression of LCA-associated dominant CRX frameshift mutations in mouse retina mimicked the Crx^Rip phenotype, which was rescued by overexpression of WT CRX. Whole-transcriptome profiling using deep RNA sequencing revealed progressive and complete loss of rod differentiation factor NRL in Crx^Rip retinas. Expression of NRL partially restored rod development in Crx^Rip/+ mice. We show that the binding of homeobox transcription factor OTX2 at the Nrl promoter was obliterated in Crx^Rip mice and ectopic expression of OTX2 rescued the rod differentiation defect. Together, our data indicate that OTX2 maintains Nrl expression in developing rods to consolidate rod fate. Our studies provide insights into CRX mutation-associated congenital blindness and should assist in therapeutic design.