Manuela Lahne

Repressing Notch Signaling and Expressing TNFα Are Sufficient to Mimic Retinal Regeneration by Inducing Müller Glial Proliferation to Generate Committed Progenitor Cells

Retinal damage in teleosts, unlike mammals, induces robust Müller glia-mediated regeneration of lost neurons. We examined whether Notch signaling regulates Müller glia proliferation in the adult zebrafish retina and demonstrated that Notch signaling maintains Müller glia in a quiescent state in the undamaged retina. Repressing Notch signaling, through injection of the γ-secretase inhibitor RO4929097, stimulates a subset of Müller glia to reenter the cell cycle without retinal damage. This RO4929097-induced Müller glia proliferation is mediated by repressing Notch signaling because inducible expression of the Notch Intracellular Domain (NICD) can reverse the effect. This RO4929097-induced proliferation requires Ascl1a expression and Jak1-mediated Stat3 phosphorylation/activation, analogous to the light-damaged retina. Moreover, coinjecting RO4929097 and TNFα, a previously identified damage signal, induced the majority of Müller glia to reenter the cell cycle and produced proliferating neuronal progenitor cells that committed to a neuronal lineage in the undamaged retina. This demonstrates that repressing Notch signaling and activating TNFα signaling are sufficient to induce Müller glia proliferation that generates neuronal progenitor cells that differentiate into retinal neurons, mimicking the responses observed in the regenerating retina.

Actin-Cytoskeleton- and Rock-Mediated INM Are Required for Photoreceptor Regeneration in the Adult Zebrafish Retina.

Loss of retinal neurons in adult zebrafish (Danio rerio) induces a robust regenerative response mediated by the reentry of the resident Müller glia into the cell cycle. Upon initiating Müller glia proliferation, their nuclei migrate along the apicobasal axis of the retina in phase with the cell cycle in a process termed interkinetic nuclear migration (INM). We examined the mechanisms governing this cellular process and explored its function in regenerating the adult zebrafish retina. Live-cell imaging revealed that the majority of Müller glia nuclei migrated to the outer nuclear layer (ONL) to divide. These Müller glia formed prominent actin filaments at the rear of nuclei that had migrated to the ONL. Inhibiting actin filament formation or Rho-associated coiled-coil kinase (Rock) activity, which is necessary for phosphorylation of myosin light chain and actin myosin-mediated contraction, disrupted INM with increased numbers of mitotic nuclei remaining in the basal inner nuclear layer, the region where Müller glia typically reside. Double knockdown of Rho-associated coiled-coil kinase 2a (Rock2a) and Rho-associated coiled-coil kinase 2b (Rock2b) similarly disrupted INM and reduced Müller glial cell cycle reentry. In contrast, Rock inhibition immediately before the onset of INM did not affect Müller glia proliferation, but subsequently reduced neuronal progenitor cell proliferation due to early cell cycle exit. Long-term, Rock inhibition increased the generation of mislocalized ganglion/amacrine cells at the expense of rod and cone photoreceptors. In summary, INM is driven by an actin-myosin-mediated process controlled by Rock2a and Rock2b activity, which is required for sufficient proliferation and regeneration of photoreceptors after light damage. SIGNIFICANCE STATEMENT The human retina does not replace lost or damaged neurons, ultimately causing vision impairment. In contrast, zebrafish are capable of regenerating lost neurons. Understanding the mechanisms that regulate retinal regeneration in these organisms will help to elucidate approaches to stimulate a similar response in humans. In the damaged zebrafish retina, Müller glia dedifferentiate and proliferate to generate neuronal progenitor cells (NPCs) that differentiate into the lost neurons. We show that the nuclei of Müller glia and NPCs migrate apically and basally in phase with the cell cycle. This migration is facilitated by the actin cytoskeleton and Rho-associated coiled-coil kinases (Rocks). We demonstrate that Rock function is required for sufficient proliferation and the regeneration of photoreceptors, likely via regulating nuclear migration.

Sox2 regulates Müller glia reprogramming and proliferation in the regenerating zebrafish retina via Lin28 and Ascl1a

ABSTRACT Sox2 is a well‐established neuronal stem cell‐associated transcription factor that regulates neural development and adult neurogenesis in vertebrates, and is one of the critical genes used to reprogram differentiated cells into induced pluripotent stem cells. We examined if Sox2 was involved in the early reprogramming‐like events that Müller glia undergo as they upregulate many pluripotency‐ and neural stem cell‐associated genes required for proliferation in light‐damaged adult zebrafish retinas. In the undamaged adult zebrafish retina, Sox2 is expressed in Müller glia and a subset of amacrine cells, similar to other vertebrates. Following 31 h of light damage, Sox2 expression significantly increased in proliferating Müller glia. Morpholino‐mediated knockdown of Sox2 expression resulted in decreased numbers of proliferating Müller glia, while induced overexpression of Sox2 stimulated Müller glia proliferation in the absence of retinal damage. Thus, Sox2 is necessary and sufficient for Müller glia proliferation. We investigated the role of Wnt/&bgr;‐catenin signaling, which is a known regulator of sox2 expression during vertebrate retinal development. While &bgr;‐catenin 2, but not &bgr;‐catenin 1, was necessary for Müller glia proliferation, neither &bgr;‐catenin paralog was required for sox2 expression following retinal damage. Sox2 expression was also necessary for ascl1a (neurogenic) and lin28a (reprogramming) expression, but not stat3 expression following retinal damage. Furthermore, Sox2 was required for Müller glial‐derived neuronal progenitor cell amplification and expression of the pro‐neural marker Tg(atoh7:EGFP). Finally, loss of Sox2 expression prevented complete regeneration of cone photoreceptors. This study is the first to identify a functional role for Sox2 during Müller glial‐based regeneration of the vertebrate retina. HIGHLIGHTSSox2 is required for Müller glia proliferation in the damaged zebrafish retina.Sox2 is sufficient for Müller glia proliferation in the absence of damage.Sox2 is required for Müller glia‐derived neuronal progenitor cell proliferation.Sox2 is required for neuronal progenitor cell commitment to a neuronal lineage.

Interkinetic Nuclear Migration in the Regenerating Retina

In the adult zebrafish, death of retinal neurons stimulates Müller glia to re-enter the cell cycle to produce neuronal progenitor cells (NPCs) that undergo further cell divisions and differentiate to replace lost neurons in the correct spatial locations. Understanding the mechanisms regulating retinal regeneration will ultimately provide avenues to overcome vision loss in human. Recently, the observation of interkinetic nuclear migration (INM) of Müller glia in the regenerating zebrafish retina resulted in the inclusion of an additional complex step to the regeneration process. The pathways regulating INM and its function in the regenerating retina have not been well studied. Here, we summarize the evidence for INM in the regenerating retina and review mechanisms that control INM during neuro-epithelial development in the context of pathways known to be critical during retinal regeneration.

Culture of Adult Transgenic Zebrafish Retinal Explants for Live-cell Imaging by Multiphoton Microscopy

An endogenous regeneration program is initiated by Müller glia in the adult zebrafish (Danio rerio) retina following neuronal damage and death. The Müller glia re-enter the cell cycle and produce neuronal progenitor cells that undergo subsequent rounds of cell divisions and differentiate into the lost neuronal cell types. Both Müller glia and neuronal progenitor cell nuclei replicate their DNA and undergo mitosis in distinct locations of the retina, i.e. they migrate between the basal Inner Nuclear Layer (INL) and the Outer Nuclear Layer (ONL), respectively, in a process described as Interkinetic Nuclear Migration (INM). INM has predominantly been studied in the developing retina. To examine the dynamics of INM in the adult regenerating zebrafish retina in detail, live-cell imaging of fluorescently-labeled Müller glia/neuronal progenitor cells is required. Here, we provide the conditions to isolate and culture dorsal retinas from Tg[gfap:nGFP]mi2004 zebrafish that were exposed to constant intense light for 35 h. We also show that these retinal cultures are viable to perform live-cell imaging experiments, continuously acquiring z-stack images throughout the thickness of the retinal explant for up to 8 h using multiphoton microscopy to monitor the migratory behavior of gfap:nGFP-positive cells. In addition, we describe the details to perform post-imaging analysis to determine the velocity of apical and basal INM. To summarize, we established conditions to study the dynamics of INM in an adult model of neuronal regeneration. This will advance our understanding of this crucial cellular process and allow us to determine the mechanisms that control INM.

Live-cell imaging: new avenues to investigate retinal regeneration

Sensing and responding to our environment requires functional neurons that act in concert. Neuronal cell loss resulting from degenerative diseases cannot be replaced in humans, causing a functional impairment to integrate and/or respond to sensory cues. In contrast, zebrafish (Danio rerio) possess an endogenous capacity to regenerate lost neurons. Here, we will focus on the processes that lead to neuronal regeneration in the zebrafish retina. Dying retinal neurons release a damage signal, tumor necrosis factor α, which induces the resident radial glia, the Müller glia, to reprogram and re-enter the cell cycle. The Müller glia divide asymmetrically to produce a Müller glia that exits the cell cycle and a neuronal progenitor cell. The arising neuronal progenitor cells undergo several rounds of cell divisions before they migrate to the site of damage to differentiate into the neuronal cell types that were lost. Molecular and immunohistochemical studies have predominantly provided insight into the mechanisms that regulate retinal regeneration. However, many processes during retinal regeneration are dynamic and require live-cell imaging to fully discern the underlying mechanisms. Recently, a multiphoton imaging approach of adult zebrafish retinal cultures was developed. We will discuss the use of live-cell imaging, the currently available tools and those that need to be developed to advance our knowledge on major open questions in the field of retinal regeneration.

Photo-regulation of rod precursor cell proliferation

ABSTRACT Teleosts are unique in their ability to undergo persistent neurogenesis and to regenerate damaged and lost retinal neurons in adults. This contrasts with the human retina, which is incapable of replacing lost retinal neurons causing vision loss/blindness in the affected individuals. Two cell populations within the adult teleost retina generate new retinal neurons throughout life. Stem cells within the ciliary marginal zone give rise to all retinal cell types except for rod photoreceptors, which are produced by the resident Müller glia that are located within the inner nuclear layer of the entire retina. Understanding the mechanisms that regulate the generation of photoreceptors in the adult teleost retina may ultimately aid developing strategies to overcome vision loss in diseases such as retinitis pigmentosa. Here, we investigated whether photic deprivation alters the proliferative capacity of rod precursor cells, which are generated from Müller glia. In dark‐adapted retinas, rod precursor cell proliferation increased, while the number of proliferating Müller glia and their derived olig2:EGFP‐positive neuronal progenitor cells was not significantly changed. Cell death of rod photoreceptors was excluded as the inducer of rod precursor cell proliferation, as the number of TUNEL‐positive cells and l‐plastin‐positive microglia in both the outer (ONL) and inner nuclear layer (INL) remained at a similar level throughout the dark‐adaptation timecourse. Rod precursor cell proliferation in response to dark‐adaptation was characterized by an increased number of EdU‐positive cells, i.e. cells that were undergoing DNA replication. These proliferating rod precursor cells in dark‐adapted zebrafish differentiated into rod photoreceptors at a comparable percentage and in a similar time frame as those maintained under standard light conditions suggesting that the cell cycle did not stall in dark‐adapted retinas. Inhibition of IGF1‐receptor signaling reduced the dark‐adaptation‐mediated proliferation response; however, caloric restriction which has been suggested to be integrated by the IGF1/growth hormone signaling axis did not influence rod precursor cell proliferation in dark‐adapted retinas, as similar numbers were observed in starved and normal fed zebrafish. In summary, photic deprivation induces cell cycle entry of rod precursor cells via IGF1‐receptor signaling independent of Müller glia proliferation. HighlightsDark‐adaptation increases rod precursor cell proliferation.Dark‐adaptation does not induce either Müller glia proliferation nor cell death.Dark‐adaptation does not affect rod precursor cell differentiation into mature rods.Dark‐adaptation recruits more rod precursors into S‐phase of the cell cycle.IGF1‐receptors regulate dark‐adaptation‐mediated rod precursor proliferation.