Substituting mouse transcription factor Pou4f2 with a sea urchin orthologue restores retinal ganglion cell development.

Pou domain transcription factor Pou4f2 is essential for the development of retinal ganglion cells (RGCs) in the vertebrate retina. A distant orthologue of Pou4f2 exists in the genome of the sea urchin (class Echinoidea) Strongylocentrotus purpuratus (SpPou4f1/2), yet the photosensory structure of sea urchins is strikingly different from that of the mammalian retina. Sea urchins have no obvious eyes, but have photoreceptors clustered around their tube feet disc. The mechanisms that are associated with the development and function of photoreception in sea urchins are largely unexplored. As an initial approach to better understand the sea urchin photosensory structure and relate it to the mammalian retina, we asked whether SpPou4f1/2 could support RGC development in the absence of Pou4f2. To answer this question, we replaced genomic Pou4f2 with an SpPou4f1/2 cDNA. In Pou4f2-null mice, retinas expressing SpPou4f1/2 were outwardly identical to those of wild-type mice. SpPou4f1/2 retinas exhibited dark-adapted electroretinogram scotopic threshold responses, indicating functionally active RGCs. During retinal development, SpPou4f1/2 activated RGC-specific genes and in S. purpuratus, SpPou4f2 was expressed in photoreceptor cells of tube feet in a pattern distinct from Opsin4 and Pax6. Our results suggest that SpPou4f1/2 and Pou4f2 share conserved components of a gene network for photosensory development and they maintain their conserved intrinsic functions despite vast morphological differences in mouse and sea urchin photosensory structures.

T-box transcription regulator Tbr2 is essential for the formation and maintenance of Opn4/melanopsin-expressing intrinsically photosensitive retinal ganglion cells.

Opsin 4 (Opn4)/melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) play a major role in non-image-forming visual system. Although advances have been made in understanding their morphological features and functions, the molecular mechanisms that regulate their formation and survival remain unknown. Previously, we found that mouse T-box brain 2 (Tbr2) (also known as Eomes), a T-box-containing transcription factor, was expressed in a subset of newborn RGCs, suggesting that it is involved in the formation of specific RGC subtypes. In this in vivo study, we used complex mouse genetics, single-cell dye tracing, and behavioral analyses to determine whether Tbr2 regulates ipRGC formation and survival. Our results show the following: (1) Opn4 is expressed exclusively in Tbr2-positive RGCs; (2) no ipRGCs are detected when Tbr2 is genetically ablated before RGC specification; and (3) most ipRGCs are eliminated when Tbr2 is deleted in established ipRGCs. The few remaining ipRGCs display abnormal dendritic morphological features and functions. In addition, some Tbr2-expressing RGCs can activate Opn4 expression on the loss of native ipRGCs, suggesting that Tbr2-expressing RGCs may serve as a reservoir of ipRGCs to regulate the number of ipRGCs and the expression levels of Opn4.

Adenosine and dopamine receptors coregulate photoreceptor coupling via gap junction phosphorylation in mouse retina.

Gap junctions in retinal photoreceptors suppress voltage noise and facilitate input of rod signals into the cone pathway during mesopic vision. These synapses are highly plastic and regulated by light and circadian clocks. Recent studies have revealed an important role for connexin36 (Cx36) phosphorylation by protein kinase A (PKA) in regulating cell-cell coupling. Dopamine is a light-adaptive signal in the retina, causing uncoupling of photoreceptors via D4 receptors (D4R), which inhibit adenylyl cyclase (AC) and reduce PKA activity. We hypothesized that adenosine, with its extracellular levels increasing in darkness, may serve as a dark signal to coregulate photoreceptor coupling through modulation of gap junction phosphorylation. Both D4R and A2a receptor (A2aR) mRNAs were present in photoreceptors, inner nuclear layer neurons, and ganglion cells in C57BL/6 mouse retina, and showed cyclic expression with partially overlapping rhythms. Pharmacologically activating A2aR or inhibiting D4R in light-adapted daytime retina increased photoreceptor coupling. Cx36 among photoreceptor terminals, representing predominantly rod-cone gap junctions but possibly including some rod-rod and cone-cone gap junctions, was phosphorylated in a PKA-dependent manner by the same treatments. Conversely, inhibiting A2aR or activating D4R in daytime dark-adapted retina decreased Cx36 phosphorylation with similar PKA dependence. A2a-deficient mouse retina showed defective regulation of photoreceptor gap junction phosphorylation, fairly regular dopamine release, and moderately downregulated expression of D4R and AC type 1 mRNA. We conclude that adenosine and dopamine coregulate photoreceptor coupling through opposite action on the PKA pathway and Cx36 phosphorylation. In addition, loss of the A2aR hampered D4R gene expression and function.

Distinct neurogenic potential in the retinal margin and the pars plana of mammalian eye.

Unlike many other vertebrates, a healthy mammalian retina does not grow throughout life and lacks a ciliary margin zone capable of actively generating new neurons. The isolation of stem-like cells from the ciliary epithelium has led to speculation that the mammalian retina and/or surrounding tissues may retain neurogenic potential capable of responding to retinal damage. Using genetically altered mouse lines with varying degrees of retinal ganglion cell loss, we show that the retinal margin responds to ganglion cell loss by prolonging specific neurogenic activity, as characterized by increased numbers of Atoh7(LacZ)-expressing cells. The extent of neurogenic activity correlated with the degree of ganglion cell deficiency. In the pars plana, but not the retinal margin, cells remain proliferative into adulthood, marking the junction of pars plana and retinal margin as a niche capable of producing proliferative cells in the mammalian retina and a potential cellular source for retinal regeneration.

Dot1a contains three nuclear localization signals and regulates the epithelial Na+ channel (ENaC) at multiple levels.

We have previously reported that Dot1a is located in the cytoplasm and nucleus (Reisenauer MR, Anderson M, Huang L, Zhang Z, Zhou Q, Kone BC, Morris AP, Lesage GD, Dryer SE, Zhang W. J Biol Chem 284: 35659-35669, 2009), widely expressed in the kidney as detected by its histone H3K79 methyltransferase activity (Zhang W, Hayashizaki Y, Kone BC. Biochem J 377: 641-651, 2004), and involved in transcriptional control of the epithelial Na(+) channel subunit-alpha gene (alphaENaC) (Zhang W, Xia X, Jalal DI, Kuncewicz T, Xu W, Lesage GD, Kone BC. Am J Physiol Cell Physiol 290: C936-C946, 2006). Aldosterone releases repression of alphaENaC by reducing expression of Dot1a and its partner AF9 (Zhang W, Xia X, Reisenauer MR, Hemenway CS, Kone BC. J Biol Chem 281: 18059-18068, 2006) and by impairing Dot1a-AF9 interaction via Sgk1-mediated AF9 phosphorylation (Zhang W, Xia X, Reisenauer MR, Rieg T, Lang F, Kuhl D, Vallon V, Kone BC. J Clin Invest 117: 773-783, 2007). This network also appears to regulate transcription of several other aldosterone target genes. Here, we provide evidence showing that Dot1a contains at least three potential nuclear localization signals (NLSs). Deletion of these NLSs causes green fluorescent protein-fused Dot1a fusions to localize almost exclusively in the cytoplasm of 293T cells as revealed by confocal microscopy. Deletion of NLSs abolished Dot1a-mediated repression of alphaENaC-promoter luciferase construct in M1 cells. AF9 is widely expressed in mouse kidney. Similar to alphaENaC, the mRNA levels of betaENaC, gammaENaC, and Sgk1 are also downregulated by Dot1a and AF9 overexpression. Small interference RNA-mediated knockdown of Dot1a and AF9 or aldosterone treatment leads to an opposite effect. Using single-cell fluorescence imaging or equivalent short-circuit current in IMCD3 and M1 cells, we show that observed transcriptional alterations correspond to changes in ENaC and Sgk1 protein levels as well as benzamil-sensitive Na(+) transport. In brief, Dot1a and AF9 downregulate Na(+) transport, most likely by regulating ENaC mRNA and subsequent protein expression and ENaC activity.

Mesendoderm cells induce oriented cell division and invagination in the marine shrimp Sicyonia ingentis.

The mesendoderm (ME) cells are the two most vegetal blastomeres in the early developing embryo of the marine shrimp Sicyonia ingentis. These two cells enter mitotic arrest for three cycles after the 5th cell cycle (32-cell stage) and ingress into the blastocoel at the 6th cycle (62-cell stage). Circumjacent to the ingressing ME cells are nine presumptive naupliar mesoderm (PNM) cells that exhibit a predictable pattern of spindle orientation into the blastopore, followed by invagination. We examined the role of ME cells and PNM cells in gastrulation using blastomere recombinations and confocal microscopy. Removal of ME progenitors prevented gastrulation. Removal of any other blastomeres, including PNM progenitors, did not interfere with normal invagination. Altered spindle orientations occurred in blastomeres that had direct contact with one of the ME cells; one spindle pole localized to the cytoplasmic region closest to ME cell contact. In recombined embryos, this resulted in an extension of the region of ME-embryo contact. Our results show that ME cells direct the spindle orientations of their adjacent cells and are consistent with a mechanism of oriented cell division being a responsible force for archenteron elongation.

Near complete loss of retinal ganglion cells in the math5/brn3b double knockout elicits severe reductions of other cell types during retinal development.

Retinal ganglion cells (RGCs) are the first cell type to differentiate during retinal histogenesis. It has been postulated that specified RGCs subsequently influence the number and fate of the remaining progenitors to produce the rest of the retinal cell types. However, several genetic knockout models have argued against this developmental role for RGCs. Although it is known that RGCs secrete cellular factors implicated in cell proliferation, survival, and differentiation, until now, limited publications have shown that reductions in the RGC number cause significant changes in these processes. In this study, we observed that Math5 and Brn3b double null mice exhibited over a 99% reduction in the number of RGCs during development. This severe reduction of RGCs is accompanied by a drastic loss in the number of all other retinal cell types that was never seen before. Unlike Brn3b null or Math5 null animals, mice null for both alleles lack an optic nerve and have severe retinal dysfunction. Results of this study support the hypothesis that RGCs play a pivotal role in the late phase of mammalian retina development.

Retinal ganglion cell death and optic nerve degeneration by genetic ablation in adult mice.

Despite the magnitude of the problem, no effective treatments exist to prevent retinal ganglion cell (RGC) death and optic nerve degeneration from occurring in diseases affecting the human eye. Animal models currently available for developing treatment strategies suffer from cumbersome procedures required to induce RGC death or rely on mutations that induce defects in developing retinas rather than in mature retinas of adults. Our objective was to develop a robust genetically engineered adult mouse model for RGC loss and optic nerve degeneration based on genetic ablation. To achieve this, we took advantage of Pou4f2 (Brn3b), a gene activated immediately as RGCs begin to differentiate and expressed throughout life. We generated adult mice whose genomes harbored a conditional Pou4f2 allele containing a floxed-lacZ-stop-diphtheria toxin A cassette and a CAGG-Cre-ER transgene. In this bigenic model, Cre recombinase is fused to a modified estrogen nuclear receptor in which the estrogen-binding domain binds preferentially to the estrogen agonist tamoxifen rather than to endogenous estradiol. Upon binding to the estrogen-binding domain, tamoxifen derepresses Cre recombinase, leading to the efficient genomic deletion of the floxed-lacZ-stop DNA sequence and expression of diphtheria toxin A. Tamoxifen administered to adult mice at different ages by intraperitoneal injection led to rapid RGC loss, reactive gliosis, progressive degradation of the optic nerve over a period of several months, and visual impairment. Perhaps more reflective of human disease, partial loss of RGCs was achieved by modulating the tamoxifen treatment. Especially relevant for RGC death and optic nerve degeneration in human retinal pathologies, RGC-ablated retinas maintained their structural integrity, and other retinal neurons and their connections in the inner and outer plexiform layers appeared unaffected by RGC ablation. These events are hallmarks of progressive optic nerve degeneration observed in human retinal pathologies and demonstrate the validity of this model for use in developing stem cell therapies for replacing dead RGCs with healthy ones.

Sequential Therapy with PARP and WEE1 Inhibitors Minimizes Toxicity while Maintaining Efficacy.

We demonstrate that concurrent administration of poly(ADP-ribose) polymerase (PARP) and WEE1 inhibitors is effective in inhibiting tumor growth but poorly tolerated. Concurrent treatment with PARP and WEE1 inhibitors induces replication stress, DNA damage, and abrogates the G2 DNA damage checkpoint in both normal and malignant cells. Following cessation of monotherapy with PARP or WEE1 inhibitors, effects of these inhibitors persist suggesting that sequential administration of PARP and WEE1 inhibitors could maintain efficacy while ameliorating toxicity. Strikingly, while sequential administration mirrored concurrent therapy in cancer cells that have high basal replication stress, low basal replication stress in normal cells protected them from DNA damage and toxicity, thus improving tolerability while preserving efficacy in ovarian cancer xenograft and patient-derived xenograft models.

Retinal ganglion cell type, size, and spacing can be specified independent of homotypic dendritic contacts.

In Brn3b(-/-) mice, where 80% of retinal ganglion cells degenerate early in development, the remaining 20% include most or all ganglion cell types. Cells of the same type cover the retinal surface evenly but tile it incompletely, indicating that a regular mosaic and normal dendritic field size can be maintained in the absence of contact among homotypic cells. In Math5(-/-) mice, where only approximately 5% of ganglion cells are formed, the dendritic arbors of at least two types among the residual ganglion cells are indistinguishable from normal in shape and size, even though throughout development they are separated by millimeters from the nearest neighboring ganglion cell of the same type. It appears that the primary phenotype of retinal ganglion cells can develop without homotypic contact; dendritic repulsion may be an end-stage mechanism that fine-tunes the dendritic arbors for more efficient coverage of the retinal surface.

Essential roles of mitochondrial biogenesis regulator Nrf1 in retinal development and homeostasis.

BACKGROUND: Mitochondrial dysfunction has been implicated in the pathologies of a number of retinal degenerative diseases in both the outer and inner retina. In the outer retina, photoreceptors are particularly vulnerable to mutations affecting mitochondrial function due to their high energy demand and sensitivity to oxidative stress. However, it is unclear how defective mitochondrial biogenesis affects neural development and contributes to neural degeneration. In this report, we investigated the in vivo function of nuclear respiratory factor 1 (Nrf1), a major transcriptional regulator of mitochondrial biogenesis in both proliferating retinal progenitor cells (RPCs) and postmitotic rod photoreceptor cells (PRs). METHODS: We used mouse genetic techniques to generate RPC-specific and rod PR-specific Nrf1 conditional knockout mouse models. We then applied a comprehensive set of tools, including histopathological and molecular analyses, RNA-seq, and electroretinography on these mouse lines to study Nrf1-regulated genes and Nrf1's roles in both developing retinas and differentiated rod PRs. For all comparisons between genotypes, a two-tailed two-sample student's t-test was used. Results were considered significant when P < 0.05. RESULTS: We uncovered essential roles of Nrf1 in cell proliferation in RPCs, cell migration and survival of newly specified retinal ganglion cells (RGCs), neurite outgrowth in retinal explants, reconfiguration of metabolic pathways in RPCs, and mitochondrial morphology, position, and function in rod PRs. CONCLUSIONS: Our findings provide in vivo evidence that Nrf1 and Nrf1-mediated pathways have context-dependent and cell-state-specific functions during neural development, and disruption of Nrf1-mediated mitochondrial biogenesis in rod PRs results in impaired mitochondria and a slow, progressive degeneration of rod PRs. These results offer new insights into the roles of Nrf1 in retinal development and neuronal homeostasis and the differential sensitivities of diverse neuronal tissues and cell types of dysfunctional mitochondria. Moreover, the conditional Nrf1 allele we have generated provides the opportunity to develop novel mouse models to understand how defective mitochondrial biogenesis contributes to the pathologies and disease progression of several neurodegenerative diseases, including glaucoma, age-related macular degeneration, Parkinson's diseases, and Huntington's disease.

Regulation of retinal cone bipolar cell differentiation and photopic vision by the CVC homeobox gene Vsx1.

Cone bipolar cells of the vertebrate retina connect photoreceptors with ganglion cells to mediate photopic vision. Despite this important role, the mechanisms that regulate cone bipolar cell differentiation are poorly understood. VSX1 is a CVC domain homeoprotein specifically expressed in cone bipolar cells. To determine the function of VSX1, we generated Vsx1 mutant mice and found that Vsx1 mutant retinal cells form but do not differentiate a mature cone bipolar cell phenotype. Electrophysiological studies demonstrated that Vsx1 mutant mice have defects in their cone visual pathway, whereas the rod visual pathway was unaffected. Thus, Vsx1 is required for cone bipolar cell differentiation and regulates photopic vision perception.

Birth of cone bipolar cells, but not rod bipolar cells, is associated with existing RGCs.

Retinal ganglion cells (RGCs) play important roles in retinogenesis. They are required for normal retinal histogenesis and retinal cell number balance. Developmental RGC loss is typically characterized by initial retinal neuronal number imbalance and subsequent loss of retinal neurons. However, it is not clear whether loss of a specific non-RGC cell type in the RGC-depleted retina is due to reduced cell production or subsequent degeneration. Taking advantage of three knockout mice with varying degrees of RGC depletion, we re-examined bipolar cell production in these retinas from various aspects. Results show that generation of the cone bipolar cells is correlated with the existing number of RGCs. However, generation of the rod bipolar cells is unaffected by RGC shortage. Results report the first observation that RGCs selectively influence the genesis of subsequent retinal cell types.

Rewiring the retinal ganglion cell gene regulatory network: Neurod1 promotes retinal ganglion cell fate in the absence of Math5.

Retinal progenitor cells (RPCs) express basic helix-loop-helix (bHLH) factors in a strikingly mosaic spatiotemporal pattern, which is thought to contribute to the establishment of individual retinal cell identity. Here, we ask whether this tightly regulated pattern is essential for the orderly differentiation of the early retinal cell types and whether different bHLH genes have distinct functions that are adapted for each RPC. To address these issues, we replaced one bHLH gene with another. Math5 is a bHLH gene that is essential for establishing retinal ganglion cell (RGC) fate. We analyzed the retinas of mice in which Math5 was replaced with Neurod1 or Math3, bHLH genes that are expressed in another RPC and are required to establish amacrine cell fate. In the absence of Math5, Math5Neurod1-KI was able to specify RGCs, activate RGC genes and restore the optic nerve, although not as effectively as Math5. By contrast, Math5Math3-KI was much less effective than Math5Neurod1-KI in replacing Math5. In addition, expression of Neurod1 and Math3 from the Math5Neurod1-KI/Math3-KI allele did not result in enhanced amacrine cell production. These results were unexpected because they indicated that bHLH genes, which are currently thought to have evolved highly specialized functions, are nonetheless able to adjust their functions by interpreting the local positional information that is programmed into the RPC lineages. We conclude that, although Neurod1 and Math3 have evolved specialized functions for establishing amacrine cell fate, they are nevertheless capable of alternative functions when expressed in foreign environments.

Retinal ganglion cell differentiation in cultured mouse retinal explants.

The availability of genetically engineered mice harboring specific mutations in genes affecting one or more retinal cell types affords new opportunities for investigating the genetic regulatory mechanisms of vertebrate retina formation. When identifying critical regulatory genes involved in retina development it is often advantageous to complement in vivo analysis with in vitro characterization. In particular, by combining classical techniques of retinal explant culturing with gene transfer procedures relying on herpes simple virus (HSV) amplicon vectors, gain-of-function analysis with genes of interest can be performed quickly and efficiently. Here, details are provided for isolating and culturing explants containing retinal progenitor cells and for infecting the explants with HSV expression vectors that perturb or rescue retinal ganglion cells, the first cell type to differentiate in the retina. In addition, the availability of sensitive techniques to monitor gene expression, including detection of reporter gene expression using antibodies and detection of endogenous marker gene expression using quantitative RT-PCR, provides an effective means for comparing wild-type and mutant retinas from genetically engineered mice.

Brn3b/Brn3c double knockout mice reveal an unsuspected role for Brn3c in retinal ganglion cell axon outgrowth.

In mice, Brn3 POU domain transcription factors play essential roles in the differentiation and survival of projection neurons within the retina, inner ear, dorsal root and trigeminal ganglia. During retinal ganglion cell differentiation, Brn3b is expressed first, followed by Brn3a and Brn3c. Targeted deletion of Brn3b, but not Brn3a or Brn3c, leads to a loss of most retinal ganglion cells before birth. However, as a few retinal ganglion cells are still present in Brn3b(-/-) mice, Brn3a and Brn3c may partially compensate for the loss of Brn3b. To examine the role of Brn3c in retinal ganglion cell development, we generated Brn3b/Brn3c double knockout mice and analyzed their retinas and optic chiasms. Retinal ganglion cell axons from double knockout mice were more severely affected than were those from Brn3b-deficient mice, indicating that Brn3c was required for retinal ganglion cell differentiation and could partially compensate for the loss of Brn3b. Moreover, Brn3c had functions in retinal ganglion cell differentiation separate from those of Brn3b. Ipsilateral and misrouted projections at the optic chiasm were overproduced in Brn3b(-/-) mice but missing were entirely in optic chiasms of Brn3b/Brn3c double knockout mice, suggesting that Brn3c controlled ipsilateral axon production. Forced expression of Brn3c in Brn3b(-/-) retinal explants restored neurite outgrowth, demonstrating that Brn3c could promote axon outgrowth in the absence of Brn3b. Our results reveal a complex genetic relationship between Brn3b and Brn3c in regulating the retinal ganglion cell axon outgrowth.