p57(Kip2) regulates progenitor cell proliferation and amacrine interneuron development in the mouse retina

Cell-intrinsic regulators of proliferation in vertebrate retinal progenitors

The proliferative expansion of retinal progenitor cells (RPCs) is a fundamental mechanism of growth during vertebrate retinal development. Over the past couple of years, significant progress has been made in identifying genes expressed in RPCs that are essential for their proliferation, and the molecular mechanisms are beginning to be resolved. In this review, we highlight recent studies that have identified regulatory components of the RPC cell cycle machinery and implicate a set of homeobox genes as key regulators of proliferative expansion in the retina.

Genomic analysis of mouse retinal development

The vertebrate retina is comprised of seven major cell types that are generated in overlapping but well-defined intervals. To identify genes that might regulate retinal development, gene expression in the developing retina was profiled at multiple time points using serial analysis of gene expression (SAGE). The expression patterns of 1,051 genes that showed developmentally dynamic expression by SAGE were investigated using in situ hybridization. A molecular atlas of gene expression in the developing and mature retina was thereby constructed, along with a taxonomic classification of developmental gene expression patterns. Genes were identified that label both temporal and spatial subsets of mitotic progenitor cells. For each developing and mature major retinal cell type, genes selectively expressed in that cell type were identified. The gene expression profiles of retinal Müller glia and mitotic progenitor cells were found to be highly similar, suggesting that Müller glia might serve to produce multiple retinal cell types under the right conditions. In addition, multiple transcripts that were evolutionarily conserved that did not appear to encode open reading frames of more than 100 amino acids in length ("noncoding RNAs") were found to be dynamically and specifically expressed in developing and mature retinal cell types. Finally, many photoreceptor-enriched genes that mapped to chromosomal intervals containing retinal disease genes were identified. These data serve as a starting point for functional investigations of the roles of these genes in retinal development and physiology.

Medaka as a model system for the characterisation of cell cycle regulators: a functional analysis of Ol-Gadd45gamma during early embryogenesis

Numerous studies, mostly performed on mammalian cell cultures, have implicated the Gadd45 family of small acidic proteins in cell cycle control (arrest and/or engagement in the apoptotic pathway). We report here the cloning, detailled expression pattern and functional characterisation in embryonic development of Ol-Gadd45gamma, the Oryzias latipes ortholog of mammalian Gadd45gamma. Its expression pattern, notably in the developing brain (optic tectum) strongly suggests that it is involved in cell cycle exit. Gain-of-function experiments (through mRNA injection) slowed down early development, and produced embryos clearly reduced in size, while morpholino knockdowns resulted in small embryos over-sensitive to DNA damage (UV irradiation). We further demonstrated that, following Ol-Gadd45gamma overexpression, cells are proliferation-arrested before both G1/S and G2/M cell cycle checkpoints, while in the MO-Ol-Gadd45 loss-of-function experiments cells are engaged in apoptosis rather than prevented from proliferating. These results show that Ol-Gadd45gamma is likely to play an important role in coordinating cell fate decisions during neurogenesis; they also demonstrate that the medakafish is a promising model to analyse in vivo the developmental control of the cell cycle.

Cell fate decisions and patterning in the vertebrate retina: the importance of timing, asymmetry, polarity and waves

The differentiation of distinct cell populations in the retina is a multi-step process that involves cell cycle exit, migration, and dramatic changes of cell morphology. All these steps are tightly controlled by multiple regulatory pathways, which involve both cell-autonomous networks of transcription factors and cell-cell signaling events. Additional regulatory inputs into cell fate decisions have been recently suggested: accumulating evidence shows that the timing of cell cycle exit, the orientation of the mitotic spindle during the last cell division, and the polarity of neuronal progenitor cells could play important roles in cell fate determination.

Activation of NF-kappaB and STAT3 in rat oval cells during 2-acetylaminofluorene/partial hepatectomy-induced liver regeneration

Proliferation and differentiation of hepatic stem cell progenies (i.e., oval cells) sustain liver regeneration when the replicative and functional capacity of hepatocytes is impaired. The signaling pathways that control stem cell activation remain poorly understood. In this study, we investigated the involvement of nuclear factor-kappa B (NF-kappaB) and signal transducer and activator of transcription 3 (STAT3) in oval cell-mediated liver regeneration induced by 2-acetylaminofluorene/partial hepatectomy (AAF/PH) protocol. Using OV1 as a marker for identification and sorting of oval cells, we established that both NF-kappaB and STAT3 were highly activated in the OV1(+) cell population. Three distinct subpopulations of oval cells were defined as OV1(low), OV1(medium), and OV1(high), based on the intensity of OV1 staining. Quantitative polymerase chain reaction analysis revealed that they represent different stages of oval cell differentiation along hepatocyte lineage. OV1(low) cells displayed the least differentiated phenotype as judged by high expression of c-kit and lack of hepatocytic differentiation markers, whereas OV1(high) cells lost c-kit expression, were more proliferative, and acquired more mature hepatocytic phenotype. Notably, NF-kappaB was activated uniformly in all three subpopulations of oval cells. In contrast, phosphorylation of STAT3 was detected only in OV1(high) cells. In conclusion, transcriptional activity supported by NF-kappaB and STAT3 is required for oval cell activation, expansion, and differentiation. The differential induction of NF-kappaB and STAT3 point to a distinct role for these transcription factors at different stages of hepatic stem cell differentiation.

The receptor protein tyrosine phosphatase mu, PTPmu, regulates histogenesis of the chick retina

The formation of laminae within the retina requires the coordinate regulation of cell differentiation and migration. The cell adhesion molecule and member of the immunoglobulin superfamily, receptor protein tyrosine phosphatase Mu, PTPmu, is expressed in precursor and early, differentiated cells of the prelaminated retina, and later becomes restricted to the inner plexiform, ganglion cell, and optic fiber layers. Since the timing of PTPmu expression correlates with the peak period of retinal lamination, we examined whether this RPTP could be regulating cell adhesion and migration within the retina, and thus influencing retinal development. Chick retinal organ cultures were infected with herpes simplex viruses encoding either an antisense sequence to PTPmu, wild-type PTPmu, or a catalytically inactive mutant form of PTPmu, and homophilic adhesion was blocked by using a function-blocking antibody. All conditions that perturbed PTPmu dramatically disrupted retinal histogenesis. Our findings demonstrate that catalytic activity and adhesion mediated by PTPmu regulate lamination of the retina, emphasizing the importance of adhesion and signaling via receptor protein tyrosine phosphatases in the developing nervous system. To our knowledge, this is the first demonstration that an Ig superfamily RPTP regulates the lamination of any neural tissue.

Neurogenesis and the cell cycle

For a long time, it has been understood that neurogenesis is linked to proliferation and thus to the cell cycle. Recently, the gears that mediate this linkage have become accessible to molecular investigation. This review describes some of the progress that has been made in understanding how the molecular machinery of the cell cycle is used in the processes of size regulation in the brain, histogenesis, neuronal differentiation, and the maintenance of stem cells.

Thyroid hormone and retinal development: an emerging field

Thyroid hormone appears to play a critical, yet not fully understood, role in the development of the neuroretina. This review focuses on recent experiments in the rodent, chicken, and amphibian, with an emphasis on how the hormone and its receptor isoforms influence retinal cell proliferation and cell fate decisions. The initial results are fueling the next generation of experiments in the retina, which promise to provide insights into the mechanisms of thyroid hormone action in a wide variety of developing neural tissue.

Transcriptional control of neuronal diversification in the retina

During embryonic development, the array of vastly different neuronal types that are incorporated into the functional architecture of the mature neuroretina derives from a common population of multipotent retinal progenitor cells (RPCs). Retinogenesis proceeds in a precise chronological order, with the seven principal cell classes generated in successive phases. Cell biological experiments established that this histogenetic order, at least in part, reflects intrinsic changes within the RPC pool. In recent years a number of molecules controlling various aspects of cell fate specification from RPCs have been identified. However, few attempts have been made to integrate previous concepts that emerged from cell biological studies and more recent results based on molecular genetic experiments. This review aims at providing an overview of recent advances in our understanding of the cellular and molecular mechanisms underlying retinal neuronal diversification, with a particular focus on cell-intrinsic factors.

Essential embryonic roles of the CKI-1 cyclin-dependent kinase inhibitor in cell-cycle exit and morphogenesis in C elegans

Following a phase of rapid proliferation, cells in developing embryos must decide when to cease division and then whether to survive and differentiate or instead undergo programmed death. In screens for genes that regulate embryonic patterning of the endoderm in Caenorhabditis elegans, we identified overlapping chromosomal deletions that define a gene required for these decisions. These deletions result in embryonic hyperplasia in multiple somatic tissues, excessive numbers of cell corpses, and profound defects in morphogenesis and differentiation. However, cell-cycle arrest of the germline is unaffected. Cell lineage analysis of these mutants revealed that cells that normally stop dividing earlier than their close relatives instead undergo an extra round of division. These deletions define a genomic region that includes cki-1 and cki-2, adjacent genes encoding members of the Cip/Kip family of cyclin-dependent kinase inhibitors. cki-1 alone can rescue the cell proliferation, programmed cell death, and differentiation and morphogenesis defects observed in these mutants. In contrast, cki-2 is not capable of significantly rescuing these phenotypes. RNA interference of cki-1 leads to embryonic lethality with phenotypes similar to, or more severe than, the deletion mutants. cki-1 and -2 gene reporters show distinct expression patterns; while both are expressed at around the time that embryonic cells exit the cell cycle, cki-2 also shows marked expression starting early in embryogenesis, when rapid cell division occurs. Our findings demonstrate that cki-1 activity plays an essential role in embryonic cell cycle arrest, differentiation and morphogenesis, and suggest that it may be required to suppress programmed cell death or engulfment of cell corpses.

Prox1 function controls progenitor cell proliferation and horizontal cell genesis in the mammalian retina

Retinal progenitor cells regulate their proliferation during development so that the correct number of each cell type is made at the appropriate time. We found that the homeodomain protein Prox1 regulates the exit of progenitor cells from the cell cycle in the embryonic mouse retina. Cells lacking Prox1 are less likely to stop dividing, and ectopic expression of Prox1 forces progenitor cells to exit the cell cycle. During retinogenesis, Prox1 can be detected in differentiating horizontal, bipolar and AII amacrine cells. Horizontal cells are absent in retinae of Prox1-/- mice and misexpression of Prox1 in postnatal progenitor cells promotes horizontal-cell formation. Thus, Prox1 activity is both necessary and sufficient for progenitor-cell proliferation and cell-fate determination in the vertebrate retina.

Determination of retinal cell fates is affected in the absence of extraocular striated muscles

Neural retinas of genetically modified mouse embryos and fetuses entirely lacking extraocular striated muscles (designated as Myf5-/-:MyoD-/- or amyogenic) are used to study in vivo the role of extraocular muscle (i.e., fetal ocular movements) in the genesis of retinal cell diversity. Although retinal lamination and the total number of cells per retinal layer appeared unaffected in amyogenic fetuses, electron microscopy and histochemistry revealed the absence of cholinergic amacrine cell type. By contrast, the amounts of other amacrine cell subpopulations (calretinin-, tyrosine hydroxylase-, and parvalbumin-expressing) were increased, whereas the amounts of Islet1/2-expressing retinal ganglion cells were decreased. Surprisingly, it was not possible to detect any change in proliferation or cell death. Consistently, the number of progenitors for retinal ganglion cells (nestin-expressing precursors) were increased, whereas the amounts of precursors for amacrine cells (syntaxin- and VC1.1-expressing precursors) were decreased in the mutant retinas. The difference in requirements for extraocular muscle support in regulation of precise ratios of retinal neuronal cell types suggests an essential role of extrinsic cues in the determination of retinal cell fates. Taken together, it appears that patterning mechanisms intrinsic to the neural retina specify the basic organization of retinal spatial organization (e.g., retinal layers and total number of cells). However, extrinsic cues seem to change intrinsic properties (e.g., competence) of retinal progenitor cells and influence the ratios of the differentiated cell types (i.e., cell fate choice) they produce.

Cell cycle and cell fate interactions in neural development

Mechanisms coupling cell cycle and cell fate operate at different steps during neural development. Intrinsic factors control the cell proliferation of distinct brain regions and changes of cell fate competence, whereas components of the cell cycle machinery could play a major role in setting the appropriate timing of the generation of different cell types.

Somatic gene targeting in the developing and adult mouse retina

Retinogenesis is a developmental process that is tightly regulated both temporally and spatially and is therefore an excellent model system for studying the molecular and cellular mechanisms of neurogenesis in the central nervous system. Understanding of these events in vivo is greatly facilitated by the availability of mouse mutant models, including those with natural or targeted mutations and those with conditional knockout or forced expression of genes. This article reviews these genetic modifications and their contribution to the study of retinogenesis in mammals, with special emphasis on conditional gene targeting approaches.

A Role for phosphoinositide 3-kinase in the control of cell division and survival during retinal development

Neurogenesis in the retina requires the concerted action of three different cellular processes: proliferation, differentiation, and apoptosis. Class IA phosphoinositide 3-kinase (PI3K) is a heterodimer composed of a p85 regulatory and a p110 catalytic subunit. p110alpha has been shown to regulate cell division and survival. Little is known of its function in development, however, as p110alpha knockout mice exhibit CNS defects, but death at early embryonic stages impairs further study. Here, we examine the role of PI3K in mouse retina development by expressing an activating form of PI3K regulatory subunit, p65(PI3K), as a transgene in the retina. Mice expressing p65(PI3K) showed severely disrupted retina morphogenesis, with ectopic cell masses in the neuroepithelium that evolved into infoldings of adult retinal cell layers. These changes correlated with an altered cell proliferation/cell death balance at early developmental stages. Nonetheless, the most affected cell layer in adult retina was that of photoreceptors, which correlated with selectively increased survival of these cells at developmental stages at which cell division has ceased. These results demonstrate the relevance of accurate PI3K regulation for normal retinal development, supporting class IA PI3K involvement in induction of cell division at early stages of neurogenesis. These data also show that, even after cell division decline, PI3K activation mediates survival of differentiated neurons in vivo.

The cyclin-dependent kinase inhibitors p19(Ink4d) and p27(Kip1) are coexpressed in select retinal cells and act cooperatively to control cell cycle exit

Cyclin-dependent kinase inhibitors (cdki's), including p19(Ink4d) and p27(Kip1), mediate exit from the cell cycle. To determine the function of these cdki's in regulating neurogenesis, we examined retina from wild-type, Ink4d-null, and Ink4d/Kip1-double null animals. Ink4d was expressed in progenitors and select neurons in the mature retina. Ink4d-null retina showed an extended period of proliferation, followed by apoptosis. Colabeling for p19(Ink4d) and p27(Kip1) revealed that a subpopulation of cells expressed both inhibitors. Deletion of Ink4d and Kip1 resulted in continued proliferation that was synergistic. This hyperproliferation led to an increase in number of horizontal cells and differentiated neurons reentering the cell cycle. Deletion of Ink4d and Kip1 also exacerbated the retinal dysplasia observed in Kip1-null mice, which was shown to be partly dependent on p53. These data indicate that select retinal cells express both p19(Ink4d) and p27(Kip1) and that they act cooperatively to ensure cell cycle exit.

Role of cyclin-dependent kinase inhibitors in the growth arrest at senescence in human prostate epithelial and uroepithelial cells

Cellular senescence has been proposed to be an in vitro and in vivo block that cells must overcome in order to immortalize and become tumorigenic. To characterize these pathways, we focused on changes in the cyclin-dependent kinase inhibitors and their binding partners that underlie the cell cycle arrest at senescence. As a model, we utilized normal human prostate epithelial cell (HPEC) and human uroepithelial cell (HUC) cultures. After 30-40 population doublings cells became growth-arrested in G0/1 with a threefold decrease in Cdk2-associated activity, a point defined as pre-senescence. Temporally following this growth arrest, the cells develop a senescence morphology and express senescence-associated beta-galactosidase (SA-beta-gal). Levels of p16(INK4a) and p57(KIP2) rise in HUCs during progressive passages, whereas only p16 increases in HPEC cultures. The induced expression of p57, similar to p16, produces a senescent-like phenotype. pRB, cyclin D, p19(INK4d) and p27(KIP1) decrease in both cell types. We find that p53, p21(CIP1) and p15(INK4b) are transiently elevated in HPECs and HUCs at the pre-senescent growth arrest, then return to low proliferating levels at terminal senescence. Analysis of p53, p21(CIP1), p15(INK4b), p16(INK4a), and p57(KIP2) reveals altered expression in immortalized, non-tumorigenic HPV16 E6 and E7 prostate lines and in tumorigenic prostate cancer cells. These results indicate: (i) the existence of a subset of growth inhibiting genes elevated at the onset of the senescence, (ii) a distinct class of genes involved in the maintenance of senescence, and (iii) the frequent inactivation of these pathways during immortalization.

Expression of Ol-KIP, a cyclin-dependent kinase inhibitor, in embryonic and adult medaka (Oryzias latipes) central nervous system

From an expression screen in a fish model, the medaka, we have isolated Ol-KIP (Oryzias latipes-kinase inhibitor protein), a new member of the KIP subfamily of cyclin-dependent kinase (Cdk) inhibitors. We have analysed its expression in the developing and adult brain by in situ hybridization and by double labeling with Ol-KIP mRNA and proliferating cell nuclear antigen (PCNA) antibodies. Ol-KIP presents a complex expression pattern in several areas of the embryonic central nervous system, most often in close vicinity to proliferative neuroepithelia. We studied in great detail its expression in the optic tectum: Ol-KIP is expressed in a ring-shaped domain lying exactly between the proliferative and the postmitotic zones of this structure and is, therefore, potentially involved in cell cycle exit. In the adult CNS, Ol-KIP expression persists in numerous nuclei, both close and distant from proliferative ventricular areas. So, Ol-KIP expression is in part compatible with a sustained "stop signal" role for proliferation, but its expression in postmitotic zones suggests that KIP proteins may have late neuronal function(s), in addition to inhibiting Cdks. This first detailed study of the expression profile of a KIP gene in a nonmammalian vertebrate, thus, opens perspectives for analysing the role of these regulators in brain development and function.

Persistent expression of cyclin D1 disrupts normal photoreceptor differentiation and retina development

The differentiation of neuronal cells in the developing mammalian retina is closely coupled to cell cycle arrest and proceeds in a highly organized manner. Cyclin D1, which regulates cell proliferation in many cells, also drives the proliferation of photoreceptor progenitors. In the mouse retina, cyclin D1 protein normally decreases as photoreceptors mature. To study the importance of the down-regulation of cyclin D1 during photoreceptor development, we generated a transgenic mouse in which cyclin D1 was persistently expressed in developing photoreceptor cells. We observed numerous abnormalities in both photoreceptors and other nonphotoreceptor cells in the retina of these transgenic mice. In particular, we observed delayed opsin expression in developing photoreceptors and alterations in their number and morphology in the mature retina. These alterations were accompanied by disorganization of the inner nuclear and plexiform layers. The expression of cyclin D1 caused excess photoreceptor cell proliferation and apoptosis. Loss of the p53 tumor suppressor gene decreased cyclin D1-induced apoptosis and led to microscopic hyperplasia in the retina. These findings are distinct from other mouse models in which the retinoblastoma gene pathway is disrupted and suggest that the IRBP-cyclin D1 mouse model may recapitulate an early step in the development of retinoblastoma.

Cloning and characterization of a novel transcription factor involved in cellular proliferation arrest: PATF

Cell cycle withdrawal involves several transcription factors such as E2Fs members that play a key role in cell growth control. Here we describe a novel putative bZIP transcription factor isolated from the retina and involved in neuronal proliferation arrest at the terminal differentiation: PATF (Proliferation Arrest Transcription Factor). We show that PATF associates with E2F4 protein and interacts with the E2F consensus site. PATF expression increases with establishment of quiescent state. Furthermore, the nuclear PATF localization like E2F4, depends on cell growth arrest. The decrease of PATF amount, using a retroviral antisense strategy, results in pursued neuroretina cell mitosis. Our results indicate that PATF could be a new molecular signal implicated in the final neuronal cell cycle withdrawal.

The perplexed and confused mutations affect distinct stages during the transition from proliferating to post-mitotic cells within the zebrafish retina

To identify and study genes essential for vertebrate retinal development, we are screening zebrafish embryos for mutations that disrupt retinal histogenesis. Key steps in retinogenesis include withdrawal from mitosis by multipotent neuroepithelial cells, specification to particular cell types, migration to the appropriate laminar positions, and molecular and morphological differentiation. In this study, we have identified two recessive mutations that affect the transition of proliferating neuroepithelial cells to postmitotic retinal cells. Both the perplexed and confused mutant phenotypes were initially detectable when the first retinal neuroepithelial cells began to leave the cell cycle. At this time, each mutant retina showed increased cell death and a lack of morphological differentiation. Cell death was found to be apoptotic in both perplexed and confused retinas based on TUNEL analysis and activation of caspase-3. TUNEL-phosphoRb-BrdU colocalization studies indicated that the perplexed mutation caused death in cells transitioning from a proliferative to postmitotic state. For the confused mutation, TUNEL-phosphoRb-BrdU analysis revealed that only a subset of postmitotic cells were induced to activate apoptosis. Mosaic analysis demonstrated that within the retina the perplexed mutation functions noncell-autonomously. Furthermore, whole lens or eye cup transplantations indicated that the retinal defect was intrinsic to the retina. Mosaic analysis with confused embryos showed this mutation acts cell-autonomously. From these studies, we conclude that the perplexed and confused genes are essential at distinct stages during the transition from proliferating to postmitotic cells within the zebrafish retina.

Regulating proliferation during retinal development

Recent studies have shown that components of the cell-cycle machinery can have diverse and unexpected roles in the retina. Cyclin-kinase inhibitors, for example, have been implicated as regulators of cell-fate decisions during histogenesis and reactive gliosis in the adult tissue after injury. Also, various mechanisms have been identified that can compensate for extra rounds of cell division when the normal timing of the cell-cycle exit is perturbed. Surprisingly, distinct components of the cell-cycle machinery seem to be used during different stages of development, and different organisms might rely on distinct pathways. Such detailed studies on the regulation of proliferation in complex multicellular tissues during development have not only advanced our knowledge of the ways in which proliferation is controlled, but might also help us to understand the degenerative disorders that are associated with gliosis and some types of tumorigenesis.

Role of QN1 protein in cell proliferation arrest and differentiation during the neuroretina development

In this report, we describe the involvement of the quail neuroretina 1 (QN1) protein in retinal development. The Qn1 cDNA was isolated as a gene specifically expressed at the onset of neuronal cell cycle withdrawal (Bidou et al., Mech. Dev. 43 (1993) 159). Qn1 is located in the cytoplasm in proliferating cells during the early stages of the development. Its distribution changes, becoming predominantly nuclear, in neurons during establishment of the quiescent state upon the differentiation. We decreased the amount of QN1 protein by an antisense strategy in vitro or in vivo. This decrease of the amount of QN1 protein results in additional mitosis and in severe abnormalities such as retinal dysplasia. Our results suggest that QN1 plays a key role at the onset of neuronal cell cycle withdrawal.

Vertebrate neural cell-fate determination: lessons from the retina

Postmitotic neurons are produced from a pool of cycling progenitors in an orderly fashion during development. Studies of cell-fate determination in the vertebrate retina have uncovered several fundamental principles by which this is achieved. Most notably, a model for vertebrate cell-fate determination has been proposed that combines findings on the relative roles of extrinsic and intrinsic regulators in controlling cell-fate choices. At the heart of the model is the proposal that progenitors pass through intrinsically determined competence states, during which they are capable of giving rise to a limited subset of cell types under the influence of extrinsic signals.

Coordinating cell proliferation and differentiation

Cell proliferation and differentiation are highly coordinated processes during development. Recent studies have revealed that this coordination may result from dual functions residing in the central regulators of proliferation, allowing them to also regulate differentiation. Studies have also shown that some terminally differentiated cells can be made to divide beyond their normal capacity.

Cell cycle and cell fate in the nervous system

Recently, a number of molecules originally thought to have a primary role in cell determination have been shown to affect the cell cycle at specific check points, while other molecules discovered for their roles in the cell cycle progression are known to affect the determination and differentiation of neurons. These discoveries have led to a more detailed investigation of the complex molecular machinery that co-ordinates proliferation and differentiation.