The disarrayed mutation results in cell cycle and neurogenesis defects during retinal development in zebrafish

Deficiency of copper responsive gene stmn4 induces retinal developmental defects

As part of the central nervous system (CNS), the retina senses light and also conducts and processes visual impulses. The damaged development of the retina not only causes visual damage, but also leads to epilepsy, dementia and other brain diseases. Recently, we have reported that copper (Cu) overload induces retinal developmental defects and down-regulates microtubule (MT) genes during zebrafish embryogenesis, but whether the down-regulation of microtubule genes mediates Cu stress induced retinal developmental defects is still unknown. In this study, we found that microtubule gene stmn4 exhibited obviously reduced expression in the retina of Cu overload embryos. Furthermore, stmn4 deficiency (stmn4-/-) resulted in retinal defects similar to those seen in Cu overload embryos, while overexpression of stmn4 effectively rescued retinal defects and cell apoptosis occurred in the Cu overload embryos and larvae. Meanwhile, stmn4 deficient embryos and larvae exhibited reduced mature retinal cells, the down-regulated expression of microtubules and cell cycle-related genes, and the mitotic cell cycle arrests of the retinal cells, which subsequently tended to apoptosis independent on p53. The results of this study demonstrate that Cu stress might lead to retinal developmental defects via down-regulating expression of microtubule gene stmn4, and stmn4 deficiency leads to impaired cell cycle and the accumulation of retinal progenitor cells (RPCs) and their subsequent apoptosis. The study provides a certain referee for copper overload in regulating the retinal development in fish.

Laminin β2 Chain Regulates Cell Cycle Dynamics in the Developing Retina

Vertebrate retinal development follows a highly stereotyped pattern, in which the retinal progenitor cells (RPCs) give rise to all retinal types in a conserved temporal sequence. Ensuring the proper control over RPC cell cycle exit and re-entry is, therefore, crucially important for the generation of properly functioning retina. In this study, we demonstrate that laminins, indispensible ECM components, at the retinal surface, regulate the mechanisms determining whether RPCs generate proliferative or post-mitotic progeny. In vivo deletion of laminin β2 in mice resulted in disturbing the RPC cell cycle dynamics, and premature cell cycle exit. Specifically, the RPC S-phase is shortened, with increased numbers of cells present in its late stages. This is followed by an accelerated G2-phase, leading to faster M-phase entry. Finally, the M-phase is extended, with RPCs dwelling longer in prophase. Addition of exogenous β2-containing laminins to laminin β2-deficient retinal explants restored the appropriate RPC cell cycle dynamics, as well as S and M-phase progression, leading to proper cell cycle re-entry. Moreover, we show that disruption of dystroglycan, a laminin receptor, phenocopies the laminin β2 deletion cell cycle phenotype. Together, our findings suggest that dystroglycan-mediated ECM signaling plays a critical role in regulating the RPC cell cycle dynamics, and the ensuing cell fate decisions.

Development and maintenance of vision's first synapse

Synapses in the outer retina are the first information relay points in vision. Here, photoreceptors form synapses onto two types of interneurons, bipolar cells and horizontal cells. Because outer retina synapses are particularly large and highly ordered, they have been a useful system for the discovery of mechanisms underlying synapse specificity and maintenance. Understanding these processes is critical to efforts aimed at restoring visual function through repairing or replacing neurons and promoting their connectivity. We review outer retina neuron synapse architecture, neural migration modes, and the cellular and molecular pathways that play key roles in the development and maintenance of these connections. We further discuss how these mechanisms may impact connectivity in the retina.

Choosing the right microscope to image mitosis in zebrafish embryos: A practical guide

Tissue growth and organismal development require orchestrated cell proliferation. To understand how cell division guides development, it is important to explore mitosis at the tissue-wide, cellular, and subcellular scale. At the tissue level this includes determining a tissue's mitotic index, at the cellular level the tracing of cell lineages, and at the subcellular level the characterization of intracellular components. These different tasks can be addressed by different imaging approaches (e.g., laser-scanning confocal, spinning disk confocal, and light-sheet fluorescence microscopy). Here, we summarize three protocols for exploring different facets of mitosis in developing zebrafish embryos. Zebrafish embryos are transparent and their rapid external development greatly facilitates the study of cellular processes and developmental dynamics using microscopy. A critical step in all imaging studies of mitosis in development is to choose the most suitable microscope for each scientific question. This choice is important in order to ensure a balance between the required temporal and spatial resolution and minimal phototoxicity that could otherwise perturb the process of interest. The use of different microscopy techniques, best suited for the purpose of each experiment, thus permits to generate a comprehensive and unbiased view on how mitosis influences development.

Live imaging of developing mouse retinal slices

BACKGROUND

Ex vivo, whole-mount explant culture of the rodent retina has proved to be a valuable approach for studying retinal development. In a limited number of recent studies, this method has been coupled to live fluorescent microscopy with the goal of directly observing dynamic cellular events. However, retinal tissue thickness imposes significant technical limitations. To obtain 3-dimensional images with high quality axial resolution, investigators are restricted to specific areas of the retina and require microscopes, such as 2-photon, with a higher level of depth penetrance. Here, we report a retinal live imaging method that is more amenable to a wider array of imaging systems and does not compromise resolution of retinal cross-sectional area.

RESULTS

Mouse retinal slice cultures were prepared and standard, inverted confocal microscopy was used to generate movies with high quality resolution of retinal cross-sections. To illustrate the ability of this method to capture discrete, physiologically relevant events during retinal development, we imaged the dynamics of the Fucci cell cycle reporter in both wild type and Cyclin D1 mutant retinal progenitor cells (RPCs) undergoing interkinetic nuclear migration (INM). Like previously reported for the zebrafish, mouse RPCs in G1 phase migrated stochastically and exhibited overall basal drift during development. In contrast, mouse RPCs in G2 phase displayed directed, apical migration toward the ventricular zone prior to mitosis. We also determined that Cyclin D1 knockout RPCs in G2 exhibited a slower apical velocity as compared to wild type. These data are consistent with previous IdU/BrdU window labeling experiments on Cyclin D1 knockout RPCs indicating an elongated cell cycle. Finally, to illustrate the ability to monitor retinal neuron differentiation, we imaged early postnatal horizontal cells (HCs). Time lapse movies uncovered specific HC neurite dynamics consistent with previously published data showing an instructive role for transient vertical neurites in HC mosaic formation.

CONCLUSIONS

We have detailed a straightforward method to image mouse retinal slice culture preparations that, due to its relative ease, extends live retinal imaging capabilities to a more diverse group of scientists. We have also shown that, by using a slice technique, we can achieve excellent lateral resolution, which is advantageous for capturing intracellular dynamics and overall cell movements during retinal development and differentiation.

Live imaging of developing mouse retinal slices

Background

Ex vivo, whole-mount explant culture of the rodent retina has proved to be a valuable approach for studying retinal development. In a limited number of recent studies, this method has been coupled to live fluorescent microscopy with the goal of directly observing dynamic cellular events. However, retinal tissue thickness imposes significant technical limitations. To obtain 3-dimensional images with high quality axial resolution, investigators are restricted to specific areas of the retina and require microscopes, such as 2-photon, with a higher level of depth penetrance. Here, we report a retinal live imaging method that is more amenable to a wider array of imaging systems and does not compromise resolution of retinal cross-sectional area.

Results

Mouse retinal slice cultures were prepared and standard, inverted confocal microscopy was used to generate movies with high quality resolution of retinal cross-sections. To illustrate the ability of this method to capture discrete, physiologically relevant events during retinal development, we imaged the dynamics of the Fucci cell cycle reporter in both wild type and Cyclin D1 mutant retinal progenitor cells (RPCs) undergoing interkinetic nuclear migration (INM). Like previously reported for the zebrafish, mouse RPCs in G1 phase migrated stochastically and exhibited overall basal drift during development. In contrast, mouse RPCs in G2 phase displayed directed, apical migration toward the ventricular zone prior to mitosis. We also determined that Cyclin D1 knockout RPCs in G2 exhibited a slower apical velocity as compared to wild type. These data are consistent with previous IdU/BrdU window labeling experiments on Cyclin D1 knockout RPCs indicating an elongated cell cycle. Finally, to illustrate the ability to monitor retinal neuron differentiation, we imaged early postnatal horizontal cells (HCs). Time lapse movies uncovered specific HC neurite dynamics consistent with previously published data showing an instructive role for transient vertical neurites in HC mosaic formation.

Conclusions

We have detailed a straightforward method to image mouse retinal slice culture preparations that, due to its relative ease, extends live retinal imaging capabilities to a more diverse group of scientists. We have also shown that, by using a slice technique, we can achieve excellent lateral resolution, which is advantageous for capturing intracellular dynamics and overall cell movements during retinal development and differentiation.

Electronic supplementary material

The online version of this article (10.1186/s13064-018-0120-y) contains supplementary material, which is available to authorized users.

Coordinating progenitor cell cycle exit and differentiation in the developing vertebrate retina

The proper development of the vertebrate retina relies heavily on producing the correct number and type of differentiated retinal cell types. To achieve this, proliferating retinal progenitor cells (RPCs) must exit the cell cycle at an appropriate time and correctly express a subset of differentiation markers that help specify retinal cell fate. Homeobox genes, which encode a family of transcription factors, have been accredited to both these processes, implicated in the transcriptional regulation of important cell cycle components, such as cyclins and cyclin-dependent kinases, and proneural genes. This dual regulation of homeobox genes allows these factors to help co-ordinate the transition from the proliferating RPC to postmitotic, differentiated cell. However, understanding the exact molecular targets of these factors remains a challenging task. This commentary highlights the current knowledge we have about how these factors regulate cell cycle progression and differentiation, with particular emphasis on a recent discovery from our lab demonstrating an antagonistic relationship between Vsx2 and Dmbx1 to control RPC proliferation. Future studies should aim to further understand the direct transcriptional targets of these genes, additional co-factors/interacting proteins and the possible recruitment of epigenetic machinery by these homeobox genes.

Midkine-A functions upstream of Id2a to regulate cell cycle kinetics in the developing vertebrate retina

Background

Midkine is a small heparin binding growth factor expressed in numerous tissues during development. The unique midkine gene in mammals has two paralogs in zebrafish: midkine-a (mdka) and midkine-b (mdkb). In the zebrafish retina, during both larval development and adult photoreceptor regeneration, mdka is expressed in retinal stem and progenitor cells and functions as a molecular component of the retina’s stem cell niche. In this study, loss-of-function and conditional overexpression were used to investigate the function of Mdka in the retina of the embryonic zebrafish.

Results

The results show that during early retinal development Mdka functions to regulate cell cycle kinetics. Following targeted knockdown of Mdka synthesis, retinal progenitors cycle more slowly, and this results in microphthalmia, a diminished rate of cell cycle exit and a temporal delay of cell cycle exit and neuronal differentiation. In contrast, Mdka overexpression results in acceleration of the cell cycle and retinal overgrowth. Mdka gain-of-function, however, does not temporally advance cell cycle exit. Experiments to identify a potential Mdka signaling pathway show that Mdka functions upstream of the HLH regulatory protein, Id2a. Gene expression analysis shows Mdka regulates id2a expression, and co-injection of Mdka morpholinos and id2a mRNA rescues the Mdka loss-of-function phenotype.

Conclusions

These data show that in zebrafish, Mdka resides in a shared Id2a pathway to regulate cell cycle kinetics in retinal progenitors. This is the first study to demonstrate the function of Midkine during retinal development and adds Midkine to the list of growth factors that transcriptionally regulate Id proteins.

Tbl3 regulates cell cycle length during zebrafish development

The regulation of cell cycle rate is essential for the correct timing of proliferation and differentiation during development. Changes to cell cycle rate can have profound effects on the size, shape and cell types of a developing organ. We previously identified a zebrafish mutant ceylon (cey) that has a severe reduction in T cells and hematopoietic stem/progenitor cells (HSPCs). Here we find that the cey phenotype is due to absence of the gene transducin (beta)-like 3 (tbl3). The tbl3 homolog in yeast regulates the cell cycle by maintaining rRNA levels and preventing p53-induced cell death. Zebrafish tbl3 is maternally expressed, but later in development its expression is restricted to specific tissues. Tissues expressing tbl3 are severely reduced in cey mutants, including HSPCs, the retina, exocrine pancreas, intestine, and jaw cartilage. Specification of these tissues is normal, suggesting the reduced size is due to a reduced number of differentiated cells. Tbl3 MO injection into either wild-type or p53-/- mutant embryos phenocopies cey, indicating that loss of tbl3 causes specific defects in cey. Progression of both hematopoietic and retinal development is delayed beginning at 3 day post fertilization due to a slowing of the cell cycle. In contrast to yeast, reduction of Tbl3 causes a slowing of the cell cycle without a corresponding increase in p53 induced cell death. These data suggest that tbl3 plays a tissue-specific role regulating cell cycle rate during development.

Analysis of cell proliferation, senescence, and cell death in zebrafish embryos

Proper control of cell proliferation is critical for normal development, growth, differentiation, and tissue homeostasis. Dysregulation of cell division and cell death underlies almost all cancers, and contributes to the pathology of birth defects and degenerative diseases. The zebrafish has proved to be an excellent system for elucidating the roles of the cell cycle in normal development, and ways in which dysregulation of cell proliferation contributes to disease. This chapter describes the methods for studying the cell cycle in zebrafish embryos, including protocols to examine cell proliferation, DNA damage, senescence, and cell death.

Id2a influences neuron and glia formation in the zebrafish retina by modulating retinoblast cell cycle kinetics

Inhibitor of differentiation (Id) family helix-loop-helix proteins regulate the proliferation, survival and differentiation of numerous cell types during development; however, their functions during retinal development have not been analyzed. Using loss-of-function and overexpression assays in zebrafish, we demonstrate that Id2a levels modulate retinoblast cell cycle kinetics and thereby influence neuron and glia formation in the retina. Id2a-deficient retinas possess increased numbers of cells occupying S phase, at the expense of mitotic cells, and kinetic analyses demonstrate that Id2a is required for S-phase progression and/or the transition from S to M phase. Id2a-dependent defects in retinoblast proliferation lead to microphthalmia and to an absence of nearly all differentiated inner and outer nuclear layer cell types. Overexpression of id2a has the opposite effect on retinoblast cell cycle kinetics: id2a-overexpressing retinoblasts progress from S to M phase more rapidly and they undergo mitosis more frequently, which results in macrophthalmia. Mosaic analyses reveal that Id2a function in facilitating both cell cycle progression and neuronal differentiation in the retina is non-cell-autonomous, suggesting that Id2a functions upstream of the extrinsic pathways that regulate retinogenesis.

Duplicate dmbx1 genes regulate progenitor cell cycle and differentiation during zebrafish midbrain and retinal development

Background

The Dmbx1 gene is important for the development of the midbrain and hindbrain, and mouse gene targeting experiments reveal that this gene is required for mediating postnatal and adult feeding behaviours. A single Dmbx1 gene exists in terrestrial vertebrate genomes, while teleost genomes have at least two paralogs. We compared the loss of function of the zebrafish dmbx1a and dmbx1b genes in order to gain insight into the molecular mechanism by which dmbx1 regulates neurogenesis, and to begin to understand why these duplicate genes have been retained in the zebrafish genome.

Results

Using gene knockdown experiments we examined the function of the dmbx1 gene paralogs in zebrafish, dmbx1a and dmbx1b in regulating neurogenesis in the developing retina and midbrain. Dose-dependent loss of dmbx1a and dmbx1b function causes a significant reduction in growth of the midbrain and retina that is evident between 48-72 hpf. We show that this phenotype is not due to patterning defects or persistent cell death, but rather a deficit in progenitor cell cycle exit and differentiation. Analyses of the morphant retina or anterior hindbrain indicate that paralogous function is partially diverged since loss of dmbx1a is more severe than loss of dmbx1b. Molecular evolutionary analyses of the Dmbx1 genes suggest that while this gene family is conservative in its evolution, there was a dramatic change in selective constraint after the duplication event that gave rise to the dmbx1a and dmbx1b gene families in teleost fish, suggestive of positive selection. Interestingly, in contrast to zebrafish dmbx1a, over expression of the mouse Dmbx1 gene does not functionally compensate for the zebrafish dmbx1a knockdown phenotype, while over expression of the dmbx1b gene only partially compensates for the dmbx1a knockdown phenotype.

Conclusion

Our data suggest that both zebrafish dmbx1a and dmbx1b genes are retained in the fish genome due to their requirement during midbrain and retinal neurogenesis, although their function is partially diverged. At the cellular level, Dmbx1 regulates cell cycle exit and differentiation of progenitor cells. The unexpected observation of putative post-duplication positive selection of teleost Dmbx1 genes, especially dmbx1a, and the differences in functionality between the mouse and zebrafish genes suggests that the teleost Dmbx1 genes may have evolved a diverged function in the regulation of neurogenesis.

Wnt5b-Ryk pathway provides directional signals to regulate gastrulation movement

The soluble ligand Wnt5b repels cells expressing the Ryk (related to tyrosine kinase) receptor, establishing directional motility during gastrulation.

Noncanonical Wnts are largely believed to act as permissive cues for vertebrate cell movement via Frizzled (Fz). In addition to Fz, Wnt ligands are known to regulate neurite outgrowth through an alternative receptor related to tyrosine kinase (Ryk). However, Wnt–Ryk signaling during embryogenesis is less well characterized. In this study, we report a role for Wnt5b as an instructive cue to regulate gastrulation movements through Ryk. In zebrafish, Ryk deficiency impairs Wnt5b-induced Ca²⁺ activity and directional cell movement. Wnt5b–Ryk signaling promotes polarized cell protrusions. Upon Wnt5b stimulation, Fz2 but not Ryk recruits Dishevelled to the cell membrane, suggesting that Fz2 and Ryk mediate separate pathways. Using co-culture assays to generate directional Wnt5b cues, we demonstrate that Ryk-expressing cells migrate away from the Wnt5b source. We conclude that full-length Ryk conveys Wnt5b signals in a directional manner during gastrulation.

Neurogenesis

For more than a decade, the zebrafish has proven to be an excellent model organism to investigate the mechanisms of neurogenesis during development. The often cited advantages, namely external development, genetic, and optical accessibility, have permitted direct examination and experimental manipulations of neurogenesis during development. Recent studies have begun to investigate adult neurogenesis, taking advantage of its widespread occurrence in the mature zebrafish brain to investigate the mechanisms underlying neural stem cell maintenance and recruitment. Here we provide a comprehensive overview of the tools and techniques available to study neurogenesis in zebrafish both during development and in adulthood. As useful resources, we provide tables of available molecular markers, transgenic, and mutant lines. We further provide optimized protocols for studying neurogenesis in the adult zebrafish brain, including in situ hybridization, immunohistochemistry, in vivo lipofection and electroporation methods to deliver expression constructs, administration of bromodeoxyuridine (BrdU), and finally slice cultures. These currently available tools have put zebrafish on par with other model organisms used to investigate neurogenesis.

Cyclin D1 fine-tunes the neurogenic output of embryonic retinal progenitor cells

BACKGROUND

Maintaining the correct balance of proliferation versus differentiation in retinal progenitor cells (RPCs) is essential for proper development of the retina. The cell cycle regulator cyclin D1 is expressed in RPCs, and mice with a targeted null allele at the cyclin D1 locus (Ccnd1-/-) have microphthalmia and hypocellular retinas, the latter phenotype attributed to reduced RPC proliferation and increased photoreceptor cell death during the postnatal period. How cyclin D1 influences RPC behavior, especially during the embryonic period, is unclear.

RESULTS

In this study, we show that embryonic RPCs lacking cyclin D1 progress through the cell cycle at a slower rate and exit the cell cycle at a faster rate. Consistent with enhanced cell cycle exit, the relative proportions of cell types born in the embryonic period, such as retinal ganglion cells and photoreceptor cells, are increased. Unexpectedly, cyclin D1 deficiency decreases the proportions of other early born retinal neurons, namely horizontal cells and specific amacrine cell types. We also found that the laminar positioning of horizontal cells and other cell types is altered in the absence of cyclin D1. Genetically replacing cyclin D1 with cyclin D2 is not efficient at correcting the phenotypes due to the cyclin D1 deficiency, which suggests the D-cyclins are not fully redundant. Replacement with cyclin E or inactivation of cyclin-dependent kinase inhibitor p27Kip1 restores the balance of RPCs and retinal cell types to more normal distributions, which suggests that regulation of the retinoblastoma pathway is an important function for cyclin D1 during embryonic retinal development.

CONCLUSION

Our findings show that cyclin D1 has important roles in RPC cell cycle regulation and retinal histogenesis. The reduction in the RPC population due to a longer cell cycle time and to an enhanced rate of cell cycle exit are likely to be the primary factors driving retinal hypocellularity and altered output of precursor populations in the embryonic Ccnd1-/- retina.

Regulation of neurogenesis by interkinetic nuclear migration through an apical-basal notch gradient

The different cell types in the central nervous system develop from a common pool of progenitor cells. The nuclei of progenitors move between the apical and basal surfaces of the neuroepithelium in phase with their cell cycle, a process termed interkinetic nuclear migration (INM). In the retina of zebrafish mikre oko (mok) mutants, in which the motor protein Dynactin-1 is disrupted, interkinetic nuclei migrate more rapidly and deeply to the basal side and more slowly to the apical side. We found that Notch signaling is predominantly activated on the apical side in both mutants and wild-type. Mutant progenitors are, thus, less exposed to Notch and exit the cell cycle prematurely. This leads to an overproduction of early-born retinal ganglion cells (RGCs) at the expense of later-born interneurons and glia. Our data indicate that the function of INM is to balance the exposure of progenitor nuclei to neurogenic versus proliferative signals.

The Tg(ccnb1:EGFP) transgenic zebrafish line labels proliferating cells during retinal development and regeneration

PURPOSE

To create the Tg(ccnb1:EGFP)(nt18) zebrafish line that spatially and temporally labels retinal progenitor cells with enhanced green fluorescent protein (EGFP) during zebrafish retinal development and regeneration.

METHODS

We cloned the 1.5 kb promoter region of the zebrafish cyclin B1 (ccnb1) gene upstream of the EGFP gene in the Tol2 vector, which was used to generate the stable Tg(ccnb1:EGFP)(nt18) transgenic zebrafish line. Immunohistochemistry and in situ hybridization techniques verified that the ccnb1:EGFP transgene was expressed in retinal progenitor cells during retinal development, in the undamaged adult retina, and in the regenerating adult retina.

RESULTS

At 36 h post-fertilization, both the enhanced green fluorescent protein (EGFP) and proliferating cell nuclear antigen (PCNA) expressions were observed throughout the developing transgenic retina, but they became restricted to the circumferential marginal zone by five days post-fertilization. In situ hybridization confirmed that this EGFP expression matched the cyclin B1 mRNA expression pattern. In comparison to the Tg(1016a1tubulin:EGFP) transgenic line that expresses EGFP in neuronal progenitor cells, the Tg(ccnb1:EGFP)(nt18) line more faithfully follows the rise and fall of PCNA expression through the developing retina and brain. In the adult retina, there are three cell types that continue to proliferate, the Müller glia in the inner nuclear layer, the rod precursor cells in the outer nuclear layer, and the stem cells in the circumferential marginal zone. In the Tg(ccnb1:EGFP)(nt18) retina, EGFP coexpressed with PCNA in all three of these proliferating cell types. Exposing the adult retina to constant intense light destroys the rod and cone photoreceptors and induces an increase in the number of proliferating Müller glia, which produces actively dividing neuronal progenitor cells that migrate to the outer nuclear layer (ONL) and replenish the lost photoreceptors. Following constant light damage, Tg(ccnb1:EGFP)(nt18) zebrafish expressed EGFP in both the proliferating Müller glia and the migrating neuronal progenitor cells.

CONCLUSIONS

The spatial and temporal patterning of EGFP expression in the Tg(ccnb1:EGFP)(nt18) line directly reflects the known locations of proliferating cells in the zebrafish retina, making it a useful marker to study the transient nature of neuronal progenitor cells during the development and regeneration of the zebrafish retina.

Nuclear migration during retinal development

In this review we focus on the mechanisms, regulation, and cellular consequences of nuclear migration in the developing retina. In the nervous system, nuclear migration is prominent during both proliferative and post-mitotic phases of development. Interkinetic nuclear migration is the process where the nucleus oscillates from the apical to basal surfaces in proliferative neuroepithelia. Proliferative nuclear movement occurs in step with the cell cycle, with M-phase being confined to the apical surface and G1-, S-, and G2-phases occurring at more basal locations. Later, following cell cycle exit, some neuron precursors migrate by nuclear translocation. In this mode of cellular migration, nuclear movement is the driving force for motility. Following discussion of the key components and important regulators for each of these processes, we present an emerging model where interkinetic nuclear migration functions to distinguish cell fates among retinal neuroepithelia.