Anisotropic Müller glial scaffolding supports a multiplex lattice mosaic of photoreceptors in zebrafish retina.

BACKGROUND: The multiplex, lattice mosaic of cone photoreceptors in the adult fish retina is a compelling example of a highly ordered epithelial cell pattern, with single cell width rows and columns of cones and precisely defined neighbor relationships among different cone types. Cellular mechanisms patterning this multiplex mosaic are not understood. Physical models can provide new insights into fundamental mechanisms of biological patterning. In earlier work, we developed a mathematical model of photoreceptor cell packing in the zebrafish retina, which predicted that anisotropic mechanical tension in the retinal epithelium orients planar polarized adhesive interfaces to align the columns as cone photoreceptors are generated at the retinal margin during post-embryonic growth. METHODS: With cell-specific fluorescent reporters and in vivo imaging of the growing retinal margin in transparent juvenile zebrafish we provide the first view of how cell packing, spatial arrangement, and cell identity are coordinated to build the lattice mosaic. With targeted laser ablation we probed the tissue mechanics of the retinal epithelium. RESULTS: Within the lattice mosaic, planar polarized Crumbs adhesion proteins pack cones into a single cell width column; between columns, N-cadherin-mediated adherens junctions stabilize Müller glial apical processes. The concentration of activated pMyosin II at these punctate adherens junctions suggests that these glial bands are under tension, forming a physical barrier between cone columns and contributing to mechanical stress anisotropies in the epithelial sheet. Unexpectedly, we discovered that the appearance of such parallel bands of Müller glial apical processes precedes the packing of cones into single cell width columns, hinting at a possible role for glia in the initial organization of the lattice mosaic. Targeted laser ablation of Müller glia directly demonstrates that these glial processes support anisotropic mechanical tension in the planar dimension of the retinal epithelium. CONCLUSIONS: These findings uncovered a novel structural feature of Müller glia associated with alignment of photoreceptors into a lattice mosaic in the zebrafish retina. This is the first demonstration, to our knowledge, of planar, anisotropic mechanical forces mediated by glial cells.

Patterning the cone mosaic array in zebrafish retina requires specification of ultraviolet-sensitive cones.

Cone photoreceptors in teleost fish are organized in precise, crystalline arrays in the epithelial plane of the retina. In zebrafish, four distinct morphological/spectral cone types occupy specific, invariant positions within a regular lattice. The cone lattice is aligned orthogonal and parallel to circumference of the retinal hemisphere: it emerges as cones generated in a germinal zone at the retinal periphery are incorporated as single-cell columns into the cone lattice. Genetic disruption of the transcription factor Tbx2b eliminates most of the cone subtype maximally sensitive to ultraviolet (UV) wavelengths and also perturbs the long-range organization of the cone lattice. In the tbx2b mutant, the other three cone types (red, green, and blue cones) are specified in the correct proportion, differentiate normally, and acquire normal, planar polarized adhesive interactions mediated by Crumbs 2a and Crumbs 2b. Quantitative image analysis of cell adjacency revealed that the cones in the tbx2b mutant primarily have two nearest neighbors and align in single-cell-wide column fragments that are separated by rod photoreceptors. Some UV cones differentiate at the dorsal retinal margin in the tbx2b mutant, although they are severely dysmorphic and are eventually eliminated. Incorporating loss of UV cones during formation of cone columns at the margin into our previously published mathematical model of zebrafish cone mosaic formation (which uses bidirectional interactions between planar cell polarity proteins and anisotropic mechanical stresses in the plane of the retinal epithelium to generate regular columns of cones parallel to the margin) reproduces many features of the pattern disruptions seen in the tbx2b mutant.

A self-renewing division of zebrafish Müller glial cells generates neuronal progenitors that require N-cadherin to regenerate retinal neurons.

Müller glia function as retinal stem cells in adult zebrafish. In response to loss of retinal neurons, Müller glia partially dedifferentiate, re-express neuroepithelial markers and re-enter the cell cycle. We show that the immunoglobulin superfamily adhesion molecule Alcama is a novel marker of multipotent retinal stem cells, including injury-induced Müller glia, and that each Müller glial cell divides asymmetrically only once to produce an Alcama-negative, proliferating retinal progenitor. The initial mitotic division of Müller glia involves interkinetic nuclear migration, but mitosis of retinal progenitors occurs in situ. Rapidly dividing retinal progenitors form neurogenic clusters tightly associated with Alcama/N-cadherin-labeled Müller glial radial processes. Genetic suppression of N-cadherin function interferes with basal migration of retinal progenitors and subsequent regeneration of HuC/D(+) inner retinal neurons.

Coupling mechanical deformations and planar cell polarity to create regular patterns in the zebrafish retina.

The orderly packing and precise arrangement of epithelial cells is essential to the functioning of many tissues, and refinement of this packing during development is a central theme in animal morphogenesis. The mechanisms that determine epithelial cell shape and position, however, remain incompletely understood. Here, we investigate these mechanisms in a striking example of planar order in a vertebrate epithelium: The periodic, almost crystalline distribution of cone photoreceptors in the adult teleost fish retina. Based on observations of the emergence of photoreceptor packing near the retinal margin, we propose a mathematical model in which ordered columns of cells form as a result of coupling between planar cell polarity (PCP) and anisotropic tissue-scale mechanical stresses. This model recapitulates many observed features of cone photoreceptor organization during retinal growth and regeneration. Consistent with the model's predictions, we report a planar-polarized distribution of Crumbs2a protein in cone photoreceptors in both unperturbed and regenerated tissue. We further show that the pattern perturbations predicted by the model to occur if the imposed stresses become isotropic closely resemble defects in the cone pattern in zebrafish lrp2 mutants, in which intraocular pressure is increased, resulting in altered mechanical stress and ocular enlargement. Evidence of interactions linking PCP, cell shape, and mechanical stresses has recently emerged in a number of systems, several of which show signs of columnar cell packing akin to that described here. Our results may hence have broader relevance for the organization of cells in epithelia. Whereas earlier models have allowed only for unidirectional influences between PCP and cell mechanics, the simple, phenomenological framework that we introduce here can encompass a broad range of bidirectional feedback interactions among planar polarity, shape, and stresses; our model thus represents a conceptual framework that can address many questions of importance to morphogenesis.

Ontogeny of cone photoreceptor mosaics in zebrafish.

Cone photoreceptors in fish are typically arranged into a precise, reiterated pattern known as a "cone mosaic." Cone mosaic patterns can vary in different fish species and in response to changes in habitat, yet their function and the mechanisms of their development remain speculative. Zebrafish (Danio rerio) have four cone subtypes arranged into precise rows in the adult retina. Here we describe larval zebrafish cone patterns and investigate a previously unrecognized transition between larval and adult cone mosaic patterns. Cone positions were determined in transgenic zebrafish expressing green fluorescent protein (GFP) in their UV-sensitive cones, by the use of multiplex in situ hybridization labelling of various cone opsins. We developed a "mosaic metric" statistical tool to measure local cone order. We found that ratios of the various cone subtypes in larval and adult zebrafish were statistically different. The cone photoreceptors in larvae form a regular heterotypic mosaic array; i.e., the position of any one cone spectral subtype relative to the other cone subtypes is statistically different from random. However, the cone spectral subtypes in larval zebrafish are not arranged in continuous rows as in the adult. We used cell birth dating to show that the larval cone mosaic pattern remains as a distinct region within the adult retina and does not reorganize into the adult row pattern. In addition, the abundance of cone subtypes relative to other subtypes is different in this larval remnant compared with that of larvae or canonical adult zebrafish retina. These observations provide baseline data for understanding the development of cone mosaics via comparative analysis of larval and adult cone development in a model species.

Genetic evidence for shared mechanisms of epimorphic regeneration in zebrafish.

In a microarray-based gene profiling analysis of Müller glia-derived retinal stem cells in light-damaged retinas from adult zebrafish, we found that 2 genes required for regeneration of fin and heart tissues in zebrafish, hspd1 (heat shock 60-kDa protein 1) and mps1 (monopolar spindle 1), were up-regulated. Expression of both genes in the neurogenic Müller glia and progenitors was independently verified by quantitative reverse transcriptase PCR and in situ hybridization. Functional analysis of temperature-sensitive mutants of hspd1 and mps1 revealed that both are necessary for Müller glia-based cone photoreceptor regeneration in adult zebrafish retina. In the amputated fin, hspd1 is required for the induction of mesenchymal stem cells and blastema formation, whereas mps1 is required at a later step for rapid cell proliferation and outgrowth. This temporal sequence of hspd1 and mps1 function is conserved in the regenerating retina. Comparison of gene expression profiles from regenerating zebrafish retina, caudal fin, and heart muscle revealed additional candidate genes potentially implicated in injury-induced epimorphic regeneration in diverse zebrafish tissues.

pak2a mutations cause cerebral hemorrhage in redhead zebrafish.

The zebrafish is a powerful model for studying vascular development, demonstrating remarkable conservation of this process with mammals. Here, we identify a zebrafish mutant, redhead (rhd(mi149)), that exhibits embryonic CNS hemorrhage with intact gross development of the vasculature and normal hemostatic function. We show that the rhd phenotype is caused by a hypomorphic mutation in p21-activated kinase 2a (pak2a). PAK2 is a kinase that acts downstream of the Rho-family GTPases CDC42 and RAC and has been implicated in angiogenesis, regulation of cytoskeletal structure, and endothelial cell migration and contractility among other functions. Correction of the Pak2a-deficient phenotype by Pak2a overexpression depends on kinase activity, implicating Pak2 signaling in the maintenance of vascular integrity. Rescue by an endothelial-specific transgene further suggests that the hemorrhage seen in Pak2a deficiency is the result of an autonomous endothelial cell defect. Reduced expression of another PAK2 ortholog, pak2b, in Pak2a-deficient embryos results in a more severe hemorrhagic phenotype, consistent with partially overlapping functions for these two orthologs. These data provide in vivo evidence for a critical function of Pak2 in vascular integrity and demonstrate a severe disease phenotype resulting from loss of Pak2 function.

Late-stage neuronal progenitors in the retina are radial Müller glia that function as retinal stem cells.

Neuronal progenitors in the mammalian brain derive from radial glia or specialized astrocytes. In developing neural retina, radial glia-like Müller cells are generated late in neurogenesis and are not considered to be neuronal progenitors, but they do proliferate after injury and can express neuronal markers, suggesting a latent neurogenic capacity. To examine the neurogenic capacity of retinal glial cells, we used lineage tracing in transgenic zebrafish with a glial-specific promoter (gfap, for glial fibrillary acid protein) driving green fluorescent protein in differentiated Müller glia. We found that all Müller glia in the zebrafish retina express low levels of the multipotent progenitor marker Pax6 (paired box gene 6), and they proliferate at a low frequency in the intact, uninjured retina. Müller glia-derived progenitors express Crx (cone rod homeobox) and are late retinal progenitors that generate the rod photoreceptor lineage in the postembryonic retina. These Müller glia-derived progenitors also remain competent to produce earlier neuronal lineages, in that they respond to loss of cone photoreceptors by specifically regenerating the missing neurons. We conclude that zebrafish Müller glia function as multipotent retinal stem cells that generate retinal neurons by homeostatic and regenerative developmental mechanisms.

Molecular characterization of retinal stem cells and their niches in adult zebrafish.

BACKGROUND: The persistence in adult teleost fish of retinal stem cells that exhibit all of the features of true 'adult stem cells'--self-renewal, multipotency, and the capacity to respond to injury by mitotic activation with the ability to regenerate differentiated tissues--has been known for several decades. However, the specialized cellular and molecular characteristics of these adult retinal stem cells and the microenvironmental niches that support their maintenance in the differentiated retina and regulate their activity during growth and regeneration have not yet been elucidated. RESULTS: Our data show that the zebrafish retina has two kinds of specialized niches that sustain retinal stem cells: 1) a neuroepithelial germinal zone at the interface between neural retina and ciliary epithelium, called the ciliary marginal zone (CMZ), a continuous annulus around the retinal circumference, and 2) the microenvironment around some Müller glia in the differentiated retina. In the uninjured retina, scattered Müller glia (more frequently those in peripheral retina) are associated with clusters of proliferating retinal progenitors that are restricted to the rod photoreceptor lineage, but following injury, the Müller-associated retinal progenitors can function as multipotent retinal stem cells to regenerate other types of retinal neurons. The CMZ has several features in common with the neurogenic niches in the adult mammalian brain, including access to the apical epithelial surface and a close association with blood vessels. Müller glia in the teleost retina have a complex response to local injury that includes some features of reactive gliosis (up-regulation of glial fibrillary acidic protein, GFAP, and re-entry into the cell cycle) together with dedifferentiation and re-acquisition of phenotypic and molecular characteristics of multipotent retinal progenitors in the CMZ (diffuse distribution of N-cadherin, activation of Notch-Delta signaling, and expression of rx1, vsx2/Chx10, and pax6a) along with characteristics associated with radial glia (expression of brain lipid binding protein, BLBP). We also describe a novel specific marker for Müller glia, apoE. CONCLUSION: The stem cell niches that support multi-lineage retinal progenitors in the intact, growing and regenerating teleost retina have properties characteristic of neuroepithelia and neurogenic radial glia. The regenerative capacity of the adult zebrafish retina with its ability to replace lost retinal neurons provides an opportunity to discover the molecular regulators that lead to functional repair of damaged neural tissue.

A moving wave patterns the cone photoreceptor mosaic array in the zebrafish retina.

In this paper, we describe the embryonic origin and patterning of the planar mosaic array of cone photoreceptor spectral subtypes in the zebrafish retina. A discussion of possible molecular mechanisms that might generate the cone mosaic array considers but discards a model that accounts for formation of neuronal mosaics in the inner retina and discusses limitations of mathematical simulations that reproduce the zebrafish cone mosaic pattern. The formation and organization of photoreceptors in the ommatidia of the compound eye of Drosophila is compared with similar features in the developing zebrafish cone mosaic, and a model is proposed that invokes spatiotemporally coordinated cell-cell interactions among cone progenitors to determine the identity and positioning of cone spectral subtypes.