Generating neuronal diversity in the retina: one for nearly all

Roles of the bHLH gene Hes1 in retinal morphogenesis

During retinal development, common precursors give rise to various types of cells in a time course specific to each cell type. Previously, we demonstrated that the bHLH gene Hes1 inhibits neuronal differentiation whereas, in Hes1-null retina, precursors prematurely differentiate into neurons and form abnormal rosette-like structures. Thus, Hes1 is essential for maintenance of precursors and morphogenesis of the neural retina. However, the precise causal link between premature differentiation and abnormal structures remains to be determined. Here, we found that misexpression of Hes1 in the developing retina promotes formation of undifferentiated precursor-like cells, whereas in Hes1-null retina, precursors are not properly maintained and prematurely differentiate into ganglion cells. Strikingly, those prematurely differentiated ganglion cells erupt into the subretinal space through the regions where precursors and the outer limiting membrane are lost. These results indicate that Hes1 maintains precursors and the outer limiting membrane and thereby regulates retinal morphogenesis.

Role of the Barhl2 homeobox gene in the specification of glycinergic amacrine cells

The mammalian retina contains numerous morphological and physiological subtypes of amacrine cells necessary for integrating and modulating visual signals presented to the output neurons. Among subtypes of amacrine cells grouped by neurotransmitter phenotypes, the glycinergic and gamma-aminobutyric acid (GABA)ergic amacrine cells constitute two major subpopulations. To date, the molecular mechanisms governing the specification of subtype identity of amacrine cells remain elusive. We report here that during mouse development, the Barhl2 homeobox gene displays an expression pattern in the nervous system that is distinct from that of its homologue Barhl1. In the developing retina, Barhl2 expression is found in postmitotic amacrine, horizontal and ganglion cells, while Barhl1 expression is absent. Forced expression of Barhl2 in retinal progenitors promotes the differentiation of glycinergic amacrine cells, whereas a dominant-negative form of Barhl2 has the opposite effect. By contrast, they exert no effect on the formation of GABAergic neurons. Moreover, misexpressed Barhl2 inhibits the formation of bipolar and Müller glial cells, indicating that Barhl2 is able to function both as a positive and negative regulator, depending on different types of cells. Taken together, our data suggest that Barhl2 may function to specify the identity of glycinergic amacrine cells from competent progenitors during retinogenesis.

Evolution of photosensory pineal organs in new light: the fate of neuroendocrine photoreceptors

Pineal evolution is envisaged as a gradual transformation of pinealocytes (a gradual regression of pinealocyte sensory capacity within a particular cell line), the so-called sensory cell line of the pineal organ. In most non-mammals the pineal organ is a directly photosensory organ, while the pineal organ of mammals (epiphysis cerebri) is a non-sensory neuroendocrine organ under photoperiod control. The phylogenetic transformation of the pineal organ is reflected in the morphology and physiology of the main parenchymal cell type, the pinealocyte. In anamniotes, pinealocytes with retinal cone photoreceptor-like characteristics predominate, whereas in sauropsids so-called rudimentary photoreceptors predominate. These have well-developed secretory characteristics, and have been interpreted as intermediaries between the anamniote pineal photoreceptors and the mammalian non-sensory pinealocytes. We have re-examined the original studies on which the gradual transformation hypothesis of pineal evolution is based, and found that the evidence for this model of pineal evolution is ambiguous. In the light of recent advances in the understanding of neural development mechanisms, we propose a new hypothesis of pineal evolution, in which the old notion 'gradual regression within the sensory cell line' should be replaced with 'changes in fate restriction within the neural lineage of the pineal field'.

Cell diversity in the retina: more than meets the eye

Over 10 years ago, Pax-6 was shown to play an evolutionarily conserved role in controlling eye formation from Drosophila to humans.1 Since then, the identification of an entire cascade of conserved eye determination genes has brought a new understanding to the developmental relationship between the insect compound eye and the vertebrate camera eye.2 Additional studies are now beginning to suggest that even late aspects of eye development, including cell type specification, also share common molecular machinery. In this commentary, I will discuss some of these findings, with a particular focus on the recent study by Dyer et al.3 describing a novel role for the Prox1 transcription factor in specifying horizontal cells in the mouse retina. As Prospero, the Drosophila homolog of Prox1, also participates in retinal cell specification, these data provide a forum for asking new questions concerning pathways that may regulate retinogenesis across evolution.

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.

RaxL regulates chick ganglion cell development

RaxL is a paired-like homeobox gene involved in vertebrate eye morphogenesis. We examined RaxL protein expression patterns during chick retinal development in combination with ganglion cell markers including the RA4 antigen, cBrn-3, Islet-1 and neuronal type III beta-tubulin. Double-immunostaining demonstrated that downregulation of RaxL protein correlates with upregulation of ganglion cell markers in the ganglion cell layer (GCL). To explore this correlation in vivo, we performed gain- and loss-of-function experiments by electroporating retroviral vectors encoding wild-type and dominant-negative-RaxL into the optic vesicles of stage 10 chick embryos. Infection with virus expressing RaxL led to a 35% decrease in Islet-1-positive ganglion cells at E5.0 and a complete loss of ganglion cells at E15, with no effect on displaced amacrine cells in the GCL. When dominant-negative RaxL was expressed, the total number of cells in the GCL increased by approximately 40% at E5.0 but was reduced to 40% at E15, due to ectopic apoptosis in the GCL from E9 to E15. These results suggest that RaxL gives an inhibitory effect on ganglion cell development and that the loss of RaxL expression is required for maintenance of ganglion cells.

Dlx1, Dlx2, Pax6, Brn3b, and Chx10 homeobox gene expression defines the retinal ganglion and inner nuclear layers of the developing and adult mouse retina

Distal-less homeobox genes are expressed in the developing forebrain. We assessed Dlx gene expression in the developing and adult mouse retina. Dlx1 and Dlx2 are detected in retinal neuroprogenitors by embryonic day (E) 12.5 (Eisenstat et al. [1999] J. Comp. Neurol. 217-237). At E13.5, the expression of four homeodomain proteins, DLX2, BRN3b, PAX6, and CHX10, define distinct yet overlapping domains in the retinal neuroepithelium. By postnatal day (P) 0, DLX2 is expressed in the neuroblastic layer and the ganglion cell layer (GCL) consisting of ganglion and displaced amacrine cells. DLX1 expression resembles DLX2 to P0 but decreases postnatally. In the adult, DLX2 is localized to ganglion, amacrine, and horizontal cells as determined by coexpression with retinal cell-specific markers. There is coincident expression of DLX2 with gamma-aminobutyric acid (GABA), glutamic acid decarboxylase (GAD)65, and GAD67 in the inner nuclear layer (INL) and GCL. In the adult, DLX2 is coexpressed with BRN3b in ganglion cells; PAX6 in amacrine, horizontal, and ganglion cells; and Chx10 in some bipolar cells. We predict that a combinatorial code of these homeobox genes and others specify retinal cell fate. Our results support a possible role for Dlx1 and Dlx2 in inner retinal development and in the terminal differentiation and/or maintenance of INL interneurons and ganglion cells in the adult. The correlation of DLX2 with GABA expression in the mouse retina closely mirrors the relationship of DLX2 to GABAergic neuronal differentiation in the embryonic forebrain, including neocortex, olfactory bulb and hippocampus, signifying a conservation of function of Dlx genes in the developing central nervous system.

N-cadherin mediates retinal lamination, maintenance of forebrain compartments and patterning of retinal neurites

The complex, yet highly ordered and predictable, structure of the neural retina is one of the most conserved features of the vertebrate central nervous system. In all vertebrate classes, retinal neurons are organized into laminae with each neuronal class adopting specific morphologies and patterns of connectivity. Using genetic analyses in zebrafish, we demonstrate that N-cadherin (Ncad) has several distinct and crucial functions during the establishment of retinal organization. Although the location of cell division is disorganized in embryos with reduced or no Ncad function, different classes of retinal neurons are generated. However, these neurons fail to organize into correct laminae, most probably owing to compromised adhesion between retinal cells. In addition, amacrine cells exhibit exuberant and misdirected outgrowth of neurites that contributes to severe disorganization of the inner plexiform layer. Retinal ganglion cells also exhibit defects in process outgrowth, with axons exhibiting fasciculation defects and adopting incorrect ipsilateral trajectories. At least some of these defects are likely to be due to a failure to maintain compartment boundaries between eye, optic nerve and brain. Although in vitro studies have implicated Fgf receptors in modulating the axon outgrowth promoting properties of Ncad, most aspects of the Ncad mutant phenotype are not phenocopied by treatments that block Fgf receptor function.

Ash1a and Neurogenin1 function downstream of Floating head to regulate epiphysial neurogenesis

The homeodomain transcription factor Floating head (Flh) is required for the generation of neurones in the zebrafish epiphysis. It regulates expression of two basic helix loop helix (bHLH) transcription factor encoding genes, ash1a (achaete/scute homologue 1a) and neurogenin1 (ngn1), in epiphysial neural progenitors. We show that ash1a and ngn1 function in parallel redundant pathways to regulate neurogenesis downstream of flh. Comparison of the epiphysial phenotypes of flh mutant and of ash1a/ngn1 double morphants reveals that reduced expression of ash1a and ngn1 can account for most of the neurogenesis defects in the flh-mutant epiphysis but also shows that Flh has additional activities. Furthermore, different cell populations show different requirements for ash1a and ngn1 within the epiphysis. These populations do not simply correspond to the two described epiphysial cell types: photoreceptors and projection neurones. These results suggest that the genetic pathways that involve ash1a and ngn1 are common to both neuronal types.

The presynaptic active zone protein bassoon is essential for photoreceptor ribbon synapse formation in the retina

The photoreceptor ribbon synapse is a highly specialized glutamatergic synapse designed for the continuous flow of synaptic vesicles to the neurotransmitter release site. The molecular mechanisms underlying ribbon synapse formation are poorly understood. We have investigated the role of the presynaptic cytomatrix protein Bassoon, a major component of the photoreceptor ribbon, in a mouse retina deficient of functional Bassoon protein. Photoreceptor ribbons lacking Bassoon are not anchored to the presynaptic active zones. This results in an impaired photoreceptor synaptic transmission, an abnormal dendritic branching of neurons postsynaptic to photoreceptors, and the formation of ectopic synapses. These findings suggest a critical role of Bassoon in the formation and the function of photoreceptor ribbon synapses of the mammalian retina.

A dorso-ventral gradient of Pax6 in the developing retina suggests a role in topographic map formation

Expression of the transcription factor Pax6 was assessed immunohistochemically in embryonic chick retina during retino-tectal map formation. A low dorsal to high ventral gradient was found that correlated with expression of the axonal guidance cue EphB2. Furthermore, transfection of Pax6 into undifferentiated P19 cells up-regulated EphB2. The results raise the possibility that Pax6 is upstream of EphB2 and that its graded expression defines the dorso-ventral axis of the retino-tectal projection.

Regulation of axial patterning of the retina and its topographic mapping in the brain

Topographic maps are a fundamental organizational feature of axonal connections in the brain. A prominent model for studying axial polarity and topographic map development is the vertebrate retina and its projection to the optic tectum (or superior colliculus). Linked processes are controlled by molecules that are graded along the axes of the retina and its target fields. Recent studies indicate that ephrin-As control the temporal-nasal mapping of the retina in the optic tectum/superior colliculus by regulating the topographically-specific interstitial branching of retinal axons along the anterior-posterior tectal axis. This branching is mediated by relative levels of EphA receptor repellent signaling. A major recent advance is the demonstration that EphB receptor forward signaling and ephrin-B reverse signaling mediate axon attraction to control dorsal-ventral retinal mapping along the lateral-medial tectal axis. In addition, several classes of regulatory proteins have been implicated in the control of the axial patterning of the retina, and its ultimate readout of topographic mapping.

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.

Putting neurons in the right places: local interactions in the genesis of retinal architecture

Development of the nervous system can be schematically summarized as (1) making the necessary cells, (2) putting these cells in the right places, and then (3) connecting them appropriately. Each of these steps represents an enormous challenge to our understanding. Focusing on the vertebrate retina, I will consider the question of what defines the right place for a neuron to go. I will illustrate data pointing to the prominent role played by short-range cellular interactions, possibly coordinated by global factors, and will discuss how a few sets of local rules could control cell positioning and proper wiring in retinal circuits.

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.

Pax6 is required for establishing naso-temporal and dorsal characteristics of the optic vesicle

The establishment of polarity is an important step during organ development. We assign a function for the paired and homeodomain transcription factor Pax6 in axis formation in the retina. Pax6 is a key factor of the highly conserved genetic network implicated in directing the initial phases of eye development. We recently demonstrated that Pax6 is also essential for later aspects of eye development, such as lens formation and retinogenesis. In this study, we present evidence that a highly conserved intronic enhancer, α, in the Pax6 gene is essential for the establishment of a distalhigh-proximallow gradient of Pax6 activity in the retina. In the mature retina, the activity mediated by the α-enhancer defines a population of retinal ganglion cells that project to two sickle-shaped domains in the superior colliculus and lateral geniculate nucleus. Deletion of the α-enhancer in vivo revealed that retinal Pax6 expression is regulated in two complementary topographic domains. We found that Pax6 activity is required for the establishment, as well as the maintenance of dorsal and nasotemporal characteristics in the optic vesicle and, later, the optic cup.

Embryonic origin of the eyes in teleost fish

The developmental history of the vertebrate eye begins at an early embryonic stage, with the formation of the body axes and induction of neural tissue. Several recent experimental embryological and genetic studies in teleost fish have produced new insights into the morphogenetic and molecular regulation of eye formation. Molecular signaling pathways and patterned expression of transcription factors implicated in eye determination are discussed, and the importance of morphogenetic cell movements is emphasized.