Genetic rescue of cell number in a mouse model of microphthalmia: interactions between Chx10 and G1-phase cell cycle regulators

Dlx1 and Dlx2 function is necessary for terminal differentiation and survival of late-born retinal ganglion cells in the developing mouse retina

Dlx homeobox genes, the vertebrate homologs of Distal-less, play important roles in the development of the vertebrate forebrain, craniofacial structures and limbs. Members of the Dlx gene family are also expressed in retinal ganglion cells (RGC), amacrine and horizontal cells of the developing and postnatal retina. Expression begins at embryonic day 12.5 and is maintained until late embryogenesis for Dlx1, while Dlx2 expression extends to adulthood. We have assessed the retinal phenotype of the Dlx1/Dlx2 double knockout mouse, which dies at birth. The Dlx1/2 null retina displays a reduced ganglion cell layer (GCL), with loss of differentiated RGCs due to increased apoptosis, and corresponding thinning of the optic nerve. Ectopic expression of Crx, the cone and rod photoreceptor homeobox gene, in the GCL and neuroblastic layers of the mutants may signify altered cell fate of uncommitted RGC progenitors. However, amacrine and horizontal cell differentiation is relatively unaffected in the Dlx1/2 null retina. Herein, we propose a model whereby early-born RGCs are Dlx1 and Dlx2 independent, but Dlx function is necessary for terminal differentiation of late-born RGC progenitors.

Chx10 repression of Mitf is required for the maintenance of mammalian neuroretinal identity

During vertebrate eye development, the cells of the optic vesicle (OV) become either neuroretinal progenitors expressing the transcription factor Chx10, or retinal pigment epithelium (RPE) progenitors expressing the transcription factor Mitf. Chx10 mutations lead to microphthalmia and impaired neuroretinal proliferation. Mitf mutants have a dorsal RPE-to-neuroretinal phenotypic transformation, indicating that Mitf is a determinant of RPE identity. We report here that Mitf is expressed ectopically in the Chx10(or-J/or-J) neuroretina (NR), demonstrating that Chx10 normally represses the neuroretinal expression of Mitf. The ectopic expression of Mitf in the Chx10(or-J/or-J) NR deflects it towards an RPE-like identity; this phenotype results not from a failure of neuroretinal specification, but from a partial loss of neuroretinal maintenance. Using Chx10 and Mitf transgenic and mutant mice, we have identified an antagonistic interaction between Chx10 and Mitf in regulating retinal cell identity. FGF (fibroblast growth factor) exposure in a developing OV has also been shown to repress Mitf expression. We demonstrate that the repression of Mitf by FGF is Chx10 dependent, indicating that FGF, Chx10 and Mitf are components of a pathway that determines and maintains the identity of the NR.

Requirement for Mab21l2 during development of murine retina and ventral body wall

The mab-21 gene was first identified because of its requirement for ray identity specification in Caenorhabditis elegans. It is now known to constitute a family of genes that are highly conserved from vertebrates to invertebrates, and two homologues Mab21l1 and Mab21l2 have been identified in many species. Here we describe the generation of Mab21l2-deficient mice, which have defects in eye and body wall formation. The mutant mouse eye has a rudimentary retina, as a result of insufficient invagination of the optic vesicle due to deficient proliferation, causing the absence of lens. The defects in optic vesicle development correlate with reduced expression of Chx10, which is also required for retina development; Rx, Lhx2, and Pax6 expression is not significantly affected. We conclude that Mab21l2 expression is essential for optic vesicle growth and formation of the optic cup, its absence causing reduced expression of Chx10. Mutant mice also display abnormal extrusion of abdominal organs, defects in ventral body wall formation, resulting in death in utero at mid-gestational stage. Our results reveal that Mab21l2 plays crucial roles in retina and in ventral body wall formation.

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.

Cell cycling through development

Regardless of the species, the development of a multicellular organism requires the precise execution of essential developmental processes including patterning, growth, proliferation and differentiation. The cell cycle, in addition to its role as coordinator of DNA replication and mitosis, is also a coordinator of developmental processes, and is a target of developmental signaling pathways. Perhaps because of its central role during development, the cell cycle mechanism, its regulation and its effects on developing tissues is remarkably complex. It was in this light that the Keystone meeting on the cell cycle and development at Snowbird, Utah in January 2004 was held.

Genetic analysis of the homeodomain transcription factor Chx10 in the retina using a novel multifunctional BAC transgenic mouse reporter

Chx10 is a homeobox-containing transcription factor critical for progenitor cell proliferation and bipolar cell determination in the developing retina. Its expression in the retina has been reported to be restricted to these cell populations. To further understand Chx10 regulation and function, a multifunctional reporter construct consisting of GFP, alkaline phosphatase, and Cre recombinase was integrated into a BAC encoding Chx10. Stable lines of transgenic mice expressing this BAC were generated and analyzed. The reporter expression was faithful to the endogenous retinal Chx10 expression pattern and revealed a previously unappreciated locus of Chx10 expression in a subset of Müller glial cells. In addition, Chx10 reporter activity was identified in mature orJ-Chx10 mutant retinas, although these retinas lack Chx10-expressing bipolar cells. Reporter and molecular analysis showed that the reporter-expressing cells in the mutant had hallmarks of progenitor cells or partially differentiated Müller glial cells. These results strongly suggest that Chx10 promotes bipolar fate by affecting differentiation of late progenitor cells. Crosses of the Chx10 BAC reporter mice to R26R mice for fate-mapping experiments revealed that Chx10 reporter-expressing progenitor cells contribute to all mature cell types of the retina. These results demonstrate the utility of these lines for generation of mosaic or complete genetic manipulations of the retina.

Specification of the retinal fate of mouse embryonic stem cells by ectopic expression of Rx/rax, a homeobox gene

With the goal of generating retinal cells from mouse embryonic stem (ES) cells by exogenous gene transfer, we introduced the Rx/rax transcription factor, which is expressed in immature retinal cells, into feeder-free mouse ES cells (CCE). CCE cells expressing Rx/rax as well as enhanced green fluorescent protein (CCE-RX/E cells) proliferated and remained in the undifferentiated state in the presence of leukemia inhibitory factor, as did parental ES cells. We made use of mouse embryo retinal explant cultures to address the differentiation ability of grafted ES cells. Dissociated embryoid bodies were treated with retinoic acid for use as donor cells and cocultured with retina explants for 2 weeks. In contrast to the parental CCE cells, which could not migrate into host retinal cultures, CCE-RX/E cells migrated into the host retina and extended their process-like structures between the host retinal cells. Most of the grafted CCE-RX/E cells became located in the ganglion cell and inner plexiform layers and expressed ganglion and horizontal cell markers. Furthermore, these grafted cells had the electrophysiological properties expected of ganglion cells. Our data thus suggest that subpopulations of retinal neurons can be generated in retinal explant cultures from grafted mouse ES cells ectopically expressing the transcription factor Rx/rax.

Cell cycle development

The Keystone Symposium on the Cell Cycle and Development brought together biologists with an interest in how cell cycle control is integrated into the ontogenetic program of multicellular organisms, and showcased research using a wide variety of systems from both animals and plants. A clear indication from the meeting is that this research is changing the conventional wisdom on both cell cycle control and development.

Transient requirement for ganglion cells during assembly of retinal synaptic layers

The inner plexiform layer (IPL) of the vertebrate retina comprises functionally specialized sublaminae, representing connections between bipolar, amacrine and ganglion cells with distinct visual functions. Developmental mechanisms that target neurites to the correct synaptic sublaminae are largely unknown. Using transgenic zebrafish expressing GFP in subsets of amacrine cells, we imaged IPL formation and sublamination in vivo and asked whether the major postsynaptic cells in this circuit, the ganglion cells, organize the presynaptic inputs. We found that in the lak/ath5 mutant retina, where ganglion cells are never born, formation of the IPL is delayed, with initial neurite outgrowth ectopically located and grossly disorganized. Over time, the majority of early neurite projection errors are corrected, and major ON and OFF sublaminae do form. However, focal regions of disarray persist where sublaminae do not form properly. Bipolar axons, which arrive later, are targeted correctly, except at places where amacrine stratification is disrupted. The lak mutant phenotype reveals that ganglion cells have a transient role organizing the earliest amacrine projections to the IPL. However, it also suggests that amacrine cells interact with each other during IPL formation; these interactions alone appear sufficient to form the IPL. Furthermore, our results suggest that amacrines may guide IPL sublamination by providing stratification cues for other cell types.

Control of late off-center cone bipolar cell differentiation and visual signaling by the homeobox gene Vsx1

Retinal bipolar cells are interneurons that transmit visual signals from photoreceptors to ganglion cells. Although the visual pathways mediated by bipolar cells have been well characterized, the genes that regulate their development and function are largely unknown. To determine the role in bipolar cell development of the homeobox gene Vsx1, whose retinal expression is restricted to a major subset of differentiating and mature cone bipolar (CB) cells, we targeted the gene in mice. Bipolar cell fate was not altered in the absence of Vsx1 function, because the pan-bipolar markers Chx10 and Ret-B1 continued to be expressed in inner nuclear layer neurons labeled by the Vsx1-targeting reporter gene, tauLacZ. The specification, number, and gross morphology of the subset of on-center and off-center (OFF)-CB cells defined by tauLacZ expression from the Vsx1 locus were also normal in Vsx1(tauLacZ)/Vsx1(tauLacZ) mice. However, the terminal differentiation of OFF-CB cells in the retina of Vsx1(tauLacZ)/Vsx1(tauLacZ) mice was incomplete, as demonstrated by a substantial reduction in the expression of at least four markers (recoverin, NK3R, Neto1, and CaB5) for these interneurons. These molecular abnormalities were associated with defects in retinal function and documented by electroretinography and in vitro ganglion cell recordings specific to cone visual signaling. In particular, there was a general reduction in the light-mediated activity of OFF, but not on-center, ganglion cells. Thus, Vsx1 is required for the late differentiation and function of OFF-CB cells and is associated with a heritable OFF visual pathway-specific retinal defect.

Retinal remodeling triggered by photoreceptor degenerations

Many photoreceptor degenerations initially affect rods, secondarily leading to cone death. It has long been assumed that the surviving neural retina is largely resistant to this sensory deafferentation. New evidence from fast retinal degenerations reveals that subtle plasticities in neuronal form and connectivity emerge early in disease. By screening mature natural, transgenic, and knockout retinal degeneration models with computational molecular phenotyping, we have found an extended late phase of negative remodeling that radically changes retinal structure. Three major transformations emerge: 1) Müller cell hypertrophy and elaboration of a distal glial seal between retina and the choroid/retinal pigmented epithelium; 2) apparent neuronal migration along glial surfaces to ectopic sites; and 3) rewiring through evolution of complex neurite fascicles, new synaptic foci in the remnant inner nuclear layer, and new connections throughout the retina. Although some neurons die, survivors express molecular signatures characteristic of normal bipolar, amacrine, and ganglion cells. Remodeling in human and rodent retinas is independent of the initial molecular targets of retinal degenerations, including defects in the retinal pigmented epithelium, rhodopsin, or downstream phototransduction elements. Although remodeling may constrain therapeutic intervals for molecular, cellular, or bionic rescue, it suggests that the neural retina may be more plastic than previously believed.

Classification of retinal ganglion cells: a statistical approach

Numerous studies have shown that retinal ganglion cells exhibit an array of responses to visual stimuli. This has led to the idea that these cells can be sorted into distinct physiological classes, such as linear versus nonlinear or on versus off. Although many classification schemes are widely accepted, few studies have provided statistical support to favor one scheme over another. Here we test whether some of the most widely used classification schemes can be statistically verified, using the mouse retina as the model system. We used a cluster analysis approach and focused on 4 standard response parameters: 1) response latency, 2) response duration, 3) relative amplitude of the on and off responses, and 4) degree of nonlinearity in the stimulus-to-response transformation. For each parameter, we plotted its distribution and tested quantitatively, using a bootstrap method, whether it divided into distinct clusters. Our analysis showed that mouse ganglion cells clustered into several groups based on response latency, duration, and relative amplitude of the on and off responses, but did not cluster into more than one group based on degree of nonlinearity-the latter formed a single, large, continuous group. Thus while some well-known schemes for classifying ganglion cells could be statistically verified, others could not. Knowledge of which schemes can be confirmed is important for building models of how retinal output is processed and how retinal circuits are built. Finally, this cluster analysis approach is general and can be used to test other classification proposals as well, both physiological and anatomical.

Neural remodeling in retinal degeneration

Mammalian retinal degenerations initiated by gene defects in rods, cones or the retinal pigmented epithelium (RPE) often trigger loss of the sensory retina, effectively leaving the neural retina deafferented. The neural retina responds to this challenge by remodeling, first by subtle changes in neuronal structure and later by large-scale reorganization. Retinal degenerations in the mammalian retina generally progress through three phases. Phase 1 initiates with expression of a primary insult, followed by phase 2 photoreceptor death that ablates the sensory retina via initial photoreceptor stress, phenotype deconstruction, irreversible stress and cell death, including bystander effects or loss of trophic support. The loss of cones heralds phase 3: a protracted period of global remodeling of the remnant neural retina. Remodeling resembles the responses of many CNS assemblies to deafferentation or trauma, and includes neuronal cell death, neuronal and glial migration, elaboration of new neurites and synapses, rewiring of retinal circuits, glial hypertrophy and the evolution of a fibrotic glial seal that isolates the remnant neural retina from the surviving RPE and choroid. In early phase 2, stressed photoreceptors sprout anomalous neurites that often reach the inner plexiform and ganglion cell layers. As death of rods and cones progresses, bipolar and horizontal cells are deafferented and retract most of their dendrites. Horizontal cells develop anomalous axonal processes and dendritic stalks that enter the inner plexiform layer. Dendrite truncation in rod bipolar cells is accompanied by revision of their macromolecular phenotype, including the loss of functioning mGluR6 transduction. After ablation of the sensory retina, Müller cells increase intermediate filament synthesis, forming a dense fibrotic layer in the remnant subretinal space. This layer invests the remnant retina and seals it from access via the choroidal route. Evidence of bipolar cell death begins in phase 1 or 2 in some animal models, but depletion of all neuronal classes is evident in phase 3. As remodeling progresses over months and years, more neurons are lost and patches of the ganglion cell layer can become depleted. Some survivor neurons of all classes elaborate new neurites, many of which form fascicles that travel hundreds of microns through the retina, often beneath the distal glial seal. These and other processes form new synaptic microneuromas in the remnant inner nuclear layer as well as cryptic connections throughout the retina. Remodeling activity peaks at mid-phase 3, where neuronal somas actively migrate on glial surfaces. Some amacrine and bipolar cells move into the former ganglion cell layer while other amacrine cells are everted through the inner nuclear layer to the glial seal. Remodeled retinas engage in anomalous self-signaling via rewired circuits that might not support vision even if they could be driven anew by cellular or bionic agents. We propose that survivor neurons actively seek excitation as sources of homeostatic Ca(2+) fluxes. In late phase 3, neuron loss continues and the retina becomes increasingly glial in composition. Retinal remodeling is not plasticity, but represents the invocation of mechanisms resembling developmental and CNS plasticities. Together, neuronal remodeling and the formation of the glial seal may abrogate many cellular and bionic rescue strategies. However, survivor neurons appear to be stable, healthy, active cells and given the evidence of their reactivity to deafferentation, it may be possible to influence their emergent rewiring and migration habits.