ISL1 and BRN3B co-regulate the differentiation of murine retinal ganglion cells

The role of homeobox genes in retinal development and disease

Homeobox genes are an evolutionarily conserved class of transcription factors that are critical for development of many organ systems, including the brain and eye. During retinogenesis, homeodomain-containing transcription factors, which are encoded by homeobox genes, play essential roles in the regionalization and patterning of the optic neuroepithelium, specification of retinal progenitors and differentiation of all seven of the retinal cell classes that derive from a common progenitor. Homeodomain transcription factors control retinal cell fate by regulating the expression of target genes required for retinal progenitor cell fate decisions and for terminal differentiation of specific retinal cell types. The essential role of homeobox genes during retinal development is demonstrated by the number of human eye diseases, including colobomas and anophthalmia, which are attributed to homeobox gene mutations. In the following review, we highlight the role of homeodomain transcription factors during retinogenesis and regulation of their gene targets. Understanding the complexities of vertebrate retina development will enhance our ability to drive differentiation of specific retinal cell types towards novel cell-based replacement therapies for retinal degenerative diseases.

Developmental wave of Brn3b expression leading to RGC fate specification is synergistically maintained by miR-23a and miR-374

Differential regulation of Brn3b is essential for the Retinal Ganglion Cell (RGC) development in the two phases of retinal histogenesis. This biphasic Brn3b regulation is required first, during early retinal histogenesis for RGC fate specification and secondly, during late histogenesis, where Brn3b is needed for RGC axon guidance and survival. Here, we have looked into how the regulation of Brn3b at these two stages happens. We identified two miRNAs, miR-23a and miR-374, as regulators of Brn3b expression, during the early stage of RGC development. Temporal expression pattern of miR-23a during E10-19, PN1-7, and adult retina revealed an inverse relation with Brn3b expression. Though miR-374 did not show such a pattern, its co-expression with miR-23a evidently inhibited Brn3b. We further substantiated these findings by ex vivo overexpression of these miRNAs in E14 mice retina and found that miR-23a and miR-374 together brings about a change in Brn3b expression pattern in ganglion cell layer (GCL) of the developing retina. From our results, it appears that the combined expression of these miRNAs could be regulating the timing of the wave of Brn3b expression required for early ganglion cell fate specification and later for its survival and maturation into RGCs. Taken together, here we provide convincing evidences for the existence of a co-ordinated mechanism by miRNAs to down regulate Brn3b that will ultimately regulate the development of RGCs from their precursors.

Delayed neurogenesis leads to altered specification of ventrotemporal retinal ganglion cells in albino mice

Background

Proper binocular vision depends on the routing at the optic chiasm of the correct proportion of retinal ganglion cell (RGC) axons that project to the same (ipsilateral) and opposite (contralateral) side of the brain. The ipsilateral RGC projection is reduced in mammals with albinism, a congenital disorder characterized by deficient pigmentation in the skin, hair, and eyes. Compared to the pigmented embryonic mouse retina, the albino embryonic mouse retina has fewer RGCs that express the zinc-finger transcription factor, Zic2, which is transiently expressed by RGCs fated to project ipsilaterally. Here, using Zic2 as a marker of ipsilateral RGCs, Islet2 as a marker of contralateral RGCs, and birthdating, we investigate spatiotemporal dynamics of RGC production as they relate to the phenotype of diminished ipsilateral RGC number in the albino retina.

Results

At embryonic day (E)15.5, fewer Zic2-positive (Zic2⁺) RGCs are found in the albino ventrotemporal (VT) retina compared with the pigmented VT retina, as we previously reported. However, the reduction in Zic2⁺ RGCs in the albino is not accompanied by a compensatory increase in Zic2-negative (Zic2⁻) RGCs, resulting in fewer RGCs in the VT retina at this time point. At E17.5, however, the number of RGCs in the VT region is similar in pigmented and albino retinae, implicating a shift in the timing of RGC production in the albino. Short-term birthdating assays reveal a delay in RGC production in the albino VT retina between E13 and E15. Specifically, fewer Zic2⁺ RGCs are born at E13 and more Zic2⁻ RGCs are born at E15. Consistent with an increase in the production of Zic2⁻ RGCs born at later ages, more RGCs at E17.5 express the contralateral marker, Islet2, in the albino VT retina compared with the pigmented retina.

Conclusions

A delay in neurogenesis in the albino retina is linked to the alteration of RGC subtype specification and consequently leads to the reduced ipsilateral projection that characterizes albinism.

Pou4f1 and pou4f2 are dispensable for the long-term survival of adult retinal ganglion cells in mice

PURPOSE

To investigate the role of Pou4f1 and Pou4f2 in the survival of adult retinal ganglion cells (RGCs).

METHODS

Conditional alleles of Pou4f1 and Pou4f2 were generated (Pou4f1loxP and Pou4f2loxP respectively) for the removal of Pou4f1 and Pou4f2 in adult retinas. A tamoxifen-inducible Cre was used to delete Pou4f1 and Pou4f2 in adult mice and retinal sections and flat mounts were subjected to immunohistochemistry to confirm the deletion of both alleles and to quantify the changes in the number of RGCs and other retinal neurons. To determine the effect of loss of Pou4f1 and Pou4f2 on RGC survival after axonal injury, controlled optic nerve crush (CONC) was performed and RGC death was assessed.

RESULTS

Pou4f1 and Pou4f2 were ablated two weeks after tamoxifen treatment. Retinal interneurons and Müller glial cells are not affected by the ablation of Pou4f1 or Pou4f2 or both. Although the deletion of both Pou4f1 and Pou4f2 slightly delays the death of RGCs at 3 days post-CONC in adult mice, it does not affect the cell death progress afterwards. Moreoever, deletion of Pou4f1 or Pou4f2 or both has no impact on the long-term viability of RGCs at up to 6 months post-tamoxifen treatment.

CONCLUSION

Pou4f1 and Pou4f2 are involved in the acute response to damage to RGCs but are dispensable for the long-term survival of adult RGC in mice.

Stem cells, retinal ganglion cells and glaucoma

Retinal ganglion cells (RGCs) represent an essential neuronal cell type for vision. These cells receive inputs from light-sensing photoreceptors via retinal interneurons and then relay these signals to the brain for further processing. RGC diseases that result in cell death, e.g. glaucoma, often lead to permanent damage since mammalian nerves do not regenerate. Stem cell differentiation can generate cells needed for replacement or can be used to generate cells capable of secreting protective factors to promote survival. In addition, stem cell-derived cells can be used in drug screening research. Here, we discuss the current state of stem cell research potential for interference in glaucoma and other optic nerve diseases with a focus on stem cell differentiation to RGCs.

Comparative expression analysis of POU4F1, POU4F2 and ISL1 in developing mouse cochleovestibular ganglion neurons

POU-homeodomain and LIM-homeodomain transcription factors are expressed in developing projection neurons within retina, inner ear, dorsal root ganglion, and trigeminal ganglion, and play synergistic roles in their differentiation and survival. Here, using immunohistochemistry, we present a comparative analysis of the spatiotemporal expression pattern of POU4F1, POU4F2, and ISL1 during the development of cochleovestibular ganglion (CVG) neurons in mouse inner ear. At early stages, when otic neurons are first detected in the otic epithelium (OE) and migrate into periotic mesenchyme to form the CVG, POU4F1 and ISL1 are co-expressed in a majority of the delaminated CVG neurons, which are marked by NEUROD1 expression, but POU4F1 is absent in the otic epithelium. The onset of POU4F2 expression starts after that of POU4F1 and ISL1, and is observed in the NEUROD1-negative, post-mitotic CVG neurons. When the CVG neurons innervate the vestibular and cochlear sensory organs, the expression of POU4F1, POU4F2, and ISL1 continues in both vestibular and spiral ganglion cells. Later in development, POU4F1 expression becomes down-regulated in a majority of spiral ganglion (SG) neurons and more neurons express POU4F2 expression while ISL1 expression is maintained. The differential as well as overlapping expression of POU4F1, POU4F2, and ISL1 combined with previous studies suggests possible functional interaction and regulatory relationship of these transcription factors in the development of inner ear neurons.

Isl1 and Pou4f2 form a complex to regulate target genes in developing retinal ganglion cells

Precise regulation of gene expression during biological processes, including development, is often achieved by combinatorial action of multiple transcription factors. The mechanisms by which these factors collaborate are largely not known. We have shown previously that Isl1, a Lim-Homeodomain transcription factor, and Pou4f2, a class IV POU domain transcription factor, co-regulate a set of genes required for retinal ganglion cell (RGC) differentiation. Here we further explore how these two factors interact to precisely regulate gene expression during RGC development. By GST pulldown assays, co-immunoprecipitation, and electrophoretic mobility shift assays, we show that Isl1 and Pou4f2 form a complex in vitro and in vivo, and identify the domains within these two proteins that are responsible for this interaction. By luciferase assay, in situ hybridization, and RNA-seq, we further demonstrate that the two factors contribute quantitatively to gene expression in the developing RGCs. Although each factor alone can activate gene expression, both factors are required to achieve optimal expression levels. Finally, we discover that Isl1 and Pou4f2 can interact with other POU and Lim-Homeodomain factors respectively, indicating the interactions between these two classes of transcription factors are prevalent in development and other biological processes.

Involvement of Bcl-2-associated transcription factor 1 in the differentiation of early-born retinal cells

Retinal progenitor proliferation and differentiation are tightly controlled by extrinsic cues and distinctive combinations of transcription factors leading to the generation of retinal cell type diversity. In this context, we have characterized Bcl-2-associated transcription factor (Bclaf1) during rodent retinogenesis. Bclaf1 expression is restricted to early-born cell types, such as ganglion, amacrine, and horizontal cells. Analysis of developing retinas in Bclaf1-deficient mice revealed a reduction in the numbers of retinal ganglion cells, amacrine cells and horizontal cells and an increase in the numbers of cone photoreceptor precursors. Silencing of Bclaf1expression by in vitro electroporation of shRNA in embryonic retina confirmed that Bclaf1 serves to promote amacrine and horizontal cell differentiation. Misexpression of Bclaf1 in late retinal progenitors was not sufficient to directly induce the generation of amacrine and horizontal cells. Domain deletion analysis indicated that the N-terminal domain of Bclaf1 containing an arginine-serine-rich and a bZip domain is required for its effects on retinal cell differentiation. In addition, analysis revealed that Bclaf1 function occurs independently of its interaction with endogenous Bcl-2-related proteins. Altogether, our data demonstrates that Bclaf1expression in postmitotic early-born cells facilitates the differentiation of early retinal precursors into retinal ganglion cells, amacrine cells, and horizontal cells rather than into cone photoreceptors.

Chemically induced specification of retinal ganglion cells from human embryonic and induced pluripotent stem cells

The loss of retinal ganglion cells (RGCs) is the primary pathological change for many retinal degenerative diseases. Although there is currently no effective treatment for this group of diseases, cell transplantation to replace lost RGCs holds great potential. However, for the development of cell replacement therapy, better understanding of the molecular details involved in differentiating stem cells into RGCs is essential. In this study, a novel, stepwise chemical protocol is described for the differentiation of human embryonic stem cells and induced pluripotent stem cells into functional RGCs. Briefly, stem cells were differentiated into neural rosettes, which were then cultured with the Notch inhibitor N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester (DAPT). The expression of neural and RGC markers (BRN3A, BRN3B, ATOH7/Math5, γ-synuclein, Islet-1, and THY-1) was examined. Approximately 30% of the cell population obtained expressed the neuronal marker TUJ1 as well the RGC markers. Moreover, the differentiated RGCs generated action potentials and exhibited both spontaneous and evoked excitatory postsynaptic currents, indicating that functional and mature RGCs were generated. In combination, these data demonstrate that a single chemical (DAPT) can induce PAX6/RX-positive stem cells to undergo differentiation into functional RGCs.

Induction of the ganglion cell differentiation program in human retinal progenitors before cell cycle exit

BACKGROUND

Despite the disease relevance, understanding of human retinal development lags behind that of other species. We compared the kinetics of gene silencing or induction during ganglion cell development in human and murine retina.

RESULTS

Induction of POU4F2 (BRN3B) marks ganglion cell commitment, and we detected this factor in S-phase progenitors that had already silenced Cyclin D1 and VSX2 (CHX10). This feature was conserved in human and mouse retina, and the fraction of Pou4f2+ murine progenitors labeled with a 30 min pulse of BrdU matched the fraction of ganglion cells predicted to be born in a half-hour period. Additional analysis of 18 markers revealed many with conserved kinetics, such as the POU4F2 pattern above, as well as the surprising maintenance of "cell cycle" proteins KI67, PCNA, and MCM6 well after terminal mitosis. However, four proteins (TUBB3, MTAP1B, UCHL1, and RBFOX3) showed considerably delayed induction in human relative to mouse retina, and two proteins (ISL1, CALB2) showed opposite kinetics, appearing on either side of terminal mitosis depending on the species.

CONCLUSION

With some notable exceptions, human and murine ganglion cell differentiation show similar kinetics, and the data add weight to prior studies supporting the existence of biased ganglion cell progenitors.

Growth hormone and retinal ganglion cell function: QNR/D cells as an experimental model

Retinal ganglion cells (RGCs) have been shown to be sites of growth hormone (GH) production and GH action in the embryonic (embryo day 7, ED7) chick neural retina. Primary RGC cell cultures were previously used to determine autocrine or paracrine actions of GH in the retina, but the antibody used in their immunopanning (anti-Thy-1) is no longer available. We have therefore characterized an immortalized neural retina (QNR/D) cell line derived from ED7 embryonic quail as a replacement experimental model. These cells express the GH gene and have GH receptor (GHR)-immunoreactivity. They are also immunoreactive for RGC markers (islet-1, calretinin, RA4) and neural fibers (neurofilament, GAP 43, vimentin) and they express the genes for Thy-1, neurotrophin 3 (NTF3), neuritin 1 (NRN1) and brn3 (POU4F). These cells are also electrically active and therefore resemble the RGCs in the neural retina. They are also similarly responsive to exogenous GH, which induces overexpression of the neurotrophin 3 and insulin-like growth factor (IGF) 1 genes and stimulates cell survival, as in the chick embryo neural retina. QNR/D cells are therefore a useful experimental model to assess the actions of GH in retinal function.

Dual role for Islet-1 in promoting striatonigral and repressing striatopallidal genetic programs to specify striatonigral cell identity

Striatal projection neurons comprise two populations of striatonigral and striatopallidal neurons. These two neuronal populations play distinct roles in controlling movement-related functions in the basal ganglia circuits. An important issue is how striatal progenitors are developmentally specified into these two distinct neuronal populations. In the present study, we characterized the function of Islet-1 (Isl1), a LIM-homeodomain transcription factor, in striatal development. Genetic fate mapping showed that Isl1(+) progeny specifically developed into a subpopulation of striatonigral neurons that transiently expressed Isl1. In Nestin-Cre;Isl1(f/f) KO mouse brain, differentiation of striatonigral neurons was defective, as evidenced by decreased expression of striatonigral-enriched genes, including substance P, prodynorphin, solute carrier family 35, member D3 (Slc35d3), and PlexinD1. Striatonigral axonal projections were also impaired, and abnormal apoptosis was observed in Isl1 KO striatum. It was of particular interest that striatopallidal-enriched genes, including dopamine D2 receptor (Drd2), proenkephalin, A2A adenosine receptor (A2aR) and G protein-coupled receptor 6 (Gpr6), were concomitantly up-regulated in Isl1 mutant striatum, suggesting derepression of striatopallidal genes in striatonigral neurons in the absence of Isl1. The suppression of striatopallidal genes by Isl1 was further examined by overexpression of Isl1 in the striatum of Drd2-EGFP transgenic mice using in utero electroporation. Ectopic Isl1 expression was sufficient to repress Drd2-EGFP signals in striatopallidal neurons. Taken together, our study suggests that Isl1 specifies the cell fate of striatonigral neurons not only by orchestrating survival, differentiation, and axonal projections of striatonigral neurons but also by suppressing striatopallidal-enriched genes. The dual action of developmental control by Isl1 in promoting appropriate striatonigral but repressing inappropriate striatopallidal genetic profiles may ensure sharpening of the striatonigral identity during development.

Hair cell overexpression of Islet1 reduces age-related and noise-induced hearing loss

Isl1 is a LIM-homeodomain transcription factor that is critical in the development and differentiation of multiple tissues. In the mouse inner ear, Isl1 is expressed in the prosensory region of otocyst, in young hair cells and supporting cells, and is no longer expressed in postnatal auditory hair cells. To evaluate how continuous Isl1 expression in postnatal hair cells affects hair cell development and cochlear function, we created a transgenic mouse model in which the Pou4f3 promoter drives Isl1 overexpression specifically in hair cells. Isl1 overexpressing hair cells develop normally, as seen by morphology and cochlear functions (auditory brainstem response and otoacoustic emissions). As the mice aged to 17 months, wild-type (WT) controls showed the progressive threshold elevation and outer hair cell loss characteristic of the age-related hearing loss (ARHL) in the background strain (C57BL/6J). In contrast, the Isl1 transgenic mice showed significantly less threshold elevation with survival of hair cells. Further, the Isl1 overexpression protected the ear from noise-induced hearing loss (NIHL): both ABR threshold shifts and hair cell death were significantly reduced when compared with WT littermates. Our model suggests a common mechanism underlying ARHL and NIHL, and provides evidence that hair cell-specific Isl1 expression can promote hair cell survival and therefore minimize the hearing impairment that normally occurs with aging and/or acoustic overexposure.

Islet-1 immunoreactivity in the developing retina of Xenopus laevis

The LIM-homeodomain transcription factor Islet1 (Isl1) has been widely used as a marker of neuronal differentiation in the developing visual system of different classes of vertebrates, including mammals, birds, reptiles, and fish. In the present study, we analyzed the spatial and temporal distribution of Isl1-immunoreactive cells during Xenopus laevis retinal development and its relation to the formation of the retinal layers, and in combination with different markers of cell differentiation. The earliest Isl1 expression appeared at St29-30 in the cell nuclei of sparse differentiating neuroblasts located in the vitreal surface of the undifferentiated retina. At St35-36, abundant Isl1-positive cells accumulated at the vitreal surface of the neuroepithelium. As development proceeded and through the postmetamorphic juveniles, Isl1 expression was identified in subpopulations of ganglion cells and in subsets of amacrine, bipolar, and horizontal cells. These data together suggest a possible role for Isl1 in the early differentiation and maintenance of different retinal cell types, and Isl1 can serve as a specific molecular marker for the study of retinal cell specification in X. laevis.

Genetic interactions between Brn3 transcription factors in retinal ganglion cell type specification

Background

Visual information is conveyed from the retina to the brain via 15–20 Retinal Ganglion Cell (RGC) types. The developmental mechanisms by which RGC types acquire their distinct molecular, morphological, physiological and circuit properties are essentially unknown, but may involve combinatorial transcriptional regulation. Brn3 transcription factors are expressed in RGCs from early developmental stages, and are restricted in adults to distinct, partially overlapping populations of RGC types. Previously, we described cell autonomous effects of Brn3b (Pou4f2) and Brn3a (Pou4f1) on RGC axon and dendrites development.

Methods and Findings

We now have investigated genetic interactions between Brn3 transcription factors with respect to RGC development, by crossing conventional knock-out alleles of each Brn3 gene with conditional knock-in reporter alleles of a second Brn3 gene, and analyzing the effects of single or double Brn3 knockouts on RGC survival and morphology. We find that Brn3b loss results in axon defects and dendritic arbor area and lamination defects in Brn3a positive RGCs, and selectively affects survival and morphology of specific Brn3c (Pou4f3) positive RGC types. Brn3a and Brn3b interact synergistically to control RGC numbers. Melanopsin positive ipRGCs are resistant to combined Brn3 loss but are under the transcriptional control of Isl1, expanding the combinatorial code of RGC specification.

Conclusions

Taken together these results complete our knowledge on the mechanisms of transcriptional control of RGC type specification. They demonstrate that Brn3b is required for the correct development of more RGC cell types than suggested by its expression pattern in the adult, but that several cell types, including some Brn3a, Brn3c or Melanopsin positive RGCs are Brn3b independent.

Heterogeneity of intrinsically photosensitive retinal ganglion cells in the mouse revealed by molecular phenotyping

Intrinsically photosensitive retinal ganglion cell (ipRGC) types can be distinguished by their dendritic tree stratification and intensity of melanopsin staining. We identified heavily stained melanopsin-positive M1 cells branching in the outermost part of the inner plexiform layer (IPL) and weakly melanopsin-positive M2 cells branching in the innermost layer of the IPL. A third type can be distinguished by the displacement of the soma to the inner nuclear layer and has morphological similarities with either M1 cells or M2 cells, and is termed here displaced or M-d cells. The aim of the present study was to examine the phenotypic traits of ipRGC types. Using whole retinae from adult mice, we performed immunohistochemistry using melanopsin immunostaining and a number of antibodies directed against proteins typically expressed in retinal ganglion cells. The majority of M1 and M2 ipRGCs expressed Isl-1, microtubule associated protein-2 (MAP2), γ-synuclein, and NeuN, whereas Brn3 transcription factor and the different neurofilaments (NF68, NF160, NF200) were able to discriminate between ipRGC subtypes. Brn3 was expressed preferentially in M2 cells and in a small subpopulation of weakly melanopsin-positive M-d cells with similarities to M2 cells. All three neurofilaments were primarily expressed in large M2 cells with similarities to the recently described alpha-like M4 cells, but not in M1 cells. Expression of NF68 and NF160 was also observed in a few large M-d ipRGCs. These findings show that ipRGCs are not a phenotypically homogenous population and that specific neuronal markers (Brn3 and neurofilament) can partly distinguish between different ipRGC subtypes.

Expression of Isl1 during mouse development

The LIM-homeodomain transcription factor Isl1 plays essential roles in cell proliferation, differentiation and survival during embryogenesis. To better visualize Isl1 expression and provide insight into the role of Isl1 during development, we generated an Isl1 nuclear LacZ (nLacZ) knockin mouse line. We have analyzed Isl1nlacZ expression during development by Xgal staining, and compared expression of Isl1nlacZ with endogenous Isl1 by coimmunostaining with antibodies to Isl1 and β-galactosidase. Results demonstrated that during development, Isl1 nLacZ is expressed in a pattern that recapitulates endogenous Isl1 protein expression. Consistent with previous in situ and immunohistochemistry data, we observed Isl1nlacZ expression in multiple tissues and cell types, including the central and peripheral nervous system, neural retina, inner ear, pharyngeal mesoderm and endoderm and their derivatives (craniofacial structures, thymus, thyroid gland and trachea), cardiovascular system (cardiac outflow tract, carotid arteries, umbilical vessels, sinoatrial node and atrial septum), gastrointestinal system (oral epithelium, stomach, pancreas, mesentery) and hindlimb.

RIT2, a neuron-specific small guanosine triphosphatase, is expressed in retinal neuronal cells and its promoter is modulated by the POU4 transcription factors

PURPOSE

Ras-like without CAAX 2 (RIT2), a member of the Ras superfamily of small guanosine triphosphatases, is involved in regulating neuronal function. RIT2 is a unique member of the Ras family in that RIT2 is preferentially expressed in various neurons, including retinal neurons. The mechanisms that regulate RIT2 expression in neurons were studied.

METHODS

Reverse transcription-quantitative PCR (RT-qPCR), immunohistochemistry, western blotting, bioinformatic prediction, electrophoretic mobility shift assay (EMSA), and cell transfection methods were used.

RESULTS

With immunohistochemistry of the mouse retina, RIT2 protein was detected in the ganglion cell layer (GCL), inner plexiform layer, inner nuclear layer, and outer plexiform layer, with the strongest staining in the GCL and the inner plexiform layer. RT-qPCR combined with laser capture microdissection detected Rit2 messenger RNA in the GCL and the inner nuclear layer. Western blot analysis showed a large increase in the RIT2 protein in the retina during maturation from newborn to adult. Transient transfection identified the 1.3 kb upstream region of human RIT2 as capable of driving expression in neuronal cell lines. Based on the known expression pattern and biological activity, we hypothesized that POU4 family factors might modulate RIT2 expression in retinal ganglion cells (RGCs). Bioinformatic analyses predicted six POU4 factor-binding sites within the 1.3 kb human RIT2 promoter region. EMSA analyses showed binding of POU4 proteins to three of the six predicted sites. Cotransfection with expression vectors demonstrated that POU4 proteins can indeed modulate the human RIT2 promoter, and that ISL1, a LIM homeodomain factor, can further modulate the activity of the POU4 factors.

CONCLUSIONS

These studies confirm the expression of RIT2 in retinal neuronal cells, including RGCs, begin to reveal the mechanisms responsible for neuronal expression of RIT2, and suggest a role for the POU4 family factors in modulating RIT2 expression in RGCs.

Transcription factors SOX4 and SOX11 function redundantly to regulate the development of mouse retinal ganglion cells

SOX family proteins belong to the high-mobility-group (HMG) domain-containing transcription factors, and function as key players to regulate embryonic development and cell fate determination. The highly related group C Sox genes Sox4 and Sox11 are widely expressed in the development of mouse retina and share a similar expression pattern with each other in this process. Here, to investigate the roles of Sox4 and Sox11 in the retinal development, Sox4, Sox11, and Sox4/Sox11 conditional knock-out (CKO) mice with deletion of Sox4, Sox11, and Sox4/Sox11 in retinas were generated. Our studies demonstrated that targeted disruption of Sox4 or Sox11 in retinas caused a moderate reduction of generation of RGCs. However, a complete loss of RGCs was observed in Sox4/Sox11-null retinas, suggesting the two genes play similar roles in the development of RGCs. Our further analysis confirms that Sox4 and Sox11 function redundantly to regulate the generation of RGCs at early embryonic stages as well as the survival of RGCs at late embryonic stages. In addition, we demonstrated that loss of Math5 impairs the expression of Sox4 and Sox11 in the ganglion cell layer while deletion of Brn3b has no effect on the expression of Sox4 and Sox11. Taken together, these findings elucidate SoxC genes as essential contributors to maintain the survival of RGCs, and imply their intermediate position between Math5 and Brn3b in the genetic hierarchy of RGC development.

Pax6 interactions with chromatin and identification of its novel direct target genes in lens and forebrain

Pax6 encodes a specific DNA-binding transcription factor that regulates the development of multiple organs, including the eye, brain and pancreas. Previous studies have shown that Pax6 regulates the entire process of ocular lens development. In the developing forebrain, Pax6 is expressed in ventricular zone precursor cells and in specific populations of neurons; absence of Pax6 results in disrupted cell proliferation and cell fate specification in telencephalon. In the pancreas, Pax6 is essential for the differentiation of α-, β- and δ-islet cells. To elucidate molecular roles of Pax6, chromatin immunoprecipitation experiments combined with high-density oligonucleotide array hybridizations (ChIP-chip) were performed using three distinct sources of chromatin (lens, forebrain and β-cells). ChIP-chip studies, performed as biological triplicates, identified a total of 5,260 promoters occupied by Pax6. 1,001 (133) of these promoter regions were shared between at least two (three) distinct chromatin sources, respectively. In lens chromatin, 2,335 promoters were bound by Pax6. RNA expression profiling from Pax6^+/− lenses combined with in vivo Pax6-binding data yielded 76 putative Pax6-direct targets, including the Gaa, Isl1, Kif1b, Mtmr2, Pcsk1n, and Snca genes. RNA and ChIP data were validated for all these genes. In lens cells, reporter assays established Kib1b and Snca as Pax6 activated and repressed genes, respectively. In situ hybridization revealed reduced expression of these genes in E14 cerebral cortex. Moreover, we examined differentially expressed transcripts between E9.5 wild type and Pax6^−/− lens placodes that suggested Efnb2, Fat4, Has2, Nav1, and Trpm3 as novel Pax6-direct targets. Collectively, the present studies, through the identification of Pax6-direct target genes, provide novel insights into the molecular mechanisms of Pax6 gene control during mouse embryonic development. In addition, the present data demonstrate that Pax6 interacts preferentially with promoter regions in a tissue-specific fashion. Nevertheless, nearly 20% of the regions identified are accessible to Pax6 in multiple tissues.

Defective FGF signaling causes coloboma formation and disrupts retinal neurogenesis

The optic fissure (OF) is a transient opening on the ventral side of the developing vertebrate eye that closes before nearly all retinal progenitor cell differentiation has occurred. Failure to close the OF results in coloboma, a congenital disease that is a major cause of childhood blindness. Although human genetic studies and animal models have linked a number of genes to coloboma, the cellular and molecular mechanisms driving the closure of the OF are still largely unclear. In this study, we used Cre-LoxP-mediated conditional removal of fibroblast growth factor (FGF) receptors, Fgfr1 and Fgfr2, from the developing optic cup (OC) to show that FGF signaling regulates the closing of the OF. Our molecular, cellular and transcriptome analyses of Fgfr1 and Fgfr2 double conditional knockout OCs suggest that FGF signaling controls the OF closure through modulation of retinal progenitor cell proliferation, fate specification and morphological changes. Furthermore, Fgfr1 and Fgfr2 double conditional mutant retinal progenitor cells fail to initiate retinal ganglion cell (RGC) genesis. Taken together, our mouse genetic studies reveal that FGF signaling is essential for OF morphogenesis and RGC development.

Adult mice transplanted with embryonic retinal progenitor cells: new approach for repairing damaged optic nerves

PURPOSE

Retinal ganglion cell (RGC) death and optic nerve degeneration are complex processes whose underlying molecular mechanisms are only vaguely understood. Treatments commonly used for optic nerve degeneration have little long-term value and only prolong degeneration. Recent advances in stem cell replacement therapy offer new ways to overcome RGC loss by transferring healthy cells into eyes of afflicted individuals. However, studies on stem cell replacement for optic nerve degeneration are hampered by limitations of the available animal models, especially genetic models. We have developed a mouse model in which RGCs are genetically ablated in adult mice with subsequent degeneration of the optic nerve. In the study reported here, we used this model to determine whether embryonic retinal progenitor cells (RPCs) removed from donor retinas when RPCs are committing to an RGC fate could restore lost RGCs.

METHODS

We used the RGC-depleted model as a host for transplanting donor green fluorescent protein (GFP)-labeled RPCs from embryonic retinas that are maximally expressing Atoh7, a basic helix-loop-helix gene essential for RGC specification. Dissociated GFP-labeled RPCs were characterized in situ by immunolabeling with antibodies against proteins known to be expressed in RPCs at embryonic day (E)14.5. Dissociated retinal cells were injected into the vitreous of one eye of RGC-depleted mice at two to six months of age. The injected and non-injected retinas were analyzed for gene expression using immunolabeling, and the morphology of optic nerves was assessed visually and with histological staining at different times up to four months after injection.

RESULTS

We demonstrate the successful transfer of embryonic GFP-labeled RPCs into the eyes of RGC-depleted mice. Many transplanted RPCs invaded the ganglion cell layer, but the efficiency of the invasion was low. GFP-labeled cells within the ganglion cell layer expressed genes associated with early and late stages of RGC differentiation, including Pou4f1, Pou4f2, NFL, Map2, and syntaxin. Several GFP-labeled cells were detected within the injected optic nerves of RGC-depleted mice, and in most cases, we observed a significant increase in the thickness of the RPC-injected optic nerves compared with non-injected controls. We also observed more bundled axons emanating from RPC-injected retinas compared with RGC-depleted controls.

CONCLUSIONS

The results offer a new approach for regenerating damaged optic nerves and indicate that a significant number of E14.5 RPCs are able to differentiate into RGCs in the foreign environment of the adult retina. However, the proportion of RPCs that populated the ganglion cell layer and contributed to the optic nerve was not sufficient to account for the increased thickness and higher number of axons. The results support the hypothesis that the injected E14.5 RPCs are contributing autonomously and non-autonomously to restoring damaged optic nerves.

Intrinsic control of mammalian retinogenesis

The generation of appropriate and diverse neuronal and glial types and subtypes during development constitutes the critical first step toward assembling functional neural circuits. During mammalian retinogenesis, all seven neuronal and glial cell types present in the adult retina are specified from multipotent progenitors by the combined action of various intrinsic and extrinsic factors. Tremendous progress has been made over the past two decades in uncovering the complex molecular mechanisms that control retinal cell diversification. Molecular genetic studies coupled with bioinformatic approaches have identified numerous transcription factors and cofactors as major intrinsic regulators leading to the establishment of progenitor multipotency and eventual differentiation of various retinal cell types and subtypes. More recently, non-coding RNAs have emerged as another class of intrinsic factors involved in generating retinal cell diversity. These intrinsic regulatory factors are found to act in different developmental processes to establish progenitor multipotency, define progenitor competence, determine cell fates, and/or specify cell types and subtypes.

Paradigm of Time-sequence Development of the Intestine of Suckling Piglets with Microarray

The interaction of the genes involved in intestinal development is the molecular basis of the regulatory mechanisms of intestinal development. The objective of this study was to identify the significant pathways and key genes that regulate intestinal development in Landrace piglets, and elucidate their rules of operation. The differential expression of genes related to intestinal development during suckling time was investigated using a porcine genome array. Time sequence profiles were analyzed for the differentially expressed genes to obtain significant expression profiles. Subsequently, the most significant profiles were assayed using Gene Ontology categories, pathway analysis, network analysis, and analysis of gene co-expression to unveil the main biological processes, the significant pathways, and the effective genes, respectively. In addition, quantitative real-time PCR was carried out to verify the reliability of the results of the analysis of the array. The results showed that more than 8000 differential expression transcripts were identified using microarray technology. Among the 30 significant obtained model profiles, profiles 66 and 13 were the most significant. Analysis of profiles 66 and 13 indicated that they were mainly involved in immunity, metabolism, and cell division or proliferation. Among the most effective genes in these two profiles, CN161469, which is similar to methylcrotonoyl-Coenzyme A carboxylase 2 (beta), and U89949.1, which encodes a folate binding protein, had a crucial influence on the co-expression network.

Mapping differentiation kinetics in the mouse retina reveals an extensive period of cell cycle protein expression in post-mitotic newborn neurons

BACKGROUND

Knowledge of gene expression kinetics around neuronal cell birth is required to dissect mechanisms underlying progenitor fate. Here, we timed cell cycle and neuronal protein silencing/induction during cell birth in the developing murine retina.

RESULTS

The pan-cell cycle markers Pcna and Mcm6 were present in the post-mitotic ganglion cell layer. Although confined to the neuroblastic layer (NBL), 6-7% of Ki67(+) cells lacked six progenitor/cell cycle markers, and expressed neuronal markers. To define protein extinction/induction timing, we defined G2/M length throughout retinogenesis, which was typically 1-2 h, but <10% cells took double this time. BrdU-chase analyses revealed that at E12.5, Tubb3 (Tuj1) appeared at M-phase, followed by Calb2 and Dcx at ~2 h, Elavl2/3/4 at ~4 h, and Map2 at ~6 h after cell birth, and these times extended with embryonic age. Strikingly, Ki67 was not extinguished until up to a day after cell cycle exit, coinciding with exit from the NBL and induction of late markers such as Map1b/Uchl1/Rbfox3.

CONCLUSIONS

A minor population of progenitors transits slowly through G2/M and, most importantly, some cell cycle proteins are retained for an unexpectedly long period in post-mitotic neurons. The high-resolution map of cell birth kinetics reported here provides a framework to better define mechanisms that regulate neurogenesis.

Math5 defines the ganglion cell competence state in a subpopulation of retinal progenitor cells exiting the cell cycle

The basic helix-loop-helix (bHLH) transcription factor Math5 (Atoh7) is transiently expressed during early retinal histogenesis and is necessary for retinal ganglion cell (RGC) development. Using nucleoside pulse-chase experiments and clonal analysis, we determined that progenitor cells activate Math5 during or after the terminal division, with progressively later onset as histogenesis proceeds. We have traced the lineage of Math5+ cells using mouse BAC transgenes that express Cre recombinase under strict regulatory control. Quantitative analysis showed that Math5+ progenitors express equivalent levels of Math5 and contribute to every major cell type in the adult retina, but are heavily skewed toward early fates. The Math5>Cre transgene labels 3% of cells in adult retina, including 55% of RGCs. Only 11% of Math5+ progenitors develop into RGCs; the majority become photoreceptors. The fate bias of the Math5 cohort, inferred from the ratio of cone and rod births, changes over time, in parallel with the remaining neurogenic population. Comparable results were obtained using Math5 mutant mice, except that ganglion cells were essentially absent, and late fates were overrepresented within the lineage. We identified Math5-independent RGC precursors in the earliest born (embryonic day 11) retinal cohort, but these precursors require Math5-expressing cells for differentiation. Math5 thus acts permissively to establish RGC competence within a subset of progenitors, but is not sufficient for fate specification. It does not autonomously promote or suppress the determination of non-RGC fates. These data are consistent with progressive and temporal restriction models for retinal neurogenesis, in which environmental factors influence the final histotypic choice.

Dynamic expression of ganglion cell markers in retinal progenitors during the terminal cell cycle

The vertebrate neural retina contains seven major cell types, which arise from a common multipotent progenitor pool. During neurogenesis, these cells stop cycling, commit to a single fate, and differentiate. The mechanism and order of these steps remain unclear. The first-born type of retinal neurons, ganglion cells (RGCs), develop through the actions of Math5 (Atoh7), Brn3b (Pou4f2) and Islet1 (Isl1) factors, whereas inhibitory amacrine and horizontal precursors require Ptf1a for differentiation. We have examined the link between these markers, and the timing of their expression during the terminal cell cycle, by nucleoside pulse-chase analysis in the mouse retina. We show that G2 phase lasts 1-2 h at embryonic (E) 13.5 and E15.5 stages. Surprisingly, we found that cells expressing Brn3b and/or Isl1 were frequently co-labeled with EdU after a short chase (<1 h) in early embryos (<E14), indicating that these factors, which mark committed RGCs, can be expressed during S or G2 phases. However, during late development (>E15), Brn3b and Isl1 were exclusively expressed in post-mitotic cells, even as new RGCs are still generated. In contrast, Ptf1a and amacrine marker AP2α were detected only after terminal mitosis, at all developmental stages. Using a retroviral tracer in embryonic retinal explants (E12-E13), we identified two-cell clones containing paired ganglion cells, consistent with RGC fate commitment prior to terminal mitosis. Thus, although cell cycle exit and fate determination are temporally correlated during retinal neurogenesis, the order of these events varies according to developmental stage and final cell type.

Diversity, phylogeny and expression patterns of Pou and Six homeodomain transcription factors in hydrozoan jellyfish Craspedacusta sowerbyi

Formation of all metazoan bodies is controlled by a group of selector genes including homeobox genes, highly conserved across the entire animal kingdom. The homeobox genes from Pou and Six classes are key members of the regulation cascades determining development of sensory organs, nervous system, gonads and muscles. Besides using common bilaterian models, more attention has recently been targeted at the identification and characterization of these genes within the basal metazoan phyla. Cnidaria as a diploblastic sister group to bilateria with simple and yet specialized organs are suitable models for studies on the sensory organ origin and the associated role of homeobox genes. In this work, Pou and Six homeobox genes, together with a broad range of other sensory-specific transcription factors, were identified in the transcriptome of hydrozoan jellyfish Craspedacusta sowerbyi. Phylogenetic analyses of Pou and Six proteins revealed cnidarian-specific sequence motifs and contributed to the classification of individual factors. The majority of the Craspedacusta sowerbyi Pou and Six homeobox genes are predominantly expressed in statocysts, manubrium and nerve ring, the tissues with sensory and nervous activities. The described diversity and expression patterns of Pou and Six factors in hydrozoan jellyfish highlight their evolutionarily conserved functions. This study extends the knowledge of the cnidarian genome complexity and shows that the transcriptome of hydrozoan jellyfish is generally rich in homeodomain transcription factors employed in the regulation of sensory and nervous functions.

Onecut 1 and Onecut 2 are potential regulators of mouse retinal development

Our current study focuses on the expression of two members of the onecut transcription factor family, Onecut1 (Oc1) and Onecut2 (Oc2), in the developing mouse retina. By immunofluorescence staining, we found that Oc1 and Oc2 had very similar expression patterns throughout retinal development. Both factors started to be expressed in the retina at around embryonic day (E) 11.5. At early stages (E11.5 and E12.5), they were expressed in both the neuroblast layer (NBL) and ganglion cell layer (GCL). As development progressed (from E14.5 to postnatal day [P] 0), expression diminished in the retinal progenitor cells and became more restricted to the GCL. By P5, Oc1 and Oc2 were expressed at very low levels in the GCL. By co-labeling with transcription factors known to be involved in retinal ganglion cell (RGC) development, we found that Oc1 and Oc2 had extensive overlap with Math5 in the NBL, and that they completely overlapped with Pou4f2 and Isl1 in the GCL, but only partially in the NBL. Co-labeling of Oc1 with cell cycle markers confirmed that Oc1 was expressed in both proliferating retinal progenitors and postmitotic retinal cells. In addition, we demonstrated that expression of Oc1 and Oc2 did not require Math5, Isl1, or Pou4f2. Thus, Oc1 and Oc2 may regulate the formation of RGCs in a pathway independent of Math5, Pou4f2, and Isl1. Furthermore, we showed that Oc1 and Oc2 were expressed in both developing and mature horizontal cells (HCs). Therefore the two factors may also function in the genesis and maintenance of HCs.

Otic mesenchyme cells regulate spiral ganglion axon fasciculation through a Pou3f4/EphA4 signaling pathway

Peripheral axons from auditory spiral ganglion neurons (SGNs) form an elaborate series of radially and spirally oriented projections that interpret complex aspects of the auditory environment. However, the developmental processes that shape these axon tracts are largely unknown. Radial bundles are comprised of dense SGN fascicles that project through otic mesenchyme to form synapses within the cochlea. Here, we show that radial bundle fasciculation and synapse formation are disrupted when Pou3f4 (DFNX2) is deleted from otic mesenchyme. Further, we demonstrate that Pou3f4 binds to and directly regulates expression of Epha4, Epha4⁻/⁻ mice present similar SGN defects, and exogenous EphA4 promotes SGN fasciculation in the absence of Pou3f4. Finally, Efnb2 deletion in SGNs leads to similar fasciculation defects, suggesting that ephrin-B2/EphA4 interactions are critical during this process. These results indicate a model whereby Pou3f4 in the otic mesenchyme establishes an Eph/ephrin-mediated fasciculation signal that promotes inner radial bundle formation.

Human Müller glia with stem cell characteristics differentiate into retinal ganglion cell (RGC) precursors in vitro and partially restore RGC function in vivo following transplantation

Müller glia with stem cell characteristics have been identified in the adult human eye, and although there is no evidence that they regenerate retina in vivo, they can be induced to grow and differentiate into retinal neurons in vitro. We differentiated human Müller stem cells into retinal ganglion cell (RGC) precursors by stimulation with fibroblast growth factor 2 together with NOTCH inhibition using the γ-secretase inhibitor N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester (DAPT). Differentiation into RGC precursors was confirmed by gene and protein expression analysis, changes in cytosolic [Ca(2+)] in response to neurotransmitters, and green fluorescent protein (GFP) expression by cells transduced with a transcriptional BRN3b-GFP reporter vector. RGC precursors transplanted onto the inner retinal surface of Lister hooded rats depleted of RGCs by N-methyl-d-aspartate aligned onto the host RGC layer at the site of transplantation but did not extend long processes toward the optic nerve. Cells were observed extending processes into the RGC layer and expressing RGC markers in vivo. This migration was observed only when adjuvant anti-inflammatory and matrix degradation therapy was used for transplantation. RGC precursors induced a significant recovery of RGC function in the transplanted eyes as determined by improvement of the negative scotopic threshold response of the electroretinogram (indicative of RGC function). The results suggest that transplanted RGC precursors may be capable of establishing local interneuron synapses and possibly release neurotrophic factors that facilitate recovery of RGC function. These cells constitute a promising source of cells for cell-based therapies to treat retinal degenerative disease caused by RGC dysfunction.

Conditional control of gene expression in the mouse retina

Conditional knockout is a powerful research tool for specific deletion of target genes, especially for the genes that are widely expressed and developmentally regulated. The development of the retina involves multiple intrinsic and extrinsic factors, many are required for embryonic development or expressed in multiple tissue or cell types. To study their roles in a spatial- or temporal-specific fashion, Cre/loxP-based gene-targeting approach has been utilized successfully. This chapter describes the methodology of conditional knockout approach in studying the development of the retina, using LIM homeobox gene Isl1 as an example. It provides details on targeting vector design and construction, introducing the vector into embryonic stem (ES) cell, screening ES cell for the recombination events, injecting ES cells, and generating chimeric and null mice. It also discusses the current issues in the use of Cre/loxP-based gene-targeting approach.

Roles of homeobox genes in retinal ganglion cell differentiation and axonal guidance

In this review, we have described four major families of homeobox genes which play important roles in RGC differentiation as well as axonal pathfinding. The mechanism underlying how these HD TFs affect axonal pathfinding is not entirely known. One possibility is that the downstream targets directly regulated by these HD TF are responsible for axonal guidance. Examples of this are the repression of EphB1 by Isl2, and Vax1/Vax2 regulation of EphB2/EphB3 expression. The roles of Dlx homeobox genes in RGC axonal guidance have not yet been reported. However, in the mouse telencephalon, Dlx1 and Dlx2 promote the tangential migration of GABAergic interneurons by repressing axonal growth (Cobos et al. 2007) and inhibiting Neuropilin-2 expression (Le et al. 2007). It is possible that the genetic program defining RGC identity also encodes a unique "sensory" network for their axons, determining how and where RGC axons respond to guidance cues en route to CNS targets.

Brn3a and Islet1 act epistatically to regulate the gene expression program of sensory differentiation

The combinatorial expression of transcription factors frequently marks cellular identity in the nervous system, yet how these factors interact to determine specific neuronal phenotypes is not well understood. Sensory neurons of the trigeminal ganglion (TG) and dorsal root ganglia (DRG) coexpress the homeodomain transcription factors Brn3a and Islet1, and past work has revealed partially overlapping programs of gene expression downstream of these factors. Here we examine sensory development in Brn3a/Islet1 double knock-out (DKO) mice. Sensory neurogenesis and the formation of the TG and DRG occur in DKO embryos, but the DRG are dorsally displaced, and the peripheral projections of the ganglia are markedly disturbed. Sensory neurons in DKO embryos show a profound loss of all early markers of sensory subtypes, including the Ntrk neurotrophin receptors, and the runt-family transcription factors Runx1 and Runx3. Examination of global gene expression in the E12.5 DRG of single and double mutant embryos shows that Brn3a and Islet1 are together required for nearly all aspects of sensory-specific gene expression, including several newly identified sensory markers. On a majority of targets, Brn3a and Islet1 exhibit negative epistasis, in which the effects of the individual knock-out alleles are less than additive in the DKO. Smaller subsets of targets exhibit positive epistasis, or are regulated exclusively by one factor. Brn3a/Islet1 double mutants also fail to developmentally repress neurogenic bHLH genes, and in vivo chromatin immunoprecipitation shows that Islet1 binds to a known Brn3a-regulated enhancer in the neurod4 gene, suggesting a mechanism of interaction between these genes.

Mouse retinal development: a dark horse model for systems biology research

The developing retina is an excellent model to study cellular fate determination and differentiation in the context of a complex tissue. Over the last decade, many basic principles and key genes that underlie these processes have been experimentally identified. In this review, we construct network models to summarize known gene interactions that underlie determination and fundamentally affect differentiation of each retinal cell type. These networks can act as a scaffold to assemble subsequent discoveries. In addition, these summary networks provide a rational segue to systems biology approaches necessary to understand the many events leading to appropriate cellular determination and differentiation in the developing retina and other complex tissues.

COL4A1 mutations cause ocular dysgenesis, neuronal localization defects, and myopathy in mice and Walker-Warburg syndrome in humans

Muscle-eye-brain disease (MEB) and Walker Warburg Syndrome (WWS) belong to a spectrum of autosomal recessive diseases characterized by ocular dysgenesis, neuronal migration defects, and congenital muscular dystrophy. Until now, the pathophysiology of MEB/WWS has been attributed to alteration in dystroglycan post-translational modification. Here, we provide evidence that mutations in a gene coding for a major basement membrane protein, collagen IV alpha 1 (COL4A1), are a novel cause of MEB/WWS. Using a combination of histological, molecular, and biochemical approaches, we show that heterozygous Col4a1 mutant mice have ocular dysgenesis, neuronal localization defects, and myopathy characteristic of MEB/WWS. Importantly, we identified putative heterozygous mutations in COL4A1 in two MEB/WWS patients. Both mutations occur within conserved amino acids of the triple-helix-forming domain of the protein, and at least one mutation interferes with secretion of the mutant proteins, resulting instead in intracellular accumulation. Expression and posttranslational modification of dystroglycan is unaltered in Col4a1 mutant mice indicating that COL4A1 mutations represent a distinct pathogenic mechanism underlying MEB/WWS. These findings implicate a novel gene and a novel mechanism in the etiology of MEB/WWS and expand the clinical spectrum of COL4A1-associated disorders.

Muscle-eye-brain disease (MEB) and Walker-Warburg Syndrome (WWS) are devastating childhood diseases that belong to a subgroup of congenital muscular dystrophies (CMDs) characterized by ocular dysgenesis, neuronal migration defects, and congenital myopathy. Genetic studies have revealed a number of genes involved in the etiology of CMDs, and subsequent studies show that alterations in dystroglycan glycosylation underlie MEB/WWS. However, over half of MEB/WWS patients do not have mutations in known genes encoding glycosyltransferases, suggesting that other genes are involved. Here, we describe a novel and genetically complex mouse model for MEB/WWS and identify putative heterozygous mutations in COL4A1 in two MEB/WWS patients. We identify a novel gene implicated in the etiology of MEB/WWS, provide evidence of mechanistic heterogeneity for this subgroup of congenital muscular dystrophies, and develop an assay to test the functional significance of putative COL4A1 mutations. Our findings represent the first evidence for a dominant mutation leading to MEB/WWS–like diseases and expand the spectrum of clinical disorders resulting from Col4a1/COL4A1 mutations.

Genetic modulation of horizontal cell number in the mouse retina

Neuronal populations display conspicuous variability in their size among individuals, but the genetic sources of this variation are largely undefined. We demonstrate a large and highly heritable variation in neuron number within the mouse retina, affecting a critical population of interneurons, the horizontal cells. Variation in the size of this population maps to the distal end of chromosome (Chr) 13, a region homologous to human Chr 5q11.1-11.2. This region contains two genes known to modulate retinal cell number. Using conditional knock-out mice, we demonstrate that one of these genes, the LIM homeodomain gene Islet-1 (Isl1), plays a role in regulating horizontal cell number. Genetic differences in Isl1 expression are high during the period of horizontal cell production, and cis-regulation of Isl1 expression within the retina is demonstrated directly. We identify a single nucleotide polymorphism in the 5' UTR of Isl1 that creates an E-box sequence as a candidate causal variant contributing to this variation in horizontal cell number.

Isl1 is required for multiple aspects of motor neuron development

The LIM homeodomain transcription factor Islet1 (Isl1) is expressed in multiple organs and plays essential roles during embryogenesis. Isl1 is required for the survival and specification of spinal cord motor neurons. Due to early embryonic lethality and loss of motor neurons, the role of Isl1 in other aspects of motor neuron development remains unclear. In this study, we generated Isl1 mutant mouse lines expressing graded doses of Isl1. Our study has revealed essential roles of Isl1 in multiple aspects of motor neuron development, including motor neuron cell body localization, motor column formation and axon growth. In addition, Isl1 is required for survival of cranial ganglia neurons.

Tissue specific characterisation of Lim-kinase 1 expression during mouse embryogenesis

The Lim-kinase (LIMK) proteins are important for the regulation of the actin cytoskeleton, in particular the control of actin nucleation and depolymerisation via regulation of cofilin, and hence may control a large number of processes during development, including cell tensegrity, migration, cell cycling, and axon guidance. LIMK1/LIMK2 knockouts disrupt spinal cord morphogenesis and synapse formation but other tissues and developmental processes that require LIMK are yet to be fully determined. To identify tissues and cell-types that may require LIMK, we characterised the pattern of LIMK1 protein during mouse embryogenesis. We showed that LIMK1 displays an expression pattern that is temporally dynamic and tissue-specific. In several tissues LIMK1 is detected in cell-types that also express Wilms' tumour protein 1 and that undergo transitions between epithelial and mesenchymal states, including the pleura, epicardium, kidney nephrons, and gonads. LIMK1 was also found in a subset of cells in the dorsal retina, and in mesenchymal cells surrounding the peripheral nerves. This detailed study of the spatial and temporal expression of LIMK1 shows that LIMK1 expression is more dynamic than previously reported, in particular at sites of tissue-tissue interactions guiding multiple developmental processes.

Neuronal transcriptional repressor REST suppresses an Atoh7-independent program for initiating retinal ganglion cell development

As neuronal progenitors differentiate into neurons, they acquire a unique set of transcription factors. The transcriptional repressor REST prevents progenitors from undergoing differentiation. Notably, REST binding sites are often associated with retinal ganglion cell (RGC) genes whose expression in the retina is positively controlled by Atoh7, a factor essential for RGC formation. The key regulators that enable a retinal progenitor cell (RPC) to commit to an RGC fate have not been identified. We show here that REST suppresses RGC gene expression in RPCs. REST inactivation causes aberrant expression of RGC transcription factors in proliferating RPCs, independent of Atoh7, resulting in increased RGC formation. Strikingly, inactivating REST in Atoh7-null retinas restores transcription factor expression, which partially activates downstream RGC genes but is insufficient to prevent RGC loss. Our results demonstrate an Atoh7-independent program for initial activation of RGC genes and suggest a novel role for REST in preventing premature expression in RPCs.

Overlapping spatiotemporal patterns of regulatory gene expression are required for neuronal progenitors to specify retinal ganglion cell fate

Retinal progenitor cells (RPCs) are programmed early in development to acquire the competence for specifying the seven retinal cell types. Acquiring competence is a complex spatiotemporal process that is still only vaguely understood. Here, our objective was to more fully understand the mechanisms by which RPCs become competent for specifying a retinal ganglion cell (RGC) fate. RGCs are the first retinal cell type to differentiate and their abnormal development leads to apoptosis and optic nerve degeneration. Previous work demonstrated that the paired domain factor Pax6 and the bHLH factor Atoh7 are required for RPCs to specify RGCs. RGC commitment is marked by the expression of the Pou domain factor Pou4f2 and the Lim domain factor Isl1. We show that three RPC subpopulations can specify RGCs: Atoh7-expressing RPCs, Neurod1-expressing RPCs, and Atoh7-Neurod1-expressing RPCs. All three RPC subpopulations were highly interspersed throughout retinal development, although each subpopulation maintained a distinct temporal pattern. Most, but not all, RPCs from each subpopulation were postmitotic. Atoh7-Neurod1 double knockout mice were generated and double-mutant retinas revealed an unexpected role for Neurod1 in specifying RGC fate. We conclude that RPCs have a complex regulatory gene expression program in which they acquire competence using highly integrated mechanisms.

Loss of LMO4 in the retina leads to reduction of GABAergic amacrine cells and functional deficits

BACKGROUND

LMO4 is a transcription cofactor expressed during retinal development and in amacrine neurons at birth. A previous study in zebrafish reported that morpholino RNA ablation of one of two related genes, LMO4b, increases the size of eyes in embryos. However, the significance of LMO4 in mammalian eye development and function remained unknown since LMO4 null mice die prior to birth.

METHODOLOGY/PRINCIPAL FINDINGS

We observed the presence of a smaller eye and/or coloboma in ∼40% LMO4 null mouse embryos. To investigate the postnatal role of LMO4 in retinal development and function, LMO4 was conditionally ablated in retinal progenitor cells using the Pax6 alpha-enhancer Cre/LMO4flox mice. We found that these mice have fewer Bhlhb5-positive GABAergic amacrine and OFF-cone bipolar cells. The deficit appears to affect the postnatal wave of Bhlhb5+ neurons, suggesting a temporal requirement for LMO4 in retinal neuron development. In contrast, cholinergic and dopaminergic amacrine, rod bipolar and photoreceptor cell numbers were not affected. The selective reduction in these interneurons was accompanied by a functional deficit revealed by electroretinography, with reduced amplitude of b-waves, indicating deficits in the inner nuclear layer of the retina.

CONCLUSIONS/SIGNIFICANCE

Inhibitory GABAergic interneurons play a critical function in controlling retinal image processing, and are important for neural networks in the central nervous system. Our finding of an essential postnatal function of LMO4 in the differentiation of Bhlhb5-expressing inhibitory interneurons in the retina may be a general mechanism whereby LMO4 controls the production of inhibitory interneurons in the nervous system.

Early B-cell factors are required for specifying multiple retinal cell types and subtypes from postmitotic precursors

The establishment of functional retinal circuits in the mammalian retina depends critically on the proper generation and assembly of six classes of neurons, five of which consist of two or more subtypes that differ in morphologies, physiological properties, and/or sublaminar positions. How these diverse neuronal types and subtypes arise during retinogenesis still remains largely to be defined at the molecular level. Here we show that all four family members of the early B-cell factor (Ebf) helix-loop-helix transcription factors are similarly expressed during mouse retinogenesis in several neuronal types and subtypes including ganglion, amacrine, bipolar, and horizontal cells, and that their expression in ganglion cells depends on the ganglion cell specification factor Brn3b. Misexpressed Ebfs bias retinal precursors toward the fates of non-AII glycinergic amacrine, type 2 OFF-cone bipolar and horizontal cells, whereas a dominant-negative Ebf suppresses the differentiation of these cells as well as ganglion cells. Reducing Ebf1 expression by RNA interference (RNAi) leads to an inhibitory effect similar to that of the dominant-negative Ebf, effectively neutralizes the promotive effect of wild-type Ebf1, but has no impact on the promotive effect of an RNAi-resistant Ebf1. These data indicate that Ebfs are both necessary and sufficient for specifying non-AII glycinergic amacrine, type 2 OFF-cone bipolar and horizontal cells, whereas they are only necessary but not sufficient for specifying ganglion cells; and further suggest that Ebfs may coordinate and cooperate with other retinogenic factors to ensure proper specification and differentiation of diverse retinal cell types and subtypes.

Development of the retina and optic pathway

Our understanding of the development of the retina and visual pathways has seen enormous advances during the past 25years. New imaging technologies, coupled with advances in molecular biology, have permitted a fuller appreciation of the histotypical events associated with proliferation, fate determination, migration, differentiation, pathway navigation, target innervation, synaptogenesis and cell death, and in many instances, in understanding the genetic, molecular, cellular and activity-dependent mechanisms underlying those developmental changes. The present review considers those advances associated with the lineal relationships between retinal nerve cells, the production of retinal nerve cell diversity, the migration, patterning and differentiation of different types of retinal nerve cells, the determinants of the decussation pattern at the optic chiasm, the formation of the retinotopic map, and the establishment of ocular domains within the thalamus.

Molecular mechanisms of pituitary organogenesis: In search of novel regulatory genes

Defects in pituitary gland organogenesis are sometimes associated with congenital anomalies that affect head development. Lesions in transcription factors and signaling pathways explain some of these developmental syndromes. Basic research studies, including the characterization of genetically engineered mice, provide a mechanistic framework for understanding how mutations create the clinical characteristics observed in patients. Defects in BMP, WNT, Notch, and FGF signaling pathways affect induction and growth of the pituitary primordium and other organ systems partly by altering the balance between signaling pathways. The PITX and LHX transcription factor families influence pituitary and head development and are clinically relevant. A few later-acting transcription factors have pituitary-specific effects, including PROP1, POU1F1 (PIT1), and TPIT (TBX19), while others, such as NeuroD1 and NR5A1 (SF1), are syndromic, influencing development of other endocrine organs. We conducted a survey of genes transcribed in developing mouse pituitary to find candidates for cases of pituitary hormone deficiency of unknown etiology. We identified numerous transcription factors that are members of gene families with roles in syndromic or non-syndromic pituitary hormone deficiency. This collection is a rich source for future basic and clinical studies.

Gene expression and differentiation characteristics in mice E13.5 and E17.5 neural retinal progenitors

PURPOSE

Retinal progenitor cells (RPCs) are the most valuable seed cells in replacement therapy for neural retinal diseases. The competence of RPCs changes with retinal development. Gene expression plays a fundamental role in determining the competence. To improve the selection of the right-timing RPCs for replacement therapy, we compared the gene expression between embryonic day (E) 13.5 and E17.5 RPCs and further explored their gene expression and differentiation capacity in vitro.

METHODS

Timed-pregnant E13.5 and E17.5 RPCs were freshly harvested and cultured in proliferation conditions for 4 days and then in differentiation conditions for 8 days. At different time points, the expression of key genes involved in retinal development was investigated by quantitative reverse transcription-PCR or immunofluorescence.

RESULTS

The expression of 14 key genes involved in retinal development was investigated in freshly harvested E13.5 and E17.5 RPCs. The freshly harvested E13.5 RPCs showed a high expression of retinal ganglion cell (RGC)-related genes, including Math5, Brn3b, Islet1, and Nfl, while the freshly harvested E17.5 RPCs displayed a high expression for Nrl, GFAP, and Thy1, the key genes involved in rod photoreceptor development, glial cell development, and synaptogenesis, respectively. During proliferation culture in vitro, the gene expression changed dramatically in both RPCs. After the 4 days of proliferation culture, the expression levels of most genes (11 of the 14 genes) in E13.5 RPCs came close to those in the freshly harvested E17.5 RPCs. Differentiation of RPCs in vitro was verified by the significant decrease in Nestin expression and BruU incorporation efficiency. After the 8 days of differentiation in vitro, the expression level of RGC-related genes (Math5, Brn3b, and Islet1) was still significantly higher in E13.5 RPCs than in E17.5 RPCs. In contrast, the expression level of Nrl and GFAP was significantly higher in E17.5 RPCs than in E13.5 RPCs. In morphology, the differentiated E13.5 RPCs displayed more robust process outgrowth than did the differentiated E17.5 RPCs. Immunofluorescence showed that, after the 8 days of differentiation, E13.5 RPCs contained more Brn3b- and Map2-positive cells, while E17.5 RPCs contained more GFAP-, GS-, and Rhodopsin-positive cells.

CONCLUSIONS

The results implied that E13.5 RPCs might be a better choice for RGC replacement therapy, while E17.5 RPCs might be better for photoreceptor replacement therapy. The duration of in vitro culture should be timed, since the expression of key genes kept changing in the proliferating RPCs.

Islet-to-LMO stoichiometries control the function of transcription complexes that specify motor neuron and V2a interneuron identity

LIM transcription factors bind to nuclear LIM interactor (Ldb/NLI/Clim) in specific ratios to form higher-order complexes that regulate gene expression. Here we examined how the dosage of LIM homeodomain proteins Isl1 and Isl2 and LIM-only protein Lmo4 influences the assembly and function of complexes involved in the generation of spinal motor neurons (MNs) and V2a interneurons (INs). Reducing the levels of Islet proteins using a graded series of mutations favored V2a IN differentiation at the expense of MN formation. Although LIM-only proteins (LMOs) are predicted to antagonize the function of Islet proteins, we found that the presence or absence of Lmo4 had little influence on MN or V2a IN specification. We did find, however, that the loss of MNs resulting from reduced Islet levels was rescued by eliminating Lmo4, unmasking a functional interaction between these proteins. Our findings demonstrate that MN and V2a IN fates are specified by distinct complexes that are sensitive to the relative stoichiometries of the constituent factors and we present a model to explain how LIM domain proteins modulate these complexes and, thereby, this binary-cell-fate decision.

Distinct effects of Hedgehog signaling on neuronal fate specification and cell cycle progression in the embryonic mouse retina

Cell-extrinsic signals can profoundly influence the production of various neurons from common progenitors. Yet mechanisms by which extrinsic signals coordinate progenitor cell proliferation, cell cycle exit, and cell fate choices are not well understood. Here, we address whether Hedgehog (Hh) signals independently regulate progenitor proliferation and neuronal fate decisions in the embryonic mouse retina. Conditional ablation of the essential Hh signaling component Smoothened (Smo) in proliferating progenitors, rather than in nascent postmitotic neurons, leads to a dramatic increase of retinal ganglion cells (RGCs) and a mild increase of cone photoreceptor precursors without significantly affecting other early-born neuronal cell types. In addition, Smo-deficient progenitors exhibit aberrant expression of cell cycle regulators and delayed G(1)/S transition, especially during the late embryonic stages, resulting in a reduced progenitor pool by birth. Deficiency in Smo function also causes reduced expression of the basic helix-loop-helix transcription repressor Hes1 and preferential elevation of the proneural gene Math5. In Smo and Math5 double knock-out mutants, the enhanced RGC production observed in Smo-deficient retinas is abolished, whereas defects in the G(1)/S transition persist, suggesting that Math5 mediates the Hh effect on neuronal fate specification but not on cell proliferation. These findings demonstrate that Hh signals regulate progenitor pool expansion primarily by promoting cell cycle progression and influence cell cycle exit and neuronal fates by controlling specific proneural genes. Together, these distinct cellular effects of Hh signaling in neural progenitor cells coordinate a balanced production of diverse neuronal cell types.

BARHL2 differentially regulates the development of retinal amacrine and ganglion neurons

Through transcriptional regulations, the BarH family of homeodomain proteins play essential roles in cell fate specification, cell differentiation, migration, and survival. Barhl2, a member of the Barh gene family, is expressed in retinal ganglion cells (RGCs), amacrine cells (ACs), and horizontal cells. Here, to investigate the role of Barhl2 in retinal development, Barhl2-deficient mice were generated. Analysis of AC subtypes in Barhl2-deficient retinas suggests that Barhl2 plays a critical role in AC subtype determination. A significant reduction of glycinergic and GABAergic ACs with a substantial increase in the number of cholinergic ACs was observed in Barhl2-null retinas. Barhl2 is also critical for the development of a normal complement of RGCs. Barhl2 deficiency resulted in a 35% increase in RGCs undergoing apoptosis during development. Genetic analysis revealed that Barhl2 functions downstream of the Atoh7-Pou4f3 regulatory pathway and regulates the maturation and/or survival of RGCs. Thus, BARHL2 appears to have numerous roles in retinal development, including regulating neuronal subtype specification, differentiation, and survival.