Pax6 Interacts with cVax and Tbx5 to Establish the Dorsoventral Boundary of the Developing Eye

Directed differentiation of human embryonic stem cells into conjunctival epithelial cells

Severe conjunctival damage can lead to extensive ocular cicatrisation, fornix shortening, and even ocular surface failure, resulting in significant vision impairment. Conjunctival reconstruction is the primary therapeutic strategy for these clinical conjunctival diseases. However, there have been limited studies on induced differentiation of conjunctival epithelial cells derived from stem cells. In this study, we established a chemical defined differentiation protocol from human embryonic stem cells (hESCs) into conjunctival epithelial cells. hES cell line H1 was used for differentiation, and RT-qPCR, immunofluorescence staining, Periodic-acid-Schiff staining (PAS), and transcriptome analysis were employed to identify the differentiated cells. Here, to imitate the development of the vertebrate conjunctiva, hESCs were induced using a three-step process involving first chetomin was used to induce ocular surface ectoderm, then nicotinamide was used to induce ocular surface epithelial progenitor cells, and finally epidermal growth factor, keratinocyte growth factor and other factors were used to differentiate mature conjunctival epithelial cells. hESC-derived conjunctival epithelial cells expressed mature conjunctival epithelial lineage markers (including PAX6, P63, K13). The presence of goblet cells was confirmed by positive PAS. Transcriptome analysis revealed that hESC-derived conjunctival epithelial cells possessed a more naïve phenotype, and exhibited greater proliferation capacity compared to mature human conjunctival epithelial cells, suggesting their potential as alternative seed cells for conjunctival reconstruction.

3D exploration of gene expression in chicken embryos through combined RNA fluorescence in situ hybridization, immunofluorescence, and clearing

BACKGROUND

Fine characterization of gene expression patterns is crucial to understand many aspects of embryonic development. The chicken embryo is a well-established and valuable animal model for developmental biology. The period spanning from the third to sixth embryonic days (E3 to E6) is critical for many organ developments. Hybridization chain reaction RNA fluorescent in situ hybridization (HCR RNA-FISH) enables multiplex RNA detection in thick samples including embryos of various animal models. However, its use is limited by tissue opacity.

RESULTS

We optimized HCR RNA-FISH protocol to efficiently label RNAs in whole mount chicken embryos from E3.5 to E5.5 and adapted it to ethyl cinnamate (ECi) tissue clearing. We show that light sheet imaging of HCR RNA-FISH after ECi clearing allows RNA expression analysis within embryonic tissues with good sensitivity and spatial resolution. Finally, whole mount immunofluorescence can be performed after HCR RNA-FISH enabling as exemplified to assay complex spatial relationships between axons and their environment or to monitor GFP electroporated neurons.

CONCLUSIONS

We could extend the use of HCR RNA-FISH to older chick embryos by optimizing HCR RNA-FISH and combining it with tissue clearing and 3D imaging. The integration of immunostaining makes possible to combine gene expression with classical cell markers, to correlate expressions with morphological differentiation and to depict gene expressions in gain or loss of function contexts. Altogether, this combined procedure further extends the potential of HCR RNA-FISH technique for chicken embryology.

Modeling a variant of unknown significance in the Drosophila ortholog of the human cardiogenic gene NKX2.5

Sequencing of human genome samples has unearthed genetic variants for which functional testing is necessary to validate their clinical significance. We used the Drosophila system to analyze a variant of unknown significance in the human congenital heart disease gene NKX2.5 (also known as NKX2-5). We generated an R321N allele of the NKX2.5 ortholog tinman (tin) to model a human K158N variant and tested its function in vitro and in vivo. The R321N Tin isoform bound poorly to DNA in vitro and was deficient in activating a Tin-dependent enhancer in tissue culture. Mutant Tin also showed a significantly reduced interaction with a Drosophila T-box cardiac factor named Dorsocross1. We generated a tinR321N allele using CRISPR/Cas9, for which homozygotes were viable and had normal heart specification, but showed defects in the differentiation of the adult heart that were exacerbated by further loss of tin function. We propose that the human K158N variant is pathogenic through causing a deficiency in DNA binding and a reduced ability to interact with a cardiac co-factor, and that cardiac defects might arise later in development or adult life.

Using Drosophila to model a variant of unknown significance in the human cardiogenic gene Nkx2.5

Sequencing of human genome samples has unearthed genetic variants for which functional testing is necessary to validate their clinical significance. We used the Drosophila system to analyze a variant of unknown significance in the human congenital heart disease gene, Nkx2 . 5 . We generated an R321N allele of the Nkx2 . 5 ortholog tinman ( tin ) to model a human K158N variant and tested its function in vitro and in vivo. The R321N Tin isoform bound poorly to DNA in vitro and was deficient in activating a Tin-dependent enhancer in tissue culture. Mutant Tin also showed a significantly reduced interaction with a Drosophila Tbox cardiac factor named Dorsocross1. We generated a tin R321N allele using CRISPR/Cas9, for which homozygotes were viable and had normal heart specification, but showed defects in the differentiation of the adult heart that were exacerbated by further loss of tin function. We conclude that the human K158N mutation is likely pathogenic through causing both a deficiency in DNA binding and a reduced ability to interact with a cardiac cofactor, and that cardiac defects might arise later in development or adult life.

SNP located in an AluJb repeat downstream of TMCO1, rs4657473, is protective for POAG in African Americans

AIMS

To determine the association of single nucleotide polymorphisms (SNPs) downstream from the TMCO1 gene with primary open-angle glaucoma (POAG) in African Americans (AA).

METHODS

AA subjects were recruited for the Primary Open-Angle African American Glaucoma Genetics (POAAGG) study from the Scheie Eye Institute and its satellite sites in Philadelphia. A region containing an AluJb repeat and seven SNPs, including rs4656461 near the TMCO1 gene, were PCR-Sanger sequenced from POAAGG cases (n=1537) and controls (n=1570). Association between POAG and SNPs near TMCO1 was investigated by logistic regression analysis. Phenotypic trait associations with these SNPs were assessed by analysis of variance. Electrophoretic mobility shift assay (EMSA) was performed to assess the affinity of human T-box 5 (TBX5) protein for a predicted binding motif in the TMCO1 region. Dual Luciferase assays were performed by transfecting recombinant plasmids containing the region surrounding the above SNPs in HEK293T and trabecular meshwork cells.

RESULTS

The SNP rs4657473 (C>T) was associated with POAG; the TT genotype was protective (OR 0.20, 95% CI 0.09 to 0.42; p<0.001). No significant associations were found between the TMCO1 variants and phenotypic traits. EMSA confirmed the affinity of TBX5 for a predicted binding motif containing TMCO1 SNP rs4657475. Luciferase assays demonstrated a regulatory function for the genomic region around SNP rs4656561, located within AluJb repeat.

CONCLUSION

Our results demonstrate that a SNP downstream of TMCO1, rs4657473, is associated with POAG in an AA population. Our studies suggest a regulatory role for the previously POAG-associated locus near the TMCO1 gene that may affect gene expression.

Eye organogenesis: A hierarchical view of ocular development

This chapter provides an overview of the early developmental origins of six ocular tissues: the cornea, lens, ciliary body, iris, neural retina, and retina pigment epithelium. Many of these tissue types are concurrently specified and undergo a complex set of morphogenetic movements that facilitate their structural interconnection. Within the context of vertebrate eye organogenesis, we also discuss the genetic hierarchies of transcription factors and signaling pathways that regulate growth, patterning, cell type specification and differentiation.

Optomotor-blind negatively regulates Drosophila eye development by blocking Jak/STAT signaling

Organ formation requires a delicate balance of positive and negative regulators. In Drosophila eye development, wingless (wg) is expressed at the lateral margins of the eye disc and serves to block retinal development. The T-box gene optomotor-blind (omb) is expressed in a similar pattern and is regulated by Wg. Omb mediates part of Wg activity in blocking eye development. Omb exerts its function primarily by blocking cell proliferation. These effects occur predominantly in the ventral margin. Our results suggest that the primary effect of Omb is the blocking of Jak/STAT signaling by repressing transcription of upd which encodes the Jak receptor ligand Unpaired.

Transcriptome analysis of newt lens regeneration reveals distinct gradients in gene expression patterns

Regeneration of the lens in newts is quite a unique process. The lens is removed in its entirety and regeneration ensues from the pigment epithelial cells of the dorsal iris via transdifferentiation. The same type of cells from the ventral iris are not capable of regenerating a lens. It is, thus, expected that differences between dorsal and ventral iris during the process of regeneration might provide important clues pertaining to the mechanism of regeneration. In this paper, we employed next generation RNA-seq to determine gene expression patterns during lens regeneration in Notophthalmus viridescens. The expression of more than 38,000 transcripts was compared between dorsal and ventral iris. Although very few genes were found to be dorsal- or ventral-specific, certain groups of genes were up-regulated specifically in the dorsal iris. These genes are involved in cell cycle, gene regulation, cytoskeleton and immune response. In addition, the expression of six highly regulated genes, TBX5, FGF10, UNC5B, VAX2, NR2F5, and NTN1, was verified using qRT-PCR. These graded gene expression patterns provide insight into the mechanism of lens regeneration, the markers that are specific to dorsal or ventral iris, and layout a map for future studies in the field.

Pax6: a multi-level regulator of ocular development

Eye development has been a paradigm for the study of organogenesis, from the demonstration of lens induction through epithelial tissue morphogenesis, to neuronal specification and differentiation. The transcription factor Pax6 has been shown to play a key role in each of these processes. Pax6 is required for initiation of developmental pathways, patterning of epithelial tissues, activation of tissue-specific genes and interaction with other regulatory pathways. Herein we examine the data accumulated over the last few decades from extensive analyses of biochemical modules and genetic manipulation of the Pax6 gene. Specifically, we describe the regulation of Pax6's expression pattern, the protein's DNA-binding properties, and its specific roles and mechanisms of action at all stages of lens and retinal development. Pax6 functions at multiple levels to integrate extracellular information and execute cell-intrinsic differentiation programs that culminate in the specification and differentiation of a distinct ocular lineage.

Compartmentalization of vertebrate optic neuroephithelium: external cues and transcription factors

The vertebrate eye is a laterally extended structure of the forebrain. It develops through a series of events, including specification and regionalization of the anterior neural plate, evagination of the optic vesicle (OV), and development of three distinct optic structures: the neural retina (NR), optic stalk (OS), and retinal pigment epithelium (RPE). Various external signals that act on the optic neuroepithelium in a spatial- and temporal-specific manner control the fates of OV subdomains by inducing localized expression of key transcription factors. Investigating the mechanisms underlying compartmentalization of these distinct optic neuroepithelium-derived tissues is therefore not only important from the standpoint of accounting for vertebrate eye morphogenesis, it is also helpful for understanding the fundamental basis of fate determination of other neuroectoderm- derived tissues. This review focuses on the molecular signatures of OV subdomains and the external factors that direct the development of tissues originating from the OV.

VAX1 mutation associated with microphthalmia, corpus callosum agenesis, and orofacial clefting: the first description of a VAX1 phenotype in humans

Vax1 and Vax2 have been implicated in eye development and the closure of the choroid fissure in mice and zebrafish. We sequenced the coding exons of VAX1 and VAX2 in 70 patients with anophthalmia/microphthalmia (A/M). In VAX1, we observed homozygosity for two successive nucleotide substitutions c.453G>A and c.454C>A, predicting p.Arg152Ser, in a proband of Egyptian origin with microphthalmia, small optic nerves, cleft lip/palate, and corpus callosum agenesis. This mutation affects an invariant residue in the homeodomain of VAX1 and was absent from 96 Egyptian controls. It is likely that the mutation results in a loss of function, as the mutation results in a phenotype similar to the Vax1 homozygous null mouse. We did not identify any mutations in VAX2. This is the first description of a phenotype associated with a VAX1 mutation in humans and establishes VAX1 as a new causative gene for A/M.

Brachyury and related Tbx proteins interact with the Mixl1 homeodomain protein and negatively regulate Mixl1 transcriptional activity

Mixl1 is a homeodomain transcription factor required for mesoderm and endoderm patterning during mammalian embryogenesis. Despite its crucial function in development, co-factors that modulate the activity of Mixl1 remain poorly defined. Here we report that Mixl1 interacts physically and functionally with the T-box protein Brachyury and related members of the T-box family of transcription factors. Transcriptional and protein analyses demonstrated overlapping expression of Mixl1 and Brachyury during embryonic stem cell differentiation. In vitro protein interaction studies showed that the Mixl1 with Brachyury associated via their DNA-binding domains and gel shift assays revealed that the Brachyury T-box domain bound to Mixl1-DNA complexes. Furthermore, luciferase reporter experiments indicated that association of Mixl1 with Brachyury and related T-box factors inhibited the transactivating potential of Mixl1 on the Gsc and Pdgfrα promoters. Our results indicate that the activity of Mixl1 can be modulated by protein-protein interactions and that T-box factors can function as negative regulators of Mixl1 activity.

miR-204 is required for lens and retinal development via Meis2 targeting

MicroRNAs (miRNAs) are small noncoding RNAs that have important roles in the regulation of gene expression. The roles of individual miRNAs in controlling vertebrate eye development remain, however, largely unexplored. Here, we show that a single miRNA, miR-204, regulates multiple aspects of eye development in the medaka fish (Oryzias latipes). Morpholino-mediated ablation of miR-204 expression resulted in an eye phenotype characterized by microphthalmia, abnormal lens formation, and altered dorsoventral (D-V) patterning of the retina, which is associated with optic fissure coloboma. Using a variety of in vivo and in vitro approaches, we identified the transcription factor Meis2 as one of the main targets of miR-204 function. We show that, together with altered regulation of the Pax6 pathway, the abnormally elevated levels of Meis2 resulting from miR-204 inactivation are largely responsible for the observed phenotype. These data provide an example of how a specific miRNA can regulate multiple events in eye formation; at the same time, they uncover an as yet unreported function of Meis2 in the specification of D-V patterning of the retina.

Ash2l interacts with Tbx1 and is required during early embryogenesis

TBX1 encodes a DNA binding transcription factor that is commonly deleted in human DiGeorge syndrome and plays an important role in heart development. Mechanisms of Tbx1 function, such as Tbx1 interacting regulatory proteins and transcriptional target specificity, are largely unknown. Ash2l is the mammalian homolog of Drosophila Ash2 (absent small homeotic 2) and is a core component of a multimeric histone methyltransferase complex that epigenetically regulates transcription via methylation of histone lysine residues. We undertook an unbiased yeast two-hybrid screen to look for functionally relevant Tbx1-interacting proteins and report a physical and functional interaction between Tbx1 and Ash2l. Tbx1 interacts with Ash2l in both yeast and mammalian cells and Ash2l acts as a transcriptional co-activator in luciferase reporter assays. Expression analysis shows that Tbx1 and Ash2l have overlapping mRNA and protein expression patterns during development. By generating an Ash2l knockout mouse utilizing gene-trap technology, we show that although Ash2l heterozygous mice are normal, Ash2l-null embryos die early during gestation. Thus, Ash2l is required for the earliest stages of embryogenesis. Furthermore, our finding of a physical interaction between Tbx1 and Ash2l suggest that at least some functions of Tbx1 may be mediated by direct interactions with a histone methyltransferase complex.

The PAX258 gene subfamily: a comparative perspective

Whole genome duplication events are thought to have substantially contributed to organismal complexity, largely via divergent transcriptional regulation. Members of the vertebrate PAX2, PAX5 and PAX8 gene subfamily derived from an ancient class of paired box genes and arose from such whole genome duplication events. These genes are critical in establishing the midbrain-hindbrain boundary, specifying interneuron populations and for eye, ear and kidney development. Also PAX2 has adopted a unique role in pancreas development, whilst PAX5 is essential for early B-cell differentiation. The contribution of PAX258 genes to their collective role has diverged across paralogues and the animal lineages, resulting in a complex wealth of literature. It is now timely to provide a comprehensive comparative overview of these genes and their ancient and divergent roles. We also discuss their fundamental place within gene regulatory networks and the likely influence of cis-regulatory elements over their differential roles during early animal development.

Progenitor cells of the rod-free area centralis originate in the anterior dorsal optic vesicle

Background

Nervous system development is dependent on early regional specification to create functionally distinct tissues within an initially undifferentiated zone. Within the retina, photoreceptors are topographically organized with rod free area centrales faithfully generated at the centre of gaze. How does the developing eye regulate this placement? Conventional wisdom indicates that the distal tip of the growing optic vesicle (OV) gives rise to the area centralis/fovea. Ectopic expression and ablation studies do not fully support this view, creating a controversy as to the origin of this region. In this study, the lineage of cells in the chicken OV was traced using DiI. The location of labelled cells was mapped onto the retina in relation to the rod-free zone at embryonic (E) 7 and E17.5. The ability to regenerate a rod free area after OV ablation was determined in conjunction with lineage tracing.

Results

Anterior OV gave rise to cells in nasal retina and posterior OV became temporal retina. The OV distal tip gave rise to cells above the optic nerve head. A dorsal and anterior region of the OV correlated with cells in the developing rod free area centralis. Only ablations including the dorsal anterior region gave rise to a retina lacking a rod free zone. DiI application after ablation indicated that cells movements were greater along the anterior/posterior axis compared with the dorsal/ventral axis.

Conclusion

Our data support the idea that the chicken rod free area centralis originates from cells located near, but not at the distal tip of the developing OV. Therefore, the hypothesis that the area centralis is derived from cells at the distal tip of the OV is not supported; rather, a region anterior and dorsal to the distal tip gives rise to the rod free region. When compared with other studies of retinal development, our results are supported on molecular, morphological and functional levels. Our data will lead to a better understanding of the mechanisms underlying the topographic organization of the retina, the origin of the rod free zone, and the general issue of compartmentalization of neural tissue before any indication of morphological differentiation.

Ocular abnormalities in mice lacking the immunoglobulin superfamily member Cdo

Vertebrate eye development requires a series of complex morphogenetic and inductive events to produce a lens vesicle centered within the bilayered optic cup and a posteriorly positioned optic stalk. Multiple congenital eye defects, including microphthalmia and coloboma, result from defects in early eye morphogenesis. Cdo is a multifunctional cell surface immunoglobulin superfamily member that interacts with and mediates signaling by cadherins and netrins to regulate myogenesis. In addition, Cdo plays an essential role in early forebrain development by functioning as coreceptor for sonic hedgehog. It is reported here that Cdo is expressed in a dynamic, but dorsally restricted, fashion during early eye development, and that mice lacking Cdo display multiple eye defects. Anomalies seen in Cdo(-/-) mice include coloboma (failure to close the optic fissure); failure to form a proper boundary between the retinal pigmented epithelium and optic stalk; defective lens formation, including failure to separate from the surface ectoderm; and microphthalmia. Consistent with this wide array of defects, developing eyes of Cdo(-/-) mice show altered expression of several regulators of dorsoventral eye patterning, including Pax6, Pax2, and Tbx5. Taken together, these findings show that Cdo is required for normal eye development and is required for normal expression of patterning genes in both the ventral and dorsal domains. The multiple eye development defects seen in Cdo(-/-) mice suggest that mutations in human Cdo could contribute to congenital eye anomalies, such as Jacobsen syndrome, which is frequently associated with ocular defects, including coloboma and Peters' anomaly.

Effects of elevated Pax6 expression and genetic background on mouse eye development

PURPOSE

To analyze the effects of Pax6 overexpression and its interaction with genetic background on eye development.

METHODS

Histologic features of eyes from hemizygous PAX77(+/-) transgenic (high Pax6 gene dose) and wild-type mice were compared on different genetic backgrounds. Experimental PAX77(+/-)<-->wild-type and control wild-type<-->wild-type chimeras were analyzed to investigate the causes of abnormal eye development in PAX77(+/-) mice.

RESULTS

PAX77(+/-) mice showed an overlapping but distinct spectrum of eye abnormalities to Pax6(+/-) heterozygotes (low Pax6 dose). Some previously reported PAX77(+/-) eye abnormalities did not occur on all three genetic backgrounds examined. Several types of eye abnormalities occurred in the experimental PAX77(+/-)<-->wild-type chimeras, and they occurred more frequently in chimeras with higher contributions of PAX77(+/-) cells. Groups of RPE cells intruded into the optic nerve sheath, indicating that the boundary between the retina and optic nerve may be displaced. Both PAX77(+/-) and wild-type cells were involved in this ingression and in retinal folds, suggesting that neither effect was cell-autonomous. Cell-autonomous effects included failure of PAX77(+/-) and wild-type cells to mix normally and overrepresentation of PAX77(+/-) in the lens epithelium and RPE.

CONCLUSIONS

The extent of PAX77(+/-) eye abnormalities depended on PAX77(+/-) genotype, genetic background, and stochastic variation. Chimera analysis identified two types of cell-autonomous effects of the PAX77(+/-) genotype. Abnormal cell mixing between PAX77(+/-) and wild-type cells suggests altered expression of cell surface adhesion molecules. Some phenotypic differences between PAX77(+/-)<-->wild-type and Pax6(+/-)<-->wild-type chimeras may reflect differences in the levels of PAX77(+/-) and Pax6(+/-) contributions to chimeric lenses.

T-box protein Tbx18 interacts with the paired box protein Pax3 in the development of the paraxial mesoderm

The compartmentalization of somites along their anterior-posterior axis is crucial to the segmental organization of the vertebral column. Anterior-posterior somite polarity is generated in the anterior presomitic mesoderm by Mesp2 and Delta/Notch signaling and is further maintained by two transcriptional regulators, Uncx4.1 and Tbx18, acting in the posterior and anterior somite compartment, respectively. Here, we report that the paired box transcription factor Pax3 cooperates with the T-box protein Tbx18 in maintaining anterior somite half identity. Our findings that both genes are co-expressed in the anterior presomitic mesoderm and in early somites, that Pax3 and Tbx18 proteins physically interact, and that the loss of Pax3 gene function enhances the vertebral defects (i.e. the gain of vertebral elements derived from posterior somite halves in Tbx18 mutant mice) suggests that the two proteins cooperatively regulate the gene expression program necessary for maintaining anterior-posterior somite polarity. Genetic interaction of Pax3 with Tbx18 and the closely related T-box gene Tbx15 was also observed in the development of the scapula blade, indicating an additional cooperative function for these genes in the paraxial mesoderm.

Anteroventrally localized activity in the optic vesicle plays a crucial role in the optic development

The vertebrate eye develops from the optic vesicle (OV), a laterally protrusive structure of the forebrain, by a coordinated interaction with surrounding tissues. The OV then invaginates to form an optic cup, and the lens placode develops to the lens vesicle at the same time. These aspects in the early stage characterize vertebrate eye formation and are controlled by appropriate dorsal-ventral coordination. In the present study, we performed surgical manipulation in the chick OV to remove either the dorsal or ventral half and examined the development of the remaining OV. The results show that the dorsal and ventral halves of the OV have a clearly different developmental pattern. When the dorsal half was removed, the remaining ventral OV developed into an entire eye, while the dorsal OV developed to a pigmented vesicle consisting of retinal pigmented epithelium alone. These results indicate that the ventral part of the OV retains the potency to develop the entire eye structure and plays an essential role in proper eye development. In subsequent manipulations of early chick embryos, it was found that only the anterior ventral quadrant of the OV has the potential to develop the entire eye and that no other part of the OV has a similar activity. Fgf8 expression was localized in this portion and no Fgf8 expression was observed within the OV when the ventral OV was removed. These results suggest that the anterior ventral portion of the OV plays a crucial role in the proper development of the eye, possibly generating the dorsal-ventral gradients of signal proteins within the eye primordium.

Introduction of silencing-inducing transgene against Fgf19 does not affect expression of Tbx5 and beta3-tubulin in the developing chicken retina

Fgf19 is known to be expressed in the developing chicken eye but its functions during retinal development have remained elusive. Since Fgf19 is expressed in the dorsal portion of the optic cup, it is intriguing to know whether FGF19 is required for expression of dorso-ventral morphogenetic genes in the eye. To clarify this, expression patterns of Tbx5 and Vax were examined in the developing eye after in ovo RNA interference targeted against Fgf19. Quantitative polymerase chain reaction (PCR) analysis showed that the short-hairpin RNAs (shRNAs) targeted against Fgf19 could reduce its expression in the eye to less than 50% of a relative amount of mRNA, compared with contralateral or untreated control eyes. However, no obvious alteration in expression domains of Tbx5 or Vax was observed. Misexpression of Tbx5 or Tbx5-RNAi did not alter the Fgf19 expression either. Furthermore, although Fgf19 is expressed in the central retina before neurogenesis occurs, beta3-tubulin, a marker for early retinal differentiation was still detected in the central retina after knockdown of Fgf19. Thus, knockdown of Fgf19 supports no obvious regulations between Fgf19 and Tbx5, or exhibits no phenotypes that perturb early retinal differentiation.

Msx1 and Msx2 are functional interacting partners of T-box factors in the regulation of Connexin43

AIMS

T-box factors Tbx2 and Tbx3 play key roles in the development of the cardiac conduction system, atrioventricular canal, and outflow tract of the heart. They regulate the gap-junction-encoding gene Connexin43 (Cx43) and other genes critical for heart development and function. Discovering protein partners of Tbx2 and Tbx3 will shed light on the mechanisms by which these factors regulate these gene programs.

METHODS AND RESULTS

Employing an yeast 2-hybrid screen and subsequent in vitro pull-down experiments we demonstrate that muscle segment homeobox genes Msx1 and Msx2 are able to bind the cardiac T-box proteins Tbx2, Tbx3, and Tbx5. This interaction, as that of the related Nkx2.5 protein, is supported by the T-box and homeodomain alone. Overlapping spatiotemporal expression patterns of Msx1 and Msx2 together with the T-box genes during cardiac development in mouse and chicken underscore the biological significance of this interaction. We demonstrate that Msx proteins together with Tbx2 and Tbx3 suppress Cx43 promoter activity and down regulate Cx43 gene activity in a rat heart-derived cell line. Using chromatin immunoprecipitation analysis we demonstrate that Msx1 can bind the Cx43 promoter at a conserved binding site located in close proximity to a previously defined T-box binding site, and that the activity of Msx proteins on this promoter appears dependent in the presence of Tbx3.

CONCLUSION

Msx1 and Msx2 can function in concert with the T-box proteins to suppress Cx43 and other working myocardial genes.

Molecular mechanisms of vertebrate retina development: Implications for ganglion cell and photoreceptor patterning

Although the neural retina appears as a relatively uniform tissue when viewed from its surface, it is in fact highly patterned along its anterior-posterior and dorso-ventral axes. The question of how and when such patterns arise has been the subject of intensive investigations over several decades. Most studies aimed at understanding retinal pattern formation have used the retinotectal map, the ordered projections of retinal ganglion cells to the brain, as a functional readout of the pattern. However, other cell types are also topographically organized in the retina. The most commonly recognized example of such a topographic cellular organization is the differential distribution of photoreceptor types across the retina. Photoreceptor patterns are highly species-specific and may represent an important adaptation to the visual niche a given species occupies. Nevertheless, few studies have addressed this functional readout of pattern to date and our understanding of its development has remained superficial. Here, we review recent advances in understanding the molecular cascades that control regionalization of the eye anlage, relate these findings to the development of photoreceptor patterns and discuss common and unique strategies involved in both aspects of retinal pattern formation.

Stage-dependent expression of Pax6 in optic vesicle/cup regulates patterning genes through signaling molecules

Dorso-ventral and proximo-distal axis formation of the optic cup is apparent from early stages of development. Pax6 is initially detectable in the optic vesicle and later shows a distal-high and proximal-low gradient of expression in the retina. To determine the early role of Pax6 in pattern formation of the optic cup, we expressed Pax6 ectopically in the optic vesicle of stages 9-10 chick embryos by in ovo electroporation, which resulted in a small eye-like phenotype. The signaling molecule fibroblast growth factor (FGF)8, which appears to be restricted to the central retina, was increased, whereas bone morphogenetic protein (BMP)4 and Tbx5, two dorsal markers, were down-regulated in Pax6-electroporated eye. Pax6 overexpression also decreased the expression of the ventral marker Vax. Electroporation with a dominant-negative form of Pax6 resulted in a decrease in FGF8 expression, but BMP4 expression was unaffected initially while it was diminished later. Our data suggest a new role for Pax6 in regulating FGF8 and BMP4 expression during pattern formation of the optic cup, and that a Pax6-regulated balance between FGF8 and BMP4 is critical for retinogenesis.

Transcriptional Repression by the T-box Proteins Tbx18 and Tbx15 Depends on Groucho Corepressors

Tbox18 (Tbx18) and Tbox15 (Tbx15) encode a closely related pair of vertebrate-specific T-box (Tbx) transcription factors. Functional analyses in the mouse have proven the requirement of Tbx15 in skin and skeletal development and of Tbx18 in the formation of the vertebral column, the ureter, and the posterior pole of the heart. Despite the accumulation of genetic data concerning the embryological roles of these genes, it is currently unclear how Tbx18 and Tbx15 exert their function on the molecular level. Here, we have initiated a molecular analysis of Tbx18 and Tbx15 proteins and have characterized functional domains for nuclear localization, DNA binding, and transcriptional modulation. We show that both proteins homo- and heterodimerize, bind to various combinations of T half-sites, and repress transcription in a Groucho-dependent manner. Competition with activating T-box proteins may constitute one mode of action as we show that Tbx18 interacts with Gata4 and Nkx2-5 and competes Tbx5-mediated activation of the cardiac Natriuretic peptide precursor type a-promoter and that ectopic expression of Tbx18 down-regulates Tbx6-activated Delta-like 1 expression in the somitic mesoderm in vivo.

Pbx homeodomain proteins pattern both the zebrafish retina and tectum

BACKGROUND

Pbx genes encode TALE class homeodomain transcription factors that pattern the developing neural tube, pancreas, and blood. Within the hindbrain, Pbx cooperates with Hox proteins to regulate rhombomere segment identity. Pbx cooperates with Eng to regulate midbrain-hindbrain boundary maintenance, and with MyoD to control fast muscle cell differentiation. Although previous results have demonstrated that Pbx is required for proper eye size, functions in regulating retinal cell identity and patterning have not yet been examined.

RESULTS

Analysis of retinal ganglion cell axon pathfinding and outgrowth in pbx2/4 null embryos demonstrated a key role for pbx genes in regulating neural cell behavior. To identify Pbx-dependent genes involved in regulating retino-tectal pathfinding, we conducted a microarray screen for Pbx-dependent transcripts in zebrafish, and detected genes that are specifically expressed in the eye and tectum. A subset of Pbx-dependent retinal transcripts delineate specific domains in the dorso-temporal lobe of the developing retina. Furthermore, we determined that some Pbx-dependent transcripts also require Meis1 and Gdf6a function. Since gdf6a expression is also dependent on Pbx, we propose a model in which Pbx proteins regulate expression of the growth factor gdf6a, which in turn regulates patterning of the dorso-temporal lobe of the retina. This, in concert with aberrant tectal patterning in pbx2/4 null embryos, may lead to the observed defects in RGC outgrowth.

CONCLUSION

These data define a novel role for Pbx in patterning the vertebrate retina and tectum in a manner required for proper retinal ganglion cell axon outgrowth.

Molecular mechanisms of optic vesicle development: complexities, ambiguities and controversies

Optic vesicle formation, transformation into an optic cup and integration with neighboring tissues are essential for normal eye formation, and involve the coordinated occurrence of complex cellular and molecular events. Perhaps not surprisingly, these complex phenomena have provided fertile ground for controversial and even contradictory results and conclusions. After presenting an overview of current knowledge of optic vesicle development, we will address conceptual and methodological issues that complicate research in this field. This will be done through a review of the pertinent literature, as well as by drawing on our own experience, gathered through recent studies of both intra- and extra-cellular regulation of optic vesicle development and patterning. Finally, and without attempting to be exhaustive, we will point out some important aspects of optic vesicle development that have not yet received enough attention.

Hepatocyte nuclear factor 4 alpha ligand binding and F domains mediate interaction and transcriptional synergy with the pancreatic islet LIM HD transcription factor Isl1

The orphan nuclear receptor HNF4alpha and the LIM homeodomain factor Isl1 are co-expressed in pancreatic beta-cells and are required for the differentiation and function of these endocrine cells. HNF4alpha activates numerous genes and mutations in its gene are associated with maturity onset diabetes of the young. Cofactors and transcription factors that interact with HNF4alpha are crucial to modulate its transcriptional activity, since the latter is not regulated by conventional ligands. These transcriptional partners interact mainly through the HNF4alpha AF-1 module and the ligand binding domain, which contains the AF-2 module. Here, we showed that Isl1 could enhance the HNF4alpha-mediated activation of transcription of the HNF1alpha, PPARalpha and insulin I promoters. Isl1 interacted with the HNF4alpha AF-2 but also required the HNF4alpha carboxy-terminal F domain for optimal interaction and transcriptional synergy. More specifically, we found that naturally occurring HNF4alpha isoforms, differing only in their F domain, exhibited different abilities to interact and synergize with Isl1, extending the crucial transcriptional modulatory role of the HNF4alpha F domain. HNF4alpha interacted with both the homeodomain and the first LIM domain of Isl1. We found that the transcriptional synergy between HNF4alpha and Isl1 involved an increase in HNF4alpha loading on promoter. The effect was more pronounced on the rat insulin I promoter containing binding sites for both HNF4alpha and Isl1 than on the human HNF1alpha promoter lacking an Isl1 binding site. Moreover, Isl1 could mediate the recruitment of the cofactor CLIM2 resulting in a further transcriptional enhancement of the HNF1alpha promoter activity.

Optic cup and lens development requires Pax6 expression in the early optic vesicle during a narrow time window

Pax6 mutations cause complex ocular malformations, but it is uncertain whether early eye development normally requires Pax6 function in both the optic vesicle (OV) and the lens epithelium, or only in the latter. To investigate this question, we electroporated the OV with anti-Pax6 or control morpholinos before the onset of lens placode formation. Pax6 downregulation was already detectable in the OV 10 h after anti-Pax6 treatment, and was accompanied by a significant increase in the death of OV cells. A small eye-like phenotype developed thereafter, whose severity was developmental stage-dependent. When treatment was applied at Hamburger Hamilton (HH) stage 10, there was no optic cup formation, and lens development was abortive despite normal Pax6 expression in the lens epithelium. Treatment at HH stage 11 resulted in structurally normal lens and optic cup, although the latter showed abnormal expression domains for several transcription factors. Early eye development therefore requires cell-autonomous Pax6 function not only in the lens but also in the optic vesicle, where it plays a hitherto unknown role in cell survival. The results, moreover, indicate that there is a critical stage during which Pax6 expression in the OV is necessary for normal lens development.

Ex ovo electroporation for gene transfer into older chicken embryos

In ovo electroporation is an excellent method to ectopically induce or inhibit gene expression in chicken embryos and to study the in vivo function of genes during embryonic development. However, the application of electroporation in ovo to date is limited to an early stage of incubation (< stage 20). In older embryos (> stage 22), the vitelline and allantoic vessels have developed extensively and the in ovo manipulation of the embryo becomes exceedingly difficult. Therefore, in this study, we validate an ex ovo electroporation system, by which the time for performing electroporation can be extended up to at least day 7 of incubation. The application of this method will help to study gene function and regulation at later stages of development in the living chicken embryo.

Vax genes ventralize the embryonic eye

The vertebrate retina and optic nerve are strikingly different in terms of their size, organization, and cellular diversity, yet these two structures develop from the same embryonic neuroepithelium. Precursor cells in the most ventral domain of this epithelium give rise only to the astrocytes of the optic nerve, whereas immediately adjacent, more dorsal precursors give rise to the myriad cell types of the retina. We provide genetic evidence that two closely related, ventrally expressed homeodomain proteins-Vax1 and Vax2-control this neuroepithelial segregation. In the absence of both proteins, we find that the optic nerve is transformed in its entirety into fully differentiated retina. We demonstrate that this transformation results from the loss of ventralizing activity in the developing eye field, and that ventralization is mediated, at least in part, via Vax repression of the Pax6 gene, a potent inducer of retinal development.

The third helix of the homeodomain of paired class homeodomain proteins acts as a recognition helix both for DNA and protein interactions

The transcription factor Pax6 is essential for the development of the eyes and the central nervous system of vertebrates and invertebrates. Pax6 contains two DNA-binding domains; an N-terminal paired domain and a centrally located homeodomain. We have previously shown that the vertebrate paired-less isoform of Pax6 (Pax6ΔPD), and several other homeodomain proteins, interact with the full-length isoform of Pax6 enhancing Pax6-mediated transactivation from paired domain-DNA binding sites. By mutation analyses and molecular modeling we now demonstrate that, surprisingly, the recognition helix for specific DNA binding of the homeodomains of Pax6 and Chx10 interacts with the C-terminal RED subdomain of the paired domain of Pax6. Basic residues in the recognition helix and the N-terminal arm of the homeodomain form an interaction surface that binds to an acidic patch involving residues in helices 1 and 2 of the RED subdomain. We used fluorescence resonance energy transfer assays to demonstrate such interactions between Pax6 molecules in the nuclei of living cells. Interestingly, two mutations in the homeodomain recognition helix, R57A and R58A, reduced protein–protein interactions, but not DNA binding of Pax6ΔPD. These findings suggest a critical role for the recognition helix and N-terminal arm of the paired class homeodomain in protein–protein interactions.

Tbx5 and Tbx20 act synergistically to control vertebrate heart morphogenesis

Members of the T-box family of proteins play a fundamental role in patterning the developing vertebrate heart; however, the precise cellular requirements for any one family member and the mechanism by which individual T-box genes function remains largely unknown. In this study, we have investigated the cellular and molecular relationship between two T-box genes, Tbx5 and Tbx20. We demonstrate that blocking Tbx5 or Tbx20 produces phenotypes that display a high degree of similarity, as judged by overall gross morphology, molecular marker analysis and cardiac physiology, implying that the two genes are required for and have non-redundant functions in early heart development. In addition, we demonstrate that although co-expressed, Tbx5 and Tbx20 are not dependent on the expression of one another, but rather have a synergistic role during early heart development. Consistent with this proposal, we show that TBX5 and TBX20 can physically interact and map the interaction domains, and we show a cellular interaction for the two proteins in cardiac development, thus providing the first evidence for direct interaction between members of the T-box gene family.