Pax: gene regulators in the developing nervous system

A Novel PAX6 Frameshift Mutation Identified in a Large Chinese Family with Congenital Aniridia

Congenital aniridia is a rare autosomal dominant congenital ocular disorder. Genetic studies suggest that heterozygous mutations in the developmental regulator PAX6 gene or the related regulatory regions leading to haploinsufficiency are the main cause of congenital aniridia. In this study, the clinical characteristics and pathogenic mutation of a four-generation Chinese family with congenital aniridia were investigated. All members recruited in this study underwent comprehensive ophthalmic examinations. Targeted gene capture sequencing and Sanger sequencing were performed to screen and confirm the candidate pathogenicity gene and its mutation. A multiple alignment of homologous sequences covering the identified mutation from different species was investigated, and the mutant protein structure was predicted using Swiss-Model. Additionally, the prediction of pathogenicity was analyzed using the ACMG Guidelines. Thirteen patients in this pedigree were diagnosed with congenital aniridia. A novel heterozygous frameshift mutation (c.391_398dupATACCAAG, p.Ser133Argfs*8) in exon 7 of the PAX6 gene was identified in all affected individuals in the family. This study demonstrates that this frameshift mutation of the PAX6 gene might be the causative genetic defect of congenital aniridia in this family. This mutation is predicted to cause the premature truncation of the PAX6 protein, leading to the functional haploinsufficiency of PAX6, which may be the major molecular mechanism underlying the aniridia phenotype. To the best of our knowledge, this is the first report of a novel pathogenic PAX6 gene variant c.391_398dupATACCAAG(p.Ser133Argfs*8) identified in a Chinese family with congenital aniridia.

A novel 4.25 kb heterozygous deletion in PAX6 in a Chinese Han family with congenital aniridia combined with cataract and nystagmus

BACKGROUND

The aim of this study is to identify the genetic defect in a Chinese family with congenital aniridia combined with cataract and nystagmus.

METHODS

Complete ophthalmic examinations, including slit-lamp biomicroscopy, dilated indirect ophthalmoscopy, anterior segment photography, and anterior segment optical coherence tomography (OCT) were performed. Blood samples were collected from all family members and genomic DNA was extracted. Genome sequencing was performed in all family members and Sanger sequencing was used to verify variant breakpoints.

RESULTS

All the thirteen members in this Chinese family, including seven patients and six normal people, were recruited in this study. The ophthalmic examination of affected patients in this family was consistent with congenital aniridia combined with cataract and nystagmus. A novel heterozygous deletion (NC_000011.10:g.31802307_31806556del) containing the 5' region of PAX6 gene was detected that segregated with the disease.

CONCLUSION

We detected a novel deletion in PAX6 responsible for congenital aniridia in the affected individuals of this Chinese family. The novel 4.25 kb deletion in PAX6 gene of our study would further broaden the genetic defects of PAX6 associated with congenital aniridia.

Pax6 expression highlights regional organization in the adult brain of lungfishes, the closest living relatives of land vertebrates

The Pax6 gene encodes a regulatory transcription factor that is key in brain development. The molecular structure of Pax6, the roles it plays and its patterns of expression in the brain have been highly conserved during vertebrate evolution. As neurodevelopment proceeds, the Pax6 expression changes from the mitotic germinal zone in the ventricular zone to become distributed in cell groups in the adult brain. Studies in various vertebrates, from fish to mammals, found that the Pax6 expression is maintained in adults in most regions that express it during development. Specifically, in amphibians, Pax6 is widely expressed in the adult brain and its distribution pattern serves to highlight regional organization of the brain. In the present study, we analyzed the detailed distribution of Pax6 cells in the adult central nervous system of lungfishes, the closest living relatives of all tetrapods. Immunohistochemistry performed using double labeling techniques with several neuronal markers of known distribution patterns served to evaluate the actual location of Pax6 cells. Our results show that the Pax6 expression is maintained in the adult brain of lungfishes, in distinct regions of the telencephalon (pallium and subpallium), diencephalon, mesencephalon, hindbrain, spinal cord, and retina. The pattern of Pax6 expression is largely shared with amphibians and helps to understand the primitive condition that would have characterized the common ancestors to all sarcopterygians (lobe-finned fishes and tetrapods), in which Pax6 would be needed to maintain specific entities of subpopulations of neurons.

Screening of PAX6 gene in Italian congenital aniridia patients revealed four novel mutations

PURPOSE

To uncover underlying mutations in a cohort of Italian patients with aniridia, a rare congenital panocular condition with an incidence ranging from 1:64,000 to 1:100,000. The disease may be found isolated or in association with other syndromes characterized by partial or complete absence of the iris and iris hypoplasia.

METHODS

We analyzed the PAX6 gene in 11 patients with aniridia fulfilling the following inclusion criteria: partial or complete absence of the iris and age < 18 years at the time of diagnosis. DNA sequence analysis was integrated with Multiple Ligation Probe Assay (MLPA) analysis.

RESULTS

We identified seven PAX6 mutations, including four novel ones. The majority of mutations lie in the DNA-binding domain and all produce a truncated protein. All tested patients did not have WT1 gene deletions thus excluding the WAGR syndrome. We present the clinical findings in the four cases harboring novel mutations. We were unable to identify mutations in four cases with complete aniridia thus indicating that other gene/s could be involved in the disease.

CONCLUSIONS

It is important to establish the molecular diagnosis early to avoid repeated and long-term screening for Wilms tumor. Our work further emphasizes that a wide range of ocular phenotypes are associated with loss of function PAX6 mutations. In addition to the possibility of stochastic variations, other genetic variations could play a role as modifier genes, thus giving rise to the observed different ocular phenotypes.

Congenital abnormalities of the optic nerve: from gene mutation to clinical expression

Optic nerve malformations are common causes of congenital blindness and are recognized with increasing prevalence. The importance of identifying these malformations lies not only in determining the cause and level of visual impairment, but also in looking for associated treatable or life threatening systemic conditions. A number of genetic mutations have been identified in the development of optic disc anomalies, such as ones in PAX2 or PAX6. Recent advances in genetic testing like array comparative genomic hybridization allow the detection of microdeletions that were previously difficult or impossible to identify.

Expression patterns of Pax6 and Pax7 in the adult brain of a urodele amphibian, Pleurodeles waltl

Expression patterns of Pax6, Pax7, and, to a lesser extent, Pax3 genes were analyzed by a combination of immunohistochemical techniques in the central nervous system of adult specimens of the urodele amphibian Pleurodeles waltl. Only Pax6 was found in the telencephalon, specifically the olfactory bulbs, striatum, septum, and lateral and central parts of the amygdala. In the diencephalon, Pax6 and Pax7 were distinct in the alar and basal parts, respectively, of prosomere 3. The distribution of Pax6, Pax7, and Pax3 cells correlated with the three pretectal domains. Pax7 specifically labeled cells in the dorsal mesencephalon, mainly in the optic tectum, and Pax6 cells were the only cells found in the tegmentum. Large populations of Pax7 cells occupied the rostral rhombencephalon, along with lower numbers of Pax6 and Pax3 cells. Pax6 was found in most granule cells of the cerebellum. Pax6 cells also formed a column of scattered neurons in the reticular formation and were found in the octavolateral area. The rhombencephalic ventricular zone of the alar plate expressed Pax7. Dorsal Pax7 cells and ventral Pax6 cells were found along the spinal cord. Our results show that the expression of Pax6 and Pax7 is widely maintained in the brains of adult urodeles, in contrast to the situation in other tetrapods. This discrepancy could be due to the generally pedomorphic features of urodele brains. Although the precise role of these transcription factors in adult brains remains to be determined, our findings support the idea that they may also function in adult urodeles.

The role of Pax6 in forebrain development

Pax6 encodes a highly conserved transcriptional regulator with two DNA-binding motifs, a paired domain and a paired-like homeodomain. Humans carrying PAX6 loss-of-function mutations suffer from abnormal development of the eyes (congenital aniridia) and brain. Small eye mice carrying Pax6 loss-of-function mutations provide a good model for these human conditions. Their analysis has demonstrated the critical importance of this transcription factor in multiple cell types and at several key stages of forebrain development. In the forebrain, Pax6 is critical for the establishment of the pallial-subpallial boundary, which separates dorsal (future cerebral cortex) and ventral (future striatum) telencephalic regions. Levels of Pax6 expression are critically important for cortical progenitor proliferation and its presence in a rostro-lateral(high) to caudo-medial(low) gradient in the cortex is necessary to establish rostro-lateral identities. Furthermore, axon guidance is disrupted in Pax6⁻/⁻ mutants: the majority of thalamocortical axons fail to enter the ventral telencephalon and those that do are unable to innervate their cortical targets. The extent to which the effects of Pax6 later in development are secondary to its effects in early patterning and proliferation remains largely unknown. This is likely to be clarified by future studies on the molecular mechanisms of action of Pax6 and, in particular, the identification of its downstream target genes. Such studies should also help generate an increasingly coherent understanding of how this pleiotropic transcription factor becomes involved in so many facets of neural development.

Maternal intake of folic acid and neural crest stem cells

Maternal folic acid (FA) intake has beneficial effects in preventing neural tube defects and may also play a role in the prevention of adult onset diseases such as Alzheimer's disease, dementia, neuropsychiatric disorders, cardiovascular diseases, and cerebral ischemia. This review will focus on the effects of maternal FA intake on neural crest stem cell proliferation and differentiation. Although FA is generally considered beneficial, it has the potential of promoting cell proliferation at the expense of differentiation. In some situations, this may lead to miscarriage or postnatal developmental abnormalities. Therefore, a blind approach such as "FA for everyone" is not necessarily the best course of action. Ultimately, the best approach for FA supplementation, and potentially other nutritional supplements, will include customized patient genomic profiles for determining dose and duration.

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.

Transcriptional activities of the Pax6 gene eyeless regulate tissue specificity of ectopic eye formation in Drosophila

Pax genes encode DNA binding proteins that play pivotal roles in the determination of complex tissues. Members of one subclass, Pax6, function as selector genes and play key roles in the retinal development of all seeing animals. Mutations within the Pax6 homologs including fly eyeless, mouse Small eye and human Pax6 lead to severe retinal defects in their respective systems. In Drosophila eyeless and twin of eyeless, play non-redundant roles in the developing retina. One particularly interesting characteristic of these genes is that, although expression of either gene can induce ectopic eye formation in non-retinal tissues, there are differences in the location and frequencies at which the eyes develop. eyeless induces much larger ectopic eyes, at higher frequencies, and in a broader range of tissues than twin of eyeless. In this report we describe a series of experiments conducted in both yeast and flies that has identified protein modules that are responsible for the differences in tissue transformation. These domains appear to contain transcriptional activator and repressor activity of distinct strengths. We propose a model in which the selective presence of these activities and their relative strengths accounts, in part, for the disparity to which ectopic eyes are induced in response to the forced expression of eyeless and twin of eyeless. The identification of both transcriptional activator and repressor activity within the Pax6 protein furthers our understanding of how this gene family regulates tissue determination.

Co-option of signaling mechanisms from neural induction to telencephalic patterning

This article provides an overview of signaling processes during early specification of the anterior neural tube, with special emphasis on the telencephalon. A series of signaling systems based on the action of distinct morphogens acts at different developmental stages, specifying interacting developmental fields that define axes of differentiation in the rostrocaudal and the dorsoventral domains. Interestingly, many of these signaling systems are co-opted for several differentiation processes. This strategy provides a simple and efficient mechanism to generate novel structures in evolution, and may have been especially important in the origin of the telencephalon and the mammalian cerebral cortex. For example, the action of fibroblast growth factor (FGF) secreted in early stages from the anterior neural ridge, but in later stages from the dorsal anterior forebrain, may have been a key factor in the early differentiation of the ventral telencephalon and in the eventual expansion of the mammalian neocortex. Likewise, bone morphogenetic proteins (BMPs) participate at several stages in neural patterning, even if early neural induction consists of the inhibition of the BMP pathway. BMPs, secreted dorsally, interact with FGFs in the frontal aspect of the hemispheres, and with PAX6-dependent signaling sources located laterally, to pattern the dorsal telencephalon. The actions of other morphogens are also described in this context, such as the ventralizing factor SHH, the dorsalizing element GLI3, and other factors related to the dorsomedial telencephalon such as WNTs and EMXs. The main conclusion we draw from this review is the well-known phylogenetic and developmental conservatism of signaling pathways, which in evolution have been applied in different embryological contexts, generating novel interactions between morphogenetic fields and leading to the generation of new morphological structures.

Molecular paleoscience: systems biology from the past

Experimental paleomolecular biology, paleobiochemistry, and paleogenetics are closely related emerging fields that infer the sequences of ancient genes and proteins from now-extinct organisms, and then resurrect them for study in the laboratory. The goal of paleogenetics is to use information from natural history to solve the conundrum of modern genomics: How can we understand deeply the function of biomolecular structures uncovered and described by modern chemical biology? Reviewed here are the first 20 cases where biomolecular resurrections have been achieved. These show how paleogenetics can lead to an understanding of the function of biomolecules, analyze changing function, and put meaning to genomic sequences, all in ways that are not possible with traditional molecular biological studies.

Structural Studies on Pax-8 Prd Domain/DNA Complex

Pax-8 is a member of the Pax family of transcription factors and is essential in the development of thyroid follicular cells. Pax-8 has two DNA-binding domains: the paired domain and the homeo domain. In this study, a preliminary X-ray diffraction analysis of the mammalian Pax-8 paired domain in complex with the C-site of the thyroglobulin promoter was achieved. The Pax-8 paired domain was crystallized by the hanging-drop vapor-diffusion method in complex with both a blunt-ended 26 bp DNA fragment and with a sticky-ended 24 bp DNA fragment with two additional overhanging bases. Crystallization experiments make clear that the growth of transparent crystals with large dimensions and regular shape is particularly influenced by ionic strength. The crystals of Pax-8 complex with blunt-ended and sticky-ended DNA, diffracted synchrotron radiation to 6.0 and 8.0 A resolution and belongs both to the C centered monoclinic system with cell dimensions: a = 89.88 A, b = 80.05 A, c = 67.73 A, and beta = 124.3 degrees and a = 256.56, b = 69.07, c = 99.32 A, and beta = 98.1 degrees , respectively. Fluorescence experiments suggest that the crystalline disorder, deduced by the poor diffraction, can be attributed to the low homogeneity of the protein-DNA sample. The theoretical comparative model of the Pax-8 paired domain complexed with the C-site of the thyroglobulin promoter shows the probable presence of some specific protein-DNA interactions already observed in other Pax proteins and the important role of the cysteine residues of PAI subdomain in the redox control of the DNA recognition.

Visual activity and cortical rewiring: activity-dependent plasticity of cortical networks

The mammalian cortex is organized anatomically into discrete areas, which receive, process, and transmit neural signals along functional pathways. These pathways form a system of complex networks that wire up through development and refine their connections into adulthood. Understanding the processes of cortical-pathway formation, maintenance, and experience-dependent plasticity has been among the major goals of contemporary neurobiology. In this chapter, we will discuss an experimental model used to investigate the role of activity in the patterning of cortical networks during development. This model involves the "rewiring" of visual inputs into the auditory thalamus and subsequent remodeling of the auditory cortex to process visual information. We review the molecular, cellular, and physiological mechanisms of visual "rewiring" and activity-dependent shaping of cortical networks.

Integrating transcription factor binding site information with gene expression datasets

MOTIVATION

Microarrays are widely used to measure gene expression differences between sets of biological samples. Many of these differences will be due to differences in the activities of transcription factors. In principle, these differences can be detected by associating motifs in promoters with differences in gene expression levels between the groups. In practice, this is hard to do.

RESULTS

We combine correspondence analysis, between group analysis and co-inertia analysis to determine which motifs, from a database of promoter motifs, are strongly associated with differences in gene expression levels. Given a database of motifs and gene expression levels from a set of arrays, the method produces a ranked list of motifs associated with any specified split in the arrays. We give an example using the Gene Atlas compendium of gene expression levels for human tissues where we search for motifs that are associated with expression in central nervous system (CNS) or muscle tissues. Most of the motifs that we find are known from previous work to be strongly associated with expression in CNS or muscle. We give a second example using a published prostate cancer dataset where we can simply and clearly find which transcriptional pathways are associated with differences between benign and metastatic samples.

AVAILABILITY

The source code is freely available upon request from the authors.

Pax7 and superior collicular polarity: insights from Pax6 (Sey) mutant mice

Pax genes are important modulators of CNS development. Pax7 and Pax6 polarise the neural tube and regionalise the brain. Pax7 is pivotal in specifying the superior colliculus/tectum, an important centre for integration of visuomotor responses and a target for Pax6+ retinal ganglion cell axons during retinocollicular mapping. Whilst initial Pax7-specification of the mesencephalon is well-established, a role in regulating polarity within the maturing mouse superior colliculus is yet to be defined, although already detailed for the chick tectum. We therefore quantified Pax7 cellular distribution and expression levels at three functionally distinct stages of superior collicular development, and analysed Pax7 expression in response to aberrant axonal input and altered forebrain/midbrain boundary placement in Pax6 mutant mice. Comparative expression profiles of ephrin-A2 and its co-localisation with Pax7 were determined in wildtype and Pax6 mutant mice. Results indicate that graded Pax7 expression in wildtype mice is perturbed in Pax6 mutant mice; changes manifest as a shift in polarity, loss of graded expression and dramatically reduced protein levels during RGC synaptogenesis. Ephrin-A2 expression is similarly altered. These results implicate Pax7 as an important determinant of polarity within the mouse superior colliculus, and suggest a role in retinotopic mapping.

Aberrant DNA demethylation in promoter region and aberrant expression of mRNA of PAX4 gene in hematologic malignancies

The PAX4 gene, a member of the paired box (PAX) gene family, is thought to be involved in regulating the fate of beta-cells in the mammalian pancreas. We observed the aberrant expression of PAX4 mRNA in 10 of 15 hematologic cell lines analyzed by RT-PCR. The restoration of PAX4 gene expression after treatment with the demethylating agent 5-aza-2'-deoxycitidine, as well as bisulfite sequencing analysis, indicated that gene overexpression was caused by DNA demethylation at the promoter region. Such DNA demethylation also was observed in primary lymphoma (20 out of 45 patients) on combined bisulfite restriction assay (COBRA). Forced expression of the PAX4 gene in the HEK293 and SHSY/610 cell lines conferred positive effects on cell growth. This profile of PAX4 thus corresponds to that of a candidate oncogene in hematologic malignancies.

Targeted deletion of AP-2alpha leads to disruption in corneal epithelial cell integrity and defects in the corneal stroma

PURPOSE

The present study was undertaken to create a conditional knockout of AP-2alpha in the corneal epithelium.

METHODS

A line of mice expressing Cre-recombinase specifically in the early lens placode was crossed with mice in which the AP-2alpha allele is flanked by two loxP sites. The resultant Le-AP-2alpha mutants exhibited a targeted deletion of AP-2alpha in lens placode derivatives, including the differentiating corneal epithelium.

RESULTS

The Le-AP-2alpha mutant mice were viable and had a normal lifespan. The adult corneal epithelium exhibited a variation in the number of stratified epithelial layers, ranging from 2 to 10 cell layers. A substantial decrease in expression of the cell-cell adhesion molecule, E-cadherin, was observed in all layers of the Le-AP-2alpha mutant corneal epithelium. The basement membrane, or Bowman's layer, was thinner in the mutant cornea and in many regions was discontinuous. These defects corresponded with altered distribution of laminin and entactin, and to a lesser degree, type IV collagen. The Le-AP-2alpha mutant cornea also exhibited stromal defects, including disrupted organization of the collagen lamellae and accumulation of fibroblasts beneath the epithelium that showed increased immunoreactivity for proliferating cell nuclear antigen (PCNA), alpha-smooth muscle actin (alpha-SMA), p-Smad2, and TGF-beta2.

CONCLUSIONS

In the absence of AP-2alpha, the corneal epithelium exhibits altered cell adhesion and integrity and defects in its underlying basement membrane. These defects likely caused the alterations in the corneal stroma.

Molecular characterization of HLH-17, a C. elegans bHLH protein required for normal larval development

The basic helix-loop-helix (bHLH) transcription factor family regulates numerous developmental events in eukaryotic cells. In the model system, C. elegans, thirty-seven bHLH proteins have been identified via genome-wide sequence analysis and fourteen have been genetically characterized to date. These proteins influence cell fate specification of neural lineages and differentiation of myogenic lineages and have distinct roles in somatic gonadogenesis. We report here on the molecular characterization of HLH-17, a protein whose putative bHLH domain is homologous to the mammalian bHLH proteins BETA3 and bHLHB5. The gene hlh-17 is transcriptionally active at all developmental stages, with the highest steady state accumulation of hlh-17 mRNA during embryogenesis. An upstream hlh-17 sequence drives expression of GFP in the sheath cells of the cephalic sensilla. Finally, animals that are defective in HLH-17 via RNAi display egg-laying defects, while those carrying null mutations in hlh-17 do not develop beyond the L2 stage and are less attracted to potassium and sodium ions. We propose that hlh-17 affects the ability of C. elegans to respond to food cues, with possible downstream effects on insulin-signaling genes involved in the normal development and reproductive viability of the worm.

Hox genes and their candidate downstream targets in the developing central nervous system

1. Homeobox (Hox) genes were originally discovered in the fruit fly Drosophila, where they function through a conserved homeodomain as transcriptional regulators to control embryonic morphogenesis. Since then over 1000 homeodomain proteins have been identified in several species. In vertebrates, 39 Hox genes have been identified as homologs of the original Drosophila complex, and like their Drosophila counterparts they are organized within chromosomal clusters. Vertebrate Hox genes have also been shown to play a critical role in embryonic development as transcriptional regulators. 2. Both the Drosophila and vertebrate Hox genes have been shown to interact with various cofactors, such as the TALE homeodomain proteins, in recognition of consensus sequences within regulatory elements of their target genes. These protein-protein interactions are believed to contribute to enhancing the specificity of target gene recognition in a cell-type or tissue- dependent manner. The regulatory activity of a particular Hox protein on a specific regulatory element is highly variable and dependent on its interacting partners within the transcriptional complex. 3. In vertebrates, Hox genes display spatially restricted patterns of expression within the developing CNS, both along the anterioposterior and dorsoventral axis of the embryo. Their restricted gene expression is suggestive of a regulatory role in patterning of the CNS, as well as in cell specification. Determining the precise function of individual Hox genes in CNS morphogenesis through classical mutational analyses is complicated due to functional redundancy between Hox genes. 4. Understanding the precise mechanisms through which Hox genes mediate embryonic morphogenesis requires the identification of their downstream target genes. Although Hox genes have been implicated in the regulation of several pathways, few target genes have been shown to be under their direct regulatory control. Development of methodologies used for the isolation of target genes and for the analysis of putative targets will be beneficial in establishing the genetic pathways controlled by Hox factors. 5. Within the developing CNS various cell adhesion molecules and signaling molecules have been identified as candidate downstream target genes of Hox proteins. These targets play a role in processes such as cell migration and differentiation, and are implicated in contributing to neuronal processes such as plasticity and/or specification. Hence, Hox genes not only play a role in patterning of the CNS during early development, but may also contribute to cell specification and identity.

How does Fgf signaling from the isthmic organizer induce midbrain and cerebellum development?

The mesencephalic/rhombomere 1 border (isthmus) is an organizing center for early development of midbrain and cerebellum. In this review, we summarize recent progress in studies of Fgf signaling in the isthmus and discuss how the isthmus instructs the differentiation of the midbrain versus cerebellum. Fgf8 is shown to play a pivotal role in isthmic organizer activity. Only a strong Fgf signal mediated by Fgf8b activates the Ras-extracellular signal-regulated kinase (ERK) pathway, and this is sufficient to induce cerebellar development. A lower level of signaling transduced by Fgf8a, Fgf17 and Fgf18 induce midbrain development. Numerous feedback loops then maintain appropriate mesencephalon/rhombomere1 and organizer gene expression.

Regulation of gene expression by Pax6 in ocular cells: a case of tissue-preferred expression of crystallins in lens

Lens development is an excellent model for genetic and biochemical studies of embryonic induction, cell cycle regulation, cellular differentiation and signal transduction. Differentiation of lens is characterized by lens-preferred expression and accumulation of water-soluble proteins, crystallins. Crystallins are required for light transparency, refraction and maintenance of lens integrity. Here, we review mechanisms of lens-preferred expression of crystallin genes by employing synergism between developmentally regulated DNA-binding transcription factors: Pax6, c-Maf, MafA/L-Maf, MafB, NRL, Sox2, Sox1, RARbeta/RXRbeta, RORalpha, Prox1, Six3, gammaFBP-B and HSF2. These factors are differentially expressed in lens precursor cells, lens epithelium and primary and secondary lens fibers. They exert their function in combination with ubiquitously expressed factors (e.g. AP-1, CREB, pRb, TFIID and USF) and co-activators/chromatin remodeling proteins (e.g. ASC-2 and CBP/p300). A special function belongs to Pax6, a paired domain and homeodomain-containing protein, which is essential for lens formation. Pax6 is expressed in lens progenitor cells before the onset of crystallin expression and it serves as an important regulatory factor required for expression of c-Maf, MafA/L-Maf, Six3, Prox1 and retinoic acid signaling both in lens precursor cells and the developing lens. The roles of these factors are illustrated by promoter studies of mouse alphaA-, alphaB-, gammaF- and guinea pig zeta-crystallins. Pax6 forms functional complexes with a number of transcription factors including the retinoblastoma protein, pRb, MafA, Mitf and Sox2. We present novel data showing that pRb antagonizes Pax6-mediated activation of the alphaA-crystallin promoter likely by inhibiting binding of Pax6 to DNA.

Neural tissue co-culture with mesenchyme to investigate patterningof peripheral nerve during murine embryonic limb development

Lateral plate mesoderm is native to the developing limb while other cells such as neurons extend migratory axonal processes from the neural tube. Questions regarding how axons migrate to their proper location in the developing limb remain unanswered. Extracellular matrix molecules expressed in developing limb cartilages, such as the versican proteoglycan, may function as inhibitory cues to nerve migration, thus facilitating its proper patterning. In the present study, a method is described for co-culture of neural tissue with high density micromass preparations of mouse limb mesenchyme in order to investigate neurite patterning during limb chondrogenesis in vitro. Comparison of hdf (heart defect) mouse limb mesenchyme, which bears an insertional mutation in the versican proteoglycan core protein, with wild type demonstrated that the described technique provides a useful method for transgenic analysis in studies of chondrogenic regulation of neurite patterning. Differentiating wild type limb mesenchyme expressed cartilage characteristic Type II collagen and versican at 1 day and exhibited numerous well defined cartilage foci by 3 days. Wild type neurites extended into central regions of host cultures between 3 and 6 days and consistently avoided versican positive chondrogenic aggregates. Wild type neural tubes cultured with hdf limb mesenchyme, which does not undergo cartilage differentiation in a wild type pattern, showed that axons exhibited no avoidance characteristics within the host culture. Results suggest that differentiating limb cartilages may limit migration of axons thus aiding in the ultimate patterning of peripheral nerve in the developing limb.

The PAX6 gene is activated by the basic helix-loop-helix transcription factor NeuroD/BETA2

PAX6 is a transcription factor that plays an important role during pancreatic morphogenesis. The aim of the present study is to identify the upstream activator(s) of the PAX6 gene possibly involved in the early stages of pancreatic differentiation. Recently, individual elements regulating PAX6 gene activity in the pancreas have been identified in a 1100 bp Spe / Hin cII fragment 4.6 kb upstream of exon 0. Preliminary sequence analysis of this region revealed some potential DNA-binding sites (E boxes) specific for the binding of basic helix-loop-helix transcription factors. By using electrophoretic mobility shift assays, we demonstrated that both nuclear protein extracts from insulin-secreting RINm5F cells and in vitro -translated NeuroD/BETA2 can bind specifically to these E boxes. Furthermore, by transient transfection experiments we demonstrated that the expression of basic helix-loop-helix transcription factor NeuroD/BETA2 can induce activation of the PAX6 promoter in the NIH-3T3 cell line. Thus we show that NeuroD/BETA2 is involved in the activation of the expression of PAX6 through E boxes in the PAX6 promoter localized in a 1.1 kb sequence within the 4.6 kb untranslated region upstream of exon 0.

Homeobox gene expression in adult dorsal root ganglia during sciatic nerve regeneration: is regeneration a recapitulation of development?

After damage of the sciatic nerve, a regeneration process is initiated. Neurons in the dorsal root ganglion regrow their axons and functional connections. The molecular mechanisms of this neuronal regenerative process have remained elusive, but a relationship with developmental processes has been conceived. This chapter discusses the applicability of the developmental hypothesis of regeneration to the dorsal root ganglion; this hypothesis states that regeneration of dorsal root ganglion neurons is a recapitulation of development. We present data on changes in gene expression upon sciatic nerve damage, and the expression and function of homeobox genes. This class of transcription factors plays a role in neuronal development. Based on these data, it is concluded that the hypothesis does not hold for dorsal root ganglion neurons, and that regeneration-specific mechanisms exist. Cytokines and the associated Jak/STAT (janus kinase/signal transducer and activator of transcription) signal transduction pathway emerge as constituents of a regeneration-specific mechanism. This mechanism may be the basis of pharmacological strategies to stimulate regeneration.

PAX3-FKHR transformation increases 26 S proteasome-dependent degradation of p27Kip1, a potential role for elevated Skp2 expression

PAX3-FKHR is an oncogenic form of the developmental regulator Pax3 transcription factor. PAX3-FKHR results from a t(2,13) chromosomal translocation, a unique genetic marker of alveolar rhabdomyosarcoma. In this study, we showed that ectopic expression of PAX3-FKHR, but not Pax3, in fibroblasts altered cell cycle control and accelerated G(0)/G(1) to S cell cycle transition. PAX3-FKHR-expressing cells had reduced expression of p27(Kip1) protein, a key cell cycle regulator. The reduction in p27(Kip1) levels by PAX3-FKHR resulted from destabilization of p27(Kip1) as shown by cycloheximide treatment and in vivo pulse-chase labeling experiments. The reduced p27(Kip1) protein level in PAX3-FKHR-expressing cells was restored to the level of control cells by treatment with chemical inhibitors that specifically blocked 26 S proteasome activity. Along with the reduction in p27(Kip1) protein, PAX3-FKHR-expressing cells exhibited elevated expression of F-box Skp2 protein, a substrate-specific component of SCF (Skp1-Cullin-F box protein) ligase involved in the cell cycle-dependent control of p27(Kip1) ubiquitination and 26 S proteasome dependent degradation. Finally, we showed that ectopic expression of p27(Kip1) in PAX3-FKHR-expressing cells significantly reduced the proliferation and colony-forming potential of these cells, implicating that down-regulation of p27(Kip1) protein played an active role in the PAX3-FKHR-directed cell transformation.

Genomic organization and promoter analysis of the human nicotinic acetylcholine receptor alpha6 subunit (CHNRA6) gene: Alu and other elements direct transcriptional repression

Nicotinic acetylcholine receptors (nAChRs) form ligand-gated ion channels involved in fast synaptic transmission. Recently mutations in nAChR genes have been reported in nocturnal frontal lobe epilepsy. We performed molecular analysis on the human neuronal nAChR alpha6 subunit (CHRNA6) gene, a member of nAChR gene family, to understand its role in disease. Genomic analysis revealed that the gene consisted of six exons with an estimated size of 16 kb. We mapped the CHRNA6 gene to chromosome 8p11.21-11.22 by fluorescence in situ hybridization, the same putative region responsible for adolescent-onset idiopathic generalized epilepsy. Examination of the 5'-regulatory region failed to identify either a TATA box or GC-rich sequences, but did highlight tandem Alu sequences, located between 910 and 370 bp upstream from a potential cap site. Analyses of transcriptional activity, performed using nested deletions of the 5'-upstream region, showed that the downstream Alu repeat and another element(s) in the promoter region, function as negative regulators. Further analyses of the tandem Alu repeats, examined by fusing them to a different ion channel gene promoter, confirmed their role as transcriptional repressors regardless of their orientation and copy number. These data may explain the limited expression of the CHRNA6 gene in the brain.

Identification of essential amino acid changes in paired domain evolution using a novel combination of evolutionary analysis and in vitro and in vivo studies

Pax genes are defined by the presence of a paired box that encodes a DNA-binding domain of 128 amino acids. They are involved in the development of the central nervous system, organogenesis, and oncogenesis. The known Pax genes are divided into five groups within two supergroups. By means of a novel combination of evolutionary analysis, in vitro binding assays and in vivo functional analyses, we have identified the key residues that determine the differing DNA-binding properties of the two supergroups and of the Pax-2, 5, 8 and Pax-6 subgroups within supergroup I. The differences in binding properties between the two supergroups are largely caused by amino acid changes at residues 20 and 121 of the paired domain. Although the paired domains of the Pax-2, 5, 8 and the Pax-6 group differ by >19 amino acids, their distinct DNA-binding properties are determined almost completely by a single amino acid change. Thus, a small number of amino acid changes can account in large part for the divergence in binding properties among the known paired domains. Our approach for selecting candidate sites responsible for the functional divergence between genes should also be useful for studying other gene families.

Structural and functional characterization of the human PAX7 5'-flanking regulatory region

The human PAX7 gene is a member of the paired box containing gene family of transcription factors implicated in development of the skeletal muscle of the trunk and limbs as well as elements of the central nervous system. To understand the molecular mechanisms involved in its expression, we have localized the transcription start sites in adult skeletal muscle and functionally characterized the 5'-flanking regulatory region responsible for PAX7 expression in this tissue. The major transcription start was identified 664 bp upstream from the ATG codon using primer extension and 5'-rapid amplification of cDNA ends (5'-RACE). Analysis of the 5'-flanking sequence revealed the absence of a TATA-box and the presence of an inverted CCAAT-box. Several consensus sites for common transcriptional regulators including Oct-1, NF1, AP2, AP4, CREB, Sp1, Nkx2.5, and MyoD are present in the promoter region. To determine the sites critical for the function of the PAX7 promoter, a series of deletion fragments of the 5'-flanking region were cloned adjacent to luciferase reporter gene and expressed in RD, Cos-7 and JAR cell lines. The maximal promoter activity was achieved by a fragment extending from the position -403 to +373. No strong positive or negative regulatory elements were discovered by adding of further sequences (up to 2.97 kb). A polymorphic (CCT)(n) repeat sequence was found 107 bp upstream of the transcription initiation site. PCR-based systematic screening for length variations in 227 unrelated individuals of a Caucasian population showed a bimodal distribution of three alleles containing 8, 10 or 11 repeat units. When different variants of this PAX7 gene-linked polymorphic region (PAX7-LPR) were fused to a luciferase reporter gene and transfected into RD cells, the variant with 11 repeat units revealed higher transcriptional efficiency compared to the 8 or 10 repeat alleles.

BMP signaling patterns the dorsal and intermediate neural tube via regulation of homeobox and helix-loop-helix transcription factors

In the spinal neural tube, populations of neuronal precursors that express a unique combination of transcription factors give rise to specific classes of neurons at precise locations along the dorsoventral axis. Understanding the patterning mechanisms that generate restricted gene expression along the dorsoventral axis is therefore crucial to understanding the creation of diverse neural cell types. Bone morphogenetic proteins (BMPs) and other transforming growth factor β (TGFβ) proteins are expressed by the dorsal-most cells of the neural tube (the roofplate) and surrounding tissues, and evidence indicates that they play a role in assigning cell identity. We have manipulated the level of BMP signaling in the chicken neural tube to show that BMPs provide patterning information to both dorsal and intermediate cells. BMP regulation of the expression boundaries of the homeobox proteins Pax6, Dbx2 and Msx1 generates precursor populations with distinct developmental potentials. Within the resulting populations, thresholds of BMP act to set expression domain boundaries of developmental regulators of the homeobox and basic helix-loop-helix (bHLH) families, ultimately leading to the generation of a diversity of differentiated neural cell types. This evidence strongly suggests that BMPs are the key regulators of dorsal cell identity in the spinal neural tube.

Identification of target genes regulated by PAX3 and PAX3-FKHR in embryogenesis and alveolar rhabdomyosarcoma

PAX3 is a transcription factor important for neural, muscle, and facial development in vertebrates. To identify genes regulated by PAX3, we used a cyclic amplification and selection of targets (CASTing) strategy to isolate cis-regulatory elements bound by PAX3. CASTing libraries were constructed with mouse DNA fragments bound by mouse PAX3, and human genomic DNA fragments bound by human PAX3 and the fusion protein PAX3-FKHR. Approximately 1000 clones were sequenced from each of these three libraries. Numerous putative targets of PAX3 and PAX3-FKHR were identified and six genes, Itm2A, Fath, FLT1, TGFA, BVES, and EN2, were examined closely. The genomic DNA fragments near these genes contain PAX3 binding sites and confer PAX3-dependent regulation. The expression levels of these genes correlate with the PAX3 expression levels in mouse embryos or with PAX3-FKHR expression levels in rhabdomyosarcoma cell lines, and indicate they may be part of the PAX3 regulatory circuitry during embryogenesis and tumor formation.

Expression of homeobox genes in cervical cancer

OBJECTIVE

Members of the homeobox (HB) gene superfamily encode transcription factors crucial for development and may be associated with tumorigenesis. In this study, we aimed to develop a procedure to survey the expression of the dispersed-type HB genes in cervical cancer cells.

METHODS

Nineteen sets of degenerate primers were designed based on conserved homeodomains of known dispersed-type HB genes. A cDNA library derived from HeLa, a cervical cancer cell line, was used. Two successive rounds of PCR were performed using a combination of the HB degenerate primers and a primer recognizing the flanking sequence of the vector used in the cDNA library construction.

RESULTS

On cloning and sequence analysis of the PCR fragments generated, 10 known and 3 putative novel HB genes were detected in HeLa. RT-PCR expression analysis further showed that HOXD9 and ATBF1 were differentially expressed in cancer cells and not in normal cervix.

CONCLUSIONS

Our data demonstrate the feasibility of using degenerate primers in PCR experiments in a collective analysis of complex gene families. Our data indicate that HOXD9 and ATBF1 are expressed in cervical cancer, but not in normal cervix.

Pax6 controls the expression of Lewis x epitope in the embryonic forebrain by regulating alpha 1,3-fucosyltransferase IX expression

Pax6 is a transcription factor involved in brain patterning and neurogenesis. Expression of Pax6 is specifically observed in the developing cerebral cortex, where Lewis x epitope that is thought to play important roles in cell interactions is colocalized. Here we examined whether Pax6 regulates localization of Lewis x using Pax6 mutant rat embryos. The Lewis x epitope disappeared in the Pax6 mutant cortex, and activity of alpha1,3-fucosyltransferase, which catalyzed the last step of Lewis x biosynthesis, drastically decreased in the mutant cortex as compared with the wild type. Furthermore, expression of a fucosyltransferase gene, FucT-IX, specifically decreased in the mutant, while no change was seen for expression of another fucosyltransferase gene, FucT-IV. These results strongly suggest that Pax6 controls Lewis x expression in the embryonic brain by regulating FucT-IX gene expression.

Early mesodermal phenotypes in splotch suggest a role for Pax3 in the formation of epithelial somites

The paired box containing transcription factor Pax3 is a crucial regulator of dermomyotome and muscle development. However, the allelic series of Pax3/Splotch mutants also displays characteristic vertebral column malformations, which do not result from defective dorsoventral somite pattern. Rather, vertebral column and sclerotomal phenotypes are reminiscent of the phenotypes observed in the segmentation/somitogenesis mutants rachiterata and pudgy. Moreover, rostrocaudal somite pattern and somitic boundaries are disturbed in Splotch as monitored by the expression of Uncx4.1 and Lunatic fringe. Alterations in EphA4, Dll1, and Uncx4.1 expression are evident already in the condensing paraxial mesoderm, correlating with the first phase of Pax3 expression before and during somite formation. This finding suggests an early function of Pax3 during the formation of epithelial somites.

Overexpression of Pax5 is not sufficient for neoplastic transformation of mouse neuroectoderm

The developmental control genes of the Pax family are essential for brain development. Several Pax genes are also involved in chromosomal translocations causing malignancies in humans, and Pax5 expression is deregulated in medulloblastomas. We have investigated whether Pax5 can induce tumors in the developing mouse brain. Primary mouse embryonic neuroectodermal cells were retrovirally transduced with mouse Pax5 and transplanted into the brain of syngeneic host mice. No tumors developed in 36 transplants after one year, and there were no alterations in the differentiation pattern of the neural transplants. We then generated transgenic mice expressing human Pax5 under control of the Engrailed-2 promoter, which is expressed in the cerebellar external granule cell layer and in medulloblastomas. Sustained expression was achieved in the cerebellum of transgenic animals throughout lifetime. Expression levels were similar to those observed in human medulloblastomas. Again, cerebellar morphogenesis was undisturbed, and no tumors arose. These results strongly argue against a dominant transforming activity of PAX5 in NEC and in cerebellar granule cell precursors of mice, and underline the restricted tissue-specificity of PAX5 related oncogenesis.

Common developmental genetic mechanisms for patterning invertebrate and vertebrate brains

Recent genetic studies on embryonic brain development in the fly Drosophila melanogaster together with investigations on early morphogenesis and patterning in the embryonic brain of the mouse revealed developmental mechanisms that are strikingly similar in insects and mammals. The homeotic (Hox) genes are expressed in a virtually colinear anteroposterior pattern in the developing posterior brain of insects and mammals, where they are required for the specification of segmental neuronal identity. The otd/Otx cephalic gap genes are expressed in the anterior brain of insects and mammals and are of central importance for its formation because in both phyla loss of otd/Otx2 causes the loss of the entire rostral brain. Specific Pax genes are involved in numerous aspects of brain development in both phyla. These developmental genetic findings reveal a striking evolutionary conservation of cephalic gap gene, homeotic gene, and Pax gene action in embryonic brain development that extends beyond gene structure to encompass patterned expression and function. This comparative evidence indicates that the genetic programs which direct embryonic brain development are remarkably conserved and lends further support to the hypothesis that a common molecular bauplan underlies brain development in invertebrates and vertebrates. In consequence, it seems increasingly likely that both modern brain types share their evolutionary origin in a common ancestral bilaterian brain which was established before the protostome-deuterostome divergence over 600 million years ago.

Differentiation therapy of human cancer: basic science and clinical applications

Current cancer therapies are highly toxic and often nonspecific. A potentially less toxic approach to treating this prevalent disease employs agents that modify cancer cell differentiation, termed 'differentiation therapy.' This approach is based on the tacit assumption that many neoplastic cell types exhibit reversible defects in differentiation, which upon appropriate treatment, results in tumor reprogramming and a concomitant loss in proliferative capacity and induction of terminal differentiation or apoptosis (programmed cell death). Laboratory studies that focus on elucidating mechanisms of action are demonstrating the effectiveness of 'differentiation therapy,' which is now beginning to show translational promise in the clinical setting.

Rostrocaudal nuclear relationships in the avian medulla oblongata: a fate map with quail chick chimeras

We present a correlative fate map of the nonsegmented caudal hindbrain down to the medullospinal boundary (medulla oblongata), as a companion to a previous fate mapping study of the hindbrain rhombomeres r2-r6 in quail chick chimeras at stages HH10/11 [Marín and Puelles (1995) Eur J Neurosci 7:1714-1738]. For reproducibility and equivalent precision of analysis, successive portions of the medulla-called pseudorhombomeres "r7" to "r11"-were delimited by transverse planes through the center of adjacent somites at stages HH10/11. These units were each grafted homotopically and isochronically from quail donors into chick hosts. The chimeric specimens were fixed at stages HH35/36 and alternate Nissl-stained sagittal sections were compared to adjacent sections in which quail cells were detected immunocytochemically. This analysis in general showed that there is little intermixing between adjacent pseudorhombomeric domains, although some neuronal populations in the vestibular and trigeminal columns, as well as in the reticular formation and pontine nuclei, do migrate selectively into the host hindbrain. Contralateral migration was scarce up to the stages examined. Several motor nuclei, i.e., the vagal motor complex, or sensory nuclei, i.e., the medial vestibular nucleus, show cytoarchitectonic limits that coincide with pseudorhombomeric ones; however, most conventional grisea were found to originate across several pseudorhombomeres. The inferior olivary complex originated between "r8" and "r11" (between the centers of somites 1 and 5). The medullospinal boundary coincided precisely with the center of the fifth somite, slightly caudal to the obex and the end of the choroidal roof, and correlated with the end of many medullary cytoarchitectonic units. In contrast, the dorsal column nuclei and the caudal subnucleus of the descending trigeminal column fell within the spinal cord. On the whole, the patterns observed were very similar to those found before within the overtly segmented part of the hindbrain, suggesting that some underlying common mechanism may account for the transverse cytoarchitectonic boundaries.

Proteoglycans in the developing brain: new conceptual insights for old proteins

Proteoglycans are a heterogeneous class of proteins bearing sulfated glycosaminoglycans. Some of the proteoglycans have distinct core protein structures, and others display similarities and thus may be grouped into families such as the syndecans, the glypicans, or the hyalectans (or lecticans). Proteoglycans can be found in almost all tissues being present in the extracellular matrix, on cellular surfaces, or in intracellular granules. In recent years, brain proteoglycans have attracted growing interest due to their highly regulated spatiotemporal expression during nervous system development and maturation. There is increasing evidence that different proteoglycans act as regulators of cell migration, axonal pathfinding, synaptogenesis, and structural plasticity. This review summarizes the most recent data on structures and functions of brain proteoglycans and focuses on new physiological concepts for their potential roles in the developing central nervous system.

Pax1/Pax9-Related genes in an agnathan vertebrate, Lampetra japonica: expression pattern of LjPax9 implies sequential evolutionary events toward the gnathostome body plan

Among the transcription factor gene families, Pax genes play important and unique roles in morphological patterning of animal body plans. Of these, Group I Pax genes (Pax1 and Pax9) are expressed in the endodermal pharyngeal pouches in many groups of deuterostomes, and vertebrates seem to have acquired more extensive expression domains in embryos. To understand the evolution of Pax1/Pax9-related genes in basal groups of vertebrates, their cognates were isolated from the Japanese marine lamprey, Lampetra japonica. RT-PCR of larval lamprey cDNA yielded two different fragments containing vertebrate Pax1- and Pax9-like paired domains. The Pax9 orthologue was isolated and named LjPax9. Whole-mount in situ hybridization revealed that this gene was expressed in endodermal pharyngeal pouches, mesenchyme of the velum (the oral pumping apparatus) and the hyoid arch, and the nasohypophysial plate, but not in the somitic mesoderm of the lamprey embryo. These expression patterns could be regarded as a link between the basal chordates and the gnathostomes and are consistent with the phylogenetic position of the lamprey. Especially, the appearance of neural crest seemed to be the basis of velar expression. Homology of the velum and the jaw is also discussed based on the LjPax9 expression in the first pharyngeal pouch and in the velar mesenchyme. We conclude that Pax9 genes have sequentially expanded into new expression domains through evolution as more complicated body plans emerged.

Xenopus Pax-2/5/8 orthologues: novel insights into Pax gene evolution and identification of Pax-8 as the earliest marker for otic and pronephric cell lineages

Pax genes are a family of transcription factors playing fundamental roles during organogenesis. We have recently demonstrated the expression of Pax-2 during Xenopus embryogenesis [Heller N, Brändli AW (1997): Mech Dev 69: 83-104]. Here we report the cloning and characterization of Xenopus Pax-5 and Pax-8, two orthologues of the Pax-2/5/8 gene family. Molecular phylogenetic analysis indicates that the amphibian Pax-2/5/8 genes are close relatives of their mammalian counterparts and that all vertebrate Pax-2/5/8 genes are derived from a single ancestral gene. Xenopus Pax-2/5/8 genes are expressed in spatially and temporally overlapping patterns during development of at least seven distinct tissues. Most strikingly, Xenopus Pax-8 was identified as the earliest marker of the prospective otic placode and of the intermediate mesoderm, indicating that Pax-8 may play a central role in auditory and excretory system development. Comparison of the expression patterns of fish, amphibian, and mammalian Pax-2/5/8 genes revealed that the tissue specificity of Pax-2/5/8 gene family expression is overall evolutionarily conserved. The expression domains of individual orthologues can however vary in a species-specific manner. For example, the thyroid glands of mammals express Pax-8, while in Xenopus Pax-2 is expressed instead. Our findings indicate that differential silencing of Pax-2/5/8 gene expression may have occurred after the different classes of vertebrates began to evolve separately.

The COOH-terminal transactivation domain plays a key role in regulating the in vitro and in vivo function of Pax3 homeodomain

Efficient transcription activation by Pax3 requires binding to a complex DNA sequence element containing binding sites for both the paired domain and the Prd type homeodomain. Previously, we have shown that this requirement is lost in PAX3-FKHR, the product of a t(2;13) chromosomal translocation associated with alveolar rhabdomyosarcoma. In contrast to Pax3, the chimeric PAX3-FKHR, which acts as an oncogene, can efficiently activate a DNA sequence element containing only a homeodomain binding site (TAATAN(2-3)ATTA), despite the presence of an intact Pax3 paired domain. Here, we showed that this alteration in sequence-specific transcription activity was determined in part by the transactivation domain. First, we demonstrated that in intact Pax3, substitution of the Pax3 transactivation domain with an unrelated viral VP16 transactivation domain enabled Pax3 to transactivate homeodomain-specific DNA sequence, as well as to transform fibroblasts. Furthermore, we could abolish the homeodomain-dependent transcription and transforming activities of PAX3-FKHR by replacing its FKHR transactivation domain with Pax3 transactivation domain. Collectively, these results suggested that the transactivation domain influences the DNA binding specificity of Pax3. The translocation process increased the oncogenic potential of Pax3 by removing the inhibitory action of Pax3 transactivation domain on its homeodomain.

Histogenesis of the cerebral cortex in rat fetuses with a mutation in the Pax-6 gene

The embryonic development of the cerebral cortex was histologically examined in rat homozygotes with a mutation of the Paired box (Pax)-6 gene, rat Small eye (rSey(2)/rSey(2)). Although the cerebral wall was thinner in rSey(2)/rSey(2) than in the wild type at embryonic day 16 (E16), cortical cells of mutants labeled with 5'-bromodeoxyuridine (BrdU) at E13 migrated as normal, settling in superficial layer at E16. Mitotic activity in the ventricular zone, estimated by immunoreactivity for proliferating cell nuclear antigen (PCNA), was also retained. On the other hand, after E20 cells were clustered in abnormally expanded ventricular and intermediate zones of the rSey(2)/rSey(2) cortex. Birthdating studies using BrdU revealed that most of these clustered cells were generated between E18 and E20. Most of clustered cells were immunoreactive for PCNA and highly polysialylated NCAM, while immunoreaction for neurofilament and microtubule-associated protein-2 (MAP-2) was hardly detected in the clusters. Furthermore, apoptosis detected with terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end labeling (TUNEL) was rarely seen, suggesting that the clustered cells remain in an undifferentiating state, but not degenerated by the end of the gestational period. Considering that Pax-6 immunoreactivity was exclusively localized in the ventricular zone of the normal rat cortex throughout the fetal period, the present results suggest that Pax-6 is crucial for differentiation and migration of late-generated cortical neurons.