Pax-5 is expressed at the midbrain-hindbrain boundary during mouse development

Evidence that FGF8 signalling from the midbrain-hindbrain junction regulates growth and polarity in the developing midbrain

The developing vertebrate mesencephalon shows a rostrocaudal gradient in the expression of a number of molecular markers and in the cytoarchitectonic differentiation of the tectum, where cells cease proliferating and differentiate in a rostral to caudal progression. Tissue grafting experiments have implicated cell signalling by the mesencephalic-metencephalic (mid-hindbrain) junction (or isthmus) in orchestrating these events. We have explored the role of Wnt-1 and FGF8 signalling in the regulation of mesencephalic polarity. Wnt-1 is expressed in the caudal mesencephalon and Fgf8 in the most rostral metencephalon. Wnt-1 regulates Fgf8 expression in the adjacent metencephalon, most likely via a secondary mesencephalic signal. Ectopic expression of Fgf8 in the mesencephalon is sufficient to activate expression of Engrailed-2 (En-2) and ELF-1, two genes normally expressed in a decreasing caudal to rostral gradient in the posterior mesencephalon. Ectopic expression of Engrailed-1 (En-1), a functionally equivalent homologue of En-2 is sufficient to activate ELF-1 expression by itself. These results indicate the existence of a molecular hierarchy in which FGF8 signalling establishes the graded expression of En-2 within the tectum. This in turn may act to specify other aspects of A-P polarity such as graded ELF-1 expression. Our studies also reveal that FGF8 is a potent mitogen within the mesencephalon: when ectopically expressed, neural precursors continue to proliferate and neurogenesis is prevented. Taken together our results suggest that FGF8 signalling from the isthmus has a key role in coordinately regulating growth and polarity in the developing mesencephalon.

Pax2 contributes to inner ear patterning and optic nerve trajectory

During gestation, the paired box-containing gene Pax2 is expressed in the mid-hindbrain area, developing eye and inner ear. We generated Pax2 null mutant mice, which show the requirement of Pax2 for the establishment of axonal pathways along the optic stalks and ventral diencephalon. In mutant brains, the optic tracts remain totally ipsilateral due to agenesis of the optic chiasma. Furthermore, Pax2 mutants show extension of the pigmented retina into the optic stalks and failure of the optic fissure to close resulting in coloboma. In the inner ear, Pax2 mutants show agenesis of the cochlea and the spiral ganglion, i.e., the parts of the organ responsible for auditory function and in whose primordium Pax2 is expressed. Our results identify Pax2 as a major regulator of patterning during organogenesis of the eye and inner ear and indicate its function in morphogenetic events required for closure of the optic fissure and neural tube.

Engrailed-1 as a target of the Wnt-1 signalling pathway in vertebrate midbrain development

A secreted signalling molecule encoded by the Wnt-1 gene is required for development of the central nervous system. In mouse embryos homozygous for a Wnt-1-null allele, the midbrain and anterior hindbrain fail to develop. This corresponds to the region where two transcription factors encoded by the mouse engrailed genes (En-1 and En-2) are normally expressed. Studies of the Drosophila orthologue of Wnt-1, wingless, indicate that Wingless signal is required to maintain engrailed expression in neighbouring cells of the developing epidermis. Here we report that expression of En-1 in the developing midbrain of Wnt-1-null embryos is sufficient to rescue early midbrain and anterior hindbrain development. This suggests that a key role of Wnt-1 signalling is to maintain En expression and that this aspect of the Wnt-1/engrailed interaction has been conserved from flies to mice.

Pax-5 (BSAP) recruits Ets proto-oncogene family proteins to form functional ternary complexes on a B-cell-specific promoter

The paired box transcription factor Pax-5 (B-cell-specific activator protein) is a key regulator of lineage-specific gene expression and differentiation in B-lymphocytes. We show that Pax-5 functions as a cell type-specific docking protein that facilitates binding of the early B-cell-specific mb-1 promoter by proteins of the Ets proto-oncogene family. Transcriptional activity of the mb-1 promoter in pre-B-cells is critically dependent on binding sites for Pax-5:Ets complexes. Ternary complex assembly requires only the Pax-5 paired box and ETS DNA-binding domains. Mutation of a single base pair in the ternary complex binding site allows for independent binding by Ets proteins but, conversely, inhibits the binding of Pax-5 by itself. Strikingly, the mutation reverses the pattern of complex assembly: Ets proteins recruit Pax-5 to bind the mutated sequence. Recruitment of Net and Elk-1, but not SAP1a, by Pax-5 defines a functional difference between closely related Ets proteins. Replacement of a valine (V68) in the ETS domain of SAP1a by aspartic acid (as found in c-Ets-1, Elk-1, and Net) enhanced ternary complex formation by more than 60-fold. Together, these observations define novel transcription factor interactions that regulate gene expression in B cells.

Inhibition of floor plate differentiation by Pax3: evidence from ectopic expression in transgenic mice

The Pax genes containing a complete paired-type homeobox are expressed in restricted dorsoventral domains of the undifferentiated neuroepithelium. Their expression respond to signals that pattern the neural tube and which emanate from the notochord, floor plate and overlying ectoderm. In order to determine whether the dorsally restricted Pax3 gene can influence the fate of cells within the neural tube, we produced transgenic mice overexpressing Pax3 in the entire neural tube under the Hoxb-4 promoter/region A enhancer. In two distinct transgenic mouse lines, we observed embryos with abnormal limb, eye, brain and neural tube development. Expression of Pax3 was not sufficient to dorsalize cells from the ventral intermediate zone, which still expressed ventral markers. However, in these embryos, expression of Pax3 at the ventral midline was associated with the absence of floor plate differentiation. Under these conditions, motor neurons still differentiated in the ventral spinal cord, although in fewer numbers.

Roles of cell-autonomous mechanisms for differential expression of region-specific transcription factors in neuroepithelial cells

Although a number of genes have been found to have restricted expression domains in the embryonic forebrain and midbrain, it remains largely unknown how the expression of these genes is regulated at the cellular level. In this study, we explored the mechanisms for the differential expression of region-specific transcription factors in neuroepithelial cells by using both primary and immortalized neuroepithelial cells from the rat brain at embryonic day 11.5. We found that differential expression patterns of Pax-3, Pax-5, Pax-6, Dlx-1, Dlx-2, Emx2, Otx1 and Dbx observed in vivo were maintained even when the cells were isolated and cultured in vitro, free from environmental influences. Furthermore, in response to Sonic hedgehog, which is a major inductive signal from the environment for regional specification, neuroepithelial cells that maintain distinct regional identities expressed different sets of ventral-specific genes including Islet-1, Nkx-2.1 and Nkx-2.2. These results suggest that certain cell-autonomous mechanisms play important roles in regulating both environmental signal-dependent and -independent expression of region-specific genes. Thus, we propose that use of the in vitro culture systems we describe in this study facilitates the understanding of regulatory mechanisms of region-specific genes in neuroepithelial cells.

Two Pax-binding sites are required for early embryonic brain expression of an Engrailed-2 transgene

The temporally and spatially restricted expression of the mouse Engrailed (En) genes is essential for development of the midbrain and cerebellum. The regulation of En-2 expression was studied using in vitro protein-DNA binding assays and in vivo expression analysis in transgenic mice to gain insight into the genetic events that lead to regionalization of the developing brain. A minimum En-2 1.0 kb enhancer fragment was defined and found to contain multiple positive and negative regulatory elements that function in concert to establish the early embryonic mid-hindbrain expression. Furthermore, the mid-hindbrain regulatory sequences were shown to be structurally and functionally conserved in humans. The mouse paired-box-containing genes Pax-2, Pax-5 and Pax-8 show overlapping expression with the En genes in the developing brain. Significantly, two DNA-binding sites for Pax-2, Pax-5 and Pax-8 proteins were identified in the 1.0 kb En-2 regulatory sequences, and mutation of the binding sites disrupted initiation and maintenance of expression in transgenic mice. These results present strong molecular evidence that the Pax genes are direct upstream regulators of En-2 in the genetic cascade controlling mid-hindbrain development. These mouse studies, taken together with others in Drosophila and zebrafish on the role of Pax genes in controlling expression of En family members, indicate that a Pax-En genetic pathway has been conserved during evolution.

BSAP: a key regulator of B-cell development and differentiation

B-cell-specific activator protein (BSAP) is a recently identified member of the Pax-gene family of transcription factors; in the lymphoid system, BSAP is produced only in B cells. Here, Markus Neurath, Eckhard Stüber and Warren Strober describe the molecular structure of BSAP and focus on the ability of this protein to regulate the expression of B-cell-specific genes. They propose that BSAP is a key protein of B cells and that it not only influence B-cell development but also influences the balance between B-cell proliferation and immunoglobulin secretion at later stages of B-cell differentiation.

Determination events in the nervous system of the vertebrate embryo

Molecular and functional data suggest that the regionalization of the caudal portion of the vertebrate embryonic brain (hindbrain) is set up through a process of segmentation. In contrast, the more rostrally located met-mesencephalic domain (mid-hindbrain junction) appears to follow a mode of specification that relies on long-range inducing and organizing activities originating from the central region of the domain. Recent studies addressing this mode of anterior/posterior determination point to Wnt-1 as a key player in this process.

Pax-2 expression in the murine neural plate precedes and encompasses the expression domains of Wnt-1 and En-1

In the Drosophila embryo, activation of wingless and engrailed in the parasegment requires paired, a member of the Pax family of transcription factors. We have explored the possible conservation of this regulatory hierarchy in the developing mouse brain. We find that Pax-2 is expressed prior to somite formation in the presumptive mid/hindbrain region. Shortly thereafter, Wnt-1 (the wingless orthologue) and Engrailed-1 are expressed in overlapping regions within the Pax-2 domain. Pax-5 expression commences later, at the 3-somite stage. Thus, the spatial and temporal expression of Pax-2 is consistent with a possible regulatory role in the activation of Wnt-1 and En-1.

Deregulated expression of PAX5 in medulloblastoma

Medulloblatoma is a pediatric brain tumor originating in the human cerebellum. A collection of 23 medulloblastomas was analyzed for expression of the developmental control genes of the PAX and EN gene families by RNase protection and in situ hybridization. Of all nine PAX genes investigated, only PAX5 and PAX6 were consistently expressed in most medulloblastomas (70 and 78% of all cases, respectively), as were the genes EN1 (57%) and EN2 (78%). EN1, EN2, and PAX6 genes were also expressed in normal cerebellar tissue, and their expression in medulloblastoma is consistent with the hypothesis that this tumor originates in the external granular layer of the developing cerebellum. PAX5 transcripts were, however, not detected in the neonatal cerebellum, indicating that this gene is deregulated in medulloblastoma. In the desmoplastic variant of medulloblastoma, PAX5 expression was restricted to the reticulin-producing proliferating tumor areas containing undifferentiated cells; PAX5 was not expressed in the reticulin-free nonproliferating islands undergoing neuronal differentiation. These data suggest that deregulated expression of PAX5 correlates positively with cell proliferation and inversely with neuronal differentiation in desmoplastic medulloblastoma.

Hematopoietic lineage commitment: role of transcription factors

This review focuses on the roles of transcription factors in hematopoietic lineage commitment. A brief introduction to lineage commitment and asymmetric cell division is followed by a discussion of several methods used to identify transcription factors important in specifying hematopoietic cell types. Next is presented a discussion of the use of embryonic stem cells in the analysis of hematopoietic gene expression and the use of targeted gene disruption to analyze the role of transcription factors in hematopoiesis. Finally, the status of our current knowledge concerning the roles of transcription factors in the commitment to erythroid, myeloid and lymphoid cell types is summarized.

Distinct functional properties of three human paired-box-protein, PAX8, isoforms generated by alternative splicing in thyroid, kidney and Wilms' tumors

The mammalian paired box (Pax) genes encode a family of transcription factors involved in embryogenesis. The murine and human Pax8 genes are expressed in developing and adult thyroid as well as in the developing secretory system and at the lower level in adult kidney. In the secretory system expression is localized to the induced, extensively differentiating parts that undergo a transition from mesenchyme to epithelium. The human PAX8 gene generates at least five different alternatively spliced transcripts encoding different PAX8 isoforms. These isoforms differ in their carboxy-terminal regions downstream of the paired domain that has been shown previously to be responsible for the DNA binding. The PAX8a isoform contains a 63 amino-acid serine-rich region that is absent in the isoform PAX8b whereas PAX8c reveals a novel 99-amino-acid proline-rich region. This proline-rich region arises due to an unusual reading-frame shift in the PAX8 transcript. RNAse protection and RT(reverse transcription)-PCR analysis show the expression of all three PAX8 transcripts in human thyroid, kidney and five Wilms' tumors. Band-shift assay indicates a greatly reduced binding affinity of the isoform PAX8c to a DNA sequence from the promoter of the thyroperoxidase gene compared to the binding of PAX8a and PAX8b to this sequence. Deletion analysis of murine PAX8a indicates that its activating domain residues at the carboxy terminus of the protein which is shared by isoforms PAX8a and PAX8b. In accordance with these data PAX8a and PAX8b activate transcription from a thyroglobulin promoter as well as from a cotransfected synthetic PAX8-specific promoter/chlorampericol acetyltransferase (CAT) reporter containing a Pax8-binding oligonucleotide in front of the basal herpes simplex virus thymidine kinase (HSV-TK) promoter (P11/12-TK-CAT). However if the basal HSV-TK promoter of this reporter is substituted by a minimal adenovirus E1b TATA element, PAX8a and PAX8b fail to activate transcription. Of the three chimaeric forms containing the GAL4 DNA-binding domain at the amino-terminal end fused to the corresponding carboxy-terminal regions of the PAX8 isoforms beginning immediately downstream of the paired domain only a GAL4-PAX8b fusion significantly activates transcription from a cotransfected GAL4-specific upstream-activating-sequence (UAS)-TK-CAT reporter. Substitution of the basal HSV-TK promoter in this reporter by the minimal E1b TATA element does not affect this activation. These results indicate that the PAX8 isoforms display different functional properties and may also function differently in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

Involvement of wnt1 and pax2 in the formation of the midbrain-hindbrain boundary in the zebrafish gastrula

The secreted signalling molecule encoded by the wnt1 gene and the paired box-containing pax2 gene are thought to play an integral role in patterning the zebrafish rostral nervous system. Using a double-label analysis, we compare the expression patterns of wnt1 RNA and pax2 protein during zebrafish embryogenesis to determine whether they were expressed in identical or overlapping patterns in individual embryos. During gastrulation, wnt1 RNA was detected in a pattern similar but not identical to the pax2 protein. Later, wnt1 and pax2 co-localize to the midbrain-hindbrain boundary. Exogenous retinoic acid, a teratogen that is known to affect the formation of the midbrain-hindbrain boundary, has a profound affect on both wnt1 and pax2 expression at gastrulation. Furthermore, when pax2 is overexpressed in zebrafish embryos, the wnt1 pattern of expression expands ventrally in the prospective rostral neuroepithelium. Despite the widespread and random distribution of exogenous pax2 RNA, it alone is unable to induce wnt1 expression in other ectopic sites. These results are consistent with the coordinate expression of wnt1 and pax2 being in a pathway responsible for establishing the midbrain-hindbrain boundary and support the earlier interpretation that pax2 may regulate wnt1 expression [Krauss et al., 1992], although only in a subset of embryonic cells. These data suggest that a predisposition for the regionalization of the central nervous system exists at gastrulation.

PAX genes

PAX genes are developmental control genes that encode transcription factors containing a DNA-binding paired domain. Mutations in three of the nine mouse genes (Pax1, Pax3 and Pax6) and two of the nine human genes (PAX3 and PAX6) are known to cause developmental defects. These defects are caused by loss-of-function alleles; pathogenesis occurs as a result of a half dosage of the PAX gene product in particular cells. Gain-of-function mutations have been implicated in cancer.

The B cell-specific transcription factor BSAP regulates B cell proliferation

The B cell-specific activator protein (BSAP) is a DNA-binding transcription factor expressed in pro-B, pre-B, and mature B cells, but not in plasma cells. In this study, we explored the role of BSAP in B cell function by assessing how the content of this protein varies in cells driven by proliferative stimuli and, conversely, how artificial manipulation of BSAP activity affects cell proliferation. We found that BSAP activity of nuclear extracts increased when B cells were activated by mitogen (lipopolysaccharide [LPS]), antigen receptor-mediated signaling (surface immunoglobulin D [IgD] cross-linking) or T cell-dependent stimulation (CD40 cross-linking). We could suppress BSAP activity by exposure of B cells to phosphorothioate oligonucleotides antisense to the BSAP translation initiation start site, whereas control oligonucleotides were virtually inactive. Antisense-induced BSAP suppression was associated with a striking reduction in LPS-induced proliferation of splenic B cells and in the spontaneous proliferation of B lymphoma cells (CH12.LX), but the antisense oligonucleotide had virtually no effect on proliferation of two cell lines lacking BSAP: the T lymphoma line EL-4 and the plasma cell line MOPC-315. Overexpression of BSAP in splenic B cells or de novo expression in MOPC-315 plasma cells induced by transfection of a BSAP expression plasmid stimulated cell proliferation. Taken together, these results suggest that BSAP activity is a rate-limiting regulator of B cell proliferation. We also found that treatment with the antisense BSAP oligonucleotide downregulated Ig class switching induced by interleukin 4 plus LPS. This effect may be secondary to reduced proliferation or could be mediated through BSAP binding sites in the IgH locus.

Regulation of gene expression at early stages of B-cell differentiation

The phenotype of B lymphocytes at various stages of differentiation is, in part, controlled at the transcriptional level. Recently, a number of B-cell lineage and stage-specific transcription factors have been identified as candidate determinants for the developmental regulation of gene expression in B lymphocytes.

Alternative splicing of Pax-8 gene transcripts is developmentally regulated and generates isoforms with different transactivation properties

Pax-8, a member of the paired box-containing gene family, was shown to be coexpressed with Pax-2 in several human kidney carcinoma cell lines. Four different Pax-8 mRNA isoforms, a to d, were cloned from one of these cell lines by polymerase chain reaction amplification, and the Pax-8 gene was isolated from a human cosmid library. Analysis of the exon-intron structure of Pax-8 revealed that the four mRNA isoforms arise by alternative splicing, resulting in inclusion or exclusion of exon 7 and/or exon 8 sequences. All four Pax-8 proteins retain the paired domain as their DNA-binding motif and recognize DNA in the same manner as do the closely related Pax-2 and BSAP (Pax-5) proteins. The Pax-8a and Pax-8b isoforms end in a serine/threonine/tyrosine-rich sequence, while the C terminus of Pax-8c and Pax-8d is translated in a different, proline-rich reading frame. Transient transfection experiments revealed that Pax-8 isoforms a and b, but not c and d, strongly stimulate transcription from a promoter containing six copies of a paired-domain recognition sequence. The same four mRNA variants were also detected by RNase protection analysis in the mouse embryo and adult kidney, thus indicating evolutionary conservation of Pax-8 mRNA splicing. A different splice pattern was observed in the developing placenta, which expresses two new variants, Pax-8e and Pax-8f, instead of transcripts b to d. Expression of these mRNAs is high at embryonic day 9.5 and is gradually reduced until Pax-8a is the predominant transcript in the 12.5-day placenta. In the embryo, however, the synthesis of mRNAs b to d is initially low and then increases relative to that of Pax-8a. Hence, alternative splicing of Pax-8 gene transcripts not only generates six different Pax-8 variants but is also temporally and spatially regulated during early mouse development.

Pax: gene regulators in the developing nervous system

In recent years, the discovery of Pax genes in mouse has played an invaluable role in furthering our understanding in mouse developmental processes and disorders. To date, eight murine paired box-containing genes have been cloned. Seven of these exhibit a distinct spatiotemporal expression pattern in the developing nervous system implying a role in the regional specification of the developing spinal cord and brain. The Pax genes encode for sequence-specific DNA binding transcription factors that play a key role in embryonic development. Three of these developmental control genes are altered in mutant mice and two are associated with human diseases. Disruption of these Pax genes leads to abnormalities in neural crest derivatives, neuroectoderm, sclerotome or myotome-derived tissues. Disruption of the Pax-3 gene causes the Splotch phenotype in mice and Waardenburg syndrome in humans. Pax-6 mutations result in Small eye mice and the human genetic disorder aniridia. The Pax-1 gene is mutated in undulated mice. Pax proteins can transform cells in culture which then form tumours following injection in nude mice. Consistent with this activity, PAX3 has been recently implicated in the generation of the tumour alveolar rhabdomyosarcoma.

Alternative splicing of Pax-8 gene transcripts is developmentally regulated and generates isoforms with different transactivation properties

Pax-8, a member of the paired box-containing gene family, was shown to be coexpressed with Pax-2 in several human kidney carcinoma cell lines. Four different Pax-8 mRNA isoforms, a to d, were cloned from one of these cell lines by polymerase chain reaction amplification, and the Pax-8 gene was isolated from a human cosmid library. Analysis of the exon-intron structure of Pax-8 revealed that the four mRNA isoforms arise by alternative splicing, resulting in inclusion or exclusion of exon 7 and/or exon 8 sequences. All four Pax-8 proteins retain the paired domain as their DNA-binding motif and recognize DNA in the same manner as do the closely related Pax-2 and BSAP (Pax-5) proteins. The Pax-8a and Pax-8b isoforms end in a serine/threonine/tyrosine-rich sequence, while the C terminus of Pax-8c and Pax-8d is translated in a different, proline-rich reading frame. Transient transfection experiments revealed that Pax-8 isoforms a and b, but not c and d, strongly stimulate transcription from a promoter containing six copies of a paired-domain recognition sequence. The same four mRNA variants were also detected by RNase protection analysis in the mouse embryo and adult kidney, thus indicating evolutionary conservation of Pax-8 mRNA splicing. A different splice pattern was observed in the developing placenta, which expresses two new variants, Pax-8e and Pax-8f, instead of transcripts b to d. Expression of these mRNAs is high at embryonic day 9.5 and is gradually reduced until Pax-8a is the predominant transcript in the 12.5-day placenta. In the embryo, however, the synthesis of mRNAs b to d is initially low and then increases relative to that of Pax-8a. Hence, alternative splicing of Pax-8 gene transcripts not only generates six different Pax-8 variants but is also temporally and spatially regulated during early mouse development.

Evolution and role of Pax genes

Pax genes encode a class of highly conserved transcription factors containing a paired-domain. These factors play important roles in Drosophila and vertebrate development, for example, in segmentation and neurogenesis. Their developmental roles are assessed in terms of their participation in conserved gene networks and mechanisms that establish positional information.

Murine Pax-2 protein is a sequence-specific trans-activator with expression in the genital system

The murine paired box containing gene Pax-2 has been proposed to be involved in kidney and central nervous system (CNS) development. In this report, we show that expression cloning of Pax-2 cDNA allowed in vitro identification of specifically bound DNA sequences. When fused to the thymidine kinase (TK) promoter in front of reporter genes, these target sequences were able to mediate trans-activation by Pax-2 protein, thus demonstrating their in vivo function. Expression studies from adult mouse tissues revealed high levels of Pax-2 transcripts in male and female genital tracts, suggesting a second phase of Pax-2 activity. Sequence-specific DNA binding and subsequent modulation of promoter activities may constitute the molecular mechanism of Pax-2 action in specific adult tissues and during development.