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

PAX8 in the Junction between Development and Tumorigenesis

Normal processes of embryonic development and abnormal transformation to cancer have many parallels, and in fact many aberrant cancer cell capabilities are embryonic traits restored in a distorted, unorganized way. Some of these capabilities are cell autonomous, such as proliferation and resisting apoptosis, while others involve a complex interplay with other cells that drives significant changes in neighboring cells. The correlation between embryonic development and cancer is driven by shared proteins. Some embryonic proteins disappear after embryogenesis in adult differentiated cells and are restored in cancer, while others are retained in adult cells, acquiring new functions upon transformation to cancer. Many embryonic factors embraced by cancer cells are transcription factors; some are master regulators that play a major role in determining cell fate. The paired box (PAX) domain family of developmental transcription factors includes nine members involved in differentiation of various organs. All paired box domain proteins are involved in different cancer types carrying pro-tumorigenic or anti-tumorigenic roles. This review focuses on PAX8, a master regulator of transcription in embryonic development of the thyroid, kidney, and male and female genital tracts. We detail the role of PAX8 in each of these organ systems, describe its role during development and in the adult if known, and highlight its pro-tumorigenic role in cancers that emerge from PAX8 expressing organs.

An Update on the Molecular Mechanism of the Vertebrate Isthmic Organizer Development in the Context of the Neuromeric Model

A crucial event during the development of the central nervous system (CNS) is the early subdivision of the neural tube along its anterior-to-posterior axis to form neuromeres, morphogenetic units separated by transversal constrictions and programed for particular genetic cascades. The narrower portions observed in the developing neural tube are responsible for relevant cellular and molecular processes, such as clonal restrictions, expression of specific regulatory genes, and differential fate specification, as well as inductive activities. In this developmental context, the gradual formation of the midbrain-hindbrain (MH) constriction has been an excellent model to study the specification of two major subdivisions of the CNS containing the mesencephalic and isthmo-cerebellar primordia. This MH boundary is coincident with the common Otx2-(midbrain)/Gbx2-(hindbrain) expressing border. The early interactions between these two pre-specified areas confer positional identities and induce the generation of specific diffusible morphogenes at this interface, in particular FGF8 and WNT1. These signaling pathways are responsible for the gradual histogenetic specifications and cellular identity acquisitions with in the MH domain. This review is focused on the cellular and molecular mechanisms involved in the specification of the midbrain/hindbrain territory and the formation of the isthmic organizer. Emphasis will be placed on the chick/quail chimeric experiments leading to the acquisition of the first fate mapping and experimental data to, in this way, better understand pioneering morphological studies and innovative gain/loss-of-function analysis.

Various biological functions of carbohydrate chains learned from glycosyltransferase-deficient mice

Carbohydrate chains are attached to various proteins and lipids and modify their functions. The complex structures of carbohydrate chains, which have various biological functions, are involved not only in regulating protein conformation, transport, and stability but also in cell-cell and cell-matrix interactions. These functional carbohydrate structures are designated as "glyco-codes." Carbohydrate chains are constructed through complex reactions of glycosyltransferases, glycosidases, nucleotide sugars, and protein and lipid substrates in a cell. To elucidate the functions of carbohydrate chains, I and my colleagues generated and characterized knockout (KO) mice of galactosyltransferase family genes. In this review, I introduce our studies about galactosyltransferase family genes together with related studies performed by other researchers, which I presented in my award lecture for the Ando-Tajima Prize of the Japanese Association for Laboratory Animal Science (JALAS) in 2019.

A single-cell transcriptomic and anatomic atlas of mouse dorsal raphe Pet1 neurons

Among the brainstem raphe nuclei, the dorsal raphe nucleus (DR) contains the greatest number of Pet1-lineage neurons, a predominantly serotonergic group distributed throughout DR subdomains. These neurons collectively regulate diverse physiology and behavior and are often therapeutically targeted to treat affective disorders. Characterizing Pet1 neuron molecular heterogeneity and relating it to anatomy is vital for understanding DR functional organization, with potential to inform therapeutic separability. Here we use high-throughput and DR subdomain-targeted single-cell transcriptomics and intersectional genetic tools to map molecular and anatomical diversity of DR-Pet1 neurons. We describe up to fourteen neuron subtypes, many showing biased cell body distributions across the DR. We further show that P2ry1-Pet1 DR neurons - the most molecularly distinct subtype - possess unique efferent projections and electrophysiological properties. These data complement and extend previous DR characterizations, combining intersectional genetics with multiple transcriptomic modalities to achieve fine-scale molecular and anatomic identification of Pet1 neuron subtypes.

The zinc-finger transcription factor GLI3 is a regulator of precerebellar neuronal migration

Hindbrain precerebellar neurons arise from progenitor pools at the dorsal edge of the embryonic hindbrain: the caudal rhombic lip. These neurons follow distinct migratory routes to establish nuclei that provide climbing or mossy fiber inputs to the cerebellum. Gli3, a zinc-finger transcription factor in the Sonic hedgehog signaling pathway, is an important regulator of dorsal brain development. We demonstrate that in Gli3-null mutant mice, disrupted neuronal migratory streams lead to a disorganization of precerebellar nuclei. Precerebellar progenitors are properly established in Gli3-null embryos and, using conditional gene inactivation, we provide evidence that Gli3 does not play a cell-autonomous role in migrating precerebellar neurons. Thus, GLI3 likely regulates the development of other hindbrain structures, such as non-precerebellar nuclei or cranial ganglia and their respective projections, which may in turn influence precerebellar migration. Although the organization of non-precerebellar hindbrain nuclei appears to be largely unaffected in absence of Gli3, trigeminal ganglia and their central descending tracts are disrupted. We show that rostrally migrating precerebellar neurons are normally in close contact with these tracts, but are detached in Gli3-null embryos.

Parvalbumin promoter hypermethylation in postmortem brain in schizophrenia

Deficits of brain parvalbumin (PV) are a consistent finding in schizophrenia and models of psychosis. We investigated whether this is associated with abnormal PV gene (PVALB) methylation in the brain in schizophrenia. Bisulfite pyrosequencing was used to determine cytosine (CpG) methylation in a PVALB promoter sequence. Greater PVALB methylation was found in schizophrenia hippocampus, while no differences were observed in prefrontal cortex. LINE-1 methylation, a measure of global methylation, was also elevated in both regions in schizophrenia, although the PVALB change was independent of this effect. These results provide the first evidence that PVALB promoter methylation is abnormal in schizophrenia and suggest that this epigenetic finding may relate to the reduction of PV expression seen in the disease.

Upregulation of Human ST8Sia VI (α2,8-Sialyltransferase) Gene Expression by Physcion in SK-N-BE(2)-C Human Neuroblastoma Cells

In this research, we firstly demonstrated that physcion, an anthraquinone derivative, specifically increased the expression of the human α2,8-sialyltransferase (hST8Sia VI) gene in SK-N-BE(2)-C human neuroblastoma cells. To establish the mechanism responsible for the up-regulation of hST8Sia VI gene expression in physcion-treated SK-N-BE(2)-C cells, the putative promoter region of the hST8Sia VI gene was functionally characterized. Promoter analysis with serially truncated fragments of the 5′-flanking region showed that the region between −320 and −240 is crucial for physcion-induced transcription of hST8Sia VI in SK-N-BE(2)-C cells. Putative binding sites for transcription factors Pax-5 and NF-Y are located at this region. The Pax-5 binding site at −262 to −256 was essential for the expression of the hST8Sia VI gene by physcion in SK-N-BE(2)-C cells. Moreover, the transcription of hST8Sia VI induced by physcion in SK-N-BE(2)-C cells was inhibited by extracellular signal-regulated protein kinase (ERK) inhibitor U0126 and p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580, but not c-Jun N-terminal kinase (JNK) inhibitor SP600125. These results suggest that physcion upregulates hST8Sia VI gene expression via ERK and p38 MAPK pathways in SK-N-BE(2)-C cells.

Fgf8 signaling for development of the midbrain and hindbrain

In this paper, we review how midbrain and hindbrain are specified. Otx2 and Gbx2 are expressed from the early phase of development, and their expression abuts at the midbrain hindbrain boundary (MHB), where Fgf8 expression is induced, and functions as an organizing molecule for the midbrain and hindbrain. Fgf8 induces En1 and Pax2 expression at the region where Otx2 is expressed to specify midbrain. Fgf8 activates Ras-ERK pathway to specify hindbrain. Downstream of ERK, Pea3 specifies isthmus (rhombomere 0, r0), and Irx2 may specify r1, where the cerebellum is formed.

Thyroid transcription factors in development, differentiation and disease

Identification of the thyroid transcription factors (TTFs), NKX2-1, FOXE1, PAX8 and HHEX, has considerably advanced our understanding of thyroid development, congenital thyroid disorders and thyroid cancer. The TTFs are fundamental to proper formation of the thyroid gland and for maintaining the functional differentiated state of the adult thyroid; however, they are not individually required for precursor cell commitment to a thyroid fate. Although knowledge of the mechanisms involved in thyroid development has increased, the full complement of genes involved in thyroid gland specification and the signals that trigger expression of the genes that encode the TTFs remain unknown. The mechanisms involved in thyroid organogenesis and differentiation have provided clues to identifying the genes that are involved in human congenital thyroid disorders and thyroid cancer. Mutations in the genes that encode the TTFs, as well as polymorphisms and epigenetic modifications, have been associated with thyroid pathologies. Here, we summarize the roles of the TTFs in thyroid development and the mechanisms by which they regulate expression of the genes involved in thyroid differentiation. We also address the implications of mutations in TTFs in thyroid diseases and in diseases not related to the thyroid gland.

Kidney regeneration: common themes from the embryo to the adult

The vertebrate kidney has an inherent ability to regenerate following acute damage. Successful regeneration of the injured kidney requires the rapid replacement of damaged tubular epithelial cells and reconstitution of normal tubular function. Identifying the cells that participate in the regeneration process as well as the molecular mechanisms involved may reveal therapeutic targets for the treatment of kidney disease. Renal regeneration is associated with the expression of genetic pathways that are necessary for kidney organogenesis, suggesting that the regenerating tubular epithelium may be "reprogrammed" to a less-differentiated, progenitor state. This review will highlight data from various vertebrate models supporting the hypothesis that nephrogenic genes are reactivated as part of the process of kidney regeneration following acute kidney injury (AKI). Emphasis will be placed on the reactivation of developmental pathways and how our understanding of the resulting regeneration process may be enhanced by lessons learned in the embryonic kidney.

Derivation of multiple cranial tissues and isolation of lens epithelium-like cells from human embryonic stem cells

Human embryonic stem cells (hESCs) provide a powerful tool to investigate early events occurring during human embryonic development. In the present study, we induced differentiation of hESCs in conditions that allowed formation of neural and non-neural ectoderm and to a lesser extent mesoderm. These tissues are required for correct specification of the neural plate border, an early embryonic transient structure from which neural crest cells (NCs) and cranial placodes (CPs) originate. Although isolation of CP derivatives from hESCs has not been previously reported, isolation of hESC-derived NC-like cells has been already described. We performed a more detailed analysis of fluorescence-activated cell sorting (FACS)-purified cell populations using the surface antigens previously used to select hESC-derived NC-like cells, p75 and HNK-1, and uncovered their heterogeneous nature. In addition to the NC component, we identified a neural component within these populations using known surface markers, such as CD15 and FORSE1. We have further exploited this information to facilitate the isolation and purification by FACS of a CP derivative, the lens, from differentiating hESCs. Two surface markers expressed on lens cells, c-Met/HGFR and CD44, were used for positive selection of multiple populations with a simultaneous subtraction of the neural/NC component mediated by p75, HNK-1, and CD15. In particular, the c-Met/HGFR allowed early isolation of proliferative lens epithelium-like cells capable of forming lentoid bodies. Isolation of hESC-derived lens cells represents an important step toward the understanding of human lens development and regeneration and the devising of future therapeutic applications.

Ontogeny of substantia nigra dopamine neurons

Understanding the ontogeny of A9 dopamine (DA) neurons is critical not only to determining basic developmental events that facilitate the emergence of the substantia nigra pars compacta (SNc) but also to the extraction and de novo generation of DA neurons as a potential cell therapy for Parkinson's disease. Recent research has identified a precise window for DA cell birth (differentiation) in the ventral mesencephalon (VM) as well as a number of factors that may facilitate this process. However, application of these factors in vitro has had limited success in specifying a dopaminergic cell fate from undifferentiated cells, suggesting that other cell/molecular signals may as yet remain undiscovered. To resolve this, current work seeks to identify particularly potent and novel DA neuron differentiation factors within the developing VM specifically at the moment of ontogeny. Through such (past and present) studies, a catalog of proteins that play a pivotal role in the generation of nigral DA neurons during normal CNS development has begun to emerge. In the future, it will be crucial to continue to evaluate the critical developmental window where DA neuron ontogeny occurs, not only to facilitate our potential to protect these cells from degeneration in the adult brain but also to mimic the developmental environment in a way that enhances our ability to generate these cells anew either in vitro or in vivo. Here we review our present understanding of factors that are thought to be involved in the emergence of the A9 dopamine neuron group from the ventral mesencephalon.

Evolution of developmental roles of Pax2/5/8 paralogs after independent duplication in urochordate and vertebrate lineages

Background

Gene duplication provides opportunities for lineage diversification and evolution of developmental novelties. Duplicated genes generally either disappear by accumulation of mutations (nonfunctionalization), or are preserved either by the origin of positively selected functions in one or both duplicates (neofunctionalization), or by the partitioning of original gene subfunctions between the duplicates (subfunctionalization). The Pax2/5/8 family of important developmental regulators has undergone parallel expansion among chordate groups. After the divergence of urochordate and vertebrate lineages, two rounds of independent gene duplications resulted in the Pax2, Pax5, and Pax8 genes of most vertebrates (the sister group of the urochordates), and an additional duplication provided the pax2a and pax2b duplicates in teleost fish. Separate from the vertebrate genome expansions, a duplication also created two Pax2/5/8 genes in the common ancestor of ascidian and larvacean urochordates.

Results

To better understand mechanisms underlying the evolution of duplicated genes, we investigated, in the larvacean urochordate Oikopleura dioica, the embryonic gene expression patterns of Pax2/5/8 paralogs. We compared the larvacean and ascidian expression patterns to infer modular subfunctions present in the single pre-duplication Pax2/5/8 gene of stem urochordates, and we compared vertebrate and urochordate expression to infer the suite of Pax2/5/8 gene subfunctions in the common ancestor of olfactores (vertebrates + urochordates). Expression pattern differences of larvacean and ascidian Pax2/5/8 orthologs in the endostyle, pharynx and hindgut suggest that some ancestral gene functions have been partitioned differently to the duplicates in the two urochordate lineages. Novel expression in the larvacean heart may have resulted from the neofunctionalization of a Pax2/5/8 gene in the urochordates. Expression of larvacean Pax2/5/8 in the endostyle, in sites of epithelial remodeling, and in sensory tissues evokes like functions of Pax2, Pax5 and Pax8 in vertebrate embryos, and may indicate ancient origins for these functions in the chordate common ancestor.

Conclusion

Comparative analysis of expression patterns of chordate Pax2/5/8 duplicates, rooted on the single-copy Pax2/5/8 gene of amphioxus, whose lineage diverged basally among chordates, provides new insights into the evolution and development of the heart, thyroid, pharynx, stomodeum and placodes in chordates; supports the controversial conclusion that the atrial siphon of ascidians and the otic placode in vertebrates are homologous; and backs the notion that Pax2/5/8 functioned in ancestral chordates to engineer epithelial fusions and perforations, including gill slit openings.

Lhx1 and Lhx5 maintain the inhibitory-neurotransmitter status of interneurons in the dorsal spinal cord

Lhx1 and Lhx5 are co-expressed in multiple interneuron cell types in the developing spinal cord. These include early-born dI4 and dI6 inhibitory interneurons, as well as late-born inhibitory dILA neurons (dILA), all of which express the paired-domain transcription factor Pax2. Although it appears that Lhx1 and Lhx5 do not control the initial specification of the neuronal cell types in which they are expressed, we have found a cell-autonomous requirement for either Lhx1 or Lhx5 to maintain the expression of Pax2, Pax5 and Pax8 in dorsal inhibitory neurons at later developmental stages. Lhx1; Lhx5 double-knockout mice exhibit a downregulation of Gad1 and Viaat (Slc32a1) from E13.5 onwards that is closely associated with a decrease in Pax2 expression. Pax2 is a key factor for dorsal GABAergic identity, with the expression of Pax5 and Pax8 being differentially dependent on Pax2 in the dorsal horn. In summary, our findings support a model in which the differentiation of GABAergic interneurons in the dorsal cord depends on Pax2, with Lhx1 and Lhx5 helping to activate and maintain Pax2 expression in these cells. Lhx1 and Lhx5 therefore function together with Pax2, Pax5 and Pax8 to establish a GABAergic inhibitory-neurotransmitter program in dorsal horn interneurons.

Pax-5 immunoexpression in various types of benign and malignant tumours: a high-throughput tissue microarray analysis

BACKGROUND

Pax-5 belongs to the Pax gene family transcription factors that play an important role in organogenesis and in B cell ontogeny. It is expressed in B cell non-Hodgkin's lymphoma (B-NHL), Hodgkin's lymphoma (HL) and neuroendocrine carcinomas. However, its expression in other tumour types is not fully explored.

AIMS AND METHODS

To determine Pax-5 expression in other tumour types, immunohistochemistry was performed on 3758 benign and malignant tumours using multiple tumour microarrays, as well as on whole sections.

RESULTS

Pax-5 was expressed in 108/118 (91.5%) B-NHLs, in 60/70 (85.7%) HLs and in 0/7 T cell lymphomas. In addition, Pax-5 was seen in 24/34 (70.6%) Merkel cell carcinomas, 42/53 (79.2%) small cell carcinomas, 1/164 (0.6%) breast carcinomas, 2/204 (1%) endometrial adenocarcinomas and 1/452 (0.2%) urothelial carcinoma of the bladder.

CONCLUSION

Despite its expression in a small subset of malignancies of epithelial origin, Pax-5 is still a good and reliable immunomarker in diagnosing B-NHL, HL and neuroendocrine carcinomas.

Genetic basis of Joubert syndrome and related disorders of cerebellar development

Over three decades have passed since Marie Joubert described the original proband for Joubert syndrome, a rare neurological disorder featuring absence of the cerebellar vermis (i.e. midline). Efforts at deciphering the molecular basis for this disease have been complicated by the clinical and genetic heterogeneity as well as extensive phenotypic overlap with other syndromes. However, progress has been made in recent years with the mapping of three genetic loci and the identification of mutations in two genes, AHI1 and NPHP1. These genes encode proteins with some shared functional domains, but their role in brain development is unclear. Clues may come from studies of related syndromes, including Bardet-Biedl syndrome and nephronophthisis, for which all of the encoded proteins localize to primary cilia. The data suggest a tantalizing connection between intraflagellar transport in cilia and brain development.

Neutralization of the chemokine CXCL10 reduces apoptosis and increases axon sprouting after spinal cord injury

Spinal cord injury (SCI) is followed by a secondary degenerative process that includes cell death. We have previously demonstrated that the chemokine CXCL10 is up-regulated following SCI and plays a critical role in T-lymphocyte recruitment to sites of injury and inhibition of angiogenesis; antibody-mediated functional blockade of CXCL10 reduced inflammation while enhancing angiogenesis. We hypothesized, based on these findings, that the injury environment established by anti-CXCL10 antibody treatment would support greater survival of neurons and enhance axon sprouting compared with the untreated, injured spinal cord. Here, we document gene array and histopathological data to support our hypothesis. Gene array analysis of treated and untreated tissue from spinal cord-injured animals revealed eight apoptosis-related genes with significant expression changes at 3 days postinjury. In support of these data, quantification of TUNEL-positive cells at 3 days postinjury indicated a 75% reduction in the number of dying cells in treated animals compared with untreated animals. Gene array analysis of treated and untreated tissue also revealed six central nervous system growth-related genes with significant expression changes in the brainstem at 14 days postinjury. In support of these data, quantification of anterograde-labeled corticospinal tract fibers indicated a 60-70% increase in axon sprouting caudal to the injury site in treated animals compared with untreated animals. These findings indicate that anti-CXCL10 antibody treatment provides an environment that reduces apoptosis and increases axon sprouting following injury to the adult spinal cord.

Specification of the meso-isthmo-cerebellar region: the Otx2/Gbx2 boundary

The midbrain/hindbrain (MH) territory containing the mesencephalic and isthmocerebellar primordial is characterized by the expression of several families of regulatory genes including transcription factors (Otx, Gbx, En, and Pax) and signaling molecules (Fgf and Wnt). At earlier stages of avian neural tube, those genes present a dynamic expression pattern and only at HH18-20 onwards, when the mesencephalic/metencephalic constriction is coincident with the Otx2/Gbx2 boundary, their expression domains become more defined. This review summarizes experimental data concerning the genetic mechanisms involved in the specification of the midbrain/hindbrain territory emphasizing the chick/quail chimeric experiments leading to the discovery of a secondary isthmic organizer. Otx2 and Gbx2 co-regulation could determine the precise location of the MH boundary and involved in the inductive events characteristic of the isthmic organizer center.

Classical Embryological Studies and Modern Genetic Analysis of Midbrain and Cerebellum Development

The brain is a remarkably complex anatomical structure that contains a diverse array of subdivisions, cell types, and synaptic connections. It is equally extraordinary in its physiological properties, as it constantly evaluates and integrates external stimuli as well as controls a complicated internal environment. The brain can be divided into three primary broad regions: the forebrain, midbrain (Mb), and hindbrain (Hb), each of which contain further subdivisions. The regions considered in this chapter are the Mb and most-anterior Hb (Mb/aHb), which are derived from the mesencephalon (mes) and rhombomere 1 (r1), respectively. The dorsal Mb consists of the laminated superior colliculus and the globular inferior colliculus (Fig. 1A and B), which modulate visual and auditory stimuli, respectively. The dorsal component of the aHb is the highly foliated cerebellum (Cb), which is primarily attributed to controlling motor skills (Fig. 1A and B). In contrast, the ventral Mb/aHb (Fig. 1B) consists of distinct clusters of neurons that together comprise a network of nuclei and projections-notably, the Mb dopaminergic and Hb serotonergic and Mb/aHb cholinergic neurons (Fig. 1G and H), which modulate a collection of behaviors, including movement, arousal, feeding, wakefulness, and emotion. Historically, the dorsal Mb and Cb have been studied using the chick as a model system because of the ease of performing both cell labeling and tissue transplants in the embryo in ovo; currently DNA electroporation techniques are also used. More recently the mouse has emerged as a powerful genetic system with numerous advantages to study events underpinning Mb/aHb development. There is a diverse array of spontaneous mutants with both Mb- and Cb-related phenotypes. In addition, numerous gene functions have been enumerated in mouse, gene expression is similar across vertebrates, and powerful genetic tools have been developed. Finally, additional insight into Mb/aHb function has been gained from studies of genetic diseases, such as Parkinson's disease, schizophrenia, cancer, and Dandy Walker syndrome, that afflict the Mb/aHb in humans and have genetic counterparts in mouse. Accordingly, this chapter discusses a spectrum of experiments, including classic embryology, in vitro assays, sophisticated genetic methods, and human diseases. We begin with an overview of Mb and aHb anatomy and physiology and mes/r1 gene expression patterns. We then provide a summary of fate-mapping studies that collectively demonstrate the complex cell behaviors that occur while the Mb and aHb primordia are established during embryogenesis and discuss the integration of both anterior-posterior (A-P) and dorsal-ventral (D-V) patterning. Finally, we describe some aspects of postnatal development and some of the insights gained from human diseases.

Cell proliferation in the forebrain and midbrain of the sea lamprey

Cell proliferation in the forebrain and midbrain of the sea lamprey (Petromyzon marinus L.) was investigated by proliferation cell nuclear antigen (PCNA) immunocytochemistry, with BrdU labeling as a complementary technique. Correspondence between proliferation regions and areas of early neuronal differentiation was also assessed using antibodies against HNK-1 early differentiation marker. The brain of late embryos shows a homogeneously thick ventricular zone (VZ) containing PCNA-immunoreactive (PCNA-ir) nuclei. In early prolarvae, several discontinuities formed by PCNA-negative cells, and differences among regions in VZ thickness, become apparent. In late prolarvae and early larvae, these differences in VZ thickness and appearance, as well as the presence of PCNA-negative discontinuities, allowed us to correlate proliferation domains and neuroanatomical regions. In larvae, the number of PCNA-ir cells in the VZs diminish gradually, although a few PCNA-ir cells are present in the ependyma of most regions. In late larvae, proliferation becomes confined to a few ventricular areas (medial pallium, caudal habenula, ventral preoptic recess near the optic nerve, and tuberal portion of the posterior hypothalamic recess). During metamorphosis there appears to be no proliferation, but in upstream adults a few PCNA-ir cells are observed in the most caudal habenula. The characteristics of the proliferative regions revealed in lamprey with PCNA immunocytochemistry show notable differences from those observed in other vertebrates, and these differences may be related to the peculiar life cycle of lampreys.

Specification of midbrain territory

The vertebrate neural plate is subdivided into four distinct territories comprising the presumptive forebrain, midbrain, hindbrain, and the spinal cord, shortly after gastrulation. Initially, this subdivision relies on a defined pattern of expression of distinct transcription and secreted factors within the newly formed neuroectoderm, even before morphological partitioning is evident. Subdivision of the neural plate into distinct territories is a complex process, which is also known as patterning or regionalisation, and involves both planar and vertical signals coming from within the neuroectoderm and from neighbouring non-neural tissues. During the course of embryogenesis, this gross subdivision of the neural plate is progressively refined by a variety of mechanisms, leading to the establishment of various subdomains that ultimately give rise to specific cell populations characteristic for the corresponding brain and spinal cord regions. Once again, a prominent feature of these later processes is the defined expression of specific genes within the developing neural tube. In the present review, we will concentrate on the genes active in the progressive refinement of the midbrain territory as a distinct subdivision of the brain. We will also give an outlook on genes that are active during early induction of the anterior neural plate and genetic mechanisms that control the generation of specific cell populations of the ventral midbrain, with special focus on the mesencephalic dopaminergic neurons.

Pax-7 Immunoreactivity in the Post-natal Chicken Central Nervous System

In this immunocytochemical study on the constitutive expression of Pax-7 protein in the postnatal chicken brain, Pax-7 showed region and cell type specific expression. In the optic tectum, only cells in grey matter showed positive immunoreactivities (IRs), whereas those in the white matters did not show any IRs. In thalamic nuclei and several pontine nuclei, we also localized Pax-7 positive IRs. On the contrary, in the cerebellum, Pax-7 was mainly localized within the Bergmann glia, whereas Purkinje cells did not show any IRs. In double immunolabelling studies, most of the Pax-7 IRs did not originate from neuroglial cells such as oligodendrocytes, microglia or astrocytes, but from neurons, with the exception of Bergmann glia in the cerebellum. The presence of Pax-7 IRs in the adult chicken brain could suggest that Pax-7 might play a role in maintaining normal physiological function in some postnatal chicken brain cells.

Regionalisation and acquisition of polarity in the optic tectum

The optic tectum differentiates from the alar plate of the mesencephalon and receives retinal fibres in a precise retinotopic manner. Here, mechanisms of tectum polarisation and regionalisation are reviewed. Misexpression of Pax2, Pax5 or En can change the fate of the presumptive diencephalon to that of the tectum. Ephrin A2 and A5 are expressed in a gradient in the tectum, caudal high and rostral low, and may play important roles in the formation of a precise retinotectal projection map. Retinal fibres that express receptors for these ligands, and which come from the temporal retina, are repulsed by the ligands and do not invade the caudal tectum. Both En1 and En2 can regulate posterior characteristics in the tectum by inducing ephrin A2 and A5. Transplantation experiments in chick have indicated that the mes/metencephalic boundary works as an organiser for the tectum and the cerebellum. Fgf8 is a candidate signalling molecule in the organiser. Pax2/5, En, and Fgf8 are in a positive feedback loop for their expression such that misexpression of one of these genes in the diencephalon turns on the feedback loop and can result in induction of an optic tectum. Otx2 and Gbx2 appear to repress each other's expression and contribute to defining the posterior border of the tectum. Misexpression of Otx2 in the metencephalon can change the fate of its alar plate to a tectum, and misexpression of Gbx2 in the mesencephalon can cause anterior shifting of the caudal limit of the tectum. The anterior border of the tectum may be determined as a result of repressive interactions between Pax6 and En1/Pax2. Along the dorsoventral axis of the mesencephalon, Shh contributes to ventralize the tissue; that is, Shh can change the fate of the presumptive tectum to that of the tegmentum that is the ventral structure. It is proposed that the brain vesicle that expresses Otx2, Pax2, and En1 may differentiate into the tectum.

Functional analyses of two alternative isoforms of the transcription factor Pax-5

The Pax-5 gene plays a central role in B cell development, activation, and differentiation. At least four different isoforms have been identified, of which isoform Pax-5a has been extensively studied, while functions for alternative isoforms were previously unknown. Here, using a transient transfection system, we provide evidence that alternative isoform Pax-5d acts as a dominant-negative regulator by suppressing activity of Pax-5a in a dose-dependent manner. In contrast, co-expression in the presence of alternative isoform Pax-5e causes an increase in Pax-5a activity. Protein studies on Pax-5e using Western blot analysis revealed that this 19-kDa isoform migrates as a 27-kDa species on SDS-polyacrylamide electrophoresis gels, while a mutant Pax-5e form in which a C-terminal cysteine residue has been mutated, runs at the expected 19 kDa. Using both Western blot and immunoprecipitation assays, we further provide evidence that this size discrepancy may be caused by a tight association between Pax-5e and a thioredoxin-like factor. Comparison of various B cell lines as well as resting and lipopolysaccharide-activated mature B lymphocytes shows that increased B cell proliferation correlates with increased levels of Pax-5e/thioredoxin, whereas increased Pax-5d amounts correlate with inhibition of cell growth. Together, our results suggest that during activation and differentiation of B lymphocytes, Pax-5a function is modulated by two alternative spliced isoforms: the dominant negative Pax-5d isoform may mediate inhibition of Pax-5a activity in resting B cells, while alternative isoform Pax-5e associated with thioredoxin may increase Pax-5a activity through an unknown (redox) mechanism.

Identification and expression of the lamprey Pax6 gene: evolutionary origin of the segmented brain of vertebrates

The Pax6 gene plays a developmental role in various metazoans as the master regulatory gene for eye patterning. Pax6 is also spatially regulated in particular regions of the neural tube. Because the amphioxus has no neuromeres, an understanding of Pax6 expression in the agnathans is crucial for an insight into the origin of neuromerism in the vertebrates. We have isolated a single cognate cDNA of the Pax6 gene, LjPax6, from a Lampetra japonica cDNA library and observed the pattern of its expression using in situ hybridization. Phylogenetic analysis revealed that LjPax6 occurs as an sister group of gnathostome Pax6. In lamprey embryos, LjPax6 is expressed in the eye, the nasohypophysial plate, the oral ectoderm and the brain. In the central nervous system, LjPax6 is expressed in clearly delineated domains in the hindbrain, midbrain and forebrain. We compared the pattern of LjPax6 expression with that of other brain-specific regulatory genes, including LjOtxA, LjPax2/5/8, LjDlx1/6, LjEmx and LjTTF1. Most of the gene expression domains showed conserved pattern, which reflects the situation in the gnathostomes, conforming partly to the neuromeric patterns proposed for the gnathostomes. We conclude that most of the segmented domains of the vertebrate brain were already established in the ancestor common to all vertebrates. Major evolutionary changes in the vertebrate brain may have involved local restriction of cell lineages, leading to the establishment of neuromeres.

Emx2directs the development of diencephalon in cooperation withOtx2

The vertebrate brain is among the most complex biological structures of which the organization remains unclear. Increasing numbers of studies have accumulated on the molecular basis of midbrain/hindbrain development, yet relatively little is known about forebrain organization. Nested expression among Otx and Emx genes has implicated their roles in rostral brain regionalization, but single mutant phenotypes of these genes have not provided sufficient information. In order to genetically determine the interaction between Emx and Otx genes in forebrain development, we have examined Emx2−/−Otx2+/− double mutants and Emx2 knock-in mutants into the Otx2 locus (Otx2+/Emx2). Emx2−/−Otx2+/− double mutants did not develop diencephalic structures such as ventral thalamus, dorsal thalamus/epithalamus and anterior pretectum. The defects were attributed to the loss of the Emx2-positive region at the three- to four-somite stage, when its expression occurs in the laterocaudal forebrain primordia. Ventral structures such as the hypothalamus, mammillary region and tegmentum developed normally. Moreover, dorsally the posterior pretectum and posterior commissure were also present in the double mutants. In contrast, Otx2+/Emx2 knock-in mutants displayed the majority of these diencephalic structures; however, the posterior pretectum and posterior commissure were specifically absent. Consequently, development of the dorsal and ventral thalamus and anterior pretectum requires cooperation between Emx2 and Otx2, whereas Emx2 expression is incompatible with development of the commissural region of the pretectum.

A gene encoding a new ONECUT class homeodomain protein in the ascidian Halocynthia roretzi functions in the differentiation and specification of neural cells in ascidian embryogenesis

Genes encoding a novel group of homeodomain transcription factors, ONECUT class homeodomain proteins, have previously been isolated from vertebrate and insect. Among them, vertebrate HNF-6 is expressed in hepatocytes and the central nervous system during embryogenesis. Although the functions of HNF-6 in hepatocytes have been well studied, the functions of HNF-6 in the central nervous system remain unknown. In this study, we isolated HrHNF-6, which encodes a new ONECUT class homeodomain protein, from an ascidian, Halocynthia roretzi. HrHNF-6 mRNA was expressed exclusively in neural cells, just posterior to the expression of Hroth during embryogenesis. One of the functions of HrHNF-6 in neural cells is the activation of the expression of HrTBB2, the ascidian beta-tubulin gene. Another is the restriction of the expression of HrPax-258 (which is expressed in the neural tube), suggesting that HrHNF-6 functions in the specification of the neural tube. These results indicate that HrHNF-6 functions in the differentiation and regional specification of neural cells during ascidian embryogenesis.

Neural plate patterning: upstream and downstream of the isthmic organizer

Two organizing centres operate at long-range distances within the anterior neural plate to pattern the forebrain, midbrain and hindbrain. Important progress has been made in understanding the formation and function of one of these organizing centres, the isthmic organizer, which controls the development of the midbrain and anterior hindbrain. Here we review our current knowledge on the identity, localization and maintenance of the isthmic organizer, as well as on the molecular cascades that underlie the activity of this organizing centre.

EN and GBX2 play essential roles downstream of FGF8 in patterning the mouse mid/hindbrain region

Fgf8, which is expressed at the embryonic mid/hindbrain junction, is required for and sufficient to induce the formation of midbrain and cerebellar structures. To address through what genetic pathways FGF8 acts, we examined the epistatic relationships of mid/hindbrain genes that respond to FGF8, using a novel mouse brain explant culture system. We found that En2 and Gbx2 are the first genes to be induced by FGF8 in wild-type E9.5 diencephalic and midbrain explants treated with FGF8-soaked beads. By examining gene expression in En1/2 double mutant mouse embryos, we found that Fgf8, Wnt1 and Pax5 do not require the En genes for initiation of expression, but do for their maintenance, and Pax6 expression is expanded caudally into the midbrain in the absence of EN function. Since E9.5 En1/2 double mutants lack the mid/hindbrain region, forebrain mutant explants were treated with FGF8 and, significantly, the EN transcription factors were found to be required for induction of Pax5. Thus, FGF8-regulated expression of Pax5 is dependent on EN proteins, and a factor other than FGF8 could be involved in initiating normal Pax5 expression in the mesencephalon/metencephalon. The En genes also play an important, but not absolute, role in repression of Pax6 in forebrain explants by FGF8. Previous Gbx2 gain-of-function studies have shown that misexpression of Gbx2 in the midbrain can lead to repression of Otx2. However, in the absence of Gbx2, FGF8 can nevertheless repress Otx2 expression in midbrain explants. In contrast, Wnt1 is initially broadly induced in Gbx2 mutant explants, as in wild-type explants, but not subsequently repressed in cells near FGF8 that normally express Gbx2. Thus GBX2 acts upstream of, or parallel to, FGF8 in repressing Otx2, and acts downstream of FGF8 in repression of Wnt1. This is the first such epistatic study performed in mouse that combines gain-of-function and loss-of-function approaches to reveal aspects of mouse gene regulation in the mesencephalon/metencephalon that have been difficult to address using either approach alone.

Regionalization of the optic tectum: combinations of gene expression that define the tectum

The optic tectum differentiates from the alar plate of the mesencephalon. Here, the molecular mechanisms for differentiation of the tectum are reviewed. Mis-expression of Pax2, Pax5 or En can change the fate of the presumptive diencephalon to become the tectum. En, Fgf8, Pax2 and Pax5, exist in a positive feedback loop for their expression so that mis-expression of any of these genes acts on the feedback loop resulting in induction of the optic tectum in the diencephalon. Otx2 and Gbx2 can repress the expression of each other and contribute to the formation of the posterior border of the tectum. Mis-expression of Otx2 in the metencephalon changed the fate of its alar plate to the tectum. The anterior border of the tectum might be determined as a result of repressive interaction of Pax6 with En1 and Pax2. Along the dorsoventral axis of the mesencephalon, Shh contributes to the ventralization of the tissue, that is, the area affected by Shh differentiates into the tegmentum. It is proposed that the brain vesicle that expresses Otx2, Pax2 and En1 might differentiate into the tectum.

Functional equivalence of the transcription factors Pax2 and Pax5 in mouse development

Pax2 and Pax5 arose by gene duplication at the onset of vertebrate evolution and have since diverged in their developmental expression patterns. They are expressed in different organs of the mouse embryo except for their coexpression at the midbrain-hindbrain boundary (MHB), which functions as an organizing center to control midbrain and cerebellum development. During MHB development, Pax2 expression is initiated prior to Pax5 transcription, and Pax2(−/−) embryos fail to generate the posterior midbrain and cerebellum, whereas Pax5(−/−) mice exhibit only minor patterning defects in the same brain regions. To investigate whether these contrasting phenotypes are caused by differences in the temporal expression or biochemical activity of these two transcription factors, we have generated a knock-in (ki) mouse, which expresses a Pax5 minigene under the control of the Pax2 locus. Midbrain and cerebellum development was entirely rescued in Pax2(5ki/5ki) embryos. Pax5 could furthermore completely substitute for the Pax2 function during morphogenesis of the inner ear and genital tracts, despite the fact that the Pax5 transcript of the Pax2(5ki)allele was expressed only at a fivefold lower level than the wild-type Pax2 mRNA. As a consequence, the Pax2(5ki)allele was able to rescue most but not all Pax2 mutant defects in the developing eye and kidney, both of which are known to be highly sensitive to Pax2 protein dosage. Together these data demonstrate that the transcription factors Pax2 and Pax5 have maintained equivalent biochemical functions since their divergence early in vertebrate evolution.

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.

Interaction between Otx2 and Gbx2 defines the organizing center for the optic tectum

Otx2 is expressed in the mesencephalon and prosencephalon, and Gbx2 is expressed in the rhombencephalon around stage 10. Loss-of-function studies of these genes in mice have revealed that Otx2 is indispensable for the development of the anterior brain segment, and that Gbx2 is required for the development of the isthmus. We carried out gain-of-function experiments of these genes in chick embryos with a newly developed gene transfer system, in ovo electroporation. When Otx2 was ectopically expressed caudally beyond the midbrain-hindbrain boundary (MHB), the alar plate of the metencephalon differentiated into the optic tectum instead of differentiating into the cerebellum. On the other hand, when Gbx2 was ectopically expressed at the mesencephalon, the caudal limit of the tectum shifted rostrally. We looked at the effects of misexpression on the isthmus- and tectum-related molecules. Otx2 and Gbx2 interacted to repress each other's expression. Ectopic Otx2 and Gbx2 repressed endogenous expression of Fgf8 in the isthmus, but induced Fgf8 expression at the interface between Otx2 and Gbx2 expression. Thus, it is suggested that interaction between Otx2 and Gbx2 determines the site of Fgf8 expression and the posterior limit of the tectum.

Two Pax2/5/8-binding sites in Engrailed2 are required for proper initiation of endogenous mid-hindbrain expression

During early brain development mouse Engrailed2 (En2) is expressed in a broad band across most of the mid-hindbrain region. Evidence from gene expression data, promoter analysis in transgenic mice and mutant phenotype analysis in mice and zebrafish has suggested that Pax2, 5 and 8 play a critical role in regulating En2 mid-hindbrain expression. Previously, we identified two Pax2/5/8-binding sites in a 1.0 kb En2 enhancer fragment that is sufficient to directed reporter gene expression to the early mid-hindbrain region and showed that the two Pax2/5/8-binding sites are essential for the mid-hindbrain expression in transgenic mice. In the present study we have examined the functional requirements of these two Pax2/5/8-binding sites in the context of the endogenous En2 gene for directing mid-hindbrain expression. The two Pax2/5/8-binding sites were deleted from the En2 locus and replaced with the bacterial neo gene by homologous recombination in mouse embryonic stem cells. After transmitting the mutation into mice, the neo gene was removed by breeding with transgenic mice expressing cre from a CMV promoter. Embryos homozygous for this En2 Pax2/5/8-binding site deletion mutation had a mild reduction in En2 expression in the presumptive mid-hindbrain region at the 5-7 somite stage, when En2 expression is normally initiated. However, from embryonic day 9.0 onwards, the mutant embryos showed En2 expression indistinguishable from that seen in wild type embryos. Furthermore, the mutants did not show the cerebellar defect seen in mice with a null mutation in En2. This result demonstrates that the two Pax2/5/8-binding sites that were deleted, while being required for mid-hindbrain expression in the context of a 1.0 kb En2 enhancer, are only required for proper initiation of expression of the endogenous En2 gene. Interestingly, a comparison of the lacZ RNA and protein expression patterns directed by the 1.0 kb enhancer fragment revealed that lacZ protein was acting as a lineage marker in the mid-hindbrain region by persisting longer than the mRNA. The transgene expression directed by the 1.0 kb enhancer fragment therefore does not mimic the entire broad domain of En2 expression. Taken together, these two studies demonstrate that DNA binding sites in addition to the two Pax2/5/8-binding sites must be necessary for En2 mid-hindbrain expression.

Fgf8 and Gbx2 induction concomitant with Otx2 repression is correlated with midbrain-hindbrain fate of caudal prosencephalon

Chick/quail transplantation experiments were performed to analyse possible factors involved in the regionalisation of the midbrain-hindbrain domain. The caudal prosomeres, expressing Otx2, were transplanted at stage HH10 into rostrocaudal levels of the midbrain-hindbrain domain, either straddling the intra-metencephalic constriction (type 1 grafts), or at rostral and medial levels of pro-rhombomere A1 (type 2 and 3 grafts, respectively); thus, in all situations, one border of the graft was in contact with the host Gbx2- and Fgf8-expressing domains. The area containing the graft, recognised by QCPN immunohistochemistry, was first analysed 48 hours after transplantation for Otx2, Gbx2, En2 and Fgf8. Although in all three situations, a large part of the graft maintained Otx2 expression, another part became Otx2 negative and was induced to express Gbx2 and Fgf8. These inductive events occurred exclusively at the interface between the Otx2-positive transplanted domain and the ipsilateral host Gbx2-positive rhombomere 1, creating a new Otx2-Gbx2 boundary within the grafted territory. In type 1 and 2 grafts, the induced Fgf8 domain is in continuity with the host Fgf8 isthmic domain, whereas for type 3 grafts, these two domains are separate. High levels of En2 expression were also induced in the area expressing Gbx2 and Fgf8, and Wnt1 and Pax2 expressions, analysed in type 3 grafts, were induced at the intragraft Otx2-Gbx2 new boundary. Moreover, at later embryonic stages, the graft developed meso-isthmo-cerebellar structures. Thus, gene expressions induced in the grafted prosencephalon not only mimicked the pattern observed in the normal midbrain-hindbrain domain, but is followed by midbrain-hindbrain cytodifferentiation, indicating that not only Fgf8 but also confrontation of Otx2 and Gbx2 may play an essential role during midbrian-hindbrain regionalisation.

The fork head transcription factor Fkh5/Mf3 is a developmental marker gene for superior colliculus layers and derivatives of the hindbrain somatic afferent zone

Fork head-5 (Fkh5; also known as Mf3 and TWH) is a transcription factor of the winged helix family. As part of an extended project to understand the function of this protein in the developing mouse brain, in the present work we have used Fkh5/Mf3 expression as a marker to study the development of the midbrain and hindbrain. In the midbrain, Fkh5/Mf3 is expressed in the superior colliculus, in the ventricular layer of the inferior colliculus and in the isthmus. In the superior colliculus, Fkh5/Mf3 is expressed by cells of layers 4a and 4c since early in development. In the hindbrain, Fkh5/Mf3 is a longitudinal marker (as opposed to a transverse or rhombomeric one), since it labels nuclei belonging to the somatic afferent zone (ventral cochlear nucleus, cuneate and external cuneate nuclei, principal and spinal nuclei of the trigeminal). In addition, Fkh5/Mf3 is expressed by the developing endopiriform nucleus and by the olivary pretectal nucleus. The results suggest that Fkh5/Mf3 has an early role in the lamination of the tectum and in the longitudinal differentiation of the hindbrain.

Role of Pax-5 in the regulation of a mid-hindbrain organizer's activity

The mes-metencephalic boundary (isthmus) has been suggested to act as an organizer in the development of the optic tectum. Pax-5 was cloned as a candidate for regulator of the organizing center. Isthmus-specific expression of Pax-5 and analogy with the genetic cascade in Drosophila suggest that Pax-5 may be at a higher hierarchical position in the gene regulation cascade of tectum development. To examine this possibility, a gain-of-function experiment on Pax-5 was carried out. In ovo electroporation on E2 chick brain with the eucaryotic expression vector that encodes chick Pax-5 cDNA was used. Not only was a considerable amount of Pax-5 expressed ectopically in the transfected brain, but irregular bulging of the neuroepithelium was induced in the diencephalon and mesencephalon. At Pax-5 misexpressing sites, uptake of BrdU was increased. Histological examination of E7 transfected brain revealed that Pax-5 caused transdifferentiation of diencephalon into the tectum-like structure. In the bulges of the E7 mesencephalon, differentiation of laminar structure was repressed when compared to the normal side. In transfected embryos, En-2, Wnt-1 and Fgf8 were up-regulated ectopically, and Otx2 was down-regulated in the diencephalon to mesencephalon. Moreover, Ephrin-A2, which is expressed specifically in the tectum with a gradient highest at the caudal end, is suggested to be involved in pathfinding of the retinal fibers, and was induced in the bulges. When the mouse Fgf8 expression vector was electroporated, Pax-5 and chick Fgf8 were also induced ectopically. These results suggest that Pax-5, together with Fgf8, hold a higher position in the genetic hierarchy of the isthmus organizing center and regulate its activity.

Characterization of three novel members of the zebrafish Pax2/5/8 family: dependency of Pax5 and Pax8 expression on the Pax2.1 (noi) function

The mammalian Pax2, Pax5 and Pax8 genes code for highly related transcription factors, which play important roles in embryonic development and organogenesis. Here we report the characterization of all members of the zebrafish Pax2/5/8 family. These genes have arisen by duplications before or at the onset of vertebrate evolution. Due to an additional genome amplification in the fish lineage, the zebrafish contains two Pax2 genes, the previously known Pax[b] gene (here renamed as Pax2.1) and a novel Pax2.2 gene. The zebrafish Pax2.1 gene most closely resembles the mammalian Pax2 gene in its expression pattern, as it is transcribed first in the midbrain-hindbrain boundary region, then in the optic stalk, otic system, pronephros and nephric ducts, and lastly in specific interneurons of the hindbrain and spinal cord. Pax2.2 differs from Pax2.1 by the absence of expression in the nephric system and by a delayed onset of transcription in other Pax2.1 expession domains. Pax8 is also expressed in the same domains as Pax2.1, but its transcription is already initiated during gastrulation in the primordia of the otic placode and pronephric anlage, thus identifying Pax8 as the earliest developmental marker of these structures. The zebrafish Pax5 gene, in contrast to its mouse orthologue, is transcribed in the otic system in addition to its prominent expression at the midbrain-hindbrain boundary. The no isthmus (noi) mutation is known to inactivate the Pax2.1 gene, thereby affecting the development of the midbrain-hindbrain boundary region, pronephric system, optic stalk and otic region. Although the different members of the Pax2/5/8 family may potentially compensate for the loss of Pax2.1 function, we demonstrate here that only the expression of the Pax2.2 gene remains unaffected in noi mutant embryos. The expression of Pax5 and Pax8 is either not initiated at the midbrain-hindbrain boundary or is later not maintained in other expression domains. Consequently, the noi mutation of zebrafish is equivalent to combined inactivation of the mouse Pax2 and Pax5 genes with regard to the loss of midbrain-hindbrain boundary development.

A series of no isthmus (noi) alleles of the zebrafish pax2.1 gene reveals multiple signaling events in development of the midbrain-hindbrain boundary

Generation of cell diversity in the vertebrate central nervous system starts during gastrulation stages in the ectodermal germ layer and involves specialized cell groups, such as the organizer located at the midbrain-hindbrain boundary (MHB). Mutations in the zebrafish no isthmus (noi) gene alter development of the MHB, and affect the pax2.1 gene (formerly pax(zf-b)). Analysis of the structure of pax2.1 reveals at least 12 normal splice variants. The noi alleles can be arranged, by molecular and phenotypic criteria, into a series of five alleles of differing strength, ranging from a null allele to weak alleles. In keeping with a role in development of the MHB organizer, gene expression is already affected in the MHB primordium of the gastrula neural ectoderm in noi mutants. eng3 activation is completely and eng2 activation is strongly dependent on noi function. In contrast, onset of wnt1, fgf8 and her5 expression occurs normally in the null mutants, but is eliminated later on. Our observations suggest that three signaling pathways, involving pax2.1, wnt1 and fgf8, are activated independently in early anterior-posterior patterning of this area. In addition, analysis of the allelic series unexpectedly suggests that noi activity is also required during dorsal-ventral patterning of the MHB in somitogenesis stages, and possibly in a later eng expression phase. We propose that noi/pax2.1 participates in sequential signaling processes as a key integrator of midbrain-hindbrain boundary development.

Identification of an evolutionarily conserved 110 base-pair cis-acting regulatory sequence that governs Wnt-1 expression in the murine neural plate

The generation of anterior-posterior polarity in the vertebrate brain requires the establishment of regional domains of gene expression at early somite stages. Wnt-1 encodes a signal that is expressed in the developing midbrain and is essential for midbrain and anterior hindbrain development. Previous work identified a 5.5 kilobase region located downstream of the Wnt-1 coding sequence which is necessary and sufficient for Wnt-1 expression in vivo. Using a transgenic mouse reporter assay, we have now identified a 110 base pair regulatory sequence within the 5.5 kilobase enhancer, which is sufficient for expression of a lacZ reporter in the approximate Wnt-1 pattern at neural plate stages. Multimers of this element driving Wnt-1 expression can partially rescue the midbrain-hindbrain phenotype of Wnt-1(−/−) embryos. The possibility that this region represents an evolutionarily conserved regulatory module is suggested by the identification of a highly homologous region located downstream of the wnt-1 gene in the pufferfish (Fugu rubripes). These sequences are capable of appropriate temporal and spatial activation of a reporter gene in the embryonic mouse midbrain; although, later aspects of the Wnt-1 expression pattern are absent. Genetic evidence has implicated Pax transcription factors in the regulation of Wnt-1. Although Pax-2 binds to the 110 base pair murine regulatory element in vitro, the location of the binding sites could not be precisely established and mutation of two putative low affinity sites did not abolish activation of a Wnt-1 reporter transgene in vivo. Thus, it is unlikely that Pax proteins regulate Wnt-1 by direct interactions with this cis-acting regulatory region. Our analysis of the 110 base pair minimal regulatory element suggests that Wnt-1 regulation is complex, involving different regulatory interactions for activation and the later maintenance of transgene expression in the dorsal midbrain and ventral diencephalon, and at the midbrain-hindbrain junction.

Follicular cells of the thyroid gland require Pax8 gene function

The thyroid gland develops from two distinct embryonic lineages: follicular cells (which produce thyroxine) and parafollicular C-cells (which produce calcitonin) are of endodermal and neural crest origin, respectively. Little is known about the molecular mechanisms governing the generation of these different cell types. Mice lacking the transcription factor Ttf1 lack both cell types and thus are unable to develop a thyroid gland. By analysis of Pax8-/- mice, we demonstrate that Pax8 is required for the formation of the follicular cells in the thyroid. We present evidence that Pax8 is necessary for providing cues for the differentiation of competent endoderm primordia into thyroxin-producing follicular cells.

Tripartite organization of the ancestral chordate brain and the antiquity of placodes: insights from ascidian Pax-2/5/8, Hox and Otx genes

Ascidians and vertebrates belong to the Phylum Chordata and both have dorsal tubular central nervous systems. The structure of the ascidian neural tube is extremely simple, containing less than 400 cells, among which less than 100 cells are neurons. Recent studies suggest that, despite its simple organization, the mechanisms patterning the ascidian neural tube are similar to those of the more complex vertebrate brain. Identification of homologous regions between vertebrate and ascidian nervous systems, however, remains to be resolved. Here we report the expression of HrPax-258 gene: an ascidian homologue of vertebrate Pax-2, Pax-5 and Pax-8 genes. Molecular phylogenetic analyses indicate that HrPax-258 is descendant from a single precursor gene that gave rise to the three vertebrate genes. The expression pattern of HrPax-258 suggests that this subfamily of Pax genes has conserved roles in regional specification of the brain. Comparison with expression of ascidian Otx (Hroth) and a Hox gene (HrHox1) by double-staining in situ hybridizations indicate that the ascidian brain region can be subdivided into three regions; the anterior region marked by Hroth probably homologous to the vertebrate forebrain and midbrain, the middle region marked by HrPax-258 probably homologous to the vertebrate anterior hindbrain (and maybe also midbrain) and the posterior region marked by Hox genes which is homologous to the vertebrate hindbrain and spinal cord. Later expression of HrPax-258 in atrial primordia implies that basal chordates such as ascidians have already acquired a sensory organ that develops from epidermal thickenings (placodes) and expresses HrPax-258; we suggest it is homologous to the vertebrate ear. Therefore, placodes are not likely to be a newly acquired feature in vertebrates, but may have already been possessed by the earliest chordates.

The characterization of novel Pax genes of the sea urchin and Drosophila reveal an ancient evolutionary origin of the Pax2/5/8 subfamily

The developmental control genes of the Pax family can be grouped into different subclasses according to structure and sequence homology. Here we describe the isolation and characterization of three novel Pax genes of the sea urchin for which no homologues are yet known in other animal phyla. One of these genes, suPaxB, codes for the previously characterized transcription factor TSAP which is involved in the developmental regulation of two pairs of late histone genes. Furthermore, conserved members of the Pax2/5/8 subfamily, which have so far been described only in vertebrates, were isolated not only from the sea urchin, but also from Drosophila and C. elegans. Hence, the Pax2/5/8 transcription factors constitute an ancient subfamily of highly conserved Pax proteins. During Drosophila embryogenesis, the Pax258 gene is shown to be expressed in the precursor cells of the external sensory organs, thus suggesting a role for Pax258 in the early development of the peripheral nervous system of insects.

Pax genes and organogenesis

Pax genes are a family of developmental control genes that encode nuclear transcription factors. They are characterized by the presence of the paired domain, a conserved amino acid motif with DNA-binding activity. Originally, paired-box-containing genes were detected in Drosophila melanogaster, where they exert multiple functions during embryogenesis. In vertebrates, Pax genes are also involved in embryogenesis. Mutations in four out of nine characterized Pax genes have been associated with either congenital human diseases such as Waardenburg syndrome (PAX3), Aniridia (PAX6), Peter's anomaly (PAX6), renal coloboma syndrome (PAX2) or spontaneous mouse mutants (undulated (Pax1), Splotch (Pax3), Small eye (Pax6), Pax2(1)Neu), which all show defects in development. Recently, analysis of spontaneous and transgenic mouse mutants has revealed that vertebrate pax genes are key regulators during organogenesis of kidney, eye, ear, nose, limb muscles, vertebral column and brain. Like their Drosophila counterparts, vertebrate Pax genes are involved in pattern formation during embryogenesis, possibly by determining the time and place of organ initiation or morphogenesis. For most tissues, however, the nature of the primary developmental action of Pax transcription factors remains to be elucidated. One predominant theme is signal transduction during tissue interactions, which may lead to a position-specific regulation of cell proliferation.

Cooperation of Pax2 and Pax5 in midbrain and cerebellum development

Midbrain and cerebellum development depends on an organizing center that is located at the midbrain-hindbrain junction of the vertebrate embryo. Expression of the two closely related transcription factors Pax2 and Pax5 overlaps spatially and temporally in this region of the developing central nervous system. To study a possible interaction of these transcription factors in midbrain and cerebellum patterning, we have generated Pax5, Krd double mutant mice. The transgene-induced Krd mutation corresponds to an approximately 7-centimorgan chromosome 19 deletion that eliminates the entire Pax2 locus. The heterozygous Krd mutation deleting one Pax2 allele had no effect on midbrain and cerebellum development. Moreover, only minor developmental defects were previously observed at the midline of the inferior colliculus and anterior cerebellum in mice that were homozygous for a targeted Pax5 mutation. Similar morphological alterations were observed in 80% of all compound heterozygous Pax5 (+/-) Krd (+/-) mice. However, in the remaining 20% of compound heterozygotes, the inferior colliculi were missing, and the vermis of the cerebellum was severely disrupted due to the failure of the cerebellar primordia to fuse at the midline. Inactivation of the second Pax5 allele in Pax5 (-/-) Krd (+/-) mice resulted in complete loss of the posterior midbrain and cerebellum, as the tissue originating from the midbrain-hindbrain boundary region was deleted in the embryo as early as day 9.5. On the basis of these data, we propose that the cooperation of Pax2 and Pax5 is essential for normal functioning of the organizing center at the midbrain-hindbrain junction.