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

The brain regulatory program predates central nervous system evolution

Understanding how brains evolved is critical to determine the origin(s) of centralized nervous systems. Brains are patterned along their anteroposterior axis by stripes of gene expression that appear to be conserved, suggesting brains are homologous. However, the striped expression is also part of the deeply conserved anteroposterior axial program. An emerging hypothesis is that similarities in brain patterning are convergent, arising through the repeated co-option of axial programs. To resolve whether shared brain neuronal programs likely reflect convergence or homology, we investigated the evolution of axial programs in neurogenesis. We show that the bilaterian anteroposterior program patterns the nerve net of the cnidarian Nematostella along the oral-aboral axis arguing that anteroposterior programs regionalized developing nervous systems in the cnidarian-bilaterian common ancestor prior to the emergence of brains. This finding rejects shared patterning as sufficient evidence to support brain homology and provides functional support for the plausibility that axial programs could be co-opted if nervous systems centralized in multiple lineages.

Delilah, prospero, and D-Pax2 constitute a gene regulatory network essential for the development of functional proprioceptors

Coordinated animal locomotion depends on the development of functional proprioceptors. While early cell-fate determination processes are well characterized, little is known about the terminal differentiation of cells within the proprioceptive lineage and the genetic networks that control them. In this work we describe a gene regulatory network consisting of three transcription factors–Prospero (Pros), D-Pax2, and Delilah (Dei)–that dictates two alternative differentiation programs within the proprioceptive lineage in Drosophila. We show that D-Pax2 and Pros control the differentiation of cap versus scolopale cells in the chordotonal organ lineage by, respectively, activating and repressing the transcription of dei. Normally, D-Pax2 activates the expression of dei in the cap cell but is unable to do so in the scolopale cell where Pros is co-expressed. We further show that D-Pax2 and Pros exert their effects on dei transcription via a 262 bp chordotonal-specific enhancer in which two D-Pax2- and three Pros-binding sites were identified experimentally. When this enhancer was removed from the fly genome, the cap- and ligament-specific expression of dei was lost, resulting in loss of chordotonal organ functionality and defective larval locomotion. Thus, coordinated larval locomotion depends on the activity of a dei enhancer that integrates both activating and repressive inputs for the generation of a functional proprioceptive organ.

Activating PAX gene family paralogs to complement PAX5 leukemia driver mutations

PAX5, one of nine members of the mammalian paired box (PAX) family of transcription factors, plays an important role in B cell development. Approximately one-third of individuals with pre-B acute lymphoblastic leukemia (ALL) acquire heterozygous inactivating mutations of PAX5 in malignant cells, and heterozygous germline loss-of-function PAX5 mutations cause autosomal dominant predisposition to ALL. At least in mice, Pax5 is required for pre-B cell maturation, and leukemic remission occurs when Pax5 expression is restored in a Pax5-deficient mouse model of ALL. Together, these observations indicate that PAX5 deficiency reversibly drives leukemogenesis. PAX5 and its two most closely related paralogs, PAX2 and PAX8, which are not mutated in ALL, exhibit overlapping expression and function redundantly during embryonic development. However, PAX5 alone is expressed in lymphocytes, while PAX2 and PAX8 are predominantly specific to kidney and thyroid, respectively. We show that forced expression of PAX2 or PAX8 complements PAX5 loss-of-function mutation in ALL cells as determined by modulation of PAX5 target genes, restoration of immunophenotypic and morphological differentiation, and, ultimately, reduction of replicative potential. Activation of PAX5 paralogs, PAX2 or PAX8, ordinarily silenced in lymphocytes, may therefore represent a novel approach for treating PAX5-deficient ALL. In pursuit of this strategy, we took advantage of the fact that, in kidney, PAX2 is upregulated by extracellular hyperosmolarity. We found that hyperosmolarity, at potentially clinically achievable levels, transcriptionally activates endogenous PAX2 in ALL cells via a mechanism dependent on NFAT5, a transcription factor coordinating response to hyperosmolarity. We also found that hyperosmolarity upregulates residual wild type PAX5 expression in ALL cells and modulates gene expression, including in PAX5-mutant primary ALL cells. These findings specifically demonstrate that osmosensing pathways may represent a new therapeutic target for ALL and more broadly point toward the possibility of using gene paralogs to rescue mutations driving cancer and other diseases.

The Pax gene family: Highlights from cephalopods

Pax genes play important roles in Metazoan development. Their evolution has been extensively studied but Lophotrochozoa are usually omitted. We addressed the question of Pax paralog diversity in Lophotrochozoa by a thorough review of available databases. The existence of six Pax families (Pax1/9, Pax2/5/8, Pax3/7, Pax4/6, Paxβ, PoxNeuro) was confirmed and the lophotrochozoan Paxβ subfamily was further characterized. Contrary to the pattern reported in chordates, the Pax2/5/8 family is devoid of homeodomain in Lophotrochozoa. Expression patterns of the three main pax classes (pax2/5/8, pax3/7, pax4/6) during Sepia officinalis development showed that Pax roles taken as ancestral and common in metazoans are modified in S. officinalis, most likely due to either the morphological specificities of cephalopods or to their direct development. Some expected expression patterns were missing (e.g. pax6 in the developing retina), and some expressions in unexpected tissues have been found (e.g. pax2/5/8 in dermal tissue and in gills). This study underlines the diversity and functional plasticity of Pax genes and illustrates the difficulty of using probable gene homology as strict indicator of homology between biological structures.

Ancestral role of Pax2/5/8 in molluscan brain and multimodal sensory system development

BACKGROUND

Mollusks represent the largest lophotrochozoan phylum and exhibit highly diverse body plans. Previous studies have demonstrated that transcription factors such as Pax genes play important roles during their development. Accordingly, in ecdysozoan and vertebrate model organisms, orthologs of Pax2/5/8 are among others involved in the formation of the midbrain/hindbrain boundary, the auditory/geosensory organ systems, and the excretory system.

METHODS

Pax2/5/8 expression was investigated by in situ hybridization during the development of representatives of the two major molluscan subclades, Aculifera and Conchifera.

RESULTS

Compared to the investigated polyplacophoran and bivalve species that lack larval statocysts as geosensory organs and elaborate central nervous systems (CNS), cephalopods possess highly centralized brains and statocysts. Pax2/5/8 is expressed in regions where sensory cells develop subsequently during ontogenesis. Expression domains include esthetes and the ampullary system in polyplacophorans as well as the eyes of cephalopods. No Pax2/5/8 expression was observed in the less centralized CNS of bivalve, polyplacophoran, and gastropod embryos, thus arguing for a loss of Pax2/5/8 involvement in CNS development in these lineages. In contrast, Pax2/5/8 is expressed among others in brain lobes along the trajectory of the esophagus that divides the cephalopod brain.

CONCLUSIONS

Our results, along with those on Otx- and Hox-gene expression, demonstrate that the cephalopod condition is similar to that in mouse and fruit fly, with Otx being expressed in the anterior-most brain region (except for the vertical lobe) and a Pax2/5/8 expression domain separating the Otx-domain from a Hox-gene expressing posterior brain region. Thus, Pax2/5/8 appears to have been recruited independently into regionalization of non-homologous complex brains of organisms as different as squid, fruit fly, and mouse. In addition, Pax2/5/8 is expressed in multimodal sensory systems in mollusks such as the esthetes and the ampullary system of polyplacophorans as well as the eyes of cephalopods. Pax2/5/8-expressing cells are present in regions where the future sensory cells such as the polyplacophoran esthetes are situated and hence Pax2/5/8 expression probably predates sensory cell development during ontogeny. In mollusks, Pax2/5/8 is only expressed in derivatives of the ectoderm and hence an ancestral role in molluscan ectoderm differentiation is inferred.

Branching out: origins of the sea urchin larval skeleton in development and evolution

It is a challenge to understand how the information encoded in DNA is used to build a three-dimensional structure. To explore how this works the assembly of a relatively simple skeleton has been examined at multiple control levels. The skeleton of the sea urchin embryo consists of a number of calcite rods produced by 64 skeletogenic cells. The ectoderm supplies spatial cues for patterning, essentially telling the skeletogenic cells where to position themselves and providing the factors for skeletal growth. Here, we describe the information known about how this works. First the ectoderm must be patterned so that the signaling cues are released from precise positions. The skeletogenic cells respond by initiating skeletogenesis immediately beneath two regions (one on the right and the other on the left side). Growth of the skeletal rods requires additional signaling from defined ectodermal locations, and the skeletogenic cells respond to produce a membrane-bound template in which the calcite crystal grows. Important in this process are three signals, fibroblast growth factor, vascular endothelial growth factor, and Wnt5. Each is necessary for explicit tasks in skeleton production.

Short-range Wnt5 signaling initiates specification of sea urchin posterior ectoderm

The border between the posterior ectoderm and the endoderm is a location where two germ layers meet and establish an enduring relationship that also later serves, in deuterostomes, as the anatomical site of the anus. In the sea urchin, a prototypic deuterostome, the ectoderm-endoderm boundary is established before gastrulation, and ectodermal cells at the boundary are thought to provide patterning inputs to the underlying mesenchyme. Here we show that a short-range Wnt5 signal from the endoderm actively patterns the adjacent boundary ectoderm. This signal activates a unique subcircuit of the ectoderm gene regulatory network, including the transcription factors IrxA, Nk1, Pax2/5/8 and Lim1, which are ultimately restricted to subregions of the border ectoderm (BE). Surprisingly, Nodal and BMP2/4, previously shown to be activators of ectodermal specification and the secondary embryonic axis, instead restrict the expression of these genes to subregions of the BE. A detailed examination showed that endodermal Wnt5 functions as a short-range signal that activates only a narrow band of ectodermal cells, even though all ectoderm is competent to receive the signal. Thus, cells in the BE integrate positive and negative signals from both the primary and secondary embryonic axes to correctly locate and specify the border ectoderm.

An RNA interference-based screen of transcription factor genes identifies pathways necessary for sensory regeneration in the avian inner ear

Sensory hair cells of the inner ear are the mechanoelectric transducers of sound and head motion. In mammals, damage to sensory hair cells leads to hearing or balance deficits. Nonmammalian vertebrates such as birds can regenerate hair cells after injury. In a previous study, we characterized transcription factor gene expression during chicken hair cell regeneration. In those studies, a laser microbeam or ototoxic antibiotics were used to damage the sensory epithelia (SE). The current study focused on 27 genes that were upregulated in regenerating SEs compared to untreated SEs in the previous study. Those genes were knocked down by siRNA to determine their requirement for supporting cell proliferation and to measure resulting changes in the larger network of gene expression. We identified 11 genes necessary for proliferation and also identified novel interactive relationships between many of them. Defined components of the WNT, PAX, and AP1 pathways were shown to be required for supporting cell proliferation. These pathways intersect on WNT4, which is also necessary for proliferation. Among the required genes, the CCAAT enhancer binding protein, CEBPG, acts downstream of Jun Kinase and JUND in the AP1 pathway. The WNT coreceptor LRP5 acts downstream of CEBPG, as does the transcription factor BTAF1. Both of these genes are also necessary for supporting cell proliferation. This is the first large-scale screen of its type and suggests an important intersection between the AP1 pathway, the PAX pathway, and WNT signaling in the regulation of supporting cell proliferation during inner ear hair cell regeneration.

Interplay between activin and Hox genes determines the formation of the kidney morphogenetic field

The kidney develops in a specific position along the anterior-posterior axis. All vertebrate kidney tissues are derived from the intermediate mesoderm (IM), and early kidney genes such as Lim1 and Pax2 are expressed in amniotes posterior to the sixth somite axial level. IM cells anterior to this level do not express kidney genes owing to changes in their competence to respond to kidney-inductive signals present along the entire axis. We aimed to understand the molecular mechanisms governing the loss of competence of anterior IM cells and the formation of the anterior border of the kidney morphogenetic field. We identified the dorsal neural tube as the potential kidney-inductive tissue and showed that activin, a secreted morphogen, is necessary but insufficient for Lim1 induction and establishment of the kidney field. Activin or activin-like and BMP signaling cascades are activated along the entire axis, including in anterior non-kidney IM, suggesting that competence to respond to these signals involves downstream or other components. Detailed expression pattern analysis of Hox genes during early chick development revealed that paralogous group four genes share the same anterior border as the kidney genes. Ectopic expression of Hoxb4 in anterior non-kidney IM, either by retinoic acid (RA) administration or plasmid-mediated overexpression, resulted in ectopic kidney gene expression. The anterior expansion of Lim1 expression was restrained when Hoxb4 was co-expressed with a truncated form of activin receptor. We suggest a model in which the competence of IM cells to respond to TGFbeta signaling and express kidney genes is driven by RA and mediated by Hoxb4.

Molecular evolution of the vertebrate mechanosensory cell and ear

The molecular basis of mechanosensation, mechanosensory cell development and mechanosensory organ development is reviewed with an emphasis on its evolution. In contrast to eye evolution and development, which apparently modified a genetic program through intercalation of genes between the master control genes on the top (Pax6, Eya1, Six1) of the hierarchy and the structural genes (rhodopsin) at the bottom, the as yet molecularly unknown mechanosensory channel precludes such a firm conclusion for mechanosensors. However, recent years have seen the identification of several structural genes which are involved in mechanosensory tethering and several transcription factors controlling mechanosensory cell and organ development; these warrant the interpretation of available data in very much the same fashion as for eye evolution: molecular homology combined with potential morphological parallelism. This assertion of molecular homology is strongly supported by recent findings of a highly conserved set of microRNAs that appear to be associated with mechanosensory cell development across phyla. The conservation of transcription factors and their regulators fits very well to the known or presumed mechanosensory specializations which can be mostly grouped as variations of a common cellular theme. Given the widespread distribution of the molecular ability to form mechanosensory cells, it comes as no surprise that structurally different mechanosensory organs evolved in different phyla, presenting a variation of a common theme specified by a conserved set of transcription factors in their cellular development. Within vertebrates and arthropods, some mechanosensory organs evolved into auditory organs, greatly increasing sensitivity to sound through modifications of accessory structures to direct sound to the specific sensory epithelia. However, while great attention has been paid to the evolution of these accessory structures in vertebrate fossils, comparatively less attention has been spent on the evolution of the inner ear and the central auditory system. Recent advances in our molecular understanding of ear and brain development provide novel avenues to this neglected aspect of auditory neurosensory evolution.

FGF signals guide migration of mesenchymal cells, control skeletal morphogenesis and regulate gastrulation during sea urchin development

The sea urchin embryo is emerging as an attractive model to study morphogenetic processes such as directed migration of mesenchyme cells and cell sheet invagination, but surprisingly, few of the genes regulating these processes have yet been characterized. We present evidence that FGFA, the first FGF family member characterized in the sea urchin, regulates directed migration of mesenchyme cells, morphogenesis of the skeleton and gastrulation during early development. We found that at blastula stages, FGFA and a novel putative FGF receptor are expressed in a pattern that prefigures morphogenesis of the skeletogenic mesoderm and that suggests that FGFA is one of the elusive signals that guide migration of primary mesenchyme cells (PMCs). We first show that fgfA expression is correlated with abnormal migration and patterning of the PMCs following treatments that perturb specification of the ectoderm along the oral-aboral and animal-vegetal axes. Specification of the ectoderm initiated by Nodal is required to restrict fgfA to the lateral ectoderm, and in the absence of Nodal, fgfA is expressed ectopically throughout most of the ectoderm. Inhibition of either FGFA, FGFR1 or FGFR2 function severely affects morphogenesis of the skeleton. Furthermore,inhibition of FGFA and FGFR1 signaling dramatically delays invagination of the archenteron, prevents regionalization of the gut and abrogates formation of the stomodeum. We identified several genes acting downstream of fgfAin these processes, including the transcription factors pea3 and pax2/5/8 and the signaling molecule sprouty in the lateral ectoderm and SM30 and SM50 in the primary mesenchyme cells. This study identifies the FGF signaling pathway as an essential regulator of gastrulation and directed cell migration in the sea urchin embryo and as a key player in the gene regulatory network directing morphogenesis of the skeleton.

MicroRNA-183 family conservation and ciliated neurosensory organ expression

MicroRNAs (miRNAs) are an integral component of the metazoan genome and affect posttranscriptional repression of target messenger RNAs. The extreme phylogenetic conservation of certain miRNAs suggests their ancient origin and crucial function in conserved developmental processes. We demonstrate that highly conserved miRNA-183 orthologs exist in both deuterostomes and protostomes and their expression is predominant in ciliated ectodermal cells and organs. The miRNA-183 family members are expressed in vertebrate sensory hair cells, in innervated regions of invertebrate deuterostomes, and in sensilla of Drosophila and C. elegans. Thus, miRNA-183 family member expression is conserved in possibly homologous but morphologically distinct sensory cells and organs. The results suggest that miR-183 family members contribute specifically to neurosensory development or function, and that extant metazoan sensory organs are derived from cells that share genetic programs of common evolutionary origin.

A procephalic territory in Drosophila exhibiting similarities and dissimilarities compared to the vertebrate midbrain/hindbrain boundary region

Background

In vertebrates, the primordium of the brain is subdivided by the expression of Otx genes (forebrain/anterior midbrain), Hox genes (posterior hindbrain), and the genes Pax2, Pax5 and Pax8 (intervening region). The latter includes the midbrain/hindbrain boundary (MHB), which acts as a key organizer during brain patterning. Recent studies in Drosophila revealed that orthologous sets of genes are expressed in a similar tripartite pattern in the late embryonic brain, which suggested correspondence between the Drosophila deutocerebral/tritocerebral boundary region and the vertebrate MHB. To gain more insight into the evolution of brain regions, and particularly the MHB, I examined the expression of a comprehensive array of MHB-specific gene orthologs in the procephalic neuroectoderm and in individually identified neuroblasts during early embryonic stages 8–11, at which the segmental organization of the brain is most clearly displayed.

Results and conclusion

I show that the early embryonic brain exhibits an anterior Otx/otd domain and a posterior Hox1/lab domain, but that Pax2/5/8 orthologs are not expressed in the neuroectoderm and neuroblasts of the intervening territory. Furthermore, the expression domains of Otx/otd and Gbx/unpg exhibit a small common interface within the anterior deutocerebrum. In contrast to vertebrates, Fgf8-related genes are not expressed posterior to the otd/unpg interface. However, at the otd/unpg interface the early expression of other MHB-specific genes (including btd, wg, en), and of dorsoventral patterning genes, closely resembles the situation at the vertebrate MHB. Altogether, these results suggest the existence of an ancestral territory within the primordium of the deutocerebrum and adjacent protocerebrum, which might be the evolutionary equivalent of the region of the vertebrate MHB. However, lack of expression of Pax2/5/8 and Fgf8-related genes, and significant differences in the expression onset of other key regulators at the otd/unpg interface, imply that genetic interactions crucial for the vertebrate organizer activity are absent in the early embryonic brain of Drosophila.

Pax-Six-Eya-Dach network during amphioxus development: conservation in vitro but context specificity in vivo

The Drosophila retinal determination gene network occurs in animals generally as a Pax-Six-Eyes absent-Dachshund network (PSEDN). For amphioxus, we describe the complete network of nine PSEDN genes, four of which-AmphiSix1/2, AmphiSix4/5, AmphSix3/6, and AmphiEya-are characterized here for the first time. For amphioxus, in vitro interactions among the genes and proteins of the network resemble those of other animals, except for the absence of Dach-Eya binding. Amphioxus PSEDN genes are expressed in highly stage- and tissue-specific patterns (sometimes conspicuously correlated with the local intensity of cell proliferation) in the gastrular organizer, notochord, somites, anterior central nervous system, peripheral nervous system, pharyngeal endoderm, and the likely homolog of the vertebrate adenohypophysis. In this last tissue, the anterior region expresses all three amphioxus Six genes and is a zone of active cell proliferation, while the posterior region expresses only AmphiPax6 and is non-proliferative. In summary, the topologies of animal PSEDNs, although considerably more variable than originally proposed, are conserved enough to be recognizable among species and among developing tissues; this conservation may reflect indispensable involvement of PSEDNs during the critically important early phases of embryology (e.g. in the control of mitosis, apoptosis, and cell/tissue motility).

A genomic view of the sea urchin nervous system

The sequencing of the Strongylocentrotus purpuratus genome provides a unique opportunity to investigate the function and evolution of neural genes. The neurobiology of sea urchins is of particular interest because they have a close phylogenetic relationship with chordates, yet a distinctive pentaradiate body plan and unusual neural organization. Orthologues of transcription factors that regulate neurogenesis in other animals have been identified and several are expressed in neurogenic domains before gastrulation indicating that they may operate near the top of a conserved neural gene regulatory network. A family of genes encoding voltage-gated ion channels is present but, surprisingly, genes encoding gap junction proteins (connexins and pannexins) appear to be absent. Genes required for synapse formation and function have been identified and genes for synthesis and transport of neurotransmitters are present. There is a large family of G-protein-coupled receptors, including 874 rhodopsin-type receptors, 28 metabotropic glutamate-like receptors and a remarkably expanded group of 161 secretin receptor-like proteins. Absence of cannabinoid, lysophospholipid and melanocortin receptors indicates that this group may be unique to chordates. There are at least 37 putative G-protein-coupled peptide receptors and precursors for several neuropeptides and peptide hormones have been identified, including SALMFamides, NGFFFamide, a vasotocin-like peptide, glycoprotein hormones and insulin/insulin-like growth factors. Identification of a neurotrophin-like gene and Trk receptor in sea urchin indicates that this neural signaling system is not unique to chordates. Several hundred chemoreceptor genes have been predicted using several approaches, a number similar to that for other animals. Intriguingly, genes encoding homologues of rhodopsin, Pax6 and several other key mammalian retinal transcription factors are expressed in tube feet, suggesting tube feet function as photosensory organs. Analysis of the sea urchin genome presents a unique perspective on the evolutionary history of deuterostome nervous systems and reveals new approaches to investigate the development and neurobiology of sea urchins.

Identification and characterization of homeobox transcription factor genes in Strongylocentrotus purpuratus, and their expression in embryonic development

A set of 96 homeobox transcription factors was identified in the Strongylocentrotus purpuratus genome using permissive blast searches with a large collection of authentic homeodomain sequences from mouse, human and fly. A phylogenetic tree was constructed to compare the sea urchin homeobox gene family to those of vertebrates, with the result that with the only a few exceptions, orthologs of all vertebrate homeodomain genes were uncovered by our search. QPCR time course measurements revealed that 65% of these genes are expressed within the first 48 h of development (late gastrula). For genes displaying sufficiently high levels of transcript during the first 24 h of development (late blastula), whole mount in situ hybridization was carried out up to 48 h to determine spatial patterns of expression. The results demonstrate that homeodomain transcription factors participate in multiple and diverse developmental functions, in that they are used at a range of time points and in every territory of the developing embryo.

Evolution of regulatory elements producing a conserved gene expression pattern in Caenorhabditis

Natural selection acts at the level of function, not at the logistical level of how organisms achieve a particular function. Consequently, significant DNA sequence and regulatory differences can achieve the same function, such as a particular gene expression pattern. To investigate how regulatory features underlying a conserved function can evolve, we compared the regulation of a conserved gene expression pattern in the related species Caenorhabditis elegans and C. briggsae. We find that both C. elegans and C. briggsae express the ovo-related zinc finger gene lin-48 in the same pattern in hindgut cells. However, the regulation of this gene by the Pax-2/5/8 protein EGL-38 differs in two important ways. First, specific differences in the regulatory sequences of lin-48 result in the presence of two redundant EGL-38 response elements in C. elegans, whereas the redundancy is absent in C. briggsae. Second, there is a single egl-38 gene in C. briggsae. In contrast, the gene is duplicated in C. elegans, with only one copy retaining the ability to regulate lin-48 in vivo. These results illustrate molecular changes that can occur despite maintenance of conserved gene function in different species.

The 5-HT receptor cell is a new member of secondary mesenchyme cell descendants and forms a major blastocoelar network in sea urchin larvae

A gene encoding the serotonin (5-hydroxytryptamine, 5-HT) receptor (5-HT-hpr) was identified from the sea urchin, Hemicentrotus pulcherrimus. Partial amino acid sequence deduced from the cDNA showed strong similarity to Aplysia californica 5-HT2 receptor. Immunoblotting analysis of this 5-HT-hpr protein (5-HT-hpr) with an antibody raised against a deduced peptide showed two bands. Their relative molecular masses were 69 and 53 kDa, respectively. The larger band alone disappeared after N-glycopeptidase F digestion, indicating the protein was N-glycosylated. Immunolocalization analysis showed that cells expressing the 5-HT-hpr (SRC) first appeared near the tip of the archenteron in 33-h post-fertilization (33 hpf) prism larvae. Their cell number doubled in 2 h, and 5-HT-hpr protein expression increased further without cell proliferation. SRC spread ventrally on the basal surface of the oral ectoderm in 36 hpf prism larvae, and then clockwise on the ventral ectoderm to the posterior region to complete formation of the SRC network in 48 hpf early plutei. The SRC network was comprised of 7 main tracts: 4 spicule system-associated tracts and 3 spicule system-independent tracts. The network extended short fibers to the larval body surface through the ectoderm, implicating a signal transmission system that receives exogenous signal. Double-stain immunohistochemistry with antibodies to primary mesenchyme cells showed that SRC were not stained by the antibody. In embryos deprived of secondary mesenchyme cell (SMC) by microsurgery, the number of SRC decreased considerably. These two data indicate that SRC are SMC descendants, adding a new member to the SMC lineage.

Zebrafish pax8 is required for otic placode induction and plays a redundant role with Pax2 genes in the maintenance of the otic placode

Vertebrate Pax2 and Pax8 proteins are closely related transcription factors hypothesized to regulate early aspects of inner ear development. In zebrafish and mouse, Pax8 expression is the earliest known marker of otic induction, and Pax2 homologs are expressed at slightly later stages of placodal development. Analysis of compound mutants has not been reported. To facilitate analysis of zebrafish pax8, we completed sequencing of the entire gene, including the 5' and 3' UTRs. pax8 transcripts undergo complex alternative splicing to generate at least ten distinct isoforms. Two different subclasses of pax8 splice isoforms encode different translation initiation sites. Antisense morpholinos (MOs) were designed to block translation from both start sites, and four additional MOs were designed to target different exon-intron boundaries to block splicing. Injection of MOs, individually and in various combinations, generated similar phenotypes. Otic induction was impaired, and otic vesicles were small. Regional ear markers were expressed correctly, but hair cell production was significantly reduced. This phenotype was strongly enhanced by simultaneously disrupting either of the co-inducers fgf3 or fgf8, or another early regulator, dlx3b, which is thought to act in a parallel pathway. In contrast, the phenotype caused by disrupting foxi1, which is required for pax8 expression, was not enhanced by simultaneously disrupting pax8. Disrupting pax8, pax2a and pax2b did not further impair otic induction relative to loss of pax8 alone. However, the amount of otic tissue gradually decreased in pax8-pax2a-pax2b-deficient embryos such that no otic tissue was detectable by 24 hours post-fertilization. Loss of otic tissue did not correlate with increased cell death, suggesting that otic cells dedifferentiate or redifferentiate as other cell type(s). These data show that pax8 is initially required for normal otic induction, and subsequently pax8, pax2a and pax2b act redundantly to maintain otic fate.

Thyroid development and its disorders: genetics and molecular mechanisms

Thyroid gland organogenesis results in an organ the shape, size, and position of which are largely conserved among adult individuals of the same species, thus suggesting that genetic factors must be involved in controlling these parameters. In humans, the organogenesis of the thyroid gland is often disturbed, leading to a variety of conditions, such as agenesis, ectopy, and hypoplasia, which are collectively called thyroid dysgenesis (TD). The molecular mechanisms leading to TD are largely unknown. Studies in murine models and in a few patients with dysgenesis revealed that mutations in regulatory genes expressed in the developing thyroid are responsible for this condition, thus showing that TD can be a genetic and inheritable disease. These studies open the way to a novel working hypothesis on the molecular and genetic basis of this frequent human condition and render the thyroid an important model in the understanding of molecular mechanisms regulating the size, shape, and position of organs.

Role of Pax genes in eye evolution: a cnidarian PaxB gene uniting Pax2 and Pax6 functions

PaxB from Tripedalia cystophora, a cubomedusan jellyfish possessing complex eyes (ocelli), was characterized. PaxB, the only Pax gene found in this cnidarian, is expressed in the larva, retina, lens, and statocyst. PaxB contains a Pax2/5/8-type paired domain and octapeptide, but a Pax6 prd-type homeodomain. Pax2/5/8-like properties of PaxB include a DNA binding specificity of the paired domain, activation and inhibitory domains, and the ability to rescue spa(pol), a Drosophila Pax2 eye mutant. Like Pax6, PaxB activates jellyfish crystallin and Drosophila rhodopsin rh6 promoters and induces small ectopic eyes in Drosophila. Pax6 has been considered a "master" control gene for eye development. Our data suggest that the ancestor of jellyfish PaxB, a PaxB-like protein, was the primordial Pax protein in eye evolution and that Pax6-like genes evolved in triploblasts after separation from Cnidaria, raising the possibility that cnidarian and sophisticated triploblastic eyes arose independently.

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

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

A novel positive transcriptional feedback loop in midbrain-hindbrain boundary development is revealed through analysis of the zebrafish pax2.1 promoter in transgenic lines

The pax2.1 gene encodes a paired-box transcription factor that is one of the earliest genes to be specifically activated in development of the midbrain and midbrain-hindbrain boundary (MHB), and is required for the development and organizer activity of this territory. To understand how this spatially restricted transcriptional activity of pax2.1 is achieved, we have isolated and characterized the pax2.1-promoter using a lacZ and a GFP reporter gene in transient injection assays and transgenic lines. Stable transgenic expression of this reporter gene shows that a 5.3-kb fragment of the 5′ region contains most, but not all, elements required for driving pax2.1 expression. The expressing tissues include the MHB, hindbrain, spinal cord, ear and pronephros. Transgene activation in the pronephros and developing ear suggests that these pax2.1-expressing tissues are composed of independently regulated subdomains. In addition, ectopic but spatially restricted activation of the reporter genes in rhombomeres 3 and 5 and in the forebrain, which do not normally express endogenous pax2.1, demonstrates the importance of negative regulation of pax2.1.

Comparison of transgene expression in wild-type and homozygous pax2.1 mutant no isthmus (noi) embryos reveals that the transgene contains control element(s) for a novel, positive transcriptional feedback loop in MHB development. Transcription of endogenous pax2.1 at the MHB is known to be initially Pax2.1 independent, during activation in late gastrulation. In contrast, transgene expression requires the endogenous Pax2.1 function. Transplantations, mRNA injections and morpholino knock-down experiments show that this feedback regulation of pax2.1 transcription occurs cell-autonomously, and that it requires eng2 and eng3 as known targets for Pax2.1 regulation. We suggest that this novel feedback loop may allow continuation of pax2.1 expression, and hence development of the MHB organizer, to become independent of the patterning machinery of the gastrula embryo.

Formation of the head-trunk boundary in the animal body plan: an evolutionary perspective

Gene expression analyses and anatomical studies suggest that the body plans of protostomes and deuterostomes are phylogenetically related. In the central nervous system (CNS), arthropods and vertebrates (as well as their closest related phyla the urochordates and cephalochordates) share a nerve cord with rostral specification: the cerebral neuromeres in Drosophila, cerebral sensory vesicle of ascidians and lancelets and the large brain of craniates. Homologous genes, in particular of the otd/Otx and Hox families, are at play in these species to specify the anterior and posterior CNS territories, respectively. In contrast, homologies in the establishment of boundary regions like those separating head and trunk structures in arthropods or mid- and hindbrain domains in chordates are still unclear. We compare in these species the formation, properties and molecular characteristics of these boundaries during embryonic development. We also discuss recent findings suggesting that insects and vertebrates might have co-opted factors of related families to control the formation of these boundary regions, the evolution of which would then appear dramatically different from that of the anterior and posterior CNS domains.

Functional equivalency of amphioxus and vertebrate Pax258 transcription factors suggests that the activation of mid-hindbrain specific genes in vertebrates occurs via the recruitment of Pax regulatory elements

Pax genes encode transcription factors that control key developmental decisions in various animal phyla. The Pax2/5/8 subfamily plays a key role in specification and/or maintenance of vertebrate mid-hindbrain boundary (MHB) region by directly regulating expression of other genes, most notably En2. In the invertebrate chordate amphioxus, expression of AmphiPax2/5/8 is found in many sites that are homologous to the regions of the vertebrate embryo expressing orthologous genes Pax2, Pax5 or Pax8. However, no co-expression of AmphiPax2/5/8 and AmphiEn is detected in the region of the neural tube that might correspond to the vertebrate MHB. Based on this observation and the absence of AmphiWnt expression in this region it appears that amphioxus does not have a MHB. Here we investigated the possibility that the AmphiPax2/5/8, as a key component of MHB development, has lost some of the properties of its vertebrate counterparts. We have analyzed both the DNA-binding and transactivation properties of AmphiPax2/5/8 as well as its ability to interact with the groucho co-repressor. In all these assays AmphiPax2/5/8 is indistinguishable from the human Pax5. In addition, we found two alternatively spliced AmphiPax2/5/8 isoforms that function similarly to the alternatively spliced isoforms of human Pax8. Analysis of the AmphiEn regulatory region provided no evidence for AmphiPax2/5/8 binding and transactivation. Therefore, in amphioxus, AmphiPax2/5/8, although capable of performing all the necessary functions has not been recruited for a developmental mechanism which usually sets up MHB development in vertebrates.

Highly conserved amino acids in Pax and Ets proteins are required for DNA binding and ternary complex assembly

Combinatorial association of DNA-binding proteins on composite binding sites enhances their nucleotide sequence specificity and functional synergy. As a paradigm for these interactions, Pax-5 (BSAP) assembles ternary complexes with Ets proteins on the B cell-specific mb-1 promoter through interactions between their respective DNA-binding domains. Pax-5 recruits Ets-1 to bind the promoter, but not the closely related Ets protein SAP1a. Here we show that, while several different mutations increase binding of SAP1a to an optimized Ets binding site, only conversion of Val68 to an acidic amino acid facilitates ternary complex assembly with Pax-5 on the mb-1 promoter. This suggests that enhanced DNA binding by SAP1a is not sufficient for recruitment by Pax-5, but instead involves protein-protein interactions mediated by the acidic side chain. Recruitment of Ets proteins by Pax-5 requires Gln22 within the N-terminal beta-hairpin motif of its paired domain. The beta-hairpin also participates in recognition of a subset of Pax-5-binding sites. Thus, Pax-5 incorporates protein-protein interaction and DNA recognition functions in a single motif. The Caenorhabditis elegans Pax protein EGL-38 also binds specifically to the mb-1 promoter and recruits murine Ets-1 or the C.elegans Ets protein T08H4.3, but not the related LIN-1 protein. Together, our results define specific amino acid requirements for Pax-Ets ternary complex assembly and show that the mechanism is conserved between evolutionarily related proteins of diverse animal species. Moreover, the data suggest that interactions between Pax and Ets proteins are an important mechanism that regulates fundamental biological processes in worms and humans.

Isolation of Cladonema Pax-B genes and studies of the DNA-binding properties of cnidarian Pax paired domains

Pax genes encode nuclear transcription factors that are involved in developmental control. They contain a conserved DNA-binding domain, the paired domain. The DNA-binding specificity of paired domains is directly related to the gene regulation function of Pax proteins. Pax genes were previously divided into five groups on the basis of a phylogenetic analysis of paired domains. In this study, two highly similar cnidarian Pax-B genes from Cladonema californicum, a jellyfish with eyes, were found and sequenced. In an effort to understand the function of the cnidarian Pax genes isolated in this and a previous study, we characterized the consensus DNA sequences bound by the cnidarian paired domains using a PCR-based method and electrophoretic mobility shift assays. The consensus DNA sequences obtained are very similar to those bound by mammalian Pax proteins. Comparison of known consensus sequences indicates that they are all partially palindromic, but this characteristic is most prominent in cnidarians, which suggests that the DNA sequences bound by the ancestral paired domain could have been palindromic. Also, cnidarian paired domains, like those of Pax-2/5/8, possess a broader binding specificity than other paired domains, which implies that the common ancestor of Pax-2/5/8 and Pax-4/6 paired domains could also have had a similar broad DNA-binding specificity. Thus far, a definitive Pax-6 gene has not been found in several cnidarian species examined, which is consistent with a later origin of the Pax-6 gene and raises two possibilities: the Pax genes of cnidarians are multifunctional and control two or more developmental pathways, including eye development, or they use a Pax-independent pathway for eye development. Whether this pathway does exist and is unique to cnidarians or it whether it represents a true master control under which Pax-6 was later included remains to be determined.

EGL-38 Pax regulates theovo-related genelin-48duringCaenorhabditis elegansorgan development

The Pax gene egl-38 plays an important role in the development of several organs in C. elegans. To understand how a Pax transcription factor influences distinct developmental choices in different cells and tissue types, we have characterized a second gene, lin-48. lin-48 functions with egl-38 in the development of one structure, the hindgut, but not in other tissues such as the egg-laying system. We show that lin-48 encodes a C2H2 zinc-finger protein that is similar to the product of the Drosophila gene ovo and is expressed in the hindgut cells that develop abnormally in lin-48 mutants. We present evidence that lin-48 is a target for EGL-38 in hindgut cells. We show that lin-48 requires egl-38 for its expression in the hindgut. Using deletion analysis, we have identified two elements in the lin-48 promoter that are necessary for lin-48 expression. We demonstrate that EGL-38 binds with high affinity to one of these elements. In addition, we have observed genetic interactions between mutations in the lin-48 promoter and specific alleles of egl-38. These experiments demonstrate a functional link between Pax and Ovo transcription factors, and provide a model for how Pax transcription factors can regulate different target genes in different cells.

A unique combination of transcription factors controls differentiation of thyroid cells

The thyroid follicular cell type is devoted to the synthesis of thyroid hormones. Several genes, whose protein products are essential for efficient hormone biosynthesis, are uniquely expressed in this cell type. A set of transcriptional regulators, unique to the thyroid follicular cell type, has been identified as responsible for thyroid specific gene expression; it comprises three transcription factors, named TTF-1, TTF-2, and Pax8, each of which is expressed also in cell types different from the thyroid follicular cells. However, the combination of these factors is unique to the thyroid hormone producing cells, strongly suggesting that they play an important role in differentiation of these cells. An overview of the molecular and biological features of these transcription factors is presented here. Data demonstrating that all three play also an important role in early thyroid development, at stages preceding expression of the differentiated phenotype, are also reviewed. The wide temporal expression, from the beginning of thyroid organogenesis to the adult state, is suggestive of a recycling of the thyroid-specific transcription factors, that is, the control of different sets of target genes at diverse developmental stages. The identification of molecular mechanisms leading to specific gene expression in thyroid cells renders this cell type an interesting model in which to address several aspects of cell differentiation and organogenesis.

Combinatorial signaling in the specification of unique cell fates

How multifunctional signals combine to specify unique cell fates during pattern formation is not well understood. Here, we demonstrate that together with the transcription factor Lozenge, the nuclear effectors of the EGFR and Notch signaling pathways directly regulate D-Pax2 transcription in cone cells of the Drosophila eye disc. Moreover, the specificity of D-Pax2 expression can be altered upon genetic manipulation of these inputs. Thus, a relatively small number of temporally and spatially controlled signals received by a set of pluripotent cells can create the unique combinations of activated transcription factors required to regulate target genes and ultimately specify distinct cell fates within this group. We expect that similar mechanisms may specify pattern formation in vertebrate developmental systems that involve intercellular communication.

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.

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.

Transcriptional repression by Pax5 (BSAP) through interaction with corepressors of the Groucho family

Pax5 (BSAP) functions as both a transcriptional activator and repressor during midbrain patterning, B-cell development and lymphomagenesis. Here we demonstrate that Pax5 exerts its repression function by recruiting members of the Groucho corepressor family. In a yeast two-hybrid screen, the groucho-related gene product Grg4 was identified as a Pax5 partner protein. Both proteins interact cooperatively via two separate domains: the N-terminal Q and central SP regions of Grg4, and the octapeptide motif and C-terminal transactivation domain of Pax5. The phosphorylation state of Grg4 is altered in vivo upon Pax5 binding. Moreover, Grg4 efficiently represses the transcriptional activity of Pax5 in an octapeptide-dependent manner. Similar protein interactions resulting in transcriptional repression were also observed between distantly related members of both the Pax2/5/8 and Groucho protein families. In agreement with this evolutionary conservation, the octapeptide motif of Pax proteins functions as a Groucho-dependent repression domain in Drosophila embryos. These data indicate that Pax proteins can be converted from transcriptional activators to repressors through interaction with corepressors of the Groucho protein family.

A novel Pax-6 binding site in rodent B1 repetitive elements: coevolution between developmental regulation and repeated elements?

Pax-6 encodes a transcription factor that is important in the development of eye and CNS. Identification of Pax-6 target genes is crucial for understanding the gene regulatory network in these developmental processes. Using an in-vitro approach of cyclic amplification of the protein binding sequences (CAPBS), we isolated a PAX6 binding sequence from a human single-copy (sc) DNA library. Characterization of this PAX6 binding sequence revealed a 15bp region (hGCalpha1BLs5) that is sufficient for PAX6 specific binding. From a homology search in the GenBank, we found that an hGCalpha1BLs5-like Pax-6 binding site exists in 21 genes (16 from rodent), 15 of which were shown to be able to bind Pax-6 in vitro. Interestingly, some of these sites occur in B1 repetitive elements. Although hGCalpha1BLs5 is highly similar to a region in B1 repetitive elements, PAX6 does not bind to the consensus sequence in B1. However, a single-step mutation in some B1 elements can lead to a gain of function for PAX6 binding. This experimental evidence and phylogenetic analysis raise an interesting speculation for the coevolution between PAX6 regulation and repeat elements. Since a (Pax-6-binding) null B1 element can be re-activated by even a single-step mutation, it has the potential to recruit gene targets for Pax-6 if it is inserted into the regulatory region, and therefore may play a role for evolutionary modification of Pax-6 regulation.

Pax2 and homeodomain proteins cooperatively regulate a 435 bp enhancer of the mouse Pax5 gene at the midbrain-hindbrain boundary

Pax and homeodomain transcription factors are essential for the formation of an organizing center at the midbrain-hindbrain boundary (mhb) which controls the genesis of the midbrain and cerebellum in the vertebrate embryo. Pax2 and Pax5 are sequentially activated in this brain region, with Pax2 expression preceding that of Pax5. Using a transgenic reporter assay, we have now identified a conserved 435 bp enhancer in the 5′ flanking region of mammalian Pax5 genes which directs lacZ expression in the correct temporal and spatial pattern at the mhb. This minimal enhancer is composed of two distinct elements, as shown by protein-binding assays with mhb-specific extracts. The proximal element contains overlapping consensus binding sites for members of the Pax2/5/8 and POU protein families, whereas a distal element is bound by homeodomain and zinc finger transcription factors. Expression analysis of transgenes carrying specific mutations in these recognition motifs identified the Pax- and homeodomain-binding sites as functional elements which cooperatively control the activity of the mhb enhancer. lacZ genes under the control of either the minimal enhancer or the endogenous Pax5 locus were normally expressed at the mhb in Pax5 mutant embryos, indicating that this enhancer does not depend on autoregulation by Pax5. In Pax2 mutant embryos, expression of the endogenous Pax5 gene was, however, delayed and severely reduced in lateral aspects of the neural plate which, on neural tube closure, becomes the dorsal mhb region. This cross-regulation by Pax2 is mediated by the Pax-binding site of the minimal enhancer which, upon specific mutation, resulted in severely reduced transgene expression in the dorsal part of the mhb. Together these data indicate that Pax2 and homeodomain proteins directly bind to and cooperatively regulate the mhb enhancer of Pax5.

Synergism between Pax-8 and lim-1 in embryonic kidney development

Pax genes encode a family of highly conserved DNA-binding transcription factors. These proteins play key roles in regulating a number of vertebrate and invertebrate developmental processes. Mutations in Pax-6 result in eye defects in flies, mice, and humans, and ectopic expression of this gene can trigger the development of ectopic compound eyes in flies. Likewise, mutation of other Pax genes in vertebrates results in the failure of specific differentiation programs-Pax-1 causes skeletal defects; Pax-2, kidney defects; Pax-3 or Pax-7, neural crest defects; Pax-4, pancreatic beta-cell defects; Pax-5, B-cell defects; Pax-8, thyroid defects; and Pax-9, tooth defects. Although this class of genes is obviously required for the normal differentiation of a number of distinct organ systems, they have not previously been demonstrated to be capable of directing the embryonic development of organs in vertebrates. In this report, it is demonstrated that Pax-8 plays such a role in the establishment of the Xenopus embryonic kidney, the pronephros. However, in order to efficiently direct cells to form pronephric kidneys, XPax-8 requires cofactors, one of which may be the homeobox transcription factor Xlim-1. These two genes are initially expressed in overlapping domains in late gastrulae, and cells expressing both genes will go on to form the kidney. Ectopic expression of either gene alone has a moderate effect on pronephric patterning, while coexpression of XPax-8 plus Xlim-1 results in the development of embryonic kidneys of up to five times normal complexity and also leads to the development of ectopic pronephric tubules. This effect was synergistic rather than additive. XPax-2 can also synergize with Xlim-1, but the expression profile of this gene indicates that it normally functions later in pronephric development than does XPax-8. Together these data indicate that the interaction between XPax-8 and Xlim-1 is a key early step in the establishment of the pronephric primordium.

DNA binding and transactivating properties of the paired and homeobox protein Pax4

Transcription factors Pax-4 and Pax-6 are known to be key regulators of pancreatic cell differentiation and development. We report on the cloning of a mouse Pax-4 gene, which contains 10 exons, spanning a 4. 7-kbp region. The gene-targeting experiments revealed that Pax-4 and Pax-6 cannot substitute for each other in tissue with overlapping expression of both genes. We identified DNA-binding specificities of Pax-4 paired domain and paired-type homeodomain. Despite the different Pax-4 amino acid residues in positions responsible for Pax-6 paired-domain specificity, the DNA-binding specificities of Pax-4 and Pax-6 are similar. The Pax-4 homeodomain was shown to preferentially dimerize on DNA sequences consisting of an inverted TAAT motif, separated by 4-nucleotide spacing. The Pax-4 transactivation domain was localized within its C-terminal region, which transactivated GAL-based reporter 2.5-fold less than the C-terminal region of Pax-6. We believe that Pax-4 can act as a Pax-6 "repressor," due to the competition for binding sites and lower transactivation potential of Pax-4.

Developmental expression of Pax1/9 genes in urochordate and hemichordate gills: insight into function and evolution of the pharyngeal epithelium

The epithelium of the pharynx contributes to the formation of gills in hemichordates, urochordates, cephalochordates and primitive vertebrates, and is therefore a key structure for understanding developmental mechanisms underlying the establishment of chordate body plans. Pax1- and Pax9-related genes encode transcription factors which are expressed in the pharyngeal region of cephalochordates as well as in the vertebrate pharyngeal pouch epithelium that forms the thymus and parathyroid glands. To explore the molecular basis underlying the occurrence and modifications of the pharyngeal epithelium during evolution, we isolated cDNA clones for Pax1- and Pax9-related genes of urochordates (HrPax1/9 of Halocynthia roretzi and CiPax1/9 of Ciona intestinalis) and a hemichordate (PfPax1/9 of Ptychodera flava) from gill cDNA libraries. Each gene is present as a single copy per haploid genome. All of the cDNAs encode typical paired domains and octapeptides but not a homeodomain, as is also true of other Pax1- and Pax9-related genes. Molecular phylogenetic analysis based on comparison of the paired domain amino-acid sequences suggests that HrPax1/9, CiPax1/9 and PfPax1/9 belong to the Pax1/9 subfamily, and that they are descendants of a single precursor of Pax1/Pax9. Screening of HrPax1/9 cDNA clones yielded six different types of transcripts which were generated by alternative splicing. Northern blot, RT-PCR/Southern and in situ hybridization analyses revealed that HrPax1/9, CiPax1/9 and PfPax1/9 are not expressed during early embryogenesis but are expressed in the epithelia of differentiating gills, suggesting that these genes encode gill-specific transcription factors. The Pax1/9 genes therefore might provide the first developmental genetic corroboration of hypotheses of organ-level homology that unifies hemichordates, urochordates and cephalochordates.

Isolation and expression of a Pax-6 gene in the regenerating and intact Planarian Dugesia(G)tigrina

The Pax-6 gene encodes a transcription factor containing both a paired and a homeodomain and is highly conserved among Metazoa. In both vertebrates and invertebrates, Pax-6 is required for eye morphogenesis, development of parts of the central nervous system, and, in some phyla, for the development of olfactory sense organs. Ectopic expression of Pax-6 from insects, mammals, cephalopods, and ascidians induces ectopic eyes in Drosophila, suggesting that Pax-6 may be a universal master control gene for eye morphogenesis. Platyhelminthes are an ancient phylum, originating from the base of spiralian protostomes, that bear primitive eyes, consisting of a group of rhabdomeric photoreceptor cells enclosed in a cup of pigment cells. The analysis of Pax-6 and its expression pattern should provide insights into the ancestral function of Pax-6 in eye morphogenesis. We have identified the Pax-6 gene of the planarian Dugesia(G)tigrina (Platyhelminthes; Turbellaria; Tricladida). This gene shares significant sequence identity and conserved genomic organization with Pax-6 proteins from other phyla. Phylogenetic analysis indicates that it clusters with the other Pax-6 genes, but in the most basal position. DtPax-6 is expressed as a single transcript in both regenerating and fully grown eyes, and electron microscopy studies show strong expression in the perykarion of both photoreceptor and pigment cells. Very low levels of expression also are detectable in other body regions. Because a bona fide Pax-6 homolog so far has not been detected in diploblastic animals, we speculate that Pax-6 may be typical for triploblasts and that the appearance of additional Pax genes may have coincided with increasingly complex body plans.

Cell fate choices and the expression of Notch, Delta and Serrate homologues in the chick inner ear: parallels with Drosophila sense-organ development

The sensory patches in the vertebrate inner ear are similar in function to the mechanosensory bristles of a fly, and consist of a similar set of cell types. If they are truly homologous structures, they should also develop by similar mechanisms. We examine the genesis of the neurons, hair cells and supporting cells that form the sensory patches in the inner ear of the chick. These all arise from the otic epithelium, and are produced normally even in otic epithelium cultured in isolation, confirming that their production is governed by mechanisms intrinsic to the epithelium. First, the neuronal sublineage becomes separate from the epithelial: between E2 and E3.5, neuroblasts delaminate from the otocyst. The neuroblasts then give rise to a mixture of neurons and neuroblasts, while the sensory epithelial cells diversify to form a mixture of hair cells and supporting cells. The epithelial patches where this occurs are marked from an early stage by uniform and maintained expression of the Notch ligand Serrate1. The Notch ligand Delta1 is also expressed, but transiently and in scattered cells: it is seen both early, during neuroblast segregation, where it appears to be in the nascent neuroblasts, and again later, in the ganglion and in differentiating sensory patches, where it appears to be in the nascent hair cells, disappearing as they mature. Delta-Notch-mediated lateral inhibition may thus act at each developmental branchpoint to drive neighbouring cells along different developmental pathways. Our findings indicate that the sensory patches of the vertebrate inner ear and the sensory bristles of a fly are generated by minor variations of the same basic developmental program, in which cell diversification driven by Delta-Notch and/or Serrate-Notch signalling plays a central part.

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.

shaven and sparkling are mutations in separate enhancers of the Drosophila Pax2 homolog

We have previously shown that the sparkling gene, which like mammalian Pax2 plays an important role in eye development, is encoded by the Drosophila homolog of Pax2. Here we demonstrate that D-Pax2 also encodes the shaven function, which is crucial during bristle development. Both sv and spa alleles, previously thought to represent different genes, are mutations in two widely separated enhancers of D-Pax2. The sv function of D-Pax2 acts in at least two distinct steps of mechanosensory bristle development: the specification of the alternative fate of shaft as opposed to socket cell, and later the differentiation of the shaft cell.