Altering the boundaries of Hox3.1 expression: evidence for antipodal gene regulation

Breaking constraint of mammalian axial formulae

The vertebral column of individual mammalian species often exhibits remarkable robustness in the number and identity of vertebral elements that form (known as axial formulae). The genetic mechanism(s) underlying this constraint however remain ill-defined. Here, we reveal the interplay of three regulatory pathways (Gdf11, miR-196 and Retinoic acid) is essential in constraining total vertebral number and regional axial identity in the mouse, from cervical through to tail vertebrae. All three pathways have differing control over Hox cluster expression, with heterochronic and quantitative changes found to parallel changes in axial identity. However, our work reveals an additional role for Hox genes in supporting axial elongation within the tail region, providing important support for an emerging view that mammalian Hox function is not limited to imparting positional identity as the mammalian body plan is laid down. More broadly, this work provides a molecular framework to interrogate mechanisms of evolutionary change and congenital anomalies of the vertebral column.

Vertebral column length and shape exhibits remarkable robustness within a species but diversity across species. Here the authors reveal the molecular logic constraining vertebral number in mouse and a novel role for posterior Hox genes in this context.

Hox Proteins in the Regulation of Muscle Development

Hox genes encode evolutionary conserved transcription factors that specify the anterior-posterior axis in all bilaterians. Being well known for their role in patterning ectoderm-derivatives, such as CNS and spinal cord, Hox protein function is also crucial in mesodermal patterning. While well described in the case of the vertebrate skeleton, much less is known about Hox functions in the development of different muscle types. In contrast to vertebrates however, studies in the fruit fly, Drosophila melanogaster, have provided precious insights into the requirement of Hox at multiple stages of the myogenic process. Here, we provide a comprehensive overview of Hox protein function in Drosophila and vertebrate muscle development, with a focus on the molecular mechanisms underlying target gene regulation in this process. Emphasizing a tight ectoderm/mesoderm cross talk for proper locomotion, we discuss shared principles between CNS and muscle lineage specification and the emerging role of Hox in neuromuscular circuit establishment.

Shark and ray genomics for disentangling their morphological diversity and vertebrate evolution

Developmental studies of sharks and rays (elasmobranchs) have provided much insight into the process of morphological evolution of vertebrates. Although those studies are supposedly fueled by large-scale molecular sequencing information, whole-genome sequences of sharks and rays were made available only recently. One compelling difficulty of elasmobranch developmental biology is the low accessibility to embryonic study materials and their slow development. Another limiting factor is the relatively large size of their genomes. Moreover, their large body sizes restrict sustainable captive breeding, while their high body fluid osmolarity prevents reproducible cell culturing for in vitro experimentation, which has also limited our knowledge of their chromosomal organization for validation of genome sequencing products. This article focuses on egg-laying elasmobranch species used in developmental biology and provides an overview of the characteristics of the shark and ray genomes revealed to date. Developmental studies performed on a gene-by-gene basis are also reviewed from a whole-genome perspective. Among the popular regulatory genes studied in developmental biology, I scrutinize shark homologs of Wnt genes that highlight vanishing repertoires in many other vertebrate lineages, as well as Hox genes that underwent an unexpected modification unique to the elasmobranch lineage. These topics are discussed together with insights into the reconstruction of developmental programs in the common ancestor of vertebrates and its subsequent evolutionary trajectories that mark the features that are unique to, and those characterizing the diversity among, cartilaginous fishes.

HOXA9 mediates and marks premalignant compartment size expansion in colonic adenomas

The transformation of normal colonic epithelium to colorectal cancer (CRC) involves a relatively ordered progression, and understanding the molecular alterations involved may aid rational design of strategies aimed at preventing or counteracting disease. Homeobox A9 (HOXA9) is an oncogene in leukemia and has been implicated in CRC pathology, although its role in disease etiology remains obscure at best. We observe that HOXA9 expression is increased in colonic adenomas compared with location-matched healthy colon epithelium. Its forced expression results in dramatic genetic and signaling changes, with increased expression of growth factors IGF1 and FLT3, super-activity of the AKT survival pathway and a concomitant increase in compartment size. Furthermore, a reduced mRNA expression of the epithelial to mesenchymal transition marker N-cadherin as well as reduced activity of the actin cytoskeletal mediator PAK was seen, which is in apparent agreement with an observed reduced migratory response in HOXA9-overexpressing cells. Thus, HOXA9 appears closely linked with adenoma growth while impairing migration and metastasis and hence is both a marker and driver of premalignant polyp growth. Colonic polyps grow but remain premalignant for up to decades. Here, we show that HOXA9 drives growth in premalignant polyps, but simultaneously prevents further transformation.

Embigin, regulated by HOXC8, plays a suppressive role in breast tumorigenesis

The transmembrane glycoprotein embigin (EMB) belongs to the immunoglobulin superfamily (IgSF) and a number of IgSF members have been identified as biomarkers for cancer progression. In this study, we show that embigin is transcriptionally regulated by Homeobox C8 (HOXC8) in breast cancer cells and embigin expression suppresses breast tumorigenesis. With aid of Western blot, luciferase reporter gene assay and chromatin immunoprecipitation, we reveal that HOXC8 binds to the EMB promoter at the region of nucleotides -2303 to -2315 and acts as a transcription inhibitor to suppress embigin expression. Depletion of embigin leads to increase in proliferation, anchorage-independent growth and migration of breast cancer cells, and the inhibitory effects mediated by HOXC8 knockdown on breast tumorigenesis can be largely rescued by depletion of embigin expression in breast cancer cells, suggesting that HOXC8 regulates breast tumorigenesis, at least partly, through regulating embigin expression. Moreover, we show that loss of embigin promotes proliferation, anchorage-independent growth, and migration ability of normal mammary epithelial MCF10A cells. The analyses of publically available human breast tumor microarray gene expression database show that low embigin levels correlate with short survival of breast tumor patients, particularly with basal-like tumor patients, and embigin expression is low specifically in patients with basal-like, ER-/HER2- tumors. Taken together, our study demonstrates that low/loss of embigin plays an important role in the progression of breast tumors.

Mutational analysis of HOXA10 gene in Chinese patients with cryptorchidism

Cryptorchidism is one of the most common congenital anomalies and affects 2-4% of full-term new born boys. Its aetiology is poorly understood at present. HOXA10 plays a pivotal role in regulation of testicular descent. Male mice mutant for Hoxa10 exhibit unilateral or bilateral cryptorchidism as a result of impaired development of the gubernaculums. In this study, we performed mutation analysis of HOXA10 gene in a cohort of 98 cryptorchid patients. And we found a mutation (N27K) in a boy with unilateral cryptorchidism. The mutation was not detected in 106 healthy controls. Both in silico analyses and functional studies showed that the mutation affected the function of HOXA10. The results demonstrated that mutation in HOXA10 gene contributes to the pathogenesis of cryptorchidism, but may not be a common cause.

Differential DNA methylation patterns of homeobox genes in proximal and distal colon epithelial cells

Region and cell-type specific differences in the molecular make up of colon epithelial cells have been reported. Those differences may underlie the region-specific characteristics of common colon epithelial diseases such as colorectal cancer and inflammatory bowel disease. DNA methylation is a cell-type specific epigenetic mark, essential for transcriptional regulation, silencing of repetitive DNA and genomic imprinting. Little is known about any region-specific variations in methylation patterns in human colon epithelial cells. Using purified epithelial cells and whole biopsies (n= 19) from human subjects, we generated epigenome-wide DNA methylation data (using the HELP-tagging assay), comparing the methylation signatures of the proximal and distal colon. We identified a total of 125 differentially methylated sites (DMS) mapping to transcription start sites of protein-coding genes, most notably several members of the homeobox (HOX) family of genes. Patterns of differential methylation were validated with MassArray EpiTYPER. We also examined DNA methylation in whole biopsies, applying a computational technique to deconvolve variation in methylation within cell types and variation in cell-type composition across biopsies. Including inferred epithelial proportions as a covariate in differential methylation analysis applied to the whole biopsies resulted in greater overlap with the results obtained from purified epithelial cells compared with when the covariate was not included. Results obtained from both approaches highlight region-specific methylation patterns of HOX genes in colonic epithelium. Regional variation in methylation patterns has implications for the study of diseases that exhibit regional expression patterns in the human colon, such as inflammatory bowel disease and colorectal cancer.

Hoxc8 initiates an ectopic mammary program by regulating Fgf10 and Tbx3 expression and Wnt/β-catenin signaling

The role of Hox genes in the formation of cutaneous accessory organs such as hair follicles and mammary glands has proved elusive, a likely consequence of overlapping function and expression among various homeobox factors. Lineage and immunohistochemical analysis of Hoxc8 in mice revealed that this midthoracic Hox gene has transient but strong regional expression in ventrolateral surface ectoderm at E10.5, much earlier than previously reported. Targeted mice were generated to conditionally misexpress Hoxc8 from the Rosa locus using select Cre drivers, which significantly expanded the domain of thoracic identity in mutant embryos. Accompanying this expansion was the induction of paired zones of ectopic mammary development in the cervical region, which generated between three and five pairs of mammary placodes anterior to the first wild-type mammary rudiment. These rudiments expressed the mammary placode markers Wnt10b and Tbx3 and were labeled by antibodies to the mammary mesenchyme markers ERα and androgen receptor. Somitic Fgf10 expression, which is required for normal mammary line formation, was upregulated in mutant cervical somites, and conditional ablation of ectodermal Tbx3 expression eliminated all normally positioned and ectopic mammary placodes. We present evidence that Hoxc8 participates in regulating the initiation stages of mammary placode morphogenesis, and suggest that this and other Hox genes are likely to have important roles during regional specification and initiation of these and other cutaneous accessory organs.

Independent regulation of vertebral number and vertebral identity by microRNA-196 paralogs

The Hox genes play a central role in patterning the embryonic anterior-to-posterior axis. An important function of Hox activity in vertebrates is the specification of different vertebral morphologies, with an additional role in axis elongation emerging. The miR-196 family of microRNAs (miRNAs) are predicted to extensively target Hox 3' UTRs, although the full extent to which miR-196 regulates Hox expression dynamics and influences mammalian development remains to be elucidated. Here we used an extensive allelic series of mouse knockouts to show that the miR-196 family of miRNAs is essential both for properly patterning vertebral identity at different axial levels and for modulating the total number of vertebrae. All three miR-196 paralogs, 196a1, 196a2, and 196b, act redundantly to pattern the midthoracic region, whereas 196a2 and 196b have an additive role in controlling the number of rib-bearing vertebra and positioning of the sacrum. Independent of this, 196a1, 196a2, and 196b act redundantly to constrain total vertebral number. Loss of miR-196 leads to a collective up-regulation of numerous trunk Hox target genes with a concomitant delay in activation of caudal Hox genes, which are proposed to signal the end of axis extension. Additionally, we identified altered molecular signatures associated with the Wnt, Fgf, and Notch/segmentation pathways and demonstrate that miR-196 has the potential to regulate Wnt activity by multiple mechanisms. By feeding into, and thereby integrating, multiple genetic networks controlling vertebral number and identity, miR-196 is a critical player defining axial formulae.

Transgenic studies on homeobox genes in nervous system development: spina bifida in Isl1 transgenic mice

To develop in vivo assays for homeobox gene function in neural development, we generated transgenic mice in which the expression of a homeobox gene is altered only within the nervous system, in neurons or neuronal precursor cells. Transgenic expression of Hoxc8 did not result in gross abnormalities, while a Hoxd4 transgene caused death shortly after birth. In neural progenitor cells, the motorneuron-specific homeodomain transcription factor Isl1 induced early developmental defects, including absence of anterior neural structures, profound defects in the neuroepithelium and defective neural tube closure. A fraction of Isl1 transgenic mice exhibited spina bifida. Isl1 transgene expression was also associated with decreased proliferation and increased Pbx1 expression in the ventral neural tube. Our results suggest a function for some homeobox genes in development of the nervous system, and that cell-type- and region-specific transgenic models will be useful to identify the cellular and molecular targets of homeobox transcription factors in nervous system development.

Ndrg2 regulates vertebral specification in differentiating somites

It is generally thought that vertebral patterning and identity are globally determined prior to somite formation. Relatively little is known about the regulators of vertebral specification after somite segmentation. Here, we demonstrated that Ndrg2, a tumor suppressor gene, was dynamically expressed in the presomitic mesoderm (PSM) and at early stage of differentiating somites. Loss of Ndrg2 in mice resulted in vertebral homeotic transformations in thoracic/lumbar and lumbar/sacral transitional regions in a dose-dependent manner. Interestingly, the inactivation of Ndrg2 in osteoblasts or chondrocytes caused defects resembling those observed in Ndrg2(-/-) mice, with a lower penetrance. In addition, forced overexpression of Ndrg2 in osteoblasts or chondrocytes also conferred vertebral defects, which were distinct from those in Ndrg2(-/-) mice. These genetic analyses revealed that Ndrg2 modulates vertebral identity in segmented somites rather than in the PSM. At the molecular level, combinatory alterations of the amount of Hoxc8-11 gene transcripts were detected in the differentiating somites of Ndrg2(-/-) embryos, which may partially account for the vertebral defects in Ndrg2 mutants. Nevertheless, Bmp/Smad signaling activity was elevated in the differentiating somites of Ndrg2(-/-) embryos. Collectively, our findings unveiled Ndrg2 as a novel regulator of vertebral specification in differentiating somites.

Boundaries, junctions and transitions in the gastrointestinal tract

Contiguous regions along the mammalian gastrointestinal tract, from the esophagus to the rectum, serve distinct digestive functions. Some organs, such as the esophagus and glandular stomach or the small bowel and colon, are separated by sharp boundaries. The duodenal, jejunal and ileal segments of the small intestine, by contrast, have imprecise borders. Because human esophageal and gastric cancers frequently arise in a background of tissue metaplasia and some intestinal disorders are confined to discrete regions, it is useful to appreciate the molecular and cellular basis of boundary formation and preservation. Here we review the anatomy and determinants of boundaries and transitions in the alimentary canal with respect to tissue morphology, gene expression, and, especially, transcriptional control of epithelial identity. We discuss the evidence for established and candidate molecular mechanisms of boundary formation, including the solitary and combinatorial actions of tissue-restricted transcription factors. Although the understanding remains sparse, genetic studies in mice do provide insights into dominant mechanisms and point the way for future investigation.

Smooth-muscle-specific expression of neurotrophin-3 in mouse embryonic and neonatal gastrointestinal tract

Vagal gastrointestinal (GI) afferents are essential for the regulation of eating, body weight, and digestion. However, their functional organization and the way that this develops are poorly understood. Neurotrophin-3 (NT-3) is crucial for the survival of vagal sensory neurons and is expressed in the developing GI tract, possibly contributing to their survival and to other aspects of vagal afferent development. The identification of the functions of this peripheral NT-3 thus requires a detailed understanding of the localization and timing of its expression in the developing GI tract. We have studied embryos and neonates expressing the lacZ reporter gene from the NT-3 locus and found that NT-3 is expressed predominantly in the smooth muscle of the outer GI wall of the stomach, intestines, and associated blood vessels and in the stomach lamina propria and esophageal epithelium. NT-3 expression has been detected in the mesenchyme of the GI wall by embryonic day 12.5 (E12.5) and becomes restricted to smooth muscle and lamina propria by E15.5, whereas its expression in blood vessels and esophageal epithelium is first observed at E15.5. Expression in most tissues is maintained at least until postnatal day 4. The lack of colocalization of beta-galactosidase and markers for myenteric ganglion cell types suggests that NT-3 is not expressed in these ganglia. Therefore, NT-3 expression in the GI tract is largely restricted to smooth muscle at ages when vagal axons grow into the GI tract, and when vagal mechanoreceptors form in smooth muscle, consistent with its role in these processes and in vagal sensory neuron survival.

Axial and appendicular skeletal transformations, ligament alterations, and motor neuron loss in Hoxc10 mutants

Vertebrate Hox genes regulate many aspects of embryonic body plan development and patterning. In particular, Hox genes have been shown to regulate regional patterning of the axial and appendicular skeleton and of the central nervous system. We have identified patterning defects resulting from the targeted mutation of Hoxc10, a member of the Hox10 paralogous family. Hoxc10 mutant mice have skeletal transformations in thoracic, lumbar, and sacral vertebrae and in the pelvis, along with alterations in the bones and ligaments of the hindlimbs. These results suggest that Hoxc10, along with other members of the Hox10 paralogous gene family, regulates vertebral identity at the transition from thoracic to lumbar and lumbar to sacral regions. Our results also suggest a general role for Hoxc10 in regulating chondrogenesis and osteogenesis in the hindlimb, along with a specific role in shaping femoral architecture. In addition, mutant mice have a reduction in lumbar motor neurons and a change in locomotor behavior. These results suggest a role for Hoxc10 in generating or maintaining the normal complement of lumbar motor neurons.

Establishment of intestinal identity and epithelial-mesenchymal signaling by Cdx2

We demonstrate that conditional ablation of the homeobox transcription factor Cdx2 from early endoderm results in the replacement of the posterior intestinal epithelium with keratinocytes, a dramatic cell fate conversion caused by ectopic activation of the foregut/esophageal differentiation program. This anterior homeotic transformation of the intestine was first apparent in the early embryonic Cdx2-deficient gut by a caudal extension of the expression domains of several key foregut endoderm regulators. While the intestinal transcriptome was severely affected, Cdx2 deficiency only transiently modified selected posterior Hox genes and the primary enteric Hox code was maintained. Further, we demonstrate that Cdx2-directed intestinal cell fate adoption plays an important role in the establishment of normal epithelial-mesenchymal interactions, as multiple signaling pathways involved in this process were severely affected. We conclude that Cdx2 controls important aspects of intestinal identity and development, and that this function is largely independent of the enteric Hox code.

Morpholino-mediated knockdown in primary chondrocytes implicates Hoxc8 in regulation of cell cycle progression

Numerous experiments in mutant and transgenic mice have implicated Hox transcription factors in development of the skeletal system, postulating a role for these proteins in cell proliferation of precursor cells and regulation of cell differentiation. Our own data from Hoxc8 and Hoxd4 transgenic mice suggest that Hoxc8 is involved in cell proliferation during cartilage development. In order to directly assess its role in cell proliferation of a specific skeletal cell type, the cartilage-producing chondrocyte, we performed morpholino-mediated knockdown experiments in normal primary chondrocytes. Through analysis of PCNA expression and staining for phosphorylated Histone 3, two cell cycle markers, we show that interference with Hoxc8 expression in chondrocytes reduces cell proliferation, but in the absence of apoptosis. Instead, cells with a knockdown in Hoxc8 expression appear to be delayed in their progression through the cell cycle. Our results provide evidence for prolonged duration of and delayed exit from M-phase, thus implicating a role for Hoxc8 in controlling cell cycle progression at this critical check point.

Comparison of diverged Hoxc8 early enhancer activities reveals modification of regulatory interactions at conserved cis-acting elements

The Hoxc8 early enhancer that controls the initiation and establishment of Hoxc8 expression in the developing mouse embryo is found in different vertebrate lineages including mammals, birds and fish. Mouse and Fugu Hoxc8 early enhancers (200 bp) have diverged in the composition of elements located towards the 3' region. However, they share cis-acting elements A-E located in the 5' region. Mutations at these elements in the context of the mouse Hoxc8 early enhancer affect reporter gene expression in the posterior neural tube, somites and lateral plate mesoderm of day 9.5 mouse embryos. Here, we demonstrate that mutations introduced at the same elements but in the context of the Fugu Hoxc8 early enhancer had different consequences on the reporter gene expression in transgenic mouse embryos. Furthermore, in contrast to the mouse enhancer the Fugu enhancer does not utilize elements D and E in achieving posterior neural tube and somite expression. These results suggest that the diverged sequences prevent regulatory interactions at conserved cis-acting elements. We propose that divergent sequences modify regulatory interactions at conserved elements by providing a "contextual change". Our finding that the enhancer elements do not act in a unitary fashion but function in the context of the surrounding sequence brings a new dimension to the study of cis-regulatory evolution.

Molecular basis for skeletal variation: insights from developmental genetic studies in mice

Skeletal variations are common in humans, and potentially are caused by genetic as well as environmental factors. We here review molecular principles in skeletal development to develop a knowledge base of possible alterations that could explain variations in skeletal element number, shape or size. Environmental agents that induce variations, such as teratogens, likely interact with the molecular pathways that regulate skeletal development.

The stomach mesenchymal transcription factor Barx1 specifies gastric epithelial identity through inhibition of transient Wnt signaling

Inductive interactions between gut endoderm and the underlying mesenchyme pattern the developing digestive tract into regions with specific morphology and functions. The molecular mechanisms behind these interactions are largely unknown. Expression of the conserved homeobox gene Barx1 is restricted to the stomach mesenchyme during gut organogenesis. Using recombinant tissue cultures, we show that Barx1 loss in the mesenchyme prevents stomach epithelial differentiation of overlying endoderm and induces intestine-specific genes instead. Additionally, Barx1 null mouse embryos show visceral homeosis, with intestinal gene expression within a highly disorganized gastric epithelium. Barx1 directs mesenchymal cell expression of two secreted Wnt antagonists, sFRP1 and sFRP2, and these factors are sufficient replacements for Barx1 function. Canonical Wnt signaling is prominent in the prospective gastric endoderm prior to epithelial differentiation, and its inhibition by Barx1-dependent signaling permits development of stomach-specific epithelium. These results define a transcriptional and signaling pathway of inductive cell interactions in vertebrate organogenesis.

Position-specific expression of Hox genes along the gastrointestinal tract

Hox genes play a critical role in morphogenesis of the early embryo along the anteroposterior axis. In mammals, 39 Hox genes with extensive homology are organized into 13 paralogous groups, forming four clusters on four separate chromosomes. The genes within each cluster are arranged in a 3' to 5' direction and expressed in a temporally and spatially coordinated manner along the anteroposterior axis in the vertebrae, limbs and viscera, including the gastrointestinal tract, but little is known about their spatial expression in the adult gastrointestinal tract. We used the quantitative polymerase chain reaction (PCR) intercalater method with SYBR Green to quantify human Hox gene expression in the adult gastrointestinal tract tissue: esophagus, stomach, duodenum, jejunum, ileum, ileocecum, cecum, ascending colon, transverse colon, descending colon and rectum. Hox gene expression was normalized to glyceraldehyde-3-phosphate-dehydrogenase (GAPDH) gene expression. The spatial expression pattern was analyzed by the multivariate method. The expression level of all 39 Hox genes could be measured in a reproducible manner. Genes with higher expression in the foregut-derived segments tended to have lower expression in hindgut-derived segments, whereas those with low expression in the former tended to have higher in the latter. Principal components analysis and permax analysis revealed a position-specific expression pattern of Hox genes along the anteroposterior axis of the adult gastrointestinal tract. The pattern recapitulates the expression pattern in the embryonic gastrointestinal tract. We suggest that Hox genes may play a pivotal role in the position-specific regenerative process of intestinal epithelial cells.

Too much of a good thing: retinoic acid as an endogenous regulator of neural differentiation and exogenous teratogen

Retinoic acid (RA) is essential for both embryonic and adult growth, activating gene transcription via specific nuclear receptors. It is generated, via a retinaldehyde intermediate, from retinol (vitamin A). RA levels require precise regulation by controlled synthesis and catabolism, and when RA concentrations deviate from normal, in either direction, abnormal growth and development occurs. This review describes: (i) how the pattern of RA metabolic enzymes controls the actions of RA; and (ii) the type of abnormalities that result when this pattern breaks down. Examples are given of RA control of the anterior/posterior axis of the hindbrain, the dorsal/ventral axis of the spinal cord, as well as certain sex-specific segments of the spinal cord, using varied animal models including mouse, quail and mosquitofish. These functions are highly sensitive to abnormal changes in RA concentration. In rodents, the control of neural patterning and differentiation are disrupted when RA concentrations are lowered, whereas inappropriately high concentrations of RA result in abnormal development of cerebellum and hindbrain nuclei. The latter parallels the malformations seen in the human embryo exposed to RA due to treatment of the mother with the acne drug Accutane (13-cis RA) and, in cases where the child survives beyond birth, a particular set of behavioural anomalies can be described. Even the adult brain may be susceptible to an imbalance of RA, particularly the hippocampus. This report shows how the properties of RA as a neural induction agent and organizer of segmentation can explain the consequences of RA depletion and overexpression.

Defects in cervical vertebrae in boric acid-exposed rat embryos are associated with anterior shifts of hox gene expression domains

BACKGROUND

Previously, we showed that prenatal exposure to boric acid (BA), an industrial agent with large production, causes alterations of the axial skeleton in rat embryos, reminiscent of homeotic transformations. Indeed, Sprague-Dawley rats exposed in utero to BA on gestation day 9 (GD 9) had only six, rather than the normal seven, cervical vertebrae. This finding, observed in 91% of GD 21 fetuses, suggests posterior transformations of vertebrae. The present study attempts to determine if these skeletal alterations could be explained by modifications of the hox code, involved in the establishment of positional information along the craniocaudal axis of the embryo.

METHODS

Pregnant rats were treated by gavage with BA (500 mg/kg, twice) on GD 9. Embryos were collected on GD 11 or GD 13.5 and processed for in situ hybridization. Several hox genes were selected according to the position of their cranial limit of expression in the cervical and thoracic region.

RESULTS

At GD 13.5, we detected a cranial shift of the anterior limit of expression of hoxc6 and hoxa6. We observed no difference between control and treated embryos in the location of the cranial limit of expression of the other genes: hoxd4, hoxa4, hoxc5, and hoxa5.

CONCLUSIONS

Our results demonstrate that following in utero exposure to BA on GD 9, a disturbance of the expression of hox genes involved inthe specification of most anterior vertebrae is observed at GD 13.5. Based on their expression domain and on their implication in the definition of the cervicothoracic vertebral boundary, it is likely that the anteriorization of hoxc6 and hoxa6 reported here is correlated to the morphological phenotype observed in BA-exposed fetuses at GD 21.

The chick transcriptional repressor Nkx3.2 acts downstream of Shh to promote BMP-dependent axial chondrogenesis

Previously, we demonstrated that Shh acts early in the development of the axial skeleton, to induce a prochondrogenic response to later BMP signaling. Here, we demonstrate that somitic expression of the transcription factor Nkx3.2 is initiated by Shh and sustained by BMP signals. Misexpression of Nkx3.2 in somitic tissue confers a prochondrogenic response to BMP signals. The transcriptional repressor activity of Nkx3.2 is essential for this factor to promote chondrogenesis. Conversely, a "reverse function" mutant of Nkx3.2 that has been converted into a transcriptional activator inhibits axial chondrogenesis in vivo. We conclude that Nkx3.2 is a critical mediator of the actions of Shh during axial cartilage formation, acting to inhibit expression of factors that interfere with the prochondrogenic effects of BMPs.

Overexpression of Hoxc13 in differentiating keratinocytes results in downregulation of a novel hair keratin gene cluster and alopecia

Studying the roles of Hox genes in normal and pathological development of skin and hair requires identification of downstream target genes in genetically defined animal models. We show that transgenic mice overexpressing Hoxc13 in differentiating keratinocytes of hair follicles develop alopecia, accompanied by a progressive pathological skin condition that resembles ichthyosis. Large-scale analysis of differential gene expression in postnatal skin of these mice identified 16 previously unknown and 13 known genes as presumptive Hoxc13 targets. The majority of these targets are downregulated and belong to a subgroup of genes that encode hair-specific keratin-associated proteins (KAPs). Genomic mapping using a mouse hamster radiation hybrid panel showed these genes to reside in a novel KAP gene cluster on mouse chromosome 16 in a region of conserved linkage with human chromosome 21q22.11. Furthermore, data obtained by Hoxc13/lacZ reporter gene analysis in mice that overexpress Hoxc13 suggest negative autoregulatory feedback control of Hoxc13 expression levels, thus providing an entry point for elucidating currently unknown mechanisms that are required for regulating quantitative levels of Hox gene expression. Combined, these results provide a framework for understanding molecular mechanisms of Hoxc13 function in hair growth and development.

Gizzard formation and the role of Bapx1

The anterior-posterior gut pattern is formed from three broad domains: fore-, mid-, and hindgut that have distinct functional, morphological, and molecular boundaries. The stomach demarcates the posterior boundary of the foregut. Avian stomachs are composed of two chambers: the anterior chamber (proventriculus) and the thick muscular posterior chamber (gizzard). Expression of candidate pattern formation control factors are restricted in the chick stomach regions such that Bmp4 and Wnt5a are not expressed in the gizzard. We previously implicated Bmp4 as controlling growth and differentiation of the gut musculature. Bmp4 is not expressed in the developing gizzard but is expressed in the rest of the gut including the adjacent proventriculus and midgut. Bapx1 (Nkx3.2) is expressed in the gizzard musculature but not in the proventriculus or midgut. We show ectopic expression of Bapx1 in the proventriculus results in a gizzard-like morphology and inhibits the normal proventricular expression of Bmp4 and Wnt5a. Overexpression of a reverse-function Bapx1 construct can result in a small stomach and ectopic extension of Bmp4 and Wnt5a expression into the gizzard. We suggest the role of Bapx1 is to regulate the expression of Bmp4 and Wnt5a to pattern the avian stomach.

Loss- and gain-of-function mutations show a polycomb group function for Ring1A in mice

The products of the Polycomb group (PcG) of genes act as transcriptional repressors involved in the maintenance of homeotic gene expression patterns throughout development, from flies to mice. Biochemical and molecular evidence suggests that the mouse Ring1A gene is a member of the PcG of genes. However, genetic evidence is needed to establish PcG function for Ring1A, since contrary to all other murine PcG genes, there is no known Drosophila PcG gene encoding a homolog of the Ring1A protein. To study Ring1A function we have generated a mouse line lacking Ring1A and mouse lines overexpressing Ring1A. Both Ring1A(−/−)and Ring1A(+/−) mice show anterior transformations and other abnormalities of the axial skeleton, which indicates an unusual sensitivity of axial skeleton patterning to Ring1A gene dosage. Ectopic expression of Ring1A also results in dose-dependent anterior transformations of vertebral identity, many of which, interestingly, are shared by Ring1A(−/−) mice. In contrast, the alterations of Hox gene expression observed in both type of mutant mice are subtle and involve a reduced number of Hox genes. Taken together, these results provide genetic evidence for a PcG function of the mouse Ring1A gene.

Plzf regulates limb and axial skeletal patterning

The promyelocytic leukaemia zinc finger (Plzf) protein (encoded by the gene Zfp145) belongs to the POZ/zinc-finger family of transcription factors. Here we generate Zfp145-/- mice and show that Plzf is essential for patterning of the limb and axial skeleton. Plzf inactivation results in patterning defects affecting all skeletal structures of the limb, including homeotic transformations of anterior skeletal elements into posterior structures. We demonstrate that Plzf acts as a growth-inhibitory and pro-apoptotic factor in the limb bud. The expression of members of the abdominal b (Abdb) Hox gene complex, as well as genes encoding bone morphogenetic proteins (Bmps), is altered in the developing limb of Zfp145-/- mice. Plzf regulates the expression of these genes in the absence of aberrant polarizing activity and independently of known patterning genes. Zfp145-/- mice also exhibit anterior-directed homeotic transformation throughout the axial skeleton with associated alterations in Hox gene expression. Plzf is therefore a mediator of anterior-to-posterior (AP) patterning in both the axial and appendicular skeleton and acts as a regulator of Hox gene expression.

Homeobox genes and gut development

The gut of vertebrates exhibits a common anteroposterior regional differentiation. The role of homeobox genes in establishing this pattern is inferred by their sites of expression. It is suggested that the primary source of positional information is in the endoderm, which subsequently establishes a 'dialogue' with the surrounding visceral layer of the lateral plate mesoderm. This results in the anatomical and physiological specialization of the adult gut.

Smad1 domains interacting with Hoxc-8 induce osteoblast differentiation

Bone morphogenetic proteins are potent osteotropic agents that induce osteoblast differentiation and bone formation. The signal transduction of bone morphogenetic proteins has recently been discovered to involve Smad proteins. Smad1 is an essential intracellular component that is specifically phosphorylated by bone morphogenetic protein receptors and translocated into the nucleus upon ligand stimulation. Previously, we have reported that Smad1 activates osteopontin gene expression in response to bone morphogenetic protein simulation through an interaction with a homeodomain transcription factor, Hoxc-8. In the present study, the interaction domains between the two proteins were characterized by deletional analysis in both yeast two-hybrid and gel shift assays. Two regions within the amino-terminal 87 amino acid residues of Smad1 were mapped to interact with Hoxc-8, one of which binds to the homeodomain. Overexpression of recombinant cDNAs encoding the Hoxc-8 interaction domains of Smad1 effectively activated osteopontin gene transcription in transient transfection assays. Furthermore, stable expression of these Smad1 fragments in 2T3 osteoblast precursor cells stimulated osteoblast differentiation-related gene expression and led to mineralized bone matrix formation. Our data suggest that the interaction of amino-terminal Smad1 with Hoxc-8 mimics bone morphogenetic protein signaling and is sufficient to induce osteoblast differentiation and bone cell formation.

Vertebrate endoderm development

Endoderm, one of the three principal germ layers, contributes to all organs of the alimentary tract. For simplicity, this review divides formation of endodermal organs into four fundamental steps: (a) formation of endoderm during gastrulation, (b) morphogenesis of a gut tube from a sheet of cells, (c) budding of organ domains from the tube, and (d) differentiation of organ-specific cell types within the growing buds. We discuss possible mechanisms that regulate how undifferentiated endoderm becomes specified into a myriad of cell types that populate the respiratory and gastrointestinal tracts.

Coordinated expression of 3' hox genes during murine embryonal gut development: an enteric Hox code

BACKGROUND & AIMS

Hox genes are highly conserved developmental control genes that may be organized and expressed in the form of a code required for correct morphogenesis. Little is known about their control of the embryonal gut. However, Hox paralogues 4 and 5, which are expressed at the sites of origin of vagal neural crest cells and splanchnic mesoderm, are likely to be important. We have studied the expression domains of these genes in the gut both spatially and temporally.

METHODS

CD1 mice embryos of embryonic days E8.5-E17.5 were studied. The spatial and temporal expression patterns of messenger RNA of Hoxa4, b4, c4, d4, a5, c5, and b5 homeoprotein were determined by in situ hybridization and immunohistochemistry in whole embryos, whole gastrointestinal tracts, and vibratome sections.

RESULTS

There were different spatial, temporal, and combinatorial expression patterns in different morphological regions: foregut, prececal gut, cecum, and postcecal gut. Two dynamic gradients, rostral and caudal, were coordinated with nested expression domains along the gut primordium. Region-specific domains were present in the stomach and cecum.

CONCLUSIONS

The expression patterns of genes in paralogous groups 4 and 5 suggest that they are organized to form a specific enteric Hox code required for correct enteric development.

Loss of Hoxa5 gene function in mice perturbs intestinal maturation

The Hox gene family of transcription factors constitutes candidate regulators in the molecular cascade of events that governs establishment of normal terminal differentiation along the duodenum to colon axis. One member of this family, Hoxa5, displays a dynamic pattern of expression during gut development. Hoxa5 transcripts are present in midgut mesenchyme at the time of remodeling, supporting a role for this gene in digestive tract specification. To study the role of Hoxa5 in proper intestinal development and maturation, we examined whether Hoxa5 mutant mice exhibit any defect in this process. We report here that even though Hoxa5 is not required for midgut morphogenesis, its loss of function perturbs the acquisition of adult mode of digestion, which normally is temporally coordinated with the process of spontaneous weaning. Impaired maturation of the digestive tract might be related to altered specification of intestinal epithelial cells. Our findings provide evidence that Hoxa5 expression in the gut mesoderm is important for the region-specific differentiation of the adjacent endoderm.

Evolution of chordate hox gene clusters

In this article, we consider the role of the Hox genes in chordate and vertebrate evolution from the viewpoints of molecular and developmental evolution. Models of Hox cluster duplication are considered with emphasis on a threefold duplication model. We also show that cluster duplication is consistent with a semiconservative model of duplication, where following duplication, one daughter cluster remains unmodified, while the other diverges and assumes a new architecture and presumably new functions. Evidence is reviewed, suggesting that Hox gene enhancers have played an important role in body plan evolution. Finally, we contrast the invertebrates and vertebrates in terms of genome and Hox cluster duplication which are present in the latter, but not the former. We question whether gene duplication has been important in vertebrates for the introduction of novel features such as limbs, a urogenital system, and specialized neuromuscular interactions.

Development of the human gastrointestinal tract: twenty years of progress

A combination of approaches has begun to elucidate the mechanisms of gastrointestinal development. This review describes progress over the last 20 years in understanding human gastrointestinal development, including data from both human and experimental animal studies that address molecular mechanisms. Rapid progress is being made in the identification of genes regulating gastrointestinal development. Genes directing initial formation of the endoderm as well as organ-specific patterning are beginning to be identified. Signaling pathways regulating the overall right-left asymmetry of the gastrointestinal tract and epithelial-mesenchymal interactions are being clarified. In searching for extrinsic developmental regulators, numerous candidate trophic factors have been proposed, but compelling evidence remains elusive. A critical gene that initiates pancreas development has been identified, as well as a number of genes regulating liver, stomach, and intestinal development. Mutations in genes affecting neural crest cell migration have been shown to give rise to Hirschsprung's disease. Considerable progress has been achieved in understanding specific phenomena, such as the transcription factors regulating expression of sucrase-isomaltase and fatty acid-binding protein. The challenge for the future is to integrate these data into a more complete understanding of the physiology of gastrointestinal development.

Evidence for regulation of cartilage differentiation by the homeobox gene Hoxc-8

Homeobox genes of the Hox class are required for proper patterning of skeletal elements, but how they regulate the differentiation of specific tissues is unclear. We show here that overexpression of a Hoxc-8 transgene causes cartilage defects whose severity depends on transgene dosage. The abnormal cartilage is characterized by an accumulation of proliferating chondrocytes and reduced maturation. Since Hoxc-8 is normally expressed in chondrocytes, these results suggest that Hoxc-8 continues to regulate skeletal development well beyond pattern formation in a tissue-specific manner, presumably by controlling the progression of cells along the chondrocyte differentiation pathway. The comparison to Hoxd-4 and Isl-1 indicates that this role in chondrogenesis is specific to proteins of the Hox class. Their capacity for regulation of cartilage differentiation suggests that Hox genes could also be involved in human chondrodysplasias or other cartilage disorders.

A conserved retinoic acid responsive element in the murine Hoxb-1 gene is required for expression in the developing gut

The murine Hoxb-1 gene contains a homeobox sequence and is expressed in a spatiotemporal specific pattern in neuroectoderm, mesoderm and gut endoderm during development. We previously identified a conserved retinoic acid (RA)-inducible enhancer, named the RAIDR5, which contains a DR5 RARE; this RAIDR5 enhancer is located 3′ of the Hoxb-1-coding region in both the mouse and chick. In the F9 murine teratocarcinoma cell line, this DR5 RARE is required for the RA response of the Hoxb-1 gene, suggesting a functional role of the DR5 RARE in Hoxb-1 gene expression during embryogenesis. From the analysis of Hoxb-1/lacZ reporter genes in transgenic mice, we have shown that a wild-type (WT) transgene with 15 kb of Hoxb-1 genomic DNA, including this Hoxb-1 3′ RAIDR5, is expressed in the same tissues and at the same times as the endogenous Hoxb-1 gene. However, a transgene construct with point mutations in the DR5 RARE (DR5mu) was not expressed in the developing foregut, which gives rise to organs such as the esophagus, lung, stomach, liver and pancreas. Like the wild-type transgene, this DR5 RARE mutated transgene was expressed in rhombomere 4 in 9.5 day postcoitum (d.p.c.) embryos. Similarly, transgene staining in the foregut of animals carrying a deletion of the entire Hox-b1 RAIDR5 enhancer (3′-del) was greatly reduced relative to that seen with the WT transgene. We also demonstrated that expression of the WT transgene in the gut increases in response to exogenous RA, resulting in anterior expansion of the expression in the gut. These observations that the Hoxb-1 gene is expressed in the developing gut and that this expression is regulated through a DR5 RARE strongly suggest a role for Hoxb-1 in the anteroposterior axis patterning of the gut and a critical role for endogenous retinoids in early gut development.

Murine Hoxc-9 gene contains a structurally and functionally conserved enhancer

Reporter gene analysis of the Hoxc-9 genomic region in transgenic mice allowed us to identify a positional enhancer in the Hoxc-9 intron that drives expression in the posterior neural tube of midgestation mouse embryos in a Hoxc-9-related manner. Sequence comparison to the chicken Choxc-9 intron revealed the existence of two highly conserved sequence elements (CSEs) in a similar spatial arrangement. These structural similarities in the mammalian and avian lineage are mirrored by conserved function of the chicken Choxc-9 intron in transgenic mice. Deletion analysis of the two introns suggests that full activity of both enhancers depends on cooperation between the two CSEs located close to the respective 5' and 3' splice sites. Following the paradigm of phylogenetically conserved developmental control mechanisms, the Hoxc-9 intragenic enhancer was tested in Drosophila. Our data show that the mouse Hoxc-9 enhancer acts in a conserved fashion in transgenic flies, conferring posteriorly restricted reporter gene expression to the developing central nervous system in third instar larvae. This finding indicates that the Hoxc-9 intragenic enhancer is involved in transcriptional regulatory circuits conserved between vertebrates and arthropods.

Misexpression of the pancreatic homeodomain protein IDX-1 by the Hoxa-4 promoter associated with agenesis of the cecum

BACKGROUND & AIMS

The endoderm-specific homeodomain transcription factor IDX-1 is critical for pancreas development and for the regulation of islet cell-specific genes. During development, IDX-1 is expressed in the epithelial cells of the endoderm in the pancreatic anlage of the foregut. The aim of this study was to determine whether IDX-1 may have potential properties of a master homeotic determinant of pancreas and/or gut development.

METHODS

Transgenic mice were generated in which the expression of IDX-1 was misdirected by a promoter of the mesoderm-specific homeodomain protein Hoxa-4 known to express in the stomach and hindgut during development. The expectation was the formation of ectopic pancreatic tissue or alterations of gut patterning or morphology.

RESULTS

Although no ectopic induction of pancreatic markers was found in these transgenic mice, they manifested an altered midgut-hindgut union and agenesis of the cecum. Further, IDX-1 binds to the gut-specific homeodomain protein Cdx-2 and inhibits transactivation of the sucrase-isomaltase promoter by Cdx-2.

CONCLUSIONS

These findings further support the emerging understanding that interactions among different classes of homeodomain proteins, expressed in a spatially and temporally restricted manner during development, determine the pattern of organogenesis. A possible mechanism for the dysmorphogenesis of the proximal colon may be an inhibition of Cdx-2 actions by IDX-1.

Epithelial-mesenchymal signaling during the regionalization of the chick gut

The development of the vertebrate gut requires signaling between the endoderm and mesoderm for establishing its normal anteroposterior (AP) axis and for tissue-specific differentiation. Factors implicated in positional specification of the AP regions of the gut include endodermally expressed Sonic hedgehog (Shh), mesodermally expressed Bmp4 and members of the Hox gene family. We have investigated the roles of these factors during AP regional specification of the chick embryonic gut. Early in gut development, the endoderm sends inductive signals to the mesoderm. Shh has been implicated as one of these signals. We find a differential response to exposure of the inductive influence of Shh along the AP axis of the gut. Virally mediated misexpression of Shh results in ectopic upregulation of its receptor Ptc and a cellular proliferation throughout the gut mesoderm. Although ectopic Shh can induce Bmp4 in the mesoderm of the midgut and hindgut, Bmp4 is not induced in the stomach region of the foregut. The stomach region has a thicker layer of mesoderm than the rest of the gut suggesting that the normal function of Bmp4 could be to limit mesodermal growth in the non-stomach regions of the gut. Ectopic Bmp4 expression in the stomach results in a reduction of the mesodermal component consistent with this hypothesis. In addition to the regional restriction on Bmp4 induction, Shh can only induce Hoxd-13 in the mesoderm of the hindgut. These findings suggest that a prepattern exists in the primitive gut mesoderm prior to expression of Shh in the endoderm. The gut mesoderm is subsequently responsible for inducing region-specific differentiation of its overlying endoderm. We tested the role of Hoxd-13, normally restricted in its mesodermal expression to the most posterior region of the hindgut (cloaca), in controlling adjacent endodermal differentiation. When virally mediated Hoxd-13 is misexpressed in the primitive midgut mesoderm, there is a transformation of the endoderm to the morphology and mucin content of the hindgut. Thus, the positionally restricted expression of a Hox gene in the gut mesoderm influences the inductive signaling that leads to regionally specific differentiation of gut endoderm.

Modification of expression and cis-regulation of Hoxc8 in the evolution of diverged axial morphology

Differential Hox gene expression between vertebrate species has been implicated in the divergence of axial morphology. To examine this relationship, we have compared expression and transcriptional regulation of Hoxc8 in chicken and mouse. In both species, expression of Hoxc8 in the paraxial mesoderm and neural tube is associated with midthoracic and brachial identities, respectively. During embryogenesis, there is a temporal delay in the activation of Hoxc8 in chicken compared with mouse. As a result, chicken Hoxc8 expression in the paraxial mesoderm is at a posterior axial level, extending over a smaller domain compared with mouse Hoxc8 expression. This finding is consistent with a shorter thoracic region in chicken compared with mouse. In addition, the chicken Hoxc8 early enhancer, differing from its mouse counterpart in only a few specific nucleotides, directs a reporter gene expression to a more posterior domain in transgenic mouse embryos. These findings are consistent with the concept that the diversification of axial morphology has been achieved through changes in cis-regulation of developmental control genes.

Region-specific expression of murine Hox genes implies the Hox code-mediated patterning of the digestive tract

BACKGROUND

Hox genes encode transcription factors which are involved in the establishment of regional identities along the anteroposterior (AP) body axis. To elucidate the AP patterning of the digestive tract, we have systematically examined the expression patterns of Hox genes belonging to paralogue groups 6, 7, 8 and 9 by whole-mount in situ hybridization and by section in situ hybridization analyses.

RESULTS

The expression patterns of these genes showed co-linearity along the wall of the digestive tract, thereby yielding the Hox code of the gut. The expression boundaries of the Hox genes at later stages (12.5 d.p.c.) corresponded to the morphological boundaries of individual gut subdomains.

CONCLUSIONS

The visceral mesoderm-restricted expression suggested that the Hox code primarily functions in the mesenchymal specification which eventually leads to the regional differentiation of gut subdomains as the result of epithelial-mesenchymal interactions. Overlapping expression patterns were found among the paralogous Hox genes, indicating that the paralogues may have redundant functions in the specification of the gut.

Anteroposterior gradient of epithelial transformation during amphibian intestinal remodeling: immunohistochemical detection of intestinal fatty acid-binding protein

To determine whether the remodeling of the well-organized intestinal epithelium during amphibian metamorphosis is regionally regulated along the anteroposterior axis of the intestine, we raised a polyclonal antibody against the Xenopus laevis intestinal fatty acid-binding protein (IFABP), which is known to be specifically expressed in intestinal absorptive cells, and examined immunohistochemically the differentiation, proliferation, and apoptosis of the epithelial cells throughout X. laevis small intestine. During pre- and prometamorphosis, IFABP-immunoreactive (ir) epithelial cells were localized only in the anterior half of the larval intestine. At the beginning of metamorphic climax, apoptotic cells detected by nick end-labeling (TUNEL) suddenly increased in number in the entire larval epithelium, concurrently with the appearance of adult epithelial primordia. Subsequently, the adult primordia in the anterior part of the intestine developed more rapidly by active cell proliferation than those in the posterior part, and replaced the larval epithelial cells earlier than those in the posterior part. IFABP-ir cells in the adult epithelium were first detectable at the tips of newly formed folds in the proximal part of the intestine. Thereafter, IFABP expression gradually progressed both in the anteroposterior direction and in the crest-trough direction of the folds. These results suggest that developmental processes of the adult epithelium in the X. laevis intestine are regionally regulated along the anteroposterior axis of the intestine, which is maintained throughout metamorphosis, and along the trough-crest axis of the epithelial folds, which is newly established during metamorphosis. Furthermore, the regional differences in IFABP expression along the anteroposterior axis of the intestine were reproduced in organ cultures in vitro. In addition, IFABP expression was first down-regulated and then reactivated in vitro when the anterior part, but not the posterior part, of the larval intestine was treated with thyroid hormone (TH) for extended periods. Therefore, it seems that, in addition to TH, an endogenous factor(s) localized in the intestine itself with an anteroposterior gradient participates in the development of the adult epithelium during amphibian metamorphosis.