Sequences 5′ of the homeobox of the Hox-1.4 gene direct tissue-specific expression of lacZ during mouse development

Etiology of Hypospadias: A Comparative Review of Genetic Factors and Developmental Processes Between Human and Animal Models

Hypospadias is a congenital anomaly of the penis with an occurrence of approximately 1 in 200 boys, but the etiology of the majority of hypospadias has remained unknown. Numerous genes have been reported as having variants in hypospadias patients, and many studies on genetic deletion of key genes in mouse genital development have also been published. Until now, no comparative analysis in the genes related literature has been reported. The basic knowledge of penile development and hypospadias is mainly obtained from animal model studies. Understanding of the differences and similarities between human and animal models is crucial for studies of hypospadias. In this review, mutations and polymorphisms of hypospadias-related genes have been compared between humans and mice, and differential genotype–phenotype relationships of certain genes between humans and mice have been discussed using the data available in PubMed and MGI online databases, and our analysis only revealed mutations in seven out of 43 human hypospadias related genes which have been reported to show similar phenotypes in mutant mice. The differences and similarities in the processes of penile development and hypospadias malformation among human and commonly used animal models suggest that the guinea pig may be a good model to study the mechanism of human penile development and etiology of hypospadias.

HOXA4, down-regulated in lung cancer, inhibits the growth, motility and invasion of lung cancer cells

The involvement of HOXA4 in colorectal cancer and epithelial ovarian cancer has been reported. Although it has been reported that the Hoxa4 gene is involved in the patterning of the mouse lung during embryonic development, little is known about the biological functions of HOXA4 in lung cancer. In the current study, HOXA4 expression was down-regulated in lung cancer tissues when compared with non-cancerous tissues. HOXA4 expression was associated with tumor size, TNM stage, lymph node metastasis and prognosis. Bioinformatics analysis revealed that HOXA4 expression was negatively correlated with cell cycle, metastasis, and the Wnt signaling pathway. Moreover, HOXA4 overexpression in lung cancer cell lines suppressed cell proliferation, migration, and invasion. HOXA4 decreased the protein expression levels of β-catenin, Cyclin D1, c-Myc and Survivin, indicating the inhibition of Wnt signaling. HOXA4 significantly increased the protein and mRNA levels of glycogen synthase kinase-3β (GSK3β) by promoting its transcription. Furthermore, inhibition of GSK3β by LiCl abolished the suppression of cell growth, migration, and invasion mediated by HOXA4. Overexpression of HOXA4 in xenograft tumors also decreased tumor growth and Wnt signaling. Collectively, these data suggest that HOXA4 is a potential diagnostic and prognostic marker in lung cancer, and its overexpression could inhibit lung cancer progression in part by promoting GSK3β transcription.

Crypto-rhombomeres of the mouse medulla oblongata, defined by molecular and morphological features

The medulla oblongata is the caudal portion of the vertebrate hindbrain. It contains major ascending and descending fiber tracts as well as several motor and interneuron populations, including neural centers that regulate the visceral functions and the maintenance of bodily homeostasis. In the avian embryo, it has been proposed that the primordium of this region is subdivided into five segments or crypto-rhombomeres (r7-r11), which were defined according to either their parameric position relative to intersomitic boundaries (Cambronero and Puelles, in J Comp Neurol 427:522-545, 2000) or a stepped expression of Hox genes (Marín et al., in Dev Biol 323:230-247, 2008). In the present work, we examine the implied similar segmental organization of the mouse medulla oblongata. To this end, we analyze the expression pattern of Hox genes from groups 3 to 8, comparing them to the expression of given cytoarchitectonic and molecular markers, from mid-gestational to perinatal stages. As a result of this approach, we conclude that the mouse medulla oblongata is segmentally organized, similarly as in avian embryos. Longitudinal structures such as the nucleus of the solitary tract, the dorsal vagal motor nucleus, the hypoglossal motor nucleus, the descending trigeminal and vestibular columns, or the reticular formation appear subdivided into discrete segmental units. Additionally, our analysis identified an internal molecular organization of the migrated pontine nuclei that reflects a differential segmental origin of their neurons as assessed by Hox gene expression.

The low expression of Dmrt7 is associated with spermatogenic arrest in cattle-yak

Dmrt7 is a member of the DM domain family of genes. Dmrt7 deficiency is also a strong candidate as a cause for male cattle-yak infertility, as it is regarded as essential for male spermatogenesis, between the pachynema and diplonema stages. In our study, the coding region sequence of yak and cattle-yak Dmrt7 was cloned by molecular cloning techniques, and the sequence, conserved domains, functional sites, and secondary and tertiary structures of the Dmrt7-encoded protein were predicted and analyzed using bioinformatics methods. The coding region sequences of the Dmrt7 gene, encoding 370 amino acids, were consistent in yak and cattle-yak. The protein encoded by yak and cattle-yak Dmrt7 contains a DM domain. We detected Dmrt7 mRNA expression in testis, but not in any other tissue. Dmrt7 mRNA and protein expression was significantly higher in testis of cattle and yak than that in cattle-yak (p < 0.01). Histological analysis indicated that seminiferous tubules in male cattle-yak were highly vacuolated and contained primarily Sertoli cells and spermatogonia, while those of cattle and yak contained abundant primary spermatocytes. Male cattle-yak testis contained a significantly larger number of apoptotic cells than those in cattle and yak assessed by terminal deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) analysis. The accumulation of SCP3-positive spermatocytes indicated the arrest of spermatogenesis at the pachynema stage in the cattle-yak. These results suggest low levels of Dmrt7 expression lead to male sterility in cattle-yak. The molecular function of Dmrt7 and the regulation of its expression warrant need to be examined in future studies.

Hox proteins drive cell segregation and non-autonomous apical remodelling during hindbrain segmentation

Hox genes encode a conserved family of homeodomain transcription factors regulating development along the major body axis. During embryogenesis, Hox proteins are expressed in segment-specific patterns and control numerous different segment-specific cell fates. It has been unclear, however, whether Hox proteins drive the epithelial cell segregation mechanism that is thought to initiate the segmentation process. Here, we investigate the role of vertebrate Hox proteins during the partitioning of the developing hindbrain into lineage-restricted units called rhombomeres. Loss-of-function mutants and ectopic expression assays reveal that Hoxb4 and its paralogue Hoxd4 are necessary and sufficient for cell segregation, and for the most caudal rhombomere boundary (r6/r7). Hox4 proteins regulate Eph/ephrins and other cell-surface proteins, and can function in a non-cell-autonomous manner to induce apical cell enlargement on both sides of their expression border. Similarly, other Hox proteins expressed at more rostral rhombomere interfaces can also regulate Eph/ephrins, induce apical remodelling and drive cell segregation in ectopic expression assays. However, Krox20, a key segmentation factor expressed in odd rhombomeres (r3 and r5), can largely override Hox proteins at the level of regulation of a cell surface target, Epha4. This study suggests that most, if not all, Hox proteins share a common potential to induce cell segregation but in some contexts this is masked or modulated by other transcription factors.

Atoh1 governs the migration of postmitotic neurons that shape respiratory effectiveness at birth and chemoresponsiveness in adulthood

Hindbrain neuronal networks serving respiratory, proprioceptive, and arousal functions share a developmental requirement for the bHLH transcription factor Atoh1. Loss of Atoh1 in mice results in respiratory failure and neonatal lethality; however, the neuronal identity and mechanism by which Atoh1-dependent cells sustain newborn breathing remains unknown. We uncovered that selective loss of Atoh1 from the postmitotic retrotrapezoid nucleus (RTN) neurons results in severely impaired inspiratory rhythm and pronounced neonatal death. Mice that escape neonatal death develop abnormal chemoresponsiveness as adults. Interestingly, the expression of Atoh1 in the RTN neurons is not required for their specification or maintenance, but is important for their proper localization and to establish essential connections with the preBötzinger Complex (preBötC). These results provide insights into the genetic regulation of neonatal breathing and shed light on the labile sites that might contribute to sudden death in newborn infants and altered chemoresponsiveness in adults.

Impairment of pachytene spermatogenesis in Dmrt7 deficient mice, possibly causing meiotic arrest

Although Dmrt7 has been reported to be essential for male spermatogenesis, the molecular mechanism underlying pachytene spermatogenesis by Dmrt7 is not known. In the present study, by detailed analysis of Dmrt7 protein distribution in spermatocytes in the first wave of spermatogenesis, we clarified the profile of Dmrt7 expression and localization in pachytene spermatogenesis. Dmrt7-deficient spermatocytes were arrested in the pachytene stage, followed by apoptosis. We analyzed to determine whether every event in the spermatogenesis at the Dmrt7-deficient mice progressed normally, because in several gene knockout mice with spermatogenic arrest described in the previous reports impairments of these events often appeared. Mutant mice showed normal synapsis and XY body formation, while impairment of meiotic sex chromosome inactivation (MSCI), decreased expression of backup genes, and increased expression of retrotransposons indicated incomplete meiotic recombination.

Cis-regulatory characterization of sequence conservation surrounding the Hox4 genes

Hox genes are key regulators of anterior-posterior axis patterning and have a major role in hindbrain development. The zebrafish Hox4 paralogs have strong overlapping activities in hindbrain rhombomeres 7 and 8, in the spinal cord and in the pharyngeal arches. With the aim to predict enhancers that act on the hoxa4a, hoxb4a, hoxc4a and hoxd4a genes, we used sequence conservation around the Hox4 genes to analyze all fish:human conserved non-coding sequences by reporter assays in stable zebrafish transgenesis. Thirty-four elements were functionally tested in GFP reporter gene constructs and more than 100 F1 lines were analyzed to establish a correlation between sequence conservation and cis-regulatory function, constituting a catalog of Hox4 CNEs. Sixteen tissue-specific enhancers could be identified. Multiple alignments of the CNEs revealed paralogous cis-regulatory sequences, however, the CNE sequence similarities were found not to correlate with tissue specificity. To identify ancestral enhancers that direct Hox4 gene activity, genome sequence alignments of mammals, teleosts, horn shark and the cephalochordate amphioxus, which is the most basal extant chordate possessing a single prototypical Hox cluster, were performed. Three elements were identified and two of them exhibited regulatory activity in transgenic zebrafish, however revealing no specificity. Our data show that the approach to identify cis-regulatory sequences by genome sequence alignments and subsequent testing in zebrafish transgenesis can be used to define enhancers within the Hox clusters and that these have significantly diverged in their function during evolution.

Expression and function of HOXA genes in normal and neoplastic ovarian epithelial cells

We studied the roles of three HOXA genes in cultured normal ovarian surface epithelial (OSE) cells and ovarian cancer cells. They included HOXA4 and HOXA7 because, by cDNA microarray analysis, these were more highly expressed in invasive ovarian carcinomas than in benign or borderline (noninvasive) ovarian tumors, and HOXA9 because it characterizes normal oviductal epithelium, which resembles ovarian serous adenocarcinomas. The three HOXA genes were more highly expressed when OSE cells were dividing and motile than when they were confluent and stationary, and also when they dispersed in response to EGF treatment or to reduced calcium concentrations in culture media. The expression of the HOXA genes varied among ovarian cancer cell lines, but was highest in lines with compact epithelial morphologies. We focused on HOXA4 as the most highly expressed in the ovarian carcinoma array. HOXA4 expression did not parallel proliferative activities of either OSE or ovarian cancer lines. Moreover, modifying HOXA4 expression in ovarian cancer cell lines did not alter either E-cadherin expression or CA125 secretion. However, HOXA4 downregulation enhanced EGFR phosphorylation and migration in serum-starved OSE and ovarian cancer cells in response to EGF, and enhanced migration of all ovarian cancer lines in 5% serum even without EGF treatment. Thus, HOXA4 expression does not correlate with proliferation or with epithelial differentiation, but it increases in response to OSE cell dispersion and negatively regulates EGFR activation and the motility of OSE and of ovarian cancer cells. HOXA4 expression was highest in cancer lines with compact epithelial growth patterns, suggesting, again, an anti-dispersion function. In summary, increased HOXA4 expression in ovarian cancer appears to constitute a tumor-suppressive, homeostatic response to aberrant cell behavior, and, in particular, to cell dispersion and migration.

Cdx protein interaction with Hoxa5 regulatory sequences contributes to Hoxa5 regional expression along the axial skeleton

Hox gene functions are intimately linked to correct developmental expression of the genes. The identification of cis-acting regulatory sequences and their associated trans-acting factors constitutes a key step in deciphering the mechanisms underlying the correct positioning of the functional domain of Hox genes along the anterior-posterior axis. We have identified DNA elements driving Hoxa5 regionalized expression in mice, using the 2.1-kb mesodermal enhancer (MES) localized in Hoxa5 3' flanking sequences as a starting point. The MES sequence comprises regulatory elements targeting Hoxa5 expression in the limbs, the urogenital and gastrointestinal tracts, and the cervical-upper thoracic region of the prevertebral column. A 164-bp DNA fragment within the MES caudally restricts Hoxa5 expression at the level of prevertebra 10, corresponding to the posterior limit of its functional domain. Cdx proteins directly bind to this element in vitro via two conserved sites. Preventing Cdx binding by mutating the sites causes caudal expansion of the transgene expression domain. Of all three murine Cdx proteins that bind this element in vitro, Cdx4 has emerged as a potential regional posterior repressor of Hoxa5 expression. The restrictive control provided by Cdx interactions with Hoxa5 regulatory sequences may be one of the critical events in cervicothoracic axial specification.

Serine-arginine-rich nuclear protein Luc7l regulates myogenesis in mice

Using a gene trap technique, we identified a murine homologue of the yeast LUC7-like gene (Luc7l), which is a serine-arginine-rich protein (SR protein) that localizes in the nucleus through its arginine-serine-rich domain (RS domain) at the C-terminus and shows a speckled distribution pattern. Although its transcripts are widely expressed in embryos and adults, they are rarely detected in adult skeletal muscle, and Luc7l expression was found to be negatively regulated during the course of development of limb skeletal muscle, as well as during in vitro differentiation of the myoblast cell lines Sol8 and C2C12. We also demonstrated that forced expression of Luc7l protein inhibited myogenesis in vitro. Based on our results, Luc7l is thought to play an important role in the regulation of muscle differentiation.

Serum response factor, an enriched cardiac mesoderm obligatory factor, is a downstream gene target for Tbx genes

We tested the idea that T-box factors direct serum response factor (SRF) gene activity early in development. Analysis of SRF-LacZ "knock-in" mice showed highly restricted expression in early embryonic cardiac and skeletal muscle mesoderm and neuroectoderm. Examination of the SRF gene for regulatory regions by linking the promoter and 5'-flanking sequences, up to 5.5 kb, failed to target LacZ transgene activity to the heart and the tail pre-somitic mesenchyme. However, linkage of a minimal SRF promoter with the SRF 3'-untranslated region (UTR), inundated with multimeric T-box binding sites (TBEs), restored robust reporter gene activity to embryonic heart and tail. Finer dissection of the 3'-UTR to a small cluster of TBEs also stimulated transgene activity in the cardiac forming region and the tail, however, when the TBEs contained within these DNA sequences were mutated, preventing Tbx binding, transgene activity was lost. Tbx2, Tbx5, and the cardiac-enriched MYST family histone acetyltransferase TIP60, were observed to be mutual interactive cofactors through the TIP60 zinc finger and the T-box of the Tbx factors. In SRF-null ES cells, TIP60, Tbx2, and Tbx5 were sufficient to stimulate co-transfected SRF reporter activity, however this activity required the presence of the SRF 3'-UTR. SRF gene transactivation was blocked by two distinct TIP60 mutants, in which either the histone acetyltransferase domain was inactivated or the Zn finger-protein binding domain was excised. Our study supports the idea that SRF embryonic cardiac gene expression is dependent upon the SRF 3'-UTR enhancer, Tbx2, Tbx5, and TIP60 histone acetyltransferase activity.

Distinct Regions of the Mouse Cyclin A1 Gene, Ccna1, Confer Male Germ-Cell Specific Expression and Enhancer Function1

The gene encoding mouse cyclin A1, Ccna1, is expressed at highest levels in late pachytene-diplotene spermatocytes, where it is required for meiotic cell division. To begin to understand the mechanisms responsible for its highly restricted pattern of expression, transgenic mouse lines carrying constructs consisting of the cyclin A1 regulatory region fused with the reporter gene lacZ were generated. Analysis of tissue-specific and testicular cell-type-specific transgene expression indicated that sequences within -1.3 kilobases (kb) of the cyclin A1 putative transcriptional start site were sufficient to direct transgene expression uniquely to late spermatocytes while maintaining repression in other tissues. However, sequences located between -4.8 kb and -1.3 kb of the putative transcriptional start site were apparently required to transcribe the reporter at levels needed for consistent X-gal staining. Comparison of the mouse, rat, and human proximal promoters revealed regions of high sequence conservation and consensus sequences both for known transcription factors, some of which are coexpressed with Ccna1, such as A-myb and Hsf2, and for elements that control expression of genes in somatic cell cycles, such as CDE, CHR, and CCAAT elements. Thus, the promoter region within 1.3 kb upstream of the putative Ccna1 transcriptional start can direct expression of lacZ to spermatocytes, while sequences located between -4.8 kb and -1.3 kb of the putative transcriptional start site may enhance expression of lacZ.

Cyclin A2 mediates cardiomyocyte mitosis in the postmitotic myocardium

Cell cycle withdrawal limits proliferation of adult mammalian cardiomyocytes. Therefore, the concept of stimulating myocyte mitotic divisions has dramatic implications for cardiomyocyte regeneration and hence, cardiovascular disease. Previous reports describing manipulation of cell cycle proteins have not shown induction of cardiomyocyte mitosis after birth. We now report that cyclin A2, normally silenced in the postnatal heart, induces cardiac enlargement because of cardiomyocyte hyperplasia when constitutively expressed from embryonic day 8 into adulthood. Cardiomyocyte hyperplasia during adulthood was coupled with an increase in cardiomyocyte mitosis, noted in transgenic hearts at all time points examined, particularly during postnatal development. Several stages of mitosis were observed within cardiomyocytes and correlated with the nuclear localization of cyclin A2. Magnetic resonance analysis confirmed cardiac enlargement. These results reveal a previously unrecognized critical role for cyclin A2 in mediating cardiomyocyte mitosis, a role that may significantly impact upon clinical treatment of damaged myocardium.

Homeobox genes in pulmonary vascular development and disease

Determining how the pulmonary vascular system is formed, maintained, or disrupted during development and disease represents a major challenge in contemporary lung biology. Whereas it is appreciated that cellular proliferation, differentiation, migration, and apoptosis need to be carefully controlled in order to attain pulmonary vascular homeostasis, knowledge of the underlying cellular and molecular mechanisms involved remains surprisingly limited. Because homeobox genes represent master regulators of organogenesis and tissue patterning, it is likely that these transcription factors play a critical role in the formation of blood vessels within the lung, as well as in pathologic states in which the highly ordered structure of the pulmonary vascular tree is compromised. The aim of this review is to discuss some of the known functions of homeobox genes in the vasculature, and to extrapolate these findings to their potential roles in developing and diseased pulmonary vessels.

Different mechanisms of syndecan-1 activation through a fibroblast-growth-factor-inducible response element (FiRE) in mucosal and cutaneous wounds

Syndecan-1 expression is enhanced in cutaneous and mucosal wounds. We have previously demonstrated that wounding-induced syndecan-1 expression in the skin occurs transcriptionally, through a fibroblast-growth-factor-inducible element (FiRE). Here, we show that FiRE is also activated in mucosal wounds. However, both the expression patterns and the activation mechanisms of FiRE are different from those in the skin. In the mucosa in vivo, the activation starts and ends earlier than in cutaneous wounds. FiRE is first detected at around 12 hours in keratinocytes, and the activation declines by the third day after wounding occurs. The activation is seen on the migrating sheet of epithelial mucosa, as in the case of cutaneous wounding. In contrast to the situation in vivo, organ-cultured mucosal wounds exhibit no FiRE activity, while organ-cultured cutaneous wounds show robust activity. Activation in mucosal wounds is enhanced, however, by the application of epidermal growth factor. This suggests that exogenous growth factor activity is required for activation of syndecan-1 in mucosal wounds but not in cutaneous wounds.

Alternatively used promoters and distinct elements direct tissue-specific expression of nephrin

Nephrin, an essential component of the glomerular ultrafilter, the slit diaphragm, has also been found to be expressed in the central nervous system and pancreas. This study examined the regulation of the nephrin gene by analyzing the expression of different length nephrin promoter-lacZ reporter constructs in transgenic mice. An upstream segment between -4 kb and -4 bp was shown to be sufficient for driving expression in all three tissues. Surprisingly, a 5.7-kb construct lacking the transcription initiation site and the immediate upstream region of the gene could drive expression in the central nervous system. This led to the identification of a novel, alternatively used exon 1B located 1871 bp upstream of the ATG codon of the previously known first exon, now termed exon 1A. The existence and functionality of exon 1B was verified in nephrin knockout mice in which exon 1A is deleted. Deletion of exon 1B and its immediate surrounding sequence, introduced in the 4-kb promoter-lacZ reporter construct, abolished the expression of the transgene in pancreas and spinal cord but not in kidney and brain in transgenic mice. Analysis of five promoter-reporter gene constructs showed that regulatory elements driving expression encoded by exon 1A in kidney and brain are localized in the region between -4 kb and 2.1 kb.

Matrix metalloproteinase-9 contributes to choroidal neovascularization

Age-related macular degeneration (AMD) is the primary cause of irreversible photoreceptors loss in adult patients and current therapies are limited. Increased levels of matrix metalloproteinases (MMPs) have been documented in neovascularization of severe ocular pathologies such as AMD and proliferative diabetic retinopathy. We report here that MMP-9 (gelatinase B) expression is induced and temporally regulated in the course of experimental choroidal neovascularization. We used transgenic mice expressing beta-galactosidase reporter gene under the dependence of MMP-9 promoter and RT-PCR analysis on choroidal neovascular structures microdissected from serial sections by laser pressure catapulting to show that MMP-9 expression is up-regulated concomitantly with the appearance of inflammatory cells in the subretinal lesion. In mice deficient in MMP-9 expression the development of choroidal neovascularization induced by laser photocoagulation still occurred, but at a reduced level.

Plzf mediates transcriptional repression of HoxD gene expression through chromatin remodeling

The molecular mechanisms that regulate coordinated and colinear activation of Hox gene expression in space and time remain poorly understood. Here we demonstrate that Plzf regulates the spatial expression of the AbdB HoxD gene complex by binding to regulatory elements required for restricted Hox gene expression and can recruit histone deacetylases to these sites. We show by scanning forced microscopy that Plzf, via homodimerization, can form DNA loops and bridge distant Plzf binding sites located within HoxD gene regulatory elements. Furthermore, we demonstrate that Plzf physically interacts with Polycomb proteins on DNA. We propose a model by which the balance between activating morphogenic signals and transcriptional repressors such as Plzf establishes proper Hox gene expression boundaries in the limb bud.

The pro- or antiangiogenic effect of plasminogen activator inhibitor 1 is dose dependent

Plasminogen activator inhibitor 1 (PAI-1) is believed to control proteolytic activity and cell migration during angiogenesis. We previously demonstrated in vivo that this inhibitor is necessary for optimal tumor invasion and vascularization. We also showed that PAI-1 angiogenic activity is associated with its control of plasminogen activation but not with the regulation of cell-matrix interaction. To dissect the role of the various components of the plasminogen activation system during angiogenesis, we have adapted the aortic ring assay to use vessels from gene-inactivated mice. The single deficiency of tPA, uPA, or uPAR, as well as combined deficiencies of uPA and tPA, did not dramatically affect microvessel formation. Deficiency of plasminogen delayed microvessel outgrowth. Lack of PAI-1 completely abolished angiogenesis, demonstrating its importance in the control of plasmin-mediated proteolysis. Microvessel outgrowth from PAI-1-/- aortic rings could be restored by adding exogenous PAI-1 (wild-type serum or purified recombinant PAI-1). Addition of recombinant PAI-1 led to a bell-shaped angiogenic response clearly showing that PAI-1 is proangiogenic at physiological concentrations and antiangiogenic at higher levels. Using specific PAI-1 mutants, we could demonstrate that PAI-1 promotes angiogenesis at physiological (nanomolar) concentrations through its antiproteolytic activity rather than by interacting with vitronectin.

Altered myelopoiesis and the development of acute myeloid leukemia in transgenic mice overexpressing cyclin A1

A mammalian A-type cyclin, cyclin A1, is highly expressed in testes of both human and mouse and targeted mutagenesis in the mouse has revealed the unique requirement for cyclin A1 in the progression of male germ cells through the meiotic cell cycle. While very low levels of cyclin A1 have been reported in the human hematopoietic system and brain, the sites of elevated levels of expression of human cyclin A1 were several leukemia cell lines and blood samples from patients with hematopoietic malignances, notably acute myeloid leukemia. To evaluate whether cyclin A1 is directly involved with the development of myeloid leukemia, mouse cyclin A1 protein was overexpressed in the myeloid lineage of transgenic mice under the direction of the human cathepsin G (hCG) promoter. The resulting transgenic mice exhibited an increased proportion of immature myeloid cells in the peripheral blood, bone marrow, and spleen. The abnormal myelopoiesis developed within the first few months after birth and progressed to overt acute myeloid leukemia at a low frequency ( approximately 15%) over the course of 7-14 months. Both the abnormalities in myelopoiesis and the leukemic state could be transplanted to irradiated SCID (severe combined immunodeficient) mice. The observations suggest that cyclin A1 overexpression results in abnormal myelopoiesis and is necessary, but not sufficient in the cooperative events inducing the transformed phenotype. The data further support an important role of cyclin A1 in hematopoiesis and the etiology of myeloid leukemia.

Hoxa4 expression in developing mouse hair follicles and skin

We have examined the expression of the Hoxa4 gene in embryonic vibrissae and developing and cycling postnatal pelage hair follicles by digoxigenin-based in situ hybridization. Hoxa4 expression is first seen in E13.5 vibrissae throughout the follicle placode. From E15.5 to E18.5 its expression is restricted to Henle's layer of the inner root sheath. Postnatally, Hoxa4 expression is observed at all stages of developing pelage follicles, from P0 to P4. Sites of expression include both inner and outer root sheaths, matrix cells, and the interfollicular epidermis. Hoxa4 is not expressed in hair follicles after P4. Hoxb4, however, is expressed both in developing follicles at P2 and in catagen at P19, suggesting differential expression of these two paralogous genes in the hair follicle cycle.

Mechanisms of Hox gene colinearity: transposition of the anterior Hoxb1 gene into the posterior HoxD complex

Transposition of Hoxd genes to a more posterior (5′) location within the HoxD complex suggested that colinearity in the expression of these genes was due, in part, to the existence of a silencing mechanism originating at the 5′ end of the cluster and extending towards the 3′ direction. To assess the strength and specificity of this repression, as well as to challenge available models on colinearity, we inserted a Hoxb1/lacZtransgene within the posterior HoxD complex, thereby reconstructing a cluster with a copy of the most anterior gene inserted at the most posterior position. Analysis of Hoxb1 expression after ectopic relocation revealed that Hoxb1-specific activity in the fourth rhombomere was totally abolished. Treatment with retinoic acid, or subsequent relocations toward more 3′ positions in theHoxD complex, did not release this silencing in hindbrain cells. In contrast, however, early and anterior transgene expression in the mesoderm was unexpectedly not suppressed. Furthermore, the transgene induced a transient ectopic activation of the neighboringHoxd13 gene, without affecting other genes of the complex. Such a local and transient break in colinearity was also observed after transposition of the Hoxd9/lacZ reporter gene, indicating that it may be a general property of these transgenes when transposed at an ectopic location. These results are discussed in the context of existing models, which account for colinear activation of vertebrate Hox genes.

Regulation of the Hoxa4 and Hoxa5 genes in the embryonic mouse lung by retinoic acid and TGFbeta1: implications for lung development and patterning

We have previously described a 5; cis-acting retinoic acid response element that is required for a subset of Hoxa4 expression, including the midgestation mouse lung. As both retinoids and Hox genes have been implicated in lung development and patterning, we have examined Hoxa4 expression in the developing mouse lung and extended our work on its regulation. At E12.5, a Hoxa4/lacZ transgene is expressed in the mesenchymal compartment of the lung. Later in development expression is restricted to the proximal mesenchyme and is also observed in smooth muscle cells, subepithelial fibroblasts, and alveolar cells. We show that both Hoxa4 and Hoxa5 are upregulated when cultured in the presence of all-trans retinoic acid. In addition, retinoic acid extends the domain of Hoxa4 and Hoxa5 expression to the periphery of the explants where the distal epithelia are developing. Interestingly, the effect of retinoic acid on Hoxa5 expression was not observed in a Hoxa4 mutant background. In contrast, TGFbeta1 was found to downregulate both Hoxa4 and Hoxa5 expression in cultured lung explants. We also establish that retinoic acid has the effect of proximalizing the mouse lung when cultured in a serum-free medium, as evidenced by reduced expression of the distal marker surfactant protein-C. Lungs from Hoxa4 mutant embryos exhibited a similar response to retinoic acid, suggesting that Hoxa4 alone is not required for the proximalizing effect. Based on their retinoid-dependent expression, we conclude that members of the group 4 and/or group 5 Hox genes are likely to be involved in patterning of the mouse lung. Dev Dyn 2000;217:62-74.

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.

Characterization of the murine A1 adenosine receptor promoter, potent regulation by GATA-4 and Nkx2.5

Adenosine acts via A1 adenosine receptors (A1ARs) in the heart and brain to potently influence mammalian physiology. A1ARs are expressed very early in embryonic development, and A1ARs are among the earliest expressed G protein coupled receptors in the heart and brain. To understand the biologic basis of A1AR expression, a genomic fragment containing the murine A1AR promoter was cloned. Reporter assay studies using DDT1 MF2 cells that express A1ARs revealed that 500 base pairs of the proximal A1AR promoter contained essential elements for A1AR gene expression. Transgenic mice with A1AR proximal promoter coupled with the beta-galactosidase reporter gene had heavy labeling of the brain and atria, consistent with normal patterns of A1AR expression. Within the proximal A1AR promoter, putative binding sites for cardiac transcription factors GATA and Nkx2.5 were identified. Co-expression studies revealed that GATA-4 and Nkx2.5 could individually drive A1AR promoter activity and act synergistically to activate A1AR expression. These observations suggest that embryonic A1AR expression involves activation of the A1AR promoter by GATA-4 and Nkx2.5.

Laminin-5 promotes adhesion and migration of epithelial cells: identification of a migration-related element in the gamma2 chain gene (LAMC2) with activity in transgenic mice

The effects of laminin-5 and its subunit gamma2 chain on cell adhesion and migration were studied, and a migration-related cis-acting element was identified in the gamma2 chain gene (LAMC2) using promoter-reporter gene constructs in transgenic mice. Intact laminin-5 molecules, but not recombinant gamma2 chain promoted cell adhesion of human keratinocytes and mouse squamous carcinoma cells, indicating that the gamma2 chain does not contain a cellular binding site. However, the gamma2 chain as such is probably involved in the process of cell locomotion, as antibodies against the short arm of the chain inhibited migration of carcinoma cells in an in vitro assay. Further evidence for the involvement of the gamma2 chain in cell migration was obtained by the identification of a cis-acting element in a promoter-lacZ reporter gene construct that was active in migratory epithelial cells of healing wounds in mice made transgenic by microinjection of the construct into fertilized oozytes. The migration active element was located in the sequence between -613 and +55. The same construct, and another one containing 5900 base pairs of the 5' flanking region, yielded very limited expression in cells of normal tissues. The limited expression was, however, only observed in epithelial cells of different tissues, i.e. cell types that normally express laminin-5 in vivo. The results show that the sequence between -613 and +55 contains elements that can drive expression during epithelial cell migration and that also partially confers more general epithelium expression. However, elements outside -5900 and +55 are needed for normal epithelium expression of the LAMC2 gene.

Murine matrix metalloproteinase 9 gene. 5'-upstream region contains cis-acting elements for expression in osteoclasts and migrating keratinocytes in transgenic mice

Knowledge about the regulation of cell lineage-specific expression of extracellular matrix metalloproteinases is limited. In the present work, the murine matrix metalloproteinase 9 (MMP-9) gene was shown to contain 13 exons, and the 2.8-kilobase pair upstream region was found to contain several common promoter elements including a TATA box-like motif, three GC boxes, four AP-1-like binding sites, an AP-2 site, and three PEA3 consensus sequences that may be important for basic activity of the gene. In order to identify cell-specific regulatory elements, constructs containing varying lengths of the upstream region in front of a LacZ reporter gene were made and studied for expression in transgenic mice generated by microinjection into fertilized oocytes. Analyses of the mice revealed that the presence of sequences between -2722 and -7745 allowed for expression in osteoclasts and migrating keratinocytes, i. e. cells that have been shown to normally express the enzyme in vivo. The results represent the first in vivo demonstration of the location of cell-specific control elements in a matrix metalloproteinase gene and show that element(s) regulating most cell-specific activities of 92-kDa type collagenase are located in the -2722 to -7745 base pair region.

Identification of upstream regulatory regions in the heart-expressed homeobox gene Nkx2-5

Nkx2-5 marks the earliest recognizable cardiac progenitor cells, and is activated in response to inductive signals involved in lineage specification. Nkx2-5 is also expressed in the developing foregut, thyroid, spleen, stomach and tongue. One approach to elucidate the signals involved in cardiogenesis was to examine the transcriptional regulation of early lineage markers such as Nkx2-5. We generated F0 transgenic mice, which carry Nkx2-5 flanking sequences linked to a lacZ reporter gene. We identified multiple regulatory regions located within the proximal 10.7 kb of the Nkx2-5 gene. In addition to a proximal promoter, we identified a second promoter and a novel upstream exon that could participate in the regulation of Nkx2-5 transcription. Although used rarely in normal development, this novel exon could be spliced into the Nkx2-5 coding region in several ways, thereby potentially creating novel Nkx2-5 protein isoforms, whose transcriptional activity is greatly diminished as compared to wild-type Nkx2-5. An enhancer that directs expression in pharynx, spleen, thyroid and stomach was identified within 3.5 kb of exon 1 between the coding exon 1 and the novel upstream exon 1a. Two or more enhancers upstream of exon 1a were capable of driving expression in the cardiac crescent, throughout the myocardium of the early heart tube, then in the outflow tract and right ventricle of the looped heart tube. A negative element was also located upstream of exon1a, which interacted in complex ways with enhancers to direct correct spatial expression. In addition, potential autoregulatory elements can be cooperatively stimulated by Nkx2-5 and GATA-4. Our results demonstrate that a complex suite of interacting regulatory domains regulate Nkx2-5 transcription. Dissection of these elements should reveal essential features of cardiac induction and positive and negative signaling within the cardiac field.

In vivo gene transfer to mouse spermatogenic cells by deoxyribonucleic acid injection into seminiferous tubules and subsequent electroporation

An in vivo gene transfer technique for living mouse testes was used to develop a novel transient expression assay system for transcriptional regulatory elements of spermatogenic specific genes. The combination of DNA injection into seminiferous tubules and subsequent in vivo electroporation resulted in an efficient and convenient assay system for gene expression during spermatogenesis. The transfer of the firefly luciferase reporting gene driven by the Protamine-1 (Prm-1) enhancer region revealed a significant increase in the activity of the reporter enzyme. Histochemical studies of the transfer of the lacZ gene driven by the Prm-1 enhancer showed specific lacZ expression only in haploid spermatid cells in adult testes, corresponding with the expression pattern of endogenous Prm-1. We were able to detect long-lasting transgene expression in the transfected spermatogenic cells. A group of spermatogenic differentiating cells maintained the transfected lacZ expression after more than 2 mo of transfection, suggesting that spermatogenic stem cells and/or spermatogonia could also incorporate foreign DNA and that the transgene could be transmitted to the progenitor cells derived from a transfected proliferating germ cell.

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.

Wound reepithelialization activates a growth factor-responsive enhancer in migrating keratinocytes

Wound reepithelialization and keratinocyte migration require strictly ordered gene expression, which is assumed to be initiated by locally released mitogens and exposure of the cells to different matrix components. The mechanisms triggering gene expression specifically during reepithelialization are poorly understood. The far upstream AP-1-driven, FGF-inducible response element (FiRE) of the syndecan-1 gene was activated during cutaneous wound healing in transgenic mice. FiRE was induced selectively in migrating but not in proliferating keratinocytes at the wound edge. The activation was initiated at the start of the cell migration, was persistent throughout the merging and stratification phases, and was terminated after completion of reepithelialization. Although FiRE has been found within the gene of syndecan-1, the proximal promoter of syndecan-1 was not required for activation of FiRE in the migrating keratinocytes. The wounding induced activation was inhibited by blocking cell surface growth factor receptors with suramin. However, the activation of FiRE in resting skin required simultaneous growth factor- and stress-induced signals, but could also be elicited by the phosphatase inhibitor, okadaic acid. The activation by both wounding and chemical stimuli was blocked by inhibiting extracellular regulated kinase and p38 MAP kinases, suggesting the involvement of at least two parallel signal transduction pathways in wounding induced gene activation. As FiRE shows specificity for migrating keratinocytes only, it can be a useful tool for future wound healing studies and for targeting genes to injured tissues.

FGF is an essential regulator of the fifth cell division in preimplantation mouse embryos

Fibroblast growth factor (FGF) signaling is required prior to gastrulation in the mouse embryo. To test for the spatial and temporal requirements of FGF signaling, a dominant negative FGF receptor (dnFGFR) was used to make transgenic mouse embryos. In mosaic embryos, cell division ceased at the fifth cell division in all cells that expressed the mutant receptor, but cell death did not increase. After the fifth cell division, the progeny of unaltered cells and cells expressing lacZ continued to accumulate at the same rate, suggesting that the FGF requirement is cell autonomous. In mosaic embryos, lacZ, but not dnFGFR expression was detected in mitotic trophoblasts adjacent to the ICM. Conversely, dnFGFR-expressing extraembryonic ectoderm cells were detected at the abembryonic pole in postmitotic cells. In blastocysts expressing the dnFGFR in all cells, the morphology appeared normal and inner cell masses (ICMs) formed, but resultant embryos had only one-third the number of cells as control embryos. In these blastocysts, cell division had also ceased at the fifth cell division, but cavitation, a concurrent morphogenetic event, initiated and progressed normally. To test for the continuing requirement of FGF, FGFR-3 was overexpressed in all cells and resulted in an increase in cell numbers after the fifth cell cycle. In a model for postimplantation development, addition of FGF-4 to blastocyst outgrowths increased the number of extraembryonic ectoderm cells, suggesting a continuing role for FGF. Thus, FGF signaling induces the cell division of embryonic and extraembryonic cells in the preimplantation mouse embryo starting at the fifth cell division. The signal requirement for FGF is cell autonomous, but is not required to prevent cell death. This provides the first evidence for the necessity of a growth factor before implantation.

Expression of the murine Hoxa4 gene requires both autoregulation and a conserved retinoic acid response element

Analysis of the regulatory regions of the Hox genes has revealed a complex array of positive and negative cis-acting elements that control the spatial and temporal pattern of expression of these genes during embryogenesis. In this study we show that normal expression of the murine Hoxa4 gene during development requires both autoregulatory and retinoic acid-dependent modes of regulation. When introduced into a Hoxa4 null background, expression of a lacZ reporter gene driven by the Hoxa4 regulatory region (Hoxa4/lacZ) is either abolished or significantly reduced in all tissues at E10. 5-E12.5. Thus, the observed autoregulation of the Drosophila Deformed gene is conserved in a mouse homolog in vivo, and is reflected in a widespread requirement for positive feedback to maintain Hoxa4 expression. We also identify three potential retinoic acid response elements in the Hoxa4 5′ flanking region, one of which is identical to a well-characterized element flanking the Hoxd4 gene. Administration of retinoic acid to Hoxa4/lacZ transgenic embryos resulted in stage-dependent ectopic expression of the reporter gene in the neural tube and hindbrain. When administered to Hoxa4 null embryos, however, persistent ectopic expression was not observed, suggesting that autoregulation is required for maintenance of the retinoic acid-induced expression. Finally, mutation of the consensus retinoic acid response element eliminated the response of the reporter gene to exogenous retinoic acid, and abolished all embryonic expression in untreated embryos, with the exception of the neural tube and prevertebrae. These data add to the evidence that Hox gene expression is regulated, in part, by endogenous retinoids and autoregulatory loops.

Hox group 3 paralogs regulate the development and migration of the thymus, thyroid, and parathyroid glands

The thymus, thyroid, and parathyroid glands in vertebrates develop from the pharyngeal region, with contributions both from pharyngeal endoderm and from neural crest cells in the pharyngeal arches. Hoxa3 mutant homozygotes have defects in the development of all three organs. Roles for the Hoxa3 paralogs, Hoxb3 and Hoxd3, were investigated by examining various mutant combinations. The thyroid defects seen in Hoxa3 single mutants are exacerbated in double mutants with either of its paralogs, although none of the double-mutant combinations resulted in thyroid agenesis. The results indicate that the primary role of these genes in thyroid development is their effect on the development and migration of the ultimobranchial bodies, which contribute the parafollicular or C-cells to the thyroid. Hoxb3, Hoxd3 double mutants show no obvious defects in the thymus or parathyroids. However, the removal of one functional copy of Hoxa3 from the Hoxb3, Hoxd3 double mutants (Hoxa3 +/-, Hoxb3-/-, Hoxd3-/-) results in the failure of the thymus and parathyroid glands to migrate to their normal positions in the throat. Very little is known about the molecular mechanisms used to mediate the movement of tissues during development. These results indicate that Hoxa3, Hoxb3, and Hoxd3 have highly overlapping functions in mediating the migration of pharyngeal organ primordia. In addition, Hoxa3 has a unique function with respect to its paralogs in thymus, parathyroid, and thyroid development. This unique function may be conferred by the expression of Hoxa3, but not Hoxb3 nor Hoxd3, in the pharyngeal pouch endoderm.

Elements both 5' and 3' to the murine Hoxd4 gene establish anterior borders of expression in mesoderm and neurectoderm

In this report, we show that a lacZ reporter spanning 12.5 kb of murine Hoxd4 genomic DNA contains the major regulatory elements controlling Hoxd4 expression in the mouse embryo. Mutational analysis revealed multiple regulatory regions both 5' and 3' to the coding region. These include a 3' enhancer region required for expression in the central nervous system (CNS) and setting the anterior border in the paraxial mesoderm, and a 5' mesodermal enhancer that directs expression in paraxial and lateral plate mesoderm. A previously defined retinoic acid response element (RARE) is a component of the 5' mesodermal enhancer. Our results support a model in which retinoic acid receptors (RARs) and HOX proteins mediate the initiation and maintenance of Hoxd4 expression.

HOXD4 and regulation of the group 4 paralog genes

From an evolutionary perspective, it is important to understand the degree of conservation of cis-regulatory mechanisms between paralogous Hox genes. In this study, we have used transgenic analysis of the human HOXD4 locus to identify one neural and two mesodermal 3′ enhancers that are capable of mediating the proper anterior limits of expression in the hindbrain and paraxial mesoderm (somites), respectively. In addition to directing expression in the central nervous system (CNS) up to the correct rhombomere 6/7 boundary in the hindbrain, the neural enhancer also mediates a three rhombomere anterior shift from this boundary in response to retinoic acid (RA), mimicking the endogenous Hoxd4 response. We have extended the transgenic analysis to Hoxa4 identifying mesodermal, neural and retinoid responsive components in the 3′ flanking region of that gene, which reflect aspects of endogenous Hoxa4 expression. Comparative analysis of the retinoid responses of Hoxd4, Hoxa4 and Hoxb4 reveals that, while they can be rapidly induced by RA, there is a window of competence for this response, which is different to that of more 3′ Hox genes. Mesodermal regulation involves multiple regions with overlapping or related activity and is complex, but with respect to neural regulation and response to RA, Hoxb4 and Hoxd4 appear to be more closely related to each other than Hoxa4. These results illustrate that much of the general positioning of 5′ and 3′ flanking regulatory regions has been conserved between three of the group 4 paralogs during vertebrate evolution, which most likely reflects the original positioning of regulatory regions in the ancestral Hox complex.

Null mutation of mCOUP-TFI results in defects in morphogenesis of the glossopharyngeal ganglion, axonal projection, and arborization

The COUP-TFs are orphan members of the steroid/thyroid hormone receptor superfamily. Multiple COUP-TF members have been cloned and they share a high degree of sequence homology between species as divergent as Drosophila and humans, suggesting a conservation of function through evolution. The COUP-TFs are highly expressed in the developing nervous systems of several species examined, indicating their possible involvement in neuronal development and differentiation. In the mouse, there are two very homologous COUP-TF genes (I and II) and their expression patterns overlap extensively. To study the physiological function of mCOUP-TFI, a gene-targeting approach was undertaken. We report here that mCOUP-TFI null animals die perinataly. Mutant embryos display an altered morphogenesis of the ninth cranial ganglion and nerve. The aberrant formation of the ninth ganglion is most possibly attributable to extra cell death in the neuronal precursor cell population. In addition, at midgestation, aberrant nerve projection and arborization were oberved in several other regions of mutant embryos. These results indicate that mCOUP-TFI is required for proper fetal development and is essential for postnatal development. Furthermore, mCOUP-TFI possesses vital physiological functions that are distinct from mCOUP-TFII despite of their high degree of homology and extensive overlapping expression patterns.

A conserved cluster of homeodomain binding sites in the mouse Hoxa-4 intron functions in Drosophila embryos as an enhancer that is directly regulated by Ultrabithorax

The evolutionary conservation of the homeodomains suggests that their in vivo DNA binding sites may also be conserved between vertebrates and invertebrates. The regulatory function of the mouse Hoxa-4 and Hoxb-4 introns were analyzed in Drosophila since they both contain a cluster of three homeodomain binding sites, the HB1 element, which was also found in the introns of other Hox genes ranging from fish to humans as well as in the Ultrabithorax (Ubx) and decapentaplegic (dpp) genes of Drosophila. The enhancer of the Hoxa-4 intron was found to respond to several homeobox genes activating a lacZ reporter gene in particular cells of the epidermis in Drosophila embryos. The enhancer activity was found to be similar to previously described autoregulatory elements of Deformed (Dfd), the Drosophila homolog of Hoxa-4, but additional expression was observed in more posterior segments activated by Ubx and repressed by abdominal-A (abd-A). Point mutations in the homeodomain binding sites in HB1 abolished the enhancer activity. A second site suppression experiment showed that UBX interacts directly with the HB1 element. When the HB1 element in the Hoxa-4 intron was replaced by that of the mesodermal enhancer of dpp, which was previously shown to be directly controlled by Ubx, Ubx-dependent activation was retained, but repression by abd-A was lost. The same result was obtained when the third binding site of HB1 was altered, suggesting that this site is responsible for abd-A-dependent repression. Finally, deletion of potential cofactor binding sites flanking the HB1 element that are also conserved in the medaka, chicken, and mouse genes revealed that they are important for enhancer function in Drosophila and that the Dfd-dependent and the Ubx-dependent expression requires different sites. The evolutionary and functional conservation of the HB1 elements indicates that not only the homeodomains but also some of their in vivo binding sites are conserved between vertebrates and invertebrates.

A sequence conserved in vertebrate Hox gene introns functions as an enhancer regulated by posterior homeotic genes in Drosophila imaginal discs

The intron of the mouse Hoxa-4 gene acts as a strong homeotic response element in Drosophila melanogaster leg imaginal discs. This activity depends on homeodomain binding sites present within a 30 bp conserved element, HB1, in the intron. A similar arrangement of homeodomain binding sites is found in many other potential homeotic target genes. HB1 activity in Drosophila imaginal discs is activated by Antennapedia and more posterior homeotic genes, but is not activated by more anterior genes. Testing a reporter gene construct with mutated binding sites in mouse embryos shows that HB1 is also active in the expression domains of posterior Hox genes in the mouse neural tube.

Function of untranslated regions in the mouse spermatogenesis-specific gene Tcp10 evaluated in transgenic mice

The mouse Tcp10 genes are transcribed exclusively in male germ cells and display multiple 5' and 3' untranslated variations generated by alternative splicing and polyadenylation signal usage. To investigate the possible role of untranslated sequences in the regulation of these genes, chimeric expression constructs with or without endogenous 5' and 3' untranslated sequences were generated and used to make transgenic mice. Analysis of these animals showed that the untranslated sequences have no effect on the transcription or translation of an attached lacZ reporter gene, thereby implying these sequences are dispensible. However, the endogenous pattern of polyadenylation site usage was altered when Tcp10 3' untranslated sequences were linked to lacZ, indicating that internal coding sequence can influence recognition of polyadenylation signals in testis. The characteristics of alternative splicing and polyadenylation signal variability reflects a common theme of promiscuity in testicular gene expression.

The compartmentalization of the cerebellum

The concept of developmental compartments originated in studies of Drosophila embryogenesis. This review examines the hypothesis that the modular structure of the vertebrate cerebellum is strongly analogous to this earlier scheme. The pattern of cerebellar development, the adult circuitry, a variety of molecular markers expressed in specific subdivisions, and the phenotypes of several neurological mutations all provide abundant evidence that the vertebrate cerebellum is organized into modules. We present the case that, as a group, these markers reveal distinct boundaries that partition the cerebellum into true developmental compartments. Although this reductionist viewpoint advances our understanding of cerebellar organization, the relationship between these compartments and the functional behavior of the cerebellum remains a mystery.

Paralogous Hox genes: function and regulation

The Hox homeobox gene family plays a pivotal role in regulating patterning and axial morphogenesis in vertebrates. Molecular characterization of the four Hox clusters has shown that they are evolutionarily related with respect to sequence, organization, and expression, suggesting they arose by duplication and divergence. Transgenic analysis has clearly demonstrated the functional roles of individual genes in a broad range of embryonic tissues, and in compound mutants has addressed the issues of cooperativity and redundancy. There is an emerging picture of the cis-regulatory elements underlying Hox expression, and for the 3' members of the clusters there is a considerable degree of conservation between paralogous genes with respect to their functional roles and regulatory control.

Transcriptional control of Hox genes in the vertebrate nervous system

The identification, in transgenic mice, of Hox gene DNA regulatory elements that can recapitulate certain aspects of the endogenous gene expression pattern has proceeded with great success. Perfect reproduction of the correct expression pattern, however, is uncommon, even when large genomic fragments spanning neighboring genes are analyzed, suggesting that important regulatory regions may be located at large distances from the genes they control or that their specific context may be important. Four classes of transcriptional regulators have been identified recently that have been shown to directly regulate Hox gene expression in the murine nervous system: retinoic acid receptors, Krox20, the Pbx/exd family, and the Hox genes themselves.

Hoxb-13: a new Hox gene in a distant region of the HOXB cluster maintains colinearity

The Hox genes are involved in patterning along the A/P axes of animals. The clustered organization of Hox genes is conserved from nematodes to vertebrates. During evolution, the number of Hox genes within the ancestral complex increased, exemplified by the five-fold amplification of the AbdB-related genes, leading to a total number of thirteen paralogs. This was followed by successive duplications of the cluster to give rise to the four vertebrate HOX clusters. A specific subset of paralogs was subsequently lost from each cluster, yet the composition of each cluster was likely conserved during tetrapod evolution. While the HOXA, HOXC and HOXD clusters contain four to five AbdB-related genes, only one gene (Hoxb-9) is found in the HOXB complex. We have identified a new member of paralog group 13 in human and mouse, and shown that it is in fact Hoxb-13. A combination of genetic and physical mapping demonstrates that the new gene is found approx. 70 kb upstream of Hoxb-9 in the same transcriptional orientation as the rest of the cluster. Despite its relatively large distance from the HOX complex, Hoxb-13 exhibits temporal and spatial colinearity in the main body axis of the mouse embryo. The onset of transcription occurs at E9.0 in the tailbud region. At later stages of development, Hoxb-13 is expressed in the tailbud and posterior domains in the spinal cord, digestive tract and urogenital system. However, it is not expressed in the secondary axes such as the limbs and genital tubercle. These results indicate that the 5′ end of the HOXB cluster has not been lost and that at least one member exists and is highly conserved among different vertebrate species. Because of its separation from the complex, Hoxb-13 may provide an important system to dissect the mechanism(s) responsible for the maintenance of colinearity.

In vitro and transgenic analysis of a human HOXD4 retinoid-responsive enhancer

Expression of vertebrate Hox genes is regulated by retinoids in cell culture and in early embryonic development. We have identified a 185-bp retinoid-responsive transcriptional enhancer 5′ of the human HOXD4 gene, which regulates inducibility of the gene in embryonal carcinoma cells through a pattern of DNA-protein interaction on at least two distinct elements. One of these elements contains a direct repeat mediating ligand-dependent interaction with retinoic acid receptors, and is necessary though not sufficient for the enhancer function. The HOXD4 enhancer directs expression of a lacZ reporter gene in the neural tube of transgenic mouse embryos in a time-regulated and regionally restricted fashion, reproducing part of the anterior neuroectodermal expression pattern of the endogenous Hoxd-4 gene. Administration of retinoic acid to developing embryos causes alterations in the spatial restriction of the transgene expression domain, indicating that the HOXD4 enhancer is also a retinoid-responsive element in vivo. The timing of the retinoic acid response differs from that seen with more 3′ Hox genes, in that it occurs much later. This shows that the temporal window of competence in the ability to respond to retinoic acid differs between Hox genes and can be linked to specific enhancers. Mutations in the direct repeat or in a second element in the enhancer affect both retinoid response in culture and developmental regulation in embryos, suggesting that co-operative interactions between different factors mediate the enhancer activity. These data provide further support for a role of endogenous retinoids in regulation and spatial restriction of Hox gene expression in the central nervous system.