Regulation of vertebrate homeobox-containing genes by morphogens

Optic Nerve Hypoplasia in Fetal Alcohol Syndrome: An Update

Optic nerve hypoplasia was detected in up to one half of a group of Swedish children born to alcoholic mothers. Using an experimental model of pre- and postnatal alcohol exposure in rats fed a liquid diet, reduced optic nerve size from gestational day 21 (294 ± 26×102 μm2 vs 502 ± 16×102 μm2; n=6; p≤0.001) to later in development was observed as a result of the daily mean blood alcohol levels achieved in dams and their offspring. Altered glial cells and degenerating and atrophic optic axons, myelin sheaths and ganglion cells were frequent in the alcohol-exposed optic nerves.

Smaller optic nerve (1,918 ± 61×102 μm2 vs 2.195 ± 40×102 μm2; n=4; p≤0.001), reduced gaglion cell and axonal densities, and ultrastructural damage to the macroglial cells and myelin sheaths were also detected in the treated group. All these changes remained in the retina and optic nerve of the oldest rats, as a consequence of the long-lasting effects of prenatal alcohol exposure. In summary, alcohol as a major teratogenic agent may induce dysmorphogenesis and irremediable damage to the retina and optic nerve, which frequently manifests itself as hypoplastic optic nerve.

Cancer cell metabolism and developmental homeodomain/POU domain transcription factors: a connecting link

The human race is afflicted with more than 100 types of cancer with diversified underlying genetic events. Still, altered metabolism (known as 'Warburg effect') and unrestrained cellular proliferation are precise traits of all cancers, being governed by the expression of genes. The obligatory energy for the proliferating neoplastic cells is endowed through the glycolytic pathway, albeit, lesser ATP is generated in this metabolic process. So, some perceptible cancer cell specific signalling is at the base of the transformed metabolism. Concurrently, the regulators of gene expression, transcription factors, have been observed to be one of the driving forces for tumourigenesis through transcriptional activation of genes involved not only in proliferation, growth and survival signalling, but also in glycolysis. This may be exemplified by the extensively studied metabolic functions of the transcriptional regulator, hypoxia inducible factor 1 (HIF1), which transactivates genes of the major enzymes of glycolysis. Preliminary investigation of a vital group of transcription factors, homeodomain transcription factors, revealed association with the process of development of an organism. The homeodomain transcription factors are, however, also found to be involved in the tumourigenesis process, with little or no information on their involvement in cancer cell metabolism. So, this is a review of the existing knowledge on homeodomain transcription factor/s for deciphering their involvement in neoplastic metabolism and it emerges that homeodomain transcription factors influence the transformed metabolic pathway in a circuitous manner.

NeuroD1 mediates nicotine-induced migration and invasion via regulation of the nicotinic acetylcholine receptor subunits in a subset of neural and neuroendocrine carcinomas

Nicotine up-regulates NeuroD1 in bronchial epithelial cells and certain undifferentiated carcinomas. NeuroD1 enhances expression of nicotinic acetylcholine receptor subunits. Increased invasion in Matrigel depends on these receptor subunits. Nicotine may induce positive feedback through NeuroD1 and increased expression of its own receptor.

Cigarette smoking is a major risk factor for acquisition of small cell lung cancer (SCLC). A role has been demonstrated for the basic helix-loop-helix transcription factor NeuroD1 in the pathogenesis of neural and neuroendocrine lung cancer, including SCLC. In the present study we investigate the possible function of NeuroD1 in established tumors, as well as actions early on in pathogenesis, in response to nicotine. We demonstrate that nicotine up-regulates NeuroD1 in immortalized normal bronchial epithelial cells and a subset of undifferentiated carcinomas. Increased expression of NeuroD1 subsequently leads to regulation of expression and function of the nicotinic acetylcholine receptor subunit cluster of α3, α5, and β4. In addition, we find that coordinated expression of these subunits by NeuroD1 leads to enhanced nicotine-induced migration and invasion, likely through changes in intracellular calcium. These findings suggest that aspects of the pathogenesis of neural and neuroendocrine lung cancers may be affected by a nicotine- and NeuroD1-induced positive feedback loop.

HOX Gene Aberrant Expression in Skin Melanoma: A Review

The homeobox family and its subset of HOX gene products represent a family of transcription factors directing DNA-protein and protein-protein interactions. In the embryo, they are central regulators in cell differentiation during morphogenesis. A series of genes of the four HOX gene clusters A, B, C, and D were reported to show aberrant expressions in oncogenesis, particularly in cutaneous malignant melanoma (CMM). They are involved in cell proliferation and progression in the CMM metastatic path. We present relevant peer-reviewed literature findings about the aberrant expression of HOX genes in CMM. The number of CMM cell nuclei exhibiting aberrant HOX protein expression appears correlated with tumour progression.

Functional cooperation between human adenovirus type 5 early region 4, open reading frame 6 protein, and cellular homeobox protein HoxB7

Human adenovirus type 5 (HAdV5) E4orf6 (early region 4 open reading frame 6 protein) is a multifunctional early viral protein promoting efficient replication and progeny production. E4orf6 complexes with E1B-55K to assemble cellular proteins into a functional E3 ubiquitin ligase complex that not only mediates proteasomal degradation of host cell substrates but also facilitates export of viral late mRNA to promote efficient viral protein expression and host cell shutoff. Recent findings defined the role of E4orf6 in RNA splicing independent of E1B-55K binding. To reveal further functions of the early viral protein in infected cells, we used a yeast two-hybrid system and identified the homeobox transcription factor HoxB7 as a novel E4orf6-associated protein. Using a HoxB7 knockdown cell line, we observed a positive role of HoxB7 in adenoviral replication. Our experiments demonstrate that the absence of HoxB7 leads to inefficient viral progeny production, as HAdV5 gene expression is highly regulated by HoxB7-mediated activation of various adenoviral promoters. We have thus identified a novel role of E4orf6 in HAdV5 gene transcription via regulation of homeobox protein-dependent modulation of viral promoter activity.

Establishment of canine hemangiosarcoma xenograft models expressing endothelial growth factors, their receptors, and angiogenesis-associated homeobox genes

BACKGROUND

Human hemangiosarcoma (HSA) tends to have a poor prognosis; its tumorigenesis has not been elucidated, as there is a dearth of HSA clinical specimens and no experimental model for HSA. However, the incidence of spontaneous HSA is relatively high in canines; therefore, canine HSA has been useful in the study of human HSA. Recently, the production of angiogenic growth factors and their receptors in human and canine HSA has been reported. Moreover, the growth-factor environment of HSA is very similar to that of pathophysiological angiogenesis, which some homeobox genes regulate in the transcription of angiogenic molecules. In the present study, we established 6 xenograft canine HSA tumors and detected the expression of growth factors, their receptors, and angiogenic homeobox genes.

METHODS

Six primary canine HSAs were xenografted to nude mice subcutaneously and serially transplanted. Subsequently, the expressions of vascular endothelial growth factor (VEGF)-A, basic fibroblast growth factors (bFGF), flt-1 and flk-1 (receptors of VEGF-A), FGFR-1, and angiogenic homeobox genes HoxA9, HoxB3, HoxB7, HoxD3, Pbx1, and Meis1 were investigated in original and xenograft tumors by histopathology, immunostaining, and reverse transcription polymerase chain reaction (RT-PCR), using canine-specific primer sets.

RESULTS

Histopathologically, xenograft tumors comprised a proliferation of neoplastic cells that were varied in shape, from spindle-shaped and polygonal to ovoid; some vascular-like structures and vascular clefts of channels were observed, similar to those in the original tumors. The expression of endothelial markers (CD31 and vWF) was detected in xenograft tumors by immunohistochemistry and RT-PCR. Moreover, the expression of VEGF-A, bFGF, flt-1, flk-1, FGFR-1, HoxA9, HoxB3, HoxB7, HoxD3, Pbx1, and Meis1 was detected in xenograft tumors. Interestingly, expressions of bFGF tended to be higher in 3 of the xenograft HSA tumors than in the other tumors.

CONCLUSION

We established 6 xenograft canine HSA tumors in nude mice and found that the expressions of angiogenic growth factors and their receptors in xenograft HSAs were similar to those in spontaneous HSA. Furthermore, we detected the expression of angiogenic homeobox genes; therefore, xenograft models may be useful in analyzing malignant growth in HSA.

Neuroembryology and functional anatomy of craniofacial clefts

The master plan of all vertebrate embryos is based on neuroanatomy. The embryo can be anatomically divided into discrete units called neuromeres so that each carries unique genetic traits. Embryonic neural crest cells arising from each neuromere induce development of nerves and concomitant arteries and support the development of specific craniofacial tissues or developmental fields. Fields are assembled upon each other in a programmed spatiotemporal order. Abnormalities in one field can affect the shape and position of developing adjacent fields. Craniofacial clefts represent states of excess or deficiency within and between specific developmental fields. The neuromeric organization of the embryo is the common denominator for understanding normal anatomy and pathology of the head and neck. Tessier's observational cleft classification system can be redefined using neuroanatomic embryology. Reassessment of Tessier's empiric observations demonstrates a more rational rearrangement of cleft zones, particularly near the midline. Neuromeric theory is also a means to understand and define other common craniofacial problems. Cleft palate, encephaloceles, craniosynostosis and cranial base defects may be analyzed in the same way.

Hoxa1 is required for the retinoic acid-induced differentiation of embryonic stem cells into neurons

The ability of embryonic stem (ES) cells to differentiate into different cell fates has been extensively evaluated, and several protocols exist for the generation of various types of cells from mouse and human ES cells. We used a differentiation protocol that involves embryoid body formation and all-trans-retinoic acid (RA, 5 microM) treatment (EB/5 microM RA) to test the ability of Hoxa1 null ES cells to adopt a neuronal fate. Hoxa1(-/-) ES cells, when treated in this EB/5 microM RA protocol, failed to differentiate along a neural lineage; Hoxa1(-/-) ES cells express severalfold lower levels of many neuronal differentiation markers, including nestin, beta-tubulin III, and MAP2, and conversely, higher levels of endodermal differentiation markers (i.e., Sox17, Col4a1) than wild type (Wt) cells. Reintroduction of exogenous Hoxa1, under the control of the metallothionein I promoter, into Hoxa1(-/-) ES cells restored their capacity to generate neurons. Moreover, overexpression of Sox17, a gene that regulates endodermal differentiation, in Wt ES cells resulted in endodermal differentiation and in a complete abolition of beta-tubulin III expression. Thus, Hoxa1 activity is essential for the neuronal differentiation of ES cells in the presence of all-trans-RA, and Hoxa1 may promote neural differentiation by inhibiting Sox17 expression. Pharmacological manipulation of Hoxa1 levels may provide a method for promoting neuronal differentiation for therapeutic uses. Furthermore, because mutations in the Hoxa1 gene can cause autism spectrum disorder in humans, these data also provide important mechanistic insights into the early developmental processes that may result in this disorder.

Tumorigenicity issues of embryonic carcinoma-derived stem cells: relevance to surgical trials using NT2 and hNT neural cells

Cell therapy is a rapidly moving field with new cells, cell lines, and tissue-engineered constructs being developed globally. As these novel cells are further developed for transplantation studies, it is important to understand their safety profiles both prior to and posttransplantation in animals and humans. Embryonic carcinoma-derived cells are considered an important alternative to stem cells. The NTera2/D1 teratocarcinoma cell-line (or NT2-N cells) gives rise to neuron-like cells called hNT neurons after exposure to retinoic acid. NT2 cells form tumors upon transplantation into the rodent. However, when the NT2 cells are treated with retinoic acid to produce hNT cells, they terminally differentiate into post-mitotic neurons with no sign of tumorigenicity. Preliminary human transplantation studies in the brain of stroke patients also demonstrated a lack of tumorigenicity of these cells. This review focuses on the use of hNT neurons in cell transplantation for the treatment in central nervous system (CNS) diseases, disorders, or injuries and on the mechanism involved in retinoic acid exposure, final differentiation state, and subsequent tumorigenicity issues that must be considered prior to widespread clinical use.

Differences in gene expression between wild type and Hoxa1 knockout embryonic stem cells after retinoic acid treatment or leukemia inhibitory factor (LIF) removal

Homeobox (Hox) genes encode a family of transcription factors that regulate embryonic patterning and organogenesis. In embryos, alterations of the normal pattern of Hox gene expression result in homeotic transformations and malformations. Disruption of the Hoxa1 gene, the most 3' member of the Hoxa cluster and a retinoic acid (RA) direct target gene, results in abnormal ossification of the skull, hindbrain, and inner ear deficiencies, and neonatal death. We have generated Hoxa1(-/-) embryonic stem (ES) cells (named Hoxa1-15) from Hoxa1(-/-) mutant blastocysts to study the Hoxa1 signaling pathway. We have characterized in detail these Hoxa1(-/-) ES cells by performing microarray analyses, and by this technique we have identified a number of putative Hoxa-1 target genes, including genes involved in bone development (e.g. Col1a1, Postn/Osf2, and the bone sialoprotein gene or BSP), genes that are expressed in the developing brain (e.g. Nnat, Wnt3a, BDNF, RhoB, and Gbx2), and genes involved in various cellular processes (e.g. M-RAS, Sox17, Cdkn2b, LamA1, Col4a1, Foxa2, Foxq1, Klf5, and Igf2). Cell proliferation assays and Northern blot analyses of a number of ES cell markers (e.g. Rex1, Oct3/4, Fgf4, and Bmp4) suggest that the Hoxa1 protein plays a role in the inhibition of cell proliferation by RA in ES cells. Additionally, Hoxa1(-/-) ES cells express high levels of various endodermal markers, including Gata4 and Dab2, and express much less Fgf5 after leukemia inhibitory factor (LIF) withdrawal. Finally, we propose a model in which the Hoxa1 protein mediates repression of endodermal differentiation while promoting expression of ectodermal and mesodermal characteristics.

Neural tube programming and craniofacial cleft formation. I. The neuromeric organization of the head and neck

This review presents a brief synopsis of neuromeric theory. Neuromeres are developmental units of the nervous system with specific anatomic content. Outlying each neuromere are tissues of ectoderm, mesoderm and endoderm that bear an anatomic relationship to the neuromere in three basic ways. This relationship is physical in that motor and sensory connections exist between a given neuromeric level and its target tissues. The relationship is also developmental because the target cells exit during gastrulation precisely at that same level. Finally the relationship is chemical because the genetic definition of a neuromere is shared with those tissues with which it interacts. The model developed by Puelles and Rubenstein is used to describe the neuroanatomy of the neuromeres. Although important details of the model are currently being refined it has immediate clinical relevance for practicing clinicians because it permits us to understand many pathologic states as relationships between the brain and the surrounding tissues. Relationships between the processes of neurulation and gastrulation have been presented to demonstrate the manner in which neuromeric anatomy is established in the embryo. We are now in a position to describe in detail the static anatomic structures that result from this system. The neuromeric 'map' of craniofacial bones, dermis, dura, muscles, and fascia will be the subject of the next part of this series.

HOXB7 expression is regulated by the transcription factors NF-Y, YY1, Sp1 and USF-1

Products of HOX genes are transcription factors responsible for developmental regulation and postnatal tissue homeostasis. Besides their well-established function played during embryonic development, we had previously demonstrated the direct role of HOXB7 in tumor progression through transactivation of several genes involved in the proliferative and angiogenic processes. This role is at first exerted through the deregulated, constitutive expression of this gene. To define the factors possibly responsible for such activation, we studied the molecular regulation of HOXB7 in embryonic and neoplastic cells. In a 1.9-kb 5' promoter region, we identified and functionally tested, at least in vitro, different regulatory sequences showing a direct binding by the NF-Y, YY1, Sp1/Sp3 and upstream stimulatory factor 1 (USF-1) transcription factors. Cell transfection and site-specific mutagenesis demonstrated Sp1/Sp3, NF-Y, YY1 and USF-1 binding to be functional and fundamental in driving HOXB7 expression. Disruption of the corresponding sites reduces gene expression of 65%, 78% and 55%, respectively. Because HOXB7 seems to play an important role in tumor proliferation and progression, the analysis of its regulatory sequences might represent an important step for gene targeting according to a new therapeutic strategy.

Transduction of HOXD3-antisense into human melanoma cells results in decreased invasive and motile activities

Homeobox genes regulate sets of genes that determine cellular fates in embryonic morphogenesis and maintenance of adult tissue architecture by regulating cellular motility and cell-cell interactions. Our previous studies showed that a specific member, HOXD3, when overexpressed, enhanced cell motility and invasiveness of human lung cancer A549 cells (Hamada et al. Int. J. Cancer 2001; 93: 516-25 [19]). In the present study, we investigated the roles of HOXD3 in motile and invasive behavior of human malignant melanoma cells. Of seven melanoma cell lines examined here, six cell lines expressed the HOXD3 gene, whereas normal melanocytes did not. We transduced the HOXD3-antisense gene expression vector into two cell lines (A375M and MMIV). The cell transduced with the HOXD3-antisense gene showed reduced in vitro invasion of Matrigel. The transduction of the HOXD3-antisense gene also decreased cell spreading, haptotactic activity to vitronectin and laminin-1, and phagokinetic activity. To find the difference of gene expression between the HOXD3-antisense-transduced A375M cells and the control A375MNeo2 cells, we carried out cDNA microarray analysis. The results of the microarray analysis indicated that the increased expression of cdc42-interacting protein 4, KIAA0554 and tropomyosin 1, which are all associated with the cytoskeletal system, may be involved in the reduction of motile and invasive activity by the HOXD3-antisense gene transduction.

Expression pattern of HOXB6 homeobox gene in myelomonocytic differentiation and acute myeloid leukemia

Homeobox genes encode transcription factors known to be important morphogenic regulators during embryonic development. An increasing body of work implies a role for homeobox genes in both hematopoiesis and leukemogenesis. In the present study we have analyzed the role of the homeobox gene, HOXB6, in the program of differentiation of the myeloid cell lines, NB4 and HL60. HOXB6 expression is transiently induced during normal granulocytopoiesis and monocytopoiesis, with an initial induction during the early phases of differentiation, followed by a blockade of expression at early maturation. The enforced expression of HOXB6 in promyelocytic NB4 cells or in myeloblastic HL60 cells elicited inhibition of the granulocytic or monocytic maturation, respectively. Furthermore, HOXB6 was frequently expressed (18 out of 49 cases) in AMLs lacking major translocations while it was expressed at very low frequency (two out of 47 cases) in AMLs characterized by PML/RAR-alpha, AML-1/ETO, CBFbeta/MYH11 fusion and rearrangements of the MLL gene at 11q23. According to these observations, we suggest that a regulated pattern of HOXB6 expression is required for normal granulopoiesis and monocytopoiesis. Abnormalities of the HOXB6 expression may contribute to the development of the leukemic phenotype.

HOX-A10 regulates hematopoietic lineage commitment: evidence for a monocyte-specific transcription factor

Homeobox genes are well known for their crucial role during embryogenesis but have also been found to be critically involved in normal and leukemic hematopoiesis. Because most previous studies focused on the role of aberrant HOX gene expression in leukemogenesis and because HOX-A10 is expressed in human CD34(+) precursor cells, this study investigated whether HOX-A10 also plays a pivotal role in normal hematopoietic-lineage determination. The effect of enforced expression of this transcription factor on hematopoietic differentiation of highly purified human cord-blood progenitors was examined by using in vitro assays. In fetal thymic organ cultures, a dramatic reduction in cells expressing high levels of HOX-A10 was observed, along with absence of thymocytes positive for CD3(+) T-cell receptor alphabeta. Furthermore, in MS-5 stromal cell cultures, there was a 7-fold reduction in the number of natural killer cells and a 9-fold reduction in the number of B cells, thus showing a profound defect in differentiation toward the lymphoid lineage in HOX-A10-transduced progenitors. In contrast, the number of CD14(+) monocytic cells in the stromal cell culture was 6-fold higher, suggesting an enhanced differentiation toward the myeloid differentiation pathway of HOX-A10-transduced progenitors. However, there was a slight reduction in the number of CD15(+) granulocytic cells, which were blocked in their final maturation. These data show that HOX-A10 can act as an important key regulator of lineage determination in human hematopoietic progenitor cells.

Hoxa-5 in mouse developing lung: cell-specific expression and retinoic acid regulation

Hoxa-5 is a homeobox gene that is highly expressed in the developing mouse lung. However, little is known about the molecular mechanisms controlling expression. We characterized the ontogeny of Hoxa-5 gene and protein expressions during lung development and then studied the cell-specific effects of retinoic acid (RA) on Hoxa-5 mRNA in fetal lung fibroblasts and MLE-12 mouse lung epithelial cells. Strong but constant Hoxa-5 gene and protein expressions were detected from mouse lung on embryonic day 13.5 to postnatal day 2. At baseline, the gene was strongly expressed in the fibroblasts of day 17.5 fetal mouse lungs. A very weak but reproducible expression was present in the MLE-12 cells. RA stimulated gene expression in both cell types in a time- and dose-dependent manner. Peak expression occurred much later in the MLE-12 cells compared with that in fibroblasts. Cycloheximide and actinomycin D treatment studies suggested that the differences in RA effect on each cell type may involve the presence of a repressor that can be overcome by RA.

Altered retinoic acid sensitivity and temporal expression of Hox genes in polycomb-M33-deficient mice

The Polycomb group genes are required for the correct expression of the homeotic complex genes and segment specification during Drosophila embryogenesis and larval development. In mouse, inactivation studies of several Polycomb group genes indicate that they are also involved in Hox gene regulation. We have used our previously generated M33 mutants to study the function of M33, the mouse homologue of the Polycomb gene of Drosophila. In this paper, we show that in the absence of M33, the window of Hoxd4 retinoic acid (RA) responsiveness is opened earlier and that Hoxd11 gene expression is activated earlier in development This indicates that M33 antagonizes the RA pathway and has a function in the establishment of the early temporal sequence of activation of Hox genes. Despite the early activation, A-P boundaries are correct in later stages, indicating a separate control mechanism for early aspects of Hox regulation. This raises a number of interesting issues with respect to the roles of both Pc-G proteins and Hox regulatory mechanisms. We propose that a function of the M33 protein is to control the accessibility of retinoic acid response elements in the vicinity of Hox genes regulatory regions by direct or indirect mechanisms or both. This could provide a means for preventing ectopic transactivation early in development and be part of the molecular basis for temporal colinearity of Hox gene expression.

All-trans retinoic acid at low concentration directly stimulates normal adult megakaryocytopoiesis in the presence of thrombopoietin or combined cytokines

In order to investigate the direct effects of retinoids on normal adult hematopoietic progenitors, purified CD34+ cells were seeded in serum-free cultures in the presence of pharmacological (10(-6)) M or physiological (10(-12)) M concentrations of all-trans retinoic acid (ATRA) and 9-cis retinoic acid (9-cis RA) plus combinations of specific cytokines. 10(-6) M ATRA and 9-cis RA significantly decreased the number of granulomacrophagic, erythroid and megakaryocytic (CFU-meg) progenitors. On the other hand, 10(-12) M ATRA significantly promoted the growth of CFU-meg, in the presence either of thrombopoietin or of IL-3+ GM-CSF, and induced a reproducible stimulation of the immature CD34+DR- subset. In conclusion, our findings suggest that retinoic acids probably play a direct role in normal adult hematopoietic development at both physiological and pharmacological concentrations. The stimulatory effect on megakaryocytopoiesis should be considered in the perspective of a potential use of low-dose ATRA, combined with thrombopoietin or other cytokines, in pathological conditions where the megakaryocytic compartment is impaired and the stimulation of megakaryocytopoiesis is requested.

The Cdx-1 and Cdx-2 homeobox genes in the intestine

The past years have witnessed an increasing number of reports relative to homeobox genes in endoderm-derived tissues. In this review, we focus on the caudal-related Cdx-1 and Cdx-2 homeobox genes to give an overview of the in vivo, in vitro, and ex vivo approaches that emphasize their primary role in intestinal development and in the control of intestinal cell proliferation, differentiation, and identity. The participation of these genes in colon tumorigenesis and their identification as important actors of the oncogenic process are also discussed.

Enforced expression of HOXB7 promotes hematopoietic stem cell proliferation and myeloid-restricted progenitor differentiation

Hematopoietic progenitor/stem cells (HPCs/HSCs) purified from human adult peripheral blood (PB) were triggered into cycling, retrovirally transduced with HOXB7 and then functionally assayed in vitro. HPCs were assayed in multi- and unilineage differentiation cultures in either liquid phase or semisolid medium, primitive HPCs in the high proliferative potential colony-forming cell (HPP-CFC) evaluation system and putative HSCs in Dexter type long-term culture (LTC) as LTC initiating cells (LTC-ICs). Control experiments ensured that the exogenous HOXB7 gene was constantly expressed, while the endogenous one was barely or not transcribed. Enforced expression of the gene markedly modulated the proliferation/differentiation program of the entire HSC/HPC population. Enforced HOXB7 expression exerted a potent stimulatory effect on the proliferation of the primitive HPC and putative HSC subsets, assayed as HPP-CFCs and LTC-ICs respectively. While not modifying the total number of HPCs, exogenous HOXB7 induced an increase of the number of granulo-monocytic (GM) HPCs [colony-forming unit GM (CFU-GM) CFU-GM, CFU-G and CFU-M, as evaluated by clonogenic assays] and markedly amplified the progeny of both CFU-G and CFU-M, which showed a sustained proliferation through at least 1-2 months (as evaluated in liquid suspension culture). The prolonged proliferative stimulus induced by HOXB7 transfer into LTC, primitive and GM oriented HPC culture was characterized by persistent proliferation of a discrete population of blast cells and a large pool of differentiated myeloid precursors. Altogether, these results suggest the hypothesis that the proliferative stimulus exerted by exogenous HOXB7 in primitive and GM-oriented HPCs may represent a preleukemic immortalization step. Consistent with the functional role of HOXB7 in the initial ontogenetic phase, these studies indicate that ectopic HOXB7 expression in early HPCs and HSCs from adult PB stimulates their self renewal, sustained proliferation and myeloid differentiation.

Retinoic acid and thyroid hormone may function through similar and competitive pathways in regenerating axolotls

The objective of this study was to determine whether thyroid hormone (TH) would interfere with retinoic acid (RA), which proximalizes axolotl larvae regenerate limb pattern. RA and TH are ligands for members of the steroid hormone thyroid hormone nuclear binding protein superfamily which form functional homodimers, but may also form stable heterodimers with the RXR protein and may recognize identical DNA sequences. TH alone does not affect limb pattern but induces metamorphosis in regenerating animals. Coinjected animals do not metamorphose, and when compared to RA controls regenerate more proximal and in some cases anteroposterior (AP) and dorsoventral (DV) duplicate limb structures. In addition, the tissues that are normally lost or changed during metamorphosis appear to be sensitized resulting in the formation of (1) new dorsal gill lamellae accompanied by bifurcation and broadening of the original gill lamellae, (2) partial resorption of the tail fin, and (3) changes in eye position and snout morphology. Bifurcation of gill lamellae tips, but not the formation of supernumerary gills, is also observed in animals treated with RA alone. These results indicate that the molecular mechanism of RA and TH function through similar and perhaps competitive pathways.

All-trans-retinoic acid upregulates the expression of COUP-TFI in early-somite mouse embryos cultured in vitro

Exposure of embryos to an excess of retinoic acid (RA) modifies the spatio-temporal pattern of expression of developmental genes. RA regulates the expression of target genes through binding of the retinoid nuclear receptors (RARs and RXRs), as heterodimers, to regulatory cis-acting elements. COUP-TF factors, which are able to dimerize with the RXRs and to compete with the retinoid receptors for their DNA binding sites, are suspected to modulate the retinoid signal transduction pathway. Therefore, COUP-TF factors may be involved in the regulation of the expression of developmental genes and/or in the modifications induced by an excess of RA in the expression of these genes. The aim of this work is to assess whether RA-induced modifications in the expression of Krox-20 and Hox genes correlate with alterations of the expression of COUP-TF genes. In addition to spatial modifications in the expression patterns of Krox-20 and Hox genes, we report here an upregulation of the expression level of COUP-TFI after RA exposure. However, this abnormality did not spatially overlap with the modifications observed in the expression of Krox-20 and Hox genes. These data suggest an involvement of COUP-TFI in the generation of RA-induced abnormalities, but do not support the hypothesis of an involvement of this factor in the regulation of the expression of Hox or Krox-20 genes.

The role of homeobox genes in hematopoiesis

Homeobox genes encode transcription factors containing a common DNA-binding motif found in virtually all animal species. Different homeobox gene families have evolved which encode homeodomains of different types or classes and thus far approximately 170 homeobox genes have been cloned. Homeoproteins are involved in the control of animal development and several lines of evidence strongly suggest that they may contribute to the regulation of hematopoiesis. Many members of this large family are expressed in blood cells. Moreover, homeobox containing genes have been involved in translocation events occurring in certain leukemias and lymphomas. Furthermore a number of studies indicate that modulation of homeobox gene expression may induce alterations in proliferative, differentiative or phenotypic characteristics of hematopoietic cells. Although the function of each individual gene has not been clearly defined there is strong evidence for cooperativity among homeoproteins indicating that regulatory combinations of homeobox genes may play a pivotal role in controlling survival, proliferation and differentiation of hematopoietic cells.

Genomic structure and sequence analysis of human HOXA-9

In order to understand the regulatory mechanisms establishing and maintaining HOXA-9 gene expression, structural information about the gene is a prerequisite. Therefore, we sequenced the 7.2-kb region of the human HOXA-9 gene and mapped the positions of two partial cDNAs consisting of one of two 5' exons, AB (358 bp) or CD (568 bp), and a common 3' exon (exon II), which are separated by 5.4- and 1.0-kb introns, respectively. When the amino acid sequence homologies were compared with those of other Hox genes belonging to the same paralogous group, exon CD exhibited the strongest homology: 73% of 91 aa residues exactly matched those of chicken Hoxa-9. An intermediate exon (90 bp) was detected within exon CD. It was surrounded by a splice acceptor and a donor at both the 5' and 3' ends, and one branchpoint site was found near the splice-acceptor site. Nucleotide sequence analysis along this region revealed two TATA boxes, one CAAT box, one GC box, and one each of the following binding sites--engrailed, eve-stripe2-hb3, and Krox20--just upstream of exon CD. A CpG island and two RARE repeats were detected within intron I. Northern blot analysis showed that at least four main transcripts were generated along this region: all fetal tissues tested (brain, lung, liver, and kidney) produced a 1.8-kb homeobox-containing transcript (HA-9A); a 2.2- and a 3.3-kb transcript were generated from exon CD and exon II (HA-9B), especially in fetal and adult kidneys as well as in adult skeletal muscle; the 1.0-kb transcript was likely to be generated by the intermediate exon in all adult and fetal tissues. Several weak bands without tissue specificity were likely to be contributed by the hybrid transcripts between HOXA-9 and the other HOXA gene(s). Together, these results may account for the unique degree of conservation of the HOX cluster in general.

Defined concentrations of a posteriorizing signal are critical for MafB/Kreisler segmental expression in the hindbrain

It has been shown by using the quail/chick chimera system that Hox gene expression in the hindbrain is influenced by positional signals arising from the environment. In order to decipher the pathway that leads to Hox gene induction, we have investigated whether a Hox gene regulator, the leucine zipper transcription factor MafB/Kr, is itself transcriptionally regulated by the environmental signals. This gene is normally expressed in rhombomeres (r) 5 and 6 and their associated neural crest. MafB/Kr expression is maintained in r5/6 when grafted into the environment of r3/4. On the contrary, the environment of rhombomeres 7/8 represses MafB/Kr expression. Thus, as previously shown for the expression of Hox genes, MafB/Kr expression is regulated by a posterior-dominant signal, which in this case induces the loss of expression of this gene. We also show that the posterior signal can be transferred to the r5/6 neuroepithelium by posterior somites (somites 7 to 10) grafted laterally to r5/6. At the r4 level, the same somites induce MafB/Kr in r4, leading it to behave like r5/6. The posterior environment regulates MafB/Kr expression in the neural crest as it does in the corresponding hindbrain level, showing that some positional regulatory mechanisms are shared by neural tube and neural crest cells. Retinoic acid beads mimic the effect produced by the somites in repressing MafB/Kr in r5/6 and progressively inducing it more rostrally as its concentration increases. We therefore propose that the MafB/Kr expression domain is defined by a molecule unevenly distributed in the paraxial mesoderm. This molecule would allow the expression of the MafB/Kr gene in a narrow window of concentration by activating its expression at a definite threshold and repressing it at higher levels, accounting for its limited domain of expression in only two rhombomeres. It thus appears that the regulation of MafB/Kr expression in the rhombomeres could be controlled by the same posteriorizing factor(s) as Hox genes.

307-bp fragment in HOXA7 upstream sequence is sufficient for anterior boundary formation

The HOX genes are expressed in a positionally and temporally restricted manner involving anteroposterior axial pattern formation during early embryogenesis. Previously, we studied the sequence and function of an upstream regulatory region of the human HOXA7 gene. To identify a critical cis-acting element, a deletion analysis was performed along the human control region (HCR) (about 1.1 kb), which was sufficient for setting the anterior boundary of expression in transgenic mice. We demonstrated that a 307-bp control region contains a cis-acting element(s) specifying an anterior boundary as well as a dorsal-ventral restriction in the neural tube at day 12.5 postconception (p.c.). The distinct anterior limit of expression was noted at the level of C7/T1 in the neural tube and spinal ganglia. In addition, our deletion experiments revealed that the HCR consisted of several cis-acting elements which were individually capable of driving regionally restricted expression patterns in the neural tube and limb buds.

Key role of the Cdx2 homeobox gene in extracellular matrix-mediated intestinal cell differentiation

To explore the role of homeobox genes in the intestine, the human colon adenocarcinoma cell line Caco2-TC7 has been stably transfected with plasmids synthesizing Cdx1 and Cdx2 sense and antisense RNAs. Cdx1 overexpression or inhibition by antisense RNA does not markedly modify the cell differentiation markers analyzed in this study. In contrast, Cdx2 overexpression stimulates two typical markers of enterocytic differentiation: sucrase-isomaltase and lactase. Cells in which the endogenous expression of Cdx2 is reduced by antisense RNA attach poorly to the substratum. Conversely, Cdx2 overexpression modifies the expression of molecules involved in cell-cell and cell-substratum interactions and in transduction process: indeed, E-cadherin, integrin-beta4 subunit, laminin-gamma2 chain, hemidesmosomal protein, APC, and alpha-actinin are upregulated. Interestingly, most of these molecules are preferentially expressed in vivo in the differentiated villi enterocytes rather than in crypt cells. Cdx2 overexpression also results in the stimulation of HoxA-9 mRNA expression, an homeobox gene selectively expressed in the colon. In contrast, Cdx2-overexpressing cells display a decline of Cdx1 mRNA, which is mostly found in vivo in crypt cells. When implanted in nude mice, Cdx2-overexpressing cells produce larger tumors than control cells, and form glandular and villus-like structures. Laminin-1 is known to stimulate intestinal cell differentiation in vitro. In the present study, we demonstrate that the differentiating effect of laminin-1 coatings on Caco2-TC7 cells is accompanied by an upregulation of Cdx2. To further document this observation, we analyzed a series of Caco2 clones in which the production of laminin-alpha1 chain is differentially inhibited by antisense RNA. We found a positive correlation between the level of Cdx2 expression, that of endogenous laminin-alpha1 chain mRNA and that of sucrase-isomaltase expression in these cell lines. Taken together, these results suggest (a) that Cdx1 and Cdx2 homeobox genes play distinct roles in the intestinal epithelium, (b) that Cdx2 provokes pleiotropic effects triggering cells towards the phenotype of differentiated villus enterocytes, and (c) that Cdx2 expression is modulated by basement membrane components. Hence, we conclude that Cdx2 plays a key role in the extracellular matrix-mediated intestinal cell differentiation.

Effects of HOX homeobox genes in blood cell differentiation

The burgeoning number of articles concerning the role of HOX genes and hematopoiesis ensures that this will continue to be an area of very active research. It seems clear that HOX genes are expressed in stage- and lineage-specific patterns during early stages of hematopoietic development and differentiation. Several lines of evidence suggest that multiple genes of the HOXB (B2, B4, B6-B9), HOXC (C6, C8), and HOXA (A5) are involved in erythropoiesis. Similarly, a number of genes of the HOXA, HOXB, and HOXC appear to play a role in lymphoid cells. Furthermore, several genes, such as A9, A10, B3, B7, and B8, may control myelomonocytic differentiation. The question arises as to whether such a multiplicity of HOX genes reflects redundancy or indicates subtlety of the regulatory machinary. A similar complexity has been observed for hematopoietic cytokines, and the current view is that, although multiple molecules may have similar or overlapping effects, each factor has a specific function and regulatory combinations appear to play a critical role in controlling hematopoietic cell processes (99). One challenge for the future is to delineate in more detail the precise expression patterns of these genes in the many distinct subpopulations of blood cells and during fetal development. Overexpression of HOX genes in hematopoietic cells can dramatically perturb the differentiation of various cell lineages and can contribute to leukemogenesis. Future studies may involve the overexpression of alternatively spliced versions of different HOX genes or of truncated versions of HOX genes to ascertain the functional domains of the proteins that mediate the biologic effects. The findings in HOX knockout mice confirm a role for these genes in normal blood cell development. Further work in this area will require careful examination of fetal hematopoiesis and of animals bearing multiple HOX gene knockouts. Involvement of HOX genes in leukemia is just beginning to be appreciated. Establishing the true extent of HOX gene mutations in human disease will require strategies such as comparative genomic hybridization (100) and analysis of high density oligonucleotide arrays (101). The holy grail of homeobox work is to discover the physiologic processes and specific target genes regulated by HOX proteins. Given the broad range of tissues in which HOX genes are expressed, they would appear to be involved in very basic cellular processes, e.g., cell proliferation and death, adhesion, and migration, etc., rather than the direct regulation of tissue-specific genes. The search for target genes may be made easier by the further characterization of cooperative DNA binding between HOX proteins and other transcription factors. We speculate that HOX proteins do not behave as conventional transcriptional activators or inhibitors but rather may mark genes for potential future activation, i.e., they may establish competency to execute specific differentiation programs, with the actual activation being accomplished by transcriptional pathways triggered by exogenous signals. This proposed function may be an architectural one, involving changes in the conformation of DNA and/or altering interactions between DNA and histones, thus making areas of the genome more or less accessible to other protein factors (102). If this is the case, we may need to develop new assays to discern the molecular action of HOX proteins. The ease of manipulating the hematopoietic systems would appear to make it a very attractive model for explicating the general functions of this remarkable family of genes.

Distinct patterns of all-trans retinoic acid dependent expression of HOXB and HOXC homeogenes in human embryonal and small-cell lung carcinoma cell lines

The expression patterns of the class I homeogenes HOXB and HOXC clusters in the presence of retinoic acid (RA) were studied in two human small-cell lung cancer (SCLC) cell lines and compared to that of NT2/D1 embryonal carcinoma cells. Contrasting with the sequential 3'-5' induction of the HOX genes observed after RA treatment of embryonic NT2/D1 cells, in the SCLC cells the responding genes (induced or down-regulated) were interspersed with insensitive genes (expressed or unexpressed), while no genomic alteration affected the corresponding clusters. These findings imply that HOX gene regulatory mechanisms are altered in non-embryonic SCLC cells, perhaps reflecting their ability to respond to more diversified stimuli, in relation with their origin from adult tissues.

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.

HOXB7 constitutively activates basic fibroblast growth factor in melanomas

Homeobox (HOX) genes control axial specification during mammalian development and also regulate skin morphogenesis. Although selected HOX genes are variably expressed in leukemias and kidney and colon cancer cell lines, their relationship with the neoplastic phenotype remains unclear. In both normal development and neoplastic transformation, HOX target genes are largely unknown. We investigated the expression and function of HOXB cluster genes in human melanoma. The HOXB7 gene was constitutively expressed in all 25 melanoma cell lines and analyzed under both normal and serum-starved conditions, as well as in in vivo primary and metastatic melanoma cells; conversely, HOXB7 was expressed in proliferating but not quiescent normal melanocytes. Treatment of melanoma cell lines with antisense oligomers targeting HOXB7 mRNA markedly inhibited cell proliferation and specifically abolished expression of basic fibroblast growth factor (bFGF) mRNA. Band shift and cotransfection experiments showed that HOXB7 directly transactivates the hFGF gene through one out of five putative homeodomain binding sites present in its promoter. These novel findings indicate a key role for constitutive HOXB7 expression in melanoma cell proliferation via bFGF. The results also raise the possibility that growth factor genes are critical HOX target genes in other developmental and/or neoplastic cell systems.

The role of HOX homeobox genes in normal and leukemic hematopoiesis

A sizable amount of new data points to a role for the HOX family of homeobox genes in hematopoiesis. Recent studies have demonstrated that HOXA and HOXB genes are expressed in human CD34+ cells, and are downregulated as cells leave the CD34+ compartment. In addition, expression of certain genes, including HOXB3 and HOXB4, is largely restricted to the long-term culture-initiating cell enriched pool, containing the putative stem cell population. Studies have also shown that HOX genes appear to be important for normal T lymphocyte and activated natural killer cell function. Overexpression of Hox-b4 in transplanted murine marrow cell results in a dramatic expansion of stem cells, while maintaining normal peripheral blood counts. In contrast, overexpression of Hox-a10 resulted in expansion of progenitor pools, accompanied by unique changes in the differentiation patterns of committed progenitors. Overexpression of Hox-a10 or Hox-b8 led to the development of myeloid leukemias, while animals transfected with marrow cells overexpressing Hox-b4 do not appear to develop malignancies. Blockade of HOX gene function using antisense oligonucleotides has revealed that several HOX genes appear to influence either myeloid or erythroid colony formation. Mice homozygous for a targeted disruption of the HOX-a9 gene show reduced numbers of granulocytes and lymphocytes, smaller spleens and thymuses, and reduced numbers of committed progenitors. These studies demonstrate that HOX homeobox genes play a role in both the early stem cell function as well as in later stages of hematopoietic differentiation, and that perturbations of HOX gene expression can be leukemogenic.

Increased bone formation and osteoblastic cell phenotype in premature cranial suture ossification (craniosynostosis)

Craniosynostosis is a heterogeneous disorder characterized by premature fusion of the skull bone sutures. To evaluate the pathogenesis of premature cranial suture ossification in craniosynostosis, we have evaluated the histologic indices of bone formation and the characteristics of osteoblastic cells derived from normal and affected cranial sutures in 47 infants and children, aged 3-18 months, with nonsyndromic craniosynostosis. The histomorphometric analysis of normal and fused sutures showed an age-related decline in the extent of endosteal bone surface covered with osteoid and osteoblasts during postnatal suture ossification. Bone formation was 20-50% higher at 3-6 months of age in fused sutures compared with normal sutures in the same patients. Cells derived from normal and fused sutures displayed characteristics of the osteoblast phenotype in culture. Analysis of [3H]thymidine incorporation into DNA from 1-14 days of culture showed an age-related decrease in osteoblastic cell growth in both normal and affected sutures. The proliferation of osteoblastic cells isolated from fused sutures was similar at all ages to that of cells isolated from normal sutures in the same patients. In contrast, alkaline phosphatase activity and osteocalcin production by osteoblastic cells cultured in basal conditions and after stimulation with 1,25-dihydroxyvitamin D (1,25[OH]2D3), were 53-74% higher in fused sutures compared with cells isolated from normal sutures in the same patients. The results indicate that bone formation activity at the suture site is locally increased in craniosynostosis, and this disorder is associated with increased in vitro parameters of osteoblastic cell differentiation, suggesting that an increased maturation of osteoblastic cells at the site of the suture leads to the premature ossification in nonsyndromic craniosynostosis.

Disruption of the murine homeobox gene Cdx1 affects axial skeletal identities by altering the mesodermal expression domains of Hox genes

Cdx1 is expressed along the embryonic axis from day 7.5 postcoitum until day 12, by which time the anterior limit of expression has regressed from the hindbrain level to the forelimb bud region. To assign a functional role for Cdx1 in murine embryonic development, we have inactivated the gene via homologous recombination. Viable fertile homozygous mutant mice were obtained that show anterior homeotic transformations of vertebrae. These abnormalities were concomitant with posterior shifts of Hox gene expression domains in the somitic mesoderm. The presence of putative Cdx1-binding sites in Hox gene control regions as well as in vitro transactivation of Hoxa-7 indicates a direct regulation.

Plasticity of transposed rhombomeres: Hox gene induction is correlated with phenotypic modifications

In this study we have analysed the expression of Hoxb-4, Hoxb-1, Hoxa-3, Hoxb-3, Hoxa-4 and Hoxd-4 in the neural tube of chick and quail embryos after rhombomere (r) heterotopic transplantations within the rhombencephalic area. Grafting experiments were carried out at the 5-somite stage, i.e. before rhombomere boundaries are visible. They were preceeded by the establishment of the precise fate map of the rhombencephalon in order to determine the presumptive territory corresponding to each rhombomere. When a rhombomere is transplanted from a caudal to a more rostral position it expresses the same set of Hox genes as in situ. By contrast in many cases, if rhombomeres are transplanted from rostral to caudal their Hox gene expression pattern is modified. They express genes normally activated at the new location of the explant, as evidenced by unilateral grafting. This induction occurs whether transplantation is carried out before or after rhombomere boundary formation. Moreover, the fate of the cells of caudally transplanted rhombomeres is modified: the rhombencephalic nuclei in the graft develop according to the new location as shown for an r5/6 to r8 transplantation. Transplantation of 5 consecutive rhombomeres (i.e. r2 to r6), to the r8 level leads to the induction of Hoxb-4 in the two posteriormost rhombomeres but not in r2,3,4. Transplantations to more caudal regions (posterior to somite 3) result in some cases in the induction of Hoxb-4 in the whole transplant. Neither the mesoderm lateral to the graft nor the notochord is responsible for the induction. Thus, the inductive signal emanates from the neural tube itself, suggesting that planar signalling and predominance of posterior properties are involved in the patterning of the neural primordium.

A novel vertebrate svp-related nuclear receptor is expressed as a step gradient in developing rhombomeres and is affected by retinoic acid

The protein encoded by the zebrafish gene svp[40] belongs to a distinct group within the steroid hormone receptor superfamily that includes Drosophila seven-up and several vertebrate orphan receptors. Svp[40] shares a particularly high degree of amino acid sequence identity (approximately 86%) with the mammalian transcription factors ARP-1 and COUP. The gene is expressed in specific regional and segmental domains within the developing brain. Correspondence between this expression pattern and early sites of neuronal differentiation and axonogenesis in the rostral brain may reflect an involvement in neural patterning. During the early embryonic stages when hindbrain rhombomeres are formed, a segmental expression pattern is established as a step gradient. The single steps of this gradient coincide directly with the four anteriormost segments suggesting a role in controlling rhombomere-specific expression of genes contributing to cell differentiation in the hindbrain. Since COUP/ARP-1 and retinoic acid receptors (RARs/RXRs) are known to have similar DNA-binding specificities, different levels of Svp[40] might modulate retinoid signaling through competition for binding to specific RAREs in the promoters of target genes. Treatment of zebrafish embryos with retinoic acid affects the svp[40] step gradient and causes an elimination of a regional expression domain in the retina. These observations are consistent with svp[40] being an integral part of the retinoid signaling network during hindbrain and eye development.

Regulation of HoxA expression in developing and regenerating axolotl limbs

Homeobox genes are important in the regulation of outgrowth and pattern formation during limb development. It is likely that homeobox genes play an equally important role during limb regeneration. We have isolated and identified 17 different homeobox-containing genes expressed by cells of regenerating axolotl limbs. Of these, nearly half of the clones represent genes belonging to the HoxA complex, which are thought to be involved in pattern formation along the proximal-distal limb axis. In this paper we report on the expression patterns of two 5′ members of this complex, HoxA13 and HoxA9. These genes are expressed in cells of developing limb buds and regenerating blastemas. The pattern of expression in developing axolotl limb buds is comparable to that in mouse and chick limb buds; the expression domain of HoxA13 is more distally restricted than that of HoxA9. As in developing mouse and chick limbs, HoxA13 likely functions in the specification of distal limb structures, and HoxA9 in the specification of more proximal structures. In contrast, during regeneration, HoxA13 and HoxA9 do not follow the rule of spatial colinearity observed in developing limbs. Instead, both genes are initially expressed in the same population of stump cells, giving them a distal Hox code regardless of the level of amputation. In addition, both are reexpressed within 24 hours after amputation, suggesting that reexpression may be synchronous rather than temporally colinear. Treatment with retinoic acid alters this Hox code to that of a more proximal region by the rapid and differential downregulation of HoxA13, at the same time that expression of HoxA9 is unaffected. HoxA reexpression occurs prior to blastema formation, 24–48 hours after amputation, and is an early molecular marker for dedifferentiation.

All-trans retinoic acid shows multiple effects on the survival, proliferation and differentiation of human fetal CD34+ haemopoietic progenitor cells

To evaluate the effect of all-trans retinoic acid (RA) on fetal haemopoiesis, we performed serum-free liquid and semisolid cultures using CD34+ cells purified from midtrimester human fetal blood samples. RA, at both physiological (10(-11) and 10(-12)M) and pharmacological (10(-6) and 10(-7)M) concentrations, significantly (P < 0.01) promoted the survival of fetal CD34+ cells in liquid cultures from day 3 onwards, by suppressing apoptosis induced by serum and growth factor deprivation. On the other hand, RA alone had no significant effect on the proliferation and differentiation of fetal haemopoietic progenitors. In the presence of optimal concentrations of recombinant interleukin-3 (IL-3), stem cell factor (SCF), granulocyte/macrophage-colony stimulating factor (GM-CSF), and erythropoietin (Epo), low and high doses of RA induced striking differential effects on CD34+ cell proliferation in liquid cultures and colony formation in semisolid assays. In fact, 10(-11)M and 10(-12)M RA were able to: (i) significantly (P < 0.05) increase 3H-thymidine uptake by fetal CD34+ cells in liquid cultures, and (ii) variably promote the growth of pluripotent (CFU-GEMM, P < 0.05), early (BFU-meg) and late (CFU-meg, P < 0.01) megakaryocyte, granulocyte/macrophage (CFU-GM, P < 0.01) and erythroid (BFU-E) progenitors in semisolid cultures. On the contrary, 10(-6) and 10(-7)M RA induced: (i) an overall inhibition (P < 0.01) of CD34+ cell growth in liquid cultures; (ii) a marked suppression of BFU-E colony formation (P < 0.01) at all Epo concentrations examined (0.002-4 IU/ml); and (iii) a significant (P < 0.01) stimulation of CFU-GM with a shift from mixed granulocyte/macrophage to pure granulocyte colonies, whereas it had little effect on the growth of CFU-GEMM, BFU-meg and CFU-meg. Our data, as a whole, demonstrate that RA has direct complex effects on the survival, growth and clonal expansion of fetal haemopoietic progenitor cells, mainly depending on the presence of recombinant cytokines, the type of progenitor and the concentrations of RA.

The expression pattern of the murine Hoxa-10 gene and the sequence recognition of its homeodomain reveal specific properties of Abdominal B-like genes

Homeobox genes of the Abdominal B (AbdB) family constitute a distinct subset of vertebrate Hox genes. Analysis of the murine Hoxa-10 gene, one member of this family, revealed several properties specific to this class. Two transcripts of Hoxa-10, a10-1 and a10-2, encode homeodomain proteins of 55 kDa (399 amino acids) and 16 kDa (96 amino acids), respectively. These proteins have identical homeodomains and C-terminal regions encoded by a common 3' exon but differ significantly in the sizes of their N-terminal regions because of the usage of alternative 5' exons. The 5' exon of the a10-2 form is also present in transcripts of Hoxa-9, the next 3' gene, indicating that splicing can occur between adjacent AbdB Hox genes within a cluster. Both Hoxa-10 transcripts demonstrated identical patterns of expression in the posterior body and proximal limb bud, differentiating them from AbdB morphogenetic and regulatory transcripts and suggesting a role with other AbdB Hox genes in the patterning of these structures. Finally, a binding site selection identified the sequence AA(A/T)TTTTATTAC as the Hoxa-10 homeodomain consensus binding site, with a TTAT core sequence. Preferential recognition of a TTAT core therefore differentiates the AbdB class from Antennapedia (Antp) class gene products which bind a TAAT core. Thus, in vertebrates, structural similarities, coordinate transcriptional regulation, sites of expression, and binding site preferences all serve to distinguish AbdB from Antp Hox genes.

Regulation of a decapentaplegic midgut enhancer by homeotic proteins

The clustered homeotic genes encode transcription factors that regulate pattern formation in all animals, conferring cell fates by coordinating the activities of downstream ‘target’ genes. In the Drosophila midgut, the Ultrabithorax (Ubx) protein activates and the abdominalA (abd-A) protein represses transcription of the decapentaplegic (dpp) gene, which encodes a secreted signalling protein of the TGF beta class. We have identified an 813 bp dpp enhancer which is capable of driving expression of a lacZ gene in a correct pattern in the embryonic midgut. The enhancer is activated ectopically in the visceral mesoderm by ubiquitous expression of Ubx or Antennapedia but not by Sex combs reduced protein. Ectopic expression of abd-A represses the enhancer. Deletion analysis reveals regions required for repression and activation. A 419 bp subfragment of the 813 bp fragment also drives reporter gene expression in an appropriate pattern, albeit more weakly. Evolutionary sequence conservation suggests other factors work with homeotic proteins to regulate dpp. A candidate cofactor, the extradenticle protein, binds to the dpp enhancer in close proximity to homeotic protein binding sites. Mutation of either this site or another conserved motif compromises enhancer function. A 45 bp fragment of DNA from within the enhancer correctly responds to both UBX and ABD-A in a largely tissue-specific manner, thus representing the smallest in vivo homeotic response element (HOMRE) identified to date.

Function of the retinoic acid receptors (RARs) during development (I). Craniofacial and skeletal abnormalities in RAR double mutants

Numerous congenital malformations have been observed in fetuses of vitamin A-deficient (VAD) dams [Wilson, J. G., Roth, C. B., Warkany, J., (1953), Am. J. Anat. 92, 189–217]. Previous studies of retinoic acid receptor (RAR) mutant mice have not revealed any of these malformations [Li, E., Sucov, H. M., Lee, K.-F., Evans, R. M., Jaenisch, R. (1993) Proc. Natl. Acad. Sci. USA 90, 1590–1594; Lohnes, D., Kastner, P., Dierich, A., Mark, M., LeMeur, M., Chambon, P. (1993) Cell 73, 643–658; Lufkin, T., Lohnes, D., Mark, M., Dierich, A., Gorry, P., Gaub, M. P., Lemeur, M., Chambon, P. (1993) Proc. Natl. Acad. Sci. USA 90, 7225–7229; Mendelsohn, C., Mark, M., Dolle, P., Dierich, A., Gaub, M.P., Krust, A., Lampron, C., Chambon, P. (1994a) Dev. Biol. in press], suggesting either that there is a considerable functional redundancy among members of the RAR family during ontogenesis or that the RARs are not essential transducers of the retinoid signal in vivo. In order to discriminate between these possibilities, we have generated a series of RAR compound null mutants. These RAR double mutants invariably died either in utero or shortly after birth and presented a number of congenital abnormalities, which are reported in this and in the accompanying study. We describe here multiple eye abnormalities which are found in various RAR double mutant fetuses and are similar to those previously seen in VAD fetuses. Interestingly, we found further abnormalities not previously reported in VAD fetuses.(ABSTRACT TRUNCATED AT 250 WORDS)

Hot spots of retinoic acid synthesis in the developing spinal cord

The embryonic spinal cord is known to be rich in retinoic acid, and several indirect lines of evidence point to a dorsoventral concentration difference of this compound. Previous measurements of dorsoventral retinoic acid levels, however, showed only minor differences. By a combination of microdissection and bioassay techniques, we compared retinoic acid levels with retinaldehyde dehydrogenase levels along spinal cords from early embryonic to postnatal mice. Both parameters vary in parallel, indicating that the principal reason for regional retinoic acid differences in the developing spinal cord is different levels of retinoic acid-generating enzyme. Consistent with previous reports, we observed overall quite high synthesis, decreasing with age, and no dorsoventral difference throughout much of the spinal cord length. In two locations, however, ventral synthesis exceeds dorsal synthesis by several orders of magnitude. These hot spots colocalize with the origins of the limb innervations. They are highest during early stages of limb innervation and disappear slowly postnatally. The synthesis hot spots are likely to create local retinoic acid diffusion halos, which may influence the survival of neurons in the limb regions of the spinal cord and which probably promote innervation of the developing limbs.