In vivo functional analysis of the Hoxa-1 3′ retinoic acid response element (3′RARE)

Hox genes in neural patterning and circuit formation in the mouse hindbrain

The mammalian hindbrain is the seat of regulation of several vital functions that involve many of the organ systems of the body. Such functions are controlled through the activity of intricate arrays of neuronal circuits and connections. The establishment of ordered patterns of neuronal specification, migration, and axonal topographic connectivity during development is crucial to build such a complex network of circuits and functional connectivity in the mature hindbrain. The early development of the vertebrate hindbrain proceeds according to a fundamental metameric partitioning along the anteroposterior axis into cellular compartments known as rhombomeres. Such an organization has been highly conserved in vertebrate evolution and has a fundamental impact on the hindbrain adult structure, nuclear organization, and connectivity. Here, we review the cellular and molecular mechanisms underlying hindbrain neuronal circuitry in the mouse, with a specific focus on the role of the homeodomain transcription factors of the Hox gene family. The Hox genes are crucial determinants of rhombomere segmental identity and anteroposterior patterning. However, recent findings suggest that, in addition to their well-known roles at early embryonic stages, the Hox genes may play important roles also in later aspect of neuronal circuit development, including stereotypic neuronal migration, axon pathfinding, and topographic mapping of connectivity.

Site-specific recombinases for manipulation of the mouse genome

Site-specific recombination systems are widespread and popular tools for all scientists interested in manipulating the mouse genome. In this chapter, we focus on the use of site-specific recombinases (SSR) to unravel the function of genes of the mouse. In the first part, we review the most commonly used SSR, Cre and Flp, as well as the newly developed systems such as Dre and PhiC31, and we present the inducible SSR systems. As experience has shown that these systems are not as straightforward as expected, particular attention is paid to facts and artefacts associated with their production and applications to study the mouse genome. In the next part of this chapter, we illustrate new applications of SSRs that allow engineering of the mouse genome with more and more precision, including the FLEX and the RMCE strategies. We conclude and suggest a workflow procedure that can be followed when using SSR to create your mouse model of interest. Together, these strategies and procedures provide the basis for a wide variety of studies that will ultimately lead to the analysis of the function of a gene at the cellular level in the mouse.

Inducing segmental aneuploid mosaicism in the mouse through targeted asymmetric sister chromatid event of recombination

Loss or gain of whole chromosomes, or parts of chromosomes, is found in various pathological conditions, such as cancer and aneuploidy, and results from the missegregation of chromosomes during cellular division or abnormal mitotic recombination. We introduce a novel strategy for determining the consequences of segmental aneuploid mosaicism, called targeted asymmetric sister chromatin event of recombination (TASCER). We took advantage of the Cre/loxP system, used extensively in embryonic stem cells for generating deletions and duplications of regions of interest, to induce recombination during the G2 phase. Using two loxP sites in a Cis configuration, we generated in vivo cells harboring microdeletions and microduplications for regions of interest covering up to 2.2 Mb. Using this approach in the mouse provides insight into the consequences of segmental aneuploidy for homologous regions of the human chromosome 21 on cell survival. Furthermore, TASCER shows that Cre-induced recombination is more efficient after DNA replication in vivo and provides an opportunity to evaluate, through genetic mosaics, the outcome of copy number variation and segmental aneuploidy in the mouse.

Genetic analysis of SH2D4A, a novel adapter protein related to T cell-specific adapter and adapter protein in lymphocytes of unknown function, reveals a redundant function in T cells

T cell-specific adapter (TSAd) protein and adapter protein in lymphocytes of unknown function (ALX) are two related Src homology 2 (SH2) domain-containing signaling adapter molecules that have both been shown to regulate TCR signal transduction in T cells. TSAd is required for normal TCR-induced synthesis of IL-2 and other cytokines in T cells and acts at least in part by promoting activation of the LCK protein tyrosine kinase at the outset of the TCR signaling cascade. By contrast, ALX functions as a negative-regulator of TCR-induced IL-2 synthesis through as yet undetermined mechanisms. In this study, we report a novel T cell-expressed adapter protein named SH2D4A that contains an SH2 domain that is highly homologous to the TSAd protein and ALX SH2 domains and that shares other structural features with these adapters. To examine the function of SH2D4A in T cells we produced SH2D4A-deficient mice by homologous recombination in embryonic stem cells. T cell development, homeostasis, proliferation, and function were all found to be normal in these mice. Furthermore, knockdown of SH2D4A expression in human T cells did not impact upon their function. We conclude that in contrast to TSAd and ALX proteins, SH2D4A is dispensable for TCR signal transduction in T cells.

In vivo functions of the proprotein convertase PC5/6 during mouse development: Gdf11 is a likely substrate

The proprotein convertase PC5/6 cleaves protein precursors after basic amino acids and is essential for implantation in CD1/129/Sv/C57BL/6 mixed-background mice. Conditional inactivation of Pcsk5 in the epiblast but not in the extraembryonic tissue bypassed early embryonic lethality but resulted in death at birth. PC5/6-deficient embryos exhibited Gdf11-related phenotypes such as altered anteroposterior patterning with extra vertebrae and lack of tail and kidney agenesis. They also exhibited Gdf11-independent phenotypes, such as a smaller size, multiple hemorrhages, collapsed alveoli, and retarded ossification. In situ hybridization revealed overlapping PC5/6 and Gdf11 mRNA expression patterns. In vitro and ex vivo analyses showed that the selectivity of PC5/6 for Gdf11 essentially resides in the presence of a P1' Asn in the RSRR downward arrowN cleavage motif. This work identifies Gdf11 as a likely in vivo specific substrate of PC5/6 and opens the way to the identification of other key substrates of this convertase.

Retinoic acid deficiency alters second heart field formation

Retinoic acid (RA), the active derivative of vitamin A, has been implicated in various steps of cardiovascular development. The retinaldehyde dehydrogenase 2 (RALDH2) enzyme catalyzes the second oxidative step in RA biosynthesis and its loss of function creates a severe embryonic RA deficiency. Raldh2(-/-) knockout embryos fail to undergo heart looping and have impaired atrial and sinus venosus development. To understand the mechanism(s) producing these changes, we examined the contribution of the second heart field (SHF) to pharyngeal mesoderm, atria, and outflow tract in Raldh2(-/-) embryos. RA deficiency alters SHF gene expression in two ways. First, Raldh2(-/-) embryos exhibited a posterior expansion of anterior markers of the SHF, including Tbx1, Fgf8, and the Mlc1v-nlacZ-24/Fgf10 reporter transgene as well as of Islet1. This occurred at early somite stages, when cardiac defects became irreversible in an avian vitamin A-deficiency model, indicating that endogenous RA is required to restrict the SHF posteriorly. Explant studies showed that this expanded progenitor population cannot differentiate properly. Second, RA up-regulated cardiac Bmp expression levels at the looping stage. The contribution of the SHF to both inflow and outflow poles was perturbed under RA deficiency, creating a disorganization of the heart tube. We also investigated genetic cross-talk between Nkx2.5 and RA signaling by generating double mutant mice. Strikingly, Nkx2.5 deficiency was able to rescue molecular defects in the posterior region of the Raldh2(-/-) mutant heart, in a gene dosage-dependent manner.

CYP26A1 knockout embryonic stem cells exhibit reduced differentiation and growth arrest in response to retinoic acid

CYP26A1, a cytochrome P450 enzyme, metabolizes all-trans-retinoic acid (RA) into polar metabolites, e.g. 4-oxo-RA and 4-OH-RA. To determine if altering RA metabolism affects embryonic stem (ES) cell differentiation, we disrupted both alleles of Cyp26a1 by homologous recombination. CYP26a1(-/-) ES cells had a 11.0+/-3.2-fold higher intracellular RA concentration than Wt ES cells after RA treatment for 48 h. RA-treated CYP26A1(-/-) ES cells exhibited 2-3 fold higher mRNA levels of Hoxa1, a primary RA target gene, than Wt ES cells. Despite increased intracellular RA levels, CYP26a1(-/-) ES cells were more resistant than Wt ES cells to RA-induced proliferation arrest. Transcripts for parietal endodermal differentiation markers, including laminin, J6(Hsp 47), and J31(SPARC, osteonectin) were expressed at lower levels in RA-treated CYP26a1(-/-) ES cells, indicating that the lack of CYP26A1 activity inhibits RA-associated differentiation. Microarray analyses revealed that RA-treated CYP26A1(-/-) ES cells exhibited lower mRNA levels than Wt ES cells for genes involved in differentiation, particularly in neural (Epha4, Pmp22, Nrp1, Gap43, Ndn) and smooth muscle differentiation (Madh3, Nrp1, Tagln Calponin, Caldesmon1). In contrast, genes involved in the stress response (e.g. Tlr2, Stk2, Fcgr2b, Bnip3, Pdk1) were expressed at higher levels in CYP26A1(-/-) than in Wt ES cells without RA. Collectively, our results show that CYP26A1 activity regulates intracellular RA levels, cell proliferation, transcriptional regulation of primary RA target genes, and ES cell differentiation to parietal endoderm.

Involvement of polycomb-group genes in establishing HoxD temporal colinearity

Temporal colinearity in mouse HoxD is dependent on repressive activity of sequences within the 5' end of the complex. We show that a 5-kb DNA fragment from this region represses transgenes when combined in mouse as well as in Drosophila melanogaster. Moreover, repressive activity in Drosophila depends on some members of the Polycomb-group (PcG) genes, for example, extra sex combs. We also showed direct association of these factors with the repressive fragment, both in transgenic flies and in the context of the native mouse HoxD complex. These results suggest that the global repressive region of the HoxD complex functions in two very different species and that some PcG genes are involved in establishing the early repressive state of the HoxD complex, thus contributing to temporal colinearity.

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

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

Apoptotic extinction of germ cells in testes of Cyp26b1 knockout mice

Cyp26b1 encodes a retinoic acid (RA) metabolizing cytochrome P450 enzyme that is expressed in embryonic tissues undergoing morphogenesis, including the testes. We have generated transgenic mice lacking Cyp26b1 and have observed increased RA levels in embryonic testes. Cyp26b1(-/-) germ cells prematurely enter meiosis at embryonic d 13.5 and appear to arrest at pachytene stage. Furthermore, after embryonic d 13.5, a rapid increase in apoptosis is observed in male germ cells derived from Cyp26b1(-/-) embryos; germ cells are essentially absent in mutant male neonates. In contrast, testicular somatic cells appear to develop normally in the absence of Cyp26b1. Moreover, ovarian germ and somatic cells appear unaffected by the lack of CYP26B1. We also show that the synthetic retinoid Am580, which is resistant to CYP26 metabolism, induces meiosis of male germ cells in cultured gonads, suggesting that abnormal development of germ cells in the Cyp26b1(-/-) testes results from excess RA rather than the absence of CYP26B1-generated metabolites of RA. These results provide evidence that CYP26B1 maintains low levels of RA in the developing testes that blocks entry into meiosis and acts as a survival factor to prevent apoptosis of male germ cells.

The TGF-beta pseudoreceptor gene Bambi is dispensable for mouse embryonic development and postnatal survival

The Bambi (Bmp and activin membrane-bound inhibitor) gene encodes a transmembrane protein highly similar in amino acid sequence to transforming growth factor-beta (TGF-beta receptors, however, the Bambi intracellular domain is short and lacks a serine/threonine-kinase domain that is essential for transducing TGF beta signaling. Previous biochemical assays showed that Bambi interacts directly with BMP receptors and antagonizes BMP signaling. Interestingly, the expression of Bambi largely overlaps, both temporally and spatially, with that of Bmp4 during early embryonic development in Xenopus, zebrafish, and mice, which led to the hypothesis that Bambi may function to regulate BMP signaling during embryogenesis. To directly analyze the roles of Bambi during embryonic development, we generated mice carrying a conditional allele of Bambi, Bambi(flox), with loxP sequences flanking the first exon that encodes the N-terminus and signal peptide region of the Bambi protein. Mice homozygous for this targeted conditional allele appear normal and fertile. We crossed the Bambi(flox)/+ mice to the EIIa-Cre transgenic mice and generated mice carrying deletion of the first exon of the Bambi gene. Surprisingly, mice homozygous for the deleted allele were viable, fertile and did not exhibit any discernible developmental defect. Our data exclude an essential role for Bambi in mouse embryonic development and postnatal survival.

Modeling the monosomy for the telomeric part of human chromosome 21 reveals haploinsufficient genes modulating the inflammatory and airway responses

Monosomy 21 is a rare human disease due to gene dosage errors disturbing a variety of physiological and morphological systems including brain, skeletal, immune and respiratory functions. Most of the human condition corresponds to partial or mosaic monosomy suggesting that Monosomy 21 may be lethal. In order to search for dosage-sensitive genes involved in the human pathology, we generated by chromosomal engineering a monosomic mouse for the Prmt2-Col6a1 interval corresponding to the most telomeric part of human chromosome 21. Haploinsufficiency of the 13 genes, located in the 0.5 Mb genetic interval and conserved in man and mouse, caused apparently no morphological defect as observed in patients. However, monosomic mice displayed an enhanced inflammatory response after local intranasal lipopolysaccharide administration with enhanced recruitment of neutrophils and secretion of cytokines such as tumor necrosis factor-alpha (TNF-alpha), IL-1beta, IL-12p70 and IFN-gamma in the lung as well increased TNF-alpha production after systemic administration. Further analysis demonstrates that monosomic macrophages were involved and that a few genes, Prmt2, Pcnt2, Mcm3ap and Lss located in the region were candidate for the inflammatory response. Altogether, these results demonstrate the existence of dosage-sensitive genes in the Prmt2-Col6a1 region that control the inflammation and the lung function. Furthermore, they point out that similar partial Monosomies 21 in human might have eluded the diagnosis due to the very specific defects observed in this murine model.

Cre/loxP-mediated chromosome engineering of the mouse genome

Together with numerous other genome modifications, chromosome engineering offers a very powerful tool to accelerate the functional analysis of the mammalian genome. The technology, based on the Cre/loxP system, is used more and more in the scientific community in order to generate new chromosomes carrying deletions, duplications, inversions and translocations in targeted regions of interest. In this review, we will present the basic principle of the technique either in vivo or in vitro and we will briefly describe some applications to provide highly valuable genetic tools, to decipher the mammalian genome organisation and to analyze human diseases in the mouse.

International Union of Pharmacology. LX. Retinoic acid receptors

Retinoid is a term for compounds that bind to and activate retinoic acid receptors (RARalpha, RARbeta, and RARgamma), members of the nuclear hormone receptor superfamily. The most important endogenous retinoid is all-trans-retinoic acid. Retinoids regulate a wide variety of essential biological processes, such as vertebrate embryonic morphogenesis and organogenesis, cell growth arrest, differentiation and apoptosis, and homeostasis, as well as their disorders. This review summarizes the considerable amount of knowledge generated on these receptors.

A mutation of spastin is responsible for swellings and impairment of transport in a region of axon characterized by changes in microtubule composition

Mutations of the spastin gene (Sp) are responsible for the most frequent autosomal dominant form of spastic paraplegia, a disease characterized by the degeneration of corticospinal tracts. We show that a deletion in the mouse Sp gene, generating a premature stop codon, is responsible for progressive axonal degeneration, restricted to the central nervous system, leading to a late and mild motor defect. The degenerative process is characterized by focal axonal swellings, associated with abnormal accumulation of organelles and cytoskeletal components. In culture, mutant cortical neurons showed normal viability and neurite density. However, they develop neurite swellings associated with focal impairment of retrograde transport. These defects occur near the growth cone, in a region characterized by the transition between stable microtubules rich in detyrosinated alpha-tubulin and dynamic microtubules composed almost exclusively of tyrosinated alpha-tubulin. Here, we show that the Sp mutation has a major impact on neurite maintenance and transport both in vivo and in vitro. These results highlight the link between spastin and microtubule dynamics in axons, but not in other neuronal compartments. In addition, it is the first description of a human neurodegenerative disease which involves this specialized region of the axon.

Expression of Hoxa2 in rhombomere 4 is regulated by a conserved cross-regulatory mechanism dependent upon Hoxb1

The Hoxa2 gene is an important component of regulatory events during hindbrain segmentation and head development in vertebrates. In this study we have used sequenced comparisons of the Hoxa2 locus from 12 vertebrate species in combination with detailed regulatory analyses in mouse and chicken embryos to characterize the mechanistic basis for the regulation of Hoxa2 in rhombomere (r) 4. A highly conserved region in the Hoxa2 intron functions as an r4 enhancer. In vitro binding studies demonstrate that within the conserved region three bipartite Hox/Pbx binding sites (PH1-PH3) in combination with a single binding site for Pbx-Prep/Meis (PM) heterodimers co-operate to regulate enhancer activity in r4. Mutational analysis reveals that these sites are required for activity of the enhancer, suggesting that the r4 enhancer from Hoxa2 functions in vivo as a Hox-response module in combination with the Hox cofactors, Pbx and Prep/Meis. Furthermore, this r4 enhancer is capable of mediating a response to ectopic HOXB1 expression in the hindbrain. These findings reveal that Hoxa2 is a target gene of Hoxb1 and permit us to develop a gene regulatory network for r4, whereby Hoxa2, along with Hoxb1, Hoxb2 and Hoxa1, is integrated into a series of auto- and cross-regulatory loops between Hox genes. These data highlight the important role played by direct cross-talk between Hox genes in regulating hindbrain patterning.

Modeling chromosomes in mouse to explore the function of genes, genomic disorders, and chromosomal organization

One of the challenges of genomic research after the completion of the human genome project is to assign a function to all the genes and to understand their interactions and organizations. Among the various techniques, the emergence of chromosome engineering tools with the aim to manipulate large genomic regions in the mouse model offers a powerful way to accelerate the discovery of gene functions and provides more mouse models to study normal and pathological developmental processes associated with aneuploidy. The combination of gene targeting in ES cells, recombinase technology, and other techniques makes it possible to generate new chromosomes carrying specific and defined deletions, duplications, inversions, and translocations that are accelerating functional analysis. This review presents the current status of chromosome engineering techniques and discusses the different applications as well as the implication of these new techniques in future research to better understand the function of chromosomal organization and structures.

Conditional (loxP-flanked) allele for the gene encoding the retinoic acid-synthesizing enzyme retinaldehyde dehydrogenase 2 (RALDH2)

Retinoic acid, the active vitamin A derivative, has pleiotropic functions during vertebrate development and postnatal life. Retinaldehyde dehydrogenase 2 (RALDH2) acts as the main retinoic acid-synthesizing enzyme during development. Mouse Raldh2 germline null mutants are early embryonic lethal and exhibit complex abnormalities that include defective heart looping morphogenesis. To investigate later functions of this enzyme, we have engineered a "floxed" (loxP-flanked) allele allowing Cre-mediated somatic gene inactivations. Mice heterozygous or homozygous for the floxed Raldh2 allele are viable and fertile. We tested whether the novel Raldh2 allele behaves as a null mutation after Cre-mediated in vivo excision by crossing the conditional mutants with CMV-Cre transgenic mice. An embryonic lethal phenotype indistinguishable from that of germline mutants was obtained. The conditional allele described herein is a genetic tool for studying tissue-specific, RALDH2-dependent functions of retinoic acid during development and in adult life.

Interplay between chromatin and trans-acting factors regulating the Hoxd4 promoter during neural differentiation

Correct patterning of the antero-posterior axis of the embryonic trunk is dependent on spatiotemporally restricted Hox gene expression. In this study, we identified components of the Hoxd4 P1 promoter directing expression in neurally differentiating retinoic acid-treated P19 cells. We mapped three nucleosomes that are subsequently remodeled into an open chromatin state upon retinoic acid-induced Hoxd4 transcription. These nucleosomes spanned the Hoxd4 transcriptional start site in addition to a GC-rich positive regulatory element located 3' to the initiation site. We further identified two major cis-acting regulatory elements. An autoregulatory element was shown to recruit HOXD4 and its cofactor PBX1 and to positively regulate Hoxd4 expression in differentiating P19 cells. Conversely, the Polycomb group (PcG) protein Ying-Yang 1 (YY1) binds to an internucleosomal linker and represses Hoxd4 transcription before and during transcriptional activation. Sequential chromatin immunoprecipitation studies revealed that the PcG protein MEL18 was co-recruited with YY1 only in undifferentiated P19 cells, suggesting a role for MEL18 in silencing Hoxd4 transcription in undifferentiated P19 cells. This study links for the first time local chromatin remodeling events that take place during transcriptional activation with the dynamics of transcription factor association and DNA accessibility at a Hox regulatory region.

Function of retinoid nuclear receptors: lessons from genetic and pharmacological dissections of the retinoic acid signaling pathway during mouse embryogenesis

Retinoic acid (RA) is involved in vertebrate morphogenesis, growth, cellular differentiation, and tissue homeostasis. The use of in vitro systems initially led to the identification of nuclear receptor RXR/RAR heterodimers as possible transducers of the RA signal. To unveil the physiological functions of RARs and RXRs, genetic and pharmacological studies have been performed in the mouse. Together, their results demonstrate that (a) RXR/RAR heterodimers in which RXR is either transcriptionally active or silent are involved in the transduction of the RA signal during prenatal development, (b) specific RXRalpha/RAR heterodimers are required at many distinct stages during early embryogenesis and organogenesis, (c) the physiological role of RA and its receptors cannot be extrapolated from teratogenesis studies using retinoids in excess. Additional cell type-restricted and temporally controlled somatic mutagenesis is required to determine the functions of RARs and RXRs during postnatal life.

Additional sex comb-like 1 (ASXL1), in cooperation with SRC-1, acts as a ligand-dependent coactivator for retinoic acid receptor

Additional sex comb-like 1 (ASXL1, 170 kDa), a mammalian homolog of Drosophila ASX, was identified as a protein that interacts with retinoic acid receptor (RAR) in the presence of retinoic acid (RA). Systematic binding assays showed that the C-terminal nuclear receptor box (LVMQLL) of ASXL1 and the activation function-2 activation domain (AF-2 AD) core of the RAR are critical for ligand-dependent interaction. The interaction was confirmed using in vitro glutathione S-transferase pulldown and in vivo immunoprecipitation (IP) assays. Confocal microscopy revealed that ASXL1 localizes in the nucleus. In addition to the intrinsic transactivation function of ASXL1, its cotransfection together with an RA-responsive luciferase reporter increased the RAR activity. This ASXL1 activity appears to be mediated through the functional cooperation with SRC-1, as shown by GST pulldown, IP, chromatin IP, and transcription assays. In the presence of ASXL1, more acetylated histone H3 was accumulated on the RA-responsive promoter in response to RA. Finally, stable expression of ASXL1 increased the expression of endogenous RA-regulated genes and enhanced the antiproliferative potential of RA. Overall, these results suggest that ASXL1 is a novel coactivator of RAR that cooperates with SRC-1 and implicates it as a potential antitumor target of RA in RA-resistant cancer cells.

Ubiquitous calpains promote caspase-12 and JNK activation during endoplasmic reticulum stress-induced apoptosis

Ubiquitously expressed mu- and m-calpain proteases are implicated in development and apoptosis. They consist of 80-kDa catalytic subunits encoded by the capn1 and capn2 genes, respectively, and a common 28-kDa regulatory subunit encoded by the capn4 gene. The regulatory subunit is required to maintain the stability and activity of mu- and m-calpains. Accordingly, genetic disruption of capn4 in the mouse eliminated both ubiquitous calpain activities. In embryonic fibroblasts derived from these mice, calpain deficiency correlated with resistance to endoplasmic reticulum (ER) stress-induced apoptosis, and this was directly related to a calpain requirement for activation of both caspase-12 and the ASK1-JNK cascade. This study provides compelling genetic evidence for calpain's role in caspase-12 activation at the ER, and reveals a novel role for the ubiquitous calpains in ER-stress induced apoptosis and JNK activation.

Deletion of the gene encoding proprotein convertase 5/6 causes early embryonic lethality in the mouse

PC5 belongs to the proprotein convertase family and activates precursor proteins by cleavage at basic sites during their transit through the secretory pathway and/or at the cell surface. These precursors include prohormones, proreceptors, growth factors, adhesion molecules, and viral glycoproteins. The Pcsk5 gene encodes two alternatively spliced isoforms, the soluble PC5A and transmembrane PC5B. We have carefully analyzed the expression of PC5 in the mouse during development and in adulthood by in situ hybridization, as well as in mouse tissues and various cell lines by quantitative reverse transcription-PCR. The data show that adrenal cortex and intestine are the richest sources of PC5A and PC5B, respectively. To better define the specific physiological roles of PC5, we have generated a mouse Pcsk5(Delta4)-deficient allele missing exon 4 that encodes the catalytic Asp173. While Delta4/+ heterozygotes were healthy and fertile, genotyping of progeny obtained from Delta4/+ interbreeding indicated that Delta4/Delta4 embryos died between embryonic days 4.5 and 7.5. These data demonstrate that Pcsk5 is an essential gene.

Retinoid signaling in inner ear development

The inner ear originates from an embryonic ectodermal placode and rapidly develops into a three-dimensional structure (the otocyst) through complex molecular and cellular interactions. Many genes and their products are involved in inner ear induction, organogenesis, and cell differentiation. Retinoic acid (RA) is an endogenous signaling molecule that may play a role during different phases of inner ear development, as shown from pathological observations. To gain insight into the function of RA during inner ear development, we have investigated the spatio-temporal expression patterns of major components of RA signaling pathway, including cellular retinoic acid binding proteins (CRABPs), cellular retinoid binding proteins (CRBPs), retinaldehyde dehydrogenases (RALDHs), catabolic enzymes (CYP26s), and nuclear receptors (RARs). Although the CrbpI, CrabpI, and -II genes are specifically expressed in the inner ear throughout development, loss-of-function studies have revealed that these proteins are dispensable for inner development and function. Several Raldh and Cyp26 gene transcripts are expressed at embryological day (E) 9.0-9.5 in the otocyst and show mainly complementary distributions in the otic epithelium and mesenchyme during following stages. From Western blot, RT-PCR, and in situ hybridization analysis, there is a low expression of Raldhs in the early otocyst at E9, while Cyp26s are strongly expressed. During the following days, there is an up-regulation of Raldhs and a down-regulation for Cyp26s. Specific RA receptor (Rar and Rxr) genes are expressed in the otocyst and during further development of the inner ear. At the otocyst stage, most of the components of the retinoid pathway are present, suggesting that the embryonic inner ear might act as an autocrine system, which is able to synthesize and metabolize RA necessary for its development. We propose a model in which two RA-dependent pathways may control inner ear ontogenesis: one indirect with RA from somitic mesoderm acting to regulate gene expression within the hindbrain neuroepithelium, and another with RA acting directly on the otocyst. Current evidence suggests that RA may regulate several genes involved in mesenchyme-epithelial interactions, thereby controlling inner ear morphogenesis. Our investigations suggest that RA signaling is a critical component not only of embryonic development, but also of postnatal maintenance of the inner ear.

Retinoic acid and hindbrain patterning

Retinoid signaling plays an important role in the developmental patterning of the hindbrain. Studies of the teratogenic effects of retinoids showed early on that the hindbrain suffered patterning defects in cases of retinoid excess or deficiency. Closer examination of these effects in animal models suggested that retinoids might play a physiological role in specifying the antero-posterior axis of the hindbrain. This idea was supported by the localization of retinoid synthetic and degradative enzymes, binding proteins, and receptors to the hindbrain and neighboring regions of the neuroepithelium and the mesoderm. In parallel, it became clear that the molecular patterning of the hindbrain, in terms of the regionalized expression of Hox genes and other developmental regulatory genes, is profoundly influenced by retinoid signaling.

Conditional disruption of ubiquitous calpains in the mouse

Ubiquitous mu- and m-calpain proteases are implicated in development and apoptosis. They are heterodimers consisting of 80-kDa catalytic subunits encoded by capn1 and capn2, respectively, and a common 28-kDa regulatory subunit encoded by capn4. The regulatory subunit is required to maintain stability and activity of mu- and m-calpains; thus, genetic disruption of capn4 was predicted to eliminate both calpain activities. Germline disruption of capn4 caused embryonic lethality, hampering the use of those mouse models to explore physiological calpain functions. Here we describe a loxP/cre conditional capn4 targeted mouse model that enables tissue-specific and temporal deletion of calpain activity. Disruption of the floxed capn4 gene using a ubiquitous cytomegalovirus promoter driven Cre recombinase transgene led to midgestation embryonic lethality. Fibroblasts from these embryos lacked detectable regulatory subunit expression, had reduced levels of the mu- and m-calpain catalytic subunits, and had no detectable mu- and m-calpain activities. These defects were corrected with a capn4-encoding lentivirus.

Conserved RARE localization in amphioxusHox clusters and implications forHox code evolution in the vertebrate neural crest

The Hox code in the neural crest cells plays an important role in the development of the complex craniofacial structures that are characteristic of vertebrates. Previously, 3' AmphiHox1 flanking region has been shown to drive gene expression in neural tubes and neural crest cells in a retinoic acid (RA)-dependent manner. In the present study, we found that the DR5-type RA response elements located at the 3' AmphiHox1 flanking region of Branchiostoma floridae are necessary and sufficient to express reporter genes in both the neural tube and neural crest cells of chick embryos, specifically at the post-otic level. The DR5 at the 3' flanking region of chick Hoxb1 is also capable of driving the same expression in chick embryos. We found that AmphiHox3 possesses a DR5-type RARE in its 5' flanking region, and this drives an expression pattern similar to the RARE element found in the 3' flanking region of AmphiHox1. Therefore, the location of these DR5-type RAREs is conserved in amphioxus and vertebrate Hox clusters. Our findings demonstrate that conserved RAREs mediate RA-dependent regulation of Hox genes in amphioxus and vertebrates, and in vertebrates this drives expression of Hox genes in both neural crest and neural tube. This suggests that Hox expression in vertebrate neural crest cells has evolved via the co-option of a pre-existing regulatory pathway that primitively regulated neural tube (and possibly epidermal) Hox expression.

Generation of a conditionally null allele of the laminin alpha1 gene

Laminins are heterotrimeric glycoproteins of the basement membranes. Laminin 1 (alpha1, beta1, gamma1) is the major laminin expressed during early mouse embryogenesis. To gain access to the physiological function of laminin alpha1 chain, we developed a conditionally null allele of its encoding gene (Lama1) using the cre/loxP system. Floxed-allele-carrying mice (Lama1(flox/flox)) display no overt phenotype. Lama1(flox/flox) mice were crossed with transgenic deleter mice (CMV-Cre) to generate Lama1-deficient mice (Lama1(Delta/Delta)). Lama1(Delta/Delta) embryos die during the early postimplantation period after embryonic day 6.5. They lack Reichert's membrane, an extraembryonic basement membrane in which laminin alpha1 is normally highly expressed. In parallel, Lama1(Delta/Delta) embryos display 1) parietal and visceral endoderm differentiation defects with altered expression of cytokeratin 19 and GATA4, respectively, and 2) an induction of apoptosis. This new mouse model is of particular interest as it will allow time- and tissue-specific inactivation of the Lama1 gene in various organs.

Expression and insights on function of potassium channel TWIK-1 in mouse kidney

Renal distribution and function of TWIK-1, a member of the two-pore-domain potassium channel family, was studied in mouse kidney. TWIK-1 is expressed in apical and subapical localizations of proximal tubule and cytoplasmic sites of thin and thick ascending limbs, distal convoluted tubules and medullary collecting duct. Studies in mice lacking intact TWIK-1 (twik-1 -/-) and wild-type mice (twik-1 +/+) revealed an attenuated ability to increase renal phosphate (Pi) reabsorption and stabilize plasma Pi concentration in response to a low Pi diet in twik-1 -/- mice. Western blot analysis and immunohistochemistry for the electrogenic 3Na(+)-1HPO(4) (2-)-cotransporter NaPi-2a revealed a reduced reno-cortical expression in twik-1 -/- mice under these conditions. Under normal diet, twik-1 -/- mice presented lower urinary flow rates. Acute pharmacologic blockade of the vasopressin V(2)-receptor revealed both an attenuated diuretic response and an attenuated internalization of aquaporin-2 in the inner medullary collecting duct in twik-1 -/- versus +/+ mice. In summary, mice deficient for TWIK-1 presented impaired regulation of (i) Pi transport in proximal tubule and (ii) water transport in medullary collecting duct. TWIK-1 may contribute to membrane trafficking/expression of transport molecules in proximal tubule and medullary collecting duct, and possibly other renal sites of expression.

Hox genes and their candidate downstream targets in the developing central nervous system

1. Homeobox (Hox) genes were originally discovered in the fruit fly Drosophila, where they function through a conserved homeodomain as transcriptional regulators to control embryonic morphogenesis. Since then over 1000 homeodomain proteins have been identified in several species. In vertebrates, 39 Hox genes have been identified as homologs of the original Drosophila complex, and like their Drosophila counterparts they are organized within chromosomal clusters. Vertebrate Hox genes have also been shown to play a critical role in embryonic development as transcriptional regulators. 2. Both the Drosophila and vertebrate Hox genes have been shown to interact with various cofactors, such as the TALE homeodomain proteins, in recognition of consensus sequences within regulatory elements of their target genes. These protein-protein interactions are believed to contribute to enhancing the specificity of target gene recognition in a cell-type or tissue- dependent manner. The regulatory activity of a particular Hox protein on a specific regulatory element is highly variable and dependent on its interacting partners within the transcriptional complex. 3. In vertebrates, Hox genes display spatially restricted patterns of expression within the developing CNS, both along the anterioposterior and dorsoventral axis of the embryo. Their restricted gene expression is suggestive of a regulatory role in patterning of the CNS, as well as in cell specification. Determining the precise function of individual Hox genes in CNS morphogenesis through classical mutational analyses is complicated due to functional redundancy between Hox genes. 4. Understanding the precise mechanisms through which Hox genes mediate embryonic morphogenesis requires the identification of their downstream target genes. Although Hox genes have been implicated in the regulation of several pathways, few target genes have been shown to be under their direct regulatory control. Development of methodologies used for the isolation of target genes and for the analysis of putative targets will be beneficial in establishing the genetic pathways controlled by Hox factors. 5. Within the developing CNS various cell adhesion molecules and signaling molecules have been identified as candidate downstream target genes of Hox proteins. These targets play a role in processes such as cell migration and differentiation, and are implicated in contributing to neuronal processes such as plasticity and/or specification. Hence, Hox genes not only play a role in patterning of the CNS during early development, but may also contribute to cell specification and identity.

Early developmental arrest of mammalian limbs lacking HoxA/HoxD gene function

Vertebrate HoxA and HoxD cluster genes are required for proper limb development. However, early lethality, compensation and redundancy have made a full assessment of their function difficult. Here we describe mice that are lacking all Hoxa and Hoxd functions in their forelimbs. We show that such limbs are arrested early in their developmental patterning and display severe truncations of distal elements, partly owing to the absence of Sonic hedgehog expression. These results indicate that the evolutionary recruitment of Hox gene function into growing appendages might have been crucial in implementing hedgehog signalling, subsequently leading to the distal extension of tetrapod appendages. Accordingly, these mutant limbs may be reminiscent of an ancestral trunk extension, related to that proposed for arthropods.

Regulation of the human tissue-nonspecific alkaline phosphatase gene expression by all-trans-retinoic acid in SaOS-2 osteosarcoma cell line

While tissue-nonspecific alkaline phosphatase (TNSALP) is a well-known indicator of bone formation and all-trans-retinoic acid a key regulator of that process, the relationship between TNSALP and retinoic acid has not yet been clearly described. The aim of the present study was therefore to clarify the mechanism by which retinoic acid modulates expression of TNSALP. After culturing SaOS-2 human osteoblastic osteosarcoma cells in the presence or absence of 10(-6) M all-trans-retinoic acid, real-time RT-PCR confirmed that retinoic acid up-regulates expression of TNSALP mRNA. Notably, this time-dependent induction of TNSALP expression was accompanied by a corresponding increase in detected catalytic activity of the enzyme. When we then isolated the 5'-upstream region of the human TNSALP gene and carried out luciferase assays with a set of deletion mutants, we found that the promoter region contains a retinoic acid response element-like motif. Moreover, electrophoretic mobility shift assays showed that the nuclear extract bound to the motif. It thus appears that retinoic acid regulates the expression of human TNSALP via a retinoic acid response element in the genes promoter region.

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.

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.

IRAG is essential for relaxation of receptor-triggered smooth muscle contraction by cGMP kinase

Signalling by cGMP-dependent protein kinase type I (cGKI) relaxes various smooth muscles modulating thereby vascular tone and gastrointestinal motility. cGKI-dependent relaxation is possibly mediated by phosphorylation of the inositol 1,4,5-trisphosphate receptor I (IP(3)RI)-associated protein (IRAG), which decreases hormone-induced IP(3)-dependent Ca(2+) release. We show now that the targeted deletion of exon 12 of IRAG coding for the N-terminus of the coiled-coil domain disrupted in vivo the IRAG-IP(3)RI interaction and resulted in hypomorphic IRAG(Delta12/Delta12) mice. These mice had a dilated gastrointestinal tract and a disturbed gastrointestinal motility. Carbachol- and phenylephrine-contracted smooth muscle strips from colon and aorta, respectively, of IRAG(Delta12/Delta12) mice were not relaxed by cGMP, while cAMP-mediated relaxation was unperturbed. Norepinephrine-induced increases in [Ca(2+)](i) were not decreased by cGMP in aortic smooth muscle cells from IRAG(Delta12/Delta12) mice. In contrast, cGMP-induced relaxation of potassium-induced smooth muscle contraction was not abolished in IRAG(Delta12/Delta12) mice. We conclude that cGMP-dependent relaxation of hormone receptor-triggered smooth muscle contraction essentially depends on the interaction of cGKI-IRAG with IP(3)RI.

Sequential histone modifications at Hoxd4 regulatory regions distinguish anterior from posterior embryonic compartments

Hox genes are differentially expressed along the embryonic anteroposterior axis. We used chromatin immunoprecipitation to detect chromatin changes at the Hoxd4 locus during neurogenesis in P19 cells and embryonic day 8.0 (E8.0) and E10.5 mouse embryos. During Hoxd4 induction in both systems, we observed that histone modifications typical of transcriptionally active chromatin occurred first at the 3' neural enhancer and then at the promoter. Moreover, the sequential distribution of histone modifications between E8.0 and E10.5 was consistent with a spreading of open chromatin, starting with the enhancer, followed by successively more 5' intervening sequences, and culminating at the promoter. Neither RNA polymerase II (Pol II) nor CBP associated with the inactive gene. During Hoxd4 induction, CBP and RNA Pol II were recruited first to the enhancer and then to the promoter. Whereas the CBP association was transient, RNA Pol II remained associated with both regulatory regions. Histone modification and transcription factor recruitment occurred in posterior, Hox-expressing embryonic tissues, but never in anterior tissues, where such genes are inactive. Together, our observations demonstrate that the direction of histone modifications at Hoxd4 mirrors colinear gene activation across Hox clusters and that the establishment of anterior and posterior compartments is accompanied by the imposition of distinct chromatin states.

Nuclear retinoid receptors and the transcription of retinoid-target genes

The pleiotropic effects of retinoids are mediated by nuclear retinoid receptors (RARs and RXRs) which are ligand-activated transcription factors. In response to retinoid binding, RAR/RXR heterodimers undergo major conformational changes and orchestrate the transcription of specific gene networks, through binding to specific DNA response elements and recruiting cofactor complexes that act to modify local chromatin structure and/or engage the basal transcription machinery. Then the degradation of RARs and RXRs by the ubiquitin-proteasome controls the magnitude and the duration of the retinoid response. RARs and RXRs also integrate a variety of signaling pathways through phosphorylation events which cooperate with the ligand for the control of retinoid-target genes transcription. These different modes of regulation reveal unexpected levels of complexity in the dynamics of retinoid-dependent transcription.

Establishment of embryonic neuroepithelial cell lines exhibiting an epiplastic expression pattern of region specific markers

Neuroepithelial b2T cells were derived from the hindbrain and the spinal cord of mouse transgenic embryos, which expressed SV40 T antigen under the control of a Hoxb2 enhancer. Strikingly, b2T cell lines of either origin exhibit a very similar gene expression pattern, including markers of the hindbrain and the spinal cord, such as Hox genes, but not of more anterior cephalic regions. In addition, the broad expression pattern of b2T cells, probably linked to culture conditions, appeared to be appropriately modulated when the cells were reimplanted at different longitudinal levels into chick host embryos, suggesting that these cells are responsive to exogenous signalling mechanisms. Further support for these allegations was obtained by culturing b2T cells in defined medium and by assessing the expression of Krox20, an odd-numbered rhombomere marker, which appeared to be modulated by a complex interplay between FGF, retinoic acid (RA), and noggin. With respect to these as yet unique properties, b2T cells constitute an original alternative tool to in vivo models for the analysis of molecular pathways involved in the patterning of the neural tube.

Retinoid receptors and vitamin A deficiency: differential patterns of transcription during early avian development and the rapid induction of RARs by retinoic acid

The functional links of specific retinoid receptors to early developmental events in the avian embryo are not known. Before such studies are undertaken, knowledge is required of the spatiotemporal expression patterns of the receptor genes and their regulation by endogenous retinoic acid levels during the early stages of development. Here, we report the expression patterns of mRNAs for RARalpha, RARalpha2, RARbeta2, RARgamma, RARgamma2, RXRalpha, and RARgamma from neurulation to HH10 in the normal and vitamin A-deficient (VAD) quail embryo. The transcripts for all retinoid receptors are detectable at HH5, except for RXRgamma, which is detected at the beginning of HH6. At the 4/5 somite stage of HH8, when retinoid signaling is initiated in the avian embryo, mRNAs of all receptors are present, with very strong and ubiquitous expression patterns for RARalpha, RARalpha2, RARgamma, RARgamma2, and RXRalpha, a persistent expression of RARgamma in the neural tissues, a strong expression of RARbeta2 in lateral plate mesoderm and somites, and an anterior expression of RXRgamma. All retinoid receptors are expressed in the heart primordia. In the VAD quail embryo, the general pattern of retinoid receptor transcript localization is similar to that of the normal, except that the expression of RARalpha2 and RARbeta2 is severely diminished. Administration of retinol or retinoic acid to VAD embryos at or before the 4/5 somite stage rescues the expression of RARalpha2 and RARbeta2 within approximately 45 min and restores normal development. RARbeta2 expression requires the expression of RARalpha2. After neurulation, the expression of all retinoid receptors in the VAD quail embryo becomes independent of vitamin A status and is similar to that of the normal. The mRNA levels and sites of expression of the key enzyme for retinoic acid biosynthesis, Raldh-2, are not affected by vitamin A status; the expression pattern is restricted and does not correspond to that of retinoid receptors at all sites. The general patterns and intensity of retinoid receptor gene expression during early quail development are comparable to those of the mammalian and thus validate the application of results from retinoid-regulated avian development studies to those of the mammalian.

TAF10 (TAF(II)30) is necessary for TFIID stability and early embryogenesis in mice

TAF10 (formerly TAF(II)30), is a component of TFIID and the TATA box-binding protein (TBP)-free TAF-containing complexes (TFTC/PCAF/STAGA). To investigate the physiological function of TAF10, we disrupted its gene in mice by using a Cre recombinase/LoxP strategy. Interestingly, no TAF10(-/-) animals were born from intercrosses of TAF10(+/-) mice, indicating that TAF10 is required for embryogenesis. TAF10(-/-) embryos developed to the blastocyst stage, implanted, but died shortly after ca. 5.5 days postcoitus. Surprisingly, trophoblast cells from TAF10(-/-) blastocysts were viable, whereas inner cell mass cells failed to survive, highlighting that TAF10 is not generally required for transcription in all cells. TAF10-deficient cells express normal levels of TBP and TAFs other than TAF10 but contain only partially formed TFIID, are endocycle arrested, and have undetectable levels of transcription. Thus, our results demonstrate that TAF10 is required for TFIID stability, cell cycle progression, and transcription in the early mouse embryo.

The role of a retinoic acid response element in establishing the anterior neural expression border of Hoxd4 transgenes

The zebrafish hoxd4a locus was compared to its murine ortholog, Hoxd4. The sequence of regulatory elements, including a DR5 type retinoic acid response element (RARE) required for Hoxd4 neural enhancer activity, are highly conserved. Additionally, zebrafish and mouse neural enhancers function identically in transgenic mouse embryos. We tested whether sequence conservation reflects functional importance by altering the spacing and sequence of the RARE in the Hoxd4 neural enhancer. Stabilizing receptor-DNA interactions did not anteriorize transgene expression. By contrast, conversion of the RARE from a DR5 to a DR2 type element decreased receptor-DNA stability and posteriorized expression. Hence, the setting of the Hox anterior expression border is not a simple function of the affinity of retinoid receptors for their cognate element.

Temporally controlled site-specific mutagenesis in the germ cell lineage of the mouse testis

We have obtained a PrP-Cre-ER(T) transgenic mouse line (28.8) that selectively expresses in testis the tamoxifen-inducible Cre-ER(T) recombinase under the control of a mouse Prion protein (PrP) promoter-containing genomic fragment. Cre-ER(T) is expressed in spermatogonia and spermatocytes, but not in Sertoli and Leydig cells. We also established reporter PrP-L-EGFP-L transgenic mice harboring a LoxP-flanked enhanced green fluorescent protein (EGFP) Cre reporter cassette under the control of the same PrP promoter-containing genomic fragment that exhibits prominent EGFP expression in brain and testis. Using the PrP-L-EGFP-L as well as other Cre-reporter mice, we demonstrate that tamoxifen administration efficiently and selectively induces Cre-mediated recombination in the germ cell lineage. The established PrP-Cre-ER(T) line should provide a valuable tool for studying functions of germ cell-expressed genes involved in spermatogenesis.

Methylation of HoxA5 and HoxB5 and its relevance to expression during mouse development

Expression and function of homeobox genes (Hox genes) in development have been subject to extensive study in a variety of organisms including mammals, however practically nothing is known regarding the methylation patterns of these genes. Here we describe the methylation patterns of HoxA5 and HoxB5 in various tissues of fetal and adult mice and their relevance to expression. Both genes exhibit tissue specific methylation patterns that are established postnatally. This methylation appears to play a role in stabilizing the newly acquired silent state of the genes. In contrast to the postimplantation wave of de novo methylation that takes place across the mammalian genome, the methylation of the Hox genes represents a different time window for de novo methylation which might be characteristic of developmental genes. In the case of HoxA5 this postnatal de novo methylation can cover a domain of at least 25 kb that includes several genes of the HoxA cluster and the CpG islands within. Our observations suggest that the establishment of tissue specific methylation patterns of HoxA5 and HoxB5 and the relationship between these methylation patterns and activity are different from what had been known for non-developmental genes. This may reflect the specialized functions played by Hox genes in development.

The role of vitamin A in mammalian reproduction and embryonic development

Since the late 1980s, there has been an explosion of information on the molecular mechanisms and functions of vitamin A. This review focuses on the essential role of vitamin A in female reproduction and embryonic development and the metabolism of vitamin A (retinol) that results in these functions. Evidence strongly supports that in situ-generated all-trans retinoic acid (atRA) is the functional form of vitamin A in female reproduction and embryonic development. This is supported by the ability to reverse most reproductive and developmental blocks found in vitamin A deficiency with atRA, the block in embryonic development that occurs in retinaldehyde dehydrogenase type 2 null mutant mice, and the essential roles of the retinoic acid receptors, at least in embryogenesis. Early studies of embryos from marginally vitamin A-deficient (VAD) pregnant rats revealed a collection of defects called the vitamin A-deficiency syndrome. The manipulation of all-trans retinoic acid (atRA) levels in the diet of VAD female rats undergoing a reproduction cycle has proved to be an important new tool in deciphering the points of atRA function in early embryos and has provided a means to generate large numbers of embryos at later stages of development with the vitamin A-deficiency syndrome. The essentiality of the retinoid receptors in mediating the activity of atRA is exemplified by the many compound null mutant embryos that now recapitulate both the original vitamin A-deficiency syndrome and exhibit a host of new defects, many of which can also be observed in the VAD-atRA-supported rat embryo model and in retinaldehyde dehydrogenase type 2 (RALDH2) mutant mice. A major task for the future is to elucidate the atRA-dependent pathways that are normally operational in vitamin A-sufficient animals and that are perturbed in deficiency, thus leading to the characteristic VAD phenotypes described above.