Dax1 is required for testis determination

The orphan nuclear receptor, Dax1, was originally proposed to act as an 'anti-testis' factor. We find, however, that Nr0b1 (also called Dax1 and Ahch, which encodes Dax1) is in fact required for testis differentiation.

Genomic and proteomic analysis of the myeloid differentiation program: global analysis of gene expression during induced differentiation in the MPRO cell line

We have used an approach using 2-dimensional gel electrophoresis with mass spectrometry analysis combined with oligonucleotide chip hybridization for a comprehensive and quantitative study of the temporal patterns of protein and mRNA expression during myeloid development in the MPRO murine cell line. This global analysis detected 123 known proteins and 29 "new" proteins out of 220 protein spots identified by tandem mass spectroscopy, including proteins in 12 functional categories such as transcription factors and cytokines. Bioinformatic analysis of these proteins revealed clusters with functional importance to myeloid differentiation. Previous analyses have found that for a substantial number of genes the absolute amount of protein in the cell is not strongly correlated to the amount of mRNA. These conclusions were based on simultaneous measurement of mRNA and protein at just a single time point. Here, however, we are able to investigate the relationship between mRNA and protein in terms of simultaneous changes in their levels over multiple time points. This is the first time such a relationship has been studied, and we find that it gives a much stronger correlation, consistent with the hypothesis that a substantial proportion of protein change is a consequence of changed mRNA levels, rather than posttranscriptional effects. Cycloheximide inhibition also showed that most of the proteins detected by gel electrophoresis were relatively stable. Specific investigation of transcription factor mRNA representation showed considerable similarity to those of mature human neutrophils and highlighted several transcription factors and other functional nuclear proteins whose mRNA levels change prominently during MPRO differentiation but which have not been investigated previously in the context of myeloid development. Data are available online at http://bioinfo.mbb.yale.edu/expression/myelopoiesis.

Liver X receptors in the central nervous system: from lipid homeostasis to neuronal degeneration

Liver X receptors (LXRalpha and -beta) are nuclear receptors abundant in the liver where they are regulators of lipid homeostasis. Both LXRs are also expressed in the brain, but their roles in this tissue remain to be clarified. We examined the brains of mice in which the genes of both LXRalpha and -beta have been disrupted and found several severe abnormalities. One of the most striking features is that the lateral ventricles are closed and lined with lipid-laden cells. In addition, there are enlarged brain blood vessels, especially in the pars reticularis of the substantia nigra and in the globus pallidus. Other features of the brains are excessive lipid deposits, proliferation of astrocytes, loss of neurons, and disorganized myelin sheaths. Electron micrographs revealed that, as mice aged, lipid vacuoles accumulated in astrocytes surrounding blood vessels. Comparison of mRNA profiles in LXR knockout mice and wild-type littermates showed that expression of several LXR target genes involved in cholesterol efflux from astrocytes was reduced. These findings show that LXRs have an important function in lipid homeostasis in the brain, and that loss of these receptors results in neurodegenerative diseases. Further characterization of the role of LXRs in the brain could lead to new insights into the etiology and treatment of some neurodegenerative disorders.

Normal fate and altered function of the cardiac neural crest cell lineage in retinoic acid receptor mutant embryos

Mouse embryos lacking the retinoic acid (RA) receptors RARalpha1 and RARbeta suffer from a failure to properly septate (divide) the early outflow tract of the heart into distinct aortic and pulmonary channels, a phenotype termed persistent truncus arteriosus. This phenotype is associated with a failure in the development of the cardiac neural crest cell lineage, which normally forms the aorticopulmonary septum. In this study, we examined the fate of the neural crest lineage in RARalpha1/RARbeta mutant embryos by crossing with the Wnt1-cre and conditional R26R alleles, which together constitute a genetic lineage marker for the neural crest. We find that the number, migration, and terminal fate of the cardiac neural crest is normal in mutant embryos; however, the specific function of these cells in forming the aorticopulmonary septum is impaired. We furthermore show that the neural crest cells themselves do not utilize retinoid receptors and do not respond to RA during this process, but rather that the phenotype is cell non-autonomous for the neural crest cell lineage. This suggests that an alternative tissue in the vicinity of the outflow tract of the heart responds directly to RA, and thereby induces or permits the neural crest cell lineage to initiate aorticopulmonary septation.

Accumulation of foam cells in liver X receptor-deficient mice

BACKGROUND

The nature of some of the target genes for liver X receptors (LXRs)-alpha and -beta, such as sterol regulatory element binding protein-1 and ATP-binding cassette transporter proteins, suggests a pivotal role of these nuclear receptors in the regulation of fatty acid and cholesterol homeostasis. The present study aimed to elucidate the physiological relevance of both LXRs with regard to lipid metabolism and macrophage cholesterol efflux.

METHODS AND RESULTS

Mice depleted for LXRalpha, LXRbeta, or both were fed low-fat rodent chow for 18 months before investigations. The combined deficiency of LXRalpha and LXRbeta was linked to impaired triglyceride metabolism, increased LDL and reduced HDL cholesterol levels, and cholesterol accumulation in macrophages (foam cells) of the spleen, lung, and arterial wall.

CONCLUSIONS

Our data demonstrate the physiological importance of both LXRs in lipid metabolism and strongly indicate that both LXRs have a protective role against the development of atherosclerosis.

Physiological and retinoid-induced proliferations of epidermis basal keratinocytes are differently controlled

To investigate the roles of retinoic acid (RA) receptors (RARs) in the physiology of epidermis that does not express RAR beta, conditional spatio-temporally controlled somatic mutagenesis was used to selectively ablate RAR alpha in keratinocytes of RAR gamma-null mice. Keratinocyte proliferation was maintained in adult mouse epidermis lacking both RAR alpha and RAR gamma, as well as in RAR beta-null mice. All RAR-mediated signalling pathways are therefore dispensable in epidermis for homeostatic keratinocyte renewal. However, topical treatment of mouse skin with selective retinoids indicated that RXR/RAR gamma heterodimers, in which RXR transcriptional activity was subordinated to that of its RAR gamma partner, were required for retinoid-induced epidermal hyperplasia, whereas RXR homodimers and RXR/RAR alpha heterodimers were not involved. RA-induced keratinocyte proliferation was studied in mutant mice in which RXR alpha, RXR alpha and RAR alpha, RAR gamma, or RXR alpha and RAR gamma genes were specifically disrupted in either basal or suprabasal keratinocytes. We demonstrate that the topical retinoid signal is transduced by RXR alpha/RAR gamma heterodimers in suprabasal keratinocytes, which, in turn, stimulate proliferation of basal keratinocytes via a paracrine signal that may be heparin-binding EGF-like growth factor.

X-linked adrenal hypoplasia congenita is caused by abnormal nuclear localization of the DAX-1 protein

Mutations in the DAX-1 [dosage-sensitive sex reversal-adrenal hypoplasia congenita (AHC) critical region on the X chromosome; NR0B1] gene cause X-linked AHC associated with hypogonadotropic hypogonadism. DAX-1 encodes an unusual orphan member of the nuclear hormone receptor superfamily, acting as a transcriptional repressor of genes involved in the steroidogenic pathway. All DAX-1 mutations found in AHC patients alter the protein C terminus, which shares similarity to the ligand binding domain of nuclear hormone receptors and bears transcriptional repressor activity. This property is invariably impaired in DAX-1 AHC mutants. Here we show that the localization of DAX-1 AHC mutant proteins is drastically shifted toward the cytoplasm, even if their nuclear localization signal, which resides in the N terminal of the protein, is intact. Cytoplasmic localization of DAX-1 AHC mutants correlates with an impairment in their transcriptional repression activity. These results reveal a critical role of an intact C terminus in determining DAX-1 subcellular localization and constitute an important example of a defect in human organogenesis caused by impaired nuclear localization of a transcription factor.

Cutting edge: organogenesis of nasal-associated lymphoid tissue (NALT) occurs independently of lymphotoxin-alpha (LT alpha) and retinoic acid receptor-related orphan receptor-gamma, but the organization of NALT is LT alpha dependent

Peyer's patch and nasal-associated lymphoid tissue (NALT) are mucosal lymphoid tissues that appear similar in structure and function. Surprisingly, we found that NALT, unlike Peyer's patch, was formed independently of lymphotoxin (LT)alpha. Furthermore, using mice deficient in the retinoic acid receptor-related orphan receptor-gamma, we found that NALT was formed in the absence of CD4+CD3- cells, which are thought to be the embryonic source of LTalpha. However, we also found that NALT of LTalpha-/- animals was disorganized and lymphopenic, suggesting that the organization and recruitment of lymphocytes within NALT remained dependent on LTalpha. Finally, we demonstrated that both the structure and function of NALT were restored in LTalpha-/- animals upon reconstitution with normal bone marrow. These results demonstrate that the organogenesis of NALT occurs through unique mechanisms.

Elevated transforming growth factor β2 enhances apoptosis and contributes to abnormal outflow tract and aortic sac development in retinoic X receptor α knockout embryos

Septation of the single tubular embryonic outflow tract into two outlet segments in the heart requires the precise integration of proliferation, differentiation and apoptosis during remodeling. Lack of proper coordination between these processes would result in a variety of congenital cardiac defects such as those seen in the retinoid X receptor α knockout (Rxra–/–) mouse. Rxra–/– embryos exhibit lethality between embryonic day (E) 13.5 and 15.5 and harbor a variety of conotruncal and aortic sac defects making it an excellent system to investigate the molecular and morphogenic causes of these cardiac malformations. At E12.5, before the embryonic lethality, we found no qualitative difference between wild type and Rxra–/– proliferation (BrdU incorporation) in outflow tract cushion tissue but a significant increase in apoptosis as assessed by both TUNEL labeling in paraffin sections and caspase activity in trypsin-dispersed hearts. Additionally, E12.5 embryos demonstrated elevated levels of transforming growth factor β2 (TGFβ2) protein in multiple cell lineages in the heart. Using a whole-mouse-embryo culture system, wild-type E11.5 embryos treated with TGFβ2 protein for 24 hours displayed enhanced apoptosis in both the sinistroventralconal cushion and dextrodorsalconal cushion in a manner analogous to that observed in the Rxra–/–. TGFβ2 protein treatment also led to malformations in both the outflow tract and aortic sac. Importantly, Rxra–/– embryos that were heterozygous for a null mutation in the Tgfb2 allele exhibited a partial restoration of the elevated apoptosis and of the malformations. This was evident at both E12.5 and E13.5. The data suggests that elevated levels of TGFβ2 can (1) contribute to abnormal outflow tract morphogenesis by enhancing apoptosis in the endocardial cushions and (2) promote aortic sac malformations by interfering with the normal development of the aorticopulmonary septum.

The role of hepatocyte RXR alpha in xenobiotic-sensing nuclear receptor-mediated pathways

Nuclear receptors constitutive androstane receptor (CAR) and pregnane X receptor (PXR) cross talk and serve as xenobiotic sensors to form a safety net against the toxic effects of harmful substances. Retinoid x receptor alpha (RXRalpha) dimerizes with CAR and PXR. In order to analyze the role of RXRalpha in these xeno-sensor-mediated pathways, hepatocyte RXRalpha-deficient mice were challenged by CAR and PXR ligands including androstanol, 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), and pregnenolone 16alpha-carbonitrile (PCN). We demonstrate that hepatocyte RXRalpha deficiency prevents TCPOBOP-induced hepatomegaly and morphological changes. We also show that in vivo the cytochrome P450 (CYP) genes including CYP2A5, CYP2B10, CYP3A1, but not CYP2E1 and CYP2D6, are the RXRalpha target genes. Androstanol, TCPOBOP, and PCN can differentially regulate the expression of these CYP450 genes. In addition, the most active peroxisome proliferator activated receptor (PPARalpha) ligand, Wy14,643, also regulates some of the xeno-sensor target genes such as CYP2A5 and CYP2B10 in vivo. Thus, the ligands of different nuclear receptors can regulate common CYP450 genes and hepatocyte RXRalpha is essential for xenobiotic metabolism in vivo.

Oxysterol stimulation of epidermal differentiation is mediated by liver X receptor-beta in murine epidermis

Liver X receptor-alpha and -beta are members of the nuclear hormone receptor superfamily that heterodimerize with retinoid X receptor and are activated by oxysterols. In recent studies we found that treatment of cultured human keratinocytes with oxysterolstimulated differentiation, as demonstrated by increased expression of involucrin and transglutaminase, and inhibited proliferation. The aims of this study were to determine: (i) whether oxysterols applied topically to the skin of mice induce differentiation in normal epidermis; (ii) whether this effect is mediated via liver X receptor-alpha and/or liver X receptor-beta; and (iii) whether oxysterols normalize epidermal morphology in an animal model of epidermal hyperplasia. Topical treatment of normal hairless mice with 22(R)-hydroxycholesterol or 24(S),25-epoxycholesterol resulted in a decrease in epidermal thickness and a decrease in keratinocyte proliferation assayed by proliferating cell nuclear antigen staining. Moreover, oxysterol treatment increased the levels of involucrin, loricrin, and profilaggrin protein and mRNA in the epidermis, indicating that oxysterols stimulate epidermal differentiation. Additionally, topical oxysterol pretreatment improved permeability barrier homeostasis. Whereas liver X receptor-alpha-/- mice revealed no alterations in epidermal differentiation, the epidermis was thinner in liver X receptor-beta-/- mice than in wild-type mice, with a reduced number of proliferating cell nuclear antigen positive cells and a modest reduction in the expression of differentiation markers. Topical oxysterol treatment induced differentiation in liver X receptor-alpha-/- mice whereas in liver X receptor-beta-/- mice there was no increase in the expression of differentiation markers. Whereas both liver X receptor-alpha and liver X receptor-beta are expressed in cultured human keratinocytes and in fetal rat skin, only liver X receptor-beta was observed on northern blotting in adult mouse epidermis. Finally, treatment of hyperproliferative epidermis with oxysterols restored epidermal homeostasis. These studies demonstrate that epidermal differentiation is regulated by liver X receptor-beta and that oxysterols, acting via liver X receptor-beta, can induce differentiation and inhibit proliferation in vivo. The ability of oxysterols to reverse epidermal hyperplasia suggests that these agents could be beneficial for the treatment of skin disorders associated with hyperproliferation and/or altered differentiation.

Aberrant growth plate development in VDR/RXR gamma double null mutant mice

VDR forms heterodimers with one of three RXRs, RXR alpha, RXR beta, and RXR gamma, and it is thought that RXR ligands can also modulate the trans-activation function of VDR/RXR heterodimers. In the present study we generated VDR/RXR gamma double null mutant mice to examine the convergent actions of vitamin D and vitamin A signaling and to explore the possibility of a functionally redundant VDR. Although RXR gamma(-/-) mice exhibited no overt abnormalities, VDR(-/-)/RXR gamma(-/-) mice appeared similar to VDR(-/-) mice, showing features typical of vitamin D-dependent rickets type II, including growth retardation, impaired bone formation, hypocalcemia, and alopecia. However, compared to VDR(-/-) mice, growth plate development in VDR(-/-)/RXR gamma(-/-) mutant mice was more severely impaired. Normalizing mineral ion homeostasis through dietary supplementation with high calcium and phosphorous effectively prevented rachitic abnormalities, except for disarranged growth plates in VDR(-/-)/RXR gamma(-/-) mutant mice, and alopecia in both VDR(-/-) and VDR(-/-)/RXR gamma(-/-) mutant mice. Histological analysis of VDR(-/-)/RXR gamma(-/-) growth plates revealed that development of the hypertrophic chondrocytes was selectively impaired. Thus, our findings indicated that the combined actions of VDR- and RXR gamma-mediated signals are essential for the normal development of growth plate chondrocytes, and raised the possibility that a functionally redundant VDR is present on chondrocytes as a heterodimer with RXR gamma.

Function of RARalpha during the maturation of neutrophils

The retinoic acid receptor alpha gene is the target of chromosomal rearrangements in all cases of acute promyelocytic leukemia (APL). This recurrent involvement of RARalpha in the pathogenesis of APL is likely to reflect an important role played by this receptor during the differentiation of immature myeloid cells to neutrophils. RARalpha is a negative regulator of promyelocyte differentiation when not complexed with RA, and stimulates this differentiation when bound to RA. Since RARs are dispensable for the generation of mature neutrophils, their role thus appears to be to modulatory, rather than obligatory, for the control of neutrophil differentiation. In vitro, retinoic acid is also a potent inducer of neutrophil cell fate, suggesting that it might play a role in the commitment of pluripotent hematopoietic progenitors to the neutrophil lineage. Thus, the APL translocations target an important regulator of myeloid cell differentiation.

Blockage of the rete testis and efferent ductules by ectopic Sertoli and Leydig cells causes infertility in Dax1-deficient male mice

DAX-1, an X-linked member of the orphan nuclear receptor superfamily of transcription factors, plays a key role in sex determination and gonadal differentiation. Dax1-deficient male mice are infertile and have small testes despite normal serum levels of T and gonadotropins. Examination of Dax1-deficient testes reveals dilated seminiferous tubules and abnormal parameters of sperm fertilizing capability consistent with a possible obstruction in the testis. To test this hypothesis, we performed a comprehensive evaluation of the male reproductive tract in Dax1-deficient mice. Light and electron microscopic examination revealed the rete testis is blocked by aberrantly located Sertoli cells, creating a tailback of necrosing sperm in the testis. Sertoli cells also obstruct the proximal and middle efferent ductules, and this is accompanied by an overgrowth of the efferent duct epithelium. Seminiferous tubules close to the rete testis contain ectopic Leydig cells, distinct from the hyperplastic Leydig cells present in the interstitial space. The peritubular tissue surrounding these tubules is frequently abnormal, containing relatively undifferentiated myoid cells and no basement membrane between the myoid cells and Sertoli cells. A third of aged (>1-yr-old) Dax1-deficient male mice develop sex cord-stromal tumors, derived from cells of the Sertoli/granulosa cell or Leydig cell lineages. Combined, these observations reveal abnormal differentiation and proliferation of Leydig cells and Sertoli cells in Dax1-deficient male mice, leading to obstruction of the rete testis and infertility.

F9 embryocarcinoma cells: a cell autonomous model to study the functional selectivity of RARs and RXRs in retinoid signaling

Mouse F9 embryocarcinoma (EC) cells constitute a well established cell-autonomous model system for investigating retinoid signaling in vitro as, depending on culture conditions, retinoic acid (RA) can induce their differentiation into either primitive, parietal or visceral extraembryonic endoderm-like cells. These RA-induced differentiations are accompanied by decreases in proliferation rates, modifications of expression of subsets of RA-target genes, and induction of apoptosis. To elucidate the roles played by the multiple retinoid receptors (RARs and RXRs) in response to RA treatments, F9 EC cells lacking one or several RARs or RXRs were engineered through homologous recombination. Mutated RARs and/or RXRs were then reexpressed in given RAR or RXR null backgrounds. WT and mutant cells were also treated with different combinations of ligands selective for RXRs and/or for each of the three RAR isotypes. These studies lead to the conclusion that most RA-induced events (e.g. primitive and visceral differentiation, growth arrest, apoptosis and activation of expression of a number of genes) are transduced by RARgamma/RXRalpha heterodimers, whereas some other events (e.g. parietal differentiation) are mediated by RARalpha/RXRalpha. heterodimers. They also demonstrate that both AF-1 and AF-2 activation functions of RARs and RXRs, as well as their phosphorylation, are differentially required in these RA-induced events. In RARgamma/RXRalpha heterodimers, the phosphorylation of RARgamma is necessary for triggering primitive differentiation, while that of RXRalpha is required for growth arrest. On the other hand, phosphorylation of RARalpha is necessary for parietal differentiation. Thus, retinoid receptors are sophisticated signal integrators that transduce not only the effects of their cognate ligands, but also those of ligands that bind to membrane receptors.

Sertoli cell-specific rescue of fertility, but not testicular pathology, in Dax1 (Ahch)-deficient male mice

DAX1 is an orphan member of the nuclear hormone receptor superfamily of transcription factors. Our recent characterization of Dax1 (Ahch)-deficient male mice revealed a primary testicular defect resulting in hypogonadism and sterility. The progressive degeneration of the germinal epithelium, independent of abnormal gonadotropin and testosterone production, suggested an intrinsic loss of Dax1 function in the Sertoli cells. To test this hypothesis, we assessed the effect of Sertoli cell-specific expression of a human DAX1 (AHC) transgene driven using the promoter of the Müllerian inhibiting substance (MIS) gene. The MIS-DAX1 transgene partially rescued the mutant phenotype of the Dax1-deficient male mice. Although testicular morphology remained abnormal, fertility was restored to levels matching that of wild-type littermates. Examination of several markers of sperm fertilizing capability revealed significant improvements in MIS-DAX1-rescued mice. Epididymal sperm count and sperm motility were greater in 12-week-old rescued mice than in age-matched Dax1-deficient mice. The ability of sperm to undergo an immediate acrosome reaction was impaired in Dax1-deficient animals, and sperm from Dax1-deficient mice fertilized only 8.2 +/- 6.8% of eggs in vitro, significantly less than rescue (67.8 +/- 19.1%) and wild-type (88.9 +/- 3.9%) sperm. These results indicate that Dax1 expression in Sertoli cells is adequate to overcome crucial thresholds related to sperm production and function. However, the failure to completely rescue the testicular pathology of Dax1-deficient mice suggests that Dax1 expression in other somatic cells is essential for normal testicular development.

A developmental transition in definitive erythropoiesis: erythropoietin expression is sequentially regulated by retinoic acid receptors and HNF4

The cytokine erythropoietin (Epo) promotes erythropoietic progenitor cell proliferation and is required for erythropoietic differentiation. We have found that the Epo gene is a direct transcriptional target gene of retinoic acid signaling during early erythropoiesis (prior to embryonic day E12.5) in the fetal liver. Mouse embryos lacking the retinoic acid receptor gene RXR alpha have a morphological and histological phenotype that is comparable with embryos in which the Epo gene itself has been mutated, and flow cytometric analysis indicates that RXR alpha-deficient embryos are deficient in erythroid differentiation. Epo mRNA levels are reduced substantially in the fetal livers of RXR alpha(-/-) embryos at E10.25 and E11.25, and genetic analysis shows that the RXR alpha and Epo genes are coupled in the same pathway. We furthermore show that the Epo gene is retinoic acid inducible in embryos, and that the Epo gene enhancer contains a DR2 sequence that represents a retinoic acid receptor-binding site and a retinoic acid receptor transcriptional response element. However, unlike Epo-deficient embryos that die from anemia, the erythropoietic deficiency in RXR alpha(-/-) embryos is transient; Epo mRNA is expressed at normal levels by E12.5, and erythropoiesis and liver morphology are normal by E14.5. We show that HNF4, like RXR alpha a member of the nuclear receptor family, is abundantly expressed in fetal liver hepatocytes, and is competitive with retinoic acid receptors for occupancy of the Epo gene enhancer DR2 element. We propose that Epo expression is regulated during the E9.5--E11.5 phase of fetal liver erythropoiesis by RXR alpha and retinoic acid, and that expression then becomes dominated by HNF4 activity from E11.5 onward. This transition may be responsible for switching regulation of Epo expression from retinoic acid control to hypoxic control, as is found throughout the remainder of life.

Impaired adipogenesis and lipolysis in the mouse upon selective ablation of the retinoid X receptor mediated by a tamoxifen-inducible chimeric Cre recombinase (Cre-ERT2) in adipocytes

Retinoid X receptor alpha (RXRalpha) is involved in multiple signaling pathways, as a heterodimeric partner of several nuclear receptors. To investigate its function in energy homeostasis, we have selectively ablated the RXRalpha gene in adipocytes of 4-week-old transgenic mice by using the tamoxifen-inducible Cre-ERT2 recombination system. Mice lacking RXRalpha in adipocytes were resistant to dietary and chemically induced obesity and impaired in fasting-induced lipolysis. Our results also indicate that RXRalpha is involved in adipocyte differentiation. Thus, our data demonstrate the feasibility of adipocyte-selective temporally controlled gene engineering and reveal a central role of RXRalpha in adipogenesis, probably as a heterodimeric partner for peroxisome proliferator-activated receptor gamma.

Peroxisome proliferator-activated receptor alpha-mediated pathways are altered in hepatocyte-specific retinoid X receptor alpha-deficient mice

Retinoid x receptor alpha (RXRalpha) serves as an active partner of peroxisome proliferator-activated receptor (PPARalpha). In order to dissect the functional role of RXRalpha and PPARalpha in PPARalpha-mediated pathways, the hepatocyte RXRalpha-deficient mice have been challenged with physiological and pharmacological stresses, fasting and Wy14,643, respectively. The data demonstrate that RXRalpha and PPARalpha deficiency are different in several aspects. At the basal untreated level, RXRalpha deficiency resulted in marked induction of apolipoprotein A-I and C-III (apoA-I and apoC-III) mRNA levels and serum cholesterol and triglyceride levels, which was not found in PPARalpha-null mice. Fasting-induced PPARalpha activation was drastically prevented in the absence of hepatocyte RXRalpha. Wy14,643-mediated pleiotropic effects were also altered due to the absence of hepatocyte RXRalpha. Hepatocyte RXRalpha deficiency did not change the basal acyl-CoA oxidase, medium chain acyl-CoA dehydrogenase, and malic enzyme mRNA levels. However, the inducibility of those genes by Wy14,643 was markedly reduced in the mutant mouse livers. In contrast, the basal cytochrome P450 4A1, liver fatty acid-binding protein, and apoA-I and apoC-III mRNA levels were significantly altered in the mutant mouse livers, but the regulatory effect of Wy14,643 on expression of those genes remained the same. Wy14,643-induced hepatomegaly was partially inhibited in hepatocyte RXRalpha-deficient mice. Wy14,643-induced hepatocyte peroxisome proliferation was preserved in the absence of hepatocyte RXRalpha. These data suggested that in comparison to PPARalpha, hepatocyte RXRalpha has its unique role in lipid homeostasis and that the effect of RXRalpha, -beta, and -gamma is redundant in certain aspects.

A molecular basis for retinoic acid-induced axial truncation

Dietary deprivation and gene disruption studies clearly demonstrate that biologically active retinoids, such as retinoic acid (RA), are essential for numerous developmental programs. Similar ontogenic processes are also affected by retinoic acid excess, suggesting that the effects of retinoid administration reflect normal retinoid-dependent events. In the mouse, exogenous retinoic acid can induce both anterior (anencephaly, exencephaly) and posterior (spina bifida) neural tube defects depending on the developmental stage of treatment. Retinoic acid receptor gamma (RARgamma) mediates these effects on the caudal neural tube at 8.5 days postcoitum, as RARgamma-/- mice are completely resistant to spina bifida induced by retinoic acid at this stage. We therefore used this null mouse as a model to examine the molecular nature of retinoid-induced caudal neural tube defects by using a panel of informative markers and comparing their expression between retinoic acid-treated wild-type and RARgamma-/- embryos. Our findings indicate that treatment of wild-type embryos led to a rapid and significant decrease in the caudal expression of all mesodermal markers examined (e.g., brachyury, wnt-3a, cdx-4), whereas somite, neuroepithelial, notochord, floorplate, and hindgut markers were unaffected. RARgamma-/- mutants exhibited normal expression patterns for all markers examined, consistent with the notion that mesodermal defects underlie the etiology of retinoid-induced spina bifida. We also found that posterior somitic, but not caudal presomitic, embryonic tissues contained detectable bioactive retinoids, an observation which correlated with the ability of caudal explants to rapidly clear exogenous RA. Interestingly, transcripts encoding mP450RAI, a cytochrome P450, the product of which is believed to catabolize retinoic acid, were abundant in the retinoid-poor region of the caudal embryo. mP450RAI was rapidly induced by retinoic acid treatment in vivo, consistent with previous studies suggesting that it plays a critical role in retinoid signaling. These data suggest that nascent mesoderm is highly sensitive to retinoic acid and that mP450RAI serves to tightly regulate retinoid levels in the caudal embryo. These findings also raise the possibility that RA may play a role in the generation of posterior mesoderm derivatives in part by affecting brachyury expression.

An essential role for retinoid receptors RARbeta and RXRgamma in long-term potentiation and depression

Hippocampal long-term potentiation (LTP) and long-term depression (LTD) are the most widely studied forms of synaptic plasticity thought to underlie spatial learning and memory. We report here that RARbeta deficiency in mice virtually eliminates hippocampal CA1 LTP and LTD. It also results in substantial performance deficits in spatial learning and memory tasks. Surprisingly, RXRgamma null mice exhibit a distinct phenotype in which LTD is lost whereas LTP is normal. Thus, while retinoid receptors contribute to both LTP and LTD, they do so in different ways. These findings not only genetically uncouple LTP and LTD but also reveal a novel and unexpected role for vitamin A in higher cognitive functions.

Downregulation of atrial markers during cardiac chamber morphogenesis is irreversible in murine embryos

Vertebrate cardiogenesis is a complex process involving multiple, distinct tissue types which interact to form a four-chambered heart. Molecules have been identified whose expression patterns co-segregate with the maturation of the atrial and ventricular muscle cell lineages. It is not currently known what role intrinsic events versus external influences play in cardiac chamber morphogenesis. We developed novel, fluorescent-based, myocardial, cellular transplantation systems in order to study these questions in murine embryos and report the irreversible nature of chamber specification with respect to the downregulation of atrial myosin light chain 2 (MLC-2a) and alpha myosin heavy chain (alpha-MHC). Grafting ventricular cells into the atrial chamber does not result in upregulation of MLC-2a expression in ventricular cells. Additionally, wild-type ventricular muscle cells grafted into the wild-type background appropriately downregulate MLC-2a and alpha-MHC. Finally, grafting of RXRalpha gene-deficient ventricular muscle cells into the ventricular chambers of wild-type embryos does not rescue the persistent expression of MLC-2a, providing further evidence that ventricular chamber maturation is an early event. These studies provide a new approach for the mechanistic dissection of critical signaling events during cardiac chamber growth, maturation and morphogenesis in the mouse, and should find utility with other approaches of cellular transplantation in murine embryos. These experiments document the irreversible nature of the downregulation of atrial markers after the onset of cardiogenesis during ventricular chamber morphogenesis and temporally define the response of cardiac muscle cells to signals regulating chamber specification.

Defects of the chorioallantoic placenta in mouse RXRalpha null fetuses

The active derivatives of vitamin A (the retinoids) play important and multiple roles in mammalian development and homeostasis. We have previously shown that specific retinoic acid receptors are expressed in the chorioallantoic placenta of the mouse and that among these, RXRalpha is strongly expressed in the developing labyrinthine zone (Sapin, V., Ward, S. J., Bronner, S., Chambon, P., Dollé, P., Dev. Dyn. 208, 199-210, 1997). Here, we show that mouse fetuses with a targeted disruption of the RXRalpha gene develop defects of the chorioallantoic placenta. Both morphological abnormalities and alterations in the expression of molecular markers were found, mostly confined to the labyrinthine zone of placentas from mid-late gestation mutants. This region exhibited edema, abnormal stasis of maternal blood, and signs of disruption of the endothelial layer of fetal vessels. We also detected a reduction in the number of lipid droplets in the trophoblastic layer and abnormal fibrin deposits in the junctional zone of the mutant placentas. These abnormalities most probably result in an impairment of the functional capacities of exchange between the maternal and fetal circulations in the mutant placentas. Thus, placental defects could represent an extraembryonic cause of lethality for RXRalpha null mutant fetuses, in addition to the previously described embryonic cardiac defects.