Postaxial polydactyly in forelimbs of CRABP-II mutant mice

Identification of cellular retinoic acid binding protein 2 (CRABP2) as downstream target of nuclear factor I/X (NFIX): implications for skeletal dysplasia syndromes

Nuclear factor I/X (NFIX) mutations are associated with 2 skeletal dysplasias, Marshall-Smith (MSS) and Malan (MAL) syndromes. NFIX encodes a transcription factor that regulates expression of genes, including Bobby sox (BBX) and glial fibrillary acidic protein (GFAP) in neural progenitor cells and astrocytes, respectively. To elucidate the role of NFIX mutations in MSS, we studied their effects in fibroblast cell lines obtained from 5 MSS unrelated patients and 3 unaffected individuals. The 5 MSS NFIX frameshift mutations in exons 6-8 comprised 3 deletions (c.819-732_1079-948del, c.819-471_1079-687del, c.819-592_1079-808del), an insertion (c.1037_1038insT), and a duplication (c.1090dupG). Quantitative reverse transcription polymerase chain reaction (qRT-PCR) and western blot analyses using MSS and unrelated control fibroblasts and in vitro expression studies in monkey kidney fibroblast (COS-7) cells showed that frameshift mutations in NFIX exons 6-8 generated mutant transcripts that were not cleared by nonsense-mediated-decay mechanisms and encoded truncated NFIX proteins. Moreover, BBX or GFAP expression was unaffected in the majority of MSS fibroblasts. To identify novel NFIX downstream target genes, RNA sequencing and proteomics analyses were performed on mouse embryonic fibroblast (MEF) cells derived from control Nfix+/+, Nfix+/Del2, Nfix+/Del24, NfixDel24/Del24, Nfix+/Del140, and NfixDel140/Del140 mice, compared with NfixDel2/Del2 mice which had developmental, skeletal, and neural abnormalities. This identified 191 transcripts and 815 proteins misregulated in NfixDel2/Del2 MEFs with ≥2-fold-change (P <0 .05). Validation studies using qRT-PCR and western blot analyses confirmed that 2 genes, cellular retinoic acid binding protein 2 (Crabp2) and vascular cell adhesion molecule 1 (Vcam1), were misregulated at the RNA and protein levels in NfixDel2/Del2 MEFs, and that CRABP2 and VCAM1 expressions were altered in 60%-100% of MSS fibroblast cells. Furthermore, in vitro luciferase reporter assays confirmed that NFIX directly regulates CRABP2 promoter activity. Thus, these altered genes and pathways may represent possible targets for drugs as potential treatments and therapies for MSS.

A regulatory circuit controlled by extranuclear and nuclear retinoic acid receptor α determines T cell activation and function

Ligation of retinoic acid receptor alpha (RARα) by RA promotes varied transcriptional programs associated with immune activation and tolerance, but genetic deletion approaches suggest the impact of RARα on TCR signaling. Here, we examined whether RARα would exert roles beyond transcriptional regulation. Specific deletion of the nuclear isoform of RARα revealed an RARα isoform in the cytoplasm of T cells. Extranuclear RARα was rapidly phosphorylated upon TCR stimulation and recruited to the TCR signalosome. RA interfered with extranuclear RARα signaling, causing suboptimal TCR activation while enhancing FOXP3+ regulatory T cell conversion. TCR activation induced the expression of CRABP2, which translocates RA to the nucleus. Deletion of Crabp2 led to increased RA in the cytoplasm and interfered with signalosome-RARα, resulting in impaired anti-pathogen immunity and suppressed autoimmune disease. Our findings underscore the significance of subcellular RA/RARα signaling in T cells and identify extranuclear RARα as a component of the TCR signalosome and a determinant of immune responses.

Plasma CRABP2 as a Novel Biomarker in Patients with Non-Small Cell Lung Cancer

BACKGROUND

Lung cancer is the most common cause of cancer-related mortality worldwide. We previously reported the identification of a new genetic marker, cellular retinoic acid binding protein 2 (CRABP2), in lung cancer tissues. The aim of this study was to assess plasma levels of CRABP2 from patients with non-small cell lung cancer (NSCLC).

METHODS

Blood samples that were collected from 122 patients with NSCLC between September 2009 and September 2013 were selected for the analysis, along with samples from age- (± 5 years), sex-, and cigarette smoking history (± 10 pack-years [PY])-matched controls from the Korea Biobank Network. The control specimens were from patients who were without malignancies or pulmonary diseases. We measured plasma levels of CRABP2 using commercially available enzyme-linked immunosorbent assay kits.

RESULTS

The mean age of the NSCLC patients was 71.8 ± 8.9 years, and the median cigarette smoking history was 32 PY (range, 0-150 PY). Plasma CRABP2 levels were significantly higher in patients with NSCLC than in the matched controls (37.63 ± 28.71 ng/mL vs. 24.09 ± 21.09 ng/mL, P < 0.001). Higher plasma CRABP2 levels were also correlated with lower survival rates in NSCLC patients (P = 0.014).

CONCLUSION

Plasma CRABP2 levels might be a novel diagnostic and prognostic marker in NSCLC.

Intracellular transport of fat-soluble vitamins A and E

Vitamins are compounds that are essential for the normal growth, reproduction and functioning of the human body. Of the 13 known vitamins, vitamins A, D, E and K are lipophilic compounds and are therefore called fat-soluble vitamins. Because of their lipophilicity, fat-soluble vitamins are solubilized and transported by intracellular carrier proteins to exert their actions and to be metabolized properly. Vitamin A and its derivatives, collectively called retinoids, are solubilized by intracellular retinoid-binding proteins such as cellular retinol-binding protein (CRBP), cellular retinoic acid-binding protein (CRABP) and cellular retinal-binding protein (CRALBP). These proteins act as chaperones that regulate the metabolism, signaling and transport of retinoids. CRALBP-mediated intracellular retinoid transport is essential for vision in human. α-Tocopherol, the main form of vitamin E found in the body, is transported by α-tocopherol transfer protein (α-TTP) in hepatic cells. Defects of α-TTP cause vitamin E deficiency and neurological disorders in humans. Recently, it has been shown that the interaction of α-TTP with phosphoinositides plays a critical role in the intracellular transport of α-tocopherol and is associated with familial vitamin E deficiency. In this review, we summarize the mechanisms and biological significance of the intracellular transport of vitamins A and E.

Retinoid pathway and cancer therapeutics

The retinoids are a class of compounds that are structurally related to vitamin A. Retinoic acid, which is the active metabolite of retinol, regulates a wide range of biological processes including development, differentiation, proliferation, and apoptosis. Retinoids exert their effects through a variety of binding proteins including cellular retinol-binding protein (CRBP), retinol-binding proteins (RBP), cellular retinoic acid-binding protein (CRABP), and nuclear receptors i.e. retinoic acid receptor (RAR) and retinoid x receptor (RXR). Because of the pleiotropic effects of retinoids, understanding the function of these binding proteins and nuclear receptors assists us in developing compounds that have specific effects. This review summarizes our current understanding of how retinoids are processed and act with an emphasis on the application of retinoids in cancer treatment and prevention.

How degrading: Cyp26s in hindbrain development

The vitamin A derivative retinoic acid performs many functions in vertebrate development and is thought to act as a diffusible morphogen that patterns the anterior-posterior axis of the hindbrain. Recent work in several systems has led to insights into how the spatial distribution of retinoic acid is regulated. These have shown local control of synthesis and degradation, and computational models suggest that degradation by the Cyp26 enzymes plays a critical role in the formation of a morphogen gradient as well as its ability to compensate for fluctuations in RA levels.

Vitamin A and infancy. Biochemical, functional, and clinical aspects

Vitamin A is a very intriguing natural compound. The molecule not only has a complex array of physiological functions, but also represents the precursor of promising and powerful new pharmacological agents. Although several aspects of human retinol metabolism, including absorption and tissue delivery, have been clarified, the type and amounts of vitamin A derivatives that are intracellularly produced remain quite elusive. In addition, their precise function and targets still need to be identified. Retinoic acids, undoubtedly, play a major role in explaining activities of retinol, but, recently, a large number of physiological functions have been attributed to different retinoids and to vitamin A itself. One of the primary roles this vitamin plays is in embryogenesis. Almost all steps in organogenesis are controlled by retinoic acids, thus suggesting that retinol is necessary for proper development of embryonic tissues. These considerations point to the dramatic importance of a sufficient intake of vitamin A and explain the consequences if intake of retinol is deficient. However, hypervitaminosis A also has a number of remarkable negative consequences, which, in same cases, could be fatal. Thus, the use of large doses of retinol in the treatment of some human diseases and the use of megavitamin therapy for certain chronic disorders as well as the growing tendency toward vitamin faddism should alert physicians to the possibility of vitamin overdose.

A new locus for postaxial polydactyly type A/B on chromosome 7q21-q34

Postaxial polydactyly (PAP) is the occurrence of one or more extra ulnar or fibular digits or parts of it. In PAP-A, the extra digit is fully developed and articulates with the fifth or an additional metacarpal/metatarsal, while it is rudimentary in PAP-B. Isolated PAP usually segregates as an autosomal dominant trait, with variable expression. Three loci are known for PAP in humans. PAPA1 (including PAP-A/B in one patient) on 7p13 caused by mutations in the GLI3 gene, PAPA2 on 13q21-q32 in a Turkish kindred with PAP-A only, and a third one (PAPA3) in a Chinese family with PAP-A/B on 19p13.1-13.2. We identified a fourth locus in a large Dutch six-generation family with 31 individuals including 11 affecteds. Their phenotype varied from either PAP-A, or PAP-B to PAP-A/B with or without the co-occurence of partial cutaneous syndactyly. We performed a whole-genome search and found linkage between PAP and markers on chromosome 7q. The highest LOD score was 3.34 obtained at D7S1799 and D7S500 with multipoint analysis.

Homozygous deletion of the CRABPI gene in AB1 embryonic stem cells results in increased CRABPII gene expression and decreased intracellular retinoic acid concentration

The cellular retinoic acid (RA) binding proteins I and II (CRABPI and CRABPII), intracellular proteins which bind retinoic acid with high affinity, are involved in the actions of RA, though their exact roles are not fully understood. We have generated several genetically engineered AB1 cell lines in which both alleles of the CRABPI gene have been deleted by homologous recombination. We have used these CRABPI knockout cell lines to examine the consequences of functional loss of CRABPI on RA-induced gene expression and RA metabolism in the murine embryonic stem cell line, AB1, which undergoes differentiation in response to RA. Complete lack of CRABPI results in decreased intracellular [3H]RA concentrations under conditions in which external concentrations of [3H]RA are low (1-10nM) and in an altered distribution of [3H] polar metabolites of [3H]RA in the cell and in the medium. Fewer [3H] polar metabolites are retained within the CRABPI(-/-) cells compared to the wild-type cells. These data suggest that CRABPI functions to regulate the intracellular concentrations of retinoic acid and to maintain high levels of oxidized retinoic acid metabolites such as 4-oxoretinoic acid within cells.

Transient modulation of cytoplasmic and nuclear retinoid receptors expression in differentiating human teratocarcinoma NT2 cells

Human embryonal carcinoma Ntera2/D1 (NT2) cells treated with retinoic acid (RA) differentiate into several cell types including post-mitotic neurons. In this study we asked if RA-induced differentiation alters the expression of RA and retinol (ROL) binding proteins. The regulation of the intracellular carrier proteins for ROL and RA, cellular retinol binding protein I (CRBP-I), and cellular retinoic acid binding protein I and II (CRABP-I, CRABP-II) were studied along with the nuclear RA receptors RARalpha, RARbeta and RARgamma2. PCR analysis of total mRNA from RA-treated cells showed a biphasic early induction of CRBP-I, CRABP-II, and RARgamma2 genes. The immediate early gene Krox-24, a zinc finger transcription factor which is up-regulated during neuronal differentiation, was also induced, but after 1 week of treatment. The induction of CRBP-I protein synthesis in differentiating NT2 cells was confirmed by western blotting and immunofluorescence experiments. Conversely, the synthetic retinoid N-(4-hydroxyphenyl)retinamide, which induces cell death, but not differentiation in different tumour cell types, did not produce the same modulation on gene expression in NT2 cells. These data suggest that the RA-specific induction of CRBP-I and CRABP-II could be an early event in the process leading to neuronal differentiation of NT2 cells.

Role and distribution of retinoic acid during CNS development

Retinoic acid (RA), the biologically active derivative of vitamin A, induces a variety of embryonal carcinoma and neuroblastoma cell lines to differentiate into neurons. The molecular events underlying this process are reviewed with a view to determining whether these data can lead to a better understanding of the normal process of neuronal differentiation during development. Several transcription factors, intracellular signaling molecules, cytoplasmic proteins, and extracellular molecules are shown to be necessary and sufficient for RA-induced differentiation. The evidence that RA is an endogenous component of the developing central nervous system (CNS) is then reviewed, data which include high-pressure liquid chromotography (HPLC) measurements, reporter systems and the distribution of the enzymes that synthesize RA. The latter is particularly relevant to whether RA signals in a paracrine fashion on adjacent tissues or whether it acts in an autocrine manner on cells that synthesize it. It seems that a paracrine system may operate to begin early patterning events within the developing CNS from adjacent somites and later within the CNS itself to induce subsets of neurons. The distribution of retinoid-binding proteins, retinoid receptors, and RA-synthesizing enzymes is described as well as the effects of knockouts of these genes. Finally, the effects of a deficiency and an excess of RA on the developing CNS are described from the point of view of patterning the CNS, where it seems that the hindbrain is the most susceptible part of the CNS to altered levels of RA or RA receptors and also from the point of view of neuronal differentiation where, as in the case of embryonal carcinoma (EC) cells, RA promotes neuronal differentiation. The crucial roles played by certain genes, particularly the Hox genes in RA-induced patterning processes, are also emphasized.

Localization of the RAR interaction domain of cellular retinoic acid binding protein-II

The pleiotropic effects of retinoic acid (RA) in mammalian cells are mediated by two classes of proteins: the retinoic acid receptors (RAR), and cellular retinoic acid binding proteins (CRABP-I and CRABP-II). The high conservation across species and the differential expression patterns of the two CRABPs suggest that they serve distinct biological functions. We previously showed that CRABP-II, but not CRABP-I, delivers RA to RAR through direct protein-protein interactions between the binding protein and the receptor. "Channeling" of RA between CRABP-II and RAR markedly facilitates the formation of the holo-receptor and, as a consequence, enhances the transcriptional activity of RAR in cells. Here, we localize the region of CRABP-II that mediates the interactions of this protein with RAR. Comparison between the electrostatic surface potential of CRABP-I and II revealed the presence of a sole region displaying a dramatic potential change between the two isoforms. Examination of the underlying model revealed that the change stemmed from CRABP-I/CRABP-II substitution of three spatially aligned residues E75Q, K81P, and E102 K, located on a protrusion above the entrance to the ligand binding pocket of the protein. Substituting the corresponding CRABP-II residues onto CRABP-I conferred upon this protein the ability to channel RA to RAR and to enhance the transcriptional activity of RAR in cells. Conversely, converting these amino acid residues in CRABP-II to the homologous CRABP-I residues resulted in loss of the ability of CRABP-II to interact with RAR and to augment the receptor's activity. The data demonstrate that the surface region of CRABP-II containing residues Gln75, Pro81, and Lys102 is necessary and sufficient for mediating the interactions of this protein with RAR, facilitating the formation of the holo-receptor, and enhancing the transcriptional activity of RA.

The molecular control of upper extremity development: implications for congenital hand anomalies

As the molecular aspects of limb development are being unraveled, more of the congenital anomalies seen by hand surgeons in the clinical setting will have an identifiable molecular basis. The majority of the data available regarding the molecular development of the upper extremity have come from experimental animal studies, specifically the mouse and chicken. These findings are being discovered by either direct surgical and molecular manipulation of the developing limb or by production of mice deficient in specific genes. Relatively few specific human mutations that cause limb abnormalities have been identified. Hand surgeons should be aware of the basic molecular pathways controlling limb development because they are in a unique position to be able to identify patients with such deformities. In turn, detailed clinical descriptions of congenital anomalies affecting the upper extremity will advance the understanding of the cellular events controlled by the molecular pathways of limb development. This review describes the general molecular basis of limb development and correlates it with disease processes affecting the upper extremity.

Perspectives on the evolutionary origin of tetrapod limbs

The study of the origin and evolution of the tetrapod limb has benefited enormously from the confluence of molecular and paleontological data. In the last two decades, our knowledge of the basic molecular mechanisms that control limb development has grown exponentially, and developmental biologists now have the possibility of combining molecular data with many available descriptions of the fossil record of vertebrate fins and limbs. This synthesis of developmental and evolutionary biology has the potential to unveil the sequence of molecular changes that culminated in the adoption of the basic tetrapod limb plan.

Bone morphogenetic proteins regulate interdigital cell death in the avian embryo

The embryonic limb bud provides an excellent model for analyzing the mechanisms that regulate programmed cell death during development. At the time of digit formation in the developing autopod, the undifferentiated distal mesodermal cells may undergo or chondrogenic differentiation or apoptosis depending whether they are incorporated into the future digital rays or into the interdigital spaces. Both chondrogenesis or apoptosis are induced by local BMPS. However, whereas the chondrogenic-promoting activity of BMPs appears to be regulated through the BMPR-1b receptor, the mechanism by which the BMPs execute the death program remains unknown. The BMP proapoptotic activity requires the expression of members of the msx family of closely related homeobox-containing genes and is finally mediated by caspase activation, but the nature of the caspase(s) directly responsible for the cell death is also unknown. Finally, other growth factors present in the developing autopod at the stages of digit formation such as members of the FGF and TGF beta families modulate the ability of BMPs to induce cell death or chondrogenesis.

Retinoic acid: its biosynthesis and metabolism

This article presents a model that integrates the functions of retinoid-binding proteins with retinoid metabolism. One of these proteins, the widely expressed (throughout retinoid target tissues and in all vertebrates) and highly conserved cellular retinol-binding protein (CRBP), sequesters retinol in an internal binding pocket that segregates it from the intracellular milieu. The CRBP-retinol complex appears to be the quantitatively major form of retinol in vivo, and may protect the promiscuous substrate from nonenzymatic degradation and/or non-specific enzymes. For example, at least seven types of dehydrogenases catalyze retinal synthesis from unbound retinol in vitro (NAD+ vs. NADP+ dependent, cytosolic vs. microsomal, short-chain dehydrogenases/reductases vs. medium-chain alcohol dehydrogenases). But only a fraction of these (some of the short-chain de-hydrogenases/reductases) have the fascinating additional ability of catalyzing retinal synthesis from CRBP-bound retinol as well. Similarly, CRBP and/or other retinoid-binding proteins function in the synthesis of retinal esters, the reduction of retinal generated from intestinal beta-carotene metabolism, and retinoic acid metabolism. The discussion details the evidence supporting an integrated model of retinoid-binding protein/metabolism. Also addressed are retinoid-androgen interactions and evidence incompatible with ethanol causing fetal alcohol syndrome by competing directly with retinol dehydrogenation to impair retinoic acid biosynthesis.

Interactions of retinoid binding proteins and enzymes in retinoid metabolism

Naturally occurring retinoids (vitamin A or retinol and its active metabolites) are vital for vision, controlling the differentiation program of epithelial cells in the digestive tract and respiratory system, skin, bone, the nervous system, the immune system, and for hematopoiesis. Retinoids are essential for growth, reproduction (conception and embryonic development), and resistance to and recovery from infection. The functions of retinoids in the embryo begin soon after conception and continue throughout the lifespan of all vertebrates. Both naturally occurring and synthetic retinoids are used in the therapy of various skin diseases, especially acne, for augmenting the treatment of diabetes, and as cancer chemopreventive agents. Retinol metabolites serve as ligands that activate specific transcription factors in the superfamily of steroid/retinoid/thyroid/vitamin D/orphan receptors and thereby control gene expression. Additionally, retinoids may also function through non-genomic actions. Various retinoid binding proteins serve as partners in retinoid function. These binding proteins show high specificity and affinity for specific retinoids and seem to control retinoid metabolism in vivo qualitatively and quantitatively by reducing 'free' retinoid concentrations, protecting retinoids from non-specific interactions, and chaperoning access of metabolic enzymes to retinoids. Implementation of the physiological effects of retinoids depends on the spatial-temporal expressions of binding proteins, receptors and metabolic enzymes. This review will discuss current understanding of the enzymes that catalyze retinol and retinoic acid metabolism and their unique and integral relationship to retinoid binding proteins.

NMR study of the binding of all-trans-retinoic acid to type II human cellular retinoic acid binding protein

Cellular RA binding proteins are thought to play important roles in the (RA), a hormonally active metabolite of vitamin A that has profound effects on cell growth, + differentiation and morphogenesis. Binding of RA to type II human cellular RA binding proteins (CRABPII) has been investigated by NMR spectroscopy. The sequential resonance assignments of +CRABPII in the presence of RA were established by heteronuclear three-dimensional NMR at pH 7.3. The resonance assignments of the bound RA were achieved by homonucl NMR. The secondary structures of holo-CRABPII determined by NMR were ess as revealed by the crystal structure of holo-CRABPII. Most of the nuclear Overhauser effects (NOEs) between CRABPII and the bound RA were consistent with those predicted crystal structure of holo-CRABPII. The results suggested that the conformations in solution and in the crystalline state are highly similar. Compared to the ligand binding pocket, especially the ligand entrance, was stabilize Ser12-Leu18, one of the structure elements that constitute the ligand binding pocket, became more mobile upon binding of RA. Intramolecular NOEs between protons of the bo the carboxylate end of the bound RA is well fixed but the &beta;-ionone

Retinoids and mammalian development

All vertebrate embryos require retinoic acid (RA) for fulfilment of the developmental program encoded in the genome. In mammals, maternal homeostatic mechanisms minimize variation of retinoid levels reaching the embryo. Retinol is transported as a complex with retinol-binding protein (RBP): transplacental transfer of retinol and its uptake by the embryonic tissues involves binding to an RBP receptor at the cell surface. Embryonic tissues in which this receptor is present also contain the retinol-binding protein CRBP I and the enzymes involved in RA synthesis; the same tissues are particularly vulnerable to vitamin A deficiency. In the nucleus, the RA signal is transduced by binding to a heterodimeric pair of retinoid receptors (RAR/RXR). In general, the receptors show functional plasticity, disruption of one RAR or RXR gene having minor or no effects on embryogenesis. However, genetic studies indicate that RXR alpha is essential for normal development of the heart and eye. Excess RA causes abnormalities of many systems; altered susceptibility to RA excess in mice lacking RAR gamma or RXR alpha suggests that the teratogenic signal is transduced through different receptors compared with physiological RA function in the same tissue.

Gene targeting of retinoid receptors

Gene targeting in embryonic stem (ES) cells has been employed to investigate the role of the retinoid receptors and binding proteins both in the mouse as well as in embryocarcinoma cells. It is a powerful technique for the modification of the mouse genome. With more recent refinements in gene targeting technology, it is now possible to introduce more subtle mutations in the murine genome, as well as to investigate gene function in a tissue and temporally-restricted manner. It should also be possible to modify genes in diverse diploid cell lines, to generate diverse model systems for analysis of retinoid receptor function. In this article, some of the basic principles for gene targeting are described.

Retinoids and their receptors in skeletal development

The embryonic vertebrate limb serves as an excellent experimental model system in which to study mechanisms that regulate morphogenesis of the skeleton. The appendicular skeleton arises through the process of endochondral ossification, whereby a cartilage template is initially formed and subsequently replaced by bone. One molecule that has a dramatic effect on these processes is the vitamin-A metabolite, retinoic acid (RA). RA functions through a class of nuclear hormone receptors, the retinoic acid receptors (RARs) and retinoid-X-receptors (RXRs), to regulate gene transcription. Experimental evidence from RA teratogenesis suggests that the presence of ligand-activated RARs and/or inappropriate expression of RARs inhibits chondrogenesis. Conversely, genetic analysis has shown that the absence of the receptors can lead to deficiencies in cartilage formation while also promoting chondrogenesis at ectopic sites. Taken together, these studies suggest that the RARs play a fundamental role in the early stages of skeletal development, specifically those involved in the formation of prechondrogenic condensations and their subsequent differentiation into chondroblasts.

Limb development: molecular dysmorphology is at hand!

We present a review of limb development integrating current molecular information and selected genetic disorders to illustrate the advances made in this field over the last few years. With this knowledge, clinical geneticists can now begin to consider molecular mechanisms and pathways when investigating patients with limb malformation syndromes.

Orphan nuclear receptor ROR alpha-deficient mice display the cerebellar defects of staggerer

It has recently been shown that the neurological mutant mouse staggerer (sg) harbors a deletion within the Rora gene that encodes the orphan nuclear receptor ROR alpha. This deletion removes an exon encoding part of the ligand binding domain of the putative receptor, generating an ROR alpha truncated protein (ROR alpha(sg)). It is unknown whether sg acts as a null or highly hypomorphic allele. To address this question, we have generated a null mutation of Rora by targeted disruption of its DNA binding domain in ES cells. The Rora-/- mice are viable but display tremor, body imbalance, small size and die between 3-4 weeks, similar to the sg mouse. Histological examination of the cerebellum of Rora-/- and sg mice showed similar defects, including small size and fewer ectopically localized Purkinje cells. Northern blot analysis of cerebellar RNA showed that ROR alpha transcripts are still expressed in the Rora-/- and sg mutants, although with altered mobilities. However, the cerebellum of the Rora-/- mutant does not express the ROR alpha protein. Attempts to complement the defect of the Rora-/- with sg failed, demonstrating conclusively that the sg defects are caused by the absence of functional ROR alpha.

The CRABP I gene contains two separable, redundant regulatory regions active in neural tissues in transgenic mouse embryos

The CRABP I gene is expressed in a spatiotemporal pattern in neural and mesenchymal tissues at the onset of organogenesis. The neural pattern of CRABP I expression includes specific rhombomeres of the hindbrain, neural crest cells and their derivatives the optic stalk, and the central area of the neural retina. We have created transgenic mouse lines with CRABP I 5' and transcribed regions fused to the lacZ structural gene that recapitulate much of this neural pattern of expression. Sequences 5' of the transcription initiation site between -7.8 and -3.2 kb confer beta-galactosidase expression to specific rhombomeres, migrating neural crest cells, trigeminal ganglion, the optic stalk, and the neural retina. We have also defined a region located between exon 1 and exon 8 that confers a portion of this expression pattern, including the mesencephalic projections of the trigeminal ganglion, the inner layer of the neural retina, and the peripheral layer of the posterior hindbrain. CRABP I expression in mesenchyme appears to require sequences in addition to or outside of those examined here.

Retinoic acid, neoplasia, differentiation and development

Retinoic acid has pronounced effects on cultures of neoplastic cells. These have attracted the attention of pathologists, but it is important to note that much of the critical data about retinoic acid has been obtained from the current extension of our knowledge in the field of development. Some of these changes are reviewed here.

A mutational analysis of the 5′ HoxD genes: dissection of genetic interactions during limb development in the mouse

Using gene targeting in mice, we have undertaken a systematic mutational analysis of the homeobox-containing 5′ HoxD genes. In particular, we have characterized the limb defects observed in mice with independent targeted disruptions of hoxd-12 and hoxd-13. Animals defective for hoxd-12 are viable, fertile, and appear outwardly normal yet have minor autopodal defects in the forelimb which include a reduction in the bone length of metacarpals and phalanges, and a malformation of the distal carpal bone d4. The limb phenotypes observed in hoxd-13 mutant mice are more extensive, including strong reductions in length, complete absences, or improper segmentations of many metacarpal and phalangeal bones. Additionally, the d4 carpal bone is not properly formed and often produces an extra rudimentary digit. To examine the genetic interactions between the 5′ HoxD genes, we bred these mutant strains with each other and with our previously characterized hoxd-11 mouse to produce a series of trans-heterozygotes. Skeletal analyses of these mice reveal that these genes interact in the formation of the vertebrate limb, since the trans-heterozygotes display phenotypes not present in the individual heterozygotes, including more severe carpal, metacarpal and phalangeal defects. Some of these phenotypes appear to be accounted for by a delay in the ossification events in the autopod, which lead to either the failure of fusion or the elimination of cartilaginous elements. Characteristically, these mutations lead to the overall truncation of digits II and V on the forelimb. Additionally, some trans-animals show the growth of an extra postaxial digit VI, which is composed of a bony element resembling a phalange. The results demonstrate that these genes interact in the formation of the limb. In addition to the previously characterized paralogous interactions, a multitude of interactions between Hox genes is used to finely sculpt the forelimb. The 5′ Hox genes could therefore act as a major permissive genetic milieu that has been exploited by evolutionary adaptation to form the tetrapod limbs.

BMP-7 is an inducer of nephrogenesis, and is also required for eye development and skeletal patterning

Bone morphogenetic proteins (BMPs) are multifunctional growth factors originally identified by their ability to induce ectopic bone formation. To investigate the function of one of the BMPs, BMP-7, we have generated BMP-7-deficient mice using embryonic stem cell technology. BMP-7-deficient mice die shortly after birth because of poor kidney development. Histological analysis of mutant embryos at several stages of development revealed that metanephric mesenchymal cells fail to differentiate, resulting in a virtual absence of glomerulus in newborn kidneys. In situ hybridization analysis showed that the absence of BMP-7 affects the expression of molecular markers of nephrogenesis, such as Pax-2 and Wnt-4 between 12.5 and 14.5 days postcoitum (dpc). This identifies BMP-7 as an inducer of nephrogenesis. In addition, BMP-7-deficient mice have eye defects that appear to originate during lens induction. Finally, BMP-7-deficient mice also have skeletal patterning defects restricted to the rib cage, the skull, and the hindlimbs.

Retinoid signaling and the generation of regional and cellular diversity in the embryonic mouse spinal cord

Retinoid-dependent gene expression accompanies the emergence of distinct regions and cell classes in the mouse spinal cord around midgestation. We asked whether changes in the expression of retinoid signaling molecules and retinoid-responsive genes reflect the establishment of this regional and cellular diversity. At E10.5, retinoic acid (RA) receptors (RAR)alpha, RAR beta, the retinoid X receptor (RXR) gamma, cellular RA binding protein (CRABP)I, CRAPBII, and cellular retinol binding protein (CRBP)I mRNAs are found throughout the entire anterior-posterior (AP) axis of the cord, as is RA (Colbert et al. [1993] Proc. Natl. Acad. Sci. U.S.A. 90:6572-6576) and RA-sensitive transgene expression (Balkan et al. [1992] Proc. Natl. Acad. Sci. U.S.A. 89:3347-3351). At E12.5, RA, transgene expression, and RAR beta become restricted to the cervical and lumbar cord. RAR alpha, CRABPI, and RXR gamma, however, are found throughout the AP extent. CRABPII and CRBPI, although expanded within the cervical and lumbar regions, are also found throughout the AP axis. Thus, several retinoid signaling molecules continue to be expressed beyond distinct regions of the spinal cord where RA is available and some RA-responsive genes are either restricted or enhanced. Exogenous RA can activate a more widespread response resulting in ectopic transgene and RAR beta expression in the thoracic and sacral cord. Not all RA-sensitive genes, however, respond; CRABPII and CRBPI expression patterns are unchanged. Finally, not every cell within the normal or exogenously induced domains of RA-dependent gene expression responds to RA, nor does every cell express RA receptors or binding proteins. Thus, regional and cellular differences in the distribution of the known retinoid receptors and binding proteins do not predict absolutely where or whether retinoid sensitive genes will be expressed or where retinoids will be available in the developing spinal cord. Instead, retinoid-mediated gene expression in the cervical and lumbar cord seems to reflect retinoid responses that rely both on the local availability of retinoids, the identity of the responding gene, and an indeterminate array of retinoid signaling molecules.

Mice lacking all isoforms of retinoic acid receptor beta develop normally and are susceptible to the teratogenic effects of retinoic acid

Retinoic acids (RA) are vitamin A derivatives essential for normal embryonic development and viability of vertebrates. The RA signal is mediated by two distinct classes of receptors, RA receptors (RARs) and retinoid X receptors (RXRs). The RAR family is composed of three genes: RAR alpha, beta, and gamma. The expression of RAR beta gene is spatially and temporally restricted in certain structures in the developing embryo, suggesting that RAR beta could play specific roles during morphogenesis. Four isoforms of the RAR beta gene (beta 1-beta 4) are generated by differential usage of promoters and alternative splicing. It has recently been demonstrated that the RAR beta 2 isoform is dispensable for normal development. To ascertain the function of all RAR beta isoforms in vivo, we have generated a mutation that disrupts all isoforms of the RAR beta gene in the mouse by gene targeting in embryonic stem cells. Mice homozygous for the mutation are viable and fertile with no externally apparent abnormalities. During development, 1/11 RAR beta mutant embryos showed fusion of the ninth and tenth cranial ganglia on both sides of the hindbrain. However, no obvious alterations in the spatial pattern of expression of Hoxb-1, Hoxb-4 and Hoxb-5 were observed in day 9.5 p.c. embryos. The RAR beta null mutation did not alter the pattern or extent of the limb and craniofacial malformations induced by RA excess, suggesting that RAR beta may not be mandatory to mediate the observed teratological effects of RA in these structures. These experiments demonstrate that RAR beta isoforms are not absolutely required for embryonic development and provide additional support to the concept of functional redundancy among members of the RAR family.

Effects of diethylstilboestrol on testicular function and luteinizing hormone receptors

Adult male Fisher-344 rats were implanted with DES-filled or empty Silastic capsules. After 14 weeks, capsules were removed and a second group of rats received DES capsules. Seven weeks later, all the rats were sacrificed. DES treatment decreased body, testes and seminal vesicle weights, and removal of the capsules partially restored the weight of these organs. The concentration of testicular LH receptors was increased by DES treatment. Circulating PRL levels were increased and gonadotropin levels were reduced in all animals having received DES at anytime. Plasma testosterone (T) levels were similar in all groups, but testicular T levels were reversibly decreased by DES. Similarly, whereas basal incubation media T levels were unchanged by DES treatment, the steroidogenic response in vitro to hCG was abolished by the presence of DES, and removal of the capsules restored this response. It appears that in this animal model DES and PRL exert opposing effects on testicular LH receptor.