GenePaint.org: an atlas of gene expression patterns in the mouse embryo

High-throughput instruments were recently developed to determine gene expression patterns on tissue sections by RNA in situ hybridization. The resulting images of gene expression patterns, chiefly of E14.5 mouse embryos, are accessible to the public at http://www.genepaint.org. This relational database is searchable for gene identifiers and RNA probe sequences. Moreover, patterns and intensity of expression in approximately 100 different embryonic tissues are annotated and can be searched using a standardized catalog of anatomical structures. A virtual microscope tool, the Zoom Image Server, was implemented in GenePaint.org and permits interactive zooming and panning across approximately 15,000 high-resolution images.

A geometric database for gene expression data

As the logical next step after sequencing the mouse genome, biologists have developed laboratory methods for rapidly determining where each of the 30K genes in the mouse genome is synthesizing protein. Applying these methods to the mouse brain, biologists are currently generating large numbers of 2D cross-sectional images that record the expression pattern for each gene in the mouse genome. In this paper, we describe the structure of a geometric database for the mouse brain that allows biologists to organize and search this gene expression data. The central component of this database is an atlas that explicitly partitions the mouse brain into key anatomical regions. This atlas is represented as a Catmull-Clark subdivision mesh with anatomical regions separated by a network of B-spline crease curves. New gene expression images are added to the database by deforming this atlas onto each image using techniques developed for fitting subdivision surfaces to scatter data. Due to this partitioning of the subdivision mesh, user queries comparing expression data between various genes can be restricted to anatomical regions without difficulty while the multi-resolution structure of the subdivision mesh allows these queries to be processed efficiently.

Murine Lix1, a novel marker for substantia nigra, cortical layer 5, and hindbrain structures

We describe the expression of Lix1 in the mouse. Starting at E8, transcripts are present in a regionalized fashion and persist throughout development. mLix1 is expressed in the cortical plate, subventricular zone, layer 5 of the postnatal cortex, the substantia nigra, dorsal root ganglia, specific nuclei of the brain stem and in spinal cord. Limb buds and facial primordia show transient expression. The prominent expression of mLix1 in the developing cerebral cortex and in the substantia nigra pars compacta makes this novel gene a candidate marker for both of these tissues.

A transcriptome atlas of the mouse brain at cellular resolution

A genome-wide expression atlas of the nervous system at cellular resolution would be a valuable resource for neurobiology, genetics, developmental biology and medicine. Progress in automation of in situ hybridization makes such an atlas possible. Standardized and computerized annotation of expression patterns will be critical for producing a searchable atlas database that can be accessed through the internet.

Development of high-throughput tools to unravel the complexity of gene expression patterns in the mammalian brain

Genomes of animals contain between 15000 (e.g. Drosophila) and 50000 (human, mouse) genes, many of which encode proteins involved in regulatory processes. The availability of sequence data for many of these genes opens up opportunities to study complex genetic and protein interactions that underlie biological regulation. Many examples demonstrate that an understanding of regulatory networks consisting of multiple components is significantly advanced by a detailed knowledge of the spatiotemporal expression pattern of each of the components. Gene expression patterns can readily be determined by RNA in situ hybridization. The unique challenge emerging from the knowledge of the sequence of entire genomes is that assignment of biological functions to genes needs to be carried out on an appropriately large scale. In terms of gene expression analysis by RNA in situ hybridization, efficient technologies need to be developed that permit determination and representation of expression patterns of thousands of genes within an acceptable time-scale. We set out to determine the spatial expression pattern of several thousand genes encoding putative regulatory proteins. To achieve this goal we have developed high-throughput technologies that allow the determination and visualization of gene expression patterns by RNA in situ hybridization on tissue sections at cellular resolution. In particular, we have invented instrumentation for robotic in situ hybridization capable of carrying out in a fully automated fashion, all steps required for detecting sites of gene expression in tissue sections. In addition, we have put together hardware and software for automated microscopic scanning of gene expression data that are produced by RNA in situ hybridization. The potential and limitations of these techniques and our efforts to build a Web-based database of gene expression patterns are discussed.

Cloning and expression analysis of chicken Lix1, a founding member of a novel gene family

Limb Expression 1 (Lix1), a founding member of a novel gene family, was identified in a screen for genes transiently and locally expressed during early chicken limb development. Most prominently, Lix1 is transiently expressed in the nascent hindlimb bud between Hamburger-Hamilton stages 15 and 19. Chicken Lix1 transcripts are also found in the basal plate of rhombomeres 3 and 5, in pharyngeal and in foregut mesenchyme and in all facial primordia except for the mandibular arches. Homologs of chick Lix1 exist in human, mouse and Drosophila.

Evidence for a role of protein kinase C in FGF signal transduction in the developing chick limb bud

In developing limbs, numerous signaling molecules have been identified but less is known about the mechanisms by which such signals direct patterning. We have explored signal transduction pathways in the chicken limb bud. A cDNA encoding RACK1, a protein that binds and stabilizes activated protein kinase C (PKC), was isolated in a screen for genes induced by retinoic acid (RA) in the chick wing bud. Fibroblast growth factor (FGF) also induced RACK1 and such induction of RACK1 expression was accompanied by a significant augmentation in the number of active PKC molecules and an elevation of PKC enzymatic activity. This suggests that PKCs mediate signal transduction in the limb bud. Application of chelerythrine, a potent PKC inhibitor, to the presumptive wing region resulted in buds that did not express sonic hedgehog (Shh) and developed into wings that were severely truncated. This observation suggests that the expression of Shh depends on PKCs. Providing ectopic SHH protein, RA or ZPA grafts overcome the effects of blocking PKC with chelerythrine and resulted in a rescue of the wing morphology. Taken together, these findings suggest that the responsiveness of Shh to FGF is mediated, at least in part, by PKCs.

Fenretinide therapy in prostate cancer: effects on tissue and serum retinoid concentration

PURPOSE

To examine the feasibility of using fenretinide (4-HPR) for the prevention and treatment of prostate cancer.

MATERIALS AND METHODS

We measured the impact of 4-HPR therapy on retinoid concentrations in vivo, in a mouse model of prostate cancer and clinically, in patients with prostate cancer who were given oral 4-HPR (200 mg/d) or placebo for 4 weeks before undergoing a radical prostatectomy.

RESULTS

Prostate tumors in mice treated with 4-HPR contained high levels of 4-HPR and of all-trans-retinoic acid (RA) and reduced levels of retinol (ROH). Patients given 4-HPR were found to have significantly higher concentrations of 4-HPR in the cancerous prostate as compared with the serum levels (463 nmol/L v 326 nmol/L; P =.049), but they were only 1/10 the levels found in mice and were far below the concentrations reported in human breast tissue. Serum and tissue ROH levels were reduced to less than half the concentrations found in untreated controls. RA concentrations in human serum and in cancerous prostates were not significantly affected by 4-HPR treatment, in contrast with the findings in mice.

CONCLUSION

The standard oral dose of 4-HPR proposed for breast cancer (200 mg/d) achieved only modest drug levels in the prostate and is unlikely to be effective for prostate cancer prevention or treatment. Higher doses need to be explored.

Complementary domains of retinoic acid production and degradation in the early chick embryo

Excess retinoids as well as retinoid deprivation cause abnormal development, suggesting that retinoid homeostasis is critical for proper morphogenesis. RALDH-2 and CYP26, two key enzymes that carry out retinoic acid (RA) synthesis and degradation, respectively, were cloned from the chick and show significant homology with their orthologs in other vertebrates. Expression patterns of RALDH-2 and CYP26 genes were determined in the early chick embryo by in situ hybridization. During gastrulation and neurulation RALDH-2 and CYP26 were expressed in nonoverlapping regions, with RALDH-2 transcripts localized to the presumptive presomitic and lateral plate mesoderm and CYP26 mRNA to the presumptive mid- and forebrain. The two domains of expression were separated by an approximately 300-micrometer-wide gap, encompassing the presumptive hindbrain. In the limb region, a similar spatial segregation of RALDH-2 and CYP26 expression was found at stages 14 and 15. Limb region mesoderm expressed RALDH-2, whereas the overlying limb ectoderm expressed CYP26. RA-synthesizing and -degrading enzymatic activities were measured biochemically in regions expressing RALDH-2 or CYP26. Regions expressing RALDH-2 generated RA efficiently from precursor retinal but degraded RA only inefficiently. Conversely, tissue expressing CYP26 efficiently degraded but did not synthesize RA. Localized regions of RA synthesis and degradation mediated by these two enzymes may therefore provide a mechanism to regulate RA homeostasis spatially in vertebrate embryos.

Temporally-regulated retinoic acid depletion produces specific neural crest, ocular and nervous system defects

Both retinoid receptor null mutants and classic nutritional deficiency studies have demonstrated that retinoids are essential for the normal development of diverse embryonic structures (e.g. eye, heart, nervous system, urogenital tract). Detailed analysis of retinoid-modulated events is hampered by several limitations of these models, including that deficiency or null mutation is present throughout gestation, making it difficult to isolate primary effects, and preventing analysis beyond embryolethality. We developed a mammalian model in which retinoid-dependent events are documented during distinct targeted windows of embryogenesis. This was accomplished through the production of vitamin A-depleted (VAD) female rats maintained on sufficient oral retinoic acid (RA) for growth and fertility. After mating to normal males, these RA-sufficient/VAD females were given oral RA doses which allowed for gestation in an RA-sufficient state; embryogenesis proceeded normally until retinoids were withdrawn dietarily to produce a sudden, acute retinoid deficiency during a selected gestational window. In this trial, final RA doses were administered on E11.5, vehicle at E12.5, and embryos analyzed on E13.5; during this 48 hour window, the last RA dose was metabolized and embryos progressed in a retinoid-deficient state. RA-sufficient embryos were normal. Retinoid-depleted embryos exhibited specific malformations of the face, neural crest, eyes, heart, and nervous system. Some defects were phenocopies of those seen in null mutant mice for RXR alpha(−/−), RXR alpha(−/−)/RAR alpha(−/−), and RAR alpha(−/−)/RAR gamma(−/−), confirming that RA transactivation of its nuclear receptors is essential for normal embryogenesis. Other defects were unique to this deficiency model, showing that complete ligand ‘knock-out’ is required to see those retinoid-dependent events previously concealed by receptor functional redundancy, and reinforcing that retinoid receptors have separate yet overlapping contributions in the embryo. This model allows for precise targeting of retinoid form and deficiency to specific developmental windows, and will facilitate studies of distinct temporal events.

Sonic hedgehog participates in craniofacial morphogenesis and is down-regulated by teratogenic doses of retinoic acid

The face is one of the most intricately patterned structures in human and yet little is known of the mechanisms by which the tissues are instructed to grow, fuse, and differentiate. We undertook a study to determine if the craniofacial primordia used the same molecular cues that mediate growth and patterning in other embryonic tissues such as the neural tube and the limb. Here we provide evidence for the presence of organizer-like tissues in the craniofacial primordia. These candidate organizers express the polarizing signal sonic hedghog (shh) and its putative receptor, patched, as well as fibroblast growth factor 8 and bone morphogeneic protein 2. Shh-expressing epithelial grafts functioned as organizing tissues in a limb bud assay system, where they evoked duplications of the digit pattern. High doses of retinoic acid, which are known to truncate the growth of the frontonasal and maxillary processes and thus produce bilateral clefting of the lip and palate, inhibited the expression of shh and patched but not fgf8, in the craniofacial primordia, and abolished polarizing activity of these tissues. From these studies we conclude that the embryonic face contains signaling centers in the epithelium that participate in craniofacial growth and patterning. In addition, we discuss a novel mechanism whereby retinoids can exert a teratogenic effect on craniofacial morphogenesis independent of its effects on Hox gene expression or neural crest cell migration.

Retinoid signaling is required for the establishment of a ZPA and for the expression of Hoxb-8, a mediator of ZPA formation

We show that retinoid receptor antagonists applied to the presumptive wing region block the formation of a zone of polarizing activity (ZPA). This suggests a direct relationship between retinoid signaling and the establishment of the ZPA. We provide evidence that the Hox gene, Hoxb-8, is a direct target of retinoid signaling since exogenously applied RA rapidly induces this gene in the absence of protein synthesis and, moreover, retinoid receptor antagonists down-regulate Hoxb-8 expression. In addition, we find that, in the lateral plate mesoderm, the domains of Hoxb-8 expression and of polarizing activity are coextensive. Taken together, these findings support the hypothesis that retinoids are required for the establishment of a ZPA, and that retinoids act, at least in part, through Hoxb-8, a gene associated with ZPA formation (Charite et al., 1994).

Ligand-bound RXR can mediate retinoid signal transduction during embryogenesis

Retinoids regulate various aspects of vertebrate development through the action of two types of receptors, the retinoic acid receptors (RARs) and the retinoid-X-receptors (RXRs). Although RXRs bind 9-cis-retinoic acid (9cRA) with high affinity, in vitro experiments suggest that RXRs are for the most part not liganded, but serve as auxiliary factors forming heterodimers with liganded partner receptors such as RAR. Here we have used RXR- and RAR-specific ligands 4-[1-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydro-2-napthyl)ethenyl]b enzoic acid (LG69) and (E)-4-[2-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-2-naphthalenyl)-1-prope nyl]benzoic acid (TTNPB), and show that, in the context of an embryo, liganded RXR can mediate retinoid signal transduction. This conclusion emerges from examining the induction of several retinoid-responsive genes in the limb bud (Hoxb-6/-8, RARbeta) and in the developing central nervous system (Hoxb-1, otx-2). RARbeta and Hoxb-1 genes were most effectively activated by a combination of TTNPB and LG69, suggesting that the activation of these genes benefits from the presence of ligand-bound RAR and ligand-bound RXR. Hoxb-6/-8 genes were most efficiently induced by LG69, suggesting that liganded RXR can activate these genes. The regulation of the expression of the otx-2 gene was complex; expression was repressed by TTNPB, but such repression was relieved when LG69 was provided together with TTNPB, suggesting that ligand-bound RXR can overcome repression of transcription exerted by liganded RAR. Based on these findings, we propose that in our experimental system in which ligands are provided exogenously, transcriptional regulation of several genes involves liganded RXR.

Abnormal level of retinoic acid in prostate cancer tissues

Vitamin A (retinol) and its derivatives, the retinoids, have been implicated as chemopreventive and differentiating agents in a variety of cancers, including that of the prostate. Very little is known about the physiological role of retinoids in the prostate. Here we show that normal prostate, benign prostate hyperplasia (BPH), and prostate carcinoma tissues contain endogenous retinol and its biologically active metabolite retinoic acid. In our studies, the concentration of retinol was 2-fold elevated in BPH compared with the other two tissues. In contrast, prostate carcinoma tissue contained five to eight times less retinoic acid than normal prostate or BPH. Moreover, we found that prostate tissue expresses dehydrogenases capable of converting retinol to retinoic acid through retinaldehyde as an intermediate. Formation of retinal from retinol takes place in microsomes, and the conversion of retinal to retinoic acid occurs in the cytosol. Furthermore, we found that the nuclear retinoic acid receptors alpha, beta, and gamma are expressed in normal and tumor samples. These studies establish a role for retinoids in the physiology of the prostate and possibly also in the pathophysiology of prostate cancer.

Retinoic acid signaling is required during early chick limb development

In the chick limb bud, the zone of polarizing activity controls limb patterning along the anteroposterior and proximodistal axes. Since retinoic acid can induce ectopic polarizing activity, we examined whether this molecule plays a role in the establishment of the endogenous zone of polarizing activity. Grafts of wing bud mesenchyme treated with physiologic doses of retinoic acid had weak polarizing activity but inclusion of a retinoic acid-exposed apical ectodermal ridge or of prospective wing bud ectoderm evoked strong polarizing activity. Likewise, polarizing activity of prospective wing mesenchyme was markedly enhanced by co-grafting either a retinoic acid-exposed apical ectodermal ridge or ectoderm from the wing region. This equivalence of ectoderm-mesenchyme interactions required for the establishment of polarizing activity in retinoic acid-treated wing buds and in prospective wing tissue, suggests a role of retinoic acid in the establishment of the zone of polarizing activity. We found that prospective wing bud tissue is a high-point of retinoic acid synthesis. Furthermore, retinoid receptor-specific antagonists blocked limb morphogenesis and down-regulated a polarizing signal, sonic hedgehog. Limb agenesis was reversed when antagonist-exposed wing buds were treated with retinoic acid. Our results demonstrate a role of retinoic acid in the establishment of the endogenous zone of polarizing activity.

Relationship between retinoic acid and sonic hedgehog, two polarizing signals in the chick wing bud

Local application of all-trans-retinoic acid (RA) to the anterior margin of chick limb buds results in pattern duplications reminescent of those that develop after grafting cells from the zone of polarizing activity (ZPA). RA may act directly by conferring positional information to limb bud cells, or it may act indirectly by creating a polarizing region in the tissue distal to the RA source. Here we demonstrate that tissue distal to an RA-releasing bead acquires polarizing activity in a dose-dependent manner. Treatments with pharmacological (beads soaked in 330 micrograms/ml) and physiological (beads soaked in 10 micrograms/ml) doses of RA are equally capable of inducing digit pattern duplication. Additionally, both treatments induce sonic hedgehog (shh; also known as vertebrate hedgehog-1, vhh-1), a putative ZPA morphogen and Hoxd-11, a gene induced by the polarizing signal. However, tissue transplantation assays reveal that pharmacological, but not physiological, doses create a polarizing region. This differential response could be explained if physiological doses induced less shh than pharmacological doses. However, our in situ hybridization analyses demonstrate that both treatments result in similar amounts of mRNA encoding this candidate ZPA morphogen. We outline a model describing the apparently disparate effects of pharmacologic and physiological doses RA on limb bud tissue.

Detection of potential ligands for nuclear receptors in cellular extracts

In response to external stimuli, steroid receptors are directly influenced to transactivate gene expression. Assuming they exist, identification of ligands for orphan steroid receptors is a key to understanding their physiology. In the orphan subgroup of the steroid receptor superfamily, the putative carboxyl terminal ligand-binding domain (LBD) is well conserved among members of the superfamily, which suggests a role in ligand binding. A consequence of ligand binding is the induction of a significant conformational change within the LBD which is necessary for the transactivation function. This characteristic conformational change can be detected by partial proteolytic digestion and has been localized by mutational analysis and epitopic mapping of the LBD using monoclonal antibodies. Based on this finding, a sensitive in vitro assay was developed for the rapid screening and identification of potential ligands for orphan receptors. We examined the patterns of conformational changes in the androgen receptor, glucocorticoid receptor, and progesterone receptor induced by binding of their cognate agonists and antagonists. We demonstrated that the conformational changes induced by ligands can serve as characteristic and reliable markers to distinguish between the ligand-bound and apoprotein states of a receptor. The sensitivity and feasibility of employing this assay to detect new endogenous ligands using fractionated cellular extracts were also tested. The results strongly suggest that unknown compounds can be defined as potential ligands for orphan receptors using this approach.

9-cis-retinoic acid, a potent inducer of digit pattern duplications in the chick wing bud

The effects of retinoids are mediated by two types of receptors, the retinoic acid receptors (RARs) and the retinoid-X-receptors (RXRs). The physiological ligand of the RARs is all-trans-retinoic acid whereas RXRs have high affinity for 9-cis-retinoic acid, a naturally occurring retinoid isomer. RXRs are broadly expressed in embryonic and adult tissues, and they are capable of forming homodimers as well as heterodimers with RARs and other nuclear hormone receptors. The role of 9-cis-retinoic acid in regulating the activity of RXR homodimers and RXR-containing heterodimers is poorly understood in vivo. To begin to explore the function of 9-cis-retinoic acid in morphogenesis, we have examined the activity of this isomer in the chick wing. Using reverse transcriptase polymerase chain reaction analyses, we show that RXR gamma is expressed in stage 20 wing buds. Similar to all-trans-retinoic acid, the 9-cis-isomer induces pattern duplications when locally applied to chick wing buds, but the 9-cis isomer is about 25 times more potent than the all-trans form. Furthermore, applied all-trans-retinoic acid is converted to the 9-cis isomer in the wing bud. The ratio of 9-cis to all-trans-retinoic acid established in the tissue is approximately 1:25. This quantitative agreement between the degree of conversion and the 25-fold higher efficacy of the 9-cis isomer, raises the possibility that, at least in part, the effects of all-trans-retinoic acid on the wing pattern result from a conversion to the 9-cis isomer.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that Hensen's node is a site of retinoic acid synthesis

Hensen's node of amniotes, like the Spemann organizer of amphibians, can induce a second body axis when grafted into a host embryo. The avian node, as well as several midline structures originating from it (notochord, floor plate), can also induce digit pattern duplications when grafted into the chick wing bud. We report here that the equivalent of Hensen's node from mouse is an effective inducer of digits in the chick wing bud. Tissues anterior and posterior to the node also evoke pattern duplications, but with a significantly lower efficiency. The finding that the murine node operates in an avian wing bud suggests that the same inducing agent(s) function in both primary and secondary embryonic fields and have been conserved during vertebrate evolution. Digit pattern duplications are also evoked by local administration of all-trans-retinoic acid. This similarity raises the possibility that Hensen's node is a source of retinoic acid. The mouse node is capable of synthesizing retinoic acid from its biosynthetic precursor all-trans-retinol at a substantially higher rate than either anterior or posterior tissues.

9-cis retinoic acid is a high affinity ligand for the retinoid X receptor

All-trans retinoic acid (RA) has previously been shown to modulate the transcriptional properties of the retinoic acid receptor (RAR) and retinoid X receptor (RXR). The inability of all-trans RA to bind to RXR suggests that it may be metabolized to a more active high affinity ligand. We report here an experimental approach that has identified 9-cis RA as an RXR ligand. It is up to 40-fold more potent than all-trans RA in transfection assays and binds with high affinity. The production of 9-cis RA in cultured cells and the identification of this molecule in liver and kidney demonstrates the existence of this molecule in living organisms. The discovery of this novel hormone points to the key role retinoid metabolism may have in generating new signaling pathways.

The open/closed conformational equilibrium of aspartate aminotransferase. Studies in the crystalline state and with a fluorescent probe in solution

Aspartate aminotransferase undergoes major shifts in the conformational equilibrium of the protein matrix during transamination. The present study defines the two conformational states of the enzyme by crystallographic analysis, examines the conditions under which the enzyme crystallizes in each of these conformations, and correlates these conditions with the conformational behaviour of the enzyme in solution, as monitored by a fluorescent reporter group. Cocrystallization of chicken mitochondrial aspartate aminotransferase with inhibitors and covalent coenzymesubstrate adducts yields three different crystal forms. Unliganded enzyme forms triclinic crystals of the open conformation, the structure of which has been solved (space group P1) [Ford, G. C., Eichele, G. & Jansonius, J. N. (1980) Proc. Natl Acad. Sci. USA 77, 2559-2563; Kirsch, J. F., Eichele, G., Ford, G. C., Vincent, M. G., Jansonius, J. N., Gehring, H. & Christen, P. (1984) J. Mol. Biol. 174, 487-525]. Complexes of the enzyme with dicarboxylate ligands form monoclinic or orthorhombic crystals of the closed conformation. The results of structure determinations of the latter two crystal forms at 0.44 nm resolution are described here. In the closed conformation, the small domain has undergone a rigid-body rotation of 12-14 which closes the active-site pocket. Shifts in the conformational equilibrium of aspartate aminotransferase in solution, as induced by substrates, substrate analogues and specific dicarboxylic inhibitors, can be monitored by changes in the relative fluoresence yield of the enzyme labelled at Cys166 with monobromotrimethylammoniobimane. The pyridoxal and pyridoxamine forms of the labelled enzyme show the same fluorescence properties, whereas in the apoenzyme the fluorescence intensity is reduced by 30%. All active-site ligands, if added to the labelled pyridoxal enzyme at saturating concentrations, cause a decrease in the fluorescence intensity by 40-70% and a blue shift of maximally 5 nm. Comparison of the fluorescence properties of the enzyme in various functional states with the crystallographic data shows that both techniques probe the same conformational equilibrium. The conformational change that closes the active site seems to be ligand-induced in the reaction of the pyridoxal form of the enzyme and syncatalytic in the reverse reaction with the pyridoxamine enzyme.

Isolation of 3,4-didehydroretinoic acid, a novel morphogenetic signal in the chick wing bud

There is increasing evidence that retinoic acid is a morphogen involved in vertebrate development. This evidence comes in part from studies of the chick wing bud, in which local application of all-trans-retinoic acid results in a duplication of the digit pattern along the anteroposterior axis. Retinoic acid may be only one of several morphogenetic signalling compounds required for limb pattern formation. To identify novel morphogenetically active compounds, fractionated extracts of whole chick embryos were tested for their ability to induce digit pattern duplications. We describe here the isolation of a new activity present in the limb bud, which we have identified as all-trans-3,4-didehydroretinoic acid. The 3,4-didehydroretinoic acid is generated in situ from retinol through a 3,4-didehydroretinol intermediate. We show that 3,4-didehydroretinoic acid and retinoic acid are equipotent in evoking digit duplications. These findings suggest that there are at least two endogenous retinoids with morphogenetic properties in the chick limb.

Polarizing activity and retinoid synthesis in the floor plate of the neural tube

In many developing organisms the establishment of axial polarity and the patterning of cells depend on local signals that derive from restricted regions of the embryo. In vertebrate embryos, the origins of tissue polarity have been examined extensively in the developing limb. The anteroposterior pattern of the chick limb seems to be controlled by a morphogen, possibly retinoic acid, that is enriched in a region of the limb known as the zone of polarizing activity (ZPA). Certain tissues other than the ZPA have also shown polarizing activity experimentally in the chick limb, raising the possibility that signalling molecules involved in pattern formation in different embryonic tissues are conserved. Here we provide evidence that a similar polarizing activity is also present in a restricted region of the developing central nervous system (CNS). We show that a specialized group of neural cells termed the floor plate, but not other regions of the CNS, mimics the ZPA in respecifying the digit pattern in the developing chick limb. In addition, using an in vitro biochemical assay, we show that the floor plate can synthesize retinoic acid and 3,4-didehydroretinol, the precursor of a second morphogenetically active retinoid, 3,4-didehydroretinoic acid. These results show that the floor plate is a local source of a ZPA-like polarizing signal, possibly a retinoid, which may regulate the pattern of cell differentiation in the developing CNS.