Retinoid-X receptor signalling in the developing spinal cord

Activation of retinoid X receptors protects retinal neurons and pigment epithelial cells from BMAA-induced death

Exposure to the non-protein amino acid cyanotoxin β-N-methylamino-L-alanine (BMAA), released by cyanobacteria found in many water reservoirs has been associated with neurodegenerative diseases. We previously demonstrated that BMAA induced cell death in both retina photoreceptors (PHRs) and amacrine neurons by triggering different molecular pathways, as activation of NMDA receptors and formation of carbamate-adducts was only observed in amacrine cell death. We established that activation of Retinoid X Receptors (RXR) protects retinal cells, including retina pigment epithelial (RPE) cells from oxidative stress-induced apoptosis. We now investigated the mechanisms underlying BMAA toxicity in these cells and those involved in RXR protection. BMAA addition to rat retinal neurons during early development in vitro increased reactive oxygen species (ROS) generation and polyADP ribose polymers (PAR) formation, while pre-treatment with serine (Ser) before BMAA addition decreased PHR death. Notably, RXR activation with the HX630 agonist prevented BMAA-induced death in both neuronal types, reducing ROS generation, preserving mitochondrial potential, and decreasing TUNEL-positive cells and PAR formation. This suggests that BMAA promoted PHR death by substituting Ser in polypeptide chains and by inducing polyADP ribose polymerase activation. BMAA induced cell death in ARPE-19 cells, a human epithelial cell line; RXR activation prevented this death, decreasing ROS generation and caspase 3/7 activity. These findings suggest that RXR activation prevents BMAA harmful effects on retinal neurons and RPE cells, supporting this activation as a broad-spectrum strategy for treating retina degenerations.

Unveiling the role of CaMKII in retinal degeneration: from biological mechanism to therapeutic strategies

Ca2+/calmodulin-dependent protein kinase II (CaMKII) is a family of broad substrate specificity serine (Ser)/threonine (Thr) protein kinases that play a crucial role in the Ca2+-dependent signaling pathways. Its significance as an intracellular Ca2+ sensor has garnered abundant research interest in the domain of neurodegeneration. Accumulating evidences suggest that CaMKII is implicated in the pathology of degenerative retinopathies such as diabetic retinopathy (DR), age-related macular degeneration (AMD), retinitis pigmentosa (RP) and glaucoma optic neuropathy. CaMKII can induce the aberrant proliferation of retinal blood vessels, influence the synaptic signaling, and exert dual effects on the survival of retinal ganglion cells and pigment epithelial cells. Researchers have put forth multiple therapeutic agents, encompassing small molecules, peptides, and nucleotides that possess the capability to modulate CaMKII activity. Due to its broad range isoforms and splice variants therapeutic strategies seek to inhibit specifically the CaMKII are confronted with considerable challenges. Therefore, it becomes crucial to discern the detrimental and advantageous aspects of CaMKII, thereby facilitating the development of efficacious treatment. In this review, we summarize recent research findings on the cellular and molecular biology of CaMKII, with special emphasis on its metabolic and regulatory mechanisms. We delve into the involvement of CaMKII in the retinal signal transduction pathways and discuss the correlation between CaMKII and calcium overload. Furthermore, we elaborate the therapeutic trials targeting CaMKII, and introduce recent developments in the zone of CaMKII inhibitors. These findings would enrich our knowledge of CaMKII, and shed light on the development of a therapeutic target for degenerative retinopathy.

Dysregulation of Aldh1a2 underlies motor neuron degeneration in spinal muscular atrophy

Lower motor neuron degeneration is the pathological hallmark of spinal muscular atrophy (SMA), a hereditary motor neuron disease caused by loss of the SMN1 gene and the resulting deficiency of ubiquitously expressed SMN protein. The molecular mechanisms underlying motor neuron degeneration, however, remain elusive. To clarify the cell-autonomous defect in developmental processes, we here performed transcriptome analyses of isolated embryonic motor neurons of SMA model mice to explore mechanisms of dysregulation of cell-type-specific gene expression. Of 12 identified genes that were differentially expressed between the SMA and control motor neurons, we focused on Aldh1a2, an essential gene for lower motor neuron development. In primary spinal motor neuron cultures, knockdown of Aldh1a2 led to the formation of axonal spheroids and neurodegeneration, reminiscent of the histopathological changes observed in human and animal cellular models. Conversely, Aldh1a2 rescued these pathological features in spinal motor neurons derived from SMA mouse embryos. Our findings suggest that developmental defects due to Aldh1a2 dysregulation enhances lower motor neuron vulnerability in SMA.

AOP Key Event Relationship report: Linking decreased retinoic acid levels with disrupted meiosis in developing oocytes

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The first case study to develop and publish an individual KER as a stand-alone unit of information under the AOP framework overseen by the OECD.

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Full description of a KER linking decreased all-trans retinoic acid (atRA) levels in developing ovaries with disrupted meiotic entry of oogonia.

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KER described is associated with an intended AOP linking inhibition of the atRA producing ALDH1A enzymes with reduced fertility in women.

The Adverse Outcome Pathway (AOP) concept is an emerging tool in regulatory toxicology that uses simplified descriptions to show cause-effect relationships between stressors and toxicity outcomes in intact organisms. The AOP structure is a modular framework, with Key Event Relationships (KERs) representing the unit of causal relationship based on existing knowledge, describing the connection between two Key Events. Because KERs are the only unit to support inference it has been argued recently that KERs should be recognized as the core building blocks of knowledge assembly within the AOP-Knowledge Base. Herein, we present a first case to support this proposal and provide a full description of a KER linking decreased all-trans retinoic acid (atRA) levels in developing ovaries with disrupted meiotic entry of oogonia. We outline the evidence to support a role for atRA in inducing meiosis in oogonia across mammals; this is important because elements of the RA synthesis/degradation pathway are recognized targets for numerous environmental chemicals. The KER we describe will be used to support an intended AOP linking inhibition of the atRA producing ALDH1A enzymes with reduced fertility in women.

Retinoid X receptor activation promotes photoreceptor survival and modulates the inflammatory response in a mouse model of retinitis pigmentosa

Photoreceptor cell (PHR) death is a hallmark of most retinal neurodegenerative diseases, in which inflammation plays a critical role. Activation of retinoid X receptors (RXR) modulates and integrates multiple cell functions, and has beneficial effects in animal models of chronic inflammatory diseases. Nonetheless, the mechanisms involved and their role in retina neuroprotection are poorly understood. In this work we assessed whether RXR activation prevents inflammation and/or PHR death in retinitis pigmentosa, an inherited retina neurodegeneration, using as an ex vivo model, retinas from the rd1 mice, a murine model of this disease. We demonstrated that rd1 retinas had lower levels of RXR alpha isoform than their wt counterparts at early developmental times, whereas its distribution pattern remained similar. In mixed neuro-glial cultures obtained from either rd1 or wt retinas, both PHR and Müller glial cells (MGC) expressed RXRalpha, and RXR activation by its synthetic pan-agonist PA024 selectively increased mRNA levels of RXRgamma isoform. PA024 decreased PHR death in rd1 mixed cultures; it reduced the amount of non-viable neurons, delayed the onset of PHR apoptosis, and decreased Bax mRNA levels. PA024 also reduced MGC reactivity in vitro before and at the onset of degeneration, decreasing GFAP expression, increasing glutamine synthetase mRNA levels, and promoting the transcription of the anti-inflammatory cytokine, Il-10. These results suggest that RXR activation rescues rd1 PHR and decreases MGC reactivity, promoting an anti-inflammatory environment in the rd1 retina, thus supporting the potential of RXR agonists as pharmacological tools for treating retina degenerative diseases.

Retinoid X receptor α downregulation is required for tail and caudal spinal cord regeneration in the adult newt

Some adult vertebrate species, such as newts, axolotls and zebrafish, have the ability to regenerate their central nervous system (CNS). However, the factors that establish a permissive CNS environment for correct morphological and functional regeneration in these species are not well understood. Recent evidence supports a role for retinoid signaling in the intrinsic ability of neurons, in these regeneration-competent species, to regrow after CNS injury. Previously, we demonstrated that a specific retinoic acid receptor (RAR) subtype, RARβ, mediates the effects of endogenous retinoic acid (RA) on neuronal growth and guidance in the adult newt CNS after injury. Here, we now examine the expression of the retinoid X receptor RXRα (a potential heterodimeric transcriptional regulator with RARβ), in newt tail and spinal cord regeneration. We show that at 21 days post-amputation (dpa), RXRα is expressed at temporally distinct periods and in non-overlapping spatial domains compared to RARβ. Whereas RARβ protein levels increase, RXRα proteins level decrease by 21 dpa. A selective agonist for RXR, SR11237, prevents both this downregulation of RXRα and upregulation of RARβ and inhibits tail and caudal spinal cord regeneration. Moreover, treatment with a selective antagonist for RARβ, LE135, inhibits regeneration with the same morphological consequences as treatment with SR11237. Interestingly, LE135 treatment also inhibits the normal downregulation of RXRα in tail and spinal cord tissues at 21 dpa. These results reveal a previously unidentified, indirect regulatory feedback loop between these two receptor subtypes in regulating the regeneration of tail and spinal cord tissues in this regeneration-competent newt.

Endogenous retinoid X receptor ligands in mouse hematopoietic cells

The retinoid X receptor α (RXRA) has been implicated in diverse hematological processes. To identify natural ligands of RXRA that are present in hematopoietic cells, we adapted an upstream activation sequence-green fluorescent protein (UAS-GFP) reporter mouse to detect natural RXRA ligands in vivo. We observed reporter activity in diverse types of hematopoietic cells in vivo. Reporter activity increased during granulocyte colony-stimulating factor (G-CSF)-induced granulopoiesis and after phenylhydrazine (PHZ)-induced anemia, suggesting the presence of dynamically regulated natural RXRA ligands in hematopoietic cells. Mouse plasma activated Gal4-UAS reporter cells in vitro, and plasma from mice treated with G-CSF or PHZ recapitulated the patterns of reporter activation that we observed in vivo. Plasma from mice with dietary vitamin A deficiency only mildly reduced RXRA reporter activity, whereas plasma from mice on a fatty acid restriction diet reduced reporter activity, implicating fatty acids as plasma RXRA ligands. Through differential extraction coupled with mass spectrometry, we identified the long-chain fatty acid C24:5 as a natural RXRA ligand that was greatly increased in abundance in response to hematopoietic stress. Together, these data suggest that natural RXRA ligands are present and dynamically increased in abundance in mouse hematopoietic cells in vivo.

An Endogenous Mammalian Retinoid X Receptor Ligand, At Last!

9-cis-Retinoic acid was identified and claimed to be the endogenous ligand of the retinoid X receptors (RXRs) in 1992. Since then, the endogenous presence of this compound has never been rigorously confirmed. Instead, concerns have been raised by other groups that have reported that 9-cis-retinoic acid is undetectable or that its presence occurs at very low levels. Furthermore, these low levels could not satisfactorily explain the physiological activation of RXR. Alternative ligands, among them various lipids, have also been identified, but also did not fulfill criteria for rigorous endogenous relevance, and their consideration as bona fide endogenous mammalian RXR ligand has likewise been questioned. Recently, novel studies claim that the saturated analogue 9-cis-13,14-dihydroretinoic acid functions as an endogenous physiologically relevant mammalian RXR ligand.

Light, lipids and photoreceptor survival: live or let die?

Due to its constant exposure to light and its high oxygen consumption the retina is highly sensitive to oxidative damage, which is a common factor in inducing the death of photoreceptors after light damage or in inherited retinal degenerations. The high content of docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in the retina, has been suggested to contribute to this sensitivity. DHA is crucial for developing and preserving normal visual function. However, further roles of DHA in the retina are still controversial. Current data support that it can tilt the scale either towards degeneration or survival of retinal cells. DHA peroxidation products can be deleterious to the retina and might lead to retinal degeneration. However, DHA has also been shown to act as, or to be the source of, a survival molecule that protects photoreceptors and retinal pigment epithelium cells from oxidative damage. We have established that DHA protects photoreceptors from oxidative stress-induced apoptosis and promotes their differentiation in vitro. DHA activates the retinoid X receptor (RXR) and the ERK/MAPK pathway, thus regulating the expression of anti and pro-apoptotic proteins. It also orchestrates a diversity of signaling pathways, modulating enzymatic pathways that control the sphingolipid metabolism and activate antioxidant defense mechanisms to promote photoreceptor survival and development. A deeper comprehension of DHA signaling pathways and context-dependent behavior is required to understand its dual functions in retinal physiology.

In vivo dynamics of estrogen receptor activity: the ERE-Luc model

In recent years several studies demonstrated the presence of estrogen receptors in mammalian tissues and significantly improved our understanding of their ability to control biological processes in reproductive as well as non-reproductive organs. Considering the manifold mechanisms and organs that are involved in estrogen action and the implication of estrogens in human female physiology, innovative approaches are required to shed light on the widespread activities of estrogen receptors in woman physiology. This is particularly relevant for the definition of novel, more efficacious hormonal replacement therapies or for the evaluation of the risk associated with the exposure to endocrine disruptors. The introduction of genetic engineering and the development and application of in vivo imaging techniques offer new tools for pre-clinical studies. The generation of the ERE-Luc mouse, a reporter animal developed for in vivo studies of the estrogen receptor activity, allows assessing the activity state of the ER signaling pathway in all target tissues and organs at once, under physiological stimuli or as a result of a pharmacological treatment. This review summarizes the main steps in the generation and appraisal of the estrogen receptor reporter mouse ERE-Luc, designed for in vivo molecular imaging studies, and describes examples demonstrating the suitability of the ERE-Luc model for drug development and for the investigation of the effects of endogenous, environmental, and dietary estrogens in vivo. This article is part of a Special Issue entitled 'Phytoestrogens'.

Unsupervised automated high throughput phenotyping of RNAi time-lapse movies

Background

Gene perturbation experiments in combination with fluorescence time-lapse cell imaging are a powerful tool in reverse genetics. High content applications require tools for the automated processing of the large amounts of data. These tools include in general several image processing steps, the extraction of morphological descriptors, and the grouping of cells into phenotype classes according to their descriptors. This phenotyping can be applied in a supervised or an unsupervised manner. Unsupervised methods are suitable for the discovery of formerly unknown phenotypes, which are expected to occur in high-throughput RNAi time-lapse screens.

Results

We developed an unsupervised phenotyping approach based on Hidden Markov Models (HMMs) with multivariate Gaussian emissions for the detection of knockdown-specific phenotypes in RNAi time-lapse movies. The automated detection of abnormal cell morphologies allows us to assign a phenotypic fingerprint to each gene knockdown. By applying our method to the Mitocheck database, we show that a phenotypic fingerprint is indicative of a gene’s function.

Conclusion

Our fully unsupervised HMM-based phenotyping is able to automatically identify cell morphologies that are specific for a certain knockdown. Beyond the identification of genes whose knockdown affects cell morphology, phenotypic fingerprints can be used to find modules of functionally related genes.

Retinoid X receptor activation is essential for docosahexaenoic acid protection of retina photoreceptors

We have established that docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in the retina, promotes survival of rat retina photoreceptors during early development in vitro and upon oxidative stress by activating the ERK/MAPK signaling pathway. Here we have investigated whether DHA turns on this pathway through activation of retinoid X receptors (RXRs) or by inducing tyrosine kinase (Trk) receptor activation. We also evaluated whether DHA release from phospholipids was required for its protective effect. Addition of RXR antagonists (HX531, PA452) to rat retinal neuronal cultures inhibited DHA protection during early development in vitro and upon oxidative stress induced with Paraquat or H2O2. In contrast, the Trk inhibitor K252a did not affect DHA prevention of photoreceptor apoptosis. These results imply that activation of RXRs was required for DHA protection whereas Trk receptors were not involved in this protection. Pretreatment with 4-bromoenol lactone, a phospholipase A2 inhibitor, blocked DHA prevention of oxidative stress-induced apoptosis of photoreceptors. It is noteworthy that RXR agonists (HX630, PA024) also rescued photoreceptors from H2O2-induced apoptosis. These results provide the first evidence that activation of RXRs prevents photoreceptor apoptosis and suggest that DHA is first released from phospholipids and then activates RXRs to promote the survival of photoreceptors.

Retinoic acid signaling in spinal cord development

Retinoic acid (RA) is an important signaling molecule mediating intercellular communication through vertebrate development. Here, we present and discuss recent information on the roles of the RA signaling pathway in spinal cord development. RA is an important player in the patterning and definition of the spinal cord territory from very early stages of development, even before the appearance of the neural plate and further serves a role in the patterning of the spinal cord both along the dorsoventral and anteroposterior axes, particularly in the promotion of neuronal differentiation. It is thus required to establish a variety of neuronal cell types at specific positions of the spinal cord. The main goal of this review is to gather information from vertebrate models, including fish, frogs, chicken and mice, and to put this information in a comparative context in an effort to visualize how the RA pathway was incorporated into the evolving vertebrate spinal cord and to identify mechanisms that are both common and different in the various vertebrate models. In doing so, we try to reconstruct how spinal cord development has been regulated by the RA signaling cascade through vertebrate diversification, highlighting areas which require further studies to obtain a better understanding of the evolutionary events that shaped this structure in the vertebrate lineage.

Nuclear hormone receptors enable macrophages and dendritic cells to sense their lipid environment and shape their immune response

A key issue in the immune system is to generate specific cell types, often with opposing activities. The mechanisms of differentiation and subtype specification of immune cells such as macrophages and dendritic cells are critical to understand the regulatory principles and logic of the immune system. In addition to cytokines and pathogens, it is increasingly appreciated that lipid signaling also has a key role in differentiation and subtype specification. In this review we explore how intracellular lipid signaling via a set of transcription factors regulates cellular differentiation, subtype specification, and immune as well as metabolic homeostasis. We introduce macrophages and dendritic cells and then we focus on a group of transcription factors, nuclear receptors, which regulate gene expression upon receiving lipid signals. The receptors we cover are the ones with a recognized physiological function in these cell types and ones which heterodimerize with the retinoid X receptor. These are as follows: the receptor for a metabolite of vitamin A, retinoic acid: retinoic acid receptor (RAR), the vitamin D receptor (VDR), the fatty acid receptor: peroxisome proliferator-activated receptor γ (PPARγ), the oxysterol receptor liver X receptor (LXR), and their obligate heterodimeric partner, the retinoid X receptor (RXR). We discuss how they can get activated and how ligand is generated and eliminated in these cell types. We also explore how activation of a particular target gene contributes to biological functions and how the regulation of individual target genes adds up to the coordination of gene networks. It appears that RXR heterodimeric nuclear receptors provide these cells with a coordinated and interrelated network of transcriptional regulators for interpreting the lipid milieu and the metabolic changes to bring about gene expression changes leading to subtype and functional specification. We also show that these networks are implicated in various immune diseases and are amenable to therapeutic exploitation.

NR4A nuclear receptors mediate carnitine palmitoyltransferase 1A gene expression by the rexinoid HX600

Retinoid X receptors (RXRs) are members of the nuclear receptor superfamily and can be activated by 9-cis retinoic acid (9CRA). RXRs form homodimers and heterodimers with other nuclear receptors such as the retinoic acid receptor and NR4 subfamily nuclear receptors, Nur77 and NURR1. Potential physiological roles of the Nur77-RXR and NURR1-RXR heterodimers have not been elucidated. In this study, we identified a gene regulated by these heterodimers utilizing HX600, a selective RXR agonist for Nur77-RXR and NURR1-RXR. While 9CRA induced many genes, including RAR-target genes, HX600 effectively induced only carnitine palmitoyltransferase 1A (CPT1A) in human teratocarcinoma NT2/D1 cells, which express RXRα, Nur77 and NURR1. HX600 also increased CPT1A expression in human embryonic kidney (HEK) 293 cells and hepatocyte-derived HepG2 cells. Although HX600 induced CPT1A less effectively than 9CRA, overexpression of Nur77 or NURR1 increased the HX600 response to levels similar to 9CRA in NT2/D1 and HEK293 cells. A dominant-negative form of Nur77 or NURR1 repressed the induction of CPT1A by HX600. A protein synthesis inhibitor did not alter HX600-dependent CPT1A induction. Thus, the rexinoid HX600 directly induces expression of CPT1A through a Nur77 or NURR1-mediated mechanism. CPT1A, a gene involved in fatty acid β-oxidation, could be a target of RXR-NR4 receptor heterodimers.

Retinoid receptors trigger neuritogenesis in retinal degenerations

Anomalous neuritogenesis is a hallmark of neurodegenerative disorders, including retinal degenerations, epilepsy, and Alzheimer's disease. The neuritogenesis processes result in a partial reinnervation, new circuitry, and functional changes within the deafferented retina and brain regions. Using the light-induced retinal degeneration (LIRD) mouse model, which provides a unique platform for exploring the mechanisms underlying neuritogenesis, we found that retinoid X receptors (RXRs) control neuritogenesis. LIRD rapidly triggered retinal neuron neuritogenesis and up-regulated several key elements of retinoic acid (RA) signaling, including retinoid X receptors (RXRs). Exogenous RA initiated neuritogenesis in normal adult retinas and primary retinal cultures and exacerbated it in LIRD retinas. However, LIRD-induced neuritogenesis was partly attenuated in retinol dehydrogenase knockout (Rdh12(-/-)) mice and by aldehyde dehydrogenase inhibitors. We further found that LIRD rapidly increased the expression of glutamate receptor 2 and β Ca(2+)/calmodulin-dependent protein kinase II (βCaMKII). Pulldown assays demonstrated interaction between βCaMKII and RXRs, suggesting that CaMKII pathway regulates the activities of RXRs. RXR antagonists completely prevented and RXR agonists were more effective than RA in inducing neuritogenesis. Thus, RXRs are in the final common path and may be therapeutic targets to attenuate retinal remodeling and facilitate global intervention methods in blinding diseases and other neurodegenerative disorders.

Juvenile hormone action through a defined enhancer motif to modulate ecdysteroid-activation of natural core promoters

We have established a model system of hormone action, in an Sf9 cell transfection system, using defined enhancer motifs and natural core promoters of metamorphosis-associated genes. The DR1 enhancer, that is an established DNA binding site for the ecdysone receptor/ultraspiracle heterodimer, was necessary for transcriptional activation by 20-OH ecdysone. For this activated transcription, a natural sequence closely 5' to the TATA box is necessary. Cotreatment with juvenile hormone III strongly suppressed the steroid activation of transcription. However, in the absence of the sequence located closely 5' to the TATA box, cotreatment with juvenile hormone instead increased transcription over that occurring due to 20-hydroxy-ecdysone alone. This sensitivity to activation by cotreatment with juvenile hormone could be transferred to a related, but otherwise unresponsive, hexamerin core promoter simply by transferring to the unresponsive promoter the five base transcription start site (ACAGT) from the responsive hexamerin gene. These are the first reports that the direction of JH action on 20-OH ecdysone-activated transcription can be reversed by removal of a sequence at the core promoter, and that modulatory action of juvenile hormone can be transferred to a different gene by transferring the transcription start site motif.

[The cloning, expression and the binding ability with TRβ1 of retinoid X receptor-α gene]

Retinoid X receptor-α (RXR-α), a member of nuclear receptor family, is capable of mediating retinoid signaling pathways and plays a critical role in regulating target gene transcription. To further study the function of RXR-α, abundant of recombinant RXR-α protein in hand is necessary. In this study an intact RXR-α coding sequence was amplified by RT-PCR and subsequently inserted into expression plasmid vector pQE-30Xa to form the recombinant construct of pQE-30Xa/RXR-α. Thereafter, competent bacteria Escherichia coli M15 [PREP4] was transformed and the expression of RXR-α was induced by adding IPTG to the medium. Bacterially expressed recombinant RXR-α was purified by Ni-NTA affinity chromatography and verified by SDS-PAGE and Western blotting analyses. The results showed that a protein, with the molecular mass around 50 kDa, could be selectively recognized by anti-RXR-α antibody. Co-immunoprecipitation assay indicated that this recombinant RXR-α could effectively bind TRβ1 to form a heterodimer, which could specifically bind the target DNA fragment. This was confirmed by EMSA. In conclusion, the recombinant human retinoid X receptor-α was prepared successfully, which makes a basic for further study of its function.

Developmental expression of a molluscan RXR and evidence for its novel, nongenomic role in growth cone guidance

It is well known that the vitamin A metabolite, retinoic acid, plays an important role in vertebrate development and regeneration. We have previously shown that the effects of RA in mediating neurite outgrowth, are conserved between vertebrates and invertebrates (Dmetrichuk et al., 2005, 2006) and that RA can induce growth cone turning in regenerating molluscan neurons (Farrar et al., 2009). In this study, we have cloned a retinoid receptor from the mollusc Lymnaea stagnalis (LymRXR) that shares about 80% amino acid identity with the vertebrate RXRalpha. We demonstrate using Western blot analysis that the LymRXR is present in the developing Lymnaea embryo and that treatment of embryos with the putative RXR ligand, 9-cis RA, or a RXR pan-agonist, PA024, significantly disrupts embryogenesis. We also demonstrate cytoplasmic localization of LymRXR in adult central neurons, with a strong localization in the neuritic (or axonal) domains. Using regenerating cultured motor neurons, we show that LymRXR is also present in the growth cones and that application of a RXR pan-agonist produces growth cone turning in isolated neurites (in the absence of the cell body and nucleus). These data support a role for RXR in growth cone guidance and are the first studies to suggest a nongenomic action for RXR in the nervous system.

A live zebrafish-based screening system for human nuclear receptor ligand and cofactor discovery

Nuclear receptors (NRs) belong to a superfamily of transcription factors that regulate numerous homeostatic, metabolic and reproductive processes. Taken together with their modulation by small lipophilic molecules, they also represent an important and successful class of drug targets. Although many NRs have been targeted successfully, the majority have not, and one third are still orphans. Here we report the development of an in vivo GFP-based reporter system suitable for monitoring NR activities in all cells and tissues using live zebrafish (Danio rerio). The human NR fusion proteins used also contain a new affinity tag cassette allowing the purification of receptors with bound molecules from responsive tissues. We show that these constructs 1) respond as expected to endogenous zebrafish hormones and cofactors, 2) facilitate efficient receptor and cofactor purification, 3) respond robustly to NR hormones and drugs and 4) yield readily quantifiable signals. Transgenic lines representing the majority of human NRs have been established and are available for the investigation of tissue- and isoform-specific ligands and cofactors.

Ligand modulated antagonism of PPARgamma by genomic and non-genomic actions of PPARdelta

Background

Members of the Peroxisome Proliferator Activated Receptor, PPAR, subfamily of nuclear receptors display complex opposing and overlapping functions and a wide range of pharmacological and molecular genetic tools have been used to dissect their specific functions. Non-agonist bound PPARδ has been shown to repress PPAR Response Element, PPRE, signalling and several lines of evidence point to the importance of PPARδ repressive actions in both cardiovascular and cancer biology.

Methodology/Principal Findings

In this report we have employed transient transfections and luciferase reporter gene technology to study the repressing effects of PPARδ and two derivatives thereof. We demonstrate for the first time that the classical dominant negative deletion of the Activation Function 2, AF2, domain of PPARδ show enhanced repression of PPRE signalling in the presence of a PPARδ agonist. We propose that the mechanism for the phenomenon is increased RXR heterodimerisation and DNA binding upon ligand binding concomitant with transcriptional co-repressor binding. We also demonstrated ligand-dependent dominant negative action of a DNA non-binding derivative of PPARδ on PPARγ1 signalling. This activity was abolished upon over-expression of RXRα suggesting a role for PPAR/cofactor competition in the absence of DNA binding.

Conclusions/Significance

These findings are important in understanding the wide spectrum of molecular interactions in which PPARδ and PPARγ have opposing biological roles and suggest novel paradigms for the design of different functional classes of nuclear receptor antagonist drugs.

Growth factor-antagonized rexinoid apoptosis involves permissive PPARgamma/RXR heterodimers to activate the intrinsic death pathway by NO

Growth factor (GF) deprivation and/or blocking of cognate signaling can induce apoptosis and is the basis of several cancer treatment paradigms. We observed that RXR agonists (rexinoids) induce apoptosis of tumor cells when GF support is abrogated. This "rexinoid apoptosis" involves activation of both iNOS and eNOS by RXR-PPARgamma and results in production of apoptogenic NO. IGF/EGF-induced IGF receptor 1-mediated MAP kinase blocks rexinoid apoptosis by RXR phosphorylation. Combining rexinoids with the MAPK inhibitor U0126 induced apoptosis in human cancer cells in vitro and ex vivo and blocked xenograft growth in vivo. Our results suggest a regulatory mechanism in which GF signaling antagonizes RXR-PPARgamma-mediated default apoptosis to sustain cell life.

Restricted patterns of Hoxd10 and Hoxd11 set segmental differences in motoneuron subtype complement in the lumbosacral spinal cord

During normal vertebrate development, Hoxd10 and Hoxd11 are expressed by differentiating motoneurons in restricted patterns along the rostrocaudal axis of the lumbosacral (LS) spinal cord. To assess the roles of these genes in the attainment of motoneuron subtypes characteristic of LS subdomains, we examined subtype complement after overexpression of Hoxd10 or Hoxd11 in the embryonic chick LS cord and in a Hoxd10 loss-of-function mouse embryo. Data presented here provide evidence that Hoxd10 defines the position of the lateral motor column (LMC) as a whole and, in rostral LS segments, specifically promotes the development of motoneurons of the lateral subdivision of the lateral motor column (LMCl). In contrast, Hoxd11 appears to impart a caudal and medial LMC (LMCm) identity to some motoneurons and molecular profiles suggestive of a suppression of LMC development in others. We also provide evidence that Hoxd11 suppresses the expression of Hoxd10 and the retinoic acid synthetic enzyme, retinaldehyde dehydrogenase 2 (RALDH2). In a normal chick embryo, Hoxd10 and RALDH2 are expressed throughout the LS region at early stages of motoneuron differentiation but their levels decline in Hoxd11-expressing caudal LS segments that ultimately contain few LMCl motoneurons. We hypothesize that one of the roles played by Hoxd11 is to modulate Hoxd10 and local retinoic acid levels and thus, perhaps define the caudal boundaries of the LMC and its subtype complement.

Nolz1 is induced by retinoid signals and controls motoneuron subtype identity through distinct repressor activities

The acquisition and maintenance of final neuronal identity depends in part upon the implementation of fate-specification programs in postmitotic neurons; however, the mechanisms involved remain unclear. In the developing spinal cord, retinoic acid (RA) signaling pathways specify the columnar and divisional identities of postmitotic motoneurons (MNs). Here we show that RA signals induce expression of the NET transcriptional regulator Nolz1 in differentiated chick MNs, where it regulates the progressive specification of prospective Lim3-negative motor columns. Nolz1 controls the initial formation of forelimb and thoracic Lim3-negative motor columns by downregulating Lim3 expression and maintaining the expression of key homeodomain proteins necessary for MN identity and survival. At forelimb levels, Nolz1 specifies lateral motor column (LMC) identity by inducing the expression of the postmitotic LMC determinant Hoxc6, and implements the partial specification of lateral LMC identity through Lim1 induction. The specificity of Nolz1 function depends upon distinct repressor activities that require, in part, the modulatory activity of Grg5, an atypical member of the Gro-TLE family of co-repressors. Thus, RA signals regulate diverse events in MN subtype specification by inducing the expression of a key transcriptional regulator that controls multiple developmental pathways via functionally distinct repressor complexes.

Gal4/UAS transgenic tools and their application to zebrafish

The ability to regulate gene expression in a cell-specific and temporally restricted manner provides a powerful means to test gene function, bypass the action of lethal genes, label subsets of cells for developmental studies, monitor subcellular structures, and target tissues for selective ablation or physiological analyses. The galactose-inducible system of yeast, mediated by the transcriptional activator Gal4 and its consensus UAS binding site, has proven to be a highly successful and versatile system for controlling transcriptional activation in Drosophila. It has also been used effectively, albeit in a more limited manner, in the mouse. While zebrafish has lagged behind other model systems in the widespread application of Gal4 transgenic approaches to modulate gene activity during development, recent technological advances are permitting rapid progress. Here we review Gal4-regulated genetic tools and discuss how they have been used in zebrafish as well as their potential drawbacks. We describe some exciting new directions, in large part afforded by the Tol2 transposition system, that are generating valuable new Gal4/UAS reagents for zebrafish research.

Locust retinoid X receptors: 9-Cis-retinoic acid in embryos from a primitive insect

The retinoid X receptor (RXR) is activated by its often elusive cognate ligand, 9-cis-retinoic acid (9-cis-RA). In flies and moths, molting is mediated by a heterodimer ecdysone receptor consisting of the ecdysone monomer (EcR) and an RXR homolog, ultraspiracle (USP); the latter is believed to have diverged from its RXR origin. In the more primitive insect, Locusta migratoria (Lm), RXR is more similar to human RXRs than to USPs. LmRXR was detected in early embryos when EcR transcripts were absent, suggesting another role apart from ecdysone signaling. Recombinant LmRXRs bound 9-cis-RA and all-trans-RA with high affinity (IC(50) = 61.2-107.7 nM; K(d) = 3 nM), similar to human RXR. To determine whether specific binding had functional significance, the presence of endogenous retinoids was assessed. Embryos were extracted by using modified Bligh and Dyer and solid-phase protocols to avoid the oily precipitate that makes this material unsuitable for assay. These extracts contained retinoids (5.4 nM) as assessed by RA-inducible Cyp26A1-promoter luciferase reporter cell lines. Furthermore, the use of HPLC and MS confirmed the presence of retinoids and identified in any embryo, 9-cis-RA, in addition to all-trans-RA. We estimate that whole embryos contain 3 nM RA, including 9-cis-RA at a concentration of 1.6 nM. These findings strongly argue for a functional role for retinoids in primitive insects and favor a model where signaling through the binding of 9-cis-RA to its RXR is established relatively early in evolution and embryonic development.

RAR/RXR and PPAR/RXR signaling in neurological and psychiatric diseases

Retinoids are important signals in brain development. They regulate gene transcription by binding to retinoic acid receptors (RAR) and, as was discovered recently, a peroxisome proliferator-activated receptor (PPAR). Traditional ligands of PPAR are best known for their functions in lipid metabolism and inflammation. RAR and PPAR are ligand-activated transcription factors, which share members of the retinoid X receptor (RXR) family as heterodimeric partners. Both signal transduction pathways have recently been implicated in the progression of neurodegenerative and psychiatric diseases. Since inflammatory processes contribute to various neurodegenerative diseases, the anti-inflammatory activity of retinoids and PPARgamma agonists recommends them as potential therapeutic targets. In addition, genetic linkage studies, transgenic mouse models and experiments with vitamin A deprivation provide evidence that retinoic acid signaling is directly involved in physiology and pathology of motoneurons, of the basal ganglia and of cognitive functions. The activation of PPAR/RXR and RAR/RXR transcription factors has therefore been proposed as a therapeutic strategy in disorders of the central nervous system.

Regulation and function of Dbx genes in the zebrafish spinal cord

Dbx homeodomain proteins are important for spinal cord dorsal/ventral patterning and the production of multiple spinal cord cell types. We have examined the regulation and function of Dbx genes in the zebrafish. We report that Hedgehog signaling is not required for the induction or maintenance of these genes; in the absence of Hedgehog signaling, dbx1a/1b/2 are expanded ventrally with concomitant increases in postmitotic neurons that differentiate from this domain. Also, we find that retinoic acid signaling is not required for the induction of Dbx expression. Furthermore, we are the first to report that knockdown of Dbx1 function causes a dorsal expansion of nkx6.2, which is thought to be the cross-repressive partner of Dbx1 in mouse. Our data confirm that the dbx1a/1b/2 domain in zebrafish spinal cord development behaves similarly to amniotes, while extending knowledge of Dbx1 function in spinal cord patterning.

Role of RXR in neurite outgrowth induced by docosahexaenoic acid

We have previously demonstrated that docosahexaenoic acid (DHA) at low micromolar concentrations has a remarkable effect on morphological differentiation of hippocampal neurons by increasing the population of neurons with more branches and longer neurites. In this study, possible involvement of the retinoid X receptor (RXR) in the DHA-induced hippocampal neurite outgrowth was evaluated as DHA is an endogenous ligand for RXR. Immunocytochemical examination revealed that all RXR isoforms, RXR-alpha, -beta(1), -beta(2), and -gamma, are expressed exclusively in neurons with distinctive intracellular distribution. The cell-based dual luciferase reporter assay indicated that DHA activates RXR-alpha at or above 10 microM but not at 1.5 microM where DHA induces neurite outgrowth. Arachidonic acid also activated RXR-alpha in a similar concentration range but with lower efficacy. Our results suggest that DHA-induced neurite outgrowth may not be mediated by direct activation of RXR-alpha, although involvement of other isoforms or DHA metabolites cannot be excluded.

Ras signaling modulates activity of the ecdysone receptor EcR during cell migration in the Drosophila ovary

Ecdysone Receptor (EcR) mediates effects of the hormone ecdysone during larval molts, pupal metamorphosis, and adult female oogenesis. In the ovary, egg chamber formation requires interactions between the somatic follicle cell (FC) epithelium and the germ line nurse cell/oocyte cyst. Previous work has shown EcR is required in the germ line for egg chamber maturation, and here we examine EcR requirements in the FC at late stages of oogenesis. EcR protein is ubiquitous in the FC but its activity is restricted, visualized by activity of the "ligand sensor" hs-GAL4-EcR ligand binding domain fusion and EcRE-lacZ reporter gene expression. GAL4-EcR is activated in the FC by an ecdysone agonist and repressed by tissue-specific Ras GTPase signals. To determine the significance of restricted sites of EcR activity in the FC, we used targeted misexpression of the dominant negative EcR (EcR-DN) molecules EcR(F645A) and EcR(W650A). EcR-DN expression at stage 10 reduced EcRE-lacZ expression in the nurse cell FC and resulted in abnormal FC migrations, including aberrant centripetal migration and dorsal appendage tube formation, leading to the formation of cup-shaped eggs with shortened, branched dorsal appendages at stage 14. Clones of FC expressing EcR-DN displayed cell-autonomous increases in DE-cadherin expression and abnormal epithelial junction formation. EcR-DN expression caused thin eggshell phenotypes that correlated with both reduced levels of chorion gene expression and reduction in chorion gene amplification. Our results indicate that tissue-specific modulation of EcR activity by the Ras signaling pathway refines temporal ecdysone signals that regulate FC differentiation and cadherin-mediated epithelial cell shape changes.

Expression of cellular retinoic acid-binding protein I is specific to neurons in adult transgenic mouse brain

Cellular retinoic acid binding protein I (CRABP-I) plays a role in retinoic acid (RA) metabolism or transport. This report shows specific neuronal expression of CRABP-I in adult transgenic mouse brain using CRABP-I promotor-driven lac-Z and neuron- and astrocyte-markers. Double staining indicates that CRABP-I is expressed in neurons and large cells (>12 microm) but to much lesser degree the astrocytes. CRABP-I-lac-Z(+) neurons were distributed throughout the brain, but in a very discreet pattern in each brain region. CRABP-I expression in specific populations of brain neurons suggests that RA is extensively metabolized in mature brains, mostly in neurons. Additionally, the genetic basis of its specific expression in these brain areas is located in the 5' regulatory region of this gene.

Cancer modeling: modern imaging applications in the generation of novel animal model systems to study cancer progression and therapy

Cancer is the result of a series of genetic and epigenetic mutations that evolve over years even decades and lead to the transformed phenotype. Paradoxically, most methods developed to study these changes are static and do not provide insights on the dynamics of the sequela of steps involved in tumorigenesis. This major shortcoming now can be overcome with the application of reporter genes and imaging technologies, which are providing tools to examine specific molecular events and their role in the carcinogenic process in single cells. In the last decade reporter-based biosensors were created to study gene transcription, protein/protein interactions, sub-cellular trafficking and protease activities; this wealth of systems enable to monitor intracellular signaling pathways at several key check points specifically involved in cancer cell development. The challenge is now to extend cell-based models to the generation of reporter mice, where non-invasive in vivo imaging technologies allow to follow single molecular events. When combined with murine models of cancer, these technologies will give an unprecedented opportunity to spatio-temporally investigate the molecular events resulting in neoplasia. The aim of the present review is to highlight the major changes occurring in this rapidly evolving field and their potential for increasing our knowledge in cancer biology and for the research of novel and more efficacious therapies.

International Union of Pharmacology. LXIII. Retinoid X receptors

The physiological effects of retinoic acids (RAs) are mediated by members of two families of nuclear receptors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs), which are encoded by three distinct human genes, RXRalpha, RXRbeta, and RXRgamma. RARs bind both all-trans- and 9-cis-RA, whereas only the 9-cis-RA stereoisomer binds to RXRs. As RXR/RAR heterodimers, these receptors control the transcription of RA target genes through binding to RA-response elements. This review is focused on the structure, mode of action, ligands, expression, and pharmacology of RXRs. Given their role as common partners to many other members of the nuclear receptor superfamily, these receptors have been the subject of intense scrutiny. Moreover, and despite numerous studies since their initial discovery, RXRs remain enigmatic nuclear receptors, and there is still no consensus regarding their role. Indeed, multiple questions about the actual biological role of RXRs and the existence of an endogenous ligand have still to be answered.

All-trans retinoic acid induces COX-2 and prostaglandin E2 synthesis in SH-SY5Y human neuroblastoma cells: involvement of retinoic acid receptors and extracellular-regulated kinase 1/2

BACKGROUND

Our recent results show that all-trans retinoic acid (ATRA), an active metabolite of vitamin A, induces COX-dependent hyperalgesia and allodynia in rats. This effect was mediated by retinoic acid receptors (RARs) and was associated with increased COX-2 expression in the spinal cord. Since ATRA also up-regulated COX-2 expression in SH-SY5Y human neuroblastoma cells, the current study was undertaken to analyze in these cells the mechanism through which ATRA increases COX activity.

METHODS

Cultured SH-SY5Y neuroblastoma cells were treated with ATRA. COX expression and kinase activity were analyzed by western blot. Transcriptional mechanisms were analyzed by RT-PCR and promoter assays. Pharmacological inhibitors of kinase activity and pan-antagonists of RAR or RXR were used to assess the relevance of these signaling pathways. Production of prostaglandin E2 (PGE2) was quantified by enzyme immunoabsorbent assay. Statistical significance between individual groups was tested using the non-parametric unpaired Mann-Whitney U test.

RESULTS

ATRA induced a significant increase of COX-2 expression in a dose- and time-dependent manner in SH-SY5Y human neuroblastoma cells, while COX-1 expression remained unchanged. Morphological features of differentiation were not observed in ATRA-treated cells. Up-regulation of COX-2 protein expression was followed by increased production of PGE2. ATRA also up-regulated COX-2 mRNA expression and increased the activity of a human COX-2 promoter construct. We next explored the participation of RARs and mitogen-activated peptide kinases (MAPK). Pre-incubation of SH-SY5Y human neuroblastoma cells with either RAR-pan-antagonist LE540 or MAP kinase kinase 1 (MEK-1) inhibitor PD98059 resulted in the abolition of ATRA-induced COX-2 promoter activity, COX-2 protein expression and PGE2 production whereas the retinoid X receptor pan-antagonist HX531, the p38 MAPK inhibitor SB203580 or the c-Jun kinase inhibitor SP600125 did not have any effect. The increase in RAR-beta expression and extracellular-regulated kinase 1/2(ERK1/2) phosphorylation in ATRA-incubated cells suggested that RARs and ERK1/2 were in fact activated by ATRA in SH-SY5Y human neuroblastoma cells.

CONCLUSION

These results highlight the importance of RAR-dependent and kinase-dependent mechanisms for ATRA-induced COX-2 expression and activity.

RXR: From Partnership to Leadership in Metabolic Regulations

Vitamin A signaling occurs through nuclear receptors recognizing diverse forms of retinoic acid (RA). The retinoic acid receptors (RARs) bind all-trans RA and its 9-cis isomer (9-cis RA). They convey most of the activity of RA, particularly during embryogenesis. The second subset of receptors, the rexinoid receptors (RXRs), binds 9-cis RA only. However, RXRs are obligatory DNA-binding partners for a number of nuclear receptors, broadening the spectrum of their biological activity to the corresponding nuclear receptor-signaling pathways. The present chapter more particularly focuses on RXR-containing transcriptional complexes for which RXR is not only a structural component necessary for DNA binding but also acts as a ligand-activated partner. After positioning RXR among the nuclear receptor superfamily in the first part, we will give an overview of three major signaling pathways involved in metabolism, which are sensitive to RXR activation: LXR:RXR, FXR:RXR, and PPAR:RXR. The third and last part is focused on RXR signaling and its potential role in metabolic regulation. Indeed, while the nature of the endogenous ligand for RXR is still in question, as we will discuss herein, a better understanding of RXR activities is necessary to envisage the potential therapeutic applications of synthetic RXR ligands.

Negative regulation of the Nrf1 transcription factor by its N-terminal domain is independent of Keap1: Nrf1, but not Nrf2, is targeted to the endoplasmic reticulum

Nrf1 (nuclear factor-erythroid 2 p45 subunit-related factor 1) and Nrf2 regulate ARE (antioxidant response element)-driven genes. At its N-terminal end, Nrf1 contains 155 additional amino acids that are absent from Nrf2. This 155-amino-acid polypeptide includes the N-terminal domain (NTD, amino acids 1-124) and a region (amino acids 125-155) that is part of acidic domain 1 (amino acids 125-295). Within acidic domain 1, residues 156-242 share 43% identity with the Neh2 (Nrf2-ECH homology 2) degron of Nrf2 that serves to destabilize this latter transcription factor through an interaction with Keap1 (Kelch-like ECH-associated protein 1). We have examined the function of the 155-amino-acid N-terminal polypeptide in Nrf1, along with its adjacent Neh2-like subdomain. Activation of ARE-driven genes by Nrf1 was negatively controlled by the NTD (N-terminal domain) through its ability to direct Nrf1 to the endoplasmic reticulum. Ectopic expression of wild-type Nrf1 and mutants lacking either the NTD or portions of its Neh2-like subdomain into wild-type and mutant mouse embryonic fibroblasts indicated that Keap1 controls neither the activity of Nrf1 nor its subcellular distribution. Immunocytochemistry showed that whereas Nrf1 gave primarily cytoplasmic staining that was co-incident with that of an endoplasmic-reticulum marker, Nrf2 gave primarily nuclear staining. Attachment of the NTD from Nrf1 to the N-terminus of Nrf2 produced a fusion protein that was redirected from the nucleus to the endoplasmic reticulum. Although this NTD-Nrf2 fusion protein exhibited less transactivation activity than wild-type Nrf2, it was nevertheless still negatively regulated by Keap1. Thus Nrf1 and Nrf2 are targeted to different subcellular compartments and are negatively regulated by distinct mechanisms.

Dynamic regulation of Drosophila nuclear receptor activity in vivo

Nuclear receptors are a large family of transcription factors that play major roles in development, metamorphosis, metabolism and disease. To determine how, where and when nuclear receptors are regulated by small chemical ligands and/or protein partners, we have used a 'ligand sensor' system to visualize spatial activity patterns for each of the 18 Drosophila nuclear receptors in live developing animals. Transgenic lines were established that express the ligand binding domain of each nuclear receptor fused to the DNA-binding domain of yeast GAL4. When combined with a GAL4-responsive reporter gene, the fusion proteins show tissue- and stage-specific patterns of activation. We show that these responses accurately reflect the presence of endogenous and exogenously added hormone, and that they can be modulated by nuclear receptor partner proteins. The amnioserosa, yolk, midgut and fat body, which play major roles in lipid storage, metabolism and developmental timing, were identified as frequent sites of nuclear receptor activity. We also see dynamic changes in activation that are indicative of sweeping changes in ligand and/or co-factor production. The screening of a small compound library using this system identified the angular psoralen angelicin and the insect growth regulator fenoxycarb as activators of the Ultraspiracle (USP) ligand-binding domain. These results demonstrate the utility of this system for the functional dissection of nuclear receptor pathways and for the development of new receptor agonists and antagonists that can be used to modulate metabolism and disease and to develop more effective means of insect control.

Gli2 and Gli3 play distinct roles in the dorsoventral patterning of the mouse hindbrain

Sonic Hedgehog (Shh) signaling plays a critical role during dorsoventral (DV) patterning of the developing neural tube by modulating the expression of neural patterning genes. Overlapping activator functions of Gli2 and Gli3 have been shown to be required for motoneuron development and correct neural patterning in the ventral spinal cord. However, the role of Gli2 and Gli3 in ventral hindbrain development is unclear. In this paper, we have examined DV patterning of the hindbrain of Shh(-/-), Gli2(-/-) and Gli3(-/-) embryos, and found that the respective role of Gli2 and Gli3 is not only different between the hindbrain and spinal cord, but also at distinct rostrocaudal levels of the hindbrain. Remarkably, the anterior hindbrain of Gli2(-/-) embryos displays ventral patterning defects as severe as those observed in Shh(-/-) embryos suggesting that, unlike in the spinal cord and posterior hindbrain, Gli3 cannot compensate for the loss of Gli2 activator function in Shh-dependent ventral patterning of the anterior hindbrain. Loss of Gli3 also results in a distinct patterning defect in the anterior hindbrain, including dorsal expansion of Nkx6.1 expression. Furthermore, we demonstrate that ventral patterning of rhombomere 4 is less affected by loss of Gli2 function revealing a different requirement for Gli proteins in this rhombomere. Taken together, these observations indicate that Gli2 and Gli3 perform rhombomere-specific function during DV patterning of the hindbrain.

Retinoids and spinal cord development

The role that RA plays in the development and patterning of the spinal cord is discussed. The morphogenetic process of neurulation is described in which RA plays a role because in the absence of RA signaling spina bifida results. Following neural induction, RA is involved in several patterning events in the spinal cord. It is one of the posteriorizing factors along with FGFs and Wnts and as such patterns the cervical spinal cord acting via the Hoxc transcription factors. It is involved in the induction of neural differentiation via genes such as NeuroM. It plays a part in patterning the dorsoventral axis of the anterior spinal cord where it interacts with FGF, Shh, and BMPs and induces an interneuronal population of neurons called V0 and V1 and a subset of motor neurons known as LMCs. To perform these actions RA is synthesized in the adjacent paraxial mesoderm by the enzyme RALDH2 and acts in a paracrine fashion on the developing neural tube. In the final action of RA, it begins to be synthesized within the neural tube at brachial and lumbar levels in the LMCs. Later-born neurons migrate through this RALDH2-expressing region and induce differentiation in these migrating neurons, which become a subset of LMC neurons known as LMCls. Thus RA acts several times over in the development of the spinal cord and not on the cells in which it is synthesized, but in adjacent cells in a paracrine manner.

Overview of retinoid metabolism and function

Retinoids (vitamin A) are crucial for most forms of life. In chordates, they have important roles in the developing nervous system and notochord and many other embryonic structures, as well as in maintenance of epithelial surfaces, immune competence, and reproduction. The ability of all-trans retinoic acid to regulate expression of several hundred genes through binding to nuclear transcription factors is believed to mediate most of these functions. The role of all-trans retinoic may extend beyond the regulation of gene transcription because a large number of noncoding RNAs also are regulated by retinoic acid. Additionally, extra-nuclear mechanisms of action of retinoids are also being identified. In organisms ranging from prokaryotes to humans, retinal is covalently linked to G protein-coupled transmembrane receptors called opsins. These receptors function as light-driven ion pumps, mediators of phototaxis, or photosensory pigments. In vertebrates phototransduction is initiated by a photochemical reaction where opsin-bound 11-cis-retinal is isomerized to all-trans-retinal. The photosensitive receptor is restored via the retinoid visual cycle. Multiple genes encoding components of this cycle have been identified and linked to many human retinal diseases. Central aspects of vitamin A absorption, enzymatic oxidation of all-trans retinol to all-trans retinal and all-trans retinoic acid, and esterification of all-trans retinol have been clarified. Furthermore, specific binding proteins are involved in several of these enzymatic processes as well as in delivery of all-trans retinoic acid to nuclear receptors. Thus, substantial progress has been made in our understanding of retinoid metabolism and function. This insight has improved our view of retinoids as critical molecules in vision, normal embryonic development, and in control of cellular growth, differentiation, and death throughout life.

Vitamin A active metabolite, all-trans retinoic acid, induces spinal cord sensitization. I. Effects after oral administration

BACKGROUND AND PURPOSE

Retinoic acid is an active metabolite of vitamin A involved in the modulation of the inflammatory and nociceptive responses. The aim of the present study was to analyze the properties of spinal cord neuronal responses of male Wistar rats treated with all-trans retinoic acid (ATRA) p.o. in the normal situation and under carrageenan-induced inflammation. We also studied the expression and distribution of cyclooxygenases (COX) in the spinal cord.

EXPERIMENTAL APPROACH

Properties of spinal cord neurons were studied by means of the single motor unit technique. The expression of COX enzymes in the spinal cord was assessed by Western blot analysis and immunohistochemistry.

KEY RESULTS

Intensity thresholds for mechanical and electrical stimulation (C-fibers) were significantly lower in animals treated with ATRA than vehicle, either in normal rats or in rats with inflammation. The size of cutaneous receptive fields was also larger in animals treated with ATRA in the normal and inflammatory conditions. The expression of COX-2 enzyme, but not COX-1, was significantly higher in animals treated with ATRA. COX-2 labeling was observed in dorsal horn cells and in ventral horn motoneurons.

CONCLUSIONS AND IMPLICATIONS

In conclusion, the oral treatment with ATRA in rats induces a sensitization-like effect on spinal cord neuronal responses similar to that observed in animals with inflammation and might explain the enhancement of allodynia and hyperalgesia observed in previously published behavioral experiments. The mechanism of action involves an over-expression of COX-2, but not COX-1, in dorsal and ventral horn areas of the lumbar spinal cord.

Retinoic acid induces neurite outgrowth and growth cone turning in invertebrate neurons

Identification of molecules involved in neurite outgrowth during development and/or regeneration is a major goal in the field of neuroscience. Retinoic acid (RA) is a biologically important metabolite of vitamin A that acts as a trophic factor and has been implicated in neurite outgrowth and regeneration in many vertebrate species. Although abundant in the CNS of many vertebrates, the precise role of RA in neural regeneration has yet to be determined. Moreover, very little information is available regarding the role of RA in invertebrate nervous systems. Here, we demonstrate for the first time that RA induces neurite outgrowth from invertebrate neurons. Using individually identified neurons isolated from the CNS of Lymnaea stagnalis, we demonstrated that a significantly greater proportion of cells produced neurite outgrowth in RA. RA also extended the duration of time that cells remained electrically excitable in vitro, and we showed that exogenously applied RA acted as a chemoattractive factor and induced growth cone turning toward the source of RA. This is the first demonstration that RA can induce turning of an individual growth cone. These data strongly suggest that the actions of RA on neurite outgrowth and cell survival are highly conserved across species.

Mesodermal and neuronal retinoids regulate the induction and maintenance of limb innervating spinal motor neurons

During embryonic development, the generation, diversification and maintenance of spinal motor neurons depend upon extrinsic signals that are tightly regulated. Retinoic acid (RA) is necessary for specifying the fates of forelimb-innervating motor neurons of the Lateral Motor Column (LMC), and the specification of LMC neurons into medial and lateral subtypes. Previous studies implicate motor neurons as the relevant source of RA for specifying lateral LMC fates at forelimb levels. However, at the time of LMC diversification, a significant amount of retinoids in the spinal cord originates from the adjacent paraxial mesoderm. Here we employ mouse genetics to show that RA derived from the paraxial mesoderm is required for lateral LMC induction at forelimb and hindlimb levels, demonstrating that mesodermally synthesized RA functions as a second source of signals to specify lateral LMC identity. Furthermore, reduced RA levels in postmitotic motor neurons result in a decrease of medial and lateral LMC neurons, and abnormal axonal projections in the limb; invoking additional roles for neuronally synthesized RA in motor neuron maintenance and survival. These findings suggest that during embryogenesis, mesodermal and neuronal retinoids act coordinately to establish and maintain appropriate cohorts of spinal motor neurons that innervate target muscles in the limb.

Methods for detecting and identifying retinoids in tissue

Methods for retinoid analysis in tissue include direct spectrophotometry or fluorometry and retinoid responsive reporter constructs in the form of cell reporter assays or transgenic reporter animals, but chromatographic methods dominate and posses several superior features in quantitative analysis. The multitude of extraction protocols used can coarsely be divided into manual liquid-liquid extraction protocols and semi- or fully automated solid phase extraction-based protocols. Liquid chromatographic separation in reversed phase dominates although normal phase is also used. Detection is mainly performed with UV detectors although electrochemical and fluorescence detection is also used. Mass spectrometry in combination with LC is more often used in retinoid analysis and is likely to dominate in the future.

Retinoic acid and hindbrain patterning

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

A Hox Regulatory Network Establishes Motor Neuron Pool Identity and Target-Muscle Connectivity

Spinal motor neurons acquire specialized "pool" identities that determine their ability to form selective connections with target muscles in the limb, but the molecular basis of this striking example of neuronal specificity has remained unclear. We show here that a Hox transcriptional regulatory network specifies motor neuron pool identity and connectivity. Two interdependent sets of Hox regulatory interactions operate within motor neurons, one assigning rostrocaudal motor pool position and a second directing motor pool diversity at a single segmental level. This Hox regulatory network directs the downstream transcriptional identity of motor neuron pools and defines the pattern of target-muscle connectivity.