Non-genomic regulation of transmitter release by retinoic acid at developing motoneurons in Xenopus cell culture

Non-canonical retinoid signaling in neural development, regeneration and synaptic function

Canonical retinoid signaling via nuclear receptors and gene regulation is critical for the initiation of developmental processes such as cellular differentiation, patterning and neurite outgrowth, but also mediates nerve regeneration and synaptic functions in adult nervous systems. In addition to canonical transcriptional regulation, retinoids also exert rapid effects, and there are now multiple lines of evidence supporting non-canonical retinoid actions outside of the nucleus, including in dendrites and axons. Together, canonical and non-canonical retinoid signaling provide the precise temporal and spatial control necessary to achieve the fine cellular coordination required for proper nervous system function. Here, we examine and discuss the evidence supporting non-canonical actions of retinoids in neural development and regeneration as well as synaptic function, including a review of the proposed molecular mechanisms involved.

Chronic Administration of 13-cis-retinoic Acid Induces Depression-Like Behavior by Altering the Activity of Dentate Granule Cells

Depression is a common but serious mental disorder and can be caused by the side effects of medications. Evidence from abundant clinical case reports and experimental animal models has revealed the association between the classic anti-acne drug 13-cis-retinoic acid (13-cis-RA) and depressive symptoms. However, direct experimental evidence of this mechanism and information on appropriate therapeutic rescue strategies are lacking. Herein, our data revealed that chronic administration of 13-cis-RA to adolescent mice induced depression-like behavior but not anxiety-like behavior. We next demonstrated that chronic 13-cis-RA application increased neural activity in the dentate gyrus (DG) using c-Fos immunostaining, which may be critically involved in some aspects of depression-like behavior. Therefore, we assessed electrophysiological functions by obtaining whole-cell patch-clamp recordings of dentate granule cells (DGCs), which revealed that chronic 13-cis-RA treatment shifted the excitatory-inhibitory balance toward excitation and increased intrinsic excitability. Furthermore, a pharmacogenetic approach was performed to repeatedly silence DGCs, and this manipulation could rescue depression-like behavior in chronically 13-cis-RA-treated mice, suggesting DGCs as a potential cellular target for the direct alleviation of 13-cis-RA-induced depression.

The multi-faceted role of retinoid X receptor in cardiovascular diseases

Retinoid X receptors (RXRs) are members of ligand-dependent transcription factors whose effects on a diversity of cellular processes, including cellular proliferation, the immune response, and lipid and glucose metabolism. Knock out of RXRα causes a hypoplasia of the myocardium which is lethal during fetal life. In addition, the heart maintains a well-orchestrated balances in utilizing fatty acids (FAs) and other substrates to meet the high energy requirements. As the master transcriptional regulators of lipid metabolism, RXRs become particularly important for the energy needs of the heart. Accumulating evidence suggested that RXRs may exert direct beneficial effects in the heart both through heterodimerization with other nuclear receptors (NRs) and homodimerization, thus standing as suitable targets for treating in cardiovascular diseases. Although compounds that target RXRs are promising drugs, their use is limited by toxicity. A better understanding of the structural biology of RXRs in cardiovascular disease should enable the rational design of more selective nuclear receptor modulators to overcome these problems. Here, this review summarizes a brief overview of RXRs structure and versatility of RXR action in the control of cardiovascular diseases. And we also discussed the therapeutic potential of RXR ligand.

Selenium and zinc: Two key players against cadmium-induced neuronal toxicity

Cadmium (Cd), a worldwide occupational pollutant, is an extremely toxic heavy metal, capable of damaging several organs, including the brain. Its toxicity has been related to neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. The neurotoxic potential of Cd has been attributed to the changes induced in the brain enzyme network involved in counteracting oxidative stress. On the other hand, it is also known that trace elements, such as zinc (Zn) and selenium (Se), required for optimal brain functions, appears to have beneficial effects on the prevention of Cd intoxication. Based on this protective effect of Zn and Se, we aimed to investigate whether these elements could protect neuronal cells from Cd-induced excitotoxicity. The experiments, firstly carried out on SH-SY5Y catecholaminergic neuroblastoma cell line, demonstrated that the treatment with 10 μM cadmium chloride (CdCl₂) for 24 h caused significant modifications both in terms of oxidative stress and neuronal sprouting, triggered by endoplasmic reticulum (ER) stress. The evaluation of the effectiveness of 50 μM of zinc chloride (ZnCl₂) and 100 nM sodium selenite (Na₂SeO₃) treatments showed that both elements were able to attenuate the Cd-dependent neurotoxicity. However, considering that following induction with retinoic acid (RA), the neuroblastoma cell line undergoes differentiation into a cholinergic neurons, our second aim was to verify the zinc and selenium efficacy also in this neuronal phenotype. Our data clearly demonstrated that, while zinc played a crucial role on neuroprotection against Cd-induced neurotoxicity independently from the cellular phenotype, selenium is ineffective in differentiated cholinergic cells, supporting the notion that the molecular events occurring in differentiated SH-SY5Y cells are critical for the response to specific stimuli.

Retinoic acid as a chemoattractant for cultured embryonic spinal cord neurons of the African Clawed Frog, Xenopus laevis

Retinoic acid (RA), an active metabolite of vitamin A, is important for neural development and regeneration and can induce neurite outgrowth. It may also act as a guidance molecule by attracting neurite processes during outgrowth. In the African Clawed Frog (Xenopus laevis (Daudin, 1802)), RA has been shown to play an important role in the development of the anterior–posterior axis. However, whether RA can act as a trophic or tropic molecule on embryonic neurons of this species has not been determined. In this study, we investigated the effects of two retinoid isomers, all-trans retinoic acid (atRA) and 9-cis retinoic acid (9-cisRA), on cultured embryonic spinal cord neurons of X. laevis. Both isomers significantly enhanced neurite outgrowth compared with the vehicle control. In addition, atRA induced growth cone turning, which was blocked with a retinoic acid receptor (RAR) antagonist, selective for the β receptor subtype. Immunostaining also revealed RAR immunoreactivity in the neurites and growth cones of these cells. Interestingly, the 9-cisRA isomer also induced significant growth cone turning and this response was inhibited by a retinoid X receptor (RXR) pan-antagonist. Overall, we have provided evidence for both trophic and chemotropic actions of two naturally occurring retinoid isomers on Xenopus embryonic spinal cord neurons in culture.

The role of retinoic acid in the formation and modulation of invertebrate central synapses

Trophic factors can influence many aspects of nervous system function, such as neurite outgrowth, synapse formation, and synapse modulation. The vitamin A metabolite, retinoic acid, can exert trophic effects to promote neuronal survival and outgrowth in many species and is also known to modulate vertebrate hippocampal synapses. However, its role in synaptogenesis has not been well studied, and whether it can modulate existing invertebrate synapses is also not known. In this study, we first examined a potential trophic effect of retinoic acid on the formation of excitatory synapses, independently of its role in neurite outgrowth, using cultured neurons of the mollusc Lymnaea stagnalis We also investigated its role in modulating both chemical and electrical synapses between various Lymnaea neurons in cell culture. Although we found no evidence to suggest retinoic acid affected short-term synaptic plasticity in the form of post-tetanic potentiation, we did find a significant cell type-specific modulation of electrical synapses. Given the prevalence of electrical synapses in invertebrate nervous systems, these findings highlight the potential for retinoic acid to modulate network function in the central nervous system of at least some invertebrates.

NEW & NOTEWORTHY

This study performed the first electrophysiological analysis of the ability of the vitamin A metabolite, retinoic acid, to exert trophic influences during synaptogenesis independently of its effects in supporting neurite outgrowth. It was also the first study to examine the ability of retinoic acid to modify both chemical and electrical synapses in any invertebrate, nonchordate species. We provide evidence that all-trans retinoic acid can modify invertebrate electrical synapses of central neurons in a cell-specific manner.

Retinoic acid affects calcium signaling in adult molluscan neurons

Retinoic acid, the active metabolite of vitamin A, is important for nervous system development, regeneration, as well as cognitive functions of the adult central nervous system. These central nervous system functions are all highly dependent on neuronal activity. Retinoic acid has previously been shown to induce changes in the firing properties and action potential waveforms of adult molluscan neurons in a dose- and isomer-dependent manner. In this study, we aimed to determine the cellular pathways by which retinoic acid might exert such effects, by testing the involvement of pathways previously shown to be affected by retinoic acid. We demonstrated that the ability of all-trans retinoic acid (atRA) to induce electrophysiological changes in cultured molluscan neurons was not prevented by inhibitors of protein synthesis, protein kinase A or phospholipase C. However, we showed that atRA was capable of rapidly reducing intracellular calcium levels in the same dose- and isomer-dependent manner as shown previously for changes in neuronal firing. Moreover, we also demonstrated that the transmembrane ion flux through voltage-gated calcium channels was rapidly modulated by retinoic acid. In particular, the peak current density was reduced and the inactivation rate was increased in the presence of atRA, over a similar time course as the changes in cell firing and reductions in intracellular calcium. These studies provide further evidence for the ability of atRA to induce rapid effects in mature neurons.

Retinoid signaling is necessary for, and promotes long-term memory formation following operant conditioning

Retinoic acid, a metabolite of vitamin A, is proposed to play an important role in vertebrate learning and memory, as well as hippocampal-dependent synaptic plasticity. However, it has not yet been determined whether retinoic acid plays a similar role in learning and memory in invertebrates. In this study, we report that retinoid signaling in the mollusc Lymnaea stagnalis, is required for long-term memory formation following operant conditioning of its aerial respiratory behaviour. Animals were exposed to inhibitors of the RALDH enzyme (which synthesizes retinoic acid), or various retinoid receptor antagonists. Following exposure to these inhibitors, neither learning nor intermediate-term memory (lasting 2 h) was affected, but long-term memory formation (tested at either 24 or 72 h) was inhibited. We next demonstrated that various retinoid receptor agonists promoted long-term memory formation. Using a training paradigm shown only to produce intermediate-term memory (lasting 2 h, but not 24 h) we found that exposure of animals to synthetic retinoids promoted memory formation that lasted up to 30 h. These findings suggest that the role of retinoids in memory formation is ancient in origin, and that retinoid signaling is also important for the formation of implicit memories, in addition to its previously demonstrated role in hippocampal-dependent memories.

Retinoic acid induces changes in electrical properties of adult neurons in a dose- and isomer-dependent manner

The electrical activity of neurons is known to play a role in neuronal development, as well as repair of adult nervous tissue. For example, the extension of neurites and motility of growth cones can be modulated by changes in the electrical firing of neurons. The vitamin A metabolite retinoic acid also plays a critical role during nervous system development and is also known to elicit regenerative responses, namely the induction, enhancement, and directionality of neurite outgrowth. However, no studies have previously reported the ability of retinoic acid to modify the electrical activity of neurons. In this study, we determined whether retinoic acid might exert effects on the nervous system by altering the electrical properties of neurons. Using cultured adult neurons from Lymnaea stagnalis, we showed that acute application of retinoic acid can rapidly elicit changes in neuronal firing properties. Retinoic acid caused the presence of atypical firing behavior such as rhythmic bursting and altered the shape of action potentials, causing increases in half-amplitude duration and decay time. Retinoic acid also caused cell silencing, whereby neuronal activity was halted within an hour. These effects of retinoic acid were shown to be both dose and isomer dependent. We then showed that the effects of retinoic acid on cell firing (but not silencing) were significantly reduced in the presence of an retinoid X receptor pan-antagonist HX531. This study suggests that some of the effects of retinoic acid during neuronal development or regeneration might possibly occur as a result of changes in electrical activity of neurons.

Retinol induces morphological alterations and proliferative focus formation through free radical-mediated activation of multiple signaling pathways

AIM

Toxicity of retinol (vitamin A) has been previously associated with apoptosis and/or cell malignant transformation. Thus, we investigated the pathways involved in the induction of proliferation, deformation and proliferative focus formation by retinol in cultured Sertoli cells of rats.

METHODS

Sertoli cells were isolated from immature rats and cultured. The cells were subjected to a 24-h treatment with different concentrations of retinol. Parameters of oxidative stress and cytotoxicity were analyzed. The effects of the p38 inhibitor SB203580 (10 μmol/L), the JNK inhibitor SP600125 (10 μmol/L), the Akt inhibitor LY294002 (10 μmol/L), the ERK inhibitor U0126 (10 μmol/L) the pan-PKC inhibitor Gö6983 (10 μmol/L) and the PKA inhibitor H89 (1 μmol/L) on morphological and proliferative/transformation-associated modifications were studied.

RESULTS

Retinol (7 and 14 μmol/L) significantly increases the reactive species production in Sertoli cells. Inhibition of p38, JNK, ERK1/2, Akt, and PKA suppressed retinol-induced [(3)H]dT incorporation into the cells, while PKC inhibition had no effect. ERK1/2 and p38 inhibition also blocked retinol-induced proliferative focus formation in the cells, while Akt and JNK inhibition partially decreased focus formation. ERK1/2 and p38 inhibition hindered transformation-associated deformation in retinol-treated cells, while other treatments had no effect.

CONCLUSION

Our results suggest that activation of multiple kinases is responsible for morphological and proliferative changes associated to malignancy development in Sertoli cells by retinol at the concentrations higher than physiological level.

Acute effects of all-trans-retinoic acid in ischemic injury

All-trans-retinoic acid (ATRA) is a vitamin A derivative that is important in neuronal patterning, survival, and neurite outgrowth. Neuroprotective effects of ATRA in ischemia have been demonstrated but its effects on glial swelling are not known. We investigated the relatively acute effects of ATRA on cell swelling in ischemic injury and on key features hypothesized to contribute to cell swelling including increased reactive oxygen species/reactive nitrogen species (ROS/RNS), depolarization of the inner mitochondrial membrane potential (ΔΨm), and increased intracellular calcium ([Ca2+]i). C6 glial cultures were subjected to 5 hr oxygen-glucose deprivation (OGD). ATRA was added to separate groups after the end of OGD. OGD increased cell volume by 43%, determined at 90 min after the end of OGD, but this increase was significantly attenuated by ATRA. OGD induced an increase in ROS/RNS production in the whole cell and mitochondria, as assessed by the fluorescent dyes CM-H2DCFDA and MitoTracker CM-H2-XROS at the end of OGD. The increase in mitochondrial ROS, but not cellular ROS, was significantly attenuated by ATRA. OGD also induced a 67% decline in mitochondrial ΔΨm but this decline was significantly attenuated by ATRA. OGD-induced increase in [Ca2+]i was also significantly attenuated by ATRA. Taken together with our previous results where calcium channel blockers reduced cell swelling, the effects of ATRA in attenuating swelling are possibly mediated through its effects in regulating [Ca2+]i. Considering the paucity of agents in attenuating brain edema in ischemia, ATRA has the potential to reduce brain edema and associated neural damage in ischemic injury.

Retinoid-regulated FGF8f secretion by osteoblasts bypasses retinoid stimuli to mediate granulocytic differentiation of myeloid leukemia cells

Signaling from the human hematopoietic stem cell (HSC) niche formed by osteoblastic cells regulates hematopoiesis. We previously found that retinoic acid receptor alpha (RARα), a transcription factor activated by retinoic acid (RA), mediates both granulocytic and osteoblastic differentiation. This effect depends on decreased phosphorylation of serine 77 of RARα (RARαS77) by the cyclin-dependent kinase-activating kinase (CAK) complex, a key cell-cycle regulator. In this article, we report that, by suppressing CAK phosphorylation of RARα, RA induces FGF8f to mediate osteosarcoma U2OS cell differentiation in an autocrine manner. By contrast, paracrine FGF8f secreted into osteoblast-conditioned medium by U2OS cells transduced with FGF8f or a phosphorylation-defective RARαS77 mutant, RARαS77A, bypasses RA stimuli to cross-mediate granulocytic differentiation of different types of human leukemic myeloblasts and normal primitive hematopoietic CD34(+) cells, possibly through modulating mitogen-activated protein kinase (MAPK) pathways. Further experiments using recombinant human FGF8f (rFGF8f) stimuli, antibody neutralization, and peptide blocking showed that paracrine FGF8f is required for mediating terminal leukemic myeloblast differentiation. These studies indicate a novel regulatory mechanism of granulocytic differentiation instigated by RA from the HSC niche, which links loss of CAK phosphorylation of RARα with paracrine FGF8f-mediated MAPK signaling to mediate leukemic myeloblast differentiation in the absence of RA. Therefore, these findings provide a compelling molecular rationale for further investigation of paracrine FGF8f regulation, with the intent of devising HSC niche-based FGF8f therapeutics for myeloid leukemia, with or without RA-resistance.

Retinoid pathway and cancer therapeutics

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

Detection of retinoic acid catabolism with reporter systems and by in situ hybridization for CYP26 enzymes

Retinoic acid (RA), an active form of vitamin A, is essential for life in vertebrates, owing to its capacity of influencing expression of a sizable fraction of all genes and proteins. It functions via two modes: (1) as controlling ligand for specific transcription factors in the nucleus it stimulates or inhibits gene expression from RA response elements in gene promoters; (2) in non-genomic pathways it activates kinase-signaling cascades that converge with additional influences to regulate gene expression and mRNA translation. RA performs a critical role in morphogenesis of the developing embryo, which is reflected in spatio-temporally changing expression patterns of RA-synthesizing and RA-degrading enzymes and in its biophysical characteristics as a small diffusible lipid. Because its histological localization cannot be directly visualized for technical reasons, its sites of action in vivo are inferred from the locations of the metabolic enzymes and through use of two kinds of RA reporter systems. Here we explain techniques for use of RA reporter cells and RA reporter mice, and we describe in situ hybridization methods for the three major RA-degrading enzymes: CYP26A1, CYP26B1, and CYP26C1. Comparisons of the different indicators for sites of RA signaling demonstrate that local RA peaks and troughs are important for inferring some but not all locations of RA actions. When integrated within cells of living mice, expression of the RA reporter construct is rarely a simple measure of local RA levels, especially in the developing brain, but it appears to provide cues to an RA involvement in site-specific regulatory networks in combination with other spatial determinants.

From carrot to clinic: an overview of the retinoic acid signaling pathway

Vitamin A is essential for the formation and maintenance of many body tissues. It is also important for embryonic growth and development and can act as a teratogen at critical periods of development. Retinoic acid (RA) is the biologically active form of vitamin A and its signaling is mediated by the RA and retinoid X receptors. In addition to its role as an important molecule during development, RA has also been implicated in clinical applications, both as a potential anti-tumor agent as well as for the treatment of skin diseases. This review presents an overview of how dietary retinoids are converted to RA, hence presenting the major players in RA metabolism and signaling, and highlights examples of treatment applications of retinoids. Moreover, we discuss the origin and diversification of the retinoid pathway, which are important factors for understanding the evolution of ligand-specificity among retinoid receptors.

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 novel, nongenomic mechanism underlies retinoic acid-induced growth cone turning

The vitamin A metabolite, retinoic acid (RA), is well known for its roles in neural development and regeneration. We have previously shown that RA can induce positive growth cone turning in regenerating neurons in vitro. In this study, we address the subcellular mechanisms underlying this chemo-attractive response, using identified central neurons from the adult mollusc, Lymnaea stagnalis. We show that the RA-induced positive growth cone turning was maintained in the presence of the transcriptional inhibitor, actinomycin D. We also physically transected the neurites from the cell body and showed that isolated growth cones retain the capacity to turn toward a gradient of RA. Moreover, this attractive turning is dependent on de novo local protein synthesis and Ca(2+) influx. Most of RA's actions during neurite outgrowth and regeneration require gene transcription, although these data show for the first time in any species, that the chemotropic action of RA in guiding neurite outgrowth, involves a novel, nongenomic mechanism.

Loss of CAK phosphorylation of RAR{alpha} mediates transcriptional control of retinoid-induced cancer cell differentiation

Although the role of the classic retinoic acid (RA)-induced genomic pathway in cancer cell differentiation is well recognized, the underlying mechanisms remain to be dissected. Retinoic acid receptor alpha (RARalpha) is a transcription factor activated by RA, and its serine 77 (RARalphaS77) is the main residue phosphorylated by the cyclin-dependent kinase (CDK)-activating kinase (CAK) complex. We report here that in both human myeloid leukemia and mouse embryonic teratocarcinoma stem cells, either RA-suppressed CAK phosphorylation of RARalpha or mutation of RARalphaS77 to alanine (RARalphaS77A) coordinates CAK-dependent G(1) arrest with cancer cell differentiation by transactivating RA-target genes. Both hypophosphorylated RARalpha and RARalphaS77A reduce binding to retinoic acid-responsive elements (RARE) in the promoters of RA-target genes while stimulating gene transcription. The enhanced transactivation and reduced RARalpha-chromatin interaction are accompanied by RARalpha dissociation from the transcriptional repressor N-CoR and are association with the coactivator NCoA-3. Such effects of decreased CAK phosphorylation of RARalphaS77 on mediating RA-dependent transcriptional control of cancer cell differentiation are examined correspondingly in both RA-resistant myeloid leukemia and embryonic teratocarcinoma stem RARalpha(-/-) cells. These studies demonstrate, for the first time, that RA couples G(1) arrest to transcriptional control of cancer cell differentiation by suppressing CAK phosphorylation of RARalpha to release transcriptional repression.-Wang, A., Alimova, I. N., Luo, P. Jong, A., Triche, T. J., Wu, L. Loss of CAK phosphorylation of RARalpha mediates transcriptional control of retinoid-induced cancer cell differentiation.

Synaptic signaling by all-trans retinoic acid in homeostatic synaptic plasticity

Normal brain function requires that the overall synaptic activity in neural circuits be kept constant. Long-term alterations of neural activity lead to homeostatic regulation of synaptic strength by a process known as synaptic scaling. The molecular mechanisms underlying synaptic scaling are largely unknown. Here, we report that all-trans retinoic acid (RA), a well-known developmental morphogen, unexpectedly mediates synaptic scaling in response to activity blockade. We show that activity blockade increases RA synthesis in neurons and that acute RA treatment enhances synaptic transmission. The RA-induced increase in synaptic strength is occluded by activity blockade-induced synaptic scaling. Suppression of RA synthesis prevents synaptic scaling. This form of RA signaling operates via a translation-dependent but transcription-independent mechanism, causes an upregulation of postsynaptic glutamate receptor levels, and requires RARalpha receptors. Together, our data suggest that RA functions in homeostatic plasticity as a signaling molecule that increases synaptic strength by a protein synthesis-dependent mechanism.

Retinol activates tyrosine hydroxylase acutely by increasing the phosphorylation of serine40 and then serine31 in bovine adrenal chromaffin cells

Tyrosine hydroxylase is the rate-limiting enzyme in the biosynthesis of the catecholamines. It has been reported that retinol (vitamin A) modulates tyrosine hydroxylase activity by increasing its expression through the activation of the nuclear retinoid receptors. In this study, we observed that retinol also leads to an acute activation of tyrosine hydroxylase in bovine adrenal chromaffin cells and this was shown to occur via two distinct non-genomic mechanisms. In the first mechanism, retinol induced an influx in extracellular calcium, activation of protein kinase C and serine40 phosphorylation, leading to tyrosine hydroxylase activation within 15 min. This effect then declined over time. The retinol-induced rise in intracellular calcium then led to a second slower mechanism; this involved an increase in reactive oxygen species, activation of extracellular signal-regulated kinase 1/2 and serine31 phosphorylation and the maintenance of tyrosine hydroxylase activation for up to 2 h. No effects were observed with retinoic acid. These results show that retinol activates tyrosine hydroxylase via two sequential non-genomic mechanisms, which have not previously been characterized. These mechanisms are likely to operate in vivo to facilitate the stress response, especially when vitamin supplements are taken or when retinol is used as a therapeutic agent.

Immunohistochemical localization of a retinoic acid-like receptor in nerve cells of two colonial anthozoans (Cnidaria)

Retinoic acid is known to induce vertebrate stem cells to differentiate into a variety of cell types, including neurons. Although retinoic acid was reported to affect morphogenetic pattern specification in the hydrozoan Hydractinia (Müller, W.A., 1984. Retinoids and pattern formation in a hydroid. J. Embryol. Exp. Morph. 81, 253-271) and a retinoid RXR receptor was cloned in the jellyfish Tripedalia (Kostrouch, Z., Kostrouchova, M., Love, W., Jannini, E., Piatigorsky, J., Rall, J.E., 1998. Retinoic acid X receptor in the diploblast, Tripedalia cystophora. Proc. Natl. Acad. Sci. U.S.A. 95, 13442-13447), the cellular targets of retinoids were not investigated. We used Western immunoblotting and immunohistochemistry to investigate the presence and cellular distribution of a RXR-like receptor in the sea pansy Renilla koellikeri and in the staghorn coral Acropora millepora (Cnidaria, Anthozoa). Western blots revealed a 64 kDa protein from a sea pansy extract in a band that co-migrated with a RXR protein from the rat brain. Using antibodies raised against an epitope of human alpha RXR, we visualized putative ectodermal sensory cells in the polyp column of the adult sea pansy. Immunoreactivity was absent in staghorn coral larvae but present in the polyp column of adult colonies in the form of clusters of neuron-like cells in the basiectoderm near the ectoderm-mesoglea interface. These observations suggest that a RXR-like receptor is involved in epithelial nerve cell specification in adult anthozoans and that this role is conserved throughout evolution.

Mechanism of β-bungarotoxin in facilitating spontaneous transmitter release at neuromuscular synapse

The mechanism of the action of beta-bungarotoxin (beta-BuTx) in the facilitation of spontaneous transmitter release at neuromuscular synapse was investigated in Xenopus cell culture using whole-cell patch clamp recording. Exposure of the culture to beta-BuTx dose-dependently enhances the frequency of spontaneous synaptic currents (SSCs). Buffering the rise of intracellular Ca2+ with BAPTA-AM hampered the facilitation of SSC frequency induced by beta-BuTx. The beta-BuTx-enhanced SSC frequency was reduced when the pharmacological Ca2+ -ATPase inhibitor thapsigargin was used to deplete intracellular Ca2+ store. Application of membrane-permeable inhibitors of inositol 1,4,5-trisphosphate (IP3) but not ryanodine receptors effectively occluded the increase of SSC frequency elicited by beta-BuTx. Treating cells with either wortmannin or LY294002, two structurally different inhibitors of phosphatidylinositol 3-kinase (PI3K) and with phospholipase C (PLC) inhibitor U73122, abolished the beta-BuTx-induced facilitation of synaptic transmission. The beta-BuTx-induced synaptic facilitation was completely abolished while there was presynaptic loading of the motoneuron with GDPbetaS, a non-hydrolyzable GDP analogue and inhibitor of G protein. Taken collectively, these results suggest that beta-BuTx elicits Ca2+ release from the IP3 sensitive intracellular Ca2+ stores of the presynaptic nerve terminal. This is done via PI3K/PLC signaling cascades and G protein activation, leading to an enhancement of spontaneous transmitter release.

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.

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 rapid, nongenomic pathway facilitates the synaptic transmission induced by retinoic acid at the developing synapse

We have previously shown that retinoic acid (RA), a factor highly expressed in spinal cord, rapidly and specifically enhances the spontaneous acetylcholine release at developing neuromuscular synapses in Xenopus cell culture, using whole-cell patch-clamp recording. We have now further investigated the underlying mechanisms that are involved in RA-induced facilitation on the frequency of spontaneous synaptic currents (SSCs). Buffering the rise of intracellular Ca2+ with BAPTA-AM hampered the facilitation of SSC frequency induced by RA. The prompt RA-enhanced SSC frequency was not abolished when Ca2+ was eliminated from the culture medium or there was bath application of the pharmacological Ca2+ channel inhibitor Cd2+, indicating that Ca2+ influx through voltage-activated Ca2+ channels are not required. Application of membrane-permeable inhibitors of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] or ryanodine receptors effectively blocked the increase of SSC frequency elicited by RA. Treating cells with either wortmannin or LY294002, two structurally different inhibitors of phosphatidylinositol 3-kinase (PI 3-kinase) and with the phospholipase Cgamma (PLCgamma) inhibitor U73122, abolished RA-induced facilitation of synaptic transmission. Preincubation of the cultures with pharmacological inhibitors, either genistein, a broad-spectrum tyrosine kinase inhibitor, or PP2, which predominantly inhibits the Src family of nonreceptor tyrosine kinase, completely abolished RA-induced synaptic facilitation. Taken collectively, these results suggest that RA elicits Ca2+ release from Ins1,4,5P3 and/or ryanodine-sensitive intracellular Ca2+ stores of the presynaptic nerve terminal. This is done via PLCgamma/PI 3-kinase signaling cascades and Src tyrosine kinase activation, leading to an enhancement of spontaneous transmitter release.