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

Retinoic acid reduces the formation of, and acutely modulates, invertebrate electrical synapses

Communication between cells in the nervous system is dependent on both chemical and electrical synapses. Factors that can affect chemical synapses have been well studied, but less is known about factors that influence electrical synapses. Retinoic acid, the vitamin A metabolite, is a known regulator of chemical synapses, but few studies have examined its capacity to regulate electrical synapses. In this study, we determine that retinoic acid is capable of rapidly altering the strength of electrical synapses in an isomer- and cell-dependent manner. Furthermore, we provide evidence that this acute effect might be independent of either the retinoid receptors or the activation of a protein kinase. In addition to the rapid modulatory effects of retinoic acid, we provide data to suggest that retinoic acid is also capable of regulating the formation of electrical synapses. Long-term exposure to both all-trans-retinoic acid or 9-cis-retinoic acid reduced the proportion of cell pairs forming electrical synapses, as well as reduced the strength of electrical synapses that did form. In summary, this study provides insights into the role that retinoids might play in both the formation and modulation of electrical synapses in the central nervous system.NEW & NOTEWORTHY Retinoids are known modulators of chemical synapses and mediate synaptic plasticity in the nervous system, but little is known of their effects on electrical synapses. Here, we show that retinoids selectively reduce electrical synapses in a cell- and isomer-dependent manner. This modulatory action on existing electrical synapses was rapid and nongenomic in nature. We also showed for the first time that longer retinoid exposures inhibit the formation of electrical synapses.

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.

A Fluorescent Vector of Carbon Dot to Deliver Rab13 and Rab14 Plasmids for Promoting Neurite Outgrowth

The complex processes of neuron differentiation and neuron repair are critical for treating nervous system injuries and neurodegenerative diseases. Neurite outgrowth plays a crucial role in these processes by enabling the formation of connections between neurons and the generation of neuroplasticity to restore the function of the nervous system. In this study, we fabricated functionalized carbon dots (CDs) with distinctive photoluminescence and low cytotoxicity for use as fluorescence imaging probes and nanocarriers to deliver plasmid DNAs to neurons effectively for inducing neurite outgrowth. CDs were prepared through a reflux process in nitric acid solution, and their surface was then modified using polyethylenimine (PEI) to obtain positively charged CDs for increasing the absorption of plasmid DNAs and the efficiency of cell uptake. Experimental results indicated that the fabricated CDs maintained a low cytotoxicity and exhibited a high neuron uptake of up to 97%. An improvement in the plasmid DNA ingestion of neurons resulted in enhanced expression of Rab13-Q67L and Rab14 proteins, which considerably promoted neurite sprouting and elongation. After the fabricated PEI-modified CDs were used to deliver the Rab13-Q67L and Rab14 plasmids, more than 56% of the neurons had a neurite length that was greater than twice the size of their soma. Thus, DNA delivery through functionalized CDs has a high potential for use in gene therapy for neuronal injuries and diseases.

Disruptions in network plasticity precede deficits in memory following inhibition of retinoid signaling

Retinoic acid, the active metabolite of vitamin A, is important for vertebrate cognition and hippocampal plasticity, but few studies have examined its role in invertebrate learning and memory, and its actions in the invertebrate central nervous system are currently unknown. Using the mollusc Lymnaea stagnalis, we examined operant conditioning of the respiratory behavior, controlled by a well-defined central pattern generator (CPG), and used citral to inhibit retinoic acid signaling. Both citral- and vehicle-treated animals showed normal learning, but citral-treated animals failed to exhibit long-term memory at 24 h. Cohorts of citral- or vehicle-treated animals were dissected into semi-intact preparations, either 1 h after training, or after the memory test 24 h later. Simultaneous electrophysiological recordings from the CPG pacemaker cell (right pedal dorsal 1; RPeD1) and an identified motorneuron (VI) were made while monitoring respiratory activity (pneumostome opening). Activity of the CPG pneumostome opener interneuron (input 3 interneuron; IP3) was also monitored indirectly. Vehicle-treated conditioned preparations showed significant changes in network parameters immediately after learning, such as reduced motorneuron bursting activity (from IP3 input), delayed pneumostome opening, and decoupling of coincident IP3 input within the network. However, citral-treated preparations failed to exhibit these network changes and more closely resembled naïve preparations. Importantly, these citral-induced differences were manifested immediately after training and before any overt changes in the behavioral response (memory impairment). These studies shed light on where and when retinoid signaling might affect a central pattern-generating network to promote memory formation during conditioning of a homeostatic behavior.NEW & NOTEWORTHY We provide novel evidence for how conditioning-induced changes in a CPG network are disrupted when retinoid signaling is inhibited. Inhibition of retinoic acid signaling prevents long-term memory formation following operant conditioning, but has no effect on learning. Simultaneous electrophysiological and behavioral analyses indicate network changes immediately following learning, but these changes are prevented with inhibition of retinoid signaling, before any overt changes in behavior. These data suggest sites for retinoid actions during memory formation.

Enteric Neural Network Assembly Was Promoted by Basic Fibroblast Growth Factor and Vitamin A but Inhibited by Epidermal Growth Factor

Extending well beyond the original use of propagating neural precursors from the central nervous system and dorsal root ganglia, neurosphere medium (NSM) and self-renewal medium (SRM) are two distinct formulas with widespread popularity in enteric neural stem cell (ENSC) applications. However, it remains unknown what growth factors or nutrients are crucial to ENSC development, let alone whether the discrepancy in their components may affect the outcomes of ENSC culture. Dispersed enterocytes from murine fetal gut were nurtured in NSM, SRM or their modifications by selective component elimination or addition to assess their effects on ENSC development. NSM generated neuriteless neurospheres, whereas SRM, even deprived of chicken embryo extract, might wire ganglia together to assemble neural networks. The distinct outcomes came from epidermal growth factor, which inhibited enteric neuronal wiring in NSM. In contrast, basic fibroblast growth factor promoted enteric neurogenesis, gangliogenesis, and neuronal wiring. Moreover, vitamin A derivatives might facilitate neuronal maturation evidenced by p75 downregulation during ENSC differentiation toward enteric neurons to promote gangliogenesis and network assembly. Our results might help to better manipulate ENSC propagation and differentiation in vitro, and open a new avenue for the study of enteric neuronal neuritogenesis and synaptogenesis.

Molluscan RXR Transcriptional Regulation by Retinoids in a Drosophila CNS Organ Culture System

Retinoic acid, the active metabolite of Vitamin A, is important for the appropriate development of the nervous system (e.g., neurite outgrowth) as well as for cognition (e.g., memory formation) in the adult brain. We have shown that many of the effects of retinoids are conserved in the CNS of the mollusc, Lymnaea stagnalis. RXRs are predominantly nuclear receptors, but the Lymnaea RXR (LymRXR) exhibits a non-nuclear distribution in the adult CNS, where it is also implicated in non-genomic retinoid functions. As such, we developed a CNS Drosophila organ culture-based system to examine the transcriptional activity and ligand-binding properties of LymRXR, in the context of a live invertebrate nervous system. The novel ligand sensor system was capable of reporting both the expression and transcriptional activity of the sensor. Our results indicate that the LymRXR ligand sensor mediated transcription following activation by both 9-cis RA (the high affinity ligand for vertebrate RXRs) as well as the vertebrate RXR synthetic agonist, SR11237. The LymRXR ligand sensor was also activated by all-trans RA, and to a much lesser extent by the vertebrate RAR synthetic agonist, EC23. This sensor also detected endogenous retinoid-like activity in the CNS of developing Drosophila larvae, primarily during the 3^rd instar larval stage. These data indicate that the LymRXR sensor can be utilized not only for characterization of ligand activation for studies related to the Lymnaea CNS, but also for future studies of retinoids and their functions in Drosophila development.

Retinoic acid treatment recruits macrophages and increases axonal regeneration after optic nerve injury in the frog Rana pipiens

Retinoic acid (RA) plays major roles during nervous system development, and during regeneration of the adult nervous system. We have previously shown that components of the RA signaling pathway are upregulated after optic nerve injury, and that exogenous application of all-trans retinoic acid (ATRA) greatly increases the survival of axotomized retinal ganglion cells (RGCs). The objective of the present study is to investigate the effects of ATRA application on the macrophages in the optic nerve after injury, and to determine whether this affects axonal regeneration. The optic nerve was crushed and treated with PBS, ATRA and/or clodronate-loaded liposomes. Nerves were examined at one and two weeks after axotomy with light microscopy, immunocytochemistry and electron microscopy. ATRA application to the optic nerve caused transient increases in the number of macrophages and microglia one week after injury. The macrophages are consistently labeled with M2-type markers, and have considerable phagocytic activity. ATRA increased ultrastructural features of ongoing phagocytic activity in macrophages at one and two weeks. ATRA treatment also significantly increased the numbers of regenerating GAP-43-labeled axons. Clodronate liposome treatment depleted macrophage numbers by 80%, completely eliminated the ATRA-mediated increase in axonal regeneration, and clodronate treatment alone decreased axonal numbers by 30%. These results suggest that the success of axon regeneration is partially dependent on the presence of debris-phagocytosing macrophages, and that the increases in regeneration caused by ATRA are in part due to their increased numbers. Further studies will examine whether macrophage depletion affects RGC survival.

Regulation of prefrontal patterning and connectivity by retinoic acid

The prefrontal cortex (PFC) and its connections with the mediodorsal thalamus are crucial for cognitive flexibility and working memory1 and are thought to be altered in disorders such as autism2,3 and schizophrenia4,5. Although developmental mechanisms that govern the regional patterning of the cerebral cortex have been characterized in rodents6-9, the mechanisms that underlie the development of PFC-mediodorsal thalamus connectivity and the lateral expansion of the PFC with a distinct granular layer 4 in primates10,11 remain unknown. Here we report an anterior (frontal) to posterior (temporal), PFC-enriched gradient of retinoic acid, a signalling molecule that regulates neural development and function12-15, and we identify genes that are regulated by retinoic acid in the neocortex of humans and macaques at the early and middle stages of fetal development. We observed several potential sources of retinoic acid, including the expression and cortical expansion of retinoic-acid-synthesizing enzymes specifically in primates as compared to mice. Furthermore, retinoic acid signalling is largely confined to the prospective PFC by CYP26B1, a retinoic-acid-catabolizing enzyme, which is upregulated in the prospective motor cortex. Genetic deletions in mice revealed that retinoic acid signalling through the retinoic acid receptors RXRG and RARB, as well as CYP26B1-dependent catabolism, are involved in proper molecular patterning of prefrontal and motor areas, development of PFC-mediodorsal thalamus connectivity, intra-PFC dendritic spinogenesis and expression of the layer 4 marker RORB. Together, these findings show that retinoic acid signalling has a critical role in the development of the PFC and, potentially, in its evolutionary expansion.

The role of Extracellular Vesicles during CNS development

With a diverse set of neuronal and glial cell populations, Central Nervous System (CNS) has one of the most complex structures in the body. Intercellular communication is therefore highly important to coordinate cell-to-cell interactions. Besides electrical and chemical messengers, CNS cells also benefit from another communication route, what is known as extracellular vesicles, to harmonize their interactions. Extracellular Vesicles (EVs) and their subtype exosomes are membranous particles secreted by cells and contain information packaged in the form of biomolecules such as small fragments of DNA, lipids, miRNAs, mRNAs, and proteins. They are able to efficiently drive changes upon their arrival to recipient cells. EVs actively participate in all stages of CNS development by stimulating neural cell proliferation, differentiation, synaptic formation, and mediating reciprocal interactions between neurons and oligodendrocyte for myelination process. The aim of the present review is to enlighten the presence and contribution of EVs at each CNS developmental milestone.

Synthesis and biological evaluation of modified laminin peptide (N2S2-KDP) with enhanced affinity for neuronal growth and targeted molecular imaging (SPECT)

An analog of γ1 laminin (RDIAEIIKDI) decapeptide has been used to augment neuronal survival and regeneration after injuries, during aging and other CNS disorder. As a prime synthetic peptide, KDI, is responsible for the neurite outgrowth of human embryonic neurons. In this study, we have designed, modified a KDI derivative and synthesized by replacing isoleucine (I) with Pro (P) amino acid at C-terminal to enhance its potency towards neurite growth. -Cys-Gly-Cys (-CGC) N2S2 motif was also incorporated in the present design for peptide radiolabeling. The modified peptide showed a better binding with the desired 3T1M receptor for neurite growth. The peptide was synthesized using solid phase peptide synthesis and Fmoc-strategy with more than 80% yield. The receptor binding studies of 99mTc-N2S2-KDP in Neuro2A cell lines showed Kd value in 31 nM range and the complex showed appreciable brain uptake in mice. The results on human SH-SY5Y indicate that the unlabeled N2S2-KDP may perhaps be useful for neurite growth in neurodegenerative disorder.

Bottom up proteomics identifies neuronal differentiation pathway networks activated by cathepsin inhibition treatment in neuroblastoma cells that are enhanced by concurrent 13-cis retinoic acid treatment

Neuroblastoma is the second most common pediatric cancer involving the peripheral nervous system in which stage IVS metastatic tumors regress due to spontaneous differentiation. 13-cis retinoic acid (13-cis RA) is currently used in the clinic for its differentiation effects and although it improves outcomes, relapse is seen in half of high-risk patients. Combinatorial therapies have been shown to be more effective in oncotherapy and since cathepsin inhibition reduces tumor growth, we explored the potential of coupling 13-cis RA with a cathepsin inhibitor (K777) to enhance therapeutic efficacy against neuroblastoma. Shotgun proteomics was used to identify proteins affected by K777 and dual (13-cis RA/K777) treatment in neuroblastoma SK-N-SH cells. Cathepsin inhibition was more effective in increasing proteins involved in neuronal differentiation and neurite outgrowth than 13-cis RA alone, but the combination of both treatments enhanced the neuronal differentiation effect. SIGNIFICANCE: As neuroblastoma can spontaneously differentiate, determining which proteins are involved in differentiation can guide development of more accurate diagnostic markers and more effective treatments. In this study, we established a differentiation proteomic map of SK-N-SH cells treated with a cathepsin inhibitor (K777) and K777/13-cis RA (dual). Bioinformatic analysis revealed these treatments enhanced neuronal differentiation and axonogenesis pathways. The most affected proteins in these pathways may become valuable biomarkers of efficacy of drugs designed to enhance differentiation of neuroblastoma [1].

Context-Dependent Role of miR-124 in Retinoic Acid-Induced Growth Cone Attraction of Regenerating Motorneurons

During development and regeneration, growth cones at the tips of extending axons navigate through a complex environment to establish accurate connections with appropriate targets. Growth cones can respond rapidly to classical and non-classical guidance cues in their environment, often requiring local protein synthesis. In vertebrate growth cones, local protein synthesis in response to classical cues can require regulation by microRNAs (miRNAs), a class of small, conserved, non-coding RNAs that post-transcriptionally regulate gene expression. However, less is known of how miRNAs mediate growth cone responses to non-classical cues (such as retinoic acid (RA)), specifically in invertebrates. Here, we utilized adult regenerating invertebrate motorneurons to study miRNA regulation of growth cone attraction to RA, shown to require local protein synthesis. In situ hybridization revealed the presence of miR-124 in growth cones of regenerating ciliary motorneurons of the mollusc Lymnaea stagnalis. Changes in the spatiotemporal distribution of miR-124 occurred following application of RA, and dysregulation of miR-124 (with mimic injection), disrupted RA-induced growth cone turning in a time-dependent manner. This behavioural regulation by miR-124 was altered when the neurite was transected, and the growth cone completely separated from the soma. miR-124 did not, however, appear to be involved in growth cone attraction to serotonin, a response independent of local protein synthesis. Finally, we provide evidence that a downstream effector of RhoGTPases, ROCK, is a potential target of miR-124 during RA-induced growth cone responses. These data advance our current understanding of how microRNAs might mediate cue- and context-dependent behaviours during axon guidance.

Gap Junction Coding Innexin in Lymnaea stagnalis: Sequence Analysis and Characterization in Tissues and the Central Nervous System

Connections between neurons called synapses are the key components underlying all nervous system functions of animals and humans. However, important genetic information on the formation and plasticity of one type, the electrical (gap junction-mediated) synapse, is understudied in many invertebrates. In the present study, we set forth to identify and characterize the gap junction-encoding gene innexin in the central nervous system (CNS) of the mollusk pond snail Lymnaea stagnalis. With PCR, 3′ and 5′ RACE, and BLAST searches, we identified eight innexin genes in the L. stagnalis genome, named Lst Inx1–Lst Inx8. Phylogenetic analysis revealed that the L. stagnalis innexin genes originated from a single copy in the common ancestor of molluskan species by multiple gene duplication events and have been maintained in L. stagnalis since they were generated. The paralogous innexin genes demonstrate distinct expression patterns among tissues. In addition, one paralog, Lst Inx1, exhibits heterogeneity in cells and ganglia, suggesting the occurrence of functional diversification after gene duplication. These results introduce possibilities to study an intriguing potential relationship between innexin paralog expression and cell-specific functional outputs such as heterogenic ability to form channels and exhibit synapse plasticity. The L. stagnalis CNS contains large neurons and functionally defined networks for behaviors; with the introduction of L. stagnalis in the gap junction gene field, we are providing novel opportunities to combine genetic research with direct investigations of functional outcomes at the cellular, synaptic, and behavioral levels.

Inhibition of Rho GTPases in Invertebrate Growth Cones Induces a Switch in Responsiveness to Retinoic Acid

During development, growth cones are essential for axon pathfinding by sensing numerous guidance cues in their environment. Retinoic acid, the metabolite of vitamin A, is important for neurite outgrowth during vertebrate development, but may also play a role in axon guidance, though little is known of the cellular mechanisms involved. Our previous studies showed that retinoid-induced growth cone turning of invertebrate motorneurons requires local protein synthesis and calcium influx. However, the signalling pathways that link calcium influx to cytoskeletal dynamics involved in retinoid-mediated growth cone turning are not currently known. The Rho GTPases, Cdc42 and Rac, are known regulators of the growth cone cytoskeleton. Here, we demonstrated that inhibition of Cdc42 or Rac not only prevented growth cone turning toward retinoic acid but could also induce a switch in growth cone responsiveness to chemorepulsion or growth cone collapse. However, the effects of Cdc42 or Rac inhibition on growth cone responsiveness differed, depending on whether the turning was induced by the all-trans or 9-cis retinoid isomer. The effects also differed depending on whether the growth cones maintained communication with the cell body. These data strongly suggest that Cdc42 and Rac are downstream effectors of retinoic acid during growth cone guidance.

Bottom up proteomics reveals novel differentiation proteins in neuroblastoma cells treated with 13-cis retinoic acid

Neuroblastoma, a cancer of the sympathetic nervous system, is the second most common pediatric cancer. A unique feature of neuroblastoma is remission in some patients due to spontaneous differentiation of metastatic tumors. 13-cis retinoic acid (13-cis RA) is currently used in the clinic to treat neuroblastoma due to its differentiation inducing effects. In this study, we used shotgun proteomics to identify proteins affected by 13-cis RA treatment in neuroblastoma SK-N-SH cells. Our results showed that 13-cis RA reduced proteins involved in extracellular matrix synthesis and organization and increased proteins involved in cell adhesion and neurofilament formation. These changes indicate that 13-cis RA induces tumor cell differentiation by decreasing extracellular matrix rigidity and increasing neurite overgrowth. Differentially-affected proteins identified in this study may be novel biomarkers of drug efficacy in the treatment of neuroblastoma. SIGNIFICANCE: As neuroblastoma can spontaneously differentiate, determining which proteins are involved in differentiation can guide development of novel treatments. 13-cis retinoic acid is currently used in the clinic as a differentiation inducer. Here we have established a proteome map of SK-N-SH cells treated with 13-cis retinoic acid. Bioinformatic analysis revealed the involvement of development, differentiation, extracellular matrix assembly, collagen biosynthesis, and neurofilament bundle association. This proteome map provides information as to which proteins are important for differentiation and identifies networks that can be targeted by drugs to treat neuroblastoma [1].

Pharmacological evidence for the role of RAR in axon guidance and embryonic development of a protostome species

Retinoic acid (RA), the active metabolite of vitamin A, functions through nuclear receptors, one of which is the retinoic acid receptor (RAR). Though the RAR is essential for various aspects of vertebrate development, little is known about the role of RAR in nonchordate invertebrates. Here, we examined the potential role of an invertebrate RAR in mediating chemotropic effects of retinoic acid. The RAR of the protostome Lymnaea stagnalis is present in the growth cones of regenerating cultured motorneurons, and a synthetic RAR agonist (EC23), was able to mimic the effects of retinoic acid in inducing growth cone turning. We also examined the ability of the natural retinoids, all-trans RA and 9-cis RA, as well as the synthetic RAR agonists, to disrupt embryonic development in Lymnaea. Developmental defects included delays in embryo hatching, arrested eye, and shell development, as well as more severe abnormalities such as halted development. Developmental defects induced by some (but not all) synthetic RAR agonists were found to mimic those induced by addition of high concentrations of the natural retinoid isomers. These pharmacological data support a possible physiological role for the RAR in axon guidance and embryonic development of an invertebrate protostome species.

The regulation of mitochondrial dynamics in neurite outgrowth by retinoic acid receptor β signaling

Neuronal regeneration is a highly energy-demanding process that greatly relies on axonal mitochondrial transport to meet the enhanced metabolic requirements. Mature neurons typically fail to regenerate after injury, partly because of mitochondrial motility and energy deficits in injured axons. Retinoic acid receptor (RAR)-β signaling is involved in axonal and neurite regeneration. Here we investigate the effect of RAR-β signaling on mitochondrial trafficking during neurite outgrowth and find that it enhances their proliferation, speed, and movement toward the growing end of the neuron via hypoxia-inducible factor 1α signaling. We also show that RAR-β signaling promotes the binding of the mitochondria to the anchoring protein, glucose-related protein 75, at the growing tip of neurite, thus allowing them to provide energy and metabolic roles required for neurite outgrowth.—Trigo, D., Goncalves, M. B., Corcoran, J. P. T. The regulation of mitochondrial dynamics in neurite outgrowth by retinoic acid receptor β signaling.

In vivo assay of the ethanol-induced embryonic hair cell loss and the protective role of the retinoic and folic acid in zebrafish larvae (Danio rerio)

In reference to the auditory manifestation of fetal alcohol syndrome, previous work has preferentially focused on the deviant neural development of the auditory system. Changes in the sensory hair cell, the ultimate sensory organ, were not well understood. In this study, we carried out an in vivo assessment of the embryonic hair cell changes on the lateral line of zebrafish upon exposure to various ethanol concentrations (0.25%, 0.5%, 0.75%, and 1.0%). A significant decrease in the hair cell count was confirmed as the ethanol concentration increased. Long-term observation (up to 240 hours post-fertilization [hpf]) suggested an irreversible hair cell loss with little chance of a simple delayed development. For an underlying biological process, a significant increase of hair cell apoptosis and a significant decrease of cytoplasmic mitochondria were confirmed as the ethanol concentration increased. Co-treatment with retinoic (0.1 nM) or folic (0.1 mM) acid with the same concentrations of ethanol resulted in significant increases in the remaining hair cells, compared to the ethanol-only treatment group, for every ethanol concentration. The retinoic acid provided more effective protection over folic acid, resulting in no significant changes in hair cell counts for every ethanol concentration (except 1.0%), compared with that of the negative control (without chemical treatment). Hair cell counts in every ethanol concentration were significantly lower than those in negative controls without chemical treatment after folic acid co-treatment. In conclusion, gestational ethanol exposure causes developmental sensory hair cell loss. Potential underlying mechanisms include retinoic or folic acid deficiency, and mitochondrial damage with subsequent hair cell apoptosis. Hair cell loss could possibly be prevented by administering either retinoic or folic acid, with retinoic acid supplementation as the preferred treatment.

The retinoic acid receptor (RAR) in molluscs: Function, evolution and endocrine disruption insights

Retinoid acid receptor (RAR)-dependent signalling pathways are essential for the regulation and maintenance of essential biological functions and are recognized targets of disruptive anthropogenic compounds. Recent studies put forward the inability of mollusc RARs to bind and respond to the canonical vertebrate ligand, retinoic acid: a feature that seems to have been lost during evolution. Yet, these studies were carried out in a limited number of molluscs. Therefore, using an in vitro transactivation assay, the present work aimed to characterize phylogenetically relevant mollusc RARs, as monomers or as functional units with RXR, not only in the presence of vertebrate bone fine ligands but also known endocrine disruptors, described to modulate retinoid-dependent pathways. In general, none of the tested mollusc RARs were able to activate reporter gene transcription when exposed to retinoic acid isomers, suggesting that the ability to respond to retinoic acid was lost across molluscs. Similarly, the analysed mollusc RAR were unresponsive towards organochloride pesticides. In contrast, transcriptional repressions were observed with the RAR/RXR unit upon exposure to retinoids or RXR-specific ligands. Loss-of-function and gain-of-function mutations further corroborate the obtained results and suggest that the repressive behaviour, observed with mollusc and human RAR/RXR heterodimers, is possibly mediated by ligand biding to RXR.

Combined use of bFGF/EGF and all-trans-retinoic acid cooperatively promotes neuronal differentiation and neurite outgrowth in neural stem cells

Neural stem cells (NSCs) as sources of new neurons in brain injuries or diseases are required to not only elicit neurons for neuronal repair, but also to enhance neurite outgrowth for neuronal network reestablishment. Various trophic or chemotropic factors have been shown to cooperatively improve NSC neurogenesis. However, effects of combined treatment of all-trans-retinoic acid (RA) with GF (Basic fibroblast growth factor and epidermal growth factor, bFGF/EGF) on neurogenesis of NSCs are poorly understood. To address this question, NSCs were isolated from the forebrains of embryonic mice, and treated with GF and RA either alone or in combination for differentiation in vitro. Neurons and astrocytes differentiated from NSCs were stained for MAP2 and GFAP separately by immunofluorescence. The results indicated that GF displayed superior efficacy in promoting neuronal differentiation, and RA showed better efficacy in advancing neurite outgrowth by increasing both neurite length and number. In addition, higher differentiation efficiency of neurons to astrocytes in RA or GF, or both acted at the early stage. However, more importantly, compared with RA alone, GF and RA in combination enhanced neuronal differentiation. Moreover, the combined use of GF and RA increased the length and number of neurites compared with GF, as well as the relative expression level of Smurf1. In addition, astrocytes induced by GF, RA, or both exhibited a radial glia-like morphology with long processes differing from serum effects, which might in part attribute to the total numbers of neurons. These findings for the first time unveil the roles of combined use of GF and RA on the neurogenesis of NSCs, suggesting that the use of this combination could be a comprehensive strategy for the functional repair of the nervous system through promoting neuronal differentiation, and advancing neurite outgrowth.

Identification and Characterization of microRNAs during Retinoic Acid-Induced Regeneration of a Molluscan Central Nervous System

Retinoic acid (RA) is the biologically active metabolite of vitamin A and has become a well-established factor that induces neurite outgrowth and regeneration in both vertebrates and invertebrates. However, the underlying regulatory mechanisms that may mediate RA-induced neurite sprouting remain unclear. In the past decade, microRNAs have emerged as important regulators of nervous system development and regeneration, and have been shown to contribute to processes such as neurite sprouting. However, few studies have demonstrated the role of miRNAs in RA-induced neurite sprouting. By miRNA sequencing analysis, we identify 482 miRNAs in the regenerating central nervous system (CNS) of the mollusc Lymnaeastagnalis, 219 of which represent potentially novel miRNAs. Of the remaining conserved miRNAs, 38 show a statistically significant up- or downregulation in regenerating CNS as a result of RA treatment. We further characterized the expression of one neuronally-enriched miRNA upregulated by RA, miR-124. We demonstrate, for the first time, that miR-124 is expressed within the cell bodies and neurites of regenerating motorneurons. Moreover, we identify miR-124 expression within the growth cones of cultured ciliary motorneurons (pedal A), whereas expression in the growth cones of another class of respiratory motorneurons (right parietal A) was absent in vitro. These findings support our hypothesis that miRNAs are important regulators of retinoic acid-induced neuronal outgrowth and regeneration in regeneration-competent species.

Retinoic acid synthesis by NG2 expressing cells promotes a permissive environment for axonal outgrowth

Stimulation of retinoic acid (RA) mediated signalling pathways following neural injury leads to regeneration in the adult nervous system and numerous studies have shown that the specific activation of the retinoic acid receptor β (RARβ) is required for this process. Here we identify a novel mechanism by which neuronal RARβ activation results in the endogenous synthesis of RA which is released in association with exosomes and acts as a positive cue to axonal/neurite outgrowth. Using an established rodent model of RARβ induced axonal regeneration, we show that neuronal RARβ activation upregulates the enzymes involved in RA synthesis in a cell specific manner; alcohol dehydrogenase7 (ADH7) in neurons and aldehyde dehydrogenase 2 (Raldh2) in NG2 expressing cells (NG2 + cells). These release RA in association with exosomes providing a permissive substrate to neurite outgrowth. Conversely, deletion of Raldh2 in the NG2 + cells in our in vivo regeneration model is sufficient to compromise axonal outgrowth. This hitherto unidentified RA paracrine signalling is required for axonal/neurite outgrowth and is initiated by the activation of neuronal RARβ signalling.

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Raldh2, the enzyme for retinoic acid synthesis, is upregulated in NG2 + cells during axonal regeneration.

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Deletion of Raldh2 in NG2 + cells prevents regeneration.

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RA signalling modulates axonal pathfinding.

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Fine-tuned regulation of RA distribution via exosome transport

Axon guidance pathways served as common targets for human speech/language evolution and related disorders

Human and several nonhuman species share the rare ability of modifying acoustic and/or syntactic features of sounds produced, i.e. vocal learning, which is the important neurobiological and behavioral substrate of human speech/language. This convergent trait was suggested to be associated with significant genomic convergence and best manifested at the ROBO-SLIT axon guidance pathway. Here we verified the significance of such genomic convergence and assessed its functional relevance to human speech/language using human genetic variation data. In normal human populations, we found the affected amino acid sites were well fixed and accompanied with significantly more associated protein-coding SNPs in the same genes than the rest genes. Diseased individuals with speech/language disorders have significant more low frequency protein coding SNPs but they preferentially occurred outside the affected genes. Such patients' SNPs were enriched in several functional categories including two axon guidance pathways (mediated by netrin and semaphorin) that interact with ROBO-SLITs. Four of the six patients have homozygous missense SNPs on PRAME gene family, one youngest gene family in human lineage, which possibly acts upon retinoic acid receptor signaling, similarly as FOXP2, to modulate axon guidance. Taken together, we suggest the axon guidance pathways (e.g. ROBO-SLIT, PRAME gene family) served as common targets for human speech/language evolution and related disorders.

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.

Morphogens and blood-brain barrier function in health and disease

The microvasculature of the brain forms a protective blood-brain barrier (BBB) that ensures a homeostatic environment for the central nervous system (CNS), which is essential for optimal brain functioning. The barrier properties of the brain endothelial cells are maintained by cells surrounding the capillaries, such as astrocytes and pericytes. Together with the endothelium and a basement membrane, these supporting cells form the neurovascular unit (NVU). Accumulating evidence indicates that the supporting cells of the NVU release a wide variety of soluble factors that induce and control barrier properties in a concentration-dependent manner. The current review provides a comprehensive overview of how such factors, called morphogens, influence BBB integrity and functioning. Since impaired BBB function is apparent in numerous CNS disorders and is often associated with disease severity, we also discuss the potential therapeutic value of these morphogens, as they may represent promising therapies for a wide variety of CNS disorders.

Evolution of retinoic acid receptors and retinoic acid signaling

Retinoic acid (RA) is a vitamin A-derived morphogen controlling important developmental processes in vertebrates, and more generally in chordates, including axial patterning and tissue formation and differentiation. In the embryo, endogenous RA levels are controlled by RA synthesizing and degrading enzymes and the RA signal is transduced by two retinoid receptors: the retinoic acid receptor (RAR) and the retinoid X receptor (RXR). Both RAR and RXR are members of the nuclear receptor superfamily of ligand-activated transcription factors and mainly act as heterodimers to activate the transcription of target genes in the presence of their ligand, all-trans RA. This signaling pathway was long thought to be a chordate innovation, however, recent findings of gene homologs involved in RA signaling in the genomes of a wide variety of non-chordate animals, including ambulacrarians (sea urchins and acorn worms) and lophotrochozoans (annelids and mollusks), challenged this traditional view and suggested that the RA signaling pathway might have a more ancient evolutionary origin than previously thought. In this chapter, we discuss the evolutionary history of the RA signaling pathway, and more particularly of the RARs, which might have experienced independent gene losses and duplications in different animal lineages. In sum, the available data reveal novel insights into the origin of the RA signaling pathway as well as into the evolutionary history of the RARs.

A mollusk retinoic acid receptor (RAR) ortholog sheds light on the evolution of ligand binding

Nuclear receptors are transcription factors that regulate networks of target genes in response to small molecules. There is a strong bias in our knowledge of these receptors because they were mainly characterized in classical model organisms, mostly vertebrates. Therefore, the evolutionary origins of specific ligand-receptor couples still remain elusive. Here we present the identification and characterization of a retinoic acid receptor (RAR) from the mollusk Nucella lapillus (NlRAR). We show that this receptor specifically binds to DNA response elements organized in direct repeats as a heterodimer with retinoid X receptor. Surprisingly, we also find that NlRAR does not bind all-trans retinoic acid or any other retinoid we tested. Furthermore, NlRAR is unable to activate the transcription of reporter genes in response to stimulation by retinoids and to recruit coactivators in the presence of these compounds. Three-dimensional modeling of the ligand-binding domain of NlRAR reveals an overall structure that is similar to vertebrate RARs. However, in the ligand-binding pocket (LBP) of the mollusk receptor, the alteration of several residues interacting with the ligand has apparently led to an overall decrease in the strength of the interaction with the ligand. Accordingly, mutations of NlRAR at key positions within the LBP generate receptors that are responsive to retinoids. Altogether our data suggest that, in mollusks, RAR has lost its affinity for all-trans retinoic acid, highlighting the evolutionary plasticity of its LBP. When put in an evolutionary context, our results reveal new structural and functional features of nuclear receptors validated by millions of years of evolution that were impossible to reveal in model organisms.

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.

Retinoic acid as a survival factor in neuronal development of the grasshopper, Locusta migratoria

Based on experience with cell cultures of adult insect neurons, we develop a serum-free culture system for embryonic locust neurons. Influences of trophic substances on survival and neurite outgrowth of developing neurons are investigated. For the first time, a positive trophic effect of 9-cis retinoic acid (9-cis RA) was shown in vitro on embryonic neurons of an insect. We observed longer cell survival of 50 % developmental stage neurons in cultures supplemented with 0.3 nM 9-cis RA. Furthermore, an influence on neuron morphology was revealed, as the addition of 9-cis RA to cell culture medium led to an increase in the number of neurites per cell. Although an RA receptor gene, LmRXR (Locusta migratoria retinoid X receptor), was expressed in the central nervous system throughout development, the influence of 9-cis RA on neuronal survival and outgrowth was restricted to 50 % stage embryonic cells.

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.

Differences in retinoid levels and metabolism among gastropod lineages: imposex-susceptible gastropods lack the ability to store retinoids in the form of retinyl esters

The presence of a complex retinoid system was long believed to be a chordate/vertebrate novelty. However, recent findings indicate otherwise since the gastropod mollusk Osilinus lineatus was found to have the capacity to store retinoids in the form of retinyl esters (REs), a key feature to maintain a homeostatic control of retinoid levels. Here, we investigated whether such a complex retinoid system is widely distributed among gastropod lineages. Additionally, since one of the most spectacular examples of endocrine disruption in the wild, the masculinization of female gastropods (imposex) by the retinoid X receptor (RXR) agonist, tributyltin (TBT), has been linked with perturbed retinoid signaling, we also investigated if retinoid storage mechanisms in the form of retinyl esters were present in imposex-susceptible gastropods. Initially, we determined the presence of both polar (active retinoic acid isomers) and nonpolar retinoids (retinol, REs) in selected gastropod species: the limpet Patella depressa and the imposex-susceptible whelks Nucella lapillus and Nassarius reticulatus. Although all species presented active retinoid forms, N. lapillus and N. reticulatus were shown to lack nonpolar retinoids. The absence of REs, which are the common retinoid storage form found in vertebrates and in O. lineatus suggest that those species are unable to use them to maintain a homeostatic control of their retinoid levels. In order to further clarify the retinoid metabolic pathways in imposex-susceptible gastropods, a retinoid exposure study was carried out with N. lapillus. The results demonstrate that although N. lapillus is able to metabolize several retinoid precursors, it lacks the capacity to store retinoids as REs. Whether the lack of retinoid storage mechanisms in the form of REs in imposex-susceptible gastropods plays an important role in the susceptibility to RXR agonists warrants additional studies.

The roles of endogenous retinoid signaling in organ and appendage regeneration

The ability to regenerate injured or lost body parts has been an age-old ambition of medical science. In contrast to humans, teleost fish and urodele amphibians can regrow almost any part of the body with seeming effortlessness. Retinoic acid is a molecule that has long been associated with these impressive regenerative capacities. The discovery 30 years ago that addition of retinoic acid to regenerating amphibian limbs causes "super-regeneration" initiated investigations into the presumptive roles of retinoic acid in regeneration of appendages and other organs. However, the evidence favoring or dismissing a role for endogenous retinoids in regeneration processes remained sparse and ambiguous. Now, the availability of genetic tools to manipulate and visualize the retinoic acid signaling pathway has opened up new routes to dissect its roles in regeneration. Here, we review the current understanding on endogenous functions of retinoic acid in regeneration and discuss key questions to be addressed in future research.

Vitamin A, folate, and choline as a possible preventive intervention to fetal alcohol syndrome

It is recognized that alcohol consumption during pregnancy is associated with fetal alcohol syndrome (FAS). Alcohol can trigger a pattern of neurodegeneration in rat brains similar to other known gamma-aminobutyric acid (GABA) specific agonists. However this does not seem to explain FAS entirely, as impoverished care-giving environments have been shown to increase the risk of FAS. Individuals living under the poverty level are at risk for micronutrient deficiencies due to insufficient intake. In particular, three nutrients commonly found to be deficient are folate, choline and vitamin A. There is evidence to suggest that ethanol alone may not explain the entire spectrum of anomalies seen in individuals with FAS. It is hypothesized that FAS may be caused more by the nutritional deficiencies that are exacerbated by alcohol than by direct alcoholic neurotoxicity. It is known that ethanol inhibits folate, choline, and vitamin A/retinoic acid metabolism at multiple steps. Additionally, mice exposed to ethanol demonstrated epigenetic changes, or variations in the methylation of DNA to control gene expression. Folate is important in the production of methyl groups, which are subsequently used to create and methylate DNA. Choline (which is metabolized to acetylcholine) is important in neurotransmission and neurodevelopment. It is also involved in an alternative pathway in the production of methyl groups. In fact a study by Thomas et al. in 2009 found that nutritional supplementation with choline in rats exposed to ethanol in utero almost completely mitigated the degenerative effects of ethanol on development and behaviour. Lastly, vitamin A and retinoic acid metabolism is associated with the regulation of one sixth of the entire proteome. Thus supplementation of folate, choline and vitamin A to mothers may mitigate the effects of the alcohol and reduce the severity or prevalence of FAS.

RAR/RXR and PPAR/RXR Signaling in Spinal Cord Injury

The retinoid acid receptors (RAR) and peroxisome proliferator-activated receptors (PPAR) have been implicated in the regulation of inflammatory reactions. Both receptor families contain ligand-activated transcription factors which form heterodimers with retinoid X receptors (RXR). We review data that imply RAR/RXR and PPAR/RXR pathways in physiological reactions after spinal cord injury. Experiments show how RAR signaling may improve axonal regeneration and modulate reactions of glia cells. While anti-inflammatory properties of PPAR are well documented in the periphery, their possible roles in the central nervous system have only recently become evident. Due to its anti-inflammatory function this transcription factor family promises to be a useful target after spinal cord or brain lesions.

Regulatory actions of neuropeptides and peptide hormones on the reproduction of molluscsThe present review is one of a series of occasional review articles that have been invited by the Editors and will feature the broad range of disciplines and expertise represented in our Editorial Advisory Board

Reproductive success of individual animals is essential for the survival of any species. Molluscs have adapted to a wide variety of environments (freshwater, brackish water, seawater, and terrestrial habits) and have evolved unique tactics for reproduction. Both of these features attract the academic interests of scientists. Because neuropeptides and peptide hormones play critical roles in neural and neurohormonal regulation of physiological functions and behaviors in this animal group, the regulatory actions of these messengers in reproduction have been extensively investigated. In this review, we will briefly summarize how peptidergic messengers are involved in various aspects of reproduction, using some peptides such as egg-laying hormone, caudo-dorsal cell hormone, APGWamide, and gonadotropin-releasing hormone as typical examples.

Endocrine interactions between plants and animals: Implications of exogenous hormone sources for the evolution of hormone signaling

Hormones are central to animal physiology, metabolism and development. Details on signal transduction systems and regulation of hormone synthesis, activation and release have only been studied for a small number of animal groups, notably arthropods and chordates. However, a significant body of literature suggests that hormonal signaling systems are not restricted to these phyla. For example, work on several echinoderm species shows that exogenous thyroid hormones (THs) affect larval development and metamorphosis and our new data provide strong evidence for endogenous synthesis of THs in sea urchin larvae. In addition to these endogenous sources, these larvae obtain THs when they consume phytoplankton. Another example of an exogenously acquired hormone or their precursors is in insect and arthropod signaling. Sterols from plants are essential for the synthesis of ecdysteroids, a crucial group of insect morphogenic steroids. The availability of a hormone or hormone precursor from food has implications for understanding hormone function and the evolution of hormonal signaling in animals. For hormone function, it creates an important link between the environment and the regulation of internal homeostatic systems. For the evolution of hormonal signaling it helps us to better understand how complex endocrine mechanisms may have evolved.

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.

Structural and proteomic analyses reveal regional brain differences during honeybee aging

Among insects, learning is particularly well studied in the fruit fly Drosophila melanogaster and the honeybee Apis mellifera. A senescence-dependent decline in classic pavlovian conditioning is demonstrated for both species. To understand how aging affects learning, genetic approaches used with Drosophila can benefit from complementary studies in Apis. Specifically, honeybees have a larger brain size allowing for compartment-specific approaches, and a unique life-history plasticity. They usually perform within-nest tasks early in life (nest bees) and later they collect food (foragers). Senescence of learning performance is a function of the bees' foraging duration but underlying causes are poorly understood. As cognitive aging is commonly associated with structural and biochemical changes in the brain, we hypothesized that brain areas implicated in learning change in synaptic and biochemical composition with increased foraging duration. First, we used synapse-specific immunohistochemistry and proteomics to screen for alterations in the calyx region of the mushroom body, a key structure for memory formation. Using proteomics, we next profiled the central brain, which comprises all higher-order integration centers. We show that, with increased foraging duration, levels of kinases, synaptic- and neuronal growth-related proteins decline in the central brain while the calyx region remains intact both in structure and biochemistry. We suggest that proteome-level changes within major anatomical sites of memory formation other than the calyx region could be central to learning dysfunction. These include large compartments of the central brain, such as the mushroom body's output regions and the antennal lobes. Our data provide novel information toward heterogeneity in the aging insect brain, and demonstrate advantages of the honeybee for invertebrate neurogerontological research.

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.

Differential gene expression during compensatory sprouting of dendrites in the auditory system of the cricket Gryllus bimaculatus

Neurones that lose their presynaptic partners because of injury usually retract or die. However, when the auditory interneurones of the cricket Gryllus bimaculatus are denervated, dendrites respond by growing across the midline and forming novel synapses with the opposite auditory afferents. Suppression subtractive hybridization was used to detect transcriptional changes 3 days after denervation. This is a stage at which we demonstrate robust compensatory dendritic sprouting. Whereas 49 unique candidates were down-regulated, no sufficiently up-regulated candidates were identified at this time point. Several candidates identified in this study are known to influence the translation and degradation of proteins in other systems. The potential role of these factors in the compensatory sprouting of cricket auditory interneurones in response to denervation is discussed.

Detection of endogenous retinoids in the molluscan CNS and characterization of the trophic and tropic actions of 9-cis retinoic acid on isolated neurons

Retinoic acid (RA) is an active metabolite of Vitamin A that plays an important role in the growth and differentiation of many cell types. All-trans RA (atRA) is the retinoic acid isomer that has been most widely studied in the nervous system, and can induce and direct neurite outgrowth from both vertebrate and invertebrate preparations. The presence and role of the 9-cis-RA isomer in the nervous system is far less well defined. Here, we used high-pressure liquid chromatography (HPLC) and mass spectrometry (MS) to show for the first time, the presence of both atRA and 9-cis-RA in the CNS of an invertebrate. We then demonstrated that 9-cis-RA was capable of exerting the same neurotrophic and chemotropic effects on cultured neurons as atRA. In this study, significantly more cells showed neurite outgrowth in 9-cis-RA versus the EtOH vehicle control, and 9-cis-RA significantly increased the number and length of neurites from identified neurons after 4 d in culture. 9-cis-RA also extended the duration of time that cells remained electrically excitable in culture. Furthermore, we showed for the first time in any species, that exogenous application of 9-cis-RA induced positive growth cone turning of cultured neurons. This study provides the first evidence for the presence of both atRA and 9-cis-RA in an invertebrate CNS and also provides the first direct evidence for a potential physiological role for 9-cis-RA in neuronal regeneration and axon pathfinding.

Identification of Aldh1a, Cyp26 and RAR orthologs in protostomes pushes back the retinoic acid genetic machinery in evolutionary time to the bilaterian ancestor

In vertebrates, retinoic acid (RA) is an important morphogenetic signal that controls embryonic development, as well as organ homeostasis in adults. RA action depends on the function of the RA-genetic machinery, which includes a metabolic module and a signaling module. The metabolic module regulates the spatiotemporal distribution of RA by the combined action of the RA-synthesizing Aldh1a enzymes, and the RA-degrading Cyp26 enzymes. The signaling module includes members of the nuclear hormone receptors family RAR and RXR, and controls the transcriptional state of RA-target genes. RA-signaling has been described primarily in chordates, but the recent finding of elements of the RA-genetic machinery in non-chordate deuterostomes has changed our perspective on the evolutionary origin of this morphogenetic signal, challenging previous assumptions that related the invention of the RA-genetic machinery with the origin of the chordate body plan. To illuminate the evolutionary origin of the RA machinery we have conducted an extensive survey of Aldh1a, Cyp26 and RAR orthologs in genomic databases of 13 non-deuterostome metazoans. Our results show for the first time the presence of Aldh1a, Cyp26 and RAR in protostomes, which implies that the components of the RA machinery may be ancient elements of animal genomes, already present in the last common ancestor of bilaterians. Interestingly, our data also reveal that independent losses of the RA toolkit have occurred multiple times during animal evolution, which may have been relevant for the evolution and developmental diversity of the current metazoan lineages.

Retinoic acid regulates murine enteric nervous system precursor proliferation, enhances neuronal precursor differentiation, and reduces neurite growth in vitro

Enteric nervous system (ENS) precursors undergo a complex process of cell migration, proliferation, and differentiation to form an integrated network of neurons and glia within the bowel wall. Although retinoids regulate ENS development, molecular and cellular mechanisms of retinoid effects on the ENS are not well understood. We hypothesized that retinoids might directly affect ENS precursor differentiation and proliferation, and tested that hypothesis using immunoselected fetal ENS precursors in primary culture. We now demonstrate that all retinoid receptors and many retinoid biosynthetic enzymes are present in the fetal bowel at about the time that migrating ENS precursors reach the distal bowel. We further demonstrate that retinoic acid (RA) enhances proliferation of subsets of ENS precursors in a time-dependent fashion and increases neuronal differentiation. Surprisingly, however, enteric neurons that develop in retinoid deficient media have dramatically longer neurites than those exposed to RA. This difference in neurite growth correlates with increased RhoA protein at the neurite tip, decreased Smurf1 (a protein that targets RhoA for degradation), and dramatically decreased Smurf1 mRNA in response to RA. Collectively these data demonstrate diverse effects of RA on ENS precursor development and suggest that altered fetal retinoid availability or metabolism could contribute to intestinal motility disorders.