Retinoid receptor-based signaling plays a role in voltage-dependent inhibition of invertebrate voltage-gated Ca2+ channels

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.

The binding of Mint/X11 PDZ domains to Ca V 2 calcium channels predates bilaterian animals

PDZ domain mediated interactions with voltage-gated calcium (Ca V ) channel C-termini play important roles in localizing and compartmentalizing membrane Ca 2+ signaling. The first such interaction discovered was between the neuronal multi-domain protein Mint-1, and the presynaptc calcium channel Ca V 2.2 in mammals. Although the physiological significance of this interaction is unclear, its occurrence in vertebrates and bilaterian invertebrates suggests important and conserved functions. In this study, we explore the evolutionary origins of Mint and its interaction with Ca V 2 channels. Phylogenetic and structural in silico analyses revealed that Mint is an animal-specific gene, like Ca V 2 channels, which bears a highly divergent N-terminus but strongly conserved C-terminus comprised of a phosphotyrosine binding domain, two tandem PDZ domains (PDZ-1 and PDZ-2), and a C-terminal auto-inhibitory element that binds and inhibits PDZ-1. Also deeply conserved are other Mint interacting proteins, namely amyloid precursor and related proteins, presenilins, neurexin, as well as CASK and Veli which form a tripartite complex with Mint in bilaterians. Through yeast 2-hybrid and bacterial 2-hybrid experiments, we show that Mint and Ca V 2 channels from cnidarians and placozoans interact in vitro , and in situ hybridization revealed co-expression of corresponding transcripts in dissociated neurons from the cnidarian Nematostella vectensis . Unexpectedly, the Mint orthologue from the ctenophore Hormiphora californiensis was able to strongly bind the divergent C-terminal ligands of cnidarian and placozoan Ca V 2 channels, despite neither the ctenophore Mint, nor the placozoan and cnidarian orthologues, binding the ctenophore Ca V 2 channel C-terminus. Altogether, our analyses provide a model for the emergence of this interaction in early animals first via adoption of a PDZ ligand by Ca V 2 channels, followed by sequence changes in the ligand that caused a modality switch for binding to Mint.

Juvenile hormones direct primordial germ cell migration to the embryonic gonad

Germ cells are essential to sexual reproduction. Across the animal kingdom, extracellular signaling isoprenoids, such as retinoic acids (RAs) in vertebrates and juvenile hormones (JHs) in invertebrates, facilitate multiple processes in reproduction. Here we investigated the role of these potent signaling molecules in embryonic germ cell development, using JHs in Drosophila melanogaster as a model system. In contrast to their established endocrine roles during larval and adult germline development, we found that JH signaling acts locally during embryonic development. Using an in vivo biosensor, we observed active JH signaling first within and near primordial germ cells (PGCs) as they migrate to the developing gonad. Through in vivo and in vitro assays, we determined that JHs are both necessary and sufficient for PGC migration. Analysis into the mechanisms of this newly uncovered paracrine JH function revealed that PGC migration was compromised when JHs were decreased or increased, suggesting that specific titers or spatiotemporal JH dynamics are required for robust PGC colonization of the gonad. Compromised PGC migration can impair fertility and cause germ cell tumors in many species, including humans. In mammals, retinoids have many roles in development and reproduction. We found that like JHs in Drosophila, RA was sufficient to impact mouse PGC migration in vitro. Together, our study reveals a previously unanticipated role of isoprenoids as local effectors of pre-gonadal PGC development and suggests a broadly shared mechanism in PGC migration.

Retinoic acid signaling in development and differentiation commitment and its regulatory topology

Retinoic acid (RA), the derivative of vitamin A/retinol, is a signaling molecule with important implications in health and disease. It is a well-known developmental morphogen that functions mainly through the transcriptional activity of nuclear RA receptors (RARs) and, uncommonly, through other nuclear receptors, including peroxisome proliferator-activated receptors. Intracellular RA is under spatiotemporally fine-tuned regulation by synthesis and degradation processes catalyzed by retinaldehyde dehydrogenases and P450 family enzymes, respectively. In addition to dictating the transcription architecture, RA also impinges on cell functioning through non-genomic mechanisms independent of RAR transcriptional activity. Although RA-based differentiation therapy has achieved impressive success in the treatment of hematologic malignancies, RA also has pro-tumor activity. Here, we highlight the relevance of RA signaling in cell-fate determination, neurogenesis, visual function, inflammatory responses and gametogenesis commitment. Genetic and post-translational modifications of RAR are also discussed. A better understanding of RA signaling will foster the development of precision medicine to improve the defects caused by deregulated RA signaling.

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.

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.

Activity-dependent modulation of neuronal KV channels by retinoic acid enhances CaV channel activity

The metabolite of vitamin A, retinoic acid (RA), is known to affect synaptic plasticity in the nervous system and to play an important role in learning and memory. A ubiquitous mechanism by which neuronal plasticity develops in the nervous system is through modulation of voltage-gated Ca²⁺ (Ca_V) and voltage-gated K⁺ channels. However, how retinoids might regulate the activity of these channels has not been determined. Here, we show that RA modulates neuronal firing by inducing spike broadening and complex spiking in a dose-dependent manner in peptidergic and dopaminergic cell types. Using patch-clamp electrophysiology, we show that RA-induced complex spiking is activity dependent and involves enhanced inactivation of delayed rectifier voltage-gated K⁺ channels. The prolonged depolarizations observed during RA-modulated spiking lead to an increase in Ca²⁺ influx through Ca_V channels, though we also show an opposing effect of RA on the same neurons to inhibit Ca²⁺ influx. At physiological levels of Ca²⁺, this inhibition is specific to Ca_V2 (not Ca_V1) channels. Examining the interaction between the spike-modulating effects of RA and its inhibition of Ca_V channels, we found that inhibition of Ca_V2 channels limits the Ca²⁺ influx resulting from spike modulation. Our data thus provide novel evidence to suggest that retinoid signaling affects both delayed rectifier K⁺ channels and Ca_V channels to fine-tune Ca²⁺ influx through Ca_V2 channels. As these channels play important roles in synaptic function, we propose that these modulatory effects of retinoids likely contribute to synaptic plasticity in the nervous system.

Antiepileptogenic Effect of Retinoic Acid

Retinoic acid, a metabolite of vitamin A, acts through either genomic or nongenomic actions. The genomic action of retinoids exerts effects on gene transcription through interaction with retinoid receptors such as retinoic acid receptors (RARα, β, and γ) and retinoid X receptors (RXRα, β, and γ) that are primarily concentrated in the amygdala, pre-frontal cortex, and hippocampal areas in the brain. In response to retinoid binding, RAR/RXR heterodimers undergo major conformational changes and orchestrate the transcription of specific gene networks. Previous experimental studies have reported that retinoic acid exerts an antiepileptogenic effect through diverse mechanisms, including the modulation of gap junctions, neurotransmitters, long-term potentiation, calcium channels and some genes. To our knowledge, there are no previous or current clinical trials evaluating the use of retinoic acid for seizure control.

Conserved biophysical features of the CaV2 presynaptic Ca2+ channel homologue from the early-diverging animal Trichoplax adhaerens

The dominant role of CaV2 voltage-gated calcium channels for driving neurotransmitter release is broadly conserved. Given the overlapping functional properties of CaV2 and CaV1 channels, and less so CaV3 channels, it is unclear why there have not been major shifts toward dependence on other CaV channels for synaptic transmission. Here, we provide a structural and functional profile of the CaV2 channel cloned from the early-diverging animal Trichoplax adhaerens, which lacks a nervous system but possesses single gene homologues for CaV1-CaV3 channels. Remarkably, the highly divergent channel possesses similar features as human CaV2.1 and other CaV2 channels, including high voltage-activated currents that are larger in external Ba2+ than in Ca2+; voltage-dependent kinetics of activation, inactivation, and deactivation; and bimodal recovery from inactivation. Altogether, the functional profile of Trichoplax CaV2 suggests that the core features of presynaptic CaV2 channels were established early during animal evolution, after CaV1 and CaV2 channels emerged via proposed gene duplication from an ancestral CaV1/2 type channel. The Trichoplax channel was relatively insensitive to mammalian CaV2 channel blockers ω-agatoxin-IVA and ω-conotoxin-GVIA and to metal cation blockers Cd2+ and Ni2+ Also absent was the capacity for voltage-dependent G-protein inhibition by co-expressed Trichoplax Gβγ subunits, which nevertheless inhibited the human CaV2.1 channel, suggesting that this modulatory capacity evolved via changes in channel sequence/structure, and not G proteins. Last, the Trichoplax channel was immunolocalized in cells that express an endomorphin-like peptide implicated in cell signaling and locomotive behavior and other likely secretory cells, suggesting contributions to regulated exocytosis.

The unlimited potential of the great pond snail, Lymnaea stagnalis

Only a limited number of animal species lend themselves to becoming model organisms in multiple biological disciplines: one of these is the great pond snail, Lymnaea stagnalis. Extensively used since the 1970s to study fundamental mechanisms in neurobiology, the value of this freshwater snail has been also recognised in fields as diverse as host-parasite interactions, ecotoxicology, evolution, genome editing and 'omics', and human disease modelling. While there is knowledge about the natural history of this species, what is currently lacking is an integration of findings from the laboratory and the field. With this in mind, this article aims to summarise the applicability of L. stagnalis and points out that this multipurpose model organism is an excellent, contemporary choice for addressing a large range of different biological questions, problems and phenomena.

Serum Levels of Vitamin A and Vitamin D and Their Association With Symptoms in Children With Attention Deficit Hyperactivity Disorder

Objective: To measure levels of vitamin A (VA) and vitamin D (VD) and the symptomatic association of their co-deficiencies on attention deficit hyperactivity disorder (ADHD) in Chinese children (6-9 years). Methods: Eighty-two children (69 boys and 13 girls; mean age = 7.1 ± 0.9 years at the time of the diagnosis) with ADHD were recruited as ADHD group. A total of 106 healthy children were recruited as the healthy control (HC) group. Serum levels of retinol and 25-hydroxyvitamin D (25(OH)D) of all children were evaluated using high-performance liquid chromatography (HPLC) and HPLC-tandem mass spectrometry. The Swanson, Nolan, and Pelham IV Rating Scale (SNAP-IV) was employed to assess the clinical symptoms of ADHD. Results: Children suffering from ADHD had significantly reduced serum levels of retinol and 25(OH)D compared with those of HCs, and the prevalence of VA deficiency and VD deficiency were higher in children suffering from ADHD. Serum concentrations of 25(OH)D and retinol were linked closely with the presence or absence of ADHD after adjustment for age, body mass index, season of blood sampling, and sun exposure. Serum concentrations of 25(OH)D and retinol showed a negative correlation with the total scores of SNAP-IV. Children with ADHD as well as VA and VD co-deficiency had increased SNAP-IV total scores and ADHD inattention subscale scores. Conclusion: VA deficiency and VD deficiency in children with ADHD were increased in comparison with that in HCs. VA and VD co-deficiency associated with ADHD symptom severity. Attention should be paid to regular testing of VA levels and VD levels. However, the mechanism of VA and VD in ADHD needs to be further studied. Interventional studies on VA and VD supplementation are recommended to further verify the relationship between VA and VD co-deficiency and ADHD.