Group I mGlu receptors regulate the expression of the neuronal calcium sensor protein VILIP-1 in vitro and in vivo: implications for mGlu receptor-dependent hippocampal plasticity?

Dimerization of Neuronal Calcium Sensor Proteins

Neuronal calcium sensor (NCS) proteins are EF-hand containing Ca²⁺ binding proteins that regulate sensory signal transduction. Many NCS proteins (recoverin, GCAPs, neurocalcin and visinin-like protein 1 (VILIP1)) form functional dimers under physiological conditions. The dimeric NCS proteins have similar amino acid sequences (50% homology) but each bind to and regulate very different physiological targets. Retinal recoverin binds to rhodopsin kinase and promotes Ca²⁺-dependent desensitization of light-excited rhodopsin during visual phototransduction. The guanylyl cyclase activating proteins (GCAP1–5) each bind and activate retinal guanylyl cyclases (RetGCs) in light-adapted photoreceptors. VILIP1 binds to membrane targets that modulate neuronal secretion. Here, I review atomic-level structures of dimeric forms of recoverin, GCAPs and VILIP1. The distinct dimeric structures in each case suggest that NCS dimerization may play a role in modulating specific target recognition. The dimerization of recoverin and VILIP1 is Ca²⁺-dependent and enhances their membrane-targeting Ca²⁺-myristoyl switch function. The dimerization of GCAP1 and GCAP2 facilitate their binding to dimeric RetGCs and may allosterically control the Ca²⁺-dependent activation of RetGCs.

Dimerization of visinin-like protein 1 is regulated by oxidative stress and calcium and is a pathological hallmark of amyotrophic lateral sclerosis

Redox control of proteins that form disulfide bonds upon oxidative challenge is an emerging topic in the physiological and pathophysiological regulation of protein function. We have investigated the role of the neuronal calcium sensor protein visinin-like protein 1 (VILIP-1) as a novel redox sensor in a cellular system. We have found oxidative stress to trigger dimerization of VILIP-1 within a cellular environment and identified thioredoxin reductase as responsible for facilitating the remonomerization of the dimeric protein. Dimerization is modulated by calcium and not dependent on the myristoylation of VILIP-1. Furthermore, we show by site-directed mutagenesis that dimerization is exclusively mediated by Cys187. As a functional consequence, VILIP-1 dimerization modulates the sensitivity of cells to an oxidative challenge. We have investigated whether dimerization of VILIP-1 occurs in two different animal models of amyotrophic lateral sclerosis (ALS) and detected soluble VILIP-1 dimers to be significantly enriched in the spinal cord from phenotypic disease onset onwards. Moreover, VILIP-1 is part of the ALS-specific protein aggregates. We show for the first time that the C-terminus of VILIP-1, containing Cys187, might represent a novel redox-sensitive motif and that VILIP-1 dimerization and aggregation are hallmarks of ALS. This suggests that VILIP-1 dimers play a functional role in integrating the cytosolic calcium concentration and the oxidative status of the cell. Furthermore, a loss of VILIP-1 function owing to protein aggregation in ALS could be relevant in the pathophysiology of the disease.

The visinin-like proteins VILIP-1 and VILIP-3 in Alzheimer's disease-old wine in new bottles

The neuronal Ca²⁺-sensor (NCS) proteins VILIP-1 and VILIP-3 have been implicated in the etiology of Alzheimer's disease (AD). Genome-wide association studies (GWAS) show association of genetic variants of VILIP-1 (VSNL1) and VILIP-3 (HPCAL1) with AD+P (+psychosis) and late onset AD (LOAD), respectively. In AD brains the expression of VILIP-1 and VILIP-3 protein and mRNA is down-regulated in cortical and limbic areas. In the hippocampus, for instance, reduced VILIP-1 mRNA levels correlate with the content of neurofibrillary tangles (NFT) and amyloid plaques, the pathological characteristics of AD, and with the mini mental state exam (MMSE), a test for cognitive impairment. More recently, VILIP-1 was evaluated as a cerebrospinal fluid (CSF) biomarker and a prognostic marker for cognitive decline in AD. In CSF increased VILIP-1 levels correlate with levels of Aβ, tau, ApoE4, and reduced MMSE scores. These findings tie in with previous results showing that VILIP-1 is involved in pathological mechanisms of altered Ca²⁺-homeostasis leading to neuronal loss. In PC12 cells, depending on co-expression with the neuroprotective Ca²⁺-buffer calbindin D28K, VILIP-1 enhanced tau phosphorylation and cell death. On the other hand, VILIP-1 affects processes, such as cyclic nucleotide signaling and dendritic growth, as well as nicotinergic modulation of neuronal network activity, both of which regulate synaptic plasticity and cognition. Similar to VILIP-1, its interaction partner α4β2 nicotinic acetylcholine receptor (nAChR) is severely reduced in AD, causing severe cognitive deficits. Comparatively little is known about VILIP-3, but its interaction with cytochrome b5, which is part of an antioxidative system impaired in AD, hint toward a role in neuroprotection. A current hypothesis is that the reduced expression of visinin-like protein (VSNLs) in AD is caused by selective vulnerability of subpopulations of neurons, leading to the death of these VILIP-1-expressing neurons, explaining its increased CSF levels. While the Ca²⁺-sensor appears to be a good biomarker for the detrimental effects of Aβ in AD, its early, possibly Aβ-induced, down-regulation of expression may additionally attenuate neuronal signal pathways regulating the functions of dendrites and neuroplasticity, and as a consequence, this may contribute to cognitive decline in early AD.

Divalent cations and redox conditions regulate the molecular structure and function of visinin-like protein-1

The NCS protein Visinin-like Protein 1 (VILIP-1) transduces calcium signals in the brain and serves as an effector of the non-retinal receptor guanylyl cyclases (GCs) GC-A and GC-B, and nicotinic acetyl choline receptors (nAchR). Analysis of the quaternary structure of VILIP-1 in solution reveals the existence of monomeric and dimeric species, the relative contents of which are affected but not exclusively regulated by divalent metal ions and Redox conditions. Using small-angle X-ray scattering, we have investigated the low resolution structure of the calcium-bound VILIP-1 dimer under reducing conditions. Scattering profiles for samples with high monomeric and dimeric contents have been obtained. The dimerization interface involves residues from EF-hand regions EF3 and EF4.

Using monolayer adsorption experiments, we show that myristoylated and unmyristoylated VILIP-1 can bind lipid membranes. The presence of calcium only marginally improves binding of the protein to the monolayer, suggesting that charged residues at the protein surface may play a role in the binding process.

In the presence of calcium, VILIP-1 undergoes a conformational re-arrangement, exposing previously hidden surfaces for interaction with protein partners. We hypothesise a working model where dimeric VILIP-1 interacts with the membrane where it binds membrane-bound receptors in a calcium-dependent manner.

Structural analysis of Mg2+ and Ca2+ binding, myristoylation, and dimerization of the neuronal calcium sensor and visinin-like protein 1 (VILIP-1)

Visinin-like protein 1 (VILIP-1) belongs to the neuronal calcium sensor family of Ca(2+)-myristoyl switch proteins that regulate signal transduction in the brain and retina. Here we analyze Ca(2+) and Mg(2+) binding, characterize metal-induced conformational changes, and determine structural effects of myristoylation and dimerization. Mg(2+) binds functionally to VILIP-1 at EF3 (ΔH = +1.8 kcal/mol and K(D) = 20 μM). Unmyristoylated VILIP-1 binds two Ca(2+) sequentially at EF2 and EF3 (K(EF3) = 0.1 μM and K(EF2) = 1-4 μM), whereas myristoylated VILIP-1 binds two Ca(2+) with lower affinity (K(D) = 1.2 μM) and positive cooperativity (Hill slope = 1.5). NMR assignments and structural analysis indicate that Ca(2+)-free VILIP-1 contains a sequestered myristoyl group like that of recoverin. NMR resonances of the attached myristate exhibit Ca(2+)-dependent chemical shifts and NOE patterns consistent with Ca(2+)-induced extrusion of the myristate. VILIP-1 forms a dimer in solution independent of Ca(2+) and myristoylation. The dimerization site is composed of residues in EF4 and the loop region between EF3 and EF4, confirmed by mutagenesis. We present the structure of the VILIP-1 dimer and a Ca(2+)-myristoyl switch to provide structural insights into Ca(2+)-induced trafficking of nicotinic acetylcholine receptors.

The role of metabotropic glutamate receptor 5 in developmental lead neurotoxicity

A complete explanation of the mechanisms of lead-induced developmental neurotoxicity remains unknown. The glutamate receptor is one of the most important targets of lead. More recently, metabotropic glutamate receptor 5 (mGluR5) has been shown to have a functional relationship with learning and memory. We investigated the impact of developmental lead exposure on hippocampal mGluR5 expression and its potential role in lead neurotoxicity. Both in vitro model of lead exposure with Pb(2+) concentrations of 0, 10 nM, 1 microM, and 100 microM in cultured rat embryonic hippocampal neurons, and the in vivo model of rat maternal lead exposure involving both gestational and lactational exposure with 0, 0.05%, 0.2%, and 0.5% lead acetate were utilized. Immunoperoxidase and immunofluorescent analyses, quantitative PCR and western blotting were used. In vitro studies revealed that expression of mGluR5 mRNA and protein was decreased dose-dependently after lead exposure, which was further confirmed by the results of in vivo studies. These data suggest that mGluR5 might be involved in lead-induced neurotoxicity by disturbing mGluR5-induced long-term depression and decreasing N-methyl-D-aspartic acid receptor (NMDAR)-dependent or protein synthesis-dependent long-term potentiation. These results might improve the understanding of the mechanism and potential treatments for moderate to severe lead poisoning in children.

Promoter regulation of the visinin-like subfamily of neuronal calcium sensor proteins by nuclear respiratory factor-1

VILIP-1 (gene name VSNL1), a member of the neuronal Ca(2+) sensor protein family, acts as a tumor suppressor gene by inhibiting cell proliferation, adhesion, and invasiveness. VILIP-1 expression is down-regulated in several types of human cancer. In human non-small cell lung cancer, we found that down-regulation was due to epigenetic changes. Consequently, in this study we analyzed the VSNL1 promoter and its regulation. Serial truncation of the proximal 2-kb VSNL1 promoter (VP-1998) from its 5' terminus disclosed that the last 3' terminal 100-bp promoter fragment maintained similar promoter activity as compared with VP-1998 and therefore was referred to as VSNL1 minimal promoter. When the 5' terminal 50 bp were deleted from the minimal promoter, the activity was dramatically decreased, suggesting that the deleted 50 bp contained a potential cis-acting element crucial for promoter activity. Deletion and site-directed mutagenesis combined with in silico transcription factor binding analysis of VSNL1 promoter identified nuclear respiratory factor (NRF)-1/alpha-PAL as a major player in regulating VSNL1 minimal promoter activity. The function of NRF-1 was further confirmed using dominant-negative NRF-1 overexpression and NRF-1 small interfering RNA knockdown. Electrophoretic mobility shift assay and chromatin immunoprecipitation provided evidence for direct NRF-1 binding to the VSNL1 promoter. Methylation of the NRF-1-binding site was found to be able to regulate VSNL1 promoter activity. Our results further indicated that NRF-1 could be a regulatory factor for gene expression of the other visinin-like subfamily members including HPCAL4, HPCAL1, HPCA, and NCALD.

Neuronal Ca2+ sensor VILIP-1 leads to the upregulation of functional alpha4beta2 nicotinic acetylcholine receptors in hippocampal neurons

The neuronal Ca2+-sensor protein VILIP-1, known to affect clathrin-dependent receptor trafficking, has been shown to interact with the cytoplasmic loop of the alpha4-subunit of the alpha4beta2 nicotinic acetylcholine receptor (nAChR), which is the most abundant nAChR subtype with high-affinity for nicotine in the brain. The alpha4beta2 nAChR is crucial for nicotine addiction and the beneficial effects of nicotine on cognition. Its dysfunction has been implicated in frontal lobe epilepsy, Alzheimer's disease and schizophrenia. Here we report that overexpression of VILIP-1 enhances ACh responsiveness, whereas siRNA against VILIP-1 reduces alpha4beta2 nAChR currents of hippocampal neurons. The underlying molecular mechanism likely involves enhanced constitutive exocytosis of alpha4beta2 nAChRs mediated by VILIP-1. The two interaction partners co-localize in a Ca2+-dependent manner with syntaxin-6, a Golgi-SNARE protein involved in trans-Golgi membrane trafficking. Thus, we speculate that regulation of VILIP-1-expression might modulate surface expression of ligand-gated ion channels, such as the alpha4beta2 nAChRs, possibly comprising a novel form of physiological up-regulation of ligand-gated ion channels.

Implication of neuronal Ca2+ -sensor protein VILIP-1 in the glutamate hypothesis of schizophrenia

Post mortem studies in the hippocampus of schizophrenia patients revealed increased expression of neuronal Ca(2+)-sensor VILIP-1 (visinin-like protein) and enhanced co-localization with alpha4beta2 nAChR in interneurons. To study the pathological role of VILIP-1, particularly in interneurons, in the context of the glutamate hypothesis of schizophrenia, we have used ketamine-treated rats, a NMDA receptor hypofunction model, and hippocampal cultures as model systems for schizophrenia. Treatment with ketamine leads to enhanced VILIP-1 expression in interneurons in rat hippocampal CA1 region. In cultures glutamate treatment led to an increase in VILIP-1-positive interneurons, which is not dependent on NMDA receptor but metabotropic glutamate receptor activation. VILIP-1 mainly co-localizes with the interneuron marker calretinin, mGluR1alpha and the VILIP-1 interaction partner alpha4beta2 nAChR in hippocampal slices. Overexpression of VILIP-1 leads to enhanced nAChR-dependent inhibitory postsynaptic current (IPSC) generation by interneurons. This novel molecular link between the pathological role of mGluRs, VILIP-1 and its interaction partner alpha4beta2 nAChR by converging pathological glutamatergic and nicotinergic transmission may underlie cognitive impairments in schizophrenia.

MGluR5 mediates the interaction between late-LTP, network activity, and learning

Hippocampal synaptic plasticity and learning are strongly regulated by metabotropic glutamate receptors (mGluRs) and particularly by mGluR5. Here, we investigated the mechanisms underlying mGluR5-modulation of these phenomena. Prolonged pharmacological blockade of mGluR5 with MPEP produced a profound impairment of spatial memory. Effects were associated with 1) a reduction of mGluR1a-expression in the dentate gyrus; 2) impaired dentate gyrus LTP; 3) enhanced CA1-LTP and 4) suppressed theta (5–10 Hz) and gamma (30–100 Hz) oscillations in the dentate gyrus. Allosteric potentiation of mGluR1 after mGluR5 blockade significantly ameliorated dentate gyrus LTP, as well as suppression of gamma oscillatory activity. CA3-lesioning prevented MPEP effects on CA1-LTP, suggesting that plasticity levels in CA1 are driven by mGluR5-dependent synaptic and network activity in the dentate gyrus. These data support the hypothesis that prolonged mGluR5-inactivation causes altered hippocampal LTP levels and network activity, which is mediated in part by impaired mGluR1-expression in the dentate gyrus. The consequence is impairment of long-term learning.

Expression of the neuronal calcium sensor visinin-like protein-1 in the rat hippocampus

Visinin-like protein-1 (VILIP-1) belongs to the neuronal calcium sensor (NCS) family of EF-hand Ca(2+)-binding proteins which are involved in a variety of Ca(2+)-dependent signal transduction processes in neurons. VILIP-1 has been implicated in the pathology of CNS disorders including Alzheimer's disease and schizophrenia, but its expression has also been found to be regulated following induction of hippocampal synaptic plasticity underlying learning and memory processes. VILIP-1 is strongly expressed in different populations of principal and non-principal neurons in the rat hippocampus. VILIP-1-containing interneurons are morphologically and neurochemically heterogeneous. On the basis of co-localizing markers, VILIP-1 is rarely present in perisomatic inhibitory parvalbumin containing cells. However, VILIP-1 is frequently expressed in mid-proximal dendritic inhibitory cells characterized by calbindin immunoreactivity, and most strongly co-expressed in calretinin-positive disinhibitory interneurons. Partial co-localization of the metabotropic glutamate receptor mGluR1alpha with VILIP-1 was often found in interneurons located in the stratum oriens of the hippocampal CA1 region and in hilar interneurons. Partial co-localization of alpha4beta2 nicotinic acetylcholine receptor with VILIP-1 was seen in stratum oriens interneurons and particularly at the border of the hilus in the dentate gyrus, where VILIP-1 also strongly co-localized with calretinin. We speculate that depending on the regulation of the expression of VILIP-1 in hippocampal pyramidal cells or defined types of interneurons, it may have different effects on hippocampal synaptic plasticity and network activity in health and disease.

Gene expression profiling of hypothalamic hamartomas: a search for genes associated with central precocious puberty

BACKGROUND

Hypothalamic hamartomas (HHs) are congenital lesions composed of neurons and astroglia. Frequently, HHs cause central precocious puberty (CPP) and/or gelastic seizures. Because HHs might express genes similar to those required for the initiation of normal puberty, we used cDNA arrays to compare the gene expression profile of an HH associated with CPP with three HHs not accompanied by sexual precocity.

METHODS

Global changes in gene expression were detected using Affymetrix arrays. The results were confirmed by semiquantitative PCR, which also served to examine the expression of selected genes in the hypothalamus of female monkeys undergoing puberty.

RESULTS

All HHs were associated with seizures. Ten genes whose expression was increased in the HH with CPP were identified. They encode proteins involved in three key cellular processes: transcriptional regulation, cell-cell signaling, and cell adhesiveness. They include IA-1 and MEF2A, two transcription factors required for neuronal development; mGluR1 and VILIP-1, which encode proteins involved in neuronal communication, and TSG-6 that encodes a protein involved in cell adhesiveness. Of these, expression of mGluR1 also increases in the female monkey hypothalamus at puberty.

CONCLUSIONS

Increased expression of these genes in HHs may be relevant to the ability of some HHs to induce sexual precocity.

Extracellular calcium regulates postsynaptic efficacy through group 1 metabotropic glutamate receptors

Bursts of synaptic transmission are known to induce transient depletion of Ca2+ within the synaptic cleft. Although Ca2+ depletion has been shown to lower presynaptic release probability, effects on the postsynaptic cell have not been reported. In this study, we show that physiologically relevant reductions in extracellular Ca2+ lead to a decrease in synaptic strength between synaptically coupled layer 2/3 cortical pyramidal neurons. Using quantal analysis and mEPSP analysis, we demonstrate that a lowered extracellular Ca2+ produces a reduction in the postsynaptic quantal size in addition to its known effect on release probability. An elevated Mg2+ level can prevent this reduction in postsynaptic efficacy at subphysiological Ca2+ levels. We show that the calcium-dependent effect on postsynaptic quantal size is mediated by group 1 metabotropic glutamate receptors, acting via CaMKII (Ca2+/calmodulin-dependent protein kinase II) and PKC. Therefore, physiologically relevant changes in extracellular Ca2+ can regulate information transfer at cortical synapses via both presynaptic and postsynaptic mechanisms.

Neuronal Ca2+ sensor protein VILIP-1 affects cGMP signalling of guanylyl cyclase B by regulating clathrin-dependent receptor recycling in hippocampal neurons

The family of neuronal Ca2+ sensor (NCS) proteins is known to influence a variety of physiological and pathological processes by affecting signalling of different receptors and ion channels. Recently, it has been shown that the NCS protein VILIP-1 influences the activity of the receptor guanylyl cyclase GC-B. In transfected cell lines, VILIP-1 performs a Ca2+-dependent membrane association, the reversible Ca2+-myristoyl switch of VILIP-1, which leads to an increase in natriuretic peptide-stimulated cGMP levels. In this study, we have investigated the effect of VILIP-1 on cGMP signalling in C6 cells and in primary hippocampal neurons, where VILIP-1 and GC-B are co-expressed in many but not all neurons and partially co-localize in the soma and in dendrites. Our data indicate that VILIP-1 modulates GC-B activity by influencing clathrin-dependent receptor recycling. These data support a general physiological role for VILIP-1 in membrane trafficking in the intact hippocampus, where the NCS protein may affect processes, such as neuronal differentiation and synaptic plasticity e.g. by influencing cGMP-signalling.

The metabotropic glutamate receptor, mGluR5, is a key determinant of good and bad spatial learning performance and hippocampal synaptic plasticity

Hippocampal synaptic plasticity is expressed to very different extents in distinct rat strains in vivo. This may correlate with differences in learning ability. We investigated whether the metabotropic glutamate receptor mGluR5 contributes to differences in long-term potentiation (LTP) and learning in freely moving hooded Lister (HL) and Wistar rats. High-frequency tetanization (HFT) generated robust CA1 LTP in Wistar rats (> 24 h) and incremental potentiation in HL rats. The mGluR5 antagonist 2-methyl-6-(phenylethynyl) pyridine (MPEP; 1.8 microg), applied intracerebrally, impaired LTP from approximately 60 min onwards in Wistar and from 24 h in HL rats. HFT generated LTP in the dentate gyrus (DG) of Wistar rats (> 24 h), which was blocked by MPEP, and MPEP-resistant short-term depression in HL rats. Training for 10 days in an eight-arm radial maze revealed no working memory differences, but better reference memory performance in Wistar compared with HL rats. Daily application of MPEP (1.8 microg) impaired working and reference memory in Wistar rats. In HL rats, working memory was impaired but reference memory was unaffected. Western blot analysis revealed lower expression of mGluR5 in HL compared with Wistar rats. MGluR1 expression was equivalent. These data reveal striking mGluR5-dependent differences in spatial learning in different rat strains, which correlate to synaptic plasticity and mGluR5 expression levels.

MGluRs regulate the expression of neuronal calcium sensor proteins NCS-1 and VILIP-1 and the immediate early gene arg3.1/arc in the hippocampus in vivo

The metabotropic glutamate receptor (mGluR) agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) is involved in several forms of hippocampal synaptic plasticity. DHPG application can induce slow-onset potentiation, a form of long-term potentiation (LTP), in the dentate gyrus and in the CA1 region in vivo. The induction of LTP correlates with increased expression levels of neuronal calcium sensor (NCS), considered as key elements for plasticity. In this study we investigated mGluR- and time-dependent changes in the expression of two different NCS proteins. Following DHPG application in vivo NCS-1 and VILIP-1 expression increased, with significant levels reached after 8 and 24h. The effect was attenuated by treatment with the group I mGluR specific antagonist S-4-carboxyphenylglycine. The immediate early gene (IEG) arg3.1/arc showed highest expression levels 2h after DHPG-treatment. Therefore, mGluRs at concentrations which induce synaptic plasticity regulate the expression of IEGs and NCS proteins in different time frames and thus contribute to late phases of synaptic plasticity.