Calcium–myristoyl switch, subcellular localization, and calcium-dependent translocation of the neuronal calcium sensor protein VILIP-3, and comparison with VILIP-1 in hippocampal neurons☆

Autophagy-dependent ferroptosis as a potential treatment for glioblastoma

Glioblastoma (GBM) is the most common malignant primary brain tumor with a poor 5-year survival rate. Autophagy is a conserved intracellular degradation system that plays a dual role in GBM pathogenesis and therapy. On one hand, stress can lead to unlimited autophagy to promote GBM cell death. On the other hand, elevated autophagy promotes the survival of glioblastoma stem cells against chemotherapy and radiation therapy. Ferroptosis is a type of lipid peroxidation-mediated regulated necrosis that initially differs from autophagy and other types of cell death in terms of cell morphology, biochemical characteristics, and the gene regulators involved. However, recent studies have challenged this view and demonstrated that the occurrence of ferroptosis is dependent on autophagy, and that many regulators of ferroptosis are involved in the control of autophagy machinery. Functionally, autophagy-dependent ferroptosis plays a unique role in tumorigenesis and therapeutic sensitivity. This mini-review will focus on the mechanisms and principles of autophagy-dependent ferroptosis and its emerging implications in GBM.

Identification of HPCAL1 as a specific autophagy receptor involved in ferroptosis

Selective macroautophagy/autophagy maintains cellular homeostasis through the lysosomal degradation of specific cellular proteins or organelles. The pro-survival effect of selective autophagy has been well-characterized, but the mechanism by which it drives cell death is still poorly understood. Here, we use a quantitative proteomic approach to identify HPCAL1 (hippocalcin like 1) as a novel autophagy receptor for the selective degradation of CDH2 (cadherin 2) during ferroptosis. HPCAL1-dependent CDH2 depletion increases susceptibility to ferroptotic death by reducing membrane tension and favoring lipid peroxidation. Site-directed mutagenesis aided by bioinformatic analyses revealed that the autophagic degradation of CDH2 requires PRKCQ (protein kinase C theta)-mediated HPCAL1 phosphorylation on Thr149, as well as a non-classical LC3-interacting region motif located between amino acids 46-51. An unbiased drug screening campaign involving 4208 small molecule compounds led to the identification of a ferroptosis inhibitor that suppressed HPCAL1 expression. The genetic or pharmacological inhibition of HPCAL1 prevented ferroptosis-induced tumor suppression and pancreatitis in suitable mouse models. These findings provide a framework for understanding how selective autophagy promotes ferroptotic cell death.Abbreviations: ANXA7: annexin A7; ARNTL: aryl hydrocarbon receptor nuclear translocator like; CCK8: cell counting kit-8; CDH2: cadherin 2; CETSAs: cellular thermal shift assays; CPT2: carnitine palmitoyltransferase 2; DAMP, danger/damage-associated molecular pattern; DPPH: 2,2-diphenyl-1-picrylhydrazyl; DFO: deferoxamine; EBNA1BP2: EBNA1 binding protein 2; EIF4G1: eukaryotic translation initiation factor 4 gamma 1; FBL: fibrillarin; FKBP1A: FKBP prolyl isomerase 1A; FTH1: ferritin heavy chain 1; GPX4: glutathione peroxidase 4; GSDMs: gasdermins; HBSS: Hanks' buffered salt solution; HMGB1: high mobility group box 1; HNRNPUL1: heterogeneous nuclear ribonucleoprotein U like 1; HPCAL1: hippocalcin like 1; H1-3/HIST1H1D: H1.3 linker histone, cluster member; IKE: imidazole ketone erastin; KD: knockdown; LDH: lactate dehydrogenase; LIR: LC3-interacting region; MAGOH: mago homolog, exon junction complex subunit; MAP1LC3B: microtubule associated protein 1 light chain 3 beta; MDA: malondialdehyde; MLKL: mixed lineage kinase domain like pseudokinase; MPO: myeloperoxidase; MTOR: mechanistic target of rapamycin kinase; OE: overexpressing; OSTM1: osteoclastogenesis associated transmembrane protein 1; PRKC/PKC: protein kinase C; PRKAR1A: protein kinase cAMP-dependent type I regulatory subunit alpha; PRDX3: peroxiredoxin 3; PTGS2: prostaglandin-endoperoxide synthase 2; ROS: reactive oxygen species; SLC7A11: solute carrier family 7 member 11; SLC40A1: solute carrier family 40 member 1; SPTAN1: spectrin alpha, non-erythrocytic 1; STS: staurosporine; UBE2M: ubiquitin conjugating enzyme E2 M; ZYX: zyxin.

Disulfide Dimerization of Neuronal Calcium Sensor-1: Implications for Zinc and Redox Signaling

Neuronal calcium sensor-1 (NCS-1) is a four-EF-hand ubiquitous signaling protein modulating neuronal function and survival, which participates in neurodegeneration and carcinogenesis. NCS-1 recognizes specific sites on cellular membranes and regulates numerous targets, including G-protein coupled receptors and their kinases (GRKs). Here, with the use of cellular models and various biophysical and computational techniques, we demonstrate that NCS-1 is a redox-sensitive protein, which responds to oxidizing conditions by the formation of disulfide dimer (dNCS-1), involving its single, highly conservative cysteine C38. The dimer content is unaffected by the elevation of intracellular calcium levels but increases to 10–30% at high free zinc concentrations (characteristic of oxidative stress), which is accompanied by accumulation of the protein in punctual clusters in the perinuclear area. The formation of dNCS-1 represents a specific Zn²⁺-promoted process, requiring proper folding of the protein and occurring at redox potential values approaching apoptotic levels. The dimer binds Ca²⁺ only in one EF-hand per monomer, thereby representing a unique state, with decreased α-helicity and thermal stability, increased surface hydrophobicity, and markedly improved inhibitory activity against GRK1 due to 20-fold higher affinity towards the enzyme. Furthermore, dNCS-1 can coordinate zinc and, according to molecular modeling, has an asymmetrical structure and increased conformational flexibility of the subunits, which may underlie their enhanced target-binding properties. In HEK293 cells, dNCS-1 can be reduced by the thioredoxin system, otherwise accumulating as protein aggregates, which are degraded by the proteasome. Interestingly, NCS-1 silencing diminishes the susceptibility of Y79 cancer cells to oxidative stress-induced apoptosis, suggesting that NCS-1 may mediate redox-regulated pathways governing cell death/survival in response to oxidative conditions.

A Novel Approach to Bacterial Expression and Purification of Myristoylated Forms of Neuronal Calcium Sensor Proteins

N-terminal myristoylation is a common co-and post-translational modification of numerous eukaryotic and viral proteins, which affects their interaction with lipids and partner proteins, thereby modulating various cellular processes. Among those are neuronal calcium sensor (NCS) proteins, mediating transduction of calcium signals in a wide range of regulatory cascades, including reception, neurotransmission, neuronal growth and survival. The details of NCSs functioning are of special interest due to their involvement in the progression of ophthalmological and neurodegenerative diseases and their role in cancer. The well-established procedures for preparation of native-like myristoylated forms of recombinant NCSs via their bacterial co-expression with N-myristoyl transferase from Saccharomyces cerevisiae often yield a mixture of the myristoylated and non-myristoylated forms. Here, we report a novel approach to preparation of several NCSs, including recoverin, GCAP1, GCAP2, neurocalcin δ and NCS-1, ensuring their nearly complete N-myristoylation. The optimized bacterial expression and myristoylation of the NCSs is followed by a set of procedures for separation of their myristoylated and non-myristoylated forms using a combination of hydrophobic interaction chromatography steps. We demonstrate that the refolded and further purified myristoylated NCS-1 maintains its Са²⁺-binding ability and stability of tertiary structure. The developed approach is generally suited for preparation of other myristoylated proteins.

HPCAL1 promotes glioblastoma proliferation via activation of Wnt/β-catenin signalling pathway

Glioblastoma (GBM) is the most prevalent primary malignancy of the central nervous system with obvious aggressiveness, and is associated with poor clinical outcome. Studies have indicated that calcium ion (Ca²⁺) can positively regulate the initiation of malignancy with regard to GBM by modulating quiescence, proliferation, migration and maintenance. Hippocalcin like‐1 protein (HPCAL1) serves as a sensor of Ca²⁺. However, the understanding of HPCAL1 activity in GBM is limited. The present study revealed that the gene HPCAL1 was up‐regulated by Ca²⁺ in the tissues and cells of GBM. Ectopic expression of HPCAL1 promoted proliferation of cells. Exhaustion of HPCAL1 inhibited cell growth not only in vivo, but also in vitro. In addition, HPCAL1 enhanced the Wnt pathway by stimulating β‐catenin accumulation and nuclear translocation in GBM cells, while β‐catenin silencing significantly inhibited the proliferation and growth of the GBM cells. Our results showed that Ser9 phosphorylation of GSK3β was significantly decreased after HPCAL1 knockdown in GBM cells, and knockdown of the gene GSK3β in GBM cells enhanced cell proliferation and promoted transcription of the genes CCND1 and c‐Myc. Furthermore, the phosphorylation of ERK was decreased in the cells with HPCAL1 knockdown, while it was promoted via overexpression of HPCAL1. The suppression or depletion of the gene ERK decreased proliferation triggered by overexpression of HPCAL1 and impaired transcription of the genes c‐Myc and CCND1. These studies elucidate the tumour‐promoting activity of HPCAL1. They also offer an innovative therapeutic strategy focusing on the HPCAL1‐Wnt/β‐catenin axis to regulate proliferation and development of GBM.

A novel method for the rapid detection of post-translationally modified visinin-like protein 1 in rat models of brain injury

BACKGROUND

Although elevated serum levels of visinin-like protein 1 (VILIP-1), a neuron-specific calcium sensor protein, are associated with ischaemic stroke, only a single study has evaluated VILIP-1 as a biomarker of traumatic brain injury (TBI). The current proof-of-concept study was designed to determine whether serum VILIP-1 levels increase post-injury in a well-characterized rat unilateral cortical contusion model.

METHODS

Lateral flow devices (LFDs) rapidly (< 20 min) detected trace serum levels (pg/mL) of VILIP-1 in a small input sample volume (10 µL). Temporal profiles of serum levels at baseline and post-injury were measured in male Sprague Dawley rats subjected to very mild-, mild unilateral-cortical contusion, or naïve surgery and in male Sprague Dawley rats following a diffuse TBI or sham surgery.

RESULTS

Mean serum levels were significantly elevated by 0.5 h post-injury and remained so throughout the temporal profile compared with baseline in very mild and mild unilateral contusions but not in naïve surgeries. Serum levels were also elevated in a small cohort of animals subjected to a diffuse TBI injury.

CONCLUSIONS

Overall, the current study demonstrates that the novel LFD is a reliable and rapid point-of-care diagnostic for the detection and quantification of serum levels of UB-VILIP-1 in a clinically relevant time frame.

Structure and Calcium Binding Properties of a Neuronal Calcium-Myristoyl Switch Protein, Visinin-Like Protein 3

Visinin-like protein 3 (VILIP-3) belongs to a family of Ca²⁺-myristoyl switch proteins that regulate signal transduction in the brain and retina. Here we analyze Ca²⁺ binding, characterize Ca²⁺-induced conformational changes, and determine the NMR structure of myristoylated VILIP-3. Three Ca²⁺ bind cooperatively to VILIP-3 at EF2, EF3 and EF4 (K_D = 0.52 μM and Hill slope of 1.8). NMR assignments, mutagenesis and structural analysis indicate that the covalently attached myristoyl group is solvent exposed in Ca²⁺-bound VILIP-3, whereas Ca²⁺-free VILIP-3 contains a sequestered myristoyl group that interacts with protein residues (E26, Y64, V68), which are distinct from myristate contacts seen in other Ca²⁺-myristoyl switch proteins. The myristoyl group in VILIP-3 forms an unusual L-shaped structure that places the C₁₄ methyl group inside a shallow protein groove, in contrast to the much deeper myristoyl binding pockets observed for recoverin, NCS-1 and GCAP1. Thus, the myristoylated VILIP-3 protein structure determined in this study is quite different from those of other known myristoyl switch proteins (recoverin, NCS-1, and GCAP1). We propose that myristoylation serves to fine tune the three-dimensional structures of neuronal calcium sensor proteins as a means of generating functional diversity.

Visinin-Like Protein-3 Modulates the Interaction Between Cytochrome b 5 and NADH-Cytochrome b 5 Reductase in a Ca2+-Dependent Manner

Visinin-like proteins (VILIPs) belong to the calcium sensor protein family. VILIP-1 has been examined as a cerebrospinal fluid biomarker and as a potential indicator for cognitive decline in Alzheimer's disease (AD). However, little is known about VILIP-3 protein biochemistry. We performed co-immunoprecipitation experiments to examine whether VILIP-3 can interact with reduced nicotine adenine dinucleotide (NADH)-cytochrome b ₅ reductase. We also evaluated the specificity of cytochrome b ₅ within the visinin-like protein subfamily and identified cytochrome P450 isoforms in the brain. In this study, we show that cytochrome b ₅ has an affinity for hippocalcin, neurocalcin-δ, and VILIP-3, but not visinin-like protein-1. VILIP-3 was also shown to interact with NADH-cytochrome b ₅ reductase in a Ca²⁺-dependent manner. These results suggest that VILIP-3, hippocalcin, and neurocalcin-δ provide a Ca²⁺-dependent modulation to the NADH-dependent microsomal electron transport. The results also suggest that future therapeutic strategies that target calcium-signaling pathways and VILIPs may be of value.

Hippocalcin-like 1 suppresses hepatocellular carcinoma progression by promoting p21(Waf/Cip1) stabilization by activating the ERK1/2-MAPK pathway

Hepatocellular carcinoma (HCC) is the third leading cause of cancer-related death. However, the underlying mechanism during hepatocarcinogenesis remains unclarified. Stable isotope labeling by amino acids in cell culture (SILAC) is a powerful quantitative strategy for proteome-wide discovery of novel biomarkers in cancers. Hippocalcin-like 1 (HPCAL1) is a calcium sensor protein. However, the biological function of HPCAL1 is poorly understood in cancers, including HCC. Herein, HPCAL1 was identified by SILAC as a novel hepatocarcinogenesis suppressor down-regulated in HCC cell lines and tissues. Importantly, lost expression of HPCAL1 was associated with worse prognosis of HCC patients. Interestingly, secreted HPCAL1 protein in the plasma dropped dramatically in HCC patients compared with healthy donors. Receiver operating characteristic curve analysis showed that serum HPCAL1 at a concentration of 8.654 ng/mL could better predict HCC. Furthermore, ectopic expression of HPCAL1 suppresses cell proliferation, while depletion of HPCAL1 led to increased cell growth both in vitro and in vivo. Mechanistically, HPCAL1 directly interacted with p21(Waf/Cip1) in the nucleus, which requires the EF-hand 4 motif of HPCAL1 and the Cy1 domain of p21. This interaction stabilized p21(Waf/Cip1) in an extracellular signal-regulated kinase 1/2-mitogen-activated protein kinase-dependent manner, which subsequently prevented p21(Waf/Cip1) proteasomal degradation by disrupting SCF(Skp2) and CRL4(Cdt2) E3 ligase complexes, resulting in increased protein stability and inhibitory effect of p21(Waf/Cip1). Notably, the tumor suppressive function of HPCAL1 was dependent on p21 in vitro and in vivo. Consistent with this observation, expression of HPCAL1 and p21(Waf/Cip1) was positively correlated in HCC tissues.

CONCLUSION

These findings highlight a novel tumor suppressor upstream of p21(Waf/Cip1) in attenuating cell cycle progression and provide a promising diagnostic and prognostic factor, as well as a potential therapeutic target for HCC.

Membrane binding of Neuronal Calcium Sensor-1 (NCS1)

Neuronal Calcium Sensor-1 (NCS1) belongs to the family of Neuronal Calcium Sensor (NCS) proteins. NCS1 is composed of four EF-hand motifs and an N-terminal myristoylation. However, the presence of a calcium-myristoyl switch in NCS1 and its role in the membrane binding are controversial. The model of Langmuir lipid monolayers is thus used to mimic the cell membrane in order to characterize the membrane interactions of NCS1. Two binding parameters are calculated from monolayer measurements: the maximum insertion pressure, up to which protein binding is energetically favorable, and the synergy, reporting attractive or repulsive interactions with the lipid monolayers. Binding membrane measurements performed in the presence of myristoylated NCS1 reveal better binding interactions for phospholipids composed of phosphoethanolamine polar head groups and unsaturated fatty acyl chains. In the absence of calcium, the membrane binding measurements are drastically modified and suggest that the protein is more strongly bound to the membrane. Indeed, the binding of calcium by three EF-hand motifs of NCS1 leads to a conformation change. NCS1 arrangement at the membrane could thus be reshuffled for better interactions with its substrates. The N-terminal peptide of NCS1 is composed of two amphiphilic helices involved in the membrane interactions of NCS1. Moreover, the presence of the myristoyl group has a weak influence on the membrane binding of NCS1 suggesting the absence of a calcium-myristoyl switch mechanism in this protein. The myristoylation could thus have a structural role required in the folding/unfolding of NCS1 which is essential to its multiple biological functions.

Comparison of VILIP-1 and VILIP-3 binding to phospholipid monolayers

The neuronal calcium sensor proteins Visinin-like Proteins 1 (VILIP-1) and 3 (VILIP-3) are effectors of guanylyl cyclase and acetyl choline receptors, and transduce calcium signals in the brain. The “calcium-myristoyl” switch, which involves a post-translationally added myristoyl moiety and calcium binding, is thought to regulate their membrane binding capacity and therefore, play a critical role in their mechanism of action. In the present study, we investigated the effect of membrane composition and solvent conditions on the membrane binding mechanisms of both VILIPs using lipid monolayers at the air/buffer interface. Results based on comparison of the adsorption kinetics of the myristoylated and non-myristoylated proteins confirm the pivotal role of calcium and the exposed myristol moiety for sustaining the membrane-bound state of both VILIPs. However, we also observed binding of both VILIP proteins in the absence of calcium and/or myristoyl conjugation. We propose a two-stage membrane binding mechanism for VILIP-1 and VILIP-3 whereby the proteins are initially attracted to the membrane surface by electrostatic interactions and possibly by specific interactions with highly negatively charged lipids head groups. The extrusion of the conjugated myristoyl group, and the subsequent anchoring in the membrane constitutes the second stage of the binding mechanism, and ensures the sustained membrane-bound form of these proteins.

CSF levels of the neuronal injury biomarker visinin-like protein-1 in Alzheimer's disease and dementia with Lewy bodies

The overlapping clinical features of Alzheimer's disease (AD) and Dementia with Lewy bodies (DLB) make differentiation difficult in the clinical environment. Evaluating the CSF levels of biomarkers in AD and DLB patients could facilitate clinical diagnosis. CSF Visinin-like protein-1 (VILIP-1), a calcium-mediated neuronal injury biomarker, has been described as a novel biomarker for AD. The aim of this study was to investigate the diagnostic utility of CSF VILIP-1 and VILIP-1/Aβ1-42 ratio to distinguish AD from DLB. Levels of CSF VILIP-1, t-tau, p-tau181P , Aβ1-42 , and α-synuclein were measured in 61 AD patients, 32 DLB patients, and 40 normal controls using commercial ELISA kits. The results showed that the CSF VILIP-1 level had significantly increased in AD patients compared with both normal controls and DLB patients. The CSF VILIP-1 and VILIP-1/Aβ1-42 levels had enough diagnostic accuracy to allow the detection and differential diagnosis of AD. Additionally, CSF VILIP-1 levels were positively correlated with t-tau and p-tau181P within each group and with α-synuclein in the AD and control groups. We conclude that CSF VILIP-1 could be a diagnostic marker for AD, differentiating it from DLB. The analysis of biomarkers, representing different neuropathologies, is an important approach reflecting the heterogeneous features of AD and DLB. Neuronal Ca(2+) -sensor protein VILIP-1 has been implicated in the calcium-mediated neuronal injury and pathological change of AD. The CSF VILIP-1 and VILIP-1/Aβ1-42 levels had enough diagnostic accuracy to allow the detection and differential diagnosis of AD. CSF VILIP-1 is a useful biomarker for AD. Evaluating the CSF levels of VILIP-1 in AD and DLB patients could facilitate clinical diagnosis.

Restricted diffusion of calretinin in cerebellar granule cell dendrites implies Ca²⁺-dependent interactions via its EF-hand 5 domain

Ca²⁺-binding proteins (CaBPs) are important regulators of neuronal Ca²⁺ signalling, acting either as buffers that shape Ca²⁺ transients and Ca²⁺ diffusion and/or as Ca²⁺ sensors. The diffusional mobility represents a crucial functional parameter of CaBPs, describing their range-of-action and possible interactions with binding partners. Calretinin (CR) is a CaBP widely expressed in the nervous system with strong expression in cerebellar granule cells. It is involved in regulating excitability and synaptic transmission of granule cells, and its absence leads to impaired motor control. We quantified the diffusional mobility of dye-labelled CR in mouse granule cells using two-photon fluorescence recovery after photobleaching. We found that movement of macromolecules in granule cell dendrites was not well described by free Brownian diffusion and that CR diffused unexpectedly slow compared to fluorescein dextrans of comparable size. During bursts of action potentials, which were associated with dendritic Ca²⁺ transients, the mobility of CR was further reduced. Diffusion was significantly accelerated by a peptide embracing EF-hand 5 of CR. Our results suggest long-lasting, Ca²⁺-dependent interactions of CR with large and/or immobile binding partners. These interactions render CR a poorly mobile Ca²⁺ buffer and point towards a Ca²⁺ sensor function of CR.

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.

Understanding the physiological roles of the neuronal calcium sensor proteins

Calcium signalling plays a crucial role in the control of neuronal function and plasticity. Changes in neuronal Ca²⁺ concentration are detected by Ca²⁺-binding proteins that can interact with and regulate target proteins to modify their function. Members of the neuronal calcium sensor (NCS) protein family have multiple non-redundant roles in the nervous system. Here we review recent advances in the understanding of the physiological roles of the NCS proteins and the molecular basis for their specificity.

Transcriptional responses of cultured rat sympathetic neurons during BMP-7-induced dendritic growth

BACKGROUND

Dendrites are the primary site of synapse formation in the vertebrate nervous system; however, relatively little is known about the molecular mechanisms that regulate the initial formation of primary dendrites. Embryonic rat sympathetic neurons cultured under defined conditions extend a single functional axon, but fail to form dendrites. Addition of bone morphogenetic proteins (BMPs) triggers these neurons to extend multiple dendrites without altering axonal growth or cell survival. We used this culture system to examine differential gene expression patterns in naïve vs. BMP-treated sympathetic neurons in order to identify candidate genes involved in regulation of primary dendritogenesis.

METHODOLOGY/PRINCIPAL FINDINGS

To determine the critical transcriptional window during BMP-induced dendritic growth, morphometric analysis of microtubule-associated protein (MAP-2)-immunopositive processes was used to quantify dendritic growth in cultures exposed to the transcription inhibitor actinomycin-D added at varying times after addition of BMP-7. BMP-7-induced dendritic growth was blocked when transcription was inhibited within the first 24 hr after adding exogenous BMP-7. Thus, total RNA was isolated from sympathetic neurons exposed to three different experimental conditions: (1) no BMP-7 treatment; (2) treatment with BMP-7 for 6 hr; and (3) treatment with BMP-7 for 24 hr. Affymetrix oligonucleotide microarrays were used to identify differential gene expression under these three culture conditions. BMP-7 significantly regulated 56 unique genes at 6 hr and 185 unique genes at 24 hr. Bioinformatic analyses implicate both established and novel genes and signaling pathways in primary dendritogenesis.

CONCLUSIONS/SIGNIFICANCE

This study provides a unique dataset that will be useful in generating testable hypotheses regarding transcriptional control of the initial stages of dendritic growth. Since BMPs selectively promote dendritic growth in central neurons as well, these findings may be generally applicable to dendritic growth in other neuronal cell types.

Interactions of Visinin-like Proteins with Phospho-inositides

The subcellular membrane localization of neuronal calcium sensor (NCS) proteins in living cells, such as Visinin-like Proteins-1 (VILIP-1) and VILIP-3, differs substantially. We have followed the hypothesis that the differential localization may be due to the specific binding capabilities of individual VILIPs for phosphatidylinositol phosphates (PIPs). Several highly conserved lysine residues in the N-terminal region could provide favourable electrostatic interactions. Molecular modelling results support a binding site for phospho-inositides in the N-terminal area of VILIP-1, and the involvement of the conserved N-terminal lysine residues in binding the phospho-inositol head group. Experimentally, the binding of VILIP-1 to inositol derivatives was tested by a PIP strip assay, which showed the requirement of phosphorylation of the inositol group for the interaction of the protein with PIPs. Monolayer adsorption measurements showed a preference of VILIP-1 binding to PI(4,5)P2 over PI(3,4,5)P3. The co-localization of VILIP-1 with PI(4,5)P2 at the cell surface membrane in hippocampal neurons further supports the idea of direct interactions of VILIP-1 with PIPs in living cells.

Visinin-like proteins (VSNLs): interaction partners and emerging functions in signal transduction of a subfamily of neuronal Ca2+ -sensor proteins

The visinin-like protein (VSNL) subfamily, including VILIP-1 (the founder protein), VILIP-2, VILIP-3, hippocalcin, and neurocalcin delta, constitute a highly homologous subfamily of neuronal calcium sensor (NCS) proteins. Comparative studies have shown that VSNLs are expressed predominantly in the brain with restricted expression patterns in various subsets of neurons but are also found in peripheral organs. In addition, the proteins display differences in their calcium affinities, in their membrane-binding kinetics, and in the intracellular targets to which they associate after calcium binding. Even though the proteins use a similar calcium-myristoyl switch mechanism to translocate to cellular membranes, they show calcium-dependent localization to various subcellular compartments when expressed in the same neuron. These distinct calcium-myristoyl switch properties might be explained by specificity for defined phospholipids and membrane-bound targets; this enables VSNLs to modulate various cellular signal transduction pathways, including cyclic nucleotide and MAPK signaling. An emerging theme is the direct or indirect effect of VSNLs on gene expression and their interaction with components of membrane trafficking complexes, with a possible role in membrane trafficking of different receptors and ion channels, such as glutamate receptors of the kainate and AMPA subtype, nicotinic acetylcholine receptors, and Ca(2+)-channels. One hypothesis is that the highly homologous VSNLs have evolved to fulfil specialized functions in membrane trafficking and thereby affect neuronal signaling and differentiation in defined subsets of neurons. VSNLs are involved in differentiation processes showing a tumor-invasion-suppressor function in peripheral organs. Finally, VSNLs play neuroprotective and neurotoxic roles and have been implicated in neurodegenerative diseases.

Regulation of P2X2 receptors by the neuronal calcium sensor VILIP1

Extracellular adenosine triphosphate (ATP) activates P2X receptors, which are involved in diverse physiological functions. Using a proteomic approach, we identified the neuronal calcium sensor VILIP1 as interacting with P2X2 receptors. We found that VILIP1 forms a signaling complex in vitro and in vivo with P2X2 receptors and regulates P2X2 receptor sensitivity to ATP, peak response, surface expression, and diffusion. VILIP1 constitutively binds to P2X2 receptors and displays enhanced interactions in an activation- and calcium-dependent manner owing to exposure of its binding segment in P2X2 receptors. VILIP1-P2X2 interactions are also enhanced in hippocampal neurons during conditions of action potential firing known to trigger P2X2 receptor activation. Our data thus reveal a previously unrecognized function for the neuronal calcium sensor protein VILIP1 and a mechanism for regulation of ATP-dependent P2X receptor signaling by neuronal calcium sensors.

Hippocalcin signaling via site-specific translocation in hippocampal neurons

Hippocalcin is a Ca(2+)-binding protein, which belongs to the family of neuronal Ca(2+) sensors. It is highly expressed in the hippocampus but molecular mechanisms underlying its action in this part of the brain have not been investigated in detail. To study whether intrinsic neuronal activity could result in hippocalcin-mediated signal transduction we examined spontaneous and action potential (AP)-dependent changes in fluorescence of yellow fluorescent protein-tagged hippocalcin (HPCA-YFP) in transiently transfected hippocampal cultured neurons. In 6-12 DIV neurons HPCA-YFP spontaneously translocated longitudinally to specific sites within diffusionally confined domains of neuronal processes. The translocations to these sites were expressed as fast, reversible increases in HPCA-YFP fluorescence coincided with a decrease in adjacent sites indicating genuine protein translocation. Physiologically relevant neuronal stimulation with short trains of action potentials also resulted in fast, simultaneous, reversible, and [Ca(2+)](i)-dependent translocations of HPCA-YFP to several sites synchronizing hippocalcin signaling in different parts of neuronal processes. The amount of translocated protein increased with the number of action potentials in a train decoding the number of APs into the amount of translocated protein. We conclude that hippocalcin may signal within diffusionally restricted domains of neuronal processes in which it might play a physiological role in Ca(2+)-dependent local activation of specific molecular targets.

Transcriptomic and proteomic analyses of mouse cerebellum reveals alterations in RasGRF1 expression following in vivo chronic treatment with delta 9-tetrahydrocannabinol

We have applied transcriptomic and proteomic techniques to identify changes in the RNA and the protein levels in the mouse cerebellum after chronic treatment with Delta(9)-tetrahydrocannabinol (THC). Among approximately 14,000 transcripts in a mouse cDNA microarray library, we found 11 genes with altered expression. RasGRF1, a neuron-specific Ras guanine nucleotide exchange factor, showed a reduction both at the RNA and protein levels with a specific decrease of the protein pool associated to cell membranes. In addition, proteomic analysis on cerebellum obtained from chronically THC-treated mice detected quantitative changes of additional 27 spots, mostly in the membranous fraction. We found enrichment of alpha (Galphao, Galphaq) and beta subunits (beta4/beta2 and beta5) of guanine nucleotide-binding proteins and of two calcium-binding proteins, calretinin and hippocalcin-like protein-1. In addition, we also detected a significant increase in the membrane fraction of proteins involved in exo-endocytosis such as septins, dynamin-1, and vesicle protein sorting 29. By western blotting, we confirmed increased membrane localization of calretinin and of dynamin-1 isoforms with higher isoelectric point, indicative for an underphosphorylated state of the molecule. In conclusion, our results indicate that chronic THC modulates the expression and subcellular localization of proteins implicated in Ras signaling, calcium-buffering potential, and trafficking.

Neuronal calcium sensor proteins: generating diversity in neuronal Ca2+ signalling

In neurons, intracellular calcium signals have crucial roles in activating neurotransmitter release and in triggering alterations in neuronal function. Calmodulin has been widely studied as a Ca(2+) sensor that has several defined roles in neuronal Ca(2+) signalling, but members of the neuronal calcium sensor protein family have also begun to emerge as key components in a number of regulatory pathways and have increased the diversity of neuronal Ca(2+) signalling pathways. The differing properties of these proteins allow them to have discrete, non-redundant functions.

Functional Contribution of Ca2+ and Mg2+ to the Intermolecular Interaction of Visinin-like Proteins

The interaction of human visinin-like protein 1 (VILIP1) and visinin-like protein 3 (VILIP3) with divalent cations (Mg2+, Ca2+, Sr2+ and Ba2+) was explored using circular dichroism and fluorescence measurement. These results showed that the four cations each induced a different subtle change in the conformation of VILIPs. Moreover, VILIP1 and VILIP3 bound with Ca2+ or Mg2+ in a cooperative manner. Studies on the truncated mutants showed that the intact EF-3 and EF-4 were essential for the binding of VILIP1 with Ca2+ and Mg2+. Pull-down assay revealed that Ca2+ and Mg2+ enhanced the intermolecular interaction of VILIPs, and led to the formation of homo- and hetero-oligomer of VILIPs. Together with previous findings that Ca2+-dependent localization of VILIPs may be involved in the regulation of distinct cascades and deprivation of Ca2+-binding capacity of VILIPs did not completely eliminate their activity, it is likely to reflect that Mg2+-bound VILIPs may play a role in regulating the biological function of VILIPs in response to a concentration fluctuation of Ca2+ in cells.

Immunoisolation of two synaptic vesicle pools from synaptosomes: a proteomics analysis

The nerve terminal proteome governs neurotransmitter release as well as the structural and functional dynamics of the presynaptic compartment. In order to further define specific presynaptic subproteomes we used subcellular fractionation and a monoclonal antibody against the synaptic vesicle protein SV2 for immunoaffinity purification of two major synaptosome-derived synaptic vesicle-containing fractions: one sedimenting at lower and one sedimenting at higher sucrose density. The less dense fraction contains free synaptic vesicles, the denser fraction synaptic vesicles as well as components of the presynaptic membrane compartment. These immunoisolated fractions were analyzed using the cationic benzyldimethyl-n-hexadecylammonium chloride (BAC) polyacrylamide gel system in the first and sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the second dimension. Protein spots were subjected to analysis by matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI TOF MS). We identified 72 proteins in the free vesicle fraction and 81 proteins in the plasma membrane-containing denser fraction. Synaptic vesicles contain a considerably larger number of protein constituents than previously anticipated. The plasma membrane-containing fraction contains synaptic vesicle proteins, components of the presynaptic fusion and retrieval machinery and numerous other proteins potentially involved in regulating the functional and structural dynamics of the nerve terminal.

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.

Traffic of Kv4 K+ channels mediated by KChIP1 is via a novel post-ER vesicular pathway

The traffic of Kv4 K⁺ channels is regulated by the potassium channel interacting proteins (KChIPs). Kv4.2 expressed alone was not retained within the ER, but reached the Golgi complex. Coexpression of KChIP1 resulted in traffic of the channel to the plasma membrane, and traffic was abolished when mutations were introduced into the EF-hands with channel captured on vesicular structures that colocalized with KChIP1(2–4)-EYFP. The EF-hand mutant had no effect on general exocytic traffic. Traffic of Kv4.2 was coat protein complex I (COPI)–dependent, but KChIP1-containing vesicles were not COPII-coated, and expression of a GTP-loaded Sar1 mutant to block COPII function more effectively inhibited traffic of vesicular stomatitis virus glycoprotein (VSVG) than did KChIP1/Kv4.2 through the secretory pathway. Therefore, KChIP1seems to be targeted to post-ER transport vesicles, different from COPII-coated vesicles and those involved in traffic of VSVG. When expressed in hippocampal neurons, KChIP1 co-distributed with dendritic Golgi outposts; therefore, the KChIP1 pathway could play an important role in local vesicular traffic in neurons.

High-affinity interaction of the N-terminal myristoylation motif of the neuronal calcium sensor protein hippocalcin with phosphatidylinositol 4,5-bisphosphate

Many proteins are associated with intracellular membranes due to their N-terminal myristoylation. Not all myristoylated proteins have the same localization within cells, indicating that other factors must determine their membrane targeting. The NCS (neuronal calcium sensor) proteins are a family of Ca2+-binding proteins with diverse functions. Most members of the family are N-terminally myristoylated and are either constitutively membrane-bound or have a Ca2+/myristoyl switch that allows their reversible membrane association in response to Ca2+ signals. In the case of hippocalcin and NCS-1, or alternatively KChIP1 (K+ channel-interacting protein 1), their N-terminal myristoylation motifs are sufficient for targeting to distinct organelles. We have shown that an N-terminal myristoylated hippocalcin peptide is able to specifically reproduce the membrane targeting of hippocalcin/NCS-1 when introduced into permeabilized cells. The peptide binds to liposomes containing phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] with high affinity (K(d) 50 nM). Full-length hippocalcin also bound preferentially to liposomes supplemented with PtdIns(4,5)P2. Co-expression of hippocalcin-(1-14)-ECFP (enhanced cyan fluorescent protein) or NCS-1-ECFP partially displaced the expressed PH (pleckstrin homology) domain of phospholipase delta1 from the plasma membrane in live cells, indicating that they have a higher affinity for PtdIns(4,5)P2 than does this PH domain. The Golgi localization of the PH domain of FAPP1 (four-phosphate-adaptor protein 1), which binds to phosphatidylinositol 4-phosphate, was unaffected. The localization of NCS-1 and hippocalcin is likely to be determined, therefore, by their interaction with PtdIns(4,5)P2.

Co-assembly of two GluR6 kainate receptor splice variants within a functional protein complex

Kainate receptors (KAR) are composed of several distinct subunits and splice variants, but the functional relevance of this diversity remains largely unclear. Here we show that two splice variants of the GluR6 subunit, GluR6a and GluR6b, which differ in their C-terminal domains, do not show distinct functional properties, but coassemble as heteromers in vitro and in vivo. Using a proteomic approach combining affinity purification and MALDI-TOF mass spectrometry, we found that GluR6a and GluR6b interact with two distinct subsets of cytosolic proteins mainly involved in Ca(2+) regulation of channel function and intracellular trafficking. Guided by these results, we provide evidence that the regulation of native KAR function by NMDA receptors depends on the heteromerization of GluR6a and GluR6b and interaction of calcineurin with GluR6b. Thus, GluR6a and GluR6b bring in close proximity two separate subsets of interacting proteins that contribute to the fine regulation of KAR trafficking and function.

Calcium-modulated ciliary membrane guanylate cyclase transduction machinery: constitution and operational principles

Odorant transduction is a biochemical process by which the odorant signal generates the electric signal. The cilia of the olfactory neuroepithelium are the sites of this process. This study documents the detailed biochemical, structural and functional description of an odorant-responsive Ca2+ -modulated membrane guanylate cyclase transduction machinery in the cilia. Myristoylated (myr)-neurocalcin delta is the Ca2+ -sensor component and the cyclase, ONE-GC, the transduction component of the machinery. Myr-neurocalcin delta senses increments in free Ca2+, binds to a defined domain of ONE-GC and stimulates the cyclase. The findings enable the formulation of an odorant transduction model in which three pivotal signaling components--Ca2+, myr-neurocalcin delta and ONE-GC--of the transduction machinery are locked. A glaring feature of the model is that its Ca2+ -dependent operational principle is opposite to the phototransduction model.

The neuronal calcium-sensor proteins

Changes in intracellular free Ca(2+) concentration ([Ca(2+)](i)) affect many different aspects of neuronal function ranging from millisecond regulation of ion channels to long term changes in gene expression. These effects of Ca(2+) are transduced by Ca(2+)-binding proteins that act as Ca(2+) sensors by binding Ca(2+), undergoing a conformational change and then modifying the function of additional target proteins. Mammalian species express 14 members of the neuronal calcium sensor (NCS) family of EF hand-containing Ca(2+)-binding proteins which are expressed mainly in photoreceptor cells or neurons. Many of the NCS proteins are membrane targeted through their N-terminal myristoylation either constitutively or following exposure of the myristoyl group after Ca(2+) binding (the Ca(2+)/myristoyl switch). The NCS proteins have been implicated in a wide range of functional roles in neuronal regulation, several of which have been confirmed though molecular genetic analyses.

Neuronal Ca2+-sensor proteins: multitalented regulators of neuronal function

Many aspects of neuronal activity are regulated by Ca2+ signals. The transduction of temporally and spatially distinct Ca2+ signals requires the action of Ca2+-sensor proteins including various EF-hand-containing Ca2+-binding proteins. The neuronal Ca2+ sensor (NCS) protein family and the related Ca2+-binding proteins (CaBPs) have begun to emerge as key players in neuronal function. Many of these proteins are expressed predominantly or only in neurons, sometimes with cell-specific patterns of expression. Their ability to associate with membranes either constitutively or in response to elevated Ca2+ concentration allows the NCS proteins to discriminate between different spatial and temporal patterns of Ca2+ signals. Recent work has established several physiological roles of these proteins, including diverse actions on gene expression, ion channel function, membrane traffic of ion channels and receptors, and the control of apoptosis.