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

Characterization of a novel recombinant calcium-binding protein from Arca subcrenata and its anti-hepatoma activities in vitro and in vivo

Previous studies demonstrated that ASP-3 was a novel calcium-binding protein from Arca subcrenata that effectively inhibited the proliferation of HepG2 cells. To further study the antitumor activity and mechanism of ASP-3, the cytotoxic effects of recombinant ASP-3 were evaluated in HepG2 cells. The results demonstrated that ASP-3 inhibited the proliferation of HepG2 cells by competitively binding to the EGF binding pocket of EGFR and inhibiting the JAK-STAT, RAS-RAF-MEK-ERK, and PI3K-Akt-mTOR signaling pathways mediated by EGFR. ASP-3 significantly inhibited tumor growth in a HepG2 cell subcutaneous xenograft nude mouse model, and its (25 mg/kg and 75 mg/kg) tumor inhibition rates were 46.92 % and 60.28 %, respectively. Furthermore, the crystal structure of ASP-3 was resolved at 1.4 Å. ASP-3 formed as a stable dimer and folded as an EF-Hand structure. ASP-3 stably bound to domain I and domain III of the EGFR extracellular region by using molecular docking and molecular dynamics simulation analysis. Compared with the endogenous ligand EGF, ASP-3 displayed a stronger interaction with EGFR. These experimental results indicated that recombinant ASP-3 possessed an effective anti-hepatoma effect. So, it might be a potential molecule for liver cancer therapy.

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.

Functional Status of Neuronal Calcium Sensor-1 Is Modulated by Zinc Binding

Neuronal calcium sensor-1 (NCS-1) protein is abundantly expressed in the central nervous system and retinal neurons, where it regulates many vital processes such as synaptic transmission. It coordinates three calcium ions by EF-hands 2-4, thereby transducing Ca2+ signals to a wide range of protein targets, including G protein-coupled receptors and their kinases. Here, we demonstrate that NCS-1 also has Zn2+-binding sites, which affect its structural and functional properties upon filling. Fluorescence and circular dichroism experiments reveal the impact of Zn2+ binding on NCS-1 secondary and tertiary structure. According to atomic absorption spectroscopy and isothermal titration calorimetry studies, apo-NCS-1 has two high-affinity (4 × 106 M-1) and one low-affinity (2 × 105 M-1) Zn2+-binding sites, whereas Mg2+-loaded and Ca2+-loaded forms (which dominate under physiological conditions) bind two zinc ions with submicromolar affinity. Metal competition analysis and circular dichroism studies suggest that Zn2+-binding sites of apo- and Mg2+-loaded NCS-1 overlap with functional EF-hands of the protein. Consistently, high Zn2+ concentrations displace Mg2+ from the EF-hands and decrease the stoichiometry of Ca2+ binding. Meanwhile, one of the EF-hands of Zn2+-saturated NCS-1 exhibits a 14-fold higher calcium affinity, which increases the overall calcium sensitivity of the protein. Based on QM/MM molecular dynamics simulations, Zn2+ binding to Ca2+-loaded NCS-1 could occur at EF-hands 2 and 4. The high-affinity zinc binding increases the thermal stability of Ca2+-free NCS-1 and favours the interaction of its Ca2+-loaded form with target proteins, such as dopamine receptor D2R and GRK1. In contrast, low-affinity zinc binding promotes NCS-1 aggregation accompanied by the formation of twisted rope-like structures. Altogether, our findings suggest a complex interplay between magnesium, calcium and zinc binding to NCS-1, leading to the appearance of multiple conformations of the protein, in turn modulating its functional status.

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.

The EHEC-host interactome reveals novel targets for the translocated intimin receptor

Enterohemorrhagic E. coli (EHEC) manipulate their human host through at least 39 effector proteins which hijack host processes through direct protein-protein interactions (PPIs). To identify their protein targets in the host cells, we performed yeast two-hybrid screens, allowing us to find 48 high-confidence protein-protein interactions between 15 EHEC effectors and 47 human host proteins. In comparison to other bacteria and viruses we found that EHEC effectors bind more frequently to hub proteins as well as to proteins that participate in a higher number of protein complexes. The data set includes six new interactions that involve the translocated intimin receptor (TIR), namely HPCAL1, HPCAL4, NCALD, ARRB1, PDE6D, and STK16. We compared these TIR interactions in EHEC and enteropathogenic E. coli (EPEC) and found that five interactions were conserved. Notably, the conserved interactions included those of serine/threonine kinase 16 (STK16), hippocalcin-like 1 (HPCAL1) as well as neurocalcin-delta (NCALD). These proteins co-localize with the infection sites of EPEC. Furthermore, our results suggest putative functions of poorly characterized effectors (EspJ, EspY1). In particular, we observed that EspJ is connected to the microtubule system while EspY1 appears to be involved in apoptosis/cell cycle regulation.

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.

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.

Mutations that disrupt PHOXB interaction with the neuronal calcium sensor HPCAL1 impede cellular differentiation in neuroblastoma

Heterozygous germline mutations in PHOX2B, a transcriptional regulator of sympathetic neuronal differentiation, predispose to diseases of the sympathetic nervous system, including neuroblastoma and congenital central hypoventilation syndrome (CCHS). Although the PHOX2B variants in CCHS largely involve expansions of the second polyalanine repeat within the C-terminus of the protein, those associated with neuroblastic tumors are nearly always frameshift and truncation mutations. To test the hypothesis that the neuroblastoma-associated variants exert their effects through loss or gain of protein-protein interactions, we performed a large-scale yeast two-hybrid screen using both wild-type (WT) and six different mutant PHOX2B proteins against over 10 000 human genes. The neuronal calcium sensor protein HPCAL1 (VILIP-3) exhibited strong binding to WT PHOX2B and a CCHS-associated polyalanine expansion mutant but only weakly or not at all to neuroblastoma-associated frameshift and truncation variants. We demonstrate that both WT PHOX2B and the neuroblastoma-associated R100L missense and the CCHS-associated alanine expansion variants induce nuclear translocation of HPCAL1 in a Ca(2+)-independent manner, while the neuroblastoma-associated 676delG frameshift and K155X truncation mutants impair subcellular localization of HPCAL1, causing it to remain in the cytoplasm. HPCAL1 did not appreciably influence the ability of WT PHOX2B to transactivate the DBH promoter, nor did it alter the decreased transactivation potential of PHOX2B variants in 293T cells. Abrogation of the PHOX2B-HPCAL1 interaction by shRNA knockdown of HPCAL1 in neuroblastoma cells expressing PHOX2B led to impaired neurite outgrowth with transcriptional profiles indicative of inhibited sympathetic neuronal differentiation. Our results suggest that certain PHOX2B variants associated with neuroblastoma pathogenesis, because of their inability to bind to key interacting proteins such as HPCAL1, may predispose to this malignancy by impeding the differentiation of immature sympathetic neurons.

Models of calcium dynamics in cerebellar granule cells

Intracellular calcium dynamics is critical for many functions of cerebellar granule cells (GrCs) including membrane excitability, synaptic plasticity, apoptosis, and regulation of gene transcription. Recent measurements of calcium responses in GrCs to depolarization and synaptic stimulation reveal spatial compartmentalization and heterogeneity within dendrites of these cells. However, the main determinants of local calcium dynamics in GrCs are still poorly understood. One reason is that there have been few published studies of calcium dynamics in intact GrCs in their native environment. In the absence of complete information, biophysically realistic models are useful for testing whether specific Ca(2+) handling mechanisms may account for existing experimental observations. Simulation results can be used to identify critical measurements that would discriminate between different models. In this review, we briefly describe experimental studies and phenomenological models of Ca(2+) signaling in GrC, and then discuss a particular biophysical model, with a special emphasis on an approach for obtaining information regarding the distribution of Ca(2+) handling systems under conditions of incomplete experimental data. Use of this approach suggests that Ca(2+) channels and fixed endogenous Ca(2+) buffers are highly heterogeneously distributed in GrCs. Research avenues for investigating calcium dynamics in GrCs by a combination of experimental and modeling studies are proposed.

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.

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.

Recoverin as a redox-sensitive protein

Recoverin is a member of the neuronal calcium sensor (NCS) family of EF-hand calcium binding proteins. In a visual cycle of photoreceptor cells, recoverin regulates activity of rhodopsin kinase in a Ca2+-dependent manner. Like all members of the NSC family, recoverin contains a conserved cysteine (Cys38) in nonfunctional EF-hand 1. This residue was shown to be critical for activation of target proteins in some members of the NCS family but not for interaction of recoverin with rhodopsin kinase. Spectrophotometric titration of Ca2+-loaded recoverin gave 7.6 for the pKa value of Cys38 thiol, suggesting partial deprotonation of the thiol in vivo conditions. An ability of recoverin to form a disulfide dimer and thiol-oxidized monomer under mild oxidizing conditions was found using SDS-PAGE in reducing and nonreducing conditions and Ellman's test. Both processes are reversible and modulated by Ca2+. Although formation of the disulfide dimer takes place only for Ca2+-loaded recoverin, accumulation of the oxidized monomer proceeds more effectively for apo-recoverin. The Ca2+ modulated susceptibility of the recoverin thiol to reversible oxidation may be of potential importance for functioning of recoverin in photoreceptor cells.