Interaction of S-100b protein with cardiolipin vesicles as monitored by electron spin resonance, pyrene fluorescence and circular dichroism

Quantitative Proteomics Reveal an Altered Pattern of Protein Expression in Brain Tissue from Mice Lacking GPR37 and GPR37L1

GPR37 and GPR37L1 are glia-enriched G protein-coupled receptors that have been implicated in several neurological and neurodegenerative diseases. To gain insight into the potential molecular mechanisms by which GPR37 and GPR37L1 regulate cellular physiology, proteomic analyses of whole mouse brain tissue from wild-type (WT) versus GPR37/GPR37L1 double knockout (DKO) mice were performed in order to identify proteins regulated by the absence versus presence of these receptors (data are available via ProteomeXchange with identifier PXD015202). These analyses revealed a number of proteins that were significantly increased or decreased by the absence of GPR37 and GPR37L1. One of the most decreased proteins in the DKO versus WT brain tissue was S100A5, a calcium-binding protein, and the reduction of S100A5 expression in KO brain tissue was validated via Western blot. Coexpression of S100A5 with either GPR37 or GPR37L1 in HEK293T cells did not result in any change in S100A5 expression but did robustly increase secretion of S100A5. To dissect the mechanism by which S100A5 secretion was enhanced, cells coexpressing S100A5 with the receptors were treated with different pharmacological reagents. These studies revealed that calcium is essential for the secretion of S100A5 downstream of GPR37 and GPR37L1 signaling, as treatment with BAPTA-AM, an intracellular Ca²⁺ chelator, reduced S100A5 secretion from transfected HEK293T cells. Collectively, these findings provide a panoramic view of proteomic changes resulting from loss of GPR37 and GPR37L1 and also impart mechanistic insight into the regulation of S100A5 by these receptors, thereby shedding light on the functions of GPR37 and GPR37L1 in brain tissue.

Cardiolipin Interactions with Proteins

Cardiolipins (CL) represent unique phospholipids of bacteria and eukaryotic mitochondria with four acyl chains and two phosphate groups that have been implicated in numerous functions from energy metabolism to apoptosis. Many proteins are known to interact with CL, and several cocrystal structures of protein-CL complexes exist. In this work, we describe the collection of the first systematic and, to the best of our knowledge, the comprehensive gold standard data set of all known CL-binding proteins. There are 62 proteins in this data set, 21 of which have nonredundant crystal structures with bound CL molecules available. Using binding patch analysis of amino acid frequencies, secondary structures and loop supersecondary structures considering phosphate and acyl chain binding regions together and separately, we gained a detailed understanding of the general structural and dynamic features involved in CL binding to proteins. Exhaustive docking of CL to all known structures of proteins experimentally shown to interact with CL demonstrated the validity of the docking approach, and provides a rich source of information for experimentalists who may wish to validate predictions.

Functions of S100 proteins

The S100 protein family consists of 24 members functionally distributed into three main subgroups: those that only exert intracellular regulatory effects, those with intracellular and extracellular functions and those which mainly exert extracellular regulatory effects. S100 proteins are only expressed in vertebrates and show cell-specific expression patterns. In some instances, a particular S100 protein can be induced in pathological circumstances in a cell type that does not express it in normal physiological conditions. Within cells, S100 proteins are involved in aspects of regulation of proliferation, differentiation, apoptosis, Ca2+ homeostasis, energy metabolism, inflammation and migration/invasion through interactions with a variety of target proteins including enzymes, cytoskeletal subunits, receptors, transcription factors and nucleic acids. Some S100 proteins are secreted or released and regulate cell functions in an autocrine and paracrine manner via activation of surface receptors (e.g. the receptor for advanced glycation end-products and toll-like receptor 4), G-protein-coupled receptors, scavenger receptors, or heparan sulfate proteoglycans and N-glycans. Extracellular S100A4 and S100B also interact with epidermal growth factor and basic fibroblast growth factor, respectively, thereby enhancing the activity of the corresponding receptors. Thus, extracellular S100 proteins exert regulatory activities on monocytes/macrophages/microglia, neutrophils, lymphocytes, mast cells, articular chondrocytes, endothelial and vascular smooth muscle cells, neurons, astrocytes, Schwann cells, epithelial cells, myoblasts and cardiomyocytes, thereby participating in innate and adaptive immune responses, cell migration and chemotaxis, tissue development and repair, and leukocyte and tumor cell invasion.

S100 calcium binding proteins and ion channels

S100 Ca(2+)-binding proteins have been associated with a multitude of intracellular Ca(2+)-dependent functions including regulation of the cell cycle, cell differentiation, cell motility and apoptosis, modulation of membrane-cytoskeletal interactions, transduction of intracellular Ca(2+) signals, and in mediating learning and memory. S100 proteins are fine tuned to read the intracellular free Ca(2+) concentration and affect protein phosphorylation, which makes them candidates to modulate certain ion channels and neuronal electrical behavior. Certain S100s are secreted from cells and are found in extracellular fluids where they exert unique extracellular functions. In addition to their neurotrophic activity, some S100 proteins modulate neuronal electrical discharge activity and appear to act directly on ion channels. The first reports regarding these effects suggested S100-mediated alterations in Ca(2+) fluxes, K(+) currents, and neuronal discharge activity. Recent reports revealed direct and indirect interactions with Ca(2+), K(+), Cl(-), and ligand activated channels. This review focuses on studies of the physical and functional interactions of S100 proteins and ion channels.

S100A1 and S100B interactions with annexins

Members of the annexin protein family interact with members of the S100 protein family thereby forming heterotetramers in which an S100 homodimer crossbridges two copies of the pertinent annexin. Previous work has shown that S100A1 and S100B bind annexin VI in a Ca(2+)-dependent manner and that annexin VI, but not annexin V, blocks the inhibitory effect of S100A1 and S100B on intermediate filament assembly. We show here that both halves of annexin VI (i.e., the N-terminal half or annexin VI-a and the C-terminal half or annexin VI-b) bind individual S100s on unique sites and that annexin VI-b, but not annexin VI-a, blocks the ability of S100A1 and S100B to inhibit intermediate filament assembly. We also show that the C-terminal extension of S100A1 (and, by analogy, S100B), that was previously demonstrated to be critical for S100A1 and S100B binding to several target proteins including intermediate filament subunits, is not part of the S100 surface implicated in the recognition of annexin VI, annexin VI-a, or annexin VI-b. Evaluation of functional properties with a liposome stability and a calcium influx assay reveals the ability of both S100 proteins to permeabilize the membrane bilayer in a similar fashion like annexins. When tested in combinations with different annexin proteins both S100 proteins mostly lead to a decrease in the calcium influx activity although not all annexin/S100 combinations behave in the same manner. Latter observation supports the hypothesis that the S100-annexin interactions differ mechanistically depending on the particular protein partners.

Functional roles of S100 proteins, calcium-binding proteins of the EF-hand type

A multigenic family of Ca2+-binding proteins of the EF-hand type known as S100 comprises 19 members that are differentially expressed in a large number of cell types. Members of this protein family have been implicated in the Ca2+-dependent (and, in some cases, Zn2+- or Cu2+-dependent) regulation of a variety of intracellular activities such as protein phosphorylation, enzyme activities, cell proliferation (including neoplastic transformation) and differentiation, the dynamics of cytoskeleton constituents, the structural organization of membranes, intracellular Ca2+ homeostasis, inflammation, and in protection from oxidative cell damage. Some S100 members are released or secreted into the extracellular space and exert trophic or toxic effects depending on their concentration, act as chemoattractants for leukocytes, modulate cell proliferation, or regulate macrophage activation. Structural data suggest that many S100 members exist within cells as dimers in which the two monomers are related by a two-fold axis of rotation and that Ca2+ binding induces in individual monomers the exposure of a binding surface with which S100 dimers are believed to interact with their target proteins. Thus, any S100 dimer is suggested to expose two binding surfaces on opposite sides, which renders homodimeric S100 proteins ideal for crossbridging two homologous or heterologous target proteins. Although in some cases different S100 proteins share their target proteins, in most cases a high degree of target specificity has been described, suggesting that individual S100 members might be implicated in the regulation of specific activities. On the other hand, the relatively large number of target proteins identified for a single S100 protein might depend on the specific role played by the individual regions that in an S100 molecule contribute to the formation of the binding surface. The pleiotropic roles played by S100 members, the identification of S100 target proteins, the analysis of functional correlates of S100-target protein interactions, and the elucidation of the three-dimensional structure of some S100 members have greatly increased the interest in S100 proteins and our knowledge of S100 protein biology in the last few years. S100 proteins probably are an example of calcium-modulated, regulatory proteins that intervene in the fine tuning of a relatively large number of specific intracellular and (in the case of some members) extracellular activities. Systems, including knock-out animal models, should be now used with the aim of defining the correspondence between the in vitro regulatory role(s) attributed to individual members of this protein family and the in vivo function(s) of each S100 protein.

Time-resolved fluorescence of S-100a protein: effect of Ca2+, Mg2+ and unilamellar vesicles of egg phosphatidylcholine

Phase-modulation fluorescence lifetime measurements were used to study the single Trp residue of the Ca(2+)-binding protein S-100a both in the absence and in the presence of Ca2+ and/or Mg2+. Trp fluorescence decay for the protein was satisfactorily described by Lorentzian lifetime distributions centered around two components (approximately 4 ns and 0.5 ns). Lifetime values were unchanged by 2 mM Ca2+, but the fractional intensity associated with longer lifetime increased up to 75%. In the presence of Mg2+, the Ca2+ induced increase of the fractional intensity associated with longer lifetime was only 57%. For the protein in buffer, about the 85% of the recovered anisotropy was associated to a rotational correlation time of 6.7 ns. After the addition of Ca2+, this value was increased to 16.08 ns. In the presence of Mg2+, Ca+2 increased the rotational correlation time to 33.75 ns. Similar studies were performed with S-100a interacting with egg phosphatidylcholine vesicles (SUV). Our data suggest that the conformation of the protein may be influenced by structural features of the lipidic membrane. Moreover, data obtained in the presence of Mg2+ indicate some interaction between lipids and S-100, likely mediated by this ion.

S-100 beta has a neuronal localisation in the rat hindbrain revealed by an antigen retrieval method

The localisation of S-100 in mammalian CNS neurons has been under debate for more than two decades. We address the question with two polyclonal and two new monoclonal antibodies. The specificity and the distribution in rat brain is based on an antigen retrieval method. We present evidence that aldehyde fixatives mask S-100 beta in neurons, and that the immunoreactivity is retrieved after trypsinisation. Neuronal S-100 beta is also detected in unfixed and ethanol fixed sections. The neuronal immunoreactivity is partly solubilised from unfixed tissue sections with 2.5 mM EDTA and is completely extracted with 2.5 mM EDTA and 1% Triton X-100. Most of the glial S-100 beta is washed out from unfixed tissue sections with saline. S-100 beta has distinct distribution in neurons of the hindbrain, i.e., the brainstem and cerebellum, but is not observed in the forebrain. One of the monoclonal antibodies immunostained neither neurons nor glia when it had been absorbed with S-100 crosslinked to nitrocellulose membranes. The distribution of neuronal S-100 beta differed from that of other neuronal calcium binding proteins, such as calbindin and parvalbumin. It was confined mainly to cholinergic neurons of the hindbrain. The presence of S-100 beta in distinct neuronal populations may indicate neurotrophic effects of S-100 beta. The notion is supported by the capability of S-100 to cause neurite outgrowth in vitro.

Time-resolved fluorescence of S-100a protein in the absence and presence of calcium and phospholipids

We have used phase-modulation fluorescence lifetime measurements to study the single Trp residue of the Ca(2+)-binding protein S-100a. Trp fluorescence decay was not exponential for the protein irrespective of the absence or presence of Ca2+. Fluorescence decay was best described by Lorentzian lifetime distributions centered around two components (approx. 3 and 0.7 ns) for protein in absence of Ca2+ and one component (approx. 2.9 ns) for the protein in presence of 2 mM Ca2+. Similar studies were performed with S-100a interacting with cardiolipin, phosphatidylserine or egg phosphatidylcholine, both in absence and in presence of 2 mM Ca2+. Our data suggest that the conformation of the protein and its Ca(2+)-binding properties vary depending on the characteristics of charge and structure of phospholipids.

Disulfide-linked S100 beta dimers and signal transduction

S100 beta is a calcium-binding protein with neurotrophic and mitogenic activities, both of which may be mediated by the protein's ability to stimulate an increase in intracellular free calcium ([Ca2+]i). These extracellular trophic activities of S100 beta require a disulfide-linked, dimeric form of the protein. In this chapter, we present a minireview on the current state of knowledge concerning extracellular functions of S100 beta, with emphasis on the potential relevance of these activities to neuropathological disorders. We also report a simplified procedure for preparation of pharmacological amounts of biologically active S100 beta dimers, based on the finding that formation of disulfide-linked S100 beta dimers can be stimulated by the presence of calcium or lipid.

Perspectives in S-100 protein biology. Review article

The S-100 protein family constitutes a subgroup of Ca(2+)-binding proteins of the EF-hand type comprising three dimeric isoforms, S-100a0, S-100a and S-100b, plus a number of structurally related proteins displaying 28-55% homology with S-100 subunits. S-100 protein was discovered in 1965; yet, its biological functions have not been fully elucidated. The present report will review the putative biological roles of S-100 protein. Both intracellular and extracellular roles have been proposed for S-100 protein. Within cells, S-100 protein has been reported to regulate protein phosphorylation, ATPase, adenylate cyclase, and aldolase activities and Ca(2+)-induced Ca2+ release. Also, cytoskeletal systems, namely microtubules and microfilaments have been reported to be regulated by the protein in the presence of Ca2+. Some molecular targets of S-100 protein within cells, have been identified. This is the case with microtubule proteins, caldesmon, and a brain aldolase. S-100 protein has been reported to be secreted; extracellular S-100 protein can stimulate neuronal differentiation, glial proliferation, and prolactin secretion. However, the mechanisms by which S-100 is secreted and stimulates the above processes are largely unknown. Future research should characterize these latter aspects of S-100 biology and find out the linkage between its intracellular effects and its extracellular activities.

Evolution of EF-hand calcium-modulated proteins. I. Relationships based on amino acid sequences

The relationships among 153 EF-hand (calcium-modulated) proteins of known amino acid sequence were determined using the method of maximum parsimony. These proteins can be ordered into 12 distinct subfamilies--calmodulin, troponin C, essential light chain of myosin, regulatory light chain, sarcoplasmic calcium binding protein, calpain, aequorin, Stronglyocentrotus purpuratus ectodermal protein, calbindin 28 kd, parvalbumin, alpha-actinin, and S100/intestinal calcium-binding protein. Eight individual proteins--calcineurin B from Bos, troponin C from Astacus, calcium vector protein from Branchiostoma, caltractin from Chlamydomonas, cell-division-cycle 31 gene product from Saccharomyces, 10-kd calcium-binding protein from Tetrahymena, LPS1 eight-domain protein from Lytechinus, and calcium-binding protein from Streptomyces--are tentatively identified as unique; that is, each may be the sole representative of another subfamily. We present dendrograms showing the relationships among the subfamilies and uniques as well as dendrograms showing relationships within each subfamily. The EF-hand proteins have been characterized from a broad range of organismal sources, and they have an enormous range of function. This is reflected in the complexity of the dendrograms. At this time we urge caution in assigning a simple scheme of gene duplications to account for the evolution of the 600 EF-hand domains of known sequence.

S-100b protein regulates aggregation and fusion of cardiolipin vesicles

We have recently shown that S-100b protein interacts with the polar surface of cardiolipin vesicles [6]. This interaction produces changes in the secondary structure of S-100b as well as changes in the structural organization of cardiolipin vesicles. We report here on the effects of S-100b on cardiolipin vesicles as investigated by turbidity, terbium-dipicolinate fluorescence and freeze-fracture. Experiments were carried out in the absence and in the presence of Ca2+. In the absence of Ca2+ (0.1 mM EDTA), S-100b favors the aggregation and fusion of vesicles to some extent. Under these conditions, electron microscope analyses reveal the presence of fused vesicles along with particles similar to those observed in protein reconstituted systems or to lipid particles observed during fusional processes. In the presence of Ca2+, S-100b counteracts the Ca2(+)-dependent tendency of vesicles to aggregate and fuse. Under these conditions, bilayer phases along with hexagonal phases can be observed by electron microscopy. The latter effects of S-100b are not due to chelation of Ca2+ because of the relative concentrations of S-100b and Ca2+ under our experimental conditions and since much larger concentrations of EDTA are required to produce the S-100b effects. We propose that the dimeric nature of S-100b plays a major role in these events. In the absence of Ca2+, the S-100b molecules probably cross-link adjacent vesicles, one subunit contacting one vesicle and the other subunit contacting another vesicle through electrostatic bonds. In the presence of Ca2+, due to the large changes occurring in the conformation of the protein (which loses about 52% of its alpha-helical content), S-100b associates strongly with the polar surface of individual vesicles, thus generating some kind of physical barrier to aggregation and fusion of vesicles.

S-100a0 protein stimulates Ca2+-induced Ca2+ release from isolated sarcoplasmic reticulum vesicles

S-100a0 protein, the alpha alpha-isoform of the S-100 family, stimulates Ca2+-induced Ca2+ release from terminal cisternae isolated from rat skeletal muscle cells. The stimulatory effect of S-100a0 is maximal at approximately 5 microM S-100a0 and half maximal at approximately 0.1 microM S-100a0, at 1.8 microM free Ca2+ in the presence of 5 mM Mg2+ plus 0.1 M KCl. The effect of the protein on Ca2+-induced Ca2+ release is completely inhibited by the calcium release blocker, ruthenium red.

Identification of S-100 proteins and S-100-binding proteins in a detergent-resistant EDTA/KCl-extractable fraction from bovine brain membranes

The Triton X-100-resistant residue of brain membranes contains appreciable amounts of S-100 proteins. This fraction of S-100 can be solubilized by high concentrations of EDTA plus or minus high concentrations of KCl. Whereas KCl (0.6 M) extracts the detergent-resistant S-100, NaCl (1 M) does not. Endogenous Ca2+ is required and is sufficient for S-100 to remain associated with the detergent-resistant residue. However, 0.6 M KCl extracts a further fraction of Triton X-100-resistant S-100. In contrast, the Triton X-100-extractable fraction of S-100 resists the action of EDTA. These data suggest that Ca2+ regulates the extent of association of S-100 with Triton X-100-resistant components in brain membranes, whereas the association of S-100 with the lipid bilayer of brain membranes and/or with some intrinsic membrane proteins is less Ca2+-regulated. Several S-100-binding proteins are identified in the detergent-resistant residue of brain membranes by an overlay procedure.

Cardiac S-100a0 protein: purification by a simple procedure and related immunocytochemical and immunochemical studies

A simple procedure is described for the purification of the alpha alpha isoform of S-100 proteins (S-100a0) from porcine heart. Purification steps include the following: i) extraction of the tissue with a hypotonic medium containing EDTA; ii) ammonium sulfate fractionation (0-50%) of the extract; iii) Ca2+-dependent affinity chromatography of the supernatant obtained through the preceding step on phenyl-sepharose and elution of absorbed proteins through a two-chamber gradient of 1.0-0.0 mM CaCl2 and 0.0--1.0 mM EGTA, respectively; and iv) chromatography of the resultant S-100-containing fractions on Sephadex G-200. The yield is 20 mg S-100a0/kg porcine heart. The whole procedure takes five days and is highly reproducible. Data obtained from the phenyl-sepharose step suggest that the affinity of Ca2+ for S-100a0 increases by several orders of magnitude once the protein had interacted with that matrix. This observation is discussed in relation to the role of S-100 proteins in amplification of the Ca2+ signal. Immunocytochemical and immunoblotting analyses indicate that S-100a0 is exclusively found at the level of the sarcolemmal membranes, the membranes of the sarcoplasmic reticulum, the external mitochondrial membranes, and in the adjacent sarcoplasm. No evidence of S-100a0 being associated with the nuclei or with myofibrils has been obtained. Finally, the cardiac tissue does not contain the Triton X-100-extractable fraction of S-100 normally detected in the brain and in adipocytes. Our data suggest that S-100a0 behaves as a peripheral membrane protein in cardiac tissue.

S-100b protein regulates the activity of skeletal muscle adenylate cyclase in vitro

We have investigated the effect of the b isoform of S-100 proteins on adenylate cyclase activity of rat skeletal muscle. S-100b inhibits the adenylate cyclase activity in the presence of Mg2+ (5.0-50 mM), while it activates the same enzyme in the presence of Ca2+ (0.1-1.0 mM) dose-dependently in both cases. S-100b counteracts the stimulatory effect of NaF on adenylate cyclase in the presence of Mg2+ and the inhibitory effect of RMI 12330 A in the presence of Ca2+.