Ca2+-Binding Proteins of the EF-Hand Superfamily: Diagnostic and Prognostic Biomarkers and Novel Therapeutic Targets.

A multitude of Ca2+-sensor proteins containing the specific Ca2+-binding motif (helix-loop-helix, called EF-hand) are of major clinical relevance in a many human diseases. Measurements of troponin, the first intracellular Ca-sensor protein to be discovered, is nowadays the "gold standard" in the diagnosis of patients with acute coronary syndrome (ACS). Mutations have been identified in calmodulin and linked to inherited ventricular tachycardia and in patients affected by severe cardiac arrhythmias. Parvalbumin, when introduced into the diseased heart by gene therapy to increase contraction and relaxation speed, is considered to be a novel therapeutic strategy to combat heart failure. S100 proteins, the largest subgroup with the EF-hand protein family, are closely associated with cardiovascular diseases, various types of cancer, inflammation, and autoimmune pathologies. The intention of this review is to summarize the clinical importance of this protein family and their use as biomarkers and potential drug targets, which could help to improve the diagnosis of human diseases and identification of more selective therapeutic interventions.

S100 proteins: Diagnostic and prognostic biomarkers in laboratory medicine.

S100 proteins are members of the superfamily of Ca2+-binding proteins characterized by the specific Ca2+-binding motif, the EF-hand. Proteins of this superfamily are of clinical use as important diagnostic and prognostic biomarkers in adult and pediatric Laboratory Medicine. For example, measurements of troponin are nowadays the 'gold standard' in the diagnosis of patients with acute coronary syndrome. Parvalbumins were identified as major fish allergens and blocking antibodies, induced by immunization with a hypoallergenic parvalbumin mutant, were shown to reduce allergic symptoms. Mutations in calmodulin are linked to inherited ventricular tachycardia, and cardiac arrhythmias. S100 proteins, the largest sub-group within the EF-hand protein family, are closely associated with cardiovascular diseases, various types of cancer, inflammation and autoimmune pathologies and brain diseases. The intention of this review is to focus on the clinical use of S100 proteins as biomarkers and potential drug targets helping to improve the diagnosis of these human diseases in children and adults leading to more selective therapeutic interventions.

Extracellular S100A4 induces smooth muscle cell phenotypic transition mediated by RAGE.

We identified S100A4 as a marker of rhomboid (R) smooth muscle cells (SMCs) in vitro (the synthetic phenotype, typical of intimal SMCs) in the porcine coronary artery and of intimal SMCs in vivo in both pigs and humans. S100A4 is an intracellular Ca²âº signaling protein and can be secreted; it has extracellular functions via the receptor for advanced glycation end products (RAGE). Our objective was to explore the role of S100A4 in SMC phenotypic change, a phenomenon characteristic of atherosclerotic plaque formation. Transfection of a human S100A4-containing plasmid in spindle-shaped (S) SMCs (devoid of S100A4) led to approximately 10% of S100A4-overexpressing SMCs, S100A4 release, and a transition towards a R-phenotype of the whole SMC population. Furthermore treatment of S-SMCs with S100A4-rich conditioned medium collected from S100A4-transfected S-SMCs induced a transition towards a R-phenotype, which was associated with decreased SMC differentiation markers and increased proliferation and migration by activating the urokinase-type plasminogen activator (uPA), matrix metalloproteinases (MMPs) and their inhibitors (TIMPs). It yielded NF-κB activation in a RAGE-dependent manner. Blockade of extracellular S100A4 in R-SMCs with S100A4 neutralizing antibody induced a transition from R- to S-phenotype, decreased proliferative activity and upregulation of SMC differentiation markers. By contrast, silencing of S100A4 mRNA in R-SMCs did not change the level of extracellular S100A4 or SMC morphology in spite of decreased proliferative activity. Our results show that extracellular S100A4 plays a pivotal role in SMC phenotypic changes. It could be a new target to prevent SMC accumulation during atherosclerosis and restenosis. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

RAGE mediates S100A4-induced cell motility via MAPK/ERK and hypoxia signaling and is a prognostic biomarker for human colorectal cancer metastasis.

Survival of colorectal cancer patients is strongly dependent on development of distant metastases. S100A4 is a prognostic biomarker and inducer for colorectal cancer metastasis. Besides exerting intracellular functions, S100A4 is secreted extracellularly. The receptor for advanced glycation end products (RAGE) is one of its interaction partners. The impact of the S100A4-RAGE interaction for cell motility and metastasis formation in colorectal cancer has not been elucidated so far. Here we demonstrate the RAGE-dependent increase in migratory and invasive capabilities of colorectal cancer cells via binding to extracellular S100A4. We show the direct interaction of S100A4 and RAGE, leading to hyperactivated MAPK/ERK and hypoxia signaling. The S100A4-RAGE axis increased cell migration (P<0.005) and invasion (P<0.005), which was counteracted with recombinant soluble RAGE and RAGE-specific antibodies. In colorectal cancer patients, not distantly metastasized at surgery, high RAGE expression in primary tumors correlated with metachronous metastasis, reduced overall (P=0.022) and metastasis-free survival (P=0.021). In summary, interaction of S100A4-RAGE mediates S100A4-induced colorectal cancer cell motility. RAGE by itself represents a biomarker for prognosis of colorectal cancer. Thus, therapeutic approaches targeting RAGE or intervening in S100A4-RAGE-dependent signaling early in tumor progression might represent alternative strategies restricting S100A4-induced colorectal cancer metastasis.

Purification and characterization of the human cysteine-rich S100A3 protein and its pseudo citrullinated forms expressed in insect cells.

High quantity and quality of recombinant Ca(2+)-binding proteins are required to study their molecular interactions, self-assembly, posttranslational modifications, and biological activities to elucidate Ca(2+)-dependent cellular signaling pathways. S100A3 is a unique member of the S100 protein family with the highest cysteine content (10%). This protein, derived from human hair follicles and cuticles, is characterized by an N-terminal acetyl group and irreversible posttranslational citrullination by peptidylarginine deiminase causing its homotetramer assembly. Insect cells, capable of introducing eukaryotic N-terminus and disulfide bonds, are an appropriate host in which to express this cysteine-rich protein. Four out of ten cysteines in the recombinant S100A3 form two intramolecular disulfide bridges that modulate its Ca(2+)-affinity. Three free thiol groups located at the C-terminus are predicted to form the high-affinity Zn(2+)-binding site. Citrullination of specific arginine residues in native S100A3 can be mimicked by site-directed mutagenic substitution of Arg/Ala. This chapter details our procedures used for the purification and characterization of the human S100A3 protein and its pseudo citrullinated forms expressed in insect cells.

Human S100A3 tetramerization propagates Ca(2+)/Zn(2+) binding states.

The S100A3 homotetramer assembles upon citrullination of a specific symmetric Arg51 pair on its homodimer interface in human hair cuticular cells. Each S100A3 subunit contains two EF-hand-type Ca(2+)-binding motifs and one (Cys)3His-type Zn(2+)-binding site in the C-terminus. The C-terminal coiled domain is cross-linked to the presumed docking surface of the dimeric S100A3 via a disulfide bridge. The aim of this study was to determine the structural and functional role of the C-terminal Zn(2+)-binding domain, which is unique to S100A3, in homotetramer assembly. The binding of either Ca(2+) or Zn(2+) reduced the α-helix content of S100A3 and modulated its affinity for the other cation. The binding of a single Zn(2+) accelerated the Ca(2+)-dependent tetramerization of S100A3 while inducing an extensive unfolding of helix IV. The Ca(2+) and Zn(2+) binding affinities of S100A3 were enhanced when the other cation bound in concert with the tetramerization of S100A3. Small angle scattering analyses revealed that the overall structure of the S100A3 tetramer bound both Ca(2+) and Zn(2+) had a similar molecular shape to the Ca(2+)-bound form in solution. The binding states of the Ca(2+) or Zn(2+) to each S100A3 subunit within a homotetramer appear to be propagated by sensing the repositioning of helix III and the rearrangement of the C-terminal tail domain. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

S100 and S100 fused-type protein families in epidermal maturation with special focus on S100A3 in mammalian hair cuticles.

Epithelial Ca(2+)-regulation, which governs cornified envelope formation in the skin epidermis and hair follicles, closely coincides with the expression of S100A3, filaggrin and trichohyalin, and the post-translational modification of these proteins by Ca(2+)-dependent peptidylarginine deiminases. This review summarizes the current nomenclature and evolutional aspects of S100 Ca(2+)-binding proteins and S100 fused-type proteins (SFTPs) classified as a separate protein family with special reference to the molecular structure and function of S100A3 dominantly expressed in hair cuticular cells. Both S100 and SFTP family members are identified by two distinct types of Ca(2+)-binding loops in an N-terminal pseudo EF-hand motif followed by a canonical EF-hand motif. Seventeen members of the S100 protein family including S100A3 are clustered with seven related genes encoding SFTPs on human chromosome 1q21, implicating their association with epidermal maturation and diseases. Human S100A3 is characterized by two disulphide bridges and a preformed Zn(2+)-pocket, and may transfer Ca(2+) ions to peptidylarginine deiminases after its citrullination-mediated tetramerization. Phylogenetic analysis utilizing current genome databases suggests that divergence of the S100A3 gene coincided with the emergence of hair, a defining feature of mammals, and that the involvement of S100A3 in epithelial Ca(2+)-cycling occurred as a result of a skin adaptation in terrestrial mammals.

The importance of Ca2+/Zn2+ signaling S100 proteins and RAGE in translational medicine.

The Receptor for Advanced Glycation Endproducts (RAGE) is a multiligand receptor involved in a large number of human disorders. Identified first as the receptor for the Advanced Glycation Endproducts (AGEs), RAGE has emerged in recent years as a major receptor for many members of the S100 calcium and zinc binding protein family. The interaction with and the signaling triggered by several S100 proteins such as S100B and S100A12 have been studied in details and have shown concentration and cell type dependent signaling cascades. The S100 protein family consists of more than 20 members which present high amino-acid sequence and structural similarities. These small EF-hand calcium binding proteins interact with a large number of protein targets and are almost all been shown to be involved in cancer. In this review we discuss the recent knowledge about the role of S100 proteins and RAGE in human disorders.

Mitochondrial matrix calcium is an activating signal for hormone secretion.

Mitochondrial Ca(2+) signals have been proposed to accelerate oxidative metabolism and ATP production to match Ca(2+)-activated energy-consuming processes. Efforts to understand the signaling role of mitochondrial Ca(2+) have been hampered by the inability to manipulate matrix Ca(2+) without directly altering cytosolic Ca(2+). We were able to selectively buffer mitochondrial Ca(2+) rises by targeting the Ca(2+)-binding protein S100G to the matrix. We find that matrix Ca(2+) controls signal-dependent NAD(P)H formation, respiration, and ATP changes in intact cells. Furthermore, we demonstrate that matrix Ca(2+) increases are necessary for the amplification of sustained glucose-dependent insulin secretion in ß cells. Through the regulation of NAD(P)H in adrenal glomerulosa cells, matrix Ca(2+) also acts as a positive signal in reductive biosynthesis, which stimulates aldosterone secretion. Our dissection of cytosolic and mitochondrial Ca(2+) signals reveals the physiological importance of matrix Ca(2+) in energy metabolism required for signal-dependent hormone secretion.

Refined crystal structures of human Ca(2+)/Zn(2+)-binding S100A3 protein characterized by two disulfide bridges.

S100A3, a member of the EF-hand-type Ca(2+)-binding S100 protein family, is unique in its exceptionally high cysteine content and Zn(2+) affinity. We produced human S100A3 protein and its mutants in insect cells using a baculovirus expression system. The purified wild-type S100A3 and the pseudo-citrullinated form (R51A) were crystallized with ammonium sulfate in N,N-bis(2-hydroxyethyl)glycine buffer and, specifically for postrefolding treatment, with Ca(2+)/Zn(2+) supplementation. We identified two previously undocumented disulfide bridges in the crystal structure of properly folded S100A3: one disulfide bridge is between Cys30 in the N-terminal pseudo-EF-hand and Cys68 in the C-terminal EF-hand (SS1), and another disulfide bridge attaches Cys99 in the C-terminal coil structure to Cys81 in helix IV (SS2). Mutational disruption of SS1 (C30A+C68A) abolished the Ca(2+) binding property of S100A3 and retarded the citrullination of Arg51 by peptidylarginine deiminase type III (PAD3), while SS2 disruption inversely increased both Ca(2+) affinity and PAD3 reactivity in vitro. Similar backbone structures of wild type, R51A, and C30A+C68A indicated that neither Arg51 conversion by PAD3 nor SS1 alters the overall dimer conformation. Comparative inspection of atomic coordinates refined to 2.15-1.40 Å resolution shows that SS1 renders the C-terminal classical Ca(2+)-binding loop flexible, which are essential for its Ca(2+) binding properties, whereas SS2 structurally shelters Arg51 in the metal-free form. We propose a model of the tetrahedral coordination of a Zn(2+) by (Cys)(3)His residues that is compatible with SS2 formation in S100A3.

Glycolaldehyde-modified ß-lactoglobulin AGEs are unable to stimulate inflammatory signaling pathways in RAGE-expressing human cell lines.

SCOPE: Advanced glycation endproducts (AGEs) are suspected to stimulate inflammatory signaling pathways in target tissues via activation of the receptor for AGEs. Endotoxins are generally recognized as potential contamination of AGE preparations and stimulate biological actions that are very similar as or identical to those induced by AGEs. METHODS AND RESULTS: In our study, we used glycolaldehyde-modified ß-lactoglobulin preparations as model AGEs and employed two methods to remove endotoxin using either affinity columns or extraction with Triton X-114 (TX-114). Affinity column-purified AGEs retained their ability to stimulate inflammatory signaling as measured by mRNA expression of inflammatory cytokines in the human lung epithelial cell line Beas2b. However, glycolaldehyde-modified AGEs purified by extraction with TX-114 did not show any stimulation of mRNA expression of inflammatory cytokines. The presence of a cell stimulating endotoxin-like activity was demonstrated in the detergent phase after extraction with TX-114, thus indicating that not AGEs but a lipophilic contamination was responsible for the stimulation of inflammatory signaling. CONCLUSION: Our results demonstrate that glycolaldehyde-modified AGEs are unable to induce inflammatory signaling in receptor for AGE-expressing cells. The observed cell-activating activity can be ascribed to an endotoxin-like lipophilic contamination present in AGE preparations and affinity column purification was insufficient to remove this contamination.

The crystal structures of human S100B in the zinc- and calcium-loaded state at three pH values reveal zinc ligand swapping.

S100B is a homodimeric zinc-, copper-, and calcium-binding protein of the family of EF-hand S100 proteins. Zn(2+) binding to S100B increases its affinity towards Ca(2+) as well as towards target peptides and proteins. Cu(2+) and Zn(2+) bind presumably to the same site in S100B. We determined the structures of human Zn(2+)- and Ca(2+)-loaded S100B at pH 6.5, pH 9, and pH 10 by X-ray crystallography at 1.5, 1.4, and 1.65Å resolution, respectively. Two Zn(2+) ions are coordinated tetrahedrally at the dimer interface by His and Glu residues from both subunits. The crystal structures revealed that ligand swapping occurs for one of the four ligands in the Zn(2+)-binding sites. Whereas at pH 9, the Zn(2+) ions are coordinated by His15, His25, His 85', and His 90', at pH 6.5 and pH 10, His90' is replaced by Glu89'. The results document that the Zn(2+)-binding sites are flexible to accommodate other metal ions such as Cu(2+). Moreover, we characterized the structural changes upon Zn(2+) binding, which might lead to increased affinity towards Ca(2+) as well as towards target proteins. We observed that in Zn(2+)-Ca(2+)-loaded S100B the C-termini of helix IV adopt a distinct conformation. Zn(2+) binding induces a repositioning of residues Phe87 and Phe88, which are involved in target protein binding. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Crosstalk between calcium, amyloid beta and the receptor for advanced glycation endproducts in Alzheimer's disease.

Hallmarks of Alzheimer's disease (AD) include the accumulation of amyloid beta peptide (Abeta), hyperphosphorylation of tau protein, and increased inflammatory activity in the hippocampus and cerebral cortex. The receptor for advanced glycation endproducts (RAGE) has been shown to interact with Abeta and to modulate Abeta transport across the blood-brain barrier. Furthermore, RAGE is upregulated at sites of inflammation and its activation results in distinct intracellular signaling cascades in respect to Abeta conformers. Besides Abeta, RAGE interacts with several members of the calcium binding S100 protein family, amphoterin and advanced glycation endproducts. Mounting evidence suggests that RAGE is a key player in the signaling pathways triggered by Abeta and S100 proteins in AD. In this review, we discuss recent discoveries about the crosstalk between RAGE, Abeta and S100 proteins in the pathophysiology of AD.