S100A1 regulates neurite organization, tubulin levels, and proliferation in PC12 cells

Mechanism of Zn2+ and Ca2+ Binding to Human S100A1

S100A1 is a member of the S100 family of small ubiquitous Ca²⁺-binding proteins, which participates in the regulation of cell differentiation, motility, and survival. It exists as homo- or heterodimers. S100A1 has also been shown to bind Zn²⁺, but the molecular mechanisms of this binding are not yet known. In this work, using ESI-MS and ITC, we demonstrate that S100A1 can coordinate 4 zinc ions per monomer, with two high affinity (K_D~4 and 770 nm) and two low affinity sites. Using competitive binding experiments between Ca²⁺ and Zn²⁺ and QM/MM molecular modeling we conclude that Zn²⁺ high affinity sites are located in the EF-hand motifs of S100A1. In addition, two lower affinity sites can bind Zn²⁺ even when the EF-hands are saturated by Ca²⁺, resulting in a 2Ca²⁺:S100A1:2Zn²⁺ conformer. Finally, we show that, in contrast to calcium, an excess of Zn²⁺ produces a destabilizing effect on S100A1 structure and leads to its aggregation. We also determined a higher affinity to Ca²⁺ (K_D~0.16 and 24 μm) than was previously reported for S100A1, which would allow this protein to function as a Ca²⁺/Zn²⁺-sensor both inside and outside cells, participating in diverse signaling pathways under normal and pathological conditions.

Direct interaction of metastasis-inducing S100P protein with tubulin causes enhanced cell migration without changes in cell adhesion

Overexpression of S100P promotes breast cancer metastasis in animals and elevated levels in primary breast cancers are associated with poor patient outcomes. S100P can differentially interact with nonmuscle myosin (NM) isoforms (IIA > IIC > IIB) leading to the redistribution of actomyosin filaments to enhance cell migration. Using COS-7 cells which do not naturally express NMIIA, S100P is now shown to interact directly with α,β-tubulin in vitro and in vivo with an equilibrium K_d of 2–3 × 10⁻⁷ M. The overexpressed S100P is located mainly in nuclei and microtubule organising centres (MTOC) and it significantly reduces their number, slows down tubulin polymerisation and enhances cell migration in S100P-induced COS-7 or HeLa cells. It fails, however, to significantly reduce cell adhesion, in contrast with NMIIA-containing S100P-inducible HeLa cells. When taxol is used to stabilise MTs or colchicine to dissociate MTs, S100P's stimulation of migration is abolished. Affinity-chromatography of tryptic digests of α and β-tubulin on S100P-bound beads identifies multiple S100P-binding sites consistent with S100P binding to all four half molecules in gel-overlay assays. When screened by NMR and ITC for interacting with S100P, four chemically synthesised peptides show interactions with low micromolar dissociation constants. The two highest affinity peptides significantly inhibit binding of S100P to α,β-tubulin and, when tagged for cellular entry, also inhibit S100P-induced reduction in tubulin polymerisation and S100P-enhancement of COS-7 or HeLa cell migration. A third peptide incapable of interacting with S100P also fails in this respect. Thus S100P can interact directly with two different cytoskeletal filaments to independently enhance cell migration, the most important step in the metastatic cascade.

S100 family proteins in inflammation and beyond

The S100 family proteins possess a variety of intracellular and extracellular functions. They interact with multiple receptors and signal transducers to regulate pathways that govern inflammation, cell differentiation, proliferation, energy metabolism, apoptosis, calcium homeostasis, cell cytoskeleton and microbial resistance. S100 proteins are also emerging as novel diagnostic markers for identifying and monitoring various diseases. Strategies aimed at targeting S100-mediated signaling pathways hold a great potential in developing novel therapeutics for multiple diseases. In this chapter, we aim to summarize the current knowledge about the role of S100 family proteins in health and disease with a major focus on their role in inflammatory conditions.

Gpr126/Adgrg6 contributes to the terminal Schwann cell response at the neuromuscular junction following peripheral nerve injury

Gpr126/Adgrg6 is an adhesion G protein-coupled receptor essential for Schwann cell (SC) myelination with important contributions to repair after nerve crush injury. Despite critical functions in myelinating SCs, the role of Gpr126 within nonmyelinating terminal Schwann cells (tSCs) at the neuromuscular junction (NMJ), is not known. tSCs have important functions in synaptic maintenance and reinnervation, and after injury tSCs extend cytoplasmic processes to guide regenerating axons to the denervated NMJ. In this study, we show that Gpr126 is expressed in tSCs, and that absence of Gpr126 in SCs (SC-specific Gpr126 knockout, cGpr126) results in a NMJ maintenance defect in the hindlimbs of aged mice, but not in young adult mice. After nerve transection and repair, cGpr126 mice display delayed NMJ reinnervation, altered tSC morphology with decreased S100β expression, and reduced tSC cytoplasmic process extensions. The immune response promoting reinnervation at the NMJ following nerve injury is also altered with decreased macrophage infiltration, Tnfα, and anomalous cytokine expression compared to NMJs of control mice. In addition, Vegfa expression is decreased in muscle, suggesting that cGpr126 non-cell autonomously modulates angiogenesis after nerve injury. In sum, cGpr126 mice demonstrated delayed NMJ reinnervation and decreased muscle mass following nerve transection and repair compared to control littermates. The integral function of Gpr126 in tSCs at the NMJ provides the framework for new therapeutic targets for neuromuscular disease.

S100 Proteins in Alzheimer's Disease

S100 proteins are calcium-binding proteins that regulate several processes associated with Alzheimer's disease (AD) but whose contribution and direct involvement in disease pathophysiology remains to be fully established. Due to neuroinflammation in AD patients, the levels of several S100 proteins are increased in the brain and some S100s play roles related to the processing of the amyloid precursor protein, regulation of amyloid beta peptide (Aβ) levels and Tau phosphorylation. S100 proteins are found associated with protein inclusions, either within plaques or as isolated S100-positive puncta, which suggests an active role in the formation of amyloid aggregates. Indeed, interactions between S100 proteins and aggregating Aβ indicate regulatory roles over the aggregation process, which may either delay or aggravate aggregation, depending on disease stage and relative S100 and Aβ levels. Additionally, S100s are also known to influence AD-related signaling pathways and levels of other cytokines. Recent evidence also suggests that metal-ligation by S100 proteins influences trace metal homeostasis in the brain, particularly of zinc, which is also a major deregulated process in AD. Altogether, this evidence strongly suggests a role of S100 proteins as key players in several AD-linked physiopathological processes, which we discuss in this review.

X-ray crystal structure of human calcium-bound S100A1

S100A1 is a member of the S100 family of Ca2+-binding proteins and regulates several cellular processes, including those involved in Ca2+ signaling and cardiac and skeletal muscle function. In Alzheimer's disease, brain S100A1 is overexpressed and gives rise to disease pathologies, making it a potential therapeutic target. The 2.25 Å resolution crystal structure of Ca2+-S100A1 is solved here and is compared with the structures of other S100 proteins, most notably S100B, which is a highly homologous S100-family member that is implicated in the progression of malignant melanoma. The observed structural differences in S100A1 versus S100B provide insights regarding target protein-binding specificity and for targeting these two S100 proteins in human diseases using structure-based drug-design approaches.

Joining S100 proteins and migration: for better or for worse, in sickness and in health

The vast diversity of S100 proteins has demonstrated a multitude of biological correlations with cell growth, cell differentiation and cell survival in numerous physiological and pathological conditions in all cells of the body. This review summarises some of the reported regulatory functions of S100 proteins (namely S100A1, S100A2, S100A4, S100A6, S100A7, S100A8/S100A9, S100A10, S100A11, S100A12, S100B and S100P) on cellular migration and invasion, established in both culture and animal model systems and the possible mechanisms that have been proposed to be responsible. These mechanisms involve intracellular events and components of the cytoskeletal organisation (actin/myosin filaments, intermediate filaments and microtubules) as well as extracellular signalling at different cell surface receptors (RAGE and integrins). Finally, we shall attempt to demonstrate how aberrant expression of the S100 proteins may lead to pathological events and human disorders and furthermore provide a rationale to possibly explain why the expression of some of the S100 proteins (mainly S100A4 and S100P) has led to conflicting results on motility, depending on the cells used.

Post-translational S-nitrosylation is an endogenous factor fine tuning the properties of human S100A1 protein

BACKGROUND

S100A1 protein is a proposed target of molecule-guided therapy for heart failure.

RESULTS

S-Nitrosylation of S100A1 is present in cells, increases Ca(2+) binding, and tunes the overall protein conformation.

CONCLUSION

Thiol-aromatic molecular switch is responsible for NO-related modification of S100A1 properties.

SIGNIFICANCE

Post-translational S-nitrosylation may provide functional diversity and specificity to S100A1 and other S100 protein family members. S100A1 is a member of the Ca(2+)-binding S100 protein family. It is expressed in brain and heart tissue, where it plays a crucial role as a modulator of Ca(2+) homeostasis, energy metabolism, neurotransmitter release, and contractile performance. Biological effects of S100A1 have been attributed to its direct interaction with a variety of target proteins. The (patho)physiological relevance of S100A1 makes it an important molecular target for future therapeutic intervention. S-Nitrosylation is a post-translational modification of proteins, which plays a role in cellular signal transduction under physiological and pathological conditions. In this study, we confirmed that S100A1 protein is endogenously modified by Cys(85) S-nitrosylation in PC12 cells, which are a well established model system for studying S100A1 function. We used isothermal calorimetry to show that S-nitrosylation facilitates the formation of Ca(2+)-loaded S100A1 at physiological ionic strength conditions. To establish the unique influence of the S-nitroso group, our study describes high resolution three-dimensional structures of human apo-S100A1 protein with the Cys(85) thiol group in reduced and S-nitrosylated states. Solution structures of the proteins are based on NMR data obtained at physiological ionic strength. Comparative analysis shows that S-nitrosylation fine tunes the overall architecture of S100A1 protein. Although the typical S100 protein intersubunit four-helix bundle is conserved upon S-nitrosylation, the conformation of S100A1 protein is reorganized at the sites most important for target recognition (i.e. the C-terminal helix and the linker connecting two EF-hand domains). In summary, this study discloses cysteine S-nitrosylation as a new factor responsible for increasing functional diversity of S100A1 and helps explain the role of S100A1 as a Ca(2+) signal transmitter sensitive to NO/redox equilibrium within cells.

Solution structure of S100A1 bound to the CapZ peptide (TRTK12)

As is typical for S100-target protein interactions, a Ca 2+-dependent conformational change in S100A1 is required to bind to a 12-residue peptide (TRTK12) derived from the actin-capping protein CapZ. In addition, the Ca 2+-binding affinity of S100A1 is found to be tightened (greater than threefold) when TRTK12 is bound. To examine the biophysical basis for these observations, we determined the solution NMR structure of TRTK12 in a complex with Ca 2+-loaded S100A1. When bound to S100A1, TRTK12 forms an amphipathic helix (residues N6 to S12) with several favorable hydrophobic interactions observed between W7, I10, and L11 of the peptide and a well-defined hydrophobic binding pocket in S100A1 that is only present in the Ca 2+-bound state. Next, the structure of S100A1-TRTK12 was compared to that of another S100A1-target complex (i.e., S100A1-RyRP12), which illustrated how the binding pocket in Ca 2+-S100A1 can accommodate peptide targets with varying amino acid sequences. Similarities and differences were observed when the structures of S100A1-TRTK12 and S100B-TRTK12 were compared, providing insights regarding how more than one S100 protein can interact with the same peptide target. Such comparisons, including those with other S100-target and S100-drug complexes, provide the basis for designing novel small-molecule inhibitors that could be specific for blocking one or more S100-target protein interactions.

S100A1: Structure, Function, and Therapeutic Potential

S100A1 is a member of the S100 family of calcium-binding proteins. As with most S100 proteins, S100A1 undergoes a large conformational change upon binding calcium as necessary to interact with numerous protein targets. Targets of S100A1 include proteins involved in calcium signaling (ryanidine receptors 1 & 2, Serca2a, phopholamban), neurotransmitter release (synapsins I & II), cytoskeletal and filament associated proteins (CapZ, microtubules, intermediate filaments, tau, mocrofilaments, desmin, tubulin, F-actin, titin, and the glial fibrillary acidic protein GFAP), transcription factors and their regulators (e.g. myoD, p53), enzymes (e.g. aldolase, phosphoglucomutase, malate dehydrogenase, glycogen phosphorylase, photoreceptor guanyl cyclases, adenylate cyclases, glyceraldehydes-3-phosphate dehydrogenase, twitchin kinase, Ndr kinase, and F1 ATP synthase), and other Ca2+-activated proteins (annexins V & VI, S100B, S100A4, S100P, and other S100 proteins). There is also a growing interest in developing inhibitors of S100A1 since they may be beneficial for treating a variety of human diseases including neurological diseases, diabetes mellitus, heart failure, and several types of cancer. The absence of significant phenotypes in S100A1 knockout mice provides some early indication that an S100A1 antagonist could have minimal side effects in normal tissues. However, development of S100A1-mediated therapies is complicated by S100A1's unusual ability to function as both an intracellular signaling molecule and as a secreted protein. Additionally, many S100A1 protein targets have only recently been identified, and so fully characterizing both these S100A1-target complexes and their resulting functions is a necessary prerequisite.

Pathologies involving the S100 proteins and RAGE

The S100 proteins are exclusively expressed in vertebrates and are the largest subgroup within the superfamily of EF-hand Ca2(+)-binding proteins Generally, S100 proteins are organized as tight homodimers (some as heterodimers). Each subunit is composed of a C-terminal, 'canonical' EF-hand, common to all EF-hand proteins, and a N-terminal, 'pseudo' EF-hand, characteristic of S100 proteins. Upon Ca2(+)-binding, the C-terminal EF-hand undergoes a large conformational change resulting in the exposure of a hydrophobic surface responsible for target binding A unique feature of this protein family is that some members are secreted from cells upon stimulation, exerting cytokine- and chemokine-like extracellular activities via the Receptor for Advanced Glycation Endproducts, RAGE. Recently, larger assemblies of some S100 proteins (hexamers, tetramers, octamers) have been also observed and are suggested to be the active extracellular species required for receptor binding and activation through receptor multimerization Most S100 genes are located in a gene cluster on human chromosome 1q21, a region frequently rearranged in human cancer The functional diversification of S100 proteins is achieved by their specific cell- and tissue-expression patterns, structural variations, different metal ion binding properties (Ca2+, Zn2+ and Cu2+) as well as their ability to form homo-, hetero- and oligomeric assemblies Here, we review the most recent developments focussing on the biological functions of the S100 proteins and we discuss the presently available S100-specific mouse models and their possible use as human disease models In addition, the S100-RAGE interaction and the activation of various cellular pathways will be discussed. Finally, the close association of S100 proteins with cardiomyopathy, cancer, inflammation and brain diseases is summarized as well as their use in diagnosis and their potential as drug targets to improve therapies in the future.

S100A1-deficient male mice exhibit increased exploratory activity and reduced anxiety-related responses

S100 proteins comprise a family of Ca(2+) binding proteins of at least 21 members. They are distinctly expressed in a variety of cell types and tissues and are thought to play unique roles, although they share a high degree of sequence homology and expression overlap. S100A1 is prominently expressed in the heart, where it takes part in Ca(2+)-cycling. Its role in the central nervous system (CNS) is largely unknown. We have generated S100A1-deficient mice by gene trap mutagenesis to study the involvement of S100A1 in the cytoarchitecture of the brain, in learning and memory, and in avoidance-approach behavior. S100A1 knock out (KO) mice develop well and their brains present with normal morphology. In wild type (Wt) mice, S100A1 protein was found in the hippocampus, cerebral cortex and amygdala, and partially co-localized with the astrocyte marker glial fibrillary acidic protein (GFAP) in the stratum radiatum of the hippocampus. Astrocytes and neurons of S100A1KO mice did not differ from those of Wt mice regarding shape, distribution and density. In the water maze, S100A1KO mice performed equally well as Wt, implying that S100A1 is not involved in spatial learning and memory. In avoidance-approach tests, predominantly male S100A1KO mice showed reduced anxiety-like responses and enhanced explorative activities. We conclude that S100A1 plays a role in modulating innate fear and exploration of novel stimuli.

Immunohistochemical distribution of S-100 protein in the cerebral cortex with regard to the cause of death in forensic autopsy

S-100 protein (S100) is an acidic calcium-binding protein, which is abundantly found in the brain. The aim of the present study was to investigate the immunohistochemical distribution of S100 in the cerebral cortex in forensic autopsy cases with regard to the cause of death and relationship to its serum levels. The cases (n = 286, 3-48 h postmortem) included fatal head injuries (n = 89), acute death from other blunt injuries (n = 29), sharp instrument injuries (n = 20), asphyxiation (n = 29: strangulation/hanging, n = 22; aspiration, n = 7), drownings (n = 22), fire fatalities (n = 68), cerebrovascular diseases (n = 9) and acute myocardial infarction/ischemia (AMI, n = 20). S100-immunopositivity was mainly observed in the gliacytes, in part, in the neurons and myelins. For S100B-immunostaining, the gliacytes and myelins were positive, whereas the neurons were negative. Positivity in astrocytes was dependent on the cause of death, and was significantly lower in acute deaths due to strangulation/hanging and drowning, and mildly low in those due to injuries. Positivity in neurons and myelins was frequently observed in delayed head injury deaths and fire fatalities, showing an inverse relationship with the positivity in astrocytes in head injury cases. For cases of acute death, there was an inverse relationship between S100-positivity in the astrocytes and the serum S100B level. These observations suggest that astrocytes are more rapidly and severely injured than neurons during fatal brain damage, thereby causing an elevation in the serum S100B level.

The three-dimensional solution structure of Ca(2+)-bound S100A1 as determined by NMR spectroscopy

S100A1 is an EF-hand-containing Ca(2+)-binding protein that undergoes a conformational change upon binding calcium as is necessary to interact with protein targets and initiate a biological response. To better understand how calcium influences the structure and function of S100A1, the three-dimensional structure of calcium-bound S100A1 was determined by multidimensional NMR spectroscopy and compared to the previously determined structure of apo. In total, 3354 nuclear Overhauser effect-derived distance constraints, 240 dihedral constraints, 160 hydrogen bond constraints, and 362 residual dipolar coupling restraints derived from a series of two-dimensional, three-dimensional, and four-dimensional NMR experiments were used in its structure determination (>21 constraints per residue). As with other dimeric S100 proteins, S100A1 is a symmetric homodimer with helices 1, 1', 4, and 4' associating into an X-type four-helix bundle at the dimer interface. Within each subunit there are four alpha-helices and a short antiparallel beta-sheet typical of two helix-loop-helix EF-hand calcium-binding domains. The addition of calcium did not change the interhelical angle of helices 1 and 2 in the pseudo EF-hand significantly; however, there was a large reorientation of helix 3 in the typical EF-hand. The large conformational change exposes a hydrophobic cleft, defined by residues in the hinge region, the C terminus, and regions of helix 3, which are important for the interaction between S100A1 and a peptide (TRTK-12) derived from the actin-capping protein CapZ.

Molecular mechanisms of S100-target protein interactions

S100 proteins have no known enzymatic activity and exert their intracellular effects via interaction with and regulation of the activity of other proteins, termed target proteins, in both a Ca(2+)-dependent and Ca(2+)-independent manner. Structural studies have identified the linker region between the two EF-hand Ca(2+) binding domains and the C-terminus as Ca(2+)-dependent target protein binding sites in several S100 family members. In fact, C-terminal aromatic residues are obligatory for interaction of S100A1 with several of its Ca(2+)-dependent target proteins. Pharmacological studies suggest the presence of additional Ca(2+)-dependent binding motifs on some family members. A minimum of seven family members interact with and regulate the activity of aldolase A in a Ca(2+)-independent manner. In the case of S100A1, Ca(2+)-independent target protein interactions utilize a binding motif distinct from the C-terminal Ca(2+)-dependent target protein binding site. Several studies suggest that ionic interactions participate in the interaction of S100 family members with Ca(2+)-independent target proteins. While some target proteins are activated by multiple family members, other target proteins exhibit family member-specific activation, i.e., they are activated by a single family member. As predicted, family member specific interactions appear to be mediated by regions that exhibit the most divergence in amino acid sequence among family members, the linker or "hinge" region and the C terminus. Further specificity in S100-target protein interactions may arise from the different biochemical/biophysical properties of the individual family members, including affinity for metal ions (Ca(2+), Zn(2+), and Cu(2+)), oligomerization properties, heterodimerization, post-translational modifications, and lipid-binding. Delineation of the structural motifs that mediate S100-target protein interactions and determination of the in vivo relevance of these interactions are needed to fully understand the role of S100 proteins in normal and diseased cells.

Intracellular and extracellular roles of S100 proteins

S100, a multigenic family of non-ubiquitous Ca(2+)-modulated proteins of the EF-hand type expressed in vertebrates exclusively, has been implicated in intracellular and extracellular regulatory activities. Members of this protein family have been shown to interact with several effector proteins within cells thereby regulating enzyme activities, the dynamics of cytoskeleton constituents, cell growth and differentiation, and Ca(2+) homeostasis. Structural information indicates that most of S100 proteins exist in the form of antiparallelly packed homodimers (in some cases heterodimers), capable of functionally crossbridging two homologous or heterologous target proteins in a Ca(2+)-dependent (and, in some instances, Ca(2+)-independent) manner. In addition, extracellular roles have been described for several S100 members, although secretion (via an unknown mechanism) has been documented for a few of them. Extracellular S100 proteins have been shown to exert regulatory effects on inflammatory cells, neurons, astrocytes, microglia, and endothelial and epithelial cells, and a cell surface receptor, RAGE, has been identified as a potential S100A12 and S100B receptor transducing the effects of these two proteins on inflammatory cells and neurons. Other cell surface molecules with ability to interact with S100 members have been identified, suggesting that RAGE might not be a universal S100 protein receptor and/or that a single S100 protein might interact with more than one receptor. Collectively, these data indicate that members of the S100 protein family are multifunctional proteins implicated in the regulation of a variety of cellular activities.

Calcyclin (S100A6) regulates pulmonary fibroblast proliferation, morphology, and cytoskeletal organization in vitro

Calcyclin (S100A6) is a member of the S100A family of calcium binding proteins. While the precise function of calcyclin is unknown, calcyclin expression is associated with cell proliferation and calcyclin is expressed in several types of cancer phenotypes. In the present study, the functional role of calcyclin was further elucidated in pulmonary fibroblasts. Antisense S100A6 RNA expression inhibited serum and mechanical strain-induced fibroblast proliferation. This attenuated proliferative response was accompanied by a flattened, spread cell morphology, and disruption of tropomyosin labeled microfilaments. Changes in cytoskeletal organization did not correspond with a decrease in tropomyosin levels. These observations suggest a role for calcyclin in modulating calcium dependent signaling events that regulate progression through the cell cycle. J. Cell. Biochem. 88: 848-854, 2003.

The zinc- and calcium-binding S100B interacts and co-localizes with IQGAP1 during dynamic rearrangement of cell membranes

The Zn(2+)- and Ca(2+)-binding S100B protein is implicated in multiple intracellular and extracellular regulatory events. In glial cells, a relationship exists between cytoplasmic S100B accumulation and cell morphological changes. We have identified the IQGAP1 protein as the major cytoplasmic S100B target protein in different rat and human glial cell lines in the presence of Zn(2+) and Ca(2+). Zn(2+) binding to S100B is sufficient to promote interaction with IQGAP1. IQ motifs on IQGAP1 represent the minimal interaction sites for S100B. We also provide evidence that, in human astrocytoma cell lines, S100B co-localizes with IQGAP1 at the polarized leading edge and areas of membrane ruffling and that both proteins relocate in a Ca(2+)-dependent manner within newly formed vesicle-like structures. Our data identify IQGAP1 as a potential target protein of S100B during processes of dynamic rearrangement of cell membrane morphology. They also reveal an additional cellular function for IQGAP1 associated with Zn(2+)/Ca(2+)-dependent relocation of S100B.

Inhibition of neurite outgrowth by reduced level of NDRG4 protein in antisense transfected PC12 cells

NDRG4, a member of the new NDRG gene family, was originally cloned as a gene that was expressed predominantly in the early postnatal rat brain. To determine whether the NDRG4 protein contributes to differentiation of neural cells, the effect of lowering the cellular NDRG4 protein level on the nerve growth factor (NGF)-induced neurite formations and transcription factor activations in PC12 cells was examined. An antisense construct of rat NDRG4 cDNA was made and transfected to PC12 cells, which constitutively express a basal level of the NDRG4 protein. Of the stably transfected antisense cell clones that expressed exogenous NDRG4 antisense RNA, six clones showed reduced levels of the NDRG4 protein, but unexpectedly two clones showed quite higher levels of NDRG4 protein than the control cells. The clones having decreased levels of the NDRG4 protein extended shorter neurites than control cells in response to NGF or dibutyryl cAMP. In contrast, the NDRG4 protein-highly expressing clones did not show suppressed neurite outgrowth induced by NGF. NGF-mediated activation of the transcription factor AP-1 was found to be suppressed in the NDRG4 protein-diminished clone and enhanced in the NDRG4 protein-upregulated clone as compared with those in the control cells. These results suggest that NDRG4 plays a role in neurite outgrowth and has an influence on an NGF-stimulated AP-1 activation by an undefined mechanism in PC12 cells.

Three-dimensional solution structure of the calcium-signaling protein apo-S100A1 as determined by NMR

S100A1, a member of the S100 protein family, is an EF-hand containing Ca(2+)-binding protein (93 residues per subunit) with noncovalent interactions at its dimer interface. Each subunit of S100A1 has four alpha-helices and a small antiparallel beta-sheet consistent with two helix-loop-helix calcium-binding domains [Baldiserri et al. (1999) J. Biomol. NMR 14, 87-88]. In this study, the three-dimensional structure of reduced apo-S100A1 was determined by NMR spectroscopy using a total of 2220 NOE distance constraints, 258 dihedral angle constraints, and 168 backbone hydrogen bond constraints derived from a series of 2D, 3D, and 4D NMR experiments. The final structure was found to be globular and compact with the four helices in each subunit aligning to form a unicornate-type four-helix bundle. Intermolecular NOE correlations were observed between residues in helices 1 and 4 from one subunit to residues in helices 1' and 4' of the other subunit, respectively, consistent with the antiparallel alignment of the two subunits to form a symmetric X-type four-helix bundle as found for other members of the S100 protein family. Because of the similarity of the S100A1 dimer interface to that found for S100B, it was possible to calculate a model of the S100A1/B heterodimer. This model is consistent with a number of NMR chemical shift changes observed when S100A1 is titrated into a sample of (15)N-labeled S100B. Helix 3 (and 3') of S100A1 was found to have an interhelical angle of -150 degrees with helix 4 (and 4') in the apo state. This crossing angle is quite different (>50 degrees ) from that typically found in other EF-hand containing proteins such as apocalmodulin and apotroponin C but more similar to apo-S100B, which has an interhelical angle of -166 degrees. As with S100B, it is likely that the second EF-hand of apo-S100A1 reorients dramatically upon the addition of Ca(2+), which can explain the Ca(2+) dependence that S100A1 has for binding several of its biological targets.

S100: a multigenic family of calcium-modulated proteins of the EF-hand type with intracellular and extracellular functional roles

S100 is a multigenic family of non-ubiquitous Ca(2+)-modulated proteins of the EF-hand type expressed in vertebrates exclusively and implicated in intracellular and extracellular regulatory activities. Within cells, most of S100 members exist in the form of antiparallelly packed homodimers (in some cases heterodimers), capable of functionally crossbridging two homologous or heterologous target proteins in a Ca(2+)-dependent (and, in some instances, Ca(2+)-independent) manner. S100 oligomers can also form, under the non-reducing conditions found in the extracellular space and/or within cells upon changes in the cell redox status. Within cells, S100 proteins have been implicated in the regulation of protein phosphorylation, some enzyme activities, the dynamics of cytoskeleton components, transcription factors, Ca(2+) homeostasis, and cell proliferation and differentiation. Certain S100 members are released into the extracellular space by an unknown mechanism. Extracellular S100 proteins stimulate neuronal survival and/or differentiation and astrocyte proliferation, cause neuronal death via apoptosis, and stimulate (in some cases) or inhibit (in other cases) the activity of inflammatory cells. A cell surface receptor, RAGE, has been identified on inflammatory cells and neurons for S100A12 and S100B, which transduces S100A12 and S100B effects. It is not known whether RAGE is a universal S100 receptor, S100 members interact with other cell surface receptors, or S100 protein interaction with other extracellular factors specifies the biological effects of a given S100 protein on a target cell. The variety of intracellular target proteins of S100 proteins and, in some cases, of a single S100 protein, and the cell specificity of expression of certain S100 members suggest that these proteins might have a role in the fine regulation of effector proteins and/or specific steps of signaling pathways/cellular functions. Future analyses should discriminate between functionally relevant S100 interactions with target proteins and in vitro observations devoid of physiological importance.

Effects of S100A1 and S100B on microtubule stability. An in vitro study using triton-cytoskeletons from astrocyte and myoblast cell lines

S100A1 and S100B are members of a multigenic family of Ca(2+)-binding proteins of the EF-hand type highly abundant in astrocyte and striated muscle cells that have been implicated in the Ca(2+)-dependent regulation of several intracellular activities including the assembly and disassembly of microtubules and type III intermediate filaments. In the present work we tested S100A1 and S100B for their ability to cause microtubule and/or intermediate filament disassembly in situ using triton-cytoskeletons obtained from U251 glioma cells and rat L6 myoblasts. Our results indicate that: (i) both proteins cause a Ca(2+)-dependent disassembly of cytoplasmic microtubules in a dose-dependent manner; (ii) the S100A1- and S100B-inhibitory peptide, TRTK-12, blocks the S100A1 and S100B effects on microtubules; (iii) S100A1Delta88-93, an S100A1 mutant lacking the C-terminal extension, does not affect microtubule stability; and (iv) no obvious S100A1- or S100B-dependent intermediate filament disassembly could be observed under the experimental conditions used in the present study, but S100A1- and S100B-dependent microtubule disassembly results in a tendency of vimentin intermediate filaments to aggregate into bundles and/or to condense. Together, these results suggest that S100A1 and S100B probably cause microtubule disassembly by interacting with the microtubule wall, and that the two proteins do not affect intermediate filament stability via interaction with preformed intermediate filaments, in agreement with previous biochemical investigation. Our present data lend support to the possibility that S100A1 and S100B might have a role in the in vivo regulation of the state of assembly of microtubules in a Ca(2+)-regulated manner and, potentially, on microtubule-based activities in astrocytes and myoblasts. Also, these data suggest that the both S100 proteins use their C-terminal extension for interacting with microtubules.

Association between the calcium-binding protein calretinin and cytoskeletal components in the human colon adenocarcinoma cell line WiDr

Calretinin (CR) is a Ca(2+)-binding protein (CaBP) of the EF-hand family expressed in a cell-type-specific manner and thought to act as a Ca(2+) buffer. Based upon previous studies, CR can undergo Ca(2+)-induced conformational changes, suggesting that it may also belong to the subfamily of Ca(2+)-sensor proteins that are characterized by their ability to interact with target ligands. To elucidate the role of CR, we used the undifferentiated colon adenocarcinoma cell line WiDr, which expresses significant amounts of CR. It has been shown previously that combined treatment with an inducer of differentiation sodium butyrate (NaBt) and a cell growth inhibitor hexamethylene bisacetamide (HMBA) or treatment with CR antisense oligonucleotides is down-regulating CR in parallel with a decrease of cell growth, suggesting a possible involvement of CR in maintaining the undifferentiated phenotype of WiDr cells. Furthermore, CR is absent from normal colon cells and from well-differentiated colon adenocarcinoma cell lines (e.g., Caco-2). Since members of the EF-hand family of proteins are interacting with cytoskeletal components, we investigated the possible association of CR with the cytoskeleton in WiDr cells. With double immunofluorescence stainings and immunoprecipitation experiments, we show close association of CR with intermediate filaments or microtubules in WiDr cells. Treatment with NaBt either disrupted or strongly diminished this interaction, respectively. The same effect was observed after elevation of [Ca(2+)](i) by applying the ionophore A-23187. These data suggest that CR may contribute to the transformation of enterocytes by interfering with the differentiation process, i.e., acting at both levels: cell shape dynamics and mitosis.

A novel kinase, AATYK induces and promotes neuronal differentiation in a human neuroblastoma (SH-SY5Y) cell line

Apoptosis Associated Tyrosine Kinase (AATYK), a novel protein recently isolated from differentiating 32D mouse myeloid cells, contains a putative tyrosine kinase domain and several binding motifs for src homology 2 (SH-2) and src homology 3 (SH-3) domain containing proteins. We observed that AATYK is expressed in different regions of the brain. Although it might play a role in normal nervous system development by modulating apoptosis, little is known regarding its function in the brain or its intracellular localization and kinase activity. Recognizing its homology with Insulin like growth factor-I (IGF-I) receptor (IGF-IR) and the critical role of IGF-I in neuronal survival, we hypothesized that AATYK plays an important role in neuronal differentiation/apoptosis. To test this hypothesis, we transfected the human adrenergic neuroblastoma (NB):SH-SY5Y cells with AATYK cDNA under a tetracycline-repressible promoter and established stable cell lines that readily express AATYK on removal of tetracycline. AATYK immunoprecipitated from these cell lysates is an active kinase. Indirect immunofluorescent staining of the clones revealed AATYK to be localized in the cytoplasm. By itself, AATYK overexpression for short duration (2-3 days) did not induce differentiation in the stable SH-SY5Y clones. On the other hand, overexpression for longer periods (7-8 days) per se, significantly (P<0.05-0.001) increased the percent of differentiated cells as well as the neurite length. AATYK-induced differentiation was in the same range as the differentiation induced by agents like all-trans retinoic acid (RA), 12-O-Tetradecanoyl phorbol 13-acetate (TPA) and IGF-I. In addition, AATYK significantly promoted the neuronal differentiation induced by these agents. Our results demonstrate for the first time that AATYK is an active, non-receptor, cytosolic kinase which induces neuronal differentiation and also promotes differentiation induced by other agents in the SH-SY5Y cells.

Depletion of a fatty acid-binding protein impairs neurite outgrowth in PC12 cells

Epithelial fatty acid-binding protein (E-FABP) is up-regulated in rat dorsal root ganglia after sciatic nerve crush and in differentiating neurons during development. The present study investigates the role of E-FABP during nerve growth factor (NGF)-mediated neurite outgrowth in PC12 cells. Undifferentiated PC12 cells express low levels of E-FABP, while NGF triggers a 6- and 8-fold induction of E-FABP mRNA and protein, respectively. Up-regulation of E-FABP mRNA occurs as early as 24 h after NGF treatment and remains highly expressed over the course of several days, corresponding to NGF-mediated neurite outgrowth. Withdrawal of NGF leads to down-regulation of E-FABP mRNA and retraction of neurites. Immunofluorescence microscopy reveals E-FABP immunoreactivity in the perinuclear cytoplasm, neurites and growth cones of NGF-differentiated cells. To examine the role of E-FABP during neurite outgrowth, PC12 cells were transfected with a constitutive antisense E-FABP vector to create the E-FABP-deficient line PC12-AS. By morphometric analysis, PC12-AS cells treated for 2, 4, and 7 days with NGF exhibited significantly decreased neurite expression relative to control (mock-transfected) cells. Taken together, these data indicate that E-FABP is important in normal NGF-mediated neurite outgrowth in PC12 cells, a finding that is consistent with a potential role in axonal development and regeneration.

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.

S100A1 utilizes different mechanisms for interacting with calcium-dependent and calcium-independent target proteins

While previous studies have identified target proteins that interact with S100A1 in a calcium-dependent manner as well as target proteins that interact in a calcium-independent manner, the molecular mechanisms of S100A1-target protein interaction have not been elucidated. In this study, point and deletion mutants of S100A1 were used to investigate the contribution of carboxyl terminal amino acids to S100A1 interaction with calcium-dependent and calcium-independent target proteins. First, a recombinant rat S100A1 protein (recS100A1) expressed in bacteria exhibited physical and chemical properties indistinguishable from native S100A1. Next, proteins lacking the carboxyl-terminal nine residues of recS100A1 (Delta85-93), or containing alanine substitutions at Phe 88 (F88A), Phe 89 (F89A), or Trp 90 (W90A), both Phe 88 and Phe 89 (F88/89A), or all three aromatic residues (F88/89A-W90A) were recombinantly expressed. Like recS100A1, F88A, F89A, and W90A proteins interacted with phenyl-Sepharose in a calcium-dependent manner. However, the Delta85-93 protein did not interact with phenyl-Sepharose, indicating that a phenyl-Sepharose-binding region (PSBR) of recS100A1 had been disrupted. The F88/89A and F88/89A-W90A proteins exhibited reduced calcium-dependent interaction with phenyl-Sepharose when compared with recS100A1, demonstrating that the carboxyl-terminal aromatic residues Phe 88, Phe 89, and Trp 90 comprise the PSBR of S100A1. Fluorescence studies showed that the Delta85-93 protein exhibited reduced calcium-dependent interaction with the dodecyl CapZ peptide, TRTK, while W90A bound TRTK with a Kd of 5.55 microM. These results demonstrate that the calcium-dependent target protein-binding site and the PSBR are indistinguishable. In contrast to the calcium-dependent target TRTK, activation of the calcium-independent target protein aldolase A by the point and deletion mutant S100A1s was indistinguishable from native S100A1. These results demonstrate that carboxyl-terminal residues are not required for S100A1 modulation of calcium-independent target protein aldolase A. Alltogether, these results indicate that S100A1 utilizes distinct mechanisms for interaction with calcium-independent and calcium-dependent target proteins.

Calcium regulation of Ndr protein kinase mediated by S100 calcium-binding proteins

Ndr is a nuclear serine/threonine protein kinase that belongs to a subfamily of kinases identified as being critical for the regulation of cell division and cell morphology. The regulatory mechanisms that control Ndr activity have not been characterized previously. In this paper, we present evidence that Ndr is regulated by EF-hand calcium-binding proteins of the S100 family, in response to changes in the intracellular calcium concentration. In vitro, S100B binds directly to and activates Ndr in a Ca2+-dependent manner. Moreover, Ndr is recovered from cell lysates in anti-S100B immunoprecipitates. The region of Ndr responsible for interaction with Ca2+/S100B is a basic/hydrophobic motif within the N-terminal regulatory domain of Ndr, and activation of Ndr by Ca2+/S100B is inhibited by a synthetic peptide derived from this region. In cultured cells, Ndr is rapidly activated following treatment with Ca2+ ionophore, and this activation is dependent upon the identified Ca2+/S100B-binding domain. Finally, Ndr activity is inhibited by W-7 in melanoma cells overexpressing S100B, but is unaffected by W-7 in melanoma cells that lack S100B. These results suggest that Ndr is regulated at least in part by changes in the intracellular calcium concentration, through binding of S100 proteins to its N-terminal regulatory domain.