The effect of S-100a and S-100b proteins and Zn2+ on the assembly of brain microtubule proteins in vitro

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.

Cajal and brain plasticity: insights relevant to emerging concepts of mind

The main legacies of Cajal are his drawings of brain structure and their connections, and his ideas of brain plasticity, not only in the mature brain but also during development and after brain injury. As the 21st century begins, many scientists are asking an old question: "how does the brain express the mind?" Although most models of mind incorporate the brain connections produced by Cajal, his ideas of plasticity are largely ignored. The purpose of this chapter is to review how some of Cajal's ideas can be useful in understanding the expression of the mind. I have also introduced several concepts and facts not available during Cajal's life. I cover the concept of homeostasis, the global projections of the monoamine neurons, and the actions of "mind-expanding" drugs. The global projecting neurons, because their monoamine transmitters have such a long history, are considered 1st order systems. The point-to-point connections are considered 2nd order systems. Their importance in theories of functional localization studies is briefly reviewed. Finally, a new model is presented called "Plastic Homeostasis," which incorporates the plastic interactions between 1st and 2nd order neurons. It is hoped that this review will encourage others to study the ideas presented by Cajal when considering functions of the brain. The emerging models of the mind would be well served by a review of the theoretical writing of Cajal.

Cajal's hypotheses on neurobiones and neurotropic factor match properties of microtubules and S-100 beta

Cajal described both the morphology and plasticity of neurons. He summarized the structure of neurons as composed of membrane, protoplasm, Golgi apparatus, nucleus, spongioplasm and neurofibrils (cytoskeleton). He initially considered the cytoskeleton as absorbing excitation energy and forming a "conductive pathway in the protoplasm" within the neuron. Later, he viewed the neurofibrillary threads as independent, living entities and called them neurobiones. Cajal recognized neuroplasticity in development, memory, sleep, injury and dementia, as well as after exposure to cold and starvation. He noted cytoskeletal changes during these events. However, he did not causatively connect the plastic changes in neurons with the changes in cytoskeleton. Finally, Cajal proposed a theory of chemoaffinity in 1892, and modified his neurotropic theory over the next 40 years. Today we accept that changes in the cytoskeleton produce changes in neuronal morphology. The properties of the cytoskeleton and neurobione as described by Cajal are similar to those of microtubules. These long intraneuronal neurofibrils are polymers of the protein tubulin and, whilst not being living entities, are highly dynamic, sensitive to environmental stimuli, and stabilized by microtubule associated proteins (MAPs). Furthermore, Cajal was very specific in his characterization of the neurotropic factor derived from Schwann cells. Initially, he thought the chemicals attracted the axonal fibers, but later he wrote that the factor was not attractant but rather was involved in assimilation, growth and ramifications. The neurotropic hypothesis described by Cajal in Degeneration and Regeneration in the Nervous System is more similar to a neurite extension factor (NEF) than to a neurotrophic growth factor or specific chemoaffinity (attractant) molecule. S-100 beta is the major NEF found in PNS Schwann cells and CNS astroglial cells. In summary, the views of Cajal on neuroplasticity, its frequency and function, agree with the modern hypothesis of neuronal instability. This concept states that MAPs regulate microtubule stability by a S-100 beta sensitive phosphorylation processes. Serotonin, by acting on the astroglial 5-HT1A receptor, releases S-100 beta and regulates neuronal morphology and apoptosis. This neuronal-glial connection provides a fresh view for linking neuroplasticity, mental illness, and memory with changes in the cytoskeleton.

Modulation of ATPase activities in the central nervous system by the S-100 proteins

The isomeric forms of bovine S-100a and S-100b have been shown to stimulate ATPase activities in fractions enriched in myelin and mitochondria isolated from the Gerbil brain and for S-100b more effectively than for calmodulin in erythrocytes or skeletal muscle. In the presence of Ca2+, S-100a produced a slight increase of ATPase activity in the mitochondrial fraction. However, S-100b in the presence of Zn2+ almost doubled the ATPase activity in brain myelin. S-100a, or S-100b, with or without Ca2+ and Zn2+ respectively, had no effect on the ATPase activity in mitochondria of the Gerbil liver. The observations may indicate a "second messenger" role for S-100b in the presence of Zn2+ in the Schwann cell.

Isolation, characterization and metal-ion-binding properties of the alpha-subunit from S-100a protein

The brain-specific S-100 protein is a mixture of two predominant components, S-100a and S-100b, with subunit compositions of alpha beta and beta beta respectively. In the present study, the alpha-subunit, isolated from S-100a by using anion-exchange chromatography in the presence of 8 M-urea, was homogeneous by the criteria of SDS/polyacrylamide-gel, urea/SDS/polyacrylamide-gel and non-SDS/polyacrylamide-gel electrophoresis. The alpha-subunit underwent a conformational change upon binding Ca2+ and Zn2+ at pH 7.5, as revealed by u.v. difference spectroscopy, c.d. and fluorescence measurements. Far-u.v. c.d. studies indicated the apparent alpha-helical content to fall when the protein bound either Ca2+ or Zn2+. Addition of Ca2+ to the alpha-subunit resulted in exposing to the solvent the single tryptophan residue and one or more tyrosine and phenylalanine residues. Zn2+ induced only a small conformational change, and among the aromatic chromophores only tyrosine residues were affected to a small extent. Ca2+ was able to bind to the alpha-subunit in the presence of Zn2+, and the two metal-ion-binding sites appeared to be different. When the apoprotein was excited at 280 nm, the fluorescence emission maximum was located at 337 nm. In the presence of Ca2+, the emission maximum occurred at 340 nm and was accompanied by a nearly 25% increase in fluorescence intensity. Fluorescence titration with Ca2+ at pH 7.5 revealed only one class of binding site, with a Kd value of 1.26 X 10(-4) M. The effect of K+ on the protein was slightly antagonistic to that of Ca2+, as indicated by u.v. difference spectroscopy and fluorescence titration.

S-100 proteins

S-100 is a group of closely related, small, acidic Ca2+-binding proteins (S-100a0, S-100a and S-100b, which are alpha alpha, alpha beta, and beta beta in composition, respectively). S-100 is structurally related to calmodulin and other Ca2+-binding proteins. S-100 is abundant in the brain and is contained in well defined cell types of both neuroectodermal and non-neuroectodermal origin, as well as in their neoplastic counterparts. In the mammalian brain, S-100a and S-100b are confined to glial cells, while S-100a0 is neuronal in localization. Single S-100 isoforms bind Ca2+ with nearly the same affinity. K+ antagonizes the binding of Ca2+ to high affinity sites on S-100. S-100 binds Zn2+ with high affinity. S-100 is found in a soluble and a membrane-bound form and has the ability to interact with artificial and natural membranes. S-100 has no enzymatic activity. S-100 has been involved in several activities including memory processes, regulation of diffusion of monovalent cations across membranes, modulation of the physical state of membranes, regulation of the phosphorylation of several proteins, control of the assembly-disassembly of microtubules. Some of these effects are strictly Ca2+-dependent, while other are not. S-100 is being secreted or released to the extracellular space. In some cases, this event is hormonally regulated. Several S-100 binding proteins are being described.

Evidence that S100 proteins regulate microtubule assembly and stability in rat brain extracts

Microtubule re-assembly in rat brain extracts was inhibited by antibodies to S100 proteins. Anti-S100 antibodies caused an increase in the cold-stability of microtubules and this effect was abolished by the presence of short lengths of microtubules formed under control conditions. Anti-S100 antibodies had no effect on the stimulation of assembly or the increase in microtubule stability caused by low zinc concentrations. Addition of exogenous S100a and S100b to brain extracts had different effects on assembly; S100a caused an inhibition of assembly while S100b stimulated the early phase of assembly. The data suggest that endogenous S100b is involved in the regulation of microtubule assembly in brain extracts.

S-100 proteins and microtubules: analysis of the effects of rat brain S-100 (S-100b) and ox brain S-100a0, S-100a and S-100b on microtubule assembly-disassembly

Rat brain S-100 (S-100b) and ox brain S-100a0, S-100a and S-100b have been tested for their ability to control the assembly and disassembly of brain microtubule proteins in the presence of either Ca2+ or Zn2+, in vitro. In the presence of Ca2+, single S-100 isoforms have similar, if not identical, effects, i.e. they inhibit assembly and promote disassembly. In the presence of Zn2+ from 0.1 to 1 mM (free concentration), rat S-100 and ox S-100a and S-100b inhibit assembly, while S-100a0 is without effect. These data are briefly discussed in relation to the cellular localization of single S-100 isoforms in the brain.

Rat brain S100b protein: purification, characterization, and ion binding properties. A comparison with bovine S100b protein

We purified to homogeneity rat brain S100b protein, which constitutes about 90% of the soluble S100 protein fraction. Purified rat S100b protein comigrates with bovine S100b protein in nondenaturant system electrophoresis but differs in its amino acid composition and in its electrophoretic mobility in urea-sodium dodecyl sulfate-polyacrylamide gel with bovine S100b protein. The properties of the Ca2+ and Zn2+ binding sites on rat S100b protein were investigated by flow dialysis and by fluorometric titration, and the conformation of rat S100b in its metal-free form as well as in the presence of Ca2+ or Zn2+ was studied. The results were compared with those obtained for the bovine S100b protein. In the absence of KCl, rat brain S100b protein is characterized by two high-affinity Ca2+ binding sites with a KD of 2 X 10(-5) M and four lower affinity sites with KD about 10(-4) M. The calcium binding properties of rat S100b protein differ from bovine S100b only by the number of low-affinity calcium binding sites whereas similar Ca2+-induced conformational changes were observed for both proteins. In the presence of 120 mM KCl rat brain S100b protein bound two Zn2+-ions/mol of protein with a KD of 10(-7) M and four other with lower affinity (KD approximately equal to 10(-6) M). The occupancy of the two high-affinity Zn2+ binding sites was responsible for most of the Zn2+-induced conformational changes in the rat S100b protein. No increase in the tyrosine fluorescence quantum yield after Zn2+ binding to rat S100b was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification, characterization and ion binding properties of human brain S100b protein

Human brain S100b (beta beta) protein was purified using zinc-dependent affinity chromatography on phenyl-Sepharose. The calcium- and zinc-binding properties of the protein were studied by flow dialysis technique and the protein conformation both in the metal-free form and in the presence of Ca2+ or Zn2+ was investigated with ultraviolet spectroscopy, sulfhydryl reactivity and interaction with a hydrophobic fluorescence probe 6-(p-toluidino)naphthalene-2-sulfonic acid (TNS). Flow dialysis measurements of Ca2+ binding to human brain S100b (beta beta) protein revealed six Ca2+-binding sites which we assumed to represent three for each beta monomer, characterized by the macroscopic association constants K1 = 0.44 X 10(5) M-1; K2 = 0.1 X 10(5) M-1 and K3 = 0.08 X 10(5) M-1. In the presence of 120 mM KCl, the affinity of the protein for calcium is drastically reduced. Zinc-binding studies on human S100b protein showed that the protein bound two zinc ions per beta monomer, with macroscopic constants K1 = 4.47 X 10(7) M-1 and K2 = 0.1 X 10(7) M-1. Most of the Zn2+-induced conformational changes occurred after the binding of two zinc ions per mole of S100b protein. These results differ significantly from those for bovine protein and cast doubt on the conservation of the S100 structure during evolution. When calcium binding was studied in the presence of zinc, we noted an increase in the affinity of the protein for calcium, K1 = 4.4 X 10(5) M-1; K2 = 0.57 X 10(5) M-1; K3 = 0.023 X 10(5) M-1. These results indicated that the Ca2+- and Zn2+-binding sites on S100b protein are different and suggest that Zn2+ may regulate Ca2+ binding by increasing the affinity of the protein for calcium.