Protein hydration during generation of coagulation factor Xa in aqueous phase and on phospholipid membranes

Age-dependent changes in brain hydration and synaptic plasticity

Aging in humans and animals is associated with gradual and variable changes in some cognitive functions, but what causes them and explains individual variations remains unclear. Hydration decreases with aging but whether dehydration contributes to cognitive dysfunction is not known. The brain hydration of aging mice was determined by colloidosmotic-pressure titration. Dehydration increased with age from ∼76 mmHg at 6 weeks to ∼105 mmHg at 40 weeks, or a progressive ∼10 percent loss of brain water but seemed to level off afterward. When we adjusted dehydration in hippocampal slices of <8-week-old mice to the levels seen in mice 40 weeks and older, their basal synaptic responses were amplified at all stimulus voltages tested, but induction of late-phase long-term potentiation was impaired. Our results document progressive brain dehydration with age in inbred mice to levels at which in vitro synaptic plasticity appears dysregulated. They also suggest that dehydration contributes to some of the changes in synaptic plasticity observed with aging, possibly due to adjustments in neuronal excitation mechanisms.

Swelling and pressure-volume relationships in the dermis measured by osmotic-stress technique

Water transfer across the extracellular matrix (ECM) involves interstitial osmotic forces in as yet unclear ways. In particular, the traditional values of Starling forces cannot adequately explain fluid transfer rates. Here, we reassess these forces by analyzing fluid transfer in live pig and human dermal explants. Pressure potentials were controlled with inert polymers adjusted by membrane osmometry (range = 3-219 mmHg), and fluid transfer in and out of the explants was followed by sequential precision weighing. Water motional freedom in the dermis was examined by NMR. In pigs, mean hydration pressure (HP; the pressure at which volume did not change) was 107 +/- 22 and 47 +/- 12 (SE) mmHg at 4 degrees C and 37 degrees C (P = 0.012, paired t-test, n = 7). Volume changes observed in response to pressure potential were reversible. The equation, Volume change = V(max)/[1+(time/T(1/2))(d)], where V(max) is maximal volume change; T(1/2), time at volume = 1/2 V(max); and d, a rate parameter, was fitted to experimental progression curves (r(2) > 0.9), yielding V(max) values linearly related to pressure, with mean slopes -3.5 +/- 0.28 and -2.6 +/- 0.21(SE) mul.g(-1).mmHg(-1) at 4 degrees C and 37 degrees C. NMR spin-spin relaxation times (T(2)) varied within 200- to 400-mum distances in directions perpendicular to the epidermis, with slopes reaching 0.03 ms/mum. Results support a mechanism in which fluid transport across the ECM is locally regulated at micrometer scales by cell- and fiber-gel-dependent osmomechanical forces. The large HP helps to explain the fast interstitial in/out flow rates observed clinically.

Osmotically induced helix-coil transition in poly(glutamic acid)

Protein folding and conformational changes are influenced by protein-water interactions and, as such, the energetics of protein function are necessarily linked to water activity. Here, we have chosen the helix-coil transition in poly(glutamic acid) as a model system to investigate the importance of hydration to protein structure by using the osmotic stress method combined with circular dichroism spectroscopy. Osmotic stress is applied using poly(ethylene glycol), molecular weight of 400, as the osmolyte. The energetics of the helix-coil transition under applied osmotic stress allows us to calculate the change in the number of preferentially included water molecules per residue accompanying the thermally induced conformational change. We find that osmotic stress raises the helix-coil transition temperature by favoring the more compact alpha-helical state over the more hydrated coil state. The contribution of other forces to alpha-helix stability also are explored by varying pH and studying a random copolymer, poly(glutamic acid-r-alanine). In this article, we clearly show the influence of osmotic pressure on the peptide folding equilibrium. Our results suggest that to study protein folding in vitro, the osmotic pressure, in addition to pH and salt concentration, should be controlled to better approximate the crowded environment inside cells.

Osmotic stress regulates the anticoagulant efficiency of dermatan sulfate

Glycosaminoglycans (GAGs) in pericellular and interstitial spaces help to maintain local water homeostasis and blood coagulation balance. This study explored whether dehydrating microenvironment conditions influence dermatan sulfate's (DS) anticoagulant activity. Water transfer during antithrombin activation by dermatan sulfate was measured using osmotic stress techniques. Anticoagulant activity was determined from the change in the rate of coagulation factor Xa (fXa) inhibition. Osmotic stress accelerated reaction rates, indicating water transfer from reactants to bulk. The net volume transferred, measured using osmotic probes similar in size to the reacting proteins, was approximately 2500 mol of water per mole of fXa inhibited. The reaction efficiency, V(sat)/K 1/2 (rate at saturation/concentration resulting in half-maximal rates), determined in titrations with monosulfated dermatan sulfate and disulfated dermatan sulfate (DDS), were 4x10(4) and 2x10(5) M-1 s-1 under osmotic stress and in the presence of calcium, corresponding to 34- and 81-fold increases over efficiency measured under standard conditions. These results indicate that dermatan sulfate can contribute significantly to antithrombin activation, and that in dehydrating environments and depending of ionic conditions, its anticoagulant efficiency can exceed that of heparan sulfate (HS).

Chondroitin sulfate anticoagulant activity is linked to water transfer: relevance to proteoglycan structure in atherosclerosis

OBJECTIVE

Changes in chondroitin sulfate (CS) proteoglycan (PG) during atherosclerosis are associated with chronic inflammatory changes and increased incidence of thrombosis. To explore how glycosaminoglycan changes could influence the thrombogenicity of atherosclerotic lesions, water-transfer reactions were examined during activation of antithrombin by CS.

METHODS AND RESULTS

Advanced type IV atherosclerotic lesions prone to thrombosis contained CSPG (versican) with undersulfated CS relative to CS of the adjacent healthy aorta. Approximately 11% of the CS disaccharide in versican from healthy arteries was oversulfated, but this proportion decreased markedly to 3% in atherosclerotic lesions. Oversulfated CS functionally bound antithrombin with a dissociation constant of 3.3+/-1.9 micromol/L. Measured by osmotic stress (OS) techniques with an approximately 26-A probe, the reaction was linked to transfer of approximately 2500 mol water per mole of coagulation factor Xa inhibited. Under OS, the anticoagulant efficiency of CS was 1.3 (micromol/L)(-1) x s(-1), approximately 5- and 15-fold higher than heparan sulfate efficiency measured under OS and standard conditions, respectively.

CONCLUSIONS

Decreased sulfation of high molecular weight CSPG in the advancing atherosclerotic lesions may predispose the lesions to thrombosis by disrupting osmotic regulation, limiting avidity for antithrombin and decreasing activation efficiency.

Regulation of glycosaminoglycan function by osmotic potentials. Measurement of water transfer during antithrombin activation by heparin

The sulfated glycosaminoglycan heparin is an important anticoagulant, widely used to treat and to prevent arterial thrombosis. Heparin triggers conformational changes in, and the functional activation of, the serine proteinase inhibitor antithrombin. We investigated water-transfer reactions during the activation process to explore the possibility that functional interaction between antithrombin and sulfated glycosaminoglycans can be regulated by osmotic potentials. Volume of water transferred upon heparin binding was measured from differences in free energy change, Delta(Delta G), with osmotic stress, pi. Osmotic stress was induced with chemically inert probes that are geometrically excluded from the water-permeable spaces of antithrombin and from intermolecular spaces formed during the association reaction. The free energy change, Delta G, for the antithrombin/heparin interaction was calculated from the dissociation constant, determined by functional titrations of heparin with antithrombin at fixed concentrations of the coagulation protease factor Xa. The effect of osmotic stress was independent of the chemical nature of osmotic probes but correlated with their radius up to radius >17 A. In mixtures including a large and a small probe, the effect of the large probe was not modified by the small probe added at a large molar excess. With an osmotic probe of 4-A radius, the Delta(Delta G)/pi slope corresponds to a transfer of 119 +/- 25 water molecules to bulk solution on formation of the complex. Analytical characterization of water-permeable volumes in x-ray-derived bound and free antithrombin structures revealed complex surfaces with smaller hydration volumes in the bound relative to the free conformation. The residue distribution in, and atomic composition of, the pockets containing atoms from residues implicated in heparin binding were distinct in the bound versus free conformer. The results demonstrate that the heparin/antithrombin interaction is linked to net water transfer and, therefore, can be regulated in biological gels by osmotic potentials.

Hydration structure of antithrombin conformers and water transfer during reactive loop insertion

The serine protease inhibitor antithrombin undergoes extensive conformational changes during functional interaction with its target proteases. Changes include insertion of the reactive loop region into a beta-sheet structure in the protein core. We explore the possibility that these changes are linked to water transfer. Volumes of water transferred during inhibition of coagulation factor Xa are compared to water-permeable volumes in the x-ray structure of two different antithrombin conformers. In one conformer, the reactive loop is largely exposed to solvent, and in the other, the loop is inserted. Hydration fingerprints of antithrombin (that is, water-permeable pockets) are analyzed to determine their location, volume, and size of access pores, using alpha shape-based methods from computational geometry. Water transfer during reactions is calculated from changes in rate with osmotic pressure. Hydration fingerprints prove markedly different in the two conformers. There is an excess of 61-76 water molecules in loop-exposed as compared to loop-inserted conformers. Quantitatively, rate increases with osmotic pressure are consistent with the transfer of 73 +/- 7 water molecules. This study demonstrates that conformational changes of antithrombin, including loop insertion, are linked to water transfer from antithrombin to bulk solution. It also illustrates the combined use of osmotic stress and analytical geometry as a new and effective tool for structure/function studies.

Large-scale preparation of human thrombin: polyethylene glycol potentiates the factor Xa-mediated activation of prothrombin

We investigated the ability of polyethylene glycol 4000 to accelerate thrombin generation in a mixture of prothrombin and factor X at concentrations of 1-30%. In the presence of 5 mM of CaCl2, polyethylene glycol 4000 promoted prothrombin activation at concentrations above 1%. The peak of activation was seen at levels of 14 and 20% of polyethylene glycol 4000. The effect of the polyethylene glycol was remarkably dependent on its molecular weight; molecular weights greater than 2000 were required for accelerating thrombin generation. Under optimal conditions, polyethylene glycol 4000, in the presence of CaCl2, promoted conversion of all of the prothrombin into thrombin and its derivatives. We conclude that polyethylene glycol 4000, at concentrations ranging from 14 to 20%, effectively accelerates thrombin generation in the presence of 5 mM of CaCl2. This new method for preparing thrombin is based on the use of polyethylene glycol 4000 and CaCl2 and is applicable to the manufacture of thrombin.

Effective electrostatic charge of coagulation factor X in solution and on phospholipid membranes: implications for activation mechanisms and structure-function relationships of the Gla domain

Electrostatic interactions during activation of coagulation factor X were analysed by comparing effects of ionic strength on reaction rates with predictions of classical electrostatic theory. Geometrical correlations were investigated using alpha-shape-based computations on the crystal structure of Ca-fragment 1 of prothrombin. The ionic strength of the reaction environment was controlled with different univalent salts including NaCl, KCl, CsCl, LiCl, NaI, NaBr and KI. Reactions were assembled in three different environments: aqueous phase, cell membranes and synthetic TF/PS/PC (tissue factor relipidated in 30% phosphatidylserine, 70% phosphatidylcholine) vesicles. Reaction rates were measured at pH 7. 2, 4 mM CaCl2 and 33 degrees C, using chromogenic substrate to follow factor Xa generation. Rates decreased with increasing concentration of univalent salt, and the magnitude of the decrease was independent of salt type. On the basis of electrostatic relationships on PS/PC vesicles, the effective charge on factor X was +1.5, and the PS/factor X stoichiometry was 2.28. Structural analysis of the gamma-carboxyglutamic acid (Gla) domain revealed three surface pockets, forming potential sites for Ca2+ binding, with distinct spatial orientations. Interpreted together, the results of the geometric analysis and the measured effective charges suggest an efficient electrostatic mechanism for capture and retention of substrates by procoagulant membranes. Non-specific and delocalized interaction between the membrane and each one of the charged facets of the Gla domain can increase the probability of substrate binding, while allowing rotational and translational mobility of substrate for specific interaction with the enzyme.