An electrostatic model for collagen fibrils. The interaction of reconstituted collagen with Ca++, Na+, and Cl-

Assembled collagen films modified using polyacrylic acid with improved mechanical properties via mineralization

The imperative task of enforcing collagen materials holds paramount significance in the field of hard tissue repair. We hereby present mineralized collagen fiber films via mineralization with improved mechanical properties. Self-extracted collagen was assembled into an array with an aligned fibrous pattern and then modified with polyacrylic acid (PAA) followed by mineralization in cationic polyacrylamide (CPAM)-SBF. Biomineralization occurred at the inner and outer surface of the assembled collagen fiber films. A tensile strength of up to 40.38 ± 3.08 MPa of mineralized collagen was obtained, for the first time, which may be attributed to the synergistic effect of polyanion and polycation on the mineralization process of assembled intrafibrillar collagen fibers. It was argued that PAA may facilitate the intra-fiber interaction of collagen, which extends the elongation at break of collagen fibers. This study introduces a pioneering approach for the preparation of mineralized collagen materials with superior mechanical properties, which would be beneficial for hard tissue repair.

Modelling intramuscular drug fate in vitro with tissue-relevant biomimetic hydrogels

Parenteral administrations are a mainstay of clinical drug delivery. Intramuscular (IM) injections deposit drug directly into skeletal muscle bellies, providing rapid systemic uptake due to the highly vascularized nature of this site. The potential to inject particulate or non-aqueous materials have also made IM injections useful for long-acting formulations. These attributes have supported a plethora of medicines being approved for IM administration. Despite these many approvals across multiple pharmaceutical categories, mechanisms that control drug release from the injection site, and thus its pharmacokinetic properties, remain poorly understood. Several pre-clinical in vivo animals have been used to model IM drug fate in patients, but these approaches have not consistently predicted clinical outcomes. This lack of a predictive in vivo model and no standardized in vitro tools have limited the options of pharmaceutical scientists to rationally design formulations for IM delivery. Here, we describe a novel, tractable in vitro model informed by dominant extracellular matrix (ECM) components present at the IM injection site. Three charge variants of green florescent protein (GFP) and the impact of three common formulation components were examined in an initial test of this in vitro model. A strongly positively charged GFP was restricted in its release from hydrogels composed of ECM components type I collagen and hyaluronic acid compared to standard and strongly negatively charged GFP. Introduction of commonly used buffers (histidine or acetate) or the non-ionic surfactant polysorbate 20 altered the release properties of these GFP variants in a manner that was dependent upon ECM element composition. In sum, this Simulator of IntraMuscular Injections, termed SIMI, demonstrated distinct release profiles of a protein biopharmaceutical surrogate that could be exploited to interrogate the impact of formulation components to expedite novel drug development and reduce current dependence on potentially non-predictive pre-clinical in vivo models.

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An initial in vitro format to model drug release from the intramuscular (IM) injection site release parameters is described.

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Mixtures of collagen type 1 (Col1) and hyaluronic acid within a semi-permeable chamber were tested.

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Green fluorescent proteins with varied charge profiles were used to model different biopharmaceutical properties.

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A Col1-dominated hydrogel format provided an initial validation of this in vitro IM injection site approach

Simulate SubQ: The Methods and the Media

For decades, there has been a growing interest in injectable subcutaneous formulations to improve the absorption of drugs into the systemic circulation and to prolong their release over a longer period. However, fluctuations in the blood plasma levels together with bioavailability issues often limit their clinical success. This warrants a closer look at the performance of long-acting depots, for example, and their dependence on the complex interplay between the dosage form and the physiological microenvironment. For this, biopredictive performance testing is used for a thorough understanding of the biophysical processes affecting the absorption of compounds from the injection site in vivo and their simulation in vitro. In the present work, we discuss in vitro methodologies including methods and media developed for the subcutaneous route of administration on the background of the most relevant absorption mechanisms. Also, we highlight some important knowledge gaps and shortcomings of the existing methodologies to provide the reader with a better understanding of the scientific evidence underlying these models.

Evaluating parameters affecting drug fate at the intramuscular injection site

Intramuscular (IM) injections are a well-established method of delivering a variety of therapeutics formulated for parenteral administration. While the wide range of commercial IM pharmaceuticals provide a wealth of pharmacokinetic (PK) information following injection, there remains an inadequate understanding of drug fate at the IM injection site that could dictate these PK outcomes. An improved understanding of injection site events could improve approaches taken by formulation scientists to identify therapeutically effective and consistent drug PK outcomes. Interplay between the typically non-physiological aspects of drug formulations and the homeostatic IM environment may provide insights into the fate of drugs at the IM injection site, leading to predictions of how a drug will behave post-injection in vivo. Immune responses occur by design after e.g. vaccine administration, however immune responses post-injection are not in the scope of this article. Taking cues from existing in vitro modelling technologies, the purpose of this article is to propose "critical parameters" of the IM environment that could be examined in hypothesis-driven studies. Outcomes of such studies might ultimately be useful in predicting and improving in vivo PK performance of IM injected drugs.

Electric Factors in Wound Healing

Significance: Electric factors such as electric charges, electrodynamic field, skin battery, and interstitial exclusion permeate wound healing physiology and physiopathology from injury to re-epithelialization. The understanding of how electric factors contribute to wound healing and how treatments may interfere with them is fundamental for the development of better strategies for the management of pathological scarring and chronic wounds. Recent Advances: Angiogenesis, cell migration, macrophage activation hemorheology, and microcirculation can interfere and be interfered with electric factors. New treatments with various types of electric currents, laser, light emitting diode, acupuncture, and weak electric fields applied directly on the wound have been developed to improve wound healing. Critical Issues: Despite the basic and clinical development, pathological scars such as keloids and chronic wounds are still a challenge. Future Directions: New treatments can be developed to improve skin wound healing taking into account the influence of electrical charges. Monitoring electrical activity during skin healing and the influence of treatments on hemorheology and microcirculation are examples of how to use knowledge of electrical factors to increase their effectiveness.

Biomineralization of Collagen-Based Materials for Hard Tissue Repair

Hydroxyapatite (HA) reinforced collagen fibrils serve as the basic building blocks of natural bone and dentin. Mineralization of collagen fibrils play an essential role in ensuring the structural and mechanical functionalities of hard tissues such as bone and dentin. Biomineralization of collagen can be divided into intrafibrillar and extrafibrillar mineralization in terms of HA distribution relative to collagen fibrils. Intrafibrillar mineralization is termed when HA minerals are incorporated within the gap zone of collagen fibrils, while extrafibrillar mineralization refers to the minerals that are formed on the surface of collagen fibrils. However, the mechanisms resulting in these two types of mineralization still remain debatable. In this review, the evolution of both classical and non-classical biomineralization theories is summarized. Different intrafibrillar mineralization mechanisms, including polymer induced liquid precursor (PILP), capillary action, electrostatic attraction, size exclusion, Gibbs-Donnan equilibrium, and interfacial energy guided theories, are discussed. Exemplary strategies to induce biomimetic intrafibrillar mineralization using non-collagenous proteins (NCPs), polymer analogs, small molecules, and fluidic shear stress are discussed, and recent applications of mineralized collagen fibers for bone regeneration and dentin repair are included. Finally, conclusions are drawn on these proposed mechanisms, and the future trend of collagen-based materials for bone regeneration and tooth repair is speculated.

Doping bioactive elements into a collagen scaffold based on synchronous self-assembly/mineralization for bone tissue engineering

Pure collagen is biocompatible but lacks inherent osteoinductive, osteoimmunomodulatory and antibacterial activities. To obtain collagen with these characteristics, we developed a novel methodology of doping bioactive elements into collagen through the synchronous self-assembly/mineralization (SSM) of collagen. In the SSM model, amorphous mineral nanoparticles (AMN) (amorphous SrCO3, amorphous Ag3PO4, etc.) stabilized by the polyampholyte, carboxymethyl chitosan (CMC), and collagen molecules were the primary components under acidic conditions. As the pH gradually increased, intrafibrillar mineralization occurred via the self-adaptive interaction between the AMNs and the collagen microfibrils, which were self-assembling; the AMNs wrapped around the microfibrils became situated in the gap zones of collagen and finally transformed into crystals. Sr-doped collagen scaffolds (Sr-CS) promoted in vitro cell proliferation and osteogenic differentiation of rat bone marrow mesenchymal stromal cells (rBMSCs) and synergistically improved osteogenesis of rBMSCs by altering the macrophage response. Ag-doped collagen scaffolds (Ag-CS) exhibited in vitro antibacterial effects on S. aureus, as well as cell/tissue compatibility. Moreover, Sr-CS implanted into the calvarial defect of a rat resulted in improved bone regeneration. Therefore, the SSM model is a de novo synthetic strategy for doping bioactive elements into collagen, and can be used to fabricate multifunctional collagen scaffolds to meet the clinical challenges of encouraging osteogenesis, boosting the immune response and fighting severe infection in bone defects.

Biomacromolecules enabled dendrite-free lithium metal battery and its origin revealed by cryo-electron microscopy

Metallic lithium anodes are highly promising for revolutionizing current rechargeable batteries because of their ultrahigh energy density. However, the application of lithium metal batteries is considerably impeded by lithium dendrite growth. Here, a biomacromolecule matrix obtained from the natural membrane of eggshell is introduced to control lithium growth and the mechanism is motivated by how living organisms regulate the orientation of inorganic crystals in biomineralization. Specifically, cryo-electron microscopy is utilized to probe the structure of lithium at the atomic level. The dendrites growing along the preferred < 111 > crystallographic orientation are greatly suppressed in the presence of the biomacromolecule. Furthermore, the naturally soluble chemical species in the biomacromolecules can participate in the formation of solid electrolyte interphase upon cycling, thus effectively homogenizing the lithium deposition. The lithium anodes employing bioinspired design exhibit enhanced cycling capability. This work sheds light on identifying substantial challenges in lithium anodes for developing advanced batteries.

Inspired by the role of proteins in regulating eggshell mineralization, here Tao, Liu and colleagues apply trifluoroethanol modified eggshell membrane to combat lithium dendrite. Cryo-electron microscopy reveals that the growth along the most favored crystallographic direction is suppressed.

The interstitium conducts extrarenal storage of sodium and represents a third compartment essential for extracellular volume and blood pressure homeostasis

The role of salt in the pathogenesis of arterial hypertension is not well understood. According to the current understanding, the central mechanism for blood pressure (BP) regulation relies on classical studies linking BP and Na⁺ balance, placing the kidney at the very centre of long-term BP regulation. To maintain BP homeostasis, the effective circulating fluid volume and thereby body Na⁺ content has to be maintained within very narrow limits. From recent work in humans and rats, the notion has emerged that Na⁺ could be stored somewhere in the body without commensurate water retention to buffer free extracellular Na⁺ and that previously unidentified extrarenal, tissue-specific regulatory mechanisms are operative regulating the release and storage of Na⁺ from a kidney-independent reservoir. Moreover, immune cells from the mononuclear phagocyte system not only function as local on-site sensors of interstitial electrolyte concentration, but also, together with lymphatics, act as systemic regulators of body fluid volume and BP. These studies have established new and unexpected targets in studies of BP control and thus the pathophysiology of hypertension: the interstitium/extracellular matrix of the skin, its inherent interstitial fluid and the lymphatic vasculature forming a vessel network in the interstitium. Aspects of the interstitium in relation to Na⁺ balance and hypertension are the focus of this review. Taken together, observations of salt storage in the skin to buffer free extracellular Na⁺ and macrophage modulation of the extracellular matrix and lymphatics suggest that electrolyte homeostasis in the body cannot be achieved by renal excretion alone, but also relies on extrarenal regulatory mechanisms.

Phase Behaviour and Miscibility Studies of Collagen/Silk Fibroin Macromolecular System in Dilute Solutions and Solid State

Miscibility is an important issue in biopolymer blends for analysis of the behavior of polymer pairs through the detection of phase separation and improvement of the mechanical and physical properties of the blend. This study presents the formulation of a stable and one-phase mixture of collagen and regenerated silk fibroin (RSF), with the highest miscibility ratio between these two macromolecules, through inducing electrostatic interactions, using salt ions. For this aim, a ternary phase diagram was experimentally built for the mixtures, based on observations of phase behavior of blend solutions with various ratios. The miscibility behavior of the blend solutions in the miscible zones of the phase diagram was confirmed quantitatively by viscosimetric measurements. Assessing the effects of biopolymer mixing ratio and salt ions, before and after dialysis of blend solutions, revealed the importance of ion-specific interactions in the formation of coacervate-based materials containing collagen and RSF blends that can be used in pharmaceutical, drug delivery, and biomedical applications. Moreover, the conformational change of silk fibroin from random coil to beta sheet, in solution and in the final solid films, was detected by circular dichroism (CD) and Fourier transform infrared spectroscopy (FTIR), respectively. Scanning electron microscopy (SEM) exhibited alterations of surface morphology for the biocomposite films with different ratios. Surface contact angle measurement illustrated different hydrophobic properties for the blended film surfaces. Differential scanning calorimetry (DSC) showed that the formation of the beta sheet structure of silk fibroin enhances the thermal stability of the final blend films. Therefore, the novel method presented in this study resulted in the formation of biocomposite films whose physico-chemical properties can be tuned by silk fibroin conformational changes by applying different component mixing ratios.

Collagen intrafibrillar mineralization as a result of the balance between osmotic equilibrium and electroneutrality

Mineralization of fibrillar collagen with biomimetic process-directing agents has enabled scientists to gain insight into the potential mechanisms involved in intrafibrillar mineralization. Here, by using polycation- and polyanion-directed intrafibrillar mineralization, we challenge the popular paradigm that electrostatic attraction is solely responsible for polyelectrolyte-directed intrafibrillar mineralization. As there is no difference when a polycationic or a polyanionic electrolyte is used to direct collagen mineralization, we argue that additional types of long-range non-electrostatic interaction are responsible for intrafibrillar mineralization. Molecular dynamics simulations of collagen structures in the presence of extrafibrillar polyelectrolytes show that the outward movement of ions and intrafibrillar water through the collagen surface occurs irrespective of the charges of polyelectrolytes, resulting in the experimentally verifiable contraction of the collagen structures. The need to balance electroneutrality and osmotic equilibrium simultaneously to establish Gibbs-Donnan equilibrium in a polyelectrolyte-directed mineralization system establishes a new model for collagen intrafibrillar mineralization that supplements existing collagen mineralization mechanisms.

High pressure processing of meat: effects on ultrastructure and protein digestibility

The effects of high pressure processing (HPP, at 175 and 600 MPa) on the ultrastructure and in vitro protein digestion of bovine longissimus dorsi muscle meat were studied. HPP caused a significant change in the visual appearance and texture of the meat subjected to HPP at 600 MPa so that it appeared similar to cooked meat, unlike the meat subjected to HPP at 175 MPa that showed no significant visible change in the colour and texture compared to the raw meat. The muscles were subjected to digestion under simulated gastric conditions for 1 h and then under simulated small-intestinal conditions for a further 2 h. The digests were analysed using gel electrophoresis (SDS-PAGE) and ninhydrin assay for amino N. The effect of the acid conditions of the stomach alone was also investigated. Reduced SDS-PAGE results showed that pepsin-digested (60 min) HPP meats showed fewer proteins or peptides of high molecular weight than the pepsin-digested untreated meat, suggesting more breakdown of the parent proteins in HPP-treated meats. This effect was more pronounced in the muscles treated at 600 MPa. These results are in accordance with microscopy results, which showed greater changes in the myofibrillar structure after simulated gastric digestion of the sample processed at 600 MPa than at 175 MPa. Transmission electron microscopy also showed the presence of protein aggregates in the former sample, resulting probably from protein denaturation of sarcoplasmic proteins, in the subcellular space and between myofibrils; along with cell contraction (similar to that caused by heating) in the former.

Effects of hydration on steric and electric charge-induced interstitial volume exclusion--a model

The presence of collagen and charged macromolecules like glycosaminoglycans (GAGs) in the interstitial space limits the space available for plasma proteins and other macromolecules. This phenomenon, known as interstitial exclusion, is of importance for interstitial fluid volume regulation. Physical/mathematical models are presented for calculating the exclusion of electrically charged and neutral macromolecules that equilibrate in the interstitium under various degrees of hydration. Here, a central hypothesis is that the swelling of highly electrically charged GAGs with increased hydration shields parts of the neutral collagen of the interstitial matrix from interacting with electrically charged macromolecules, such that exclusion of charged macromolecules exhibits change due to steric and charge effects. GAGs are also thought to allow relatively small neutral, but also charged macromolecules neutralized by a very high ionic strength, diffuse into the interior of GAGs, whereas larger macromolecules may not. Thus, in the model, relatively small electrically charged macromolecules, such as human serum albumin, and larger neutral macromolecules such as IgG, will have quite similar total volume exclusion properties in the interstitium. Our results are in agreement with ex vivo and in vivo experiments, and suggest that the charge of GAGs or macromolecular drugs may be targeted to increase the tissue uptake of macromolecular therapeutic agents.

Electricity and colloidal stability: how charge distribution in the tissue can affects wound healing

The role of endogenous electric fields in wound healing is still not fully understood. Electric fields are of fundamental importance in various biological processes, ranging from embryonic development to disease progression, as described by many investigators in the last century. This hypothesis brings together some relevant literature on the importance of electric fields in physiology and pathology, the theory of biologically closed electric circuits, skin battery (a phenomenon that occurs after skin injury and seems to be involved in tissue repair), the relationship between electric charge and interstitial exclusion, and how skin tissues can be regarded as colloidal systems. The importance of electric charges, as established in the early works on the subject and the relevance of zeta potential and colloid stability are also analyzed, and together bring a new light for the physics involved in the wound repair of all the body tissues.

Interstitial Fluid and Lymph Formation and Transport: Physiological Regulation and Roles in Inflammation and Cancer

The interstitium describes the fluid, proteins, solutes, and the extracellular matrix (ECM) that comprise the cellular microenvironment in tissues. Its alterations are fundamental to changes in cell function in inflammation, pathogenesis, and cancer. Interstitial fluid (IF) is created by transcapillary filtration and cleared by lymphatic vessels. Herein we discuss the biophysical, biomechanical, and functional implications of IF in normal and pathological tissue states from both fluid balance and cell function perspectives. We also discuss analysis methods to access IF, which enables quantification of the cellular microenvironment; such methods have demonstrated, for example, that there can be dramatic gradients from tissue to plasma during inflammation and that tumor IF is hypoxic and acidic compared with subcutaneous IF and plasma. Accumulated recent data show that IF and its convection through the interstitium and delivery to the lymph nodes have many and diverse biological effects, including in ECM reorganization, cell migration, and capillary morphogenesis as well as in immunity and peripheral tolerance. This review integrates the biophysical, biomechanical, and biological aspects of interstitial and lymph fluid and its transport in tissue physiology, pathophysiology, and immune regulation.

Ex vivo assessment of mouse cervical remodeling through pregnancy via 23Na MRS

Preterm birth occurs in 12.5% of births in the United States and can lead to risk of infant death or to lifelong serious health complications. A greater understanding by which the two main processes, uterine contraction and cervical remodeling are regulated is required to reduce rates of preterm birth. The cervix must undergo extensive remodeling through pregnancy in preparation for parturition, the process of labor and delivery of young. One key aspect of this dynamic process is a change in the composition and abundance of glycosaminoglycans (GAGs) and proteoglycans within the extracellular matrix, which influences the loss of tensile strength or stiffness of the cervix during labor. 23Na NMR spectroscopy has previously been validated as a method to quantify GAGs in tissues. In the current study, the Na+ concentration was measured at several time points through pregnancy in mouse cervices using 23Na NMR spectroscopy. The Na+ concentration increased progressively during pregnancy and peaked one day before birth followed by a rapid decline after birth. The same trend was seen in GAGs as measured by a biochemical assay using independent cervix samples over the course of pregnancy. We suggest that monitoring the Na+ concentration via 23Na NMR spectroscopy can serve as an informative physiological marker in evaluating the stages of cervical remodeling ex vivo and warrants further investigation to determine its utility as a diagnostic tool for the identification of women at risk for impending preterm birth.

Effects of pH on transport properties of articular cartilages

Articular cartilages swell and shrink depending on the ionic strength of the electrolyte they are in contact with. This electro-chemo-mechanical coupling is due to the presence of fixed electrical charges on proteoglycans (PGs). In addition, at nonphysiological pH, collagen fibers become charged. Therefore, variation of the pH of the electrolyte has strong implications on the electrical charge of cartilages and, by the same token, on their transport and mechanical properties. Articular cartilages are viewed as three-phase multi-species porous media. The constitutive framework is phrased in the theory of thermodynamics of porous media. Acid-base reactions, as well as calcium binding, are embedded in this framework. Although macroscopic in nature, the model accounts for a number of biochemical details defining collagen and PGs. The change of the electrical charge is due to the binding of hydrogen ions on specific sites of PGs and collagen. Simulations are performed mimicking laboratory experiments where either the ionic strength or the pH of the bath, the cartilage piece is in contact with, is varied. They provide the evolutions of the chemical compositions of mobile ions, of the sites of acid-base reactions and calcium binding, and of the charges of collagen and glycosaminoglycans, at constant volume fraction of water. Emphasis is laid on the effects of pH, ionic strength and calcium binding on the transport properties of cartilages, and, in particular, on the electrical conductivity and electro-osmotic coefficient.

Electrostatic interactions modulate the conformation of collagen I

The pH- and electrolyte-dependent charging of collagen I fibrils was analyzed by streaming potential/streaming current experiments using the Microslit Electrokinetic Setup. Differential scanning calorimetry and circular dichroism spectroscopy were applied in similar electrolyte solutions to characterize the influence of electrostatic interactions on the conformational stability of the protein. The acid base behavior of collagen I was found to be strongly influenced by the ionic strength in KCl as well as in CaCl(2) solutions. An increase of the ionic strength with KCl from 10(-4) M to 10(-2) M shifts the isoelectric point (IEP) of the protein from pH 7.5 to 5.3. However, a similar increase of the ionic strength in CaCl(2) solutions shifts the IEP from 7.5 to above pH 9. Enhanced thermal stability with increasing ionic strength was observed by differential scanning calorimetry in both electrolyte systems. In line with this, circular dichroism spectroscopy results show an increase of the helicity with increasing ionic strength. Better screening of charged residues and the formation of salt bridges are assumed to cause the stabilization of collagen I with increasing ionic strength in both electrolyte systems. Preferential adsorption of hydroxide ions onto intrinsically uncharged sites in KCl solutions and calcium binding to negatively charged carboxylic acid moieties in CaCl(2) solutions are concluded to shift the IEP and influence the conformational stability of the protein.

Effect of hydration on interstitial distribution of charged albumin in rat dermis in vitro

At physiological pH, negatively charged glycosaminoglycans in the extracellular matrix may influence distribution volume of macromolecular probes, a phenomenon of importance for hydration of the interstitium and therefore for body fluid balance. We hypothesized that such charge effect was dependent on hydration. Human serum albumin (HSA) (the pH value for the isoelectric point (pI) = 4.9) was made neutral by cationization (cHSA) (pI = 7.6). Rat dermis was studied in vitro in a specially designed equilibration cell allowing control of hydration. Using a buffer containing labelled native HSA and cHSA, the distribution volumes were calculated relative to that of 51Cr-EDTA, an extracellular tracer. During changes in hydration (H), defined as (wet weight - dry weight) (dry weight)(-1)), the slope of the equation describing the relationship between extracellular fluid volume (V(x)) (in g H2O (g dry weight)(-1)) and H (V(x) = 0.925 H + 0.105) differed significantly from that for available volumes of cHSA (V(a,cHSA) = 0.624 H - 0.538) and HSA (V(a,HSA) = 0.518 H - 0.518). A gradual reduction in H led to a reduction in difference between available volumes for the two albumin species. Screening the fixed charges by 1 m NaCl resulted in similar available and excluded volumes of native HSA and neutral cHSA. We conclude that during gradual dehydration, there is a reduced effect of fixed negative charges on interstitial exclusion of charged macromolecules. This effect may be explained by a reduced hydration domain surrounding tissue and probe macromolecules in conditions of increased electrostatic interactions. Furthermore, screening of negative charges suggested that hyaluronan associated with collagen may influence intrafibrillar volume of collagen and thereby available and excluded volume fraction.

Mechanical effects of ionic replacements in articular cartilage. Part I: The constitutive model

A three-phase multi-species electro-chemo-mechanical model of articular cartilage is developed that accounts for the effect of two water compartments, namely intra-fibrillar water stored in between collagen fibrils and extra-fibrillar water covering proteoglycans. The collagen fibers constitute the solid phase while intra-fibrillar water and dissolved NaCl and CaCl(2) on one hand and extra-fibrillar water, ions Na(+), Ca(2+) and Cl(-) and proteoglycans on the other hand, form the two fluid phases. The complete picture that includes time-dependent mass transfers between the two fluid phases, diffusion of water and ions and electrical flow emerges from the Clausius-Duhem inequality but it is deferred to further study. The analysis is restricted to equilibrium states. The present work complements the mechanical model developed in Loret and Simões (Mech Material 36(5-6): 515-541, 2004a) where the presence of the sole NaCl was considered. In its current version, the model can handle mechanical and chemical loadings and unloadings involving the two salts, NaCl and CaCl(2). In order to reproduce experimental data, the shielding effects are made cation-dependent. Strong orientation of collagen fibers parallel to the joint surface implies anisotropic mechanical properties. Electro-chemo-mechanical couplings result in a chemistry-dependent apparent tensile Poisson's ratio, that increases to large values as the solution gets fresher. The model captures these aspects as well. The features of the model are first exposed in an infinitesimal strain context. Subsequently, large strains that typically occur in uniaxial traction under deionized water are accounted for, and a nonlinear anisotropic hyper-elastic behavior is developed. Parametric identification and simulations of actual loading processes are described in a companion paper, Loret and Simões (Biomech Model Mechanobiol, in press, DOI 10.1007/s10237-004-0063-6).

The interstitial distribution of macromolecules in rat tumours is influenced by the negatively charged matrix components

Knowledge of macromolecular distribution volumes is essential in understanding fluid transport within normal and pathological tissues. In this study in vivo we determined the distribution volumes of several macromolecules, including one monoclonal antibody, in tumours and tested whether charges associated with the tumour extracellular matrix influence their available volumes. Steady state levels of the monoclonal antibody trastuzumab (Herceptin) (pI = 9.2), IgG (pI = 7.6) as well as native (pI = 5.0) and cationized albumin (pI = 7.6) were established in rats bearing dimethylbenzanthracene (DMBA)-induced mammary tumours by continuous infusion using osmotic minipumps. After a 5-7 day infusion period, the rats were nephrectomized and the extracellular volume was determined with 51Cr-labelled EDTA. Plasma volumes were measured with 125I-labelled human serum albumin or rat IgM in a separate series. Steady state concentrations of probes were determined in the interstitial fluid that was isolated by centrifugation from tumours or by post mortem wick implantation in the back skin. Calculations were made for interstitial fluid volume (Vi), along with the available (Va/Vi) and excluded (Ve/Vi) relative interstitial volume fractions. The Ve/Vi for the positively charged trastuzumab in tumours averaged 0.29 +/- 0.03 (n = 16), a value which was significantly lower than the corresponding one for IgG of 0.36 +/- 0.02 (n = 16). Native albumin was excluded from 38% of the tumour interstitial fluid, whereas cationization of albumin reduced the excluded volume by approximately 50%. Our experiments suggest that the tumour interstitium acts as a negatively charged matrix and is an important factor in determining the macromolecular distribution volume.

Effect of charge on interstitial distribution of albumin in rat dermis in vitro

At physiological pH, negatively charged glycosaminoglycans in the extracellular matrix may influence distribution volume of a probe. We hypothesized that by varying the probe charge we would be able to observe a graded response of available volume fraction. Human serum albumin (HSA) (isoelectric point (pI) 5.0) was made more positive by cationization. Using reaction times of 10, 45 and 60 min, cationized HSA (cHSA) with respective pIs of 6.5, 7.3 and 8.0 were made. After eight days of equilibration in a buffer containing labelled native HSA and cHSA, the distribution volumes were calculated relative to that of 51Cr-EDTA, an extracellular tracer. The available volume in fully swollen dermis for native albumin relative to that of the extracellular tracer averaged 0.485+/-0.008 (n=49), with corresponding volumes for cHSA-10 min, cHSA-45 min and cHSA-60 min of 0.554+/-0.012 (n=17), 0.647+/-0.026 (n=17) and 0.718+/-0.021 (n=12), respectively. Increasing the ionic strength of the bathing solution to 1 M NaCl, thereby screening the fixed charges of tissue elements and probes alike, resulted in similar available and thereby excluded volumes of native HSA and neutral cHSA-45 min. These experiments suggest that fixed negative charges, most likely glycosaminoglycans, contribute significantly to interstitial exclusion of charged macromolecules, a phenomenon of importance for hydration of the interstitial fluid phase and therefore for body fluid balance. Moreover, the data indicate that previous findings of similar excluded volumes for the two differently sized major plasma proteins albumin (molecular mass 66 kDa) and IgG (molecular mass 160 kDa) may be explained by a more pronounced electrostatic repulsion of the former by the extracellular matrix.

Interstitial exclusion of positively and negatively charged IgG in rat skin and muscle

Volume exclusion, i.e., the space not available for a specific probe, may be dependent on the probe charge. Therefore, interstitial exclusion was measured for positively and negatively charged immunoglobulin (IgG) in skin and muscle of rats by using a continuous infusion method (30). Steady-state concentration of (125)I-labeled IgG 1 (pI = 8.7) and (131)I- labeled IgG 4 (pI = 6.6) was maintained by infusion of tracer for 120-168 h with an implanted osmotic pump. At the end of the infusion period and before tissue sampling, the rat was anesthetized and nephrectomized, and (51)Cr-labeled EDTA was injected and allowed 4 h for equilibration to measure interstitial fluid volume (V(i)). Interstitial fluid was isolated from skin and muscle by using nylon wicks implanted post mortem. The relative IgG available space was measured as the ratio between labeled IgG and (51)Cr-labeled EDTA wick fluid equivalent spaces, and relative excluded volume fraction (V(e)/V(i)) was calculated as 1--V(a)/V(i). V(e)/V(i) in hindlimb skin averaged 0.37 +/- 0.05 (SE) and 0.65 +/- 0.06 (P < 0.01) for IgG 1 and 4, respectively, with corresponding figures of 0.24 +/- 0.05 and 0.51 +/- 0.04 (P < 0.01) in hindlimb muscle (n = 9 for both tissues). These experiments suggest that fixed negative charges, most likely glycosaminoglycans, influence distribution of macromolecules in the interstitium and therefore affect interstitial fluid balance.

Time-dependent changes in the response of cartilage to static compression suggest interstitial pH is not the only signaling mechanism

The goal of the present study was to reexamine the role of interstitial pH in regulating the biosynthetic rate in cartilage tissue by addressing two research questions: (a) Do small, short-term changes in interstitial pH, induced independently by two different mechanisms (namely, by controlling the pH of the medium or by mechanical compression), result in biosynthetic rates commensurate with those expected from the "natural" relationship between interstitial pH and biosynthesis? and (b) Are the effects of changes in the pH of the medium or in compression the same for short-term (14-hour) and long-term (60-hour) exposures? Biosynthetic rates were estimated from incorporation of sulfate and proline into explants of bovine epiphyseal cartilage during the final 14 hours of culture. These rates decreased with decreasing pH of the medium, with increasing compression, and with decreasing native glycosaminoglycan content; or, expressed in terms of interstitial pH, acidification induced by compression or by lowering the pH of the medium resulted in a decreased biosynthetic rate, whereas interstitial acidification effected by increasing glycosaminoglycan content enhanced it. When the time for which tissue was exposed to changes in the pH of the medium was increased from 14 to 60 hours, the relationship between the biosynthetic rate and the pH remained constant whereas the relationship between the biosynthetic rate and compression was reversed. These data suggest that the transduction mechanisms underlying the response to pH of the medium and compression differ and that some adaptation or stimulation by modest levels of compression can occur with longer exposures. Interstitial pH is not the sole determinant of biosynthesis, and it cannot really account for the long-term response of cartilage tissue to static compression.

Temperature-favoured assembly of collagen is driven by hydrophilic not hydrophobic interactions

It has become almost axiomatic that protein folding and assembly are dominated by the hydrophobic effect. The contributions from this, and other, hydrophilic interactions can now be better distinguished by direct measurement of forces between proteins. Here we report the measurement of forces between triple helices of type I collagen at different temperatures, pH and solute concentrations. We separate repulsive and attractive components of the net force and analyze the origin of the attraction responsible for the collagen self-assembly. In this case the role of the hydrophobic effect appears to be negligible. Instead, water-mediated hydrogen bonding between polar residues is the most consistent explanation.

Determination of fixed charge density in cartilage using nuclear magnetic resonance

Many biomechanical and chemical properties of cartilage are dependent on the fixed charge density (FCD) of the extracellular matrix. In this study, nuclear magnetic resonance (NMR) spectroscopy was investigated as a nondestructive technique for determining FCD in cartilage. Sodium content was measured by NMR in cartilage explants and was compared with sodium content measured by inductively coupled plasma emission spectroscopy (ICP) in order to verify the total NMR visibility of sodium in cartilage. The ratio of NMR to ICP results was 1.02 +/- 0.04 (calf, mean +/- SD, n = 7) and 1.04 +/- 0.11 (adult bovine, n = 8). Sodium concentration as measured by NMR was then used with ideal Donnan theory to compute estimates of FCD. For calf articular cartilage (AC) near physiological conditions, calculated FCD was -0.28 +/- 0.03 M (n = 10). NMR measurements were then made for individual cartilage specimens sequentially equilibrated in baths of differing salt composition, pH, or ionic strength. For calf and adult AC, calculated FCD decreased dramatically between pH 3 and 2, with adult specimens becoming positively charged but calf tissue retaining a net negative charge. For calf AC equilibrated in 0.3-0.015 M NaCl, calculated FCD was observed to decrease slightly with decreasing bath ionic strength. For epiphyseal cartilage, FCD varied with the position of origin of the explant within the joint, ranging from -0.19 to -0.35 M in a manner that correlated with tissue glycosaminoglycan content. Preliminary NMR imaging experiments demonstrated similar variations of sodium concentration in intact ulnar epiphyseal cartilage. Collectively, these results demonstrate the ability of NMR to nondestructively follow FCD in cartilage. The technique is applicable to dynamic studies as well as to both in vitro and in vivo studies on living tissue.

The effect of osmotic and mechanical pressures on water partitioning in articular cartilage

X-ray diffraction measurements on native and proteoglycan-free articular cartilage have been made in order to test the dependence of the lateral packing of the collagen molecules on the osmotic pressure gradient, either naturally occurring or externally applied, between the intra- and extrafibrillar compartments. From the information on collagen packing we have been able to calculate, albeit with several assumptions, the amount of intrafibrillar water as a function of pressure. In parallel with the above measurements, we have quantitated, using serum albumin partitioning, the intrafibrillar water in proteoglycan-free cartilage, as a function of mechanically applied pressure. The results of both sets of experiments lead to the conclusion that the molecular packing density, and hence the intrafibrillar water content, are a function of the osmotic pressure difference between the extrafibrillar and intrafibrillar spaces or the equivalent mechanically applied pressure. The determination of intrafibrillar water has enabled us to calculate, from measured values of fixed charge density, the internal osmotic pressure of cartilage specimens, both in compressed and uncompressed states.

On the adaptive structures of the collagen fibrils of bone and cartilage

In order to investigate how bone and cartilage respond at the molecular level to changing demands of the skeleton, the influence of endogenous and external chemical stresses on collagen fibrillar structure as a function of locale in the femoral articular cartilage from 2 to 14 year old cows has been studied by x-ray diffraction. The fibrils were found to be osmotically compressed by vicinal proteoglycans. For most locales, the molecular packing density was less at the articular surface and increased to a maximum adjacent to the bone. This gradient in structure became more accentuated with animal age. For all ages, the packing density gradients could be almost completely eliminated upon enzymatic removal of the proteoglycans. However, in the regions of cartilage which had experienced the greatest stress in locomotion, the fibrils had a hyperswollen structure. This tendency towards hyperswelling increased with animal age. We have concluded that the collagen fibrils in articular cartilage adapt a structure that is in response to their respective mechanico-chemical histories.

The uptake of ions and neutral solutes by the artery and by artery wall preparations

Radiotracer methods have been used to measure the distributions of inorganic ions in the aorta. Data on fresh tissue were generally consistent with previous measurements and showed UCa greater than UI greater than UNa. In order to relate these measurements to the macromolecular composition of the arterial wall, the effects of various interventions--cell lysis, autoclaving, and NaOH digestion--were investigated. Elastin was found to be the major determinant of ionic distribution coefficients, to possess a slight positive charge under physiological conditions, and to display a high affinity for Ca++. The distribution coefficients did not conform to the predictions of the Donnan theory of ionic equilibrium and the degree of nonideality, as measured by the fixed charge density and the ion selectivity coefficients, varied with ionic strength. Data are also presented on the distribution of neutral solutes of different molecular weights in the various arterial preparation, in order to provide a basis for comparison of the behaviour of charged solutes.

Cartilage electromechanics--I. Electrokinetic transduction and the effects of electrolyte pH and ionic strength

Articular cartilage contains a high fixed charge density under physiological conditions associated primarily with the ionized proteoglycan molecules of the extracellular matrix. Oscillatory compression of cartilage using physiological loads produces electrical potentials that have been shown previously to be the result of an electrokinetic (streaming) transduction mechanism. We have now observed two additional electromechanical phenomena not previously seen in cartilage or other soft tissues: 'streaming current' and 'current-generated stress'. Sinusoidal mechanical compression induced a sinusoidal streaming current density through cartilage disks when the Ag/AgCl electrodes that were used to compress the cartilage were shorted together externally. Conversely, a sinusoidal current density applied to the tissue generated a sinusoidal mechanical stress within the tissue. Both these phenomena were found to be consistent with the same electrokinetic transduction mechanism responsible for the streaming potential. Changes in the measured streaming potential response that resulted from modification of bath ionic strength and pH have provided additional insights into the molecular origins of these transduction processes. Finally, we have now observed streaming potentials in living cartilage maintained in organ culture, as well as in previously frozen tissue.

Extrafibrillar proteoglycans osmotically regulate the molecular packing of collagen in cartilage

The molecular packing density of collagen and hence the intrafibrillar water content appears to be regulated in cartilage by the osmotic pressure gradient existing between the extrafibrillar and the intrafibrillar compartments.

Organization of extracellularly mineralized tissues: a comparative study of biological crystal growth

Biological mineralization processes are extremely diverse and, to date, it is an act of faith rather than an established principle that organisms utilize common mechanisms for forming crystals. A systematic analysis of the structural organization, as far as possible at the molecular level, of five different extracellularly mineralized tissues is presented to demonstrate that at least these mineralization processes are all part of the same continuum. The degrees of control exercised over crystal nucleation and crystal growth modulation are the basic variables. The five tissues, extracellularly mineralizing algae, radial and granular foraminifera, mammalian bone, mammalian enamel, and mollusk shell nacre, probably span the entire spectrum. Their crystal shapes, sizes, and the relations between the mineral phase and the organic phase, are primarily used to assess probable degrees of control exercised over crystal nucleation and modulation. Three different types of nucleation processes can be recognized: nonspecific, stereochemical, and epitaxial. Modulation of crystal growth after nucleation is either absent, achieved by adsorption of macromolecules onto specific crystal faces, or occurs by the prepositioning of matrix surfaces which interrupt crystal growth. The tissues in which active control is exercised over crystal growth all contain similar types of acidic matrix macromolecules. Significantly, the framework matrix macromolecules are all quite different and hence probably perform some tissue-specific functions. The study shows that there is a common basis for understanding these mineralization processes which is reflected in the nature of the protein-crystal interactions which occur in each tissue.

Influence of cartilage conformation on its equilibrium water partition

The spatial and bulk water equilibrium partition and fluid content were determined for normal adult bovine articular cartilage as a function of pH, temperature, and geometric confinement. Water partition averaged 60 +/- 7% at neutral pH and 37 degrees C and increased with decreasing pH and increasing temperature without a concomitant change in fluid content. The variation in water partition appeared to be a result of local conformation changes in the collagen fibril ultrastructure causing a transfer between free and trapped water volume. Removal of the lateral and subchondral bone geometric constraints caused an increase in both the water partition and fluid content. However, this partition variation could be accounted for solely from a change in free fluid volume without a change in the trapped fluid volume. These results suggest that in articular cartilage the proteoglycan-collagen interaction may be an important mechanism for controlling the partition of water between a freely exchangeable space and a space allowing no fluid exchange.

Characterization of calcium binding to brush-border membranes from rat duodenum

The Ca2+-binding properties of isolated brush-border membranes at physiological ionic strength and pH were examined by rapid Millipore filtration. A comprehensive analysis of the binding data suggested the presence of two types of Ca2+-binding sites. The high-affinity sites, Ka = (6.3 +/- 3.3) X 10(5) M-1 (mean +/- S.E.M.), bound 0.8 +/- 0.1 nmol of Ca2+/mg of protein and the low-affinity sites, Ka = (2.8 +/- 0.3) X 10(2) M-1, bound 33 +/- 3.5 nmol of Ca2+/mg of protein. The high-affinity site exhibited a selectivity for Ca2+, since high concentrations of competing bivalent cations were required to inhibit Ca2+ binding. The relative effectiveness of the competing cations (1 and 10 mM) for the high-affinity site was Mn2+ approximately equal to Sr2+ greater than Ba2+ greater than Mg2+. Data from the pH studies, treatment of the membranes with carbodi-imide and extraction of phospholipids with aqueous acetone and NH3 provided evidence that the low-affinity sites were primarily phospholipids and the high-affinity sites were either phosphoprotein or protein with associated phospholipid. Two possible roles for the high-affinity binding sites are suggested. Either high-affinity Ca2+ binding is involved with specific enzyme activities or Ca2+ transport across the luminal membrane occurs via a Ca2+ channel which contains a high-affinity Ca2+-specific binding site that may regulate the intracellular Ca2+ concentration and gating of the channel.

The effects of dipalmitoyl phosphatidyl choline on the precipitation of native fibrils and segment-long-spacing aggregates from collagen solution

The effect of dipalmitoyl phosphatidyl choline (DPPC), the major phospholipid component of pulmonary surfactant, on the precipitation of collagen in the form of native fibrils and segment-long-spacing (SLS) aggregates was studied in vitro. The effects of DPPC on both phases of collagen fibrillogenesis were analyzed spectrophotometrically, and alterations in the morphology of precipitated fibrils and SLS aggregates were ascertained by transmission electron microscopy (TEM). Low concentrations of DPPC inhibited the growth phase of fibrillogenesis, while higher concentrations were required to inhibit nucleation. Both the meshwork density and mean width of precipitated fibrils were altered by DPPC, as was the size of SLS aggregates. Segment-long-spacing aggregates prepared from pepsin-treated collagen were inhibited to a greater degree than SLS aggregates prepared from untreated collagen, indicating that the pepsin-susceptible residues of the telopeptide extensions of tropocollagen molecules stabilize SLS aggregates against the effects of DPPC. Based on these results and the inhibition of the growth phase at lower concentrations than those which inhibited the nucleation phase of fibrillogenesis, it was concluded that the primary mechanism of DPPC inhibition is electrostatic interference between the positively charged phospholipid molecules and the net positive charge of collagen. It is proposed that pathological conditions involving the pulmonary epithelium may allow interaction between surfactant and collagen, which could further weaken the interstitial connective tissue.

The visualization of individual collagen molecules and aggregates in collagen solution: the effect of preparation and sampling techniques

The use of electron microscopic observations to characterize the state of aggregation of collagen molecules in solution has been examined from the point of view that the observed structures must be compatible with the physical chemical properties of the solution. The common procedure of addition of droplets of a collagen solution to a grid followed by negative staining produces aggregates on the grid which could not have represented the aggregation state in solution. Closer representation of fibril diameters can be achieved by freeze cleave-etch methods. The best representation of the state of aggregation in collagen solutions can be achieved by use of a careful drop-washing procedure. This procedure, described in detail, permits correlation between intrinsic viscosity and electron microscopic observations.

On the state of anionic groups of demineralized matrices of bone and dentine

Calcium-binding and biochemical studies have been applied to characterize the state of the carboxylate and protein-bound phosphate groups in the EDTA-demineralized matrices of rat bone and dentine. The organic phosphate and carboxylate content of demineralized bone is virtually identical to that of purified steer skin collagen whereas demineralized dentine has a significantly higher phosphate and carboxylate content, presumably due to the presence of an acidic non-collagenous phosphoprotein. Two classes of calcium-binding sites can be detected in demineralized bone, demineralized dentine, and purified, reconstituted collagen. The number of strong calcium-binding sites correlates with the number of protein-bound phosphate groups. Depending on the preparative procedure, seven to nine such sites (per collagen molecule) are present in dentine, and one to two in the purified reconstituted collagen and in bone. The binding constant for the dentinal sites (1.1 X 10(4) M-1), however, is 20 times greater than that for bone or reconstituted collagen fibrils from skin. We tentatively conclude that the strong calcium-binding site in bone and reconstituted collagen is of the form protein-PO-4Ca++ whereas in dentine it is of the form protein (formula: see text); the weak binding sites in bone and dentine are of the form protein-COO-Ca++; and that approximately 160 of the 217 carboxylate groups of the collagen molecules of dentine or bone are present as electrostatic linkages of the form protein-COO-+H3N-protein.