Fundamental Studies and Applications of Tissue Sciences

Eunice Kennedy Shriver National Institute Of Child Health & Human Development

Investigators

Linked publications & trials

Abstract

Water-ion-biopolymer interactions define the molecular organization of tissue components and govern the macroscopic mechanical/osmotic and functional properties of tissues. Well-defined model systems designed to mimic the behavior of living systems, enable us to study the physicochemical foundation of these systems, which are often composed of polyelectrolytes whose characteristics are largely determined by the properties of water hydrating the polyelectrolyte molecules, the counter-ions present in the solution, and the interaction between and among the components (polyelectrolytes, water molecules and counterions). Systematic studies conducted on model systems may open new avenues to understand the molecular origin of the properties and function of tissue. It has been demonstrated that in many systems, especially in the vicinity of ion induced phase transition, the physical response is governed by universal scaling principles. There are various physical forces and interactions implicated in these processes, including electrostatic repulsion and attraction, hydrophobic and hydrophilic interactions, hydrogen bonding and van der Waals forces. The important role that polymer physics plays in the behavior of biological systems offers a powerful framework to identify variables required to describe certain biological phenomena. Divalent cations, particularly calcium ions (Ca+2) play a key role in biopolymeric system behavior. In general, experiments to determine the interactions between ions and biopolymers are difficult to perform, because above a relatively low ion concentration multivalent cations cause phase separation (or precipitation) of the charged macromolecules. Since macroscopic phase separation does not occur in gels, we can overcome this limitation by cross-linking these polymers, i.e., extending the range of ion concentrations over which the systems remain stable. We use various new non-destructive procedures to investigate cross-linked gels of a model synthetic polymer, polyacrylic acid (PAA), and different biopolymers such as DNA and hyaluronic acid (HA) to determine the size of the structural elements that govern the osmotic concentration fluctuations. We combine SANS and SAXS to estimate the osmotic modulus of HA in the presence of both monovalent and divalent counterions. We also study the dynamic properties (e.g., diffusion processes) of these systems by dynamic light scattering and determine the osmotic modulus from the relaxation response. We developed an experimental procedure to determine the distribution of counter-ions around charged biopolymer molecules using anomalous small-angle X-ray scattering (ASAXS) measurements. We analyze and compare various network elasticity models that address essential physical aspects of biopolymer systems. We are particularly interested in understanding the mechanisms of interactions of biologically important divalent cations like Ca+2 with negatively charged biopolymers. Moreover, while there is some understanding of water-ion-biopolymer interactions in low and high ionic strength regimes, there is little known about these in the intermediate ionic strength regime which characterizes the conditions in physiological salt solutions. Because most functional biopolymers are polyelectrolytes, studying this regime is critical for understanding a myriad of biological transport processes, including transport of mass, momentum, charge, and energy. A basic feature of charged polymers is that their properties are affected by many factors, such as the intrinsic properties of the polymer (e.g., chain rigidity), polymer conformation, molecular mass, the concentration and valence of counterions, as well as inter- and intramolecular interactions (e.g., ion condensation, hydration, hydrogen bonding). To date, no satisfactory theoretical framework embodying the relationship between molecular/supramolecular structure, macroscopic properties and biological function of either synthetic or natural polyelectrolyte molecules has been developed. Therefore, making systematic experimental studies on well-defined model systems under well-controlled conditions is critically important to advance our understanding of the mechanisms governing complex biological processes. In many cases, these model systems become the basis of novel translational NMR and MRI phantoms, and non-invasivem methods to measure and map ECM properties. We also focus on ion-polymer interactions in bottlebrush polymers comprised of densely grafted chains tethered to a polymer backbone. These molecules are essential components of living systems. Typical examples are cartilage and viscoelastic mucin layers forming protective coatings in the lungs, orifices and digestive tract of humans and animals. Our aim is to better understand the physicochemical behavior of bottlebrush molecules. We have systematically investigated solutions of synthetic polyelectrolyte bottlebrushes through a combination of measurements of solution properties (osmometry, neutron and dynamic light scattering, and molecular dynamics simulations). We found that bottlebrush polymers differ strongly from their linear polymer counterparts, not only in their architecture, but also in their physical properties that are primarily controlled by the length of the main and side-chains, the grafting density, and the steric repulsion of the side-chains. In collaboration with Prof. Xia (Department of Chemistry, Stanford University) we designed and synthesized novel bottlebrush polymers and determined the physicochemical properties of these model polymer solutions (neutralized polyacrylic acid bottlebrushes) having well-controlled molecular architecture. A Despite the tremendous potential of bottlebrush polymers in both materials science and biology, the effects of the charged bottlebrush topology on the physicochemical behavior of their solutions have not yet been fully elucidated. For example, the effect of charges on the conformation of bottlebrush molecules is poorly understood. The effect of ions is particularly important in the biological milieu where both mono- and divalent counter-ions are present. Elucidating these basic physical properties is of fundamental importance to fully understand the basic molecular physics underlying numerous biological functions. We made systematic studies on a family of bottlebrush structures by varying (i) the length of the main chains (at constant length of the side chains) and (ii) the length of the side chains (at constant length of the main chains). Structural investigations of charged bottlebrush polymers have been complemented by molecular dynamics simulations performed by Dr. Alexandros Chremos in collaboration with Dr. Jack Douglas (National Institute of Standard and Technology). These studies reveal the role of the organization of biopolymers that gives rise to the unique behavior of the tissue. Knowledge of the molecular origin of tissue properties is essential for developing successful tissue engineered regenerative medicine strategies. With Dr. Matan Mussel, we developed a multicomponent model that imposes conservation laws and constitutive relations for polymer chains, water, and ions to predict swelling behavior of polyelectrolyte materials. We have demonstrated that understanding the organization of charged macromolecules in near-physiological environment may shed light on the mechanism of structure formation in living systems in which the complexity of the structure and interactions makes it difficult to perform conclusive experiments under well-controlled conditions.