Solution Structure and Dynamics of Cartilage Aggrecan

Abstract

We studied the structure and dynamics of porcine laryngeal aggrecan in solution using a range of noninvasive techniques: dynamic light scattering (DLS), small-angle neutron scattering (SANS), video particle tracking (VPT) microrheology, and diffusing wave spectroscopy (DWS). The data are analyzed within the framework of a combined static and dynamic scaling model, and evidence is found for reptation of the comb backbones with unentangled side-chain dynamics. Small-angle neutron scattering indicated standard polyelectrolyte scaling of the mesh size (xi) with concentration (c) in semidilute solutions for the whole aggrecan aggregate, xi = Ac(-0.47+/-0.04), with the prefactor (A) implying there is on average 60 nm between the aggrecan subunits along the backbone. VPT demonstrated large exponents for the power law dependence of the intrinsic viscosity (eta) on the polymer concentration in the semidilute concentration regime, eta approximately c(alpha); with alpha equal to 2.04 +/- 0.06 and 1.95 +/- 0.08 for the assembled and disassembled aggrecan aggregates, respectively. DWS at high frequencies (10(4)-10(5) Hz) gave evidence for internal Rouse modes of the aggrecan monomers, independent of the degree of self-assembly of the molecules.

Biological and Biomimetic Comb Polyelectrolytes

“…A recent combination of diffusing wave spectroscopy and fast particle tracking measurements provided a range of 0.1-10 6 Hz for the complex shear moduli [6] as a function of polymer concentration. At high frequencies evidence is found for flexible polymeric Rouse modes in aggrecan in agreement with dynamic light scattering studies [7]. Aggrecan chains do not gel in their native state, providing their solutions with high viscosity and low elasticity.…”

“…Our lack of understanding is not representative of the comb motif's ubiquity, since the motif is found in a vast range of biomolecular systems. Indeed many molecules from two huge divisions of molecular biology, glycoproteins [3][4][5] and proteoglycans [6,7], carry the comb motif. These are some of the worst understood biological molecules due to their heterogeneity and the aforementioned problems in structural determination (primarily due to their non-crystallinity).…”

“…(a) Gelling mucins, e.g., porcine MUC6 [11]. (b) Proteoglycans, e.g., bovine aggrecan [6,7]. (c) Epithelial mucins [13].…”

Ions in hyaluronic acid solutions

“…For example, knowledge of the osmotic modulus of cartilage and the interactions between HA and aggrecan molecules offer a possible explanation of the ability of the HA-aggrecan complexes to resist compressive forces. 35 The structural properties of the HA molecule, namely, the hydrophilic character of the backbone, the extended chain configuration, and its ionizability give rise to a unique behavioral pattern that distinguishes this molecule from all other biopolymers and explains its abundance and multiple functions. These structural features, accompanied by a set of essential physical properties, exceptional ion insensitivity, fast diffusion, and high osmotic modulus, make this molecule a universal structural component in living systems.…”

“…Hyaluronic acid has also been shown to behave like a flexible polyelectrolyte the persistence length of which significantly increases and becomes on the order of ϳ100 nm when aggrecan monomers are attached. 34,35 Experimental observations of the effect of ionic environment on the osmotic modulus of the HA molecules alone in solution are an essential step in placing these results in context and in understanding cartilage load bearing properties.…”