Aggrecan, an unusual polyelectrolyte: review of solution behavior and physiological implications

Abstract

Aggrecan is a high-molecular-weight, bottlebrush-shaped, negatively charged biopolymer that forms supermolecular complexes with hyaluronic acid. In the extracellular matrix of cartilage, aggrecan-hyaluronic acid complexes are interspersed in a collagen meshwork and provide the osmotic properties required to resist deswelling under compressive load. In this review we compile aggrecan solution behavior from different experimental techniques, and discuss them in the context of concentration regimes that were identified in osmotic pressure experiments. At low concentrations, aggrecan exhibits microgel-like behavior. With increasing concentration, the bottlebrushes self-assemble into large complexes. In the physiological concentration range (2<c(aggrecan)<8% w/w), the physical properties of the solution are dominated by repulsive electrostatic interactions between aggrecan complexes. We discuss the consequences of the bottlebrush architecture on the polyelectrolyte characteristics of the aggrecan molecule, and its implications for cartilage properties and function.

Fig. 1. The aggrecan monomer

(a) 2D schematic representation of the structure of aggrecan monomer, along with the length/radius dimensions of the components [1, 3, 37, 79]. (b) AFM imaging of aggrecan monomer on APTES-coated mica.

Fig. 2

Osmotic Pressure vs. aggrecan monomer concentration compiled from different publications. Data from Horkay et al. [6] (inset) in the concentration range of 0.001 – 0.1 g/cm³, shows three osmotic distinct regimes which are not affected by the Ca⁺⁺ concentration.

Fig. 3

Osmotic pressure vs. concentration of aggrecan solutions with representative data from Horkay et al. [6] (black filled squares) and Werner et al. [34] (black unfilled squares). Four concentration regimes are distinguishable, where the osmotic pressure shows different power-law dependence on concentration. Shown alongside in grey squares, is the estimated separation distance between aggrecan protein cores. The separation distances were estimated assuming average aggrecan MW of 3 KDa, and an idealized cylinder shape of length 400 nm.

Fig. 4

The diffusion coefficient of aggrecan monomers as determined from Dynamic Light Scattering, by assuming a single relaxing entity [45] (black unfilled) and two relaxing entities [24] (black filled). Also shown (dashed lines) is the concentration range where the aggrecan monomers in solution are assembled into fractal clusters [6, 7].

Fig. 5

The self-diffusion coefficient of aggrecan monomers reproduced from Gribbon et al. [55] (black filled) and Comper et al.[48] (black unfilled). The self-diffusion coefficient shows different power-law dependence in the different concentration regimes.

Fig. 6

Rheology of aggrecan monomers in two concentration regimes compiled from Papagiannopoulos et al. [24] and Soby et al.[57]. The light grey plots shows data for 0.007 g/ml aggrecan concentration (‘dilute-transition’ regime) [24]. The dark grey plots show data for 0.032 mg/ml aggrecan concentration (‘physiological’ regime) [57] The storage modulus (G′) and the loss modulus (G″) are in units of kPa. η is the viscosity in kPa sec.

Fig. 7

The reduced viscosity of aggrecan monomers at different concentrations, culled from Papagiannopoulos et al. [24] (black filled circles), Hardingham et al. [64] (black filled squares) and Mow et al.[59] (black unfilled squares). The storage modulus at 10⁵ rad/s (black cross) is also shown [63]. The units for reduced viscosity and storage modulus are (mg/ml)⁻¹ and Pa, respectively.