Abstract 455: Extracellular Matrix Incorporation of Thrombospondins Depends on Intermolecular Recruitment by Their L-lectin Domains: A Novel Process in ECM Organization

Abstract

Thrombospondins (TSPs) are conserved, multi-domain, extracellular glycoproteins, that participate in many biological processes. TSP1 is present in low levels in the extracellular matrix (ECM) of healthy vessel walls but is up-regulated during neointima and plaque formation. TSP1 is responsible for inducing vascular smooth muscle cell (VSMC) proliferation and migration upon vascular injury or during atherosclerosis. We have examined the organisation of elastic fibril layers in the aortae of 10 to 12-week old wild-type and Thbs1 -/- female and male mice. The total width of elastic layers was measured from medial cross sections of the ascending arch and thoracic aorta. 1055 measurements were made from Thbs1 -/- and 671 from wild-type sections. The total width of layers is increased in Thbs1 -/- mice (p<0.05) and organisation of layers is more irregular. Similar perturbations were identified in aortae from 12-week old transgenic mice that specifically over-express TSP1 in arterial smooth muscle cells (based on 878 and 1503 measurements of TSP1 transgenic mice and mice expressing cre in SMC as control, respectively). Thus, ECM organisation in the aortic wall is calibrated to the physiological level of TSP1: either increasing or decreasing TSP1 perturbs the aortic wall ECM. The mechanisms by which TSP1 is deposited into ECM are poorly understood. Previous research from this laboratory identified a conserved site on the outer face of the C-terminal L-lectin domain that is necessary for accumulation of TSP-1 or TSP-5 into insoluble ECM. We present data from ECM assays, western blotting and immunofluorescence that identify a novel mechanism of trans inter-molecular interactions between TSP molecules within ECM. Trans-interactions can be homophilic (TSP1/TSP1 or TSP5/TSP5) or heterophilic (TSP1/TSP5). The mechanism is L-lectin domain specific and is also supported by glycosaminoglycan-dependent interactions that alter the patterning and rate of accumulation of TSP1 in ECM. Concentration-dependent matrix trapping of TSPs may explain how a low abundance component, TSP1, can impact on large-scale ECM organisation, as in the wall of the aorta. This mechanism could provide a new starting point for controlling TSP deposition in ECM under normal or pathological conditions.