3D distribution of perlecan within intervertebral disc chondrons suggests novel regulatory roles for this multifunctional modular heparan sulphate proteoglycan

Perlecan is a modular, multifunctional heparan sulphate-proteoglycan (HS-PG) that is present in the pericellular and wider extracellular matrix of connective tissues. In the present study, confocal microscopy was used to study perlecan distribution within intervertebral disc chondrons. Perlecan immunolabel was demonstrated intracellularly and in close association with the cell nucleus within chondrons of both the annulus fibrosus (AF) and nucleus pulposus (NP). This observation is consistent with earlier studies that have localised HS-PGs with nuclear cytoskeletal components. Nuclear HS-PGs have been proposed to transport fibroblast growth factor (FGF)-1, FGF-2 and FGFR-1 into the cell nucleus, influencing cell proliferation and the cell-cycle. Perlecan has well-known interactive properties with FGF family members in the pericellular and extracellular matrix. Perinuclear perlecan may also participate in translocation events with FGFs. The glycosaminoglycan side chains of HS-PGs can modulate chromatin structure by regulating the access of transcription factors to DNA. These mechanisms are consistent with the distribution patterns identified here and previously reported for other HS-PGs, introducing a potentially-novel arena for perlecan in gene regulation. Whilst much is known of the structure and function of perlecan in the pericellular and extracellular matrix, very little is known of any intracellular forms of perlecan. The perlecan labelling patterns described here suggest the possibility of involvement of this HS-PG in an intracrine regulatory system. Future studies should further explore this possibility and the potential for this HS-PG as a novel therapeutic target.

A role of heparan sulphate proteoglycan in the cellular uptake of lipocalins ß-lactoglobulin and allergen Fel d 4

Lipocalins, small extracellular hydrophobic molecule carriers, can be internalized by a variety of different cells. However, to date receptors have only been identified for human lipocalins. Here, we specifically investigated uptake mechanisms for lipocalins ß-lactoglobulin and Fel d 4 in HeLa and Chinese hamster ovary (CHO) cells. We provide evidence that cell surface heparan sulphate proteoglycan is essential for internalization of these lipocalins. In HeLa cells, lipocalin uptake was inhibited by competition with soluble heparin, enzymatic digestion of cellular heparan sulphate by heparinase and inhibition of its biosynthesis by sodium chlorate. Biochemical studies by heparin affinity chromatography and colocalization studies further supported a role of heparan sulphate proteoglycan in lipocalin uptake. Finally, lipocalin uptake was blocked in CHO mutant cells defective in glycosaminoglycan biosynthesis whereas in wild-type cells it was clearly detectable. Thus, cell surface heparan sulphate proteoglycan represents a novel component absolutely participating in the cellular uptake of some lipocalins.