Identification and characterization of glycanated and non-glycanated forms of biglycan and decorin in the human intervertebral disc.

Immunological studies revealed the presence of several different forms of biglycan and decorin in human intervertebral-disc tissues (annulus fibrosus, nucleus pulposus and cartilage end-plate). In the young intervertebral disc, glycosaminoglycan-containing (glycanated) forms of both biglycan and decorin represented a greater proportion of the total proteoglycan population present in extracts of annulus fibrosus and cartilage end-plate compared with extracts of nucleus pulposus, in which they were barely detectable. In older discs the glycanated forms of biglycan and decorin represented only a small proportion of the total proteoglycan present. Immunochemical analyses with an antibody to chondroitin/dermatan sulphate isomers indicated differences in the glycosaminoglycans substituted on glycanated forms of small proteoglycans found in different disc tissues. Dermatan sulphate was the predominant glycosaminoglycan present on biglycan and decorin in annulus fibrosus extracts, whereas chondroitin 4-sulphate was present in both small proteoglycans isolated from cartilage end-plate. In addition, immunochemical analyses with antibodies against core protein epitopes identified two non-glycanated forms of both biglycan and decorin. These non-glycanated forms of the small proteoglycans were found in all three regions of the disc. The two nonglycanated forms of biglycan had estimated molecular masses of 37 and 41 kDa and those of decorin were 43 and 45 kDa, respectively. These non-glycanated forms of biglycan and decorin increased in proportion with aging. N-terminal sequence analysis indicated that the larger non-glycanated form of decorin was a degradation product of its glycanated precursor. However, no N-terminal sequence information was obtainable from the other non-glycanated form of decorin or the two non-glycanated forms of biglycan. These data are consistent with the hypothesis that some of the non-glycanated forms of decorin and biglycan are degradation products of native precursors. However, the possibility remains that several different post-translationally modified forms of decorin and biglycan are synthesized by intervertebral-disc tissues.