Proteoglycans are secreted into the extracellular matrix of virtually all cell types and function in several cellular processes. They consist of a core
protein onto which
glycosaminoglycans (e.g., heparan or
chondroitin sulphates), are attached.
Proteoglycans are important modulators of gradient formation and signal transduction. Impaired biosynthesis of heparan sulphate
glycosaminoglycans causes
osteochondroma, the most common bone tumour to occur during adolescence. Cytochemical staining with positively charged
dyes (e.g.,
polyethyleneimine-PEI) allows, visualisation of
proteoglycans and provides a detailed description of how
proteoglycans are distributed throughout the cartilage matrix. PEI staining was studied by electron and reflection contrast microscopy in human growth plates,
osteochondromas and five different
proteoglycan-deficient zebrafish mutants displaying one of the following skeletal phenotypes: dackel (dak/ext2), lacking heparan sulphate and identified as a model for human
multiple osteochondromas; hi307 (β3gat3), deficient for most
glycosaminoglycans; pinscher (pic/slc35b2), presenting with defective sulphation of
glycosaminoglycans; hi954 (uxs1), lacking most
glycosaminoglycans; and knypek (kny/gpc4), missing the
protein core of the
glypican-4 proteoglycan. The panel of genetically well-characterized
proteoglycan-deficient zebrafish mutants serves as a convincing and comprehensive study model to investigate
proteoglycan distribution and the relation of this distribution to the model mutation status. They also provide insight into the distributions and gradients that can be expected in the human homologue. Human growth plate, wild-type zebrafish and fish mutants with mild
proteoglycan defects (hi307 and kny) displayed
proteoglycans distributed in a gradient throughout the matrix. Although the mutants pic and hi954, which had severely impaired
proteoglycan biosynthesis, showed no PEI staining, dak mutants demonstrated reduced PEI staining and no gradient formation. Most chondrocytes from human
osteochondromas showed normal PEI staining. However, approximately 10% of tumour chondrocytes were similar to those found in the dak mutant (e.g., lack of PEI gradients). The cells in the reduced PEI-stained areas are likely associated with loss-of-function mutations in the EXT genes, and they might contribute to tumour initiation by disrupting the gradients.