Function of heparan sulfate proteoglycans (HSPGs) and heparanase (HPSE) in endochondral bone formation
Endochondral bone formation is a tightly regulated process involving coordination among cell-cell, cell-matrix and growth factor signaling that eventually results in the production of mineralized bone from a cartilage template. Chondrogenic and osteogenic differentiation occur in sequence during this process, and the temporospatial patterning clearly requires the activities of heparan sulfate proteoglycans (HSPGs), heparin binding growth factors (HBGFs) and their receptors. Previous studies revealed that the glycosaminoglycan-bearing domain I of perlecan (HSPG2), a large, multidomain, multifunctional heparan sulfate proteoglycan (HSPG), supports early chondrogenesis and growth factor delivery. Although domain I of HSPG2 continues to be well studied in the area of endochondral bone formation, the role of the additional domains which have sequence similarity to known molecules remains to be determined. A bioinformatics approach to predict regions likely to be on the surface of the protein was employed to identify the functional potential of domain IV of HSPG2. A novel peptide sequence from an immunoglobin (Ig) repeat in domain IV supported cell adhesion, spreading, and focal adhesion kinase (FAK) activation when compared to control peptides.
Also, heparanase (HPSE), a HSPG2 modifier known to cleave heparan sulfate (HS) chains, has been shown to play a role in osteogenesis, but the mechanism by which it functions is incompletely understood. I employed a combination of ex vivo and in vitro approaches and a well described HPSE inhibitor, PI-88, to study HPSE in endochondral bone formation. In situ hybridization and immunolocalization with HPSE antibodies revealed that HPSE is expressed in the perichondrium (Pc), periosteum (Po), and at the chondro-osseous junction (COJ), all sites of key signaling events and tissue morphogenesis. Interestingly, the COJ is the site at which both perlecan/HSPG2 and HS abruptly disappear from developing bone, suggesting that HPSE activity plays a key role in the release of HS chains during HSPG2 disappearance and the onset of mineralization, an idea consistent with published findings that HPSE overexpression increases osteogenesis. Addition of PI-88 to metatarsal organ cultures reduced growth and suggested that HPSE activity aids the transition from chondrogenic to osteogenic in growing long bones. To identify the mechanism by which HPSE influences the processes of endochondral bone formation, I employed high density cultures of ATDC5 prechondrogenic cells grown under conditions favoring chondrogenesis or osteogenesis. Under chondrogenic conditions, Both HPSE protein and mRNA was expressed at high levels during the mid-culture period, at the onset of terminal chondrogenesis. PI-88 addition reduced chondrogenesis and accelerated osteogenesis, as demonstrated by a dramatic increase of osteocalcin (Ocn) transcript levels. In normal growth medium, adecrease in HPSE activity reduced migration of ATDC-5 cells, suggesting a decrease in the number of proliferative chondrocytes and increase in differentiated cells. Together, these studies demonstrate the importance of HS chains of HSPGs and HPSE activity in the chondrogenic and osteogenic processes of endochondral bone formation.