Most elements of the vertebrate skeleton are formed by endochondral ossification. This process is initiated with mesenchymal cells that condense and differentiate into chondrocytes. These undergo several steps of differentiation from proliferating into hypertrophic chondrocytes, which are subsequently replaced by bone. Chondrocyte proliferation and differentiation are tightly controlled by a complex network of signaling molecules. During recent years, it has become increasingly clear that heparan sulfate (HS) carrying proteoglycans play a critical role in controlling the distribution and activity of these secreted factors. In this review we summarize the current understanding of the role of HS in regulating bone formation. In human, mutations in the HS synthetizing enzymes Ext1 and Ext2 induce the Multiple Osteochondroma syndrome, a skeletal disorder characterized by short stature and the formation of benign cartilage-capped tumors. We review the current insight into the origin of the disease and discuss its possible molecular basis. In addition, we summarize the existing insight into the role of HS as a regulator of signal propagation and signaling strength in the developing skeleton.

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Most elements of the vertebrate skeleton are formed by endochondral ossification. This process is initiated with mesenchymal cells that condense and differentiate into chondrocytes. These undergo several steps of differentiation from proliferating into hypertrophic chondrocytes, which are subsequently replaced by bone. Chondrocyte proliferation and differentiation are tightly controlled by a complex network of signaling molecules. During recent years, it has become increasingly clear that heparan sulfate (HS) carrying proteoglycans play a critical role in controlling the distribution and activity of these secreted factors. In this review we summarize the current understanding of the role of HS in regulating bone formation. In human, mutations in the HS synthetizing enzymes Ext1 and Ext2 induce the Multiple Osteochondroma syndrome, a skeletal disorder characterized by short stature and the formation of benign cartilage-capped tumors. We review the current insight into the origin of the disease and discuss its possible molecular basis. In addition, we summarize the existing insight into the role of HS as a regulator of signal propagation and signaling strength in the developing skeleton.

Objective: Osteoarthritis (OA) is a progressive degenerative disease of the articular cartilage caused by an unbalanced activity of proteases, cytokines and other secreted proteins. Since heparan sulfate (HS) determines the activity of many extracellular factors, we investigated its role in OA progression. Methods: To analyze the role of the HS level, OA was induced by anterior cruciate ligament transection (ACLT) in transgenic mice carrying a loss-of-function allele of Ext1 in clones of chondrocytes (Col2-rtTA-Cre;Ext1 e2fl/e2fl ). To study the impact of the HS sulfation pattern, OA was surgically induced in mice with a heterozygous (Ndst1 þ/À ) or chondrocyte-specific (Col2-Cre;Ndst1 fl/fl ) loss-of-function allele of the sulfotransferase Ndst1. OA progression was evaluated using the OARSI scoring system. To investigate expression and activity of cartilage degrading proteases, femoral head explants of Ndst1 þ/À mutants were analyzed by qRT-PCR, Western Blot and gelatin zymography. Results: All investigated mouse strains showed reduced OA scores (Col2-rtTA-Cre;Ext1 e2fl/e2fl : 0.83; 95% HDI 0.72e0.96; Ndst1 þ/À : 0.83, 95% HDI 0.74e0.9; Col2-Cre;Ndst1 fl/fl : 0.87, 95% HDI 0.76e1). Using cartilage explant cultures of Ndst1 animals, we detected higher amounts of aggrecan degradation products in wildtype samples (NITEGE 4.24-fold, 95% HDI 1.05e18.55; VDIPEN 1.54-fold, 95% HDI 1.54 e2.34). Accordingly, gelatin zymography revealed lower Mmp2 activity in mutant samples upon RAtreatment (0.77-fold, 95% HDI: 0.60e0.96). As expression of major proteases and their inhibitors was not altered, HS seems to regulate cartilage degeneration by affecting protease activity. Conclusion: A decreased HS content or a reduced sulfation level protect against OA progression by regulating protease activity rather than expression.

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