Osteoblastic heparan sulfate regulates osteoprotegerin function and bone mass

Area of Science:

• Biochemistry
• Bone Biology
• Glycobiology

Background:

• Bone remodeling involves formation and resorption; imbalances cause osteoporosis.
• Heparin therapy is linked to osteoporosis, suggesting a role for heparan sulfate (HS).
• The function of endogenous HS in adult bone homeostasis is not well understood.

Purpose of the Study:

• To investigate the role of endogenous heparan sulfate (HS) in adult bone homeostasis.
• To elucidate the mechanism by which HS influences bone mass regulation.

Main Methods:

• Conditional ablation of Ext1, essential for HS biosynthesis, in osteoblasts of mice.
• Analysis of bone formation and resorption in resultant conditional mutant mice.
• Investigation of the interaction between HS and osteoprotegerin (OPG) in regulating RANKL/RANK signaling.

Main Results:

• Conditional Ext1 ablation in osteoblasts led to severe osteopenia in mice.
• The osteopenia resulted from enhanced bone resorption, not impaired bone formation.
• Heparan sulfate (HS) acts as a cell surface binding partner for osteoprotegerin (OPG), facilitating its inhibition of RANKL/RANK signaling.
• Reduced bone mineral density was observed in patients with multiple hereditary exostoses (EXT gene mutations).

Conclusions:

• Endogenous heparan sulfate (HS) is essential for maintaining bone mass in adult bone.
• HS regulates bone homeostasis by mediating the inhibitory function of osteoprotegerin (OPG) on bone resorption.
• Dysregulation of HS-OPG interaction may contribute to low bone mass conditions in humans, including genetic disorders like multiple hereditary exostoses.