Cartilage and bone are rich in extracellularmatrix and the remodeling
of this matrix is critical for development and repair. MMPs are enzymes
mediating localized proteolytic modification of this extracellular matrix,
allowing (i) cartilage and bone remodeling and (ii) release of stored
angiogenic signaling molecules and subsequent blood vessel invasion.
During skeletal development, mice lacking membrane-type 1 (MT1)
MMP or MMP-9 display growth plate abnormalities in the long bones
and impaired bone formation, concomitantwith a delay in angiogenesis
[36–38]. The function of MMP-9 during development was mirrored
during fracture repair. Mmp9−/− mice display delayed bone healing,
linked to a defect in the removal ofmatrix deposited by the hypertrophic
chondrocytes and a delay in vascular invasion. Surprisingly, this defect
was not explained by a decrease in Vegf expression, nor its receptor. In
contrast,MMP-9 likely regulates the bioavailability of VEGF as exogenous
application of VEGF compensated for the lack of Mmp9 [39], although
decisive proof is lacking.
In addition, identifying other angiogenic growth factors and
proteases will contribute in establishing a more complete model of
how remodeling of the extracellular matrix and angiogenesis are
linked during bone healing.