Project Summary
Sclerostin is a potent inhibitor of bone formation and has been shown to be a valuable drug
target for treating osteoporosis. Mechanistically, sclerostin functions by binding to LRP5/6 on
the osteoblast lineage cells to antagonize canonical Wnt signaling, thus negatively regulates
bone formation. Presumably, after sclerostin is secreted by osteocytes, they reach the target
osteoblasts at the bone surfaces by diffusion. However, to date it remains unknown how
secreted sclerostin is regulated on the cell surface and in the extracellular matrix. To address
this significant gap in sclerostin biology, we focus our study on sclerostin–heparan sulfate (HS)
interaction. HS, a universal glycosaminoglycan found at the cell surface and in the extracellular
matrix, is known to bind sclerostin and might regulate the bioactivity and diffusion of sclerostin.
Our central hypothesis is that HS can regulate the biological functions of sclerostin in bone
formation. To test this hypothesis, our overall objective here is to elucidate how HS interacts
with sclerostin and how HS–sclerostin interaction regulates bone formation. We plan to pursue
the following two specific aims: Aim 1. Determine the biological significance of HS–sclerostin
interactions in vitro. We postulate that HS helps concentrate sclerostin on the osteoblast cell
surface and facilitates the binding of sclerostin to its receptor LRP5/6 by forming ternary
complex. We also hypothesize that HS serves a storage depot of sclerostin on the cell surface
of osteocytes after it is secreted, and protects it from proteolytic degradation. We will manipulate
HS–sclerostin interactions biochemically at the surface of both osteoblasts and osteocytes to
determine the mechanisms by which HS regulates the function of sclerostin in these cellular
contexts. Aim 2. Determine the role of HS–sclerostin interaction in bone formation in vivo. Our
working hypothesis is that dampening HS–sclerostin interactions impairs the inhibitory potency
of sclerostin towards LRP5/6, which leads to enhanced bone formation. Using a novel sclerostin
knock-in mouse strain, we will examine the consequence of altering HS–sclerostin interactions
in bone formation in vivo. Our contribution will be significant because we will identify multiple
molecular mechanisms by which HS regulates sclerostin and elucidate how such interactions
regulate bone formation. Results from the proposed experiments will substantially advance our
understanding of the cellular regulation of sclerostin on both osteoblasts and osteocytes by
elucidating the role of HS in the system. Importantly, these results are expected to have positive
translational impact because by identifying how HS regulates the bioavailability of sclerostin, we
may be able to provide new perspective for manipulating sclerostin in bone diseases.