Credit: Dorling Kindersley

A new study published in Nature Medicine identifies osteoblast–endothelial cell crosstalk via Slit homolog 3 protein (SLIT3) as central to bone formation. “Osteoblasts produce high levels of the axon guidance cue SLIT3,” says corresponding author Matthew Greenblatt, “which increases bone formation by acting indirectly as an angiogenic factor to promote the growth of a subtype of vascular endothelium.”

The bone marrow-localized CD31hiendomucin (EMCN)hi subtype of endothelium has been implicated previously in regulating bone turnover in response to platelet-derived growth factor type BB secreted by pre-osteoclasts. These new data, however, are the first evidence of an osteoblast-derived signal that instructs the microvasculature to initiate bone growth.

The researchers found an increase in CD31hiEMCNhi vasculature in the bones of osteoblast-specific inducible Shn3 (also known as Hivep3)-knockout mice, which have excess bone formation. Osteoblast cultures from Shn3-/- mice were then used to make conditioned medium to screen for angiogenic factors by RNA-seq, and SLIT3 was the only major differentially expressed factor when compared with medium from wild-type osteoblast cultures.

Bone marrow endothelial progenitors treated with recombinant SLIT3 proliferated, and had increased migratory potential and tube forming activity.

recombinant SLIT3 was as effective as parathyroid hormone

By contrast, osteoblast-specific Slit3-/- mice had less CD31hiEMCNhi skeletal endothelium and bone mass than wild-type mice. SLIT proteins signal via the roundabout homolog (ROBO) family; the researchers identified ROBO1 as the main endothelial SLIT3 receptor.

Deletion of SLIT3 in mice with an open femoral midshaft fracture resulted in non-union, whereas deletion of Shn3 had the opposite effect, as did treatment with recombinant SLIT3 delivered in a collagen sponge to the fracture site, a method designed to avoid off-target effects.

Finally the researchers also showed that recombinant SLIT3 was as effective as parathyroid hormone in therapeutic application to reverse the bone-loss effects of ovariectomy, a mouse model of postmenopausal osteoporosis, hinting at future therapeutic applications for bone diseases.