A small molecular inhibitor of LRRK1 identified by homology modeling and virtual screening suppresses osteoclast function, but not osteoclast differentiation, in vitro

We used TGFβ activation kinase 1 as a template to build a 3D structure of the human LRRK1 kinase domain (hLRRK1 KD) and performed small molecule docking. One of the chemicals (IN04) that docked into the pocket was chosen for evaluation of biological effects on osteoclasts (OCs) in vitro. INO4 at 16 nM completely blocked ATP binding to hLRRK1 KD in an in vitro pulldown assay. In differentiation and pit assays, while the number of OCs on bone slices were comparable for OCs treated with IN04 and DMSO, IN04 treatment of OCs significantly impaired their ability to resorb bone. The area of pits on bone slices was reduced by 43% at 5 μM and 83% at 10 μM as compared to DMSO. Individual pits appeared smaller and shallower. F-actin staining revealed that DMSO-treated OCs displayed clear actin rings, and F-actin forms a peripheral sealing zone. By contrast, IN04-treated OCs showed disarranged F-actin in the cytoplasm, and F-actin failed to form a sealing zone on bone slices. IN04 treatment had no effects on OC-derived coupling factor production nor on osteoblast nodule formation. Our data indicate IN04 is a potent inhibitor of LRRK1, suppressing OC function with no effect on OC formation.