Uncoupling Osteoclast Fusion and Resorptive Activity Through Differential Talin1-Rap1 Signalling Pathways

Bone remodelling is essential for skeletal growth and repair and depends on a balance between osteoblast-driven bone formation and osteoclast-mediated bone resorption. Integrin activation, regulated by Rap1 and Talin1, is critical for osteoclast function by coordinated adhesion and actin cytoskeleton organization. In this study, we investigated Rap1 effectors controlling Talin1 in osteoclasts. RIAM and Lamellipodin (Lpd) are expressed and localise to actin-rich structures, including tunneling nanotubes (TNTs) and podosomes, supporting roles in cytoskeletal organisation and intercellular connectivity. Functional analyses revealed distinct, stage-specific functions. Lpd deficiency impaired early osteoclastogenesis by reducing TNT formation, disrupting precursor fusion, decreasing multinucleation, and lowering bone resorption. In contrast, RIAM deficiency has minimal impact on fusion but significantly reduced resorption activity in mature osteoclasts, indicating a role at later stages. Disruption of the direct Rap1-Talin1 interaction caused broader defects affecting both early and late stages, including reduced TNT formation, delayed fusion, altered multinucleation, and impaired bone degradation. In vivo, only disruption of this direct interaction increased bone mass and reduced osteoclast number and/or activity, whereas RIAM or Lpd deletion had no significantly impact. These results demonstrate that RIAM and Lpd perform distinct but complementary functions and identify the direct Rap1-Talin1 interaction as a central mechanism controlling osteoclast fusion and bone resorption.