RKIP regulates bone marrow macrophage differentiation to mediate osteoclastogenesis and H-type vessel formation

Area of Science:

• Osteoimmunology
• Bone Biology
• Macrophage Differentiation

Background:

• Bone homeostasis relies on the balance between bone formation and resorption, regulated by specialized cells within the bone microenvironment.
• Macrophages in the bone niche play critical roles in osteoimmunology, with their differentiation pathways influencing bone remodeling.
• Understanding the mechanisms governing macrophage differentiation is crucial for developing effective treatments for bone diseases like osteoporosis.

Purpose of the Study:

• To investigate the role of Raf kinase inhibitor protein (RKIP) in regulating bone homeostasis.
• To elucidate the mechanisms by which RKIP influences macrophage differentiation in the bone niche.
• To explore the therapeutic potential of targeting RKIP for treating bone loss and osteoporosis.

Main Methods:

• Global and macrophage-specific knockout mouse models were utilized to study RKIP function.
• Macrophage differentiation assays were performed to assess osteoclastogenesis and pro-angiogenic subcluster formation.
• Molecular mechanisms involving RKIP interactions with ARHGAP, CDC42, HIF-1α, and VHL were investigated.
• Ovariectomy (OVX)-induced bone loss models were used to evaluate RKIP inhibition in vivo.

Main Results:

• RKIP knockout significantly increased bone mass in mice, attributed to reduced bone resorption and enhanced bone formation.
• RKIP deficiency inhibited osteoclast differentiation while promoting differentiation towards pro-angiogenic macrophage subclusters, enhancing H-type vessel formation.
• RKIP was found to promote osteoclastogenesis by suppressing CDC42 inactivation via interaction with ARHGAP.
• Intranuclear RKIP was shown to reduce HIF-1α protein stability by bridging HIF-1α and VHL, thereby suppressing angiogenic gene expression.
• Deletion or inhibition of RKIP rescued ovariectomy-induced bone loss in vivo.

Conclusions:

• RKIP plays a dual role in regulating bone niche macrophage differentiation, impacting both bone resorption and formation.
• Extranuclear and intranuclear RKIP have distinct functions in controlling macrophage polarization and vascularization within the bone microenvironment.
• Targeting RKIP presents a promising therapeutic strategy for managing bone homeostasis-related diseases, including osteoporosis.