Profilin 1 Negatively Regulates Osteoclast Migration in Postnatal Skeletal Growth, Remodeling, and Homeostasis in Mice

Jumpei Shirakawa,Ralph T Böttcher,Tadayoshi Hayata,Yoichi Ezura,Masaki Noda,Yayoi Izu,Shuhei Kajikawa,Mercedes Costell

doi:10.1002/jbm4.10130

Abstract

Profilin 1 (Pfn1), a regulator of actin polymerization, controls cell movement in a context-dependent manner. Pfn1 supports the locomotion of most adherent cells by assisting actin-filament elongation, as has been shown in skeletal progenitor cells in our previous study. However, because Pfn1 has also been known to inhibit migration of certain cells, including T cells, by suppressing branched-end elongation of actin filaments, we hypothesized that its roles in osteoclasts may be different from that of osteoblasts. By investigating the osteoclasts in culture, we first verified that Pfn1-knockdown (KD) enhances bone resorption in preosteoclastic RAW264.7 cells, despite having a comparable number and size of osteoclasts. Pfn1-KD in bone marrow cells showed similar results. Mechanistically, Pfn1-KD osteoclasts appeared more mobile than in controls. In vivo, the osteoclast-specific conditional Pfn1-deficient mice (Pfn1-cKO) by CathepsinK-Cre driver demonstrated postnatal skeletal phenotype, including dwarfism, craniofacial deformities, and long-bone metaphyseal osteolytic expansion, by 8 weeks of age. Metaphyseal and diaphyseal femurs were drastically expanded with suppressed trabecular bone mass as indicated by μCT analysis. Histologically, TRAP-positive osteoclasts were increased at endosteal metaphysis to diaphysis of Pfn1-cKO mice. The enhanced movement of Pfn1-cKO osteoclasts in culture was associated with a slight increase in cell size and podosome belt length, as well as an increase in bone-resorbing activity. Our study, for the first time, demonstrated that Pfn1 has critical roles in inhibiting osteoclast motility and bone resorption, thereby contributing to essential roles in postnatal skeletal homeostasis. Our study also provides novel insight into understanding skeletal deformities in human disorders.

Highlights

Skeletal homeostasis is maintained by various factors, including hormones, cytokines, growth factors, and coupling factors.[1,2] Those factors influence cell–cell communication, signaling, gene expression, and behaviors including division, proliferation, vesicular transportation, and migration
This study demonstrated that Pfn1 functions negatively in osteoclast movement and bone-resorbing activity
A novel aspect of Pfn1 function was demonstrated by postnatal growth failure in craniofacial and limb bones associated with an osteolytic expansion in osteoclast-specific Pfn1-deficient mice

Summary

Introduction

Skeletal homeostasis is maintained by various factors, including hormones, cytokines, growth factors, and coupling factors.[1,2] Those factors influence cell–cell communication, signaling, gene expression, and behaviors including division, proliferation, vesicular transportation, and migration. Osteoclasts are critical players in bone remodeling and belong to the macrophage lineage cells. Their differentiation and activation requires cell–cell fusion, vesicle trafficking, transcytosis, and sealing zone formation, etc.—all of which require cytoskeletal reorganization.[14,15] Characteristically, the osteoclasts utilize a cellular process, named podosome, with specific arrangements for their movement or attachment, depending on the differentiation/activation status.[16] Most characteristically, the activated osteoclasts at the terminal differentiation stage create the structure, the podosome belt, which is required for active bone resorption. We propose that observed skeletal phenotypes in these model mice might be related to the pathogenesis of certain osteolytic disorders that are possibly influenced by the modulation of the actin cytoskeleton

Methods

Results

Conclusion