An SNX10-dependent mechanism downregulates fusion between mature osteoclasts

Maayan Barnea-Zohar,Moien Kanaan,Benjamin Geiger,Nina Reuven,Fadi Thalji,Sabina E Winograd-Katz,Moran Shalev,Esther Arman,Ofra Golani,Ari Elson,Merle Stein,Lee Roth,Jan Tuckermann

doi:10.1242/jcs.254979

Abstract

ABSTRACTHomozygosity for the R51Q mutation in sorting nexin 10 (SNX10) inactivates osteoclasts (OCLs) and induces autosomal recessive osteopetrosis in humans and in mice. We show here that the fusion of wild-type murine monocytes to form OCLs is highly regulated, and that its extent is limited by blocking fusion between mature OCLs. In contrast, monocytes from homozygous R51Q SNX10 mice fuse uncontrollably, forming giant dysfunctional OCLs that can become 10- to 100-fold larger than their wild-type counterparts. Furthermore, mutant OCLs display reduced endocytotic activity, suggesting that their deregulated fusion is due to alterations in membrane homeostasis caused by loss of SNX10 function. This is supported by the finding that the R51Q SNX10 protein is unstable and exhibits altered lipid-binding properties, and is consistent with a key role for SNX10 in vesicular trafficking. We propose that OCL size and functionality are regulated by a cell-autonomous SNX10-dependent mechanism that downregulates fusion between mature OCLs. The R51Q mutation abolishes this regulatory activity, leading to excessive fusion, loss of bone resorption capacity and, consequently, to an osteopetrotic phenotype in vivo.This article has an associated First Person interview with the joint first authors of the paper.

Highlights

Bone growth and remodeling requires coordinated regulation of osteoblasts, which deposit new bone matrix, and osteoclasts (OCLs) that degrade it
Cultured RQ/RQ OCLs are gigantic and unstable In order to monitor osteoclastogenesis of homozygous R51Q sorting nexin 10 (SNX10) (RQ/RQ) OCLs, splenocytes from RQ/RQ and wild-type (+/+) mice were induced to differentiate in culture for 5 to 7 days in the presence of M-CSF and RANKL
Deregulated fusion of RQ/RQ OCLs is not caused by aberrant RANKL signaling Cell fusion during osteoclastogenesis is driven by the continuous presence of RANKL; we examined whether the deregulated fusion of RQ/RQ OCLs might be caused by increased sensitivity to this cytokine

Summary

Introduction

Bone growth and remodeling requires coordinated regulation of osteoblasts, which deposit new bone matrix, and osteoclasts (OCLs) that degrade it. Disrupting the fine balance between these two cellular activities can lead to severe pathological states, such as osteoporosis and osteopetrosis OCLs adhere to mineralized tissue via integrin-based actin-rich adhesion structures, namely podosomes (Blair et al, 2009; Nakamura et al, 2007; Rodan and Rodan, 1997), that assemble into a robust sealing zone belt (SZ) that confines the bone surface area that is being degraded. The cells secrete proteases and protons from a specialized region of their ventral membrane known as the ruffled border, thereby degrading the protein and mineral components of the underlying bone (Novack and Teitelbaum, 2008)

Methods

Results

Conclusion