Osteoclast-specific Plastin 3 knockout in mice fail to develop osteoporosis despite dramatic increased osteoclast resorption activity

Abstract

PLS3 loss-of-function mutations in humans and mice cause X-linked primary osteoporosis. However, it remains largely unknown how PLS3 mutations cause osteoporosis and which function PLS3 plays in bone homeostasis. A recent study showed that the ubiquitous Pls3 KO in mice results in osteoporosis with decreased bone thickness and stiffness. In these mice, mainly osteoclasts were impacted in their function, exhibiting increased resorptive activity and altered podosome formation through a misregulation of the NFκB pathway. Specifically, Pls3 KO caused the decreased nuclear localization of its interaction partner NFκB repressing factor, NKRF, thereby augmenting Nfatc1 transcription. However, it has not been proven if, indeed, the osteoclasts are the major cell type affected and responsible for the osteoporosis development in ubiquitous Pls3 KO mice. Here, we generated osteoclast-specific Pls3 KO female (Pls3fl/fl; LysMCretg/0) and male (Pls3fl;LysMCretg/0) mice and demonstrate specific PLS3 loss in cultured osteoclasts. In addition, we developed a novel polyclonal PLS3 antibody that showed for the first time specific PLS3 loss in immunofluorescence staining of osteoclasts in contrast to previously available antibodies against PLS3 that failed to show PLS3-specificity in mouse cells. Moreover, we demonstrate that the osteoclast-specific Pls3 KO causes a dramatic increase in the resorptive activity of osteoclasts in vitro. Despite this pronounced effect on osteoclast resorption activity, osteoclast-specific Pls3 KO in vivo failed to cause any osteoporotic phenotype in 12-, 24-, and 48-week-old mice as proven by micro-CT and three-point bending test. These results demonstrate that the pathomechanism of PLS3-associated osteoporosis is highly complex and cannot be reproduced in a system singularly focused on one cell type, leading us to conclude that the loss of PLS3 in alternative bone cell types, such as osteoblasts and osteocytes contributes to the osteoporosis phenotype in ubiquitous Pls3 KO mice.