Hematopoietic or Osteoclast-Specific Deletion of Syk Leads to Increased Bone Mass in Experimental Mice

Abstract

Syk is a non-receptor tyrosine kinase critically involved in signaling by various immunoreceptors including B-cell-receptors and activating Fc-receptors. We have previously shown that Syk also mediates immunoreceptor-like signals required for the <i>in vitro</i> development and function of osteoclasts. However, the perinatal lethality of <i>Syk</i>^-/- mice precluded the analysis of the role of Syk in <i>in vivo</i> bone metabolism. To overcome that problem, we generated mice with osteoclast-specific (<i>Syk</i>^Δ<i>OC</i> ) or hematopoietic (<i>Syk</i>^{Δ<i>Haemo</i>} ) Syk deficiency by conditional deletion of Syk using Cre recombinase expressed under the control of the Ctsk or Vav1 promoter, respectively. Micro-CT analysis revealed increased bone trabecular density in both <i>Syk</i>^Δ<i>OC</i> and <i>Syk</i>^{Δ<i>Haemo</i>} mice, although hematopoietic Syk deficiency caused a more severe phenotype than osteoclast-specific Syk deficiency. Osteoclast-specific Syk deficiency reduced, whereas hematopoietic Syk deficiency completely blocked <i>in vitro</i> development of osteoclasts. Both interventions inhibited the resorptive activity of osteoclasts and osteoclast-specific gene expression. Kinetic analysis of Syk protein levels, Cre expression and the genomic deletion of the <i>Syk</i>^flox allele revealed complete and early deletion of Syk from <i>Syk</i>^{Δ<i>Haemo</i>} osteoclasts whereas Syk was incompletely deleted at a later stage of osteoclast development from <i>Syk</i>^Δ<i>OC</i> cultures. Those results provide an explanation for the <i>in vivo</i> and <i>in vitro</i> difference between the <i>Syk</i>^Δ<i>OC</i> and <i>Syk</i>^{Δ<i>Haemo</i>} mutant strains and suggest late activation of, and incomplete target gene deletion upon, osteoclast-specific Cre expression driven by the Ctsk promoter. Taken together, our results indicate that Syk plays an indispensable role in osteoclast-mediated <i>in vivo</i> bone resorption and suggest that Syk-specific inhibitors may provide therapeutic benefit in inflammatory and other diseases characterized by excessive osteoclast-mediated bone resorption.