OR29-01 Deleting Salt Inducible Kinases In Osteoprogenitor Cells Increases Trabecular Bone

Disclosure: G. Papaioannou: None. A. Morellato Alcantara: None. K. Strauss: None. H.M. Kronenberg: None. M.N. Wein: Other; Self; Radius Health, Inc, research funding and patent royalties, holds equity in and is a scientific advisory board member of Relation Therapeutics, consultant for AstraZeneca, Guidepoint LLC, and Aditum Bio. Introduction: Osteoblasts are continuously generated by the differentiation of skeletal progenitor cells. Osx positive cells are committed osteoprogenitors. Ebf3 positive cells are a group of bone marrow stromal cells marked by CXCL12 expression that are thought to contribute to osteoblasts in adult mice. Salt-inducible kinases (SIKs) are key kinases that control the skeletal actions of parathyroid hormone. We previously reported that deletion of SIK genes in osteoblasts/osteocytes (using Dmp1-Cre) or ubiquitously (using Ub-Cre(ERt2)) causes dramatic increases of trabecular bone and stromal cells. However, the specific osteoblast progenitor populations through which SIKs control bone homeostasis downstream of PTH signaling are still to be determined. Methods: SIK genes were deleted from Osx or Ebf3 positive cells in 3-day-old, 6- or 9-week-old mice using the Cre(ERt2) (tamoxifen inducible) system. OsxCre(ERt2) mice were crossed with animals harboring floxed Sik2 and Sik3 genes. Ebf3Cre(ERt2) mice were crossed with Sik1, Sik2, and Sik3 floxed mice. These mice also bear a tdTomato reporter that labels cells and their descendants upon Cre recombination. After sacrifice, tibiae were fixed in paraformaldehyde or formalin and decalcified in EDTA for frozen or paraffin sections respectively. Results: In OsxCre(ERt2); Sik2(f/f);Sik3(f/f);R26R-Tomato mice tamoxifen was injected at 6 weeks old or P3 and tibias were harvested 4 weeks later. In both cases there was a dramatic increase in trabecular bone mass, that filled a large part of the bone marrow, and an increase in stromal cells. Most of the osteoblasts and many of the stromal cells were tomato positive, indicating that they derive from the osteoprogenitors in which SIKs were deleted. In Ebf3Cre(ERt2);Sik1(f/f);Sik2(f/f);Sik3(f/f);R26R-Tomato mice, tamoxifen was injected at 9-weeks of age and tibiae examined 3 and 9 weeks later, or tamoxifen was given at 3 days of age and tibiae examined 4 weeks later. There were no obvious differences in H/E staining and no significant differences in the number or localization of cells labeled with tomato reporter. The number of tomato positive cells was small (compared to the OsxCreER model) at all ages examined. Some of these cells, were lining the bone, presumably becoming osteoblasts, but interestingly did not increase their number despite SIK deletion and in contrast with the OsxCreER model. Conclusion: Conditional deletion of SIK genes in Osx positive cells leads to a dramatic increase of trabecular bone and stromal cells in long bones suggesting a crucial role for SIKs in more committed osteoprogenitors and their descendants. The SIK pathway may not play a role in the subset of mesenchymal progenitor cells labeled with Ebf3Cre(ERt2). Presentation: Sunday, June 18, 2023