Abstract Skeletal homeostasis reflects a balanced state of bone remodeling, requiring a combination of osteoclast- dependent bone resorption and osteoblast-mediated bone formation. In turn, pathologic bone remodeling precipitates a variety of bone-destructive disease states, ranging from osteoporosis to rheumatoid arthritis. In addition to the traditional role of osteoclasts as "bone eaters", these cells also participate in the maintenance of bone homeostasis via the release of signaling molecules that establish crosstalk with osteoblast-lineage cells. Despite advances in the field, however, the molecular mechanisms that allow osteoclasts to both degrade and induce new bone formation in a timely fashion remain to be defined. In new studies, we have found that cathepsin K (Ctsk)-null osteoclasts dramatically upregulate expression of the membrane-anchored matrix metalloproteinase, Mmp14/MT1-MMP, raising the possibility that, working together, the two proteinases play a previously unrecognized role in regulating osteoclast function. As such, we generated myeloid-specific Csf1r- Cre/Mmp14f/f/Ctskf/f mice where we find that the double knockout (DKO) mice unexpectedly display striking increases in bone mass and osteogenic coupling activity relative to myeloid-specific cathepsin K-null (CtskΔM/ΔM) or Mmp14-null (Mmp14ΔM/ΔM) mice. Further, the up-regulated bone formation parameters observed in these mice occur while leaving bone resorptive activity comparable to that found in CtskΔM/ΔM mice. In an effort to define the mechanisms underlying a combined requirement for Mmp14 and Ctsk in osteoclast-mediated bone coupling activity, the transcriptome of DKO osteoclasts reveals unexpected alterations in TGFβ receptor signaling in tandem with the decreased expression of the tetraspanin family member, Tspan4. These findings led us to posit a new model wherein Mmp14 and Ctsk co-regulate the TGFβ/Smad-dependent control of Tspan4 whose expression directs the formation of osteoclast migrasomes, a new type of organelle/extracellular vesicle deposited by migrating cells. Further, we provide supporting evidence that wild-type osteoclast-derived migrasomes negatively regulate skeletal stem/progenitor cell recruitment and osteoblastogenesis, thereby preventing premature or unrestrained bone formation. In turn, migrasome-deficient DKO osteoclasts create conditions permissive for excessive osteogenesis and bone formation. Given these preliminary findings, we propose to i) characterize the cooperative role played by a new Mmp14/Ctsk couplet in controlling osteoclast- mediated osteogenic bone coupling functions in vitro and in vivo, utilizing both myeloid- and osteoclast-specific conditional knockout mice; ii) define a novel Mmp14/Ctsk-TGF-β/Smad-Tspan4 axis that regulates osteoclast migrasome formation and bone coupling activity in vitro and in vivo, and iii) identify the osteoclast-derived migrasome component(s) that control osteoblastogenic recruitment and commitment of sk...