Microenvironmental expansion of hematopoietic stem cells (HSCs) is induced by treatment with parathyroid hormone (PTH)or activation of the PTH receptor (PTH1R) in osteoblastic cells; however, the osteoblastic subset mediating this action of PTH is unknown. Osteocytes are terminally differentiated osteoblasts embedded in mineralized bone matrix but are connected with the BM. Activation of PTH1R in osteocytes increases osteoblastic number and bone mass. To establish whether osteocyte-mediated PTH1R signaling expands HSCs, we studied mice expressing a constitutively active PTH1R in osteocytes (TG mice
IntroductionTo survive throughout the life of an individual, hematopoietic stem cells (HSCs) must balance self-renewal and differentiation. 1 This essential regulation of stem cells is thought to be determined at least in part by the environment, or niche, in which these cells reside. 2 Hormonal stimulation by parathyroid hormone (PTH) results in HSC expansion through the niche, 3 but the PTH receptor (PTH1R) is not expressed in HSCs. 4 In the BM microenvironment, PTH1R is expressed in cells in the osteoblastic lineage, including Nestin ϩ cells, thought to represent mesenchymal stem cells (MSCs), which recent data suggest are regulatory components of the HSC niche. 5 Because PTH treatment expands the Nestin ϩ cell pool and causes it to more rapidly differentiate into its progeny, 5 it is not known which cells in the mesenchymal/osteoblastic lineage are responsible for the PTH-dependent signals that result in HSC expansion. We have demonstrated previously that expression of a constitutively active PTH receptor (caPTH1R) in immature and mature osteoblasts was sufficient to expand HSCs. 3 Therefore, the cell population capable of initiating microenvironmental changes that expand HSCs must comprise osteoblastic cells targeted by the 2.3-kb fragment of the mouse collagen I gene promoter (hereafter referred to as 2.3Col1 osteoblastic cells) and their progeny, which includes osteocytic cells.It was demonstrated recently that PTH stimulates osteocytes, in which activation of PTH1R signaling down-regulates the expression of the Wnt antagonist Sclerostin. 6 This effect of PTH on Sclerostin has also been demonstrated in postmenopausal women treated with PTH, 7 suggesting that Sclerostin down-regulation may be an important mediator of the bone anabolic action of PTH.Using an in vivo model in which a constitutively active PTH1R is targeted to osteocytes (dentin matrix protein-1 [DMP1]-caPTH1R mice, hereafter referred to as TG mice), we demonstrated that PTH-dependent signals in osteocytes control bone mass and bone remodeling. 8 Osteocytes are former osteoblasts buried into bone matrix but connected to the endosteum via cytoplasmic processes, 9 which sense mechanical stimuli and regulate both osteoblastic and osteoclastic numbers. 10 It was unknown whether osteocytes affect hematopoiesis and/or HSCs. In the present study, we examined whether signaling downstream of the PTH1R in osteocytes supports the expansion of HSCs by analyzi...