Parathyroid Hormone Remodels Bone Transitional Vessels and the Leptin Receptor-Positive Pericyte Network in Mice

Abstract

Intermittent parathyroid hormone (iPTH) is anti‐osteoporotic and affects bone vessels. Transitional capillaries close to the bone surface, which express both endomucin (Edm) and CD31, bear leptin receptor‐expressing (LepR) perivascular cells that may differentiate into osteoblasts. Increased numbers of type H endothelial cells (THEC; ie, Edmhi/CD31hi cells assessed by flow cytometry, FACS) are associated with higher bone formation in young mice. We hypothesized that iPTH administration impacts transitional vessels by expanding THECs. Four‐month‐old C57/Bl6J female mice were injected with PTH 1–84 (100 μg/kg/d) or saline (CT) for 7 or 14 days. We quantified LepR+, CD31+, Edm+ cells and THECs by FACS in hindlimb bone marrow, and Edm/LepR double immunolabelings on tibia cryosections. Additionally, we analyzed bone mRNA expression of 87 angiogenesis‐related genes in mice treated with either intermittent or continuous PTH (iPTH/cPTH) or saline (CT) for 7, 14, and 28 days. iPTH dramatically decreased the percentage of THECs by 78% and 90% at days 7 and 14, respectively, and of LepR+ cells at day 14 (–46%) versus CT. Immunolabeling quantification showed that the intracortical Edm+‐vessel density increased at day 14 under iPTH. In the bone marrow, perivascular LepR+ cells, connected to each other via a dendrite network, were sparser under iPTH at day 14 (–58%) versus CT. iPTH decreased LepR+ cell coverage of transitional vessels only (–51%), whereas the number of LepR+ cells not attached to vessels increased in the endocortical area only (+ 49%). Transcriptomic analyses showed that iPTH consistently upregulated PEDF, Collagen‐18α1, and TIMP‐1 mRNA expression compared with CT and cPTH. Finally, iPTH increased immunolabeling of endostatin, a Collagen‐18 domain that can be cleaved and become antiangiogenic, in both endocortical (79%) and peritrabecular transitional microvessels at day 14. Our results show that iPTH specifically remodels transitional vessels and suggest that it promotes LepR+ cell mobilization from these vessels close to the bone surface. © 2019 American Society for Bone and Mineral Research.

Perivascular osteoprogenitors are associated with transcortical channels of long bones

“…Intermittent PTH is also known to remodel vessels and induces perivascular cells to subsequently migrate to bone surfaces; it is also a strong inducer of bone formation. We wanted to test whether the iDMP/T+;ColGFP− cortical progenitor cells respond to intermittent PTH treatment and whether this would influence bone regeneration.…”

“…They are important in maintaining the haematopoietic niche as depletion of Nestin‐positive cells leads to significant reduction of haematopoietic progenitors . Leptin‐receptor Cre marks a mesenchymal stromal progenitor cell with potential to become predominantly osteoblasts and adipocytes . Although generated postnatally, these cells are quiescent in nature until bone injury occurs which triggers their proliferation and differentiation …”

Gli1+ Cells Couple with Type H Vessels and Are Required for Type H Vessel Formation

“…The above angiogenic proteins have shown pharmacological potential as osteoanabolic therapies. In addition, a recent study has reported that intermittent parathyroid hormone treatment expands type H ECs and mobilizes perivascular LepR + cells, indicating extrinsic hormonal regulation of type H vessels in relation to MSC-based anti-osteoporotic actions ( Caire et al., 2019 ). Recent studies have also indicated that distinct MSC subsets reside adjacent to type H or type L vessels ( Sivaraj and Adams, 2016 ), e.g., platelet-derived growth factor receptor beta (PDGFRβ) + and NG2 + subpopulations are associated with the type H vasculature ( Kusumbe et al., 2016 ), while the type L sinusoids are surrounded by subpopulations expressing LepR and CXCL12 ( Ding et al., 2012 ; Sugiyama et al., 2006 ).…”

“…Importantly, the type H endothelium has been revealed as a crucial mediator of bone regeneration and a pharmacological target to counteract bone loss and enhance fracture healing ( Kusumbe et al., 2014 ; Xu et al., 2018 ). Mechanistically, a series of cellular and molecular basis has been reported to regulate type H vessel formation ( Caire et al., 2019 ; Huang et al., 2016 ; Ramasamy et al., 2014 ; Xie et al., 2014 ; Xu et al., 2018 ; Yang et al., 2017 ). Nevertheless, the MSC-mediated regulation of type H vessels is not clear.…”

Loss of Adenylyl Cyclase 6 in Leptin Receptor‐Expressing Stromal Cells Attenuates Loading‐Induced Endosteal Bone Formation

“…Recently, leptin receptor‐positive (LepR + ) cells were identified as being perivascular and a major source of the stem cell fraction within the bone marrow. ( 3,15–19 ) Additionally, these LepR + cells were found to express the bone marrow MSC markers PDGFRα and CD51, and to be highly enriched for fibroblast colony‐forming units. Moreover, analyses indicated that LepR + cells in the bone marrow largely overlap with Nestin, an intermediate filament protein that is known as a neural stem/progenitor marker in adult bone marrow.…”