Parathyroid Hormone Directs Bone Marrow Mesenchymal Cell Fate

Area of Science:

• Endocrinology
• Bone Biology
• Stem Cell Biology

Background:

• Intermittent parathyroid hormone (PTH) administration is a recognized therapy for increasing bone mass and preventing fractures.
• The precise cellular mechanisms underlying PTH's anabolic effects on bone remain incompletely understood.

Purpose of the Study:

• To elucidate the role of the PTH/PTHrP receptor (PTH1R) in mesenchymal stem cells (MSCs) in regulating bone metabolism and marrow adipose tissue (BMAT).
• To investigate the contribution of BMAT to bone resorption and the potential therapeutic effects of PTH on BMAT.

Main Methods:

• Genetic deletion of PTH1R in Prx1-expressing MSCs using a Prx1Cre mouse model.
• Analysis of bone formation, resorption, and BMAT content in mutant and control mice.
• Cell sorting and characterization of bone marrow progenitors, including Pref1 and RANKL expression.
• Assessment of RANKL levels in bone marrow supernatant and serum.
• Evaluation of the effect of intermittent PTH administration on BMAT in mice and male osteoporotic patients.

Main Results:

• Genetic deletion of PTH1R in MSCs led to reduced bone formation, increased bone resorption, and significantly elevated BMAT.
• Bone marrow adipocytes were identified as originating from Prx1-expressing progenitors and exhibited high expression of adipogenic markers and RANKL.
• Elevated RANKL levels were detected in bone marrow supernatant and serum of mutant mice.
• The population of Pref1+RANKL+ marrow progenitors was significantly increased in mice lacking PTH1R in MSCs.
• Intermittent PTH administration effectively reduced BMAT in control mice and showed a similar trend in male osteoporotic patients.

Conclusions:

• Marrow adipocytes possess both osteogenic and adipogenic potential and are uniquely responsive to PTH.
• PTH administration can decrease BMAT, suggesting a novel mechanism for its therapeutic action in bone diseases.
• These findings highlight the critical role of PTH1R signaling in MSCs for maintaining bone homeostasis and suggest that targeting BMAT is a viable therapeutic strategy for osteoporosis.