Osteoporosis is a skeletal condition that weakens bones, making them porous, fragile and prone to breakage. Osteoporosis causes 8.9 million fractures each year, with one fracture occurring every three seconds! The elderly population is most vulnerable to primary osteoporosis, as their frailty often requires long-term treatment and support. Advances in healthcare and a corresponding increase in the ageing population have put pressure on available resources, highlighting the need for effective treatments against osteoporosis.
Induction of parathyroid hormone (PTH) signaling using a PTH-derived peptide – teriparatide – has demonstrated strong bone-promoting effects in patients with osteoporosis. These effects are mediated by osteogenesis, the process of bone formation that involves the differentiation and maturation of bone-forming cells, called osteoblasts. However, PTH induction is also associated with the differentiation of macrophages into osteoclasts, which are specialized cells responsible for bone resorption. Although bone remodeling by osteoblasts and osteoclasts is critical for maintaining skeletal health, PTH-induced osteoclast differentiation may reduce treatment efficacy in patients with osteoporosis. However, the exact molecular mechanisms underlying the dual action of PTH signaling in bone remodeling are not well understood.
To fill this gap, Professor Tadayoshi Hayata and Ms. Chisato Sampei from Tokyo University of Science, together with their colleagues, conducted a series of experiments to identify druggable target genes downstream of PTH signaling in osteoblasts. Explaining the rationale behind their study, published on May 20, 2024, Journal of Cellular PhysiologyCorresponding author, Prof. Hayata says, “In Japan, it is estimated that 12.8 million people, or one in ten people, suffer from osteoporosis, which can significantly impair their quality of life. Teriparatide is classified as a drug that promotes bone formation, but it also promotes bone resorption, which may limit bone formation. However, the full scope of its pharmacological action is unknown.”
The researchers treated cultured mouse osteoblast cells and mice with teriparatide. They then assessed gene expression changes induced by PTH in both cultured cells and bone cells isolated from the femur of treated animals, using advanced RNA-sequencing analysis. Among the many upregulated genes, they identified a new PTH-induced gene – 'GPRC5A', which encodes an orphan G protein-coupled receptor, previously discovered as a therapeutic target. However, its exact role in osteoblast differentiation was not fully understood.
PTH induction is known to activate the cyclic adenosine monophosphate (cAMP) and protein kinase C (PKC) signaling pathways. Interestingly, the team found that in addition to PTH induction, activation of cAMP and PKC also resulted in overexpression of Gprc5a, although to a lesser extent, which underlines the possible involvement of other molecular pathways. Notably, the upregulation of Gprc5a was suppressed upon inhibition of transcription, but, remained unaffected when protein synthesis was suppressed, suggesting that Gprc5a may be transcribed as early as possible in response to PTH signaling and that it serves as a direct target gene.
In addition, the researchers examined the effect of Gprc5a downregulation on osteoblast proliferation and differentiation. Notably, while PTH induction alone did not affect cell proliferation, Gprc5a knockdown resulted in increased expression of cell-cycle-related genes and osteoblast differentiation markers. These findings suggest that Gprc5a suppresses osteoblast proliferation and differentiation.
Diving deeper into the molecular mechanisms underlying the effects of GPRC5A in PTH-induced osteogenesis, the researchers identified activin receptor-like kinase 3 (ALK3) – a bone morphogenetic protein (BMP) signaling pathway receptor, as an interacting partner of GPRC5A. According to their speculation, overexpression of GPRC5A indeed led to suppression of BMP signaling through receptors including ALK3.
Overall, these findings suggest that GPRC5A – a novel induced target gene of PTH, negatively regulates osteoblast proliferation and differentiation by partially suppressing BMP signaling. Thus, GPRC5A could be adopted as a novel therapeutic target when designing treatments against osteoporosis. The study sheds light on the complex process of bone remodeling and explains the bone promoting and bone resorption effects of PTH signaling.
“Our study suggests that Gprc5a may act as a negative feedback factor for the bone formation-promoting effect of teriparatide. Therefore, suppressing Gprc5a function may increase the effectiveness of teriparatide in non-responsive patients. In the future, we hope that our research will lead to improved quality of life and healthy longevity for people with osteoporosis,” concluded Prof. Hayata.
We also hope that these findings will pave the way for the development of effective treatments that can improve the lives of people suffering from osteoporosis.