NMUR1 in the NMU-Mediated Regulation of Bone Remodeling.

Yu-Tin Hsiao,Gabriella Born-Evers,Corinne E Metzger,Matthew R Allen,Maria E Squire,Julia M Hum,Kelli J Manikowski,Ashley L Orr,Tara Zukosky,Nicole Griffin,Elizabeth Q Hulsey,Jonathan W Lowery,Samantha Snyder

doi:10.3390/life11101028

Abstract

Neuromedin-U (NMU) is an evolutionarily conserved peptide that regulates varying physiologic effects including blood pressure, stress and allergic responses, metabolic and feeding behavior, pain perception, and neuroendocrine functions. Recently, several lines of investigation implicate NMU in regulating bone remodeling. For instance, global loss of NMU expression in male and female mice leads to high bone mass due to elevated bone formation rate with no alteration in bone resorption rate or observable defect in skeletal patterning. Additionally, NMU treatment regulates the activity of osteoblasts in vitro. The downstream pathway utilized by NMU to carry out these effects is unknown as NMU signals via two G-protein-coupled receptors (GPCRs), NMU receptor 1 (NMUR1), and NMU receptor 2 (NMUR2), and both are expressed in the postnatal skeleton. Here, we sought to address this open question and build a better understanding of the downstream pathway utilized by NMU. Our approach involved the knockdown of Nmur1 in MC3T3-E1 cells in vitro and a global knockout of Nmur1 in vivo. We detail specific cell signaling events (e.g., mTOR phosphorylation) that are deficient in the absence of NMUR1 expression yet trabecular bone volume in femora and tibiae of 12-week-old male Nmur1 knockout mice are unchanged, compared to controls. These results suggest that NMUR1 is required for NMU-dependent signaling in MC3T3-E1 cells, but it is not required for the NMU-mediated effects on bone remodeling in vivo. Future studies examining the role of NMUR2 are required to determine the downstream pathway utilized by NMU to regulate bone remodeling in vivo.

Highlights

Two additional factors differed in the ratio of the phosphorylated isoform isoform relative to that target’s total expression level: the level of Src homology and collagen relative to that target’s total expression level: the level of Src homology and collagen adaptor adaptor protein (Shc) phosphorylated at Tyrosine-427 was increased, while the mechanistic protein (Shc) phosphorylated at Tyrosine-427 was increased, while the mechanistic target of target of rapamycin kinase phosphorylated at Serine-2448 was decreased in Nmu rapamycin kinase phosphorylated at Serine-2448 was decreased in Nmu knockout knockout tibiae, compared to wild-type controls (Figure 1C,D and Table S2)
We are unaware of reports implicating these factors as regulated by NMU signaling, but notably, treatment of MC3T3-E1 cells with exogenous NMU25 led to increased phosphorylation of mTOR at Serine-2448 and this response was absent in Nmur1-KD cells
Several other factors were differentially regulated in Nmur1-KD cells, compared to controls, but, besides mTOR, none of those changes were reflected in Nmu knockout tibiae compared to controls

Summary

Introduction

Bone mass generally declines after the third decade of life due to the rate of bone resorption, carried out by osteoclasts, exceeding the rate of bone formation, carried out by osteoblasts [1]. Osteoporosis, characterized by low bone mass, places individuals at greater risk for fracture, disability, and death [2]. In the US, hospitalizations for osteoporotic fractures exceed those for heart attack, stroke, and breast cancer combined [3]. Osteoporosis rates are expected to rise significantly in the coming decades, with an estimated 3 million osteoporotic fractures per year by 2025 in the US [4,5]. A more complete understanding of the molecular pathways regulating the balance of bone resorption and bone formation may reveal new therapeutic approaches for improving bone mass and decreasing fracture risk in patients

Methods

Results

Conclusion