Glucose- and glutamine-dependent bioenergetics sensitize bone mechanoresponse after unloading by modulating osteocyte calcium dynamics
Xiyu Liu, … , Liangliang Shen, Da Jing
Xiyu Liu, … , Liangliang Shen, Da Jing
Published December 13, 2022
Citation Information: J Clin Invest. 2023;133(3):e164508. https://doi.org/10.1172/JCI164508.
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Research Article Bone biology

Glucose- and glutamine-dependent bioenergetics sensitize bone mechanoresponse after unloading by modulating osteocyte calcium dynamics

Abstract

Disuse osteoporosis is a metabolic bone disease resulting from skeletal unloading (e.g., during extended bed rest, limb immobilization, and spaceflight), and the slow and insufficient bone recovery during reambulation remains an unresolved medical challenge. Here, we demonstrated that loading-induced increase in bone architecture/strength was suppressed in skeletons previously exposed to unloading. This reduction in bone mechanosensitivity was directly associated with attenuated osteocytic Ca2+ oscillatory dynamics. The unloading-induced compromised osteocytic Ca2+ response to reloading resulted from the HIF-1α/PDK1 axis–mediated increase in glycolysis, and a subsequent reduction in ATP synthesis. HIF-1α also transcriptionally induced substantial glutaminase 2 expression and thereby glutamine addiction in osteocytes. Inhibition of glycolysis by blockade of PDK1 or glutamine supplementation restored the mechanosensitivity in those skeletons with previous unloading by fueling the tricarboxylic acid cycle and rescuing subsequent Ca2+ oscillations in osteocytes. Thus, we provide mechanistic insight into disuse-induced deterioration of bone mechanosensitivity and a promising therapeutic approach to accelerate bone recovery after long-duration disuse.

Authors

Xiyu Liu, Zedong Yan, Jing Cai, Dan Wang, Yongqing Yang, Yuanjun Ding, Xi Shao, Xiaoxia Hao, Erping Luo, X. Edward Guo, Peng Luo, Liangliang Shen, Da Jing

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Figure 2

The intracellular Ca2+ response to reloading in osteocytes exposed to previous unloading is markedly weakened both in vitro and in situ.

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The intracellular Ca2+ response to reloading in osteocytes exposed to pr...
(A) The experimental protocol of osteocyte rotation to establish the in vitro model of simulated microgravity (SMG) with 24 rpm for 48 hours and subsequent real-time Ca2+ imaging under fluid shear stress (FSS) stimulation. (B and C) Comparison of intracellular Ca2+ dynamics in normal and SMG-exposed MLO-Y4 osteocytic cells in vitro in response to FSS at 2 Pa, and the corresponding quantitative data (n = 90 cells for Ctrl and n = 120 cells for SMG). (D) The experimental protocol of 4-week tail suspension to establish the HU model and subsequent real-time Ca2+ imaging in tibial osteocytes in situ under uniaxial cyclic compressive loading based on a novel synchronized cyclic loading/confocal imaging technique. (E and F) Comparison of intracellular Ca2+ signaling in tibial osteocytes in situ in normal and tail-suspended mice in response to subsequent uniaxial cyclic compressive loading at 1,500 με tensile strain on the antemedial surface of the tibia, and the corresponding quantitative data (n = 72 cells for Ctrl and n = 60 cells for HU). (G and H) Effects of the inhibition of mitochondrial ATP generation using oligomycin (1 μM; a mitochondrial ATP synthase inhibitor) or FCCP (1 μM; a mitochondrial oxidative phosphorylation uncoupler) on intracellular Ca2+ dynamics in MLO-Y4 cells in vitro under fluid flow stimulation, and the corresponding quantitative data (n = 85 cells for vehicle, n = 90 cells for oligomycin, and n = 70 cells for FCCP). Graphs represent mean ± SD. ***P < 0.001 by 2-tailed unpaired Student’s t test. Scale bars: B and top panel of G, 40 μm; E and bottom panel of G, 25 μm.