The Role of Parathyroid Hormone-Related Protein (PTHrP) in Osteoblast Response to Microgravity: Mechanistic Implications for Osteoporosis Development

Prolonged skeletal unloading through bedrest results in bone loss similar to that observed in elderly osteoporotic patients, but with an accelerated timeframe. This rapid effect on weight-bearing bones is also observed in astronauts who can lose up to 2% of their bone mass per month spent in Space. Despite the important implications for Spaceflight travelers and bedridden patients, the exact mechanisms involved in disuse osteoporosis have not been elucidated. Parathyroid hormone-related protein (PTHrP) regulates many physiological processes including skeletal development, and has been proposed as a mechanosensor. To investigate the role of PTHrP in microgravity-induced bone loss, trabecular and calvarial osteoblasts (TOs and COs) from Pthrp +/+ and -/- mice were subjected to actual Spaceflight for 6 days (Foton M3 satellite). Pthrp +/+, +/- and -/- osteoblasts were also exposed to simulated microgravity for periods varying from 6 days to 6 weeks. While COs displayed little change in viability in 0g, viability of all TOs rapidly decreased in inverse proportion to PTHrP expression levels. Furthermore, Pthrp+/+ TOs displayed a sharp viability decline after 2 weeks at 0g. Microarray analysis of Pthrp+/+ TOs after 6 days in simulated 0g revealed expression changes in genes encoding prolactins, apoptosis/survival molecules, bone metabolism and extra-cellular matrix composition proteins, chemokines, insulin-like growth factor family members and Wnt-related signalling molecules. 88% of 0g-induced expression changes in Pthrp+/+ cells overlapped those caused by Pthrp ablation in normal gravity, and pulsatile treatment with PTHrP1-36 not only reversed a large proportion of 0g-induced effects in Pthrp+/+ TOs but maintained viability over 6-week exposure to microgravity. Our results confirm PTHrP efficacy as an anabolic agent to prevent microgravity-induced cell death in TOs.

长期卧床引发的骨骼去负荷状态，会导致与老年骨质疏松症患者相似的骨丢失，但进程更为迅速。这种对负重骨的快速影响同样见于宇航员——在太空驻留期间，他们每月可流失高达2%的骨量。尽管该现象对航天旅行者与卧床患者具有重要意义，但废用性骨质疏松的确切发病机制尚未阐明。甲状旁腺激素相关蛋白（Parathyroid Hormone-related Protein, PTHrP）可调控包括骨骼发育在内的多种生理过程，且被认为是一种机械感受器。为探究PTHrP在微重力诱导骨丢失中的作用，本研究将来自Pthrp基因野生型（Pthrp+/+）、杂合敲除（Pthrp+/-）与纯合敲除（Pthrp-/-）小鼠的小梁骨成骨细胞（Trabecular Osteoblasts, TOs）及颅骨成骨细胞（Calvarial Osteoblasts, COs），搭载于光子-M3卫星开展了为期6天的实际太空飞行实验。同时，还将上述三种基因型的成骨细胞暴露于模拟微重力环境，处理时长从6天至6周不等。实验结果显示，颅骨成骨细胞在微重力环境下的细胞活力几乎无变化，但所有组别小梁骨成骨细胞的活力均迅速下降，且下降幅度与PTHrP表达水平呈负相关。此外，Pthrp+/+小梁骨成骨细胞在微重力环境中暴露2周后，细胞活力出现急剧下降。对暴露于模拟微重力环境6天的Pthrp+/+小梁骨成骨细胞进行基因芯片分析后发现，催乳素家族、凋亡/存活相关分子、骨代谢与细胞外基质组成蛋白、趋化因子、胰岛素样生长因子家族成员及Wnt相关信号分子的编码基因均出现表达变化。在正常重力环境下，Pthrp基因敲除所引发的表达变化，与88%的微重力诱导的Pthrp+/+细胞表达变化存在重叠；而采用PTHrP1-36片段进行脉冲处理，不仅可逆转Pthrp+/+小梁骨成骨细胞中大部分微重力诱导的效应，还可在长达6周的微重力暴露过程中维持细胞活力。本研究结果证实，PTHrP作为一种合成代谢制剂，可有效预防微重力诱导的小梁骨成骨细胞死亡。