Ranieri Marianna, Venneri Maria, Storlino Giuseppina, Ferrulli Angela, D'Agostino Mariagrazia, Centrone Mariangela, Di Mise Annarita, Zerlotin Roberta, Tamma Grazia, Grano Maria, Valenti Giovanna
Department of Biosciences, Biotechnologies and Environment, University of Bari, Bari, Italy.
Istituti Clinici Scientifici Maugeri SPA SB IRCCS, Bari, Italy.
Front Physiol. 2025 Apr 15;16:1535053. doi: 10.3389/fphys.2025.1535053. eCollection 2025.
Murine hindlimb unloading (HU) is considered a model of choice for simulating the physiological effects of microgravity on several functions, including fluid and electrolyte homeostasis. Microgravity causes changes in blood redistribution, modulating vasopressin secretion, a major hormone controlling water reabsorption through the vasopressin-sensitive water channel AQP2. In this study, mice were hindlimb suspended over 4 weeks or rested in the ground as controls, and vasopressin levels, along with renal aquaporins expression were investigated. Copeptin, a stable precursor of the hormone vasopressin, significantly increased as early as 1 week of unloading which correlated with a significant increase in AQP2 total protein expression and decrease in serum osmolality, suggesting early activation of the vasopressin/AQP2 axis in this model. Conversely, in 4 weeks HU suspended mice, copeptin decreased significantly and both AQP2 mRNA and AQP2 total protein expression were significantly reduced. Consistent with a downregulation of the vasopressin/AQP2 axis an increase in serum osmolality was observed at 4 weeks HU. The basolateral water channels AQP3 and AQP4 were, on the other hand, unaffected. Immunolocalization studies confirmed reduced expression of AQP2 in renal collecting ducts of HU mice at 4 weeks. A significantly increased amount of the expressed AQP2 was found phosphorylated at Ser261, a site regulating AQP2 protein stability and degradation. In line, p38-MAPK, committed to phosphorylate Ser261 and to increase miR137 expression, an AQP2 mRNA-targeted microRNA, was significantly increased in HU, suggesting that reduced AQP2 expression was mainly due to increased protein degradation and downregulation of AQP2-mRNA translation. Our results suggest that vasopressin/AQP2 axis is upregulated as early as 1 week and may be involved in the antidiuretic response also observed in early spaceflight period in astronauts. Contrariwise, the vasopressin-AQP2 system is downregulated after 4 weeks HU, likely to counteract the persistent central venous pressure due to cephalic shift of fluids.
小鼠后肢卸载（HU）被认为是模拟微重力对多种功能（包括体液和电解质稳态）生理影响的首选模型。微重力会导致血液重新分布发生变化，调节抗利尿激素分泌，抗利尿激素是一种通过抗利尿激素敏感水通道蛋白2（AQP2）控制水重吸收的主要激素。在本研究中，将小鼠后肢悬吊4周，或作为对照置于地面休息，并研究抗利尿激素水平以及肾水通道蛋白的表达。抗利尿激素的稳定前体copeptin早在卸载1周时就显著增加，这与AQP2总蛋白表达的显著增加以及血清渗透压的降低相关，表明该模型中抗利尿激素/AQP2轴的早期激活。相反，在悬吊4周的HU小鼠中，copeptin显著降低，AQP2 mRNA和AQP2总蛋白表达均显著减少。与抗利尿激素/AQP2轴的下调一致，在悬吊4周的HU小鼠中观察到血清渗透压升高。另一方面，基底侧水通道蛋白AQP3和AQP4未受影响。免疫定位研究证实，4周时HU小鼠肾集合管中AQP2的表达减少。发现表达的AQP2中有显著增加的量在Ser261位点发生磷酸化，该位点调节AQP2蛋白的稳定性和降解。同样，负责磷酸化Ser261并增加miR137表达（一种靶向AQP2 mRNA的微小RNA）的p38丝裂原活化蛋白激酶（p38-MAPK）在HU小鼠中显著增加，表明AQP2表达降低主要是由于蛋白降解增加和AQP2-mRNA翻译下调。我们的结果表明，抗利尿激素/AQP2轴早在1周时就上调，可能参与了宇航员在太空飞行早期也观察到的抗利尿反应。相反，悬吊4周的HU后抗利尿激素-AQP2系统下调，可能是为了抵消由于液体向头部转移导致的持续中心静脉压。
