Molecular mechanisms of adaptation to intrauterine and perinatal changes of oxygen partial pressure: The liver-to-kidney switch of Erythropoietin production as model system

导语：充足的氧平衡对胎儿宫内正常发育至关重要。对胎盘氧合的适应受到复杂的分子机制的精确调控，这些机制控制着氧气感知和氧气依赖基因的表达。对宫内缺氧的反应改变了发育程序，从个体遗传学上讲，只有在足月身体准备切换到呼吸空气的高氧分压(PO2)时，该程序才会发生巨大变化。在极早产儿中，氧依赖基因如血管内皮生长因子(VEGF)或促红细胞生成素(EPO)的正常表达立即停止会导致特征性疾病，这些疾病可能会对神经发育结局和重大疾病产生终生后果。为了研究氧稳态紊乱的病理生理学，EPO的调节是一个有趣的模型系统。EPO基因转录受氧感应系统的调控，氧感应系统包括脯氨酸羟基酶(PhDS)和低氧诱导因子(HIF)。依赖于HIF的EPO的表达不仅受到氧气供应(肾脏的高生产能力)反应的显著影响，而且还受到血液灌流(肝到肾的切换)的影响。因此，EPO模型将被用来阐明胎儿和新生儿在与早产儿疾病高度相关的低氧和高氧条件下的氧感应系统的适应。方法和工作方案：在发育过程中，通过使用PHD抑制剂稳定HIF(从而模拟低氧)或高氧来破坏HIF的稳定，将研究来自计时妊娠小鼠的胚胎/胎儿在调制氧气感应的条件下。HIF稳定和高氧均可模拟胎儿或早期新生儿氧平衡失调。这类治疗的效果以前从未被研究过。详细地，我们将检查宫内和围产期PO2的变化对HIF-PHD氧气传感系统适应性反应的影响。我们将比较怀孕小鼠短期或长期暴露在HIF稳定或降解状态下与正常情况下发育期间急性和慢性PO2变化的影响。将胎儿氧合紊乱与围产期急性缺氧或高氧症结合起来也是有意义的。在HIF-PHD系统的下游，我们将剖析EPO的产生以及EPO表达的两个发育开关的分子机制：1)在肝脏发育过程中从高表达，主要是低氧非依赖于低氧，但低氧依赖，以及2)从肝脏到肾脏。目的：通过这个研究项目，我们将了解氧传感和EPO产生在发育过程中的复杂调控。研究结果对了解早产儿特有疾病的一般病理生理学有重要意义。这最终将激发新的治疗概念，以改善长期结果。

INTRODUCTION: The adequate oxygen homeostasis is essential for proper intrauterine development. The adaptation to placental oxygenation is precisely regulated by complex molecular mechanisms that control oxygen sensing and expression of oxygen-dependent genes. The response to intrauterine hypoxia modifies a developmental program that should, ontogenetically, only dramatically change at term when the body is prepared to switch to high oxygen partial pressure (pO2) of air-breathing life. In very preterm infants, the immediate stop of proper expression of oxygen-dependent genes such as Vascular Endothelial Growth Factor (VEGF) or Erythropoietin (EPO) causes characteristic diseases that may have life-long consequences for neurodevelopmental outcome and significant morbidities. To study the pathophysiology of disorders of oxygen homeostasis, regulation of EPO is an intriguing model system. EPO gene transcription is modulated by the oxygen sensing system which includes the Prolyl Hydroxylases (PHDs) and Hypoxia Inducible Factor (HIF). The HIF-dependent EPO expression is subject to significant changes in the response to oxygen availability (higher production capacity in kidneys), but also to blood perfusion (liver-to-kidney switch). Thus, the EPO model will be utilized to elucidate the adaptation of the oxygen sensing system of the fetus and neonate under hypoxia and hyperoxia that are highly relevant for diseases of very preterm infants. METHODS and WORK PROGRAMME: Embryos/fetuses from timed-pregnant mice will be studied under conditions of modulated oxygen sensing during development by using a PHD inhibitor to stabilize HIF (and thereby mimic hypoxia) or hyperoxia to destabilize HIF. HIF stabilization and hyperoxia both imitate disturbed fetal or early neonatal oxygen homeostasis. The effects of such treatments have not been studied before. In detail, we will examine the impact of intrauterine and perinatal changes of the pO2 on the adaptive response of the HIF-PHD oxygen sensing system. We will contrast the influence of acute and chronic pO2 changes during development by short- or long-term exposure of pregnant mice to HIF stabilization or degradation vs. normal conditions. It will also be of interest to combine disturbed fetal oxygenation with acute perinatal hypoxia or hyperoxia. Downstream of the HIF-PHD system, we will dissect the Epo production and the molecular mechanisms of the two developmental switches of Epo expression: 1) from high, mostly hypoxia-independent to lower, but hypoxia-dependent expression during liver development, and 2) from the liver to the kidney. OBJECTIVES: With this research project, we will gain knowledge on the complex regulation of oxygen sensing and Epo production during development. The results will be highly relevant for the understanding of the general pathophysiology of characteristic diseases of very premature infants. This will ultimately inspirit novel treatment concepts for improved long-term outcomes.