NEONATAL BILIRUBIN NEUROTOXICITY AND P-GLYCOPROTEIN

非结合型高胆红素血症是新生儿期最常见的临床疾病，严重时可导致神经损伤，伴长期不良神经发育后遗症，如早期出院的近足月婴儿再次出现核黄疸所证明。 高胆红素血症性脑病的发病机制尚不清楚，但其发展的核心是胆红素穿过血脑屏障（BBB）进入中枢神经系统（CNS）。 最近的研究表明，胆红素是ATP依赖性完整质膜外排泵磷酸糖蛋白或P-gp的底物。 P-gp在脑毛细血管内皮细胞的腔面上大量表达，并限制多种亲脂性化合物的脑内流。 我们已经观察到：i）成年P-gp缺陷转基因小鼠的脑胆红素摄取显著增加，和ii）与成年小鼠相比，胎儿和新生野生型小鼠脑中的P-gp表达（mRNA和蛋白质）显著降低。这些研究结果表明，P-gp起着重要的作用，在防止胆红素流入中枢神经系统和有限的P-gp表达在新生儿脑微血管系统可能会提高脑胆红素摄取在这个年龄组。 拟定的实验旨在检验三个假设：1）胆红素与P-gp相互作用并由P-gp转运; 2）脑P-gp表达、水平和功能以时间和空间方式调节; 3）代谢抑制、缺氧和/或缺氧/缺血组合损害P-gp功能。 我们将在体外使用（放射性配体结合和光亲和标记），细胞（小鼠脑毛细血管内皮细胞，和用人或小鼠P-gp基因转染的LLC-PK 1细胞），和体内（野生型和P-gp缺陷小鼠）模型，以表征i）胆红素和P-gp之间的相互作用，ii）CNS P-gp的个体发育和区域表达，以及iii）代谢抑制对P-gp的影响。除了我们的初步研究，没有关于P-gp在CNS中发育表达的信息，只有有限的数据胆红素和P-gp之间的相互作用和影响P-gp功能的因素。 研究结果将充分验证我们上述假设。所获得的信息将提供新的见解P-gp在衰减脑胆红素摄取的作用，并作为一种推动力，发展模式，增加BBB P-gp表达的新生儿，从而提高对新生儿胆红素神经毒性的保护。

Unconjugated hyperbilirubinemia is the most common clinical condition in the newborn period and when severe can result in neurologic injury with long term adverse neurodevelopment sequelae as evidenced by the reemergence of kernicterus in near-term infants subject to early hospital discharge. The pathogenesis of hyperbilirubinemic encephalopathy remains unclear but central to its development is the passage of bilirubin across the blood-brain barrier (BBB) into the central nervous system (CNS). Recent studies suggest that bilirubin is a substrate for the ATP dependent integral plasma membrane efflux pump phosphoglycoprotein or P-gp. P-gp is expressed in abundance on the luminal aspect of brain capillary endothelial cells and limits the brain influx of a variety of lipophilic compounds. We have observed i) that brain bilirubin uptake is significantly increased in adult P-gp deficient transgenie mice, and ii) that P-gp expression (mRNA and protein) is markedly lower in the fetal and neonatal wild type mouse brain as compared with adults. These findings suggest that P-gp plays an important role in preventing the influx of bilirubin into the CNS and that limited P-gp expression in the newborn brain microvasculature may enhance brain bilirubin uptake in this age group. The proposed experiments are designed to test three hypotheses: 1) that bilirubin interacts with and is transported by P-gp; 2) that brain P-gp expression, levels, and function are regulated in a temporal and spatial fashion, and 3) that metabolic inhibition, hypoxia, and/or a combination of hypoxia/ischemia impairs P-gp function. We will use in vitro (radioligand binding and photoaffinity labeling), cellular (mouse brain capillary endothelial cells, and LLC-PK1 cells transfected with the gene for human or mouse P-gp), and in vivo (wild type and P-gp deficient mice) models to characterize the i) interaction between bilirubin and P-gp, ii) the ontogeny and regional expression of CNS P-gp, and iii) the effects of metabolic inhibition on P-gp. Other than our preliminary studies, there is no information regarding the developmental expression of P-gp in the CNS, and only limited data on the interaction between bilirubin and P-gp and the factors that impact P-gp function. Results of the proposed studies will fully test our above stated hypotheses. The information obtained will provide novel insights regarding the role P-gp plays in attenuating brain bilirubin uptake and serve as an impetus towards developing modalities that increase BBB P-gp expression in newborns thereby enhancing protection against neonatal bilirubin neurotoxicity.