Functional characterization and relevance of carnitine transporter OCTN2 to secondary carnitine deficiency.

在本研究中，从人和小鼠中鉴定了新的Na^+依赖性肉碱/有机阳离子转运蛋白家族，OCTNs，并对其进行了功能表征。我们分别从人和小鼠中分离到2个和3个OCTN家族成员。octn存在于多种组织中，包括肾脏、心脏、骨骼肌和胎盘，以及几种人类来源的癌细胞系。通过免疫组化分析，小鼠肾小管上皮细胞的OCTNs通常定位于管腔膜。大多数人和小鼠的octn具有转运肉碱和有机阳离子四乙基铵（TEA）的多功能性。此外，钠离子是人类和小鼠OCTN1和2转运肉毒碱所必需的。在系统性肉毒碱缺乏症（SCD）表型小鼠模型中，发现OCTN2基因突变。此外，在人类SCD患者中发现了几种突变，表明OCTN2是生理上重要的肉毒碱转运蛋白。有趣的是，TEA转运与钠无关。此外，OCTN2转运多种阳离子药物，如奎尼丁、异拉帕米和放射线霉素d。此外，由于人类OCTN2的一个突变丧失了肉毒碱转运活性，但保留了TEA转运活性，这表明OCTN2对肉毒碱和有机阳离子具有不同的功能位点。因此，octn被认为是一种多功能转运体，可分别以钠离子依赖和独立的方式转运肉毒碱和有机阳离子，对有机阳离子药物和肉毒碱的处置具有重要意义。此外，由于通过OCTN2的肉毒碱转运受到多种化合物的抑制，包括有机阳离子、有机阴离子（如丙戊酸）和中性化合物（如皮质酮），预计药物诱导的继发性肉毒碱缺乏可能是由OCTN2的相互作用引起的。

In the present study, novel Na^+-dependent carnitine/organic cation transporter family, OCTNs from human and mouse were identified and functionally characterized. We isolated two and three members of OCTN family from human and mouse, respectively. OCTNs are present in various tissues, including kidney, heart, skeletal muscle and placenta strongly, and in several human-derived cancer cell lines. By immunohistochemical analysis in kidney, mouse OCTNs localized commonly in luminal membrane of tubular epithelial cells. Most of human and mouse OCTNs exhibited multifunctionality by transporting both of carnitine and organic cation, tetraethylammonium (TEA). Furthermore, sodium ions were essential for carnitine transport by human and mouse OCTN1 and 2. In systemic carnitine deficiency (SCD) phenotype mouse model, juvenile visceral steotosis (jvs) mouse, mutation in OCTN2 gene was found. Furthermore, several kinds of mutation in human SCD patients were found, demonstrating that OCTN2 is a physiologically important carnitine transporter. Interestingly, TEA transport was sodium independent. In addition, OCTNs transporterd various cationic drugs such as quinidine, verapamil, and actinomycin D.Furthermore, since one mutation of human OCTN2 lost carnitine transport activity but retained TEA transport activity, it was suggested that OCTN2 have differential functional sites for carnitine and organic cations. So, OCTNs are thought to be multifunctional transporters by transporting carnitine and organic cations by sodium ion dependent and independent manner, respectively, and would be important for disposition of organic cationic drugs as well as carnitine. Furthermore, since carnitine transport via OCTN2 is inhibited various compounds, including organic cations, organic anions such as valproic acid and neutral compound such as cortisole, it is anticipated that drug-induced secondary carnitine deficiency could be caused by interaction on OCTN2.