類雌激素受體在斑馬魚離子細胞分化及功能調控之角色

Abstract

相較於陸生哺乳動物而言，魚類必須面對更多滲透壓及離子平衡調控上的挑戰，因此專職於離子運輸的離子細胞尤其得耗費更多的能量去掌控離子運輸的生理功能以面對不同滲透壓環境下對魚體造成的壓力。過去研究類雌激素受體(ERR)的主要功能多為調控能量上的代謝及粒線體之生理功能，而調控離子平衡的角色或其所需的能量運作上鮮少研究提及。離子細胞為一種高能量需求且富含粒線體的細胞，在魚類的滲透壓及離子平衡調控上扮演重要角色。本實驗目的研究ERR在斑馬魚離子運輸及滲透壓平衡中所扮演的角色及調控機制。過去的研究指出ERR中僅兩型受體α及δ之mRNA呈現胡椒鹽狀分布於斑馬魚胚胎表皮上，這種分布情形極為類似離子細胞散布狀態，故本研究針對此兩型受體作了以下實驗，實驗中發現在酸環境下會刺激ERRα及δ表現上升，且此兩型mRNA皆表現於富氫幫浦細胞(H+-ATPase-rich cell, HRC)，但在分別抑制此兩型受體蛋白表現後，僅α型會影響氫離子運輸功能，而δ型則否；另一影響HRC分化之轉錄因子膠細胞迷失因子2 (glial cell missing 2)表現量上升試圖彌補因ERRα抑制所造成的HRC數量的下滑。綜合以上所述，ERRα經由調控斑馬魚胚胎時期HRC分化參與了氫離子運輸之功能，而ERRδ在HRC中所參與的功能仍待研究。

Compared to terrestrial mammals, fish have to cope with more challenging osmotic and ionic gradients from aquatic environment with variant ion composition and pH values. Ionocytes, are highly energy-demanding and studied cells in research on fish ionoregulation, contribute an excellent model to study the regulatory mechanisms of ion transport. Estrogen-related receptors (ERRs) are members of orphan nuclear receptors and ubiquitously express in high-energy demanding organs for energy metabolism. Recently, ERRα was especially studied in mammalian kidney for sodium and potassium homeostasis through renin-angiotensin aldosterone system. In fact, skin/gill ionocytes in zebrafish are analogous to several types of the ion-transporting cells in mammalian kidney in terms of the expression and function of ion transporters. The purpose of the present study is to explore the role of zebrafish ERRs in ion regulatory mechanism by using molecular physiological approaches. zERRα and δ mRNA distributions showed “salt-and-pepper” patterns similar to ionocytes expression on the skin of zebrafish embryos so they would be the targets of the present study. The zERRα and δ mRNA expression were found to be stimulated by low pH ambience and also localized in HRC. However, in the loss-of-function experiments, the impairment of proton secretion only occurred when zERRα, but not zERRδ. Further, knockdown of zERRα decreased the cell density of H+-ATPase-rich cell (HRC) even with a concomitant increase of zebrafish glial cell missing 2 (zgcm2; a transcription factor specific for HRC differentiation) expression in zebreafish embryos. Hence, zERRα may play control zebrafish acid secretion function through regulation of zGCM2-mediated cell differentiation of HRC in the skin/gill. On the other hand, it will be an interesting and challenging issue to see if zERRδ, the other zERR member expressing in HRC, play an indirect role in acid secretion or is involved in other functions of zebrafish HRC.