轉錄因子TFAP2A與TFAP2C在斑馬魚表皮離子細胞分化中扮演之角色

Abstract

魚類具有能調節離子及維持酸鹼平衡的離子細胞，目前已知有四型，分別為富鈉鉀幫浦細胞(Na+-K+-ATPase-riched cell)、富氫幫浦細胞(H+-ATPase-riched cell)、鈉氯離子運輸蛋白細胞(Na+-Cl--cotransporter cell)與泌鉀細胞(K+-secreting cell)。過去於離子細胞上的研究多著重於功能性的分析，較少著墨其細胞分化機制。目前已知魚類胚胎表皮發育完成於受精後14小時; 並且離子細胞於24小時發育成熟，而兩種基因，forkhead box class I3 與glial cell missing 2 已經證實分別參與其分化調控。此研究目的藉由尋找更上游的調控機制，以使離子細胞分化的模型更臻完整。transcription factor activator protein 2a (tfap2a) 與transcription factor activator protein 2c (tfap2c) 兩基因已知與Notch signaling共同調控老鼠皮膚發育。而於斑馬魚的研究中指出tfap2a與tfap2c最早期表現於非神經外胚層中，並且影響神經脊細胞及其衍生細胞的發育。先前研究並無探究tfap2a與tfap2c與魚類表皮發育的關連。魚類中已知調控離子細胞發育的醣皮質素cortisol，當其受器(glucocorticoid receptor)轉譯遭抑制時，會造成tfap2a基因表現顯著降低。 此篇研究發現tfap2a與tfap2c至少表現於兩類的離子細胞。並且藉由抑制tfap2a與tfap2c的轉譯，發現離子細胞的密度因此下降，此結果暗示tfap2a與tfap2c極有可能參與離子細胞的分化。並且以外源性cortisol浸泡魚類胚胎時，tfap2a與tfap2c基因表現皆會於第一天顯著增加 ; 而於第三天時，tfap2a的表現依然顯著上升，然而tfap2c則下降。總結以上所述，本篇研究發現tfap2a與tfap2c極有可能為cortisol的下游基因，並且受其調控而參與離子細胞的發育。

It has been reported for years that ionocytes, a specific cell group in fish skin epithelium and gill, are function on ion-regulation. Four subtypes of ionocytes, which are Na+-K+-ATPase-riched (NaR) cells, H+-ATPase-riched (HR) cells, Na+-Cl--cotransporter (NCC) cells and K+-secreting (KS) cells have been identified in zebrafish so far. Classification of ionocyte subtypes is based on expression of different transporters and enzymes. Even though ionocytes play important role in balancing osmolarity and pH value. Little is known about their developmental mechanism. From the very beginning, embryonic ectoderm completely develop into epidermis by 14 hours postfertilization (hpf), not only provide the barrier function between the external environment and the internal organ, but also differentiate to ionocytes later in 1 day postfertilization (dpf). During the germ layer formation, BMP signaling stimulates the expression of downstream target, p63, in epidermal stem cells and segregates epithelium cell linage from neural ectoderm. In the skin stem cell pool, delta-Notch signaling separates the ionocyte progenitors from keratinocyte progenitors via lateral inhibition. But the mechanism that regulates stem cell specification is uncertain. These ionocyte progenitors express two duplicated forkhead transcription factor, foxi3a and foxi3b, at tail-bud stage and then downregulate the p63 expression later at 14-somite stage. foxi3a-foxi3b promote a feed-back regulatory loop that may contribute to the subsequent differentiation of NaR and HR cells. Transcription factor activator protein 2a and 2c, Tfap2a and Tfap2c, are known to express in early embryonic ectoderm. In mice model, tfap2a and tfap2c redundantly control skin development and regulate the transition from skin basal proliferation to suprabasal differentiation. The present study demonstrates that tfap2a and tfap2c are expressed in at least two subtypes of ionocytes, which are NaR cells and HR cells. And the expression of tfap2a and tfap2c is related to the differentiation of ionocytes. Moreover, tfap2a and tfap2c are found to be the potential downstream targets of cortisol signaling, and they may mediate the regulation of ionocyte differentiation.