纖維母細胞生長因子訊息對於斑馬魚始基生殖細胞遷移時期的重要性

Abstract

始基生殖細胞（primordial germ cell, PGC）是配子的前驅細胞。由於 PGC 與體細胞分歧於發育早期，因此需要橫越胚胎遷移至未來性腺形成的位置。在脊索動物中，體細胞分泌的趨化因子 Cxcl12 能夠激發 PGC 細胞膜上的受器 Cxcr4，引導 PGC 沿著趨化因子的濃度梯度移動。 利用斑馬魚為動物模式，我發現另一種訊息傳遞途徑——纖維母細胞生長因子 （fibroblast growth fator, Fgf）訊息傳遞也參與在 PGC 的遷移。首先，為了瞭解 PGC 是否能夠感知 Fgf 訊息，我將 PGC 專一表現綠螢光蛋白之轉基因魚 Tg(kop:egfp-F-nanos3-3'UTR) 的 PGC 分選出來，進行 RNA 表現量分析。我們發現斑馬魚 PGC 表現了 Fgf 受器（fgfr1a, fgfr1b, fgfr2, and fgfr4），其中 fgfr4 表現量最高，暗示斑馬魚 PGC 可以偵測 Fgf 訊息。 接著，為了檢驗 PGC 對於 Fgf 訊息的需求，我們利用了顯性失活形式（dominant negative form）的 Fgf 受器 dn-Fgfr3 來競爭內源性 Fgf 訊息傳遞，為了專一在 PGC 中過量表現 dn-Fgfr3，在其序列中連接了 nanos3 基因的 3 端非轉譯區（3'-UTR），並在斑馬魚胚發育一細胞時期顯微注射入細胞中。在對照組中，PGC 在受精後六小時分佈在胚盤的邊緣，並在受精後一日齡時抵達生殖脊；當 Fgf 訊息傳遞受干擾時，有些 PGC 在受精後六小時脫離了胚盤邊緣，往動物極（animal pole）移動，並在受精後一日齡時散落在胚的各處，表示 PGC 仰賴接受 Fgf 訊息完成定向遷移。此失能表現型可以透過同時讓 PGC 表現持續激活形式（constitutively active form）的 Fgf 受器 ca-Fgfr4 拯救之，表示 dn-Fgfr3 導致的 PGC 分佈異常為專一性抑制 Fgf 訊息傳遞的結果。 總結來說，我們發現斑馬魚 PGC 可能具有接收 Fgf 訊息的能力，而 Fgf 訊息參與在 PGC 的遷移中。據我們所知，這是第一個脊索動物 PGC 與 Fgf 訊息在體內直接互動的證據。

Gametes are derived from primordial germ cells (PGCs). As one of the first cell lineages specified, PGCs migrate across the embryo toward where the gonad forms during development. In vertebrates, somatic cell-secreted chemokine Cxcl12 is known to guide PGCs up the chemokine gradient. Using zebrafish as the animal model, I report here that fibroblast growth factor (Fgf) signaling is participated in PGC migration. To determine the competence of PGCs to respond to Fgf signals, I isolated PGCs from Tg(kop:egfp-F-nanos3-3'UTR) embryos to measure the gene expressions of Fgf receptors. I identified all five Fgf receptors (fgfr1a, fgfr1b, and fgfr2-­4) expressed in zebrafish PGCs while the fgfr4 transcript is the most abundant. It suggests PGCs may be able to perceive Fgf signals. Next, to assess the necessity of Fgf signaling by PGCs themselves, I performed loss­-of­-function experiments by introducing mRNA encoding a dominant negative form of Fgf receptor 3 (dn-­Fgfr3) with the 3'-UTR of nanos3, which inhibits its mRNA translation in the soma. In the control embryos, representative PGCs were aligned at the blastoderm margin during early gastrulation (6 hours post­-fertilization, hpf) and accumulated at the gonadal ridge at the end of somitogenesis (24 hpf). In those embryos in which the Fgf signaling in PGCs was repressed, however, some of PGCs were centralized toward the animal pole at 6 hpf and mislocalized throughout the embryo at 24 hpf. The loss-of-function phenotypes could be rescued by co-expression of constitutively-active Fgfr4 (ca-Fgfr4), suggesting the specific effects of dn-Fgfr3. Taken together, Fgf signals are required for directional migration of PGCs in zebrafish. My study illustrates the direct participation of Fgf signaling in PGC migration in vivo. To my knowledge, it is the first in vivo evidence to show that Fgf signaling plays a direct role in the migration of vertebrate PGCs.