線蟲dpy-24基因整合在DTC細胞遷移過程中時間與空間的訊息調控

Abstract

細胞遷移在生物發育過程中扮演著重要的角色，細胞在什麼時間遷移到哪個地點，都必須經過嚴格的控制；現階段對細胞遷移的研究，僅針對時間或空間調控的個別探討，而整合兩者調控的相關機制目前瞭解得很少。在雌雄同體的線蟲個體中，有兩顆DTC細胞在特定的發育時期會進行特定的遷移路徑，這樣的遷移路徑也決定日後生殖腺的形狀。在DTC爬行過程中，荷爾蒙接收器DAF-12、F box蛋白質DRE-1及轉錄調控子LIN-29負責共同調控DTC在三齡幼蟲時期的遷移，其中包含了DTC從腹側到背側的背向遷移；此背向遷移需要DTC細胞表現UNC-5接收器，只要DTC表現UNC-5接收器，DTC便會接收到位在腹側Netrin蛋白質的排斥訊息，而遷移到線蟲的背側。在本研究中，我們分析dpy-24突變株、研究dpy-24基因，發現dpy-24能藉由整合三齡幼蟲發育時期的時間訊息及UNC-5腹側排斥的空間訊息，來調控DTC的背向遷移；當dpy-24發生突變時，DTC背向遷移會提早發生，而當dpy-24過度表現時，DTC的背向遷移則會延遲發生。DPY-24是一個具有zinc fingers的蛋白質，和哺乳類的轉錄抑制子Blimp-1和PRDI-BF1非常相似。藉由對unc-5轉錄表現的觀察，我們發現dpy-24會抑制unc-5的轉錄來阻止DTC提早進行背向遷移，之後免疫染色的結果顯現DPY-24在DTC中僅表現在DTC進行背向遷移之前，當DTC開始進行背向遷移後，DPY-24的表現便消失了。我們發現dre-1、daf-12與lin-29共同負責DPY-24在三齡幼蟲時期的削減，DPY-24在二齡幼蟲時期表現量最高，因此才能阻止DTC在不正確時間的背向遷移；而三齡幼蟲時期的時間訊息讓DPY-24消失，於是DPY-24不再抑制unc-5的轉錄，再加上此時LIN-29和DAF-12對於unc-5的活化，使得unc-5得以開始表現，在UNC-5順利表現後，DTC才能進行背向遷移。這個研究成果提供了一個在發育過程中，藉由整合時間與空間的訊息來調控特定細胞遷移的分子機制。

Cell migration plays a key role in animal development and must be temporally and spatially regulated. However, little is known about how temporal and directional signals are integrated to give rise to specific cell migration patterns. In the C. elegans hermaphrodite, two somatic distal tip cells (DTC) undergo a developmental stage- and direction-specific migration pattern which determines the shape of the gonad. The heterochronic genes daf-12, dre-1 and lin-29, encoding steroid hormone receptor, F-box protein, and zinc finger transcription factor, respectively, act together to control the third larval (L3) developmental program of the gonad, including the ventral-to-dorsal migration of the DTCs. The guidance receptor unc-5 is both necessary and sufficient for dorsal migration of the DTCs away from the ventrally concentrated extracellular cue netrin. Here, we identify and characterize dpy-24 mutants and show that dpy-24 links the L3 temporal signal to the spatial regulator unc-5 and thereby controls the timing of DTC dorsal migration. Mutations in dpy-24 results in precocious dorsalward turning of the DTCs, whereas constitutive expression of dpy-24 leads to retarded DTC dorsal turn. dpy-24 encodes a zinc-finger-containing protein, similar to mammalian transcription repressors Blimp-1 and PRDI-BF1. Using an unc-5 transcriptional gfp reporter, we show that dpy-24 prevents the DTCs from precocious dorsal turn by transcriptional repression of unc-5. The immunostaining data reveals that DPY-24 is present in the DTCs prior to their dorsalward turning and disappears during and after the dorsal turn. Further studies indicate that dre-1, daf-12 and lin-29 are responsible for DPY-24 down-regulation at the L3 stage. DPY-24 protein level is high in L2 and thus prevents DTC from dorsalward turning. The timely disappearance of DPY-24, which is regulated by L3-specific temporal signal, leads to concomitant unc-5 transcriptional up-regulation, likely mediated through transcription activities of LIN-29 and DAF-12, hence allowing DTC migration to switch towards the dorsal direction. These results provide a molecular mechanism by which temporal and spatial signals are integrated to control the precise cell migration pattern during development.