Ephrin-B家族過度表現誘導斑馬魚胚胎神經纖維之生長

Abstract

Ephrin與其受體Eph皆為細胞膜上之蛋白，為不同細胞之間傳遞訊息的媒介，能夠調控細胞的遷移與神經系統的發育。在許多例子中，ephrin對於神經的發育扮演著負向的調控者，它們能夠刺激神經生長錐的萎縮而抑制神經纖維的生長。在本實驗中，為了釐清ephrinBs對於斑馬魚胚胎發育時期之神經纖維生長的影響，我選殖了斑馬魚的四種ephrinB基因，包括ephrinB1、B2a、B2b以及B3，並且藉由兩種具有神經專一性之啟動子表現於斑馬魚胚胎中。實驗結果發現，GFAP啟動子表現這四種ephrinBs時，皆能促進神經纖維的過度生長；而同樣的現象也可以在HuC啟動子表現ephrinB2a以及ephrinB2b時被觀察到；然而，若為HuC啟動子表現ephrinB1以及ephrinB3時，則並沒有神經纖維過度生長的現象發生。為了進一步釐清ephrinBs是否藉由調控small Rho family GTPases的活性而造成神經纖維的過度生長，我將ephrinBs與下列各small Rho family GTPases之突變型同時表現，包括Rac1 T17N顯性抑制突變、RhoA G14V持續性表現突變、或者Cdc42 T17N顯性抑制突變，結果不論是在GFAP啟動子表現四種ephrinBs或者HuC啟動子表現ephrinB2a以及ephrinB2b之情況下，這些small Rho family GTPases之突變型皆能夠造成神經纖維過度生長的程度有明顯的下降。並且，利用Rac1 inhibitor處理GFAP或HuC啟動子表現ephrinBs之斑馬魚胚胎，其神經纖維過度生長的程度也有下降的趨勢；另外，處理ROCK inhibitor則是能夠恢復因表現RhoA G14V突變而受抑制之神經纖維過度生長。因此，這些實驗的結果說明，在斑馬魚胚胎發育時期，ephrinBs之過度表現能夠透過調控small Rho family GTPases的活性而造成神經纖維過度生長的發生。

The ephrins and their Eph receptors are membrane-bound molecules that mediate cell-to-cell signals implicated in the regulation of cell migration and development of nervous system. In many cases, ephrins act as negative regulators that stimulate growth cone collapse and inhibit neurite outgrowth. In this study, to investigate the effects of ephrinBs on neurite outgrowth, I cloned ephrinB genes of zebrafish including ephrinB1, B2a, B2b, B3 and driven them by two neural-specific promoters in zebrafish embryos. I found that overexpression of all the ephrinBs by GFAP promoter stimulated neurite outgrowth. Furthermore, both ephrinB2a and ephrinB2b expressed by HuC promoter also stimulated neurtie outgrowth, while ephrinB1 and ephrinB3 had no effect on neurite outgrowth. To investigate the possibility that ephrinBs stimulated neurite outgrowth via regulating small Rho family GTPases, expressing Rac1 T17N dominant negative mutant, RhoA G14V constitutively active mutant, or Cdc42 T17N dominant negative mutant, respectively, decreased the level of neurite outgrowth which was induced by GFAP and HuC promoter driving ephrinBs. Moreover, treatment with Rac1 inhibitor decreased the level of neurite outgrowth which was induced by ephrinBs, and ROCK inhibitor also abolished the effect of RhoA G14V constitutively active mutant on inhibiting ephrinBs-induced neurite outgrowth. These results indicated that overexpression of ephrinBs induced neurite outgrowth by regulating small Rho family GTPases in zebrafish embryos.