Rab18透過Wnt訊息傳遞路徑調控出生後小鼠側腦室下區神經幹細胞功能

Abstract

成年哺乳類動物的大腦中有兩個腦區存在神經幹細胞，且終其一生負責成年神經元新生。其中一個腦區位於側腦室的腦室下區 (subventricular zone, SVZ)，位於此處的神經幹細胞 (neural stem cell, NSC) 會產生大量的神經母細胞 (neuroblast)，而這群神經母細胞會沿著吻測遷移流 (rostral migratory stream)遷移至嗅球 (olfactory bulb, OB)，進而分化成中間神經元 (interneuron)。胚胎發育時期，Wnt訊息傳遞路徑會維持神經前驅細胞 (neural progenitor cell) 的增殖，且近期研究發現Wnt訊息傳遞路徑對於成年神經元新生是必要的。Rab18是Ras相關的小GTP酶Rab家族中的成員之一，實驗室先前研究發現Rab18會活化Wnt訊息傳遞路徑。因此我們提出Rab18可能透過Wnt訊息傳遞路徑調控出生後小鼠的神經元新生的假設。我們取出生後第七天小鼠SVZ的神經幹細胞培養成初代神經球，發現Rab18會透過Wnt訊息傳遞路徑促進出生後小鼠SVZ神經幹細胞的增殖。銀杏內脂A (Ginkgolide A, GA) 以及銀杏內脂B (Ginkgolide B, GB) 是銀杏萃取物含量較高的成分。過去實驗室發現，GA和GB皆會活化Wnt訊息傳遞路徑，且GA會促進Wnt目標基因的表達，而GB則會透過Wnt訊息傳遞路徑促進神經幹細胞分化成神經元。因此我們提出當Rab18缺失時，GA和GB能夠挽救出生後小鼠SVZ神經元新生的假設。發現當Rab18缺失的情況下，GA能夠挽救出生後小鼠SVZ神經幹細胞增殖，而GB能夠挽救出生後小鼠SVZ神經幹細胞分化成神經元。綜合以上結果，Rab18會透過Wnt訊息傳遞路徑調控出生後小鼠SVZ神經元新生。Neural stem cells (NSCs) exist in two regions of the adult mammalian brain responsible for neurogenesis throughout life. One of the brain areas is at the subventricular zone (SVZ) of the lateral ventricle, where neural stem cells produce a large number of neuroblasts, which migrate through the rostral migratory stream (RMS) to the olfactory bulb (OB) and differentiate into interneurons. During embryonic development, Wnt signaling pathway maintains neural progenitor cell proliferation. Recent studies have found that the Wnt signaling pathway is necessary for adult neurogenesis. Rab18 is a member of the Ras-related small GTPases superfamily. Previous we find that Rab18 activates the Wnt signaling pathway and is necessary and sufficient for the expression of the Wnt target gene Cyclin D1. Therefore, we hypothesize that Rab18 may regulate neurogenesis in postnatal mouse SVZ through the Wnt signaling pathway. We overexpressed Rab18 and knocked down the Wnt effector β-catenin in primary neurospheres from NSCs of postnatal day seven SVZ and found that Rab18 promoted NSC proliferation through the Wnt signaling pathway. Ginkgolide A (GA) and Ginkgolide B (GB) are components from the Ginkgo biloba extract. Previous, we find that GA and GB activate the Wnt signaling pathway that GA promotes the expression of β-catenin and Cyclin D1, whereas GB promotes neuronal differentiation in postnatal NSCs. Therefore, we hypothesize that GA and GB may rescue neurogenesis defects in Rab18 deficient condition. We found that in Rab18 knockdown NSC cultures, GA rescued NSC proliferation and GB rescued neuronal differentiation. Taken together, our results suggest that Rab18 regulates adult neurogenesis through the Wnt signaling pathway in the postnatal SVZ.