Rab18調控出生後小鼠神經幹細胞的功能及成年小鼠的行為

Abstract

成年哺乳動物的大腦中有兩個區域存在神經幹細胞，分別位於側腦室 (lateral ventricle)的腦室下區 (subventricular zone, SVZ) 以及海馬迴之齒狀回 (dentate gyrus, DG) 的顆粒下區 (subgranular zone, SGZ)。腦室下區及顆粒下區的神經幹細胞分別會在嗅球 (olfactory bulb, OB) 及齒狀回中產生新的神經元，過去研究指出嗅球與齒狀回的神經元新生對成年哺乳類動物的嗅覺行為及空間學習與記憶是必要的。Rab18是Ras相關的小GTP酶Rab家族之成員，實驗室先前的研究中發現Rab18對成年小鼠齒狀回的神經元新生是必要的，因此我們想知道Rab18是否會透過調控成年小鼠齒狀回的神經元新生來影響小鼠的空間學習與記憶。我們利用了巴恩斯迷宮 (Barnes maze task) 測試老鼠的空間學習、空間短期以及長期記憶的能力，並且分別針對公鼠及母鼠進行實驗。從實驗結果我們發現，Rab18對於成年公鼠的空間學習以及公鼠和母鼠的長期記憶是必要的。過去的研究也發現Rab18對成年小鼠嗅球的神經元新生是必要的，因此我們想知道Rab18對成年小鼠嗅覺功能為何。透過氣味辨識和氣味敏感度實驗我們發現，Rab18對小鼠的氣味辨識能力沒有影響。過去的研究發現Sonic hedgehog (Shh) 和Wnt訊息傳遞路徑可以調控神經元新生，而實驗室過去也發現Rab18可以活化這兩個訊息傳遞路徑，因此我們首先好奇Rab18是否會透過活化Shh訊息傳遞路徑來調控神經元新生。我們發現Rab18會透過Shh訊息傳遞路徑促進神經幹細胞增殖，但不會影響神經幹細胞的自我更新及分化成神經元的能力。接著我們想知道Rab18是否會促進神經幹細胞分泌Wnt來活化Wnt訊息傳遞路徑，利用酵素結合免疫吸附分析法 (Enzyme-linked immunosorbent assay, ELISA) 發現Rab18可能可以促進神經幹細胞分泌Wnt7a。綜合以上結果，我們得知Rab18對於公鼠的空間學習與長期記憶、母鼠的空間長期記憶是必要的，但不影響公鼠的氣味辨識能力。此外，Rab18可以透過Shh訊息傳遞路徑調控神經幹細胞的增殖，也可能可以促進神經幹細胞分泌Wnt7a。Neural stem cells (NSCs) exist in two places in the adult mammalian brain, the subventricular zone (SVZ) of the lateral ventricle and the subgranular zone (SGZ) of the dentate gyrus (DG) in the hippocampus. NSCs in the SGZ and SVZ give rise to new neurons in the olfactory bulb (OB) and DG, respectively. Adult OB and hippocampal neurogenesis is required for olfactory behaviors and spatial learning and memory. Rab18 is a member of Rab proteins, which belong to Ras-related superfamily of small GTPase. Previously, we found that Rab18 was required for adult neurogenesis in the SGZ. We hypothesized that Rab18 played roles in spatial learning and memory in mice. Barnes maze task was carried out to examine spatial learning and memory of wildtype and Rab18 knockout mice. We found that Rab18 was required for spatial learning and long-term memory in male mice, as well as spatial long-term memory in female mice. We also found that Rab18 was required for adult neurogenesis in the OB. We hypothesized that Rab18 played roles in olfactory abilities in mice. We found that Rab18 was not required for olfactory discrimination. Previous studies have reported that the Sonic hedgehog (Shh) and Wnt signaling pathways regulate adult neurogenesis. It was previously discovered that Rab18 activated both of these signaling pathways. Therefore, we hypothesized that Rab18 regulated adult neurogenesis through the Shh signaling pathway. We found that Rab18 promoted neural stem cell proliferation via the Shh signaling pathway. Next, we examined whether Rab18 promoted the secretion of Wnt. We found that Rab18 might promote the secretion of Wnt7a in neural stem cells by using enzyme-linked immunosorbent assay (ELISA). Taken together, our results suggest that Rab18 is require for spatial learning and long-term memory in male mice and spatial long-term memory in female mice. In addition, Rab18 may promote the secretion of Wnt7a by neural stem cells and regulates the proliferation of neural stem cells through the Shh signaling pathway.