NCS-1與Auxilin-1在神經細胞中對於訊息傳遞的影響

Abstract

鈣離子在神經細胞訊息傳遞中扮演了非常重要的角色，而這些功能多半是由具鈣離子結合功能的蛋白質進行作用。Neuronal calcium sensor-1 (NCS-1)是表現在神經細胞中且具有細胞膜結合能力的鈣離子感應蛋白，有三個能和鈣離子結合的 EF-hand 結構。之前的研究證實 NCS-1在牛腎上腺嗜鉻細胞 (adrenal chromaffin cells) 與大鼠腎上腺嗜鉻瘤细胞株 (PC12)中會影響胞內囊泡的循環再利用機轉。我們實驗室先前由酵母菌雙雜合實驗中發現，NCS-1與協助神經細胞胞吞作用的Auxilin-1 (Aux-1)具有交互作用，因此觀察此交互作用是否為神經細胞調控胞吞與胞吐作用的關鍵。我們以GST標定NCS-1來進行Pulldown Assay，在不同鈣離子濃度中無法觀察到明確的交互作用，我們推測NCS-1與Aux-1之間的作用可能只發生於特定的實驗條件之下。為瞭解 NCS-1與Aux-1分別對神經傳導的影響，我們將蛋白質表現在初級培養的神經胚胎細胞中，測量神經細胞受刺激後，胞内鈣離子的濃度變化。結果顯示過量表現NCS-1會使目標細胞及下游細胞的反應顯著的下降，NCS-1的變異株皆會促進下游細胞的鈣離子反應，對目標細胞的影響各有不同。表現Aux-1的目標細胞反應下降，並不影響下游細胞的反應。我們推測NCS-1可能在神經細胞的囊泡回收機轉扮演抑制性的角色，NCS-1變異株則是與內生性的NCS-1競爭來影響細胞的反應，而Aux-1雖會抑制神經細胞的鈣離子反應，仍能夠協助囊泡回收機轉。

Calcium influx in neurons is an important event in regulating various signaling pathways like neurotransmitter release and neurite outgrowth. Neuronal calcium sensor-1 (NCS-1) is a member of the EF-hand Ca2+ binding proteins and expressed mainly in nervous system. NCS-1 overexpression affects the vesicle recycling in bovine adrenal chromaffin cells (BC) and has little effect on Ca2+ channels. In our lab, we have also found that NCS-1 and Auxilin-1 (Aux-1), a protein assists endocytosis, have interaction via yeast-two-hybrid. We believe this interaction can be the key to regulate exocytosis and endocytosis. However, our results in protein pulldown assay shows that NCS-1 and Aux-1 have no direct binding activity. We suppose the interaction only exist under specific condition. To observe the effect of NCS-1 and Aux-1 on neurotransmission, we use Ca2+ imaging technique to visualize Ca2+ response after excitation. Our results demonstrate that NCS-1 overexpression inhibits [Ca2+]i elevation in both presynaptic and postsynaptic neurons, but NCS-1 mutants overexpression can enhance Ca2+ response in postsynaptic neurons and have different effect on target neurons. In the Aux-1 expressing neuron, the [Ca2+]i elevation is reduced, while in postsynaptic neurons is unaltered. Therefore, NCS-1 may play an inhibitory role in neurotransmission by regulating synaptic vesicle recycling, and Aux-1 may suppress Ca2+ response and support synaptic vesicle recycling.