探討GEF-H1調控嗜中性球胞外網狀結構之釋放

Abstract

嗜中性球在免疫系統扮演非常重要的角色，近年來有研究發現發現嗜中性白血球有個特殊的機制，會將細胞核的染色體結構解旋，將上面帶有抗微生物蛋白的DNA釋放到胞外，稱為嗜中性胞外網狀結構(neutrophil extracellular traps, NETs)，這種網狀結構會纏繞病原，阻止其擴散並促進清除和分解。由於過去已知細胞骨架會調控NETs的形成，而guanine nucleotide exchange factor H1 (GEF-H1)可以藉由調節RhoA GTPases的活性來影響下游微小管(microtubule)和肌動蛋白(actin)的動態平衡，因此本論文欲探討GEF-H1在NETs形成時所扮演的角色。我們從缺乏GEF-H1的基因改造小鼠分離出嗜中性球，經實驗確認GEF-H1缺陷並不會影響嗜中性球的分化以及穩定狀態下的細胞骨架型態。我們以phorbol 12-myristate 13-acetate (PMA)刺激嗜中性球，發現GEF-H1缺陷的嗜中性球有較低的胞外DNA訊號，雖然顆粒球(granules)中的蛋白能夠遷移到細胞核內，缺少GEF-H1的嗜中性球卻無法將其釋放到細胞外。且經由定量分析，得知GEF-H1缺陷嗜中性球所生成之NETs大多為擴散型(diffused)而非散佈型(spread)。實驗結果顯示GEF-H1並非藉由調控活性氧化物(reactive oxygen species, ROS)影響NETs的產生。進一步的研究發現GEF-H1會與散佈型NETs結合，並一起被釋放到胞外。綜合以上結果，可推論GEF-H1會調控NETs的釋放。

Neutrophils play an important role in the first line of immune defense. Apart from conventional pathways, neutrophil extracellular trap (NET), a newly discovered mechanism that neutrophil releases chromosomal DNA and specific anti-microbial proteins to the extracellular space and captures pathogens was demonstrated. The formation of NET, a process called NETosis, is accepted as a special form of cell death. However, the detailed mechanism of NETosis has not been fully understood. Previous studies suggest that actin and microtubule networks are important for NETosis. Given that the guanine nucleotide exchange factor GEF-H1 is crucial in coupling microtubule dynamics to RhoA GTPase activation and actin polymerization, we hypothesized GEF-H1 plays a role in NETosis by regulating cytoskeleton dynamics. Here we found that the area of extracellular DNA was significantly reduced in GEF-H1-deficient mouse neutrophils after phorbol 12-myristate 13-acetate (PMA) incubation. While neutrophil elastase (NE), myeloperoxidase (MPO) and citrullinated histone H3 (citH3) were detected in the nucleus of PMA-incubated GEF-H1-deficient neutrophils, those neutrophils failed to release granule proteins and chromatins extracellularly. Quantification analysis further indicated they had a significantly decreased percentage of spread form NET when compared to wild-type (WT) neutrophils in the presence of PMA. Moreover, we found that GEF-H1 was released and bound to NET components in response to PMA stimulation. Interestingly, the amount of ROS production was comparable between PMA-incubated WT and GEF-H1-deficient neutrophils further indicated that there were profound defects in the late stage of spread NET formation in GEF-H1-deficient neutrophils. Overall, our data suggest that GEF-H1 is the critical regulator of NET release.