中科院水生所張永安團隊在魚類病毒的免疫逃逸機制研究領域取得新進展

水產品提供了我國人民餐桌上約1/3的動物蛋白。然而，水生病毒往往給水產養殖業帶來重大經濟損失，其中鯉春病毒血症病毒（Spring viremia of carp virus，SVCV）和草魚出血病病毒（Grass carp reovirus，GCRV）是魚類病毒病的重要病原。一直以來，關於魚類病毒是如何躲避魚體免疫系統的逃逸機制尚不清楚。中國科學院水生生物研究所魚類分子免疫學學科組近幾年針對SVCV和GCRV的免疫逃逸機制開展了一系列的深入研究。近日，中科院水生所張永安團隊在魚類病毒的免疫逃逸機制研究領域取得新進展，研究結果以「Spring viraemia of carp virus modulates p53 expression using two distinct mechanisms」為題，在線發表在PLoS Pathogens上。

干擾素（Interferon，IFN）反應是魚類天然免疫的重要組成部分，也是細胞抵抗病毒入侵的一道重要防線。該學科組前期工作發現SVCV的N蛋白能夠通過泛素-蛋白酶體途徑降解MAVS進而抑制宿主細胞IFN的產生（Journal of Immunology，2016），該研究結果被雜誌以「Fish Virus Is Unfriendly to IFN」為題重點推薦。緊接著，該學科組又發現SVCV的P蛋白能夠模擬TBK1的底物，進而抑制TBK1的激酶活性從而阻斷IFN的產生（Journal of Virology，2016）。另外，還發現了草魚出血病病毒GCRV的S8節段編碼蛋白能夠阻斷MITA的磷酸化，導致IFN減少（Journal of Virology，2017）。以上研究確認了魚類病毒能夠降低宿主IFN的產生，從而利於病毒增殖，這些內容為低等脊椎動物病毒免疫逃逸提供了強有力的證據。

p53作為一個腫瘤抑制蛋白，調控各種基因的表達，包括細胞凋亡、細胞生長抑制、細胞周期進程等。由於細胞周期調控以及凋亡與病毒感染的關係密不可分，從病毒的角度來講，在感染早期控制細胞狀態穩定能夠獲得更多的細胞內資源進行病毒複製；在感染晚期，則是促進細胞凋亡，促進病毒的釋放。近期，該學科組發現SVCV的N蛋白和P蛋白能夠分別降解和穩定p53的蛋白表達，從而為病毒的增殖提供最優條件。在SVCV感染的初期，細胞停滯在S期數量明顯增多，同時SVCV的N蛋白與p53結合併引起p53蛋白水平的減少；在SVCV感染的後期，細胞產生明顯凋亡現象，同時SVCV的P蛋白與p53結合併引起p53的蛋白表達增加；進一步發現p53的K358位點是N蛋白和P蛋白引起p53發生K63型多聚泛素化修飾的關鍵位點，從而精確控制p53表達量。揭示了魚類病毒通過調控p53實現免疫逃逸，發現了病毒針對p53的兩種截然不同的調控方式。以上研究內容第一作者為李順副研究員和（或）陸龍鳳助理研究員，通訊作者為張永安研究員。這些研究成果從病毒與細胞相互作用的角度闡明魚病的致病機理，對於魚病的檢測、預防和清除，疫苗及診斷試劑的研製開發都具有重要意義。

Abstract

p53, which regulates cell-cycle arrest and apoptosis, is a crucial target for viruses to release cells from cell-cycle checkpoints or to protect cells from apoptosis for their own benefit. Viral evasion mechanisms of aquatic viruses remain mysterious. Here, we report the spring viremia of carp virus (SVCV) degrading and stabilizing p53 in the ubiquitin-proteasome pathway by the N and P proteins, respectively. Early in an SVCV infection, significant induction was observed in the S phase and p53 was decreased in the protein level. Further experiments demonstrated that p53 interacted with SVCV N protein and was degraded by suppressing the K63-linked ubiquitination. However, the increase of p53 was observed late in the infection and experiments suggested that p53 was bound to SVCV P protein and stabilized by enhancing the K63-linked ubiquitination. Finally, lysine residue 358 was the key site for p53 K63-linked ubiquitination by the N and P proteins. Thus, our findings suggest that fish p53 is modulated by SVCV N and P protein in two distinct mechanisms, which uncovers the strategy for the subversion of p53-mediated host innate immune responses by aquatic viruses.

Author summary

Upon viral infiltration, host cells employ p53 to defend against infection. Thus, viruses need to inhibit these antiviral surveillance mechanisms in the host to efficiently spread to new hosts. To date, the evasion mechanisms against fish p53 remain unclear. In this study, we reveal that SVCV modulates host p53 expression by two distinct mechanisms. Through a series of experiments, we show that SVCV N protein bound and degraded host p53 through suppressing the K63-linked ubiquitination; SVCV P protein interacted with and stabilized p53 while enhancing the K63-linked ubiquitination; lysine residue 358 was the key site for p53 ubiquitination by the N and P proteins. Our findings shed light on the special evasion mechanisms of fish virus and expand our knowledge of the virus–host interactions that are responsible for regulating p53 in lower vertebrates.

本期編輯：Tony

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