Yin Xin, Ambardekar Charuta, Lu Yurong, Feng Zongdi
Center for Vaccines and Immunity, The Research Institute at Nationwide Children's Hospital, Columbus, Ohio, USA.
Center for Vaccines and Immunity, The Research Institute at Nationwide Children's Hospital, Columbus, Ohio, USA
J Virol. 2016 Mar 28;90(8):4232-4242. doi: 10.1128/JVI.02804-15. Print 2016 Apr.
The hepatitis E virus (HEV) sheds into feces as nonenveloped virions but circulates in the blood in a membrane-associated, quasi-enveloped form (eHEV). Since the eHEV virions lack viral proteins on the surface, we investigated the entry mechanism for eHEV. We found that compared to nonenveloped HEV virions, eHEV attachment to the cell was much less efficient, requiring a longer inoculation time to reach its maximal infectivity. A survey of cellular internalization pathways identified clathrin-mediated endocytosis as the main route for eHEV entry. Unlike nonenveloped HEV virions, eHEV entry requires Rab5 and Rab7, small GTPases involved in endosomal trafficking, and blocking endosomal acidification abrogated eHEV infectivity. However, low pH alone was not sufficient for eHEV uncoating, suggesting that additional steps are required for entry. Supporting this concept, eHEV infectivity was substantially reduced in cells depleted of Niemann-Pick disease type C1, a lysosomal protein required for cholesterol extraction from lipid, or in cells treated with an inhibitor of lysosomal acid lipase. These data support a model in which the quasi-envelope is degraded within the lysosome prior to virus uncoating, a potentially novel mechanism for virus entry.
The recent discovery of quasi-enveloped viruses has shifted the paradigm of virus-host interactions. The impact of quasi-envelopment in the virus life cycle and pathogenesis is largely unknown. HEV is a highly relevant model to study these questions. HEV circulates as quasi-enveloped virions in the blood that are hidden from neutralizing antibodies. eHEV particles most likely are responsible for the cell-to-cell spread of the virus. Given the increasing concerns about persistent HEV infection and its potential for transmission via the blood supply, understanding how eHEV infects cells is important for understanding its pathogenesis and developing therapies. Our data provide evidence that eHEV uses a potentially novel mechanism for cellular entry. Several steps critical to eHEV entry were identified and may provide a basis for developing treatments for hepatitis E. Because quasi-enveloped viruses resemble exosomes, these data also may provide insights into the exosome-mediated intercellular communications.
戊型肝炎病毒（HEV）以无包膜病毒粒子的形式排入粪便，但以膜相关的准包膜形式（eHEV）在血液中循环。由于eHEV病毒粒子表面缺乏病毒蛋白，我们研究了eHEV的进入机制。我们发现，与无包膜的HEV病毒粒子相比，eHEV与细胞的附着效率要低得多，需要更长的接种时间才能达到其最大感染性。对细胞内化途径的调查确定网格蛋白介导的内吞作用是eHEV进入的主要途径。与无包膜的HEV病毒粒子不同，eHEV的进入需要Rab5和Rab7，这两种参与内体运输的小GTP酶，并且阻断内体酸化会消除eHEV的感染性。然而，仅低pH值不足以使eHEV脱壳，这表明进入还需要其他步骤。支持这一概念的是，在缺乏尼曼-皮克病C1型（一种从脂质中提取胆固醇所需的溶酶体蛋白）的细胞中，或在用溶酶体酸性脂肪酶抑制剂处理的细胞中，eHEV的感染性显著降低。这些数据支持了一种模型，即准包膜在病毒脱壳之前在溶酶体内降解，这是一种潜在的病毒进入新机制。
准包膜病毒的最新发现改变了病毒-宿主相互作用的模式。准包膜在病毒生命周期和发病机制中的影响在很大程度上尚不清楚。HEV是研究这些问题的高度相关模型。HEV以准包膜病毒粒子的形式在血液中循环，这些病毒粒子对中和抗体具有隐匿性。eHEV颗粒很可能负责病毒的细胞间传播。鉴于对戊型肝炎持续感染及其通过血液供应传播的可能性的担忧日益增加，了解eHEV如何感染细胞对于理解其发病机制和开发治疗方法很重要。我们的数据提供了证据，表明eHEV使用一种潜在的新型细胞进入机制。确定了对eHEV进入至关重要的几个步骤，可能为开发戊型肝炎治疗方法提供基础。由于准包膜病毒类似于外泌体，这些数据也可能为外泌体介导的细胞间通讯提供见解。
