Virus entry: open sesame

Abstract

Detailed information about the replication cycle of viruses and their interactions with host organisms is required to develop strategies to stop them. Cell biology studies, live-cell imaging, and systems biology have started to illuminate the multiple and subtly different pathways that animal viruses use to enter host cells. These insights are revolutionizing our understanding of endocytosis and the movement of vesicles within cells. In addition, such insights reveal new targets for attacking viruses before they can usurp the host-cell machinery for replication.

Figure 1

Steps in the Endocytic Entry Program of a Typical Animal Virus Whether enveloped or nonenveloped, many viruses depend on the host cell's endocytic pathways for entry. They follow a multistep entry and uncoating program that allows them to move from the cell periphery to the perinuclear space. In this example, the virus proceeds to deliver its uncoated genome into the nucleoplasm. The interaction between the virus and the host cell starts with virus binding to attachment factors and receptors on the cell surface, followed by lateral movement of the virus-receptor complexes and the induction of signals that result in the endocytic internalization of the virus particle. After vesicular trafficking and delivery into the lumen of endosomes, caveosomes, or the ER, a change in the virus conformation is induced by cellular cues. This alteration results in the penetration of the virus or its capsids through the vacuole membrane into the cytosolic compartment. Enveloped viruses use membrane fusion for penetration, whereas nonenveloped viruses induce lysis or pore formation. After targeting and transport along microtubules, the virus or the capsid binds, as in this example, to the nuclear pore complex, undergoes a final conversion, and releases the viral genome into the nucleus. The details in the entry program vary for different viruses and cell types, but many of the key steps shown here are general.

Figure 2

Endocytic Pathways Used by Viruses In mammalian cells, many different mechanisms are available for the endocytic internalization of virus particles. Some of these mechanisms, such as clathrin-mediated endocytosis, are ongoing, whereas others, such as caveolae, are ligand and cargo induced. Currently, there is evidence for six pathways. (A) Macropinocytosis is involved in the entry of adenoviruses. (B) A clathrin-independent pathway from the plasma membrane has been shown to exist for influenza virus and arenaviruses. (C) The clathrin-mediated pathway is the most commonly observed uptake pathway for viruses. The viruses are transported via early endosomes to late endosomes and eventually to lysosomes. (D) The caveolar pathway is one of several closely related, cholesterol-dependent pathways that bring viruses including SV40, coxsackie B, mouse polyoma, and Echo 1 to caveosomes, from which many of them continue, by a second vesicle transport step, to the ER. (E) A cholesterol-dependent endocytic pathway devoid of clathrin and caveolin-1, used by polyomavirus and SV40. (F) A pathway similar to (D) except dependent on dynamin-2. It is used by Echo virus 1. Depending on the virus and cell type, penetration reactions occur in five locations: the plasma membrane, early and late endosomes, caveosomes, and the ER. Note that the additional endocytic mechanism of phagocytosis also operates in many cells but has not as yet been linked to virus entry and is not included here.

Figure 3

Electron Micrographs Showing Virus Internalization by Clathrin- or Caveolar/Raft-Mediated Endocytosis (A and B) Semliki Forest virus, a 70 nm diameter enveloped alphavirus, is internalized by clathrin-coated pits (A) and vesicles (B) for endocytosis and infection. Here, the virus is interacting with a BHK-21 cell. (C and D) Simian virus 40, a small 50 nm diameter nonenveloped DNA virus, binds to gangliosides in the plasma membrane of CV-1 cells and enters via caveolae (C) and tight-fitting small vesicles. The viruses are transported through caveosomes to the ER, where many accumulate in smooth membrane domains (D). Scale bar in (A) and (C) = 100 nm, (B) = 200 nm, and (D) = 250 nm. (A), (C), and (D) are courtesy of J. Kartenbeck and A.H. (B) is reproduced from Helenius et al. (1980), The Journal of Cell Biology, 1980, volume 84, pp. 404–420 by copyright permission of The Rockefeller University Press.

Figure 4

Swine Pox Viruses at the Tip of Projections in the Plasma Membrane of the Infected Cell After release from the cell, the viruses remain associated with the plasma membrane, where they induce assembly of actin filaments. The projections that are formed are motile and push the viruses into contact with neighboring cells. In this way, they promote infection within tissues. Note the actin filaments emanating from the area of virus cell contact. Courtesy of J. Kartenbeck. Scale bar = 500 nm.