Visualizing infection of individual influenza viruses

Abstract

Influenza is a paradigm for understanding viral infections. As an opportunistic pathogen exploiting the cellular endocytic machinery for infection, influenza is also a valuable model system for exploring the cell's constitutive endocytic pathway. We have studied the transport, acidification, and fusion of single influenza viruses in living cells by using real-time fluorescence microscopy and have dissected individual stages of the viral entry pathway. The movement of individual viruses revealed a striking three-stage active transport process that preceded viral fusion with endosomes starting with an actin-dependent movement in the cell periphery, followed by a rapid, dynein-directed translocation to the perinuclear region, and finally an intermittent movement involving both plus-and minus-end-directed microtubule-based motilities in the perinuclear region. Surprisingly, the majority of viruses experience their initial acidification in the perinuclear region immediately following the dynein-directed rapid translocation step. This finding suggests a previously undescribed scenario of the endocytic pathway toward late endosomes: endosome maturation, including initial acidification, largely occurs in the perinuclear region.T he infection pathway of influenza is a complex, multistep process: viruses enter cells by receptor-mediated endocytosis, then move from endocytic vesicles to early endosomes and finally to late endosomes where the viruses fuse with the endosomal membrane to release viral genes (1-7), a pathway followed by many other medically important viruses (3, 4). Exploiting the cell's constitutive endocytic machinery for infection, influenza is an ideal probe for exploring the cell's endocytic pathway. However, despite intensive efforts in investigating influenza infection, many critical properties of the endocytosis and endocytic traffic of the virus remain elusive. Among these are important and general questions for cellular endocytosis: what are the transport mechanisms of endocytic compartments at different stages on the endocytic pathway, and how do these compartments and the enclosed endocytic cargo mature (6-8)?The difficulty in real-time imaging of the endocytic pathway presents one of the major hurdles in addressing the above questions. In this work, by tracking the behavior of single influenza viruses in real-time (9-12), we have dissected individual stages of the viral entry process, observed transient, previously unobserved steps in endocytic viral trafficking, and obtained a dynamic picture of the cell's endocytic pathway exploited by influenza. We have discovered that virus-bearing endosomes are transported in cells in three distinct stages, each with a distinct cytoskeleton-and motor-protein-dependent mechanism. We have revealed surprising endocytic acidification dynamics: the initial acidification of endocytic cargo mainly occurs in the perinuclear region, bringing into question the previous picture that early endosomes in the cell periphery are the early acidification sites for endocytic ca...

Virus trafficking – learning from single-virus tracking

“…2). This was observed for Reovirus, Adenovirus, HSV, influenza virus and HIV 5,[42][43][44][45] . Viral transport is often highly regulated as with, for example, HSV in neurons.…”

“…For example, whereas hundreds to thousands of dye molecules can be attached to a relatively large influenza virion without affecting its infectivity 5 , only 30-50 dye molecules can be attached to a polio capsid owing to its small size (B.B. et al, unpublished data).…”

“…For example, fusion between viral and cellular membranes allows the virus to release its contents into the cell and is therefore a crucial infection step for enveloped viruses. So monitoring the fusion event of enveloped viruses, by fluorescence dequenching 5 , enables the researcher to selectively focus on a subgroup of viruses that will have a productive infection. For non-enveloped viruses, viral genome release can be monitored as a marker of a productive infection.…”

“…The trajectories of single influenza viruses in live cells revealed actin-dependent movement of the virus before the particles were taken up by endocytosis 5,13 . Several viruses, such as Murine Leukaemia virus, Avian Leukosis virus, HIV and Vesicular Stomatitis virus (VSV) use the cortical actin cytoskeleton, together with myosin II, for directed movement along microvilli or filopodia towards entry 'hot spots' at the cell body 15 .…”

Cofilin-1 is involved in regulation of actin reorganization during influenza A virus assembly and budding

“…Actin filament depolymerization with CytoD and stabilization with jasplakinolide both inhibited influenza entry, suggesting that dynamic actin cytoskeletal reorganization is required for successful internalization of IAV into cells [5]. Actin cytoskeleton is also responsible for intracellular trafficking of IAV after entry, since treatment of cells with CytoD restricted intracellular viral movement [7].…”