A Ca(2+)-dependent signalling circuit regulates influenza A virus internalization and infection.

Various viruses enter host cells via endocytosis, but the molecular mechanisms underlying the specific internalization pathways remain unclear. Here we show that influenza A viruses (IAVs) enter cells via redundant pathways of clathrin-mediated and clathrin-independent endocytosis, with intracellular Ca(2+) having a central role in regulation of both pathways by activating a signalling axis comprising RhoA, Rho-kinase, phosphatidylinositol 4-phosphate 5-kinase (PIP5K) and phospholipase C (PLC). IAV infection induces oscillations in the cytosolic Ca(2+) concentration of host cells, the prevention of which markedly attenuates virus internalization and infection. The small GTPase RhoA is found both to function downstream of the virus-induced Ca(2+) response and itself to induce Ca(2+) oscillations in a manner dependent on Rho-kinase and subsequent PIP5K-PLC signalling. This signalling circuit regulates both clathrin-mediated and clathrin-independent endocytosis during virus infection and seems to constitute a key mechanism for regulation of IAV internalization and infection.

Pathological examination of lung tissues in influenza a virus-infected mice.

This study examined pathological changes in the lung tissues of young and aged mice infected with influenza virus. Young mice inoculated with influenza virus showed body weight loss at 4 days post-infection (dpi), meanwhile body weight decrease started from 9 dpi in the aged mice. We histopathologically examined the lungs of these mice. Immunohistochemical examination revealed that viral antigen-positive bronchiolar and alveolar epithelial cell numbers at 3 dpi were significantly higher in young mice than in the aged ones. Further, viral antigen-positive cells were observed at 9 dpi in the aged mice, but not in the young ones. Diffuse and severe bronchointerstitial pneumonia characterized by the accumulation of polymorphonuclear leukocytes (PMNs) was observed in young mice at 6 dpi. Histopathological changes in the aged mice were milder than those in the young mice. Moreover, T cell and macrophage accumulation in the lungs was significantly higher in the young mice than in the aged mice at 9 dpi. These results suggest that there may be a correlation between the relatively low level of infiltration of PMNs, macrophages, and T lymphocytes and the delayed body weight loss and longer lasting infections observed in the lungs of the aged mice. These findings provide detailed insights into the age-specific course of infection in young and aged populations with associated differences in lung pathology.

The Ras-PI3K signaling pathway is involved in clathrin-independent endocytosis and the internalization of influenza viruses.

BACKGROUND: Influenza virus infection causes highly contagious, severe respiratory disorders and gives rise to thousands of deaths every year; however, the efficacy of currently approved defense strategies, including vaccines and neuraminidase inhibitors, is limited because the virus frequently acquires resistance via antigen drift and reassortment. It is therefore important to establish a novel, effective therapeutic strategy that is effective irrespective of viral subtype. METHODOLOGY/PRINCIPAL FINDINGS: Here, we identify the Ras-phosphoinositide 3-kinase (PI3K) signaling pathway as a host-cell regulatory mechanism for influenza virus entry. The binding of Ras to PI3K is specifically involved in clathrin-independent endocytosis, endosomal maturation, and intracellular transport of viruses, which result in decreased infectious efficacy of different subtypes of influenza viruses in cells lacking the Ras-PI3K interaction. Moreover, influenza virus infection indeed triggered Ras activation and subsequent PI3K activation in early endosomes. CONCLUSIONS/SIGNIFICANCE: Taken together, these results demonstrate that the Ras-PI3K signaling axis acts as a host-oriented mechanism for viral internalization. Given that virus incorporation is a process conserved among virus subtypes and species, this signaling pathway may provide a target for potent, well-tolerated prophylactics and therapeutics against a broad range of viruses.

Fern constituents: dryocrassy formate, sitostanyl formate and 12alpha-hydroxyfern-9(11)-ene from Cyathea podophylla.

Dryocrassyl formate, sitostanyl formate, and 12alpha-hydroxyfern-9(11)-ene were isolated from the fresh fronds of Cyathea podophylla. Their structures were elucidated by spectroscopic techniques and synthesis. Ten known triterpenoids, three derivatives of phytol, a stanol, and beta-tocopherol were also identified from this fern.