Phosphatidylserine is involved in gene expression from Sindbis virus subgenomic promoter

Melon necrotic spot virus Replication Occurs in Association with Altered Mitochondria

Melon necrotic spot virus (MNSV) (genus Carmovirus, family Tombusviridae) is a single-stranded, positive-sense RNA virus that has become an experimental model for the analysis of cell-to-cell virus movement and translation of uncapped viral RNAs, whereas little is known about its replication. Analysis of the cytopathology after MNSV infection showed the specific presence of modified organelles that resemble mitochondria. Immunolocalization of the glycine decarboxylase complex (GDC) P protein in these organelles confirmed their mitochondrial origin. In situ hybridization and immunolocalization experiments showed the specific localization of positive-sense viral RNA, capsid protein (CP), and double-stranded (ds)RNA in these organelles meaning that replication of the virus takes place in association with them. The three-dimensional reconstructions of the altered mitochondria showed the presence of large, interconnected, internal dilations which appeared to be linked to the outside cytoplasmic environment through pores and/or complex structures, and with lipid bodies. Transient expression of MNSV p29 revealed that its specific target is mitochondria. Our data document the extensive reorganization of host mitochondria induced by MNSV, which provides a protected environment to viral replication, and show that the MNSV p29 protein is the primary determinant of this effect in the host.

Forkhead box transcription factor regulation and lipid accumulation by hepatitis C virus

We have previously shown that hepatitis C virus (HCV) infection modulates the expression of forkhead box transcription factors, including FoxO1 and FoxA2, which play key roles in gluconeogenesis and β-oxidation of fatty acid, respectively. The aim of the present study was to determine the role of forkhead box transcription factors in modulating lipid metabolism. HCV infection or core protein expression alone in transfected Huh7.5 cells increased expression of sterol regulatory element binding protein 1c (SREBP-1c) and its downstream target, fatty acid synthase (FASN), which are key proteins involved in lipid synthesis. Knockdown of FoxO1 by small interfering RNA in HCV-infected cells significantly decreased SREBP-1c and FASN expression. Further, HCV infection or core protein expression in Huh7.5 cells significantly decreased the expression of medium-chain acyl coenzyme A dehydrogenase (MCAD) and short-chain acyl coenzyme A dehydrogenase (SCAD), involved in the regulation of β-oxidation of fatty acids. Ectopic expression of FoxA2 in HCV-infected cells rescued the expression of MCAD and SCAD. Oil red O and neutral lipid staining indicated that HCV infection significantly increases lipid accumulation compared to that in the mock-infected control. This was further verified by the increased expression of perilipin-2 and decreased activity of hormone-sensitive lipase (HSL) in HCV-infected hepatocytes, implying increased accumulation of neutral lipids. Knockdown of FoxO1 and ectopic expression of FoxA2 significantly decreased HCV replication. Taken together, these results suggest that HCV modulates forkhead box transcription factors which together increase lipid accumulation and promote viral replication.

IMPORTANCE

Hepatic steatosis is a frequent complication associated with chronic HCV infection. Its presence is a key prognostic indicator associated with the progression to hepatic fibrosis and hepatocellular carcinoma. Several mechanisms have been proposed to account for the development of steatosis and fatty liver during HCV infection. We observed that HCV infection increases expression of both SREBP-1c and FASN. Further investigation suggested that the expression of SREBP-1c and FASN is controlled by the transcription factor FoxO1 during HCV infection. In addition, HCV infection significantly decreased both MCAD and SCAD expression, which is controlled by FoxA2. HCV infection also increased lipid droplet accumulation, increased perilipin-2 expression, and decreased HSL activity. Thus, knockdown of FoxO1 (decreased lipogenesis) and overexpression of FoxA2 (increased β-oxidation) resulted in a significant disruption of the platform and, hence, a decrease in HCV genome replication. Thus, targeting of FoxO1 and FoxA2 might be useful in developing a therapeutic approach against HCV infection.

Role of cellular lipids in positive-sense RNA virus replication complex assembly and function

Positive-sense RNA viruses are responsible for frequent and often devastating diseases in humans, animals, and plants. However, the development of effective vaccines and anti-viral therapies targeted towards these pathogens has been hindered by an incomplete understanding of the molecular mechanisms involved in viral replication. One common feature of all positive-sense RNA viruses is the manipulation of host intracellular membranes for the assembly of functional viral RNA replication complexes. This review will discuss the interplay between cellular membranes and positive-sense RNA virus replication, and will focus specifically on the potential structural and functional roles for cellular lipids in this process.