Date Published: August 17, 2018
Publisher: Public Library of Science
Author(s): Rebekah C. Gullberg, J. Jordan Steel, Venugopal Pujari, Joel Rovnak, Dean C. Crick, Rushika Perera, Glenn Randall.
Positive strand RNA viruses, such as dengue virus type 2 (DENV2) expand and structurally alter ER membranes to optimize cellular communication pathways that promote viral replicative needs. These complex rearrangements require significant protein scaffolding as well as changes to the ER chemical composition to support these structures. We have previously shown that the lipid abundance and repertoire of host cells are significantly altered during infection with these viruses. Specifically, enzymes in the lipid biosynthesis pathway such as fatty acid synthase (FAS) are recruited to viral replication sites by interaction with viral proteins and displayed enhanced activities during infection. We have now identified that events downstream of FAS (fatty acid desaturation) are critical for virus replication. In this study we screened enzymes in the unsaturated fatty acid (UFA) biosynthetic pathway and found that the rate-limiting enzyme in monounsaturated fatty acid biosynthesis, stearoyl-CoA desaturase 1 (SCD1), is indispensable for DENV2 replication. The enzymatic activity of SCD1, was required for viral genome replication and particle release, and it was regulated in a time-dependent manner with a stringent requirement early during viral infection. As infection progressed, SCD1 protein expression levels were inversely correlated with the concentration of viral dsRNA in the cell. This modulation of SCD1, coinciding with the stage of viral replication, highlighted its function as a trigger of early infection and an enzyme that controlled alternate lipid requirements during early versus advanced infections. Loss of function of this enzyme disrupted structural alterations of assembled viral particles rendering them non-infectious and immature and defective in viral entry. This study identifies the complex involvement of SCD1 in DENV2 infection and demonstrates that these viruses alter ER lipid composition to increase infectivity of the virus particles.
Phospholipids are critical for membrane structure, function and stability of eukaryotic cells. Specific distributions of lipids within these membranes define their characteristics such as curvature, fluidity, leakiness and the interactions between membranes and membrane-bound protein complexes. A key approach to alter the architecture of a membrane is to incorporate unsaturated fatty acyl chains, to induce curvature and fluidity in a lipid bilayer, altering its functional capacity [1, 2]. Unsaturated fatty acids (UFA) are generated in the cytoplasm and after their initial desaturation they are further elongated, desaturated and shunted towards triglyceride, cholesterol ester or phospholipid synthesis. This initial desaturation event is the rate-limiting step in UFA biosynthesis and is catalyzed at the Δ9 position in the carbon chain by stearoyl CoA desaturase (SCD) [3, 4]. In humans, it has two isoforms: SCD1 is ubiquitously expressed and preferentially converts stearic and palmitic acids into oleic and palmitoleic acids, respectively. SCD5, is restricted to the brain and pancreas . SCD1 is a 40 kD integral membrane protein in the endoplasmic reticulum (ER) and is highly conserved from bacteria to mammals . It regulates the balance between saturated and monounsaturated fatty acids (MUFA) in the cell.
Previously we showed that lipid biosynthesis was upregulated in DENV-infected cells through the activation and relocalization of fatty acid synthase (FAS), an enzyme critical to the production of palmitic and stearic acids that are structural components of complex lipids . Here, we investigated the next step following FAS-catalyzed fatty acid production in the lipid biosynthesis pathway and demonstrated that desaturation of these fatty acids plays a critical role in the viral life cycle. Specifically, DENV2 infection resulted in upregulated monounsaturated fatty acid (MUFA) biosynthesis, catalyzed by SCD1 at early time points post-infection. Inhibition of this process impaired virion maturation and particle infectivity and stability.