The endoplasmic reticulum (ER): a crucial cellular hub in flavivirus infection and potential target site for antiviral interventions

Abstract

Dengue virus (DENV) is the most prevalent arthropod-borne flavivirus and imposes a significant healthcare threat worldwide. At present no FDA-approved specific antiviral treatment is available, and the safety of a vaccine against DENV is still on debate. Following its entry into the host cell, DENV takes advantage of the cellular secretory pathway to produce new infectious particles. The key organelle of the host cell in DENV infections is the endoplasmic reticulum (ER) which supports various stages throughout the entire life cycle of flaviviruses. This review delves into the intricate interplay between flaviviruses and the ER during their life cycle with a focus on the molecular mechanisms underlying viral replication, protein processing and virion assembly. Emphasizing the significance of the ER in the flavivirus life cycle, we highlight potential antiviral targets in ER-related steps during DENV replication and summarize the current antiviral drugs that are in (pre)clinical developmental stage. Insights into the exploitation of the ER by DENV offer promising avenues for the development of targeted antiviral strategies, providing a foundation for future research and therapeutic interventions against flaviviruses.

Fig. 1. The role of the endoplasmic reticulum (ER) in the dengue virus (DENV) life cycle.

A DENV (flavivirus) life cycle. DENV can attach to a broad range of receptors and enter the cell via clathrin-mediated endocytosis. Subsequent fusion of viral and endosomal membranes results in release of the viral RNA genome. The RNA is translated into one large polyprotein that is inserted at the ER membrane where it is further processed into its individual structural (Ca: capsid, E: envelope and prM: pre-membrane) and non-structural (NS1, NS2A, NS2B, NS3, NS4A, NS4B, NS5) proteins. The NS proteins orchestrates the assembly of the replication complexes (RC) and replicate the viral RNA genome. The ER is a large and highly interconnected membranous network that extends from the nuclear envelope. The ER cisternae that harbour RCs would be interconnected with the ER that harbours the virion assembly sites. The newly generated viral RNA complexes with the Ca protein units and the resulting nucleocapsid is engulfed by the viral membrane (containing prM and E proteins) through budding into the ER lumen, predominantly in regions opposite to the RC. Maturation of new virions by furin-mediated cleavage of prM proteins occurs along the secretory pathway within the Golgi apparatus. CM: Convoluted membranes. B DENV polyprotein topology and predicted transmembrane domains at the host cell’s ER membrane. The polyprotein is processed by cellular signal peptidase at the ER lumen and by viral protease (NS2B/NS3) at the cytoplasmic side. The prM proteins further mature by furin cleavage and the additional NS1/NS2A cleavage is carried out by a yet unknown enzyme. C Model of the DENV-induced RC at the ER membrane. NS4A and NS4B might induce a negative membrane curvature which is stabilized by the homo- and hetero-oligomerization of NS4A and NS4B (not depicted) along with the association of NS1 at the luminal side of the ER. NS2A and other host factors (not depicted) may also contribute to the membrane reformations. RdRp: RNA-dependent RNA polymerase, MTase: NS5 methyltransferase, Hel: NS3 Helicase, Pro: NS2B-NS3 viral protease, +: plus RNA strand (red) and −: minus RNA strand (black). The figure was created with Biorender.com.