Ebola Life Cycle: A Comprehensive Molecular Overview

Ebola virus is a highly virulent pathogen that causes severe hemorrhagic fever with high fatality rates. Its rapid progression and ability to evade the immune system make it a serious public health concern, as seen in multiple outbreaks across Africa. Understanding its molecular life cycle is crucial for developing targeted antiviral therapies and improving outbreak response strategies.

This article provides an in-depth look at the key stages of Ebola virus replication within host cells, highlighting the mechanisms that drive infection and viral propagation.

Attachment And Receptor Binding

Ebola virus initiates infection by targeting host cells through an interaction between its surface glycoprotein (GP) and cellular receptors. The viral GP, a trimeric transmembrane protein, mediates attachment by binding to host cell surface molecules. C-type lectins such as DC-SIGN and L-SIGN facilitate initial adhesion by interacting with the glycosylated regions of GP, increasing viral concentration at the cell surface. Macrophages and dendritic cells, which express these lectins in abundance, are among the first targets, contributing to rapid dissemination (Takada et al., 2004, Journal of Virology).

Following adhesion, the virus engages specific host receptors that mediate internalization. The most well-characterized is Niemann-Pick C1 (NPC1), an endosomal cholesterol transporter essential for viral entry. Unlike traditional viral receptors at the plasma membrane, NPC1 operates within the late endosome, requiring viral uptake before receptor engagement. Structural studies show that after endocytosis, host cathepsins cleave GP, exposing the receptor-binding domain (RBD), which then binds NPC1, triggering conformational changes necessary for membrane fusion (Miller et al., 2012, Nature).

Genetic studies confirm NPC1’s critical role, as cells lacking functional NPC1 are resistant to infection. Côté et al. (2011, Nature) demonstrated that NPC1-deficient fibroblasts exhibit a complete block in viral entry. Small-molecule inhibitors targeting the GP-NPC1 interaction have shown promise in preclinical models, highlighting potential therapeutic avenues.

Fusion And Entry

Ebola virus relies on the acidic environment of the late endosome and lysosome to trigger membrane fusion. After internalization via macropinocytosis, the virus moves through the endosomal network, encountering progressively acidic conditions that facilitate GP processing. Host cathepsins, particularly cathepsin B and cathepsin L, cleave the mucin-like domain of GP, exposing the RBD for NPC1 interaction (Chandran et al., 2005, Science).

GP-NPC1 binding triggers further conformational changes in GP2, which contains a fusion loop that inserts into the host membrane, initiating lipid bilayer destabilization (Gong et al., 2016, Cell). The fusion loop, a conserved hydrophobic region, penetrates the endosomal membrane, facilitating membrane merging. This process requires multiple GP trimers to overcome the energy barrier of fusion.

GP2 then undergoes a dramatic refolding event, forming a six-helix bundle that stabilizes the fusion pore, allowing the viral nucleocapsid to enter the cytoplasm (Harrison, 2015, Annual Review of Cell and Developmental Biology). Endosomal lipid composition, particularly cholesterol levels, influences this process. Disrupting cholesterol homeostasis significantly impairs entry, revealing a potential therapeutic target (Spence et al., 2019, PLOS Pathogens).

Replication Of Viral RNA

Once the genome enters the cytoplasm, replication begins via the viral RNA-dependent RNA polymerase (RdRp). Unlike cellular polymerases, which use a DNA template, the viral RdRp transcribes and replicates the single-stranded, negative-sense RNA genome entirely in the cytoplasm. The enzyme is part of the ribonucleoprotein (RNP) complex, which includes nucleoprotein (NP) and co-factors that stabilize the viral RNA in a helical conformation.

Early in infection, the polymerase primarily conducts transcription, producing distinct mRNA transcripts for each viral gene. These transcripts receive a 5′ cap and 3′ polyadenylation signal, mimicking host mRNAs for efficient translation. The polymerase follows a stop-start mechanism, pausing at intergenic junctions before resuming synthesis, creating a transcriptional gradient where genes near the 3′ end are produced in higher quantities. NP accumulation triggers a switch from transcription to genome replication.

Once sufficient NP levels are reached, the polymerase shifts to full-length genome replication, bypassing transcriptional termination signals to generate complementary positive-sense RNA intermediates. These antigenomes serve as templates for new negative-sense genomes. Unlike mRNA transcripts, full-length copies remain tightly associated with NP, forming stable replication complexes that protect the RNA from degradation. VP30, a transcriptional activator, is required for mRNA synthesis but dispensable for genome replication, highlighting distinct regulatory mechanisms.

Viral Protein Synthesis

Ebola virus hijacks the host’s translational machinery to synthesize its proteins. Unlike host mRNAs, which undergo processing in the nucleus, Ebola virus mRNAs are immediately accessible to ribosomes in the cytoplasm. Each transcript contains a 5′ cap and 3′ polyadenylation signal, ensuring recognition by the host’s translation initiation factors.

Protein synthesis follows a temporal pattern. Early in infection, NP and polymerase co-factors VP35 and VP30 are produced in high quantities to facilitate genome replication. As infection progresses, matrix proteins VP40 and VP24, along with glycoprotein (GP), are synthesized for virion assembly. GP undergoes extensive post-translational modifications, including glycosylation in the endoplasmic reticulum and Golgi apparatus, which is essential for proper folding and functionality.

Assembly And Packaging

As replication progresses, newly synthesized components must be assembled into functional virions. Assembly occurs at the inner surface of the plasma membrane, where VP40 and VP24 play key roles. VP40, the most abundant structural protein, drives virion budding by interacting with lipid rafts in the host membrane, creating a platform for recruiting additional viral components. It can independently form virus-like particles (VLPs), demonstrating its intrinsic ability to facilitate membrane curvature and particle formation (Noda et al., 2002, Journal of Virology).

Encapsulation of viral RNA requires NP and the polymerase complex. NP binds viral genomic RNA in a helical arrangement, shielding it from degradation. The RNA-bound NP complex is transported to the membrane, guided by interactions with VP35 and VP30. GP, after post-translational modifications in the endoplasmic reticulum and Golgi, is trafficked to the plasma membrane, where it integrates into budding virions. Proper GP localization depends on its cytoplasmic tail, which interacts with VP40 to ensure efficient particle formation. Mutations in VP40 or GP impair assembly and reduce infectivity.

Budding And Release

Following assembly, newly formed virions exit the host cell. This process, known as budding, is driven by VP40, which facilitates membrane scission and virion release. VP40 hijacks the host’s endosomal sorting complex required for transport (ESCRT) machinery, typically involved in membrane remodeling and vesicle trafficking. By recruiting ESCRT components such as Tsg101 and VPS4, VP40 induces membrane deformation, leading to the pinching off of viral particles (Silvestri et al., 2007, Proceedings of the National Academy of Sciences). Inhibition of ESCRT function significantly reduces viral egress.

Once released, virions remain structurally intact and highly infectious. The lipid envelope, derived from the host membrane, provides a protective barrier, while embedded glycoproteins ensure efficient targeting of new host cells. Viral egress influences disease severity, as higher viral loads correlate with increased transmission potential.

Host Immune Evasion Strategies

Ebola virus has evolved mechanisms to evade host immune responses, allowing it to establish infection before an effective defense can be mounted. One primary strategy involves suppressing interferon (IFN) signaling. VP35 inhibits the activation of RIG-I-like receptors, which detect viral RNA and trigger IFN production. By binding double-stranded RNA intermediates, VP35 prevents the activation of interferon regulatory factor 3 (IRF3), blocking IFN-stimulated gene transcription (Hartman et al., 2008, Cell Host & Microbe).

Additionally, VP24 disrupts nuclear import pathways, preventing the translocation of phosphorylated STAT1, a transcription factor essential for IFN-mediated gene expression. By blocking STAT1 signaling, VP24 reduces major histocompatibility complex (MHC) molecule expression, impairing antigen presentation and adaptive immune activation. Excessive production of soluble glycoprotein (sGP), an alternative GP product, acts as a decoy by binding neutralizing antibodies, diverting immune responses from infectious virions.