Viruses are microscopic entities that cannot replicate on their own. They must enter host cells to multiply and spread, delivering their genetic material. A cellular compartment known as an “endosome” often plays a direct role in this entry process, as viruses are commonly engulfed by the host cell.
The Entry Process: Endocytosis
The process by which cells take in external substances, including viruses, by engulfing them within a membrane-bound vesicle is called endocytosis. This is a fundamental cellular mechanism that allows cells to absorb nutrients, regulate surface receptors, and neutralize pathogens. During endocytosis, the cell membrane forms an inward pocket around the virus. This pocket then pinches off, creating a small, sealed sac called an endosome, which contains the engulfed virus. Viruses often rely on this host cell uptake machinery to initiate an infection.
Diverse Pathways of Viral Endocytosis
While endocytosis is the overarching term, viruses exploit several distinct pathways to enter host cells. Each pathway involves specific cellular machinery and membrane dynamics, demonstrating how viruses hijack host cell functions for replication.
Clathrin-Mediated Endocytosis
One common pathway is clathrin-mediated endocytosis, involving specific proteins called clathrin. Clathrin molecules assemble on the inner surface of the cell membrane, forming a coated pit that invaginates and pinches off to create a clathrin-coated vesicle. This vesicle, containing the virus, then fuses with an early endosome after the clathrin coat is removed. Influenza A virus and Hepatitis C virus are examples that utilize this pathway.
Caveolae-Mediated Endocytosis
Another pathway is caveolae-mediated endocytosis, involving flask-shaped invaginations of the plasma membrane called caveolae. These structures are rich in cholesterol and specific proteins called caveolins. Caveolae-mediated uptake often occurs upon specific cell stimulation. Simian virus 40 (SV40) is an example of a virus that enters cells via this pathway.
Macropinocytosis
Macropinocytosis is a broader, less specific engulfment process involving large-scale membrane ruffling. Cells use this pathway for the non-selective uptake of extracellular fluid and larger particles. Viruses trigger this process by activating signaling pathways, leading to actin-dependent membrane protrusions and the formation of large vacuoles called macropinosomes. Vaccinia virus, Ebola virus, and some strains of influenza A virus can utilize macropinocytosis for entry.
Uncoating Within the Endosome
Once a virus is inside an endosome, a crucial step known as “uncoating” must occur for the infection to proceed. The endosomal compartment undergoes progressive acidification as it matures, with the pH decreasing from approximately 7.5 in early endosomes to around 5.0–5.5 in late endosomes or lysosomes. This acidic environment acts as a trigger for many viruses.
The drop in pH causes conformational changes in viral proteins, particularly those on the viral surface or capsid. For enveloped viruses, this pH change can lead to the fusion of the viral envelope with the endosomal membrane, releasing the viral genetic material and associated proteins into the host cell’s cytoplasm. For non-enveloped viruses, the acidic pH can induce structural modifications in the capsid, leading to pore formation or rupture of the endosomal membrane, allowing the viral genome to escape. This release of genetic material into the cytoplasm is necessary for the virus to begin its replication cycle.
Targeting Endosomal Entry for Antiviral Therapies
Understanding the steps of viral endosomal entry and uncoating is important for medical advancements. These processes represent vulnerable points in the viral life cycle that can be targeted for antiviral interventions. Therapies could block the initial binding of viruses to cell surface receptors, preventing their internalization.
Other strategies involve inhibiting endocytic pathways, such as compounds that interfere with clathrin or caveolin formation, or those that disrupt macropinocytosis. Preventing the pH-triggered uncoating step within the endosome also offers an avenue for drug development. For instance, drugs that neutralize endosomal pH or interfere with conformational changes of viral proteins could stop the infection before the viral genetic material is released, halting viral replication.