Question

A.  Explain the connection between endocytosis, endosomes, and lysosomes. Explain what endosome maturation involves.

B.  Once a virus has been endocytosed and is in an endosome, how does it avoid being digested by the lysosome?

Answer 1

a.  

The endocytic pathway of mammalian cells consists of distinct membrane compartments, which internalize molecules from the plasma membrane and recycle them back to the surface (as in early endosomes and recycling endosomes), or sort them to degradation (as in late endosomes and lysosomes). The principal components of the endocytic pathway are:

•             Early endosomes are the first compartment of the endocytic pathway. Early endosomes are often located in the periphery of the cell, and receive most types of vesicles coming from the cell surface. They have a characteristic tubulo-vesicular structure (vesicles up to 1 µm in diameter with connected tubules of approx. 50 nm diameter) and a mildly acid pH. They are principally sorting organelles where many endocytosed ligands dissociate from their receptors in the acid pH of the compartment, and from which many of the receptors recycle to the cell surface (via tubules). It is also the site of sorting into transcytotic pathway to later compartments (like late endosomes or lysosomes) via transvesicular compartments (like multivesicular bodies (MVB) or endosomal carrier vesicles (ECVs)).

•             Late endosomes receive endocytosed material en route to lysosomes, usually from early endosomes in the endocytic pathway, from trans-Golgi network (TGN) in the biosynthetic pathway, and from phagosomes in the phagocytic pathway.Late endosomes often contain proteins characteristic of lysosomes, including lysosomal membrane glycoproteins and acid hydrolases. They are acidic (approx. pH 5.5), and are part of the trafficking pathway of mannose-6-phosphate receptors. Late endosomes are thought to mediate a final set of sorting events prior to delivery of material to lysosomes.

•             Lysosomes are the last compartment of the endocytic pathway. Their chief function is to break down cellular waste products, fats, carbohydrates, proteins, and other macromolecules into simple compounds. These are then returned to the cytoplasm as new cell-building materials. To accomplish this, lysosomes use some 40 different types of hydrolytic enzymes, all of which are manufactured in the endoplasmic reticulum, modified in the Golgi apparatus and function in an acidic environment. The approximate pH of a lysosome is 4.8 and by electron microscopy (EM) usually appear as large vacuoles (1-2 µm in diameter) containing electron-dense material. They have a high content of lysosomal membrane proteins and active lysosomal hydrolases, but no mannose-6-phosphate receptor. They are generally regarded as the principal hydrolytic compartment of the cell.

B.

Viruses can also have surface receptors which can be specific to those on the macrophage. Viruses need to access the host cell’s cytoplasm or nucleus in order to replicate and cause an infection, so they use their surface receptors to interact with immune system cells and exploit the immune response for entry into the cell. Sometimes, when a virus and a host cell interact, the host cell is able to successfully destroy the virus and stop the spread of infection. Other times, the host cell engulfs the virus, and the virus tricks the cell, gaining access to what it needs to replicate. Once this happens, the infected cell is identified and destroyed by other cells of the immune system in order to stop viral replication and infection.

Simple viruses use a most efficient method of releasing their genome into the cytosol: It crosses the plasma membrane after the viruses bind to specific cell surface receptors . In the case of complex viruses, they enter cells by classical endocytosis. After internalization, complex viruses that are localized in endosomes can escape endocytic vesicles under suitable conditions, as they have developed ways to mediate their escape. The escape strategies employed by viruses depend on the type of virus. Enveloped viruses fuse with endosomal membranes, whereas non enveloped viruses lyse or form pores to release the viral genome . Thus, an elegant strategy has evolved, whereby viruses avoid lysosomal degradation, which is a dead end for many particles in a classic endocytic pathway. As a result of their communication with cells, complex viruses have developed other ways to exploit nonclassic endocytosis, such as a unique caveolar pathway to move into the cell. This internalization route is comprised of a neutral pH compartment, which allows viruses to avoid degradation at low pH. Interestingly, this internalization pathway also mediates trafficking of viruses, such as Simian virus 40 (SV40), to intracellular organelles, including the endoplasmic reticulum (ER), before the viral genome enters the nucleus . Therefore, it is evident that viral evolution over millions of years has resulted in the acquisition of strategies whereby viruses utilize and control cell functions.

Phagolysosome lowers the pH to break down its contents.

A lysosome or phagolysosome is able to break down the stuff inside of itself by drastically lowering the pH of its internal environment. Lowering the pH makes the environment inside the phagolysosome makes it very acidic. This is an effective way of killing or neutralizing whatever is inside the phagolysosome so it cannot infect the cell.

Some viruses actually exploit the lowered pH to escape the phagolysosome and start replicating inside the cell. For example, influenza (the flu virus) uses the drop in pH to activate a conformational change, allowing it to escape into the cytoplasm.