Viruses can enter cells through a variety of different pathways. Many enter through endocytosis, and there are actually several endocytic pathways: clathrin mediated, caveolin mediated, phago- and pinocytosis, and the rather mysterious “non-clathrin, non-caveolin mediated endocytosis.”
Ebola virus causes a severe hemorrhagic disease with 90% mortality. Its an obviously frightening virus which makes it difficult to study, but knowing the details of its replication cycle may provide important clues on how to treat or prevent the disease. A recent paper demonstrates that Ebola probably uses clathrin-mediated endocytosis. Clathrin is a protein that forms a polyhedral lattice on the inside of the cell membrane helping to form vesicles. Virus attachment induces this vesicle formation, giving the virus access to the cell by entering through these vesicles. Among other experiments, they found that if you use the drug chorpromazine, which inhibits clathrin function, you can block Ebola entry.
The paper raises some interesting questions. First, they didnt actually use Ebola virus. They used a modified HIV that expresses the Ebola virus glycoprotein involved in attachment and entry. Does the natural virus enter in the same way? They used several different cells in culture and found clathrin dependence in all of them, but is it the same in an infected animal? Finally is the drug chlorpromazine one that could be used clinically? Presumably not since disrupting clathrin mediated endocytosis would probably have a broadly toxic effect on the host, but it is an interesting lead compound.
Interesting! It does seem that chlorpromazine is an anti-psychotic used for the above mentioned conditions, as well as schizophrenia and even hiccups. The NIH has some info on its uses: http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0000553 I wonder if its anti-psychocitic properties are related to its effect on clathrin.
Experimental conditions can sometimes force a virus to enter through means that would not normally be used. Using multiple cell line helps support that this may be the natural pathway of entry. Its probable that only the Ebola glycoprotein is required for normal entry, but I honestly dont know how well the recombinant HIV system mimics the natural infection. Ultimately, experiments with Ebola itself should be done. But not a lot of people want to handle a virus with a 90% mortality rate and no vaccine or treatment, and few facilities are appropriately equipped to use it. This is a good stand in until we know exactly what the most important and informative experiments will be.
When we consider pathogenesis, or how the virus actually causes disease, we absolutely need to consider the immune system and use whole animal experiments. However the cellular and molecular events of viral replication are independent of the immune system and typically are the same in vivo as in vitro.
Wouldn’t the fact that the cells used were in a culture mean that the role of the immune system wouldn’t be accounted for? I remember when we were discussing the Spanish flu, that it was the reaction of the immune system, which flooded the lungs with white blood cells (and hence blood) that made the virus so deadly. Are cell culture results generally considered valid for determining the effects of a virus on a whole organism?
Since an animal would have more complicated processes than cells in a culture, it seems likely that the reception proteins may also differ. Is this the case?
I don’t think that their experiments’ methods are sufficient to safely assume that an Ebola virus would be hindered by the lack of clathrin functionality. We’ve been learning how diverse viruses are and how they have so many clever ways to gain entry to a cell; the only way to know for sure what could stop it would be to test with Ebola itself, not just a modified version of HIV with Ebola’s glycoprotein. Though barring that glycoprotein might – I’d go so far as to say would probably – stop entry, I don’t think it’s reasonable to be satisfied with that conclusion without further testing.
Infected animal cells are also dependent on Clathrin because Clathrin plays a role in protein transport for eukaryotic cells, which include animal cells. For the last question, I researched whether or not Chloropromazine was actually clinically prescribed and it is clinically used. Also known as Thorazine, Chloroprzine is used for treating nausea and vomiting, as well as certain symptoms of people who have bipolar disorder and severe deficit hyperactivity disorder in children.