Investigating how ebola kills - one receptor at a time

  
   The emergence of a deadly Ebola virus (EBOV) strain into human populations has been our constant worry for over 40 years now. And news of the most recent case where a 12-year-old girl was killed near a major trade hub in Uganda earlier this month only serves to remind us of the devastating impact it can have. Infection with the Zaire strain of EBOV, the one that caused the death of the girl, initially causes a rapid onset of fever and headaches culminating in internal and extrernal bleeding, vomiting and diarrhea and eventually, death. No specific therapy or vaccines are available.

   While Ebola virus Zaire is known to cause up to 90% mortality the exact mechanisms of how it causes disease in humans are not understood. Kondratowicz et al, publishing recently in the journal PNAS, add to our knowledge of EBOV pathogenesis and biology by identifying the receptor molecule (TIM-1) on the surface of our cells that the virus uses to infect humans. 



What is a virus receptor?

EBOV glycoprotein (GP) synthesis and functions. http://www.bio.davidson.edu/

   Viruses are parasites of cells; that is, in order to survive they need to enter our cells and replicate. One major barrier to this, however, is the cell's plasma membrane: an outer covering made up of fats and proteins that protects the cell from the harsh outside environment. On the other hand, viruses have evolved diverse strategies to bypass this barrier and EBOV is not any different (reviewed in TWiV here). Ebola glycoproteins (GP) 1 and 2 lie on the outside of the virus particle and mediate both attachment to the cell via a receptor molecule AND fusion of the virus membrane with the cell membrane releasing the infectious virus genome into the host cytoplasm. This process also appears to involve a peculiar process known as macropinocytosis.


   Viruses do not infect every cell within the human body - they are pretty selective, having adapted their replication cycle to a particular host over hundreds or thousands of years. What then dictates this selectivity? and what role does this host cell choice have on the replication of the virus? The answer lies upon the particular receptor moleculas the viruses use.

EBOV GP structure. http://www.als.lbl.gov/

What about the ebola receptor?


   A number of cell surface molecules have been identified that play a role in the complex process of EBOV entry, all mainly only increasing its efficiency. Although none have been shown to specifically interact and bind to the virus GP 1/2, these may play more of a general role in enhancing entry. How then are we meant to identify what molecules act as specific receptors? More specifically, what does a virus receptor behave like? if we saw one, what would it look like?


   To prove that a particular molecule is a receptor for a specific virus, it should fulfil a set of predictions. Primarily, it must:

• Physically interact with virus proteins (GP 1/2) on it's surface
• Expression of it must significantly enhance infection (shoudn't be found on any cells it can't infect) and if it cannot infect a particular cell, does forced expression restore infectability?
• Removal or blocking of receptor molecule will prevent infection of cell previously susceptable

How did the group do this? 


   To search for the receptor molecule that EBOV uses, Kondratowicz et al sought to correlate EBOV infection with the expression of the receptor molecule. Using a non-related virus (Vesicular Stromatitis virus) that was engineered to express EBOV glycoproteins as well as a green fluorescent protein (GFP), they infected a panel of 54 tumour cell lines. By counting the number of green cells (i.e those that EBOV could enter) they could see which cells were most easily infected and hence expressed the required receptor molecule. 

Percent transduction (GFP expression) of the VSV-EBOV-GP virus (A), Non-EBOV-VSV (B) in a range of cell lines. Correlated with TIM-1 expression date (C).

TIM-1 on the cell surface

   The group then searched through gene expression data corresponding to the cell lines and attempted to pull out a set of candidate receptors common to all susceptible cells, i.e. those that generated a lot of green cells. One gene that they found to be significantly correlated with infection was TIM-1, a type one transmembrane protein expressed on dividing kidney cells and some immune cells. They do note that TIM-1 is not found on all the susceptible cells, suggesting a role for other unknown factors in EBOV entry. Alas the hunt may continue.


   Kondratowicz et al further provides ample evidence that TIM-1 plays a significant role in EBOV glycoprotein mediated entry: he team showed that if you knockdown expression of TIM-1 on the surface of infectable cells with siRNAs, no longer does EBOV infect the cells; they also showed that expressing TIM-1 in cells not previously able to be infected, or ones that weren't all that good at it, allowed the entry of EBOV. Soluble TIM-1 is able to outcompete cell surface TIM-1 for binding to EBOV GP thus causing an inhibition of entry. TIM-1 EBOV GP binding is also demonstrated, fulfilling the requirement of direct interaction between the two. They go on to identify the expression of TIM-1 on the epithelial cells lining the human respiratory tract, a fact important as the means that EBOV initially infects us. Interestingly, the receptor is also found in conjunctiva around the eye. And, finally, infection with viable 'live' EBOV (not the VSV + EBOV glycoproteins) was blocked by administration of an anti-TIM-1 antibody. 

   We may safely conclude that TIM-1 is at least one of the receptors used by EBOV to infect human cells. These results shed light on the possible early events of EBOV infection in humans: aerosolised virus particles entering via the respiratory tract bind to an infect the epithelial cells lining it. From here, further cells are infected, like immune cells, which may allow spread throughout the body. Interestingly, TIM-1 was also found on the conjunctiva indicating a role for hand/aerosol-eye contact mediated infection. 

Death from ebola may soon be a thing of the past. http://www.documentingreality.com/

   This discovery however, does not end the story of the EBOV receptor. Not all cells that can be infected express TIM-1. What receptors are being used in this case? we just don't know yet. What we do know is that we may be on to a worthwile antiviral against ebola. For if we bock virus access to TIM-1 (like what was done in with the antibodies in this experiment) we may be able to severely inhibit EBOV infection and disease.




Kondratowicz AS, Lennemann NJ, Sinn PL, Davey RA, Hunt CL, Moller-Tank S, Meyerholz DK, Rennert P, Mullins RF, Brindley M, Sandersfeld LM, Quinn K, Weller M, McCray PB Jr, Chiorini J, & Maury W (2011). From the Cover: T-cell immunoglobulin and mucin domain 1 (TIM-1) is a receptor for Zaire Ebolavirus and Lake Victoria Marburgvirus. Proceedings of the National Academy of Sciences of the United States of America, 108 (20), 8426-31 PMID: 21536871