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Highlights
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Hantaviruses are carried by small mammal hosts such as rodents, persistently infecting them, with little or no deleterious effect on the animals. |
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Hantaviruses are transmitted to humans by means of aerosolized excreted virus. |
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Humans are not viral hosts but are accidentally and transiently infected. |
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Hantaviruses, of which there are several pathogenic and non-pathogenic types, cause both hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome; the latter has a 40% mortality rate. |
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Hantaviruses primarily infect animal and human endothelial cells, although the cells are not destroyed by the infection; how the cells are altered is under investigation. |
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Pathogenic hantaviruses use b3 integrins as cellular receptors, although non-pathogenic ones do not; the importance of b3 integrins in platelet and endothelial cell functions suggests that integrin usage may contribute to pulmonary edema and hemorrhage caused by pathogenic hantaviruses. |
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The virus may interact with host cells in an RGD-independent manner, which is a novel means of interaction with b3 integrins. |
B3 integrins: key to hantavirus pathogenesis?
Erich R. Mackow's talk focused on his laboratory's efforts to understand hantavirus pathogenesis by analyzing hantavirus interactions with key cellular receptors that regulate vascular permeability, studying the function of hantavirus proteins, and defining cellular responses that contribute to disease. Specifically, the team is investigating the unique use of b3 integrins by pathogenic—but not non-pathogenic-hantaviruses, and defining a means by which such integrin usage dysregulates normal endothelial cell functions.
"The importance of b3 integrins in platelet and endothelial cell functions suggests that integrin usage may contribute to pulmonary edema and hemorrhage caused by pathogenic hantaviruses," Mackow said. "Recently, we demonstrated that endothelial cell migration on b3 but not b1 integrin ligands was blocked by only pathogenic hantaviruses. This defines a means by which hantavirus-integrin interactions selectively alter endothelial cell functions that maintain capillary integrity."
B3 integrins may contribute to the pulmonary edema and hemorrhage caused by hantaviruses
For viruses such as Ebola, this mechanism isn't a concern, Mackow continued: "Ebola lyses endothelial cells, causing bleeding by a completely different mechanism from that of the hantaviruses. Since hantaviruses are not lytic infections, the question is, what is the mechanism by which hemorrhage or vascular leakage may occur? The b3 integrin that pathogenic hantaviruses use, dysregulate, and may direct antibodies to, is the perfect way by which hantavirus might cause disease."
In fact, pathogenic hantaviruses interact with a very small, 53 amino-acid domain present at the apex of the bent conformation of avb3 integrins, Mackow said. This affects b3 integrin function in both platelets and capillaries, setting the stage for vascular disorders, including hemorrhage and edema. By contrast, non-pathogenic hantaviruses—for example, Prospect Hill virus and Tula virus—interact with b1, not b3, integrins.
Moreover, pathogenic hantaviruses interact with b3 integrins in an RGD-independent way. "That's a novel means by which substances interact with b3 integrins, and that has also been part of the problem in looking at how the virus binds. Most substances that bind to b3 integrins are ligands that bind to the extended (not bent) conformation and via RGD binding to the integrin," Mackow explained. "The real key that pulled all the data together is that we showed the interaction was RGD-independent-there was no RGD motif in the virus-and that calcium and manganese work opposite to what activates the integrins. Calcium activates viral infection and manganese inhibits infection. That's counterintuitive to what is normally the state of ligand binding to the integrin."
Calcium activates viral infection and manganese inhibits infection.
Another key is the structural data showing that two conformations of the integrins exist: bent (in the presence of calcium) and extended (in the presence of magnesium). "If we didn't have that information, we'd be hard-pressed to explain how the virus is binding," Mackow said. "That revelation is what made this come together and permitted us to test a number of interesting things about the binding of hantaviruses to a protein 53 amino-acid piece N-terminal domain of b3 that we discovered. This structure shows that the b3 integrin is dynamically regulated into two different conformations that are either active or inactive. The inactive bent conformer provides a means by which viral binding interactions can inactivate b3 and maintain the integrin in an inactive ligand binding conformation. Viral binding to the bent integrin may act very much like an antibody would act by binding to that integrin to dysregulate its function."
 Next steps
"One possibility is that the 53 amino-acid N-terminus of b3 we've identified, which interacts with hantaviruses, is important not only for hantavirus binding but for other antibody responses that direct hemorrhagic disease. This small domain also provides a viable target for devising a means to block hantavirus infection," Mackow said. "Can we design antibodies that are directed at this protein? Can we develop other ligands that block it, thereby blocking hantavirus infection? This is very interesting for potential therapeutics."
The focus of future work will be on the types of therapeutics that can be used to block hantavirus infection—for example, antibodies that can be directed to the b3-integrin domain and ways of using that domain to identify binding partners for the viral glycoprotein.
In addition, said Mackow, "there are more viruses to look at. We've looked at many of the pathogenic hantaviruses, but not all of them. We need to determine whether the other pathogenic hantaviruses use the b3 integrin and whether blocking hantavirus interactions with b3 integrins inhibits disease in an animal model."
Work on therapeutics is also progressing. "We've been defining even smaller pieces of the target," Mackow noted. "We have three small targets for use in recombinatorial libraries, and we're setting up screens for drugs that might block binding to b3 polypeptides, and then testing substances that bind for function. First, we'll find out if they bind, and second, we'll see if they block hantavirus infection or disease. We have a target, and so we have the potential to define therapeutics," he concluded. |
