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DRAFT--EDITING IN PROGRESS
Dengue Envelope Proteins
Dengue is a term for a group of illnesses caused by four viruses, which are extremely relevant because they can infect humans. (Dengue,Epidemiology) The Dengue viruses belong to the flavivirus family.(Pierson, 2012) The viruses are known as DENV 1, DENV 2, DENV 3, and DENV 4. The primary vector of transmission is one of two types of mosquitos, Aedes aegypti and Aedes albopictus. Although the viruses developed in humans between 100 and 800 years ago, dengue was not a prominent illness until recently. (Epidemiology)
Dengue can manifest in two forms: dengue fever and dengue hemorrhagic fever (DHF). Currently, there is no medicine to target either type of infection. (Frequently) It is important to know the symptoms symptoms of dengue infection and educate oneself about the illness. In order to design a treatment for dengue infection, it is important to understand the mechanism of infection and the structure of the dengue virus family.
Envelope proteins (E proteins) allow the dengue viruses to interact with host cells. They are is about 53 KDa and composed of three domains. In certain types of viruses, they are glycoproteins.(Pierson, 2012)
Structural Variation
Crystal structure
The four different types of dengue virus have slightly different genomes, which means that they may have different protein structures. Antibodies that neutralize one type of dengue virus may be ineffective against another dengue strain due to these structural differences so understanding structural differences is key to treating the virus. Crystallography can show the folded structure of the proteins and allow for comparison of proteins from the different strains. Modis and collaborators used crystallography to learn about the structure of DENV 3 E protein and compare it to the previously characterized analogue in DENV 2. (Modis, 2005)
The researchers were able to crystallize a soluble DENV 3 E protein and determine the structure. Comparison of the E protein from DENV 3 and DENV 2 (strains CH53498 and PR159S1, respectively) lead to interesting insights. The two strains compared had 68% sequence agreement, and folded in similar ways. In solution, the proteins dimerized. (Modis, 2005)
Modis and collaborators also explored the two N-linked glycosylation sites on the glycoproteins. The two resides where this occurs are Asn-67 and Asn-153. Asn-67 is unique to dengue viruses, while the second site can be found in a larger class of viruses. They are interested in these glycosylation sites because the glycans on the protein helps the viruses successfully attack cells. In DENV 3, the protein loop with Asn-153 is shorter than in the other dengue viruses. Both Asn sites have glycans attached to them, which means that at least in Drosophila, the organism from which the protein was isolated, they are both necessary for infection. When DENV 2 was crystallized from Aedes allobpicus, there was no sugar at the 153 position. The group explained that the DENV 3 glycans may have crystallized well due to the other parts of the protein that they link to. The group notes that the most major difference between E proteins on the two types of dengue is the spatial oritentation of domains I and II. The change in orientation between the two viruses is large enough that it must be attributed to a structural difference and not to protein movement in the original solutions. They note that within the domains, the high sequence agreement leads to some structural similarity. There are however, (Modis, 2005)
Genomic Sequences
An earlier study examined the structural properties of the dengue virus that affect their degree of pathogenicity. Specifically, the authors looked at factors that determined whether the infection would manifest as dengue fever or dengue hemorrhagic fever. They noted that both immune system factors and viral factors determined the severity of symptoms, and focused their work on the viral factors. Using reverse transcriptase PCR, they compared the RNA genomes from different strains of DENV 2 obtained through patient plasma. Eleven strains were compared, some of which caused dengue fever and others which cause dengue hemorrhagic fever. The researchers noted that in most viruses related to dengue, “the molecular markers for pathogenicity have been localized in the E gene.” (Leitmeyer, 1999)
The group sequenced the genomes and compared sequences to determine degree of similarity and mutation rates in samples from Southeast Asia and samples from the Americas. The Southeast Asian strains tended to be associated with dengue hemorrhagic fever while the American strains were not. The group showed that amino acids that related to the activity of the viruses were highly conserved, but that approximately fifty amino acid changes could be found. The phylogenetic tree produced from the results was consistent with previous predictions except that there was a high statistical significance for the separation of two genotypic groups. In the Southeast Asian group, there was no segregation based on resulting symptoms, and within the American group there was so segregation and all produced the same symptoms. The researchers were unable to find specific sequences that corresponded to type of symptom. This means that all viruses from certain groups, such as the Southeast Asian group, are capable of inducing dengue hemorrhagic fever. (Leitmeyer, 1999)
Within the two sample groups, there was an amino acid difference at the 390 position of the E protein; mutations in the protein had previously been shown to alter the virulence of dengue and viruses of the same family. The mutation occurred in a portion of the protein that tends to be folded towards the outer surface and that has been shown to interact with the host cell surface. In the American variety, which did not produce hemorrhagic fever, the 370 position was an Asp, while in the Southeast Asian variety, which could produce hemorrhagic fever, the 730 position was an Asn and is believed to interact more strongly with host cells. (Leitmeyer, 1999).
Structural Changes of the Protein
Fusion with Host Cell
E proteins have been implicated in the virus’s ability to bind to host cells, the mechanism for which is important for understanding the attack on the host. In order to better understand the mechanism, Klein and coworkers studied crystal structures in order to understand the final step of membrane fusion. The protein transitions from a dimeric to a trimeric form in the process and the group was able to crysalize portions of this transition. (Klein, 2013)
Other Changes
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Targeting Dengue
Immune Recognition
The structures of E proteins have implications for viral interactions. Antibodies to the dengue viruses may have different affinities to different types of dengue due to structural features on the E proteins. Some of these features may be due variation of 56 amino acid residues in parts of the E proteins, especially variation in a portion of the protein that interacts with cell receptors. If antibodies cannot properly neutralize to the dengue virus, the presence of the antibody on the virus may enhance the virus’s ability to infiltrate the target cells because the host cells will recognize the antibodies as host material. This is called “antibody-dependent enhancement” of infection and can cause more severe symptoms. Other antibodies inhibit the viruses at regions that are conserved in the different types of dengue and tend work by inhibiting membrane fusion. (Modis, 2005)
Drug Design
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Conclusion
Dengue is a group of viruses that infects humans via mosquito vectors. A structural feature of the virus that is essential for infection is the envelope protein, or E protein. While there are currently no drugs to specifically target the dengue viruses, substantial work is being done in order to understand the proteins that enable attack. Hopefully, these insights can lead to drug development be providing information on specific drug targets.
References
Cited in text
Dengue. Center for Disease Control and Prevention. http://www.cdc.gov/Dengue/
Epidemiology. Dengue. Center for Disease Control and Prevention. http://www.cdc.gov/dengue/epidemiology/index.html
Frequently Asked Questions. Dengue. Center for Disease Control and Prevention. http://www.cdc.gov/Dengue/faqFacts/index.html
Klein, Daryl E., et al. (2013) Structure of a Dengue Virus Envelope Protein Late-Stage Fusion Intermediate. Journal of Virology. 87(4): 2287-2293.
Leitmeyer, Katrin C. et al. (1999). Dengue Virus Structural Difference That Correlate with Pathogenesis. Journal of Virology. 4738-4747.
Modis, Yorgo, et al. (2005). Variable Surface Epitopes in the Crystal structure of Dengue Virus Type 3 Envelope Glycoprotein. Journal of Virology. 1223-1231.
Pierson, Theodore C., et al. (2012) Degrees of maturity: the complex structure and biology of flaviviruses.Current Opinion in Virology. 2: 168-175.
Photo credits
photos are still awaiting permissions
Link Credits
"Comparison Between Main Dengue Vectors". Entomology. Dengue. Center for Disease Control and Prevention. http://www.cdc.gov/dengue/resources/30Jan2012/comparisondenguevectors.pdf
"Symptoms & Treatment". Dengue. Center for Disease Control and Prevention. http://www.cdc.gov/Dengue/symptoms/index.html/
Edited by (your name here), a student of Nora Sullivan in BIOL187S (Microbial Life) in The Keck Science Department of the Claremont Colleges Spring 2013.
