Dengue virus

Dengue virus (DENV) in one of four serotypes is the cause of dengue fever. It is a mosquito-borne single positive-stranded RNA virus of the family Flaviviridae; genus Flavivirus. All four serotypes can cause the full spectrum of disease.

Its genome is about 11000 bases that codes for three structural proteins, capsid protein C, membrane protein M, envelope protein E; seven nonstructural proteins, NS1, NS2a, NS2b, NS3, NS4a, NS4b, NS5; and short non-coding regions on both the 5' and 3' ends. Further classification of each serotype into genotypes often relates to the region where particular strains are commonly found or were first found.

Evolution
The Dengue type 1 virus appears to have evolved in the early 19th century Based on the analysis of the envelope protein there are at least four genotypes (1 to 4). The rate of nucleotide substitution for this virus has been estimated to be 6.5×10−4 per nucleotide per year, a rate similar to other RNA viruses. The American African genotype has been estimated to have evolved between 1907 to 1949. This period includes World War I and II which were associated with considerable movement of populations and environmental disturbance, factors known to promote the evolution of new vector borne viral species.

Life cycle
The primary life cycle of the dengue virus involves humans and mosquitoes of the genus Aedes. There is also another cycle in Africa and also on Asia, which involves non-human primates.

E protein
The DENV E (envelope) protein, found on the viral surface, is important in the initial attachment of the viral particle to the host cell. Several molecules which interact with the viral E protein (ICAM3-grabbing non-integrin., CD209, Rab 5, GRP 78, and The Mannose Receptor )have been shown to be important factors mediating attachment and viral entry.

prM/M protein
The DENV prM (membrane) protein, which is important in the formation and maturation of the viral particle, consists of seven antiparallel β-strands stabilized by three disulfide bonds.

The glycoprotein shell of the mature DENV virion consists of 180 copies each of the E protein and M protein. The immature virion starts out with the E and prM proteins forming 90 heterodimers that give a spiky exterior to the viral particle. This immature viral particle buds into the endoplasmic reticulum and eventually travels via the secretory pathway to the Golgi apparatus. As the virion passes through the trans-Golgi Network (TGN) it is exposed to low pH. This acidic environment causes a conformational change in the E protein which disassociates it from the prM protein and causes it to form E homodimers. These homodimers lie flat against the viral surface giving the maturing virion a smooth appearance. During this maturation pr peptide is cleaved from the M peptide by the host protease, furin. The M protein then acts as a transmembrane protein under the E-protein shell of the mature virion. The pr peptide stays associated with the E protein until the viral particle is released into the extracellular environment. This pr peptide acts like a cap, covering the hydrophobic fusion loop of the E protein until the viral particle has exited the cell.

NS3 protein
The DENV NS3 is a serine protease, as well as an RNA helicase and RTPase/NTPase. The protease domain consists of six β-strands arranged into two β-barrels formed by residues 1-180 of the protein. The catalytic triad (His-51, Asp-75 and Ser-135), is found between these two β-barrels, and its activity is dependent on the presence of the NS2B cofactor. This cofactor wraps around the NS3 protease domain and becomes part of the active site. The remaining NS3 residues (180-618), form the three subdomains of the DENV helicase. A six-stranded parallel β-sheet surrounded by four α-helices make up subdomains I and II, and subdomain III is composed of 4 α-helices surrounded by three shorter α-helices and two antiparallel β-strands.

NS5 protein
The DENV NS5 protein is a 900 residue peptide with a methyltransferase domain at its N-terminal end (residues 1-296) and a RNA-dependent RNA polymerase (RdRp) at its C-terminal end (residues 320–900). The methyltransferase domain consists of an α/β/β sandwich flanked by N-and C-terminal subdomains. The DENV RdRp is similar to other RdRps containing palm,, finger, and thumb subdomains and a GDD motif for incorporating nucleotides.

Severe disease
The reason that some people suffer from more severe forms of dengue, such as dengue hemorrhagic fever, is multifactorial. Different strains of viruses interacting with people with different immune backgrounds lead to a complex interaction. Among the possible causes are cross-serotypic immune response, through a mechanism known as antibody-dependent enhancement, which happens when a person who has been previously infected with dengue gets infected for the second, third or fourth time. The previous antibodies to the old strain of dengue virus now interfere with the immune response to the current strain, leading paradoxically to more virus entry and uptake.

Immune system interaction
In recent years, many studies have shown that flaviviruses, especially dengue virus has the ability to inhibit the innate immune response during the infection. Indeed, the dengue virus has many nonstructural proteins that allow the inhibition of various mediators of the innate immune system response. These proteins act on two levels :

Inhibition of interferon signaling by blocking signal transducer
NS4B it is a small hydrophobic protein located in association with the endoplasmic reticulum. It may block the phosphorylation of STAT 1 after induction by interferons type I alpha, beta. In fact, the activity of Tyk2 kinase decreases with the dengue virus, so STAT 1 phosphorylation decreases too. Therefore, the innate immune system response may be blocked. Thus there is no production of ISG. NS2A and NS4A cofactor may also take part in the STAT 1 inhibition.

NS5 : the presence of this 105 kDa protein results in inactivation of STAT2 (via the signal transduction of the response to interferon) when it is expressed alone. When NS5 is cleaved with NS4B by a protease (NS2B3) it can degrade STAT2. In fact, after the cleavage of NS5 by the protease, there is a E3 ligase association with STAT2, and the E3 ligase targets STAT2 for the degradation.

Inhibition of the type I interferon response
NS2B3 protease complex: It’s a proteolytic core consisting of the last 40 amino acids of NS2B and the first 180 amino acids of NS3. It’s a cleavage on the NS2B3 precursor which active the protease complex. This protease complex allows the inhibition of the production of type I interferon by reduce the activity of IFN-beta promoter : studies have shown that NS2B3 protease complex is involved in inhibiting the phosphorylation of IRF3. A recent study shows that the NS2B3 protease complex inhibits (by cleaving) protein MITA which allows the IRF3 activation.

Vaccine research
There currently is no human vaccine available. Developing a vaccine against the disease is challenging. With four different serotypes of the dengue virus that can cause the disease, the vaccine must immunize against all four types to be effective. Vaccination against only one serotype could possibly lead to severe DHS (Dengue hemorrhagic shock) when infected with another serotype due to Antibody-Dependent Enhancement. It is estimated that a vaccine will be available in 2014.