Dengue Viruses

Introduction

Viruses are tiny agents that can infect a variety of living organisms, including bacteria, plants, and animals. Like other viruses, the dengue virus is a microscopic structure that can only replicate inside a host organism. Who discovered the dengue virus? How many types of dengue viruses are there, and what do we know about them? How does the dengue virus infect a cell and replicate itself? In this section, we will explore the answers to these questions.

Discovery of the Dengue Viruses

The dengue viruses are members of the genus Flavivirus in the family Flaviviridae. Along with the dengue virus, this genus also includes a number of other viruses transmitted by mosquitoes and ticks that are responsible for human diseases. Flavivirus includes the yellow fever, West Nile, Japanese encephalitis, and tick-borne encephalitis viruses.
In 1943, Ren Kimura and Susumu Hotta first isolated the dengue virus. These two scientists were studying blood samples of patients taken during the 1943 dengue epidemic in Nagasaki, Japan. A year later, Albert B. Sabin and Walter Schlesinger independently isolated the dengue virus. Both pairs of scientists had isolated the virus now referred to as dengue virus 1 (DEN-1). Is DEN-1 the only type of dengue virus?

The Dengue Serotypes

Dengue infections are caused by four closely related viruses named DEN-1, DEN-2, DEN-3, and DEN-4. These four viruses are called serotypes because each has different interactions with the antibodies in human blood serum. The four dengue viruses are similar — they share approximately 65% of their genomes — but even within a single serotype, there is some genetic variation. Despite these variations, infection with each of the dengue serotypes results in the same disease and range of clinical symptoms.
Are these four viruses all found in the same regions of the world? In the 1970s, both DEN-1 and DEN-2 were found in Central America and Africa, and all four serotypes were present in Southeast Asia. By 2004, however, the geographical distribution of the four serotypes had spread widely. Now all four dengue serotypes circulate together in tropical and subtropical regions around the world (Figure 1). The four dengue serotypes share the same geographic and ecological niche. Where did the dengue viruses first come from? Scientists hypothesize that the dengue viruses evolved in nonhuman primates and jumped from these primates to humans in Africa or Southeast Asia between 500 and 1,000 years ago.

A world map is shown in each of panels A and B. The countries are shaded green and outlined by a thin black line. Labeled white boxes representing the four dengue virus serotypes are shown on the map. Panel A shows the geographic distribution of dengue serotypes in 1970. Panel B shows the geographic distribution of dengue serotypes in 2004. In panel A, the DEN-1 and DEN-2 serotypes are shown in Central America and Western Africa. The DEN-1, DEN-2, DEN-3, and DEN-4 serotypes are shown in Southeast Asia. In panel B, all four serotypes are shown in Central America, the Caribbean, South America, Africa, India, Southeast Asia, Australia, and the Pacific Islands. Only DEN-1 and DEN-2 are shown in the Middle East.

Figure 1: The change in distribution of dengue serotypes

The distribution of dengue serotypes in 1970 (a) and 2004 (b).

After recovering from an infection with one dengue serotype, a person has immunity against that particular serotype. Does infection with one serotype protect against future dengue infections with the other serotypes? Individuals are protected from infections with the remaining three serotypes for two to three months after the first dengue infection. Unfortunately, it is not long-term protection. After that short period, a person can be infected with any of the remaining three dengue serotypes. Researchers have noticed that subsequent infections can put individuals at a greater risk for severe dengue illnesses than those who have not been previously infected.

Dengue Virus Genome and Structure

The dengue virus genome is a single strand of RNA. It is referred to as positive-sense RNA because it can be directly translated into proteins. The viral genome encodes ten genes (Figure 2). The genome is translated as a single, long polypeptide and then cut into ten proteins.

A diagram shows the dengue virus RNA genome with its structural and non-structural regions labeled. The RNA is depicted as a horizontal cylinder separated into several colored sections of varying sizes. A thin black coiled line representing untranslated RNA extends from the cylinder's lefthand terminus and is labeled the 5 prime UTR. From left to right, the genes encoded by the dengue virus genome are: the capsid, labeled C and colored light brown; the membrane, labeled M and colored orange; the envelope, labeled E and colored blue; and several non-structural genes, including NS1 (green), NS2A (red), NS2B (dark brown), NS3 (yellow), NS4A (dark orange), NS4B (teal), and NS5 (purple). NS5 is the longest gene; NS2A and NS2B have the shortest lengths. A region of untranslated RNA at the cylinder's righthand terminus is labeled the 3 prime UTR.

Figure 2: Dengue virus genome

The dengue virus genome encodes three structural (capsid [C], membrane [M], and envelope [E]) and seven nonstructural (NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5) proteins.

What are the roles of these ten proteins? Three are structural proteins: the capsid (C), envelope (E), and membrane (M) proteins. Seven are nonstructural proteins: NS1, NS2A, NS2B, NS3, NS4A, NS4B, and NS5. These nonstructural proteins play roles in viral replication and assembly.

A schematic of the dengue virus shows its primary structural features. The virus is depicted as an orange hexagon encapsulated inside a light brown circle. The hexagon is the nucleocapsid and the brown circle is the viral envelope. Thin red material coiled up inside the nucleocapsid represents the viral genome. Seven red lines radiate outward from the viral envelope in a symmetrical orientation. Each line has a green sphere at the end. These protrusions are E and M proteins.

Figure 3: Dengue virus structure

The dengue virus has a roughly spherical shape. Inside the virus is the nucleocapsid, which is made of the viral genome and C proteins. The nucleocapsid is surrounded by a membrane called the viral envelope, a lipid bilayer that is taken from the host. Embedded in the viral envelope are E and M proteins that span through the lipid bilayer. These proteins form a protective outer layer that controls the entry of the virus into human cells.

The structure of the dengue virus is roughly spherical, with a diameter of approximately 50 nm (1 nm is one millionth of 1 mm) (Figure 3). The core of the virus is the nucleocapsid, a structure that is made of the viral genome along with C proteins. The nucleocapsid is surrounded by a membrane called the viral envelope, a lipid bilayer that is taken from the host. Embedded in the viral envelope are 180 copies of the E and M proteins that span through the lipid bilayer. These proteins form a protective outer layer that controls the entry of the virus into human cells.

Dengue Virus Replication and Infectious Cycle

How does the virus behave once it enters the human body? The dengue viral replication process begins when the virus attaches to a human skin cell (Figure 4). After this attachment, the skin cell's membrane folds around the virus and forms a pouch that seals around the virus particle. This pouch is called an endosome. A cell normally uses endosomes to take in large molecules and particles from outside the cell for nourishment. By hijacking this normal cell process, the dengue virus is able to enter a host cell.

A schematic shows a dengue virus particle infecting a eukaryotic host cell, using the eukaryotic cell machinery to replicate the viral genome, and then exiting the host cell as a mature virion. The eukaryotic host cell is depicted as an irregular-shaped sphere with a single nucleus and many organelles. The virus is depicted as a small circle approximately 1/200th the size of the eukaryotic cell.

Figure 4: Dengue virus replication

The dengue virus attaches to the surface of a host cell and enters the cell by a process called endocytosis. Once deep inside the cell, the virus fuses with the endosomal membrane and is released into the cytoplasm. The virus particle comes apart, releasing the viral genome. The viral RNA (vRNA) is translated into a single polypeptide that is cut into ten proteins, and the viral genome is replicated. Virus assembly occurs on the surface of the endoplasmic reticulum (ER) when the structural proteins and newly synthesized RNA bud out from the ER. The immature viral particles are transported through the trans-Golgi network (TGN), where they mature and convert to their infectious form. The mature viruses are then released from the cell and can go on to infect other cells.

© 2005 Nature Publishing Group Mukhopadhyay, S., Kuhn, R. J., & Rossmann M. G. A structural perspective of the flavivirus life cycle. Nature Reviews Microbiology 3, 13–22 (2005). doi:10.1038/nrmicro1067 All rights reserved.

Figure Detail

Once the virus has entered a host cell, the virus penetrates deeper into the cell while still inside the endosome. How does the virus exit the endosome, and why? Researchers have learned that two conditions are needed for the dengue virus to exit the endosome:

The endosome must be deep inside the cell where the environment is acidic.
The endosomal membrane must gain a negative charge.

These two conditions allow the virus envelope to fuse with the endosomal membrane, and that process releases the dengue nucleocapsid into the cytoplasm of the cell.
Once it is released into the cell cytoplasm, how does the virus replicate itself? In the cytoplasm, the nucleocapsid opens to uncoat the viral genome. This process releases the viral RNA into the cytoplasm. The viral RNA then hijacks the host cell's machinery to replicate itself. The virus uses ribosomes on the host's rough endoplasmic reticulum (ER) to translate the viral RNA and produce the viral polypeptide. This polypeptide is then cut to form the ten dengue proteins.
The newly synthesized viral RNA is enclosed in the C proteins, forming a nucleocapid. The nucleocapsid enters the rough ER and is enveloped in the ER membrane and surrounded by the M and E proteins. This step adds the viral envelope and protective outer layer. The immature viruses travel through the Golgi apparatus complex, where the viruses mature and convert into their infectious form. The mature dengue viruses are then released from the cell and can go on to infect other cells.

Summary

The dengue virus is a tiny structure that can only replicate inside a host organism. The four closely related dengue viruses — DEN-1, DEN-2, DEN-3, and DEN-4 — are found in the same regions of the world. The dengue virus is a roughly spherical structure composed of the viral genome and capsid proteins surrounded by an envelope and a shell of proteins. After infecting a host cell, the dengue virus hijacks the host cell's machinery to replicate the viral RNA genome and viral proteins. After maturing, the newly synthesized dengue viruses are released and go on to infect other host cells.

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