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A Brief Passage Of Time In The Deadly Ebola Virus - Health - Nairaland

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A Brief Passage Of Time In The Deadly Ebola Virus by ruffcoin09(m): 11:46pm On Jul 29, 2014
As if from a science fiction movie, the Ebola virus is one of the most lethal viruses known to man. The haste with which it dispatches its victims is truly gruesome! Ebola was first identified in Africa in1976, and can be caused by any four of the five Ebola viruses: Bundibugyo virus, Ebola virus, Sudan virus, and Taï forest virus. The fifth virus, Reston virus is not thought to be disease causing for humans. Ebola is a member of the family Filoviridae in the order Mononegavirales. EVD has a case fatality rate of 90%. Infections of the virus cause a rapidly fatal haemorrhagic fever. Humans become infected by contact with bodily fluids from an infected person or objects that have been contaminated. The virus can also be contracted from infected animals. The reservoir of Ebola is still unknown, but scientists believe fruit bats are the most likely hosts.

Incubation spans two to twenty-one days and symptoms include: weakness, fever, aches, diarrhoea, vomiting and stomach pain, throat soreness, difficulty breathing, or swallowing and bleeding. Within days the virus causes a condition known as disseminated intravascular coagulation, which is marked by both blood clots and haemorrhaging. Patients exhibit symptoms of spontaneous bleeding from body orifices and any breaks in the skin, including injection sites. The virus attacks the gastrointestinal tract, skin, and internal organs. Death is brought on by haemorrhaging, shock, or renal failure and occurs within 8 to 17 days. No FDA approved therapy for Ebola virus exists, though the FDA is currently fast tracking a RNA interference therapy specific to the Ebola virus being developed by Tekmira Pharmaceuticals.


So what does the Ebola virus look like? In fact, it has a quite menacing look. To start with, it has a threadlike structure, which is characteristic of all filoviruses. The virons are tubular and have a diameter around 80 nm. To increase its menacing appearance, lipid bilayer anchors called glycoproteins, project 10 nm spikes from the viral envelope. They’re generally somewhere between 800 nm and 1000nm long. The nucleocapsid in the center of the viron is made of a RNA wound helically with the proteins NP, VP35, VP30, and last but not least L. The viral proteins VP40 and VP24 are found in the viral tegument or the area between the envelope and nucleocapsid.


Each viron of Ebola virus has a single stranded, negative sense RNA linear genome. It’s between 18,959 and 18,961 nucleotides in length. One frightening aspect of the virus is that replication of the virus can occur with only 427 nucleotides from the 3’ end and 731 nucleotides from the 5’ end. The virus codes for seven structural proteins and one non-structural protein. Having a negative sense polarity, the virus’ RNA structure begins in the 3’ position and ends in the 5’ position. Following the 3’ position is a non-transcribed region known as the leader. Next, a nucleoprotein of 739 amino acids that plays a central role in the virus’ replication. Then comes a protein known as VP35, which is responsible for binding a double stranded RNA and inhibiting the host cells alpha/beta interferon. After this comes the VP40 protein, which uses the COPII transport system for intracellular transport. Then comes the sGP that serves as a structural protein in the virus and then the VP30 protein, which is a RNA binding protein. Next comes the VP24 protein that the virus uses to prevent Heterogeneous ribonucleoprotien particles C1/C2 binding to Karyopherin Alpha-1 and partially alter its nuclear import. This is followed by an L protein which caps and Polyadenylates mRNA’s. Next comes a non-transcribed trailer, and finally 3’. What’s interesting about the leader and trailer is that they carry signals which control transcription, replication, and package the viral genomes into new virons.

As I mentioned earlier the virus’s lipid bilayer anchors glycoproteins, which project 10 nm spikes. What I didn’t mention was that these spikes mediate the entry of the virus into the cell. The spikes attach viral particles to the cell surface. The viral particles adhere to lectin proteins and are taken up and transported to endosomes which contain host proteins cathepsin B and Niemann Pick C1 (NPC1) and catalyze fusion between viral and the endosomes membranes. In a study published in Nature by the MIT’s Whitehead Institute for Biomedical Research, it was found that for the Ebola virus to enter it required the cholesterol transporter Niemann-Pick C1 (NPC1). In the study, cells with mutated NPC1 proteins were exposed to the Ebola virus and something very, very interesting happened: The cells survived and appeared to be immune to the virus. This is interesting for that fact that this particular mutation causes a naturally occurring genetic disease (unfortunately terminal). So technically these people with Niemann-Pick disease type C, would be immune to some of the earth’s most deadly viruses. Also significant, it shows that the Ebola uses the NPC1 protein to enter the cell. The same immunity was found with Ebola’s filovirus cousin the Marburg virus. So the NPC1 protein is essential for the entry of filoviruses and acts as a receptor for the virus that mediates the infection by binding directly to the viral envelope glycoprotein.


The Ebola virus, like all viruses, must reproduce its components and assemble itself within cells, by hijacking cellular machinery to reproduce its infectious virons. So let’s go over the replication process and then look further into the proteins that affect this. The virus begins by attaching itself to the host’s receptor through its glycoprotein spike (peplomer) Then, it goes through the process of macropinocytososis forming a pocket around the virus when it enters the host. The viral membrane then fuses with the vesicle membrane, and the nucleocapsid is released into the host cells cytoplasm. Then the negative-sense genomic ssRNA, which is enclosed in a protein shell, is used as a template for synthesis (3’-5’) of the polyadenylated messenger RNA. The cellular machinery begins translating mRNA into viral proteins. Viral proteins are processed, and the glycoprotein precursor is cleaved to heavily glycosylated GP1 and GP2. The GP1 and GP2 molecules assemble first into heterodimers, and then into trimmers to give the surface glycoprotein spikes. The secreted glycoprotein precursor is cleaved to sGP and delta peptide both of which are released from the cell. As the number of viral proteins in the cell rise, a switch happens. Things go from translation to replication… The negative sense genomic RNA is used as a template and a extra single stranded RNA is synthesized then used as a template for the synthesis of new genomic ssRNA which is rapidly encapsidated. Then things start to get really bad for the cell: Newly formed nucleocapsids and envelope proteins associate at the host cell’s plasma membrane. Budding occurs, and the cell is destroyed.

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