Herpes simplex virus

Classification

The herpes simplex virus is one of the herpes, DNA viruses that are important in causing human disease.

It is "composed of an inner core containing linear double-stranded DNA, surrounded concentrically by an icosahedral capsid of approximately 100 nm, an amorphous layer termed the tegument, and an outer envelope composed of lipids and glycoproteins." (Paula W. Annunziato: Chapter 15 – Herpes Simplex Virus Infections inGershon: Krugman's Infectious Diseases of Children, 11th ed. Mosby, 2004)

Herpes is a large viral group. Within it are up to 8 Herpes simplex viruses, Varicella-zoster virus, Epstein-Barr virus, and more recently the Human herpes virus.

Using the Baltimore classification (which is based on the mechanism of mRNA production) is a Group 1 virus (dsDNA) from the family Herpesviridae, subfamily Alphaherpesvirinae, Genus Simplexvirus and Species Herpes simplex virus 1. (Another species can be Herpes simplex virus 2).

Brief History

Herpes can be found throughout human history by reading texts from the past that describe the characteristics of the fever blisters. It was first discovered back in the time of the Ancient Greeks. Ancient documents show that Greek doctors described the symptoms of herpes as sores that would "creep" onto the skin. This is where the name herpes originated from because it means "to creep" in Greek. During the Roman Empire, a wild outbreak of sores on common folk's mouths began and to cease the spread Emperor Tiberius placed a ban on kissing at public ceremonies. Of course no one at the time new what the cause of these sores were but in both cases they understood that direct skin to skin contact would result of spreading of these blisters on the face.

In 1906, the first time the term herpes simplex was used. At this time, however, it as believed that the virus that caused the oral blisters was the same that caused the genital ones.

Description and significance

The Herpes simplex virus is a large, circular virus that is about 150-200 nm in diameter. HSV is surrounded by an envelope and has a protein-bound caspid which holds the genetic information. The envelope that surround the virus is made up of glycoproteins. These glycoproteins come from the host cell the virus is invading. When the virus is done replicating within the nucleus of the nerve's cell body, the virus buds off and incorporates part of the nuclear membrane. By doing this, the virus is able to use these glycoproteins and move about the host without being identified as a pathogen by the immune system. In addition, the glycoproteins are used to invade other cells by matching with receptors on the cell membrane. The envelope also acts as a protective covering. Once the envelope is breached, the virus is unable to thrive, is not infectious and can readily fall apart. Being in moist environments keeps the viral envelope intact. This is why the virus infects the oral or genital regions of the human body. These mucoepithial cells of the body are the target cells the virus look to infect. Between the envelope and caspid is the area known as the tegument. This space contains viral proteins and enzymes that are involved in the initiation process of replication. The caspid has a icosahedral shape and is about 100-150 nm in diameter. It is made up of about 162 capsomeres which are the building blocks, or subunits, of the caspid.

In 1873, Vidal was the first to inoculate the virus. I could not find how he actually inoculated HSV or what type he inoculated first.

Genome structure

For HSV, it has a double stranded DNA located in the protein caspid. This dsDNA is about 152,000 base pairs long. When the virus first enters the body, the caspid of the virus is passed into the cell by matching the glycoproteins of the virus' envelope to the receptors of the host's cells membrane. The caspid then migrates to the nuclear membrane where the viral dsDNA embeds itself into the host's DNA. When the virus is in its latent stage in the nerve cells of the body, it is said that the viral genome produces latency-associated transcripts (LATs). It has been unknown what these long, repeating segments of coding actually are for and producing. It has been believed that they code for proteins that prevent programmed cell death. Speculations on why it is hard to find any proteins is because the virus needs to remain hidden within the cell's genome and if viral proteins are discovered by the body's immune system, it could be killed. However, Dr. Nigel W. Haser of at the University of Pennsylvania School of Medicine discovered such a protein that is produced during latency and its purpose. Early in 2006, Dr. Haser and his team discovered a small RNA molecule (microRNA) to be the reason herpes is able to stay in a dormant state and stay with the infected human for the rest of his/her life. This mircoRNA is coded in the LAT and works through a process called RNA interference to prevent apoptosis. Apoptosis is a process cells in multicellular organisms go through to "commit suicide" by changing the cell membrane's shape and alter the DNA structure until the cell cannot function. However, the microRNA produced by the viral genome prevents this from happening. Through his research, Dr. Haser found that the microRNA disrupts the production of two proteins: TGF-b and SMAD-3. These two proteins are required for cell death. The microRNA binds to the mRNA that was coded from the genes of both TGF-b and SMAD-3 and degrades the mRNA until it is unreadable. Dr. Haser found this microRNA in HSV-1 and is looking for the same in HSV-2.

What is known of the virus' genome is that it encodes of at least 80 proteins. Interestingly, near 50% of these proteins are only used between the parasitic-like relationship between the host cell/immune system and the virus genome. The rest of the proteins are used to produce the structure of the virus and control the process of replication. Enzymes that are made are DNA-dependent DNA polymerase, thymidine kinase, ribonucleotide reductase, and approximately 11 surface glycoproteins. Thymidine kinase's function is to phosphorylate thymidine and ribonucleotide reductase converts ribonucleotides to nucleotides. Of the glycoproteins produced, two are used primarily for protection against the human immune system. This type of protection is called immune escape. What HSV does is two of the glycoproteins (gE and gI) bind to the IgG (immunoglobulin G) on immunoglobulin and use this to hide from the immune system.

Replication Process

First, the HSV binds to the cell membrane of the host cell. It is able to accomplish this through matching the glycoproteins on its viral envelope to the receptors on the hosts cell. Next, the nucleocaspid migrates through the cytoplasm of the host cell until it reaches the nuclear membrane. Here, the viral dsDNA is injected into the nucleus. Following, three types of proteins need to be produced through RNA transcription for the virus to replicate. For RNA transcription to take place, a cellular enzyme, DNA-dependent RNA polymerase I, needs to be present. These three proteins are called alpha, beta, and gamma proteins. Alpha proteins are used for transcriptional regulation and are not found in the mature virus. DNA polymerase is part of the beta proteins and are used in DNA replication. Gamma proteins are the structural proteins that make up the virus. With DNA synthesis, the next step, HSV encodes for their own DNA-dependent DNA polymerase. Thymidine kinase may also be produced if the virus develops in a non-dividing cell that does not have the right mechanisms for DNA synthesis. After assembly of the viral pieces, DNA is inserted. The newly developed viruses move out of the nucleus and leave through exocytosis pathway or bud through the cell membrane.

Ecology

In previous sections I stated where the virus lives within humans. I could not find any other information stating if HSV affects other mammals or if herpes is strictly a human pathogen.

Pathology

Herpes simplex virus is contracted by direct skin to skin contact. The infected person would need to have a visible sore and have that sore make contact with another human for the virus to be transmitted. This type of contact is due to the unstable envelope surrounding the virus. Once the envelope is breached, the virus is unviable and will soon die. Once you have contracted the virus, it will take several days to a week for the virus to act ont he body and produce sores. Along with the sores, additional symptoms are fevers, sore throat, and pain in the area of the blisters. The initial outbreak is typically the worst and most painful and can last up to two weeks. After the initial outbreak, the virus retreats into the nerves of the body. Here it will lay dormant until it looks to resurface. It is unknown how to predict when a person will get another outbreak of the blisters but some triggers are known to be physical or psychological stress, ultraviolet light, local injury to infected area, and other viral infections. The Herpes virus does not just infected any area of the body. It can be located in one's oral or genital areas. Each area is infected by a particular strain of the virus. HSV-1 usually infects the oral region (90% of coldsore cases) and HSV-2 infects the genitals (90% of genital herpes cases). However, due to increasing variety of sexual encounters between partners (oral sex) HSV-1 can be found in the genital area and HSV-2 in the oral area. In HSV-1, the sores are normally found around the mouth or on the face in close proximity to the mouth. In HSV-2, the sores can be found on the shaft of the penis in men and along the inner thigh and anus area and for women on the labia, clitoris, vulva, and anus. Women tend to have more painful and frequent outbreaks. However, new polls state that 90% of newly infected humans say they contracted the virus without their partner showing any visible symptoms of an outbreak. Now known as asymptomatic transmission, or viral shedding, the virus can at times migrate from the nerves to the surface of the skin. Here the virus can be transmitted through direct skin contact through microscopic abrasions on the skin.

In addition to affecting the oral and genital areas of the body, HSV can infect other areas. Herpes keratitis is an infection of the eye by the HSV-1. HSV-1 can spread to the eye by immediate touching of a finger from a blister on the mouth. It can be recurrent and may lead to blindness. It is a leading cause of corneal blindness in the United States. Also, neonatal HSV can be found. This is when a pregnant woman contracts the HSV virus within the third stage of development of the fetus. Because of the amount of time it takes the mother to build antibodies for HSV, the mother will not have enough time to pass the antibodies to the baby. In a case like this, the mother must give birth through a c-section or the baby could be seriously affected. If live birth occurs, the baby has a chance to contract the virus while passing through the birth canal and die. It is an even greater risk if the mother has a outbreak of genital lesions during the birth.

Application to Biotechnology

Does this organism produce any useful compounds or enzymes? What are they and how are they used?

I couldn't find any information relating to biotechnology as you may have wanted. With HSV, many of the compounds and enzymes produced are unknown or their true function is unknown. What I stated it other parts of this webpage. What I am going to describe is the process in which one is diagnosed with the virus besides obvious physical symptoms. First, a sample can be taken from the visible lesions. The problem that comes from this is that the lesions need to be very fresh for high viral numbers. If the lesions is a few days old and/or begins to crust over, there will not be enough of a sample and the lab results will not be accurate. Secondly, a blood test can be done. From the blood, the immunoglobulin-G (IgG) count can show if there is a viral infection in the body. However, this type of test may come back negative. The reason is that if the person is having an initial outbreak, the body's immune system has not had time to develop a high IgG count. Typically, it takes approximately 3 months for an accurate test to come back. The flaw with this test is that it does not distinguish between a person having HSV-1 or HSV-2. Another blood test is done called a "type-specific" serologic herpes test. Still, each one of these tests has its flaws with diagnosing a person. This is why most doctors will tell if a person has HSV based on the physical symptoms. If the blisters follow the pattern of HSV outbreaks then even will lab tests having negative results, it is safe to say that a person has the virus.

Prevention is the key in not contracting this virus. Knowing when someone has an outbreak and limiting physical contact during these times is only method in keeping oneself from getting HSV. It is said that when an infected individual is about to get an outbreak of blisters, either oral or genital, a tingling and burning sensation is felt. This sensation will come about 1-2 days prior to visible blisters. When it comes to treatments, certain medications have been developed to treat the virus. There is nothing that can destroy the viral genome that lays dormant in the ganglions of the nerves but the medications available are affective in reducing the effectiveness that it reproduces. Valtrex (valacyclovir HCl) is the current advertised and prescribed drug for people with HSV. What it does is it disrupts the replication process of the virus. This drug has been clinically tested and it is proven it reduces the amount of outbreaks significantly a person may get as well as makes them less painful. It is mainly given to those with genital herpes to take as a daily suppressive therapy but those with oral herpes can take the drug as well. Usually, a doctor will give it to a patient with HSV-1 when the person believes an outbreak is about to occur and subscribes a 5 day supply. However, no vaccine is available for HSV. The virus is highly unique in each individual so it is hard to obtain the right information on how a vaccine should be developed.

Current Research

One research experiment already summarized in a previous section. Topic referred to microRNA. With that experiment, could not find actually write-up. Only found magazine article explaining the experiment.

In this research experiment, Becket Feierbach and his associates looked to determine how HSV-1 develops spatial compartments within the host's nucleus where replication takes place. They found that nuclear actin filaments were produced and acted as walls where the virus can replicate. Serial block-face scanning electron microscopy and confocal microscopy were used to visually see the filaments. In uninfected nerve cells, these filaments were not present. To determine that these filaments were associated with the virus development of the caspid, green fluorescent protein were placed into a specific viral caspid protein: VP26. By doing this, Feierbach was able to watch the development of these nuclear actin filaments formed prior to assembly of the caspid. In addition, the experiment showed how the filaments were essential in making virus assembly sites. Also found in the experiment is that the process of viral transcription, not viral DNA replication, is necessary for the nuclear actin filaments to be produced.

The third research article I found dealt with a possible vaccine that is being developed to prevent being infected with HSV-2. Dr. Lawrence R. Stanberry and associates conducted the experiment for the GlaxoSmithKline Herpes Vaccine Efficacy Study Group. The name of the vaccine was the glycoprotein-D-subunit vaccine. I could not obtain the full research experiment but from what I read from the brief abstract and methods was that the doctors were developing a vaccine that would disrupt one of the glycoproteins on the viral envelope. If this could be accomplished the virus would not be able to gain entry into the nerve cells since its glycoproteins will not match the receptors on the nerve's cell outer membrane. The vaccine was given to random individuals in the study at the beginning of the experiment, 1 month after and 6 months after that. The subjects were evaluated over a 19 month timeframe. Of the subjects tested, those whom were seronegative (did not have any previous physical symptoms of HSV-1 or HSV-2) showed signs the vaccine worked. Approximately 40% of females did not contract the virus. One problem with the vaccine is that it does work for men or women who are seropositive for HSV-1 and seronegative for HSV-2. No reasoning was stated in why the vaccine was ineffective on males, regardless if they have shown signs of HSV or not.

References

University of Pennsylvania School of Medicine. "Infected for life: How the Herpes Simplex Virus Uses MicroRNA to Hide Out in Cells." ScienceDaily 13 June 2006. 16 April 2008 <http://www.sciencedaily.com /releases/2006/06/060612184600.htm>.

"Scientific Herpes Information." Herpes-Coldsores.com. 2008. 05 Apr 2008 <http://www.herpes-coldsores.com/>.

Hunt, Dr. Richard. "Virology Chapter 11 - Herpes Viruses." Microbiology and Immunology Online. 10 July 2007. University of South Carolina School of Medicine. 07 Apr 2008 <http://pathmicro.med.sc.edu/virol/herpes.htm>.

Feierbach B, Piccinotti S, Bisher M, Denk W, Enquist LW (2006) Alpha-Herpesvirus Infection Induces the Formation of Nuclear Actin Filaments . PLoS Pathog 2(8): e85. doi:10.1371/journal.ppat.0020085

Stanberry, Lawrence. R, Spotswood L. Spruance, and Anthony L. Cunningham. "Glycoprotein-D–Adjuvant Vaccine to Prevent Genital Herpes." The New England Journal of Medicin 347(2002): 1652-1661.