Prions, Viriods, Virusoids




Prions




A prion is thought to be an infectious agent that is comprised entirely of a propagated, mis-folded (rouge) protein.







The rogue prion protein has been implicated in a number of diseases in a variety of mammals, including bovine spongiform encephalopathy (BSE, also known as "mad cow disease") in cattle and Creutzfeldt-Jakob disease (CJD) in humans.




All hypothesized prion diseases affect the structure of the brain or other neural tissue, and all are currently untreatable and are always fatal.




PrPC refers to the normal cellular protein, which is found in a multitude of tissues, while PrPSC refers to the prion form of PrPC, and is responsible for clogging brain cells, and causing cells to die eventually.




Hypothetically, Prions infect and propagate by refolding converting normal molecules of the protein into the abnormally structured form.







All known prions induce the formation of tightly packed beta sheets. This altered structure is extremely stable and accumulates in infected tissue, causing cell death and tissue damage.




This stability means that prions are resistant to denaturation by chemical and physical agents, making disposal and containment of these particles difficult.

Mechanism of Prion Propagation


Prion Protein Structure



How Prions Arise

Prions Madcow mechanism

Viriods

Viroids are infectious agents composed of a single piece of circular single stranded RNA which has some double-stranded regions.

Because of their simplified structures both prions and viroids are sometimes called subviral particles.

Viroids mainly cause plant diseases but have recently been reported to cause a human disease (Hepatitis D)

Human diseases caused by Viroids

The only human disease known to be caused by a viroid is hepatitis D. For hepatitis D to occur there must be simultaneous infection of a cell with both the hepatitis B virus and the hepatitis D viroid (co-infection)

There is extensive sequence complementarity between the hepatitis D viroid RNA and human liver cell 7S RNA, a small cytoplasmic RNA that is a component of the signal recognition particle, the structure involved in the translocation of secretory and membrane-associated particles.

The hepatitis D viroid causes liver cell death via sequestering this 7S RNA and/or cleaving it.

Transmission

The hepatitis D viroid can only enter a human liver cell if it is enclosed in a capsid that contains a binding protein. It obtains this from the hepatitis B virus.

The viroid then enters the blood stream and can be transmitted via blood or serum transfusions.

Virusoids

Virusoids are circular single-stranded RNAs that are dependent on plant viruses(Viroids) for replication and encapsidation.

The genome of virusoids consist of several hundred nucleotides and only encodes structural proteins.

Virusoids are not considered as viruses but as subviral particles. Since they depend on helper viruses, they are classified as satellites

Done By: Alyssa Huang

Virus Host interaction

1st Stage- Attachment

It is a specific binding between viral surface proteins and their receptors on the host cellular surface.

This specificity determines the host range of a virus.

Example, the Human Immunodeficiency Virus (HIV) attacks only human's immune cells (mainly T cells), because its surface protein, gp120, can interact with CD4 and chemokine receptors on the T cell's surface.


2nd Stage- Entry, Direct Cell Membrane Fusion

Viruses take advantage of these processes in a number of ways, after they have attached to the cell surface via binding to a specific receptor.

The simplest is DIRECT MEMBRANE FUSION, where the virion membrane fuses with the cell membrane, and the virion nucleoprotein complex is delivered into the cell cytoplasm directly.

This is generally a pH-independent process, and requires only that the membrane be fluid (ie: temperature in the physiological range), and generally that some divalent cations be present.

The entry process for HIV is shown in the graphic below.


3rd Stage-Uncoating

Uncoating is a process that viral capsid is degraded by viral enzymes or host enzymes.


4th Stage-Replication

Replication involves assembly of viral proteins and genetic materials produced in the host cell.


5th Stage- Assembly

Assembly of infectious virions is dependent on the action of: (a) an aspartyl protease encoded by the viral pol gene and responsible for cleavage of the gag and gag-pol precursors into mature proteins (1,2) and (b) cellular N-protein myristoyl transferase (NMT) which adds myristic acid to the N-terminus of gag, gag-pol and nef viral polyprotein precursors(3).


6th Stage- Release

Viruses may escape from the host cell by causing cell rupture (lysis).

Enveloped viruses such as HIV typically "bud" from the host cell. During the budding process, a virus acquires the phospholipid envelope containing the embedded viral glycoproteins.


Done by: Toh Wan Nee

Class VII

Class VII viruses are Double stranded DNA viruses that replicate though a single stranded RNA intermediate.

An example of a virus from class VII is hepadnaviridae.


Hepadnaviridae

Hepadnaviruses are a family of viruses which can cause liver infections in humans and animals.

There are two recognized genera:

• Genus Orthohepadnavirus; type species: Hepatitis B virus
• Genus Avihepadnavirus; type species: Duck hepatitis B virus


Virology

Genome

Hepadnaviruses have very small genomes of partially double-stranded, partially single stranded circular DNA.

The genome consists of two uneven strands of DNA.

One has a negative-sense orientation, and the other, shorter, strand has a positive-sense orientation.

It has a endogeneous DNA which depends on DNA polymerase. It make use of overlapping reading frame (ORF) and has an RNA intermediate.


Replication



Hepadnaviruses replicate through an RNA intermediate (which they transcribe back into cDNA using reverse transcriptase).

The reverse transcriptase becomes covalently linked to a short 3- or 4-nucleotide primer.

Most hepadnaviruses will only replicate in specific hosts, and this makes experiments using in vitro methods very difficult.

The virus binds to specific receptors on cells and the core particle enters the cell cytoplasm.

This is then translocated to the nucleus, where the partially double stranded DNA is 'repaired' by the cell to form a complete circle of DNA.

It then undergoes transcription by the host cell RNA polymerase and the transcript is translated by host cell ribosomes.

New virus particles are formed, which acquire lipid from the endoplasmic reticulum of the host cell, and the genome is packaged within these particles, which then bud off from the cell.

Hepadnavirus-infected cells translate the protein known as the virus surface antigen many times until there is too much protein to coat the virions formed.


These proteins then aggregate to form rod shapes, and it is this antigen, known as the Australian or hepatitis B surface antigen, which is released from the cell and which leads to a very strong immune response from the host.



Pathogenesis

Hepadna virus can cause chronic or acute infection. Chronic or acute infection will on the age of infection. 90% of neonates and 50% of young children infected will have chronic infection whereas only 5% to 10% of Immuno-competent adults who were infected will develop chronic infection.


Clinical Features

Acute

Acute clinical features can be from subclinical to fulminant.

Acute symptoms:

• Loss of appetite
• Nausea
• Vomiting
• Fever
• Abdominal Pain
• Jaundice

About 90% to 95% of acutely infected adults recover without squeal and about 5% to 10% of acutely infected adults become chronically infected.

Chronic

Patients who are chronically infected will be in a chronic carrier state.

They are still potentially infectious but have not symptoms and no abnormalities on laboratory testing.

Others will have clinically apparent chronic hepatitis.

Some will go on to develop cirrosis, which is the hardening of the liver and finally, hepatocellular carcinoma.



Lab Diagnosis

Diagnosis of Hepatitis B virus infection generally made on the basis of serology.

For infected patients, there will be detectable serum hepatitis B surface antigen (HBsAg).

Hepatitis B envelop surface Antigen (HbeAg) is also detectable in acute infection which is characterised by a high rate of viral replication.

IgM antibodies against core antigen are also detectable in serum.

Subsequently, IgG antibodies against the core are produced.

As acute infection resolves, IgG antibodies against core antigen persist and IgM antibodies and HbsAg becomes undetectable.

Most people who have had acute infection that resolves continue to have IgG antibodies against core antigen for life

Acutely infected patients who do not clear Hepatitis B Virus continues to have serum HbsAg.

Diagnosis of Hepatitis B is confirmed and prognosis is assessed by liver autopsy.


Treatment


The treatment for hepatitis B includes a course of alpha interferon; this is a very expensive treatment which lasts for around 12-15 weeks and makes the patient very sick.

It seems that the interferon treatment aims at kick-starting the host immune response to clear the infection; it is not the drugs that clear the infection.

There are reverse transcriptase inhibitors available as treatment; these drugs target the virus replication strategy by (as the name suggests) inhibiting reverse transcription.

Hepatitis B infection can also be prevented by means of a recombinant Hepatitis B surface antigen vaccine.






Done by: Jeremy Lee

Retroviridae


A retrovirus is a virus with an RNA genome that replicates by using a viral reverse transcriptase enzyme to transcribe its RNA into DNA in the host cell.


The DNA is then incorporated into the host's genome by an integrase enzyme.


The virus thereafter replicates as part of the host cell's DNA. Retroviruses are enveloped viruses that belong to the viral family Retroviridae.



Description of virus

The virus itself stores its nucleic acid, in the form of a +mRNA genome and serves as a means of delivery of that genome into cells it targets as an obligate parasite, and constitutes the infection.

Once in the host's cell, the RNA strands undergo reverse transcription in the cytosol and are integrated into the host's genome, at which point the retroviral DNA is referred to as a provirus.


It is difficult to detect the virus until it has infected the host.





Virion Structure

The main virion components are:
• Envelope: lipid bilayer which obtained from host plasma membrane
• RNA
• Proteins: consisted of gag proteins, protease (PR), pol proteins and env proteins.


Gag proteins are major components of the viral capsid which are about 2000-4000 copies per virion. Protease, on one hand, is expressed differently in different viruses.


It functions in proteolytic cleavages during virion maturation to make mature gag and pol proteins. Pol proteins are responsible for synthesis of viral DNA and integration into host DNA after infection.


Finally, env proteins play role in association and entry of virion into the host cell.



Properties:

It is a spherical virion and has a ribonucleoprotein in central nucleoid within icohedral capsid.

It has an envelope with glycoprotein peplomers and has 2 copies of linear (+) sense ssRNA.

The virus have a 3’ polyadenylated tail and 5’ cap and has the enzyme reverse transcriptase which reverse transcribe the viral RNA to DNA.

The virus will form long terminal repeats before provirus DNA is being inserted into host genome.

Gag, pol, env genes, some regulatory genes and some oncogenes are present in the virus.



Multiplication




When retroviruses have integrated their own genome into the germ line, their genome is passed on to a following generation.

While transcription was classically thought to only occur from DNA to RNA, reverse transcriptase transcribes RNA into DNA.

The term "retro" in retrovirus refers to this reversal (making DNA from RNA) of the central dogma of molecular biology.

These inserts are transcriped by host's enzymes into new RNA molecules which enter the cytosol. Next, some of these RNA molecules are translated into viral proteins. For example, the gag gene is translated into molecules of the capsid protein, the pol gene is transcribed into molecules of reverse transcriptase, and the env gene is translated into molecules of the envelope protein.

It is important to note that a retrovirus must "bring" its own reverse transcriptase in its capsid, otherwise it is unable to utilize the infected cell's enzymes to carry out the task, due to the unusual nature of producing DNA from RNA.

Because reverse transcription lacks the usual proofreading of DNA replication, a retrovirus mutates very often.

This enables the virus to grow resistant to antiviral pharmaceuticals quickly, and impedes the development of effective vaccines and inhibitors for the retrovirus.


Genes

Retrovirus genomes commonly contain three open reading frames that encode for proteins that can be found in the mature virus:

• group-specific antigen (gag) codes for core and structural proteins of the virus;
• polymerase (pol) codes for reverse transcriptase, protease and integrase; and,
• envelope (env) codes for the retroviral coat proteins.


Provirus

This DNA can be incorporated into host genome as a provirus that can be passed on to progeny cells. In this way some retroviruses can convert normal cells into cancer cells. Some provirus remains latent in the cell for a long period of time before it is activated by the change in cell environment.


Seven subfamilies

Subfamily: Orthoretrovirinae

• Alpharetrovirus
• Betaretrovirus
• Gammaretrovirus
• Deltaretrovirus
• Epsilonretrovirus
• Lentivirus

Subfamily: Spumaretrovirinae

• Spumavirus
  

Lets look at Human Immunodeficiency Virus (HIV), a type of Lentivirus.

Transmission

HIV can be transmitted via sexual contact, through blood and blood products and secretion of other bodily fluids like semen and vaginal secretion.

A mother can pass HIV to a child via the placenta, mucosa and breast milk.

Primary infection

During the acute stages, flu-like symptoms, fever, skin rash and swollen lymph nodes may be observable.

Virulence factors include rate of replication, propensity of mutate and cytopathogenicity.

The host immune system can reisist infection via supression by CD8 T supressor cells and presence of cytotoxic T-lymphocytes.

Asymptomatic stage
During asymptomatic stage, there is no apparent disease. But there will be a fall in CD4 T lymphocytes (primary target cells).

Possible signs includes fatigue, depression, weight loss and memory disorder.

Symptomatic stage

AIDS-related complex will occur. They are diseases not considered difinitive of AIDS but may be attributed to HIV infection. It is a indication of defects in cell-mediate immunity.

Opportunistic infections as a result of fall in CD4 T lymphocytes in AIDS patients.

Therapy

Non-specific therapeutic management is used to boost general health. This includes vitamins, mineral, antioxidants and others.

Specific therapeutic management involves in antiretroviral therapy like:

• Neucleoside reverse transcriptase inhibitors includes AZT (azidothymidine) and 3TC (lamivudine)
• Non-nucleoside reverse transcriptase inhibitors includes Efavirenz and Nevirapine
• Protease inhobitors include Indinavir and Ritonavir

The inefficiency of reverse transcriptase causes the virus to rapidly mutate.
A combine therapy is used to combat resistance.

Immunomodulation include enhancement of immune system through treatment with potential drugs that are still under study.

Vaccines that are still under development.

Diseases

Diseases that can occur in AIDS patients includes Kaposi’s Sarcoma, Oral Hairy Leukoplakia and Candidiasis.

Done by: Jeremy Lee

Flaviviridae

It is a spherical enveloped virion with the size of 40-50nm. The inner core contains protein C and the enveloped is made up of glycoprotein peplomers (E).

It is single linear (+) ssRNA which is very infectious.

Dengue is one of the symptoms for flaviviridae. The most important arbovirus is present in the in it and it started from Southeast Asia to Americas to pacific to Africa.










There are 4 different class of Dengue. They are:

- Den-1
- Den-2
- Den-3
- Den-4

Class Den-2 shows the greatest antigenic and genotypic distance from the others. Is has a protective immunity after infection homotypic.


Dengue Life cycle




Symtoms for Dengue Fever

- Many infectious asymptomatic.

- Acute infectious resulting in fever, severe headache, retro-orbital pain, nausea and vomiting

- Severe muscle and bone pain

- Maculopapular rash just before recovery


Severe Dengue fever might lead to DENGUE HAEMORRAHGIC FEVER (DHF) /DENGUE SHOCK SYNDROME (DSS).


DHF/DSS

The properties of DHF/DSS are:

- Prior infection and age key factors
- Seldom occurs in individuals above 15 years
- Similar to yellow fever in biphasic nature:

Initial symptoms similar to DF followed by remission

Sudden deterioration of patent condition

- Severe prostration, hypotension, circulatory collapse, bleeding and shock
- Bleeding

Injection and punction sites

Gastrointestinal bleeding

Petechiae in skin, mucous membranes (mouth)

There are also 4 different classes grading of DHF:

- Grade I: Fever with non-specific, constitutional symptoms and only haemorrhagic manifestations bring positive tourniquet test

- Grade II: As for grade I, but with specific haemorrhagic manifestations

- Grade III: Signs of circulatory failure or hypotension

- Grade IV: Profound shock with pulse and blood pressure undectable
Pathogenesis of DHF/DSS

- Not well understood despite intensive study-2 theories

- Virulent strain theory:

Some strains more virulent than others

Molecular studies show variations in sequences amongst different strains, within serotypes
Early evidence pointed to DEN-2.

- Antibody enhancement:

Main theory for DHF/DSS

Main Cell target of DEN: monocytes/ macrophages

Most cases of DHF/DSS had prior infection or infants below 1 year had maternal Ab

Monkey experiments showed similar enhancement



Cause

- Immune system overreacting
- Sever Acute Respiratory Syndrome



By: Toh Wan Nee

picornaviridae



-The largest virus family found on earth
-it is a naked virus
-Very infectious due to (+) RNA as genome information (it can be directly use as an mRNA for translation and replicate)
(e.g. rhino virus)

• Why It cause infection throughout the year:
àVirus is abundant in nasal discharge
àInfected people go to crowded area
àPeople sneeze without using tissue worse the hand contact to other people

Rhino virus only infect upper respiratory tract
No vaccination
-our body can produce immune (IgG) toward it but is it only last for a few years
-impossible to create vaccine due to:
àFirstly, there is no vaccination
àSecondly, there are over 100 types of rhino virus
àLastly, our body can produce immune toward it (free), therefore, not worthy to create (if there is vaccine toward it)

• Symptom
-Running nose
-sore throat

• Lab diagnosis
- Virus culture
- PCR (polymerase chain reaction)
- Nasal washings

Control
• Avoid people that have cold
• Sneeze into tissue
• If you are infected, stay at home


Done by: Pow Ze Liang

Group Members:

Alyssa Huang
Jeremy Lee
Nicolina Ng
Pow Ze Liang
Toh Wan Nee

Module Group:

MB0803

ShoutMix chat widget