number Done by Corrected by Doctor Ashraf Khasawneh

Transcription

1 number 3 Done by Mahdi Sharawi Corrected by Doctor Ashraf Khasawneh

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3 *Note: Please go back to the slides to view the information that the doctor didn t mention. Prions Definition: Prions are rather ill-defined Infectious agents consisting of a single type of protein which doesn't have any nucleic acid component. *Confusion arises from the fact that the prion protein & the gene which encodes it are also found in normal 'uninfected' cells. *prions cause infections in humans and animals These infectious agents are associated with diseases like Creutzfeldt-Jakob disease in humans, Scrapie in Sheep, and bovine spongiform encephalopathy (BSE) in cattle (in animals). *Human Prion Diseases Creutzfeldt-Jakob Disease (CJD) Variant Creutzfeldt-Jakob Disease (vcjd) Gerstmann-Straussler-Scheinker Syndrome Fatal Familial Insomnia Kuru So we can say that prions are transmissible pathogens responsible for series of fatal neurodegenerative diseases, like the ones we mentioned before. *Spongiform Encephalopathy: Brain becomes sponge-like whether in humans or animals. What's the difference between a normal protein and a prion? Prions are proteins with pathological conformation; they infect other proteins and propagate conformational changes of normal (native) proteins into the abnormally structured form.

4 Infection mechanism: We have a gene that encodes this prion, which function is unknown, when a mutation occurs to the prion protein in our cells, it changes from a normal prion into the abnormal form of the prion(infectious form), and this abnormal form of the prion changes normal prions into the abnormal form upon contact. Prions are eitherprp C, which are the cell's normal form of the prions (healthy prion), or PrP SC which is the infectious form the prion (abnormal conformation). This abnormal form of prion can come from mutations occurring in the cells (endogenous source), or it can come from exogenous sources through ingestion of proteins containing abnormal conformation of prions, and as soon as it comes in contact with normal prions they infect them. How do prions get infected? 1. Sporadic (85%): Occurs through mutation in the prion protein, no underlying cause, like creutzfeldt-jakob disease 2. Familial (inherited-15%): Family related, genes favour the transformation from normal prions to abnormal prion conformation. Examples: fatal familial insomnia (FFI), Gertsmann-strausslerscheinker disease (GSS) 3. Transmissible: Exogenous source (explained earlier) Examples: Kuru diease, BSE, vcjd (CJD is sporadic) *note: sporadic and familial are (endogenous source). **Propagation of kuru disease in New Guinea natives is through ritualistic cannibalism. Recently, it has been discovered that BSE had been transmitted to humans in Europe after consumption of infected beef, producing a variant of the CJD called vcjd

5 Transmissible spongioform encephalopathy (TSE) A group of progressive conditions that affect the brain and nervous system of humans and animals and are transmitted by prions The pathology: vascular degeneration, neuronal loss, astrocytosis and amyloid plaque formation The normal prions are soluble, they can be cut by proteases. However, the scrapie form which is infected isn't water soluble, it isn't affected by proteases and it aggregate and accumulates. The clinical signs: loss of motor functions (lack of coordination, ataxia, involuntary jerking movements), personality changes, depression, insomnia, confusion, memory problems, dementia, progressive tonic paralysis, death Definitive diagnostic test: biopsy of brain tissue (histopathological examination and immunostaining for PrPSc), infection of a prion doesn't cause an inflammation. There is no cure *normal prion protein is an alpha helix but the abnormal form is a beta sheet. (look to the figure) There's a comparison table between normal and infectious prions, you should read it from the slides

6 *from slide there are summary to the most important topics please look to them Viral Replication Multiplicity of infection (MOI): ratio of infectious agents (e.g. phage or virus) to infection targets MOI = Infectious Agents Infection Targets Eclipse phase: period during which the input virus becomes uncoated, this is because the virus goes into the cell as a nucleocapsid complex; 10-12h Synthetic phase: time during which new virus particles are assembled (protein synthesis and genome synthesis); 4-6h Latent period: no extracellular virus can be detected, because all the viruses are now inside the cell, they can't be detected outside. Burst size: amount of infectious virus produced, per infected cell; 10-10,000 (the cap is 10,000 because that's the maximum cell ability to produce viruses because of the cell limitations: the ribosomes capacity and so) *Numbers aren t accurate (concerning amount of time) Viral Replication Cycle: - Virus replication can be divided into eight arbitrary stages. - Regardless of their hosts, all viruses must undergo each of these stages in some form to complete their replication cycle.

7 - Not all the steps described here are detectable as distinct stages for all viruses. Note: The doctor says that these stages do not have to occur as individual steps, any set amount of steps may occur at the same time in the cycle (Step 4, 5 and 6 occurring at the same time for instance) Viral life Cycle: 1- Attachment: Spikes and glycoproteins attach to the receptors on the target cells. This causes the envelope to get closer to the cell membrane, causing repulsion which leads to small pore formation, and then this pore becomes a large pore, allowing the virus to get into the cell. 2- Penetration (Entry). 3- Uncoating (disassembly): The capsid disintegrates and we get a free floating nucleic material in the cytoplasm. 4- Synthetic Phase: This phase is divided into Genome replication, which is the replication of the genetic material of the virus, and Protein synthesis, which includes the formation of the viral proteins. 5- Assembly: After the genetic and protein components of the virus have been synthesized, the virus assembles itself into a virus, the spike/glycoproteins are found on the outer surface of the cell membrane to envelope the virus when it migrates out of the cell through budding. 6- Maturation: Might occur inside the cell (before release) or outside the cell(after release), before maturation occurs, the virus is not yet

8 complete or infectious, this process might take minutes up to hours to be completed. 7- Release. *This was an overview of the viral life cycle; it is further explained ahead and in the slides. 1- Attachment: Virus attachment consists of specific binding of a virus-attachment protein (or 'antireceptor') to a cellular receptor molecule. Target receptor molecules on cell surfaces may be proteins (usually glycoproteins), or the carbohydrate residues present on glycoproteins or glycolipids. Some complex viruses (e.g. poxviruses, herpes viruses) use more than one receptor and have alternative routes of uptake into cells. 2- Adsorption: In this step, the route taken by a virus differs depending on the virus whether it's enveloped or a naked virus. A- Enveloped Viruses: It has glycoproteins/spikes which are going to attach to the receptor on the target cells, now this glycoprotein can be composed of a single protein, or a complex composed of multiple units, for instance HIV virus has a complex glycoprotein which is composed of two protein units, Called GP160, which is composed of a transmembrane unit called GP41 and GP120 on top of it, GP120 is the part which attaches to the target cell's receptor, for HIV the receptor is CDR4 on T- Cells. However, HIV requires a co-receptor, single receptor attachment is not sufficient to initiate infection, so when HIV GP120 attaches to the CDR4 Receptor it disintegrates and allows the GP41 to attach to the cellular membrane, to bind to the co-receptor which CCR5 or CXCR4. Do all viruses have a unique receptor or can they share the same receptor?

9 The general rule is that each virus has a separate receptor. However, in some cases some viruses can share a certain receptor. B- Naked viruses: They also have Surface proteins which interact with cellular membrane receptors to initiate infection. - Host range: the collection of hosts that an organism can utilize as a partner(humans, animals, plants) - Cellular (tissue) tropism: the cells and tissues of a host which support growth of a particular virus ( Cell which have the needed receptors to be infected by this virus such as influenza virus which can only infect the upper respiratory cells, but cannot infect the liver for instance). **Slide 12 shows each virus and its receptors, the doctor said we should know that for instance the sialic acid receptor is shared by 3 viruses. *Each virus has approximately 15 glycoproteins/spikes/surface proteins around its surface, we need binding for 3-5 of these proteins to the cellular receptor to initiate proper infection and to overcome the repulsion forces and allow proper entry of the virus into the host cell. 3- Penetration: Penetration of the target cell normally occurs a very short time after attachment of the virus to its receptor in the cell membrane. Unlike attachment, cell penetration is generally an energy-dependent process, i.e. the cell must be metabolically active for this to occur. Three main mechanisms are involved, which differ from naked to enveloped viruses: A. Translocation: The doctor says it doesn t occur with viruses and it's mainly for nutrients. B. Endocytosis: Naked viruses' method of entry, but can also occur in enveloped viruses for entry, it's considered as receptor-mediated endocytosis.

10 How do those viruses escape the endocytic vesicle? They changed the vesicle's ph causing conformational changes to the protein allowing the virus to escape the vesicle or they inject their genetic material outwards leaving the capsid inside but this occurs rarely or even in some cases the virus could release lytic enzymes that destroy the vesicle and escape. C. Fusion: Enveloped viruses' method of entry. Doesn't occur in naked viruses. 4- Uncoating: Uncoating is a general term for the events which occur after penetration. Uncoating is one of the stages of virus replication that has been least studied and is relatively poorly understood. The product of uncoating depends on the structure of the virus nucleocapsid. The structure and chemistry of the nucleocapsid determines the subsequent steps in replication. "The Greatest enemy of knowledge is not ignorance; it is the illusion of knowledge." Stephen Hawking