PRIONS

Defn: small proteinaceous infectious particles that resist

inactivation by procedures that modify viruses and nucleic

acids

Prion / Amyloid Diseases

ScrapieBSEKuruCreutzfeldt-Jakob disease

Alzheimer’s (~ 4.5 million, US)Parkinson’s (~ 500,000, US)Huntington’s (~ 30,000 US)

Prion Diseases•High levels of misfolded prion proteins•Transmissible

Amyloid Diseases•Amyloid Fibers Found in Brain•Not Transmissible

Prion /Amyloid diseases are usually neurodegenerative.

Courtesy of Sid Taylor, MUW

CJD is a neurological disease

Occurs sporadically in humans at a ratio of 1 per 1 million people

Estimated that 1 per 10,000 people have CJD at the time of death(estimate could be inaccurate as CJD could be mistakenfor similar neurological diseases)

Two types of CJD (genetic—hereditary and infectious)

1. CJD naturally occurring due to a mutation in a gene that encodes the PrPc neural protein.

2. vCJD infectious caused by consuming beef from cattle with BSEwho are infected with the PrPsc protein

The neural proteins PrPc and PrPsc look different so one can tell the difference between infectious and hereditary CJD

PrPsc is only transmissible from one human to another viacorneal transplantsbrain surgery with contaminated instrumentscontaminated brain probes

CJD--PathologyPost-mortem examination of brain and neural tissue andcells show

amyloid (starch-like) protein deposits—plaques between cells

Non-inflammatory lesions

Strange formations of neurons

Vacuoles—large membrane bound inclusion bodies

CJD—clinical manifestations

Naturally occurring CJD affects humans at the age of 50-60vCJD from transmissible prions can affect people as early as 14

Disease manifestationsshakingloss of motor controlDementia—memory lossparalysispneumoniadeath—a few months to 1.5 years after first symptoms

Prion Properties • Prions are misfolded proteins.

– Prion conformation is rich in -sheets.

• Prions aggregate in amyloids.• Prions are infectious.

– Prion proteins induce conformational changes in other like proteins.

• Prions can propagate into other cells.

Courtesy of Sid Taylor/MUW

What are proteinsProteins are made up of amino acids—each amino acid has its own Chemical properties

The amino acids are linked together in by peptide bonds to form a Primary Sequence like “beads on a string”--polypeptide

The primary sequence can form higher ordered structures called Secondary Structures—these can be alpha helices or beta sheets

Region that has formed secondary structures can fold further tobring these secondary structures together to form a tertiary structure.

Two or more identical or different tertiary structures come together to form quaternary structures (more than one subunit)

Formation of a peptide bond

Formation of an alpha helix—2o structure

Formation of a beta sheet—2o structure

Within one chain alpha helices and beta sheets come together to form a 3o structure—

this can be a functional protein

If more than one tertiary structure form interactions a 4o structure is formed

Where proteins come from

How proteins are made in eukaryotic cells

Where do proteins end up?

1. Stay in the cytoplasm2. Can be transported to the surface

of the cell—membrane protein3. Can be secreted from the cell and

into the external milieu--they may stay near the cells that

secreted them--they may be circulated throughout the

body

Summary of previous notes

1. CJD genetic anomaly vs. vCJD caused by consuming cows withBSE.2. PrPc (Prion protein cellular), alpha helical makeup, associated with brain cell surface. Function unknown. 3. PrPsc (Prion protein that looks like protein that causes scrapies in sheep), beta sheet structure, not associated with the cell surface.

Summary of previous notesCJD genetic anomaly in humans

1. PrPc is normally found on the surface of brain cells.2. In some individuals a rare mutation on chromosome 20 can causePrPc to misfold into PrPsc .3. PrPsc dissociates from the cell membrane but causes PrPc that isassociated with the cell membrane to misfold and dissociate from thecell surface. 4. As PrPsc dissociates from the cell surface, more PrPc is translated in brain cell and translocated to the surface/ PrPsc induces misfolding andrelease of these proteins. Etcetera.5. The accumulation of cell free PrPsc forms proteinaceous plaquesbetween the brain cells6. Aggregated PrPsc is finally internalized into cells giving cells the spongiform appearance.

What happens on a molecular

level ?

Summary of previous notes

vCJD variant CJD from consuming infected beef from cows with BSE

• BSE in cattle is analogous to CJD in humans in that it is a geneticanomaly that leads to misfolding cell associated prion proteins intopathological cell free proteins.2. In later stages of the disease process this is manifested as MadCow Disease.3. The PrPsc from the cow can be transmitted to humans when humansconsume contaminated meat.4. The misfolded protein enters the human’s nerve cells and travelsto the brain tissue. 5. The pathological proteins from the cow causes the PrPc that isnaturally associated with the human cell membrane to misfold and dissociate from the cell surface.

Epidemiology

PrPsc so far only transmissible to man from cows with BSE

Cattle can also transmit disease to domestic cats, sheep and pigs

Transmission from sheep and pigs to humans so far not documented

CWD –chronic wasting disease in elk and muledeer--a disease similar to BSE in cattle not documented as transmissible to humans

PrPsc—not transmissible through milk or milk products

Probable cause

Hereditary BSE occurs naturally in cows at a low rate as CJD doesin humans.

1. Cows are butchered before they show manifestations ofdisease such that PrPsc enters the human population at a lowrate

2. Can be passed to the offspring and found associated withplacenta that contaminates grass that cows graze on

3. Farmers put ground up cattle in cattle feed, spreading BSEin the bovine community—PrPsc enters the human populationat a high rate.

PrPsc is spread from cows with BSE to humans to cause vCJD by consumption of such cows

PrPsc has been found concentrated in the following areas in cows

brainspinal cordretina (eye)distal ileum (small intestines)neurons near the backbonebone marrowlymphatic tissue

Transmission after beef consumptionPrPsc is taken up into Peyer’s patches—(mucousa associated lymphoidTissue (AKA MALT)—associated with the small intestines

T-cells induce its uptake by phagocytic calls that do not destroythe protein

Cells leave MALT and enter the lymphatic tissue where they become associated with

lymph nodesspleen tonsils

Cells can leave the lymphatic system and enter the blood circulatorysystem via the thoracic duct

Also lymphatic tissue is highly enervated so PrPsc can enter nerve cells

PrPsc moves up the axon of nerve cells to spinal cordeventually the brain

Route of transmission:

: MALT to lymphatic

system

Lymphatic system to

blood circulatory

system

Prevention and CurePrevention

1. Don’t feed cows to cows2. Destroy cow population once BSE found3. Implement sensitive diagnostic tests to identify cows with BSE before they show symptoms

There is no good way to destroy PrPsc in living material

Cure

SO FAR NO CURE!!!!

Future treatments might include treatments currently used forAlzheimer’s Disease.