Themes > Science > Life Sciences > General Biology > Microbiology > Prions and Viroids > Prions


Structure

Prions are infectious agents composed exclusively of a single sialoglycoprotein called PrP 27-30. They contain no nucleic acid. PrP 27-30 has a mass of 27,000 - 30,000 daltons and is composed of 145 amino acids with glycosylation at or near amino acids 181 and 197. The carboxy terminus contains a phosphatidylinositol glycolipid whose components are ethanolamine, phosphate, myo-inositol and stearic acid. This protein polymerizes into rods possessing the ultrastructural and histochemical characteristics of amyloid. Amyloid is a generic term referring to any optically homogenous, waxy, translucent glycoprotein; it is deposited intercellularly and/or intracellularly in many human diseases such as:
  • Alzheimer's disease
  • Creutzfeldt-Jakob disease
  • Down's syndrome
  • Fatal familial insomnia
  • Gerstmann-Straussler syndrome
  • Kuru Leprosy

Replication

The prion is a product of a human gene, termed the PrP gene, found on chromosome 20. This gene contains two exons separated by a single intron. Exon I and Exon II are transcribed and the two RNAs ligated into a single mRNA. This mRNA contains an open reading frame (ORF) or protein coding region which is translated into the PrP protein. The PrP protein is a precursor of the prion protein. It is termed PrP 33-35.
 
Replication

The PrP 33-35 undergoes several post-translational events to become the prion protein (PrP 27-30):

    1.    Glycosylation - at two sites.

    2.    Formation of a disulfide bond between two cysteine residues.

    3.    Removal of the N-terminal signal peptide.

    4.    Removal of the C-terminal hydrophobic segment.

    5.    Addition of a phosphatidylinositol glycolipid at the C-terminal.

    6.    Removal of the N-terminal first 57 amino acids.
     

    Structure of the prion protein
In normal cells only the PrP 33-35 protein is synthesized. It is found in the neural cell membrane where it's function is to sequester Cu++ ions. In abnormal ("infected") cells, the PrP 27-30 is produced from the PrP 33-35 protein. The PrP 27-30 triggers a series of reactions that produce more PrP 27-30 proteins, i.e., PrP 27-30 induces its own synthesis. In addition to the post translational modifications, the PrP 27-30 protein differs from the PrP 33-35 protein in a single amino acid residue. Residue 178 in the PrP 27-30 contains an asparagine residue whereas the PrP 33-35 protein has an aspartate residue at this position. This causes a conformational change in the PrP 27-30 protein from an a-helix to a b-sheet. This conformational change in the PrP 27-30 protein has three effects:
    1.    It imparts to the PrP 27-30 protein the ability to induce the same a-helix to b-sheet conformation in the PrP 33-35
            protein. This is a permanent conformational change. It thus induces its own "replication."

    2.    The b-sheet-forming peptides aggregate to form amyloid fibrils.

    3.    The amyloid fibrils kill thalamus neurons through apoptosis, a programmed series of events that leads to cell death.

Pathologies induced by prions

All diseases known to be of prion etiology, in animals and humans, are neurodegenerative diseases. In the human this includes:
  • Creutzfeldt-Jakob disease (CJD)
  • Fatal Familial Insomnia
  • Gerstmann-Straussler syndrome
  • Kuru
The pathological and clinical signs of these diseases suggest that they are closely related. In fact they may be variants of the same disorder. All pathological features are confined to the central nervous system. The prion protein accumulates selectively and abnormally in CNS nerve cells during the course of the disease. PrP 27-30 accumulates within the neuropil where it causes:
    1.    Astrocyte gliosis (an increase in the number of astrocytes).

    2.    Depletion of dendritic spines in neurons.

    3.    Formation of numerous vacuoles in the cerebellar cortex (spongiform encephalopathy).

    4.    Amyloidosis - deposition of amyloid in the cerebellar cortex, thalamus, brain stem and in the lumen of blood vessels
            within the brain. These amyloid plaques consist of discrete eosinophilic glassy-appearing masses, often having
            radiating amyloid fibrils at their periphery. The plaques are primarily subependymal, subpial and perivascular.

Note that the pathology does NOT include any signs of inflammation or fever. This is evidence that the immune system does not respond to the prion protein. Since the prion protein is derived from self this is what you would expect.

These pathologies give rise to the clinical symptomology seen in these patients. These are:

1.    A long incubation period (several years) which has given rise to the term "slow infection."

2.    Loss of muscle coordination which leads to a difficulty in walking, indicating a functional disorder of the cerebellum.

3.    Dementia characterized initially by loss of memory, diminished intellect and poor judgement.

4.    Progressive insomnia characterized by a marked reduction or loss of the slow-wave and rapid-eye-movement phases.

Transmission

Spread of the disease is via horizontal transmission, i.e., transmission from one person to another, either directly or by fomites or by ingestion of contaminated meat.

Diagnosis

In the past, diagnosis of prion disease was made through examination of brain biopsies taken from patients in advanced stages of the disease or, more commonly, after they had died.  In January of 1999 it was found that the prion protein accumulated in the tonsils and could be detected by an immunofluorescence test on tonsilar biopsies.  A second test was simultaneously developed which was based on a Western blot.  Later that year a third test was developed that had the high sensitivity necessary to detect the prion protein in blood.  This test is based on capillary electrophoresis with laser-induced fluorescence.  It detects as little as 10-18 mole.


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