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Prions-friends or Enemies


Blessy Jacob*, Lata Khani Bisht, Dr. Vineeth Chandy
T John College of Pharmacy, Gottigere, Bangalore-560083, INDIA

ABSRACT

Prion are infectious neurodegenerative and fatal diseases in humans and animals. Prions are altered type of normal protein known as cellular Prion proteins (PrPc), which are found majorily on the surface of CNS cells.They to play an important role in protecting cells and helping cells in responding to oxygen deficiency and metabolism. Prion accumulation in the brain leads to neuronal cell death or degeneration of neuronal tissue. The misfolding of cellular prion protein (PrP(C)) into pathogenic conformers and protease-resistant isoform PrP(Sc), which leads to a group of fatal neurodegenerative Prion diseases, many diseases like Kuru, Creutzfeldt Jakob disease, and Gerstmann Straussler Scheinker diseases are caused because of them. But on the other side prions are indicated to be beneficial in the process of memory formation.

Keywords: Prion proteins (PrPc), CNS, cellular prion protein (PrP(C)), protease-resistant isoform PrP(Sc).

INTRODUCTION

The word prion has been derived from proteinaceous infectious particle which is found to be responsible for transmissible spongiform encephalopathies (TSE) which are infectious neurodegenerative and fatal diseases in humans and animals. And until today no treatment is available with clinical efficacy.

They are very small and unique pathogens in that they don’t possess any nucleic acid and are resistant to methods taken to either break or destroy nucleic acid. Their size is even less than virus are visible in aggregated form when observed through electron microscope, not only that prion distinguish themselves from other pathogens in that they are responsible for genetic, sporadic, and acquired form of neurodegenerative diseases and do not leads to an immune response in the host as other infection causing foreign agents.

Prions are altered type of normal protein known as cellular Prion proteins (PrPc), which are found majorily on the surface of CNS cells as surface glycoprotein and also on other tissues of the body and appears to play an important role in protecting cells and helping cells in responding to oxygen deficiency. It is also found that prion protein is binded to copper and thus also involved in its metabolism.

The misfolding of cellular prion protein (PrP(C)) into pathogenic conformers and protease-resistant isoform PrP(Sc), which leads to a group of fatal neurodegenerative Prion diseases. They are expressed in other species like birds, fishes and reptiles as well beside mammals but cause diseases only in mammals. The prion protein present in bird (chPrPC) shares almost 30% of similarity with prion protein present in mammals (PrPC) as in both of them N-terminal domain have same type of amino acids which is proven by NMR studies. In spite of  similarity on structure, the normal isoform of human prion protein can be completely degraded by proteinase K, but bird prion protein cannot be degraded.

Figure: 1 Normal and Diseased Prions

Figure 1. A schematic of the two possible states of a prion protein. On the left, the protein is in its normal shape (PrPC). In this state, the protein carries out a certain function in the cell. In the middle, the protein has converted to the aggregated prion form (PrPSC). Prions have the ability to recruit normal PrPC proteins to the ends of the aggregates and convert them to prions as well. The aggregates grow larger in this way and when they get too big, they will break apart into smaller aggregates, which then convert even more normal protein to prions. 

Prions can be killed by implementing extreme temperature & pressure through incineration (temperature-900 F-4hours) in an autoclave, can be deactivated (temperature -270 F at 21 psi pressure maintained for 90 min). By using solution of sodium hydroxide for 1 hour at 250 F and 21 psi, prions can be deactivated.

Figure 3: Creutzfeldt- Jakob Disease Symptoms

Table 1: Genral Description of Diseases

Diseases symptoms Route of acquisition distribution Span of overt illness
Kuru Loss of coordination, followed by dementia Infection (probably through cannibalism) Known only in highlands of Papua New Guinea Three months to one year
Creutzfeldt Jakob disease Dementia,followed by loss of coordination Unknown

Sometimes (in 10 to 15 percent of cases) inheritence of mutation in the gene coding for the prion protein PrP.

 

Sporadic form: 1 person million worldwide Typically about one year, it can range from one month to 10 years
Fatal familial insomnia Trouble sleeping and disturbance of autonomic nervous system, followed by insomnia and dementia Inheritance of mutation in the PrP gene Nine extended families have been identified Typically about one year
Gerstmann Straussler Scheinker diseases Loss of coordination, often followed by dementia Inheritance of mutation in the PrP gene Some 50 extended families have been identified Typically two to six years

The prions which are protein aggregates or clumps are infectious to near by cells and can spread. They lead to degeneration of brain cells. The mechanism of replication of prions is still under research and study but it is believed to induce normal prion protein (PrPC) and convert into pathogenic shape that is resistant to attack of enzymes like proteases. The normal prion protein structure consists of several flexible coils known as alpha helices prone to breakdown by proteases but in pathogenic form these helices are in the form of flat sheets (beta sheets) resistant to proteases and thus concentrate in the tissues of brain.

Prion accumulation in the brain leads to neuronal cell death or degeneration of neuronal tissue by accumulation of protein called amyloid. This occurs due to disruption of normal cell processes by prions and accumulation of faulty proteinswhich results in cell death. It is termed as spongiform disease because of appearance of tiny holes in brain tissue that look like sponge under the microscope. The individuals affected by it develop dementia that keeps on worsening progressively and ultimately causes death. One of the example is Kuru disease in Papua New Guinea, that spreads among tribals on eating the brain of dead during funereal rituals. It is thought that sheep may have transferred the prions to cows as they were first found to be infected in a fatal disease known as sheep scrapie.

The recent research links infection to consuming TSE infected material that is taken up by lymphoid tissue. From lymphoid tissue it finds way to gastrointestinal tract, appendix, spleen and tonsil. In the lymphatic system prions replicate and translocate to Autonomic Nervous System followed by Central Nervous System. They can also enter via lesions and wounds in oral cavity and then to vagus nerve which carries them to brain.

But inspite of this,  few recent researches in this field of prions shown that prions were found in our primitive relatives, fungi and yeast, revealing that they accompained us during evolution where yeast was thought to harbor them and in return these prions help them to survive the harsh environment. Thus Prions played powerful role in the survival and evolution of wild yeast strains. They are found to be useful in formation of memory in higher organisms, ass on experiencing a encounter

As it is shown that yeast strains that can form prions are more resistant to antifungal drugs or heat/chemical stress than strains that cannot form prions.

The prions are indicated to be beneficial in the process of memory formation. Synapses present in between the neurons are encoded with memories. Memory is formed when there is communication in between the neurons that results in strengthening previous connections and establishment of newer connections referred to as “happens once and persists” characteristic feature of a memory. This is synonymous with prions where they once formed will persist in the being for which evidence is collected by research on sea slugs. The protein named CPEB present in brain functions like prion by facilitating synapse formation and storing new memory. CPEB replicates and aggregates like prion but does not cause neuronal death like prions. It changes its shape from normal to prion like that is better at communicating with nearby neurons and establish memory.

CONCLUSION

Prions are harmful as well as useful to humans but the benefits done by prions are far less by harmful effects incurred by them. Many people in the past lost their lives because of this protinaceous protein misfolding. Hence prions in humans can be explained as moiety which can be fatal which can eventually leads to dementia and death to element of life in yeast and memory formation in humans.

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