21.4 Other Acellular Entities: Prions and Viroids
2 min read•june 14, 2024
and are unique infectious agents that challenge our understanding of disease. Prions, misfolded proteins, cause brain disorders in animals, while viroids, tiny RNA molecules, infect plants. Both lack genetic material yet can replicate and spread.
These demonstrate how simple structures can have devastating effects. Prions misfold normal proteins, leading to brain damage, while viroids disrupt plant gene expression. Their unusual nature makes them fascinating subjects in the study of infectious diseases.
Prions and Viroids
Prions in neurodegenerative diseases
Top images from around the web for Prions in neurodegenerative diseases
Frontiers | Common mechanisms underlying axonal transport deficits in neurodegenerative diseases ... View original
Is this image relevant?
Frontiers | Mathematical Modeling of Protein Misfolding Mechanisms in Neurological Diseases: A ... View original
Is this image relevant?
Frontiers | Case Report: Histopathology and Prion Protein Molecular Properties in Inherited ... View original
Is this image relevant?
Frontiers | Common mechanisms underlying axonal transport deficits in neurodegenerative diseases ... View original
Is this image relevant?
Frontiers | Mathematical Modeling of Protein Misfolding Mechanisms in Neurological Diseases: A ... View original
Is this image relevant?
1 of 3
Top images from around the web for Prions in neurodegenerative diseases
Frontiers | Common mechanisms underlying axonal transport deficits in neurodegenerative diseases ... View original
Is this image relevant?
Frontiers | Mathematical Modeling of Protein Misfolding Mechanisms in Neurological Diseases: A ... View original
Is this image relevant?
Frontiers | Case Report: Histopathology and Prion Protein Molecular Properties in Inherited ... View original
Is this image relevant?
Frontiers | Common mechanisms underlying axonal transport deficits in neurodegenerative diseases ... View original
Is this image relevant?
Frontiers | Mathematical Modeling of Protein Misfolding Mechanisms in Neurological Diseases: A ... View original
Is this image relevant?
1 of 3
- Infectious protein particles lacking nucleic acids ()
- Composed of misfolded proteins called ( protein scrapie) an abnormal isoform of the normal cellular prion protein ()
- Cause neurodegenerative diseases by inducing conformational changes in normal prion proteins
- interacts with causing it to misfold and aggregate
- Misfolded proteins accumulate in the brain forming which disrupt normal brain function and cause neuronal death
- Examples of prion diseases ():
- transmitted through ritualistic cannibalism (Papua New Guinea)
- (BSE or "mad cow disease") transmitted to cattle through contaminated feed containing prion-infected meat and bone meal
- (CJD) human equivalent of BSE acquired through consumption of infected beef or inherited mutation in the
- is the key mechanism in prion disease progression
Structure and replication of viroids
- Small single-stranded molecules lacking a protein coat
- Consist of 246-401 nucleotides
- Highly structured with self-complementary regions forming double-stranded segments
- Replication occurs in the nucleus or chloroplasts of infected plant cells
- transcribes the circular RNA producing linear multimeric copies
- Multimeric RNA is cleaved by host enzymes into monomeric units
- Monomeric RNA circularizes through self-ligation or host enzymes
- Circular RNA serves as a template for further replication
- Interfere with plant gene expression causing disease symptoms
- Interact with host proteins disrupting normal cellular processes
- Induce pathways leading to degradation of host mRNAs
- Viroids are important affecting various crops
Transmission of prions vs viroids
- Prion transmission:
- Acquired through consumption of infected tissue (cannibalism, contaminated feed)
- Can be inherited through genetic mutations in the prion protein gene
- No evidence of transmission through casual contact or environmental exposure
- transmission:
- Primarily transmitted through mechanical means (contaminated tools or equipment)
- Can be spread by grafting infected plant material onto healthy plants
- Some viroids are transmitted through pollen or seeds
- Both prions and viroids do not require vectors for transmission
- Prions are transmitted directly between animals through ingestion of infected tissue
- Viroids are transmitted directly between plants through mechanical means or plant propagation
Key Terms to Review (22)
Amyloid plaques: Amyloid plaques are abnormal clusters of protein fragments that accumulate between nerve cells in the brain, primarily associated with neurodegenerative diseases like Alzheimer's. These plaques are formed when beta-amyloid peptides aggregate and misfold, disrupting cell function and leading to cognitive decline and neuronal death. Their presence is a hallmark of certain nervous system disorders, highlighting their critical role in understanding disease mechanisms.
Bovine spongiform encephalopathy: Bovine spongiform encephalopathy (BSE), commonly known as 'mad cow disease,' is a progressive neurodegenerative disorder found in cattle that results from the accumulation of abnormal prion proteins in the brain. This disease is part of a group of transmissible spongiform encephalopathies (TSEs) caused by misfolded proteins called prions, which lead to brain damage and characteristic sponge-like changes in brain tissue. The connection between BSE and human health concerns emerged when variant Creutzfeldt-Jakob disease (vCJD), a related prion disease, was linked to the consumption of infected beef products, raising significant public health alarms.
Circular RNA: Circular RNA (circRNA) refers to a type of RNA molecule that forms a covalently closed loop, meaning it has no free ends. This unique structure can influence gene expression and plays various roles in cellular processes, making circRNA significant in the study of other acellular entities like prions and viroids. Unlike linear RNA, circRNA can evade degradation by exonucleases, allowing it to persist longer in cells and potentially exert regulatory effects on gene expression or act as a sponge for microRNAs.
Conformational Change: Conformational change refers to the alteration of the structure of a protein or other macromolecule, resulting from interactions with other molecules, environmental changes, or internal dynamics. This process is crucial for the function of proteins and plays a significant role in various biological signaling pathways and the behavior of acellular entities like prions and viroids.
Creutzfeldt-Jakob disease: Creutzfeldt-Jakob disease (CJD) is a rare, degenerative neurological disorder caused by prions, which are misfolded proteins that induce abnormal folding of normal cellular proteins in the brain. This condition leads to rapid cognitive decline and severe neurological symptoms, ultimately resulting in death. CJD is one of the most well-known examples of prion diseases, highlighting the unique and dangerous nature of these acellular entities.
Kuru: Kuru is a rare and fatal neurodegenerative disorder caused by prions, which are misfolded proteins that lead to brain damage. It primarily affected the Fore people of Papua New Guinea and was transmitted through ritualistic cannibalism, particularly the consumption of infected human brains. The study of Kuru has been instrumental in understanding prion diseases and their mechanisms.
Nucleic acid-free infectious agents: Nucleic acid-free infectious agents are unconventional pathogens that lack nucleic acids, such as DNA or RNA, which are essential for typical biological replication. These agents primarily include prions and viroids, which challenge traditional concepts of infectious diseases due to their unique modes of action and structure. Prions are misfolded proteins that induce abnormal folding in other proteins, while viroids consist of short, circular RNA molecules that can cause disease in plants.
Pathogens: Pathogens are microorganisms or agents that cause disease in their host. They can be bacteria, viruses, prions, viroids, or other acellular entities.
Plant pathogens: Plant pathogens are organisms that cause diseases in plants, leading to detrimental effects on their growth, development, and yield. These pathogens can be bacteria, fungi, viruses, nematodes, or even viroids and prions, which are acellular entities that can affect plant health. Understanding plant pathogens is crucial for managing agricultural practices and ensuring crop productivity.
Prion: A prion is a type of infectious agent composed solely of protein that can induce abnormal folding of normal cellular proteins, leading to neurodegenerative diseases. Unlike bacteria or viruses, prions do not contain nucleic acids, making them unique among pathogens. They are known for their ability to cause diseases that are often fatal and are notoriously resistant to conventional methods of sterilization.
Prion protein gene: The prion protein gene encodes for the prion protein, a protein that can misfold and cause neurodegenerative diseases. This gene is critical in understanding how certain proteins can lead to infectious diseases through abnormal conformations, linking it to acellular entities like prions.
Prions: Prions are misfolded proteins that can induce other normal proteins to also misfold, causing a chain reaction. They are known to cause a variety of neurodegenerative diseases.
Protein misfolding: Protein misfolding refers to the incorrect folding of proteins, leading to dysfunctional or toxic conformations. This phenomenon is crucial in the context of certain acellular entities, like prions and viroids, where misfolded proteins can induce further misfolding in other proteins, causing a cascade of pathological effects.
PrPc: PrPc is the normal, cellular isoform of the prion protein found in healthy animals and humans. It is primarily located on the cell membrane and plays roles in cellular processes such as signaling and protecting cells from oxidative stress.
PrPC: PrPC refers to the normal cellular prion protein, which is a glycoprotein found primarily in the brain and is involved in various cellular functions. This protein is essential for maintaining neuronal health and can undergo misfolding to become the pathogenic form, PrPSc, which is associated with prion diseases. Understanding PrPC is crucial as it highlights the delicate balance between normal protein function and the consequences of its aberrant conformations.
PrPsc: PrPsc is an abnormal, pathogenic form of the prion protein. It is responsible for causing transmissible spongiform encephalopathies (TSEs) in humans and animals.
PrPSc: PrPSc is an abnormal, misfolded form of the prion protein (PrP) that is associated with transmissible spongiform encephalopathies (TSEs), a group of neurodegenerative diseases. Unlike normal prion proteins, which are harmless, PrPSc is infectious and can induce the misfolding of normal proteins into the pathological form, leading to brain damage and ultimately death. This makes PrPSc a critical component in understanding how prions propagate disease and affect nervous systems.
RNA polymerase II: RNA polymerase II is a crucial enzyme in eukaryotic cells responsible for synthesizing messenger RNA (mRNA) from DNA during the transcription process. It plays a key role in gene expression by converting genetic information encoded in DNA into RNA, which can then be translated into proteins. This enzyme also facilitates the processing of pre-mRNA, including capping and polyadenylation, which are vital for mRNA stability and translation efficiency.
RNA silencing: RNA silencing is a biological process that regulates gene expression and defends against viral infections by using small RNA molecules to target and degrade specific messenger RNA (mRNA) transcripts. This mechanism is crucial in controlling various cellular processes, including development and response to stress, and it plays a significant role in the interactions between organisms and certain acellular entities such as viroids.
Transmissible spongiform encephalopathies: Transmissible spongiform encephalopathies (TSEs) are a group of progressive neurodegenerative disorders caused by infectious agents known as prions, which are misfolded proteins that induce abnormal folding of normal proteins in the brain. These diseases lead to brain damage, resulting in a sponge-like appearance and severe neurological symptoms. TSEs affect both humans and animals, emphasizing the unique properties of prions as acellular entities that can be transmitted between hosts.
Viroid: A viroid is a small, infectious agent composed solely of a short strand of circular RNA without any protein coating. Unlike viruses, viroids do not encode proteins and rely entirely on the host's cellular machinery for replication. They primarily infect plants, leading to various diseases that can significantly impact agricultural productivity.
Viroids: Viroids are small, circular RNA molecules that infect plants. Unlike viruses, they do not encode proteins and rely entirely on the host's cellular machinery for replication.