MMI 133 Session 5: Viruses and Prions

Viruses: An Overview

Introduction to Viruses

• Definition: Viruses are infectious particles, not considered living cells.
• Prevalence: They are responsible for the majority of visits to family physicians.
• Transmission: Viruses can be transmitted from host to host in many ways, including:
  • Blood transfusions
  • Mucus droplets
  • Aerosols
  • Fomites
  • Air (airborne transmission)
  • Water
  • Food
  • Vectors
• Therapeutic Strategies: Due to their unique makeup and structure, viruses require different therapeutic strategies compared to other microorganisms.
• Analogy (Spaceship):
  • A virus is like a "spaceship without fuel tanks" (lacking energy reserves).
  • It's "destined to drift in space" until it can "dock with the appropriate 'mother-ship'" (a host cell) and "offload cargo."
  • The "cargo" is the viral genome, which "takes over the host cell machinery and produces many 'clones'" (new viruses).
  • These new viruses then "leave the host cell and drift, looking for their own host cells to infect," sometimes taking a "souvenir coat" (lipid envelope) when they depart.

Viruses Contrasted with Bacteria

• Size:
  • Virus: Smaller, typically ranging from $10$ to $500 ext{ nm}$.
  • Bacteria: Larger, typically ranging from $500 ext{ nm}$ to approximately $2000 ext{ nm}$.
• Replication:
  • Virus: Multiply only within host cells (obligate intracellular parasites).
  • Bacteria: Most are free-living and multiply outside of host cells.
• Nucleic Acid Content:
  • Virus: Contain either DNA or RNA, but never both.
  • Bacteria: Always contain both DNA and RNA.
• Enzymes and Metabolism:
  • Virus: Possess few enzymes; lack ribosomes and enzymes for metabolizing nutrients.
  • Bacteria: Contain many enzymes; possess ribosomes and enzymes for metabolism of nutrients.
• Examples of Viral Shapes: Coronavirus, Influenza virus, Ebola virus, Rabies virus.

Viral Structure

• Virion: A complete infectious viral particle consisting of nucleic acid surrounded by a protein coat.
• Nucleic Acid Core: Contains either RNA or DNA (single-stranded (ss) or double-stranded (ds)).
• Capsid: The protein coat that surrounds the nucleic acid.
  • Comprised of protein molecules called "capsomeres."
• Nucleocapsid: The complete unit formed by the nucleic acid and its capsid.
• Lipid Envelopes:
  • Some viruses possess lipid envelopes outside of the nucleocapsid.
  • These envelopes are derived from the host cell's membranes.
  • Examples: Influenza virus, Herpes virus, Hepatitis B virus (HBV), Human Immunodeficiency Virus (HIV).
• Glycoproteins or Spikes: Structural proteins inserted into the envelopes or capsid, often involved in attachment to host cells.
• Example: Influenza A virus structure: Includes a lipid envelope with Neuraminidase and Hemagglutinin glycoproteins, M2 ion channel, M1 matrix protein, and eight RNA segments within.
• Viral Naming: Viral naming conventions are different from those for bacteria.
  • Examples: Herpes simplex type 1 (HSV-1), Herpes simplex type 2 (HSV-2), Cytomegalovirus (CMV), Smallpox (Variola major).

Host Range and Specificity

• Broad Infectivity: Different viruses can infect nearly every life form, including bacteria, fungi, plants, animals, and humans.
• Specificity: Viruses usually exhibit a fairly specific "host range" or "specificity."
  • Example: Bacteriophages infect only bacteria.

Bacteriophages

• Definition: Viruses that specifically infect bacterial cells.
• Genetic Transfer: Can transfer new genes from one bacterium to another.
  • Tool for Genetic Engineering: Can be used in genetic engineering (e.g., in the production of human insulin from E. coli).
  • Toxin Gene Transfer: Can transfer genes for toxin production (e.g., in Corynebacterium diphtheriae through a process called lysogeny).

Classifying Viruses

• Viruses are grouped into families based on their genome composition (DNA or RNA).
• DNA Viruses:
  • Families (6 to learn): Papillomaviridae (HPV), Adenoviridae (adenovirus), Hepadnaviridae (hepatitis B virus), Herpesviridae (HSV-1, HSV-2, HHV-3), Poxviridae (smallpox, monkeypox), Parvoviridae (B-19).
• RNA Viruses:
  • All other viruses not classified as DNA viruses are RNA viruses.
  • Key Characteristic: RNA viruses are known for their ability to mutate quickly.
  • Families/Examples: Retroviridae (HIV), Flaviviridae (Yellow fever virus, West Nile virus), Orthomyxoviridae (Influenza A, B, C), Picornoviridae (Enterovirus), Coronavirus.

Events in Viral Infection

1. Attachment and Penetration into Host Cell

• Attachment: The virus must first attach to the host cell, typically by binding to specific host cell receptors.
• Penetration Mechanisms: Two main processes for the virus to enter the host cell:
  • Fusion: The viral envelope fuses directly with the host cell membrane, releasing the nucleocapsid into the cytoplasm (common for enveloped viruses).
  • Pinocytosis (Endocytosis): The host cell engulfs the virus through a vesicle, internalizing the virus.

2. Replication of the Genome

• Central Dogma Reminder: In organisms, DNA forms the genome. The sequence of nucleotides (A, T, C, G) determines genes. Transcription of DNA produces mRNA, which then undergoes translation at ribosomes to synthesize proteins.
• Viral Goal: The primary goal of a virus is to replicate itself.
• DNA Viruses:
  • Most DNA viruses replicate in the host cell nucleus.
  • They often "hijack" the host cell's DNA polymerase to produce new viral DNA: ext{viral DNA}
    ightarrow ext{viral DNA}.
  • Transcription of viral DNA leads to mRNA, which then translates into structural proteins and viral enzymes.
• RNA Viruses (Most):
  • Host cells lack an enzyme that can produce RNA from an RNA template.
  • Therefore, RNA viruses must carry their own RNA-dependent RNA polymerase enzyme into the host cell to produce new RNA from viral RNA: ext{viral RNA}
    ightarrow ext{viral RNA}.
  • Positive (+) RNA viruses: The viral RNA itself can serve as mRNA, directly translated into structural proteins and enzymes. It also serves as a template to make negative (-) RNA, which then makes more positive (+) RNA.
  • Negative (-) RNA viruses: The viral RNA cannot be directly translated. It must first be transcribed into positive (+) RNA (mRNA) by the viral RNA-dependent RNA polymerase, and then this mRNA is translated into proteins.
• RNA Retroviruses (e.g., HIV):
  • These viruses carry their own reverse transcriptase enzyme within the virus capsid.
  • This enzyme synthesizes DNA from the viral RNA template during reverse transcription: ext{viral RNA}
    ightarrow ext{viral DNA}.
  • The newly synthesized viral DNA is then integrated into the host cell's chromosome.
  • From this integrated viral DNA (provirus), host machinery transcribes new viral RNA, which is translated into viral proteins and serves as new viral genomes.

3. Assembly and Release of New Virus

• Assembly: Newly replicated viral nucleic acids (RNA or DNA) associate with newly made viral proteins to form new nucleocapsids.
• Release:
  • Enveloped Viruses: Envelope components (viral glycoproteins) are produced by the host cell and inserted into the host cell's plasma membrane. The viral particle then attaches to these modified membrane regions and buds through the membrane, acquiring its lipid envelope as it exits.
  • Naked Viruses: Typically released by lysis (bursting) of the host cell.

Antiviral Drugs: Targets in Viral Infection

Antiviral drugs specifically target different stages of the viral infection cycle:

• Fuzeon: Stops the entry (fusion) of HIV into cells.
• Acyclovir: Stops the replication of herpes viruses by interfering with the viral DNA polymerase.
• Tamiflu (Oseltamivir): Stops the budding/release of influenza virus from the host cell.

Host-Virus Interactions

• Acute (Productive) Infection:
  • The virus replicates extensively, producing many new virions.
  • The host cell is often killed (lytic infection).
  • Examples: Adenovirus, Poliovirus.
• Latent Infection:
  • The viral genome persists within the host cell but does not actively replicate.
  • The viral genome in this dormant state is called a "provirus."
  • Examples: Herpesviruses, HIV.
• Chronic Infection:
  • The virus replicates continuously over a long period without immediately causing host cell lysis.
  • The infection can persist for extended durations.
  • Examples: Hepatitis C virus, Hepatitis B virus.

Chickenpox and Shingles (HHV-3/Varicella-Zoster Virus)

• Causative Agent: Human Herpesvirus 3 (HHV-3), also known as varicella-zoster virus.
  • It is an enveloped DNA virus.

Chickenpox (Acute Infection)

• Symptoms: Fever, itchy (pruritic) rash lasting approximately $5$ days.
• Rash Characteristics:
  • Spread primarily on the trunk and head, with fewer lesions on the arms and legs.
  • Unique characteristic: Skin vesicles are always present in different stages of lesion formation (papules, vesicles, scabs coexist).
• Transmission: Virus particles in the rash can escape and infect others; classified as an airborne virus.
• Latency Establishment: When lesions heal, the virus travels up the nerve innervating the skin at the lesion site and takes residence in the peripheral nerve ganglion, where it remains latent.

Shingles or Zoster (Reactivation of Latent Infection)

• Reactivation: The latent HHV-3 virus reactivates and travels back down the nerve to the skin from its site of latency.
  • Triggers: Reactivation can be triggered by factors like age or a decline in the immune system.
• Symptoms:
  • Begins as a local skin rash, usually following a dermatome (area of skin supplied by a single nerve).
  • Characterized by a confluent (blistering and merging) rash and significant pain.
  • If untreated, the rash typically lasts for $2$ to $5$ weeks.
• Infectiousness: Infectious to individuals who have not had chickenpox.

Complications of Chickenpox

• Most Common (Immunocompromised): Secondary bacterial infection (e.g., often leading to concerns about "flesh-eating disease" if chickenpox parties are held).
• Central Nervous System (CNS) Disease: Encephalitis, meningitis, myelitis.
• Hemorrhagic Complications: Rare but serious.
• Primary Varicella Pneumonia: Very rare but can be very serious.

Complications of Zoster (Shingles)

• Post Herpetic Neuralgia (PHN): The most common complication.
  • Affects 25-50 ext{%} of patients over $50$ years of age.
  • Involves pain that can persist for months or even years after the rash has disappeared.
  • Can cause permanent nerve damage.
  • Treatment: Important to receive treatment (e.g., Acyclovir) within $48-72$ hours of symptom onset to minimize severity and duration of PHN.
• Other Complications: Meningoencephalitis, myelitis, cranial nerve palsies, vasculopathy, gastrointestinal ulcers, pancreatitis, hepatitis.
• Immunosuppressed Zoster: In immunocompromised patients (e.g., HIV-infected), more than one dermatome may be involved, or specific manifestations like ophthalmic zoster (involving the trigeminal nerve) may occur.

Prevention of Chickenpox (HHV-3)

• Active Immunization:
  • Live attenuated vaccine, highly protective, induces the production of memory cells.
  • Now often included with the Measles, Mumps, Rubella vaccine (MMRV).
• Passive Immunization:
  • Varicella-Zoster Immunoglobulin (VZIG): Contains antibodies specific to the virus.
  • Effective up to $3$ days post-exposure.
  • Normally used for immunocompromised exposed children and neonates born to mothers with active varicella.

Differentiating Chickenpox and Smallpox

• Chickenpox: Lesions are observed in different stages of development simultaneously (e.g., papules, vesicles, scabs all present).
• Smallpox: All lesions are in the same stage of development (more details in Lecture 13).

Oncogenic Viruses

• Transformation: Some viruses have the ability to "transform" normal host cells into cancer cells.
• Caveat: Not all "transformed" cells necessarily become cancerous (e.g., wart viruses often cause benign tumors).
• Types: Both RNA and DNA viruses can be cancer-producing.
• Example: Burkitt's Lymphoma is associated with a herpesvirus known as Epstein-Barr virus.

Viral Diagnosis

• Polymerase Chain Reaction (PCR): A common molecular biology technique used to detect viral genes in an organism.
• Cell Culture (Older, Less Common):
  • Patient specimens are added to cultured cells in a laboratory setting.
  • After a period for viral growth, the cells are observed for characteristic "cytopathic effects" (CPE), which are changes the virus causes in the infected cells.
  • Disadvantages: Labor-intensive and slow, as it depends on viral growth rates.
  • Example of CPE: Syncytia, where infected cells merge to form multi-nucleated giant cells (e.g., caused by Measles virus in the lung).

Prions

• Discovery: Defined as late as $1982$ by Stanley Prusiner.
• Nature: Prions are misfolded proteins that act as infectious agents in susceptible exposed animals.
• Pathogenesis: Involves a "protein misfolding disease" where a self-protein becomes changed and nonfunctional.
• Protein Folding Overview:
  • Primary Structure: Sequence of amino acids.
  • Secondary Structure: Occurs when amino acid chains are linked by hydrogen bonds, forming alpha helices and pleated sheets.
  • Tertiary Structure: Attractions between alpha helices and pleated sheets lead to a specific three-dimensional shape.
  • Quaternary Structure: A protein consisting of more than one amino acid chain.

Examples of Prion Diseases

• Animals:
  • Mad Cow Disease (BSE): Bovine Spongiform Encephalitis in cattle.
  • Scrapie: In sheep.
  • CWD: Chronic Wasting Disease in deer.
• Humans:
  • Kuru: Found in New Guinea, historically transmitted by cannibalism.
  • CJD: Creutzfeldt-Jakob Disease.
  • vCJD: Variant Creutzfeldt-Jakob Disease (linked to consumption of BSE-infected products).
  • Fatal Familial Insomnia: A rare hereditary prion disease.

Human Prion Disease Characteristics

• Outcome: Always fatal.
• Signs/Symptoms:
  • Ataxia (loss of coordination).
  • Cortical visual symptoms.
  • Progressive dementia.
  • Myoclonus (involuntary muscle jerks).
  • Akinetic mutism (a state of unresponsiveness).
• Prognosis: Average time to death after onset of symptoms is approximately $4$ months.
• Histological Picture: Brain tissue of CJD patients shows characteristic spongiform (microscopic holes) changes.