Viruses and Prions: Structure, Replication, Evolution, and Clinical Aspects

General Virus Characteristics

Viruses as Nonliving Microbes

Viruses are unique infectious agents that are classified as nonliving because they lack the fundamental characteristics of cellular life.

• Acellular Structure: Viruses do not possess cellular organization; they lack organelles, cytoplasm, and a plasma membrane.

• No Independent Metabolism: Viruses cannot carry out metabolic processes on their own and do not generate or store energy.

• Obligate Intracellular Pathogens: Viruses must invade living host cells to replicate, relying entirely on the host's biochemical machinery.

Viral Structure: Capsids, Envelopes, and Spikes

Viruses are composed of genetic material encased in a protein shell, with some possessing additional structural features.

• Virion: A single, complete infectious virus particle.

• Capsid: The protein shell that encloses the viral genome, made of protein subunits called capsomeres. Capsids can be helical, icosahedral, or complex in shape.

• Envelope: Some viruses have a lipid-based envelope derived from the host cell membrane during viral budding. Naked viruses lack this envelope and are typically released by cell lysis.

• Spikes (Peplomers): Glycoprotein extensions on the surface of enveloped (and some naked) viruses. They mediate attachment to host cell receptors, determine host specificity (tropism), and can evolve to evade immune detection.

Viral Genomic Variations

Viral genomes are highly diverse in their composition and structure, influencing their replication strategies.

• Nucleic Acid Type: DNA or RNA, which may be single-stranded (ss) or double-stranded (ds).

• Genome Structure: Linear, circular, or segmented molecules; typically small, with fewer than 300 genes.

• Replication Strategies:

  • dsDNA viruses: Use host RNA polymerase to transcribe mRNA.

  • ssDNA viruses: Convert to dsDNA before transcription.

  • ssRNA (+) sense viruses: Genome acts directly as mRNA.

  • ssRNA (–) sense viruses: Require RNA-dependent RNA polymerase to synthesize mRNA.

  • Retroviruses: Use reverse transcriptase to convert RNA to dsDNA, which integrates into the host genome.

  • dsRNA viruses: Must unwind genome and use RNA-dependent RNA polymerase to produce mRNA.

Viral Replication and Evolution

Strategies for mRNA Production

Viruses employ different mechanisms to generate mRNA, depending on their genome type.

• dsDNA viruses: Host RNA polymerase transcribes DNA to mRNA.

• ssDNA viruses: Convert to dsDNA, then transcribe to mRNA.

• ssRNA (+) sense viruses: Genome serves as mRNA for direct translation.

• ssRNA (–) sense viruses: Synthesize mRNA using viral RNA-dependent RNA polymerase.

• dsRNA viruses: Unwind genome and use RNA-dependent RNA polymerase for mRNA synthesis.

• Retroviruses: Reverse transcription to dsDNA, integration into host genome, then transcription to mRNA.

Viral Genome Evolution

Viral genomes evolve rapidly due to high replication rates and genetic changes.

• Mutation: Random changes in the viral genome, especially frequent in RNA viruses due to lack of proofreading by RNA polymerases.

• Reassortment: Exchange of genome segments between related viruses during coinfection, producing new strains.

• Consequences: Mutations can be neutral, beneficial (e.g., increased infectivity, immune evasion), or deleterious (attenuation).

• Antigenic Drift: Minor mutations in antigen-coding genes (e.g., influenza HA and NA spikes) leading to seasonal outbreaks.

• Antigenic Shift: Major genetic changes from reassortment, potentially causing pandemics due to lack of population immunity.

Antigenic Drift vs. Antigenic Shift in Influenza

Influenza virus evolution is driven by two main mechanisms:

• Antigenic Drift: Gradual accumulation of mutations in HA and NA genes, resulting in new strains that evade existing immunity and necessitate annual vaccine updates.

• Antigenic Shift: Abrupt reassortment of genome segments when two different influenza viruses coinfect a host, creating novel HA and/or NA combinations. This can lead to pandemics.

Classifying and Naming Viruses

Criteria for Virus Classification

Viruses are classified based on several structural and genetic features:

• Nucleic Acid Type: DNA or RNA

• Capsid Symmetry: Helical, icosahedral, or complex

• Envelope Presence: Enveloped or naked

• Genome Architecture: Single- or double-stranded, linear, circular, or segmented

• Other Factors: Virus size, host range, tissue/cell tropism, and disease characteristics

Host Range and Tropism

• Host Range: The spectrum of species a virus can infect (narrow or broad).

• Tropism: The specificity of a virus for particular tissues or cell types within a host, determined by viral surface proteins and host cell receptors.

Naming Conventions for Viruses

Virus taxonomy follows standardized rules set by the International Committee on Taxonomy of Viruses (ICTV):

• Order: Italicized, capitalized, ends in “-virales” (e.g., Mononegavirales).

• Family: Italicized, capitalized, ends in “-viridae” (e.g., Herpesviridae).

• Subfamily: Italicized, capitalized, ends in “-virinae” (e.g., Alphaherpesvirinae).

• Genus: Italicized, capitalized, ends in “-virus” (e.g., Simplexvirus).

• Species: Italicized, first word and proper nouns capitalized, not abbreviated (e.g., Human immunodeficiency virus 1).

• Common Names: Not italicized; proper nouns capitalized; may be abbreviated after first use.

Viral Replication Pathways

Bacteriophage Lytic and Lysogenic Cycles

Bacteriophages (viruses that infect bacteria) can replicate via two main pathways:

• Lytic Cycle:

  1. Attachment: Phage binds to bacterial cell wall proteins.

  2. Penetration: Phage injects genetic material into host.

  3. Replication: Host machinery produces viral components.

  4. Assembly: New virions are assembled.

  5. Release: Host cell lyses, releasing new phages.

• Lysogenic Cycle:

  1. Attachment and penetration as above.

  2. Integration: Phage genome integrates into host chromosome (prophage).

  3. Replication: Prophage is copied with host genome during cell division.

  4. Induction: Under stress, prophage excises and enters lytic cycle.

  Temperate phages can switch between lysogenic and lytic cycles.

Animal Virus Replication

Animal viruses follow a generalized replication pathway:

1. Attachment: Viral proteins or spikes bind to host cell receptors.

2. Penetration: Enveloped viruses enter by membrane fusion or endocytosis; naked viruses typically enter by endocytosis.

3. Uncoating: Host enzymes degrade the capsid, releasing the viral genome.

4. Replication: DNA viruses usually replicate in the nucleus; most RNA viruses replicate in the cytoplasm.

5. Assembly: New virions are assembled from replicated genomes and capsid proteins.

6. Release: Enveloped viruses bud from the host membrane; naked viruses are released by cell lysis.

Enveloped vs. Naked Virus Replication

• Entry: Enveloped viruses use membrane fusion or endocytosis; naked viruses use endocytosis only.

• Release: Enveloped viruses bud from the host cell, acquiring an envelope; naked viruses are released by lysis, destroying the host cell.

Persistent Infections: Chronic and Latent Mechanisms

• Persistent Infection: Virus remains in the host for extended periods, evading immune clearance.

• Chronic Infection: Continuous, low-level viral replication and release; slow disease progression.

• Latent Infection: Virus remains dormant with no active replication; can reactivate, causing symptoms.

Oncogenic Viruses

Some viruses can induce cancer by disrupting normal cell cycle regulation.

• Mechanisms: Stimulate uncontrolled cell division or inhibit apoptosis, leading to immortalized cells.

• Examples:

  • Human papillomaviruses (HPVs): Cervical, oropharyngeal, anal, vaginal, and penile cancers.

  • Human herpesvirus-8: Kaposi sarcoma.

  • Epstein–Barr virus: B and T cell lymphomas, Hodgkin’s disease.

  • Human T-lymphotropic viruses (HTLVs): Adult T cell leukemia.

  • Hepatitis B and C viruses: Liver cancer via chronic inflammation and DNA damage.

Clinical Aspects of Viruses and Prions

Laboratory Methods for Growing Viruses

• Bacteriophages: Grown in bacterial cultures (liquid or solid agar). Plaque assay is used to quantify phages by counting clear zones (plaques) where bacteria are lysed.

• Animal Viruses: Require living host cells, grown in tissue culture (e.g., HeLa cells), primary cell lines, live animals, or embryonated chicken eggs.

Plaque Assay

The plaque assay is a quantitative method for measuring lytic virus concentration.

• Serially diluted virus is mixed with host cells and agar, poured onto plates.

• After incubation, plaques (clear zones) form where viruses have lysed cells.

• Each plaque represents one infectious virus particle; results are expressed as plaque-forming units (PFUs) per milliliter.

Detection of Viral Proteins and Genetic Material

• Protein Detection:

  • Agglutination Assays: Antibody-coated beads clump in presence of viral antigens or patient antibodies.

  • ELISA: Enzyme-linked immunosorbent assay detects antigen-antibody binding via color change.

  • Advantages: High sensitivity and specificity.

  • Limitations: May miss early infections (seroconversion window); affected by antigenic drift/shift.

• Genetic Material Detection:

  • PCR/RT-PCR: Amplifies viral DNA or RNA for detection.

  • Sequencing and Probes: Identify specific viral genes.

  • Advantages: Detects early infections, identifies new viruses.

  • Limitations: Requires specialized equipment and expertise.

Antiviral Drug Strategies

Antiviral drugs target specific steps in the viral life cycle:

• Attachment/Entry Inhibitors: HRIG, maraviroc (block receptor binding); enfuvirtide, docosanol (block membrane fusion).

• Uncoating Inhibitors: Amantadine, rimantadine, vapendavir (prevent genome release).

• Replication Inhibitors: Nucleoside analogs (acyclovir, ribavirin, remdesivir, AZT) disrupt genome synthesis; protease inhibitors block protein processing.

• Release Inhibitors: Oseltamivir, zanamivir (prevent influenza virion budding).

• Immune Modulators: Interferon-alfa limits viral spread.

• Note: Most antivirals limit infection rather than cure; hepatitis C is a notable exception.

Prions: Infectious Proteins

Prions are unique infectious agents composed solely of misfolded protein, causing fatal neurodegenerative diseases.

• Nature: Proteinaceous infectious particles lacking genetic material; do not replicate like viruses.

• Mechanism: Induce misfolding of normal prion proteins in the brain, leading to clumping and tissue degeneration (spongiform encephalopathies).

• Human Diseases:

  • Creutzfeldt–Jakob disease (CJD): Variant, sporadic, inherited, and iatrogenic forms.

  • Gerstmann–Sträussler–Scheinker syndrome

  • Fatal familial insomnia

• Transmission: Inherited (mutated gene), acquired (e.g., contaminated meat for variant CJD), or iatrogenic (medical procedures with contaminated instruments or tissue transplants).