Lecture B6 - microsporidia as intracellular fungal pathogens

What are Microsporidia?

Microsporidia are obligate intracellular fungal pathogens that infect a wide range of animals, including humans. They are among the smallest eukaryotic pathogens, with highly reduced genomes and specialized unicellular spores.

How are Microsporidia transmitted?

Microsporidia transmission occurs through:

• Oral-fecal contamination (contaminated food and water)

• Inhalation of spores

• Zoonotic transmission (from infected animals)

• Vector transmission (e.g., Trachipleistophora hominis via mosquitoes)

What are the most common Microsporidia species in humans

• Enterocytozoon bieneusi: Causes chronic diarrhea and malabsorption

• Encephalitozoon intestinalis: Infects intestines, kidneys, and brain

What are the key immune cells involved in fighting Microsporidia infections?

• CD4+ T cells: Prime and sustain CD8+ T cell responses (via IL-2 and IL-21).

• CD8+ T cells: Execute cytotoxic destruction of infected cells.

• Dendritic cells (DCs): Antigen-presenting cells that produce IL-12 and IFN-γ to activate T cells.

• Macrophages: Recognize spores via pattern recognition receptors (PRRs) and initiate phagocytosis.

How do intraepithelial lymphocytes (IELs) contribute to mucosal immunity against Microsporidia?

• CD8+ αβ IELs: Recognize and eliminate infected cells.

• CD8+ γδ IELs: Respond to stressed epithelial cells and recognize stress ligands (e.g., MICA, RAE-1).

• IELs migrate to the epithelium after activation in Peyer’s patches (PPs) and mesenteric lymph nodes (MLNs).

What is the role of IL-21 in Microsporidia infections?

IL-21 is significantly upregulated during peak CD8+ T cell responses to Encephalitozoon cuniculi infection, enhancing cytotoxic activity.

How do Microsporidia invade host cells?

1. Polar Tube Discharge: A unique mechanism where the spore rapidly everts its polar tube, injecting the sporoplasm into the host cell cytoplasm.

2. Intracellular Proliferation: The sporoplasm replicates via merogony, forming multinucleate plasmodia, which divide by binary fission.

3. Sporogony: New spores form and lyse host cells, continuing the infection cycle.

What genomic adaptations enable Microsporidia to survive as intracellular parasites?

• Extreme genome reduction (~2.5 Mb in E. cuniculi).

• Loss of genes for amino acid, nucleotide, and lipid biosynthesis.

• Expansion of nucleotide transporters (NTTs) to steal host ATP.

• Retention of genes for host cell invasion (polar tube proteins) and Fe-S cluster biosynthesis.

What are de novo genes, and how do they contribute to Microsporidia evolution?

• De novo genes may have evolved recently and are under positive selection, particularly in host-parasite interactions.

• Found in T. hominis and V. culicis, they may:

  1. Localize to the parasite cell surface.

  2. Be part of the spore protein coat.

  3. Function in the polar tube structure (e.g., PTP2/3).

  4. Be secreted as effectors into the host cell.

What are mitosomes, and why are they important in Microsporidia?

• Mitosomes are highly reduced mitochondrial remnants in Microsporidia.

• They lack ATP production but retain iron-sulfur (Fe-S) cluster biosynthesis, essential for cellular function.

What proteins are involved in Fe-S cluster biosynthesis in Microsporidia?

• Nfs/Lsd11 (Desulphurase complex): Converts cysteine to sulfur.

• Frataxin: Facilitates iron incorporation into Fe-S clusters.

• Hsp70 and Isu: Assist Fe-S assembly.

• Ferredoxin (Yah1): Provides electrons for Fe-S cluster formation.

How do yeast and Microsporidia differ in Fe-S cluster biosynthesis?

• Yeast mitochondria export Fe-S intermediates via Atm1 for cytosolic and nuclear Fe-S protein assembly.

• Microsporidia retain Fe-S clusters within mitosomes, using them for essential functions without export.

What essential substrates are needed for Fe-S cluster biosynthesis in Microsporidia?

• ATP

• NADH (electron donor)

• Cysteine (sulfur source)

• Iron (Fe2+)

How do Microsporidia obtain energy?

• Microsporidia lack de novo ATP production and depend entirely on host-derived ATP.

• Nucleotide Transporters (NTTs) exchange ATP, GTP, and NAD+ across the membrane.

• NTTs function as ATP/ADP exchangers or H+-symporters, enabling survival.

What are the current treatment options for Microsporidia infections?

• Albendazole: Targets tubulin polymerization, effective against Encephalitozoon spp.

• Fumagillin: Inhibits MetAP2, effective against Enterocytozoon bieneusi.

What are emerging drug targets for Microsporidia infections?

1. Alternative Oxidase (AOX) inhibitors: Suppress T. hominis proliferation.

2. Blocking NTTs: Prevents ATP uptake, starving the parasite.

3. Disrupting polar tube formation: Prevents host cell invasion.

What key concepts summarize Microsporidia pathogenesis and treatment?

• Microsporidia are obligate intracellular parasites with extreme genome reduction.

• They invade cells via polar tube ejection and survive by hijacking host ATP.

• Mitosomes replace mitochondria, retaining only Fe-S cluster biosynthesis.

• Host immune responses rely on CD4+, CD8+ T cells, IELs, and macrophages.

• Therapeutic strategies focus on targeting energy metabolism and invasion mechanisms.

How do Microsporidia avoid detection by the host immune system?

• Intracellular lifestyle prevents exposure to extracellular immune components.

• Polar tube ejection allows direct invasion of host cytoplasm, bypassing immune defenses.

• Modulation of host apoptosis to prolong intracellular survival.

• Downregulation of pro-inflammatory cytokines (e.g., IFN-γ, TNF-α) to suppress immune response.

What are the structural components of Microsporidia spores?

• Thick chitin-rich spore wall provides environmental resistance.

• Anchoring disc assists in host cell attachment.

• Polar tube apparatus is essential for infection.

• Sporoplasm contains infectious material injected into host cells.

What triggers Microsporidia spore germination?

• Host environmental signals such as pH shifts, osmotic pressure changes, and exposure to digestive enzymes.

• Contact with host cell receptors may also trigger ejection of the polar tube.

What role do heat shock proteins (HSPs) play in Microsporidia survival?

• HSPs act as molecular chaperones, assisting in protein folding and stress response.

• Hsp70 and Hsp90 protect against oxidative and heat stress, enhancing intracellular survival.

What are the major differences between Microsporidia and other intracellular parasites?

• Microsporidia lack mitochondria, using mitosomes instead.

• Highly compact genomes with minimal metabolic pathways.

• Polar tube-mediated invasion, a unique entry mechanism.

• Energy parasitism via nucleotide transporters (NTTs) instead of glycolysis or oxidative phosphorylation.

How does lateral gene transfer (LGT) contribute to Microsporidia evolution?

• LGT from bacteria and fungi has allowed adaptation to intracellular parasitism.

• Acquired genes encode transporters, metabolic enzymes, and stress-response proteins.

• Some transferred genes compensate for lost biosynthetic pathways.

What alternative oxidase (AOX) pathways exist in Microsporidia?

• AOX provides an alternative respiratory mechanism, bypassing traditional oxidative phosphorylation.

• Helps regenerate NAD+, supporting glycolytic energy production.

• AOX inhibitors are potential therapeutic targets for Microsporidia.

How does iron homeostasis affect Microsporidia survival?

• Microsporidia rely on host iron sources, lacking independent iron acquisition pathways.

• Fe-S cluster biosynthesis in mitosomes is essential for redox balance.

• Disrupting host iron availability may impair Microsporidia replication.

What are the potential vaccine strategies against Microsporidia?

• Subunit vaccines targeting polar tube proteins.

• Live-attenuated Microsporidia strains with impaired infection capability.

• mRNA-based vaccines designed to prime T cell immunity.

Why are Microsporidia difficult to culture in the lab?

• Strict intracellular dependency prevents growth in standard media.

• Rapid host cell lysis complicates long-term culture.

• Limited genetic tools hinder molecular studies.

What is the significance of tRNA gene loss in Microsporidia genomes?

• Microsporidia have lost many tRNA genes, relying on host tRNAs for protein synthesis.

• Genome reduction has led to codon bias, restricting adaptation to specific hosts.

What is the role of Microsporidia in insect population control?

• Some species (Nosema spp.) infect bees and agricultural pests.

• Used as biocontrol agents to manage insect populations in pest control programs.

How do Microsporidia impact global ecosystems?

• Influence on host population dynamics through chronic infections.

• Potential role in emerging zoonotic diseases.

• Pathogenicity in aquaculture and beekeeping affects food security.