Apicomplexan parasites

Question 1

apicomplexa

Answer 1

• ancient - origin 400-800mya
• diverse
• eukaryotes
• obligate intracellular life stage
• likely evolution from colpodellids - single cell photosynthetic symbionts

Question 2

features of apicomplexan cells

Answer 2

• polarised with apical complex
• dense granules, micronemes and rhoptries (secretory)
• apicoplasts
• pellicle
• sub-pellicular microtubules

Question 3

apicoplast

Answer 3

• relic chloroplast-like organelle
• can’t photosynthesise
• fatty acid biosynthesis
• potential pesticide target

Question 4

pellicle

Answer 4

• plasma membrane surrounding double membrane
• double membrane = inner membrane complex
• defines cell movement

Question 5

sub-pellicular microtubules

Answer 5

• highly stable and sheet-like skeleton that defines cell shape
• long polymers of alpha and beta tubulin
• lie under outer compartment
• between 22 and 26 microtubules
• maintain cell surface integrity
• microtubule inhibtors often have no effect (stable)
• shape changes during invasion or division

Question 6

paired apical rhoptries

Answer 6

• sac-like and club-shaped
• protein and lipid
• anterior region (apex)
• variable number, usually 2, changes in life cycle
• vacuole formation during invasion
  
  • no rhoptries - no invasion
• secrete contents upon invasion

Question 7

micronemes

Answer 7

• small secretory organelles
• secrete to extracellular environment or cell surface
• role in motility - abundance in gliding cells
• at apical pole
• adhesive proteins released for motility and attachmnet

Question 8

conoid vs aconoid

Answer 8

• conoid - toxoplasma
  
  • microtubule rich basket like structure at cell tip
  • role unknown
  • when extruded cell moves
  • when withdrawn cell stops
• aconoid - plasmodium

Question 9

lifecyle features

Answer 9

• typically 2 hosts
• one is definitive host

Question 10

definitive host

Answer 10

• organism in which pathogen undergoes sexual reproduction
• putative evolutionary origin

Question 11

toxoplasma lifecycle

Answer 11

• sporozoite oocyst in external environment e.g. soil
• oocyst ingested and bursts
• sporozoites penetrate midgut → tachyzoites
• dissemination throughout body
• bradyzoite cyst formation can occur
• carnivorous ingestion by definitive host of cyst
• cyst rupture
• trophozoite invasion of midgut epithelium → merozoites
• sporogony forming oocysts

Question 12

cryptosporidium lifecycle

Answer 12

• extracellular environmental stage
  
  • highly resistant aquatic spores
• one host stage
• taken in through stomach lining
• sporulation and epithelium invasion
• replication and sexual development

Question 13

apicomplexan innovations

Answer 13

• antigenic variation
• host cell remodelling
• tight junction
• motility

Question 14

host cell remodelling

Answer 14

• plasmodium secretion of proteins into RBC
• alters RBS to provide parasite with what it needs to survive
• requires signal sequence PEXEL

Question 15

PEXEL

Answer 15

• plasmodium export signal
• RxLxE/Q/D sequence
• required for entry into host cell using protein release
  
  • mutation of R to A prevents entry
• cleaved in ER by aspartic acid protease between L and E
  
  • acetylated product recognised and translocated by translocon complex
• translocon sits in vacuole membrane
  
  • unique to plasmodium
• host cell is effective biological extension of parasite
• conserved - similar motifs in other apicomplexa e.g. toxoplasma shuttlign ot host cell nucleus

Question 16

tight junction

Answer 16

• forms at meeting point between host cell membrane and parasite
• transient seal required for invasion
• RON2 and AMA1 (microneme proteins) injected, anchored and interact
  
  • handshake between AMA1 and RON4 pulls parasite in to host cell
• RON 4 - rhoptry neck protein 4
  
  • localised in ring of tight junction

Question 17

role of tight junction

Answer 17

• definite anchor point
  
  • potential molecular sieve
• coat proteins are shed but why?
• prevent bursting of host cell