Lecture 5: Parasite glycobiology Flashcards
True or false: trypanosomatids all have a kinetoplast?
True
Which trypanosoma species is Leishmania clade closely related to?
Trypanosoma cruzi and Trypanosoma brucei
What disease is caused by T. brucei?
Human African Trypanosomiasis (HAT) or Sleeping sickness
What is the vector for the following parasites:
1) T. brucei
2) T. cruzi
3) Leishmania sp.
1) Tsetse fly
2) Triatomine bug
3) Sandfly
Which form of the parasite is taken up by the Tsetse fly and which form of the parasite is injected by the Tsetse fly into the human?
Taken up: stumpy form that rapidly differentiates to procyclic form
Injects: metacyclic form
True or false: T. brucei is strictly extracellular?
True
True or false: Leishmania sp. is strictly extracellular?
False, they are intracellular (phagocytosed by macrophages and proliferate within parasitophorous vacuole as amastigotes)
How does the triatomine bug transmit the T.cruzi parasite to the human host?
Bites around eyes and mouth (soft tissue parts), defecates parasite close to wound site, which is irritated and the parasite is rubbed into the wound
True or false: T. cruzi is strictly extracellular?
False: it is intracellular - cross epithelial barrier so can enter tissues (mainly smooth muscles particularly of the heart, skeleton and oesophagus)
What is the name of the disease caused by T. cruzi?
Chagas disease
what does the surface coat of the T. brucei parasite contain in the human host?
mainly VSGs
what does the surface coat of the Leishmania parasite contain in the human host?
LPG
Surface proteases (PSP)
GIPLs
what does the surface coat of the T. cruzi parasite contain in the human host?
Mucins
Trans- sialidases
GIPLs
What are the T. brucei glycoconjugates and what is the structure of each?
VSG = homodimer, each monomer is N-glycoslyated and GPI anchored to the plasma membrane
Transferrin receptor = heterodimer, both monomers are N-glycosylated but only one monomer is GPI anchored to the plasma membrane
True or false: the T. brucei transferrin receptor (TfR) is structurally related to VSG?
True, expressed form the same expression site (ES) by RNA pol1
What is the role of the T. brucei TfR?
heterodimer that recognises and internalises host transferrin
What is the role of VSG and TfR glycosylation in T. brucei?
shield the plasma membrane from complement and prevent exposure of conserved GPI anchor
What glycoproteins are expressed in the surface of the T.brucei parasite in the Tsetse fly mid-gut?
Procyclins (Aka procyclic acidic repetitive proteins (PARPs)
Describe the structure of procyclins in T brucei
GPI-anchor that is heavily modified by addition of long glycan chain that masks the surface
Contains a single N-glycosylation site for GlcNAc2Man5 glycan
- pack together to form a dense surface coat.
- multiple procyclin isoforms that are classified as either EP (containing EP repeats) or GPEET (containing GPEEpT repeats) procyclins.
How does the expression of procyclin isoforms differ during tsetse transmission?
Developmentally expressed
- GPEET expressed in early infection
- EP expressed in late infection
How do procyclins promote survival of parasite in tsetse fly midgut?
N-terminal domain is cleaved by fly proteases and the remaining glycoconjugate is protease-resistant and provides a tough coat against gut hydrolases
How does procyclin expression allow the T. brucei parasite to be injected into the host?
Once in midgut, the parasite needs to migrate across the peritrophic matrix into the salivary glands to be injected back out
- procyclin-null mutants are not as able to escape from the mid-hut into the salivary glands
Describe the structure and function of the Leishmania LPG?
GPI-anchored phosphoglycan that dominates the cell surface of the Leishmania parasite
- GPI-anchor (conserved)
- glycan core (conserved)
- repeat unit made of mannose and phosphorylated galactose which may or may not be bound by different sugars (hexoses or pentoses) - variation
- cap (variation)
Function:
- required during sandfly transmission
- prevent lysosome-phagosome fusion during metacyclic-to-amastigote differentiation
Describe the structure and function of the Leishmania PSP?
GPI-anchored glycoprotein and is a protease widely known as GP63
important for the phagocytosis of metacyclic Leishmania by host macrophages - part of invasion process
What are GIPLs on the Leishmania parasite surface and what is their role?
Small glycoinositolphospholipids
have role in protection for the otherwise exposed plasma membrane
Where does variation in the Leishmania LPG phosphoglycan arise?
variation in glycosylation pattern and length of the repeat unit between species, lifecycle stages and strains
What is the function of Leishmania LPG in the promastigote stage?
required for attachment to mid-gut epithelium in the sandfly, which is mediated by the insect lectin (galectin) as the receptor for the LPG ligand (recognises surface galactose)
- if attachment does not occur, parasite would be excreted
How does the Leishmania LPG structure change as they differentiate form the procyclic to the metacyclic promastigote form in the sandfly midgut?
- cap the galactose residues with different sugars so they are no longer able to attach to the galectin in the sandfly midgut
–> this allows them to be injected with the blood meal into the human host
The difference in Leishmania LPG strucute is thought to define what in terms of sand fly species and the human host?
Thought to define:
- the interaction of Leishmania species with different sandflies
- the interaction and severity of disease within the human host
What is Leishmania PSG?
Promastigote secretory gel - a mix of extracellular glycoconjugates with a mucin-like filamentous proteophosphoglycan (fPPG) as a major component
- secreted in the anterior mid-gut to form a gel-like substance within which infectious metacyclic promastigotes accumulate
Which form is the Leishmania parasite in when it is uptaken by the sandfly?
Amastigote (then differentiates to procyclic promastigote that attached to mid-gut, then differentiates to metacyclic promastigote that allows it injection again into the mammalian host)
What is the major component of Leishmania PSG?
filamentous proteophosphoglycan (fPPG)
What is the function and role of Leishmania PSG?
secreted in anterior mid-gut by metacyclic promastigotes which forms a plug that blocks flies from feeding effectively
–> alters the feeding behaviour of the sandfly - results in continued re-probing of the host as it cannot take a full blood meal so thinks it hasn’t been able to inject into somewhere where it can feed
- eventually regurgitates the PSG plug containing metacyclic promastigotes in order to increase feeding
–> result in increased potential for parasite transmission
What role does Leishmania PSG play in infectivity within the mammalian host?
role in the infectivity within the mammalian host
- inoculation of mice with parasite with addition of PSG results in increased size of lesion and infectivity
- glycan moieties on glycosylated fPPG is sufficient for lesion exacerbation
What component of the fPPG is responsible for increased lesion size and infectivity in mammalian host?
the glycan moieties on glycosylated fPPG (but not eh peptide backbone of fPPG)
What is the major component of the T. cruzi surface coat and what is their structure?
Mucins
- glycoproteins rich in Ser/Thr with multiple O-linked glycans
Explain the expression of mucins in T. cruzi
Mucins are encoded by an expanded multi-gene family formed of 800 genes
- the different classes of mucins are developmentally regulated (different classes expressed in the epimastigotes and mammalian forms)
True or false: Trans-sialidases in T. cruzi are encoded by an expanded multi-gene family?
True
What are the three main processes in which T. cruzi mucins are involved in?
- Protection
- Attachment
- Host cell invasion
How are T. cruzi mucins modified?
Addition of host sialic acid by the parasite derived trans-sialidase bound to cell surface
- trans-sialidase transfers sialic acid from host serum sialoglycoconjugates to the mucins on their surface
What is the purpose of sialic acid modification of T. cruzi mucins?
sialic acid is quite specific to mammalian cells so addition of sialic acid to mucins allows evasion of immune system and recognition by host immune system as ‘self’
What are the three different trans-sialidases of T.cruzi and how do they differ?
aTS = active trans-sialidases
iTS = inactive trans-sialidases
soluble aTS
aTS transfers sialic acid from host serum sialoglycoconjugates
iTS recognises and binds sialic acid but does not transfer it - this is part of the recognition of host cells for invasion process to occur
soluble aTS is used to cause confusion - transfer sialic acid from host serum glycoproteins and remodel the host glycophenotype to cause immune response to self cells - can not distinguish between normal glycophenotype and the parasite. - escape detection from immune system whilst it becomes intracellular
