Giardia AV Flashcards
What phylum does Giardia belong to?
Metamonida
Is giardia intracellular or extracellular?
Extracellular
Giardia colonises?
The small intestine
Giardia was first observed by?
Antoni Van Leeuwenhoek who identified the trophozoites in his faeces
Giardia causes?
Giardiasis
Giardiasis can be caused by?
Contaminated drinking water, consuming cysts
Giardia is spread via which route?
The fecal oral route
What are the two forms of Giardia?
Trophozoites and cysts
Excystation occurs?
Due to the low pH of the stomach
Encystation occurs?
In the colon
Which form of Giardia can survive for months in cold water?
The cyst form
What is released during excystation?
Two trophozoites
Describe the lifecycle?
1) Cysts are consumed
2) Excystation occurs in response to the low pH of the stomach
3) Trophozoites attach to the small intestine via an adhesive disc and replicate via binary fission
4) Encystation occurs in the colon
5) Cysts are released in the faeces
Can trophozoites survive in the environment?
No
How do trophozoites replicate?
Via binary fission
What are the features of trophozoites?
- Not environmentally hardy
- Contain two transcriptionally active nuclei
- Have 8 flagella in 4 pairs
- Replicate via binary fission
- Look like smiley faces
- Extracellular parasites
- Pear shaped
- No obvious mitochondria or golgi bodies
- Metabolically active
How do trophozoites attach to the small intestine?
Via an adhesive disk
How do trophozoites replicate?
Binary fission
How many nuclei are present in trophozoites?
2 transcriptionally active nuclei
What are the features of cysts?
- Cysts are present in the faeces
- Cysts are environmentally hardy
- Cysts are able to survive in cold water in the environment for months
- Cysts each give rise to two trophozoites
- Cysts contain 4 nuclei and 4 axonemes due to duplication of organelles without cell division
- Not metabolically active
What shape are trophozoites?
Pear shaped
Why was it thought that giardia was a primitive eukaryote?
Appear to be amitochondrion- appear to lack a mitochondria. Thought that giardia was a primitive eukaryote which branched off before the acquisition of the mitochondrion
Thought to be a molecular fossil
However, now we know that giardia lacks the mitochondrial structures due to?
Reductive evolution
What has reductive evolution resulted in?
Has resulted in the mitosomes
The mitosomes are mitochondrion derived
What occurs in the mitosomes?
Iron sulfur cluster synthesis
What occurs in mitochondria?
There is ATP synthesis
Where is ATP made in giardia?
Cytosol
What are mitosomes?
Double membrane bound mitochondrial relics
Formed via reductive evolution
Site of iron-sulfur cluster synthesis
How are the mitosomes different to mitochondria?
Do not contain genes within them
Mitosomes do not contain DNA
Any mitosomal proteins are trafficked there
How are mitosomes similar to mitochondria?
Double membrane bound
Associated with ATP synthesis
The mitochondria degenerated into the mitosome through a process known as?
Reductive evolution
Antigen variation of what occurs in Giardia?
VSP
What is VSP?
Variant Specific Surface Protein
VSP is known as?
Variant Specific Surface Protein
Expression of VSP is?
Monoallelic
How many VSP variants are there?
~200
Are VSP genes telomeric?
Not necessarily
How is monoallelic expression of VSPs achieved?
RNAi
RNAi general process?
All VSP genes are transcriptionally active
Transcripts of all the VSP genes are continually being produced
There is silencing of all VSP transcript variants aside from one
The transcript that is not silenced is the variant of VSP that will be expressed
How is it decided which transcript will not be silenced?
Unknown
Thought it is due to the abundance of the transcripts
There will be certain transcripts of a VSP variant which are more abundant and may be able to escape RdRP and silencing= this is the transcript that is expressed
What is the specific process of RNAi silencing?
mRNA of the VSP
Antisense RNA is produced by RdRP
The dsRNA/ RNA duplex is recognised by Dicer
Cleavage into siRNA duplex by dicer
siRNA is recognised by RISC and Argonaute complex
Directed to mRNA that is complementary, silencing via the RISC complex and mRNA degradation
How many VSP genes are there?
~200
What is the VSP structure?
Variable N terminus
Semi conserved C terminus
Transmembrane domain
Invariant cytoplasmic tail
VSP is expressed in which fashion?
VSP is expressed in a monoallelic fashion
Evidence for RNAi being the regulator of monoallelic expression?
If you disrupt the RNAi pathway you notice that more VSPs are expressed, expression is no longer monoallelic
How does switching occur?
Change in the levels of VSP transcript abundance
Why might switching occur in daughter cells?
May be possible that upon cell division the daughter cels receive varying abundances of VSP transcripts or siRNAs which may cause a switch to a different VSP variation.
How does Giardia mediate attachment?
Trophozoites attach to the small intestine via the adhesive disc
Giardiasis does not usually need to be treated as it is?
Self-limiting
How is the mitosome similar to mitochondria?
Double membrane bound
Involved in ATP synthesis
Contains mitochondrial proteins
How does the mitosome differ to mitochondria?
Site of iron sulfur cluster synthesis but ATP synthesis occurs in the cytosol
Does not contain any DNA
All the proteins must be trafficked to the mitosome
Giardia was thought to be?
A primitive eukaryote
A molecular fossil
Thought it was an amitochondriate which diverged before the acquisition of the mitochondrion
How many VSP genes are there?
~200
Are VSPs telomeric?
Not necessarily
Does VSP switching require DNA rearrangements?
No
Control of VSP is?
Post-transcriptional
VSP structure?
Variable N terminus
Semi-conserved C terminus
Transmembrane domain
Invariant cytoplasmic tail
How can you alter the monoallelic expression and get multiple VSPs to be expressed?
Disrupt RNAi e.g. knockout gene encoding dicer
How might division into daughter cells mediate vsp switch?
During daughter cell production there may be variable amounts of vsp transcripts or vsp siRNAs delivered to the daughter cells. This may result in a switch to the expression of a different vsp variant e.g. in the daughter cells a different vsp transcript may be more abundant and escape silencing
