W8L2 Discovery and fuction of piRNAs Flashcards

1
Q

Piwi mutants

A

Piwi (P-element induced wimpy testis) mutants have defects in gametogenesis and are sterile
Piwi involved in processes underlying germline development – but may also be involved in a pathway involved in hybrid dysgenesis

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2
Q

evidence suggesting that Piwi is involved in regulating TE activity

A

TE activity is elevated in the germline of Piwi mutants – mainly affects retrotransposons but some DNA transposons

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3
Q

Hybrid dysgenesis

A
  • Crossing a WT female with a mutant L male lead to infertile offspring
    -Crossing mutant L female with male lead to fertile offspring
    -This is due to maternal factor present
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4
Q

Piwi genes encode Ago-like protein

A

Two group of Argonaute protein: Ago (animals, plant, fungus) and Piwi Ago (animal germ line specific)
Piwi/Aub/Ago3 expressed in BOTH male/female germline and closely associated cells
Aub also accumulates in pole plasm of the oocyte before fertilization

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5
Q

Small RNAs associated with Piwi proteins

A

Characterize RNAs associated with fly Ago proteins following crosslinking + immunopreciptation (CLIP) procedure:
Non-Piwi Ago (act as control): Ago1 around 21-22nt
Piwi-Ago: Piwi, Ago3, Aub around 24-30nt

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6
Q

diversity Result of small RNA associated with PIWI protein test

A

Incredibly diverse - 1.5 million piRNAs in fly oocytes
Most derived from retrotransposons, transposons and repetitive elements
Called Piwi-interacting RNAs: piRNAs

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7
Q

Mammalian Piwis and piRNAs

A

§ Three Piwis in mice (Mili/Miwi/Miwi2) – mainly expressed in male germline
§ Arrest of spermatogenesis and complete sterility in male mili/miwi2 mice mutants associated with activation of LINE and retrotransposons

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8
Q

Two type of mammalian piwis

A

Pre-pachytene piRNAs are derived from TE (retrotransposons)
Pachytene piRNAs lack sequences matching TEs

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9
Q

Pi-RNA cluster

A

§ 80% piRNAs map to specific genomic loci – bidirectional vs unidirectional fly – pericentromeric/sub-telomeric regions
mouse – small clusters of complex DNA in euchromatic regions
§ Cluster length varies (2-200kb) – 40-4000 piRNAs present
§ Variable piRNA density within cluster
§ Enriched for transposon and other repeats – but inactive remnants
§ Associated with H3K9me3 marks – required for transcription

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10
Q

Production of piRNAs

A

pi-cluster in genome – sense/antisense remnants of TE/repetitive sequences
* piRNA precursor is large, single-stranded and lacks a foldback loop
* No 5’ Cap, polyA tail and no promoter
* Dicer not involved in production of piRNAs
* Processing of piRNA precursor into mature piRNAs occurs in perinuclear RNA granules called nuage or Yb bodies – U-enriched at position 1 and antisense to TE sequences
-processed by Piwi-Argo and exonucleases in nuage, loaded into Aub and Piwi (24-30nt antisense)

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11
Q

piRNAs – mode of action

A
  1. Target retrotransposon/transposon mRNA for endonucleolytic cleavage (slicing) in nuage (post-transcriptional gene silencing) - Aub
  2. Chromatin modification of TEs in nucleus (transcriptional gene silencing) - Piwi
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12
Q

Amplification of piRNAs

A

Amplification cycle increases the abundance of piRNAs in the germline that target active TEs and repetitive elements – ping-pong
§ piRNAs associated with Aub-RISC are antisense to TE sequences and have U at position 1
§ piRNAs associated with Ago3-RISC are mainly identical to sense TE sequences and have a bias for A at position 10
§ 10nt overlap between piRNAs associated with Aub-RISC and Ago3-RISC

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13
Q

piRNA biogenesis – the ping-pong cycle

A

-Aub-containing piRISC binds to complementary TE mRNA
-Slicing inactivates the TE mRNA, Creates a recognition site for Ago3
-Ago3 binds to the cleaved TE transcrip
-Ago3-bound TE transcript is processed into a sense piRNA
-Ago3-containing piRISC binds to the piRNA precursor
-Aub binds to the cleaved piRNA precursor
-Aub-bound piRNA precursor is processed into an antisense piRNA

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14
Q

How does hybrid dysgeneisis work

A

As all RNA present during the early stages of embryogenesis is maternally-derived – look at the piRNAs in embryo vs maternal parent

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15
Q

Characterising maternally inherited piRNAs

A

piRNA profile of F1 embryo and female ovary are identical
Aub-associated piRNAs to I elements are deposited into the developing oocyte of I females

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16
Q

Maternally inherited piRNAs prime defence against TEs

A

Following fertilisation maternally-inherited Aub-piRNAs:
Trigger the ping-pong cycle in the nuage of pole cells of the embryo
Enter the nuclei of pole cells and install H3K9me3 marks on the pi-RNA clusters – enhance transcription of piRNA clusters

17
Q

Key issues facing genomic defense mechanism:

A
  1. Diversity of TE with little similarity at the primary sequence level
  2. TE evolve rapidly
  3. TE can jump species barriers e.g. D melanogaster (1900’s), D. simulans (1990)
18
Q

pi-RNA clusters as TE traps

A

pi-RNA clusters function as TE traps - programmable silencing loci
Acquire new content through innate mobility of TEs
When TEs land in a pi-RNA cluster immunity to that TE is immediately established –adaptive immunity

19
Q

Test of the TE trap model

A
  1. If TE depression occurs – piRNA clusters play an important role in TE control
  2. If TE repression is maintained – piRNA clusters may not be the principal regulators of TE activity
20
Q

piRNA clusters are not required for TE control

A

Delete three most highly expressed germline pi-RNA clusters
No change in fertility
Massive decrease in piRNAs
No change in TE expression

21
Q

TE control is mediated in cis theory

A

For many TEs, primary source for piRNA production is provided by dispersed full-length TE insertions rather than the germline-expressed piRNA clusters
TE regulation may be achieved in cis by piRNAs derived from dispersed elements

22
Q

Invasion dynamics of P-element in D. erecta

A

Introduction of P element into a naïve genome
Maintain flies for 50 generations:
genome sequencing, RNA-seq and small RNA-seqs
Repeated 3 time:
R1/R4 – P element numbers remain constant from gen 20 onwards (siRNA increasing in abundance, piRNA low, afterward piRNA increasing in abundant)
R2 - P element numbers increases exponentially from gen 20 onwards()

23
Q

Crank up model

A
  1. TE insertion in the genome triggers endo-siRNA production
  2. Cleaved TE RNA substrates for piRNA processing (v. small number)
  3. Piwi-associated piRNAs enter the nucleus and convert a TE insertion into a piRNA-producing locus
  4. Endogenously produced piRNAs amplified by the ping-pong cycle
  5. Control of TEs established