Interaction Between Niche Intrinsic and Extrinsic Signals Flashcards
Summarise how factors in the drosophila ovary niche determine cell fate?
- DPP is a diffusible factor secreted by the niche, promoting GSC renewal and survival
- Stem cells are retained by adherans junction, keeping it in the niche to receive signals
- The adherans junction also orients cell division, so that cystoblasts are pushed out away from strong DPP signaling
- This is amplified by the fact collagen restricts DPP diffusion further afield
- Cystoblast moving away is receiving less DPP signal → allows it to turn on Bam
- Bam represses DPP signal, allowing more Bam to be switched on
- When Bam is switched on, it represses Pumilio
- Pumilio blocks translation of differentiation factors in GSC by binding to the 3’ region of mRNA – in the cystoblasts, Bam is repressing Pumilio so you get translation of the mRNA of the differentiation factors.
An extra mechanism for Bam has been published:
The ‘competetive cell fate switch’
• Revolves around the COP9 complex
− Multi-subunit complex involved in regulating ubiquitination and protein turnover of certain targets.
− When Csn4 is present in the complex → the complex functions to promote self-renewal, independent of DPP
− In the cystoblast, Csn4 is sequestered by Bam → this switches the substrates of the COP9 complex, and so differentiation is promoted.
➢ Bam in complex with Csn4 promotes differentiation
➢ Bam in complex with Bgcn promotes differentiation (this represses pumilio)
➢ Both of these complexes allow COP9 to function as a pro-differentiation factor.
How does the drosophila ovary niche signals control cell proliferation?
How do SCs maintain proliferation in the niche when niche cells aren’t cycling, even though they are exposed to similar signals?
• Suggests a distinct mechanism of cell-cycle regulation
Are there mutations which affect GSC division but not survival?
Hatfield et al, 2005
• Dicer-1 mutant GSCs show reduced ability to proliferate, but are not lost from the niche
− Dicer-1 mutants are anchored normally, have a spectrosome and respond to DPP signaling
− However, Dicer-1 GSCs are arrested at the G1-S checkpoint of the cell cycle
− Dicer-1 GSCs have high staining for cyclin E
− Cyclin E normally has to be degraded for progression to occur → P21 kianse does this
− An inhibitor of P21 kinase, Dacapo, prevents this happening
− Dicer is required to generate miRNA and siRNA by cleavage of precursor RNAs
− Dicer-1 mutants aren’t making the miRNA
Model:
• Niche cells are held in cell cycle arrest by a signal that requires the p21/Dacapo complex inhibiting cyclin E
• GSCs normally express an miRNA which blocks Dacapo expression, a negative regulator of the cell cycle, overcoming the break on proliferation.
• Dicer-1 mutants cant express this miRNA - Dacapo isn’t blocked, so cell cycle is halted
How do signals in other niches regulate cell fate?
The GSC Niche in C.Elegans
Notch Signalling:
• Delta is expressed by distal tip cells
• Delta binding to Notch results in proteolytic cleavage of Notch.
• The intracellular domain moves to the nucleus, and activates the TF Suppressor of Hairless
• As the daughter products are pushed away from the niche, they are not in contact with the distal tip, so cant receive the Notch ligand → they will differentiate
FBF:
• The pumilio-like protein FBF is required for GSC self-renewal
• FBF blocks translation of a downstream regulator, Gld-1 (Gld-1 promotes differentiation)
• FBF-dependent Gld-1 repression is regulated by Delta/Notch
→ Notch signaling promotes FBF function to downstream Gld-1
→ As the stem cell divides, one of its daughters is pushed away. IT loses Notch signaling, FBF is down-regulated, and Gld-1 switched on.
The GSC Niche in Mouse Testis
• If you have a mutant of the Pumilio homologue Pum2, you get degenerating seminiferous tubules
• Pum2 contributes to mouse GSC maintenance, but it is not essential
• Functional analysis is complicated by the presence of Pum-1 – a gene duplication which may lead to functional overlap between Pum1 and Pum2
The HSC Niche in the Mouse
• Pumilio-like genes are expressed in HSCs (controlled by Notch) but not analysis of function has been published.
Drosophila Intestinal Stem Cell Niche
• Proliferation and maintenance of Drosophila intestinal stem cells is regulated by Wingless (Wnt) diffusing across the ECM from adjacent circular muscle cells
How do these stem cells undergo asymmetric cell division?
• Delta in the stem cell signals to the daughter cell to promote differentiation
• When they divide, they divide in such a way that one of the daughter cells receives Notch signaling, pushing it to become an enteroblast.
Notch also has other roles
• Enteroblasts can become either an enteroendocrine cell, or an enterocyte
• Low notch → enteroendocrine
• High notch → enterocyte
Mammalian Skin
• Dermal papilla seems to be a key niche component and a source of signals that stimulate the activity of matrix stem cells
• Hair follicles do not develop, persist or function without a papilla
• FGF-7 is produced by the papilla and stimulates keratinocyte growth
• Disruption of a-catenin leads to hyperproliferation
• Shh is required in developing hair follicles
• Wnt signaling also required to form hair follicles
Mammalian Intestine
• Several cell types have been ablated to see if they contribute to niche signals
• Paneth cells reside near the stem cells in the basal region of the crypt → secrete antimicrobial peptides. Ablation doesn’t change cell proliferation
• Targeted ablation of secretin producing cells has no effect
• Signals from mesencymal cells thought to regulate → Tcf-4 mediated signals needed
Do systemic signals regulate stem cell behaviour in the niche?
Nutritional Response of Egg Production in Drosophila
• Flies fed on a rich diet will lay a lot of eggs per day
• If you switch them to poor food, within 24 hours, their egg laying capacity wil reduce from around 100 to almost 0. Their ovaries also shrink
Lafaver et al, 2005:
• GSCs mutant for the insulin receptor show decreased division rates compared to wild-type
− Ablated neural cells required for insulin expression in fly brain by over expression a cell death inducing gene specifically in brain cell insulin producing cells
− Resulted in decreased egg production, decreased GSCs and follicle division rates
− The signal acts via the insulin receptor on GSCs, which then signal by an unknown mechanism to regulate somatic follicle cells.
• Insulin also acts on the Cap cell niche by promoting Notch signaling and cadherin expression
• So diet (through insulin) controls both SC proliferation and the size of the niche
Bonfini et al, 2015
Dietary effects on the niche via Notch signaling are reversible
• Temperature sensitive mutant disrupted Notch signaling → decline in Cap cells
• If you shift them back to correct temp and restore Notch signaling → Cap cells recover
• However, there were no apoptotic markers to account for the loss of the Cap niche cells, we we know niche cells don’t proliferate, so this doesn’t explain their increase in number
− Leaves us with the possibility that niche cells trans-differentiate
− Lose their expression of Cap specific markers and ability to act as niche cells
− Flanking cells then become Cap cells to restore niche upon nutrient restoration
What seems to be common properties between all niches?
• Many niches use specialized cell groups, such as hub, dermal papilla and cap cells
• A principle signal received by the stem cells in the niche often controls their behavior and many originate from the specialized cells
− eg) Unpaired in drosophila testis
− eg) DPP in drosophila ovary
− eg) Hedgehog in drosophila epithelial cell
• A basement membrane is part of most niches discussed
• ECM may help structre niches spatially and modulate concentration of signaling molecules
How do niches change regulation during development?
• Niches modulate their numbers during development and in response to environmental factors
• Normally, hair follicle number doesn’t increase after birth
• Crypts duplicate by branching, possibly when number of SCs passes a threshold
• Certain signals can induce de novo procuction of niches
− New hair follicles form in adult skin when Wnt is upregulated
− Excess hedgehog signaling in drosophila ovary increases size of SSC niche
• Niches also modify properties in response to changing conditions
− Bulge SCs proliferate in response to wounding
− After irradiation of the testis, remaining stem cells induced to divide equally to regenerate stem cells and spermatogonia
