Age Related Decline in Niche Function

Question 1

Examples of SC loss during ageing

Answer 1

Steingrimsson et al, 2005
• Melanocyte stem cells lost during ageing, resulting in hair greying

Anversa et al, 2005
• It was thought until recently that cardiomyocytes were not replaced, and their loss over time impairs the performance of the heart
• But now clusters of undifferentiated cardiac stem cells have been found which can self renew and renew heart muscle
• Increased cell senescence and death has been proposed as a contributing factor of heart failure in the elderly
− In ageing hearts, more cardiac stem cells express the p16INK4a senescence marker

Question 2

How is telomere shortening linked with replicative potential of SCs?

Answer 2

• Linear chromosome replication has a problem → due to 5’ primer requirement, every round of replication leaves a 5’ gap, so every round of replication means chromosomes would get shorter.
• Telomerase attaches telomeric repeats to ends of chromosomes to overcomes this
• However, stem cells are not immortal
• Serial transplants of bone marrow stem cells from donor to recipient mice can only be performed about 4 successive times

Blasco 2003: Telomere length and cell ageing
• Level of telomerase activity determines cell longevity
• High telomerase – germline stem cells
• Insufficient telomerase – somatic stem cells

Premature ageing syndromes are linked to telomere dysfunction
• Werner syndrome
− mutation in gene associated with telomere maintenance
− accelerated ageing effects include cancer, osteoporosis, T2 diabetes, ocular cataracts, athereosclerosis → developing when an individual reaches 20-30
• Ataxia telangiectasia
− results from a mutation in the ATM gene, and in humans results in premature ageing syndromes
− Linked to shortening of telomeres and genome instability due to failure to initiate double strand break repair
− Wong et al, found ability of stem cells to proliferate and renew tissues was impaired

Jaskelioff et al, 2010
Telomerase re-activation reverses tissue degeneration in aged telomerase deficient mice
• Induced a tamoxifen inducible telomerase gene into a mouse mutant line otherwise deficient in telomerase
➢ In the G4 mice with no telomerase → widespread tissue atrophy and premature aging
➢ There is restoration of telomeres in G4 cells when telomerase expression restored
➢ There is recovery from tissue atrophy (brain shrinkage) when telomerase expression is restored
➢ Neurosphere assays of neural stem cells from G4 mice show restored NSC activity by telomerase induction
− Restored capacity for NSC self renewal
− Presence of DNA damage p53 activation
− Restore capacity for NSC differentiation

Question 3

What is the role of tumours, stem cells and ageing?

Answer 3

Role of p53 in cancer and ageing
• p53 is the guardian of the genome → tumour suppressor involved in DNA repair/promotion of apoptosis if damage is too great
• p53 loss of function mutation found in many human cancers
• Constitutively active gain of function mutations in mice:
− suppress tumour formation
reduce life span
− Generate phenotypes in multiple organs with similarity to ageing tissue and failure of tissue renewal – possibly due to senescence of stem cells

Is ageing the price paid for tumour suppression?
• With low p53 activity, there is increased mortality due to cancer
• With high p53 activity, there is increased mortality due to ageing
• There may therefore be an optimum p53 activity

Park et al, 2003
Bmi-1 as an oncogene which suppresses stem cell senescence
• Bmi-1 is a transcriptional regulatory factor that functions by altering chromatin structure and repressing transcription of target genes
• Found to be expressed in HSCs but downregulated in differentiating progeny
• Bmi-1 mutant mice are defective in haemopoeisis and die of bone marrow failure
• Bmi-1 mutant bone marrow cells only transiently repopulate the bone marrow of an irradiated mouse – fail to maintain the stem cell population.

→ so Bmi-1 is expressed in the stem cell and is required for self-renewal of HSCs

• Bmi-1 normally acts in stem cells to repress the transcription of genes involved in inducing apoptosis and senescence.
• Over expression of Bmi-1 will increase stem cell life, however it is oncogenic → associated with generation of lymphomas in cooperation with other oncogenic mutations (results in the repression of the transcription of tumour suppressors p16INK4a and p19ARF)

Question 4

Describe how extrinsic factors are involved in ageing in the drosophila ovary?

Answer 4

Pan et al, 2007:
Extrinsic factors:
• Decline of niche signals (DPP) results in reduced BMP pathway signaling
• Decline of cadherin expression results in reduced anchorage of GSCs to the niche
• Genetic manipulation to upregulate the BMP pathway activation or increase cadherion expression partly reverses age-related decline

Notch
• Low notch – loses the niche
• High notch – increases the niche

Insulin
• Mutants with reduced insulin receptor function show accelerated loss of GSCs and Cap cells with age
• So insulin impacts on the niche both with diet, and with age

Question 5

Describe how extrinsic factors are involve din ageing of muscle stem cells?

Answer 5

Collins et al, 2005
• Satellite cells are self-renewing muscle stem cells responsible for muscle repair, giving rise to differentiated muscle cells
• We have a decline in the ability to repair muscles with age, however the numbers of satellite cells are not significantly different in aged mouse muscles
• We do however, see fewer proliferating satellite derived daughter cells in aged muscle in response to injury → so repair does not go as well with age

This is due to changes in Notch receptor signaling

Conboy and Rando, 2002
The regulation of Notch signaling controls satellite cell activation and cell fate determination in postnatal myogenesis
• Agonists stimulate Notch activity and increase muscle repair in old mice
• Antagonists block Notch activity and muscle repair in young mice

How does this happen
• Generated pairs of mice where their circulatory systems were connected
− Young/young → injured leg muscle regenerates
− Old/old → impaired regeneration
− Young/old → regenerated muscle
• A circulating factor in the blood of young mice maintains muscle repair capacity via maintaining their capacity for Notch signaling

So, ageing reduces Notch signaling to satellite cells:
• Notch signaling is high in young muscle → promotes activation of satellite muscle stem cells
• Notch signaling is low in aged muscle

• Frailty in aged humans is a problem
• Being able to repair your muscles is necessary to help prevent fraily

Is the decline in Notch signaling happening in humans too?

Carlson et al, 2009
• Did an experiment using volunteers – damage the muscle by immobilization – leads to atrophy due to lack of use
• Re-build the muscle with physiotherapy and take a biopsy
• Compared between young and old volunteers

→ Age related increase in muscle atrophy and decreased repair after injury

Question 6

How can we manipulate niche signalling in stem cells for therapeutics?

Answer 6

Androutsellis-Theotokis et al, 2006
• Induced stroke in a region of rat brain, and measure the resultant motor deficit
• Manipulated the stem cell activity in the brain by activating Notch (injected Delta-like 4 into the brain)
• Claim this increases self-renewal of stem cells
• Improved the motor deficit over time – it isn’t as big, and it doesn’t get worse
• Also did with the growth factor FGF – and when they combined the two signals, they got a recovery in the stroke induced motor