stem cells and ageing

Question 1

what is the stem cell theory of ageing?

Answer 1

that a decline in the new and functionality stem cells pools of renewable tissues contributes to ageing

Question 2

why is the hierarchy of the stem cell progenitor system good?

Answer 2

the stem cell only divides once to produce a daughter cell- this then undergoes consequitive stages of differentiation and proliferation. This protects the stem cell from having to undergo many divisions that could induce mutations

Question 3

what is the point of stem cells going into a quiescent state?

Answer 3

they are less metabolically active- this means they are protected from the damage caused b y metabolic side products such as ROS

Question 4

how can stem cell pool decrease in functionality but not number?

Answer 4

they may accumulate damage that only activates tumour suppressor pathways when the cell divides to produce a daughter cell- this results in the progenitor undergoing sen or apoptosis but not the stem cell as this may maintain the intact DNA or not activate the tumour suppressor

Question 5

why is it hard to determine if stem cell pools decline with age?

Answer 5

stem cells can be heterogeneous and there are not universal markers for stem cells or even subsets of stem ells such as HSCs

Question 6

which stem cells are the best to observe whether stem cell number declines with age and why?

Answer 6

HSC- it is possible purify to neat homogeneity and assay their function using validated assay

Question 7

what are the ageing signs of the blood organ?

Answer 7

• decreased immunity, increased incidence of bone marrow failure and haematological neoplasia and moderate anaemia - reduced marrow regenerative capacity

Question 8

what is a particularly telling example of HSCs reducing in functionality with age?

Answer 8

• age of stem cell donor in a transplant is a predictor of transplant-related mortality suggesting that the diminished reconstituting ability go HSCs from elderly donors is partly cell autonomous

Question 9

what has been a surprising finding in rodents about stem cell pool size in elation to age?

Answer 9

the number of stem cells in the pool does not decrease but the function clearly does and sometimes the number actually expands

Question 10

how can the observation that HSC can stay the same or even increase with age in mice be explained?

Answer 10

• they may increase in number due to the fact they become functionally defective and can’t asymmetrically divide
• sen and apoptosis mechanisms are no engaged until old HSCs attempt to divide asymmetrically- this means that the progenitor number will be effected by not the HSC
• the mice in the lab for these lines are not exposed to sufficient stress in the lab to mirror normal ageing
• ageing might be caused by cell intrinsic functioning such as lineage bia- this is seen in HSCs which are bias to myeloid lineages not lymphoid

Question 11

what is the lineage change that occurs in ageing HSCs?

Answer 11

they become bias towards the myeloid not lymphoid lineages

Question 12

what tissues suffer from age related functional decline in their stem cell pools? (4)

Answer 12

HSC, neural stem cells, pancreatic islet, melanocyte

Question 13

what has the decline in the neural stem cell pool been linked to? (2)

Answer 13

parkinsons and impairment of olfactory discrimination

Question 14

what has hair greying been linked to

Answer 14

melanocyte stem cell depletion

Question 15

how has diabetes type 2 been linked to exhaustion of stem cell pools?

Answer 15

• redcued b-cell mass has been found among adults with diabetes
• islet replication appears to decline with human ageing
• diabetes type II may arise from failure of islet replications and of increased in insulin resistants

Question 16

describe an experiment which demonstrated that the niche that a muscle stem cell is in can affect it. and the same for HSCs

Answer 16

• satellite cells from aged mice were rejuvenated by exposure to a young blood supply
• HSCs from a young mouse was transplanted into a mouse that was older with shorter telomere- this animal was less able to support lymphopoieses than younger mice with longer telomeres

Question 17

what generally causes SC to age?

Answer 17

• mitochondria
• DNA damage (telomere or non telomere)
• ROS production
• proteome homeostasis
• environment - the niche that it is in
• tumour suppressors

Question 18

how can tumour suppressors cause senescence and defer it?

Answer 18

• assisted cycling involved thee extension of the cell cycle which allows more time for DNA repair- this can be mediated by p53. The difference between assisted cycling and sen is unknown ut likely to be tissues and stressor dependent .

Question 19

describe the evidence for increased DNA damage causing a functional decline over time and hence ageing, in stem cell pools.. (2)

Answer 19

• HSCs expressing alterations in the proteins involved in the DNA response- suffer from age induced decline in HSC number and function ( so DNA damage can alter stem cell functioning )
• there is an accumulation go H2AX foci (that form when there is DNA damage) in the DNA

Question 20

where is the DNA damage in stem cells thought to come from/ arise from? (2)

Answer 20

• from proliferation over time

- from ROS increase

Question 21

what is the evidence showing that increased proliferation in stem cell pools can cause functional defect in the pool and eventual exhaustion?

Answer 21

• ATR mutant is toxic to proliferating cells . this was expressed in a mouse. But after a few weeks, some of the cells which had not recombined with the Atr allele did not die and so survived and these were able to reconstitute and the mouse was normal. but then these mice developer progeroid phenotypes a few months later- this suggests that the increase proliferation but string on the stem cells and made them age- this may be due to increase DNA replication which can induce damage or from the production of toxic metabolites

Question 22

what is the DNA-damage accrual model of ageing ?

Answer 22

• this is the theory that over time unprepared or improperly repaired demonic damage accumulates in stem cell compartments. at some point accumulated damage can result in a few possible fates: transformation, senesce, apoptosis or dysfunction (dysfunction would be the loss of ability to robustly produce progeny or an impaired potential for multi-lineage differentiation, myelodysplasia). As this proceeds over time, depleted and or dysfunctional stem-cell compartments canot match the regenerative needs of a given organ and homeostatic failure ensues. Likewise, if oncogenic DNA-damage-induced lesions accumulate, self-renew- ing clones that contain such lesions undergo positive selection, leading to cancer. Therefore, it is tempting to speculate that cancer and ageing are related endpoints of accumulating DNA damage within self-renewing compartments.

Question 23

what is the different between human telomeres and mice telomeres?

Answer 23

• murine telomeres are very long and most cells express telomerase

Question 24

describe an experiment which looked suggested that telomere induced apoptosis/ senescence can cause ageing in stem cells

Answer 24

• telomere shortening activates sen or apoptosis
• in mice that have short telomeres- there are many extreme phenotypic changes but when you suppress p53 and therefore suppress the sen or pop pathway, the stem cell phenotypes are rescued- suggesting that telomere attrition and the apop or sen that it induces can damage stem cells pools by altering their proliferative ability maybe

Question 25

how does CR affect stem cells?

Answer 25

enhances stem cell function and decreases p16- may slow ageing in stem cells by preventing the sen response

Question 26

how have ROS been shown to affect a specific stem cell type?

Answer 26

increase ROS levels has been shown to comprimise HSC function

Question 27

what have studies on FOXO transcription factors in mice revealed?

Answer 27

• FOXO transcription factors were shown o promote stem cell longevity by decreasing levels of ROS

Question 28

How was sen pathways linked to the increased longevity of three stem cell types and what were the tissues?

Answer 28

• p16 deficiency in pancreatic islets, neural stem cells and HSCs resulted in attenuated age-induced decline in proliferation and function and over expression of p16 attenuated HSC function and islet proliferation in an age dependent manner. This suggests that p16 limits replicative function by inducing senescence in response to relevant cues or by decreasing cell cycle entry

Question 29

what occurred HSCs from p21 defiance time and how can this be explained ?

Answer 29

stem cells underwent premature exhaustion- this suggests that cycle assistance mediated by p53/p21 is required for DNA damage repair and stem cell longevity

Question 30

how is p16 generally thought to cause stem cell ageing? how is this cell autonomous view supported?

Answer 30

• it increases in cells with time which means that it can increase sen stimulation. this is supported by the fact that p16 deficient HSCs have an enhanced function even when transplanted into p16 positive mice

Question 31

why is the role of ROS in stem cell sen tricky to analyse?

Answer 31

because ROS are involved in signalling pathways such as the quiescent to proliferation pathway so determining heher ROS directly or indirectly cause stem cell ageing is hard

Question 32

describe the evidence in support of the hypothesis that ROS generation may promote stem cell ageing (4)

Answer 32

• studies in aged human mesenchymal stem cells have found elevated ROS
• the frequency of blood-forming hematopoietic stem cells with low ROS levels decline with time in mice
• in mouse HSCs and NSCs excessive cellular ROS concentrations lead to abnormal proliferatio, malignancy and compromised stem cell self renewal
• mice that lack genes encoding SOD in HSCS showed increased oxidative stress. SOD is downstream of FOXO3
• age dependent alterations to SOD activity has been show to contribute to loss of HSC function

Question 33

describe experiments which looked at the role of FOXO transcription factors in (2)

Answer 33

• conditional abldation in the mouse hematopoeitc system of the transcription factor FOXO1, FOXO3 and FOXO4, increases makred increase in ROS. This disrupts HSC quiescence (proliferation is stimulated by high ROS), increases apoptosis, and causes defects in HSC repopulating abilities
• similar results were seen for NSCs lacking FOXOs TFs in mice
• FOXOs are implicated in inhibiting ROS which means that they can stimulate quiescence, prevent apoptosis and facilitate self renewal which is inhibited by ROS

Question 34

what is thought to be a role of the IIS pathway in stem cell ageing?

Answer 34

• IIS inhibits FOXO transcription factors which has been shown to be able to promote stem, cell longevity- so maybe the IIS pathway causes ageing by preventing the FOXO transcription factors from metabolising ROS.

Question 35

what transcription factor os FOXO3 downstream of?

Answer 35

FOXO3

Question 36

how have SIRT genes been implicated in ROS related stem cell ageing

Answer 36

SIRT3 controls ROS regulation in cells . ectopic induction of SIRT3 has been shown to improve function go aged HSCs by increasing the activity of SOD2

Question 37

what can be applied to target ROS production to prevent stem cell ageing ?

Answer 37

n-acetyl-L-cysteine (NAC)

Question 38

how has NAC been used to improve the damaging effects of ROS on stem cel ageing??

Answer 38

ROS levels increase in HSCs with age, and prolonged treatment with the antioxidant N-acetyl-L-cysteine increases the replicative potential of HSCs upon serial transplantation in irradiated mice
- in FOXO deficient mice, NAC rescues the repopulating abilities of HSC cells

Question 39

how can increased DNA damage with age be viewed in stem cells?

Answer 39

HSCs and muscle stem cells show an increased number of nuclear foci which serve as a marker of double strand breaks with age

Question 40

what are the ways in which DNA damage can accumulate in cells over time?

Answer 40

an accumulation of damage over time, an increase in the rat of dmane, a decrease in the rate of repaor in response to DNA damage or all three

Question 41

what is evidence for DNA damage accumulation occurring in aged stem cells?

Answer 41

• human HSC cells show compromised capacity to repair experimentally introduced DNA damage, such as that produced by ionising radiation
• naturally occurring mutations that disrupt the ability of cels to detect, respond and repair to DNA damage frequently result in progressed phenotypes- suggesting that increased DNA damage causes stem cell ageing in some way

Question 42

what is an alternative view as to how DNA damage can cause cell ageing in stem cells in the absence of a reduced ability of the machinery to identify dn respond to damage?

Answer 42

that DNA damage is able to accumulate in quiescent cells and is only repaired in HSCs when they try to proliferate- this suggests that if the accumulation was too great, the cell wouldn’t be able to repair and the stem cell function would be impaired in old stem cells in old animals- becausee quiescent cells can only repair using error prone non homologues end joing- damage could be great in quiescent cells

Question 43

why can old quiescent cells gain a large about of damage?

Answer 43

they are exposed to toxins etc but can only repair by using non homologues end joing- this means this can induce more mutations

Question 44

what is protein homeostasis?

Answer 44

the cellular processes responsible for the synthesis, folding and turnover of proteins.

Question 45

what is proteostasis important for?

Answer 45

DNA replication, catalysis of metabolic reactions etc etc

Question 46

what does the process of proteostasis involve?

Answer 46

• chaperones and folding enzymes, pathways that degrade proteins such as proteasome, lysosomes and autophagy pathways

Question 47

how is protein misfloding linked to ageing?

Answer 47

as proteins becomes damaged by ROS or mutation, protein misfiling can occur which can lead to diseases such as alzheimers or parkinsons. aged cells may be more prone to form and accumulate misfolded protein

Question 48

how is increased protein misfolding etc thought to occur in aged stem cells- by what processes?

Answer 48

reported age-dependent deficits in the activities of both the autophagy-lysosomal and the ubiquitin-proteasome systems, which represent the principal proteolytic systems implicated in protein quality control. Yet other studies have challenged the view that proteolytic activity declines with age, showing instead that autophagic potential or proteasome activity remains intact in aged cells and attributing age-related disruptions in proteostasis mainly to increasing damage caused by metabolic stress that overwhelms the protective capacity of proteolytic systems.

Question 49

who has autophagy been implicated in ageing and proteostasis?

Answer 49

As regards stem cell function, proteostasis has been implicated as an important determinant of stem cell maintenance through studies in HSCs showing that deletion of autophagy-related gene 7 (Atg7) increases ROS levels and depletes HSCs. This notion has been further supported by Warr et al., who reported autophagy induction by FoxO3A to be an essential mechanism that protects HSCs from metabolic stress in aging76. Also, mTOR, a potent activator of protein translation and inhibitor of autophagy, has been shown to regulate the self-renewal and differentiation of HSCs and intestinal stem cells

Question 50

how has mitochondrial function between implicated in stem cell ageing?

Answer 50

• age-dependent reductions in mitochondrial function lead to respiratory chain dysfunction has been observed in cell systems and is thought to occur from mutations in mDNA which can occur via the exposure to high ROS levels or by replication machinery.. Supporting this view, a genetically modified mtDNA mutator mouse, harboring a proofreading-defective mtDNA polymerase g, displays severe respiratory chain deficiency and premature aging pheno- types. Furthermore, hematopoietic progenitor cells of the mtDNA mutator mice are affected during fetal development, and their neural stem cells exhibit declining quiescence and decreased self-renewal capacity in response to mtDNA mutation accumulation.However, in adult mouse HSCs, rapidly accumulating mtDNA mutations induced by the presence of proofreading-defective mtDNA polymerase have little functional effect on the HSC pool and instead cause differentiation blocks and/or disappearance of downstream progenitors

Question 51

describe how increased ROS could upset the profile quiescent balance and contribute to ageing

Answer 51

Changes in intracellular levels of ROS may also determine the balance between quiescence and proliferation of HSCs, and perhaps other adult stem cells. Low levels of ROS are required for quiescence and stem cell maintenance, whereas ROS induction leads to proliferation and differen- tiation programs26. When combined with the prevailing notion that ROS increases with aging, this result provides a possible explanation for how stem cells lose quiescence and are depleted in aged tissuesThe balance between quiescence and proliferation in aged stem cells might also be disrupted by an increased demand for replacement of mature cells in aged tissues, as suggested by observations in disease mod- els such as Duchenne muscular dystrophy (DMD). Repetitive damage, caused by continual degeneration of structurally unsound DMD fibers, elicits constant demand for regeneration, diminishing the regenerative capacity of muscle stem cells as a consequence of replicative aging13

Question 52

what disease demonstrates how over proliferation o a stem cell pool can induce exhaustion and death

Answer 52

The balance between quiescence and proliferation in aged stem cells might also be disrupted by an increased demand for replacement of mature cells in aged tissues, as suggested by observations in disease mod- els such as Duchenne muscular dystrophy (DMD). Repetitive damage, caused by continual degeneration of structurally unsound DMD fibers, elicits constant demand for regeneration, diminishing the regenerative capacity of muscle stem cells as a consequence of replicative aging13

Question 53

give an example of whena stem cell niche has become defective over time (2)

Answer 53

imilarly, expression of glial cell–derived neurotrophic factor, which is required for normal regulation of spermatogonial stem cell self-renewal and differentiation, is markedly reduced with age in Sertoli cells, the nurse cells of the testes154. Moreover, consecutive transplantation of spermatogonial stem cells to the testes of young male mice was sufficient to maintain stem cell function for extended periods of time, underscoring a major role of aging of the niche in GSC functional decline and suggesting the potential for reversing aging phenotypes by targeting the niche
- fibro/adipogenic progenitors (FAPs), stromal constituents of skeletal muscle, facilitate muscle regeneration and serve as a principal source of soluble extracellular matrix regulators158,159. The presence of FAPs in the muscle is essential for proper muscle repair after injury158, and this regula- tory relationship is likely to involve direct communication between FAPs and satellite cells, as co-culturing FAPs with satellite cells enhances the rate of differentiation of primary myogenic progenitors159. Muscle fibers also serve as an important source of niche signals to regulate satellite cell function with age. Aged muscle fibers fail to adequately induce the Notch ligand delta, and they overproduce fibroblast growth factor 2 (refs. 132, 160).

Question 54

how does differentiation change in neural stem cells?

Answer 54

-decrease in neurogenesis and upregulation og gliogenesis

Question 55

how does differentiation change in muscle stem cells?

Answer 55

decrease myogenic and increase in firbogenic and adipogenci

Question 56

how does the melanocyte stem cell population change with age?

Answer 56

they undergo terminal differentiation

Question 57

in drosophila, how has improved stem cell pools been shown to extend lifespan?

Answer 57

in Drosophila genetic changes that improve homeostasis in the intestinal epithelium by blocking stem cell overproliferation and differentiation defects during aging do extend life span

Question 58

How is the increase in tumour suppressor expression induced in stem cells and what does this say about this mechanism and why?

Answer 58

• stem cells increase the expression of tumour suppressors over time and it is thought that this i regulated by heterochornic genes which are involved in mediating the developmental transitions of c.elegans- this suggests that tumor suppressor expression and the temporal changes in stem cell function in aging mammalian tissues are partly developmentally programmed.

Question 59

how does elevated p53 expression affect stem cell pools?

Answer 59

Elevated p53 expression or constitutive p53 activation can deplete stem cells , cause premature aging, and shorten life span despite reducing cancer incidence

Question 60

how can p53 respond to damage dna? (2)

Answer 60

Activation of the DNA damage response network can transiently halt the cell cycle and repair damaged DNA through p53-dependent mechanisms. If the damage is too extensive to be repaired, the network can trigger the onset of senescence or cell death to eliminate the cells.

Question 61

describe an experiment which demonstrated tat reduced DNA repair in stem cells can result in depletion of the pool (6)

Answer 61

• Loss of the DNA damage sensor ATM depletes HSCs, exacerbates the loss of melanocyte stem cells in response to low dose radiation, and promotes the loss of undifferentiated spermatogonia . - Loss of a related DNA damage sensor, ATR, depletes HSCs and hair follicle stem cells.
• Mice deficient in nucleotide excision repair (XpdTTD), mismatch repair (Msh2−/−), nonhomologous end joining (Lig4(Y288C) and Ku80−/−), or homologous recombination (Brca2−/−) all exhibit reduced HSC function.

Question 62

what is the mechanism by which dna repair machinary mutant stem cell pools results in stem cell dysfunction?

Answer 62

The mechanism by which deficiency for DNA repair genes depletes stem cells involves accumulation of DNA damage, induction of p53 and p21cip1, elevated ROS levels, and premature differentiation

Question 63

mice have long telomeres and express telomerase in most cells, what is the phenotype of mice with a loss of telomerase and what does this tell us about human stem cell ageing?

Answer 63

Loss of telomerase function in mice reduces the regenerative capacity of proliferative organs , accelerates the development of aging phenotypes (like hair graying), reduces life span, and increases cancer incidence (particularly in the absence of p53). Telomerase-deficient mice exhibit defects in stem cell function in the forebrain, epidermis, intestinal epithelium, and hematopoietic system through cell-autonomous. human ells have shorter telomeres and no telomerase mostly- so this suggests that telomere attrition can cause again in mice and so this is likely what happens in human stem cells

Question 64

what disorders have been associated with shortened telomeres?

Answer 64

Accelerated shortening of telomeres has also been observed in other conditions with premature aging phenotypes, including trisomy 21

Question 65

describe how changes in stem cell niches can affect stem cell viability of the germline stem cell pools in flies? (2)

Answer 65

• In the male testis, these changes are partially caused by changes within the niche, as hub cells from older animals express reduced levels of DE-cadherin and Unpaired, both of which are necessary for germline stem cell maintenance . Reduced Unpaired expression is caused partly by an increase in mRNA degradation from let-7-targeting of IGF-II messenger RNA binding protein (IMP) expression in aging hub cells
• Similarly, in the Drosophila ovary, E-cadherin and BMP expression within the niche decline with age, and genetically increasing expression can enhance the function of old stem cells

Question 66

describe how changes in stem cell niches can affect stem cell viability of the muscle stem cell pools in mammals? (2)

Answer 66

Aging is associated with a reduced capacity for muscle regeneration after injury, partly as a result of reduced expression of Notch ligand by satellite muscle cells, which reduces satellite cell proliferation after injury . Aging muscles also produce elevated levels of TGF-β, which impedes regeneration and satellite cell proliferation. However, exposure of old mice to young systemic factors by making old and young mice parabiotic can rejuvenate stem cell function

Question 67

how can the presence of other sen cells in the stem cell pool contribute to ageing?

Answer 67

The accumulation of senescent cells in aging tissues can also non-cell-autonomously affect the function of other cells. Senescence is a cellular state associated with an irreversible loss in the ability to divide. Senescent cells undergo a series of changes, including the secretion of inflammatory factors, growth regulators, proteases, and other signaling molecules (Coppé et al., 2010). These secreted factors affect other cells in the local environment, promoting senescence and inflammation and promoting or inhibiting tumor growth.

Question 68

how has an experiment looking at the effect of the presence of sen cell in stem cell pools shown that they can be inhibitory to stem cell funcitiong? (2)

Answer 68

Life-long clearance of senescent cells from adult mouse tissues resulting from genetic ablation of p16Ink4a-expressing cells delays the onset of pathologies in multiple aging tissues (Baker et al., 2011). Clearance of senescent cells only late in life did not improve age-related pathologies but did attenuate their progression. This raises the question of the extent to which p16Ink4a non-cell-autonomously or cell-autonomously influences stem cell function in aging tissues.

Question 69

what is some evidence that shows that DR can increase stem cel functioning? (2)

Answer 69

• Short-term dietary restriction increases the frequency and function of satellite cells in skeletal muscle of both young and old mice, partly by increasing mitochondrial content and promoting oxidative metabolism
• In at least one short-lived mouse strain, dietary restriction attenuates age-related declines in HSC frequency and reconstituting activity

Question 70

how can the TOR pathway been linked to the deletion of stem cells?

Answer 70

Autophagy is likely to be important for HSC maintenance, because deletion of either Atg7 or Fip200 , both of which are necessary for autophagy, increases ROS levels and depletes HSCs. Decreasing translation by reducing ribosomal protein levels in yeast increases replicative life span.- TOR pathway is involved this both of these things