Increasing p16INK4a expression decreases forebrain progenitors and neurogenesis during ageing

Adult hippocampal neurogenesis and aging

The demographic changes in the foreseeable future stress the need for research on successful cognitive aging. Advancing age constitutes a primary risk factor for disease of the central nervous system most notably neurodegenerative disorders. The hippocampus is one of the brain regions that is prominently affected by neurodegeneration and functional decline even in what is still considered "normal aging". Plasticity is the basis for how the brain adapts to changes over time. The discovery of adult hippocampal neurogenesis has added a whole new dimension to research on structural plasticity in the adult and aging hippocampus. In this article, we briefly summarize and discuss recent findings on the regulation of adult neurogenesis with relevance to aging. Aging is an important co-variable for many regulatory mechanisms affecting adult neurogenesis but so far, only few studies have specifically addressed this interaction. We hypothesize that adult neurogenesis contributes to a neural reserve, i.e. the maintained potential for structural plasticity that allows compensation in situations of functional losses with aging. As such we propose that adult neurogenesis might contribute to the structural correlates of successful aging.

Sox17 dependence distinguishes the transcriptional regulation of fetal from adult hematopoietic stem cells

Fetal stem cells differ phenotypically and functionally from adult stem cells in diverse tissues. However, little is known about how these differences are regulated. To address this we compared the gene expression profiles of fetal versus adult hematopoietic stem cells (HSCs) and discovered that the Sox17 transcriptional regulator is specifically expressed in fetal and neonatal but not adult HSCs. Germline deletion of Sox17 led to severe fetal hematopoietic defects, including a lack of detectable definitive HSCs. Conditional deletion of Sox17 from hematopoietic cells led to the loss of fetal and neonatal but not adult HSCs. HSCs stopped expressing Sox17 approximately 4 weeks after birth. During this transition, loss of Sox17 expression correlated with slower proliferation and the acquisition of an adult phenotype by individual HSCs. Sox17 is thus required for the maintenance of fetal and neonatal HSCs and distinguishes their transcriptional regulation from adult HSCs.

Rapid asymmetric evolution of a dual-coding tumor suppressor INK4a/ARF locus contradicts its function

INK4a/ARF tumor suppressor locus encodes two protein products, INK4a and ARF, essential for controlling tumorigenesis and mutated in more than half of human cancers. There is no resemblance between the two proteins: their coding regions are assembled by alternative splicing of two mutually exclusive 5' exons into a constitutive one containing overlapping out-of-phase reading frames. We show that the dual-coding arrangement conflicts with the high cost of mutations within INK4a/ARF. Unexpectedly, the locus evolves rapidly and asymmetrically, with ARF accumulating the majority of amino acid replacements. Rapid evolution drives both INK4a and ARF proteins out of sync with other members of the RB and p53 tumor suppressor pathways, both of which are controlled by the locus. Yet, the asymmetric behavior may be an intrinsic property of dual-coding exons: INK4a/ARF closely mimics the evolution of 90 newly identified genes with similar dual-coding structure. Thus, the strong link between mutations in INK4a/ARF and cancer may be a direct consequence of the architecture of the locus.

Deficit of circulating stem--progenitor cells in opiate addiction: a pilot study

A substantial literature describes the capacity of all addictive drugs to slow cell growth and potentiate apoptosis. Flow cytometry was used as a means to compare two lineages of circulating progenitor cells in addicted patients. Buprenorphine treated opiate addicts were compared with medical patients. Peripheral venous blood CD34(+) CD45(+) double positive cells were counted as haemopoietic stem cells (HSC's), and CD34(+) KDR(+) (VEGFR2(+)) cells were taken as endothelial progenitor cells (EPC's). 10 opiate dependent patients with substance use disorder (SUD) and 11 non-addicted (N-SUD) were studied. The ages were (mean + S.D.) 36.2 + 8.6 and 56.4 + 18.6 respectively (P <0.01). HSC's were not different in the SUD (2.38 + 1.09 Vs. 3.40 + 4.56 cells/mcl). EPC's were however significantly lower in the SUD (0.09 + 0.14 Vs. 0.26 + 0.20 cells/mcl; No. > 0.15, OR = 0.09, 95% C.I. 0.01-0.97), a finding of some interest given the substantially older age of the N-SUD group. These laboratory data are thus consistent with clinical data suggesting accelerated ageing in addicted humans and implicate the important stem cell pool in both addiction toxicology and ageing. They carry important policy implications for understanding the fundamental toxicology of addiction, and suggest that the toxicity both of addiction itself and of indefinite agonist maintenance therapies may have been seriously underestimated.

Cancer as a manifestation of aberrant chromatin structure

In this article we review many important epigenetic changes in early carcinogenesis and discuss the possibility of these alterations being targeted for therapeutic intervention in the future. Both regional DNA methylation and global chromatin packaging are interrelated partners that function in concert to control gene transcription. We first summarize briefly DNA methylation and its role in gene expression. Then, we focus on how the DNA is packaged into chromatin and the tight relationship between chromatin and DNA methylation. A more complete understanding of these key, regulatory events is vital in approaching a more rational drug therapy to various malignancies.

Genetic cooperation between p21Cip1 and INK4 inhibitors in cellular senescence and tumor suppression

Cell-cycle inhibitors of the Cip/Kip and INK4 families are involved in cellular senescence and tumor suppression. Some of these proteins, p21(Cip1), p16(INK4a) and p15(INK4b), are coexpressed in response to antiproliferative signals such as cellular senescence resulting in cell-cycle arrest. To understand the roles of these inhibitors and their synergistic effect, we have characterized the growth properties and senescent behavior of primary cells deficient in p21(Cip1) and expressing an endogenous Cdk4(R24C) (cyclin-dependent kinase) mutant (Cdk4(R24C) knock-in cells) insensitive to INK4 proteins. Inactivation of both p21(Cip1) and INK4 pathways strongly cooperate in suppressing cellular senescence in vitro. These double mutant cells behavior as immortal cultures and display high sensitivity to cellular transformation by oncogenes. Moreover, mice double mutant in the INK4 and p21(Cip1) pathways (Cdk4(R24C); p21(Cip1)-null mice) display an increased incidence of specific sarcomas, suggesting a significant cooperation between these two families of cell-cycle inhibitors in senescence responses and tumor suppression in vivo.

Aging and cancer cell biology, 2007

This Hot Topics review, the second in a new Aging Cell series, discusses articles published in the last year that have stimulated new ideas about the tangled relationship of aging to cancer cell biology. The year's highlights include reports on the ability of Mdm2 mutations to diminish risks of cancer in aging mice, on proliferative competition between oncogenic cells and bone marrow stem cells, and on the role of metalloproteinases in overcoming age-associated barriers to tumor invasion. Of particular interest were three articles showing that diminished activity of the tumor-suppressor gene p16/INK4a, while increasing the risk of cancer mortality, can lead to improved function in several varieties of age-sensitive stem cells.

Loss of stem cell regenerative capacity within aged niches

This work uncovers novel mechanisms of aging within stem cell niches that are evolutionarily conserved between mice and humans and affect both embryonic and adult stem cells. Specifically, we have examined the effects of aged muscle and systemic niches on key molecular identifiers of regenerative potential of human embryonic stem cells (hESCs) and post-natal muscle stem cells (satellite cells). Our results reveal that aged differentiated niches dominantly inhibit the expression of Oct4 in hESCs and Myf-5 in activated satellite cells, and reduce proliferation and myogenic differentiation of both embryonic and tissue-specific adult stem cells (ASCs). Therefore, despite their general neoorganogenesis potential, the ability of hESCs, and the more differentiated myogenic ASCs to contribute to tissue repair in the old will be greatly restricted due to the conserved inhibitory influence of aged differentiated niches. Significantly, this work establishes that hESC-derived factors enhance the regenerative potential of both young and, importantly, aged muscle stem cells in vitro and in vivo; thus, suggesting that the regenerative outcome of stem cell-based replacement therapies will be determined by a balance between negative influences of aged tissues on transplanted cells and positive effects of embryonic cells on the endogenous regenerative capacity. Comprehensively, this work points toward novel venues for in situ restoration of tissue repair in the old and identifies critical determinants of successful cell-replacement therapies for aged degenerating organs.

Remodeling of chromatin structure in senescent cells and its potential impact on tumor suppression and aging

Cellular senescence is an important tumor suppression process, and a possible contributor to tissue aging. Senescence is accompanied by extensive changes in chromatin structure. In particular, many senescent cells accumulate specialized domains of facultative heterochromatin, called Senescence-Associated Heterochromatin Foci (SAHF), which are thought to repress expression of proliferation-promoting genes, thereby contributing to senescence-associated proliferation arrest. This article reviews our current understanding of the structure, assembly and function of these SAHF at a cellular level. The possible contribution of SAHF to tumor suppression and tissue aging is also critically discussed.

Degenerative phenotypes caused by the combined deficiency of murine HIP1 and HIP1r are rescued by human HIP1

The members of the huntingtin-interacting protein-1 (HIP1) family, HIP1 and HIP1-related (HIP1r), are multi-domain proteins that interact with inositol lipids, clathrin and actin. HIP1 is over-expressed in a variety of cancers and both HIP1 and HIP1r prolong the half-life of multiple growth factor receptors. To better understand the physiological importance of the HIP1 family in vivo, we have analyzed a large cohort of double Hip1/Hip1r knockout (DKO) mice. All DKO mice were dwarfed, afflicted with severe vertebral defects and died in early adulthood. These phenotypes were not observed during early adulthood in the single Hip1 or Hip1r knockouts, indicating that HIP1 and HIP1r compensate for one another. Despite the ability of HIP1 and HIP1r to modulate growth factor receptor levels when over-expressed, studies herein using DKO fibroblasts indicate that the HIP1 family is not necessary for endocytosis but is necessary for the maintenance of diverse adult tissues in vivo. To test if human HIP1 can function similar to mouse HIP1, transgenic mice with 'ubiquitous' expression of the human HIP1 cDNA were generated and crossed with DKO mice. Strikingly, the compound human HIP1 transgenic DKO mice were completely free from dwarfism and spinal defects. This successful rescue demonstrates that the human HIP1 protein shares some interchangeable functions with both HIP1 and HIP1r in vivo. In addition, we conclude that the degenerative phenotypes seen in the DKO mice are due mainly to HIP1 and HIP1r protein deficiency rather than altered expression of neighboring genes or disrupted intronic elements.

The epigenomics of cancer

Aberrant gene function and altered patterns of gene expression are key features of cancer. Growing evidence shows that acquired epigenetic abnormalities participate with genetic alterations to cause this dysregulation. Here, we review recent advances in understanding how epigenetic alterations participate in the earliest stages of neoplasia, including stem/precursor cell contributions, and discuss the growing implications of these advances for strategies to control cancer.

A common variant of the p16(INK4a) genetic region is associated with physical function in older people

p16(INK4a) is active in cell senescence, ageing and tumor suppression. Deletion of the small p16(INK4a)/ARF/p15(INK4b) region occurs in many cancers. We screened 25 common polymorphisms across the region and three related genes for associations with physical functioning in older people. In an initial sample of 938 (aged 65-80 years) from the EPIC study (Norfolk, UK), the rs2811712 SNP minor allele (located between the shared p16(INK4a)/ARF locus and p15(INK4b)) was associated with reduced physical impairment. This association remained after testing an additional 1319 EPIC-Norfolk samples (p-value=0.013, total n=2257), and on independent replication in the InCHIANTI study (n=709, p=0.015), and at one-sided significance in Iowa-EPESE (n=419, p=0.079). Overall (n=3372), the prevalence of severely limited physical function was 15.0% in common homozygotes and 7.0% in rare homozygotes (per minor allele odds ratio=1.48, 95% CI: 1.17-1.88, p=0.001, adjusted for age, sex and study). This estimate was similar excluding screening set 1 (OR=1.45, 95% CI: 1.09-1.92, p=0.010, n=2434). These findings require further replication, but provide the first direct evidence that the p16(INK4a)/ARF/p15(INK4b) genetic region and the senescence machinery are active in physical ageing in heterogeneous human populations. The mechanism involved may be via greater cellular restorative activity and reduced stem cell senescence.

Gene expression profiling of aging reveals activation of a p53-mediated transcriptional program

BACKGROUND

Aging has been associated with widespread changes at the gene expression level in multiple mammalian tissues. We have used high density oligonucleotide arrays and novel statistical methods to identify specific transcriptional classes that may uncover biological processes that play a central role in mammalian aging.

RESULTS

We identified 712 transcripts that are differentially expressed in young (5 month old) and old (25-month old) mouse skeletal muscle. Caloric restriction (CR) completely or partially reversed 87% of the changes in expression. Examination of individual genes revealed a transcriptional profile indicative of increased p53 activity in the older muscle. To determine whether the increase in p53 activity is associated with transcriptional activation of apoptotic targets, we performed RT-PCR on four well known mediators of p53-induced apoptosis: puma, noxa, tnfrsf10b and bok. Expression levels for these proapoptotic genes increased significantly with age (P < 0.05), while CR significantly lowered expression levels for these genes as compared to control fed old mice (P < 0.05). Age-related induction of p53-related genes was observed in multiple tissues, but was not observed in young SOD2+/- and GPX4+/- mice, suggesting that oxidative stress does not induce the expression of these genes. Western blot analysis confirmed that protein levels for both p21 and GADD45a, two established transcriptional targets of p53, were higher in the older muscle tissue.

CONCLUSION

These observations support a role for p53-mediated transcriptional program in mammalian aging and suggest that mechanisms other than reactive oxygen species are involved in the age-related transcriptional activation of p53 targets.

The molecular basis of neurosensory cell formation in ear development: a blueprint for hair cell and sensory neuron regeneration?

The inner ear of mammals uses neurosensory cells derived from the embryonic ear for mechanoelectric transduction of vestibular and auditory stimuli (the hair cells) and conducts this information to the brain via sensory neurons. As with most other neurons of mammals, lost hair cells and sensory neurons are not spontaneously replaced and result instead in age-dependent progressive hearing loss. We review the molecular basis of neurosensory development in the mouse ear to provide a blueprint for possible enhancement of therapeutically useful transformation of stem cells into lost neurosensory cells. We identify several readily available adult sources of stem cells that express, like the ectoderm-derived ear, genes known to be essential for ear development. Use of these stem cells combined with molecular insights into neurosensory cell specification and proliferation regulation of the ear, might allow for neurosensory regeneration of mammalian ears in the near future.

The Polycomb group proteins bind throughout the INK4A-ARF locus and are disassociated in senescent cells

The p16INK4A and p14ARF proteins, encoded by the INK4A-ARF locus, are key regulators of cellular senescence, yet the mechanisms triggering their up-regulation are not well understood. Here, we show that the ability of the oncogene BMI1 to repress the INK4A-ARF locus requires its direct association and is dependent on the continued presence of the EZH2-containing Polycomb-Repressive Complex 2 (PRC2) complex. Significantly, EZH2 is down-regulated in stressed and senescing populations of cells, coinciding with decreased levels of associated H3K27me3, displacement of BMI1, and activation of transcription. These results provide a model for how the INK4A-ARF locus is activated and how Polycombs contribute to cancer.

Cellular senescence and chromatin organisation

Despite the potential importance of senescence in tumour suppression, its effector mechanism is poorly understood. Recent studies suggest that alterations in the chromatin environment might add an additional layer of stability to the phenotype. In this review, recent discoveries on the interplay between senescence and chromatin biology are overviewed.

Vascular cell senescence: contribution to atherosclerosis

Cardiologists and most physicians believe that aging is an independent risk factor for human atherosclerosis, whereas atherosclerosis is thought to be a characteristic feature of aging in humans by many gerontologists. Because atherosclerosis is among the age-associated changes that almost always escape the influence of natural selection in humans, it might be reasonable to regard atherosclerosis as a feature of aging. Accordingly, when we investigate the pathogenesis of human atherosclerosis, it may be more important to answer the question of how we age than what specifically promotes atherosclerosis. Recently, genetic analyses using various animal models have identified molecules that are crucial for aging. These include components of the DNA-repair system, the tumor suppressor pathway, the telomere maintenance system, the insulin/Akt pathway, and other metabolic pathways. Interestingly, most of the molecules that influence the phenotypic changes of aging also regulate cellular senescence, suggesting a causative link between cellular senescence and aging. For example, DNA-repair defects can cause phenotypic changes that resemble premature aging, and senescent cells that show DNA damage accumulate in the elderly. Excessive calorie intake can cause diabetes and hyperinsulinemia, whereas dysregulation of the insulin pathway has been shown to induce cellular senescence in vitro. Calorie restriction or a reduction of insulin signals extends the lifespan of various species and decreases biomarkers of cellular senescence in vivo. There is emerging evidence that cellular senescence contributes to the pathogenesis of human atherosclerosis. Senescent vascular cells accumulate in human atheroma tissues and exhibit various features of dysfunction. In this review, we examine the hypothesis that cellular senescence might contribute to atherosclerosis, which is a characteristic of aging in humans.

Mutant alpha-synuclein exacerbates age-related decrease of neurogenesis

In Parkinson disease, wild-type alpha-synuclein accumulates during aging, whereas alpha-synuclein mutations lead to an early onset and accelerated course of the disease. The generation of new neurons is decreased in regions of neurogenesis in adult mice overexpressing wild-type human alpha-synuclein. We examined the subventricular zone/olfactory bulb neurogenesis in aged mice expressing either wild-type human or A53T mutant alpha-synuclein. Aging wild-type and mutant alpha-synuclein-expressing animals generated significantly fewer new neurons than their non-transgenic littermates. This decreased neurogenesis was caused by a reduction in cell proliferation within the subventricular zone of mutant alpha-synuclein mice. In contrast, no difference was detected in mice overexpressing the wild-type allele. Also, more TUNEL-positive profiles were detected in the subventricular zone, following mutant alpha-synuclein expression and in the olfactory bulb, following wild-type and mutant alpha-synuclein expression. The impaired neurogenesis in the olfactory bulb of different transgenic alpha-synuclein mice during aging highlights the need to further explore the interplay between olfactory dysfunction and neurogenesis in Parkinson disease.

The cancer epigenome--components and functional correlates

It is increasingly apparent that cancer development not only depends on genetic alterations but on an abnormal cellular memory, or epigenetic changes, which convey heritable gene expression patterns critical for neoplastic initiation and progression. These aberrant epigenetic mechanisms are manifest in both global changes in chromatin packaging and in localized gene promoter changes that influence the transcription of genes important to the cancer process. An exciting emerging theme is that an understanding of stem cell chromatin control of gene expression, including relationships between histone modifications and DNA methylation, may hold a key to understanding the origins of cancer epigenetic changes. This possibility, coupled with the reversible nature of epigenetics, has enormous significance for the prevention and control of cancer.

Definition of pRB- and p53-dependent and -independent steps in HIRA/ASF1a-mediated formation of senescence-associated heterochromatin foci

Cellular senescence is an irreversible proliferation arrest triggered by short chromosome telomeres, activated oncogenes, and cell stress and mediated by the pRB and p53 tumor suppressor pathways. One of the earliest steps in the senescence program is translocation of a histone chaperone, HIRA, into promyelocytic leukemia (PML) nuclear bodies. This relocalization precedes other markers of senescence, including the appearance of specialized domains of facultative heterochromatin called senescence-associated heterochromatin foci (SAHF) and cell cycle exit. SAHF represses expression of proliferation-promoting genes, thereby driving exit from the cell cycle. HIRA bound to another histone chaperone, ASF1a, drives formation of SAHF. Here, we show that HIRA's translocation to PML bodies occurs in response to all senescence triggers tested. Dominant negative HIRA mutants that block HIRA's localization to PML bodies prevent formation of SAHF, as does a PML-RARalpha fusion protein which disrupts PML bodies, directly supporting the idea that localization of HIRA to PML bodies is required for formation of SAHF. Significantly, translocation of HIRA to PML bodies occurs in the absence of functional pRB and p53 tumor suppressor pathways. However, our evidence indicates that downstream of HIRA's localization to PML bodies, the HIRA/ASF1a pathway cooperates with pRB and p53 to make SAHF, with the HIRA/ASF1a and pRB pathways acting in parallel. We present evidence that convergence of the HIRA/ASF1a and pRB pathways occurs through a DNAJ-domain protein, DNAJA2.

Molecular dissection of formation of senescence-associated heterochromatin foci

Senescence is characterized by an irreversible cell proliferation arrest. Specialized domains of facultative heterochromatin, called senescence-associated heterochromatin foci (SAHF), are thought to contribute to the irreversible cell cycle exit in many senescent cells by repressing the expression of proliferation-promoting genes such as cyclin A. SAHF contain known heterochromatin-forming proteins, such as heterochromatin protein 1 (HP1) and the histone H2A variant macroH2A, and other specialized chromatin proteins, such as HMGA proteins. Previously, we showed that a complex of histone chaperones, histone repressor A (HIRA) and antisilencing function 1a (ASF1a), plays a key role in the formation of SAHF. Here we have further dissected the series of events that contribute to SAHF formation. We show that each chromosome condenses into a single SAHF focus. Chromosome condensation depends on the ability of ASF1a to physically interact with its deposition substrate, histone H3, in addition to its cochaperone, HIRA. In cells entering senescence, HP1gamma, but not the related proteins HP1alpha and HP1beta, becomes phosphorylated on serine 93. This phosphorylation is required for efficient incorporation of HP1gamma into SAHF. Remarkably, however, a dramatic reduction in the amount of chromatin-bound HP1 proteins does not detectably affect chromosome condensation into SAHF. Moreover, abundant HP1 proteins are not required for the accumulation in SAHF of histone H3 methylated on lysine 9, the recruitment of macroH2A proteins, nor other hallmarks of senescence, such as the expression of senescence-associated beta-galactosidase activity and senescence-associated cell cycle exit. Based on our results, we propose a stepwise model for the formation of SAHF.

Age-dependent increase in side population distribution within hematopoiesis: implications for our understanding of the mechanism of aging

It is thought that, as we age, damage to our stem cells may lead to diminished stem cell pool function and, consequently, a reduced organ regeneration potential that contributes to somatic senescence. Stem cells have evolved many antitoxicity mechanisms, and certain mechanisms may be utilized to isolate hematopoietic stem cells. One method exploits the activity of the ATP-binding cassette/G2 transporter to efflux Hoechst 33,342 and results in a stem cell population known as the side population (SP). The SP subset represents a remarkable enrichment for hematopoietic stem cells and provides an opportunity to re-evaluate age-based changes in hematopoietic stem cells. We report here that the frequency of SP cells steadily increases with age, as does the proportion of Lin(-)/Sca-1(+)/c-kit(+) cells that is capable of Hoechst efflux. Phenotyping, progenitor, and long-term repopulation assays have indicated that SP cells in older mice are still stem cells, albeit with a lower homing efficiency than SP cells from younger mice. Analysis of apoptosis within SP cells has revealed an apoptosis-resistant population in SP cells from old mice. Gene expression analysis has determined that SP cells from old mice have a reduced expression of apoptosis-promoting genes than SP cells from young mice. This increase in SP cells with age seems to be an intrinsic property that may be independent of the age of the microenvironment (niche), and our data might provide some clues as to how this alteration in the proportion of stem/progenitor cells occurs. A possible selection-based mechanism of stem cell pool aging is discussed.

The regulation of INK4/ARF in cancer and aging

Loss of the INK4a/ARF/INK4b locus on chromosome 9p21 is among the most frequent cytogenetic events in human cancer. The products of the locus--p15(INK4b), p16(INK4a), and ARF--play widespread and independent roles in tumor suppression. Recent data also suggest that expression of p16(INK4a) induces an age-dependent decrease in the proliferative capacity of certain tissue-specific stem cells and unipotent progenitors. Here, we discuss the regulation and role of p16(INK4a), ARF, and p15(INK4b) in cancer and aging.

The molecular basis of ageing in stem cells

Ageing is often defined in the context of telomerase activity and telomere length regulation. Most somatic cells have limited replication ability and undergo senescence eventually. Stem cells are unique as they possess more abundant telomerase activity and are able to maintain telomere lengths for a longer period. Embryonic stem cells are particularly resistant to ageing and can be propagated indefinitely. Remarkably, adult somatic cells can be reprogrammed to an ESC-like state by various means including cell fusion, exposure to ESC cell-free extracts, enforced expression of specific molecules, and somatic cell nuclear transfer. Thus, the rejuvenation of an 'aged' state can be effected by the activation of specific key molecules in the cell. Here, we argue that cellular ageing is a reversible process, and this is determined by the balance of biological molecules which directly or indirectly control telomere length and telomerase activity, either through altering gene expression and/or modulating the epigenetic state of the chromatin.

The impact of altered p53 dosage on hematopoietic stem cell dynamics during aging

A temporal decline in tissue stem cell functionality may be a key component of mammalian aging. The tumor suppressor p53 has recently been implicated as a potential regulator of aging. We examined age-associated hematopoietic stem cell (HSC) dynamics in mice with varying p53 activities. Reduced p53 activity in p53+/- mice was associated with higher numbers of proliferating hematopoietic stem and progenitor cells in old age compared with aged wild-type (p53+/+) mice. We also assessed HSC dynamics in a p53 mutant mouse model (p53+/m) with higher apparent p53 activity than wild-type mice. The p53 hypermorphic (p53+/m) mice display phenotypes of premature aging. Many aged p53+/m organs exhibit reduced cellularity and atrophy, suggesting defects in stem-cell regenerative capacity. HSC numbers from old p53+/m mice fail to increase with age, unlike those of their p53+/+ and p53+/- counterparts. Moreover, transplantation of 500 HSCs from old p53+/m mice into lethally irradiated recipients resulted in reduced engraftment compared with old wild-type p53+/+ and p53+/- HSCs. Thus, alteration of p53 activity affects stem-cell numbers, proliferation potential, and hematopoiesis in older organisms, supporting a model in which aging is caused in part by a decline in tissue stem cell regenerative function.

p16INK4a induces an age-dependent decline in islet regenerative potential

The p16INK4a tumour suppressor accumulates in many tissues as a function of advancing age. p16INK4a is an effector of senescence and a potent inhibitor of the proliferative kinase Cdk4 (ref. 6), which is essential for pancreatic beta-cell proliferation in adult mammals. Here we show that p16INK4a constrains islet proliferation and regeneration in an age-dependent manner. Expression of the p16INK4a transcript is enriched in purified islets compared with the exocrine pancreas, and islet-specific expression of p16INK4a, but not other cyclin-dependent kinase inhibitors, increases markedly with ageing. To determine the physiological significance of p16INK4a accumulation on islet function, we assessed the impact of p16INK4a deficiency and overexpression with increasing age and in the regenerative response after exposure to a specific beta-cell toxin. Transgenic mice that overexpress p16INK4a to a degree seen with ageing demonstrated decreased islet proliferation. Similarly, islet proliferation was unaffected by p16INK4a deficiency in young mice, but was relatively increased in p16(INK4a)-deficient old mice. Survival after toxin-mediated ablation of beta-cells, which requires islet proliferation, declined with advancing age; however, mice lacking p16INK4a demonstrated enhanced islet proliferation and survival after beta-cell ablation. These genetic data support the view that an age-induced increase of p16INK4a expression limits the regenerative capacity of beta-cells with ageing.

Stem-cell ageing modified by the cyclin-dependent kinase inhibitor p16INK4a

Stem-cell ageing is thought to contribute to altered tissue maintenance and repair. Older humans experience increased bone marrow failure and poorer haematologic tolerance of cytotoxic injury. Haematopoietic stem cells (HSCs) in older mice have decreased per-cell repopulating activity, self-renewal and homing abilities, myeloid skewing of differentiation, and increased apoptosis with stress. Here we report that the cyclin-dependent kinase inhibitor p16INK4a, the level of which was previously noted to increase in other cell types with age, accumulates and modulates specific age-associated HSC functions. Notably, in the absence of p16INK4a, HSC repopulating defects and apoptosis were mitigated, improving the stress tolerance of cells and the survival of animals in successive transplants, a stem-cell-autonomous tissue regeneration model. Inhibition of p16INK4a may ameliorate the physiological impact of ageing on stem cells and thereby improve injury repair in aged tissue.

Molecular signaling and genetic pathways of senescence: Its role in tumorigenesis and aging

In response to progressive telomere shortening in successive cell divisions, normal somatic cells enter senescence, during which they cease to proliferate irreversibly and undergo dramatic changes in gene expression. Senescence can also be activated by various types of stressful stimuli, including aberrant oncogenic signaling, oxidative stress, and DNA damage. Because of the limited proliferative capacity imposed by senescence, as well as the ability of senescent cells to influence neighboring non-senescent cells, senescence has been proposed to play an important role in tumorigenesis and to contribute to aging. Considerable effort has been put into elucidating the molecular mechanisms of senescence, including the signals that trigger senescence, the molecular pathways by which cells enter senescence, and evidence that supports its role in tumorigenesis and aging.