Hot topics in stem cells and self-renewal: 2010

In many tissues, mammalian aging is associated with a decline in the replicative and functional capacity of somatic stem cells and other self-renewing compartments. Understanding the basis of this decline is a major goal of aging research. In particular, therapeutic approaches to ameliorate or reverse the age-associated loss of stem function could be of use in clinical geriatrics. Such approaches include attempts to protect stem cells from age-promoting damage, to ‘rejuvenate’ stem cells through the use of pharmacologic agents that mitigate aging-induced alterations in signaling, and to replace lost stem cells through regenerative medicine approaches. Some headway has been made in each of these arenas over the last 18 months including advances in the production of donor-specific totipotent stem cells through induced pluripotency (iPS), gains in our understanding of how tumor suppressor signaling is controlled in self-renewing compartments to regulate aging, and further demonstration of extracellular ‘milieu’ factors that perturb stem cell function with age. This period has also been marked by the recent award of the Nobel Prize in Physiology or Medicine for elucidation of telomeres and telomerase, a topic of critical importance to stem cell aging

Tumor suppressor mechanisms in immune aging

The cancer-ageing hypothesis suggests that the activation of some tumor suppressor mechanisms beneficially prevent cancer but also untowardly promote mammalian ageing. Along these lines, activation of tumor suppressor mechanisms that inhibit the cell cycle (e.g. p16INK4a and p53) in response to DNA damage and other age-promoting stimuli have taken center stage in immune-ageing research. Immune cells are intrinsically susceptible to transforming events due to V(D)J recombination, a high rate of cellular turnover and requisite long-term self-renewal. Therefore, the DNA damage response and cell cycle regulation play a clear role in maintaining homeostasis without neoplastic progression. Here we will argue based on recent advances in our understanding of tumor suppressor mechanisms in immune cells, however, that aspects of these same beneficial pathways have the potential to induce intrinsic immune ageing

Coming of age: molecular drivers of aging and therapeutic opportunities

Aging is like the weather: everyone talks about it, but no one seems to do anything about it. We believe this may soon change, as an improved understanding of the molecular and genetic pathways underlying aging suggests it is possible to therapeutically target the aging process and increase health span. This Review series focuses on fundamental cellular mechanisms of aging and their relationship to human disease. These pathways include telomere dysfunction in cellular senescence and induction of the senescence-associated secretory phenotype (SASP) in systemic aging, sirtuin family regulation of metabolism and aging-associated diseases, mitochondrial metabolism in aging, the mechanistic target of rapamycin (mTOR) signaling pathway and the use of mTOR inhibitors to increase longevity, the progressive decline of the immune system with age, and aging-associated changes to pancreatic islet β cells that may contribute to diabetes. Together, these articles explore pathways affecting aging and possible interventional targets to slow or delay the onset of age-related pathologies

The Regulation of INK4/ARF in Cancer and Aging

Loss of the // locus on chromosome 9p21 is among the most frequent cytogenetic events in human cancer. The products of the locus—p15, p16, and ARF—play widespread and independent roles in tumor suppression. Recent data also suggest that expression of p16 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, ARF, and p15 in cancer and aging

p16INK4a protects against dysfunctional telomere–induced ATR-dependent DNA damage responses

Dysfunctional telomeres limit cellular proliferative capacity by activating the p53-p21– and p16INK4a-Rb–dependent DNA damage responses (DDRs). The p16INK4a tumor suppressor accumulates in aging tissues, is a biomarker for cellular senescence, and limits stem cell function in vivo. While the activation of a p53-dependent DDR by dysfunctional telomeres has been well documented in human cells and mouse models, the role for p16INK4a in response to telomere dysfunction remains unclear. Here, we generated protection of telomeres 1b p16–/– mice (Pot1bΔ/Δ;p16–/–) to address the function of p16INK4a in the setting of telomere dysfunction in vivo. We found that deletion of p16INK4a accelerated organ impairment and observed functional defects in highly proliferative organs, including the hematopoietic system, small intestine, and testes. Pot1bΔ/Δ;p16–/– hematopoietic cells exhibited increased telomere loss, increased chromosomal fusions, and telomere replication defects. p16INK4a deletion enhanced the activation of the ATR-dependent DDR in Pot1bΔ/Δ hematopoietic cells, leading to p53 stabilization, increased p21-dependent cell cycle arrest, and elevated p53-dependent apoptosis. In contrast to p16INK4a, deletion of p21 did not activate ATR, rescued proliferative defects in Pot1bΔ/Δ hematopoietic cells, and significantly increased organismal lifespan. Our results provide experimental evidence that p16INK4a exerts protective functions in proliferative cells bearing dysfunctional telomeres

Detecting and characterizing circular RNAs

Circular RNA transcripts were first identified in the early 1990s but knowledge of these species has remained limited, as their study has been difficult through traditional methods of RNA analysis. Now, novel bioinformatic approaches coupled with biochemical enrichment strategies and deep sequencing have allowed comprehensive studies of circular RNA species. Recent studies have revealed thousands of endogenous circular RNAs (circRNAs) in mammalian cells, some of which are highly abundant and evolutionarily conserved. Evidence is emerging that some circRNAs might regulate microRNA (miRNA) function, and roles in transcriptional control have also been suggested. Therefore, study of this class of non-coding RNAs has potential implications for therapeutic and research applications. We believe the key future challenge to the field will be to understand the regulation and function of these unusual molecules

Cells exhibiting strong p16INK4a promoter activation in vivo display features of senescence

The activation of cellular senescence throughout the lifespan promotes tumor suppression, whereas the persistence of senescent cells contributes to aspects of aging. This theory has been limited, however, by an inability to identify and isolate individual senescent cells within an intact organism. Toward that end, we generated a murine reporter strain by “knocking-in” a fluorochrome, tandem-dimer Tomato (tdTom), into exon 1α of the p16 INK4a locus. We used this allele (p16 tdTom ) for the enumeration, isolation, and characterization of individual p16 INK4a -expressing cells (tdTom + ). The half-life of the knocked-in transcript was shorter than that of the endogenous p16 INK4a mRNA, and therefore reporter expression better correlated with p16 INK4a promoter activation than p16 INK4a transcript abundance. The frequency of tdTom + cells increased with serial passage in cultured murine embryo fibroblasts from p16 tdTom/+ mice. In adult mice, tdTom + cells could be readily detected at low frequency in many tissues, and the frequency of these cells increased with aging. Using an in vivo model of peritoneal inflammation, we compared the phenotype of cells with or without activation of p16 INK4a and found that tdTom + macrophages exhibited some features of senescence, including reduced proliferation, senescence-associated β-galactosidase (SA-β-gal) activation, and increased mRNA expression of a subset of transcripts encoding factors involved in SA-secretory phenotype (SASP). These results indicate that cells harboring activation of the p16 INK4a promoter accumulate with aging and inflammation in vivo, and display characteristics of senescence

Defining the toxicology of aging

Mammalian aging is complex and incompletely understood. While significant effort has been spent addressing the genetics or, more recently, the pharmacology of aging, the toxicology of aging has been relatively understudied. Just as an understanding of `carcinogens' has proven critical to modern cancer biology, an understanding of environmental toxicants that accelerate aging (`gerontogens') will inform gerontology. In this review, we discuss the evidence for the existence of mammalian gerontogens, as well as describe biomarkers needed to measure the age-promoting activity of a given toxicant. We focus on the effects of putative gerontogens on the in vivo accumulation of senescent cells, a characteristic feature of aging that plays a causal role in some age-associated phenotypes

Review: A Meta-Analysis of GWAS Studies and Age-Associated Diseases

Genome-Wide Association studies (GWAS) offer an unbiased means to understand the genetic basis of traits by identifying single nucleotide polymorphisms (SNPs) linked to causal variants of complex phenotypes. GWAS have identified a host of susceptibility SNPs associated with many important human diseases, including diseases associated with aging. In an effort to understand the genetics of broad resistance to age-associated diseases (i.e. ‘wellness’), we performed a meta-analysis of human GWAS. Toward that end, we compiled 372 GWAS that identified 1,775 susceptibility SNPs to 105 unique diseases and used these SNPs to create a genomic landscape of disease susceptibility. This map was constructed by partitioning the genome into 200 kb ‘bins’ and mapping the 1,775 susceptibility SNPs to bins based on their genomic location. Investigation of these data revealed significant heterogeneity of disease association within the genome, with 92% of bins devoid of disease-associated SNPs. In contrast, 10 bins (0.06%) were significantly (pINK4/ARF (CDKN2a/b) tumor suppressor locus on 9p21.3. Provocatively, all 10 significantly enriched bins contained genes linked to either inflammation or cellular senescence pathways, and SNPs near regulators of senescence were particularly associated with disease of aging (e.g. cancer, atherosclerosis, type 2 diabetes, glaucoma). This analysis suggests that germline genetic heterogeneity in the regulation of immunity and cellular senescence influences the human health span

The Journal of the Friends' Historical Society vol. 3 No. 3

1. Notices. 2. Notes and Queries. 3. Letter from William Penn. 4. David Lloyd II. 5. Friends on the Atlantic. 6. King's Briefs the Forerunners of Mutual Insurance Societies. 7. Whitefield's Estimate of Quakerism. 8. Earlham College Library, Richmond, Ind. 9. Editors' Notes. 10. Large Gatherings of Friends. 11. Meeting Records I. 12. Friends in Current Literature. 13. A Quotation in the London Y.M. Epistle. 14. List of Officers for 1906-7. 15. Accounts and Balance Sheet for 1905