Aging Cell

Replicative senescence limits the proliferation of somatic cells passaged in culture and may reflect cellular aging in vivo. The most widely used biomarker for senescent and aging cells is senescence‐associated β‐galactosidase (SA‐β‐gal), which is defined as β‐galactosidase activity detectable at pH 6.0 in senescent cells, but the origin of SA‐β‐gal and its cellular roles in senescence are not known. We demonstrate here that SA‐β‐gal activity is expressed from GLB1, the gene encoding lysosomal β‐D‐galactosidase, the activity of which is typically measured at acidic pH 4.5. Fibroblasts from patients with autosomal recessive G_M1‐gangliosidosis, which have defective lysosomal β‐galactosidase, did not express SA‐β‐gal at late passages even though they underwent replicative senescence. In addition, late passage normal fibroblasts expressing small‐hairpin interfering RNA that depleted GLB1 mRNA underwent senescence but failed to express SA‐β‐gal. GLB1 mRNA depletion also prevented expression of SA‐β‐gal activity in HeLa cervical carcinoma cells induced to enter a senescent state by repression of their endogenous human papillomavirus E7 oncogene. SA‐β‐gal induction during senescence was due at least in part to increased expression of the lysosomal β‐galactosidase protein. These results also indicate that SA‐β‐gal is not required for senescence.

Fat tissue, frequently the largest organ in humans, is at the nexus of mechanisms involved in longevity and age‐related metabolic dysfunction. Fat distribution and function change dramatically throughout life. Obesity is associated with accelerated onset of diseases common in old age, while fat ablation and certain mutations affecting fat increase life span. Fat cells turn over throughout the life span. Fat cell progenitors, preadipocytes, are abundant, closely related to macrophages, and dysdifferentiate in old age, switching into a pro‐inflammatory, tissue‐remodeling, senescent‐like state. Other mesenchymal progenitors also can acquire a pro‐inflammatory, adipocyte‐like phenotype with aging. We propose a hypothetical model in which cellular stress and preadipocyte overutilization with aging induce cellular senescence, leading to impaired adipogenesis, failure to sequester lipotoxic fatty acids, inflammatory cytokine and chemokine generation, and innate and adaptive immune response activation. These pro‐inflammatory processes may amplify each other and have systemic consequences. This model is consistent with recent concepts about cellular senescence as a stress‐responsive, adaptive phenotype that develops through multiple stages, including major metabolic and secretory readjustments, which can spread from cell to cell and can occur at any point during life. Senescence could be an alternative cell fate that develops in response to injury or metabolic dysfunction and might occur in nondividing as well as dividing cells. Consistent with this, a senescent‐like state can develop in preadipocytes and fat cells from young obese individuals. Senescent, pro‐inflammatory cells in fat could have profound clinical consequences because of the large size of the fat organ and its central metabolic role.

Many conditions that shift cells from states of nutrient utilization and growth to states of cell maintenance extend lifespan. We have carried out a systematic lifespan analysis of conditions that inhibit protein synthesis. We find that reducing the levels of ribosomal proteins, ribosomal‐protein S6 kinase or translation‐initiation factors increases the lifespan of Caenorhabditis elegans. These perturbations, as well as inhibition of the nutrient sensor target of rapamycin (TOR), which is known to increase lifespan, all increase thermal‐stress resistance. Thus inhibiting translation may extend lifespan by shifting cells to physiological states that favor maintenance and repair. Interestingly, different types of translation inhibition lead to one of two mutually exclusive outputs, one that increases lifespan and stress resistance through the transcription factor DAF‐16/FOXO, and one that increases lifespan and stress resistance independently of DAF‐16. Our findings link TOR, but not sir‐2.1, to the longevity response induced by dietary restriction (DR) in C. elegans, and they suggest that neither TOR inhibition nor DR extends lifespan simply by reducing protein synthesis.

We generated mice that overexpress the sirtuin, SIRT1. Transgenic mice have been generated by knocking in SIRT1 cDNA into the β‐actin locus. Mice that are hemizygous for this transgene express normal levels of β‐actin and higher levels of SIRT1 protein in several tissues. Transgenic mice display some phenotypes similar to mice on a calorie‐restricted diet: they are leaner than littermate controls; are more metabolically active; display reductions in blood cholesterol, adipokines, insulin and fasted glucose; and are more glucose tolerant. Furthermore, transgenic mice perform better on a rotarod challenge and also show a delay in reproduction. Our findings suggest that increased expression of SIRT1 in mice elicits beneficial phenotypes that may be relevant to human health and longevity.

The impact of cellular senescence onto aging of organisms is not fully clear, not at least because of the scarcity of reliable data on the mere frequency of senescent cells in aging tissues. Activation of a DNA damage response including formation of DNA damage foci containing activated H2A.X (γ‐H2A.X) at either uncapped telomeres or persistent DNA strand breaks is the major trigger of cell senescence. Therefore, γ‐H2A.X immunohistochemistry (IHC) was established by us as a reliable quantitative indicator of senescence in fibroblasts in vitro and in hepatocytes in vivo and the age dependency of DNA damage foci accumulation in ten organs of C57Bl6 mice was analysed over an age range from 12 to 42 months. There were significant increases with age in the frequency of foci‐containing cells in lung, spleen, dermis, liver and gut epithelium. In liver, foci‐positive cells were preferentially found in the centrilobular area, which is exposed to higher levels of oxidative stress. Foci formation in the intestine was restricted to the crypts. It was not associated with either apoptosis or hyperproliferation. That telomeres shortened with age in both crypt and villus enterocytes, but telomeres in the crypt epithelium were longer than those in villi at all ages were confirmed by us. Still, there was no more than random co‐localization between γ‐H2A.X foci and telomeres even in crypts from very old mice, indicating that senescence in the crypt enterocytes is telomere independent. The results suggest that stress‐dependent cell senescence could play a causal role for aging of mice.

Senescent cells produce and secrete various bioactive molecules including interleukins, growth factors, matrix‐degrading enzymes and reactive oxygen species (ROS). Thus, it has been proposed that senescent cells can damage their local environment, and a stimulatory effect on tumour cell growth and invasiveness has been documented. However, it was unknown what effect, if any, senescent cells have on their normal, proliferation‐competent counterparts. We show here that senescent cells induce a DNA damage response, characteristic for senescence, in neighbouring cells via gap junction‐mediated cell–cell contact and processes involving ROS. Continuous exposure to senescent cells induced cell senescence in intact bystander fibroblasts. Hepatocytes bearing senescence markers clustered together in mice livers. Thus, senescent cells can induce a bystander effect, spreading senescence towards their neighbours in vitro and, possibly, in vivo.

Mesenchymal stem cells (MSC) are capable of differentiating into bone, fat, cartilage, tendon and other organ progenitor cells. Despite the abundance of MSC within the organism, little is known about their in vivo properties or about their corresponding in vivo niches. We therefore isolated MSC from spongy (cancellous) bone biopsies of healthy adults. When compared with the surrounding marrow, a fourfold higher number of colony‐forming units was found within the tight meshwork of trabecular bone surface. At these sites, oxygen concentrations range from 1% to 7%. In MSC cultured at oxygen as low as 3%, rates for cell death and hypoxia‐induced gene transcription remained unchanged, while in vitro proliferative lifespan was significantly increased, with about 10 additional population doublings before reaching terminal growth arrest. However, differentiation capacity into adipogenic progeny was diminished and no osteogenic differentiation was detectable at 3% oxygen. In turn, MSC that had previously been cultured at 3% oxygen could subsequently be stimulated to successfully differentiate at 20% oxygen. These data support our preliminary finding that primary MSC are enriched at the surface of spongy bone. Low oxygen levels in this location provide a milieu that extends cellular lifespan and furthermore is instructive for the stemness of MSC allowing proliferation upon stimulation while suppressing differentiation.

The global population of individuals over the age of 65 is growing at an unprecedented rate and is expected to reach 1.6 billion by 2050. Most older individuals are affected by multiple chronic diseases, leading to complex drug treatments and increased risk of physical and cognitive disability. Improving or preserving the health and quality of life of these individuals is challenging due to a lack of well‐established clinical guidelines. Physicians are often forced to engage in cycles of “trial and error” that are centered on palliative treatment of symptoms rather than the root cause, often resulting in dubious outcomes. Recently, geroscience challenged this view, proposing that the underlying biological mechanisms of aging are central to the global increase in susceptibility to disease and disability that occurs with aging. In fact, strong correlations have recently been revealed between health dimensions and phenotypes that are typical of aging, especially with autophagy, mitochondrial function, cellular senescence, and DNA methylation. Current research focuses on measuring the pace of aging to identify individuals who are “aging faster” to test and develop interventions that could prevent or delay the progression of multimorbidity and disability with aging. Understanding how the underlying biological mechanisms of aging connect to and impact longitudinal changes in health trajectories offers a unique opportunity to identify resilience mechanisms, their dynamic changes, and their impact on stress responses. Harnessing how to evoke and control resilience mechanisms in individuals with successful aging could lead to writing a new chapter in human medicine.