Live Fast, Die Young: New Lessons in Mammalian Longevity

Crosstalk Between Senescent Bone Cells and the Bone Tissue Microenvironment Influences Bone Fragility During Chronological Age and in Diabetes

Bone is a complex organ serving roles in skeletal support and movement, and is a source of blood cells including adaptive and innate immune cells. Structural and functional integrity is maintained through a balance between bone synthesis and bone degradation, dependent in part on mechanical loading but also on signaling and influences of the tissue microenvironment. Bone structure and the extracellular bone milieu change with age, predisposing to osteoporosis and increased fracture risk, and this is exacerbated in patients with diabetes. Such changes can include loss of bone mineral density, deterioration in micro-architecture, as well as decreased bone flexibility, through alteration of proteinaceous bone support structures, and accumulation of senescent cells. Senescence is a state of proliferation arrest accompanied by marked morphological and metabolic changes. It is driven by cellular stress and serves an important acute tumor suppressive mechanism when followed by immune-mediated senescent cell clearance. However, aging and pathological conditions including diabetes are associated with accumulation of senescent cells that generate a pro-inflammatory and tissue-destructive secretome (the SASP). The SASP impinges on the tissue microenvironment with detrimental local and systemic consequences; senescent cells are thought to contribute to the multimorbidity associated with advanced chronological age. Here, we assess factors that promote bone fragility, in the context both of chronological aging and accelerated aging in progeroid syndromes and in diabetes, including senescence-dependent alterations in the bone tissue microenvironment, and glycation changes to the tissue microenvironment that stimulate RAGE signaling, a process that is accelerated in diabetic patients. Finally, we discuss therapeutic interventions targeting RAGE signaling and cell senescence that show promise in improving bone health in older people and those living with diabetes.

Peptidylprolyl Isomerases as In Vivo Carriers for Drugs That Target Various Intracellular Entities

Analyses of sequences and structures of the cyclosporine A (CsA)-binding proteins (cyclophilins) and the immunosuppressive macrolide FK506-binding proteins (FKBPs) have revealed that they exhibit peculiar spatial distributions of charges, their overall hydrophobicity indexes vary within a considerable level whereas their points isoelectric (pIs) are contained from 4 to 11. These two families of peptidylprolyl cis/trans isomerases (PPIases) have several distinct functional attributes such as: (1) high affinity binding to some pharmacologically-useful hydrophobic macrocyclic drugs; (2) diversified binding epitopes to proteins that may induce transient manifolds with altered flexibility and functional fitness; and (3) electrostatic interactions between positively charged segments of PPIases and negatively charged intracellular entities that support their spatial integration. These three attributes enhance binding of PPIase/pharmacophore complexes to diverse intracellular entities, some of which perturb signalization pathways causing immunosuppression and other system-altering phenomena in humans.

Animal and human models to understand ageing

Human ageing is the gradual decline in organ and tissue function with increasing chronological time, leading eventually to loss of function and death. To study the processes involved over research-relevant timescales requires the use of accessible model systems that share significant similarities with humans. In this review, we assess the usefulness of various models, including unicellular yeasts, invertebrate worms and flies, mice and primates including humans, and highlight the benefits and possible drawbacks of each model system in its ability to illuminate human ageing mechanisms. We describe the strong evolutionary conservation of molecular pathways that govern cell responses to extracellular and intracellular signals and which are strongly implicated in ageing. Such pathways centre around insulin-like growth factor signalling and integration of stress and nutritional signals through mTOR kinase. The process of cellular senescence is evaluated as a possible underlying cause for many of the frailties and diseases of human ageing. Also considered is ageing arising from systemic changes that cannot be modelled in lower organisms and instead require studies either in small mammals or in primates. We also touch briefly on novel therapeutic options arising from a better understanding of the biology of ageing.

Reversal of phenotypes of cellular senescence by pan-mTOR inhibition

Cellular senescence, a state of essentially irreversible proliferation arrest, serves as a potent tumour suppressor mechanism. However, accumulation of senescent cells with chronological age is likely to contribute to loss of tissue and organ function and organismal aging. A crucial biochemical modulator of aging is mTOR; here, we have addressed the question of whether acute mTORC inhibition in near-senescent cells can modify phenotypes of senescence. We show that acute short term treatment of human skin fibroblasts with low dose ATP mimetic pan-mTORC inhibitor AZD8055 leads to reversal of many phenotypes that develop as cells near replicative senescence, including reduction in cell size and granularity, loss of SA-β-gal staining and reacquisition of fibroblastic spindle morphology. AZD8055 treatment also induced rearrangement of the actin cytoskeleton, providing a possible mechanism of action for the observed rejuvenation. Importantly, short-term drug exposure had no detrimental effects on cell proliferation control across the life-course of the fibroblasts. Our findings suggest that combined inhibition of both mTORC1 and mTORC2 may provide a promising strategy to reverse the development of senescence-associated features in near-senescent cells.

The role of DNA exonucleases in protecting genome stability and their impact on ageing

Exonucleases are key enzymes involved in many aspects of cellular metabolism and maintenance and are essential to genome stability, acting to cleave DNA from free ends. Exonucleases can act as proof-readers during DNA polymerisation in DNA replication, to remove unusual DNA structures that arise from problems with DNA replication fork progression, and they can be directly involved in repairing damaged DNA. Several exonucleases have been recently discovered, with potentially critical roles in genome stability and ageing. Here we discuss how both intrinsic and extrinsic exonuclease activities contribute to the fidelity of DNA polymerases in DNA replication. The action of exonucleases in processing DNA intermediates during normal and aberrant DNA replication is then assessed, as is the importance of exonucleases in repair of double-strand breaks and interstrand crosslinks. Finally we examine how exonucleases are involved in maintenance of mitochondrial genome stability. Throughout the review, we assess how nuclease mutation or loss predisposes to a range of clinical diseases and particularly ageing.

NAD(+) and metabolic regulation of age-related proteoxicity: A possible role for methylglyoxal?

A common biochemical characteristic of aging is proteotoxicity i.e. the accumulation of altered proteins. While many studies have shown that NAD(+) is important when lifespan is modulated by dietary means, it is uncertain whether or how NAD(+) affects either formation or elimination of altered proteins. It is suggested here that changes in NAD(+) availability can affect generation of methylglyoxal (MG) from glycolytic intermediates, which in turn can damage proteins, promote generation of reactive oxygen species and induce mitochondrial dysfunction. The proposal can also help to explain how, by altering NAD(+) regeneration from NADH, dietary supplementation with glycerol or glucose, which decreases lifespan in the nematode Caenorhabditis elegans, could cause MG generation to increase, whereas oxaloacetate supplementation, which increases lifespan, could lower the potential for MG formation. The proposal also suggests how upregulation of mitogenesis and mitochondrial activity, and increased aerobic activity, help to decrease the potential for MG formation and resultant proteotoxicity, with consequential beneficial effects with respect to aging.

Increasing longevity through caloric restriction or rapamycin feeding in mammals: common mechanisms for common outcomes?

Significant extension of lifespan in important mammalian species is bound to attract the attention not only of the aging research community, but also the media and the wider public. Two recent papers published by Harrison et al. (2009) in Nature and by Colman et al. (2009) in Science report increased longevity of mice fed with rapamycin and of rhesus monkeys undergoing caloric restriction, respectively. These papers have generated considerable debate in the aging community. Here we assess what is new about these findings, how they fit with our knowledge of lifespan extension from other studies and what prospects this new work holds out for improvements in human longevity and human health span.