Genetics of aging

Genetics of longevity

Recent studies on the genetics of aging in the yeast Saccharomyces cerevisiae, the roundworm Caenorhabditis elegans, and the fruit fly Drosophila melanogaster have converged revealing the central role of metabolic capacity and resistance to stress in determining life span. Signal transduction has emerged from these studies as an important molecular mechanism underlying longevity. In their broad features, the results obtained in these genetic models are applicable to the dietary restriction paradigm in mammals, suggesting a general significance. It will be of interest to determine whether many of the molecular details will also pertain. The examination of centenarian populations for the frequency of certain alleles of pertinent genes may provide insights into the relevance of the conclusions of studies in invertebrates to human aging. These population genetic studies can be augmented by mechanistic studies in transgenic mice.

Tau protein pathology in neurodegenerative diseases

Abundant tau-positive neurofibrillary lesions constitute a defining neuropathological characteristic of Alzheimer's disease. Filamentous tau pathology is also central to a number of other dementing disorders, such as Pick's disease, progressive supranuclear palsy, corticobasal degeneration and familial frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17). The discovery of mutations in the tau gene in FTDP-17 has firmly established the relevance of tau pathology for the neurodegenerative process. Experimental studies have provided a system for the assembly of full-length tau into Alzheimer-like filaments, providing an assay for the testing of compounds that inhabit the formation of tau filaments.

Roles of lipid peroxidation in modulation of cellular signaling pathways, cell dysfunction, and death in the nervous system

Free radicals are known to occur as natural by-products under physiological conditions and have been implicated in the neuronal loss observed in a variety of neuropathological conditions including Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD), and ischemia. Oxyradical-induced cytotoxicity arises from both chronic and acute increases in reactive oxygen species which give rise to subsequent lipid peroxidation (LP). By reacting with polyunsaturated fatty acids in the the various cellular membranes, oxyradicals such as hydroxyl (OH.) and peroxynitrite (ONOO) give rise to a variety of lipid peroxidation products (LPP), including 4-hydroxynonenal (HNE) and malondialdehyde (MD). Once formed, these peroxidation metabolites have been demonstrated to have relatively long half-lives within cells (minutes to hours), allowing for multiple interactions with cellular components. Emerging data suggest that LP and LPP may underlie the neuronal alterations and neurotoxicity observed in numerous neurodegenerative conditions. Data supporting this involvement include the detection of LP and formation of LPP in a variety of neuropathological conditions including AD, ALS, PD, and ischemia. Secondly, direct application of LPP, either in vivo or in vitro, has been shown to be cytotoxic and mimic neuronal alterations observed in neuropathological conditions. Furthermore, prevention of LP and subsequent LPP formation have been demonstrated to be neuroprotective in a variety of neurodegenerative paradigms. Additionally, LP and LPP have been implicated in the modulation of a wide array of activities within the central nervous system including long term potentiation, neurite outgrowth, and proliferation. Understanding the mechanism(s) and involvement of LP in these processes will greatly enhance the understanding of oxyradical and ion homeostasis in neurophysiological and neuropathological conditions. The focus of this review is to describe the process by which lipid peroxidation occurs and establish a framework for its involvement in the central nervous system.

Dual modes of aging in Mediterranean fruit fly females

The life history of medflies is characterized by two physiological modes with different demographic schedules of fertility and survival: a waiting mode in which both mortality and reproduction are low and a reproductive mode in which mortality is very low at the onset of egg laying but accelerates as eggs are laid. Medflies stay in waiting mode when they are fed only sugar. When fed protein, a scarce resource in the wild, medflies switch to reproductive mode. Medflies that switch from waiting to reproductive mode survive longer than medflies kept in either mode exclusively. An understanding of the physiological shift that occurs between the waiting and reproductive modes may yield information about the fundamental processes that determine longevity.

Mechanisms of neurofibrillary degeneration and the formation of neurofibrillary tangles

Alzheimer disease (AD) has polyetiology. Independent of the etiology the disease is characterized histopathologically by the intraneuronal accumulation of paired helical filaments (PHF), forming neurofibrillary tangles, neuropil threads and dystrophic neurites surrounding the extracellular deposits of beta-amyloid in plaques, the second major lesion. The clincal expression of AD correlates with the presence of neurofibrillary degeneration; beta-amyloid alone does not produce the disease clinically. Thus arresting neurofibrillary degeneration offers a promising key target for therapeutic intervention of AD. The major protein subunit of PHF is the microtubule-associated protein tau. Tau in AD brain, especially PHF, is abnormally hyperphosphorylated and glycosylated. With maturation, the tangles are increasingly ubiquitinated. Levels of tau and conjugated ubiquitin are elevated both in AD brain and CSF. The AD abnormally phosphorylated tau (AD P-tau) does not promote microtubule assembly, but on dephosphorylation its microtubule promoting activity is restored to approximately that of the normal tau. The AD P-tau competes with tubulin in binding to normal tau, MAP1 and MAP2 and inhibits their microtubule assembly promoting activities. Furthermore, the AD P-tau sequesters normal MAPs from microtubules. The association of AD P-tau with normal tau but not with MAP1 or MAP2 results in the formation of tangles of 3.3 +/- 0.5 mm filaments. Deglycosylation of Alzheimer neurofibrillary tangles with endoglycosidase F/N-glycosidase F untwists the PHF resulting in tangles of thin filaments similar to those formed by association between the AD P-tau and normal tau. Dephosphorylation or deglycosylation plus dephosphorylation but not deglycosylation alone restores the microtubule assembly promoting activity of tau. In vitro AD P-tau can be dephosphorylated by protein phosphatases PP-2B, PP-2A and PP-1 but not PP-2C and all the three tau phosphatases are present in brain neurons. Tau phosphatase activity is decreased by approximately 30% in AD brain. Inhibition of PP-2A and PP-1 activities in SY5Y neuroblastoma by 10 nM okadaic acid causes breakdown of microtubules and the degeneration of these cells. It is suggested (I) that a defect(s) in the protein phosphorylation/dephosphorylation system(s) leads to a hyperphosphorylation of tau, (ii) that this altered tau causes disassembly of microtubules and consequently a retrograde neuronal degeneration; (iii) a pharmacological approach to AD is to enhance the tau phosphatase activity; and (iv) that CSF tau and conjugated ubiquitin levels are promising markers of AD brain pathology.

Aging and apoptosis control

The phenomenon of aging is distinct from processes associated with advanced age known to increase risk of diseases, such as cancer. Furthermore, the process of aging is not necessarily related to phenomena such as in vitro replicative senescence; however, any unifying hypothesis of aging must account for all age-dependent phenomena, including senescence. It is proposed that apoptosis forms the ultimate protective process for preservation of phenotypic fidelity in multicellular organisms since it is the process by which the organism detects damage and replaces the defective cell. Time-dependent degeneration of apoptosis control is the rate-limiting step in the process of aging.

Biodemographic trajectories of longevity

Old-age survival has increased substantially since 1950. Death rates decelerate with age for insects, worms, and yeast, as well as humans. This evidence of extended postreproductive survival is puzzling. Three biodemographic insights--concerning the correlation of death rates across age, individual differences in survival chances, and induced alterations in age patterns of fertility and mortality--offer clues and suggest research on the failure of complicated systems, on new demographic equations for evolutionary theory, and on fertility-longevity interactions. Nongenetic changes account for increases in human life-spans to date. Explication of these causes and the genetic license for extended survival, as well as discovery of genes and other survival attributes affecting longevity, will lead to even longer lives.