Longevity, genes, and aging

The prohibitin family of mitochondrial proteins regulate replicative lifespan

Cellular senescence is determined by multiple factors, including the genetic regulation of metabolism and responses to endogenous and exogenous stresses [1-4]. Recent studies implicate a limited number of gene products in elongating lifespan in yeast and Caenorhabditis elegans [2-4]; these include the C, elegans gene cik-1, a central regulator of metabolism [5], and yeast RAS2, which controls the response to ultraviolet irradiation and other stresses [3]. Another gene postulated to effect senescence is PHB1, the yeast homologue of prohibitin [3], a rodent gene initially identified as a potential regulator of growth arrest and tumour suppressor [6-8]. Highly conserved prohibitin homologues have been identified in mammals [9], Drosophila [10], C. elegans [9], plants [11] and yeast. A second mammalian gene, encoding BAP37, a protein with sequence similarity to prohibitin, is thought to be involved in lymphocyte function [9]. Here, we show that the nuclear-encoded mammalian prohibitin and BAP37 proteins are present in mitochondria, are co-expressed, and interact physically with each other. Deletion of the Saccharomyces cerevisiae homologues, PHB1 and PHB2, results in a decreased replicative lifespan and a defect in mitochondrial membrane potential. Our observations highlight the relationship between the metabolic efficiency of cells and the ageing process, and provide evidence for its evolutionary conservation.

Genetic control of programmed cell death and aging in the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has been used extensively as a model system for the study of basic biological processes. In this species, apoptosis and aging are both under genetic control. Molecular studies have shown that the death machinery that kills C. elegans cells has remained conserved through evolution and also functions to promote apoptotic death in mammalian cells. At least some of the genes that affect C. elegans life span are also evolutionarily conserved; whether the vertebrate homologs of these genes also influence life span remains to be determined. Although a large number of mutations have been isolated that affect either apoptosis or aging in C. elegans, there is so far no evidence that the genetic pathways that control these processes might overlap.

The heterochromatin loss model of aging

There are significant changes in gene expression that occur with cellular senescence and organismic aging. Genes residing in compacted heterochromatin domains are typically silenced due to an altered accessibility to transcription factors. Heterochromatin domains and gene silencing are set up in early development and were initially believed to be maintained for the remainder of the lifespan. Recent data suggest that there may be a net loss of heterochromatin with advancing age in both yeast and mice. The gradual loss of heterochromatin-induced gene silencing could explain the changes in gene expression that are closely linked with aging. A general model is proposed for heterochromatin loss as a major factor in generating alterations in gene expression with age. The heterochromatin loss model is supported by several lines of evidence and suggests that a fundamental genetic mechanism underlies most of the changes in gene expression observed with senescence.

Regulation of apoptosis and cell cycle arrest by Zac1, a novel zinc finger protein expressed in the pituitary gland and the brain

The proliferation rate of a cell population reflects a balance between cell division, cell cycle arrest, differentiation and apoptosis. The regulation of these processes is central to development and tissue homeostasis, whereas dysregulation may lead to overt pathological outcomes, notably cancer and neurodegenerative disorders. We report here the cloning of a novel zinc finger protein which regulates apoptosis and cell cycle arrest and was accordingly named Zac1. In vitro Zac1 inhibited proliferation of tumor cells, as evidenced by measuring colony formation, growth rate and cloning in soft agar. In vivo Zac1 abrogated tumor formation in nude mice. The antiproliferative activity of Zac1 was due to induction of extensive apoptosis and of G1 arrest, which proceeded independently of retinoblastoma protein and of regulation of p21(WAF1/Cip1), p27Kip1, p57Kip2 and p16INK4a expression. Zac1-mediated apoptosis was unrelated to cell cycle phase and G1 arrest was independent of apoptosis, indicating separate control of apoptosis and cell cycle arrest. Zac1 is thus the first gene besides p53 which concurrently induces apoptosis and cell cycle arrest.

Early mortality surge in protein-deprived females causes reversal of sex differential of life expectancy in Mediterranean fruit flies

Experiments based on over 400,000 medflies revealed that females maintained on a normal diet (sucrose plus protein) have a higher life expectancy than males maintained on a normal diet, with a difference of 1.30 +/- 0.27 days in favor of females. However, this sex differential reverses under protein deprivation, with a difference of 2.24 +/- 0.18 days in favor of males. The reversal of the male-female life expectancy differential is caused by a sustained surge in early female mortality under protein deprivation that is tied to egg-laying and physiological processes. In contrast, male mortality and life expectancy are only mildly affected by protein deprivation. The surge in early mortality for female medfly cohorts is an instance of a vulnerable period. These vulnerable periods are linked with patterns in hazard rates.

Characterization of two age-induced intracisternal A-particle-related transcripts in the mouse liver. Transcriptional read-through into an open reading frame with similarities to the yeast ccr4 transcription factor

Intracisternal A-particle (IAP) sequences are endogenous retrovirus-like elements present at 1,000 copies in the mouse genome. We had previously identified IAP-related transcripts of unusual size (6 and 10 kilobases (kb)), which are observed exclusively in the liver of the aging mouse. In this report, using cDNA libraries that we have constructed from the liver mRNAs of an aged DBA/2 mouse, we have cloned and entirely sequenced the corresponding cDNAs. Both are initiated within the 5' long terminal repeat of a type IDelta1 IAP sequence, and correspond to a read-through into a unique flanking cellular sequence containing a 966-nucleotide open reading frame, located 3' to the IAP sequence. The 6-kb IAP-related transcript corresponds to a post-transcriptional modification of the 10-kb mRNA, and is generated by a splicing event with the donor site in the IAP sequence, and the acceptor site 5' to the open reading frame. This open reading frame is located on chromosome 3, is evolutionarily conserved, and discloses significant similarity to the yeast CCR4 transcription factor at the amino acid level. The specific expression of these age-induced transcripts, which account for more than 50% of the IAP-related transcripts in the liver of old mice, is therefore entirely consistent with the induction of a single genomic locus, thus strengthening the importance of position effects for the expression of transposable elements. Characterization of this locus should now allow studies on its chromatin and methylation status, and on the "molecular factors of senescence" possibly involved in its induction.

What demographers can learn from fruit fly actuarial models and biology

Historically demographers have viewed the results of actuarial studies of nonhuman species, particularly those on invertebrates such as fruit flies, as largely irrelevant to investigations on human populations. In this paper I present life table data from large scale studies on the Mediterranean fruit fly, and show that they provide important insights into fundamental aspects of mortality relevant to human populations: the trajectory of mortality at older ages, sex mortality differentials, the concept of maximal life span, and demographic heterogeneity and selection. An overriding theme of the paper is the need for demographers to acquire a heightened awareness of new developments in biology including areas such as evolutionary ecology, experimental demography, and molecular medicine.