The evolution of aging

Stem cell aging in the Drosophila ovary

Accumulating evidence suggests that with time human stem cells may become defective or depleted, thereby contributing to aging and aging-related diseases. Drosophila provides a convenient model system in which to study stem cell aging. The adult Drosophila ovary contains two types of stem cells: the germ-line stem cells give rise to the oocyte and its supporting nurse cells, while the somatic stem cells give rise to the follicular epithelium-a highly differentiated tissue that surrounds each oocyte as it develops. Genetic and transgenic analyses have identified several conserved signaling pathways that function in the ovary to regulate stem cell maintenance, division and differentiation, including the wingless, hedgehog, JAK/STAT, insulin and TGF-β pathways. During Drosophila aging the division of the stem cells decreases dramatically, coincident with reduced egg production. It is unknown if this reproductive senescence is due to a defect in the stem cells themselves, or due to the lack of signals normally sent to the stem cells from elsewhere in the animal, such as from the central nervous system or the stem cell niche. Methods are being developed to genetically mark stem cells in adult Drosophila and measure their survival, division rate and function during aging.

Superoxide dismutase evolution and life span regulation

Superoxide is among the most abundant reactive oxygen species (ROS) produced by the mitochondria, and is involved in cellular signaling pathways. Superoxide and other ROS can damage cellular macromolecules and levels of oxidative damage products are positively correlated with aging. Superoxide dismutase (SOD) enzymes catalyze the breakdown of superoxide into hydrogen peroxide and water and are therefore central regulators of ROS levels. Genetic and transgenic manipulation of SOD activities in model systems such as S. cereviseae, mouse and Drosophila are consistent with a central role for SOD enzymes in regulating oxidative stress resistance. Over-expression of SOD in S. cereviseae and Drosophila can reduce oxidative damage and extend life span, but the mechanism(s) are not yet clear. A phylogenetic analysis of publicly available SOD protein sequences suggests several additional conserved gene families. For example, in addition to the well-characterized soluble Cu/Zn enzyme (Sod) and mitochondrial manganese-containing form (Sod2), Drosophila melanogaster is found to contain a putative copper chaperone (CCS), an extracellular Cu/Zn enzyme (Sod3), and an extracellular protein distantly related to the Cu/Zn forms (Sodq). C. elegans and blue crab are unusual in having two Mn-containing SODs, and A. gambiae contains an unusual internally repeated SOD. The most parsimonius conclusion from the analysis of the extracellular SODs is that they evolved independently multiple times by addition of a signal peptide to cytoplasmic SOD.

Effects of simultaneous over-expression of Cu/ZnSOD and MnSOD on Drosophila melanogaster life span

The FLP-out technique, based on yeast FLP recombinase, allows induced over-expression of transgenes in Drosophila adults. With FLP-out control and over-expressing flies have identical genetic backgrounds and therefore differences in life span must result from transgene induction. The amount of over-expression achieved varies between independent transgenic lines, and previously for both Cu/ZnSOD and MnSOD life span was found to be increased in proportion to the increase in enzyme activity. To determine if greater increases in enzyme and life span could be achieved with FLP-out, enzyme over-expression and life span were analyzed in eight lines containing two MnSOD transgenes, three lines containing three MnSOD transgenes, and three lines containing a MnSOD transgene plus a Cu/ZnSOD transgene. Life span was again found to be increased in proportion to the increase in MnSOD enzyme activity, with increases of up to 40% in mean and maximum life span. However the increases in enzyme activity and life span conferred per transgene were reduced when more than one transgene was present at the same time. When the reduced efficiency of enzyme over-expression per transgene was taken into account, simultaneous over-expression of MnSOD and Cu/ZnSOD was found to have partially additive effects on life span.

Induced overexpression of mitochondrial Mn-superoxide dismutase extends the life span of adult Drosophila melanogaster

A transgenic system ("FLP-out") based on yeast FLP recombinase allowed induced overexpression of MnSOD enzyme in adult Drosophila melanogaster. With FLP-out a brief heat pulse (HP) of young, adult flies triggered the rearrangement and subsequent expression of a MnSOD transgene throughout the adult life span. Control (no HP) and overexpressing (HP) flies had identical genetic backgrounds. The amount of MnSOD enzyme overexpression achieved varied among six independent transgenic lines, with increases up to 75%. Life span was increased in proportion to the increase in enzyme. Mean life span was increased by an average of 16%, with some lines showing 30-33% increases. Maximum life span was increased by an average of 15%, with one line showing as much as 37% increase. Simultaneous overexpression of catalase with MnSOD had no added benefit, consistent with previous observations that catalase is present in excess in the adult fly with regard to life span. Cu/ZnSOD overexpression also increases mean and maximum life span. For both MnSOD and Cu/ZnSOD lines, increased life span was not associated with decreased metabolic activity, as measured by O2 consumption.

Transgenic methods for increasing Drosophila life span

At least five different transgenic approaches have been applied to the study of Drosophila aging. There are two single component systems: transgenes with native (normal) promoters and transgenes with heterologous promoters; as well as three binary systems: 'GAL4/UAS', 'FLP-out' and 'tet-on'. These approaches vary in ability to meet several technical challenges, and the relative advantages and disadvantages of each are discussed. Using these techniques, over-expression of the hsp70, Cu/ZnSOD and MnSOD genes has each been demonstrated to increase Drosophila life span.

Sequence requirements for upregulated expression of Drosophila hsp70 transgenes during aging

hsp70 protein and hsp70:lacZ fusion reporters are upregulated during aging and in response to oxidative stress in the thorax of Drosophila. hsp70 expression was increased during aging in each of seven different Drosophila genetic backgrounds tested, 2.6-4.8-fold. DNA sequence requirements were investigated by analysis of nine distinct hsp70:lacZ fusion reporter constructs in multiple independent transgenic lines. hsp70 sequences -194 to +276 supported an average 2.7-fold increase during aging. This increase was reduced or eliminated by deletion or point mutation of the heat shock response elements, consistent with a transcriptional mechanism. Similar sequence requirements were observed for increased expression in response to catalase null mutation as a model of oxidative stress. hsp70 5'UTR sequences were required for efficient basal expression of transgenes, but were not sufficient to confer detectable upregulation during aging. Inclusion of additional hsp70 coding region sequences from +276 to + 1011 created a larger hsp70:lacZ fusion protein and had two effects: dramatic reduction of the overall expression level of the fusion protein, and an additional three to fourfold upregulation during aging. These results suggest that the coding region sequences reduce fusion protein abundance and that this negative effect decreases as a function of age. The data support a model for increased expression of hsp70 transgenes during aging involving both transcriptional and posttranscriptional components.

Invertebrate gerontology: the age mutations of Caenorhabditis elegans

Ageing is a complex phenomenon which remains a major challenge to modern biology. Although the evolutionary biology of ageing is well understood, the mechanisms that limit lifespan are unknown. The isolation and analysis of single-gene mutations which extend lifespan (Age mutations) is likely to reveal processes which influence ageing. Caenorhabditis elegans is the only metazoan in which Age mutations have been identified. The Age mutations not only prolong life, but also confer a complex array of other phenotypes. Some of these phenotypes provide clues to the evolutionary origins of these genes while others allude to mechanisms of lifespan-extension. Many of the Age genes interact and share a second common phenotype, that of stress resistance. Rather than invertebrate ageing being determined by a 'clock mechanism', a picture is emerging of ageing as a non-adaptive process determined, in part, by resistance to intrinsic stress mediated by stress-response genes.