Whether there are viable therapeutic targets that would delay or slow the aging process is currently being discussed. To date, the only intervention consistently shown to affect aging is caloric restriction (CR) which has been shown to increase lifespan in a diverse range of species from yeast to monkeys (Mair and Dillin 2008
). In Rhesus monkeys, CR reduces the incidence of age-related deaths and delays the onset of age-associated diseases including cancer, diabetes, cardiovascular diseases, and brain atrophy (Colman et al. 2009
). In humans, although no definitive study has shown an increase in lifespan, CR does improve cardiovascular functions, and reduces inflammation and age-related diseases such as atherosclerosis and diabetes (Holloszy and Fontana 2007
One of the ultimate goals of the aging field is to develop therapeutic agents that may extend human lifespan, and, perhaps more importantly, improve the quality of life or ‘healthspan’ (Nass et al. 2009
). To accomplish this, targets, or biomarkers of aging, must be identified which can be predictive of the aging process. Biomarkers of aging are biological characteristics (e.g., physiological and molecular) that change as a function of age and, therefore, are indicative of a given age period. Traditional physiological markers for old age include gray hair, cataracts (Klein et al. 2002
; Harper et al. 2003
), reduced skin resistance to stress (James et al. 2007
), and reduced immunity (Miller 1996
). Other physiological variables that are used to evaluate aging include changes in blood pressure, forced expiratory capacity, hematocrit, serum albumin, and blood urea nitrogen (Nakamura and Miyao 2007
). In this regard, it has been found that systolic blood pressure and blood urea nitrogen increase with chronological age, whereas forced expiratory volume, hematocrit, and albumin decrease (Nakamura and Miyao 2007
). Another aging biomarker is dehydroepiandrosterone sulfate, an adrenal steroid sulphate, which declines during normal aging in humans (Orentreich et al. 1992
). Lastly, humans and other animals including mice show declined physical activity as they age, partly due to reduced neurotransmission in the central dopamine system (Ingram 2000
). Although these physiological markers are available to evaluate the aging process, they are largely descriptive. Additional biomarkers, ideally specific proteins, could provide potential therapeutic targets and additional insight into the mechanisms of aging.
Serum/plasma proteins are relatively easy to access and are widely used for disease diagnosis. Serum proteins that change as a function of age (i.e., biomarkers of aging) may facilitate the elucidation of the physiological processes of aging and provide insight in therapeutic targets and intervention strategies to impede aging.
Proteomic techniques including two-dimensional gel electrophoresis (2-DE) have been used to study aging (Ballesteros et al. 2001
; Gromov et al. 2003
; Sato et al. 2006
; Miura et al. 2007
) and have led to the identification of many candidate proteins that are altered by aging. For example, older human skin expresses reduced levels of certain proteins, including manganese-superoxide dismutase, tryptophanyl-tRNA synthetase, the p85ß subunit of phosphatidylinositol 3-kinase, and proteasomal proteins PA28-α (Gromov et al. 2003
). In rats, peroxiredoxin-2, an antioxidant enzyme, is able to be induced by irradiation in cultured astrocytes from young but less so from old rats (Miura et al. 2007
). Phosphorylation of α-tubulin is increased with age in rat astrocytes (Miura et al. 2007
) and N-glycosylated proteins including cathepsin D, a lysosomal protease, are found to accumulate in aged rat cerebral cortex (Sato et al. 2006
). Additionally, oxidized proteins have been found to be increased in senescent bacteria cells (Ballesteros et al. 2001
). Since proteomics reveals the total detectable proteins in a given sample, this approach makes it possible to identify additional proteins involved in the aging process. In the present study, 2-DE followed by mass spectrometry (MS), MS/MS and liquid chromatography (LC) MS/MS was used to identify plasma biomarkers of mouse aging.