Mitochondrial Metabolism and Aging in Yeast.

Mitochondrial functionality is one of the main factors involved in cell survival, and mitochondrial dysfunctions have been identified as an aging hallmark. In particular, the insurgence of mitochondrial dysfunctions is tightly connected to mitochondrial metabolism. During aging, both mitochondrial oxidative and biosynthetic metabolisms are progressively altered, with the development of malfunctions, in turn affecting mitochondrial functionality. In this context, the relation between mitochondrial pathways and aging is evolutionarily conserved from single-celled organisms, such as yeasts, to complex multicellular organisms, such as humans. Useful information has been provided by the yeast Saccharomyces cerevisiae, which is being increasingly acknowledged as a valuable model system to uncover mechanisms underlying cellular longevity in humans. On this basis, we review the impact of specific aspects of mitochondrial metabolism on aging supported by the contributions brought by numerous studies performed employing yeast. Initially, we will focus on the tricarboxylic acid cycle and oxidative phosphorylation, describing how their modulation has consequences on cellular longevity. Afterward, we will report information regarding the importance of nicotinamide adenine dinucleotide (NAD) metabolism during aging, highlighting its relation with mitochondrial functionality. The comprehension of these key points regarding mitochondrial metabolism and their physiological importance is an essential first step for the development of therapeutic interventions that point to increase life quality during aging, therefore promoting "healthy aging," as well as lifespan itself.

Altered Expression of Mitochondrial NAD+ Carriers Influences Yeast Chronological Lifespan by Modulating Cytosolic and Mitochondrial Metabolism.

Nicotinamide adenine dinucleotide (NAD+) represents an essential cofactor in sustaining cellular bioenergetics and maintaining cellular fitness, and has emerged as a therapeutic target to counteract aging and age-related diseases. Besides NAD+ involvement in multiple redox reactions, it is also required as co-substrate for the activity of Sirtuins, a family of evolutionary conserved NAD+-dependent deacetylases that regulate both metabolism and aging. The founding member of this family is Sir2 of Saccharomyces cerevisiae, a well-established model system for studying aging of post-mitotic mammalian cells. In this context, it refers to chronological aging, in which the chronological lifespan (CLS) is measured. In this paper, we investigated the effects of changes in the cellular content of NAD+ on CLS by altering the expression of mitochondrial NAD+ carriers, namely Ndt1 and Ndt2. We found that the deletion or overexpression of these carriers alters the intracellular levels of NAD+ with opposite outcomes on CLS. In particular, lack of both carriers decreases NAD+ content and extends CLS, whereas NDT1 overexpression increases NAD+ content and reduces CLS. This correlates with opposite cytosolic and mitochondrial metabolic assets shown by the two types of mutants. In the former, an increase in the efficiency of oxidative phosphorylation is observed together with an enhancement of a pro-longevity anabolic metabolism toward gluconeogenesis and trehalose storage. On the contrary, NDT1 overexpression brings about on the one hand, a decrease in the respiratory efficiency generating harmful superoxide anions, and on the other, a decrease in gluconeogenesis and trehalose stores: all this is reflected into a time-dependent loss of mitochondrial functionality during chronological aging.

Skin infections are eliminated by cooperation of the fibrinolytic and innate immune systems.

Nuclear factor of activated T cells (NFAT) is activated in innate immune cells downstream of pattern recognition receptors, but little is known about NFAT's functions in innate immunity compared with adaptive immunity. We show that early activation of NFAT balances the two major phases of the innate response to Candida albicans skin infections: the protective containment (abscess) and the elimination (expulsion) phases. During the early containment phase, transforming growth factor-ß (TGF-ß) induces the deposit of collagen around newly recruited polymorphonuclear cells to prevent microbial spreading. During the elimination phase, interferon-γ (IFN-γ) blocks differentiation of fibroblasts into myofibroblasts by antagonizing TGF-ß signaling. IFN-γ also induces the formation of plasmin that, in turn, promotes abscess capsule digestion and skin ulceration for microbial discharge. NFAT controls innate IFN-γ production and microbial expulsion. This cross-talk between the innate immune and the fibrinolytic systems also occurs during infection with Staphylococcus aureus and is a protective response to minimize tissue damage and optimize pathogen elimination.

During yeast chronological aging resveratrol supplementation results in a short-lived phenotype Sir2-dependent.

Resveratrol (RSV) is a naturally occurring polyphenolic compound endowed with interesting biological properties/functions amongst which are its activity as an antioxidant and as Sirtuin activating compound towards SIRT1 in mammals. Sirtuins comprise a family of NAD+-dependent protein deacetylases that are involved in many physiological and pathological processes including aging and age-related diseases. These enzymes are conserved across species and SIRT1 is the closest mammalian orthologue of Sir2 of Saccharomyces cerevisiae. In the field of aging researches, it is well known that Sir2 is a positive regulator of replicative lifespan and, in this context, the RSV effects have been already examined. Here, we analyzed RSV effects during chronological aging, in which Sir2 acts as a negative regulator of chronological lifespan (CLS). Chronological aging refers to quiescent cells in stationary phase; these cells display a survival-based metabolism characterized by an increase in oxidative stress. We found that RSV supplementation at the onset of chronological aging, namely at the diauxic shift, increases oxidative stress and significantly reduces CLS. CLS reduction is dependent on Sir2 presence both in expired medium and in extreme Calorie Restriction. In addition, all data point to an enhancement of Sir2 activity, in particular Sir2-mediated deacetylation of the key gluconeogenic enzyme phosphoenolpyruvate carboxykinase (Pck1). This leads to a reduction in the amount of the acetylated active form of Pck1, whose enzymatic activity is essential for gluconeogenesis and CLS extension.