Signaling pathways in eukaryotic stress, aging, and senescence: Common and distinct pathways

Abstract

Aging has been often described as a set of intricate changes occurring in an organism leading to a decline in its physiological functions, ultimately promoting disease and death. Senescence is also a term commonly associated with wear and tear of an organism that occurs with age. Aging as a process has long been thought to be a random process without any strict molecular basis. However, discovery of various conserved signaling pathways controlling longevity has provided strong proof for aging to be under the control of a programmed pathway. Multiple signaling cascades such as insulin signaling and TOR (target of rapamycin) pathway have been shown to be critical regulators of lifespan and aging-related processes across species. Improved longevity has also been associated with increased stress resistance suggesting cross talk between longevity and stress signaling pathways. Plant aging and senescence differ from that of other eukaryotes in terms of a broader range of lifespan observed across plant species that could be explained by different modes of nutrient accumulation and means of reproduction. Different signaling cascades such as those involved in sugar sensing and nutrient sensing in general have been found to be playing an important role in plant longevity. Existence of similar homologues of these proteins in animal kingdom that perform similar roles in aging-associated functions suggests some degree of conservation in pathways controlling aging across plant and animal kingdom. TOR pathway is one such signaling pathway, which is a well-known regulator of lifespan in animals and has been recently shown to be important for plant longevity as well. Besides nutrient signaling, different classes of hormones have also been implicated in plant stresses and senescence suggesting the existence of a complex interplay between these different physiological and environmental signals in regulating plant aging. With the advent of functional genomic approaches such as whole genome microarray, proteomics and ChIP-based (chromatin immunoprecipitation) sequencing have been utilized to understand the molecular mechanisms underlying the aging process. In this chapter, we attempt to summarize such findings that are relevant to aging and senescence in different organisms such as animal and plant model system