Cells and organisms must coordinate their metabolic activity with changes in their environment to ensure their growth only when conditions are favourable. In order to maintain cellular homoeostasis, a tight regulation between the synthesis and degradation of cellular components is essential. At the epicentre of the cellular nutrient sensing is the mechanistic target of rapamycin complex 1 (mTORC1) which connects environmental cues, including nutrient and growth factor availability as well as stress, to metabolic processes in order to preserve cellular homoeostasis. Under nutrient-rich conditions mTORC1 promotes cell growth by stimulating biosynthetic pathways, including synthesis of proteins, lipids and nucleotides, and by inhibiting cellular catabolism through repression of the autophagic pathway. Its close signalling interplay with the energy sensor AMP-activated protein kinase (AMPK) dictates whether the cell actively favours anabolic or catabolic processes. Underlining the role of mTORC1 in the coordination of cellular metabolism, its deregulation is linked to numerous human diseases ranging from metabolic disorders to many cancers. Although mTORC1 can be modulated by a number of different inputs, amino acids represent primordial cues that cannot be compensated for by any other stimuli. The understanding of how amino acids signal to mTORC1 has increased considerably in the last years; however this area of research remains a hot topic in biomedical sciences. The current ideas and models proposed to explain the interrelationship between amino acid sensing, mTORC1 signalling and autophagy is the subject of the present review.
Essays in Biochemistry volume 61 (issue 6), entitled Signalling Mechanisms in Autophagy , covers a range of topics in autophagy signalling, touching on emerging new details on the mechanisms of autophagy regulation, novel aspects of selective autophagy and how autophagy functions in organelle homeostasis. It also looks at how autophagy research is leading to better understanding of human disease and plant biology that can be exploited for the benefit of society.
Autophagy is a process of lysosome-dependent intracellular degradation that participates in the liberation of resources including amino acids and energy to maintain homoeostasis. Autophagy is particularly important in stress conditions such as nutrient starvation and any perturbation in the ability of the cell to activate or regulate autophagy can lead to cellular dysfunction and disease. An area of intense research interest is the role and indeed the fate of autophagy during cellular and organismal ageing. Age-related disorders are associated with increased cellular stress and assault including DNA damage, reduced energy availability, protein aggregation and accumulation of damaged organelles. A reduction in autophagy activity has been observed in a number of ageing models and its up-regulation via pharmacological and genetic methods can alleviate age-related pathologies. In particular, autophagy induction can enhance clearance of toxic intracellular waste associated with neurodegenerative diseases and has been comprehensively demonstrated to improve lifespan in yeast, worms, flies, rodents and primates. The situation, however, has been complicated by the identification that autophagy up-regulation can also occur during ageing. Indeed, in certain situations, reduced autophagosome induction may actually provide benefits to ageing cells. Future studies will undoubtedly improve our understanding of exactly how the multiple signals that are integrated to control appropriate autophagy activity change during ageing, what affect this has on autophagy and to what extent autophagy contributes to age-associated pathologies. Identification of mechanisms that influence a healthy lifespan is of economic, medical and social importance in our ‘ageing’ world.