The regulation of autophagy by calcium signals: Do we have a consensus?

Macroautophagy (hereafter called 'autophagy') is a cellular process for degrading and recycling cellular constituents, and for maintenance of cell function. Autophagy initiates via vesicular engulfment of cellular materials and culminates in their degradation via lysosomal hydrolases, with the whole process often being termed 'autophagic flux'. Autophagy is a multi-step pathway requiring the interplay of numerous scaffolding and signalling molecules. In particular, orthologs of the family of ∼30 autophagy-regulating (Atg) proteins that were first characterised in yeast play essential roles in the initiation and processing of autophagic vesicles in mammalian cells. The serine/threonine kinase mTOR (mechanistic target of rapamycin) is a master regulator of the canonical autophagic response of cells to nutrient starvation. In addition, AMP-activated protein kinase (AMPK), which is a key sensor of cellular energy status, can trigger autophagy by inhibiting mTOR, or by phosphorylating other downstream targets. Calcium (Ca²⁺) has been implicated in autophagic signalling pathways encompassing both mTOR and AMPK, as well as in autophagy seemingly not involving these kinases. Numerous studies have shown that cytosolic Ca²⁺ signals can trigger autophagy. Moreover, introduction of an exogenous chelator to prevent cytosolic Ca²⁺ signals inhibits autophagy in response to many different stimuli, with suggestions that buffering Ca²⁺ affects not only the triggering of autophagy, but also proximal and distal steps during autophagic flux. Observations such as these indicate that Ca²⁺ plays an essential role as a pro-autophagic signal. However, cellular Ca²⁺ signals can exert anti-autophagic actions too. For example, Ca²⁺ channel blockers induce autophagy due to the loss of autophagy-suppressing Ca²⁺ signals. In addition, the sequestration of Ca²⁺ by mitochondria during physiological signalling appears necessary to maintain cellular bio-energetics, thereby suppressing AMPK-dependent autophagy. This article attempts to provide an integrated overview of the evidence for the proposed roles of various Ca²⁺ signals, Ca²⁺ channels and Ca²⁺ sources in controlling autophagic flux.