Regulation of basal autophagy by calmodulin availability

Macroautophagy (hereafter autophagy) is a process that degrades cellular components to maintain homeostasis. The Ca2+ sensor calmodulin (CaM) regulates numerous cell functions but is a limiting factor due to its insufficient availability for all target proteins. However, evidence that CaM availability regulates basal autophagy is lacking. Here, we have tested this hypothesis. CaM antagonists W‐7, trifluoperazine and CGS9343b cause autophagosome accumulation and inhibit basal autophagic flux in the same manner as does chloroquine. These reagents promote the activity of AMP‐activated protein kinase (AMPK) but not that of the mechanistic target of rapamycin (mTOR). Competitive binding assays using CaM sensors with different Ca2+ dependencies showed that chloroquine directly binds CaM in a Ca2+‐dependent fashion. The CaM antagonists have disparate effects on cytoplasmic Ca2+, triggering from none to robust signals, indicating that their consistent inhibition of autophagy is due to inhibition of CaM and not Ca2+. Chelating intracellular Ca2+ reduces the effect of the CaM antagonists to accumulate LC3‐II, indicating that they do so by inhibiting CaM‐dependent activities at basal Ca2+ level. The CaM antagonists cause lysosomal alkalinisation. Consistently, buffering CaM with a high‐affinity CaM‐binding protein that binds CaM at resting Ca2+ level increases lysosomal pH. Enhanced CaM buffering using a chimeric protein that contains two high‐affinity CaM‐binding sites that can collectively bind CaM at a large range of Ca2+ further increases lysosomal pH and increases LC3‐II accumulation and AMPK activity, but not that of mTOR. These data demonstrate that CaM availability is required for basal autophagy. Autophagy is a process that degrades unnecessary cellular components to maintain homeostasis. Calmodulin is the ubiquitous calcium sensor and is required for the activities of numerous proteins. However, it is not produced sufficiently for all its target proteins, rendering a universal status of limiting calmodulin. This work demonstrates that the availability of calmodulin is required for basal autophagy. Factors that regulate calmodulin availability are thus predicted to regulate this process.