Exercise is an effective intervention for lifestyle-related diseases with clear benefits in type 2 diabetes (T2D)1. Exercise causes extensive molecular perturbations which initiate protein phosphorylation events that lead to acute and long-term adaptations2. Fortuitously, these adaptations improve processes that are defective in T2D including insulin sensitivity, glucose uptake, mitochondrial biogenesis and defence against a range of intracellular stresses. Unfortunately, many individuals affected by T2D are not capable of meeting exercise recommendations due to comorbidities such as cardiovascular disease and sarcopenia. Tapping into the benefits of exercise at a molecular level could be of major utility for combating T2D.
The most well-characterised exercise-responsive kinase is AMP-activated protein kinase (AMPK). Mice lacking AMPK in skeletal muscle are exercise intolerant4 and prone to developing insulin resistance5. However, the full repertoire of downstream AMPK phosphotargets is not known. We have recently used mass spectrometry to expand the signalling network in exercised human skeletal muscle, including several novel AMPK phosphotargets3. Among these novel AMPK phosphotargets is stromal interaction molecule 1 (STIM1), which is required for store-operated Ca2+ entry (SOCE), the process of refilling the sarcoplasmic reticulum with Ca2+ following muscle contraction. We demonstrate that phosphorylation of this site on STIM1 is dependent on AMPK in AMPK knockout cells. Using an in vitro kinase assay, we confirm that AMPK directly phosphorylates STIM1 at this site. Furthermore, using site-directed mutagenesis and fluorescence lifetime imaging microscopy, we show this phosphorylation affects STIM1 activation.
This discovery is intriguing in light of the role of Ca2+ as a potent signalling molecule which regulates diverse processes including muscle contraction, nutrient metabolism, and antioxidant defence6. Dysregulated Ca2+ in skeletal muscle is one of the most consistent features of T2D and is proposed to underlie many of the disease pathologies7. This project makes possible development of new targeted therapies to recapitulate exercise benefits.