Sixth significant progress has been made during the

Sixth, significant progress has been made during the last few years regarding the molecular mechanisms underlying the activation of AMPK and this should aid to develop more potent and specific compounds. Information from crystallographic structures combined with computer-based drug design hold promise as a rational approach for discovering isoform-specific small molecule AMPK activators [129], [130]. Identification of novel β1-selective AMPK activators by virtual screening using molecular docking was recently reported [131] and holds promise on structure-based design of novel isoform-selective AMPK activators. In addition, improvement of screening methods based on the measure of direct allosteric activation, protection from dephosphorylation or a combination of both can contribute to fasten the identification of new hits [73], [74], [132]. Also, the use of specially-configured biosensors to monitor AMPK activation or conformational changes induced by ligands opens possibilities for identification and characterization of AMPK activators in cell-based assays [133].
Conclusion Given the pleiotropic action of AMPK signaling on many cellular processes (e.g., increase glucose uptake in skeletal muscle), AMPK has gained much attention as a therapeutic target and there has been a major drive to develop pharmacological activators. The first described pharmacological AMPK activator was the pro-drug AICAR, metabolized into ZMP to mimick the allosteric effects of AMP on the AMPK system owing to its structural analogy with the Pentostatin nucleotide. However, its therapeutic use in humans have so far been inconclusive, given to its relatively high threshold for AMPK activation in skeletal muscle [134]. In 2006, discovery of the first direct AMPK activator, A-769662, provided a novel insight into the development of direct AMPK activators by demonstrating that AMPK activation with non-nucleotide ligands was possible [75]. Several new direct AMPK activators have been reported (Fig. 3). They exhibit different mechanisms of action through binding to a novel allosteric site formed between the small lobe of the α-KD and the β-GBD (ADaM site) or a site located within the solvent-accessible core of the AMPK-γ subunit. Interestingly, combination strategies involving compounds binding at these different sites resulted in a synergistic effect on the phosphorylation of AMPK and physiological effects in intact cells [71], [96], [135], [136], [137], [138]. These findings are highly relevant from a pharmaceutical point of view for the development of combined treatment of various AMPK agonists such as direct small molecule AMPK activators and the indirect AMPK activator metformin. These combinatorial treatments would be of value to enhance AMPK activation in patients and could help to reduce the amount of drugs administrated for a better tolerability and efficacy. While some small molecules are potent pan-activators for all 12 heterotrimeric AMPK complexes, other show isoform-specific activation for AMPK complexes containing the α1- or β1-isoforms. These findings indicate the possibility of developing isoform-selective AMPK activators for tissue-specific therapeutic interventions. For instance, one interesting therapeutic aspect will be to target the AMPK α2β2γ3 heterotrimer which is specifically expressed in skeletal muscle and is predominantly activated by exercise compared to other AMPK heterotrimers [64] and thus, limit potential adverse effect on cardiac hypertrophy [111].
Acknowledgements This work was supported by grants from Inserm, CNRS, Université Paris Descartes, Société Francophone du Diabète (SFD) and Fondation pour la Recherche Médicale (FRM) grant number DEQ20150934718. This work was performed within the Département Hospitalo-Universitaire (DHU) AUToimmune and HORmonal diseaseS (AUTHORS). S.O. holds a doctoral fellowship from the Région Ile-de-France (CORDDIM).