Neural, hormonal and nutritional control of autophagy

Abstract

Urocortin 2 (Ucn2) is a peptide of 38 amino acids, member of the corticotrophin releasing factor (CRF) neuropeptide family. The skeletal muscle express both Ucn2 and its specific receptor CRF2R² (corticotrophin releasing factor 2² receptor), although this notary autocrine/paracrine action of this peptide, practically, nothing is known about its skeletal muscles actions (Reyes et al., 2001, Hsu & Hsueh, 2001, Chen et al., 2004, Chen et al., 2006). CRF2R² is known to be a Gs protein-bound receptor, which activates adenylyl cyclase and PKA (Chen et al., 2006; Perrin & Vale, 1999). In fact, Ucn2 stimulated cAMP production in differentiated myotubes and muscle isolated tissue (Hinkle et al., 2003). Considering that cAMP is the second messenger that mediates muscle hypertrophy induced by beta-2 agonists (Koopman et al., 2010), one of the objectives of our laboratory has been to investigate the action of new peptides that act through this signaling molecule in the attempt to combat atrophy. Such an anabolic effect was associated with increased muscle cAMP content and PKA activity as well as Akt phosphorylation (Ser473) suggesting that these muscles may have developed greater insulin sensitivity. Very little is known about how cAMP can lead to Akt phosphorylation, but Epac protein (downstream effector of cAMP), a non-classical target of PKA, may be the mediator of the cross-talk between the Ucn2 canonical pathway of Ucn2 (cAMP/PKA/CREB) and the insulin/IGF-1 signaling. In fact, in vitro studies from our laboratory demonstrated that Epac, via activation of PI3K, promotes phosphorylation and activation of Akt in skeletal muscles of normal rats (Baviera et al., 2010). Consistent with these results, our experimental model of hypertrophy also presented higher content of Epac and two Akt downstream targets: Foxo1 (Ser256) and S6 (Ser235/236) (unpublished data). Once phosphorylated by Akt, Foxo translocates from the nucleus to the cytosol and becomes inactive, resulting in the suppression of different components of the proteolytic systems Ub-proteasome (atrogin-1 and MuRF1) and lysosomal/autophagic (LC3, Gabarap and cathepsin L) (Nader, 2005; Sandri et al., 2004). Corroborating these findings, Ucn2 was able to reduce lysosomal proteolysis in vitro in EDL muscles isolated from normal rats, as well as reduces atrophy induced by motor denervation in vivo (unpublished data). The anti-atrophic and antiautophagic action of Ucn2 may have a positive impact on the maintenance of motor plaques since the increase in AChR turnover in the atrophic denervation model was associated with a higher interaction between MuRF-1 and Bif-1 (autophagy regulating factor) ( Rudolf et al., 2013). In addition, our group demonstrated that the hypertrophic effect of Ucn2 in vivo involves the participation of ERK1/2, targets kinases of insulin phosphorylation and that have been implicated in the control of AChR subunit gene expression in JNM (Tansey et al. 1996, Altiok et al., 1997, Si & Meu, 1999). Continuing these studies, the present project will test the hypothesis that Ucn2, through the modulation of autophagic machinery, can regulate the remodeling process of muscle synapses in denervated muscles. Considering previous evidence that skeletal muscles of KO Atg7”sm animals develop greater insulin sensitivity in the hyperlipid diet model (Kim et al, 2013), we also intend to combat protein catabolism and improve insulin sensitivity in this model of diabetes in transfected muscles with Ucn2.