Modulation of mTOR signaling as a strategy for the treatment of Pompe disease.

Jeong‐A Lim,Kyle M Trudeau,Lishu Li,Nina Raben,Rosa Puertollano,Orian S Shirihai

doi:10.15252/emmm.201606547

Abstract

Mechanistic target of rapamycin (mTOR) coordinates biosynthetic and catabolic processes in response to multiple extracellular and intracellular signals including growth factors and nutrients. This serine/threonine kinase has long been known as a critical regulator of muscle mass. The recent finding that the decision regarding its activation/inactivation takes place at the lysosome undeniably brings mTOR into the field of lysosomal storage diseases. In this study, we have examined the involvement of the mTOR pathway in the pathophysiology of a severe muscle wasting condition, Pompe disease, caused by excessive accumulation of lysosomal glycogen. Here, we report the dysregulation of mTOR signaling in the diseased muscle cells, and we focus on potential sites for therapeutic intervention. Reactivation of mTOR in the whole muscle of Pompe mice by TSC knockdown resulted in the reversal of atrophy and a striking removal of autophagic buildup. Of particular interest, we found that the aberrant mTOR signaling can be reversed by arginine. This finding can be translated into the clinic and may become a paradigm for targeted therapy in lysosomal, metabolic, and neuromuscular diseases.

Highlights

Mechanistic target of rapamycin, a highly conserved serine/threonine kinase, forms two multiprotein complexes, mTOR complex 1 (TORC1) and mTOR complex 2 (TORC2).Rapamycin-sensitive mTORC1 complex responds to multiple signals, and when activated, changes the cell metabolism from catabolic to anabolic program, promoting protein synthesis and cell growth while repressing autophagy
When cells have sufficient amino acids, V-ATPase promotes the guanine nucleotide exchange factor (GEF) activity of Ragulator leading to the formation of active RagA/BÁGTP complex at the lysosome; in this active configuration, Rag binds to and delivers mTORC1 to the lysosome where the kinase is activated by Rheb (Ras homolog enriched in brain), a small GTPase that is fixed to the lysosomal surface (Sancak et al, 2010; Zoncu et al, 2011; reviewed in Bar-Peled & Sabatini, 2014)
Since mTOR kinase is a principal regulator of protein synthesis, we evaluated the rate of protein synthesis in KO cells by using a surface sensing of translation (SUnSET) method, which relies on the incorporation of puromycin into nascent peptide chains resulting in the termination of their elongation (Goodman et al, 2011)

Summary

Introduction

Mechanistic target of rapamycin (mTOR), a highly conserved serine/threonine kinase, forms two multiprotein complexes, mTOR complex 1 (TORC1) and mTOR complex 2 (TORC2).Rapamycin-sensitive mTORC1 complex responds to multiple signals, and when activated, changes the cell metabolism from catabolic to anabolic program, promoting protein synthesis and cell growth while repressing autophagy. The role of lysosome in controlling metabolic programs is emphasized by the discovery that activation of this potent anabolic regulator happens at the lysosome in a process mediated through an amino acid-sensing cascade involving V-ATPase, Ragulator, and Rag GTPases. The recent view of the lysosomes as a site of the mTORC1 activation, along with the long-established role of this kinase in the control of muscle mass, has made the study of mTORC1 signaling of particular interest to research on Pompe disease, a severe muscle wasting disorder characterized by altered lysosomal function. Absence of the enzyme leads to a rapidly fatal cardiomyopathy and skeletal muscle myopathy in infants; low levels of residual enzyme activity are associated with childhood and adult-onset progressive skeletal muscle myopathy usually without cardiac involvement (Van der Ploeg & Reuser, 2008)

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Results

Conclusion