Mechanisms underlying the predictive power of high skeletal muscle uptake of FDG in Amyotrophic Lateral Sclerosis
Authors: Marini Cecilia , Cossu Vanessa , Bonifacino Tiziana , Bauckneht Matteo , Torazza Carola , Bruno Silvia , Castellani Patrizia , Ravera Silvia , Milanese Marco , Venturi Consuelo , Carlone Sebastiano , Piccioli Patrizia , Emionite Laura , Morbelli Silvia , Orengo Anna Maria , Donegani Maria Isabella , Miceli Alberto , Raffa Stefano , Marra Stefano , Cortese Katia , Grillo Federica , Fiocca Roberto , Bonanno Giambattista , Sambuceti Gianmario 
Published: April 2020

Abstract
BackgroundUsing a computational approach to PET/CT images, we recently reported that enhanced 18F-fluorodeoxyglucose (FDG) uptake in skeletal muscles predicts aggressiveness and outcome in patients with amyotrophic lateral sclerosis (ALS). The present experimental study aimed to assess the mechanisms underlying the predictive potential of this metabolic shift in SOD1G93A mice as a model of ALS.Methods.The study included 15 SOD1G93A mice and 15 wild-type mice (around 120-days-old). Mice were submitted to micro-PET imaging. Enzymatic pathways and response to oxidative stress were evaluated in harvested quadriceps and hearts by biochemical, immunohistochemical and immunofluorescence analysis. Colocalization between the endoplasmic reticulum (ER) and the fluorescent FDG analog 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxyglucose (2-NBDG) were performed in fresh skeletal muscle sections. Finally, mitochondrial ultrastructure and bioenergetic evaluation were evaluated in harvested quadriceps and hearts.ResultsFDG retention was significantly higher in hindlimb skeletal muscles of symptomatic SOD1G93A mice with respect to control ones. This difference was not explained by any acceleration in cytosolic glucose degradation through glycolysis or pentose phosphate pathway (PPP). Similarly, it was independent of inflammatory infiltration. Rather, the high FDG retention in SOD1G93A skeletal muscle was associated with an accelerated generation of reactive oxygen species. This redox stress selectively involved the ER and the local PPP triggered by hexose-6P-dehydrogenase. ER involvement was confirmed by the colocalization of the 2-NBDG with a vital ER tracker. The oxidative damage in transgenic skeletal muscle was associated with a severe impairment in the crosstalk between ER and mitochondria combined with alterations in mitochondrial ultrastructure and fusion/fission balance. The expected respiratory damage was confirmed by a deceleration in ATP synthesis and oxygen consumption rate. These same abnormalities were represented to markedly lower degree in the myocardium, as a sample of non-voluntary skeletal muscle.ConclusionSkeletal muscle of SOD1G93A mice reproduces the increased FDG uptake observed in ALS patients. This finding reflects the selective activation of the ER-PPP in response to a significant redox stress associated with alterations of mitochondrial ultrastructure, networking and connection with the ER itself. This scenario is less severe in cardiomyocytes suggesting a relevant role for either communication with synaptic plaque or contraction dynamics.