Mechanisms underlying the predictive power of high skeletal muscle uptake of FDG in Amyotrophic Lateral Sclerosis

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.

Metabolic Changes in an Animal Model of Amyotrophic Lateral Sclerosis Evaluated by [18F]-FDG Positron Emission Tomography

“…The effects of ALS on metabolic function in humans and in animal models of ALS are yet to be fully understood. In animals, only one study has measured the impact of SOD1 G93A genotype on glucose uptake using [ 18 F]-FDG, however focus was specifically placed onto metabolic changes in skeletal muscles (7). Marini et al found that mice expressing mutant SOD1 G93A had a higher [ 18 F]-FDG uptake in quadriceps when compared with WT animals, indicative of a higher metabolic rate.…”

“…Marini et al found that mice expressing mutant SOD1 G93A had a higher [ 18 F]-FDG uptake in quadriceps when compared with WT animals, indicative of a higher metabolic rate. These changes were accompanied by increased inflammatory profiles and mitochondrial dysfunction in muscle (7). In humans, however, the assessment of brain metabolism in ALS patients with [ 18 F]-FDG PET has been studied with a focus on prediction of ALS onset and characterization of disease progression, both with mixed results (5).…”