Adapt & Overcome: Hypoxia Resistance is a Novel Phenotype of the Flexor Digitorum Brevis Skeletal Muscle

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Minchew, Everett C.

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East Carolina University


Skeletal muscle comprises the largest mammalian organ system by mass that enables movement, respiration, and thermogenesis. Each of these processes requires the presence of molecular oxygen (O2). Inadequate O2 bioavailability (i.e., hypoxia) is detrimental to muscle contractile function and morphology, and in chronic cases can result in muscle wasting. Importantly, modern therapeutics and surgical approaches have proven largely ineffective to rescue skeletal muscle from hypoxic damage. Such a gap in the field is attributed predominantly to the fact that known skeletal muscles are highly susceptible to hypoxic insults. Thus, current knowledge of specific cellular properties that promote hypoxia resistance is sparse. However, our lab has identified a murine skeletal muscle that retains physiological function in the complete absence of O2. Using models of in vivo hindlimb ischemia and ex vivo muscle hypoxia exposure, we observed the preservation of force production and plasma membrane stability in the flexor digitorum brevis (FDB). In contrast, the extensor digitorum longus (EDL) and soleus muscles suffered functional decline and structural damage following the same hypoxic insults. A critical aspect of hypoxia resistance in the FDB is the ability to sustain energetic charge. Hypoxia decreases the mass action ratio of adenosine triphosphate ([ATP]) to adenosine diphosphate ([ADP]), which determines the capacity of a cell to perform work. However, the [ATP]/[ADP] ratio is maintained by the hypoxic FDB. Contrary to the EDL and soleus, we found that the FDB does not depend upon the mitochondria but requires glycolysis to function during hypoxia. Furthermore, the hypoxic FDB preserves stability of the plasma membrane whereas hypoxia causes membrane damage in the EDL and soleus. Following an unbiased, discovery-based interrogation of muscle proteomes, we identified significantly higher expression of multiple protein targets in the FDB as compared to the EDL and soleus that related to our findings. Due to the loss of force output in the FDB with inhibition of glycolysis, we selected the transmembrane glucose transporter GLUT1 as our first target. Loss- and gain-of-function experiments surrounding GLUT1 were employed to further characterize the FDB phenotype. Skeletal muscle-specific deletion of GLUT1 accelerated the loss of force output in the FDB during hypoxia exposure. Thus, to test the sufficiency of GLUT1 to attenuate hypoxia-induced damage in other skeletal muscles, we utilized an adeno-associated virus (AAV) to overexpress GLUT1 in the EDL. During these experiments we also manipulated concentrations of glucose and the oxidized form of ascorbic acid as these are the primary substrates that are transported by GLUT1. However, all conditions failed to prevent the loss of force production of the EDL during hypoxia exposure. Collectively, our results demonstrate that GLUT1 is necessary for the FDB phenotype, but insufficient to rescue other skeletal muscles from functional decline in a hypoxic environment. Due to the preservation of structural integrity in the FDB during hypoxia, we selected the membrane repair protein Annexin A2 and the extracellular matrix protein Col5a1 as our second and third targets, respectively. In separate sets of experiments, FDBs lacking Annexin A2 and Col5a1 did not demonstrate increased susceptibility to hypoxic damage as compared to control muscles. Importantly, however, using the same experimental approach that revealed the FDB’s ability to resist hypoxia, we have observed that the phenotype is adapted over time as opposed to an innate feature of the muscle. Hypoxic FDBs from young mice (4 weeks) exhibit a loss of force output and structural integrity in a manner that is similar to the hypoxic EDL. Following our observation of this apparent adaptation, we employed a proteomic analysis of FDBs from adult and young mice to further characterize differences between age groups. We subsequently integrated this data with our initial discovery-based proteomics analysis of the FDB, EDL, and soleus with the goal of improving the precision of our future target selection. Collectively, the data suggest that the FDB’s ability to survive hypoxia may be conferred by protection against structural damage via stabilization of the dystrophin-glycoprotein protein complex (DGC), which links the muscle plasma membrane to the extracellular matrix. In turn, structural protection likely promotes the maintenance of physiological function in the hypoxic FDB. Defining the FDB’s unique ability to resist hypoxic damage remains of critical importance for the muscle biology field to develop novel therapeutic approaches that promote the survival of skeletal muscle during times of limited O2 availability.