Progress Update: Novel Therapeutics for FSHD: Mitochondria-targeted Antioxidants

Update by Dr. Heher
see grant Novel Therapeutics for FSHD: Mitochondria-targeted Antioxidants

FSHD muscles suffer from metabolic stress, a condition where the muscles’ energy factories, the so-called mitochondria, do not work efficiently. Our previously reported work in this project has identified dysfunctional mitochondria as the major source of oxidative stress through generation of reactive oxygen species (ROS), small but highly reactive molecules that can damage the muscle irreversibly and can even lead to muscle death. This work has shown that antioxidants that specifically localize to the mitochondria and scavenge excess ROS, thereby alleviating FSHD pathology more efficiently than conventional antioxidants.

An interesting observation that we have made is that FSHD muscles show higher sensitivity to hypoxia, a condition of low O2 availability that naturally occurs in working muscles, for example when we exercise. When O2 becomes limited, muscles usually adapt their metabolism accordingly, meaning that they use other metabolites to generate energy than O2 which is normally used efficiently in the mitochondria. This process is called metabolic switching and is governed by a cellular sensing mechanism referred to as the molecular response to hypoxia. Hypoxia-inducible factor 1α (HIF1α) is a key player orchestrating this response, but is deregulated by FSHD-causing DUX4 protein. Strikingly, hypoxia not only disproportionally increases oxidative stress in FSHD muscle cells, but also DUX4 protein levels in a DUX4-inducible muscle cell line. Consequently, DUX4 causes more muscle cell death (apoptosis) in hypoxia than at ambient O2 levels (normoxia). Mitochondria-targeted, but not conventional non-targeted antioxidants can reduce DUX4-induced apoptosis in hypoxia to the level observed in normoxia, demonstrating that hypoxic stress (and cell death) in FSHD muscle cells operates through excess ROS production from the mitochondria. In addition, we found that DUX4 impairs the expression of genes involved in glucose uptake, a central metabolite in muscle energy homeostasis that can be used to generate energy under both anaerobic (low O2 availability) and aerobic (high O2 availability) conditions. This could mean that FSHD muscle cells are not only impaired in their ability to use the comparably energy efficient mitochondrial oxidative metabolism, but also fail to properly utilize other means of energy production that are dependent on glucose uptake and usage, especially when O2 becomes limited. This is supported by our preliminary metabolomic analysis showing that lactate, the end product of anaerobic glucose metabolism (anaerobic glycolysis), is present in much higher levels in FSHD muscle cells compared to controls, even when enough O2 for mitochondrial oxidative metabolism is available. We have previously found that improvement of mitochondrial function through use of mitochondria-targeted antioxidants alleviates oxidative stress in FSHD muscle cells. We hypothesize that FSHD muscle cells are characterized by a much broader metabolic stress than previously thought, where impaired mitochondrial function forces them to chronically utilize metabolic pathways that are less efficient, and maybe also deregulated by DUX4. We are thus now investigating whether improving mitochondrial health through administration of mitochondria-targeted antioxidants can restore FSHD muscle cell function by allowing these cells to overall operate at a healthier and more efficient bioenergetic state.SHD muscles suffer from metabolic stress, a condition where the muscles’ energy factories, the so-called mitochondria, do not work efficiently. Our previously reported work in this project has identified dysfunctional mitochondria as the major source of oxidative stress through generation of reactive oxygen species (ROS), small but highly reactive molecules that can damage the muscle irreversibly and can even lead to muscle death. This work has shown that antioxidants that specifically localize to the mitochondria and scavenge excess ROS, thereby alleviating FSHD pathology more efficiently than conventional antioxidants. An interesting observation that we have made is that FSHD muscles show higher sensitivity to hypoxia, a condition of low O2 availability that naturally occurs in working muscles, for example when we exercise. When O2 becomes limited, muscles usually adapt their metabolism accordingly, meaning that they use other metabolites to generate energy than O2 which is normally used efficiently in the mitochondria. This process is called metabolic switching and is governed by a cellular sensing mechanism referred to as the molecular response to hypoxia. Hypoxia-inducible factor 1α (HIF1α) is a key player orchestrating this response, but is deregulated by FSHD-causing DUX4 protein. Strikingly, hypoxia not only disproportionally increases oxidative stress in FSHD muscle cells, but also DUX4 protein levels in a DUX4-inducible muscle cell line. Consequently, DUX4 causes more muscle cell death (apoptosis) in hypoxia than at ambient O2 levels (normoxia). Mitochondria-targeted, but not conventional non-targeted antioxidants can reduce DUX4-induced apoptosis in hypoxia to the level observed in normoxia, demonstrating that hypoxic stress (and cell death) in FSHD muscle cells operates through excess ROS production from the mitochondria. In addition, we found that DUX4 impairs the expression of genes involved in glucose uptake, a central metabolite in muscle energy homeostasis that can be used to generate energy under both anaerobic (low O2 availability) and aerobic (high O2 availability) conditions. This could mean that FSHD muscle cells are not only impaired in their ability to use the comparably energy efficient mitochondrial oxidative metabolism, but also fail to properly utilize other means of energy production that are dependent on glucose uptake and usage, especially when O2 becomes limited. This is supported by our preliminary metabolomic analysis showing that lactate, the end product of anaerobic glucose metabolism (anaerobic glycolysis), is present in much higher levels in FSHD muscle cells compared to controls, even when enough O2 for mitochondrial oxidative metabolism is available.

We have previously found that improvement of mitochondrial function through use of mitochondria-targeted antioxidants alleviates oxidative stress in FSHD muscle cells. We hypothesize that FSHD muscle cells are characterized by a much broader metabolic stress than previously thought, where impaired mitochondrial function forces them to chronically utilize metabolic pathways that are less efficient, and maybe also deregulated by DUX4. We are thus now investigating whether improving mitochondrial health through administration of mitochondria-targeted antioxidants can restore FSHD muscle cell function by allowing these cells to overall operate at a healthier and more efficient bioenergetic state.