Stimulating mitochondria to protect the brain following traumatic brain injury

Traumatic brain injury (TBI) is an acquired injury to the brain that occurs with sudden trauma that can range from mild (concussive) to severe. TBI is considered a leading cause of death in children and young adults, with the Centers for Disease Control and Prevention estimating that approximately 1.7 million cases of TBI occur in the United States annually (Faul et al., 2010). Further, since the beginning of the global war on terrorism, the Department of Defense has reported over 344,000 U.S. Service Members have been diagnosed with traumatic brain injury from penetrating injuries to mild forms of TBI. TBI, caused by a sudden impact, penetration, or rapid movement of the brain, interrupts the normal functioning of the brain. While the intracranial location and severity of injury contribute to the extent of functional deficits, there are also contributions from physiological changes that occur during hours to days after injury. The progression of molecular changes following TBI leads to a number of secondary injuries including: mitochondrial dysfunction, neuronal degeneration, inflammation, blood-brain barrier (BBB) dysfunction, and edema formation. These changes in the cellular microenvironment contribute to the secondary injury phase when neuronal loss either permanently disables victims, decreasing their quality of life, or results in death. While many preclinical trials have shown positive results in treating TBI, none have resulted in effective neuroprotective treatments in the clinical setting. This likely reflects the multifaceted nature of TBI and incomplete understanding of the contributing mechanisms underlying the progression of events during the secondary injury phase that results in functional deficits. Due to the inability to reverse the damage to the brain during the initial injury, clinicians focus on stabilizing the patient and try to prevent further injury. The need to understand the pathophysiology and subsequently develop and test novel neuroprotective treatments following brain injury has been the driving force in our laboratory. Mitochondria are considered essential organelles for cell survival as they generate the majority of the cells’ energy production by oxidative phosphorylation. Mitochondria are sensitive to changes in the physiological state of a cell, and their dysfunction has been implicated in numerous neuropathological diseases, including TBI. Increasing evidence suggests that TBI leads to both direct mechanical damage and functional disturbances in mitochondria, and plays a key role in contributing to apoptotic and necrotic cell death. As the brain is a highly aerobic, energy-demanding tissue, an imbalance in energy demand for repair of cellular damage and concomitant decrease in energy generation caused by mitochondrial dysfunction leads to increased cellular damage. Excitotoxicity, production of reactive oxygen species (ROS), Ca overload, apoptosis-inducing factor, and caspases are some of the primary candidates that induce mitochondrial damage following TBI (Vink et al., 1990; Singh et al., 2006). During injury, the surge in ROS facilitates a vicious cycle that accelerates mitochondrial damage, excitotoxicity, lipid peroxidation, and inflammation. Mitochondrial membrane potential is a key indicator of damage to the mitochondria and may be linked to early-stage apoptosis. Thus, mitochondrial targeting strategies in TBI have been increasingly studied, as their maintenance could potentially preserve brain function. Determining the link between mitochondrial dysfunction and vascular physiology may also provide additional potential therapeutic targets following brain injury. One of our primary goals was to determine whether improving mitochondrial function would reduce lesion volume following TBI and result in improved functional outcomes. To accomplish this we focused on the use of methylene blue (MB), as it is a U.S. Food and Drug Administration-grandfathered drug that has shown promise in reducing neurobehavioral impairment in other disease states such as Parkinson’s disease, Alzheimer’s disease and ischemic stroke, where mitochondria may also play a critical role in disease progression (Rojas et al., 2012). In addition, low doses of MB have been shown in vivo to increase global glucose uptake, oxygen consumption, cerebral blood flow (Lin et al., 2012) and evoked responses (Huang et al., 2013) in rats. Consistent with these beneficial effects, MB has been shown to be effective in salvaging at-risk tissue (i.e., perfusion-diffusion mismatch which approximates the ischemic penumbra), ultimately reducing infarct size and behavioral deficits in models of ischemic stroke compared to vehicle-treated groups (Shen et al., 2013).