The role of mitochondrial uncoupling in the regulation of mitostasis after traumatic brain injury

Abstract

Mitostasis, the maintenance of healthy mitochondria, plays a critical role in brain health. The brain’s high energy demands and reliance on mitochondria for energy production make mitostasis vital for neuronal function. Traumatic brain injury (TBI) disrupts mitochondrial homeostasis, leading to secondary cellular damage, neuronal degeneration, and cognitive deficits. Mild mitochondrial uncoupling, which dissociates ATP production from oxygen consumption, offers a promising avenue for TBI treatment. Accumulating evidence, from endogenous and exogenous mitochondrial uncoupling, suggests that mitostasis is closely regulating by mitochondrial uncoupling and cellular injury environments may be more sensitive to uncoupling. Mitochondrial uncoupling can mitigate calcium overload, reduce oxidative stress, and induce mitochondrial proteostasis and mitophagy, a process that eliminates damaged mitochondria. The interplay between mitochondrial uncoupling and mitostasis is ripe for further investigation in the context of TBI. These multi-faceted mechanisms of action for mitochondrial uncoupling hold promise for TBI therapy, with the potential to restore mitochondrial health, improve neurological outcomes, and prevent long-term TBI-related pathology.

The role of mitostasis in brain health

Mitochondrial homeostasis, or mitostasis, is the maintenance of bioenergetically-competent (healthy) mitochondria to supply and respond to the needs of the cell. Mitostasis is regulated in a symphony of related processes, including mitochondrial trafficking, mitochondrial fission/fusion, mitophagy, mitochondrial biogenesis as well as ROS- and calcium- mediated pathways [1]. It is well known that the brain is only 2% of the total mass of the body but consumes ~20% of the body’s supply of oxygen at rest, which is driven by oxidative phosphorylation in the mitochondrion [2]. As neurons are the highest oxygen-consuming cell in the brain [3], control of mitochondrial dynamics, the coordinated balance of fission and fusion to maintain size, shape, and distribution, is critical for proper neuronal function.

Mitostasis in neurons is integral for meeting the unique demands of neurotransmission and metabolism; this concept is detailed in a review by Misgeld and Schwarz [4]. As such, there are specialized areas, such as synapses and nodes of Ranvier, in neurons that require more ATP and thereby, a stronger mitochondrial presence [4, 5]. For instance, synaptic vesicle cycling, which is driven by efficient calcium uptake, relies on high levels of ATP production [6]. Mitochondrial ATP is also required to drive sodium-potassium pumps which are key to maintaining neuronal activity and allowing the firing of action potentials in neurons.

The mitochondrial membrane potential (ΔΨ) is generated by the translocation of protons across the inner mitochondria membrane via the electron transport system (ETS) culminating in the reduction of O₂ to H₂O. This store of potential energy (the electrochemical gradient) can then be coupled to ATP production as protons flow back through the ATP synthase and complete the proton circuit. The potential can also be used to drive Ca²⁺ into the mitochondrial matrix via the electrogenic uniporter when cytosolic levels increase [7]. Maintenance of low intracellular Ca²⁺ ([Ca²⁺]_i) is necessary for proper cell function and to allow brief pulses of it to initiate second-messenger pathways, allowing intracellular communication. Mitochondria sequester Ca²⁺ to ensure that homeostasis is maintained [8–10]. Changes in mitochondrial Ca²⁺ cycling, which is driven by ΔΨ, have been demonstrated to alter both LTP and LTD as well [11].

Mitochondrial uncoupling is an endogenous process that is regulated by uncoupling proteins (UCPs), which are found in the mitochondrial inner membrane. UCPs regulate the proton gradient generated by the ETC to initiate functions like thermogenesis and the maintenance of redox balance and ROS. UCPs are subject to regulation at both the transcriptional and mitochondrial levels [12, 13] including dietary modifications [14].

A central question in the concept of mitostasis is: how are damaged mitochondria dealt with? One such mechanism is mitochondrial protein turnover. When there is oxidative damage to mitochondrial proteins, a degradation process can be led by resident mitochondrial proteases, which serve to preserve mitochondrial activity by removing and remodeling protein molecules [4, 15]. Indeed, this process is also exploited by cytosolic proteostasis by importing damaged proteins into the mitochondrion [16]. Mitophagy, however, is the destruction of entire mitochondria in response to a severe insult. Mitophagy selectively eliminates mitochondria using autophagic machinery and serves as a universal mechanism for degrading malfunctioning mitochondria [17]. Mitochondrial fission has a principal role in generating depolarized mitochondria that are the eventual target of mitophagy [17] but can also modulate mitochondrial size to enact biogenesis [18]. Importantly, under neuronal stress, axonal mitochondria are shuttled retrograde to undergo somatic mitophagy [19]. This review will explore these mitochondrial processes, specifically the relationship between mitochondrial uncoupling and mitostasis, in the context of TBI and brain trauma.

Mitochondrial deficits after traumatic brain injury

Neuronal degeneration following TBI is believed to evolve in two interwoven phases: 1) primary mechanical insult and 2) progressive secondary cellular damage [20–22]. Secondary injury mechanisms, involving excitotoxicity, the disruption of Ca²⁺ homeostasis, increased reactive oxygen species (ROS) production, and mitochondrial dysfunction are major factors contributing to neuropathology and persistent cognitive impairment [23–36]. Deficits in mitochondrial function and metabolism are considered driving forces in progressive energy crisis developed after TBI [37–40]. This loss of mitochondrial homeostasis following TBI results in disruption of synaptic homeostasis [41–45], implicating a pivotal role for mitochondria in the sequalae of TBI-related neuropathology.

Mitochondria are key to the regulation of cell death or apoptosis [46, 47]. Excitatory amino acid (EAA)-mediated neurotoxicity [48–54] causes high amounts of intracellular Ca²⁺ ions that mitochondria will sequester [52, 55–62]. During injury, Ca²⁺ uptake into mitochondria increases ROS production, inhibit ATP synthesis and induce mitochondrial permeability transitions and the release of pro-apoptotic proteins [48, 53, 63–65]. ΔΨm is the driving force for Ca²⁺ influx from the cytoplasm to the mitochondrion. Some mitochondria undergo a recoverable Ca²⁺ load, which produces a ΔΨm around 110-140 mV [66] (Fig. 1). Once calcium overload reaches a certain threshold (~100-110 mV), there is an opening of the mitochondrial permeability transition pore (mPTP), which can signal cytochrome c release and/or mitophagy [7, 67]. The mPTP is a non-selective, Ca²⁺-dependent channel in the inner mitochondrial membrane that allows for the passage of small molecules under conditions of high Ca²⁺ and oxidative stress. The composition of the mPTP is debated but evidence shows involvement of F-ATP synthase and the adenine nucleotide translocase (ANT) with matrix cyclophilin D (CypD) [68]. Indeed, our group demonstrated the role of CypD in early mitochondrial deficits and secondary tissue loss following TBI [69]. Further, our laboratories and others have demonstrated that therapeutic intervention with cyclosporin A following experimental TBI significantly reduces mitochondrial dysfunction [43] and cortical damage [43, 70, 71], as well as cytoskeletal changes and axonal dysfunction [72].

Figure 1. The effect of mild uncoupling on dysfunctional and damaged mitochondria after traumatic brain injury.

(Top Panel) Mild uncoupling of mitochondria in dysfunctional mitochondria (−140 to −120 mV) post-TBI will shift ΔΨm slightly to enabled mitochondrial Ca²⁺ cycling back into the cytosol based on electrochemical gradients. Further, mild uncoupling can reduce reactive oxygen species (ROS) by decreasing free radical generation. Mitochondrial proteostasis, via focal ubiquitination, is initiated to repair oxidized mitochondrial proteins and lipids. (Bottom Panel) When mitochondrial have high levels of mitochondrial Ca²⁺, ΔΨm will plummet to below −120 mV and mitochondria enter a damaged state after TBI. In this damaged state, mild uncoupling will drop ΔΨm to a level that activates the PINK1/Parkin pathway and may induce mitochondrial permeability transition. The mitochondrion will undergo mitophagy, which can make way for biogenesis of bioenergetically-competent mitochondria for overall cellular health.

The other major intertwined secondary process involved in mitochondrial dysfunction after TBI is oxidative damage. Free radical production is a byproduct of ATP generation in mitochondria via the electron transport chain (ETC). Electrons escape from the chain and reduce O₂ to O₂^−.. Normally cells convert O₂^−. to H₂O₂ utilizing both manganese superoxide dismutase, which is localized to the mitochondria, and copper-zinc superoxide dismutase found in the cytosol. H₂O₂ is normally rapidly converted to H₂O, but has the potential to be converted to the highly reactive hydroxyl radical (OH^.) via the Fenton reaction, underlying ROS neurotoxicity. OH^. rapidly attacks unsaturated fatty acids in membranes causing lipid peroxidation and the production of 4-Hydroxynonenal (HNE) that conjugates to membrane proteins, impairing their function [73–76]. Such oxidative injury results in significant alterations in cellular function. In particular, ROS induction of lipid peroxidation and protein oxidation products may be particularly important in neurodegeneration (for review see [77]) and TBI [23, 24, 42, 78]. Mitochondrial ROS production is intimately linked to ΔΨ such that hyperpolarization (high ΔΨ) increases and promotes ROS production [79–81]. At a high ΔΨ, protons can no longer be pumped out of the matrix (against the electrochemical proton gradient), electron flow thru the ETS slows and begin to slip out of the complexes and ultimately resulting in increased ROS production.

Recent findings highlight deficits in mitochondrial dynamics after TBI. These findings include altered mitochondrial length, increased mitochondrial fission, and decreased mitochondrial copy number [82–86]. Interestingly, it is found that the alteration of mitochondrial fission and fusion processes following TBI is injury severity dependent [87]. Mitochondrial fission after TBI is a major initiator of the mitophagy process [17]. Mitochondrial trafficking and transport along the axon, which is ATP-dependent, is integral for proper mitochondrial dynamics and mitostasis in the neuron. On average, mitochondria move at ~0.5 μm/s along the axon and have a slight preference to move in anterograde [88, 89]. Others have postulated that deficits occur in axonal transport of mitochondria after TBI [90] and this may represent a roadblock in the mitostasis machinery in response to brain trauma.

The role of endogenous mitochondrial uncoupling in traumatic brain injury

Endogenous mitochondrial uncoupling is mediated by members of the UCP family, which function to dissociate ATP production from oxygen consumption in mitochondria of muscle and fat tissues [91], leading to heat generation. This is a normal cellular process and typically relies on protonophore activity at the inner mitochondrial membrane. Several hypotheses have been put forth concerning possible physiological roles of the UCPs, including: energy partitioning, energy balance, and control of metabolism which may be pivotal in obesity and diabetes [92, 93]. Skulachev was the first to hypothesize that mild uncoupling could be beneficial in certain disease states since it causes a decrease in ROS production [79, 94]. Indeed, it is well known that UCPs in the central nervous system function to regulate ROS generation and energy metabolism based on needs of the cellular environment [95, 96]. We will refer the readers to several comprehensive reviews that fully describe the role of UCPs, including UCP-2, in health and disease [97, 98]. UCPs are critical in neuronal metabolism and are a therapeutic target for cellular metabolic dysfunction in neurological diseases, such as Parkinson’s disease [99] and epilepsy [100]. The energy needs are high in a post-traumatic neuronal environment and UCP-2, in particular, was identified to have a neuroprotective role after trauma via endogenous mild mitochondrial uncoupling and subsequent inhibition of cytochrome c release, caspase activation and apoptosis [67]. We have reported a neuroprotective role for UCP-2 in excitotoxic cell death in vivo [101]. Our findings demonstrate that reducing UCP-2 expression and UCP activity, increases induced mitochondrial ROS production and neuronal cell loss in p12 rats pups [101]. Further studies have demonstrated that fasting or ketone bodies administration, can induce UCP expression, alter mPTP activity and improve outcome after TBI and in seizure models [102–108]. Based on these studies, upregulation of UCPs occurs over time after TBI and therefore cannot target the initial consequences of excitotoxity. The concept of supplemental mitochondrial uncouplers that could be administered immediately after injury and target initial mitochondrial calcium overload was developed. This idea initiated decades of work examining the role of mild chemical mitochondrial uncoupling to treat TBI [109–112].

Mitochondrial uncoupling drugs to treat traumatic brain injury

The economic impact of TBI coupled with numerous neurological consequences has initiated an enormous focus on the development and discovery of neuroprotective and/or pro-regenerative agents which might have clinical relevance following TBI [113–115]. To this end, strategies to target mitochondrial-related secondary outcomes of TBI are promising with many aimed towards increasing cellular energy supply or blocking cell death [69, 103, 107, 116–118]. Our group has identified mitochondrial-targeting pharmaceuticals, or ‘mitoceuticals,’ that can restore secondary mitochondrial, behavioral, and pathological outcomes in preclinical models of TBI [14, 111, 112, 119, 120]. One such beneficial approach has been the use of exogenous mild mitochondrial uncoupling to supplement the endogenous response to brain injury. For this approach, mild, or low concentration administration, is key as high dosing of mitochondrial uncouplers can lead to ATP, redox, and energy supply dysregulation and imbalances [121]. Mild mitochondrial uncoupling with pharmaceuticals is an approach that has been studied in many neurological disease states. Mild mitochondrial uncoupling has proven efficacious in Alzheimer’s’ disease (AD) in preserving short-term memory and enhancing cognition. The repurposing of popular uncouplers, such as DNP, at low doses, in addition to the development of novel uncoupling agents, including MP201 [122], provides opportunities for new breakthrough therapeutic interventions in a range of neurodegenerative conditions, chief of these being TBI.

Targeting mitochondrial dysfunction after TBI have proven successful using pharmacological modulation of mitochondrial physiology by uncoupling agents (i.e. 2,4-dinitrophenol (DNP). In brief, this platform of extrinsic mitochondrial uncoupling agents can increase proton leakage across, thereby lowering ΔΨm, reducing oxidative stress, mitigating Ca²⁺ overloading, and increasing mitochondrial respiration [119, 123, 124]. Our group has published reports outlining neuroprotective therapy of mitochondrial uncouplers carbonyl cyanide-4-phenylhydrazone (FCCP) and DNP [111, 112, 125, 126], by imparting greater tissue and neuronal sparing, as well as improved behavioral outcomes compared to placebo groups [111, 112].

MP201 is a novel prodrug synthesized with a ~20x lower C_max and ~3x longer elimination time compared to the parent drug DNP [122]. The compound MP201 can be metabolized to 2,4-DNP, a lipophilic weak acid, which readily crosses the blood-brain barrier and translocates protons across the inner mitochondrial membrane. The activity of certain mitochondrial proteins intricately involved with energy production are differentially compromised in synaptic (neuronal) mitochondria after TBI in rodents [76, 111, 112, 127–132] and this dysfunction is amendable to MP201 treatment [133]. MP201 is shown to provide cell protection previously in a model of optic neuritis, including prevention of vision loss, preservation of retinal ganglion cells and protection of the axon from demyelination [134]. In our previous reports, MP201 treatment affords improvement in mitochondrial bioenergetics and reduction in oxidative damage after experimental TBI [133]. MP201 provides mitochondrial rescue in both ipsilateral cortex and ipsilateral hippocampus as well as a decrease in oxidative damage markers, protein carbonyls (PC) and HNE, following CCI [133]. Importantly, for these studies, we utilized a clinically relevant therapeutic window for initiation of treatment of 2h post-injury, suggesting that MP201 treatment strategy would be effective given at delayed time points after injury. Treatment of TBI with MP201 (80 mg/kg) beginning at 2 hrs post-injury and given once daily, confers neuroprotection, as evidenced by increased cortical sparing at 2 weeks post-injury [110]. Indeed, our group previously used mitochondrial uncouplers (FCCP) administered at 24h post-injury and this dosing paradigm still conferred neuroprotection [111].

Mild TBI does not produce widespread neuronal death [37, 135–138] and the onset of cognitive deficits likely depends on subtle cellular changes, such as mitochondrial dysfunction and metabolic crisis [109, 139], which drive worsened outcomes after rmTBI. The importance that mitochondria play in cellular vulnerability has been implied by an exacerbated and prolonged posttraumatic drop in CMRglc [39, 140] and bioenergetics [37] after repeated mTBI. Together, these studies point to a pivotal role of mitochondria in the long-term pathophysiology and cognitive impairment after mild TBI.

Our previous work shows that rmTBI can result in brain mitochondrial deficits and oxidative stress. Indeed, we have published seminal findings showing that mitochondrial dysfunction in the hippocampus and cortex occurs acutely after mTBI [37]. When a second mTBI is sustained during the period of decreased mitochondrial function, oxidative damage is propagated in these vulnerable regions of the brain. Based on these findings, we recently found that mild mitochondrial uncoupling imparted by MP201 provide significantly reduced oxidative damage after repeated mild TBI (rmTBI), even when administered at delayed time points after injury [109]. Further , we show that there is significant synaptic mitochondrial impairment after rmbTBI that can be rescued by MP201 [109]. Therapeutic restoration of mitochondrial dysfunction by mild mitochondrial uncoupling therefore is a promising treatment for even mild TBI. Future experiments are planned to demonstrate that targeting mitochondria reduces the neurological morbidity associated with rmTBI. Therapeutics are needed to combat neuronal energy crisis and aid in prevention on long-term rmTBI pathology. As there are also other phenotypes of mild TBI, and TBI in general, such as vascular damage and blood-brain barrier dysfunction [141], future directions could consider the effect of low concentrations of chemical mitochondrial uncouplers in endothelial cells and other cell types.

Multi-faceted mechanisms of action for mitochondrial uncoupling in TBI

Mild mitochondrial uncoupling is a powerful and translational approach as it modulates mitochondrial biophysics, rather than targeting a specific protein or cytokine. There are a multitude of cellular responses to mitochondrial uncoupling, including calcium regulation and ROS reduction in the mitochondrion [142]. Mitochondrial uncoupling lowers ΔΨm, thereby reducing toxicity caused by Ca²⁺ influx into the mitochondrial matrix. The reduction in ΔΨm can also diminish free radical pathways leading to mitigation of downstream events such as ROS production and cell death.

Intracellular calcium regulation

It is also important to note that inhibition of mitochondrial Ca²⁺ uptake by reducing ΔΨ (chemical uncoupling) following excitotoxic insults is neuroprotective, emphasizing the pivotal role of mitochondrial Ca²⁺ uptake in EAA neuronal cell death [49, 51, 52]. Mild mitochondrial uncoupling lowers mitochondrial Ca²⁺ overload, which prevents the opening of the mPTP. Given the driving potential for Ca²⁺ uptake into mitochondrial, even a small depolarization of the mitochondrial membrane potential can reduce driving force of Ca²⁺ uptake several fold.

Mitochondrial Ca²⁺ is regulated by two transporters; the mitochondrial calcium uniporter (MCU), which facilitates Ca²⁺ influx, is electrogenic and responds rapidly to changes in ΔΨ, and the mitochondrial Na+/Ca²⁺/Li exchanger (NCLX), which mediates Ca2+ efflux [143, 144]. Ca²⁺ uptake by MCU is determined by the Ca²⁺ gradient between the mitochondrial matrix and the cytoplasm and ultimately by ΔΨm [145]. NCLX-mediated mitochondrial Ca2+ efflux is several orders of magnitude slower than the MCU-mediated Ca²⁺ influx. Thus, NCLX is the rate-limiting system to overcome huge mitochondrial Ca²⁺ surges [143, 146]. There are few studies that examine the role of mild mitochondrial uncoupling on NCLX activity, Kostic, et al. found that the mild mitochondrial depolarization induced by chemical uncoupling downregulates and inhibits NCLX activity [146]. Reports demonstrate that NCLX activity is impaired or NCLX potentially operates in reverse in the post-traumatic brain [147, 148]. However, it is clear that, in the post-traumatic brain where in excitotoxicity overloads mitochondrial Ca²⁺ [149], mild mitochondrial uncoupling has a greater effect on MCU activity than it does on NCLX function. Mild depolarization by exogenous uncoupling significantly decreases mitochondrial Ca²⁺ uptake to provide neuroprotective and cellular recovery after brain trauma [150, 151].

Balance of reactive oxygen species

ROS production correlates with ΔΨ; therefore, mitochondrial uncoupling, which increases proton conductance (decreases ΔΨ and increases ETC activity), reduces the ROS generation rate [79, 81, 152]. Increasing mitochondrial respiration via synthetic uncoupling contributes to reduction in mitochondrial oxidants [153]. Of importance, the physiological benefit of mild mitochondrial uncoupling on decreased ROS production is not without controversy [154]. However, mitochondrial uncoupling in the context of disease or injury is likely more plausible and one of the main avenues of cellular protection [101, 119, 155, 156].

Induction of mitochondrial dynamics, mitophagy and mitochondrial-derived vesicle pathways

PINK1/Parkin-independent mitophagy and mitochondrial-derived vesicle (MDV) pathways are critical for mitochondrial quality control and mitostasis, especially in the contest of neurological disease [157]. The activation of the PINK1/Parkin pathway occurs when PINK1 becomes stabilized on the outer surface of impaired mitochondria due to a reduction in ΔΨm, enabling the recruitment of cytosolic Parkin [158]. Mitophagy occurs under excessive mitochondrial damage, such as high dose addition of any agent that leads to the dissipation of ΔΨm [142]. It has been shown that DNP can stimulate autophagic responses in the cerebral cortex [159], which may be mediated via mTOR signaling. Critically, mitochondrial biogenesis is activated in response to PINK1/Parkin-mediated mitophagy [160], which supports the role of uncoupling-induced mitophagy in the process of mitostasis (Fig. 1). It is shown that mitochondria-selective mitophagy due to mild mitochondrial uncoupling of oxidative phosphorylation by mitochondria-targeting cations may underlie their therapeutic effects [161]. We show that mild mitochondrial uncoupling using MP201 can significantly alter mitochondrial dynamics, including increased mitochondrial volume and number, in the cerebral cortex at 1d after administration (Fig. 2). This process may be a result of mitochondrial fission-induced biogenesis [18]. We hypothesize that uncoupling-induced mitophagy after brain trauma will allow for compensatory mitochondrial biogenesis of functional mitochondria. This process is delicate as modulation of mitochondrial dynamics by uncoupling in a rapidly-changing cellular environment after TBI may drop ΔΨm enough to lead to abundant cytochrome c release that induces neuronal apoptosis. For instance, pioglitazone, which is a known inducer of mitochondrial biogenesis, does not modulate mitochondrial outcomes when given early post-TBI but produces beneficial mitochondrial and behavioral function under delayed administration following experimental TBI [162].

Figure. 2. Mild mitochondrial uncoupling-induced changes in mitochondrial dynamics.

Mice (B6;129S-Gt (ROSA)26Sortm1.1 (CAG-COX8A/Dendra2) Dcc/J) expressing a mitochondrial-specific version of Dendra-2 green fluorescent protein (mtD2g, Strain#: 018397, Jackson Laboratories) were used at the age of 12–16 weeks (5-6 mice/group; mixed sex). A single dose of MP201, a prodrug for 2,4 dinitrophenol, (80mg/Kg bw) or vehicle was given orally to naive mice, sacrificed after 24 hours and brain samples were collected. Paraffin sections (5 μm) were mounted using a vectashield antifade mounting medium with DAPI (H-1800, vector laboratories, USA) after dewaxing. Using a Nikon A1R confocal microscope Z-stack images were taken from the cortex region of the brain and mitochondrial shape, size, and count were quantified using Imaris (X64 9.6.1) as described previously [85]. Confocal images were processed using the bulk processing option in Imaris with the same settings. The volume, count and size of individual mitochondria detected using Imaris were exported to an Excel file for further analysis. A. Representative confocal maximum intensity projected micrograph (100X oil) of brain section showing mitochondria-specific fluorescence. B. Percentage frequency distribution of mitochondrial volumes (μm3). C. Fold change of mitochondrial number (each data point represents an average number of the mitochondria calculated from the individual animal normalized to vehicle-treated). D. Average volume of the mitochondria (each data point represents the average volume of the mitochondria calculated from the individual animal). Data are represented as mean ± SEM. p ≤ 0.05 *; p ≤ 0.01 **; using unpaired t-test (C, D).

Beyond the process of mitophagy, there are several homeostatic mitochondrial proteostasis mechanisms to deal with relatively lower levels of lipid and protein oxidation. Mitochondrial protein shuttling network and mitochondrial unfolded protein response are intricate processes [163]. As these are steady-state pathways, we do not hypothesize that these processes are altered in post-traumatic cellular environments. The MDV cascade consists of removal of oxidized mitochondrial protein complexes and lipids and shuttles to peroxisomes or lysosomes in a manner that is independent of classical mitophagy mechanism [164]. We hypothesize that mild uncoupling of dysfunctional mitochondria will quell oxidative stress and activate MDV-mediated mitostasis, consisting of selective removal of focally damaged portions of the mitochondrion (Fig. 1). This is also hypothesized to promote mitochondrial biogenesis (Fig. 2). Overall, prevailing literature and our results demonstrate that mild mitochondrial uncoupling can induce mitophagy in damaged mitochondria while rescuing dysfunctional mitochondrial and promoting biogenesis of healthy mitochondria. This “MitoSwap” process [165] represents an important mechanism of action for how mild mitochondrial uncoupling can improve cellular outcomes in TBI as well as other neurodegenerative diseases.

Conclusions

Although TBI is a devastating healthcare problem in the US, there are currently no pharmacological treatments approved for clinical treatment of TBI. The fine line between cell survival and cell death relies on mitochondrial integrity and ultimately the state of ΔΨ, which the absence of is cell death. Mild mitochondrial uncoupling is a novel therapeutic approach for TBI and accumulating evidence suggests that it can restore neurobehavioral outcomes. We propose in this article that mitochondrial uncoupling-induced mitophagy and mitochondrial proteostasis is important for overall mitochondrial recovery after TBI. Along with the regulation of intracellular calcium and reactive oxygen species, modulation of mitophagy and mitochondrial dynamics represents a critical mechanism by which mild mitochondrial uncoupling induces a beneficial mitostasis response following TBI.

Mitochondrial dysfunction is a hallmark of traumatic brain injury

Mitochondrial homeostasis or mitostasis is important for recovery after TBI

Mild mitochondrial uncoupling can alter mitochondrial dynamics, including mitophagy

Mild uncoupling of mitochondria can induce a “MitoSwap” mechanism after TBI