Advanced and High-Throughput Method for Mitochondrial Bioenergetics Evaluation in Neurotrauma

Increased Prolylcarboxypeptidase Expression Can Serve as a Biomarker of Senescence in Culture

Prolylcarboxypeptidase (PRCP, PCP, Lysosomal Pro-X-carboxypeptidase, Angiotensinase C) controls angiotensin- and kinin-induced cell signaling. Elevation of PRCP appears to be activated in chronic inflammatory diseases [cardiovascular disease (CVD), diabetes] in proportion to severity. Vascular endothelial cell senescence and mitochondrial dysfunction have consistently been shown in models of CVD in aging. Cellular senescence, a driver of age-related dysfunction, can differentially alter the expression of lysosomal enzymes due to lysosomal membrane permeability. There is a lack of data demonstrating the effect of age-related dysfunction on the expression and function of PRCP. To explore the changes in PRCP, the PRCP-dependent prekallikrein (PK) pathway was characterized in early- and late-passage human pulmonary artery endothelial cells (HPAECs). Detailed kinetic analysis of cells treated with high molecular weight kininogen (HK), a precursor of bradykinin (BK), and PK revealed a mechanism by which senescent HPAECs activate the generation of kallikrein upon the assembly of the HK-PK complex on HPAECs in parallel with an upregulation of PRCP and endothelial nitric oxide (NO) synthase (eNOS) and NO formation. The NO production and expression of both PRCP and eNOS increased in early-passage HPAECs and decreased in late-passage HPAECs. Low activity of PRCP in late-passage HPAECs was associated with rapid decreased telomerase reverse transcriptase mRNA levels. We also found that, with an increase in the passage number of HPAECs, reduced PRCP altered the respiration rate. These results indicated that aging dysregulates PRCP protein expression, and further studies will shed light into the complexity of the PRCP-dependent signaling pathway in aging.

Time-Course Evaluation of Brain Regional Mitochondrial Bioenergetics in a Pre-Clinical Model of Severe Penetrating Traumatic Brain Injury

Mitochondrial dysfunction is a pivotal target for neuroprotection strategies for traumatic brain injury (TBI). However, comprehensive time-course evaluations of mitochondrial dysfunction are lacking in the pre-clinical penetrating TBI (PTBI) model. The current study was designed to characterize temporal responses of mitochondrial dysfunction from 30 min to 2 weeks post-injury after PTBI. Anesthetized adult male rats were subjected to either PTBI or sham craniectomy (n = 6 animals per group × 7 time points). Animals were euthanized at 30 min, 3 h, 6 h, 24 h, 3 days, 7 days, and 14 days post-PTBI, and mitochondria were isolated from the ipsilateral hemisphere of brain regions near the injury core (i.e., frontal cortex [FC] and striatum [ST]) and a more distant region from the injury core (i.e., hippocampus [HIP]). Mitochondrial bioenergetics parameters were measured in real time using the high-throughput procedures of the Seahorse Flux Analyzer (Agilent Technologies, Santa Clara, CA). The post-injury time course of FC + ST showed a biphasic mitochondrial bioenergetics dysfunction response, indicative of reduced adenosine triphosphate synthesis rate and maximal respiratory capacity after PTBI. An initial phase of energy crisis was detected at 30 min (-42%; p < 0.05 vs. sham), which resolved to baseline levels between 3 and 6 h (non-significant vs. sham). This was followed by a second and more robust phase of bioenergetics dysregulation detected at 24 h that remained unresolved out to 14 days post-injury (-55% to -90%; p < 0.05 vs. sham). In contrast, HIP mitochondria showed a delayed onset of mitochondrial dysfunction at 7 days (-74%; p < 0.05 vs. sham) that remained evident out to 14 days (-51%; p < 0.05 vs. sham) post-PTBI. Collectively, PTBI-induced mitochondrial dysfunction responses were time and region specific, evident differentially at the injury core and distant region of PTBI. The current results provide the basis that mitochondrial dysfunction may be targeted differentially based on region specificity post-PTBI. Even more important, these results suggest that therapeutic interventions targeting mitochondrial dysfunction may require extended dosing regimens to achieve clinical efficacy after TBI.

Temporal changes in inflammatory mitochondria-enriched microRNAs following traumatic brain injury and effects of miR-146a nanoparticle delivery

MicroRNAs (miRNAs) are small non-coding RNA molecules that regulate post-transcriptional gene expression and contribute to all aspects of cellular function. We previously reported that the activities of several mitochondria-enriched miRNAs regulating inflammation (i.e., miR-142-3p, miR-142-5p, and miR-146a) are altered in the hippocampus at 3-12 hours following a severe traumatic brain injury. In the present study, we investigated the temporal expression profile of these inflammatory miRNAs in mitochondria and cytosol fractions at more chronic post-injury times following severe controlled cortical impact injury in rats. In addition, several inflammatory genes were analyzed in the cytosol fractions. The analysis showed that while elevated levels were observed in cytoplasm, the mitochondria-enriched miRNAs, miR-142-3p and miR-142-5p continued to be significantly reduced in mitochondria from injured hippocampi for at least 3 days and returned to near normal levels at 7 days post-injury. Although not statistically significant, miR-146a also remained at reduced levels for up to 3 days following controlled cortical impact injury, and recovered by 7 days. In contrast, miRNAs that are not enriched in mitochondria, including miR-124a, miR-150, miR-19b, miR-155, and miR-223 were either increased or demonstrated no change in their levels in mitochondrial fractions for 7 days. The one exception was that miR-223 levels were reduced in mitochondria at 1 day following injury. No major alterations were observed in sham operated animals. This temporal pattern was unique to mitochondria-enriched miRNAs and correlated with injury-induced changes in mitochondrial bioenergetics as well as expression levels of several inflammatory markers. These observations suggested a potential compartmental re-distribution of the mitochondria-enriched inflammatory miRNAs and may reflect an intracellular mechanism by which specific miRNAs regulate injury-induced inflammatory signaling. To test this, we utilized a novel peptide-based nanoparticle strategy for in vitro and in vivo delivery of a miR-146a mimic as a potential therapeutic strategy for targeting nuclear factor-kappaB inflammatory modulators in the injured brain. Nanoparticle delivery of miR-146a to BV-2 or SH-SY5Y cells significantly reduced expression of TNF receptor-associated factor 6 (TRAF6) and interleukin-1 receptor-associated kinase 1 (IRAK1), two important modulators of the nuclear factor-kappaB (NF-κB) pro-inflammatory pathway. Moreover, injections of miR-146a containing nanoparticles into the brain immediately following controlled cortical impact injury significantly reduced hippocampal TNF receptor-associated factor 6 and interleukin-1 receptor-associated kinase 1 levels. Taken together, our studies demonstrate the subcellular alteration of inflammatory miRNAs after traumatic brain injury and establish proof of principle that nanoparticle delivery of miR-146a has therapeutic potential for modulating pro-inflammatory effectors in the injured brain. All of the studies performed were approved by the University of Kentucky Institutional Animal Care and Usage Committee (IACUC protocol # 2014-1300) on August 17, 2017.

A method for assessing tissue respiration in anatomically defined brain regions

The survival and function of brain cells requires uninterrupted ATP synthesis. Different brain structures subserve distinct neurological functions, and therefore have different energy production/consumption requirements. Typically, mitochondrial function is assessed following their isolation from relatively large amounts of starting tissue, making it difficult to ascertain energy production/failure in small anatomical locations. In order to overcome this limitation, we have developed and optimized a method to measure mitochondrial function in brain tissue biopsy punches excised from anatomically defined brain structures, including white matter tracts. We describe the procedures for maintaining tissue viability prior to performing the biopsy punches, as well as provide guidance for optimizing punch size and the drug doses needed to assess various aspects of mitochondrial respiration. We demonstrate that our method can be used to measure mitochondrial respiration in anatomically defined subfields within the rat hippocampus. Using this method, we present experimental results which show that a mild traumatic brain injury (mTBI, often referred to as concussion) causes differential mitochondrial responses within these hippocampal subfields and the corpus callosum, novel findings that would have been difficult to obtain using traditional mitochondrial isolation methods. Our method is easy to implement and will be of interest to researchers working in the field of brain bioenergetics and brain diseases.

Formoterol, a β2-adrenoreceptor agonist, induces mitochondrial biogenesis and promotes cognitive recovery after traumatic brain injury

Traumatic brain injury (TBI) leads to acute necrosis at the site of injury followed by a sequence of secondary events lasting from hours to weeks and often years. Targeting mitochondrial impairment following TBI has shown improvements in brain mitochondrial bioenergetics and neuronal function. Recently formoterol, a highly selective β₂-adrenoreceptor agonist, was found to induce mitochondrial biogenesis (MB) via Gβγ-Akt-eNOS-sGC pathway. Activation of MB is a novel approach that has been shown to restore mitochondrial function in several disease and injury models. We hypothesized that activation of MB as a target of formoterol after TBI would mitigate mitochondrial dysfunction, enhance neuronal function and improve behavioral outcomes. TBI-injured C57BL/6 male mice were injected (i.p.) with vehicle (normal saline) or formoterol (0.3 mg/kg) at 15 min, 8 h, 16 h, 24 h and then daily after controlled cortical impact (CCI) until euthanasia. After CCI, mitochondrial copy number and bioenergetic function were decreased in the ipsilateral cortex of the CCI-vehicle group. Compared to CCI-vehicle, cortical and hippocampal mitochondrial respiration rates as well as cortical mitochondrial DNA copy number were increased in the CCI-formoterol group. Mitochondrial Ca²⁺ buffering capacity in the hippocampus was higher in the CCI-formoterol group compared to CCI-vehicle group. Both assessments of cognitive performance, novel object recognition (NOR) and Morris water maze (MWM), decreased following CCI and were restored in the CCI-formoterol group. Although no changes were seen in the amount of cortical tissue spared between CCI-formoterol and CCI-vehicle groups, elevated levels of hippocampal neurons and improved white matter sparing in the corpus callosum were observed in CCI-formoterol group. Collectively, these results indicate that formoterol-mediated MB activation may be a potential therapeutic target to restore mitochondrial bioenergetics and promote functional recovery after TBI.

Age- and Organ-Specific Differences in Mitochondrial Bioenergetics in Brown Norway Rats

Mitochondria play a central role in energy homeostasis and act as regulatory checkpoints for downstream metabolic responses and cell senescence processes during an entire life span. Acute or chronic environmental toxicant exposures have shown deleterious organ-specific human health issues at various life stages. Since mitochondria are a prime target for ensuing cellular bioenergetics responses and senescence, it is essential to understand mitochondrial bioenergetic responses in different organs over multiple life stages. Therefore, in the present study, we evaluated mitochondrial bioenergetic parameters in the liver, lung, and heart in four diverse age groups (young: 1 month; adult: 4 months; middle-aged: 12 months; old-aged: 24 month) using male Brown Norway rats as a model of aging (n = 5 sample size/organ/age group) and compared them with our previously published results on brain. Real-time mitochondrial bioenergetic parameters (i.e., State III, State IV, and State V) were measured using the Seahorse Extracellular Flux Analyzer. Additionally, mitochondrial enzyme pyruvate dehydrogenase complex (PDHC), Complex I, Complex II, and Complex IV activities were measured using Synergy HT plate reader. Our results indicated that nearly in all parameters, significant age- and organ-specific interactions were observed. We observed age-specific declines in State III (i.e., ATP synthesis rate) responses in both the heart and lung, where opposite was observed in the liver as age advances. Across the age, the heart has highest enzyme activities than the liver and lung. Interestingly, heart and liver mitochondrial bioenergetic rates and enzyme activities remain higher than the lung, which specifies their higher metabolic capabilities than the lung. Amongst all, bioenergetic rates and enzyme activities in the lung remain lowest suggesting the lung may display higher vulnerability and lower resilience to environmental toxicants during aging than other organs tested here. Overall, these age- and organ-specific findings may facilitate a more contextualized understanding of mitochondrial bioenergetic outcomes when considering the interactions of age-related sensitivities with exposure to chemical stressors from the environment.

Bioenergetic restoration and neuroprotection after therapeutic targeting of mitoNEET: New mechanism of pioglitazone following traumatic brain injury

Mitochondrial dysfunction is a pivotal event in many neurodegenerative disease states including traumatic brain injury (TBI) and spinal cord injury (SCI). One possible mechanism driving mitochondrial dysfunction is glutamate excitotoxicity leading to Ca²⁺-overload in neuronal or glial mitochondria. Therapies that reduce calcium overload and enhance bioenergetics have been shown to improve neurological outcomes. Pioglitazone, an FDA approved compound, has shown neuroprotective properties following TBI and SCI, but the underlying mechanism(s) are unknown. We hypothesized that the interaction between pioglitazone and a novel mitochondrial protein called mitoNEET was the basis for neuroprotection following CNS injury. We discovered that mitoNEET is an important mediator of Ca²⁺-mediated mitochondrial dysfunction and show that binding mitoNEET with pioglitazone can prevent Ca²⁺-induced dysfunction. By utilizing wild-type (WT) and mitoNEET null mice, we show that pioglitazone mitigates mitochondrial dysfunction and provides neuroprotection in WT mice, though produces no restorative effects in mitoNEET null mice. We also show that NL-1, a novel mitoNEET ligand, is neuroprotective following TBI in both mice and rats. These results support the crucial role of mitoNEET for mitochondrial bioenergetics, its importance in the neuropathological sequelae of TBI and the necessity of mitoNEET for pioglitazone-mediated neuroprotection. Since mitochondrial dysfunction is a pathobiological complication seen in other diseases such as diabetes, motor neuron disease and cancer, targeting mitoNEET may provide a novel mitoceutical target and therapeutic intervention for diseases that expand beyond TBI.

Comprehensive Profile of Acute Mitochondrial Dysfunction in a Preclinical Model of Severe Penetrating TBI

Mitochondria constitute a central role in brain energy metabolism, and play a pivotal role in the development of secondary pathophysiology and subsequent neuronal cell death following traumatic brain injury (TBI). Under normal circumstances, the brain consumes glucose as the preferred energy source for adenosine triphosphate (ATP) production over ketones. To understand the comprehensive picture of substrate-specific mitochondrial bioenergetics responses following TBI, adult male rats were subjected to either 10% unilateral penetrating ballistic-like brain injury (PBBI) or sham craniectomy (n = 5 animals per group). At 24 h post-injury, mitochondria were isolated from pooled brain regions (frontal cortex and striatum) of the ipsilateral hemisphere. Mitochondrial bioenergetics parameters were measured ex vivo in the presence of four sets of metabolic substrates: pyruvate+malate (PM), glutamate+malate (GM), succinate (Succ), and β-hydroxybutyrate+malate (BHBM). Additionally, mitochondrial matrix dehydrogenase activities [i.e., pyruvate dehydrogenase complex (PDHC), alpha-ketoglutarate dehydrogenase complex (α-KGDHC), and glutamate dehydrogenase (GDH)] and mitochondrial membrane-bound dehydrogenase activities [i.e., electron transport chain (ETC) Complex I, II, and IV] were compared between PBBI and sham groups. Furthermore, mitochondrial coenzyme contents, including NAD_(t) and FAD_(t), were quantitatively measured in both groups. Collectively, PBBI led to an overall significant decline in the ATP synthesis rates (43-50%; ^* p < 0.05 vs. sham) when measured using each of the four sets of substrates. The PDHC and GDH activities were significantly reduced in the PBBI group (42-53%; ^* p < 0.05 vs. sham), whereas no significant differences were noted in α-KGDHC activity between groups. Both Complex I and Complex IV activities were significantly reduced following PBBI (47-81%; ^* p < 0.05 vs. sham), whereas, Complex II activity was comparable between groups. The NAD_(t) and FAD_(t) contents were significantly decreased in the PBBI group (27-35%; ^* p < 0.05 vs. sham). The decreased ATP synthesis rates may be due to the significant reductions in brain mitochondrial dehydrogenase activities and coenzyme contents observed acutely following PBBI. These results provide a basis for the use of "alternative biofuels" for achieving higher ATP production following severe penetrating brain trauma.

Acute Mitochondrial Impairment Underlies Prolonged Cellular Dysfunction after Repeated Mild Traumatic Brain Injuries

Mild traumatic brain injuries (mTBI), accounting for more than 80% of TBIs, can cause cognitive and behavioral impairments, the severity and duration of which increase after additional mTBIs. While mTBI does not cause widespread neuronal death, the mechanisms underlying increased cellular susceptibility to subsequent head impacts remain unknown. To investigate the hypothesis that altered mitochondrial bioenergetics underlie cellular vulnerability to repeated insults, we employed a mouse model of mild closed head injury (CHI) to examine mitochondrial function and oxidative stress, because these mechanisms are often intertwined. Mitochondrial respiration was assayed (Seahorse XFe24 Flux Analyzer) from cortex and hippocampus collected at 6 h, 24 h, 48 h, and 96 h post-injury. State III (adenosine diphosphate [ADP]-mediated) respiration was significantly decreased in the hippocampal mitochondria of the CHI group compared with sham at 48 h post-injury. Further, cortex-derived mitochondria exhibited a decrease in State III respiration at 24 h and 48 h post-injury. No significant differences were observed at 6 h or 96 h post-injury in either region of interest. A second CHI repeated either 48 h or 96 h after the first did not worsen State III respiration at 48 h after the final injury compared with a single CHI, but CHI repeated at a 48 h interval prolonged cortical mitochondrial dysfunction to 96 h after the final injury. Markers of oxidative stress were significantly elevated after two CHIs delivered 48 h apart, but not after single CHI or two CHI delivered 96 h apart. This study establishes that mTBI results in early mitochondrial dysfunction, which may be a determinant for cellular vulnerability to repeated head impacts. Thus, therapies targeting mitochondrial impairment could improve outcomes after repeated mTBI.

Mitochondrial uncoupling prodrug improves tissue sparing, cognitive outcome, and mitochondrial bioenergetics after traumatic brain injury in male mice

Traumatic brain injury (TBI) results in cognitive impairment, which can be long-lasting after moderate to severe TBI. Currently, there are no FDA-approved therapeutics to treat the devastating consequences of TBI and improve recovery. This study utilizes a prodrug of 2,4-dinitrophenol, MP201, a mitochondrial uncoupler with extended elimination time, that was administered after TBI to target mitochondrial dysfunction, a hallmark of TBI. Using a model of cortical impact in male C57/BL6 mice, MP201 (80 mg/kg) was provided via oral gavage 2-hr post-injury and daily afterwards. At 25-hr post-injury, mice were euthanized and the acute rescue of mitochondrial bioenergetics was assessed demonstrating a significant improvement in both the ipsilateral cortex and ipsilateral hippocampus after treatment with MP201. Additionally, oxidative markers, 4-hydroxyneneal and protein carbonyls, were reduced compared to vehicle animals after MP201 administration. At 2-weeks post-injury, mice treated with MP201 post-injury (80 mg/kg; q.d.) displayed significantly increased cortical sparing (p = .0059; 38% lesion spared) and improved cognitive outcome (p = .0133) compared to vehicle-treated mice. Additionally, vehicle-treated mice had significantly lower (p = .0019) CA3 neuron count compared to sham while MP201-treated mice were not significantly different from sham levels. These results suggest that acute mitochondrial dysfunction can be targeted to impart neuroprotection from reactive oxygen species, but chronic administration may have an added benefit in recovery. This study highlights the potential for safe, effective therapy by MP201 to alleviate negative outcomes of TBI.

Combination Drug Therapy of Pioglitazone and D-cycloserine Attenuates Chronic Orofacial Neuropathic Pain and Anxiety by Improving Mitochondrial Function Following Trigeminal Nerve Injury

OBJECTIVES

The study aim was to determine how peripheral trigeminal nerve injury affects mitochondrial respiration and to test efficacy of combined treatment with 2 Federal Drug Administration approved drugs with potential for improving mitochondrial bioenergetics, pain and anxiety-related behaviors in a chronic orofacial neuropathic pain mouse model.

METHODS

Efficacy of (R)-(+)-4-amino-3-isoxazolidinone (D-cycloserine, DCS), an N-Methyl-D-aspartate antagonist/agonist, and Pioglitazone (PIO), a selective agonist of nuclear receptor peroxisome proliferator-activated receptor gamma was investigate in the trigeminal inflammatory compression (TIC) neuropathic nerve injury mouse model. Combined low doses of these drugs (80 mg/kg DCS and 100 mg/kg PIO) were given as a single bolus or daily for 7 days post-TIC to test ability to attenuate neuropathic nociceptive and associated cognitive dependent anxiety behaviors. In addition, beneficial effects of the DCS/PIO drug combination were explored ex vivo in isolated cortex/brainstem mitochondria at 28 weeks post-TIC.

RESULTS

The DCS/PIO combination not only attenuated orofacial neuropathic pain and anxiety-related behaviors associated with trigeminal nerve injury, but it also improved mitochondrial bioenergetics.

DISCUSSION

The DCS/PIO combination uncoupled mitochondrial respiration in the TIC model to improve cortical mitochondrial dysfunction, as well as reduced nociceptive and anxiety behaviors present in mice with centralized chronic neuropathic nerve injury. Combining these drugs could be a beneficial treatment for patients with depression, anxiety, or other psychological conditions due to their chronic pain status.

Targeting mitochondrial dysfunction in CNS injury using Methylene Blue; still a magic bullet?

Complex, multi-factorial secondary injury cascades are initiated following traumatic brain injury, which makes this a difficult disease to treat. The secondary injury cascades following the primary mechanical tissue damage, are likely where effective therapeutic interventions may be targeted. One promising therapeutic target following brain injury are mitochondria. Mitochondria are complex organelles found within the cell, which act as powerhouses within all cells by supplying ATP. These organelles are also necessary for calcium cycling, redox signaling and play a major role in the initiation of cell death pathways. When mitochondria become dysfunctional, there is a tendency for the cell to loose cellular homeostasis and can lead to eventual cell death. Targeting of mitochondrial dysfunction in various diseases has proven a successful approach, lending support to mitochondria as a pivotal player in TBI cell death and loss of behavioral function. Within this mixed mini review/research article there will be a general discussion of mitochondrial bioenergetics, followed by a brief discussion of traumatic brain injury and how mitochondria play an integral role in the neuropathological sequelae following an injury. We will also give an overview of one relatively new TBI therapeutic approach, Methylene Blue, currently being studied to ameliorate mitochondrial dysfunction following brain injury. We will also present novel experimental findings, that for the first time, characterize the ex vivo effect of Methylene Blue on mitochondrial function in synaptic and non-synaptic populations of mitochondria.