Brain cortex mitochondrial bioenergetics in synaptosomes and non-synaptic mitochondria during aging

Mitochondrial metabolism and neuroinflammation in the cerebral cortex and cortical synapses of rats: effect of milk intake through DNA methylation

Brain plasticity and cognitive functions are tightly influenced by foods or nutrients, which determine a metabolic modulation having a long-term effect on health, involving also epigenetic mechanisms. Breast milk or formula based on cow milk is the first food for human beings, who, throughout their lives, are then exposed to different types of milk. We previously demonstrated that rats fed with milk derived from distinct species, with different compositions and nutritional properties, display selective modulation of systemic metabolic and inflammatory profiles through changes of mitochondrial functions and redox state in liver, skeletal and cardiac muscle. Here, in a rat model, we demonstrated that isoenergetic supplementation of milk from cow (CM), donkey (DM) or human (HM) impacts mitochondrial functions and redox state in the brain cortex and cortical synapses, affecting neuroinflammation and synaptic plasticity. Interestingly, we found that the administration of different milk modulates DNA methylation in rat brain cortex and consequently affects gene expression. Our results emphasize the importance of nutrition in brain and synapse physiology, and highlight the key role played in this context by mitochondria, nutrient-sensitive organelles able to orchestrate metabolic and inflammatory responses.

Apoptosis Due to After-effects of Acute Ethanol Exposure in Brain Cortex: Intrinsic and Extrinsic Signaling Pathways

Alcohol hangover is the combination of negative mental and physical symptoms which can be experienced after a single episode of alcohol consumption, starting when blood alcohol concentration approaches zero. We previously demonstrated that hangover provokes mitochondrial dysfunction, oxidative stress, imbalance in antioxidant defenses, and impairment in cellular bioenergetics. Chronic and acute ethanol intake induces neuroapoptosis but there are no studies which evaluated apoptosis at alcohol hangover. The aim of the present work was to study alcohol residual effects on intrinsic and extrinsic apoptotic signaling pathways in mice brain cortex. Male Swiss mice received i.p. injection of ethanol (3.8 g/kg) or saline. Six hours after injection, at alcohol hangover onset, mitochondria and tissue lysates were obtained from brain cortex. Results indicated that during alcohol hangover a loss of granularity of mitochondria and a strong increment in mitochondrial permeability were observed, indicating the occurrence of swelling. Alcohol-treated mice showed a significant 35% increase in Bax/Bcl-2 ratio and a 5-fold increase in the ratio level of cytochrome c between mitochondria and cytosol. Caspase 3, 8 and 9 protein expressions were 32%, 33% and 20% respectively enhanced and the activity of caspase 3 and 6 was 30% and 20% increased also due to the hangover condition. Moreover, 38% and 32% increments were found in PARP1 and p53 protein expression respectively and on the contrary, SIRT-1 was almost 50% lower than controls due to the hangover condition. The present work demonstrates that alcohol after-effects could result in the activation of mitochondrial and non-mitochondrial apoptosis pathways.

Long-Term Region-Specific Mitochondrial Functionality Changes in Both Cerebral Hemispheres after fMCAo Model of Ischemic Stroke

Cerebral ischemia/reperfusion (I/R) refers to a secondary brain injury that results in mitochondrial dysfunction of variable extent, leading to neuronal cell damage. The impact of this process has mainly been studied in the short term, from the early hours up to one week after blood flow reperfusion, and in the ischemic hemisphere only. The focus of this study was to assess the long-term impacts of I/R on mitochondrial functionality using high-resolution fluorespirometry to evaluate state-dependent activities in both ischemic (ipsilateral) and non-ischemic (contralateral) hemispheres of male mice 60, 90, 120, and 180 days after I/R caused by 60-min-long filament-induced middle cerebral artery occlusion (fMCAo). Our results indicate that in cortical tissues, succinate-supported oxygen flux (Complex I&II OXPHOS state) and H2O2 production (Complex II LEAK state) were significantly decreased in the fMCAo (stroke) group ipsilateral hemisphere compared to measurements in the contralateral hemisphere 60 and 90 days after stroke. In hippocampal tissues, during the Complex I&II ET state, mitochondrial respiration was generally lower in the ipsilateral compared to the contralateral hemisphere 90 days following stroke. An aging-dependent impact on mitochondria oxygen consumption following I/R injury was observed 180 days after surgery, wherein Complex I&II activities were lowest in both hemispheres. The obtained results highlight the importance of long-term studies in the field of ischemic stroke, particularly when evaluating mitochondrial bioenergetics in specific brain regions within and between separately affected cerebral hemispheres.

Stable isotope tracing reveals disturbed cellular energy and glutamate metabolism in hippocampal slices of aged male mice

Neurons and astrocytes work in close metabolic collaboration, linking neurotransmission to brain energy and neurotransmitter metabolism. Dysregulated energy metabolism is a hallmark of the aging brain and may underlie the progressive age-dependent cognitive decline. However, astrocyte and neurotransmitter metabolism remains understudied in aging brain research. In particular, how aging affects metabolism of glutamate, being the primary excitatory neurotransmitter, is still poorly understood. Here we investigated critical aspects of cellular energy metabolism in the aging male mouse hippocampus using stable isotope tracing in vitro. Metabolism of [U-13C]glucose demonstrated an elevated glycolytic capacity of aged hippocampal slices, whereas oxidative [U-13C]glucose metabolism in the TCA cycle was significantly reduced with aging. In addition, metabolism of [1,2-13C]acetate, reflecting astrocyte energy metabolism, was likewise reduced in the hippocampal slices of old mice. In contrast, uptake and subsequent metabolism of [U-13C]glutamate was elevated, suggesting increased capacity for cellular glutamate handling with aging. Finally, metabolism of [15N]glutamate was maintained in the aged slices, demonstrating sustained glutamate nitrogen metabolism. Collectively, this study reveals fundamental alterations in cellular energy and neurotransmitter metabolism in the aging brain, which may contribute to age-related hippocampal deficits.

Mitochondrial bioenergetics and cytometric characterization of a synaptosomal preparation from mouse brain cortex

Mitochondrial function at synapses can be assessed in isolated nerve terminals. Synaptosomes are structures obtained in vitro by detaching the nerve endings from neuronal bodies under controlled homogenization conditions. Several protocols have been described for the preparation of intact synaptosomal fractions. Herein a fast and economical method to obtain synaptosomes with optimal intrasynaptic mitochondria functionality was described. Synaptosomal fractions were obtained from mouse brain cortex by differential centrifugation followed by centrifugation in a Ficoll gradient. The characteristics of the subcellular particles obtained were analyzed by flow cytometry employing specific tools. Integrity and specificity of the obtained organelles were evaluated by calcein and SNAP-25 probes. The proportion of positive events of the synaptosomal preparation was 75 ± 2 % and 48 ± 7% for calcein and Synaptosomal-Associated Protein of 25 kDa (SNAP-25), respectively. Mitochondrial integrity was evaluated by flow cytometric analysis of cardiolipin content, which indicated that 73 ± 1% of the total events were 10 N-nonylacridine orange (NAO)-positive. Oxygen consumption, ATP production and mitochondrial membrane potential determinations showed that mitochondria inside synaptosomes remained functional after the isolation procedure. Mitochondrial and synaptosomal enrichment were determined by measuring synaptosomes/ homogenate ratio of specific markers. Functionality of synaptosomes was verified by nitric oxide detection after glutamate addition. As compared with other methods, the present protocol can be performed briefly, does not imply high economic costs, and provides an useful tool for the isolation of a synaptosomal preparation with high mitochondrial respiratory capacity and an adequate integrity and function of intraterminal mitochondria.

Neuronal Plasticity and Age-Related Functional Decline in the Motor Cortex

Physiological aging causes a decline of motor function due to impairment of motor cortex function, losses of motor neurons and neuromuscular junctions, sarcopenia, and frailty. There is increasing evidence suggesting that the changes in motor function start earlier in the middle-aged stage. The mechanism underlining the middle-aged decline in motor function seems to relate to the central nervous system rather than the peripheral neuromuscular system. The motor cortex is one of the responsible central nervous systems for coordinating and learning motor functions. The neuronal circuits in the motor cortex show plasticity in response to motor learning, including LTP. This motor cortex plasticity seems important for the intervention method mechanisms that revert the age-related decline of motor function. This review will focus on recent findings on the role of plasticity in the motor cortex for motor function and age-related changes. The review will also introduce our recent identification of an age-related decline of neuronal activity in the primary motor cortex of middle-aged mice using electrophysiological recordings of brain slices.

Axonal energy metabolism, and the effects in aging and neurodegenerative diseases

Human studies consistently identify bioenergetic maladaptations in brains upon aging and neurodegenerative disorders of aging (NDAs), such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and Amyotrophic lateral sclerosis. Glucose is the major brain fuel and glucose hypometabolism has been observed in brain regions vulnerable to aging and NDAs. Many neurodegenerative susceptible regions are in the topological central hub of the brain connectome, linked by densely interconnected long-range axons. Axons, key components of the connectome, have high metabolic needs to support neurotransmission and other essential activities. Long-range axons are particularly vulnerable to injury, neurotoxin exposure, protein stress, lysosomal dysfunction, etc. Axonopathy is often an early sign of neurodegeneration. Recent studies ascribe axonal maintenance failures to local bioenergetic dysregulation. With this review, we aim to stimulate research in exploring metabolically oriented neuroprotection strategies to enhance or normalize bioenergetics in NDA models. Here we start by summarizing evidence from human patients and animal models to reveal the correlation between glucose hypometabolism and connectomic disintegration upon aging/NDAs. To encourage mechanistic investigations on how axonal bioenergetic dysregulation occurs during aging/NDAs, we first review the current literature on axonal bioenergetics in distinct axonal subdomains: axon initial segments, myelinated axonal segments, and axonal arbors harboring pre-synaptic boutons. In each subdomain, we focus on the organization, activity-dependent regulation of the bioenergetic system, and external glial support. Second, we review the mechanisms regulating axonal nicotinamide adenine dinucleotide (NAD+) homeostasis, an essential molecule for energy metabolism processes, including NAD+ biosynthetic, recycling, and consuming pathways. Third, we highlight the innate metabolic vulnerability of the brain connectome and discuss its perturbation during aging and NDAs. As axonal bioenergetic deficits are developing into NDAs, especially in asymptomatic phase, they are likely exaggerated further by impaired NAD+ homeostasis, the high energetic cost of neural network hyperactivity, and glial pathology. Future research in interrogating the causal relationship between metabolic vulnerability, axonopathy, amyloid/tau pathology, and cognitive decline will provide fundamental knowledge for developing therapeutic interventions.

Coenzyme Q10 supplementation improves the motor function of middle-aged mice by restoring the neuronal activity of the motor cortex

Physiological aging causes motor function decline and anatomical and biochemical changes in the motor cortex. We confirmed that middle-aged mice at 15-18 months old show motor function decline, which can be restored to the young adult level by supplementing with mitochondrial electron transporter coenzyme Q₁₀ (CoQ₁₀) as a water-soluble nanoformula by drinking water for 1 week. CoQ₁₀ supplementation concurrently improved brain mitochondrial respiration but not muscle strength. Notably, we identified an age-related decline in field excitatory postsynaptic potential (fEPSP) amplitude in the pathway from layers II/III to V of the primary motor area of middle-aged mice, which was restored to the young adult level by supplementing with CoQ₁₀ for 1 week but not by administering CoQ₁₀ acutely to brain slices. Interestingly, CoQ₁₀ with high-frequency stimulation induced NMDA receptor-dependent long-term potentiation (LTP) in layer V of the primary motor cortex of middle-aged mice. Importantly, the fEPSP amplitude showed a larger input‒output relationship after CoQ₁₀-dependent LTP expression. These data suggest that CoQ₁₀ restores the motor function of middle-aged mice by improving brain mitochondrial function and the basal fEPSP level of the motor cortex, potentially by enhancing synaptic plasticity efficacy. Thus, CoQ₁₀ supplementation may ameliorate the age-related decline in motor function in humans.

Ageing and the Autonomic Nervous System

The vertebrate nervous system is divided into central (CNS) and peripheral (PNS) components. In turn, the PNS is divided into the autonomic (ANS) and enteric (ENS) nervous systems. Ageing implicates time-related changes to anatomy and physiology in reducing organismal fitness. In the case of the CNS, there exists substantial experimental evidence of the effects of age on individual neuronal and glial function. Although many such changes have yet to be experimentally observed in the PNS, there is considerable evidence of the role of ageing in the decline of ANS function over time. As such, this chapter will argue that the ANS constitutes a paradigm for the physiological consequences of ageing, as well as for their clinical implications.

Isolated Mitochondrial Preparations and In organello Assays: A Powerful and Relevant Ex vivo Tool for Assessment of Brain (Patho)physiology

Mitochondria regulate multiple aspects of neuronal development, physiology, plasticity, and pathology through their regulatory roles in bioenergetic, calcium, redox, and cell survival/death signalling. While several reviews have addressed these different aspects, a comprehensive discussion focussing on the relevance of isolated brain mitochondria and their utilities in neuroscience research has been lacking. This is relevant because the employment of isolated mitochondria rather than their in situ functional evaluation, offers definitive evidence of organelle-specificity, negating the interference from extra mitochondrial cellular factors/signals. This mini-review was designed primarily to explore the commonly employed in organello analytical assays for the assessment of mitochondrial physiology and its dysfunction, with a particular focus on neuroscience research. The authors briefly discuss the methodologies for biochemical isolation of mitochondria, their quality assessment, and cryopreservation. Further, the review attempts to accumulate the key biochemical protocols for in organello assessment of a multitude of mitochondrial functions critical for neurophysiology, including assays for bioenergetic activity, calcium and redox homeostasis, and mitochondrial protein translation. The purpose of this review is not to examine each and every method or study related to the functional assessment of isolated brain mitochondria, but rather to assemble the commonly used protocols of in organello mitochondrial research in a single publication. The hope is that this review will provide a suitable platform aiding neuroscientists to choose and apply the required protocols and tools to address their particular mechanistic, diagnostic, or therapeutic question dealing within the confines of the research area of mitochondrial patho-physiology in the neuronal perspective.

Natural products regulate mitochondrial function in cognitive dysfunction-A scoping review

Medicines from natural products can not only treat neurodegenerative diseases but also improve the cognitive dysfunction caused by treatments with western medicines. This study reviews the literature related to the regulation of mitochondrial participation in cognitive function by natural products. In this study, we focused on English articles in PubMed, Web of Science, and Google Scholar, from 15 October 2017, to 15 October 2022. Fourteen studies that followed the inclusion criteria were integrated, analyzed, and summarized. Several studies have shown that natural products can improve or reduce cognitive dysfunction by ameliorating mitochondrial dysfunction. These results suggest that natural products may serve as new therapeutic targets for neurodegenerative diseases.

Effects of aging and long-term physical activity on mitochondrial physiology and redox state of the cortex and cerebellum of female rats

We investigated the effects of aging and long-term physical activity on markers of mitochondrial function and dynamics in the cortex and cerebellum of female rats. Additionally, we interrogated markers of oxidative damage and antioxidants. Thirty-four female Lewis rats were separated into three groups. A young group (YNG, n = 10) was euthanized at 6 months of age. Two other groups were aged to 15 months and included a physical activity group (MA-PA, n = 12) and a sedentary group (MA-SED, n = 12). There were no age effects for any of the variables investigated, except for SOD2 protein levels in the cortex (+6.5%, p = 0.012). Long-term physical activity increased mitochondrial complex IV activity in the cortex compared to YNG (+85%, p = 0.016) and MA-SED (+82%, p = 0.023) and decreased carbonyl levels in the cortex compared to YNG (-12.49%, p = 0.034). Our results suggest that the mitochondrial network and redox state of the brain of females may be more resilient to the aging process than initially thought. Further, voluntary wheel running had minimal beneficial effects on brain markers of oxidative damage and mitochondrial physiology.

Divergent Cellular Energetics, Glutamate Metabolism, and Mitochondrial Function Between Human and Mouse Cerebral Cortex

Disruptions of brain energy and neurotransmitter metabolism are associated with several pathological conditions including neurodegenerative diseases such as Alzheimer's disease. Transgenic rodent models, and in vitro preparations hereof, are often applied for studying pathological aspects of brain metabolism. However, despite the conserved cerebral development across mammalian species, distinct differences in cellular composition and structure may influence metabolism of the rodent and human brain. To address this, we investigated the metabolic function of acutely isolated brain slices and non-synaptic mitochondria obtained from the cerebral cortex of mice and neurosurgically resected neocortical tissue of humans. Utilizing dynamic isotope labeling with ¹³C-enriched metabolic substrates, we show that metabolism of glucose, acetate, β-hydroxybutyrate, and glutamine operates at lower rates in human cerebral cortical slices when compared to mouse slices. In contrast, human cerebral cortical slices display a higher capacity for converting exogenous glutamate into glutamine, which subsequently supports neuronal GABA synthesis, whereas mouse slices primarily convert glutamate into aspartate. In line with the reduced metabolic rate of the human brain slices, isolated non-synaptic mitochondria of the human cerebral cortex have a lower oxygen consumption rate when provided succinate as substrate. However, when provided pyruvate and malate, human mitochondria display a higher coupled respiration and lower proton leak, signifying a more efficient mitochondrial coupling compared to mouse mitochondria. This study reveals key differences between mouse and human brain metabolism concerning both neurons and astrocytes, which must be taken into account when applying in vitro rodent preparations as a model system of the human brain.

The compartmentalised nature of neuronal mitophagy: molecular insights and implications

The maintenance of a healthy mitochondrial network and the ability to adjust organelle population in response to internal or external stimuli are essential for the function and the survival of eukaryotic cells. Over the last two decades several studies have demonstrated the paramount importance of mitophagy, a selective form of autophagy that removes damaged and/or superfluous organelles, in organismal physiology. Post-mitotic neuronal cells are particularly vulnerable to mitochondrial damage, and mitophagy impairment has emerged as a causative factor in multiple neurodegenerative pathologies, including Alzheimer's disease and Parkinson's disease among others. Although mitochondrial turnover is a multifaceted process, neurons have to tackle additional complications, arising from their pronounced bioenergetic demands and their unique architecture and cellular polarisation that render the degradation of distal organelles challenging. Mounting evidence indicates that despite the functional conservation of mitophagy pathways, the unique features of neuronal physiology have led to the adaptation of compartmentalised solutions, which serve to ensure seamless mitochondrial removal in every part of the cell. In this review, we summarise the current knowledge concerning the molecular mechanisms that mediate mitophagy compartmentalisation and discuss their implications in various human pathologies.

Simultaneous Alteration of the Circadian Variation of Memory, Hippocampal Synaptic Plasticity, and Metabolism in a Triple Transgenic Mouse Model of Alzheimer’s Disease

Alzheimer’s disease (AD) is characterized by progressive memory deficits accompanied by synaptic and metabolic deficits, namely of mitochondrial function. AD patients also display a disrupted circadian pattern. Thus, we now compared memory performance, synaptic plasticity, and mitochondria function in 24-week-old non-transgenic (non-Tg) and triple transgenic male mice modeling AD (3xTg-AD) at Zeitgeber 04 (ZT-4, inactive phase) and ZT-16 (active phase). Using the Morris water maze test to minimize the influence of circadian-associated locomotor activity, we observed a circadian variation in hippocampus-dependent learning performance in non-Tg mice, which was impaired in 3xTg-AD mice. 3xTg-AD mice also displayed a lack of circadian variation of their performance in the reversal spatial learning task. Additionally, the amplitude of hippocampal long-term potentiation also exhibited a circadian profile in non-Tg mice, which was not observed in 3xTg-AD mice. Moreover, cerebral cortical synaptosomes of non-Tg mice also displayed a circadian variation of FCCP-stimulated oxygen consumption as well as in mitochondrial calcium retention that were blunted in 3xTg-AD mice. In sum, this multidimensional study shows that the ability to maintain a circadian oscillation in brain behavior, synaptic plasticity, and synaptic mitochondria function are simultaneously impaired in 3xTg-AD mice, highlighting the effects of circadian misalignment in AD.

Phosphorylated tau as a toxic agent in synaptic mitochondria: implications in aging and Alzheimer's disease

During normal aging, there is a decline in all physiological functions in the organism. One of the most affected organs is the brain, where neurons lose their proper synaptic function leading to cognitive impairment. Aging is one of the main risk factors for the development of neurodegenerative diseases, such as Alzheimer’s disease. One of the main responsible factors for synaptic dysfunction in aging and neurodegenerative diseases is the accumulation of abnormal proteins forming aggregates. The most studied brain aggregates are the senile plaques, formed by Aβ peptide; however, the aggregates formed by phosphorylated tau protein have gained relevance in the last years by their toxicity. It is reported that neurons undergo severe mitochondrial dysfunction with age, with a decrease in adenosine 5′-triphosphate production, loss of the mitochondrial membrane potential, redox imbalance, impaired mitophagy, and loss of calcium buffer capacity. Interestingly, abnormal tau protein interacts with several mitochondrial proteins, suggesting that it could induce mitochondrial dysfunction. Nevertheless, whether tau-mediated mitochondrial dysfunction occurs indirectly or directly is still unknown. A recent study of our laboratory shows that phosphorylated tau at Ser396/404 (known as PHF-1), an epitope commonly related to pathology, accumulates inside mitochondria during normal aging. This accumulation occurs preferentially in synaptic mitochondria, which suggests that it may contribute to the synaptic failure and cognitive impairment seen in aged individuals. Here, we review the main tau modifications promoting mitochondrial dysfunction, and the possible mechanism involved. Also, we discuss the evidence that supports the possibility that phosphorylated tau accumulation in synaptic mitochondria promotes synaptic and cognitive impairment in aging. Finally, we show evidence and argue about the presence of phosphorylated tau PHF-1 inside mitochondria in Alzheimer’s disease, which could be considered as an early event in the neurodegenerative process. Thus, phosphorylated tau PHF-1 inside the mitochondria could be considered such a potential therapeutic target to prevent or attenuate age-related cognitive impairment.

Directly Reprogrammed Human Neurons to Understand Age-Related Energy Metabolism Impairment and Mitochondrial Dysfunction in Healthy Aging and Neurodegeneration

Brain aging is characterized by several molecular and cellular changes grouped as the hallmarks or pillars of aging, including organelle dysfunction, metabolic and nutrition-sensor changes, stem cell attrition, and macromolecular damages. Separately and collectively, these features degrade the most critical neuronal function: transmission of information in the brain. It is widely accepted that aging is the leading risk factor contributing to the onset of the most prevalent pathological conditions that affect brain functions, such as Alzheimer's, Parkinson's, and Huntington's disease. One of the limitations in understanding the molecular mechanisms involved in those diseases is the lack of an appropriate cellular model that recapitulates the “aged” context in human neurons. The advent of the cellular reprogramming of somatic cells, i.e., dermal fibroblasts, to obtain directly induced neurons (iNs) and induced pluripotent stem cell- (iPSC-) derived neurons is technical sound advances that could open the avenues to understand better the contribution of aging toward neurodegeneration. In this review, we will summarize the commonalities and singularities of these two approaches for the study of brain aging, with an emphasis on the role of mitochondrial dysfunction and redox biology. We will address the evidence showing that iNs retain age-related features in contrast to iPSC-derived neurons that lose the aging signatures during the reprogramming to pluripotency, rendering iNs a powerful strategy to deepen our knowledge of the processes driving normal cellular function decline and neurodegeneration in a human adult model. We will finally discuss the potential utilization of these novel technologies to understand the differential contribution of genetic and epigenetic factors toward neuronal aging, to identify and develop new drugs and therapeutic strategies.

Ionizing Radiation-Induced Brain Cell Aging and the Potential Underlying Molecular Mechanisms

Population aging is occurring rapidly worldwide, challenging the global economy and healthcare services. Brain aging is a significant contributor to various age-related neurological and neuropsychological disorders, including Alzheimer's disease and Parkinson's disease. Several extrinsic factors, such as exposure to ionizing radiation, can accelerate senescence. Multiple human and animal studies have reported that exposure to ionizing radiation can have varied effects on organ aging and lead to the prolongation or shortening of life span depending on the radiation dose or dose rate. This paper reviews the effects of radiation on the aging of different types of brain cells, including neurons, microglia, astrocytes, and cerebral endothelial cells. Further, the relevant molecular mechanisms are discussed. Overall, this review highlights how radiation-induced senescence in different cell types may lead to brain aging, which could result in the development of various neurological and neuropsychological disorders. Therefore, treatment targeting radiation-induced oxidative stress and neuroinflammation may prevent radiation-induced brain aging and the neurological and neuropsychological disorders it may cause.

Temporal Profiling of the Cortical Synaptic Mitochondrial Proteome Identifies Ageing Associated Regulators of Stability

Synapses are particularly susceptible to the effects of advancing age, and mitochondria have long been implicated as organelles contributing to this compartmental vulnerability. Despite this, the mitochondrial molecular cascades promoting age-dependent synaptic demise remain to be elucidated. Here, we sought to examine how the synaptic mitochondrial proteome (including strongly mitochondrial associated proteins) was dynamically and temporally regulated throughout ageing to determine whether alterations in the expression of individual candidates can influence synaptic stability/morphology. Proteomic profiling of wild-type mouse cortical synaptic and non-synaptic mitochondria across the lifespan revealed significant age-dependent heterogeneity between mitochondrial subpopulations, with aged organelles exhibiting unique protein expression profiles. Recapitulation of aged synaptic mitochondrial protein expression at the Drosophila neuromuscular junction has the propensity to perturb the synaptic architecture, demonstrating that temporal regulation of the mitochondrial proteome may directly modulate the stability of the synapse in vivo.

Brain aging mechanisms with mechanical manifestations

Brain aging is a complex process that affects everything from the subcellular to the organ level, begins early in life, and accelerates with age. Morphologically, brain aging is primarily characterized by brain volume loss, cortical thinning, white matter degradation, loss of gyrification, and ventricular enlargement. Pathophysiologically, brain aging is associated with neuron cell shrinking, dendritic degeneration, demyelination, small vessel disease, metabolic slowing, microglial activation, and the formation of white matter lesions. In recent years, the mechanics community has demonstrated increasing interest in modeling the brain's (bio)mechanical behavior and uses constitutive modeling to predict shape changes of anatomically accurate finite element brain models in health and disease. Here, we pursue two objectives. First, we review existing imaging-based data on white and gray matter atrophy rates and organ-level aging patterns. This data is required to calibrate and validate constitutive brain models. Second, we review the most critical cell- and tissue-level aging mechanisms that drive white and gray matter changes. We focuse on aging mechanisms that ultimately manifest as organ-level shape changes based on the idea that the integration of imaging and mechanical modeling may help identify the tipping point when normal aging ends and pathological neurodegeneration begins.

Irisin Rescues Blood-Brain Barrier Permeability following Traumatic Brain Injury and Contributes to the Neuroprotection of Exercise in Traumatic Brain Injury

Traumatic brain injury (TBI) has a high incidence, mortality, and morbidity all over the world. One important reason for its poor clinical prognosis is brain edema caused by blood-brain barrier (BBB) dysfunction after TBI. The mechanism may be related to the disorder of mitochondrial morphology and function of neurons in damaged brain tissue, the decrease of uncoupling protein 2 (UCP2) activity, and the increase of inflammatory reaction and oxidative stress. In this study, we aimed to investigate the effects of exogenous irisin on BBB dysfunction after TBI and its role in the neuroprotective effects of endurance exercise (EE) in mice. The concentrations of irisin in cerebrospinal fluid (CSF) and plasma of patients with mild to severe TBI were measured by ELISA. Then, male C57BL/6J mice and UCP2 knockout mice with C57BL/6J background were used to establish the TBI model. The BBB structure and permeability were examined by transmission electron microscopy and Evans blue extravasation, respectively. The protein expressions of irisin, occludin, claudin-5, zonula occludens-1 (ZO-1), nuclear factor E2-related factor 2(Nrf2), quinine oxidoreductase (NQO-1), hemeoxygenase-1 (HO-1), cytochrome C (Cyt-C), cytochrome C oxidase (COX IV), BCL2-associated X protein (Bax), cleaved caspase-3, and UCP2 were detected by western blot. The production of reactive oxygen species (ROS) was evaluated by the dihydroethidium (DHE) staining. The levels of inflammatory factors were detected by ELISA. In this study, we found that the CSF irisin levels were positively correlated with the severity of disease in patients with TBI and both EE and exogenous irisin could reduce BBB damage in a mouse model of TBI. In addition, we used UCP2-/- mice and further found that irisin could improve the dysfunction of BBB after TBI by promoting the expression of UCP2 on the mitochondrial membrane of neurons, reducing the damage of mitochondrial structure and function, thus alleviating the inflammatory response and oxidative stress. In conclusion, the results of this study suggested that irisin might alleviate brain edema after TBI by promoting the expression of UCP2 on the mitochondrial membrane of neurons and contribute to the neuroprotection of EE against TBI.

The Cerebral Effect of Ammonia in Brain Aging: Blood-Brain Barrier Breakdown, Mitochondrial Dysfunction, and Neuroinflammation

Aging occurs along with multiple pathological problems in various organs. The aged brain, especially, shows a reduction in brain mass, neuronal cell death, energy dysregulation, and memory loss. Brain aging is influenced by altered metabolites both in the systemic blood circulation and the central nervous system (CNS). High levels of ammonia, a natural by-product produced in the body, have been reported as contributing to inflammatory responses, energy metabolism, and synaptic function, leading to memory function in CNS. Ammonia levels in the brain also increase as a consequence of the aging process, ultimately leading to neuropathological problems in the CNS. Although many researchers have demonstrated that the level of ammonia in the body alters with age and results in diverse pathological alterations, the definitive relationship between ammonia and the aged brain is not yet clear. Thus, we review the current body of evidence related to the roles of ammonia in the aged brain. On the basis of this, we hypothesize that the modulation of ammonia level in the CNS may be a critical clinical point to attenuate neuropathological alterations associated with aging.

Changes in synaptic proteins of the complex PSD-95/NMDA receptor/nNOS and mitochondrial dysfunction after levocabastine treatment

Nitric oxide generation is related to the activity of certain proteins located at synaptic sites. Previous findings show that NOS activity, nNOS protein expression, respiratory parameters and mitochondrial complex activities are altered in rat cerebral cortex by administration of levocabastine, an antagonist of histamine H1 and neurotensin NTS2 receptors. ATP provision by mitochondria may play an important role in the functional interaction between synaptic proteins NMDA receptor and PSD-95 with NO synthesis. In this context, our purpose was to evaluate the effect of levocabastine administration on protein expression of PSD-95, GluN2B and iNOS, as well as on mitochondrial ATP production. Male Wistar rats received a single (i.p.) dose of levocabastine (50 μg/kg) or saline solution (controls) and were decapitated 18 h later. Mitochondrial and synaptosomal membrane fractions were isolated from cerebral cortex by differential and sucrose gradient centrifugation. Expression of synaptic proteins was evaluated by Western blot assays in synaptosomal membrane fractions. Oxygen consumption, mitochondrial membrane potential and ATP production rate were determined in fresh crude mitochondrial fractions. After levocabastine treatment, protein expression of PSD-95, GluN2B and β-actin decreased 97, 45 and 55%, respectively, whereas that of iNOS enhanced 3.5-fold versus controls. In crude mitochondrial fractions levocabastine administration reduced roughly 15% respiratory control rate as assayed with malate-glutamate or succinate as substrates, decreased mitochondrial membrane potential (21%), and ATP production rates (57%). Results suggested that levocabastine administration induces alterations in synaptic proteins of the protein complex PSD-95/NMDA receptor/nNOS and in neuron cytoskeleton. Mitochondrial bioenergetics impairment may play a role in the functional link between synaptic proteins and NO synthesis.

Molecular and cellular pathways contributing to brain aging

Aging is the leading risk factor for several age-associated diseases such as neurodegenerative diseases. Understanding the biology of aging mechanisms is essential to the pursuit of brain health. In this regard, brain aging is defined by a gradual decrease in neurophysiological functions, impaired adaptive neuroplasticity, dysregulation of neuronal Ca2+ homeostasis, neuroinflammation, and oxidatively modified molecules and organelles. Numerous pathways lead to brain aging, including increased oxidative stress, inflammation, disturbances in energy metabolism such as deregulated autophagy, mitochondrial dysfunction, and IGF-1, mTOR, ROS, AMPK, SIRTs, and p53 as central modulators of the metabolic control, connecting aging to the pathways, which lead to neurodegenerative disorders. Also, calorie restriction (CR), physical exercise, and mental activities can extend lifespan and increase nervous system resistance to age-associated neurodegenerative diseases. The neuroprotective effect of CR involves increased protection against ROS generation, maintenance of cellular Ca2+ homeostasis, and inhibition of apoptosis. The recent evidence about the modem molecular and cellular methods in neurobiology to brain aging is exhibiting a significant potential in brain cells for adaptation to aging and resistance to neurodegenerative disorders.

The old guard: Age-related changes in microglia and their consequences

Among all major organs, the brain is one of the most susceptible to the inexorable effects of aging. Throughout the last decades, several studies in human cohorts and animal models have revealed a plethora of age-related changes in the brain, including reduced neurogenesis, oxidative damage, mitochondrial dysfunction and cell senescence. As the main immune effectors and first responders of the nervous tissue, microglia are at the center of these events. These cells experience irrevocable changes as a result from cumulative exposure to environmental triggers, such as stress, infection and metabolic dysregulation. The age-related immunosenescent phenotype acquired by microglia is characterized by profound modifications in their transcriptomic profile, secretome, morphology and phagocytic activity, which compromise both their housekeeping and defensive functions. As a result, aged microglia are no longer capable of establishing effective immune responses and sustaining normal synaptic activity, directly contributing to age-associated cognitive decline and neurodegeneration. This review discusses how lifestyle and environmental factors drive microglia dysfunction at the molecular and functional level, also highlighting possible interventions to reverse aging-associated damage to the nervous and immune systems.

Alcohol hangover induces nitric oxide metabolism changes by impairing NMDA receptor-PSD95-nNOS pathway

Alcohol hangover is defined as the combination of mental and physical symptoms experienced the day after a single episode of heavy drinking, starting when blood alcohol concentration approaches zero. We previously evidenced increments in free radical generation and an imbalance in antioxidant defences in non-synaptic mitochondria and synaptosomes during hangover. It is widely known that acute alcohol exposure induces changes in nitric oxide (NO) production and blocks the binding of glutamate to NMDAR in central nervous system. Our aim was to evaluate the residual effect of acute ethanol exposure (hangover) on NO metabolism and the role of NMDA receptor-PSD95-nNOS pathway in non-synaptic mitochondria and synaptosomes from mouse brain cortex. Results obtained for the synaptosomes fraction showed a 37% decrease in NO total content, a 36% decrease in NOS activity and a 19% decrease in nNOS protein expression. The in vitro addition of glutamate to synaptosomes produced a concentration-dependent enhancement of NO production which was significantly lower in samples from hangover mice than in controls for all the glutamate concentrations tested. A similar patter of response was observed for nNOS activity being decreased both in basal conditions and after glutamate addition. In addition, synaptosomes exhibited a 64% and 15% reduction in NMDA receptor subunit GluN2B and PSD-95 protein expression, respectively. Together with this, glutamate-induced calcium entry was significant decreased in synaptosomes from alcohol-treated mice. On the other hand, in non-synaptic mitochondria, no significant differences were observed in NO content, NOS activity or nNOS protein expression. The expression of iNOS remained unaltered in synaptosomes and non-synaptic mitochondria. Here we demonstrated that hangover effects on NO metabolism are strongly evidenced in synaptosomes probably due to a disruption in NMDAR/PSD-95/nNOS pathway.

Pathologically phosphorylated tau at S396/404 (PHF-1) is accumulated inside of hippocampal synaptic mitochondria of aged Wild-type mice

Brain aging is a natural process characterized by cognitive decline and memory loss. This impairment is related to mitochondrial dysfunction and has recently been linked to the accumulation of abnormal proteins in the hippocampus. Age-related mitochondrial dysfunction could be induced by modified forms of tau. Here, we demonstrated that phosphorylated tau at Ser 396/404 sites, epitope known as PHF-1, is increased in the hippocampus of aged mice at the same time that oxidative damage and mitochondrial dysfunction are observed. Most importantly, we showed that tau PHF-1 is located in hippocampal mitochondria and accumulates in the mitochondria of old mice. Finally, since two mitochondrial populations were found in neurons, we evaluated tau PHF-1 levels in both non-synaptic and synaptic mitochondria. Interestingly, our results revealed that tau PHF-1 accumulates primarily in synaptic mitochondria during aging, and immunogold electron microscopy and Proteinase K protection assays demonstrated that tau PHF-1 is located inside mitochondria. These results demonstrated the presence of phosphorylated tau at PHF-1 commonly related to tauopathy, inside the mitochondria from the hippocampus of healthy aged mice for the first time. Thus, this study strongly suggests that synaptic mitochondria could be damaged by tau PHF-1 accumulation inside this organelle, which in turn could result in synaptic mitochondrial dysfunction, contributing to synaptic failure and memory loss at an advanced age.

Impaired GABAergic and glutamatergic neurometabolic activity in aged mice brain as measured by 1 H-[13 C]-NMR spectroscopy

Healthy aging is associated with a decline in cognitive function, and is a major risk factor for many neurodegenerative diseases. Although, there are several evidence that brain mitochondrial function is altered with aging its significance at the cellular level is elusive. In this study, we have investigated mitochondrial TCA cycle and neurotransmitter cycle fluxes associated with glutamatergic, GABAergic neurons and astroglia in the cerebral cortex and hippocampus of young (6 months) and aged (24 months) C57BL6 mice by using ¹ H-[¹³ C]-NMR spectroscopy together with timed infusion of ¹³ C-labeled glucose and acetate. The ratio V_Cyc /V_TCA was determined from a steady-state [2-¹³ C]acetate experiment. Metabolic fluxes were obtained by fitting a three-compartment metabolic model to ¹³ C turnover of amino acids from glucose. Levels of glutamate, aspartate and taurine were reduced in the cerebral cortex, while glutamine and choline were elevated in the hippocampus of aged mice. Interestingly, the rate of acetate oxidation increased in the cerebral cortex, while the flux of mitochondrial TCA cycle of glutamatergic neurons decreased in the cerebral cortex (P < .0001) and hippocampus (P = .025) of aged mice. The glutamate-glutamine neurotransmitter cycle flux was reduced in the cerebral cortex (P < .0001). The GABAergic TCA cycle flux was reduced in the cerebral cortex (P = .0008), while GABA-glutamine neurotransmitter cycling flux was also reduced in the cerebral cortex (P = .011) and hippocampus (P = .042) of aged brain. In conclusion, the reduction in excitatory and inhibitory neurotransmitter activity of glutamatergic and GABAergic neurons in the cerebral cortex and hippocampus correlates qualitatively with declined cognitive function in aged mice.

The Synaptic Dysregulation in Adolescent Rats Exposed to Maternal Immune Activation

Maternal immune activation (MIA) is a risk factor for neurodevelopmental disorders in offspring, but the pathomechanism is largely unknown. The aim of our study was to analyse the molecular mechanisms contributing to synaptic alterations in hippocampi of adolescent rats exposed prenatally to MIA. MIA was evoked in pregnant female rats by i.p. administration of lipopolysaccharide at gestation day 9.5. Hippocampi of offspring (52-53-days-old rats) were analysed using transmission electron microscopy (TEM), qPCR and Western blotting. Moreover, mitochondrial membrane potential, activity of respiratory complexes, and changes in glutathione system were measured. It was found that MIA induced changes in hippocampi morphology, especially in the ultrastructure of synapses, including synaptic mitochondria, which were accompanied by impairment of mitochondrial electron transport chain and decreased mitochondrial membrane potential. These phenomena were in agreement with increased generation of reactive oxygen species, which was evidenced by a decreased reduced/oxidised glutathione ratio and an increased level of dichlorofluorescein (DCF) oxidation. Activation of cyclin-dependent kinase 5, and phosphorylation of glycogen synthase kinase 3β on Ser9 occurred, leading to its inhibition and, accordingly, to hypophosphorylation of microtubule associated protein tau (MAPT). Abnormal phosphorylation and dysfunction of MAPT, the manager of the neuronal cytoskeleton, harmonised with changes in synaptic proteins. In conclusion, this is the first study demonstrating widespread synaptic changes in hippocampi of adolescent offspring prenatally exposed to MIA.

Combination of ellagic acid and trans-cinnamaldehyde alleviates aging-induced cognitive impairment via modulation of mitochondrial function and inflammatory and apoptotic mediators in the prefrontal cortex of aged rats

Cognitive impairments are associated with advancing age. Trans-cinnamaldehyde (CIN) and ellagic acid (ELA) have multiplex activities to reduce various age-related cognitive disorders. In this study, we investigated the effects of these compounds separately or in combination on the cognitive outcomes, mitochondrial function, and inflammatory and apoptotic mediators in aged male Wistar rats. Thirty-two old (22 months old) and eight young (5 months old) rats were randomly allocated to five groups of young control, aged control, ELA-aged, CIN-aged, and ELA + CIN-aged. ELA (15 mg/kg, orally) and CIN (50 mg/kg, intraperitoneally) separately or in combination were administered for 1 month in aged animals. Spatial memory and cognitive activity were evaluated by the Barnes maze and novel object recognition tests. Mitochondrial function (its reactive oxygen species [ROS], mitochondrial membrane potential and ATP level), pro-inflammatory cytokines such as interleukin (IL)-1β and IL-6 and pro-apoptotic caspase 3 and Bax, and anti-apoptotic Bcl2 levels and their ratio were assessed in the prefrontal cortex. Behavioral results revealed that CIN separately or in combination with ELA significantly alleviates aging-induced memory impairment. Moreover, co-administration of agents effectively decreased inflammatory cytokines, cleaved-caspase 3, Bax and Bax/Bcl2 levels, mitochondrial ROS production, and mitochondrial membrane depolarization and increased Bcl2 and ATP level as compared with untreated aged control rats. Combination therapy was greater than those of individual treatments in all parameters. Therefore, combination therapy with CIN and ELA improved aging-induced cognitive impairment through anti-inflammatory, anti-apoptotic, and mitochondrial-boosting effects in aged rats.

The aging brain: impact of heavy metal neurotoxicity

The aging process is accompanied by critical changes in cellular and molecular functions, which upset the homeostatic balance in the central nervous system. Accumulation of metals renders the brain susceptible to neurotoxic insults by mechanisms such as mitochondrial dysfunction, neuronal calcium-ion dyshomeostasis, buildup of damaged molecules, compromised DNA repair, reduction in neurogenesis, and impaired energy metabolism. These hallmarks have been identified to be responsible for neuronal injuries, resulting in several neurological disorders. Various studies have shown solid associations between metal accumulation, abnormal protein expressions, and pathogenesis of neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, Huntington's disease, and Amyotrophic lateral sclerosis. This review highlights metals (such as manganese, zinc, iron, copper, and nickel) for their accumulation, and consequences in the development of neurological disorders, in relation to the aging brain.

The Mitochondrial Unfolded Protein Response: A Hinge Between Healthy and Pathological Aging

Aging is the time-dependent functional decline that increases the vulnerability to different forms of stress, constituting the major risk factor for the development of neurodegenerative diseases. Dysfunctional mitochondria significantly contribute to aging phenotypes, accumulating particularly in post-mitotic cells, including neurons. To cope with deleterious effects, mitochondria feature different mechanisms for quality control. One such mechanism is the mitochondrial unfolded protein response (UPRMT), which corresponds to the transcriptional activation of mitochondrial chaperones, proteases, and antioxidant enzymes to repair defective mitochondria. Transcription of target UPRMT genes is epigenetically regulated by Histone 3-specific methylation. Age-dependency of this regulation could explain a differential UPRMT activity in early developmental stages or aged organisms. At the same time, precise tuning of mitochondrial stress responses is crucial for maintaining neuronal homeostasis. However, compared to other mitochondrial and stress response programs, the role of UPRMT in neurodegenerative disease is barely understood and studies in this topic are just emerging. In this review, we document the reported evidence characterizing the evolutionarily conserved regulation of the UPRMT and summarize the recent advances in understanding the role of the pathway in neurodegenerative diseases and aging.

Interplay between Peripheral and Central Inflammation in Obesity-Promoted Disorders: The Impact on Synaptic Mitochondrial Functions

The metabolic dysfunctions induced by high fat diet (HFD) consumption are not limited to organs involved in energy metabolism but cause also a chronic low-grade systemic inflammation that affects the whole body including the central nervous system. The brain has been considered for a long time to be protected from systemic inflammation by the blood–brain barrier, but more recent data indicated an association between obesity and neurodegeneration. Moreover, obesity-related consequences, such as insulin and leptin resistance, mitochondrial dysfunction and reactive oxygen species (ROS) production, may anticipate and accelerate the physiological aging processes characterized by systemic inflammation and higher susceptibility to neurological disorders. Here, we discussed the link between obesity-related metabolic dysfunctions and neuroinflammation, with particular attention to molecules regulating the interplay between energetic impairment and altered synaptic plasticity, for instance AMP-activated protein kinase (AMPK) and Brain-derived neurotrophic factor (BDNF). The effects of HFD-induced neuroinflammation on neuronal plasticity may be mediated by altered brain mitochondrial functions. Since mitochondria play a key role in synaptic areas, providing energy to support synaptic plasticity and controlling ROS production, the negative effects of HFD may be more pronounced in synapses. In conclusion, it will be emphasized how HFD-induced metabolic alterations, systemic inflammation, oxidative stress, neuroinflammation and impaired brain plasticity are tightly interconnected processes, implicated in the pathogenesis of neurological diseases.

Normal ageing of the brain: Histological and biological aspects

All the hallmarks of ageing are observed in the brain, and its cells, especially neurons, are characterized by their remarkably long lifetime. Like any organ or system, the brain is exposed to ageing processes which affect molecules, cells, blood vessels, gross morphology and, uniquely for this organ, cognition. The preponderant cerebral structures are characterized by the cellular processes of neurons and glial cells and while the quantity of cerebral interstitial fluid is limited, it is now recognized as playing a crucial role in maintaining cerebral homeostasis. Most of our current knowledge of the ageing brain derives from studies of neurodegenerative disorders. It is interesting to note that common features of these disorders, like Tau, phosphoTau and amyloid peptide accumulation, can begin relatively early in life as a result of physiological ageing and are present in subclinical cases while also being used as early-stage markers of neurodegenerative diseases in progression. In this article, we review tissue and cellular modifications in the ageing brain. Commonly described macroscopic, microscopic and vascular changes that in the ageing brain are contrasted with those seen in neurodegenerative contexts. We also review the molecular changes that occur with age in the brain, such as modifications in gene expression, insulin/insulin-like growth factor 1 signalling dysfunction, post-translational protein modifications, mitochondrial dysfunction, autophagy and calcium conductance changes.

Developmental lead (Pb)-induced deficits in redox and bioenergetic status of cerebellar synapses are ameliorated by ascorbate supplementation

Neurotoxicity induced by exposure to heavy metal lead (Pb) is a concern of utmost importance particularly for countries with industrial-based economies. The developing brain is especially sensitive to exposure to even minute quantities of Pb which can alter neurodevelopmental trajectory with irreversible effects on motor, emotive-social and cognitive attributes even into later adulthood. Chemical synapses form the major pathway of inter-neuronal communications and are prime candidates for higher order brain (motor, memory and behavior) functions and determine the resistance/susceptibility for neurological disorders, including neuropsychopathologies. The synaptic pathways and mechanisms underlying Pb-mediated alterations in neuronal signaling and plasticity are not completely understood. Employing a biochemically isolated synaptosomal fraction which is enriched in synaptic terminals and synaptic mitochondria, this study aimed to analyze the alterations in bioenergetic and redox/antioxidant status of cerebellar synapses induced by developmental exposure to Pb (0.2 %). Moreover, we test the efficacy of vitamin C (ascorbate; 500 mg/kg body weight), a neuroprotective and neuromodulatory antioxidant, in mitigation of Pb-induced neuronal deficits. Our results implicate redox and bioenergetic disruptions as an underlying feature of the synaptic dysfunction observed in developmental Pb neurotoxicity, potentially contributing to consequent deficits in motor, behavioral and psychological attributes of the organisms. In addition, we establish ascorbate as a key ingredient for therapeutic approach against Pb induced neurotoxicity, particularly for early-life exposures.

Premature synaptic mitochondrial dysfunction in the hippocampus during aging contributes to memory loss

Aging is a process characterized by cognitive impairment and mitochondrial dysfunction. In neurons, these organelles are classified as synaptic and non-synaptic mitochondria depending on their localization. Interestingly, synaptic mitochondria from the cerebral cortex accumulate more damage and are more sensitive to swelling than non-synaptic mitochondria. The hippocampus is fundamental for learning and memory, synaptic processes with high energy demand. However, it is unknown if functional differences are found in synaptic and non-synaptic hippocampal mitochondria; and whether this could contribute to memory loss during aging. In this study, we used 3, 6, 12 and 18 month-old (mo) mice to evaluate hippocampal memory and the function of both synaptic and non-synaptic mitochondria. Our results indicate that recognition memory is impaired from 12mo, whereas spatial memory is impaired at 18mo. This was accompanied by a differential function of synaptic and non-synaptic mitochondria. Interestingly, we observed premature dysfunction of synaptic mitochondria at 12mo, indicated by increased ROS generation, reduced ATP production and higher sensitivity to calcium overload, an effect that is not observed in non-synaptic mitochondria. In addition, at 18mo both mitochondrial populations showed bioenergetic defects, but synaptic mitochondria were prone to swelling than non-synaptic mitochondria. Finally, we treated 2, 11, and 17mo mice with MitoQ or Curcumin (Cc) for 5 weeks, to determine if the prevention of synaptic mitochondrial dysfunction could attenuate memory loss. Our results indicate that reducing synaptic mitochondrial dysfunction is sufficient to decrease age-associated cognitive impairment. In conclusion, our results indicate that age-related alterations in ATP produced by synaptic mitochondria are correlated with decreases in spatial and object recognition memory and propose that the maintenance of functional synaptic mitochondria is critical to prevent memory loss during aging.

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Hippocampus-dependent learning and memory are impaired with age, which correlated with synaptic mitochondrial dysfunction.

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Synaptic mitochondria fail before non-synaptic mitochondria, indicating premature synaptic mitochondrial damage in aging.

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Reducing synaptic mitochondrial dysfunction, with MitoQ or Curcumin, decrease age-associated hippocampal memory impairment.

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Age-related changes in ATP production of synaptic mitochondria correlated with decreased hippocampal memory.

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Maintenance of functional synaptic mitochondria is critical to prevent memory loss during aging.

Regulation and roles of mitophagy at synapses

Maintenance of synaptic homeostasis is a challenging task, due to the intricate spatial organization and intense activity of synapses. Typically, synapses are located far away from the neuronal cell body, where they orchestrate neuronal signalling and communication, through neurotransmitter release. Stationary mitochondria provide energy required for synaptic vesicle cycling, and preserve ionic balance by buffering intercellular calcium at synapses. Thus, synaptic homeostasis is critically dependent on proper mitochondrial function. Indeed, defective mitochondrial metabolism is a common feature of several neurodegenerative and psychiatric disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), bipolar disorders and schizophrenia among others, which are also accompanied by excessive synaptic abnormalities. Specialized and compartmentalized quality control mechanisms have evolved to restore and maintain synaptic energy metabolism. Here, we survey recent advances towards the elucidation of the pivotal role of mitochondria in neurotransmission and implicating mitophagy in the maintenance of synaptic homeostasis during ageing.

High-Fat Diet Induces Neuroinflammation and Mitochondrial Impairment in Mice Cerebral Cortex and Synaptic Fraction

Brain mitochondrial dysfunction is involved in the development of neurological and neurodegenerative diseases. Mitochondria specifically located at synapses play a key role in providing energy to support synaptic functions and plasticity, thus their defects may lead to synaptic failure, which is a common hallmark of neurodegenerative diseases. High-Fat Diet (HFD) consumption increases brain oxidative stress and impairs brain mitochondrial functions, although the underlying mechanisms are not completely understood. The aim of our study is to analyze neuroinflammation and mitochondrial dysfunctions in brain cortex and synaptosomal fraction isolated from a mouse model of diet-induced obesity. Male C57Bl/6 mice were divided into two groups fed a standard diet or HFD for 18 weeks. At the end of the treatment, inflammation (detected by ELISA), antioxidant state (measured by enzymatic activity), mitochondrial functions and efficiency (detected by oxidative capacity and Seahorse analysis), and brain-derived neurotrophic factor (BDNF) pathway (analyzed by western blot) were determined in brain cortex and synaptosomal fraction. In HFD animals, we observed an increase in inflammatory parameters and oxidative stress and a decrease in mitochondrial oxidative capacity both in the brain cortex and synaptosomal fraction. These alterations parallel with modulation of BDNF, a brain key signaling molecule that is linking synaptic plasticity and energy metabolism. Neuroinflammation HFD-dependent negatively affects BDNF pathway and mitochondrial activity in the brain cortex. The effect is even more pronounced in the synaptic region, where the impaired energy supply may have a negative impact on neuronal plasticity.

Mitochondrial Dysfunction as a Predictor and Driver of Alzheimer's Disease-Like Pathology in OXYS Rats

Growing evidence suggests that mitochondrial dysfunction is an early event in sporadic Alzheimer's disease (AD), but the impact of mitochondrial dysfunction on the transition from healthy aging to AD remains elusive. Here we estimated the influence of mitochondrial dysfunction on the initiation of AD signs in OXYS rats, which simulate key characteristics of sporadic AD. We assessed the mitochondrial ultrastructure of pyramidal neurons of the hippocampus at the age preceding the development (age 20 days), during manifestation (4-5 months), and at the well-pronounced stages (18-24 months) of the AD-like pathology in OXYS rats. Ultrastructural alterations were collated with the amounts of proteins mediating mitochondrial dynamics [mitofusins (MFN1 and MFN2) and dynamin-1-like protein (DRP1)]; with activity of respiratory chain complexes I, IV, and V in the hippocampal mitochondria; with reactive oxygen species (ROS) production; and with expression of uncoupling protein 2 (UCP2) regulating ROS production. Already at the preclinical stage, OXYS rats showed some characteristic changes in hippocampal mitochondria, which increased in size with the manifestation and progression of AD-like pathology, including decreased activity of respiratory complexes against the background of greater fusion and formation of larger mitochondria. Signs of AD developed simultaneously with increasing dysfunction of mitochondria, with a dramatic decrease in their number, and with increased fission but without upregulation of ROS production (observed only in 20-day-old OXYS rats). Summarizing the data from our present and previous studies, we conclude that mitochondrial dysfunction appears to mediate or possibly even initiate pathological molecular cascades of AD-like pathology in OXYS rats and can be considered a predictor of the early development of the late-onset form of AD in humans.

Alcohol hangover effects on brain cortex non-synaptic mitochondria and synaptosomes bioenergetics

Alcohol hangover (AH) has been associated with oxidative stress and mitochondrial dysfunction. We herein postulate that AH-induced mitochondrial alterations can be due to a different pattern of response in synaptosomes and non-synaptic (NS) mitochondria. Mice received intraperitoneal (i.p.) injections of ethanol (3.8 g/kg) or saline and were sacrificed 6 h afterward. Brain cortex NS mitochondria and synaptosomes were isolated by Ficoll gradient. Oxygen consumption rates were measured in NS mitochondria and synaptosomes by high-resolution respirometry. Results showed that NS-synaptic mitochondria from AH animals presented a 26% decrease in malate-glutamate state 3 respiration, a 64% reduction in ATP content, 28-37% decrements in ATP production rates (malate-glutamate or succinate-dependent, respectively), and 44% inhibition in complex IV activity. No changes were observed in mitochondrial transmembrane potential (ΔΨ) or in UCP-2 expression in NS-mitochondria. Synaptosome respiration driving proton leak (in the presence of oligomycin), and spare respiratory capacity (percentage ratio between maximum and basal respiration) were 30% and 15% increased in hangover condition, respectively. Synaptosomal ATP content was 26% decreased, and ATP production rates were 40-55% decreased (malate-glutamate or succinate-dependent, respectively) in AH mice. In addition, a 24% decrease in ΔΨ and a 21% increase in UCP-2 protein expression were observed in synaptosomes from AH mice. Moreover, mitochondrial respiratory complexes I-III, II-III, and IV activities measured in synaptosomes from AH mice were decreased by 18%, 34%, and 50%, respectively. Results of this study reveal that alterations in bioenergetics status during AH could be mainly due to changes in mitochondrial function at the level of synapses.

Hallmarks of Brain Aging: Adaptive and Pathological Modification by Metabolic States

During aging, the cellular milieu of the brain exhibits tell-tale signs of compromised bioenergetics, impaired adaptive neuroplasticity and resilience, aberrant neuronal network activity, dysregulation of neuronal Ca2+ homeostasis, the accrual of oxidatively modified molecules and organelles, and inflammation. These alterations render the aging brain vulnerable to Alzheimer's and Parkinson's diseases and stroke. Emerging findings are revealing mechanisms by which sedentary overindulgent lifestyles accelerate brain aging, whereas lifestyles that include intermittent bioenergetic challenges (exercise, fasting, and intellectual challenges) foster healthy brain aging. Here we provide an overview of the cellular and molecular biology of brain aging, how those processes interface with disease-specific neurodegenerative pathways, and how metabolic states influence brain health.

Exogenous Gene Transmission of Isocitrate Dehydrogenase 2 Mimics Ischemic Preconditioning Protection

Ischemic preconditioning confers organ-wide protection against subsequent ischemic stress. A substantial body of evidence underscores the importance of mitochondria adaptation as a critical component of cell protection from ischemia. To identify changes in mitochondria protein expression in response to ischemic preconditioning, we isolated mitochondria from ischemic preconditioned kidneys and sham-treated kidneys as a basis for comparison. The proteomic screen identified highly upregulated proteins, including NADP+-dependent isocitrate dehydrogenase 2 (IDH2), and we confirmed the ability of this protein to confer cellular protection from injury in murine S3 proximal tubule cells subjected to hypoxia. To further evaluate the role of IDH2 in cell protection, we performed detailed analysis of the effects of Idh2 gene delivery on kidney susceptibility to ischemia-reperfusion injury. Gene delivery of IDH2 before injury attenuated the injury-induced rise in serum creatinine (P<0.05) observed in controls and increased the mitochondria membrane potential (P<0.05), maximal respiratory capacity (P<0.05), and intracellular ATP levels (P<0.05) above those in controls. This communication shows that gene delivery of Idh2 can confer organ-wide protection against subsequent ischemia-reperfusion injury and mimics ischemic preconditioning.

The low affinity neurotensin receptor antagonist levocabastine impairs brain nitric oxide synthesis and mitochondrial function by independent mechanisms

Neurotensin is known to inhibit neuronal Na⁺ , K⁺ -ATPase, an effect that is rescued by nitric oxide (NO) synthase inhibition. However, whether the neurotensinergic and the nitrergic systems are independent pathways, or are mechanistically linked, remains unknown. Here, we addressed this issue and found that the administration of low affinity neurotensin receptor (NTS2) antagonist, levocabastine (50 μg/kg, i.p.) inhibited NO synthase (NOS) activity by 74 and 42% after 18 h in synaptosomal and mitochondrial fractions isolated from the Wistar rat cerebral cortex, respectively; these effects disappeared 36 h after levocabastine treatment. Intriguingly, whereas neuronal NOS protein abundance decreased (by 56%) in synaptosomes membranes, it was enhanced (by 86%) in mitochondria 18 h after levocabastine administration. Levocabastine enhanced the respiratory rate of synaptosomes in the presence of oligomycin, but it failed to alter the spare respiratory capacity; furthermore, the mitochondrial respiratory chain (MRC) complexes I-IV activities were severely diminished by levocabastine administration. The inhibition of NOS and MRC complexes activities were also observed after incubation of synaptosomes and mitochondria with levocabastine (1 μM) in vitro. These data indicate that the NTS2 antagonist levocabastine regulates NOS expression and activity at the synapse, suggesting an interrelationship between the neurotensinergic and the nitrergic systems. However, the bioenergetics effects of NTS2 activity inhibition are likely to be independent from the regulation of NO synthesis.

Brain aging and neurodegeneration: from a mitochondrial point of view

Aging is defined as a progressive time-related accumulation of changes responsible for or at least involved in the increased susceptibility to disease and death. The brain seems to be particularly sensitive to the aging process since the appearance of neurodegenerative diseases, including Alzheimer's disease, is exponential with the increasing age. Mitochondria were placed at the center of the 'free-radical theory of aging', because these paramount organelles are not only the main producers of energy in the cells, but also to main source of reactive oxygen species. Thus, in this review, we aim to look at brain aging processes from a mitochondrial point of view by asking: (i) What happens to brain mitochondrial bioenergetics and dynamics during aging? (ii) Why is the brain so sensitive to the age-related mitochondrial impairments? (iii) Is there a sex difference in the age-induced mitochondrial dysfunction? Understanding mitochondrial physiology in the context of brain aging may help identify therapeutic targets against neurodegeneration. This article is part of a series "Beyond Amyloid".

Mitochondrial dysfunction in the APP/PSEN1 mouse model of Alzheimer's disease and a novel protective role for ascorbate

Mitochondrial dysfunction is elevated in very early stages of Alzheimer's disease and exacerbates oxidative stress, which contributes to disease pathology. Mitochondria were isolated from 4-month-old wild-type mice, transgenic mice carrying the APP_SWE and PSEN1_dE9 mutations, mice with decreased brain and mitochondrial ascorbate (vitamin C) via heterozygous knockout of the sodium dependent vitamin C transporter (SVCT2^+/-) and transgenic APP/PSEN1 mice with heterozygous SVCT2 expression. Mitochondrial isolates from SVCT2^+/- mice were observed to consume less oxygen using high-resolution respirometry, and also exhibited decreased mitochondrial membrane potential compared to wild type isolates. Conversely, isolates from young (4 months) APP/PSEN1 mice consumed more oxygen, and exhibited an increase in mitochondrial membrane potential, but had a significantly lower ATP/ADP ratio compared to wild type isolates. Greater levels of reactive oxygen species were also produced in mitochondria isolated from both APP/PSEN1 and SVCT2^+/- mice compared to wild type isolates. Acute administration of ascorbate to mitochondria isolated from wild-type mice increased oxygen consumption compared with untreated mitochondria suggesting ascorbate may support energy production. This study suggests that both presence of amyloid and ascorbate deficiency can contribute to mitochondrial dysfunction, even at an early, prodromal stage of Alzheimer's disease, although occurring via different pathways. Ascorbate may, therefore, provide a useful preventative strategy against neurodegenerative disease, particularly in populations most at risk for Alzheimer's disease in which stores are often depleted through mitochondrial dysfunction and elevated oxidative stress.

Mitochondrial Bioenergetics Is Altered in Fibroblasts from Patients with Sporadic Alzheimer's Disease

The identification of an early biomarker to diagnose Alzheimer's disease (AD) remains a challenge. Neuropathological studies in animal and AD patients have shown that mitochondrial dysfunction is a hallmark of the development of the disease. Current studies suggest the use of peripheral tissues, like skin fibroblasts as a possibility to detect the early pathological alterations present in the AD brain. In this context, we studied mitochondrial function properties (bioenergetics and morphology) in cultured fibroblasts obtained from AD, aged-match and young healthy patients. We observed that AD fibroblasts presented a significant reduction in mitochondrial length with important changes in the expression of proteins that control mitochondrial fusion. Moreover, AD fibroblasts showed a distinct alteration in proteolytic processing of OPA1, a master regulator of mitochondrial fusion, compared to control fibroblasts. Complementary to these changes AD fibroblasts showed a dysfunctional mitochondrial bioenergetics profile that differentiates these cells from aged-matched and young patient fibroblasts. Our findings suggest that the human skin fibroblasts obtained from AD patients could replicate mitochondrial impairment observed in the AD brain. These promising observations suggest that the analysis of mitochondrial bioenergetics could represent a promising strategy to develop new diagnostic methods in peripheral tissues of AD patients.

The path from mitochondrial ROS to aging runs through the mitochondrial permeability transition pore

Excessive production of mitochondrial reactive oxygen species (mROS) is strongly associated with mitochondrial and cellular oxidative damage, aging, and degenerative diseases. However, mROS also induces pathways of protection of mitochondria that slow aging, inhibit cell death, and increase lifespan. Recent studies show that the activation of the mitochondrial permeability transition pore (mPTP), which is triggered by mROS and mitochondrial calcium overloading, is enhanced in aged animals and humans and in aging-related degenerative diseases. mPTP opening initiates further production and release of mROS that damage both mitochondrial and nuclear DNA, proteins, and phospholipids, and also releases matrix NAD that is hydrolyzed in the intermembrane space, thus contributing to the depletion of cellular NAD that accelerates aging. Oxidative damage to calcium transporters leads to calcium overload and more frequent opening of mPTP. Because aging enhances the opening of the mPTP and mPTP opening accelerates aging, we suggest that mPTP opening drives the progression of aging. Activation of the mPTP is regulated, directly and indirectly, not only by the mitochondrial protection pathways that are induced by mROS, but also by pro-apoptotic signals that are induced by DNA damage. We suggest that the integration of these contrasting signals by the mPTP largely determines the rate of cell aging and the initiation of cell death, and thus animal lifespan. The suggestion that the control of mPTP activation is critical for the progression of aging can explain the conflicting and confusing evidence regarding the beneficial and deleterious effects of mROS on health and lifespan.