Motile axonal mitochondria contribute to the variability of presynaptic strength

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.

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.

Fluid intelligence and the locus coeruleus-norepinephrine system

The last decade has seen significant progress identifying genetic and brain differences related to intelligence. However, there remain considerable gaps in our understanding of how cognitive mechanisms that underpin intelligence map onto various brain functions. In this article, we argue that the locus coeruleus-norepinephrine system is essential for understanding the biological basis of intelligence. We review evidence suggesting that the locus coeruleus-norepinephrine system plays a central role at all levels of brain function, from metabolic processes to the organization of large-scale brain networks. We connect this evidence with our executive attention view of working-memory capacity and fluid intelligence and present analyses on baseline pupil size, an indicator of locus coeruleus activity. Using a latent variable approach, our analyses showed that a common executive attention factor predicted baseline pupil size. Additionally, the executive attention function of disengagement--not maintenance--uniquely predicted baseline pupil size. These findings suggest that the ability to control attention may be important for understanding how cognitive mechanisms of fluid intelligence map onto the locus coeruleus-norepinephrine system. We discuss how further research is needed to better understand the relationships between fluid intelligence, the locus coeruleus-norepinephrine system, and functionally organized brain networks.

Energy matters: presynaptic metabolism and the maintenance of synaptic transmission

Synaptic activity imposes large energy demands that are met by local adenosine triphosphate (ATP) synthesis through glycolysis and mitochondrial oxidative phosphorylation. ATP drives action potentials, supports synapse assembly and remodelling, and fuels synaptic vesicle filling and recycling, thus sustaining synaptic transmission. Given their polarized morphological features - including long axons and extensive branching in their terminal regions - neurons face exceptional challenges in maintaining presynaptic energy homeostasis, particularly during intensive synaptic activity. Recent studies have started to uncover the mechanisms and signalling pathways involved in activity-dependent and energy-sensitive regulation of presynaptic energetics, or 'synaptoenergetics'. These conceptual advances have established the energetic regulation of synaptic efficacy and plasticity as an exciting research field that is relevant to a range of neurological disorders associated with bioenergetic failure and synaptic dysfunction.

Quantitative analysis of neuronal mitochondrial movement reveals patterns resulting from neurotoxicity of rotenone and 6-hydroxydopamine

Alterations in mitochondrial dynamics, including their intracellular trafficking, are common early manifestations of neuronal degeneration. However, current methodologies used to study mitochondrial trafficking events rely on parameters that are primarily altered in later stages of neurodegeneration. Our objective was to establish a reliable applied statistical analysis to detect early alterations in neuronal mitochondrial trafficking. We propose a novel quantitative analysis of mitochondria trajectories based on innovative movement descriptors, including straightness, efficiency, anisotropy, and kurtosis. We evaluated time- and dose-dependent alterations in trajectory descriptors using biological data from differentiated SH-SY5Y cells treated with the mitochondrial toxicants 6-hydroxydopamine and rotenone. MitoTracker Red CMXRos-labelled mitochondria movement was analyzed by total internal reflection fluorescence microscopy followed by computational modelling to describe the process. Based on the aforementioned trajectory descriptors, this innovative analysis of mitochondria trajectories provides insights into mitochondrial movement characteristics and can be a consistent and sensitive method to detect alterations in mitochondrial trafficking occurring in the earliest time points of neurodegeneration.

Oligodendrocytes enhance axonal energy metabolism by deacetylation of mitochondrial proteins through transcellular delivery of SIRT2

Neurons require mechanisms to maintain ATP homeostasis in axons, which are highly vulnerable to bioenergetic failure. Here, we elucidate a transcellular signaling mechanism by which oligodendrocytes support axonal energy metabolism via transcellular delivery of NAD-dependent deacetylase SIRT2. SIRT2 is undetectable in neurons but enriched in oligodendrocytes and released within exosomes. By deleting sirt2, knocking down SIRT2, or blocking exosome release, we demonstrate that transcellular delivery of SIRT2 is critical for axonal energy enhancement. Mass spectrometry and acetylation analyses indicate that neurons treated with oligodendrocyte-conditioned media from WT, but not sirt2-knockout, mice exhibit strong deacetylation of mitochondrial adenine nucleotide translocases 1 and 2 (ANT1/2). In vivo delivery of SIRT2-filled exosomes into myelinated axons rescues mitochondrial integrity in sirt2-knockout mouse spinal cords. Thus, our study reveals an oligodendrocyte-to-axon delivery of SIRT2, which enhances ATP production by deacetylating mitochondrial proteins, providing a target for boosting axonal bioenergetic metabolism in neurological disorders.

Mechanisms governing activity-dependent synaptic pruning in the developing mammalian CNS

Almost 60 years have passed since the initial discovery by Hubel and Wiesel that changes in neuronal activity can elicit developmental rewiring of the central nervous system (CNS). Over this period, we have gained a more comprehensive picture of how both spontaneous neural activity and sensory experience-induced changes in neuronal activity guide CNS circuit development. Here we review activity-dependent synaptic pruning in the mammalian CNS, which we define as the removal of a subset of synapses, while others are maintained, in response to changes in neural activity in the developing nervous system. We discuss the mounting evidence that immune and cell-death molecules are important mechanistic links by which changes in neural activity guide the pruning of specific synapses, emphasizing the role of glial cells in this process. Finally, we discuss how these developmental pruning programmes may go awry in neurodevelopmental disorders of the human CNS, focusing on autism spectrum disorder and schizophrenia. Together, our aim is to give an overview of how the field of activity-dependent pruning research has evolved, led to exciting new questions and guided the identification of new, therapeutically relevant mechanisms that result in aberrant circuit development in neurodevelopmental disorders.

A novel integrated approach to estimate the mitochondrial content of neuronal cells and brain tissues

BACKGROUND

Mitochondria and their dynamics fuel most cellular processes both in physiological and pathological conditions. In the central nervous system, mitochondria sustain synaptic transmission and plasticity via multiple mechanisms which include their redistribution and/or expansion to higher energy demanding sites, sustaining activity changes and promoting morphological circuit adaptations.

NEW METHOD

To be able to evaluate changes in mitochondrial number and protein phenotype, we propose a novel methodological approach where the simultaneous analysis of both mitochondrial DNA and protein content is performed on each individual microsample, avoiding non-homogeneous loss of material.

RESULTS

We validated this method on neuronal-like cells and tissue samples and obtained estimates for the mitochondrial/genomic DNA ratio as well as for the abundance of protein counterparts. When the mitochondrial content per cell was evaluated in different brain areas, our results matched the known regional variation in aerobic-anaerobic metabolism. When long-term potentiation (LTP) was induced on hippocampal neurons, we detected increases in the abundance of mitochondria that correlated with the degree of synaptic enhancement.

CONCLUSIONS

Our approach can be effectively used to study the mitochondrial content andits changes in different brain cells and tissues.

Neural oscillatory activity serving sensorimotor control is predicted by superoxide-sensitive mitochondrial redox environments

Mitochondrial integrity and associated redox profiles have long been revered as key contributors to a host of age- and disease-related pathologies, which eventually lead to neuronal and behavioral dysfunction in the sensorimotor and other systems. However, the precise role of the mitochondrial redox environment in human sensorimotor brain systems and motor behavior remains poorly understood. Herein, we provide evidence for a strong predictive capacity of superoxide and its scavenger, superoxide dismutase, on the neural oscillatory dynamics serving motor planning and execution above and beyond the effects of mitochondrial respiratory capacities alone. Importantly, these data provide insight regarding the impact of the redox environment on the population-level neural oscillations that serve motor function in healthy humans.

Motor control requires a coordinated ensemble of spatiotemporally precise neural oscillations across a distributed motor network, particularly in the beta range (15 to 30 Hz) to successfully plan and execute volitional actions. While substantial evidence implicates beta activity as critical to motor control, the molecular processes supporting these microcircuits and their inherent oscillatory dynamics remain poorly understood. Among these processes are mitochondrial integrity and the associated redox environments, although their direct impact on human neurophysiological function is unknown. Herein, 40 healthy adults completed a motor sequence paradigm during magnetoencephalography (MEG). MEG data were imaged in the time–frequency domain using a beamformer to evaluate beta oscillatory profiles during distinct phases of motor control (i.e., planning and execution) and subsequent behavior. To comprehensively quantify features of the mitochondrial redox environment, we used state-of-the-art systems biology approaches including Seahorse Analyzer to assess mitochondrial respiration and electron paramagnetic resonance spectroscopy to measure superoxide levels in whole blood as well as antioxidant activity assays. Using structural equation modeling, we tested the relationship between mitochondrial function and sensorimotor brain-behavior dynamics through alterations in the redox environment (e.g., generation of superoxide and alteration in antioxidant defenses). Our results indicated that superoxide-sensitive but not hydrogen peroxide–sensitive features of the redox environment had direct and mediating effects on the bioenergetic–neural pathways serving motor performance in healthy adults. Importantly, our results suggest that alterations in the redox environment may directly impact behavior above and beyond mitochondrial respiratory capacities alone and further may be effective targets for age- and disease-related declines in cognitive–motor function.

Presynaptic stochasticity improves energy efficiency and helps alleviate the stability-plasticity dilemma

When an action potential arrives at a synapse there is a large probability that no neurotransmitter is released. Surprisingly, simple computational models suggest that these synaptic failures enable information processing at lower metabolic costs. However, these models only consider information transmission at single synapses ignoring the remainder of the neural network as well as its overall computational goal. Here, we investigate how synaptic failures affect the energy efficiency of models of entire neural networks that solve a goal-driven task. We find that presynaptic stochasticity and plasticity improve energy efficiency and show that the network allocates most energy to a sparse subset of important synapses. We demonstrate that stabilising these synapses helps to alleviate the stability-plasticity dilemma, thus connecting a presynaptic notion of importance to a computational role in lifelong learning. Overall, our findings present a set of hypotheses for how presynaptic plasticity and stochasticity contribute to sparsity, energy efficiency and improved trade-offs in the stability-plasticity dilemma.

Bioenergetic Requirements and Spatiotemporal Profile of Nerve Growth Factor Induced PI3K-Akt Signaling Along Sensory Axons

Nerve Growth Factor (NGF) promotes the elaboration of axonal filopodia and branches through PI3K-Akt. NGF activates the TrkA receptor resulting in an initial transient high amplitude burst of PI3K-Akt signaling followed by a maintained lower steady state, hereafter referred to as initiation and steady state phases. Akt initially undergoes phosphorylation at T308 followed by phosphorylation at S473, resulting in maximal kinase activation. We report that during the initiation phase the localization of PI3K signaling, reported by visualizing sites of PIP3 formation, and Akt signaling, reflected by Akt phosphorylation at T308, correlates with the positioning of axonal mitochondria. Mitochondrial oxidative phosphorylation but not glycolysis is required for Akt phosphorylation at T308. In contrast, the phosphorylation of Akt at S473 is not spatially associated with mitochondria and is dependent on both oxidative phosphorylation and glycolysis. Under NGF steady state conditions, maintenance of phosphorylation at T308 shows dual dependence on oxidative phosphorylation and glycolysis. Phosphorylation at S473 is more dependent on glycolysis but also requires oxidative phosphorylation for maintenance over longer time periods. The data indicate that NGF induced PI3K-Akt signaling along axons is preferentially initiated at sites containing mitochondria, in a manner dependent on oxidative phosphorylation. Steady state signaling is discussed in the context of combined contributions by mitochondria and the possibility of glycolysis occurring in association with endocytosed signalosomes.

Developmental shift to mitochondrial respiration for energetic support of sustained transmission during maturation at the calyx of Held

A considerable amount of energy is expended following presynaptic activity to regenerate electrical polarization and maintain efficient release and recycling of neurotransmitter. Mitochondria are the major suppliers of neuronal energy, generating ATP via oxidative phosphorylation. However, the specific utilization of energy from cytosolic glycolysis rather than mitochondrial respiration at the presynaptic terminal during synaptic activity remains unclear and controversial. We use a synapse specialized for high-frequency transmission in mice, the calyx of Held, to test the sources of energy used to maintain energy during short activity bursts (<1 s) and sustained neurotransmission (30-150 s). We dissect the role of presynaptic glycolysis versus mitochondrial respiration by acutely and selectively blocking these ATP-generating pathways in a synaptic preparation where mitochondria and synaptic vesicles are prolific, under near-physiological conditions. Surprisingly, if either glycolysis or mitochondrial ATP production is intact, transmission during repetitive short bursts of activity is not affected. In slices from young animals before the onset of hearing, where the synapse is not yet fully specialized, both glycolytic and mitochondrial ATP production are required to support sustained, high-frequency neurotransmission. In mature synapses, sustained transmission relies exclusively on mitochondrial ATP production supported by bath lactate, but not glycolysis. At both ages, we observe that action potential propagation begins to fail before defects in synaptic vesicle recycling. Our data describe a specific metabolic profile to support high-frequency information transmission at the mature calyx of Held, shifting during postnatal synaptic maturation from glycolysis to rely on monocarboxylates as a fuel source.NEW & NOTEWORTHY We dissect the role of presynaptic glycolysis versus mitochondrial respiration in supporting high-frequency neurotransmission, by acutely blocking these ATP-generating pathways at a synapse tuned for high-frequency transmission. We find that massive energy expenditure is required to generate failure when only one pathway is inhibited. Action potential propagation is lost before impaired synaptic vesicle recycling. Synaptic transmission is exclusively dependent on oxidative phosphorylation in mature synapses, indicating presynaptic glycolysis may be dispensable for ATP maintenance.

Mitochondria in Neuronal Health: From Energy Metabolism to Parkinson's Disease

Mitochondria are the main suppliers of neuronal adenosine triphosphate and play a critical role in brain energy metabolism. Mitochondria also serve as Ca²⁺ sinks and anabolic factories and are therefore essential for neuronal function and survival. Dysregulation of neuronal bioenergetics is increasingly implicated in neurodegenerative disorders, particularly Parkinson's disease. This review describes the role of mitochondria in energy metabolism under resting conditions and during synaptic transmission, and presents evidence for the contribution of neuronal mitochondrial dysfunction to Parkinson's disease.

Aging: All roads lead to mitochondria

Mitochondria were described as early as 1890 as ubiquitous intracellular structures by Ernster and Schatz (1981) [1]. Since then, the accretion of knowledge in the past century has revealed much of the molecular details of mitochondria, ranging from mitochondrial origin, structure, metabolism, genetics, and signaling, and their implications in health and disease. We now know that mitochondria are remarkably multifunctional and deeply intertwined with many vital cellular processes. They are quasi-self organelles that still possess remnants of its bacterial ancestry, including an independent genome. The mitochondrial free radical theory of aging (MFRTA), which postulated that aging is a product of oxidative damage to mitochondrial DNA, provided a conceptual framework that put mitochondria on the map of aging research. However, several studies have more recently challenged the general validity of the theory, favoring novel ideas based on emerging evidence to understand how mitochondria contribute to aging and age-related diseases. One prominent topic of investigation lies on the fact that mitochondria are not only production sites for bioenergetics and macromolecules, but also regulatory hubs that communicate and coordinate many vital physiological processes at the cellular and organismal level. The bi-directional communication and coordination between the co-evolved mitochondrial and nuclear genomes is especially interesting in terms of cellular regulation. Mitochondria are dynamic and adaptive, rendering their function sensitive to cellular context. Tissues with high energy demands, such as the brain, seem to be uniquely affected by age-dependent mitochondrial dysfunction, providing a foundation for the development of novel mitochondrial-based therapeutics and diagnostics.

Astaxanthin-s-allyl cysteine diester against high glucose-induced neuronal toxicity in vitro and diabetes-associated cognitive decline in vivo: Effect on p53, oxidative stress and mitochondrial function

Neuroprotective effect of astaxanthin-s-allyl cysteine diester (AST-SAC) against high glucose (HG)-induced oxidative stress in in vitro and cognitive decline under diabetes conditions in in vivo has been explored. Pretreatment of AST-SAC (5, 10 and 15 μM) dose-dependently preserved the neuronal cells (SH-SY5Y) viability against HG toxicity through i) decreasing oxidative stress (decreasing reactive oxygen species generation and increasing endogenous antioxidants level); ii) protecting mitochondrial function [oxidative phosphorylation (OXPHOS) complexes activity and mitochondrial membrane potential (MMP)]; and iii) decreasing p53 level thereby subsequently decreasing the level of apoptotic marker proteins. Male Spraque-Dawley rats were orally administered AST-SAC (1 mg/kg/day) for 45 days in streptozotocin-induced diabetes mellitus (DM) rats. AST-SAC administration prevented the loss of spatial memory in DM rats as determined using the novel object location test. AST-SAC administration alleviated the DM-induced injury in brain such as increased cholinesterases activity, elevated oxidative stress and mitochondrial dysfunction. Altogether, the results from the present study demonstrated that AST-SAC averted the neuronal apoptosis and preserved the cognitive function against HG toxicity under DM conditions.

Reprogramming an energetic AKT-PAK5 axis boosts axon energy supply and facilitates neuron survival and regeneration after injury and ischemia

Mitochondria supply adenosine triphosphate (ATP) essential for neuronal survival and regeneration. Brain injury and ischemia trigger acute mitochondrial damage and a local energy crisis, leading to degeneration. Boosting local ATP supply in injured axons is thus critical to meet increased energy demand during nerve repair and regeneration in adult brains, where mitochondria remain largely stationary. Here, we elucidate an intrinsic energetic repair signaling axis that boosts axonal energy supply by reprogramming mitochondrial trafficking and anchoring in response to acute injury-ischemic stress in mature neurons and adult brains. P21-activated kinase 5 (PAK5) is a brain mitochondrial kinase with declined expression in mature neurons. PAK5 synthesis and signaling is spatiotemporally activated within axons in response to ischemic stress and axonal injury. PAK5 signaling remobilizes and replaces damaged mitochondria via the phosphorylation switch that turns off the axonal mitochondrial anchor syntaphilin. Injury-ischemic insults trigger AKT growth signaling that activates PAK5 and boosts local energy supply, thus protecting axon survival and facilitating regeneration in in vitro and in vivo models. Our study reveals an axonal mitochondrial signaling axis that responds to injury and ischemia by remobilizing damaged mitochondria for replacement, thereby maintaining local energy supply to support central nervous system (CNS) survival and regeneration.

Estrogen Deficiency Induces Mitochondrial Damage Prior to Emergence of Cognitive Deficits in a Postmenopausal Mouse Model

Background: Estrogen deficiency contributes to the development of Alzheimer's disease (AD) in menopausal women. In the current study, we examined the impact of estrogen deficiency on mitochondrial function and cognition using a postmenopausal mouse model. Methods: Bilateral ovariectomy was conducted in adult females C57BL/6J. Cognitive function was examined using the Morris water maze (MWM) test at 2 weeks, 1, 2, and 3 months after ovariectomy. Neurodegeneration was assessed using an immunofluorescence assay of microtubule-associated protein 2 (MAP2) in the hippocampus and immunoblotting against postsynaptic density-95 (PSD95). Mitochondrial function in the hippocampus was assessed using immunoblotting for NDUFB8, SDHB, UQCRC2, MTCO1, and ATP5A1. Mitochondrial biogenesis was examined using immunoblotting for PGC-1α, NRF1, and mtTFA. Mitochondrion fission was assessed with immunoblotting for Drp1, whereas mitochondrion fusion was analyzed with immunoblotting for OPA1 and Mfn2. Mitophagy was examined with immunoblotting for PINK1 and LC3B. Mice receiving sham surgery were used as controls. Results: Ovariectomy resulted in significant learning and memory deficits in the MWM test at 3 months, but not at any earlier time points. At 2 weeks after ovariectomy, levels of Drp1 phosphorylated at Ser637 decreased in the hippocampus. At 1 month after ovariectomy, hippocampal levels of NDUFB8, SDHB, PGC-1α, mtTFA, OPA1, and Mfn2 were significantly reduced. At 2 months after ovariectomy, hippocampal levels of MAP2, PSD95, MTCO1, NRF1, and Pink1 were also reduced. At 3 months, levels of LC3B-II were reduced. Conclusions: The cognitive decline associated with estrogen deficiency is preceded by mitochondrial dysfunction, abnormal mitochondrial biogenesis, irregular mitochondrial dynamics, and decreased mitophagy. Thus, mitochondrial damage may contribute to cognitive impairment associated with estrogen deficiency.

Sensory neurons derived from diabetic rats exhibit deficits in functional glycolysis and ATP that are ameliorated by IGF-1

OBJECTIVE

The distal dying-back of the longest nerve fibres is a hallmark of diabetic neuropathy, and impaired provision of energy in the form of adenosine triphosphate (ATP) may contribute to this neurodegenerative process. We hypothesised that energy supplementation via glycolysis and/or mitochondrial oxidative phosphorylation is compromised in cultured dorsal root ganglion (DRG) sensory neurons from diabetic rodents, thus contributing to axonal degeneration. Functional analysis of glycolysis and mitochondrial respiration and real-time measurement of ATP levels in live cells were our specific means to test this hypothesis.

METHODS

DRG neuron cultures from age-matched control or streptozotocin (STZ)-induced type 1 diabetic rats were used for in vitro studies. Three plasmids containing ATP biosensors of varying affinities were transfected into neurons to study endogenous ATP levels in real time. The Seahorse XF analyser was used for glycolysis and mitochondrial respiration measurements.

RESULTS

Fluorescence resonance energy transfer (FRET) efficiency (YFP/CFP ratio) of the ATP biosensors AT1.03 (low affinity) and AT1.03YEMK (medium affinity) were significantly higher than that measured using the ATP-insensitive construct AT1.03R122/6K in both cell bodies and neurites of DRG neurons (p < 0.0001). The ATP level was homogenous along the axons but higher in cell bodies in cultured DRG neurons from both control and diabetic rats. Treatment with oligomycin (an ATP synthase inhibitor in mitochondria) decreased the ATP levels in cultured DRG neurons. Likewise, blockade of glycolysis using 2-deoxy-d-glucose (2-DG: a glucose analogue) reduced ATP levels (p < 0.001). Cultured DRG neurons derived from diabetic rats showed a diminishment of ATP levels (p < 0.01), glycolytic capacity, glycolytic reserve and non-glycolytic acidification. Application of insulin-like growth factor-1 (IGF-1) significantly elevated all the above parameters in DRG neurons from diabetic rats. Oligomycin pre-treatment of DRG neurons, to block oxidative phosphorylation, depleted the glycolytic reserve and lowered basal respiration in sensory neurons derived from control and diabetic rats. Depletion was much higher in sensory neurons from diabetic rats compared to control rats. In addition, an acute increase in glucose concentration, in the presence or absence of oligomycin, elevated parameters of glycolysis by 1.5- to 2-fold while having no impact on mitochondrial respiration.

CONCLUSION

We provide the first functional evidence for decreased glycolytic capacity in DRG neurons derived from type 1 diabetic rats. IGF-1 protected against the loss of ATP supplies in DRG cell bodies and axons in neurons derived from diabetic rats by augmenting various parameters of glycolysis and mitochondrial respiration.

Tracking Mitochondrial Density and Positioning along a Growing Neuronal Process in Individual C. elegans Neuron Using a Long-Term Growth and Imaging Microfluidic Device

The long cellular architecture of neurons requires regulation in part through transport and anchoring events to distribute intracellular organelles. During development, cellular and subcellular events such as organelle additions and their recruitment at specific sites on the growing axons occur over different time scales and often show interanimal variability thus making it difficult to identify specific phenomena in population averages. To measure the variability in subcellular events such as organelle positions, we developed a microfluidic device to feed and immobilize Caenorhabditis elegans for high-resolution imaging over several days. The microfluidic device enabled long-term imaging of individual animals and allowed us to investigate organelle density using mitochondria as a testbed in a growing neuronal process in vivo Subcellular imaging of an individual neuron in multiple animals, over 36 h in our microfluidic device, shows the addition of new mitochondria along the neuronal process and an increase in the accumulation of synaptic vesicles (SVs) at synapses. Long-term imaging of individual C. elegans touch receptor neurons (TRNs) shows that the addition of new mitochondria takes place along the entire neuronal process length at a rate of ∼0.6 mitochondria/h. The threshold for the addition of a new mitochondrion occurs when the average separation between the two preexisting mitochondria exceeds 24 μm. Our assay provides a new opportunity to move beyond simple observations obtained from in vitro assays to allow the discovery of genes that regulate positioning of mitochondria in neurons.

Miro1-dependent mitochondrial dynamics in parvalbumin interneurons

The spatiotemporal distribution of mitochondria is crucial for precise ATP provision and calcium buffering required to support neuronal signaling. Fast-spiking GABAergic interneurons expressing parvalbumin (PV+) have a high mitochondrial content reflecting their large energy utilization. The importance for correct trafficking and precise mitochondrial positioning remains poorly elucidated in inhibitory neurons. Miro1 is a Ca²⁺-sensing adaptor protein that links mitochondria to the trafficking apparatus, for their microtubule-dependent transport along axons and dendrites, in order to meet the metabolic and Ca²⁺-buffering requirements of the cell. Here, we explore the role of Miro1 in PV+ interneurons and how changes in mitochondrial trafficking could alter network activity in the mouse brain. By employing live and fixed imaging, we found that the impairments in Miro1-directed trafficking in PV+ interneurons altered their mitochondrial distribution and axonal arborization, while PV+ interneuron-mediated inhibition remained intact. These changes were accompanied by an increase in the ex vivo hippocampal γ-oscillation (30–80 Hz) frequency and promoted anxiolysis. Our findings show that precise regulation of mitochondrial dynamics in PV+ interneurons is crucial for proper neuronal signaling and network synchronization.

TSG101 negatively regulates mitochondrial biogenesis in axons

There is a tight association between mitochondrial dysfunction and neurodegenerative diseases and axons that are particularly vulnerable to degeneration, but how mitochondria are maintained in axons to support their physiology remains poorly defined. In an in vivo forward genetic screen for mutants altering axonal mitochondria, we identified tsg101 Neurons mutant for tsg101 exhibited an increase in mitochondrial number and decrease in mitochondrial size. TSG101 is best known as a component of the endosomal sorting complexes required for transport (ESCRT) complexes; however, loss of most other ESCRT components did not affect mitochondrial numbers or size, suggesting TSG101 regulates mitochondrial biology in a noncanonical, ESCRT-independent manner. The TSG101-mutant phenotype was not caused by lack of mitophagy, and we found that autophagy blockade was detrimental only to the mitochondria in the cell bodies, arguing mitophagy and autophagy are dispensable for the regulation of mitochondria number in axons. Interestingly, TSG101 mitochondrial phenotypes were instead caused by activation of PGC-1ɑ/Nrf2-dependent mitochondrial biogenesis, which was mTOR independent and TFEB dependent and required the mitochondrial fission-fusion machinery. Our work identifies a role for TSG101 in inhibiting mitochondrial biogenesis, which is essential for the maintenance of mitochondrial numbers and sizes, in the axonal compartment.

Re-emphasizing early Alzheimer's disease pathology starting in select entorhinal neurons, with a special focus on mitophagy

The entorhinal-hippocampal system contains distinct networks subserving declarative memory. This system is selectively vulnerable to changes of ageing and pathological processes. The entorhinal cortex (EC) is a pivotal component of this memory system since it serves as the interface between the neocortex and the hippocampus. EC is heavily affected by the proteinopathies of Alzheimer's disease (AD). These appear in a stereotypical spatiotemporal manner and include increased levels of intracellular amyloid-beta Aβ (iAβ), parenchymal deposition of Aβ plaques, and neurofibrillary tangles (NFTs) containing abnormally processed Tau. Increased levels of iAβ and the formation of NFTs are seen very early on in a population of neurons belonging to EC layer II (EC LII), and recent evidence leads us to believe that this population is made up of highly energy-demanding reelin-positive (RE+) projection neurons. Mitochondria are fundamental to the energy supply, metabolism, and plasticity of neurons. Evidence from AD postmortem brain tissues supports the notion that mitochondrial dysfunction is one of the initial pathological events in AD, and this is likely to take place in the vulnerable RE + EC LII neurons. Here we review and discuss these notions, anchored to the anatomy of AD, and formulate a hypothesis attempting to explain the vulnerability of RE + EC LII neurons to the formation of NFTs. We attempt to link impaired mitochondrial clearance to iAβ and signaling involving both apolipoprotein 4 and reelin, and argue for their relevance to the formation of NFTs specifically in RE + EC LII neurons during the prodromal stages of AD. We believe future studies on these interactions holds promise to advance our understanding of AD etiology and provide new ideas for drug development.

The Multifaceted Roles of Zinc in Neuronal Mitochondrial Dysfunction

Zinc is a highly abundant cation in the brain, essential for cellular functions, including transcription, enzymatic activity, and cell signaling. However, zinc can also trigger injurious cascades in neurons, contributing to the pathology of neurodegenerative diseases. Mitochondria, critical for meeting the high energy demands of the central nervous system (CNS), are a principal target of the deleterious actions of zinc. An increasing body of work suggests that intracellular zinc can, under certain circumstances, contribute to neuronal damage by inhibiting mitochondrial energy processes, including dissipation of the mitochondrial membrane potential (MMP), leading to ATP depletion. Additional consequences of zinc-mediated mitochondrial damage include reactive oxygen species (ROS) generation, mitochondrial permeability transition, and excitotoxic calcium deregulation. Zinc can also induce mitochondrial fission, resulting in mitochondrial fragmentation, as well as inhibition of mitochondrial motility. Here, we review the known mechanisms responsible for the deleterious actions of zinc on the organelle, within the context of neuronal injury associated with neurodegenerative processes. Elucidating the critical contributions of zinc-induced mitochondrial defects to neurotoxicity and neurodegeneration may provide insight into novel therapeutic targets in the clinical setting.

Mitochondrial calcium at the synapse

Mitochondria are dynamic organelles, which serve various purposes, including but not limited to the production of ATP and various metabolites, buffering ions, acting as a signaling hub, etc. In recent years, mitochondria are being seen as the central regulators of cellular growth, development, and death. Since neurons are highly specialized cells with a heavy metabolic demand, it is not surprising that neurons are one of the most mitochondria-rich cells in an animal. At synapses, mitochondrial function and dynamics is tightly regulated by synaptic calcium. Calcium influx during synaptic activity causes increased mitochondrial calcium influx leading to an increased ATP production as well as buffering of synaptic calcium. While increased ATP production is required during synaptic transmission, calcium buffering by mitochondria is crucial to prevent faulty neurotransmission and excitotoxicity. Interestingly, mitochondrial calcium also regulates the mobility of mitochondria within synapses causing mitochondria to halt at the synapse during synaptic transmission. In this review, we summarize the various roles of mitochondrial calcium at the synapse.

Developmental regulation of microtubule-based trafficking and anchoring of axonal mitochondria in health and diseases

Mitochondria are cellular power plants that supply most of the ATP required in the brain to power neuronal growth, function, and regeneration. Given their extremely polarized structures and extended long axons, neurons face an exceptional challenge to maintain energy homeostasis in distal axons, synapses, and growth cones. Anchored mitochondria serve as local energy sources; therefore, the regulation of mitochondrial trafficking and anchoring ensures that these metabolically active areas are adequately supplied with ATP. Chronic mitochondrial dysfunction is a hallmark feature of major aging-related neurodegenerative diseases, and thus, anchored mitochondria in aging neurons need to be removed when they become dysfunctional. Investigations into the regulation of microtubule (MT)-based trafficking and anchoring of axonal mitochondria under physiological and pathological circumstances represent an important emerging area. In this short review article, we provide an updated overview of recent in vitro and in vivo studies showing (1) how mitochondria are transported and positioned in axons and synapses during neuronal developmental and maturation stages, and (2) how altered mitochondrial motility and axonal energy deficits in aging nervous systems link to neurodegeneration and regeneration in a disease or injury setting. We also highlight a major role of syntaphilin as a key MT-based regulator of axonal mitochondrial trafficking and anchoring in mature neurons.

Biomaterials reinforced MSCs transplantation for spinal cord injury repair

Due to the complex pathophysiological mechanism, spinal cord injury (SCI) has become one of the most intractable central nervous system (CNS) diseases to therapy. Stem cell transplantation, mesenchymal stem cells (MSCs) particularly, appeals to more and more attention along with the encouraging therapeutic results for the functional regeneration of SCI. However, traditional cell transplantation strategies have some limitations, including the unsatisfying survival rate of MSCs and their random diffusion from the injection site to ambient tissues. The application of biomaterials in tissue engineering provides a new horizon. Biomaterials can not only confine MSCs in the injured lesions with higher cell viability, but also promote their therapeutic efficacy. This review summarizes the strategies and advantages of biomaterials reinforced MSCs transplantation to treat SCI in recent years, which are clarified in the light of various therapeutic effects in pathophysiological aspects of SCI.

Quantifying mitochondrial volume density in phrenic motor neurons

BACKGROUND

Previous assessments of mitochondrial volume density within motor neurons used electron microscopy (EM) to image mitochondria. However, adequate identification and sampling of motor neurons within a particular motor neuron pool is largely precluded using EM. Here, we present an alternative method for determining mitochondrial volume density in identified motor neurons within the phrenic motor neuron (PhMN) pool, with greatly increased sampling.

NEW METHOD

This novel method for assessing mitochondrial volume density in PhMNs uses a combination of intrapleural injection of Alexa 488-conjugated cholera toxin B (CTB) to retrogradely label PhMNs, followed by intrathecal application of MitoTracker Red to label mitochondria. This technique was validated by comparison to 3D EM determination of mitochondrial volume density as a "gold standard".

RESULTS

A mean mitochondrial volume density of ∼11 % was observed across PhMNs using the new MitoTracker Red method. This compared favourably with mitochondrial volume density (∼11 %) measurements using EM.

COMPARISON WITH EXISTING METHOD

The range, mean and variance of mitochondrial volume density estimates in PhMNs were not different between EM and fluorescent imaging techniques.

CONCLUSIONS

Fluorescent imaging may be used to estimate mitochondrial volume density in a large sample of motor neurons, with results similar to EM, although EM did distinguish finer mitochondrion morphology compared to MitoTracker fluorescence. Compared to EM methods, the assessment of a larger sample size and unambiguous identification of motor neurons belonging to a specific motor neuron pool represent major advantages over previous methods.

Mitochondrial Functioning and the Relations among Health, Cognition, and Aging: Where Cell Biology Meets Cognitive Science

Cognitive scientists have determined that there is a set of mechanisms common to all sensory, perceptual, and cognitive abilities and correlated with age- and disease-related declines in cognition. These mechanisms also contribute to the development and functional coherence of the large-scale brain networks that support complex forms of cognition. At the same time, these brain and cognitive patterns are correlated with myriad health outcomes, indicating that at least some of the underlying mechanisms are common to all biological systems. Mitochondrial functions, including cellular energy production and control of oxidative stress, among others, are well situated to explain the relations among the brain, cognition, and health. Here, I provide an overview of the relations among cognitive abilities, associated brain networks, and the importance of mitochondrial energy production for their functioning. These are then linked to the relations between cognition, health, and aging. The discussion closes with implications for better integrating research in cognitive science and cell biology in the context of developing more sensitive measures of age- and disease-related declines in cognition.

TRAK1-Mediated Abnormality of Mitochondrial Fission Increases Seizure Susceptibility in Temporal Lobe Epilepsy

Mitochondrial dysfunction is closely related to the occurrence of epilepsy. Homeostasis of mitochondrial fusion and division can alleviate mitochondrial dysfunction. The trafficking kinesin protein 1 (TRAK1) is a key regulator of mitochondrial movement and regulates mitochondrial fusion-fission balance. The pathogenic variants in TRAK1 result in the severe neurodevelopmental disorders. However, the role of TRAK1 in epilepsy remains unclear. In the present study, we report that TRAK1 has a crucial function in regulation of epileptogenesis in temporal lobe epilepsy (TLE). TRAK1 expression is decreased in the patient specimens and animal model of TLE. Knockdown of TRAK1 causes an increase in mitochondrial fission factor (MFF) in vitro and the susceptibility to seizures in vivo. Exogenous overexpression of TRAK1 can rescue the dysfunction caused by TRAK1 knockdown. These findings provide new insights into the fundamental mechanisms of TRAK1 in TLE and have important implications for understanding and treating TLE via targeting mitochondrion.

Concepts of Neuroinflammation and Their Relationship With Impaired Mitochondrial Functions in Bipolar Disorder

Bipolar disorder (BD) is a chronic psychiatric disease, characterized by frequent behavioral episodes of depression and mania, and neurologically by dysregulated neurotransmission, neuroplasticity, growth factor signaling, and metabolism, as well as oxidative stress, and neuronal apoptosis, contributing to chronic neuroinflammation. These abnormalities result from complex interactions between multiple susceptibility genes and environmental factors such as stress. The neurocellular abnormalities of BD can result in gross morphological changes, such as reduced prefrontal and hippocampal volume, and circuit reorganization resulting in cognitive and emotional deficits. The term "neuroprogression" is used to denote the progressive changes from early to late stages, as BD severity and loss of treatment response correlate with the number of past episodes. In addition to circuit and cellular abnormalities, BD is associated with dysfunctional mitochondria, leading to severe metabolic disruption in high energy-demanding neurons and glia. Indeed, mitochondrial dysfunction involving electron transport chain (ETC) disruption is considered the primary cause of chronic oxidative stress in BD. The ensuing damage to membrane lipids, proteins, and DNA further perpetuates oxidative stress and neuroinflammation, creating a perpetuating pathogenic cycle. A deeper understanding of BD pathophysiology and identification of associated biomarkers of neuroinflammation are needed to facilitate early diagnosis and treatment of this debilitating disorder.

Axonal Organelles as Molecular Platforms for Axon Growth and Regeneration after Injury

Investigating the molecular mechanisms governing developmental axon growth has been a useful approach for identifying new strategies for boosting axon regeneration after injury, with the goal of treating debilitating conditions such as spinal cord injury and vision loss. The picture emerging is that various axonal organelles are important centers for organizing the molecular mechanisms and machinery required for growth cone development and axon extension, and these have recently been targeted to stimulate robust regeneration in the injured adult central nervous system (CNS). This review summarizes recent literature highlighting a central role for organelles such as recycling endosomes, the endoplasmic reticulum, mitochondria, lysosomes, autophagosomes and the proteasome in developmental axon growth, and describes how these organelles can be targeted to promote axon regeneration after injury to the adult CNS. This review also examines the connections between these organelles in developing and regenerating axons, and finally discusses the molecular mechanisms within the axon that are required for successful axon growth.

In-depth quantitative proteomic characterization of organotypic hippocampal slice culture reveals sex-specific differences in biochemical pathways

Sex differences in the brain of mammals range from neuroarchitecture through cognition to cellular metabolism. The hippocampus, a structure mostly associated with learning and memory, presents high vulnerability to neurodegeneration and aging. Therefore, we explored basal sex-related differences in the proteome of organotypic hippocampal slice culture, a major in vitro model for studying the cellular and molecular mechanisms related to neurodegenerative disorders. Results suggest a greater prevalence of astrocytic metabolism in females and significant neuronal metabolism in males. The preference for glucose use in glycolysis, pentose phosphate pathway and glycogen metabolism in females and high abundance of mitochondrial respiration subunits in males support this idea. An overall upregulation of lipid metabolism was observed in females. Upregulation of proteins responsible for neuronal glutamate and GABA synthesis, along with synaptic associated proteins, were observed in males. In general, the significant spectrum of pathways known to predominate in neurons or astrocytes, together with the well-known neuronal and glial markers observed, revealed sex-specific metabolic differences in the hippocampus. TEM qualitative analysis might indicate a greater presence of mitochondria at CA1 synapses in females. These findings are crucial to a better understanding of how sex chromosomes can influence the physiology of cultured hippocampal slices and allow us to gain insights into distinct responses of males and females on neurological diseases that present a sex-biased incidence.

Targeting Mitophagy in Alzheimer's Disease

Mitochondria perform many essential cellular functions including energy production, calcium homeostasis, transduction of metabolic and stress signals, and mediating cell survival and death. Maintaining viable populations of mitochondria is therefore critical for normal cell function. The selective disposal of damaged mitochondria, by a pathway known as mitophagy, plays a key role in preserving mitochondrial integrity and quality. Mitophagy reduces the formation of reactive oxygen species and is considered as a protective cellular process. Mitochondrial dysfunction and deficits of mitophagy have important roles in aging and especially in neurodegenerative disorders such as Alzheimer's disease (AD). Targeting mitophagy pathways has been suggested to have potential therapeutic effects against AD. In this review, we aim to briefly discuss the emerging concepts on mitophagy, molecular regulation of the mitophagy process, current mitophagy detection methods, and mitophagy dysfunction in AD. Finally, we will also briefly examine the stimulation of mitophagy as an approach for attenuating neurodegeneration in AD.

The therapeutic potential of mitochondrial transplantation for the treatment of neurodegenerative disorders

Mitochondrial activity is essential to support neural functions, and changes in the integrity and activity of the mitochondria can contribute to synaptic damage and neuronal death, especially in degenerative diseases associated with age, such as Alzheimer's and Parkinson's disease. Currently, different approaches are used to treat these conditions, and one strategy under research is mitochondrial transplantation. For years, mitochondria have been shown to be transferred between cells of different tissues. This process has allowed several attempts to develop transplantation schemes by isolating functional mitochondria and introducing them into damaged tissue in particular to counteract the harmful effects of myocardial ischemia. Recently, mitochondrial transfer between brain cells has also been reported, and thus, mitochondrial transplantation for disorders of the nervous system has begun to be investigated. In this review, we focus on the relevance of mitochondria in the nervous system, as well as some mitochondrial alterations that occur in neurodegenerative diseases associated with age. In addition, we describe studies that have performed mitochondrial transplantation in various tissues, and we emphasize the advances in mitochondrial transplantation aimed at treating diseases of the nervous system.

Inter-spike mitochondrial Ca2+ release enhances high frequency synaptic transmission

KEY POINTS

Presynaptic mitochondria not only absorb but also release Ca²⁺ during high frequency stimulation (HFS) when presynaptic [Ca²⁺ ] is kept low (<500 nm) by high cytosolic Ca²⁺ buffer or strong plasma membrane calcium clearance mechanisms under physiological external [Ca²⁺ ]. Mitochondrial Ca²⁺ release (MCR) does not alter the global presynaptic Ca²⁺ transients. MCR during HFS enhances short-term facilitation and steady state excitatory postsynaptic currents by increasing vesicular release probability. The intra-train MCR may provide residual calcium at interspike intervals, and thus support high frequency neurotransmission at central glutamatergic synapses.

ABSTRACT

Emerging evidence indicates that mitochondrial Ca²⁺ buffering contributes to local regulation of synaptic transmission. It is unknown, however, whether mitochondrial Ca²⁺ release (MCR) occurs during high frequency synaptic transmission. Confirming the previous notion that 2 μm tetraphenylphosphonium (TPP⁺ ) is a specific inhibitor of the mitochondrial Na⁺ /Ca²⁺ exchanger (mNCX), we studied the role of MCR via mNCX in short-term plasticity during high frequency stimulation (HFS) at the calyx of Held synapse of the rat. TPP⁺ reduced short-term facilitation (STF) and steady state excitatory postsynaptic currents during HFS at mature calyx synapses under physiological extracellular [Ca²⁺ ] ([Ca²⁺ ]_o  = 1.2 mm), but not at immature calyx or at 2 mm [Ca²⁺ ]_o . The inhibitory effects of TPP⁺ were stronger at synapses with morphologically complex calyces harbouring many swellings and at 32°C than at simple calyx synapses and at room temperature. These effects of TPP⁺ on STF were well correlated with those on the presynaptic mitochondrial [Ca²⁺ ] build-up during HFS. Mitochondrial [Ca²⁺ ] during HFS was increased by TPP⁺ at mature calyces under 1.2 mm [Ca²⁺ ]_o , and further enhanced at 32°C, but not under 2 mm [Ca²⁺ ]_o or at immature calyces. The close correlation of the effects of TPP⁺ on mitochondrial [Ca²⁺ ] with those on STF suggests that mNCX contributes to STF at the calyx of Held synapses. The intra-train MCR enhanced vesicular release probability without altering global presynaptic [Ca²⁺ ]. Our results suggest that MCR during HFS elevates local [Ca²⁺ ] near synaptic sites at interspike intervals to enhance STF and to support stable synaptic transmission under physiological [Ca²⁺ ]_o .

The mitochondrial calcium uniporter is crucial for the generation of fast cortical network rhythms

The role of the mitochondrial calcium uniporter (MCU) gene (Mcu) in cellular energy homeostasis and generation of electrical brain rhythms is widely unknown. We investigated this issue in mice and rats using Mcu-knockout and -knockdown strategies in vivo and in situ and determined the effects of these genetic manipulations on hippocampal gamma oscillations (30-70 Hz) and sharp wave-ripples. These physiological network states require precise neurotransmission between pyramidal cells and inhibitory interneurons, support spike-timing and synaptic plasticity and are associated with perception, attention and memory. Absence of the MCU resulted in (i) gamma oscillations with decreased power (by >40%) and lower synchrony, including less precise neural action potential generation ('spiking'), (ii) sharp waves with decreased incidence (by about 22%) and decreased fast ripple frequency (by about 3%) and (iii) lack of activity-dependent pyruvate dehydrogenase dephosphorylation. However, compensatory adaptation in gene expression related to mitochondrial function and glucose metabolism was not detected. These data suggest that the neuronal MCU is crucial for the generation of network rhythms, most likely by influences on oxidative phosphorylation and perhaps by controlling cytoplasmic Ca²⁺ homeostasis. This work contributes to an increased understanding of mitochondrial Ca²⁺ uptake in cortical information processing underlying cognition and behaviour.

Brain energy metabolism and multiple sclerosis: progress and prospects

Multiple sclerosis (MS) is an autoimmune disease accompanied with nerve pain and paralysis. Although various pathogenic causes of MS have been suggested, including genetic and environmental factors, how MS occurs remains unclear. Moreover, MS should be diagnosed based on clinical experiences because of no disease-specific biomarker and currently available treatments for MS just can reduce relapsing frequency or severity with little effects on disease disability. Therefore, more efforts are required to identify pathophysiology of MS and diagnosis markers. Recent evidence indicates another aspect of MS pathogenesis, energy failure in the central nervous system (CNS). For instance, inflammation that is a characteristic MS symptom and occurs frequently in the CNS of MS patients can result into energy failure in mitochondria and cytosol. Indeed, metabolomics studies for MS have reported energy failure in oxidative phosphorylation and alteration of aerobic glycolysis. Therefore, studies on the metabolism in the CNS may provide another insight for understanding complexity of MS and pathogenesis, which would facilitate the discovery of promising strategies for developing therapeutics to treat MS. This review will provide an overview on recent progress of metabolomic studies for MS, with a focus on the fluctuation of energy metabolism in MS.

A Computational Model of Mitochondria Motility in Axons

Mitochondria play a critical role in regulating cellular processes including ATP production, intracellular calcium signaling and generation of reactive oxidative species (ROS). Neurons rely on mitochondrial function to perform a range of complex processes, and mitochondrial dysfunctions have been shown to have an impact in pathologies of the nervous system. Yet, neurons contain a finite number of mitochondria, and their location is known to change in response to a number of factors including age and cellular activity, thereby impacting neuronal response. In this paper, we introduce a novel computational model of mitochondria motility that focuses on their movements along the axon. We describe the biological processes involved and the main parameters of the model. We use the model to investigate how some of these parameters affect the ability of mitochondria to position themselves in regions of high energy demand. Finally, we discuss the significance of our work and its downstream applications in further understanding pathologies of the nervous system such as Alzheimer's disease, and help identify potential novel therapeutic targets.

The cross-talk of energy sensing and mitochondrial anchoring sustains synaptic efficacy by maintaining presynaptic metabolism

Mitochondria supply ATP essential for synaptic transmission. Neurons face exceptional challenges in maintaining energy homoeostasis at synapses. Regulation of mitochondrial trafficking and anchoring is critical for neurons to meet increased energy consumption during sustained synaptic activity. However, mechanisms recruiting and retaining presynaptic mitochondria in sensing synaptic ATP levels remain elusive. Here we reveal an energy signalling axis that controls presynaptic mitochondrial maintenance. Activity-induced presynaptic energy deficits can be rescued by recruiting mitochondria through the AMP-activated protein kinase (AMPK)-p21-activated kinase (PAK) energy signalling pathway. Synaptic activity induces AMPK activation within axonal compartments and AMPK-PAK signalling triggers phosphorylation of myosin VI, which drives mitochondrial recruitment and syntaphilin-mediated anchoring on presynaptic filamentous actin. This pathway maintains presynaptic energy supply and calcium clearance during intensive synaptic activity. Disrupting this signalling cross-talk triggers local energy deficits and intracellular calcium build-up, leading to impaired synaptic efficacy during trains of stimulation and reduced recovery from synaptic depression after prolonged synaptic activity. Our study reveals a mechanistic cross-talk between energy sensing and mitochondria anchoring to maintain presynaptic metabolism, thus fine-tuning short-term synaptic plasticity and prolonged synaptic efficacy.

The bioenergetics of neuronal morphogenesis and regeneration: Frontiers beyond the mitochondrion

The formation of axons and dendrites during development, and their regeneration following injury, are energy intensive processes. The underlying assembly and dynamics of the cytoskeleton, axonal transport mechanisms, and extensive signaling networks all rely on ATP and GTP consumption. Cellular ATP is generated through oxidative phosphorylation (OxP) in mitochondria, glycolysis and "regenerative" kinase systems. Recent investigations have focused on the role of the mitochondrion in axonal development and regeneration emphasizing the importance of this organelle and OxP in axon development and regeneration. In contrast, the understanding of alternative sources of ATP in neuronal morphogenesis and regeneration remains largely unexplored. This review focuses on the current state of the field of neuronal bioenergetics underlying morphogenesis and regeneration and considers the literature on the bioenergetics of non-neuronal cell motility to emphasize the potential contributions of non-mitochondrial energy sources.

Chronic intermittent hypoxia alters the dendritic mitochondrial structure and activity in the pre-Bötzinger complex of rats

Mitochondrial bioenergetics is dynamically coupled with neuronal activities, which are altered by hypoxia-induced respiratory neuroplasticity. Here we report structural features of postsynaptic mitochondria in the pre-Bötzinger complex (pre-BötC) of rats treated with chronic intermittent hypoxia (CIH) simulating a severe condition of obstructive sleep apnea. The subcellular changes in dendritic mitochondria and histochemistry of cytochrome c oxidase (CO) activity were examined in pre-BötC neurons localized by immunoreactivity of neurokinin 1 receptors. Assays of mitochondrial electron transport chain (ETC) complex I, IV, V activities, and membrane potential were performed in the ventrolateral medulla containing the pre-BötC region. We found significant decreases in the mean length and area of dendritic mitochondria in the pre-BötC of CIH rats, when compared to the normoxic control and hypoxic group with daily acute intermittent hypoxia (dAIH) that evokes robust synaptic plasticity. Notably, these morphological alterations were mainly observed in the mitochondria in close proximity to the synapses. In addition, the proportion of mitochondria presented with enlarged compartments and filamentous cytoskeletal elements in the CIH group was less than the control and dAIH groups. Intriguingly, these distinct characteristics of structural adaptability were observed in the mitochondria within spatially restricted dendritic spines. Furthermore, the proportion of moderately to darkly CO-reactive mitochondria was reduced in the CIH group, indicating reduced mitochondrial activity. Consistently, mitochondrial ETC enzyme activities and membrane potential were lowered in the CIH group. These findings suggest that hypoxia-induced respiratory plasticity was characterized by spatially confined mitochondrial alterations within postsynaptic spines in the pre-BötC neurons. In contrast to the robust plasticity evoked by dAIH preconditioning, a severe CIH challenge may weaken the local mitochondrial bioenergetics that the fuel postsynaptic activities of the respiratory motor drive.

Inter and Intracellular mitochondrial trafficking in health and disease

Neurons and glia maintain central nervous system (CNS) homeostasis through diverse mechanisms of intra- and intercellular signaling. Some of these interactions include the exchange of soluble factors between cells via direct cell-to-cell contact for both short and long-distance transfer of biological materials. Transcellular transfer of mitochondria has emerged as a key example of this communication. This transcellular transfer of mitochondria are dynamically involved in the cellular and tissue response to CNS injury and play beneficial roles in recovery. This review highlights recent research addressing the cause and effect of intra- and intercellular mitochondrial transfer with a specific focus on the future of mitochondrial transplantation therapy. We believe that mitochondrial transfer plays a crucial role during bioenergetic crisis/deficit, but the quality, quantity and mode of mitochondrial transfer determines the protective capacity for the receiving cells. Mitochondrial transplantation is a new treatment paradigm and will overcome the major bottleneck of traditional approach of correcting mitochondria-related disorders.

The Molecular Determinants of Mitochondrial Membrane Contact With ER, Lysosomes and Peroxisomes in Neuronal Physiology and Pathology

Membrane tethering is an important communication method for membrane-packaged organelles. Mitochondria are organelles with a bilayer membrane, and the membrane contact between mitochondria and other organelles is indispensable for maintaining cellular homeostasis. Increased levels of molecular determinants that mediate the membrane contact between mitochondria and other organelles, and their functions, have been revealed in recent years. In this review article, we aim to summarize the findings on the tethering between mitochondria and other organelles in physiological or pathological conditions, and discuss their roles in cellular homeostasis, neural activity, and neurodegenerative diseases.

Mesenchymal Stem/Stromal Cell-Mediated Mitochondrial Transfer and the Therapeutic Potential in Treatment of Neurological Diseases

Mesenchymal stem/stromal cells (MSCs) are multipotent stem cells that can be derived from various tissues. Due to their regenerative and immunomodulatory properties, MSCs have been extensively researched and tested for treatment of different diseases/indications. One mechanism that MSCs exert functions is through the transfer of mitochondria, a key player involved in many biological processes in health and disease. Mitochondria transfer is bidirectional and has an impact on both donor and recipient cells. In this review, we discussed how MSC-mediated mitochondrial transfer may affect cellular metabolism, survival, proliferation, and differentiation; how this process influences inflammatory processes; and what is the molecular machinery that mediates mitochondrial transfer. In the end, we summarized recent advances in preclinical research and clinical trials for the treatment of stroke and spinal cord injury, through application of MSCs and/or MSC-derived mitochondria.

Pleiotropic Mitochondria: The Influence of Mitochondria on Neuronal Development and Disease

Mitochondria play many important biological roles, including ATP production, lipid biogenesis, ROS regulation, and calcium clearance. In neurons, the mitochondrion is an essential organelle for metabolism and calcium homeostasis. Moreover, mitochondria are extremely dynamic and able to divide, fuse, and move along microtubule tracks to ensure their distribution to the neuronal periphery. Mitochondrial dysfunction and altered mitochondrial dynamics are observed in a wide range of conditions, from impaired neuronal development to various neurodegenerative diseases. Novel imaging techniques and genetic tools provide unprecedented access to the physiological roles of mitochondria by visualizing mitochondrial trafficking, morphological dynamics, ATP generation, and ultrastructure. Recent studies using these new techniques have unveiled the influence of mitochondria on axon branching, synaptic function, calcium regulation with the ER, glial cell function, neurogenesis, and neuronal repair. This review provides an overview of the crucial roles played by mitochondria in the CNS in physiological and pathophysiological conditions.

Mechanisms and roles of mitochondrial localisation and dynamics in neuronal function

Neurons are highly polarised, complex and incredibly energy intensive cells, and their demand for ATP during neuronal transmission is primarily met by oxidative phosphorylation by mitochondria. Thus, maintaining the health and efficient function of mitochondria is vital for neuronal integrity, viability and synaptic activity. Mitochondria do not exist in isolation, but constantly undergo cycles of fusion and fission, and are actively transported around the neuron to sites of high energy demand. Intriguingly, axonal and dendritic mitochondria exhibit different morphologies. In axons mitochondria are small and sparse whereas in dendrites they are larger and more densely packed. The transport mechanisms and mitochondrial dynamics that underlie these differences, and their functional implications, have been the focus of concerted investigation. Moreover, it is now clear that deficiencies in mitochondrial dynamics can be a primary factor in many neurodegenerative diseases. Here, we review the role that mitochondrial dynamics play in neuronal function, how these processes support synaptic transmission and how mitochondrial dysfunction is implicated in neurodegenerative disease.

Sustained Ultrastructural Changes in Rat Hippocampal Formation After Repeated Electroconvulsive Seizures

BACKGROUND

Electroconvulsive therapy (ECT) is a highly effective and fast-acting treatment for depression used in the clinic. Its mechanism of therapeutic action remains uncertain. Previous studies have focused on documenting neuroplasticity in the early phase following electroconvulsive seizures (ECS), an animal model of ECT. Here, we investigate whether changes in synaptic plasticity and nonneuronal plasticity (vascular and mitochondria) are sustained 3 months after repeated ECS trials.

METHODS

ECS or sham treatment was given daily for 1 day or 10 days to a genetic animal model of depression: the Flinders Sensitive and Resistant Line rats. Stereological principles were employed to quantify numbers of synapses and mitochondria as well as length of microvessels in the hippocampus 24 hours after a single ECS. Three months after 10 ECS treatments (1 per day for 10 days) and sham-treatment, brain-derived neurotrophic factor and vascular endothelial growth factor protein levels were quantified with immunohistochemistry.

RESULTS

A single ECS treatment significantly increased the volume of hippocampal CA1-stratum radiatum, the total length of microvessels, mitochondria number, and synapse number. Observed changes were sustained as shown in the multiple ECS treatment group analyzed 3 months after the last of 10 ECS treatments.

CONCLUSION

A single ECS caused rapid effects of synaptic plasticity and nonneuronal plasticity, while repeated ECS induced long-lasting changes in the efficacy of synaptic plasticity and nonneuronal plasticity at least up to 3 months after ECS.

Restoring Cellular Energetics Promotes Axonal Regeneration and Functional Recovery after Spinal Cord Injury

Axonal regeneration in the central nervous system (CNS) is a highly energy-demanding process. Extrinsic insults and intrinsic restrictions lead to an energy crisis in injured axons, raising the question of whether recovering energy deficits facilitates regeneration. Here, we reveal that enhancing axonal mitochondrial transport by deleting syntaphilin (Snph) recovers injury-induced mitochondrial depolarization. Using three CNS injury mouse models, we demonstrate that Snph-/- mice display enhanced corticospinal tract (CST) regeneration passing through a spinal cord lesion, accelerated regrowth of monoaminergic axons across a transection gap, and increased compensatory sprouting of uninjured CST. Notably, regenerated CST axons form functional synapses and promote motor functional recovery. Administration of the bioenergetic compound creatine boosts CST regenerative capacity in Snph-/- mice. Our study provides mechanistic insights into intrinsic regeneration failure in CNS and suggests that enhancing mitochondrial transport and cellular energetics are promising strategies to promote regeneration and functional restoration after CNS injuries.

Development of novel phosphodiesterase 5 inhibitors for the therapy of Alzheimer's disease

Nitric oxide (NO) is a gaseous molecule that plays a multifactorial role in several cellular processes. In the central nervous system, the NO dual nature in neuroprotection and neurotoxicity has been explored to unveil its involvement in Alzheimer's disease (AD). A growing body of research shows that the activation of the NO signaling pathway leading to the phosphorylation of the transcription factor cyclic adenine monophosphate responsive element binding protein (CREB) (so-called NO/cGMP/PKG/CREB signaling pathway) ameliorates altered neuroplasticity and memory deficits in AD animal models. In addition to NO donors, several other pharmacological agents, such as phosphodiesterase 5 (PDE5) inhibitors have been used to activate the pathway and rescue memory disorders. PDE5 inhibitors, including sildenafil, tadalafil and vardenafil, are marketed for the treatment of erectile dysfunction and arterial pulmonary hypertension due to their vasodilatory properties. The ability of PDE5 inhibitors to interfere with the NO/cGMP/PKG/CREB signaling pathway by increasing the levels of cGMP has prompted the hypothesis that PDE5 inhibition might be used as an effective therapeutic strategy for the treatment of AD. To this end, newly designed PDE5 inhibitors belonging to different chemical classes with improved pharmacologic profile (e.g. higher potency, improved selectivity, and blood-brain barrier penetration) have been synthesized and evaluated in several animal models of AD. In addition, recent medicinal chemistry effort has led to the development of agents concurrently acting on the PDE5 enzyme and a second target involved in AD. Both marketed and investigational PDE5 inhibitors have shown to reverse cognitive defects in young and aged wild type mice as well as transgenic mouse models of AD and tauopathy using a variety of behavioral tasks. These studies confirmed the therapeutic potential of PDE5 inhibitors as cognitive enhancers. However, clinical studies assessing cognitive functions using marketed PDE5 inhibitors have not been conclusive. Drug discovery efforts by our group and others are currently directed towards the development of novel PDE5 inhibitors tailored to AD with improved pharmacodynamic and pharmacokinetic properties. In summary, the present perspective reports an overview of the correlation between the NO signaling and AD, as well as an outline of the PDE5 inhibitors used as an alternative approach in altering the NO pathway leading to an improvement of learning and memory. The last two sections describe the preclinical and clinical evaluation of PDE5 inhibitors for the treatment of AD, providing a comprehensive analysis of the current status of the AD drug discovery efforts involving PDE5 as a new therapeutic target.