Mitostasis in Neurons: Maintaining Mitochondria in an Extended Cellular Architecture

Ultrastructural heterogeneity of layer 4 excitatory synaptic boutons in the adult human temporal lobe neocortex

Synapses are fundamental building blocks controlling and modulating the ‘behavior’ of brain networks. How their structural composition, most notably their quantitative morphology underlie their computational properties remains rather unclear, particularly in humans. Here, excitatory synaptic boutons (SBs) in layer 4 (L4) of the temporal lobe neocortex (TLN) were quantitatively investigated. Biopsies from epilepsy surgery were used for fine-scale and tomographic electron microscopy (EM) to generate 3D-reconstructions of SBs. Particularly, the size of active zones (AZs) and that of the three functionally defined pools of synaptic vesicles (SVs) were quantified. SBs were comparatively small (~2.50 μm²), with a single AZ (~0.13 µm²); preferentially established on spines. SBs had a total pool of ~1800 SVs with strikingly large readily releasable (~20), recycling (~80) and resting pools (~850). Thus, human L4 SBs may act as ‘amplifiers’ of signals from the sensory periphery, integrate, synchronize and modulate intra- and extracortical synaptic activity.

DLK Activation Synergizes with Mitochondrial Dysfunction to Downregulate Axon Survival Factors and Promote SARM1-Dependent Axon Degeneration

Axon degeneration is a prominent component of many neurological disorders. Identifying cellular pathways that contribute to axon vulnerability may identify new therapeutic strategies for maintenance of neural circuits. Dual leucine zipper kinase (DLK) is an axonal stress response MAP3K that is chronically activated in several neurodegenerative diseases. Activated DLK transmits an axon injury signal to the neuronal cell body to provoke transcriptional adaptations. However, the consequence of enhanced DLK signaling to axon vulnerability is unknown. We find that stimulating DLK activity predisposes axons to SARM1-dependent degeneration. Activating DLK reduces levels of the axon survival factors NMNAT2 and SCG10, accelerating their loss from severed axons. Moreover, mitochondrial dysfunction independently decreases the levels of NMNAT2 and SCG10 in axons, and in conjunction with DLK activation, leads to a dramatic loss of axonal NMNAT2 and SCG10 and evokes spontaneous axon degeneration. Hence, enhanced DLK activity reduces axon survival factor abundance and renders axons more susceptible to trauma and metabolic insult.

SQSTM1/p62 promotes mitochondrial ubiquitination independently of PINK1 and PRKN/parkin in mitophagy

The ubiquitination of mitochondrial proteins labels damaged mitochondria for degradation through mitophagy. We recently developed an in vivo system in which mitophagy is slowed by inhibiting mitochondrial division through knockout of Dnm1l/Drp1, a dynamin related GTPase that mediates mitochondrial division. Using this system, we revealed that the ubiquitination of mitochondrial proteins required SQSTM1/p62, but not the ubiquitin E3 ligase PRKN/parkin, during mitophagy. Here, we tested the role of PINK1, a mitochondrial protein kinase that activates mitophagy by phosphorylating ubiquitin, in mitochondrial ubiquitination by knocking out Pink1 in dnm1l-knockout liver. We found mitochondrial ubiquitination did not decrease in the absence of PINK1; instead, PINK1 was required for the degradation of MFN1 (mitofusin 1) and MFN2, two homologous outer membrane proteins that mediate mitochondrial fusion in dnm1l-knockout hepatocytes. These data suggest that mitochondrial ubiquitination is promoted by SQSTM1 independently of PINK1 and PRKN during mitophagy. PINK1 and PRKN appear to control the balance between mitochondrial division and fusion in vivo. Abbreviations: DNM1L/DRP1: dynamin 1-like; KEAP1: kelch-like ECH-associated protein 1; KO: knockout; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MFN1/2: mitofusin 1/2; OPA1: OPA1, mitochondrial dynamin like GTPase; PDH: pyruvate dehydrogenase E1; PINK1: PTEN induced putative kinase 1; PRKN/parkin: parkin RBR E3 ubiquitin protein ligase.

Glial mitochondrial function and dysfunction in health and neurodegeneration

Mitochondria play essential metabolic roles in neural cells. Mitochondrial dysfunction has profound effects on the brain. In primary mitochondrial diseases, mutations that impair specific oxidative phosphorylation (OXPHOS) proteins or OXPHOS assembly factors lead to isolated biochemical defects and a heterogeneous group of clinical phenotypes, including mitochondrial encephalopathies. A broader defect of OXPHOS function, due to mutations in proteins involved in mitochondrial DNA maintenance, mitochondrial biogenesis, or mitochondrial tRNAs can also underlie severe mitochondrial encephalopathies. While primary mitochondrial dysfunction causes rare genetic forms of neurological disorders, secondary mitochondrial dysfunction is involved in the pathophysiology of some of the most common neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. Many studies have investigated mitochondrial function and dysfunction in bulk central nervous system (CNS) tissue. However, the interpretation of these studies has been often complicated by the extreme cellular heterogeneity of the CNS, which includes many different types of neurons and glial cells. Because neurons are especially dependent on OXPHOS for ATP generation, mitochondrial dysfunction is thought to be directly involved in cell autonomous neuronal demise. Despite being metabolically more flexible than neurons, glial mitochondria also play an essential role in the function of the CNS, and have adapted specific metabolic and mitochondrial features to support their diversity of functions. This review analyzes our current understanding and the gaps in knowledge of mitochondrial properties of glia and how they affect neuronal functions, in health and disease.

Relation between activity-induced intracellular sodium transients and ATP dynamics in mouse hippocampal neurons

KEY POINTS

Employing quantitative Na⁺ -imaging and Förster resonance energy transfer-based imaging with ATeam1.03^YEMK (ATeam), we studied the relation between activity-induced Na⁺ influx and intracellular ATP in CA1 pyramidal neurons of the mouse hippocampus. Calibration of ATeam in situ enabled a quantitative estimate of changes in intracellular ATP concentrations. Different paradigms of stimulation that induced global Na⁺ influx into the entire neuron resulted in decreases in [ATP] in the range of 0.1-0.6 mm in somata and dendrites, while Na⁺ influx that was locally restricted to parts of dendrites did not evoke a detectable change in dendritic [ATP]. Our data suggest that global Na⁺ transients require global cellular activation of the Na⁺ /K⁺ -ATPase resulting in a consumption of ATP that transiently overrides its production. For recovery from locally restricted Na⁺ influx, ATP production as well as fast intracellular diffusion of ATP and Na⁺ might prevent a local drop in [ATP].

ABSTRACT

Excitatory neuronal activity results in the influx of Na⁺ through voltage- and ligand-gated channels. Recovery from accompanying increases in intracellular Na⁺ concentrations ([Na⁺ ]_i ) is mainly mediated by the Na⁺ /K⁺ -ATPase (NKA) and is one of the major energy-consuming processes in the brain. Here, we analysed the relation between different patterns of activity-induced [Na⁺ ]_i signalling and ATP in mouse hippocampal CA1 pyramidal neurons by Na⁺ imaging with sodium-binding benzofurane isophthalate (SBFI) and employing the genetically encoded nanosensor ATeam1.03^YEMK (ATeam). In situ calibrations demonstrated a sigmoidal dependence of the ATeam Förster resonance energy transfer ratio on the intracellular ATP concentration ([ATP]_i ) with an apparent K_D of 2.6 mm, indicating its suitability for [ATP]_i measurement. Induction of recurrent network activity resulted in global [Na⁺ ]_i oscillations with amplitudes of ∼10 mm, encompassing somata and dendrites. These were accompanied by a steady decline in [ATP]_i by 0.3-0.4 mm in both compartments. Global [Na⁺ ]_i transients, induced by afferent fibre stimulation or bath application of glutamate, caused delayed, transient decreases in [ATP]_i as well. Brief focal glutamate application that evoked transient local Na⁺ influx into a dendrite, however, did not result in a measurable reduction in [ATP]_i . Our results suggest that ATP consumption by the NKA following global [Na⁺ ]_i transients temporarily overrides its availability, causing a decrease in [ATP]_i . Locally restricted Na⁺ transients, however, do not result in detectable changes in local [ATP]_i , suggesting that ATP production, together with rapid intracellular diffusion of both ATP and Na⁺ from and to unstimulated neighbouring regions, counteracts a local energy shortage under these conditions.

The Overcrowded Crossroads: Mitochondria, Alpha-Synuclein, and the Endo-Lysosomal System Interaction in Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder worldwide, mainly affecting the elderly. The disease progresses gradually, with core motor presentations and a multitude of non-motor manifestations. There are two neuropathological hallmarks of PD, the dopaminergic neuronal loss and the alpha-synuclein-containing Lewy body inclusions in the substantia nigra. While the exact pathomechanisms of PD remain unclear, genetic investigations have revealed evidence of the involvement of mitochondrial function, alpha-synuclein (α-syn) aggregation, and the endo-lysosomal system, in disease pathogenesis. Due to the high energy demand of dopaminergic neurons, mitochondria are of special importance acting as the cellular powerhouse. Mitochondrial dynamic fusion and fission, and autophagy quality control keep the mitochondrial network in a healthy state. Should defects of the organelle occur, a variety of reactions would ensue at the cellular level, including disrupted mitochondrial respiratory network and perturbed calcium homeostasis, possibly resulting in cellular death. Meanwhile, α-syn is a presynaptic protein that helps regulate synaptic vesicle transportation and endocytosis. Its misfolding into oligomeric sheets and fibrillation is toxic to the mitochondria and neurons. Increased cellular oxidative stress leads to α-syn accumulation, causing mitochondrial dysfunction. The proteasome and endo-lysosomal systems function to regulate damage and unwanted waste management within the cell while facilitating the quality control of mitochondria and α-syn. This review will analyze the biological functions and interactions between mitochondria, α-syn, and the endo-lysosomal system in the pathogenesis of PD.

Microscopic Viscosity of Neuronal Plasma Membranes Measured Using Fluorescent Molecular Rotors: Effects of Oxidative Stress and Neuroprotection

Molecular mobility in neuronal plasma membranes is a crucial factor in brain function. Microscopic viscosity is an important parameter that determines molecular mobility. This study presents the first direct measurement of the microviscosity of plasma membranes of live neurons. Microviscosity maps were obtained using fluorescence lifetime imaging of environment-sensing dyes termed "molecular rotors". Neurons were investigated both in the basal state and following common neurodegenerative stimuli, excitotoxicity, or oxidative stress. Both types of neurotoxic challenges induced microviscosity decrease in cultured neurons, and oxidant-induced membrane fluidification was counteracted by the wide-spectrum neuroprotectant, the H3 peptide. These results provide new insights into molecular mobility in neuronal membranes, paramount for basic brain function, and suggest that preservation of membrane stability may be an important aspect of neuroprotection in brain insults and neurodegenerative disorders.

Neuronal vulnerability in Parkinson disease: Should the focus be on axons and synaptic terminals?

While current effective therapies are available for the symptomatic control of PD, treatments to halt the progressive neurodegeneration still do not exist. Loss of dopamine neurons in the SNc and dopamine terminals in the striatum drive the motor features of PD. Multiple lines of research point to several pathways which may contribute to dopaminergic neurodegeneration. These pathways include extensive axonal arborization, mitochondrial dysfunction, dopamine's biochemical properties, abnormal protein accumulation of α-synuclein, defective autophagy and lysosomal degradation, and synaptic impairment. Thus, understanding the essential features and mechanisms of dopaminergic neuronal vulnerability is a major scientific challenge and highlights an outstanding need for fostering effective therapies against neurodegeneration in PD. This article, which arose from the Movement Disorders 2018 Conference, discusses and reviews the possible mechanisms underlying neuronal vulnerability and potential therapeutic approaches in PD. © 2019 International Parkinson and Movement Disorder Society.

The Good and the Bad of Mitochondrial Breakups

Mitochondrial morphology is a crucial determinant of mitochondrial and cellular function. Opposing fusion and fission events shape the tubular mitochondrial reticulum and ensure mitochondrial transport within cells. Cellular stress and pathophysiological conditions can lead to fragmentation of the mitochondrial network, which facilitates mitophagy and is associated with cell death. However, mitochondrial shape changes are also intertwined with the cellular metabolism, and metabolic switches can induce but also result from alterations in mitochondrial morphology. Here, we discuss recent advances in the field of mitochondrial dynamics, demonstrating cell- and tissue-specific effects of mitochondrial fragmentation on cellular metabolism, cell survival, and mitochondrial quality control.

Mitochondrial movement in Aralar/Slc25a12/AGC1 deficient cortical neurons

The elevated energy demands in the brain are fulfilled mainly by glucose catabolism. In highly polarized neurons, about 10-50% of mitochondria are transported along microtubules using mitochondrial-born ATP to locations with high energy requirements. In this report, we have investigated the impact of Aralar deficiency on mitochondrial transport in cultured cortical neurons. Aralar/slc25a12/AGC1 is the neuronal isoform of the aspartate-glutamate mitochondrial carrier, a component of the malate-aspartate shuttle (MAS) which plays an important role in redox balance, which is essential to maintain glycolytic pyruvate supply to neuronal mitochondria. Using live imaging microscopy we observed that the lack of Aralar does not affect the number of moving mitochondria nor the Ca²⁺-induced stop, the only difference being a 10% increase in mitochondrial velocity in Aralar deficient neurons. Therefore, we evaluated the possible fuels used in each case by studying the relative contribution of oxidative phosphorylation and glycolysis to mitochondrial movement using specific inhibitors. We found that the ATP synthase inhibitor oligomycin caused a smaller inhibition of mitochondrial movement in Aralar-KO than control neurons, whereas the glycolysis inhibitor iodoacetate had similar effects in neurons from both genotypes. In line with these findings, the decrease in cytosolic ATP/ADP ratio caused by oligomycin was more pronounced in control than in Aralar-KO neurons, but no differences were observed with iodoacetate. Oligomycin effect was reverted by aralar re-expression in knock out cultures. As mitochondrial movement is not reduced in Aralar-KO neurons, these results suggest that these neurons may use an additional pathway for mitochondria movement and ATP/ADP ratio maintenance.

Temporal Control of Axonal Transport: The Extreme Case of Organismal Ageing

A fundamental question in cell biology is how cellular components are delivered to their destination with spatial and temporal precision within the crowded cytoplasmic environment. The long processes of neurons represent a significant spatial challenge and make these cells particularly dependent on mechanisms for long-range cytoskeletal transport of proteins, RNA and organelles. Although many studies have substantiated a role for defective transport of axonal cargoes in the pathogenesis of neurodevelopmental and neurodegenerative diseases, remarkably little is known about how transport is regulated throughout ageing. The scale of the challenge posed by ageing is considerable because, in this case, the temporal regulation of transport is ultimately dictated by the length of organismal lifespan, which can extend to days, years or decades. Recent methodological advances to study live axonal transport during ageing in situ have provided new tools to scratch beneath the surface of this complex problem and revealed that age-dependent decline in the transport of mitochondria is a common feature across different neuronal populations of several model organisms. In certain instances, the molecular pathways that affect transport in ageing animals have begun to emerge. However, the functional implications of these observations are still not fully understood. Whether transport decline is a significant determinant of neuronal ageing or a mere consequence of decreased cellular fitness remains an open question. In this review, we discuss the latest developments in axonal trafficking in the ageing nervous system, along with the early studies that inaugurated this new area of research. We explore the possibility that the interplay between mitochondrial function and motility represents a crucial driver of ageing in neurons and put forward the hypothesis that declining axonal transport may be legitimately considered a hallmark of neuronal ageing.

Axonal Transport and Mitochondrial Function in Neurons

The complex and elaborate architecture of a neuron poses a great challenge to the cellular machinery which localizes proteins and organelles, such as mitochondria, to necessary locations. Proper mitochondrial localization in neurons is particularly important as this organelle provides energy and metabolites essential to form and maintain functional neural connections. Consequently, maintenance of a healthy pool of mitochondria and removal of damaged organelles are essential for neuronal homeostasis. Long distance transport of the organelle itself as well as components necessary for maintaining mitochondria in distal compartments are important for a constant supply of healthy mitochondria at the right time and place. Accordingly, many neurodegenerative diseases have been associated with mitochondrial abnormalities. Here, we review our current understanding on transport-dependent mechanisms that regulate mitochondrial replenishment. We focus on axonal transport and import of mRNAs and proteins destined for mitochondria as well as mitochondrial fusion and fission to maintain mitochondrial homeostasis in distal compartments of the neuron.

Loss of the mitochondrial i-AAA protease YME1L leads to ocular dysfunction and spinal axonopathy

Disturbances in the morphology and function of mitochondria cause neurological diseases, which can affect the central and peripheral nervous system. The i‐AAA protease YME1L ensures mitochondrial proteostasis and regulates mitochondrial dynamics by processing of the dynamin‐like GTPase OPA1. Mutations in YME1L cause a multi‐systemic mitochondriopathy associated with neurological dysfunction and mitochondrial fragmentation but pathogenic mechanisms remained enigmatic. Here, we report on striking cell‐type‐specific defects in mice lacking YME1L in the nervous system. YME1L‐deficient mice manifest ocular dysfunction with microphthalmia and cataracts and develop deficiencies in locomotor activity due to specific degeneration of spinal cord axons, which relay proprioceptive signals from the hind limbs to the cerebellum. Mitochondrial fragmentation occurs throughout the nervous system and does not correlate with the degenerative phenotype. Deletion of Oma1 restores tubular mitochondria but deteriorates axonal degeneration in the absence of YME1L, demonstrating that impaired mitochondrial proteostasis rather than mitochondrial fragmentation causes the observed neurological defects.

Mechanisms for the maintenance and regulation of axonal energy supply

The unique polarization and high-energy demand of neurons necessitates specialized mechanisms to maintain energy homeostasis throughout the cell, particularly in the distal axon. Mitochondria play a key role in meeting axonal energy demand by generating adenosine triphosphate through oxidative phosphorylation. Recent evidence demonstrates how axonal mitochondrial trafficking and anchoring are coordinated to sense and respond to altered energy requirements. If and when these mechanisms are impacted in pathological conditions, such as injury and neurodegenerative disease, is an emerging research frontier. Recent evidence also suggests that axonal energy demand may be supplemented by local glial cells, including astrocytes and oligodendrocytes. In this review, we provide an updated discussion of how oxidative phosphorylation, aerobic glycolysis, and oligodendrocyte-derived metabolic support contribute to the maintenance of axonal energy homeostasis.

Pathologic Alterations in the Proteome of Synaptosomes from a Mouse Model of Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a human genetic disorder characterized by muscle weakness, muscle atrophy, and death of motor neurons. SMA is caused by mutations or deletions in a gene called survival motor neuron 1 (SMN1). SMN1 is a housekeeping gene, but the most prominent pathologies in SMA are atrophy of myofibers and death of motor neurons. Further, degeneration of neuromuscular junctions, of synapses, and of axonal regions are features of SMA disease. Here, we have investigated the proteome dynamics of central synapses in P14 Smn^2B/- mice, a model of SMA. Label-free quantitative proteomics on isolated synaptosomes from spinal cords of these animals identified 2030 protein groups. Statistical data analysis revealed 65 specific alterations in the proteome of the central synapses at the early onset stage of disease. Functional analysis of the dysregulated proteins indicated a significant enrichment of proteins associated with mitochondrial dynamics, cholesterol biogenesis, and protein clearance. These pathways represent potential targets for therapy development with the goal of providing stability to the central synapses, thereby preserving neuronal integrity in the context of SMA disease. Data are available via ProteomeXchange with identifier PXD012850.

Quality Control in Neurons: Mitophagy and Other Selective Autophagy Mechanisms

The cargo-specific removal of organelles via selective autophagy is important to maintain neuronal homeostasis. Genetic studies indicate that deficits in these pathways are implicated in neurodegenerative diseases, including Parkinson's and amyotrophic lateral sclerosis. Here, we review our current understanding of the pathways that regulate mitochondrial quality control, and compare these mechanisms to those regulating turnover of the endoplasmic reticulum and the clearance of protein aggregates. Research suggests that there are multiple mechanisms regulating the degradation of specific cargos, such as dysfunctional organelles and protein aggregates. These mechanisms are critical for neuronal health, as neurons are uniquely vulnerable to impairment in organelle quality control pathways due to their morphology, size, polarity, and postmitotic nature. We highlight the consequences of dysregulation of selective autophagy in neurons and discuss current challenges in correlating noncongruent findings from in vitro and in vivo systems.

Glutathione S-Transferase Regulates Mitochondrial Populations in Axons through Increased Glutathione Oxidation

Mitochondria are essential in long axons to provide metabolic support and sustain neuron integrity. A healthy mitochondrial pool is maintained by biogenesis, transport, mitophagy, fission, and fusion, but how these events are regulated in axons is not well defined. Here, we show that the Drosophila glutathione S-transferase (GST) Gfzf prevents mitochondrial hyperfusion in axons. Gfzf loss altered redox balance between glutathione (GSH) and oxidized glutathione (GSSG) and initiated mitochondrial fusion through the coordinated action of Mfn and Opa1. Gfzf functioned epistatically with the thioredoxin peroxidase Jafrac1 and the thioredoxin reductase 1 TrxR-1 to regulate mitochondrial dynamics. Altering GSH:GSSG ratios in mouse primary neurons in vitro also induced hyperfusion. Mitochondrial changes caused deficits in trafficking, the metabolome, and neuronal physiology. Changes in GSH and oxidative state are associated with neurodegenerative diseases like Alzheimer's. Our demonstration that GSTs are key in vivo regulators of axonal mitochondrial length and number provides a potential mechanistic link.

Nanoresolution real-time 3D orbital tracking for studying mitochondrial trafficking in vertebrate axons in vivo

We present the development and in vivo application of a feedback-based tracking microscope to follow individual mitochondria in sensory neurons of zebrafish larvae with nanometer precision and millisecond temporal resolution. By combining various technical improvements, we tracked individual mitochondria with unprecedented spatiotemporal resolution over distances of >100 µm. Using these nanoscopic trajectory data, we discriminated five motional states: a fast and a slow directional motion state in both the anterograde and retrograde directions and a stationary state. The transition pattern revealed that, after a pause, mitochondria predominantly persist in the original direction of travel, while transient changes of direction often exhibited longer pauses. Moreover, mitochondria in the vicinity of a second, stationary mitochondria displayed an increased probability to pause. The capability of following and optically manipulating a single organelle with high spatiotemporal resolution in a living organism offers a new approach to elucidating their function in its complete physiological context.

Neuronal Mitophagy: Lessons from a Pathway Linked to Parkinson's Disease

Neurons are specialized cells with complex and extended architecture and high energy requirements. Energy in the form of adenosine triphosphate, produced essentially by mitochondrial respiration, is necessary to preserve neuronal morphology, maintain resting potential, fire action potentials, and ensure neurotransmission. Pools of functional mitochondria are required in all neuronal compartments, including cell body and dendrites, nodes of Ranvier, growth cones, axons, and synapses. The mechanisms by which old or damaged mitochondria are removed and replaced in neurons remain to be fully understood. Mitophagy has gained considerable interest since the discovery of familial forms of Parkinson's disease caused by dysfunction of PINK1 and Parkin, two multifunctional proteins cooperating in the regulation of this process. Over the past 10 years, the molecular mechanisms by which PINK1 and Parkin jointly promote the degradation of defective mitochondria by autophagy have been dissected. However, our understanding of the relevance of mitophagy to mitochondrial homeostasis in neurons remains poor. Insight has been recently gained thanks to the development of fluorescent reporter systems for tracking mitochondria in the acidic compartment of the lysosome. Using these tools, mitophagy events have been visualized in primary neurons in culture and in vivo, under basal conditions and in response to toxic insults. Despite these advances, whether PINK1 and Parkin play a major role in promoting neuronal mitophagy under physiological conditions in adult animals and during aging remains a matter of debate. Future studies will have to clarify in how far dysfunction of neuronal mitophagy is central to the pathophysiology of Parkinson's disease.

Proteomic Analysis and Biochemical Correlates of Mitochondrial Dysfunction after Low-Intensity Primary Blast Exposure

Service members during military actions or combat training are frequently exposed to primary blasts by weaponry. Most studies have investigated moderate or severe brain injuries from blasts generating overpressures >100 kPa, whereas understanding the pathophysiology of low-intensity blast (LIB)-induced mild traumatic brain injury (mTBI) leading to neurological deficits remains elusive. Our recent studies, using an open-field LIB-induced mTBI mouse model with a peak overpressure at 46.6 kPa, demonstrated behavioral impairments and brain nanoscale damages, notably mitochondrial and axonal ultrastructural changes. In this study, we used tandem mass tagged (TMT) quantitative proteomics and bioinformatics analysis to seek insights into the molecular mechanisms underlying ultrastructural pathology. Changes in global- and phospho-proteomes were determined at 3 and 24 h and at 7 and 30 days post injury (DPI), in order to investigate the biochemical and molecular correlates of mitochondrial dysfunction. Results showed striking dynamic changes in a total of 2216 proteins and 459 phosphorylated proteins at vary time points after blast. Disruption of key canonical pathways included evidence of mitochondrial dysfunction, oxidative stress, axonal/cytoskeletal/synaptic dysregulation, and neurodegeneration. Bioinformatic analysis identified blast-induced trends in networks related to cellular growth/development/movement/assembly and cell-to-cell signaling interactions. With observations of proteomic changes, we found LIB-induced oxidative stress associated with mitochondrial dysfunction mainly at 7 and 30 DPI. These dysfunctions included impaired fission-fusion dynamics, diminished mitophagy, decreased oxidative phosphorylation, and compensated respiration-relevant enzyme activities. Insights on the early pathogenesis of primary LIB-induced brain damage provide a template for further characterization of its chronic effects, identification of potential biomarkers, and targets for intervention.

Mitochondrial Dysfunction in Parkinson's Disease-Cause or Consequence?

James Parkinson first described the motor symptoms of the disease that took his name over 200 years ago. While our knowledge of many of the changes that occur in this condition has increased, it is still unknown what causes this neurodegeneration and why it only affects some individuals with advancing age. Here we review current literature to discuss whether the mitochondrial dysfunction we have detected in Parkinson’s disease is a pathogenic cause of neuronal loss or whether it is itself a consequence of dysfunction in other pathways. We examine research data from cases of idiopathic Parkinson’s with that from model systems and individuals with familial forms of the disease. Furthermore, we include data from healthy aged individuals to highlight that many of the changes described are also present with advancing age, though not normally in the presence of severe neurodegeneration. While a definitive answer to this question may still be just out of reach, it is clear that mitochondrial dysfunction sits prominently at the centre of the disease pathway that leads to catastrophic neuronal loss in those affected by this disease.

The light-sensitive dimerizer zapalog reveals distinct modes of immobilization for axonal mitochondria

Controlling cellular processes with light can help elucidate their underlying mechanisms. Here we present zapalog, a small-molecule dimerizer that undergoes photolysis when exposed to blue light. Zapalog dimerizes any two proteins tagged with the FKBP and DHFR domains until exposure to light causes its photolysis. Dimerization can be repeatedly restored with uncleaved zapalog. We implement this method to investigate mitochondrial motility and positioning in cultured neurons. Using zapalog, we tether mitochondria to constitutively active kinesin motors, forcing them down the axon towards microtubule (+) ends until their instantaneous release via blue light, which results in full restoration of their endogenous motility. We find that one-third of stationary mitochondria cannot be pulled away from their position and that these firmly anchored mitochondria preferentially localize to VGLUT1-positive presynapses. Furthermore, inhibition of actin polymerization with latrunculin A reduces this firmly anchored pool. On release from exogenous motors, mitochondria are preferentially recaptured at presynapses.

Phosphoregulation on mitochondria: Integration of cell and organelle responses

Mitochondria are highly integrated organelles that are crucial to cell adaptation and mitigating adverse physiology. Recent studies demonstrate that fundamental signal transduction pathways incorporate mitochondrial substrates into their biological programs. Reversible phosphorylation is emerging as a useful mechanism to modulate mitochondrial function in accordance with cellular changes. Critical serine/threonine protein kinases, such as the c-Jun N-terminal kinase (JNK), protein kinase A (PKA), PTEN-induced kinase-1 (PINK1), and AMP-dependent protein kinase (AMPK), readily translocate to the outer mitochondrial membrane (OMM), the interface of mitochondria-cell communication. OMM protein kinases phosphorylate diverse mitochondrial substrates that have discrete effects on organelle dynamics, protein import, respiratory complex activity, antioxidant capacity, and apoptosis. OMM phosphorylation events can be tempered through the actions of local protein phosphatases, such as mitogen-activated protein kinase phosphatase-1 (MKP-1) and protein phosphatase 2A (PP2A), to regulate the extent and duration of signaling. The central mediators of OMM signal transduction are the scaffold proteins because the relative abundance of these accessory proteins determines the magnitude and duration of a signaling event on the mitochondrial surface, which dictates the biological outcome of a local signal transduction pathway. The concentrations of scaffold proteins, such as A-kinase anchoring proteins (AKAPs) and Sab (or SH3 binding protein 5-SH3BP5), have been shown to influence neuronal survival and vulnerability, respectively, in models of Parkinson's disease (PD), highlighting the importance of OMM signaling to health and disease. Despite recent progress, much remains to be discovered concerning the mechanisms of OMM signaling. Nonetheless, enhancing beneficial OMM signaling events and inhibiting detrimental protein-protein interactions on the mitochondrial surface may represent highly selective approaches to restore mitochondrial health and homeostasis and mitigate organelle dysfunction in conditions such as PD.

Restoring mitofusin balance prevents axonal degeneration in a Charcot-Marie-Tooth type 2A model

Mitofusin-2 (MFN2) is a mitochondrial outer-membrane protein that plays a pivotal role in mitochondrial dynamics in most tissues, yet mutations in MFN2, which cause Charcot-Marie-Tooth disease type 2A (CMT2A), primarily affect the nervous system. We generated a transgenic mouse model of CMT2A that developed severe early onset vision loss and neurological deficits, axonal degeneration without cell body loss, and cytoplasmic and axonal accumulations of fragmented mitochondria. While mitochondrial aggregates were labeled for mitophagy, mutant MFN2 did not inhibit Parkin-mediated degradation, but instead had a dominant negative effect on mitochondrial fusion only when MFN1 was at low levels, as occurs in neurons. Finally, using a transgenic approach, we found that augmenting the level of MFN1 in the nervous system in vivo rescued all phenotypes in mutant MFN2R94Q-expressing mice. These data demonstrate that the MFN1/MFN2 ratio is a key determinant of tissue specificity in CMT2A and indicate that augmentation of MFN1 in the nervous system is a viable therapeutic strategy for the disease.

Powerhouse of the mind: mitochondrial plasticity at the synapse

Neurons are highly polarized cells with extraordinary energy demands, which are mainly fulfilled by mitochondria. In response to altered neuronal energy state, mitochondria adapt to enable energy homeostasis and nervous system function. This adaptation, also called mitochondrial plasticity, can be observed as alterations in the form, function and position. The primary site of energy consumption in neurons is localized at the synapse, where mitochondria are critical for both pre- and postsynaptic functions. In this review, we will discuss molecular mechanisms regulating mitochondrial plasticity at the synapse and how they contribute to information processing within neurons.

Optical excitation and detection of neuronal activity

Optogenetics has emerged as an exciting tool for manipulating neural activity, which in turn, can modulate behavior in live organisms. However, detecting the response to the optical stimulation requires electrophysiology with physical contact or fluorescent imaging at target locations, which is often limited by photobleaching and phototoxicity. In this paper, we show that phase imaging can report the intracellular transport induced by optogenetic stimulation. We developed a multimodal instrument that can both stimulate cells with subcellular spatial resolution and detect optical pathlength (OPL) changes with nanometer scale sensitivity. We found that OPL fluctuations following stimulation are consistent with active organelle transport. Furthermore, the results indicate a broadening in the transport velocity distribution, which is significantly higher in stimulated cells compared to optogenetically inactive cells. It is likely that this label-free, contactless measurement of optogenetic response will provide an enabling approach to neuroscience.

Surveillance and transportation of mitochondria in neurons

Neurons need to allocate and sustain mitochondria to provide adequate energy and sufficient Ca²⁺-buffering capacity in each sub specialization of their extensive arborizations. Damaged mitochondria, which are highly deleterious to the neuron, must be rapidly repaired or eliminated, even when they are left at terminals extremely far away from the soma. The unique shape of neurons complicates the tasks of both transporting and clearing mitochondria. Errors in the underlying molecular regulations cause detrimental neurodegeneration. Here, we review the molecular machinery and regulatory mechanisms employed to transport, distribute, and clear mitochondria in neurons, and how these fundamental regulations are compromised in neurological disorders.

Mitochondrial Populations Exhibit Differential Dynamic Responses to Increased Energy Demand during Exocytosis In Vivo

Mitochondria are dynamic organelles undergoing fission, fusion, and translocation. These processes have been studied in cultured cells; however, little is known about their regulation in cells within tissues in vivo. We applied four-dimensional intravital microscopy to address this in secretory cells of the salivary gland. We found that mitochondria are organized in two populations: one juxtaposed to the basolateral plasma membrane and the other dispersed in the cytosol. Under basal conditions, central mitochondria exhibit microtubule-dependent motility and low fusion rate, whereas basolateral mitochondria are static and display high fusion rate. Increasing cellular energy demand by β-adrenergic stimulation of regulated exocytosis selectively enhanced motility and fusion of central mitochondria. Inhibition of microtubule polymerization led to inhibition of central mitochondrial motility and fusion and a marked reduction in exocytosis. This study reveals a conserved heterogeneity in mitochondrial positioning and dynamics in exocrine tissues that may have fundamental implications in organ pathophysiology.

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In the salivary glands, mitochondria exist in two populations: basolateral and central

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Basolateral mitochondria are static and frequently fuse

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Central mitochondria are highly motile and rarely fuse

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Exocytosis elicits selective, microtubule-dependent response in central mitochondria

Targeting the proteostasis network in Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a polyglutamine expansion mutation in the huntingtin protein. Expansions above 40 polyglutamine repeats are invariably fatal, following a symptomatic period characterised by choreiform movements, behavioural abnormalities, and cognitive decline. While mutant huntingtin (mHtt) is widely expressed from early life, most patients with HD present in mid-adulthood, highlighting the role of ageing in disease pathogenesis. mHtt undergoes proteolytic cleavage, misfolding, accumulation, and aggregation into inclusion bodies. The emerging model of HD pathogenesis proposes that the chronic production of misfolded mHtt overwhelms the chaperone machinery, diverting other misfolded clients to the proteasome and the autophagy pathways, ultimately leading to a global collapse of the proteostasis network. Multiple converging hypotheses also implicate ageing and its impact in the dysfunction of organelles as additional contributing factors to the collapse of proteostasis in HD. In particular, mitochondrial function is required to sustain the activity of ATP-dependent chaperones and proteolytic machinery. Recent studies elucidating mitochondria-endoplasmic reticulum interactions and uncovering a dedicated proteostasis machinery in mitochondria, suggest that mitochondria play a more active role in the maintenance of cellular proteostasis than previously thought. The enhancement of cytosolic proteostasis pathways shows promise for HD treatment, protecting cells from the detrimental effects of mHtt accumulation. In this review, we consider how mHtt and its post translational modifications interfere with protein quality control pathways, and how the pharmacological and genetic modulation of components of the proteostasis network impact disease phenotypes in cellular and in vivo HD models.

Mitochondrial Dynamics in Physiology and Pathology of Myelinated Axons

Mitochondria play essential roles in neurons and abnormal functions of mitochondria have been implicated in neurological disorders including myelin diseases. Since mitochondrial functions are regulated and maintained by their dynamic behavior involving localization, transport, and fusion/fission, modulation of mitochondrial dynamics would be involved in physiology and pathology of myelinated axons. In fact, the integration of multimodal imaging in vivo and in vitro revealed that mitochondrial localization and transport are differentially regulated in nodal and internodal regions in response to the changes of metabolic demand in myelinated axons. In addition, the mitochondrial behavior in axons is modulated as adaptive responses to demyelination irrespective of the cause of myelin loss, and the behavioral modulation is partly through interactions with cytoskeletons and closely associated with the pathophysiology of demyelinating diseases. Furthermore, the behavior and functions of axonal mitochondria are modulated in congenital myelin disorders involving impaired interactions between axons and myelin-forming cells, and, together with the inflammatory environment, implicated in axonal degeneration and disease phenotypes. Further studies on the regulatory mechanisms of the mitochondrial dynamics in myelinated axons would provide deeper insights into axo-glial interactions mediated through myelin ensheathment, and effective manipulations of the dynamics may lead to novel therapeutic strategies protecting axonal and neuronal functions and survival in primary diseases of myelin.

Chain length of saturated fatty acids regulates mitochondrial trafficking and function in sensory neurons

Dyslipidemia associated with T2D leads to diabetic neuropathy, a complication characterized by sensory neuronal dysfunction and peripheral nerve damage. Sensory dorsal root ganglion (DRG) neurons are dependent on axonal mitochondrial energy production facilitated by mitochondrial transport mechanisms that distribute mitochondria throughout the axon. Because long-chain saturated FAs (SFAs) damage DRG neurons and medium-chain SFAs are reported to improve neuronal function, we evaluated the impact of SFA chain length on mitochondrial trafficking, mitochondrial function, and apoptosis. DRG neurons were exposed to SFAs with C12:0-C18:0 chain lengths and evaluated for changes in mitochondrial trafficking, mitochondrial polarization, and apoptosis. DRG neurons treated with C16:0 and C18:0 SFAs showed a significant decrease in the percentage of motile mitochondria and velocity of mitochondrial trafficking, whereas C12:0 and C14:0 SFAs had no impact on motility. Treatment with C16:0 and C18:0 SFAs exhibited mitochondrial depolarization correlating with impaired mitochondrial motility; the C12:0- and C14:0-treated neurons retained mitochondrial polarization. The reduction in mitochondrial trafficking and function in C16:0- and C18:0-treated DRG neurons correlated with apoptosis that was blocked in C12:0 and C14:0 SFA treatments. These results suggest that SFA chain length plays an important role in regulating axonal mitochondrial trafficking and function in DRG neurons.

Mechanisms Orchestrating Mitochondrial Dynamics for Energy Homeostasis

To maintain homeostasis, every cell must constantly monitor its energy level and appropriately adjust energy, in the form of ATP, production rates based on metabolic demand. Continuous fulfillment of this energy demand depends on the ability of cells to sense, metabolize, and convert nutrients into chemical energy. Mitochondria are the main energy conversion sites for many cell types. Cellular metabolic states dictate the mitochondrial size, shape, function, and positioning. Mitochondrial shape varies from singular discrete organelles to interconnected reticular networks within cells. The morphological adaptations of mitochondria to metabolic cues are facilitated by the dynamic events categorized as transport, fusion, fission, and quality control. By changing their dynamics and strategic positioning within the cytoplasm, mitochondria carry out critical functions and also participate actively in inter-organelle cross-talk, assisting metabolite transfer, degradation, and biogenesis. Mitochondrial dynamics has become an active area of research because of its particular importance in cancer, metabolic diseases, and neurological disorders. In this review, we will highlight the molecular pathways involved in the regulation of mitochondrial dynamics and their roles in maintaining energy homeostasis.

High Content, Phenotypic Assays and Screens for Compounds Modulating Cellular Processes in Primary Neurons

High content, phenotypic screens offer a powerful approach to systems biology at the cellular level. The approach employs cells carrying fluorescently labeled molecules or organelles in 384- or 1536-well microplates, and an automated confocal screening microscope for capturing images from each well. Although some specifics vary according to the assay type, each will apply some degree of image processing and feature extraction followed by a data analysis pipeline to identify the perturbations (small molecules, etc.) of interest. We describe and discuss the advantages and limitations of high content assays and screens using the specific example of assaying mitochondrial dynamics in primary neurons. We provide a detailed description of our culturing methods, imaging and data analysis techniques and provide an open source, ready to use CellProfiler pipeline for high-throughput image segmentation and quantification tool for mitochondrial parameters.

Long-Term Potentiation Requires a Rapid Burst of Dendritic Mitochondrial Fission during Induction

Synaptic transmission is bioenergetically demanding, and the diverse processes underlying synaptic plasticity elevate these demands. Therefore, mitochondrial functions, including ATP synthesis and Ca²⁺ handling, are likely essential for plasticity. Although axonal mitochondria have been extensively analyzed, LTP is predominantly induced postsynaptically, where mitochondria are understudied. Additionally, though mitochondrial fission is essential for their function, signaling pathways that regulate fission in neurons remain poorly understood. We found that NMDAR-dependent LTP induction prompted a rapid burst of dendritic mitochondrial fission and elevations of mitochondrial matrix Ca²⁺. The fission burst was triggered by cytosolic Ca²⁺ elevation and required CaMKII, actin, and Drp1, as well as dynamin 2. Preventing fission impaired mitochondrial matrix Ca²⁺ elevations, structural LTP in cultured neurons, and electrophysiological LTP in hippocampal slices. These data illustrate a novel pathway whereby synaptic activity controls mitochondrial fission and show that dynamic control of fission regulates plasticity induction, perhaps by modulating mitochondrial Ca²⁺ handling.

Maintenance mechanisms of circuit-integrated axons

Adult, circuit-integrated neurons must be maintained and supported for the life span of their host. The attenuation of either maintenance or plasticity leads to impaired circuit function and ultimately to neurodegenerative disorders. Over the last few years, significant discoveries of molecular mechanisms were made that mediate the formation and maintenance of axons. Here, we highlight intrinsic and extrinsic mechanisms that ensure the health and survival of axons. We also briefly discuss examples of mutations associated with impaired axonal maintenance identified in specific neurological conditions. A better understanding of these mechanisms will therefore help to define targets for therapeutic interventions.

Morphology of mitochondria in spatially restricted axons revealed by cryo-electron tomography

Neurons project axons to local and distal sites and can display heterogeneous morphologies with limited physical dimensions that may influence the structure of large organelles such as mitochondria. Using cryo-electron tomography (cryo-ET), we characterized native environments within axons and presynaptic varicosities to examine whether spatial restrictions within these compartments influence the morphology of mitochondria. Segmented tomographic reconstructions revealed distinctive morphological characteristics of mitochondria residing at the narrowed boundary between presynaptic varicosities and axons with limited physical dimensions (approximately 80 nm), compared to mitochondria in nonspatially restricted environments. Furthermore, segmentation of the tomograms revealed discrete organizations between the inner and outer membranes, suggesting possible independent remodeling of each membrane in mitochondria at spatially restricted axonal/varicosity boundaries. Thus, cryo-ET of mitochondria within axonal subcompartments reveals that spatial restrictions do not obstruct mitochondria from residing within them, but limited available space can influence their gross morphology and the organization of the inner and outer membranes. These findings offer new perspectives on the influence of physical and spatial characteristics of cellular environments on mitochondrial morphology and highlight the potential for remarkable structural plasticity of mitochondria to adapt to spatial restrictions within axons.

Neurons are complex cells that communicate with each other via axons that can extend over distances of a meter or longer. Axons place enormous demands on neuronal energy production, and to maintain connections with local and distal targets, neurons have efficient systems that transport mitochondria to areas of high energy consumption. However, axons show variable dimensions, sometimes thinning to a diameter significantly smaller than the standard diameter of mitochondria, raising the question of how mitochondrial structures can adapt to the local spatial environment. In the present study, we employed electron tomography to investigate the physical and structural relationships between thin axons and the mitochondria that reside within them. We discovered that mitochondria exhibit a remarkable constriction of outer and inner membrane structure in regions of restricted physical dimensions of the axonal space. These findings highlight the remarkable structural plasticity of mitochondria and the potential influence of available space within cells on the structure of mitochondria. Given that maintaining a population of properly localized mitochondria is necessary to support synaptic function, the findings also suggest an adaptive role for mitochondrial structure in facilitating efficient axonal transport.

Cellular Sources and Regional Variations in the Expression of the Neuroinflammatory Marker Translocator Protein (TSPO) in the Normal Brain

The inducible expression of the mitochondrial translocator protein 18 kDa (TSPO) by activated microglia is a prominent, regular feature of acute and chronic-progressive brain pathology. This expression is also the rationale for the continual development of new TSPO binding molecules for the diagnosis of "neuroinflammation" by molecular imaging. However, there is in the normal brain an ill-defined, low-level constitutive expression of TSPO. Taking advantage of healthy TSPO knockout mouse brain tissue to validate TSPO antibody specificity, this study uses immunohistochemistry to determine the regional distribution and cellular sources of TSPO in the normal mouse brain. Fluorescence microscopy revealed punctate TSPO immunostaining in vascular endothelial cells throughout the brain. In the olfactory nerve layers and glomeruli of the olfactory bulb, choroid plexus and ependymal layers, we confirm constitutive TSPO expression levels similar to peripheral organs, while some low TSPO expression is present in regions of known neurogenesis, as well as cerebellar Purkinje cells. The distributed-sparse expression of TSPO in endothelial mitochondria throughout the normal brain can be expected to give rise to a low baseline signal in TSPO molecular imaging studies. Finally, our study emphasises the need for valid and methodologically robust verification of the selectivity of TSPO ligands through the use of TSPO knockout tissues.

Image-Based Analysis of Mitochondrial Area and Counting from Adult Mouse Dopaminergic Neurites

Mitochondria form dynamic cytoplasmic networks which undergo morphological changes in order to adapt to cellular stresses and signals. These changes can include alterations in size and number within a given cell. Analysis of the whole network can be a useful metric to assess overall mitochondrial health, particularly in neurons, which are highly sensitive to mitochondrial dysfunction. Here we describe a method which combines immunofluorescence and computerized image analysis to measure mitochondrial morphology (quantification of number, density, and area) in dopaminergic neurites of mice expressing mitochondrially-targeted eYFP.

Local Oxidative Damage in the Soma and Dendrites Quarantines Neuronal Mitochondria at the Site of Insult

Neurons are highly dependent on mitochondria, but little is known about how they react to a local mitochondrial oxidative insult. We therefore developed a protocol in primary hippocampal cultures that combines the photosensitizer mito-KillerRed with fluorescent biosensors and photoactivatable GFP. We found in both the soma and dendrites that neurons restrict the local increase in mitochondria-derived reactive oxygen species and the decrease in ATP production to the damaged compartment, by quarantining mitochondria. Although the cytosol of both the soma and dendrites became oxidized after mito-KillerRed activation, dendrites were more sensitive to the oxidative insult. Importantly, the impaired mitochondria exhibited decreased motility and fusion, thereby avoiding the spread of oxidation throughout the neuron. These results establish how neurons manage oxidative damage and increase our understanding about the somatodendritic regulation of mitochondrial functions after a local oxidative insult.

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An oxidative insult is contained locally to the damaged region of a neuron

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ATP levels decrease only in the damaged region of the soma or dendrite

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ATP levels increase in the regions distal to the oxidative insult

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Stressed mitochondria are fragmented, with a decreased motility and fusion rate

Mitochondria and Reactive Oxygen Species in Aging and Age-Related Diseases

Aging has been linked to several degenerative processes that, through the accumulation of molecular and cellular damage, can progressively lead to cell dysfunction and organ failure. Human aging is linked with a higher risk for individuals to develop cancer, neurodegenerative, cardiovascular, and metabolic disorders. The understanding of the molecular basis of aging and associated diseases has been one major challenge of scientific research over the last decades. Mitochondria, the center of oxidative metabolism and principal site of reactive oxygen species (ROS) production, are crucial both in health and in pathogenesis of many diseases. Redox signaling is important for the modulation of cell functions and several studies indicate a dual role for ROS in cell physiology. In fact, high concentrations of ROS are pathogenic and can cause severe damage to cell and organelle membranes, DNA, and proteins. On the other hand, moderate amounts of ROS are essential for the maintenance of several biological processes, including gene expression. In this review, we provide an update regarding the key roles of ROS-mitochondria cross talk in different fundamental physiological or pathological situations accompanying aging and highlighting that mitochondrial ROS may be a decisive target in clinical practice.

Mitochondrial-cytoskeletal interactions: dynamic associations that facilitate network function and remodeling

Mitochondria are dynamic organelles that can form complex networks in the cell. These networks can be rapidly remodeled in response to environmental changes or to support cellular needs. Mitochondrial dynamics are dependent on interactions with the cellular cytoskeleton - both microtubules and actin filaments. Mitochondrial-cytoskeletal interactions have a well-established role in mitochondrial motility. Recent progress indicates that these interactions also regulate the balance of mitochondrial fission/fusion, as well as mitochondria turnover and mitochondrial inheritance during cell division. We review these advances, and how this work has deepened our understanding of mitochondrial dynamics in the cell.

Mitochondrial Quality Control in Neurodegenerative Diseases: Focus on Parkinson's Disease and Huntington's Disease

In recent years, several important advances have been made in our understanding of the pathways that lead to cell dysfunction and death in Parkinson's disease (PD) and Huntington's disease (HD). Despite distinct clinical and pathological features, these two neurodegenerative diseases share critical processes, such as the presence of misfolded and/or aggregated proteins, oxidative stress, and mitochondrial anomalies. Even though the mitochondria are commonly regarded as the "powerhouses" of the cell, they are involved in a multitude of cellular events such as heme metabolism, calcium homeostasis, and apoptosis. Disruption of mitochondrial homeostasis and subsequent mitochondrial dysfunction play a key role in the pathophysiology of neurodegenerative diseases, further highlighting the importance of these organelles, especially in neurons. The maintenance of mitochondrial integrity through different surveillance mechanisms is thus critical for neuron survival. Mitochondria display a wide range of quality control mechanisms, from the molecular to the organellar level. Interestingly, many of these lines of defense have been found to be altered in neurodegenerative diseases such as PD and HD. Current knowledge and further elucidation of the novel pathways that protect the cell through mitochondrial quality control may offer unique opportunities for disease therapy in situations where ongoing mitochondrial damage occurs. In this review, we discuss the involvement of mitochondrial dysfunction in neurodegeneration with a special focus on the recent findings regarding mitochondrial quality control pathways, beyond the classical effects of increased production of reactive oxygen species (ROS) and bioenergetic alterations. We also discuss how disturbances in these processes underlie the pathophysiology of neurodegenerative disorders such as PD and HD.

Mitochondrial Ultrastructure Is Coupled to Synaptic Performance at Axonal Release Sites

Mitochondrial function in neurons is tightly linked with metabolic and signaling mechanisms that ultimately determine neuronal performance. The subcellular distribution of these organelles is dynamically regulated as they are directed to axonal release sites on demand, but whether mitochondrial internal ultrastructure and molecular properties would reflect the actual performance requirements in a synapse-specific manner, remains to be established. Here, we examined performance-determining ultrastructural features of presynaptic mitochondria in GABAergic and glutamatergic axons of mice and human. Using electron-tomography and super-resolution microscopy we found, that these features were coupled to synaptic strength: mitochondria in boutons with high synaptic activity exhibited an ultrastructure optimized for high rate metabolism and contained higher levels of the respiratory chain protein cytochrome-c (CytC) than mitochondria in boutons with lower activity. The strong, cell type-independent correlation between mitochondrial ultrastructure, molecular fingerprints and synaptic performance suggests that changes in synaptic activity could trigger ultrastructural plasticity of presynaptic mitochondria, likely to adjust their performance to the actual metabolic demand.

Highlighting the endoplasmic reticulum-mitochondria connection: Focus on Mitofusin 2

The endoplasmic reticulum (ER) and the mitochondrial network are two highly interconnected cellular structures. By proteinaceous tethers, specialized membrane domains of the ER are tightly associated with the outer membrane of mitochondria, allowing the assembly of signaling platforms where different cell functions take place or are modulated, such as lipid biosynthesis, Ca²⁺ homeostasis, inflammation, autophagy and apoptosis. The ER-mitochondria coupling is highly dynamic and contacts between the two organelles can be modified in their number, extension and thickness by different stimuli. Importantly, several pathological conditions, such as cancer, neurodegenerative diseases and metabolic syndromes show alterations in this feature, underlining the key role of ER-mitochondria crosstalk in cell physiology. In this contribution, we will focus on one of the major modulator of ER-mitochondria apposition, Mitofusin 2, discussing the structure of the protein and its debated role on organelles tethering. Moreover, we will critically describe different techniques commonly used to investigate this crucial issue, highlighting their advantages, drawbacks and limits.

Mitochondrial Chaperones in the Brain: Safeguarding Brain Health and Metabolism?

The brain orchestrates organ function and regulates whole body metabolism by the concerted action of neurons and glia cells in the central nervous system. To do so, the brain has tremendously high energy consumption and relies mainly on glucose utilization and mitochondrial function in order to exert its function. As a consequence of high rate metabolism, mitochondria in the brain accumulate errors over time, such as mitochondrial DNA (mtDNA) mutations, reactive oxygen species, and misfolded and aggregated proteins. Thus, mitochondria need to employ specific mechanisms to avoid or ameliorate the rise of damaged proteins that contribute to aberrant mitochondrial function and oxidative stress. To maintain mitochondria homeostasis (mitostasis), cells evolved molecular chaperones that shuttle, refold, or in coordination with proteolytic systems, help to maintain a low steady-state level of misfolded/aggregated proteins. Their importance is exemplified by the occurrence of various brain diseases which exhibit reduced action of chaperones. Chaperone loss (expression and/or function) has been observed during aging, metabolic diseases such as type 2 diabetes and in neurodegenerative diseases such as Alzheimer's (AD), Parkinson's (PD) or even Huntington's (HD) diseases, where the accumulation of damage proteins is evidenced. Within this perspective, we propose that proper brain function is maintained by the joint action of mitochondrial chaperones to ensure and maintain mitostasis contributing to brain health, and that upon failure, alter brain function which can cause metabolic diseases.