Mitochondrial trafficking and the provision of energy and calcium buffering at excitatory synapses

Mitochondrial multifaceted dysfunction in schizophrenia; complex I as a possible pathological target

Mitochondria are key players in various essential cellular processes beyond being the main energy supplier of the cell. Accordingly, they are involved in neuronal synaptic transmission, neuronal growth and sprouting and consequently neuronal plasticity and connectivity. In addition, mitochondria participate in the modulation of gene transcription and inflammation as well in physiological responses in health and disease. Schizophrenia is currently regarded as a neurodevelopmental disorder associated with impaired immune system, aberrant neuronal differentiation and abnormalities in various neurotransmitter systems mainly the dopaminergic, glutaminergic and GABAergic. Ample evidence has been accumulated over the last decade indicating a multifaceted dysfunction of mitochondria in schizophrenia. Indeed, mitochondrial deficit can be of relevance for the majority of the pathologies observed in this disease. In the present article, we overview specific deficits of the mitochondria in schizophrenia, with a focus on the first complex (complex I) of the mitochondrial electron transport chain (ETC). We argue that complex I, being a major factor in the regulation of mitochondrial ETC, is a possible key modulator of various functions of the mitochondria. We review biochemical, molecular, cellular and functional evidence for mitochondrial impairments and their possible convergence to impact in-vitro neuronal differentiation efficiency in schizophrenia. Mitochondrial function in schizophrenia may advance our knowledge of the disease pathophysiology and open the road for new treatment targets for the benefit of the patients.

Ubiquitination at the mitochondria in neuronal health and disease

The preservation of mitochondrial function is of particular importance in neurons given the high energy requirements of action potential propagation and synaptic transmission. Indeed, disruptions in mitochondrial dynamics and quality control are linked to cellular pathology in neurodegenerative diseases, such as Alzheimer's and Parkinson's disease. Here, we will discuss the role of ubiquitination by the E3 ligases: Parkin, MARCH5 and Mul1, and how they regulate mitochondrial homeostasis. Furthermore, given the role of Parkin and Mul1 in the formation of mitochondria-derived vesicles we give an overview of this area of mitochondrial homeostasis. We highlight how through the activity of these enzymes and MDV formation, multiple facets of mitochondrial biology can be regulated, ensuring the functionality of the mitochondrial network thus preserving neuronal health.

PINK1 regulates mitochondrial trafficking in dendrites of cortical neurons through mitochondrial PKA

Mitochondrial Protein Kinase A (PKA) and PTEN-induced kinase 1 (PINK1), which is linked to Parkinson's disease, are two neuroprotective serine/threonine kinases that regulate dendrite remodeling and mitochondrial function. We have previously shown that PINK1 regulates dendrite morphology by enhancing PKA activity. Here, we show the molecular mechanisms by which PINK1 and PKA in the mitochondrion interact to regulate dendrite remodeling, mitochondrial morphology, content, and trafficking in dendrites. PINK1-deficient cortical neurons exhibit impaired mitochondrial trafficking, reduced mitochondrial content, fragmented mitochondria, and a reduction in dendrite outgrowth compared to wild-type neurons. Transient expression of wild-type, but not a PKA-binding-deficient mutant of the PKA-mitochondrial scaffold dual-specificity A Kinase Anchoring Protein 1 (D-AKAP1), restores mitochondrial trafficking, morphology, and content in dendrites of PINK1-deficient cortical neurons suggesting that recruiting PKA to the mitochondrion reverses mitochondrial pathology in dendrites induced by loss of PINK1. Mechanistically, full-length and cleaved forms of PINK1 increase the binding of the regulatory subunit β of PKA (PKA/RIIβ) to D-AKAP1 to enhance the autocatalytic-mediated phosphorylation of PKA/RIIβ and PKA activity. D-AKAP1/PKA governs mitochondrial trafficking in dendrites via the Miro-2/TRAK2 complex and by increasing the phosphorylation of Miro-2. Our study identifies a new role of D-AKAP1 in regulating mitochondrial trafficking through Miro-2, and supports a model in which PINK1 and mitochondrial PKA participate in a similar neuroprotective signaling pathway to maintain dendrite connectivity.

Mitochondrial dysfunction in Autism Spectrum Disorder: clinical features and perspectives

Autism Spectrum Disorder (ASD) is a prototypic pervasive developmental disorder characterized by social interaction, and communication deficits, repetitive, stereotypic patterns of behavior, and impairments in language and development. Clinical studies have identified mitochondrial disturbances at the levels of DNA, activity, complexes, oxidative stress, and metabolites in blood and urine of ASD patients. However, these observations from postmortem brains or peripheral tissues do not provide a direct link between autism and mitochondria. The synaptic abnormality of autistic patients has not been investigated yet. Here we review the findings of clinical studies investigating mitochondrial involvement in ASD patients, focusing particularly on the brain and the limitations and future directions needed in order to fully understand the role of mitochondria in ASD pathology.

Biogenetic and morphofunctional heterogeneity of mitochondria: the case of synaptic mitochondria

The mitochondria of different cells are different in their morphological and biochemical properties. These organelles generate free radicals during activity, leading inevitably to mitochondrial DNA damage. It is not clear how this problem is addressed in long-lived cells, such as neurons. We propose the hypothesis that mitochondria within the same cell also differ in lifespan and ability to divide. According to our suggestion, cells have a pool of ‘stem’ mitochondria with low metabolic activity and a pool of ‘differentiated’ mitochondria with significantly shorter lifespans and high metabolic activity. We consider synaptic mitochondria as a possible example of ‘differentiated’ mitochondria. They are significantly smaller than mitochondria from the cell body, and they are different in key enzyme activity levels, proteome, and lipidome. Synaptic mitochondria are more sensitive to different damaging factors. It has been established that neurons have a sorting mechanism that sends mitochondria with high membrane potential to presynaptic endings. This review describes the properties of synaptic mitochondria and their role in the regulation of synaptic transmission.

Amplifying mitochondrial function rescues adult neurogenesis in a mouse model of Alzheimer's disease

Adult hippocampal neurogenesis is strongly impaired in Alzheimer's disease (AD). In several mouse models of AD, it was shown that adult-born neurons exhibit reduced survival and altered synaptic integration due to a severe lack of dendritic spines. In the present work, using the APPxPS1 mouse model of AD, we reveal that this reduced number of spines is concomitant of a marked deficit in their neuronal mitochondrial content. Remarkably, we show that targeting the overexpression of the pro-neural transcription factor Neurod1 into APPxPS1 adult-born neurons restores not only their dendritic spine density, but also their mitochondrial content and the proportion of spines associated with mitochondria. Using primary neurons, a bona fide model of neuronal maturation, we identified that increases of mitochondrial respiration accompany the stimulating effect of Neurod1 overexpression on dendritic growth and spine formation. Reciprocally, pharmacologically impairing mitochondria prevented Neurod1-dependent trophic effects. Thus, since overexpression of Neurod1 into new neurons of APPxPS1 mice rescues spatial memory, our present data suggest that manipulating the mitochondrial system of adult-born hippocampal neurons provides neuronal plasticity to the AD brain. These findings open new avenues for far-reaching therapeutic implications towards neurodegenerative diseases associated with cognitive impairment.

CDK5 inhibitors prevent astroglial apoptosis and reactive astrogliosis by regulating PKA and DRP1 phosphorylations in the rat hippocampus

Status epilepticus (SE) results in the unique pattern of dynamin-related protein 1 (DRP1)-mediated mitochondrial dynamics, which is associated with astroglial apoptosis and reactive astrogliosis in the regional-specific pattern representing the differential astroglial properties. However, less defined are the epiphenomena/upstream effecters for DRP1 phosphorylation in this process. Since cyclin-dependent kinase 5 (CDK5) is involved in reactive astrogliosis, CDK5 is one of the possible upstream regulators for DRP1 phosphorylation. In the present study, both olomoucine and roscovitine (CDK5 inhibitors) effectively ameliorated SE-induced astroglial apoptosis in the dentate gyrus without changed seizure susceptibility. In addition, they inhibited reactive astrogliosis in the CA1 region independent of neuronal death induced by SE. These effects of CDK5 inhibitors were relevant to abrogation of altered DRP1 phosphorylation ratio and mitochondrial length induced by SE. CDK5 inhibitors also negatively regulated protein kinase A (PKA) activity in astrocytes. Therefore, our findings suggest that CDK5 inhibitors may mitigate astroglial apoptosis and reactive astrogliosis accompanied by modulations of DRP1-mediated mitochondrial dynamics.

Drive the Car(go)s-New Modalities to Control Cargo Trafficking in Live Cells

Synaptic transmission is a fundamental molecular process underlying learning and memory. Successful synaptic transmission involves coupled interaction between electrical signals (action potentials) and chemical signals (neurotransmitters). Defective synaptic transmission has been reported in a variety of neurological disorders such as Autism and Alzheimer’s disease. A large variety of macromolecules and organelles are enriched near functional synapses. Although a portion of macromolecules can be produced locally at the synapse, a large number of synaptic components especially the membrane-bound receptors and peptide neurotransmitters require active transport machinery to reach their sites of action. This spatial relocation is mediated by energy-consuming, motor protein-driven cargo trafficking. Properly regulated cargo trafficking is of fundamental importance to neuronal functions, including synaptic transmission. In this review, we discuss the molecular machinery of cargo trafficking with emphasis on new experimental strategies that enable direct modulation of cargo trafficking in live cells. These strategies promise to provide insights into a quantitative understanding of cargo trafficking, which could lead to new intervention strategies for the treatment of neurological diseases.

Exercise increases mitochondrial complex I activity and DRP1 expression in the brains of aged mice

Exercise is known to have numerous beneficial effects. Recent studies indicate that exercise improves mitochondrial energetics not only in skeletal muscle but also in other tissues. While exercise elicits positive effects on memory, neurogenesis, and synaptic plasticity, the effects of exercise on brain mitochondrial energetics remain relatively unknown. Herein, we studied the effects of exercise training in old and young mice on brain mitochondrial energetics, in comparison to known effects on peripheral tissues that utilize fatty acid oxidation. Exercise improved the capacity for muscle and liver to oxidize palmitate in old mice, but not young mice. In the brain, exercise increased rates of respiration and reactive oxygen species (ROS) production in the old group only while utilizing complex I substrates, effects that were not seen in the young group. Coupled complex I to III enzymatic activity was significantly increased in old trained versus untrained mice with no effect on coupled II to III enzymatic activity. Mitochondrial protein content and markers of mitochondrial biogenesis (PGC-1α and TFAM) were not affected by exercise training in the brain, in contrast to the skeletal muscle of old mice. Brain levels of the autophagy marker LC3-II and protein levels of other signaling proteins that regulate metabolism or transport (BDNF, HSP60, phosphorylated mTOR, FNDC5, SIRT3) were not significantly altered. Old exercised mice showed a significant increase in DRP1 protein levels in the brain without changes in phosphorylation, while MFN2 and OPA1 protein levels were unchanged. Our results suggest that exercise training in old mice can improve brain mitochondrial function through effects on electron transport chain function and mitochondrial dynamics without increasing mitochondrial biogenesis.

Mitochondrial Dysfunction in Depression

Abstract: Background

Depression is the most debilitating neuropsychiatric disorder with significant impact on socio-occupational and well being of individual. The exact pathophysiology of depression is still enigmatic though various theories have been put forwarded. There are evidences showing that mitochondrial dysfunction in various brain regions is associated with depression. Recent findings have sparked renewed appreciation for the role of mitochondria in many intracellular processes coupled to synaptic plasticity and cellular resilience. New insights in depression pathophysiology are revolving around the impairment of neuroplasticity. Mitochondria have potential role in ATP production, intracellular Ca²⁺ signalling to establish membrane stability, reactive oxygen species (ROS) balance and to execute the complex processes of neurotransmission and plasticity. So understanding the various concepts of mitochondrial dysfunction in pathogenesis of depression indubitably helps to generate novel and more targeted therapeutic approaches for depression treatment.

Objective

The review was aimed to give a comprehensive insight on role of mitochondrial dysfunction in depression.

Result

Targeting mitochondrial dysfunction and enhancing the mitochondrial functions might act as potential target for the treatment of depression.

Conclusion

Literature cited in this review highly supports the role of mitochondrial dysfunction in depression. As impairment in the mitochondrial functions lead to the generation of various insults that exaggerate the pathogenesis of depression. So, it is useful to study mitochondrial dysfunction in relation to mood disorders, synaptic plasticity, neurogenesis and enhancing the functions of mitochondria might show promiscuous effects in the treatment of depressed patients.

Synaptic Mitochondria Sustain More Damage than Non-Synaptic Mitochondria after Traumatic Brain Injury and Are Protected by Cyclosporine A

Currently, there are no Food and Drug Administration (FDA)-approved pharmacotherapies for the treatment of those with traumatic brain injury (TBI). As central mediators of the secondary injury cascade, mitochondria are promising therapeutic targets for prevention of cellular death and dysfunction after TBI. One of the most promising and extensively studied mitochondrial targeted TBI therapies is inhibition of the mitochondrial permeability transition pore (mPTP) by the FDA-approved drug, cyclosporine A (CsA). A number of studies have evaluated the effects of CsA on total brain mitochondria after TBI; however, no study has investigated the effects of CsA on isolated synaptic and non-synaptic mitochondria. Synaptic mitochondria are considered essential for proper neurotransmission and synaptic plasticity, and their dysfunction has been implicated in neurodegeneration. Synaptic and non-synaptic mitochondria have heterogeneous characteristics, but their heterogeneity can be masked in total mitochondrial (synaptic and non-synaptic) preparations. Therefore, it is essential that mitochondria targeted pharmacotherapies, such as CsA, be evaluated in both populations. This is the first study to examine the effects of CsA on isolated synaptic and non-synaptic mitochondria after experimental TBI. We conclude that synaptic mitochondria sustain more damage than non-synaptic mitochondria 24 h after severe controlled cortical impact injury (CCI), and that intraperitoneal administration of CsA (20 mg/kg) 15 min after injury improves synaptic and non-synaptic respiration, with a significant improvement being seen in the more severely impaired synaptic population. As such, CsA remains a promising neuroprotective candidate for the treatment of those with TBI.

Upstream Pathways Controlling Mitochondrial Function in Major Psychosis: A Focus on Bipolar Disorder

Mitochondrial dysfunction is commonly observed in bipolar disorder (BD) and schizophrenia (SCZ) and may be a central feature of psychosis. These illnesses are complex and heterogeneous, which is reflected by the complexity of the processes regulating mitochondrial function. Mitochondria are typically associated with energy production; however, dysfunction of mitochondria affects not only energy production but also vital cellular processes, including the formation of reactive oxygen species, cell cycle and survival, intracellular Ca(2+) homeostasis, and neurotransmission. In this review, we characterize the upstream components controlling mitochondrial function, including 1) mutations in nuclear and mitochondrial DNA, 2) mitochondrial dynamics, and 3) intracellular Ca(2+) homeostasis. Characterizing and understanding the upstream factors that regulate mitochondrial function is essential to understand progression of these illnesses and develop biomarkers and therapeutics.

DISC1 is a coordinator of intracellular trafficking to shape neuronal development and connectivity

The long, asymmetric and specialised architecture of neuronal processes necessitates a properly regulated transport network of molecular motors and cytoskeletal tracks. This allows appropriate distribution of cargo for correct formation and activity of the synapse, and thus normal neuronal communication. This communication is impaired in psychiatric disease, and ongoing studies have proposed that Disrupted in schizophrenia 1 (DISC1) is an important genetic risk factor for these disorders. The mechanisms by which DISC1 dysfunction might increase propensity to psychiatric disease are not completely understood; however, an emerging theme is that DISC1 can function as a key regulator of neuronal intracellular trafficking. Transport of a wide range of potential cargoes - including mRNAs, neurotransmitter receptors, vesicles and mitochondria - can be modulated by DISC1, and therefore is susceptible to DISC1 dysfunction. This theme highlights the importance of understanding precisely how DISC1 can regulate intracellular trafficking, and suggests that a novel approach to the treatment of psychiatric disorders could be provided by targeting this protein and the trafficking machinery with which it interacts.

The Differential DRP1 Phosphorylation and Mitochondrial Dynamics in the Regional Specific Astroglial Death Induced by Status Epilepticus

The response and susceptibility to astroglial degenerations are relevant to the distinctive properties of astrocytes in a hemodynamic-independent manner following status epilepticus (SE). Since impaired mitochondrial fission plays an important role in mitosis, apoptosis and programmed necrosis, we investigated whether the unique pattern of mitochondrial dynamics is involved in the characteristics of astroglial death induced by SE. In the present study, SE induced astroglial apoptosis in the molecular layer of the dentate gyrus, accompanied by decreased mitochondrial length. In contrast, clasmatodendritic (autophagic) astrocytes in the CA1 region showed mitochondrial elongation induced by SE. Mdivi-1 (an inhibitor of mitochondrial fission) effectively attenuated astroglial apoptosis, but WY14643 (an enhancer of mitochondrial fission) aggravated it. In addition, Mdivi-1 accelerated clasmatodendritic changes in astrocytes. These regional specific mitochondrial dynamics in astrocytes were closely correlated with dynamin-related protein 1 (DRP1; a mitochondrial fission protein) phosphorylation, not optic atrophy 1 (OPA1; a mitochondrial fusion protein) expression. To the best of our knowledge, the present data demonstrate for the first time the novel role of DRP1-mediated mitochondrial fission in astroglial loss. Thus, the present findings suggest that the differential astroglial mitochondrial dynamics may participate in the distinct characteristics of astroglial death induced by SE.

Polo Kinase Phosphorylates Miro to Control ER-Mitochondria Contact Sites and Mitochondrial Ca(2+) Homeostasis in Neural Stem Cell Development

Mitochondria play central roles in buffering intracellular Ca²⁺ transients. While basal mitochondrial Ca²⁺ (Ca²⁺ mito) is needed to maintain organellar physiology, Ca²⁺ mito overload can lead to cell death. How Ca²⁺ mito homeostasis is regulated is not well understood. Here we show that Miro, a known component of the mitochondrial transport machinery, regulates Drosophila neural stem cell (NSC) development through Ca²⁺ mito homeostasis control, independent of its role in mitochondrial transport. Miro interacts with Ca²⁺ transporters at the ER-mitochondria contact site (ERMCS). Its inactivation causes Ca²⁺ mito depletion and metabolic impairment, whereas its overexpression results in Ca²⁺ mito overload, mitochondrial morphology change, and apoptotic response. Both conditions impaired NSC lineage progression. Ca²⁺ mito homeostasis is influenced by Polo-mediated phosphorylation of a conserved residue in Miro, which positively regulates Miro localization to, and the integrity of, ERMCS. Our results elucidate a regulatory mechanism underlying Ca²⁺ mito homeostasis and how its dysregulation may affect NSC metabolism/development and contribute to disease.

Miro1 Regulates Activity-Driven Positioning of Mitochondria within Astrocytic Processes Apposed to Synapses to Regulate Intracellular Calcium Signaling

It is fast emerging that maintaining mitochondrial function is important for regulating astrocyte function, although the specific mechanisms that govern astrocyte mitochondrial trafficking and positioning remain poorly understood. The mitochondrial Rho-GTPase 1 protein (Miro1) regulates mitochondrial trafficking and detachment from the microtubule transport network to control activity-dependent mitochondrial positioning in neurons. However, whether Miro proteins are important for regulating signaling-dependent mitochondrial dynamics in astrocytic processes remains unclear. Using live-cell confocal microscopy of rat organotypic hippocampal slices, we find that enhancing neuronal activity induces transient mitochondrial remodeling in astrocytes, with a concomitant, transient reduction in mitochondrial trafficking, mediated by elevations in intracellular Ca(2+). Stimulating neuronal activity also induced mitochondrial confinement within astrocytic processes in close proximity to synapses. Furthermore, we show that the Ca(2+)-sensing EF-hand domains of Miro1 are important for regulating mitochondrial trafficking in astrocytes and required for activity-driven mitochondrial confinement near synapses. Additionally, activity-dependent mitochondrial positioning by Miro1 reciprocally regulates the levels of intracellular Ca(2+) in astrocytic processes. Thus, the regulation of intracellular Ca(2+) signaling, dependent on Miro1-mediated mitochondrial positioning, could have important consequences for astrocyte Ca(2+) wave propagation, gliotransmission, and ultimately neuronal function.

Purinergic signaling and energy homeostasis in psychiatric disorders

Purinergic signaling regulates numerous vital biological processes in the central nervous system (CNS). The two principle purines, ATP and adenosine act as excitatory and inhibitory neurotransmitters, respectively. Compared to other classical neurotransmitters, the role of purinergic signaling in psychiatric disorders is not well understood or appreciated. Because ATP exerts its main effect on energy homeostasis, neuronal function of ATP has been underestimated. Similarly, adenosine is primarily appreciated as a precursor of nucleotide synthesis during active cell growth and division. However, recent findings suggest that purinergic signaling may explain how neuronal activity is associated neuronal energy charge and energy homeostasis, especially in mental disorders. In this review, we provide an overview of the synaptic function of mitochondria and purines in neuromodulation, synaptic plasticity, and neuron-glia interactions. We summarize how mitochondrial and purinergic dysfunction contribute to mental illnesses such as schizophrenia, bipolar disorder, autism spectrum disorder (ASD), depression, and addiction. Finally, we discuss future implications regarding the pharmacological targeting of mitochondrial and purinergic function for the treatment of psychiatric disorders.

Lipid transfer and metabolism across the endolysosomal-mitochondrial boundary

Lysosomes and mitochondria occupy a central stage in the maintenance of cellular homeostasis, by playing complementary roles in nutrient sensing and energy metabolism. Specifically, these organelles function as signaling hubs that integrate environmental and endogenous stimuli with specific metabolic responses. In particular, they control various lipid biosynthetic and degradative pipelines, either directly or indirectly, by regulating major cellular metabolic pathways, and by physical and functional connections established with each other and with other organelles. Membrane contact sites allow the exchange of ions and molecules between organelles, even without membrane fusion, and are privileged routes for lipid transfer among different membrane compartments. These inter-organellar connections typically involve the endoplasmic reticulum. Direct membrane contacts have now been described also between lysosomes, autophagosomes, lipid droplets, and mitochondria. This review focuses on these recently identified membrane contact sites, and on their role in lipid biosynthesis, exchange, turnover and catabolism. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Age-Related Phasic Patterns of Mitochondrial Maintenance in Adult Caenorhabditis elegans Neurons

Aging is associated with cognitive decline and increasing risk of neurodegeneration. Perturbation of mitochondrial function, dynamics, and trafficking are implicated in the pathogenesis of several age-associated neurodegenerative diseases. Despite this fundamental importance, the critical understanding of how organismal aging affects lifetime neuronal mitochondrial maintenance remains unknown, particularly in a physiologically relevant context. To address this issue, we performed a comprehensive in vivo analysis of age-associated changes in mitochondrial morphology, density, trafficking, and stress resistance in individual Caenorhabditis elegans neurons throughout adult life. Adult neurons display three distinct stages of increase, maintenance, and decrease in mitochondrial size and density during adulthood. Mitochondrial trafficking in the distal neuronal processes declines progressively with age starting from early adulthood. In contrast, long-lived daf-2 mutants exhibit delayed age-associated changes in mitochondrial morphology, constant mitochondrial density, and maintained trafficking rates during adulthood. Reduced mitochondrial load at late adulthood correlates with decreased mitochondrial resistance to oxidative stress. Revealing aging-associated changes in neuronal mitochondria in vivo is an essential precedent that will allow future elucidation of the mechanistic causes of mitochondrial aging. Thus, our study establishes the critical foundation for the future analysis of cellular pathways and genetic and pharmacological factors regulating mitochondrial maintenance in aging- and disease-relevant conditions.

SIGNIFICANCE STATEMENT

Using Caenorhabditis elegans as a model, we address long-standing questions: How does aging affect neuronal mitochondrial morphology, density, trafficking, and oxidative stress resistance? Are these age-related changes amenable to genetic manipulations that slow down the aging process? Our study illustrates that mitochondrial trafficking declines progressively from the first day of adulthood, whereas mitochondrial size, density, and resistance to oxidative stress undergo three distinct stages: increase in early adulthood, maintenance at high levels during mid-adulthood, and decline during late adulthood. Thus, our study characterizes mitochondrial aging profile at the level of a single neuron in its native environment and establishes the critical foundation for the future genetic and pharmacological dissection of factors that influence long-term mitochondrial maintenance in neurons.

Movement and structure of mitochondria in oligodendrocytes and their myelin sheaths

Mitochondria play several crucial roles in the life of oligodendrocytes. During development of the myelin sheath they are essential providers of carbon skeletons and energy for lipid synthesis. During normal brain function their consumption of pyruvate will be a key determinant of how much lactate is available for oligodendrocytes to export to power axonal function. Finally, during calcium-overload induced pathology, as occurs in ischemia, mitochondria may buffer calcium or induce apoptosis. Despite their important functions, very little is known of the properties of oligodendrocyte mitochondria, and mitochondria have never been observed in the myelin sheaths. We have now used targeted expression of fluorescent mitochondrial markers to characterize the location and movement of mitochondria within oligodendrocytes. We show for the first time that mitochondria are able to enter and move within the myelin sheath. Within the myelin sheath the highest number of mitochondria was in the cytoplasmic ridges along the sheath. Mitochondria moved more slowly than in neurons and, in contrast to their behavior in neurons and astrocytes, their movement was increased rather than inhibited by glutamate activating NMDA receptors. By electron microscopy we show that myelin sheath mitochondria have a low surface area of cristae, which suggests a low ATP production. These data specify fundamental properties of the oxidative phosphorylation system in oligodendrocytes, the glial cells that enhance cognition by speeding action potential propagation and provide metabolic support to axons.

The impact of mitochondrial aldehyde dehydrogenase (ALDH2) activation by Alda-1 on the behavioral and biochemical disturbances in animal model of depression

The etiology of depression remains still unclear. Recently, it has been proposed, that mitochondrial dysfunction may be associated with development of mood disorders, such as depression, bipolar disorder and anxiety disorders. Mitochondrial aldehyde dehydrogenase (ALDH2), an enzyme responsible for the detoxification of reactive aldehydes, is considered to exert protective function in mitochondria. We investigated the influence of Alda-1, a small-molecule activator of ALDH2, on depressive- and anxiety-like behaviors in an animal model of depression - the prenatally stressed rats - using behavioral, molecular and proteomic methods. Prolonged Alda-1 administration significantly increased the climbing time, tended to reduce the immobility time and increased the swimming time of the prenatally stressed rats in the forced swim test. Moreover, treatment of prenatally stressed rats with Alda-1 significantly increased number of entries into the open arms of the maze and the time spent therein, as assessed by elevated plus-maze test. Such actions were associated with reduction of plasma 4-HNE-protein content, decrease of TNF-α mRNA and increase of PGC-1α (regulator of mitochondrial biogenesis) mRNA level in the frontal cortex and hippocampus of the prenatally stressed rats as well as with normalization of peripheral immune parameters and significant changes in expression of 6 and 4 proteins related to mitochondrial functions in the frontal cortex and hippocampus, respectively. Collectively, ALDH2 activation by Alda-1 led to a significant attenuation of depressive- and anxiety-like behaviors in the prenatally stressed rats. The pattern of changes suggested mitoprotective effect of Alda-1, however the exact functional consequences of the revealed alterations require further investigation.

DISC1-dependent Regulation of Mitochondrial Dynamics Controls the Morphogenesis of Complex Neuronal Dendrites

The DISC1 protein is implicated in major mental illnesses including schizophrenia, depression, bipolar disorder, and autism. Aberrant mitochondrial dynamics are also associated with major mental illness. DISC1 plays a role in mitochondrial transport in neuronal axons, but its effects in dendrites have yet to be studied. Further, the mechanisms of this regulation and its role in neuronal development and brain function are poorly understood. Here we have demonstrated that DISC1 couples to the mitochondrial transport and fusion machinery via interaction with the outer mitochondrial membrane GTPase proteins Miro1 and Miro2, the TRAK1 and TRAK2 mitochondrial trafficking adaptors, and the mitochondrial fusion proteins (mitofusins). Using live cell imaging, we show that disruption of the DISC1-Miro-TRAK complex inhibits mitochondrial transport in neurons. We also show that the fusion protein generated from the originally described DISC1 translocation (DISC1-Boymaw) localizes to the mitochondria, where it similarly disrupts mitochondrial dynamics. We also show by super resolution microscopy that DISC1 is localized to endoplasmic reticulum contact sites and that the DISC1-Boymaw fusion protein decreases the endoplasmic reticulum-mitochondria contact area. Moreover, disruption of mitochondrial dynamics by targeting the DISC1-Miro-TRAK complex or upon expression of the DISC1-Boymaw fusion protein impairs the correct development of neuronal dendrites. Thus, DISC1 acts as an important regulator of mitochondrial dynamics in both axons and dendrites to mediate the transport, fusion, and cross-talk of these organelles, and pathological DISC1 isoforms disrupt this critical function leading to abnormal neuronal development.

Mitochondrial dynamics in astrocytes

Astrocytes exhibit cellular excitability through variations in their intracellular calcium (Ca²⁺) levels in response to synaptic activity. Astrocyte Ca²⁺ elevations can trigger the release of neuroactive substances that can modulate synaptic transmission and plasticity, hence promoting bidirectional communication with neurons. Intracellular Ca²⁺ dynamics can be regulated by several proteins located in the plasma membrane, within the cytosol and by intracellular organelles such as mitochondria. Spatial dynamics and strategic positioning of mitochondria are important for matching local energy provision and Ca²⁺ buffering requirements to the demands of neuronal signalling. Although relatively unresolved in astrocytes, further understanding the role of mitochondria in astrocytes may reveal more about the complex bidirectional relationship between astrocytes and neurons in health and disease. In the present review, we discuss some recent insights regarding mitochondrial function, transport and turnover in astrocytes and highlight some important questions that remain to be answered.

Hyperglycemia / hypoglycemia-induced mitochondrial dysfunction and cerebral ischemic damage in diabetics

Enhancement of ischemic brain damage is one of the most serious complications of diabetes. Studies from various in vivo and in vitro models of cerebral ischemia have led to an understanding of the role of mitochondria and complex interrelated mitochondrial biochemical pathways leading to the aggravation of ischemic neuronal damage. Advancements in the elucidation of the mechanisms of ischemic brain damage in diabetic subjects have revealed a number of key mitochondrial targets that have been hypothesized to participate in enhancement of brain damage. The present review initially discusses the neurobiology of ischemic neuronal injury, with special emphasis on the central role of mitochondria in mediating its pathogenesis and therapeutic targets. Later it further details the potential role of various biochemical mediators and second messengers causing widespread ischemic brain damage among diabetics via mitochondrial pathways. The present review discusses preclinical data which validates the significance of mitochondrial mechanisms in mediating the aggravation of ischemic cerebral injury in diabetes. Exploitation of these targets may provide effective therapeutic agents for the management of diabetes-related aggravation of ischemic neuronal damage.

ATP binding to synaspsin IIa regulates usage and clustering of vesicles in terminals of hippocampal neurons

Synaptic transmission is expensive in terms of its energy demands and was recently shown to decrease the ATP concentration within presynaptic terminals transiently, an observation that we confirm. We hypothesized that, in addition to being an energy source, ATP may modulate the synapsins directly. Synapsins are abundant neuronal proteins that associate with the surface of synaptic vesicles and possess a well defined ATP-binding site of undetermined function. To examine our hypothesis, we produced a mutation (K270Q) in synapsin IIa that prevents ATP binding and reintroduced the mutant into cultured mouse hippocampal neurons devoid of all synapsins. Remarkably, staining for synaptic vesicle markers was enhanced in these neurons compared with neurons expressing wild-type synapsin IIa, suggesting overly efficient clustering of vesicles. In contrast, the mutation completely disrupted the capability of synapsin IIa to slow synaptic depression during sustained 10 Hz stimulation, indicating that it interfered with synapsin-dependent vesicle recruitment. Finally, we found that the K270Q mutation attenuated the phosphorylation of synapsin IIa on a distant PKA/CaMKI consensus site known to be essential for vesicle recruitment. We conclude that ATP binding to synapsin IIa plays a key role in modulating its function and in defining its contribution to hippocampal short-term synaptic plasticity.

Mitochondria: key players in the neurotoxic effects of amphetamines

Amphetamines are a class of psychotropic drugs with high abuse potential, as a result of their stimulant, euphoric, emphathogenic, entactogenic, and hallucinogenic properties. Although most amphetamines are synthetic drugs, of which methamphetamine, amphetamine, and 3,4-methylenedioxymethamphetamine ("ecstasy") represent well-recognized examples, the use of natural related compounds, namely cathinone and ephedrine, has been part of the history of humankind for thousands of years. Resulting from their amphiphilic nature, these drugs can easily cross the blood-brain barrier and elicit their well-known psychotropic effects. In the field of amphetamines' research, there is a general consensus that mitochondrial-dependent pathways can provide a major understanding concerning pathological processes underlying the neurotoxicity of these drugs. These events include alterations on tricarboxylic acid cycle's enzymes functioning, inhibition of mitochondrial electron transport chain's complexes, perturbations of mitochondrial clearance mechanisms, interference with mitochondrial dynamics, as well as oxidative modifications in mitochondrial macromolecules. Additionally, other studies indicate that amphetamines-induced neuronal toxicity is closely regulated by B cell lymphoma 2 superfamily of proteins with consequent activation of caspase-mediated downstream cell death pathway. Understanding the molecular mechanisms at mitochondrial level involved in amphetamines' neurotoxicity can help in defining target pathways or molecules mediating these effects, as well as in developing putative therapeutic approaches to prevent or treat the acute- or long-lasting neuropsychiatric complications seen in human abusers.

Permeabilization of brain tissue in situ enables multiregion analysis of mitochondrial function in a single mouse brain

KEY POINTS

Mitochondrial function in the brain is traditionally assessed through analysing respiration in isolated mitochondria, a technique that possesses significant tissue and time requirements while also disrupting the cooperative mitochondrial reticulum. We permeabilized brain tissue in situ to permit analysis of mitochondrial respiration with the native mitochondrial morphology intact, removing the need for isolation time and minimizing tissue requirements to ∼2 mg wet weight. The permeabilized brain technique was validated against the traditional method of isolated mitochondria and was then further applied to assess regional variation in the mouse brain with ischaemia-reperfusion injuries. A transgenic mouse model overexpressing catalase within mitochondria was applied to show the contribution of mitochondrial reactive oxygen species to ischaemia-reperfusion injuries in different brain regions. This technique enhances the accessibility of addressing physiological questions in small brain regions and in applying transgenic mouse models to assess mechanisms regulating mitochondrial function in health and disease.

ABSTRACT

Mitochondria function as the core energy providers in the brain and symptoms of neurodegenerative diseases are often attributed to their dysregulation. Assessing mitochondrial function is classically performed in isolated mitochondria; however, this process requires significant isolation time, demand for abundant tissue and disruption of the cooperative mitochondrial reticulum, all of which reduce reliability when attempting to assess in vivo mitochondrial bioenergetics. Here we introduce a method that advances the assessment of mitochondrial respiration in the brain by permeabilizing existing brain tissue to grant direct access to the mitochondrial reticulum in situ. The permeabilized brain preparation allows for instant analysis of mitochondrial function with unaltered mitochondrial morphology using significantly small sample sizes (∼2 mg), which permits the analysis of mitochondrial function in multiple subregions within a single mouse brain. Here this technique was applied to assess regional variation in brain mitochondrial function with acute ischaemia-reperfusion injuries and to determine the role of reactive oxygen species in exacerbating dysfunction through the application of a transgenic mouse model overexpressing catalase within mitochondria. Through creating accessibility to small regions for the investigation of mitochondrial function, the permeabilized brain preparation enhances the capacity for examining regional differences in mitochondrial regulation within the brain, as the majority of genetic models used for unique approaches exist in the mouse model.

Analysis of mitochondrial network by imaging: proof of technique in schizophrenia

Mitochondria, similar to living cells and organelles, have negative membrane potential and can therefore accumulate permeable lipophilic cations. Those cations which exhibit fluorescence activity after accumulation into energized systems are widely used to decipher changes in membrane potential by imaging techniques. Here we describe the use of the lipophilic cation 5,5',6,6'tetrachloro-1,1',3,3'-tetraethylbenzimidazol-carbocyanine iodide (JC-1), which alters reversibly its color from green (J-monomer, at its low concentration in the cytosol) to red (J-aggregates, at its high concentration in active mitochondria) with increasing mitochondrial membrane potential (Δψm). We show that in addition to changes in Δψm, this specific dye can be used to follow alterations in mitochondrial distribution and mitochondrial network connectivity. We suggest that JC-1 is a preferable probe to compare between treatment groups, as the ratio of green to red fluorescence intensities is used for analysis. This ratio depends only on the mitochondrial membrane potential and not on other mitochondrial dependent or independent factors. We demonstrate various applications of JC-1 staining to study mitochondrial abnormalities in different cell types derived from schizophrenia patients and healthy subjects.

PSEA-Quant: a protein set enrichment analysis on label-free and label-based protein quantification data

The majority of large-scale proteomics quantification methods yield long lists of quantified proteins that are often difficult to interpret and poorly reproduced. Computational approaches are required to analyze such intricate quantitative proteomics data sets. We propose a statistical approach to computationally identify protein sets (e.g., Gene Ontology (GO) terms) that are significantly enriched with abundant proteins with reproducible quantification measurements across a set of replicates. To this end, we developed PSEA-Quant, a protein set enrichment analysis algorithm for label-free and label-based protein quantification data sets. It offers an alternative approach to classic GO analyses, models protein annotation biases, and allows the analysis of samples originating from a single condition, unlike analogous approaches such as GSEA and PSEA. We demonstrate that PSEA-Quant produces results complementary to GO analyses. We also show that PSEA-Quant provides valuable information about the biological processes involved in cystic fibrosis using label-free protein quantification of a cell line expressing a CFTR mutant. Finally, PSEA-Quant highlights the differences in the mechanisms taking place in the human, rat, and mouse brain frontal cortices based on tandem mass tag quantification. Our approach, which is available online, will thus improve the analysis of proteomics quantification data sets by providing meaningful biological insights.

Mitochondrial trafficking in neurons and the role of the Miro family of GTPase proteins

Correct mitochondrial dynamics are essential to neuronal function. These dynamics include mitochondrial trafficking and quality-control systems that maintain a precisely distributed and healthy mitochondrial network, so that local energy demands or Ca2+-buffering requirements within the intricate architecture of the neuron can be met. Mitochondria make use of molecular machinery that couples these organelles to microtubule-based transport via kinesin and dynein motors, facilitating the required long-range movements. These motors in turn are associated with a variety of adaptor proteins allowing additional regulation of the complex dynamics demonstrated by these organelles. Over recent years, a number of new motor and adaptor proteins have been added to a growing list of components implicated in mitochondrial trafficking and distribution. Yet, there are major questions that remain to be addressed about the regulation of mitochondrial transport complexes. One of the core components of this machinery, the mitochondrial Rho GTPases Miro1 (mitochondrial Rho 1) and Miro2 have received special attention due to their Ca2+-sensing and GTPase abilities, marking Miro an exceptional candidate for co-ordinating mitochondrial dynamics and intracellular signalling pathways. In the present paper, we discuss the wealth of literature regarding Miro-mediated mitochondrial transport in neurons and recently highlighted involvement of Miro proteins in mitochondrial turnover, emerging as a key process affected in neurodegeneration.

Targeting mitochondrially mediated plasticity to develop improved therapeutics for bipolar disorder

INTRODUCTION

Bipolar disorder (BPD) is a severe illness with few treatments available. Understanding BPD pathophysiology and identifying potential relevant targets could prove useful for developing new treatments. Remarkably, subtle impairments of mitochondrial function may play an important role in BPD pathophysiology.

AREAS COVERED

This article focuses on human studies and reviews evidence of mitochondrial dysfunction in BPD as a promising target for the development of new, improved treatments. Mitochondria are crucial for energy production, generated mainly through the electron transport chain (ETC) and play an important role in regulating apoptosis and calcium (Ca²⁺) signaling as well as synaptic plasticity. Mitochondria move throughout the neurons to provide energy for intracellular signaling. Studies showed polymorphisms of mitochondria-related genes as risk factors for BPD. Postmortem studies in BPD also show decreased ETC activity/expression and increased nitrosative and oxidative stress (OxS) in patient brains. BPD has been also associated with increased OxS, Ca²⁺ dysregulation and increased proapoptotic signaling in peripheral blood. Neuroimaging studies consistently show decreased energy levels and pH in brains of BPD patients.

EXPERT OPINION

Targeting mitochondrial function, and their role in energy metabolism, synaptic plasticity and cell survival, may be an important avenue for development of new mood-stabilizing agents.

Organelle biogenesis and interorganellar connections: Better in contact than in isolation

Membrane contact sites (MCSs) allow the exchange of molecules and information between organelles, even when their membranes cannot fuse directly. In recent years, a number of functions have been attributed to these contacts, highlighting their critical role in cell homeostasis. Although inter-organellar connections typically involve the endoplasmic reticulum (ER), we recently reported the presence of a novel MCSs between melanosomes and mitochondria. Melanosome-mitochondrion contacts appear mediated by fibrillar bridges resembling the protein tethers linking mitochondria and the ER, both for their ultrastructural features and the involvement of Mitofusin 2. The frequency of these connections correlates spatially and timely with melanosome biogenesis, suggesting a functional link between the 2 processes and in general that organelle biogenesis in the secretory pathway requires interorganellar crosstalks at multiple steps. Here, we summarize the different functions attributed to MCSs, and discuss their possible relevance for the newly identified melanosome-mitochondrion liaison.

Integrated regulation of motor-driven organelle transport by scaffolding proteins

Intracellular trafficking pathways, including endocytosis, autophagy, and secretion, rely on directed organelle transport driven by the opposing microtubule motor proteins kinesin and dynein. Precise spatial and temporal targeting of vesicles and organelles requires the integrated regulation of these opposing motors, which are often bound simultaneously to the same cargo. Recent progress demonstrates that organelle-associated scaffolding proteins, including Milton/TRAKs (trafficking kinesin-binding protein), JIP1, JIP3 (JNK-interacting proteins), huntingtin, and Hook1, interact with molecular motors to coordinate activity and sustain unidirectional transport. Scaffolding proteins also bind to upstream regulatory proteins, including kinases and GTPases, to modulate transport in the cell. This integration of regulatory control with motor activity allows for cargo-specific changes in the transport or targeting of organelles in response to cues from the complex cellular environment.

Mitochondria modify exercise-induced development of stem cell-derived neurons in the adult brain

Neural stem cells in the adult mammalian hippocampus continuously generate new functional neurons, which modify the hippocampal network and significantly contribute to cognitive processes and mood regulation. Here, we show that the development of new neurons from stem cells in adult mice is paralleled by extensive changes to mitochondrial mass, distribution, and shape. Moreover, exercise-a strong modifier of adult hippocampal neurogenesis-accelerates neuronal maturation and induces a profound increase in mitochondrial content and the presence of mitochondria in dendritic segments. Genetic inhibition of the activity of the mitochondrial fission factor dynamin-related protein 1 (Drp1) inhibits neurogenesis under basal and exercise conditions. Conversely, enhanced Drp1 activity furthers exercise-induced acceleration of neuronal maturation. Collectively, these results indicate that adult hippocampal neurogenesis requires adaptation of the mitochondrial compartment and suggest that mitochondria are targets for enhancing neurogenesis-dependent hippocampal plasticity.

Neuronal activity and glutamate uptake decrease mitochondrial mobility in astrocytes and position mitochondria near glutamate transporters

Within neurons, mitochondria are nonuniformly distributed and are retained at sites of high activity and metabolic demand. Glutamate transport and the concomitant activation of the Na(+)/K(+)-ATPase represent a substantial energetic demand on astrocytes. We hypothesized that mitochondrial mobility within astrocytic processes might be regulated by neuronal activity and glutamate transport. We imaged organotypic hippocampal slice cultures of rat, in which astrocytes maintain their highly branched morphologies and express glutamate transporters. Using time-lapse confocal microscopy, the mobility of mitochondria within individual astrocytic processes and neuronal dendrites was tracked. Within neurons, a greater percentage of mitochondria were mobile than in astrocytes. Furthermore, they moved faster and farther than in astrocytes. Inhibiting neuronal activity with tetrodotoxin (TTX) increased the percentage of mobile mitochondria in astrocytes. Mitochondrial movement in astrocytes was inhibited by vinblastine and cytochalasin D, demonstrating that this mobility depends on both the microtubule and actin cytoskeletons. Inhibition of glutamate transport tripled the percentage of mobile mitochondria in astrocytes. Conversely, application of the transporter substrate d-aspartate reversed the TTX-induced increase in the percentage of mobile mitochondria. Inhibition of reversed Na(+)/Ca(2+) exchange also increased the percentage of mitochondria that were mobile. Last, we demonstrated that neuronal activity increases the probability that mitochondria appose GLT-1 particles within astrocyte processes, without changing the proximity of GLT-1 particles to VGLUT1. These results imply that neuronal activity and the resulting clearance of glutamate by astrocytes regulate the movement of astrocytic mitochondria and suggest a mechanism by which glutamate transporters might retain mitochondria at sites of glutamate uptake.

Mitochondria and melanosomes establish physical contacts modulated by Mfn2 and involved in organelle biogenesis

BACKGROUND

To efficiently supply ATP to sites of high-energy demand and finely regulate calcium signaling, mitochondria adapt their metabolism, shape, and distribution within the cells, including relative positioning with respect to other organelles. However, physical contacts between mitochondria and the secretory/endocytic pathway have been demonstrated so far only with the ER, through structural and functional interorganellar connections.

RESULTS

Here we show by electron tomography that mitochondria physically contact melanosomes, specialized lysosome-related organelles of pigment cells, through fibrillar bridges resembling the protein tethers linking mitochondria and the ER. Mitofusin (Mfn) 2, which bridges ER to mitochondria, specifically localizes also to melanosome-mitochondrion contacts, and its knockdown significantly reduces the interorganellar connections. Contacts are associated to the melanogenesis process, as indicated by the fact that they are reduced in a model of aberrant melanogenesis whereas they are enhanced both where melanosome biogenesis takes place in the perinuclear area and when it is actively stimulated by OA1, a G protein-coupled receptor implicated in ocular albinism and organellogenesis. Consistently, Mfn2 knockdown prevents melanogenesis activation by OA1, and the pharmacological inhibition of mitochondrial ATP synthesis severely reduces contact formation and impairs melanosome biogenesis, by affecting in particular the developing organelles showing the highest frequency of contacts.

CONCLUSIONS

Altogether, our findings reveal the presence of an unprecedented physical and functional connection between mitochondria and the secretory/endocytic pathway that goes beyond the ER-mitochondria linkage and is spatially and timely associated to secretory organelle biogenesis.

Mitochondria localize to the cleavage furrow in mammalian cytokinesis

Mitochondria are dynamic organelles with multiple cellular functions, including ATP production, calcium buffering, and lipid biosynthesis. Several studies have shown that mitochondrial positioning is regulated by the cytoskeleton during cell division in several eukaryotic systems. However, the distribution of mitochondria during mammalian cytokinesis and whether the distribution is regulated by the cytoskeleton has not been examined. Using live spinning disk confocal microscopy and quantitative analysis of mitochondrial fluorescence intensity, we demonstrate that mitochondria are recruited to the cleavage furrow during cytokinesis in HeLa cells. After anaphase onset, the mitochondria are recruited towards the site of cleavage furrow formation, where they remain enriched as the furrow ingresses and until cytokinesis completion. Furthermore, we show that recruitment of mitochondria to the furrow occurs in multiple mammalian cells lines as well as in monopolar, bipolar, and multipolar divisions, suggesting that the mechanism of recruitment is conserved and robust. Using inhibitors of cytoskeleton dynamics, we show that the microtubule cytoskeleton, but not actin, is required to transport mitochondria to the cleavage furrow. Thus, mitochondria are specifically recruited to the cleavage furrow in a microtubule-dependent manner during mammalian cytokinesis. Two possible reasons for this could be to localize mitochondrial function to the furrow to facilitate cytokinesis and / or ensure accurate mitochondrial inheritance.

Characterization of early pathogenesis in the SOD1(G93A) mouse model of ALS: part II, results and discussion

Pathological events are well characterized in amyotrophic lateral sclerosis (ALS) mouse models, but review of the literature fails to identify a specific initiating event that precipitates disease pathology. There is now growing consensus in the field that axon and synapses are first cellular sites of degeneration, but controversy exists over whether axon and synapse loss is initiated autonomously at those sites or by pathology in the cell body, in nonneuronal cells or even in nonmotoneurons (MNs). Previous studies have identified pathological events in the mutant superoxide dismutase 1 (SOD1) models involving spinal cord, peripheral axons, neuromuscular junctions (NMJs), or muscle; however, few studies have systematically examined pathogenesis at multiple sites in the same study. We have performed ultrastructural examination of both central and peripheral components of the neuromuscular system in the SOD1(G93A) mouse model of ALS. Twenty percent of MNs undergo degeneration by P60, but NMJ innervation in fast fatigable muscles is reduced by 40% by P30. Gait alterations and muscle weakness were also found at P30. There was no change in axonal transport prior to initial NMJ denervation. Mitochondrial morphological changes are observed at P7 and become more prominent with disease progression. At P30 there was a significant decrease in excitatory axo-dendritic and axo-somatic synapses with an increase in C-type axo-somatic synapses. Our study examined early pathology in both peripheral and central neuromuscular system. The muscle denervation is associated with functional motor deficits and begins during the first postnatal month in SOD1(G93A) mice. Physiological dysfunction and pathology in the mitochondria of synapses and MN soma and dendrites occur, and disease onset in these animals begins more than 2 months earlier than originally thought. This information may be valuable for designing preclinical trials that are more likely to impact disease onset and progression.

Biased inheritance of mitochondria during asymmetric cell division in the mouse oocyte

A fundamental rule of cell division is that daughter cells inherit half the DNA complement and an appropriate proportion of cellular organelles. The highly asymmetric cell divisions of female meiosis present a different challenge because one of the daughters, the polar body, is destined to degenerate, putting at risk essential maternally inherited organelles such as mitochondria. We have therefore investigated mitochondrial inheritance during the meiotic divisions of the mouse oocyte. We find that mitochondria are aggregated around the spindle by a dynein-mediated mechanism during meiosis I, and migrate together with the spindle towards the oocyte cortex. However, at cell division they are not equally segregated and move instead towards the oocyte-directed spindle pole and are excluded from the polar body. We show that this asymmetrical inheritance in favour of the oocyte is not caused by bias in the spindle itself but is dependent on an intact actin cytoskeleton, spindle-cortex proximity, and cell cycle progression. Thus, oocyte-biased inheritance of mitochondria is a variation on rules that normally govern organelle segregation at cell division, and ensures that essential maternally inherited mitochondria are retained to provide ATP for early mammalian development.

Synaptic energy use and supply

Neuronal computation is energetically expensive. Consequently, the brain's limited energy supply imposes constraints on its information processing capability. Most brain energy is used on synaptic transmission, making it important to understand how energy is provided to and used by synapses. We describe how information transmission through presynaptic terminals and postsynaptic spines is related to their energy consumption, assess which mechanisms normally ensure an adequate supply of ATP to these structures, consider the influence of synaptic plasticity and changing brain state on synaptic energy use, and explain how disruption of the energy supply to synapses leads to neuropathology.

Motility of astrocytic mitochondria is arrested by Ca2+-dependent interaction between mitochondria and actin filaments

Motility of mitochondria, as well as their activity-dependent immobilization ("trapping"), is essential for neuronal function, but its regulation by cytoskeleton and relevance for glial cell signalling are unknown. Using time-lapse fluorescence imaging in rat cultured astrocytes, we evaluated the role of microtubules and actin filaments in motility of mitochondria in resting cells and during physiological or pathological Ca(2+) elevations. We found that mitochondria were significantly more aligned with microtubules than with actin filaments. Mitochondria were highly mobile under resting conditions at low intracellular free Ca(2+) concentrations ([Ca(2+)](i)). Activation of a moderate increase in [Ca(2+)](i) by either low-dose ionomycin or ATP immobilized mitochondria significantly but reversibly, without affecting mitochondrial morphology. A larger dose of ionomycin caused irreversible arrest and fragmentation of mitochondria. Disruption of microtubules completely arrested mitochondrial motility, while disruption of actin filaments had no effect on the basal mitochondrial motility at resting [Ca(2+)](i) levels but significantly reduced mitochondrial immobilization during [Ca(2+)](i) elevations. These results suggest that: (i) motility of astrocytic mitochondria is inversely related to [Ca(2+)](i), (ii) mitochondria require intact microtubules for their motility, and (iii) elevated [Ca(2+)](i) immobilizes mitochondria by strengthening their interaction with actin filaments.

Mitochondrial dynamics: the intersection of form and function

Mitochondria within a cell exist as a population in a dynamic -morphological continuum. The balance of mitochondrial fusion and fission dictates a spectrum of shapes from interconnected networks to fragmented individual units. This plasticity bestows the adaptive flexibility needed to adjust to changing cellular stresses and metabolic demands. The mechanisms that regulate mitochondrial dynamics, their importance in normal cell biology, and the roles they play in disease conditions are only beginning to be understood. Dysfunction of mitochondrial dynamics has been identified as a possible disease mechanism in Parkinson's disease. This chapter will introduce the budding field of mitochondrial dynamics and explore unique characteristics of affected neurons in Parkinson's disease that increase susceptibility to disruptions in mitochondrial dynamics.

BAD-dependent regulation of fuel metabolism and K(ATP) channel activity confers resistance to epileptic seizures

Neuronal excitation can be substantially modulated by alterations in metabolism, as evident from the anticonvulsant effect of diets that reduce glucose utilization and promote ketone body metabolism. We provide genetic evidence that BAD, a protein with dual functions in apoptosis and glucose metabolism, imparts reciprocal effects on metabolism of glucose and ketone bodies in brain cells. These effects involve phosphoregulation of BAD and are independent of its apoptotic function. BAD modifications that reduce glucose metabolism produce a marked increase in the activity of metabolically sensitive K(ATP) channels in neurons, as well as resistance to behavioral and electrographic seizures in vivo. Seizure resistance is reversed by genetic ablation of the K(ATP) channel, implicating the BAD-K(ATP) axis in metabolic control of neuronal excitation and seizure responses.