Mitochondrial movement and positioning in axons: the role of growth factor signaling

Spinal motor neuron development and metabolism are transcriptionally regulated by Nuclear Factor IA

Neural circuits governing all motor behaviors in vertebrates rely on the proper development of motor neurons and their precise targeting of limb muscles. Transcription factors are essential for motor neuron development, regulating their specification, migration, and axonal targeting. While transcriptional regulation of the early stages of motor neuron specification is well-established, much less is known about the role of transcription factors in the later stages of maturation and terminal arborization. Defining the molecular mechanisms of these later stages is critical for elucidating how motor circuits are constructed. Here, we demonstrate that the transcription factor Nuclear Factor-IA (NFIA) is required for motor neuron positioning, axonal branching, and neuromuscular junction formation. Moreover, we find that NFIA is required for proper mitochondrial function and ATP production, providing a new and important link between transcription factors and metabolism during motor neuron development. Together, these findings underscore the critical role of NFIA in instructing the assembly of spinal circuits for movement.

Micro-and mesoscale aspects of neurodegeneration in engineered human neural networks carrying the LRRK2 G2019S mutation

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene have been widely linked to Parkinson's disease, where the G2019S variant has been shown to contribute uniquely to both familial and sporadic forms of the disease. LRRK2-related mutations have been extensively studied, yet the wide variety of cellular and network events related to these mutations remain poorly understood. The advancement and availability of tools for neural engineering now enable modeling of selected pathological aspects of neurodegenerative disease in human neural networks in vitro. Our study revealed distinct pathology associated dynamics in engineered human cortical neural networks carrying the LRRK2 G2019S mutation compared to healthy isogenic control neural networks. The neurons carrying the LRRK2 G2019S mutation self-organized into networks with aberrant morphology and mitochondrial dynamics, affecting emerging structure-function relationships both at the micro-and mesoscale. Taken together, the findings of our study points toward an overall heightened metabolic demand in networks carrying the LRRK2 G2019S mutation, as well as a resilience to change in response to perturbation, compared to healthy isogenic controls.

Unique Role of Vimentin in the Intermediate Filament Proteins Family

Intermediate filaments (IFs), being traditionally the least studied component of the cytoskeleton, have begun to receive more attention in recent years. IFs are found in different cell types and are specific to them. Accumulated data have shifted the paradigm about the role of IFs as structures that merely provide mechanical strength to the cell. In addition to this role, IFs have been shown to participate in maintaining cell shape and strengthening cell adhesion. The data have also been obtained that point out to the role of IFs in a number of other biological processes, including organization of microtubules and microfilaments, regulation of nuclear structure and activity, cell cycle control, and regulation of signal transduction pathways. They are also actively involved in the regulation of several aspects of intracellular transport. Among the intermediate filament proteins, vimentin is of particular interest for researchers. Vimentin has been shown to be associated with a range of diseases, including cancer, cataracts, Crohn's disease, rheumatoid arthritis, and HIV. In this review, we focus almost exclusively on vimentin and the currently known functions of vimentin intermediate filaments (VIFs). This is due to the structural features of vimentin, biological functions of its domains, and its involvement in the regulation of a wide range of basic cellular functions, and its role in the development of human diseases. Particular attention in the review will be paid to comparing the role of VIFs with the role of intermediate filaments consisting of other proteins in cell physiology.

Mitochondrial trafficking as a protective mechanism against chemotherapy drug-induced peripheral neuropathy: Identifying the key site of action

AIMS

Chemotherapy induced peripheral neuropathy (CIPN) is a common side effect seen in patients who have undergone most chemotherapy treatments to which there are currently no treatment methods. CIPN has been shown to cause axonal degeneration leading to Peripheral Neuropathy (PN), which can lead to major dosage reduction and may prevent further chemotherapy treatment due to oftentimes debilitating pain. Previously, we have determined the site-specific action of Paclitaxel (PTX), a microtubule targeting agent, as well as the neuroprotective effect of Fluocinolone Acetonide (FA) against Paclitaxel Induced Peripheral Neuropathy (PIPN).

MAIN METHODS

Mitochondrial trafficking analysis was determined for all sample sets, wherein FA showed enhanced anterograde (axonal) mitochondrial trafficking leading to neuroprotective effects for all samples.

KEY FINDINGS

Using this system, we demonstrate that PTX, Monomethyl auristatin E (MMAE), and Vincristine (VCR), are toxic at clinically prescribed levels when treated focally to axons. However, Cisplatin (CDDP) was determined to have a higher toxicity when treated to cell bodies. Although having different targeting mechanisms, the administration of FA was determined to have a significant neuroprotective effect for against all chemotherapy drugs tested.

SIGNIFICANCE

This study identifies key insights regarding site of action and neuroprotective strategies to further development as potential therapeutics against CIPN. FA was treated alongside each chemotherapy drug to identify the neuroprotective effect against CIPN, where FA was found to be neuroprotective for all drugs tested. This study found that treatment with FA led to an enhancement in the anterograde movement of mitochondria based on fluorescent imaging.

Misregulation of mitochondria-lysosome contact dynamics in Charcot-Marie-Tooth Type 2B disease Rab7 mutant sensory peripheral neurons

Inter-organelle contact sites between mitochondria and lysosomes mediate the crosstalk and bidirectional regulation of their dynamics in health and disease. However, mitochondria-lysosome contact sites and their misregulation have not been investigated in peripheral sensory neurons. Charcot-Marie-Tooth type 2B disease is an autosomal dominant axonal neuropathy affecting peripheral sensory neurons caused by mutations in the GTPase Rab7. Using live super-resolution and confocal time-lapse microscopy, we showed that mitochondria-lysosome contact sites dynamically form in the soma and axons of peripheral sensory neurons. Interestingly, Charcot-Marie-Tooth type 2B mutant Rab7 led to prolonged mitochondria-lysosome contact site tethering preferentially in the axons of peripheral sensory neurons, due to impaired Rab7 GTP hydrolysis-mediated contact site untethering. We further generated a Charcot-Marie-Tooth type 2B mutant Rab7 knock-in mouse model which exhibited prolonged axonal mitochondria-lysosome contact site tethering and defective downstream axonal mitochondrial dynamics due to impaired Rab7 GTP hydrolysis as well as fragmented mitochondria in the axon of the sciatic nerve. Importantly, mutant Rab7 mice further demonstrated preferential sensory behavioral abnormalities and neuropathy, highlighting an important role for mutant Rab7 in driving degeneration of peripheral sensory neurons. Together, this study identifies an important role for mitochondria-lysosome contact sites in the pathogenesis of peripheral neuropathy.

Highly Specialized Mechanisms for Mitochondrial Transport in Neurons: From Intracellular Mobility to Intercellular Transfer of Mitochondria

The highly specialized structure and function of neurons depend on a sophisticated organization of the cytoskeleton, which supports a similarly sophisticated system to traffic organelles and cargo vesicles. Mitochondria sustain crucial functions by providing energy and buffering calcium where it is needed. Accordingly, the distribution of mitochondria is not even in neurons and is regulated by a dynamic balance between active transport and stable docking events. This system is finely tuned to respond to changes in environmental conditions and neuronal activity. In this review, we summarize the mechanisms by which mitochondria are selectively transported in different compartments, taking into account the structure of the cytoskeleton, the molecular motors and the metabolism of neurons. Remarkably, the motor proteins driving the mitochondrial transport in axons have been shown to also mediate their transfer between cells. This so-named intercellular transport of mitochondria is opening new exciting perspectives in the treatment of multiple diseases.

Mitochondria as central hubs in synaptic modulation

Mitochondria are present in the pre- and post-synaptic regions, providing the energy required for the activity of these very specialized neuronal compartments. Biogenesis of synaptic mitochondria takes place in the cell body, and these organelles are then transported to the synapse by motor proteins that carry their cargo along microtubule tracks. The transport of mitochondria along neurites is a highly regulated process, being modulated by the pattern of neuronal activity and by extracellular cues that interact with surface receptors. These signals act by controlling the distribution of mitochondria and by regulating their activity. Therefore, mitochondria activity at the synapse allows the integration of different signals and the organelles are important players in the response to synaptic stimulation. Herein we review the available evidence regarding the regulation of mitochondrial dynamics by neuronal activity and by neuromodulators, and how these changes in the activity of mitochondria affect synaptic communication.

Spatial and temporal dynamics of ATP synthase from mitochondria toward the cell surface

Ectopic ATP synthase complex (eATP synthase), located on cancer cell surface, has been reported to possess catalytic activity that facilitates the generation of ATP in the extracellular environment to establish a suitable microenvironment and to be a potential target for cancer therapy. However, the mechanism of intracellular ATP synthase complex transport remains unclear. Using a combination of spatial proteomics, interaction proteomics, and transcriptomics analyses, we find ATP synthase complex is first assembled in the mitochondria and subsequently delivered to the cell surface along the microtubule via the interplay of dynamin-related protein 1 (DRP1) and kinesin family member 5B (KIF5B). We further demonstrate that the mitochondrial membrane fuses to the plasma membrane in turn to anchor ATP syntheses on the cell surface using super-resolution imaging and real-time fusion assay in live cells. Our results provide a blueprint of eATP synthase trafficking and contribute to the understanding of the dynamics of tumor progression.

Inter and intracellular mitochondrial transfer: Future of mitochondrial transplant therapy in Parkinson's disease

Parkinson's disease (PD) is marked by the gradual degeneration of dopaminergic neurons and the intracellular build-up of Lewy bodies rich in α-synuclein protein. This impairs various aspects of the mitochondria including the generation of ROS, biogenesis, dynamics, mitophagy etc. Mitochondrial dynamics are regulated through the inter and intracellular movement which impairs mitochondrial trafficking within and between cells. This inter and intracellular mitochondrial movement plays a significant role in maintaining neuronal dynamics in terms of energy and growth. Kinesin, dynein, myosin, Mitochondrial rho GTPase (Miro), and TRAK facilitate the retrograde and anterograde movement of mitochondria. Enzymes such as Kinases along with Calcium (Ca²⁺), Adenosine triphosphate (ATP) and the genes PINK1 and Parkin are also involved. Extracellular vesicles, gap junctions, and tunneling nanotubes control intercellular movement. The knowledge and understanding of these proteins, enzymes, molecules, and movements have led to the development of mitochondrial transplant as a therapeutic approach for various disorders involving mitochondrial dysfunction such as stroke, ischemia and PD. A better understanding of these pathways plays a crucial role in establishing extracellular mitochondrial transplant therapy for reverting the pathology of PD. Currently, techniques such as mitochondrial coculture, mitopunch and mitoception are being utilized in the pre-clinical stages and should be further explored for translational value. This review highlights how intercellular and intracellular mitochondrial dynamics are affected during mitochondrial dysfunction in PD. The field of mitochondrial transplant therapy in PD is underlined in particular due to recent developments and the potential that it holds in the near future.

Miro GTPase domains regulate the assembly of the mitochondrial motor-adaptor complex

Mitochondrial transport relies on a motor-adaptor complex containing Miro1, a mitochondrial outer membrane protein with two GTPase domains, and TRAK1/2, kinesin-1, and dynein. Using a peroxisome-directed Miro1, we quantified the ability of GTPase mutations to influence the peroxisomal recruitment of complex components. Miro1 whose N-GTPase is locked in the GDP state does not recruit TRAK1/2, kinesin, or P135 to peroxisomes, whereas the GTP state does. Similarly, the expression of the MiroGAP VopE dislodges TRAK1 from mitochondria. Miro1 C-GTPase mutations have little influence on complex recruitment. Although Miro2 is thought to support mitochondrial motility, peroxisome-directed Miro2 did not recruit the other complex components regardless of the state of its GTPase domains. Neurons expressing peroxisomal Miro1 with the GTP-state form of the N-GTPase had markedly increased peroxisomal transport to growth cones, whereas the GDP-state caused their retention in the soma. Thus, the N-GTPase domain of Miro1 is critical for regulating Miro1's interaction with the other components of the motor-adaptor complex and thereby for regulating mitochondrial motility.

Mitochondrial heterogeneity and homeostasis through the lens of a neuron

Mitochondria are vital organelles with distinct morphological features and functional properties. The dynamic network of mitochondria undergoes structural and functional adaptations in response to cell-type-specific metabolic demands. Even within the same cell, mitochondria can display wide diversity and separate into functionally distinct subpopulations. Mitochondrial heterogeneity supports unique subcellular functions and is crucial to polarized cells, such as neurons. The spatiotemporal metabolic burden within the complex shape of a neuron requires precisely localized mitochondria. By travelling great lengths throughout neurons and experiencing bouts of immobility, mitochondria meet distant local fuel demands. Understanding mitochondrial heterogeneity and homeostasis mechanisms in neurons provides a framework to probe their significance to many other cell types. Here, we put forth an outline of the multifaceted role of mitochondria in regulating neuronal physiology and cellular functions more broadly.

Mitochondrial calcium uniporter deletion prevents painful diabetic neuropathy by restoring mitochondrial morphology and dynamics

ABSTRACT

Painful diabetic neuropathy (PDN) is an intractable complication affecting 25% of diabetic patients. Painful diabetic neuropathy is characterized by neuropathic pain accompanied by dorsal root ganglion (DRG) nociceptor hyperexcitability, resulting in calcium overload, axonal degeneration, and loss of cutaneous innervation. The molecular pathways underlying these effects are unknown. Using high-throughput and deep-proteome profiling, we found that mitochondrial fission proteins were elevated in DRG neurons from mice with PDN induced by a high-fat diet (HFD). In vivo calcium imaging revealed increased calcium signaling in DRG nociceptors from mice with PDN. Furthermore, using electron microscopy, we showed that mitochondria in DRG nociceptors had fragmented morphology as early as 2 weeks after starting HFD, preceding the onset of mechanical allodynia and small-fiber degeneration. Moreover, preventing calcium entry into the mitochondria, by selectively deleting the mitochondrial calcium uniporter from these neurons, restored normal mitochondrial morphology, prevented axonal degeneration, and reversed mechanical allodynia in the HFD mouse model of PDN. These studies suggest a molecular cascade linking neuropathic pain to axonal degeneration in PDN. In particular, nociceptor hyperexcitability and the associated increased intracellular calcium concentrations could lead to excessive calcium entry into mitochondria mediated by the mitochondrial calcium uniporter, resulting in increased calcium-dependent mitochondrial fission and ultimately contributing to small-fiber degeneration and neuropathic pain in PDN. Hence, we propose that targeting calcium entry into nociceptor mitochondria may represent a promising effective and disease-modifying therapeutic approach for this currently intractable and widespread affliction. Moreover, these results are likely to inform studies of other neurodegenerative disease involving similar underlying events.

Mitochondria, energy, and metabolism in neuronal health and disease

Mitochondria are associated with various cellular activities critical to homeostasis, particularly in the nervous system. The plastic architecture of the mitochondrial network and its dynamic structure play crucial roles in ensuring that varying energetic demands are rapidly met to maintain neuronal and axonal energy homeostasis. Recent evidence associates ageing and neurodegeneration with anomalous neuronal metabolism, as age-dependent alterations of neuronal metabolism are now believed to occur prior to neurodegeneration. The brain has a high energy demand, which makes it particularly sensitive to mitochondrial dysfunction. Distinct cellular events causing oxidative stress or disruption of metabolism and mitochondrial homeostasis can trigger a neuropathology. This review explores the bioenergetic hypothesis for the neurodegenerative pathomechanisms, discussing factors leading to age-related brain hypometabolism and its contribution to cognitive decline. Recent research on the mitochondrial network in healthy nervous system cells, its response to stress and how it is affected by pathology, as well as current contributions to novel therapeutic approaches will be highlighted.

Serine/Threonine Protein Phosphatase 2A Regulates the Transport of Axonal Mitochondria

Microtubule-based transport provides mitochondria to distant regions of neurons and is essential for neuronal health. To identify compounds that increase mitochondrial motility, we screened 1,641 small-molecules in a high-throughput screening platform. Indirubin and cantharidin increased mitochondrial motility in rat cortical neurons. Cantharidin is known to inhibit protein phosphatase 2A (PP2A). We therefore tested two other inhibitors of PP2A: LB-100 and okadaic acid. LB-100 increased mitochondrial motility, but okadaic acid did not. To resolve this discrepancy, we knocked down expression of the catalytic subunit of PP2A (PP2CA). This long-term inhibition of PP2A more than doubled retrograde transport of axonal mitochondria, confirming the importance of PP2A as a regulator of mitochondrial motility and as the likely mediator of cantharidin's effect.

Miro1-dependent mitochondrial dynamics in parvalbumin interneurons

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

Stressed mitochondria: A target to intrude alzheimer's disease

Alzheimer's disease (AD) is the inoperable, incapacitating, neuropsychiatric, and degenerative manifestation that drastically affects human life quality. The current medications target extra-neuronal senile plaques, oxidative stress, neuroinflammation, intraneuronal neurofibrillary tangles, cholinergic deficits, and excitotoxicity. Among novel pathways and targets, bioenergetic and resultant mitochondrial dysfunction has been recognized as essential factors that decide the neuronal fate and consequent neurodegeneration in AD. The crucial attributes of mitochondria, including bioenergesis, signaling, sensing, integrating, and transmitting biological signals contribute to optimum networking of neuronal dynamics and make them indispensable for cell survival. In AD, mitochondrial dysfunction and mitophagy are a preliminary and critical event that aggravates the pathological cascade. Stress is known to promote and exaggerate the neuropathological alteration during neurodegeneration and metabolic impairments, especially in the cortico-limbic system, besides adversely affecting the normal physiology and mitochondrial dynamics. Stress involves the allocation of energy resources for neuronal survival. Chronic and aggravated stress response leads to excessive release of glucocorticoids by activation of the hypothalamic-pituitaryadrenal (HPA) axis. By acting through their receptors, glucocorticoids influence adverse mitochondrial changes and alter mtDNA transcription, mtRNA expression, hippocampal mitochondrial network, and ultimately mitochondrial physiology. Chronic stress also affects mitochondrial dynamics by changing metabolic and neuro-endocrinal signalling, aggravating oxidative stress, provoking inflammatory mediators, altering tropic factors, influencing gene expression, and modifying epigenetic pathways. Thus, exploring chronic stress-induced glucocorticoid dysregulation and resultant bio-behavioral and psychosomatic mitochondrial alterations may be a feasible narrative to investigate and unravel the mysterious pathobiology of AD.

Mitochondrial transfer from mesenchymal stem cells improves neuronal metabolism after oxidant injury in vitro: The role of Miro1

Stroke-induced cerebral ischemia is a major cause of death and disability. The disruption of blood flow results in neuronal and glial cell death leading to brain injury. Reperfusion restores oxygen to the affected tissue, but can also cause damage through an enhanced oxidative stress and inflammatory response. This study examines mitochondrial transfer from MSC to neurons and the role it plays in neuronal preservation after oxidant injury. We observed the transfer of mitochondria from MSC to mouse neurons in vitro following hydrogen peroxide exposure. The observed transfer was dependent on cell-to-cell contact and led to increased neuronal survival and improved metabolism. A number of pro-inflammatory and mitochondrial motility genes were upregulated in neurons after hydrogen peroxide exposure. This included Miro1 and TNFAIP2, linking inflammation and mitochondrial transfer to oxidant injury. Increasing Miro1 expression in MSC improved the metabolic benefit of mitochondrial transfer after neuronal oxidant injury. Decreasing Miro1 expression had the opposite effect, decreasing the metabolic benefit of MSC co-culture. MSC transfer of mitochondria to oxidant-damaged neurons may help improve neuronal preservation and functional recovery after stroke.

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

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

The Functional Impact of Mitochondrial Structure Across Subcellular Scales

Mitochondria are key determinants of cellular health. However, the functional role of mitochondria varies from cell to cell depending on the relative demands for energy distribution, metabolite biosynthesis, and/or signaling. In order to support the specific needs of different cell types, mitochondrial functional capacity can be optimized in part by modulating mitochondrial structure across several different spatial scales. Here we discuss the functional implications of altering mitochondrial structure with an emphasis on the physiological trade-offs associated with different mitochondrial configurations. Within a mitochondrion, increasing the amount of cristae in the inner membrane improves capacity for energy conversion and free radical-mediated signaling but may come at the expense of matrix space where enzymes critical for metabolite biosynthesis and signaling reside. Electrically isolating individual cristae could provide a protective mechanism to limit the spread of dysfunction within a mitochondrion but may also slow the response time to an increase in cellular energy demand. For individual mitochondria, those with relatively greater surface areas can facilitate interactions with the cytosol or other organelles but may be more costly to remove through mitophagy due to the need for larger phagophore membranes. At the network scale, a large, stable mitochondrial reticulum can provide a structural pathway for energy distribution and communication across long distances yet also enable rapid spreading of localized dysfunction. Highly dynamic mitochondrial networks allow for frequent content mixing and communication but require constant cellular remodeling to accommodate the movement of mitochondria. The formation of contact sites between mitochondria and several other organelles provides a mechanism for specialized communication and direct content transfer between organelles. However, increasing the number of contact sites between mitochondria and any given organelle reduces the mitochondrial surface area available for contact sites with other organelles as well as for metabolite exchange with cytosol. Though the precise mechanisms guiding the coordinated multi-scale mitochondrial configurations observed in different cell types have yet to be elucidated, it is clear that mitochondrial structure is tailored at every level to optimize mitochondrial function to meet specific cellular demands.

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

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

A High-Content Screen Identifies TPP1 and Aurora B as Regulators of Axonal Mitochondrial Transport

Dysregulated axonal trafficking of mitochondria is linked to neurodegenerative disorders. We report a high-content screen for small-molecule regulators of the axonal transport of mitochondria. Six compounds enhanced mitochondrial transport in the sub-micromolar range, acting via three cellular targets: F-actin, Tripeptidyl peptidase 1 (TPP1), or Aurora Kinase B (AurKB). Pharmacological inhibition or small hairpin RNA (shRNA) knockdown of each target promotes mitochondrial axonal transport in rat hippocampal neurons and induced pluripotent stem cell (iPSC)-derived human cortical neurons and enhances mitochondrial transport in iPSC-derived motor neurons from an amyotrophic lateral sclerosis (ALS) patient bearing one copy of SOD1^A4V mutation. Our work identifies druggable regulators of axonal transport of mitochondria, provides broadly applicable methods for similar image-based screens, and suggests that restoration of proper axonal trafficking of mitochondria can be achieved in human ALS neurons.

Shlevkov et al. establish a high-content screen for enhancers of axonal mitochondrial trafficking. Identified compounds act through three cellular targets: F-Actin, Tripeptidyl peptidase 1, and Aurora Kinase B. Motor neurons derived from a SOD1^+/A4VALS patient have decreased mitochondrial motility, which can be reversed by inhibitors of these targets.

Cofilin dysregulation alters actin turnover in frataxin-deficient neurons

Abnormalities in actin cytoskeleton have been linked to Friedreich's ataxia (FRDA), an inherited peripheral neuropathy characterised by an early loss of neurons in dorsal root ganglia (DRG) among other clinical symptoms. Despite all efforts to date, we still do not fully understand the molecular events that contribute to the lack of sensory neurons in FRDA. We studied the adult neuronal growth cone (GC) at the cellular and molecular level to decipher the connection between frataxin and actin cytoskeleton in DRG neurons of the well-characterised YG8R Friedreich's ataxia mouse model. Immunofluorescence studies in primary cultures of DRG from YG8R mice showed neurons with fewer and smaller GCs than controls, associated with an inhibition of neurite growth. In frataxin-deficient neurons, we also observed an increase in the filamentous (F)-actin/monomeric (G)-actin ratio (F/G-actin ratio) in axons and GCs linked to dysregulation of two crucial modulators of filamentous actin turnover, cofilin-1 and the actin-related protein (ARP) 2/3 complex. We show how the activation of cofilin is due to the increase in chronophin (CIN), a cofilin-activating phosphatase. Thus cofilin emerges, for the first time, as a link between frataxin deficiency and actin cytoskeleton alterations.

Drp1 overexpression induces desmin disassembling and drives kinesin-1 activation promoting mitochondrial trafficking in skeletal muscle

Mitochondria change distribution across cells following a variety of pathophysiological stimuli. The mechanisms presiding over this redistribution are yet undefined. In a murine model overexpressing Drp1 specifically in skeletal muscle, we find marked mitochondria repositioning in muscle fibres and we demonstrate that Drp1 is involved in this process. Drp1 binds KLC1 and enhances microtubule-dependent transport of mitochondria. Drp1-KLC1 coupling triggers the displacement of KIF5B from kinesin-1 complex increasing its binding to microtubule tracks and mitochondrial transport. High levels of Drp1 exacerbate this mechanism leading to the repositioning of mitochondria closer to nuclei. The reduction of Drp1 levels decreases kinesin-1 activation and induces the partial recovery of mitochondrial distribution. Drp1 overexpression is also associated with higher cyclin-dependent kinase-1 (Cdk-1) activation that promotes the persistent phosphorylation of desmin at Ser-31 and its disassembling. Fission inhibition has a positive effect on desmin Ser-31 phosphorylation, regardless of Cdk-1 activation, suggesting that induction of both fission and Cdk-1 are required for desmin collapse. This altered desmin architecture impairs mechanotransduction and compromises mitochondrial network stability priming mitochondria transport through microtubule-dependent trafficking with a mechanism that involves the Drp1-dependent regulation of kinesin-1 complex.

Poly(ADP-Ribose) Polymerase-3 Regulates Regeneration in Planarians

Protein ADP-ribosylation is a reversible post-translational modification (PTM) process that plays fundamental roles in cell signaling. The covalent attachment of ADP ribose polymers is executed by PAR polymerases (PARP) and it is essential for chromatin organization, DNA repair, cell cycle, transcription, and replication, among other critical cellular events. The process of PARylation or polyADP-ribosylation is dynamic and takes place across many tissues undergoing renewal and repair, but the molecular mechanisms regulating this PTM remain mostly unknown. Here, we introduce the use of the planarian Schmidtea mediterranea as a tractable model to study PARylation in the complexity of the adult body that is under constant renewal and is capable of regenerating damaged tissues. We identified the evolutionary conservation of PARP signaling that is expressed in planarian stem cells and differentiated tissues. We also demonstrate that Smed-PARP-3 homolog is required for proper regeneration of tissues in the anterior region of the animal. Furthermore, our results demonstrate, Smed-PARP-3(RNAi) disrupts the timely location of injury-induced cell death near the anterior facing wounds and also affects the regeneration of the central nervous system. Our work reveals novel roles for PARylation in large-scale regeneration and provides a simplified platform to investigate PARP signaling in the complexity of the adult body.

Crosstalk between Mitochondria and Cytoskeleton in Cardiac Cells

Elucidation of the mitochondrial regulatory mechanisms for the understanding of muscle bioenergetics and the role of mitochondria is a fundamental problem in cellular physiology and pathophysiology. The cytoskeleton (microtubules, intermediate filaments, microfilaments) plays a central role in the maintenance of mitochondrial shape, location, and motility. In addition, numerous interactions between cytoskeletal proteins and mitochondria can actively participate in the regulation of mitochondrial respiration and oxidative phosphorylation. In cardiac and skeletal muscles, mitochondrial positions are tightly fixed, providing their regular arrangement and numerous interactions with other cellular structures such as sarcoplasmic reticulum and cytoskeleton. This can involve association of cytoskeletal proteins with voltage-dependent anion channel (VDAC), thereby, governing the permeability of the outer mitochondrial membrane (OMM) to metabolites, and regulating cell energy metabolism. Cardiomyocytes and myocardial fibers demonstrate regular arrangement of tubulin beta-II isoform entirely co-localized with mitochondria, in contrast to other isoforms of tubulin. This observation suggests the participation of tubulin beta-II in the regulation of OMM permeability through interaction with VDAC. The OMM permeability is also regulated by the specific isoform of cytolinker protein plectin. This review summarizes and discusses previous studies on the role of cytoskeletal proteins in the regulation of energy metabolism and mitochondrial function, adenosine triphosphate (ATP) production, and energy transfer.

MTP18 is a Novel Regulator of Mitochondrial Fission in CNS Neuron Development, Axonal Growth, and Injury Responses

The process of mitochondrial fission-fusion has been implicated in diverse neuronal roles including neuronal survival, axon degeneration, and axon regeneration. However, whether increased fission or fusion is beneficial for neuronal health and/or axonal growth is not entirely clear, and is likely situational and cell type-dependent. In searching for mitochondrial fission-fusion regulating proteins for improving axonal growth within the visual system, we uncover that mitochondrial fission process 1,18 kDa (MTP18/MTFP1), a pro-fission protein within the CNS, is critical to maintaining mitochondrial size and volume under normal and injury conditions, in retinal ganglion cells (RGCs). We demonstrate that MTP18’s expression is regulated by transcription factors involved in axonal growth, Kruppel-like factor (KLF) transcription factors-7 and -9, and that knockdown of MTP18 promotes axon growth. This investigation exposes MTP18’s previously unexplored role in regulating mitochondrial fission, implicates MTP18 as a downstream component of axon regenerative signaling, and ultimately lays the groundwork for investigations on the therapeutic efficacy of MTP18 expression suppression during CNS axon degenerative events.

Mitochondrial function and abnormalities implicated in the pathogenesis of ASD

Mitochondria are the powerhouse that generate over 90% of the ATP produced in cells. In addition to its role in energy production, the mitochondrion also plays a major role in carbohydrate, fatty acid, amino acid and nucleotide metabolism, programmed cell death (apoptosis), generation of and protection against reactive oxygen species (ROS), immune response, regulation of intracellular calcium ion levels and even maintenance of gut microbiota. With its essential role in bio-energetic as well as non-energetic biological processes, it is not surprising that proper cellular, tissue and organ function is dependent upon proper mitochondrial function. Accordingly, mitochondrial dysfunction has been shown to be directly linked to a variety of medical disorders, particularly neuromuscular disorders and increasing evidence has linked mitochondrial dysfunction to neurodegenerative and neurodevelopmental disorders such as Alzheimer's Disease (AD), Parkinson's Disease (PD), Rett Syndrome (RS) and Autism Spectrum Disorders (ASD). Over the last 40 years there has been a dramatic increase in the diagnosis of ASD and, more recently, an increasing body of evidence indicates that mitochondrial dysfunction plays an important role in ASD development. In this review, the latest evidence linking mitochondrial dysfunction and abnormalities in mitochondrial DNA (mtDNA) to the pathogenesis of autism will be presented. This review will also summarize the results of several recent `approaches used for improving mitochondrial function that may lead to new therapeutic approaches to managing and/or treating ASD.

Differential effect of amyloid beta peptides on mitochondrial axonal trafficking depends on their state of aggregation and binding to the plasma membrane

Inhibition of mitochondrial axonal trafficking by amyloid beta (Aβ) peptides has been implicated in early pathophysiology of Alzheimer's Disease (AD). Yet, it remains unclear whether the loss of motility inevitably induces the loss of mitochondrial function, and whether restoration of axonal trafficking represents a valid therapeutic target. Moreover, while some investigations identify Aβ oligomers as the culprit of trafficking inhibition, others propose that fibrils play the detrimental role. We have examined the effect of a panel of Aβ peptides with different mutations found in familial AD on mitochondrial motility in primary cortical mouse neurons. Peptides with higher propensity to aggregate inhibit mitochondrial trafficking to a greater extent with fibrils inducing the strongest inhibition. Binding of Aβ peptides to the plasma membrane was sufficient to induce trafficking inhibition where peptides with reduced plasma membrane binding and internalization had lesser effect on mitochondrial motility. We also found that Aβ peptide with Icelandic mutation A673T affects axonal trafficking of mitochondria but has very low rates of plasma membrane binding and internalization in neurons, which could explain its relatively low toxicity. Inhibition of mitochondrial dynamics caused by Aβ peptides or fibrils did not instantly affect mitochondrial bioenergetic and function. Our results support a mechanism where inhibition of axonal trafficking is initiated at the plasma membrane by soluble low molecular weight Aβ species and is exacerbated by fibrils. Since trafficking inhibition does not coincide with the loss of mitochondrial function, restoration of axonal transport could be beneficial at early stages of AD progression. However, strategies designed to block Aβ aggregation or fibril formation alone without ensuring the efficient clearance of soluble Aβ may not be sufficient to alleviate the trafficking phenotype.

MCU Interacts with Miro1 to Modulate Mitochondrial Functions in Neurons

Mitochondrial Ca²⁺ uptake is gated by the mitochondrial calcium uniplex, which is comprised of mitochondrial calcium uniporter (MCU), the Ca²⁺ pore-forming subunit of the complex, and its regulators. Ca²⁺ influx through MCU affects both mitochondrial function and movement in neurons, but its direct role in mitochondrial movement has not been explored. In this report, we show a link between MCU and Miro1, a membrane protein known to regulate mitochondrial movement. We find that MCU interacts with Miro1 through MCU's N-terminal domain, previously thought to be the mitochondrial targeting sequence. Our results show that the N-terminus of MCU has a transmembrane domain that traverses the outer mitochondrial membrane, which is dispensable for MCU localization into mitochondria. However, this domain is required for Miro1 interaction and is critical for Miro1 directed movement. Together, our findings reveal Miro1 as a new component of the MCU complex, and that MCU is an important regulator of mitochondrial transport.SIGNIFICANCE STATEMENT Mitochondrial calcium level is critical for mitochondrial metabolic activity and mitochondrial transport in neurons. While it has been established that calcium influx into mitochondria is modulated by mitochondrial calcium uniporter (MCU) complex, how MCU regulates mitochondrial movement still remains unclear. Here, we discover that the N-terminus of MCU plays a different role than previously thought; it is not required for mitochondrial targeting but is essential for interaction with Miro1, an outer mitochondrial membrane protein important for mitochondrial movement. Furthermore, we show that MCU-Miro1 interaction is required to maintain mitochondrial transport. Our data identify that Miro1 is a novel component of the mitochondrial calcium uniplex and demonstrate that coupling between MCU and Miro1 as a novel mechanism modulating both mitochondrial Ca²⁺ uptake and mitochondrial transport.

Spatial Cytoskeleton Organization Supports Targeted Intracellular Transport

The efficiency of intracellular cargo transport from specific sources to target locations is strongly dependent upon molecular motor-assisted motion along the cytoskeleton. Radial transport along microtubules and lateral transport along the filaments of the actin cortex underneath the cell membrane are characteristic for cells with a centrosome. The interplay between the specific cytoskeleton organization and the motor performance results in a spatially inhomogeneous intermittent search strategy. To analyze the efficiency of such intracellular search strategies, we formulate a random velocity model with intermittent arrest states. We evaluate efficiency in terms of mean first passage times for three different, frequently encountered intracellular transport tasks: 1) the narrow escape problem, which emerges during cargo transport to a synapse or other specific region of the cell membrane; 2) the reaction problem, which considers the binding time of two particles within the cell; and 3) the reaction-escape problem, which arises when cargo must be released at a synapse only after pairing with another particle. Our results indicate that cells are able to realize efficient search strategies for various intracellular transport tasks economically through a spatial cytoskeleton organization that involves only a narrow actin cortex rather than a cell body filled with randomly oriented actin filaments.

Transitional correlation between inner-membrane potential and ATP levels of neuronal mitochondria

The importance of highly active mitochondria and their contribution to neuronal function has been of recent interest. In most cases, however, mitochondrial activity is estimated using measurements of mitochondrial inner membrane potential (IMP_mito), and little is known about the dynamics of native mitochondrial ATP (ATP_mito). This study conducted simultaneous imaging of IMP_mito and ATP_mito in neurons to explore their behaviour and their correlation during physiological mitochondrial/neuronal activity. We found that mitochondrial size, transport velocity and transport direction are not dependent on ATP_mito or IMP_mito. However, changes in ATP_mito and IMP_mito during mitochondrial fission/fusion were found; IMP_mito depolarized via mitochondrial fission and hyperpolarized via fusion, and ATP_mito levels increased after fusion. Because the density of mitochondria is higher in growth cones (GCs) than in axonal processes, integrated ATP_mito signals (density × ATP_mito) were higher in GCs. This integrated signal in GCs correlated with axonal elongation. However, while the averaged IMP_mito was relatively hyperpolarized in GCs, there was no correlation between IMP_mito in GCs and axonal elongation. A detailed time-course analysis performed to clarify the reason for these discrepancies showed that IMP_mito and ATP_mito levels did not always correlate accurately; rather, there were several correlation patterns that changed over time.

Miro proteins coordinate microtubule- and actin-dependent mitochondrial transport and distribution

In the current model of mitochondrial trafficking, Miro1 and Miro2 Rho-GTPases regulate mitochondrial transport along microtubules by linking mitochondria to kinesin and dynein motors. By generating Miro1/2 double-knockout mouse embryos and single- and double-knockout embryonic fibroblasts, we demonstrate the essential and non-redundant roles of Miro proteins for embryonic development and subcellular mitochondrial distribution. Unexpectedly, the TRAK1 and TRAK2 motor protein adaptors can still localise to the outer mitochondrial membrane to drive anterograde mitochondrial motility in Miro1/2 double-knockout cells. In contrast, we show that TRAK2-mediated retrograde mitochondrial transport is Miro1-dependent. Interestingly, we find that Miro is critical for recruiting and stabilising the mitochondrial myosin Myo19 on the mitochondria for coupling mitochondria to the actin cytoskeleton. Moreover, Miro depletion during PINK1/Parkin-dependent mitophagy can also drive a loss of mitochondrial Myo19 upon mitochondrial damage. Finally, aberrant positioning of mitochondria in Miro1/2 double-knockout cells leads to disruption of correct mitochondrial segregation during mitosis. Thus, Miro proteins can fine-tune actin- and tubulin-dependent mitochondrial motility and positioning, to regulate key cellular functions such as cell proliferation.

NGF protects endothelial cells from indomethacin-induced injury through activation of mitochondria and upregulation of IGF-1

BACKGROUND/AIMS

Endothelial cells (ECs) lining blood vessels are critical for delivery of oxygen and nutrients to all tissues and organs and play a crucial role in the regeneration of blood vessel following tissue injury. ECs are also major targets of injury by a variety of noxious factors [e.g., ethanol and nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin, indomethacin, diclofenac], especially in gastric mucosa that has direct exposure to these agents. In this study, we investigated whether nerve growth factor (NGF) can protect gastric microvascular ECs (GECs) from injury by indomethacin (INDO) and the mechanisms involved.

METHODS

GECs were isolated from rat gastric mucosa and pre-treated with either vehicle or NGF (100ng/ml) for 30min to 4h followed by treatment with vehicle or 0.25mM INDO for 4h.

STUDIES

1) cell viability using Calcein AM live cell tracking dye, 2) mitochondrial structure and function using MitoTracker, molecular probe that stains mitochondria in live cells in a manner dependent on mitochondrial membrane potential (MMP), 3) in vitro angiogenesis - endothelial tube formation on Matrigel, 4) expression and subcellular localization of NGF receptor, TrkA, and 5) expression of IGF-1 protein.

RESULTS

Treatment with INDO reduced GEC viability and in vitro angiogenesis and induced mitochondrial injury and MMP depolarization. NGF pre-treatment protected GECs from INDO-induced injury preventing both INDO-induced MMP depolarization and reduced in vitro angiogenesis. The NGF high affinity receptor, TrkA, was localized in GECs to both cell membrane and mitochondria. NGF treatment of GECs also resulted in increased IGF-1 protein expression.

CONCLUSIONS

1) NGF protects GECs against IND-induced injury. 2) Mitochondria are major targets of both INDO-induced injury and NGF afforded protection of GECs. 3) TrkA expression in the mitochondria of GECs indicates that the protection afforded by NGF is partly mediated by its direct action on mitochondria. 4) NGF prevents MMP depolarization and increases expression of IGF-1 protein in GECs. These studies indicate that NGF may play a protective role against injury to GECs; and, that maintenance of mitochondrial structure and function is one of the mechanisms.

Crosstalk of cell polarity signaling pathways

Cell polarity, the asymmetric organization of cellular components along one or multiple axes, is present in most cells. From budding yeast cell polarization induced by pheromone signaling, oocyte polarization at fertilization to polarized epithelia and neuronal cells in multicellular organisms, similar mechanisms are used to determine cell polarity. Crucial role in this process is played by signaling lipid molecules, small Rho family GTPases and Par proteins. All these signaling circuits finally govern the cytoskeleton, which is responsible for oriented cell migration, cell shape changes, and polarized membrane and organelle trafficking. Thus, typically in the process of cell polarization, most cellular constituents become polarized, including plasma membrane lipid composition, ion concentrations, membrane receptors, and proteins in general, mRNA, vesicle trafficking, or intracellular organelles. This review gives a brief overview how these systems talk to each other both during initial symmetry breaking and within the signaling feedback loop mechanisms used to preserve the polarized state.

Live Imaging Mitochondrial Transport in Neurons

Mitochondria are among a cell's most vital organelles. They not only produce the majority of the cell's ATP but also play a key role in Ca²⁺ buffering and apoptotic signaling. While proper allocation of mitochondria is critical to all cells, it is particularly important for the highly polarized neurons. Because mitochondria are mainly synthesized in the soma, they must be transported long distances to be distributed to the far-flung reaches of the neuron-up to 1 m in the case of some human motor neurons. Furthermore, damaged mitochondria can be detrimental to neuronal health, causing oxidative stress and even cell death, therefore the retrograde transport of damaged mitochondria back to the soma for proper disposal, as well as the anterograde transport of fresh mitochondria from the soma to repair damage, are equally critical. Intriguingly, errors in mitochondrial transport have been increasingly implicated in neurological disorders. Here, we describe how to investigate mitochondrial transport in three complementary neuronal systems: cultured induced pluripotent stem cell-derived neurons, cultured rat hippocampal and cortical neurons, and Drosophila larval neurons in vivo. These models allow us to uncover the molecular and cellular mechanisms underlying transport issues that may occur under physiological or pathological conditions.

Spatial organization of the cytoskeleton enhances cargo delivery to specific target areas on the plasma membrane of spherical cells

Intracellular transport is vital for the proper functioning and survival of a cell. Cargo (proteins, vesicles, organelles, etc) is transferred from its place of creation to its target locations via molecular motor assisted transport along cytoskeletal filaments. The transport efficiency is strongly affected by the spatial organization of the cytoskeleton, which constitutes an inhomogeneous, complex network. In cells with a centrosome microtubules grow radially from the central microtubule organizing center towards the cell periphery whereas actin filaments form a dense meshwork, the actin cortex, underneath the cell membrane with a broad range of orientations. The emerging ballistic motion along filaments is frequently interrupted due to constricting intersection nodes or cycles of detachment and reattachment processes in the crowded cytoplasm. In order to investigate the efficiency of search strategies established by the cell's specific spatial organization of the cytoskeleton we formulate a random velocity model with intermittent arrest states. With extensive computer simulations we analyze the dependence of the mean first passage times for narrow escape problems on the structural characteristics of the cytoskeleton, the motor properties and the fraction of time spent in each state. We find that an inhomogeneous architecture with a small width of the actin cortex constitutes an efficient intracellular search strategy.

Quercetin Exerts Differential Neuroprotective Effects Against H2O2 and Aβ Aggregates in Hippocampal Neurons: the Role of Mitochondria

Amyloid-β peptide (Aβ) is one of the major players in the pathogenesis of Alzheimer's disease (AD). Despite numerous studies, the mechanisms by which Aβ induces neurodegeneration are not completely understood. Oxidative stress is considered a major contributor to the pathogenesis of AD, and accumulating evidence indicates that high levels of reactive oxygen species (ROS) are involved in Aβ-induced neurodegeneration. Moreover, Aβ can induce the deregulation of calcium homeostasis, which also affects mitochondrial function and triggers neuronal cell death. In the present study, we analyzed the effects of quercetin, a plant flavonoid with antioxidant properties, on oxidative stress- and Aβ-induced degeneration. Our results indicate that quercetin efficiently protected against H₂O₂-induced neuronal toxicity; however, this protection was only partial in rat hippocampal neurons that were treated with Aβ. Treatment with quercetin decreased ROS levels, recovered the normal morphology of mitochondria, and prevented mitochondrial dysfunction in neurons that were treated with H₂O₂. By contrast, quercetin treatment partially rescued hippocampal neurons from Aβ-induced mitochondrial injury. Most importantly, quercetin treatment prevented the toxic effects that are induced by H₂O₂ in hippocampal neurons and, to a lesser extent, the Aβ-induced toxicity that is associated with the superoxide anion, which is a precursor of ROS production in mitochondria. Collectively, these results indicate that quercetin exerts differential effects on the prevention of H₂O₂- and Aβ-induced neurotoxicity in hippocampal neurons and may be a powerful tool for dissecting the molecular mechanisms underlying Aβ neurotoxicity.

Imaging of neuronal mitochondria in situ

Neuronal mitochondria are receiving a rapidly increasing level of attention. This is to a significant part due to the ability to visualize neuronal mitochondria in novel ways, especially in vivo. Such an approach allows studying neuronal mitochondria in an intact tissue context, during different developmental states and in various genetic backgrounds and disease conditions. Hence, in vivo imaging of mitochondria in the nervous system can reveal aspects of the 'mitochondrial life cycle' in neurons that hitherto have remained obscure or could only be inferred indirectly. In this survey of the current literature, we review the new insights that have emerged from studies using mitochondrial imaging in intact neural preparations ranging from worms to mice.

Transporting mitochondria in neurons

Neurons demand vast and vacillating supplies of energy. As the key contributors of this energy, as well as primary pools of calcium and signaling molecules, mitochondria must be where the neuron needs them, when the neuron needs them. The unique architecture and length of neurons, however, make them a complex system for mitochondria to navigate. To add to this difficulty, mitochondria are synthesized mainly in the soma, but must be transported as far as the distant terminals of the neuron. Similarly, damaged mitochondria-which can cause oxidative stress to the neuron-must fuse with healthy mitochondria to repair the damage, return all the way back to the soma for disposal, or be eliminated at the terminals. Increasing evidence suggests that the improper distribution of mitochondria in neurons can lead to neurodegenerative and neuropsychiatric disorders. Here, we will discuss the machinery and regulatory systems used to properly distribute mitochondria in neurons, and how this knowledge has been leveraged to better understand neurological dysfunction.

Dynamics of Mitochondrial Transport in Axons

The polarized structure and long neurites of neurons pose a unique challenge for proper mitochondrial distribution. It is widely accepted that mitochondria move from the cell body to axon ends and vice versa; however, we have found that mitochondria originating from the axon ends moving in the retrograde direction never reach to the cell body, and only a limited number of mitochondria moving in the anterograde direction from the cell body arrive at the axon ends of mouse hippocampal neurons. Furthermore, we have derived a mathematical formula using the Fokker-Planck equation to characterize features of mitochondrial transport, and the equation could determine altered mitochondrial transport in axons overexpressing parkin. Our analysis will provide new insights into the dynamics of mitochondrial transport in axons of normal and unhealthy neurons.

Mitochondrial biogenesis is required for axonal growth

During early development, neurons undergo complex morphological rearrangements to assemble into neuronal circuits and propagate signals. Rapid growth requires a large quantity of building materials, efficient intracellular transport and also a considerable amount of energy. To produce this energy, the neuron should first generate new mitochondria because the pre-existing mitochondria are unlikely to provide a sufficient acceleration in ATP production. Here, we demonstrate that mitochondrial biogenesis and ATP production are required for axonal growth and neuronal development in cultured rat cortical neurons. We also demonstrate that growth signals activating the CaMKKβ, LKB1-STRAD or TAK1 pathways also co-activate the AMPK-PGC-1α-NRF1 axis leading to the generation of new mitochondria to ensure energy for upcoming growth. In conclusion, our results suggest that neurons are capable of signalling for upcoming energy requirements. Earlier activation of mitochondrial biogenesis through these pathways will accelerate the generation of new mitochondria, thereby ensuring energy-producing capability for when other factors for axonal growth are synthesized.

Glutaminase Increases in Rat Dorsal Root Ganglion Neurons after Unilateral Adjuvant-Induced Hind Paw Inflammation

Glutamate is a neurotransmitter used at both the peripheral and central terminals of nociceptive primary sensory neurons, yet little is known concerning regulation of glutamate metabolism during peripheral inflammation. Glutaminase (GLS) is an enzyme of the glutamate-glutamine cycle that converts glutamine into glutamate for neurotransmission and is implicated in producing elevated levels of glutamate in central and peripheral terminals. A potential mechanism for increased levels of glutamate is an elevation in GLS expression. We assessed GLS expression after unilateral hind paw inflammation by measuring GLS immunoreactivity (ir) with quantitative image analysis of L4 dorsal root ganglion (DRG) neurons after one, two, four, and eight days of adjuvant-induced arthritis (AIA) compared to saline injected controls. No significant elevation in GLS-ir occurred in the DRG ipsilateral to the inflamed hind paw after one or two days of AIA. After four days AIA, GLS-ir was elevated significantly in all sizes of DRG neurons. After eight days AIA, GLS-ir remained elevated in small (<400 µm²), presumably nociceptive neurons. Western blot analysis of the L4 DRG at day four AIA confirmed the elevated GLS-ir. The present study indicates that GLS expression is increased in the chronic stage of inflammation and may be a target for chronic pain therapy.

Inner membrane fusion mediates spatial distribution of axonal mitochondria

In eukaryotic cells, mitochondria form a dynamic interconnected network to respond to changing needs at different subcellular locations. A fundamental yet unanswered question regarding this network is whether, and if so how, local fusion and fission of individual mitochondria affect their global distribution. To address this question, we developed high-resolution computational image analysis techniques to examine the relations between mitochondrial fusion/fission and spatial distribution within the axon of Drosophila larval neurons. We found that stationary and moving mitochondria underwent fusion and fission regularly but followed different spatial distribution patterns and exhibited different morphology. Disruption of inner membrane fusion by knockdown of dOpa1, Drosophila Optic Atrophy 1, not only increased the spatial density of stationary and moving mitochondria but also changed their spatial distributions and morphology differentially. Knockdown of dOpa1 also impaired axonal transport of mitochondria. But the changed spatial distributions of mitochondria resulted primarily from disruption of inner membrane fusion because knockdown of Milton, a mitochondrial kinesin-1 adapter, caused similar transport velocity impairment but different spatial distributions. Together, our data reveals that stationary mitochondria within the axon interconnect with moving mitochondria through fusion and fission and that local inner membrane fusion between individual mitochondria mediates their global distribution.

Poly(ADP-ribose) polymerase 1 is a novel target to promote axonal regeneration

Therapeutic options for the restoration of neurological functions after acute axonal injury are severely limited. In addition to limiting neuronal loss, effective treatments face the challenge of restoring axonal growth within an injury environment where inhibitory molecules from damaged myelin and activated astrocytes act as molecular and physical barriers. Overcoming these barriers to permit axon growth is critical for the development of any repair strategy in the central nervous system. Here, we identify poly(ADP-ribose) polymerase 1 (PARP1) as a previously unidentified and critical mediator of multiple growth-inhibitory signals. We show that exposure of neurons to growth-limiting molecules--such as myelin-derived Nogo and myelin-associated glycoprotein--or reactive astrocyte-produced chondroitin sulfate proteoglycans activates PARP1, resulting in the accumulation of poly(ADP-ribose) in the cell body and axon and limited axonal growth. Accordingly, we find that pharmacological inhibition or genetic loss of PARP1 markedly facilitates axon regeneration over nonpermissive substrates. Together, our findings provide critical insights into the molecular mechanisms of axon growth inhibition and identify PARP1 as an effective target to promote axon regeneration.

A close look at axonal transport: Cargos slow down when crossing stationary organelles

The bidirectional transport of cargos along the thin axon is fundamental for the structure, function and survival of neurons. Defective axonal transport has been linked to the mechanism of neurodegenerative diseases. In this paper, we study the effect of the local axonal environment to cargo transport behavior in neurons. Using dual-color fluorescence imaging in microfluidic neuronal devices, we quantify the transport dynamics of cargos when crossing stationary organelles such as non-moving endosomes and stationary mitochondria in the axon. We show that the axonal cargos tend to slow down, or pause transiently within the vicinity of stationary organelles. The slow-down effect is observed in both retrograde and anterograde transport directions of three different cargos (TrkA, lysosomes and TrkB). Our results agree with the hypothesis that bulky axonal structures can pose as steric hindrance for axonal transport. However, the results do not rule out the possibility that cellular mechanisms causing stationary organelles are also responsible for the delay in moving cargos at the same locations.

Evidence of Mitochondrial Dysfunction within the Complex Genetic Etiology of Schizophrenia

Genetic evidence has supported the hypothesis that schizophrenia (SZ) is a polygenic disorder caused by the disruption in function of several or many genes. The most common and reproducible cellular phenotype associated with SZ is a reduction in dendritic spines within the neocortex, suggesting alterations in dendritic architecture may cause aberrant cortical circuitry and SZ symptoms. Here, we review evidence supporting a multifactorial model of mitochondrial dysfunction in SZ etiology and discuss how these multiple paths to mitochondrial dysfunction may contribute to dendritic spine loss and/or underdevelopment in some SZ subjects. The pathophysiological role of mitochondrial dysfunction in SZ is based upon genomic analyses of both the mitochondrial genome and nuclear genes involved in mitochondrial function. Previous studies and preliminary data suggest SZ is associated with specific alleles and haplogroups of the mitochondrial genome, and also correlates with a reduction in mitochondrial copy number and an increase in synonymous and nonsynonymous substitutions of mitochondrial DNA. Mitochondrial dysfunction has also been widely implicated in SZ by genome-wide association, exome sequencing, altered gene expression, proteomics, microscopy analyses, and induced pluripotent stem cell studies. Together, these data support the hypothesis that SZ is a polygenic disorder with an enrichment of mitochondrial targets.

Vimentin is involved in regulation of mitochondrial motility and membrane potential by Rac1

In this study we show that binding of mitochondria to vimentin intermediate filaments (VIF) is regulated by GTPase Rac1. The activation of Rac1 leads to a redoubling of mitochondrial motility in murine fibroblasts. Using double-mutants Rac1(G12V, F37L) and Rac1(G12V, Y40H) that are capable to activate different effectors of Rac1, we show that mitochondrial movements are regulated through PAK1 kinase. The involvement of PAK1 kinase is also confirmed by the fact that expression of its auto inhibitory domain (PID) blocks the effect of activated Rac1 on mitochondrial motility. The observed effect of Rac1 and PAK1 kinase on mitochondria depends on phosphorylation of the Ser-55 of vimentin. Besides the effect on motility Rac1 activation also decreases the mitochondrial membrane potential (MMP) which is detected by ∼20% drop of the fluorescence intensity of mitochondria stained with the potential sensitive dye TMRM. One of important consequences of the discovered regulation of MMP by Rac1 and PAK1 is a spatial differentiation of mitochondria in polarized fibroblasts: at the front of the cell they are less energized (by ∼25%) than at the rear part.