Principles of the mitochondrial fusion and fission cycle in neurons

The role of mitochondrial dysfunction in the pathogenesis of Alzheimer's disease and future strategies for targeted therapy

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive cognitive decline, behavioral impairments, and psychiatric comorbidities. The pathogenesis of AD remains incompletely elucidated, despite advances in dominant hypotheses such as the β-amyloid (Aβ) cascade, tauopathy, cholinergic deficiency, and neuroinflammation mechanisms. However, these hypotheses inadequately explain the multifactorial nature of AD, which exposes limitations in our understanding of its mechanisms. Mitochondrial dysfunction is known to play a pivotal role in AD, and since patients exhibit intracellular mitochondrial dysfunction and structural changes in the brain at an early stage, correcting the imbalance of mitochondrial homeostasis and the cytopathological changes caused by it may be a potential target for early treatment of AD. Mitochondrial structural abnormalities accelerate AD pathogenesis. For instance, structural and functional alterations in the mitochondria-associated endoplasmic reticulum membrane (MAM) can disrupt intracellular Ca2⁺ homeostasis and cholesterol metabolism, consequently promoting Aβ accumulation. In addition, the overaccumulation of Aβ and hyperphosphorylated tau proteins can further damage neurons by disrupting mitochondrial integrity and mitophagy, thereby amplifying pathological aggregation and exacerbating neurodegeneration in AD. Furthermore, Aβ deposition and abnormal tau proteins can disrupt mitochondrial dynamics through dysregulation of fission/fusion proteins, leading to excessive mitochondrial fragmentation and subsequent dysfunction. Additionally, hyperphosphorylated tau proteins can impair mitochondrial transport, resulting in axonal dysfunction in AD. This article reviews the biological significance of mitochondrial structural morphology, dynamics, and mitochondrial DNA (mtDNA) instability in AD pathology, emphasizing mitophagy abnormalities as a critical contributor to AD progression. Additionally, mitochondrial biogenesis and proteostasis are critical for maintaining mitochondrial function and integrity. Impairments in these processes have been implicated in the progression of AD, further highlighting the multifaceted role of mitochondrial dysfunction in neurodegeneration. It further discusses the therapeutic potential of mitochondria-targeted strategies for AD drug development.

Dynamins combine mechano-constriction and membrane remodeling to enable two-step mitochondrial fission via a 'snap-through' instability

Mitochondrial fission is controlled by dynamin proteins, the dysregulation of which is correlated with diverse diseases. Fission dynamins are GTP hydrolysis-driven mechanoenzymes that self-oligomerize into helical structures that constrict membrane to achieve fission, but details are not well understood. However, dynamins can also remodel membranes by inducing negative Gaussian curvature, the type of curvature required for completion of fission. Here, we examine how these drastically different mechanisms synergistically exert their influences on a membrane, via a mechanical model calibrated with small-angle X-ray scattering structural data. We find that free dynamin can trigger a "snap-through instability" that enforces a shape transition from an oligomer-confined cylindrical membrane to a drastically narrower catenoid-shaped neck within the spontaneous hemi-fission regime, in a manner that depends critically on the length of the confined tube. These results indicate how the combination of dynamin assembly, and paradoxically disassembly, can lead to diverse pathways to scission.

Computation of the mitochondrial age distribution along the axon length

We describe a compartmental model of mitochondrial transport in axons, which we apply to compute mitochondrial age at different distances from the soma. The model predicts that at the tip of an axon that has a length of 1 cm, the average mitochondrial age is approximately 22 h. The mitochondria are youngest closest to the soma and their age scales approximately linearly with distance from the soma. To the best of the authors' knowledge, this is the first attempt to predict the spatial distribution of mitochondrial age within an axon. A sensitivity study of the mean age of mitochondria to various model parameters is also presented.

Acquired disorders of mitochondrial metabolism and dynamics in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is an orphan disease of the cardiopulmonary unit that reflects an obstructive pulmonary vasculopathy and presents with hypertrophy, inflammation, fibrosis, and ultimately failure of the right ventricle (RVF). Despite treatment using pulmonary hypertension (PH)-targeted therapies, persistent functional impairment reduces the quality of life for people with PAH and death from RVF occurs in approximately 40% of patients within 5 years of diagnosis. PH-targeted therapeutics are primarily vasodilators and none, alone or in combination, are curative. This highlights a need to therapeutically explore molecular targets in other pathways that are involved in the pathogenesis of PAH. Several candidate pathways in PAH involve acquired mitochondrial dysfunction. These mitochondrial disorders include: 1) a shift in metabolism related to increased expression of pyruvate dehydrogenase kinase and pyruvate kinase, which together increase uncoupled glycolysis (Warburg metabolism); 2) disruption of oxygen-sensing related to increased expression of hypoxia-inducible factor 1α, resulting in a state of pseudohypoxia; 3) altered mitochondrial calcium homeostasis related to impaired function of the mitochondrial calcium uniporter complex, which elevates cytosolic calcium and reduces intramitochondrial calcium; and 4) abnormal mitochondrial dynamics related to increased expression of dynamin-related protein 1 and its binding partners, such as mitochondrial dynamics proteins of 49 kDa and 51 kDa, and depressed expression of mitofusin 2, resulting in increased mitotic fission. These acquired mitochondrial abnormalities increase proliferation and impair apoptosis in most pulmonary vascular cells (including endothelial cells, smooth muscle cells and fibroblasts). In the RV, Warburg metabolism and induction of glutaminolysis impairs bioenergetics and promotes hypokinesis, hypertrophy, and fibrosis. This review will explore our current knowledge of the causes and consequences of disordered mitochondrial function in PAH.

Mitochondrial fission in hepatocytes as a potential therapeutic target for nonalcoholic steatohepatitis

AIM

The mitochondria are highly plastic and dynamic organelles; mitochondrial dysfunction has been reported to play causative roles in diabetes, cardiovascular diseases, and nonalcoholic fatty liver disease (NAFLD). However, the relationship between mitochondrial fission and NAFLD pathogenesis remains unknown. We aimed to investigate whether alterations in mitochondrial fission could play a role in the progression of NAFLD.

METHODS

Mice were fed a standard diet or choline-deficient, L-amino acid-defined (CDAA) diet with vehicle or mitochondrial division inhibitor-1.

RESULTS

Substantial enhancement of mitochondrial fission in hepatocytes was triggered by 4 weeks of feeding and was associated with changes reflecting the early stage of human nonalcoholic steatohepatitis (NASH), steatotic change with liver inflammation, and hepatocyte ballooning. Excessive mitochondrial fission inhibition in hepatocytes and lipid metabolism dysregulation in adipose tissue attenuated liver inflammation and fibrogenesis but not steatosis and the systemic pathological changes in the early and chronic fibrotic NASH stages (4- and 12-week CDAA feeding). These beneficial changes due to the suppression of mitochondrial fission against the liver and systemic injuries were associated with decreased autophagic responses and endoplasmic reticulum stress in hepatocytes. Injuries to other liver cells, such as endothelial cells, Kupffer cells, and hepatic stellate cells, were also attenuated by the inhibition of mitochondrial fission in hepatocytes.

CONCLUSIONS

Taken together, these findings suggest that excessive mitochondrial fission in hepatocytes could play a causative role in NAFLD progression by liver inflammation and fibrogenesis through altered cell cross-talk. This study provides a potential therapeutic target for NAFLD.

Simple to Complex: The Role of Actin and Microtubules in Mitochondrial Dynamics in Amoeba, Yeast, and Mammalian Cells

Mitochondria are complex organelles that provide energy for the cell in the form of adenosine triphosphate (ATP) and have very specific structures. For most organisms, this is a reticular or tubular mitochondrial network, while others have singular oval-shaped organelles. Nonetheless, maintenance of this structure is dependent on the mitochondrial dynamics, fission, fusion, and motility. Recently, studies have shown that the cytoskeleton has a significant role in the regulation of mitochondrial dynamics. In this review, we focus on microtubules and actin filaments and look at what is currently known about the cytoskeleton’s role in mitochondrial dynamics in complex models like mammals and yeast, as well as what is known in the simple model system, Dictyostelium discoideum. Understanding how the cytoskeleton is involved in mitochondrial dynamics increases our understanding of mitochondrial disease, especially neurodegenerative diseases. Increases in fission, loss of fusion, and fragmented mitochondria are seen in several neurodegenerative diseases such as Parkinson’s, Alzheimer’s, and Huntington’s disease. There is no known cure for these diseases, but new therapeutic strategies using drugs to alter mitochondrial fusion and fission activity are being considered. The future of these therapeutic studies is dependent on an in-depth understanding of the mechanisms of mitochondrial dynamics. Understanding the cytoskeleton’s role in dynamics in multiple model organisms will further our understanding of these mechanisms and could potentially uncover new therapeutic targets for these neurodegenerative diseases.

Mitochondrial behavior when things go wrong in the axon

Axonal homeostasis is maintained by processes that include cytoskeletal regulation, cargo transport, synaptic activity, ionic balance, and energy supply. Several of these processes involve mitochondria to varying degrees. As a transportable powerplant, the mitochondria deliver ATP and Ca2+-buffering capabilities and require fusion/fission to maintain proper functioning. Taking into consideration the long distances that need to be covered by mitochondria in the axons, their transport, distribution, fusion/fission, and health are of cardinal importance. However, axonal homeostasis is disrupted in several disorders of the nervous system, or by traumatic brain injury (TBI), where the external insult is translated into physical forces that damage nervous tissue including axons. The degree of damage varies and can disconnect the axon into two segments and/or generate axonal swellings in addition to cytoskeletal changes, membrane leakage, and changes in ionic composition. Cytoskeletal changes and increased intra-axonal Ca2+ levels are the main factors that challenge mitochondrial homeostasis. On the other hand, a proper function and distribution of mitochondria can determine the recovery or regeneration of the axonal physiological state. Here, we discuss the current knowledge regarding mitochondrial transport, fusion/fission, and Ca2+ regulation under axonal physiological or pathological conditions.

Neurodegeneration in Multiple Sclerosis: The Role of Nrf2-Dependent Pathways

Multiple sclerosis (MS) encompasses a chronic, irreversible, and predominantly immune-mediated disease of the central nervous system that leads to axonal degeneration, neuronal death, and several neurological symptoms. Although various immune therapies have reduced relapse rates and the severity of symptoms in relapsing-remitting MS, there is still no cure for this devastating disease. In this brief review, we discuss the role of mitochondria dysfunction in the progression of MS, focused on the possible role of Nrf2 signaling in orchestrating the impairment of critical cellular and molecular aspects such as reactive oxygen species (ROS) management, under neuroinflammation and neurodegeneration in MS. In this scenario, we propose a new potential downstream signaling of Nrf2 pathway, namely the opening of hemichannels and pannexons. These large-pore channels are known to modulate glial/neuronal function and ROS production as they are permeable to extracellular Ca²⁺ and release potentially harmful transmitters to the synaptic cleft. In this way, the Nrf2 dysfunction impairs not only the bioenergetics and metabolic properties of glial cells but also the proper antioxidant defense and energy supply that they provide to neurons.

Effect of Titanium Dioxide and Silver Nanoparticles on Mitochondrial Dynamics in Mouse Testis Tissue

This study was performed to investigate whether the toxicity of nanoparticles (Ag NPs or TiO₂ NPs) affected mitochondrial dynamics (mitochondrial fusion and fission mechanisms) in testicular cells of mice. Animals were assigned into three groups (ten mice per group): control group (distilled water), TiO₂ NP group (5 mg/kg per dose), and Ag NP group (5 mg/kg per dose). NPs were administered intravenously (via tail vein) to mice with 3-day intervals. To determine the possible toxic effect of NPs on mitochondrial dynamics, the expression levels of mitochondrial fission (Drp1)- and fusion (Mfn1, Mfn2, OPA1)-related genes were analyzed. The results showed that both Ag NPs and TiO₂ NPs entered the testis via the blood-testis barier and accumulated in mouse testis tissue. Experiments showed that administration of Ag NPs neither alters testicular weight and testicular index nor causes significant toxic effect on sperm parameters. RT-PCR analysis demonstrated that Ag NP treatment did not disrupt mitochondrial dynamics in testicular cells. Conversely, administration of TiO₂ NPs (anatase, < 25 nm) decreased the sperm motility and the percentages of sperms with swollen tail. Furthermore, RT-PCR and western blot analyses showed that TiO₂ NPs disrupted mitochondrial dynamics by causing excess mitochondrial fission (excess expression of Drp1 gene and DRP1 protein). This is the first report on the toxicity of nanoparticles on mitochondrial dynamics (fusion and fission mechanisms) in testicular cells.

Mechanisms of Acupuncture in Improving Alzheimer's Disease Caused by Mitochondrial Damage

Alzheimer's disease (AD) is one of the most common neurodegenerative diseases among the elderly and it accounts for nearly 80% of all dementias. The pathogenesis of AD is complicated and enigmatic thus far. The mitochondrial cascade hypothesis assumes that mitochondrial damage may mediate, drive, or contribute to a variety of AD pathologies and may be the main factor in late-onset AD. Currently, there are no widely recognized drugs able to attenuate mitochondrial damage in AD. Notably, increasing evidence supports the efficacy of acupuncture for improving the mitochondrial structure and protecting mitochondrial functions in AD. This review reports the mechanisms by which acupuncture regulates mitochondrial dynamics, energy metabolism, calcium homeostasis and apoptosis. In conclusion, these findings suggest that AD mitochondrial dysfunction represents a reasonable therapeutic target and acupuncture could play a significant role in preventing and treating AD.

Possible relationship between mitochondrial changes and oxidative stress under low dose-rate irradiation

Objectives: High dose-rate ionizing radiation (IR) causes severe DSB damage, as well as reactive oxygen species (ROS) accumulation and oxidative stress. However, it is unknown what biological processes are affected by low dose-rate IR; therefore, the molecular relationships between mitochondria changes and oxidative stress in human normal cells was investigated after low dose-rate IR.Methods: We compared several cellular response between high and low dose-rate irradiation using cell survival assay, ROS/RNS assay, immunofluorescence and western blot analysis.Results: Reduced DSB damage and increased levels of ROS, with subsequent oxidative stress responses, were observed in normal cells after low dose-rate IR. Low dose-rate IR caused several mitochondrial changes, including morphology mass, and mitochondrial membrane potential, suggesting that mitochondrial damage was caused. Although damaged mitochondria were removed by mitophagy to stop ROS leakage, the mitophagy-regulatory factor, PINK1, was reduced following low dose-rate IR. Although mitochondrial dynamics (fission/fusion events) are important for the proper mitophagy process, some mitochondrial fusion factors decreased following low dose-rate IR.Discussion: The dysfunction of mitophagy pathway under low dose-rate IR increased ROS and the subsequent activation of the oxidative stress response.

Alteration of mitochondrial homeostasis is an early event in a C. elegans model of human tauopathy

Tauopathies are a group of progressive neurodegenerative disorders characterized by the presence of insoluble intracellular tau filaments in the brain. Evidence suggests that there is a tight connection between mitochondrial dysfunction and tauopathies, including Alzheimer’s disease. However, whether mitochondrial dysfunction occurs prior to the detection of tau aggregates in tauopathies remains elusive. Here, we utilized transgenic nematodes expressing the full length of wild type tau in neuronal cells and monitored mitochondrial morphology alterations over time. Although tau-expressing nematodes did not accumulate detectable levels of tau aggregates during larval stages, they displayed increased mitochondrial damage and locomotion defects compared to the control worms. Chelating calcium restored mitochondrial activity and improved motility in the tau-expressing larvae suggesting a link between mitochondrial damage, calcium homeostasis and neuronal impairment in these animals. Our findings suggest that defective mitochondrial function is an early pathogenic event of tauopathies, taking place before tau aggregation and undermining neuronal homeostasis and organismal fitness. Understanding the molecular mechanisms causing mitochondrial dysfunction early in tauopathy will be of significant clinical and therapeutic value and merits further investigation.

Acetylation in Mitochondria Dynamics and Neurodegeneration

Mitochondria are a unique intracellular organelle due to their evolutionary origin and multifunctional role in overall cellular physiology and pathophysiology. To meet the specific spatial metabolic demands within the cell, mitochondria are actively moving, dividing, or fusing. This process of mitochondrial dynamics is fine-tuned by a specific group of proteins and their complex post-translational modifications. In this review, we discuss the mitochondrial dynamics regulatory enzymes, their adaptor proteins, and the effect of acetylation on the activity of fusion and fission machinery as a ubiquitous response to metabolic stresses. Further, we discuss the role of intracellular cytoskeleton structures and their post-translational modifications in the modulation of mitochondrial fusion and fission. Finally, we review the role of mitochondrial dynamics dysregulation in the pathophysiology of acute brain injury and the treatment strategies based on modulation of NAD⁺-dependent deacetylation.

The Expression of RAAS Key Receptors, Agtr2 and Bdkrb1, Is Downregulated at an Early Stage in a Rat Model of Wolfram Syndrome

Wolfram syndrome (WS) 1 is a rare monogenic neurodegenerative disorder caused by mutations in the gene encoding WFS1. Knowledge of the pathophysiology of WS is incomplete and to date, there is no treatment available. Here, we describe early deviations in the renin-angiotensin-aldosterone system (RAAS) and bradykinin pathway (kallikrein kinin system, KKS) observed in a rat model of WS (Wfs1 KO) and the modulative effect of glucagon-like peptide-1 receptor agonist liraglutide (LIR) and anti-epileptic drug valproate (VPA), which have been proven effective in delaying WS progression in WS animal models. We found that the expression of key receptors of the RAAS and KKS, Agtr2 and Bdkrb1, were drastically downregulated both in vitro and in vivo at an early stage in a rat model of WS. Moreover, in Wfs1, KO serum aldosterone levels were substantially decreased and bradykinin levels increased compared to WT animals. Neither treatment nor their combination affected the gene expression levels seen in the Wfs1 KO animals. However, all the treatments elevated serum aldosterone and decreased bradykinin in the Wfs1 KO rats, as well as increasing angiotensin II levels independent of genotype. Altogether, our results indicate that Wfs1 deficiency might disturb the normal functioning of RAAS and KKS and that LIR and VPA have the ability to modulate these systems.

Optimizing mitochondrial maintenance in extended neuronal projections

Neurons rely on localized mitochondria to fulfill spatially heterogeneous metabolic demands. Mitochondrial aging occurs on timescales shorter than the neuronal lifespan, necessitating transport of fresh material from the soma. Maintaining an optimal distribution of healthy mitochondria requires an interplay between a stationary pool localized to sites of high metabolic demand and a motile pool capable of delivering new material. Interchange between these pools can occur via transient fusion / fission events or by halting and restarting entire mitochondria. Our quantitative model of neuronal mitostasis identifies key parameters that govern steady-state mitochondrial health at discrete locations. Very infrequent exchange between stationary and motile pools optimizes this system. Exchange via transient fusion allows for robust maintenance, which can be further improved by selective recycling through mitophagy. These results provide a framework for quantifying how perturbations in organelle transport and interactions affect mitochondrial homeostasis in neurons, a key aspect underlying many neurodegenerative disorders.

Motor proteins at the mitochondria-cytoskeleton interface

Mitochondria are multifunctional organelles that not only produce energy for the cell, but are also important for cell signalling, apoptosis and many biosynthetic pathways. In most cell types, they form highly dynamic networks that are constantly remodelled through fission and fusion events, repositioned by motor-dependent transport and degraded when they become dysfunctional. Motor proteins and their tracks are key regulators of mitochondrial homeostasis, and in this Review, we discuss the diverse functions of the three classes of motor proteins associated with mitochondria - the actin-based myosins, as well as the microtubule-based kinesins and dynein. In addition, Miro and TRAK proteins act as adaptors that link kinesin-1 and dynein, as well as myosin of class XIX (MYO19), to mitochondria and coordinate microtubule- and actin-based motor activities. Here, we highlight the roles of motor proteins and motor-linked track dynamics in the transporting and docking of mitochondria, and emphasize their adaptations in specialized cells. Finally, we discuss how motor-cargo complexes mediate changes in mitochondrial morphology through fission and fusion, and how they modulate the turnover of damaged organelles via quality control pathways, such as mitophagy. Understanding the importance of motor proteins for mitochondrial homeostasis will help to elucidate the molecular basis of a number of human diseases.

Plasticity in parental effects confers rapid larval thermal tolerance in the estuarine anemone Nematostella vectensis

Parental effects can prepare offspring for different environments and facilitate survival across generations. We exposed parental populations of the estuarine anemone, Nematostella vectensis, from Massachusetts to elevated temperatures and quantified larval mortality across a temperature gradient. We found that parental exposure to elevated temperatures resulted in a consistent increase in larval thermal tolerance, as measured by the temperature at which 50% of larvae die (LT50), with a mean increase in LT50 of 0.3°C. Larvae from subsequent spawns returned to baseline thermal thresholds when parents were returned to normal temperatures, indicating plasticity in these parental effects. Histological analyses of gametogenesis in females suggested that these dynamic shifts in larval thermal tolerance may be facilitated by maternal effects in non-overlapping gametic cohorts. We also compared larvae from North Carolina (a genetically distinct population with higher baseline thermal tolerance) and Massachusetts parents, and observed that larvae from heat-exposed Massachusetts parents had thermal thresholds comparable to those of larvae from unexposed North Carolina parents. North Carolina parents also increased larval thermal tolerance under the same high-temperature regime, suggesting that plasticity in parental effects is an inherent trait for N. vectensis Overall, we find that larval thermal tolerance in N. vectensis shows a strong genetic basis and can be modulated by parental effects. Further understanding of the mechanisms behind these shifts can elucidate the fate of thermally sensitive ectotherms in a rapidly changing thermal environment.

Opa1 Deficiency Leads to Diminished Mitochondrial Bioenergetics With Compensatory Increased Mitochondrial Motility

Purpose

Retinal ganglion cells (RGCs) are susceptible to mitochondrial deficits and also the major cell type affected in patients with mutations in the OPA1 gene in autosomal dominant optic atrophy (ADOA). Here, we characterized mitochondria in RGCs in vitro from a heterozygous B6; C3-Opa1^Q285STOP (Opa1^+/−) mouse model to investigate mitochondrial changes underlying the pathology in ADOA.

Methods

Mouse RGCs were purified from wild-type and Opa1^+/− mouse retina by two-step immunopanning. The mitochondria in neurites of RGCs were labeled with MitoTracker Red for structure and motility measurement by time-lapse imaging. Mitochondrial bioenergetics were determined by the real-time measurement of oxygen consumption rate using a Seahorse XF^e 96 Extracellular Flux Analyzer.

Results

We observed a significant decrease in mitochondrial length in Opa1^+/− RGCs with a remarkably higher proportion and density of motile mitochondria along the neurites. We also observed an increased transport velocity with a higher number of contacts between mitochondria in Opa1^+/− RGC neurites. The oxygen consumption assays showed a severe impairment in basal respiration, Adenosine triphosphate-linked (ATP-linked) oxygen consumption, as well as reserve respiratory capacity, in RGCs from Opa1^+/− mouse retina.

Conclusions

Opa1 deficiency leads to significant fragmentation of mitochondrial morphology, activation of mitochondrial motility and impaired respiratory function in RGCs from the B6; C3-Opa1^Q285STOP mouse model. This highlights the significant alterations in the intricate interplay between mitochondrial morphology, motility, and energy production in RGCs with Opa1 deficiency long before the onset of clinical symptoms of the pathology.

Miro (Mitochondrial Rho GTPase), a key player of mitochondrial axonal transport and mitochondrial dynamics in neurodegenerative diseases

Miro (mitochondrial Rho GTPases) a mitochondrial outer membrane protein, plays a vital role in the microtubule-based mitochondrial axonal transport, mitochondrial dynamics (fusion and fission) and Mito-Ca2+ homeostasis. It forms a major protein complex with Milton (an adaptor protein), kinesin and dynein (motor proteins), and facilitates bidirectional mitochondrial axonal transport such as anterograde and retrograde transport. By forming this protein complex, Miro facilitates the mitochondrial axonal transport and fulfills the neuronal energy demand, maintain the mitochondrial homeostasis and neuronal survival. It has been demonstrated that altered mitochondrial biogenesis, improper mitochondrial axonal transport, and mitochondrial dynamics are the early pathologies associated with most of the neurodegenerative diseases (NDs). Being the sole mitochondrial outer membrane protein associated with mitochondrial axonal transport-related processes, Miro proteins can be one of the key players in various NDs such as Alzheimer's disease (AD), Parkinson's disease (PD), Amyotrophic lateral sclerosis (ALS) and Huntington's disease (HD). Thus, in the current review, we have discussed the evolutionarily conserved Miro proteins and its role in the pathogenesis of the various NDs. From this, we indicated that Miro proteins may act as a potential target for a novel therapeutic intervention for the treatment of various NDs.

Dysbindin-1 regulates mitochondrial fission and gamma oscillations

Mitochondria are cellular ATP generators. They are dynamic structures undergoing fission and fusion. While much is known about the mitochondrial fission machinery, the mechanism of initiating fission and the significance of fission to neurophysiology are largely unclear. Gamma oscillations are synchronized neural activities that impose a great energy challenge to synapses. The cellular mechanism of fueling gamma oscillations has yet to be defined. Here, we show that dysbindin-1, a protein decreased in the brain of individuals with schizophrenia, is required for neural activity-induced fission by promoting Drp1 oligomerization. This process is engaged by gamma-frequency activities and in turn, supports gamma oscillations. Gamma oscillations and novel object recognition are impaired in dysbindin-1 null mice. These defects can be ameliorated by increasing mitochondrial fission. These findings identify a molecular mechanism for activity-induced mitochondrial fission, a role of mitochondrial fission in gamma oscillations, and mitochondrial fission as a potential target for improving cognitive functions.

Abnormalities of synaptic mitochondria in autism spectrum disorder and related neurodevelopmental disorders

Autism spectrum disorder (ASD) is a neurodevelopmental condition primarily characterized by an impairment of social interaction combined with the occurrence of repetitive behaviors. ASD starts in childhood and prevails across the lifespan. The variability of its clinical presentation renders early diagnosis difficult. Mutations in synaptic genes and alterations of mitochondrial functions are considered important underlying pathogenic factors, but it is obvious that we are far from a comprehensive understanding of ASD pathophysiology. At the synapse, mitochondria perform diverse functions, which are clearly not limited to their classical role as energy providers. Here, we review the current knowledge about mitochondria at the synapse and summarize the mitochondrial disturbances found in mouse models of ASD and other ASD-related neurodevelopmental disorders, like DiGeorge syndrome, Rett syndrome, Tuberous sclerosis complex, and Down syndrome.

An impedimetric assay for the identification of abnormal mitochondrial dynamics in living cells

Mitochondrial dynamics (fission and fusion) plays an important role in cell functions. Disruption in mitochondrial dynamics has been associated with diseases such as neurobiological disorders and cardiovascular diseases. Analysis of mitochondrial fission/fusion has been mostly achieved through direct visualization of the fission/fusion events in live-cell imaging of fluorescently labeled mitochondria. In this study, we demonstrated a label-free, non-invasive Electrical Impedance Spectroscopy (EIS) approach to analyze mitochondrial dynamics in a genetically modified human neuroblastoma SH-SY5Y cell line with no huntingtin protein expression. Huntingtin protein has been shown to regulate mitochondria dynamics. We performed EIS studies on normal SH-SY5Y cells and two independent clones of huntingtin-null cells. The impedance data was used to determine the suspension conductivity and further cytoplasmic conductivity and relate to the abnormal mitochondrial dynamics. For instance, the cytoplasm conductivity value was increased by 11% from huntingtin-null cells to normal cells. Results of this study demonstrated that EIS is sensitive to characterize the abnormal mitochondrial dynamics that can be difficult to quantify by the conventional microscopic method.

Regulation of peroxisomal trafficking and distribution

Peroxisomes are organelles that perform a wide range of essential metabolic processes. To ensure that peroxisomes are optimally positioned in the cell, they must be transported by both long- and short-range trafficking events in response to cellular needs. Here, we review our current understanding of the mechanisms by which the cytoskeleton and organelle contact sites alter peroxisomal distribution. Though the focus of the review is peroxisomal transport in mammalian cells, findings from flies and fungi are used for comparison and to inform the gaps in our understanding. Attention is given to the apparent overlap in regulatory mechanisms for mitochondrial and peroxisomal trafficking, along with the recently discovered role of the mitochondrial Rho-GTPases, Miro, in peroxisomal dynamics. Moreover, we outline and discuss the known pathological and pharmacological conditions that perturb peroxisomal positioning. We conclude by highlighting several gaps in our current knowledge and suggest future directions that require attention.

Getting around the cell: physical transport in the intracellular world

Eukaryotic cells face the challenging task of transporting a variety of particles through the complex intracellular milieu in order to deliver, distribute, and mix the many components that support cell function. In this review, we explore the biological objectives and physical mechanisms of intracellular transport. Our focus is on cytoplasmic and intra-organelle transport at the whole-cell scale. We outline several key biological functions that depend on physically transporting components across the cell, including the delivery of secreted proteins, support of cell growth and repair, propagation of intracellular signals, establishment of organelle contacts, and spatial organization of metabolic gradients. We then review the three primary physical modes of transport in eukaryotic cells: diffusive motion, motor-driven transport, and advection by cytoplasmic flow. For each mechanism, we identify the main factors that determine speed and directionality. We also highlight the efficiency of each transport mode in fulfilling various key objectives of transport, such as particle mixing, directed delivery, and rapid target search. Taken together, the interplay of diffusion, molecular motors, and flows supports the intracellular transport needs that underlie a broad variety of biological phenomena.

Teriflunomide preserves peripheral nerve mitochondria from oxidative stress-mediated alterations

Mitochondrial dysfunction is a common pathological hallmark in various inflammatory and degenerative diseases of the central nervous system, including multiple sclerosis (MS). We previously showed that oxidative stress alters axonal mitochondria, limiting their transport and inducing conformational changes that lead to axonal damage. Teriflunomide (TFN), an oral immunomodulatory drug approved for the treatment of relapsing forms of MS, reversibly inhibits dihydroorotate dehydrogenase (DHODH). DHODH is crucial for de novo pyrimidine biosynthesis and is the only mitochondrial enzyme in this pathway, thus conferring a link between inflammation, mitochondrial activity and axonal integrity. Here, we investigated how DHODH inhibition may affect mitochondrial behavior in the context of oxidative stress. We employed a model of transected murine spinal roots, previously developed in our laboratory. Using confocal live imaging of axonal mitochondria, we showed that in unmanipulated axons, TFN increased significantly the mitochondria length without altering their transport features. In mitochondria challenged with 50 µM hydrogen peroxide (H₂O₂) to induce oxidative stress, the presence of TFN at 1 µM concentration was able to restore mitochondrial shape, motility, as well as mitochondrial oxidation potential to control levels. No effects were observed at 5 µM TFN, while some shape and motility parameters were restored to control levels at 50 µM TFN. Thus, our data demonstrate an undescribed link between DHODH and mitochondrial dynamics and point to a potential neuroprotective effect of DHODH inhibition in the context of oxidative stress-induced damage of axonal mitochondria.

Role of Mitochondria-Cytoskeleton Interactions in the Regulation of Mitochondrial Structure and Function in Cancer Stem Cells

Despite the promise of cancer medicine, major challenges currently confronting the treatment of cancer patients include chemoresistance and recurrence. The existence of subpopulations of cancer cells, known as cancer stem cells (CSCs), contributes to the failure of cancer therapies and is associated with poor clinical outcomes. Of note, one of the recently characterized features of CSCs is augmented mitochondrial function. The cytoskeleton network is essential in regulating mitochondrial morphology and rearrangement, which are inextricably linked to its functions, such as oxidative phosphorylation (OXPHOS). The interaction between the cytoskeleton and mitochondria can enable CSCs to adapt to challenging conditions, such as a lack of energy sources, and to maintain their stemness. Cytoskeleton-mediated mitochondrial trafficking and relocating to the high energy requirement region are crucial steps in epithelial-to-mesenchymal transition (EMT). In addition, the cytoskeleton itself interplays with and blocks the voltage-dependent anion channel (VDAC) to directly regulate bioenergetics. In this review, we describe the regulation of cellular bioenergetics in CSCs, focusing on the cytoskeleton-mediated dynamic control of mitochondrial structure and function.

Promising Molecular Targets for Pharmacological Therapy of Neurodegenerative Pathologies

Drug development for the treatment of neurodegenerative diseases has to confront numerous problems occurring, in particular, because of attempts to address only one of the causes of the pathogenesis of neurological disorders. Recent advances in multitarget therapy research are gaining momentum by utilizing pharmacophores that simultaneously affect different pathological pathways in the neurodegeneration process. The application of such a therapeutic strategy not only involves the treatment of symptoms, but also mainly addresses prevention of the fundamental pathological processes of neurodegenerative diseases and the reduction of cognitive abilities. Neuroinflammation and oxidative stress, mitochondrial dysfunction, dysregulation of the expression of histone deacetylases, and aggregation of pathogenic forms of proteins are among the most common and significant pathological features of neurodegenerative diseases. In this review, we focus on the molecular mechanisms and highlight the main aspects, including reactive oxygen species, the cell endogenous antioxidant system, neuroinflammation triggers, metalloproteinases, α-synuclein, tau proteins, neuromelanin, histone deacetylases, presenilins, etc. The processes and molecular targets discussed in this review could serve as a starting point for screening leader compounds that could help prevent or slow down the development of neurodegenerative diseases.

Mechanisms and roles of mitochondrial localisation and dynamics in neuronal function

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

Mitochondrial dynamics in yeast with repressed adenine nucleotide translocator AAC2

The mitochondrial network structure dynamically adapts to cellular metabolic challenges. Mitochondrial depolarisation, particularly, induces fragmentation of the network. This fragmentation may be a result of either a direct regulation of the mitochondrial fusion machinery by transmembrane potential or an indirect effect of metabolic remodelling. Activities of ATP synthase and adenine nucleotide translocator (ANT) link the mitochondrial transmembrane potential with the cytosolic NTP/NDP ratio. Given that mitochondrial fusion requires cytosolic GTP, a decrease in the NTP/NDP ratio might also account for protonophore-induced mitochondrial fragmentation. For evaluating the contributions of direct and indirect mechanisms to mitochondrial remodelling, we assessed the morphology of the mitochondrial network in yeast cells with inhibited ANT. We showed that the repression of AAC2 (PET9), a major ANT gene in yeast, increases mitochondrial transmembrane potential. However, the mitochondrial network in this strain was fragmented. Meanwhile, AAC2 repression did not prevent mitochondrial fusion in zygotes; nor did it inhibit mitochondrial hyperfusion induced by Dnm1p inhibitor mdivi-1. These results suggest that the inhibition of ANT, rather than preventing mitochondrial fusion, facilitates mitochondrial fission. The protonophores were not able to induce additional mitochondrial fragmentation in an AAC2-repressed strain and in yeast cells with inhibited ATP synthase. Importantly, treatment with the ATP synthase inhibitor oligomycin A also induced mitochondrial fragmentation and hyperpolarization. Taken together, our data suggest that ATP/ADP translocation plays a crucial role in shaping of the mitochondrial network and exemplify that an increase in mitochondrial membrane potential does not necessarily oppose mitochondrial fragmentation.

Axonal Endoplasmic Reticulum Dynamics and Its Roles in Neurodegeneration

The physical continuity of axons over long cellular distances poses challenges for their maintenance. One organelle that faces this challenge is endoplasmic reticulum (ER); unlike other intracellular organelles, this forms a physically continuous network throughout the cell, with a single membrane and a single lumen. In axons, ER is mainly smooth, forming a tubular network with occasional sheets or cisternae and low amounts of rough ER. It has many potential roles: lipid biosynthesis, glucose homeostasis, a Ca2+ store, protein export, and contacting and regulating other organelles. This tubular network structure is determined by ER-shaping proteins, mutations in some of which are causative for neurodegenerative disorders such as hereditary spastic paraplegia (HSP). While axonal ER shares many features with the tubular ER network in other contexts, these features must be adapted to the long and narrow dimensions of axons. ER appears to be physically continuous throughout axons, over distances that are enormous on a subcellular scale. It is therefore a potential channel for long-distance or regional communication within neurons, independent of action potentials or physical transport of cargos, but involving its physiological roles such as Ca2+ or organelle homeostasis. Despite its apparent stability, axonal ER is highly dynamic, showing features like anterograde and retrograde transport, potentially reflecting continuous fusion and breakage of the network. Here we discuss the transport processes that must contribute to this dynamic behavior of ER. We also discuss the model that these processes underpin a homeostatic process that ensures both enough ER to maintain continuity of the network and repair breaks in it, but not too much ER that might disrupt local cellular physiology. Finally, we discuss how failure of ER organization in axons could lead to axon degenerative diseases, and how a requirement for ER continuity could make distal axons most susceptible to degeneration in conditions that disrupt ER continuity.

Tuning mitochondrial structure and function to criticality by fluctuation-driven mechanotransduction

Cells in vascular walls are exposed to blood pressure variability (BPV)-induced cycle-by-cycle fluctuations in mechanical forces which vary considerably with pathology. For example, BPV is elevated in hypertension but reduced under anesthesia. We hypothesized that the extent of mechanical fluctuations applied to vascular smooth muscle cells (VSMCs) regulates mitochondrial network structure near the percolation transition, which also influences ATP and reactive oxygen species (ROS) production. We stretched VSMCs in culture with cycle-by-cycle variability in area strain ranging from no variability (0%), as in standard laboratory conditions, through abnormally small (6%) and physiological (25%) to pathologically high (50%) variability mimicking hypertension, superimposed on 0.1 mean area strain. To explore how oxidative stress and ATP-dependent metabolism affect mitochondria, experiments were repeated in the presence of hydrogen peroxide and AMP-PNP, an ATP analog and competitive inhibitor of ATPases. Physiological 25% variability maintained activated mitochondrial cluster structure at percolation with a power law distribution and exponent matching the theoretical value in 2 dimensions. The 25% variability also maximized ATP and minimized cellular and mitochondrial ROS production via selective control of fission and fusion proteins (mitofusins, OPA1 and DRP1) as well as through stretch-sensitive regulation of the ATP synthase and VDAC1, the channel that releases ATP into the cytosol. Furthermore, pathologically low or high variability moved mitochondria away from percolation which reduced the effectiveness of the electron transport chain by lowering ATP and increasing ROS productions. We conclude that normal BPV is required for maintaining optimal mitochondrial structure and function in VSMCs.

The FtsZ Homolog, FszB, Inhibits Mitochondrial Dynamics in Dictyostelium discoideum

Dictyostelium discoideum is a well-established mitochondrial model system for both disease and dynamics, yet we still do not understand the actual mechanism of mitochondrial dynamics in this system. The FtsZ proteins are known to mediate membrane remodeling events such as cytokinesis in bacteria and fission of chloroplasts; D. discoideum has two FtsZ proteins, FszA and FszB. To determine the role of these proteins in mitochondrial dynamics we overexpressed FszB-GFP and determined its effect on fission, fusion, and motility in the presence of intact and disrupted cytoskeletal filaments. Here we show that overexpression of FszB-GFP decreases mitochondrial dynamics and suggest that actin may play a positive role driving fission in the context of excessive inhibition by overexpressed FszB-GFP.

Lipid droplet motility and organelle contacts

Lipid droplets (LDs) are fat storage organelles integral to energy homeostasis and a wide range of cellular processes. LDs physically and functionally interact with many partner organelles, including the ER, mitochondria, lysosomes, and peroxisomes. Recent findings suggest that the dynamics of LD inter-organelle contacts is in part controlled by LD intracellular motility. LDs can be transported directly by motor proteins along either actin filaments or microtubules, via Kinesin-1, Cytoplasmic Dynein, and type V Myosins. LDs can also be propelled indirectly, by hitchhiking on other organelles, cytoplasmic flows, and potentially actin polymerization. Although the anchors that attach motors to LDs remain elusive, other regulators of LD motility have been identified, ranging from modification of the tracks to motor co-factors to members of the perilipin family of LD proteins. Manipulating these regulatory pathways provides a tool to probe whether altered motility affects organelle contacts and has revealed that LD motility can promote interactions with numerous partners, with profound consequences for metabolism. LD motility can cause dramatic redistribution of LDs between a clustered and a dispersed state, resulting in altered organelle contacts and LD turnover. We propose that LD motility can thus promote switches in the metabolic state of a cell. Finally, LD motility is also important for LD allocation during cell division. In a number of animal embryos, uneven allocation results in a large difference in LD content in distinct daughter cells, suggesting cell-type specific LD needs.

Astrocytes autophagy in aging and neurodegenerative disorders

Astrocytes can serve multiple functions in maintaining cellular homeostasis of the central nervous system (CNS), and normal functions for autophagy in astrocytes is considered to have very vital roles in the pathogenesis of aging and neurodegenerative diseases. Autophagy is a major intracellular lysosomal (or its yeast analog, vacuolar) clearance pathways involved in the degradation and recycling of long-lived proteins, oxidatively damaged proteins and dysfunctional organelles by lysosomes. Current evidence has shown that autophagy might influence inflammation, oxidative stress, aging and function of astrocytes. Although the interrelation between autophagy and inflammation, oxidative stress, aging or neurological disorders have been addressed in detail, the influence of astrocytes mediated-autophagy in aging and neurodegenerative disorders has yet to be fully reviewed. In this review, we will summarize the most up-to-date findings and highlight the role of autophagy in astrocytes and link autophagy of astrocytes to aging and neurodegenerative diseases. Due to the prominent roles of astrocytic autophagy in age-related neurodegenerative diseases, we believe that we can provide new suggestions for the treatment of these disorders.

Monitoring and Determining Mitochondrial Network Parameters in Live Lung Cancer Cells

Mitochondria are dynamic organelles that constantly fuse and divide, forming dynamic tubular networks. Abnormalities in mitochondrial dynamics and morphology are linked to diverse pathological states, including cancer. Thus, alterations in mitochondrial parameters could indicate early events of disease manifestation or progression. However, finding reliable and quantitative tools for monitoring mitochondria and determining the network parameters, particularly in live cells, has proven challenging. Here, we present a 2D confocal imaging-based approach that combines automatic mitochondrial morphology and dynamics analysis with fractal analysis in live small cell lung cancer (SCLC) cells. We chose SCLC cells as a test case since they typically have very little cytoplasm, but an abundance of smaller mitochondria compared to many of the commonly used cell types. The 2D confocal images provide a robust approach to quantitatively measure mitochondrial dynamics and morphology in live cells. Furthermore, we performed 3D reconstruction of electron microscopic images and show that the 3D reconstruction of the electron microscopic images complements this approach to yield better resolution. The data also suggest that the parameters of mitochondrial dynamics and fractal dimensions are sensitive indicators of cellular response to subtle perturbations, and hence, may serve as potential markers of drug response in lung cancer.

Mitochondrial Network State Scales mtDNA Genetic Dynamics

Mitochondrial DNA (mtDNA) mutations cause severe congenital diseases but may also be associated with healthy aging. mtDNA is stochastically replicated and degraded, and exists within organelles which undergo dynamic fusion and fission. The role of the resulting mitochondrial networks in the time evolution of the cellular proportion of mutated mtDNA molecules (heteroplasmy), and cell-to-cell variability in heteroplasmy (heteroplasmy variance), remains incompletely understood. Heteroplasmy variance is particularly important since it modulates the number of pathological cells in a tissue. Here, we provide the first wide-reaching theoretical framework which bridges mitochondrial network and genetic states. We show that, under a range of conditions, the (genetic) rate of increase in heteroplasmy variance and de novo mutation are proportionally modulated by the (physical) fraction of unfused mitochondria, independently of the absolute fission-fusion rate. In the context of selective fusion, we show that intermediate fusion:fission ratios are optimal for the clearance of mtDNA mutants. Our findings imply that modulating network state, mitophagy rate, and copy number to slow down heteroplasmy dynamics when mean heteroplasmy is low could have therapeutic advantages for mitochondrial disease and healthy aging.

Alzheimer's Disease: Recent Concepts on the Relation of Mitochondrial Disturbances, Excitotoxicity, Neuroinflammation, and Kynurenines

The pathomechanism of Alzheimer's disease (AD) certainly involves mitochondrial disturbances, glutamate excitotoxicity, and neuroinflammation. The three main aspects of mitochondrial dysfunction in AD, i.e., the defects in dynamics, altered bioenergetics, and the deficient transport, act synergistically. In addition, glutamatergic neurotransmission is affected in several ways. The balance between synaptic and extrasynaptic glutamatergic transmission is shifted toward the extrasynaptic site contributing to glutamate excitotoxicity, a phenomenon augmented by increased glutamate release and decreased glutamate uptake. Neuroinflammation in AD is predominantly linked to central players of the innate immune system, with central nervous system (CNS)-resident microglia, astroglia, and perivascular macrophages having been implicated at the cellular level. Several abnormalities have been described regarding the activation of certain steps of the kynurenine (KYN) pathway of tryptophan metabolism in AD. First of all, the activation of indolamine 2,3-dioxygenase, the first and rate-limiting step of the pathway, is well-demonstrated. 3-Hydroxy-L-KYN and its metabolite, 3-hydroxy-anthranilic acid have pro-oxidant, antioxidant, and potent immunomodulatory features, giving relevance to their alterations in AD. Another metabolite, quinolinic acid, has been demonstrated to be neurotoxic, promoting glutamate excitotoxicity, reactive oxygen species production, lipid peroxidation, and microglial neuroinflammation, and its abundant presence in AD pathologies has been demonstrated. Finally, the neuroprotective metabolite, kynurenic acid, has been associated with antagonistic effects at glutamate receptors, free radical scavenging, and immunomodulation, giving rise to potential therapeutic implications. This review presents the multiple connections of KYN pathway-related alterations to three main domains of AD pathomechanism, such as mitochondrial dysfunction, excitotoxicity, and neuroinflammation, implicating possible therapeutic options.

Two separate functions of NME3 critical for cell survival underlie a neurodegenerative disorder

We report a patient who presented with congenital hypotonia, hypoventilation, and cerebellar histopathological alterations. Exome analysis revealed a homozygous mutation in the initiation codon of the NME3 gene, which encodes an NDP kinase. The initiation-codon mutation leads to deficiency in NME3 protein expression. NME3 is a mitochondrial outer-membrane protein capable of interacting with MFN1/2, and its depletion causes dysfunction in mitochondrial dynamics. Consistently, the patient's fibroblasts were characterized by a slow rate of mitochondrial dynamics, which was reversed by expression of wild-type or catalytic-dead NME3. Moreover, glucose starvation caused mitochondrial fragmentation and cell death in the patient's cells. The expression of wild-type and catalytic-dead but not oligomerization-attenuated NME3 restored mitochondrial elongation. However, only wild-type NME3 sustained ATP production and viability. Thus, the separate functions of NME3 in mitochondrial fusion and NDP kinase cooperate in metabolic adaptation for cell survival in response to glucose starvation. Given the critical role of mitochondrial dynamics and energy requirements in neuronal development, the homozygous mutation in NME3 is linked to a fatal mitochondrial neurodegenerative disorder.

Parvalbumin alters mitochondrial dynamics and affects cell morphology

The Ca2+-binding protein parvalbumin (PV) and mitochondria play important roles in Ca2+ signaling, buffering and sequestration. Antagonistic regulation of PV and mitochondrial volume is observed in in vitro and in vivo model systems. Changes in mitochondrial morphology, mitochondrial volume and dynamics (fusion, fission, mitophagy) resulting from modulation of PV were investigated in MDCK epithelial cells with stable overexpression/downregulation of PV. Increased PV levels resulted in smaller, roundish cells and shorter mitochondria, the latter phenomenon related to reduced fusion rates and decreased expression of genes involved in mitochondrial fusion. PV-overexpressing cells displayed increased mitophagy, a likely cause for the decreased mitochondrial volumes and the smaller overall cell size. Cells showed lower mobility in vitro, paralleled by reduced protrusions. Constitutive PV down-regulation in PV-overexpressing cells reverted mitochondrial morphology and fractional volume to the state present in control MDCK cells, resulting from increased mitochondrial movement and augmented fusion rates. PV-modulated, bi-directional and reversible mitochondrial dynamics are key to regulation of mitochondrial volume.

Miro proteins prime mitochondria for Parkin translocation and mitophagy

The Parkinson's disease‐associated protein kinase PINK1 and ubiquitin ligase Parkin coordinate the ubiquitination of mitochondrial proteins, which marks mitochondria for degradation. Miro1, an atypical GTPase involved in mitochondrial trafficking, is one of the substrates tagged by Parkin after mitochondrial damage. Here, we demonstrate that a small pool of Parkin interacts with Miro1 before mitochondrial damage occurs. This interaction does not require PINK1, does not involve ubiquitination of Miro1 and also does not disturb Miro1 function. However, following mitochondrial damage and PINK1 accumulation, this initial pool of Parkin becomes activated, leading to the ubiquitination and degradation of Miro1. Knockdown of Miro proteins reduces Parkin translocation to mitochondria and suppresses mitophagic removal of mitochondria. Moreover, we demonstrate that Miro1 EF‐hand domains control Miro1's ubiquitination and Parkin recruitment to damaged mitochondria, and they protect neurons from glutamate‐induced mitophagy. Together, our results suggest that Miro1 functions as a calcium‐sensitive docking site for Parkin on mitochondria.

Modulation of Mitochondrial Dynamics in Neurodegenerative Diseases: An Insight Into Prion Diseases

Mitochondrial dysfunction is a common and prominent feature of prion diseases and other neurodegenerative disorders. Mitochondria are dynamic organelles that constantly fuse with one another and subsequently break apart. Defective or superfluous mitochondria are usually eliminated by a form of autophagy, referred to as mitophagy, to maintain mitochondrial homeostasis. Mitochondrial dynamics are tightly regulated by processes including fusion and fission. Dysfunction of mitochondrial dynamics can lead to the accumulation of abnormal mitochondria and contribute to cellular damage. Neurons are among the cell types that consume the most energy, have a highly complex morphology, and are particularly dependent on mitochondrial functions and dynamics. In this review article, we summarize the molecular mechanisms underlying the mitochondrial dynamics and the regulation of mitophagy and discuss the dysfunction of these processes in the progression of prion diseases and other neurodegenerative disorders. We have also provided an overview of mitochondrial dynamics as a therapeutic target for neurodegenerative diseases.

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

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

Mitochondrial stress management: a dynamic journey

Mitochondria undergo continuous challenges in the course of their life, from their generation to their degradation. These challenges include the management of reactive oxygen species, the proper assembly of mitochondrial respiratory complexes and the need to balance potential mutations in the mitochondrial DNA. The detection of damage and the ability to keep it under control is critical to fine-tune mitochondrial function to the organismal energy needs. In this review, we will analyze the multiple mechanisms that safeguard mitochondrial function in light of in crescendo damage. This sequence of events will include initial defense against excessive reactive oxygen species production, compensation mechanisms by the unfolded protein response (UPR^mt), mitochondrial dynamics and elimination by mitophagy.

Hypertonia-linked protein Trak1 functions with mitofusins to promote mitochondrial tethering and fusion

Hypertonia is a neurological dysfunction associated with a number of central nervous system disorders, including cerebral palsy, Parkinson's disease, dystonia, and epilepsy. Genetic studies have identified a homozygous truncation mutation in Trak1 that causes hypertonia in mice. Moreover, elevated Trak1 protein expression is associated with several types of cancers and variants in Trak1 are linked to childhood absence epilepsy in humans. Despite the importance of Trak1 in health and disease, the mechanisms of Trak1 action remain unclear and the pathogenic effects of Trak1 mutation are unknown. Here we report that Trak1 has a crucial function in regulation of mitochondrial fusion. Depletion of Trak1 inhibits mitochondrial fusion, resulting in mitochondrial fragmentation, whereas overexpression of Trak1 elongates and enlarges mitochondria. Our analyses revealed that Trak1 interacts and colocalizes with mitofusins on the outer mitochondrial membrane and functions with mitofusins to promote mitochondrial tethering and fusion. Furthermore, Trak1 is required for stress-induced mitochondrial hyperfusion and pro-survival response. We found that hypertonia-associated mutation impairs Trak1 mitochondrial localization and its ability to facilitate mitochondrial tethering and fusion. Our findings uncover a novel function of Trak1 as a regulator of mitochondrial fusion and provide evidence linking dysregulated mitochondrial dynamics to hypertonia pathogenesis.

ROS as Regulators of Mitochondrial Dynamics in Neurons

Mitochondrial dynamics is a complex process, which involves the fission and fusion of mitochondrial outer and inner membranes. These processes organize the mitochondrial size and morphology, as well as their localization throughout the cells. In the last two decades, it has become a spotlight due to their importance in the pathophysiological processes, particularly in neurological diseases. It is known that Drp1, mitofusin 1 and 2, and Opa1 constitute the core of proteins that coordinate this intricate and dynamic process. Likewise, changes in the levels of reactive oxygen species (ROS) lead to modifications in the expression and/or activity of the proteins implicated in the mitochondrial dynamics, suggesting an involvement of these molecules in the process. In this review, we discuss the role of ROS in the regulation of fusion/fission in the nervous system, as well as the involvement of mitochondrial dynamics proteins in neurodegenerative diseases.

Alterations in the E3 ligases Parkin and CHIP result in unique metabolic signaling defects and mitochondrial quality control issues

E3 ligases are essential scaffold proteins, facilitating the transfer of ubiquitin from E2 enzymes to lysine residues of client proteins via isopeptide bonds. The specificity of substrate binding and the expression and localization of E3 ligases can, however, endow these proteins with unique features with variable effects on mitochondrial, metabolic and CNS function. By comparing and contrasting two E3 ligases, Parkin and C-terminus of HSC70-Interacting protein (CHIP) we seek to highlight the biophysical properties that may promote mitochondrial dysfunction, acute stress signaling and critical developmental periods to cease in response to mutations in these genes. Encoded by over 600 human genes, RING-finger proteins are the largest class of E3 ligases. Parkin contains three RING finger domains, with R1 and R2 separated by an in-between region (IBR) domain. Loss-of-function mutations in Parkin were identified in patients with early onset Parkinson's disease. CHIP is a member of the Ubox family of E3 ligases. It contains an N-terminal TPR domain and forms unique asymmetric homodimers. While CHIP can substitute for mutated Parkin and enhance survival, CHIP also has unique functions. The differences between these proteins are underscored by the observation that unlike Parkin-deficient animals, CHIP-null animals age prematurely and have significantly impaired motor function. These properties make these E3 ligases appealing targets for clinical intervention. In this work, we discuss how biophysical and metabolic properties of these E3 ligases have driven rapid progress in identifying roles for E3 ligases in development, proteostasis, mitochondrial biology, and cell health, as well as new data about how these proteins alter the CNS proteome.

Local and global influences on protein turnover in neurons and glia

Regulation of protein turnover allows cells to react to their environment and maintain homeostasis. Proteins can show different turnover rates in different tissue, but little is known about protein turnover in different brain cell types. We used dynamic SILAC to determine half-lives of over 5100 proteins in rat primary hippocampal cultures as well as in neuron-enriched and glia-enriched cultures ranging from <1 to >20 days. In contrast to synaptic proteins, membrane proteins were relatively shorter-lived and mitochondrial proteins were longer-lived compared to the population. Half-lives also correlate with protein functions and the dynamics of the complexes they are incorporated in. Proteins in glia possessed shorter half-lives than the same proteins in neurons. The presence of glia sped up or slowed down the turnover of neuronal proteins. Our results demonstrate that both the cell-type of origin as well as the nature of the extracellular environment have potent influences on protein turnover.

Human immunodeficiency virus Tat impairs mitochondrial fission in neurons

Human immunodeficiency virus-1 (HIV) infection of the central nervous system promotes neuronal injury that culminates in HIV-associated neurocognitive disorders. Viral proteins, including transactivator of transcription (Tat), have emerged as leading candidates to explain HIV-mediated neurotoxicity, though the mechanisms remain unclear. Tat transgenic mice or neurons exposed to Tat, which show neuronal loss, exhibit smaller mitochondria as compared to controls. To provide an experimental clue as to which mechanisms are used by Tat to promote changes in mitochondrial morphology, rat cortical neurons were exposed to Tat (100 nM) for various time points. Within 30 min, Tat caused a significant reduction in mitochondrial membrane potential, a process that is regulated by fusion and fission. To further assess whether Tat changes these processes, fission and fusion proteins dynamin-related protein 1 (Drp1) and mitofusin-2 (Mfn2), respectively, were measured. We found that Drp1 levels increased beginning at 2 h after Tat exposure while Mfn2 remained unchanged. Moreover, increased levels of an active form of Drp1 were found to be present following Tat exposure. Furthermore, Drp1 and calcineurin inhibitors prevented Tat-mediated effects on mitochondria size. These findings indicate that mitochondrial fission is likely the leading factor in Tat-mediated alterations to mitochondrial morphology. This disruption in mitochondria homeostasis may contribute to the instability of the organelle and ultimately neuronal cell death following Tat exposure.