Mutant huntingtin aggregates impair mitochondrial movement and trafficking in cortical neurons

Mutant huntingtin disrupts mitochondrial proteostasis by interacting with TIM23

Mutant huntingtin (mHTT), the causative protein in Huntington's disease (HD), associates with the translocase of mitochondrial inner membrane 23 (TIM23) complex, resulting in inhibition of synaptic mitochondrial protein import first detected in presymptomatic HD mice. The early timing of this event suggests that it is a relevant and direct pathophysiologic consequence of mHTT expression. We show that, of the 4 TIM23 complex proteins, mHTT specifically binds to the TIM23 subunit and that full-length wild-type huntingtin (wtHTT) and mHTT reside in the mitochondrial intermembrane space. We investigated differences in mitochondrial proteome between wtHTT and mHTT cells and found numerous proteomic disparities between mHTT and wtHTT mitochondria. We validated these data by quantitative immunoblotting in striatal cell lines and human HD brain tissue. The level of soluble matrix mitochondrial proteins imported through the TIM23 complex is lower in mHTT-expressing cell lines and brain tissues of HD patients compared with controls. In mHTT-expressing cell lines, membrane-bound TIM23-imported proteins have lower intramitochondrial levels, whereas inner membrane multispan proteins that are imported via the TIM22 pathway and proteins integrated into the outer membrane generally remain unchanged. In summary, we show that, in mitochondria, huntingtin is located in the intermembrane space, that mHTT binds with high-affinity to TIM23, and that mitochondria from mHTT-expressing cells and brain tissues of HD patients have reduced levels of nuclearly encoded proteins imported through TIM23. These data demonstrate the mechanism and biological significance of mHTT-mediated inhibition of mitochondrial protein import, a mechanism likely broadly relevant to other neurodegenerative diseases.

Quality Control in Huntington's Disease: a Therapeutic Target

Huntington's disease (HD) is a fatal autosomal dominantly inherited brain disease caused by excessively expanded CAG repeats in gene which encodes huntingtin protein. These abnormally encoded huntingtin proteins and their truncated fragments result in disruption of cellular quality mechanism ultimately triggering neuronal death. Despite great efforts, a potential causative agent leading to genetic mutation in HTT, manifesting the neurons more prone to oxidative stress, cellular inflammation, energy depletion and apoptotic death, has not been established yet. Current scenario concentrates on symptomatic pathologies to improvise the disease progression and to better the survival. Most of the therapeutic developments have been converged to rescue the protein homeostasis. In HD, abnormal expansion of glutamine repeats in the protein huntingtin leads to toxic aggregation of huntingtin which in turn impairs the quality control mechanism of cells through damaging the machineries involved in removal of aggregated abnormal protein. Therapeutic approaches to improve the efficiency of aggregate clearance through quality control mechanisms involve protein folding machineries such as chaperones and protein degradation machineries such as proteasome and autophagy. Also, to reduce protein aggregation by enhancing proper folding, to degrade and eliminate the aggregates are suggested to negatively regulate the HD progression associated with the disruption of protein homeostasis. This review focuses on the collection of therapeutic strategies targeting enhancement of protein quality control activity to delay the HD pathogenesis.

The role of the protein-RNA recognition code in neurodegeneration

MicroRNAs are small endogenous RNAs that pair and bind to sites on mRNAs to direct post-transcriptional repression. However, there is a possibility that microRNAs directly influence protein structure and activity, and this influence can be termed post-translational riboregulation. This conceptual review explores the literature on neurodegenerative disorders. Research on the association between neurodegeneration and RNA-repeat toxicity provides data that support a protein-RNA recognition code. For example, this code explains why hnRNP H and SFPQ proteins, which are involved in amyotrophic lateral sclerosis, are sequestered by the (GGGGCC)n repeat sequence. Similarly, it explains why MNBL proteins and (CTG)n repeats in RNA, which are involved in myotonic dystrophy, are sequestered into RNA foci. Using this code, proteins involved in diseases can be identified. A simple protein BLAST search of the human genome for amino acid repeats that correspond to the nucleotide repeats reveals new proteins among already known proteins that are involved in diseases. For example, the (CAG)n repeat sequence, when transcribed into possible peptide sequences, leads to the identification of PTCD3, Rem2, MESP2, SYPL2, WDR33, COL23A1, and others. After confirming this approach on RNA repeats, in the next step, the code was used in the opposite manner. Proteins that are involved in diseases were compared with microRNAs involved in those diseases. For example, a reasonable correspondence of microRNA 9 and 107 with amyloid-β-peptide (Aβ42) was identified. In the last step, a miRBase search for micro-nucleotides, obtained by transcription of a prion amino acid sequence, revealed new microRNAs and microRNAs that have previously been identified as involved in prion diseases. This concept provides a useful key for designing RNA or peptide probes.

Superlocalized Three-Dimensional Live Imaging of Mitochondrial Dynamics in Neurons Using Plasmonic Nanohole Arrays

We investigated the transport of neuronal mitochondria using superlocalized near-fields with plasmonic nanohole arrays (PNAs). Compared to traditional imaging techniques, PNAs create a massive array of superlocalized light beams and allow 3D mitochondrial dynamics to be sampled and extracted almost in real time. In this work, mitochondrial fluorescence excited by the PNAs was captured by an optical microscope using dual objective lenses, which produced superlocalized dynamics while minimizing light scattering by the plasmonic substrate. It was found that mitochondria move with an average velocity 0.33 ± 0.26 μm/s, a significant part of which, by almost 50%, was contributed by the movement along the depth axis ( z-axis). Mitochondrial positions were acquired with superlocalized precision (σ _x = 5.7 nm and σ _y = 11.8 nm) in the lateral plane and σ _z = 78.7 nm in the z-axis, which presents an enhancement by 12.7-fold in resolution compared to confocal fluorescence microscopy. The approach is expected to serve as a way to provide 3D information on molecular dynamics in real time.

Mitochondrial transport serves as a mitochondrial quality control strategy in axons: Implications for central nervous system disorders

Axonal mitochondrial quality is essential for neuronal health and functions. Compromised mitochondrial quality, reflected by loss of membrane potential, collapse of ATP production, abnormal morphology, burst of reactive oxygen species generation, and impaired Ca²⁺ buffering capacity, can alter mitochondrial transport. Mitochondrial transport in turn maintains axonal mitochondrial homeostasis in several ways. Newly generated mitochondria are anterogradely transported along with axon from soma to replenish axonal mitochondrial pool, while damaged mitochondria undergo retrograde transport for repair or degradation. Besides, mitochondria are also arrested in axon to quarantine damages locally. Accumulating evidence suggests abnormal mitochondrial transport leads to mitochondrial dysfunction and axon degeneration in a variety of neurological and psychiatric disorders. Further investigations into the details of this process would help to extend our understanding of various neurological diseases and shed light on the corresponding therapies.

Prionoid Proteins in the Pathogenesis of Neurodegenerative Diseases

There is a growing body of evidence that prionoid protein behaviors are a core element of neurodegenerative diseases (NDs) that afflict humans. Common elements in pathogenesis, pathological effects and protein-level behaviors exist between Alzheimer's Disease (AD), Parkinson's Disease (PD), Huntington's Disease (HD) and Amyotrophic Lateral Sclerosis (ALS). These extend beyond the affected neurons to glial cells and processes. This results in a complicated system of disease progression, which often takes advantage of protective processes to promote the propagation of pathological protein aggregates. This review article provides a current snapshot of knowledge on these proteins and their intrinsic role in the pathogenesis and disease progression seen across NDs.

Targeting the proteostasis network in Huntington's disease

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

Mitochondria modulate programmed neuritic retraction

Neuritic retraction in the absence of overt neuronal death is a shared feature of normal aging and neurodegenerative disorders, but the intracellular mechanisms modulating this process are not understood. We propose that cumulative distal mitochondrial protein damage results in impaired protein import, leading to mitochondrial dysfunction and focal activation of the canonical apoptosis pathway in neurites. This is a controlled process that may not lead to neuronal death and, thus, we term this phenomenon "neuritosis." Consistent with our hypothesis, we show that in primary cerebrocortical neurons, mitochondrial distance from the soma correlates with increased mitochondrial protein damage, PINK1 accumulation, reactive oxygen species production, and decreased mitochondrial membrane potential and depolarization threshold. Furthermore, we demonstrate that the distance-dependent mitochondrial membrane potential gradient exists in vivo in mice. We demonstrate that impaired distal mitochondria have a lower threshold for focal/nonlethal neuritic caspase-3 activation in normal neurons that is exacerbated in aging, stress, and neurodegenerative conditions, thus delineating a fundamental mechanistic underpinning for synaptic vulnerability.

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.

Towards an Understanding of Energy Impairment in Huntington's Disease Brain

This review systematically examines the evidence for shifts in flux through energy generating biochemical pathways in Huntington’s disease (HD) brains from humans and model systems. Compromise of the electron transport chain (ETC) appears not to be the primary or earliest metabolic change in HD pathogenesis. Rather, compromise of glucose uptake facilitates glucose flux through glycolysis and may possibly decrease flux through the pentose phosphate pathway (PPP), limiting subsequent NADPH and GSH production needed for antioxidant protection. As a result, oxidative damage to key glycolytic and tricarboxylic acid (TCA) cycle enzymes further restricts energy production so that while basal needs may be met through oxidative phosphorylation, those of excessive stimulation cannot. Energy production may also be compromised by deficits in mitochondrial biogenesis, dynamics or trafficking. Restrictions on energy production may be compensated for by glutamate oxidation and/or stimulation of fatty acid oxidation. Transcriptional dysregulation generated by mutant huntingtin also contributes to energetic disruption at specific enzymatic steps. Many of the alterations in metabolic substrates and enzymes may derive from normal regulatory feedback mechanisms and appear oscillatory. Fine temporal sequencing of the shifts in metabolic flux and transcriptional and expression changes associated with mutant huntingtin expression remain largely unexplored and may be model dependent. Differences in disease progression among HD model systems at the time of experimentation and their varying states of metabolic compensation may explain conflicting reports in the literature. Progressive shifts in metabolic flux represent homeostatic compensatory mechanisms that maintain the model organism through presymptomatic and symptomatic stages.

Synaptic Mitochondria are More Susceptible to Traumatic Brain Injury-induced Oxidative Damage and Respiratory Dysfunction than Non-synaptic Mitochondria

Traumatic brain injury (TBI) results in mitochondrial dysfunction and induction of lipid peroxidation (LP). Lipid peroxidation-derived neurotoxic aldehydes such as 4-HNE and acrolein bind to mitochondrial proteins, inducing additional oxidative damage and further exacerbating mitochondrial dysfunction and LP. Mitochondria are heterogeneous, consisting of both synaptic and non-synaptic populations. Synaptic mitochondria are reported to be more vulnerable to injury; however, this is the first study to characterize the temporal profile of synaptic and non-synaptic mitochondria following TBI, including investigation of respiratory dysfunction and oxidative damage to mitochondrial proteins between 3 and 120 h following injury. These results indicate that synaptic mitochondria are indeed the more vulnerable population, showing both more rapid and severe impairments than non-synaptic mitochondria. By 24 h, synaptic respiration is significantly impaired compared to synaptic sham, whereas non-synaptic respiration does not decline significantly until 48 h. Decreases in respiration are associated with increases in oxidative damage to synaptic and non-synaptic mitochondrial proteins at 48 h and 72 h, respectively. These results indicate that the therapeutic window for mitochondria-targeted pharmacological neuroprotectants to prevent respiratory dysfunction is shorter for the more vulnerable synaptic mitochondria than for the non-synaptic population.

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

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

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

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

Huntington's disease: novel therapeutic perspectives hanging in the balance

INTRODUCTION

Huntington's disease (HD), an autosomal dominant neurodegenerative disorder caused by an expansion of CAG repeats in the huntingtin gene, has long been characterized by the presence of motor symptoms due to the loss of striatal projection neurons. Cognitive dysfunction and neuropsychiatric symptoms are also present and they occur in the absence of cell death in most mouse models, pointing to neuronal dysfunction and abnormal synaptic plasticity as causative mechanisms. Areas covered: Here, we focus on those common mechanisms altered by the presence of mutant huntingtin affecting corticostriatal and hippocampal function as therapeutic targets that could prove beneficial to ameliorate both cognitive and motor function in HD. Specifically, we discuss the importance of reestablishing the balance in (1) synaptic/extrasynaptic N-methyl-D-aspartate receptor signaling, (2) mitochondrial dynamics/trafficking, (3) TrkB/p75^NTR signaling, and (4) transcriptional activity. Expert opinion: Mutant huntingtin has a broad impact on multiple cellular processes, which makes it very challenging to design a curative therapeutic strategy. As we point out here, novel therapeutic interventions should look for multi-purpose drugs targeting common and early affected processes leading to corticostriatal and hippocampal dysfunction that additionally operate in a feedforward vicious cycle downstream the activation of extrasynaptic N-methyl-D-aspartate receptor.

Brain pericyte activation occurs early in Huntington's disease

Microvascular changes have recently been described for several neurodegenerative disorders, including Huntington's disease (HD). HD is characterized by a progressive neuronal cell loss due to a mutation in the Huntingtin gene. However, the temporal and spatial microvascular alterations in HD remain unclear. Also, knowledge on the implication of pericytes in HD pathology is still sparse and existing findings are contradictory. Here we examine alterations in brain pericytes in the R6/2 mouse model of HD and in human post mortem HD brain sections. To specifically track activated pericytes, we crossbred R6/2 mice with transgenic mice expressing the Green fluorescent protein gene under the Regulator of G-protein signaling 5 (Rgs5) promoter. We demonstrate an increase in activated pericytes in the R6/2 brain and in post mortem HD brain tissue. Importantly, pericyte changes are already detected before striatal neuronal cell loss, weight loss or behavioural deficits occur in R6/2 mice. This is associated with vascular alterations, whereby striatal changes precede cortical changes. Our findings suggest that pericyte activation may be one of the initial steps contributing to the observed vascular modifications in HD. Thus, pericytes may constitute an important target to address early microvascular changes contributing to disease progression in HD.

The viral protein gp120 decreases the acetylation of neuronal tubulin: potential mechanism of neurotoxicity

The human immunodeficiency virus (HIV) envelope protein gp120 promotes axonal damage and neurite pruning, similar to that observed in HIV-positive subjects with neurocognitive disorders. Thus, gp120 has been used to examine molecular and cellular pathways underlying HIV-mediated neuronal dysfunction. Gp120 binds to tubulin beta III, a component of neuronal microtubules. Microtubule function, which modulates the homeostasis of neurons, is regulated by polymerization and post-translational modifications. Based on these considerations, we tested the hypothesis that gp120 induces dynamic instability of neuronal microtubules. We first observed that gp120 prevents the normal polymerization of tubulin in vitro. We then tested whether gp120 alters the post-translational modifications in tubulin by examining the ability of gp120 to change the levels of acetylated tubulin in primary rat neuronal cultures. Gp120 elicited a time-dependent decrease in tubulin acetylation that was reversed by Helix-A peptide, a compound that competitively displaces the binding of gp120 to neuronal microtubules. To determine whether post-translational modifications in tubulin also occur in vivo, we measured acetylated tubulin in the cerebral cortex of HIV transgenic rats (HIV-tg). We observed a decrease in tubulin acetylation in 5- and 9-month-old HIV-tg rats when compared to age-matched wild type. Neither changes in microglia morphology nor alterations in mRNA levels for interleukin-1β and tumor necrosis factor α were detected in 5-month-old animals. Our findings propose neuronal microtubule instability as a novel mechanism of HIV neurotoxicity, without evidence of enhanced inflammation.

Endocytic Trafficking of HIV gp120 is Mediated by Dynamin and Plays a Role in gp120 Neurotoxicity

Neurons that endocytose the human immunodeficiency virus-1 (HIV) protein gp120 exhibit neurite retraction and activation of caspase-3, suggesting that the endocytic process may be crucial for gp120-mediated neuronal injury. The goal of this study is to demonstrate that internalization and accumulation of gp120 play a role in its neurotoxic effects. In mammalian cells, endocytosis is primarily a dynamin-dependent process. To establish whether gp120 is endocytosed in a dynamin-dependent manner, we used fibroblasts in which deletion of dynamins was induced by tamoxifen. We observed a robust reduction of intracellular gp120 immunoreactivity in tamoxifen-treated cells. To examine whether endocytosis of gp120 is crucial for its neurotoxic effect, we blocked gp120 internalization into primary rat cortical neurons by dynasore, an inhibitor of the dynamin GTP-ase activity. We found that dynasore blocks both gp120 internalization and neurotoxicity. We then utilized gp120-loaded mesoporous silica nanoparticles to deliver gp120 intracellularly. We established that once internalized, gp120 is neurotoxic regardless of chemokine receptor activation. Our data suggest that dynamin-dependent endocytosis of gp120 is critical for its neurotoxicity.

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.

Developmental changes in trak-mediated mitochondrial transport in neurons

Previous studies established that the kinesin adaptor proteins, TRAK1 and TRAK2, play an important role in mitochondrial transport in neurons. They link mitochondria to kinesin motor proteins via a TRAK acceptor protein in the mitochondrial outer membrane, the Rho GTPase, Miro. TRAKs also associate with enzyme, O-linked N-acetylglucosamine transferase (OGT), to form a quaternary, mitochondrial trafficking complex. A recent report suggested that TRAK1 preferentially controls mitochondrial transport in axons of hippocampal neurons whereas TRAK2 controls mitochondrial transport in dendrites. However, it is not clear whether the function of any of these proteins is exclusive to axons or dendrites and if their mechanisms of action are conserved between different neuronal populations and also, during maturation. Here, a comparative study was carried out into TRAK-mediated mitochondrial mobility in axons and dendrites of hippocampal and cortical neurons during maturation in vitro using a shRNA gene knockdown approach. It was found that in mature hippocampal and cortical neurons, TRAK1 predominantly mediates axonal mitochondrial transport whereas dendritic transport is mediated via TRAK2. In young, maturing neurons, TRAK1 and TRAK2 contribute similarly in mitochondrial transport in both axons and dendrites in both neuronal types. These findings demonstrate maturation regulation of mitochondrial transport which is conserved between at least two distinct neuronal subtypes.

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Mitochondrial transport and velocity changes during neuronal maturation.

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TRAK1 and TRAK2 contribute to transport in axons and dendrites of immature neurons.

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In mature neurons TRAK1 controls axonal mitochondrial transport.

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In mature neurons TRAK2 controls dendritic mitochondrial transport.

A link between chromatin condensation mechanisms and Huntington's disease: connecting the dots

Huntington's disease is a rare neurodegenerative disorder whose complex pathophysiology exhibits system-wide changes in the body, with striking and debilitating clinical features targeting the central nervous system. Among the various molecular functions affected in this disease, mitochondrial dysfunction and transcriptional dysregulation are some of the most studied aspects of this disease. However, there is evidence of the involvement of a mutant Huntingtin protein in the processes of DNA damage, chromosome condensation and DNA repair. This review attempts to briefly recapitulate the clinical features, model systems used to study the disease, major molecular processes affected, and, more importantly, examines recent evidence for the involvement of the mutant Huntingtin protein in the processes regulating chromosome condensation, leading to DNA damage response and neuronal death.

Possible role of mitochondrial permeability transition pore in the pathogenesis of Huntington disease

Huntington disease (HD) is a devastating neurological disorder that affects the striatum and cortex of patients. HD patients develop progressive motor dysfunction and psychiatric disturbances with gradual dementia. HD is caused by a pathological expansion of CAG repeats in the huntingtin gene that codifies for a protein called huntingtin (Htt), which principal function is not completely understood. Accumulative evidence shows that this pathological expansion modifies Htt function affecting different neuronal targets, including mitochondrial function which is an important factor that contributes to HD. Interestingly, several groups have shown mitochondrial disturbances including calcium handling defects, depolarization, decrease of mitochondrial transport, ATP reduction, and increase of reactive oxygen species (ROS) in cellular and murine HD models. Systematic analysis of this evidence indicates that a mitochondrial structure, the mitochondrial permeability transition pore (mPTP), could be responsible for these changes that affect mitochondria. The mPTP plays an important role in apoptosis and neurodegeneration. It has also been reported to have some physiological functions in heart development and synaptic communication. In HD, the presence of mutant huntingtin (mHtt) activates this mechanism producing a significant compromise of mitochondrial metabolism and bioenergetics. Considering these findings this review explores the evidence that suggests the important role of mPTP in the mitochondrial impairment induced by mHtt, which leads to calcium derangement and contributes to neuronal dysfunction in HD.

Novel Metabolic Abnormalities in the Tricarboxylic Acid Cycle in Peripheral Cells From Huntington's Disease Patients

Metabolic dysfunction is well-documented in Huntington's disease (HD). However, the link between the mutant huntingtin (mHTT) gene and the pathology is unknown. The tricarboxylic acid (TCA) cycle is the main metabolic pathway for the production of NADH for conversion to ATP via the electron transport chain (ETC). The objective of this study was to test for differences in enzyme activities, mRNAs and protein levels related to the TCA cycle between lymphoblasts from healthy subjects and from patients with HD. The experiments utilize the advantages of lymphoblasts to reveal new insights about HD. The large quantity of homogeneous cell populations permits multiple dynamic measures to be made on exactly comparable tissues. The activities of nine enzymes related to the TCA cycle and the expression of twenty-nine mRNAs encoding for these enzymes and enzyme complexes were measured. Cells were studied under baseline conditions and during metabolic stress. The results support our recent findings that the activities of the pyruvate dehydrogenase complex (PDHC) and succinate dehydrogenase (SDH) are elevated in HD. The data also show a large unexpected depression in MDH activities. Furthermore, message levels for isocitrate dehydrogenase 1 (IDH1) were markedly increased in in HD lymphoblasts and were responsive to treatments. The use of lymphoblasts allowed us to clarify that the reported decrease in aconitase activity in HD autopsy brains is likely due to secondary hypoxic effects. These results demonstrate the mRNA and enzymes of the TCA cycle are critical therapeutic targets that have been understudied in HD.

Mitochondrial Oxidative Phosphorylation System (OXPHOS) Deficits in Schizophrenia: Possible Interactions with Cellular Processes

Mitochondria are key players in the generation and regulation of cellular bioenergetics, producing the majority of adenosine triphosphate molecules by the oxidative phosphorylation system (OXPHOS). Linked to numerous signaling pathways and cellular functions, mitochondria, and OXPHOS in particular, are involved in neuronal development, connectivity, plasticity, and differentiation. Impairments in a variety of mitochondrial functions have been described in different general and psychiatric disorders, including schizophrenia (SCZ), a severe, chronic, debilitating illness that heavily affects the lives of patients and their families. This article reviews findings emphasizing the role of OXPHOS in the pathophysiology of SCZ. Evidence accumulated during the past few decades from imaging, transcriptomic, proteomic, and metabolomic studies points at OXPHOS deficit involvement in SCZ. Abnormalities have been reported in high-energy phosphates generated by the OXPHOS, in the activity of its complexes and gene expression, primarily of complex I (CoI). In addition, cellular signaling such as cAMP/protein kinase A (PKA) and Ca(+2), neuronal development, connectivity, and plasticity have been linked to OXPHOS function and are reported to be impaired in SCZ. Finally, CoI has been shown as a site of interaction for both dopamine (DA) and antipsychotic drugs, further substantiating its role in the pathology of SCZ. Understanding the role of mitochondria and the OXPHOS in particular may encourage new insights into the pathophysiology and etiology of this debilitating disorder.

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.

Impaired GAPDH-induced mitophagy contributes to the pathology of Huntington's disease

Mitochondrial dysfunction is implicated in multiple neurodegenerative diseases. In order to maintain a healthy population of functional mitochondria in cells, defective mitochondria must be properly eliminated by lysosomal machinery in a process referred to as mitophagy. Here, we uncover a new molecular mechanism underlying mitophagy driven by glyceraldehyde-3-phosphate dehydrogenase (GAPDH) under the pathological condition of Huntington’s disease (HD) caused by expansion of polyglutamine repeats. Expression of expanded polyglutamine tracts catalytically inactivates GAPDH (iGAPDH), which triggers its selective association with damaged mitochondria in several cell culture models of HD. Through this mechanism, iGAPDH serves as a signaling molecule to induce direct engulfment of damaged mitochondria into lysosomes (micro-mitophagy). However, abnormal interaction of mitochondrial GAPDH with long polyglutamine tracts stalled GAPDH-mediated mitophagy, leading to accumulation of damaged mitochondria, and increased cell death. We further demonstrated that overexpression of inactive GAPDH rescues this blunted process and enhances mitochondrial function and cell survival, indicating a role for GAPDH-driven mitophagy in the pathology of HD.

Mutant Huntingtin and Elusive Defects in Oxidative Metabolism and Mitochondrial Calcium Handling

Elongation of a polyglutamine (polyQ) stretch in huntingtin protein (Htt) is linked to Huntington's disease (HD) pathogenesis. The mutation in Htt correlates with neuronal dysfunction in the striatum and cerebral cortex and eventually leads to neuronal cell death. The exact mechanisms of the injurious effect of mutant Htt (mHtt) on neurons are not completely understood but might include aberrant gene transcription, defective autophagy, abnormal mitochondrial biogenesis, anomalous mitochondrial dynamics, and trafficking. In addition, deficiency in oxidative metabolism and defects in mitochondrial Ca(2+) handling are considered essential contributing factors to neuronal dysfunction in HD and, consequently, in HD pathogenesis. Since the discovery of the mutation in Htt, the questions whether mHtt affects oxidative metabolism and mitochondrial Ca(2+) handling and, if it does, what mechanisms could be involved were in focus of numerous investigations. However, despite significant research efforts, the detrimental effect of mHtt and the mechanisms by which mHtt might impair oxidative metabolism and mitochondrial Ca(2+) handling remain elusive. In this paper, I will briefly review studies aimed at clarifying the consequences of mHtt interaction with mitochondria and discuss experimental results supporting or arguing against the mHtt effects on oxidative metabolism and mitochondrial Ca(2+) handling.

Oxidative metabolism and Ca2+ handling in isolated brain mitochondria and striatal neurons from R6/2 mice, a model of Huntington's disease

Alterations in oxidative metabolism and defects in mitochondrial Ca²⁺ handling have been implicated in the pathology of Huntington's disease (HD), but existing data are contradictory. We investigated the effect of human mHtt fragments on oxidative metabolism and Ca²⁺ handling in isolated brain mitochondria and cultured striatal neurons from the R6/2 mouse model of HD. Non-synaptic and synaptic mitochondria isolated from the brains of R6/2 mice had similar respiratory rates and Ca²⁺ uptake capacity compared with mitochondria from wild-type (WT) mice. Respiratory activity of cultured striatal neurons measured with Seahorse XF24 flux analyzer revealed unaltered cellular respiration in neurons derived from R6/2 mice compared with neurons from WT animals. Consistent with the lack of respiratory dysfunction, ATP content of cultured striatal neurons from R6/2 and WT mice was similar. Mitochondrial Ca²⁺ accumulation was also evaluated in cultured striatal neurons from R6/2 and WT animals. Our data obtained with striatal neurons derived from R6/2 and WT mice show that both glutamate-induced increases in cytosolic Ca²⁺ and subsequent carbonilcyanide p-triflouromethoxyphenylhydrazone-induced increases in cytosolic Ca²⁺ were similar between WT and R6/2, suggesting that mitochondria in neurons derived from both types of animals accumulated comparable amounts of Ca²⁺ Overall, our data argue against respiratory deficiency and impaired Ca²⁺ handling induced by human mHtt fragments in both isolated brain mitochondria and cultured striatal neurons from transgenic R6/2 mice.

The HIV Protein gp120 Alters Mitochondrial Dynamics in Neurons

Neurotoxicity of human immunodeficiency virus-1 (HIV) includes synaptic simplification and neuronal apoptosis. However, the mechanisms of HIV-associated neurotoxicity remain unclear, thus precluding an effective treatment of the neurological complications. The present study was undertaken to characterize novel mechanisms of HIV neurotoxicity that may explain how HIV subjects develop neuronal degeneration. Several neurodegenerative disorders are characterized by mitochondrial dysfunction; therefore, we hypothesized that HIV promotes mitochondrial damage. We first analyzed brains from HIV encephalitis (HIVE) by electron microscopy. Several sections of HIVE subjects contained enlarged and damaged mitochondria compared to brains from HIV subjects with no neurological complications. Similar pathologies were observed in mice overexpressing the HIV protein gp120, suggesting that this viral protein may be responsible for mitochondrial pathology found in HIVE. To gain more information about the cellular mechanisms of gp120 neurotoxicity, we exposed rat cortical neurons to gp120 and we determined cellular oxygen consumption rate, mitochondrial distribution, and trafficking. Our data show that gp120 evokes impairment in mitochondrial function and distribution. These data suggest that one of the mechanisms of HIV neurotoxicity includes altered mitochondrial dynamics in neurons.

The role of mitochondrial dysfunction in age-related diseases

The aging process is accompanied by the onset of disease and a general decline in wellness. Insights into the aging process have revealed a number of cellular hallmarks of aging, among these epigenetic alterations, loss of proteostasis, mitochondrial dysfunction, cellular senescence, and stem cell exhaustion. Mitochondrial dysfunction increasingly appears to be a common factor connecting several of these hallmarks, driving the aging process and afflicting tissues throughout the body. Recent research has uncovered a much more complex involvement of mitochondria in the cell than has previously been appreciated and revealed novel ways in which mitochondrial defects feed into disease pathology. In this review we evaluate ways in which problems in mitochondria contribute to disease beyond the well-known mechanisms of oxidative stress and bioenergetic deficits, and we predict the direction that mitochondrial disease research will take in years to come.

Mitochondrial division inhibitor 1 protects against mutant huntingtin-induced abnormal mitochondrial dynamics and neuronal damage in Huntington's disease

The objective of this study was to determine the protective effects of the mitochondrial division inhibitor 1 (Mdivi1) in striatal neurons that stably express mutant Htt (STHDhQ111/Q111) and wild-type (WT) Htt (STHDhQ7/Q7). Using gene expression analysis, biochemical methods, transmission electron microscopy (TEM) and confocal microscopy methods, we studied (i) mitochondrial and synaptic activities by measuring mRNA and the protein levels of mitochondrial and synaptic genes, (ii) mitochondrial function and (iii) ultra-structural changes in mutant Htt neurons relative to WT Htt neurons. We also studied these parameters in Mdivil-treated and untreated WT and mutant Htt neurons. Increased expressions of mitochondrial fission genes, decreased expression of fusion genes and synaptic genes were found in the mutant Htt neurons relative to the WT Htt neurons. Electron microscopy of the mutant Htt neurons revealed a significantly increased number of mitochondria, indicating that mutant Htt fragments mitochondria. Biochemical analysis revealed defective mitochondrial functioning. In the Mdivil-treated mutant Htt neurons, fission genes were down-regulated, and fusion genes were up-regulated, suggesting that Mdivil decreases fission activity. Synaptic genes were up-regulated, and mitochondrial function was normal in the Mdivi1-treated mutant Htt neurons. Immunoblotting findings of mitochondrial and synaptic proteins agreed with mRNA findings. The TEM studies revealed that increased numbers of structurally intact mitochondria were present in Mdivi1-treated mutant Htt neurons. Increased synaptic and mitochondrial fusion genes and decreased fission genes were found in the Mdivi1-treated WT Htt neurons, indicating that Mdivi1 beneficially affects healthy neurons. Taken together, these findings suggest that Mdivi1 is protective against mutant Htt-induced mitochondrial and synaptic damage in HD neurons and that Mdivi1 may be a promising molecule for the treatment of HD patients.

Mitochondrial dynamics and quality control in Huntington's disease

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by polyglutamine expansion mutations in the huntingtin protein. Despite its ubiquitous distribution, expression of mutant huntingtin (mHtt) is particularly detrimental to medium spiny neurons within the striatum. Mitochondrial dysfunction has been associated with HD pathogenesis. Here we review the current evidence for mHtt-induced abnormalities in mitochondrial dynamics and quality control, with a particular focus on brain and neuronal data pertaining to striatal vulnerability. We address mHtt effects on mitochondrial biogenesis, protein import, complex assembly, fission and fusion, mitochondrial transport, and on the degradation of damaged mitochondria via autophagy (mitophagy). For an integrated perspective on potentially converging pathogenic mechanisms, we also address impaired autophagosomal transport and abnormal mHtt proteostasis in HD.

Electron Transport Disturbances and Neurodegeneration: From Albert Szent-Györgyi's Concept (Szeged) till Novel Approaches to Boost Mitochondrial Bioenergetics

Impaired function of certain mitochondrial respiratory complexes has long been linked to the pathogenesis of chronic neurodegenerative disorders such as Parkinson's and Huntington's diseases. Furthermore, genetic alterations of mitochondrial genome or nuclear genes encoding proteins playing essential roles in maintaining proper mitochondrial function can lead to the development of severe systemic diseases associated with neurodegeneration and vacuolar myelinopathy. At present, all of these diseases lack effective disease modifying therapy. Following a brief commemoration of Professor Albert Szent-Györgyi, a Nobel Prize laureate who pioneered in the field of cellular respiration, antioxidant processes, and the roles of free radicals in health and disease, the present paper overviews the current knowledge on the involvement of mitochondrial dysfunction in central nervous system diseases associated with neurodegeneration including Parkinson's and Huntington's disease as well as mitochondrial encephalopathies. The review puts special focus on the involvement and the potential therapeutic relevance of peroxisome proliferator-activated receptor-gamma coactivator 1-alpha (PGC-1α), a nuclear-encoded master regulator of mitochondrial biogenesis and antioxidant responses in these disorders, the transcriptional activation of which may hold novel therapeutic value as a more system-based approach aiming to restore mitochondrial functions in neurodegenerative processes.

Ca(2+) handling in isolated brain mitochondria and cultured neurons derived from the YAC128 mouse model of Huntington's disease

We investigated Ca(2+) handling in isolated brain synaptic and non-synaptic mitochondria and in cultured striatal neurons from the YAC128 mouse model of Huntington's disease. Both synaptic and non-synaptic mitochondria from 2- and 12-month-old YAC128 mice had larger Ca(2+) uptake capacity than mitochondria from YAC18 and wild-type FVB/NJ mice. Synaptic mitochondria from 12-month-old YAC128 mice had further augmented Ca(2+) capacity compared with mitochondria from 2-month-old YAC128 mice and age-matched YAC18 and FVB/NJ mice. This increase in Ca(2+) uptake capacity correlated with an increase in the amount of mutant huntingtin protein (mHtt) associated with mitochondria from 12-month-old YAC128 mice. We speculate that this may happen because of mHtt-mediated sequestration of free fatty acids thereby increasing resistance of mitochondria to Ca(2+)-induced damage. In experiments with striatal neurons from YAC128 and FVB/NJ mice, brief exposure to 25 or 100 μM glutamate produced transient elevations in cytosolic Ca(2+) followed by recovery to near resting levels. Following recovery of cytosolic Ca(2+), mitochondrial depolarization with FCCP produced comparable elevations in cytosolic Ca(2+), suggesting similar Ca(2+) release and, consequently, Ca(2+) loads in neuronal mitochondria from YAC128 and FVB/NJ mice. Together, our data argue against a detrimental effect of mHtt on Ca(2+) handling in brain mitochondria of YAC128 mice. We demonstrate that mutant huntingtin (mHtt) binds to brain synaptic and nonsynaptic mitochondria and the amount of mitochondria-bound mHtt correlates with increased mitochondrial Ca(2+) uptake capacity. We propose that this may happen due to mHtt-mediated sequestration of free fatty acids thereby increasing resistance of mitochondria to Ca(2+)-induced damage.

Assessing Mitochondrial Movement Within Neurons: Manual Versus Automated Tracking Methods

Owing to the small size of mitochondria and the complexity of their motility patterns, mitochondrial tracking is technically challenging. Mitochondria are often tracked manually; however, this is time-consuming and prone to measurement error. Here, we examined the suitability of four commercial and open-source software alternatives for automated mitochondrial tracking in neurons compared with manual measurements. We show that all the automated tracking tools dramatically underestimated track length, mitochondrial displacement and movement duration, with reductions ranging from 45 to 77% of the values obtained manually. In contrast, mitochondrial velocity was generally overestimated. Only the number of motile mitochondria and their directionality were similar between strategies. Despite these discrepancies, we show that automated tools successfully detected transport alterations after applying an oxidant agent. Thus, automated methods appear to be suitable for assessing relative transport differences between experimental groups, but not for absolute quantification of mitochondrial dynamics. Although useful for objective and time-efficient measurements of mitochondrial movements, results provided by automated methods should be interpreted with caution.

Disruption of the nuclear membrane by perinuclear inclusions of mutant huntingtin causes cell-cycle re-entry and striatal cell death in mouse and cell models of Huntington's disease

Accumulation of N-terminal fragments of mutant huntingtin (mHTT) in the cytoplasm, nuclei and axons of neurons is a hallmark of Huntington's disease (HD), although how these fragments negatively impact neurons remains unclear. We followed the distribution of mHTT in the striata of transgenic R6/2-J2 HD mice as their motor function declined. The fraction of cells with diffuse, perinuclear or intranuclear mHTT changed in parallel with decreasing motor function. In transgenic mice, medium spiny neurons (MSNs) that exhibited perinuclear inclusions expressed cell-cycle markers typically not seen in the striata of normal mice, and these cells are preferentially lost as disease progresses. Electron microscopy reveals that perinuclear inclusions disrupt the nuclear envelope. The progression of perinuclear inclusions being accompanied by cell-cycle activation and culminating in cell death was also observed in 1° cortical neurons. These observations provide a strong correlation between the subcellular location of mHTT, disruption of the nucleus, re-entry into the cell-cycle and eventual neuronal death. They also highlight the fact that the subcellular distribution of mHTT is highly dynamic such that the distribution of mHTT observed depends greatly on the stage of the disease being examined.

From pathways to targets: understanding the mechanisms behind polyglutamine disease

The history of polyglutamine diseases dates back approximately 20 years to the discovery of a polyglutamine repeat in the androgen receptor of SBMA followed by the identification of similar expansion mutations in Huntington's disease, SCA1, DRPLA, and the other spinocerebellar ataxias. This common molecular feature of polyglutamine diseases suggests shared mechanisms in disease pathology and neurodegeneration of disease specific brain regions. In this review, we discuss the main pathogenic pathways including proteolytic processing, nuclear shuttling and aggregation, mitochondrial dysfunction, and clearance of misfolded polyglutamine proteins and point out possible targets for treatment.

Microtubule-stabilizing agents as potential therapeutics for neurodegenerative disease

Microtubules (MTs), cytoskeletal elements found in all mammalian cells, play a significant role in cell structure and in cell division. They are especially critical in the proper functioning of post-mitotic central nervous system neurons, where MTs serve as the structures on which key cellular constituents are trafficked in axonal projections. MTs are stabilized in axons by the MT-associated protein tau, and in several neurodegenerative diseases, including Alzheimer's disease, frontotemporal lobar degeneration, and Parkinson's disease, tau function appears to be compromised due to the protein dissociating from MTs and depositing into insoluble inclusions referred to as neurofibrillary tangles. This loss of tau function is believed to result in alterations of MT structure and function, resulting in aberrant axonal transport that likely contributes to the neurodegenerative process. There is also evidence of axonal transport deficiencies in other neurodegenerative diseases, including amyotrophic lateral sclerosis and Huntington's disease, which may result, at least in part, from MT alterations. Accordingly, a possible therapeutic strategy for such neurodegenerative conditions is to treat with MT-stabilizing agents, such as those that have been used in the treatment of cancer. Here, we review evidence of axonal transport and MT deficiencies in a number of neurodegenerative diseases, and summarize the various classes of known MT-stabilizing agents. Finally, we highlight the growing evidence that small molecule MT-stabilizing agents provide benefit in animal models of neurodegenerative disease and discuss the desired features of such molecules for the treatment of these central nervous system disorders.

Antibodies and protein misfolding: From structural research tools to therapeutic strategies

Protein misfolding disorders, including the neurodegenerative conditions Alzheimer's disease (AD) and Parkinson's disease (PD) represent one of the major medical challenges or our time. The underlying molecular mechanisms that govern protein misfolding and its links with disease are very complex processes, involving the formation of transiently populated but highly toxic molecular species within the crowded environment of the cell and tissue. Nevertheless, much progress has been made in understanding these events in recent years through innovative experiments and therapeutic strategies, and in this review we present an overview of the key roles of antibodies and antibody fragments in these endeavors. We discuss in particular how these species are being used in combination with a variety of powerful biochemical and biophysical methodologies, including a range of spectroscopic and microscopic techniques applied not just in vitro but also in situ and in vivo, both to gain a better understanding of the mechanistic nature of protein misfolding and aggregation and also to design novel therapeutic strategies to combat the family of diseases with which they are associated. This article is part of a Special Issue entitled: Recent advances in molecular engineering of antibody.

Expression, pharmacology and functional activity of adenosine A1 receptors in genetic models of Huntington's disease

Adenosine A1 receptor (A1R) stimulation exerts beneficial effects in response to various insults to the brain and, although it was found neuroprotective in a lesional model of Huntington's disease (HD), the features of this receptor in genetic models of HD have never been explored. In the present study we characterized the expression, affinity and functional effects of A1Rs in R6/2 mice (the most widely used transgenic model of HD) and in a cellular model of HD. Binding studies revealed that the density of A1Rs was significantly reduced in the cortex and the striatum of R6/2 mice compared to age-matched wild-type (WT), while receptor affinity was unchanged. The selective A1R agonist cyclopentyladenosine (CPA, 300nM) was significantly more effective in reducing synaptic transmission in corticostriatal slices from symptomatic R6/2 than in age-matched WT mice. Such an effect was due to a stronger inhibition of glutamate release from the pre-synaptic terminal. The different functional activities of A1Rs in HD mice were associated also to a different intracellular signaling pathway involved in the synaptic effect of CPA. In fact, while the PKA pathway was involved in both genotypes, p38 MAPK inhibitor SB203580 partially prevented synaptic effects of CPA in R6/2, but not in WT, mice; moreover, CPA differently modulated the phosphorylation status of p38 in the two genotypes. In vitro studies confirmed a different behavior of A1Rs in HD: CPA (100 nM for 5h) modulated cell viability in STHdh(Q111/Q111) (mhttHD cells), without affecting the viability of STHdh(Q7/Q7) (wthtt cells). This effect was prevented by the application of SB203580. Our results demonstrate that in the presence of the HD mutation A1Rs undergo profound changes in terms of expression, pharmacology and functional activity. These changes have to be taken in due account when considering A1Rs as a potential therapeutic target for this disease.

Axons degenerate in the absence of mitochondria in C. elegans

Many neurodegenerative disorders are associated with mitochondrial defects [1-3]. Mitochondria can play an active role in degeneration by releasing reactive oxygen species and apoptotic factors [4-7]. Alternatively, mitochondria can protect axons from stress and insults, for example by buffering calcium [8]. Recent studies manipulating mitochondria lend support to both of these models [9-13]. Here, we identify a C. elegans mutant, ric-7, in which mitochondria are unable to exit the neuron cell bodies, similar to the kinesin-1/unc-116 mutant. When axons lacking mitochondria are cut with a laser, they rapidly degenerate. Some neurons even spontaneously degenerate in ric-7 mutants. Degeneration can be suppressed by forcing mitochondria into the axons of the mutants. The protective effect of mitochondria is also observed in the wild-type: a majority of axon fragments containing a mitochondrion survive axotomy, whereas those lacking mitochondria degenerate. Thus, mitochondria are not required for axon degeneration and serve a protective role in C. elegans axons.

The regulation of autophagosome dynamics by huntingtin and HAP1 is disrupted by expression of mutant huntingtin, leading to defective cargo degradation

Autophagy is an essential cellular pathway for degrading defective organelles and aggregated proteins. Defects in autophagy have been implicated in the neurodegenerative disorder Huntington's disease (HD), in which polyglutamine-expanded huntingtin (polyQ-htt) is predominantly cleared by autophagy. In neurons, autophagosomes form constitutively at the axon tip and undergo robust retrograde axonal transport toward the cell body, but the factors regulating autophagosome dynamics and autophagosome maturation are not well understood. Here, we show that both huntingtin (htt) and its adaptor protein huntingtin-associated protein-1 (HAP1) copurify and colocalize with autophagosomes in neurons. We use live-cell imaging and RNAi in primary neurons from GFP-LC3 transgenic mice to show that htt and HAP1 control autophagosome dynamics, regulating dynein and kinesin motors to promote processive transport. Expression of polyQ-htt in either primary neurons or striatal cells from HD knock-in mice is sufficient to disrupt the axonal transport of autophagosomes. Htt is not required for autophagosome formation or cargo loading. However, the defective autophagosome transport observed in both htt-depleted neurons and polyQ-htt-expressing neurons is correlated with inefficient degradation of engulfed mitochondrial fragments. Together, these studies identify htt and HAP1 as regulators of autophagosome transport in neurons and suggest that misregulation of autophagosome transport in HD leads to inefficient autophagosome maturation, potentially due to inhibition of autophagosome/lysosome fusion along the axon. The resulting defective clearance of both polyQ-htt aggregates and dysfunctional mitochondria by neuronal autophagosomes may contribute to neurodegeneration and cell death in HD.