Disrupted-in-schizophrenia 1 (DISC1) and Syntaphilin collaborate to modulate axonal mitochondrial anchoring

Gcap14 is a microtubule plus-end-tracking protein coordinating microtubule-actin crosstalk during neurodevelopment

Regulation of microtubule dynamics is required to properly control various steps of neurodevelopment. In this study, we identified granule cell antiserum-positive 14 (Gcap14) as a microtubule plus-end-tracking protein and as a regulator of microtubule dynamics during neurodevelopment. Gcap14 knockout mice exhibited impaired cortical lamination. Gcap14 deficiency resulted in defective neuronal migration. Moreover, nuclear distribution element nudE-like 1 (Ndel1), an interacting partner of Gcap14, effectively corrected the downregulation of microtubule dynamics and the defects in neuronal migration caused by Gcap14 deficiency. Finally, we found that the Gcap14-Ndel1 complex participates in the functional link between microtubule and actin filament, thereby regulating their crosstalks in the growth cones of cortical neurons. Taken together, we propose that the Gcap14-Ndel1 complex is fundamental for cytoskeletal remodeling during neurodevelopmental processes such as neuronal processes elongation and neuronal migration.

DYRK3 phosphorylates SNAPIN to regulate axonal retrograde transport and neurotransmitter release

Among the five members of the dual-specificity tyrosine-phosphorylation-regulated kinase (DYRK) family, the cellular functions of DYRK3 have not been fully elucidated. Some studies have indicated limited physiological roles and substrates of DYRK3, including promotion of glioblastoma, requirement in influenza virus replication, and coupling of stress granule condensation with mammalian target of rapamycin complex 1 signaling. Here, we demonstrate that serum deprivation causes a decrease in intracellular DYRK3 levels via the proteolytic autophagy pathway, as well as the suppression of DYRK3 gene expression. To further demonstrate how DYRK3 affects cell viability, especially in neurons, we used a yeast two-hybrid assay and identified multiple DYRK3-binding proteins, including SNAPIN, a SNARE-associated protein implicated in synaptic transmission. We also found that DYRK3 directly phosphorylates SNAPIN at the threonine (Thr) 14 residue, increasing the interaction of SNAPIN with other proteins such as dynein and synaptotagmin-1. In central nervous system neurons, SNAPIN is associated with and mediate the retrograde axonal transport of diverse cellular products from the distal axon terminal to the soma and the synaptic release of neurotransmitters, respectively. Moreover, phosphorylation of SNAPIN at Thr-14 was found to positively modulate mitochondrial retrograde transport in mouse cortical neurons and the recycling pool size of synaptic vesicles, contributing to neuronal viability. In conclusion, the present study demonstrates that DYRK3 phosphorylates SNAPIN, positively regulating the dynein-mediated retrograde transport of mitochondria and SNARE complex-mediated exocytosis of synaptic vesicles within the neurons. This finding further suggests that DYRK3 affects cell viability and provides a novel neuroprotective mechanism.

The compartmentalised nature of neuronal mitophagy: molecular insights and implications

The maintenance of a healthy mitochondrial network and the ability to adjust organelle population in response to internal or external stimuli are essential for the function and the survival of eukaryotic cells. Over the last two decades several studies have demonstrated the paramount importance of mitophagy, a selective form of autophagy that removes damaged and/or superfluous organelles, in organismal physiology. Post-mitotic neuronal cells are particularly vulnerable to mitochondrial damage, and mitophagy impairment has emerged as a causative factor in multiple neurodegenerative pathologies, including Alzheimer's disease and Parkinson's disease among others. Although mitochondrial turnover is a multifaceted process, neurons have to tackle additional complications, arising from their pronounced bioenergetic demands and their unique architecture and cellular polarisation that render the degradation of distal organelles challenging. Mounting evidence indicates that despite the functional conservation of mitophagy pathways, the unique features of neuronal physiology have led to the adaptation of compartmentalised solutions, which serve to ensure seamless mitochondrial removal in every part of the cell. In this review, we summarise the current knowledge concerning the molecular mechanisms that mediate mitophagy compartmentalisation and discuss their implications in various human pathologies.

Restoration of mitochondria axonal transport by adaptor Disc1 supplementation prevents neurodegeneration and rescues visual function

Deficits in mitochondrial transport are a common feature of neurodegenerative diseases. We investigated whether loss of components of the mitochondrial transport machinery impinge directly on metabolic stress, neuronal death, and circuit dysfunction. Using multiphoton microscope live imaging, we showed that ocular hypertension, a major risk factor in glaucoma, disrupts mitochondria anterograde axonal transport leading to energy decline in vulnerable neurons. Gene- and protein-expression analysis revealed loss of the adaptor disrupted in schizophrenia 1 (Disc1) in retinal neurons subjected to high intraocular pressure. Disc1 gene delivery was sufficient to rescue anterograde transport and replenish axonal mitochondria. A genetically encoded ATP sensor combined with longitudinal live imaging showed that Disc1 supplementation increased ATP production in stressed neurons. Disc1 gene therapy promotes neuronal survival, reverses abnormal single-cell calcium dynamics, and restores visual responses. Our study demonstrates that enhancing anterograde mitochondrial transport is an effective strategy to alleviate metabolic stress and neurodegeneration.

Quantification of a Neurological Protein in a Single Cell Without Amplification

Proteins are key biomolecules that not only play various roles in the living body but also are used as biomarkers. If these proteins can be quantified at the level of a single cell, understanding the role of proteins will be deepened and diagnosing diseases and abnormality will be further upgraded. In this study, we quantified a neurological protein in a single cell using atomic force microscopy (AFM). After capturing specifically disrupted-in-schizophrenia 1 (DISC1) in a single cell onto a microspot immobilizing the corresponding antibody on the surface, force mapping with AFM was followed to visualize individual DISC1. Although a large variation of the number of DISC1 in a cell was observed, the average number is 4.38 × 10³, and the number agrees with the ensemble-averaged value. The current AFM approach for the quantitative analysis of proteins in a single cell should be useful to study molecular behavior of proteins in depth and to follow physiological change of individual cells in response to external stimuli.

LIS1 and NDEL1 Regulate Axonal Trafficking of Mitochondria in Mature Neurons

Defective mitochondrial dynamics in axons have been linked to both developmental and late-onset neurological disorders. Axonal trafficking is in large part governed by the microtubule motors kinesin-1 and cytoplasmic dynein 1 (dynein). Dynein is the primary retrograde transport motor in axons, and mutations in dynein and many of its regulators also cause neurological diseases. Depletion of LIS1, famous for linking dynein deregulation to lissencephaly (smooth brain), in adult mice leads to severe neurological phenotypes, demonstrating post-developmental roles. LIS1 stimulates retrograde transport of acidic organelles in cultured adult rat dorsal root ganglion (DRG) axons but findings on its role in mitochondrial trafficking have been inconsistent and have not been reported for adult axons. Here we report that there is an increased number of mitochondria in cross-sections of sciatic nerve axons from adult LIS1^+/– mice. This is probably related to reduced dynein activity as axons from adult rat nerves exposed to the dynein inhibitor, ciliobrevin D also had increased numbers of mitochondria. Moreover, LIS1 overexpression (OE) in cultured adult rat DRG axons stimulated retrograde mitochondrial transport while LIS1 knockdown (KD) or expression of a LIS1 dynein-binding mutant (LIS1-K147A) inhibited retrograde transport, as did KD of dynein heavy chain (DHC). These findings are consistent with our report on acidic organelles. However, KD of NDEL1, a LIS1 and dynein binding protein, or expression of a LIS1 NDEL1-binding mutant (LIS1-R212A) also dramatically impacted retrograde mitochondrial transport, which was not the case for acidic organelles. Manipulations that disrupted retrograde mitochondrial transport also increased the average length of axonal mitochondria, suggesting a role for dynein in fusion or fission events. Our data point to cargo specificity in NDEL1 function and raise the possibility that defects in the LIS1/NDEL1 dynein regulatory pathway could contribute to mitochondrial diseases with axonal pathologies.

Mutations in DISC1 alter IP3R and voltage-gated Ca2+ channel functioning, implications for major mental illness

Disrupted in Schizophrenia 1 (DISC1) participates in a wide variety of developmental processes of central neurons. It also serves critical roles that underlie cognitive functioning in adult central neurons. Here we summarize DISC1’s general properties and discuss its use as a model system for understanding major mental illnesses (MMIs). We then discuss the cellular actions of DISC1 that involve or regulate Ca²⁺ signaling in adult central neurons. In particular, we focus on the tethering role DISC1 plays in transporting RNA particles containing Ca²⁺ channel subunit RNAs, including IP3R1, CACNA1C and CACNA2D1, and in transporting mitochondria into dendritic and axonal processes. We also review DISC1’s role in modulating IP₃R1 activity within mitochondria-associated ER membrane (MAM). Finally, we discuss DISC1-glycogen synthase kinase 3β (GSK3β) signaling that regulates functional expression of voltage-gated Ca²⁺ channels (VGCCs) at central synapses. In each case, DISC1 regulates the movement of molecules that impact Ca²⁺ signaling in neurons.

Mitochondrial Miro GTPases coordinate mitochondrial and peroxisomal dynamics

Mitochondria and peroxisomes are highly dynamic, multifunctional organelles. Both perform key roles for cellular physiology and homoeostasis by mediating bioenergetics, biosynthesis, and/or signalling. To support cellular function, they must be properly distributed, of proper size, and be able to interact with other organelles. Accumulating evidence suggests that the small atypical GTPase Miro provides a central signalling node to coordinate mitochondrial as well as peroxisomal dynamics. In this review, I summarize our current understanding of Miro-dependent functions and molecular mechanisms underlying the proper distribution, size and function of mitochondria and peroxisomes.

FHL2 anchors mitochondria to actin and adapts mitochondrial dynamics to glucose supply

Mitochondrial movement and distribution are fundamental to their function. Here we report a mechanism that regulates mitochondrial movement by anchoring mitochondria to the F-actin cytoskeleton. This mechanism is activated by an increase in glucose influx and the consequent O-GlcNAcylation of TRAK (Milton), a component of the mitochondrial motor-adaptor complex. The protein four and a half LIM domains protein 2 (FHL2) serves as the anchor. FHL2 associates with O-GlcNAcylated TRAK and is both necessary and sufficient to drive the accumulation of F-actin around mitochondria and to arrest mitochondrial movement by anchoring to F-actin. Disruption of F-actin restores mitochondrial movement that had been arrested by either TRAK O-GlcNAcylation or forced direction of FHL2 to mitochondria. This pathway for mitochondrial immobilization is present in both neurons and non-neuronal cells and can thereby adapt mitochondrial dynamics to changes in glucose availability.

Schizophrenia-associated dysbindin modulates axonal mitochondrial movement in cooperation with p150glued

Mitochondrial movement in neurons is finely regulated to meet the local demand for energy and calcium buffering. Elaborate transport machinery including motor complexes is required to deliver and localize mitochondria to appropriate positions. Defects in mitochondrial transport are associated with various neurological disorders without a detailed mechanistic information. In this study, we present evidence that dystrobrevin-binding protein 1 (dysbindin), a schizophrenia-associated factor, plays a critical role in axonal mitochondrial movement. We observed that mitochondrial movement was impaired in dysbindin knockout mouse neurons. Reduced mitochondrial motility caused by dysbindin deficiency decreased the density of mitochondria in the distal part of axons. Moreover, the transport and distribution of mitochondria were regulated by the association between dysbindin and p150glued. Furthermore, altered mitochondrial distribution in axons led to disrupted calcium dynamics, showing abnormal calcium influx in presynaptic terminals. These data collectively suggest that dysbindin forms a functional complex with p150glued that regulates axonal mitochondrial transport, thereby affecting presynaptic calcium homeostasis.

Astrocyte DISC1 contributes to cognitive function in a brain region-dependent manner

Our understanding of the contribution of genetic risk factors to neuropsychiatric diseases is limited to abnormal neurodevelopment and neuronal dysfunction. Much less is known about the mechanisms whereby risk variants could affect the physiology of glial cells. Our prior studies have shown that a mutant (dominant-negative) form of a rare but highly penetrant psychiatric risk factor, Disrupted-In-Schizophrenia-1 (DISC1), impairs metabolic functions of astrocytes and leads to cognitive dysfunction. In order to overcome the limitations of the mutant DISC1 model and understand the putative regional properties of astrocyte DISC1, we assessed whether knockdown of Disc1 (Disc1-KD) in mature mouse astrocytes of the prefrontal cortex (PFC) or the hippocampus would produce behavioral abnormalities that could be attributed to astrocyte bioenergetics. We found that Disc1-KD in the hippocampus but not PFC impaired trace fear conditioning in adult mice. Using the innovative deep learning approach and convolutional deep neural networks (cDNNs), ResNet50 or ResNet18, and single cell-based analysis, we found that Disc1-KD decreased the spatial density of astrocytes associated with abnormal levels and distribution of the mitochondrial markers and the glutamate transporter, GLAST. Disc1-KD in astrocytes also led to decreased expression of the glutamatergic and increased expression of the GABA-ergic synaptic markers, possibly via non-apoptotic activation of caspase 3 in neurons located within the individual territories of Disc1-KD astrocytes. Our results indicate that altered expression of DISC1 in astrocytes could impair astrocyte bioenergetics, leading to abnormalities in synaptic neurotransmission and cognitive function in a region-dependent fashion.

DISC1 Regulates Mitochondrial Trafficking in a Miro1-GTP-Dependent Manner

The disrupted in schizophrenia 1 (DISC1) protein is implicated in major mental illnesses including schizophrenia and bipolar disorder. A key feature of psychiatric disease is aberrant synaptic communication. Correct synaptic transmission is dependent on spatiotemporally regulated energy provision and calcium buffering. This can be achieved by precise distribution of mitochondria throughout the elaborate architecture of the neuron. Central to this process is the calcium sensor and GTPase Miro1, which allows mitochondrial trafficking by molecular motors. While the role of Miro1-calcium binding in mitochondrial transport is well described, far less is known regarding the functions of the two GTPase domains. Here, we investigate the effects of a psychiatric disease-associated mutation in DISC1 on mitochondrial trafficking. We show that this DISC1 mutation impairs Miro1’s ability to transport mitochondria. We also demonstrate the necessity of the first Miro1 GTPase domain in determining direction of mitochondrial transport and the involvement of DISC1 in this process. Finally, we describe the effects of mutant DISC1 on positioning of mitochondria at synapses.

Regulation and roles of mitophagy at synapses

Maintenance of synaptic homeostasis is a challenging task, due to the intricate spatial organization and intense activity of synapses. Typically, synapses are located far away from the neuronal cell body, where they orchestrate neuronal signalling and communication, through neurotransmitter release. Stationary mitochondria provide energy required for synaptic vesicle cycling, and preserve ionic balance by buffering intercellular calcium at synapses. Thus, synaptic homeostasis is critically dependent on proper mitochondrial function. Indeed, defective mitochondrial metabolism is a common feature of several neurodegenerative and psychiatric disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), bipolar disorders and schizophrenia among others, which are also accompanied by excessive synaptic abnormalities. Specialized and compartmentalized quality control mechanisms have evolved to restore and maintain synaptic energy metabolism. Here, we survey recent advances towards the elucidation of the pivotal role of mitochondria in neurotransmission and implicating mitophagy in the maintenance of synaptic homeostasis during ageing.

Epigenomics of being bullied: changes in DNA methylation following bullying exposure

Bullying among children is ubiquitous and associated with pervasive mental health problems. However, little is known about the biological pathways that change after exposure to bullying. Epigenome-wide changes in DNA methylation in peripheral blood were studied from pre- to post measurement of bullying exposure, in a longitudinal study of the population-based Generation R Study and Avon Longitudinal Study of Parents and Children (combined n = 1,352). Linear mixed-model results were meta-analysed to estimate how DNA methylation changed as a function of exposure to bullying. Sensitivity analyses including co-occurring child characteristics and risks were performed, as well as a Gene Ontology analysis. A candidate follow-up was employed for CpG (cytosine-phosphate-guanine) sites annotated to 5-HTT and NR3C1. One site, cg17312179, showed small changes in DNA methylation associated to bullying exposure (b = -2.67e-03, SE = 4.97e-04, p = 7.17e-08). This site is annotated to RAB14, an oncogene related to Golgi apparatus functioning, and its methylation levels decreased for exposed but increased for non-exposed. This result was consistent across sensitivity analyses. Enriched Gene Ontology pathways for differentially methylated sites included cardiac function and neurodevelopmental processes. Top CpG sites tended to have overall low levels of DNA methylation, decreasing in exposed, increasing in non-exposed individuals. There were no gene-wide corrected findings for 5-HTT and NR3C1. This is the first study to identify changes in DNA methylation associated with bullying exposure at the epigenome-wide significance level. Consistent with other population-based studies, we do not find evidence for strong associations between bullying exposure and DNA methylation.

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.

The TRAX, DISC1, and GSK3 complex in mental disorders and therapeutic interventions

Psychiatric disorders (such as bipolar disorder, depression, and schizophrenia) affect the lives of millions of individuals worldwide. Despite the tremendous efforts devoted to various types of psychiatric studies and rapidly accumulating genetic information, the molecular mechanisms underlying psychiatric disorder development remain elusive. Among the genes that have been implicated in schizophrenia and other mental disorders, disrupted in schizophrenia 1 (DISC1) and glycogen synthase kinase 3 (GSK3) have been intensively investigated. DISC1 binds directly to GSK3 and modulates many cellular functions by negatively inhibiting GSK3 activity. The human DISC1 gene is located on chromosome 1 and is highly associated with schizophrenia and other mental disorders. A recent study demonstrated that a neighboring gene of DISC1, translin-associated factor X (TRAX), binds to the DISC1/GSK3β complex and at least partly mediates the actions of the DISC1/GSK3β complex. Previous studies also demonstrate that TRAX and most of its interacting proteins that have been identified so far are risk genes and/or markers of mental disorders. In the present review, we will focus on the emerging roles of TRAX and its interacting proteins (including DISC1 and GSK3β) in psychiatric disorders and the potential implications for developing therapeutic interventions.

Mechanisms underlying the role of DISC1 in synaptic plasticity

Disrupted in schizophrenia 1 (DISC1) is an important hub protein, forming multimeric complexes by self-association and interacting with a large number of synaptic and cytoskeletal molecules. The synaptic location of DISC1 in the adult brain suggests a role in synaptic plasticity, and indeed, a number of studies have discovered synaptic plasticity impairments in a variety of different DISC1 mutants. This review explores the possibility that DISC1 is an important molecule for organizing proteins involved in synaptic plasticity and examines why mutations in DISC1 impair plasticity. It concentrates on DISC1's role in interacting with synaptic proteins, controlling dendritic structure and cellular trafficking of mRNA, synaptic vesicles and mitochondria. N-terminal directed mutations appear to impair synaptic plasticity through interactions with phosphodiesterase 4B (PDE4B) and hence protein kinase A (PKA)/GluA1 and PKA/cAMP response element-binding protein (CREB) signalling pathways, and affect spine structure through interactions with kalirin 7 (Kal-7) and Rac1. C-terminal directed mutations also impair plasticity possibly through altered interactions with lissencephaly protein 1 (LIS1) and nuclear distribution protein nudE-like 1 (NDEL1), thereby affecting developmental processes such as dendritic structure and spine maturation. Many of the same molecules involved in DISC1's cytoskeletal interactions are also involved in intracellular trafficking, raising the possibility that impairments in intracellular trafficking affect cytoskeletal development and vice versa. While the multiplicity of DISC1 protein interactions makes it difficult to pinpoint a single causal signalling pathway, we suggest that the immediate-term effects of N-terminal influences on GluA1, Rac1 and CREB, coupled with the developmental effects of C-terminal influences on trafficking and the cytoskeleton make up the two main branches of DISC1's effect on synaptic plasticity and dendritic spine stability.

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

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

DISC1 regulates lactate metabolism in astrocytes: implications for psychiatric disorders

Our knowledge of how genetic risk variants contribute to psychiatric disease is mainly limited to neurons. However, the mechanisms whereby the same genetic risk factors could affect the physiology of glial cells remain poorly understood. We studied the role of a psychiatric genetic risk factor, Disrupted-In-Schizophrenia-1 (DISC1), in metabolic functions of astrocytes. We evaluated the effects of knockdown of mouse endogenous DISC1 (DISC1-KD) and expression of a dominant-negative, C-terminus truncated human DISC1 (DN-DISC1) on the markers of energy metabolism, including glucose uptake and lactate production, in primary astrocytes and in mice with selective expression of DN-DISC1 in astrocytes. We also assessed the effects of lactate treatment on altered affective behaviors and impaired spatial memory in DN-DISC1 mice. Both DISC1-KD and DN-DISC1 comparably decreased mRNA and protein levels of glucose transporter 4 and glucose uptake by primary astrocytes. Decreased glucose uptake was associated with reduced oxidative phosphorylation and glycolysis as well as diminished lactate production in vitro and in vivo. No significant effects of DISC1 manipulations in astrocytes were observed on expression of the subunits of the electron transport chain complexes or mitofilin, a neuronal DISC1 partner. Lactate treatment rescued the abnormal behaviors in DN-DISC1 male and female mice. Our results suggest that DISC1 may be involved in the regulation of lactate production in astrocytes to support neuronal activity and associated behaviors. Abnormal expression of DISC1 in astrocytes and resulting abnormalities in energy supply may be responsible for aspects of mood and cognitive disorders observed in patients with major psychiatric illnesses.

Mitochondrial roles of the psychiatric disease risk factor DISC1

Ion transport during neuronal signalling utilizes the majority of the brain's energy supply. Mitochondria are key sites for energy provision through ATP synthesis and play other important roles including calcium buffering. Thus, tightly regulated distribution and function of these organelles throughout the intricate architecture of the neuron is essential for normal synaptic communication. Therefore, delineating mechanisms coordinating mitochondrial transport and function is essential for understanding nervous system physiology and pathology. While aberrant mitochondrial transport and dynamics have long been associated with neurodegenerative disease, they have also more recently been linked to major mental illness including schizophrenia, autism and depression. However, the underlying mechanisms have yet to be elucidated, due to an incomplete understanding of the combinations of genetic and environmental factors contributing to these conditions. Consequently, the DISC1 gene has undergone intense study since its discovery at the site of a balanced chromosomal translocation, segregating with mental illness in a Scottish pedigree. The precise molecular functions of DISC1 remain elusive. Reported functions of DISC1 include regulation of intracellular signalling pathways, neuronal migration and dendritic development. Intriguingly, a role for DISC1 in mitochondrial homeostasis and transport is fast emerging. Therefore, a major function of DISC1 in regulating mitochondrial distribution, ATP synthesis and calcium buffering may be disrupted in psychiatric disease. In this review, we discuss the links between DISC1 and mitochondria, considering both trafficking of these organelles and their function, and how, via these processes, DISC1 may contribute to the regulation of neuronal behavior in normal and psychiatric disease states.

Neurobiology of axonal transport defects in motor neuron diseases: Opportunities for translational research?

Intracellular trafficking of cargoes is an essential process to maintain the structure and function of all mammalian cell types, but especially of neurons because of their extreme axon/dendrite polarisation. Axonal transport mediates the movement of cargoes such as proteins, mRNA, lipids, membrane-bound vesicles and organelles that are mostly synthesised in the cell body and in doing so is responsible for their correct spatiotemporal distribution in the axon, for example at specialised sites such as nodes of Ranvier and synaptic terminals. In addition, axonal transport maintains the essential long-distance communication between the cell body and synaptic terminals that allows neurons to react to their surroundings via trafficking of for example signalling endosomes. Axonal transport defects are a common observation in a variety of neurodegenerative diseases, and mutations in components of the axonal transport machinery have unequivocally shown that impaired axonal transport can cause neurodegeneration (reviewed in El-Kadi et al., 2007, De Vos et al., 2008; Millecamps and Julien, 2013). Here we review our current understanding of axonal transport defects and the role they play in motor neuron diseases (MNDs) with a specific focus on the most common form of MND, amyotrophic lateral sclerosis (ALS).

Regulation of mitochondrial dynamics by DISC1, a putative risk factor for major mental illness

Mitochondria are dynamic organelles that are essential to power the process of neurotransmission. Neurons must therefore ensure that mitochondria maintain their functional integrity and are efficiently transported along the full extent of the axons and dendrites, from soma to synapses. Mitochondrial dynamics (trafficking, fission and fusion) co-ordinately regulate mitochondrial quality control and function. DISC1 is a component of the mitochondrial transport machinery and regulates mitochondrial dynamics. DISC1's role in this is adversely affected by sequence variants connected to brain structure/function and disease risk, and by mutant truncation. The DISC1 interactors NDE1 and GSK3β are also involved, indicating a convergence of putative risk factors for psychiatric illness upon mitochondrial dynamics.

Role of DISC1 in Neuronal Trafficking and its Implication in Neuropsychiatric Manifestation and Neurotherapeutics

Disrupted-in-schizophrenia 1 (DISC1) was initially identified as a gene disrupted by a translocation mutation co-segregating with a variety of psychotic and mood disorders in a Scottish pedigree. In agreement with this original finding, mouse models that perturb Disc1 display deficits of behaviors in specific dimensions, such as cognition and emotion, but not a motor dimension. Although DISC1 is not a risk gene for sporadic cases of specific psychiatric disorders defined by categorical diagnostic criteria (e.g., schizophrenia and major depressive disorder), DISC1 is now regarded as an important molecular lead to decipher molecular pathology for specific dimensions relevant to major mental illnesses. Emerging evidence points to the role of DISC1 in the regulation of intracellular trafficking of a wide range of neuronal cargoes. We will review recent progress in this aspect of DISC1 biology and discuss how we could utilize this body of knowledge to better understand the pathophysiology of mental illnesses.