Targeted disruption of mouse conventional kinesin heavy chain, kif5B, results in abnormal perinuclear clustering of mitochondria

Kinesin-1 Regulates Extrasynaptic Targeting of NMDARs and Neuronal Vulnerability Toward Excitotoxicity

N-methyl-D-aspartate (NMDA) receptor (NMDAR) is highly compartmentalized in neurons, and its dysfunction has been implicated in various neuropsychiatric and neurodegenerative disorders. Recent failure to exploit NMDAR antagonization as a potential therapeutic target has driven the need to identify molecular mechanisms that regulate NMDAR compartmentalization. Here, we report that the reduction of Kif5b, the heavy chain of kinesin-1, protected neurons against NMDA-induced excitotoxicity and ischemia-provoked neurodegeneration. Direct binding of kinesin-1 to the GluN2B cytoplasmic tails regulated the levels of NMDAR at extrasynaptic sites and the subsequent influx of calcium mediated by extrasynaptic NMDAR by regulating the insertion of NMDARs into neuronal surface. Transient increase of Kif5b restored the surface levels of NMDAR and the decreased neuronal susceptibility to NMDA-induced excitotoxicity. The expression of Kif5b was repressed in cerebral ischemia preconditioning. Our findings reveal that kinesin-1 regulates extrasynaptic NMDAR targeting and signaling, and the reduction of kinesin-1 could be exploited to defer neurodegeneration.

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Kif5b directly binds with GluN2B-containing NMDAR

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Kinesin-1 mediates extrasynaptic NMDAR targeting and function

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Reduction of kinesin-1 protects neurons against NMDAR-elicited excitotoxicity

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Reduction of kinesin-1 protects brain against ischemia-elicited neurodegeneration

Ciliary kinesins beyond IFT: Cilium length, disassembly, cargo transport and signalling

Cilia and flagella are microtubule-based antenna which are highly conserved among eukaryotes. In vertebrates, primary and motile cilia have evolved to exert several key functions during development and tissue homoeostasis. Ciliary dysfunction in humans causes a highly heterogeneous group of diseases called ciliopathies, a class of genetic multisystemic disorders primarily affecting kidney, skeleton, retina, lung and the central nervous system. Among key ciliary proteins, kinesin family members (KIF) are microtubule-interacting proteins involved in many diverse cellular functions, including transport of cargo (organelles, proteins and lipids) along microtubules and regulating the dynamics of cytoplasmic and spindle microtubules through their depolymerising activity. Many KIFs are also involved in diverse ciliary functions including assembly/disassembly, motility and signalling. We here review these ciliary kinesins in vertebrates and focus on their involvement in ciliopathy-related disorders.

The integral function of the endocytic recycling compartment is regulated by RFFL-mediated ubiquitylation of Rab11 effectors

Endocytic trafficking is regulated by ubiquitylation (also known as ubiquitination) of cargoes and endocytic machineries. The role of ubiquitylation in lysosomal delivery has been well documented, but its role in the recycling pathway is largely unknown. Here, we report that the ubiquitin (Ub) ligase RFFL regulates ubiquitylation of endocytic recycling regulators. An RFFL dominant-negative (DN) mutant induced clustering of endocytic recycling compartments (ERCs) and delayed endocytic cargo recycling without affecting lysosomal traffic. A BioID RFFL interactome analysis revealed that RFFL interacts with the Rab11 effectors EHD1, MICALL1 and class I Rab11-FIPs. The RFFL DN mutant strongly captured these Rab11 effectors and inhibited their ubiquitylation. The prolonged interaction of RFFL with Rab11 effectors was sufficient to induce the clustered ERC phenotype and to delay cargo recycling. RFFL directly ubiquitylates these Rab11 effectors in vitro, but RFFL knockout (KO) only reduced the ubiquitylation of Rab11-FIP1. RFFL KO had a minimal effect on the ubiquitylation of EHD1, MICALL1, and Rab11-FIP2, and failed to delay transferrin recycling. These results suggest that multiple Ub ligases including RFFL regulate the ubiquitylation of Rab11 effectors, determining the integral function of the ERC.

Trafficking of a multifunctional protein by endosomal microautophagy: linking two independent unconventional secretory pathways

During physiologic stresses, like micronutrient starvation, infection, and cancer, the cytosolic moonlighting protein glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is trafficked to the plasma membrane (PM) and extracellular milieu (ECM). Our work demonstrates that GAPDH mobilized to the PM, and the ECM does not utilize the classic endoplasmic reticulum-Golgi route of secretion; instead, it is first selectively translocated into early and late endosomes from the cytosol via microautophagy. GAPDH recruited to this common entry point is subsequently delivered into multivesicular bodies, leading to its membrane trafficking through secretion via exosomes and secretory lysosomes. We present evidence that both pathways of GAPDH membrane trafficking are up-regulated upon iron starvation, potentially by mobilization of intracellular calcium. These pathways also play a role in clearance of misfolded intracellular polypeptide aggregates. Our findings suggest that cells build in redundancy for vital cellular pathways to maintain micronutrient homeostasis and prevent buildup of toxic intracellular misfolded protein refuse.-Chauhan, A. S., Kumar, M., Chaudhary, S., Dhiman, A., Patidar, A., Jakhar, P., Jaswal, P., Sharma, K., Sheokand, N., Malhotra, H., Raje, C. I., Raje. M. Trafficking of a multifunctional protein by endosomal microautophagy: linking two independent unconventional secretory pathways.

Tau and Axonal Transport Misregulation in Tauopathies

Tau is a microtubule-associated protein that is involved in both normal and pathological processes in neurons. Since the discovery and characterization of tau over 40 years ago, our understanding of tau's normal functions and toxic roles in neurodegenerative tauopathies has continued to expand. Fast axonal transport is a critical process for maintaining axons and functioning synapses, critical subcellular compartments underlying neuronal connectivity. Signs of fast axonal transport disruption are pervasive in Alzheimer's disease and other tauopathies and various mechanisms have been proposed for regulation of fast axonal transport by tau. Post-translational modifications of tau including phosphorylation at specific sites, FTDP-17 point mutations, and oligomerization, confer upon tau a toxic effect on fast axonal transport. Consistent with the well-established dependence of axons on fast axonal transport, these disease-related modifications are closely associated temporally and spatially with axonal degeneration in the early disease stages. These factors position tau as a potentially critical factor mediating the disruption of fast axonal transport that precedes synaptic dysfunction and axonal degeneration at later disease stages. In this chapter, we review the evidence that tau affects fast axonal transport and examine several potential mechanisms proposed to underlie this toxicity.

Kinesin-2 and IFT-A act as a complex promoting nuclear localization of β-catenin during Wnt signalling

Wnt/Wg-signalling is critical signalling in all metazoans. Recent studies suggest that IFT-A proteins and Kinesin-2 modulate canonical Wnt/Wg-signalling independently of their ciliary role. Whether they function together in Wnt-signalling and their mechanistic role in the pathway remained unresolved. Here we demonstrate that Kinesin-2 and IFT-A proteins act as a complex during Drosophila Wg-signalling, affecting pathway activity in the same manner, interacting genetically and physically, and co-localizing with β-catenin, the mediator of Wnt/Wg-signalling on microtubules. Following pathway activation, Kinesin-2/IFT-A mutant cells exhibit high cytoplasmic β-catenin levels, yet fail to activate Wg-targets. In mutant tissues in both, Drosophila and mouse/MEFs, nuclear localization of β-catenin is markedly reduced. We demonstrate a conserved, motor-domain dependent function of the Kinesin-2/IFT-A complex in promoting nuclear translocation of β-catenin. We show that this is mediated by protecting β-catenin from a conserved cytoplasmic retention process, thus identifying a mechanism for Kinesin-2/IFT-A in Wnt-signalling that is independent of their ciliary role.

IFT-A proteins and Kinesin-2 modulate canonical Wnt/Wg-signalling independent of their ciliary role, but how is unclear. Here, the authors show that Kinesin-2 and IFT-A act as a complex to promote nuclear translocation of β-catenin in Drosophila and mouse MEF Wnt signalling independent of its ciliary role.

Multimodal transport and dispersion of organelles in narrow tubular cells

Intracellular components explore the cytoplasm via active motor-driven transport in conjunction with passive diffusion. We model the motion of organelles in narrow tubular cells using analytical techniques and numerical simulations to study the efficiency of different transport modes in achieving various cellular objectives. Our model describes length and time scales over which each transport mode dominates organelle motion, along with various metrics to quantify exploration of intracellular space. For organelles that search for a specific target, we obtain the average capture time for given transport parameters and show that diffusion and active motion contribute to target capture in the biologically relevant regime. Because many organelles have been found to tether to microtubules when not engaged in active motion, we study the interplay between immobilization due to tethering and increased probability of active transport. We derive parameter-dependent conditions under which tethering enhances long-range transport and improves the target capture time. These results shed light on the optimization of intracellular transport machinery and provide experimentally testable predictions for the effects of transport regulation mechanisms such as tethering.

Retraction of rod-like mitochondria during microtubule-dependent transport

Molecular motors play relevant roles on the regulation of mitochondria size and shape, essential properties for the cell homeostasis. In this work, we tracked single rod-shaped mitochondria with nanometer precision to explore the performance of microtubule motor teams during processive anterograde and retrograde transport. We analyzed simultaneously the organelle size and verified that mitochondria retracted during retrograde transport with their leading tip moving slower in comparison with the rear tip. In contrast, mitochondria preserved their size during anterograde runs indicating a different performance of plus-end directed teams. These results were interpreted considering the different performance of dynein and kinesin teams and provide valuable information on the collective action of motors during mitochondria transport.

Kinesin-dependent transport of keratin filaments: a unified mechanism for intermediate filament transport

Keratin intermediate filaments (IFs) are the major cytoskeletal component in epithelial cells. The dynamics of keratin IFs have been described to depend mostly on the actin cytoskeleton, but the rapid transport of fully polymerized keratin filaments has not been reported. In this work, we used a combination of photoconversion experiments and clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeats-associated protein 9 genome editing to study the role of microtubules and microtubule motors in keratin filament transport. We found that long keratin filaments, like other types of IFs, are transported along microtubules by kinesin-1. Our data revealed that keratin and vimentin are nonconventional kinesin-1 cargoes because their transport did not require kinesin light chains, which are a typical adapter for kinesin-dependent cargo transport. Furthermore, we found that the same domain of the kinesin heavy chain tail is involved in keratin and vimentin IF transport, strongly suggesting that multiple types of IFs move along microtubules using an identical mechanism.-Robert, A., Tian, P., Adam, S. A., Kittisopikul, M., Jaqaman, K., Goldman, R. D., Gelfand, V. I. Kinesin-dependent transport of keratin filaments: a unified mechanism for intermediate filament transport.

ROS Control Mitochondrial Motility through p38 and the Motor Adaptor Miro/Trak

Mitochondrial distribution and motility are recognized as central to many cellular functions, but their regulation by signaling mechanisms remains to be elucidated. Here, we report that reactive oxygen species (ROS), either derived from an extracellular source or intracellularly generated, control mitochondrial distribution and function by dose-dependently, specifically, and reversibly decreasing mitochondrial motility in both rat hippocampal primary cultured neurons and cell lines. ROS decrease motility independently of cytoplasmic [Ca²⁺], mitochondrial membrane potential, or permeability transition pore opening, known effectors of oxidative stress. However, multiple lines of genetic and pharmacological evidence support that a ROS-activated mitogen-activated protein kinase (MAPK), p38α, is required for the motility inhibition. Furthermore, anchoring mitochondria directly to kinesins without involvement of the physiological adaptors between the organelles and the motor protein prevents the H₂O₂-induced decrease in mitochondrial motility. Thus, ROS engage p38α and the motor adaptor complex to exert changes in mitochondrial motility, which likely has both physiological and pathophysiological relevance.

UNC-16/JIP3 and UNC-76/FEZ1 limit the density of mitochondria in C. elegans neurons by maintaining the balance of anterograde and retrograde mitochondrial transport

We investigate the role of axonal transport in regulating neuronal mitochondrial density. We show that the density of mitochondria in the touch receptor neuron (TRN) of adult Caenorhabditis elegans is constant. Mitochondrial density and transport are controlled both by the Kinesin heavy chain and the Dynein-Dynactin complex. However, unlike in other models, the presence of mitochondria in C. elegans TRNs depends on a Kinesin light chain as well. Mutants in the three C. elegans miro genes do not alter mitochondrial density in the TRNs. Mutants in the Kinesin-1 associated proteins, UNC-16/JIP3 and UNC-76/FEZ1, show increased mitochondrial density and also have elevated levels of both the Kinesin Heavy and Light Chains in neurons. Genetic analyses suggest that, the increased mitochondrial density at the distal end of the neuronal process in unc-16 and unc-76 depends partly on Dynein. We observe a net anterograde bias in the ratio of anterograde to retrograde mitochondrial flux in the neuronal processes of unc-16 and unc-76, likely due to both increased Kinesin-1 and decreased Dynein in the neuronal processes. Our study shows that UNC-16 and UNC-76 indirectly limit mitochondrial density in the neuronal process by maintaining a balance in anterograde and retrograde mitochondrial axonal transport.

Analysis of the function of KIF3A and KIF3B in the spermatogenesis in Boleophthalmus pectinirostris

Spermatogenesis represents one of the most complicated morphological transformation procedures. During this process, the assembly and maintenance of the flagella and intracellular transport of membrane-bound organelles required KIF3A and KIF3B. Our main goal was to test KIF3A and KIF3B location during spermatogenesis of Boleophthalmus pectinirostris. We cloned complete cDNA of KIF3A/3B from the testis of B. pectinirostris by PCR and rapid amplification of cDNA ends (RACE). The predicted secondary and tertiary structures of B. pectinirostris KIF3A/3B contained three domains: (a) the head region, (b) the stalk region, and (c) the tail region. Real-time quantitative PCR (qPCR) results revealed that KIF3A and KIF3B mRNA were presented in all the tissues examined, with the highest expression seen in the testis. In situ hybridization (ISH) showed that KIF3A and KIF3B were distributed in the periphery of the nuclear in the spermatocyte and the early spermatid. In the late spermatid and mature sperm, the KIF3A and KIF3B mRNA were gradually gathered to one side where the flagella formed. Immunofluorescence (IF) showed that KIF3A, tubulin, and mitochondria were co-localized in different stages during spermiogenesis in B. pectinirostris. The temporal and spatial expression dynamics of KIF3A/3B indicate that KIF3A and KIF3B might be involved in flagellar assembly and maintenance at the mRNA and protein levels. Moreover, these proteins may transport the mitochondria resulting in flagellum formation in B. pectinirostris.

Mitochondrial dynamics in Parkinson's disease: a role for α-synuclein?

The distinctive pathological hallmarks of Parkinson's disease are the progressive death of dopaminergic neurons and the intracellular accumulation of Lewy bodies enriched in α-synuclein protein. Several lines of evidence from the study of sporadic, familial and pharmacologically induced forms of human Parkinson's disease also suggest that mitochondrial dysfunction plays an important role in disease progression. Although many functions have been proposed for α-synuclein, emerging data from human and animal models of Parkinson's disease highlight a role for α-synuclein in the control of neuronal mitochondrial dynamics. Here, we review the α-synuclein structural, biophysical and biochemical properties that influence relevant mitochondrial dynamic processes such as fusion-fission, transport and clearance. Drawing on current evidence, we propose that α-synuclein contributes to the mitochondrial defects that are associated with the pathology of this common and progressive neurodegenerative disease.

Summary: The authors review the α-synuclein structural, biophysical and biochemical properties that influence relevant mitochondrial physiological processes such as fusion-fission, transport and clearance, and propose that α-synuclein contributes to the mitochondrial defects that are associated with Parkinson's disease.

MIRO-1 Determines Mitochondrial Shape Transition upon GPCR Activation and Ca2+ Stress

Mitochondria shape cytosolic calcium ([Ca2+]c) transients and utilize the mitochondrial Ca2+ ([Ca2+]m) in exchange for bioenergetics output. Conversely, dysregulated [Ca2+]c causes [Ca2+]m overload and induces permeability transition pore and cell death. Ablation of MCU-mediated Ca2+ uptake exhibited elevated [Ca2+]c and failed to prevent stress-induced cell death. The mechanisms for these effects remain elusive. Here, we report that mitochondria undergo a cytosolic Ca2+-induced shape change that is distinct from mitochondrial fission and swelling. [Ca2+]c elevation, but not MCU-mediated Ca2+ uptake, appears to be essential for the process we term mitochondrial shape transition (MiST). MiST is mediated by the mitochondrial protein Miro1 through its EF-hand domain 1 in multiple cell types. Moreover, Ca2+-dependent disruption of Miro1/KIF5B/tubulin complex is determined by Miro1 EF1 domain. Functionally, Miro1-dependent MiST is essential for autophagy/mitophagy that is attenuated in Miro1 EF1 mutants. Thus, Miro1 is a cytosolic Ca2+ sensor that decodes metazoan Ca2+ signals as MiST.

Tau-Induced Pathology in Epilepsy and Dementia: Notions from Patients and Animal Models

Patients with dementia present epilepsy more frequently than the general population. Seizures are more common in patients with Alzheimer’s disease (AD), dementia with Lewy bodies (LBD), frontotemporal dementia (FTD) and progressive supranuclear palsy (PSP) than in other dementias. Missense mutations in the microtubule associated protein tau (MAPT) gene have been found to cause familial FTD and PSP, while the P301S mutation in MAPT has been associated with early-onset fast progressive dementia and the presence of seizures. Brains of patients with AD, LBD, FTD and PSP show hyperphosphorylated tau aggregates, amyloid-β plaques and neuropil threads. Increasing evidence suggests the existence of overlapping mechanisms related to the generation of network hyperexcitability and cognitive decline. Neuronal overexpression of tau with various mutations found in FTD with parkinsonism-linked to chromosome 17 (FTDP-17) in mice produces epileptic activity. On the other hand, the use of certain antiepileptic drugs in animal models with AD prevents cognitive impairment. Further efforts should be made to search for plausible common targets for both conditions. Moreover, attempts should also be made to evaluate the use of drugs targeting tau and amyloid-β as suitable pharmacological interventions in epileptic disorders. The diagnosis of dementia and epilepsy in early stages of those diseases may be helpful for the initiation of treatments that could prevent the generation of epileptic activity and cognitive deterioration.

Genome-wide Analyses Identify KIF5A as a Novel ALS Gene

To identify novel genes associated with ALS, we undertook two lines of investigation. We carried out a genome-wide association study comparing 20,806 ALS cases and 59,804 controls. Independently, we performed a rare variant burden analysis comparing 1,138 index familial ALS cases and 19,494 controls. Through both approaches, we identified kinesin family member 5A (KIF5A) as a novel gene associated with ALS. Interestingly, mutations predominantly in the N-terminal motor domain of KIF5A are causative for two neurodegenerative diseases: hereditary spastic paraplegia (SPG10) and Charcot-Marie-Tooth type 2 (CMT2). In contrast, ALS-associated mutations are primarily located at the C-terminal cargo-binding tail domain and patients harboring loss-of-function mutations displayed an extended survival relative to typical ALS cases. Taken together, these results broaden the phenotype spectrum resulting from mutations in KIF5A and strengthen the role of cytoskeletal defects in the pathogenesis of ALS.

The role of the mitochondria and the endoplasmic reticulum contact sites in the development of the immune responses

Mitochondria and endoplasmic reticulum (ER) contact sites (MERCs) are dynamic modules enriched in subset of lipids and specialized proteins that determine their structure and functions. The MERCs regulate lipid transfer, autophagosome formation, mitochondrial fission, Ca2+ homeostasis and apoptosis. Since these functions are essential for cell biology, it is therefore not surprising that MERCs also play a critical role in organ physiology among which the immune system stands by its critical host defense function. This defense system must discriminate and tolerate host cells and beneficial commensal microorganisms while eliminating pathogenic ones in order to preserve normal homeostasis. To meet this goal, the immune system has two lines of defense. First, the fast acting but unspecific innate immune system relies on anatomical physical barriers and subsets of hematopoietically derived cells expressing germline-encoded receptors called pattern recognition receptors (PRR) recognizing conserved motifs on the pathogens. Second, the slower but very specific adaptive immune response is added to complement innate immunity. Adaptive immunity relies on another set of specialized cells, the lymphocytes, harboring receptors requiring somatic recombination to be expressed. Both innate and adaptive immune cells must be activated to phagocytose and process pathogens, migrate, proliferate, release soluble factors and destroy infected cells. Some of these functions are strongly dependent on lipid transfer, autophagosome formation, mitochondrial fission, and Ca2+ flux; this indicates that MERCs could regulate immunity.

Mitochondrial Aspects of Synaptic Dysfunction in Alzheimer's Disease

Alzheimer's disease (AD) is characterized by brain deposition of amyloid plaques and tau neurofibrillary tangles along with steady cognitive decline. Synaptic damage, an early pathological event, correlates strongly with cognitive deficits and memory loss. Mitochondria are essential organelles for synaptic function. Neurons utilize specialized mechanisms to drive mitochondrial trafficking to synapses in which mitochondria buffer Ca2+ and serve as local energy sources by supplying ATP to sustain neurotransmitter release. Mitochondrial abnormalities are one of the earliest and prominent features in AD patient brains. Amyloid-β (Aβ) and tau both trigger mitochondrial alterations. Accumulating evidence suggests that mitochondrial perturbation acts as a key factor that is involved in synaptic failure and degeneration in AD. The importance of mitochondria in supporting synaptic function has made them a promising target of new therapeutic strategies for AD. Here, we review the molecular mechanisms regulating mitochondrial function at synapses, highlight recent findings on the disturbance of mitochondrial dynamics and transport in AD, and discuss how these alterations impact synaptic vesicle release and thus contribute to synaptic pathology associated with AD.

Mitochondria are transported along microtubules in membrane nanotubes to rescue distressed cardiomyocytes from apoptosis

Membrane nanotubes (MNTs) act as “highways” between cells to facilitate the transfer of multiple signals and play an important role in many diseases. Our previous work reported on the transfer of mitochondria via MNTs between cardiomyocytes (CMs) and cardiac myofibroblasts (MFs); however, the elucidation of the underlying mechanism and pathophysiological significance of this transfer requires additional study. In this study, we determined that the mean movement velocity of mitochondria in MNTs between CMs and MFs was approximately 17.5 ± 2.1 nm/s. Meanwhile, treatment with microtubule polymerisation inhibitors nocodazole or colcemid in cell culture decreased mitochondrial velocity, and knockdown of the microtubule motor protein kinesin family member 5B (KIF5B) led to a similar effect, indicating that mitochondrial movement was dependent on microtubules and the motor protein KIF5B. Furthermore, we showed that hypoxia/reoxygenation-induced CM apoptosis was attenuated by coculture with intact or hypoxia/reoxygenation-treated MFs, which transferred mitochondria to CMs. This rescue was prevented either by separating the cells using Transwell culture or by impairing mitochondrial transfer with nocodazole or colcemid treatment. In conclusion, as a novel means of intercellular communication, MNTs rescue distressed CMs from apoptosis by transporting mitochondria along microtubules via KIF5B.

The Arf-like GTPase Arl8b is essential for three-dimensional invasive growth of prostate cancer in vitro and xenograft formation and growth in vivo

Cancer is a multistep process that requires cells to respond appropriately to the tumor microenvironment, both in early proliferative stages and in later invasive disease. Arl8b is a lysosome localized Arf-like GTPase that controls the spatial distribution of lysosomes via recruitment of kinesin motors. Common features of the tumor microenvironment such as acidic extracellular pH and various growthfactors stimulate lysosome trafficking to the cell periphery (anterograde), which is critical for tumor invasion by facilitating the release of lysosomal proteases to promote matrix remodeling. Herein we report for the first time that Arl8b regulates anterograde lysosome trafficking in response to hepatocyte growth factor, epidermal growth factor, and acidic extracellular pH. Depletion of Arl8b results in juxtanuclear lysosome aggregation, and this effect corresponds with both diminished invasive growth and proteolytic extracellular matrix degradation in a three-dimensional model of prostate cancer. Strikingly, we found that depletion of Arl8b abolishes the ability of prostate cancer cells to establish subcutaneous xenografts in mice. We present evidence that Arl8b facilitates lipid hydrolysis to maintain efficient metabolism for a proliferative capacity in low nutrient environments, suggesting a likely explanation for the complete inability of Arl8b-depleted tumor cells to grow in vivo. In conclusion, we have identified two mechanisms by which Arl8b regulates cancer progression: 1) through lysosome positioning and protease release leading to an invasive phenotype and 2) through control of lipid metabolism to support cellular proliferation. These novel roles highlight that Arl8b is a potential target for the development of novel anti-cancer therapeutics.

TUFT1 interacts with RABGAP1 and regulates mTORC1 signaling

The mammalian target of rapamycin (mTOR) pathway is commonly activated in human cancers. The activity of mTOR complex 1 (mTORC1) signaling is supported by the intracellular positioning of cellular compartments and vesicle trafficking, regulated by Rab GTPases. Here we showed that tuftelin 1 (TUFT1) was involved in the activation of mTORC1 through modulating the Rab GTPase-regulated process. TUFT1 promoted tumor growth and metastasis. Consistently, the expression of TUFT1 correlated with poor prognosis in lung, breast and gastric cancers. Mechanistically, TUFT1 physically interacted with RABGAP1, thereby modulating intracellular lysosomal positioning and vesicular trafficking, and promoted mTORC1 signaling. In addition, expression of TUFT1 predicted sensitivity to perifosine, an alkylphospholipid that alters the composition of lipid rafts. Perifosine treatment altered the positioning and trafficking of cellular compartments to inhibit mTORC1. Our observations indicate that TUFT1 is a key regulator of the mTORC1 pathway and suggest that it is a promising therapeutic target or a biomarker for tumor progression.

Monitoring the Mitochondrial Dynamics in Mammalian Cells

Mitochondria exist in a dynamic state inside mammalian cells. They undergo processes of fusion and fission to adjust their shape according to the different cell needs. Different proteins tightly regulate these dynamics: Opa-1 and Mitofusin-1 and Mitofusin-2 are the main profusion proteins, while Drp1 and its different receptors (Mff, Fis1, MiD49, MiD51) regulate mitochondrial fission. The dynamic nature of the mitochondrial network has become evident and detectable, thanks to recent advances in live imaging video microscopy and to the availability of mitochondria-tagged fluorescent proteins. High-resolution confocal reconstruction of mitochondria over time allows researchers to visualize mitochondria shape changes in living cells, under different experimental conditions. Moreover, in recent years, different techniques in living cells have been developed to study the process of mitochondria fusion in more details. Among them are fluorescence recovery after photobleaching (FRAP) of mitochondria-tagged GFP (mtGFP), use of photoactivatable mtGFP, polyethylene glycol (PEG)-based fusion of mtGFP and mtRFP cells, and Renilla luciferase assay (for population studies). In addition, in combination with imaging, the analysis of the expression levels of the different mitochondria-shaping proteins, along with that of their activation status, represents a powerful tool to investigate potential modulations of the mitochondrial network. Here, we review this aspect and then mention a number of techniques, with particular attention to their relative protocols.

Regulation of long-distance transport of mitochondria along microtubules

Mitochondria are cellular organelles of crucial importance, playing roles in cellular life and death. In certain cell types, such as neurons, mitochondria must travel long distances so as to meet metabolic demands of the cell. Mitochondrial movement is essentially microtubule (MT) based and is executed by two main motor proteins, Dynein and Kinesin. The organization of the cellular MT network and the identity of motors dictate mitochondrial transport. Tight coupling between MTs, motors, and the mitochondria is needed for the organelle precise localization. Two adaptor proteins are involved directly in mitochondria-motor coupling, namely Milton known also as TRAK, which is the motor adaptor, and Miro, which is the mitochondrial protein. Here, we discuss the active mitochondria transport process, as well as motor-mitochondria coupling in the context of MT organization in different cell types. We focus on mitochondrial trafficking in different cell types, specifically neurons, migrating cells, and polarized epithelial cells.

Identification of Miro as a mitochondrial receptor for myosin XIX

Mitochondrial distribution in cells is critical for cellular function and proper inheritance during cell division. In mammalian cells, mitochondria are transported predominantly along microtubules by kinesin and dynein motors that bind indirectly via TRAK1/2 to outer mitochondrial membrane proteins Miro1/2. Here, using proximity labeling, we identified Miro1/2 as potential binding partners of myosin XIX (Myo19). Interaction studies show that Miro1 binds directly to a C-terminal fragment of the Myo19 tail region and that Miro recruits the Myo19 tail in vivo. This recruitment is regulated by the nucleotide-state of the N-terminal Rho-like GTPase domain of Miro. Notably, Myo19 protein stability in cells depends on its association with Miro. Downregulation of Miro or overexpression of the adapter proteins TRAK1 and TRAK2 caused a reduction in Myo19 protein levels. Finally, Myo19 regulates the subcellular distribution of mitochondria. Downregulation, as well as overexpression, of Myo19 induces perinuclear collapse of mitochondria, phenocopying the loss of kinesin KIF5, dynein or their mitochondrial receptor Miro. These results suggest that Miro coordinates microtubule- and actin-based mitochondrial movement.

A small-molecule activator of kinesin-1 drives remodeling of the microtubule network

The microtubule motor kinesin-1 interacts via its cargo-binding domain with both microtubules and organelles, and hence plays an important role in controlling organelle transport and microtubule dynamics. In the absence of cargo, kinesin-1 is found in an autoinhibited conformation. The molecular basis of how cargo engagement affects the balance between kinesin-1's active and inactive conformations and roles in microtubule dynamics and organelle transport is not well understood. Here we describe the discovery of kinesore, a small molecule that in vitro inhibits kinesin-1 interactions with short linear peptide motifs found in organelle-specific cargo adaptors, yet activates kinesin-1's function of controlling microtubule dynamics in cells, demonstrating that these functions are mechanistically coupled. We establish a proof-of-concept that a microtubule motor-cargo interface and associated autoregulatory mechanism can be manipulated using a small molecule, and define a target for the modulation of microtubule dynamics.

Loss of Dendritic Complexity Precedes Neurodegeneration in a Mouse Model with Disrupted Mitochondrial Distribution in Mature Dendrites

Correct mitochondrial distribution is critical for satisfying local energy demands and calcium buffering requirements and supporting key cellular processes. The mitochondrially targeted proteins Miro1 and Miro2 are important components of the mitochondrial transport machinery, but their specific roles in neuronal development, maintenance, and survival remain poorly understood. Using mouse knockout strategies, we demonstrate that Miro1, as opposed to Miro2, is the primary regulator of mitochondrial transport in both axons and dendrites. Miro1 deletion leads to depletion of mitochondria from distal dendrites but not axons, accompanied by a marked reduction in dendritic complexity. Disrupting postnatal mitochondrial distribution in vivo by deleting Miro1 in mature neurons causes a progressive loss of distal dendrites and compromises neuronal survival. Thus, the local availability of mitochondrial mass is critical for generating and sustaining dendritic arbors, and disruption of mitochondrial distribution in mature neurons is associated with neurodegeneration.

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Miro1 deletion alters mitochondrial distribution in dendrites but not axons

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Mitochondrial distribution impacts dendritic development

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Correct mitochondrial distribution is key to maintain complex dendritic geometries

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Aberrant dendritic mitochondrial distribution triggers neurodegeneration

Regulation of mitochondrial dynamics in astrocytes: Mechanisms, consequences, and unknowns

Astrocytes are the major glial cell in the central nervous system. These polarized cells possess numerous processes that ensheath the vasculature and contact synapses. Astrocytes play important roles in synaptic signaling, neurotransmitter synthesis and recycling, control of nutrient uptake, and control of local blood flow. Many of these processes depend on local metabolism and/or energy utilization. While astrocytes respond to increases in neuronal activity and metabolic demand by upregulating glycolysis and glycogenolysis, astrocytes also possess significant capacity for oxidative (mitochondrial) metabolism. Mitochondria mediate energy supply and metabolism, cellular survival, ionic homeostasis, and proliferation. These organelles are dynamic structures undergoing extensive fission and fusion, directed movement along cytoskeletal tracts, and degradation. While many of the mechanisms underlying the dynamics of these organelles and their physiologic roles have been characterized in neurons and other cells, the roles that mitochondrial dynamics play in glial physiology is less well understood. Recent work from several laboratories has demonstrated that mitochondria are present within the fine processes of astrocytes, that their movement is regulated, and that they contribute to local Ca²⁺ signaling within the astrocyte. They likely play a role in local ATP production and metabolism, particularly that of glutamate. Here we will review these and other findings describing the mechanism by which mitochondrial dynamics are regulated in astrocytes, how mitochondrial dynamics might influence astrocyte and brain metabolism, and draw parallels to mitochondrial dynamics in neurons. Additionally, we present new analyses of the size, distribution, and dynamics of mitochondria in astrocytes performed using in vivo using 2-photon microscopy.

Lysosome trafficking is necessary for EGF-driven invasion and is regulated by p38 MAPK and Na+/H+ exchangers

Background

Tumor invasion through a basement membrane is one of the earliest steps in metastasis, and growth factors, such as Epidermal Growth Factor (EGF) and Hepatocyte Growth Factor (HGF), stimulate this process in a majority of solid tumors. Basement membrane breakdown is one of the hallmarks of invasion; therefore, tumor cells secrete a variety of proteases to aid in this process, including lysosomal proteases. Previous studies demonstrated that peripheral lysosome distribution coincides with the release of lysosomal cathepsins.

Methods

Immunofluorescence microscopy, western blot, and 2D and 3D cell culture techniques were performed to evaluate the effects of EGF on lysosome trafficking and cell motility and invasion.

Results

EGF-mediated lysosome trafficking, protease secretion, and invasion is regulated by the activity of p38 mitogen activated protein kinase (MAPK) and sodium hydrogen exchangers (NHEs). Interestingly, EGF stimulates anterograde lysosome trafficking through a different mechanism than previously reported for HGF, suggesting that there are redundant signaling pathways that control lysosome positioning and trafficking in tumor cells.

Conclusions

These data suggest that EGF stimulation induces peripheral (anterograde) lysosome trafficking, which is critical for EGF-mediated invasion and protease release, through the activation of p38 MAPK and NHEs. Taken together, this report demonstrates that anterograde lysosome trafficking is necessary for EGF-mediated tumor invasion and begins to characterize the molecular mechanisms required for EGF-stimulated lysosome trafficking.

Electronic supplementary material

The online version of this article (10.1186/s12885-017-3660-3) contains supplementary material, which is available to authorized users.

Impairments in age-dependent ubiquitin proteostasis and structural integrity of selective neurons by uncoupling Ran GTPase from the Ran-binding domain 3 of Ranbp2 and identification of novel mitochondrial isoforms of ubiquitin-conjugating enzyme E2I (ubc9) and Ranbp2

The Ran-binding protein 2 (Ranbp2/Nup358) is a cytoplasmic and peripheral nucleoporin comprised of 4 Ran-GTP-binding domains (RBDs) that are interspersed among diverse structural domains with multifunctional activities. Our prior studies found that the RBD2 and RBD3 of Ranbp2 control mitochondrial motility independently of Ran-GTP-binding in cultured cells, whereas loss of Ran-GTP-binding to RBD2 and RBD3 are essential to support cone photoreceptor development and the survival of mature retinal pigment epithelium (RPE) in mice. Here, we uncover that loss of Ran-GTP-binding to RBD3 alone promotes the robust age-dependent increase of ubiquitylated substrates and S1 subunit (Pmsd1) of the 19S cap of the proteasome in the retina and RPE and that such loss in RBD3 also compromises the structural integrity of the outer segment compartment of cone photoreceptors only and without affecting the viability of these neurons. We also found that the E2-ligase and partner of Ranbp2, ubc9, is localized prominently in the mitochondrial-rich ellipsoid compartment of photoreceptors, where Ranbp2 is also known to localize with and modulate the activity of mitochondrial proteins. However, the natures of Ranbp2 and ubc9 isoforms to the mitochondria are heretofore elusive. Subcellular fractionation, co-immunolocalization and immunoaffinity purification of Ranbp2 complexes show that novel isoforms of Ranbp2 and ubc9 with molecular masses distinct from the large Ranbp2 and unmodified ubc9 isoforms localize specifically to the mitochondrial fraction or associate with mitochondrial components, whereas unmodified and SUMOylated Ran GTPase are excluded from the mitochondrial fraction. Further, liposome-mediated intracellular delivery of an antibody against a domain shared by the mitochondrial and nuclear pore isoforms of Ranbp2 causes the profound fragmentation of mitochondria and their delocalization from Ranbp2 and without affecting Ranbp2 localization at the nuclear pores. Collectively, the data support that Ran GTPase-dependent and independent and moonlighting roles of Ranbp2 or domains thereof and ubc9 control selectively age-dependent, neural-type and mitochondrial functions.

Glycerol kinase-like proteins cooperate with Pld6 in regulating sperm mitochondrial sheath formation and male fertility

Spermatids undergo the final steps of maturation during spermiogenesis, a process that necessitates extensive rearrangement of organelles such as the mitochondria. Male infertility has been linked to mitochondrial disorder, for example, hypospermatogenesis and asthenozoospermia. However, the mechanisms that regulate mitochondrial dynamics during spermiogenesis remain largely unknown. We found the glycerol kinase (Gyk)-like proteins glycerol kinase-like 1 (Gykl1) and glycerol kinase 2 (Gk2) were specifically localized to the mitochondria in spermatids. Male mice deficient in either Gykl1 or Gk2 were infertile due to dysfunctional spermatozoa, which exhibited unregulated ATP production, disordered mitochondrial sheath formation, abnormal mitochondrial morphology, and defective sperm tail. We demonstrated that the unique C-terminal sequences found in Gykl1 and Gk2 mediated their targeting to the mitochondrial outer membrane. Furthermore, both Gykl1 and Gk2 could interact with Pld6 (MitoPLD) and induce Pld6 and phosphatidic acid (PA)-dependent mitochondrial clustering in cells. Taken together, our study has revealed previously unsuspected functions of Gyk-like proteins in spermiogenesis, providing new insight into the potential mechanisms that lead to spermatozoa dysfunction and male infertility.

T-Cell Intracellular Antigens and Hu Antigen R Antagonistically Modulate Mitochondrial Activity and Dynamics by Regulating Optic Atrophy 1 Gene Expression

Mitochondria undergo frequent morphological changes to control their function. We show here that T-cell intracellular antigens (TIA1b/TIARb) and Hu antigen R (HuR) have antagonistic roles in mitochondrial function by modulating the expression of mitochondrial shaping proteins. Expression of TIA1b/TIARb alters the mitochondrial dynamic network by enhancing fission and clustering, which is accompanied by a decrease in respiration. In contrast, HuR expression promotes fusion and cristae remodeling and increases respiratory activity. Mechanistically, TIA proteins downregulate the expression of optic atrophy 1 (OPA1) protein via switching of the splicing patterns of OPA1 to facilitate the production of OPA1 variant 5 (OPA1v5). Conversely, HuR enhances the expression of OPA1 mRNA isoforms through increasing steady-state levels and targeting translational efficiency at the 3' untranslated region. Knockdown of TIA1/TIAR or HuR partially reversed the expression profile of OPA1, whereas knockdown of OPA1 or overexpression of OPA1v5 provoked mitochondrial clustering. Middle-term expression of TIA1b/TIARb triggers reactive oxygen species production and mitochondrial DNA damage, which is accompanied by mitophagy, autophagy, and apoptosis. In contrast, HuR expression promotes mitochondrion-dependent cell proliferation. Collectively, these results provide molecular insights into the antagonistic functions of TIA1b/TIARb and HuR in mitochondrial activity dynamics and suggest that their balance might contribute to mitochondrial physiopathology.

Moving and positioning the endolysosomal system

The endolysosomal system is extremely dynamic, yet highly organized. The spatio-temporal distribution of endolysosomal organelles depends on transport driven by microtubule motors such as kinesins and dynein, and by actin-based myosin motors. It has recently become appreciated that interactions with motors are controlled by contacts with other organelles, particularly the endoplasmic reticulum (ER). The ER also controls the concentration of endolysosomal organelles in the perinuclear area, as well as their fission and fusion, through a complex system of tethering proteins. Dynamic interactions go both ways, as contacts with endosomes can influence the movement of the ER and peroxisomes. The dynamics of endolysosomal organelles should thus no longer be studied in isolation, but in the context of the whole endomembrane system.

An Emerging Role for Tubulin Isotypes in Modulating Cancer Biology and Chemotherapy Resistance

Tubulin proteins, as components of the microtubule cytoskeleton perform critical cellular functions throughout all phases of the cell cycle. Altered tubulin isotype composition of microtubules is emerging as a feature of aggressive and treatment refractory cancers. Emerging evidence highlighting a role for tubulin isotypes in differentially influencing microtubule behaviour and broader functional networks within cells is illuminating a complex role for tubulin isotypes regulating cancer biology and chemotherapy resistance. This review focuses on the role of different tubulin isotypes in microtubule dynamics as well as in oncogenic changes that provide a survival or proliferative advantage to cancer cells within the tumour microenvironment and during metastatic processes. Consideration of the role of tubulin isotypes beyond their structural function will be essential to improving the current clinical use of tubulin-targeted chemotherapy agents and informing the development of more effective cancer therapies.

Kinesin-1 promotes chondrocyte maintenance during skeletal morphogenesis

During skeletal morphogenesis diverse mechanisms are used to support bone formation. This can be seen in the bones that require a cartilage template for their development. In mammals the cartilage template is removed, but in zebrafish the cartilage template persists and the bone mineralizes around the cartilage scaffold. Remodeling of unmineralized cartilage occurs via planar cell polarity (PCP) mediated cell rearrangements that contribute to lengthening of elements; however, the mechanisms that maintain the chondrocyte template that supports perichondral ossification remain unclear. We report double mutants disrupting two zebrafish kinesin-I genes (hereafter kif5Blof) that we generated using CRISPR/Cas9 mutagenesis. We show that zygotic Kif5Bs have a conserved function in maintaining muscle integrity, and are required for cartilage remodeling and maintenance during craniofacial morphogenesis by a PCP-distinct mechanism. Further, kif5Blof does not activate ER stress response genes, but instead disrupts lysosomal function, matrix secretion, and causes deregulated autophagic markers and eventual chondrocyte apoptosis. Ultrastructural and transplantation analysis reveal neighboring cells engulfing extruded kif5Blof chondrocytes. Initial cartilage specification is intact; however, during remodeling, kif5Blof chondrocytes die and the cartilage matrix devoid of hypertrophic chondrocytes remains and impedes normal ossification. Chimeric and mosaic analyses indicate that Kif5B functions cell-autonomously in secretion, nuclear position, cell elongation and maintenance of hypertrophic chondrocytes. Interestingly, large groups of wild-type cells can support elongation of neighboring mutant cells. Finally, mosaic expression of kif5Ba, but not kif5Aa in cartilage rescues the chondrocyte phenotype, further supporting a specific requirement for Kif5B. Cumulatively, we show essential Kif5B functions in promoting cartilage remodeling and chondrocyte maintenance during zebrafish craniofacial morphogenesis.

Miro1-mediated mitochondrial positioning shapes intracellular energy gradients required for cell migration

The ratio of ATP:ADP is highest at perinuclear sites, where mitochondria are dense, and dissipates toward the periphery. Miro1 positions mitochondria toward the cortical cytoskeleton. Deletion of Miro1 results in perinuclear clustering of mitochondria, altering intracellular ATP:ADP gradients, and impairs energy-expensive cell migratory processes.

It has long been postulated, although never directly demonstrated, that mitochondria are strategically positioned in the cytoplasm to meet local requirements for energy production. Here we show that positioning of mitochondria in mouse embryonic fibroblasts (MEFs) determines the shape of intracellular energy gradients in living cells. Specifically, the ratio of ATP to ADP was highest at perinuclear areas of dense mitochondria and gradually decreased as more-peripheral sites were approached. Furthermore, the majority of mitochondria were positioned at the ventral surface of the cell, correlating with high ATP:ADP ratios close to the ventral membrane, which rapidly decreased toward the dorsal surface. We used cells deficient for the mitochondrial Rho-GTPase 1 (Miro1), an essential mediator of microtubule-based mitochondrial motility, to study how changes in mitochondrial positioning affect cytoplasmic energy distribution and cell migration, an energy-expensive process. The mitochondrial network in Miro1^−/− MEFs was restricted to the perinuclear area, with few mitochondria present at the cell periphery. This change in mitochondrial distribution dramatically reduced the ratio of ATP to ADP at the cell cortex and disrupted events essential for cell movement, including actin dynamics, lamellipodia protrusion, and membrane ruffling. Cell adhesion status was also affected by changes in mitochondrial positioning; focal adhesion assembly and stability was decreased in Miro1^−/− MEFs compared with Miro1^+/+ MEFs. Consequently Miro1^−/− MEFs migrated slower than control cells during both collective and single-cell migration. These data establish that Miro1-mediated mitochondrial positioning at the leading edge provides localized energy production that promotes cell migration by supporting membrane protrusion and focal adhesion stability.

Kinesin 1 Drives Autolysosome Tubulation

Autophagic lysosome reformation (ALR) plays an important role in maintaining lysosome homeostasis. During ALR, lysosomes are reformed by recycling lysosomal components from autolysosomes. The most noticeable step of ALR is autolysosome tubulation, but it is currently unknown how the process is regulated. Here, using an approach combining in vivo studies and in vitro reconstitution, we found that the kinesin motor protein KIF5B is required for autolysosome tubulation and that KIF5B drives autolysosome tubulation by pulling on the autolysosomal membrane. Furthermore, we show that KIF5B directly interacts with PtdIns(4,5)P2. Kinesin motors are recruited and clustered on autolysosomes via interaction with PtdIns(4,5)P2 in a clathrin-dependent manner. Finally, we demonstrate that clathrin promotes formation of PtdIns(4,5)P2-enriched microdomains, which are required for clustering of KIF5B. Our study reveals a mechanism by which autolysosome tubulation was generated.

A Rab3a-dependent complex essential for lysosome positioning and plasma membrane repair

Encarnação et al. show that Rab3a, together with its newly identified effector NMHC IIA, mediates the positioning of peripheral lysosomes in nonsecretory cells, thereby promoting lysosome exocytosis and plasma membrane repair.

Lysosome exocytosis plays a major role in resealing plasma membrane (PM) disruptions. This process involves two sequential steps. First, lysosomes are recruited to the periphery of the cell and then fuse with the damaged PM. However, the trafficking molecular machinery involved in lysosome exocytosis and PM repair (PMR) is poorly understood. We performed a systematic screen of the human Rab family to identify Rabs required for lysosome exocytosis and PMR. Rab3a, which partially localizes to peripheral lysosomes, was one of the most robust hits. Silencing of Rab3a or its effector, synaptotagmin-like protein 4a (Slp4-a), leads to the collapse of lysosomes to the perinuclear region and inhibition of PMR. Importantly, we have also identified a new Rab3 effector, nonmuscle myosin heavy chain IIA, as part of the complex formed by Rab3a and Slp4-a that is responsible for lysosome positioning at the cell periphery and lysosome exocytosis.

Inefficient recruitment of kinesin-1 to melanosomes precludes it from facilitating their transport

Microtubules and F-actin, and their associated motor proteins, are considered to play complementary roles in long- and short-range organelle transport. However, there is growing appreciation that myosin/F-actin networks can drive long-range transport. In melanocytes, myosin-Va and kinesin-1 have both been proposed as long-range centrifugal transporters moving melanosomes into the peripheral dendrites. Here, we investigated the role of kinesin-1 heavy chain (Kif5b) and its suggested targeting factor Rab1a in transport. We performed confocal microscopy and subcellular fractionation, but did not detect Kif5b or Rab1a on melanosomes. Meanwhile functional studies, using siRNA knockdown and dominant negative mutants, did not support a role for Kif5b or Rab1a in melanosome transport. To probe the potential of Kif5b to function in transport, we generated fusion proteins that target active Kif5b to melanosomes and tested their ability to rescue perinuclear clustering in myosin-Va-deficient cells. Expression of these chimeras, but not full-length Kif5b, dispersed melanosomes with similar efficiency to myosin-Va. Our data indicate that kinesin and microtubules can compensate for defects in myosin-Va and actin-based transport in mammals, but that endogenous Kif5b does not have an important role in transport of melanocytes due to its inefficient recruitment to melanosomes.

Highlighted Article: We show that Kif5b can compensate for defects in myosin-Va-based transport in mammals, but that endogenous Kif5b plays a minimal role in transport in melanocytes due to inefficient recruitment to melanosomes.

Common general anesthetic propofol impairs kinesin processivity

Propofol is the most widely used i.v. general anesthetic to induce and maintain anesthesia. It is now recognized that this small molecule influences ligand-gated channels, including the GABA_A receptor and others. Specific propofol binding sites have been mapped using photoaffinity ligands and mutagenesis; however, their precise target interaction profiles fail to provide complete mechanistic underpinnings for the anesthetic state. These results suggest that propofol and other common anesthetics, such as etomidate and ketamine, may target additional protein networks of the CNS to contribute to the desired and undesired anesthesia end points. Some evidence for anesthetic interactions with the cytoskeleton exists, but the molecular motors have received no attention as anesthetic targets. We have recently discovered that propofol inhibits conventional kinesin-1 KIF5B and kinesin-2 KIF3AB and KIF3AC, causing a significant reduction in the distances that these processive kinesins can travel. These microtubule-based motors are highly expressed in the CNS and the major anterograde transporters of cargos, such as mitochondria, synaptic vesicle precursors, neurotransmitter receptors, cell signaling and adhesion molecules, and ciliary intraflagellar transport particles. The single-molecule results presented show that the kinesin processive stepping distance decreases 40-60% with EC₅₀ values <100 nM propofol without an effect on velocity. The lack of a velocity effect suggests that propofol is not binding at the ATP site or allosteric sites that modulate microtubule-activated ATP turnover. Rather, we propose that a transient propofol allosteric site forms when the motor head binds to the microtubule during stepping.

Connecting mitochondrial dynamics and life-or-death events via Bcl-2 family proteins

The morphology of a population of mitochondria is the result of several interacting dynamical phenomena, including fission, fusion, movement, elimination and biogenesis. Each of these phenomena is controlled by underlying molecular machinery, and when defective can cause disease. New understanding of the relationships between form and function of mitochondria in health and disease is beginning to be unraveled on several fronts. Studies in mammals and model organisms have revealed that mitochondrial morphology, dynamics and function appear to be subject to regulation by the same proteins that regulate apoptotic cell death. One protein family that influences mitochondrial dynamics in both healthy and dying cells is the Bcl-2 protein family. Connecting mitochondrial dynamics with life-death pathway forks may arise from the intersection of Bcl-2 family proteins with the proteins and lipids that determine mitochondrial shape and function. Bcl-2 family proteins also have multifaceted influences on cells and mitochondria, including calcium handling, autophagy and energetics, as well as the subcellular localization of mitochondrial organelles to neuronal synapses. The remarkable range of physical or functional interactions by Bcl-2 family proteins is challenging to assimilate into a cohesive understanding. Most of their effects may be distinct from their direct roles in apoptotic cell death and are particularly apparent in the nervous system. Dual roles in mitochondrial dynamics and cell death extend beyond BCL-2 family proteins. In this review, we discuss many processes that govern mitochondrial structure and function in health and disease, and how Bcl-2 family proteins integrate into some of these processes.

Multiple kinesin-14 family members drive microtubule minus end-directed transport in plant cells

Minus end-directed cargo transport along microtubules (MTs) is exclusively driven by the molecular motor dynein in a wide variety of cell types. Interestingly, during evolution, plants have lost the genes encoding dynein; the MT motors that compensate for dynein function are unknown. Here, we show that two members of the kinesin-14 family drive minus end-directed transport in plants. Gene knockout analyses of the moss Physcomitrella patens revealed that the plant-specific class VI kinesin-14, KCBP, is required for minus end-directed transport of the nucleus and chloroplasts. Purified KCBP directly bound to acidic phospholipids and unidirectionally transported phospholipid liposomes along MTs in vitro. Thus, minus end-directed transport of membranous cargoes might be driven by their direct interaction with this motor protein. Newly nucleated cytoplasmic MTs represent another known cargo exhibiting minus end-directed motility, and we identified the conserved class I kinesin-14 (ATK) as the motor involved. These results suggest that kinesin-14 motors were duplicated and developed as alternative MT-based minus end-directed transporters in land plants.

Regulation of motor proteins, axonal transport deficits and adult-onset neurodegenerative diseases

Neurons affected in a wide variety of unrelated adult-onset neurodegenerative diseases (AONDs) typically exhibit a "dying back" pattern of degeneration, which is characterized by early deficits in synaptic function and neuritic pathology long before neuronal cell death. Consistent with this observation, multiple unrelated AONDs including Alzheimer's disease, Parkinson's disease, Huntington's disease, and several motor neuron diseases feature early alterations in kinase-based signaling pathways associated with deficits in axonal transport (AT), a complex cellular process involving multiple intracellular trafficking events powered by microtubule-based motor proteins. These pathogenic events have important therapeutic implications, suggesting that a focus on preservation of neuronal connections may be more effective to treat AONDs than addressing neuronal cell death. While the molecular mechanisms underlying AT abnormalities in AONDs are still being analyzed, evidence has accumulated linking those to a well-established pathological hallmark of multiple AONDs: altered patterns of neuronal protein phosphorylation. Here, we present a short overview on the biochemical heterogeneity of major motor proteins for AT, their regulation by protein kinases, and evidence revealing cell type-specific AT specializations. When considered together, these findings may help explain how independent pathogenic pathways can affect AT differentially in the context of each AOND.

BORC/kinesin-1 ensemble drives polarized transport of lysosomes into the axon

The ability of lysosomes to move within the cytoplasm is important for many cellular functions. This ability is particularly critical in neurons, which comprise vast, highly differentiated domains such as the axon and dendrites. The mechanisms that control lysosome movement in these domains, however, remain poorly understood. Here we show that an ensemble of BORC, Arl8, SKIP, and kinesin-1, previously shown to mediate centrifugal transport of lysosomes in nonneuronal cells, specifically drives lysosome transport into the axon, and not the dendrites, in cultured rat hippocampal neurons. This transport is essential for maintenance of axonal growth-cone dynamics and autophagosome turnover. Our findings illustrate how a general mechanism for lysosome dispersal in nonneuronal cells is adapted to drive polarized transport in neurons, and emphasize the importance of this mechanism for critical axonal processes.