Kinesin-mediated organelle translocation revealed by specific cellular manipulations

A detection method for neuronal death indicates abnormalities in intracellular membranous components in neuronal cells that underwent delayed death

Acute neuronal degeneration is always preceded under the light and electron microscopes by a stage called microvacuolation, which is characterized by a finely vacuolar alteration in the cytoplasm of the neurons destined to death. In this study, we reported a method for detecting neuronal death using two membrane-bound dyes, rhodamine R6 and DiOC₆(3), which may be associated with the so-called microvacuolation. This new method produced a spatiotemporally similar staining pattern to Fluoro-Jade B in kainic acid-damaged brains in mice. Further experiments showed that increased staining of rhodamine R6 and DiOC₆(3) was observed only in degenerated neurons, but not in glia, erythrocytes, or meninges. Different from Fluoro-Jade-related dyes, rhodamine R6 and DiOC₆(3) staining is highly sensitive to solvent extraction and detergent exposure. Staining with Nile red for phospholipids and filipin III for non-esterified cholesterol supports that the increased staining of rhodamine R6 and DiOC₆(3) might be associated with increased levels of phospholipids and free cholesterol in the perinuclear cytoplasm of damaged neurons. In addition to kainic acid-injected neuronal death, rhodamine R6 and DiOC₆(3) were similarly useful for detecting neuronal death in ischemic models either in vivo or in vitro. As far as we know, the staining with rhodamine R6 or DiOC₆(3) is one of a few histochemical methods for detecting neuronal death whose target molecules have been well defined and therefore may be useful for explaining experimental results as well as exploring the mechanisms of neuronal death. (250 words).

Algorithm for Modern Electron Microscopic Examination of the Golgi Complex

The Golgi complex (GC) is an essential organelle of the eukaryotic exocytic pathway. It has a very complexed structure and thus localization of its resident proteins is not trivial. Fast development of microscopic methods generates a huge difficulty for Golgi researchers to select the best protocol to use. Modern methods of light microscopy, such as super-resolution light microscopy (SRLM) and electron microscopy (EM), open new possibilities in analysis of various biological structures at organelle, cell, and organ levels. Nowadays, new generation of EM methods became available for the study of the GC; these include three-dimensional EM (3DEM), correlative light-EM (CLEM), immune EM, and new estimators within stereology that allow realization of maximal goal of any morphological study, namely, to achieve a three-dimensional model of the sample with optimal level of resolution and quantitative determination of its chemical composition. Methods of 3DEM have partially overlapping capabilities. This requires a careful comparison of these methods, identification of their strengths and weaknesses, and formulation of recommendations for their application to cell or tissue samples. Here, we present an overview of 3DEM methods for the study of the GC and some basics for how the images are formed and how the image quality can be improved.

Transmembrane Protein TMEM230, a Target of Glioblastoma Therapy

Glioblastomas (GBM) are the most aggressive tumors originating in the brain. Histopathologic features include circuitous, disorganized, and highly permeable blood vessels with intermittent blood flow. These features contribute to the inability to direct therapeutic agents to tumor cells. Known targets for anti-angiogenic therapies provide minimal or no effect in overall survival of 12–15 months following diagnosis. Identification of novel targets therefore remains an important goal for effective treatment of highly vascularized tumors such as GBM. We previously demonstrated in zebrafish that a balanced level of expression of the transmembrane protein TMEM230/C20ORF30 was required to maintain normal blood vessel structural integrity and promote proper vessel network formation. To investigate whether TMEM230 has a role in the pathogenesis of GBM, we analyzed its prognostic value in patient tumor gene expression datasets and performed cell functional analysis. TMEM230 was found necessary for growth of U87-MG cells, a model of human GBM. Downregulation of TMEM230 resulted in loss of U87 migration, substratum adhesion, and re-passaging capacity. Conditioned media from U87 expressing endogenous TMEM230 induced sprouting and tubule-like structure formation of HUVECs. Moreover, TMEM230 promoted vascular mimicry-like behavior of U87 cells. Gene expression analysis of 702 patients identified that TMEM230 expression levels distinguished high from low grade gliomas. Transcriptomic analysis of patients with gliomas revealed molecular pathways consistent with properties observed in U87 cell assays. Within low grade gliomas, elevated TMEM230 expression levels correlated with reduced overall survival independent from tumor subtype. Highest level of TMEM230 correlated with glioblastoma and ATP-dependent microtubule kinesin motor activity, providing a direction for future therapeutic intervention. Our studies support that TMEM230 has both glial tumor and endothelial cell intracellular and extracellular functions. Elevated levels of TMEM230 promote glial tumor cell migration, extracellular scaffold remodeling, and hypervascularization and abnormal formation of blood vessels. Downregulation of TMEM230 expression may inhibit both low grade glioma and glioblastoma tumor progression and promote normalization of abnormally formed blood vessels. TMEM230 therefore is both a promising anticancer and antiangiogenic therapeutic target for inhibiting GBM tumor cells and tumor-driven angiogenesis.

Machine Learning Based Real-Time Image-Guided Cell Sorting and Classification

Cell classification based on phenotypical, spatial, and genetic information greatly advances our understanding of the physiology and pathology of biological systems. Technologies derived from next generation sequencing and fluorescent activated cell sorting are cornerstones for cell- and genomic-based assays supporting cell classification and mapping. However, there exists a deficiency in technology space to rapidly isolate cells based on high content image information. Fluorescence-activated cell sorting can only resolve cell-to-cell variation in fluorescence and optical scattering. Utilizing microfluidics, photonics, computation microscopy, real-time image processing and machine learning, we demonstrate an image-guided cell sorting and classification system possessing the high throughput of flow cytometer and high information content of microscopy. We demonstrate the utility of this technology in cell sorting based on (1) nuclear localization of glucocorticoid receptors, (2) particle binding to the cell membrane, and (3) DNA damage induced γ-H2AX foci. © 2019 International Society for Advancement of Cytometry.

Role of Intracellular Transport in the Centriole-Dependent Formation of Golgi Ribbon

The intracellular transport is the most confusing issue in the field of cell biology. The Golgi complex (GC) is the central station along the secretory pathway. It contains Golgi glycosylation enzymes, which are responsible for protein and lipid glycosylation, and in many cells, it is organized into a ribbon. Position and structure of the GC depend on the position and function of the centriole. Here, we analyze published data related to the role of centriole and intracellular transport (ICT) for the formation of Golgi ribbon and specifically stress the importance of the delivery of membranes containing cargo and membrane proteins to the cell centre where centriole/centrosome is localized. Additionally, we re-examined the formation of Golgi ribbon from the point of view of different models of ICT.

Ca2+ signals initiate at immobile IP3 receptors adjacent to ER-plasma membrane junctions

IP₃ receptors (IP₃Rs) release Ca²⁺ from the ER when they bind IP₃ and Ca²⁺. The spatial organization of IP₃Rs determines both the propagation of Ca²⁺ signals between IP₃Rs and the selective regulation of cellular responses. Here we use gene editing to fluorescently tag endogenous IP₃Rs, and super-resolution microscopy to determine the geography of IP₃Rs and Ca²⁺ signals within living cells. We show that native IP₃Rs cluster within ER membranes. Most IP₃R clusters are mobile, moved by diffusion and microtubule motors. Ca²⁺ signals are generated by a small population of immobile IP₃Rs. These IP₃Rs are licensed to respond, but they do not readily mix with mobile IP₃Rs. The licensed IP₃Rs reside alongside ER-plasma membrane junctions where STIM1, which regulates store-operated Ca²⁺ entry, accumulates after depletion of Ca²⁺ stores. IP₃Rs tethered close to ER-plasma membrane junctions are licensed to respond and optimally placed to be activated by endogenous IP₃ and to regulate Ca²⁺ entry.

IP₃ receptors mediate Ca²⁺ release from the endoplasmic reticulum. Here the authors show that only a small fraction of IP₃ receptors initiate Ca²⁺ signals; these immobile IP₃ receptors adjacent to the plasma membrane are optimally placed to control STIM1-dependent Ca²⁺ entry.

Mutant spastin proteins promote deficits in axonal transport through an isoform-specific mechanism involving casein kinase 2 activation

Mutations of various genes cause hereditary spastic paraplegia (HSP), a neurological disease involving dying-back degeneration of upper motor neurons. From these, mutations in the SPAST gene encoding the microtubule-severing protein spastin account for most HSP cases. Cumulative genetic and experimental evidence suggests that alterations in various intracellular trafficking events, including fast axonal transport (FAT), may contribute to HSP pathogenesis. However, the mechanisms linking SPAST mutations to such deficits remain largely unknown. Experiments presented here using isolated squid axoplasm reveal inhibition of FAT as a common toxic effect elicited by spastin proteins with different HSP mutations, independent of microtubule-binding or severing activity. Mutant spastin proteins produce this toxic effect only when presented as the tissue-specific M1 isoform, not when presented as the ubiquitously-expressed shorter M87 isoform. Biochemical and pharmacological experiments further indicate that the toxic effects of mutant M1 spastins on FAT involve casein kinase 2 (CK2) activation. In mammalian cells, expression of mutant M1 spastins, but not their mutant M87 counterparts, promotes abnormalities in the distribution of intracellular organelles that are correctable by pharmacological CK2 inhibition. Collectively, these results demonstrate isoform-specific toxic effects of mutant M1 spastin on FAT, and identify CK2 as a critical mediator of these effects.

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.

Endoplasmic reticulum-mediated signalling in cellular microdomains

The endoplasmic reticulum (ER) is a prime mediator of cellular signalling due to its functions as an internal cellular store for calcium, as well as a site for synthesis of proteins and lipids. Its peripheral network of sheets and tubules facilitates calcium and lipid signalling, especially in areas of the cell that are more distant to the main cytoplasmic network. Specific membrane proteins shape the peripheral ER architecture and influence the network stability to project into restricted spaces. The signalling microdomains are anatomically separate from the cytoplasm as a whole and exhibit localized protein, ion channel and cytoskeletal element expression. Signalling can also occur between the ER and other organelles, such as the Golgi or mitochondria. Lipids made in the ER membrane can be sent to the Golgi via specialized transfer proteins and specific phospholipid synthases are enriched at ER-mitochondria junctions to more efficiently expedite phospholipid transfer. As a hub for protein and lipid synthesis, a store for intracellular calcium [Ca²⁺ ]_i and a mediator of cellular stress, the ER is an important cellular organelle. Its ability to organize into tubules and project into restricted spaces allows for discrete and temporal signalling, which is important for cellular physiology and organism homoeostasis.

Centrosome positioning in non-dividing cells

Centrioles and centrosomes are found in almost all eukaryotic cells, where they are important for organising the microtubule cytoskeleton in both dividing and non-dividing cells. The spatial location of centrioles and centrosomes is tightly controlled and, in non-dividing cells, plays an important part in cell migration, ciliogenesis and immune cell functions. Here, we examine some of the ways that centrosomes are connected to other organelles and how this impacts on cilium formation, cell migration and immune cell function in metazoan cells.

Peroxisomes, lipid droplets, and endoplasmic reticulum "hitchhike" on motile early endosomes

Intracellular transport is mediated by molecular motors that bind cargo to be transported along the cytoskeleton. Here, we report, for the first time, that peroxisomes (POs), lipid droplets (LDs), and the endoplasmic reticulum (ER) rely on early endosomes (EEs) for intracellular movement in a fungal model system. We show that POs undergo kinesin-3- and dynein-dependent transport along microtubules. Surprisingly, kinesin-3 does not colocalize with POs. Instead, the motor moves EEs that drag the POs through the cell. PO motility is abolished when EE motility is blocked in various mutants. Most LD and ER motility also depends on EE motility, whereas mitochondria move independently of EEs. Covisualization studies show that EE-mediated ER motility is not required for PO or LD movement, suggesting that the organelles interact with EEs independently. In the absence of EE motility, POs and LDs cluster at the growing tip, whereas ER is partially retracted to subapical regions. Collectively, our results show that moving EEs interact transiently with other organelles, thereby mediating their directed transport and distribution in the cell.

Form follows function: the importance of endoplasmic reticulum shape

The endoplasmic reticulum (ER) has a remarkably complex structure, composed of a single bilayer that forms the nuclear envelope, along with a network of sheets and dynamic tubules. Our understanding of the biological significance of the complex architecture of the ER has improved dramatically in the last few years. The identification of proteins and forces required for maintaining ER shape, as well as more advanced imaging techniques, has allowed the relationship between ER shape and function to come into focus. These studies have also revealed unexpected new functions of the ER and novel ER domains regulating alterations in ER dynamics. The importance of ER structure has become evident as recent research has identified diseases linked to mutations in ER-shaping proteins. In this review, we discuss what is known about the maintenance of ER architecture, the relationship between ER structure and function, and diseases associated with defects in ER structure.

The late endosomal p14-MP1 (LAMTOR2/3) complex regulates focal adhesion dynamics during cell migration

Late endosomes locally regulate cell migration by transporting the p14–MP1 scaffold complex to the vicinity of focal adhesions.

Cell migration is mediated by the dynamic remodeling of focal adhesions (FAs). Recently, an important role of endosomal signaling in regulation of cell migration was recognized. Here, we show an essential function for late endosomes carrying the p14–MP1 (LAMTOR2/3) complex in FA dynamics. p14–MP1-positive endosomes move to the cell periphery along microtubules (MTs) in a kinesin1- and Arl8b-dependent manner. There they specifically target FAs to regulate FA turnover, which is required for cell migration. Using genetically modified fibroblasts from p14-deficient mice and Arl8b-depleted cells, we demonstrate that MT plus end–directed traffic of p14–MP1-positive endosomes triggered IQGAP1 disassociation from FAs. The release of IQGAP was required for FA dynamics. Taken together, our results suggest that late endosomes contribute to the regulation of cell migration by transporting the p14–MP1 scaffold complex to the vicinity of FAs.

Protein tyrosine phosphatase PTP1B in cell adhesion and migration

Cell migration requires a highly coordinated interplay between specialized plasma membrane adhesion complexes and the cytoskeleton. Protein phosphorylation/dephosphorylation modifications regulate many aspects of the integrin-cytoskeleton interdependence, including their coupling, dynamics, and organization to support cell movement. The endoplasmic reticulum-bound protein tyrosine phosphatase PTP1B has been implicated as a regulator of cell adhesion and migration. Recent results from our laboratory shed light on potential mechanisms, such as Src/FAK signaling through Rho GTPases and integrin-cytoskeletal coupling.

Protein tyrosine phosphatase PTP1B is involved in hippocampal synapse formation and learning

ER-bound PTP1B is expressed in hippocampal neurons, and accumulates among neurite contacts. PTP1B dephosphorylates ß-catenin in N-cadherin complexes ensuring cell-cell adhesion. Here we show that endogenous PTP1B, as well as expressed GFP-PTP1B, are present in dendritic spines of hippocampal neurons in culture. GFP-PTP1B overexpression does not affect filopodial density or length. In contrast, impairment of PTP1B function or genetic PTP1B-deficiency leads to increased filopodia-like dendritic spines and a reduction in mushroom-like spines, while spine density is unaffected. These morphological alterations are accompanied by a disorganization of pre- and post-synapses, as judged by decreased clustering of synapsin-1 and PSD-95, and suggest a dynamic synaptic phenotype. Notably, levels of ß-catenin-Tyr-654 phosphorylation increased ∼5-fold in the hippocampus of adult PTP1B^−/− (KO) mice compared to wild type (WT) mice and this was accompanied by a reduction in the amount of ß-catenin associated with N-cadherin. To determine whether PTP1B-deficiency alters learning and memory, we generated mice lacking PTP1B in the hippocampus and cortex (PTP1B^fl/fl–Emx1-Cre). PTP1B^fl/fl–Emx1-Cre mice displayed improved performance in the Barnes maze (decreased time to find and enter target hole), utilized a more efficient strategy (cued), and had better recall compared to WT controls. Our results implicate PTP1B in structural plasticity within the hippocampus, likely through modulation of N-cadherin function by ensuring dephosphorylation of ß-catenin on Tyr-654. Disruption of hippocampal PTP1B function or expression leads to elongation of dendritic filopodia and improved learning and memory, demonstrating an exciting novel role for this phosphatase.

ER-bound protein tyrosine phosphatase PTP1B interacts with Src at the plasma membrane/substrate interface

PTP1B is an endoplasmic reticulum (ER) anchored enzyme whose access to substrates is partly dependent on the ER distribution and dynamics. One of these substrates, the protein tyrosine kinase Src, has been found in the cytosol, endosomes, and plasma membrane. Here we analyzed where PTP1B and Src physically interact in intact cells, by bimolecular fluorescence complementation (BiFC) in combination with temporal and high resolution microscopy. We also determined the structural basis of this interaction. We found that BiFC signal is displayed as puncta scattered throughout the ER network, a feature that was enhanced when the substrate trapping mutant PTP1B-D181A was used. Time-lapse and co-localization analyses revealed that BiFC puncta did not correspond to vesicular carriers; instead they localized at the tip of dynamic ER tubules. BiFC puncta were retained in ventral membrane preparations after cell unroofing and were also detected within the evanescent field of total internal reflection fluorescent microscopy (TIRFM) associated to the ventral membranes of whole cells. Furthermore, BiFC puncta often colocalized with dark spots seen by surface reflection interference contrast (SRIC). Removal of Src myristoylation and polybasic motifs abolished BiFC. In addition, PTP1B active site and negative regulatory tyrosine 529 on Src were primary determinants of BiFC occurrence, although the SH3 binding motif on PTP1B also played a role. Our results suggest that ER-bound PTP1B dynamically interacts with the negative regulatory site at the C-terminus of Src at random puncta in the plasma membrane/substrate interface, likely leading to Src activation and recruitment to adhesion complexes. We postulate that this functional ER/plasma membrane crosstalk could apply to a wide array of protein partners, opening an exciting field of research.

Autophagy, and BiP level decrease are early key events in retrograde degeneration of motoneurons

Disconnection of the axon from the soma of spinal motoneurons (MNs) leads either to a retrograde degenerative process or to a regenerative reaction, depending on the severity and the proximity to the soma of the axonal lesion. The endoplasmic reticulum (ER) is a continuous membranous network that extends from the nucleus to the entire cytoplasm of the neuronal soma, axon and dendrites. We investigated whether axonal injury is sensed by the ER and triggers the activation of protective mechanisms, such as the unfolded protein response (UPR) and autophagy. We found early (at 3 days) accumulation of beclin1, LC3II and Lamp-1, hallmarks of autophagy, in both degenerating MNs after spinal root avulsion and in non-degenerating MNs after distal nerve section, although Lamp-1 disappeared by 5 days only in the former. In contrast, only degenerating MNs presented early activation of IRE1α, revealed by an increase of the spliced isoform of Xbp1 and accumulation of ATF4 in their nucleus, two branches of the UPR, and late BiP downregulation in association with cytoskeletal and organelle disorganization. We conclude that BiP decrease is a signature of the degenerating process, as its overexpression led to an increase in MN survival after root avulsion. Besides, Bcl2 is strongly implicated in the survival pathway activated by BiP overexpression.

SKIP, the host target of the Salmonella virulence factor SifA, promotes kinesin-1-dependent vacuolar membrane exchanges

In Salmonella-infected cells, the bacterial effector SifA forms a functional complex with the eukaryotic protein SKIP (SifA and kinesin-interacting protein). The lack of either partner has important consequences on the intracellular fate and on the virulence of this pathogen. In addition to SifA, SKIP binds the microtubule-based motor kinesin-1. Yet the absence of SifA or SKIP results in an unusual accumulation of kinesin-1 on the bacterial vacuolar membrane. To understand this apparent contradiction, we investigated the interaction between SKIP and kinesin-1 and the function of this complex. We show that the C-terminal RUN (RPIP8, UNC-14 and NESCA) domain of SKIP interacted specifically with the tetratricopeptide repeat (TPR) domain of the kinesin light chain. Overexpression of SKIP induced a microtubule- and kinesin-1-dependent anterograde movement of late endosomal/lysosomal compartments. In infected cells, SifA contributed to the fission of vesicles from the bacterial vacuole and the SifA/SKIP complex was required for the formation and/or the anterograde transport of kinesin-1-enriched vesicles. These observations reflect the role of SKIP as a linker and/or an activator for kinesin-1.

How and why does the endoplasmic reticulum move?

The ER (endoplasmic reticulum) is a fascinating organelle that is highly dynamic, undergoing constant movement and reorganization. It has many key roles, including protein synthesis, folding and trafficking, calcium homoeostasis and lipid synthesis. It can expand in size when needed, and the balance between tubular and lamellar regions can be altered. The distribution and organization of the ER depends on both motile and static interactions with microtubules and the actin cytoskeleton. In the present paper, we review how the ER moves, and consider why this movement may be important for ER and cellular function.

Mechanism of membrane nanotube formation by molecular motors

Membrane nanotubes are ubiquitous in eukaryotic cells due to their involvement in the communication between many different membrane compartments. They are very dynamical structures, which are generally extended along the microtubule network. One possible mechanism of tube formation involves the action of molecular motors, which can generate the necessary force to pull the tubes along the cytoskeleton tracks. However, it has not been possible so far to image in living organisms simultaneously both tube formation and the molecular motors involved in the process. The reasons for this are mainly technological. To overcome these limitations and to elucidate in detail the mechanism of tube formation, many experiments have been developed over the last years in cell-free environments. In the present review, we present the results, which have been obtained in vitro either in cell extracts or with purified and artificial components. In particular, we will focus on a biomimetic system, which involves Giant Unilamellar Vesicles, kinesin-1 motors and microtubules in the presence of ATP. We present both theoretical and experimental results based on fluorescence microscopy that elucidate the dynamics of membrane tube formation, growth and stalling.

A missense mutation in the Arabidopsis COPII coat protein Sec24A induces the formation of clusters of the endoplasmic reticulum and Golgi apparatus

How the endoplasmic reticulum (ER) and the Golgi apparatus maintain their morphological and functional identity while working in concert to ensure the production of biomolecules necessary for the cell's survival is a fundamental question in plant biology. Here, we isolated and characterized an Arabidopsis thaliana mutant that partially accumulates Golgi membrane markers and a soluble secretory marker in globular structures composed of a mass of convoluted ER tubules that maintain a connection with the bulk ER. We established that the aberrant phenotype was due to a missense recessive mutation in sec24A, one of the three Arabidopsis isoforms encoding the coat protomer complex II (COPII) protein Sec24, and that the mutation affects the distribution of this critical component at ER export sites. By contrast, total loss of sec24A function was lethal, suggesting that Arabidopsis sec24A is an essential gene. These results produce important insights into the functional diversification of plant COPII coat components and the role of these proteins in maintaining the dynamic identity of organelles of the early plant secretory pathway.

Molecular motors and the Golgi complex: staying put and moving through

The Golgi apparatus is a highly dynamic organelle through which nascent proteins released from the endoplasmic reticulum (ER) are trafficked. Proteins are post-translationally modified within the Golgi and subsequently packaged into carriers for transport to a variety of cellular destinations. This transit of proteins, as well as the maintenance of Golgi structure and position, is highly dependent upon the actin and microtubule cytoskeletons and their associated molecular motors. Here we review how motors contribute to the correct functioning of the Golgi in higher eukaryotes and discuss the secretory pathway as a model system for studying cooperation between motor proteins.

Role of kinesin-1 and cytoplasmic dynein in endoplasmic reticulum movement in VERO cells

Generating the extended endoplasmic reticulum (ER) network depends on microtubules, which act as tracks for motor-driven ER tubule movement, generate the force to extend ER tubules by means of attachment to growing microtubule plus-ends and provide static attachment points. We have analysed ER dynamics in living VERO cells and find that most ER tubule extension is driven by microtubule motors. Surprisingly, we observe that approximately 50% of rapid ER tubule movements occur in the direction of the centre of the cell, driven by cytoplasmic dynein. Inhibition of this movement leads to an accumulation of lamellar ER in the cell periphery. By expressing dominant-negative kinesin-1 constructs, we show that kinesin-1 drives ER tubule extension towards the cell periphery and that this motility is dependent on the KLC1B kinesin light chain splice form but not on KLC1D. Inhibition of kinesin-1 promotes a shift from tubular to lamellar morphology and slows down the recovery of the ER network after microtubule depolymerisation and regrowth. These observations reconcile previous conflicting studies of kinesin-1 function in ER motility in vivo. Furthermore, our data reveal that cytoplasmic dynein plays a role in ER motility in a mammalian cultured cell, demonstrating that ER motility is more complex than previously thought.

Microtubules regulate dynamic organization of vacuoles in Physcomitrella patens

Eukaryotic cells have developed several essential membrane components. In flowering plants, appropriate structures and distributions of the major membrane components are predominantly regulated by actin microfilaments. In this study, we have focused on the regulatory mechanism of vacuolar structures in the moss, Physcomitrella patens. The high ability of P. patens to undergo homologous recombination enabled us stably to express green fluorescent protein (GFP) or red fluorescent protein (RFP) fusion proteins, and the simple body structure of P. patens enabled us to perform detailed visualization of the intracellular vacuolar and cytoskeletal structures. Three-dimensional analysis and high-speed time-lapse observations revealed surprisingly complex structures and dynamics of the vacuole, with inner sheets and tubular protrusions, and frequent rearrangements by separation and fusion of the membranes. Depolymerization of microtubules dramatically affected these structures and movements. Dual observation of microtubules and vacuolar membranes revealed that microtubules induced tubular protrusions and cytoplasmic strands of the vacuoles, indicative of interactions between microtubules and vacuolar membranes. These results demonstrate a novel function of microtubules in maintaining the distribution of the vacuole and suggest a functional divergence of cytoskeletal functions in land plant evolution.

p31 deficiency influences endoplasmic reticulum tubular morphology and cell survival

p31, the mammalian orthologue of yeast Use1p, is an endoplasmic reticulum (ER)-localized soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE) that forms a complex with other SNAREs, particularly syntaxin 18. However, the role of p31 in ER function remains unknown. To determine the role of p31 in vivo, we generated p31 conditional knockout mice. We found that homozygous deletion of the p31 gene led to early embryonic lethality before embryonic day 8.5. Conditional knockout of p31 in brains and mouse embryonic fibroblasts (MEFs) caused massive apoptosis accompanied by upregulation of ER stress-associated genes. Microscopic analysis showed vesiculation and subsequent enlargement of the ER membrane in p31-deficient cells. This type of drastic disorganization in the ER tubules has not been demonstrated to date. This marked change in ER structure preceded nuclear translocation of the ER stress-related transcription factor C/EBP homologous protein (CHOP), suggesting that ER stress-induced apoptosis resulted from disruption of the ER membrane structure. Taken together, these results suggest that p31 is an essential molecule involved in the maintenance of ER morphology and that its deficiency leads to ER stress-induced apoptosis.

Calsyntenins mediate TGN exit of APP in a kinesin-1-dependent manner

Kinesin motors are required for the export of membranous cargo from the trans-Golgi network (TGN), yet information about how kinesins are recruited to forming transport intermediates is sparse. Here we show that the Kinesin-1 docking protein calsyntenin-1 localizes to the TGN in vivo and directly and specifically recruits Kinesin-1 to Golgi/TGN membranes as well as to dynamic post-Golgi carriers. Overexpression of various calsyntenin chimeras and kinesin light chain 1 (KLC1) at high levels caused the formation of aberrant membrane stacks at the endoplasmic reticulum (ER) or the Golgi, disrupted overall Golgi structure and blocked exit of calsyntenin from the TGN. Intriguingly, this blockade of calsyntenin exit strongly and selectively impeded TGN exit of amyloid precursor protein (APP). Using live cell microscopy we found that calsyntenins exit the TGN in Kinesin-1-decorated tubular structures which may serve as carriers for calsyntenin-1-mediated post-TGN transport of APP. Abrogation of this pathway via virus-mediated knockdown of calsyntenin-1 expression in primary cultured neurons caused a marked elevation of APP C-terminal fragments. Together, these results indicate a role for calsyntenin-1 in Kinesin-1-dependent TGN exit and post-Golgi transport of APP-containing organelles and further suggest that distinct intracellular routes may exhibit different capacities for proteolytic processing of APP.

Microtubule and cell contact dependency of ER-bound PTP1B localization in growth cones

PTP1B is an ER-bound protein tyrosine phosphatase implied in the regulation of cell adhesion. Here we investigated mechanisms involved in the positioning and dynamics of PTP1B in axonal growth cones and evaluated the role of this enzyme in axons. In growth cones, PTP1B consistently localizes in the central domain, and occasionally at the peripheral region and filopodia. Live imaging of GFP-PTP1B reveals dynamic excursions of fingerlike processes within the peripheral region and filopodia. PTP1B and GFP-PTP1B colocalize with ER markers and coalign with microtubules at the peripheral region and redistribute to the base of the growth cone after treatment with nocodazole, a condition that is reversible. Growth cone contact with cellular targets is accompanied by invasion of PTP1B and stable microtubules in the peripheral region aligned with the contact axis. Functional impairment of PTP1B causes retardation of axon elongation, as well as reduction of growth cone filopodia lifetime and Src activity. Our results highlight the role of microtubules and cell contacts in the positioning of ER-bound PTP1B to the peripheral region of growth cones, which may be required for the positive role of PTP1B in axon elongation, filopodia stabilization, and Src activity.

Kinesin-1 (uKHC/KIF5B) is required for bidirectional motility of ER exit sites and efficient ER-to-Golgi transport

Transport of proteins and lipids between intracellular compartments is fundamental to the organization and function of eukaryotic cells. The efficiency of this process is greatly enhanced through coupling of membranes to microtubules. This serves two functions, organelle positioning and vesicular transport. In this study, we show that in addition to the well-known role for the minus-end motor dynein in endoplasmic reticulum (ER)-to-Golgi transport, the plus-end-directed motor kinesin-1 is involved in positioning coat protein II-coated ER exit sites (ERES) in cells as well as the formation of transport carriers and their movement to the Golgi. Using two-dimensional Gaussian fitting to determine their location at high spatial resolution, we show that ERES undergo short-range bidirectional movements. Bidirectionality depends on both kinesin-1 and dynein. Suppression of kinesin-1 (KIF5B) also inhibits ER-to-Golgi transport and affects the morphology of ER-to-Golgi transport carriers. Furthermore, we show that suppression of dynein heavy chain expression increases the range of movement of ERES, suggesting that dynein might anchor ERES, or the ER itself, to microtubules. These data implicate kinesin-1 in the spatial organization of the ER/Golgi interface as well as in traffic outside the ER.

Conventional kinesin holoenzymes are composed of heavy and light chain homodimers

Conventional kinesin is a major microtubule-based motor protein responsible for anterograde transport of various membrane-bounded organelles (MBO) along axons. Structurally, this molecular motor protein is a tetrameric complex composed of two heavy (kinesin-1) chains and two light chain (KLC) subunits. The products of three kinesin-1 (kinesin-1A, -1B, and -1C, formerly KIF5A, -B, and -C) and two KLC (KLC1, KLC2) genes are expressed in mammalian nervous tissue, but the functional significance of this subunit heterogeneity remains unknown. In this work, we examine all possible combinations among conventional kinesin subunits in brain tissue. In sharp contrast with previous reports, immunoprecipitation experiments here demonstrate that conventional kinesin holoenzymes are formed of kinesin-1 homodimers. Similar experiments confirmed previous findings of KLC homodimerization. Additionally, no specificity was found in the interaction between kinesin-1s and KLCs, suggesting the existence of six variant forms of conventional kinesin, as defined by their gene product composition. Subcellular fractionation studies indicate that such variants associate with biochemically different MBOs and further suggest a role of kinesin-1s in the targeting of conventional kinesin holoenzymes to specific MBO cargoes. Taken together, our data address the combination of subunits that characterize endogenous conventional kinesin. Findings on the composition and subunit organization of conventional kinesin as described here provide a molecular basis for the regulation of axonal transport and delivery of selected MBOs to discrete subcellular locations.

p180 is involved in the interaction between the endoplasmic reticulum and microtubules through a novel microtubule-binding and bundling domain

p180 was originally reported as a ribosome-binding protein on the rough endoplasmic reticulum membrane, although its precise role in animal cells has not yet been elucidated. Here, we characterized a new function of human p180 as a microtubule-binding and -modulating protein. Overexpression of p180 in mammalian cells induced an elongated morphology and enhanced acetylated microtubules. Consistently, electron microscopic analysis clearly revealed microtubule bundles in p180-overexpressing cells. Targeted depletion of endogenous p180 by small interfering RNAs led to aberrant patterns of microtubules and endoplasmic reticulum in mammalian cells, suggesting a specific interaction between p180 and microtubules. In vitro sedimentation assays using recombinant polypeptides revealed that p180 bound to microtubules directly and possessed a novel microtubule-binding domain (designated MTB-1). MTB-1 consists of a predicted coiled-coil region and repeat domain, and strongly promoted bundle formation both in vitro and in vivo when expressed alone. Overexpression of p180 induced acetylated microtubules in cultured cells in an MTB-1-dependent manner. Thus, our data suggest that p180 mediates interactions between the endoplasmic reticulum and microtubules mainly through the novel microtubule-binding and -bundling domain MTB-1.

Changes in endoplasmic reticulum structure during mouse oocyte maturation are controlled by the cytoskeleton and cytoplasmic dynein

Oocyte maturation in mouse is associated with a dramatic reorganisation of the endoplasmic reticulum (ER) from a network of cytoplasmic accumulations in the germinal vesicle-stage oocyte (GV) to a network of distinctive cortical clusters in the metaphase II egg (MII). Multiple lines of evidence suggest that this redistribution of the ER is important to prepare the oocyte for the generation of repetitive Ca2+ transients which trigger egg activation at fertilisation. The aim of the current study was therefore to investigate the timecourse and mechanism of ER reorganisation during oocyte maturation. The ER is first restructured at the time of GV-breakdown (GVBD) into a dense network of membranes which envelop and invade the developing meiotic spindle. GVBD is essential for the initiation of ER reorganisation, since ER structure does not change in GV-arrested oocytes. ER reorganisation is also prevented by the microtubule inhibitor nocodazole and by the inhibition of cytoplasmic dynein, a microtubule-associated motor protein. ER redistribution at GVBD is therefore dynein-driven and cell cycle-dependent. Following GVBD the dense network of ER surrounds the spindle during its migration to the oocyte cortex. Cortical clusters of ER are formed close to the time of, but independently of the metaphase I-metaphase II transition. Formation of the characteristic ER clusters is prevented by the depolymerisation of microfilaments, but not of microtubules. These experiments reveal that ER reorganisation during oocyte maturation is a complex multi-step process involving distinct microtubule- and microfilament-dependent phases and indicate a role for dynein in the cytoplasmic changes which prepare the oocyte for fertilisation.

Cellular Motors for Molecular Manufacturing

Cells are composed of macromolecular structures of various sizes that act individually or collectively to maintain their viability and perform their function within the organism. This review focuses on one structure, the microtubule, and one of the motor proteins that move along it, conventional kinesin (kinesin 1). Recent work on the cellular functions of kinesins, such as the organization of microtubules during cellular division and the movement of the organelles and vesicles, offers insights into how biological motors might prove useful for organizing structures in engineered environments.

Cargo selection by specific kinesin light chain 1 isoforms

Kinesin-1 drives the movement of diverse cargoes, and it has been proposed that specific kinesin light chain (KLC) isoforms target kinesin-1 to these different structures. Here, we test this hypothesis using two in vitro motility assays, which reconstitute the movement of rough endoplasmic reticulum (RER) and vesicles present in a Golgi membrane fraction. We generated GST-tagged fusion proteins of KLC1B and KLC1D that included the tetratricopeptide repeat domain and the variable C-terminus. We find that preincubation of RER with KLC1B inhibits RER motility, whereas KLC1D does not. In contrast, Golgi fraction vesicle movement is inhibited by KLC1D but not KLC1B reagents. Both RER and vesicle movement is inhibited by preincubation with the GST-tagged C-terminal domain of ubiquitous kinesin heavy chain (uKHC), which binds to the N-terminal domain of uKHC and alters its interaction with microtubules. We propose that although the TRR domains are required for cargo binding, it is the variable C-terminal region of KLCs that are vital for targeting kinesin-1 to different cellular structures.

Morphogenesis of the endoplasmic reticulum: beyond active membrane expansion

The endoplasmic reticulum (ER) of higher eukaryotic cells is a dynamic network of interconnected membrane tubules that pervades almost the entire cytoplasm. On the basis of the morphological changes induced by the disruption of the cytoskeleton or molecular motor proteins, the commonly accepted model has emerged that microtubules and conventional kinesin (kinesin-1) are essential determinants in establishing and maintaining the structure of the ER by active membrane expansion. Surprisingly, very similar ER phenotypes have now been observed when the cytoskeleton-linking ER membrane protein of 63 kDa (CLIMP-63) is mutated, revealing stable attachment of ER membranes to the microtubular cytoskeleton as a novel requirement for ER maintenance. Additional recent findings suggest that ER maintenance also requires ongoing homotypic membrane fusion, possibly controlled by the p97/p47/VICP135 protein complex. Work on other proteins proposed to regulate ER structure, including huntingtin, the EF-hand Ca(2+)-binding protein p22, the vesicle-associated membrane protein-associated protein B and kinectin isoforms further contribute to the new emerging concept that ER shape is not only determined by motor driven processes but by a variety of different mechanisms.

Regulation of intracellular trafficking of huntingtin-associated protein-1 is critical for TrkA protein levels and neurite outgrowth

Mutant huntingtin can affect vesicular and receptor trafficking via its abnormal protein interactions, suggesting that impairment of intracellular trafficking may contribute to Huntington's disease. There is growing evidence that huntingtin-associated protein-1 (HAP1) also interacts with microtubule-dependent transporters and is involved in intracellular trafficking. However, it remains unclear how the trafficking of HAP1 is regulated and contributes to neuronal function. Here we report that phosphorylation of HAP1 decreases its association with microtubule-dependent transport proteins dynactin p150 and kinesin light chain and reduces its localization in neurite tips. Suppressing HAP1 expression by RNA interference reduces neurite outgrowth and the level of tropomyosin-related kinase A receptor tyrosine kinase (TrkA), a nerve growth factor receptor whose internalization and trafficking are required for neurite outgrowth. HAP1 maintains the normal level of membrane TrkA by preventing the degradation of internalized TrkA. Mutant huntingtin also reduces the association of HAP1 with dynactin p150 and kinesin light chain and thereby decreases the intracellular level of TrkA. These findings suggest that HAP1 trafficking is critical for the stability of TrkA and neurite function, both of which can be attenuated by mutant huntingtin.

JNK mediates pathogenic effects of polyglutamine-expanded androgen receptor on fast axonal transport

Expansion of the polyglutamine (polyQ) stretch in the androgen receptor (AR) protein leads to spinal and bulbar muscular atrophy (SBMA), a neurodegenerative disease characterized by lower motor neuron degeneration. The pathogenic mechanisms underlying SBMA remain unknown, but recent experiments show that inhibition of fast axonal transport (FAT) by polyQ-expanded proteins, including polyQ-AR, represents a new cytoplasmic pathogenic lesion. Using pharmacological, biochemical and cell biological experiments, we found a new pathogenic pathway that is affected in SBMA and results in compromised FAT. PolyQ-AR inhibits FAT in a human cell line and in squid axoplasm through a pathway that involves activation of cJun N-terminal kinase (JNK) activity. Active JNK phosphorylated kinesin-1 heavy chains and inhibited kinesin-1 microtubule-binding activity. JNK inhibitors prevented polyQ-AR-mediated inhibition of FAT and reversed suppression of neurite formation by polyQ-AR. We propose that JNK represents a promising target for therapeutic interventions in SBMA.

The organization, structure, and inheritance of the ER in higher and lower eukaryotes

The endoplasmic reticulum (ER) is a fundamental organelle required for protein assembly, lipid biosynthesis, and vesicular traffic, as well as calcium storage and the controlled release of calcium from the ER lumen into the cytosol. Membranes functionally linked to the ER by vesicle-mediated transport, such as the Golgi complex, endosomes, vacuoles-lysosomes, secretory vesicles, and the plasma membrane, originate largely from proteins and lipids synthesized in the ER. In this review we will discuss the structural organization of the ER and its inheritance.

Interaction of Huntingtin-associated Protein-1 with Kinesin Light Chain

Huntingtin-associated protein-1 (HAP1) was initially identified as an interacting partner of huntingtin, the Huntington disease protein. Unlike huntingtin that is ubiquitously expressed throughout the brain and body, HAP1 is enriched in neurons, suggesting that its dysfunction could contribute to Huntington disease neuropathology. Growing evidence has demonstrated that HAP1 and huntingtin are anterogradely transported in axons and that the abnormal interaction between mutant huntingtin and HAP1 may impair axonal transport. However, the exact role of HAP1 in anterograde transport remains unclear. Here we report that HAP1 interacts with kinesin light chain, a subunit of the kinesin motor complex that drives anterograde transport along microtubules in neuronal processes. The interaction of HAP1 with kinesin light chain is demonstrated via a yeast two-hybrid assay, glutathione S-transferase pull down, and coimmunoprecipitation. Furthermore, HAP1 is colocalized with kinesin in growth cones of neuronal cells. We also demonstrated that knocking down HAP1 via small interfering RNA suppresses neurite outgrowth of PC12 cells. Analysis of live neuronal cells with fluorescence microscopy and fluorescence recovery after photobleaching demonstrates that suppressing the expression of HAP1 or deleting the HAP1 gene inhibits the kinesin-dependent transport of amyloid precursor protein vesicles. These studies provide a molecular basis for the participation of HAP1 in anterograde transport in neuronal cells.

Dynamics and inheritance of the endoplasmic reticulum

The endoplasmic reticulum (ER) consists of a polygonal array of interconnected tubules and sheets that spreads throughout the eukaryotic cell and is contiguous with the nuclear envelope. This elaborate structure is created and maintained by a constant remodeling process that involves the formation of new tubules, their cytoskeletal transport and homotypic fusion. Since the ER is a large, single-copy organelle, it must be actively segregated into daughter cells during cell division. Recent analysis in budding yeast indicates that ER inheritance involves the polarized transport of cytoplasmic ER tubules into newly formed buds along actin cables by a type V myosin. The tubules then become anchored to a site at the bud tip and this requires the Sec3p subunit of the exocyst complex. The ER is then propagated along the cortex of the bud to yield a cortical ER structure similar to that of the mother cell. In animal cells, the ER moves predominantly along microtubules, whereas actin fibers serve a complementary role. It is not yet clear to what extent the other components controlling ER distribution in yeast might be conserved in animal cells.

Polyglutamine expansion diseases: failing to deliver

Polyglutamine (polyQ)-expansion diseases are dominantly inherited adult-onset neurodegenerative diseases with unknown pathogenic mechanisms. Current models for pathogenesis include potential toxic effects of polyQ proteins, interference with survival pathways and deregulated gene transcription. Recently, nuclear and aggregate-independent alterations in fast axonal transport (FAT) have been demonstrated for several different polyQ disease models. Given the unique vulnerability of neurons to decrements in FAT, we propose an alternative model for polyQ disease pathogenesis. In this model, FAT is compromised because polyQ proteins affect enzymatic activities involved in FAT regulation. Decrements in FAT ultimately result in a failure to deliver essential cargos to specific neuronal subdomains, including presynaptic terminals, eventually leading to neuronal dysfunction and death. Pharmacological manipulation of such activities might provide the basis for new therapeutic strategies for treating polyQ diseases.

Golgins and GTPases, giving identity and structure to the Golgi apparatus

In this review we will focus on the recent advances in how coiled-coil proteins of the golgin family give identity and structure to the Golgi apparatus in animal cells. A number of recent studies reveal a common theme for the targeting of golgins containing the ARL-binding GRIP domain, and the related ARF-binding GRAB domain. Similarly, other golgins such as the vesicle tethering factor p115 and Bicaudal-D are targeted by the Rab GTPases, Rab1 and Rab6, respectively. Together golgins and their regulatory GTPases form a complex network, commonly known as the Golgi matrix, which organizes Golgi membranes and regulates membrane trafficking.

The Intracellular Fate of Salmonella Depends on the Recruitment of Kinesin

Salmonella enterica causes a variety of diseases, including gastroenteritis and typhoid fever. The success of this pathogen depends on its capacity to proliferate within host cells in a membrane-bound compartment. We found that the Salmonella-containing vacuole recruited the plus-end-directed motor kinesin. Bacterial effector proteins translocated into the host cell by a type III secretion system antagonistically regulated this event. Among these effectors, SifA targeted SKIP, a host protein that down-regulated the recruitment of kinesin on the bacterial vacuole and, in turn, controlled vacuolar membrane dynamics.

Phosphorylation controls CLIMP-63-mediated anchoring of the endoplasmic reticulum to microtubules

The microtubule-binding 63-kDa cytoskeleton-linking membrane protein (CLIMP-63) is an integral membrane protein that links the endoplasmic reticulum (ER) to microtubules. Here, we tested whether this interaction is regulated by phosphorylation. Metabolic labeling with (32)P showed that CLIMP-63 is a phosphoprotein with increased phosphorylation during mitosis. CLIMP-63 of mitotic cells is unable to bind to microtubules in vitro. Mitotic phosphorylation can be prevented by mutation of serines 3, 17, and 19 in the cytoplasmic domain of CLIMP-63. When these residues are mutated to glutamic acid, and hence mimic mitotic phosphorylation, CLIMP-63 does no longer bind to microtubules in vitro. Overexpression of the phospho-mimicking mitotic form of CLIMP-63 in interphase cells leads to a collapse of the ER around the nucleus, leaving the microtubular network intact. The results suggest that CLIMP-63-mediated stable anchoring of the ER to microtubules is required to maintain the spatial distribution of the ER during interphase and that this interaction is abolished by phosphorylation of CLIMP-63 during mitosis.

In search of membrane receptors for microtubule-based motors - is kinectin a kinesin receptor?

The past few years have seen an explosion in the number of molecular motors reported in the literature. By us the energy of hydrolysis, these motors move various organelles along cytoskeletal 'tracks' within the cell. It is thought that some of the specificity of movement resides in receptors on the surface of the cargo organelles, but, in general, little is known about these molecules. In this article, Janis Burkhardt discusses the evidence that the protein kinectin serves as a membrane receptor for kinesin, and describes how motor-receptor proteins may interact with other components of the motility machinery to generate regulated movement of membrane organelles.

Organelle transport along microtubules - the role of KIFs

Organelle transporters are very important for cellular morphogenesis and other cellular functions, conveying and targeting important materials to the correct destination, often at considerable velocities. One of the first proteins to be identified as a motor was kinesin, and recently at least 10 new kinesin superfamily proteins (KIFs) have been described. Characterization of some of them reveals that each member can convey a specific organelle or cargo, although there is some redundancy. It has also become clear that there are distinct subclasses of KIFs that form monomeric, heterodimeric and homodimeric motors. Here, Nobutaka Hirokawa reviews what is known about the kinesin superfamily and discusses how a study of the different types of motors is helping to elucidate the mechanism of mechanical force generation.

Interaction of microtubule-associated protein-2 and p63: a new link between microtubules and rough endoplasmic reticulum membranes in neurons

Neurons are polarized cells presenting two distinct compartments, dendrites and an axon. Dendrites can be distinguished from the axon by the presence of rough endoplasmic reticulum (RER). The mechanism by which the structure and distribution of the RER is maintained in these cells is poorly understood. In the present study, we investigated the role of the dendritic microtubule-associated protein-2 (MAP2) in the RER membrane positioning by comparing their distribution in brain subcellular fractions and in primary hippocampal cells and by examining the MAP2-microtubule interaction with RER membranes in vitro. Subcellular fractionation of rat brain revealed a high MAP2 content in a subfraction enriched with the endoplasmic reticulum markers ribophorin and p63. Electron microscope morphometry confirmed the enrichment of this subfraction with RER membranes. In cultured hippocampal neurons, MAP2 and p63 were found to concomitantly compartmentalize to the dendritic processes during neuronal differentiation. Protein blot overlays using purified MAP2c protein revealed its interaction with p63, and immunoprecipitation experiments performed in HeLa cells showed that this interaction involves the projection domain of MAP2. In an in vitro reconstitution assay, MAP2-containing microtubules were observed to bind to RER membranes in contrast to microtubules containing tau, the axonal MAP. This binding of MAP2c microtubules was reduced when an anti-p63 antibody was added to the assay. The present results suggest that MAP2 is involved in the association of RER membranes with microtubules and thereby could participate in the differential distribution of RER membranes within a neuron.

Bidirectional transport along microtubules

Active transport by microtubule motors has a plethora of crucial roles in eukaryotic cells. Organelles often move bidirectionally, employing both plus-end and minus-end directed motors. Bidirectional motion is widespread and may allow dynamic regulation, error correction and the establishment of polarized organelle distributions. Emerging evidence suggests that motors for both directions are simultaneously present on cellular 'cargo', but that their activity is coordinated so that when plus-end motors are active, minus-end motors are not, and vice versa. Both the dynein cofactor dynactin and the Klarsicht (Klar) protein appear to be important for such coordination. The direction of net transport depends on the balance between plus-end directed and minus-end directed motion. In several model systems, factors crucial for setting this balance have now been identified, setting the stage for a molecular dissection of the underlying regulatory mechanisms. These analyses will likely provide insight into motor cooperation in general.