CLIP-170-dependent capture of membrane organelles by microtubules initiates minus-end directed transport

The p25 subunit of the dynactin complex is required for dynein-early endosome interaction

The p25 subunit of the dynactin complex is required for the interaction between cytoplasmic dynein and early endosomes but is not required for dynein-mediated nuclear distribution.

Cytoplasmic dynein transports various cellular cargoes including early endosomes, but how dynein is linked to early endosomes is unclear. We find that the Aspergillus nidulans orthologue of the p25 subunit of dynactin is critical for dynein-mediated early endosome movement but not for dynein-mediated nuclear distribution. In the absence of NUDF/LIS1, p25 deletion abolished the localization of dynein–dynactin to the hyphal tip where early endosomes abnormally accumulate but did not prevent dynein–dynactin localization to microtubule plus ends. Within the dynactin complex, p25 locates at the pointed end of the Arp1 filament with Arp11 and p62, and our data suggest that Arp11 but not p62 is important for p25–dynactin association. Loss of either Arp1 or p25 significantly weakened the physical interaction between dynein and early endosomes, although loss of p25 did not apparently affect the integrity of the Arp1 filament. These results indicate that p25, in conjunction with the rest of the dynactin complex, is important for dynein–early endosome interaction.

Stimulation of the CLIP-170--dependent capture of membrane organelles by microtubules through fine tuning of microtubule assembly dynamics

Cytoplasmic microtubules (MTs) continuously grow and shorten at their free plus ends, a behavior that allows them to capture membrane organelles destined for MT minus end-directed transport. In Xenopus melanophores, the capture of pigment granules (melanosomes) involves the +TIP CLIP-170, which is enriched at growing MT plus ends. Here we used Xenopus melanophores to test whether signals that stimulate minus end MT transport also enhance CLIP-170-dependent binding of melanosomes to MT tips. We found that these signals significantly (>twofold) increased the number of growing MT plus ends and their density at the cell periphery, thereby enhancing the likelihood of interaction with dispersed melanosomes. Computational simulations showed that local and global increases in the density of CLIP-170-decorated MT plus ends could reduce the half-time of melanosome aggregation by ~50%. We conclude that pigment granule aggregation signals in melanophores stimulate MT minus end-directed transport by the increasing number of growing MT plus ends decorated with CLIP-170 and redistributing these ends to more efficiently capture melanosomes throughout the cytoplasm.

Controlled and stochastic retention concentrates dynein at microtubule ends to keep endosomes on track

Bidirectional transport of early endosomes (EEs) involves microtubules (MTs) and associated motors. In fungi, the dynein/dynactin motor complex concentrates in a comet-like accumulation at MT plus-ends to receive kinesin-3-delivered EEs for retrograde transport. Here, we analyse the loading of endosomes onto dynein by combining live imaging of photoactivated endosomes and fluorescent dynein with mathematical modelling. Using nuclear pores as an internal calibration standard, we show that the dynein comet consists of ∼55 dynein motors. About half of the motors are slowly turned over (T(1/2): ∼98 s) and they are kept at the plus-ends by an active retention mechanism involving an interaction between dynactin and EB1. The other half is more dynamic (T(1/2): ∼10 s) and mathematical modelling suggests that they concentrate at MT ends because of stochastic motor behaviour. When the active retention is impaired by inhibitory peptides, dynein numbers in the comet are reduced to half and ∼10% of the EEs fall off the MT plus-ends. Thus, a combination of stochastic accumulation and active retention forms the dynein comet to ensure capturing of arriving organelles by retrograde motors.

CK1 activates minus-end-directed transport of membrane organelles along microtubules

This study shows that the signal transduction pathway responsible for the initiation of minus-end–directed movement of membrane-bounded pigment granules in melanophores involves sequential activation of protein phosphatase 2A and casein kinase 1 and that this activation correlates with increased phosphorylation of the dynein intermediate chain.

Microtubule (MT)-based organelle transport is driven by MT motor proteins that move cargoes toward MT minus-ends clustered in the cell center (dyneins) or plus-ends extended to the periphery (kinesins). Cells are able to rapidly switch the direction of transport in response to external cues, but the signaling events that control switching remain poorly understood. Here, we examined the signaling mechanism responsible for the rapid activation of dynein-dependent MT minus-end–directed pigment granule movement in Xenopus melanophores (pigment aggregation). We found that, along with the previously identified protein phosphatase 2A (PP2A), pigment aggregation signaling also involved casein kinase 1ε (CK1ε), that both enzymes were bound to pigment granules, and that their activities were increased during pigment aggregation. Furthermore we found that CK1ε functioned downstream of PP2A in the pigment aggregation signaling pathway. Finally, we discovered that stimulation of pigment aggregation increased phosphorylation of dynein intermediate chain (DIC) and that this increase was partially suppressed by CK1ε inhibition. We propose that signal transduction during pigment aggregation involves successive activation of PP2A and CK1ε and CK1ε-dependent phosphorylation of DIC, which stimulates dynein motor activity and increases minus-end–directed runs of pigment granules.

Leucine-rich repeat kinase LRRK1 regulates endosomal trafficking of the EGF receptor

Activation of the epidermal growth factor receptor (EGFR) not only initiates multiple signal-transduction pathways, including the MAP kinase (MAPK) pathway, but also triggers trafficking events that relocalize receptors from the cell surface to intracellular endocytic compartments. In this paper, we demonstrate that leucine-rich repeat kinase LRRK1, which contains a MAPKKK-like kinase domain, forms a complex with activated EGFR through an interaction with Grb2. Subsequently, LRRK1 and epidermal growth factor (EGF) are internalized and co-localized in early endosomes. LRRK1 regulates EGFR transport from early to late endosomes and regulates the motility of EGF-containing early endosomes in a manner dependent on its kinase activity. Furthermore, LRRK1 serves as a scaffold facilitating the interaction of EGFR with the endosomal sorting complex required for transport-0 complex, thus enabling efficient sorting of EGFR to the inner vesicles of multivesicular bodies. Our findings provide the first evidence that a MAPKKK-like protein regulates the endosomal trafficking of EGFR.

Activation of the epidermal growth factor receptor can result in its internalization and subsequent intracellular trafficking. In this study, the authors show that leucine-rich repeat kinase-1 can bind to the receptor and regulate its trafficking between different endosomal compartments.

Directed microtubule growth, +TIPs, and kinesin-2 are required for uniform microtubule polarity in dendrites

BACKGROUND

in many differentiated cells, microtubules are organized into polarized noncentrosomal arrays, yet few mechanisms that control these arrays have been identified. For example, mechanisms that maintain microtubule polarity in the face of constant remodeling by dynamic instability are not known. Drosophila neurons contain uniform-polarity minus-end-out microtubules in dendrites, which are often highly branched. Because undirected microtubule growth through dendrite branch points jeopardizes uniform microtubule polarity, we have used this system to understand how cells can maintain dynamic arrays of polarized microtubules.

RESULTS

we find that growing microtubules navigate dendrite branch points by turning the same way, toward the cell body, 98% of the time and that growing microtubules track along stable microtubules toward their plus ends. Using RNAi and genetic approaches, we show that kinesin-2, and the +TIPS EB1 and APC, are required for uniform dendrite microtubule polarity. Moreover, the protein-protein interactions and localization of Apc2-GFP and Apc-RFP to branch points suggests that these proteins work together at dendrite branches. The functional importance of this polarity mechanism is demonstrated by the failure of neurons with reduced kinesin-2 to regenerate an axon from a dendrite.

CONCLUSIONS

we conclude that microtubule growth is directed at dendrite branch points and that kinesin-2, APC, and EB1 are likely to play a role in this process. We propose that kinesin-2 is recruited to growing microtubules by +TIPS and that the motor protein steers growing microtubules at branch points. This represents a newly discovered mechanism for maintaining polarized arrays of microtubules.

Plus-end-tracking proteins and their interactions at microtubule ends

Microtubules are cytoskeletal elements that are essential for a large number of intracellular processes, including mitosis, cell differentiation and migration, and vesicle transport. In many cells, the microtubule network is organized in a radial manner, with one end of a microtubule (the minus end) embedded near the nucleus and the other end (the plus end) exploring cytoplasmic space, switching between episodes of growth and shrinkage. Mammalian plus-end-tracking proteins (+TIPs) localize to the ends of growing microtubules and regulate both the dynamic behavior of microtubules as well as the interactions of microtubules with other cellular components. Because of these crucial roles, +TIPs and the mechanisms underlying their association with microtubule ends have been intensively investigated. Results indicate that +TIPs reach microtubule ends by motor-mediated transport or diffusion. Individual +TIP molecules exchange rapidly on microtubule end-binding sites that are formed during microtubule polymerization and that have a slower turnover. Most +TIPs associate with the end-binding (EB) proteins, and appear to require these 'core' +TIPs for localization at microtubule ends. Accumulation of +TIPs may also involve structural features of the microtubule end and interactions with other +TIPs. This complexity makes it difficult to assign discrete roles to specific +TIPs. Given that +TIPs concentrate at microtubule ends and that each +TIP binds in a conformationally distinct manner, I propose that the ends of growing microtubules are 'nano-platforms' for productive interactions between selected proteins and that these interactions might persist and be functional elsewhere in the cytoplasm than at the microtubule end at which they originated.

Integrin-linked kinase controls microtubule dynamics required for plasma membrane targeting of caveolae

Caveolae are specialized compartments of the plasma membrane that are involved in signaling, endocytosis, and cholesterol transport. Their formation requires the transport of caveolin-1 to the plasma membrane, but the molecular mechanisms regulating the transport are largely unknown. Here, we identify a critical role for adhesion-mediated signaling through β1 integrins and integrin-linked kinase (ILK) in caveolae formation. Mice lacking β1 integrins or ILK in keratinocytes have dramatically reduced numbers of plasma membrane caveolae in vivo, which is due to impaired transport of caveolin-1-containing vesicles along microtubules (MT) to the plasma membrane. Mechanistically, ILK promotes the recruitment of the F-actin binding protein IQGAP1 to the cell cortex, which, in turn, cooperates with its effector mDia1 to locally stabilize MTs and to allow stable insertion of caveolae into the plasma membrane. Our results assign an important role to the integrin/ILK complex for caveolar trafficking to the cell surface.

The microtubule plus-end localization of Aspergillus dynein is important for dynein-early-endosome interaction but not for dynein ATPase activation

Cytoplasmic dynein in filamentous fungi accumulates at microtubule plus-ends near the hyphal tip, which is important for minus-end-directed transport of early endosomes. It was hypothesized that dynein is switched on at the plus-end by cargo association. Here, we show in Aspergillus nidulans that kinesin-1-dependent plus-end localization is not a prerequisite for dynein ATPase activation. First, the Walker A and Walker B mutations in the dynein heavy chain AAA1 domain implicated in blocking different steps of the ATPase cycle cause different effects on dynein localization to microtubules, arguing against the suggestion that ATPase is inactive before arriving at the plus-end. Second, dynein from ΔkinA (kinesin 1) mutant cells has normal ATPase activity despite the absence of dynein plus-end accumulation. In ΔkinA hyphae, dynein localizes along microtubules and does not colocalize with abnormally accumulated early endosomes at the hyphal tip. This is in contrast to the colocalization of dynein and early endosomes in the absence of NUDF/LIS1. However, the Walker B mutation allows dynein to colocalize with the hyphal-tip-accumulated early endosomes in the ΔkinA background. We suggest that the normal ability of dyenin to interact with microtubules as an active minus-end-directed motor demands kinesin-1-mediated plus-end accumulation for effective interactions with early endosomes.

Phosphorylation controls autoinhibition of cytoplasmic linker protein-170

CLIP-170 conformational changes are regulated by phosphorylation on S³⁰⁹ and S³¹¹ residues resulting in diminished binding of CLIP-170 for growing MT ends and p150Glued.

Cytoplasmic linker protein (CLIP)-170 is a microtubule (MT) plus-end-tracking protein that regulates MT dynamics and links MT plus ends to different intracellular structures. We have shown previously that intramolecular association between the N and C termini results in autoinhibition of CLIP-170, thus altering its binding to MTs and the dynactin subunit p150^Glued (J. Cell Biol. 2004: 166, 1003–1014). In this study, we demonstrate that conformational changes in CLIP-170 are regulated by phosphorylation that enhances the affinity between the N- and C-terminal domains. By using site-directed mutagenesis and phosphoproteomic analysis, we mapped the phosphorylation sites in the third serine-rich region of CLIP-170. A phosphorylation-deficient mutant of CLIP-170 displays an “open” conformation and a higher binding affinity for growing MT ends and p150^Glued as compared with nonmutated protein, whereas a phosphomimetic mutant confined to the “folded back” conformation shows decreased MT association and does not interact with p150^Glued. We conclude that phosphorylation regulates CLIP-170 conformational changes resulting in its autoinhibition.

Dsl1p/Zw10: common mechanisms behind tethering vesicles and microtubules

Fusion of Golgi-derived COP (coat protein)-I vesicles with the endoplasmic reticulum (ER) is initiated by specific tethering complexes: the Dsl1 (depends on SLY1-20) complex in yeast and the syntaxin 18 complex in mammalian cells. Both tethering complexes are firmly associated with soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) at the ER. The structure of the Dsl1 tethering complex has been determined recently. The complex seems to be designed to expose an unstructured domain of Dsl1p at its top, which is required to capture vesicles. The subunit composition and the interactions within the equivalent mammalian complex are similar. Interestingly, some of the mammalian counterparts have additional functions during mitosis in animal cells. Zw10, the metazoan homolog of Dsl1p, is an important component of a complex that monitors the correct tethering of microtubules to kinetochores during cell division. This review brings together evidence to suggest that there could be common mechanisms behind these different activities, giving clues as to how they might have evolved.

Dynamics of peptidergic secretory granule transport are regulated by neuronal stimulation

Background

Peptidergic neurons store and secrete the contents of large dense core vesicles (LDCVs) from axon terminals and from dendrites. Secretion of peptides requires a highly regulated exocytotic mechanism, plus coordinated synthesis and transport of LDCVs to their sites of release. Although these trafficking events are critical to function, little is known regarding the dynamic behavior of LDCVs and the mechanisms by which their transport is regulated. Sensory neurons also package opiate receptors in peptide-containing LDCVs, which is thought to be important in pain sensation. Since peptide granules cannot be refilled locally after their contents are secreted, it is particularly important to understand how neurons support regulated release of peptides.

Results

A vector encoding soluble peptidylglycine α-hydroxylating monooxygenase fused to green fluorescent protein was constructed to address these questions in cultured primary peptidergic neurons of the trigeminal ganglion using time lapse confocal microscopy. The time course of release differs with secretagogue; the secretory response to depolarization with K⁺ is rapid and terminates within 15 minutes, while phorbol ester stimulation of secretion is maintained over a longer period. The data demonstrate fundamental differences between LDCV dynamics in axons and growth cones under basal conditions.

Conclusions

Under basal conditions, LDCVs move faster away from the soma than toward the soma, but fewer LDCVs travel anterograde than retrograde. Stimulation decreased average anterograde velocity and increases granule pausing. Data from antibody uptake, quantification of enzyme secretion and appearance of pHluorin fluorescence demonstrate distributed release of peptides all along the axon, not just at terminals.

Microtubule dynamics at the cell cortex probed by TIRF microscopy

Total internal reflection fluorescence (TIRF) microscopy is a technique that allows selective excitation of fluorescence at a liquid/solid interface within a short distance from the boundary. The penetration depth of TIRF microscopy depends on the angle of illumination resulting in a range of depths, which typically vary from approximately similar 70-200 nm up to reverse approximately 500 nm. The advantages of TIRF microscopy include excellent signal-to-noise ratio, high sensitivity, low photobleaching, and low photodamage. TIRF microscopy is widely used for studying cell adhesion, exo- and endocytosis, and the dynamics of plasma membrane-associated molecules. TIRF microscopy can also be applied for selective visualization of any other cellular processes that occur near the basal membrane even if their localization is not restricted to this part of the cell. For example, microtubules are distributed throughout the cytoplasm, but the use of TIRF microscopy makes it possible to visualize specifically the microtubule subpopulation in the vicinity of the basal cortex and thus study cortical microtubule attachment and stabilization, interactions between microtubules and matrix adhesion structures, and the behavior of specific molecules involved in these processes. In this chapter we describe the application of a commercially available setup to analyze microtubule behavior in live mammalian cells using TIRF microscopy.

Melanophores for microtubule dynamics and motility assays

Microtubules (MTs) are cytoskeletal structures essential for cell division, locomotion, intracellular transport, and spatial organization of the cytoplasm. In most interphase cells, MTs are organized into a polarized radial array with minus-ends clustered at the centrosome and plus-ends extended to the cell periphery. This array directs transport of organelles driven by MT-based motor proteins that specifically move either to plus- or to minus-ends. Along with using MTs as tracks for cargo, motor proteins can organize MTs into a radial array in the absence of the centrosome. Transport of organelles and motor-dependent radial organization of MTs require MT dynamics, continuous addition and loss of tubulin subunits at minus- and plus-ends. A unique experimental system for studying the role of MT dynamics in these processes is the melanophore, which provides a useful tool for imaging of both dynamic MTs and moving membrane organelles. Melanophores are filled with pigment granules that are synchronously transported by motor proteins in response to hormonal stimuli. The flat shape of the cell and the radial organization of MTs facilitate imaging of dynamic MT plus-ends and monitoring of their interaction with membrane organelles. Microsurgically produced cytoplasmic fragments of melanophores are used to study the centrosome-independent rearrangement of MTs into a radial array. Here we describe the experimental approaches to study the role of MT dynamics in intracellular transport and centrosome-independent MT organization in melanophores. We focus on the preparation of cell cultures, microsurgery and microinjection, fluorescence labeling, and live imaging of MTs.

Use of virtual cell in studies of cellular dynamics

The Virtual Cell (VCell) is a unique computational environment for modeling and simulation of cell biology. It has been specifically designed to be a tool for a wide range of scientists, from experimental cell biologists to theoretical biophysicists. The models created with VCell can range from the simple, to evaluate hypotheses or to interpret experimental data, to complex multilayered models used to probe the predicted behavior of spatially resolved, highly nonlinear systems. In this chapter, we discuss modeling capabilities of VCell and demonstrate representative examples of the models published by the VCell users.

Regulators of the cytoplasmic dynein motor

Eukaryotic cells use cytoskeletal motor proteins to transport many different intracellular cargos. Numerous kinesins and myosins have evolved to cope with the various transport needs that have arisen during eukaryotic evolution. Surprisingly, a single cytoplasmic dynein (a minus end-directed microtubule motor) carries out similarly diverse transport activities as the many different types of kinesin. How is dynein coupled to its wide range of cargos and how is it spatially and temporally regulated? The answer could lie in the several multifunctional adaptors, including dynactin, lissencephaly 1, nuclear distribution protein E (NUDE) and NUDE-like, Bicaudal D, Rod-ZW10-Zwilch and Spindly, that regulate dynein function and localization.