Interaction of the microtubule cytoskeleton with endocytic vesicles and cytoplasmic dynein in cultured rat hepatocytes

Raft-dependent endocytic movement and intracellular cluster formation during T cell activation triggered by concanavalin A

Certain food ingredients can stimulate the human immune system. A lectin, concanavalin A (ConA), from Canavalia ensiformis (jack bean) is one of the most well-known food-derived immunostimulants and mediates activation of cell-mediated immunity through T cell proliferation. Generally, T cell activation is known to be triggered by the interaction between T cells and antigen-presenting cells (APCs) via a juxtacrine (contact-dependent) signaling pathway. The mechanism has been well characterized and is referred to as formation of the immunological synapse (IS). We were interested in the mechanism behind the T cell activation by food-derived ConA which might be different from that of T cell activation by APCs. The purpose of this study was to characterize T cell activation by ConA with regard to (i) movement of raft domain, (ii) endocytic vesicular transport, (iii) the cytoskeleton (actin and microtubules), and (iv) cholesterol composition. We found that raft-dependent endocytic movement was important for T cell activation by ConA and this movement was dependent on actin, microtubules, and cholesterol. The T cell signaling mechanism triggered by ConA can be defined as endocrine signaling which is distinct from the activation process triggered by interaction between T cells and APCs by juxtacrine signaling. Therefore, we hypothesized that T cell activation by ConA includes both two-dimensional superficial raft movement on the membrane surface along actin filaments and three-dimensional endocytic movement toward the inside of the cell along microtubules. These findings are important for developing new methods for immune stimulation and cancer therapy based on the function of ConA.

GAPDH binds Akt to facilitate cargo transport in the early secretory pathway

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) undergoes numerous post-translational modifications, which impart new function and influence intracellular location. For example, atypical PKC ι/λ phosphorylates GAPDH that locates to vesicular tubular clusters and is required for retrograde membrane trafficking in the early secretory pathway. GAPDH is also required in the endocytic pathway; substitution of Pro²³⁴ to Ser (Pro²³⁴Ser) rendered CHO cells defective in endocytosis. To determine if GAPDH (Pro²³⁴Ser) could inhibit endoplasmic reticulum to Golgi trafficking, we introduced the recombinant mutant enzyme into several biochemical and morphological transport assays. The mutant protein efficiently blocked vesicular stomatitis virus-G protein transport. Because GAPDH binds to microtubules (MTs), we evaluated MT binding and MT intracellular distribution in the presence of the mutant. Although these properties were not changed relative to wild-type, GAPDH (Pro²³⁴Ser) altered Golgi complex morphology. We determined that the GAPDH point mutation disrupted association between the enzyme and the serine/threonine kinase Akt. Interestingly Rab1, which functions in anterograde-directed trafficking, stimulates GAPDH-Akt association with membranes in a quantitative binding assay. In contrast, Rab2 does not stimulate GAPDH-Akt membrane binding but instead recruits GAPDH-aPKC. We propose a mechanism whereby the association of GAPDH with Akt or with aPKC serves as a switch to discriminate between anterograde directed cargo and recycling cargo retrieved back to the ER, respectively.

Characterisation of detergent-insoluble membranes in pollen tubes of Nicotiana tabacum (L.)

Pollen tubes are the vehicle for sperm cell delivery to the embryo sac during fertilisation of Angiosperms. They provide an intriguing model for unravelling mechanisms of growing to extremes. The asymmetric distribution of lipids and proteins in the pollen tube plasma membrane modulates ion fluxes and actin dynamics and is maintained by a delicate equilibrium between exocytosis and endocytosis. The structural constraints regulating polarised secretion and asymmetric protein distribution on the plasma membrane are mostly unknown. To address this problem, we investigated whether ordered membrane microdomains, namely membrane rafts, might contribute to sperm cell delivery. Detergent insoluble membranes, rich in sterols and sphingolipids, were isolated from tobacco pollen tubes. MALDI TOF/MS analysis revealed that actin, prohibitins and proteins involved in methylation reactions and in phosphoinositide pattern regulation are specifically present in pollen tube detergent insoluble membranes. Tubulins, voltage-dependent anion channels and proteins involved in membrane trafficking and signalling were also present. This paper reports the first evidence of membrane rafts in Angiosperm pollen tubes, opening new perspectives on the coordination of signal transduction, cytoskeleton dynamics and polarised secretion.

Basal body proteins regulate Notch signaling through endosomal trafficking

Proteins associated with primary cilia and basal bodies mediate numerous signaling pathways, but little is known about their role in Notch signaling. Here, we report that loss of the Bardet-Biedl syndrome proteins BBS1 or BBS4 produces increased Notch-directed transcription in a zebrafish reporter line and in human cell lines. Pathway overactivation is accompanied by reduced localization of Notch receptor at both the plasma membrane and the cilium. In Drosophila mutants, overactivation of Notch can result from receptor accumulation in endosomes, and recent studies implicate ciliary proteins in endosomal trafficking, suggesting a possible mechanism by which overactivation occurs in BBS mutants. Consistent with this, we observe genetic interaction of BBS1 and BBS4 with the endosomal sorting complexes required for transport (ESCRT) gene TSG101 and accumulation of receptor in late endosomes, reduced endosomal recycling and reduced receptor degradation in lysosomes. We observe similar defects with disruption of BBS3. Loss of another basal body protein, ALMS1, also enhances Notch activation and the accumulation of receptor in late endosomes, but does not disrupt recycling. These findings suggest a role for these proteins in the regulation of Notch through endosomal trafficking of the receptor.

Oatp1a1 requires PDZK1 to traffic to the plasma membrane by selective recruitment of microtubule-based motor proteins

Previous studies identified a family of organic anion transport proteins (OATPs), many of which have C-terminal PDZ binding consensus sequences. In particular, the C-terminal four amino acids of Oatp1a1, a transporter on rat and mouse hepatocytes, comprise a consensus binding site for PDZK1. In PDZK1 knockout mice and in transfected cells where PDZK1 expression was knocked down, Oatp1a1 accumulates in intracellular vesicles. The present study tests the hypothesis that Oatp1a1 traffics to and from the cell surface in vesicles along microtubules, and that PDZK1 guides recruitment of specific motors to these vesicles. Oatp1a1-containing vesicles were prepared from wild-type and PDZK1 knockout mice. As seen by immunofluorescence, kinesin-1, a microtubule plus-end directed motor, was largely associated with vesicles from wild-type mouse liver, whereas dynein, a minus-end directed motor, was largely associated with vesicles from PDZK1 knockout mouse liver. Quantification of motility on directionally marked microtubules following addition of 50 µM ATP showed that wild-type vesicles moved equally toward the plus and minus ends whereas PDZK1 knockout vesicles moved predominantly toward the minus end, consistent with net movement toward the cell interior. These studies provide a novel mechanism by which PDZK1 regulates intracellular trafficking of Oatp1a1 by recruiting specific motors to Oatp1a1-containing vesicles. In the absence of PDZK1, Oatp1a1-containing vesicles cannot recruit kinesin-1 and associate with dynein as a predominant minus-end directed motor. Whether this is a result of direct interaction of the Oatp1a1 cytoplasmic domain with dynein or with a dynein-containing protein complex remains to be established.

Microtubule motors mediate endosomal sorting by maintaining functional domain organization

Many microtubule motors have been shown to couple to endosomal membranes. These motors include dynein in addition to many different kinesin family members. Sorting nexins (SNXs) are central to the organization and function of endosomes. These proteins can actively shape endosomal membranes and couple directly or indirectly to the minus-end microtubule motor dynein. Motor proteins acting on endosomes drive their motility, dictate their morphology and affect cargo segregation. We have used well-characterized members of the SNX family to elucidate motor coupling using high-resolution light microscopy coupled with depletion of specific microtubule motors. Endosomal domains labelled with SNX1, SNX4 and SNX8 couple to discrete combinations of dynein and kinesin motors. These specific combinations govern the structure and motility of each SNX-coated membrane in addition to the segregation of distinct functional endosomal subdomains. Taken together, our data show that these key features of endosome dynamics are governed by the same set of opposing microtubule motors. Thus, microtubule motors help to define the mosaic layout of endosomes that underpins cargo sorting.

Albumin uptake in OK cells exposed to rotenone: a model for studying the effects of mitochondrial dysfunction on endocytosis in the proximal tubule?

Background

The renal proximal tubule (PT) is clinically vulnerable to mitochondrial dysfunction; sub-lethal injury can lead to the Fanconi syndrome, with elevated urinary excretion of low-molecular-weight proteins. As the mechanism that couples mitochondrial dysfunction to impaired PT low-molecular weight protein uptake is unknown, we investigated the effect of respiratory chain (RC) inhibitors on endocytosis of FITC-albumin in PT-derived OK cells.

Methods

Uptake of FITC-albumin was quantified using confocal microscopy. Cytosolic ATP levels were measured in real time using both luciferin/luciferase assays and measurements of free [Mg²⁺]. Reactive oxygen species production was measured using mitosox.

Results

RC blockade produced only a small decrease in cytosolic ATP levels and had minimal effect on FITC-albumin uptake. Inhibition of glycolysis caused a much bigger decrease in both cytosolic ATP levels and FITC-albumin endocytosis. Rotenone led to higher rates of reactive oxygen species production than other RC inhibitors. Rotenone also caused widespread structural damage on electron microscopy, which was mimicked by colchicine and prevented by taxol; consistent with inhibition of microtubule polymerisation as the underlying mechanism.

Conclusions

Endocytosis of FITC-albumin is ATP-dependent in OK cells, but the cells are very glycolytic and therefore represent a poor metabolic model of the PT. Rotenone has toxic extra-mitochondrial structural effects.

Direct binding of cytosolic NDP kinases to membrane lipids is regulated by nucleotides

In spite of their complete lack of any structural features that characterize membrane proteins, cytosolic nucleoside-diphosphate kinases (NDPKs) have been found repeatedly to associate with membranes. In some instances the recruitment of cytosolic NDPKs to membranes was attributed to interactions with peripheral or integral membrane proteins, but in many cases the mechanism underlying the association of NDPKs with membranes remained unknown. We show here that cytosolic NDPKs bind directly to membrane lipids in a dynamic process that is controlled by its substrates, nucleoside tri- and diphosphates, and can be fully reconstituted with chemically defined, protein-free phospholipids and recombinant NDPK, or with purified NDPK. Our results uncover a novel mechanism for the reversible targeting of soluble NDPKs to membranes, where they may act as a reservoir of high energy phosphate, supporting the operation of membrane-based processes that utilize nucleotides other than ATP, such as intracellular traffic and phospholipid biosynthesis.

Topological assessment of oatp1a1: a 12-transmembrane domain integral membrane protein with three N-linked carbohydrate chains

Organic anion transport protein 1a1 (oatp1a1), a prototypical member of the oatp family of highly homologous transport proteins, is expressed on the basolateral (sinusoidal) surface of rat hepatocytes. The organization of oatp1a1 within the plasma membrane has not been well defined, and computer-based models have predicted possible 12- as well as 10-transmembrane domain structures. Which of oatp1a1's four potential N-linked glycosylation sites are actually glycosylated and their influence on transport function have not been investigated in a mammalian system. In the present study, topology of oatp1a1 in the rat hepatocyte plasma membrane was examined by immunofluorescence analysis using an epitope-specific antibody designed to differentiate a 10- from a 12-transmembrane domain model. To map glycosylation sites, the asparagines at the each of the four N-linked glycosylation consensus sites were mutagenized to glutamines. Mutagenized oatp1a1 constructs were expressed in HeLa cells, and effects on protein expression and transport activity were assessed. These studies revealed that oatp1a1 is a 12-transmembrane-domain protein in which the second and fifth extracellular loops are glycosylated at asparagines 124, 135, and 492, whereas the potential glycosylation site at asparagine 62 is not utilized, consistent with its position in a transmembrane domain. Constructs in which more than one glycosylation site were eliminated had reduced transport activity but not necessarily reduced transporter expression. This was in accord with the finding that fully unglycosylated oatp1a1 was well expressed but located intracellularly with limited transport ability as a consequence of its reduced cell surface expression.

Single vesicle analysis of endocytic fission on microtubules in vitro

Following endocytosis, internalized molecules are found within intracellular vesicles and tubules that move along the cytoskeleton and undergo fission, as demonstrated here using primary cultured rat hepatocytes. Although the use of depolymerizing drugs has shown that the cytoskeleton is not required to segregate endocytic protein, many studies suggest that the cytoskeleton is involved in the segregation of protein in normal cells. To investigate whether cytoskeletal-based movement results in the segregation of protein, we tracked the contents of vesicles during in vitro microscopy assays. These studies showed that the addition of ATP causes fission of endocytic contents along microtubules, resulting in the segregation of proteins that are targeted for different cellular compartments. The plasma membrane proteins, sodium (Na+) taurocholate cotransporting polypeptide (ntcp) and transferrin receptor, segregated from asialoorosomucoid (ASOR), an endocytic ligand that is targeted for degradation. Epidermal growth factor receptor, which is degraded, and the asialoglycoprotein receptor, which remains partially bound to ASOR, segregated less efficiently from ASOR. Vesicles containing ntcp and transferrin receptor had reduced fission in the absence of ASOR, suggesting that fission is regulated to allow proteins to segregate. A single round of fission resulted in 6.5-fold purification of ntcp from ASOR, and 25% of the resulting vesicles were completely depleted of the endocytic ligand.

Requirement for the dynein light chain km23-1 in a Smad2-dependent transforming growth factor-beta signaling pathway

We have identified km23-1 as a novel transforming growth factor-beta (TGFbeta) receptor (TbetaR)-interacting protein that is also a light chain of the motor protein dynein (dynein light chain). Herein, we demonstrate by sucrose gradient analyses that, in the presence of TGFbeta but not in the absence, km23-1 was present in early endosomes with the TbetaRs. Further, confocal microscopy studies indicate that endogenous km23-1 was co-localized with endogenous Smad2 at early times after TGFbeta treatment, prior to Smad2 translocation to the nucleus. In addition, immunoprecipitation/blot analyses showed that TGFbeta regulated the interaction between endogenous km23-1 and endogenous Smad2 in vivo. Blockade of km23-1 using a small interfering RNA approach resulted in a reduction in both total intracellular Smad2 levels and in nuclear levels of phosphorylated Smad2 after TGFbeta treatment. This decrease was reversed by lactacystin, a specific inhibitor of the 26 S proteasome, suggesting that knockdown of km23-1 causes proteasomal degradation of phosphorylated (i.e. activated) Smad2. Blockade of km23-1 also resulted in a reduction in TGFbeta/Smad2-dependent ARE-Lux transcriptional activity, which was rescued by a km23-1 small interfering RNA-resistant construct. In contrast, a reduction in TGFbeta/Smad3-dependent SBE2-Luc transcriptional activity did not occur under similar conditions. Furthermore, overexpression of the dynactin subunit dynamitin, which is known to disrupt dynein-mediated intracellular transport, blocked TGFbeta-stimulated nuclear translocation of Smad2. Collectively, our findings indicate for the first time that a dynein light chain is required for a Smad2-dependent TGFbeta signaling pathway.

Intracellular transport of hepatitis B virus

For genome multiplication hepadnaviruses use the transcriptional machinery of the cell that is found within the nucleus. Thus the viral genome has to be transported through the cytoplasm and nuclear pore. The intracytosolic translocation is facilitated by the viral capsid that surrounds the genome and that interacts with cellular microtubules. The subsequent passage through the nuclear pore complexes (NPC) is mediated by the nuclear transport receptors importin α and β. Importin α binds to the C-terminus of the capsid protein that comprises a nuclear localization signal (NLS). The exposure of the NLS is regulated and depends upon genome maturation and/or phosphorylation of the capsid protein. As for other karyophilic cargos using this pathway importin α interacts with importin β that facilitates docking of the import complex to the NPC and the passage through the pore. Being a unique strategy, the import of the viral capsid is incomplete in that it becomes arrested inside the nuclear basket, which is a cage-like structure on the karyoplasmic face of the NPC. Presumably only this compartment provides the factors that are required for capsid disassembly and genome release that is restricted to those capsids comprising a mature viral DNA genome.

Adaptor heat shock protein complex formation regulates trafficking of the asialoglycoprotein receptor

In the asialoglycoprotein receptor (ASGPR) endocytic pathway, internalized receptors pass through early, recycling, and sorting endosomal compartments before returning to the cell surface. Sorting motifs in the cytoplasmic domain (CD) and protein interactions with these sequences presumably direct receptor trafficking. Previous studies have shown that association of a potential sorting heat shock protein (HSP) heterocomplex with the ASGPR-CD was regulated by casein kinase 2 (CK2)-mediated phosphorylation. Mass spectrometry and immunoblot analyses identified five of these ASGPR-CD-associated proteins as the molecular chaperones glycoprotein 96, HSP70, HSP90, cyclophilin A, and FK 506 binding protein. The present study was undertaken to determine whether any of the adaptor protein complexes (AP1, AP2, or AP3) were selectivity associated with the ASGPR-CD. In conjunction with molecular chaperones, AP2 and AP1 were recovered from a CK2 phosphorylated agarose-GSH-GST-ASGPR-CD matrix. Binding of AP3 was independent of the phosphorylation status of the CD matrix. Inhibition of CK2-mediated phosphorylation with tetrabromobenzotriazole prevented AP recovery within an immunoadsorbed ASGPR complex. Rapamycin, which dissociates the HSP heterocomplex from ASGPR-CD, thereby altering receptor trafficking also, inhibited AP association. Similar results were obtained with an inhibitor of HSP90 heterocomplex formation, geldanmycin. The data presented provide evidence that recruitment of AP1 and AP2, which is necessary for appropriate receptor trafficking, is mediated by the interaction of AP with the ASGPR-CD-bound HSP complex.

Distribution and functions of kinectin isoforms

Kinectin is an integral transmembrane protein on the endoplasmic reticulum, binding to kinesin, interacting with Rho GTPase and anchoring the translation elongation factor-1 complex. There has been debate on the specific role(s) of kinectin in different species and cell types. Here we identified 15 novel kinectin isoforms in the mouse nervous system, constituting a family of alternatively spliced carboxyl-terminal variants. Isoform expression is subject to cell type- and developmental stage-specific regulation. We raised specific antibodies to the kinectin variants to characterise their differential intracellular localisation and discovered that certain kinectin isoforms are found in axons where kinectin was previously believed to be absent. We also demonstrated in vivo by overexpression and RNA interference assay that kinectin is selectively involved in the transport of specific types of organelles. A 160 kDa kinectin species is mainly concentrated in the endoplasmic reticulum, anchored via its transmembrane domain and is essential for endoplasmic reticulum membrane extension. A 120 kDa kinectin species is specifically associated with mitochondria, and its interaction with kinesin was found to influence mitochondrial dynamics. These findings contribute to a more unified view of kinectin function. They suggest that different cellular processes use specific kinectin isoforms to mediate intracellular motility and targeting by transient interaction with different motor proteins or other binding partners.

Role of the microtubule cytoskeleton in traffic of EGF through the lacrimal acinar cell endomembrane network

We have previously documented a novel biphasic traffic pattern for epidermal growth factor (EGF) in the acinar epithelial cell of the lacrimal gland. Different from the typical paradigm observed in many other cell types, EGF initially accumulates in the acinar basal-lateral recycling endosome, then is re-directed to the prelysosomes and lysosomes and degraded. While the cellular content of intact EGF decreases by 40% between 20 and 120 m of continuous incubation at 37 degrees C, the EGF receptor (EGFR) content decreases only modestly [J. Cell Physiol. 199 (2004) 108]. The purpose of the present study was to investigate the role of the microtubule cytoskeleton in this traffic. Primary cultured rabbit lacrimocytes were incubated with [(125)I]-EGF, lysed, and analyzed by subcellular fractionation on sorbitol density gradients. Nocodazole treatment appeared to slightly decrease the initial uptake rate but to have no significant effect on the total amount of [(125)I] accumulation. However, it enhanced accumulation of [(125)I]-EGF and EGFR in the basal-lateral recycling endosome, and it enhanced accumulation of prepro- and pro- cathepsin B in fractions containing late endosomes and prelysosomes. Nocodazole permitted the time-dependent release of [(125)I]-EGF from the recycling endosome, but it partially inhibited [(125)I]-EGF degradation and decreased accumulation of [(125)I]-labeled degradation products in the lysosome. The microtubule-based molecular motors, cytoplasmic dynein and kinesin, were localized in compartments containing the late endosomes, prelysosomes, and lysosomes, consistent with the suggestion that microtubule-based molecular motors play important roles in traffic within the lysosomal pathway. Confocal fluorescence microscopy imaging of FITC-EGF substantiated the effects observed in biochemical studies by demonstrating that nocodazole increased accumulation in a peripheral compartment and decreased traffic to a perinuclear compartment. These data suggest that initial accumulation in the basal-lateral recycling endosome and subsequent release from the recycling endosome to the late endosomes and prelysosome are not microtubule-dependent. On the other hand, microtubule-based motors are more critical for traffic from the prelysosome to the lysosome.

Microtubule-dependent movement of late endocytic vesicles in vitro: requirements for Dynein and Kinesin

Our previous studies demonstrated that fluorescent early endocytic vesicles prepared from rat liver after injection of Texas red asialoorosomucoid contain asialoglycoprotein and its receptor and move and undergo fission along microtubules using kinesin I and KIFC2, with Rab4 regulating KIFC2 activity (J. Cell Sci. 116, 2749, 2003). In the current study, procedures to prepare fluorescent late endocytic vesicles were devised. In addition, flow cytometry was utilized to prepare highly purified fluorescent endocytic vesicles, permitting validation of microscopy-based experiments as well as direct biochemical analysis. These studies revealed that late vesicles bound to and moved along microtubules, but in contrast to early vesicles, did not undergo fission. As compared with early vesicles, late vesicles had reduced association with receptor, Rab4, and kinesin I but were highly associated with dynein, Rab7, dynactin, and KIF3A. Dynein and KIF3A antibodies inhibited late vesicle motility, whereas kinesin I and KIFC2 antibodies had no effect. Dynamitin antibodies prevented the association of late vesicles with microtubules. These results indicate that acquisition and exchange of specific motor and regulatory proteins characterizes and may regulate the transition of early to late endocytic vesicles. Flow cytometric purification should ultimately facilitate detailed proteomic analysis and mapping of endocytic vesicle-associated proteins.

Receptor-mediated endocytosis by the asialoglycoprotein receptor: effect of ethanol administration on endosomal distribution of receptor and ligand

Using the asialoglycoprotein receptor (ASGP-R) and a representative ligand, asialoorosomucoid (ASOR), we have previously shown ethanol-induced impairment of endosomal acidification, receptor recycling and ligand binding, internalization, and degradation. In the current study, we further investigated ethanol-induced alterations in receptor/ligand trafficking by labeling endosomes in vivo with either Texas-Red-ASOR or 125I-ASOR, and then assessing the receptor/ligand content of endosomes. We assessed two fractions after both 5 and 25 min of labeling: 'early endosomes' (EEs; endosomes from the cell periphery) and 'late endosomes' (LEs; endosomes farther into the cell interior). At both time points, significantly more ligand was found in EE fractions isolated from chow- and pair-fed controls (3:1, EE to LE, respectively). However, endosomes isolated from ethanol-fed animals showed a shift over time toward a more equal ligand distribution between endosome fractions (P < or = 0.05). Analysis of the ASGP-R content revealed a distribution pattern between the endosome fractions similar to that observed for ligand distribution. Impairment of receptor-ligand dissociation was assessed in endosome fractions by determining bound/free ligand ratios. Analysis showed that most of the ligand present in both endosome fractions was free (56-99%), although more was bound to receptor in EE vs LE of both control and ethanol animals (P < or = 0.05). At 5 min, more ligand remained bound in endosomes from ethanol-fed animals compared with control endosomes (P < or = 0.05), and the same pattern was observed at the latter time point. These results suggest that delayed dissociation may cause the receptor ligand complexes to travel farther into the cell interior, which may impair proper trafficking of the ligand to lysosomes and alter the receptor recycling.

Roles of the cytoskeleton and motor proteins in endocytic sorting

After internalization, endocytic material is actively transported through the cytoplasm, predominantly by microtubule motor proteins. Microtubule-based endocytic transport facilitates sorting of endocytic contents, vesicle fusion and fission, delivery to lysosomes, cytosolic dispersal, as well as nuclear uptake and cytosolic egress of pathogens. Endosomes, like most organelles, move bidirectionally through the cytosol and regulate their cellular location by controlling the activity of motor proteins, and potentially by controlling microtubule and actin polymerization. Control of motor protein activity is manifest by increased microtubule "run lengths", and the binding of motor proteins to organelles can be regulated by motor protein receptors. A mechanistic understanding of how organelles control motor protein activity to allow for endocytic sorting presents an exciting avenue for future research.

Exploitation of microtubule cytoskeleton and dynein during parvoviral traffic toward the nucleus

Canine parvovirus (CPV), a model virus for the study of parvoviral entry, enters host cells by receptor-mediated endocytosis, escapes from endosomal vesicles to the cytosol, and then replicates in the nucleus. We examined the role of the microtubule (MT)-mediated cytoplasmic trafficking of viral particles toward the nucleus. Immunofluorescence and immunoelectron microscopy showed that capsids were transported through the cytoplasm into the nucleus after cytoplasmic microinjection but that in the presence of MT-depolymerizing agents, viral capsids were unable to reach the nucleus. The nuclear accumulation of capsids was also reduced by microinjection of an anti-dynein antibody. Moreover, electron microscopy and light microscopy experiments demonstrated that viral capsids associate with tubulin and dynein in vitro. Coprecipitation studies indicated that viral capsids interact with dynein. When the cytoplasmic transport process was studied in living cells by microinjecting fluorescently labeled capsids into the cytoplasm of cells containing fluorescent tubulin, capsids were found in close contact with MTs. These results suggest that intact MTs and the motor protein dynein are required for the cytoplasmic transport of CPV capsids and contribute to the accumulation of the capsid in the nucleus.

Receptor activation regulates cortical, but not vesicular localization of NDP kinase

We used immunofluorescence techniques to determine the localization of nucleoside diphosphate (NDP) kinase in NIH-3T3 fibroblasts. We found that cytoplasmic NDP kinase can be separated into two populations according to subcellular localization and response to extracellular stimuli. Specifically, within minutes of stimulation of resting fibroblasts with serum, growth factors or bombesin, a portion of NDP kinase becomes associated with membrane ruffles and lamellipodia. Another pool of NDP kinase accumulates independently of stimulation around intracellular vesicles. Transfection of cells with activated Rac mimics, whereas expression of dominant negative Rac inhibits, the effects of extracellular stimulation on the translocation of NDP kinase to the cell cortex. Neither Rac mutant affects the vesicle-associated pool. Association of NDP kinase with vesicles depends on microtubule integrity and is disrupted by nocodazole. In cell-free assays NDP kinase binds tightly to membrane vesicles associated with taxol-stabilized microtubules. Binding of NDP kinase to this fraction is reduced by ATP and abolished by GTP, as well as guanine nucleotides that are NDP kinase substrates. Thus, the localization of the two NDP kinase pools identified here is regulated independently by distinct cellular components: the appearance of cortical NDP kinase is a consequence of Rac activation, whereas vesicular NDP kinase is responsive to microtubule dynamics and nucleotides, in particular GTP. These results suggest that in fibroblasts NDP kinase participates in Rac-related cortical events and in GTP-dependent processes linked to intracellular vesicle trafficking.

A role for epsin N-terminal homology/AP180 N-terminal homology (ENTH/ANTH) domains in tubulin binding

The epsin N-terminal homology (ENTH) domain is a protein module of approximately 150 amino acids found at the N terminus of a variety of proteins identified in yeast, plants, nematode, frog, and mammals. ENTH domains comprise multiple alpha-helices folded upon each other to form a compact globular structure that has been implicated in interactions with lipids and proteins. In characterizing this evolutionarily conserved domain, we isolated and identified tubulin as an ENTH domain-binding partner. The interaction, which is direct and has a dissociation constant of approximately 1 microm, was observed with ENTH domains of proteins present in various species. Tubulin is co-immunoprecipitated from rat brain extracts with the ENTH domain-containing proteins, epsins 1 and 2, and punctate epsin staining is observed along the microtubule cytoskeleton of dissociated cortical neurons. Consistent with a role in microtubule processes, the over-expression of epsin ENTH domain in PC12 cells stimulates neurite outgrowth. These data demonstrate an evolutionarily conserved property of ENTH domains to interact with tubulin and microtubules.

Regulation of early endocytic vesicle motility and fission in a reconstituted system

We previously established conditions to reconstitute kinesin-dependent early endocytic vesicle motility and fission on microtubules in vitro. The present study examined the question whether motility and fission are regulated in this system. Screening for proteins by immunofluorescence microscopy revealed that the small G protein, Rab4, was associated with 80% of hepatocyte-derived early endocytic vesicles that contain the ligand asialoorosomucoid (ASOR). By contrast, other markers for early endocytic vesicles including clathrin, Rab5 and EEA1 were present in the preparation but did not colocalize with the ASOR vesicles. Guanine nucleotides exchanged into the Rab4 present on the vesicles as shown by solubilization of Rab4 by Rab-GDI; solubilization was inhibited by incubation with GTP-gamma-S and promoted by GDP. Pre-incubation of vesicles with GDP increased the number of vesicles moving on microtubules and markedly increased vesicle fission. This increase in motility from GDP was shown to be towards the minus end of microtubules, possibly through activation of the minus-end-directed kinesin, KIFC2. Pre-incubation of vesicles with GTP-gamma-S, by contrast, repressed motility. Addition of exogenous GST-Rab4- GTP-gamma-S led to a further repression of motility and fission. Repression was not seen with addition of GST-Rab4-GDP. Treatment of vesicles with Rab4 antibody also repressed motility, and repression was not seen when vesicles were pre-incubated with GDP. Based on these results we hypothesize that endogenous Rab4-GTP suppresses motility of ASOR-containing vesicles in hepatocytes and that conversion of Rab4-GTP to Rab4-GDP serves as a molecular switch that activates minus-end kinesin-based motility, facilitating early endosome fission and consequent receptor-ligand segregation.

Transcytosis: crossing cellular barriers

Transcytosis, the vesicular transport of macromolecules from one side of a cell to the other, is a strategy used by multicellular organisms to selectively move material between two environments without altering the unique compositions of those environments. In this review, we summarize our knowledge of the different cell types using transcytosis in vivo, the variety of cargo moved, and the diverse pathways for delivering that cargo. We evaluate in vitro models that are currently being used to study transcytosis. Caveolae-mediated transcytosis by endothelial cells that line the microvasculature and carry circulating plasma proteins to the interstitium is explained in more detail, as is clathrin-mediated transcytosis of IgA by epithelial cells of the digestive tract. The molecular basis of vesicle traffic is discussed, with emphasis on the gaps and uncertainties in our understanding of the molecules and mechanisms that regulate transcytosis. In our view there is still much to be learned about this fundamental process.

Late endosome motility depends on lipids via the small GTPase Rab7

We report that lipids contribute to regulate the bidirectional motility of late endocytic compartments. Late endocytic vesicles loaded with cholesterol lose their dynamic properties, and become essentially immobile, including in cells from Niemann-Pick C patients. These vesicles then retain cytoplasmic dynein activity, but seem to be unable to acquire kinesin activity, eventually leading to paralysis. Our data suggest that this defect depends on the small GTPase Rab7, since the motility of vesicles loaded with cholesterol can be restored by the Rab7 inhibitory mutant N125I. Conversely, wild-type Rab7 overexpression mimics the effects of cholesterol on motility in control cells. Consistently, cholesterol accumulation increases the amounts of membrane-associated Rab7, and inhibits Rab7 membrane extraction by the guanine nucleotide dissociation inhibitor. Our observations thus indicate that cholesterol contributes to regulate the Rab7 cycle, and that Rab7 in turn controls the net movement of late endocytic elements. We conclude that motor functions can be regulated by the membrane lipid composition via the Rab7 cycle.

Cellular and molecular biology of the liver

Recently, several lines of investigation focused on basic mechanisms governing cellular and molecular aspects of liver biology have intersected at the study of the hepatic stem cell. Despite years of study, the very question of the existence of the hepatic stem cell has yet to be unequivocally established. A second field of investigation into the cellular and molecular aspects of liver biology is aimed at liver-directed gene therapy in which several new vehicles have been devised to mediate gene transfer. Gene therapy is no longer thought of in the limited framework of a means to correct inherited disorders; it is now expanding into new therapeutic applications. A third major area of investigation includes studies of mechanisms that regulate membrane protein traffic necessary to maintain the integrity of differentiated liver cell function. In this review, some of the most recent advances and applications in these three areas are highlighted, and, where appropriate, points of interaction and potential therapeutic importance are emphasized.

Plasma membrane localization of palmitoylated tubulin

PC12 pheochromocytoma cells incorporate [(3)H]palmitic acid into tubulin in a time- and cell-density-dependent manner. The plasma membrane-enriched fraction contains most of the radioactivity of the membrane pellet. While palmitoylated tubulin is found in both the cytoplasm and particulate fraction, the bulk of [(3)H]palmitic acid bound to tubulin is present in the crude membrane pellet and the tubulin extracted from the plasma membrane is more heavily palmitoylated than that extracted from endoplasmic reticulum. Detergent-extracted tubulin from plasma membrane is, to a large extent, polymerization competent; a substantial fraction, increasing as a function of labeling time, is not hydroxylamine-labile. The requirement for detergent extraction, the accompanying changes in tubulin properties and the present findings of preferential incorporation of labeled tubulin into plasma membranes, make it clear that direct incorporation of tubulin into the plasma membrane can occur.

Molecular motors and membrane traffic in Dictyostelium

Phagocytosis and membrane traffic in general are largely dependent on the cytoskeleton and their associated molecular motors. The myosin family of motors, especially the unconventional myosins, interact with the actin cortex to facilitate the internalization of external materials during the early steps of phagocytosis. Members of the kinesin and dynein motor families, which mediate transport along microtubules (MTs), facilitate the intracellular processing of the internalized materials and the movement of membrane. Recent studies indicate that some unconventional myosins are also involved in membrane transport, and that the MT- and actin-dependent transport systems might interact with each other. Studies in Dictyostelium have led to the discovery of many motors involved in critical steps of phagocytosis and membrane transport. With the ease of genetic and biochemical approaches, the established functional analysis to test phagocytosis and vesicle transport, and the effort of the Dictyostelium cDNA and Genome Projects, Dictyostelium will continue to be a superb model system to study phagocytosis in particular and cytoskeleton and motors in general.

The small GTPase Rab4A interacts with the central region of cytoplasmic dynein light intermediate chain-1

Rab4 belongs to the Rab family of small GTPases involved in the regulation of intracellular transport, and has been localized to early endosomes. We have employed the yeast two-hybrid system to identify proteins that specifically interact with Rab4AQ67L, a GTPase-deficient mutant form of Rab4A. Screening a mouse embryo cDNA library identified a clone (M449) that interacted with Rab4A in a nucleotide-dependent fashion. Data base searches identified this clone as the mouse cytoplasmic dynein light intermediate chain-1 (LIC-1). Based on this finding, the full-length equivalent human cytoplasmic dynein LIC-1 was isolated by PCR. When Rab4A was overexpressed together with either M449 or dynein LIC-1 in HeLa cells, the proteins were found to colocalize in the perinuclear region. We characterize the localization of both overexpressed human dynein LIC-1 and the endogenous protein with respect to microtubules and show that it concentrates to the microtubule-organizing center and mitotic spindle. Additionally, GFPRab4A endosomes localize to microtubules and are redistributed by nocodazole treatment. This is the first described interaction between cytoplasmic dynein, a retrograde motor protein, and a Rab protein.

Modulation of cytoplasmic dynein ATPase activity by the accessory subunits

The microtubule-based motor molecule cytoplasmic dynein has been proposed to be regulated by a variety of mechanisms, including phosphorylation and specific interaction with the organelle-associated complex, dynactin. In this study, we examined whether the intermediate chain subunits of cytoplasmic dynein are involved in modulation of ATP hydrolysis, and thereby affect motility. Treatment of testis cytoplasmic dynein under hypertonic salt conditions resulted in separation of the intermediate chains from the remainder of the dynein molecule, and led to a 4-fold enhancement of ATP hydrolysis. This result suggests that the accessory subunits act as negative regulators of dynein heavy chain activity. Comparison of ATPase activities of dyneins with differing intermediate chain isoforms showed significant differences in basal ATP hydrolysis rates, with testis dynein 7-fold more active than dynein from brain. Removal of the intermediate chain subunits led to an equalization of ATPase activity between brain and testis dyneins, suggesting that the accessory subunits are responsible for the observed differences in tissue activity. Finally, our preparative procedures have allowed for the identification and purification of a 1:1 complex of dynein with dynactin. As this interaction is presumed to be mediated by the dynein intermediate chain subunits, we now have defined experimental conditions for further exploration of dynein enzymatic and motility regulation.

Microtubule and motor-dependent endocytic vesicle sorting in vitro

Endocytic vesicles undergo fission to sort ligand from receptor. Using quantitative immunofluorescence and video imaging, we provide the first in vitro reconstitution of receptor-ligand sorting in early endocytic vesicles derived from rat liver. We show that to undergo fission, presegregation vesicles must bind to microtubules (MTs) and move upon addition of ATP. Over 13% of motile vesicles elongate and are capable of fission. After fission, one vesicle continues to move, whereas the other remains stationary, resulting in their separation. On average, almost 90% receptor is found in one daughter vesicle, whereas ligand is enriched by approximately 300% with respect to receptor in the other daughter vesicle. Although studies performed on polarity marked MTs showed approximately equal plus and minus end-directed motility, immunofluorescence microscopy revealed that kinesins, but not dynein, were associated with these vesicles. Motility and fission were prevented by addition of 1 mM 5'-adenylylimido-diphosphate (AMP-PNP, an inhibitor of kinesins) or incubation with kinesin antibodies, but were unaffected by addition of 5 microM vanadate (a dynein inhibitor) or dynein antibodies. These studies indicate an essential role of kinesin-based MT motility in endocytic vesicle sorting, providing a system in which factors required for endocytic vesicle processing can be identified and characterized.

Optimal development of Chlamydophila psittaci in L929 fibroblast and BGM epithelial cells requires the participation of microfilaments and microtubule-motor proteins

The cytoskeleton is involved in several cellular activities, including internalization and transport of foreign particles. Although particular functions to each cytoskeleton component have been described, interactions between those components seem to occur. The involvement of the different host cell cytoskeletal components in uptake and development of Chlamydophila psittaci is incompletely understood. In this study, the participation of the microfilament network along with the kinesin and dynein microtubule motor proteins in the internalization and further development of Chlamydophila psittaci were investigated in L929 fibroblast and BGM epithelial cells. Cytochalasin D disruption of actin filaments, and blockage of the motor proteins through the introduction of monoclonal antibodies into the host cells were carried out, either single or combined, at different moments around bacterial inoculation, and Chlamydophila infectivity determined 24 h post- inoculation by direct immunofluorescence. Our results show that, although Chlamydophila Ipsittaci can make use of both microfilament-dependent and independent entry pathways in both cell types, Chlamydophila internalization and development in the fibroblast cells mainly concerned processes mediated by microfilaments while in the epithelial cells mechanisms that require microtubule motor proteins were the ones predominantly involved. Evidence that mutual participation of the actin and tubulin networks in both host cells are required for optimal growth of Chlamydophila psittaci is also presented.

Reconstitution of ATP-dependent movement of endocytic vesicles along microtubules in vitro: an oscillatory bidirectional process

We have previously used the asialoglycoprotein receptor system to elucidate the pathway of hepatocytic processing of ligands such as asialoorosomucoid (ASOR). These studies suggested that endocytic vesicles bind to and travel along microtubules under the control of molecular motors such as cytoplasmic dynein. We now report reconstitution of this process in vitro with the use of a microscope assay to observe the interaction of early endocytic vesicles containing fluorescent ASOR with fluorescent microtubules. We find that ASOR-containing endosomes bind to microtubules and translocate along them in the presence of ATP. This represents the first time that mammalian endosomes containing a well-characterized ligand have been directly observed to translocate on microtubules in vitro. The endosome movement does not require cytosol or exogenous motor protein, is oscillatory, and is directed toward the plus and minus ends at equal frequencies. We also observe endosomes being stretched in opposite directions along microtubules, suggesting that microtubules could provide a mechanical basis for endocytic sorting events. The movement of endosomes in vitro is consistent with the hypothesis that microtubules actively participate in the sorting and distribution of endocytic contents.

Significance of host cell kinesin in the development of Chlamydia psittaci

The influence of the microtubule-associated motor protein kinesin on Chlamydia psittaci inclusion development in epithelial and fibroblast cell lines was addressed. Kinesin was blocked early after chlamydial internalization (4 h postinfection [p.i.]) and before the initiation of active chlamydial multiplication (8 h p.i.). Chlamydia development was monitored by fluorescence and transmission electron microscopy at different times during the cycle. In both host cell lines, kinesin blockage restricted mitochondria from the chlamydial vacuole. The effects of kinesin blockage on the C. psittaci replication cycle included the presence of multiple inclusions up to late in the cycle, the presence of enlarged pleomorphic reticulate bodies, and a delayed reappearance of elementary bodies. The last effect seems to be greater when kinesin is blocked early after infection. Our results show that kinesin activity is required for optimal development of these microorganisms, most probably acting through the apposition of mitochondria to the C. psittaci inclusions.

Canalicular export pumps traffic with polymeric immunoglobulin A receptor on the same microtubule-associated vesicle in rat liver

Basolateral to apical vesicular transcytosis in the hepatocyte is an essential pathway for the delivery of compounds from the sinusoidal blood to the bile and to traffic newly synthesized resident apical membrane proteins to their site of function at the canalicular membrane front. To characterize this pathway better, microtubules in a hepatocyte homogenate were polymerized by addition of taxol, and associated membrane-bound vesicles were isolated. This fraction was enriched in polymeric immunoglobulin A receptor and contained apical membrane proteins. Immunoelectron microscopy demonstrated that polymeric immunoglobulin A receptor was localized predominantly on vesicles ranging from 100 to 160 nm and that the multidrug resistance protein 2 and the bile salt export pump co-localized on these vesicles. The minus-ended microtubule motor, dynein, was highly enriched in the fraction, and its intermediate chain could be released effectively by incubation with 1 mM ATP or GTP. However, the association of the transcytotic vesicles with the microtubules was not sensitive to hydrolyzable or non-hydrolyzable nucleotides. This study characterizes a fraction of microtubule-associated vesicles from rat hepatocytes and demonstrates that several resident apical membrane transport proteins and the polymeric immunoglobulin A receptor traffic on the same vesicle.

Targeting endocytosis and motor proteins to enhance DNA persistence

Gene therapy provides a major new therapeutic strategy for the treatment of disease. Despite its potential for the inhibition of disease progression at the molecular level, gene therapy has faced numerous challenges. Foremost amongst these is the introduction of a sufficient amount of DNA-based drug to the target cell under conditions that encourage persistence of the introduced DNA. Because many DNA-based drugs enter cells via receptor-mediated endocytosis, clearly modulation of this process is a key issue in maximizing DNA persistence. In this review, a particular protein that participates in receptor-mediated endocytosis, the microtubule-based motor protein, cytoplasmic dynein, is introduced. In addition, recent advances in the study of cytoplasmic dynein in receptor-mediated endocytosis are discussed, and there is consideration of the potential of cytoplasmic dynein as a critical target for the regulation of DNA-uptake and persistence.

Opposing motor activities of dynein and kinesin determine retention and transport of MHC class II-containing compartments

MHC class II molecules exert their function at the cell surface by presenting to T cells antigenic fragments that are generated in the endosomal pathway. The class II molecules are targetted to early lysosomal structures, termed MIIC, where they interact with antigenic fragments and are subsequently transported to the cell surface. We previously visualised vesicular transport of MHC class II-containing early lysosomes from the microtubule organising centre (MTOC) region towards the cell surface in living cells. Here we show that the MIIC move bidirectionally in a ‘stop-and-go’ fashion. Overexpression of a motor head-deleted kinesin inhibited MIIC motility, showing that kinesin is the motor that drives its plus end transport towards the cell periphery. Cytoplasmic dynein mediates the return of vesicles to the MTOC area and effectively retains the vesicles at this location, as assessed by inactivation of dynein by overexpression of dynamitin. Our data suggest a retention mechanism that determines the perinuclear accumulation of MIIC, which is the result of dynein activity being superior over kinesin activity. The bidirectional nature of MIIC movement is the result of both kinesin and dynein acting reciprocally on the MIIC during its transport. The motors may be the ultimate targets of regulatory kinases since the protein kinase inhibitor staurosporine induces a massive release of lysosomal vesicles from the MTOC region that is morphologically similar to that observed after inactivation of the dynein motor.

Gene knockouts reveal separate functions for two cytoplasmic dyneins in Tetrahymena thermophila

In many organisms, there are multiple isoforms of cytoplasmic dynein heavy chains, and division of labor among the isoforms would provide a mechanism to regulate dynein function. The targeted disruption of somatic genes in Tetrahymena thermophila presents the opportunity to determine the contributions of individual dynein isoforms in a single cell that expresses multiple dynein heavy chain genes. Substantial portions of two Tetrahymena cytoplasmic dynein heavy chain genes were cloned, and their motor domains were sequenced. Tetrahymena DYH1 encodes the ubiquitous cytoplasmic dynein Dyh1, and DYH2 encodes a second cytoplasmic dynein isoform, Dyh2. The disruption of DYH1, but not DYH2, resulted in cells with two detectable defects: 1) phagocytic activity was inhibited, and 2) the cells failed to distribute their chromosomes correctly during micronuclear mitosis. In contrast, the disruption of DYH2 resulted in a loss of regulation of cell size and cell shape and in the apparent inability of the cells to repair their cortical cytoskeletons. We conclude that the two dyneins perform separate tasks in Tetrahymena.

Molecular aspects of the endocytic pathway

Observation of the flow of material along the endocytic pathway has lead to the description of the basic architecture of the pathway and provided insight into the relationship between compartments. Significant advances have been made in the study of endocytic transport steps at the molecular level, of which studies of cargo selection, vesicle budding and membrane fusion events comprise the major part. Progress in this area has been driven by two approaches, yeast genetics and in vitro or cell-free assays, which reconstitute particular transport steps and allow biochemical manipulation. The complex protein machineries that control vesicle budding and fusion are significantly conserved between the secretory and endocytic pathways such that proteins that regulate particular steps are often part of a larger family of proteins which exercise a conserved function at other locations within the cell. Well characterized examples include vesicle coat proteins, rabs (small GTPases) and soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein (SNAP) receptors (SNAREs). Intracompartmental pH, lipid composition and cytoskeletal organization have also been identified as important determinants of the orderly flow of material within the endocytic pathway.

Taxol inhibits endosomal-lysosomal membrane trafficking at two distinct steps in CV-1 cells

Although taxol inhibits membrane trafficking, the nature of this inhibition has not been well defined. In this study, we define the effects of taxol on endocytosis in CV-1 cells using density gradient centrifugation of membranes over sorbitol density gradients. After taxol treatment, resident endosomal enzymes and the epidermal growth factor (EGF) receptor (EGFR) showed significant (P </= 0.05) enrichment in membranes with properties of early endosomes (fractions 4 and 5); the EGFR and Na+-K+-ATPase were also significantly (P </= 0.05) depleted in lysosomal fractions (fractions 10 and 11). The suggestion that taxol specifically reduces movement of endosomal constituents to lysosomes was supported by fluorescence microscopy studies revealing restriction of EGF to the peripheries of taxol-treated cells, in contrast to the perinuclear lysosomal-like distribution of EGF seen in controls. Kinetic studies with 125I-labeled EGF were also consistent with a taxol-induced block in traffic from endosomes and lysosomes after 15 min of uptake but also suggested an additional taxol-sensitive step in trafficking that involved redistribution of 125I-EGF within high-density compartments after 150 min. Related changes in cytoplasmic dynein distribution were observed within high-density compartments from taxol-treated cells, suggesting that this motor might participate in this later taxol-sensitive trafficking event. Electron microscopic examination of high-density membranes (fraction 12) showed that taxol increased the numbers of small (<500 nm) dense vesicles, with a relative depletion of the larger (>500 nm) vesicles found in controls. These data demonstrate that disruption of endocytic events by taxol includes the early accumulation of protein and endocytic markers in endosomes and the later accumulation in a dense compartment that we propose is a subdomain of the lysosomes.

KIF2beta, a new kinesin superfamily protein in non-neuronal cells, is associated with lysosomes and may be implicated in their centrifugal translocation

Lysosomes concentrate juxtanuclearly in the region around the microtubule-organizing center by interaction with microtubules. Different experimental and physiological conditions can induce these organelles to move to the cell periphery by a mechanism implying a plus-end-directed microtubule-motor protein (a kinesin-like motor). The responsible kinesin-superfamily protein, however, is unknown. We have identified a new mouse isoform of the kinesin superfamily, KIF2beta, an alternatively spliced isoform of the known, neuronal kinesin, KIF2. Developmental expression pattern and cell-type analysis in vivo and in vitro reveal that KIF2beta is abundant at early developmental stages of the hippocampus but is then downregulated in differentiated neuronal cells, and it is mainly or uniquely expressed in non-neuronal cells while KIF2 remains exclusively neuronal. Electron microscopy of mouse fibroblasts and immunofluorescence of KIF2beta-transiently-transfected fibroblasts show KIF2 and KIF2beta primarily associated with lysosomes, and this association can be disrupted by detergent treatment. In KIF2beta-overexpressing cells, lysosomes (labeled with anti-lysosome-associated membrane protein-1) become abnormally large and peripherally located at some distance from their usual perinuclear positions. Overexpression of KIF2 or KIF2beta does not change the size or distribution of early, late and recycling endosomes nor does overexpression of different kinesin superfamily proteins result in changes in lysosome size or positioning. These results implicate KIF2beta as a motor responsible for the peripheral translocation of lysosomes.

Microtubule-dependent vesicle transport: modulation of channel and transporter activity in liver and kidney

Microtubule-based vesicle transport driven by kinesin and cytoplasmic dynein motor proteins facilitates several membrane-trafficking steps including elements of endocytosis and exocytosis in many different cell types. Most early studies on the role of microtubule-dependent vesicle transport in membrane trafficking focused either on neurons or on simple cell lines. More recently, other work has considered the role of microtubule-based vesicle transport in other physiological systems, including kidney and liver. Investigation of the role of microtubule-based vesicle transport in membrane trafficking in cells of the kidney and liver suggests a major role for microtubule-based vesicle transport in the rapid and directed movement of ion channels and transporters to and from the apical plasma membranes, events essential for kidney and liver function and homeostasis. This review discusses the evidence supporting a role for microtubule-based vesicle transport and the motor proteins, kinesin and cytoplasmic dynein, in different aspects of membrane trafficking in cells of the kidney and liver, with emphasis on those functions such as maintenance of ion channel and transporter composition in apical membranes that are specialized functions of these organs. Evidence that defects in microtubule-based transport contribute to diseases of the kidney and liver is also discussed.

A screen for dynein synthetic lethals in Aspergillus nidulans identifies spindle assembly checkpoint genes and other genes involved in mitosis

Cytoplasmic dynein is a ubiquitously expressed microtubule motor involved in vesicle transport, mitosis, nuclear migration, and spindle orientation. In the filamentous fungus Aspergillus nidulans, inactivation of cytoplasmic dynein, although not lethal, severely impairs nuclear migration. The role of dynein in mitosis and vesicle transport in this organism is unclear. To investigate the complete range of dynein function in A. nidulans, we searched for synthetic lethal mutations that significantly reduced growth in the absence of dynein but had little effect on their own. We isolated 19 sld (synthetic lethality without dynein) mutations in nine different genes. Mutations in two genes exacerbate the nuclear migration defect seen in the absence of dynein. Mutations in six other genes, including sldA and sldB, show a strong synthetic lethal interaction with a mutation in the mitotic kinesin bimC and, thus, are likely to play a role in mitosis. Mutations in sldA and sldB also confer hypersensitivity to the microtubule-destabilizing drug benomyl. sldA and sldB were cloned by complementation of their mutant phenotypes using an A. nidulans autonomously replicating vector. Sequencing revealed homology to the spindle assembly checkpoint genes BUB1 and BUB3 from Saccharomyces cerevisiae. Genetic interaction between dynein and spindle assembly checkpoint genes, as well as other mitotic genes, indicates that A. nidulans dynein plays a role in mitosis. We suggest a model for dynein motor action in A. nidulans that can explain dynein involvement in both mitosis and nuclear distribution.

Ethanol-induced alterations of the microtubule cytoskeleton in hepatocytes

Ethanol has been predicted to alter vesicle-based protein traffic in hepatocytes, in part, via a disruption of the microtubule (MT) cytoskeleton. However, information on the effects of chronic ethanol exposure on MT function in vivo is sparse. Therefore the goal of this study was to test for ethanol-induced changes in rat liver tubulin expression, assembly, and cellular organization, using molecular, biochemical and morphological methods. The results of this study showed that tubulin mRNA and protein levels were not altered by ethanol. Tubulin, isolated from control and ethanol-fed rats, showed similar polymerization characteristics as assessed by calculation of the critical concentration for assembly and morphological structure. In contrast, the total amount of assembly-competent tubulin was reduced in livers from ethanol-fed rats compared with control rats when assessed by quantitative immunoblot analysis using a tubulin antibody. In addition, we observed that MT regrowth and organization in cultured hepatocytes treated with cold and nocodazole was markedly impaired by chronic ethanol exposure. In summary, these results indicate that tubulin levels in liver are not reduced by ethanol exposure. While there is a substantial amount of tubulin protein capable of assembling into functional MTs in ethanol-damaged livers, a marked portion of this tubulin is polymerization incompetent. This may explain why these hepatocytes exhibit a reduced number of MTs with an altered organization.

Molecular motors and their role in membrane traffic

Recent investigations support a role for the vesicle motor proteins (kinesin, cytoplasmic dynein, and myosin) in numerous membrane trafficking events including endocytosis and transcytosis. Kinesin and cytoplasmic dynein are responsible for movement of membrane vesicles along cellular microtubules to and from cellular membrane compartments, while certain members of the myosin family also appear to drive membrane vesicles along actin filaments to and from membrane compartments. In this review, our current understanding of the role of these vesicle motors in membrane trafficking is highlighted. Future areas of interest which may be able to make use of these vesicle motors as potential targets for drug delivery are also discussed.

Evidence for four cytoplasmic dynein heavy chain isoforms in rat testis

Recent studies have revealed the expression of multiple putative cytoplasmic dynein heavy chain (DHC) genes in several organisms, with each gene encoding a separate protein isoform. This finding is consistent with the hypothesis that different isoforms do different things, as is the case for the axonemal dyneins. Furthermore, the large number of tasks ascribed to cytoplasmic dynein suggests that there may be additional isoforms not yet identified. Two of the mammalian cytoplasmic dynein heavy chains are DHC1a and DHC1b. DHC1a is conventional cytoplasmic dynein and is found in all organisms examined. DHC1b is expressed in organisms that have multiple dyneins, and has been implicated in the intracellular trafficking of molecules in unciliated and ciliated cells. In the present study, we examined the DHC1b protein from rat testis. Testis cytoplasmic dynein contains a large amount of dynein heavy chain reactive with an antibody raised against a peptide sequence of rat DHC1b. The testis anti-DHC1b immunoreactive protein is slightly smaller than testis DHC1a, as assessed by SDS-PAGE. In Northern blots, the DHC1b mRNA is smaller than the DHC1a mRNA. In sucrose gradients made in low ionic strength, DHC1a sedimented at approximately 20S, and the anti-1b immunoreactive heavy chains sedimented in a broad band centered at approximately 14S. The V1-photolysis reaction of individual sucrose gradient fractions revealed three distinct patterns of photolysis, suggesting that there are at least three separate 1b-like heavy chain isoforms in testis. Using a high-stringency Western blotting protocol, the anti-1b antibody and the anti-DHC2 antibody recognized the same heavy chain and specifically bound to one of the three 1b-like heavy chains. We conclude that rat testis contains three 1b-like dynein heavy chains, and one of these is the product of the DHC1b/DHC2 gene previously identified.

Identification of cytoskeleton-associated proteins in isolated rat liver endosomes

The polypeptides of three highly purified endosomal fractions isolated from the livers of oestradiol-treated rats were analysed by Western blotting, and the amount and distribution of intrinsic and cytoskeletal-associated proteins were quantified and studied. The 'late' endosomes [multivesicular bodies (MVBs)] had the lowest content of cytoskeletal-associated proteins, the most significant being the presence of 25% of the total dynein found in endosomes. The 'early' endosome [compartment of uncoupling receptors and ligands (CURL)] fraction contained kinesin (40% of the total in endosomes), dynein (23%), actin (15%) and tubulin (10%). The receptor-recycling compartment (RRC), also demonstrated to be involved in transcytosis, contained the largest number and enrichment of cytoskeletal proteins: actin (84% of the total in endosomes), alpha-actinin (90%), dynein (52%), tubulin (91%) and kinesin (45%). We also analysed and compared the presence of different endosomal markers such as Rab4, Rab5 and cellubrevin (vesicle soluble NSF attachment protein receptor) in CURL (41%, 15% and 60%) and in RRC (44%, 75% and 30% respectively). Finally, the expression of annexins I, II, IV and VI was studied: annexin I was equally distributed between MVBs and CURL; annexin II was highly enriched in RRC (95%), annexin IV was equally distributed between CURL and RRC, and annexin VI was enriched in CURL (57%). The results indicate that isolated rat liver endosomes contain all the required molecular machinery for the achievement of their role in intracellular trafficking.