Construction of the endoplasmic reticulum

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

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

Cytoplasmic factories, virus assembly, and DNA replication kinetics collectively constrain the formation of poxvirus recombinants

Poxviruses replicate in cytoplasmic structures called factories and each factory begins as a single infecting particle. Sixty-years ago Cairns predicted that this might have effects on vaccinia virus (VACV) recombination because the factories would have to collide and mix their contents to permit recombination. We've since shown that factories collide irregularly and that even then the viroplasm mixes poorly. We’ve also observed that while intragenic recombination occurs frequently early in infection, intergenic recombination is less efficient and happens late in infection. Something inhibits factory fusion and viroplasm mixing but what is unclear. To study this, we’ve used optical and electron microscopy to track factory movement in co-infected cells and correlate these observations with virus development and recombinant formation. While the technical complexity of the experiments limited the number of cells that are amenable to extensive statistical analysis, these studies do show that intergenic recombination coincides with virion assembly and when VACV replication has declined to ≤10% of earlier levels. Along the boundaries between colliding factories, one sees ER membrane remnants and other cell constituents like mitochondria. These collisions don't always cause factory fusion, but when factories do fuse, they still entrain cell constituents like mitochondria and ER-wrapped microtubules. However, these materials wouldn’t seem to pose much of a further barrier to DNA mixing and so it’s likely that the viroplasm also presents an omnipresent impediment to DNA mixing. Late packaging reactions might help to disrupt the viroplasm, but packaging would sequester the DNA just as the replication and recombination machinery goes into decline and further reduce recombinant yields. Many factors thus appear to conspire to limit recombination between co-infecting poxviruses.

ER stress-induced aggresome trafficking of HtrA1 protects against proteotoxicity

High temperature requirement A1 (HtrA1) belongs to an ancient protein family that is linked to various human disorders. The precise role of exon 1-encoded N-terminal domains and how these influence the biological functions of human HtrA1 remain elusive. In this study, we traced the evolutionary origins of these N-terminal domains to a single gene fusion event in the most recent common ancestor of vertebrates. We hypothesized that human HtrA1 is implicated in unfolded protein response. In highly secretory cells of the retinal pigmented epithelia, endoplasmic reticulum (ER) stress upregulated HtrA1. HtrA1 co-localized with vimentin intermediate filaments in highly arborized fashion. Upon ER stress, HtrA1 tracked along intermediate filaments, which collapsed and bundled in an aggresome at the microtubule organizing center. Gene silencing of HtrA1 altered the schedule and amplitude of adaptive signaling and concomitantly resulted in apoptosis. Restoration of wild-type HtrA1, but not its protease inactive mutant, was necessary and sufficient to protect from apoptosis. A variant of HtrA1 that harbored exon 1 substitutions displayed reduced efficacy in rescuing cells from proteotoxicity. Our results illuminate the integration of HtrA1 in the toolkit of mammalian cells against protein misfolding and the implications of defects in HtrA1 in proteostasis.

Naturally Occurring Hepatitis B Virus Mutations Leading to Endoplasmic Reticulum Stress and Their Contribution to the Progression of Hepatocellular Carcinoma

Hepatitis B virus (HBV) infection is a global health problem that causes a wide range of pathological outcomes, including cirrhosis and hepatocellular carcinoma (HCC). Endoplasmic reticulum (ER) stress induction by HBV infection has been implicated in liver carcinogenesis and disease progression with chronic inflammation via enhanced inflammation, oxidative stress-mediated DNA damage, and hepatocyte proliferation. In the natural course of HBV infection, the accumulation of naturally occurring mutations in the HBV genome can generate several mutant types of HBV-encoded proteins, including three different proteins in the S ORF (SHBs, MHBs, and LHBs) and HBcAg in the C ORF, which could contribute to enhanced ER stress in infected hepatocytes mainly via increased ER accumulation of mutant proteins. However, it seems that there may be distinct capacity and pathway in ER stress-induction and distinct resulting clinical outcomes between HBV variants. In addition, the role of HBxAg mutations in ER stress remains unknown. However, it has been reported that HBxAg itself could exert ER stress in infected cells, resulting in HCC generation in chronic HBV patients. To date, review papers regarding ER stress-mediated HBV mutation have been limited into a specific mutation type: preS2 deletion. So, in this review, we will discuss details about various mutation types in all four regions of the HBV genome (preS1, preS2, S, and C) related to ER stress and their distinct ER stress mechanisms and clinical outcomes in terms of mutation types.

Shaping the endoplasmic reticulum in vitro

Organelles in eukaryotic cells often have complex shapes that deviate significantly from simple spheres. A prime example is the endoplasmic reticulum (ER) that forms an extensive network of membrane tubules in many mammalian cell types and in reconstitution assays in vitro. Despite the successful hunt for molecular determinants of ER shape we are still far from having a comprehensive understanding of ER network morphogenesis. Here, we have studied the hitherto neglected influence of the host substrate when reconstituting ER networks in vitro as compared to ER networks in vivo. In culture cells we observed cytoplasm-spanning ER networks with tubules being connected almost exclusively by three-way junctions and segment lengths being narrowly distributed around a mean length of about 1μm. In contrast, networks reconstituted from purified ER microsomes on flat glass or gel substrates of varying stiffness showed significantly broader length distributions with an up to fourfold larger mean length. Self-assembly of ER microsomes on small oil droplets, however, yielded networks that resembled more closely the native ER network of mammalian cells. We conclude from these observations that the ER microsomes' inherent self-assembly capacity is sufficient to support network formation with a native geometry if the influence of the host substrate's surface chemistry becomes negligible. We hypothesize that under these conditions the networks' preference for three-way junctions follows from creating 'starfish-shaped' vesicles when ER microsomes with a protein-induced spontaneous curvature undergo fusion.

Membrane tethering by the atlastin GTPase depends on GTP hydrolysis but not on forming the cross-over configuration

The atlastin GTPase couples nucleotide hydrolysis to formation of a trans cross-over dimer to catalyze homotypic endoplasmic reticulum membrane fusion. Assays that separate tethering from fusion reveal that stable trans contact for tethering depends on GTP hydrolysis. In contrast, cross-over formation is required only for the fusion step.

The membrane-anchored atlastin GTPase couples nucleotide hydrolysis to the catalysis of homotypic membrane fusion to form a branched endoplasmic reticulum network. Trans dimerization between atlastins anchored in opposing membranes, accompanied by a cross-over conformational change, is thought to draw the membranes together for fusion. Previous studies on the conformational coupling of atlastin to its GTP hydrolysis cycle have been carried out largely on atlastins lacking a membrane anchor. Consequently, whether fusion involves a discrete tethering step and, if so, the potential role of GTP hydrolysis and cross-over in tethering remain unknown. In this study, we used membrane-anchored atlastins in assays that separate tethering from fusion to dissect the requirements for each. We found that tethering depended on GTP hydrolysis, but, unlike fusion, it did not depend on cross-over. Thus GTP hydrolysis initiates stable head-domain contact in trans to tether opposing membranes, whereas cross-over formation plays a more pivotal role in powering the lipid rearrangements for fusion.

Valosin-containing protein-interacting membrane protein (VIMP) links the endoplasmic reticulum with microtubules in concert with cytoskeleton-linking membrane protein (CLIMP)-63

The distribution and morphology of the endoplasmic reticulum (ER) in mammalian cells depend on both dynamic and static interactions of ER membrane proteins with microtubules (MTs). Cytoskeleton-linking membrane protein (CLIMP)-63 is exclusively localized in sheet-like ER membranes, typical structures of the rough ER, and plays a pivotal role in the static interaction with MTs. Our previous study showed that the 42-kDa ER-residing form of syntaxin 5 (Syn5L) regulates ER structure through the interactions with both CLIMP-63 and MTs. Here, we extend our previous study and show that the valosin-containing protein/p97-interacting membrane protein (VIMP)/SelS is also a member of the family of proteins that shape the ER by interacting with MTs. Depletion of VIMP causes the spreading of the ER to the cell periphery and affects an MT-dependent process on the ER. Although VIMP can interact with CLIMP-63 and Syn5L, it does not interact with MT-binding ER proteins (such as Reep1) that shape the tubular smooth ER, suggesting that different sets of MT-binding ER proteins are used to organize different ER subdomains.

Identification of discrete sites in Yip1A necessary for regulation of endoplasmic reticulum structure

The endoplasmic reticulum (ER) of specialized cells can undergo dramatic changes in structural organization, including formation of concentric whorls. We previously reported that depletion of Yip1A, an integral membrane protein conserved between yeast and mammals, caused ER whorl formation reminiscent of that seen in specialized cells. Yip1A and its yeast homologue Yip1p cycle between the ER and early Golgi, have been implicated in a number of distinct trafficking steps, and interact with a conserved set of binding partners including Yif1p/Yif1A and the Ypt1/Ypt31 Rab GTPases. Here, we carried out a mutational analysis of Yip1A to obtain insight into how it regulates ER whorl formation. Most of the Yip1A cytoplasmic domain was dispensable, whereas the transmembrane (TM) domain, especially residues within predicted TM helices 3 and 4, were sensitive to mutagenesis. Comprehensive analysis revealed two discrete functionally required determinants. One was E95 and flanking residues L92 and L96 within the cytoplasmic domain; the other was K146 and nearby residue V152 within the TM domain. Notably, the identified determinants correspond closely to two sites previously found to be essential for yeast viability (E76 and K130 in Yip1p corresponding to E95 and K146 in Yip1A, respectively). In contrast, a third site (E89) also essential for yeast viability (E70 in Yip1p) was dispensable for regulation of whorl formation. Earlier work showed that E76 (E95) was dispensable for binding Yif1p or Ypt1p/Ypt31p, whereas E70 (E89) was required. Collectively, these findings suggest that the ability of Yip1A to bind its established binding partners may be uncoupled from its ability to control ER whorl formation. In support, Yif1A knockdown did not cause ER whorl formation. Thus Yip1A may use the sites identified herein to interact with a novel binding partner to regulate ER membrane organization.

Contribution of the long form of syntaxin 5 to the organization of the endoplasmic reticulum

The SNARE protein syntaxin 5 exists as long (42 kDa) and short (35 kDa) isoforms. The short form is principally localized in the Golgi complex, whereas the long form resides not only in the Golgi but also in the endoplasmic reticulum (ER). Although the Golgi-localized short form has been extensively investigated, little is known about the long form. In the present study, we demonstrate that the long form of syntaxin 5 functions to shape the ER. We found that overexpression of the long form of syntaxin 5 induces rearrangement and co-alignment of the ER membrane with microtubules, the pattern of which is quite similar to that observed in cells overexpressing CLIMP-63, a linker between the ER membrane and microtubules. The ability of syntaxin 5 to induce ER-microtubule rearrangement is not related to its SNARE function, but correlates with its binding affinities for CLIMP-63, and CLIMP-63 is essential for the induction of this rearrangement. Microtubule co-sedimentation assays demonstrated that the long form of syntaxin 5 has a substantial microtubule-binding activity. These results suggest that the long form of syntaxin 5 contributes to the regulation of ER structure by interacting with both CLIMP-63 and microtubules. Indeed, depletion of syntaxin 5 caused the spreading of the ER to the cell periphery, similar to the phenotype observed in cells treated with the microtubule-depolymerizing reagent nocodazole. Our results disclose a previously undescribed function of the long form of syntaxin 5 that is not related to its function as a SNARE.

An intramolecular salt bridge drives the soluble domain of GTP-bound atlastin into the postfusion conformation

Endoplasmic reticulum (ER) network branching requires homotypic tethering and fusion of tubules mediated by the atlastin (ATL) guanosine triphosphatase (GTPase). Recent structural studies on the ATL soluble domain reveal two dimeric conformers proposed to correspond to a tethered prefusion state and a postfusion state. How the prefusion conformer transitions to the postfusion conformer is unknown. In this paper, we identify an intramolecular salt bridge mediated by two residues outside the GTPase domain near the point of rotation that converts the prefusion dimer to the postfusion state. Charge reversal of either residue blocked ER network branching, whereas a compensatory charge reversal to reestablish electrostatic attraction restored function. In vitro assays using the soluble domain revealed that the salt bridge was dispensable for GTP binding and hydrolysis but was required for forming the postfusion dimer. Unexpectedly, the postfusion conformation of the soluble domain was achieved when bound to the nonhydrolyzable GTP analogue guanosine 5'-[β,γ-imido]triphosphate, suggesting that nucleotide hydrolysis might not be required for the prefusion to postfusion conformational change.

Taxol induces paraptosis independent of both protein synthesis and MAPK pathway

Our recent studies have shown that high concentration of taxol induced a caspase-independent paraptosis-like cell death and cytoplasmic vacuolization derived predominantly from endoplasmic reticulum (ER) swelling in human lung carcinoma cell lines (ASTC-a-1). In this report, we further explored the relationship between taxol-induced cell death and vacuolization, and the roles of protein synthesis, mitogen-activated protein kinase kinases (MEK), c-jun N-terminal kinase (JNK) and P38 in taxol-induced paraptosis. Enhanced green fluorescent protein (EGFP) was used to probe the cell morphological change, while ER-targeted red fluorescent protein (er-RFP) was used to probe ER spatial distribution. Real-time monitoring of the ER swelling dynamics during the formation of vacuolization inside single living cells co-expressing EGFP and er-RFP further demonstrated that taxol-induced cytoplasmic vacuolization was from the ER restructuring due to fusion and swelling. PI staining showed that taxol-induced vacuolization was not necrosis. These results further demonstrated that the taxol-induced cell death was neither apoptosis nor necrosis, and fitted the criteria of paraptosis characterized by cytoplasmic vacuolization, caspase-independence, lack of apoptotic morphology and insensitivity to broad caspase inhibitor. Our data further indicated that taxol-induced paraptosis required neither protein synthesis nor the participation of MEK, JNK, and P38, which was different from the insulin-like growth factor I receptor (IGFIR)-induced paraptosis. These results suggest that high concentration of taxol activates an alternative paraptotic cell death pathway.

Mechanism of membrane nanotube formation by molecular motors

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

Hydrophobic profiles of the tail anchors in SLMAP dictate subcellular targeting

BACKGROUND

Tail anchored (TA) membrane proteins target subcellular structures via a C-terminal transmembrane domain and serve prominent roles in membrane fusion and vesicle transport. Sarcolemmal Membrane Associated Protein (SLMAP) possesses two alternatively spliced tail anchors (TA1 or TA2) but their specificity of subcellular targeting remains unknown.

RESULTS

TA1 or TA2 can direct SLMAP to reticular structures including the endoplasmic reticulum (ER), whilst TA2 directs SLMAP additionally to the mitochondria. Despite the general structural similarity of SLMAP to other vesicle trafficking proteins, we found no evidence for its localization with the vesicle transport machinery or a role in vesicle transport. The predicted transmembrane region of TA2 is flanked on either side by a positively charged amino acid and is itself less hydrophobic than the transmembrane helix present in TA1. Substitution of the positively charged amino acids, in the regions flanking the transmembrane helix of TA2, with leucine did not alter its subcellular targeting. The targeting of SLMAP to the mitochondria was dependent on the hydrophobic nature of TA2 since targeting of SLMAP-TA2 was prevented by the substitution of leucine (L) for moderately hydrophobic amino acid residues within the transmembrane region. The SLMAP-TA2-4L mutant had a hydrophobic profile that was comparable to that of SLMAP-TA1 and had identical targeting properties to SLMAP-TA1.

CONCLUSION

Thus the overall hydrophobicity of the two alternatively spliced TAs in SLMAP determines its subcellular targeting and TA2 predominantly directs SLMAP to the mitochondira where it may serve roles in the function of this organelle.

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

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

Nucleoside diphosphate kinase B (NDKB) scaffolds endoplasmic reticulum membranes in vitro

The mechanisms that structure the mammalian endoplasmic reticulum (ER) network are not fully understood. Here we show that salt extraction of semi-intact normal rat kidney (NRK) fibroblasts and subsequent incubation of the extracted cells with ATP resulted in dramatic ER network retraction. Under these conditions, addition of a single protein, Nucleoside Diphosphate Kinase B (NDKB), was sufficient to reverse the retraction and to promote ER network extension. The underlying mechanism of membrane extension involved direct lipid binding, as NDKB bound phosphatidylinositol (PtdIns)(4)P, PtdIns(4,5)P(2) and phosphatidic acid (PA); binding to these anionic lipids required clusters of basic residues on the surface of the NDKB hexamer; and amino acid changes in NDKB that blocked lipid binding also blocked ER network extension. Remarkably, purified NDKB transformed a uniform population of synthetic lipid vesicles into extensive membrane networks, and this also required its phospholipid-binding activity. Altogether these results identify a protein sufficient to scaffold extended membrane networks, and suggest a possible role for NDKB-like proteins, as well as phosphoinositides and/or acidic phospholipids, in modulating ER network morphogenesis.

Microtubule/organelle motility assays

This unit describes an in vitro assay that uses video-enhanced differential interference contrast (VE-DIC) microscopy to examine the motile interactions between isolated organelle fractions and microtubules (MTs). The method can be used to dissect the molecular requirements for organelle movement and membrane trafficking. A field of axoneme-nucleated MTs, growing and shortening as they would in a living cell (dynamic MTs), is generated in a simple microscope perfusion chamber. Various combinations of isolated endoplasmic reticulum (ER) and Golgi apparatus organelles, cytosol containing motor proteins and other soluble factors, nucleotides, and specific pharmacological reagents are then added to the dynamic MT, and the motile interactions between the organelles and MTs are observed by VE-DIC microscopy.

Sequestration of mutated alpha1-antitrypsin into inclusion bodies is a cell-protective mechanism to maintain endoplasmic reticulum function

A variant alpha1-antitrypsin with E342K mutation has a high tendency to form intracellular polymers, and it is associated with liver disease. In the hepatocytes of individuals carrying the mutation, alpha1-antitrypsin localizes both to the endoplasmic reticulum (ER) and to membrane-surrounded inclusion bodies (IBs). It is unclear whether the IBs contribute to cell toxicity or whether they are protective to the cell. We found that in hepatoma cells, mutated alpha1-antitrypsin exited the ER and accumulated in IBs that were negative for autophagosomal and lysosomal markers, and contained several ER components, but not calnexin. Mutated alpha1-antitrypsin induced IBs also in neuroendocrine cells, showing that formation of these organelles is not cell type specific. In the presence of IBs, ER function was largely maintained. Increased levels of calnexin, but not of protein disulfide isomerase, inhibited formation of IBs and lead to retention of mutated alpha1-antitrypsin in the ER. In hepatoma cells, shift of mutated alpha1-antitrypsin localization to the ER by calnexin overexpression lead to cell shrinkage, ER stress, and impairment of the secretory pathway at the ER level. We conclude that segregation of mutated alpha1-antitrypsin from the ER to the IBs is a protective cell response to maintain a functional secretory pathway.

Disruption of microtubule organization and centrosome function by expression of tobacco mosaic virus movement protein

The movement protein (MP) of Tobacco mosaic virus mediates the cell-to-cell transport of viral RNA through plasmodesmata, cytoplasmic cell wall channels for direct cell-to-cell communication between adjacent cells. Previous in vivo studies demonstrated that the RNA transport function of the protein correlates with its association with microtubules, although the exact role of microtubules in the movement process remains unknown. Since the binding of MP to microtubules is conserved in transfected mammalian cells, we took advantage of available mammalian cell biology reagents and tools to further address the interaction in flat-growing and transparent COS-7 cells. We demonstrate that neither actin, nor endoplasmic reticulum (ER), nor dynein motor complexes are involved in the apparent alignment of MP with microtubules. Together with results of in vitro coprecipitation experiments, these findings indicate that MP binds microtubules directly. Unlike microtubules associated with neuronal MAP2c, MP-associated microtubules are resistant to disruption by microtubule-disrupting agents or cold, suggesting that MP is a specialized microtubule binding protein that forms unusually stable complexes with microtubules. MP-associated microtubules accumulate ER membranes, which is consistent with a proposed role for MP in the recruitment of membranes in infected plant cells and may suggest that microtubules are involved in this process. The ability of MP to interfere with centrosomal gamma-tubulin is independent of microtubule association with MP, does not involve the removal of other tested centrosomal markers, and correlates with inhibition of centrosomal microtubule nucleation activity. These observations suggest that the function of MP in viral movement may involve interaction with the microtubule-nucleating machinery.

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

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

Dynamics and inheritance of the endoplasmic reticulum

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

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

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

Macromolecular Transport and Signaling Through Plasmodesmata

Plasmodesmata (Pd) are channels in the plant cell wall that in conjunction with associated phloem form an intercellular communication network that supports the cell-to-cell and long-distance trafficking of a wide spectrum of endogenous proteins and ribonucleoprotein complexes. The trafficking of such macromolecules is of importance in the orchestration of non-cell autonomous developmental and physiological processes. Plant viruses encode movement proteins (MPs) that subvert this communication network to facilitate the spread of infection. These viral proteins thus represent excellent experimental keys for exploring the mechanisms involved in intercellular trafficking and communication via Pd.

Membrane tube formation from giant vesicles by dynamic association of motor proteins

The tubular morphology of intracellular membranous compartments is actively maintained through interactions with motor proteins and the cytoskeleton. Moving along cytoskeletal elements, motor proteins exert forces on the membranes to which they are attached, resulting in the formation of membrane tubes and tubular networks. To study the formation of membrane tubes by motor proteins, we developed an in vitro assay consisting of purified kinesin proteins directly linked to the lipids of giant unilamellar vesicles. When the vesicles are brought into contact with a network of immobilized microtubules, membrane tubes and tubular networks are formed. Through systematic variation of the kinesin concentration and membrane composition we study the mechanism involved. We show that a threshold concentration of motor proteins is needed and that a low membrane tension facilitates tube formation. Forces involved in tube formation were measured directly with optical tweezers and are shown to depend only on the tension and bending rigidity of the membrane. The forces were found to be higher than can be generated by individual motor proteins, indicating that multiple motors were working together to pull tubes. We propose a simple mechanism by which individual motor proteins can dynamically associate into clusters that provide the force needed for the formation of tubes, explaining why, in contrast to earlier findings [Roux, A., Cappello, G., Cartaud, J., Prost, J., Goud, B. & Bassereau, P. (2002) Proc. Natl. Acad. Sci. USA 99, 5394-5399], motor proteins do not need to be physically linked to each other to be able to pull tubes.

The EF-hand Ca2+-binding protein p22 plays a role in microtubule and endoplasmic reticulum organization and dynamics with distinct Ca2+-binding requirements

We have reported that p22, an N-myristoylated EF-hand Ca(2+)-binding protein, associates with microtubules and plays a role in membrane trafficking. Here, we show that p22 also associates with membranes of the early secretory pathway membranes, in particular endoplasmic reticulum (ER). On binding of Ca(2+), p22's ability to associate with membranes increases in an N-myristoylation-dependent manner, which is suggestive of a nonclassical Ca(2+)-myristoyl switch mechanism. To address the intracellular functions of p22, a digitonin-based "bulk microinjection" assay was developed to load cells with anti-p22, wild-type, or mutant p22 proteins. Antibodies against a p22 peptide induce microtubule depolymerization and ER fragmentation; this antibody-mediated effect is overcome by preincubation with the respective p22 peptide. In contrast, N-myristoylated p22 induces the formation of microtubule bundles, the accumulation of ER structures along the bundles as well as an increase in ER network formation. An N-myristoylated Ca(2+)-binding p22 mutant, which is unable to undergo Ca(2+)-mediated conformational changes, induces microtubule bundling and accumulation of ER structures along the bundles but does not increase ER network formation. Together, these data strongly suggest that p22 modulates the organization and dynamics of microtubule cytoskeleton in a Ca(2+)-independent manner and affects ER network assembly in a Ca(2+)-dependent manner.

Structural organization of the endoplasmic reticulum

The endoplasmic reticulum (ER) is a continuous membrane system but consists of various domains that perform different functions. Structurally distinct domains of this organelle include the nuclear envelope (NE), the rough and smooth ER, and the regions that contact other organelles. The establishment of these domains and the targeting of proteins to them are understood to varying degrees. Despite its complexity, the ER is a dynamic structure. In mitosis it must be divided between daughter cells and domains must be re-established, and even in interphase it is constantly rearranged as tubules extend along the cytoskeleton. Throughout these rearrangements the ER maintains its basic structure. How this is accomplished remains mysterious, but some insight has been gained from in vitro systems.

Morphological studies on the mechanisms of pigmentary organelle transport in fish xanthophores and melanophores

Pigmentary organelle translocations within fish chromatophores undergo physiological color changes when exposed to external signals. Chromatophores can be isolated in high yields, and their pigmentary organelles can be tracked readily by microscopy. The combined efforts of morphology and biomolecular chemistry have led to the identification of and determination of the interrelationships between cytoskeletal elements and accessory proteins, motor molecules, cytomatrix, and pigmentary organelles of various sizes. Fish chromatophores have been classified as fast, intermediate, and slow translocators, based on the relative numbers of microtubules. Studies on cultured goldfish (Carassius auratus L.) xanthophores for over 20 years have demonstrated that in this slow translocator, tubulovesicular structures of the smooth endoplasmic reticular (SER) cisternae are involved in the disperson and aggregation of associated carotenoid droplets (CD) with some involvement of cytoskeletal elements. Killifish (Fundulus heteroclitus L.) melanophore, a fast translocator, was also examined. Recent work demonstrates a bright fluorescent "starburst"-like spot that we call an actin filament-organizing center (AFOC) with radiating microfilaments, akin to the microtubule-organizing center (MTOC) with radiating microtubules. Melanosomes translocate single-file on microtubules and are not associated with SER cisternae. Slower CD dispersion or aggregation in goldfish xanthophores seems to be predominantly microfilament-based transport, or microfilament- and microtubule-based transport, respectively. Faster melanosome translocations in killifish melanophores are based on microtubules, with our evidence indicating microfilament involvement. Neural crest-derived chromatophores are models for vesicular transport in axons, and immunocytochemical and imaging technologies may help to elucidate the cellular transport mechanisms.

Dynein supports motility of endoplasmic reticulum in the fungus Ustilago maydis

The endoplasmic reticulum (ER) of most vertebrate cells is spread out by kinesin-dependent transport along microtubules, whereas studies in Saccharomyces cerevisiae indicated that motility of fungal ER is an actin-based process. However, microtubules are of minor importance for organelle transport in yeast, but they are crucial for intracellular transport within numerous other fungi. Herein, we set out to elucidate the role of the tubulin cytoskeleton in ER organization and dynamics in the fungal pathogen Ustilago maydis. An ER-resident green fluorescent protein (GFP)-fusion protein localized to a peripheral network and the nuclear envelope. Tubules and patches within the network exhibited rapid dynein-driven motion along microtubules, whereas conventional kinesin did not participate in ER motility. Cortical ER organization was independent of microtubules or F-actin, but reformation of the network after experimental disruption was mediated by microtubules and dynein. In addition, a polar gradient of motile ER-GFP stained dots was detected that accumulated around the apical Golgi apparatus. Both the gradient and the Golgi apparatus were sensitive to brefeldin A or benomyl treatment, suggesting that the gradient represents microtubule-dependent vesicle trafficking between ER and Golgi. Our results demonstrate a role of cytoplasmic dynein and microtubules in motility, but not peripheral localization of the ER in U. maydis.

Paclitaxel affects cytosolic calcium signals by opening the mitochondrial permeability transition pore

We have characterized the effects of the antimitotic drug paclitaxel (Taxol(TM)) on the Ca(2+) signaling cascade of terminally differentiated mouse pancreatic acinar cells. Using single cell fluorescence techniques and whole-cell patch clamping to record cytosolic Ca(2+) and plasma membrane Ca(2+)-dependent Cl(-) currents, we find that paclitaxel abolishes cytosolic Ca(2+) oscillations and in more than half of the cells it also induces a rapid, transient cytosolic Ca(2+) response. This response is not affected by removal of extracellular Ca(2+) indicating that paclitaxel releases Ca(2+) from an intracellular Ca(2+) store. Using saponin-permeabilized cells, we show that paclitaxel does not affect Ca(2+) release from an inositol trisphosphate-sensitive store. Furthermore, up to 15 min after paclitaxel application, there is no significant effect on either microtubule organization or on endoplasmic reticulum organization. The data suggest a non-endoplasmic reticulum source for the intracellular Ca(2+) response. Using the mitochondrial fluorescent dyes, JC-1 and Rhod-2, we show that paclitaxel evoked a rapid decline in the mitochondrial membrane potential and a loss of mitochondrial Ca(2+). Cyclosporin A, a blocker of the mitochondrial permeability transition pore, blocked both the paclitaxel-induced loss of mitochondrial Ca(2+) and the effect on Ca(2+) spikes. We conclude that paclitaxel exerts rapid effects on the cytosolic Ca(2+) signal via the opening of the mitochondrial permeability transition pore. This work indicates that some of the more rapidly developing side effects of chemotherapy might be due to an action of antimitotic drugs on mitochondrial function and an interference with the Ca(2+) signal cascade.

Endoplasmic reticulum of animal cells and its organization into structural and functional domains

The endoplasmic reticulum (ER) in animal cells is an extensive, morphologically continuous network of membrane tubules and flattened cisternae. The ER is a multifunctional organelle; the synthesis of membrane lipids, membrane and secretory proteins, and the regulation of intracellular calcium are prominent among its array of functions. Many of these functions are not homogeneously distributed throughout the ER but rather are confined to distinct ER subregions or domains. This review describes the structural and functional organization of the ER and highlights the dynamic properties of the ER network and the mechanisms that support the positioning of ER membranes within the cell. Furthermore, we outline processes involved in the establishment and maintenance of an anisotropic distribution of ER-resident proteins and, thus, in the organization of the ER into functionally and morphologically different subregions.

Centrosome maturation

In the past, centrosome maturation has been described as the change in microtubule nucleation potential that occurs as cells pass through specific phases of the cell cycle. It is suggested that the idea of centrosome maturation be expanded to include gain of functions that are not necessarily related to microtubule nucleation. Some of these functions could be transient and dependent on the temporary association of molecules with the centrosome as cells progress through the cell cycle. Thus, the centrosome may best be viewed as a site for mediating macromolecular interactions, perhaps as a central processing station within the cell. The centromatrix, a relatively stable lattice of polymers within the centrosome's PCM, could serve as a scaffold for the transient binding of mediator molecules, as well as allow the dynamic exchange of centrosome constituents with a soluble cytoplasmic pool. New evidence adds support to the idea that centrioles are crucial for the maintenance of PCM structure. However, significant evidence indicates that aspects of centrosome structure and function can be maintained in the absence of centrioles. In the case of paternal centrosome maturation, sperm centrioles may not contain an associated centromatrix. It is proposed that regulation of paternal centrioles or centriole associated proteins could mediate centriole-dependent centromatrix assembly following fertilization. Thus, regulation of centromatrix-centriole interactions could be involved in maintaining the integrity of the centrosome's PCM and play an important role in centrosome disassembly during cell differentiation and morphogenesis.

Attributes and dynamics of the endoplasmic reticulum in mammalian eggs

The endoplasmic reticulum is a multifunctional continuous network of membrane-enclosed sacs and tubules that extends throughout the cell. The endoplasmic reticulum is the site of protein synthesis and assembly, as well as lipid and membrane synthesis. Additionally, the endoplasmic reticulum contains calcium pumps, intraluminal calcium storage proteins, and specific calcium-releasing channels. Thus, this membrane system plays a central role in intracellular signaling through the storage and release of calcium. At fertilization, the sperm triggers a large and dramatic release of calcium from the endoplasmic reticulum, which activates the egg to begin development. The ability of the egg to fully elevate calcium depends on biochemical and structural changes during oocyte maturation. The sensitivity of the calcium-releasing system increases and the endoplasmic reticulum is reorganized during maturation of the oocyte; together, these dynamic changes place a substantial calcium storage compartment just beneath the membrane, near the site of sperm-egg fusion. Localization of the calcium store may also contribute to the long-lasting calcium oscillations that are characteristic of mammalian fertilization. Examination of the endoplasmic reticulum in living eggs is leading to a better understanding of calcium release at fertilization.

Immobilization of the early secretory pathway by a virus glycoprotein that binds to microtubules

Membrane trafficking from the endoplasmic reticulum (ER) to the Golgi complex is mediated by pleiomorphic carrier vesicles that are driven along microtubule tracks by the action of motor proteins. Here we describe how NSP4, a rotavirus membrane glycoprotein, binds to microtubules and blocks ER-to-Golgi trafficking in vivo. NSP4 accumulates in a post-ER, microtubule-associated membrane compartment and prevents targeting of vesicular stomatitis virus glycoprotein (VSV-G) at a pre-Golgi step. NSP4 also redistributes beta-COP and ERGIC53, markers of a vesicular compartment that dynamically cycles between the ER and Golgi, to structures aligned along linear tracks radiating throughout the cytoplasm. This block in membrane trafficking is released when microtubules are depolymerized with nocodazole, indicating that vesicles containing NSP4 are tethered to the microtubule cytoskeleton. Disruption of microtubule-mediated membrane transport by a viral glycoprotein may represent a novel pathogenic mechanism and provides a new experimental tool for the dissection of early steps in exocytic transport.

Association between the calcium-binding protein calretinin and cytoskeletal components in the human colon adenocarcinoma cell line WiDr

Calretinin (CR) is a Ca(2+)-binding protein (CaBP) of the EF-hand family expressed in a cell-type-specific manner and thought to act as a Ca(2+) buffer. Based upon previous studies, CR can undergo Ca(2+)-induced conformational changes, suggesting that it may also belong to the subfamily of Ca(2+)-sensor proteins that are characterized by their ability to interact with target ligands. To elucidate the role of CR, we used the undifferentiated colon adenocarcinoma cell line WiDr, which expresses significant amounts of CR. It has been shown previously that combined treatment with an inducer of differentiation sodium butyrate (NaBt) and a cell growth inhibitor hexamethylene bisacetamide (HMBA) or treatment with CR antisense oligonucleotides is down-regulating CR in parallel with a decrease of cell growth, suggesting a possible involvement of CR in maintaining the undifferentiated phenotype of WiDr cells. Furthermore, CR is absent from normal colon cells and from well-differentiated colon adenocarcinoma cell lines (e.g., Caco-2). Since members of the EF-hand family of proteins are interacting with cytoskeletal components, we investigated the possible association of CR with the cytoskeleton in WiDr cells. With double immunofluorescence stainings and immunoprecipitation experiments, we show close association of CR with intermediate filaments or microtubules in WiDr cells. Treatment with NaBt either disrupted or strongly diminished this interaction, respectively. The same effect was observed after elevation of [Ca(2+)](i) by applying the ionophore A-23187. These data suggest that CR may contribute to the transformation of enterocytes by interfering with the differentiation process, i.e., acting at both levels: cell shape dynamics and mitosis.

Mutants affecting the structure of the cortical endoplasmic reticulum in Saccharomyces cerevisiae

We find that the peripheral ER in Saccharomyces cerevisiae forms a dynamic network of interconnecting membrane tubules throughout the cell cycle, similar to the ER in higher eukaryotes. Maintenance of this network does not require microtubule or actin filaments, but its dynamic behavior is largely dependent on the actin cytoskeleton. We isolated three conditional mutants that disrupt peripheral ER structure. One has a mutation in a component of the COPI coat complex, which is required for vesicle budding. This mutant has a partial defect in ER segregation into daughter cells and disorganized ER in mother cells. A similar phenotype was found in other mutants with defects in vesicular trafficking between ER and Golgi complex, but not in mutants blocked at later steps in the secretory pathway. The other two mutants found in the screen have defects in the signal recognition particle (SRP) receptor. This receptor, along with SRP, targets ribosome-nascent chain complexes to the ER membrane for protein translocation. A conditional mutation in SRP also disrupts ER structure, but other mutants with translocation defects do not. We also demonstrate that, both in wild-type and mutant cells, the ER and mitochondria partially coalign, and that mutations that disrupt ER structure also affect mitochondrial structure. Our data suggest that both trafficking between the ER and Golgi complex and ribosome targeting are important for maintaining ER structure, and that proper ER structure may be required to maintain mitochondrial structure.

Microtubules regulate local Ca2+ spiking in secretory epithelial cells

The role of the cytoskeleton in regulating Ca(2+) release has been explored in epithelial cells. Trains of local Ca(2+) spikes were elicited in pancreatic acinar cells by infusion of inositol trisphosphate through a whole cell patch pipette, and the Ca(2+)-dependent Cl(-) current spikes were recorded. The spikes were only transiently inhibited by cytochalasin B, an agent that acts on microfilaments. In contrast, nocodazole (5-100 micrometer), an agent that disrupts the microtubular network, dose-dependently reduced spike frequency and decreased spike amplitude leading to total blockade of the response. Consistent with an effect of microtubular disruption, colchicine also inhibited spiking but neither Me(2)SO nor beta-lumicolchicine, an inactive analogue of colchicine, had any effect. The microtubule-stabilizing agent, taxol, also inhibited spiking. The nocodazole effects were not due to complete loss of function of the Ca(2+) signaling apparatus, because supramaximal carbachol concentrations were still able to mobilize a Ca(2+) response. Finally, as visualized by 2-photon excitation microscopy of ER-Tracker, nocodazole promoted a loss of the endoplasmic reticulum in the secretory pole region. We conclude that microtubules specifically maintain localized Ca(2+) spikes at least in part because of the local positioning of the endoplasmic reticulum.

Association of spectrin with a subcompartment of the endoplasmic reticulum in honeybee photoreceptor cells

The endoplasmic reticulum (ER) in honeybee photoreceptors is organized into structurally distinct subregions. The most prominent of these, the submicrovillar network of ER cisternae, is tightly associated with actin filaments. Electron microscopic techniques have demonstrated that the ER-associated actin filaments are regularly spaced at 60-80 nm and cross-bridged by filamentous structures. A polyclonal antibody against Drosophila alpha-spectrin has been used to examine the distribution of spectrin in the photoreceptors. On Western blots of bee retina, the antibody identifies a 260-kDa protein that exhibits biochemical and immunological properties characteristic of alpha-spectrin. Immunofluorescence microscopy has shown that alpha-spectrin codistributes with the submicrovillar ER but not with other ER subdomains. After cytochalasin-B-induced depolymerization of the ER-associated F-actin system, alpha-spectrin remains colocalized with the ER, indicating that alpha-spectrin is bound to the ER membrane. The F-actin/spectrin system associated with the submicrovillar ER may stabilize the shape of this ER subcompartment and may play a role in maintaining functional ER subregions.

Reactivation of cell surface transport in Reticulomyxa

Granuloreticulosean protists transport particles (e.g., bacteria, algae, and sand grains) along the outer surfaces of their pseudopodia. This cell surface transport plays a vital role in feeding, reproduction, shell construction, and locomotion and can be visualized by the movements of extracellularly adherent polystyrene microspheres (i.e., latex beads). Our videomicroscopic analyses of transport associated with the pseudopodia of Reticulomyxa filosa revealed two distinct types of both intracellular and cell surface transport: (1) saltatory, bidirectional transport of individual or clustered organelles and/or surface-attached particles, and (2) continuous, unidirectional bulk or "resolute" motion of aggregated organelles and/or surface-bound particles. Organelles and surface-attached polystyrene microspheres remained firmly attached to the microtubule cytoskeletons of detergent-extracted pseudopodia. Both saltatory and resolute organelle and surface transport reactivated upon the addition of 0.01-1.0 mM ATP. At 1 mM ATP, the velocities of reactivated saltatory transport were indistinguishable from those observed in vivo. The reactivated transport was microtubule-dependent and was not inhibited by incubation with Ca(2+)-gelsolin under conditions that abolish rhodamine-phalloidin detection of actin filaments. These findings provide further support that both intracellular organelle and membrane surface transport are mediated by a common mechanism, and establish Reticulomyxa as a unique model system to further study the mechanochemistry of cell surface transport in vitro.

Dynein and plus-end microtubule-dependent motors are associated with specialized Sertoli cell junction plaques (ectoplasmic specializations)

The mechanism responsible for spermatid translocation in the mammalian seminiferous epithelium was proposed to be the microtubule-based transport of specialized junction plaques (ectoplasmic specializations) that occur in Sertoli cell regions attached to spermatid heads. These plaques each consist of a cistern of endoplasmic reticulum, a layer of actin filaments and the adjacent plasma membrane. It is predicted that motor proteins function to move the junction plaques, and hence the attached spermatids, first towards the base and then back to the apex of the epithelium, along microtubules. If this hypothesis is true, motor proteins should be associated with the cytoplasmic face of the endoplasmic reticulum component of ectoplasmic specializations. In addition, isolated junction plaques should support microtubule movement both in the plus and minus directions to account for the bidirectional translocation of spermatids in vivo. To determine if cytoplasmic dynein is localized to the endoplasmic reticulum of the plaques, perfusion-fixed rat testes were immunologically probed, at the ultrastructural level, for the intermediate chain of cytoplasmic dynein (IC74). In addition, testicular fractions enriched for spermatid/junction complexes were incubated with and without gelsolin, centrifuged and the supernatants compared, by western blot analysis, for Glucose Regulated Protein 94 (a marker for endoplasmic reticulum) and IC74. At the ultrastructural level, the probe for IC74 clearly labelled material associated with the cytoplasmic face of the endoplasmic reticulum component of the junction plaques. In the gelsolin experiments, both probes reacted more strongly with appropriate bands from the gelsolin-treated supernatants than with corresponding bands from controls. To determine if the junction plaques support microtubule transport in both directions, polarity-labelled microtubules were bound to isolated spermatid/junction complexes and then assessed for motility in the presence of ATP and testicular cytosol (2 mg/ml). Of 25 recorded motility events, 17 were in a direction consistent with a plus-end directed motor being present, and 8 were in the minus-end direction. The results are consistent with the conclusion that the junction plaques have the potential for moving along microtubules in both the plus and minus directions and that both a kinesin-type and a dynein-type motor may be associated with the junction plaques. The data also indicate that cytoplasmic dynein is localized to the cytoplasmic face of the endoplasmic reticulum component of the plaques.

Reconstitution of microtubule nucleation potential in centrosomes isolated from Spisula solidissima oocytes

Treatment of isolated Spisula solidissima centrosomes with KI removes (gamma)-tubulin, 25 nm rings, and their microtubule nucleation potential, revealing the presence of a filamentous lattice, the ‘centromatrix’. Treatment of this centromatrix with Spisula oocyte extract results in the binding of (gamma)-tubulin and 25 nm rings, and the recovery of microtubule nucleation potential. Fractionation of this extract resulted in the separation of elements that are required for the recovery of microtubule nucleation potential. We show that some, but not all, of the elements needed cosediment with microtubules. Further, extracts prepared from activated (meiotic) and non-activated (interphase) Spisula oocytes, CHO cells blocked in S phase, Drosophila embryos and Xenopus oocytes all support the recovery of microtubule nucleation potential by the Spisula centromatrix. These results demonstrate that components necessary for centrosome-dependent microtubule nucleation are functionally conserved and abundant in both interphase and meiotic/mitotic cytoplasm.

In vitro formation of the endoplasmic reticulum occurs independently of microtubules by a controlled fusion reaction

We have established an in vitro system for the formation of the endoplasmic reticulum (ER). Starting from small membrane vesicles prepared from Xenopus laevis eggs, an elaborate network of membrane tubules is formed in the presence of cytosol. In the absence of cytosol, the vesicles only fuse to form large spheres. Network formation requires a ubiquitous cytosolic protein and nucleoside triphosphates, is sensitive to N-ethylmaleimide and high cytosolic Ca(2+) concentrations, and proceeds via an intermediate stage in which vesicles appear to be clustered. Microtubules are not required for membrane tubule and network formation. Formation of the ER network shares significant similarities with formation of the nuclear envelope. Our results suggest that the ER network forms in a process in which cytosolic factors modify and regulate a basic reaction of membrane vesicle fusion.

Unique and multifunctional adhesion junctions in the testis: ectoplasmic specializations

In this paper, we review the structure and function of a unique type of actin-related intercellular adhesion junctions in the testis. Based on their ultrastructure, the junctions are divided into five distinct domains. The currently identified molecular components of each domain are summarized. In addition, the architecture of the mammalian system is compared with that of non-mammalian vertebrates. Functionally, the junctions are related to the turnover of adhesion between Sertoli cells, to the attachment of spermatids to the seminiferous epithelium, and to sperm release. They also are part of the mechanism by which spermatids are moved through the epithelium. Evidence consistent with adhesion and motility related functions is discussed. Control, both of junction turnover and of microtubule-based transport, is identified as an important avenue for future research.

Microtubule-based endoplasmic reticulum motility in Xenopus laevis: activation of membrane-associated kinesin during development

The endoplasmic reticulum (ER) in animal cells uses microtubule motor proteins to adopt and maintain its extended, reticular organization. Although the orientation of microtubules in many somatic cell types predicts that the ER should move toward microtubule plus ends, motor-dependent ER motility reconstituted in extracts of Xenopus laevis eggs is exclusively a minus end-directed, cytoplasmic dynein-driven process. We have used Xenopus egg, embryo, and somatic Xenopus tissue culture cell (XTC) extracts to study ER motility during embryonic development in Xenopus by video-enhanced differential interference contrast microscopy. Our results demonstrate that cytoplasmic dynein is the sole motor for microtubule-based ER motility throughout the early stages of development (up to at least the fifth embryonic interphase). When egg-derived ER membranes were incubated in somatic XTC cytosol, however, ER tubules moved in both directions along microtubules. Data from directionality assays suggest that plus end-directed ER tubule extensions contribute approximately 19% of the total microtubule-based ER motility under these conditions. In XTC extracts, the rate of ER tubule extensions toward microtubule plus ends is lower ( approximately 0.4 microm/s) than minus end-directed motility ( approximately 1.3 microm/s), and plus end-directed motility is eliminated by a function-blocking anti-conventional kinesin heavy chain antibody (SUK4). In addition, we provide evidence that the initiation of plus end-directed ER motility in somatic cytosol is likely to occur via activation of membrane-associated kinesin.

Spermatid translocation in the rat seminiferous epithelium: coupling membrane trafficking machinery to a junction plaque

In this study, we demonstrate that specialized junction plaques that occur between Sertoli cells and spermatids in the rat testis support microtubule translocation in vitro. During spermatogenesis, Sertoli cells are attached to spermatids by specialized adhesion junctions termed ectoplasmic specializations (ESs). These structures consist of regions of the plasma membrane adherent to the spermatid head, a submembrane layer of tightly packed actin filaments, and an attached cistern of endoplasmic reticulum. It has been proposed that motor proteins on the endoplasmic reticulum interact with adjacent microtubules to translocate the junction plaques, and hence the attached spermatids, within the epithelium. If this hypothesis is true, then isolated junctions should support microtubule transport. To verify this prediction, we have mechanically isolated rat spermatids, together with their attached ESs, and tested them for their ability to transport microtubules in vitro. Most assays were done in the presence of 2 mg/ml testicular cytosol and at room temperature. ESs attached to spermatids supported microtubule translocation. In some cases in which motility events were detected, microtubules moved smoothly over the junction site. In others, the movement was slow but progressive, saltatory and "inch-worm-like." No motility was detected in the absence of exogenous ATP or in the presence of apyrase (an enzyme that catalyses the breakdown of ATP). Our results are consistent with the microtubule-based motility hypothesis of spermatid translocation.

Focal agonist stimulation results in spatially restricted Ca2+ release and capacitative Ca2+ entry in bovine vascular endothelial cells

1. Intracellular Ca2+ ([Ca2+]i) signals were studied with spatial resolution in bovine vascular endothelial cells using the fluorescent Ca2+ indicator fluo-3 and confocal laser scanning microscopy. Single cells were stimulated with the purinergic receptor agonist ATP resulting in an increase of [Ca2+]i due to intracellular Ca2+ release from inositol 1,4,5-trisphosphate (IP3)-sensitive stores. ATP-induced Ca2+ release was quantal, i.e. submaximal concentrations mobilized only a fraction of the intracellularly stored Ca2+. 2. Focal receptor stimulation in Ca2+-free solution by pressure application of agonist-containing solution through a fine glass micropipette resulted in a spatially restricted increase in [Ca2+]i. Ca2+ release was initiated at the site of stimulation and frequently propagated some tens of micrometres into non-stimulated regions. 3. Local Ca2+ release caused activation of capacitative Ca2+ entry (CCE). CCE was initially colocalized with Ca2+ release. Following repetitive focal stimulation, however, CCE became detectable at remote sites where no Ca2+ release had been observed. In addition, the rate of Ca2+ store depletion with repetitive local activation of release in Ca2+-free solution was markedly slower than that elicited by ATP stimulation of the entire cell. 4. From these experiments it is concluded that both intracellular IP3-dependent Ca2+ release and activation of CCE are controlled locally at the subcellular level. Moreover, redistribution of intracellular Ca2+ stored within the endoplasmic reticulum efficiently counteracts local store depletion and accounts for the spatial spread of CCE activation.

The disassembly and reassembly of functional centrosomes in vitro

Animal cells contain a single centrosome that nucleates and organizes a polarized array of microtubules which functions in many cellular processes. In most cells the centrosome is composed of two centrioles surrounded by an ill-defined "cloud" of pericentriolar material. Recently, gamma-tubulin-containing 25-nm diameter ring structures have been identified as likely microtubule nucleation sites within the pericentriolar material of isolated centrosomes. Here we demonstrate that when Spisula centrosomes are extracted with 1.0 M KI they lose their microtubule nucleation potential and appear by three-dimensional electron microscopy as a complex lattice, built from 12- to 15-nm thick elementary fiber(s), that lack centrioles and 25-nm rings. Importantly, when these remnants are incubated in extracts prepared from Spisula oocytes they recover their 25-nm rings, gamma-tubulin, and microtubule nucleation potential. This recovery process occurs in the absence of microtubules, divalent cations, and nucleotides. Thus, in animals the centrosome is structurally organized around a KI-insoluble filament-based "centromatrix" that serves as a scaffold to which those proteins required for microtubule nucleation bind, either directly or indirectly, in a divalent cation and nucleotide independent manner.