Activation of myosin V-based motility and F-actin-dependent network formation of endoplasmic reticulum during mitosis

A tubule-sheet continuum model for the mechanism of nuclear envelope assembly

Nuclear envelope (NE) assembly defects cause chromosome fragmentation, cancer, and aging. However, major questions about the mechanism of NE assembly and its relationship to nuclear pathology are unresolved. In particular, how cells efficiently assemble the NE starting from vastly different, cell type-specific endoplasmic reticulum (ER) morphologies is unclear. Here, we identify a NE assembly mechanism, "membrane infiltration," that defines one end of a continuum with another NE assembly mechanism, "lateral sheet expansion," in human cells. Membrane infiltration involves the recruitment of ER tubules or small sheets to the chromatin surface by mitotic actin filaments. Lateral sheet expansion involves actin-independent envelopment of peripheral chromatin by large ER sheets that then extend over chromatin within the spindle. We propose a "tubule-sheet continuum" model that explains the efficient NE assembly from any starting ER morphology, the cell type-specific patterns of nuclear pore complex (NPC) assembly, and the obligatory NPC assembly defect of micronuclei.

Intertwined and Finely Balanced: Endoplasmic Reticulum Morphology, Dynamics, Function, and Diseases

The endoplasmic reticulum (ER) is an organelle that is responsible for many essential subcellular processes. Interconnected narrow tubules at the periphery and thicker sheet-like regions in the perinuclear region are linked to the nuclear envelope. It is becoming apparent that the complex morphology and dynamics of the ER are linked to its function. Mutations in the proteins involved in regulating ER structure and movement are implicated in many diseases including neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and amyotrophic lateral sclerosis (ALS). The ER is also hijacked by pathogens to promote their replication. Bacteria such as Legionella pneumophila and Chlamydia trachomatis, as well as the Zika virus, bind to ER morphology and dynamics-regulating proteins to exploit the functions of the ER to their advantage. This review covers our understanding of ER morphology, including the functional subdomains and membrane contact sites that the organelle forms. We also focus on ER dynamics and the current efforts to quantify ER motion and discuss the diseases related to ER morphology and dynamics.

A double-blind randomized controlled trial of the effects of eicosapentaenoic acid supplementation on muscle inflammation and physical function in patients undergoing colorectal cancer resection

BACKGROUND

Resection of colorectal cancer (CRC) initiates inflammation, mediated at least partly by NFĸB (nuclear factor kappa-light-chain-enhancer of activated B-cells), leading to muscle catabolism and reduced physical performance. Eicosapentaenoic acid (EPA) has been shown to modulate NFĸB, but evidence for its benefit around the time of surgery is limited.

OBJECTIVE

To assess the effect of EPA supplementation on muscle inflammation and physical function around the time of major surgery.

DESIGN

In a double-blind randomized control trial, 61 patients (age: 68.3 ± 0.95 y; 42 male) scheduled for CRC resection, received 3 g per day of EPA (n = 32) or placebo (n = 29) for 5-days before and 21-days after operation. Lean muscle mass (LMM) (via dual energy X-ray absorptiometry (DXA)), anaerobic threshold (AT) (via cardiopulmonary exercise testing (CPET)) and hand-grip strength (HG) were assessed before and 4-weeks after surgery, with muscle biopsies (m. vastus lateralis) obtained for the assessment of NF-ĸB protein expression.

RESULTS

There were no differences in muscle NFĸB between EPA and placebo groups (mean difference (MD) -0.002; 95% confidence interval (CI) -0.19 to 0.19); p = 0.98). There was no difference in LMM (MD 704.77 g; 95% CI -1045.6 g-2455.13 g; p = 0.42) or AT (MD 1.11 mls/kg/min; 95% CI -0.52 mls/kg/min to 2.74 mls/kg/min; p = 0.18) between the groups. Similarly, there was no difference between the groups in HG at follow up (MD 0.1; 95% CI -1.88 to 2.08; p = 0.81). Results were similar when missing data was imputed.

CONCLUSION

EPA supplementation confers no benefit in terms of inflammatory status, as judged by NFĸB, or preservation of LMM, aerobic capacity or physical function following major colorectal surgery.

CLINICAL TRIALS REFERENCE

NCT01320319.

F-actin cytoskeleton and the fate of organelles in chromaffin cells

In addition to playing a fundamental structural role, the F-actin cytoskeleton in neuroendocrine chromaffin cells has a prominent influence on governing the molecular mechanism and regulating the secretory process. Performing such roles, the F-actin network might be essential to first transport, and later locate the cellular organelles participating in the secretory cycle. Chromaffin granules are transported from the internal cytosolic regions to the cell periphery along microtubular and F-actin structures. Once in the cortical region, they are embedded in the F-actin network where these vesicles experience restrictions in motility. Similarly, mitochondria transport is affected by both microtubule and F-actin inhibitors and suffers increasing motion restrictions when they are located in the cortical region. Therefore, the F-actin cortex is a key factor in defining the existence of two populations of cortical and perinuclear granules and mitochondria which could be distinguished by their different location and mobility. Interestingly, other important organelles for controlling intracellular calcium levels, such as the endoplasmic reticulum network, present clear differences in distribution and much lower mobility than chromaffin vesicles and mitochondria. Nevertheless, both mitochondria and the endoplasmic reticulum appear to distribute in the proximity of secretory sites to fulfill a pivotal role, forming triads with calcium channels ensuring the fine tuning of the secretory response. This review presents the contributions that provide the basis for our current view regarding the influence that F-actin has on the distribution of organelles participating in the release of catecholamines in chromaffin cells, and summarizes this knowledge in simple models. In chromaffin cells, organelles such as granules and mitochondria distribute forming cortical and perinuclear populations whereas others like the ER present homogenous distributions. In the present review we discuss the role of transport systems and the existence of an F-actin cortical structure as the main factors behind the formation of organelle subpopulations in this neuroendocrine cell model. This article is part of a mini review series on Chromaffin cells (ISCCB Meeting, 2015). Cover image for this issue: doi: 10.1111/jnc.13322.

Dynamic phase microscopy reveals periodic oscillations of endoplasmic reticulum during network formation

Dynamic phase microscopy was used to study the dynamic events of formation of the endoplasmic reticulum (ER) in interphase-arrested Xenopus egg extract. We have shown that the ER periodically oscillated in an ATP-dependent manner in the frequency range of 1.6-2.2 Hz, while the tubular membrane network formed in vitro. The spectral density, i.e. the pattern of a given frequency component in the Fourier spectrum, was strongly correlated with the dynamic events during microtubule-dependent and microtubule-independent ER network formation observed by video-enhanced contrast differential interference contrast and fluorescence microscopy. Because the 1.6-2.2 Hz frequency of oscillation during the network formation was detected both in the presence and absence of microtubules, it appears to be an intrinsic ATP-dependent ER membrane property. Several characteristic active and inactive stages of ER network formation were observed both in the presence and absence of microtubules. However, data analysis of these stages indicated that microtubules and dynein motor activity have a strong influence and a cooperative effect on the kinetics of ER formation by controlled fusion reaction.

The calcium-dependent ribonuclease XendoU promotes ER network formation through local RNA degradation

In both Xenopus laevis egg extract and human cells, an increase in cytosolic calcium activates the endogenous ribonuclease XendoU/hEndoU, which localizes to the ER, promotes RNA cleavage and RNP removal, and induces ER network assembly.

How cells shape and remodel organelles in response to cellular signals is a poorly understood process. Using Xenopus laevis egg extract, we found that increases in cytosolic calcium lead to the activation of an endogenous ribonuclease, XendoU. A fraction of XendoU localizes to the endoplasmic reticulum (ER) and is required for nuclear envelope assembly and ER network formation in a catalysis-dependent manner. Using a purified vesicle fusion assay, we show that XendoU functions on the surface of ER membranes to promote RNA cleavage and ribonucleoprotein (RNP) removal. Additionally, RNA removal from the surface of vesicles by RNase treatment leads to increased ER network formation. Using human tissue culture cells, we found that hEndoU localizes to the ER, where it promotes the formation of ER tubules in a catalysis-dependent manner. Together, these results demonstrate that calcium-activated removal of RNA from membranes by XendoU promotes and refines ER remodeling and the formation of tubular ER.

Microtubules as key coordinators of nuclear envelope and endoplasmic reticulum dynamics during mitosis

During mitosis, cells comprehensively restructure their interior to promote the faithful inheritance of DNA and cytoplasmic contents. In metazoans, this restructuring entails disassembly of the nuclear envelope, redistribution of its components into the endoplasmic reticulum (ER) and eventually nuclear envelope reassembly around the segregated chromosomes. The microtubule cytoskeleton has recently emerged as a critical regulator of mitotic nuclear envelope and ER dynamics. Microtubules and associated molecular motors tear open the nuclear envelope in prophase and remove nuclear envelope remnants from chromatin. Additionally, two distinct mechanisms of microtubule-based regulation of ER dynamics operate later in mitosis. First, association of the ER with microtubules is reduced, preventing invasion of ER into the spindle area, and second, organelle membrane is actively cleared from metaphase chromosomes. However, we are only beginning to understand the role of microtubules in shaping and distributing ER and other organelles during mitosis.

Myosin XI-dependent formation of tubular structures from endoplasmic reticulum isolated from tobacco cultured BY-2 cells

The reticular network of the endoplasmic reticulum (ER) consists of tubular and lamellar elements and is arranged in the cortical region of plant cells. This network constantly shows shape change and remodeling motion. Tubular ER structures were formed when GTP was added to the ER vesicles isolated from tobacco (Nicotiana tabacum) cultured BY-2 cells expressing ER-localized green fluorescent protein. The hydrolysis of GTP during ER tubule formation was higher than that under conditions in which ER tubule formation was not induced. Furthermore, a shearing force, such as the flow of liquid, was needed for the elongation/extension of the ER tubule. The shearing force was assumed to correspond to the force generated by the actomyosin system in vivo. To confirm this hypothesis, the S12 fraction was prepared, which contained both cytosol and microsome fractions, including two classes of myosins, XI (175-kD myosin) and VIII (BY-2 myosin VIII-1), and ER-localized green fluorescent protein vesicles. The ER tubules and their mesh-like structures were arranged in the S12 fraction efficiently by the addition of ATP, GTP, and exogenous filamentous actin. The tubule formation was significantly inhibited by the depletion of 175-kD myosin from the S12 fraction but not BY-2 myosin VIII-1. Furthermore, a recombinant carboxyl-terminal tail region of 175-kD myosin also suppressed ER tubule formation. The tips of tubules moved along filamentous actin during tubule elongation. These results indicated that the motive force generated by the actomyosin system contributes to the formation of ER tubules, suggesting that myosin XI is responsible not only for the transport of ER in cytoplasm but also for the reticular organization of cortical ER.

Myosin-Va transports the endoplasmic reticulum into the dendritic spines of Purkinje neurons

Extension of the endoplasmic reticulum (ER) into dendritic spines of Purkinje neurons (PNs) is required for cerebellar synaptic plasticity and is disrupted in animals with null mutations in Myo5a, the gene encoding myosin-Va^1–3. Notably, the mechanism ensuring the ER's localization to spines has not been unraveled. While it has been proposed that animal class V myosins localize organelles by tethering them to the actin cytoskeleton^4–7, we demonstrate here that myosin-Va acts as a point-to-point organelle transporter to pull ER as cargo into PN spines. Specifically, the myosin accumulates at the ER tip as the organelle moves into spines, and the myosin's ability to hydrolyze ATP is required for spine ER targeting. Moreover, myosin-Va is responsible for the vast majority of spine ER insertional events. Finally, attenuation of the myosin's ability to move along actin filaments reduces the maximum velocity of ER movement into spines, providing direct evidence that myosin-Va drives ER motility. Thus, we establish that an actin-based motor moves ER within animal cells, and we uncover the mechanism that mediates ER localization to PN spines, a prerequisite for synaptic plasticity.

Myosin Va participates in acrosomal formation and nuclear morphogenesis during spermatogenesis of Chinese mitten crab Eriocheir sinensis

Background

The Chinese mitten crab Eriocheir sinensis belongs to the Class Crustacea, Decapoda, Brachyura. The spermatozoon of this species is of aflagellated type, it has a spherical acrosome surrounded by the cup-shaped nucleus, which are unique to brachyurans. For the past several decades, studies on the spermatogenesis of the mitten crab mainly focus on the morphology. Compared with the extensive study of molecular mechanism of spermatogenesis in mammals, relatively less information is available in crustacean species. Myosin Va, a member of Class V myosin, has been implicated in acrosome biogenesis and vesicle transport during spermatogenesis in mammals. In the present study we demonstrate the expression and cellular localization of myosin Va during spermatogenesis in E. sinensis.

Methodology/Principal Findings

Western blot demonstrated that myosin Va is expressed during spermatogenesis. Immunocytochemical and ultrastructural analyses showed that myosin Va mainly localizes in the cytoplasm in spermatocytes. At the early stage of spermiogenesis, myosin Va binds to the endoplasmic reticulum vesicle (EV) and proacrosomal granule (PG). Subsequently, myosin Va localizes within the proacrosomal vesicle (PV) formed by PG and EV fusion and locates in the membrane complex (MC) at the mid spermatid stage. At the late spermatid stage, myosin Va is associated with the shaping nucleus and mitochondria. In mature spermatozoon, myosin Va predominates in acrosomal tubule (AT) and nucleus.

Conclusions/Significance

Our study demonstrates that myosin Va may be involved in acrosome biogenesis and nuclear morphogenesis during spermatogenesis in E. sinensis. Considering the distribution and molecular characteristics of myosin Va, we also propose a hypothesis of AT formation in this species. It is the first time to uncover the role of myosin Va in crustacean spermatogenesis.

Networking in the endoplasmic reticulum

The network of the ER (endoplasmic reticulum) is set up by cytoskeletal control of the movement and remodelling of polygonal rings of tubules, bundles of tubules and cisternal regions. We have developed a new image analysis tool, persistency mapping, to understand the framework upon which the plant ER remodels. With this new tool, we have explored the network nodes, called anchor/growth sites, that may anchor the network by attachment to the plasma membrane. We have determined how the polygonal ring structure depends on myosin XI-K for 'opening' and 'closing'. With latrunculin B treatment, we have investigated the involvement of actin in the elongation and persistency of the tubules. We also show how the cytoskeleton is involved in directional diffusion within the membrane. This observation may lead to an answer to the question of what function this network structure serves in the cell. We propose that the ER acts as a trafficking network, delivering lipid, protein, calcium and signalling molecules to different regions of the cell. It does so by directional reduced dimensional diffusion. The ER network of tubules restricts the dimensionality of diffusion to near one-dimensional, whereas the cisternae reduce it to near two-dimensional. The cytoskeleton does not drive the diffusion, but participates by providing directionality to the diffusion.

Both actin and myosin inhibitors affect spindle architecture in PtK1 cells: does an actomyosin system contribute to mitotic spindle forces by regulating attachment and movements of chromosomes in mammalian cells?

Immunocytochemical techniques are used to analyze the effects of both an actin and myosin inhibitor on spindle architecture in PtK(1) cells to understand why both these inhibitors slow or block chromosome motion and detach chromosomes. Cytochalasin J, an actin inhibitor and a myosin inhibitor, 2, 3 butanedione 2-monoxime, have similar effects on changes in spindle organization. Using primary antibodies and stains, changes are studied in microtubule (MT), actin, myosin, and chromatin localization. Treatment of mitotic cells with both inhibitors results in detachment or misalignment of chromosomes from the spindle and a prominent buckling of MTs within the spindle, particularly evident in kinetochore fibers. Evidence is presented to suggest that an actomyosin system may help to regulate the initial and continued attachment of chromosomes to the mammalian spindle and could also influence spindle checkpoint(s).

How and why does the endoplasmic reticulum move?

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

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.

Cell and molecular biology of the fastest myosins

Chara myosin is a class XI plant myosin in green algae Chara corallina and responsible for fast cytoplasmic streaming. The Chara myosin exhibits the fastest sliding movement of F-actin at 60 mum/s as observed so far, 10-fold of the shortening speed of muscle. It has some distinct properties differing from those of muscle myosin. Although knowledge about Chara myosin is very limited at present, we have tried to elucidate functional bases of its characteristics by comparing with those of other myosins. In particular, we have built the putative atomic model of Chara myosin by using the homology-based modeling system and databases. Based on the putative structure of Chara myosin obtained, we have analyzed the relationship between structure and function of Chara myosin to understand its distinct properties from various aspects by referring to the accumulated knowledge on mechanochemical and structural properties of other classes of myosin, particularly animal and fungal myosin V. We will also discuss the functional significance of Chara myosin in a living cell.

Endoplasmic reticulum remains continuous and undergoes sheet-to-tubule transformation during cell division in mammalian cells

The endoplasmic reticulum (ER) is a multifaceted cellular organelle both structurally and functionally, and its cell cycle–dependent morphological changes are poorly understood. Our quantitative confocal and EM analyses show that the ER undergoes dramatic reorganization during cell division in cultured mammalian cells as mitotic ER profiles become shorter and more branched. 3D modeling by electron tomography reveals that the abundant interphase structures, sheets, are lost and subsequently transform into a branched tubular network that remains continuous. This is confirmed by observing the most prominent ER subdomain, the nuclear envelope (NE). A NE marker protein spreads to the mitotic ER tubules, although it does not show a homogenous distribution within the network. We mimicked the mitotic ER reorganization using puromycin to strip the membrane-bound ribosomes from the interphase ER corresponding to the observed loss of ribosomes normally occurring during mitosis. We propose that the structural changes in mitotic ER are linked to ribosomal action on the ER membranes.

Membrane associated nonmuscle myosin II functions as a motor for actin-based vesicle transport in clam oocyte extracts

Nonmuscle myosin II (Myo2) has been shown to associate with membranes of the trans-Golgi network and to be involved in Golgi to ER retrograde protein transport. Here, we provide evidence that Myo2 not only associates with membranes but functions to transport vesicles on actin filaments (AFs). We used extracts from unactivated clam oocytes for these studies. AFs assembled spontaneously in these extracts and myosin-dependent vesicle transport was observed upon activation. In addition, actin bundles formed and moved relative to each other at an average speed of 0.30 microm/s. Motion analysis revealed that vesicles moved on the spontaneously assembled AFs at speeds greater than 1 microm/s. The motor on these vesicles was identified as a member of the nonmuscle Myo2 family based on sequence determination by Edman chemistry. Vesicles in these extracts were purified by sucrose gradient centrifugation and movement was reconstituted in vitro using skeletal muscle actin coated coverslips. When peripheral membrane proteins of vesicles including Myo2 were removed by salt stripping or when extracts were treated with an antibody specific to clam oocyte nonmuscle Myo2, vesicle movement was inhibited. Blebbistatin, a Myo2 specific inhibitor, also blocked vesicle movement. Myo2 light chain kinase activity was found to be essential for vesicle movement and sliding of actin bundles. Together, our data provide direct evidence that nonmuscle Myo2 is involved in actin-dependent vesicle transport in clam oocytes.

Differential organelle movement on the actin cytoskeleton in lily pollen tubes

We have examined the arrangement and movement of three major compartments, the endoplasmic reticulum (ER), mitochondria, and the vacuole during oscillatory, polarized growth in lily pollen tubes. These movements are dependent on the actin cytoskeleton, because they are strongly perturbed by the anti-microfilament drug, latrunculin-B, and unaffected by the anti-microtubule agent, oryzalin. The ER, which has been labeled with mGFP5-HDEL or cytochalasin D tetramethylrhodamine, displays an oscillatory motion in the pollen tube apex. First it moves apically in the cortical region, presumably along the cortical actin fringe, and then periodically folds inward creating a platform that transects the apical domain in a plate-like structure. Finally, the ER reverses its direction and moves basipetally through the central core of the pollen tube. When subjected to cross-correlation analysis, the formation of the platform precedes maximal growth rates by an average of 3 s (35-40 degrees ). Mitochondria, labeled with Mitotracker Green, are enriched in the subapical region, and their movement closely resembles that of the ER. The vacuole, labeled with carboxy-dichlorofluorescein diacetate, consists of thin tubules arranged longitudinally in a reticulate network, which undergoes active motion. In contrast to the mitochondria and ER, the vacuole is located back from the apex, and never extends into the apical clear zone. We have not been able to decipher an oscillatory pattern in vacuole motion. Because this motion is dependent on actin and not tubulin, we think this is due to a different myosin from that which drives the ER and mitochondria.

Endoplasmic reticulum targeted GFP reveals ER organization in tobacco NT-1 cells during cell division

The endoplasmic reticulum (ER) of plant cells undergoes a drastic reorganization during cell division. In tobacco NT-1 cells that stably express a GFP construct targeted to the ER, we have mapped the reorganization of ER that occurs during mitosis and cytokinesis with confocal laser scanning microscopy. During division, the ER and nuclear envelope do not vesiculate. Instead, tubules of ER accumulate around the chromosomes after the nuclear envelope breaks down, with these tubules aligning parallel to the microtubules of the mitotic spindle. In cytokinesis, the phragmoplast is particularly rich in ER, and the transnuclear channels and invaginations present in many interphase cells appear to develop from ER tubules trapped in the developing phragmoplast. Drug studies, using oryzalin and latrunculin to disrupt the microtubules and actin microfilaments, respectively, demonstrate that during division, the arrangement of ER is controlled by microtubules and not by actin, which is the reverse of the situation in interphase cells.

Dynamics and inheritance of the endoplasmic reticulum

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

Endoplasmic reticulum positioning and partitioning in mitotic HeLa cells

Cell division serves to distribute chromosomes and organelles into two daughter cells, but the mechanism of rough endoplasmic reticulum (RER) segregation in animal cell mitosis is poorly understood. Here we study the distribution of RER in mitotic HeLa cells and its relation to the cytoskeleton. At metaphase, the RER was located in the cell cortex and was most concentrated in two locations. Close to the plasma membrane the RER was closely associated with cortical actin, and after treatment with Latrunculin A RER elements retracted to the deep cortex and became more tubular. Positioning was therefore dependent on cortical F-actin. Deeper in the cortex cisternae were wrapped tightly around the contours of the spindle body and orientated along microtubules close the spindle poles. Stereology revealed a close correlation between RER volume and cell volume in telophase daughter cells. These results suggest that the RER is positioned at the outer and inner regions of metaphase cortex by association with cytoskeleton. This arrangement combined with a disposition in concentric layers, deep to the plasma membrane, appears to distribute the RER evenly in the cortex and may help to couple quantities of RER and cell constituents.

Alpha-kinase 1, a new component in apical protein transport

A key aspect in the structure of epithelial cells is the maintenance of a polarized organization based on a highly specific sorting machinery for cargo destined for the apical or the basolateral membrane domain at the exit site of the trans-Golgi network. We could recently identify two distinct post-trans-Golgi network vesicle populations that travel along separate routes to the plasma membrane, a lipid raft-dependent and a lipid raft-independent pathway. A new component of raft-carrying apical vesicles is alpha-kinase 1 (ALPK1), which was identified in immunoisolated vesicles carrying raft-associated sucrase-isomaltase (SI). This kinase was absent from vesicles carrying raft-non-associated lactase-phlorizin hydrolase. The expression of ALPK1 increases by the time of epithelial cell differentiation, whereas the intracellular localization of ALPK1 on apical transport vesicles was confirmed by confocal analysis. A phosphorylation assay on isolated SI-carrying vesicles revealed the phosphorylation of a protein band of about 105 kDa, which could be identified as the motor protein myosin I. Finally, a specific reduction of ALPK1-expression by RNA interference results in a significant decrease in the apical delivery of SI. Taken together, our data suggest that the phosphorylation of myosin I by ALPK1 is an essential process in the apical trafficking of raft-associated SI.

Rab GTPases and myosin motors in organelle motility

The actin cytoskeleton is essential to ensure the proper location of, and communication between, intracellular organelles. Some actin-based myosin motors have been implicated in this process, particularly members of the class V myosins. We discuss here the emerging role of the Ras-like GTPases of the Rab family as regulators of myosin function in organelle transport. Evidence from yeast secretory vesicles and mitochondria, and mammalian melanosomes and endosomes suggests that Rab GTPases are crucial components of the myosin organelle receptor machinery. Better understood is the case of the melanosome where Rab27a recruits a specific effector called melanophilin, which in turn binds myosin Va. The presence of a linker protein between a Rab and a myosin may represent a general mechanism. We argue that Rabs are ideally suited to perform this role as they are exquisite organelle markers. Furthermore, the molecular switch property of Rabs may enable them to regulate the timing of the myosin association with the target organelle.

Redox-regulated Turnover of Nrf2 Is Determined by at Least Two Separate Protein Domains, the Redox-sensitive Neh2 Degron and the Redox-insensitive Neh6 Degron

The Nrf2 transcription factor is more rapidly turned over in cells grown under homeostatic conditions than in those experiencing oxidative stress. The variable turnover of Nrf2 is accomplished through the use of at least two degrons and its redox-sensitive interaction with the Kelch-repeat protein Keap1. In homeostatic COS1 cells, the Neh2 degron confers on Nrf2 a half-life of less than 10 min. Analyses of deletion mutants of a Gal4(HA)mNeh2 fusion protein and full-length mNrf2 indicate that full redox-sensitive Neh2 destabilizing activity depends upon two separate sequences within this N-terminal domain. The DIDLID element (amino acids 17-32) is indispensable for Neh2 activity and appears necessary to recruit a ubiquitin ligase to the fusion protein. A second motif within Neh2, the ETGE tetrapeptide (amino acids 79-82), allows the redox-sensitive recruitment of Nrf2 to Keap1. This interaction, which occurs only in homeostatic cells, enhances the capacity of the Neh2 degron to direct degradation by functioning downstream of ubiquitination mediated by the DIDLID element. By contrast with the situation under homeostatic conditions, the Neh2 degron is neither necessary nor sufficient to account for the characteristic half-life of Nrf2 in oxidatively stressed cells. Instead, the previously uncharacterized, redox-insensitive Neh6 degron (amino acids 329-379) is essential to ensure that the transcription factor is still appropriately turned over in stressed cells, albeit with an increased half-life of 40 min. A model can now be proposed to explain how the turnover of this protein adapts in response to alterations in cellular redox state.

Myosin Va and microtubule-based motors are required for fast axonal retrograde transport of tetanus toxin in motor neurons

Using a novel assay based on the sorting and transport of a fluorescent fragment of tetanus toxin, we have investigated the cytoskeletal and motor requirements of axonal retrograde transport in living mammalian motor neurons. This essential process ensures the movement of neurotrophins and organelles from the periphery to the cell body and is crucial for neuronal survival. Unlike what is observed in sympathetic neurons, fast retrograde transport in motor neurons requires not only intact microtubules, but also actin microfilaments. Here, we show that the movement of tetanus toxin-containing carriers relies on the nonredundant activities of dynein as well as kinesin family members. Quantitative kinetic analysis indicates a role for dynein as the main motor of these carriers. Moreover, this approach suggests the involvement of myosin(s) in retrograde movement. Immunofluorescence screening with isoform-specific myosin antibodies reveals colocalization of tetanus toxin-containing retrograde carriers with myosin Va. Motor neurons from homozygous myosin Va null mice showed slower retrograde transport compared with wild-type cells, establishing a unique role for myosin Va in this process. On the basis of our findings, we propose that coordination of myosin Va and microtubule-dependent motors is required for fast axonal retrograde transport in motor neurons.

Deficiencies in the endoplasmic reticulum (ER)-membrane protein Gab1p perturb transfer of glycosylphosphatidylinositol to proteins and cause perinuclear ER-associated actin bar formation

The essential GAB1 gene, which encodes an endoplasmic reticulum (ER)-membrane protein, was identified in a screen for mutants defective in cellular morphogenesis. A temperature-sensitive gab1 mutant accumulates complete glycosylphosphatidylinositol (GPI) precursors, and its temperature sensitivity is suppressed differentially by overexpression of different subunits of the GPI transamidase, from strong suppression by Gpi8p and Gpi17p, to weak suppression by Gaa1p, and to no suppression by Gpi16p. In addition, both Gab1p and Gpi17p localize to the ER and are in the same protein complex in vivo. These findings suggest that Gab1p is a subunit of the GPI transamidase with distinct relationships to other subunits in the same complex. We also show that depletion of Gab1p or Gpi8p, but not Gpi17p, Gpi16p, or Gaa1p causes accumulation of cofilin-decorated actin bars that are closely associated with the perinuclear ER, which highlights a functional interaction between the ER network and the actin cytoskeleton.

Myo4p and She3p are required for cortical ER inheritance in Saccharomyces cerevisiae

Myo4p is a nonessential type V myosin required for the bud tip localization of ASH1 and IST2 mRNA. These mRNAs associate with Myo4p via the She2p and She3p proteins. She3p is an adaptor protein that links Myo4p to its cargo. She2p binds to ASH1 and IST2 mRNA, while She3p binds to both She2p and Myo4p. Here we show that Myo4p and She3p, but not She2p, are required for the inheritance of cortical ER in the budding yeast Saccharomyces cerevisiae. Consistent with this observation, we find that cortical ER inheritance is independent of mRNA transport. Cortical ER is a dynamic network that forms cytoplasmic tubular connections to the nuclear envelope. ER tubules failed to grow when actin polymerization was blocked with the drug latrunculin A (Lat-A). Additionally, a reduction in the number of cytoplasmic ER tubules was observed in Lat-A–treated and myo4Δ cells. Our results suggest that Myo4p and She3p facilitate the growth and orientation of ER tubules.

Myosin V and the endoplasmic reticulum: the connection grows

In this issue, Estrada et al. (2003) provide new and important insights into how the endoplasmic reticulum (ER) of budding yeast cells is inherited. Together with other studies in plant and animal cells, the results of Estrada et al. (2003) support the idea that myosin V acts as a universal motor for the transport of ER membranes.

Fast anterograde transport of herpes simplex virus: role for the amyloid precursor protein of alzheimer's disease

Anterograde transport of herpes simplex virus (HSV) from its site of synthesis in the neuronal cell body out the neuronal process to the mucosal membrane is crucial for transmission of the virus from one person to another, yet the molecular mechanism is not known. By injecting GFP-labeled HSV into the giant axon of the squid, we reconstitute fast anterograde transport of human HSV and use this as an assay to uncover the underlying molecular mechanism. HSV travels by fast axonal transport at velocities four-fold faster (0.9 microm/sec average, 1.2 microm/sec maximal) than that of mitochondria moving in the same axon (0.2 microm/sec) and ten-fold faster than negatively charged beads (0.08 microm/sec). Transport of HSV utilizes cellular transport mechanisms because it appears to be driven from inside cellular membranes as revealed by negative stain electron microscopy and by the association of TGN46, a component of the cellular secretory pathway, with GFP-labeled viral particles. Finally, we show that amyloid precursor protein (APP), a putative receptor for the microtubule motor, kinesin, is a major component of viral particles, at least as abundant as any viral encoded protein, while another putative motor receptor, JIP 1/2, is not detected. Conventional kinesin is also associated with viral particles. This work links fast anterograde transport of the common pathogen, HSV, with the neurodegenerative Alzheimer's disease. This novel connection should prompt new ideas for treatment and prevention strategies.

Dynamics of the apical plasma membrane recycling system during cell division

The members of the family of Rab11 small GTPases are critical regulators of the plasma membrane vesicle recycling system. While previous studies have determined that the Golgi apparatus disperses during mitosis and reorganizes after cytokinesis, the fate of the recycling system during the cell cycle is more obscure. We have now studied in MDCK cells the fate during mitosis of an apical recycling system cargo, the polymeric IgA receptor (pIgAR), and regulators of the recycling system, Rab11a and its interacting proteins myosin Vb, Rab11-FIP1, Rab11-FIP2 and pp75/Rip11. Rab11a, pIgAR and myosin Vb containing vesicles dispersed into diffuse puncta in the cytosol during prophase and then became clustered near the spindle poles after metaphase, increasing in intensity throughout telophase. A similar pattern was observed for Rab11-FIP1 and Rab11-FIP2. However, Rab11-FIP1 lost colocalization with other recycling system markers during late prophase, relocating to the pericentriolar material. During telophase, Rab11-FIP1 returned to recycling system vesicles. Western blot analysis indicated that both Rab11a and pIgAR remained associated with membrane vesicles throughout the cell cycle. This behavior of the Rab11a-containing apical recycling endosome system during division was distinct from that of the Golgi apparatus. These results indicate that critical components of the apical recycling system remain associated on vesicles throughout the cell cycle and may provide a means for rapid re-establishment of plasma membrane components after mitosis.

Endoplasmic reticulum reorganizations and Ca2+ signaling in maturing and fertilized oocytes of marine protostome worms: the roles of MAPKs and MPF

Before a proper Ca(2+) response is produced at fertilization, oocytes typically undergo a maturation process during which their endoplasmic reticulum (ER) is restructured. In marine protostome worms belonging to the phylum Nemertea, the ER of maturing oocytes forms numerous distinct clusters that are about 5 micro m in diameter. After fertilization, mature oocytes with such aggregates generate a normal series of Ca(2+) oscillations and eventually disassemble their ER clusters at around the time that the oscillations cease. Immature oocytes, however, lack prominent ER clusters and fail to exhibit repetitive Ca(2+) oscillations upon insemination, collectively suggesting that cell cycle-related changes in ER structure may play a role in Ca(2+) signaling. To assess the effects of meiotic regulators on the morphology of the ER and the type of Ca(2+) response that is produced at fertilization, nemertean oocytes were treated with pharmacological modulators of mitogen-activated protein kinases (MAPKs) or maturation-promoting factor (MPF) prior to confocal microscopic analyses. Based on such imaging studies and correlative assays of kinase activities, MAPKs of the ERK1/2 type (extracellular signal regulated kinases 1/2) do not seem to be essential for either structural reorganizations of the ER or repetitive Ca(2+) signaling at fertilization. Conversely, MPF levels appear to modulate both ER structure and the capacity to produce normal Ca(2+) oscillations. The significance of these findings is discussed with respect to other reports on ER structure, MPF cycling and Ca(2+) signaling in oocytes of deuterostome animals.