Transport of macromolecules from the nucleus to the cytoplasm is essential for nearly all cellular and developmental events, and when mis‐regulated, is associated with diseases, tumor formation/growth, and cancer progression. Nuclear Envelope (NE)‐budding is a newly appreciated nuclear export pathway for large macromolecular machineries, including those assembled to allow co‐regulation of functionally related components, that bypasses canonical nuclear export through nuclear pores. In this pathway, large macromolecular complexes are enveloped by the inner nuclear membrane, transverse the perinuclear space, and (...) then exit through the outer nuclear membrane to release its contents into the cytoplasm. NE‐budding is a conserved process and shares many features with nuclear egress mechanisms used by herpesviruses. Despite its biological importance and clinical relevance, little is yet known about the regulatory and structural machineries that allow NE‐budding to occur in any system. Here we summarize what is currently known or proposed for this intriguing nuclear export process. (shrink)

This review explores the hypothesis that regulation of nucleocytoplasmic shuttling is a means of driving differentiation, using spermatogenesis as a model. The transition from undifferentiated spermatogonial stem cell to terminally differentiated spermatozoon is, at its most basic, a change in the repertoire of expressed genes. To effect this, the complement of nuclear proteins, such as transcription factors and chromatin remodelling components must change. Current knowledge of the nuclear proteins and nucleocytoplasmic transport machinery relevant to spermatogenesis is consolidated (...) in this review, and their functional linkages are highlighted not only as a means of regulating nuclear protein composition, but also as a key mechanism regulating gene transcription and hence cell fate. Through this, we hypothesize that male germ cell differentiation is mediated through regulation of nuclear transport machinery components, and thereby of the access of critical factors to the nucleus. The importance of nucleocytoplasmic trafficking to male germ cell differentiation is discussed, using the sex-determining factors Sry and SOX9, cell cycle regulators, CREM and cofactors and the Smads as specific examples, together with the roles in gametogenesis for particular nuclear transport factors in Caenorhabditis elegans and Drosophila. BioEssays 27:1011–1025, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

The cilium/flagellum is a sensory-motile organelle ancestrally present in eukaryotic cells. For assembly cilia universally rely on intraflagellar transport (IFT), a specialised bidirectional transport process mediated by the ancestral and conserved IFT complex. Based on the homology of IFT complex proteins to components of coat protein I (COPI) and clathrin-coated vesicles, we propose that the non- vesicular, membrane-bound IFT evolved as a specialised form of coated vesicle transport from a protocoatomer complex. IFT thus shares common (...) ancestry with all protocoatomer derivatives, including all vesicle coats and the nuclear pore complex (NPC). This has major implications for the evolutionary origin of the cilium. First, it reinforces the tenet that duplication and divergence of pre-existing structures, rather than symbiosis, were the major themes during cilium evolution. Second, it suggests that the initial step in the autogenous origin of the cilium was the establishment of a membrane patch with transmembrane proteins transported by the ancestral vesicle-coating IFT complex. We propose a scenario for how the initial membrane patch gradually protruded to enhance exposure to the environment, then started to move, and finally compartmentalised to render receptor signalling and ciliary beating more efficient. BioEssays 28: 191–198, 2006. © 2006 Wiley periodicals, Inc. (shrink)

Receptor‐mediated endocytosis occurs via clathrin‐coated pits and is therefore coupled to the dynamic cycle of assembly and disassembly of the coat constituents. These coat proteins comprise part, but certainly not all, of the machinery involved in the recognition of membrane receptors and their selective packaging into transport vesicles for internalization. Despite considerable knowledge about the biochemistry of coated vesicles and purified coat proteins, little is known about the mechanisms of coated pit assembly, receptor‐sorting and coated vesicle formation. (...) Cell‐free assays which faithfully reconstitute these events provide powerful new tools with which to elucidate the overall mechanism of receptor‐mediated endocytosis. (shrink)

The Segregation Distorter (SD) system of Drosophila melanogaster is one the best‐characterized meiotic drive complexes known. SD gains an unfair transmission advantage through heterozygous SD/SD+ males by incapacitating SD+‐bearing spermatids so that virtually all progeny inherit SD. Segregation distorter (Sd), the primary distorting locus in the SD complex, is a truncated duplication of the RanGAP gene, a major regulator of the small GTPase Ran, which has several functions including the maintenance of the nucleocytoplasmic RanGTP concentration gradient that mediates nuclear (...) transport. The truncated Sd‐RanGAP protein is enzymatically active but mislocalizes to the nucleus where it somehow causes distortion. Here I present data consistent with the idea that wild‐type RanGAP, and possibly other loci able to influence the RanGTP gradient, has been caught up in an ancient genetic conflict that predates the SD complex. The legacy of this conflict could include the unexpectedly rapid evolution of nuclear transport‐related proteins, the accumulation of chromosomal inversions, the recruitment of gene duplications, and the turnover of repetitive sequences in the centric heterochromatin. BioEssays 29:386–391, 2007. © 2007 Wiley Periodicals, Inc. (shrink)

A fast growing family of ATPases has recently been highlighted. It was named the AAA family, for ATPases Associated to a variety of cellular Activities. The key feature of the family is a highly conserved module of 230 amino acids present in one or two copies in each protein. Despite extensive sequence conservation, the members of the family fulfil a large diversity of cellular functions: cell cycle regulation, gene expression in yeast and HIV, vesicle‐mediated transport, peroxisome assembly, (...) 26S protease function etc. In addition, several members of this family can be found in the same organism (up to 17 in S. cerevisiae). The contrast between functional diversity and structural conservation of the module, from archaebacteria to mammals, suggests that it plays an essential, but as yet unknown, role at key points of the cellular machinery. Two (non‐exclusive) such possibilities are: (1) ATP‐dependent proteasome function and (2) ATP‐dependent anchorage of proteins. Finally, the basic biochemical activity of the AAA module is still a matter of speculation, and we propose that it acts as an ATP‐dependent protein clamp. (shrink)

Endosomes shuttle select cargoes between cellular compartments and, in doing so, maintain intracellular homeostasis and enable interactions with the extracellular space. Directionality of endosomal transport critically impinges on cargo fate, as retrograde (microtubule minus‐end directed) traffic delivers vesicle contents to the lysosome for proteolysis, while the opposing anterograde (plus‐end directed) movement promotes recycling and secretion. Intriguingly, the endoplasmic reticulum (ER) is emerging as a key player in spatiotemporal control of late endosome and lysosome transport, through the establishment (...) of physical contacts with these organelles. Earlier studies have described how minus‐end‐directed motor proteins become discharged from vesicles engaged at such contact sites. Now, Raiborg et al. implicate ER‐mediated interactions, induced by protrudin, in loading plus‐end‐directed motor kinesin‐1 onto endosomes, thereby stimulating their transport toward the cell's periphery. In this review, we recast the prevailing concepts on bidirectional late endosome transport and discuss the emerging paradigm of inter‐compartmental regulation from the ER‐endosome interface viewpoint. (shrink)

The rate‐limiting step in the uptake and metabolism of Dglucose by insulin target cells is thought to be glucose transport mediated by glucose transporters (primarily the GLUT4 isoform) localized to the plasma membrane. However, subcellular fractionation, photolabelling and immunocytochemical studies have shown that the pool of GLUT4 present in the plasma membrane is only one of many subcellular pools of this protein. GLUT4 has been found in occluded vesicles at the plasma membrane, clathrin‐coated pits and vesicles, early endosomes, (...) and tubulo‐vesicular structures; the latter are analogous to known specialized secretory compartments. Tracking the movement of GLUT4 through these compartments, and defining the mechanism and site of action of insulin in stimulating this subcellular trafficking, are major topics of current investigation. Recent evidence focuses attention on the exocytosis of GLUT4 as the major site of insulin action. Increased exocytosis may be due to decreased retention of glucose transporters in an intracellular pool, or possibly to increased assembly of a vesicle docking and fusion complex. Although details are unknown, the presence in GLUT4 vesicles of a synaptobrevin homologue leads us to propose that a process analogous to that occurring in synaptic vesicle trafficking is involved in the assembly of GLUT4 vesicles into a form suitable for fusion with the plasma membrane. Evidence that the pathways of signalling from the insulin receptor and of GLUT4 vesicle exocytosis may converge at the level of the key signalling enzyme, phosphatidylinositol 3‐kinase, is discussed. (shrink)

The cellular machinery responsible for conveying proteins between the endoplasmic reticulum and the Golgi is being investigated using genetics and biochemistry. A role for vesicles in mediating protein traffic between the ER and the Golgi has been established by characterizing yeast mutants defective in this process, and by using recently developed cell‐free assays that measure ER to Golgi transport. These tools have also allowed the identification of several proteins crucial to intracellular protein trafficking. The characterization and possible functions of (...) several GTP‐binding proteins, peripheral membrane proteins, and an integral membrane protein during ER to Golgi transport are discussed here. (shrink)

An entirely different mechanism and localization were recently proposed for the COPII coat complex, challenging its well‐accepted function to select and concentrate cargo into small COPII‐coated spherical transport vesicles. Instead, the COPII complex is suggested to form a dynamic yet stationary collar that forms a boundary between the ER and the ER export membrane domain. This membrane domain, the ER exit site (ERES), is the site of COPII‐mediated sorting and concentration of transport competent proteins. Subsequently, the ERES (...) is implicated to mature and bud to form a sizeable pleiomorphic transport carrier that translocate on microtubules to fuse with the Golgi apparatus. Despite this drastic mechanistic dogma shift, most of the underlying protein‐protein and protein‐membrane interactions remain unchanged. Here, we attempt to provide a detailed description of the newly proposed model of how ER to Golgi transport works by describing the role of several essential proteins of the transport machinery. (shrink)

Ras homolog enriched in the striatum (Rhes) is a striatal enriched protein that promotes the formation of thin membranous tubes resembling tunneling nanotubes (TNT)—“Rhes tunnels”—that connect neighboring cell and transport cargoes: vesicles and proteins between the neuronal cells. Here the literature on TNT‐like structures is reviewed, and the implications of Rhes‐mediated TNT, the mechanisms of its formation, and its potential in novel cell‐to‐cell communication in regulating striatal biology and disease are emphasized. Thought‐provoking ideas regarding how Rhes‐mediated TNT, (...) if it exists, in vivo, would radically change the way neurons communicate in the brain are discussed. (shrink)

Cytolkinesis ensures proper partitioning of the nucleocytoplasmic contents into two daughter cells. It has generally been thought that cytokinesis is accomplished differently in animals and plants because of the differences in the preparatory phases, into the centrosomal or acentrosomal nature of the process, the presence or absence of rigid cell walls, and on the basis of 'outside-in' or 'inside-out' mechanism. However, this long-standing paradigm needs further reevaluation based on new findings. Recent advances reveal that plant cells, similarly to animal (...) cells, possess astral microtubuies that regulate the cell division plane. Furthermore, endocytosis has been found to be important for cytokinesis in animal and plant cells: vesicles containing endocytosed cargo provide material for the cell plate formation in plants and for closure of the midbody channel in animals. Thus, although the preparatory phases of the cell division process differ between plant and animal cells, the later phases show similarities. We unify these findings in a model that suggests a conserved mode of cytokinesis. (shrink)

Cytoplasmic dynein is the major minus‐end‐directed motor protein in eukaryotes, and has functions ranging from organelle and vesicle transport to spindle positioning and orientation. The mode of regulation of dynein in the cell remains elusive, but a tantalising possibility is that dynein is maintained in an inhibited, non‐motile state until bound to cargo. In vivo, stable attachment of dynein to the cell membrane via anchor proteins enables dynein to produce force by pulling on microtubules and serves to organise (...) the nuclear material. Anchor proteins of dynein assume diverse structures and functions and differ in their interaction with the membrane. In yeast, the anchor protein has come to the fore as one of the key mediators of dynein activity. In other systems, much is yet to be discovered about the anchors, but future work in this area will prove invaluable in understanding dynein regulation in the cell. (shrink)

Proteins and RNAs move between the nucleus and cytoplasm by translocation through nuclear pore complexes in the nuclear envelope. To do this, they require specific targeting signals, energy, and a cellular apparatus that catalyzes their transport. Several of the factors involved in nucleocytoplasmic trafficking of proteins have been identified and characterized in some detail. The emerging picture for nuclear transport proposes a central role for the small GTPase Ran and proteins with which it interacts. In particular, asymmetric (...) distribution of these proteins between nucleus and cytoplasm appears to be responsible for the vectorial nature of nucleocytoplasmic transport. Here, we summarize the role of Ran and Ran-binding proteins in nuclear trafficking of proteins with classical nuclear localisation signals. We also discuss examples of the growing number of alternative pathways that are involved in transport of proteins across the nuclear envelope. BioEssays 21:579–589, 1999. © 1999 John Wiley & Sons, Inc. (shrink)

Increasing evidence indicates that extracellular vesicles (EVs) secreted from tumor cells play a key role in the overall progression of the disease state. EVs such as exosomes are secreted by a wide variety of cells and transport a varied population of proteins, lipids, DNA, and RNA species within the body. Gliomas constitute a significant proportion of all primary brain tumors and majority of brain malignancies. Glioblastoma multiforme (GBM) represents grade IV glioma and is associated with very poor prognosis despite (...) the cumulative advances in diagnostic procedures and treatment strategies. Here, the authors describe the progress in understanding the role of EVs, especially exosomes, in overall glioma progression, and how new research is unraveling the utilities of exosomes in glioma diagnostics and development of next‐generation therapeutic systems. Finally, based on an understanding of the latest scientific literature, a model for the possible working of therapeutic exosomes in glioma treatment is proposed. (shrink)

Once regarded as cellular ‘debris’ extracellular vesicles (EVs) emerge as one of the most intriguing entities in cancer pathogenesis. Intercellular trafficking of EVs challenges the notion of cancer cell autonomy, and highlights the multicellular nature of such fundamental processes as stem cell niche formation, tumour stroma generation, angiogenesis, inflammation or immunity. Recent studies reveal that intercellular exchange mediated by EVs runs deeper than expected, and includes molecules causative for cancer progression, such as oncogenes (epidermal growth factor receptor, Ras), and (...) tumour suppressors (PTEN). The uptake of oncogenic EVs (oncosomes) by various cells may profoundly change their biology, signalling patterns and gene expression, and in some cases cause their overt tumorigenic conversion. Moreover, EVs circulating in blood and present in body fluids provide an unprecedented access to the molecular circuitry driving cancer cells, and new technologies are being developed to exploit this property as a source of unique cancer biomarkers. (shrink)

Recently, Gao et al. and Chappie et al. elucidated the crystal structures of the polytetrameric stalk domain of the dynamin‐like virus resistance protein, MxA, and of the G‐domain dimer of the large, membrane‐deforming GTPase, dynamin, respectively. Combined, they provide a hypothetical oligomeric structure for the complete dynamin protein. Here, it is discussed how the oligomers are expected to form and how they participate in dynamin mediated vesicle fission during the process of endocytosis. The proposed oligomeric structure is compared (...) with the novel mechanochemical model of dynamin function recently proposed by Bashkirov et al. and Pucadyil and Schmid. In conclusion, the new model of the dynamin oligomer has the potential to explain how short self‐limiting fissogenic dynamin assemblies are formed and how concerted GTP hydrolysis is achieved. The oligomerisation of two other dynamin superfamily proteins, the guanylate binding proteins (GBPs) and the immunity‐related GTPases (IRGs), is addressed briefly. (shrink)

The Endosomal Sorting Complexes Required for Transport (ESCRTs) drive membrane remodeling in a variety of cellular processes that include the formation of endosomal intralumenal vesicles (ILVs) during multivesicular body (MVB) biogenesis. During MVB sorting, ESCRTs recognize ubiquitin (Ub) attached to membrane protein cargo and execute ILV formation by controlling the activities of ESCRT‐III polymers regulated by the AAA‐ATPase Vps4. Exactly how these events are coordinated to ensure proper cargo loading into ILVs remains unclear. Here we discuss recent work documenting (...) the ability of Bro1, an ESCRT‐associated Ub‐binding protein, to coordinate ESCRT‐III and Vps4‐dependent ILV biogenesis with upstream events such as cargo recognition. (shrink)

Clathrin and the endocytosis machinery has recently been described as being required in mammalian cells for the internalization of large particles including pathogenic bacteria, fungi, and large viruses. These apparently unexpected observations, within the framework of the classical mechanisms for the formation of clathrin‐coated vesicles, are now considered as examples of a new non‐classical function of clathrin, which can promote the internalization of membrane domains associated to planar clathrin lattices. The role of actin downstream of clathrin seems to be critical (...) for this still poorly characterized process. The historical frontier between endocytosis and phagocytosis is vanishing in the light of this new role for clathrin. (shrink)

Recent findings in several organ systems show that cytoneme‐mediated signaling transports signaling proteins along cellular extensions and targets cell‐to‐cell exchanges to synaptic contacts. This mechanism of paracrine signaling may be a general one that is used by many (or all) cell types in many (or all) organs. We briefly review these findings in this perspective. We also describe the properties of several signaling systems that have previously been interpreted to support a passive diffusion mechanism of signaling protein dispersion, but (...) can now be understood in the context of the cytoneme mechanism.Also watch the Video Abstract. (shrink)

The assessment of a technology with such a huge societal impact as the Superphoenix project cannot be anything but a transdisciplinary and political event. This project of course is quite extraordinary, a case of Big Science and High Technology. The Superphoenix project belongs to the travaux publics, more particularly the grands travaux, those necessities of every state’s sovereignty and power. As such they represent greatness and even glory, they breathe a sense of mystery. But from the perspective of public disbelief (...) these cathedrals of modern times breathe nothing but deceit and thievery.The Superphoenix project is very specific. Nevertheless it suits well as an example of how new technologies should not be conceived and implemented: it functions as a bad example. Professor Hériard shows the tragedy of a socalled technology push: the development of a new technology pushed by preoccupied engineers and a single-minded interest group. It is evident that, for an innovation to be successful, should be implemented otherwise. The technology push option is very risky: the innovation threatens to fossilize even before functioning.According to Hériard, instead of the monologue of this technology push, a dialogue should take place: a debate on the various technical, economic, political and cultural dimensions of a technology, a debate in which professionals and the public should participate. The key concept in Hériard’s position is mediation: the necessity of the mediation of the various dimensions, technical and non-technical, within the process of technological innovation. Mediation: in French médiation, in German: Vermittlung.Dialogue and debate are the vehicles that transport this mediation. Dialogue and debate are indeed of great value in the best possible world of technology. The implementation of a new technology and even the conception itself, the technological idea, are of a social character, are societal phenomenona. The mediation of various dimensions however is not and cannot be proposed as a simple answer. This mediation confronts us with the problem of complexity, i.e, the complexity of the technological innovation itself, and the complexity of professional ethics.Hériard demonstrates with his case-study of the Superphoenix project the importance of mediation. I am supposed to criticize Hériard’s statements, but I prefer to stress that with regard to technology, with regard to the way we handle and evaluate technologies, we do indeed have to think in terms of mediation. With my remarks, I intend to elaborate a little bit on this mediation. I will try to give a sort of philosophical explanation of this concept. And perhaps professor Hériard doesn’t agree with the way I understand his intentions. In that case my remarks are perhaps part of a critical reflection after all. (shrink)

It was not until the emergence, in the 19th century, of new technical devices – such as the telegraph and the phonograph – that the term medius came to serve as a collective noun (media) for advanced communication technologies. Although mediation is extensively theorized in philosophy and sociology, and is approached by medium theory and media studies, the concept remains undertheorized in the field of communication theory.By exploring the problem of mediation and by challenging its representationalist and transmissive accounts, this (...) paper posits mediation beyond the medium. It traces mediation as a relational principle which can be best understood as modulation involving inherent translation, inflection and movement. Mediation is not a simple transportation of meaning through a passive intermediary. It is not just something which stands or comes between things otherwise separated or opposed. Mediation is neither a middle-thing linking separate entities, nor do things get mediated: They are already forms of mediation. Mediation is, foremost, a connecting mean that generates modified experiences and relations, a modulating process that re-connects at the moment it separates. (shrink)

Transport through the nuclear pore complex (NPC), a keystone of the eukaryotic building plan, is known to involve a large channel and an abundance of phenylalanine–glycine (FG) protein domains serving as binding sites for soluble nuclear transport receptors and their cargo complexes. However, the conformation of the FG domains in vivo, their arrangement in relation to the transport channel and their function(s) in transport are still vividly debated. Here, we revisit a number of representative transport (...) models—specifically Brownian affinity gating, selective phase gating, reversible FG domain collapse, and reduction of dimensionality (ROD)—in the light of new data obtained by optical single transporter recording, optical superresolution microscopy, artificial nanopores, and many other techniques. The analysis suggests that a properly adapted, simplified version of the ROD model accounts well for the available data. This has implications for nucleocytoplasmic transport in general. (shrink)

Adenosine transport in cultured human umbilical vein endothelial cells (HUVEC) was characterized and shown to be mediated by a single facilitated diffusion mechanism. Initial rates of adenosine influx at 22 degrees C were saturable [apparent Michaelis constant, 69 +/- 10 mu M; maximum velocity (V-max), 600 +/- 70 pmol.10(6) cells(-1).s(-1)] and inhibited by nitrobenzylthioinosine (NBMPR). Formycin B had an unusually high affinity [inhibitory constant K-i), 18 +/- 4.3 mu M], whereas inosine had a low affinity (K-i, 440 +/- (...) 68 mu M) and nucleobases were without effect on adenosine influx. The number of transporters (1.2 x 10(6) sites/cell) was estimated by NBMPR equilibrium binding (apparent dissociation constant, 0.11 +/- 0.01 nM; maximum binding, 2.0 +/- 0.15 pmol/10(6) cells). In addition, we compared these endothelial cells with those obtained from cords from pregnancies complicated by diabetes (HUVEC-D), since embriopathy may occur in these conditions. HUVEC-D exhibited a 2.3-fold reduction in both the V-max for adenosine influx and the maximum number of NBMPR binding sites (260 +/- 40 pmol.10(6) cells(-1).s(-1) and 0.86 +/- 0.08 pmol/10(6) cells, respectively). However, the turnover number for each nucleoside transporter in normal and diabetic HUVEC was similar (approximate to 300 adenosine molecules/s). Adenosine metabolism at 10 mu M in HUVEC-D was modified compared with normal cells. Intracellular phosphorylation (> 90%) was the predominant pathway in normal HUVEC, whereas in HUVEC-D, substantial levels of adenine and adenosine were detected. The present results demonstrate therefore the downregulation of the NBMPR-sensitive nucleoside transporter and changes in adenosine metabolism in HUVEC from diabetic pregnancies. (shrink)

Remote driving, i.e., the capacity of controlling road vehicles at a distance, is an innovative transportation technology often associated with potential ethical benefits, especially when deployed to tackle urban traffic issues. However, prospected benefits could only be reaped if remote driving can be executed in a safe and responsible way. This paper builds on notions elaborated in the philosophical literature on technological mediation to offer a systematic examination of the extent to which current and emerging Human–Machine Interfaces contribute to hindering (...) or supporting the exercise of responsibility behind the remote wheel. More specifically, the analysis discusses how video, audio, and haptic interfaces co-shape the remote driving experience and, at the same time,the operators’ capacity to drive responsibly. The multidisciplinary approach explored in this research offers a novel methodological framework to structure future empirical inquiries while identifying finely tuned multi-sensory HMIs and dedicated training as critical presuppositions to the remote drivers’ exercise of responsibility. (shrink)

When cells are starved of their substrate, many nutrient transporters are induced. These undergo rapid endocytosis and redirection of their intracellular trafficking when their substrate becomes available again. The discovery that some of these transporters also act as receptors, or transceptors, suggests that at least part of the sophisticated controls governing the trafficking of these proteins has to do with their signaling function rather than with control of transport. In yeast, the general amino acid permease Gap1 mediates signaling to (...) the protein kinase A pathway. Its endocytic internalization and intracellular trafficking are subject to amino acid control. Other nutrient transceptors controlling this signal transduction pathway appear to be subject to similar trafficking regulation. Transporters with complex regulatory control have also been suggested to function as transceptors in other organisms. Hence, precise regulation of intracellular trafficking in nutrient transporters may be related to the need for tight control of nutrient‐induced signaling. (shrink)

The tetratricopeptide repeat (TPR) motif is a protein-protein interaction module found in multiple copies in a number of functionally different proteins that facilitates specific interactions with a partner protein(s). Three-dimensional structural data have shown that a TPR motif contains two antiparallel α-helices such that tandem arrays of TPR motifs generate a right-handed helical structure with an amphipathic channel that might accommodate the complementary region of a target protein. Most TPR-containing proteins are associated with multiprotein complexes, and there is extensive evidence (...) indicating that TPR motifs are important to the functioning of chaperone, cell-cycle, transcription, and protein transport complexes. The TPR motif may represent an ancient protein-protein interaction module that has been recruited by different proteins and adapted for specific functions. BioEssays 1999;21:932–939. © 1999 John Wiley & Sons, Inc. (shrink)

Following the implementation of temporary pop-up bike lanes in Berlin, traffic counts by the city administration show an increased number of cyclists. This present paper aims to understand reasons behind this observation. To this end, we focus on the role of mobility-related descriptive social norms as mediators of this effect. Results from one correlational and two experimental online studies are reported. The correlational study confirms the expected association of mobility-related descriptive social norms and self-reported mobility behavior. Moreover, it demonstrates that, (...) as expected, mobility-related descriptive social norms reliably reflect differences in cities’ objective transport structure and mediate the impact of these infrastructural differences on mobility behavior. Results from two online experiments provide additional causal evidence that participants use the visual cues provided by manipulated photos to form their perceived mobility-related descriptive social norms. Furthermore, the second online experiment provides evidence that the combination of infrastructural cues and observable mobility behavior has the strongest impact on perceived mobility-related descriptive social norms. (shrink)

Regulated transport of the plant hormone auxin is central to many aspects of plant development. Directional transport, mediated by membrane transporters, produces patterns of auxin distribution in tissues that trigger developmental processes, such as vascular patterning or leaf formation. Experimentation has produced many, largely qualitative, data providing strong evidence for multiple feedback systems between auxin and its transport. However, the exact mechanisms concerned remain elusive and the experiments required to evaluate alternative hypotheses are challenging. Because of (...) this, computational modelling now plays an important role in auxin transport research. Here we review some current approaches and underlying assumptions of computational auxin transport models. We focus on self‐organising models for polar auxin transport and on recent attempts to unify conflicting mechanistic explanations. In addition, we discuss in general how these computer simulations are proving to be increasingly effective in hypothesis generation and testing, and how simulation can be used to direct future experiments.Editor's suggested further reading in BioEssays Local auxin production: a small contribution to a big field Abstract. (shrink)

As it is essential to explore the influence of social communications on transportation routes in tourism, this article aims to examine the impacts of social communications on transportation routes in the field of tourism and to further explore the relationship between tourism destinations and their psychological perceptions. In terms of links between different tourism destinations in space and time dimensions, our empirical analysis draws the following conclusions: the behavior of tourist flow is a mediating variable on the links between tourist (...) psychological perceptions and tour routes; three modes of point-line interaction are presented in the space and time of tourism destinations; and the scenic city’s location, name, and features are important to tourists’ psychological perceptions. (shrink)

Illness and trauma challenge self-narratives. Traumatized individuals, unable to speak about their experiences, suffer in isolation. In this paper, I explore Kristeva’s theories of the speaking subject and signification, with its symbolic and semiotic modalities, to understand how a person comes to speak the unspeakable. In discussing the origin of the speaking subject, Kristeva employs Plato’s chora (related to choreo , “to make room for”). The chora reflects the mother’s preparation of the child’s entry into language and forms an interior (...) darkroom, the reservoir of lived experience, from which self-narratives issue. Unable to speak of their suffering, traumatized individuals need someone to help them make room for a time of remembrance, someone who is a willing and capable listener. I call such a person a healing witness . Through the mediating presence of the healing witness, fragmented memories of trauma are recreated and incorporated into self-narratives that are sharable with others. Unfortunately, opportunities for witnessing are vanishing. In the last section, I examine the failure of modern media and communication technologies to bear (“hold,” “carry,” “transport”) acts of witnessing. I argue that they perturb the semiotic. According to Kristeva, meaning arises from the dialectical tension between the semiotic (drives and affects) and the symbolic (logic and rules) and is threatened by arid discourse, psychosomatic illnesses, and outbreaks of violence when the semiotic is not represented . Unless we open technology to the imaginary, we risk losing the capacity to bear witness to one another and to create narratives and connections that are meaningful. (shrink)

With the discovery of the phytohormone auxin in the late 1920s, it became possible to link the regulation of complex plant growth responses to a single biologically active compound. Among all the plant growth regulators characterised so far, only auxin appears to be actively transported throughout the plant to create complex variations in concentration patterns and flow directions over time. This stimulated interest in the specific mechanisms underlying auxin transport as key factors in plant growth responses. Research in the (...) last decade revealed several genes involved in the controlled transport of auxin and greatly improved our understanding of the basic principles of auxin‐mediated responses. We are at this point, however, only starting to understand the complex interplay and control of factors that ultimately underlie the observed spatiotemporal variations in auxin transport and thus mediate plant growth and environmental responses. This review highlights important findings that provide us with a framework of molecular players and potential regulatory mechanisms that should contribute to the formulation of a comprehensive dynamic model of spatiotemporal auxin distribution. BioEssays 27:1246–1255, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

Extracellular ATP released from necrotic cells in inflamed tissues activates the P2X7 receptor, stimulates the exposure of phosphatidylserine, and causes cell lysis. Recent findings indicated that XK, a paralogue of XKR8 lipid scramblase, forms a complex with VPS13A at the plasma membrane of T cells. Upon engagement by ATP, an unidentified signal(s) from the P2X7 receptor activates the XK‐VPS13A complex to scramble phospholipids, followed by necrotic cell death. P2X7 is expressed highly in CD25+CD4+ T cells but weakly in CD8+ T (...) cells, suggesting a role of this system in the activation of the immune system to prevent infection. On the other hand, a loss‐of‐function mutation in XK or VPS13A causes neuroacanthocytosis, indicating the crucial involvement of XK‐VPS13A‐mediated phospholipid scrambling at plasma membranes in the maintenance of homeostasis in the nervous and red blood cell systems. (shrink)

This article summarizes the study of anion exchange mechanisms in bacteria. Along with defining at least two different families of anion exchange, an examination of such carrier‐mediated antiport reactions has led to techniques that considerably broaden the scope of biochemical methods for examining membrane proteins. Such advances have been exploited to show that anion exchange itself forms the mechanistic base of an entirely new kind of proton pump, one which may shed light on a variety of bacterial events, including (...) methanogenesis. Perhaps most important, the study of exchange provided the final link in a chain of evidence pointing to a structural [rhythm] that seems to characterize membrane carriers. These three issues ‐ a biochemical tool, a new proton pump, and a common structural rhythm ‐ are briefly examined in the context of their origins in the analysis of bacterial anion exchange. (shrink)

Fission of cellular membranes is ubiquitous and essential for life. Complex protein machineries, such as the dynamin and ESCRT spirals, have evolved to mediate membrane fission during diverse cellular processes, for example, vesicle budding. A new study suggests that non-specialized membrane-bound proteins can induce membrane fission through mass action due to protein crowding. Because up to 2/3 of the mass of cellular membranes is contributed by proteins, membrane protein crowding is an important physiological parameter. Considering the complexity of membrane (...) shape transitions during a fission reaction, spatial and temporal variability in protein distribution, and the abundance of intrinsically disordered regions in proteins on an invaginating membrane, protein crowding can have diverse consequences for fission in the cell. The question is, how and to what extent this mechanism combines with the action of dedicated fission machineries. Membrane fission, which is required for formation of vesicles or cytokinesis, can be carried out by specialized proteins like dynamin and the ESCRT complex. Recent work suggests that fission of cellular membranes, which are densely packed with proteins, can also be induced by protein crowding due to high entropic tension. (shrink)

Cytokinesis ensures proper partitioning of the nucleocytoplasmic contents into two daughter cells. It has generally been thought that cytokinesis is accomplished differently in animals and plants because of the differences in the preparatory phases, into the centrosomal or acentrosomal nature of the process, the presence or absence of rigid cell walls, and on the basis of ‘outside‐in’ or ‘inside‐out’ mechanism. However, this long‐standing paradigm needs further reevaluation based on new findings. Recent advances reveal that plant cells, similarly to animal (...) cells, possess astral microtubules that regulate the cell division plane. Furthermore, endocytosis has been found to be important for cytokinesis in animal and plant cells: vesicles containing endocytosed cargo provide material for the cell plate formation in plants and for closure of the midbody channel in animals. Thus, although the preparatory phases of the cell division process differ between plant and animal cells, the later phases show similarities. We unify these findings in a model that suggests a conserved mode of cytokinesis. BioEssays 29:371–381, 2007. © 2007 Wiley Periodicals, Inc. (shrink)

The chromosomes undergo a condensation‐decondensation cycle within the life cycle of mammalian cells. Chromosome condensation is a complex and critical event that is necessary for the equal distribution of genetic material between the two daughter cells. Although chromosome condensation‐decondensation and segregation is mechanistically complex, it proceeds with high fidelity during the eukaryotic cell division cycle. Cell fusion studies have indicated the presence of chromosome condensation factors in mammalian cells during mitosis. If extracts from mitotic cells are injected into immature oocytes (...) of Xenopus laevis, they induce meiotic maturation (i.e. germinal vesicle breakdown and chromosome condensation) within 2–3 hours. Recently, we showed that the maturation‐promoting activity of the mitotic cell extracts is inactivated by certain protein factors present in cells during the G1 period. The activity of the G1 factors coincides with the process of chromosome decondensation that begins at telophase and continues throughout the G1 period. These studies have revealed that the mitotic factors and the G1 factors play a pivotal role in the regulation of condensation and decondensation of chromosomes. Furthermore, our studies strongly suggest that nonhistone protein phosphorylation and dephosphorylation may mediate chromosome condensation and decondensation, respectively. (shrink)

The cellular processes of transport, division and, possibly, early development all involve microtubule‐based motors. Recent work shows that, unexpectedly, many of these cellular functions are carried out by different types of kinesin and kinesin‐related motor proteins. The kinesin proteins are a large and rapidly growing family of microtubule‐motor proteins that share a 340‐amino‐acid motor domain. Phylogenetic analysis of the conserved motor domains groups the kinesin proteins into a number of subfamilies, the members of which exhibit a common molecular organization (...) and related functions. The kinesin proteins that belong to different subfamilies differ in their rates and polarity of movement along microtubules, and probably in the particles/organelles that they transport. The kinesins arose early in eukaryotic evolution and gene duplication has allowed functional specialization to occur, resulting in a surprisingly large number of different classes of these proteins adapted for intracellular transport of vesicles and organelles, and for assembly and force generation in the meiotic and mitotic spindles. (shrink)

In a companion review1 we discussed the data supporting the conclusion that at least two subtypes of spectrin exist in mammalian brain. One form is found in the cell bodies, dendrites, and post‐synaptic terminals of neurons (brain spectrin(240/235E)) and the other subtype is located in the axons and presynaptic terminals (brain spectrin(240/235)). Our recent understanding of brain spectrin subtype localization suggests a possible explanation for a conundrum concerning brain 4.1 localization. Amelin, an immunoreactive analogue of red blood cell (rbc) cytoskeletal (...) protein 4.1, is localized in neuronal cell bodies and dendrites when brain sections are stained with antibody against rbc protein 4.1. However, it has recently been suggested that synapsin I, a neuron‐specific phosphoprotein associated with the cytoplasmic surface of small synaptic vesicles, is related to erythrocyte 4.1. In this review we hypothesize that there are at least two forms of brain 4.1: a cell body/dendritic form (amelin) which is detected with rbc protein 4.1 antibody, and a unique form found exclusively in the presynaptic terminal (synapsin I). The binding of synapsin I to brain spectrin(240/235), and its ability to stimulate the spectrin/F‐actin interaction in a phosphorylation‐dependent manner suggests a model for the regulation of synaptic transmission mediated by the neuronal cytoskeleton. (shrink)

Adipose tissue (AT) inflammation, a hallmark of the metabolic syndrome, is triggered by overburdened adipocytes sending out immune cell recruitment signals during obesity development. An AT immune landscape persistent throughout weight loss and regain constitutes an immune‐obesogenic memory that hinders long‐term weight loss management. Lipid‐associated macrophages (LAMs) are emerging as major players in diseased, inflamed metabolic tissues and may be key contributors to an obesogenic memory in AT. Our previous study found that LAM abundance increases with weight loss via intermittent (...) fasting (IF) in obese mice, which is driven by adipocyte p53 signalling. However, the specific signals causing LAM accumulation in AT under IF remain unknown. In this piece, we hypothesise on a range of adipocyte‐secreted signals that can harbor immune‐attractive features upon fasting/refeeding cycles. We highlight possible mechanisms including cell death signalling, matrikines, and other damage‐associated molecular patterns (DAMPs), as well as adipo(‐cyto)kines, lipid mediators, metabolites, extracellular vesicles, and epigenetic rewiring. Finally, we consider how advances in mechanisms of AT LAM recruitment gleaned from preclinical models might be translatable to long‐term weight management in humans. Thus, we provide vantage points to study signals driving monocyte recruitment, polarisation towards LAMs, and LAM retention, to harness the therapeutic potential of modulating AT LAM levels by impacting the immune‐obesogenic memory in metabolic disease. (shrink)

Lysosomes are the site of degradation of obsolete intracellular material during autophagy and of extracellular macromolecules following endocytosis and phagocytosis. The membrane of lysosomes and late endosomes is enriched in highly glycosylated transmembrane proteins of largely unknown function. Significant progress has been made in recent years towards elucidating the pathways by which these lysosomal membrane proteins are delivered to late endosomes and lysosomes. While some lysosomal membrane proteins follow the constitutive secretory pathway and reach lysosomes indirectly via the cell surface (...) and endocytosis, others exit the trans‐Golgi network in clathrin‐coated vesicles for direct delivery to endosomes and lysosomes. Sorting from the Golgi or the plasma membrane into the endosomal system is mediated by signals encoded by the short cytosolic domain of these proteins. This review will discuss the role of lysosomal membrane proteins in the biogenesis of the late endosomal and lysosomal membranes, with particular emphasis on the structural features and molecular mechanisms underlying the intracellular trafficking of these proteins. (shrink)

The cellular capacity to internalise objects, involving attachment, engulfment and uptake, exists in virtually all organisms. Many uptake reactions are associated with cell signalling. However, the mechanical forces that form endocytotic vesicles are not known. We propose a ‘leverage‐mediated’ uptake mechanism involving lateral cross‐linking processes on the cell surface that can generate the configurational energy to create an inverse curvature of the membrane. BioEssays 26:1344–1350, 2004. © 2004 Wiley Periodicals, Inc.

In circulation, T cells are spherical with selectin enriched dynamic microvilli protruding from the surface. Following extravasation, these microvilli serve another role, continuously surveying their environment for antigen in the form of peptide‐MHC (pMHC) expressed on the surface of antigen presenting cells (APCs). Upon recognition of their cognate pMHC, the microvilli are initially stabilized and then flatten into F‐actin dependent microclusters as the T cell spreads over the APC. Within 1–5 min, clathrin is recruited by the ESCRT‐0 component Hrs to (...) mediate release of T cell receptor (TCR) loaded vesicles directly from the plasma membrane by clathrin and ESCRT‐mediated ectocytosis (CEME). After 5‐10 min, Hrs is displaced by the endocytic clathrin adaptor epsin‐1 to induce clathrin‐mediated trans‐endocytosis (CMTE) of TCR‐pMHC conjugates. Here we discuss some of the functional properties of the clathrin machinery which enables it to control these topologically opposite modes of membrane transfer at the immunological synapse, and how this might be regulated during T cell activation. (shrink)

Sorting of membrane proteins is generally mediated by cytosolic coats, which create a scaffold to form coated buds and vesicles and to selectively concentrate cargo by interacting with cytosolic signals. The classical paradigm is the interaction between clathrin coats and associated adaptor proteins, which cluster receptors with characteristic tyrosine and dileucine motifs during endocytosis. Clathrin in association with different sets of adaptors is found in addition at the trans-Golgi network and endosomes. Sequences similar to internalization signals also direct lysosomal (...) and basolateral sorting, which implicates related clathrin-adaptor coats in the respective sorting pathways. This review concentrates on the recognition of sorting signals by clathrin-associated adaptor proteins, an area of significant recent progress due to new methodological and conceptual approaches. BioEssays 21:558–567, 1999. © 1999 John Wiley & Sons, Inc. (shrink)