THE JOURNAL OF http://www.jcb.org/cgi/doi/ The JournalofCell Biology, Vol. 14,20087–11 180,No.1,January © TheRockefellerUniversity Press ulrik.uio.no @iimcb.gov.pl; orHaraldStenmark:stenmark@ CorrespondencetoMartaMiaczynska: miaczynska three kineticallydistinctpopulationsof tion ontheplasmamembrane.Inthisway, track thedynamicsofCCP/CCVforma- quantitative computationalanalysesto Scripps ResearchInstitute)described onds tominutes.SandraSchmid(The reported intheliteraturerangefromsec- has remainedcontroversial, asvalues coated pits(CCPs)andvesicles (CCVs) toxins, viruses,andbacteria. teins, but alsotomediateentryofcertain extracellular moleculesorcellularpro- that thispathway isusednotonlytotraffi nalization was wellillustratedbythefact The plasticityofclathrin-mediatedinter- ture anddynamicsoftheproteinsinvolved. to moleculardetailsregarding thestruc- nisms, withanunprecedentedscaledown novel insightsintoitsunderlying mecha- new technologieskeep providing uswith tion hasbeeninvestigated formany years, Although -mediatedinternaliza- dependent endocytosis Mechanismsofclathrin- neurotransmission. from developmenttoimmunityand multiple aspectsofbiology, ranging revealed howendocytosiscontrols to studyendocytosis.Themeeting approaches andmodelsystemsused land), showcasedthemultifaceted Riezman inVillars-sur-Ollon(Switzer- and Function,organizedbyHoward titled EndocyticSystems:Mechanism ArecentEMBO-FEBSworkshopen- 2 1 Marta Mechanismsandfunctionsofendocytosis Centre for Cancer Biomedicine, Faculty of Medicine, University of Oslo, and Department ofBiochemistry, RadiumHo CentreforCancerBiomedicine,FacultyofMedicine,UniversityOslo,andDepartment theNorwegian Laboratory ofCellBiology, Laboratory InstituteofMolecularandCellBiology, International 02-109Warsaw, Poland The issueoflifetimes ofclathrin-

Miaczynska

1 and 10.1083/jcb.200711073 Harald $30.00

Stenmark c dependent andbecomesevident during in synapticvesicle endocytosis isactivity this proteininvivo. The roleofdynamin-1 would have predictedacrucialrolefor of dynaminmutantsincultured cells ( Ferguson et al.,2007 ), althoughstudies endocytic recycling ofsynapticvesicles to beessentialforthebiogenesisand strated thatdynamin-1appearednot University SchoolofMedicine),demon- reported byPietroDeCamilli(Yale tem. Knockout (KO) studiesinmice, ious tissues,includingthenervous sys- serves somespecializedfunctionsinvar- CCV formation. ment atthetimeofvesicle pinchingand trated attheplaceoftheiroriginalrecruit- while retainingtheadaptorsconcen- and earlyphasesofcoatedpitassembly, they arecapturedduringthenucleation restricted localizationofadaptors,as CCV. This mayresultfromaninitially nonsymmetrically withinanindividual AP-2 adaptorproteinsarealsolocalized fl cells basedontotalinternalrefl Moreover, high-resolution imagingoflive buried insidetheshell( Cheng etal., 2007 ). metrically locatedmembranevesicles to organize aclathrinlattice,withasym- revealed abroadrangeofpatternsused electron tomographyofindividual CCVs hausen (Harvard MedicalSchool).Cryo- CCVs was emphasizedby Tomas Kirch- be regulated bytheactivity ofdynamin. fecting theirlifetimes,whichcaninturn tive, long-lived CCPs/CCVswithoutaf- appears toincreaseanumberofproduc- for over oneminute.Interestingly, cargo long-lived productive populationstable times intherangeofseconds)andone lived (early-andlate-abortive withlife- CCPs couldbedistinguished,two short- uorescence technologyindicates that

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Cahi-eitd endocytosis Clathrin-mediated Morphological heterogeneityof ection interest. Geneticstudiesinyeast have actin andclathrinfunctionsare of special clathrin inendocytosis, proteinsthatlink the actinmachinery. profi copy todefine theprimaryendocytic and theuseofimmuno-electronmicros- pate intheformationofendocytic profi the complex actinstructuresthatpartici- scribed aninvitroassaytoreconstitute de Biolog í a MoleculardeBarcelona)de- endocytosis. Maria-IsabelGeli(Instituto scission duringclathrin/actin-mediated tion, actin-meshwork assembly, and vesicle ate coatformation,membraneinvagina- into fourfunctionalmodulesthatmedi- cating thattheseproteinscanbegrouped cytic sites.Drubinpresenteddataindi- disappearance oftheseproteinsatendo- dynamic appearance,movement, and of California,Berkeley) hasanalyzedthe cytic proteins,David Drubin(University tagged versions ofmorethan40endo- of yeastcellsexpressing fl cytosis. Usingreal-timeimageanalysis functions togetherwithclathrininendo- and akey questionconcernshow actin tial roleforactininendocytosis inyeast, Genetic studieshave revealed anessen- molecular machineriesofendocytosis. has beenvery usefulfordissectingthe tions, theyeast Because oftheeasegeneticmanipula- dependent endocytosis Roleofactininclathrin- to theplasmamembrane. capped byclathrin-coatedpitsthatopen coat components,formingtubular networks apses arefi tridimensional reconstruction.Suchsyn- alized byEMtomographyfollowed by phology ofKO nerve terminalswas visu- strong stimulationofneurons. The mor- les inyeastandthelocalizationof JCB: Given theimportanceofactinand spital, Montebello, N-0310 Oslo, Norway spital, Montebello,N-0310Oslo,Norway lled withclustersofclathrin- MEETINGREVIEW Saccharomyces cerevisiae uorescently les les

JCB 7 indicated that clathrin light chain may regulate the ability of Sla2 to control ac- tin dynamics in endocytosis (Newpher et al., 2006 ). Frances Brodsky (Univer- sity of California, San Francisco) de- scribed a study of Hip1 and Hip1R, the mammalian homologues of Sla2, which have overlapping but not identical func- tions in endocytosis. Brodsky presented evidence that Hip proteins interact se- quentially with clathrin and actin rather than functioning as bridges between the two.

Clathrin-dependent endocytosis and pathogen entry Certain toxins and pathogens harness clathrin-mediated internalization to enter cells. Endocytosis of anthrax toxin, de- scribed by Gisou van der Goot (Federal Polytechnic School of Lausanne), is Figure 1. Distinct endocytic internalization pathways discussed at the conference, with examples of clathrin- and dynamin-mediated but key regulators. Color code: blue, coat proteins; red, kinases; orange, GTPases; green, adaptors; requires also the presence of lipid rafts, a black, other proteins. The various functions of endocytosis are indicated on top. classical hallmark of clathrin-independent entry routes. The protective (PA) However, the clathrin-mediated involved and cargos entering via these subunit of the toxin binds to cell surface pathway appears to be exploited not only mechanisms. This allows us now to re- receptors (TEM8 and CMG2) and induces by viruses, but also by much larger bac- defi ne these pathways more precisely and their multiple post-translational modifi - terial pathogens such as Listeria mono- in positive terms, in contrast to their initial cations, such as palmitoylation, phos- cytogenes, as reported by Pascale Cossart collective description as “ non-clathrin phorylation, and ubiquitination, which (Pasteur Institute of Paris). In a process endocytosis” . Some classifi cation schemes differentially regulate toxin internaliza- induced by the bacterial surface protein have been already proposed ( Mayor and tion (Abrami et al., 2006). Yet another InlB, clathrin and auxilin are recruited Pagano, 2007 ), based primarily on the new player implicated in this process around the entering bacteria, followed dependence on dynamin and various small appears to be the Wnt coreceptor LRP6, by actin polymerization. Nevertheless, GTPases; however, the exact number of which interacts with TEM8 and CMG2 despite certain similarities and common clathrin-independent pathways and their and its depletion results in reduced toxin players involved (dynamin, Eps15, and mutual relations remain unclear. uptake. Anthrax toxin thus provides an the E3 ubiquitin ligase Cbl), this process Satyajit Mayor (National Centre for interesting example of using complex appears mechanistically and kinetically Biological Sciences, Bangalore) reported intracellular endocytic and signaling different from the canonical clathrin- progress toward further characterization mechanisms for precise regulation of its mediated internalization of macro- of a clathrin-, dynamin-, and caveolae- internalization in time and space. molecules. This in turn argues that the independent internalization route that Semliki forest virus (SFV) was one networks of protein– protein and protein– transports GPI-anchored proteins (GPI- of the fi rst viral pathogens identifi ed to lipid interactions involved in clathrin- APs) and involves GEECs (GPI-AP – exploit clathrin-dependent internalization mediated internalization can assemble in enriched early endosomal compartments) mode, as recalled by Ari Helenius (ETH various combinations. This plasticity may as intermediates. This constitutive pino- Zurich) in his plenary lecture, along with be exploited not only by pathogens, but cytic pathway is initiated by a cholesterol- a number of viruses using other pathways also under physiological conditions by dependent recruitment and stabilization (see Fig. 1). Further viruses, such as spe- different types of cells or tissues with of active Cdc42 on the plasma mem- cies C adenovirus type 2 (Ad2) and Ad5 particular needs for specialized forms brane, which leads to localized actin reported by Urs Greber (UZH Zurich), of internalization. polymerization ( Chadda et al., 2007 ). enter cells via clathrin- and dynamin- Interestingly, the activity of Cdc42 is dependent mechanisms but escape the Clathrin-independent controlled by an Arf1-dependent re- classical Rab5-Rab7-EEA1-Hrs pathway endocytosis pathways cruitment of RhoGAP, such as ARH- from early to late endosomes and are re- Once obscure, clathrin-independent in- GAP10. This mechanism represents directed to trans-Golgi compartments in ternalization routes are a focus of intense an interesting example of a cross-talk an Arf1-dependent process. research revealing the molecular players between different small GTPases of

8JCB • VOLUME 180 • NUMBER 1 • 2008 the Ras superfamily in regulation fi ssion in other transport steps, assumes potential upon sealing of the phagocytic of endocytosis. this role also in macropinocytosis. CtBP1/ cup, due mainly to hydrolysis of phospho- Flotillins appear to be essential BARS is locally recruited to the closure inositides ( Yeung et al., 2006 ). This structural components of the internaliza- site of a macropinocytic cup where it is affects binding of several signaling and tion pathway independent of clathrin and phosphorylated by p21-activated kinase-1 regulatory molecules (e.g., K-Ras, Rac1, caveolae, as reported by Ben Nichols (Pak1). This is a key event in the fi s- c-Src) recruited to the plasma membrane (MRC Laboratory of Molecular Biology). sion process in which CtBP1/BARS acts via electrostatic interactions. Internal Flotillin-1 and -2 coassemble on the as a scaffold in a larger complex coupling endocytic compartments have lower nega- plasma membrane into microdomains, membrane rearrangements with cyto- tive surface charge than the plasma which are laterally mobile, distinct from skeletal machinery. membrane, so depending on the strength CCPs or caveolae but bearing certain fea- The morphological features and of the cationic targeting sequences, pro- tures of lipid rafts ( Frick et al., 2007 ). the molecular players involved in the teins can be differentially recruited to the Electron microscopy and live-cell imaging entry of vaccinia virus make it a new addi- plasma membrane or endocytic compart- demonstrated that these microdomains tion to the list of cargo internalized via ments, whereas proteins lacking such can induce membrane invaginations and macropinocytosis, as presented by Ari motifs localize increasingly to the endo- eventually bud off from the plasma mem- Helenius (ETH Zurich). Besides exploit- plasmic reticulum. brane in a process stimulated by over- ing features of regular macropinocytosis expression of fl otillins. The resulting (dependence on actin, Rho GTPases, Systems biology primary endocytic structures contain chol- cholesterol, and Pak1), the virus uses approaches to study era toxin B subunit, but not transferrin. additional specialized mechanisms, involv- endocytosis They appear morphologically different to ing formation of transient blebs on the Current technologies enable global anal- GEECs, although patched GPI-APs partly plasma membrane induced upon virus yses of physiological pathways, and a colocalize with fl otillins. binding and preceding its internalization. few systems biology approaches to Shiga toxin B-subunit (STxB) uses This surprising effect was tracked down study endocytosis in various organisms clathrin-dependent and -independent to the presence of phosphatidylserine were presented. Genome-wide analysis modes of internalization. Ludger Johannes (PS) on the viral particle, as a key factor of endocytic recycling in S. cerevisiae (Institut Curie) reported that STxB can for successful infection. PS externaliza- was reported by Liz Conibear (Univer- induce tubular invaginations, possibly tion is a known hallmark of apoptosis, sity of British Columbia). The screen acting as endocytic internalization inter- resulting in extensive plasma membrane involved measuring an increased or de- mediates. Their formation does not re- blebbing. Thus, this “ apoptotic mimicry ” creased presence of the v-SNARE Snc1p quire clathrin, actin, dynamin, or caveolins, used by the virus causes a transient stim- on the plasma membrane, thus identify- and is also observed on energy-depleted ulation of membrane rearrangements to ing both exocytic and endocytic defects cells. Interestingly, the STxB-induced ensure an effi cient entry. among the mutant collection encom- tubulation could be reproduced in vitro Even within one family of viruses, passing deletions of all nonessential on giant unilamellar vesicles in the ab- different species can use various internal- genes. Genetic interaction analysis of sence of cytosolic protein machinery. ization routes. In contrast to its relatives the top hits established a network of While the exact mechanisms underlying entering via clathrin-dependent mecha- gene clusters involved in various intra- this phenomenon await further character- nisms (see above), the species B adenovirus cellular processes contributing to Snc1p ization, the fi ndings indicate that some type 3 (Ad3) described by Urs Greber traffi cking, containing both known and endocytic internalization events can be (UZH Zurich) induces macropinocytosis novel components. mediated by protein-induced rearrange- to mediate its own entry. This process Endocytic internalization of trans- ments of a lipid bilayer as a driving force requires cell surface receptor CD46 and ferrin (destined for recycling) and epi- of membrane deformation. ␣v integrins, in addition to other proteins dermal growth factor (EGF; directed for Macropinocytosis represents clath- regulating macropino cytosis, including degradation) in mammalian cells has rin-independent internalization of vast CtBP1/BARS. been a target process for a multiparame- areas of plasma membrane and large vol- Although phagocytosis represents ter, genome-wide RNAi screen under- umes of extracellular fl uid that are en- a very distinct and specialized class of taken by Marino Zerial (Max Planck closed in macropinosomes, big ( > 1 ␮ m) endocytic internalization, it shares certain Institute of Molecular Cell Biology and vacuolar structures. Such voluminous mechanisms and machinery with other Genetics) and colleagues. Given the high membrane rearrangements involve actin pathways. Sergio Grinstein (Hospital for frequency of off-target effects by com- ruffl es and are driven by Rac1 and Src Sick Children) reported that lipid re- mercially available siRNA libraries, the kinases. Fission of nascent macropino- modeling and the resulting changes in screen has been performed using at least somes does not involve dynamin, but no membrane surface charge can regulate seven independent siRNA oligonucle- alternative mechanisms have been pro- protein recruitment in phagocytosis, a otides and a mixture of endoribonuclease- posed. As reported by Prisca Liberali mechanism that likely could be extended prepared siRNAs (esiRNA) per gene, (Consorzio Mario Negri Sud), CtBP1/ to endocytosis in general. Novel genetic making this an effort of unprecedented BARS, a protein controlling membrane probes revealed a decrease of surface scale. The initial analysis of the screen

MECHANISMS AND FUNCTIONS OF EARLY ENDOCYTOSIS • MIACZYNSKA AND STENMARK 9 results included over 60 highly quantita- Traffi cking to lysosomes ESCRT-I is required for back-fusion, and tive parameters describing the morphol- Many endocytosed membrane proteins, Alix depletion stimulates this process. ogy, intracellular distribution, and cargo including receptors for growth factors, He proposed that Alix stabilizes a post- content of transferrin- and EGF-bearing cytokines and hormones, are transported fusion, pre-fi ssion intermediate between endosomes. In addition to the identifi ca- to lysosomes for degradation. Conjuga- ILVs and the limiting MVB membrane, tion of novel genes regulating endocytosis, tion with ubiquitin serves as a signal for whereas ESCRT-I controls ILV biogene- the study revealed interesting correla- this pathway, and endosomal ubiquitin- sis directly. tions and general principles pertaining to binding proteins have consequently been Even though the canonical func- the organization of the endocytic path- sought as components of the sorting ma- tion of lysosomes is in degradation of way in mammalian cells. chinery. Through genetic and biochemi- endocytosed and autophagocytosed ma- Another effort based also on siRNA cal studies of yeast mutants defective in terial, these organelles do have additional high-throughput screening technology vacuolar protein sorting, Scott Emr (Cor- functions. Norma Andrews (Yale Uni- was described by Lucas Pelkmans (ETH nell Institute for Cell and Molecular versity School of Medicine) explained Zurich) to determine infectomes, and Biology) has identifi ed three endosomal that lysosomes are the major vesicle type reveal detailed infection pathways for sorting complexes required for transport, responsible for calcium-dependent exo- 15 mammalian viruses. This approach ESCRTs, that mediate degradative sort- cytosis in nonsecretory cells to re-seal allows for identifi cation of critical host ing of ubiquitinated membrane proteins the plasma membrane after injury. Other genes and grouping together viruses us- (Saksena et al., 2007 ). Emr showed evi- cell types contain specialized lysosomes ing similar intracellular components for dence that ESCRT-II may nucleate the that fuse with the plasma membrane establishing infection. However, the effi - formation of ESCRT-III multimers and independently of cell damage. Such ciency of viral infection depends strongly suggested that some ESCRT-III subunits secretory lysosomes are exemplifi ed by on several parameters such as local cell may function as capping proteins that granules in cytotoxic T lymphocytes density, cell and colony size, or number prevent chain elongation. This controlled (CTLs), which contain enzymes that kill of cells. These factors, describing cell multimerization, reversed by Vps4, may target cells, as presented by Gillian Griffi ths population properties of infection, are drive the membrane rearrangements that (University of Cambridge). also quantitatively assessed as a part of underlie MVB biogenesis. In support of a tri-dimensional dataset along with this hypothesis, Phyllis Hanson (Wash- Endocytosis in signaling the corresponding viruses and siRNA ington University, St. Louis) presented and development phenotypes. Secondary screens for endo- a poster showing that overexpressed In addition to its well-described roles in cytosis of various cargo types will ESCRT-III subunits assemble on mem- nutrient uptake and receptor down-regu- complement the infection screens in branes to form curved fi laments that asso- lation, endocytosis plays a direct role order to pinpoint the common machin- ciate into circular arrays. The ESCRTs in the modulation of cell signaling re- ery involved. are evolutionarily conserved, and consis- sponses ( Miaczynska et al., 2004 ). Marcos Presenting a genome-wide RNAi tent with their role in lysosomal receptor Gonzalez-Gait á n (University of Geneva) screen for endocytic regulators in Cae- down-regulation they function as tumor presented studies on Smad anchor for norhabditis elegans, Barth Grant (Rut- suppressors in Drosophila (Hariharan receptor activation, SARA, which links gers University) identifi ed 168 candidate and Bilder, 2006 ). Harald Stenmark Dpp (TGF ␤) receptors with Smad sig- genes. Surprisingly, a group of proteins (University of Oslo) presented evidence naling adaptor proteins and is a central known for their role in embryonic and that these complexes may also protect component of the Dpp signaling pathway epithelial polarity, PAR-3, PAR-6, PKC-3, against neurotoxicity by facilitating auto- in Drosophila. SARA is located to a sub- and CDC-42, were identified in this phagic degradation of toxic protein aggre- set of endosomes that mediate cellular screen. A closer analysis revealed that gates ( Rusten et al., 2007 ). memory of Dpp signaling in wing imagi- ablation of these proteins disrupted the Jean Gruenberg (University of nal discs by distributing evenly between morphology and function of recycling Geneva) showed that back-fusion of intra- the two daughter cells during cell divi- endosomes. Consistent with these data, lumenal vesicles (ILVs) with the limiting sion ( Bokel et al., 2006 ). Using develop- CDC-42 and its mammalian homologue membrane of the MVB is exploited by ment of the sensory organ precursor (SOP) Cdc42 were found on recycling endo- vesicular stomatitis virus (VSV), which as a model for asymmetrical cell division, somes in C. elegans and mammalian enters MVBs through endocytosis and Gonzalez-Gait á n showed that SARA- cells. One possible function of the polar- subsequent traffi cking and releases its containing endosomes accumulate at the ity proteins may be, together with actin, nucleocapsid to the cytosol when VSV- central spindle during cell division and to mediate scission of vesicles or tubular containing ILVs fuse with the limiting then partition asymmetrically into the elements from the recycling endosome. MVB membrane. This appears to hap- signal-receiving daughter cell. Even These fi ndings are consistent with the pen in late MVBs and depends on the though SARA is a mediator of Dpp sig- view that membrane traffi cking, and in late endosome –specifi c lipid LBPA and naling, the SARA-containing endosomes particular endocytic recycling, may con- its effector protein Alix. Gruenberg pre- could well function as vehicles for other tribute to epithelial polarity ( Balklava sented an in vitro assay to study back- signaling pathways, and Gonzalez-Gait á n et al., 2007 ). fusion of ILVs and showed evidence that is currently investigating the possibility

10 JCB • VOLUME 180 • NUMBER 1 • 2008 that asymmetrical Notch-Delta signal- Among clathrin-independent endo- genes in Drosophila . Annu. Rev. Genet. 40 : 335 – 361 . ing, strongly implicated in binary fate cytic mechanisms, an internalization Mayor , S. , and R.E. Pagano . 2007 . Pathways of decisions, could be mediated via SARA- route involving caveolae has been inves- clathrin-independent endocytosis. Nat. Rev. containing endosomes. Another evidence tigated in much detail. Unlike constitu- Mol. Cell Biol. 8 : 603 – 612 . that endocytosis regulates SOP develop- tive clathrin-dependent internalization, Miaczynska , M. , L. Pelkmans , and M. Zerial . 2004 . Not just a sink: endosomes in control of ment was provided by Roland Le Borgne caveolar endocytosis is inducible upon signal transduction. Curr. Opin. Cell Biol. (Université de Rennes), who proposed signaling triggers. As presented by Miguel 16 : 400 – 406 . Minina , S. , M. Reichman-Fried , and E. Raz . that transcytosis of Delta may be re- Del Pozo (Universidad Complutense de 2007 . Control of receptor internalization, quired for Notch-Delta signaling in Madrid), cell adhesion and integrin sig- signaling level, and precise arrival at the target in guided cell migration. Curr. Biol. the SOP. naling play a crucial role in this process, 17 : 1164 – 1172 . Recent studies have shown that re- with global implications for cell polarity, Newpher , T.M. , F.Z. Idrissi , M.I. Geli , and S.K. cycling endosomes contribute membrane motility, and invasiveness. Lemmon . 2006 . Novel function of clathrin light chain in promoting endocytic vesicle to the advancing cleavage furrow during formation. Mol. Biol. Cell . 17 : 4343 – 4352 . the cytokinesis phase of cell division Conclusion Rusten , T.E. , T. Vaccari , K. Lindmo , L.M. Rodahl , (Strickland and Burgess, 2004 ). How In conclusion, signifi cant progress has I.P. Nezis , C. Sem-Jacobsen , F. Wendler, J.P. Vincent , A. Brech , D. Bilder, and H. endosomes are targeted specifi cally to the now been made regarding the mecha- Stenmark . 2007 . ESCRTs and Fab1 regu- cleavage furrow and the midbody is still nisms of endocytosis and post-endocytic late distinct steps of autophagy. Curr. Biol. 17 : 1817 – 1825 . an open question. Philippe Chavrier traffi cking, and their roles in cell signal- Saksena , S. , J. Sun , T. Chu , and S.D. Emr . 2007 . (Institut Curie) presented evidence that a ing, development, and host – pathogen ESCRTing proteins in the endocytic path- novel family of effectors of the endocytic interactions. The recent introduction of way. Trends Biochem. Sci. 32 : 561 – 573 . Strickland , L.I. , and D.R. Burgess . 2004 . Pathways small GTPase Arf6 are involved in the systems biology approaches promises to for membrane traffi cking during cytokine- completion of cytokinesis via control of provide a deeper understanding of how sis. Trends Cell Biol. 14 : 115 – 118 . membrane delivery at the midbody through endocytosis works, and how it serves to Yeung , T. , M. Terebiznik , L. Yu , J. Silvius , W.M. Abidi , M. Philips , T. Levine , A. Kapus , and interactions with microtubule motors. orchestrate biological processes. S. Grinstein . 2006 . Receptor activation alters One interesting function of endo- inner surface potential during phagocytosis. We thank Howard Riezman for having organized Science . 313 : 347 – 351 . cytosis in developing tissues is its involve- an excellent conference. We apologize to those ment in cell migration, and transparent speakers whose interesting talks could not be cited zebrafi sh embryos have been one of the due to space restrictions. favorite models for such studies. Carl- Philipp Heisenberg (Max Planck Institute References of Molecular Cell Biology and Genetics) Abrami , L. , S.H. Leppla , and F.G. van der Goot. showed that mesoderm polarization and 2006 . Receptor palmitoylation and ubiqui- tination regulate anthrax toxin endocytosis. directed migration strongly depend on J. Cell Biol. 172 : 309 – 320 . Rab5-dependent traffi cking and cellu- Balklava , Z. , S. Pant , H. Fares , and B.D. Grant . 2007 . brevin-mediated recycling, indicating Genome-wide analysis identifi es a general requirement for polarity proteins in endo- that the endocytic cycle is required for cytic traffi c. Nat. Cell Biol. 9 : 1066 – 1073 . proper mesoderm cell migration. Endo- Bokel , C. , A. Schwabedissen, E. Entchev , O. cytosis and recycling may be important Renaud , and M. Gonzalez-Gait á n . 2006 . Sara endosomes and the maintenance of for membrane rearrangements during cell Dpp signaling levels across mitosis. Science . migration, but could also play a role in 314 : 1135 – 1139 . Chadda , R. , M.T. Howes , S.J. Plowman , J.F. signal sensing during migration. An Hancock , R.G. Parton , and S. Mayor . 2007 . interesting example of the latter was discus- Cholesterol-sensitive Cdc42 activation regu- lates actin polymerization for endocytosis sed by Erez Raz (University of M ü nster), via the GEEC pathway. Traffi c . 8 : 702 – 717 . who has been studying the involvement Cheng , Y. , W. Boll , T. Kirchhausen , S.C. Harrison , of the chemokine SDF-1a and its receptor and T. Walz . 2007 . Cryo-electron tomogra- phy of clathrin-coated vesicles: structural CXCR4b in germ cell migration in zebra- implications for coat assembly. J. Mol. Biol. fi sh. The receptor is expressed on the 365 : 892 – 899 . surface of the primordial germ cells, which Ferguson , S.M. , G. Brasnjo , M. Hayashi , M. Wolfel , C. Collesi , S. Giovedi , A. Raimondi , L.W. receive directional cues from somatic Gong , P. Ariel , S. Paradise , et al. 2007 . A tissues that secrete SDF-1a. While wild- selective activity-dependent requirement for dynamin 1 in synaptic vesicle endocytosis. type germ cells put on the brakes as they Science . 316 : 570 – 574 . approach their target tissue where the go- Frick , M. , N.A. Bright , K. Riento , A. Bray, nad develops, cells expressing a non- C. Merrifi ed , and B.J. Nichols . 2007 . Coassembly of fl otillins induces formation internalizable CXCR4b mutant fail to of membrane microdomains, membrane down-regulate signaling, which causes curvature, and vesicle budding. Curr. Biol. 17 : 1151 – 1156 . longer runs and less precise targeting Hariharan , I.K. , and D. Bilder. 2006. Regulation of ( Minina et al., 2007 ). imaginal disc growth by tumor-suppressor

MECHANISMS AND FUNCTIONS OF EARLY ENDOCYTOSIS • MIACZYNSKA AND STENMARK 11