The Journal of Cell Biology recent issues

Prostaglandin E2 regulates B cell proliferation through a candidate tumor suppressor, Ptger4

Murn, J., Alibert, O., Wu, N., Tendil, S., Gidrol, X. — 2008-12-29

Cardio(lipin) workout beefs up mitochondrial imports

Leslie, M. — 2008-12-29

DNA, just chill

Leslie, M. — 2008-12-29

Leptin's long-distance call to the pancreas

Leslie, M. — 2008-12-29

Matrix fragments trigger fatal excitement

Leslie, M. — 2008-12-29

Slow down, enzymes at work

Leslie, M. — 2008-12-29

Kai Simons: Membrane master

Sedwick, C. — 2008-12-29

EBs clip CLIPs to growing microtubule ends

Wittmann, T. — 2008-12-29

Proteins that track growing microtubule (MT) ends are important for many aspects of intracellular MT function, but the mechanism by which these +TIPs accumulate at MT ends has been the subject of a long-standing controversy. In this issue, Bieling et al. (Bieling, P., S. Kandels-Lewis, I.A. Telley, J. van Dijk, C. Janke, and T. Surrey. 2008. J. Cell Biol. 183:1223–1233) reconstitute plus end tracking of EB1 and CLIP-170 in vitro, which demonstrates that CLIP-170 plus end tracking is EB1-dependent and that both +TIPs rapidly exchange between a soluble and a plus end–associated pool. This strongly supports the hypothesis that plus end tracking depends on a biochemical property of growing MT ends, and that the characteristic +TIP comets result from the generation of new +TIP binding sites through MT polymerization in combination with the exponential decay of these binding sites.

Nanotubes, exosomes, and nucleic acid-binding peptides provide novel mechanisms of intercellular communication in eukaryotic cells: implications in health and disease

Belting, M., Wittrup, A. — 2008-12-29

The prevailing view that eukaryotic cells are restrained from intercellular exchange of genetic information has been challenged by recent reports on nanotubes, exosomes, apoptotic bodies, and nucleic acid–binding peptides that provide novel pathways for cell–cell communication, with implications in health and disease.

Three-dimensional localization of CENP-A suggests a complex higher order structure of centromeric chromatin

Marshall, O. J., Marshall, A. T., Choo, K.H. A. — 2008-12-29

The histone H3 variant centromere protein A (CENP-A) is central to centromere formation throughout eukaryotes. A long-standing question in centromere biology has been the organization of CENP-A at the centromere and its implications for the structure of centromeric chromatin. In this study, we describe the three-dimensional localization of CENP-A at the inner kinetochore plate through serial-section transmission electron microscopy of human mitotic chromosomes. At the kinetochores of normal centromeres and at a neocentromere, CENP-A occupies a compact domain at the inner kinetochore plate, stretching across two thirds of the length of the constriction but encompassing only one third of the constriction width and height. Within this domain, evidence of substructure is apparent. Combined with previous chromatin immunoprecipitation results (Saffery, R., H. Sumer, S. Hassan, L.H. Wong, J.M. Craig, K. Todokoro, M. Anderson, A. Stafford, and K.H.A. Choo. 2003. Mol. Cell. 12:509–516; Chueh, A.C., L.H. Wong, N. Wong, and K.H.A. Choo. 2005. Hum. Mol. Genet. 14:85–93), our data suggest that centromeric chromatin is arranged in a coiled 30-nm fiber that is itself coiled or folded to form a higher order structure.

PARP-1 ensures regulation of replication fork progression by homologous recombination on damaged DNA

Sugimura, K., Takebayashi, S.-i., Taguchi, H., Takeda, S., Okumura, K. — 2008-12-29

Poly-ADP ribose polymerase 1 (PARP-1) is activated by DNA damage and has been implicated in the repair of single-strand breaks (SSBs). Involvement of PARP-1 in other DNA damage responses remains controversial. In this study, we show that PARP-1 is required for replication fork slowing on damaged DNA. Fork progression in PARP-1^–/– DT40 cells is not slowed down even in the presence of DNA damage induced by the topoisomerase I inhibitor camptothecin (CPT). Mammalian cells treated with a PARP inhibitor or PARP-1–specific small interfering RNAs show similar results. The expression of human PARP-1 restores fork slowing in PARP-1^–/– DT40 cells. PARP-1 affects SSB repair, homologous recombination (HR), and nonhomologous end joining; therefore, we analyzed the effect of CPT on DT40 clones deficient in these pathways. We find that fork slowing is correlated with the proficiency of HR-mediated repair. Our data support the presence of a novel checkpoint pathway in which the initiation of HR but not DNA damage delays the fork progression.

The translocator maintenance protein Tam41 is required for mitochondrial cardiolipin biosynthesis

2008-12-29

The mitochondrial inner membrane contains different translocator systems for the import of presequence-carrying proteins and carrier proteins. The translocator assembly and maintenance protein 41 (Tam41/mitochondrial matrix protein 37) was identified as a new member of the mitochondrial protein translocator systems by its role in maintaining the integrity and activity of the presequence translocase of the inner membrane (TIM23 complex). Here we demonstrate that the assembly of proteins imported by the carrier translocase, TIM22 complex, is even more strongly affected by the lack of Tam41. Moreover, respiratory chain supercomplexes and the inner membrane potential are impaired by lack of Tam41. The phenotype of Tam41-deficient mitochondria thus resembles that of mitochondria lacking cardiolipin. Indeed, we found that Tam41 is required for the biosynthesis of the dimeric phospholipid cardiolipin. The pleiotropic effects of the translocator maintenance protein on preprotein import and respiratory chain can be attributed to its role in biosynthesis of mitochondrial cardiolipin.

CLIP-170 tracks growing microtubule ends by dynamically recognizing composite EB1/tubulin-binding sites

2008-12-29

The microtubule cytoskeleton is crucial for the internal organization of eukaryotic cells. Several microtubule-associated proteins link microtubules to subcellular structures. A subclass of these proteins, the plus end–binding proteins (+TIPs), selectively binds to the growing plus ends of microtubules. Here, we reconstitute a vertebrate plus end tracking system composed of the most prominent +TIPs, end-binding protein 1 (EB1) and CLIP-170, in vitro and dissect their end-tracking mechanism. We find that EB1 autonomously recognizes specific binding sites present at growing microtubule ends. In contrast, CLIP-170 does not end-track by itself but requires EB1. CLIP-170 recognizes and turns over rapidly on composite binding sites constituted by end-accumulated EB1 and tyrosinated -tubulin. In contrast to its fission yeast orthologue Tip1, dynamic end tracking of CLIP-170 does not require the activity of a molecular motor. Our results demonstrate evolutionary diversity of the plus end recognition mechanism of CLIP-170 family members, whereas the autonomous end-tracking mechanism of EB family members is conserved.

The sympathetic tone mediates leptin's inhibition of insulin secretion by modulating osteocalcin bioactivity

2008-12-29

The osteoblast-secreted molecule osteocalcin favors insulin secretion, but how this function is regulated in vivo by extracellular signals is for now unknown. In this study, we show that leptin, which instead inhibits insulin secretion, partly uses the sympathetic nervous system to fulfill this function. Remarkably, for our purpose, an osteoblast-specific ablation of sympathetic signaling results in a leptin-dependent hyperinsulinemia. In osteoblasts, sympathetic tone stimulates expression of Esp, a gene inhibiting the activity of osteocalcin, which is an insulin secretagogue. Accordingly, Esp inactivation doubles hyperinsulinemia and delays glucose intolerance in ob/ob mice, whereas Osteocalcin inactivation halves their hyperinsulinemia. By showing that leptin inhibits insulin secretion by decreasing osteocalcin bioactivity, this study illustrates the importance of the relationship existing between fat and skeleton for the regulation of glucose homeostasis.

Coordinated control of self-renewal and differentiation of neural stem cells by Myc and the p19ARF-p53 pathway

Nagao, M., Campbell, K., Burns, K., Kuan, C.-Y., Trumpp, A., Nakafuku, M. — 2008-12-29

The modes of proliferation and differentiation of neural stem cells (NSCs) are coordinately controlled during development, but the underlying mechanisms remain largely unknown. In this study, we show that the protooncoprotein Myc and the tumor suppressor p19^ARF regulate both NSC self-renewal and their neuronal and glial fate in a developmental stage–dependent manner. Early-stage NSCs have low p19^ARF expression and retain a high self-renewal and neurogenic capacity, whereas late-stage NSCs with higher p19^ARF expression possess a lower self-renewal capacity and predominantly generate glia. Overexpression of Myc or inactivation of p19^ARF reverts the properties of late-stage NSCs to those of early-stage cells. Conversely, inactivation of Myc or forced p19^ARF expression attenuates self-renewal and induces precocious gliogenesis through modulation of the responsiveness to gliogenic signals. These actions of p19^ARF in NSCs are mainly mediated by p53. We propose that opposing actions of Myc and the p19^ARF–p53 pathway have important functions in coordinated developmental control of self-renewal and cell fate choices in NSCs.

UBF levels determine the number of active ribosomal RNA genes in mammals

2008-12-29

In mammals, the mechanisms regulating the number of active copies of the ~200 ribosomal RNA (rRNA) genes transcribed by RNA polymerase I are unclear. We demonstrate that depletion of the transcription factor upstream binding factor (UBF) leads to the stable and reversible methylation-independent silencing of rRNA genes by promoting histone H1–induced assembly of transcriptionally inactive chromatin. Chromatin remodeling is abrogated by the mutation of an extracellular signal-regulated kinase site within the high mobility group box 1 domain of UBF1, which is required for its ability to bend and loop DNA in vitro. Surprisingly, rRNA gene silencing does not reduce net rRNA synthesis as transcription from remaining active genes is increased. We also show that the active rRNA gene pool is not static but decreases during differentiation, correlating with diminished UBF expression. Thus, UBF1 levels regulate active rRNA gene chromatin during growth and differentiation.

The peroxisomal membrane protein import receptor Pex3p is directly transported to peroxisomes by a novel Pex19p- and Pex16p-dependent pathway

Matsuzaki, T., Fujiki, Y. — 2008-12-29

Two distinct pathways have recently been proposed for the import of peroxisomal membrane proteins (PMPs): a Pex19p- and Pex3p-dependent class I pathway and a Pex19p- and Pex3p-independent class II pathway. We show here that Pex19p plays an essential role as the chaperone for full-length Pex3p in the cytosol. Pex19p forms a soluble complex with newly synthesized Pex3p in the cytosol and directly translocates it to peroxisomes. Knockdown of Pex19p inhibits peroxisomal targeting of newly synthesized full-length Pex3p and results in failure of the peroxisomal localization of Pex3p. Moreover, we demonstrate that Pex16p functions as the Pex3p-docking site and serves as the peroxisomal membrane receptor that is specific to the Pex3p–Pex19p complexes. Based on these novel findings, we suggest a model for the import of PMPs that provides new insights into the molecular mechanisms underlying the biogenesis of peroxisomes and its regulation involving Pex3p, Pex19p, and Pex16p.

The microtubule-binding protein CLIP-170 coordinates mDia1 and actin reorganization during CR3-mediated phagocytosis

2008-12-29

Microtubule dynamics are modulated by regulatory proteins that bind to their plus ends (+TIPs [plus end tracking proteins]), such as cytoplasmic linker protein 170 (CLIP-170) or end-binding protein 1 (EB1). We investigated the role of +TIPs during phagocytosis in macrophages. Using RNA interference and dominant-negative approaches, we show that CLIP-170 is specifically required for efficient phagocytosis triggered by Mβ2 integrin/complement receptor activation. This property is not observed for EB1 and EB3. Accordingly, whereas CLIP-170 is dynamically enriched at the site of phagocytosis, EB1 is not. Furthermore, we observe that CLIP-170 controls the recruitment of the formin mDia1, an actin-nucleating protein, at the onset of phagocytosis and thereby controls actin polymerization events that are essential for phagocytosis. CLIP-170 directly interacts with the formin homology 2 domain of mDia1. The interaction between CLIP-170 and mDia1 is negatively regulated during Mβ2-mediated phagocytosis. Our results unravel a new microtubule/actin cooperation that involves CLIP-170 and mDia1 and that functions downstream of Mβ2 integrins.

Proteolytic fragments of laminin promote excitotoxic neurodegeneration by up-regulation of the KA1 subunit of the kainate receptor

Chen, Z.-L., Yu, H., Yu, W.-M., Pawlak, R., Strickland, S. — 2008-12-29

Degradation of the extracellular matrix (ECM) protein laminin contributes to excitotoxic cell death in the hippocampus, but the mechanism of this effect is unknown. To study this process, we disrupted laminin 1 (lam1) expression in the hippocampus. Lam1 knockout (KO) and control mice had similar basal expression of kainate (KA) receptors, but the lam1 KO mice were resistant to KA-induced neuronal death. After KA injection, KA1 subunit levels increased in control mice but were unchanged in lam1 KO mice. KA1 levels in tissue plasminogen activator (tPA)–KO mice were also unchanged after KA, indicating that both tPA and laminin were necessary for KA1 up-regulation after KA injection. Infusion of plasmin-digested laminin-1 into the hippocampus of lam1 or tPA KO mice restored KA1 up-regulation and KA-induced neuronal degeneration. Interfering with KA1 function with a specific anti-KA1 antibody protected against KA-induced neuronal death both in vitro and in vivo. These results demonstrate a novel pathway for neurodegeneration involving proteolysis of the ECM and KA1 KA receptor subunit up-regulation.

Uniting to save cancer cells

Leslie, M. — 2008-12-29

A false sense of security

Rossner, M. — 2008-12-29

Deriving a germinal center lymphocyte migration model from two-photon data

2008-12-15

An essential role for the MAL protein in targeting Lck to the plasma membrane of human T lymphocytes

2008-12-15

Control of the adaptive response of the heart to stress via the Notch1 receptor pathway

2008-12-15

Multiple axons and actions with PSD-95

Williams, R. — 2008-12-15

Junction protein goes on the road

Williams, R. — 2008-12-15

CK2: channel controller

Williams, R. — 2008-12-15

The actin flow paradox

Williams, R. — 2008-12-15

FHL1 adds some muscle

Short, B. — 2008-12-15

Andrea Ladd: Getting to the heart of alternative splicing control

Williams, R. — 2008-12-15

Announcing the JCB DataViewer, a browser-based application for viewing original image files

Hill, E. — 2008-12-15

Over the lifetime of the JCB, the technology available to cell biologists has advanced immensely, producing more complex data with higher information content. Despite the advent of the Internet, the presentation of data in publications has not kept pace. Authors still present single, static, "representative" images and compressed movie files. I am therefore excited and proud to announce the launch of the JCB DataViewer (http://jcb-dataviewer.rupress.org).

Regulating polarity by directing traffic: Cdc42 prevents adherens junctions from Crumblin' aPart

Duncan, M. C., Peifer, M. — 2008-12-15

The GTPase Cdc42 was among the original genes identified with roles in cell polarity, and interest in its cellular roles from yeast to humans remains high. Cdc42 is a well-known regulator of the actin cytoskeleton, but also plays important roles in vesicular trafficking. In this issue, Harris and Tepass (Harris, K.P, and U. Tepass. 2008. J. Cell. Biol. 183:1129–1143) provide new insights into how Cdc42 and Par proteins work together to modulate cell adhesion and polarity during embryonic morphogenesis by regulating the traffic of key cell junction proteins.

Na+ channels get anchored...with a little help

Rasband, M. N. — 2008-12-15

Neurons have high densities of voltage-gated Na⁺ channels that are restricted to axon initial segments and nodes of Ranvier, where they are responsible for initiating and propagating action potentials. New findings (Bréchet, A., M.-P. Fache, A. Brachet, G. Ferracci, A. Baude, M. Irondelle, S. Pereira, C. Leterrier, and B. Dargent. 2008. J. Cell Biol. 183:1101–1114) reveal that phosphorylation of several key serine residues by the protein kinase CK2 regulates Na⁺ channel interactions with ankyrin G. The presence of CK2 at the axon initial segment and nodes of Ranvier provides a mechanism to regulate the specific accumulation and retention of Na⁺ channels within these important domains.

Assembly of normal actomyosin rings in the absence of Mid1p and cortical nodes in fission yeast

Huang, Y., Yan, H., Balasubramanian, M. K. — 2008-12-15

Cytokinesis in many eukaryotes depends on the function of an actomyosin contractile ring. The mechanisms regulating assembly and positioning of this ring are not fully understood. The fission yeast Schizosaccharomyces pombe divides using an actomyosin ring and is an attractive organism for the study of cytokinesis. Recent studies in S. pombe (Wu, J.Q., V. Sirotkin, D.R. Kovar, M. Lord, C.C. Beltzner, J.R. Kuhn, and T.D. Pollard. 2006. J. Cell Biol. 174:391–402; Vavylonis, D., J.Q. Wu, S. Hao, B. O'Shaughnessy, and T.D. Pollard. 2008. Science. 319:97–100) have suggested that the assembly of the actomyosin ring is initiated from a series of cortical nodes containing several components of this ring. These studies have proposed that actomyosin interactions bring together the cortical nodes to form a compacted ring structure. In this study, we test this model in cells that are unable to assemble cortical nodes. Although the cortical nodes play a role in the timing of ring assembly, we find that they are dispensable for the assembly of orthogonal actomyosin rings. Thus, a mechanism that is independent of cortical nodes is sufficient for the assembly of normal actomyosin rings.

{alpha}-E-catenin binds to dynamitin and regulates dynactin-mediated intracellular traffic

Lien, W.-H., Gelfand, V. I., Vasioukhin, V. — 2008-12-15

–Epithelial catenin (E-catenin) is an important cell–cell adhesion protein. In this study, we show that –E-catenin also regulates intracellular traffic by binding to the dynactin complex component dynamitin. Dynactin-mediated organelle trafficking is increased in –E-catenin^–/– keratinocytes, an effect that is reversed by expression of exogenous –E-catenin. Disruption of adherens junctions in low-calcium media does not affect dynactin-mediated traffic, indicating that –E-catenin regulates traffic independently from its function in cell–cell adhesion. Although neither the integrity of dynactin–dynein complexes nor their association with vesicles is affected by –E-catenin, –E-catenin is necessary for the attenuation of microtubule-dependent trafficking by the actin cytoskeleton. Because the actin-binding domain of –E-catenin is necessary for this regulation, we hypothesize that –E-catenin functions as a dynamic link between the dynactin complex and actin and, thus, integrates the microtubule and actin cytoskeleton during intracellular trafficking.

Traction stress in focal adhesions correlates biphasically with actin retrograde flow speed

Gardel, M. L., Sabass, B., Ji, L., Danuser, G., Schwarz, U. S., Waterman, C. M. — 2008-12-15

How focal adhesions (FAs) convert retrograde filamentous actin (F-actin) flow into traction stress on the extracellular matrix to drive cell migration is unknown. Using combined traction force and fluorescent speckle microscopy, we observed a robust biphasic relationship between F-actin speed and traction force. F-actin speed is inversely related to traction stress near the cell edge where FAs are formed and F-actin motion is rapid. In contrast, larger FAs where the F-actin speed is low are marked by a direct relationship between F-actin speed and traction stress. We found that the biphasic switch is determined by a threshold F-actin speed of 8–10 nm/s, independent of changes in FA protein density, age, stress magnitude, assembly/disassembly status, or subcellular position induced by pleiotropic perturbations to Rho family guanosine triphosphatase signaling and myosin II activity. Thus, F-actin speed is a fundamental regulator of traction force at FAs during cell migration.

CaM kinase II initiates meiotic spindle depolymerization independently of APC/C activation

Reber, S., Over, S., Kronja, I., Gruss, O. J. — 2008-12-15

Altered spindle microtubule dynamics at anaphase onset are the basis for chromosome segregation. In Xenopus laevis egg extracts, increasing free calcium levels and subsequently rising calcium-calmodulin–dependent kinase II (CaMKII) activity promote a release from meiosis II arrest and reentry into anaphase. CaMKII induces the activation of the anaphase-promoting complex/cyclosome (APC/C), which destines securin and cyclin B for degradation to allow chromosome separation and mitotic exit.

In this study, we investigated the calcium-dependent signal responsible for microtubule depolymerization at anaphase onset after release from meiotic arrest in Xenopus egg extracts. Using Ran–guanosine triphosphate–mediated microtubule assemblies and quantitative analysis of complete spindles, we demonstrate that CaMKII triggers anaphase microtubule depolymerization. A CaMKII-induced twofold increase in microtubule catastrophe rates can explain reduced microtubule stability. However, calcium or constitutively active CaMKII promotes microtubule destabilization even upon APC/C inhibition and in the presence of high cyclin-dependent kinase 1 activity. Therefore, our data demonstrate that CaMKII turns on parallel pathways to activate the APC/C and to induce microtubule depolymerization at meiotic anaphase onset.

A handcuff model for the cohesin complex

Zhang, N., Kuznetsov, S. G., Sharan, S. K., Li, K., Rao, P. H., Pati, D. — 2008-12-15

The cohesin complex is responsible for the accurate separation of sister chromatids into two daughter cells. Several models for the cohesin complex have been proposed, but the one-ring embrace model currently predominates the field. However, the static configuration of the embrace model is not flexible enough for cohesins to perform their functions during DNA replication, transcription, and DNA repair. We used coimmunoprecipitation, a protein fragment complement assay, and a yeast two-hybrid assay to analyze the protein–protein interactions among cohesin subunits. The results show that three of the four human cohesin core subunits (Smc1, Smc3, and Rad21) interact with themselves in an Scc3 (SA1/SA2)-dependent manner. These data support a two-ring handcuff model for the cohesin complex, which is flexible enough to establish and maintain sister chromatid cohesion as well as ensure the fidelity of chromosome segregation in higher eukaryotes.

Identification of FHL1 as a regulator of skeletal muscle mass: implications for human myopathy

2008-12-15

Regulators of skeletal muscle mass are of interest, given the morbidity and mortality of muscle atrophy and myopathy. Four-and-a-half LIM protein 1 (FHL1) is mutated in several human myopathies, including reducing-body myopathy (RBM). The normal function of FHL1 in muscle and how it causes myopathy remains unknown. We find that FHL1 transgenic expression in mouse skeletal muscle promotes hypertrophy and an oxidative fiber-type switch, leading to increased whole-body strength and fatigue resistance. Additionally, FHL1 overexpression enhances myoblast fusion, resulting in hypertrophic myotubes in C2C12 cells, (a phenotype rescued by calcineurin inhibition). In FHL1-RBM C2C12 cells, there are no hypertrophic myotubes. FHL1 binds with the calcineurin-regulated transcription factor NFATc1 (nuclear factor of activated T cells, cytoplasmic, calcineurin-dependent 1), enhancing NFATc1 transcriptional activity. Mutant RBM-FHL1 forms aggregate bodies in C2C12 cells, sequestering NFATc1 and resulting in reduced NFAT nuclear translocation and transcriptional activity. NFATc1 also colocalizes with mutant FHL1 to reducing bodies in RBM-afflicted skeletal muscle. Therefore, via NFATc1 signaling regulation, FHL1 appears to modulate muscle mass and strength enhancement.

FoxK mediates TGF-{beta} signalling during midgut differentiation in flies

Casas-Tinto, S., Gomez-Velazquez, M., Granadino, B., Fernandez-Funez, P. — 2008-12-15

Inductive signals across germ layers are important for the development of the endoderm in vertebrates and invertebrates (Tam, P.P., M. Kanai-Azuma, and Y. Kanai. 2003. Curr. Opin. Genet. Dev. 13:393–400; Nakagoshi, H. 2005. Dev. Growth Differ. 47:383–392). In flies, the visceral mesoderm secretes signaling molecules that diffuse into the underlying midgut endoderm, where conserved signaling cascades activate the Hox gene labial, which is important for the differentiation of copper cells (Bienz, M. 1997. Curr. Opin. Genet. Dev. 7:683–688). We present here a Drosophila melanogaster gene of the Fox family of transcription factors, FoxK, that mediates transforming growth factor β (TGF-β) signaling in the embryonic midgut endoderm. FoxK mutant embryos fail to generate midgut constrictions and lack Labial in the endoderm. Our observations suggest that TGF-β signaling directly regulates FoxK through functional Smad/Mad-binding sites, whereas FoxK, in turn, regulates labial expression. We also describe a new cooperative activity of the transcription factors FoxK and Dfos/AP-1 that regulates labial expression in the midgut endoderm. This regulatory activity does not require direct labial activation by the TGF-β effector Mad. Thus, we propose that the combined activity of the TGF-β target genes FoxK and Dfos is critical for the direct activation of lab in the endoderm.

Assembly of the PtdIns 4-kinase Stt4 complex at the plasma membrane requires Ypp1 and Efr3

Baird, D., Stefan, C., Audhya, A., Weys, S., Emr, S. D. — 2008-12-15

The phosphoinositide phosphatidylinositol 4-phosphate (PtdIns4P) is an essential signaling lipid that regulates secretion and polarization of the actin cytoskeleton. In Saccharomyces cerevisiae, the PtdIns 4-kinase Stt4 catalyzes the synthesis of PtdIns4P at the plasma membrane (PM). In this paper, we identify and characterize two novel regulatory components of the Stt4 kinase complex, Ypp1 and Efr3. The essential gene YPP1 encodes a conserved protein that colocalizes with Stt4 at cortical punctate structures and regulates the stability of this lipid kinase. Accordingly, Ypp1 interacts with distinct regions on Stt4 that are necessary for the assembly and recruitment of multiple copies of the kinase into phosphoinositide kinase (PIK) patches. We identify the membrane protein Efr3 as an additional component of Stt4 PIK patches. Efr3 is essential for assembly of both Ypp1 and Stt4 at PIK patches. We conclude that Ypp1 and Efr3 are required for the formation and architecture of Stt4 PIK patches and ultimately PM-based PtdIns4P signaling.

Plasma membrane microdomains regulate turnover of transport proteins in yeast

2008-12-15

In this study, we investigate whether the stable segregation of proteins and lipids within the yeast plasma membrane serves a particular biological function. We show that 21 proteins cluster within or associate with the ergosterol-rich membrane compartment of Can1 (MCC). However, proteins of the endocytic machinery are excluded from MCC. In a screen, we identified 28 genes affecting MCC appearance and found that genes involved in lipid biosynthesis and vesicle transport are significantly overrepresented. Deletion of Pil1, a component of eisosomes, or of Nce102, an integral membrane protein of MCC, results in the dissipation of all MCC markers. These deletion mutants also show accelerated endocytosis of MCC-resident permeases Can1 and Fur4. Our data suggest that release from MCC makes these proteins accessible to the endocytic machinery. Addition of arginine to wild-type cells leads to a similar redistribution and increased turnover of Can1. Thus, MCC represents a protective area within the plasma membrane to control turnover of transport proteins.

Binding interactions control SNARE specificity in vivo

Yang, H.-J., Nakanishi, H., Liu, S., McNew, J. A., Neiman, A. M. — 2008-12-15

Saccharomyces cerevisiae contains two SNAP25 paralogues, Sec9 and Spo20, which mediate vesicle fusion at the plasma membrane and the prospore membrane, respectively. Fusion at the prospore membrane is sensitive to perturbation of the central ionic layer of the SNARE complex. Mutation of the central glutamine of the t-SNARE Sso1 impaired sporulation, but does not affect vegetative growth. Suppression of the sporulation defect of an sso1 mutant requires expression of a chimeric form of Spo20 carrying the SNARE helices of Sec9. Mutation of two residues in one SNARE domain of Spo20 to match those in Sec9 created a form of Spo20 that restores sporulation in the presence of the sso1 mutant and can replace SEC9 in vegetative cells. This mutant form of Spo20 displayed enhanced activity in in vitro fusion assays, as well as tighter binding to Sso1 and Snc2. These results demonstrate that differences within the SNARE helices can discriminate between closely related SNAREs for function in vivo.

Protein kinase CK2 contributes to the organization of sodium channels in axonal membranes by regulating their interactions with ankyrin G

2008-12-15

In neurons, generation and propagation of action potentials requires the precise accumulation of sodium channels at the axonal initial segment (AIS) and in the nodes of Ranvier through ankyrin G scaffolding. We found that the ankyrin-binding motif of Na_v1.2 that determines channel concentration at the AIS depends on a glutamate residue (E1111), but also on several serine residues (S1112, S1124, and S1126). We showed that phosphorylation of these residues by protein kinase CK2 (CK2) regulates Na_v channel interaction with ankyrins. Furthermore, we observed that CK2 is highly enriched at the AIS and the nodes of Ranvier in vivo. An ion channel chimera containing the Na_v1.2 ankyrin-binding motif perturbed endogenous sodium channel accumulation at the AIS, whereas phosphorylation-deficient chimeras did not. Finally, inhibition of CK2 activity reduced sodium channel accumulation at the AIS of neurons. In conclusion, CK2 contributes to sodium channel organization by regulating their interaction with ankyrin G.

PSD-95 promotes synaptogenesis and multiinnervated spine formation through nitric oxide signaling

Nikonenko, I., Boda, B., Steen, S., Knott, G., Welker, E., Muller, D. — 2008-12-15

Postsynaptic density 95 (PSD-95) is an important regulator of synaptic structure and plasticity. However, its contribution to synapse formation and organization remains unclear. Using a combined electron microscopic, genetic, and pharmacological approach, we uncover a new mechanism through which PSD-95 regulates synaptogenesis. We find that PSD-95 overexpression affected spine morphology but also promoted the formation of multiinnervated spines (MISs) contacted by up to seven presynaptic terminals. The formation of multiple contacts was specifically prevented by deletion of the PDZ₂ domain of PSD-95, which interacts with nitric oxide (NO) synthase (NOS). Similarly, PSD-95 overexpression combined with small interfering RNA–mediated down-regulation or the pharmacological blockade of NOS prevented axon differentiation into varicosities and multisynapse formation. Conversely, treatment of hippocampal slices with an NO donor or cyclic guanosine monophosphate analogue induced MISs. NOS blockade also reduced spine and synapse density in developing hippocampal cultures. These results indicate that the postsynaptic site, through an NOS–PSD-95 interaction and NO signaling, promotes synapse formation with nearby axons.

Cdc42 and Par proteins stabilize dynamic adherens junctions in the Drosophila neuroectoderm through regulation of apical endocytosis

Harris, K. P., Tepass, U. — 2008-12-15

Cell rearrangements require dynamic changes in cell–cell contacts to maintain tissue integrity. We investigated the function of Cdc42 in maintaining adherens junctions (AJs) and apical polarity in the Drosophila melanogaster neuroectodermal epithelium. About one third of cells exit the epithelium through ingression and become neuroblasts. Cdc42-compromised embryos lost AJs in the neuroectoderm during neuroblast ingression. In contrast, when neuroblast formation was suppressed, AJs were maintained despite the loss of Cdc42 function. Loss of Cdc42 function caused an increase in the endocytotic uptake of apical proteins, including apical polarity factors such as Crumbs, which are required for AJ stability. In addition, Cdc42 has a second function in regulating endocytotic trafficking, as it is required for the progression of apical cargo from the early to the late endosome. The Par complex acts as an effector for Cdc42 in controlling the endocytosis of apical proteins. This study reveals functional interactions between apical polarity proteins and endocytosis that are critical for stabilizing dynamic basolateral AJs.

The angiogenic response is dictated by {beta}3 integrin on bone marrow-derived cells

2008-12-15

Angiogenesis is dependent on the coordinated action of numerous cell types. A key adhesion molecule expressed by these cells is the _vβ₃ integrin. Here, we show that although this receptor is present on most vascular and blood cells, the key regulatory function in tumor and wound angiogenesis is performed by β₃ integrin on bone marrow–derived cells (BMDCs) recruited to sites of neovascularization. Using knockin mice expressing functionally stunted β₃ integrin, we show that bone marrow transplantation rescues impaired angiogenesis in these mice by normalizing BMDC recruitment. We demonstrate that _vβ₃ integrin enhances BMDC recruitment and retention at angiogenic sites by mediating cellular adhesion and transmigration of BMDCs through the endothelial monolayer but not their release from the bone niche. Thus, β₃ integrin has the potential to control processes such as tumor growth and wound healing by regulating BMDC recruitment to sites undergoing pathological and adaptive angiogenesis.

JAM-L-mediated leukocyte adhesion to endothelial cells is regulated in cis by {alpha}4{beta}1 integrin activation

Luissint, A.-C., Lutz, P. G., Calderwood, D. A., Couraud, P.-O., Bourdoulous, S. — 2008-12-15

Junctional adhesion molecules (JAMs) are endothelial and epithelial adhesion molecules involved in the recruitment of circulating leukocytes to inflammatory sites. We show here that JAM-L, a protein related to the JAM family, is restricted to leukocytes and promotes their adhesion to endothelial cells. Cis dimerization of JAM-L is required to engage in heterophilic interactions with its cognate counter-receptor CAR (coxsackie and adenovirus receptor). Interestingly, JAM-L expressed on neutrophils binds CAR independently of integrin activation. However, on resting monocytes and T lymphocytes, which express the integrin VLA-4, JAM-L molecules engage in complexes with VLA-4 and mainly accumulate in their monomeric form. Integrin activation is required for the dissociation of JAM-L–VLA-4 complexes and the accumulation of functional JAM-L dimers, which indicates that the leukocyte integrin VLA-4 controls JAM-L function in cis by controlling its dimerization state. This provides a mechanism through which VLA-4 and JAM-L functions are coordinately regulated, allowing JAM-L to strengthen integrin-dependent adhesion of leukocytes to endothelial cells.

Quantitative analysis of autophagy-related protein stoichiometry by fluorescence microscopy

Geng, J., Baba, M., Nair, U., Klionsky, D. J. — 2008-12-15

Aire controls the differentiation program of thymic epithelial cells in the medulla for the establishment of self-tolerance

2008-12-01

TIMs: central regulators of immune responses

Hafler, D. A., Kuchroo, V. — 2008-12-01

The Two-Membrane Model of Epithelial Transport:Koefoed-Johnsen and Ussing (1958)

Palmer, L. G., Andersen, O. S. — 2008-12-01

Claudin 11 stops the leaks

Sedwick, C. — 2008-12-01

Parkin cleans house

Sedwick, C. — 2008-12-01

Dictyostelium cells lay a breadcrumb trail

Sedwick, C. — 2008-12-01

Cofilin activity is a total coincidence

Sedwick, C. — 2008-12-01

Dynein gives the "all clear"

Sedwick, C. — 2008-12-01

Brett Lindenbach: Deconstructing and reconstructing hepatitis C

Williams, R. — 2008-12-01

Parkin mitochondria in the autophagosome

McBride, H. M. — 2008-12-01

Narendra et al. (see p. 795 of this issue) have made an exciting new discovery that links the fields of mitochondrial quality control and the genetics of Parkinson's disease (PD). Through an elegant series of high-resolution imaging experiments, they are the first to provide evidence that the PARK2 gene product Parkin is selectively recruited to damaged or uncoupled mitochondria. This recruitment leads to the clearance of the organelles through the autophagosome, demonstrating a primary function for Parkin in the regulation of mitochondrial turnover. This work significantly increases our understanding of PD and provides a new framework for the development of therapeutic interventions.

The Mad2 partial unfolding model: regulating mitosis through Mad2 conformational switching

Skinner, J. J., Wood, S., Shorter, J., Englander, S. W., Black, B. E. — 2008-12-01

The metamorphic Mad2 protein acts as a molecular switch in the checkpoint mechanism that monitors proper chromosome attachment to spindle microtubules during cell division. The remarkably slow spontaneous rate of Mad2 switching between its checkpoint inactive and active forms is catalyzed onto a physiologically relevant time scale by a self–self interaction between its two forms, culminating in a large pool of active Mad2. Recent structural, biochemical, and cell biological advances suggest that the catalyzed conversion of Mad2 requires a major structural rearrangement that transits through a partially unfolded intermediate.

Recruitment of RNA polymerase II cofactor PC4 to DNA damage sites

2008-12-01

The multifunctional nuclear protein positive cofactor 4 (PC4) is involved in various cellular processes including transcription, replication, and chromatin organization. Recently, PC4 has been identified as a suppressor of oxidative mutagenesis in Escherichia coli and Saccharomyces cerevisiae. To investigate a potential role of PC4 in mammalian DNA repair, we used a combination of live cell microscopy, microirradiation, and fluorescence recovery after photobleaching analysis. We found a clear accumulation of endogenous PC4 at DNA damage sites introduced by either chemical agents or laser microirradiation. Using fluorescent fusion proteins and specific mutants, we demonstrated that the rapid recruitment of PC4 to laser-induced DNA damage sites is independent of poly(ADP-ribosyl)ation and H2AX but depends on its single strand binding capacity. Furthermore, PC4 showed a high turnover at DNA damages sites compared with the repair factors replication protein A and proliferating cell nuclear antigen. We propose that PC4 plays a role in the early response to DNA damage by recognizing single-stranded DNA and may thus initiate or facilitate the subsequent steps of DNA repair.

Multiple autophosphorylation sites are dispensable for murine ATM activation in vivo

2008-12-01

Cellular responses to both physiological and pathological DNA double-strand breaks are initiated through activation of the evolutionarily conserved ataxia telangiectasia mutated (ATM) kinase. Upon DNA damage, an activation mechanism involving autophosphorylation has been reported to allow ATM to phosphorylate downstream targets important for cell cycle checkpoints and DNA repair. In humans, serine residues 367, 1893, and 1981 have been shown to be autophosphorylation sites that are individually required for ATM activation. To test the physiological importance of these sites, we generated a transgenic mouse model in which all three conserved ATM serine autophosphorylation sites (S367/1899/1987) have been replaced with alanine. In this study, we show that ATM-dependent responses at both cellular and organismal levels are functional in mice that express a triple serine mutant form of ATM as their sole ATM species. These results lend further support to the notion that ATM autophosphorylation correlates with the DNA damage–induced activation of the kinase but is not required for ATM function in vivo.

Schizosaccharomyces pombe Pak-related protein, Pak1p/Orb2p, phosphorylates myosin regulatory light chain to inhibit cytokinesis

Loo, T.-H., Balasubramanian, M. — 2008-12-01

p21-activated kinases (Paks) have been identified in a variety of eukaryotic cells as key effectors of the Cdc42 family of guanosine triphosphatases. Pak kinases play important roles in regulating the filamentous actin cytoskeleton. In this study, we describe a function for the Schizosaccharomyces pombe Pak-related protein Pak1p/Orb2p in cytokinesis. Pak1p localizes to the actomyosin ring during mitosis and cytokinesis. Loss of Pak1p function leads to accelerated cytokinesis. Pak1p mediates phosphorylation of myosin II regulatory light chain Rlc1p at serine residues 35 and 36 in vivo. Interestingly, loss of Pak1p function or substitution of serine 35 and serine 36 of Rlc1p with alanines, thereby mimicking a dephosphorylated state of Rlc1p, leads to defective coordination of mitosis and cytokinesis. This study reveals a new mechanism involving Pak1p kinase that helps ensure the fidelity of cytokinesis.

Parkin is recruited selectively to impaired mitochondria and promotes their autophagy

Narendra, D., Tanaka, A., Suen, D.-F., Youle, R. J. — 2008-12-01

Loss-of-function mutations in Park2, the gene coding for the ubiquitin ligase Parkin, are a significant cause of early onset Parkinson's disease. Although the role of Parkin in neuron maintenance is unknown, recent work has linked Parkin to the regulation of mitochondria. Its loss is associated with swollen mitochondria and muscle degeneration in Drosophila melanogaster, as well as mitochondrial dysfunction and increased susceptibility to mitochondrial toxins in other species. Here, we show that Parkin is selectively recruited to dysfunctional mitochondria with low membrane potential in mammalian cells. After recruitment, Parkin mediates the engulfment of mitochondria by autophagosomes and the selective elimination of impaired mitochondria. These results show that Parkin promotes autophagy of damaged mitochondria and implicate a failure to eliminate dysfunctional mitochondria in the pathogenesis of Parkinson's disease.

Genome-wide analysis reveals a cell cycle-dependent mechanism controlling centromere propagation

2008-12-01

Centromeres are the structural and functional foundation for kinetochore formation, spindle attachment, and chromosome segregation. In this study, we isolated factors required for centromere propagation using genome-wide RNA interference screening for defects in centromere protein A (CENP-A; centromere identifier [CID]) localization in Drosophila melanogaster. We identified the proteins CAL1 and CENP-C as essential factors for CID assembly at the centromere. CID, CAL1, and CENP-C coimmunoprecipitate and are mutually dependent for centromere localization and function. We also identified the mitotic cyclin A (CYCA) and the anaphase-promoting complex (APC) inhibitor RCA1/Emi1 as regulators of centromere propagation. We show that CYCA is centromere localized and that CYCA and RCA1/Emi1 couple centromere assembly to the cell cycle through regulation of the fizzy-related/CDH1 subunit of the APC. Our findings identify essential components of the epigenetic machinery that ensures proper specification and propagation of the centromere and suggest a mechanism for coordinating centromere inheritance with cell division.

Phosphorylation regulates targeting of cytoplasmic dynein to kinetochores during mitosis

2008-12-01

Cytoplasmic dynein functions at several sites during mitosis; however, the basis of targeting to each site remains unclear. Tandem mass spectrometry analysis of mitotic dynein revealed a phosphorylation site in the dynein intermediate chains (ICs) that mediates binding to kinetochores. IC phosphorylation directs binding to zw10 rather than dynactin, and this interaction is needed for kinetochore dynein localization. Phosphodynein associates with kinetochores from nuclear envelope breakdown to metaphase, but bioriented microtubule (MT) attachment and chromosome alignment induce IC dephosphorylation. IC dephosphorylation stimulates binding to dynactin and poleward streaming. MT depolymerization, release of kinetochore tension, and a PP1- mutant each inhibited IC dephosphorylation, leading to the retention of phosphodynein at kinetochores and reduced poleward streaming. The depletion of kinetochore dynactin by moderate levels of p50(dynamitin) expression disrupted the ability of dynein to remove checkpoint proteins by streaming at metaphase but not other aspects of kinetochore dynein activity. Together, these results suggest a new model for localization of kinetochore dynein and the contribution of kinetochore dynactin.

FAM29A promotes microtubule amplification via recruitment of the NEDD1-{gamma}-tubulin complex to the mitotic spindle

Zhu, H., Coppinger, J. A., Jang, C.-Y., Yates, J. R., Fang, G. — 2008-12-01

Microtubules (MTs) are nucleated from centrosomes and chromatin. In addition, MTs can be generated from preexiting MTs in a -tubulin–dependent manner in yeast, plant, and Drosophila cells, although the underlying mechanism remains unknown. Here we show the spindle-associated protein FAM29A promotes MT-dependent MT amplification and is required for efficient chromosome congression and segregation in mammalian cells. Depletion of FAM29A reduces spindle MT density. FAM29A is not involved in the nucleation of MTs from centrosomes and chromatin, but is required for a subsequent increase in MT mass in cells released from nocodazole. FAM29A interacts with the NEDD1–-tubulin complex and recruits this complex to the spindle, which, in turn, promotes MT polymerization. FAM29A preferentially associates with kinetochore MTs and knockdown of FAM29A reduces the number of MTs in a kinetochore fiber, activates the spindle checkpoint, and delays the mitotic progression. Our study provides a biochemical mechanism for MT-dependent MT amplification and for the maturation of kinetochore fibers in mammalian cells.

CaMKII locally encodes L-type channel activity to signal to nuclear CREB in excitation-transcription coupling

Wheeler, D. G., Barrett, C. F., Groth, R. D., Safa, P., Tsien, R. W. — 2008-12-01

Communication between cell surface proteins and the nucleus is integral to many cellular adaptations. In the case of ion channels in excitable cells, the dynamics of signaling to the nucleus are particularly important because the natural stimulus, surface membrane depolarization, is rapidly pulsatile. To better understand excitation–transcription coupling we characterized the dependence of cAMP response element–binding protein phosphorylation, a critical step in neuronal plasticity, on the level and duration of membrane depolarization. We find that signaling strength is steeply dependent on depolarization, with sensitivity far greater than hitherto recognized. In contrast, graded blockade of the Ca²⁺ channel pore has a remarkably mild effect, although some Ca²⁺ entry is absolutely required. Our data indicate that Ca²⁺/CaM-dependent protein kinase II acting near the channel couples local Ca²⁺ rises to signal transduction, encoding the frequency of Ca²⁺ channel openings rather than integrated Ca²⁺ flux—a form of digital logic.

Cofilin is a pH sensor for actin free barbed end formation: role of phosphoinositide binding

2008-12-01

Newly generated actin free barbed ends at the front of motile cells provide sites for actin filament assembly driving membrane protrusion. Growth factors induce a rapid biphasic increase in actin free barbed ends, and we found both phases absent in fibroblasts lacking H⁺ efflux by the Na-H exchanger NHE1. The first phase is restored by expression of mutant cofilin-H133A but not unphosphorylated cofilin-S3A. Constant pH molecular dynamics simulations and nuclear magnetic resonance (NMR) reveal pH-sensitive structural changes in the cofilin C-terminal filamentous actin binding site dependent on His133. However, cofilin-H133A retains pH-sensitive changes in NMR spectra and severing activity in vitro, which suggests that it has a more complex behavior in cells. Cofilin activity is inhibited by phosphoinositide binding, and we found that phosphoinositide binding is pH-dependent for wild-type cofilin, with decreased binding at a higher pH. In contrast, phosphoinositide binding by cofilin-H133A is attenuated and pH insensitive. These data suggest a molecular mechanism whereby cofilin acts as a pH sensor to mediate a pH-dependent actin filament dynamics.

{micro}2 adaptin facilitates but is not essential for synaptic vesicle recycling in Caenorhabditis elegans

2008-12-01

Synaptic vesicles must be recycled to sustain neurotransmission, in large part via clathrin-mediated endocytosis. Clathrin is recruited to endocytic sites on the plasma membrane by the AP2 adaptor complex. The medium subunit (µ2) of AP2 binds to cargo proteins and phosphatidylinositol-4,5-bisphosphate on the cell surface. Here, we characterize the apm-2 gene (also called dpy-23), which encodes the only µ2 subunit in the nematode Caenorhabditis elegans. APM-2 is highly expressed in the nervous system and is localized to synapses; yet specific loss of APM-2 in neurons does not affect locomotion. In apm-2 mutants, clathrin is mislocalized at synapses, and synaptic vesicle numbers and evoked responses are reduced to 60 and 65%, respectively. Collectively, these data suggest AP2 µ2 facilitates but is not essential for synaptic vesicle recycling.

Synapses are regulated by the cytoplasmic tyrosine kinase Fer in a pathway mediated by p120catenin, Fer, SHP-2, and {beta}-catenin

2008-12-01

Localization of presynaptic components to synaptic sites is critical for hippocampal synapse formation. Cell adhesion–regulated signaling is important for synaptic development and function, but little is known about differentiation of the presynaptic compartment. In this study, we describe a pathway that promotes presynaptic development involving p120catenin (p120ctn), the cytoplasmic tyrosine kinase Fer, the protein phosphatase SHP-2, and β-catenin. Presynaptic Fer depletion prevents localization of active zone constituents and synaptic vesicles and inhibits excitatory synapse formation and synaptic transmission. Depletion of p120ctn or SHP-2 similarly disrupts synaptic vesicle localization with active SHP-2, restoring synapse formation in the absence of Fer. Fer or SHP-2 depletion results in elevated tyrosine phosphorylation of β-catenin. β-Catenin overexpression restores normal synaptic vesicle localization in the absence of Fer or SHP-2. Our results indicate that a presynaptic signaling pathway through p120ctn, Fer, SHP-2, and β-catenin promotes excitatory synapse development and function.

Tight junctions potentiate the insulative properties of small CNS myelinated axons

Devaux, J., Gow, A. — 2008-12-01

Claudin family proteins form the physical barriers of tight junctions (TJs) and regulate paracellular diffusion across polarized epithelia. In addition to these heterotypic TJs, claudin 11 forms autotypic TJs comprising the radial component of central nervous system myelin. The exact function of these TJs has been unclear, although their location at the membrane perimeter is well sited to regulate diffusion between the interstitium and intramyelinic space. In this study, we demonstrate that claudin 11 affords rapid nerve conduction principally for small diameter myelinated axons. Claudin 11–null mice have preserved myelin and axonal architecture, but as much as a 60% decrease in conduction. They also have increased action potential thresholds and activated internodal potassium channels. These data indicate that TJs modulate the biophysical properties of myelin. Computational modeling reveals that claudin 11 reduces current flow through myelin and moderates its capacitive charging. Together, our data shed new light on myelin structural components and our understanding of the biology and pathophysiology of this membrane.

Molecular architecture of inner dynein arms in situ in Chlamydomonas reinhardtii flagella

Bui, K. H., Sakakibara, H., Movassagh, T., Oiwa, K., Ishikawa, T. — 2008-12-01

The inner dynein arm regulates axonemal bending motion in eukaryotes. We used cryo-electron tomography to reconstruct the three-dimensional structure of inner dynein arms from Chlamydomonas reinhardtii. All the eight different heavy chains were identified in one 96-nm periodic repeat, as expected from previous biochemical studies. Based on mutants, we identified the positions of the AAA rings and the N-terminal tails of all the eight heavy chains. The dynein f dimer is located close to the surface of the A-microtubule, whereas the other six heavy chain rings are roughly colinear at a larger distance to form three dyads. Each dyad consists of two heavy chains and has a corresponding radial spoke or a similar feature. In each of the six heavy chains (dynein a, b, c, d, e, and g), the N-terminal tail extends from the distal side of the ring. To interact with the B-microtubule through stalks, the inner-arm dyneins must have either different handedness or, more probably, the opposite orientation of the AAA rings compared with the outer-arm dyneins.

Regulation of EphB2 activation and cell repulsion by feedback control of the MAPK pathway

Poliakov, A., Cotrina, M. L., Pasini, A., Wilkinson, D. G. — 2008-12-01

In this study, we investigated whether the ability of Eph receptor signaling to mediate cell repulsion is antagonized by fibroblast growth factor receptor (FGFR) activation that can promote cell invasion. We find that activation of FGFR1 in EphB2-expressing cells prevents segregation, repulsion, and collapse responses to ephrinB1 ligand. FGFR1 activation leads to increased phosphorylation of unstimulated EphB2, which we show is caused by down-regulation of the leukocyte common antigen–related tyrosine phosphatase receptor that dephosphorylates EphB2. In addition, FGFR1 signaling inhibits further phosphorylation of EphB2 upon stimulation with ephrinB1, and we show that this involves a requirement for the mitogen-activated protein kinase (MAPK) pathway. In the absence of activated FGFR1, EphB2 activates the MAPK pathway, which in turn promotes EphB2 activation in a positive feedback loop. However, after FGFR1 activation, the induction of Sprouty genes inhibits the MAPK pathway downstream of EphB2 and decreases cell repulsion and segregation. These findings reveal a novel feedback loop that promotes EphB2 activation and cell repulsion that is blocked by transcriptional targets of FGFR1.

Collective cell migration requires vesicular trafficking for chemoattractant delivery at the trailing edge

Kriebel, P. W., Barr, V. A., Rericha, E. C., Zhang, G., Parent, C. A. — 2008-12-01

Chemoattractant signaling induces the polarization and directed movement of cells secondary to the activation of multiple effector pathways. In addition, chemotactic signals can be amplified and relayed to proximal cells via the synthesis and secretion of additional chemoattractant. The mechanisms underlying such remarkable features remain ill defined. We show that the asymmetrical distribution of adenylyl cyclase (ACA) at the back of Dictyostelium discoideum cells, an essential determinant of their ability to migrate in a head-to-tail fashion, requires vesicular trafficking. This trafficking results in a local accumulation of ACA-containing intracellular vesicles and involves intact actin, microtubule networks, and de novo protein synthesis. We also show that migrating cells leave behind ACA-containing vesicles, likely secreted as multivesicular bodies and presumably involved in the formation of head-to-tail arrays of migrating cells. We propose that similar compartmentalization and shedding mechanisms exist in mammalian cells during embryogenesis, wound healing, neuron growth, and metastasis.

Tim-3 expression defines a novel population of dysfunctional T cells with highly elevated frequencies in progressive HIV-1 infection

2008-11-17

Tpl2 kinase regulates T cell interferon-g production and host resistance to Toxoplasma gondii

2008-11-17