Cortactin, an 80/85-kilodalton pp60src substrate, is a filamentous actin-binding protein enriched in the cell cortex

Cortactin regulates podosome formation: roles of the protein interaction domains

Cortactin, a multi-domain scaffolding protein involved in actin polymerization, is enriched in podosomes induced by phorbol ester in vascular smooth muscle cells. We generated several functional and truncation mutants of cortactin to probe the roles of various protein interaction domains in the regulation of the dynamics of podosome formation. At the onset of podosome genesis, cortactin clustered near the ends of stress fibers that appeared to act as nucleation platforms onto which the actin polymerization machinery assembled. Translocation of cortactin to these pre-podosome clusters required the intact N-WASp-binding SH3 domain. Overexpression of the C-terminal third of cortactin containing the intact SH3 domain inhibited podosome formation presumably by sequestering of N-WASp and prevented cortactin clustering. Subsequent assembly of the actin-rich core of podosomes required translocation of additional cortactin to the actin core, a process that required the actin-binding repeats, but not the Arp2/3-binding N-terminal acidic region nor the SH3 domain. These results suggest that the SH3 domain and the actin-binding repeat region are involved, respectively, in the early and late stages of podosome formation process.

Signaling Mechanisms Regulating Endothelial Permeability

The microvascular endothelial cell monolayer localized at the critical interface between the blood and vessel wall has the vital functions of regulating tissue fluid balance and supplying the essential nutrients needed for the survival of the organism. The endothelial cell is an exquisite “sensor” that responds to diverse signals generated in the blood, subendothelium, and interacting cells. The endothelial cell is able to dynamically regulate its paracellular and transcellular pathways for transport of plasma proteins, solutes, and liquid. The semipermeable characteristic of the endothelium (which distinguishes it from the epithelium) is crucial for establishing the transendothelial protein gradient (the colloid osmotic gradient) required for tissue fluid homeostasis. Interendothelial junctions comprise a complex array of proteins in series with the extracellular matrix constituents and serve to limit the transport of albumin and other plasma proteins by the paracellular pathway. This pathway is highly regulated by the activation of specific extrinsic and intrinsic signaling pathways. Recent evidence has also highlighted the importance of the heretofore enigmatic transcellular pathway in mediating albumin transport via transcytosis. Caveolae, the vesicular carriers filled with receptor-bound and unbound free solutes, have been shown to shuttle between the vascular and extravascular spaces depositing their contents outside the cell. This review summarizes and analyzes the recent data from genetic, physiological, cellular, and morphological studies that have addressed the signaling mechanisms involved in the regulation of both the paracellular and transcellular transport pathways.

Critical role of cortactin in actin ring formation and osteoclastic bone resorption

Tyrosine kinase c-Src plays an essential role in ruffled border formation and bone resorption in osteoclasts; however, it is unclear how c-Src controls ruffled border formation during bone resorption. To address this question, we investigated the role of cortactin, a c-Src substrate, in osteoclasts. We found that cortactin showed colocalization with c-Src and actin rings in osteoclasts. Overexpression of cortactin stimulated actin ring formation in RAW 264.7 cells. In contrast, overexpression of Csk inhibited tyrosine phosphorylation of cortactin and binding of cortactin to c-Src. More importantly, overexertion of a mutant cortactin strongly suppressed actin ring formation and bone resorbing activity in osteoclasts. Collectively, our data indicate that cortactin controls osteoclastic bone resorption by regulating actin organization.

Cortactin and phagocytosis in isolated Sertoli cells

Background

Cortactin, an actin binding protein, has been associated with Sertoli cell ectoplasmic specializations in vivo, based on its immunolocalization around the heads of elongated spermatids, but not previously identified in isolated Sertoli cells. In an in vitro model of Sertoli cell-spermatid binding, cortactin was identified around debris and dead germ cells. Based on this observation, we hypothesized that this actin binding protein may be associated with a non-junction-related physiological function, such as phagocytosis. The purpose of this study was to identify the presence and distribution of cortactin in isolated rat Sertoli cells active in phagocytic activity following the addition of 0.8 μm latex beads.

Results

Sertoli cell monocultures were incubated with or without follicle stimulating hormone (FSH; 0.1 μg/ml) in the presence or absence of cytochalasin D (2 μM), as an actin disrupter. Cortactin was identified by standard immunostaining with anti-cortactin, clone 4F11 (Upstate) after incubation times of 15 min, 2 hr, and 24 hr with or without beads. Cells exposed to no hormone and no beads appeared to have a ubiquitous distribution of cortactin throughout the cytoplasm. In the presence of cytochalasin D, cortactin immunostaining was punctate and distributed in a pattern similar to that reported for actin in cells exposed to cytochalasin D. Sertoli cells not exposed to FSH, but activated with beads, did not show cortactin immunostaining around the phagocytized beads at any of the time periods. FSH exposure did not alter the distribution of cortactin within Sertoli cells, even when phagocytic activity was upregulated by the presence of beads.

Conclusion

Results of this study suggest cortactin is not associated with peripheralized actin at junctional or phagocytic sites. Further studies are necessary to clarify the role of cortactin in Sertoli cells.

Cortactin is required for integrin-mediated cell spreading

Cortactin is an SH3 domain-containing protein that contributes to the formation of dynamic cortical actin-associated structures, such as lamellipodia and membrane ruffles. Here we show that expression of either the GFP-tagged N-terminal or the C-teminal halves of cortactin inhibits significantly the spreading of COS7 cells on fibronectin. Introducing inactivating point mutation into the SH3 domain of the C-terminal half of cortactin suspends the dominant negative effect of the construct. In addition, a vector-based RNA interference was used to knock-down endogenous level of cortactin in cells. We demonstrate that cortactin deficient cells were not able to spread. These results suggest that cortactin is required for integrin-mediated signalling pathways.

Bi-directional regulation of postsynaptic cortactin distribution by BDNF and NMDA receptor activity

Abstract Cortactin is an F-actin-associated protein which interacts with the postsynaptic scaffolding protein Shank at the SH3 domain and is localized within the dendritic spine in the mouse neuron. Green fluorescent protein (GFP)-based time-lapse imaging revealed cortactin redistribution from dendritic cytoplasm to postsynaptic sites by application of brain-derived neurotrophic factor (BDNF). This response was mediated by mitogen-activated protein (MAP) kinase activation and was dependent on the C-terminal SH3 domain. In contrast, activation of N-methyl-D-aspartate (NMDA) receptors induced loss of cortactin from postsynaptic sites. This NMDA-dependent redistribution was blocked by an Src family kinase inhibitor. Conversely, increasing Src family kinase activity induced cortactin phosphorylation and loss of cortactin from the postsynaptic sites. Finally, blocking of endogenous BDNF reduced the amount of cortactin at the postsynaptic sites and an NMDA receptor antagonist prevented this reduction. These results indicate the importance of counterbalance between BDNF and NMDA receptor-mediated signalling in the reorganization of the postsynaptic actin cytoskeleton during neuronal development.

Two mink parvoviruses use different cellular receptors for entry into CRFK cells

Mink enteritis virus (MEV) and Aleutian mink disease parvovirus (ADV) are two mink parvoviruses that replicate permissively in Crandell feline kidney (CRFK) cells. We have used this cell model to examine if these two mink parvoviruses use the same cellular receptor. Whereas the cellular receptor for MEV is expected to be the transferrin receptor (TfR), the cellular receptor for ADV has not been clearly identified. We used short hairpin RNAs (shRNAs) produced from plasmids to trigger RNA interference (RNAi), specifically and effectively reducing TfR expression in CRFK cells. TfR expression was reduced to levels undetectable by immunofluorescence in the majority of cells. In viral infection assays, we show that TfR expression was necessary for MEV infection but was not required for ADV infection. Thus, our results demonstrate that TfR is the cellular receptor for MEV, but not the cellular receptor for ADV. The use of two different receptors by MEV and ADV to infect the same cell line is yet another difference between these two parvoviruses that may contribute to their unique pathogenesis in mink.

Dominant expression of 85-kDa form of cortactin in colorectal cancer

PURPOSE

Cortactin is commonly expressed in several human cancers, which may alter their invasive or metastatic properties. Eighty five kilodalton form (p85) and 80-kDa form (p80) of cortactin are two separate bands in SDS-PAGE representing different conformational states. The objective of this study was to investigate cortactin expression in colorectal cancer (CRC).

EXPERIMENTAL DESIGN

Cortactin expression was studied in an eight paired laser capture microdissection (LCM) CRC tissues and matched non-cancerous epithelia by immunoblotting. The expression in 58 CRC and two cell lines, HCT8 and HCT116, was studied respectively by immunohistochemistry and confocal laser scanning immunofluorescence.

RESULTS

Dominant expression of p85 was identified in LCM-procured CRC tissues compared with equal intensity of p85 and p80 forms in non-cancerous tissues, while the amount of total cortactin was approximate. Immunohistochemistry analysis demonstrated that cortactin located in the cytoplasm of tumor cells and adjacent non-cancerous cells, and its expression was negatively correlated with TNM staging and lymphatic invasion status. However, the invasion fronts in 3 of 58 primary tumors and 28 of 39 available lymph node metastases were intensively stained. Further, immunofluorescence analysis showed that cortactin was distributed in cytoplasm and enriched in the front of the extending lamellipodia at adhering side of cultured cancer cells.

CONCLUSIONS

Our results demonstrated the dominant expression of p85 form of cortactin in CRC for the first time. The enrichment of cortactin in the invasion front of some tumor cells and in the extending lamellipodia of cultured cancer cells suggests that cortactin may help cancer cell movement.

Cortactin promotes cell motility by enhancing lamellipodial persistence

BACKGROUND

Lamellipodial protrusion, which is the first step in cell movement, is driven by actin assembly and requires activity of the Arp2/3 actin-nucleating complex. However, it is unclear how actin assembly is dynamically regulated to support effective cell migration.

RESULTS

Cells deficient in cortactin have impaired cell migration and invasion. Kymography analyses of live-cell imaging studies demonstrate that cortactin-knockdown cells have a selective defect in the persistence of lamellipodial protrusions. The motility and protrusion defects are fully rescued by cortactin molecules, provided both the Arp2/3 complex and F-actin binding sites are intact. Consistent with this requirement for simultaneous contacts with Arp2/3 and F-actin, cortactin is recruited by Arp2/3 complex to lamellipodia and binds with a higher affinity to ATP/ADP-Pi-F-actin than to ADP-F-actin. In situ labeling of lamellipodia revealed that the relative levels of free barbed ends of actin filaments are reduced by over 30% in the cortactin-knockdown cells; however, there is no change in Arp2/3-complex localization to lamellipodia. Cortactin-knockdown cells also have a selective defect in the assembly of new adhesions in protrusions, as assessed by analysis of GFP-paxillin dynamics in living cells.

CONCLUSIONS

Cortactin enhances lamellipodial persistence, at least in part through regulation of Arp2/3 complex. The presence of cortactin also enhances the rate of new adhesion formation in lamellipodia. In vivo, these functions may be important during directed cell motility.

Phosphorylation of N-cadherin-associated cortactin by Fer kinase regulates N-cadherin mobility and intercellular adhesion strength

Cortactin regulates the strength of nascent N-cadherin-mediated intercellular adhesions through a tyrosine phosphorylation-dependent mechanism. Currently, the functional significance of cortactin phosphorylation and the kinases responsible for the regulation of adhesion strength are not defined. We show that the nonreceptor tyrosine kinase Fer phosphorylates cadherin-associated cortactin and that this process is involved in mediating intercellular adhesion strength. In wild-type fibroblasts N-cadherin ligation-induced transient phosphorylation of Fer, indicating that junction formation activates Fer kinase. Tyrosine phosphorylation of cortactin after N-cadherin ligation was strongly reduced in fibroblasts expressing only catalytically inactive Fer (D743R), compared with wild-type cells. In wild-type cells, N-cadherin-coated bead pull-off assays induced fourfold greater endogenous N-cadherin association than in D743R cells. Fluorescence recovery after photobleaching showed that GFP-N-cadherin mobility at nascent contacts was 50% faster in wild-type than D743R cells. In shear wash-off assays, nascent intercellular adhesion strength was twofold higher in wild-type than D743R cells. Cortactin recruitment to adhesions was independent of Fer kinase activity, but was impacted by N-cadherin ligation-provoked Rac activation. We conclude that N-cadherin ligation induces Rac-dependent cortactin recruitment and Fer-dependent cortactin phosphorylation, which in turn promotes enhanced mobilization and interaction of surface expressed N-cadherin in contacting cells.

Effects of tyrosine phosphorylation of cortactin on podosome formation in A7r5 vascular smooth muscle cells

Cortactin, a predominant substrate of Src family kinases, plays an important role in Arp2/3-dependent actin polymerization in lamellipodia and membrane ruffles and was recently shown to be enriched in podosomes induced by either c-Src or phorbol ester. However, the mechanisms by which cortactin regulates podosome formation have not been determined. In this study, we showed that cortactin is required for podosome formation, using siRNA knockdown of cortactin expression in smooth muscle A7r5 cells. Treatment with phorbol ester or expression of constitutively active c-Src induced genesis of cortactin-containing podosomes as well as increase in phosphorylation of cortactin at Y421 and Y466, the Src phosphorylation sites on cortactin. The Src kinase inhibitor SU-6656 significantly inhibited formation of podosomes induced by phorbol ester and phosphorylation of cortactin, whereas PKCalpha inhibitor did not affect podosome formation in c-Src-transfected cells. Unexpectedly, expression of cortactin mutants containing Y421F, Y421D, Y466F, or Y466D mutated sites did not affect podosome formation or cortactin translocation to podosomes, although endogenous tyrosine-phosphorylated cortactin at Y421 and Y466 was present in podosomes. Our data indicate that 1) PKCalpha acts upstream of Src in phosphorylation of cortactin and podosome formation in smooth muscle cells; 2) expression of cortactin is essential for genesis of podosomes; 3) phosphorylation at Y421 and Y466 is not required for translocation of cortactin to podosomes, although phosphorylation at these sites appears to be enriched in podosomes; and 4) tyrosine phosphorylation of cortactin may be involved in regulation of stability and turnover of podosomes, rather than targeting this protein to the site of podosome formation.

The coiled-coil domain is required for HS1 to bind to F-actin and activate Arp2/3 complex

HS1 (hematopoietic lineage cell-specific protein 1), a substrate of protein tyrosine kinases in lymphocytes, binds to F-actin, and promotes Arp2/3 complex-mediated actin polymerization. However, the mechanism for the interaction between HS1 and F-actin has not yet been fully characterized. HS1 contains 3.5 tandem repeats, a coiled-coil region, and an SH3 domain at the C terminus. Unlike cortactin, which is closely related to HS1 and requires absolutely the repeat domain for F-actin binding, an HS1 mutant with deletion of the repeat domain maintains a significant F-actin binding activity. On the other hand, deletion of the coiled-coil region abolished the ability of HS1 to bind to actin filaments and to activate the Arp2/3 complex for actin nucleation and actin branching. Furthermore, a peptide containing the coiled-coil sequence only was sufficient for F-actin binding. Within cells overexpressing green fluorescent protein-tagged HS1 proteins, wild type HS1 co-localizes with cortical F-actin at the cell leading edge, whereas mutants with deletion of either the coiled-coil region or the repeat domain diffuse in the cytoplasm. Immunoprecipitation analysis reveals that the coiled-coil deletion mutant binds poorly to F-actin, whereas the mutant without the repeat domain fails to bind to both Arp2/3 complex and F-actin. These data suggest that the HS1 coiled-coil region acts synergistically with the repeat domain in the modulation of the Arp2/3 complex-mediated actin polymerization.

Expression analysis of neuroleukin, calmodulin, cortactin, and Rho7/Rnd2 in the intact and injured mouse brain

Subtracted cDNA libraries from the mouse developing inferior colliculus were previously constructed between postnatal day (P) 6 and 10. In the P10-P6 subtracted library, neuroleukin, calmodulin I, cortactin, and Rho7 were identified. The goal of the present study was to analyze their distribution, at the mRNA and protein levels, in both the adult and the developing mouse brain. The four molecules showed a wide expression throughout the brain, with a neuronal-enriched localization in structures such as the cortex, the hippocampus, the cerebellum, and the inferior colliculus. The level of expression of their corresponding mRNAs increased during brain postnatal development. The expression of these molecules was also investigated 2 weeks after a mechanical lesion in the adult cerebral cortex. Neuroleukin and cortactin were found to be expressed by reactive astrocytes, while there were no changes in the expression of calmodulin and Rho7. The expression of neuroleukin, calmodulin, cortactin, and Rho7 is discussed in the context of their putative role in the maturation of the brain and in the axonal regeneration process.

Bacteria-host-cell interactions at the plasma membrane: stories on actin cytoskeleton subversion

Exploitation of the host-cell actin cytoskeleton is pivotal for many microbial pathogens to enter cells, to disseminate within and between infected tissues, to prevent their uptake by phagocytic cells, or to promote intimate attachment to the cell surface. To accomplish this, these pathogens have evolved common as well as unique strategies to modulate actin dynamics at the plasma membrane, which will be discussed here, exemplified by a number of well-studied bacterial pathogens.

Invasion of endothelial cells by Neisseria meningitidis requires cortactin recruitment by a phosphoinositide-3-kinase/Rac1 signalling pathway triggered by the lipo-oligosaccharide

Type-IV-pilus-mediated adhesion of Neisseria meningitidis (also known as meningococcus) to human endothelial cells induces the formation of membrane protrusions leading to bacterial uptake. We have previously shown that these protrusions result from a Rho- and Cdc42-dependent cortical actin polymerization, and from the activation of the ErbB2 tyrosine-kinase receptor and the Src kinase, leading to tyrosine phosphorylation of cortactin. We report here that N. meningitidis mutants expressing a deglycosylated lipo-oligosaccharide are poorly invasive. These mutants show structurally altered actin polymerization. Moreover, although they efficiently recruit and activate ErbB2 and Src, these mutants are defective in the recruitment and phosphorylation of cortactin. We demonstrate that phosphorylated cortactin controls the cortical actin polymerization, which leads to membrane protrusion formation. In addition, we show that cortactin recruitment is dependent on the activation of a phosphoinositide-3-kinase/Rac1-GTPase signalling pathway, which is required for actin polymerization and internalization of N. meningitidis, and is not activated by the mutant strains. Altogether, these results define a new role for the lipo-oligosaccharide in triggering a phosphoinositide-3-kinase/Rac1 signalling required to elicit an efficient uptake of N. meningitidis in non-phagocytic cells.

Cortactin: an Achilles' heel of the actin cytoskeleton targeted by pathogens

Cortactin is an actin-binding protein and a central regulator of the actin cytoskeleton. Importantly, cortactin is also a common target exploited by microbes during infection. Its involvement in disease development is exemplified by a variety of pathogenic processes, such as pedestal formation [enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC)], invasion (Shigella, Neisseria, Rickettsia, Chlamydia, Staphylococcus and Cryptosporidium), actin-based motility (Listeria, Shigella and vaccinia virus) and cell scattering (Helicobacter). Recent progress turns our attention to how cortactin function can be regulated by serine and tyrosine phosphorylation. This has an important impact on how pathogens abuse cortactin to modulate the architecture of the host actin cytoskeleton.

The subcellular organization of cortactin in hippocampus

Spines may undergo rapid, activity-dependent changes in shape and size, reflecting reorganization of the actin cytoskeleton. This remodeling is implicated in development and also in the late phase of long-term potentiation. However, the cellular mechanisms that convert activity into morphological change remain poorly understood, and little is known about the anatomical distribution of the actin-regulating proteins that mediate this remodeling. Using immunocytochemistry, we demonstrate here that cortactin (a protein implicated in actin filament nucleation, branching, and stabilization) is concentrated in hippocampal spines, where it colocalizes with F-actin. Cortactin has a Shank-binding domain; recent studies report that synaptic activity may trigger actin remodeling via this interaction with Shank. However, our immunogold electron microscopic data show that cortactin concentrates within the spine core, 100-150 nm away from the postsynaptic density (PSD); only a small fraction of the cortactin in spines lies adjacent to the PSD. These data suggest that the adult dendritic spine contains two functional pools of cortactin: a large pool in the spine core that may help to mediates changes in spine shape and a small synaptic pool that may modify the PSD in response to synaptic activity.

Actin Dynamics: Growth from Dendritic Branches

The dendritic nucleation model was devised to explain the cycle of actin dynamics resulting in actin filament network assembly and disassembly in two contexts--at the leading edge of motile cells and in the actin comet tails of intracellular pathogenic bacteria and viruses. Due to the detailed nature of its biochemical predictions, the model has provided an excellent focus for subsequent experimentation. This review summarizes recent work on actin dynamics in the context of the dendritic nucleation model. One outcome of this research is the possibility that additional proteins, as well as the six proteins included in the original model, might increase the efficiency of dendritic nucleation or modify the resulting actin network. In addition, actin dynamics at the leading edge might be influenced by a second actin filament network, independent of dendritic nucleation.

Differential regulation of cortactin and N-WASP-mediated actin polymerization by missing in metastasis (MIM) protein

Missing in metastasis (MIM) gene encodes an actin binding protein that is expressed at low levels in a subset of malignant cell lines. MIM protein tagged by green fluorescent protein (GFP) colocalizes with cortactin, an Arp2/3 complex activator, and interacts directly with the SH3 domain of cortactin. Recombinant full-length MIM promotes markedly cortactin and Arp2/3 complex-mediated actin polymerization in an SH3 dependent manner. In contrast, MIM-CT, a short splicing variant of MIM, binds poorly to cortactin in vitro and is unable to enhance actin polymerization. Full-length MIM binds to G-actin with a similar affinity as N-WASP-VCA, a constitutively active form of N-WASP, and inhibits N-WASP-VCA-mediated actin polymerization as analysed in vitro. The significance of the association of MIM with cortactin and G-actin was evaluated in NIH3T3 cells expressing several MIM constructs. Overexpression of full-length wild-type MIM-GFP inhibited markedly the motility of NIH3T3 cells induced by PDGF and that of human vein umbilical endothelial cells induced by sphingosine 1 phosphate. However, an MIM mutant with deletion of the WH2 domain, which is responsible for G-actin binding, enhanced cell motility. The motility inhibition imposed by MIM was compromised in the cells overexpressing N-WASP. In contrast, deletion of an MIM proline-rich domain, which is required for an optimal binding to cortactin, substantiated the MIM-mediated inhibition of cell motility. These data imply that MIM regulates cell motility by modulating different Arp2/3 activators in a distinguished manner.

The actin cytoskeleton in normal and pathological cell motility

Cell motility is crucial for tissue formation and for development of organisms. Later on cell migration remains essential throughout the lifetime of the organism for wound healing and immune responses. The actin cytoskeleton is the cellular engine that drives cell motility downstream of a complex signal transduction cascade. The basic molecular machinery underlying the assembly and disassembly of actin filaments consists of a variety of actin binding proteins that regulate the dynamic behavior of the cytoskeleton in response to different signals. The multitude of proteins and regulatory mechanisms partaking in this system makes it vulnerable to mutations and alterations in expression levels that ultimately may cause diseases. The most familiar one is cancer that in later stages is characterized by active aberrant cell migration. Indeed tumor invasion and metastasis are increasingly being associated with deregulation of the actin system.

A dynamin-3 spliced variant modulates the actin/cortactin-dependent morphogenesis of dendritic spines

Immature dendrites extend many actin-rich filopodial structures that can be replaced by synapse-containing dendritic spines as the neuron matures. The large GTPase dynamin-3 (Dyn3) is a component of the postsynapse in hippocampal neurons but its function is undefined. Here, we demonstrate that a specific Dyn3 variant (Dyn3baa) promotes the formation of immature dendritic filopodia in cultured neurons. This effect is dependent upon Dyn3 GTPase activity and a direct interaction with the F-actin-binding protein cortactin. Consistent with these findings, Dyn3baa binds to cortactin with a 200% higher affinity than Dyn3aaa, a near identical isoform that does not induce dendritic filopodia when expressed in cultured neurons. Finally, levels of Dyn3baa-encoding mRNA are tightly regulated during neuronal maturation and are markedly upregulated during synaptogenesis. Together, these findings provide the first evidence that an enhanced interaction between a specific Dyn3 splice variant and cortactin modulate actin-membrane dynamics in developing neurons to regulate the morphogenesis of dendritic spines.

Cortactin signalling and dynamic actin networks

Cortactin was first identified over a decade ago, and its initial characterization as both an F-actin binding protein and v-Src substrate suggested that it was likely to be a key regulator of actin rearrangements in response to tyrosine kinase signalling. The recent discovery that cortactin binds and activates the actin related protein (Arp)2/3 complex, and thus regulates the formation of branched actin networks, together with the identification of multiple protein targets of the cortactin SH3 domain, have revealed diverse cellular roles for this protein. This article reviews current knowledge regarding the role of cortactin in signalling to the actin cytoskeleton in the context of these developments.

Comparative genome analysis of cortactin and HS1: the significance of the F-actin binding repeat domain

BACKGROUND

In human carcinomas, overexpression of cortactin correlates with poor prognosis. Cortactin is an F-actin-binding protein involved in cytoskeletal rearrangements and cell migration by promoting actin-related protein (Arp)2/3 mediated actin polymerization. It shares a high amino acid sequence and structural similarity to hematopoietic lineage cell-specific protein 1 (HS1) although their functions differ considerable. In this manuscript we describe the genomic organization of these two genes in a variety of species by a combination of cloning and database searches. Based on our analysis, we predict the genesis of the actin-binding repeat domain during evolution.

RESULTS

Cortactin homologues exist in sponges, worms, shrimps, insects, urochordates, fishes, amphibians, birds and mammalians, whereas HS1 exists in vertebrates only, suggesting that both genes have been derived from an ancestor cortactin gene by duplication. In agreement with this, comparative genome analysis revealed very similar exon-intron structures and sequence homologies, especially over the regions that encode the characteristic highly conserved F-actin-binding repeat domain. Cortactin splice variants affecting this F-actin-binding domain were identified not only in mammalians, but also in amphibians, fishes and birds. In mammalians, cortactin is ubiquitously expressed except in hematopoietic cells, whereas HS1 is mainly expressed in hematopoietic cells. In accordance with their distinct tissue specificity, the putative promoter region of cortactin is different from HS1.

CONCLUSIONS

Comparative analysis of the genomic organization and amino acid sequences of cortactin and HS1 provides inside into their origin and evolution. Our analysis shows that both genes originated from a gene duplication event and subsequently HS1 lost two repeats, whereas cortactin gained one repeat. Our analysis genetically underscores the significance of the F-actin binding domain in cytoskeletal remodeling, which is of importance for the major role of HS1 in apoptosis and for cortactin in cell migration.

Cortactin regulates cell migration through activation of N-WASP

Cortactin is an actin-associated scaffolding protein that regulates cell migration. Amplification of the human gene, EMS1, has been detected in breast, head and neck tumors, where it correlates with increased invasiveness. Cortactin can regulate actin dynamics directly via its N-terminal half, which can bind and activate the Arp2/3 complex. The C-terminal portion of cortactin, however, is thought to have limited function in its regulation of the actin polymerization machinery. In this report, we identify a role for the cortactin C-terminus in regulating cell migration and, more specifically, actin dynamics. Overexpression of either full-length cortactin or cortactin C-terminus is sufficient to enhance migration of mammary epithelial cells. In vitro, cortactin binds to and activates, via its SH3 domain, a regulator of the Arp2/3 complex, neural Wiskott Aldrich Syndrome protein (N-WASP). This in vitro activation of N-WASP is likely to be important in vivo, as cortactin-enhanced migration is dependent upon N-WASP. Thus, our results suggest that cortactin has multiple mechanisms by which it can recruit and modulate the actin machinery and ultimately regulate cell migration.

Src phosphorylates ezrin at tyrosine 477 and induces a phosphospecific association between ezrin and a kelch-repeat protein family member

Ezrin, a linker between plasma membrane and actin cytoskeleton possesses morphogenic properties and can promote dissemination of tumor cells. Ezrin is phosphorylated on tyrosine, but a detailed picture of the signaling pathways involved in this modification is lacking. The transforming tyrosine kinase Src has various cytoskeletal substrates and is involved in regulation of cellular adhesion. We studied the role of Src in tyrosine phosphorylation of ezrin in adherent cells. We show that ezrin is phosphorylated in human embryonic kidney 293 cells in a Src family-dependent way. In SYF cells lacking Src, Yes, and Fyn, ezrin was not tyrosine-phosphorylated but reintroduction of wild-type Src followed by Src activation or introduction of active Src restored phosphorylation. Mapping of the Src-catalyzed tyrosine in vitro and in vivo by site-directed mutagenesis demonstrated Tyr(477) as the primary target residue. We generated a pTyr(477)-phosphospecific antibody, which confirmed that Tyr(477) becomes phosphorylated in cells in a Src-dependent manner. Tyr(477) phosphorylation did not affect ezrin head-to-tail association or phosphorylation of ezrin on threonine 566, indicating that the function of Tyr(477) phosphorylation is not related to the intramolecular regulation of ezrin. A modified yeast two-hybrid screen in which ezrin bait was phosphorylated by Src identified a novel interaction with a kelch-repeat protein family member, KBTBD2 (Kelch-repeat and BTB/POZ domain containing 2). The Src dependence of the interaction was further verified by affinity precipitation assays. Identification of a functional interplay with Src opens novel avenues for further characterization of the biological activities of ezrin.

Characterization of an ankyrin repeat-containing Shank2 isoform (Shank2E) in liver epithelial cells

Shank proteins are a family of multidomain scaffolding proteins best known for their role in organizing the postsynaptic density region in neurons. Unlike Shank1 and Shank3, Shank2 [also known as Pro-SAP1 (proline-rich synapse-associated protein 1), CortBP1 (cortactin binding protein 1) or Spank-3] has been described as a truncated family member without an N-terminal ankyrin repeat domain. The present study utilized bioinformatics to demonstrate the presence of exons encoding ankyrin repeats in the region preceding the previously described Shank2 gene. cDNA sequencing of mRNA from epithelial cells revealed a novel spliceoform of Shank2, termed Shank2E, that encodes a predicted 200 kDa protein with six N-terminal ankyrin repeats. Shank2 mRNA from epithelial tissues was larger than transcripts in brain. Likewise, the apparent mass of Shank2 protein was larger in epithelial tissues (230 kDa) when compared with brain (165/180 kDa). Immunofluorescence and membrane fractionation found Shank2E concentrated at the apical membrane of liver epithelial cells. In cultured cholangiocytes, co-immunoprecipitation and detergent solubility studies revealed Shank2E complexed with actin and co-distributed with actin in detergent-insoluble lipid rafts. These findings indicate epithelial cells express an ankyrin repeat-containing Shank2 isoform, termed Shank2E, that is poised to co-ordinate actin-dependent events at the apical membrane.

Actin depolymerization-induced tyrosine phosphorylation of cortactin: the role of Fer kinase

The F-actin-binding protein cortactin is an important regulator of cytoskeletal dynamics, and a prominent target of various tyrosine kinases. Tyrosine phosphorylation of cortactin has been suggested to reduce its F-actin cross-linking capability. In the present study, we investigated whether a reciprocal relationship exists, i.e. whether the polymerization state of actin impacts on the cortactin tyrosine phosphorylation. Actin depolymerization by LB (latrunculin B) induced robust phosphorylation of C-terminal tyrosine residues of cortactin. In contrast, F-actin stabilization by jasplakinolide, which redistributed cortactin to F-actin-containing patches, prevented cortactin phosphorylation triggered by hypertonic stress or LB. Using cell lines deficient in candidate tyrosine kinases, we found that the F-actin depolymerization-induced cortactin phosphorylation was mediated by the Fyn/Fer kinase pathway, independent of Src and c-Abl. LB caused modest Fer activation and strongly facilitated the association between Fer and cortactin. Interestingly, the F-actin-binding region within the cortactin N-terminus was essential for the efficient phosphorylation of C-terminal tyrosine residues. Investigating the structural requirements for the Fer-cortactin association, we found that (i) phosphorylation-incompetent cortactin still bound to Fer; (ii) the isolated N-terminus associated with Fer; and (iii) the C-terminus alone was insufficient for binding. Thus the cortactin N-terminus participates in the Fer-cortactin interaction, which cannot be fully due to the binding of the Fer Src homology 2 domain to C-terminal tyrosine residues of cortactin. Taken together, F-actin stabilization prevents cortactin tyrosine phosphorylation, whereas depolymerization promotes it. Depolymerization-induced phosphorylation is mediated by Fer, and requires the actin-binding domain of cortactin. These results define a novel F-actin-dependent pathway that may serve as a feedback mechanism during cytoskeleton remodelling.

Early signaling events involved in the entry of Rickettsia conorii into mammalian cells

Rickettsia conorii, the causative agent of Mediterranean spotted fever, is able to attach to and invade a variety of cell types both in vitro and in vivo. Although previous studies show that entry of R. conorii into non-phagocytic cells relies on actin polymerization, little else is known about the molecular details governing Rickettsia-host cell interactions and actin rearrangements. We determined that R. conorii recruits the Arp2/3 complex to the site of entry foci and that expression of an Arp 2/3 binding derivative of the WASP-family member, Scar, inhibited bacterial entry into Vero cells, establishing that Arp2/3 is an active component of this process. Using transient transfection with plasmids expressing dominant negative versions of small GTPases, we showed that Cdc42, but not Rac1 is involved in R. conorii invasion into Vero cells. Using pharmacological approaches, we show that this invasion is dependent on phosphoinositide (PI) 3-kinase and on protein tyrosine kinase (PTK) activities, in particular Src-family kinases. C-Src and its downstream target, p80/85 cortactin, colocalize at entry sites early in the infection process. R. conorii internalization correlated with the tyrosine phosphorylation of several other host proteins, including focal adhesion kinase (FAK), within minutes of R. conorii infection. Our results reveal that R. conorii entry into nonphagocytic cells is dependent on the Arp2/3 complex and that the interplay of pathways involving Cdc42, PI 3-kinase, c-Src, cortactin and tyrosine-phosphorylated proteins regulates Arp2/3 activation leading to the localized actin rearrangements observed during bacterial entry. This is the first report that documents the mechanism of entry of a rickettsial species into mammalian cells.

Overexpression of cortactin is involved in motility and metastasis of hepatocellular carcinoma

BACKGROUND/AIMS

The molecular basis of the metastasis of hepatocellular carcinoma (HCC) is not fully understood. The aim of this study was to elucidate the crucial genes involved in metastasis of HCC.

METHODS

We compared expression profiles among highly metastatic HCC cell lines and non-metastatic HCC cell lines by using oligonucleotide array to identify genes associated with metastasis. We further investigated the effect of identified gene on cell motility and metastasis in vitro and in vivo. Finally, we examined immunohistochemistry in human tissue samples.

RESULTS

We identified 39 genes whose expression levels were significantly correlated with metastatic ability (P<0.05). Of these genes, we further investigated cortactin, because this cortical actin-associated protein is a substrate of Src, whose activation has been shown to be involved in HCC cell migration and metastasis. Overexpression of cortactin in a non-metastatic HCC cell line increased cell motility, and resulted in metastasis in an orthotopic model. Furthermore, immunohistochemical expression of cortactin revealed its significant overexpression in HCC with intrahepatic metastasis compared with HCC without intrahepatic metastasis (P<0.005).

CONCLUSIONS

Overexpression of cortactin may play a role in the metastasis of HCC by influencing cell motility, and cortactin could be a sensitive marker for HCC with intrahepatic metastasis.

Foot and mouth: podosomes, invadopodia and circular dorsal ruffles

The plasma membrane of many motile cells undergoes highly regulated protrusions and invaginations that support the formation of podosomes, invadopodia and circular dorsal ruffles. Although they are similar in appearance and in their formation--which is mediated by a highly conserved actin-membrane apparatus--these transient surface membrane distortions are distinct. Their function is to help the cell as it migrates, attaches and invades.

Cellular and molecular characterization of early and late retinal stem cells/progenitors: Differential regulation of proliferation and context dependent role of Notch signaling

Retinal stem cells/progenitors that define the evolutionarily conserved early and late stages of retinal histogenesis are known to have distinct competence to give rise to stage-specific retinal cell types. However, the information regarding their innate proliferative behavior and phenotypic potential in terms of generating neurons and glia is lacking. Here we demonstrate that, like their counterparts in other central nervous system (CNS) regions during early and late stages of embryonic development, the early and late retinal stem cells/progenitors display different proliferative response to fibroblast growth factor 2 (FGF2) and epidermal growth factor (EGF) and bias towards generating neurons or glia. Although the former predominantly generate neurons, the latter are partial towards giving rise to glia. Transcription profiling identified classes of genes that are differentially expressed in early and late retinal stem cells/progenitors in proliferating conditions and suggested that the distinct proliferative response to FGF2 and EGF is likely due to differential expression of FGF receptor 1 (FGFR1) and EGF receptor (EGFR). However, the proliferative maintenance of retinal stem cells/progenitors is likely to include other signaling pathways such as those mediated by insulin-like growth factors (IGFs) and stem cell factor (SCF). Transcription profiling of early and late retinal stem cells/progenitors in proliferating and differentiating conditions suggested a context dependent role for Notch signaling, which may constitute one of the mechanisms underlying the stage-dependent phenotypic potential of retinal stem cells/progenitors.

Cortactin and Crk cooperate to trigger actin polymerization during Shigella invasion of epithelial cells

Shigella, the causative agent of bacillary dysentery, invades epithelial cells in a process involving Src tyrosine kinase signaling. Cortactin, a ubiquitous actin-binding protein present in structures of dynamic actin assembly, is the major protein tyrosine phosphorylated during Shigella invasion. Here, we report that RNA interference silencing of cortactin expression, as does Src inhibition in cells expressing kinase-inactive Src, interferes with actin polymerization required for the formation of cellular extensions engulfing the bacteria. Shigella invasion induced the recruitment of cortactin at plasma membranes in a tyrosine phosphorylation–dependent manner. Overexpression of wild-type forms of cortactin or the adaptor protein Crk favored Shigella uptake, and Arp2/3 binding–deficient cortactin derivatives or an Src homology 2 domain Crk mutant interfered with bacterial-induced actin foci formation. Crk was shown to directly interact with tyrosine-phosphorylated cortactin and to condition cortactin-dependent actin polymerization required for Shigella uptake. These results point at a major role for a Crk–cortactin complex in actin polymerization downstream of tyrosine kinase signaling.

Cortactin is necessary for E-cadherin-mediated contact formation and actin reorganization

Classical cadherin adhesion molecules are key determinants of cell–cell recognition during development and in post-embryonic life. A decisive step in productive cadherin-based recognition is the conversion of nascent adhesions into stable zones of contact. It is increasingly clear that such contact zone extension entails active cooperation between cadherin adhesion and the force-generating capacity of the actin cytoskeleton. Cortactin has recently emerged as an important regulator of actin dynamics in several forms of cell motility. We now report that cortactin is recruited to cell–cell adhesive contacts in response to homophilic cadherin ligation. Notably, cortactin accumulates preferentially, with Arp2/3, at cell margins where adhesive contacts are being extended. Recruitment of cortactin is accompanied by a ligation-dependent biochemical interaction between cortactin and the cadherin adhesive complex. Inhibition of cortactin activity in cells blocked Arp2/3-dependent actin assembly at cadherin adhesive contacts, significantly reduced cadherin adhesive contact zone extension, and perturbed both cell morphology and junctional accumulation of cadherins in polarized epithelia. Together, our findings identify a necessary role for cortactin in the cadherin–actin cooperation that supports productive contact formation.

Interaction of cortactin and Arp2/3 complex is required for sphingosine-1-phosphate-induced endothelial cell remodeling

Sphingosine-1-phosphate (S1P) induces capillary formation of endothelial cells on Matrigel in accompany with actin assembly and accumulation of cortactin and Arp2/3 complex at the cell-leading edge. Suppression of cortactin expression with a cortactin antisense oligo significantly impaired S1P-induced capillary formation, migration of endothelial cells, and actin assembly at the cell periphery. Overexpression of wild-type cortactin tagged by green fluorescent protein (GFP) increased the S1P-induced tube formation and cell motility, whereas the cells overexpressing the mutant formed poorly capillary network and became less motile in response to S1P. Analysis of distribution in Triton X-100 insoluble fractions demonstrated that the cortactin mutant inhibited the association of wild-type cortactin and Arp2/3 complex with the actin-enriched complex. Furthermore, actin polymerization at and distribution of Arp2/3 complex as well as endogenous cortactin into the cell-leading edge mediated by S1P was disturbed. These data suggest that the interaction between cortactin and Arp2/3 complex plays an important role in S1P-mediated remodeling of endothelial cells.

Cortactin modulates cell migration and ring canal morphogenesis during Drosophila oogenesis

Cortactin is a Src substrate that interacts with F-actin and can stimulate actin polymerization by direct interaction with the Arp2/3 complex. We have isolated complete loss-of-function mutants of the single Drosophila cortactin gene. Mutants are viable and fertile, showing that cortactin is not an essential gene. However, cortactin mutants show distinct defects during oogenesis. During oogenesis, Cortactin protein is enriched at the F-actin rich ring canals in the germ line, and in migrating border cells. In cortactin mutants, the ring canals are smaller than normal. A similar phenotype has been observed in Src64 mutants and in mutants for genes encoding Arp2/3 complex components, supporting that these protein products act together to control specific processes in vivo. Cortactin mutants also show impaired border cell migration. This invasive cell migration is guided by Drosophila EGFR and PDGF/VEGF receptor (PVR). We find that accumulation of Cortactin protein is positively regulated by PVR. Also, overexpression of Cortactin can by itself induce F-actin accumulation and ectopic filopodia formation in epithelial cells. We present evidence that Cortactin is one of the factors acting downstream of PVR and Src to stimulate F-actin accumulation. Cortactin is a minor contributor in this regulation, consistent with the cortactin gene not being essential for development.

v-Src accelerates spontaneous motility via phosphoinositide 3-kinase, phospholipase C and phospholipase D, but abrogates chemotaxis in Rat-1 and MDCK cells

In Rat-1 fibroblasts, v-Src causes a profound remodelling of cortical actin cytoskeleton. This transformation includes membrane ruffling, a hallmark of the leading edge in migrating cells, and results from activation of phosphoinositide 3-kinase (PI 3-kinase), phospholipase C (PLC) and phospholipase D (PLD). We therefore reexamined whether motility is constitutively triggered by v-Src and studied whether this response is controlled by the same signalling pathway. The study was performed using Rat-1/tsLA29 and MDCK/tsLA31 cells, each harbouring a different thermosensitive v-Src kinase, active at 34 degrees C but inactivated at 40 degrees C. In both cell lines, overnight v-Src activation induced transformation and accelerated spontaneous motility by approximately twofold, as evidenced by wound-healing assay and by single-cell track, time-lapse recording in Dunn chambers. Inhibitors of PI 3-kinase, PLC and PLD selectively abrogated acceleration of motility by v-Src. Since mechanisms that co-ordinate spontaneous, as distinct from oriented, cell migration are separable, we further analysed in Dunn chambers chemotactic response of Rat-1/tsLA29 cells to PDGF and of MDCK/tsLA31 cells to EGF. In both cases, v-Src decreased the steady-state level of growth factor receptors at the cell surface twofold, and abrogated movement directionality at comparable level of occupancy as in non-transformed cells. The burst of pinocytosis in response to growth factors was also abolished by v-Src. Altogether, these results indicate that v-Src triggers motility in a PI 3-kinase-, PLC- and PLD-dependent manner, but abrogates directionality by suppressing polarised signalling downstream of growth factor receptors.

Filamentous actin binding ability of cortactin isoforms is responsible for their cell-cell junctional localization in epithelial cells

Cortactin is an F-actin binding protein that contributes to cytoskeleton remodelling. We identified five isoforms of mouse cortactin that differ in the number of tandem 37-amino acid repeats, named cortactin repeats. The transcription of minor isoforms with 4.5, 3.5 or 2.5 cortactin repeats was low in most adult tissues whereas an isoform with 4.5 cortactin repeats was highly transcribed in the adult brain. In accordance with the brain-specific upregulation of a minor isoform, a brain-specific novel 72-kDa cortactin protein was identified. Major isoforms with 6.5 or 5.5 cortactin repeats bound F-actin more robustly than minor isoforms in vitro. All isoforms were concentrated at cell-cell junction sites in epithelial cells. Deletion mutants lacking whole cortactin repeats did not bind F-actin and were not concentrated at cell-cell junction sites. Thus, the F-actin binding ability is mostly correlated with the number of cortactin repeats and is required for the cell-cell junctional localization.

Erk/Src phosphorylation of cortactin acts as a switch on-switch off mechanism that controls its ability to activate N-WASP

The Arp2/3 complex can be independently activated to initiate actin polymerization by the VCA domain of WASP family members and by the acidic N-terminal and F-actin-binding repeat region of cortactin, which possesses a C-terminal SH3 domain. Cortactin is a target for phosphorylation by Src tyrosine kinases and by serine/threonine kinases that include Erk. Here we demonstrate that cortactin binds N-WASP and WASP via its SH3 domain, induces in vitro N-WASP-mediated actin polymerization, and colocalizes with N-WASP and WASP at sites of active actin polymerization. Erk phosphorylation and a mimicking S405,418D double mutation enhanced cortactin binding and activation of N-WASP. In contrast, Src phosphorylation inhibited the ability of cortactin previously phosphorylated by Erk, and that of S405,418D double mutant cortactin, to bind and activate N-WASP. Furthermore, Y-->D mutation of three tyrosine residues targeted by Src (Y421, Y466, and Y482) inhibited the ability of S405,418D cortactin to activate N-WASP. We propose that Erk phosphorylation liberates the SH3 domain of cortactin from intramolecular interactions with proline-rich regions, causing it to synergize with WASP and N-WASP in activating the Arp2/3 complex, and that Src phosphorylation terminates cortactin activation of N-WASP and WASP.

Endocytosis as a mechanism for tyrosine kinase-dependent suppression of a voltage-gated potassium channel

The voltage-gated potassium channel Kv1.2 undergoes tyrosine phosphorylation-dependent suppression of its ionic current. However, little is known about the physical mechanism behind that process. We have found that the Kv1.2 alpha-subunit protein undergoes endocytosis in response to the same stimuli that evoke suppression of Kv1.2 ionic current. The process is tyrosine phosphorylation-dependent because the same tyrosine to phenylalanine mutation in the N-terminus of Kv1.2 that confers resistance to channel suppression (Y132F) also confers resistance to channel endocytosis. Overexpression of a dominant negative form of dynamin blocked stimulus-induced Kv1.2 endocytosis and also blocked suppression of Kv1.2 ionic current. These data indicate that endocytosis of Kv1.2 from the cell surface is a key mechanism for channel suppression by tyrosine kinases.

Matrix-specific activation of Src and Rho initiates capillary morphogenesis of endothelial cells

Interstitial collagen I stimulates microvascular endothelial cells to form solid cords that imitate precapillary structures found during angiogenesis. Time-lapse microscopy identified cell retraction and disruption of cell-cell contacts as early critical steps in collagen I-induced capillary morphogenesis. These early stages paralleled collagen I activation of Src kinase and GTPase Rho through beta1 integrins. The Src inhibitor PP2, dominant-negative Src, and Rho inhibitor exoenzyme C3 transferase each inhibited collagen I induction of actin stress fibers that mediate cell retraction and each inhibited capillary morphogenesis. Collagen I also disrupted VE-cadherin from intercellular junctions through a Src-dependent mechanism; both the Src inhibitor PP2 and dominant-negative Src preserved VE-cadherin localization to regions of cell-cell contact. An active Src mutant disrupted VE-cadherin and cell-cell contacts similarly to collagen I. In sharp contrast, laminin-1 did not induce capillary morphogenesis, and laminin-1 did not induce activation of Src or Rho. Rather, laminin-1 induced persistent activation of the GTPase Rac. Thus, these studies identify activation of Src and Rho as key mechanisms by which collagen I provokes capillary morphogenesis of microvascular endothelial cells, and they define marked differences between the functions of collagen I and laminin-1 in regulating endothelial cell morphogenesis.

Regulating Actin Dynamics at Membranes: A Focus on Dynamin

Dynamin, the large guanosine triphosphatase, is generally considered to have a key role in deforming membranes to create tubules or vesicles. Dynamin, particularly dynamin2 isoforms, also are localized with actin filaments, often at locations where cellular membranes undergo remodeling. Perturbing dynamin function interferes with endocytic traffic and actin function. Thus, dynamin may regulate actin filaments coordinately with its activities that remodel membranes. This review will highlight recent observations that provide clues to mechanisms whereby dynamin might coordinate membrane remodeling and actin filament dynamics during endocytic traffic, cell morphogenesis and cell migration.

Analysis of the adaptor function of the LIM domain-containing protein FHL2 using an affinity chromatography approach

Containing four LIM domains and an N-terminal half LIM domain, FHL2 has been predicted to have an adaptor function in the formation of higher order molecular complexes in the nucleus and the cytoplasm of cells. We expressed recombinant FHL2 in insect cells using the baculovirus system and used it to isolate direct or indirect interaction partners from the cytosolic fraction of fibroblasts by affinity chromatography. These were identified by their peptide mass fingerprints using MALDI-TOF mass spectrometry. Cytoskeleton-associated proteins present among the bound proteins were shown to co-localise with FHL2 in cell lamellipodia by indirect immunofluorescence staining.

Actin-related protein 2/3 complex is required for actin ring formation

Actin rings are vital for osteoclastic bone resorption, and actin-related protein 2/3 complex is a pivotal regulator of actin polymerization. Actin-related protein 2/3 complex was found in the podosomes of actin rings. A short interfering RNA knocked down expression of actin-related protein 2 in osteoclasts and disrupted actin rings, suggesting that the complex is crucial to actin ring formation.

INTRODUCTION

To resorb bone, osteoclasts form an extracellular acidic compartment segregated by a sealing zone. This is dependent on an actin ring that is composed of filamentous actin organized into dynamic structures called podosomes. The actin-related protein 2/3 (Arp2/3) complex is a vital regulator of actin polymerization. We tested whether the Arp2/3 complex is a component of actin rings and is important for actin ring formation.

MATERIALS AND METHODS

Western blot analysis was used to determine levels of Arp2 and Arp3, two components of the Arp2/3 complex in osteoclast-like cells. Confocal microscopy studies using antibodies for immunocytochemistry demonstrated localization of Arp2/3 complex in osteoclasts. Short interfering RNA oligonucleotides (siRNAs) were made against Arp2 and used to knock down its expression.

RESULTS

A 3-fold increase in Arp2 and Arp3 was detected during RANKL-induced differentiation of RAW 264.7 cells into osteoclast-like cells. Arp2/3 complex was concentrated in actin rings and enriched near the sealing zone. Arp2/3 complex co-localized with cortactin, a component of podosomes, but not vinculin, which surrounds podosomes. siRNA against Arp2, transfected into RAW 264.7 cells 5 days after stimulation with RANKL, reduced Arp2 protein levels 70% compared with cells transfected with ineffective siRNAs. Cytochemical characterization of RAW 264.7 osteoclast-like cells and marrow osteoclasts in which Arp2 was knocked down revealed fewer podosomes and no actin rings, although many cells remained well spread.

CONCLUSIONS

These data show that Arp2/3 complex is a component of actin rings and that the presence of Arp2/3 complex is vital to the formation of actin rings. In addition, the results show the use of siRNAs for the study of RAW 264.7 osteoclast-like cells.

SHN-1, a Shank homologue inC. elegans, affects defecation rhythm via the inositol-1,4,5-trisphosphate receptor

Protein localization in the postsynaptic density (PSD) of neurons is mediated by scaffolding proteins such as PSD-95 and Shank, which ensure proper function of receptors at the membrane. The Shank family of scaffolding proteins contain PDZ (PSD-95, Dlg, and ZO-1) domains and have been implicated in the localizations of many receptor proteins including glutamate receptors in mammals. We have identified and characterized shn-1, the only homologue of Shank in Caenorhabditis elegans. The shn-1 gene shows approximately 40% identity over 1000 amino acids to rat Shanks. SHN-1 protein is localized in various tissues including neurons, pharynx and intestine. RNAi suppression of SHN-1 did not cause lethality or developmental abnormality. However, suppression of SHN-1 in the itr-1 (sa73) mutant, which has a defective inositol-1,4,5-trisphosphate (IP(3)) receptor, resulted in animals with altered defecation rhythm. Our data suggest a possible role of SHN-1 in affecting function of IP(3) receptors in C. elegans.

Actin assembly and endocytosis: from yeast to mammals

Internalization of receptors, lipids, pathogens, and other cargo at the plasma membrane involves several different pathways and requires coordinated interactions between a variety of protein and lipid molecules. The actin cytoskeleton is an integral part of the cell cortex, and there is growing evidence that F-actin plays a direct role in these endocytic events. Genetic studies in yeast have firmly established a functional connection between actin and endocytosis. Identification of several proteins that may function at the interface between actin and the endocytic machinery has provided further evidence for this association in both yeast and mammalian cells. Several of these proteins are directly involved in regulating actin assembly and could thus harness forces produced during actin polymerization to facilitate specific steps in the endocytic process. Recent microscopy studies in mammalian cells provide powerful evidence that localized recruitment and polymerization of actin occurs at endocytic sites. In this review, we focus on progress made in elucidating the functions of the actin cytoskeleton in endocytosis.

Activity-dependent redistribution and essential role of cortactin in dendritic spine morphogenesis

The number and shape of dendritic spines are influenced by activity and regulated by molecules that organize the actin cytoskeleton of spines. Cortactin is an F-actin binding protein and activator of the Arp2/3 actin nucleation machinery that also interacts with the postsynaptic density (PSD) protein Shank. Cortactin is concentrated in dendritic spines of cultured hippocampal neurons, and the N-terminal half of the protein containing the Arp2/3 and F-actin binding domains is necessary and sufficient for spine targeting. Knockdown of cortactin protein by short-interfering RNA (siRNA) results in depletion of dendritic spines in hippocampal neurons, whereas overexpression of cortactin causes elongation of spines. In response to synaptic stimulation and NMDA receptor activation, cortactin redistributes rapidly from spines to dendritic shaft, correlating with remodeling of the actin cytoskeleton, implicating cortactin in the activity-dependent regulation of spine morphogenesis.

Actin filament organization of foot processes in rat podocytes

The foot processes of podocytes possess abundant microfilaments and modulate glomerular filtration. We investigated the actin filament organization of foot processes in adult rat podocytes and the formation of the actin cytoskeletal system of immature podocytes during glomerulogenesis. Electron microscopy revealed two populations of actin cytoskeletons in foot processes of adult podocytes. One is the actin bundle running above the level of slit diaphragms and the other is the cortical actin network located beneath the plasmalemma. Immunogold labeling for actin-binding proteins demonstrated that alpha-actinin and synaptopodin were localized in the actin bundle, whereas cortactin was in the cortical actin network. Immunofluorescence labeling for actin-binding proteins in immature podocyte showed that alpha-actinin was localized at the level of the junctional complex, whereas cortactin was distributed beneath the entire plasmalemma. Synaptopodin was first observed along the basal plasmalemma from the advanced S-shaped body to the capillary loop stage. We conclude that foot processes have specialized actin filamentous organization and that its establishment is associated with the expression and redistribution of actin-binding proteins during development.

The novel dominant mutation Dspd leads to a severe spermiogenesis defect in mice

Spermiogenesis is a complex process that is regulated by a plethora of genes and interactions between germ and somatic cells. Here we report a novel mutant mouse strain that carries a transgene insertional/translocational mutation and exhibits dominant male sterility. We named the mutation dominant spermiogenesis defect (Dspd). In the testes of Dspd mutant mice, spermatids detached from the seminiferous epithelium at different steps of the differentiation process before the completion of spermiogenesis. Microinsemination using spermatids collected from the mutant testes resulted in the birth of normal offspring. These observations indicate that the major cause of Dspd infertility is (are) a defect(s) in the Sertoli cell-spermatid interaction or communication in the seminiferous tubules. Fluorescent in situ hybridization (FISH) analysis revealed a translocation between chromosomes 7F and 14C at the transgene insertion site. The deletion of a genomic region of chromosome 7F greater than 1 megabase and containing at least six genes (Cttn, Fadd, Fgf3, Fgf4, Fgf15, and Ccnd1) was associated with the translocation. Cttn encodes the actin-binding protein cortactin. Immunohistochemical analysis revealed localization of cortactin beside elongated spermatids in wild-type testes; abnormality of cortactin localization was found in mutant testes. These data suggest an important role of cortactin in Sertoli cell-spermatid interactions and in the Dspd phenotype.

Activity-dependent redistribution and essential role of cortactin in dendritic spine morphogenesis

The number and shape of dendritic spines are influenced by activity and regulated by molecules that organize the actin cytoskeleton of spines. Cortactin is an F-actin binding protein and activator of the Arp2/3 actin nucleation machinery that also interacts with the postsynaptic density (PSD) protein Shank. Cortactin is concentrated in dendritic spines of cultured hippocampal neurons, and the N-terminal half of the protein containing the Arp2/3 and F-actin binding domains is necessary and sufficient for spine targeting. Knockdown of cortactin protein by short-interfering RNA (siRNA) results in depletion of dendritic spines in hippocampal neurons, whereas overexpression of cortactin causes elongation of spines. In response to synaptic stimulation and NMDA receptor activation, cortactin redistributes rapidly from spines to dendritic shaft, correlating with remodeling of the actin cytoskeleton, implicating cortactin in the activity-dependent regulation of spine morphogenesis.