Proteolysis of cortactin by calpain regulates membrane protrusion during cell migration

Calpain-mediated proteolysis of paxillin negatively regulates focal adhesion dynamics and cell migration

The dynamic turnover of integrin-mediated adhesions is important for cell migration. Paxillin is an adaptor protein that localizes to focal adhesions and has been implicated in cell motility. We previously reported that calpain-mediated proteolysis of talin1 and focal adhesion kinase mediates adhesion disassembly in motile cells. To determine whether calpain-mediated paxillin proteolysis regulates focal adhesion dynamics and cell motility, we mapped the preferred calpain proteolytic site in paxillin. The cleavage site is between the paxillin LD1 and LD2 motifs and generates a C-terminal fragment that is similar in size to the alternative product paxillin delta. The calpain-generated proteolytic fragment, like paxillin delta, functions as a paxillin antagonist and impairs focal adhesion disassembly and migration. We generated mutant paxillin with a point mutation (S95G) that renders it partially resistant to calpain proteolysis. Paxillin-deficient cells that express paxillin S95G display increased turnover of zyxin-containing adhesions using time-lapse microscopy and also show increased migration. Moreover, cancer-associated somatic mutations in paxillin are common in the N-terminal region between the LD1 and LD2 motifs and confer partial calpain resistance. Taken together, these findings suggest a novel role for calpain-mediated proteolysis of paxillin as a negative regulator of focal adhesion dynamics and migration that may function to limit cancer cell invasion.

Calpain 2 is required for glioblastoma cell invasion: regulation of matrix metalloproteinase 2

Invasion of glioblastoma cells significantly reduces the effectiveness of current treatments, highlighting the importance of understanding dispersal mechanisms and characteristics of the invasive population. Induction of calcium fluxes into glioblastoma cells by autocrine glutamate is critical for invasion. However, the target(s) by which calcium acts to stimulate the dispersal of glioblastoma cells is not clear. In this study, we tested the hypothesis that the calcium-activated protease calpain 2 is required for glioblastoma cell invasion. Knockdown of calpain 2 expression using shRNA or chemical inhibition of calpain activity reduced glioblastoma cell invasion by 90%. Interestingly, decreased expression of calpain 2 did not influence morphology or migration, suggesting regulation of invasion specific mechanisms. Consistent with this idea, 39% less extracellular MMP2 was measured from knockdown cells identifying one mechanism by which calpain 2 mediates glioblastoma cell invasion. This is the first report demonstrating that calpain 2 is required for glioblastoma cell invasion.

Regulation of calpain-2 in neurons: implications for synaptic plasticity

The family of calcium-dependent neutral proteases, calpains, was discovered more than 30 years ago, but their functional roles in the nervous system under physiological or pathological conditions still remain unclear. Although calpain was proposed to participate in synaptic plasticity and in learning and memory in the early 1980s, the precise mechanism regarding its activation, its target(s) and the functional consequences of its activation have remained controversial. A major issue has been the identification of roles of the two major calpain isoforms present in the brain, calpain-1 and calpain-2, and the calcium requirement for their activation, which exceeds levels that could be reached intracellularly under conditions leading to changes in synaptic efficacy. In this review, we discussed the features of calpains that make them ideally suited to link certain patterns of presynaptic activity to the structural modifications of dendritic spines that could underlie synaptic plasticity and learning and memory. We then summarize recent findings that provide critical answers to the various questions raised by the initial hypothesis, and that further support the idea that, in brain, calpain-2 plays critical roles in developmental and adult synaptic plasticity.

Brain-derived neurotrophic factor and epidermal growth factor activate neuronal m-calpain via mitogen-activated protein kinase-dependent phosphorylation

Calpain is a calcium-dependent protease that plays a significant role in synaptic plasticity, cell motility, and neurodegeneration. Two major calpain isoforms are present in brain, with mu-calpain (calpain1) requiring micromolar calcium concentrations for activation and m-calpain (calpain2) needing millimolar concentrations. Recent studies in fibroblasts indicate that epidermal growth factor (EGF) can activate m-calpain independently of calcium via mitogen-activated protein kinase (MAPK)-mediated phosphorylation. In neurons, MAPK is activated by both brain-derived neurotrophic factor (BDNF) and EGF. We therefore examined whether these growth factors could activate m-calpain by MAPK-dependent phosphorylation using cultured primary neurons and HEK-TrkB cells, both of which express BDNF and EGF receptors. Calpain activation was monitored by quantitative analysis of spectrin degradation and by a fluorescence resonance energy transfer (FRET)-based assay, which assessed the truncation of a calpain-specific peptide flanked by the FRET fluorophore pair DABCYL and EDANS. In both cell types, BDNF and EGF rapidly elicited calpain activation, which was completely blocked by MAPK and calpain inhibitors. BDNF stimulated m-calpain but not mu-calpain serine phosphorylation, an effect also blocked by MAPK inhibitors. Remarkably, BDNF- and EGF-induced calpain activation was preferentially localized in dendrites and dendritic spines of hippocampal neurons and was associated with actin polymerization, which was prevented by calpain inhibition. Our results indicate that, in cultured neurons, both BDNF and EGF activate m-calpain by MAPK-mediated phosphorylation. These results strongly support a role for calpain in synaptic plasticity and may explain why m-calpain, although widely expressed in CNS, requires nonphysiological calcium levels for activation.

Calpain regulates sensitivity to trastuzumab and survival in HER2-positive breast cancer

Resistance to anti-HER2 (human epithelial growth factor receptor 2) trastuzumab therapy occurs commonly in HER2-positive breast cancer and involves overactivation of HER2 and/or AKT1. Using the model of trastuzumab-sensitive or trastuzumab-resistant HER2-positive cells with wild-type PTEN, negative regulator of AKT1, we explore the involvement of cysteine protease calpain in mechanisms of trastuzumab resistance. Overexpression of calpain1 or activation of endogenous calpain during adhesion or trastuzumab treatment of trastuzumab-sensitive cells induces cleavage of cytoplasmic domains of HER2/phospho-HER2; cleavage occurs in HER2-positive tumors. Expression of the catalytically inactive mutant of calpain1 reduces the cleavage to enhance the activity of HER2, inactivates PTEN to enhance the activation of AKT1, induces desensitization to trastuzumab and promotes survival of trastuzumab-sensitive cells. In the model of trastuzumab resistance, constitutive overactivation of HER2 and AKT1 correlates with reduced activation of calpain. Moreover, inhibitors of the catalytic site of calpain reduce the increase in constitutive activity of AKT1 and survival of trastuzumab-resistant cells selectively. Together, by regulating the activation of HER2 and PTEN/AKT1, calpain regulates trastuzumab sensitivity and survival, and the deregulation of the activation of calpain promotes trastuzumab resistance. Trastuzumab-resistant cells activate AKT1 in a mechanism dependent on the residual calpain activity, inhibition of which restores trastuzumab sensitivity and rescues resistance. These data identify calpain as a new therapeutic target in HER2-positive breast cancer.

Actin-binding protein-1 interacts with WASp-interacting protein to regulate growth factor-induced dorsal ruffle formation

The authors show that the mammalian actin binding protein-1 (mAbp1) is required for PDGF-induced dorsal ruffle formation. mAbp1 interacts directly with WASp Interacting Protein (WIP) through its SH3 domain, and this interaction is important for regulating dorsal ruffle formation.

Growth factor stimulation induces the formation of dynamic actin structures known as dorsal ruffles. Mammalian actin-binding protein-1 (mAbp1) is an actin-binding protein that has been implicated in regulating clathrin-mediated endocytosis; however, a role for mAbp1 in regulating the dynamics of growth factor–induced actin-based structures has not been defined. Here we show that mAbp1 localizes to dorsal ruffles and is necessary for platelet-derived growth factor (PDGF)-mediated dorsal ruffle formation. Despite their structural similarity, we find that mAbp1 and cortactin have nonredundant functions in the regulation of dorsal ruffle formation. mAbp1, like cortactin, is a calpain 2 substrate and the preferred cleavage site occurs between the actin-binding domain and the proline-rich region, generating a C-terminal mAbp1 fragment that inhibits dorsal ruffle formation. Furthermore, mAbp1 directly interacts with the actin regulatory protein WASp-interacting protein (WIP) through its SH3 domain. Finally, we demonstrate that the interaction between mAbp1 and WIP is important in regulating dorsal ruffle formation and that WIP-mediated effects on dorsal ruffle formation require mAbp1. Taken together, these findings identify a novel role for mAbp1 in growth factor–induced dorsal ruffle formation through its interaction with WIP.

Filamin A is required for vimentin-mediated cell adhesion and spreading

Cell adhesion and spreading are regulated by complex interactions involving the cytoskeleton and extracellular matrix proteins. We examined the interaction of the intermediate filament protein vimentin with the actin cross-linking protein filamin A in regulation of spreading in HEK-293 and 3T3 cells. Filamin A and vimentin-expressing cells were well spread on collagen and exhibited numerous cell extensions enriched with filamin A and vimentin. By contrast, cells treated with small interfering RNA (siRNA) to knock down filamin A or vimentin were poorly spread; both of these cell populations exhibited >50% reductions of cell adhesion, cell surface beta1 integrin expression, and beta1 integrin activation. Knockdown of filamin A reduced vimentin phosphorylation and blocked recruitment of vimentin to cell extensions, whereas knockdown of filamin and/or vimentin inhibited the formation of cell extensions. Reduced vimentin phosphorylation, cell spreading, and beta1 integrin surface expression, and activation were phenocopied in cells treated with the protein kinase C inhibitor bisindolylmaleimide; cell spreading was also reduced by siRNA knockdown of protein kinase C-epsilon. By immunoprecipitation of cell lysates and by pull-down assays using purified proteins, we found an association between filamin A and vimentin. Filamin A also associated with protein kinase C-epsilon, which was enriched in cell extensions. These data indicate that filamin A associates with vimentin and to protein kinase C-epsilon, thereby enabling vimentin phosphorylation, which is important for beta1 integrin activation and cell spreading on collagen.

Semaphorin 5B mediates synapse elimination in hippocampal neurons

BACKGROUND

Semaphorins are known to play an important role in axon guidance and growth by triggering dynamic rearrangements of the actin cytoskeleton in the neuronal growth cone. Intriguingly, some of these guidance molecules are persistently expressed after axonal pathfinding and target recognition are completed. Although their function at these later stages is poorly understood, recent findings suggest a role for these proteins in regulating synaptic connections.

RESULTS

Here we demonstrate that semaphorin 5B (Sema5B) regulates the elimination of synaptic connections in cultured hippocampal neurons. We show that Sema5B is proteolytically processed in neonatal brains and primary hippocampal cultures, resulting in the secretion of Sema5B fragments that include the biologically active semaphorin domain. Overexpression of full-length Sema5B in hippocampal neurons reduces synapse number while expression of a Sema5B construct lacking the semaphorin domain has no effect. Moreover, bath application with the proteolytically processed, secreted fragments containing the semaphorin domain of Sema5B, results in a rapid elimination of synaptic connections as demonstrated by time-lapse imaging. Conversely, depletion of endogenous Sema5B using RNA interference results in a significant increase in synapse number as well as a significant increase in the size of presynaptic and postsynaptic compartments.

CONCLUSION

Our results demonstrate that in addition to its role as a guidance cue, Sema5B regulates the development and maintenance of synapse size and number in hippocampal neurons. In addition, proteolytic cleavage of Sema5B results in the release of a potentially diffusible semaphorin domain that is a necessary component for its biological function in the regulation of synapse morphology.

FAK alters invadopodia and focal adhesion composition and dynamics to regulate breast cancer invasion

Focal adhesion kinase (FAK) is important for breast cancer progression and invasion and is necessary for the dynamic turnover of focal adhesions. However, it has not been determined whether FAK also regulates the dynamics of invasive adhesions formed in cancer cells known as invadopodia. In this study, we report that endogenous FAK functions upstream of cellular Src (c-Src) as a negative regulator of invadopodia formation and dynamics in breast cancer cells. We show that depletion of FAK induces the formation of active invadopodia but impairs invasive cell migration. FAK-deficient MTLn3 breast cancer cells display enhanced assembly and dynamics of invadopodia that are rescued by expression of wild-type FAK but not by FAK that cannot be phosphorylated at tyrosine 397. Moreover, our findings demonstrate that FAK depletion switches phosphotyrosine-containing proteins from focal adhesions to invadopodia through the temporal and spatial regulation of c-Src activity. Collectively, our findings provide novel insight into the interplay between FAK and Src to promote invasion.

HL-1 mouse cardiomyocyte injury and death after simulated ischemia and reperfusion: roles of pH, Ca2+-independent phospholipase A2, and Na+/H+ exchange

The Ca(2+)-independent phospholipase A(2) VI (iPLA(2)-VI) and the Na(+)/H(+) exchanger isoform 1 (NHE1) are highly pH-sensitive proteins that exert both protective and detrimental effects in cardiac ischemia-reperfusion. Here, we investigated the role of extracellular pH (pH(o)) in ischemia-reperfusion injury and death and in regulation and function of iPLA(2)-VI and NHE1 under these conditions. HL-1 cardiomyocytes were exposed to simulated ischemia (SI; 0.5% O(2), 8 mM K(+), and 20 mM lactate) at pH(o) 6.0 and 7.4, with or without 4 or 8 h of reperfusion (SI/R). Cytochrome c release and caspase-3 activation were reduced after acidic compared with neutral SI, whereas necrotic death, estimated as glucose-6-phosphate dehydrogenase release, was similar in the two conditions. Inhibition of iPLA(2)-VI activity by bromoenol lactone (BEL) elicited cardiomyocyte necrosis during normoxia and after acidic, yet not after neutral, SI. The isoform-selective enantiomers R- and S-BEL both mimicked the effect of racemic BEL after acidic SI. In contrast, inhibition of NHE activity by EIPA had no significant effect on necrosis after SI. Both neutral and acidic SI were associated with a reversible loss of F-actin and cortactin integrity. Inhibition of iPLA(2)-VI disrupted F-actin, cortactin, and mitochondrial integrity, whereas inhibition of NHE slightly reduced stress fiber content. iPLA(2)-VIA and NHE1 mRNA levels were reduced during SI and upregulated in a pH(o)-dependent manner during SI/R. This also affected the subcellular localization of iPLA(2)-VIA. Thus, the mode of cell death and the roles and regulation of iPLA(2)-VI and NHE1 are at least in part determined by the pH(o) during SI. In addition to having clinically relevant implications, these findings can in part explain the contradictory results obtained from previous studies of iPLA(2)-VIA and NHE1 during cardiac I/R.

Non-muscle myosin IIA differentially regulates intestinal epithelial cell restitution and matrix invasion

Epithelial cell motility is critical for self-rejuvenation of normal intestinal mucosa, wound repair, and cancer metastasis. This process is regulated by the reorganization of the F-actin cytoskeleton, which is driven by a myosin II motor. However, the role of myosin II in regulating epithelial cell migration remains poorly understood. This study addressed the role of non-muscle myosin (NM) IIA in two different modes of epithelial cell migration: two-dimensional (2-D) migration that occurs during wound closure and three-dimensional (3-D) migration through a Matrigel matrix that occurs during cancer metastasis. Pharmacological inhibition or siRNA-mediated knockdown of NM IIA in SK-CO15 human colonic epithelial cells resulted in decreased 2-D migration and increased 3-D invasion. The attenuated 2-D migration was associated with increased cell adhesiveness to collagen and laminin and enhanced expression of beta1-integrin and paxillin. On the 2-D surface, NM IIA-deficient SK-CO15 cells failed to assemble focal adhesions and F-actin stress fibers. In contrast, the enhanced invasion of NM IIA-depleted cells was dependent on Raf-ERK1/2 signaling pathway activation, enhanced calpain activity, and increased calpain-2 expression. Our findings suggest that NM IIA promotes 2-D epithelial cell migration but antagonizes 3-D invasion. These observations indicate multiple functions for NM IIA, which, along with the regulation of the F-actin cytoskeleton and cell-matrix adhesions, involve previously unrecognized control of intracellular signaling and protein expression.

Supervillin reorganizes the actin cytoskeleton and increases invadopodial efficiency

Tumor cells use actin-rich protrusions called invadopodia to degrade extracellular matrix (ECM) and invade tissues; related structures, termed podosomes, are sites of dynamic ECM interaction. We show here that supervillin (SV), a peripheral membrane protein that binds F-actin and myosin II, reorganizes the actin cytoskeleton and potentiates invadopodial function. Overexpressed SV induces redistribution of lamellipodial cortactin and lamellipodin/RAPH1/PREL1 away from the cell periphery to internal sites and concomitantly increases the numbers of F-actin punctae. Most punctae are highly dynamic and colocalize with the podosome/invadopodial proteins, cortactin, Tks5, and cdc42. Cortactin binds SV sequences in vitro and contributes to the formation of enhanced green fluorescent protein (EGFP)-SV induced punctae. SV localizes to the cores of Src-generated podosomes in COS-7 cells and with invadopodia in MDA-MB-231 cells. EGFP-SV overexpression increases average numbers of ECM holes per cell; RNA interference-mediated knockdown of SV decreases these numbers. Although SV knockdown alone has no effect, simultaneous down-regulation of SV and the closely related protein gelsolin reduces invasion through ECM. Together, our results show that SV is a component of podosomes and invadopodia and that SV plays a role in invadopodial function, perhaps as a mediator of cortactin localization, activation state, and/or dynamics of metalloproteinases at the ventral cell surface.

Neurite consolidation is an active process requiring constant repression of protrusive activity

During development, neurons extend projections that pathfind to reach their appropriate targets. These projections are composed of two distinct domains: a highly dynamic growth cone and a stable neurite shaft, which is considered to be consolidated. Although the regulation of these domains is critical to the appropriate formation of neural networks, the molecular mechanisms that regulate neurite shape remain poorly understood. Here, we show that calpain protease activity localizes to the neurite shaft, where it is essential for the repression of protrusive activity by limiting cortactin levels and inhibiting actin polymerization. Correspondingly, inhibition of calpain by branching factors induces the formation of new growth cones along the neurite shaft through cAMP elevation. These findings demonstrate that neurite consolidation is an active process requiring constant repression of protrusive activity. We also show that sprouting is, at least in part, accomplished by turning off the mechanism of consolidation.

Profiling distinct mechanisms of tumour invasion for drug discovery: imaging adhesion, signalling and matrix turnover

Recent advances in microscopic imaging technology, fluorescent reporter reagents, 3-dimensional (3D) cell models and multiparametric image analysis have enhanced our ability to model and understand complex cell physiology. Extension of these approaches to live cell, kinetic studies allows further spatial and temporal understanding of a multitude of dynamic functional events, including tumour cell invasion. Recent in vivo and 3D in vitro studies reveal how tumour cells utilize a diverse variety of mechanisms to permit invasion through 3D tissue environments. Such high degrees of diversity and plasticity between invasion mechanisms present a significant challenge to the successful treatment of malignant cancer. This review examines how advances in time-resolved imaging has contributed to the characterization of distinct modes of invasion and their associated molecular mechanisms. Specifically, we highlight the development of fluorescent reporter molecules and their incorporation into more predictive 3D in vitro and in vivo models, to enhance mechanistic analysis of tumour invasion. We also highlight the latest advances in kinetic imaging instrumentation applicable to in vitro and in vivo models of tumour invasion. We discuss how multiparametric image analysis can be used to interpret image data generated by these approaches. We further discuss how these approaches can be integrated into drug discovery pipelines to facilitate evaluation and selection of candidate drugs and novel pharmaceutical compositions, targeting multiple invasive mechanisms.

Cortactin branches out: roles in regulating protrusive actin dynamics

Since its discovery in the early 1990's, cortactin has emerged as a key signaling protein in many cellular processes, including cell adhesion, migration, endocytosis, and tumor invasion. While the list of cellular functions influenced by cortactin grows, the ability of cortactin to interact with and alter the cortical actin network is central to its role in regulating these processes. Recently, several advances have been made in our understanding of the interaction between actin and cortactin, providing insight into how these two proteins work together to provide a framework for normal and altered cellular function. This review examines how regulation of cortactin through post-translational modifications and interactions with multiple binding partners elicits changes in cortical actin cytoskeletal organization, impacting the regulation and formation of actin-rich motility structures.

Adhesions ring: a structural comparison between podosomes and the immune synapse

Podosomes and the immune synapse are integrin-mediated adhesive structures that share a common ring-like morphology. Both podosomes and immune synapses have a central core surrounded by a peripheral ring containing talin, vinculin and paxillin. Recent progress suggests significant parallels between the regulatory mechanisms that contribute to the formation of these adhesive structures. In this review, we compare the structures, functions and regulation of podosomes and the immune synapse.

Calpain 2 and PTP1B function in a novel pathway with Src to regulate invadopodia dynamics and breast cancer cell invasion

Invasive cancer cells form dynamic adhesive structures associated with matrix degradation called invadopodia. Calpain 2 is a calcium-dependent intracellular protease that regulates adhesion turnover and disassembly through the targeting of specific substrates such as talin. Here, we describe a novel function for calpain 2 in the formation of invadopodia and in the invasive abilities of breast cancer cells through the modulation of endogenous c-Src activity. Calpain-deficient breast cancer cells show impaired invadopodia formation that is rescued by expression of a truncated fragment of protein tyrosine phosphatase 1B (PTP1B) corresponding to the calpain proteolytic fragment, which indicates that calpain modulates invadopodia through PTP1B. Moreover, PTP1B activity is required for efficient invadopodia formation and breast cancer invasion, which suggests that PTP1B may modulate breast cancer progression through its effects on invadopodia. Collectively, our experiments implicate a novel signaling pathway involving calpain 2, PTP1B, and Src in the regulation of invadopodia and breast cancer invasion.

Podosome-type adhesions and focal adhesions, so alike yet so different

Cell-matrix adhesions are essential for cell migration, tissue organization and differentiation, therefore playing central roles in embryonic development, remodeling and homeostasis of tissues and organs. Matrix adhesion-dependent signals cooperate with other pathways to regulate biological functions such as cell survival, cell proliferation, wound healing, and tumorigenesis. Cell migration and invasion are integrated processes requiring the continuous, coordinated assembly and disassembly of integrin-mediated adhesions. An understanding of how integrins regulate cell migration and invasiveness through the dynamic regulation of adhesions is fundamental to both physiological and pathological situations. A variety of cell-matrix adhesions has been identified, namely, focal complexes, focal adhesions, fibrillar adhesions, podosomes, and invadopodia (podosome-type adhesions). These adhesion sites contain integrin clusters able to develop specialized structures, which are different in their architecture and dynamics although they share almost the same proteins. Here we compare recent advances and developments in the elucidation of the organization and dynamics of focal adhesions and podosome-type adhesions, in order to understand how such subcellular sites - though closely related in their composition - can be structurally and functionally different. The underlying question is how their respective physiological or pathological roles are related to their distinct organization.

Cortactin in tumor invasiveness

Cortactin is a cytoskeletal protein and src kinase substrate that is frequently overexpressed in cancer. Animal studies suggest that cortactin overexpression increases tumor aggressiveness, possibly through promotion of tumor invasion and metastasis. Recently, many studies have documented a role for cortactin in promoting cell motility and invasion, including a critical role in invadopodia, actin rich-subcellular protrusions associated with degradation of the extracellular matrix by cancer cells. Here, I review the evidence and potential mechanisms for cortactin as a critical mediator of tumor cell invasion.

The R-Ras interaction partner ORP3 regulates cell adhesion

Oxysterol-binding protein (OSBP)-related protein 3 (ORP3) is highly expressed in epithelial, neuronal and hematopoietic cells, as well as in certain forms of cancer. We assessed the function of ORP3 in HEK293 cells and in human macrophages. We show that ORP3 interacts with R-Ras, a small GTPase regulating cell adhesion, spreading and migration. Gene silencing of ORP3 in HEK293 cells results in altered organization of the actin cytoskeleton, impaired cell-cell adhesion, enhanced cell spreading and an increase of beta1 integrin activity--effects similar to those of constitutively active R-Ras(38V). Overexpression of ORP3 leads to formation of polarized cell-surface protrusions, impaired cell spreading and decreased beta1 integrin activity. In primary macrophages, overexpression of ORP3 leads to the disappearance of podosomal structures and decreased phagocytotic uptake of latex beads, consistent with a role in actin regulation. ORP3 is phosphorylated when cells lose adhesive contacts, suggesting that it is subject to regulation by outside-in signals mediated by adhesion receptors. The present findings demonstrate a new function of ORP3 as part of the machinery that controls the actin cytoskeleton, cell polarity and cell adhesion.

Phosphoinositide binding to the substrate regulates susceptibility to proteolysis by calpain

Calpain-mediated proteolysis regulates cytoskeletal dynamics and is altered during aging and the progression of numerous diseases or pathological conditions. Although several cytoskeletal proteins have been identified as substrates, how localized calpain activity is regulated and the mechanisms controlling substrate recognition are not clear. In this study, we report that phosphoinositide binding regulates the susceptibility of the cytoskeletal adhesion protein alpha-actinin to proteolysis by calpains 1 and 2. At first, alpha-actinin did not appear to be a substrate for calpain 2; however, phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) binding to alpha-actinin resulted in nearly complete proteolysis of the full-length protein, producing stable breakdown products. Calpain 1 was able to cleave alpha-actinin in the absence of phosphoinositide binding; however, PtdIns(3,4,5)P(3) binding increased the rate of proteolysis, and phosphatidylinositol 4,5-diphosphate (PtdIns(4,5)P(2)) binding significantly inhibited cleavage. Phosphoinositide binding appeared to regulate calpain proteolysis of alpha-actinin by modulating the exposure of a highly sensitive cleavage site within the calponin homology 2 domain. In U87MG glioblastoma cells, which contain elevated levels of PtdIns(3,4,5)P(3), alpha-actinin colocalized with calpain within dynamic actin cytoskeletal structures. Furthermore, proteolysis of alpha-actinin producing stable breakdown products was observed in U87MG cells treated with calcium ionophore to activate the calcium-dependent calpains. Additional evidence of PtdIns(3,4,5)P(3)-mediated calpain proteolysis of alpha-actinin was observed in rat embryonic fibroblasts. These results suggest that PtdIns(3,4,5)P(3) binding is a critical determinant for alpha-actinin proteolysis by calpain. In conclusion, phosphoinositide binding to the substrate is a potential mechanism for regulating susceptibility to proteolysis by calpain.

Specific knockdown of m-calpain blocks myogenesis with cDNA deduced from the corresponding RNAi

Fusion of mononuclear myoblast to multinucleated myotubes is crucial for myogenesis. Both mu- and m-calpain are ubiquitously expressed in most cells and are particularly abundant in muscle cells. Knockout of calpain-1 (catalytic subunit of mu-calpain) induced moderate platelet dysaggregation, preserving the normal development and growth, although knockout of calpain-2 (m-calpain) is lethal in mice. Therefore, there should be muscle-specific function of m-calpain per se. Previous methods lack direct evidence for the involvement of m-calpain, because the specific inhibitor to m-calpain has not been developed yet and the inhibition was less potent. Here, we show that screened RNA interference (RNAi) specifically blocked the m-calpain expression by 95% at both the protein and the activity levels. After transfection of adenovirus vector-mediated cDNA corresponding to the RNAi-induced short hairpin RNA, m-calpain in C(2)C(12) myoblasts was knocked down with no compensatory overexpression of mu-calpain or calpain-3. The specific knockdown strongly inhibited the fusion to multinucleated myotubes. In addition, the knockdown modestly blocked ubiquitous effects, including cell migration, cell spreading, and alignment of central stress fiberlike structures. These results may indicate that m-calpain requiring millimolar Ca(2+) level for the full activation plays specific roles in myogenesis, independent of mu-calpain, and leave us challenging problems in the future.

Calcium-dependent proteolytic system and muscle dysfunctions: a possible role of calpains in sarcopenia

The calcium-dependent proteolytic system is composed of cysteine proteases named calpains. They are ubiquitous or tissue-specific enzymes. The two best characterised isoforms are the ubiquitously expressed mu- and m-calpains. Besides its regulation by calcium, calpain activity is tightly controlled by calpastatin, the specific endogenous inhibitor, binding to phospholipids, autoproteolysis and phosphorylation. Calpains are responsible for limited proteolytic events. Among the multitude of substrates identified so far are cytoskeletal and membrane proteins, enzymes and transcription factors. Calpain activity is involved in a large number of physiological and pathological processes. In this review, we will particularly focus on the implication of the calcium-dependent proteolytic system in relation to muscle physiology. Because of their ability to remodel cytoskeletal anchorage complexes, calpains play a major role in the regulation of cell adhesion, migration and fusion, three key steps of myogenesis. Calcium-dependent proteolysis is also involved in the control of cell cycle. In muscle tissue, in particular, calpains intervene in the regeneration process. Another important class of calpain substrates belongs to apoptosis regulating factors. The proteases may thus play a role in muscle cell death, and as a consequence in muscle atrophy. The relationships between calcium-dependent proteolysis and muscle dysfunctions are being further developed in this review with a particular emphasis on sarcopenia.

Integrins in Cell Migration

Integrins are cell-surface adhesion receptors that play a central role in regulating cell migration by mediating interactions between the extracellular matrix and the actin cytoskeleton. Substantial progress has been made in understanding the mechanisms by which the formation and breakdown of adhesions are regulated. Here we describe general methods used to study integrin-mediated cell migration. Furthermore, we outline detailed procedures to examine focal adhesion assembly and disassembly using time-lapse fluorescent microscopy. Finally, we provide methods for the analysis of podosomes, which are highly dynamic adhesive structures that form in immune cells and invasive cancer cells.

Epidermal growth factor receptor-dependent PI3K-activation promotes restitution of wounded human gastric epithelial monolayers

Restitution is a crucial event during the healing of superficial injury of the gastric mucosa involving epithelial cell sheet movement into the damaged area. We demonstrated that growth factors promote the restitution of human gastric epithelial cells. However, the intracellular signaling pathways that transmit extracellular cues as well as regulate basal and growth factor-stimulated gastric epithelial cell migration are still unclear. Herein, confluent human gastric epithelial cell monolayers (HGE-17) or primary cultures of gastric epithelial cells were wounded with a razor blade and the migration response was analyzed in presence or absence of TGFalpha or of pharmacological inhibitors of signaling proteins. Kinase activation profile analysis and phase-contrast microscopy were also performed in parallel. We report that ERK1/2 and Akt activities are rapidly stimulated following wounding of HGE-17 cells. Treatment of confluent HGE-17 cells or primary cultures of gastric epithelial cells with the phosphatidylinositol 3-kinase inhibitor LY294002, but not the MEK1 inhibitor, PD98059, significantly inhibits basal and TGFalpha-induced migration following wounding. Conversely, treatment of wounded HGE-17 cells with phosphatidylinositol(3,4,5)-triphosphate is sufficient to stimulate basal cell migration by 235%. In addition, pp60c-src kinase activity and tyrosine phosphorylation of epidermal growth factor receptors (EGFR) are also rapidly enhanced after wounding and pharmacological inhibition of both these activities strongly attenuates basal and TGFalpha-induced migration as well as Akt phosphorylation levels. In conclusion, the present results indicate that EGFR-dependent PI3K activation promotes restitution of wounded human gastric epithelial monolayers.

Serine phosphorylation regulates paxillin turnover during cell migration

Background

Paxillin acts as an adaptor protein that localizes to focal adhesion. This protein is regulated during cell migration by phosphorylation on tyrosine, serine and threonine residues. Most of these phosphorylations have been implicated in the regulation of different steps of cell migration. The two major phosphorylation sites of paxillin in response to adhesion to an extracellular matrix are serines 188 and 190. However, the function of this phosphorylation event remains unknown. The purpose of this work was to determine the role of paxillin phosphorylation on residues S188 and S190 in the regulation of cell migration.

Results

We used NBT-II epithelial cells that can be induced to migrate when plated on collagen. To examine the role of paxillin serines 188/190 in cell migration, we constructed an EGFP-tagged paxillin mutant in which S188/S190 were mutated into unphosphorylatable alanine residues. We provide evidence that paxillin is regulated by proteasomal degradation following polyubiquitylation of the protein. During active cell migration on collagen, paxillin is protected from proteasome-dependent degradation. We demonstrate that phosphorylation of serines 188/190 is necessary for the protective effect of collagen. In an effort to understand the physiological relevance of paxillin protection from degradation, we show that cells expressing the paxillin S188/190A interfering mutant spread less, have reduced protrusive activity but migrate more actively.

Conclusion

Our data demonstrate for the first time that serine-regulated degradation of paxillin plays a key role in the modulation of membrane dynamics and consequently, in the control of cell motility.

Cortactin: the gray eminence of the cytoskeleton

Cortactin, an actin filament-binding protein and target of multiple kinases, has emerged as a central element connecting signaling pathways with cytoskeleton restructuring. It is involved in a perplexingly diverse array of cellular processes, including cell motility, invasiveness, synaptogenesis, endocytosis, intercellular contact assembly, and host-pathogen interactions, where the common denominator appears to be a role in the coordination of membrane dynamics with cytoskeletal remodeling. Although in recent years our knowledge about cortactin has increased exponentially, the exact mechanisms underlying its fundamental roles remain to be defined.

Calpain involvement in the remodeling of cytoskeletal anchorage complexes

Cells offer different types of cytoskeletal anchorages: transitory structures such as focal contacts and perennial ones such as the sarcomeric cytoskeleton of muscle cells. The turnover of these structures is controlled with different timing by a family of cysteine proteases activated by calcium, the calpains. The large number of potential substrates present in each of these structures imposes fine tuning of the activity of the proteases to avoid excessive action. This phenomenon is thus guaranteed by various types of regulation, ranging from a relatively high calcium concentration necessary for activation, phosphorylation of substrates or the proteases themselves with either a favorable or inhibitory effect, possible intervention of phospholipids, and the presence of a specific inhibitor and its possible degradation before activation. Finally, formation of multiprotein complexes containing calpains offers a new method of regulation.

A role for cortactin in Listeria monocytogenes invasion of NIH 3T3 cells, but not in its intracellular motility

Cortactin is an F-actin binding protein that binds to the Arp2/3 complex, stimulates its actin nucleation activity, and inhibits actin filament debranching. Using RNA interference directed against cortactin, we explored the importance of cortactin for several processes involving dynamic actin assembly. Silencing cortactin expression was efficiently achieved in HeLa and NIH 3T3 cells, with less than 5% of cortactin expression in siRNA-treated cells. Surprisingly, endocytosis in HeLa and NIH 3T3 cells, and cell migration rates, were not altered by RNAi-mediated cortactin silencing. Listeria utilizes actin-based motility to move within and spread among mammalian host cells; its actin-clouds and tails recruit cortactin. We explored the role of cortactin during the Listeria life cycle in cortactin "knockdown" NIH 3T3 cells. Interestingly, cortactin siRNA-treated cells showed a significant reduction in the efficiency of the bacteria invasion in NIH 3T3 cells. However, cortactin depletion did not interfere with assembly of Listeria actin clouds or actin tails, or Listeria intracellular motility or speed. Therefore, our findings suggest that cortactin plays a role in Listeria internalization, but not in the formation of actin clouds and tails, or in bacteria intracellular motility.

Regulation of the actin cytoskeleton in cancer cell migration and invasion

Malignant cancer cells utilize their intrinsic migratory ability to invade adjacent tissues and the vasculature, and ultimately to metastasize. Cell migration is the sum of multi-step processes initiated by the formation of membrane protrusions in response to migratory and chemotactic stimuli. The driving force for membrane protrusion is localized polymerization of submembrane actin filaments. Recently, several studies revealed that molecules that link migratory signals to the actin cytoskeleton are upregulated in invasive and metastatic cancer cells. In this review, we summarize recent progress on molecular mechanisms of formation of invasive protrusions used by tumor cells, such as lamellipodia and invadopodia, with regard to the functions of key regulatory proteins of the actin cytoskeleton; WASP family proteins, Arp2/3 complex, LIM-kinase, cofilin, and cortactin.

Use of recombinant calpain-2 siRNA adenovirus to assess calpain-2 modulation of lung endothelial cell migration and proliferation

In this study, we developed an adenoviral vector harboring calpain-2 siRNA expression unit in which sense and anti-sense strands composing the siRNA duplex were connected by a loop and transcribed into a siRNA in porcine pulmonary artery endothelial cells (PAEC). We screened one efficient adenoviral vector Ad/si-m187 and found that Ad/si-m187 successfully exerted a gene knockdown effect on calpain-2 mRNA transcription and protein expression levels. The protein content of calpain-2 was reduced by 30%-80% in PAEC infected with Ad/si-m187 in comparison to a control adenoviral vector Ad/si-luc. The mRNA levels of calpain-2 were measured by real-time PCR and were decreased by 60%-100% and in a dose dependent manner. In correspondence to silencing calpain-2 gene expression, calpain-2 activity was decreased significantly. We further evaluated the role of calpain-2 in endothelial cell migration and proliferation. PAEC infected with Ad/si-m187 displayed impaired migration and cell proliferation in comparison to cells infected with control adenoviral vector (Ad/si-luc). These results indicate that adenoviral vector harboring calpain-2 siRNA expression unit is a valuable tool to study the biology of calpains and that calpain-2 plays an important role in lung endothelial cell migration and proliferation.