The nematode Caenorhabditis elegans is an excellent model system in which to study long-distance cell migration in vivo. This chapter describes methods used to study a subset of migratory cells in the hermaphrodite nematode, the distal tip cells. These methods take advantage of the organism’s transparent body and the expression of green fluorescent protein to observe cell migration and behavior. Additionally, the availability of nematode mutants and gene knockdown techniques that affect cell migration allow the analysis and comparison of wild-type and aberrant migratory paths. Methods for nematode growth and maintenance, strain acquisition, observation and live imaging, gene knockdown, and analysis of cell migration defects are covered.
C. elegans; Cell migration; Mutants; RNAi; Distal tip cells; Live imaging; Green fluorescent protein
Interaction of vimentin filaments (vIFs) and force-bearing adhesions is essential for endoplasm spreading. For adhesions to be connected to a contractile network involved in endoplasm spreading, vIFs are needed. Thus endoplasm spreading and microtubule stabilization in the periphery require a multicomponent actin network anchored at adhesions.
For cells to develop long-range forces and carry materials to the periphery, the microtubule and organelle-rich region at the center of the cell—the endoplasm—needs to extend to near the cell edge. Depletion of the actin cross-linking protein filamin A (FlnA) causes a collapse of the endoplasm into a sphere around the nucleus of fibroblasts and disruption of matrix adhesions, indicating that FlnA is involved in endoplasmic spreading and adhesion growth. Here, we report that treatment with the calpain inhibitor N-[N-(N-acetyl-l-leucyl)-l-leucyl]-l-norleucine (ALLN) restores endoplasmic spreading as well as focal adhesion (FA) growth on fibronectin-coated surfaces in a Fln-depleted background. Addback of calpain-uncleavable talin, not full-length talin, achieves a similar effect in Fln-depleted cells and indicates a crucial role for talin in endoplasmic spreading. Because FA maturation involves the vimentin intermediate filament (vIF) network, we also examined the role of vIFs in endoplasmic spreading. Wild-type cells expressing a vimentin variant incapable of polymerization exhibit deficient endoplasmic spreading as well as defects in FA growth. ALLN treatment restores FA growth despite the lack of vIFs but does not restore endoplasmic spreading, implying that vIFs are essential for endoplasm spreading. Consistent with that hypothesis, vIFs are always displaced from adhesions when the endoplasm does not spread. In Fln-depleted cells, vIFs extend beyond adhesions, nearly to the cell edge. Finally, inhibiting myosin II–mediated contraction blocks endoplasmic spreading and adhesion growth. Thus we propose a model in which myosin II–mediated forces and coalescence of vIFs at mature FAs are required for endoplasmic spreading.
Fibronectin (FN) is a multidomain protein with the ability to bind
simultaneously to cell surface receptors, collagen, proteoglycans, and other FN
molecules. Many of these domains and interactions are also involved in the
assembly of FN dimers into a multimeric fibrillar matrix. When, where, and how
FN binds to its various partners must be controlled and coordinated during
fibrillogenesis. Steps in the process of FN fibrillogenesis including FN
self-association, receptor activities, and intracellular pathways have been
under intense investigation for years. In this review, the domain organization
of FN including the extra domains and variable region that are controlled by
alternative splicing are described. We discuss how FN–FN and
cell–FN interactions play essential roles in the initiation and
progression of matrix assembly using complementary results from cell culture and
embryonic model systems that have enhanced our understanding of this
Traditional tissue regeneration approaches to activate cell behaviours on biomaterials rely on the use of extracellular matrix based or soluble growth factor cues. In this article, we highlight a novel approach to dynamically steer cellular phenomena such as cell motility based on nanoscale substratum features of biological ligands. Albumin derived nanocarriers (ANCs) of variable nanoscale size features were functionalized with fibronectin III9–10 matrix ligand and effects on primary human keratinocyte activation were investigated. The display of fibronectin fragment from ANCs significantly enhanced cell migration compared to free ligands at equivalent concentrations. Notably, cell migration was influenced by the size of underlying ANCs even for variably sized ANCs presenting comparable levels of fibronectin fragment. For equivalent ligand concentrations, cell migration on the smaller-sized ANCs (30 nm and 50 nm) was significantly more enhanced compared to that on larger-sized ANCs (75 nm and 100 nm). In contrast, the enhancement of cell migration on nanocarriers was abolished by the use of immobilized biofunctionalized ANCs, indicating that “dynamic” nanocarrier internalization events underlie the role of nanocarrier geometry on the differential regulation of cell migration kinetics. Uptake studies using fluorescent ANCs indicated that larger-sized ANCs showed delayed endocytic kinetics and hence could present barriers for internalization during the cell adhesion and motility processes. Motile cells exhibited diminished migration upon exposure to clathrin-inhibitors, but not caveolin-inhibitors, suggesting the role of clathrin-mediated endocytosis in facilitating cell migratory responsiveness to the nanocarriers. Overall, a monotonic relationship was found between the degree of nanocarrier cytointernalization rate and cell migration rate, suggesting the possibility of designing biointerfacial features for dynamic control of cell migration. Thus, the major findings of this study are that (a) the presentation of a biorelevant ligand on a mobile nanocarrier can be used to sensitize cellular motility activation to the adhesion ligands; and (b) such nanocarrier interfaces can dynamically attune cell migration kinetics by “modulating” the uptake of the ligand-nanocarrier complex via nanocarrier size.
nanocomposites; cell motility; nanobiotechnology; biological interfaces
Integrin receptors for extracellular matrix (ECM) are critical determinants of biological processes. Regulation of integrin expression is one way for cells to respond to changes in the ECM, to integrate intracellular signals, and to obtain appropriate adhesion for cell motility, proliferation, and differentiation. Transcriptional and post-translational mechanisms for changing the integrin repertoire at the cell surface have recently been described. These mechanisms work through transcriptional regulation that alters the proportions of one integrin relative to another, referred to as integrin switching, or through localized regulation of integrin-ECM interactions, thus providing exquisite control over cell rearrangements during tissue morphogenesis and remodeling. These integrin regulatory pathways may also be important targets in such emerging fields as tissue engineering and regenerative medicine.
The mammary gland consists of a polarized epithelium surrounded by a basement membrane matrix that forms a series of branching ducts ending in hollow, sphere-like acini. Essential roles for the epithelial basement membrane during acinar differentiation, in particular laminin and its integrin receptors, have been identified using mammary epithelial cells cultured on a reconstituted basement membrane. Contributions from fibronectin, which is abundant in the mammary gland during development and tumorigenesis, have not been fully examined. Here, we show that fibronectin expression by mammary epithelial cells is dynamically regulated during the morphogenic process. Experiments with synthetic polyacrylamide gel substrates implicate both specific extracellular matrix components, including fibronectin itself, and matrix rigidity in this regulation. Alterations in fibronectin levels perturbed acinar organization. During acinar development, increased fibronectin levels resulted in overproliferation of mammary epithelial cells and increased acinar size. Addition of fibronectin to differentiated acini stimulated proliferation and reversed growth arrest of mammary epithelial cells negatively affecting maintenance of proper acinar morphology. These results show that expression of fibronectin creates a permissive environment for cell growth that antagonizes the differentiation signals from the basement membrane. These effects suggest a link between fibronectin expression and epithelial cell growth during development and oncogenesis in the mammary gland.
Diastrophic dysplasia sulfate transporter (DTDST) is a sulfate/chloride antiporter whose function is impaired in several human chondrodysplasias. We show that DTDST is upregulated by dexamethasone stimulation of HT1080 fibrosarcoma cells and is required for fibronectin (FN) extracellular matrix deposition by these cells. DTDST imports sulfate for the modification of glycosaminoglycans. We find that N-sulfation of these chains is important for FN matrix assembly and that sulfation of cell surface proteoglycans is reduced in the absence of DTDST. Of the candidate HT1080 cell surface proteoglycans, only loss of syndecan-2 compromises FN assembly, as shown by syndecan-2 small interfering RNA knockdown. DTDST is both necessary and sufficient to induce FN matrix assembly in HT1080 cells. Knockdown of DTDST ablates FN matrix, whereas its overexpression increases assembly without dexamethasone stimulation. These results identify a previously unrecognized regulatory pathway for matrix assembly via modulation of a sulfate transporter and proteoglycan sulfation. These data raise the possibility that FN assembly defects contribute to chondrodysplasias.
Fibronectin (FN) assembly into a fibrillar extracellular matrix is a stepwise process requiring participation from multiple FN domains. Fibril formation is regulated in part by segments within the first seven type III repeats (III1–7). To define the specific function(s) of this region, recombinant FNs (recFNs) containing an overlapping set of deletions were tested for the ability to assemble into fibrils. Surprisingly, recFN lacking type III repeat III1 (FNΔIII1), which contains a cryptic FN binding site and has been suggested to be essential for fibril assembly, formed a matrix identical in all respects to a native FN matrix. Similarly, displacement of the cell binding domain in repeats III9–10 to a position close to the NH2-terminal assembly domain, as well as a large deletion spanning repeats III4–7, had no effect on assembly. In contrast, two deletions that included repeat III2, ΔIII1–2 and ΔIII2–5, caused significant reductions in fibril elongation, although binding of FN to the cell surface and initiation of assembly still proceeded. Using individual repeats in binding assays, we show that III2 but not III1 contains an FN binding site. Thus, these results pinpoint repeat III2 as an important module for FN–FN interactions during fibril growth.
fibronectin; matrix assembly; type III repeats; RGD sequence; self-association
This study shows that fibrillar collagen I is the physiological inducer of a novel class of invadosomes, which we named “linear invadosomes.” They are dependent on the scaffold protein Tks5 and are able to degrade extracellular matrix elements. Moreover, we demonstrate that they are β1- and β3-integrin independent, unlike classical invadosomes.
Invadosomes are F-actin structures capable of degrading the matrix through the activation of matrix metalloproteases. As fibrillar type I collagen promotes pro-matrix metalloproteinase 2 activation by membrane type 1 matrix metalloproteinase, we aimed at investigating the functional relationships between collagen I organization and invadosome induction. We found that fibrillar collagen I induced linear F-actin structures, distributed along the fibrils, on endothelial cells, macrophages, fibroblasts, and tumor cells. These structures share features with conventional invadosomes, as they express cortactin and N-WASP and accumulate the scaffold protein Tks5, which proved essential for their formation. On the basis of their ability to degrade extracellular matrix elements and their original architecture, we named these structures “linear invadosomes.” Interestingly, podosomes or invadopodia were replaced by linear invadosomes upon contact of the cells with fibrillar collagen I. However, linear invadosomes clearly differ from classical invadosomes, as they do not contain paxillin, vinculin, and β1/β3 integrins. Using knockout mouse embryonic fibroblasts and RGD peptide, we demonstrate that linear invadosome formation and activity are independent of β1 and β3 integrins. Finally, linear invadosomes also formed in a three-dimensional collagen matrix. This study demonstrates that fibrillar collagen I is the physiological inducer of a novel class of invadosomes.
Invadopodia are actin-rich membrane protrusions formed by tumor cells that degrade the extracellular matrix for invasion. Invadopodia formation involves membrane protrusions driven by Arp2/3-mediated actin polymerization and secretion of matrix metalloproteinases (MMPs) at the focal degrading sites. The exocyst mediates the tethering of post-Golgi secretory vesicles at the plasma membrane for exocytosis and has recently been implicated in regulating actin dynamics during cell migration. Here, we report that the exocyst plays a pivotal role in invadopodial activity. With RNAi knockdown of the exocyst component Exo70 or Sec8, MDA-MB-231 cells expressing constitutively active c-Src failed to form invadopodia. On the other hand, overexpression of Exo70 promoted invadopodia formation. Disrupting the exocyst function by siEXO70 or siSEC8 treatment or by expression of a dominant negative fragment of Exo70 inhibited the secretion of MMPs. We have also found that the exocyst interacts with the Arp2/3 complex in cells with high invasion potential; blocking the exocyst-Arp2/3 interaction inhibited Arp2/3-mediated actin polymerization and invadopodia formation. Together, our results suggest that the exocyst plays important roles in cell invasion by mediating the secretion of MMPs at focal degrading sites and regulating Arp2/3-mediated actin dynamics.
α6β4 integrin, a component of hemidesmosomes, also plays a role in keratinocyte migration via signaling through Rac1 to the actin-severing protein cofilin. Here, we tested the hypothesis that the β4 integrin-associated plakin protein, bullous pemphigoid antigen 1e (BPAG1e) functions as a scaffold for Rac1/cofilin signal transduction. We generated keratinocyte lines exhibiting a stable knockdown in BPAG1e expression. Knockdown of BPAG1e does not affect expression levels of other hemidesmosomal proteins, nor the amount of β4 integrin expressed at the cell surface. However, the amount of Rac1 associating with β4 integrin and the activity of both Rac1 and cofilin are significantly lower in BPAG1e-deficient cells compared with wild-type keratinocytes. In addition, keratinocytes deficient in BPAG1e exhibit loss of front-to-rear polarity and display aberrant motility. These defects are rescued by inducing expression of constitutively active Rac1 or active cofilin. These data indicate that the BPAG1e is required for efficient regulation of keratinocyte polarity and migration by determining the activation of Rac1.
Secretion and assembly of the extracellular matrix protein fibronectin regulates a number of normal cell and tissue functions and is dysregulated in disease states such as fibrosis, diabetes, and cancer. We found that mislocalized scaffolding by the plasma membrane Na-H exchanger NHE1 suppresses fibronectin expression, secretion, and assembly. In fibroblasts, wild-type NHE1 localizes to the distal margin of membrane protrusions or lamellipodia but a mutant NHE1-KRA2 lacking binding sites for PI(4,5)P2 and the ERM proteins ezrin, radixin, and moesin is mislocalized and found uniformly along the plasma membrane. Although NHE1 regulates intracellular pH homeostasis, fibronectin production is not regulated by changes in intracellular pH, nor is it attenuated in NHE1-deficient cells, indicating fibronectin expression is independent of NHE1 activity. However, fibronectin production is nearly absent in cells expressing NHE1-KRA2 because scaffolding by NHE1 is mislocalized. Additionally, secretion of active but not latent TGF-β is reduced and exogenous TGF-β restores fibronectin secretion and assembly. Our data indicate that scaffolding by NHE1-KRA2 dominantly suppresses fibronectin synthesis and TGF-β activation, and they suggest that NHE1-KRA2 can be used for obtaining a mechanistic understanding of how fibronectin production is regulated and speculatively for therapeutic control of dysregulated production in pathological conditions.
Membrane type-1 matrix metalloproteinase (MT1-MMP) supports tumor cell invasion through extracellular matrix barriers containing fibrin, collagen, fibronectin, and other proteins. Here, we show that simultaneous knockdown of two or three members of the tetraspanin family (CD9, CD81, and TSPAN12) markedly decreases MT1-MMP proteolytic functions in cancer cells. Affected functions include fibronectin proteolysis, invasion and growth in three-dimensional fibrin and collagen gels, and MMP-2 activation. Tetraspanin proteins (CD9, CD81, and TSPAN2) selectively coimmunoprecipitate and colocalize with MT1-MMP. Although tetraspanins do not affect the initial biosynthesis of MT1-MMP, they do protect the newly synthesized protein from lysosomal degradation and support its delivery to the cell surface. Interfering with MT1-MMP-tetraspanin collaboration may be a useful therapeutic approach to limit cancer cell invasion and metastasis.
Adhesion modulatory proteins are important effectors of cell–matrix interactions during tissue remodeling and regeneration. They comprise a diverse group of matricellular proteins that confer antiadhesive properties to the extracellular matrix (ECM). We compared the inhibitory effects of two adhesion modulatory proteins, fibulin-1 and tenascin-C, both of which bind to the C-terminal heparin-binding (HepII) domain of fibronectin (FN) but are structurally distinct. Here, we report that, like tenascin-C, fibulin-1 inhibits fibroblast spreading and cell-mediated contraction of a fibrin–FN matrix. These proteins act by modulation of focal adhesion kinase and extracellular signal-regulated kinase signaling. The inhibitory effects were bypassed by lysophosphatidic acid, an activator of RhoA GTPase. Fibroblast response to fibulin-1, similar to tenascin-C, was dependent on expression of the heparan sulfate proteoglycan syndecan-4, which also binds to the HepII domain. Therefore, blockade of HepII-mediated signaling by competitive binding of fibulin-1 or tenascin-C represents a shared mechanism of adhesion modulation among disparate modulatory proteins.
Fibrillins constitute the major backbone of multifunctional microfibrils in elastic and nonelastic extracellular matrices. Proper assembly mechanisms are central to the formation and function of these microfibrils, and their properties are often compromised in pathological circumstances such as in Marfan syndrome and in other fibrillinopathies. Here, we have used human dermal fibroblasts to analyze the assembly of fibrillin-1 in dependence of other matrix-forming proteins. siRNA knockdown experiments demonstrated that the assembly of fibrillin-1 is strictly dependent on the presence of extracellular fibronectin fibrils. Immunolabeling performed at the light and electron microscopic level showed colocalization of fibrillin-1 with fibronectin fibrils at the early stages of the assembly process. Protein-binding assays demonstrated interactions of fibronectin with a C-terminal region of fibrillin-1, -2, and -3 and with an N-terminal region of fibrillin-1. The C-terminal half of fibrillin-2 and -3 had propensities to multimerize, as has been previously shown for fibrillin-1. The C-terminal of all three fibrillins interacted strongly with fibronectin as multimers, but not as monomers. Mapping studies revealed that the major binding interaction between fibrillins and fibronectin involves the collagen/gelatin-binding region between domains FNI6 and FNI9.
Epidermal growth factor (EGF) receptor-mediated cell migration plays a vital role in invasion of many tumor types. EGF receptor ligands increase invasiveness in vivo, but it remains unclear how consequent effects on intrinsic cell motility behavior versus effects on extrinsic matrix properties integrate to result in net increase of translational speed and/or directional persistence of migration in a 3D environment. Understanding this convolution is important for therapeutic targeting of tumor invasion, as key regulatory pathways for intrinsic versus extrinsic effects may not be coincident. Accordingly, we have undertaken a quantitative single-cell imaging study of glioblastoma cell movement in 3D matrices and on 2D substrata across a range of collagen densities with systematic variation of protease-mediated matrix degradation. In 3D, EGF induced a mild increase in cell speed and a strong increase in directional persistence, the latter depending heavily on matrix density and EGF-stimulated protease activity. In contrast, in 2D, EGF induced a similarly mild increase in speed but conversely a decrease in directional persistence (both independent of protease activity). Thus, the EGF-enhanced 3D tumor cell migration results only partially from cell-intrinsic effects, with override of cell-intrinsic persistence decrease by protease-mediated cell-extrinsic reduction of matrix steric hindrance.
The Caenorhabditis elegans teneurin ortholog, ten-1, plays an important role in gonad and pharynx development. We found that lack of TEN-1 does not affect germline proliferation but leads to local basement membrane deficiency and early gonad disruption. Teneurin is expressed in the somatic precursor cells of the gonad that appear to be crucial for gonad epithelialization and basement membrane integrity. Ten-1 null mutants also arrest as L1 larvae with malformed pharynges and disorganized pharyngeal basement membranes. The pleiotropic phenotype of ten-1 mutant worms is similar to defects found in basement membrane receptor mutants ina-1 and dgn-1 as well as in the mutants of the extracellular matrix component laminin, epi-1. We show that the ten-1 mutation is synthetic lethal with mutations of genes encoding basement membrane components and receptors due to pharyngeal or hypodermal defects. This indicates that TEN-1 could act redundantly with integrin INA-1, dystroglycan DGN-1, and laminin EPI-1 in C. elegans development. Moreover, ten-1 deletion sensitizes worms to loss of nidogen nid-1 causing a pharynx unattached phenotype in ten-1;nid-1 double mutants. We conclude that TEN-1 is important for basement membrane maintenance and/or adhesion in particular organs and affects the function of somatic gonad precursor cells.
Zirconium tetra(tert-butoxide) reacts with surface amide groups of polyamide nylon 6/6 to give (η2-amidate)zirconium complexes in high yield. These surface complexes react to bond the cell-adhesive peptide arginine-glycine-aspartic acid (RGD) to the polymer surface. A surface loading of 0.18 nmol/cm2 of RGD is achieved, which is 20−1000 times higher than previously reported attainable on natural or synthetic polymers by other strategies. Approximately 40% of the nylon surface is covered by the RGD which gives a surface that is both stable to hydrolysis and highly active for cell adhesion and spreading in vitro.
Integrin-mediated cell adhesion is essential for development of multicellular organisms. In worms, flies, and vertebrates, talin forms a physical link between integrin cytoplasmic domains and the actin cytoskeleton. Loss of either integrins or talin leads to similar phenotypes. In vertebrates, talin is also a key regulator of integrin affinity. We used a ligand-mimetic Fab fragment, TWOW-1, to assess talin's role in regulating Drosophila αPS2βPS affinity. Depletion of cellular metabolic energy reduced TWOW-1 binding, suggesting αPS2βPS affinity is an active process as it is for vertebrate integrins. In contrast to vertebrate integrins, neither talin knockdown by RNA interference nor talin head overexpression had a significant effect on TWOW-1 binding. Furthermore, replacement of the transmembrane or talin-binding cytoplasmic domains of αPS2βPS with those of human αIIbβ3 failed to enable talin regulation of TWOW-1 binding. However, substitution of the extracellular and transmembrane domains of αPS2βPS with those of αIIbβ3 resulted in a constitutively active integrin whose affinity was reduced by talin knockdown. Furthermore, wild-type αIIbβ3 was activated by overexpression of Drosophila talin head domain. Thus, despite evolutionary conservation of talin's integrin/cytoskeleton linkage function, talin is not sufficient to regulate Drosophila αPS2βPS affinity because of structural features inherent in the αPS2βPS extracellular and/or transmembrane domains.
Syndecan-4 is a ubiquitously expressed heparan sulfate proteoglycan that modulates cell interactions with the extracellular matrix. It is transiently up-regulated during tissue repair by cells that mediate wound healing. Here, we report that syndecan-4 is essential for optimal fibroblast response to the three-dimensional fibrin-fibronectin provisional matrix that is deposited upon tissue injury. Interference with syndecan-4 function inhibits matrix contraction by preventing cell spreading, actin stress fiber formation, and activation of focal adhesion kinase and RhoA mediated-intracellular signaling pathways. Tenascin-C is an extracellular matrix protein that regulates cell response to fibronectin within the provisional matrix. Syndecan-4 is also required for tenascin-C action. Inhibition of syndecan-4 function suppresses tenascin-C activity and overexpression of syndecan-4 circumvents the effects of tenascin-C. In this way, tenascin-C and syndecan-4 work together to control fibroblast morphology and signaling and regulate events such as matrix contraction that are essential for efficient tissue repair.
A provisional matrix consisting of fibrin and fibronectin (FN) is deposited at sites of tissue damage and repair. This matrix serves as a scaffold for fibroblast migration into the wound where these cells deposit new matrix to replace lost or damaged tissue and eventually contract the matrix to bring the margins of the wound together. Tenascin-C is expressed transiently during wound repair in tissue adjacent to areas of injury and contacts the provisional matrix in vivo. Using a synthetic model of the provisional matrix, we have found that tenascin-C regulates cell responses to a fibrin-FN matrix through modulation of focal adhesion kinase (FAK) and RhoA activation. Cells on fibrin-FN+tenascin-C redistribute their actin to the cell cortex, downregulate focal adhesion formation, and do not assemble a FN matrix. Cells surrounded by a fibrin-FN+tenascin-C matrix are unable to induce matrix contraction. The inhibitory effect of tenascin-C is circumvented by downstream activation of RhoA. FAK is also required for matrix contraction and the absence of FAK cannot be overcome by activation of RhoA. These observations show dual requirements for both FAK and RhoA activities during contraction of a fibrin-FN matrix. The effects of tenascin-C combined with its location around the wound bed suggest that this protein regulates fundamental processes of tissue repair by limiting the extent of matrix deposition and contraction to fibrin-FN-rich matrix in the primary wound area.
Cell binding to extracellular matrix (ECM) components changes cytoskeletal organization by the activation of Rho family GTPases. Tenascin-C, a developmentally regulated matrix protein, modulates cellular responses to other matrix proteins, such as fibronectin (FN). Here, we report that tenascin-C markedly altered cell phenotype on a three-dimensional fibrin matrix containing FN, resulting in suppression of actin stress fibers and induction of actin-rich filopodia. This distinct morphology was associated with complete suppression of the activation of RhoA, a small GTPase that induces actin stress fiber formation. Enforced activation of RhoA circumvented the effects of tenascin. Effects of active Rho were reversed by a Rho inhibitor C3 transferase. Suppression of GTPase activation allows tenascin-C expression to act as a regulatory switch to reverse the effects of adhesive proteins on Rho function. This represents a novel paradigm for the regulation of cytoskeletal organization by ECM.
tenascin-C; provisional matrix; fibronectin; Rho GTPase; filopodia
Initiation of fibronectin (FN) matrix assembly is dependent on specific interactions between FN and cell surface integrin receptors. Here, we show that de novo FN matrix assembly exhibits a slow phase during initiation of fibrillogenesis followed by a more rapid growth phase. Mn2+, which acts by enhancing integrin function, increased the rate of FN fibril growth, but only after the initial lag phase. The RGD cell-binding sequence in type III repeat 10 is an absolute requirement for initiation by α5β1 integrin. To investigate the role of the cell-binding synergy site in the adjacent repeat III9, a full-length recombinant FN containing a synergy mutation, FN(syn−), was tested for its ability to form fibrils. Mutation of this site drastically reduced FN assembly by CHOα5 cells. Only sparse short fibrils were formed even after prolonged incubation, indicating that FN(syn−) is defective in progression of the assembly process. These results show that the synergy site is essential for α5β1-mediated accumulation of a FN matrix. However, the incorporation of FN(syn−) into fibrils and the deoxycholate-insoluble matrix could be stimulated by Mn2+. Therefore, exogenous activation of integrin receptors can overcome the requirement for FN’s synergy site as well as modulate the rate of FN matrix formation.
Starvation of Dictyostelium induces a developmental program in which cells form an aggregate that eventually differentiates into a multicellular structure. The aggregate formation is mediated by directional migration of individual cells that quickly transition to group migration in which cells align in a head-to-tail manner to form streams. Cyclic AMP acts as a chemoattractant and its production, secretion, and degradation are highly regulated. A key protein is the extracellular phosphodiesterase PdsA. In this study we examine the role and localization of PdsA during chemotaxis and streaming. We find that pdsA− cells respond chemotactically to a narrower range of chemoattractant concentrations compared with wild-type (WT) cells. Moreover, unlike WT cells, pdsA− cells do not form streams at low cell densities and form unusual thick and transient streams at high cell densities. We find that the intracellular pool of PdsA is localized to the endoplasmic reticulum, which may provide a compartment for storage and secretion of PdsA. Because we find that cAMP synthesis is normal in cells lacking PdsA, we conclude that signal degradation regulates the external cAMP gradient field generation and that the group migration behavior of these cells is compromised even though their signaling machinery is intact.