Quantification of Cell Movement Reveals Distinct Edge Motility Types During Cell Spreading

Actin-based motility is central to cellular processes such as migration, bacterial engulfment, and cancer metastasis, and requires precise spatial and temporal regulation of the cytoskeleton. We studied one such process, fibroblast spreading, which involves three temporal phases: early, middle, and late spreading, distinguished by differences in cell area growth. In these studies, aided by improved algorithms for analyzing edge movement, we observed that each phase was dominated by a single, kinematically and biochemically distinct cytoskeletal organization, or motility type. Specifically, early spreading was dominated by periodic blebbing; continuous protrusion occurred predominantly during middle spreading; and periodic contractions were prevalent in late spreading. Further characterization revealed that each motility type exhibited a distinct distribution of the actin-related protein VASP, while inhibition of actin polymerization by cytochalasin D treatment revealed different dependences on barbed-end polymerization. Through this detailed characterization and graded perturbation of the system, we observed that although each temporal phase of spreading was dominated by a single motility type, in general cells exhibited a variety of motility types in neighboring spatial domains of the plasma membrane edge. These observations support a model in which global signals bias local cytoskeletal biochemistry in favor of a particular motility type

The Vibrio cholerae Minor Pilin TcpB Initiates Assembly and Retraction of the Toxin- Coregulated Pilus

Type IV pilus (T4P) systems are complex molecular machines that polymerize major pilin proteins into thin filaments displayed on bacterial surfaces. Pilus functions require rapid extension and depolymerization of the pilus, powered by the assembly and retraction ATPases, respectively. A set of low abundance minor pilins influences pilus dynamics by unknown mechanisms. The Vibrio cholerae toxin-coregulated pilus (TCP) is among the simplest of the T4P systems, having a single minor pilin TcpB and lacking a retraction ATPase. Here we show that TcpB, like its homolog CofB, initiates pilus assembly. TcpB co-localizes with the pili but at extremely low levels, equivalent to one subunit per pilus. We used a micropillars assay to demonstrate that TCP are retractile despite the absence of a retraction ATPase, and that retraction relies on TcpB, as a V. cholerae tcpB Glu5Val mutant is fully piliated but does not induce micropillars movements. This mutant is impaired in TCP-mediated autoagglutination and TcpF secretion, consistent with retraction being required for these functions. We propose that TcpB initiates pilus retraction by incorporating into the growing pilus in a Glu5-dependent manner, which stalls assembly and triggers processive disassembly. These results provide a framework for understanding filament dynamics in more complex T4P systems and the closely related Type II secretion system

The MSX1 allele 4 homozygous child exposed to smoking at periconception is most sensitive in developing nonsyndromic orofacial clefts

Nonsyndromic orofacial clefts (OFC) are common birth defects caused by certain genes interacting with environmental factors. Mutations and association studies indicate that the homeobox gene MSX1 plays a role in human clefting. In a Dutch case-control triad study (mother, father, and child), we investigated interactions between MSX1 and the parents' periconceptional lifestyle in relation to the risk of OFC in their offspring. We s

Quantification of cell edge velocities and traction forces reveals distinct motility modules during cell spreading.

Actin-based cell motility and force generation are central to immune response, tissue development, and cancer metastasis, and understanding actin cytoskeleton regulation is a major goal of cell biologists. Cell spreading is a commonly used model system for motility experiments -- spreading fibroblasts exhibit stereotypic, spatially-isotropic edge dynamics during a reproducible sequence of functional phases: 1) During early spreading, cells form initial contacts with the surface. 2) The middle spreading phase exhibits rapidly increasing attachment area. 3) Late spreading is characterized by periodic contractions and stable adhesions formation. While differences in cytoskeletal regulation between phases are known, a global analysis of the spatial and temporal coordination of motility and force generation is missing. Implementing improved algorithms for analyzing edge dynamics over the entire cell periphery, we observed that a single domain of homogeneous cytoskeletal dynamics dominated each of the three phases of spreading. These domains exhibited a unique combination of biophysical and biochemical parameters -- a motility module. Biophysical characterization of the motility modules revealed that the early phase was dominated by periodic, rapid membrane blebbing; the middle phase exhibited continuous protrusion with very low traction force generation; and the late phase was characterized by global periodic contractions and high force generation. Biochemically, each motility module exhibited a different distribution of the actin-related protein VASP, while inhibition of actin polymerization revealed different dependencies on barbed-end polymerization. In addition, our whole-cell analysis revealed that many cells exhibited heterogeneous combinations of motility modules in neighboring regions of the cell edge. Together, these observations support a model of motility in which regions of the cell edge exhibit one of a limited number of motility modules that, together, determine the overall motility function. Our data and algorithms are publicly available to encourage further exploration

The <i>Vibrio cholerae</i> Minor Pilin TcpB Initiates Assembly and Retraction of the Toxin-Coregulated Pilus

<div><p>Type IV pilus (T4P) systems are complex molecular machines that polymerize major pilin proteins into thin filaments displayed on bacterial surfaces. Pilus functions require rapid extension and depolymerization of the pilus, powered by the assembly and retraction ATPases, respectively. A set of low abundance minor pilins influences pilus dynamics by unknown mechanisms. The <i>Vibrio cholerae</i> toxin-coregulated pilus (TCP) is among the simplest of the T4P systems, having a single minor pilin TcpB and lacking a retraction ATPase. Here we show that TcpB, like its homolog CofB, initiates pilus assembly. TcpB co-localizes with the pili but at extremely low levels, equivalent to one subunit per pilus. We used a micropillars assay to demonstrate that TCP are retractile despite the absence of a retraction ATPase, and that retraction relies on TcpB, as a <i>V</i>. <i>cholerae tcpB</i> Glu5Val mutant is fully piliated but does not induce micropillars movements. This mutant is impaired in TCP-mediated autoagglutination and TcpF secretion, consistent with retraction being required for these functions. We propose that TcpB initiates pilus retraction by incorporating into the growing pilus in a Glu5-dependent manner, which stalls assembly and triggers processive disassembly. These results provide a framework for understanding filament dynamics in more complex T4P systems and the closely related Type II secretion system.</p></div

TCP are produced in very low numbers in the <i>V</i>. <i>cholerae</i> Δ<i>tcpB</i> strain.

<p><b>(A)</b> The left TEM image is representative of WT O395, in which TCP bundles are abundant, indicated by arrows. In contrast, very few TCP bundles are observed for the Δ<i>tcpB</i> strain. One small bundle is shown in the image on the right. Flagella are indicated with arrowheads. <b>(B)</b> TEM images of TCP labeled with anti-TcpA primary antibody and gold-labeled secondary antibody. The gold particles are 6 nm in diameter. A section of each image is magnified in the inset to show the gold particles attached to the pili.</p

Complementation of <i>V</i>. <i>cholerae</i> Δ<i>tcpB</i> with <i>tcpB</i>-E5V mutants results in impaired autoagglutination and TcpF secretion without disrupting pilus assembly.

<p><b>(A)</b> TEM images of <i>V</i>. <i>cholerae</i> Δ<i>tcpB</i> complemented with <i>tcpB</i> mutants encoding Glu5 substitutions show abundant pilus production. Arrows point to TCP bundles and arrowheads point to flagella. <b>(B)</b> Immunoblots of <i>V</i>. <i>cholerae</i> fractions for the Δ<i>tcpB</i> strain complemented with WT <i>tcpB</i> or <i>tcpB</i> mutants. TcpF secretion is disrupted for the TcpB Glu5 variants despite them producing TcpB levels comparable to that of the WT <i>tcpB</i>-complemented Δ<i>tcpB</i> strain. <b>(C)</b> Autoagglutination is impaired in the <i>V</i>. <i>cholerae</i> Δ<i>tcpB</i> strain rescued with TcpB Glu5 mutants. The lower the OD₆₀₀ values the more complete the autoagglutination. Values are averaged for three replicates; error bars represent standard deviations.</p

Comparison of TcpB and the ETEC minor pilin CofB and model for TcpB-mediated assembly and retraction.

<p><b>(A)</b> Crystal structure of N-terminally truncated CofB (4QS4 [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1006109#ppat.1006109.ref098" target="_blank">98</a>]) shown in cartoon representation (top), and as a schematic (bottom) with the full N-terminal α-helix. Cysteines are colored cyan. <b>(B)</b> Amino acid sequence alignment of TcpB and CofB (NCBI Accession BAB62898). Alignment was first performed using Clustal Omega [<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1006109#ppat.1006109.ref110" target="_blank">110</a>] then adjusted to align the cysteines. Identical residues are shown in boldface type. The conserved Glu5 (red with white text) and cysteines (cyan) are indicated. Discrete domains are shaded based on the coloring of CofB shown in (A). <b>(C)</b> Proposed schematic of the TcpB structure based on sequence alignment with CofB. <b>(D)</b> Model for TcpB-mediated initiation of pilus assembly and retraction. TcpB is represented as a pilin domain (red stick and small oval) with an additional C-terminal domain (large oval). Incorporation of TcpB into the growing pilus may block passage of the pilus through the secretin complex as shown (Steps 3, 4) or may prevent further incorporation of TcpA. If pilus assembly cannot proceed, the pilin subunits will melt back into the membrane, one subunit at a time, retracting the pilus.</p