Characterization of Chromatic Effects in Small Star Trackers

This paper explores how star color affects the design, calibration, and operation of star trackers. We develop a series of tools that characterize the chromatic performance of a star tracker and suggest how this information can be used in the design (e.g., selection of detector, lens, and catalog stars), and calibration (e.g., focusing, and camera calibration) of a star tracker to maximize sensor performance. We also present a simple per-star focal length correction aimed at minimizing arc length errors due to chromatic effects. Evaluating these corrections with on-orbit data from six different star trackers shows these corrections typically provide improvements of 3 − 15% in the arc-length RMSE ratio. The tools and corrections illustrate the many ways that engineers must account for color in the design of high performance star trackers

Boston Hospitality Review: Fall 2012

Lodging Update: Greater Boston by Rachel Rogisnky and Matthew Arrants -- A Sense of Place by Rachel Black -- Hospitality, Tourism, and Politics by Stephen W. Litvin -- Paris in Boston by photo essay by Jack Dzamba -- The Cradle of American Hospitality by Bradford Hudson -- Thompon’s Spa: The Most Famous Lunch Counter in the World by Peter Szende and Heather Rule -- The Restaurant as Hybrid: Lean Manufacturer and Service Provider by Christopher Mulle

Multiscale computational analysis of Xenopus laevis morphogenesis reveals key insights of systems-level behavior

<p>Abstract</p> <p>Background</p> <p>Tissue morphogenesis is a complex process whereby tissue structures self-assemble by the aggregate behaviors of independently acting cells responding to both intracellular and extracellular cues in their environment. During embryonic development, morphogenesis is particularly important for organizing cells into tissues, and although key regulatory events of this process are well studied in isolation, a number of important systems-level questions remain unanswered. This is due, in part, to a lack of integrative tools that enable the coupling of biological phenomena across spatial and temporal scales. Here, we present a new computational framework that integrates intracellular signaling information with multi-cell behaviors in the context of a spatially heterogeneous tissue environment.</p> <p>Results</p> <p>We have developed a computational simulation of mesendoderm migration in the <it>Xenopus laevis </it>explant model, which is a well studied biological model of tissue morphogenesis that recapitulates many features of this process during development in humans. The simulation couples, via a JAVA interface, an ordinary differential equation-based mass action kinetics model to compute intracellular Wnt/β-catenin signaling with an agent-based model of mesendoderm migration across a fibronectin extracellular matrix substrate. The emergent cell behaviors in the simulation suggest the following properties of the system: maintaining the integrity of cell-to-cell contact signals is necessary for preventing fractionation of cells as they move, contact with the Fn substrate and the existence of a Fn gradient provides an extracellular feedback loop that governs migration speed, the incorporation of polarity signals is required for cells to migrate in the same direction, and a delicate balance of integrin and cadherin interactions is needed to reproduce experimentally observed migratory behaviors.</p> <p>Conclusion</p> <p>Our computational framework couples two different spatial scales in biology: intracellular with multicellular. In our simulation, events at one scale have quantitative and dynamic impact on events at the other scale. This integration enables the testing and identification of key systems-level hypotheses regarding how signaling proteins affect overall tissue-level behavior during morphogenesis in an experimentally verifiable system. Applications of this approach extend to the study of tissue patterning processes that occur during adulthood and disease, such as tumorgenesis and atherogenesis.</p

Tissue Transglutaminase Is an Integrin-Binding Adhesion Coreceptor for Fibronectin

The protein cross-linking enzyme tissue transglutaminase binds in vitro with high affinity to fibronectin via its 42-kD gelatin-binding domain. Here we report that cell surface transglutaminase mediates adhesion and spreading of cells on the 42-kD fibronectin fragment, which lacks integrin-binding motifs. Overexpression of tissue transglutaminase increases its amount on the cell surface, enhances adhesion and spreading on fibronectin and its 42-kD fragment, enlarges focal adhesions, and amplifies adhesion-dependent phosphorylation of focal adhesion kinase. These effects are specific for tissue transglutaminase and are not shared by its functional homologue, a catalytic subunit of factor XIII. Adhesive function of tissue transglutaminase does not require its cross-linking activity but depends on its stable noncovalent association with integrins. Transglutaminase interacts directly with multiple integrins of β1 and β3 subfamilies, but not with β2 integrins. Complexes of transglutaminase with integrins are formed inside the cell during biosynthesis and accumulate on the surface and in focal adhesions. Together our results demonstrate that tissue transglutaminase mediates the interaction of integrins with fibronectin, thereby acting as an integrin-associated coreceptor to promote cell adhesion and spreading

A tectorin-based matrix and planar-cell-polarity genes are required for normal collagen-fibril orientation in the developing tectorial membrane

The tectorial membrane is an extracellular structure of the cochlea. It develops on the surface of an epithelium and contains collagen fibrils embedded in a tectorin-based matrix. The collagen fibrils are oriented radially with an apically-directed slant - a feature considered critical for hearing. To determine how this pattern is generated, collagen-fibril formation was examined in mice lacking a tectorin-based matrix, epithelial cilia, or the planar-cell-polarity genes Vangl2 and Ptk7. In wild-type mice, collagen-fibril bundles appear within a tectorin-based matrix at E15.5 and, as fibril-number rapidly increases, become co-aligned and correctly oriented. Epithelial-width measurements and data from Kif3acKO mice suggest, respectively, radial stretch and cilia play little, if any, role in determining normal collagen-fibril orientation, but evidence from tectorin-knockout mice indicates confinement is important. PRICKLE2 distribution reveals the planar-cell-polarity axis in the underlying epithelium is organised along the length of the cochlea and, in mice in which this polarity is disrupted, the apically-directed collagen offset is no longer observed. These results highlight the importance of the tectorin-based matrix and epithelial signals for precise collagen organisation in the tectorial membran

Rho-mediated Contractility Exposes a Cryptic Site in Fibronectin and Induces Fibronectin Matrix Assembly

Many factors influence the assembly of fibronectin into an insoluble fibrillar extracellular matrix. Previous work demonstrated that one component in serum that promotes the assembly of fibronectin is lysophosphatidic acid (Zhang, Q., W.J. Checovich, D.M. Peters, R.M. Albrecht, and D.F. Mosher. 1994. J. Cell Biol. 127:1447–1459). Here we show that C3 transferase, an inhibitor of the low molecular weight GTP-binding protein Rho, blocks the binding of fibronectin and the 70-kD NH2-terminal fibronectin fragment to cells and blocks the assembly of fibronectin into matrix induced by serum or lysophosphatidic acid. Microinjection of recombinant, constitutively active Rho into quiescent Swiss 3T3 cells promotes fibronectin matrix assembly by the injected cells. Investigating the mechanism by which Rho promotes fibronectin polymerization, we have used C3 to determine whether integrin activation is involved. Under conditions where C3 decreases fibronectin assembly we have only detected small changes in the state of integrin activation. However, several inhibitors of cellular contractility, that differ in their mode of action, inhibit cell binding of fibronectin and the 70-kD NH2-terminal fibronectin fragment, decrease fibronectin incorporation into the deoxycholate insoluble matrix, and prevent fibronectin's assembly into fibrils on the cell surface. Because Rho stimulates contractility, these results suggest that Rho-mediated contractility promotes assembly of fibronectin into a fibrillar matrix. One mechanism by which contractility could enhance fibronectin assembly is by tension exposing cryptic self-assembly sites within fibronectin that is being stretched. Exploring this possibility, we have found a monoclonal antibody, L8, that stains fibronectin matrices differentially depending on the state of cell contractility. L8 was previously shown to inhibit fibronectin matrix assembly (Chernousov, M.A., A.I. Faerman, M.G. Frid, O.Y. Printseva, and V.E. Koteliansky. 1987. FEBS (Fed. Eur. Biochem. Soc.) Lett. 217:124–128). When it is used to stain normal cultures that are developing tension, it reveals a matrix indistinguishable from that revealed by polyclonal anti-fibronectin antibodies. However, the staining of fibronectin matrices by L8 is reduced relative to the polyclonal antibody when the contractility of cells is inhibited by C3. We have investigated the consequences of mechanically stretching fibronectin in the absence of cells. Applying a 30–35% stretch to immobilized fibronectin induced binding of soluble fibronectin, 70-kD fibronectin fragment, and L8 monoclonal antibody. Together, these results provide evidence that self-assembly sites within fibronectin are exposed by tension

Tissue stiffening coordinates morphogenesis by triggering collective cell migration in vivo.

Collective cell migration is essential for morphogenesis, tissue remodelling and cancer invasion. In vivo, groups of cells move in an orchestrated way through tissues. This movement involves mechanical as well as molecular interactions between cells and their environment. While the role of molecular signals in collective cell migration is comparatively well understood, how tissue mechanics influence collective cell migration in vivo remains unknown. Here we investigated the importance of mechanical cues in the collective migration of the Xenopus laevis neural crest cells, an embryonic cell population whose migratory behaviour has been likened to cancer invasion. We found that, during morphogenesis, the head mesoderm underlying the cephalic neural crest stiffens. This stiffening initiates an epithelial-to-mesenchymal transition in neural crest cells and triggers their collective migration. To detect changes in their mechanical environment, neural crest cells use mechanosensation mediated by the integrin-vinculin-talin complex. By performing mechanical and molecular manipulations, we show that mesoderm stiffening is necessary and sufficient to trigger neural crest migration. Finally, we demonstrate that convergent extension of the mesoderm, which starts during gastrulation, leads to increased mesoderm stiffness by increasing the cell density underneath the neural crest. These results show that convergent extension of the mesoderm has a role as a mechanical coordinator of morphogenesis, and reveal a link between two apparently unconnected processes-gastrulation and neural crest migration-via changes in tissue mechanics. Overall, we demonstrate that changes in substrate stiffness can trigger collective cell migration by promoting epithelial-to-mesenchymal transition in vivo. More broadly, our results raise the idea that tissue mechanics combines with molecular effectors to coordinate morphogenesis

A Novel Fibronectin Binding Motif in MSCRAMMs Targets F3 Modules

BBK32 is a surface expressed lipoprotein and fibronectin (Fn)-binding microbial surface component recognizing adhesive matrix molecule (MSCRAMM) of Borrelia burgdorferi, the causative agent of Lyme disease. Previous studies from our group showed that BBK32 is a virulence factor in experimental Lyme disease and located the Fn-binding region to residues 21-205 of the lipoprotein.Studies aimed at identifying interacting sites between BBK32 and Fn revealed an interaction between the MSCRAMM and the Fn F3 modules. Further analysis of this interaction showed that BBK32 can cause the aggregation of human plasma Fn in a similar concentration-dependent manner to that of anastellin, the superfibronectin (sFn) inducing agent. The resulting Fn aggregates are conformationally distinct from plasma Fn as indicated by a change in available thermolysin cleavage sites. Recombinant BBK32 and anastellin affect the structure of Fn matrices formed by cultured fibroblasts and inhibit endothelial cell proliferation similarly. Within BBK32, we have located the sFn-forming activity to a region between residues 160 and 175 which contains two sequence motifs that are also found in anastellin. Synthetic peptides mimicking these motifs induce Fn aggregation, whereas a peptide with a scrambled sequence motif was inactive, suggesting that these motifs represent the sFn-inducing sequence.We conclude that BBK32 induces the formation of Fn aggregates that are indistinguishable from those formed by anastellin. The results of this study provide evidence for how bacteria can target host proteins to manipulate host cell activities