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  • br traction forces and the


    traction forces and the migration behavior under iden-tical matrix conditions compensates for the potential disadvantages.
    Under both interventions, the percentage of motile 109244-58-8 within the collagen gel remains high (>70%). Although the mean invasion distance of the stiffened cells is decreased by up to 18%, these findings suggest that a substantial in-crease in steric hindrance does not prevent cells from invading the surrounding matrix.
    3-D force measurements reveal two distinct compensa-tory mechanisms. Under conditions of a moderately increased steric hindrance, as in the case of MDA-lamA cells, the cells increase the force polarity and thus employ their contractile forces more efficiently. Under conditions of greatly increased steric hindrance as in the case of MDA-beads cells, the cells increase their contractile forces. In addition, this force increase is accompanied by a reduc-tion of the cell aspect ratio and force polarity back to control levels.
    The elastic strain energy stored in the collagen matrix is not increased in cells carrying beads and is even reduced un-der conditions of increased nucleus stiffness. Instead, we find a more pronounced collagen matrix stiffening with increasing levels of steric hindrance in the vicinity of the cells, in line with previous results (22,23). This matrix stiff-ening is the combined effect of an altered contractile force magnitude, force polarity, and force vector distribution.
    Importantly, matrix stiffening does not arise from out-ward-directed squeezing forces as the cells attempt to migrate through the confining pores of the matrix. Pore widening leads to a dramatic stiffening of the circumferen-tial collagen fibers that constitute the pore, but this stiffening is highly localized and not detectable at the resolution of our 
    measurements (14). Rather, the observed matrix stiffening arises exclusively from the cell-generated inward-directed contractile forces. As the collagen fibers that carry the con-tractile stress are stretched, they cause an anisotropic and long-range stiffening of the collagen matrix that we are able to detect with our method (14). We argue that this force-induced matrix stiffening, because it is not causing any further steric hindrance, promotes cell migration and hence is an important mechanism that cells employ the counteract steric hindrance.
    In summary, our data demonstrate that cells can compensate for an increased steric hindrance of a dense 3-D network and maintain their invasive behavior. The parameter repertoire of cells for this compensation includes alterations in cell shape, force magnitude, and force distribution. The specific strategy of individual cells for adjusting those parameters, however, appears to depend on individual cell mechanical properties and local mechanical properties as well as struc-tural properties of the matrix.
    Co´ndor et al.
    We thank Michael Davidson for providing us with the tdTomato Farnesyl construct and Ingo Thievessen, Julian Steinwachs, Astrid Mainka, and Jan-ina Lange for valuable help.
    References (24,25) appear in the Supporting Material.
    5. Miron-Mendoza, M., J. Seemann, and F. Grinnell. 2010. The differen-tial regulation of cell motile activity through matrix stiffness and porosity in three dimensional collagen matrices. Biomaterials. 31:6425–6435.
    24. Yang, Y. L., and L. J. Kaufman. 2009. Rheology and confocal reflec-tance microscopy as probes of mechanical properties and structure dur-ing collagen and collagen/hyaluronan self-assembly. Biophys. J. 96:1566–1585.
    Breast Cancer Classification Based on Proteotypes Obtained by SWATH Mass Spectrometry
    Graphical Abstract
    96 BrCa tissues mRNA
    5 conventional subtypes
    protein level
    Set of SWATH-MS
    breast cancer proteotypes
    subtype correlation based classification
    d Proteotyping of 96 breast tumors by SWATH mass spectrometry
    d Three key proteins for breast tumor classification
    d Varying degrees of heterogeneity within conventional breast cancer subtypes
    d Generally modest correlation between protein and transcript levels in tumor tissue 
    Pavel Bouchal, Olga T. Schubert, Jakub Faktor, ..., Eva Budinska, Rudolf Nenutil, Ruedi Aebersold
    [email protected] (P.B.), [email protected] (R.A.)
    In Brief
    Bouchal et al. explore and confirm the suitability of SWATH-MS for proteotyping of human tumor samples at relatively high throughput. Results indicate that proteotype-based classification resolves more variability than is apparent from conventional subtyping and potentially improves current classification.