Extracellular matrix is integral to tissue architecture and regulates many aspects of cell behavior. Fibronectin matrix assembly involves the actin cytoskeleton and the small GTPase RhoA, but downstream signaling is not understood. Here, down-regulation of either rho kinase isoform (ROCK I or -II) by small interfering RNA treatment blocked fibronectin matrix assembly, although the phenotypes were distinct and despite persistence of the alternate kinase. Remnant fibronectin on ROCK-deficient fibroblasts was mostly punctate and more deoxycholate soluble compared with controls. Fibronectin matrix assembly defects in ROCK-deficient cells did not result from decreased synthesis/secretion, altered fibronectin mRNA splicing, metalloproteinase activity, or ␣5␤1 integrin dysfunction. Rescue could be effected by ROCK protein restoration or phosphomimetic myosin light chain expression. However, the effect of ROCK I deficiency on fibronectin matrix assembly was secondary to altered cell surface morphology, rich in filopodia, resulting from high GTP-Cdc42 levels. Total internal reflection microscopy revealed that a submembranous pool of myosin light chain in control cells was missing in ROCK II-deficient cells and replaced by stress fibers. Together, two rho kinases contribute to fibronectin matrix assembly in a different manner and cortical myosin II-driven contractility, but not stress fibers, may be critical in this activity. INTRODUCTIONCells such as fibroblasts are responsible for synthesis, assembly, and turnover of extracellular matrix. The assembly of fibronectin (FN)-rich matrices at the cell surface is integral to this process because FN can interact with cell surface receptors such as integrins and proteoglycans, and with extracellular matrix collagen, fibrin, and FN itself. The process of FN matrix assembly is known to depend on several factors (for review, see Mao and Schwarzbauer, 2005). The binding of secreted FN dimers to integrin is an early step, accompanied by an unfolding mechanism, because soluble FN is compact and not competent in polymerization. For many cells, the ␣5␤1 integrin fills this role, and microscopy indicates that it can be translocated from focal adhesions into fibrillar adhesions, rich in FN and tensin, but poor in the focal adhesion protein paxillin (Katz et al., 2000). Conformational changes in FN may be assisted by tension that unfolds molecules (Zhong et al., 1998;Baneyx et al., 2002), allowing self-association through several sites, but the 70-kDa amino-terminal fragment is particularly important. Binding of other cell surface molecules such as glycosaminoglycans may additionally potentiate FN matrix formation (Klass et al., 2000). Also known to be essential is an intact actin cytoskeleton, and FN matrix assembly is promoted by GTP-RhoA (Zhong et al., 1998) that itself promotes actin microfilament bundle and focal adhesion formation (Etienne-Manneville and Hall, 2002;Burridge and Wennerberg, 2004). Lysophosphatidic acid, present in serum, acting through cell surface receptors can sign...

The lack of radiation sources in the frequency range of 7-10 THz is associated with strong absorption of the THz waves on optical phonons within the GaAs Reststrahlen band. To avoid such absorption, we propose to use HgCdTe as an alternative material for THz quantum cascade lasers thanks to a lower phonon energy than in III-V semiconductors. In this work, HgCdTe-based quantum cascade lasers operating in the GaAs phonon Reststrahlen band with a target frequency of 8.3 THz have been theoretically investigated using the balance equation method. The optimized active region designs, which are based on three and two quantum wells, exhibit the peak gain exceeding 100 cm−1 at 150 K. We have analyzed the temperature dependence of the peak gain and predicted the maximum operating temperatures of 170 K and 225 K for three- and two-well designs, respectively. At temperatures exceeding 120 K, the better temperature performance has been obtained for the two-well design, which is associated with a larger spatial overlap of weakly localized lasing wavefunctions, as well as, a higher population inversion. We believe that the findings of this work can open a pathway towards the development of THz quantum cascade lasers featuring a high level of optical gain due to the low electron effective mass in HgCdTe.

The density of energy states is calculated for alloyed semiconductor superlattices for different excitation levels with cllowance for the effects of screening and fluctuatio.zs of impurity -oncentrations. The influence of the state density tails on spontaneous-emission spectra is investigated for cross-over transitions and in a model without a selection rule for the electron wave vector. Account for the state density tails allows one to describe the longwave wing and shape of spontaneous-emission spectra in accordance with experimental data.

Optical and electric properties of doping superlattices, or n-i-p-i crystals, can be varied in a wide range under excitation and through the choice of the thicknesses and doping of the crystal layers. Some basic results concerned thc transformation of the electron energy spectrum of doping superlattices are summarized. Parameters and characteristics of doping superlattices related to optoelectronics devices, such as photodetectors, laser diodes, and optical modulators, are presented.