Bronchial smooth muscle (SM) mesenchymal cell precursors change their shape from round to spread/elongated while undergoing differentiation. Here we show that this change in cell shape induces the expression of laminin (LM) α2 chain not present in round mesenchymal cells. LM α2 expression is reversible and switched on and off by altering the cell's shape in culture. In comparison, the expression of LM β1 and γ1 remains unchanged. Functional studies showed that mesenchymal cell spreading and further differentiation into SM are inhibited by an antibody against LM α2. Dy/dy mice express very low levels of LM α2 and exhibit congenital muscular dystrophy. Lung SM cells isolated from adult dy/dy mice spread defectively and synthesized less SM α-actin, desmin, and SM-myosin than controls. These deficiencies were completely corrected by exogenous LM-2. On histological examination, dy/dy mouse airways and gastrointestinal tract had shorter SM cells, and lungs from dy/dy mice contained less SM-specific protein. The intestine, however, showed compensatory hyperplasia, perhaps related to its higher contractile activity. This study therefore demonstrated a novel role for the LM α2 chain in SM myogenesis and showed that its decrease in dy/dy mice results in abnormal SM.

The binding of Escherichia coli LexA repressor to the recA operator was examined as a function of the concentration of NaCl, KCl, NaF, and MgCl2 at pH 7.5, 21 degrees C. The effects of pH at 100 mM NaCl were also examined. Changes both in the qualitative appearance of the binding isotherms and in the magnitude of the apparent binding affinity with changes in solution conditions suggest that binding of anions and protons by LexA repressor is linked to oligomerization and/or operator binding. Binding of LexA repressor to the recA operator in the presence of NaCl ranging from 25 to 400 mM at picomolar DNA concentration showed a broad, apparently noncooperative, binding isotherm. Binding of LexA repressor in NaF at the same [DNA] yielded binding isotherms with a narrow transition, reflecting an apparently cooperative binding process. Also, the apparent binding affinity was weaker in NaF than in NaCl. Furthermore, the binding affinity and also the apparent binding mode, cooperative vs noncooperative, were pH dependent. The binding affinity of LexA repressor for operator was greatest near neutral pH. The apparent binding mode was noncooperative at pH 7-9 but was cooperative at pH 6 or 9.3. These observations suggest that the specific cation and anion composition and concentrations must be considered in understanding the details of regulation of the SOS system.

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Undifferentiated embryonic mesenchymal cells are round/cuboidal in shape. During development, visceral myogenesis is shortly preceded by mesenchymal cell elongation. To determine the role of the cell's shape on smooth muscle development, undifferentiated embryonic mesenchymal cells from intestine (abundant visceral muscle), lung (some visceral muscle) or kidney (no visceral muscle) were plated under conditions that maintained cell rounding or promoted elongation. Regardless of their fate in vivo, all the cells differentiated into smooth muscle upon elongation as indicated by the expression of smooth muscle-specific proteins and the development of membrane potentials of −60 mV and voltage-dependent Ca2+ currents, characteristic of excitable cells. Smooth muscle differentiation occurred within 24 hours and was independent of cell proliferation. Regardless of their fate in vivo, all the round cells remained negative for smooth muscle markers, had membrane potentials of −30 mV and showed no voltage-activated current. These cells, however, differentiated into smooth muscle upon elongation. The role of the cell's shape in controlling smooth muscle differentiation was not overcome by treatment with retinoic acid, TGF-beta1, PDGF BB or epithelial-conditioned medium (all modulators of smooth muscle differentiation). These studies suggest that the mesenchymal cell shape plays a main role in visceral myogenesis.

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