Adhesive interactions among the extracellular matrix protein laminin, cell surface receptors known as integrins, and the microfilament network play a fundamental role in the regulation of neural cell migration during brain development. The disturbed neuronal migration that occurs when thyroid hormone is lacking during early neonatal life contributes to the profound morphological alterations characteristic of the cretinous brain. We have previously shown that thyroid hormone determines the organization of the microfilament network in astrocytes by regulating the polymerization of F-actin fibers. In this paper, we examined whether T4-dependent alterations in microfilament organization affected astrocyte-laminin interactions. We show that T4-treated astrocytes readily attached to laminin, whereas attachment of thyroid hormone-deficient cells to laminin was delayed. T4-dependent cell attachment to laminin was completely abolished by blocking integrin recognition sites with site-specific peptides or by depolymerizing the microfilaments with dihydrocytochalasin B. We also show that T4 was required for integrin clustering and focal contact formation in astrocytes attached to laminin. Thus, T4 dynamically regulates interactions between integrins and laminin via modulation of microfilament organization in astrocytes. The T4-dependent regulation of laminin-integrin interactions provides a mechanism by which this morphogenic hormone can influence neuronal migration and development.

Abstract. We have investigated the association of actin with membranes isolated from rat liver. A plasma membrane-enriched fraction prepared by homogenization in a low salt/CaCl2 buffer was found to contain a substantial amount of residual actin which could be removed by treatment with 1 M Na2CO3/NaHCO3, pH 10.5. Using a sedimentation binding assay that uses gelsolin to shorten actin filaments and render membrane binding saturable (Schwartz, M. A., and E. J. Luna. 1986. J. Cell Biol. 102:2067-2075, we found that membranes stripped of endogenous actin bound ~25I-actin in a specific and saturable manner. Scatchard plots of binding data were linear, indicating a single class of binding sites with a Kd of 1.6 #m; 66 #g actin bound/mg membrane protein at saturation. Binding of actin to liver cell membranes was negligible with unstripped membranes, was competed by excess unlabeled actin, and was greatly reduced by preheating or proteolytic digestion of the membranes. Kinetic measurements showed that binding had an initial lag phase and was strongly temperature dependent. The binding of actin to liver cell membranes was also found to be competitively inhibited by ATP and other nucleotides, including the nonhydrolyzable analogue AMP-PNP. We conclude that we have reconstituted an interaction between actin and integral membrane proteins from the rat liver. This interaction exhibits a number of distinctive features which have not been observed in other actin-membrane systems.

Abstract. Previous work has shown that actin binds specifically and saturably to liver membranes stripped of endogenous actin (Tranter, M. P., S. P. Sugrue, and M. A. Schwartz. 1989.

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