Function of the Cortical Actin Lattice in the Motor of Macrophages

Macrophages are cells of the hematopoietic system with important defensive functions in the tissues of higher animals. Movement is an important aspect of the execution of these functions, which include locomotion, spreading on surfaces and the phagocytosis of foreign debris and microorganisms. These cells also secrete lysosomal enzymes by the process of exocytosis, and the released enzymes may play a part in the pathogenesis of tissue destruction during inflammation.The “motor” region of macrophages appears to reside in the peripheral cytoplasm beneath the plasma membrane. This cortical region ordinarily excludes lysosomes and other organelles, and, although evident throughout the entire cell periphery, it is particularly prominent in veils, pleats and pseudopodia extended by the cell ( 1 ).</jats:p

Nonideality of volume flows and phase transitions of F-actin solutions in response to osmotic stress

Ovalbumin and G-actin solutions decreased their volume in a concentration-dependent manner in response to an osmotic stress, arising from an osmotic pressure gradient of 5–20cm H2O at 25 degrees C, at protein concentrations as high as 20 mg/ml. In contrast, solutions of F-actin exhibited a concentration-dependent decrease in their rate of volume change in response to the osmotic stress. Shortening of F-actin by gelsolin did not affect this decrease, suggesting that the elastic response of the filaments underlies the osmotically nonideal behavior. However, above a critical actin concentration of approximately 7 mg/ml, no volume change occurred in response to osmotic gradients as high as 20cm H2O. The concentration at which this critical phenomenon occurred and its abolition by shortening of F-actin by gelsolin suggest that a transition of diffusible rods to a glassy state is the cause of this critical phenomenon. Above the critical concentration, an increase in the osmotic pressure applied to an F-actin solution to greater than 20cm H2O produced a transient increase in flow rate to that expected for a solution containing no polymer. This finding may represent a transition from an isotropic glassy state to an anisotropic and heterogeneous one wherein regions of pure solvent coexist with domains of pure polymer

Regulation of water flow by actin-binding protein-induced actin gelatin

Actin filaments inhibit osmotically driven water flow (Ito, T., K.S. Zaner, and T.P. Stossel. 1987. Biophys. J. 51: 745–753). Here we show that the actin gelation protein, actin-binding protein (ABP), impedes both osmotic shrinkage and swelling of an actin filament solution and reduces markedly the concentration of actin filaments required for this inhibition. These effects depend on actin filament immobilization, because the ABP concentration that causes initial impairment of water flow by actin filaments corresponds to the gel point measured viscometrically and because gelsolin, which noncovalently severs actin filaments, solates actin gels and restores water flow in a solution of actin cross-linked by ABP. Since ABP gels actin filaments in the periphery of many eukaryotic cells, such actin networks may contribute to physiological cell volume regulation