Phagocytic and macropinocytic activity in MARCKS-deficient macrophages andfibroblasts

Macrophages express high levels of the myristoylated, alanine-rich, C kinas e substrate (MARCKS), an actin cross-linking protein. To investigate a poss ible role of MARCKS in macrophage function, fetal liver-derived macrophages were generated from wild-type and MARCKS knockout mouse embryos. No differ ences between the wild-type and MARCKS-deficient macrophages with respect t o morphology (Wright's stain) or actin distribution (staining with rhodamin e-phalloidin, under basal conditions or after treatment with phorbol esters , lipopolysaccharide, or both) were observed. We then evaluated phagocytosi s mediated by different receptors: Fc receptors tested with IgG-coated shee p red blood cells, complement C3b receptors tested with C3b-coated yeast, m annose receptors tested with unopsonized zymosan, and nonspecific phagocyto sis tested with latex beads. We also studied fluid phase endocytosis in mac rophages and mouse embryo fibroblasts by using FITC-dextran to quantitate t his process. In most cases, there were no differences between the cells der ived from wild-type and MARCKS-deficient mice. However, a minor but signifi cant and reproducible difference in rates of zymosan phagocytosis at 45-60 min was observed, with lower rates of phagocytosis in the MARCKS-deficient cells. Our data indicate that MARCKS deficiency may lead to slightly decrea sed rates of zymosan phagocytosis.