Cytoskeletal F-Actin Polymerization from Cytosolic G-Actin Occurs in the Phagocytosing Immunocytes of Arthropods (Limulus polyphemusandGromphadorhina portentosa): Does [cAMP]iPlay Any Role?

Abstract

Phagocytosis is a major defense reaction in arthropods and is accomplished by two blood cells (hemocytes), the granulocyte (GRs) and plasmatocytes (PLs), collectively called immunocytes. Immunocytes (principally the GRs) from two arthropods,Limulus polyphemus(horseshoe crab) andGromphadorhina portentosa(Madagascar hissing cockroach) effectively phagocytose fluorescein isothiocyanate (FITC)-conjugated fluoresbrite microspheres (FITC-FM) and chicken (Gallus domesticus) erythrocytes within 1 hr of incubation. Although actin polymerization and changes in intracellular cAMP ([cAMP]i) levels occur during the early stages of phagocytosis in vertebrates, these two phenomena have not been studied in arthropod immunocytes. Using the DNase I inhibition assay, we found a decrease in cytosolic G-actin and an increase in the cytoskeletal F-actin in the phagocytosing immunocytes; the total actin in both resting and phagocytosing immunocytes remained constant. These results showed an 86% increase in F-actin inG. portentosaimmunocytes and a 29% increase in those ofL. polyphemusafter 1 hr of initial incubation with FITC-FM. As in some vertebrates, the role of [cAMP]iin the early stages of phagocytosis in these two animals—and perhaps in arthropods in general—is variable; although we detected some negligible amounts of [cAMP]i(0.10–0.80 pmol/cell at different time intervals) inL. polyphemusimmunocytes, it was inconclusive whether those inG. portentosaalso contained [cAMP]i. Even inL. polyphemus,the difference in the amounts of [cAMP]iin resting and phagocytosing cells was insignificant (P>0.05). It was also inconclusive whether [Ca2+]iand/or [Mg2+]iplay any roles in the early stages of phagocytosis in the two arthropods in this study. These results suggest that the two phenomena (F-actin polymerization and levels of [cAMP]iin arthropods) are basically similar to those in vertebrate neutrophils and macrophages, which suggests that certain immunological mechanisms are conserved in nature.