CHARACTERIZATION AND IMMUNOCYTOCHEMICAL LOCALIZATION OF ACTIN AND FIBRONECTIN IN HEMOCYTES OF THE MUSSEL MYTILUS-GALLOPROVINCIALIS

Cell-extracellular matrix interactions are recognized to be important for human leucocyte functions, including chemotaxis and phagocytosis. These activities depend on a reorganization of the microfilament actin (F-actin) promoted by fibronectin, one of the major components of ext racellular matrices. Although invertebrate haemocytes are, in many asp ects, similar to the human granulocyte-monocyte-macrophage cell Lineag e, actin and fibronectin have not been well studied in these cells. Co nsequently, the characterization and structural organization of actin and fibronectin in mussel (Mytilus galloprovincialis) haemocytes was i nvestigated using Western blotting analysis, indirect immunofluorescen ce and immunoelectron microscopy. Actin was immunocharacterized by an anti-total actin monoclonal antibody. Fibronectin was immunocharacteri zed by an autologous polyclonal antiserum directed against the protein of mussel haemolymph. Actin was mainly localized along the peripheral cytoplasm of the haemocyte. The distribution of the F-actin microfila ments was assayed with Rhodamine-labelled phalloidin. F-actin was asso ciated mainly with stress-fibres of spreading haemocytes and with micr ospikes at the adhesion sites. The labelling by the anti-fibronectin a ntiserum of the haemocyte rough endoplasmic reticulum vesicles, reveal ed by immunoelectron microscopy, suggests that these cells are involve d in fibronectin biosynthesis. Gold particles were also present along the outer surfaces of the cell plasma membrane and its protrusions. Mu ssel fibronectin was localized immunohistochemically at the adhesion s ites and in the extracellular matrix fibrils. The relationships betwee n fibronectin and the actin cystoskeleton in Mytilus galloprovincialis haemocytes are discussed.