Activation of extracellular-signal regulated kinase is required for phagocytosis by Lymnaea stagnalis Haemocytes

Haemocytes are the primary defence cells of molluscs. In the present study, extracellular-signal regulated kinase (ERK) 1/2-like proteins were identified within Lymnaea stagnalis haemocytes, with apparent molecular weights of 44 and 43 kDa, respectively. Mitogen-activated protein kinase (MAPK) activity assays have confirmed that the L. stagnalis ERK possesses kinase activity towards Elk-1. Challenge of haemocytes with bacterial lipopolysaccharide (LPS) resulted in a transient activation of ERK, and immunocytochemistry revealed that phospho-ERK was present in both the perinuclear region and the nucleus following challenge. MAPK/ERK kinase (MEK) inhibitors blocked ERK activation confirming that MEK lies upstream of ERK in haemocytes. Moreover, phagocytosis assays, using various inhibitors, showed that ERK activity was vital for efficient phagocytosis and that ERK may be activated by both Ras-dependent and Ras-independent mechanisms. Overall, this study has furthered knowledge of ERK signalling in molluscan immunity and has shown that the ERK pathway regulates the phagocytic activity of molluscan haemocytes.