Cellular Receptors and Signal Transduction in Molluscan Hemocytes: Connections with the Innate Immune System of Vertebrates

Abstract

The involvement of circulating hemocytes as the principal cellular effector mediating molluscan immune responses is well established. They participate in a variety of internal defense-related activities including microbial phagocytosis, multicellular encapsulation, and cell-mediated cytotoxicity reactions that are presumed to be initiated through foreign ligand binding to hemocyte receptors and subsequent transduction of the binding signal through the cell resulting in appropriate (or in some cases, inappropriate) hemocyte responses. At present, however, although functional evidence abounds as to the existence of hemocyte "recognition" receptors, few have been characterized at the molecular level. Similarly, signal transduction systems associated with various receptor-mediated hemocyte functions in molluscs are only beginning to be investigated and understood. This review examines what is currently known about the molluscan hemocyte receptors and the putative signal transduction pathways involved in regulating their cellular behaviors/activities. The cumulative data implies the presence of various hemocyte-associated receptors capable of binding specific carbohydrates, extracellular matrix proteins, growth factors, hormones, and cytokines. Moreover, receptor-ligand interactions appear to involve signaling molecules similar to those already recognized in vertebrate immunocyte signal transduction pathways, such as protein kinases A and C, focal adhesion kinase, Src, Ca(2+) and mitogen-activated protein kinase. Overall, the experimental evidence suggests that molluscan immune responses rely on molecules that share homology with those of vertebrate signaling systems. As more information regarding the molecular nature of hemocyte recognition receptors and their associated signaling molecules is accumulated, a clearer picture of how hemocyte immune responses to invading organisms are regulated will begin to emerge.

Larval excretory-secretory products from the parasite Schistosoma mansoni modulate HSP70 protein expression in defence cells of its snail host, Biomphalaria glabrata

“…Parasite components such as ESPs play an important role in suppressing snail host immune defence mechanisms, mainly by interfering with the physiology and behaviour of host haemocytes (reviewed by Humphries and Yoshino 2003). Our previous study showed that S. mansoni ESPs are capable of attenuating ERK signalling in susceptible snail haemocytes (Zahoor et al 2008); in the current study, we have found that the inhibition of the ERK pathway also influences HSP70 protein levels in these cells.…”

Dynamics of haemocytes from Pseudosuccinea columella circulating infected by Fasciola hepatica

“…By histology, sporocysts near the radula were found. According to Humphries & Yoshino (2003), the elimination of metazoan parasites, such as trematodes, by molluscs would be directly associated with the formation of multicellular capsules of haemocytes, in a coordinated action with soluble factors of the haemolymph. In the Biomphalaria spp.…”

The Uniqueness of Achatina fulica in its Evolutionary Success

“…If attention is focused on the functional attributes of hemocytes, several reports in this direction revealed diverse immunological functions such as phagocytosis [34], cytotoxicity [35], aggregation [36] and pathogen encapsulation [37,38]. In addition to hemocytes, hemolymph, the humoral component of the molluscan immune system, is reported to exhibit the activities of superoxide dismutase [39], catalase [40] and acid [41] and alkaline phosphatases [42].…”

“…Acid and alkaline phosphatases are functionally involved in pathogen destruction in phagolysosome which bear immunological significance [50]. Several reports also demonstrated a range of receptors which bind carbohydrates, extracellular matrix proteins, hormones, growth factors and cytokines resulting specific immunocyte signals not only in vertebrates but also in molluscs [37]. Thus, it can be surmised that signaling systems are evolutionary conserved functions of immunocytes in the animal kingdom.…”