Exposure to the Neurotoxic Dinoflagellate, Alexandrium catenella, Induces Apoptosis of the Hemocytes of the Oyster, Crassostrea gigas

Walid Medhioub,Zouher Amzil,Veronique Savar,Simon Ramondenc,Agnes Vergnes,Mohamed Laabir,Estelle Masseret,Jean Rolland,Audrey Vanhove

doi:10.3390/md11124799

Abstract

This study assessed the apoptotic process occurring in the hemocytes of the Pacific oyster, Crassostrea gigas, exposed to Alexandrium catenella, a paralytic shellfish toxins (PSTs) producer. Oysters were experimentally exposed during 48 h to the toxic algae. PSTs accumulation, the expression of 12 key apoptotic-related genes, as well as the variation of the number of hemocytes in apoptosis was measured at time intervals during the experiment. Results show a significant increase of the number of hemocytes in apoptosis after 29 h of exposure. Two pro-apoptotic genes (Bax and Bax-like) implicated in the mitochondrial pathway were significantly upregulated at 21 h followed by the overexpression of two caspase executor genes (caspase-3 and caspase-7) at 29 h, suggesting that the intrinsic pathway was activated. No modulation of the expression of genes implicated in the cell signaling Fas-Associated protein with Death Domain (FADD) and initiation-phase (caspase-2) was observed, suggesting that only the extrinsic pathway was not activated. Moreover, the clear time-dependent upregulation of five (Bcl2, BI-1, IAP1, IAP7B and Hsp70) inhibitors of apoptosis-related genes associated with the return to the initial number of hemocytes in apoptosis at 48 h of exposure suggests the involvement of strong regulatory mechanisms of apoptosis occurring in the hemocytes of the Pacific oyster.

Highlights

Apoptosis or type I programmed cell death was reported to play an important role in organism immunity, especially in mollusks [1,2]
The aim of the present study was to investigate if exposure to a dense concentration of the toxic dinoflagellate, A. catenella, induces the apoptosis of the effective cells involved in the immune responses of oyster, the hemocytes
In C. gigas, key genes involved in this process appear to be similar to those of the vertebrate model

Summary

Introduction

Apoptosis or type I programmed cell death was reported to play an important role in organism immunity, especially in mollusks [1,2]. Extracellular signals activate the extrinsic pathway (receptor-mediated) through death receptors. Fas-Associated protein with Death Domain (FADD), forming the death-inducing signaling complex (DISC), inducing the activation of initiator cysteine proteases of the caspase family. B-cell lymphoma 2 (Bcl-2) family proteins regulate this process by releasing apoptotic signals from the mitochondria [6,7,8,9]. Members of this family are conserved in invertebrate [10]. Pro-apoptosis can be further subdivided into more fully conserved, “multi-domain” members with homology in the BH1–BH3 domains and BH3-only Bcl-2 family proteins. The cell death process is regulated by inhibitors of caspase, inhibitors of apoptosis (IAPs) [6] and heat shock proteins (Hsps) [12,13,14]

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Results

Conclusion