Deep sea mussel bathymodiolus azoricus exposure to vibrio diabolicus induces the expression of apoptotic genes

Apoptosis is a biological process known as programmed cell death that is found broadly in an array of organisms from invertebrates to vertebrates. This process has a key role in the maintenance of the immune system balance; in the normal cellular development and in the responses to environmental stress. The extreme conditions to which deep sea vent organisms have come to adapt represent an outstanding model to study immune responses and how they may be affected by the apoptosis pathway. At such chemosynthetic habitats, surrounding microorganisms, reduced toxic chemical compounds from venting activity and high hydrostatic pressure constitute ecological and physiological challenges that require the symbiont bearing vent mussel Bathymodiolus azoricus to engage critical signalling pathways leading to immune and apoptosis survival mechanisms. Here we demonstrate how vent mussels when challenged with the hydrothermal vent-related Vibrio diabolicus bacterium and submitted to similar hydro- static pressure found at Menez Gwen vent site, are able to regulate their immune and apoptosis-related genes. Fifteen differentially expressed immune and apoptosis genes were analysed by means of quantitative-Polymerase Chain Reaction (qPCR) in B. azoricus gill tissues from 3h, 12h, 24h and 48h post- Vibrio challenges and from seawater control mussels. Our results demonstrated that Caspase 8 was the highest up-regulated gene in 3h and 24h post-Vibrio challenge gill samples whereas, 12h and 24h post- Vibrio challenge gill samples showed remarkably different profiles for LITAF and TRAF6 genes whose expression was down-regulated. However, TRAF6 showed a contrasting up-regulation in Vibrio challenged mussels comparing to all the remaining genes at 48h post- Vibrio challenge. Control and V. diabolicus challenged mussels showed a clear significant expression difference for all genes tested. These results suggested that bacterial inoculation and IPOCAMP stimuli induced initial stress reactions leading to up-regulation of Caspase 8, MyD88, LITAF and GpxI genes. After 48h post-Vibrio challenge, gene expression was down-regulated, as shown for apoptotic genes including Caspases 8 and 9, BAX, and members of the Bcl-2 family. These novel results suggest that the down-regulation of apoptotic genes is involved in keeping active the immune gene transcriptional activity in deep-sea vent mussel B. azoricus possibly intertwining apoptotic signalling pathways.