Summary
We investigated effects of acute heat- and cold stress on cell viability, lysosome membrane stability, double- and single-stranded DNA breakage, and signalling mechanisms involved in cellular homeostasis and apoptosis in hemocytes of native and invasive mussels, Mytilus californianus and M. galloprovincialis, respectively. Both heat stress (28ºC, 32ºC) and cold stress (2ºC, 6ºC) led to significant double- and single-stranded breaks in DNA. The types and extents of DNA damage were temperature- and time-dependent, as was caspase-3 activation, an indicator of apoptosis, which may occur in response to DNA damage. Hemocyte viability and lysosomal membrane stability decreased significantly under heat stress. Western blot analyses of hemocyte extracts with antibodies for proteins associated with cell signalling and stress responses [including members of the phospho-specific Mitogen Activated Protein Kinase (MAPK) family (c-JUN NH(2)-terminal kinase (JNK) and p38-MAPK) and apoptosis executor caspase-3] revealed that heat- and cold stress induced a time-dependent activation of JNK, p38-MAPK and caspase-3 and that these signalling and stress responses differed between species. Thermal limits for activation of cell signalling processes linked to repair of stress-induced damage may help determine cellular thermal tolerance limits. Our results show similarities in responses to cold- and heat stress and suggest causal linkages between levels of DNA damage at both extremes of temperature and downstream regulatory responses, including induction of apoptosis. Compared to M. californianus, M. galloprovincialis might have a wider temperature tolerance due to a lower amount of double-stranded DNA damage, faster signalling activation and transduction, and stronger repair ability against temperature stress.
