Palliative effect of abalone insulin-related peptide 2 on emersion stress-induced hyperglycemia in the Pacific abalone (Haliotis discus hannai)

The Pacific abalone (Haliotis discus hannai), an economically important marine mollusk in East Asia, is often exposed to extreme environments, such as high water temperatures during summer and air exposure during handling or transportation. These stressful conditions lead to hyperglycemia and the generation of reactive oxygen species (ROS), resulting in oxidative stress and subsequent cell damage in the organism. To understand the role of insulin-related peptides in the regulation of elevated hemolymph glucose, hyperglycemia was confirmed in abalone under high water temperature and air (emersion) exposure (p < 0.05). To prove the existence of a glucose regulatory system in abalone, changes in hemolymph glucose were measured after bovine insulin injection, and hemolymph glucose was lowered by bovine insulin injection in hyperglycemia caused by both stressors (p < 0.05), supporting the existence of a glucose regulatory system functionally similar to that of vertebrates. Among the three abalone insulin-related peptides (AIPs), which were previously reported to be upregulated along with hemolymph glucose levels upon feeding, the relative mRNA expression of AIP2 in the cerebral ganglion showed a close correlation with high water temperature and emersion stress, which peaked 12 h after treatment (p < 0.05). Based on these results, we focused on the function of AIP2 in glucose homeostasis and characterized the localization of AIP2 mRNA-expressing cells in the cerebral ganglion using in situ hybridization. Positive signals from RNA probes were detected in neural cortex cells in the cerebral ganglion. Furthermore, suppressing AIP2 mRNA expression using RNA interference with double-stranded RNA (dsRNA) revealed that dsRNA-AIP2 injected abalone maintained high hemolymph glucose levels after 24 h of recovery following 1 h of emersion stress (p < 0.01). In contrast, abalones that received dsRNA-GFP and molluscan saline showed relatively rapid clearance of hemolymph glucose. This study demonstrates that AIP2 gene expression affects the recovery rate from emersion stress-induced hyperglycemia by regulating hemolymph glucose.