Coordination of intra- and extracellular signaling during salinity stress
Von Willebrand factor A domain-containing (VFA) proteins are extracellular proteins involved in cell-cell adhesion and signaling as well as blood clot formation after injury. VFA domain-containing protein 8 (VFA8) was the third most highly regulated protein (FC=6.41, p=.0001) while VFA/sushi/EGF and pentraxin domain-containing protein 1 was the second most significantly regulated mRNA (FC=-17.4, p=1.02E-6). This opposite regulation pattern suggests that different VFA proteins are associated with renal function in FW versus BW acclimated tilapia. In one study, red blood cell count was significantly reduced in O. niloticus exposed to 12ppt saline water for 14 days (Elarabany, Bahnasawy, Edrees, & Alkazagli, 2017). Osmotic stress is known to induce erythrocyte cell death by opening Ca2+-permeable cation channels, increasing cytosolic Ca2+ activity and triggering erythrocyte apoptosis (Lang et al., 2003). Therefore, BW acclimation may increase erythrocyte cell death in the kidney. This conclusion is supported by the significant reduction in hemoglobin subunit beta-1 (FC=-7.27, p=.0051) and increase in haptoglobin (FC=2.56, p=.03), which neutralizes the oxidative effects of free heme groups. Furthermore, VFA8 has been reported to translocate to the inner membrane of mitochondria in response to calcium-induced signaling and to influence mitochondrial energy metabolism (Luo et al., 2020). These data suggest that VFA proteins are involved in linking extracellular osmotic stress sensing with intracellular responses such as increased mitochondrial energy production.