Thermal preconditioning shifts cell signaling in favor of pro-life genes pBcl-2 and pBI-1
Expression of pro-survival genes pBI-1 and pBcl-2 and pro-death genes pBAK and pBAX increased during first six hours of the stress (Fig. 2A). After preconditioning, the increase of pBI-1 , pBcl-2 and pBAX expression was even more pronounced (p(BI-1,3h) = 0.006, p(Bcl-2, 0h) = 0.076, p(Bcl-2, 3h) = 0.002, p(BAX, 1h) = 0.027, p(BAX, 3h) = 0.014), but the expression of pro-death BAK gene in PC corals dropped to control levels at 6 hours, differing from the NPC corals (p = 0.084). Upon proper signaling, BAK and BAX heterodimerize and permeabilize mitochondrial membrane, releasing cytochrome c and considerably progressing towards cell death (Galluzzi et al., 2018; Karch et al., 2017). Bcl-2 can antagonize this process via direct protein-protein interaction with both BAK and BAX. BI-1 acts as an apoptosis inhibitor but the exact mechanism is still not fully understood. PCD signaling consists of a complex protein networking where single gene expression profile does not reflect the entirety of the process. For this reason, we calculated the expression ratio of pro-survival vs. pro-death genes to better express the prevalent signalization in the cell. After preconditioning, the pBcl/pBAK, pBcl/pBAX, and pBI/pBAX ratios shift towards the pro-survival partner during the early heat-stress response (Fig. 2B). pBI-1 expression level is higher than the pBAXexpression level in PC vs. NPC corals at the first hour after stress (p = 0.058). pBcl-2 is expressed considerably more than the pBAK gene at 3 hours (p = 0.002), while compared to pBAX , it’s the basal expression of pBcl-2 in non-stressed PC corals and the expression around 1h of the stress that is higher (p(0h) = 0.005, p(1h) = 0.084). All these results suggest that during thermal preconditioning, coral PCD signaling is manipulated to favor the pro-survival genes.