The expression of pNFkB in PC and NPC corals supports the
involvement of autophagy/symbiophagy in P. acuta bleaching
pBcl-2 and other involved proteins are the effectors in the PCD
signaling, not the triggers. That’s why we analyzed the expression rate
of NFkB, a pleiotropic transcription factor (TF) involved in many
crucial cellular processes. In response to various stressors, NFkB is
translocated to nucleus where it usually triggers transcription of
pro-survival genes such as Bcl-2 (Luo, Kamata, & Karin, 2005).
Increased expression levels of NFkB as a result of hyperthermal
shock have been observed in coral Acropora palmata (DeSalvo,
Sunagawa, Voolstra, & Medina, 2010). Moreover, Bellantuono et al.
observed increased levels of NFkB inhibitor (NFKBI ) in thermally
PC corals (Bellantuono et al., 2012). Our results also show increasedNFKBI expression levels in PC vs NPC corals (p(0h) = 0.003, p(1h) = 0.013) (Fig. 5B).
It suggests the role of NFkB in corals is to promote, not to prevent
cell death. It is known that NFkB is involved in innate immune system
across species (Gilmore & Wolenski, 2012). It promotes organismal
immune reaction to get rid of the pathogen while it enhances the
capability of the host cells to survive. One of the algal-coral
symbiosis theories suggests that coral symbiont forms an intracellular
organelle called the symbiosome, which is a late endosome in the state
of arrested phagocytosis (Davy, Allemand, & Weis, 2012). Under stress
conditions, the host innate immune response is reactivated leading to a
specific type of autophagy called symbiophagy. Autophagy and NFkB
pathway share common upstream signals and can control each other
(Trocoli & Djavaheri-Mergny, 2011). During signaling, NFkB represses
autophagy through activation of mTOR, but under specific conditions such
as presence of reactive oxygen species (ROS) or after heat shock, NFkB
directly activates autophagy (Trocoli & Djavaheri-Mergny, 2011). It has
been well documented that symbionts release increased ROS during heat
stress due to overexcitation of their photosynthetic systems, and
symbiophagy is one of the main observed mechanisms of algal loss in
hyperthermia (Downs et al., 2009; Dunn et al., 2007). NFkB has the
potential to link these two processes.