II.3. SOME METAZOANS CAN ADAPT TO SEVERE HYPOXIA AND ANOXIA
II.3.4. Postconditioning and preconditioning
Finally, to cope with the increased ROS production during the reperfusion period following an oxygen deprivation, anoxia-tolerant animals significantly increase synthesis of specific antioxidant enzymes during the severe hypoxia or anoxia incubation (9, 21).
II.3.4. Postconditioning and preconditioning
Postconditioning is a relatively new concept that consists of a series of mechanical interruptions of reperfusion after ischemia. Such a reperfusion prevents IR injury (114, 115).
Ischemic preconditioning (IP) is defined as an increased tolerance to ischemia and reperfusion induced by a previous sublethal period of ischemia (figure 12 a).
Such a protection has been shown to occur in every species tested so far (116, 117).
Basically, IP induces tolerance both to ischemia itself and to reperfusion. Indeed, IP induces channel arrest (118), maintenance of intra-cellular ion homeostasis and
acid-base balance during prolonged ischemia (119). Moreover, IP gives the cell the ability to function with less ATP during ischemia (120), possibly through the AMPK (121).
Together, these observations suggest that preconditioning adopts strategies that can be compared to those used by metazoans to resist anoxia. Moreover, during reperfusion, preconditioned cells show greater resistance to ROS (122) and an inhibition of MPTP opening (123), resulting in less mitochondrial apoptosis-inducing factors been released compared to non-preconditioned cells (124).
IP offers two windows of protection: an initial strong protective stimulus (early or classical preconditioning) that is brief and a later less powerful but longer lasting protection (late or delayed preconditioning), which may require de novo protein synthesis. There is a transient loss of protection between the two protection windows (figure 12 a and b) (116, 121, 125).
Figure 12: Schematic representation of IP. a. Episodes of non-lethal ischemia (isch) immediately preceding ischemia (early IP) or separated by a 24 h delay before ischemia (late IP). b. Biphasic protection induced by IP. Note that the ischemic protection conferred by IP is lost between the early and late preconditioning (modified from 121).
Importantly, it has been shown that many stressors induce protection against IR (116). For instance, brief period of acute volume loading resulting in myocardial stretch (126) and transient hyperthermia preceding a sustained period of total oxygen deprivation (127-129) have both been shown to limit IR injury. It is thought that similar signaling pathways are shared between classical ischemic preconditioning and
“stress” preconditioning (130). Briefly, adenosine binds to the G-protein-coupled receptor A1 and A3, resulting in the phospholipase C (PLC) activation. The hydrolysis of phosphatidyl-4,5-bisphosphate (PIP2) produces inositol-1,4,5-triphsphate (IP3) and diacylglycerol (DAG) that activate the protein kinase C (PKC). After its activation, PKC may effect protection by activation of protein tyrosine kinase (PTK), which activates map kinases (MAPK). MAPK consist of three major families (p38MAPK, p46 and p54 c-Jun N-terminal kinases (JNK) and extracellular signal-regulated
kinases (ERK)). Activation of p38MAPK leads to the activation of heat-shock protein 27 (HSP27) that promotes cytoskeleton stability, and thus may protect against reperfusion injury (125). PKC also activates both KATP channels that inhibit apoptosis induced by ROS and the ecto-5’-nucleotidase that transforms adenosine mono-phosphate into adenosine, which increases the preconditioning signal (figure 13) (116, 125).
Figure 13: Schematic representation of the signal transduction cascade induced by IP. See main text (modified from 125).
Interestingly, the bioactive ceramide (Cer) has been shown to play a crucial role in IP of rat heart (131) and adding Cer to fibroblasts mimics the effect of IP (132). Importantly, ceramidase (CDase) transforms Cer to the bioactive sphingosine, which is the precursor of another essential mediator of IP: the bioactive lipid sphingosine-1-phosphate (S1P). S1P is known to be a survival factor for a variety of cell types (133-135) and is formed by the phosphorylation of sphingosine catalyzed by the sphingosine kinase (SK) (136). Local S1P protect the heart against IR injury and mediates preconditioning (137). Importantly, it has been shown that IP is able to protect remote cells and organs, which have not been preconditioned by themselves, a phenomenon called remote preconditioning (138, 139). Increasing evidence suggest that high-density lipoproteins (HDL) are a direct agent of the protection conferred by
the remote preconditioning and that S1P is responsible for the beneficial effect of HDL (137). Mechanistically, PKCε is recruited by IP and induces the activation of SK, which phosphorylates the sphingosine to form S1P (140). S1P is then secreted and binds to one of the five S1P receptors (S1PR1 to 5), which is a G protein-coupled receptor (141). Different receptors are differentially expressed in different cells and tissues and coupled to different G proteins, leading to distinct signaling pathways and cellular responses (141, 142) (figure 14).
Figure 14: Schematic representation of the S1P formation following IP. Following IP, the PKCε is activated and phosphorylates SK. Such phosphorylation activates SK, which phosphorylates sphingosine into S1P. S1P is exported outside the cell and binds to one of the five G-coupled receptor S1PR1 to 5 that in turn activates different intracellular pathways depending on the receptor (modified from 142).
II.4. CERAMIDES
Eukaryotic cell membranes possess three main kinds of lipid families: the glycerolipids, the sterols and the sphingolipids (SL) (143). In 1884, J. Thudichum originally named the latter for their enigmatic nature in reference to the sphinx of the mythology. In addition to their structural role in membrane formation, molecules belonging to the SL family have been shown to function as bioactive signaling molecules. Indeed, they are known to have essential roles in membrane microdomains (lipid raft) and to act as both first and second messengers in different signaling pathways controlling, for instance, regulation of cell growth, stress response, apoptosis, differentiation and senescence (144). Ceramide (Cer) is the simplest SL that occupies a central position in sphingolipid biosynthesis/catabolism. It serves as the precursor for all major sphingolipid species in eukaryotes (142). Basically, Cer consists of a sphingoid base to which a fatty acid is attached at C-2 via N-acetylation (figure 15). It serves as a backbone for all complex SL, which are formed by attachment of different head groups at C-1 (145).
Figure 15: “Generic” structural representation of a ceramide. R represents the attachment of any head group at C-1 (from Wikipedia, searching for “ceramide”).