Proteostasis During Cerebral Ischemia

Abstract

Cerebral ischemia is a complex pathology involving a cascade of cellular mechanisms, which deregulate proteostasis and lead to neuronal death. Proteostasis refers to the equilibrium between protein synthesis, folding, transport, and protein degradation. Within the brain proteostasis plays key roles in learning and memory by controlling protein synthesis and degradation. Two important pathways are implicated in the regulation of proteostasis: the unfolded protein response (UPR) and macroautophagy (called hereafter autophagy). Both are necessary for cell survival, however, their over-activation in duration or intensity can lead to cell death. Moreover, UPR and autophagy can activate and potentiate each other to worsen the issue of cerebral ischemia. A better understanding of autophagy and ER stress will allow the development of therapeutic strategies for stroke, both at the acute phase and during recovery. This review summarizes the latest therapeutic advances implicating ER stress or autophagy in cerebral ischemia. We argue that the processes governing proteostasis should be considered together in stroke, rather than focusing either on ER stress or autophagy in isolation.

Keywords: ER stress; autophagy; mTOR; proteostasis; stroke.

FIGURE 1

Schematic model of autophagy. (A) Macroautophagy is the result of the fusion of an autophagosome, a double membrane vacuole transporting cellular cargo that has been targeted for degradation, with a lysosome. (B) Microautophagy is a non-selective lysosomal degradative process that involves direct engulfment of cytoplasmic materials by the invagination of the lysosome. (C) Chaperone-mediated autophagy involves the direct translocation, through LAMP2A, of specific proteins containing a motif recognized by a chaperone named hsc70 (heat-shock cognate protein of 70 kDa).

FIGURE 2

Autophagy signaling pathway. Autophagy is induced by an energetic stress, leading to AMPK activation; or by growth factors starvation, leading to the inhibition of the class I phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway. Two complexes can be involved in autophagy induction: ULK1 complex composed of autophagy-regulated 101 (Agt1), Atg101, FIP200, and ULK1 and class III PI3K complex form by Beclin-1, vacuolar protein sorting 34 (VPS34) and vacuolar protein sorting 15 (VPS15). (A) Schematic illustration of autophagy initiation and autophagosome formation. Different Atg proteins including Atg5-Atg12-Atg16L and the microtubule-associated protein LC3-II are involved in double membrane elongation and in autophagosome-lysosome fusion. P62 binds to ubiquitinated proteins targeting them for degradation in the autolysosomes. Rapamycin can activate autophagy by inhibiting mTOR Complex 1. Both 3-MA (3-methyladenine), chloroquine and Bafilomycin are autophagy inhibitors by, respectively, inhibiting Class III PI3K complex and autophagosome/lysosome fusion. (B) Schematic illustration of cross talk and interaction among autophagy, apoptosis and necroptosis during brain ischemia. The dissociation of Beclin1 and Bcl-2 is also responsible of autophagy increase. The receptor interacting protein (RIP) Kinase 1 activation by death receptors, such as Fas, TLR4 and TNFR1, is responsible of Jun N-terminal kinase 1 (JNK1) pathway activation, ROS (reactive oxygen species) release and necroptosis. RIP1 interacts with FADD (Fas-Associated protein with Death Domain) and TRADD (Tumor necrosis factor receptor type 1-associated DEATH domain protein) to induce extrinsic apoptosis. RIP1 is also involved in intrinsic (mitochondrial) apoptosis through cytochrome C release which activates caspase-9 and capase-3.

FIGURE 3

Endoplasmic reticulum (ER) stress signaling. Under physiological conditions, the Glucose Related Protein 78 (GRP78) binds to the three ER transmembrane sensors; protein kinase RNA-like ER kinase (PERK), inositol-requiring kinase 1α (IRE1α) and activating transcription factor 6 (ATF6), and maintains them inactive. Ischemic stroke induces ER stress, leading to the activation of the UPR. Under stress conditions, GRP78 dissociates from the ER sensors to bind to misfolded proteins, allowing the trans-autophosphorylation of PERK. P-PERK phosphorylates the eukaryotic translation initiation factor 2α (eIF2α), which decreases the global protein translation. Paradoxically, P-eIF2α allows the transcription of activating transcription factor 4 (ATF4), CCAT/enhancer binding protein homologous protein (CHOP). Both P-eIF2α and CHOP can activate the translation of growth arrest and DNA damage inducible gene/protein 34 (GADD34). The latter creates a feedback mechanism by dephosphorylating eiF2α. PERK is associated to apoptosis, autophagy and redox control. IRE1α dimerizes and trans-autophosphorylates, to splice the unspliced mRNA XBP1 (uXBP1) into spliced XBP1 (sXBP1). IRE1α activates c-jun-N-terminal-inhibiting kinase (JIK) and allows the recruitment of TNF receptor-associated factor 2 (TRAF2), leading to the activation of nuclear factor κ-B (NFκ-B), caspase 12 and c-jun-N-terminal kinase (JNK) pathway. IRE1α activates Regulated IRE1-Dependent Decay (RIDD) leading to the degradation of selected mRNAs. IRE1α is also involved in apoptosis, ER-associated degradation (ERAD) and lipid synthesis. The activation of ATF6 is dependent of its cleavage by S1P and S2P endopeptidases in the Golgi apparatus. Cleaved ATF6 induces the transcription of GRP78, nitric oxide synthase (NOS) and CHOP. Moreover, cleaved ATF6 controls ERAD, apoptosis and protein folding.