Differential profile of Nix upregulation and translocation during hypoxia/ischaemia in vivo versus in vitro

Mitophagy Regulation Following Myocardial Infarction

Mitophagy, which mediates the selective elimination of dysfunctional mitochondria, is essential for cardiac homeostasis. Mitophagy is regulated mainly by PTEN-induced putative kinase protein-1 (PINK1)/parkin pathway but also by FUN14 domain-containing 1 (FUNDC1) or Bcl2 interacting protein 3 (BNIP3) and BNIP3-like (BNIP3L/NIX) pathways. Several studies have shown that dysregulated mitophagy is involved in cardiac dysfunction induced by aging, aortic stenosis, myocardial infarction or diabetes. The cardioprotective role of mitophagy is well described, whereas excessive mitophagy could contribute to cell death and cardiac dysfunction. In this review, we summarize the mechanisms involved in the regulation of cardiac mitophagy and its role in physiological condition. We focused on cardiac mitophagy during and following myocardial infarction by highlighting the role and the regulation of PI NK1/parkin-; FUNDC1-; BNIP3- and BNIP3L/NIX-induced mitophagy during ischemia and reperfusion.

BNIP3L/NIX-mediated mitophagy: molecular mechanisms and implications for human disease

Mitophagy is a highly conserved cellular process that maintains the mitochondrial quantity by eliminating dysfunctional or superfluous mitochondria through autophagy machinery. The mitochondrial outer membrane protein BNIP3L/Nix serves as a mitophagy receptor by recognizing autophagosomes. BNIP3L is initially known to clear the mitochondria during the development of reticulocytes. Recent studies indicated it also engages in a variety of physiological and pathological processes. In this review, we provide an overview of how BNIP3L induces mitophagy and discuss the biological functions of BNIP3L and its regulation at the molecular level. We further discuss current evidence indicating the involvement of BNIP3L-mediated mitophagy in human disease, particularly in cancer and neurological disorders.

TMEM175 mediates Lysosomal function and participates in neuronal injury induced by cerebral ischemia-reperfusion

As the main organelles for the clearance of damaged proteins and damaged organelles, the function of lysosomes is crucial for maintaining the intracellular homeostasis of long-lived neurons. A stable acidic environment is essential for lysosomes to perform their functions. TMEM175 has been identified as a new K⁺ channel that is responsible for regulating lysosomal membrane potential and pH stability in neurons. This study aimed to understand the role of TMEM175 in lysosomal function of neurons and neuronal injury following cerebral ischemia-reperfusion (I/R). A middle-cerebral-artery occlusion/reperfusion (MCAO/R) model was established in adult male Sprague-Dawley rats in vivo, and cultured neurons were exposed to oxygen-glucose deprivation/reoxygenation (OGD/R) to mimic ischemia-reperfusion (I/R) injury in vitro. We found that the protein level of TMEM175 decreased after cerebral I/R injury and that TMEM175 overexpression ameliorated MCAO/R-induced brain-cell death and neurobehavioral deficits in vivo. Furthermore, these results were recapitulated in cultured neurons. Acridine orange (AO) staining, as well as LysoSensor Green DND-189, cathepsin-B (CTSB), and cathepsin-D (CTSD) activities, showed that TMEM175 deficiency inhibited the hydrolytic function of lysosomes by affecting lysosomal pH. In contrast, TMEM175 upregulation reversed OGD/R-induced lysosomal dysfunction and impaired mitochondrial accumulation in cultured neurons. TMEM175 deficiency induced by cerebral I/R injury leads to compromised lysosomal pH stability, thus inhibiting the hydrolytic function of lysosomes. Consequently, lysosomal-dependent degradation of damaged mitochondria is suppressed and thereby exacerbates brain damage. Exogenous up-regulation of TMEM175 protein level could reverse the neuronal lysosomal dysfunction after ischemia-reperfusion.

BNIP3L/NIX-mediated mitophagy protects against ischemic brain injury independent of PARK2

Cerebral ischemia induces massive mitochondrial damage. These damaged mitochondria are cleared, thus attenuating brain injury, by mitophagy. Here, we identified the involvement of BNIP3L/NIX in cerebral ischemia-reperfusion (I-R)-induced mitophagy. Bnip3l knockout (bnip3l^-/-) impaired mitophagy and aggravated cerebral I-R injury in mice, which can be rescued by BNIP3L overexpression. The rescuing effects of BNIP3L overexpression can be observed in park2^-/- mice, which showed mitophagy deficiency after I-R. Interestingly, bnip3l and park2 double-knockout mice showed a synergistic mitophagy deficiency with I-R treatment, which further highlighted the roles of BNIP3L-mediated mitophagy as being independent from PARK2. Further experiments indicated that phosphorylation of BNIP3L serine 81 is critical for BNIP3L-mediated mitophagy. Nonphosphorylatable mutant BNIP3L^S81A failed to counteract both mitophagy impairment and neuroprotective effects in bnip3l^-/- mice. Our findings offer insights into mitochondrial quality control in ischemic stroke and bring forth the concept that BNIP3L could be a potential therapeutic target for ischemic stroke, beyond its accepted role in reticulocyte maturation.

Neuronal HIF-1α and HIF-2α deficiency improves neuronal survival and sensorimotor function in the early acute phase after ischemic stroke

Hypoxia-inducible factors mediate adaptive responses to ischemia, among others, by induction of anti- and pro-survival genes. Thus, the impact of HIF on neuronal survival upon stroke is controversial. Therefore, neuron-specific knockout mice deficient for Hif1a and Hif2a were exposed to inspiratory hypoxia or ischemia-reperfusion injury. Both Hif1a- and Hif2a-deficient mice showed no altered infarct and edema size, suggesting that both HIF-α subunits might compensate for each other. Accordingly, hypoxic HIF-target gene regulation was marginally affected with exception of anti-survival Bnip3 and pro-survival erythropoietin. In the early acute stage upon stroke, Hif1a/Hif2a double knockout mice exhibited significantly reduced expression of the anti-survival Bnip3, Bnip3L, and Pmaip1 Accordingly, global cell death and edema were significantly reduced upon 24 h but not 72 h reperfusion. Behavioral assessment indicated that Hif1a/Hif2a-deficient mice initially performed better, but became significantly more impaired after 72 h accompanied by increased apoptosis and reduced angiogenesis. Our findings suggest that in neurons HIF-1 and HIF-2 have redundant functions for cellular survival under ischemic conditions. By contrast, lack of anti-survival factors in Hif1a/Hif2a-deficient mice might protect from early acute neuronal cell death and neurological impairment, indicating a benefit of HIF-pathway inhibition in neurons in the very acute phase after ischemic stroke.

rAAV8-733-Mediated Gene Transfer of CHIP/Stub-1 Prevents Hippocampal Neuronal Death in Experimental Brain Ischemia

Brain ischemia is a major cause of adult disability and death, and it represents a worldwide health problem with significant economic burden for modern society. The identification of the molecular pathways activated after brain ischemia, together with efficient technologies of gene delivery to the CNS, may lead to novel treatments based on gene therapy. Recombinant adeno-associated virus (rAAV) is an effective platform for gene transfer to the CNS. Here, we used a serotype 8 rAAV bearing the Y733F mutation (rAAV8-733) to overexpress co-chaperone E3 ligase CHIP (also known as Stub-1) in rat hippocampal neurons, both in an oxygen and glucose deprivation model in vitro and in a four-vessel occlusion model of ischemia in vivo. We show that CHIP overexpression prevented neuronal degeneration in both cases and led to a decrease of both eIF2α (serine 51) and AKT (serine 473) phosphorylation, as well as reduced amounts of ubiquitinated proteins following hypoxia or ischemia. These data add to current knowledge of ischemia-related signaling in the brain and suggest that gene therapy based on the role of CHIP in proteostasis may provide a new venue for brain ischemia treatment.

The critical roles of mitophagy in cerebral ischemia

Mitochondria play a key role in various cell processes including ATP production, Ca²⁺ homeostasis, reactive oxygen species (ROS) generation, and apoptosis. The selective removal of impaired mitochondria by autophagosome is known as mitophagy. Cerebral ischemia is a common form of stroke caused by insufficient blood supply to the brain. Emerging evidence suggests that mitophagy plays important roles in the pathophysiological process of cerebral ischemia. This review focuses on the relationship between ischemic brain injury and mitophagy. Based on the latest research, it describes how the signaling pathways of mitophagy appear to be involved in cerebral ischemia.

Hypoxia Induces autophagic cell death through hypoxia-inducible factor 1α in microglia

As phagocytic cells of central nervous system, excessive activation or cell death of microglia is involved in a lot of nervous system injury and degenerative disease, such as stroke, epilepsy, Parkinson's disease, Alzheimer's disease. Accumulating evidence indicates that hypoxia upregulates HIF-1α expression leading to cell death of microglia. However, the exact mechanism of cell death induced by hypoxia in microglia is not clear. In the current study, we showed that hypoxia induced cell death and autophagy in microglia. The suppression of autophagy using either pharmacologic inhibitors (3-methyladenine, bafilomycin A1) or RNA interference in essential autophagy genes (BECN1 and ATG5) decreased the cell death induced by hypoxia in microglia cells. Moreover, the suppression of HIF-1α using either pharmacologic inhibitors (3-MA, Baf A1) or RNA interference decreased the microglia death and autophagy in vitro. Taken together, these data indicate that hypoxia contributes to autophagic cell death of microglia through HIF-1α, and provide novel therapeutic interventions for cerebral hypoxic diseases associated with microglia activation.

MicroRNA-137 is a novel hypoxia-responsive microRNA that inhibits mitophagy via regulation of two mitophagy receptors FUNDC1 and NIX

Mitophagy receptors mediate the selective recognition and targeting of damaged mitochondria by autophagosomes. The mechanism for the regulation of these receptors remains unknown. Here, we demonstrated that a novel hypoxia-responsive microRNA, microRNA-137 (miR-137), markedly inhibits mitochondrial degradation by autophagy without affecting global autophagy. miR-137 targets the expression of two mitophagy receptors NIX and FUNDC1. Impaired mitophagy in response to hypoxia caused by miR-137 is reversed by re-expression of FUNDC1 and NIX expression vectors lacking the miR-137 recognition sites at their 3' UTR. Conversely, miR-137 also suppresses the mitophagy induced by fundc1 (CDS+3'UTR) but not fundc1 (CDS) overexpression. Finally, we found that miR-137 inhibits mitophagy by reducing the expression of the mitophagy receptor thereby leads to inadequate interaction between mitophagy receptor and LC3. Our results demonstrated the regulatory role of miRNA to mitophagy receptors and revealed a novel link between miR-137 and mitophagy.

miRNA expression is modulated over time after focal ischaemia: up-regulation of miR-347 promotes neuronal apoptosis

Despite the large number of molecules reported as being over-expressed after ischaemia, little is known regarding their regulation. miRNAs are potent post-transcriptional regulators of gene expression, and reports have shown differentially miRNA expression in response to focal cerebral ischaemia. The present study analysed miRNA expression from acute to late phases of ischaemia to identify specific ischaemia-related miRNAs, elucidate their role, and identify potential targets involved in stroke pathophysiology. Of 112 miRNAs, 32 showed significant changes and different expression profiles. In addition to the previously reported differentially expressed miRNAs, new ischaemia-regulated miRNAs have been found, including miR-347. Forty-seven genes involved in brain functions or related to ischaemia are predicted to be potential targets of the differentially expressed miRNAs after middle cerebral artery occlusion. Analysis of four of these targets (Acsl4, Arf3, Btg2 and Dpysl5) showed them to be differentially regulated by ischaemia at the transcriptional or post-transcriptional level. Acsl4, Bnip3l and Phyhip, potential targets of miR-347, were up-regulated after miR-347 over-expression, inducing neuronal apoptotic death. Our findings suggest that miR-347 plays an important role in regulating neuronal cell death, identify Acsl4 as a new protein requiring study in ischaemia, and provide an important resource for future functional studies of miRNAs after ischaemia.

Differential expression of BNIP family members of BH3-only proteins during the development and after axotomy in the rat

The BNIPs (BCL2 and adenovirus E1B 19 kDa interacting proteins) are a subfamily of BCL2 family proteins typically containing a single BCL2 homology 3 (BH3) domain. BNIPs exert important roles in two major degradation processes in cells - apoptosis and autophagy. Although it is known that the function of BNIPs is transcriptionally regulated under hypoxic conditions in tumors, their regulation in the developing brain and neurons following the induction of apoptosis/autophagy is largely unknown. In this study, we demonstrate that three members of the BNIP family, BNIP1, BNIP3 and BNIP3L, are expressed in the developing brain with distinct brain region specificity. BNIP3 mRNA was especially enriched in the entorhinal cortex, raising a possibility that it may have additional biological functions in addition to its apoptotic and autophagic functions. Following starvation-induced autophagy induction, BNIP1 mRNA was selectively increased in cultured neurons. However, the apoptogenic chemical staurosporine failed to modulate the expression of BNIPs, which is in contrast to the marked induction of all BNIPs by glucose-oxygen deprivation. Finally, neonatal nerve axotomy, which triggers apoptosis in motoneurons, selectively enhanced BNIP3 mRNA expression. Collectively, these results suggest that the expression of BNIPs is differentially regulated depending on the stimuli, and BNIPs may exert unique biological functions.

Mitochondrial pruning by Nix and BNip3: an essential function for cardiac-expressed death factors

Programmed cardiac myocyte death via the intrinsic, or mitochondrial, pathway is a mechanism of pathological ventricular remodeling after myocardial infarction and during chronic pressure overload hypertrophy. Transcriptional upregulation of the closely related proapoptotic Bcl2 family members BNip3 in ischemic myocardium and Nix in hypertrophied myocardium suggested a molecular mechanism by which programmed cell death can be initiated by cardiac stress and lead to dilated cardiomyopathy. Studies using transgenic and gene knockout mice subsequently demonstrated that expression of BNip3 and Nix is both sufficient for cardiomyopathy development and necessary for cardiac remodeling after reversible coronary occlusion and transverse aortic banding, respectively. Here, these data are reviewed in the context of recent findings showing that Nix not only stimulates cardiomyocyte apoptosis but also induces mitochondrial autophagy (mitophagy) and indirectly activates the mitochondrial permeability transition pore, causing cell necrosis. New findings are presented suggesting that Nix and BNip3 have an essential function, "mitochondrial pruning," that restrains mitochondrial proliferation in cardiomyocytes and without which an age-dependent mitochondrial cardiomyopathy develops.

Characterization of the endocannabinoid system in human neuronal cells and proteomic analysis of anandamide-induced apoptosis

Anandamide (AEA) is an endogenous agonist of type 1 cannabinoid receptors (CB1R) that, along with metabolic enzymes of AEA and congeners, compose the "endocannabinoid system." Here we report the biochemical, morphological, and functional characterization of the endocannabinoid system in human neuroblastoma SH-SY5Y cells that are an experimental model for neuronal cell damage and death, as well as for major human neurodegenerative disorders. We also show that AEA dose-dependently induced apoptosis of SH-SY5Y cells. Through proteomic analysis, we further demonstrate that AEA-induced apoptosis was paralleled by an approximately 3 to approximately 5-fold up-regulation or down-regulation of five genes; IgG heavy chain-binding protein, stress-induced phosphoprotein-1, and triose-phosphate isomerase-1, which were up-regulated, are known to act as anti-apoptotic agents; actin-related protein 2/3 complex subunit 5 and peptidylprolyl isomerase-like protein 3 isoform PPIL3b were down-regulated, and the first is required for actin network formation whereas the second is still function-orphan. Interestingly, only the effect of AEA on BiP was reversed by the CB1R antagonist SR141716, in SH-SY5Y cells as well as in human neuroblastoma LAN-5 cells (that express a functional CB1R) but not in SK-NBE cells (which do not express CB1R). Silencing or overexpression of BiP increased or reduced, respectively, AEA-induced apoptosis of SH-SY5Y cells. In addition, the expression of BiP and of the BiP-related apoptotic markers p53 and PUMA was increased by AEA through a CB1R-dependent pathway that engages p38 and p42/44 mitogen-activated protein kinases. Consistently, this effect of AEA was minimized by SR141716. In conclusion, we identified BiP as a key protein in neuronal apoptosis induced by AEA.

Apoptosis induced by staurosporine alters chaperone and endoplasmic reticulum proteins: Identification by quantitative proteomics

Apoptosis contributes to cell death after cerebral ischaemia. A quantitative proteomics approach has been employed to define alterations in protein levels in apoptosis induced with staurosporine (STS). Human neuroblastoma derived SH-SY5Y cells were treated with STS (500 nM for 6 h) to induce apoptosis. Quantitative 2-DE was used to determine the changing protein levels with MALDI-TOF MS identification of proteins. Of the 154 proteins analysed, 13 proteins were significantly altered as a result of the apoptotic stimulus; ten of the proteins showed an increase in level with STS and were identified as heat shock cognate 71 (Hsc71), two isoforms of heat shock protein 70 (Hsp70), glucose regulated protein 78 (GRP78), F-actin capping protein, stress-induced phosphoprotein 1, chromatin assembly factor 1 (CAF-1), protein disulphide isomerase A3 (PDI A3) precursor, transitional ER ATPase and actin interacting protein 1 (AIP 1). Three proteins which displayed significant decrease in levels with STS were identified as tubulin, vimentin and glucose regulated protein 94 (GRP94). The functional roles and subcellular locations of these proteins collectively indicate that STS-induced apoptosis provokes induces an unfolded protein response involving molecular chaperones, cochaperones and structural proteins indicative of ER stress.

1H, 13C and 15N resonance assignments for proapoptotic protein Nix (residue 1 approximately 156) from Danio rerio

Nix protein is a BH3-only pro-apoptotic mitochondrial protein. Here we reported the 1H, 13C and 15N resonance assignments of zebrafish Nix protein for further understanding the structure and function relationship.

Proapoptotic Nix activates the JNK pathway by interacting with POSH and mediates death in a Parkinson disease model

Nix, a pro-apoptotic BH3-only protein, promotes apoptosis of non-neuronal cells, although the mechanisms involved remain incompletely understood. Using a yeast two-hybrid screen with POSH (plenty of SH3 domains, a scaffold involved in activation of the apoptotic JNK/c-Jun pathway) as the bait, we identified an interaction between POSH and Nix. Co-immunoprecipitation and in vitro binding studies confirmed a direct interaction between POSH and Nix in mammalian cells. When overexpressed in HEK293 cells, Nix promotes apoptosis along with enhanced phosphorylation/activation of JNKs and their target c-Jun. These effects appear to be dependent on POSH because Nix does not promote either JNK/c-Jun phosphorylation or apoptosis of 293 cells that do not express POSH. Nix and POSH appear to mutually stabilize one another and this effect could contribute to their promotion of death. Past work showed induction of Nix transcripts in a cellular model of Parkinson disease based on neuronal PC12 cells exposed to 6-hydroxydopamine. Here, we confirm elevation of Nix protein in this model and that Nix over-expression causes apoptotic death of PC12 cells by a mechanism dependent on c-Jun activation. Expression of s-Nix, a dominant-negative form of Nix, protects neuronal PC12 cells from 6-hydroxydopamine but not from nerve growth factor deprivation. These results indicate that Nix promotes cell death via interaction with POSH and activation of the JNK/c-Jun pathway and that Nix protein is induced and contributes to cell death in a cellular model of Parkinson disease.

Nucleophosmin is a novel Bax chaperone that regulates apoptotic cell death

The proapoptotic B-cell lymphoma-2 family protein Bax is a key regulatory point in the intrinsic apoptotic pathway. However, the factors controlling the process of Bax activation and translocation to mitochondria have yet to be fully identified and characterized. We performed affinity chromatography using peptides corresponding to the mitochondrial-targeting region of Bax, which is normally sequestered within the inactive structure. The molecular chaperone nucleophosmin was identified as a novel Bax-binding protein by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Reciprocal co-immunoprecipitation and proximity assays confirmed the Bax-nucleophosmin protein-protein interaction and verified that nucleophosmin only bound to activated conformationally altered Bax. Confocal microscopy in a cell-based apoptosis model, demonstrated that nucleophosmin translocation from nucleolus to cytosol preceded Bax movement. Specific knockdown of nucleophosmin expression using RNAi attenuated apoptosis as measured by mitochondrial cytochrome c release and activation of the caspase cascade. In a mouse model of ischaemic stroke, subcellular fractionation studies verified that nucleophosmin translocation occurred within 3 h, at a time before Bax translocation but after Bax conformational changes have occurred. Thus, we have elucidated a novel molecular mechanism whereby Bax becomes activated and translocates to the mitochondria to orchestrate mitochondrial dysfunction and apoptotic cell death, which opens new avenues for therapeutic intervention.

Angiogenesis in gliomas: biology and molecular pathophysiology

Glioblastoma multiforme (GBM) is characterized by exuberant angiogenesis, a key event in tumor growth and progression. The pathologic mechanisms driving this change and the biological behavior of gliomas remain unclear. One mechanism may involve cooption of native blood vessels by glioma cells inducing expression of angiopoietin-2 by endothelial cells. Subsequently, vascular apoptosis and involution leads to necrosis and hypoxia. This in turn induces angiogenesis that is associated with expression of hypoxia-inducible factor (HIF)-1alpha and vascular endothelial growth factor (VEGF) in perinecrotic pseudopalisading glioma cells. Here we review the molecular and cellular mechanisms implicated in HIF-1-dependent and HIF-1-independent glioma-associated angiogenesis. In GBMs, both tumor hypoxia and genetic alterations commonly occur and act together to induce the expression of HIF-1. The angiogenic response of the tumor to HIF-1 is mediated by HIF-1-regulated target genes leading to the upregulation of several proangiogenic factors such as VEGF and other adaptive response molecules. Understanding the roles of these regulatory processes in tumor neovascularization, tumor growth and progression, and resistance to therapy will ultimately lead to the development of improved antiangiogenic therapies for GBMs.