Increased autophagy reduces endoplasmic reticulum stress after neonatal hypoxia-ischemia: role of protein synthesis and autophagic pathways

Melatonin modulates neonatal brain inflammation through endoplasmic reticulum stress, autophagy, and miR-34a/silent information regulator 1 pathway

Maternal infection/inflammation represents one of the most important factors involved in the etiology of brain injury in newborns. We investigated the modulating effect of prenatal melatonin on the neonatal brain inflammation process resulting from maternal intraperitoneal (i.p.) lipopolysaccharide (LPS) injections. LPS (300 μg/kg) was administered to pregnant rats at gestational days 19 and 20. Melatonin (5 mg/kg) was administered i.p. at the same time as LPS. Melatonin counteracted the LPS sensitization to a second ibotenate-induced excitotoxic insult performed on postnatal day (PND) 4. As melatonin succeeded in reducing microglial activation in neonatal brain at PND1, pathways previously implicated in brain inflammation regulation, such as endoplasmic reticulum (ER) stress, autophagy and silent information regulator 1 (SIRT1), a melatonin target, were assessed at the same time-point in our experimental groups. Results showed that maternal LPS administrations resulted in an increase in CHOP and Hsp70 protein expression and eIF2α phosphorylation, indicative of activation of the unfolded protein response consequent to ER stress, and a slighter decrease in the autophagy process, determined by reduced lipidated LC3 and increased p62 expression. LPS-induced inflammation also reduced brain SIRT1 expression and affected the expression of miR-34a, miR146a, and miR-126. All these effects were blocked by melatonin. Cleaved-caspase-3 apoptosis pathway did not seem to be implicated in the noxious effect of LPS on the PND1 brain. We conclude that melatonin is effective in reducing maternal LPS-induced neonatal inflammation and related brain injury. Its role as a prophylactic/therapeutic drug deserves to be investigated by clinical studies.

Adaptive preconditioning in neurological diseases - therapeutic insights from proteostatic perturbations

In neurological disorders, both acute and chronic neural stress can disrupt cellular proteostasis, resulting in the generation of pathological protein. However in most cases, neurons adapt to these proteostatic perturbations by activating a range of cellular protective and repair responses, thus maintaining cell function. These interconnected adaptive mechanisms comprise a 'proteostasis network' and include the unfolded protein response, the ubiquitin proteasome system and autophagy. Interestingly, several recent studies have shown that these adaptive responses can be stimulated by preconditioning treatments, which confer resistance to a subsequent toxic challenge - the phenomenon known as hormesis. In this review we discuss the impact of adaptive stress responses stimulated in diverse human neuropathologies including Parkinson׳s disease, Wolfram syndrome, brain ischemia, and brain cancer. Further, we examine how these responses and the molecular pathways they recruit might be exploited for therapeutic gain. This article is part of a Special Issue entitled SI:ER stress.

Hydrogen-rich saline mediates neuroprotection through the regulation of endoplasmic reticulum stress and autophagy under hypoxia-ischemia neonatal brain injury in mice

Hydrogen as a new medical gas exerts organ-protective effects through regulating oxidative stress, inflammation and apoptosis. Multiple lines of evidence reveal the protective effects of hydrogen in various models of brain injury. However, the exact mechanism underlying this protective effect of hydrogen against hypoxic-ischemic brain damage (HIBD) is not fully understood. The present study was designed to investigate whether hydrogen-rich saline (HS) attenuates HIBD in neonatal mice and whether the observed protection is associated with reduced endoplasmic reticulum (ER) stress and regulated autophagy. The results showed that HS treatment significantly improved brain edema and decreased infarct volume. Furthermore, HS significantly attenuated HIBD-induced ER stress responses, including the decreased expression of glucose-regulated protein 78, C/EBP homologous protein, and down-regulated transcription factor. Additionally, we demonstrated that HS induced autophagy, including increased LC3B and Beclin-1 expression and decreased phosphorylation of mTOR and Stat3, as well as phosphorylation of ERK. Taken together, HS exerts neuroprotection against HIBD in neonatal mouse, mediated in part by reducing ER stress and increasing autophagy machinery.

The role of autophagy in cardiac hypertrophy

Autophagy is conserved in nature from lower eukaryotes to mammals and is an important self-cannibalizing, degradative process that contributes to the elimination of superfluous materials. Cardiac hypertrophy is primarily characterized by excess protein synthesis, increased cardiomyocyte size, and thickened ventricular walls and is a major risk factor that promotes arrhythmia and heart failure. In recent years, cardiomyocyte autophagy has been considered to play a role in controlling the hypertrophic response. However, the beneficial or aggravating role of cardiomyocyte autophagy in cardiac hypertrophy remains controversial. The exact mechanism of cardiomyocyte autophagy in cardiac hypertrophy requires further study. In this review, we summarize the controversies associated with autophagy in cardiac hypertrophy and provide insights into the role of autophagy in the development of cardiac hypertrophy. We conclude that future studies should emphasize the relationship between autophagy and the different stages of cardiac hypertrophy, as well as the autophagic flux and selective autophagy. Autophagy will be a potential therapeutic target for cardiac hypertrophy.

Autophagy Promotes Peripheral Nerve Regeneration and Motor Recovery Following Sciatic Nerve Crush Injury in Rats

Autophagy maintains cellular homeostasis by stimulating the lysosomal degradation of cytoplasmic structures, including damaged organelles and dysfunctional proteins. The role of autophagy in the renewal and regeneration of injured peripheral nerves remains poorly understood. The current study investigated the role of autophagy in peripheral nerve regeneration and motor function recovery following sciatic nerve crush injury in rats by stimulating or suppressing autophagy and detecting the presence of autophagosomes and LC3-II expression by electron microscopy and Western blotting, respectively. Neurobehavioral function was tested by CatWalk gait analysis 1, 2, 3, and 6 weeks after injury, and the expression of neurofilament (NF)-200 and myelin basic protein (MBP) at the injury site was examined by immunocytochemistry. Apoptosis at the lesion site was determined by the terminal deoxynucleotidyl transferase dUTP nick end labeling assay. Treatment of injured rats with the autophagy inducer rapamycin increased the number of autophagosomes and LC3-II expression while reducing the number of apoptotic cells at the lesion; this was associated with an upregulation of MBP and NF-200 expression and increased motor function recovery as compared to sham-operated rats and those that were subjected to crush injury but untreated. The opposite effects were observed in rats treated with the autophagy inhibitor 3-methyladenine. These data indicate that the modulation of autophagy in peripheral nerve injury could be an effective pharmacological approach to promote nerve regeneration and reestablish motor function.

Decreased Energy Capacity and Increased Autophagic Activity in Optic Nerve Axons With Defective Anterograde Transport

PURPOSE

Autophagy is a critical process, compromised in neurodegenerative disease, by which terminally differentiated cells like neurons manage cytoskeletal and organelle turnover. How autophagy relates to associated neurodegenerative pathologies remain unclear. We examined autophagy in optic neuropathy by investigating cytoskeletal degradation, mitochondria, and autophagic vesicles in the DBA2/J mouse model of glaucoma exhibiting differing levels of axon transport functionality.

METHODS

DBA/2J and DBA/2J(wt-gpnmb) control mice 11 to 14 months of age were injected with cholera toxin-B (CTB) to assay anterograde axonal transport. Axonal mitochondria and autophagic vesicles were analyzed with respect to transport integrity in proximal and distal optic nerve using serial block face scanning electron microscopy (3D EM).

RESULTS

Several indices varied significantly between the DBA/2J and DBA/2J(wt-gpnmb) mice, including mitochondrial volume, average number of autophagic vesicles per axon, and mitochondrial cristae. However, there were no differences in mitochondrial cristae for axons with functional versus dysfunctional CTB transport, suggesting that mitochondrial dysfunction precedes overt transport blockade. Anterograde transport failure was accompanied by a dissociation of the relationship between mitochondrial and axon volumes. Autophagic vesicle profiles were significantly increased in optic nerve with transport deficit, consistent with greater autophagic activity. Mitochondria within autophagosomes, indicative of mitophagy, were observed in both proximal and distal axons.

CONCLUSIONS

Loss of anterograde transport in DBA/2J optic nerve is concomitant with diminished mitochondrial volume, increased cytoskeletal breakdown and autophagic activity, and accumulation of autophagic profiles, including signs of mitophagy, in proximal optic nerve. Axons with transport deficit are metabolically underserved, though not necessarily from mitophagy.

Endoplasmic reticulum pathology and stress response in neurons precede programmed necrosis after neonatal hypoxia-ischemia

The endoplasmic reticulum (ER) is tasked, among many other functions, with preventing excitotoxicity from killing neurons following neonatal hypoxia-ischemia (HI). With the search for delayed therapies to treat neonatal HI, the study of delayed ER responses becomes relevant. We hypothesized that ER stress is a prominent feature of delayed neuronal death via programmed necrosis after neonatal HI. Since necrostatin-1 (Nec-1), an inhibitor of programmed necrosis, provides delayed neuroprotection against neonatal HI in male mice, Nec-1 is an ideal tool to study delayed ER responses. C57B6 male mice were exposed to right carotid ligation followed by exposure to FiO2=0.08 for 45 min at p7. Mice were treated with vehicle or Nec-1 (0.1 μl of 8 μmol) intracerebroventricularly with age-matched littermates as controls. Biochemistry assays at 3 and 24h and electron microscopy (EM) and immunohistochemistry at 96 h after HI were performed. EM showed ER dilation and mitochondrial swelling as apparent early changes in neurons. With advanced neurodegeneration, large cytoplasmic fragments containing dilated ER "shed" into the surrounding neuropil and calreticulin immunoreactivity was lost concurrent with nuclear features suggestive of programmed necrosis. Nec-1 attenuated biochemical markers of ER stress after neonatal HI, including PERK and eIF2α phosphorylation, and unconventional XBP-1 splicing, consistent with the mitigation of later ER pathology. ER pathology may be an indicator of severity of neuronal injury and potential for recovery characterized by cytoplasmic shedding, distinct from apoptotic blebbing, that we term neuronal macrozeiosis. Therapies to attenuate ER stress applied at delayed stages may rescue stressed neurons after neonatal HI.

Chondrocyte Apoptosis in the Pathogenesis of Osteoarthritis

Apoptosis is a highly-regulated, active process of cell death involved in development, homeostasis and aging. Dysregulation of apoptosis leads to pathological states, such as cancer, developmental anomalies and degenerative diseases. Osteoarthritis (OA), the most common chronic joint disease in the elderly population, is characterized by progressive destruction of articular cartilage, resulting in significant disability. Because articular cartilage depends solely on its resident cells, the chondrocytes, for the maintenance of extracellular matrix, the compromising of chondrocyte function and survival would lead to the failure of the articular cartilage. The role of subchondral bone in the maintenance of proper cartilage matrix has been suggested as well, and it has been proposed that both articular cartilage and subchondral bone interact with each other in the maintenance of articular integrity and physiology. Some investigators include both articular cartilage and subchondral bone as targets for repairing joint degeneration. In late-stage OA, the cartilage becomes hypocellular, often accompanied by lacunar emptying, which has been considered as evidence that chondrocyte death is a central feature in OA progression. Apoptosis clearly occurs in osteoarthritic cartilage; however, the relative contribution of chondrocyte apoptosis in the pathogenesis of OA is difficult to evaluate, and contradictory reports exist on the rate of apoptotic chondrocytes in osteoarthritic cartilage. It is not clear whether chondrocyte apoptosis is the inducer of cartilage degeneration or a byproduct of cartilage destruction. Chondrocyte death and matrix loss may form a vicious cycle, with the progression of one aggravating the other, and the literature reveals that there is a definite correlation between the degree of cartilage damage and chondrocyte apoptosis. Because current treatments for OA act only on symptoms and do not prevent or cure OA, chondrocyte apoptosis would be a valid target to modulate cartilage degeneration.

Endoplasmic reticulum stress in brain ischemia

Endoplasmic reticulum (ER) stress is an intricate mechanism that mediates numerous responses during brain ischemia, thus being essential to determine the fate of neurons. In recent years, studies of the mechanisms of brain ischemic injury have centered on ER stress, glutamate excitotoxicity, dysfunction of mitochondria, inflammatory reactions, calcium overload and death receptor pathways. The role of ER stress is highly important. In addition to resulting in neuronal cell death through calcium toxicity and apoptotic pathways, ER stress also triggers a series of adaptive responses including unfolded protein response (UPR), autophagy, the expression of pro-survival proteins and the enhancement of ER self-repair ability, leading to less ischemic brain damage. This paper provides an overview of recent advances in understanding of the relations between ER stress and brain ischemia.

Neuronal death after perinatal cerebral hypoxia-ischemia: Focus on autophagy-mediated cell death

Neonatal hypoxic-ischemic encephalopathy is a critical cerebral event occurring around birth with high mortality and neurological morbidity associated with long-term invalidating sequelae. In view of the great clinical importance of this condition and the lack of very efficacious neuroprotective strategies, it is urgent to better understand the different cell death mechanisms involved with the ultimate aim of developing new therapeutic approaches. The morphological features of three different cell death types can be observed in models of perinatal cerebral hypoxia-ischemia: necrotic, apoptotic and autophagic cell death. They may be combined in the same dying neuron. In the present review, we discuss the different cell death mechanisms involved in neonatal cerebral hypoxia-ischemia with a special focus on how autophagy may be involved in neuronal death, based: (1) on experimental models of perinatal hypoxia-ischemia and stroke, and (2) on the brains of human neonates who suffered from neonatal hypoxia-ischemia.

Interplay between unfolded protein response and autophagy promotes tumor drug resistance

The endoplasmic reticulum (ER) is involved in the quality control of secreted protein via promoting the correct folding of nascent protein and mediating the degradation of unfolded or misfolded protein, namely ER-associated degradation. When the unfolded or misfolded proteins are abundant, the unfolded protein response (UPR) is elicited, an adaptive signaling cascade from the ER to the nucleus, which restores the homeostatic functions of the ER. Autophagy is a conserved catabolic process where cellular long-lived proteins and damaged organelles are engulfed and degraded for recycling to maintain homeostasis. The UPR and autophagy occur simultaneously and are involved in pathological processes, including tumorigenesis, chemoresistance of malignancies and neurodegeneration. Accumulative data has indicated that the UPR may induce autophagy and that autophagy is able to alleviate the UPR. However, the detailed mechanism of interplay between autophagy and UPR remains to be fully understood. The present review aimed to depict the core pathways of the two processes and to elucidate how autophagy and UPR are regulated. Moreover, the review also discusses the molecular mechanism of crosstalk between the UPR and autophagy and their roles in malignant survival and drug resistance.

Poloxamer 188 Attenuates Cerebral Hypoxia/Ischemia Injury in Parallel with Preventing Mitochondrial Membrane Permeabilization and Autophagic Activation

While the previous studies have shown poloxamer 188 (P188)'s neuroprotection in cultured HT22 cells under oxygen-glucose deprivation (OGD) insults, we investigated whether P188 is a potential neuroprotective agent in primary cortical neurons (PCNs) and in cerebral ischemia in vivo and whether the possible underlying mechanisms correlate with regulating mitochondrial membrane permeability (MMP) and autophagy. The protective effects of P188 were tested in PCNs in vitro exposed to OGD, as well as in cerebral ischemia in vivo. Cell death and viability were detected with LDH and MTT assay, and mitochondrial membrane potential was assessed using fluorescence microscopy. The apoptosis and autophagy of PCNs were investigated by expressions of cyt-c, caspase-3, light chain 3 (LC3), and Beclin-1. In addition, a mouse middle cerebral artery occlusion (MCAO) model was used to produce focal ischemia, and mice were treated with P188 and rapamycin after MCAO 10 min. The infarct volume, neurological scores, and phosphorylation of mammalian target of rapamycin (mTOR) were evaluated. The in vitro results showed that P188 prevented OGD-induced primary cerebrocortical neuron death and inhibited loss of mitochondrial membrane potential, the release of mitochondrial apoptogenic factor cytochrome c from mitochondria to the cytoplasm, and activation of caspase-3. P188 suppressed the activation of autophagy by decreasing LC3-II and Beclin-1 levels under OGD accordingly. Moreover, the in vivo results showed that P188 and rapamycin remarkably reduced the infarct volume and neurological deficits in the MCAO mouse model of cerebral ischemia, respectively. Both P188 and rapamycin induced phosphorylation of mTOR and reversed the decreased level upon MCAO. These data indicate that P188 prevents neuronal cell death resulting from ischemic brain injury and that its neuroprotective effects are mediated by preventing mitochondrial membrane integrity damage and autophagic activation. Given that it has low toxicity, P188 might become a potential novel therapy for ischemic injury.

Signaling pathways and mechanisms of hypoxia-induced autophagy in the animal cells

Hypoxia occurs in a series of supraphysiological circumstances, for instance, sleep disorders, myocardial infarction and cerebral stroke, that can induce a systematic inflammatory response. Such a response may then lead to a widespread dysfunction and cell injury. Autophagy, a cellular homeostatic process that governs the turnover of damaged organelles and proteins, can be triggered by multiple forms of extra- and intracellular stress, for example, hypoxia, nutrient deprivation and reactive oxygen specie. Central to this process is the formation of double-membrane vesicles, thereby autophagosomes sequester portions of cytosol and deliver them to the lysosomes for a breakdown. In recent years, several distinct oxygen-sensing pathways that regulate the cellular response to autophagy have been defined. For instance, hypoxia influences autophagy in part through the activation of the hypoxia-inducible factor (HIF)-dependent pathways. In chronic and moderate hypoxia, autophagy plays a protective role by mediating the removal of the damaged organelles and protein. Moreover, three additional oxygen-sensitive signaling pathways are also associated with the activation of autophagy. These include mammalian target of rapamycin (mTOR) kinase, unfolded protein response (UPR)- and PKCδ-JNK1-dependent pathways. Contrary to the protective effects of autophagy, during rapid and severe oxygen fluctuations, autophagy may be detrimental and induce cell death. In this review, we highlight a serious of recent advances on how autophagy is regulated at the molecular level and on final consequences of cell under different hypoxic environment.

Oxidative stress, unfolded protein response, and apoptosis in developmental toxicity

Physiological development requires precise spatiotemporal regulation of cellular and molecular processes. Disruption of these key events can generate developmental toxicity in the form of teratogenesis or mortality. The mechanism behind many developmental toxicants remains unknown. While recent work has focused on the unfolded protein response (UPR), oxidative stress, and apoptosis in the pathogenesis of disease, few studies have addressed their relationship in developmental toxicity. Redox regulation, UPR, and apoptosis are essential for physiological development and can be disturbed by a variety of endogenous and exogenous toxicants to generate lethality and diverse malformations. This review examines the current knowledge of the role of oxidative stress, UPR, and apoptosis in physiological development as well as in developmental toxicity, focusing on studies and advances in vertebrates model systems.

Thioredoxin 1 and glutaredoxin 2 contribute to maintain the phenotype and integrity of neurons following perinatal asphyxia

BACKGROUND

Thioredoxin (Trx) family proteins are crucial mediators of cell functions via regulation of the thiol redox state of various key proteins and the levels of the intracellular second messenger hydrogen peroxide. Their expression, localization and functions are altered in various pathologies. Here, we have analyzed the impact of Trx family proteins in neuronal development and recovery, following hypoxia/ischemia and reperfusion.

METHODS

We have analyzed the regulation and potential functions of Trx family proteins during hypoxia/ischemia and reoxygenation of the developing brain in both an animal and a cellular model of perinatal asphyxia. We have analyzed the distribution of 14 Trx family and related proteins in the cerebellum, striatum, and hippocampus, three areas of the rat brain that are especially susceptible to hypoxia. Using SH-SY5Y cells subjected to hypoxia and reoxygenation, we have analyzed the functions of some redoxins suggested by the animal experiment.

RESULTS AND CONCLUSIONS

We have described/discovered a complex, cell-type and tissue-specific expression pattern following the hypoxia/ischemia and reoxygenation. Particularly, Grx2 and Trx1 showed distinct changes during tissue recovery following hypoxia/ischemia and reoxygenation. Silencing of these proteins in SH-SY5Y cells subjected to hypoxia-reoxygenation confirmed that these proteins are required to maintain the normal neuronal phenotype.

GENERAL SIGNIFICANCE

These findings demonstrate the significance of redox signaling in cellular pathways. Grx2 and Trx1 contribute significantly to neuronal integrity and could be clinically relevant in neuronal damage following perinatal asphyxia and other neuronal disorders.

Melatonin reduces endoplasmic reticulum stress and preserves sirtuin 1 expression in neuronal cells of newborn rats after hypoxia-ischemia

Conditions that interfere with the endoplasmic reticulum (ER) functions cause accumulation of unfolded proteins in the ER lumen, referred to as ER stress, and activate a homeostatic signaling network known as unfolded protein response (UPR). We have previously shown that in neonatal rats subjected to hypoxia-ischemia (HI), melatonin administration significantly reduces brain damage. This study assessed whether attenuation of ER stress is involved in the neuroprotective effect of melatonin after neonatal HI. We found that the UPR was strongly activated after HI. Melatonin significantly reduced the neuron splicing of XBP-1 mRNA, the increased phosphorylation of eIF2α, and elevated expression of chaperone proteins GRP78 and Hsp70 observed after HI in the brain. CHOP, which plays a convergent role in the UPR, was reduced as well. Melatonin also completely prevented the depletion of SIRT-1 induced by HI, and this effect was observed in the same neurons that over-express CHOP. These results demonstrate that melatonin reduces ER stress induced by neonatal HI and preserves SIRT-1 expression, suggesting that SIRT-1, due to its action in the modulation of a wide variety of signaling pathways involved in neuroprotection, may play a key role in the reduction of ER stress and neuroprotection observed after melatonin.