An autophagic mechanism is involved in the 6-hydroxydopamine-induced neurotoxicity in vivo

Trehalose Attenuates In Vitro Neurotoxicity of 6-Hydroxydopamine by Reducing Oxidative Stress and Activation of MAPK/AMPK Signaling Pathways

The effects of trehalose, an autophagy-inducing disaccharide with neuroprotective properties, on the neurotoxicity of parkinsonian mimetics 6-hydroxydopamine (6-OHDA) and 1-methyl-4-phenylpiridinium (MPP+) are poorly understood. In our study, trehalose suppressed 6-OHDA-induced caspase-3/PARP1 cleavage (detected by immunoblotting), apoptotic DNA fragmentation/phosphatidylserine externalization, oxidative stress, mitochondrial depolarization (flow cytometry), and mitochondrial damage (electron microscopy) in SH-SY5Y neuroblastoma cells. The protection was not mediated by autophagy, autophagic receptor p62, or antioxidant enzymes superoxide dismutase and catalase. Trehalose suppressed 6-OHDA-induced activation of c-Jun N-terminal kinase (JNK), p38 mitogen-activated protein kinase (MAPK), and AMP-activated protein kinase (AMPK), as revealed by immunoblotting. Pharmacological/genetic inhibition of JNK, p38 MAPK, or AMPK mimicked the trehalose-mediated cytoprotection. Trehalose did not affect the extracellular signal-regulated kinase (ERK) and mechanistic target of rapamycin complex 1 (mTORC1)/4EBP1 pathways, while it reduced the prosurvival mTORC2/AKT signaling. Finally, trehalose enhanced oxidative stress, mitochondrial damage, and apoptosis without decreasing JNK, p38 MAPK, AMPK, or AKT activation in SH-SY5Y cells exposed to MPP+. In conclusion, trehalose protects SH-SY5Y cells from 6-OHDA-induced oxidative stress, mitochondrial damage, and apoptosis through autophagy/p62-independent inhibition of JNK, p38 MAPK, and AMPK. The opposite effects of trehalose on the neurotoxicity of 6-OHDA and MPP+ suggest caution in its potential development as a neuroprotective agent.

Multifunctional roles of γ-enolase in the central nervous system: more than a neuronal marker

Enolase, a multifunctional protein with diverse isoforms, has generally been recognized for its primary roles in glycolysis and gluconeogenesis. The shift in isoform expression from α-enolase to neuron-specific γ-enolase extends beyond its enzymatic role. Enolase is essential for neuronal survival, differentiation, and the maturation of neurons and glial cells in the central nervous system. Neuron-specific γ-enolase is a critical biomarker for neurodegenerative pathologies and neurological conditions, not only indicating disease but also participating in nerve cell formation and neuroprotection and exhibiting neurotrophic-like properties. These properties are precisely regulated by cysteine peptidase cathepsin X and scaffold protein γ1-syntrophin. Our findings suggest that γ-enolase, specifically its C-terminal part, may offer neuroprotective benefits against neurotoxicity seen in Alzheimer's and Parkinson's disease. Furthermore, although the therapeutic potential of γ-enolase seems promising, the effectiveness of enolase inhibitors is under debate. This paper reviews the research on the roles of γ-enolase in the central nervous system, especially in pathophysiological events and the regulation of neurodegenerative diseases.

Re-routing Metabolism by the Mitochondrial Pyruvate Carrier Inhibitor MSDC-0160 Attenuates Neurodegeneration in a Rat Model of Parkinson's Disease

A growing body of evidence supports the idea that mitochondrial dysfunction might represent a key feature of Parkinson's disease (PD). Central regulators of energy production, mitochondria, are also involved in several other essential functions such as cell death pathways and neuroinflammation which make them a potential therapeutic target for PD management. Interestingly, recent studies related to PD have reported a neuroprotective effect of targeting mitochondrial pyruvate carrier (MPC) by the insulin sensitizer MSDC-0160. As the sole point of entry of pyruvate into the mitochondrial matrix, MPC plays a crucial role in energetic metabolism which is impacted in PD. This study therefore aimed at providing insights into the mechanisms underlying the neuroprotective effect of MSDC-0160. We investigated behavioral, cellular, and metabolic impact of chronic MSDC-0160 treatment in unilateral 6-OHDA PD rats. We evaluated mitochondrially related processes through the expression of pivotal mitochondrial enzymes in dorsal striatal biopsies and the level of metabolites in serum samples using nuclear magnetic resonance spectroscopy (NMR)-based metabolomics. MSDC-0160 treatment in unilateral 6-OHDA rats improved motor behavior, decreased dopaminergic denervation, and reduced mTOR activity and neuroinflammation. Concomitantly, MSDC-0160 administration strongly modified energy metabolism as revealed by increased ketogenesis, beta oxidation, and glutamate oxidation to satisfy energy needs and maintain energy homeostasis. MSDC-0160 exerts its neuroprotective effect through reorganization of multiple pathways connected to energy metabolism.

Re-routing Metabolism by the Mitochondrial Pyruvate Carrier Inhibitor MSDC-0160 Attenuates Neurodegeneration in a Rat Model of Parkinson's Disease

A growing body of evidence supports the idea that mitochondrial dysfunction might represent a key feature of Parkinson's disease (PD). Central regulators of energy production, mitochondria, are also involved in several other essential functions such as cell death pathways and neuroinflammation which make them a potential therapeutic target for PD management. Interestingly, recent studies related to PD have reported a neuroprotective effect of targeting mitochondrial pyruvate carrier (MPC) by the insulin sensitizer MSDC-0160. As the sole point of entry of pyruvate into the mitochondrial matrix, MPC plays a crucial role in energetic metabolism which is impacted in PD. This study therefore aimed at providing insights into the mechanisms underlying the neuroprotective effect of MSDC-0160. We investigated behavioral, cellular, and metabolic impact of chronic MSDC-0160 treatment in unilateral 6-OHDA PD rats. We evaluated mitochondrially related processes through the expression of pivotal mitochondrial enzymes in dorsal striatal biopsies and the level of metabolites in serum samples using nuclear magnetic resonance spectroscopy (NMR)-based metabolomics. MSDC-0160 treatment in unilateral 6-OHDA rats improved motor behavior, decreased dopaminergic denervation, and reduced mTOR activity and neuroinflammation. Concomitantly, MSDC-0160 administration strongly modified energy metabolism as revealed by increased ketogenesis, beta oxidation, and glutamate oxidation to satisfy energy needs and maintain energy homeostasis. MSDC-0160 exerts its neuroprotective effect through reorganization of multiple pathways connected to energy metabolism.

Toxic Feedback Loop Involving Iron, Reactive Oxygen Species, α-Synuclein and Neuromelanin in Parkinson's Disease and Intervention with Turmeric

Parkinson's disease (PD) is a movement disorder associated with severe loss of mainly dopaminergic neurons in the substantia nigra. Pathological hallmarks include Lewy bodies, and loss of neuromelanin, due to degeneration of neuromelanin-containing dopaminergic neurons. Despite being described over 200 years ago, the etiology of PD remains unknown. Here, we highlight the roles of reactive oxygen species (ROS), iron, alpha synuclein (α-syn) and neuromelanin in a toxic feedback loop culminating in neuronal death and spread of the disease. Dopaminergic neurons are particularly vulnerable due to decreased antioxidant concentration with aging, constant exposure to ROS and presence of neurotoxic compounds (e.g. ortho-quinones). ROS and iron increase each other's levels, creating a state of oxidative stress. α-Syn aggregation is influenced by ROS and iron but also increases ROS and iron via its induced mitochondrial dysfunction and ferric-reductase activity. Neuromelanin's binding affinity is affected by increased ROS and iron. Furthermore, during neuronal death, neuromelanin is degraded in the extracellular space, releasing its bound toxins. This cycle of events continues to neighboring neurons in the form of a toxic loop, causing PD pathology. The increase in ROS and iron may be an important target for therapies to disrupt this toxic loop, and therefore diets rich in certain 'nutraceuticals' may be beneficial. Turmeric is an attractive candidate, as it is known to have anti-oxidant and iron chelating properties. More studies are needed to test this theory and if validated, this would be a step towards development of lifestyle-based therapeutic modalities to complement existing PD treatments.

Arsenic induces autophagy-dependent apoptosis via Akt inactivation and AMPK activation signaling pathways leading to neuronal cell death

Inorganic arsenic (As³⁺), a well-known worldwide industrial and environmental pollutant, has been linked to neurodegenerative disorders (NDs). Autophagy plays an important role in controlling neuronal cell survival/death. However, limited information is available regarding the toxicological mechanism at the interplay between autophagy and As³⁺-induced neurotoxicity. The present study found that As³⁺ exposure induced a concomitant activation of apoptosis and autophagy in Neuro-2a cells, which was accompanied with the increase of phosphatidylserine exposure on outer membrane leaflets and apoptotic cell population, and the activation of caspase-3, -7, and PARP as well as the elevation of protein expressions of LC3-II, Atg-5, and Beclin-1, and the accumulation of autophagosome. Pretreatment of cells with autophagy inhibitor 3-MA, but not that of Z-VAD-FMK (a pan-caspase inhibitor), effectively prevented the As³⁺-induced autophagic and apoptotic responses, indicating that As³⁺-triggered autophagy was contributing to neuronal cell apoptosis. Furthermore, As³⁺ exposure evoked the dephosphorylation of Akt. Pretreatment with SC79, an Akt activator, could significantly attenuated As³⁺-induced Akt inactivation as well as autophagic and apoptotic events. Expectedly, inhibition of Akt signaling with LY294002 obviously enhanced As³⁺-triggered autophagy and apoptosis. Exposure to As³⁺ also dramatically increased the phosphorylation level of AMPKα. Pretreatment of AMPK inhibitor (Compound C) could markedly abrogate the As³⁺-induced phosphorylated AMPKα expression, and autophagy and apoptosis activation. Taken together, these results indicated that As³⁺ exerted its cytotoxicity in neuronal cells via the Akt inactivation/AMPK activation downstream-regulated autophagy-dependent apoptosis pathways, which ultimately lead to cell death. Our findings suggest that the regulation of Akt/AMPK signals may be a promising intervention to against As³⁺-induced neurotoxicity and NDs.

Dysregulated autophagy is linked to BAX oligomerization and subsequent cytochrome c release in 6-hydroxydopmaine-treated neuronal cells

Autophagy and apoptosis are essential physiological pathways that are required to maintain cellular homeostasis. Therefore, it is suggested that dysregulation in both pathways is linked to several disease states. Moreover, the crosstalk between autophagy and apoptosis plays an important role in pathophysiological processes associated with several neurodegenerative disorders. We have previously reported that 6-hydroxydopamine (6-OHDA)-triggered reactive oxygen species (ROS) induces dysregulated autophagy, and that a dysregulated autophagic flux contributes to caspase-dependent neuronal apoptosis. Based on our previous findings, we specifically aimed to elucidate the molecular mechanisms underlying the potential role of dysregulated autophagy in apoptotic neurodegeneration. The disuccinimidyl suberate (DSS) cross-linking assay and immunological analyses indicated that exposure of several types of cells to 6-OHDA resulted in BAX activation and subsequent oligomerization. Pharmacological inhibition and genetic perturbation of autophagy prevented 6-OHDA-induced BAX oligomerization and subsequent release of mitochondrial cytochrome c into the cytosol and caspase activation. These events were independent of expression levels of XIAP. Taken together, our results suggest that BAX oligomerization comprises a critical step by which 6-OHDA-induced dysregulated autophagy mediates neuronal apoptosis.

Acylated Ghrelin is Protective Against 6-OHDA-induced Neurotoxicity by Regulating Autophagic Flux

Parkinson’s disease (PD) is one of the most common neurodegenerative disorders, and our previous study revealed that autophagic flux dysfunction contributes to the neuron death in 6-OHDA-induced PD models. Acylated ghrelin is a neuropeptide that has a variety of actions in the central nervous system. In the current study, we aimed to investigate whether ghrelin is neuroprotective in 6-OHDA-induced rat model and SH-SY5Y cell model and whether it is related to autophagic flux regulation. We observed that ghrelin could effectively reduce apomorphine-induced contralateral rotation in 6-OHDA-induced PD rats, preserve the expression of tyrosine hydroxylase (TH) and increase the cell viability. It could upregulate the expression of autophagy related proteins like Atg7 and LC3-II and downregulate p62, and downregulate apoptosis related proteins like bax and cleaved caspase 3. SH-SY5Y cells transfected with adenovirus Ad-mCherry-GFP-LC3B further revealed that ghrelin could relieve the autophagic flux dysfunction induced by 6-OHDA. Lysotracker staining showed that ghrelin could reverse the decrease in lysosomes induced by 6-OHDA and immunofluorescence staining revealed a reverse of TFEB level in SH-SY5Y cells. Blocking autophagy activation with 3-methyladenine (3-MA) in rats treated with ghrelin and 6-OHDA showed no notable change in apoptosis-related markers, while blocking autophagosome fusion with lysosomes with chloroquine could notably reverse the downregulation of bax/bcl-2 ratio and cleaved caspase three expression by ghrelin. Additionally, knockdown ATG7, the upstream regulator of autophagy, with siRNA could further decrease the number of apoptotic cells in SH-SY5Y cells exposed to 6-OHDA and treated with ghrelin, while knockdown TFEB, a key transcription factor for lysosome biosynthesis and function, with siRNA could completely abolish the anti-apoptosis effect of ghrelin. These data suggest that ghrelin is neuroprotective in 6-OHDA-induced PD models via improving autophagic flux dysfunction and restoration of TFEB level.

Autophagy Induction Contributes to the Neuroprotective Impact of Intermittent Fasting on the Acutely Injured Spinal Cord

Spinal cord injury (SCI) is one of the leading causes of neurological disability and death. So far, there is no satisfactory treatment for SCI, because of its complex and ill-defined pathophysiology. Recently, autophagy has been implicated as protective in acute SCI rat models. Here, we investigated the therapeutic value of a dietary intervention, namely, intermittent fasting (IF), on neuronal survival after acute SCI in rats, and its underlying mechanism related to autophagy regulation. We found remarkable improvement in both behavioral performance and neuronal survival at the injured segment of the spinal cord of animals previously subjected to IF. Western blotting revealed a marked decrease in apoptosis-related markers such as cleaved caspase 3 levels and the bax/bcl-2 ratio in the IF group, which suggested an inhibition of the intrinsic apoptosis pathway. In addition, the expression of the autophagy markers LC3-II and beclin 1 was also increased in the IF group compared with ad libitum fed animals. In parallel, IF decreased the levels of the substrate protein of autophagy, p62, indicative of an upregulation of the autophagic processes. Treatment with 3-methyladenine (3-MA), a selective inhibitor of autophagy, reversed the downregulated apoptosis-related markers by IF. Finally, IF could activate the adenosine monophosphate (AMP)-activated protein kinase (AMPK)/mammalian target of rapamycin (mTOR) pathway and enhance lysosome function by upregulating transcription factor (TF)EB expression. Altogether, the present findings suggest that IF exerts a neuroprotective effect after acute SCI via the upregulation of autophagy, and further points to dietary interventions as a promising combinatorial treatment for SCI.

Berberine attenuated the cytotoxicity induced by t-BHP via inhibiting oxidative stress and mitochondria dysfunction in PC-12 cells

Neurodegenerative diseases all share several common features such as involvement of oxidative damage and mitochondrial dysfunction in pathogenesis. Oxidative stress induced by overproduction of mitochondrial reactive oxygen species (ROS) or impairment of the antioxidant deficiency results in mitochondrial dysfunction and initiation of the cell death cascade. Berberine (BBR), a traditional Chinese medicine, has been reported to exert anti-oxidative stress and anti-apoptosis effect in CNS diseases. However, the mechanism of BBR on regulating mitophagy and protecting mitochondrial function under oxidative stress remains unclear. In present study, we evaluated the beneficial effects of BBR on the tert-butyl hydroperoxide (t-BHP)-induced cytotoxicity. Furthermore, we explored the protective role of BBR in mitochondrial function and mitophagy under oxidative stress in PC-12 cells. Our results demonstrated that BBR effectively inhibited t-BHP-induced apoptosis which is associated with the decreased leakage of lactate dehydrogenase (LDH) and ROS overproduction. Moreover, BBR significantly suppressed cytochrome c expression, upregulated the ratio of Bcl-2/Bax, and ameliorated mitochondrial dysfunction by optimizing mitochondria membrane potential (ΔΨm) status and ATP production. In addition, BBR reduced the expression of autophagy-specific marker LC3, SQTM1/p62, and maintained lysosome normal function which involved the restoration of upstream signaling pathway AKT and mTOR phosphorylation level. Collectively, these findings suggested that BBR protects PC-12 cells from oxidative injury through inhibiting ROS level, mitochondria dysfunction, and mitophagy via PI3K/AKT/mTOR signaling pathways, which suggest a potential therapeutic strategy for oxidative stress and neurotoxic damages.

Iron chelator Deferoxamine protects human neuroblastoma cell line SH-SY5Y from 6-Hydroxydopamine-induced apoptosis and autophagy dysfunction

BACKGROUND

Intracellular iron involves in Fenton's reaction-mediated Hydroxyl radical (OH·) generation by reacting with the neurotoxic agent 6-Hydroxydopamine (6-OHDA) autoxidation derivative Hydrogen Peroxide (H₂O₂). Several studies have been conducted so far on the neuroprotective activities of the iron chelator Deferoxamine (DFO) but little or no clear evidence about the underlying cellular mechanism is available.

METHODS

The present study was conducted on Human neuroblastoma cell line SH-SY5Y in the absence or presence of 6-OHDA or H₂O₂ and / or DFO. Following incubation, cell viability assay, intracellular reactive oxygen species (ROS) determination, flow cytometric quantification of apoptotic cells followed by nuclear staining, intracellular tracking of transfected fusion construct of microtubule-associated protein 1B-light chain with Green fluorescent protein - Red fluorescent protein (LC3B-GFP-RFP reporters) and immunocytochemistry of intracellular Cathepsin protein by confocal microscopy, were conducted. In addition, western blotting was carried out to detect expressions of apoptotic and autophagy related proteins.

RESULTS

This study confirmed the neuroprotective potential of DFO by inhibiting 6-OHDA-mediated cell death and ROS generation. Reduced percentage of apoptotic cells and appearance of altered nuclei architecture followed by a reduced expression of cleaved PARP (Poly-ADP-ribose Polymerase) and cleaved Caspase-3 were observed upon DFO treatment against 6-OHDA, and as well as against H₂O₂ in SH-SY5Y cell lines. Besides, DFO induced the intracellular autophagolysosome formation (red puncta) rather than autophagosome (yellow puncta) only. Thereafter it was observed that DFO restored the expression of intracellular lysosomal protease Cathepsin and reduced the expression of the LC3-II.

CONCLUSION

Taken together, this study clearly demonstrated that the anti-Fenton activity of DFO inhibited apoptosis and caused blockade in ALP or autophagy dysfunction in SH-SY5Y cell lines. These outcomes further suggest that DFO provides neuroprotection by inhibiting apoptosis and inducing the progression of Autophagy- lysosomal pathway (ALP).

Ghrelin Attenuates Neuroinflammation and Demyelination in Experimental Autoimmune Encephalomyelitis Involving NLRP3 Inflammasome Signaling Pathway and Pyroptosis

Multiple sclerosis (MS) is a chronic autoimmune and degenerative disease of the central nervous system, and conventional treatments have limited efficacy or side effects. Ghrelin, a 28-amino acid octanoylated peptide, has been reported to have neuroprotective effects, including anti-oxidation, anti-inflammation, and anti-apoptosis. Pyroptosis, also called inflammatory cell death, is triggered by overly active inflammasomes and accompanied by the production of numerous cytokines. As immune dysfunction is primarily involved in the pathogenesis of MS, this study aimed to explore the therapeutic effects and precise functional mechanisms of ghrelin against the nod-like receptor protein 3 (NLRP3) inflammasome and pyroptosis in experimental autoimmune encephalomyelitis (EAE). Sprague Dawley rats were immunized with guinea pig spinal cord homogenates and pertussis toxin to develop an EAE model. All rats were randomly divided into four groups: normal control group, EAE group, EAE + ghrelin group, and ghrelin control group. EAE rats showed abnormal behavioral scores and body weight changes. Histologic analysis displayed severe inflammatory infiltration and demyelination in the brain and spinal cord of EAE rats. Ghrelin treatments potently restored these abnormal changes. In addition, the ghrelin-treated EAE group showed significantly downregulated expression of inflammatory cytokines. The expression of proteins involved in the NLRP3 signaling pathway and pyroptosis was decreased as well. We also found that the anti-inflammatory effect of ghrelin was associated with inhibition of nuclear factor (NF)-κB activation. Compared with rats in the healthy control group, rats in the ghrelin control group did not show statistically significant changes in histologic examinations, pro-inflammatory cytokines production, or molecules involved in the NLRP3 signaling pathway, which indicated that ghrelin induced no side effects in the animals of our study. Our findings provide more insight into the use of ghrelin as a novel candidate for MS.

Dysregulated autophagy contributes to caspase-dependent neuronal apoptosis

Autophagy is a regulated, intracellular degradation process that delivers unnecessary or dysfunctional cargo to the lysosome. Autophagy has been viewed as an adaptive survival response to various stresses, whereas in other cases, it promotes cell death. Therefore, both deficient and excessive autophagy may lead to cell death. In this study, we specifically attempted to explore whether and how dysregulated autophagy contributes to caspase-dependent neuronal cell death induced by the neurotoxin 6-hydroxydopamine (6-OHDA). Ultrastructural and biochemical analyses indicated that MN9D neuronal cells and primary cultures of cortical neurons challenged with 6-OHDA displayed typical features of autophagy. Cotreatment with chloroquine and monitoring autophagic flux by a tandem mRFP-EGFP-tagged LC3 probe indicated that the autophagic phenomena were primarily caused by dysregulated autophagic flux. Consequently, cotreatment with an antioxidant but not with a pan-caspase inhibitor significantly blocked 6-OHDA-stimulated dysregulated autophagy. These results indicated that 6-OHDA-induced generation of reactive oxygen species (ROS) played a critical role in triggering neuronal death by causing dysregulated autophagy and subsequent caspase-dependent apoptosis. The results of the MTT reduction, caspase-3 activation, and TUNEL assays indicated that pharmacological inhibition of autophagy using 3-methyladenine or deletion of the autophagy-related gene Atg5 significantly inhibited 6-OHDA-induced cell death. Taken together, our results suggest that abnormal induction of autophagic flux promotes apoptotic neuronal cell death, and that the treatments limiting dysregulated autophagy may have a strong neuroprotective potential.

pH-Independent two-photon fluorescent lysotrackers for real-time monitoring autophagy

Two carbazole-based two-photon fluorescent lysotrackers with different electron-donating groups (a methoxyl group for Lyso-MCO and a dimethylamino group for Lyso-NCO, respectively) have been developed from simple starting materials via an only 2-step procedure. Both of them exhibit pH-independent and specific lysosome location with a rapid staining rate, high photostability and deep issue penetration along with large two-photon absorption action cross-sections. By virtue of the better two-photon absorption properties of Lyso-NCO, it was chosen to visually monitor lysosomal tracking and autophagy. Compared with the approach of GFP-LC3 for autophagy detection, lysotracker Lyso-NCO achieved efficient and real-time visualization of the membrane fusion period in the autophagy process through detecting the level of the co-localization coefficients between Lyso-NCO and Mitotracker Red FM (MTR) in live cells.

6-Hydroxydopamine induces autophagic flux dysfunction by impairing transcription factor EB activation and lysosomal function in dopaminergic neurons and SH-SY5Y cells

Autophagy deregulation has been implicated in Parkinson's disease (PD), yet the role of autophagy in neuronal survival remains controversial. In this study, we comprehensively investigated the time-course of autophagy-related markers in 6-OHDA-induced Parkinsonian rat models and assessed its effect on the state of autophagic flux both in vivo and in vitro. We observed an early activation of autophagy followed by autophagic flux impairment, which was confirmed with autophagy inhibitor chloroquine in vivo and Ad-GFP-mCherry-LC3-infected SH-SY5Y cells in vitro. In addition, 6-OHDA not only remarkably reduced the expression level of lysosome-associated membrane protein 1 (Lamp1), but also impaired the hydrolase activities of lysosomal proteases. Transcription factor EB (TFEB), a key transcription factor controlling lysosome biogenesis, was also significantly downregulated by 6-OHDA and its nuclear translocation was inhibited as well, which could account for the impaired lysosomal function. Promoting lysosome biogenesis through TFEB overexpression could protect SH-SY5Y cells against 6-OHDA-induced neurotoxicity. The above findings demonstrated that autophagic flux dysfunction was closely associated with 6-OHDA-induced neurotoxicity and highlighted the importance of functional lysosomes and homeostatic autophagic flux in developing therapeutic agents for PD.