Effect of the cross-talk between autophagy and endoplasmic reticulum stress on Mn-induced alpha-synuclein oligomerization

Heavy Metal Exposure: Molecular Pathways, Clinical Implications, and Protective Strategies

Heavy metals are often found in soil and can contaminate drinking water, posing a serious threat to human health. Molecular pathways and curation therapies for mitigating heavy metal toxicity have been studied for a long time. Recent studies on oxidative stress and aging have shown that the molecular foundation of cellular damage caused by heavy metals, namely, apoptosis, endoplasmic reticulum stress, and mitochondrial stress, share the same pathways as those involved in cellular senescence and aging. In recent aging studies, many types of heavy metal exposures have been used in both cellular and animal aging models. Chelation therapy is a traditional treatment for heavy metal toxicity. However, recently, various antioxidants have been found to be effective in treating heavy metal-induced damage, shifting the research focus to investigating the interplay between antioxidants and heavy metals. In this review, we introduce the molecular basis of heavy metal-induced cellular damage and its relationship with aging, summarize its clinical implications, and discuss antioxidants and other agents with protective effects against heavy metal damage.

Manganese activates autophagy and microglia M2 polarization against endoplasmic reticulum stress-induced neuroinflammation: Involvement of GSK-3β signaling

BACKGROUND

Endoplasmic reticulum (ER) stress-induced nerve cell damage has been known to be a hallmark feature of Mn-induced parkinsonism pathogenesis. However, several compensatory machineries, such as unfolded protein response (UPR), autophagy, and immune response, play an essential role in this damage, and the underlying molecular mechanisms are poorly understood.

METHODS

Neurobehavioral impairment was assessed using catwalk gait analysis and open field test. RNA-seq analyzed the differentially expressed genes (DEGs). TUNEL staining and immunohistochemical analysis evaluated the nerve cells apoptosis and microglial cell activation. Flow cytometry assay measured microglia M1/M2 polarization. Western blotting measured protein expression. Immunofluorescence staining was used to observe the target molecules' subcellular localization.

RESULTS

The study revealed that Mn caused a reduction in motor capacity, nerve cell apoptosis, and microglia activation with an imbalance in M1/M2 polarization, coupled with NF-κB signaling and PERK signaling activation. 4-PBA pretreatment could counteract these effects, while 3-MA administration exacerbated them. Additionally, autophagy could be activated by Mn. This activation could be further upregulated by 4-PBA pretreatment, whereas it was suppressed under 3-MA administration. Mn also decreased inactive GSK-3β, increased STAT3 signaling activation, and increased colocalization of GSK-3β and STAT3. These effects were strengthened by 4-PBA pretreatment, while 3-MA administration reversed them.

DISCUSSION

This study suggests that autophagy and M2 microglia polarization might be protective in Mn-induced ER stress damage, possibly through GSK-3β-ULK1 autophagy signaling and STAT3 signaling activation.

Manganese-induced neuronal apoptosis: new insights into the role of endoplasmic reticulum stress in regulating autophagy-related proteins

Manganese (Mn) is an essential trace element that participates in various physiological and pathological processes. However, epidemiological observations indicate that overexposure to Mn is strongly associated with neurodegenerative disorders and has been recognized as a potential risk factor of neuronal apoptosis. Many mechanisms are involved in the pathogenesis of Mn-induced neuronal apoptosis, such as reactive oxygen species generation, neuroinflammation reactions, protein accumulation, endoplasmic reticulum stress (ER stress), and autophagy, all of which collectively accelerate the process of nerve cell damage. As sophisticated cellular processes for maintaining intracellular homeostasis, ER-mediated unfolded protein response and autophagy both play bilateral roles including cell protection and cell injury under pathophysiological conditions, which might interact with each other. Although emerging evidence suggests that ER stress is involved in regulating the compensatory activation of autophagy to promote cell survival, the inherent relationship between ER stress and autophagy on Mn-induced neurotoxicity remains obscure. Here, our review focuses on discussing the existing mechanisms and connections between ER stress, autophagy, and apoptosis, which provide a new perspective on Mn-induced neuronal apoptosis, and the pathogenesis of neurodegenerative diseases.

Dynamics of a model for the degradation mechanism of aggregated α-synuclein in Parkinson's disease

Accumulation of the misfolded synaptic protein α-synuclein (αSyn^*) is a hallmark of neurodegenerative disease in Parkinson's disease (PD). Recent studies suggest that the autophagy lysosome pathway (ALP) including both the Beclin1-associated and mTOR-signaling pathways is involved in the αSyn^* clearance mechanism. In this study, a mathematical model is proposed for the degradation of αSyn^* by ALP with the crosstalk element of mTOR. Using codimension-1 bifurcation analysis, the tri-stability of αSyn^* is surveyed under three different stress signals and, in addition, consideration is given to the regulatory mechanisms for the Beclin1- and mTOR-dependent rates on αSyn^* degradation using the codimension-1 and-2 bifurcation diagrams. It was found that, especially under internal and external oxidative stresses (S ₁), the bistable switch of the aggregation of αSyn^* can be transformed from an irreversible to a reversible condition through the ALP degradation pathways. Furthermore, the robustness of the tri-stable state for the stress S ₁ to the parameters related to mTOR-mediated ALP was probed. It was confirmed that mTOR-mediated ALP is important for maintaining the essential dynamic features of the tri-stable state. This study may provide a promising avenue for conducting further experiments and simulations of the degradation mechanism of dynamic modeling in PD.

Silencing of miR-497-5p inhibits cell apoptosis and promotes autophagy in Parkinson's disease by upregulation of FGF2

Parkinson's disease (PD) is a progressive neurodegenerative disorder with increasing prevalence in elderly individuals globally. MicroRNAs (miRNAs) have been confirmed to participate in the pathogenesis of various neurodegenerative diseases, including PD. MiR-497-5p is previously reported to be upregulated in PD. The present study was designed to further explore the function of miR-497-5p in PD. MiR-497-5p was significantly upregulated in 1-methyl-4-phenylpyridinium (MPP⁺ )-treated SH-SY5Y cells. Inhibition of miR-497-5p suppressed the cell apoptosis and triggered autophagy of MPP⁺ -treated SH-SY5Y cells. Further, miR-497-5p targeted fibroblast growth factor-2 (FGF2) in MPP⁺ -treated SH-SY5Y cells. Subsequently, rescue assays revealed that miR-497-5p regulated apoptosis and autophagy of MPP⁺ -treated SH-SY5Y cells by mediation on FGF2. In addition, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induced PD mice models were established. The results exhibited that silencing of miR-497-5p improved mice bradykinesia, reduced cell apoptosis and induced autophagy in PD mice by FGF2. In conclusion, silencing of miR-497-5p alleviates PD by suppressing cell apoptosis and promoting autophagy in a FGF2 dependent manner, which will provide a novel target for Parkinson's disease management.

Manganese-induced alpha-synuclein overexpression aggravates mitochondrial damage by repressing PINK1/Parkin-mediated mitophagy

Chronic manganese (Mn) exposure is related to elevated risks of neurodegenerative diseases, and mitochondrial dysfunction is considered a critical pathophysiological feature of Mn neurotoxicity. Although previous research has demonstrated Mn-induced alpha-synuclein (α-Syn) overexpression, the role of α-Syn in mitochondrial dysfunction remains unclear. Here, we used Wistar rats and human neuroblastoma cells (SH-SY5Y cells) to elucidate the molecular mechanisms underlying how α-Syn overexpression induced by different doses of Mn (15, 30, and 60 mg/kg) results in mitochondrial dysfunction. We found that Mn-induced neural cell injury was associated with mitochondrial damage. Furthermore, Mn upregulated α-Syn protein levels and increased the interaction between α-Syn and mitochondria. We then used a lentivirus vector containing α-Syn shRNA to examine the effect of Mn-induced α-Syn protein on PINK1/Parkin-mediated mitophagy in SH-SY5Y cells. Our data demonstrated that the knockdown of α-Syn decreased the interaction between α-Syn and PINK1. The enhanced level of phosphorylated Parkin (p-Parkin) was due to the decrease of the interaction between α-Syn and PINK1. Moreover, the knockdown of α-Syn increased recruitment of p-Parkin to mitochondria. Collectively, these observations revealed that Mn-induced α-Syn overexpression repressed PINK1/Parkin-mediated mitophagy and exacerbated mitochondrial damage.

Molecular Targets of Manganese-Induced Neurotoxicity: A Five-Year Update

Understanding of the immediate mechanisms of Mn-induced neurotoxicity is rapidly evolving. We seek to provide a summary of recent findings in the field, with an emphasis to clarify existing gaps and future research directions. We provide, here, a brief review of pertinent discoveries related to Mn-induced neurotoxicity research from the last five years. Significant progress was achieved in understanding the role of Mn transporters, such as SLC39A14, SLC39A8, and SLC30A10, in the regulation of systemic and brain manganese handling. Genetic analysis identified multiple metabolic pathways that could be considered as Mn neurotoxicity targets, including oxidative stress, endoplasmic reticulum stress, apoptosis, neuroinflammation, cell signaling pathways, and interference with neurotransmitter metabolism, to name a few. Recent findings have also demonstrated the impact of Mn exposure on transcriptional regulation of these pathways. There is a significant role of autophagy as a protective mechanism against cytotoxic Mn neurotoxicity, yet also a role for Mn to induce autophagic flux itself and autophagic dysfunction under conditions of decreased Mn bioavailability. This ambivalent role may be at the crossroad of mitochondrial dysfunction, endoplasmic reticulum stress, and apoptosis. Yet very recent evidence suggests Mn can have toxic impacts below the no observed adverse effect of Mn-induced mitochondrial dysfunction. The impact of Mn exposure on supramolecular complexes SNARE and NLRP3 inflammasome greatly contributes to Mn-induced synaptic dysfunction and neuroinflammation, respectively. The aforementioned effects might be at least partially mediated by the impact of Mn on α-synuclein accumulation. In addition to Mn-induced synaptic dysfunction, impaired neurotransmission is shown to be mediated by the effects of Mn on neurotransmitter systems and their complex interplay. Although multiple novel mechanisms have been highlighted, additional studies are required to identify the critical targets of Mn-induced neurotoxicity.

The Role of Autophagy in Manganese-Induced Neurotoxicity

Manganese (Mn), an essential micronutrient, acts as a cofactor for multiple enzymes. Epidemiological investigations have shown that an excessive level of Mn is an important environmental factor involved in neurotoxicity. Frequent pollution of air and water by Mn is a serious threat to the health of the population. Overexposure to Mn is particularly detrimental to the central nervous system, leading to symptoms similar to several neurological disorders. Many different mechanisms have been implicated in Mn-induced neurotoxicity, including oxidative/nitrosative stress, toxic protein aggregation, endoplasmic reticulum (ER) stress, mitochondrial dysfunction, dysregulation of autophagy, and the apoptotic cascade, which together promote the progressive neurodegeneration of nerve cells. As a compensatory regulatory mechanism, autophagy plays dual roles in various biological activities under pathological stress conditions. Dysregulation of autophagy is involved in the development of neurodegenerative disorders, with recent emerging evidence indicating a strong, complex relationship between autophagy and Mn-induced neurotoxicity. This review discusses the connection between autophagy and Mn-induced neurotoxicity, especially alpha-synuclein oligomerization, ER stress, and aberrated protein S-nitrosylation, which will provide new insights to profoundly explore the precise mechanisms of Mn-induced neurotoxicity.

Kinetic and protective role of autophagy in manganese-exposed BV-2 cells

Manganese (Mn) plays an important role in many physiological processes. Nevertheless, Mn accumulation in the brain can cause a parkinsonian-like syndrome known as manganism. Unfortunately, the therapeutic options for this disease are scarce and of limited efficacy. For this reason, a great effort is being made to understand the cellular and molecular mechanisms involved in Mn toxicity in neuronal and glial cells. Even though evidence indicates that Mn activates autophagy in microglia, the consequences of this activation in cell death remain unknown. In this study, we demonstrated a key role of reactive oxygen species in Mn-induced damage in microglial cells. These species generated by Mn²⁺ induce lysosomal alterations, LMP, cathepsins release and cell death. Besides, we described for the first time the kinetic of Mn²⁺-induced autophagy in BV-2 microglial cells and its relevance to cell fate. We found that Mn promotes a time-dependent increase in LC3-II and p62 expression levels, suggesting autophagy activation. Possibly, cells trigger autophagy to neutralize the risks associated with lysosomal rupture. In addition, pre-treatment with both Rapamycin and Melatonin enhanced autophagy and retarded Mn²⁺ cytotoxicity. In summary, our results demonstrated that, despite the damage inflicted on a subset of lysosomes, the autophagic pathway plays a protective role in Mn-induced microglial cell death. We propose that 2 h Mn²⁺ exposure will not induce disturbances in the autophagic flux. However, as time passes, the accumulated damage inside the cell could trigger a dysfunction of this mechanism. These findings may represent a valuable contribution to future research concerning manganism therapies.

IRE1 signaling pathway mediates protective autophagic response against manganese-induced neuronal apoptosis in vivo and in vitro

Overexposure to manganese (Mn) can result in neurotoxicity and is associated with manganism, a Parkinson's-like neurological disorder. In addition, Mn can induce endoplasmic reticulum (ER) stress and autophagy. In this study, we used C57BL/6 mice to establish a model of manganism and found that Mn could induce cell injury. Our results also showed that Mn could initiate the unfolded protein response (UPR) signaling and autophagy, via initiation of the UPR signaling occurring earlier than autophagy. We further investigated the intrinsic relationship between the endoplasmic reticulum to nucleus 1(ERN1, also known as inositol requiring enzyme 1, IRE1) signaling pathway and autophagy induction in SH-SY5Y cells exposed to Mn. Our results revealed that autophagy activation was a protective response in Mn-induced toxicity. Additionally, we found that Jun N-terminal kinase (JNK) inhibition downregulated autophagy and interaction of c-Jun with the Beclin1 promoter. In addition, knockdown of IRE1 with the LV-IRE1 shRNA suppressed the expression of IRE1, TRAF2, p-ASK1, and p-JNK in Mn-treated SH-SY5Y cells. Furthermore, the expression of proteins associated with ASK1-TRAF2 complex formation and autophagy activation were reversed by the LV-IRE1 shRNA. These findings suggest that IRE1 was involved in the activation of JNK through the formation of the ASK1-TRAF2 complex, and JNK activation led to the induction of autophagy, which required Beclin1 transcription by c-Jun. In this study, we demonstrated that the IRE1 signaling pathway mediated the activation of JNK signaling via the formation of the ASK1-TRAF2 complex which could initiate autophagy and the protein c-Jun which regulates Beclin1 transcription in Mn-induced neurotoxicity.

Manganese induces autophagy dysregulation: The role of S-nitrosylation in regulating autophagy related proteins in vivo and in vitro

Exposure to excess levels of manganese (Mn) may lead to nitrosative stress and neurotoxic effects on the central nervous system (CNS). The dysfunction of autophagy correlates with Mn-induced nitrosative stress; however, the exact mechanism of Mn-mediated autophagy dysfunction is still unclear. Three S-nitrosylated target proteins, namely, JNK, Bcl-2, and IKKβ, were classified as the pivotal signaling pathway mediators that could play a role in the regulation of autophagy. To reveal whether these three proteins were involved in Mn-mediated autophagy dysregulation, we studied the effects of Mn on C57/BL6 mice and human neuroblastoma cells. Exposing the mice or cells, to 300 μmol/kg or 200 μM Mn, inhibited the degradation system of the autophagy-lysosome pathway. Additionally, in Mn-treated mice or cells, S-nitrosylated JNK, Bcl-2, and IKKβ increased while the level of their phosphorylation reduced. The interaction of Beclin1 and Bcl-2 significantly increased in response to 200 μM Mn, whereas the decrease in phosphorylation of AMPK activated the mTOR pathway. We then used 20 μM 1400 W, an iNOS-specific inhibitor, to neutralize the nitrosative stress induced by Mn. Our results show that 1400 W reduced the S-nitrosylated JNK, Bcl-2, and Ikkβ and relieved their downstream signaling molecular functions. Moreover, pretreatment with 20 μM 1400 W alleviated Mn-induced autophagic dysregulation and nerve cell injury. These findings revealed that S-nitrosylated JNK, Bcl-2, and IKKβ are crucial signaling molecules in the Mn-mediated autophagic dysfunction.

Protective effects of trehalose against Mn-induced α-synuclein oligomerization in mice: Involvement of oxidative stress and autophagy

Overexposure to manganese (Mn) is widely known to induce alpha-synuclein (α-Syn) oligomerization, which has been attributed to the oxidative damage of α-Syn protein. Trehalose has been shown to induce autophagy and serve as a chemical chaperone, but little information has been reported about its effect on Mn-induced α-Syn oligomerization. In this study, we investigate whether trehalose can effectively interfere with Mn-induced α-Syn oligomerization, using different concentrations of trehalose (2% and 4% (g/vol [mL])) in a mouse model of manganism. After 6 weeks of exposure to Mn, both oxidative stress and autophagy were activated and resulted in α-Syn oligomerization and neuronal cell damage in the mouse brain tissue. Our results also revealed that pretreatment with trehalose significantly reduced the oxidative damage to α-Syn protein and increased autophagy activation. These findings clearly demonstrated that trehalose can relieve Mn-induced α-Syn oligomerization and neuronal cell damage through its anti-oxidative and autophagy-inducing effects.

Mechanisms of titanium dioxide nanoparticle-induced oxidative stress and modulation of plasma glucose in mice

Titanium dioxide nanoparticles (TiO₂ NPs) are reported to increase plasma glucose levels in mice at specific doses. The production and accumulation of reactive oxygen species (ROS) is potentially the most important factor underlying the biological toxicity of TiO₂ NPs but the underlying mechanisms are unclear at present. Data from genome-wide analyses showed that TiO₂ NPs induce endoplasmic reticulum (ER) stress and ROS generation, leading to the inference that TiO₂ NP-induced ER stress contributes to enhancement of ROS in mice. Resveratrol (Res) effectively relieved TiO₂ NP-induced ER stress and ROS generation by ameliorating expression of a common set of activated genes for both processes, signifying that ER stress and ROS are closely related. TiO₂ NP-induced ER stress occurred earlier than ROS generation. Upon treatment with 4-phenylbutyric acid to relieve ER stress, plasma glucose levels tended toward normal and TiO₂ NP increased ROS production was inhibited. These results suggest that TiO2 NP-induced ER stress promotes the generation of ROS, in turn, triggering increased plasma glucose levels in mice. In addition, Res that displays the ability to reduce ER stress presents a dietary polyphenol antioxidant that can effectively prevent the toxicological effects of TiO₂ NPs on plasma glucose metabolism.

GLAST Activity is Modified by Acute Manganese Exposure in Bergmann Glial Cells

Glutamate is the major excitatory amino acid neurotransmitter in the vertebrate brain. It exerts its actions through the activation of specific plasma membrane receptors expressed in neurons and glial cells. Overactivation of glutamate receptors results in neuronal death, known as excitotoxicity. A family of sodium-dependent glutamate transporters enriched in glial cells are responsible of the vast majority of the removal of this amino acid form the synaptic cleft. Therefore, a precise and exquisite regulation of these proteins is required not only for a proper glutamatergic transmission but also for the prevention of an excitotoxic insult. Manganese is a trace element essential as a cofactor for several enzymatic systems, although in high concentrations is involved in the disruption of brain glutamate homeostasis. The molecular mechanisms associated to manganese neurotoxicity have been focused on mitochondrial function, although energy depletion severely compromises the glutamate uptake process. In this context, in this contribution we analyze the effect of manganese exposure in glial glutamate transporters function. To this end, we used the well-established model of chick cerebellar Bergmann glia cultures. A time and dose dependent modulation of [³H]-D-aspartate uptake was found. An increase in the transporter catalytic efficiency, most probably linked to a discrete increase in the affinity of the transporter was detected upon manganese exposure. Interestingly, glucose uptake was reduced by this metal. These results favor the notion of a direct effect of manganese on glial cells, this in turn alters their coupling with neurons and might lead to changes in glutamatergic transmission.

Overexpression of miR-138-5p suppresses MnCl2 -induced autophagy by targeting SIRT1 in SH-SY5Y cells

The mechanism of manganism caused by manganese (Mn), an important environmental risk factor for Parkinson's disease, is still unclear. Recent evidence suggested that autophagy participated in neurodegenerative diseases, in which microRNA played a crucial role. However, roles of microRNA in the aberrant autophagy that occurs in neurodegenerative diseases remains controversial. In nervous system, miRNA-138-5p is highly expressed and plays a key role in regulating memory and axon regeneration. Importantly, we also found that miR-138-5p expression decreased significantly after SH-SY5Y cells exposed to manganese chloride (MnCl₂ ) in previous study. To explore the role of miR-138-5p in Mn-induced autophagy, autophagy associated indicators were detected. And we found that MnCl₂ could induce autophagic dysregulation and inhibit expression of miR-138-5p. While the levels of LC3-II/LC3-I, Beclin1, and p62, the number of autophagosome formation significantly decreased after miR-138-5p over-expression, which demonstrated that miR-138-5p could clearly retard Mn-induced autophagy. In additional, we found there were classical and evolutionarily conserved miR-138-5p binding sites in 3'-UTR region of SIRT1, which was inhibited when overexpression of miR-138-5p. Therefore, it was speculated that elevated expression of SIRT1 may be resulted from inhibition of miR-138-5p after cells exposed to MnCl₂ . Finally, we found that SIRT1 inhibitor EX-527 suppressed Mn-induced autophagy as well as miR-138-5p, while the suppression was reversed by SIRT1-specific activator SRT1720. These results indicated that overexpression of miR-138-5p suppressed Mn-induced autophagy by targeting SIRT1.

The role of melatonin in targeting cell signaling pathways in neurodegeneration

Neurodegenerative diseases are typified by neuronal loss associated with progressive dysfunction and clinical presentation. Neurodegenerative diseases are characterized by the intra- and extracellular conglomeration of misfolded proteins that occur because of abnormal protein dynamics and genetic manipulations; these trigger processes of cell death in these disorders. The disrupted signaling mechanisms involved are oxidative stress-mediated mitochondrial and calcium signaling deregulation, alterations in immune and inflammatory signaling, disruption of autophagic integrity, proteostasis dysfunction, and anomalies in the insulin, Notch, and Wnt/β-catenin signaling pathways. Herein, we accentuate some of the contemporary translational approaches made in characterizing the underlying mechanisms of neurodegeneration. Melatonin-induced cognitive enhancement and inhibition of oxidative signaling substantiates the efficacy of melatonin in combating neurodegenerative processes. Our review considers in detail the possible roles of melatonin in understanding the synergistic pathogenic mechanisms between aggregated proteins and in regulating, modulating, and preventing the altered signaling mechanisms discovered in cellular and animal models along with clinical evaluations pertaining to neurodegeneration. Furthermore, this review showcases the therapeutic potential of melatonin in preventing and treating neurodegenerative diseases with optimum prognosis.

Endoplasmic reticulum stress-mediated autophagy contributes to 5-ethylamino-9-diethylaminobenzo[a]phenoselenazinium-mediated photodynamic therapy via the PERK-eIF2α pathway

Introduction

5-ethylamino-9-diethylaminobenzo[a]phenoselenazinium (EtNBSe) is a novel synthetic bipolar photosensitizer with many promising applications. This study investigated the impact of EtNBSe-mediated photodynamic therapy (EtNBSe-PDT) on the autophagy and endoplasmic reticulum (ER) stress of squamous carcinoma cells (A-431 cells), as well as the related molecular mechanisms.

Methods

The potency of EtNBSe-PDT against squamous cell carcinoma was evaluated in BALB/c nude mice. Cell viability was evaluated using MTT. Western blotting and immunofluorescence were used to determine the expression levels of ER stress- and autophagy-related proteins.

Results

Both morphological and microscopic findings showed that the tumor on the xenograft mice exhibited an apparent reduction in volume and was replaced with fibrosis 20 days after EtNBSe-PDT. Additionally, in an in vitro study using A-431 cells, EtNBSe-PDT was found to inhibit A-431 cell survival in an EtNBSe concentration- and light dose- dependent manner, and to induce ER stress via the PERK-eIF2α signaling pathway. Additionally, EtNBSe-PDT could also induce autophagy of A-431 cells. Furthermore, the ER stress inhibitor 4-PBA and the eIF2α inhibitor salubrinal were found to inhibit the autophagy induced by EtNBSe-PDT.

Conclusion

This study demonstrated that the PERK-eIF2α signaling pathway was involved in the ER stress induced by EtNBSe-PDT. Meanwhile, the ER stress via the PERK-eIF2α pathway promoted the occurrence of autophagy in A-431 cells.