Oxidized low-density lipoprotein induced mouse hippocampal HT-22 cell damage via promoting the shift from autophagy to apoptosis

LDL Exposure Disrupts Mitochondrial Function and Dynamics in a Hippocampal Neuronal Cell Line

Hypercholesterolemia has been associated with cognitive dysfunction and neurodegenerative diseases. Moreover, this metabolic condition disrupts the blood-brain barrier, allowing low-density lipoprotein (LDL) to enter the central nervous system. Thus, we investigated the effects of LDL exposure on mitochondrial function in a mouse hippocampal neuronal cell line (HT-22). HT-22 cells were exposed to human LDL (50 and 300 μg/mL) for 24 h. After this, intracellular lipid droplet (LD) content, cell viability, cell death, and mitochondrial parameters were assessed. We found that the higher LDL concentration increases LD content compared with control. Both concentrations increased the number of Annexin V-positive cells, indicating apoptosis. Moreover, in mitochondrial parameters, the LDL exposure on hippocampal neuronal cell line leads to a decrease in mitochondrial complexes I and II activities in both concentrations tested and a reduction in Mitotracker™ Red fluorescence and Mitotracker™ Red and Mitotracker™ Green ratio in the higher concentration, indicating mitochondrial impairment. The LDL incubation induces mitochondrial superoxide production and decreases superoxide dismutase activity in the lower concentration in HT-22 cells. Finally, LDL exposure increases the expression of genes associated with mitochondrial fusion (OPA1 and mitofusin 2) in the lower concentration. In conclusion, our findings suggest that LDL exposure induces mitochondrial dysfunction and modulates mitochondrial dynamics in the hippocampal neuronal cells.

Hypercholesterolemia, oxidative stress, and low-grade inflammation: a potentially dangerous scenario to blood-brain barrier

For more than a century, hypercholesterolemia has been linked to atherosclerotic cardiovascular disease. Notably, this metabolic condition has also been pointed out as a risk factor for neurodegenerative diseases, such as Alzheimer's disease (AD). Oxidative stress seems to be the connective factor between hypercholesterolemia and cardio and neurological disorders. By disturbing redox homeostasis, hypercholesterolemia impairs nitric oxide (NO) availability, an essential vasoprotective element, and jeopardizes endothelial function and selective permeability. The central nervous system (CNS) is partially protected from peripheral insults due to an arrangement between endothelial cells, astrocytes, microglia, and pericytes that form the blood-brain barrier (BBB). The endothelial dysfunction related to hypercholesterolemia increases the risk of developing cardiovascular diseases and also initiates BBB breakdown, which is a cause of brain damage characterized by neuroinflammation, oxidative stress, mitochondrial dysfunction, and, ultimately, neuronal and synaptic impairment. In this regard, we reviewed the mechanisms by which hypercholesterolemia-induced oxidative stress affects peripheral vessels, BBB, and leads to memory deficits. Finally, we suggest oxidative stress as the missing link between hypercholesterolemia and dementia.

Acidosis induces autophagic cell death through ASIC1-mediated Akt/mTOR signaling in HT22 neurons

Although it has been confirmed that acid-sensing ion channel 1 (ASIC1) plays a critical role in acidosis-induced neuronal injury and death, its underlying mechanisms remain largely unclear. In the present study, we investigated the involvement of ASIC1 in acidosis-induced neuronal death and its underlying mechanisms in HT22 neurons. The neurons were cultured in acidic medium to mimic extracellular acidosis. Cell viability and death, autophagy, ASIC1 expression, and the phosphorylation of Akt and mTOR were evaluated. Our results demonstrated that acidosis markedly increased the cell death rate, which was profoundly reversed by 3-MA (an autophagy inhibitor) but exacerbated by rapamycin (an autophagy activator). Moreover, our results indicated that acidosis induced excessive autophagy by increasing the expression and translocation of ASIC1, and decreasing the phosphorylation of the Akt and mTOR proteins. Intriguingly, inhibiting the activation of ASIC1 with its blocker PcTx-1 not only significantly decreased acidosis-induced neurotoxicity but also markedly compromised acidosis-induced autophagy and Akt/mTOR signaling inactivation, as evidenced by a decrease in the neuronal death rate, LC3Ⅱ/LC3Ⅰ ratio, and autophagosome number as well as p62 degradation and an increase in the phosphorylation of Akt and mTOR. Collectively, these results indicate that acidosis exerts its cytotoxic effects on HT22 neurons by inducing autophagic cell death through the ASIC1-related Akt/mTOR signaling pathway.

Acid-Sensing Ion Channel 1/Calpain1 Activation Impedes Macrophage ATP-Binding Cassette Protein A1-Mediated Cholesterol Efflux Induced by Extracellular Acidification

Background

Extracellular acidification is a common feature of atherosclerotic lesions, and such an acidic microenvironment impedes ATP-binding cassette transporter A1 (ABCA1)-mediated cholesterol efflux and promotes atherogenesis. However, the underlying mechanism is still unclear. Acid-sensing ion channel 1 (ASIC1) is a critical H⁺ receptor, which is responsible for the perception and transduction of extracellular acidification signals.

Aim

In this study, we explored whether or how ASIC1 influences extracellular acidification-induced ABCA1-mediated cholesterol efflux from macrophage-derived foam cells.

Methods

RAW 264.7 macrophages were cultured in an acidic medium (pH 6.5) to generate foam cells. Then the intracellular lipid deposition, cholesterol efflux, and ASIC1/calpain1/ABCA1 expressions were evaluated.

Results

We showed that extracellular acidification enhanced ASIC1 expression and translocation, promoted calpain1 expression and lipid accumulation, and decreased ABCA1 protein expression as well as ABCA1-mediated cholesterol efflux. Of note, inhibiting ASIC1 activation with amiloride or Psalmotoxin 1 (PcTx-1) not only lowered calpain1 protein level and lipid accumulation but also enhanced ABCA1 protein levels and ABCA1-mediated cholesterol efflux of macrophages under extracellular acidification conditions. Furthermore, similar results were observed in macrophages treated with calpain1 inhibitor PD150606.

Conclusion

Extracellular acidification declines cholesterol efflux via activating ASIC1 to promote calpain1-mediated ABCA1 degradation. Thus, ASIC1 may be a novel therapeutic target for atherosclerosis.

Regulation of hippocampal neuronal apoptosis and autophagy in mice with sepsis-associated encephalopathy by immunity-related GTPase M1

Aims

Sepsis‐associated encephalopathy (SAE) is a common complication of severe sepsis. Our goal was to investigate the role of immunity‐related GTPase M1 (IRGM1) in SAE and its underlying mechanism.

Methods

A mouse sepsis model was established by cecal ligation and perforation. SAE was diagnosed by behavior, electroencephalography, and somatosensory evoked potentials. Wild‐type mice with SAE were treated with SB203580 to block the p38 mitogen‐activated protein kinase (MAPK) signaling pathway. We assessed hippocampal histological changes and the expression of IRGM1, interferon‐γ (IFN‐γ), and p38 MAPK signaling pathway‐related proteins.

Results

Immunity‐related GTPase M1 and IFN‐γ levels increased in the hippocampus, with apoptosis, autophagy, and the p38 MAPK signaling pathway activated in neurons. Administration of SB203580 to mice with SAE reduced apoptosis and autophagy. Relative to wild‐type mice with SAE, the general condition of Irgm1^‐/‐ mice with SAE was worsened, the p38 MAPK signaling pathway was inhibited, and neuronal apoptosis and autophagy were reduced. The absence of IRGM1 exacerbated SAE, with higher p38 MAPK signaling pathway activity and increased apoptosis and autophagy.

Conclusions

During SAE, IRGM1 can at least partially regulate apoptosis and autophagy in hippocampal neurons through the p38 MAPK signaling pathway.

Corticosterone induces neurotoxicity in PC12 cells via disrupting autophagy flux mediated by AMPK/mTOR signaling

Aims

Our previous study indicated that chronic stress caused autophagy impairment and subsequent neuron apoptosis in hippocampus. However, the mechanism underlying the stress‐induced damage to neurons is unclear. In present work, we investigated whether stress‐level glucocorticoids (GCs) GCs promoted PC12 cell damage via AMPK/mTOR signaling‐mediated autophagy.

Methods

Chronic stress‐induced PC12 cell injury model was built by treatment with high level corticosterone (CORT). Cell injury was evaluated by flow cytometry assay and transmission electron microscopy observation.

Results

Autophagy flux was measured based on the changes in LC3‐II and P62 protein expressions, and the color alteration of mCherry‐GFP‐LC3‐II transfection. Our results showed that CORT not only increased cell injury and apoptosis, but also dysregulated AMPK/mTOR signaling‐mediated autophagy flux, as indicated by the upregulated expression of LC3‐II and P62 proteins, and the lowered ration of autolysosomes to autophagosomes. Mechanistically, our results demonstrated that autophagy activation by AMPK activator metformin or mTOR inhibitor rapamycin obviously promotes cell survival and autophagy flux, improved mitochondrial ultrastructure, and reduced expression of Cyt‐C and caspase‐3 in CORT‐induced PC12 cells.

Conclusion

These results indicate that high CORT triggers PC12 cell damage through disrupting AMPK/mTOR‐mediated autophagy flux. Targeting this signaling may be a promising approach to protect against high CORT and chronic stress‐induced neuronal impairment.

A Genome-wide Expression Association Analysis Identifies Genes and Pathways Associated with Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease with strong genetic components. To identity novel risk variants for ALS, utilizing the latest genome-wide association studies (GWAS) and eQTL study data, we conducted a genome-wide expression association analysis by summary data-based Mendelian randomization (SMR) method. Summary data were derived from a large-scale GWAS of ALS, involving 12577 cases and 23475 controls. The eQTL annotation dataset included 923,021 cis-eQTL for 14,329 genes and 4732 trans-eQTL for 2612 genes. Genome-wide single gene expression association analysis was conducted by SMR software. To identify ALS-associated biological pathways, the SMR analysis results were further subjected to gene set enrichment analysis (GSEA). SMR single gene analysis identified one significant and four suggestive genes associated with ALS, including C9ORF72 (P value = 7.08 × 10-6), NT5C3L (P value = 1.33 × 10-5), GGNBP2 (P value = 1.81 × 10-5), ZNHIT3(P value = 2.94 × 10-5), and KIAA1600(P value = 9.97 × 10-5). GSEA identified 7 significant biological pathways, such as PEROXISOME (empirical P value = 0.006), GLYCOLYSIS_GLUCONEOGENESIS (empirical P value = 0.043), and ARACHIDONIC_ACID_ METABOLISM (empirical P value = 0.040). Our study provides novel clues for the genetic mechanism studies of ALS.

Oxidized low-density lipoprotein induced mouse hippocampal HT-22 cell damage via promoting the shift from autophagy to apoptosis

AIMS

Although oxidized low-density lipoprotein (ox-LDL) in the brain induces neuronal death, the mechanism underlying the damage effects remains largely unknown. Given that the ultimate outcome of a cell is depended on the balance between autophagy and apoptosis, this study was performed to explore whether ox-LDL induced HT-22 neuronal cell damage via autophagy impairment and apoptosis enhancement.

METHODS

Flow cytometry and transmission electron microscopy (TEM) were used to evaluate changes in cell apoptosis and autophagy, respectively. The protein expression of LC3-II, p62, Bcl-2, and Bax in HT-22 cells was measured by Western bolt analysis.

RESULTS

Our study confirmed that 100 μg/mL of ox-LDL not only promoted TH-22 cell apoptosis, characterized by elevated cell apoptosis rate and Bax protein expression, decreased Bcl-2 protein expression, and damaged cellular ultrastructures, but also impaired autophagy as indicated by the decreased LC3-II levels and the increased p62 levels. Importantly, all of these effects of ox-LDL were significantly aggravated by cotreatment with chloroquine (an inhibitor of autophagy flux). In contrast, cotreatment with rapamycin (an inducer of autophagy) remarkably reversed these effects of ox-LDL.

CONCLUSIONS

Taken together, our results indicated that ox-LDL-induced shift from autophagy to apoptosis contributes to HT-22 cell damage.