Doo Sin Jo

Inhibition of Drp1 Ameliorates Synaptic Depression, Aβ Deposition, and Cognitive Impairment in an Alzheimer's Disease Model

Excessive mitochondrial fission is a prominent early event and contributes to mitochondrial dysfunction, synaptic failure, and neuronal cell death in the progression of Alzheimers disease (AD). However, it remains to be determined whether inhibition of excessive mitochondrial fission is beneficial in mammal models of AD. To determine whether dynamin-related protein 1 (Drp1), a key regulator of mitochondrial fragmentation, can be a disease-modifying therapeutic target for AD, we examined the effects of Drp1 inhibitor on mitochondrial and synaptic dysfunctions induced by oligomeric amyloid-β (Aβ) in neurons and neuropathology and cognitive functions in Aβ precursor protein/presenilin 1 double-transgenic AD mice. Inhibition of Drp1 alleviates mitochondrial fragmentation, loss of mitochondrial membrane potential, reactive oxygen species production, ATP reduction, and synaptic depression in Aβ-treated neurons. Furthermore, Drp1 inhibition significantly improves learning and memory and prevents mitochondrial fragmentation, lipid peroxidation, BACE1 expression, and Aβ deposition in the brain in the AD model. These results provide evidence that Drp1 plays an important role in Aβ-mediated and AD-related neuropathology and in cognitive decline in an AD animal model. Therefore, inhibiting excessive Drp1-mediated mitochondrial fission may be an efficient therapeutic avenue for AD. SIGNIFICANCE STATEMENT Mitochondrial fission relies on the evolutionary conserved dynamin-related protein 1 (Drp1). Drp1 activity and mitochondria fragmentation are significantly elevated in the brains of sporadic Alzheimers disease (AD) cases. In the present study, we first demonstrated that the inhibition of Drp1 restored amyloid-β (Aβ)-mediated mitochondrial dysfunctions and synaptic depression in neurons and significantly reduced lipid peroxidation, BACE1 expression, and Aβ deposition in the brain of AD mice. As a result, memory deficits in AD mice were rescued by Drp1 inhibition. These results suggest that neuropathology and combined cognitive decline can be attributed to hyperactivation of Drp1 in the pathogenesis of AD. Therefore, inhibitors of excessive mitochondrial fission, such as Drp1 inhibitors, may be a new strategy for AD.

Mitochondrial dynamics regulate melanogenesis through proteasomal degradation of MITF via ROS‐ERK activation

Mitochondrial dynamics control mitochondrial functions as well as their morphology. However, the role of mitochondrial dynamics in melanogenesis is largely unknown. Here, we show that mitochondrial dynamics regulate melanogenesis by modulating the ROS‐ERK signaling pathway. Genetic and chemical inhibition of Drp1, a mitochondrial fission protein, increased melanin production and mitochondrial elongation in melanocytes and melanoma cells. In contrast, down‐regulation of OPA1, a mitochondria fusion regulator, suppressed melanogensis but induced massive mitochondrial fragmentation in hyperpigmented cells. Consistently, treatment with CCCP, a mitochondrial fission chemical inducer, also efficiently repressed melanogenesis. Furthermore, we found that ROS production and ERK phosphorylation were increased in cells with fragmented mitochondria. And inhibition of ROS or ERK suppressed the antimelanogenic effect of mitochondrial fission in α‐MSH‐treated cells. In addition, the activation of ROS‐ERK pathway by mitochondrial fission induced phosphorylation of serine73 on MITF accelerating its proteasomal degradation. In conclusion, mitochondrial dynamics may regulate melanogenesis by modulating ROS‐ERK signaling pathway.

Heterogeneous nuclear ribonucleoprotein A1 post-transcriptionally regulates Drp1 expression in neuroblastoma cells.

Excessive mitochondrial fission is associated with the pathogenesis of neurodegenerative diseases. Dynamin-related protein 1 (Drp1) possesses specific fission activity in the mitochondria and peroxisomes. Various post-translational modifications of Drp1 are known to modulate complex mitochondrial dynamics. However, the post-transcriptional regulation of Drp1 remains poorly understood. Here, we show that the heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) regulates Drp1 expression at the post-transcriptional level. hnRNP A1 directly interacts with Drp1 mRNA at its 3′UTR region, and enhances translation potential without affecting mRNA stability. Down-regulation of hnRNP A1 induces mitochondrial elongation by reducing Drp1 expression. Moreover, depletion of hnRNP A1 suppresses 3-NP-mediated mitochondrial fission and dysfunction. In contrast, over-expression of hnRNP A1 promotes mitochondrial fragmentation by increasing Drp1 expression. Additionally, hnRNP A1 significantly exacerbates 3-NP-induced mitochondrial dysfunction and cell death in neuroblastoma cells. Interestingly, treatment with 3-NP induces subcellular translocation of hnRNP A1 from the nucleus to the cytoplasm, which accelerates the increase in Drp1 expression in hnRNP A1 over-expressing cells. Collectively, our findings suggest that hnRNP A1 controls mitochondrial dynamics by post-transcriptional regulation of Drp1.

O-GlcNAcylation of ATG4B positively regulates autophagy by increasing its hydroxylase activity

Autophagy is a catabolic degradation process and maintains cellular homeostasis. And autophagy is activated in response to various stress conditions. Although O-GlcNAcylation functions a sensor for nutrient and stress, the relationship between O-GlcNAcylation and autophagy is largely unknown. Here, we identified that ATG4B is novel target for O-GlcNAcylation under metabolic stress condition. Treatment with PugNAc, an O-GlcNAcase inhibitor increased activation of autophagy in SH-SY5Y cells. Both bimolecular fluorescence complementation and immunoprecipitation assay indicated that OGT directly interacts with ATG4B in SH-SY5Y cells. We also found that the O-GlcNAcylated ATG4B was increased in autophagy activation conditions, and down-regulation of OGT reduces O-GlcNAcylation of ATG4B under low glucose condition. Furthermore, the proteolytic activity of ATG4B for LC3 cleavage was enhanced in PugNAc-treated cells. Taken together, these results imply that O-GlcNAcylation of ATG4B regulates autophagy activation by increasing its proteolytic activity under metabolic stress condition.

Autophagy Regulates Formation of Primary Cilia in Mefloquine-Treated Cells.

Primary cilia have critical roles in coordinating multiple cellular signaling pathways. Dysregulation of primary cilia is implicated in various ciliopathies. To identify specific regulators of autophagy, we screened chemical libraries and identified mefloquine, an anti-malaria medicine, as a potent regulator of primary cilia in human retinal pigmented epithelial (RPE) cells. Not only ciliated cells but also primary cilium length was increased in mefloquine-treated RPE cells. Treatment with mefloquine strongly induced the elongation of primary cilia by blocking disassembly of primary cilium. In addition, we found that autophagy was increased in mefloquine-treated cells by enhancing autophagic flux. Both chemical and genetic inhibition of autophagy suppressed ciliogenesis in mefloquine-treated RPE cells. Taken together, these results suggest that autophagy induced by mefloquine positively regulates the elongation of primary cilia in RPE cells.

Inhibition of never in mitosis A (NIMA)-related kinase-4 reduces survivin expression and sensitizes cancer cells to TRAIL-induced cell death.

The tumor necrosis factor-related apoptosis inducing ligand (TRAIL) preferentially induces apoptosis in cancer cells. However, many tumors are resistant to TRAIL-induced apoptosis, and resistance mechanisms are not fully understood. To identify novel regulatory molecules of TRAIL resistance, we screened a siRNA library targeting the human kinome, and NEK4 (NIMA-related kinase-4) was identified. Knockdown of NEK4 sensitized TRAIL-resistant cancer cells and in vivo xenografts to cell death. In contrast, over expression of NEK4 suppressed TRAIL-induced cell death in TRAIL-sensitive cancer cells. In addition, loss of NEK4 resulted in decrease of the anti-apoptotic protein survivin, but an increase in apoptotic cell death. Interestingly, NEK4 was highly upregulated in tumor tissues derived from patients with lung cancer and colon cancer. These results suggest that inhibition of NEK4 sensitizes cancer cells to TRAIL-induced apoptosis by regulation of survivin expression.

Pexophagy is induced by increasing peroxisomal reactive oxygen species in 1'10-phenanthroline-treated cells.

Although autophagy regulates the quality and quantity of cellular organelles, the regulatory mechanisms of peroxisomal autophagy remain largely unknown. In this study, we developed a cell-based image screening assay, and identified 1,10-phenanthroline (Phen) as a novel pexophagy inducer from chemical library screening. Treatment with Phen induces selective loss of peroxisomes but not endoplasmic reticulum and Golgi apparatus in hepatocytes. In addition, Phen increases autophagic engulfment of peroxisomes in an ATG5 dependent manner. Interestingly, treatment of Phen excessively produces peroxisomal reactive oxygen species (ROS), and inhibition of the ROS suppresses loss of peroxisome in Phen-treated cells. Taken together, these results suggest that Phen triggers pexophagy by enhancing peroxisomal ROS.

Suppression of Cpn10 Increases Mitochondrial Fission and Dysfunction in Neuroblastoma Cells

To date, several regulatory proteins involved in mitochondrial dynamics have been identified. However, the precise mechanism coordinating these complex processes remains unclear. Mitochondrial chaperones regulate mitochondrial function and structure. Chaperonin 10 (Cpn10) interacts with heat shock protein 60 (HSP60) and functions as a co-chaperone. In this study, we found that down-regulation of Cpn10 highly promoted mitochondrial fragmentation in SK-N-MC and SH-SY5Y neuroblastoma cells. Both genetic and chemical inhibition of Drp1 suppressed the mitochondrial fragmentation induced by Cpn10 reduction. Reactive oxygen species (ROS) generation in 3-NP-treated cells was markedly enhanced by Cpn10 knock down. Depletion of Cpn10 synergistically increased cell death in response to 3-NP treatment. Furthermore, inhibition of Drp1 recovered Cpn10-mediated mitochondrial dysfunction in 3-NP-treated cells. Moreover, an ROS scavenger suppressed cell death mediated by Cpn10 knockdown in 3-NP-treated cells. Taken together, these results showed that down-regulation of Cpn10 increased mitochondrial fragmentation and potentiated 3-NP-mediated mitochondrial dysfunction in neuroblastoma cells.

Attenuation of Aβ toxicity by promotion of mitochondrial fusion in neuroblastoma cells by liquiritigenin

Mitochondrial dynamics control mitochondrial morphology and function, and aberrations in these are associated with various neurodegenerative diseases including Alzheimer’s disease and Parkinson’s disease. To identify novel regulators of mitochondrial dynamics, we screened a phytochemical library and identified liquiritigenin as a potent inducer of mitochondrial fusion. Treatment with liquiritigenin induced an elongated mitochondrial morphology in SK-N-MC cells. In addition, liquiritigenin rescued mitochondrial fragmentation induced by knockout of mitochondrial fusion mediators such as Mfn1, Mfn2, and Opa1. Furthermore, we found that treatment with liquiritigenin notably inhibited mitochondrial fragmentation and cytotoxicity induced by Aβ in SK-N-MC cells.

Anti-melanogenic activity of schaftoside in Rhizoma Arisaematis by increasing autophagy in B16F1 cells

Skin pigmentation involves multiple processes, including melanin synthesis, transport, and melanosome release. Melanin content determines skin color and protects against UV radiation-induced damage. Autophagy is a cooperative process between autophagosomes and lysosomes that degrades cellular components and organelles. In the present study, B16F1 cells were treated with Rhizoma Arisaematis extract (RA) and assessed for pigmentation and autophagy regulation. RA treatment suppressed the α-MSH-stimulated increase of melanogenesis and down-regulated the expression of tyrosinase and TRP1 proteins in B16F1 cells. In addition, autophagy was activated in RA-treated cells. Inhibition of autophagy reduced the anti-melanogenic activity of RA in α-MSH-treated B16F1 cells. We identified schaftoside as an effector molecule by LC-MS analysis of RA. Consistently, treatment of schaftoside showed anti-melanogenic effect and induced autophagy activation in B16F1 cells. Inhibition of autophagy by 3 MA treatment reduced the anti-melanogenic effect of the schaftoside and recovered expression level of melanogenesis regulators in α-MSH-treated B16F1 cells. Taken together, our results suggest that schaftoside from RA inhibits skin pigmentation through modulation of autophagy.