Focusing on mitochondria in the brain: from biology to therapeutics.

Mitochondria have multiple functions such as supplying energy, regulating the redox status, and producing proteins encoded by an independent genome. They are closely related to the physiology and pathology of many organs and tissues, among which the brain is particularly prominent. The brain demands 20% of the resting metabolic rate and holds highly active mitochondrial activities. Considerable research shows that mitochondria are closely related to brain function, while mitochondrial defects induce or exacerbate pathology in the brain. In this review, we provide comprehensive research advances of mitochondrial biology involved in brain functions, as well as the mitochondria-dependent cellular events in brain physiology and pathology. Furthermore, various perspectives are explored to better identify the mitochondrial roles in neurological diseases and the neurophenotypes of mitochondrial diseases. Finally, mitochondrial therapies are discussed. Mitochondrial-targeting therapeutics are showing great potentials in the treatment of brain diseases.

CircEPS15, as a sponge of MIR24-3p ameliorates neuronal damage in Parkinson disease through boosting PINK1-PRKN-mediated mitophagy.

Despite growing evidence that has declared the importance of circRNAs in neurodegenerative diseases, the clinical significance of circRNAs in dopaminergic (DA) neuronal degeneration in the pathogenesis of Parkinson disease (PD) remains unclear. Here, we performed rRNA-depleted RNA sequencing and detected more than 10,000 circRNAs in the plasma samples of PD patients. In consideration of ROC and the correlation between Hohen-Yahr stage (H-Y stage) and Unified Parkinson Disease Rating Scale-motor score (UPDRS) of 40 PD patients, circEPS15 was selected for further research. Low expression of circEPS15 was found in PD patients and there was a negative positive correlation between the circEPS15 level and severity of PD motor symptoms, while overexpression of circEPS15 protected DA neurons against neurotoxin-induced PD-like neurodegeneration in vitro and in vivo. Mechanistically, circEPS15 acted as a MIR24-3p sponge to promote the stable expression of target gene PINK1, thus enhancing PINK1-PRKN-dependent mitophagy to eliminate damaged mitochondria and maintain mitochondrial homeostasis. Thus, circEPS15 rescued DA neuronal degeneration through the MIR24-3p-PINK1 axis-mediated improvement of mitochondrial function. This study reveals that circEPS15 exerts a critical role in participating in PD pathogenesis, and may give us an insight into the novel avenue to develop potential biomarkers and therapeutic targets for PD.Abbreviations: AAV: adeno-associated virus; DA: dopaminergic; FISH: fluorescence in situ hybridizations; HPLC: high-performance liquid chromatography; H-Y stage: Hohen-Yahr stage; LDH: lactate dehydrogenase; MMP: mitochondrial membrane potential; MPTP/p: 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/probenecid; NC: negative control; PD: Parkinson disease; PINK1: PTEN induced kinase 1; PBS: phosphate-buffered saline; ROS: reactive oxygen species; SNpc: substantia nigra pars compacta; TEM: transmission electron microscopy; UPDRS: Unified Parkinson's Disease Rating Scale-motor score.

Corrigendum: Induced expression of kir6.2 in A1 astrocytes propagates inflammatory neurodegeneration via Drp1-dependent mitochondrial fission.

[This corrects the article DOI: 10.3389/fphar.2020.618992.].

Impeding the combination of astrocytic ASCT2 and NLRP3 by talniflumate alleviates neuroinflammation in experimental models of Parkinson's disease.

Alanine-serine-cysteine transporter 2 (ASCT2) is reported to participate in the progression of tumors and metabolic diseases. It is also considered to play a crucial role in the glutamate-glutamine shuttle of neuroglial network. However, it remains unclear the involvement of ASCT2 in neurological diseases such as Parkinson's disease (PD). In this study, we demonstrated that high expression of ASCT2 in the plasma samples of PD patients and the midbrain of MPTP mouse models is positively correlated with dyskinesia. We further illustrated that ASCT2 expressed in astrocytes rather than neurons significantly upregulated in response to either MPP+ or LPS/ATP challenge. Genetic ablation of astrocytic ASCT2 alleviated the neuroinflammation and rescued dopaminergic (DA) neuron damage in PD models in vitro and in vivo. Notably, the binding of ASCT2 to NLRP3 aggravates astrocytic inflammasome-triggered neuroinflammation. Then a panel of 2513 FDA-approved drugs were performed via virtual molecular screening based on the target ASCT2 and we succeed in getting the drug talniflumate. It is validated talniflumate impedes astrocytic inflammation and prevents degeneration of DA neurons in PD models. Collectively, these findings reveal the role of astrocytic ASCT2 in the pathogenesis of PD, broaden the therapeutic strategy and provide a promising candidate drug for PD treatment.

An imbalance of netrin-1 and DCC during nigral degeneration in experimental models and patients with Parkinson's disease.

AIMS: Multiple guidance cues, such as netrin-1 (NTN-1)/deleted in colorectal carcinoma (DCC), control the guidance of axons and help establish functional neural circuits during development. However, the function of these guidance molecules during the neurodegenerative process is unclear. METHODS: To access the alterations of NTN-1 and DCC during the onset and progression of PD, we first established two subacute and one chronic PD model. Then, we investigated the relationship between the NTN-1/DCC pathway and cell death in SH-SY5Y cells. Finally, we conducted correlation studies between plasma NTN-1 and parkinsonian symptoms in patients to understand how this pathway contributes to PD. RESULTS: We found that the imbalance of NTN-1 and DCC was a common feature of nigral DA neuron injury in PD mouse models. We investigated that MPP+ inhibited NTN-1 expression and increased DCC expression in a concentration- and time-dependent manner. We further discovered a significant decrease in plasma NTN-1 levels and a positive correlation with UPDRS scores in PD patients. CONCLUSION: Our findings confirmed the imbalance of NTN-1/DCC signaling during nigral degeneration in experimental PD models and found for the first time a correlation of plasma NTN-1 with PD symptoms in patients.

ß-Arrestin2-biased Drd2 agonist UNC9995 alleviates astrocyte inflammatory injury via interaction between ß-arrestin2 and STAT3 in mouse model of depression.

BACKGROUND: Major depressive disorder (MDD) is a prevalent and devastating psychiatric illness. Unfortunately, the current therapeutic practice, generally depending on the serotonergic system for drug treatment is unsatisfactory and shows intractable side effects. Multiple evidence suggests that dopamine (DA) and dopaminergic signals associated with neuroinflammation are highly involved in the pathophysiology of depression as well as in the mechanism of antidepressant drugs, which is still in the early stage of study and well worthy of investigation. METHODS: We established two chronic stress models, including chronic unpredictable mild stress (CUMS), and chronic social defeat stress (CSDS), to complementarily recapitulate depression-like behaviors. Then, hippocampal tissues were used to detect inflammation-related molecules and signaling pathways. Pathological changes in depressive mouse hippocampal astrocytes were examined by RNA sequencing. After confirming the dopamine receptor 2 (Drd2)/ß-arrestin2 signaling changes in the depressive mice brain, we then established the depressive mouse model using the ß-arrestin2 knockout mice or administrating the ß-arrestin2-biased Drd2 agonist to investigate the roles. Label-free mass spectrometry was used to identify the ß-arrestin2-binding proteins as the underlying mechanisms. We modeled neuroinflammation with interleukin-6 (IL-6) and corticosterone treatment and characterized astrocytes using multiple methods including cell viability assay, flow cytometry, and confocal immunofluorescence. RESULTS: Drd2-biased ß-arrestin2 pathway is significantly changed in the progression of depression, and genetic deletion of ß-arrestin2 aggravates neuroinflammation and depressive-like phenotypes. Mechanistically, astrocytic ß-arrestin2 retains STAT3 in the cytoplasm by structural combination with STAT3, therefore, inhibiting the JAK-STAT3 pathway-mediated inflammatory activation. Furtherly, pharmacological activation of Drd2/ß-arrestin2 pathway by UNC9995 abolishes the inflammation-induced loss of astrocytes and ameliorates depressive-like behaviors in mouse model for depression. CONCLUSIONS: Drd2/ß-arrestin2 pathway is a potential therapeutic target for depression and ß-arrestin2-biased Drd2 agonist UNC9995 is identified as a potential anti-depressant strategy for preventing astrocytic dysfunctions and relieving neuropathological manifestations in mouse model for depression, which provides insights for the therapy of depression.

NLRP3/caspase-1/GSDMD-mediated pyroptosis exerts a crucial role in astrocyte pathological injury in mouse model of depression.

Emerging evidence suggests that astrocyte loss is one of the most important pathological features in the hippocampus of patients with major depressive disorder (MDD) and depressive mice. Pyroptosis is a recently discovered form of programmed cell death depending on Caspase-gasdermin D (Casp-GSDMD), which is involved in multiple neuropsychiatric diseases. However, the involvement of pyroptosis in the onset of MDD and glial pathological injury remains obscure. Here, we observed that depressive mice showed astrocytic pyroptosis, which was responsible for astrocyte loss, and selective serotonin reuptake inhibitor (SSRI) treatment could attenuate the pyroptosis induced by the chronic mild stress (CMS) model. Genetic KO of GSDMD, Casp-1, and astrocytic NOD-like receptor protein 3 (NLRP3) inflammasome in mice alleviated depression-like behaviors and inhibited the pyroptosis-associated protein expression. In contrast, overexpression of astrocytic GSDMD-N-terminal domain (GSDMD-N) in the hippocampus could abolish the improvement of behavioral alterations in GSDMD-deficient mice. This work illustrates that targeting the NLRP3/Casp-1/GSDMD-mediated pyroptosis may provide potential therapeutic benefits to stress-related astrocyte loss in the pathogenesis of depression.

Kir6.2 is essential to maintain neurite features by modulating PM20D1-reduced mitochondrial ATP generation.

Kir6.2, a pore-forming subunit of the ATP-sensitive potassium (KATP) channels, regulates the functions of metabolically active tissues and acts as an ideal therapeutic target for multiple diseases. Previous studies have been conducted on peripheral kir6.2, but its precise physiological roles in the central nervous system (CNS) have rarely been revealed. In the current study, we evaluated the neurophenotypes and neuroethology of kir6.2 knockout (kir6.2-/-) mice. We demonstrated the beneficial effects of kir6.2 on maintaining the morphology of mesencephalic neurons and controlling the motor coordination of mice. The mechanisms underlying the abnormal neurological features of kir6.2 deficiency were analyzed by RNA sequencing (RNA-seq). Pm20d1, a gene encoding PM20D1 secretase that promotes the generation of endogenous mitochondria uncouplers in vivo, was dramatically upregulated in the midbrain of kir6.2-/- mice. Further investigations verified that PM20D1-induced increase of N-acyl amino acids (N-AAAs) from circulating fatty acids and amino acids promoted mitochondrial impairments and cut down the ATP generation, which mediated the morphological defects of the mesencephalic neurons and thus led to the behavioral impairments of kir6.2 knockout mice. This study is the first evidence to demonstrate the roles of kir6.2 in the morphological maintenance of neurite and motor coordination control of mice, which extends our understanding of kir6.2/KATP channels in regulating the neurophysiological function.

Opposing functions of ß-arrestin 1 and 2 in Parkinson's disease via microglia inflammation and Nprl3.

Although ß-arrestins (ARRBs) regulate diverse physiological and pathophysiological processes, their functions and regulation in Parkinson's disease (PD) remain poorly defined. In this study, we show that the expression of ß-arrestin 1 (ARRB1) and ß-arrestin 2 (ARRB2) is reciprocally regulated in PD mouse models, particularly in microglia. ARRB1 ablation ameliorates, whereas ARRB2 knockout aggravates, the pathological features of PD, including dopaminergic neuron loss, neuroinflammation and microglia activation in vivo, and microglia-mediated neuron damage in vitro. We also demonstrate that ARRB1 and ARRB2 produce adverse effects on inflammation and activation of the inflammatory STAT1 and NF-κB pathways in primary cultures of microglia and macrophages and that two ARRBs competitively interact with the activated form of p65, a component of the NF-κB pathway. We further find that ARRB1 and ARRB2 differentially regulate the expression of nitrogen permease regulator-like 3 (Nprl3), a functionally poorly characterized protein, as revealed by RNA sequencing, and that in the gain- and loss-of-function studies, Nprl3 mediates the functions of both ARRBs in microglia inflammatory responses. Collectively, these data demonstrate that two closely related ARRBs exert opposite functions in microglia-mediated inflammation and the pathogenesis of PD which are mediated at least in part through Nprl3 and provide novel insights into the understanding of the functional divergence of ARRBs in PD.

Pyridoxine induces glutathione synthesis via PKM2-mediated Nrf2 transactivation and confers neuroprotection.

Oxidative stress is a major pathogenic mechanism in Parkinson's disease (PD). As an important cellular antioxidant, glutathione (GSH) balances the production and incorporation of free radicals to protect neurons from oxidative damage. GSH level is decreased in the brains of PD patients. Hence, clarifying the molecular mechanism of GSH deficiency may help deepen our knowledge of PD pathogenesis. Here we report that the astrocytic dopamine D2 receptor (DRD2) regulates GSH synthesis via PKM2-mediated Nrf2 transactivation. In addition we find that pyridoxine can dimerize PKM2 to promote GSH biosynthesis. Further experiments show that pyridoxine supplementation increases the resistance of nigral dopaminergic neurons to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced neurotoxicity in wild-type mice as well as in astrocytic Drd2 conditional knockout mice. We conclude that dimerizing PKM2 may be a potential target for PD treatment.

Induced Expression of kir6.2 in A1 Astrocytes Propagates Inflammatory Neurodegeneration via Drp1-dependent Mitochondrial Fission.

Glia-mediated inflammatory processes are crucial in the pathogenesis of Parkinson's disease (PD). As the most abundant cells of the brain and active participants in neuroinflammatory responses, astrocytes largely propagate inflammatory signals and amplify neuronal loss. Hence, intensive control of astrocytic activation is necessary to prevent neurodegeneration. In this study, we report that the astrocytic kir6.2, as a abnormal response after inflammatory stimuli, promotes the reactivity of A1 neurotoxic astrocytes. Using kir6.2 knockout (KO) mice, we find reversal effects of kir6.2 deficiency on A1-like astrocyte activation and death of dopaminergic neurons in lipopolysaccharide (LPS)-induced mouse models for PD. Further in vitro experiments show that aberrant kir6.2 expression induced by inflammatory irritants in astrocytes mediates the dynamin-related protein 1 (Drp1)-dependent excessive mitochondrial fragmentation and results in mitochondrial malfunctions. By deleting kir6.2, astrocytic activation is reduced and astrocytes-derived neuronal injury is prevented. We therefore conclude that astrocytic kir6.2 can potentially elucidate the pathology of PD and promote the development of therapeutic strategies for PD.

Small molecule-driven NLRP3 inflammation inhibition via interplay between ubiquitination and autophagy: implications for Parkinson disease.

Aging-related, nonresolving inflammation in both the central nervous system (CNS) and periphery predisposes individuals to the development of neurodegenerative disorders (NDDs). Inflammasomes are thought to be especially relevant to immune homeostasis, and their dysregulation contributes to inflammation and NDDs. However, few agents have been clinically shown to reduce NDD incidence by targeting inflammasomes. Our study indicated that NLRP3 (NLR family, pyrin domain containing 3) inflammasome is involved in Parkinson disease (PD) progression in patients and various murine models. In addition, the small molecule kaempferol (Ka) protected mice against LPS- and SNCA-induced neurodegeneration by inhibiting NLRP3 inflammasome activation as evidenced by the fact that Ka reduced cleaved CASP1 expression and disrupted NLRP3-PYCARD-CASP1 complex assembly with concomitant decreased IL1B secretion. Mechanically, Ka promoted macroautophagy/autophagy in microglia, leading to reduced NLRP3 protein expression, which in turn deactivated the NLRP3 inflammasome. Intriguingly, ubiquitination was involved in Ka-induced autophagic NLRP3 degradation. These findings were further confirmed in vivo as knockdown of Atg5 expression or autophagy inhibitor treatment significantly inhibited the Ka-mediated NLRP3 inflammasome inhibition and neurodegeneration amelioration. Thus, we demonstrated that Ka promotes neuroinflammatory inhibition via the cooperation of ubiquitination and autophagy, suggesting that Ka is a promising therapeutic strategy for the treatment of NDDs. Abbreviations: 3-MA: 3-methyladenine; AAV: adeno-associated virus; ACTB: actin, beta; AIF1/IBA1: allograft inflammatory factor 1; ATG5: autophagy related 5; ATG7: autophagy related 7; BafA1: bafilomycin A1; BECN1: beclin 1, autophagy related; CASP1: caspase 1; CNS: central nervous system; CQ: chloroquine; DA neurons: dopaminergic neurons; DAMPS: damage-associated molecular patterns; DAPI: 4',6-diamidino-2-phenylindole; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GFP: green fluorescent protein; GFAP: glial fibrillary acidic protein; IP: immunoprecipitation; i.p.: intraperitoneally; Ka: kaempferol; KD: knockdown; KO: knockout; LPS: lipopolysaccharide; IL1B: interleukin 1 beta; IL6: interleukin 6; Ly: lysate; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; MPTP: 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine; NC: negative control; NDD: neurodegenerative diseases; NLRP3: NLR family, pyrin domain containing 3; OE: overexpression; PD: Parkinson disease; poly-Ub: poly-ubiquitin; PTM: post-translational modification; PYCARD/ASC: PYD and CARD domain containing; Rapa: rapamycin; RFP: red fluorescent protein; SN: supernatant; SNCA: synuclein alpha; SNpc: substantia nigra pars compacta; SQSTM1: sequestosome 1; TH: tyrosine hydroxylase; TNF/TNF-alpha: tumor necrosis factor; Ub: ubiquitin; WT: wild type.

Salmeterol, agonist of ß2-aderenergic receptor, prevents systemic inflammation via inhibiting NLRP3 inflammasome.

ß2-Aderenergic receptor (ß2AR) agonist, Salmeterol exhibits anti-inflammatory activities. However, the inhibitory effects of Salmeterol on inflammasome activation are elusive and the underlying mechanisms need to be explored. In this study, we established inflammatory model in primary bone marrow-derived macrophages (BMDM) from C57BL/6J mice and ß-arrestin2 knockout (ß-arrestin2-/-) mice in vitro. In vivo study by LPS intraperitoneally (i.p.) in C57BL/6J mice was carried out to ascertain its roles in systemic inflammation. We found that Salmeterol (10-10 M-10-7 M) prevented the cleavage of caspase-1 and the activation of NLRP3 inflammasome, reduced the release of pro-inflammatory cytokines tumor necrosis factor-α (TNF-α) and interleukin-1ß (IL-1ß) in vitro. Blockade of adenosine3',5'cyclic monophosphate (cAMP)/protein kinase A (PKA) pathway with cAMP or PKA inhibitors inhibited anti-inflammatory effects of Salmeterol only at 10-7 M. Depletion of ß-arrestin2 compromised the inhibitory effects of Salmeterol at both 10-10 M and 10-7 M. Salmeterol increased the interaction of ß-arrestin2 and NLRP3. In vivo study showed that Salmeterol decreased the serum concentrations of pro-inflammatory cytokines IL-1ß and TNF-α, blocked cleavage of caspase-1 and release of IL-1ß in BMDM. These findings imply that Salmeterol at low concentrations (10-10 M-10-7 M) shows anti-inflammatory effect via inhibiting NLRP3 inflammasome. The underlying mechanisms is dosage-dependent: Salmeterol at 10-10 M shows anti-inflammatory effects through ß-arrestin2 pathway, and 10-7 M Salmeterol inhibits inflammation via both classical G-protein coupled receptor (GPCR)/cAMP pathway and ß-arrestin2 pathway. These results provide new ideas for the future treatment of systemic inflammation and other inflammatory diseases.