Glial mitochondrial function and dysfunction in health and neurodegeneration

Astrocyte Mitochondria Are a Sensitive Target of PCB52 and its Human-Relevant Metabolites

Polychlorinated biphenyls (PCBs) are industrial chemicals that are ubiquitously found in the environment. Exposure to these compounds has been associated with neurotoxic outcomes; however, the underlying mechanisms for such outcomes remain to be fully understood. Recent studies have shown that astrocytes, the most abundant glial cell type in the brain, are susceptible to PCB exposure as well as exposure to human-relevant metabolites of PCBs. Astrocytes are critical for maintaining healthy brain function due to their unique functional attributes and positioning within the neuronal networks in the brain. In this study, we assessed the toxicity of PCB52, one of the most abundantly found PCB congeners in outdoor and indoor air, and two of its human-relevant metabolites, on astrocyte mitochondria. We exposed C6 cells, an astrocyte cell line, to PCB52 or its human-relevant metabolites and found that all the compounds showed increased toxicity in galactose-containing media compared to that in the glucose-containing media, indicating the involvement of mitochondria in observed toxicity. Additionally, we also found increased oxidative stress upon exposure to PCB52 metabolites. All three compounds caused a loss of mitochondrial membrane potential, distinct changes in the mitochondrial structure, and impaired mitochondrial function. The hydroxylated metabolite 4-OH-PCB52 likely functions as an uncoupler of mitochondria. This is the first study to report the adverse effects of exposure to PCB52 and its human-relevant metabolites on the mitochondrial structure and function in astrocytes.

Interleukin-6-elicited chronic neuroinflammation may decrease survival but is not sufficient to drive disease progression in a mouse model of Leigh syndrome

BACKGROUND

Mitochondrial diseases (MDs) are genetic disorders characterized by dysfunctions in mitochondria. Clinical data suggest that additional factors, beyond genetics, contribute to the onset and progression of this group of diseases, but these influencing factors remain largely unknown. Mounting evidence indicates that immune dysregulation or distress could play a role. Clinical observations have described the co-incidence of infection and the onset of the disease as well as the worsening of symptoms following infection. These findings highlight the complex interactions between MDs and immunity and underscore the need to better understand their underlying relationships.

RESULTS

We used Ndufs4 KO mice, a well-established mouse model of Leigh syndrome (one of the most relevant MDs), to test whether chronic induction of a neuroinflammatory state in the central nervous system before the development of neurological symptoms would affect both the onset and progression of the disease in Ndufs4 KO mice. To this aim, we took advantage of the GFAP-IL6 mouse, which overexpresses interleukin-6 (IL-6) in astrocytes and produces chronic glial reactivity, by generating a mouse line with IL-6 overexpression and NDUFS4 deficiency. IL-6 overexpression aggravated the mortality of female Ndufs4 KO mice but did not alter the main motor and respiratory phenotypes measured in any sex. Interestingly, an abnormal region-dependent microglial response to IL-6 overexpression was observed in Ndufs4 KO mice compared to controls.

CONCLUSION

Overall, our data indicate that chronic neuroinflammation may worsen the disease in Ndufs4 KO female mice, but not in males, and uncovers an abnormal microglial response due to OXPHOS dysfunction, which may have implications for our understanding of the effect of OXPHOS dysfunction in microglia.

Glial metabolic alterations during glaucoma pathogenesis

Glaucoma is the leading cause of irreversible blindness. Current treatment options are limited and often only slow disease progression. Metabolic dysfunction has recently been recognized as a key early and persistent mechanism in glaucoma pathophysiology. Several intrinsic metabolic dysfunctions have been identified and treated in retinal ganglion cells to provide neuroprotection. Growing pre-clinical and clinical evidence has confirmed that metabolic alterations in glaucoma are widespread, occurring across visual system tissues, in ocular fluids, in blood/serum, and at the level of genomic and mitochondrial DNA. This suggests that metabolic dysfunction is not constrained to retinal ganglion cells and that metabolic alterations extrinsic to retinal ganglion cells may contribute to their metabolic compromise. Retinal ganglion cells are reliant on glial metabolic support under normal physiological conditions, but the implications of metabolic dysfunction in glia are underexplored. We highlight emerging evidence that has demonstrated metabolic alterations occurring within glia in glaucoma, and how this may affect neuro-glial metabolic coupling and the metabolic vulnerability of retinal ganglion cells. In other neurodegenerative diseases which share features with glaucoma, several other glial metabolic alterations have been identified, suggesting that similar mechanisms and therapeutic targets may exist in glaucoma.

Modulating mitochondrial calcium channels (TRPM2/MCU/NCX) as a therapeutic strategy for neurodegenerative disorders

Efficient cellular communication is essential for the brain to regulate diverse functions like muscle contractions, memory formation and recall, decision-making, and task execution. This communication is facilitated by rapid signaling through electrical and chemical messengers, including voltage-gated ion channels and neurotransmitters. These messengers elicit broad responses by propagating action potentials and mediating synaptic transmission. Calcium influx and efflux are essential for releasing neurotransmitters and regulating synaptic transmission. Mitochondria, which are involved in oxidative phosphorylation, and the energy generation process, also interact with the endoplasmic reticulum to store and regulate cytoplasmic calcium levels. The number, morphology, and distribution of mitochondria in different cell types vary based on energy demands. Mitochondrial damage can cause excess reactive oxygen species (ROS) generation. Mitophagy is a selective process that targets and degrades damaged mitochondria via autophagosome-lysosome fusion. Defects in mitophagy can lead to a buildup of ROS and cell death. Numerous studies have attempted to characterize the relationship between mitochondrial dysfunction and calcium dysregulation in neurodegenerative diseases such as Alzheimer's Disease, Parkinson's Disease, Huntington's Disease, Amyotrophic lateral sclerosis, spinocerebellar ataxia, and aging. Interventional strategies to reduce mitochondrial damage and accumulation could serve as a therapeutic target, but further research is needed to unravel this potential. This review offers an overview of calcium signaling related to mitochondria in various neuronal cells. It critically examines recent findings, exploring the potential roles that mitochondrial dysfunction might play in multiple neurodegenerative diseases and aging. Furthermore, the review identifies existing gaps in knowledge to guide the direction of future research.

Mechanistic insights into the role of herpes simplex virus 1 in Alzheimer's disease

Alzheimer's Disease (AD) is an aging-associated neurodegenerative disorder, threatening millions of people worldwide. The onset and progression of AD can be accelerated by environmental risk factors, such as bacterial and viral infections. Human herpesviruses are ubiquitous infectious agents that underpin numerous inflammatory disorders including neurodegenerative diseases. Published studies concerning human herpesviruses in AD imply an active role HSV-1 in the pathogenesis of AD. This review will summarize the current understanding of HSV-1 infection in AD and highlight some barriers to advance this emerging field.

FDXR-associated disease in a Chinese cohort: Unraveling expanded ocular phenotypes and genetic spectrum

FDXR: associated disease is characterized by optic atrophy, acoustic neuropathy, and developmental delays. This study evaluated the ocular phenotypes and genetic features of patients with biallelic FDXR variants. Five individuals from unrelated non-consanguineous Chinese families with biallelic FDXR variants were identified using whole exome sequencing, Sanger sequencing, and co-segregation validation. In addition to optic atrophy and diverse extraocular manifestations, all patients presented with retinal dystrophy, and electroretinogram showed severely impaired cone and rod functions in their first decades. Three of the five patients showed attenuated retinal vessels that appeared as white lines on the fundus, and fundus fluorescein angiography (FFA) further revealed vascular abnormalities including delayed filling, completely occluded retinal vasculature, and severe retinal vascular nonperfusion of the peripheral retina. Five novel FDXR variants were identified: c.383C > T (p.A128V), c.963delG (p.R322fs*7), c.1052_1053delTC (p.L351Pfs*12), c.394-11T > G and c.1002+1G > A. Retinal dystrophy with attenuated retinal vessels appearing as white lines was observed in this cohort, and the FFA images revealed that retinal vascular occlusion could be a distinct clinical characteristic of FDXR-associated disease. Probands with FDXR revealed severe early onset ophthalmic features with rapid-progression, indicating the importance of early diagnosis and treatment. Moreover, this is the first study to report FFA manifestations in an FDXR cohort, expanding the FDXR-associated ocular disease phenotype and genetic spectrum.

Reverse Electron Transport at Mitochondrial Complex I in Ischemic Stroke, Aging, and Age-Related Diseases

Stroke is one of the leading causes of morbidity and mortality worldwide. A main cause of brain damage by stroke is ischemia-reperfusion (IR) injury due to the increased production of reactive oxygen species (ROS) and energy failure caused by changes in mitochondrial metabolism. Ischemia causes a build-up of succinate in tissues and changes in the mitochondrial NADH: ubiquinone oxidoreductase (complex I) activity that promote reverse electron transfer (RET), in which a portion of the electrons derived from succinate are redirected from ubiquinol along complex I to reach the NADH dehydrogenase module of complex I, where matrix NAD+ is converted to NADH and excessive ROS is produced. RET has been shown to play a role in macrophage activation in response to bacterial infection, electron transport chain reorganization in response to changes in the energy supply, and carotid body adaptation to changes in the oxygen levels. In addition to stroke, deregulated RET and RET-generated ROS (RET-ROS) have been implicated in tissue damage during organ transplantation, whereas an RET-induced NAD+/NADH ratio decrease has been implicated in aging, age-related neurodegeneration, and cancer. In this review, we provide a historical account of the roles of ROS and oxidative damage in the pathogenesis of ischemic stroke, summarize the latest developments in our understanding of RET biology and RET-associated pathological conditions, and discuss new ways to target ischemic stroke, cancer, aging, and age-related neurodegenerative diseases by modulating RET.

A comprehensive perspective of Huntington's disease and mitochondrial dysfunction

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease. It is caused by the expansion of the CAG trinucleotide repeat sequence in the HTT gene. HD mainly manifests as involuntary dance-like movements and severe mental disorders. As it progresses, patients lose the ability to speak, think, and even swallow. Although the pathogenesis is unclear, studies have found that mitochondrial dysfunctions occupy an important position in the pathogenesis of HD. Based on the latest research advances, this review sorts out and discusses the role of mitochondrial dysfunction on HD in terms of bioenergetics, abnormal autophagy, and abnormal mitochondrial membranes. This review provides researchers with a more complete perspective on the mechanisms underlying the relationship between mitochondrial dysregulation and HD.

SIRT3 alleviates mitochondrial dysfunction induced by recurrent low glucose and improves the supportive function of astrocytes to neurons

Hypoglycemia is an independent risk factor of cognitive impairment in patients with diabetes. Our previous study indicated that dysfunction of astrocytic mitochondria induced by recurrent low glucose (RLG) may account for hypoglycemia-associated neuronal injury and cognitive decline. Sirtuin 3 (SIRT3) is a key deacetylase for mitochondrial proteins and has recently been demonstrated to be an important regulator of mitochondrial function. However, whether mitochondrial dysfunction due to hypoglycemia is associated with astrocytic SIRT3 remains unclear, and few studies have focused on the impact of astrocytic SIRT3 on neuronal survival. In the present work, primary mouse cortical astrocytes cultured in normal glucose (5.5 mM) and high glucose (16.5 mM) were treated with five rounds of RLG (0.1 mM). The results showed that RLG suppressed SIRT3 expression in a glucose-dependent manner. High-glucose culture considerably increased the vulnerability of SIRT3 to RLG, leading to disrupted mitochondrial morphology in astrocytes. Overexpression of SIRT3 markedly improved astrocytic mitochondrial function and reduced RLG-induced oxidative stress. Moreover, SIRT3 suppressed a shift towards a neuroinflammatory A1-like reactive phenotype of astrocytes in response to RLG with reduced IL-1β, IL-6, and TNFα levels. Furthermore, it elevated brain-derived neurotrophic factor (BDNF) levels and promoted neurite growth by activating BDNF/TrkB signaling in the co-cultured neurons. The present study reveals the probable crosstalk between neurons and astrocytes after hypoglycemic exposure and provides a potential target in treating hypoglycemia-associated neuronal injury.

Mitochondrial dysfunction in microglia: a novel perspective for pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease in the elderly globally. Emerging evidence has demonstrated microglia-driven neuroinflammation as a key contributor to the onset and progression of AD, however, the mechanisms that mediate neuroinflammation remain largely unknown. Recent studies have suggested mitochondrial dysfunction including mitochondrial DNA (mtDNA) damage, metabolic defects, and quality control (QC) disorders precedes microglial activation and subsequent neuroinflammation. Therefore, an in-depth understanding of the relationship between mitochondrial dysfunction and microglial activation in AD is important to unveil the pathogenesis of AD and develop effective approaches for early AD diagnosis and treatment. In this review, we summarized current progress in the roles of mtDNA, mitochondrial metabolism, mitochondrial QC changes in microglial activation in AD, and provide comprehensive thoughts for targeting microglial mitochondria as potential therapeutic strategies of AD.

Dental Pulp-Derived Stem Cells Preserve Astrocyte Health During Induced Gliosis by Modulating Mitochondrial Activity and Functions

Astrocytes have been implicated in the onset and complication of various central nervous system (CNS) injuries and disorders. Uncontrolled astrogliosis (gliosis), while a necessary process for recovery after CNS trauma, also causes impairments in CNS performance and functions. The ability to preserve astrocyte health and better regulate the gliosis process could play a major role in controlling damage in the aftermath of acute insults and during chronic dysfunction. Here in, we demonstrate the ability of dental pulp-derived stem cells (DPSCs) in protecting the health of astrocytes during induced gliosis. First of all, we have characterized the expression of genes in primary astrocytes that are relevant to the pathological conditions of CNS by inducing gliosis. Subsequently, we found that astrocytes co-cultured with DPSCs reduced ROS production, NRF2 and GCLM expressions, mitochondrial membrane potential, and mitochondrial functions compared to the astrocytes that were not co-cultured with DPSCs in gliosis condition. In addition, hyperactive autophagy was also decreased in astrocytes that were co-cultured with DPSCs compared to the astrocytes that were not co-cultured with DPSCs during gliosis. This reversal and mitigation of gliosis in astrocytes were partly due to induction of neurogenesis in DPSCs through enhanced expressions of the neuronal genes like GFAP, NeuN, and Synapsin in DPSCs and by secretion of higher amounts of neurotropic factors, such as BDNF, GDNF, and TIMP-2. Protein-Protein docking analysis suggested that BDNF and GDNF can bind with CSPG4 and block the downstream signaling. Together these findings demonstrate novel functions of DPSCs to preserve astrocyte health during gliosis.

Recent advances in the use of CRISPR/Cas for understanding the early development of molecular gaps in glial cells

Glial cells are non-neuronal elements of the nervous system (NS) and play a central role in its development, maturation, and homeostasis. Glial cell interest has increased, leading to the discovery of novel study fields. The CRISPR/Cas system has been widely employed for NS understanding. Its use to study glial cells gives crucial information about their mechanisms and role in the central nervous system (CNS) and neurodegenerative disorders. Furthermore, the increasingly accelerated discovery of genes associated with the multiple implications of glial cells could be studied and complemented with the novel screening methods of high-content and single-cell screens at the genome-scale as Perturb-Seq, CRISP-seq, and CROPseq. Besides, the emerging methods, GESTALT, and LINNAEUS, employed to generate large-scale cell lineage maps have yielded invaluable information about processes involved in neurogenesis. These advances offer new therapeutic approaches to finding critical unanswered questions about glial cells and their fundamental role in the nervous system. Furthermore, they help to better understanding the significance of glial cells and their role in developmental biology.

Mitochondrial and Neuronal Dysfunctions in L1 Mutant Mice

Adhesion molecules regulate cell proliferation, migration, survival, neuritogenesis, synapse formation and synaptic plasticity during the nervous system’s development and in the adult. Among such molecules, the neural cell adhesion molecule L1 contributes to these functions during development, and in synapse formation, synaptic plasticity and regeneration after trauma. Proteolytic cleavage of L1 by different proteases is essential for these functions. A proteolytic fragment of 70 kDa (abbreviated L1-70) comprising part of the extracellular domain and the transmembrane and intracellular domains was shown to interact with mitochondrial proteins and is suggested to be involved in mitochondrial functions. To further determine the role of L1-70 in mitochondria, we generated two lines of gene-edited mice expressing full-length L1, but no or only low levels of L1-70. We showed that in the absence of L1-70, mitochondria in cultured cerebellar neurons move more retrogradely and exhibit reduced mitochondrial membrane potential, impaired Complex I activity and lower ATP levels compared to wild-type littermates. Neither neuronal migration, neuronal survival nor neuritogenesis in these mutants were stimulated with a function-triggering L1 antibody or with small agonistic L1 mimetics. These results suggest that L1-70 is important for mitochondrial homeostasis and that its absence contributes to the L1 syndrome phenotypes.

Astrocytic Changes in Mitochondrial Oxidative Phosphorylation Protein Levels in Parkinson's Disease

BACKGROUND

Mitochondrial dysfunction within neurons, particularly those of the substantia nigra, has been well characterized in Parkinson's disease and is considered to be related to the pathogenesis of this disorder. Dysfunction within this important organelle has been suggested to impair neuronal communication and survival; however, the reliance of astrocytes on mitochondria and the impact of their dysfunction on this essential cell type are less well characterized.

OBJECTIVE

This study aimed to uncover whether astrocytes harbor oxidative phosphorylation (OXPHOS) deficiencies in Parkinson's disease and whether these deficiencies are more likely to occur in astrocytes closely associated with neurons or those more distant from them.

METHODS

Postmortem human brain sections from patients with Parkinson's disease were subjected to imaging mass cytometry for individual astrocyte analysis of key OXPHOS proteins across all five complexes.

RESULTS

We show the variability in the astrocytic expression of mitochondrial proteins between individuals. In addition, we found that there is evidence of deficiencies in respiratory chain subunit expression within these important glia and changes, particularly in mitochondrial mass, associated with Parkinson's disease and that are not simply a consequence of advancing age.

CONCLUSION

Our data show that astrocytes, like neurons, are susceptible to mitochondrial defects and that these could have an impact on their reactivity and ability to support neurons in Parkinson's disease.

Rescuing mitochondria in traumatic brain injury and intracerebral hemorrhages - A potential therapeutic approach

Mitochondria are dynamic organelles responsible for cellular energy production. Besides, regulating energy homeostasis, mitochondria are responsible for calcium homeostasis, signal transmission, and the fate of cellular survival in case of injury and pathologies. Accumulating reports have suggested multiple roles of mitochondria in neuropathologies, neurodegeneration, and immune activation under physiological and pathological conditions. Mitochondrial dysfunction, which occurs at the initial phase of brain injury, involves oxidative stress, inflammation, deficits in mitochondrial bioenergetics, biogenesis, transport, and autophagy. Thus, development of targeted therapeutics to protect mitochondria may improve functional outcomes following traumatic brain injury (TBI) and intracerebral hemorrhages (ICH). In this review, we summarize mitochondrial dysfunction related to TBI and ICH, including the mechanisms involved, and discuss therapeutic approaches with special emphasis on past and current clinical trials.

Genome-wide data mining to construct a competing endogenous RNA network and reveal the pivotal therapeutic targets of Parkinson's disease

Parkinson's disease (PD) is one of the most common neurodegenerative movement disorders, for which there has been no effective treatments. To clarify the pathogenesis of PD, we constructed a competing endogenous RNA (ceRNA) network based on the genome-wide RNA sequencing data. It was found that 92 RNAs were differentially expressed, including 50 mRNAs, 25 miRNAs and 17 lncRNAs, based on which a ceRNA network was constructed and evaluated from 4 aspects of number of nodes, topological coefficients, closeness centrality and betweenness centrality. The functional annotation and enrichment analysis suggested that 6 functional modules, particularly the peripheral nervous system development and toxin metabolic process, dominated the development of PD. To validate the assumption, the gene set enrichment analysis (GSEA) was conducted basing on the genome-wide RNAs regardless whether they were differentially expressed or not. Consistently, the results revealed that dysregulation of MAG, HOXB3, MYRF and PLP1 led to metabolic disorders of sphingolipid and glutathione, which contributed to the pathogenesis of PD. Also, in-depth mining of previous literature confirmed a pivotal role of these dysregulated RNAs, which had been indicated to be potential diagnostic and therapeutic biomarkers of PD. Overall, we constructed a ceRNA network based on the dysregulated mRNAs, lncRNAs and miRNAs in PD, and the aberrant expression of MAG, HOXB3, MYRF and PLP1 caused metabolism disorder of sphingolipid and glutathione, and these genes are of great significance for the diagnosis and treatment of PD.

The PINK1-Mediated Crosstalk between Neural Cells and the Underlying Link to Parkinson's Disease

Mitochondrial dysfunction has a fundamental role in the development of idiopathic and familiar forms of Parkinson's disease (PD). The nuclear-encoded mitochondrial kinase PINK1, linked to familial PD, is responsible for diverse mechanisms of mitochondrial quality control, ATP production, mitochondrial-mediated apoptosis and neuroinflammation. The main pathological hallmark of PD is the loss of dopaminergic neurons. However, novel discoveries have brought forward the concept that a disruption in overall brain homeostasis may be the underlying cause of this neurodegeneration disease. To sustain this, astrocytes and microglia cells lacking PINK1 have revealed increased neuroinflammation and deficits in physiological roles, such as decreased wound healing capacity and ATP production, which clearly indicate involvement of these cells in the physiopathology of PD. PINK1 executes vital functions within mitochondrial regulation that have a detrimental impact on the development and progression of PD. Hence, in this review, we aim to broaden the horizon of PINK1-mediated phenotypes occurring in neurons, astrocytes and microglia and, ultimately, highlight the importance of the crosstalk between these neural cells that is crucial for brain homeostasis.

Astrocyte-Neuron Metabolic Crosstalk in Neurodegeneration: A Mitochondrial Perspective

Converging evidence made clear that declining brain energetics contribute to aging and are implicated in the initiation and progression of neurodegenerative disorders such as Alzheimer's and Parkinson's disease. Indeed, both pathologies involve instances of hypometabolism of glucose and oxygen in the brain causing mitochondrial dysfunction, energetic failure and oxidative stress. Importantly, recent evidence suggests that astrocytes, which play a key role in supporting neuronal function and metabolism, might contribute to the development of neurodegenerative diseases. Therefore, exploring how the neuro-supportive role of astrocytes may be impaired in the context of these disorders has great therapeutic potential. In the following, we will discuss some of the so far identified features underlining the astrocyte-neuron metabolic crosstalk. Thereby, special focus will be given to the role of mitochondria. Furthermore, we will report on recent advancements concerning iPSC-derived models used to unravel the metabolic contribution of astrocytes to neuronal demise. Finally, we discuss how mitochondrial dysfunction in astrocytes could contribute to inflammatory signaling in neurodegenerative diseases.

Mitochondria of teleost radial glia: A novel target of neuroendocrine disruption by environmental chemicals?

In teleost fish, radial glial cells (RGCs) are progenitor cells for neurons and the major cell type synthesizing neuroestrogens. We hypothesized that chemical exposure impairs mitochondrial bioenergetics of RGCs, which then may lead to downstream consequences for neuroestrogen production. Here we provide proof of concept that mitochondria of RGCs can be perturbed by fungicides. We isolated RGCs from a mixed sex population of goldfish (Carassius auratus) and measured metabolic capacity of primary cells to a model mitotoxin fluazinam, a broad-spectrum fungicide that inhibits mitochondria electron transport chain (or ETC) Complex I. Using immunocytochemistry and real-time PCR, we demonstrate that the goldfish primary cell cultures are highly enriched for glia after multiple passages. Cytotoxicity assays revealed that glia treated with >25 μM fluazinam for 24 and 48-h showed reduced viability. As such, metabolic assays were conducted with non-cytotoxic concentrations (0.25-12.5 μM). Fluazinam did not affect oxygen consumption rates of RGCs at 24 h, but after 48 h, oligomycin induced ATP-linked respiration was decreased by both 6.25 and 12.5 μM fluazinam. Moreover, concentrations as low as 0.25 μM disrupted the mitochondrial membrane potential of RGCs, reflecting strong uncoupling effects of the fungicide on mitochondria. Here we provide proof of concept that mitochondrial bioenergetics of teleostean RGCs can be responsive to agrochemicals. Additional studies are required to address low-dose exposures in vivo and to determine if metabolic disruption impairs neuroendocrine functions of RGCs. We propose this mechanism constitutes a novel aspect of neuroendocrine disruption, significant because dysregulation of neuron-glia communication is expected to contribute to neuroendocrine disruption.

SGK1 inhibition in glia ameliorates pathologies and symptoms in Parkinson disease animal models

Astrocytes and microglia are brain-resident glia that can establish harmful inflammatory environments in disease contexts and thereby contribute to the progression of neuronal loss in neurodegenerative disorders. Correcting the diseased properties of glia is therefore an appealing strategy for treating brain diseases. Previous studies have shown that serum/ glucocorticoid related kinase 1 (SGK1) is upregulated in the brains of patients with various neurodegenerative disorders, suggesting its involvement in the pathogenesis of those diseases. In this study, we show that inhibiting glial SGK1 corrects the pro-inflammatory properties of glia by suppressing the intracellular NFκB-, NLRP3-inflammasome-, and CGAS-STING-mediated inflammatory pathways. Furthermore, SGK1 inhibition potentiated glial activity to scavenge glutamate toxicity and prevented glial cell senescence and mitochondrial damage, which have recently been reported as critical pathologic features of and therapeutic targets in Parkinson disease (PD) and Alzheimer disease (AD). Along with those anti-inflammatory/neurotrophic functions, silencing and pharmacological inhibition of SGK1 protected midbrain dopamine neurons from degeneration and cured pathologic synuclein alpha (SNCA) aggregation and PD-associated behavioral deficits in multiple in vitro and in vivo PD models. Collectively, these findings suggest that SGK1 inhibition could be a useful strategy for treating PD and other neurodegenerative disorders that share the common pathology of glia-mediated neuroinflammation.

Continuous assessment of metabolic activity of mitochondria using resonance Raman microspectroscopy

Dysfunctional mitochondrial activity can lead to a variety of different diseases. As such, there exists a need to quantify changes in mitochondria function as it relates to these specific diseased states. Here, we present the use of resonance Raman (RR) spectroscopy as a tool to determine changes in isolated mitochondrial activity. RR spectroscopy, using 532 nm as the excitation source, specifically provides information on the reduction and oxidation (RedOx) state of cytochrome c, which is determined by the activity of protein complexes in the electron transport chain (ETC). In this model, injection of the substrate succinate into the mitochondrial sample is used to drive the ETC, which causes a subsequent change in cytochrome c RedOx state. This change in RedOx state is tracked by RR spectroscopy. This tool gives real-time information on the rise and fall of the amount of reduced cytochrome c within the mitochondrial sample, providing a method for rapid assessment of mitochondrial metabolism that has broad applications in both basic science and medical research.

Bi-phasic dose response in the preclinical and clinical developments of sigma-1 receptor ligands for the treatment of neurodegenerative disorders

Introduction: The sigma-1 receptor (S1R) is attracting much attention for disease-modifying therapies in neurodegenerative diseases. It is a conserved protein, present in plasma and endoplasmic reticulum (ER) membranes and enriched in mitochondria-associated ER membranes (MAMs). It modulates ER-mitochondria Ca2+ transfer and ER stress pathways. Mitochondrial and MAM dysfunctions contribute to neurodegenerative processes in diseases such as Alzheimer, Parkinson, Huntington or Amyotrophic Lateral Sclerosis. Interestingly, the S1R can be activated by small druggable molecules and accumulating preclinical data suggest that S1R agonists are effective protectants in these neurodegenerative diseases.Area covered: In this review, we will present the data showing the high therapeutic potential of S1R drugs for the treatment of neurodegenerative diseases, focusing on pridopidine as a potent and selective S1R agonist under clinical development. Of particular interest is the bi-phasic (bell-shaped) dose-response effect, representing a common feature of all S1R agonists and described in numerous preclinical models in vitro, in vivo and in clinical trials.Expert opinion: S1R agonists modulate inter-organelles communication altered in neurodegenerative diseases and activate intracellular survival pathways. Research will continue growing in the future. The particular cellular nature of this chaperone protein must be better understood to facilitate the clinical developement of promising molecules.

Involvement of Mitophagy in Aluminum Oxide Nanoparticle-Induced Impairment of Learning and Memory in Mice

Aluminum oxide nanoparticles (nano-aluminum) have been known to be widespread in the environment for decades. Exposure to nano-aluminum may impair learning and memory, but the potential mechanism has not yet been elucidated. In neurons, efficient clearance of damaged mitochondria through mitophagy plays an important role in mitochondrial energy supply, neuronal survival, and health. However, abnormal mitophagy induces accumulation of damaged mitochondria, which induces cellular dysfunction, contributing to the impairment of learning and memory. It is currently unclear whether nano-aluminum interferes with the function of nerve cells through mitophagy, leading to learning and memory disorders. Institute of Cancer Research (ICR) female mice were randomly divided into four groups, and treated with normal saline (control) and 50 nm nano-aluminum at concentrations of 25, 50, and 75 mg/kg for 30 days. Our results showed that exposure to nano-aluminum impaired the spatial learning and memory of mice. Superoxide dismutase levels decreased, whereas the levels of malondialdehyde increased. Moreover, there were significant pathological changes in the ultra-structure and function of mitochondria. Finally, expression of autophagy-related proteins LC3-II and Beclin-1 was upregulated and p62 expression decreased, but the expression of apoptotic and necrosis-related proteins had no significant difference among groups. Our results suggest that learning and memory impairment induced by nano-aluminum could be related to mitophagy.

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.

Contribution of astrocytes to neuropathology of neurodegenerative diseases

Classically, the loss of vulnerable neuronal populations in neurodegenerative diseases was considered to be the consequence of cell autonomous degeneration of neurons. However, progress in the understanding of glial function, the availability of improved animal models recapitulating the features of the human diseases, and the development of new approaches to derive glia and neurons from induced pluripotent stem cells obtained from patients, provided novel information that altered this view. Current evidence strongly supports the notion that non-cell autonomous mechanisms contribute to the demise of neurons in neurodegenerative disorders, and glia causally participate in the pathogenesis and progression of these diseases. In addition to microglia, astrocytes have emerged as key players in neurodegenerative diseases and will be the focus of the present review. Under the influence of pathological stimuli present in the microenvironment of the diseased CNS, astrocytes undergo morphological, transcriptional, and functional changes and become reactive. Reactive astrocytes are heterogeneous and exhibit neurotoxic (A1) or neuroprotective (A2) phenotypes. In recent years, single-cell or single-nucleus transcriptome analyses unraveled new, disease-specific phenotypes beyond A1/A2. These investigations highlighted the complexity of the astrocytic responses to CNS pathology. The present review will discuss the contribution of astrocytes to neurodegenerative diseases with particular emphasis on Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and frontotemporal dementia. Some of the commonalties and differences in astrocyte-mediated mechanisms that possibly drive the pathogenesis or progression of the diseases will be summarized. The emerging view is that astrocytes are potential new targets for therapeutic interventions. A comprehensive understanding of astrocyte heterogeneity and disease-specific phenotypic complexity could facilitate the design of novel strategies to treat neurodegenerative disorders.

Akkouh I, Hughes T, Frei E, A. Andreassen O, Djurovic S. Using iPSC Models to Understand the Role of Estrogen in Neuron-Glia Interactions in Schizophrenia and Bipolar Disorder

Schizophrenia (SCZ) and bipolar disorder (BIP) are severe mental disorders with a considerable disease burden worldwide due to early age of onset, chronicity, and lack of efficient treatments or prevention strategies. Whilst our current knowledge is that SCZ and BIP are highly heritable and share common pathophysiological mechanisms associated with cellular signaling, neurotransmission, energy metabolism, and neuroinflammation, the development of novel therapies has been hampered by the unavailability of appropriate models to identify novel targetable pathomechanisms. Recent data suggest that neuron-glia interactions are disturbed in SCZ and BIP, and are modulated by estrogen (E2). However, most of the knowledge we have so far on the neuromodulatory effects of E2 came from studies on animal models and human cell lines, and may not accurately reflect many processes occurring exclusively in the human brain. Thus, here we highlight the advantages of using induced pluripotent stem cell (iPSC) models to revisit studies of mechanisms underlying beneficial effects of E2 in human brain cells. A better understanding of these mechanisms opens the opportunity to identify putative targets of novel therapeutic agents for SCZ and BIP. In this review, we first summarize the literature on the molecular mechanisms involved in SCZ and BIP pathology and the beneficial effects of E2 on neuron-glia interactions. Then, we briefly present the most recent developments in the iPSC field, emphasizing the potential of using patient-derived iPSCs as more relevant models to study the effects of E2 on neuron-glia interactions.

Metabolic regulation of aging and age-related disease

Inquiry into relationships between energy metabolism and brain function requires a uniquely interdisciplinary mindset, and implementation of anti-aging lifestyle strategies based on this work also involves consistent mental and physical discipline. Dr. Mark P. Mattson embodies both of these qualities, based on the breadth and depth of his work on neurobiological responses to energetic stress, and on his own diligent practice of regular exercise and caloric restriction. Dr. Mattson created a neurotrophic niche in his own laboratory, allowing trainees to grow their skills, form new connections, and eventually migrate, forming their own labs while remaining part of the extended lab family. In this historical review, we highlight Dr. Mattson's many contributions to understanding neurobiological responses to physical exercise and dietary restriction, with an emphasis on the mechanisms that may underlie neuroprotection in ageing and age-related disease. On the occasion of Dr. Mattson's retirement from the National Institute on Aging, we highlight his foundational work on metabolism and neuroplasticity by reviewing the context for these findings and considering their impact on future research on the neuroscience of aging.

p62-Nrf2-p62 Mitophagy Regulatory Loop as a Target for Preventive Therapy of Neurodegenerative Diseases

Turnover of the mitochondrial pool due to coordinated processes of mitochondrial biogenesis and mitophagy is an important process in maintaining mitochondrial stability. An important role in this process is played by the Nrf2/ARE signaling pathway, which is involved in the regulation of the expression of genes responsible for oxidative stress protection, regulation of mitochondrial biogenesis, and mitophagy. The p62 protein is a multifunctional cytoplasmic protein that functions as a selective mitophagy receptor for the degradation of ubiquitinated substrates. There is evidence that p62 can positively regulate Nrf2 by binding to its negative regulator, Keap1. However, there is also strong evidence that Nrf2 up-regulates p62 expression. Thereby, a regulatory loop is formed between two important signaling pathways, which may be an important target for drugs aimed at treating neurodegeneration. Constitutive activation of p62 in parallel with Nrf2 would most likely result in the activation of mTORC1-mediated signaling pathways that are associated with the development of malignant neoplasms. The purpose of this review is to describe the p62-Nrf2-p62 regulatory loop and to evaluate its role in the regulation of mitophagy under various physiological conditions.

The Role of Glial Mitochondria in α-Synuclein Toxicity

The abnormal accumulation of alpha-synuclein (α-syn) aggregates in neurons and glial cells is widely known to be associated with many neurodegenerative diseases, including Parkinson's disease (PD), Dementia with Lewy bodies (DLB), and Multiple system atrophy (MSA). Mitochondrial dysfunction in neurons and glia is known as a key feature of α-syn toxicity. Studies aimed at understanding α-syn-induced toxicity and its role in neurodegenerative diseases have primarily focused on neurons. However, a growing body of evidence demonstrates that glial cells such as microglia and astrocytes have been implicated in the initial pathogenesis and the progression of α-Synucleinopathy. Glial cells are important for supporting neuronal survival, synaptic functions, and local immunity. Furthermore, recent studies highlight the role of mitochondrial metabolism in the normal function of glial cells. In this work, we review the complex relationship between glial mitochondria and α-syn-mediated neurodegeneration, which may provide novel insights into the roles of glial cells in α-syn-associated neurodegenerative diseases.

Transcriptomic Analysis of Human Astrocytes In Vitro Reveals Hypoxia-Induced Mitochondrial Dysfunction, Modulation of Metabolism, and Dysregulation of the Immune Response

Hypoxia is a feature of neurodegenerative diseases, and can both directly and indirectly impact on neuronal function through modulation of glial function. Astrocytes play a key role in regulating homeostasis within the central nervous system, and mediate hypoxia-induced changes in response to reduced oxygen availability. The current study performed a detailed characterization of hypoxia-induced changes in the transcriptomic profile of astrocytes in vitro. Human astrocytes were cultured under normoxic (5% CO₂, 95% air) or hypoxic conditions (1% O₂, 5% CO₂, 94% N₂) for 24 h, and the gene expression profile assessed by microarray analysis. In response to hypoxia 4904 genes were significantly differentially expressed (1306 upregulated and 3598 downregulated, FC ≥ 2 and p ≤ 0.05). Analysis of the significant differentially expressed transcripts identified an increase in immune response pathways, and dysregulation of signalling pathways, including HIF-1 (p = 0.002), and metabolism, including glycolysis (p = 0.006). To assess whether the hypoxia-induced metabolic gene changes observed affected metabolism at a functional level, both the glycolytic and mitochondrial flux were measured using an XF bioanalyser. In support of the transcriptomic data, under physiological conditions hypoxia significantly reduced mitochondrial respiratory flux (p = 0.0001) but increased basal glycolytic flux (p = 0.0313). However, when metabolically stressed, hypoxia reduced mitochondrial spare respiratory capacity (p = 0.0485) and both glycolytic capacity (p = 0.0001) and glycolytic reserve (p < 0.0001). In summary, the current findings detail hypoxia-induced changes in the astrocyte transcriptome in vitro, identifying potential targets for modifying the astrocyte response to reduced oxygen availability in pathological conditions associated with ischaemia/hypoxia, including manipulation of mitochondrial function, metabolism, and the immune response.

Proteomic analysis links alterations of bioenergetics, mitochondria-ER interactions and proteostasis in hippocampal astrocytes from 3xTg-AD mice

The pathogenesis of Alzheimer’s disease (AD), a slowly-developing age-related neurodegenerative disorder, is a result of the action of multiple factors including deregulation of Ca²⁺ homeostasis, mitochondrial dysfunction, and dysproteostasis. Interaction of these factors in astrocytes, principal homeostatic cells in the central nervous system, is still poorly understood. Here we report that in immortalized hippocampal astrocytes from 3xTg-AD mice (3Tg-iAstro cells) bioenergetics is impaired, including reduced glycolysis and mitochondrial oxygen consumption, and increased production of reactive oxygen species. Shotgun proteomics analysis of mitochondria-ER-enriched fraction showed no alterations in the expression of mitochondrial and OxPhos proteins, while those related to the ER functions and protein synthesis were deregulated. Using ER- and mitochondria-targeted aequorin-based Ca²⁺ probe we show that, in 3Tg-iAstro cells, ER was overloaded with Ca²⁺ while Ca²⁺ uptake by mitochondria upon ATP stimulation was reduced. This was accompanied by the increase in short distance (≈8–10 nm) contact area between mitochondria and ER, upregulation of ER-stress/unfolded protein response genes Atf4, Atf6 and Herp, and reduction of global protein synthesis rate. We suggest that familial AD mutations in 3Tg-iAstro cells induce mitochondria-ER interaction changes that deregulate astrocytic bioenergetics, Ca²⁺ homeostasis and proteostasis. These factors may interact, creating a pathogenic loop compromising homeostatic and defensive functions of astroglial cells predisposing neurons to dysfunction.

Gender Differences in Neurodegeneration, Neuroinflammation and Na+-Ca2+ Exchangers in the Female A53T Transgenic Mouse Model of Parkinson's Disease

Twelve-month-old male mice expressing the human A53T variant of α-synuclein (A53T) develop dopamine neuron degeneration, neuroinflammation, and motor deficits, along with dysfunctions of the mitochondrial Na⁺-Ca²⁺ exchanger (NCX) isoforms 1 (NCX1) and 3 (NCX3) in the nigrostriatal system. Since gender is thought to play a role in the etiology of Parkinson's disease (PD), we characterized neurochemical and behavioral alterations in 12-month-old female A53T transgenic mice. We investigated the presence of dopaminergic degeneration, astrogliosis and microgliosis using immunohistochemistry for tyrosine hydroxylase (TH), glial fibrillary acidic protein (GFAP) and ionized calcium-binding adaptor molecule-1 (IBA-1) in both the substantia nigra pars compacta (SNc) and striatum. In the same regions, we also evaluated the co-localization of NCX1 in cells positive for IBA-1 and the co-localization of NCX3 in TH-positive neurons and fibers. Furthermore, in both male and female mice, we performed motor (beam walking and pole tests) and memory [novel object recognition (NOR) and spontaneous alternation] tasks, together with tests to evaluate peripheral deficits (olfactory and stool collection tests). Female A53T transgenic mice displayed degeneration of nigral dopaminergic neurons, but neither microgliosis nor astrogliosis in the SNc and striatum. Moreover, female A53T transgenic mice displayed co-localization between NCX1 and IBA-1 positive cells in the striatum but not SNc, whereas NCX3 did not co-localize with either TH-positive terminals or neuronal bodies in the nigrostriatal system. Furthermore, female A53T transgenic mice showed increased crossing time in the beam walking test, but no impairments in the pole or memory tests, and in tests that evaluated peripheral deficits, whereas male A53T transgenic mice displayed motor, memory and peripheral deficits. Immunohistochemical and behavioral results obtained here in the female mice differ from those previously observed in males, and suggest a dissimilar influence of NCX1 and NCX3 on dopaminergic function in female and male A53T transgenic mice, strengthening the validity of these mice as a model for studying the etiological factors of PD.

Klotho Pathways, Myelination Disorders, Neurodegenerative Diseases, and Epigenetic Drugs

In this review we outline a rationale for identifying neuroprotectants aimed at inducing endogenous Klotho activity and expression, which is epigenetic action, by definition. Such an approach should promote remyelination and/or stimulate myelin repair by acting on mitochondrial function, thereby heralding a life-saving path forward for patients suffering from neuroinflammatory diseases. Disorders of myelin in the nervous system damage the transmission of signals, resulting in loss of vision, motion, sensation, and other functions depending on the affected nerves, currently with no effective treatment. Klotho genes and their single-pass transmembrane Klotho proteins are powerful governors of the threads of life and death, true to the origin of their name, Fates, in Greek mythology. Among its many important functions, Klotho is an obligatory co-receptor that binds, activates, and/or potentiates critical fibroblast growth factor activity. Since the discovery of Klotho a little over two decades ago, it has become ever more apparent that when Klotho pathways go awry, oxidative stress and mitochondrial dysfunction take over, and age-related chronic disorders are likely to follow. The physiological consequences can be wide ranging, potentially wreaking havoc on the brain, eye, kidney, muscle, and more. Central nervous system disorders, neurodegenerative in nature, and especially those affecting the myelin sheath, represent worthy targets for advancing therapies that act upon Klotho pathways. Current drugs for these diseases, even therapeutics that are disease modifying rather than treating only the symptoms, leave much room for improvement. It is thus no wonder that this topic has caught the attention of biomedical researchers around the world.