Guidelines on experimental methods to assess mitochondrial dysfunction in cellular models of neurodegenerative diseases

The Biosynthesis, Signaling, and Neurological Functions of Bile Acids

Bile acids (BA) are amphipathic steroid acids synthesized from cholesterol in the liver. They act as detergents to expedite the digestion and absorption of dietary lipids and lipophilic vitamins. BA are also considered to be signaling molecules, being ligands of nuclear and cell-surface receptors, including farnesoid X receptor and Takeda G-protein receptor 5. Moreover, BA also activate ion channels, including the bile acid-sensitive ion channel and epithelial Na⁺ channel. BA regulate glucose and lipid metabolism by activating these receptors in peripheral tissues, such as the liver and brown and white adipose tissue. Recently, 20 different BA have been identified in the central nervous system. Furthermore, BA affect the function of neurotransmitter receptors, such as the muscarinic acetylcholine receptor and γ-aminobutyric acid receptor. BA are also known to be protective against neurodegeneration. Here, we review recent findings regarding the biosynthesis, signaling, and neurological functions of BA.

Mitochondrial dysfunction in the pathogenesis of chemotherapy-induced peripheral neuropathy

Several first-line chemotherapeutic agents, including taxanes, platinum agents and proteasome inhibitors, are associated with the dose-limiting side effect of chemotherapy-induced peripheral neuropathy (CIPN). CIPN predominantly manifests as sensory symptoms, which are likely due to drug accumulation within peripheral nervous tissues rather than the central nervous system. No treatment is currently available to prevent or reverse CIPN. The causal mechanisms underlying CIPN are not yet fully understood. Mitochondrial dysfunction has emerged as a major factor contributing to the development and maintenance of CIPN. This chapter will provide an overview of both clinical and preclinical data supporting this hypothesis. We will review the studies reporting the nature of mitochondrial dysfunction evoked by chemotherapy in terms of changes in mitochondrial morphology, bioenergetics and reactive oxygen species (ROS) generation. Furthermore, we will discuss the in vivo effects of pharmacological interventions that counteract chemotherapy-evoked mitochondrial dysfunction and ameliorate pain-like behavior.

Systems biology identifies preserved integrity but impaired metabolism of mitochondria due to a glycolytic defect in Alzheimer's disease neurons

Mitochondrial dysfunction is implicated in most neurodegenerative diseases, including Alzheimer's disease (AD). We here combined experimental and computational approaches to investigate mitochondrial health and bioenergetic function in neurons from a double transgenic animal model of AD (PS2APP/B6.152H). Experiments in primary cortical neurons demonstrated that AD neurons had reduced mitochondrial respiratory capacity. Interestingly, the computational model predicted that this mitochondrial bioenergetic phenotype could not be explained by any defect in the mitochondrial respiratory chain (RC), but could be closely resembled by a simulated impairment in the mitochondrial NADH flux. Further computational analysis predicted that such an impairment would reduce levels of mitochondrial NADH, both in the resting state and following pharmacological manipulation of the RC. To validate these predictions, we utilized fluorescence lifetime imaging microscopy (FLIM) and autofluorescence imaging and confirmed that transgenic AD neurons had reduced mitochondrial NAD(P)H levels at rest, and impaired power of mitochondrial NAD(P)H production. Of note, FLIM measurements also highlighted reduced cytosolic NAD(P)H in these cells, and extracellular acidification experiments showed an impaired glycolytic flux. The impaired glycolytic flux was identified to be responsible for the observed mitochondrial hypometabolism, since bypassing glycolysis with pyruvate restored mitochondrial health. This study highlights the benefits of a systems biology approach when investigating complex, nonintuitive molecular processes such as mitochondrial bioenergetics, and indicates that primary cortical neurons from a transgenic AD model have reduced glycolytic flux, leading to reduced cytosolic and mitochondrial NAD(P)H and reduced mitochondrial respiratory capacity.

Live-Imaging Readouts and Cell Models for Phenotypic Profiling of Mitochondrial Function

Mitochondria are best known as the powerhouses of the cells but their cellular role goes far beyond energy production; among others, they have a pivotal function in cellular calcium and redox homeostasis. Mitochondrial dysfunction is often associated with severe and relatively rare disorders with an unmet therapeutic need. Given their central integrating role in multiple cellular pathways, mitochondrial dysfunction is also relevant in the pathogenesis of various other, more common, human pathologies. Here we discuss how live-cell high content microscopy can be used for image-based phenotypic profiling to assess mitochondrial (dys) function. From this perspective, we discuss a selection of live-cell fluorescent reporters and imaging strategies and discuss the pros/cons of human cell models in mitochondrial research. We also present an overview of live-cell high content microscopy applications used to detect disease-associated cellular phenotypes and perform cell-based drug screening.

Distinct differences in rates of oxygen consumption and ATP synthesis of regionally isolated non-synaptic mouse brain mitochondria

Brain mitochondrial dysfunction has been implicated in several neurodegenerative diseases. The distribution and efficiency of mitochondria display large heterogeneity throughout the regions of the brain. This may imply that the selective regional susceptibility of neurodegenerative diseases could be mediated through inherent differences in regional mitochondrial function. To investigate regional cerebral mitochondrial energetics, the rates of oxygen consumption and adenosine-5'-triphosphate (ATP) synthesis were assessed in isolated non-synaptic mitochondria of the cerebral cortex, hippocampus, and striatum of the male mouse brain. Oxygen consumption rates were assessed using a Seahorse XFe96 analyzer and ATP synthesis rates were determined by an online luciferin-luciferase coupled luminescence assay. Complex I- and complex II-driven respiration and ATP synthesis, were investigated by applying pyruvate in combination with malate, or succinate, as respiratory substrates, respectively. Hippocampal mitochondria exhibited the lowest basal and adenosine-5'-diphosphate (ADP)-stimulated rate of oxygen consumption when provided pyruvate and malate. However, hippocampal mitochondria also exhibited an increased proton leak and an elevated relative rate of oxygen consumption in response to the uncoupler carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP), showing a large capacity for uncoupled respiration in the presence of pyruvate. When the complex II-linked substrate succinate was provided, striatal mitochondria exhibited the highest respiration and ATP synthesis rate, whereas hippocampal mitochondria had the lowest. However, the mitochondrial efficiency, determined as ATP produced/O₂ consumed, was similar between the three regions. This study reveals inherent differences in regional mitochondrial energetics and may serve as a tool for further investigations of regional mitochondrial function in relation to neurodegenerative diseases.

Effects of Cimicifuga racemosa extract Ze450 on mitochondria in models of oxidative stress in neuronal cells

This data article describes the influence of Cimicifuga racemosa extract Ze 450 on neuronal cells in models of glutamate-induced excitotoxicity and cell death induced by oxidative stress. Effects of Ze 450 on glutamate-mediated excitotoxicity were assessed in primary cortical rat and mouse neurons and, further, glutamate-mediated oxidative stress was analyzed in HT22 cells lacking ionotropic glutamate receptors. This study especially focusses on mitochondrial parameters like mitochondrial ROS formation, mitochondrial membrane potential, ATP production and mitochondrial integrity. Further the effects of Ze 450 on lipid-peroxidation, metabolic activity, cell proliferation and cell death were assessed under control conditions and oxidative challenge evoked by millimolar concentrations of glutamate in HT22 cells. These data support the findings in HT22, mHypo and HepG2 liver cells (Rabenau et al., 2018) [1].

High glycolytic activity of tumor cells leads to underestimation of electron transport system capacity when mitochondrial ATP synthase is inhibited

This study sought to elucidate how oligomycin, an ATP synthase blocker, leads to underestimation of maximal oxygen consumption rate (_maxOCR) and spare respiratory capacity (SRC) in tumor cells. T98G and U-87MG glioma cells were titrated with the protonophore CCCP to induce _maxOCR. The presence of oligomycin (0.3-3.0 µg/mL) led to underestimation of _maxOCR and a consequent decrease in SRC values of between 25% and 40% in medium containing 5.5 or 11 mM glucose. The inhibitory effect of oligomycin on CCCP-induced _maxOCR did not occur when glutamine was the metabolic substrate or when the glycolytic inhibitor 2-deoxyglucose was present. ATP levels were reduced and ADP/ATP ratios increased in cells treated with CCCP, but these changes were minimized when oligomycin was used to inhibit reverse activity of ATP synthase. Exposing digitonin-permeabilized cells to exogenous ATP, but not ADP, resulted in partial inhibition of CCCP-induced _maxOCR. We conclude that underestimation of _maxOCR and SRC in tumor cells when ATP synthase is inhibited is associated with high glycolytic activity and that the glycolytic ATP yield may have an inhibitory effect on the metabolism of respiratory substrates and cytochrome c oxidase activity. Under CCCP-induced _maxOCR, oligomycin preserves intracellular ATP by inhibiting ATP synthase reverse activity.

Critical role of mitosis in spontaneous late-onset Alzheimer's disease; from a Shugoshin 1 cohesinopathy mouse model

From early-onset Alzheimer's disease (EOAD) studies, the amyloid-beta hypothesis emerged as the foremost theory of the pathological causes of AD. However, how amyloid-beta accumulation is triggered and progresses toward senile plaques in spontaneous late-onset Alzheimer's disease (LOAD) in humans remains unanswered. Various LOAD facilitators have been proposed, and LOAD is currently considered a complex disease with multiple causes. Mice do not normally develop LOAD. Possibly due to the multiple causes, proposed LOAD facilitators have not been able to replicate spontaneous LOAD in mice, representing a disease modeling issue. Recently, we reported spontaneous late-onset development of amyloid-beta accumulation in brains of Shugoshin 1 (Sgo1) haploinsufficient mice, a cohesinopathy-mediated chromosome instability model. The result for the first time expands disease relevance of mitosis studies to a major disease other than cancers. Reverse-engineering of the model would shed light on the process of late-onset amyloid-beta accumulation in the brain and spontaneous LOAD development, and contribute to development of interventions for LOAD. This review will discuss the Sgo1 model, our current "three-hit hypothesis" regarding LOAD development with an emphasis on critical role of prolonged mitosis in amyloid-beta accumulation, and implications for human LOAD intervention and treatment. Abbreviations: Alzheimer's disease (AD); Late-onset Alzheimer's disease (LOAD); Early-onset Alzheimer's disease (EOAD); Shugoshin-1 (Sgo1); Chromosome Instability (CIN); apolipoprotein (Apoe); Central nervous system (CNS); Amyloid precursor protein (APP); N-methyl-d-aspartate (NMDA); Hazard ratio (HR); Cyclin-dependent kinase (CDK); Chronic Atrial Intestinal Dysrhythmia (CAID); beta-secretase 1 (BACE); phosphor-Histone H3 (p-H3); Research and development (R&D); Non-steroidal anti-inflammatory drugs (NSAIDs); Brain blood barrier (BBB).

Clinical spectrum and genetic landscape for hereditary spastic paraplegias in China

Background

Hereditary spastic paraplegias (HSP) is a heterogeneous group of rare neurodegenerative disorders affecting the corticospinal tracts. To date, more than 78 HSP loci have been mapped to cause HSP. However, both the clinical and mutational spectrum of Chinese patients with HSP remained unclear. In this study, we aim to perform a comprehensive analysis of clinical phenotypes and genetic distributions in a large cohort of Chinese HSP patients, and to elucidate the primary pathogenesis in this population.

Methods

We firstly performed next-generation sequencing targeting 149 genes correlated with HSP in 99 index cases of our cohort. Multiplex ligation-dependent probe amplification testing was further carried out among those patients without known disease-causing gene mutations. We simultaneously performed a retrospective study on the reported patients exhibiting HSP in other Chinese cohorts. All clinical and molecular characterization from above two groups of Chinese HSP patients were analyzed and summarized. Eventually, we further validated the cellular changes in fibroblasts of two major spastic paraplegia (SPG) patients (SPG4 and SPG11) in vitro.

Results

Most patients of ADHSP (94%) are pure forms, whereas most patients of ARHSP (78%) tend to be complicated forms. In ADHSP, we found that SPG4 (79%) was the most prevalent, followed by SPG3A (11%), SPG6 (4%) and SPG33 (2%). Subtle mutations were the common genetic cause for SPG4 patients and most of them located in AAA cassette domain of spastin protein. In ARHSP, the most common subtype was SPG11 (53%), followed by SPG5 (32%), SPG35 (6%) and SPG46 (3%). Moreover, haplotype analysis showed a unique haplotype was shared in 14 families carrying c.334C > T (p.R112^*) mutation in CYP7B1 gene, suggesting the founder effect. Functionally, we observed significantly different patterns of mitochondrial dynamics and network, decreased mitochondrial membrane potential (Δψm), increased reactive oxygen species and reduced ATP content in SPG4 fibroblasts. Moreover, we also found the enlargement of LAMP1-positive organelles and abnormal accumulation of autolysosomes in SPG11 fibroblasts.

Conclusions

Our study present a comprehensive clinical spectrum and genetic landscape for HSP in China. We have also provided additional evidences for mitochondrial and autolysosomal-mediated pathways in the pathogenesis of HSP.

Electronic supplementary material

The online version of this article (10.1186/s13024-018-0269-1) contains supplementary material, which is available to authorized users.

Cellular and Molecular Basis of Neurodegeneration in Parkinson Disease

It has been 200 years since Parkinson disease (PD) was described by Dr. Parkinson in 1817. The disease is the second most common neurodegenerative disease characterized by a progressive loss of dopaminergic neurons in the substantia nigra pars compacta. Although the pathogenesis of PD is still unknown, the research findings from scientists are conducive to understand the pathological mechanisms. It is well accepted that both genetic and environmental factors contribute to the onset of PD. In this review, we summarize the mutations of main seven genes (α-synuclein, LRRK2, PINK1, Parkin, DJ-1, VPS35 and GBA1) linked to PD, discuss the potential mechanisms for the loss of dopaminergic neurons (dopamine metabolism, mitochondrial dysfunction, endoplasmic reticulum stress, impaired autophagy, and deregulation of immunity) in PD, and expect the development direction for treatment of PD.

Highlighting the endoplasmic reticulum-mitochondria connection: Focus on Mitofusin 2

The endoplasmic reticulum (ER) and the mitochondrial network are two highly interconnected cellular structures. By proteinaceous tethers, specialized membrane domains of the ER are tightly associated with the outer membrane of mitochondria, allowing the assembly of signaling platforms where different cell functions take place or are modulated, such as lipid biosynthesis, Ca²⁺ homeostasis, inflammation, autophagy and apoptosis. The ER-mitochondria coupling is highly dynamic and contacts between the two organelles can be modified in their number, extension and thickness by different stimuli. Importantly, several pathological conditions, such as cancer, neurodegenerative diseases and metabolic syndromes show alterations in this feature, underlining the key role of ER-mitochondria crosstalk in cell physiology. In this contribution, we will focus on one of the major modulator of ER-mitochondria apposition, Mitofusin 2, discussing the structure of the protein and its debated role on organelles tethering. Moreover, we will critically describe different techniques commonly used to investigate this crucial issue, highlighting their advantages, drawbacks and limits.