Mitochondrial Ca2+ uniporter haploinsufficiency enhances long-term potentiation at hippocampal mossy fibre synapses.

Long-term changes in synaptic strength form the basis of learning and memory. These changes rely upon energy-demanding mechanisms, which are regulated by local Ca2+ signalling. Mitochondria are optimised for providing energy and buffering Ca2+. However, our understanding of the role of mitochondria in regulating synaptic plasticity is incomplete. Here, we have used optical and electrophysiological techniques in cultured hippocampal neurons and ex vivo hippocampal slices from mice with haploinsufficiency of the mitochondrial Ca2+ uniporter (MCU+/-) to address whether reducing mitochondrial Ca2+ uptake alters synaptic transmission and plasticity. We found that cultured MCU+/- hippocampal neurons have impaired Ca2+ clearance, and consequently enhanced synaptic vesicle fusion at presynapses occupied by mitochondria. Furthermore, long-term potentiation (LTP) at mossy fibre (MF) synapses, a process which is dependent on presynaptic Ca2+ accumulation, is enhanced in MCU+/- slices. Our results reveal a previously unrecognised role for mitochondria in regulating presynaptic plasticity of a major excitatory pathway involved in learning and memory.

Mitochondria at the neuronal presynapse in health and disease.

Synapses enable neurons to communicate with each other and are therefore a prerequisite for normal brain function. Presynaptically, this communication requires energy and generates large fluctuations in calcium concentrations. Mitochondria are optimized for supplying energy and buffering calcium, and they are actively recruited to presynapses. However, not all presynapses contain mitochondria; thus, how might synapses with and without mitochondria differ? Mitochondria are also increasingly recognized to serve additional functions at the presynapse. Here, we discuss the importance of presynaptic mitochondria in maintaining neuronal homeostasis and how dysfunctional presynaptic mitochondria might contribute to the development of disease.

Miro1-dependent mitochondrial positioning drives the rescaling of presynaptic Ca2+ signals during homeostatic plasticity.

Mitochondrial trafficking is influenced by neuronal activity, but it remains unclear how mitochondrial positioning influences neuronal transmission and plasticity. Here, we use live cell imaging with the genetically encoded presynaptically targeted Ca2+ indicator, SyGCaMP5, to address whether presynaptic Ca2+ responses are altered by mitochondria in synaptic terminals. We find that presynaptic Ca2+ signals, as well as neurotransmitter release, are significantly decreased in terminals containing mitochondria. Moreover, the localisation of mitochondria at presynaptic sites can be altered during long-term activity changes, dependent on the Ca2+-sensing function of the mitochondrial trafficking protein, Miro1. In addition, we find that Miro1-mediated activity-dependent synaptic repositioning of mitochondria allows neurons to homeostatically alter the strength of presynaptic Ca2+ signals in response to prolonged changes in neuronal activity. Our results support a model in which mitochondria are recruited to presynaptic terminals during periods of raised neuronal activity and are involved in rescaling synaptic signals during homeostatic plasticity.

Developmental regulation of tau splicing is disrupted in stem cell-derived neurons from frontotemporal dementia patients with the 10 + 16 splice-site mutation in MAPT.

The alternative splicing of the tau gene, MAPT, generates six protein isoforms in the adult human central nervous system (CNS). Tau splicing is developmentally regulated and dysregulated in disease. Mutations in MAPT that alter tau splicing cause frontotemporal dementia (FTD) with tau pathology, providing evidence for a causal link between altered tau splicing and disease. The use of induced pluripotent stem cell (iPSC)-derived neurons has revolutionized the way we model neurological disease in vitro. However, as most tau mutations are located within or around the alternatively spliced exon 10, it is important that iPSC-neurons splice tau appropriately in order to be used as disease models. To address this issue, we analyzed the expression and splicing of tau in iPSC-derived cortical neurons from control patients and FTD patients with the 10 + 16 intronic mutation in MAPT. We show that control neurons only express the fetal tau isoform (0N3R), even at extended time points of 100 days in vitro. Neurons from FTD patients with the 10 + 16 mutation in MAPT express both 0N3R and 0N4R tau isoforms, demonstrating that this mutation overrides the developmental regulation of exon 10 inclusion in our in vitro model. Further, at extended time points of 365 days in vitro, we observe a switch in tau splicing to include six tau isoforms as seen in the adult human CNS. Our results demonstrate the importance of neuronal maturity for use in in vitro modeling and provide a system that will be important for understanding the functional consequences of altered tau splicing.

α-Synuclein binds to the ER-mitochondria tethering protein VAPB to disrupt Ca2+ homeostasis and mitochondrial ATP production.

α-Synuclein is strongly linked to Parkinson's disease but the molecular targets for its toxicity are not fully clear. However, many neuronal functions damaged in Parkinson's disease are regulated by signalling between the endoplasmic reticulum (ER) and mitochondria. This signalling involves close physical associations between the two organelles that are mediated by binding of the integral ER protein vesicle-associated membrane protein-associated protein B (VAPB) to the outer mitochondrial membrane protein, protein tyrosine phosphatase-interacting protein 51 (PTPIP51). VAPB and PTPIP51 thus act as a scaffold to tether the two organelles. Here we show that α-synuclein binds to VAPB and that overexpression of wild-type and familial Parkinson's disease mutant α-synuclein disrupt the VAPB-PTPIP51 tethers to loosen ER-mitochondria associations. This disruption to the VAPB-PTPIP51 tethers is also seen in neurons derived from induced pluripotent stem cells from familial Parkinson's disease patients harbouring pathogenic triplication of the α-synuclein gene. We also show that the α-synuclein induced loosening of ER-mitochondria contacts is accompanied by disruption to Ca2+ exchange between the two organelles and mitochondrial ATP production. Such disruptions are likely to be particularly damaging to neurons that are heavily dependent on correct Ca2+ signaling and ATP.

Miro sculpts mitochondrial dynamics in neuronal health and disease.

Neurons are highly polarised cells with an elaborate and diverse cytoarchitecture. But this complex architecture presents a major problem: how to appropriately distribute metabolic resources where they are most needed within the cell. The solution comes in the form of mitochondria: highly dynamic organelles subject to a repertoire of trafficking, fission/fusion and quality control systems which work in concert to orchestrate a precisely distributed and healthy mitochondrial network. Mitochondria are critical for maintaining local energy supply and buffering Ca(2+) flux within neurons, and are increasingly recognised as being essential for healthy neuronal function. Mitochondrial movements are facilitated by their coupling to microtubule-based transport via kinesin and dynein motors. Adaptor proteins are required for this coupling and the mitochondrial Rho GTPases Miro1 and Miro2 are core components of this machinery. Both Miros have Ca(2+)-sensing and GTPase domains, and are therefore ideally suited to coordinating mitochondrial dynamics with intracellular signalling pathways and local energy turnover. In this review, we focus on Miro's role in mediating mitochondrial transport in neurons, and the relevance of these mechanisms to neuronal health and disease.

A cell culture model for monitoring α-synuclein cell-to-cell transfer.

The transfer of α-synuclein (α-syn) between cells has been proposed to be the primary mechanism of disease spreading in Parkinson's disease. Several cellular models exist that monitor the uptake of recombinant α-syn from the culture medium. Here we established a more physiologically relevant model system in which α-syn is produced and transferred between mammalian neurons. We generated cell lines expressing either α-syn tagged with fluorescent proteins or fluorescent tags alone then we co-cultured these cell lines to measure protein uptake. We used live-cell imaging to demonstrate intercellular α-syn transfer and used flow cytometry and high content analysis to quantify the transfer. We then successfully inhibited intercellular protein transfer genetically by down-regulating dynamin or pharmacologically using dynasore or heparin. In addition, we differentiated human induced pluripotent stem cells carrying a triplication of the α-syn gene into dopaminergic neurons. These cells secreted high levels of α-syn, which was taken up by neighboring neurons. Collectively, our co-culture systems provide simple but physiologically relevant tools for the identification of genetic modifiers or small molecules that inhibit α-syn cell-to-cell transfer.

Characterization of two human induced pluripotent stem cell lines derived from Batten disease patient fibroblasts harbouring CLN5 mutations.

The neuronal ceroid lipofuscinoses (NCLs) are a group of common inherited neurodegenerative disorders of childhood. All forms of NCLs are life-limiting with no curative treatments. Most of the 13 NCL genes encode proteins residing in endolysosomal pathways, such as CLN5, a potential lysosomal enzyme. Two induced pluripotent stem cell lines (hiPSCs) were generated from skin fibroblasts of CLN5 disease patients via non-integrating Sendai virus reprogramming. They demonstrate typical stem cell morphology, express pluripotency markers, exhibit trilineage differentiation potential and also successfully differentiate into neurons. These hiPSCs represent a potential resource to model CLN5 disease in a human context and investigate potential therapies.

Pathogenic Parkinson's disease mutations across the functional domains of LRRK2 alter the autophagic/lysosomal response to starvation.

LRRK2 is one of the most important genetic contributors to Parkinson's disease (PD). Point mutations in this gene cause an autosomal dominant form of PD, but to date no cellular phenotype has been consistently linked with mutations in each of the functional domains (ROC, COR and Kinase) of the protein product of this gene. In this study, primary fibroblasts from individuals carrying pathogenic mutations in the three central domains of LRRK2 were assessed for alterations in the autophagy/lysosomal pathway using a combination of biochemical and cellular approaches. Mutations in all three domains resulted in alterations in markers for autophagy/lysosomal function compared to wild type cells. These data highlight the autophagy and lysosomal pathways as read outs for pathogenic LRRK2 function and as a marker for disease, and provide insight into the mechanisms linking LRRK2 function and mutations.

Emotion in psychogenic nonepileptic seizures: responses to affective pictures.

We examined emotional responses to standard affective pictures in 18 psychogenic nonepileptic seizure (PNES) patients. Given reports of trauma and posttraumatic stress symptoms (PTS) in many PNES patients, comparison groups were seizure-free individuals high and low in PTS (PTS-high, PTS-low; n=18 per group). Patients with psychogenic nonepileptic seizures (1) reported more emotional intensity to neutral and pleasant pictures than PTS-low and more intensity to neutral pictures than PTS-high, and (2) showed less positive emotional behavior to pleasant pictures than PTS-high. Groups did not differ in pleasantness/unpleasantness ratings, negative emotional behavior, cardiac interbeat interval, or respiratory sinus arrhythmia (RSA) reactivity to the pictures. Patients with psychogenic nonepileptic seizures reported more general emotion regulation difficulties and showed lower baseline RSA than PTS-low but not PTS-high. In sum, intense emotional experience and diminished positive emotional behavior characterized PNES patients' emotional responses.

Parkinson's disease and α-synuclein expression.

Genetic studies of Parkinson's disease over the last decade or more have revolutionized our understanding of this condition. α-Synuclein was the first gene to be linked to Parkinson's disease, and is arguably the most important: the protein is the principal constituent of Lewy bodies, and variation at its locus is the major genetic risk factor for sporadic disease. Intriguingly, duplications and triplications of the locus, as well as point mutations, cause familial disease. Therefore, subtle alterations of α-synuclein expression can manifest with a dramatic phenotype. We outline the clinical impact of α-synuclein locus multiplications, and the implications that this has for Parkinson's disease pathogenesis. Finally, we discuss potential strategies for disease-modifying therapies for this currently incurable disorder.

Alpha synuclein aggregation drives ferroptosis: an interplay of iron, calcium and lipid peroxidation.

Protein aggregation and abnormal lipid homeostasis are both implicated in neurodegeneration through unknown mechanisms. Here we demonstrate that aggregate-membrane interaction is critical to induce a form of cell death called ferroptosis. Importantly, the aggregate-membrane interaction that drives ferroptosis depends both on the conformational structure of the aggregate, as well as the oxidation state of the lipid membrane. We generated human stem cell-derived models of synucleinopathy, characterized by the intracellular formation of α-synuclein aggregates that bind to membranes. In human iPSC-derived neurons with SNCA triplication, physiological concentrations of glutamate and dopamine induce abnormal calcium signaling owing to the incorporation of excess α-synuclein oligomers into membranes, leading to altered membrane conductance and abnormal calcium influx. α-synuclein oligomers further induce lipid peroxidation. Targeted inhibition of lipid peroxidation prevents the aggregate-membrane interaction, abolishes aberrant calcium fluxes, and restores physiological calcium signaling. Inhibition of lipid peroxidation, and reduction of iron-dependent accumulation of free radicals, further prevents oligomer-induced toxicity in human neurons. In summary, we report that peroxidation of polyunsaturated fatty acids underlies the incorporation of ß-sheet-rich aggregates into the membranes, and that additionally induces neuronal death. This suggests a role for ferroptosis in Parkinson's disease, and highlights a new mechanism by which lipid peroxidation causes cell death.

Correction: Alpha synuclein aggregation drives ferroptosis: an interplay of iron, calcium and lipid peroxidation.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Using stem cell-derived neurons in drug screening for neurological diseases.

Induced pluripotent stem cells and their derivatives have become an important tool for researching disease mechanisms. It is hoped that they could be used to discover new therapies by providing the most reliable and relevant human in vitro disease models for drug discovery. This review will summarize recent efforts to use stem cell-derived neurons for drug screening. We also explain the current hurdles to using these cells for high-throughput pharmaceutical screening and developments that may help overcome these hurdles. Finally, we critically discuss whether induced pluripotent stem cell-derived neurons will come to fruition as a model that is regularly used to screen for drugs to treat neurological diseases.

High-Content Autophagy Analysis in iPSC-Derived Neurons Using Immunofluorescence.

Autophagy is the process by which cellular proteins and organelles are degraded and recycled and is essential to the survival of cells. Defective autophagic degradation has been linked to many neurodegenerative diseases and in particular lysosomal storage diseases. Here we describe a high-content assay to detect defects in the autophagy pathway in induced pluripotent stem cell-derived neurons. This assay utilizes immunofluorescence to stain autophagosomes and uses automated image analysis to measure changes in autophagosome levels in response to modulators of autophagy.

High-Content Analysis of Mitochondrial Function in iPSC-Derived Neurons.

Mitochondrial dysfunction is linked to many neurological diseases; therefore, the ability to measure mitochondrial function is of great use for researching disease and testing potential therapeutics. Here we describe a high-content assay to simultaneously measure mitochondrial membrane potential, morphology and cell viability in iPSC-derived neurons. Neurons are seeded into plates suitable for fluorescent microscopy, stained with the mitochondrial membrane potential-dependent dye TMRM, cytoplasmic dye Calcein AM, and nuclear stain Hoechst 33342. Images are acquired in live cells and analyzed using automated image analysis software.

Seeding Induced Pluripotent Stem Cell-Derived Neurons onto 384-Well Plates.

Induced pluripotent stem cell (iPSC) derived neurons are an excellent in vitro model of neurological diseases that are often used in early stage drug discovery projects. Thus far, the use of iPSC-derived cells in small molecule drug screening has been limited, and one of the reasons for this has been the challenge of miniaturization of iPSC culture and differentiation in low volume microwell plate formats. Here we describe a method of seeding iPSC-derived neurons into 384-well plates towards the end of the differentiation procedure. This method covers coating the plates with substrates to aid attachment, dissociation of the cells into a single cell suspension, and seeding onto 384-well plates to give an even distribution of neurons. This method facilitates the use of iPSC-derived neurons for high-content imaging, whole-well assays, and small-molecule drug screening.

Nanoscale tweezers for single-cell biopsies.

Much of the functionality of multicellular systems arises from the spatial organization and dynamic behaviours within and between cells. Current single-cell genomic methods only provide a transcriptional 'snapshot' of individual cells. The real-time analysis and perturbation of living cells would generate a step change in single-cell analysis. Here we describe minimally invasive nanotweezers that can be spatially controlled to extract samples from living cells with single-molecule precision. They consist of two closely spaced electrodes with gaps as small as 10-20 nm, which can be used for the dielectrophoretic trapping of DNA and proteins. Aside from trapping single molecules, we also extract nucleic acids for gene expression analysis from living cells without affecting their viability. Finally, we report on the trapping and extraction of a single mitochondrion. This work bridges the gap between single-molecule/organelle manipulation and cell biology and can ultimately enable a better understanding of living cells.

A single cell high content assay detects mitochondrial dysfunction in iPSC-derived neurons with mutations in SNCA.

Mitochondrial dysfunction is implicated in many neurodegenerative diseases including Parkinson's disease (PD). Induced pluripotent stem cells (iPSCs) provide a unique cell model for studying neurological diseases. We have established a high-content assay that can simultaneously measure mitochondrial function, morphology and cell viability in iPSC-derived dopaminergic neurons. iPSCs from PD patients with mutations in SNCA and unaffected controls were differentiated into dopaminergic neurons, seeded in 384-well plates and stained with the mitochondrial membrane potential dependent dye TMRM, alongside Hoechst-33342 and Calcein-AM. Images were acquired using an automated confocal screening microscope and single cells were analysed using automated image analysis software. PD neurons displayed reduced mitochondrial membrane potential and altered mitochondrial morphology compared to control neurons. This assay demonstrates that high content screening techniques can be applied to the analysis of mitochondria in iPSC-derived neurons. This technique could form part of a drug discovery platform to test potential new therapeutics for PD and other neurodegenerative diseases.