A transcriptomic taxonomy of mouse brain-wide spinal projecting neurons

The brain controls nearly all bodily functions via spinal projecting neurons (SPNs) that carry command signals from the brain to the spinal cord. However, a comprehensive molecular characterization of brain-wide SPNs is still lacking. Here we transcriptionally profiled a total of 65,002 SPNs, identified 76 region-specific SPN types, and mapped these types into a companion atlas of the whole mouse brain1. This taxonomy reveals a three-component organization of SPNs: (1) molecularly homogeneous excitatory SPNs from the cortex, red nucleus and cerebellum with somatotopic spinal terminations suitable for point-to-point communication; (2) heterogeneous populations in the reticular formation with broad spinal termination patterns, suitable for relaying commands related to the activities of the entire spinal cord; and (3) modulatory neurons expressing slow-acting neurotransmitters and/or neuropeptides in the hypothalamus, midbrain and reticular formation for 'gain setting' of brain-spinal signals. In addition, this atlas revealed a LIM homeobox transcription factor code that parcellates the reticulospinal neurons into five molecularly distinct and spatially segregated populations. Finally, we found transcriptional signatures of a subset of SPNs with large soma size and correlated these with fast-firing electrophysiological properties. Together, this study establishes a comprehensive taxonomy of brain-wide SPNs and provides insight into the functional organization of SPNs in mediating brain control of bodily functions.

The molecular cytoarchitecture of the adult mouse brain

The function of the mammalian brain relies upon the specification and spatial positioning of diversely specialized cell types. Yet, the molecular identities of the cell types and their positions within individual anatomical structures remain incompletely known. To construct a comprehensive atlas of cell types in each brain structure, we paired high-throughput single-nucleus RNA sequencing with Slide-seq1,2-a recently developed spatial transcriptomics method with near-cellular resolution-across the entire mouse brain. Integration of these datasets revealed the cell type composition of each neuroanatomical structure. Cell type diversity was found to be remarkably high in the midbrain, hindbrain and hypothalamus, with most clusters requiring a combination of at least three discrete gene expression markers to uniquely define them. Using these data, we developed a framework for genetically accessing each cell type, comprehensively characterized neuropeptide and neurotransmitter signalling, elucidated region-specific specializations in activity-regulated gene expression and ascertained the heritability enrichment of neurological and psychiatric phenotypes. These data, available as an online resource ( www.BrainCellData.org ), should find diverse applications across neuroscience, including the construction of new genetic tools and the prioritization of specific cell types and circuits in the study of brain diseases.

Brain-wide correspondence of neuronal epigenomics and distant projections

Single-cell analyses parse the brain's billions of neurons into thousands of 'cell-type' clusters residing in different brain structures1. Many cell types mediate their functions through targeted long-distance projections allowing interactions between specific cell types. Here we used epi-retro-seq2 to link single-cell epigenomes and cell types to long-distance projections for 33,034 neurons dissected from 32 different regions projecting to 24 different targets (225 source-to-target combinations) across the whole mouse brain. We highlight uses of these data for interrogating principles relating projection types to transcriptomics and epigenomics, and for addressing hypotheses about cell types and connections related to genetics. We provide an overall synthesis with 926 statistical comparisons of discriminability of neurons projecting to each target for every source. We integrate this dataset into the larger BRAIN Initiative Cell Census Network atlas, composed of millions of neurons, to link projection cell types to consensus clusters. Integration with spatial transcriptomics further assigns projection-enriched clusters to smaller source regions than the original dissections. We exemplify this by presenting in-depth analyses of projection neurons from the hypothalamus, thalamus, hindbrain, amygdala and midbrain to provide insights into properties of those cell types, including differentially expressed genes, their associated cis-regulatory elements and transcription-factor-binding motifs, and neurotransmitter use.

Dopamine subsystems that track internal states

Food and water are rewarding in part because they satisfy our internal needs1,2. Dopaminergic neurons in the ventral tegmental area (VTA) are activated by gustatory rewards3-5, but how animals learn to associate these oral cues with the delayed physiological effects of ingestion is unknown. Here we show that individual dopaminergic neurons in the VTA respond to detection of nutrients or water at specific stages of ingestion. A major subset of dopaminergic neurons tracks changes in systemic hydration that occur tens of minutes after thirsty mice drink water, whereas different dopaminergic neurons respond to nutrients in the gastrointestinal tract. We show that information about fluid balance is transmitted to the VTA by a hypothalamic pathway and then re-routed to downstream circuits that track the oral, gastrointestinal and post-absorptive stages of ingestion. To investigate the function of these signals, we used a paradigm in which a fluid's oral and post-absorptive effects can be independently manipulated and temporally separated. We show that mice rapidly learn to prefer one fluid over another based solely on its rehydrating ability and that this post-ingestive learning is prevented if dopaminergic neurons in the VTA are selectively silenced after consumption. These findings reveal that the midbrain dopamine system contains subsystems that track different modalities and stages of ingestion, on timescales from seconds to tens of minutes, and that this information is used to drive learning about the consequences of ingestion.

Diverse processing underlying frequency integration in midbrain neurons of barn owls

Emergent response properties of sensory neurons depend on circuit connectivity and somatodendritic processing. Neurons of the barn owl’s external nucleus of the inferior colliculus (ICx) display emergence of spatial selectivity. These neurons use interaural time difference (ITD) as a cue for the horizontal direction of sound sources. ITD is detected by upstream brainstem neurons with narrow frequency tuning, resulting in spatially ambiguous responses. This spatial ambiguity is resolved by ICx neurons integrating inputs over frequency, a relevant processing in sound localization across species. Previous models have predicted that ICx neurons function as point neurons that linearly integrate inputs across frequency. However, the complex dendritic trees and spines of ICx neurons raises the question of whether this prediction is accurate. Data from in vivo intracellular recordings of ICx neurons were used to address this question. Results revealed diverse frequency integration properties, where some ICx neurons showed responses consistent with the point neuron hypothesis and others with nonlinear dendritic integration. Modeling showed that varied connectivity patterns and forms of dendritic processing may underlie observed ICx neurons’ frequency integration processing. These results corroborate the ability of neurons with complex dendritic trees to implement diverse linear and nonlinear integration of synaptic inputs, of relevance for adaptive coding and learning, and supporting a fundamental mechanism in sound localization.

Neurons at higher stages of sensory pathways often display selectivity for properties of sensory stimuli that result from computations performed within the nervous system. These emergent response properties can be produced by patterns of neural connectivity and processing that occur within individual cells. Here we investigated whether neural connectivity and single-neuron computation may contribute to the emergence of spatial selectivity in auditory neurons in the barn owl’s midbrain. We used data from in vivo intracellular recordings to test the hypothesis from previous modeling work that these cells function as point neurons that perform a linear sum of their inputs in their subthreshold responses. Results indicate that while some neurons show responses consistent with the point neuron hypothesis, others match predictions of nonlinear integration, indicating a diversity of frequency integration properties across neurons. Modeling further showed that varied connectivity patterns and forms of single-neuron computation may underlie observed responses. These results demonstrate that neurons with complex morphologies may implement diverse integration of synaptic inputs, relevant for adaptive coding and learning.

Adult Neural Stem Cells from Midbrain Periventricular Regions Show Limited Neurogenic Potential after Transplantation into the Hippocampal Neurogenic Niche

The regulation of adult neural stem or progenitor cell (aNSC) proliferation and differentiation as an interplay of cell-intrinsic and local environmental cues remains in part unclear, impeding their role in putative regenerative therapies. aNSCs with all major properties of NSCs in vitro have been identified in a variety of brain regions beyond the classic neurogenic niches, including the caudal periventricular regions (PVRs) of the midbrain, though active neurogenesis is either limited or merely absent in these regions. To elucidate cell-intrinsic properties of aNSCs from various PVRs, we here examined the proliferation and early differentiation capacity of murine aNSCs from non-neurogenic midbrain PVRs (PVRMB) compared to aNSCs from the neurogenic ventricular-subventricular zone (PVRV-SVZ) 7 days after transplantation into the permissive pro-neurogenic niche of the dentate gyrus (DG) of the hippocampus in mice. An initial in vitro characterization of the transplants displayed very similar characteristics of both aNSC grafts after in vitro expansion with equal capacities of terminal differentiation into astrocytes and Tuj1+ neurons. Upon the allogenic transplantation of the respective aNSCs into the DG, PVRMB grafts showed a significantly lower graft survival and proliferative capacity compared to PVRV-SVZ transplants, whereby the latter are exclusively capable of generating new neurons. Although these differences might be-in part-related to the transplantation procedure and the short-term study design, our data strongly imply important cell-intrinsic differences between aNSCs from neurogenic compared to non-neurogenic PVRs with respect to their neurogenic potential and/or their sensitivity to neurogenic cues.

Inactivation mode of sodium channels defines the different maximal firing rates of conventional versus atypical midbrain dopamine neurons

Two subpopulations of midbrain dopamine (DA) neurons are known to have different dynamic firing ranges in vitro that correspond to distinct projection targets: the originally identified conventional DA neurons project to the dorsal striatum and the lateral shell of the nucleus accumbens, whereas an atypical DA population with higher maximum firing frequencies projects to prefrontal regions and other limbic regions including the medial shell of nucleus accumbens. Using a computational model, we show that previously identified differences in biophysical properties do not fully account for the larger dynamic range of the atypical population and predict that the major difference is that originally identified conventional cells have larger occupancy of voltage-gated sodium channels in a long-term inactivated state that recovers slowly; stronger sodium and potassium conductances during action potential firing are also predicted for the conventional compared to the atypical DA population. These differences in sodium channel gating imply that longer intervals between spikes are required in the conventional population for full recovery from long-term inactivation induced by the preceding spike, hence the lower maximum frequency. These same differences can also change the bifurcation structure to account for distinct modes of entry into depolarization block: abrupt versus gradual. The model predicted that in cells that have entered depolarization block, it is much more likely that an additional depolarization can evoke an action potential in conventional DA population. New experiments comparing lateral to medial shell projecting neurons confirmed this model prediction, with implications for differential synaptic integration in the two populations.

We developed a theoretical and mathematical framework that could explain the major electrophysiological differences between the conventional midbrain dopamine (DA) neurons with a low maximum firing rate, and the more recently identified atypical DA neurons. Testable predictions from this framework were then verified with in vitro patch-clamp recordings from DA neurons with identified phenotypes and projection targets. Since different subpopulations of DA neurons participate in different circuits, and these circuits are likely differentially dysregulated in diseases such as addiction, Parkinson disease, and schizophrenia, it is important to identify the differences of their intrinsic electrophysiological properties as a prelude to developing more precisely targeted therapies.

Thyroid hormone deficiency during zebrafish development impairs central nervous system myelination

Thyroid hormones are messengers that bind to specific nuclear receptors and regulate a wide range of physiological processes in the early stages of vertebrate embryonic development, including neurodevelopment and myelogenesis. We here tested the effects of reduced T3 availability upon the myelination process by treating zebrafish embryos with low concentrations of iopanoic acid (IOP) to block T4 to T3 conversion. Black Gold II staining showed that T3 deficiency reduced the myelin density in the forebrain, midbrain, hindbrain and the spinal cord at 3 and 7 dpf. These observations were confirmed in 3 dpf mbp:egfp transgenic zebrafish, showing that the administration of IOP reduced the fluorescent signal in the brain. T3 rescue treatment restored brain myelination and reversed the changes in myelin-related gene expression induced by IOP exposure. NG2 immunostaining revealed that T3 deficiency reduced the amount of oligodendrocyte precursor cells in 3 dpf IOP-treated larvae. Altogether, the present results show that inhibition of T4 to T3 conversion results in hypomyelination, suggesting that THs are part of the key signaling molecules that control the timing of oligodendrocyte differentiation and myelin synthesis from very early stages of brain development.

Ncx3-Induced Mitochondrial Dysfunction in Midbrain Leads to Neuroinflammation in Striatum of A53t-α-Synuclein Transgenic Old Mice

The exact mechanism underlying selective dopaminergic neurodegeneration is not completely understood. The complex interplay among toxic alpha-synuclein aggregates, oxidative stress, altered intracellular Ca²⁺-homeostasis, mitochondrial dysfunction and disruption of mitochondrial integrity is considered among the pathogenic mechanisms leading to dopaminergic neuronal loss. We herein investigated the molecular mechanisms leading to mitochondrial dysfunction and its relationship with activation of the neuroinflammatory process occurring in Parkinson’s disease. To address these issues, experiments were performed in vitro and in vivo in mice carrying the human mutation of α-synuclein A53T under the prion murine promoter. In these models, the expression and activity of NCX isoforms, a family of important transporters regulating ionic homeostasis in mammalian cells working in a bidirectional way, were evaluated in neurons and glial cells. Mitochondrial function was monitored with confocal microscopy and fluorescent dyes to measure mitochondrial calcium content and mitochondrial membrane potential. Parallel experiments were performed in 4 and 16-month-old A53T-α-synuclein Tg mice to correlate the functional data obtained in vitro with mitochondrial dysfunction and neuroinflammation through biochemical analysis. The results obtained demonstrated: 1. in A53T mice mitochondrial dysfunction occurs early in midbrain and later in striatum; 2. mitochondrial dysfunction occurring in the midbrain is mediated by the impairment of NCX3 protein expression in neurons and astrocytes; 3. mitochondrial dysfunction occurring early in midbrain triggers neuroinflammation later into the striatum, thus contributing to PD progression during mice aging.

Transcriptomic encoding of sensorimotor transformation in the midbrain

Sensorimotor transformation, a process that converts sensory stimuli into motor actions, is critical for the brain to initiate behaviors. Although the circuitry involved in sensorimotor transformation has been well delineated, the molecular logic behind this process remains poorly understood. Here, we performed high-throughput and circuit-specific single-cell transcriptomic analyses of neurons in the superior colliculus (SC), a midbrain structure implicated in early sensorimotor transformation. We found that SC neurons in distinct laminae expressed discrete marker genes. Of particular interest, Cbln2 and Pitx2 were key markers that define glutamatergic projection neurons in the optic nerve (Op) and intermediate gray (InG) layers, respectively. The Cbln2+ neurons responded to visual stimuli mimicking cruising predators, while the Pitx2+ neurons encoded prey-derived vibrissal tactile cues. By forming distinct input and output connections with other brain areas, these neuronal subtypes independently mediated behaviors of predator avoidance and prey capture. Our results reveal that, in the midbrain, sensorimotor transformation for different behaviors may be performed by separate circuit modules that are molecularly defined by distinct transcriptomic codes.

High-throughput generation of midbrain dopaminergic neuron organoids from reporter human pluripotent stem cells

Here, we describe a high-throughput 3D differentiation protocol for deriving midbrain dopaminergic neurons from human pluripotent stem cells. The use of organoids has become prevalent in disease modeling, but there is a high demand for more homogeneous cultures. Our approach is advantageous for large-scale production of uniform midbrain organoids that can be maintained in diverse formats, and our reporters allow for sorting of dopaminergic neurons. The maturing long-term organoid cultures can be used as a model for the entire midbrain. For complete details on the use and execution of this protocol, please refer to Ahfeldt et al. (2020).

Midbrain dopaminergic inputs gate amygdala intercalated cell clusters by distinct and cooperative mechanisms in male mice

Dopaminergic signaling plays an important role in associative learning, including fear and extinction learning. Dopaminergic midbrain neurons encode prediction error-like signals when threats differ from expectations. Within the amygdala, GABAergic intercalated cell (ITC) clusters receive one of the densest dopaminergic projections, but their physiological consequences are incompletely understood. ITCs are important for fear extinction, a function thought to be supported by activation of ventromedial ITCs that inhibit central amygdala fear output. In mice, we reveal two distinct novel mechanisms by which mesencephalic dopaminergic afferents control ITCs. Firstly, they co-release GABA to mediate rapid, direct inhibition. Secondly, dopamine suppresses inhibitory interactions between distinct ITC clusters via presynaptic D1 receptors. Early extinction training augments both GABA co-release onto dorsomedial ITCs and dopamine-mediated suppression of dorso- to ventromedial inhibition between ITC clusters. These findings provide novel insights into dopaminergic mechanisms shaping the activity balance between distinct ITC clusters that could support their opposing roles in fear behavior.

Population Coding of Natural Electrosensory Stimuli by Midbrain Neurons

Natural stimuli display spatiotemporal characteristics that typically vary over orders of magnitude, and their encoding by sensory neurons remains poorly understood. We investigated population coding of highly heterogeneous natural electrocommunication stimuli in Apteronotus leptorhynchus of either sex. Neuronal activities were positively correlated with one another in the absence of stimulation, and correlation magnitude decayed with increasing distance between recording sites. Under stimulation, we found that correlations between trial-averaged neuronal responses (i.e., signal correlations) were positive and higher in magnitude for neurons located close to another, but that correlations between the trial-to-trial variability (i.e., noise correlations) were independent of physical distance. Overall, signal and noise correlations were independent of stimulus waveform as well as of one another. To investigate how neuronal populations encoded natural electrocommunication stimuli, we considered a nonlinear decoder for which the activities were combined. Decoding performance was best for a timescale of 6 ms, indicating that midbrain neurons transmit information via precise spike timing. A simple summation of neuronal activities (equally weighted sum) revealed that noise correlations limited decoding performance by introducing redundancy. Using an evolution algorithm to optimize performance when considering instead unequally weighted sums of neuronal activities revealed much greater performance values, indicating that midbrain neuron populations transmit information that reliably enable discrimination between different stimulus waveforms. Interestingly, we found that different weight combinations gave rise to similar discriminability, suggesting robustness. Our results have important implications for understanding how natural stimuli are integrated by downstream brain areas to give rise to behavioral responses.SIGNIFICANCE STATEMENT We show that midbrain electrosensory neurons display correlations between their activities and that these can significantly impact performance of decoders. While noise correlations limited discrimination performance by introducing redundancy, considering unequally weighted sums of neuronal activities gave rise to much improved performance and mitigated the deleterious effects of noise correlations. Further analysis revealed that increased discriminability was achieved by making trial-averaged responses more separable, as well as by reducing trial-to-trial variability by eliminating noise correlations. We further found that multiple combinations of weights could give rise to similar discrimination performances, which suggests that such combinatorial codes could be achieved in the brain. We conclude that the activities of midbrain neuronal populations can be used to reliably discriminate between highly heterogeneous stimulus waveforms.

Single-Cell Profiling of Coding and Noncoding Genes in Human Dopamine Neuron Differentiation

Dopaminergic (DA) neurons derived from human pluripotent stem cells (hPSCs) represent a renewable and available source of cells useful for understanding development, developing disease models, and stem-cell therapies for Parkinson's disease (PD). To assess the utility of stem cell cultures as an in vitro model system of human DA neurogenesis, we performed high-throughput transcriptional profiling of ~20,000 ventral midbrain (VM)-patterned stem cells at different stages of maturation using droplet-based single-cell RNA sequencing (scRNAseq). Using this dataset, we defined the cellular composition of human VM cultures at different timepoints and found high purity DA progenitor formation at an early stage of differentiation. DA neurons sharing similar molecular identities to those found in authentic DA neurons derived from human fetal VM were the major cell type after two months in culture. We also developed a bioinformatic pipeline that provided a comprehensive long noncoding RNA landscape based on temporal and cell-type specificity, which may contribute to unraveling the intricate regulatory network of coding and noncoding genes in DA neuron differentiation. Our findings serve as a valuable resource to elucidate the molecular steps of development, maturation, and function of human DA neurons, and to identify novel candidate coding and noncoding genes driving specification of progenitors into functionally mature DA neurons.

Polysynaptic inhibition between striatal cholinergic interneurons shapes their network activity patterns in a dopamine-dependent manner

Striatal activity is dynamically modulated by acetylcholine and dopamine, both of which are essential for basal ganglia function. Synchronized pauses in the activity of striatal cholinergic interneurons (ChINs) are correlated with elevated activity of midbrain dopaminergic neurons, whereas synchronous firing of ChINs induces local release of dopamine. The mechanisms underlying ChIN synchronization and its interplay with dopamine release are not fully understood. Here we show that polysynaptic inhibition between ChINs is a robust network motif and instrumental in shaping the network activity of ChINs. Action potentials in ChINs evoke large inhibitory responses in multiple neighboring ChINs, strong enough to suppress their tonic activity. Using a combination of optogenetics and chemogenetics we show the involvement of striatal tyrosine hydroxylase-expressing interneurons in mediating this inhibition. Inhibition between ChINs is attenuated by dopaminergic midbrain afferents acting presynaptically on D2 receptors. Our results present a novel form of interaction between striatal dopamine and acetylcholine dynamics.

Heterogeneous expression of dopaminergic markers and Vglut2 in mouse mesodiencephalic dopaminergic nuclei A8-A13

Co-transmission of glutamate by brain dopaminergic (DA) neurons was recently proposed as a potential factor influencing cell survival in models of Parkinson's disease. Intriguingly, brain DA nuclei are differentially affected in Parkinson's disease. Whether this is associated with different patterns of co-expression of the glutamatergic phenotype along the rostrocaudal brain axis is unknown in mammals. We hypothesized that, as in zebrafish, the glutamatergic phenotype is present preferentially in the caudal mesodiencephalic DA nuclei. Here, we used in mice a cell fate mapping strategy based on reporter protein expression (ZsGreen) consecutive to previous expression of the vesicular glutamate transporter 2 (Vglut2) gene, coupled with immunofluorescence experiments against tyrosine hydroxylase (TH) or dopamine transporter (DAT). We found three expression patterns in DA cells, organized along the rostrocaudal brain axis. The first pattern (TH-positive, DAT-positive, ZsGreen-positive) was found in A8-A10. The second pattern (TH-positive, DAT-negative, ZsGreen-positive) was found in A11. The third pattern (TH-positive, DAT-negative, ZsGreen-negative) was found in A12-A13. These patterns should help to refine the establishment of the homology of DA nuclei between vertebrate species. Our results also uncover that Vglut2 is expressed at some point during cell lifetime in DA nuclei known to degenerate in Parkinson's disease and largely absent from those that are preserved, suggesting that co-expression of the glutamatergic phenotype in DA cells influences their survival in Parkinson's disease.

Carbon Fibers as a New Type of Scaffold for Midbrain Organoid Development

The combination of induced pluripotent stem cell (iPSC) technology and 3D cell culture creates a unique possibility for the generation of organoids that mimic human organs in in vitro cultures. The use of iPS cells in organoid cultures enables the differentiation of cells into dopaminergic neurons, also found in the human midbrain. However, long-lasting organoid cultures often cause necrosis within organoids. In this work, we present carbon fibers (CFs) for medical use as a new type of scaffold for organoid culture, comparing them to a previously tested copolymer poly-(lactic-co-glycolic acid) (PLGA) scaffold. We verified the physicochemical properties of CF scaffolds compared to PLGA in improving the efficiency of iPSC differentiation within organoids. The physicochemical properties of carbon scaffolds such as porosity, microstructure, or stability in the cellular environment make them a convenient material for creating in vitro organoid models. Through screening several genes expressed during the differentiation of organoids at crucial brain stages of development, we found that there is a correlation between PITX3, one of the key regulators of terminal differentiation, and the survival of midbrain dopaminergic (mDA) neurons and tyrosine hydroxylase (TH) gene expression. This makes organoids formed on carbon scaffolds an improved model containing mDA neurons convenient for studying midbrain-associated neurodegenerative diseases such as Parkinson's disease.

Age-dependent neuroprotective effect of an SK3 channel agonist on excitotoxity to dopaminergic neurons in organotypic culture

Background

Small conductance, calcium-activated (SK3) potassium channels control the intrinsic excitability of dopaminergic neurons (DN) in the midbrain and modulate their susceptibility to toxic insults during development.

Methods

We evaluated the age-dependency of the neuroprotective effect of an SK3 agonist, 1-Ethyl-1,3-dihydro-2H-benzimidazol-2-one (1-EBIO), on Amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) excitotoxicity to DN in ventral mesencephalon (VM) organotypic cultures.

Results

Most tyrosine hydroxylase (TH)+ neurons were also SK3+; SK3+/TH- cells (DN+) were common at each developmental stage but more prominently at day in vitro (DIV) 8. Young DN+ neurons were small bipolar and fusiform, whereas mature ones were large and multipolar. Exposure of organotypic cultures to AMPA (100 μm, 16 h) had no effect on the survival of DN+ at DIV 8, but caused significant toxicity at DIV 15 (n = 15, p = 0.005) and DIV 22 (n = 15, p<0.001). These results indicate that susceptibility of DN to AMPA excitotoxicity is developmental stage-dependent in embryonic VM organotypic cultures. Immature DN+ (small, bipolar) were increased after AMPA (100 μm, 16 h) at DIV 8, at the expense of the number of differentiated (large, multipolar) DN+ (p = 0.039). This effect was larger at DIV 15 (p<<<0.0001) and at DIV 22 (p<<<0.0001). At DIV 8, 30 μM 1-EBIO resulted in a large increase in DN+. At DIV 15, AMPA toxicity was prevented by exposure to 30 μM, but not 100 μM 1-EBIO. At DIV 22, excitotoxicity was unaffected by 30 μM 1-EBIO, and partially reduced by 100 μM 1-EBIO.

Conclusion

The effects of the SK3 channel agonist 1-EBIO on the survival of SK3-expressing dopaminergic neurons were concentration-dependent and influenced by neuronal developmental stage.

Acquisition of the Midbrain Dopaminergic Neuronal Identity

The mesodiencephalic dopaminergic (mdDA) group of neurons comprises molecularly distinct subgroups, of which the substantia nigra (SN) and ventral tegmental area (VTA) are the best known, due to the selective degeneration of the SN during Parkinson’s disease. However, although significant research has been conducted on the molecular build-up of these subsets, much is still unknown about how these subsets develop and which factors are involved in this process. In this review, we aim to describe the life of an mdDA neuron, from specification in the floor plate to differentiation into the different subsets. All mdDA neurons are born in the mesodiencephalic floor plate under the influence of both SHH-signaling, important for floor plate patterning, and WNT-signaling, involved in establishing the progenitor pool and the start of the specification of mdDA neurons. Furthermore, transcription factors, like Ngn2, Ascl1, Lmx1a, and En1, and epigenetic factors, like Ezh2, are important in the correct specification of dopamine (DA) progenitors. Later during development, mdDA neurons are further subdivided into different molecular subsets by, amongst others, Otx2, involved in the specification of subsets in the VTA, and En1, Pitx3, Lmx1a, and WNT-signaling, involved in the specification of subsets in the SN. Interestingly, factors involved in early specification in the floor plate can serve a dual function and can also be involved in subset specification. Besides the mdDA group of neurons, other systems in the embryo contain different subsets, like the immune system. Interestingly, many factors involved in the development of mdDA neurons are similarly involved in immune system development and vice versa. This indicates that similar mechanisms are used in the development of these systems, and that knowledge about the development of the immune system may hold clues for the factors involved in the development of mdDA neurons, which may be used in culture protocols for cell replacement therapies.

A chromosomal connectome for psychiatric and metabolic risk variants in adult dopaminergic neurons

BACKGROUND

Midbrain dopaminergic neurons (MDN) represent 0.0005% of the brain's neuronal population and mediate cognition, food intake, and metabolism. MDN are also posited to underlay the neurobiological dysfunction of schizophrenia (SCZ), a severe neuropsychiatric disorder that is characterized by psychosis as well as multifactorial medical co-morbidities, including metabolic disease, contributing to markedly increased morbidity and mortality. Paradoxically, however, the genetic risk sequences of psychosis and traits associated with metabolic disease, such as body mass, show very limited overlap.

METHODS

We investigated the genomic interaction of SCZ with medical conditions and traits, including body mass index (BMI), by exploring the MDN's "spatial genome," including chromosomal contact landscapes as a critical layer of cell type-specific epigenomic regulation. Low-input Hi-C protocols were applied to 5-10 × 103 dopaminergic and other cell-specific nuclei collected by fluorescence-activated nuclei sorting from the adult human midbrain.

RESULTS

The Hi-C-reconstructed MDN spatial genome revealed 11 "Euclidean hot spots" of clustered chromatin domains harboring risk sequences for SCZ and elevated BMI. Inter- and intra-chromosomal contacts interconnecting SCZ and BMI risk sequences showed massive enrichment for brain-specific expression quantitative trait loci (eQTL), with gene ontologies, regulatory motifs and proteomic interactions related to adipogenesis and lipid regulation, dopaminergic neurogenesis and neuronal connectivity, and reward- and addiction-related pathways.

CONCLUSIONS

We uncovered shared nuclear topographies of cognitive and metabolic risk variants. More broadly, our PsychENCODE sponsored Hi-C study offers a novel genomic approach for the study of psychiatric and medical co-morbidities constrained by limited overlap of their respective genetic risk architectures on the linear genome.

In Vitro Induction of Human Embryonic Stem Cells into the Midbrain Dopaminergic Neurons and Transplantation in Cynomolgus Monkey

A simple, rapid, efficient, and specialized culture system was successfully developed in this study to induce human embryonic stem cells into dopaminergic neurons in vitro. It only took 5 days to generate quickly and directly a large number of homogeneous neural stem cell (NSC) spheres by the introduction of small molecules LDN (inhibitor of BMP [bone morphogenetic protein] pathway that inhibits BMP type I receptors ALK₂ and ALK₃), SB431542 (inhibitor of TGF-β/Activin/Dodal pathway that inhibits ALK₄, ALK₅, and ALK₇), CHIR99021 (inhibitors of GSK-3 [glycogen synthase kinase 3]), and basic fibroblast growth factor (bFGF). The dopaminergic neurons were successfully induced at day 25 (tyrosine hydroxylase [TH] expressed) and at day 32 (TH highly expressed) with high purity (TH/Tuj1: 84.14% and 93.15%, respectively) by the addition of FGF8 (fibroblast growth factor 8), sonic hedgehog (SHH), and Purmorphamine after the generation of NSC at day 5. And, the dopaminergic neurons induced by this system successfully survived and integrated into the striatum of cynomolgus monkey brain after transplantation, which verified the efficiency of the induction system developed in this study, suggesting the potential clinical application in cell therapy for neurological diseases.

The noradrenergic system is necessary for survival of vulnerable midbrain dopaminergic neurons: implications for development and Parkinson's disease

The cause of midbrain dopaminergic (mDA) neuron loss in sporadic Parkinson's disease (PD) is multifactorial, involving cell autonomous factors, cell-cell interactions, and the effects of environmental toxins. Early loss of neurons in the locus coeruleus (LC), the main source of ascending noradrenergic (NA) projections, is an important feature of PD and other neurodegenerative disorders. We hypothesized that NA afferents provide trophic support for vulnerable mDA neurons. We demonstrate that depriving mDA neurons of NA input increases postnatal apoptosis and decreases cell survival in young adult rodents, with relative sparing of calbindin-positive subpopulations known to be resistant to degeneration in PD. As a mechanism, we propose that the neurotrophin brain-derived neurotrophic factor (BDNF) modulates anterograde survival effects of LC inputs to mDA neurons. We demonstrate that the LC is rich in BDNF mRNA in postnatal and young adult brains. Early postnatal NA denervation reduces both BDNF protein and activation of TrkB receptors in the ventral midbrain. Furthermore, overexpression of BDNF in NA afferents in transgenic mice increases mDA neuronal survival. Finally, increasing NA activity in primary cultures of mDA neurons improves survival, an effect that is additive or synergistic in the presence of different concentrations of BDNF. Taken together, our results point to a novel mechanism whereby LC afferents couple BDNF effects and NA activity to provide anterograde trophic support for vulnerable mDA neurons. Early loss of NA activity and anterograde neurotrophin support may contribute to degeneration of vulnerable neurons in PD and other neurodegenerative disorders.

Loss of CELF6 RNA binding protein impairs cocaine conditioned place preference and contextual fear conditioning

In addition to gene expression differences in distinct cell types, there is substantial post-transcriptional regulation driven in part by RNA binding proteins (RBPs). Loss-of-function RBP mutations have been associated with neurodevelopmental disorders, such as Fragile-X syndrome and syndromic autism. Work performed in animal models to elucidate the influence of neurodevelopmental disorder-associated RBPs on distinct behaviors has showed a connection between normal post-transcriptional regulation and conditioned learning. We previously reported cognitive inflexibility in a mouse model null for the RBP CUG-BP, Elav-like factor 6 (CELF6), which we also found to be associated with human autism. Specifically, these mice failed to potentiate exploratory hole-poking behavior in response to familiarization to a rewarding stimuli. Characterization of Celf6 gene expression showed high levels in monoaminergic populations such as the dopaminergic midbrain populations. To better understand the underlying behavioral disruption mediating the resistance to change exploratory behavior in the holeboard task, we tested three hypotheses: Does Celf6 loss lead to global restricted patterns of behavior, failure of immediate response to reward or failure to alter behavior in response to reward (conditioning). We found the acute response to reward was intact, yet Celf6 mutant mice exhibited impaired conditioned learning to both reward and aversive stimuli. Thus, we found that the resistance to change by the Celf6 mutant in the holeboard was most parsimoniously explained as a failure of conditioning, as the mice had blunted responses even to potent rewarding stimuli such as cocaine. These findings further support the role of RBPs in conditioned learning.

Bright and photostable chemigenetic indicators for extended in vivo voltage imaging

Genetically encoded voltage indicators (GEVIs) enable monitoring of neuronal activity at high spatial and temporal resolution. However, the utility of existing GEVIs has been limited by the brightness and photostability of fluorescent proteins and rhodopsins. We engineered a GEVI, called Voltron, that uses bright and photostable synthetic dyes instead of protein-based fluorophores, thereby extending the number of neurons imaged simultaneously in vivo by a factor of 10 and enabling imaging for significantly longer durations relative to existing GEVIs. We used Voltron for in vivo voltage imaging in mice, zebrafish, and fruit flies. In the mouse cortex, Voltron allowed single-trial recording of spikes and subthreshold voltage signals from dozens of neurons simultaneously over a 15-minute period of continuous imaging. In larval zebrafish, Voltron enabled the precise correlation of spike timing with behavior.

Defensive Behaviors Driven by a Hypothalamic-Ventral Midbrain Circuit

The paraventricular hypothalamus (PVH) regulates stress, feeding behaviors and other homeostatic processes, but whether PVH also drives defensive states remains unknown. Here we showed that photostimulation of PVH neurons in mice elicited escape jumping, a typical defensive behavior. We mapped PVH outputs that densely terminate in the ventral midbrain (vMB) area, and found that activation of the PVH→vMB circuit produced profound defensive behavioral changes, including escape jumping, hiding, hyperlocomotion, and learned aversion. Electrophysiological recordings showed excitatory postsynaptic input onto vMB neurons via PVH fiber activation, and in vivo studies demonstrated that glutamate transmission from PVH→vMB was required for the evoked behavioral responses. Photostimulation of PVH→vMB fibers induced cFos expression mainly in non-dopaminergic neurons. Using a dual optogenetic-chemogenetic strategy, we further revealed that escape jumping and hiding were partially contributed by the activation of midbrain glutamatergic neurons. Taken together, our work unveils a hypothalamic-vMB circuit that encodes defensive properties, which may be implicated in stress-induced defensive responses.

Identification of midbrain dopamine neurons using features from spontaneous spike activity patterns. Annu Int Conf IEEE Eng Med Biol Soc 2019;2019:2990-2993. [PMID: 31946517 DOI: 10.1109/embc.2019.8857574] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Although dopamine neurons are considered essential in various brain functions, their specific roles are under debate, partially due to the difficulty to identify dopamine neurons among surrounding neurons deep in the brain based only on the extracellularly recorded electric activities. Thus, a handy method to identify dopamine and non-dopamine neurons based on the spontaneous activity patterns is desired. Here we tried to discriminate optogenetically-identified dopamine neurons from other types of neurons and obtained 86.0% success.

Necrosis, apoptosis, necroptosis, three modes of action of dopaminergic neuron neurotoxins

Most of the Parkinson’s disease (PD) cases are sporadic, although several genes are directly related to PD. Several pathways are central in PD pathogenesis: protein aggregation linked to proteasomal impairments, mitochondrial dysfunctions and impairment in dopamine (DA) release. Here we studied the close crossing of mitochondrial dysfunction and aggregation of α-synuclein (α-syn) and in the extension in the dopaminergic neuronal death. Here, using rat primary cultures of mesencephalic neurons, we induced the mitochondrial impairments using “DA-toxins” (MPP⁺, 6OHDA, rotenone). We showed that the DA-Toxins induced dopaminergic cell death through different pathways: caspase-dependent cell death for 6OHDA; MPP⁺ stimulated caspase-independent cell death, and rotenone activated both pathways. In addition, a decrease in energy production and/or a development of oxidative stress were observed and were linked to α-syn aggregation with generation of Lewy body-like inclusions (found inside and outside the dopaminergic neurons). We demonstrated that any of induced mitochondrial disturbances and processes of death led to α-syn protein aggregation and finally to cell death. Our study depicts the cell death mechanisms taking place in in vitro models of Parkinson’s disease and how mitochondrial dysfunctions is at the cross road of the pathologies of this disease.

Mesencephalic origin of the inferior lobe in zebrafish

BACKGROUND

Although the overall brain organization is shared in vertebrates, there are significant differences within subregions among different groups, notably between Sarcopterygii (lobe-finned fish) and Actinopterygii (ray-finned fish). Recent comparative studies focusing on the ventricular morphology have revealed a large diversity of the hypothalamus. Here, we study the development of the inferior lobe (IL), a prominent structure forming a bump on the ventral surface of the teleost brain. Based on its position, IL has been thought to be part of the hypothalamus (therefore forebrain).

RESULTS

Taking advantage of genetic lineage-tracing techniques in zebrafish, we reveal that cells originating from her5-expressing progenitors in the midbrain-hindbrain boundary (MHB) participate in the formation of a large part of the IL. 3D visualization demonstrated how IL develops in relation to the ventricular system. We found that IL is constituted by two developmental components: the periventricular zone of hypothalamic origin and the external zone of mesencephalic origin. The mesencephalic external zone grows progressively until adulthood by adding new cells throughout development.

CONCLUSION

Our results disprove a homology between the IL and the mammalian lateral hypothalamus. We suggest that the IL is likely to be involved in multimodal sensory integration rather than feeding motivation. The teleost brain is not a simpler version of the mammalian brain, and our study highlights the evolutionary plasticity of the brain which gives rise to novel structures.

Engineering synucleinopathy-resistant human dopaminergic neurons by CRISPR-mediated deletion of the SNCA gene

An emerging treatment for Parkinson's disease (PD) is cell replacement therapy. Authentic midbrain dopaminergic (mDA) neuronal precursors can be differentiated from human embryonic stem cells (hESCs) and human induced pluripotent stem cells (iPSCs). These laboratory-generated mDA cells have been demonstrated to mature into functional dopaminergic neurons upon transplantation into preclinical models of PD. However, clinical trials with human fetal mesenchephalic cells have shown that cell replacement grafts in PD are susceptible to Lewy body formation suggesting host-to-graft transfer of α-synuclein pathology. Here, we have used CRISPR/Cas9n technology to delete the endogenous SNCA gene, encoding for α-synuclein, in a clinical-grade hESC line to generate SNCA+/- and SNCA-/- cell lines. These hESC lines were first differentiated into mDA neurons, and then challenged with recombinant α-synuclein preformed fibrils (PFFs) to seed the formation for Lewy-like pathology as measured by phosphorylation of serine-129 of α-synuclein (pS129-αSyn). Wild-type neurons were fully susceptible to the formation of protein aggregates positive for pS129-αSyn, while SNCA+/- and SNCA-/- neurons exhibited significant resistance to the formation of this pathological mark. This work demonstrates that reducing or completely removing SNCA alleles by CRISPR/Cas9n-mediated gene editing confers a measure of resistance to Lewy pathology.

Region-Specific Neuroprotective Features of Astrocytes against Oxidative Stress Induced by 6-Hydroxydopamine

In previous studies, we found regional differences in the induction of antioxidative molecules in astrocytes against oxidative stress, postulating that region-specific features of astrocytes lead region-specific vulnerability of neurons. We examined region-specific astrocytic features against dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA) as an oxidative stress using co-culture of mesencephalic neurons and mesencephalic or striatal astrocytes in the present study. The 6-OHDA-induced reduction of mesencephalic dopamine neurons was inhibited by co-culturing with astrocytes. The co-culture of midbrain neurons with striatal astrocytes was more resistant to 6-OHDA than that with mesencephalic astrocytes. Furthermore, glia conditioned medium from 6-OHDA-treated striatal astrocytes showed a greater protective effect on the 6-OHDA-induced neurotoxicity and oxidative stress than that from mesencephalic astrocytes. The cDNA microarray analysis showed that the number of altered genes in both mesencephalic and striatal astrocytes was fewer than that changed in either astrocyte. The 6-OHDA treatment, apparently up-regulated expressions of Nrf2 and some anti-oxidative or Nrf2-regulating phase II, III detoxifying molecules related to glutathione synthesis and export in the striatal astrocytes but not mesencephalic astrocytes. There is a profound regional difference of gene expression in astrocytes induced by 6-OHDA. These results suggest that protective features of astrocytes against oxidative stress are more prominent in striatal astrocytes, possibly by secreting humoral factors in striatal astrocytes.

Neural binaural sensitivity at high sound speeds: Single cell responses in cat midbrain to fast-changing interaural time differences of broadband sounds

Relative motion between the body and the outside world is a rich source of information. Neural selectivity to motion is well-established in several sensory systems, but is controversial in hearing. This study examines neural sensitivity to changes in the instantaneous interaural time difference of sounds at the two ears. Midbrain neurons track such changes up to extremely high speeds, show only a coarse dependence of firing rate on speed, and lack directional selectivity. These results argue against the presence of selectivity to auditory motion at the level of the midbrain, but reveal an acuity which enables coding of fast-fluctuating binaural cues in realistic sound environments.

Menthol Stereoisomers Exhibit Different Effects on α4β2 nAChR Upregulation and Dopamine Neuron Spontaneous Firing

Menthol contributes to poor cessation rates among smokers, in part because menthol enhances nicotine reward and reinforcement. Mentholated tobacco products contain (-)-menthol and (+)-menthol, in varying proportions. We examined these two menthol stereoisomers for their ability to upregulate α4β2 nAChRs and to alter dopamine neuron firing frequency using long-term, low-dose (≤500 nm) exposure that is pharmacologically relevant to smoking. We found that (-)-menthol upregulates α4β2 nAChRs while (+)-menthol does not. We also found that (-)-menthol decreases dopamine neuron baseline firing and dopamine neuron excitability, while (+)-menthol exhibits no effect. We then examined both stereoisomers for their ability to inhibit α4β2 nAChR function at higher concentrations (>10 µm) using the Xenopus oocyte expression system. To probe for the potential binding site of menthol, we conducted flooding simulations and site-directed mutagenesis. We found that menthol likely binds to the 9´ position on the TM2 (transmembrane M2) helix. We found that menthol inhibition is dependent on the end-to-end distance of the side chain at the 9´ residue. Additionally, we have found that (-)-menthol is only modestly (∼25%) more potent than (+)-menthol at inhibiting wild-type α4β2 nAChRs and a series of L9´ mutant nAChRs. These data reveal that menthol exhibits a stereoselective effect on nAChRs and that the stereochemical effect is much greater for long-term, submicromolar exposure in mice than for acute, higher-level exposure. We hypothesize that of the two menthol stereoisomers, only (-)-menthol plays a role in enhancing nicotine reward through nAChRs on dopamine neurons.

Millifluidic culture improves human midbrain organoid vitality and differentiation

Human midbrain-specific organoids (hMOs) serve as an experimental in vitro model for studying the pathogenesis of Parkinson's disease (PD). In hMOs, neuroepithelial stem cells (NESCs) give rise to functional midbrain dopaminergic (mDA) neurons that are selectively degenerating during PD. A limitation of the hMO model is an under-supply of oxygen and nutrients to the densely packed core region, which leads eventually to a "dead core". To reduce this phenomenon, we applied a millifluidic culture system that ensures media supply by continuous laminar flow. We developed a computational model of oxygen transport and consumption in order to predict oxygen levels within the hMOs. The modelling predicts higher oxygen levels in the hMO core region under millifluidic conditions. In agreement with the computational model, a significantly smaller "dead core" was observed in hMOs cultured in a bioreactor system compared to those ones kept under conventional shaking conditions. Comparing the necrotic core regions in the organoids with those obtained from the model allowed an estimation of the critical oxygen concentration necessary for ensuring cell vitality. Besides the reduced "dead core" size, the differentiation efficiency from NESCs to mDA neurons was elevated in hMOs exposed to medium flow. Increased differentiation involved a metabolic maturation process that was further developed in the millifluidic culture. Overall, bioreactor conditions that improve hMO quality are worth considering in the context of advanced PD modelling.

Dopamine neurons create Pavlovian conditioned stimuli with circuit-defined motivational properties

Environmental cues, through Pavlovian learning, become conditioned stimuli that guide animals toward the acquisition of rewards (for example, food) that are necessary for survival. We tested the fundamental role of midbrain dopamine neurons in conferring predictive and motivational properties to cues, independent of external rewards. We found that brief phasic optogenetic excitation of dopamine neurons, when presented in temporal association with discrete sensory cues, was sufficient to instantiate those cues as conditioned stimuli that subsequently both evoked dopamine neuron activity on their own and elicited cue-locked conditioned behavior. Notably, we identified highly parcellated functions for dopamine neuron subpopulations projecting to different regions of striatum, revealing dissociable dopamine systems for the generation of incentive value and conditioned movement invigoration. Our results indicate that dopamine neurons orchestrate Pavlovian conditioning via functionally heterogeneous, circuit-specific motivational signals to create, gate, and shape cue-controlled behaviors.

Combinatorial Expression of Grp and Neurod6 Defines Dopamine Neuron Populations with Distinct Projection Patterns and Disease Vulnerability

Midbrain dopamine neurons project to numerous targets throughout the brain to modulate various behaviors and brain states. Within this small population of neurons exists significant heterogeneity based on physiology, circuitry, and disease susceptibility. Recent studies have shown that dopamine neurons can be subdivided based on gene expression; however, the extent to which genetic markers represent functionally relevant dopaminergic subpopulations has not been fully explored. Here we performed single-cell RNA-sequencing of mouse dopamine neurons and validated studies showing that Neurod6 and Grp are selective markers for dopaminergic subpopulations. Using a combination of multiplex fluorescent in situ hybridization, retrograde labeling, and electrophysiology in mice of both sexes, we defined the anatomy, projection targets, physiological properties, and disease vulnerability of dopamine neurons based on Grp and/or Neurod6 expression. We found that the combinatorial expression of Grp and Neurod6 defines dopaminergic subpopulations with unique features. Grp+/Neurod6+ dopamine neurons reside in the ventromedial VTA, send projections to the medial shell of the nucleus accumbens, and have noncanonical physiological properties. Grp+/Neurod6- dopamine neurons are found in the VTA as well as in the ventromedial portion of the SNc, where they project selectively to the dorsomedial striatum. Grp-/Neurod6+ dopamine neurons represent a smaller VTA subpopulation, which is preferentially spared in a 6-OHDA model of Parkinson's disease. Together, our work provides detailed characterization of Neurod6 and Grp expression in the midbrain and generates new insights into how these markers define functionally relevant dopaminergic subpopulations.

Tyrosyl-tRNA synthetase: A potential kyotorphin synthetase in mammals

Kyotorphin (KTP; L-tyrosyl-l-arginine), an opioid-like analgesic discovered in the bovine brain, is potentially a neuromodulator because of its localization in synaptosomes, the existence of a specific KTP receptor, and the presence of its biosynthetic enzyme in the brain. KTP is formed in the brain from its constituent amino acids, L-tyrosine and L-arginine, by an enzyme termed KTP synthetase. However, the latter has never been identified. We aimed to test the hypothesis that tyrosyl-tRNA synthetase (TyrRS) is also KTP synthetase. We found that recombinant hTyrRS synthesizes KTP from tyrosine, arginine, and ATP, with Km = 1400 μM and 200 μM for arginine and tyrosine, respectively. TyrRS knockdown of PC12 cells with a small interfering RNA (siRNA) in the presence of 1.6 mM tyrosine, arginine, proline, or tryptophan significantly reduced the level of KTP, but not those of tyrosine-tyrosine, tyrosine-proline, or tyrosine-tryptophan. siRNA treatment did not affect cell survival or proliferation. In mice, TyrRS levels were found to be greater in the midbrain and medulla oblongata than in other brain regions. When arginine was administered 2 h prior to brain dissection, the KTP levels in these regions plus olfactory bulb significantly increased, although basal brain KTP levels remained relatively even. Our conclusion is further supported by a positive correlation across brain regions between TyrRS expression and arginine-accelerated KTP production.

Long 3'UTR of Nurr1 mRNAs is targeted by miRNAs in mesencephalic dopamine neurons

The development of mesencephalic dopamine neurons and their survival later in life requires the continuous presence of the transcription factor Nurr1 (NR4A2). Nurr1 belongs to the nuclear receptors superfamily. However, it is an orphan member that does not require a ligand to regulate the transcription of its target genes. Therefore, controlling the expression of Nurr1 is an important manner to control its function. Several reports have shown that microRNAs (miRNAs) regulate Nurr1 expression. However, Nurr1 has several splicing variants, posing the question what variants are subjected to miRNA regulation. In this work, we identified a long 3'UTR variant of rat Nurr1 mRNA. We used bioinformatics analysis to identify miRNAs with the potential to regulate Nurr1 expression. Reporter assays performed with the luciferase gene fused to the short (658 bp) or long (1,339 bp) 3'UTR of rat Nurr1 mRNAs, showed that miR-93, miR-204 and miR-302d selectively regulate the mRNA with the longest 3'UTR. We found that the longest variant of Nurr1 mRNA expresses in the rat mesencephalon as assessed by PCR. The transfection of rat mesencephalic neurons with mixed miR-93, miR-204 and miR-302d resulted in a significant reduction of Nurr1 protein levels. In conclusion, Nurr1 mRNA variant with the longest 3'UTR undergoes a specific regulation by miRNAs. It is discussed the importance of fine-tuning Nurr1 protein levels in mesencephalic dopamine neurons.

CCAAT/Enhancer binding protein β silencing mitigates glial activation and neurodegeneration in a rat model of Parkinson's disease

The CCAAT/Enhancer binding protein β (C/EBPβ) is a transcription factor involved in numerous physiological as well as pathological conditions in the brain. However, little is known regarding its possible role in neurodegenerative disorders. We have previously shown that C/EBPβ regulates the expression of genes involved in inflammatory processes and brain injury. Here, we have analyzed the effects of C/EBPβ interference in dopaminergic cell death and glial activation in the 6-hydroxydopamine model of Parkinson's disease. Our results showed that lentivirus-mediated C/EBPβ deprivation conferred marked in vitro and in vivo neuroprotection of dopaminergic cells concomitant with a significant attenuation of the level of the inflammatory response and glial activation. Additionally, C/EBPβ interference diminished the induction of α-synuclein in the substantia nigra pars compacta of animals injected with 6-hydroxydopamine. Taking together, these results reveal an essential function for C/EBPβ in the pathways leading to inflammatory-mediated brain damage and suggest novel roles for C/EBPβ in neurodegenerative diseases, specifically in Parkinson's disease, opening the door for new therapeutic interventions.

A PITX3-EGFP Reporter Line Reveals Connectivity of Dopamine and Non-dopamine Neuronal Subtypes in Grafts Generated from Human Embryonic Stem Cells

Development of safe and effective stem cell-based therapies for brain repair requires an in-depth understanding of the in vivo properties of neural grafts generated from human stem cells. Replacing dopamine neurons in Parkinson's disease remains one of the most anticipated applications. Here, we have used a human PITX3-EGFP embryonic stem cell line to characterize the connectivity of stem cell-derived midbrain dopamine neurons in the dopamine-depleted host brain with an unprecedented level of specificity. The results show that the major A9 and A10 subclasses of implanted dopamine neurons innervate multiple, developmentally appropriate host targets but also that the majority of graft-derived connectivity is non-dopaminergic. These findings highlight the promise of stem cell-based procedures for anatomically correct reconstruction of specific neuronal pathways but also emphasize the scope for further refinement in order to limit the inclusion of uncharacterized and potentially unwanted cell types.

Pitx3 and En1 determine the size and molecular programming of the dopaminergic neuronal pool

Mesodiencephalic dopaminergic (mdDA) neurons are located in the ventral midbrain. These neurons form the substantia nigra (SNc) and the ventral tegmental area (VTA). Two transcription factors that play important roles in the process of terminal differentiation and subset-specification of mdDA neurons, are paired-like homeodomain transcription factor 3 (Pitx3), and homeobox transcription factor Engrailed 1 (En1). We previously investigated the single Pitx3KO and En1KO and observed important changes in the survival of mdDA neurons of the SNc and VTA as well as altered expression of pivotal rostral- and caudal-markers, Ahd2 and Cck, respectively. To refine our understanding of the regional-specific relationships between En1 and Pitx3 and their (combined) role in the programming mdDA neurons on the rostral-to-caudal axis, we created double En1tm1Alj/tm1Alj;Pitx3gfp/gfp (En1KO;Pitx3GFP/GFP) animals. Here we report, that in absence of En1 and Pitx3, only a limited number of mdDA neurons are present at E14.5. These mdDA neurons have a rudimentary dopaminergic cell fate, as they express Nurr1, Pbx3 and Otx2 but have lost their rostral or caudal subset identity. Furthermore, we report that the expression of Cck depends on En1 expression, while (in contrast) both Pitx3 and En1 are involved in the initiation of Ahd2 expression. Thus we reveal in this manuscript that regulated levels of Pitx3 and En1 control the size and rostral/caudal-identity of the mdDA neuronal population.

3D Differentiation of LUHMES Cell Line to Study Recovery and Delayed Neurotoxic Effects

Current neurotoxicity testing and the study of molecular mechanisms in neurodegeneration in vitro usually focuses on acute exposures to compounds. 3D Lund human mesencephalic (LUHMES) cells allow long-term treatment or pulse exposure in combination with compound washout to study delayed neurotoxic effects as well as recovery and neurodegeneration pathways. In this unit we describe 3D LUHMES culture and characterization. Characterization of the model involves immunocytochemistry, flow cytometry, and qPCR measurements. Studying the delayed effects of compounds is more relevant to human exposures and neurodegenerative diseases with a strong genetic or environmental component. Most assays for molecular endpoints have been developed for monolayer cell culture and therefore need to be adapted for 3D models. In this unit, we further describe toxicological assays for molecular endpoints such as ATP levels, mitochondrial viability, and neurite outgrowth, which have been adapted for use in 3D LUHMES cultures. © 2017 by John Wiley & Sons, Inc.

Brain infusion of α-synuclein oligomers induces motor and non-motor Parkinson's disease-like symptoms in mice

Parkinson's disease (PD) is characterized by motor dysfunction, which is preceded by a number of non-motor symptoms including olfactory deficits. Aggregation of α-synuclein (α-syn) gives rise to Lewy bodies in dopaminergic neurons and is thought to play a central role in PD pathology. However, whether amyloid fibrils or soluble oligomers of α-syn are the main neurotoxic species in PD remains controversial. Here, we performed a single intracerebroventricular (i.c.v.) infusion of α-syn oligomers (α-SYOs) in mice and evaluated motor and non-motor symptoms. Familiar bedding and vanillin essence discrimination tasks showed that α-SYOs impaired olfactory performance of mice, and decreased TH and dopamine levels in the olfactory bulb early after infusion. The olfactory deficit persisted until 45days post-infusion (dpi). α- SYO-infused mice behaved normally in the object recognition and forced swim tests, but showed increased anxiety-like behavior in the open field and elevated plus maze tests 20 dpi. Finally, administration of α-SYOs induced late motor impairment in the pole test and rotarod paradigms, along with reduced TH and dopamine content in the caudate putamen, 45 dpi. Reduced number of TH-positive cells was also seen in the substantia nigra of α-SYO-injected mice compared to control. In conclusion, i.c.v. infusion of α-SYOs recapitulated some of PD-associated non-motor symptoms, such as increased anxiety and olfactory dysfunction, but failed to recapitulate memory impairment and depressive-like behavior typical of the disease. Moreover, α-SYOs i.c.v. administration induced motor deficits and loss of TH and dopamine levels, key features of PD. Results point to α-syn oligomers as the proximal neurotoxins responsible for early non-motor and motor deficits in PD and suggest that the i.c.v. infusion model characterized here may comprise a useful tool for identification of PD novel therapeutic targets and drug screening.

Combining Neuroprotective Treatment of Embryonic Nigral Donor Tissue with Mild Hypothermia of the Graft Recipient

Around 80–95% of the immature dopaminergic neurons die when embryonic ventral mesencephalic tissue is transplanted. Cell death occurs both during the preparation of donor tissue and after graft implantation, but the effect of combining successful neuroprotective treatments before and after transplantation has not been extensively investigated. We therefore treated embryonic rat mesencephalic tissue with a combination of the lipid peroxidation inhibitor tirilazad mesylate (3 μM) and the caspase inhibitor Ac.YVAD.cmk (500 μM) and transplanted the tissue into hemiparkinsonian rats kept hypothermic (32–33°C) or normothermic (37°C) during, and 90 min following, graft surgery. Suspension cell number did not differ between untreated or tirilazad/YVAD-treated preparations prior to transplantation. When graft survival was evaluated 6 weeks after implantation, both tirilazad/YVAD pretreatment and mild hypothermia increased the survival of transplanted dopaminergic neurons. Approximately 50–57% of the embryonic dopaminergic neurons survived the dissociation and grafting procedure in rats rendered hypothermic, but there was no significant additive effect on graft survival with a combined treatment. All groups of rats exhibited behavioral recovery in the amphetamine-induced rotation test. There was a significantly enhanced functional capacity of grafts placed in hypothermic as compared to normothermic rats. However, tirilazad/YVAD pretreated implants did not afford greater behavioral improvement than control-treated grafts. Our results suggest that neuroprotective treatments administered prior to and immediately after neural graft implantation may under certain conditions rescue, at least in part, the same subset of dopaminergic neurons. The study also emphasizes the importance of the immediate time after grafting for transplant survival, with relevance both for primary mesencephalic implants and stem cell grafts.

Comparison of Neurotoxicity following Repeated Administration of L-Dopa, D-Dopa, and Dopamine to Embryonic Mesencephalic Dopamine Neurons in Cultures Derived from Fisher 344 and Sprague-Dawley Donors

Levodopa is the most efficacious and widely used symptomatic drug for Parkinson's disease (PD). There is currently, however, a great deal of interest focused on the possibility that levodopa-induced increases in dopamine (DA) turnover may increase oxidative damage derived from the breakdown of DA. Increased oxidative damage following levodopa may contribute to the progressive degeneration of remaining host nigral neurons as well as interfere with development and function of embryonic nigral neurons in neural grafting trials. There is abundant evidence that levodopa is toxic to embryonic nigral DA neurons in both cell culture and neural grafting models. These findings have prompted a number of studies on mechanisms of levodopa toxicity to identify effective means of ameliorating potential oxidative stress related to levodopa in PD. In the current study we have utilized cultures of embryonic nigral DA neurons to address the fundamental question of whether levodopa-induced toxicity is related to DA production or whether dopa itself contributes to cell death. We compared the degree of nigral DA cell death following chronic administration of: 1) levodopa (e.g.: l-dopa); 2) its less active stereoisomer d-dopa; and 3) DA. We examined the rank order of toxicity of these compounds in two species of rats, Fisher 344 (F344) and Sprague-Dawley (SD). Results indicate a toxicity profile of: DA > l-dopa > > d-dopa. In addition, although there was no difference in response of F344 and SD cultures to l-dopa, the SD cultures were significantly more susceptible to the neurotoxic effects of DA. Taken collectively, these results suggest that levodopa-induced toxicity is related primarily to DA production rather than oxidation of dopa to toxic metabolites, and that some strain related differences do exist.

Metabolic Energy Capacity of Dopaminergic Grafts and the Implanted Striatum in Parkinsonian Nonhuman Primates as Visualized with Cytochrome Oxidase Histochemistry

Histochemistry for visualization of the mitochondrial enzyme cytochrome oxidase has been used to detect cellular and regional differences in brain energy metabolism. We have examined the pattern of cytochrome oxidase (CO) staining in grafts of embryonic ventral mesencephalic tissue, and in the implanted striatum, of MPTP-treated monkeys as one index of the functional activity of grafted tissue and its influence on the host brain. Four monkeys were selected for study based on interesting variations in dopamine (DA) neuron content of their bilateral grafts as demonstrated with tyrosine-hydroxylase (TH) immunocytochemistry. The results suggest that grafts rich in DA neurons increase the metabolic activity of the implanted striatum of DA-depleted monkeys, and that this improvement of local energy metabolism is greater in the vicinity of grafts containing greater numbers of DA neurons. In addition, the pattern of CO staining within tissue transplants indicates that DA neurons exhibit the highest rate of metabolic activity among all cell types contained in the ventral mesencephalic grafts, and that the transplants receive metabolically active innervation from outside or within the grafted tissue.

The Effects of Transforming Growth Factor-β2 on Dopaminergic Graft Survival

Dopaminergic cell transplantation is a promising therapeutic approach for the treatment of Parkinson's disease, the potential of which is limited due to poor survival and low dopamine content within engrafted tissue. In this study, the ability of transforming growth factor-β2 (TGF-β2) to influence transplant survival was evaluated. Cell suspensions containing fetal rat ventral mesencephalon (VM) cells were incubated prior to surgery with vehicle (DPBS), varying concentrations of TGF-β2 (5–1000 ng/ml), or a pan-specific antibody against TGF-β (1D11, 100 ng/ml). VM cell suspensions (200,000 cells) were unilaterally implanted into the striatum of adult Sprague-Dawley rats (n = 5–11 animals/group). Following a 3-week survival period, small but viable VM grafts containing tyrosine hydroxylase-positive (TH+) neurons and fibers were present in all animals. Addition of TGF-β2 resulted in a steep, bell-shaped dose-response curve with a significant effect on TH+/dopamine cell survival. At 50 ng/ml TGF-β2, the number of surviving dopamine neurons was increased twofold compared with controls. Addition of TGF-β2 or 1D11 did not significantly influence graft volume. Further studies, possibly in combination with other neurotrophic factors, need to be performed to obtain a greater understanding of the effects of TGF-β on dopamine neurons and fetal VM cell engraftment.

Individual Human Serum Differs in the Amount of Antibodies with Affinity for Pig Fetal Ventral Mesencephalic Cells and the Ability to Lyse These Cells by Complement Activation

Xenografting pig fetal ventral mesencephalic (pfVM) cells to repair the dopamine deficit in patients with Parkinson's disease is the focus of both experimental and clinical investigations. Although there have been marked advances in the experimental and even clinical application of these xenogeneic transplantations, questions regarding the host's xenospecific immune response remain unanswered. It has been shown that human serum is able to lyse pfVM tissue by both anti-gal-gal and non-anti-gal-gal antibodies by complement activation. The aim of this study was to investigate whether interindividual differences exist in the levels of pfVM cell-specific IgM and IgG subclass antibodies, their ability to lyse pfVM cells in vitro and the relationship between both. Pig fetal VM cells were incubated with heat-inactivated serum from 10 different individuals and binding of IgM antibodies and IgG subclass antibodies to pfVM cells was analyzed by flow cytometry. The ability to lyse pfVM cells was analyzed exposing 51Cr-labeled pfVM cells to fresh serum or isolated IgM and IgG from the same individuals and subsequent determination of released 51Cr from lysed cells. Strong differences were found between individuals in the levels of pfVM cell-specific IgM antibodies: antibody levels differed up to 40-fold. pfVM-specific IgG1 and IgG2 levels were only detectable in a few individuals. The ability to lyse pfVM cells ranged from negligible lysis up to 66.5% specific lysis. There was a strong correlation between the levels of individual pfVM-specific IgM antibodies and the ability to lyse pfVM cells in vitro. Isolated IgM, but not IgG, was able to lyse pfVM cells in the presence of complement. In conclusion, the interindividual differences in the levels of IgM with affinity for pfVM cells and their ability to lyse pfVM cells in vitro are considerable. Only few individuals possessed IgG1 and IgG2 subclass antibodies with affinity for pfVM. These findings may influence patient selection for porcine transplants and chances of graft survival in individual patients.

The Effects of Storage Conditions and Trophic Supplementation on the Survival of Fetal mesencephalic cells

It is estimated that only 5-10% of dopamine (DA) neurons implanted into the striatum of patients undergoing fetal-nigral transplantation as a treatment for Parkinson's Disease survive. Because it is often necessary to store fetal tissue prior to transplantation, we evaluated various storage parameters that could influence DA neuron viability in rostral mesencephalic tegmentum (RMT) cultures using tyrosine hydroxylase immunoreactive (THir) cell counts as an index of DA neuron survival. A high K+ hibernation media (HM) was used in all studies. We found that RMT cell viability and THir cell counts decreased as storage duration increased (up to 120 h). Storage at 37°C in HM killed all cells, while storage at 10°C yielded higher survival rates than 4° C. In comparison to trypsinization, mechanical dissociation of tissue increased cell viability. Neutral pH and a storage density of at least 1 × 106 cells/mL were found to be optimal, while striatal coculture of RMT cells with striatal feeder layers increased THir viability up to 16-fold in comparison to monocultures. The nurturing effect of striatal coculture may be explained by the release of autotrophic factors, and we tested this hypothesis by supplementing the HM with human placental cord serum (HPCS, 8%), glial-derived neurotrophic factor (GDNF; 10 μg/mL), and brain-derived neurotrophic factor (BDNF; 10 μg/mL). GDNF and HPCS supplements increased RMT cell viability by 10-15%, while GDNF, BDNF, and HPCS increased viability of THir cells by approximately 40% at all time points studied. As Klenow enzyme labeling technique indicated that 33% of stored RMT cells were undergoing apoptosis, we found that GDNF, BDNF, and HPCS reduced apoptosis by 50%. DNA laddering and DAPI nuclear stain confirmed the presence of apoptosis in hibernated RMT cells, leading us to postulate that the high viability counts seen with trypan blue exclusion are misleading.

Bridging Nigrostriatal Pathway with Fibroblast Growth Factor-Primed Peripheral Nerves and Fetal Ventral Mesencephalon Transplant Recuperates from Deficits in Parkinsonian Rats

Previous studies have indicated that the nigrostriatal dopaminergic (DA) pathway can be reconstructed in hemiparkinsonian rats with a bridge transplantation technique involving fetal ventral mesencephalic transplants and glial cell line-derived neurotrophic factor. In this study, we examined if the nigrostriatal pathway can be restored by combining peripheral nervous tissue with the fetal ventral mesencephalon transplants. Adult rats were injected with 6-hydroxydopamine into left median forebrain bundle. Those with marked rotational behavior, which has been previously shown to indicate complete DA dennervtion, were used for transplant treatments. One month after the lesion, fetal ventral mesencephalic cells were transplanted into the nigral region followed by nigral-striatal grafting of peripheral nerves as a bridge. The bridging nerves (sciatic or intercostals) were pretreated with basic fibrous growth factor (nerve+bFGF+) or Hank's saline (nerve+bFGF-). We found that (a) animals receiving transplants of VM and bFGF+ nerve had a reduction in rotational behavior; (b) animals receiving bFGF- nerve bridge only had a partial improvement in rotation. Reinnervation of tyrosine hydroxylase (TH)-immunoreactive (ir) fibers into the striatum was found in both of the above groups with more innervation in the former than in the latter. No TH-ir fibers in lesioned striatum or reduction in rotational behavior were found in animals receiving VM only, or VM plus bFGF. Taken together, our data indicate that peripheral nerve, with the aid of bFGF, greatly facilitates the reconstitution of the TH pathway from nigra to striatum and improves motor function in hemiparkinsonian rats.

In Vivo PET Measurements with [11C]PE2I to Evaluate Fetal Mesencephalic Transplantations to Unilateral 6-OHDA-Lesioned Rats

Positron emission tomography (PET) is a useful tool to assess and visualize neurotransmissions in vivo. In this study, we performed repeated PET scans with [11C]PE2I, a tracer of the dopamine transporter, to evaluate the alteration of the expression of dopamine (DA) transmission component after a fetal mesencephalic transplantation. The fetal mesencephalic cells were transplanted into the striatum of unilateral 6-OHDA-lesioned rats. PET scans with [11C]PE2I were performed to evaluate the DA transporter before and 2 and 4 weeks after the transplantation. Rotation behavior tests, in vitro autoradiography, measurements of DA contents in the striatum by high-performance liquid chromatography (HPLC), and tyrosine hydroxylase (TH) immuno-histological examinations were performed at the same time points and examined for their relationship to changes in the dopamine transporter. The number of ipsilateral rotations induced by methamphetamine injections decreased. DA contents in the striatum measured with HPLC significantly increased. In the PET study, the binding potential of [11C]PE2I increased at 4 weeks. The results of the in vitro autoradiography study corresponded with those of the PET study. The degrees of the change in the binding potentials correlated with those of the numbers of rotations in the behavioral study and the DA contents in the striatum. In the histological examination, TH-positive cells with axons were observed at 2 and 4 weeks after the transplantation. As the dopamine transporter exists only in the axon terminal of DA neurons, these results suggested that PET measurements of [11C]PE2I binding indicated not only survival, but maturity and functioning of the transplanted cells. Repeated PET measurements of DA transporters are a useful tool in assessing the effectiveness of neural transplantations.