Glutamate controls growth rate and branching of dopaminergic axons

Tet controls axon guidance in early brain development through glutamatergic signaling

Mutations in ten-eleven translocation (TET) proteins are associated with human neurodevelopmental disorders. We find a function of Tet in regulating Drosophila early brain development. The Tet DNA-binding domain (TetAXXC) is required for axon guidance in the mushroom body (MB). Glutamine synthetase 2 (Gs2), a key enzyme in glutamatergic signaling, is significantly down-regulated in the TetAXXC brains. Loss of Gs2 recapitulates the TetAXXC phenotype. Surprisingly, Tet and Gs2 act in the insulin-producing cells (IPCs) to control MB axon guidance, and overexpression of Gs2 in IPCs rescues the defects of TetAXXC. Feeding TetAXXC with metabotropic glutamate receptor antagonist MPEP rescues the phenotype while glutamate enhances it. Mutants in Tet and Drosophila Fmr1, the homolog of human FMR1, have similar defects, and overexpression of Gs2 in IPCs also rescues the Fmr1 phenotype. We provide the first evidence that Tet controls the guidance of developing brain axons by modulating glutamatergic signaling.

Untangling Tau: Molecular Insights into Neuroinflammation, Pathophysiology, and Emerging Immunotherapies

Neuroinflammation, a core pathological feature observed in several neurodegenerative diseases, including Alzheimer's disease (AD), is rapidly gaining attention as a target in understanding the molecular underpinnings of these disorders. Glial cells, endothelial cells, peripheral immune cells, and astrocytes produce a variety of pro-inflammatory mediators that exacerbate the disease progression. Additionally, microglial cells play a complex role in AD, facilitating the clearance of pathological amyloid-beta peptide (Aβ) plaques and aggregates of the tau protein. Tau proteins, traditionally associated with microtubule stabilization, have come under intense scrutiny for their perturbed roles in neurodegenerative conditions. In this narrative review, we focus on recent advances from molecular insights that have revealed aberrant tau post-translational modifications, such as phosphorylation and acetylation, serving as pathological hallmarks. These modifications also trigger the activation of CNS-resident immune cells, such as microglia and astrocytes substantially contributing to neuroinflammation. This intricate relationship between tau pathologies and neuroinflammation fosters a cascading impact on neural pathophysiology. Furthermore, understanding the molecular mechanisms underpinning tau's influence on neuroinflammation presents a frontier for the development of innovative immunotherapies. Neurodegenerative diseases have been relatively intractable to conventional pharmacology using small molecules. We further comprehensively document the many alternative approaches using immunotherapy targeting tau pathological epitopes and structures with a wide array of antibodies. Clinical trials are discussed using these therapeutic approaches, which have both promising and disappointing outcomes. Future directions for tau immunotherapies may include combining treatments with Aβ immunotherapy, which may result in more significant clinical outcomes for neurodegenerative diseases.

Tet Controls Axon Guidance in Early Brain Development through Glutamatergic Signaling

Mutations in human TET proteins have been found in individuals with neurodevelopmental disorders. Here we report a new function of Tet in regulating Drosophila early brain development. We found that mutation in the Tet DNA-binding domain ( Tet AXXC ) resulted in axon guidance defects in the mushroom body (MB). Tet is required in early brain development during the outgrowth of MB β axons. Transcriptomic study shows that glutamine synthetase 2 (Gs2), a key enzyme in glutamatergic signaling, is significantly downregulated in the Tet AXXC mutant brains. CRISPR/Cas9 mutagenesis or RNAi knockdown of Gs2 recapitulates the Tet AXXC mutant phenotype. Surprisingly, Tet and Gs2 act in the insulin-producing cells (IPCs) to control MB axon guidance, and overexpression of Gs2 in these cells rescues the axon guidance defects of Tet AXXC . Treating Tet AXXC with the metabotropic glutamate receptor antagonist MPEP can rescue while treating with glutamate enhances the phenotype confirming Tet function in regulating glutamatergic signaling. Tet AXXC and the Drosophila homolog of Fragile X Messenger Ribonucleoprotein protein mutant ( Fmr1 3 ) have similar axon guidance defects and reduction in Gs2 mRNA levels. Interestingly, overexpression of Gs2 in the IPCs also rescues the Fmr1 3 phenotype, suggesting functional overlapping of the two genes. Our studies provide the first evidence that Tet can control the guidance of axons in the developing brain by modulating glutamatergic signaling and the function is mediated by its DNA-binding domain.

Leptin Promotes Striatal Dopamine Release via Cholinergic Interneurons and Regionally Distinct Signaling Pathways

Dopamine (DA) is a critical regulator of striatal network activity and is essential for motor activation and reward-associated behaviors. Previous work has shown that DA is influenced by the reward value of food, as well as by hormonal factors that reguate food intake and energy expenditure. Changes in striatal DA signaling also have been linked to aberrant eating patterns. Here we test the effect of leptin, an adipocyte-derived hormone involved in feeding and energy homeostasis regulation, on striatal DA release and uptake. Immunohistochemical evaluation identified leptin receptor (LepR) expression throughout mouse striatum, including on striatal cholinergic interneurons (ChIs) and their extensive processes. Using fast-scan cyclic voltammetry (FSCV), we found that leptin causes a concentration-dependent increase in evoked extra-cellular DA concentration ([DA]o) in dorsal striatum (dStr) and nucleus accumbens (NAc) core and shell in male mouse striatal slices, and also an increase in the rate of DA uptake. Further, we found that leptin increases ChI excitability, and that the enhancing effect of leptin on evoked [DA]o is lost when nicotinic acetylcholine (ACh) receptors are antagonized or when examined in striatal slices from mice lacking ACh synthesis. Evaluation of signaling pathways underlying leptin's action revealed a requirement for intracellular Ca2+, and the involvement of different downstream pathways in dStr and NAc core versus NAc shell. These results provide the first evidence for dynamic regulation of DA release and uptake by leptin within brain motor and reward pathways, and highlight the involvement of ChIs in this process.SIGNIFICANCE STATEMENT Given the importance of striatal dopamine (DA) in reward, motivation, motor behavior and food intake, identifying the actions of metabolic hormones on DA release in striatal subregions should provide new insight into factors that influence DA-dependent motivated behaviors. We find that one of these hormones, leptin, boosts striatal DA release through a process involving striatal cholinergic interneurons (ChIs) and nicotinic acetylcholine (ACh) receptors. Moreover, we find that the intracellular cascades downstream from leptin receptor (LepR) activation that lead to enhanced DA release differ among striatal subregions. Thus, we not only show that leptin regulates DA release, but also identify characteristics of this process that could be harnessed to alter pathologic eating behaviors.

Recent Investigations on Neurotransmitters' Role in Acute White Matter Injury of Perinatal Glia and Pharmacotherapies-Glia Dynamics in Stem Cell Therapy

Periventricular leukomalacia (PVL) and cerebral palsy are two neurological disease conditions developed from the premyelinated white matter ischemic injury (WMI). The significant pathophysiology of these diseases is accompanied by the cognitive deficits due to the loss of function of glial cells and axons. White matter makes up 50% of the brain volume consisting of myelinated and non-myelinated axons, glia, blood vessels, optic nerves, and corpus callosum. Studies over the years have delineated the susceptibility of white matter towards ischemic injury especially during pregnancy (prenatal, perinatal) or immediately after child birth (postnatal). Impairment in membrane depolarization of neurons and glial cells by ischemia-invoked excitotoxicity is mediated through the overactivation of NMDA receptors or non-NMDA receptors by excessive glutamate influx, calcium, or ROS overload and has been some of the well-studied molecular mechanisms conducive to the injury of white matter. Expression of glutamate receptors (GluR) and transporters (GLT1, EACC1, and GST) has significant influence in glial and axonal-mediated injury of premyelinated white matter during PVL and cerebral palsy. Predominantly, the central premyelinated axons express extensive levels of functional NMDA GluR receptors to confer ischemic injury to premyelinated white matter which in turn invoke defects in neural plasticity. Several underlying molecular mechanisms are yet to be unraveled to delineate the complete pathophysiology of these prenatal neurological diseases for developing the novel therapeutic modalities to mitigate pathophysiology and premature mortality of newborn babies. In this review, we have substantially discussed the above multiple pathophysiological aspects of white matter injury along with glial dynamics, and the pharmacotherapies including recent insights into the application of MSCs as therapeutic modality in treating white matter injury.

Subcellular proteomics of dopamine neurons in the mouse brain

Dopaminergic neurons modulate neural circuits and behaviors via dopamine (DA) release from expansive, long range axonal projections. The elaborate cytoarchitecture of these neurons is embedded within complex brain tissue, making it difficult to access the neuronal proteome using conventional methods. Here, we demonstrate APEX2 proximity labeling within genetically targeted neurons in the mouse brain, enabling subcellular proteomics with cell-type specificity. By combining APEX2 biotinylation with mass spectrometry, we mapped the somatodendritic and axonal proteomes of midbrain dopaminergic neurons. Our dataset reveals the proteomic architecture underlying proteostasis, axonal metabolism, and neurotransmission in these neurons. We find that most proteins encoded by DA neuron-enriched genes are localized within striatal dopaminergic axons, including ion channels with previously undescribed axonal localization. These proteomic datasets provide a resource for neuronal cell biology, and this approach can be readily adapted for study of other neural cell types.

Possible Mechanisms of Tau Spread and Toxicity in Alzheimer's Disease

Tau is a protein that associates with microtubules (MTs) and promotes their assembly and stability. The protein loses its ability to bind MTs in tauopathies, and detached tau can misfold and induce the pathological changes that characterize Alzheimer’s disease (AD). A growing body of evidence indicates that tauopathies can spread between cells or connected regions. Pathological tau transmission in the brain of patients with AD and other tauopathies is due to the spread of various tau species along neuroanatomically connected regions in a “prion-like” manner. This complex process involves multiple steps of secretion, cellular uptake, transcellular transfer, and/or seeding, but the precise mechanisms of tau pathology propagation remain unclear. This review summarizes the current evidence on the nature of propagative tau species and the possible steps involved in the process of tau pathology spread, including detachment from MTs, degradations, and secretion, and discusses the different mechanisms underlying the spread of tau pathology.

Acetylation-dependent glutamate receptor GluR signalosome formation for STAT3 activation in both transcriptional and metabolism regulation

Besides their original regulating roles in the brain, spinal cord, retina, and peripheral nervous system for mediating fast excitatory synaptic transmission, glutamate receptors consisting of metabotropic glutamate receptors (GluRs) and ionotropic glutamate receptors (iGluRs) have emerged to have a critical role in the biology of cancer initiation, progression, and metastasis. However, the precise mechanism underpinning the signal transduction mediated by ligand-bound GluRs is not clearly elucidated. Here, we show that iGluRs, GluR1 and GluR2, are acetylated by acetyltransferase CREB-binding protein upon glutamate stimulation of cells, and are targeted by lysyl oxidase-like 2 for deacetylation. Acetylated GluR1/2 recruit β-arrestin1/2 and signal transducer and activator of transcription 3 (STAT3) to form a protein complex. Both β-arrestin1/2 and STAT3 are subsequently acetylated and activated. Simultaneously, activated STAT3 acetylated at lysine 685 translocates to mitochondria to upregulate energy metabolism-related gene transcription. Our results reveal that acetylation-dependent formation of GluR1/2-β-arrestin1/2-STAT3 signalosome is critical for glutamate-induced cell proliferation.

Impairment of motor coordination and interneuron migration in perinatal exposure to glufosinate-ammonium

Glufosinate-ammonium (GLA) is a broad-spectrum herbicide for agricultural weed control and crop desiccation. Due to many GLA-resistant crops being developed to effectively control weeds and increase harvest yields, herbicide usage and the residual GLA in food has increased significantly. Though perinatal exposure by the residual GLA in food might affect brain development, the developmental neurotoxicity of GLA is still unclear. Therefore, this study aimed to investigate the effects of perinatal exposure to GLA on cortical development. The analysis revealed that perinatal GLA exposure altered behavioral changes in offspring, especially motor functional behavior. Moreover, perinatal GLA exposure affected cortical development, particularly by disrupting interneuron migration. These results provide new evidence that early life exposure to GLA alters cortical development.

VGluT2 Expression in Dopamine Neurons Contributes to Postlesional Striatal Reinnervation

A subset of adult ventral tegmental area dopamine (DA) neurons expresses vesicular glutamate transporter 2 (VGluT2) and releases glutamate as a second neurotransmitter in the striatum, while only few adult substantia nigra DA neurons have this capacity. Recent work showed that cellular stress created by neurotoxins such as MPTP and 6-hydroxydopamine can upregulate VGluT2 in surviving DA neurons, suggesting the possibility of a role in cell survival, although a high level of overexpression could be toxic to DA neurons. Here we examined the level of VGluT2 upregulation in response to neurotoxins and its impact on postlesional plasticity. We first took advantage of an in vitro neurotoxin model of Parkinson's disease and found that this caused an average 2.5-fold enhancement of Vglut2 mRNA in DA neurons. This could represent a reactivation of a developmental phenotype because using an intersectional genetic lineage-mapping approach, we find that >98% of DA neurons have a VGluT2+ lineage. Expression of VGluT2 was detectable in most DA neurons at embryonic day 11.5 and was localized in developing axons. Finally, compatible with the possibility that enhanced VGluT2 expression in DA neurons promotes axonal outgrowth and reinnervation in the postlesional brain, we observed that DA neurons in female and male mice in which VGluT2 was conditionally removed established fewer striatal connections 7 weeks after a neurotoxin lesion. Thus, we propose here that the developmental expression of VGluT2 in DA neurons can be reactivated at postnatal stages, contributing to postlesional plasticity of dopaminergic axons.SIGNIFICANCE STATEMENT A small subset of dopamine neurons in the adult, healthy brain expresses vesicular glutamate transporter 2 (VGluT2) and thus releases glutamate as a second neurotransmitter in the striatum. This neurochemical phenotype appears to be plastic as exposure to neurotoxins, such as 6-OHDA or MPTP, that model certain aspects of Parkinson's disease pathophysiology, boosts VGluT2 expression in surviving dopamine neurons. Here we show that this enhanced VGluT2 expression in dopamine neurons drives axonal outgrowth and contributes to dopamine neuron axonal plasticity in the postlesional brain. A better understanding of the neurochemical changes that occur during the progression of Parkinson's disease pathology will aid the development of novel therapeutic strategies for this disease.

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.

A Whole-brain Map of Long-range Inputs to GABAergic Interneurons in the Mouse Caudal Forelimb Area

The caudal forelimb area (CFA) of the mouse cortex is essential in many forelimb movements, and diverse types of GABAergic interneuron in the CFA are distinct in the mediation of cortical inhibition in motor information processing. However, their long-range inputs remain unclear. In the present study, we combined the monosynaptic rabies virus system with Cre driver mouse lines to generate a whole-brain map of the inputs to three major inhibitory interneuron types in the CFA. We discovered that each type was innervated by the same upstream areas, but there were quantitative differences in the inputs from the cortex, thalamus, and pallidum. Comparing the locations of the interneurons in two sub-regions of the CFA, we discovered that their long-range inputs were remarkably different in distribution and proportion. This whole-brain mapping indicates the existence of parallel pathway organization in the forelimb subnetwork and provides insight into the inhibitory processes in forelimb movement to reveal the structural architecture underlying the functions of the CFA.

Developmental Co-expression of Vglut2 and Nurr1 in a Mes-Di-Encephalic Continuum Preceeds Dopamine and Glutamate Neuron Specification

Midbrain dopamine (DA) neurons exist as several subtypes and are found in a heterogeneous environment including GABAergic and glutamatergic neurons as well as various types of co-releasing neurons. Developmental programs underlying this heterogeneity have remained elusive. In this study, combinatorial mRNA analysis was performed at stages when neuronal phenotypes are first specified. Vesicular transporters for dopamine and other monoamines (VMAT2), GABA (VIAAT), and glutamate (VGLUT2) were assessed by systematically applying fluorescent in situ hybridization through the mes-di-encephalon of the mouse embryo at embryonal days (E) 9.5–14.5. The results show that early differentiating dopamine neurons express the gene encoding VGLUT2 before onset of any dopaminergic markers. Prior to its down-regulation in maturing dopamine neurons, Vglut2 mRNA co-localizes extensively with Tyrosine hydroxylase (Th) and Nurr1, commonly used as markers for DA neurons. Further, Vglut2 and Nurr1 mRNAs are shown to overlap substantially in diencephalic neurons that maintain a glutamatergic phenotype. The results suggest that Vglut2/Nurr1-double positive cells give rise both to dopaminergic and glutamatergic neurons within the mes-di-encephalic area. Finally, analysis of markers representing subtypes of dopamine neurons, including the newly described NeuroD6 subtype, shows that certain subtype specifications arise early. Histological findings are outlined in the context of neuroanatomical concepts and the prosomeric model of brain development. The study contributes to the current decoding of the recently discovered heterogeneity among neurons residing along the cephalic flexure.

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.

In Vitro Models of Brain Disorders

The brain is the most complex organ of the body, and many pathological processes underlying various brain disorders are poorly understood. Limited accessibility hinders observation of such processes in the in vivo brain, and experimental freedom is often insufficient to enable informative manipulations. In vitro preparations (brain slices or cultures of dissociated neurons) offer much better accessibility and reduced complexity and have yielded valuable new insights into various brain disorders. Both types of preparations have their advantages and limitations with regard to lifespan, preservation of in vivo brain structure, composition of cell types, and the link to behavioral outcome is often unclear in in vitro models. While these limitations hamper general usage of in vitro preparations to study, e.g., brain development, in vitro preparations are very useful to study neuronal and synaptic functioning under pathologic conditions. This chapter addresses several brain disorders, focusing on neuronal and synaptic functioning, as well as network aspects. Recent progress in the fields of brain circulation disorders, excitability disorders, and memory disorders will be discussed, as well as limitations of current in vitro models.

NMDA receptors inhibit axonal outgrowth by inactivating Akt and activating GSK-3β via calcineurin in cultured immature hippocampal neurons

Neurons are highly polarized cells with an axon and dendritic arbors. It is still not well studied that how formation and elaboration of axon and dendrites is controlled by diffusible signaling factors such as glutamate via specific receptors. We found that N-methyl-D-aspartate (NMDA) receptors were enriched (stage 2-3) but decreased expression (stage 4-5) at tip of axon of cultured hippocampal neurons during distinct development stages. Inhibition of NMDA receptor activity by competitive antagonist DL-2-amino-5-phosphonovalerate (APV) or channel blocker MK801 promoted axonal outgrowth at the early stages, whereas inhibited dendritic development in later stages. Meanwhile, knockdown of NMDA receptors also promoted axonal outgrowth and branch in immature neurons. Furthermore, GluN2B but not GluN2A subunit inhibited axonal outgrowth in immature hippocampal neurons. Finally, we found that NMDA receptors inhibited axonal outgrowth by inactivating Akt and activating GSK-3β signaling in a calcineurin-dependent manner. Taken together, our results demonstrate that stabilization GSK-3β activation in the axon growth cone by Ca²⁺ influx through NMDA receptors may be involved in regulation of axon formation in immature neurons at early stages.

Evolution of Excitation-Inhibition Ratio in Cortical Cultures Exposed to Hypoxia

In the core of a brain infarct, neuronal death occurs within minutes after loss of perfusion. In the penumbra, a surrounding area with some residual perfusion, neurons initially remain structurally intact, but hypoxia-induced synaptic failure impedes neuronal activity. Penumbral activity may recover or further deteriorate, reflecting cell death. Mechanisms leading to either outcome remain ill-understood, but may involve changes in the excitation to inhibition (E/I) ratio. The E/I ratio is determined by structural (relative densities of excitatory and inhibitory synapses) and functional factors (synaptic strengths). Clinical studies demonstrated excitability alterations in regions surrounding the infarct core. These may be related to structural E/I changes, but the effects of hypoxia /ischemia on structural connectivity have not yet been investigated, and the role of structural connectivity changes in excitability alterations remains unclear. We investigated the evolution of the structural E/I ratio and associated network excitability in cortical cultures exposed to severe hypoxia of varying duration. 6–12 h of hypoxia reduced the total synaptic density. In particular, the inhibitory synaptic density dropped significantly, resulting in an elevated E/I ratio. Initially, this does not lead to increased excitability due to hypoxia-induced synaptic failure. Increased excitability becomes apparent upon reoxygenation after 6 or 12 h, but not after 24 h. After 24 h of hypoxia, structural patterns of vesicular glutamate stainings change. This possibly reflects disassembly of excitatory synapses, and may account for the irreversible reduction of activity and stimulus responses seen after 24 h.

Neuroprotection and neurorestoration as experimental therapeutics for Parkinson's disease

Disease-modifying treatments remain an unmet medical need in Parkinson's disease (PD). Such treatments can be operationally defined as interventions that slow down the clinical evolution to advanced disease milestones. A treatment may achieve this outcome by either inhibiting primary neurodegenerative events ("neuroprotection") or boosting compensatory and regenerative mechanisms in the brain ("neurorestoration"). Here we review experimental paradigms that are currently used to assess the neuroprotective and neurorestorative potential of candidate treatments in animal models of PD. We review some key molecular mediators of neuroprotection and neurorestoration in the nigrostriatal dopamine pathway that are likely to exert beneficial effects on multiple neural systems affected in PD. We further review past and current strategies to therapeutically stimulate these mediators, and discuss the preclinical evidence that exercise training can have neuroprotective and neurorestorative effects. A future translational task will be to combine behavioral and pharmacological interventions to exploit endogenous mechanisms of neuroprotection and neurorestoration for therapeutic purposes. This type of approach is likely to provide benefit to many PD patients, despite the clinical, etiological, and genetic heterogeneity of the disease.

Loss and recovery of functional connectivity in cultured cortical networks exposed to hypoxia

In the core of a brain infarct, loss of neuronal function is followed by neuronal death within minutes. In an area surrounding the core (penumbra), some perfusion remains. Here, neurons initially remain structurally intact, but massive synaptic failure strongly reduces neural activity. Activity in the penumbra may eventually recover or further deteriorate toward massive cell death. Besides activity recovery, return of brain functioning requires restoration of connectivity. However, low activity has been shown to initiate compensatory mechanisms that affect network connectivity. We investigated the effect of transient hypoxia and compensatory mechanisms on activity and functional connectivity using cultured cortical networks on multielectrode arrays. Networks were exposed to hypoxia of controlled depth (10-90% of normoxia) and duration (6-48 h). First, we determined how hypoxic depth and duration govern activity recovery. Then, we investigated connectivity changes during and after hypoxic incidents, mild enough for activity to recover. Shortly after hypoxia onset, activity and connectivity decreased. Following 4-6 h of ongoing hypoxia, we observed partial recovery. Only if the hypoxic burden was limited did connectivity show further recovery upon return to normoxia. Partial recovery during hypoxia was dominated by restored baseline connections, rather than newly formed ones. Baseline strengths of surviving (persisting or recovered) and lost connections did not differ nor did baseline activity at their "presynaptic" electrodes. However, "postsynaptic" electrodes of surviving connections were significantly more active during baseline than those of lost connections. This implies that recovery during hypoxia reflects an effective mechanism to restore network activity, which does not necessarily conserve prehypoxia connectivity.NEW & NOTEWORTHY Hypoxia reduced the firing rates of cultured neurons. Depending on hypoxic depth and duration, activity recovered during hypoxia and upon return to normoxia. Recovery (partial) during hypoxia was associated with restored baseline connections rather than newly formed ones. Predominantly, baseline connections with most active postsynaptic electrodes recovered, supporting the notion of effective activity homeostasis. This compensatory mechanism remained effective during ~20 h of hypoxia. Beyond 20 h of compensation, loss of activity and connectivity became irreversible.

Defects of the Glycinergic Synapse in Zebrafish

Glycine mediates fast inhibitory synaptic transmission. Physiological importance of the glycinergic synapse is well established in the brainstem and the spinal cord. In humans, the loss of glycinergic function in the spinal cord and brainstem leads to hyperekplexia, which is characterized by an excess startle reflex to sudden acoustic or tactile stimulation. In addition, glycinergic synapses in this region are also involved in the regulation of respiration and locomotion, and in the nociceptive processing. The importance of the glycinergic synapse is conserved across vertebrate species. A teleost fish, the zebrafish, offers several advantages as a vertebrate model for research of glycinergic synapse. Mutagenesis screens in zebrafish have isolated two motor defective mutants that have pathogenic mutations in glycinergic synaptic transmission: bandoneon (beo) and shocked (sho). Beo mutants have a loss-of-function mutation of glycine receptor (GlyR) β-subunit b, alternatively, sho mutant is a glycinergic transporter 1 (GlyT1) defective mutant. These mutants are useful animal models for understanding of glycinergic synaptic transmission and for identification of novel therapeutic agents for human diseases arising from defect in glycinergic transmission, such as hyperekplexia or glycine encephalopathy. Recent advances in techniques for genome editing and for imaging and manipulating of a molecule or a physiological process make zebrafish more attractive model. In this review, we describe the glycinergic defective zebrafish mutants and the technical advances in both forward and reverse genetic approaches as well as in vivo visualization and manipulation approaches for the study of the glycinergic synapse in zebrafish.

Abnormal Development of Glutamatergic Synapses Afferent to Dopaminergic Neurons of the Pink1(-/-) Mouse Model of Parkinson's Disease

In a preceding study, we showed that in adult pink1^−/− mice, a monogenic animal model of Parkinson’s disease (PD), striatal neurons display aberrant electrical activities that precede the onset of overt clinical manifestations. Here, we tested the hypothesis that the maturation of dopaminergic (DA) neurons of the pink1^−/− substantia nigra compacta (SNc) follows, from early stages on, a different developmental trajectory from age-matched wild type (wt) SNc DA neurons. We used immature (postnatal days P2–P10) and young adult (P30–P90) midbrain slices of pink1^−/− mice expressing the green fluorescent protein in tyrosine hydroxylase (TH)-positive neurons. We report that the developmental sequence of N-Methyl-D-aspartic acid (NMDA) spontaneous excitatory postsynaptic currents (sEPSCs) is altered in pink1^−/− SNc DA neurons, starting from shortly after birth. They lack the transient episode of high NMDA receptor-mediated neuronal activity characteristic of the immature stage of wt SNc DA neurons. The maturation of the membrane resistance of pink1^−/− SNc DA neurons is also altered. Collectively, these observations suggest that electrical manifestations occurring shortly after birth in SNc DA neurons might lead to dysfunction in dopamine release and constitute an early pathogenic mechanism of PD.

Progression of Neuronal Damage in an In Vitro Model of the Ischemic Penumbra

Improvement of neuronal recovery in the ischemic penumbra around a brain infarct has a large potential to advance clinical recovery of patients with acute ischemic stroke. However, pathophysiological mechanisms leading to either recovery or secondary damage in the penumbra are not completely understood. We studied neuronal dynamics in a model system of the penumbra consisting of networks of cultured cortical neurons exposed to controlled levels and durations of hypoxia. Short periods of hypoxia (pO₂≈20mmHg) reduced spontaneous activity, due to impeded synaptic function. After ≈6 hours, activity and connectivity partially recovered, even during continuing hypoxia. If the oxygen supply was restored within 12 hours, changes in network connectivity were completely reversible. For longer periods of hypoxia (12–30 h), activity levels initially increased, but eventually decreased and connectivity changes became partially irreversible. After ≈30 hours, all functional connections disappeared and no activity remained. Since this complete silence seemed unrelated to hypoxic depths, but always followed an extended period of low activity, we speculate that irreversible damage (at least partly) results from insufficient neuronal activation. This opens avenues for therapies to improve recovery by neuronal activation.

Glutamate Stimulates Local Protein Synthesis in the Axons of Rat Cortical Neurons by Activating α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptors and Metabotropic Glutamate Receptors

Glutamate is the principal excitatory neurotransmitter in the mammalian CNS. By analyzing the metabolic incorporation of azidohomoalanine, a methionine analogue, in newly synthesized proteins, we find that glutamate treatments up-regulate protein translation not only in intact rat cortical neurons in culture but also in the axons emitting from cortical neurons before making synapses with target cells. The process by which glutamate stimulates local translation in axons begins with the binding of glutamate to the ionotropic AMPA receptors and metabotropic glutamate receptor 1 and members of group 2 metabotropic glutamate receptors on the plasma membrane. Subsequently, the activated mammalian target of rapamycin (mTOR) signaling pathway and the rise in Ca(2+), resulting from Ca(2+) influxes through calcium-permeable AMPA receptors, voltage-gated Ca(2+) channels, and transient receptor potential canonical channels, in axons stimulate the local translation machinery. For comparison, the enhancement effects of brain-derived neurotrophic factor (BDNF) on the local protein synthesis in cortical axons were also studied. The results indicate that Ca(2+) influxes via transient receptor potential canonical channels and activated the mTOR pathway in axons also mediate BDNF stimulation to local protein synthesis. However, glutamate- and BDNF-induced enhancements of translation in axons exhibit different kinetics. Moreover, Ca(2+) and mTOR signaling appear to play roles carrying different weights, respectively, in transducing glutamate- and BDNF-induced enhancements of axonal translation. Thus, our results indicate that exposure to transient increases of glutamate and more lasting increases of BDNF would stimulate local protein synthesis in migrating axons en route to their targets in the developing brain.

Premyelinated central axons express neurotoxic NMDA receptors: relevance to early developing white-matter injury

Ischemic-type injury to developing white matter is associated with the significant clinical condition cerebral palsy and with the cognitive deficits associated with premature birth. Premyelinated axons are the major cellular component of fetal white matter and loss of axon function underlies the disability, but the cellular mechanisms producing ischemic injury to premyelinated axons have not previously been described. Injury was found to require longer periods of modelled ischemia than at latter developmental points. Ischemia produced initial hyperexcitability in axons followed by loss of function after Na(+) and Ca(2+) influx. N-methyl-D-aspartate- (NMDA) type glutamate receptor (GluR) agonists potentiated axon injury while antagonists were protective. The NMDA GluR obligatory Nr1 subunit colocalized with markers of small premyelinated axons and expression was found at focal regions of axon injury. Ischemic injury of glial cells present in early developing white matter was NMDA GluR independent. Axons in human postconception week 18 to 23 white matter had a uniform prediameter expansion phenotype and postembedded immuno-gold labelling showed Nr1 subunit expression on the membrane of these axons, demonstrating a shared key neuropathologic feature with the rodent model. Premyelinated central axons therefore express high levels of functional NMDA GluRs that confer sensitivity to ischemic injury.

Homeostatic regulation of excitatory synapses on striatal medium spiny neurons expressing the D2 dopamine receptor

Striatal medium spiny neurons (MSNs) are contacted by glutamatergic axon terminals originating from cortex, thalamus and other regions. The striatum is also innervated by dopaminergic (DAergic) terminals, some of which release glutamate as a co-transmitter. Despite evidence for functional DA release at birth in the striatum, the role of DA in the establishment of striatal circuitry is unclear. In light of recent work suggesting activity-dependent homeostatic regulation of glutamatergic terminals on MSNs expressing the D2 DA receptor (D2-MSNs), we used primary co-cultures to test the hypothesis that stimulation of DA and glutamate receptors regulates the homeostasis of glutamatergic synapses on MSNs. Co-culture of D2-MSNs with mesencephalic DA neurons or with cortical neurons produced an increase in spines and functional glutamate synapses expressing VGLUT2 or VGLUT1, respectively. The density of VGLUT2-positive terminals was reduced by the conditional knockout of this gene from DA neurons. In the presence of both mesencephalic and cortical neurons, the density of synapses reached the same total, compatible with the possibility of a homeostatic mechanism capping excitatory synaptic density. Blockade of D2 receptors increased the density of cortical and mesencephalic glutamatergic terminals, without changing MSN spine density or mEPSC frequency. Combined blockade of AMPA and NMDA glutamate receptors increased the density of cortical terminals and decreased that of mesencephalic VGLUT2-positive terminals, with no net change in total excitatory terminal density or in mEPSC frequency. These results suggest that DA and glutamate signaling regulate excitatory inputs to striatal D2-MSNs at both the pre- and postsynaptic level, under the influence of a homeostatic mechanism controlling functional output of the circuit.

Structural plasticity in mesencephalic dopaminergic neurons produced by drugs of abuse: critical role of BDNF and dopamine

Mesencephalic dopaminergic neurons were suggested to be a critical physiopathology substrate for addiction disorders. Among neuroadaptive processes to addictive drugs, structural plasticity has attracted attention. While structural plasticity occurs at both pre- and post-synaptic levels in the mesolimbic dopaminergic system, the present review focuses only on dopaminergic neurons. Exposures to addictive drugs determine two opposite structural responses, hypothrophic plasticity produced by opioids and cannabinoids (in particular during the early withdrawal phase) and hypertrophic plasticity, mostly driven by psychostimulants and nicotine. In vitro and in vivo studies identified BDNF and extracellular dopamine as two critical factors in determining structural plasticity, the two molecules sharing similar intracellular pathways involved in cell soma and dendrite growth, the MEK-ERK1/2 and the PI3K-Akt-mTOR, via preferential activation of TrkB and dopamine D3 receptors, respectively. At present information regarding specific structural changes associated to the various stages of the addiction cycle is incomplete. Encouraging neuroimaging data in humans indirectly support the preclinical evidence of hypotrophic and hypertrophic effects, suggesting a possible differential engagement of dopamine neurons in parallel and partially converging circuits controlling motivation, stress, and emotions.

Hydrogen sulfide functions as a neuromodulator to regulate striatal neurotransmission in a mouse model of Parkinson's disease

Hydrogen sulfide (H2S), a novel endogenous gasotransmitter, has been considered a neuromodulator to enhance hippocampal long-term potentiation and exerts neuroprotective effects against neurotoxin-induced neurodegeneration in rodent models of Parkinson's disease (PD). However, whether H2S can function as a neuromodulator to regulate the levels of nigrostriatal neurotransmitters and then impact the vulnerability of dopaminergic (DA) neurons in response to neurotoxins remains unknown. For this study, we prepared a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine plus probenecid (MPTP/p)-induced mouse subacute model of PD to explore the modulatory effect of H2S on monoamine and amino acid neurotransmitters in the striatum of MPTP-treated mice. This study shows that NaHS (an H2S donor, 5.6 mg/kg/day, i.p.) administration improves the survival rate and significantly ameliorates the weight loss of MPTP-treated mice. NaHS treatment attenuated MPTP-induced neuronal damage, restored the diminution of DA neurons, and suppressed the overactivation of astrocytes in the mouse striatum. Additionally, NaHS upregulated striatal serotonin levels and modulated the balance of excitatory glutamate and the inhibitory γ-aminobutyric acid system in response to MPTP challenge. The current study indicates that H2S may function as an effective neuromodulator to regulate striatal neurotransmission and provides insight into the potential of H2S for PD therapy.

Nimodipine enhances neurite outgrowth in dopaminergic brain slice co-cultures

Calcium ions (Ca(2+)) play important roles in neuroplasticity and the regeneration of nerves. Intracellular Ca(2+) concentrations are regulated by Ca(2+) channels, among them L-type voltage-gated Ca(2+) channels, which are inhibited by dihydropyridines like nimodipine. The purpose of this study was to investigate the effect of nimodipine on neurite growth during development and regeneration. As an appropriate model to study neurite growth, we chose organotypic brain slice co-cultures of the mesocortical dopaminergic projection system, consisting of the ventral tegmental area/substantia nigra and the prefrontal cortex from neonatal rat brains. Quantification of the density of the newly built neurites in the border region (region between the two cultivated slices) of the co-cultures revealed a growth promoting effect of nimodipine at concentrations of 0.1μM and 1μM that was even more pronounced than the effect of the growth factor NGF. This beneficial effect was absent when 10μM nimodipine were applied. Toxicological tests revealed that the application of nimodipine at this higher concentration slightly induced caspase 3 activation in the cortical part of the co-cultures, but did neither affect the amount of lactate dehydrogenase release or propidium iodide uptake nor the ratio of bax/bcl-2. Furthermore, the expression levels of different genes were quantified after nimodipine treatment. The expression of Ca(2+) binding proteins, immediate early genes, glial fibrillary acidic protein, and myelin components did not change significantly after treatment, indicating that the regulation of their expression is not primarily involved in the observed nimodipine mediated neurite growth. In summary, this study revealed for the first time a neurite growth promoting effect of nimodipine in the mesocortical dopaminergic projection system that is highly dependent on the applied concentrations.

Integrating pathways of Parkinson's disease in a molecular interaction map

Parkinson's disease (PD) is a major neurodegenerative chronic disease, most likely caused by a complex interplay of genetic and environmental factors. Information on various aspects of PD pathogenesis is rapidly increasing and needs to be efficiently organized, so that the resulting data is available for exploration and analysis. Here we introduce a computationally tractable, comprehensive molecular interaction map of PD. This map integrates pathways implicated in PD pathogenesis such as synaptic and mitochondrial dysfunction, impaired protein degradation, alpha-synuclein pathobiology and neuroinflammation. We also present bioinformatics tools for the analysis, enrichment and annotation of the map, allowing the research community to open new avenues in PD research. The PD map is accessible at http://minerva.uni.lu/pd_map .

The multilingual nature of dopamine neurons

The ability of dopamine (DA) neurons to release other transmitters in addition to DA itself has been increasingly recognized, hence the concept of their multilingual nature. A subset of DA neurons, mainly found in the ventral tegmental area, express VGLUT2, allowing them to package and release glutamate onto striatal spiny projection neurons and cholinergic interneurons. Some dopaminergic axon terminals release GABA. Glutamate release by DA neurons has a developmental role, facilitating axonal growth and survival, and may determine in part the critical contribution of the ventral striatum to psychostimulant-induced behavior. Vesicular glutamate coentry may have synergistic effects on vesicular DA filling. The multilingual transmission of DA neurons across multiple striatal domains and the increasing insight into the role of glutamate cotransmission in the ventral striatum highlight the importance of analyzing DA neuron transmission at the synaptic level.

Glycine transporter-1 inhibition promotes striatal axon sprouting via NMDA receptors in dopamine neurons

NMDA receptor activity is involved in shaping synaptic connections throughout development and adulthood. We recently reported that brief activation of NMDA receptors on cultured ventral midbrain dopamine neurons enhanced their axon growth rate and induced axonal branching. To test whether this mechanism was relevant to axon regrowth in adult animals, we examined the reinnervation of dorsal striatum following nigral dopamine neuron loss induced by unilateral intrastriatal injections of the toxin 6-hydroxydopamine. We used a pharmacological approach to enhance NMDA receptor-dependent signaling by treatment with an inhibitor of glycine transporter-1 that elevates levels of extracellular glycine, a coagonist required for NMDA receptor activation. All mice displayed sprouting of dopaminergic axons from spared fibers in the ventral striatum to the denervated dorsal striatum at 7 weeks post-lesion, but the reinnervation in mice treated for 4 weeks with glycine uptake inhibitor was approximately twice as dense as in untreated mice. The treated mice also displayed higher levels of striatal dopamine and a complete recovery from lateralization in a test of sensorimotor behavior. We confirmed that the actions of glycine uptake inhibition on reinnervation and behavioral recovery required NMDA receptors in dopamine neurons using targeted deletion of the NR1 NMDA receptor subunit in dopamine neurons. Glycine transport inhibitors promote functionally relevant sprouting of surviving dopamine axons and could provide clinical treatment for disorders such as Parkinson's disease.

A simple rule for dendritic spine and axonal bouton formation can account for cortical reorganization after focal retinal lesions

Lasting alterations in sensory input trigger massive structural and functional adaptations in cortical networks. The principles governing these experience-dependent changes are, however, poorly understood. Here, we examine whether a simple rule based on the neurons' need for homeostasis in electrical activity may serve as driving force for cortical reorganization. According to this rule, a neuron creates new spines and boutons when its level of electrical activity is below a homeostatic set-point and decreases the number of spines and boutons when its activity exceeds this set-point. In addition, neurons need a minimum level of activity to form spines and boutons. Spine and bouton formation depends solely on the neuron's own activity level, and synapses are formed by merging spines and boutons independently of activity. Using a novel computational model, we show that this simple growth rule produces neuron and network changes as observed in the visual cortex after focal retinal lesions. In the model, as in the cortex, the turnover of dendritic spines was increased strongest in the center of the lesion projection zone, while axonal boutons displayed a marked overshoot followed by pruning. Moreover, the decrease in external input was compensated for by the formation of new horizontal connections, which caused a retinotopic remapping. Homeostatic regulation may provide a unifying framework for understanding cortical reorganization, including network repair in degenerative diseases or following focal stroke.

The adult brain is less hard-wired than traditionally thought. About ten percent of synapses in the mature visual cortex is continually replaced by new ones (structural plasticity). This percentage greatly increases after lasting changes in visual input. Due to the topographically organized nerve connections from the retina in the eye to the primary visual cortex in the brain, a small circumscribed lesion in the retina leads to a defined area in the cortex that is deprived of input. Recent experimental studies have revealed that axonal sprouting and dendritic spine turnover are massively increased in and around the cortical area that is deprived of input. However, the driving forces for this structural plasticity remain unclear. Using a novel computational model, we examine whether the need for activity homeostasis of individual neurons may drive cortical reorganization after lasting changes in input activity. We show that homeostatic growth rules indeed give rise to structural and functional reorganization of neuronal networks similar to the cortical reorganization observed experimentally. Understanding the principles of structural plasticity may eventually lead to novel treatment strategies for stimulating functional reorganization after brain damage and neurodegeneration.

Calcium signals and FGF-2 induced neurite growth in cultured parasympathetic neurons: spatial localization and mechanisms of activation

The growth of neuritic processes in developing neurons is tightly controlled by a wide set of extracellular cues that act by initiating downstream signaling cascades, where calcium signals play a major role. Here we analyze the calcium dependence of the neurite growth promoted by basic fibroblast growth factor (bFGF or FGF-2) in chick embryonic ciliary ganglion neurons, taking advantage of dissociated, organotypic, and compartmentalized cultures. We report that signals at both the growth cone and the soma are involved in the promotion of neurite growth by the factor. Blocking calcium influx through L- and N-type voltage-dependent calcium channels and transient receptor potential canonical (TRPC) channels reduces, while release from intracellular stores does not significantly affect, the growth of neuritic processes. Simultaneous recordings of calcium signals elicited by FGF-2 at the soma and at the growth cone show that the factor activates different patterns of responses in the two compartments: steady and sustained responses at the former, oscillations at the latter. At the soma, both voltage-dependent channel and TRPC blockers strongly affect steady-state levels. At the growth cone, the changes in the oscillatory pattern are more complex; therefore, we used a tool based on wavelet analysis to obtain a quantitative evaluation of the effects of the two classes of blockers. We report that the oscillatory behavior at the growth cone is dramatically affected by all the blockers, pointing to a role for calcium influx through the two classes of channels in the generation of signals at the leading edge of the elongating neurites.

Physiological release of endogenous tau is stimulated by neuronal activity

This report provides evidence that stimulation of neuronal activity, or AMPA receptor activation, induces tau release from cortical neurons via a calcium-dependent mechanism. Dysregulation of this process could lead to the spread of tau pathology in disease.

Propagation of tau pathology is linked with progressive neurodegeneration, but the mechanism underlying trans-synaptic spread of tau is unknown. We show that stimulation of neuronal activity, or AMPA receptor activation, induces tau release from healthy, mature cortical neurons. Notably, phosphorylation of extracellular tau appears reduced in comparison with intracellular tau. We also find that AMPA-induced release of tau is calcium-dependent. Blocking pre-synaptic vesicle release by tetanus toxin and inhibiting neuronal activity with tetrodotoxin both significantly impair AMPA-mediated tau release. Tau secretion is therefore a regulatable process, dysregulation of which could lead to the spread of tau pathology in disease.

Glutamate corelease promotes growth and survival of midbrain dopamine neurons

Recent studies have proposed that glutamate corelease by mesostriatal dopamine (DA) neurons regulates behavioral activation by psychostimulants. How and when glutamate release by DA neurons might play this role remains unclear. Considering evidence for early expression of the type 2 vesicular glutamate transporter in mesencephalic DA neurons, we hypothesized that this cophenotype is particularly important during development. Using a conditional gene knock-out approach to selectively disrupt the Vglut2 gene in mouse DA neurons, we obtained in vitro and in vivo evidence for reduced growth and survival of mesencephalic DA neurons, associated with a decrease in the density of DA innervation in the nucleus accumbens, reduced activity-dependent DA release, and impaired motor behavior. These findings provide strong evidence for a functional role of the glutamatergic cophenotype in the development of mesencephalic DA neurons, opening new perspectives into the pathophysiology of neurodegenerative disorders involving the mesostriatal DA system.

Importance of the brain Angiotensin system in Parkinson's disease

Parkinson's disease (PD) has become a major health problem affecting 1.5% of the world's population over 65 years of age. As life expectancy has increased so has the occurrence of PD. The primary direct consequence of this disease is the loss of dopaminergic (DA) neurons in the substantia nigra and striatum. As the intensity of motor dysfunction increases, the symptomatic triad of bradykinesia, tremors-at-rest, and rigidity occur. Progressive neurodegeneration may also impact non-DA neurotransmitter systems including cholinergic, noradrenergic, and serotonergic, often leading to the development of depression, sleep disturbances, dementia, and autonomic nervous system failure. L-DOPA is the most efficacious oral delivery treatment for controlling motor symptoms; however, this approach is ineffective regarding nonmotor symptoms. New treatment strategies are needed designed to provide neuroprotection and encourage neurogenesis and synaptogenesis to slow or reverse this disease process. The hepatocyte growth factor (HGF)/c-Met receptor system is a member of the growth factor family and has been shown to protect against degeneration of DA neurons in animal models. Recently, small angiotensin-based blood-brain barrier penetrant mimetics have been developed that activate this HGF/c-Met system. These compounds may offer a new and novel approach to the treatment of Parkinson's disease.

Calcium - a central regulator of pollen germination and tube growth

Pollen tubes grow rapidly by very fast rates and reach extended lengths to bring about fertilization during plant reproduction. The pollen tube grows exclusively at its tip. Fundamental for such local, tip-focused growth are the presence of internal gradients and transmembrane fluxes of ions. Consequently, vegetative pollen tube cells are an excellent single cell model system to investigate cell biological processes of vesicle transport, cytoskeleton reorganization and regulation of ion transport. The second messenger Ca(2+) has emerged as a central and crucial modulator that not only regulates but also integrates the coordination each of these processes. In this review we reflect on recent advances in our understanding of the mechanisms of Ca(2+) function in pollen tube growth, focusing on its role in basic cellular processes such as control of cell growth, vesicular transport and intracellular signaling by localized gradients of second messengers. In particular we discuss new insights into the identity and role of Ca(2+) conductive ion channels and present experimental addressable hypotheses about their regulation. This article is part of a Special Issue entitled:12th European Symposium on Calcium.

Dopaminergic axons preferentially innervate dendritic spines with hyperactive glutamatergic synapses in the rat striatum

Dopaminergic and glutamatergic afferents simultaneously innervate median spiny neurons (MSNs) and interact to mediate basal ganglia functions. However, the association between dopaminergic and glutamatergic axons is not clear. In the present study, nigrostriatal, corticostriatal, and thalamostriatal axons were anterogradely traced with biotinylated dextran amines (BDA) in rats, and MSNs were labeled with chloromethylbenzamido-DiI for neurogeometric analysis. Results showed that nigrostriatal, but not corticostriatal or thalamostriatal, axons were biased to a target on dendritic spines of the MSNs. In addition, the MSN dendritic spines, which were innervated by tyrosine hydroxylase-immunoreactive (TH-IR) axons and vesicular glutamate transporter 1 or 2-immunoreactive (VGluT-IR) terminals, were significantly larger than dendritic spines innervated by VGluT-IR terminals alone. Under electron microscopy, glutamatergic synapses on the dendritic spines were located with TH-IR terminals and displayed longer postsynaptic density. In addition, these synapses were more perforated than those on dendritic spines lacking innervated TH-IR terminals. These results demonstrated that dopaminergic axons were biased to a target and preferred to innervate dendritic spines with hyperactive and high-efficacy glutamatergic synapses in the striatum.

Distribution of extrasynaptic NMDA receptors on neurons

NMDA receptors are found in both synaptic and extrasynaptic locations on neurons. NMDA receptors also can be found on neurons in early stages prior to synaptogenesis, where they may be involved in migration and differentiation. Extrasynaptic NMDA receptors typically are associated with contacts with adjacent processes such as axons and glia. Extrasynaptic NMDA receptor clusters vary in size and may form associations with scaffolding proteins such as PSD-95 and SAP102. The best-characterized extrasynaptic NMDA receptors contain NR1 and NR2B subunits. Extrasynaptic NMDA receptors may be activated by glutamate spillover from synapses or from ectopic release of glutamate. Consequently, extrasynaptic NMDA receptor activation may occur under different circumstances than that for synaptic NMDA receptors, indicating different functional consequences for the neuron. In some cases, activation of extrasynaptic NMDA receptors may have a negative influence on the neuron, leading to cell damage and death, as may occur in some major diseases of the nervous system.

Regulation of striatal dopamine release by presynaptic auto- and heteroreceptors

Striatal dopamine neurotransmission is critical for normal voluntary movement, affect and cognition. Dysfunctions of its regulation are implicated in a broad range of behaviors and disorders including Parkinson's disease, schizophrenia and drug abuse. Extracellular dopamine levels result from a dynamic equilibrium between release and reuptake by dopaminergic terminals. Both processes are regulated by multiple mechanisms. Here we review data characterizing how dopamine levels are regulated by presynaptic autoreceptors and heteroreceptors, an area intensively investigated due to advances in real time electrochemical detection of extracellular dopamine, i.e., fast-scan cyclic voltammetry and amperometry, and the development of mutant mouse lines with deletions for specific receptors.

Medium spiny neurons of the neostriatal matrix exhibit specific, stereotyped changes in dendritic arborization during a critical developmental period in mice

In mice, the matrix compartment of the striatum (caudate/putamen) undergoes major developmental changes during the second postnatal week, including the establishment of corticostriatal and nigrostriatal afferents, the maturation of parvalbumin-positive interneurons and the appearance of perineuronal nets. It is not known if any of these events influence the dendritic structure of medium spiny neurons, the principal output cells of the striatum. To determine whether any measurable changes in the dendrites of matrix medium spiny neurons occur during this important developmental period, we labeled individual cells at different time points flanking the second postnatal week. These cells exhibit distinct dendritic morphologies from the earliest postnatal time points examined. Furthermore, our data show that the dendritic arbors of these neurons change in length, branch points, diameter and tortuosity, regardless of morphological type. The increase in dendritic length is accompanied by a decrease in the number of branch points that occur in different, but consistent, parts of the dendritic arbor. All of these changes are most pronounced during the second postnatal week, coinciding with a number of developmental events considered important for consolidating circuitry within the striatal matrix. Our results further support the critical importance of this early postnatal period in striatal development.

Modulation of neurite outgrowth by activation of calcium-permeable kainate receptors expressed by rat nociceptive-like dorsal root ganglion neurons

Neurite outgrowth is a fundamental step in establishing proper neuronal connections in the developing central nervous system. Dynamic control of outgrowth has been attributed to changes in growth cone Ca2+ levels in response to extracellular cues. Here we have investigated a possible role for Ca2+ permeable kainate (KA) receptors in regulating neurite outgrowth of nociceptive-like dorsal root ganglion (DRG) neurons. To identify KA receptor subunits likely to be involved, we used quantitative RT-PCR on acutely dissociated DRG and dorsal horn neurons. DRG neurons expressed more GluK1, particularly the GluK1b spice variant, than dorsal horn neurons. Conversely, dorsal horn neurons expressed more GluK2, particularly GluK2a, than DRG neurons. Further, an RNA editing assay indicated that the majority of GluK1 and GluK2 mRNA transcripts in DRG were unedited. Imaging Ca2+ transients following application of a KA receptor agonist to DRG and dorsal horn co-cultures revealed increases in intracellular Ca2+ in the growth cones of DRG neurons. In the majority of cases, this increase in Ca2+ was partly or completely blocked by Joro spider toxin (JSTX), an antagonist for Ca2+-permeable AMPA and KA receptors. Treatment of DRG/dorsal horn co-cultures with KA for 18 hours suppressed neurite outgrowth while application of the rapidly desensitizing KA receptor agonist SYM 2081, the competitive AMPA/KA receptor antagonist, CNQX, and JSTX or philanthotoxin enhanced neurite outgrowth and prevented KA effects on neurite outgrowth. Thus, Ca2+ entry through KA receptors at the growth cone of DRG neurons may be an important regulator of neurite outgrowth.

Functional NMDA receptors at axonal growth cones of young hippocampal neurons

NMDA receptors (NMDARs) are critical to the development of the nervous system, although their roles at axonal growth cones are unclear. We examined NMDAR localization and function at axonal growth cones of young hippocampal neurons. Our immunocytochemical data showed that native and transfected NMDAR subunits are expressed in axons and growth cones of young (days in vitro 3-6) hippocampal rat neurons. Moreover, immunogold electron microscopy showed that NR1 is expressed in growth cones of postnatal day 2 rat hippocampus. Local application of NMDAR agonists to growth cones of voltage-clamped neurons evoked inward currents that were blocked by bath application of an NMDAR antagonist (dl-APV), indicating that these NMDARs are functional. In addition, calcium imaging experiments indicated that NMDARs present in growth cones mediate calcium influx. Calcium transients in growth cones persisted despite pharmacological blockade of voltage-sensitive calcium channels and depletion of intracellular calcium stores. Our findings reveal the presence of functional NMDARs in axons and growth cones of young neurons, suggesting a role for these receptors in axonal guidance and synapse formation during neuronal development.

Glutamatergic signaling by midbrain dopaminergic neurons: recent insights from optogenetic, molecular and behavioral studies

Although the notion that dopaminergic neurons utilize glutamate as a co-transmitter has long been supported by tantalizing molecular, immunocytochemical and electrophysiological evidence it has only been with the recent addition of optogenetic and other approaches that the existence and functional relevance of this mechanism could be unambiguously demonstrated. Here we discuss the possible mechanisms of action of glutamate released from mesoaccumbens dopaminergic neurons based on recently accumulated evidence. Surprisingly, rather then to confirm a role in conventional fast excitatory transmission, the latest evidence suggests that glutamate released from dopaminergic neurons may primarily act through different unconventional presynaptic and postsynaptic mechanisms.

Physiological properties of neurons derived from human embryonic stem cells using a dibutyryl cyclic AMP-based protocol

Neurons derived from human embryonic stem cells hold promise for the therapy of neurological diseases. Quality inspection of human embryonic stem cell-derived neurons has often been based on immunolabeling for neuronal markers. Here we put emphasis on their physiological properties. Electrophysiological measurements were carried out systematically at different stages of neuronal in vitro development, including the very early stage, neuroepithelial rosettes. Developing human neurons are able to generate action potentials (APs) as early as 10 days after the start of differentiation. Tyrosine hydroxylase (TH)-positive (putative dopaminergic, DA) neurons tend to aggregate into clumps, and their overall yield per coverslip is relatively low (8.3%) because of areas void of DA neurons. On the same in vitro day, neighboring neurons can be in very different stages of differentiation, including repetitive AP firing, single full-size AP, and abortive AP. Similarly, the basic electrophysiological parameters (resting membrane potential, input resistance, peak sodium, and peak potassium currents) are scattered in a wide range. Visual appearance of differentiating neurons, and number of primary and secondary dendrites cannot be used to predict the peak sodium current or AP firing properties of cultured neurons. Approximately 13% of neurons showed evidence of hyperpolarization-induced current (I(h)), a characteristic of DA neurons; however, no neurons with repetitive APs showed I(h). The electrophysiological measurements thus indicate that a standard DA differentiation (dibutyryl cyclic AMP-based) protocol, applied for 2-5 weeks, produces a heterogeneous ensemble of mostly immature neurons. The overall quality of human neurons under present conditions (survival factors were not used) begins to deteriorate after 12 days of differentiation.