Biophysical properties and ionic signature of neuronal progenitors of the postnatal subventricular zone in situ

Altered Cl- homeostasis hinders forebrain GABAergic interneuron migration in a mouse model of intellectual disability

Impairments of inhibitory circuits are at the basis of most, if not all, cognitive deficits. The impact of OPHN1, a gene associate with intellectual disability (ID), on inhibitory neurons remains elusive. We addressed this issue by analyzing the postnatal migration of inhibitory interneurons derived from the subventricular zone in a validated mouse model of ID (OPHN1^-/y mice). We found that the speed and directionality of migrating neuroblasts were deeply perturbed in OPHN1^-/y mice. The significant reduction in speed was due to altered chloride (Cl^-) homeostasis, while the overactivation of the OPHN1 downstream signaling pathway, RhoA kinase (ROCK), caused abnormalities in the directionality of the neuroblast progression in mutants. Blocking the cation-Cl^- cotransporter KCC2 almost completely rescued the migration speed while proper directionality was restored upon ROCK inhibition. Our data unveil a strong impact of OPHN1 on GABAergic inhibitory interneurons and identify putative targets for successful therapeutic approaches.

Emerging intersections between neuroscience and glioma biology

The establishment of neuronal and glial networks in the brain depends on the activities of neural progenitors, which are influenced by cell-intrinsic mechanisms, interactions with the local microenvironment and long-range signaling. Progress in neuroscience has helped identify key factors in CNS development. In parallel, studies in recent years have increased our understanding of molecular and cellular factors in the development and growth of primary brain tumors. To thrive, glioma cells exploit pathways that are active in normal CNS progenitor cells, as well as in normal neurotransmitter signaling. Furthermore, tumor cells of incurable gliomas integrate into communicating multicellular networks, where they are interconnected through neurite-like cellular protrusions. In this Review, we discuss evidence that CNS development, organization and function share a number of common features with glioma progression and malignancy. These include mechanisms used by cells to proliferate and migrate, interact with their microenvironment and integrate into multicellular networks. The emerging intersections between the fields of neuroscience and neuro-oncology considered in this review point to new research directions and novel therapeutic opportunities.

Excitable Adult-Generated GABAergic Neurons Acquire Functional Innervation in the Cortex after Stroke

Ischemic stroke enhances the proliferation of adult-generated precursor cells that ectopically migrate toward the infarct. Studies have correlated precursor cell proliferation and subsequent adult neurogenesis with enhanced stroke recovery, yet it remains unclear whether stroke can generate new neurons capable of functional integration into the injured cortex. Here, using single and bitransgenic reporter mice, we identify spatial and temporal features of a multilineage cellular response to focal ischemia. We reveal that a small population of stroke-induced immature neurons accumulate within the peri-infarct region of the adult sensorimotor cortex, exhibit voltage-dependent conductances, fire action potentials, express GABAergic markers, and receive sparse GABAergic synaptic inputs. Collectively, these findings reveal that GABAergic neurons arising from the lateral ventricle have the capacity to integrate into the stroke-injured cortex, although their limited number and exiguous synaptic integration may limit their ability to participate in stroke recovery.

Pilocarpine-Induced Status Epilepticus Increases the Sensitivity of P2X7 and P2Y1 Receptors to Nucleotides at Neural Progenitor Cells of the Juvenile Rodent Hippocampus

Patch-clamp recordings indicated the presence of P2X7 receptors at neural progenitor cells (NPCs) in the subgranular zone of the dentate gyrus in hippocampal brain slices prepared from transgenic nestin reporter mice. The activation of these receptors caused inward current near the resting membrane potential of the NPCs, while P2Y1 receptor activation initiated outward current near the reversal potential of the P2X7 receptor current. Both receptors were identified by biophysical/pharmacological methods. When the brain slices were prepared from mice which underwent a pilocarpine-induced status epilepticus or when brain slices were incubated in pilocarpine-containing external medium, the sensitivity of P2X7 and P2Y1 receptors was invariably increased. Confocal microscopy confirmed the localization of P2X7 and P2Y1 receptor-immunopositivity at nestin-positive NPCs. A one-time status epilepticus in rats caused after a latency of about 5 days recurrent epileptic fits. The blockade of central P2X7 receptors increased the number of seizures and their severity. It is hypothesized that P2Y1 receptors after a status epilepticus may increase the ATP-induced proliferation/ectopic migration of NPCs; the P2X7 receptor-mediated necrosis/apoptosis might counteract these effects, which would otherwise lead to a chronic manifestation of recurrent epileptic fits.

Optogenetic stimulation of glutamatergic neuronal activity in the striatum enhances neurogenesis in the subventricular zone of normal and stroke mice

Neurogenesis in the subventricular zone (SVZ) of the adult brain may contribute to tissue repair after brain injuries. Whether SVZ neurogenesis can be upregulated by specific neuronal activity in vivo and promote functional recovery after stroke is largely unknown. Using the spatial and cell type specific optogenetic technique combined with multiple approaches of in vitro, ex vivo and in vivo examinations, we tested the hypothesis that glutamatergic activation in the striatum could upregulate SVZ neurogenesis in the normal and ischemic brain. In transgenic mice expressing the light-gated channelrhodopsin-2 (ChR2) channel in glutamatergic neurons, optogenetic stimulation of the glutamatergic activity in the striatum triggered glutamate release into SVZ region, evoked membrane currents, Ca²⁺ influx and increased proliferation of SVZ neuroblasts, mediated by AMPA receptor activation. In ChR2 transgenic mice subjected to focal ischemic stroke, optogenetic stimuli to the striatum started 5days after stroke for 8days not only promoted cell proliferation but also the migration of SVZ neuroblasts into the peri-infarct cortex with increased neuronal differentiation and improved long-term functional recovery. These data provide the first morphological and functional evidence showing a unique striatum-SVZ neuronal regulation via a semi-phasic synaptic mechanism that can boost neurogenic cascades and stroke recovery. The benefits from stimulating endogenous glutamatergic activity suggest a novel regenerative strategy after ischemic stroke and other brain injuries.

Regulation of adult neural progenitor cell functions by purinergic signaling

Extracellular purines are signaling molecules in the neurogenic niches of the brain and spinal cord, where they activate cell surface purinoceptors at embryonic neural stem cells (NSCs) and adult neural progenitor cells (NPCs). Although mRNA and protein are expressed at NSCs/NPCs for almost all subtypes of the nucleotide-sensitive P2X/P2Y, and the nucleoside-sensitive adenosine receptors, only a few of those have acquired functional significance. ATP is sequentially degraded by ecto-nucleotidases to ADP, AMP, and adenosine with agonistic properties for distinct receptor-classes. Nucleotides/nucleosides facilitate or inhibit NSC/NPC proliferation, migration and differentiation. The most ubiquitous effect of all agonists (especially of ATP and ADP) appears to be the facilitation of cell proliferation, usually through P2Y1Rs and sometimes through P2X7Rs. However, usually P2X7R activation causes necrosis/apoptosis of NPCs. Differentiation can be initiated by P2Y2R-activation or P2X7R-blockade. A key element in the transduction mechanism of either receptor is the increase of the intracellular free Ca²⁺ concentration, which may arise due to its release from intracellular storage sites (G protein-coupling; P2Y) or due to its passage through the receptor-channel itself from the extracellular space (ATP-gated ion channel; P2X). Further research is needed to clarify how purinergic signaling controls NSC/NPC fate and how the balance between the quiescent and activated states is established with fine and dynamic regulation. GLIA 2017;65:213-230.

The multifaceted subventricular zone astrocyte: From a metabolic and pro-neurogenic role to acting as a neural stem cell

A few decades ago it was discovered that two regions of the adult brain retain the ability to generate new neurons. These regions include the subgranular zone of the hippocampal dentate gyrus and the ventricular-subventricular zone (V-SVZ) located at the border of the lateral ventricle. In the V-SVZ, it was discovered that neural progenitor cells (NPCs) share many features of mature astrocytes and are often referred as V-SVZ astrocytes. We will first describe the markers, the morphology, and the neurophysiological characteristics of the mouse V-SVZ astrocytes. We will then discuss the fact that V-SVZ astrocytes constitute a mixed population with respect to their neurogenic properties, e.g., quiescent versus activated state, neurogenic fate, and transcription factors expression. Finally, we will describe two functions of V-SVZ astrocytes, their metabolic coupling to blood vessels and their neurogenic-supportive role consisting of providing guidance and survival cues to migrating newborn neurons.

Purinergic receptors in embryonic and adult neurogenesis

ATP (adenosine 5'-triphosphate), one of the most ancient neurotransmitters, exerts essential functions in the brain, including neurotransmission and modulation of synaptic activity. Moreover, this nucleotide has been attributed with trophic properties and experimental evidence points to the participation of ATP-activated P2X and P2Y purinergic receptors in embryonic brain development as well as in adult neurogenesis for maintenance of normal brain functions and neuroregeneration upon brain injury. We discuss here the available data on purinergic P2 receptor expression and function during brain development and in the neurogenic zones of the adult brain, as well as the insights based on the use of in vitro stem cell cultures. While several P2 receptor subtypes were shown to be expressed during in vitro and in vivo neurogenesis, specific functions have been proposed for P2Y1, P2Y2 metabotropic as well as P2X2 ionotropic receptors to promote neurogenesis. Further, the P2X7 receptor is suggested to function in the maintenance of pools of neural stem and progenitor cells through induction of proliferation or cell death, depending on the microenvironment. Pathophysiological actions have been proposed for this receptor in worsening damage in brain disease. The P2X7 receptor and possibly additional P2 receptor subtypes have been implicated in pathophysiology of neurological diseases including Parkinson's disease, Alzheimer's disease and epilepsy. New strategies in cell therapy could involve modulation of purinergic signaling, either in the achievement of more effective protocols to obtain viable and homogeneous cell populations or in the process of functional engraftment of transplanted cells into the damaged brain. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

A time course analysis of the electrophysiological properties of neurons differentiated from human induced pluripotent stem cells (iPSCs)

Many protocols have been designed to differentiate human embryonic stem cells (ESCs) and human induced pluripotent stem cells (iPSCs) into neurons. Despite the relevance of electrophysiological properties for proper neuronal function, little is known about the evolution over time of important neuronal electrophysiological parameters in iPSC-derived neurons. Yet, understanding the development of basic electrophysiological characteristics of iPSC-derived neurons is critical for evaluating their usefulness in basic and translational research. Therefore, we analyzed the basic electrophysiological parameters of forebrain neurons differentiated from human iPSCs, from day 31 to day 55 after the initiation of neuronal differentiation. We assayed the developmental progression of various properties, including resting membrane potential, action potential, sodium and potassium channel currents, somatic calcium transients and synaptic activity. During the maturation of iPSC-derived neurons, the resting membrane potential became more negative, the expression of voltage-gated sodium channels increased, the membrane became capable of generating action potentials following adequate depolarization and, at day 48–55, 50% of the cells were capable of firing action potentials in response to a prolonged depolarizing current step, of which 30% produced multiple action potentials. The percentage of cells exhibiting miniature excitatory post-synaptic currents increased over time with a significant increase in their frequency and amplitude. These changes were associated with an increase of Ca²⁺ transient frequency. Co-culturing iPSC-derived neurons with mouse glial cells enhanced the development of electrophysiological parameters as compared to pure iPSC-derived neuronal cultures. This study demonstrates the importance of properly evaluating the electrophysiological status of the newly generated neurons when using stem cell technology, as electrophysiological properties of iPSC-derived neurons mature over time.

P2X7 receptors at adult neural progenitor cells of the mouse subventricular zone

Neurogenesis requires the balance between the proliferation of newly formed progenitor cells and subsequent death of surplus cells. RT-PCR and immunocytochemistry demonstrated the presence of P2X7 receptor mRNA and immunoreactivity in cultured neural progenitor cells (NPCs) prepared from the adult mouse subventricular zone (SVZ). Whole-cell patch-clamp recordings showed a marked potentiation of the inward current responses both to ATP and the prototypic P2X7 receptor agonist dibenzoyl-ATP (Bz-ATP) at low Ca(2+) and zero Mg(2+) concentrations in the bath medium. The Bz-ATP-induced currents reversed their polarity near 0 mV; in NPCs prepared from P2X7(-/-) mice, Bz-ATP failed to elicit membrane currents. The general P2X/P2Y receptor antagonist PPADS and the P2X7 selective antagonists Brilliant Blue G and A-438079 strongly depressed the effect of Bz-ATP. Long-lasting application of Bz-ATP induced an initial current, which slowly increased to a steady-state response. In combination with the determination of YO-PRO uptake, these experiments suggest the dilation of a receptor-channel and/or the recruitment of a dye-uptake pathway. Ca(2+)-imaging by means of Fura-2 revealed that in a Mg(2+)-deficient bath medium Bz-ATP causes [Ca(2+)](i) transients fully depending on the presence of external Ca(2+). The MTT test indicated a concentration-dependent decrease in cell viability by Bz-ATP treatment. Correspondingly, Bz-ATP led to an increase in active caspase 3 immunoreactivity, indicating a P2X7-controlled apoptosis. In acute SVZ brain slices of transgenic Tg(nestin/EGFP) mice, patch-clamp recordings identified P2X7 receptors at NPCs with pharmacological properties identical to those of their cultured counterparts. We suggest that the apoptotic/necrotic P2X7 receptors at NPCs may be of particular relevance during pathological conditions which lead to increased ATP release and thus could counterbalance the ensuing excessive cell proliferation.

KCa3.1 modulates neuroblast migration along the rostral migratory stream (RMS) in vivo

From the subventricular zone (SVZ), neuronal precursor cells (NPCs), called neuroblasts, migrate through the rostral migratory stream (RMS) to become interneurons in the olfactory bulb (OB). Ion channels regulate neuronal migration during development, yet their role in migration through the adult RMS is unknown. To address this question, we utilized Nestin-CreER(T2)/R26R-YFP mice to fluorescently label neuroblasts in the adult. Patch-clamp recordings from neuroblasts reveal K(+) currents that are sensitive to intracellular Ca(2+) levels and blocked by clotrimazole and TRAM-34, inhibitors of intermediate conductance Ca(2+)-activated K(+) (KCa3.1) channels. Immunolabeling and electrophysiology show KCa3.1 expression restricted to neuroblasts in the SVZ and RMS, but absent in OB neurons. Time-lapse confocal microscopy in situ showed inhibiting KCa3.1 prolonged the stationary phase of neuroblasts' saltatory migration, reducing migration speed by over 50%. Both migration and KCa3.1 currents could also be inhibited by blocking Ca(2+) influx via transient receptor potential (TRP) channels, which, together with positive immunostaining for transient receptor potential canonical 1 (TRPC1), suggest that TRP channels are an important Ca(2+) source modulating KCa3.1 activity. Finally, injecting TRAM-34 into Nestin-CreER(T2)/R26R-YFP mice significantly reduced the number of neuroblasts that reached the OB, suggesting an important role for KCa3.1 in vivo. These studies describe a previously unrecognized protein in migration of adult NPCs.

Rheb activation in subventricular zone progenitors leads to heterotopia, ectopic neuronal differentiation, and rapamycin-sensitive olfactory micronodules and dendrite hypertrophy of newborn neurons

Mammalian target of rapamycin (mTOR) hyperactivity in perinatal neural progenitor cells (NPCs) of tuberous sclerosis complex 1 (Tsc1) heterozygote mice leads to heterotopia and abnormal neuronal morphogenesis as seen in patients with tuberous sclerosis. Considering that pathological hyperactive mTOR also occurs in individuals carrying no genetic mutations, we examined whether increasing mTOR activity in neonatal NPCs of wild-type mice would recapitulate the above phenotypes. Electroporation of a plasmid encoding constitutively active Ras-homolog enriched in brain (Rheb(CA)) into subventricular zone NPCs increased mTOR activity in newborn cells. At 19 d post-electroporation (dpe), heterotopia and ectopic cells with a neuronal morphology were observed along the migratory path [rostral migratory stream (RMS)] and in the olfactory bulb (OB). These ectopic cells displayed action potentials and received synaptic inputs identifying them as synaptically integrated neurons. RMS heterotopias contained astrocytes, neurons, and entrapped neuroblasts. Immunostaining at 3 dpe revealed the presence of Mash1(+) Olig2(-) cells in the migratory route accompanied by ectopic neuronal differentiation and altered direction and speed of neuroblast migration at 7 dpe, suggesting a non-cell-autonomous disruption of migration. At >19 dpe, newborn Rheb(CA)-expressing neurons displayed altered distribution and formed micronodules in the OB. In addition, they displayed increased dendritic complexity along with altered membrane biophysics and increased frequency of GABAergic synaptic inputs. OB heterotopia, micronodules, and dendrite hypertrophy were notably prevented by rapamycin treatment, suggesting their mTOR dependence. Collectively, these data show that increasing mTOR activity in neonatal NPCs of wild-type mice recapitulate the pathologies observed in Tsc1 mutant mice. In addition, increased mTOR activity in individuals without known mutations could significantly impact neurogenesis and circuit formation.

Human glioma-initiating cells show a distinct immature phenotype resembling but not identical to NG2 glia

Glioma-initiating cells (GICs) represent a potential important therapeutic target because they are likely to account for the frequent recurrence of malignant gliomas; however, their identity remains unsolved. Here, we characterized the cellular lineage fingerprint of GICs through a combination of electrophysiology, lineage marker expression, and differentiation assays of 5 human patient-derived primary GIC lines. Most GICs coexpressed nestin, NG2 proteoglycan, platelet-derived growth factor receptor-α, and glial fibrillary acidic protein. Glioma-initiating cells could be partially differentiated into astrocytic but not oligodendroglial or neural lineages. We also demonstrate that GICs have a characteristic electrophysiologic profile distinct from that of well-characterized tumor bulk cells. Together, our results suggest that GICs represent a unique type of cells reminiscent of an immature phenotype that closely resembles but is not identical to NG2 glia with respect to marker expression and functional membrane properties.

Preparation of acute subventricular zone slices for calcium imaging

The subventricular zone (SVZ) is one of the two neurogenic zones in the postnatal brain. The SVZ contains densely packed cells, including neural progenitor cells with astrocytic features (called SVZ astrocytes), neuroblasts, and intermediate progenitor cells. Neuroblasts born in the SVZ tangentially migrate a great distance to the olfactory bulb, where they differentiate into interneurons. Intercellular signaling through adhesion molecules and diffusible signals play important roles in controlling neurogenesis. Many of these signals trigger intercellular calcium activity that transmits information inside and between cells. Calcium activity is thus reflective of the activity of extracellular signals and is an optimal way to understand functional intercellular signaling among SVZ cells. Calcium activity has been studied in many other regions and cell types, including mature astrocytes and neurons. However, the traditional method to load cells with calcium indicator dye (i.e. bath loading) was not efficient at loading all SVZ cell types. Indeed, the cellular density in the SVZ precludes dye diffusion inside the tissue. In addition, preparing sagittal slices will better preserve the three-dimensional arrangement of SVZ cells, particularly the stream of neuroblast migration on the rostral-caudal axis. Here, we describe methods to prepare sagittal sections containing the SVZ, the loading of SVZ cells with calcium indicator dye, and the acquisition of calcium activity with time-lapse movies. We used Fluo-4 AM dye for loading SVZ astrocytes using pressure application inside the tissue. Calcium activity was recorded using a scanning confocal microscope allowing a precise resolution for distinguishing individual cells. Our approach is applicable to other neurogenic zones including the adult hippocampal subgranular zone and embryonic neurogenic zones. In addition, other types of dyes can be applied using the described method.

Local generation of glia is a major astrocyte source in postnatal cortex

Glial cells constitute nearly 50% of the cells in the human brain. Astrocytes, which make up the largest glial population, are crucial to the regulation of synaptic connectivity during postnatal development. Because defects in astrocyte generation are associated with severe neurological disorders such as brain tumours, it is important to understand how astrocytes are produced. Astrocytes reportedly arise from two sources: radial glia in the ventricular zone and progenitors in the subventricular zone, with the contribution from each region shifting with time. During the first three weeks of postnatal development, the glial cell population, which contains predominantly astrocytes, expands 6-8-fold in the rodent brain. Little is known about the mechanisms underlying this expansion. Here we show that a major source of glia in the postnatal cortex in mice is the local proliferation of differentiated astrocytes. Unlike glial progenitors in the subventricular zone, differentiated astrocytes undergo symmetric division, and their progeny integrate functionally into the existing glial network as mature astrocytes that form endfeet with blood vessels, couple electrically to neighbouring astrocytes, and take up glutamate after neuronal activity.

Stem cells as a potential therapy for epilepsy

Neural stem cells and neural progenitors (NSC/NPs) hold great promise in neuro-restorative therapy due to their remarkable capacity for self-renewal, plasticity, and ability to integrate into host brain circuitry. Some types of epilepsy would appear to be excellent targets for this type of therapy due to known alterations in local circuitry based on loss or malfunction of specific types of neurons in specific brain structures. Potential sources for NSC/NPs include the embryonic blastocyst, the fetal brain, and adult brain and non-neural tissues. Each of these cell types has potential strengths and weaknesses as candidates for clinical therapeutic agents. This article reviews some of the major types of NSC/NPs and how they have been studied with regard to synaptic integration into host brain circuits. It also reviews how these transplanted cells develop and interact with host brain cells in animal models of epilepsy. The field is still wide open with a number of very promising results but there are also some major challenges that will need to be addressed prior to considering clinical applications for epilepsy.

Developmental activation of the proteolipid protein promoter transgene in neuronal and oligodendroglial cells of neostriatum in mice

Prior studies suggest that non-canonical proteolipid protein (PLP) gene expression occurs during development in non-myelinating neurons as well as myelinating oligodendroglia in mammalian brain. To assess this possibility in neostriatum, a region of uncertain PLP gene expression in neurons, morphological and electrophysiological tools were used to determine phenotypes of cells with activation of a PLP promoter transgene during the early postnatal period in mice. PLP gene expression is evident in both neuronal and oligodendroglial phenotypes in developing neostriatum, a conclusion based on three novel observations: (1) An enhanced green fluorescent protein (EGFP) reporter of PLP promoter activation was localized in two distinct populations of cells, which exhibit collective, developmental differences of morphological and electrophysiological characteristics in accord with neuronal and oligodendroglial phenotypes of neostriatal cells found during the early postnatal period in both transgenic and wild-type mice. (2) The EGFP reporter of PLP promoter activation was appropriately positioned to serve as a regulator of PLP gene expression. It colocalized with native PLP proteins in both neuronal and oligodendroglial phenotypes; however, only soma-restricted PLP protein isoforms were found in the neuronal phenotype, while classic and soma-restricted PLP protein isoforms were found in the oligodendroglial phenotype. (3) As shown by EGFP reporter, PLP promoter activation was placed to regulate PLP gene expression in only one neuronal phenotype among the several that constitute neostriatum. It was localized in medium spiny neurons, but not large aspiny neurons. These outcomes have significant implications for the non-canonical functional roles of PLP gene expression in addition to myelinogenesis in mammalian brain, and are consistent with potentially independent pathologic loci in neurons during the course of human mutational disorders of PLP gene expression.

Neurotransmitters couple brain activity to subventricular zone neurogenesis

Adult neurogenesis occurs in two privileged microenvironments, the hippocampal subgranular zone of the dentate gyrus and the subventricular zone (SVZ) along the lateral ventricle. This review focuses on accumulating evidence suggesting that the activity of specific brain regions or bodily states influences SVZ cell proliferation and neurogenesis. Neuromodulators such as dopamine and serotonin have been shown to have long-range effects through neuronal projections into the SVZ. Local γ-aminobutyric acid and glutamate signaling have demonstrated effects on SVZ proliferation and neurogenesis, but an extra-niche source of these neurotransmitters remains to be explored and options will be discussed. There is also accumulating evidence that diseases and bodily states such as Alzheimer's disease, seizures, sleep and pregnancy influence SVZ cell proliferation. With such complex behavior and environmentally-driven factors that control subregion-specific activity, it will become necessary to account for overlapping roles of multiple neurotransmitter systems on neurogenesis when developing cell therapies or drug treatments.

From progenitors to integrated neurons: role of neurotransmitters in adult olfactory neurogenesis

Adult neurogenesis is due to the persistence of pools of constitutive stem cells able to give rise to a progeny of proliferating progenitors. In rodents, adult neurogenic niches have been found in the subventricular zone (SVZ) along the lateral ventricles and in the subgranular zone of the dentate gyrus in the hippocampus. SVZ progenitors undergo a unique process of tangential migration from the lateral ventricle to the olfactory bulb (OB) where they differentiate mainly into GABAergic interneurons in the granule and glomerular layers. SVZ progenitor proliferation, migration and differentiation into fully integrated neurons, are strictly related processes regulated by complex interactions between cell intrinsic and extrinsic influences. Numerous observations demonstrate that neurotrasmitters are involved in all steps of the adult neurogenic process, but the understanding of their role is hampered by their intricate mechanism of action and by the highly complex network in which neurotransmitters work. By considering the three main steps of olfactory adult neurogenesis (proliferation, migration and integration), this review will discuss recent advances in the study of neurotransmitters, highlighting the regulatory mechanisms upstream and downstream their action.

Synergistic effects of sodium butyrate, a histone deacetylase inhibitor, on increase of neurogenesis induced by pyridoxine and increase of neural proliferation in the mouse dentate gyrus

We previously observed that pyridoxine (vitamin B(6)) significantly increased cell proliferation and neuroblast differentiation without any neuronal damage in the hippocampus. In this study, we investigated the effects of sodium butyrate, a histone deacetylase (HDAC) inhibitor which serves as an epigenetic regulator of gene expression, on pyridoxine-induced neural proliferation and neurogenesis induced by the increase of neural proliferation in the mouse dentate gyrus. Sodium butyrate (300 mg/kg, subcutaneously), pyridoxine (350 mg/kg, intraperitoneally), or combination with sodium butyrate were administered to 8-week-old mice twice a day and once a day, respectively, for 14 days. The administration of sodium butyrate significantly increased acetyl-histone H3 levels in the dentate gyrus. Sodium butyrate alone did not show the significant increase of cell proliferation in the dentate gyrus. But, pyridoxine alone significantly increased cell proliferation. Sodium butyrate in combination with pyridoxine robustly enhanced cell proliferation and neurogenesis induced by the increase of neural proliferation in the dentate gyrus, showing that sodium butyrate treatment distinctively enhanced development of neuroblast dendrites. These results indicate that an inhibition of HDAC synergistically promotes neurogenesis induced by a pyridoxine and increase of neural proliferation.

How, when, and where new inhibitory neurons release neurotransmitters in the adult olfactory bulb

Adult-born neurons continuously incorporate into the olfactory bulb where they rapidly establish contacts with a variety of synaptic inputs. Little is known, however, about the functional properties of their output. Characterization of synaptic outputs from new neurons is essential to assess the functional impact of adult neurogenesis on mature circuits. Here, we used optogenetics to control neurotransmitter release from new neurons. We found that light-induced synaptic GABA release from adult-born neurons leads to profound modifications of postsynaptic target firing patterns. We revealed that functional output synapses form just after new cells acquire the faculty to spike, but most synapses were made a month later. Despite discrepancies in the timing of new synapse recruitment, the properties of postsynaptic signals remain constant. Remarkably, we found that all major cell types of the olfactory bulb circuit, including output neurons and several distinct subtypes of local interneurons, were contacted by adult-born neurons. Thus, this study provides new insights into how new neurons integrate into the adult neural network and may influence the sense of smell.

Integration of neuronally predifferentiated human dental pulp stem cells into rat brain in vivo

Pluripotency and their neural crest origin make dental pulp stem cells (DPSCs) an attractive donor source for neuronal cell replacement. Despite recent encouraging results in this field, little is known about the integration of transplanted DPSC derived neuronal pecursors into the central nervous system. To address this issue, neuronally predifferentiated DPSCs, labeled with a vital cell dye Vybrant DiD were introduced into postnatal rat brain. DPSCs were transplanted into the cerebrospinal fluid of 3-day-old male Wistar rats. Cortical lesion was induced by touching a cold (-60°C) metal stamp to the calvaria over the forelimb motor cortex. Four weeks later cell localization was detected by fluorescent microscopy and neuronal cell markers were studied by immunohistochemistry. To investigate electrophysiological properties of engrafted, fluorescently labeled DPSCs, 300 μm-thick horizontal brain slices were prepared and the presence of voltage-dependent sodium and potassium channels were recorded by patch clamping. Predifferentiated donor DPSCs injected into the cerebrospinal fluid of newborn rats migrated as single cells into a variety of brain regions. Most of the cells were localized in the normal neural progenitor zones of the brain, the subventricular zone (SVZ), subgranular zone (SGZ) and subcallosal zone (SCZ). Immunohistochemical analysis revealed that transplanted DPSCs expressed the early neuronal marker N-tubulin, the neuronal specific intermediate filament protein NF-M, the postmitotic neuronal marker NeuN, and glial GFAP. Moreover, the cells displayed TTX sensitive voltage dependent (VD) sodium currents (I(Na)) and TEA sensitive delayed rectifier potassium currents (K(DR)). Four weeks after injury, fluorescently labeled cells were detected in the lesioned cortex. Neurospecific marker expression was increased in DPSCs found in the area of the cortical lesions compared to that in fluorescent cells of uninjured brain. TTX sensitive VD sodium currents and TEA sensitive K(DR) significantly increased in labeled cells of the cortically injured area. In conclusion, our data demonstrate that engrafted DPSC-derived cells integrate into the host brain and show neuronal properties not only by expressing neuron-specific markers but also by exhibiting voltage dependent sodium and potassium channels. This proof of concept study reveals that predifferentiated hDPSCs may serve as useful sources of neuro- and gliogenesis in vivo, especially when the brain is injured.

Prostaglandin E2 induces glutamate release from subventricular zone astrocytes

It was recently reported that in one of the adult neurogenetic zones, the subventricular zone (SVZ), astrocyte-like cells release glutamate upon intracellular Ca2+ increases. However, the signals that control Ca2+ activity and glutamate release from SVZ astrocytes are not known. Here, we examined whether prostaglandin E2 (PGE2), which induces glutamate release from mature astrocytes, is such a signal. Using the gramicidin-perforated patch-clamp technique, we show that the activity of N-Methyl-D-Aspartate receptor (NMDAR) channel in neuroblasts is a high fidelity sensor of ambient glutamate levels. Using such sensors, we found that application of PGE2 led to increased ambient glutamate levels in the SVZ. In parallel experiments, PGE2 induced an increase in intracellular Ca2+ levels in SVZ cells, in particular astrocyte-like cells, as shown using Ca2+ imaging. Finally, a PGE2 enzyme immunoassay showed that the choroid plexus of the lateral ventricle and to a lesser extent the SVZ (ten-fold less) released PGE2. These findings suggest that PGE2 is a physiological signal for inducing glutamate release from SVZ astrocytes that is important for controlling neuroblast survival and proliferation. This signal may be accentuated following ischemia or injury-induced PGE2 release and may contribute to the injury-associated increased neurogenesis.

Distinct effects of sonic hedgehog and Wnt-7a on differentiation of neonatal neural stem/progenitor cells in vitro

Sonic hedgehog (Shh) and Wnt-7a are morphogens involved in embryonic as well as ongoing adult neurogenesis. Their effects on the differentiation and membrane properties of neonatal neural stem/progenitor cells (NS/PCs) were studied in vitro using NS/PCs transduced with either Shh or Wnt-7a. Eight days after the onset of in vitro differentiation the cells were analyzed for the expression of neuronal and glial markers using immunocytochemical and Western blot analysis, and their membrane properties were characterized using the patch-clamp technique. Our results showed that both Shh and Wnt-7a increased the numbers of cells expressing neuronal markers; however, quantitative immunocytochemical analysis showed that only Wnt-7a enhanced the outgrowth and the development of processes in these cells. In addition, Wnt-7a markedly suppressed gliogenesis. The electrophysiological analysis revealed that Wnt-7a increased, while Shh decreased the incidence of cells displaying a neuron-like current pattern, represented by outwardly rectifying K(+) currents and tetrodotoxin-sensitive Na(+) currents. Additionally, Wnt-7a increased cell proliferation only at the early stages of differentiation, while Shh promoted proliferation within the entire course of differentiation. Thus we can conclude that Shh and Wnt-7a interfere differently with the process of neuronal differentiation and that they promote distinct stages of neuronal differentiation in neonatal NS/PCs.

Imaging and recording subventricular zone progenitor cells in live tissue of postnatal mice

The subventricular zone (SVZ) is one of two regions where neurogenesis persists in the postnatal brain. The SVZ, located along the lateral ventricle, is the largest neurogenic zone in the brain that contains multiple cell populations including astrocyte-like cells and neuroblasts. Neuroblasts migrate in chains to the olfactory bulb where they differentiate into interneurons. Here, we discuss the experimental approaches to record the electrophysiology of these cells and image their migration and calcium activity in acute slices. Although these techniques were in place for studying glial cells and neurons in mature networks, the SVZ raises new challenges due to the unique properties of SVZ cells, the cellular diversity, and the architecture of the region. We emphasize different methods, such as the use of transgenic mice and in vivo electroporation that permit identification of the different SVZ cell populations for patch clamp recording or imaging. Electroporation also permits genetic labeling of cells using fluorescent reporter mice and modification of the system using either RNA interference technology or floxed mice. In this review, we aim to provide conceptual and technical details of the approaches to perform electrophysiological and imaging studies of SVZ cells.

Olfactory ensheathing cell membrane properties are shaped by connectivity

Olfactory ensheathing cells (OECs) have been repeatedly implicated in mediating plasticity, particularly in situ in the olfactory nerve in which they support the extension of olfactory sensory neuron (OSN) axons from the olfactory epithelium to the olfactory bulb (OB). OECs are specialized glia whose processes surround OSN axon fascicles within the olfactory nerve and across the OB surface. Despite their purported importance in promoting axon extension, and following transplants, little is known about either morphology or biophysical properties of OECs in situ. In particular, cell-cell interactions that may influence OEC function are largely unexplored. Here, we studied OEC connectivity and morphology in slice preparations, preserving tissue structure and cell-cell interactions. Our analyses showed that OECs form a matrix of cellular projections surrounding axons, unique among glia, and express high levels of connexin-43. Lucifer Yellow injections revealed selective dye coupling among small subgroups of OECs. Two types of OECs were biophysically distinguished with whole-cell voltage-clamp recordings: (1) with low-input resistance (R(i)), linear current profiles, and frequently dye coupled; and (2) with high R(i), nonlinear current profiles, and infrequent dye coupling. Pharmacological blockade of gap junctions changed OEC membrane properties such that linear OECs became nonlinear. Double recordings indicated that the appearance of the nonlinear current profile was associated with the loss of electrical coupling between OECs. We conclude that the diversity of OEC current profiles can be explained by differences in gap-junction connectivity and discuss implications of this diversity for OEC influences on axon growth and excitability.

Physiological roles of ion channels in adult neural stem cells and their progeny

Elucidation of the machinery of adult neurogenesis is indispensable for the treatment of neurodegenerative diseases by therapeutic drugs and/or by neural stem cell (NSC) transplantation. It is well known that membrane ion channels play a critical role in cell function, including proliferation, apoptosis and migration in a wide range of cells. In NSC research, interdisciplinary collaboration between cell biologists and membrane physiologists has been pursued principally to monitor ion channel and synaptic currents as a hallmark of neuronal differentiation and maturation of NSC progeny. Nevertheless, less attention had been paid to a functional role of ion channels in NSCs or their immature progeny. Recently, however, evidence regarding their functional relevance has started to accumulate. In focusing on the early stages of the neurogenic process during which NSCs give rise to neuroblasts, this review highlights the latent ability of ion channels to act as functional regulators of adult neurogenesis.

Electrophysiological properties of subventricular zone cells in adult mouse brain

The subventricular zone (SVZ) is a principal site of adult neurogenesis and appears to participate in the brain's response to injury. Thus, measures that enhance SVZ neurogenesis may have a role in treatment of neurological disease. To better characterize SVZ cells and identify potential targets for therapeutic intervention, we studied electrophysiological properties of SVZ cells in adult mouse brain slices using patch-clamp techniques. Electrophysiology was correlated with immunohistochemical phenotype by injecting cells with lucifer yellow and by studying transgenic mice carrying green fluorescent protein under control of the doublecortin (DCX) or glial fibrillary acidic protein (GFAP) promoter. We identified five types of cells in the adult mouse SVZ: type 1 cells, with 4-aminopyridine (4-AP)/tetraethylammonium (TEA)-sensitive and CdCl(2)-sensitive inward currents; type 2 cells, with Ca(2+)-sensitive K(+) and both 4-AP/TEA-sensitive and -insensitive currents; type 3 cells, with 4-AP/TEA-sensitive and -insensitive K(+) and small Na(+) currents; type 4 cells, with slowly activating, large linear outward current and sustained outward current without fast-inactivating component; and type 5 cells, with a large outward rectifying current with a fast inactivating component. Type 2 and 3 cells expressed DCX, types 4 and 5 cells expressed GFAP, and type 1 cells expressed neither. We propose that SVZ neurogenesis involves a progression of electrophysiological cell phenotypes from types 4 and 5 cells (astrocytes) to type 1 cells (neuronal progenitors) to types 2 and 3 cells (nascent neurons), and that drugs acting on ion channels expressed during neurogenesis might promote therapeutic neurogenesis in the injured brain.

NMDA receptors activated by subventricular zone astrocytic glutamate are critical for neuroblast survival prior to entering a synaptic network

Even before integrating into existing circuitry, adult-born neurons express receptors for neurotransmitters, but the intercellular mechanisms and their impact on neurogenesis remain largely unexplored. Here, we show that neuroblasts born in the postnatal subventricular zone (SVZ) acquire NMDA receptors (NMDARs) during their migration to the olfactory bulb. Along their route, neuroblasts are ensheathed by astrocyte-like cells expressing vesicular glutamate release machinery. Increasing calcium in these specialized astrocytes induced NMDAR activity in neuroblasts, and blocking astrocytic vesicular release eliminated spontaneous NMDAR activity. Single-cell knockout of NMDARs using neonatal electroporation resulted in neuroblast apoptosis at the time of NMDAR acquisition. This cumulated in a 40% loss of neuroblasts along their migratory route, demonstrating that NMDAR acquisition is critical for neuroblast survival prior to entering a synaptic network. In addition, our findings suggest an unexpected mechanism wherein SVZ astrocytes use glutamate signaling through NMDARs to control the number of adult-born neurons reaching their final destination.

GABA(A) Increases Calcium in Subventricular Zone Astrocyte-Like Cells Through L- and T-Type Voltage-Gated Calcium Channels

In the adult neurogenic subventricular zone (SVZ), the behavior of astrocyte-like cells and some of their functions depend on changes in intracellular Ca²⁺ levels and tonic GABA_A receptor activation. However, it is unknown whether, and if so how, GABA_A receptor activity regulates intracellular Ca²⁺ dynamics in SVZ astrocytes. To monitor Ca²⁺ activity selectively in astrocyte-like cells, we used two lines of transgenic mice expressing either GFP fused to a Gq-coupled receptor or DsRed under the human glial fibrillary acidic protein (hGFAP) promoter. GABA_A receptor activation induced Ca²⁺ increases in 40–50% of SVZ astrocytes. GABA_A-induced Ca²⁺ increases were prevented with nifedipine and mibefradil, blockers of L- and T-type voltage-gated calcium channels (VGCC). The L-type Ca²⁺ channel activator BayK 8644 increased the percentage of GABA_A-responding astrocyte-like cells to 75%, suggesting that the majority of SVZ astrocytes express functional VGCCs. SVZ astrocytes also displayed spontaneous Ca²⁺ activity, the frequency of which was regulated by tonic GABA_A receptor activation. These data support a role for ambient GABA in tonically regulating intracellular Ca²⁺ dynamics through GABA_A receptors and VGCC in a subpopulation of astrocyte-like cells in the postnatal SVZ.

Analysis of in Vitro and in Vivo Characteristics of Human Embryonic Stem Cell-Derived Neural Precursors

During the last decade, much progress has been made in developing protocols for the differentiation of human embryonic stem cells (hESCs) into a neural phenotype. The appropriate agent for cell therapy is neural precursors (NPs). Here, we demonstrate the derivation of highly enriched and expandable populations of proliferating NPs from the CCTL14 line of hESCs. These NPs could differentiate in vitro into functionally active neurons, as confirmed by immunohistochemical staining and electrophysiological analysis. Neural cells differentiated in vitro from hESCs exhibit broad cellular heterogeneity with respect to developmental stage and lineage specification. To analyze the population of the derived NPs, we used fluorescence-activated cell sorting (FACS) and characterized the expression of several pluripotent and neural markers, such as Nanog, SSEA-4, SSEA-1, TRA-1-60, CD24, CD133, CD56 (NCAM), β-III-tubulin, NF70, nestin, CD271 (NGFR), CD29, CD73, and CD105 during long-term propagation. The analyzed cells were used for transplantation into the injured rodent brain; the tumorigenicity of the transplanted cells was apparently eliminated following long-term culture. These results complete the characterization of the CCTL14 line of hESCs and provide a framework for developing cell selection strategies for neural cell-based therapies.

A symphony of signals conducts early and late stages of adult neurogenesis

Neurogenesis is continually occurring in two regions within the mammalian central nervous system (CNS) and increasing evidence suggests that it is important for selective learning and memory. How this plasticity is maintained in isolated niches within mature networks has been extensively studied in recent years, and a large body of evidence has accumulated describing many different regulatory factors and points of regulation. In this review, we attempt to organize the current research by summarizing findings affecting early neurogenesis: during proliferation, fate commitment and migration, versus late neurogenesis: including dendritic development, synaptic integration, and survival. We discuss the roles of three different classes of factors regulating early and late phases of neurogenesis: intrinsic factors, extrinsic factors, and neurotransmitters. Finally, we suggest that neurotransmitters may act upstream from extracellular other factors and cell-intrinsic mechanisms by coupling network activity to the niche microenvironment and intracellular machinery to ultimately regulate neurogenesis.

Calcium-permeable AMPA receptors mediate glutamatergic signaling in neural precursor cells of the postnatal olfactory bulb

Neural precursor cells (NPCs) in the mammalian olfactory bulb give rise to local inhibitory neurons that integrate into existing circuitry throughout adult life. However, the functional properties of neurotransmitter receptors expressed by NPCs are not well understood. In this study, we use patch-clamp recording and calcium imaging to explore the properties of glutamate receptors expressed by NPCs in the olfactory bulb subependymal layer. We find that calcium-permeable AMPA receptors (AMPARs) are the major receptor type underlying glutamatergic signaling in olfactory bulb NPCs. We also show that when transmitter uptake is reduced, glutamate spillover from distant nerve terminals in the olfactory bulb can activate nonsynaptic NPC AMPARs and generate increases in intracellular calcium. Together, these results suggest that Ca(2+) influx via AMPARs may contribute to calcium-dependent processes that govern NPC differentiation and maturation.

Analysis of stem cell lineage progression in the neonatal subventricular zone identifies EGFR+/NG2- cells as transit-amplifying precursors

In the adult subventricular zone (SVZ), astroglial stem cells generate transit-amplifying precursors (TAPs). Both stem cells and TAPs form clones in response to epidermal growth factor (EGF). However, in vivo, in the absence of sustained EGF receptor (EGFR) activation, TAPs divide a few times before differentiating into neuroblasts. The lack of suitable markers has hampered the analysis of stem cell lineage progression and associated functional changes in the neonatal germinal epithelium. Here we purified neuroblasts and clone-forming precursors from the neonatal SVZ using expression levels of EGFR and polysialylated neural cell adhesion molecule (PSANCAM). As in the adult SVZ, most neonatal clone-forming precursors did not express the neuroglia proteoglycan 2 (NG2) but displayed characteristics of TAPs, and only a subset exhibited antigenic characteristics of astroglial stem cells. Both precursors and neuroblasts were PSANCAM(+); however, neuroblasts also expressed doublecortin and functional voltage-dependent Ca(2+) channels. Neuroblasts and precursors had distinct outwardly rectifying K(+) current densities and passive membrane properties, particularly in precursors contacting each other, because of the contribution of gap junction coupling. Confirming the hypothesis that most are TAPs, cell tracing in brain slices revealed that within 2 days the majority of EGFR(+) cells had exited the cell cycle and differentiated into a progenitor displaying intermediate antigenic and functional properties between TAPs and neuroblasts. Thus, distinct functional and antigenic properties mark stem cell lineage progression in the neonatal SVZ.

GABA increases Ca2+ in cerebellar granule cell precursors via depolarization: implications for proliferation

The amino acids glutamate and gamma-aminobutyric acid (GABA) have primarily been characterized as the most prevalent excitatory and inhibitory, respectively, neurotransmitters in the vertebrate central nervous system. However, the role of these signaling molecules extends far beyond the synapse. GABA, glutamate, and their complement of receptors are essential signaling molecules that regulate developmental processes in both embryonic and young adult mammals. In this review, we describe the current knowledge on the role of GABA and glutamate in development, focusing on the perinatal cerebellum. We will then present novel data suggesting that GABA depolarizes granule cell precursors via GABA(A) receptors, which leads to calcium increases in these cells. Finally, we will consider the role of GABA and glutamate signaling on cell proliferation and perhaps neural cancers. From our review of the literature and these data, we hypothesize that GABA(A) receptors and metabotropic glutamate receptors may be a novel target for the pharmacological regulation of the cerebellar tumors, medulloblastomas.

Increase of intracellular Ca2+ by adenine and uracil nucleotides in human midbrain-derived neuronal progenitor cells

Nucleotides play an important role in brain development and may exert their action via ligand-gated cationic channels or G protein-coupled receptors. Patch-clamp measurements indicated that in contrast to AMPA, ATP did not induce membrane currents in human midbrain derived neuronal progenitor cells (hmNPCs). Various nucleotide agonists concentration-dependently increased [Ca(2+)](i) as measured by the Fura-2 method, with the rank order of potency ATP>ADP>UTP>UDP. A Ca(2+)-free external medium moderately decreased, whereas a depletion of the intracellular Ca(2+) storage sites by cyclopiazonic acid markedly depressed the [Ca(2+)](i) transients induced by either ATP or UTP. Further, the P2Y(1) receptor antagonistic PPADS and MRS 2179, as well as the nucleotide catalyzing enzyme apyrase, allmost abolished the effects of these two nucleotides. However, the P2Y(1,2,12) antagonistic suramin only slightly blocked the action of ATP, but strongly inhibited that of UTP. In agreement with this finding, UTP evoked the release of ATP from hmNPCs in a suramin-, but not PPADS-sensitive manner. Immunocytochemistry indicated the co-localization of P2Y(1,2,4)-immunoreactivities (IR) with nestin-IR at these cells. In conclusion, UTP may induce the release of ATP from hmNPCs via P2Y(2) receptor-activation and thereby causes [Ca(2+)](i) transients by stimulating a P2Y(1)-like receptor.

L-type calcium channels govern calcium signaling in migrating newborn neurons in the postnatal olfactory bulb

Newborn inhibitory neurons migrate into existing neural circuitry in the olfactory bulb throughout the lifetime of adult mammals. While many factors contribute to the maturation of neural circuits, intracellular calcium is believed to play an important role in regulating cell migration and the development of neural systems. However, the factors underlying calcium signaling within newborn neurons in the postnatal olfactory bulb are not well understood. Here, we show that migrating, immature neurons in the olfactory bulb subependymal layer (SEL) undergo spontaneous and depolarization-evoked intracellular calcium transients mediated by high-voltage-activated L-type calcium channels. In contrast to migrating immature neurons in other brain regions, modulation of calcium transients in SEL cells does not alter their rate of migration.

AQP4 knockout impairs proliferation, migration and neuronal differentiation of adult neural stem cells

Aquaporin-4 (AQP4), a key molecule for maintaining water and ion homeostasis in the central nervous system, is expressed in adult neural stem cells (ANSCs) as well as astrocytes. However, little is known about the functions of AQP4 in the ANSCs in vitro. Here we show that AQP4 knockout inhibits the proliferation, survival, migration and neuronal differentiation of ANSCs derived from the subventricular zone of adult mice. Flow cytometric cell cycle analysis revealed that AQP4 knockout increased the basal apoptosis and induced a G2-M arrest in ANSCs. Using Fluo-3 Ca2+ imaging, we show that AQP4 knockout alters the spontaneous Ca2+ oscillations by frequency enhancement and amplitude suppression, and suppresses KCl-induced Ca2+ influx. AQP4 knockout downregulated the expression of connexin43 and the L-type voltage-gated Ca2+ channel CaV1.2 subtype in ANSCs. Together, these findings suggest that AQP4 plays a crucial role in regulating the proliferation, migration and differentiation of ANSCs, and this function of AQP4 is probably mediated by its action on intracellular Ca2+ dynamics.

Tonic activation of GLUK5 kainate receptors decreases neuroblast migration in whole-mounts of the subventricular zone

In the postnatal subventricular zone (SVZ), neuroblasts migrate in chains along the lateral ventricle towards the olfactory bulb. AMPA/kainate receptors as well as metabotropic glutamate receptors subtype 5 (mGluR5) are expressed in SVZ cells. However, the cells expressing these receptors and the function of these receptors remain unexplored. We thus examined whether SVZ neuroblasts express mGluR5 and Ca(2+)-permeable kainate receptors in mouse slices. Doublecortin (DCX)-immunopositive cells (i.e. neuroblasts) immunostained positive for mGluR5 and GLU(K5-7)-containing kainate receptors. RT-PCR from approximately 10 GFP-fluorescent cell aspirates obtained in acute slices from transgenic mice expressing green fluorescent protein (GFP) under the DCX promoter showed mGluR5 and GLU(K5) receptor mRNA in SVZ neuroblasts. Patch-clamp data suggest that approximately 60% of neuroblasts express functional GLU(K5)-containing receptors. Activation of mGluR5 and GLU(K5)-containing receptors induced Ca(2+) increases in 50% and 60% of SVZ neuroblasts, respectively, while most neuroblasts displayed GABA(A)-mediated Ca(2+) responses. To examine the effects of these receptors on the speed of neuroblast migration, we developed a whole-mount preparation of the entire lateral ventricle from postnatal day (P) 20-25 DCX-GFP mice. The GABA(A) receptor (GABA(A)R) antagonist bicuculline increased the speed of neuroblast migration by 27%, as previously reported in acute slices. While the mGluR5 antagonist MPEP did not affect the speed of neuroblast migration, the homomeric and heteromeric GLU(K5) receptor antagonists, NS3763 and UB302, respectively, increased the migration speed by 38%. These data show that although both GLU(K5) receptor and mGluR5 activations increase Ca(2+) in neuroblasts, only GLU(K5) receptors tonically reduce the speed of neuroblast migration along the lateral ventricle.

Connexin 43 delimits functional domains of neurogenic precursors in the spinal cord

The cells lining the central canal (CC) of the spinal cord derive from the ventral part of the neural tube and, in some vertebrates, are responsible for the functional recovery after spinal cord injury. The region that surrounds the CC in the turtle contains proliferating cells that seem to generate both glia and neurons. Understanding the biology of spinal progenitors with the potential to generate new neurons "in situ" is important for cell replacement therapies. Here, we aimed to identify and characterize precursor cells in the spinal cord of Trachemys dorbignyi. To evaluate the population of proliferating cells, 5-bromo-2'-deoxyuridine (BrdU) was injected every 4 h (50 microg/g, i.p.) during 24 h. We found BrdU(+) nuclei around the CC with a higher density in the lateral quadrants, in which whole-cell patch-clamp recordings showed extensive dye coupling of cells. Carbenoxolone (100 microM) increased the input resistance, suggesting strong gap junction coupling among precursors. The expression of brain lipid binding protein (a marker of a subtype of radial glia) and Pax6 matched the location of clusters, suggesting these cells belonged to a domain of neurogenic precursors. These domains were delimited by a high density of connexin 43 (Cx43) located on the endfeet of CC contacting cells. Our findings indicate that spinal precursors share basic properties with those in the embryo and neurogenic niches of the adult brain, and support a key role of functional clustering via Cx43 in spinal cord neurogenesis.

Delayed rectifier and A-type potassium channels associated with Kv 2.1 and Kv 4.3 expression in embryonic rat neural progenitor cells

BACKGROUND

Because of the importance of voltage-activated K(+) channels during embryonic development and in cell proliferation, we present here the first description of these channels in E15 rat embryonic neural progenitor cells derived from the subventricular zone (SVZ). Activation, inactivation, and single-channel conductance properties of recorded progenitor cells were compared with those obtained by others when these Kv gene products were expressed in oocytes.

METHODOLOGY/PRINCIPAL FINDINGS

Neural progenitor cells derived from the subventricular zone of E15 embryonic rats were cultured under conditions that did not promote differentiation. Immunocytochemical and Western blot assays for nestin expression indicated that almost all of the cells available for recording expressed this intermediate filament protein, which is generally accepted as a marker for uncommitted embryonic neural progenitor cells. However, a very small numbers of the cells expressed GFAP, a marker for astrocytes, O4, a marker for immature oligodendrocytes, and betaIII-tubulin, a marker for neurons. Using immunocytochemistry and Western blots, we detected consistently the expression of Kv2.1, and 4.3. In whole-cell mode, we recorded two outward currents, a delayed rectifier and an A-type current.

CONCLUSIONS/SIGNIFICANCE

We conclude that Kv2.1, and 4.3 are expressed in E15 SVZ neural progenitor cells, and we propose that they may be associated with the delayed-rectifier and the A-type currents, respectively, that we recorded. These results demonstrate the early expression of delayed rectifier and A-type K(+) currents and channels in embryonic neural progenitor cells prior to the differentiation of these cells.

Electrophysiological characterization of neural stem/progenitor cells during in vitro differentiation: Study with an immortalized neuroectodermal cell line

Despite the accumulating data on the molecular and cell biological characteristics of neural stem/progenitor cells, their electrophysiological properties are not well understood. In the present work, changes in the membrane properties and current profiles were investigated in the course of in vitro-induced neuron formation in NE-4C cells. Induction by retinoic acid resulted in neuronal differentiation of about 50% of cells. Voltage-dependent Na+ currents appeared early in neuronal commitment, often preceding any morphological changes. A-type K+ currents were detected only at the stage of network formation by neuronal processes. Flat, epithelial- like, nestin-expressing progenitors persisted beside differentiated neurons and astrocytes. Stem/progenitor cells were gap junction coupled and displayed large, symmetrical, voltage-independent currents. By the blocking of gap junction communication, voltage-independent conductance was significantly reduced, and delayed-rectifying K+ currents became detectable. Our data indicate that voltage-independent symmetrical currents and gap junction coupling are characteristic physiological features of neural stem and progenitor cells regardless of the developmental state of their cellular environment.

Local origin and activity-dependent generation of nestin-expressing protoplasmic astrocytes in CA1

Since reports that precursor cells in the adult subventricular zone (SVZ) contribute to regenerative neuro- and gliogenesis in CA1, we wondered whether a similar route of migration might also exist under physiological conditions. Permanent labeling of SVZ precursor cells with a lentiviral vector for green fluorescent protein did not reveal any migration from the SVZ into CA1 in the intact murine brain. However, in a nestin-GFP reporter mouse we found proliferating cells within the corpus callosum/alveus region expressing nestin and glial fibrillary acidic protein similar to precursor cells in the neighboring neurogenic region of the adult dentate gyrus. Within 3 weeks of BrdU administration, BrdU-positive nestin-GFP-expressing protoplasmic astrocytes emerged in CA1. Similar to precursor cells isolated from the dentate gyrus and the SVZ, nestin-GFP-expressing cells from corpus callosum/alveus were self-renewing and multipotent in vitro, whereas cells isolated from CA1 were not. Nestin-GFP-expressing cells in CA1 differentiated into postmitotic astrocytes characterized by S100beta expression. No new neurons were found in CA1. The number of nestin-GFP-expressing astrocytes in CA1 was increased by environmental enrichment. We conclude that astrogenesis in CA1 is influenced by environmental conditions. However, SVZ precursor cells do not contribute to physiological cellular plasticity in CA1.

The doublecortin-expressing population in the developing and adult brain contains multipotential precursors in addition to neuronal-lineage cells

Doublecortin (DCX) has recently been promulgated as a selective marker of cells committed to the neuronal lineage in both the developing and the adult brain. To explore the potential of DCX-positive (DCX+) cells more stringently, these cells were isolated by flow cytometry from the brains of transgenic mice expressing green fluorescent protein under the control of the DCX promoter in embryonic, early postnatal, and adult animals. It was found that virtually all of the cells (99.9%) expressing high levels of DCX (DCX(high)) in the embryonic brain coexpressed the neuronal marker betaIII-tubulin and that this population contained no stem-like cells as demonstrated by lack of neurosphere formation in vitro. However, the DCX+ population from the early postnatal brain and the adult subventricular zone and hippocampus, which expressed low levels of DCX (DCX(low)), was enriched for neurosphere-forming cells, with only a small subpopulation of these cells coexpressing the neuronal markers betaIII-tubulin or microtubule-associated protein 2. Similarly, the DCX(low) population from embryonic day 14 (E14) brain contained neurosphere-forming cells. Only the postnatal cerebellum and adult olfactory bulb contained some DCX(high) cells, which were shown to be similar to the E14 DCX(high) cells in that they had no stem cell activity. Electrophysiological studies confirmed the heterogeneous nature of DCX+ cells, with some cells displaying characteristics of immature or mature neurons, whereas others showed no neuronal characteristics whatsoever. These results indicate that DCX(high) cells, regardless of location, are restricted to the neuronal lineage or are bone fide neurons, whereas some DCX(low) cells retain their multipotentiality.

Selective blockage of Kv1.3 and Kv3.1 channels increases neural progenitor cell proliferation

The modulation of cell proliferation in neural progenitor cells (NPCs) is believed to play a role in neuronal regeneration. Recent studies showed that K(+) channel activity influenced cell proliferation. Therefore, we examined NPCs for K(+) channels and tested whether NPC self renewing can be modulated by synthetic K(+) channel modulators. The whole-cell K(+) current was partly K(+) dependent and showed a cumulative inactivating component. Two tetra-ethyl-ammonium ion (TEA)-sensitive K(+) currents with different voltage dependencies ( = 65 microm, E(50) = -0.3 +/- 1.3 mV and = 8 mm, E(50) = -15.2 +/- 2.8 mV) and an almost TEA-insensitive current were identified. Kaliotoxin blocked approximately 50% of the entire K(+) currents (IC(50) = 0.25 nm). These properties resembled functional characteristics of K(v)1.4, K(v)1.3 and K(v)3.1 channels. Transcripts for these channels, as well as proteins for K(v)1.3 and K(v)3.1, were identified. Immunocytochemical staining revealed K(v)1.3 and K(v)3.1 K(+) channel expression in almost all NPCs. The blockage of K(v)3.1 by low concentrations of TEA, as well as the blockage of K(v)1.3 by Psora-4, increased NPC proliferation. These findings underline the regulatory role of K(+) channels on the cell cycle and imply that K(+) channel modulators might be used therapeutically to activate endogenous NPCs.