Changes of KCl sensitivity of proliferating neural progenitors during in vitro neurogenesis

Using Human iPSC-Derived Neurons to Uncover Activity-Dependent Non-Coding RNAs

Humans are arguably the most complex organisms present on Earth with their ability to imagine, create, and problem solve. As underlying mechanisms enabling these capacities reside in the brain, it is not surprising that the brain has undergone an extraordinary increase in size and complexity within the last few million years. Human induced pluripotent stem cells (hiPSCs) can be differentiated into many cell types that were virtually inaccessible historically, such as neurons. Here, we used hiPSC-derived neurons to investigate the cellular response to activation at the transcript level. Neuronal activation was performed with potassium chloride (KCl) and its effects were assessed by RNA sequencing. Our results revealed the involvement of long non-coding RNAs and human-specific genetic variants in response to neuronal activation and help validate hiPSCs as a valuable resource for the study of human neuronal networks. In summary, we find that genes affected by KCl-triggered activation are implicated in pathways that drive cell proliferation, differentiation, and the emergence of specialized morphological features. Interestingly, non-coding RNAs of various classes are amongst the most highly expressed genes in activated hiPSC-derived neurons, thus suggesting these play crucial roles in neural pathways and may significantly contribute to the unique functioning of the human brain.

Differentiation-Dependent Motility-Responses of Developing Neural Progenitors to Optogenetic Stimulation

During neural tissue genesis, neural stem/progenitor cells are exposed to bioelectric stimuli well before synaptogenesis and neural circuit formation. Fluctuations in the electrochemical potential in the vicinity of developing cells influence the genesis, migration and maturation of neuronal precursors. The complexity of the in vivo environment and the coexistence of various progenitor populations hinder the understanding of the significance of ionic/bioelectric stimuli in the early phases of neuronal differentiation. Using optogenetic stimulation, we investigated the in vitro motility responses of radial glia-like neural stem/progenitor populations to ionic stimuli. Radial glia-like neural stem cells were isolated from CAG^loxpStop^loxpChR2(H134)-eYFP transgenic mouse embryos. After transfection with Cre-recombinase, ChR2(channelrhodopsin-2)-expressing and non-expressing cells were separated by eYFP fluorescence. Expression of light-gated ion channels were checked by patch clamp and fluorescence intensity assays. Neurogenesis by ChR2-expressing and non-expressing cells was induced by withdrawal of EGF from the medium. Cells in different (stem cell, migrating progenitor and maturing precursor) stages of development were illuminated with laser light (λ = 488 nm; 1.3 mW/mm²; 300 ms) in every 5 min for 12 h. The displacement of the cells was analyzed on images taken at the end of each light pulse. Results demonstrated that the migratory activity decreased with the advancement of neuronal differentiation regardless of stimulation. Light-sensitive cells, however, responded on a differentiation-dependent way. In non-differentiated ChR2-expressing stem cell populations, the motility did not change significantly in response to light-stimulation. The displacement activity of migrating progenitors was enhanced, while the motility of differentiating neuronal precursors was markedly reduced by illumination.

Retinoid machinery in distinct neural stem cell populations with different retinoid responsiveness

Retinoic acid (RA) is present at sites of neurogenesis in both the embryonic and adult brain. While it is widely accepted that RA signaling is involved in the regulation of neural stem cell differentiation, little is known about vitamin A utilization and biosynthesis of active retinoids in the neurogenic niches, or about the details of retinoid metabolism in neural stem cells and differentiating progenies. Here we provide data on retinoid responsiveness and RA production of distinct neural stem cell/neural progenitor populations. In addition, we demonstrate differentiation-related changes in the expression of genes encoding proteins of the retinoid machinery, including components responsible for uptake (Stra6) and storage (Lrat) of vitamin A, transport of retinoids (Rbp4, CrbpI, CrabpI-II), synthesis (Rdh10, Raldh1-4), degradation of RA (Cyp26a1-c1) and RA signaling (Rarα,β,γ, Rxrα,β,γ). We show that both early embryonic neuroectodermal (NE-4C) stem cells and late embryonic or adult derived radial glia like progenitors (RGl cells) are capable to produce bioactive retinoids but respond differently to retinoid signals. However, while neuronal differentiation of RGl cells can not be induced by RA, neuron formation by NE-4C cells is initiated by both RA and RA-precursors (retinol or retinyl acetate). The data indicate that endogenous RA production, at least in some neural stem cell populations, may result in autocrine regulation of neuronal differentiation.

Prolonged membrane depolarization enhances midbrain dopamine neuron differentiation via epigenetic histone modifications

Understanding midbrain dopamine (DA) neuron differentiation is of importance, because of physiological and clinical implications of this neuronal subtype. We show that prolonged membrane depolarization induced by KCl treatment promotes DA neuron differentiation from neural precursor cells (NPCs) derived from embryonic ventral midbrain (VM). Interestingly, the depolarization-induced increase of DA neuron yields was not abolished by L-type calcium channel blockers, along with no depolarization-mediated change of intracellular calcium level in the VM-derived NPCs (VM-NPCs), suggesting that the depolarization effect is due to a calcium-independent mechanism. Experiments with labeled DA neuron progenitors indicate that membrane depolarization acts at the differentiation fate determination stage and promotes the expression of DA phenotype genes (tyrosine hydroxylase [TH] and DA transporter [DAT]). Recruitment of Nurr1, a transcription factor crucial for midbrain DA neuron development, to the promoter of TH gene was enhanced by depolarization, along with increases of histone 3 acetylation (H3Ac) and trimethylation of histone3 on lysine 4 (H3K4m3), and decreases of H3K9m3 and H3K27m3 in the consensus Nurr1 binding regions of TH promoter. Depolarization stimuli on differentiating VM-NPCs also induced dissociation of methyl CpG binding protein 2 and related repressor complex molecules (repressor element-1 silencing transcription factor corepressor and histone deacetylase 1) from the CpG sites of TH and DAT promoters. Based on these findings, we suggest that membrane depolarization promotes DA neuron differentiation by opening chromatin structures surrounding DA phenotype genes and inhibiting the binding of corepressors, thus allowing transcriptional activators such as Nurr1 to access DA neuron differentiation gene promoter regions.

Survival and differentiation of neuroectodermal cells with stem cell properties at different oxygen levels

Freeze-lesioned regions of the forebrain cortex provide adequate environment for growth of non-differentiated neural progenitors, but do not support their neuron formation. Reduced oxygen supply, among numerous factors, was suspected to impair neuronal cell fate commitment. In the present study, proliferation and differentiation of neural stem/progenitor cells were investigated at different oxygen levels both in vitro and in vivo. Low (1% atmospheric) oxygen supply did not affect the in vitro viability and proliferation of stem cells or the transcription of "stemness" genes but impaired the viability of committed neuronal progenitors and the expression of proneural and neuronal genes. Consequently, the rate of in vitro neuron formation was markedly reduced under hypoxic conditions. In vivo, neural stem/progenitor cells survived and proliferated in freeze-lesioned adult mouse forebrains, but did not develop into neurons. Hypoperfusion-caused hypoxia in lesioned cortices was partially corrected by hyperbaric oxygen treatment (HBOT). HBOT, while reduced the rate of cell proliferation at the lesion site, resulted in sporadic neuron formation from implanted neural stem cells. The data indicate that in hypoxic brain areas, neural stem cells survive and proliferate, but neural tissue-type differentiation can not proceed. Oxygenation renders the damaged brain areas more permissive for tissue-type differentiation and may help the integration of neural stem/progenitor cells.

Effects of extracellular acidic-alkaline stresses on trigeminal ganglion neurons in the mouse embryo in vivo

Acidic-alkaline stresses caused by ischemia and hypoglycemia induce neuronal cell death resulting from intracellular pH disturbance. The effects of acidic-alkaline disturbance on the trigeminal ganglion (TG) neurons of the embryonic mouse were investigated by caspase-3-immunohistochemistry and Nissl staining. TG neurons exhibited apoptosis in 3.08 ± 0.55% of neurons in intact embryos at day 16. Intraperitoneal injection of alkaline solution (pH 8.97; 0.005-0.1 M K₂HPO₄ or 0.01-0.04 M KOH) into the embryo at embryonic day 15 significantly increased the number of apoptotic neurons in the TG at embryonic day 16 with dependence on concentration (3.40-6.05 and 2.93-5.55%, respectively). On the other hand, acidic solutions (pH 4.4; 0.01-0.2 M KH₂PO₄ slightly, but not significantly, increased the number of apoptotic cells (3.64-5.15%, without dependence on concentration). Neutral solutions (pH 7.4; 0.01-0.2 M potassium phosphate buffer) had no effect on neuronal survival in the TG (2.89-3.48%). The results indicated that alkaline stress significantly increased apoptosis in the developing nervous system, but acidic stress did not.

Na,K-ATPase activity regulates AMPA receptor turnover through proteasome-mediated proteolysis

Neuronal activity largely depends on two key components on the membrane: the Na,K-ATPase (NKA) that maintains the ion gradients and sets the foundation of excitability, and the ionotropic glutamatergic AMPA receptors (AMPARs) through which sodium influx forms the driving force for excitation. Because the frequent sodium transients from glutamate receptor activity need to be efficiently extruded, a functional coupling between NKA and AMPARs should be a necessary cellular device for synapse physiology. We show that NKA is enriched at synapses and associates with AMPARs. NKA dysfunction induces a rapid reduction in AMPAR cell-surface expression as well as total protein abundance, leading to a long-lasting depression in synaptic transmission. AMPAR proteolysis requires sodium influx, proteasomal activity and receptor internalization. These data elucidate a novel mechanism by which NKA regulates AMPAR turnover and thereby synaptic strength and brain function.

Different behaviour of implanted stem cells in intact and lesioned forebrain cortices

Cell-replacement therapy promises a useful tool to regenerate compromised brain tissue, but the interaction between grafted cells and host tissues is not well understood. In these studies, the fates of neuroectodermal stem cells were compared in 'healthy' or damaged mouse forebrains. One-cell derived, fluorescent GFP-4C neural stem cells were implanted into normal and cold-lesioned mouse cortices. The fates of implanted cells were followed by histological and immunocytochemical assays for a 55-day postimplantation period. Cells were recultivated from lesioned cortices and characterized by cell cycle parameters, chromosome numbers, immunocytochemical markers and in vitro inducibility. Their intracerebral fates were checked upon re-implanting into 'healthy' mouse brain cortices. GFP-4C cells, giving rise to neurones and astrocytes upon in vitro induction, failed to differentiate in either normal or lesioned cortical tissues. The rate of proliferation and the length of the survival, however, depended on the host environment, markedly. In intact cortices, implanted cells formed compact, isolated aggregates and their survival did not exceed 4 weeks. In compromised cortices, GFP-4C cells survived longer than 8 weeks and repopulated the decayed region. The morphology, viability, immunocytochemical properties, in vitro inducibility and chromosome number of cells recultivated from lesioned cortices were identical to those of the master cells. Long-term survival and repopulating capability were due to signals present in the lesioned, but missing from the intact cortical environment. The results underline the importance of host environment in the fate determination of grafted cells and emphasize the need to understand the 'roles' of recipient tissues for potential cell-replacement methodologies.

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.

Transplantation of embryonic neuroectodermal progenitor cells into the site of a photochemical lesion: Immunohistochemical and electrophysiological analysis

GFP labeled/NE-4C neural progenitor cells cloned from primary neuroectodermal cultures of p53- mouse embryos give rise to neurons when exposed to retinoic acid in vitro. To study their survival and differentiation in vivo, cells were transplanted into the cortex of 6-week-old rats, 1 week after the induction of a photochemical lesion or into noninjured cortex. The electrophysiological properties of GFP/NE-4C cells were studied in vitro (8-10 days after differentiation induction) and 4 weeks after transplantation using the whole-cell patch-clamp technique, and immunohistochemical analyses were carried out. After transplantation into a photochemical lesion, a large number of cells survived, some of which expressed the astrocytic marker GFAP. GFP/GFAP-positive cells, with an average resting membrane potential (Vrest) of -71.9 mV, displayed passive time- and voltage-independent K+ currents and, additionally, voltage-dependent A-type K+ currents (KA) and/or delayed outwardly rectifying K+ currents (KDR). Numerous GFP-positive cells expressed NeuN, betaIII-tubulin, or 68 kD neurofilaments. GFP/betaIII-tubulin-positive cells, with an average Vrest of -61.6 mV, were characterized by the expression of KA and KDR currents and tetrodotoxin-sensitive Na+ currents. GFP/NE-4C cells also gave rise to oligodendrocytes, based on the detection of oligodendrocyte-specific markers. Our results indicate that GFP/NE-4C neural progenitors transplanted into the site of a photochemical lesion give rise to neurons and astrocytes with membrane properties comparable to those transplanted into noninjured cortex. Therefore, GFP/NE-4C cells provide a suitable model for studying neuro- and gliogenesis in vivo. Further, our results suggest that embryonic neuroectodermal progenitor cells may hold considerable promise for the repair of ischemic brain lesions.

SCL, GATA-2 and Lmo2 expression in neurogenesis

SCL, Lmo2 and GATA factors form common transcription complexes during hematopoietic differentiation. The overlapping expression of SCL with GATA-2 and GATA-3 in the developing brain indicated that these factors might collaborate also in the course of neural tissue differentiation. The expression pattern of Lmo2 in the developing CNS, however, is not well understood. Here, we show that neural cells in the early embryonic chick mid- and hindbrain express SCL and GATA-2, while Lmo2 is expressed only in vascular elements. The lack of Lmo2 transcripts in neural cells demonstrated that SCL and GATA-2 cannot form common complexes with Lmo2 in the developing brain. In the course of neural tissue genesis, GATA-2 mRNA appeared prior to the SCL transcript. While GATA-2 expression decreased with maturation, SCL expression persisted at a high level also in post-neurogenic periods. The temporal pattern of SCL and GATA-2/3 expression was investigated also in vitro, in the course of induced neurogenesis by NE-4C neural stem cells. While GATA-2 expression increased from the very beginning of differentiation, SCL expression appeared only in more differentiated cells expressing proneural genes. GATA-3 expression, on the other hand, was detected only in advanced stages of the neuronal maturation, which were characterised by the activation of the Math2 neuronal gene. Similarly to the hematopoietic differentiation, GATA-2 expression precedes the activation of both SCL and GATA-3, and may play roles in the activation of the SCL gene in neuronal development. In contrast to hematopoietic differentiation, however, our results failed to demonstrate co-assembling of GATA factors or SCL with Lmo2. While overlapping expression of GATA-2/3 and SCL was detected, Lmo2 activation could not be demonstrated in neural cells in the investigated period of neuronal development.

Studies on the use of NE-4C embryonic neuroectodermal stem cells for targeting brain tumour

Neural stem cells were suggested to migrate to and invade intracranial gliomas. In the presented studies, interactions of NE-4C embryonic neural stem cells were investigated with C6 and Gl261, LL and U87, glioblastoma cells or with primary astrocytes. Glioma-derived humoral factors did not influence the proliferation of stem cells. NE-4C-derived humoral factors did not alter the proliferation of Gl261 and U87 cells, but increased the mitotic activity of C6 cells and that of astrocytes. In chimera-aggregates, all types of glioma cells co-aggregated with astrocytes, but most of them segregated from stem cells. Complete intercalation of stem and tumour cells was detected only in chimera-aggregates of Gl261 glioma and NE-4C cells. If mixed suspensions of NE-4C and Gl261 cells were injected into the brain, stem cells survived and grew inside the tumour mass. NE-4C stem cells, however, did not migrate towards the tumour, if implanted near to Gl261 tumours established in the adult mouse forebrain. The observations indicate that not all types of stem cells could be used for targeting all sorts of brain tumours.

Induction of GABAergic phenotype in a neural stem cell line for transplantation in an excitotoxic model of Huntington's disease

The implementation of cell replacement therapies for Huntington's disease using multipotent neural stem cells (NSCs) requires the specific differentiation into gamma-aminobutyric acid (GABA) neuronal subtype before transplantation. Here we present an efficient culture procedure that induces stable GABAergic neurons from the immortalized striatal neural stem cell line ST14A. This process requires sequential retinoic acid treatment and KCl depolarization. Initial addition of 10 microM retinoic acid increased cell survival and promoted neuronal differentiation. Subsequent stimulation with 40 mM KCl induced specific differentiation into GABAergic neurons, yielding 74% of total cultured cells. KCl-evoked Ca(2+) influx reduced cell proliferation and nestin expression, and induced neurite outgrowth and GABAergic markers as well as GABA contents, release, and uptake. Characterization of the integration, survival, and phenotype of these predifferentiated GABAergic neurons following transplantation into the adult brain in a model of Huntington's disease revealed long-term survival in quinolinate-lesioned striata. Under these conditions, cells maintained their GABAergic phenotype and elaborated neurite processes with synaptic contacts with endogenous neurons. In conclusion, we have generated a homogeneous population of functional GABAergic neurons from a neural stem cell line, which survive and maintain their acquired fate in vivo. These data may lend support to the possibility of cell replacement therapies for Huntington's disease using neural stem cells.

Ischemia-induced increase in long-term potentiation is warded off by specific calpain inhibitor PD150606

In the present study, the effect of specific, membrane-permeable calpain inhibitor, PD150606, was analysed on synaptic efficacy in in vitro brain slices experiments after ischemic insult of rats in vivo, and on cell viability in a glutamate excitotoxicity test in mouse cell culture. Bilateral common carotid artery ligation (BCCL) for 24 h markedly increased calpain activity and enhanced LTP induction in rat hippocampus, although the CA1 layer significantly shrank. The enhancement of LTP could be diminished by short-term application of PD150606 (40 microM) into the perfusion solution. Intracerebroventricular administration of PD150606 (100 microM) parallel with ischemic insult prevented LTP and effectively inhibited hippocampal calpain activity. Intracerebroventricularly applied PD150606 inhibited the CA1 layer shrinkage after common carotid ligation. High level of exogenous glutamate caused marked decrease of cell viability in mouse cerebellar granule cell cultures, which could be partly warded off by 20 microM PD150606. Our data witness that calpain action is intricately involved in the regulation of synaptic efficacy.

Fate of cloned embryonic neuroectodermal cells implanted into the adult, newborn and embryonic forebrain

NE-4C, one-cell derived neuroectodermal stem cells expressing a reporter gene--green fluorescent protein (GFP) or heat-resistant alkaline phosphatase (PLAP)--or prelabeled with bromodeoxyuridine (BrdU) were implanted into the forebrain of adult, new-born and fetal mice and into the mid- and forebrain vesicles of early chick embryos. The fate of implanted cells in the mouse and chick hosts was followed up to 6 and 2 weeks, respectively. Neural differentiation was monitored by detecting the expression of neuron-specific markers and GFAP. NE-4C cells integrated into the early embryonic brain tissue and developed into morphologically differentiated neurons. The same cells produced expanding tumor-like aggregates in the newborn forebrain and were expelled from the adult forebrain parenchyma. In the adult brain, long-term survival and integration of stem cells were revealed only in neurogenic zones. The data suggest that noncommitted, proliferating neuroectodermal progenitors can integrate into the brain tissue at time and site of tissue genesis.

Role of γ-aminobutyric acid in early neuronal development: Studies with an embryonic neuroectodermal stem cell clone

gamma-Aminobutyric acid (GABA) has been known to function as an autocrine/paracrine signal molecule in addition to its well-known inhibitory neurotransmitter function. Studies on the developing brain and on primary brain cell cultures provided evidence for a variety of GABA functions in periods preceding the formation of synapses. The exact role of GABA in the early neural development, however, is still not well understood. In this study, one-cell-derived NE-4C neuroectodermal stem cells were induced to form neurons and astrocytes in vitro, and the role of GABA was investigated in defined phases of neurogenesis. Noninduced NE-4C cells contained GABA, expressed GABA(A)R alpha subunits, and carried functional GABA(A) ion channels. A moderate cytoplasmic GABA content was detected during the entire period of differentiation. By the time of the formation of differentiated neurons, neuron-like cells with both high and low GABA content were clearly distinguishable. HPLC analysis indicated that NE-4C cells released GABA into their fluid environment during all stages of neuronal development. By using the patch-clamp technique, GABA-evoked currents were recorded during the entire proliferation/differentiation period, whereas a GABA-evoked increase in intracellular Ca(2+) was detected only during the maturation of postmitotic neuronal precursors. Bicuculline blocked both the ion currents and the [Ca(2+)](i) increase in response to GABA. Neuron formation was facilitated by GABA through GABA(A) ion channels during postmitotic differentiation, but not earlier during the phases of cell fate commitment. Although the data clearly demonstrate an early responsiveness to GABA, understanding the significance of GABA influence in early neural cell fate decisions will require further investigation.

Stage-dependent effects of cell-to-cell connections on in vitro induced neurogenesis

NE-4C, a p53-deficient, immortalized neuroectodermal progenitor cell line, was used to investigate the role and importance of cellular interactions in neural commitment and differentiation. NE-4C cells give rise to neurons and astrocytes in the presence of all-trans retinoic acid, if they can establish intercellular contacts. Aggregation per se, however, was insufficient to induce large-scale neuron formation. In the absence of RA, the majority of the aggregated cells died. For neuron formation, therefore, concerted actions of RA and cellular interaction were needed. Electron microscopic and electrophysiological studies revealed that gap junctions were formed between the cells. Persistent blockage of communication via gap junctions with gap junction blockers, however, had no effects on neuron formation. If cell-to-cell connections were disrupted on the fourth day after induction, the rate of neuron production increased significantly. The contact interactions formed between already committed progenitor cells seemed to hinder the formation of novel neurons. The process resembled the phenomenon called "lateral inhibition" first observed in the course of neurogenesis in Drosophila. Our results indicate that NE-4C cells provide a useful model system to investigate the role of contact communication during some early steps of neurogenesis.