Expression of Rab3A GTPase and other synaptic proteins is induced in differentiated NT2N neurons

Identification of β-Dystrobrevin as a Direct Target of miR-143: Involvement in Early Stages of Neural Differentiation

Duchenne Muscular Dystrophy, a genetic disorder that results in a gradual breakdown of muscle, is associated to mild to severe cognitive impairment in about one-third of dystrophic patients. The brain dysfunction is independent of the muscular pathology, occurs early, and is most likely due to defects in the assembly of the Dystrophin-associated Protein Complex (DPC) during embryogenesis. We have recently described the interaction of the DPC component β-dystrobrevin with members of complexes that regulate chromatin dynamics, and suggested that β-dystrobrevin may play a role in the initiation of neuronal differentiation. Since oxygen concentrations and miRNAs appear as well to be involved in the cellular processes related to neuronal development, we have studied how these factors act on β-dystrobrevin and investigated the possibility of their functional interplay using the NTera-2 cell line, a well-established model for studying neurogenesis. We followed the pattern of expression and regulation of β-dystrobrevin during the early stages of neuronal differentiation induced by exposure to retinoic acid (RA) under hypoxia as compared with normoxia, and found that β-dystrobrevin expression is regulated during RA-induced differentiation of NTera-2 cells. We also found that β-dystrobrevin pattern is delayed under hypoxic conditions, together with a delay in the differentiation and an increase in the proliferation rate of cells. We identified miRNA-143 as a direct regulator of β-dystrobrevin expression, demonstrated that β-dystrobrevin is expressed in the nucleus and showed that, in line with our previous in vitro results, β-dystrobrevin is a repressor of synapsin I in live cells. Altogether the newly identified regulatory pathway miR-143/β-dystrobrevin/synapsin I provides novel insights into the functions of β-dystrobrevin and opens up new perspectives for elucidating the molecular mechanisms underlying the neuronal involvement in muscular dystrophy.

Characterization of the NT2-derived neuronal and astrocytic cell lines as alternative in vitro models for primary human neurons and astrocytes

Primary human fetal neurons and astrocytes (HFNs and HFAs, respectively) provide relevant cell types with which to study in vitro the mechanisms involved in various human neurological diseases, such as multiple sclerosis, Parkinson's disease, and Alzheimer's disease. However, the limited availability of human fetal cells poses a significant problem for the study of these diseases when a human cell model system is required. Thus, generating a readily available alternative cell source with the essential features of human neurons and astrocytes is necessary. The human teratoma-derived NTera2/D1 (NT2) cell line is a promising tool from which both neuronal and glial cells can be generated. Nevertheless, a direct comparison of NT2 neurons and primary HFNs in terms of their morphology physiological and chemical properties is still missing. This study directly compares NT2-derived neurons and primary HFNs using immunocytochemistry, confocal calcium imaging, high-performance liquid chromatography, and high-content analysis techniques. We investigated the morphological similarities and differences, levels of relevant amino acids, and internal calcium fluctuations in response to certain neurotransmitters/stimuli. We also compared NT2-derived astrocytes and HFAs. In most of the parameters tested, both neuronal and astrocytic cell types exhibited similarities to primary human fetal neurons and astrocytes. NT2-derived neurons and astrocytes are reliable in vitro tools and a renewable cell source that can serve as a valid alternative to HFNs/HFAs for mechanistic studies of neurological diseases.

Nucleoside drugs induce cellular differentiation by caspase-dependent degradation of stem cell factors

BACKGROUND

Stem cell characteristics are an important feature of human cancer cells and play a major role in the therapy resistance of tumours. Strategies to target cancer stem cells are thus of major importance for cancer therapy. Differentiation therapy by nucleoside drugs represents an attractive approach for the elimination of cancer stem cells. However, even if it is generally assumed that the activity of these drugs is mediated by their ability to modulate epigenetic pathways, their precise mode of action remains to be established. We therefore analysed the potential of three nucleoside analogues to induce differentiation of the embryonic cancer stem cell line NTERA 2 D1 and compared their effect to the natural ligand retinoic acid.

METHODOLOGY/PRINCIPAL FINDINGS

All nucleoside analogues analyzed, but not retinoic acid, triggered proteolytic degradation of the Polycomb group protein EZH2. Two of them, 3-Deazaneplanocin A (DZNep) and 2'-deoxy-5-azacytidine (decitabine), also induced a decrease in global DNA methylation. Nevertheless, only decitabine and 1beta-arabinofuranosylcytosine (cytarabine) effectively triggered neuronal differentiation of NT2 cells. We show that drug-induced differentiation, in contrast to retinoic acid induction, is caused by caspase activation, which mediates depletion of the stem cell factors NANOG and OCT4. Consistent with this observation, protein degradation and differentiation could be counteracted by co-treatment with caspase inhibitors or by depletion of CASPASE-3 and CASPASE-7 through dsRNA interference. In agreement with this, OCT4 was found to be a direct in-vitro-target of CASPASE-7.

CONCLUSIONS/SIGNIFICANCE

We show that drug-induced differentiation is not a consequence of pharmacologic epigenetic modulation, but is induced by the degradation of stem-cell-specific proteins by caspases. Our results thus uncover a novel pathway that induces differentiation of embryonic cancer stem cells and is triggered by the established anticancer drugs cytarabine and decitabine. These findings suggest new approaches for directly targeting the stem cell fraction of human tumours.

Killing of cancer cells by the photoactivatable protein kinase C inhibitor, calphostin C, involves induction of endoplasmic reticulum stress

Calphostin C (cal-C) is a photoactivatable inhibitor that binds to the regulatory domain of protein kinase C (PKC) and to other proteins that contain diacylglycerol/phorbol ester binding sites. Cal-C is cytotoxic against many types of cancer cells, yet the basis for this activity remains poorly understood. Here, we show that one of the earliest effects of cal-C is an impairment of glycoprotein export from the endoplasmic reticulum (ER), accompanied by formation of ER-derived vacuoles. Vacuolization of the ER is correlated with induction of an ER stress response that includes activation of c-Jun N-terminal kinase and protein kinase R-like ER kinase, as well as increased expression of CCAAT/enhancer binding protein homologous transcription factor (CHOP; GADD153). These effects of cal-C are not mimicked by staurosporine, an inhibitor of PKC catalytic activity, indicating that ER stress is due to interaction of cal-C with targets other than PKC. In conjunction with the induction of ER stress, breast carcinoma cells undergo caspase-dependent cell death with early activation of caspases 9 and 7 and cleavage of poly(ADP-ribose)polymerase. Reduction of CHOP expression by short hairpin RNA decreases the sensitivity of the cells to cal-C, suggesting that induction of apoptosis by cal-C is related, at least in part, to ER stress triggered by disruption of ER morphology and transport function. Antineoplastic drugs that work by inducting ER stress have shown promise in preclinical and clinical trials. Thus, the present findings raise the possibility that cal-C may be useful for photodynamic therapy based on induction of ER stress in some forms of cancer.

Nitric oxide and cyclic nucleotide signal transduction modulates synaptic vesicle turnover in human model neurons

The human Ntera2 (NT2) teratocarcinoma cell line can be induced to differentiate into post-mitotic neurons. Here, we report that the human NT2 neurons generated by a spherical aggregate cell culture method express increasing levels of typical pre-synaptic proteins (synapsin and synaptotagmin I) along the neurite depending on the length of in vitro culture. By employing an antibody directed against the luminal domain of synaptotagmin I and the fluorescent dye N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide, we show that depolarized NT2 neurons display calcium-dependent exo-endocytotic synaptic vesicle recycling. NT2 neurons express the neuronal isoform of neuronal nitric oxide synthase and soluble guanylyl cyclase (sGC), the major receptor for nitric oxide (NO). We tested whether NO signal transduction modulates synaptic vesicle turnover in human NT2 neurons. NO donors and cylic guanosine-monophosphate analogs enhanced synaptic vesicle recycling while a sGC inhibitor blocked the effect of NO donors. Two NO donors, sodium nitroprusside, and and N-Ethyl-2-(1-ethyl-2-hydroxy-2-nitrosohydrazino) ethanamine evoked vesicle exocytosis which was partially blocked by the sGC inhibitor. The activator of adenylyl cyclase, forskolin, and a cAMP analog induced synaptic vesicle recycling and exocytosis via a parallel acting protein kinase A pathway. Our data from NT2 neurons suggest that NO/cyclic nucleotide signaling pathways may facilitate neurotransmitter release in human brain cells.

Motifs and cis-regulatory modules mediating the expression of genes co-expressed in presynaptic neurons

An integrative strategy of comparative genomics, experimental and computational approaches reveals aspects of a regulatory network controlling neuronal-specific expression in presynaptic neurons.

Background

Hundreds of proteins modulate neurotransmitter release and synaptic plasticity during neuronal development and in response to synaptic activity. The expression of genes in the pre- and post-synaptic neurons is under stringent spatio-temporal control, but the mechanism underlying the neuronal expression of these genes remains largely unknown.

Results

Using unbiased in vivo and in vitro screens, we characterized the cis elements regulating the Rab3A gene, which is expressed abundantly in presynaptic neurons. A set of identified regulatory elements of the Rab3A gene corresponded to the defined Rab3A multi-species conserved elements. In order to identify clusters of enriched transcription factor binding sites, for example, cis-regulatory modules, we analyzed intergenic multi-species conserved elements in the vicinity of nine presynaptic genes, including Rab3A, that are highly and specifically expressed in brain regions. Sixteen transcription factor binding motifs were over-represented in these multi-species conserved elements. Based on a combined occurrence for these enriched motifs, multi-species conserved elements in the vicinity of 107 previously identified presynaptic genes were scored and ranked. We then experimentally validated the scoring strategy by showing that 12 of 16 (75%) high-scoring multi-species conserved elements functioned as neuronal enhancers in a cell-based assay.

Conclusions

This work introduces an integrative strategy of comparative genomics, experimental, and computational approaches to reveal aspects of a regulatory network controlling neuronal-specific expression of genes in presynaptic neurons.

Amyloid precursor protein is involved in staurosporine induced glial differentiation of neural progenitor cells

Staurosporine (STS) has been reported as not only a pro-apoptotic agent, but also a terminal differentiation inducer in several neuroblastoma cell lines. Here, we report involvement of amyloid precursor protein (APP) in a STS induced astrocytic differentiation of human neural progenitor cells (NT-2/D1). We found that STS-treated NT-2/D1 cells expressed astrocyte-specific glial fibrillary acidic protein (GFAP), aspartate transporter, and glutamate transporter-1 with a distinctive astrocytic morphology. STS treatment increased GFAP promoter activity and increased expression and secretion of APP in NT-2/D1 cell culture. Overexpressed APP enhanced GFAP promoter activity and expression of GFAP, while gene silencing of APP by RNA interference decreased GFAP expression. These results indicate involvement of APP in STS induced astrocytic differentiation of NT-2/D1 cells. Furthermore, suppression of ERK1/2 phosphorylation, which is known to regulate APP expression by a MEK1 inhibitor, PD098059, reduced both APP and GFAP expression in STS treated NT-2/D1 cells. Thus, STS may induce astrocytic differentiation of NT-2/D1 by increasing APP levels associate with activation of ERK pathway.

Neuronal differentiation of cultured human NTERA-2cl.D1 cells leads to increased expression of synapsins

The synapsin family consists of three neuronal-specific phosphoproteins associated with dynamic reorganization of the neuronal cytoskeleton. Synapsin I and II are implicated in axonal and synaptic differentiation, formation and maintenance, whereas the function of synapsin III is not as well defined. We report a significant transcriptional upregulation of all three synapsins (synapsin I, 2.1-fold; synapsin II, 2.6-fold; and synapsin III, 5.5-fold) by retinoic acid-induced differentiation of NTera-2cl.D1 cells, a human paradigm for neuronal differentiation. The observed stronger regulation of synapsin III might be due to still active neurite elongation and a rather early state of presynaptic maturation at the time-point investigated, as synapsin III was previously found to be highly enriched in growth cones and during early synaptic development.

Positive effect of transplantation of hNT neurons (NTera 2/D1 cell-line) in a model of familial amyotrophic lateral sclerosis

Transplantation of hNT Neurons derived from the human teratocarcinoma cell-line (NTera2/D1) has been shown to ameliorate motor dysfunction in a number of injury or disease models in which the deficits are fairly localized. However, these cells have not been used before in a model with more extensive neurodegeneration. The aim of this study is to determine the effects of hNT Neuron transplants on motor neuron function in a mouse model of familial amyotrophic lateral sclerosis (FALS) in which there is a substitution of Alanine for Glycine at position 93 of the human SOD1 gene (G93A). Amyotrophic lateral sclerosis is a fatal degenerative motor neuron disease affecting the spinal cord, brainstem, and cortex. This disease clinically manifests as progressive muscular weakness and atrophy, leading to paralysis and death within 3-5 years of diagnosis. The FALS represents 10-13% of all cases. A range of behavioral tests was used to examine spontaneous locomotor activity, coordination, and muscle strength of mice. Long-term (10-11 weeks) transplantation of hNT Neurons into the L(4)-L(5) segments of the ventral horn spinal cord of FALS(G93A) mice at 7 weeks of age (before onset of overt behavioral symptoms of disease) delayed the onset of motor dysfunction for at least 3 weeks. The average lifespan of the transplanted mice was 128 days compared to 106 days for media-injected group. The last mouse in the hNT Neuron transplanted group was euthanized at 135 days of age when it display partial paralysis of the hindlimbs. Immunohistochemical analysis of the implanted spinal cords demonstrated the survival of grafted hNT Neurons and showed many healthy-appearing motor neurons near the implant site. These results suggest that hNT Neuron transplantation may be a promising therapeutic strategy for ALS.

Novel cellular approaches to repair of neurodegenerative disease: from Sertoli cells to umbilical cord blood stem cells

Neural transplantation is a promising approach to the treatment of neurodegenerative diseases and brain injury that has been shown to be efficacious in many animal models. However, the use of fetal tissue limits the acceptability and widespread application of this technique. In this review we discuss possible alternative cell sources that may be used to repair the brain and spinal cord, with a focus on Sertoli cells, hNT Neurons, bone marrow and umbilical cord blood derived stem cells.

Neuronal formation of free radicals plays a minor role in hypoxic cell death in human NT2-N neurons

Free radicals are suggested to play an important role in hypoxic-ischemic neuronal death. However, the importance in human disease is not known. Furthermore, whether posthypoxic free radical formation mainly occurs in endothelium and neutrophils, or whether neuronal production is important, is not finally determined. To study this we differentiated human Ntera2 teratocarcinoma cells into postmitotic NT2-N neurons and exposed them to free radicals, hypoxia, or oxygen and glucose deprivation. These cells are devoid of nitric oxide synthase, and we hypothesized that free radicals are important mediators downstream of N-methyl-D-aspartate stimulation. Production of free radicals, evaluated with the fluorescent dyes dihydrorhodamine and 2',7'-dichlorodihydrofluorescein, was significantly higher in neurons deprived of oxygen and glucose after 40 min of reoxygenation than in normoxic cells. The antioxidant trolox, the flavonoid quercetin, thiopental, and the N-methyl-D-aspartate-glutamate receptor antagonist MK-801 reduced the formation of free radicals. Treatment with the flavonoid rutin (86 +/- 16% of hypoxic cells without drug, p < 0.01), trolox (86 +/- 20%, p < 0.01), and MK-801 (57 +/- 12%, p < 0.01) reduced lactate dehydrogenase release after 6 h of hypoxia. Trolox, salicylate, and quercetin also significantly reduced lactate dehydrogenase release after 3 h of oxygen and glucose deprivation. The protection offered by these antioxidants was, however, limited compared with the effect of MK-801. We conclude that oxygen and glucose deprivation causes a moderate increase in the formation of free radicals in NT2-N neurons that can be inhibited by antioxidants and by blocking of the N-methyl-D-aspartate-glutamate receptor. Although MK-801 conveys profound protection, antioxidants provide only a limited improvement in neuronal survival. Thus in this model, mechanisms downstream of the N-methyl-D-aspartate-glutamate receptor other than free radicals and nitric oxide have to be invoked.

Single-Cell Antisense RNA Amplification and Microarray Analysis as a Tool for Studying Neurological Degeneration and Restoration

Neurodegenerative diseases typically affect subpopulations of neurons. Characterizing these vulnerable cells and identifying the factors that make them susceptible to damage while neighboring cells remain resistant are essential to the understanding of molecular pathogenesis that underlies neurodegenerative diseases. Classically, molecular analysis of the central nervous system involves the identification and isolation of an anatomic region of interest; next, the relevant tissue is pulverized, and the resulting homogenate is analyzed. Although this method provides useful data, its effectiveness diminishes when used in areas of high cellular diversity or in instances in which one cell type is lost as a consequence of selective cell death or quiescence. A technique that affords the ability to assess molecular events in a very precise anatomical site would provide a powerful tool for this research discipline. In this review, we discuss the amplification of messenger RNA from single neural cells and the subsequent use of the RNA to probe DNA microarrays in an effort to create cell-specific molecular profiles. Specifically, recent work in single-cell expression profiling in Alzheimer's and Huntington's diseases is discussed. We also review some new work with neural stem cells and their application to restorative neurobiology. Finally, we discuss the use of cell-specific molecular profiles to better understand the basics of neuronal cell biology.

Retention of the Alzheimer's amyloid precursor fragment C99 in the endoplasmic reticulum prevents formation of amyloid beta-peptide

gamma-Secretase is a membrane-associated endoprotease that catalyzes the final step in the processing of Alzheimer's beta-amyloid precursor protein (APP), resulting in the release of amyloid beta-peptide (Abeta). The molecular identity of gamma-secretase remains in question, although recent studies have implicated the presenilins, which are membrane-spanning proteins localized predominantly in the endoplasmic reticulum (ER). Based on these observations, we have tested the hypothesis that gamma-secretase cleavage of the membrane-anchored C-terminal stump of APP (i.e. C99) occurs in the ER compartment. When recombinant C99 was expressed in 293 cells, it was localized mainly in the Golgi apparatus and gave rise to abundant amounts of Abeta. Co-expression of C99 with mutant forms of presenilin-1 (PS1) found in familial Alzheimer's disease resulted in a characteristic elevation of the Abeta(42)/Abeta(40) ratio, indicating that the N-terminal exodomain of APP is not required for mutant PS1 to influence the site of gamma-secretase cleavage. Biogenesis of both Abeta(40) and Abeta(42) was almost completely eliminated when C99 was prevented from leaving the ER by addition of a di-lysine retention motif (KKQN) or by co-expression with a dominant-negative mutant of the Rab1B GTPase. These findings indicate that the ER is not a major intracellular site for gamma-secretase cleavage of C99. Thus, by inference, PS1 localized in this compartment does not appear to be active as gamma-secretase. The results suggest that presenilins may acquire the characteristics of gamma-secretase after leaving the ER, possibly by assembling with other proteins in peripheral membranes.

Different Rab GTPases associate preferentially with alpha or beta GDP-dissociation inhibitors

GDIs (GDP-dissociation inhibitors) bind to Rab GTPases and mediate their membrane targeting and recycling. In vitro, most Rabs can bind to either of the major isoforms of GDI, leading to the assumption that the proportion of each specific Rab/GDI complex in vivo reflects the relative abundance of the alpha versus beta forms of GDI. Here we show that when human teratocarcinoma cells (Ntera2) are induced to differentiate into postmitotic neurons (NT2N), there is a major change in the proportion of GDIalpha relative to GDIbeta. Under these conditions, certain Rab GTPases associate preferentially with either GDIalpha or GDIbeta, irrespective of the relative abundance of the GDI isoform. These findings suggest that heretofore unrecognized functional specificity may exist between the two major forms of GDI.

Increased expression of Galpha(q/11) and of phospholipase-Cbeta1/4 in differentiated human NT2-N neurons: enhancement of phosphoinositide hydrolysis

The CNS is enriched in phosphoinositide-specific phospholipase C (PLC) and in the G proteins linked to its activation. Although the regional distributions of these signaling components within the brain have been determined, neither their cell type-specific localizations (i.e., neuronal versus glial) nor the functional significance of their high expression has been definitively established. In this study, we have examined the expression of phosphoinositide signaling proteins in human NT2-N cells, a well characterized model system for CNS neurons. Retinoic acid-mediated differentiation of NT2 precursor cells to the neuronal phenotype resulted in five- to 15-fold increases in the expression of PLC-beta1, PLC-beta4, and Galpha(q/11) (the prime G protein activator of these isozymes). In contrast, the expression of PLC-beta3 and PLC-gamma1 was markedly reduced following neuronal differentiation. Similar alterations in cell morphology and in the expression of PLC-beta1, PLC-beta3, and Galpha(q/11) expression were observed when NT2 cells were differentiated with berberine, a compound structurally unrelated to retinoic acid. NT2-N neurons exhibited a significantly higher rate of phosphoinositide hydrolysis than NT2 precursor cells in response to direct activation of either G proteins or PLC. These results indicate that neuronal differentiation of NT2 cells is associated with dramatic changes in the expression of proteins of the phosphoinositide signaling system and that, accordingly, differentiated NT2-N neurons possess an increased ability to hydrolyze inositol lipids.

Rab24 is an atypical member of the Rab GTPase family. Deficient GTPase activity, GDP dissociation inhibitor interaction, and prenylation of Rab24 expressed in cultured cells

The function of Rab24 is currently unknown, but other members of the Rab GTPase family are known to participate in various protein trafficking pathways. Rab proteins are thought to cycle on and off vesicle membranes in conjunction with changes in their guanine nucleotide state. The present studies indicate that Rab24 possesses several unusual characteristics that distinguish it from other Rab proteins. 1) Based on [(32)P]orthophosphate labeling of protein-bound nucleotide, Rab24 exists predominantly in the GTP state when expressed in cultured cells. The low GTPase activity is related to the presence of serine instead of glutamine at the position cognate to Ras Gln-61. 2) Posttranslational geranylgeranylation of Rab24, determined by metabolic labeling or detergent partitioning assays, is inefficient when compared with other Rabs ending with the common CXC and CC carboxyl-terminal motifs. This is partly due to the presence of two histidines distal to the target cysteines, but also involves other unidentified features. 3) Most of the Rab24 in the cytoplasmic compartment of cultured cells is not associated with Rab GDP dissociation inhibitors. These findings indicate that, if Rab24 functions in vesicular transport processes, it may operate through a novel mechanism that does not depend on GTP hydrolysis or GDP dissociation inhibitor-mediated recycling.