Absence of binding activity of neuron-restrictive silencer factor is necessary, but not sufficient for transcription of NMDA receptor subunit type 1 in neuronal cells

REST Is Not Resting: REST/NRSF in Health and Disease

Chromatin modifications play a crucial role in the regulation of gene expression. The repressor element-1 (RE1) silencing transcription factor (REST), also known as neuron-restrictive silencer factor (NRSF) and X2 box repressor (XBR), was found to regulate gene transcription by binding to chromatin and recruiting chromatin-modifying enzymes. Earlier studies revealed that REST plays an important role in the development and disease of the nervous system, mainly by repressing the transcription of neuron-specific genes. Subsequently, REST was found to be critical in other tissues, such as the heart, pancreas, skin, eye, and vascular. Dysregulation of REST was also found in nervous and non-nervous system cancers. In parallel, multiple strategies to target REST have been developed. In this paper, we provide a comprehensive summary of the research progress made over the past 28 years since the discovery of REST, encompassing both physiological and pathological aspects. These insights into the effects and mechanisms of REST contribute to an in-depth understanding of the transcriptional regulatory mechanisms of genes and their roles in the development and progression of disease, with a view to discovering potential therapeutic targets and intervention strategies for various related diseases.

Inhibitory Effect of Memantine on Streptozotocin-Induced Insulin Receptor Dysfunction, Neuroinflammation, Amyloidogenesis, and Neurotrophic Factor Decline in Astrocytes

Our earlier studies showed that insulin receptor (IR) dysfunction along with neuroinflammation and amyloidogenesis played a major role in streptozotocin (STZ)-induced toxicity in astrocytes. N-methyl-D-aspartate (NMDA) receptor antagonist-memantine shows beneficial effects in Alzheimer's disease (AD) pathology. However, the protective molecular and cellular mechanism of memantine in astrocytes is not properly understood. Therefore, the present study was undertaken to investigate the effect of memantine on insulin receptors, neurotrophic factors, neuroinflammation, and amyloidogenesis in STZ-treated astrocytes. STZ (100 μM) treatment for 24 h in astrocytes resulted significant decrease in brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), and insulin-degrading enzyme (IDE) expression in astrocytes. Treatment with memantine (1-10 μM) improved STZ-induced neurotrophic factor decline (BDNF, GDNF) along with IR dysfunction as evidenced by a significant increase in IR protein expression, phosphorylation of IRS-1, Akt, and GSK-3 α/β in astrocytes. Further, memantine attenuated STZ-induced amyloid precursor protein (APP), β-site APP-cleaving enzyme-1 and amyloid-β_1-42 expression and restored IDE expression in astrocytes. In addition, memantine also displays protective effects against STZ-induced astrocyte activation showed by reduction of inflammatory markers, nuclear factor kappa-B translocation, glial fibrillary acidic protein, cyclooxygenase-2, tumor necrosis factor-α level, and oxidative-nitrostative stress. The results suggest that besides the NMDA receptor antagonisic activity, effect on astroglial IR and neurotrophic factor may also be an important factor in the beneficial effect of memantine in AD pathology. Graphical Abstract Novel neuroprotective mechanisms of memenatine in streptozotocin-induced toxicity in astrocytes.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Memantine lowers amyloid-beta peptide levels in neuronal cultures and in APP/PS1 transgenic mice

Memantine is a moderate-affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist that stabilizes cognitive, functional, and behavioral decline in patients with moderate to severe Alzheimer's disease (AD). In AD, the extracellular deposition of fibrillogenic amyloid-beta peptides (Abeta) occurs as a result of aberrant processing of the full-length Abeta precursor protein (APP). Memantine protects neurons from the neurotoxic effects of Abeta and improves cognition in transgenic mice with high brain levels of Abeta. However, it is unknown how memantine protects cells against neurodegeneration and affects APP processing and Abeta production. We report the effects of memantine in three different systems. In human neuroblastoma cells, memantine, at therapeutically relevant concentrations (1-4 muM), decreased levels of secreted APP and Abeta(1-40). Levels of the potentially amylodogenic Abeta(1-42) were undetectable in these cells. In primary rat cortical neuronal cultures, memantine treatment lowered Abeta(1-42) secretion. At the concentrations used, memantine treatment was not toxic to neuroblastoma or primary cultures and increased cell viability and/or metabolic activity under certain conditions. In APP/presenilin-1 (PS1) transgenic mice exhibiting high brain levels of Abeta(1-42), oral dosing of memantine (20 mg/kg/day for 8 days) produced a plasma drug concentration of 0.96 microM and significantly reduced the cortical levels of soluble Abeta(1-42). The ratio of Abeta(1-40)/Abeta(1-42) increased in treated mice, suggesting effects on the gamma-secretase complex. Thus, memantine reduces the levels of Abeta peptides at therapeutic concentrations and may inhibit the accumulation of fibrillogenic Abeta in mammalian brains. Memantine's ability to preserve neuronal cells against neurodegeneration, to increase metabolic activity, and to lower Abeta level has therapeutic implications for neurodegenerative disorders.

Transcription factor REST negatively influences the protein kinase C-dependent up-regulation of human mu-opioid receptor gene transcription

Mu-opioid receptor expression increases during neurogenesis, regulates the survival of maturing neurons and is implicated in ischemia-induced neuronal death. The repressor element 1 silencing transcription factor (REST), a regulator of a subset of genes in differentiating and post-mitotic neurons, is involved in its transcriptional repression. Extracellular signaling molecules and mechanisms that control the human mu-opioid receptor (hMOR) gene transcription are not clearly understood. We examined the role of protein kinase C (PKC) on hMOR transcription in a model of neuronal cells and in the context of the potential influence of REST. In native SH-SY5Y neuroblastoma cells, PKC activation with phorbol 12-myristate 13-acetate (PMA, 16 nM, 24h) down-regulated hMOR transcription and concomitantly elevated the REST binding activity to repressor element 1 of the hMOR promoter. In contrast, PMA activated hMOR gene transcription when REST expression was knocked down by an antisense strategy or by retinoic acid-induced cell differentiation. PMA acts through a PKC-dependent pathway requiring downstream MAP kinases and the transcription factor AP-1. In a series of hMOR-luciferase promoter/reporter constructs transfected into SH-SY5Y cells and PC12 cells, PMA up-regulated hMOR transcription in PC12 cells lacking REST, and in SH-SY5Y cells either transfected with constructs deficient in the REST DNA binding element or when REST was down-regulated in retinoic acid-differentiated cells. These findings help explain how hMOR transcription is regulated and may clarify its contribution to epigenetic modifications and reprogramming of differentiated neuronal cells exposed to PKC-activating agents.

Regulated expression of the Ras effector Rin1 in forebrain neurons

The Ras effector Rin1 is induced concomitant with synaptogenesis in forebrain neurons, where it inhibits fear conditioning and amygdala LTP. In epithelial cells, lower levels of Rin1 orchestrate receptor endocytosis. A 945 bp Rin1 promoter fragment was active in hippocampal neurons and directed accurate tissue-specific and temporal expression in transgenic mice. Regulated expression in neurons and epithelial cells was mediated in part by Snail transcriptional repressors: mutation of a conserved Snail site increased expression and endogenous Snai1 was detected at the Rin1 promoter. We also describe an element closely related to, but distinct from, the consensus site for REST, a master repressor of neuronal genes. Conversion to a consensus REST sequence reduced expression in both cell types. These results provide insight into regulated expression of a neuronal Ras effector, define a promoter useful in telencephalic neuron studies, and describe a novel REST site variant directing expression to mature neurons.

Transcriptional regulation of PSA-NCAM expression by NMDA receptor activation in RA-differentiated C6 glioma cultures

N-Methyl-d-aspartate (NMDA) receptors exhibit a dichotomy of signaling with both toxic and plastic responses. Recent reports have shown that exposure to subtoxic concentration of NMDA results in a neuroprotective state that was measured when these neurons were subsequently challenged with toxic doses of glutamate or kainate. Control of polysialylated neural cell adhesion molecule (PSA-NCAM) expression by NMDA receptor activation has been described in several systems, suggesting a functional link between these two proteins. The perception of glial role in CNS function has changed dramatically over the past few years from simple trophic functions to that of cells with important roles in development and maintenance of CNS in cooperation with neurons. We report here the transcriptional regulation of PSA-NCAM expression by subtoxic dose of NMDA in retinoic acid differentiated C6 glioma cell cultures. C6 glioma cell cultures differentiated with retinoic acid (10microM) were exposed to NMDA (100microM) or to antagonist MK-801 (200nM) prior to treatment with NMDA and cells were harvested after 24h of treatment to study the expression of total NCAM, PSA-NCAM, nuclear factor kappaB (NF-kappaB) and activator protein-1 (AP-1) by Western blotting and dual immunocytofluorescence and expression of PST mRNA by fluorescent in situ hybridization (FISH). Significant increase in the levels of PSA-NCAM, NF-kappaB, AP-1 and PST mRNA was observed in NMDA treated cultures. Treatment of cultures with MK-801, a non-competitive NMDA receptor antagonist, prior to NMDA exposure prevented the NMDA-mediated changes indicating the involvement of NMDA receptor activation. The results elucidate the possible cellular and molecular mechanisms of regulation of PSA-NCAM expression in astroglial cultures by extracellular signals.

Transcription of the chicken Grin1 gene is regulated by the activity of SP3 and NRSF in undifferentiated cells and neurons

The NMDA (N-methyl-D-aspartate) receptors are important in the regulation of neuronal development, synaptic plasticity, learning and memory, and are involved in several brain pathologies. The NR1 subunit is essential for the assembly of functional receptors, as it forms the calcium-permeable ion channel and contains the obligatory co-agonist binding site. Previous studies have shown that NR1 gene (Grin1) expression is up-regulated during neuronal differentiation and its expression is widespread in the central nervous system. We have previously cloned the chicken Grin1 gene and 1.9 kb of the 5'-regulatory region. In the present study, we analysed the molecular mechanisms that regulate chicken Grin1 gene transcription in undifferentiated cells and neurons. By functional analysis of chicken Grin1-luciferase gene 5'-regulatory region constructs, we demonstrate that the basal promoter is delimited within 210 bp upstream from the main transcription initiation site. DNA-protein binding and functional assays revealed that the 5'-UTR (untranslated region) has one consensus NRSE (neuron-restrictive silencing element) that binds NRSF (neuron-restrictive silencing factor), and one SP (stimulating protein transcription factor) element that binds SP3, both repressing Grin1 gene transcription in undifferentiated P19 cells (embryonic terato-carcinoma cells) and PC12 cells (phaeochromocytoma cells). The promoter region lacks a consensus TATA box, but contains one GSG/SP (GSG-like box near a SP-consensus site) that binds SP3 and up-regulates gene transcription in embryonic chicken cortical neurons. Taken together, these results demonstrate a dual role of SP3 in regulating the expression of the Grin1 gene, by repressing transcription in the 5'-UTR in undifferentiated cells as well as acting as a transcription factor, increasing Grin1 gene transcription in neurons.

Targeting adenoviral transgene expression to neurons

Adenovirus (Ad) is an efficient and safe vector for CNS gene delivery since it infects non-replicating neurons and does not cause insertional mutagenesis of host cell genomes. However, the promiscuous Ad CAR receptor targets cells non-specifically and activates a host immune response. Using Ad5 containing an expression cassette encoding the gene for green fluorescent protein, gfp, regulated by the neuron specific promoter synapsin-1 and the woodchuck post-transcriptional regulatory element (WPRE), we demonstrate efficient, prolonged and promoter-restricted gfp expression in neurons of mixed primary adult rat dorsal root ganglion (DRG) and retinal cell cultures. We also demonstrate restricted gfp expression in DRG neurons after direct injections of Ad5 containing the synapsin-1(gfp)/WPRE construct into L4 DRG in vivo, while Ad5 CMV(gfp) transfected both DRG glia and neurons. Moreover, since the effective titres of delivered Ad5 are reduced with this neuron specific promoter/WPRE expression cassette, the viral immune challenge should be attenuated when used in vivo.

Integrating Synapse Proteomics with Transcriptional Regulation

The mammalian postsynaptic proteome (PSP) comprises a highly interconnected set of approximately 1,000 proteins. The PSP is organized into macromolecular complexes that have a modular architecture defined by protein interactions and function. Signals initiated by neurotransmitter receptors are integrated by these complexes and their constituent enzymes to orchestrate multiple downstream cellular changes, including transcriptional regulation of genes at the nucleus. Genome wide transcriptome studies are beginning to map the sets of genes regulated by the synapse proteome. Conversely, understanding the transcriptional regulation of genes encoding the synapse proteome will shed light on synapse formation. Mutations that disrupt synapse signalling complexes result in cognitive impairments in mice and humans, and recent evidence indicates that these mutation change gene expression profiles. We discuss the need for global approaches combining genetics, transcriptomics and proteomics in order to understand cognitive function and disruption in diseases.

Amantadine and memantine induce the expression of the glial cell line-derived neurotrophic factor in C6 glioma cells

Aminoadamantanes are commonly used in the treatment of Parkinson's and Alzheimer's diseases. While these drugs are shown to antagonise ionotropic glutamate receptors on neuronal cells, additional mechanisms could contribute to their neuroprotective properties. The aim of the present study was to investigate the effect of aminoadamantanes on the production of the glial cell line-derived neurotrophic factor (GDNF) in glial cells. For this purpose, we measured the modulation of GDNF release in C6 glioma cell cultures treated for 24 h with amantadine and memantine. Both drugs dose-dependently increased GDNF level in the culture medium with similar potency (submicromolar range) and efficacy (three to four-fold induction). RT-PCR studies revealed that both compounds also increased GDNF mRNA levels and their influence on the GDNF gene transcription was further evidenced using a rat GDNF promoter luciferase reporter assay. Together, these results demonstrate that the neuroprotective effect of amantadine and memantine could involve the regulation of GDNF production by glial cells.

Expression of the repressor element-1 silencing transcription factor (REST) is influenced by insulin-like growth factor-I in differentiating human neuroblastoma cells

The repressor element-1 (RE-1) silencing transcription factor (REST) interacts with an RE-1 cis element and represses the transcription of neuron-specific genes in neuronal progenitors but is down-regulated in post-mitotic neurons. We report that REST expression is modified, in a time-dependent manner, in SH-SY5Y neuroblastoma cells exposed to insulin-like growth factor I (IGF-I), a polypeptide hormone affecting various aspects of neuronal induction and maturation. REST is increased in cells treated with IGF-I for 2 days and then declines in 5-day-treated cells concomitant with a progressive neurite extension. To investigate any role played by REST in neurodifferentiation by IGF-I, we employed an antisense oligonucleotide (AS-ODN) complementary to REST mRNA. In AS-ODN-treated cells, the effects elicited by IGF-I on cell proliferation are not influenced whereas a marked decrease of REST significantly increases neurite elongation without any gross perturbation of neurogenesis. Synapsin I and betaIII-tubulin gene promoters contain an RE-1 motif and their transcription is repressed by REST; both of them are increased in cells exposed to IGF-I for 5 days and further elevated by AS-ODN treatment. A parallel increase of growth cone-associated protein 43, a protein chosen as a neuronal marker not directly regulated by REST, is also observed. Therefore, REST is elevated during early steps of neural induction by IGF-I and could contribute to down-regulate genes not yet required by the differentiation program while it declines later for the acquisition of neural phenotypes. These results suggest a model in which differentiating neuroblastoma cells determine their extent of neurite outgrowth on the basis of REST disappearance.

Neuron-restrictive silencer factor regulates the N-methyl-D-aspartate receptor 2B subunit gene in basal and ethanol-induced gene expression in fetal cortical neurons

Neuron-restrictive silencer factor (NRSF) is a transcriptional repressor of multiple neuronal genes. This study addressed the role of NRSF in N-methyl-D-aspartate (NMDA) receptor NR2B promoter activity and the molecular mechanisms of ethanol-induced NR2B up-regulation in fetal cortical neurons. The 5'-flanking region of the NR2B gene contains five NRSE-like elements. Functional analysis of the upstream regions of the NR2B gene by transient transfection of neurons revealed that neuron-restrictive silencer element (NRSE) motifs located between base pair -1407 and -2741 represses transcription of the gene. Analysis by electrophoretic mobility shift assay and reporter gene assay identified NRSE2 and 3 as responsible for repressing NR2B gene transcription. The identity of NRSF as the functional binding factor is suggested by the specific binding of in vitro synthesized NRSF or cell lysate to the labeled probes and the specific antibody-induced supershift. Furthermore, whereas mutations of NRSE2 and 3 motifs increased the promoter activity, overexpression of NRSF reduced it significantly. The pattern of NRSF expression during development was investigated and demonstrated that the highest expression is on embryonic day 14 with moderate expression on postnatal day 0, reflecting a possible role of NRSF as a regulator during development. Treatment of cultured cortical neurons with 100 mM ethanol for 5 days caused a significant decrease in the NRSF mRNA and protein levels, NRSF/NRSE binding activity, and an increase in the promoter activity. Therefore, our studies suggest that NRSF is a negative regulator of NR2B expression and may contribute to the ethanol-induced up-regulation of the NR2B gene in fetal cortical neurons.

GABAA receptors: building the bridge between subunit mRNAs, their promoters, and cognate transcription factors

The type A gamma-aminobutyric acid (GABA(A)) receptors mediate the majority of fast inhibitory neurotransmission in the CNS, and alterations in GABA(A) receptor function is believed to be involved in the pathology of several neurological and psychiatric illnesses, such as epilepsy, anxiety, Alzheimer's disease, and schizophrenia. GABA(A) receptors can be assembled from eight distinct subunit families defined by sequence similarity: alpha(1-6), beta(1-3), gamma(1-3), delta, pi, theta, and rho(1-3). The regulation of GABA(A) receptor function in the brain is a highly compensating system, influencing both the number and the composition of receptors at the cell surface. While transcriptional and translational points of control operate in parallel, it is becoming increasingly evident that many functional changes in GABA(A) receptors reflect the differential gene regulation of its subunits. The fact that certain GABA(A) receptor subunit genes are transcribed in distinct cell types during specific periods of development strongly suggests that genetic control plays a major role in the choice of subunit variants available for receptor assembly. This review focuses on the physiological conditions that alter subunit mRNA levels, the promoters that may control such levels, and the use of a conceptual framework created by bioinformatics to study coordinate and independent GABA(A) receptor subunit gene regulation. As this exciting field moves closer to identifying the language hidden inside the chromatin of GABA(A) receptor subunit gene clusters, future experiments will be aimed at testing models generated by computational analysis with biologically relevant in vivo and in vitro assays. It is hoped that through this functional genomic approach there will be the identification of new targets for therapeutic intervention.

The role of the RE1 element in activation of the NR1 promoter during neuronal differentiation

To understand the genetic mechanism controlling the expression of the NMDA subtype of glutamate receptors during neuronal differentiation, we studied activation of the N-methyl-D-aspartate receptor subunit 1 (NR1) gene and the role of the repressor element-1 (RE1) element in NR1 promoter activation. Following neuronal differentiation of P19 embryonic carcinoma cells, the NR1 transcription rate and mRNA level were significantly increased, while the nuclear level of the repressor RE1 silencing transcription factor (REST)/neuron-restriction silencer factor (NRSF) was reduced. Nuclear REST/NRSF from undifferentiated cells formed a large complex with the NR1 RE1 element. While this complex was significantly reduced after the differentiation, REST/NRSF from differentiated cells formed a new, faster migrating complex. In transient transfections, deletion of the RE1 element increased activity of the 5.4-kb NR1 promoter sixfold in undifferentiated cells, but only induced approximately 1.4-fold increase in differentiated cells. Forced expression of REST/NRSF in differentiated cells suppressed the promoter, while forced expression of a dominant-negative REST/NRSF induced promoter activity as well as the mRNA of the NR1 gene in undifferentiated cells. In stable transfectants, the wild-type promoter showed a robust increase in activity following differentiation in a pattern similar to the NR1 mRNA increase. Conversely, the promoter lacking the RE1 element showed only a moderate increase. Our data suggest that the NR1 gene up-regulation during neuronal differentiation is controlled by its promoter activation, which is largely determined by the interaction between the RE1 element and the repressor REST/NRSF.

The N-methyl-D-aspartate receptor subunit NR2B: localization, functional properties, regulation, and clinical implications

The N-methyl-D-aspartate (NMDA) receptor is an example of a heteromeric ligand-gated ion channel that interacts with multiple intracellular proteins by way of different subunits. NMDA receptors are composed of seven known subunits (NR1, NR2A-D, NR3A-B). The present review focuses on the NR2B subunit of the receptor. Over the last several years, an increasing number of reports have demonstrated the importance of the NR2B subunit in a variety of synaptic signaling events and protein-protein interactions. The NR2B subunit has been implicated in modulating functions such as learning, memory processing, pain perception, and feeding behaviors, as well as being involved in a number of human disorders. The following review provides a summary of recent findings regarding the structural features, localization, functional properties, and regulation of the NR2B subunit. The review concludes with a section discussing the role of NR2B in human diseases.

Effect of the ubiquitous transcription factors, SP1 and MAZ, on NMDA receptor subunit type 1 (NR1) expression during neuronal differentiation

The silencer factor NRSF/REST has been reported to restrict expression to neurons of a variety of genes, including that encoding NMDA receptor subunit type 1 (NR1), by suppressing transcription in nonneuronal cells. However, we recently reported that in addition to the absence of NRSF/REST-binding activity, another neuron-specific mechanism is necessary for high level expression of the NR1 gene in neurons. In this study, we explored the mechanism of induction of NR1 promoter activity during neuronal differentiation of the P19 cell line. We identified a 27 base pair GC-rich region in the promoter as an important element responsible for induction of the NR1 gene after neuronal differentiation. We found that the ubiquitous transcription factors SP1 and MAZ bind to this GC-rich region. Surprisingly, the binding activities of SP1 and MAZ are not remarkably changed after neuronal differentiation. Mutations in the SP1 and MAZ sites impair binding of SP1 and MAZ proteins and also decrease NR1 promoter activity. These findings suggest that SP1 and MAZ mediate enhancement of NR1 promoter activity during neuronal differentiation despite the fact that their binding activity does not change.

REST mRNA expression in normal and regenerating avian auditory epithelium

Hair cells (HCs) and supporting cells (SCs) in the auditory epithelium initially arise from a sheet of undifferentiated cells. Although much has been learned about the initial steps leading to the fate determination of HCs and SCs, respectively, little is known about what molecular events 'finalize' cell fate determination. We investigated the role of repressor element-1 (RE-1) silencing transcription factor (REST), whose inactivation is known to be a requirement for a cell to assume a neuronal identity. Here we show by in situ hybridization (ISH) that SCs express REST messenger RNA (mRNA) but sensory HCs lack detectable expression. Using a more sensitive reverse transcription-polymerase chain reaction assay, however, we detected the presence of a neuron-specific splice variant in the epithelium, suggesting that HCs express REST mRNA at levels too low to be detectable by ISH. In regenerating auditory epithelium, we found that REST mRNA was expressed and upregulated in all remaining cells in the damaged region of the epithelium, consistent with its expression pattern during development prior to neurogenesis. Surprisingly, REST mRNA was also upregulated in SCs in the apical, undamaged region of the epithelium, and readily detectable by ISH in the HCs in this region. This finding suggests that the grossly undamaged region of the epithelium is in fact biochemically altered towards a 'less developed' state. Our results indicate that REST inactivation is an important step in finalizing HC fate in the chick inner ear.

Antiapoptotic role of the p38 mitogen-activated protein kinase-myocyte enhancer factor 2 transcription factor pathway during neuronal differentiation

Myocyte enhancer factor 2 (MEF2) is in the MADS (MCM1agamous-deficiens-serum response factor) family of transcription factors. Although MEF2 is known as a myogenic factor, the expression pattern of the MEF2 family of genes (MEF2A-D) in developing brain also suggests a role in neurogenesis. Here we show that transfection with MEF2C, the predominant form in mammalian cerebral cortex, induces a mixed neuronal/myogenic phenotype in undifferentiated P19 precursor cells. During retinoic acid-induced neurogenesis of these cells, a dominant negative form of MEF2 enhances apoptosis but does not affect cell division. The mitogen-activated protein kinase p38alpha activates MEF2C. Dominant negative p38alpha also enhances apoptotic death of differentiating neurons, but these cells can be rescued from apoptosis by coexpression of constitutively active MEF2C. These findings suggest that the p38alpha/MEF2 pathway prevents cell death during neuronal differentiation.