Chromatin structure as a molecular marker of cell lineage and developmental potential in neural crest-derived chromaffin cells

Transdifferentiation and its applicability for inner ear therapy

During normal development, cells divide, then differentiate to adopt their individual form and function in an organism. Under most circumstances, mature cells cannot transdifferentiate, changing their fate to adopt a different form and function. Because differentiated cells cannot usually divide, the repair of injuries as well as regeneration largely depends on the activation of stem cell reserves. The mature cochlea is an exception among epithelial cell layers in that it lacks stem cells. Consequently, the sensory hair cells that receive sound information cannot be replaced, and their loss results in permanent hearing impairment. The lack of a spontaneous cell replacement mechanism in the organ of Corti, the mammalian auditory sensory epithelium, has led researchers to investigate circumstances in which transdifferentiation does occur. The hope is that this information can be used to design therapies to replace lost hair cells and restore impaired hearing in humans.

Interaction of the repressor element 1-silencing transcription factor (REST) with target genes

The repressor element 1-silencing transcription factor (REST) has been proposed to restrict expression of repressor element 1 (RE1) bearing genes to differentiated neurons by silencing their expression in non-neural tissue. Here, we have examined the interaction of REST with the M(4) muscarinic acetylcholine receptor gene. We show that REST binds to the RE1 of the M(4) gene in those cell lines and brain regions where the M(4) gene is expressed but not in those where the M(4) is not expressed. Furthermore, in cells that express M(4), the presence of REST represses but is insufficient to silence transcription of M(4). In non-neural cells REST is absent from the RE1 of the silent M(4) gene and perturbation of REST function fails to induce M(4) expression. We propose that REST acts to regulate expression levels of some RE1-bearing genes in neural cells, thereby playing an important role in defining neuronal activity.

REST repression of neuronal genes requires components of the hSWI.SNF complex

A function of the transcription factor REST is to block the expression of neuronal phenotypic traits in non-neuronal cells. Previous studies have shown that REST-mediated repression requires histone deacetylase activity and that recruitment of deacetylases is mediated by two co-repressors, Sin3A and CoREST. In this study, we show that a repressor domain in CoREST interacts with BRG1-associated factor (BAF) 57, a component of the hSWI.SNF complex. In vivo, BAF57 occupies the neuronal sodium channel gene (Nav1.2) promoter, and targeting to this gene requires REST. In addition to BAF57, the ATPase BRG1 and BAF170, other members of the hSWI.SNF complex, are also present in the REST.CoREST repressor complex. Microinjection of specific antibodies against BRG1, BAF57, or BAF170 into Rat1 fibroblasts relieves repression of RE1 reporter genes. Together, our data suggest that ATP-dependent chromatin remodeling, as well as histone deacetylation, is needed for REST-mediated repression.

Neural restrictive silencer factor recruits mSin3 and histone deacetylase complex to repress neuron-specific target genes

Accumulative evidence suggests that more than 20 neuron-specific genes are regulated by a transcriptional cis-regulatory element known as the neural restrictive silencer (NRS). A trans-acting repressor that binds the NRS, NRSF [also designated RE1-silencing transcription factor (REST)] has been cloned, but the mechanism by which it represses transcription is unknown. Here we show evidence that NRSF represses transcription of its target genes by recruiting mSin3 and histone deacetylase. Transfection experiments using a series of NRSF deletion constructs revealed the presence of two repression domains, RD-1 and RD-2, within the N- and C-terminal regions, respectively. A yeast two-hybrid screen using the RD-1 region as a bait identified a short form of mSin3B. In vitro pull-down assays and in vivo immunoprecipitation-Western analyses revealed a specific interaction between NRSF-RD1 and mSin3 PAH1-PAH2 domains. Furthermore, NRSF and mSin3 formed a complex with histone deacetylase 1, suggesting that NRSF-mediated repression involves histone deacetylation. When the deacetylation of histones was inhibited by tricostatin A in non-neuronal cells, mRNAs encoding several neuronal-specific genes such as SCG10, NMDAR1, and choline acetyltransferase became detectable. These results indicate that NRSF recruits mSin3 and histone deacetylase 1 to silence neural-specific genes and suggest further that repression of histone deacetylation is crucial for transcriptional activation of neural-specific genes during neuronal terminal differentiation.

NRSF/REST is required in vivo for repression of multiple neuronal target genes during embryogenesis

The neuron-restrictive silencer factor NRSF (also known as REST and XBR) can silence transcription from neuronal promoters in non-neuronal cell lines, but its function during normal development is unknown. In mice, a targeted mutation of Rest, the gene encoding NRSF, caused derepression of neuron-specific tubulin in a subset of non-neural tissues and embryonic lethality. Mosaic inhibition of NRSF in chicken embryos, using a dominant-negative form of NRSF, also caused derepression of neuronal tubulin, as well as of several other neuronal target genes, in both non-neural tissues and central nervous system neuronal progenitors. These results indicate that NRSF is required to repress neuronal gene expression in vivo, in both extra-neural and undifferentiated neural tissue.

Silencing is golden: negative regulation in the control of neuronal gene transcription

Recent work has identified negative-acting DNA regulatory elements that function to prevent the expression of neuronal genes in non-neuronal cell types or in inappropriate neuronal subtypes. In some cases, the protein factors that interact with these silencer elements have been isolated and characterized. For example, the recently cloned silencer-binding factor NRSF/REST is a novel zinc-finger protein that interacts with silencer elements in a number of neuron-specific genes. These data suggest that negative regulation plays a major role in determining the diverse patterns of gene expression within the nervous system.

A PCR analysis of rhodopsin gene transcription in rat pineal photoreceptor differentiation

A polymerase chain reaction (PCR) method was used to detect rhodopsin transcripts in rat pineals both in vivo and in vitro. Only very low levels of transcripts were detected in preparations from adult rat pineal tissue, but fairly large amounts were detected in cDNA preparations from cultures of newborn rat pineals. The transcript level was reduced significantly if the cells had been cultured in the presence of 10 microM norepinephrine (NE). This concentration of NE had previously been shown to abolish almost completely the rhodopsin immunoreactivity normally seen in such cultures. The present work indicates that part, but not all, of the effect of NE is probably at the level of translation.

Statistical evidence for a random commitment of pluripotent cephalic neural crest cells

The neural crest (NC) of vertebrate embryos yields cell types belonging to the neural, melanocytic and mesectodermal lineages. To test the possibility that the precursors of these lineages segregate from pluripotent cells by a process involving stochastic determinants, we have analyzed with statistical methods the associations between six differentiated cell types in 201 clones obtained in vitro from migrating cephalic NC cells. Our analysis suggests that neuronal, adrenergic and Schwann cells are not randomly associated, whereas these neural cell types differentiate in the clones independently of both melanocytes and cartilage. These results raise the possibility that pluripotent NC progenitors give rise to the precursors of the major NC-derived lineages (neural, melanocytic and mesectodermal) by a process involving stochastic restrictions of their developmental potentialities.

A common silencer element in the SCG10 and type II Na+ channel genes binds a factor present in nonneuronal cells but not in neuronal cells

We have localized a cell type-specific silencer element in the SCG10 gene by deletion analysis. This neural-restrictive silencer element (NRSE) selectively represses SCG10 expression in nonneuronal cells and tissues. The NRSE contains a 21 bp region with striking homology to a sequence present in a silencer domain of the rat type II sodium channel (NaII), another neuron-specific gene. We have identified a sequence-specific protein(s) that binds the SCG10 NRSE, as well as the homologous element in the NaII gene. A point mutation in the NRSE that abolishes binding of this neural-restrictive silencer-binding factor (NRSBF) in vitro also eliminates silencing activity in vivo. NRSBF is present in nuclear extracts from nonneuronal cells but not in extracts from neuronal cells, suggesting that the neuron-specific expression of SCG10 reflects, at least in part, the absence or inactivity of this protein. These data identify the NRSE as a potentially general DNA element for the control of neuron-specific gene expression in vertebrates.

A transcriptional regulatory element of the gene encoding the 67,000-M(r) form of human glutamate decarboxylase is similar to a Drosophila regulatory element

We have isolated the 5' flanking DNA sequences of the human gene encoding the 67,000-M(r) form of glutamate decarboxylase (GAD67), the gamma-aminobutyric acid synthetic enzyme. Transcription begins at a single promoter (P1) in adult brain but at two tandem promoters, P1 and P2, in fetal brain. P1, which is 3' to P2, resembles the promoter regions of many constitutively expressed genes, whereas P2 resembles a tissue-specific promoter. P1 contains a 10-base sequence (dec-1) that closely matches the element I cis-regulatory sequence identified in the promoter region of Drosophila 3,4-dihydroxyphenylalanine decarboxylase. Gel shift and transient expression assays demonstrate that the dec-1 sequence plays a role in the transcription of the human GAD67 gene.

Interaction of multiple nuclear proteins with the promoter region of the mouse 68-kDa neurofilament gene

Four brain-specific, DNase I hypersensitive sites (HSS) have been mapped to the 5' flanking region of the mouse 68-kDa neurofilament gene. These sites are contained within a 1.7-kb sequence that confers neuronal specificity of expression of this gene in transgenic mice. To identify DNA sequences that might be involved in gene regulation, the HSS situated near the promoter region has been analyzed by gel mobility shift assays and DNase I footprinting to investigate protein binding sequences. Of particular interest are two footprints localized to a 9-nucleotide sequence that flanks both the light and medium neurofilament gene in mouse and to a sequence that demonstrates partial homology to several promoter regions, including element-1, a motif required for neuron specificity in Drosophila. A prominent footprint was also detected at a sequence that contains a near-perfect palindrome centered at a PstI restriction site.

Co-expression of multiple neurotransmitter enzyme genes in normal and immortalized sympathoadrenal progenitor cells

We have examined the expression of mRNAs encoding five major neurotransmitter-synthesizing enzymes in MAH cells, a clonal cell line derived by retroviral immortalization of a rat embryonic sympathoadrenal progenitor cell. These mRNAs include tyrosine hydroxylase (TH), choline acetyltransferase (ChAT), tryptophan hydroxylase (TpH), and glutamic acid decarboxylases (GADs) 1 and 2. We find that MAH cells express high levels of TH mRNA and low levels of ChAT and TpH mRNAs. Neither GAD1 nor GAD2 mRNAs are detectable using an RNase protection assay with a detection limit of less than one transcript per cell. A similar pattern of mRNA expression is observed in postnatal superior cervical ganglia, adrenal medulla, and in PC12 cells. Transmitter synthesis and accumulation assays indicate that MAH cells can synthesize both catecholamines and acetylcholine. Thus the TH and ChAT mRNAs detected in these cells are likely to be translated into active enzyme. To corroborate these data obtained using MAH cells, we performed similar transmitter synthesis and accumulation assays on sympathoadrenal progenitors directly isolated from E14.5 fetal adrenal glands by fluorescence-activated cell sorting. These progenitor cells also synthesize and accumulate both catecholamines and acetylcholine, albeit to different extents than MAH cells. Both MAH cells and their nonimmortal counterparts are able to increase slightly their cholinergic function upon short-term exposure to CDF/LIF, a factor known to induce acetylcholine synthesis in postmitotic sympathetic neurons. Taken together, these data suggest that progenitor cells in the sympathoadrenal lineage acquire the ability to simultaneously transcribe several different neurotransmitter enzyme genes early in development, prior to their choice of final cell fate. At the same time, the progenitors possess receptors which regulate expression of these genes in response to environmental factors. This ability may permit the cells to choose from several different transmitter phenotypes in response to different environments, as they migrate through the embryo. The persistent transcription of these genes in adult cells, moreover, may in part account for the phenotypic plasticity of cells in this lineage.

A cell type-preferred silencer element that controls the neural-specific expression of the SCG10 gene

SCG10 is a growth-associated protein that is expressed early in the development of neuronal derivatives of the neural crest. We describe here the isolation of the SCG10 chromosomal gene and the identification of regulatory regions that control its expression. The SCG10 transcription unit spans approximately 40 kb. Like other neural-specific genes, SCG10 contains multiple transcription initiation sites. The gene contains a constitutive enhancer-like element in the promoter-proximal region and a silencer located farther upstream. This silencer preferentially suppresses the activity of the enhancer in nonneuronal cells. Furthermore, the silencer is able to confer such preferential suppression upon a heterologous promoter in an orientation-independent manner. These data suggest that the expression of SCG10 in neuronal cells depends predominantly upon specific derepression.

Analysis of SCG10 gene expression in transgenic mice reveals that neural specificity is achieved through selective derepression

SCG10 is a neural-specific, growth-associated protein that is broadly expressed in the embryonic central and peripheral nervous systems. Transgenic mice harboring a chimeric gene containing 4 kb of SCG10 5' flanking DNA fused to the bacterial CAT gene exhibit expression in brain but not in nonneuronal tissues. A low level of expression is detected in adrenal gland as well, consistent with the behavior of endogenous SCG10. Such a transgene is also activated at the same relative stage of embryonic development as its endogenous counterpart. Deletion of the 5'-most 3.7 kb of SCG10 sequence yields deregulated expression of the transgene in numerous nonneuronal tissues, although expression remains highest in brain. In contrast to other tissue-specific genes, therefore, the specificity of SCG10 expression appears to be achieved predominantly through selective repression in nonneuronal tissues.

A v-myc-immortalized sympathoadrenal progenitor cell line in which neuronal differentiation is initiated by FGF but not NGF

Sympathetic neurons differentiate from a developmentally restricted progenitor cell in the neural crest-derived sympathoadrenal lineage. We have isolated these progenitors by fluorescence-activated cell sorting and immortalized them using a v-myc-containing retrovirus. The complement of antigenic markers expressed by these lines suggests that they have retained many of the properties of their normal counterparts. These lines initiate neuronal differentiation in response to basic FGF, but not to NGF, and do not contain NGF receptor mRNA. In NGF plus FGF, however, a small percentage of the cells differentiate to NGF-dependent postmitotic neurons. Furthermore, an induction of NGF receptor mRNA can be observed in response to FGF. Thus, the development of sympathetic neurons may involve a relay, in which FGF both initiates differentiation and induces the NGF receptor, which in turn controls further maturation and survival.