Nerve growth factor-induced derepression of peripherin gene expression is associated with alterations in proteins binding to a negative regulatory element

An intact intermediate filament network is required for collateral sprouting of small diameter nerve fibers

Expression of the intermediate filament (IF) protein peripherin is initiated during development at the time of axonal extension and increases during regeneration of nerve fibers. To test whether the IF network is essential for neuron process outgrowth in the mature organism in vivo, we disrupted endogenous peripherin IF in small-sized dorsal root ganglion (DRG) neurons in transgenic mice via expression of a mutant peripherin transgene under control of peripherin gene regulatory sequences. Anatomical and functional analyses showed that these neurons send peripheral and central axonal projections to correct targets, express correct neuropeptides, and mediate acute pain responses normally. However, disruption of IF significantly impaired the ability of uninjured small-sized DRG neurons to sprout collateral axons into adjacent denervated skin, indicating a critical role for intact IF in plasticity, specifically in compensatory nociceptive nerve sprouting.

Intron 1 is required for cell type-specific, but not injury-responsive, peripherin gene expression

The "primitive" neurons of the peripheral nervous system (PNS) have the remarkable ability to regenerate new fibers. This regenerative process requires a sequence of gene activation and repression that is poorly understood. One gene that is almost exclusively expressed in neurons of the PNS and is activated after nerve injury is the peripherin intermediate filament gene, but little is known about the genomic elements that control either its restricted expression or its response to nerve injury in adult mice. Previous studies suggested that both 5' flanking sequence and intragenic regions were required for cell type-specific and injury-specific expression. To determine which intragenic regions were critical, mice were generated that expressed peripherin transgenes lacking different introns. Analyses of these mice revealed that deletion of introns 2-8 had no effect on either the cell type-specific or injury-specific expression of the peripherin gene; however, the remaining intron, intron 1, differentially bound Sp1 transcription-related proteins/protein complexes in extracts from peripherin-expressing and nonexpressing tissues. Furthermore, a transgene that lacked intron 1 was not expressed in many neurons that contain endogenous peripherin but was activated after injury. Thus, accurate cell type-specific peripherin gene expression in the PNS depends on elements within intron 1, but other sequences, most likely in the 5'flanking region, are required for activating the peripherin gene in response to nerve injury.

Normal stimulation of the synthesis, phosphorylation and DNA binding activity of c-Fos and Zif268 proteins by nerve growth factor is not sufficient to mediate neuronal differentiation of PC12 cells

Early-response genes (ERGs) are rapidly induced by nerve growth factor (NGF) in the PC12 rat pheochromocytoma cell line. To analyze the possible role of Ras and ERGs in neuronal differentiation, experiments were carried out to study the involvement of Ras proteins in the NGF-stimulated expression of two ERG-coded proteins (c-Fos and Zif268) implicated in NGF signaling. Using PC12 subclones expressing the dominant negative Ha-Ras Asn-17 protein, NGF-induced expression, phosphorylation and DNA-binding of these ERG products were found to be not sufficient to convey the biological response of PC12 cells to NGF.

Protein kinase A-dependent derepression of the human prodynorphin gene via differential binding to an intragenic silencer element

Induction of the prodynorphin gene has been implicated in medium and long-term adaptation during memory acquisition and pain. By 5' deletion mapping and site-directed mutagenesis of the human prodynorphin promoter, we demonstrate that both basal transcription and protein kinase A (PKA)-induced transcription in NB69 and SK-N-MC human neuroblastoma cells are regulated by the GAGTCAAGG sequence centered at position +40 in the 5' untranslated region of the gene (named the DRE, for downstream regulatory element). The DRE repressed basal transcription in an orientation-independent and cell-specific manner when placed downstream from the heterologous thymidine kinase promoter. Southwestern blotting and UV cross-linking experiments with nuclear extracts from human neuroblastoma cells or human brain revealed a protein complex of approximately 110 kDa that specifically bound to the DRE. Forskolin treatment reduced binding to the DRE, and the time course paralleled that for an increase in prodynorphin gene expression. Our results suggest that under basal conditions, expression of the prodynorphin gene is repressed by occupancy of the DRE site. Upon PKA stimulation, binding to the DRE is reduced and transcription increases. We propose a model for human prodynorphin activation through PKA-dependent derepression at the DRE site.

Differential Ras-dependence of gene induction by nerve growth factor and second messenger analogs in PC12 cells

Induction of neurite formation by nerve growth factor (NGF) in PC12 pheochromocytoma cells can be efficiently inhibited by expressing a dominant negative mutant form of the small guanine nucleotide binding Ha-Ras protein in these cells. The block in NGF-induced neuritogenesis caused by inhibition of endogenous Ras proteins was found to be partially relieved by simultaneous stimulation of cAMP- or Ca++-dependent signaling pathways. Since expression of certain genes is believed to be involved in NGF-signaling leading to morphological differentiation, we decided to study the combined effects of NGF and second messenger analogs on gene expression in PC12 cell lines expressing different levels of the interfering Ras protein. We found NGF-second messenger combinations that induced normal c-fos, zif268 and nur77 early-response gene expression without neuritogenesis, and, conversely, cell lines in which certain combination treatments caused partial neuronal differentiation in the absence of substantial activation of these genes. Similarly, neurite outgrowth induced by combination treatments does not seem to require the activation of the late-response transin gene. Our results thus suggest a lack of strong correlation between NGF-stimulated early- and secondary-response gene induction and morphological differentiation.

Regulation of the neural-specific gene VGF in PC12 cells. Identification of transcription factors interacting with NGF-responsive elements

Nerve growth factor (NGF) is an important regulator of differentiation and survival in both the peripheral and central nervous systems. We have begun to analyze the mechanism by which NGF induces the expression of a neural specific gene, VGF, in PC12 cells. Using DNase I footprinting and transient transfection analysis, we identified two VGF promoter regions, V1 and V2, that are required for basal promoter expression as well as gene induction by NGF, epidermal growth factor (EGF), and cAMP. The V1 element is essential for VGF promoter function, but it is not sufficient to confer NGF responsiveness to a heterologous promoter. In contrast, the V2 element can independently stimulate the expression of a linked herpes simplex virus (HSV) thymidine kinase promoter in response to NGF. We showed that the V2 region also contains a sequence that acts as a promoter-specific negative regulator of basal VGF gene expression. As determined by gel mobility shift and Southwestern analysis, the V1 sequence is recognized by a novel PC12 nuclear protein of about 110-kDa molecular mass. Using oligonucleotide competition and antibody supershift assays, we demonstrated that the cAMP-response element (CRE) motif within the V2 element interacted specifically with proteins related to cAMP-response element binding (CREB), JunB, and JunD transcription factors. The JunB-related binding activities were transiently induced by NGF, suggesting that part of the mechanism utilized by NGF to activate VGF transcription includes increased synthesis of a V2 binding protein. Taken together, our analysis suggests that the VGF promoter is regulated by a complex mechanism, and its activation requires combinatorial action of several transcription factors interacting with multiple promoter elements.

Ubiquitous and neuronal DNA-binding proteins interact with a negative regulatory element of the human hypoxanthine phosphoribosyltransferase gene

The hypoxanthine phosphoribosyltransferase (HPRT) gene is constitutively expressed at low levels in all tissues but at higher levels in the brain; the significance and mechanism of this differential expression are unknown. We previously identified a 182-bp element (hHPRT-NE) within the 5'-flanking region of the human HPRT (hHPRT) gene, which is involved not only in conferring neuronal specificity but also in repressing gene expression in nonneuronal tissues. Here we report that this element interacts with different nuclear proteins, some of which are present specifically in neuronal cells (complex I) and others of which are present in cells showing constitutive expression of the gene (complex II). In addition, we found that complex I factors are expressed in human NT2/D1 cells following induction of neuronal differentiation by retinoic acid. This finding correlates with an increase of HPRT gene transcription following neuronal differentiation. We also mapped the binding sites for both complexes to a 60-bp region (Ff; positions -510 to -451) which, when analyzed in transfection assays, functioned as a repressor element analogous to the full-length hHPRT-NE sequence. Methylation interference footprintings revealed a minimal unique DNA motif, 5'-GGAAGCC-3', as the binding site for nuclear proteins from both neuronal and nonneuronal sources. However, site-directed mutagenesis of the footprinted region indicated that different nucleotides are essential for the associations of these two complexes. Moreover, UV cross-linking experiments showed that both complexes are formed by the association of several different proteins. Taken together, these data suggest that differential interaction of DNA-binding factors with this regulatory element plays a crucial role in the brain-preferential expression of the gene, and they should lead to the isolation of transcriptional regulators important in neuronal expression of the HPRT gene.

The presence of both negative and positive elements in the 5'-flanking sequence of the rat Na,K-ATPase alpha 3 subunit gene are required for brain expression in transgenic mice

The Na,K-ATPase is an integral plasma membrane protein consisting of alpha and beta subunits, each of which has discrete isoforms expressed in a tissue-specific manner. Of the three functional alpha isoform genes, the one encoding the alpha 3 isoform is the most tissue-restricted in its expression, being found primarily in the brain. To identify regions of the alpha 3 isoform gene that are involved in directing expression in the brain, a 1.6 kb 5'-flanking sequence was attached to a reporter gene, chloramphenicol acetyltransferase (CAT). The alpha 3-CAT chimeric gene construct was microinjected into fertilized mouse eggs, and transgenic mice were produced. Analysis of adult transgenic mice from different lines revealed that the transgene is expressed primarily in the brain. To further delineate regions that are needed for conferring expression in this tissue, systematic deletions of the 5'-flanking sequence of the alpha 3-CAT fusion constructs were made and analyzed, again using transgenic mice. The results from these analyses indicate that DNA sequences required for mediating brain-specific expression of the alpha 3 isoform gene are present within 210 bp upstream of the transcription initiation site. alpha 3-CAT promoter constructs containing scanning mutations in this region were also assayed in transgenic mice. These studies have identified both a functional neural-restrictive silencer element as well as a positively acting cis element.

Identification and characterization of a suppressor element in the 5'-flanking region of the mouse androgen receptor gene

Androgens play an important role in the development and maintenance of male reproductive organs through the androgen receptor (AR). In order to study the mechanism of regulation of AR at the molecular level, a 1571 bp fragment in the 5'-flanking region of the mouse androgen receptor (mAR) gene was isolated and sequenced. Transfection of 5'-deletion constructs cloned into vectors containing the chloramphenicol acetyl transferase (CAT) gene indicated the presence of a promoter in the sequence -146 to +131. These experiments also suggested the presence of a suppressor element. Further characterization indicated that the suppressor is present between -486 to -351. It is functional in the context of the natural AR promoter and the heterologous thymidine kinase promoter. Transfection of a -546/ + 131 construct in which the putative suppressor element (-421 to -448) had been deleted caused increased basal CAT activity suggesting that the suppressor is limited to this 28 bp element in the 5'-flanking region of the mouse AR gene.