The ability of the inhibitory domain of the POU family transcription factor Oct-2 to interfere with promoter activation by different classes of activation domains is dependent upon the nature of the basal promoter elements

Glutamate regulates Oct-2 DNA-binding activity through α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors in cultured chick Bergmann glia cells

Ionotropic glutamate receptors in cerebellar Bergmann glial cells are linked to transcriptional regulation and, by these means, are thought to play an important role in plasticity, learning and memory and in several neuropathologies. Within the CNS, the transcription factors of the POU family bind their target DNA sequences after a growth factor-dependent phosphorylation-dephosphorylation cascade. Exposure of cultured Bergmann glial cells to glutamate leads to a time- and dose-dependent increase in Oct-2 DNA-binding activity. The use of specific pharmacological tools established the involvement of Ca2+-permeable alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors. Furthermore, the signaling cascade includes phosphatidyl inositol 3-kinase as well as protein kinase C activation. Interestingly, transcriptional as well as translational inhibitors abolish the glutamate effect, suggesting a transcriptional up-regulation of the oct-2 gene. These data demonstrate that Oct-2 expression is not restricted to neurons and further strengthen the notion that the glial glutamate receptors participate in the modulation of glutamatergic cerebellar neurotransmission.

The basic helix-loop-helix protein, sharp-1, represses transcription by a histone deacetylase-dependent and histone deacetylase-independent mechanism

Many aspects of neurogenesis and neuronal differentiation are controlled by basic helix-loop-helix (bHLH) proteins. One such factor is SHARP-1, initially identified on the basis of its sequence similarity to hairy. Unlike hairy, and atypically for bHLHs, SHARP-1 is expressed late in development, suggestive of a role in terminal aspects of differentiation. Nevertheless, the role of SHARP-1 and the identity of its target genes remain unknown. During the course of a one-hybrid screen for transcription factors that bind to regulatory domains of the M1 muscarinic acetylcholine receptor gene, we isolated the bHLH transcription factor SHARP-1. In this study, we investigated the functional role of SHARP-1 in regulating transcription. Fusion proteins of SHARP-1 tethered to the gal4 DNA binding domain repress both basal and activated transcription when recruited to either a TATA-containing or a TATAless promoter. Furthermore, we identified two independent repression domains that operate via distinct mechanisms. Repression by a domain in the C terminus is sensitive to the histone deacetylase inhibitor trichostatin A, whereas repression by the bHLH domain is insensitive to TSA. Furthermore, overexpression of SHARP-1 represses transcription from the M(1) promoter. This study represents the first report to assign a function to, and to identify a target gene for, the bHLH transcription factor SHARP-1.

POU domain factors in the neuroendocrine system: lessons from developmental biology provide insights into human disease

POU domain factors are transcriptional regulators characterized by a highly conserved DNA-binding domain referred to as the POU domain. The structure of the POU domain has been solved, facilitating the understanding of how these proteins bind to DNA and regulate transcription via complex protein-protein interactions. Several members of the POU domain family have been implicated in the control of development and function of the neuroendocrine system. Such roles have been most clearly established for Pit-1, which is required for formation of somatotropes, lactotropes, and thyrotropes in the anterior pituitary gland, and for Brn-2, which is critical for formation of magnocellular and parvocellular neurons in the paraventricular and supraoptic nuclei of the hypothalamus. While genetic evidence is lacking, molecular biology experiments have implicated several other POU factors in the regulation of gene expression in the hypothalamus and pituitary gland. Pit-1 mutations in humans cause combined pituitary hormone deficiency similar to that found in mice deleted for the Pit-1 gene, providing a striking example of how basic developmental biology studies have provided important insights into human disease.

POU family transcription factors in the nervous system

The POU (Pit-Oct-Unc) family of transcription factors was originally defined on the basis of a common DNA binding domain in the mammalian factors Pit-1, Oct-1, and Oct-2 as well as the nematode protein Unc-86. Subsequently, a number of other POU family factors have been identified in both vertebrates and invertebrates. Many of these original and subsequently isolated members of the family have been shown to play critical roles in the development and functioning of the nervous system. To exemplify this, studies are described involving the functional characterisation of the Oct-2 factor, one of the original POU factors, and of the Brn-3 factors, which were isolated subsequently and are the mammalian factors most closely related to Unc-86.

Dictyostelium TRFA homologous to yeast Ssn6 is required for normal growth and early development

The TPR (tetratricopeptide repeat) family became widespread during evolution, having been found from bacteria to mammals. By means of restriction enzyme-mediated integration, we have identified a Dictyostelium gene (trfA) highly homologous to a Saccharomyces cerevisiae gene encoding a TPR protein, Ssn6 (Cyc8), which functions as a global transcriptional repressor for diverse genes. The deduced amino acid sequence of the Dictyostelium gene product, TRFA, contains 10 consecutive TPR units as well as Gln repeats, Asn repeats, and a region rich in Glu, Lys, Ser, and Thr. The sequences of some of the 10 TPR units in TRFA are more than 70% identical to the corresponding units in Ssn6. The trfA- cells produced smooth plaques on a bacterial lawn and failed to aggregate normally when starved on a plain agar plate. Individual trfA- cells also failed to correctly respond to cAMP, although the adenylyl cyclase of trfA- cells was expressed upon starvation and activated by stimulation with cAMP as in the wild-type cells. When cultured in a rich medium in suspension, they grew more slowly and stopped growing at a lower density than the wild-type cells. Furthermore, they divided into cells of various sizes and tended to be much smaller than the wild-type cells. These pleiotropic defects of the trfA- cells suggest the possibility that Dictyostelium TRFA may regulate the transcription of diverse genes required for normal growth and early development.

Adjacent proline residues in the inhibitory domain of the Oct-2 transcription factor play distinct functional roles

A 40 amino acid region of Oct-2 from amino acids 142 to 181 functions as an active repressor domain capable of inhibiting both basal activity and activation of promoters containing a TATA box, but not of those that contain an initiator element. Based on our observation that the equivalent region of the closely related Oct-1 factor does not act as an inhibitory domain, we have mutated specific residues in the Oct-2 domain in an attempt to probe their importance in repressor domain function. Although mutations of several residues have no or minimal effect, mutation of proline 175 to arginine abolishes the ability to inhibit both basal and activated transcription. In contrast, mutation of proline 174 to arginine confers upon the domain the ability to repress activation of an initiator-containing promoter by acidic activation domains, and also suppresses the effect of the proline 175 mutation. Hence, adjacent proline residues play key roles in the functioning of the inhibitory domain and in limiting its specificity to TATA-box-containing promoters.

The different inhibitory domains of the Oct-2 transcription factor have distinct functional activities

The Oct-2 POU family transcription factor contains three distinct regions whose deletion reduces its ability to inhibit transcription via its octamer binding site. Here we show that only one of these inhibitory domains is capable of also inhibiting the activity of activating molecules bound at adjacent sites upstream of a TATA box-containing promoter whereas the other two regions are inactive in this assay. None of the three regions is able to achieve this effect when located upstream of the same promoter containing an initiator motif. The mechanisms of action of these domains and their role in the functioning of the Oct-2 factor are discussed.

Differential regulation of the two neuronal nitric-oxide synthase gene promoters by the Oct-2 transcription factor

The Oct-2 transcription factor has been shown previously to repress both the cellular tyrosine hydroxylase and the herpes simplex virus immediate-early genes in neuronal cells. Here we identify the gene encoding the neuronal nitric-oxide synthase (nNOS) as the first example of a gene activated in neuronal cells by Oct-2. The levels of the nNOS mRNA and protein are greatly reduced in neuronal cell lines in which Oct-2 levels have been reduced by an antisense method, although these cells have enhanced levels of tyrosine hydroxylase. Moreover, the nNOS gene regulatory region is activated by Oct-2 expression vectors upon cotransfection into both neuronal and non-neuronal cells, and this response is dependent upon a 20-amino acid region within the COOH-terminal activation domain of Oct-2. Of the two closely linked promoters that drive nNOS gene expression, only the downstream 5.1 promoter is activated by Oct-2, whereas the 5.2 promoter is unaffected. These effects are discussed in terms of the potential role of Oct-2 in regulating nNOS expression in the nervous system.