Transcription of the rat and mouse proenkephalin genes is initiated at distinct sites in spermatogenic and somatic cells

Regulation of male fertility by the opioid system

Endogenous opioid peptides are substances involved in cell communication. They are present in various organs and tissues of the male and female reproductive tract, suggesting that they may regulate some of the processes involved in reproductive function. In fact, the opioid system that operates as a multi-messenger system can participate in the regulation of reproductive physiology at multiple levels, for example, at the levels of the central nervous system, at the testes level and at sperm level. A better understanding of the implication of the opioid system in reproductive processes may contribute to clarifying the etiology of many cases of infertility and the effect of opiate abuse on fertility. Indeed, a novel biochemical tool for the diagnosis and treatment of male infertility could be based upon components of the opioid system. The presence of the opioid system in sperm cells also represents a novel opportunity for reproductive management, for either enhancing the probability of fertilization or reducing it through the development of novel targeted contraceptives.

Widespread expression and estrogen regulation of PPEIA-3' nuclear RNA in the rat brain

We previously identified a novel nuclear RNA species derived from the preproenkephalin (PPE) gene. This transcript, which we have named PPEIA-3' RNA, hybridizes with probes directed at a region of PPE intron A downstream of an alternative germ-cell transcription start site, but does not contain PPE protein coding sequences. We now report that estrogen treatment of ovariectomized rats increases the expression of conventional PPE heteronuclear RNA, and also induces the expression of PPEIA-3' RNA, apparently in separate cell populations within the ventromedial nucleus of the hypothalamus. Further, we show that cells expressing PPEIA-3' are found in several neuronal groups in the rat forebrain and brainstem, with a distinct topographical distribution. High densities of PPEIA-3' containing cells are found in the reticular thalamic nucleus, the basal forebrain, the vestibular complex, the deep cerebellar nuclei, and the trapezoid body, a pattern that parallels the distribution of atypical nuclear RNAs described by other groups. These results suggest that this diverse neuronal population shares a common set of nuclear factors responsible for the expression and retention of this atypical RNA transcript. The implication of these results for cell-specific gene transcription and regulation in the brain and the possible relationship of PPEIA-3' RNA and other atypical nuclear RNAs is discussed.

The effect of cycloheximide on the regulation of proenkephalin and prodynorphin gene expressions induced by kainic acid in rat hippocampus

The effect of cycloheximide (CHX), a protein synthesis inhibitor, on the regulation of proenkephalin (proENK) and prodynorphin (proDYN) mRNA levels, proto-oncogenes, such as c-fos, 35-kDa fra and c-jun mRNA, and the levels of their products induced by kainic acid (KA) in rat hippocampus was studied. The proENK and proDYN mRNA levels were markedly increased 4 and 8 h after KA (10 mg/kg i.p.) administration. However, the intracellular proENK protein level was not affected by KA. The elevations of both proENK and proDYN mRNA levels induced by KA were inhibited by pre-administration of CHX (15 mg/kg i.p.). The increases of proENK and proDYN mRNA levels induced by KA were well-correlated with the increases of c-Fos, 35-kDa Fra and c-Jun protein levels. KA administration increased the hippocampal levels of c-Fos, 35-kDa Fra and c-Jun proteins with the time. The increases of c-Fos, 35-kDa Fra and c-Jun protein levels induced by KA administration were also inhibited by CHX pre-administration. KA administration markedly increased both c-fos and c-jun mRNA levels during 1 and 4 h and the increased levels of these proto-oncogene mRNA were further prolonged by the treatment with CHX. In addition, CHX alone increased both c-fos and c-jun mRNA levels although the onset times of induction were different. In electrophoretic mobility shift-assay, both AP-1 and ENKCRE-2 DNA-binding activities were increased by KA. KA-induced increases of AP-1 and ENKCRE-2 DNA-binding activities were also attenuated by CHX. In addition, KA-induced AP-1 and ENKCRE-2 DNA-binding activities were diminished by the antibodies against Fos and Jun family proteins. Furthermore, the cross-competition studies revealed that AP-1 proteins actively participated in ENKCRE-2 DNA domain. The results suggest that KA-induced proENK and proDYN mRNA expressions may require on-going synthesis of proteins, such as c-Fos, c-Jun and 35-kDa Fra, which may have a possible role in the up-regulation of proENK and proDYN gene expression through the binding with AP-1 and ENKCRE-2 DNA-binding motifs.

Interpreting cDNA sequences: some insights from studies on translation

This review discusses some rules for assessing the completeness of a cDNA sequence and identifying the start site for translation. Features commonly invoked-such as an ATG codon in a favorable context for initiation, or the presence of an upstream in-frame terminator codon, or the prediction of a signal peptide-like sequence at the amino terminus-have some validity; but examples drawn from the literature illustrate limitations to each of these criteria. The best advice is to inspect a cDNA sequence not only for these positive features but also for the absence of certain negative indicators. Three specific warning signs are discussed and documented: (i) The presence of numerous ATG codons upstream from the presumptive start site for translation often indicates an aberration (sometimes a retained intron) at the 5' end of the cDNA. (ii) Even one strong, upstream, out-of-frame ATG codon poses a problem if the reading frame set by the upstream ATG overlaps the presumptive start of the major open reading frame. Many cDNAs that display this arrangement turn out to be incomplete; that is, the out-of-frame ATG codon is within, rather than upstream from, the protein coding domain. (iii) A very weak context at the putative start site for translation often means that the cDNA lacks the authentic initiator codon. In addition to presenting some criteria that may aid in recognizing incomplete cDNA sequences, the review includes some advice for using in vitro translation systems for the expression of cDNAs. Some unresolved questions about translational regulation are discussed by way of illustrating the importance of verifying mRNA structures before making deductions about translation.

An alternatively spliced form of the transcription factor Sp1 containing only a single glutamine-rich transactivation domain

Protein-protein interactions involving specific transactivation domains play a central role in gene transcription and its regulation. The promoter-specific transcription factor Sp1 contains two glutamine-rich transcriptional activation domains (A and B) that mediate direct interactions with the transcription factor TFIID complex associated with RNA polymerase II and synergistic effects involving multiple Sp1 molecules. In the present study, we report the complementary DNA sequence for an alternatively spliced form of mouse Sp1 (mSp1-S) that lacks one of the two glutamine-rich activation regions present in the full-length protein. Corresponding transcripts were identified in mouse tissues and cell lines, and an Sp1-related protein identical in size to that predicted for mSp1-S was detected in mouse nuclear extracts. Cotransfection analysis revealed that mSp1-S lacks appreciable activity at promoters containing a single Sp1 response element but is active when multiple Sp1 sites are present, suggesting synergistic interactions between multiple mSp1-S molecules. The absence of a single glutamine-rich domain does not fully explain the properties of the smaller protein and indicates that additional structural features account for its unique transcriptional activity. The functional implications of this alternatively spliced form of Sp1 are discussed.

The transcriptional regulation of the preproenkephalin gene

Images

Structure and expression of the guinea pig preproenkephalin gene: site-specific cleavage in the 3' untranslated region yields truncated mRNA transcripts in specific brain regions

We isolated the guinea pig preproenkephalin gene from a genomic library by hybridization to a rat cDNA probe. The entire nucleotide sequence of the gene was determined. Genomic Southern blot hybridization demonstrated that the gene exists in a single copy within the genome. On the basis of RNase protection transcript mapping and homology comparisons with known preproenkephalin sequences from other species and assuming a poly(A) tail length of 100 residues, we predicted an mRNA transcript of approximately 1,400 nucleotides encoded by three exons. Northern (RNA) blot analysis of total RNA from several brain regions showed high levels of preproenkephalin mRNA in the caudate putamen, nucleus accumbens, and hypothalamus, with detectable levels in the amygdala, ventral tegmental area, and central gray and also in the pituitary. Unexpectedly, in several brain regions, the mRNA appeared not only in the 1,400-nucleotide length but also in a shorter length of approximately 1,130 bases. Significant amounts of the shorter mRNA were found in the caudate putamen, nucleus accumbens, and amygdala. The longer, but not the shorter, transcripts from the caudate putamen were found to be polyadenylated, but the difference in size was not due solely to the presence of poly(A) tails. Northern gel analysis of total RNA from the caudate putamen with probes from each exon, together with RNase protection mapping of the 3' end of the mRNA demonstrated that the 1,400-base preproenkephalin mRNA transcripts are cleaved in a site-specific manner in some brain regions, yielding a 1,130-base transcript and a 165-base polyadenylated fragment derived from the terminal end of the 3' untranslated region of the mRNA. This cleavage may serve as a preliminary step in RNA degradation and provide a mechanism for control of preproenkephalin mRNA abundance through selective degradation.

Heterogeneity in the 5' untranslated region of mouse cytochrome cT mRNAs leads to altered translational status of the mRNAs

Previous studies have shown that the differential regulation of mouse somatic cytochrome c (cyt cS) and testicular cytochrome c (cyt cT) during spermatogenesis is accompanied by changes in mRNA length [Hake et al. (1990) Development, 110, 249-257]. When analyzed by polysomal gradient sedimentation, cytochrome cT sediments in two broad size classes: non-polysomal mRNAs are about 0.6 to 0.75 kb and polysomal mRNAs range from 0.7 to 0.9 kb. Both classes of mRNAs shorten to about 0.5 kb following deadenylation. Oligonucleotide-directed cleavage of the cytochrome cT RNAs by RNase H reveals that the size heterogeneity of cytochrome cT mRNAs resides in the 5' untranslated regions (UTRs). Ribonuclease protection assays reveal that multiple cytochrome cT mRNAs are transcribed from six different transcriptional start sites spanning a region of 59 nucleotides in the 5'UTR from +1 to +59. Transcripts derived from the first and second transcriptional initiation sites are not loaded onto polysomes as efficiently as those transcripts initiated from the other start sites. Each of the longer mRNAs has an upstream open reading frame, which starts at +8 and ends at +136 in the 5'UTR of the cytochrome cT transcript. Computer analysis suggests that the lengthened 5'UTR sequences allow additional hairpin structures to be formed. Since the upstream open reading frame and the additional stem loop structure are absent in the 5' UTRs of the cytochrome cT mRNAs initiated from the four downstream start sites, we suggest that these sequences in the two longest cytochrome cT transcripts hinder their loading onto polysomes.

Preproenkephalin promoter yields region-specific and long-term expression in adult brain after direct in vivo gene transfer via a defective herpes simplex viral vector

We have previously used a defective herpes simplex virus vector to express a foreign gene in the adult rat brain. One application of this technology would be the in vivo analysis of promoter function in brain after de novo transfer, which would allow the rapid generation of vectors with localized application in a broad range of mammalian species while avoiding influences of other nearby promoters. A 2.7-kb fragment of the rat preproenkephalin promoter was placed upstream of the bacterial lacZ gene in our herpes simplex virus amplicon. A restricted pattern of lacZ expression was observed in vivo, which follows previously observed patterns of endogenous preproenkephalin expression. These results, from the direct gene transfer into an adult animal brain for in vivo promoter analysis, demonstrate that sequence information that influences restricted expression of preproenkephalin is located within 2.7 kb upstream of transcriptional initiation. lacZ expression was also observed in rat brain for 2 months after direct transfer, and PCR analysis confirmed the continued presence of amplicon DNA in lacZ-positive sections. Restricted and long-term expression observed with an endogenous promoter has important implications for gene therapy using viral vectors.

Protein-DNA interactions associated with the onset of testis-specific expression of the mammalian Pgk-2 gene

We have identified difference in protein-DNA interactions associated with the promoter of the mammalian spermatogenesis-specific Pgk-2 gene in expressing and nonexpressing cells, using a band shift assay. We compared DNA-binding activities in nuclear protein extracts from expressing adult testis cells versus nonexpressing prepuberal testis cells and nonexpressing somatic cells. One or two DNA-binding activities were found to be uniquely associated with the expressed state of Pgk-2, while a third appears to be associated with the nonexpressed state. All three of these activities map to a region within the first 40 bp upstream from the core promoter of this gene. The Pgk-2 core promoter lacks a TATA box but contain a GC box and a CAAT box. We show that the GC box binds the ubiquitous transcription factor Sp1 and that the CAAT box binds CTF-1, both of which are present in extracts from all three tissue types tested. These results suggest that tissue-specific transcription of the Pgk-2 gene is associated with changes in protein-DNA interactions occurring within a 40-bp enhancer region and that different arrays of protein-DNA interactions in this region are associated with the actively expressed state of the Pgk-2 gene in spermatocytes and spermatids and with the terminally repressed state of Pgk-2 in somatic cells.

Protein-DNA interactions associated with the onset of testis-specific expression of the mammalian Pgk-2 gene

We have identified difference in protein-DNA interactions associated with the promoter of the mammalian spermatogenesis-specific Pgk-2 gene in expressing and nonexpressing cells, using a band shift assay. We compared DNA-binding activities in nuclear protein extracts from expressing adult testis cells versus nonexpressing prepuberal testis cells and nonexpressing somatic cells. One or two DNA-binding activities were found to be uniquely associated with the expressed state of Pgk-2, while a third appears to be associated with the nonexpressed state. All three of these activities map to a region within the first 40 bp upstream from the core promoter of this gene. The Pgk-2 core promoter lacks a TATA box but contain a GC box and a CAAT box. We show that the GC box binds the ubiquitous transcription factor Sp1 and that the CAAT box binds CTF-1, both of which are present in extracts from all three tissue types tested. These results suggest that tissue-specific transcription of the Pgk-2 gene is associated with changes in protein-DNA interactions occurring within a 40-bp enhancer region and that different arrays of protein-DNA interactions in this region are associated with the actively expressed state of the Pgk-2 gene in spermatocytes and spermatids and with the terminally repressed state of Pgk-2 in somatic cells.

An analysis of vertebrate mRNA sequences: intimations of translational control

Five structural features in mRNAs have been found to contribute to the fidelity and efficiency of initiation by eukaryotic ribosomes. Scrutiny of vertebrate cDNA sequences in light of these criteria reveals a set of transcripts--encoding oncoproteins, growth factors, transcription factors, and other regulatory proteins--that seem designed to be translated poorly. Thus, throttling at the level of translation may be a critical component of gene regulation in vertebrates. An alternative interpretation is that some (perhaps many) cDNAs with encumbered 5' noncoding sequences represent mRNA precursors, which would imply extensive regulation at a posttranscriptional step that precedes translation.

Localization of pre-messenger RNA at discrete nuclear sites

We have studied the nuclear localization of rhodamine-labeled pre-mRNA after microinjection into nuclei of cultured rat kidney epithelial cells. Intranuclear localization of the injected RNA was followed in the living cells by fluorescence microscopy and digital image processing. Injected human beta-globin pre-mRNA became localized in 30-60 discrete nuclear sites that were coincident with loci defined by monoclonal antibodies against small nuclear ribonucleoproteins (Sm) or another spliceosome component (SC-35) in parallel immunocytochemical studies on the same nuclei. Similar patterns of nuclear localization were observed with a rat proenkephalin pre-mRNA. Nuclear microinjection of an intronlacking beta-globin RNA, a splicing-defective beta-globin mutant pre-mRNA, or an antisense beta-globin pre-mRNA did not result in localization at discrete sites. These results indicate that pre-mRNA binds preferentially to nuclear Sm and SC-35 antibody-reactive sites in vivo and that the binding requires intron sequences.

Proenkephalin products are stored in the sperm acrosome and may function in fertilization

Previous studies have shown that spermatogenic cells are a major source of testicular RNA encoding the opioid peptide precursor proenkephalin, suggesting that proenkephalin-derived peptides may function as intratesticular paracrine factors produced by male germ cells. However, direct evidence for the production of proenkephalin by spermatogenic cells has been lacking. In this report, we have used polysome profile analysis, peptide quantitation, and immunocytochemistry to show that proenkephalin products are synthesized during spermatogenesis and are retained within spermatozoa of humans, hamsters, rats, and sheep. We further show that these peptides are stored in the sperm acrosome and are depleted from sperm following the acrosome reaction, an exocytotic event required for fertilization. Proenkephalin products thus may serve a dual function as sperm acrosomal factors released during the fertilization process as well as intratesticular regulators secreted by spermatogenic cells.