Cell-specific expression of preproenkephalin intronic heteronuclear RNA in the rat forebrain

Opiate Drugs with Abuse Liability Hijack the Endogenous Opioid System to Disrupt Neuronal and Glial Maturation in the Central Nervous System

The endogenous opioid system, comprised of multiple opioid neuropeptide and receptor gene families, is highly expressed by developing neural cells and can significantly influence neuronal and glial maturation. In many central nervous system (CNS) regions, the expression of opioid peptides and receptors occurs only transiently during development, effectively disappearing with subsequent maturation only to reemerge under pathologic conditions, such as with inflammation or injury. Opiate drugs with abuse liability act to modify growth and development by mimicking the actions of endogenous opioids. Although typically mediated by μ-opioid receptors, opiate drugs can also act through δ- and κ-opioid receptors to modulate growth in a cell-type, region-specific, and developmentally regulated manner. Opioids act as biological response modifiers and their actions are highly contextual, plastic, modifiable, and influenced by other physiological processes or pathophysiological conditions, such as neuro-acquired immunodeficiency syndrome. To date, most studies have considered the acute effects of opiates on cellular maturation. For example, activating opioid receptors typically results in acute growth inhibition in both neurons and glia. However, with sustained opioid exposure, compensatory factors become operative, a concept that has been largely overlooked during CNS maturation. Accordingly, this article surveys prior studies on the effects of opiates on CNS maturation, and also suggests new directions for future research in this area. Identifying the cellular and molecular mechanisms underlying the adaptive responses to chronic opiate exposure (e.g., tolerance) during maturation is crucial toward understanding the consequences of perinatal opiate exposure on the CNS.

Actions of sex steroids on behaviours beyond reproductive reflexes

The actions of sex steroids in the brain have been shown, from molecular to systems levels, to control reproductive behaviour in a wide range of vertebrates. It has become increasingly clear that gonadal steroid hormones have regulatory functions which extend far beyond the direct coordination of an animal's physiological state and its display of sexual behaviour. While some of these actions may include changes in mood or other behavioural measures, such as exploration or excitability, sex steroid hormones also influence neural plasticity, neuronal activity and, possibly, learning and memory, as reflected by long-term potentiation or age-related deficits. Here we describe two systems that have been used to explore the non-reproductive roles of gonadal steroid hormones. The first of these is to examine the oestrogen-sensitive opioid peptide gene expression in the hypothalamus. Currently, we are attempting to identify the types of behaviour which may be altered consequent to the oestrogenic induction of the preproenkephalin gene. The second approach involves studying the effects of progesterone at the neuronal cell membrane and characterizing the metabolites of progesterone which have benzodiazepine-like actions in the brain. A number of studies suggest that this may provide an alternative mechanism through which progesterone can influence mood or behaviour.

Expression of an intron-containing beta-tubulin mRNA in catfish olfactory epithelium

Beta-tubulin genes code for very similar proteins, sharing extensive identity in amino acid sequence within and across species, each of which manifests characteristic patterns of cell and tissue expression. While searching for olfactory specific mRNAs in the channel catfish (Ictalurus punctatus), we isolated a novel beta-tubulin cDNA. In the putative ORF, 1298 nucleotides were 80-88% identical to cloned cDNAs from zebrafish to human for beta-tubulin isotype IVb. This ORF is interrupted by an insert of 111 nucleotides located between the regions corresponding to exons 2 and 3 in other species. This insert lacks similarity to any sequence in the NCBI databases. We showed that this novel cDNA fragment hybridizes specifically to catfish olfactory epithelium mRNA on Northern analysis. Here we demonstrate by in situ analysis of catfish olfactory epithelium that the expression of this mRNA is spatially restricted to the outer two-thirds of each olfactory lamella where olfactory receptor neurons reside. These results suggest that this nucleotide sequence is the result of incomplete RNA transcript processing. The growing awareness of the regulatory roles played by RNAs transcribed from intronic regions of genes suggests that this observation may have relevance to regulation of gene expression in olfactory tissue during development and axon targeting.

Primary structure of guinea pig preprodynorphin and preproenkephalin mRNAs: multiple transcription initiation sites for preprodynorphin

Preprodynorphin and preproenkephalin are protein precursors from which are derived two classes of opioid neurotransmitter peptides. Dynorphin A((1-17)) is produced by proteolytic processing of prodynorphin, and processing of proenkephalin yields the enkephalin peptides. We report here on the isolation and sequencing of multiple clones for these two mRNAs from a cDNA library. Two cDNA clones of preprodynorphin contained the full-length sequence (2.35 kb) with the primary structure predicted from the guinea pig gene sequence. In contrast, one clone encoded the full-length sequence but also an additional 192 nt at the 5' end. This sequence has high homology to the 5' flanking region of the human preprodynorphin gene, and RNase protection assays demonstrated that in addition to a primary initiation site, transcription of this mRNA is initiated at several sites 160-190 nt 5' with respect to the primary site. This difference may alter translational efficiency or mRNA stability. The sequence of preproenkephalin cDNA clones confirmed the structure predicted from the gene sequence. One clone, however, contained sequences encoded by exons 2 and 3, and initiated within the first intron (intron A) of the gene. We used RNase protection mapping to assess the abundance in the brain and pituitary of preproenkephalin transcripts that initiate within intron A. These studies confirmed that the primary transcription start site is 28 nucleotides downstream from the TATAA site, and that intron A sequences are not present in significant amounts in these tissues.

Systemic immune challenge induces preproenkephalin gene transcription in distinct autonomic structures of the rat brain

The involvement of enkephalins in the immune response was investigated in rats injected intravenously with interleukin-1beta (2 microg/kg). In situ hybridization with a riboprobe complementary to intron A of the preproenkephalin (ppENK) gene showed distinct transcriptional activation within several brain regions known to be activated by immune stimuli, including the nucleus of the solitary tract, the area postrema, the paraventricular hypothalamic nucleus, and the oval nucleus of the bed nucleus of the stria terminalis, and dual labeling confirmed that a large proportion of the intron expressing neurons co-expressed c-fos mRNA. Rats injected with saline (controls) showed little or no heteronuclear transcript in these structures. The induced signal was strongest after 1 hour but was present in some structures 30 minutes after interleukin-1beta injection. At 3 hours, transcriptional activity returned to basal levels. High basal expression of the heteronuclear transcript that appeared unchanged by the immune stimulus was seen in regions not primarily involved in the immune response, such as the striatum, the olfactory tubercle, and the islands of Calleja and in the immune activated central nucleus of the amygdala. The heteronuclear transcript colocalized with ppENK mRNA, demonstrating that it occurred in enkephalinergic neurons and was not the result of alternative transcription from the ppENK gene in other cells. These results demonstrated that enkephalin transcription is induced in central autonomic neurons during immune challenge, suggesting that enkephalins are involved in the centrally orchestrated response to such stimuli.

Localization of the secretory granule marker protein chromogranin B in the nucleus. Potential role in transcription control

Chromogranins A (CGA) and B (CGB) are two major Ca(2+) storage proteins of the secretory granules of neuroendocrine cells. Nevertheless, we found in the present study that CGB was also localized in the nucleus. In immunogold electron microscopy using bovine adrenal medullary chromaffin cells, it was found that the number of CGB-labeled gold particles localized per microm(2) of the nucleus was equivalent to 20% that of CGB-labeled gold particles localized per microm(2) of the secretory granules. Considering that CGB is estimated to exist in the 0.1-0.2-mm range in the secretory granules of bovine chromaffin cells, 20% of these amounts to 20-40 microm. In addition, transfection of CGA and CGB into nonneuroendocrine COS-7 and NIH3T3 cells repeatedly indicated the nuclear localization of CGB in addition to its usual localization in the cytoplasm. Moreover, immunoblot and immunogold electron microscopy analyses of neuroendocrine PC12 cells also showed the existence of endogenous CGB in both the cytosol and the nucleus. Nuclear routing of CGB did not appear to depend entirely upon the nuclear localization signal as some of the nuclear localization signal mutant CGB were still targeted to the nucleus. In gene array assay, CGB was shown to either induce or suppress transcription of many genes including those of transcription factors. Of these we have analyzed eight genes, four induced (zinc finger protein, MEF2C, hCRP2, abLIM) and four suppressed (hcKrox, T3-receptor, troponin C, integrin) using the quantitative reverse transcription-PCR method and spectrophotometry to determine the transcription levels of each mRNA. CGB was shown to increase the transcription of zinc finger protein, MEF2C, hCRP2, and abLIM by 2.5-5-fold while suppressing that of hcKrox, T3-receptor, troponin C, and integrin by 60-75%. Given that MEF2C and hcKrox genes are transcription factors, these results pointed to the transcription control role of CGB in the nucleus.

Opioids intrinsically inhibit the genesis of mouse cerebellar granule neuron precursors in vitro: differential impact of mu and delta receptor activation on proliferation and neurite elongation

Although opioids are known to affect neurogenesis in vivo, it is uncertain the extent to which opioids directly or indirectly affect the proliferation, differentiation or death of neuronal precursors. To address these questions, the intrinsic role of the opioid system in neurogenesis was systematically explored in cerebellar external granular layer (EGL) neuronal precursors isolated from postnatal mice and maintained in vitro. Isolated neuronal precursors expressed proenkephalin-derived peptides, as well as specific mu and delta, but negligible kappa, opioid receptors. The developmental effects of opioids were highly selective. Morphine-induced mu receptor activation inhibited DNA synthesis, while a preferential delta2-receptor agonist ([D-Ala2]-deltorphin II) or Met-enkephalin, but not the delta1 agonist [D-Pen2, D-Pen5]-enkephalin, inhibited differentiation within the same neuronal population. If similar patterns occur in the developing cerebellum, spatiotemporal differences in endogenous mu and delta opioid ligand-receptor interactions may coordinate distinct aspects of granule neuron maturation. The data additionally suggest that perinatal exposure to opiate drugs of abuse directly interfere with cerebellar maturation by disrupting normal opioid signalling and inhibiting the proliferation of granule neuron precursors.

Changes in neurotensin mRNA by estrogen in the female rat preoptic area during aging: an in situ hybridization histochemistry study

The present study examined changes in the response of neurotensin mRNA to estrogen during aging at the single cell level. Ten days after ovariectomy, 3-, 10-, and 15-month-old female rats were implanted with estrogen or cholesterol and sacrificed 4 days later. An in situ hybridization study revealed that estrogen significantly increased the number of cells expressing neurotensin mRNA in the preoptic area of all age groups. Furthermore, frequency analysis indicated that estrogen significantly increased the proportion of heavily labeled cells in older rats but not in younger rats. Distributions of the grains/cells between cholesterol- and estrogen-treated rats suggested that older rats were at least as responsive to estrogen as young rats and possibly even more responsive. The result suggests that, at least as reflected by neurotensin mRNA, reproductive senescence in rats is not due to a general decrease in sensitivity to estrogen. Indeed, there is evidence of an increased responsiveness to estrogen with age.

Novel transcriptional mechanisms are involved in regulating preproenkephalin gene expression in vivo

For the dissection of the temporal and spatial patterns of cell- and tissue-specific gene expression an understanding of the contributing regulating mechanisms is required. We now confirm that there are novel mechanisms regulating preproenkephalin gene expression in basal as well as cholinergic agonist treated rats. Moreover, we demonstrate that these novel transcriptional mechanisms are consistent with RNA intragenic elongation pausing, alternate promoter usage, and small sense and antisense RNA transcription from the preproenkephalin gene locus. We report that while basal striatal and olfactory bulb proenkephalin RNA transcripts are initiated from the "normal" proximal promoter, in cerebellum de novo RNA transcription appears to be initiated from the distal so-called "germ-cell" promoter. Furthermore, "normally" initiated olfactory bulb proenkephalin RNA transcripts appear to be down-regulated by the time the RNA polymerase II complex reaches the first preproenkephalin intron, in a way that is consistent with RNA elongation pausing. As the pattern of small sense and antisense transcripts found associated with this gene's expression is tissue-specific, we suggest that they may also play a role in regulating gene expression. The understanding of this gene's regulation should have widespread importance, not only to those interested in opioid gene expression, but also to those interested in gene regulation, in general.

Opioid receptor and peptide mRNA expression in proliferative zones of fetal rat central nervous system

There is increasing evidence to suggest that opioid peptides may have widespread effects as regulators of growth. To evaluate the hypothesis that endogenous opioids control cellular proliferation during neural development, we have used in situ hybridization to examine opioid peptide and receptor mRNA expression in neuroepithelial zones of fetal rat brain and spinal cord. Our data show that proenkephalin mRNA is widely expressed in forebrain germinal zones and choroid plexus during the second half of gestation. In contrast, prodynorphin mRNA expression is restricted to the periventricular region of the ventral spinal cord. Little µ or delta receptor mRNA expression was detected in any regions of neuronal proliferation prior to birth. However, kappa receptor mRNA is widely expressed in hindbrain germinal zones during the 3rd week of gestation. Our present findings support the hypothesis that endogenous opioids may regulate proliferation of both neuronal and non-neuronal cells during central nervous system development. Given the segregated expression of proenkephalin mRNA in forebrain neuroepithelium and kappa receptor mRNA within hindbrain, different opioid mechanisms may regulate cell division in rostral and caudal brain regions.Key words: enkephalin, dynorphin, ontogeny, neurogenesis.

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.

Estrogen regulation of preproenkephalin messenger RNA in the forebrain of female mice

We studied the effects of 10 micrograms of 17-beta-estradiol-3-benzoate treatment on preproenkephalin (PPE) mRNA expression in female ovariectomized (OVX) Swiss Webster mice after 0, 1, 6, 12, 24, or 48 h, using the in situ hybridization technique. The VMH showed a 1.6- and 3.3-fold increase in PPE mRNA levels after 24 and 48 h of estrogen treatment (respectively) when compared to OVX females. No differences at 1, 6 or 12 h of estrogen treatment groups were observed compared to control groups. PPE mRNA levels were also increased at 24 and 48 h after estrogen treatment in the posterior medial nucleus of the amygdala (MeAmyg) by 3.3- and 2.5-fold, respectively, and in the arcuate nucleus (ARC) by 2- and 1.9-fold, respectively. No effects of estrogen were observed on PPE mRNA levels in the caudate-putamen (CPu) or the posterior lateral cortical nucleus of the amygdala (plCoAmyg). Furthermore, basal levels of PPE mRNA expression in the VMH and MeAmyg of female mice were lower than those observed in rats, although levels in the CPu, plCoAmyg, and ARC were similar between females of the two species. In conclusion, we have found two differences between the species. First, Swiss mice demonstrated a slower time course of estrogen induction of PPE mRNA in the VMH, ARC. and MeAmyg compared to female rats. Second, there are differences in basal levels of PPE in the MeAmyg and VMH.

Anesthesia during hormone administration abolishes the estrogen induction of preproenkephalin mRNA in ventromedial hypothalamus of female rats

Estrogen treatment increases preproenkephalin (PPE) mRNA levels in the ventromedial nucleus of the hypothalamus (VMH). Roy et al. (Brain Res., 337 (1985) 163-166) discovered that anesthesia during estrogen priming could reduce female rat sexual receptivity. In the present study we tested whether the action of estrogen to induce PPE gene expression in the VMH could be similarly affected by anesthesia. By quantitative in situ hybridization and slot-blot analysis techniques we found a 1.8-fold increase in PPE mRNA levels in the VMH after 1 hour of estrogen treatment in ovariectomized (OVX) Sprague-Dawley female rats. Anesthetizing the rats with pentobarbital for 1 h during the exposure to estrogen blocked the estrogen induction of PPE mRNA in the VMH. By way of contrast no changes in the PPE mRNA levels were observed in the caudate putamen. A similar trend was seen using chloral hydrate. It appears that neuronal activity is required for the early phase of estrogen induction of PPE mRNA levels in the VMH. This in turn could be correlated with changes in female sociosexual behaviors.

Estrogen modulation of mRNA levels for the two forms of glutamic acid decarboxylase (GAD) in female rat brain

Two separate forms of glutamic acid decarboxylase, now termed GAD65 and GAD67, are the rate limiting enzymes for synthesis of gamma-aminobutyric acid (GABA). Because of the significance of GABA to neuroendocrine processes, numerous attempts have been made to determine the impact of gonadal steroids on enzyme functioning with inconclusive results. Therefore, we attempted to determine the impact of estradiol on mRNA levels for each form of GAD by quantitative in situ hybridization histochemistry in various brain regions. Ovariectomized rats were treated with estradiol benzoate or oil vehicle on 2 consecutive days and the brains collected on the third day. DNA probes specific for GAD65 and GAD67 were radiolabeled with CTP32 using asymmetric polymerase chain reaction. Results of in situ hybridizations for each probe on alternate sections from the same animals were analyzed for magnocellular preoptic area (McPOA), dorsal medial nucleus of the hypothalamus (DMN), zona incerta (ZI), and midbrain central gray (MCG). In the McPOA, estradiol exerted opposite effects on the frequency distribution of pixels per cell for two GAD mRNA probes, significantly increasing GAD65 (P < .05) and decreasing GAD67 (P < .01; Kolmogorov-Smirnov). In the DMN, estradiol treatment significantly increased GAD67 by 60% (P < .05; two-way ANOVA) but decreased GAD65 mRNA by 73% (P < .01). Note the direction of effects are opposite between McPOA and DMN. In MCG, analysis showed no estradiol effect on GAD mRNA levels/cells, but the proportion of cells expressing detectable levels of GAD65 or GAD67 increased by 33-40% in estradiol-treated rats (chi 2, P < .001).

Proenkephalin is a nuclear protein responsive to growth arrest and differentiation signals

Neuropeptide precursors are traditionally viewed as molecules destined to be cleaved into bioactive peptides, which are then released from the cell to act on target cell surface receptors. In this report we demonstrate nuclear localization of the enkephalin precursor, proenkephalin, in rodent and human embryonic fibroblasts (Swiss 3T3 and MRC-5 cells) and in rodent myoblasts (C2C12 cells). Nuclear proenkephalin, detected by immunofluorescence with a panel of antiproenkephalin monoclonal antibodies, is distributed predominantly in three patterns. Selective abolition of these patterns with salt, nuclease, or methanol is associated with liberation of immunoprecipitable proenkephalin into the extraction supernatant. Proenkephalin antigenic domains, mapped using phage display libraries and synthetic peptides, are differentially revealed in the three distribution patterns. Selective epitope revelation may reflect different conformational forms of proenkephalin or its existence in complexes with other nuclear proteins, forms which therefore have different biochemical associations with the nuclear substructure. In fibroblast cell populations in transition to growth arrest, nuclear proenkephalin responds promptly to mitogen withdrawal and cell-cell contact by transient, virtually synchronous unmasking of multiple antigenic domains in a fine punctate distribution. A similar phenomenon is observed in myoblasts undergoing differentiation. The acknowledgment of growth arrest and differentiation signals by nuclear proenkephalin suggests its integration with transduction pathways mediating these signals. To begin to address the mechanism of nuclear targeting, we have transfected mutated and nonmutated proenkephalin into COS (African green monkey kidney) cells. Nonmutated proenkephalin is localized exclusively in the cytoplasm; however, proenkephalin mutated at the first ATG codon, or devoid of its signal peptide sequence, is targeted to the nucleus as well as to the cytoplasm. From this we speculate that nuclear proenkephalin arises from a primary translation product that lacks a signal peptide sequence because of initiation at a different site.

The transcriptional regulation of the preproenkephalin gene

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Changes in preproenkephalin messenger RNA level in the rat ventromedial hypothalamus during the estrous cycle

To gain a better understanding of the relationship between the female rat reproductive system and preproenkephalin (PPE) expressing neurons under physiological conditions, we examined changes in PPE mRNA levels in the mediobasal hypothalamus during the rat estrous cycle by means of northern blotting and in situ hybridization histochemistry (ISHH). In the Northern blot studies, we found that PPE mRNA levels in the mediobasal hypothalamus were significantly increased by noon of proestrus compared to those in the morning and stayed high until diestrus day 1, and returned toward low levels on diestrous day 2. In contrast, measured as controls, glyceraldehyde-3-phosphate-dehydrogenase mRNA levels were significantly higher on proestrus regardless of time of day compared to diestrus day 2, and levels of calcineurin mRNA on proestrous and estrous were significantly lower than diestrous day 1 and day 2. ISHH studies revealed that these changes in PPE mRNA levels were specific in the ventromedial hypothalamic nucleus pars ventrolateralis (VMHVL), since we could not see any significant changes in signal in other parts including ventromedial hypothalamic nucleus pars dorsomedialis and arcuate hypothalamic nucleus. In the VMHVL, PPE mRNA levels in the afternoon of proestrous were significantly higher than those in the afternoon of diestrous day 2 whereas no significant change in PPE mRNA was observed in the caudate-putamen. The present study provides additional information relevant to possible implications of PPE gene expression in female reproductive systems, since changes in PPE mRNA levels may be associated with estrogen as well as progesterone or other hormonal concentrations during the estrous cycle.(ABSTRACT TRUNCATED AT 250 WORDS)