Interleukin-1 beta regulates proenkephalin gene expression in astrocytes cultured from rat cortex

Prefrontal dopaminergic and enkephalinergic synaptic accommodation in HIV-associated neurocognitive disorders and encephalitis

Changes in synapse structure occur in frontal neocortex with HIV encephalitis (HIVE) and may contribute to HIV-associated neurocognitive disorders (HAND). A postmortem survey was conducted to determine if mRNAs involved in synaptic transmission are perturbed in dorsolateral prefrontal cortex (DLPFC) in subjects with HIVE or HAND. Expression of the opioid neurotransmitter preproenkephalin mRNA (PENK) was significantly decreased in a sampling of 446 brain specimens from HIV-1 infected people compared to 67 HIV negative subjects. Decreased DLPFC PENK was most evident in subjects with HIVE and/or increased expression of interferon regulatory factor 1 mRNA (IRF1). Type 2 dopamine receptor mRNA (DRD2L) was decreased significantly, but not in the same set of subjects with PENK dysregulation. DRD2L downregulation occurred primarily in the subjects without HIVE or neurocognitive impairment. Subjects with neurocognitive impairment often failed to significantly downregulate DRD2L and had abnormally high IRF1 expression. Conclusion: Dysregulation of synaptic preproenkephalin and DRD2L in frontal neocortex can occur with and without neurocognitive impairment in HIV-infected people. Downregulation of DRD2L in the prefrontal cortex was associated with more favorable neuropsychological and neuropathological outcomes; the failure to downregulate DRD2L was significantly less favorable. PENK downregulation was related neuropathologically to HIVE, but was not related to neuropsychological outcome independently. Emulating endogenous synaptic plasticity pharmacodynamically could enhance synaptic accommodation and improve neuropsychological and neuropathological outcomes in HIV/AIDS.

Interactive comorbidity between opioid drug abuse and HIV-1 Tat: chronic exposure augments spine loss and sublethal dendritic pathology in striatal neurons

HIV-1 infection predisposes the central nervous system to damage by opportunistic infections and environmental insults. Such maladaptive plasticity may underlie the exaggerated comorbidity seen with HIV-1 infection and opioid abuse. Although morphine and HIV-1 Tat synergize at high concentrations to increase neuronal death in vitro, we questioned whether chronic low Tat exposure in vivo might contribute to the spectrum of neuropathology through sublethal neuronal injury. We used a doxycycline-driven, inducible, HIV-1 Tat transgenic mouse, in which striatal neuron death was previously shown to be absent, to examine effects of differential Tat expression, alone and combined with morphine. Low constitutive Tat expression caused neurodegeneration; higher levels induced by 7 days of doxycycline significantly reduced dendritic spine numbers. Moreover, Tat expression widely disrupted the endogenous opioid system, altering mu and kappa, but not delta, opioid receptor and proopiomelanocortin, proenkephalin, and prodynorphin transcript levels in cortex, hippocampus, and striatum. In addition to markedly reducing spine density by itself, morphine amplified the effect of higher levels of Tat on spines, and also potentiated Tat-mediated dendritic pathology, thus contributing to maladaptive neuroplasticity at multiple levels. The dendritic pathology and reductions in spine density suggest that sustained Tat +/- morphine exposure underlie key aspects of chronic neurodegenerative changes in neuroAIDS, which may contribute to the exacerbated neurological impairment in HIV patients who abuse opioids.

Methionine-enkephalin and leucine-enkephalin increase interleukin-1 beta release in mixed glia cultures

Interleukin-1 beta (IL-1 beta) is synthesized in the brain in response to LPS. Excessive IL-1 beta expression is observed in neurodegenerative diseases. The aim of this study was to evaluate the effects of methionine-enkephalin (ME) and leucine-enkephalin (LE) on the baseline and LPS-activated release of IL-1 beta in rat mixed glia cultures. ME and LE increased LPS-induced IL-1 beta release, which was not blocked by naloxone. Both ME and LE increased the baseline release of IL-1 beta, which was completely blocked by naloxone pretreatment. Mixed glia cultures deprived of microglia (by shaking and incubating with L-leucine methyl ester) did not release IL-1 beta, which indicates microglia as a source of the changes in IL-1 beta release. The results of the study suggest that neurons may regulate glial activity through releasing enkephalins.

Opioid receptor gene: cytokine response element and the effect of cytokines

Previous studies demonstrated that cytokines regulate opioid and opioid receptor gene expression in neuronal and immune cells. The gene sequence analysis of opioid receptors revealed that mu-, delta- and kappa-opioid receptor promoter regions contain potential cytokine response elements (NF-IL6 binding sites). It was postulated that the response elements present in opioid receptor promoter regions may have a role in the cytokine effects on opioid receptor gene expression through cis-trans interaction. The present study investigated whether cytokines have an effect on opioid receptor gene expression by cytokine-induced transcription factor, NF-IL6, using a number of immune cell lines which respond to cytokines. Further investigation was made to determine whether the potential cytokine response element DNA sequences on opioid receptor promoter region have functional significance which may be affected by the DNA context of the opioid receptor promoter in immune cell lines. Tandem repeats of conserved cytokine response elements of IL-6 gene fused to a heteropromoter SV40 were utilized as a positive control and expressed 2-fold increased promoter activity after cytokine stimulation. Transient transfection studies in time courses (24-72 h) and different dose treatments (100-500 U/ml for IL-6 and 50-200 U/ml for IL-1 alpha+beta) were also carried out to investigate the possibility that the upregulated gene expression may be transient or cytokine-dose-dependent. Our data demonstrated that there was no significant cytokine-stimulated opioid receptor gene expression in immune cells tested. In addition, the cytokine response elements (NF-IL6 binding sites) in opioid receptor genes are not functional. These results contradict the previous reports in which cytokines modulated the expression of opioid and opioid receptors in neuronal and immune cells. The possible reasons regarding the different results were discussed.

Regulation of nociceptin/orphanin FQ gene expression by neuropoietic cytokines and neurotrophic factors in neurons and astrocytes

We have identified the gene encoding nociceptin/orphanin FQ (N/OFQ), the novel opioid-like neuropeptide, as responsive to ciliary neurotrophic factor (CNTF). N/OFQ mRNA levels were induced five- and ninefold by CNTF in striatal and cortical neurons. In primary astrocytes CNTF also increased N/OFQ mRNA levels. CNTF is a multifunctional cytokine that mediates the development and differentiation of both neurons and astrocytes and supports the survival of various neurons. CNTF is also an injury-induced factor in the brain playing a crucial role in astrogliosis. The mechanism by which CNTF elicits these effects is not well understood, but it is likely to involve regulation of specific genes. CNTF regulation of N/OFQ expression was sensitive to the kinase inhibitors H-7 and genistein but not to inhibition of protein synthesis. This pharmacological profile is consistent with CNTF activating the Janus protein tyrosine kinase (JAK)/ signal transducers and activators of transcription (STAT) pathway to induce N/OFQ transcription. In nuclear extracts of CNTF-treated striatal neurons DNA binding of STAT proteins was increased. Radioimmunoassays revealed elevated N/OFQ immunoreactivity in striatal neurons after CNTF treatment. Expression of the related proenkephalin gene was not affected by CNTF in either neuronal or glial cultures. Regulation of N/OFQ expression by CNTF might point to a possible function of N/OFQ during development and after neural injury.

The interleukin-1beta-mediated regulation of proenkephalin and opioid receptor messenger RNA in primary astrocyte-enriched cultures

Opioids have been found to modulate the function of the immune system by regulating the biochemical and proliferative properties of its cellular components. The interaction of opioid and immune systems, however, is not unidirectional, but rather, bidirectional in nature. In the CNS, one cellular target of immune system activation is the astrocytes, glial cells known to synthesize proenkephalin. We have recently shown that these cells also express the messenger RNA transcripts for the opioid receptors mu, delta and kappa, raising the question of the functional significance of this opioid peptide and the related receptors in the astrocytes. That is, why do astrocytes express proenkephalin and opioid receptors, and are these molecules responsive to a factor to which the astrocytes could be exposed in vivo? Furthermore, do these molecules respond to this factor in a region-specific fashion? In the present study, in order to characterize the astrocytic opioid response to an immune factor, we examined the concomitant regulation of mu, delta, kappa and proenkephalin messenger RNAs by interleukin-1beta (1 ng/ml=60 pM, 24 h) in primary astrocyte-enriched cultures derived from the rat (post-natal day 1-2) cortex, striatum, cerebellum, hippocampus and hypothalamus. Interleukin-1beta treatment was found to increase by 55-75% the level of mu receptor messenger RNA in striatal, cerebellar and hippocampal cultures, but not in cultures derived from the cortex or hypothalamus. However, the cytokine had no effect on the level of delta receptor messenger RNA in any of the five cultures examined. In marked contrast to its stimulatory effects on mu receptor messenger RNA levels and its lack of an effect on 6 receptor messenger RNA expression, interleukin-1beta reduced to 10-30% of control levels the kappa receptor messenger RNA levels in all cultures. Interleukin-1beta had no effect on the level of proenkephalin messenger RNA in cortical, striatal, cerebellar and hypothalamic cultures, but did significantly decrease the expression of proenkephalin messenger RNA in hippocampal cultures to 40% of the control level. Therefore, interleukin-1beta differentially regulated opioid receptor messenger RNA in astrocyte-enriched cultures in a manner dependent upon both the receptor type and the brain region from which the culture was derived. The cytokine also differentially regulated proenkephalin messenger RNA in a region-dependent fashion. These findings suggest a capacity for astrocytes to differentially regulate opioid peptide and receptor messenger RNAs in response to an immune factor, supporting the potential existence of a novel immune-opioid system interaction in the CNS.

Interleukin-1 enhances the ATP-evoked release of arachidonic acid from mouse astrocytes

During neuropathological states associated with inflammation, the levels of cytokines such as interleukin-1beta (IL-1beta) are increased. Several studies have suggested that the neuronal damage observed in pathogenesis implicating IL-1beta are caused by an alteration in the neurochemical interactions between neurons and astrocytes. We report here that treating striatal astrocytes in primary culture with IL-1beta for 22-24 hr enhances the ATP-evoked release of arachidonic acid (AA) with no effect on the ATP-induced accumulation of inositol phosphates. The molecular mechanism responsible for this effect involves the expression of P2Y2 receptors (a subtype of purinoceptor activated by ATP) and cytosolic phospholipase A2 (cPLA2, an enzyme that mediates AA release). Indeed, P2Y2 antisense oligonucleotides reduce the ATP-evoked release of AA only from IL-1beta-treated astrocytes. Further, both the amount of cPLA2 (as assessed by Western blotting) and the release of AA resulting from direct activation of cPLA2 increased fourfold in cells treated with IL-1beta. We also report evidence indicating that the coupling of newly expressed P2Y2 receptors to cPLA2 is dependent on PKC activity. These results suggest that during inflammatory conditions, IL-1beta reveals a functional P2Y2 signaling pathway in astrocytes that results in a dramatic increase in the levels of free AA. This pathway may thus contribute to the neuronal loss associated with cerebral ischemia or traumatic brain injury.

Transcription factor AP-2 regulates human apolipoprotein E gene expression in astrocytoma cells

Apolipoprotein E (apoE), one of the major plasma lipoproteins, also is expressed in a variety of cell types, including the glial cells of the nervous system. apoE is involved in processes of degeneration and regeneration after nerve lesions as well as in the pathogenesis of Alzheimer's disease (AD). Glial synthesis of apoE is activated in response to injury both in the peripheral and central nervous system. We now report that the activity of the proximal apoE promoter in astrocytes is upregulated by cAMP and retinoic acid, which act synergistically. Sequence analysis of the apoE promoter indicated the presence of several AP-2 consensus sequences that could mediate the stimulatory effect of cAMP and retinoic acid. The possible functional role of AP-2 was examined by cotransfection of AP-2-deficient HepG2 cells with an apoE promoter construct and a human AP-2 expression construct. Cotransfection with AP-2 significantly elevated apoE promoter activity. DNase I footprinting technique revealed the existence of two binding sites for recombinant AP-2 in regions from -48 to -74 and from -107 to -135 of the apoE promoter. Mutations in these regions markedly impaired the trans-stimulatory effect of AP-2. These results indicate the existence of functional AP-2 sites in the promoter region of apoE that could contribute to the complex regulation of this gene in developmental, degenerative, and regenerative processess of the nervous system.

Induction of NF-kB-like transcription factors in brain areas susceptible to kainate toxicity

Administration of kainate (KA), a glutamate receptor agonist, to rats causes neuronal damage in the CA1/CA3 fields of the hippocampus and in the pyriform/ entorhinal cortex. Reactive gliosis also occurs and activated astrocytes upregulate their expression of a large number of molecules. Since NF-kB transcription factors are involved in cellular responses to diverse pathogenic stimuli and have been shown to be induced in astrocytes in vitro in response to cytokines and growth factors, we investigated their possible involvement in the changes in gene expression subsequent to KA-induced lesions. Immunoreactivity to the p65 subunit of NF-kB was markedly increased in non-neuronal cells 2 days after KA administration (8 mg/kg i.p.) in the areas of selective neuronal degeneration. This increase was not observed 3 h or 1 day after injection, but was still present 7-10 days after KA injection. By gel mobility-shift assay, a protein complex binding to the kB consensus sequence was found to be induced by 2 days after KA, which correlated with immunohistochemical findings. This NF-kB-protein complex seemed to be localized in reactive astrocytes, as indicated by the morphological similarity of NF-kB-positive cells and reactive astrocytes stained with glial fibrillary acidic protein (GFAP) antibody, and the parallelism between the time course of NF-kB induction and appearance of gliosis after KA treatment. Double immunocytochemistry experiments demonstrated the colocalization of NF-kB positive cells and reactive astrocytes. Our results suggest that activated NF-kB in astrocytes participates in delayed and long-term responses of glia to injury.

Neuron-glia interactions in the release of oxytocin and vasopressin from the rat neural lobe: the role of opioids, other neuropeptides and their receptors

The release of the neurohormones oxytocin and vasopressin from the neural lobe into the circulation is regulated in a complex manner, which has only been partly elucidated. At the level of the neural lobe, regulation of release can occur by various endogenous compounds that act on specific receptors present on the nerve terminals themselves. In addition, release may be modulated by an alternative pathway in which the local glia cells, the pituicytes, are involved. It is especially the latter pathway that is discussed in detail in this commentary.

Preproenkephalin mRNA expression in the developing and adult rat brain

[Met5]-Enkephalin is derived from the protein precursor, proenkephalin A, which in turn is encoded by the preproenkephalin (PPE) gene. [Met5]-Enkephalin is not only a putative neuromodulatory substance, but also serves as a growth factor (= opioid growth factor, OGF). OGF exerts an inhibitory influence on the developing nervous system and is especially targeted to cell proliferative and differentiative events. This study examined the relationship of PPE mRNA expression to late prenatal and postnatal rat brain development. Northern blot analysis of the whole brain and cerebellum showed that message is present in the fetal nervous system on prenatal day 15 (the earliest timepoint examined), is expressed at relatively similar levels within each tissue during the first 2 postnatal weeks, and reaches adult levels by the beginning of the 3rd postnatal week. In situ hybridization methodology revealed that PPE mRNA was prominent in areas associated with cell generation. Message was found in sites of primary (i.e., ventricular region) and secondary (e.g., external germinal layer of the cerebellum) cellular replication, as well as in discrete foci of cell proliferation (e.g., medullary layer of the cerebellum). PPE mRNA was also present for varying periods of time in postmitotic cells. During development, a number of patterns (decrease, increase, and no perceptible change) of PPE mRNA could be detected in relationship to the fetal/neonatal period. Given the strong evidence (e.g., regulation of cell proliferation and differentiation, temporal and spatial patterns of peptide and zeta opioid receptor) that enkephalin immunoreactivity is associated with proliferating and differentiating neurons and glia, these results suggest that the source of [Met5]-enkephalin is both autocrine and paracrine in nature.

Molecular profile of reactive astrocytes--implications for their role in neurologic disease

The central nervous system responds to diverse neurologic injuries with a vigorous activation of astrocytes. While this phenomenon is found in many different species, its function is obscure. Understanding the molecular profile characteristic of reactive astrocytes should help define their function. The purpose of this review is to provide a summary of molecules whose levels of expression differentiate activated from resting astrocytes and to use the molecular profile of reactive astrocytes as the basis for speculations on the functions of these cells. At present, reactive astrocytosis is defined primarily as an increase in the number and size of cells expressing glial fibrillary acidic protein. In vivo, this increase in glial fibrillary acidic protein-positive cells reflects predominantly phenotypic changes of resident astroglia rather than migration or proliferation of such cells. Upon activation, astrocytes upmodulate the expression of a large number of molecules. From this molecular profile it becomes apparent that reactive astrocytes may benefit the injured nervous system by participating in diverse biological processes. For example, upregulation of proteases and protease inhibitors could help remodel the extracellular matrix, regulate the concentration of different proteins in the neuropil and clear up debris from degenerating cells. Cytokines are key mediators of immunity and inflammation and could play a critical role in the regulation of the blood-central nervous system interface. Neurotrophic factors, transporter molecules and enzymes involved in the metabolism of excitotoxic amino acids or in the antioxidant pathway may help protect neurons and other brain cells by controlling neurotoxin levels and contributing to homeostasis within the central nervous system. Therefore, an impairment of astroglial performance has the potential to exacerbate neuronal dysfunction. Based on the synopsis of studies presented, a number of issues become apparent that deserve a more extensive analysis. Among them are the relative contribution of microglia and astrocytes to early wound repair, the characterization of astroglial subpopulations, the specificity of the astroglial response in different diseases as well as the analysis of reactive astrocytes with techniques that can resolve fast physiologic processes. Differences between reactive astrocytes in vivo and primary astrocytes in culture are discussed and underline the need for the development and exploitation of models that will allow the analysis of reactive astrocytes in the intact organism.