Cooperative interactions among afferents govern the induction of homosynaptic long-term depression in the hippocampus.

Prolonged periods of low-frequency stimulation have been shown to produce a robust, long-term synaptic depression (LTD) in both hippocampus and visual cortex. In the present study we have examined the extent to which interactions among afferents govern the induction of homosynaptic LTD in young-adult rats in hippocampal region CA1 in vitro. Field excitatory postsynaptic potentials were assessed before and after conditioning stimulation consisting of two 10-min trains of low-frequency stimulation (LFS; 1 Hz) of the Schaffer collateral/commissural pathway. LFS at an intensity producing a 0.5-mV response did not produce significant synaptic depression. However, LFS administered at a higher intensity resulted in significant input-specific LTD of a 0.5-mV test response. Picrotoxin, which also facilitates depolarization of CA1 neurons, significantly enhanced the magnitude of LTD after LFS at 0.5 mV. In addition, LFS at 0.5 mV in normal perfusion medium (no picrotoxin) produced only small changes in synaptic efficacy when either of two converging pathways was conditioned separately but produced a robust LTD when both pathways were conditioned simultaneously. This cooperative LTD was reversibly blocked by prior administration of 100 microM DL-aminophosphonovaleric acid but not by 20 microM nimodipine. Taken together, these results suggest that cooperative interactions among afferents contribute to voltage-dependent processes underlying the induction of homosynaptic LTD.

cAMP-dependent SOS induction and mutagenesis in resting bacterial populations.

The inducible SOS system increases the survival of bacteria exposed to DNA-damaging agents by increasing the capacity of error-free and error-prone DNA repair systems. The inducible mutator effect is expected to contribute to the adaptation of bacterial populations to these adverse life conditions by increasing their genetic variability. The evolutionary impact of the SOS system would be even greater if it was also induced under conditions common in nature, such as in resting bacterial populations. The results presented here show that SOS induction and mutagenesis do occur in bacteria in aging colonies on agar plates. The observed SOS induction and mutagenesis are controlled by the LexA repressor and are RecA- and cAMP-dependent.

Recovery of respiration following the SOS response of Escherichia coli requires RecA-mediated induction of 2-keto-4-hydroxyglutarate aldolase.

Agents that damage DNA in Escherichia coli or interfere with its replication induce DNA repair and mutagenesis via the SOS response. This well-known activity is regulated by the RecA protein and the LexA repressor. Following repair or bypass of the DNA lesion, the cell returns to its resting state by a largely unknown process. We found that 2-keto-4-hydroxyglutarate aldolase (4-hydroxy-2-oxoglutarate aldolase; EC 4.1.3.16) is necessary for the recovery of respiration and that it is regulated by the SOS response. This protein was induced by DNA-damaging agents. Induction required RecA activation. When the LexA regulon was repressed, activation of RecA was not sufficient for induction, indicating the requirement for an additional protein under LexA control. Finally, a mutant in the corresponding hga gene was UV sensitive. 2-Keto-4-hydroxyglutarate aldolase also plays a role in respiratory metabolic pathways, which suggests a mechanism for respiration resumption during the termination of the SOS response.

Multiple protein kinase A-regulated events are required for transcriptional induction by cAMP.

The second messenger cAMP stimulates the expression of numerous genes via the protein kinase A-mediated phosphorylation of the cAMP response element-binding protein (CREB) at Ser-133. Ser-133 phosphorylation, in turn, appears to induce target gene expression by promoting interaction between CREB and CBP, a 265-kDa nuclear phospho-CREB-binding protein. It is unclear, however, whether Ser-133 phosphorylation per se is sufficient for CREB-CBP complex formation and for target gene induction in vivo. Here we examine CREB activity in Jurkat T cells after stimulation of the T-cell receptor (TCR), an event that leads to calcium entry and diacylglycerol production. Triggering of the TCR stimulated Ser-133 phosphorylation of CREB with high stoichiometry, but TCR activation did not promote CREB-CBP complex formation or target gene induction unless suboptimal doses of cAMP agonist were provided as a costimulus. Our results demonstrate that, in addition to mediating Ser-133 phosphorylation of CREB, protein kinase A regulates additional proteins that are required for recruitment of the transcriptional apparatus to cAMP-responsive genes.

Thyroid-stimulating hormone (TSH)-directed induction of the CREM gene in the thyroid gland participates in the long-term desensitization of the TSH receptor.

Thyroid gland function is regulated by the hypothalamic-pituitary axis via the secretion of TSH, according to environmental, developmental, and circadian stimuli. TSH modulates both the secretion of thyroid hormone and gland trophism through interaction with a specific guanine nucleotide-binding protein-coupled receptor (TSH receptor; TSH-R), which elicits the activation of the cAMP-dependent signaling pathway. After TSH stimulation, the levels of TSH-R RNA are known to decrease dramatically within a few hours. This phenomenon ultimately leads to homologous long-term desensitization of the TSH-R. Here we show that TSH drives the induction of the inducible cAMP early repressor (ICER) isoform of the cAMP response element (CRE) modulator gene both in rat thyroid gland and in the differentiated thyroid cell line FRTL-5. The kinetics of ICER protein induction mirrors the down-regulation of TSH-R mRNA. ICER binds to a CRE-like sequence in the TSH-R promoter and represses its expression. Thus, ICER induction by TSH in the thyroid gland represents a paradigm of the molecular mechanism by which pituitary hormones elicit homologous long-term desensitization.

Induction of a retinoblastoma phosphatase activity by anticancer drugs accompanies p53-independent G1 arrest and apoptosis.

DNA-damaging agents induce accumulation of the tumor suppressor and G1 checkpoint protein p53, leading cells to either growth arrest in G1 or apoptosis (programmed cell death). The p53-dependent G1 arrest involves induction of p21 (also called WAF1/CIP1/SDI1), which prevents cyclin kinase-mediated phosphorylation of retinoblastoma protein (RB). Recent studies suggest a p53-independent G1 checkpoint as well; however, little is known about its molecular mechanisms. We report that induction of a protein-serine/threonine phosphatase activity by DNA damage signals is at least one of the mechanisms responsible for p53-independent, RB-mediated G1 arrest and consequent apoptosis. When two p53-null human leukemic cell lines (HL-60 and U-937) were treated with a variety of anticancer agents, RB became hypophosphorylated, accompanied with G1 arrest. This was followed immediately (in less than 30 min) by apoptosis, as determined by the accumulation of pre-G1 apoptotic cells and the internucleosomal fragmentation of DNA. Addition of calyculin A or okadaic acid (specific serine/threonine phosphatase inhibitors) or zinc chloride (apoptosis inhibitor) prevented the G1 arrest- and apoptosis-specific RB dephosphorylation. The levels of cyclin E- and cyclin A-associated kinase activities remained high during RB dephosphorylation, supporting the involvement of a chemotherapy-induced serine/threonine phosphatase(s) rather than p21. Furthermore, the induced phosphatase activity coimmunoprecipitated with the hyperphosphorylated RB and was active in a cell-free system that reproduced the growth arrest- and apoptosis-specific RB dephosphorylation, which was inhibitable by calyculin A but not zinc. We propose that the RB phosphatase(s) might be one of the p53-independent G1 checkpoint regulators.

Critical role of the interleukin 2 (IL-2) receptor gamma-chain-associated Jak3 in the IL-2-induced c-fos and c-myc, but not bcl-2, gene induction.

The interleukin 2 receptor (IL-2R) consists of three subunits, the IL-2R alpha, IL-2R beta c, and IL-2R gamma c chains. Two Janus family protein tyrosine kinases (PTKs), Jak1 and Jak3, were shown to associate with IL-2R beta c and IL-2R gamma c, respectively, and their PTK activities are increased after IL-2 stimulation. A Jak3 mutant with truncation of the C-terminal PTK domain lacks its intrinsic kinase activity but can still associate with IL-2R gamma c. In a hematopoietic cell line, F7, that responds to either IL-2 or IL-3, overexpression of this Jak3 mutant results in selective inhibition of the IL-2-induced activation of Jak1/Jak3 PTKs and of cell proliferation. Of the three target nuclear protooncogenes of the IL-2 signaling, c-fos and c-myc genes, but not the bcl-2 gene, were found to be impaired. On the other hand, overexpression of the dominant negative form of the IL-2R gamma c chain, which lacks most of its cytoplasmic domain, in F7 cells resulted in the inhibition of all three protooncogenes. These results provide a further molecular basis for the critical role of Jak3 in IL-2 signaling and also suggest a Jak PTK-independent signaling pathway(s) for the bcl-2 gene induction by IL-2R.

Specific induction of cAMP in Langerhans cells by calcitonin gene-related peptide: relevance to functional effects.

Epidermal Langerhans cells (LC) are associated anatomically with epidermal nerves, and a product of these nerves, calcitonin gene-related peptide (CGRP), inhibits the antigen-presenting capacity of LC and macrophages. As the CGRP receptor appears to be coupled to Gs alpha protein, which in turn activates adenylate cyclase, the ability of CGRP to induce cAMP in LC was examined and correlated with functional effects. LC were isolated from murine epidermal cells using antibodies on magnetic microspheres. Exposure to CGRP induced a significant increase in cAMP content, which could be inhibited by coculture with a truncated form of CGRP [CGRP-(8-37)] that is a specific competitive inhibitor of CGRP. Substance P and calcitonin failed to induce cAMP in LC. Although culture in CGRP reduced the ability of murine epidermal cells enriched for LC content to present pigeon cytochrome c to a responsive clone or to present antigen for elicitation of delayed-type hypersensitivity in immune mice, culture in forskolin had little or no effect on antigen presentation despite increased cAMP content of LC as much or more than that induced by CGRP. The effect of CGRP on antigen presentation in these systems could be blocked with CGRP-(8-37). CGRP inhibited the induction of B7-2 by lipopolysaccharide on peritoneal macrophages and a LC line, whereas calcitonin did not. CGRP induces specific accumulation of cAMP in LC and inhibits LC antigen-presenting function by a receptor-mediated event. However, the induction of cAMP by itself does not account for inhibition of antigen presentation. Suppression of the expression of B7-2 may be one mechanism by which CGRP inhibits antigen presentation.

Induction of mammary epithelial hyperplasias and mammary tumors in transgenic mice expressing a murine mammary tumor virus/activated c-src fusion gene.

Activation of the c-Src tyrosine kinase has been implicated as an important step in the induction of mammary tumors in both mice and humans. To directly assess the effect of mammary gland-specific expression of activated c-Src, we established transgenic mice that carry a constitutively activated form of c-src under transcriptional control of the murine mammary tumor virus long terminal repeat. Female mice derived from several independent transgenic lines lactate poorly as a consequence of an impairment in normal mammary epithelial development. In addition to this lactation defect, female mice frequently develop mammary epithelial hyperplasias, which occasionally progress to frank neoplasias. Taken together, these observations suggest that expression of activated c-Src in the mammary epithelium of transgenic mice is not sufficient for induction of mammary tumors.

Adaptation of Spodoptera exigua larvae to plant proteinase inhibitors by induction of gut proteinase activity insensitive to inhibition.

Tobacco plants were transformed with a cDNA clone of chymotrypsin/trypsin-specific potato proteinase inhibitor II (PI2) under the control of a constitutive promoter. Although considerable levels of transgene expression could be demonstrated, the growth of Spodoptera exigua larvae fed with detached leaves of PI2-expressing plants was not affected. Analysis of the composition of tryptic gut activity demonstrated that only 18% of the proteinase activity of insects reared on these transgenic plants was sensitive to inhibition by PI2, whereas 78% was sensitive in insects reared on control plants. Larvae had compensated for this loss of tryptic activity by a 2.5-fold induction of new activity that was insensitive to inhibition by PI2. PI2-insensitive proteolytic activity was also induced in response to endogenous proteinase inhibitors of tobacco; therefore, induction of such proteinase activity may represent the mechanism by which insects that feed on plants overcome plant proteinase inhibitor defense.

Induction of antigen-specific cytolytic T cells in situ in human melanoma by immunization with synthetic peptide-pulsed autologous antigen presenting cells.

Human melanoma cells can process the MAGE-1 gene product and present the processed nonapeptide EADPTGHSY on their major histocompatibility complex class I molecules, HLA-A1, as a determinant for cytolytic T lymphocytes (CTLs). Considering that autologous antigen presenting cells (APCs) pulsed with the synthetic nonapeptide might, therefore, be immunogenic, melanoma patients whose tumor cells express the MAGE-1 gene and who are HLA-A1+ were immunized with a vaccine made of cultured autologous APCs pulsed with the synthetic nonapeptide. Analyses of the nature of the in vivo host immune response to the vaccine revealed that the peptide-pulsed APCs are capable of inducing autologous melanoma-reactive and the nonapeptide-specific CTLs in situ at the immunization site and at distant metastatic disease sites.

Ergosteroids: induction of thermogenic enzymes in liver of rats treated with steroids derived from dehydroepiandrosterone.

Dehydroepiandrosterone (DHEA), an intermediate in the biosynthesis of testosterone and estrogens, exerts several physiological effects not involving the sex hormones. When fed to rats it induces the thermogenic enzymes mitochondrial sn-glycerol-3-phosphate dehydrogenase and cytosolic malic enzyme in their livers. Animals and humans, and their excised tissues, are known to hydroxylate DHEA at several positions and to interconvert 7 alpha-hydroxy-DHEA, 7 beta-hydroxy-DHEA, 7-oxo-DHEA, and the corresponding derivatives of androst-5-enediol. We report here that these 7-oxygenated derivatives are active inducers of these thermogenic enzymes in rats and that the 7-oxo derivatives are more active than the parent steroids. We postulate that the 7 alpha-hydroxy and 7-oxo derivatives are on a metabolic pathway from DHEA to more active steroid hormones. These 7-oxo steroids have potential as therapeutic agents because of their increased activity and because they are not convertible to either testosterone or estrogens.

T-cell receptor (TCR) usage in Lewis rat experimental autoimmune encephalomyelitis: TCR beta-chain-variable-region V beta 8.2-positive T cells are not essential for induction and course of disease.

Predominant usage of V beta 8.2 gene segments, encoding a T-cell receptor (TCR) beta chain variable region, has been reported for pathogenic Lewis rat T cells reactive to myelin basic protein (MBP). However, up to 75% of the alpha/beta T cells in a panel of MBP-specific T-cell lines did not display TCR V beta 8.2, V beta 8.5, V beta 10, or V beta 16 elements. To further investigate TCR usage, we sorted the T-cell lines for V beta 8.2- and V beta 10-positive T cells or depleted the lines of cells with these TCRs. V beta 8.2-positive T cells and one of the depleted T-cell lines strongly reacted against the MBP peptide MBP-(68-88). The depleted T-cell line caused marked experimental autoimmune encephalomyelitis (EAE) even in Lewis rats in which endogenous V beta 8.2-positive T cells had been eliminated by neonatal treatment with anti-V beta 8.2 monoclonal antibodies. T-cell hybridomas generated from this line predominantly used V beta 3 TCR genes coexpressed with TCR V alpha 2 transcripts, which were also used by V beta 8.2-positive T cells. Furthermore, V beta 10-positive T cells reactive to MBP-(44-67) were encephalitogenic when injected immediately after positive selection. After induction of EAE by sorted V beta 8.2- or V beta 10-positive T-cell lines, immunocytochemical analysis of the spinal cord tissue showed a predominance of the injected TCR or of nontypable alpha/beta T cells after injection of the depleted line. Our results demonstrate heterogeneity of TCR beta-chain usage even for a single autoantigen in an inbred strain. Moreover, V beta 8.2-positive T cells are not essential for the induction and progression of adoptive-transfer EAE.

Efficient induction of point mutations allowing recovery of specific locus mutations in zebrafish.

A technique is described that greatly increases the efficiency of recovering specific locus point mutations in zebrafish (Danio rerio). Founder individuals that were mosaic for point mutations were produced by mutagenizing postmeiotic gametes with the alkylating agent N-ethyl-N-nitrosourea. Under optimal conditions, each founder carried an average of 10 mutations affecting genes required for embryogenesis. Moreover, approximately 2% of these founders transmitted new mutations at any prespecified pigmentation locus. Analyses of new pigmentation mutations confirmed that most were likely to be point mutations. Thus, mutagenesis of postmeiotic gametes with N-ethyl-N-nitrosourea yielded frequencies of point mutations at specific loci that were 10- to 15-fold higher than previously achieved in zebrafish. Our procedure should, therefore, greatly facilitate recovery of multiple mutant alleles at any locus of interest.

Accelerated induction of experimental allergic encephalomyelitis in PL/J mice by a non-V beta 8-specific superantigen.

Superantigens such as the staphylococcal enterotoxins can play an important role in exacerbation of autoimmune disorders such as experimental allergic encephalomyelitis (EAE) in mice. In fact, superantigens can reactivate EAE in PL/J mice that have been sensitized to rat myelin basic protein (MBP). The T-cell subset predominantly responsible for disease in PL/J mice bears the V beta 8+ T-cell antigen receptor (TCR). The question arises as to whether T cells bearing other V beta specificities are involved in induction or reactivation of EAE with superantigen. Thus, we have investigated the ability of a non-V beta 8-specific superantigen, staphylococcal enterotoxin A (SEA) (V beta specificities 1, 3, 10, 11, and 17), to induce EAE in PL/J mice that have been previously protected from disease by anergy and deletion of V beta 8+ T cells. PL/J mice were first pretreated with the V beta 8-specific superantigen staphylococcal enterotoxin B (SEB) and then immunized with MBP. These mice exhibited V beta 8-specific anergy and depletion and did not develop EAE, even when further treated with SEB. However, administration of SEA to these same mice induced an initial episode of EAE which was characterized by severe hindleg paralysis and accelerated onset of disease. In contrast to SEB pretreatment, PL/J mice pretreated with SEA did develop EAE when immunized with MBP, and after resolution of clinical signs of disease these mice were susceptible to relapse of EAE induced by SEB but not by SEA. Thus, superantigens can activate encephalitogenic MBP-specific non-V beta 8+ T cells to cause EAE in PL/J mice. These data suggest that superantigens can play a central role in autoimmune disorders and that they introduce a profound complexity to autoimmune diseases such as EAE, akin to the complexity seen in multiple sclerosis.