Singing, but not seizure, induces synaptotagmin IV in zebra finch song circuit nuclei

Synaptotagmins are a family of proteins that function in membrane fusion events, including synaptic vesicle exocytosis. Within this family, synaptotagmin IV (Syt IV) is unique in being a depolarization-induced immediate early gene (IEG). Experimental perturbation of Syt IV modulates neurotransmitter release in mice, flies, and PC12 cells, and modulates learning in mice. Despite these features, induction of Syt IV expression by a natural behavior has not been previously reported. We used the zebra finch, a songbird species, to investigate Syt IV because song is a naturally learned behavior whose neuroanatomical basis is largely identified. We observed that, similar to rodents, Syt IV is inducible in songbirds. This induction was selective and depended on the nature of neuronal depolarization. Generalized seizures caused by the GABA(A) receptor antagonist, metrazole, induced the IEG, ZENK, in zebra finch brain. However, these same seizures failed to induce Syt IV in song control areas. In contrast, when nontreated birds sang, three song control areas showed striking Syt IV induction. Further, this induction appeared sensitive to the social context in which song was sung. Together, these data suggest that neural activity during singing can drive Syt IV expression within song circuitry whereas generalized seizure activity fails to do so even though song control areas are depolarized. Our findings indicate that, within this neural circuit for a procedurally learned sensorimotor behavior, Syt IV is selective and requires precisely patterned neural activity and/or neuromodulation associated with singing.

Monitoring adenoviral DNA delivery, using a mutant herpes simplex virus type 1 thymidine kinase gene as a PET reporter gene

Current gene therapy protocols often suffer from an inability to monitor the site, level and persistence of gene expression following somatic DNA delivery. Herpes simplex virus 1 thymidine kinase (HSV1-tk) is currently under intensive investigation as a reporter gene for in vivo imaging of reporter gene expression. The presence of the HSV1-tk reporter gene is repetitively and non-invasively monitored by systemic injection of positron-emitting, radionuclide-labeled thymidine analogues or acycloguanosine HSV1-TK substrates and subsequent detection, by positron emission tomography, of trapped, phosphorylated product. To improve the efficacy of the HSV1-tk PET reporter gene system, both alternative substrates and mutations in the HSV1-tk gene have been described. We used a replication defective adenovirus to deliver the HSV1-sr39tk mutant enzyme and the wild-type HSV1-tk enzyme to mice. HSV1-sr39TK demonstrates greater sensitivity than wild-type HSV1-TK enzyme in vivo, using 9-[(4-[(18)F]fluoro-3-hydroxymethylbutyl)guanine as probe, following adenovirus-mediated hepatic expression in mice. Using this adenoviral delivery system, the location, magnitude and duration of HSV1-sr39tk PET reporter gene expression could be non-invasively, quantitatively and repetitively monitored for over 3 months by microPET.

Monitoring gene therapy with reporter gene imaging

Rapid advances in imaging technologies and gene transfer strategies offer a great opportunity to optimize clinical trials of human gene therapy. Reporter genes are emerging as very powerful tools to monitor the delivery, magnitude, and time variation of therapeutic gene transfer in vivo. Several reporter genes, such as the herpes simplex virus type 1 thymidine kinase, the dopamine type 2 receptor, and the somatostatin receptor type 2, are currently being successfully used with gamma camera, single photon emission computed tomography, and positron emission tomography imaging. These reporter genes can be coupled with a therapeutic gene of interest to indirectly monitor the expression of the therapeutic gene. Finally, applications of the reporter gene technology to other areas, such as cell trafficking studies and transgenic animal models, are now possible.

Quantitative imaging of gene induction in living animals

Methods to repeatedly, non-invasively, and quantitatively image gene expression in living animals are rapidly emerging and should fundamentally change studies of gene expression in vivo. We previously developed assays utilizing positron emission tomography (PET) to image reporter gene expression. In this paper we: (1) describe a new bi-directional, tetracycline-inducible system that can be used to pharmacologically induce target gene expression and to quantitatively image induced expression by using a PET reporter gene; (2) demonstrate the potential of this system in transient and stable cell transfection assays; and (3) demonstrate the ability to repetitively and quantitatively image tetracycline and tetracycline analog induction of gene expression in living animals. We utilize the dopamine type-2 receptor (D(2)R) and the mutant herpes-simplex virus type 1 thymidine kinase (HSV1-sr39tk) reporter genes to validate this system. We utilize microPET technology to show that quantitative tomographic imaging of gene induction is possible. We find a high correlation (r(2) = 0.98) between 'target' and reporter gene expression. This work establishes a new technique for imaging time-dependent variation of gene expression both from vectors with inducible promoters and in transgenic animals in which pharmacologic induction of gene expression must be monitored. These techniques may be applied both in gene therapy and for the study of gene expression in transgenic animals.

Noninvasive, quantitative imaging in living animals of a mutant dopamine D2 receptor reporter gene in which ligand binding is uncoupled from signal transduction

The dopamine D2 receptor (D2R) has been used in adenoviral delivery systems and in tumor cell xenografts as an in vivo reporter gene. D2R reporter gene expression has been non-invasively, repetitively and quantitatively imaged by positron emission tomography (PET), following systemic injection of a positron-labeled ligand (3-(2'-[18F]-fluoroethyl)-spiperone; FESP) and subsequent D2R-dependent sequestration. However, dopamine binding to the D2R can modulate cyclic AMP levels. For optimal utilization of D2R as a reporter gene, it is important to uncouple ligand-binding from Gi-protein-mediated inhibition of cAMP production. Mutation of Asp80 or Ser194 produces D2Rs that still bind [3H]spiperone in transfected cells. The D2R80A mutation completely eliminates the ability of the D2R to suppress forskolin-stimulated cAMP accumulation in response to dopamine, in cells transfected with a D2R80A expression plasmid and in cells infected with replication-defective adenovirus expressing D2R80A. The D2R194A mutation substantially reduces, but does not completely eliminate, dopamine modulation of cAMP levels. Cultured cells infected with adenoviruses expressing D2R and D2R80A demonstrated equivalent [3H]spiperone binding activity. Moreover, hepatic FESP sequestration is equivalent, following intravenous injection of adenoviruses expressing D2R and D2R80A. The D2R80A mutant, which can no longer modulate cAMP levels following ligand binding, has full capability as a PET reporter gene.

Human pharmacokinetic and dosimetry studies of [(18)F]FHBG: a reporter probe for imaging herpes simplex virus type-1 thymidine kinase reporter gene expression

9-[4-[(18)F]fluoro-3-(hydroxymethyl)butyl]guanine ([(18)F]FHBG) has been used as a reporter probe to image expression of herpes simplex virus type-1 thymidine kinase (HSV1-tk) reporter gene in living animals. Our aim was to study the kinetics, biodistribution, stability, dosimetry, and safety of [(18)F]FHBG in healthy human volunteers, preparatory to imaging patients undergoing HSV1-tk gene therapy.

METHODS

[(18)F]FHBG was synthesized with a specific activity of 37,000--444,000 GBq/mmol and a radiochemical purity > 99%. Ten healthy volunteers consented to participate in the study. A transmission scan was obtained before bolus injection of 70.3--229.4 MBq [(18)F]FHBG into a hand vein, followed by dynamic PET imaging with 4 consecutive emission scans. Warmed hand-vein blood was withdrawn at various times after injection for blood time--activity measurements. Electrocardiography, blood pressure, and blood and urine pharmacologic parameters were measured before and after injection of the [(18)F]FHBG tracer (n = 5). The stability of [(18)F]FHBG in the urine was analyzed. Attenuation-corrected images were reconstructed using the ordered-subsets expectation maximization algorithm. Image region-of-interest time-activity data were used with the MIRD program to estimate absorbed radiation dosages.

RESULTS

[(18)F]FHBG had rapid blood clearance; only 8.42% +/- 4.76% (mean +/- SD) of the peak blood activity remained at approximately 30 min. The average ratio of plasma activity to whole-blood activity during the study was 0.91 +/- 0.04. Penetration of [(18)F]FHBG across the blood-brain barrier was not observed. The primary routes of clearance were renal and hepatobiliary. High activities were observed in the bladder, gut, liver, and kidneys, but <0.0002% of the injected dose per gram was observed in other tissues. In the urine, 83% of activity 180 min after injection was stable [(18)F]FHBG. Blood and urine pharmacologic parameters did not change significantly after injection of the [(18)F]FHBG tracer. The bladder absorbed the highest radiation dose.

CONCLUSION

[(18)F]FHBG has the desirable in vivo characteristics of stability, rapid blood clearance, low background signal, biosafety, and acceptable radiation dosimetry in humans. This study forms the foundation for using [(18)F]FHBG in applications to monitor HSV1-tk reporter gene expression.

Direct correlation between positron emission tomographic images of two reporter genes delivered by two distinct adenoviral vectors

Biodistribution, magnitude and duration of a therapeutic transgene's expression may be assessed by linking it to the expression of a positron emission tomography (PET) reporter gene (PRG) and then imaging the PRG's expression by a PET reporter probe (PRP) in living animals. We validate the simple approach of co-administering two distinct but otherwise identical adenoviruses, one expressing a therapeutic transgene and the other expressing the PRG, to track the therapeutic gene's expression. Two PET reporter genes, a mutant herpes simplex virus type 1 thymidine kinase (HSV1-sr39tk) and dopamine-2 receptor (D(2)R), each regulated by the same cytomegalovirus (CMV) promoter, have been inserted into separate adenoviral vectors (Ad). We demonstrate that cells co-infected with equivalent titers of Ad-CMV-HSV1-sr39tk and Ad-CMV-D(2)R express both reporter genes with good correlation (r(2) = 0.93). Similarly, a high correlation (r(2) = 0.97) was observed between the expression of both PRGs in the livers of mice co-infected via tail-vein injection with equivalent titers of these two adenoviruses. Finally, microPET imaging of HSV1-sr39tk and D(2)R expression with 9-(4-[(18)F]fluoro-3-hydroxymethylbutyl) guanine ([(18)F]FHBG) and 3-(2-[(18)F]fluoroethyl)spiperone ([(18)F]FESP), utilizing several adenovirus-mediated delivery routes, illustrates the feasibility of evaluating relative levels of transgene expression in living animals, using this approach.

NID67, a small putative membrane protein, is preferentially induced by NGF in PC12 pheochromocytoma cells

In an effort to identify genes involved in neuronal differentiation, we have used representational difference analysis (RDA) to clone cDNAs that are preferentially induced by nerve growth factor (NGF) vs. epidermal growth factor (EGF) in PC12 pheochromocytoma cells. We now report the cloning of a previously unknown primary response gene, NID67. In addition to a robust induction by NGF and FGF, both of which cause PC12 cells to differentiate, NID67 is strongly induced by forskolin, A23187 and ATP. EGF, TPA and KCl induce NID67 only weakly. NID67 mRNA is most abundant in heart, ovary and adrenal. Modest levels are present in most brain regions, testis, thyroid, thymus, pituitary, kidney and intestine; little NID67 is present in skeletal muscle and cerebellum. The NID67 cDNA contains a 180 bp open reading frame (ORF) that encodes a 60 amino acid protein. The central 29 amino acids are very hydrophobic and very likely comprise a transmembrane domain. Mouse and human NID67 cDNAs contain an ORF similar to NID67; the rat and human protein sequences are 85% identical whereas the rat and mouse sequences are 92% identical. In vitro transcription and translation reactions confirmed that the ORF we identified produces a 6000 Da protein product. Several small membrane proteins are similar to NID67; they contain a transmembrane domain and little more. All of these proteins participate in forming or regulating ion channels. NID67 may play a similar role in cellular physiology.

Synaptotagmin IV: biochemistry, genetics, behavior, and possible links to human psychiatric disease

We isolated the rat synaptotagmin IV (Syt IV) cDNA in a screen for sequences that are specifically induced in neuronal cells. The Syts are a large family of genes thought to mediate synaptic function. Syt IV is brain-specific, induced in hippocampus by depolarization, and predominantly vesicular. To assess the function role of Syt IV in vivo, we generated Syt IV(-/-) mutant mice. Syt IV (-/-) mice are viable and appear normal, indicating this gene is not essential for survival or gross development. However, Syt IV (-/-) mutants, when compared to wild-type littermates, have deficits in fine motor coordination and hippocampus-dependent memory, suggesting Syt IV has a role in normal brain function. The human Syt IV ortholog maps to a region of chromosome 18 previously associated with the human psychiatric disorders, schizophrenia and bipolar disease. These results suggest that Syt IV is required in certain types of neurons for optimal functionality, that perturbations in the levels of Syt IV can result in memory loss in mice, and that Syt IV alterations may lead to psychiatric disease in humans.

Arginine methylation of STAT1 modulates IFNalpha/beta-induced transcription

Transcriptional induction by interferons requires the tyrosine and serine phosphorylation of STAT transcription factors. The N-terminal region is highly homologous among the STAT proteins and surrounds a completely conserved arginine residue. Here we demonstrate arginine methylation of STAT1 by the protein arginine methyl-transferase PRMT1 as a novel requirement for IFNalpha/beta-induced transcription. Methyl-thioadenosine, a methyl-transferase inhibitor that accumulates in many transformed cells, inhibits STAT1-mediated IFN responses. This inhibition arises from impaired STAT1-DNA binding due to an increased association of the STAT inhibitor PIAS1 with phosphorylated STAT1 dimers in the absence of arginine methylation. Thus, arginine methylation of STAT1 is an additional posttranslational modification regulating transcription factor function, and alteration of arginine methylation might be responsible for the lack of interferon responsiveness observed in many malignancies.

Expression of the urokinase plasminogen activator receptor is transiently required during "priming" of PC12 cells in nerve growth factor-directed cellular differentiation

We previously identified the urokinase plasminogen activator receptor (UPAR) as a gene induced by nerve growth factor (NGF), but not by epidermal growth factor (EGF), in PC12 cells (Farias-Eisner et al. [2000] J. Neurosci. 20:230-239). Antisense oligonucleotides for the UPAR mRNA or an antibody directed against UPAR protein, added simultaneously with NGF, block NGF-induced morphological and biochemical differentiation of PC12 cells. In this report, we show that anti-UPAR antibody blocks morphological differentiation and the expression of two NGF-specific secondary response genes, collagenase-1 and transin, in PC12 cells only during the first 2 hr following NGF exposure. These data suggest that induced UPAR expression is required only over a short period of time following exposure to NGF for the differentiation program in PC12 cells to proceed. For two models of "primed" PC12 cells, we found that UPAR expression and function are not required for NGF-induced differentiation. UPAR and the secondary response genes collagenase-1 and transin are not induced in "primed" PC12 cells in response to NGF, and anti-UPAR antibody does not block morphological differentiation in these cells. Our data suggests that UPAR is required only transiently during the "priming" of PC12 cells in NGF-induced PC12 cell differentiation.

8-[18F]Fluoropenciclovir: an improved reporter probe for imaging HSV1-tk reporter gene expression in vivo using PET

We have synthesized and evaluated 8-[18F]fluoropenciclovir (FPCV) and compared it with 8-[18F]fluoroganciclovir (FGCV) for monitoring the expression of herpes simplex virus type 1 thymidine kinase (HSV1 -tk) reporter gene in cell culture and in vivo.

METHODS

C6 rat glioma cells stably transfected with HSV1-tk (C6-stb-tk+) and control C6 cells were evaluated for their ability to accumulate FGCV versus FPCV. For in vivo studies, 15 mice were injected by tail vein with increasing levels of an adenoviral vector carrying HSV1-tk. Forty-eight hours later the mice were injected with FPCV and killed 3 h later. The percentage injected dose per gram (%ID/g) liver was then determined. Two additional mice were studied by microPET and autoradiography using FPCV to image adenoviral-mediated hepatic HSV1-tk reporter gene expression. A tumor-bearing mouse (C6 control and C6-stb-tk+) was imaged with FDG, FGCV, and FPCV. Two mice carrying tumors expressing two different reporter genes, HSV1-tk and dopamine type 2 receptor (D2R), were also imaged by microPET using FPCV (day 1) and 3-(2'-[18F]fluoroethyl)spiperone (FESP) (day 2).

RESULTS

FPCV shows a significantly greater accumulation in C6-stb-tk+ cells than does FGCV (P < 0.05). Over identical ranges of adenoviral administration, mouse liver shows a higher %ID/g liver for FPCV (0%-9%) compared with our previously reported results with FGCV (0%-3%). In C6 control and C6-stb-tk+ tumor-bearing mice, FPCV has a greater accumulation than does FGCV for equal levels of HSV1-tk gene expression. In mice carrying tumors expressing either HSV1-tk or D2R reporter genes, there is a corresponding retention of FPCV and FESP, respectively.

CONCLUSION

These results indicate that FPCV is a better reporter probe than is FGCV for imaging lower levels of HSV1 -tk gene expression in vivo. The results also reveal the ability to monitor the expression of two distinct reporter genes in the same animal using reporter probes specific for each gene.

The human synaptotagmin IV gene defines an evolutionary break point between syntenic mouse and human chromosome regions but retains ligand inducibility and tissue specificity

Rat synaptotagmin IV (SYT IV) is a depolarization-inducible synaptic vesicle protein. SYT IV homozygous mutant mice are viable and have deficits in fine motor coordination and some forms of memory. In this study, we report the identification of a human SYT IV orthologue. The predicted amino acid sequence of the human SYT IV clone is nearly 90% identical to the rat and mouse SYT IV proteins. In addition, human SYT IV has a characteristic serine for aspartate substitution within the first C2 domain that is conserved among Drosophila, Caenorhabditis elegans, mouse, and rat SYT IV sequences. The human SYT IV gene maps to chromosome band 18q12.3, a region that defines a break point in the synteny with mouse chromosome 18 and has been implicated by associated markers in two human psychiatric disorders. In the human neuroblastoma cell line SK-N-SH, SYT IV is an immediate-early gene inducible by elevated intracellular calcium and by forskolin, an activator of adenylyl cyclase. Expression of human SYT IV mRNA is restricted to brain and is not detectable in non-neuronal tissues. Within brain, human SYT IV mRNA is most highly expressed in hippocampus, with lower levels present in amygdala and thalamus. These results suggest a role for SYT IV in human brain function and in human neurological disease.

PET imaging of transgene expression

A vital step in transgenic animal study and gene therapy is the ability to assay the extent of transgene expression. Unfortunately, classic methods of assaying transgene expression require biopsies or death of the subject. We are developing techniques to noninvasively and repetitively determine the location, duration, and magnitude of transgene expression in living animals. This will allow investigators and clinicians to assay the effectiveness of their particular experimental and therapeutic paradigms. Of radionuclide (single photon emission computed tomography, positron emission tomography [PET]), optical (green fluorescent protein, luciferase), and magnetic (magnetic resonance imaging) approaches, only the radionuclide approach has sufficient sensitivity and quantitation to measure the expression of genes in vivo. We describe the instrumentation involved in high resolution PET scanning. We also describe the principles of PET reporter gene/reporter probe in vivo imaging, the development of two in vivo reporter gene imaging systems, and the validation of our ability to noninvasively, quantitatively, and repetitively image gene expression in murine viral gene transfer and transgenic models. We compare the two reporter gene systems and discuss their utility for the study of transgenic animals and gene therapies. Finally, we mention alternative approaches to image gene expression by using radiolabeled antibody fragments to image specific proteins and radiolabeled oligonucleotides to image RNA messages directly.

Protein-arginine methyltransferase I, the predominant protein-arginine methyltransferase in cells, interacts with and is regulated by interleukin enhancer-binding factor 3

Arginine methylation is a common post-translation modification found in many proteins. Protein-arginine methyltransferase I (PRMT1) contributes >90% of type I protein-arginine methyltransferase activity in cells and tissues. To expand our knowledge on the regulation and role of PRMT1 in cells, we used the yeast two-hybrid system to identify proteins that interact with PRMT1. One of the interacting proteins we cloned is interleukin enhancer-binding factor 3 (ILF3), also known as M phase phosphoprotein 4. ILF3 is closely related to nuclear factor 90 (NF90). Using an immunofluorescence analysis, we determined that ILF3 and PRMT1 co-localize in the nucleus. Moreover, PRMT1 and ILF3 co-precipitate in immunoprecipitation assays and can be isolated together in "pull-down" experiments using recombinant fusion proteins. ILF3 is a robust substrate for methylation by PRMT1 and can modulate PRMT1 activity in in vitro methylation assays. Deletion studies demonstrated that the COOH-terminal region of ILF3, which is rich in arginine, glycine, and serine, is responsible for the strong interaction between PRMT1 and ILF3 and is the site of ILF3 methylation by PRMT1. Although ILF3 and NF90 are highly similar, they differ in their carboxyl-terminal regions. Because of this difference, NF90 does not interact with PRMT1, is a much poorer substrate than ILF3 for PRMT1-dependent methylation, and does not modulate PRMT1 enzyme activity.

The salt-inducible kinase, SIK, is induced by depolarization in brain

Membrane depolarization of neurons is thought to lead to changes in gene expression that modulate neuronal plasticity. We used representational difference analysis to identify a group of cDNAs that are induced by membrane depolarization or by forskolin, but not by neurotrophins or growth factors, in PC12 pheochromocytoma cells. One of these genes, SIK (salt-inducible kinase), is a member of the sucrose-nonfermenting 1 protein kinase/AMP-activated protein kinase protein kinase family that was also recently identified from the adrenal gland of rats treated with high-salt diets. SIK mRNA is induced up to eightfold in specific regions of the hippocampus and cortex in rats, following systemic kainic acid administration and seizure induction.

Deficits in memory and motor performance in synaptotagmin IV mutant mice

Synaptotagmin (Syt) IV is a synaptic vesicle protein. Syt IV expression is induced in the rat hippocampus after systemic kainic acid treatment. To examine the functional role of this protein in vivo, we derived Syt IV null [Syt IV(-/-)] mutant mice. Studies with the rotorod revealed that the Syt IV mutants have impaired motor coordination, a result consistent with constitutive Syt IV expression in the cerebellum. Because Syt IV is thought to modulate synaptic function, we also have examined Syt IV mutant mice in learning and memory tests. Our studies show that the Syt IV mutation disrupts contextual fear conditioning, a learning task sensitive to hippocampal and amygdala lesions. In contrast, cued fear conditioning is normal in the Syt IV mutants, suggesting that this mutation did not disrupt amygdala function. Conditioned taste aversion, which also depends on the amygdala, is normal in the Syt IV mutants. Consistent with the idea that the Syt IV mutation preferentially affects hippocampal function, Syt IV mutant mice also display impaired social transmission of food preference. These studies demonstrate that Syt IV is critical for brain function and suggest that the Syt IV mutation affects hippocampal-dependent learning and memory, as well as motor coordination.

The RGG domain in hnRNP A2 affects subcellular localization

The heterogeneous nuclear ribonucleoproteins (hnRNP) associate with pre-mRNA in the nucleus and play an important role in RNA processing and splice site selection. In addition, hnRNP A proteins function in the export of mRNA to the cytoplasm. Although the hnRNP A proteins are predominantly nuclear, hnRNP A1 shuttles rapidly between the nucleus and the cytoplasm. HnRNP A2, whose cytoplasmic overexpression has been identified as an early biomarker of lung cancer, has been less well studied. Cytosolic hnRNP A2 overexpression has also been noted in brain tumors, in which it has been correlated with translational repression of Glucose Transporter-1 expression. We now examine the role of arginine methylation on the nucleocytoplasmic localization of hnRNP A2 in the HEK-293 and NIH-3T3 mammalian cell lines. Treatment of either cell line with the methyltransferase inhibitor adenosine dialdehyde dramatically shifts hnRNP A2 localization from the nuclear to the cytoplasmic compartment, as shown both by immunoblotting and by immunocytochemistry. In vitro radiolabeling with [(3)H]AdoMet of GST-tagged hnRNP A2 RGG mutants, using recombinant protein arginine methyltransferase (PRMT1), shows (i) that hnRNP A2 is a substrate for PRMT1 and (ii) that methylated residues are found only in the RGG domain. Deletion of the RGG domain (R191-G253) of hnRNP A2 results in a cytoplasmic localization phenotype, detected both by immunoblotting and by immunocytochemistry. These studies indicate that the RGG domain of hnRNP A2 contains sequences critical for cellular localization of the protein. The data suggest that hnRNP A2 may contain a novel nuclear localization sequence, regulated by arginine methylation, that lies in the R191-G253 region and may function independently of the M9 transportin-1-binding region in hnRNP A2.

rTLE3, a newly identified transducin-like enhancer of split, is induced by depolarization in brain

The transducin-like enhancers of split are a family of mammalian proteins that share sequence homology with the Drosophila protein Groucho. Using representational difference analysis, we isolated the cDNA for a previously unidentified gene, rTLE3 (rat transducin-like enhancer of split 3), as a sequence induced by depolarization and forskolin, but not by neurotrophins or growth factors, in PC12 pheochromocytoma cells. rTLE3 encodes the protein rTLE3, a 764-amino acid orthologue of mouse and human TLE3. R-esp2, the gene encoding the closest related rat protein, is not induced by any of the four treatments in PC12 cells. rTLE3 and R-esp2 have different patterns of expression in the adult rat CNS and other tissues. After systemic administration of kainic acid, rTLE3 is induced specifically in the dentate gyrus of the hippocampus. We propose that members of the transducin-like enhancer of split family of proteins may have distinct functions in the mature CNS, in addition to their functions during development.

Searching for depolarization-induced genes that modulate synaptic plasticity and neurotrophin-induced genes that mediate neuronal differentiation

We identify and characterize two classes of immediate-early genes: (i) genes, induced by depolarization in neurons, that play a role in depolarization-induced neuronal plasticity and (ii) genes, induced in neuronal precursors by neurotrophins, that play a causal role in neurotrophin-directed neuronal differentiation. We use rat PC12 pheochromocytoma cells to identify (i) genes preferentially induced by [depolarization or forskolin] versus [Nerve Growth Factor (NGF) or Epidermal Growth Factor (EGF)] and (ii) genes preferentially induced by NGF versus EGF. We describe (i) a collection of genes preferentially induced by depolarization/forskolin in PC12 cells and by kainic acid in vivo, and (ii) a collection of genes preferentially induced by NGF. The synaptotagmin IV gene encodes a synaptic vesicle protein whose level is modulated by depolarization. NGF preferentially induces the urokinase-plasminogen activator receptor in PC12 cells. Antisense oligonucleotide and anti-UPAR antibody experiments demonstrate that NGF-induced UPAR expression is required for NGF-driven PC12 cell differentiation.

PRMT1 is the predominant type I protein arginine methyltransferase in mammalian cells

Type I protein arginine methyltransferases catalyze the formation of asymmetric omega-N(G),N(G)-dimethylarginine residues by transferring methyl groups from S-adenosyl-L-methionine to guanidino groups of arginine residues in a variety of eucaryotic proteins. The predominant type I enzyme activity is found in mammalian cells as a high molecular weight complex (300-400 kDa). In a previous study, this protein arginine methyltransferase activity was identified as an additional activity of 10-formyltetrahydrofolate dehydrogenase (FDH) protein. However, immunodepletion of FDH activity in RAT1 cells and in murine tissue extracts with antibody to FDH does not diminish type I methyltransferase activity toward the methyl-accepting substrates glutathione S-transferase fibrillarin glycine arginine domain fusion protein or heterogeneous nuclear ribonucleoprotein A1. Similarly, immunodepletion with anti-FDH antibody does not remove the endogenous methylating activity for hypomethylated proteins present in extracts from adenosine dialdehyde-treated RAT1 cells. In contrast, anti-PRMT1 antibody can remove PRMT1 activity from RAT1 extracts, murine tissue extracts, and purified rat liver FDH preparations. Tissue extracts from FDH(+/+), FDH(+/-), and FDH(-/-) mice have similar protein arginine methyltransferase activities but high, intermediate, and undetectable FDH activities, respectively. Recombinant glutathione S-transferase-PRMT1, but not purified FDH, can be cross-linked to the methyl-donor substrate S-adenosyl-L-methionine. We conclude that PRMT1 contributes the major type I protein arginine methyltransferase enzyme activity present in mammalian cells and tissues.

Seeing is believing: non-invasive, quantitative and repetitive imaging of reporter gene expression in living animals, using positron emission tomography

The ability to monitor reporter gene expression in living animals and in patients will permit longitudinal examinations both of somatically transferred DNA in experimental animals and patients and of transgenic constructs expressed in experimental animals. If investigators can non-invasively monitor the organ and tissue specificity, the magnitude and the duration of gene expression from somatically transferred DNA and from transgenes, conceptually new experimental paradigms will be possible. If clinicians can non-invasively monitor the location, extent and duration of somatically transferred genes, they will be better able to determine the correlations between expression of therapeutic genes and clinical outcomes. We have developed two reporter gene systems for in vivo reporter gene imaging in which the protein products of the reporter genes sequester positron-emitting reporter probes. The "PET reporter gene" dependent sequestration of the "PET reporter probes" is subsequently measured in living animals by Positron Emission Tomography (PET). We describe here the principles of PET reporter gene/PET reporter probe in vivo imaging, the development of two imaging systems, and the validation of their ability to non-invasively, quantitatively and repetitively image reporter gene expression in murine viral gene transfer and transgenic models.

A mutant herpes simplex virus type 1 thymidine kinase reporter gene shows improved sensitivity for imaging reporter gene expression with positron emission tomography

We are developing assays for noninvasive, quantitative imaging of reporter genes with positron emission tomography (PET), for application both in animal models and in human gene therapy. We report here a method to improve the detection of lower levels of PET reporter gene expression by utilizing a mutant herpes simplex virus type 1 thymidine kinase (HSV1-sr39tk) as a PET reporter gene. The HSV1-sr39tk mutant was identified from a library of site-directed mutants. Accumulation (net uptake) of the radioactively labeled substrates [8-(3)H]penciclovir ([8-(3)H]PCV), and 8-[(18)F]fluoropenciclovir (FPCV) in C6 rat glioma cells expressing HSV1-sr39tk is increased by a factor of approximately 2.0 when compared with C6 cells expressing wild-type HSV1-tk. The increased imaging sensitivity of HSV1-sr39tk when FPCV is used is also demonstrated in vivo both with tumor cells stably transfected with either HSV1-tk or HSV1-sr39tk, and after hepatic delivery of HSV1-tk or HSV1-sr39tk by using adenoviral vectors. The use of HSV1-sr39tk as a PET reporter gene and FPCV as a PET reporter probe results in significantly enhanced sensitivity for imaging reporter gene expression in vivo.

Transcriptional activation of the cyclooxygenase-2 gene in endotoxin-treated RAW 264.7 macrophages

Cyclooxygenase-2 (COX-2), the enzyme primarily responsible for induced prostaglandin synthesis, is an immediate early gene induced by endotoxin in macrophages. We investigated the cis-acting elements of the COX-2 5'-flanking sequence, the transcription factors and signaling pathways responsible for transcriptional activation of the COX-2 gene in endotoxin-treated murine RAW 264.7 macrophages. Luciferase reporter constructs with alterations in presumptive cis-acting transcriptional regulatory elements demonstrate that the cyclic AMP-response element and two nuclear factor interleukin-6 (CCAAT/enhancer-binding protein (C/EBP)) sites of the COX-2 promoter are required for optimal endotoxin-dependent induction. In contrast, the E-box and NF-kappaB sites are not required for endotoxin-dependent induction. Inhibition of endotoxin-induced NF-kappaB activation by expression of an inhibitor-kappaB alpha mutant does not block endotoxin-dependent COX-2 reporter activity. Overexpression of c-Jun, C/EBPbeta, and C/EBPdelta enhances induction of the COX-2 reporter, while overexpression of cyclic AMP-response element-binding protein or "dominant negative" C/EBPbeta represses COX-2 induction. In addition, endotoxin rapidly and transiently elicits c-Jun phosphorylation in RAW 264.7 macrophages. Cotransfection of the COX-2 reporter with dominant negative expression vectors shows that endotoxin-induced COX-2 gene expression requires signaling through a Ras-independent pathway involving the adapter protein ECSIT and the signaling kinases MEKK1 and JNK. In contrast, endotoxin-induced COX-2 reporter activity is not blocked by overexpression of dominant-negative forms of Raf-1, ERK1, or ERK2.

Transcriptional regulation of the cyclooxygenase-2 gene in activated mast cells

Activation of mast cells by aggregation of their IgE receptors induces rapid and transient synthesis of cyclooxygenase-2 (COX-2). In this study we investigated (i) the cis-acting response elements and transcription factors active at the COX-2 promoter and (ii) the signal transduction pathways mediating COX-2 induction following aggregation of mast cell IgE receptors. Transient transfection assays with COX-2 promoter/luciferase constructs suggest that a consensus cyclic AMP response element is essential for induced COX-2 expression. Cotransfection studies with plasmids expressing c-Jun, dominant negative Ras, dominant negative c-Jun NH(2)-terminal kinase, and dominant negative MEKK1 demonstrate that activation of the Ras/MEKK1/c-Jun NH(2)-terminal kinase/c-Jun pathway is required for COX-2 promoter-mediated luciferase expression. Attenuation of COX-2 promoter activity by dominant negative constructs for Raf-1, ERK1, and ERK2 suggests that the Ras/Raf-1/extracellular signal-regulated kinase pathway is also necessary for COX-2 induction. Although mutating the two NF-IL6 sites individually did not affect COX-2 promoter activity, mutating both NF-IL6 sites substantially inhibits COX-2 promoter activity. Moreover, overexpression of wild type CCAAT/enhancer-binding protein-beta (C/EBPbeta) augments COX-2 promoter activity in activated mast cells and cotransfection of a dominant negative C/EBPbeta construct completely blocks COX-2 promoter/luciferase expression. Our data suggest that in activated mast cells, a Ras/MEKK1/c-Jun NH(2)-terminal kinase signal transduction pathway activating c-Jun, a Ras/Raf-1/extracellular signal-regulated kinase pathway, and activated C/EBPbeta facilitate COX-2 induction via the cyclic AMP response element and NF-IL6 sites of the COX-2 promoter.

Synthesis of 8-[(18)F]fluoroguanine derivatives: in vivo probes for imaging gene expression with positron emission tomography

A new method for the preparation of 8-[(18)F]fluoroguanine derivatives based on a direct radiofluorination reaction has been developed. The radiofluorination of ganciclovir (1a) with [(18)F]F(2) was carried out in absolute ethanol in the presence of tetraethylammonium hydroxide at room temperature to give 8-[(18)F]fluoroganciclovir (3a) in an approximately 1% radiochemical yield. Similarly, 8-[(18)F]fluoropenciclovir (3b), 8-[(18)F]fluoroacyclovir (3c), and 8-[(18)F]fluoroguanosine (3d) were synthesized from penciclovir (1b), acyclovir (1c), and guanosine (1d), respectively, using [(18)F]F(2). The structural analyses of the final products (3a, 3b, 3c, and 3d) were carried out after (18)F decay by (1)H, (13)C, and (19)F nuclear magnetic resonance and high resolution mass spectroscopy.

Imaging transgene expression with radionuclide imaging technologies

A variety of imaging technologies are being investigated as tools for studying gene expression in living subjects. Noninvasive, repetitive and quantitative imaging of gene expression will help both to facilitate human gene therapy trials and to allow for the study of animal models of molecular and cellular therapy. Radionuclide approaches using single photon emission computed tomography (SPECT) and positron emission tomography (PET) are the most mature of the current imaging technologies and offer many advantages for imaging gene expression compared to optical and magnetic resonance imaging (MRI)-based approaches. These advantages include relatively high sensitivity, full quantitative capability (for PET), and the ability to extend small animal assays directly into clinical human applications. We describe a PET scanner (microPET) designed specifically for studies of small animals. We review "marker/reporter gene" imaging approaches using the herpes simplex type 1 virus thymidine kinase (HSV1-tk) and the dopamine type 2 receptor (D2R) genes. We describe and contrast several radiolabeled probes that can be used with the HSV1-tk reporter gene both for SPECT and for PET imaging. We also describe the advantages/disadvantages of each of the assays developed and discuss future animal and human applications.

The urokinase plasminogen activator receptor (UPAR) is preferentially induced by nerve growth factor in PC12 pheochromocytoma cells and is required for NGF-driven differentiation

Nerve growth factor (NGF)-driven differentiation of PC12 pheochromocytoma cells is a well studied model used both to identify molecular, biochemical, and physiological correlates of neurotrophin-driven neuronal differentiation and to determine the causal nature of specific events in this differentiation process. Although epidermal growth factor (EGF) elicits many of the same early biochemical and molecular changes in PC12 cells observed in response to NGF, EGF does not induce molecular or morphological differentiation of PC12 cells. The identification of genes whose expression is differentially regulated by NGF versus EGF in PC12 cells has, therefore, been considered a source of potential insight into the molecular specificity of neurotrophin-driven neuronal differentiation. A "second generation" representational difference analysis procedure now identifies the urokinase plasminogen activator receptor (UPAR) as a gene that is much more extensively induced by NGF than by EGF in PC12 cells. Both an antisense oligonucleotide for the UPAR mRNA and an antibody directed against UPAR protein block NGF-induced morphological and biochemical differentiation of PC12 cells; NGF-induced UPAR expression is required for subsequent NGF-driven differentiation.

Transcriptional regulation of the cyclooxygenase-2 gene by diverse ligands in murine osteoblasts

Osteoblasts produce prostaglandins in response to a wide variety of stimuli. Induced prostaglandin synthesis is generally the consequence of elevated cyclooxygenase-2 (COX-2) expression. Agents as diverse as serum, bFGF, PDGF, PGE(2), or [TNFalpha + IL1beta] rapidly induce expression of COX-2 protein in murine MC3T3-E1 osteogenic cells. Transient transfection studies using reporter constructs containing either wild-type COX-2 regulatory sequences or mutated cis-acting sequences linked to a luciferase reporter gene identify a CRE site and two NF-IL6 (C/EBP) sites which play important roles in the regulation of COX-2 expression in response to all these agents in osteoblasts. Induction of wild-type COX-2 reporter gene expression in MC3T3-E1 cells by all these agents involves signaling through the MEKK/JNK pathway and activation of both c-Jun and the C/EBP family of transcription factors.

Genetic evidence for distinct roles of COX-1 and COX-2 in the immediate and delayed phases of prostaglandin synthesis in mast cells

Activation of mast cells by aggregation of their high-affinity IgE receptors stimulates prostaglandin (PG) D(2) synthesis and secretion. An immediate phase of PGD(2) synthesis, complete within 30 min, is followed by a delayed, second phase of PGD(2) production that reaches a maximum 4 to 8 h after activation. Activation of mast cells from COX-2 (-/-) mice stimulates the release of PGD(2) during the first 30 min, whereas activation of mast cells from COX-1 (-/-) mice does not generate any PGD(2) in the first 2 h. On the other hand, COX-2 (-/-) cells do not participate in delayed phase of PGD(2) synthesis, while COX-1 (-/-) cells secrete low levels of PGD(2) between 2 and 4 h after activation. These data demonstrate that (i) the first phase of PG synthesis is COX-1 dependent and (ii) the second, delayed phase of PG synthesis is dependent on activation-induced synthesis and activity of COX-2.

Functional and biochemical analysis of the C2 domains of synaptotagmin IV

Synaptotagmins (Syts) are a family of vesicle proteins that have been implicated in both regulated neurosecretion and general membrane trafficking. Calcium-dependent interactions mediated through their C2 domains are proposed to contribute to the mechanism by which Syts trigger calcium-dependent neurotransmitter release. Syt IV is a novel member of the Syt family that is induced by cell depolarization and has a rapid rate of synthesis and a short half-life. Moreover, the C2A domain of Syt IV does not bind calcium. We have examined the biochemical and functional properties of the C2 domains of Syt IV. Consistent with its non-calcium binding properties, the C2A domain of Syt IV binds syntaxin isoforms in a calcium-independent manner. In neuroendocrine pheochromocytoma (PC12) cells, Syt IV colocalizes with Syt I in the tips of the neurites. Microinjection of the C2A domain reveals that calcium-independent interactions mediated through this domain of Syt IV inhibit calcium-mediated neurotransmitter release from PC12 cells. Conversely, the C2B domain of Syt IV contains calcium binding properties, which permit homo-oligomerization as well as hetero-oligomerization with Syt I. Our observation that different combinatorial interactions exist between Syt and syntaxin isoforms, coupled with the calcium stimulated hetero-oligomerization of Syt isoforms, suggests that the secretory machinery contains a vast repertoire of biochemical properties for sensing calcium and regulating neurotransmitter release accordingly.

Assays for noninvasive imaging of reporter gene expression

Repeated, noninvasive imaging of reporter gene expression is emerging as a valuable tool for monitoring the expression of genes in animals and humans. Monitoring of organ/cell transplantation in living animals and humans, and the assessment of environmental, behavioral, and pharmacologic modulation of gene expression in transgenic animals should soon be possible. The earliest clinical application is likely to be monitoring human gene therapy in tumors transduced with the herpes simplex virus type 1 thymidine kinase (HSV1-tk) suicide gene. Several candidate assays for imaging reporter gene expression have been studied, utilizing cytosine deaminase (CD), HSV1-tk, and dopamine 2 receptor (D2R) as reporter genes. For the HSV1-tk reporter gene, both uracil nucleoside derivatives (e.g., 5-iodo-2'-fluoro-2'-deoxy-1-beta-D-arabinofuranosyl-5-iodouracil [FIAU] labeled with 124I, 131I) and acycloguanosine derivatives [e.g., 8-[18F]fluoro-9-[[2-hydroxy-1-(hydroxymethyl)ethoxy]methyl]guanine (8-[18F]-fluoroganciclovir) ([18F]FGCV), 9-[(3-[18F]fluoro-1-hydroxy-2-propoxy)methyl]guanine ([18F]FHPG)] have been investigated as reporter probes. For the D2R reporter gene, a derivative of spiperone [3-(2'-[18F]-Fluoroethyl)spiperone ([18F]FESP)] has been used with positron emission tomography (PET) imaging. In this review, the principles and specific assays for imaging reporter gene expression are presented and discussed. Specific examples utilizing adenoviral-mediated delivery of a reporter gene as well as tumors expressing reporter genes are discussed.

Unusual sites of arginine methylation in Poly(A)-binding protein II and in vitro methylation by protein arginine methyltransferases PRMT1 and PRMT3

Arginine methylation is a post-translational modification found mostly in RNA-binding proteins. Poly(A)-binding protein II from calf thymus was shown by mass spectrometry and sequencing to contain NG, NG-dimethylarginine at 13 positions in its amino acid sequence. Two additional arginine residues were partially methylated. Almost all of the modified residues were found in Arg-Xaa-Arg clusters in the C terminus of the protein. These motifs are distinct from Arg-Gly-Gly motifs that have been previously described as sites and specificity determinants for asymmetric arginine dimethylation. Poly(A)-binding protein II and deletion mutants expressed in Escherichia coli were in vitro substrates for two mammalian protein arginine methyltransferases, PRMT1 and PRMT3, with S-adenosyl-L-methionine as the methyl group donor. Both PRMT1 and PRMT3 specifically methylated arginines in the C-terminal domain corresponding to the naturally modified sites.

Repetitive, non-invasive imaging of the dopamine D2 receptor as a reporter gene in living animals

Reporter genes (e.g. beta-galactosidase, chloramphenicol-acetyltransferase, green fluorescent protein, luciferase) play critical roles in investigating mechanisms of gene expression in transgenic animals and in developing gene delivery systems for gene therapy. However, measuring expression of these reporter genes requires biopsy or death. We now report a procedure to image reporter gene expression repetitively and non-invasively in living animals with positron emission tomography (PET), using the dopamine type 2 receptor (D2R) as a reporter gene and 3-(2'-[18F]fluoroethyl)spiperone (FESP) as a reporter probe. We use a viral delivery system to demonstrate the ability of this PET reporter gene/PET reporter probe system to image reporter gene expression following somatic gene transfer. In mice injected intravenously with replication-deficient adenovirus carrying a D2R reporter gene, PET in vivo measures of hepatic [18F] retention are proportional to in vitro measures of hepatic FESP retention, D2R ligand binding and D2R mRNA. We use tumor-forming cells carrying a stably transfected D2R gene to demonstrate imaging of this PET reporter gene/PET reporter probe system in 'tissues'. Tumors expressing the transfected D2R reporter gene retain substantially more FESP than control tumors. The D2R/FESP reporter gene/reporter probe system should be a valuable technique to monitor, in vivo, expression from both gene therapy vectors and transgenes.

Synthesis degradation, and subcellular localization of synaptotagmin IV, a neuronal immediate early gene product

Synaptotagmin IV (Syt IV) is an immediate early gene induced by depolarization in rat PC12 cells and in rat hippocampus. We prepared an antiserum to Syt IV protein. The 46-kDa Syt IV protein is nearly undetectable by western blotting in unstimulated PC12 cells. After depolarization, Syt IV increases rapidly, peaks at 4 h, and decays to near baseline levels by 12 h. Forskolin stimulation also leads to rapid Syt IV protein accumulation. The rate of Syt IV protein synthesis, determined by labeling with radioactive amino acids and immunoprecipitation, is low in unstimulated PC12 cells, but increases over the first 3 h after forskolin stimulation and remains elevated for several hours. Syt IV protein is relatively labile; metabolically labeled Syt IV has a half-life of approximately 2 h in PC12 cells. Sucrose density gradient fractionation and vesicle immunoisolation experiments suggest that Syt IV protein is present in both synaptic-like microvesicles and secretory granules. Vesicles immunoisolated from forskolin-treated PC12 cells with anti-Syt I antibody contain radioactively labeled Syt IV, demonstrating that Syt I and Syt IV colocalize in common vesicles. These results suggest that Syt IV protein, after its stimulation-induced synthesis, is rapidly transported to secretory vesicles where it may transiently modulate the exocytotic machinery.

Imaging adenoviral-directed reporter gene expression in living animals with positron emission tomography

We are developing quantitative assays to repeatedly and noninvasively image expression of reporter genes in living animals, using positron emission tomography (PET). We synthesized positron-emitting 8-[18F]fluoroganciclovir (FGCV) and demonstrated that this compound is a substrate for the herpes simplex virus 1 thymidine kinase enzyme (HSV1-TK). Using positron-emitting FGCV as a PET reporter probe, we imaged adenovirus-directed hepatic expression of the HSV1-tk reporter gene in living mice. There is a significant positive correlation between the percent injected dose of FGCV retained per gram of liver and the levels of hepatic HSV1-tk reporter gene expression (r2 > 0.80). Over a similar range of HSV1-tk expression in vivo, the percent injected dose retained per gram of liver was 0-23% for ganciclovir and 0-3% for FGCV. Repeated, noninvasive, and quantitative imaging of PET reporter gene expression should be a valuable tool for studies of human gene therapy, of organ/cell transplantation, and of both environmental and behavioral modulation of gene expression in transgenic mice.

Imaging of adenoviral-directed herpes simplex virus type 1 thymidine kinase reporter gene expression in mice with radiolabeled ganciclovir

We are developing procedures to repeatedly and noninvasively image the expression of transplanted reporter genes in living animals and in patients, using PET. We have investigated the use of the Herpes Simplex Virus type 1 thymidine kinase gene (HSV1-tk) as a reporter gene and [8-14C]-ganciclovir as a reporter probe. HSV1-tk, when expressed, leads to phosphorylation of [8-14C]-ganciclovir. As a result, specific accumulation of phosphorylated [8-14C]-ganciclovir should occur almost exclusively in tissues expressing the HSV1-tk gene.

METHODS

An adenoviral vector was constructed carrying the HSV1-tk gene along with a control vector. C6 rat glioma cells were infected with either viral vector and uptake of [8-3H]-ganciclovir was determined. In addition, 12 mice were injected with varying levels of either viral vector. Adenovirus administration in mice leads primarily to liver infection. Forty-eight hours later the mice were injected with [8-14C]-ganciclovir, and 1 hr later the mice were sacrificed and biodistribution studies performed. Digital whole-body autoradiography also was performed on separate animals. HSV1-tk expression was assayed, using both normalized HSV1-tk mRNA levels and relative HSV1-TK enzyme levels, in both the cell culture and murine studies.

RESULTS

Cell culture, murine tissue biodistribution and murine in vivo digital whole-body autoradiography all demonstrate the feasibility of HSV1-tk as a reporter gene and [8-14C]-ganciclovir as an imaging reporter probe. A good correlation (r2 = 0.86) between the [8-14C]-ganciclovir percent injected dose per gram tissue from HSV1-tk positive tissues and HSV1-TK enzyme levels in vivo was found. An initial study in mice with [8-18F]-fluoroganciclovir and microPET imaging supports further investigation of [8-18F]-fluoroganciclovir as a PET reporter probe for imaging HSV1-tk gene expression.

CONCLUSION

These results demonstrate the feasibility of using [8-14C]-ganciclovir as a reporter probe for the HSV1-tk reporter gene, using an in vivo adenoviral mediated gene delivery system in a murine model. The results form the foundation for further investigation of [8-18F]-fluoroganciclovir for noninvasive and repeated imaging of gene expression with PET.

The murine neutrophil-chemoattractant chemokines LIX, KC, and MIP-2 have distinct induction kinetics, tissue distributions, and tissue-specific sensitivities to glucocorticoid regulation in endotoxemia

Lipopolysaccharide-induced CXC chemokine (LIX) is a novel murine neutrophil-chemoattractant CXC chemokine cloned as a glucocorticoid-attenuated response gene. We investigated LIX message expression in an acute endotoxemia model. LIX message peaks later than KC or macrophage inflammatory protein-2 (MIP-2) and remains elevated longer in almost all tissues. Induced LIX message expression in heart is 5- to 6-fold greater than in lung and spleen, and 20-fold greater than in liver. In contrast, KC expression is equal in heart, lung, and liver, whereas MIP-2 expression is strongest in the lung. Glucocorticoid regulation of these genes also differs. Endotoxemia-induced LIX message expression in the lung is markedly enhanced in adrenalectomized mice and strongly attenuated by dexamethasone, whereas lung KC and MIP-2 expression are unaffected by glucocorticoids. It is surprising to note that endotoxemia-induced brain expression of LIX (but not KC or MIP-2) is increased by dexamethasone. These observations suggest that LIX may have biological roles distinct from KC and MIP-2.

Nurr1 mRNA expression in neonatal and adult rat brain following kainic acid-induced seizure activity

Nurr1 is an immediate early gene encoding a member of the steroid-thyroid hormone receptor family. In PC12 cells, Nurr1 is readily induced by membrane depolarization, but not by growth factors. Nurr1 is predominantly expressed in the brain, and is essential to the differentiation of midbrain dopaminergic neurons. However, Nurr1 is also expressed in brain regions unrelated to dopaminergic neurons, e.g., hippocampus and cerebral cortex, and its immediate induction following seizure activity suggests a potential involvement of this transcription factor in modulating gene expression in the nervous system. To investigate the response of Nurr1 to neuronal activation, we analyzed Nurr1 mRNA expression in neonatal and adult rat brain following kainic acid (KA)-induced seizure. In P7 animals, systemic injection of KA increased Nurr1 mRNA levels in a few hilar cells of the dentate gyrus and some pyramidal cells of the CA3 region of the hippocampus. In older animals, Nurr1 induction progressively expanded to all hippocampal regions (P14, P21) and eventually to cortical regions (adult). The increase was rapid and transient in the dentate gyrus, a structure resistant to the neurotoxic effect of KA, and was more prolonged in other regions more susceptible to KA toxicity. Induction of Nurr1 at early postnatal stages and rapid increase in the dentate gyrus following KA-induced seizure, suggest that Nurr1 expression is modulated by neuronal activity. On the other hand, prolonged Nurr1 induction in regions sensitive to KA toxicity indicates a possible involvement of Nurr1 in selective neuronal vulnerability.

Seizure activity induces PIM-1 expression in brain

We recently identified KID-1, a previously undescribed protein kinase induced by depolarization in PC12 cells and brain (Feldman et al., 1998). KID-1 shares a high degree of sequence homology with PIM-1, a proto-oncogene previously reported to be expressed in hematopoietic and germ cells. We examined PIM-1 expression in stimulated PC12 cells, brains of kainic acid-treated rats, and a number of tissues from untreated rats. We now report that forskolin, but not depolarization or growth factors, induces PIM-1 expression in PC12 cells. PIM-1 is an immediate early gene induced in response to forskolin stimulation. We detect PIM-1 mRNA in a number of unstimulated tissues and at low levels in unstimulated brain. Systemic kainic acid administration to adult rats induces PIM-1 expression in the dentate gyrus region of the hippocampus.

PRMT 3, a type I protein arginine N-methyltransferase that differs from PRMT1 in its oligomerization, subcellular localization, substrate specificity, and regulation

Methylation is one of the many post-translational modifications that modulate protein function. Although asymmetric NG,NG-dimethylation of arginine residues in glycine-arginine-rich domains of eucaryotic proteins, catalyzed by type I protein arginine N-methyltransferases (PRMT), has been known for some time, members of this enzyme class have only recently been cloned. The first example of this type of enzyme, designated PRMT1, cloned because of its ability to interact with the mammalian TIS21 immediate-early protein, was then shown to have protein arginine methyltransferase activity. We have now isolated rat and human cDNA orthologues that encode proteins with substantial sequence similarity to PRMT1. A recombinant glutathione S-transferase (GST) fusion product of this new rat protein, named PRMT3, asymmetrically dimethylates arginine residues present both in the designed substrate GST-GAR and in substrate proteins present in hypomethylated extracts of a yeast rmt1 mutant that lacks type I arginine methyltransferase activity; PRMT3 is thus a functional type I protein arginine N-methyltransferase. However, rat PRMT1 and PRMT3 glutathione S-transferase fusion proteins have distinct enzyme specificities for substrates present in both hypomethylated rmt1 yeast extract and hypomethylated RAT1 embryo cell extract. TIS21 protein modulates the enzymatic activity of recombinant GST-PRMT1 fusion protein but not the activity of GST-PRMT3. Western blot analysis of gel filtration fractions suggests that PRMT3 is present as a monomer in RAT1 cell extracts. In contrast, PRMT1 is present in an oligomeric complex. Immunofluorescence analysis localized PRMT1 predominantly to the nucleus of RAT1 cells. In contrast, PRMT3 is predominantly cytoplasmic.

KID-1, a protein kinase induced by depolarization in brain

Membrane depolarization leads to changes in gene expression that modulate neuronal plasticity. Using representational difference analysis, we have identified a previously undiscovered cDNA, KID-1 (kinase induced by depolarization), that is induced by membrane depolarization or forskolin, but not by neurotrophins or growth factors, in PC12 pheochromocytoma cells. KID-1 is an immediate early gene that shares a high degree of sequence similarity with the family of PIM-1 serine/threonine protein kinases. Recombinant KID-1 fusion protein is able to catalyze both histone phosphorylation and autophosphorylation. KID-1 mRNA is present in a number of unstimulated tissues, including brain. In response to kainic acid and electroconvulsive shock-induced seizures, KID-1 is induced in specific regions of the hippocampus and cortex.

Noninvasive methods for quantitating blood time-activity curves from mouse PET images obtained with fluorine-18-fluorodeoxyglucose

The mouse model is currently being explored for various applications with PET imaging. Low resolution of current animal scanners relative to mouse size leads to difficulty in quantitating data from mouse PET images. We have, therefore, investigated methods for determining blood time-activity curves (TACs) from mouse PET studies done with fluorine-18-fluorodeoxyglucose (FDG).

METHODS

Eight mice were fasted, the tail vein was injected with 150-300 microCi of FDG and dynamic images were acquired with a CTI/Siemens (Knoxville, TN) animal tomograph for 64.5 min. Concurrently, 11-14 left ventricle (LV) blood samples were drawn directly from the LV chamber. Organ TACs were obtained by drawing circular regions of interest (ROIs) of various sizes on images of the heart, liver and brain. For each mouse, the FDG model parameter K = (K1 x k3)/(k2 + k3) was estimated by a Patlak algorithm with various estimates of the blood TAC and, as a reference tissue TAC, the brain TAC.

RESULTS

Most partial-volume-corrected heart ROI TACs overestimated the LV samples. Blood TACs from heart images produced statistically different estimates of K than did the LV samples. The liver image-derived blood TACs yielded estimates of K that were comparable to those yielded by the LV samples. Estimates of K determined with two directly sampled LV points in conjunction with the liver image-derived TAC were not statistically different from the estimates obtained with the LV samples. The size and location of ROIs on images of the liver minimally affected the TACs.

CONCLUSION

We have shown that it is experimentally possible to obtain a blood TAC from mouse studies by repeatedly sampling from the LV. We have also shown that images of the liver can be used to reliably estimate the blood TAC. Future FDG PET studies with the mouse model will benefit from this demonstrated ability to noninvasively quantitate blood TACs directly from FDG PET images.

Sequence similarities of a subgroup of CXC chemokines related to murine LIX: implications for the interpretation of evolutionary relationships among chemokines

The murine CXC chemokine LIX has distinctive sequence features that suggest it is a novel chemokine. Among known human chemokines, ENA-78 and GCP-2 are the two most closely related to LIX. We have recently cloned the human GCP-2 gene. Phylogenetic analysis shows that the LIX coding region is more distant from both human GCP-2 and ENA-78 than is porcine AMCF-II, the chemokine with greatest sequence similarity to LIX. Human GCP-2 and ENA-78 have very high nucleotide similarity in non-coding as well as coding sequences, which suggests that these genes are the result of an evolutionarily recent gene duplication event. If this duplication occurred during primate evolution, then non-primate species may have only a single chemokine corresponding to this pair of human genes. This example shows that a one-to-one genetic correspondence does not necessarily exist between all the chemokine genes in two different species. These observations may have important implications for other chemokines that belong to clusters of closely related genes.

Gastrin-releasing peptide-induced expression of prostaglandin synthase-2 in Swiss 3T3 cells

Prostaglandins, produced in response to mitogens and cytokines, are potent modulators of gastrointestinal physiology and pathophysiology. We investigated modulation of Prostaglandin synthase 2 (PGS-2) expression by the gastrin-releasing peptide (GRP) receptor in Swiss 3T3 cells. PGS-2 mRNA expression in Swiss 3T3 cells was determined by Northern blot analysis. PGS-2 protein expression in Swiss 3T3 cells was measured by Western blot analysis. GRP caused a transient induction of PGS-2 mRNA in Swiss 3T3 cells that resulted in GRP-dependent expression of PGS-2 protein. Transcriptional activation of PGS-2 by GRP was independent of de novo protein synthesis and was not affected by pertussis toxin. Comparison of signaling pathways used by PMA or EGF to those used by GRP showed that PGS-2 induction by GRP increased under conditions that inhibit PKC activity. Dexamethasone, which blocks PMA and EGF induction of PGS-2, also inhibited GRP-induced accumulation of PGS-2 mRNA. These results show that PGS-2 expression in Swiss 3T3 cells is not only controlled by PKC and receptor tyrosine kinase pathways but also by G-protein coupled receptor signaling pathways.