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.

UNLABELLED: 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.

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.

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

UNLABELLED: 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.

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.

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.

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.

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.

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.

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.

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 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.

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.