Human cytomegalovirus (HCMV) immediate-early enhancer/promoter specificity during embryogenesis defines target tissues of congenital HCMV infection

The baculovirus promoter OpIE2 sequence has inhibitory effect on the activity of the cytomegalovirus (CMV) promoter in HeLa and HEK-293 T cells

Understanding how promoters work in non-host cells is complex. Nonetheless, understanding this process is crucial while performing gene expression modulation studies. This study began with the process of constructing a shuttle vector with CMV and OpIE2 promoters in a tandem arrangement to achieve gene expression in both mammalian and insect cells, respectively. In this system, inhibitory regions in the 5' end of the OpIE2 insect viral promoter were found to be blocking the activity of the CMV promoter in mammalian cells. Initially, the OpIE2 promoter was cloned downstream of the CMV promoter and upstream of the EGFP reporter gene. After introducing the constructed shuttle vector to insect and mammalian cells, a significant drop in the CMV promoter activity in mammalian cells was observed. To enhance the CMV promoter activity, several modifications were made to the shuttle vector including site-directed mutagenesis to remove all ATG codons from the downstream promoter (OpIE2), separating the two promoters to eliminate the effect of transcription interference between them, and finally, identifying some inhibitory regions in the OpIE2 promoter sequence. When these inhibitory regions were removed, high expression levels in insect and mammalian cells were maintained. In conclusion, a shuttle vector was constructed that works efficiently in both mammalian and insect cell lines in the absence of baculovirus infection or gene expression. Moreover, the shuttle vector can be used as a platform to further study the reason for this inhibition, which may give new insights about transcription and promoters' mode of action in both insect and mammalian hosts.

Human cytomegalovirus infection modulates thrombospondins 1 and 2 in primary fetal astrocytes

Transmission of human cytomegalovirus (HCMV) to the fetus is the most common type of intrauterine infection; the mechanism of HCMV pathogenesis in the developing central nervous system remains unclear. Thrombospondins 1 and 2 (TSP1, TSP2) produced by immature astrocytes are critical for fetal synaptogenesis. To examine the effect of HCMV on fetal astrocytes, human fetal astrocytes were isolated and cultured with HCMV AD169. Cells were harvested at different time points. Protein and mRNA expressions of TSP1 and TSP2 were determined using RT-qPCR, western blotting analysis, and enzyme-linked immunosorbent assay. The results showed that HCMV infection induced time-dependent decreases in mRNA and protein expressions of both TSP1 and TSP2 in astrocytes. Flow cytometry was used to detect apoptosis of HCMV-infected astrocytes, and the result indicated that there was no linkage between cell apoptosis and the decrease in TSP1 and TSP2 expressions induced by HCMV infection. When ganciclovir treatment was performed on HCMV-infected astrocytes, results showed that ganciclovir treatment inhibited the reduction of TSP1 and TSP2 expression in astrocytes. In the further study, pEGFP-N3-IE1 was transfected into astrocytes to identify that it was not IE1 but active viral replication that was essential in the continuous decrease of TSP1 and TSP2 expressions in HCMV-infected astrocytes.

Tissue and cell-specific transcriptional activity of the human cytomegalovirus immediate early gene promoter (UL123) in zebrafish

The human cytomegalovirus (CMV) is a member of the herpesvirus superfamily and causes different diseases including encephalitis, gastrointestinal diseases, pneumonitis, hepatitis, and retinitis. The immediate early (IE) gene of the human cytomegalovirus is essential to the viral replication. The proximal promoter region of this gene behaves as a strong enhancer and was commonly used to overexpress genes in vitro and in vivo in numerous cell types and species. However, there was no detailed report on the spatial and temporal transcriptional activity of the human CMV-IE gene promoter in zebrafish. In the present study, we generated stable transgenic zebrafish lines carrying the eGFP reporter gene under the control of the human CMV-IE gene promoter (-602/-14). We demonstrated that the hCMV-IE:eGFP transgene was expressed in numerous tissues but transgene expression was either regionalized or restricted to specific cell types as embryo and larval development progressed. In adult, the global expression pattern was similar but not identical to that described for the simian CMV-IE gene promoter in stable zebrafish with high transgene expression in the spinal cord, olfactory organs, central nervous system, neuromasts, retina, and skeletal muscles. However, we describe additional major expression sites in the hepatocytes, the epithelial cells of the intestine, the epithelial cells of the renal tubules, and the oocytes. Interestingly, our study shows that the tissue and cell specific expression pattern of the human CMV-IE gene promoter is rather well conserved in stable transgenic zebrafish compared to that observed in mouse. The major expression sites described in zebrafish are in agreement with the targeted cells and symptoms resulting from CMV infections in human. Finally, the hCMV:eGFP transgenic lines described in the present study will be valuable tools to trace specific cell lineages in adult zebrafish.

Congenital cytomegalovirus infection: molecular mechanisms mediating viral pathogenesis

Human cytomegalovirus (CMV) is responsible for approximately 40,000 congenital infections in the United States each year. Congenital CMV disease frequently produces serious neurodevelopmental disability, as well as vision impairment and sensorineural hearing loss. Development of a CMV vaccine is therefore considered to be a major public health priority. The mechanisms by which CMV injures the fetus are complex and likely include a combination of direct fetal injury induced by pathologic virally-encoded gene products, an inability of the maternal immune response to control infection, and the direct impact of infection on placental function. CMV encodes gene products that function, both at the RNA and the protein level, to interfere with many cellular processes. These include gene products that modify the cell cycle; interfere with apoptosis; induce an inflammatory response; mediate vascular injury; induce site-specific breakage of chromosomes; promote oncogenesis; dysregulate cellular proliferation; and facilitate evasion of host immune responses. This minireview summarizes current concepts regarding these aspects of the molecular virology of CMV and the potential pathogenic impact of viral gene expression on the developing fetus. Areas for potential development of novel therapeutic intervention are suggested for improving the outcome of this disabling congenital infection.

The activator protein 1 binding motifs within the human cytomegalovirus major immediate-early enhancer are functionally redundant and act in a cooperative manner with the NF-{kappa}B sites during acute infection

Human cytomegalovirus (HCMV) infection causes a rapid induction of c-Fos and c-Jun, the major subunits of activator protein 1 (AP-1), which in turn have been postulated to activate the viral immediate-early (IE) genes. Accordingly, the major IE promoter (MIEP) enhancer, a critical control region for initiating lytic HCMV infection and reactivation from the latent state, contains one well-characterized AP-1 site and a second candidate interaction site. In this study we explored the role of these AP-1 elements in the context of the infection. We first show that the distal candidate AP-1 motif binds c-Fos/c-Jun heterodimers (AP-1 complex) and confers c-Fos/c-Jun-mediated activity to a core promoter. Site-directed mutagenesis studies indicate that both AP-1 response elements are critical for 12-O-tetradecanoylphorbol-13-acetate (TPA)-enhanced MIEP activity in transient-transfection assays. In marked contrast to the results obtained with the isolated promoter, disruption of the AP-1 recognition sites of the MIEP in the context of the infectious HCMV genome has no significant influence on the expression of the MIE protein IE1 or viral replication in different cell types. Moreover, a chimeric murine CMV driven by the HCMV MIEP (hMCMV-ES) with the two AP-1 binding sites mutated is not compromised in virulence, is able to grow and disseminate to different organs of the newborn mice as efficiently as the parental virus, and is competent in reactivation. We show, however, that combined inactivation of the enhancer AP-1 and NF-κB recognition sites leads to an attenuation of the hMCMV-ES in the neonatal murine infection model, not observed when each single element is abolished. Altogether, these results underline the functional redundancy of the MIEP elements, highlighting the plasticity of this region, which probably evolved to ensure maximal transcriptional performance across many diverse environments.

In vivo competence of murine cytomegalovirus under the control of the human cytomegalovirus major immediate-early enhancer in the establishment of latency and reactivation

The human cytomegalovirus (HCMV) major immediate-early enhancer has been postulated to play a pivotal role in the control of latency and reactivation. However, the absence of an animal model has obstructed a direct test of this hypothesis. Here we report on the establishment of an in vivo, experimentally tractable system for quantitatively investigating physiological functions of the HCMV enhancer. Using a neonate BALB/c mouse model, we show that a chimeric murine CMV under the control of the HCMV enhancer is competent in vivo, replicating in key organs of mice with acute CMV infection and exhibiting latency/reactivation features comparable for the most part to those of the parental and revertant viruses.

Msx2 exerts bone anabolism via canonical Wnt signaling

Msx2 is a homeodomain transcription factor first identified in craniofacial bone and human femoral osteoblasts. We hypothesized that Msx2 might activate skeletal Wnt signaling. Therefore, we analyzed the effects of CMV-Msx2 transgene (Msx2Tg) expression on skeletal physiology and composition. Skeletal Msx2 expression was increased 2-3-fold by Msx2Tg, with expanded protein accumulation in marrow, secondary ossification centers, and periosteum. Microcomputed tomography established increased bone volume in Msx2Tg mice, with increased numbers of plate-like trabeculae. Histomorphometry revealed increased bone formation in Msx2Tg mice versus non-Tg siblings, arising from increased osteoblast numbers. While decreasing adipogenesis, Msx2Tg increased osteogenic differentiation via mechanisms inhibited by Dkk1, an antagonist of Wnt receptors LRP5 and LRP6. Bone from Msx2Tg mice elaborated higher levels of Wnt7 canonical agonists, with diminished Dkk1, changes that augment canonical signaling. Analysis of non-Tg and Msx2Tg siblings possessing the TOPGAL reporter confirmed this; Msx2Tg up-regulated skeletal beta-galactosidase expression (p </= 0.01), along with Wnt7a and Wnt7b, and reduced circulating Dkk1. To better understand molecular mechanisms, we studied C3H10T1/2 osteoprogenitor cells. As in bone, Msx2 increased Wnt7 genes and down-regulated Dkk1, while inducing the osteoblast gene alkaline phosphatase. Msx2-directed RNA interference increased Dkk1 expression and promoter activity, while reducing Wnt7a, Wnt7b, and alkaline phosphatase. Moreover, Msx2 inhibited Dkk1 promoter activity and reduced RNA polymerase association with Dkk1 chromatin. RNA interference-mediated knockdown of Wnt7a, Wnt7b, and LRP6 significantly reduced Msx2-induced alkaline phosphatase. Msx2 exerts bone anabolism in part by reducing Dkk1 expression and enhancing Wnt signaling, thus promoting osteogenic differentiation of skeletal progenitors.

Anticapsid immunity level, not viral persistence level, correlates with the progression of Theiler's virus-induced demyelinating disease in viral P1-transgenic mice

Intracranial infection of Theiler's murine encephalomyelitis virus (TMEV) induces demyelination and a neurological disease in susceptible SJL/J (SJL) mice that resembles multiple sclerosis. While the virus is cleared from the central nervous system (CNS) of resistant C57BL/6 (B6) mice, it persists in SJL mice. To investigate the role of viral persistence and its accompanying immune responses in the development of demyelinating disease, transgenic mice expressing the P1 region of the TMEV genome (P1-Tg) were employed. Interestingly, P1-Tg mice with the B6 background showed severe reductions in both CD4(+) and CD8(+) T-cell responses to capsid epitopes, while P1-Tg mice with the SJL background displayed transient reductions following viral infection. Reduced antiviral immune responses in P1-Tg mice led to >100- to 1,000-fold increases in viral persistence at 120 days postinfection in the CNS of mice with both backgrounds. Despite the increased CNS TMEV levels in these P1-Tg mice, B6 P1-Tg mice developed neither neuropathological symptoms nor demyelinating lesions, and SJL P1-Tg mice developed significantly less severe TMEV-induced demyelinating disease. These results strongly suggest that viral persistence alone is not sufficient to induce disease and that the level of T-cell immunity to viral capsid epitopes is critical for the development of demyelinating disease in SJL mice.

Human cytomegalovirus immediate-early-gene expression disrupts embryogenesis in transgenic Drosophila

Intrauterine infection with human cytomegalovirus (HCMV) is the leading viral cause of birth defects involving the central nervous system. Due to the highly species specific nature of the virus, its course of natural infection cannot be studied in animal models. Here we introduce a novel transgenic Drosophila model system for studying the effects of the major viral regulatory genes, the immediate-early genes, on normal embryonic development. We show that ectopic expression of the immediate-early genes in Drosophila led to increased embryonic lethality manifested in disintegration of the embryos. Further analysis suggested that immediate-early gene expression interfered with adherens junction maintenance, leading to the disruption of embryonic epithelial integrity. Owing to the evolutionary conservation of developmental mechanisms from invertebrates to mammals, we anticipate that the studies in Drosophila will be relevant also to humans and will ultimately provide a versatile system for studying different aspects of viral-host interactions.

Cytomegalovirus induces interferon-stimulated gene expression and is attenuated by interferon in the developing brain

Cytomegalovirus (CMV) is considered the most common infectious agent causing permanent neurological dysfunction in the developing brain. We have previously shown that CMV infects developing brain cells more easily than it infects mature brain cells and that this preference is independent of the host B- and T-cell responses. In the present study, we examined the innate antiviral defenses against mouse (m) and human (h) CMVs in developing and mature brain and brain cells. mCMV infection induced interferon (IFN)-stimulated gene expression by 10- to 100-fold in both glia- and neuron-enriched cultures. Treatment of primary brain cultures with IFN-alpha, -beta, and -gamma or a synthetic RNA, poly(I:C), reduced the number of mCMV-infected cells, both in older cells and in fresh cultures from embryonic mouse brains. When a viral dose that killed almost all unprotected cells was used, IFN-protected cells had a natural appearance, and when they were tested with whole-cell patch clamp recording, they appeared physiologically normal with typical resting membrane potentials and action potentials. mCMV infection increased expression of representative IFN-stimulated genes (IFIT3, OAS, LMP2, TGTP, and USP18) in both neonatal and adult brains to similarly large degrees. The robust upregulation of gene expression in the neonatal brain was associated with a much higher degree of viral replication at this stage of development. In contrast to the case for downstream gene induction, CMV upregulated IFN-alpha/beta expression to a greater degree in the adult brain than in the neonatal brain. Similar to the case with cultured brain cells, IFN treatment of the developing brain in vivo depressed mCMV replication. In parallel work with cultured primary human brain cells, IFN and poly(I:C) treatment reduced hCMV infection and prevented virus-mediated cell death. These results suggest that coupling IFN administration with current treatments may reduce CMV infections in the developing brain.

Nonprimate models of congenital cytomegalovirus (CMV) infection: gaining insight into pathogenesis and prevention of disease in newborns

Congenital and perinatal infections with cytomegalovirus (CMV) are responsible for considerable short- and long- term morbidity in infants. CMV is the most common congenital viral infection in the developed world, and is a common cause of neurodevelopmental injury, including mental retardation and sensorineural hearing loss (SNHL). Antiviral therapy has been shown to be valuable in ameliorating the severity of SNHL, but CMV disease control in newborns ultimately depends on successful development of a vaccine. Because CMVs are extremely species specific, preclinical evaluation of vaccines must be performed in animal models using the appropriate CMV of the animal being studied. Several small animal models available for CMV vaccine and pathogenesis research are described. The discussion focuses on the guinea pig model because guinea pig cytomegalovirus (GPCMV), which crosses the placenta and causes infection in utero, is uniquely useful. Examination of vaccines in the GPCMV and other nonprimate models should provide insights into the determinants of the host response that protect the fetus, and may help to prioritize potential vaccine strategies for use in human clinical trials related to this important public health problem.

Generation of biologically active retro-genes upon interaction of mouse spermatozoa with exogenous DNA

Mature spermatozoa of most animal species can spontaneously take up foreign DNA molecules which can be delivered to embryos upon fertilization. Following this procedure, transgenic animals of various species have been generated. We recently discovered a reverse transcriptase (RT) activity in mouse spermatozoa that can reverse-transcribe exogenous RNA molecules into cDNA copies. These cDNA copies are transferred to embryos at fertilization, mosaic propagated as non-integrated structures in tissues of founder individuals and further transmitted to F1 progeny. Reverse-transcribed sequences behave as functional genes, being correctly expressed in tissues of F0 and F1 animals. To learn more about this mechanism and further characterize the reverse transcription step, we have now incubated spermatozoa with a plasmid harboring a green fluorescent protein (EGFP) retrotransposition cassette interrupted by an intron in the opposite orientation to the EGFP gene. We found that reverse-transcribed spliced EGFP DNA sequences are generated in sperm cells and transmitted to embryos in IVF assays. After implantation in foster mothers, embryos developed into mice that expressed EGFP in the blood vessel endothelia of a variety of organs. The EGFP-encoding cDNA sequences were detected in positive tissues as extrachromosomal mosaic-propagated structures, maintained in low-copy number (<1 copy/genome), and mosaic transmitted from founders to the F1 progeny. These results indicate that an efficient machinery is present in mature spermatozoa, which can transcribe, splice, and reverse-transcribe exogenous DNA molecules. This mechanism is implicated in the genesis and non-Mendelian propagation of new genetic information besides that contained in chromosomes.

Viral infections in the developing and mature brain

A number of different RNA and DNA viruses can invade the brain and cause neurological dysfunction. These range from the tiny polio picornavirus, which has only 7kb of RNA genetic code that preferentially infects motor neurons, to the relatively large cytomegalovirus, which has >100 genes in its 235kb DNA genome and causes various neurological problems in the developing brain but is comparatively harmless to adults. This brief overview of some aspects of neurovirology addresses the complex problems that underlie an appreciation of the contribution of viral infections to brain disease. [This review is part of the INMED/TINS special issue "Nature and nurture in brain development and neurological disorders", based on presentations at the annual INMED/TINS symposium (http://inmednet.com/).]

Distinct cardiac malformations caused by absence of connexin 43 in the neural crest and in the non-crest neural tube

Connexin 43 (Cx43) is expressed in the embryonic heart, cardiac neural crest (CNC) and neural tube, and germline knockout (KO) of Cx43 results in aberrant cardiac outflow tract (OFT) formation and abnormal coronary deployment. Prior studies suggest a vital role for CNC expression of Cx43 in heart development. Surprisingly, we found that conditional knockout (CKO) of Cx43 in the dorsal neural tube and CNC mediated by Wnt1-Cre failed to recapitulate the Cx43-null OFT phenotype, although coronary vasculature was abnormal in this mutant line. A broader CKO mediated by P3pro (Pax3)-Cre, involving both ventral and dorsal aspects of the thoracic neural tube and CNC, resulted in infundibular bulging and coronary anomalies similar to those seen in germline Cx43-null hearts. P3pro-Cre-mediated loss of Cx43 in the neural tube was characterized by a late phase of cellular delamination from the dorsal and lateral neural tube, a markedly increased abundance of neuroepithelium-derived cells outside of the neural tube and an excess of such cells infiltrating the heart and infundibulum. Thus, expression of Cx43 in the CNC is crucial for normal coronary deployment, but Cx43 is not required in the CNC for normal OFT morphogenesis. Rather, this study suggests a novel function for Cx43 in which Cx43 acts through non-crest neuroepithelial cells to suppress cellular delamination from the neural tube and thereby preserve normal OFT development.

Transduction of avian cells with recombinant baculovirus

Transduction of chicken and duck cells was examined by Ac-CMV-eGFP, a recombinant baculovirus capable of expressing an eGFP reporter gene under the control of the CMV promoter. The results showed that chicken and duck cells were transducible, as demonstrated by a flow cytometry assay. The transduction efficiency of duck cells was higher than that of chicken cells. The addition of histone deacetylase inhibitor sodium butyrate enhanced the expression levels of eGFP both in chicken and duck cells. Ac-CMV-eGFP is capable of transducing genes into a variety of chicken cells in organs such as liver, lung and kidney. Of three cells from different organs tested, the highest transduction was observed in lung cells (49.8%), followed by kidney cells (44%) and liver cells (43%). Only in chicken liver cells AcMNPV transduction was in a dose-dependent manner. It also showed that baculovirus enters the avian cells by endocytosis and is released into the cytoplasm by acid-induced fusion.

Circadian Regulation of a Viral Gene Promoter in Live Transgenic Mice Expressing Firefly Luciferase

PURPOSE

This study was conducted to test for possible circadian control of viral infection in live animals using bioluminescence imaging of a firefly luciferase transgene.

METHODS

Transgenic mice expressing the firefly luciferase gene under the control of the promoter and enhancer of the human cytomegalovirus major immediate-early gene (CMV::luc) were examined through whole-animal imaging. Mice were crossed with HRS/J hairless albino mice to improve imaging of deep structures.

RESULTS

Transgene expression in the extremities and head was elevated around dusk in mice maintained in cycles of light and dark. Signal was also elevated during the animal's night in mice maintained in extended darkness. The viral promoter was induced during the active phase of the circadian locomotor rhythm in several tissues. Both the acinar cells and islets expressed the transgene in dissociated pancreas cultures.

CONCLUSIONS

These results suggest that viruses may exploit the circadian system for optimal timing of infection at particular phases in several tissue types.

Generation of reversible Rb-knockdown mice

This study describes the generation of reversible Rb-knockdown mice using Tet-off system coupled with Rb-deficient mice currently available. Mice expressing pRB conditionally in Rb-/- background were generated by crossings P(hCMV)-tTA/TRE-Rb transgenic mice with conventional Rb+/- mice. Transgenic Rb was tightly controlled with reversibility and biologically effective as exemplified by cyclin E expression in a doxycycline-dependent manner in mouse embryonic fibroblasts. However, its ectopic expression was not sufficient to rescue the phenotypes of Rb-/- embryos at organismal level, suggesting the requirement of more sophisticated regulation of pRB. With all, these results demonstrate that our experimental strategy can be an alternative way to convert classical gene-disrupted mice into reversible conditional ones.

Neuropathogenesis in cytomegalovirus infection: indication of the mechanisms using mouse models

Cytomegalovirus (CMV) is the most frequent infectious cause of developmental brain disorders and also causes brain damage in immunocompromised individuals. Although the brain is one of the main targets of CMV infection, little is known about the neuropathogenesis of the brain disorders caused by CMV in humans because of the limitations in studying human subjects. Murine CMV (MCMV) is similar to human CMV (HCMV) in terms of genome structure, pattern of gene expressions, cell tropism and infectious dynamics. In mouse models, it has been shown that neural stem/progenitor cells are the most susceptible to CMV infection in developing brains. During brain development, lytic infection tends to occur in immature glial cells, presumably causing structural disorders of the brain. In the prolonged phase of infection, CMV preferentially infects neuronal cells. Infection of neurons may tend to become persistent by evasion of immune reactions, anti-apoptotic effects and neuron-specific activation of the e1-promoter, presumably causing functional neuronal disorders. It has also been shown that CMV infection in developing brains may become latent in neural immature cells. Brain disorders may occur long after infection by reactivation of the latent infection.

The Development of the Embryonic Outflow Tract Provides Novel Insights into Cardiac Differentiation and Remodeling

The embryonic cardiac outflow tract (OFT) connects the developing ventricles with the aortic sac. In birds and mammals, OFT cardiomyocytes are generated from a "secondary (anterior)," heart-forming field well after the formation of the primitive heart tube. The OFT cardiomyocytes have unique properties and developmental fates as compared with the myocytes of the atrial and ventricular chambers. Many of the OFT cardiomyocytes of the avian embryo are eliminated by programmed cell death (PCD) during OFT remodeling in the transition from a single- to a dual-series circulation. Targeted PCD gain and loss-of-function studies indicate that PCD drives the shortening and rotation of the OFT required for the aorta and pulmonary artery to connect with the left and right ventricles, respectively. Defects in this process model aspects of the relatively common and often life-threatening congenital human conotruncal heart defects. Using indicators of tissue hypoxia, we suggest that OFT myocardial hypoxia may be the trigger for the PCD-dependent remodeling of the OFT. This review discusses these aspects of the formation and remodeling of the embryonic OFT in the context of the broader questions of cardiac muscle biology.

The herpes simplex virus type 1 early gene (thymidine kinase) promoter is activated in neurons of brain, but not trigeminal ganglia, of transgenic mice in the absence of viral proteins

Latent infection of sensory neurons and reactivation are necessary for maintenance of herpes simplex virus type 1 (HSV-1) in its host population. It has been proposed that the HSV-1 early gene, thymidine kinase (TK), may play an important regulatory role in this process. The authors used reporter transgenic mice to test whether sensory ganglia neurons could activate the HSV-1 TK reporter transgene in the absence of viral proteins. The reporter transgene was activated in subsets of neurons in the brain but was not activated in sensory ganglia neurons following a variety of experimental manipulations. These results do not support a role for TK in regulation of the latent viral genome.

Morphologic characterization of green fluorescent protein in embryonic, neonatal, and adult transgenic rats

Transgenic (Tg) animals with reporter genes are useful models in which to study cell lineage and the process of differentiation into tissues. We developed the green fluorescent protein (GFP)-Tg rat, which is more suitable for transplantation and stem cell research because it is larger than mice. We found that marker gene expression was dependent on each organ and developmental stage. In this study we describe GFP expression in various tissues from embryonic, neonatal, and adult animals. GFP expression in brain, lung, liver, and islet tissues was restricted to early developmental stages, but it was continuously strong in the exocrine pancreas, kidney, and cardiac and skeletal muscles. The CAG promoter that was presumed to induce ubiquitous protein expression might be responsible for the differences in expression.

Monitoring immediate-early gene expression through firefly luciferase imaging of HRS/J hairless mice

BACKGROUND

Gene promoters fused to the firefly luciferase gene (luc) are useful for examining gene regulation in live transgenic mice and they provide unique views of functioning organs. The dynamics of gene expression in cells and tissues expressing luciferase can be observed by imaging this enzyme's bioluminescent oxidation of luciferin. Neural pathways involved in specific behaviors have been identified by localizing expression of immediate-early genes such as c-fos. A transgenic mouse line with luc controlled by the human c-fos promoter (fos::luc) has enabled gene expression imaging in brain slice cultures. To optimize imaging of immediate-early gene expression throughout intact mice, the present study examined fos::luc mice and a second transgenic mouse containing luc controlled by the human cytomegalovirus immediate-early gene 1 promoter and enhancer (CMV::luc). Because skin pigments and hair can significantly scatter light from underlying structures, the two transgenic lines were crossed with a hairless albino mouse (HRS/J) to explore which deep structures could be imaged. Furthermore, live anesthetized mice were compared with overdosed mice.

RESULTS

Bioluminescence imaging of anesthetized mice over several weeks corresponded with expression patterns in mice imaged rapidly after a lethal overdose. Both fos::luc and CMV::luc mice showed quantifiable bright bioluminescence in ear, nose, paws, and tail whether they were anesthetized or overdosed. CMV::luc and fos::luc neonates had bioluminescence patterns similar to those of adults, although intensity was significantly higher in neonates. CMV::luc mice crossed with HRS/J mice had high expression in bone, claws, head, pancreas, and skeletal muscle, but less in extremities than haired CMV::luc mice. Imaging of brain bioluminescence through the neonatal skull was also practical. By imaging luciferin autofluorescence it was clear that substrate distribution did not restrict bioluminescence imaging to capillaries after injection. Luciferin treatment and anesthesia during imaging did not adversely affect circadian rhythms in locomotor activity.

CONCLUSIONS

Imaging of gene expression patterns with luciferase can be extended from studies of live animals to rapid imaging of mice following a pentobarbital overdose before significant effects from postmortem changes occurs. Bioluminescent transgenic mice crossed with HRS/J mice are valuable for examining gene expression in deep tissues.

The latency-associated transcript promoter of pseudorabies virus directs neuron-specific expression in trigeminal ganglia of transgenic mice

The latency-associated transcript (LAT) promoter of pseudorabies virus (PRV) is unique among viral promoters in that it remains active in trigeminal ganglia during the latent state. It is not known which the viral or host proteins regulate expression of the PRV LAT gene in latently infected neurons. To determine whether host transcriptional proteins in neurons can regulate the PRV LAT promoter in vivo, three transgenic mouse lines containing the PRV LAT promoter (LAP; LAP1 and LAP2) linked to the chloramphenicol acetyltransferase (CAT) gene were generated. All of the transgenic mouse lines, in the absence of the viral proteins, displayed strong expression of the transgene in trigeminal ganglia in addition to other neuronal tissues such as cerebral cortex, cerebellum, hippocampus and olfactory bulb. Expression of the transgene in neurons of trigeminal ganglia was demonstrated by in situ hybridization. These data provide direct evidence that neuronal transcription factors are sufficient to activate the PRV LAP in vivo and that the promoter is neuron-specific.

Domains in tropoelastin that mediate elastin deposition in vitro and in vivo

Elastic fiber assembly is a complicated process involving multiple different proteins and enzyme activities. However, the specific protein-protein interactions that facilitate elastin polymerization have not been defined. To identify domains in the tropoelastin molecule important for the assembly process, we utilized an in vitro assembly model to map sequences within tropoelastin that facilitate its association with fibrillin-containing microfibrils in the extracellular matrix. Our results show that an essential assembly domain is located in the C-terminal region of the molecule, encoded by exons 29-36. Fine mapping studies using an exon deletion strategy and synthetic peptides identified the hydrophobic sequence in exon 30 as a major functional element in this region and suggested that the assembly process is driven by the propensity of this sequence to form beta-sheet structure. Tropoelastin molecules lacking the C-terminal assembly domain expressed as transgenes in mice did not assemble nor did they interfere with assembly of full-length normal mouse elastin. In addition to providing important information about elastin assembly in general, the results of this study suggest how removal or alteration of the C terminus through stop or frameshift mutations might contribute to the elastin-related diseases supravalvular aortic stenosis and cutis laxa.

An essential role of the enhancer for murine cytomegalovirus in vivo growth and pathogenesis

The transcription of cytomegalovirus (CMV) immediate-early (IE) genes is regulated by a large and complex enhancer containing an array of binding sites for a variety of cellular transcription factors. Previously, using bacterial artificial chromosome recombinants of the virus genome, it was reported that the enhancer region of murine CMV (MCMV) is dispensable but performs a key function for viral multiplication (A. Angulo, M. Messerle, U. H. Koszinowski, and P. Ghazal, J. Virol. 72:8502-8509, 1998). In the present study, we defined, through the reconstitution of infectious enhancerless MCMVs, the growth requirement for the enhancer in tissue culture and explored its significance for steering a productive infection in vivo. A comparison of cis and trans complementation systems for infection of enhancerless virus in permissive fibroblasts revealed a multiplicity-dependent growth phenotype that is severely compromised in the rate of infectious-virus multiplication. The in vivo impact of viruses that have an amputated enhancer was investigated in an extremely sensitive model of MCMV infection, the SCID mouse. Histological examination of spleens, livers, lungs, and salivary glands from animals infected with enhancer-deficient MCMV demonstrated an absence of tissue damage associated with CMV infection. The lack of pathogenic lesions correlated with a defect in replication competence. Enhancerless viruses were not detectable in major target organs harvested from SCID mice. The pathogenesis and growth defect reverted upon restoration of the enhancer. Markedly, while SCID mice infected with 5 PFU of parental MCMV died within 50 days postinfection, all mice infected with enhancerless virus survived for the duration of the experiment (1 year) after infection with 5 x 10(5) PFU. Together, these results clarify the importance of the enhancer for MCMV growth in cell culture and underscore the in vivo significance of this region for MCMV virulence and pathogenesis.

The P4 promoter of the parvovirus minute virus of mice is developmentally regulated in transgenic P4-LacZ mice

Activation of the minute virus of mice (MVM) P4 promoter is a key step in the life cycle of the virus and is completely dependent on host transcription factors. Since transcription-factor composition varies widely in different cell types, there is the possibility that only some cell types in the host organism have the capacity to initiate expression from the P4 promoter and therefore that the promoter may be a factor in determining the tropism of MVM. In this study, the ability of various cell types to activate P4, independent of the other virus-host interactions, was examined in transgenic mouse lines bearing a beta-galactosidase reporter sequence driven by the P4 promoter. It was found that lacZ was expressed during embryogenesis and in the adult in a cell-type-specific and differentiation-dependent pattern. The data are consistent with cell-type and stage-specific activation of the P4 promoter having a role in determining the host cell-type range of MVM. The ability of some parvoviruses to replicate in, and kill oncogenically transformed cells, and to destroy induced tumors in laboratory animals is the basis of recent approaches to use MVM-based vectors in cancer gene therapy. Since these vectors rely on the activation of the P4 promoter by the target tissues, understanding the promoter dependence on cell-type and differentiation status is important for their design and potential use.

Recombinant adeno-associated virus vector design and gene expression in the mammalian brain

Efficiency and stability of recombinant adeno-associated virus (rAAV)-mediated gene expression within the mammalian brain are determined by several factors. These include the dose of infectious particles, the purity of the vector stock, the serotype of rAAV, the route of administration, and the intrinsic properties, most notably the rAAV receptor density, of the targeted area. Furthermore, the choice of appropriate regulatory elements in rAAV vector design is of fundamental importance to achieve high-level sustained in vivo transcription and translation. This review summarizes the characteristics of various transcriptional and posttranscriptional regulatory elements, and highlights their influence on the expression performance of rAAV vectors in the mammalian brain.

Enhanced cytomegalovirus infection of developing brain independent of the adaptive immune system

Cytomegalovirus (CMV) has been suggested as the most prevalent infectious agent causing neurological dysfunction in the developing brain; in contrast, CMV infections are rare in the adult brain. One explanation generally given for the developmental susceptibility to the virus is that the developing immune system is too immature to protect the central nervous system from viral infection, but as the immune system develops it can protect the brain. We suggest an alternate view: that developing brain cells are inherently more susceptible to CMV infection, independent of the immune system. We used a recombinant mouse CMV that leads to green fluorescent protein expression in infected cells. Control experiments demonstrated a high correlation between the number of cells detected with the viral GFP reporter gene and with immunocytochemical detection of the virus. After intracerebral inoculation, the number of CMV-infected cells in neonatal brains was many times greater than in mature control or mature immunodepressed SCID mice, and the mortality rate of neonates was substantially greater than SCID or control adults. Parallel experiments with live brain slices inoculated in vitro, done in the absence of the systemic immune system, generated similar data, with immature hippocampus, hypothalamus, cortex, striatum, and cerebellum showing substantially greater numbers of infected cells (100-fold) than found in adult slices in these same regions. Interestingly, in the cerebellar cortex, CMV-infected cells were more prevalent in the postmitotic Purkinje cell layer than in the mitotic granule cell layer, suggesting a selective infection of some cell types not dependent on cell division. Together, these data support the view that CMV has an intrinsic preference for infection of developing brain cells, independent, but not mutually exclusive, of the developmental status of the systemic immune system in controlling CMV infection.

Human cytomegalovirus protects endothelial cells from apoptosis induced by growth factor withdrawal

BACKGROUND

The placental and umbilical blood vessel endothelia may play an important role in human cytomegalovirus (CMV) transmission and viral propagation in the fetus.

OBJECTIVES

We propose that CMV infection promotes endothelial cell viability for virus replication. Furthermore, we suggest that certain viral mechanisms are established to delay or inhibit the cellular response to undergo apoptosis.

STUDY DESIGN

We have established a model of CMV infection in primary endothelial cells (HUVECs) in which productive infection with the endothelial-adapted strain of CMV, strain 4010, inhibits apoptosis induced by growth factor withdrawal. Apoptosis was measured by assessment of nuclear changes by TUNEL staining.

RESULTS

Our results indicate that CMV infection at 48-96 h significantly protects the endothelial cell from apoptosis induced by growth factor withdrawal. We found that uninfected cells in the CMV-infected cell cultures, but not CMV-infected cells, were susceptible to apoptosis during growth factor withdrawal. Our studies examine the pro-apoptotic pathways that are inhibited and the anti-apoptotic pathways that are activated during CMV infection. The pro-apoptotic protein, caspase-3, is activated during growth factor withdrawal in mock-infected HUVECs, but activation is reduced in CMV-infected HUVECs.

CONCLUSIONS

During infection, the CMV-encoded chemokine receptor US28 is expressed on the infected cell surface. Previously, we have shown that stimulation of US28 with the chemokine RANTES activates cellular proliferative responses in endothelial cells. It is likely, therefore, that US28 might activate anti-apoptotic mechanisms during CMV infection.

Efficient ectopic gene expression targeting chick mesoderm

The chick model has been instrumental in illuminating genes that regulate early vertebrate development and pattern formation. Targeted ectopic gene expression is critical to dissect further the complicated gene interactions that are involved. In an effort to develop a consistent method to ectopically introduce and focally express genes in chick mesoderm, we evaluated and optimized several gene delivery methods, including implantation of 293 cells laden with viral vectors, direct adenoviral injection, and electroporation (EP). We targeted the mesoderm of chick wing buds between stages 19 and 21 (Hamburger and Hamilton stages) and used beta-galactosidase and green fluorescent protein (GFP) to document gene transfer. Expression constructs using the cytomegalovirus (CMV) promoter, the beta-actin promoter, and vectors with an internal ribosomal entry sequence linked to GFP (IRES-GFP) were also compared. After gene transfer, we monitored expression for up to 3 days. The functionality of ectopic expression was demonstrated with constructs containing the coding sequences for Shh, a secreted signaling protein, or Hoxb-8, a transcription factor, both of which can induce digit duplication when ectopically expressed in anterior limb mesoderm. We identified several factors that enhance mesodermal gene transfer. First, the use of a vector with the beta-actin promoter coupled to the 69% fragment of the bovine papilloma virus yielded superior mesodermal expression both by markers and functional results when compared with several CMV-driven vectors. Second, we found the use of mineral oil to be an important adjuvant for EP and direct viral injection to localize and contain vector within the mesoderm at the injection site. Lastly, although ectopic expression could be achieved with all three methods, we favored EP confined to the mesoderm with insulated microelectrodes (confined microelectroporation- CMEP), because vector construction is rapid, the method is efficient, and results were consistent and reproducible.

Efficient method for expressing transgenes in nonhuman primate embryos using a stable episomal vector

Transgenesis in the nonhuman primate can enhance the study of human biology by providing animal models for the study of primate-specific physiology, pathophysiology, and embryonic development. Progress with this technology has been hindered by the inherent inefficiency of transgenesis, transgene silencing, and practical restrictions on the production of sufficient pronuclear stage nonhuman primate zygotes. We have developed a novel technique using an Epstein Barr virus (EBV)-based episomal vector to produce rhesus monkey (Macaca mulatta) embryos expressing a transgene. Plasmid DNA containing the latent origin of replication, oriP, and Epstein Barr Nuclear Antigen-1 (EBNA-1) of EBV, as well as a CMV IE-enhanced green fluorescent protein (eGFP) expression cassette, was introduced into rhesus embryos by direct pronuclear microinjection. We detected eGFP in early cleavage stage embryos (4-8 cell) and throughout the duration of culture (day 8-9 blastocysts) by epifluorescent microscopy. A 50% transduction rate was obtained with the EBV-based vector. Microinjected embryos expressed eGFP and retained their developmental capacity as evidenced by development to the blastocyst stage. EBV-based vectors present a novel and efficient means of delivering transgenes for the study of the molecular control of primate embryonic development.

Transcriptional activation of the promoter of human cytomegalovirus immediate early gene (CMV-IE) by the hepatitis B viral X protein (HBx) through the NF-kappaB site

The reactivation of latent cytomegalovirus (CMV) in a human by another viral infection may induce virus-related symptoms. Based on this presumption, we investigated the effect of HBx on the activation of the CMV-IE, which is also known as a transactivator and potential oncogene. The HBx transactivated the CMV-IE promoter by up to 4- and 18-fold factors in human liver HepG2 and lung fibroblast MRC-5 cells, respectively. Cotransfection of HBx with each transcription factor presented in the CMV-IE promoter showed that only NF-kappaB synergistically activated the promoter by up to a 14-fold factor. Serial deletion assays and point mutation analysis showed that the third NF-kappaB site (nt -267 to -258) and the second one (nt -162 to -153) appeared as the major responsible site and minor one, respectively, for the transactivation. These results suggest the possibility that the HBV infection of a cell previously infected by CMV would exert influence on the reactivation of the latent cytomegalovirus in a human to induce virus-related symptoms.

Neurons differentially activate the herpes simplex virus type 1 immediate-early gene ICP0 and ICP27 promoters in transgenic mice

Herpes simplex virus type 1 (HSV-1) immediate-early (IE) proteins are required for the expression of viral early and late proteins. It has been hypothesized that host neuronal proteins regulate expression of HSV-1 IE genes that in turn control viral latency and reactivation. We investigated the ability of neuronal proteins in vivo to activate HSV-1 IE gene promoters (ICP0 and ICP27) and a late gene promoter (gC). Transgenic mice containing IE (ICP0 and ICP27) and late (gC) gene promoters of HSV-1 fused to the Escherichia coli beta-galactosidase coding sequence were generated. Expression of the ICP0 and ICP27 reporter transgenes was present in anatomically distinct subsets of neurons in the absence of viral proteins. The anatomic locations of beta-galactosidase-positive neurons in the brains of ICP0 and ICP27 reporter transgenic mice were similar and included cerebral cortex, lateral septal nucleus, cingulum, hippocampus, thalamus, amygdala, and vestibular nucleus. Trigeminal ganglion neurons were positive for beta-galactosidase in adult ICP0 and ICP27 reporter transgenic mice. The ICP0 reporter transgene was differentially regulated in trigeminal ganglion neurons depending upon age. beta-galactosidase-labeled cells in trigeminal ganglia and cerebral cortex of ICP0 and ICP27 reporter transgenic mice were confirmed as neurons by double labeling with antineurofilament antibody. Nearly all nonneuronal cells in ICP0 and ICP27 reporter transgenic mice and all neuronal and nonneuronal cells in gC reporter transgenic mice were negative for beta-galactosidase labeling in the absence of HSV-1. We conclude that factors in neurons are able to differentially regulate the HSV-1 IE gene promoters (ICP0 and ICP27) in transgenic mice in the absence of viral proteins. These findings are important for understanding the regulation of the latent and reactivated stages of HSV-1 infection in neurons.

Regulated gene expression in the chicken embryo by using replication-competent retroviral vectors

Rous sarcoma virus (RSV)-derived retroviral vector could efficiently deliver the green fluorescent protein (GFP), which is driven by the internal cytomegalovirus enhancer/promoter, into restricted cell populations in the chicken embryo. RSV-derived vectors coupled with the tet regulatory elements also revealed doxycycline-dependent inducible GFP expression in the chicken embryo in ovo.

The transgenic ICP4 promoter is activated in Schwann cells in trigeminal ganglia of mice latently infected with herpes simplex virus type 1

Herpes simplex virus type 1 (HSV-1) establishes a latent infection in neurons of sensory ganglia, including those of the trigeminal ganglia. Latent viral infection has been hypothesized to be regulated by restriction of viral immediate-early gene expression in neurons. Numerous in situ hybridization studies in mice and in humans have shown that transcription from the HSV-1 genome in latently infected neurons is limited to the latency-associated transcripts. In other studies, immediate-early gene (ICP4) transcripts have been detected by reverse transcription-PCR (RT-PCR) in homogenates of latently infected trigeminal ganglia of mice. We used reporter transgenic mice containing the HSV-1(F) ICP4 promoter fused to the coding sequence of the beta-galactosidase gene to determine whether neurons in latently infected trigeminal ganglia activated the ICP4 promoter. Mice were inoculated via the corneal route with HSV-1(F). At 5, 11, 23, and 37 days postinfection (dpi), trigeminal ganglia were examined for beta-galactosidase-positive cells. The numbers of beta-galactosidase-positive neurons and nonneuronal cells were similar at 5 dpi. The number of positive neurons decreased at 11 dpi and returned to the level of mock-inoculated transgenic controls at 23 and 37 dpi. The number of positive nonneuronal cells increased at 11 and 23 dpi and remained elevated at 37 dpi. Viral proteins were detected in neurons and nonneuronal cells in acutely infected ganglia, but were not detected in latently infected ganglia. Colabeling experiments confirmed that the transgenic ICP4 promoter was activated in Schwann cells during latent infection. These findings suggest that the cells that express the HSV-1 ICP4 gene in latently infected ganglia are not neurons.

Regulated control by granulocyte-macrophage colony-stimulating factor AU-rich element during mouse embryogenesis

In vitro studies have indicated that the granulocyte-macrophage colony-stimulating factor (GM-CSF) gene expression is regulated at the posttranscriptional level by the AU-rich element (ARE) sequence present in its 3' untranslated region (UTR). This study investigated the importance of the ARE in the control of GM-CSF gene expression in vivo. For this purpose, transgenic mice bearing GM-CSF gene constructs containing or lacking the ARE (GM-CSF AU(+) or GM-CSF AU(-), respectively) were generated. Both transgenes were under the transcriptional control of the immediate early promoter of the cytomegalovirus (CMV) to ensure their early, widespread, and constitutive expression. The regulation imposed by the ARE was revealed by comparing transgene expression at day 14 of embryonic development (E14); only the ARE-deleted but not the ARE-containing construct was expressed. Although GM-CSF AU(+) embryos were phenotypically normal, overexpression of GM-CSF in E14 GM-CSF AU(-) embryos led to severe hematopoietic alterations such as abnormal proliferation of granulocytes and macrophages accompanied by an increased number of peroxidase-expressing cells, their putative progenitor cells. These abnormalities compromise development because no viable GM-CSF AU(-) transgenic pups could be obtained. Surprisingly, by E18, significant accumulation of transgene messenger RNA was also observed in GM-CSF AU(+) embryos leading to similar phenotypic abnormalities. Altogether, these observations reveal that GM-CSF ARE is a developmentally controlled regulatory element and highlight the consequences of GM-CSF overexpression on myeloid cell proliferation and differentiation.

Tet-system for the regulation of gene expression during embryonic development

The ability to control gene expression in a temporal and spatial manner provides a new tool for the study of mammalian gene function particularly during development and oncogenesis. In this study the suitability of the tet-system for investigating embryogenesis was tested in detail. The tTACMV(M1) and rTACMV-3 (reverse Tc-controlled transactivator) transgenic mice were bred with NZL-2 bi-reporter mice containing the vector with a tTA/rTA responsive bidirectional promoter that allows simultaneous regulation of expression of two reporter genes encoding luciferase and beta-galactosidase. In both cases reporter genes were found to be expressed in a wide spectrum of tissues of double transgenic embryos and adult mice. The earliest expression was detected in tTACMV(M1)/NZL-2 embryos at embryonic day 10.5 (E10.5) and rTACMV-3/NZL-2 embryos at E13.5. Doxycycline abolished beta-gal expression in tTACMV(M1)/NZL-2 but induced it in rTACMV-3/NZL-2 embryos including late stages of embryo-genesis. The tTA and rtTA transactivators thus revealed a partially complementary mode of action during second half of embryonic development. These experiments demonstrated that both Tet regulatory systems function during embryonic development. We conclude that the Tet systems allows regulation of gene expression during embryonic development and that 'double reporter' animals like the NZL-2 mice are useful tools for the characterization of newly generated tet transactivator lines expressing tTA (or rtTA) in embryonic as well as in adult tissues.

Herpesvirus saimiri pathogenicity enhanced by thymidine kinase of herpes simplex virus

Herpesvirus saimiri can be used as an efficient gene expression vector for human T lymphocytes and thus may allow applications in experimental leukemia therapy. We constructed recombinant viruses for the functional expression of the thymidine kinase (TK) of herpes simplex virus type 1 (HSV) as a suicide gene. These viruses reliably allowed the targeted elimination of transduced nonpermissive human T cells in vitro after the administration of ganciclovir. To test the reliability of this function under the most stringent permissive conditions, in this study we analyzed the influence of the prodrugs ganciclovir and acyclovir in common marmosets on the acute leukemogenesis induced by either wild-type herpesvirus saimiri C488 or by a recombinant derivative expressing TK of HSV. Antiviral drug treatment did not influence the rapid development of acute disease. In contrast, the presence of the HSV tk gene resulted in a faster disease progression. In addition, HSV TK-expressing viruses showed faster replication than wild-type virus in culture at low serum concentrations. Thus, HSV TK accelerates the replication of herpesvirus saimiri and enhances its pathogenicity. This should be generally considered when HSV TK is applied as a transgene in replication-competent DNA virus vectors for gene therapy.

Predominant transgene expression in exocrine pancreas directed by the CMV promoter

The enhancer/promoter of the human cytomegalovirus gene encoding the major immediate-early protein (CMVp) is reputed to be one of the strongest and most promiscuous regulatory elements for directing transcription of heterologous genes in vitro. However, transgene expression under the promoter in adult transgenic mice is often more restricted. We selected a CMVp segment from position -350 to +59 to control expression of transgenes for two secretory fusion proteins. Expression was analyzed by immunohistology staining and quantified by Northern blot, Western blot, and ELISA of secretions from explanted tissues. In all six lines of transgenic mice, the highest expression of transgenes at the mRNA and protein level was observed in the exocrine tissue of the pancreas, although the levels of expression varied among the lines. The results indicate not only that CMVp is not a universal promoter in vivo but indeed that it can be relatively specific for the exocrine pancreas, where expression of the gene it controlled was consistently very high.

Human cytomegalovirus IE1 promoter/enhancer drives variable gene expression in all fiber types in transgenic mouse skeletal muscle

BACKGROUND

Versatile transgenic manipulation of skeletal muscle requires knowledge of the expression profiles of diverse promoter/enhancer elements in the transcriptionally specialized fiber types of which muscle is composed. "Universal" viral promoters/enhancers, e.g., cytomegalovirus IE1 (CMV IE1), are of interest as reagents that may drive broad expression. However, a previous study noted a marked heterogeneity of CMV IE1-driven transgene expression among muscle fibers, raising the possibility of fiber-type-restricted expression. The purpose of the present study was to characterize CMV IE1-driven expression in terms of fiber type.

RESULTS

We produced two lines of transgenic mice carrying the CMV IE1/ beta-galactosidase construct CMVLacZ, and analyzed transgene expression and fiber type by histochemical analysis of hindlimb muscle sections. In both lines CMVLacZ was expressed in all four major fiber types: type I (slow) and types IIA, IIB and IIX (fast). There was no unique pattern of fiber-type-preferential expression; fiber-type quantitative differences were observed but details varied between muscle regions and between lines. Both lines showed similar fiber-type-independent regional differences in overall expression levels, and a high level of within-fiber-type variability of expression, even among nearby fibers. The soleus muscle showed strong expression and comparatively little within-fiber-type or between-fiber-type variability.

CONCLUSIONS

The CMV IE1 promoter/enhancer is not fiber-type-restricted and can be useful for driving germ-line transgene expression in all four fiber types. However, not all fibers express the gene at high levels due in part to regional differences in overall expression levels, and to a high level of within-fiber-type variability. Given the multinucleate syncitial nature of muscle fibers, it is not likely that this variability is due to variegating heterochromatinization. The soleus muscle would make a suitable subject for near-uniform experimental gene expression driven by CMV IE1 elements.

Identification of a boundary domain adjacent to the potent human cytomegalovirus enhancer that represses transcription of the divergent UL127 promoter

Transcriptional repression within a complex modular promoter may play a key role in determining the action of enhancer elements. In human cytomegalovirus, the major immediate-early promoter (MIEP) locus contains a highly potent and complex modular enhancer. Evidence is presented suggesting that sequences of the MIEP between nucleotide positions -556 and -673 function to prevent transcription activation by enhancer elements from the UL127 open reading frame divergent promoter. Transient transfection assays of reporter plasmids revealed repressor sequences located between nucleotides -556 and -638. The ability of these sequences to confer repression in the context of an infection was shown using recombinant viruses generated from a bacterial artificial chromosome containing an infectious human cytomegalovirus genome. In addition to repressor sequences between -556 and -638, infection experiments using recombinant virus mutants indicated that sequences between -638 and -673 also contribute to repression of the UL127 promoter. On the basis of in vitro transcription and transient transfection assays, we further show that interposed viral repressor sequences completely inhibit enhancer-mediated activation of not only the homologous but also heterologous promoters. These and other experiments suggest that repression involves an interaction of host-encoded regulatory factors with defined promoter sequences that have the property of proximally interfering with upstream enhancer elements in a chromatin-independent manner. Altogether, our findings establish the presence of a boundary domain that efficiently blocks enhancer-promoter interactions, thus explaining how the enhancer can work to selectively activate the MIEP.

Cytomegalovirus cell tropism, replication, and gene transfer in brain

Cytomegalovirus (CMV) infects a majority of adult humans. During early development and in the immunocompromised adult, CMV causes neurological deficits. We used recombinant murine cytomegalovirus (mCMV) expressing either green fluorescent protein (GFP) or beta-galactosidase under control of human elongation factor 1 promoter or CMV immediate early-1 promoter as reporter genes for infected brain cells. In vivo and in vitro studies revealed that neurons and glial cells supported strong reporter gene expression after CMV exposure. Brain cultures selectively enriched in either glia or neurons supported viral replication, leading to process degeneration and cell death within 2 d of viral exposure. In addition, endothelial cells, tanycytes, radial glia, ependymal cells, microglia, and cells from the meninges and choroid were infected. Although mCMV showed no absolute brain cell preference, relative cell preferences were detected. Radial glia cells play an important role in guiding migrating neurons; these were viral targets in the developing brain, suggesting that cortical problems including microgyria that are a consequence of CMV may be caused by compromised radial glia. Although CMV is a species-specific virus, recombinant mCMV entered and expressed reporter genes in both rat and human brain cells, suggesting that mCMV might serve as a vector for gene transfer into brain cells of non-murine species. GFP expression was sufficiently strong that long axons, dendrites, and their associated spines were readily detected in both living and fixed tissue, indicating that mCMV reporter gene constructs may be useful for labeling neurons and their pathways.

Cytomegalovirus cell tropism, replication, and gene transfer in brain

Cytomegalovirus (CMV) infects a majority of adult humans. During early development and in the immunocompromised adult, CMV causes neurological deficits. We used recombinant murine cytomegalovirus (mCMV) expressing either green fluorescent protein (GFP) or beta-galactosidase under control of human elongation factor 1 promoter or CMV immediate early-1 promoter as reporter genes for infected brain cells. In vivo and in vitro studies revealed that neurons and glial cells supported strong reporter gene expression after CMV exposure. Brain cultures selectively enriched in either glia or neurons supported viral replication, leading to process degeneration and cell death within 2 d of viral exposure. In addition, endothelial cells, tanycytes, radial glia, ependymal cells, microglia, and cells from the meninges and choroid were infected. Although mCMV showed no absolute brain cell preference, relative cell preferences were detected. Radial glia cells play an important role in guiding migrating neurons; these were viral targets in the developing brain, suggesting that cortical problems including microgyria that are a consequence of CMV may be caused by compromised radial glia. Although CMV is a species-specific virus, recombinant mCMV entered and expressed reporter genes in both rat and human brain cells, suggesting that mCMV might serve as a vector for gene transfer into brain cells of non-murine species. GFP expression was sufficiently strong that long axons, dendrites, and their associated spines were readily detected in both living and fixed tissue, indicating that mCMV reporter gene constructs may be useful for labeling neurons and their pathways.

In vivo replication of recombinant murine cytomegalovirus driven by the paralogous major immediate-early promoter-enhancer of human cytomegalovirus

Transcription of the major immediate-early (MIE) genes of cytomegaloviruses (CMV) is driven by a strong promoter-enhancer (MIEPE) complex. Transactivator proteins encoded by these MIE genes are essential for productive infection. Accordingly, the MIEPE is a crucial control point, and its regulation by activators and repressors is pertinent to virus replication. Since the MIEPE contains multiple regulatory elements, it was reasonable to assume that specific sequence motifs are irreplaceable for specifying the cell-type tropism and replication pattern. Recent work on murine CMV infectivity (A. Angulo, M. Messerle, U. H. Koszinowski, and P. Ghazal, J. Virol. 72:8502-8509, 1998) has documented the proposed enhancing function of the enhancer in that its resection or its replacement by a nonregulatory stuffer sequence resulted in a significant reduction of infectivity, even though replication competence was maintained by a basal activity of the spared authentic MIE promoter. Notably, full capacity for productive in vitro infection of fibroblasts was restored in recombinant viruses by the human CMV enhancer. Using two-color in situ hybridization with MIEPE-specific polynucleotide probes, we demonstrated that a murine CMV recombinant in which the complete murine CMV MIEPE is replaced by the paralogous human CMV core promoter and enhancer (recombinant virus mCMVhMIEPE) retained the potential to replicate in vivo in all tissues relevant to CMV disease. Notably, mCMVhMIEPE was also found to replicate in the liver, a site at which transgenic hCMV MIEPE is silenced. We conclude that productive in vivo infection with murine CMV does not strictly depend on a MIEPE type-specific regulation.

Murine cytomegalovirus immediate-early promoter directs astrocyte-specific expression in transgenic mice

Murine cytomegalovirus (MCMV), which causes acute, latent, and persistent infection of the natural host, is used as an animal model of human cytomegalovirus (HCMV) infection. Transcription of MCMV immediate-early (IE) genes is required for expression of the early and late genes and is dependent on host cell transcription factors. Cell-type-specific expression activity of the MCMV IE promoter was analyzed in transgenic mice generated with the major IE (MIE) enhancer/promoter involving nucleotides -1343 to -6 (1338 bp) connected to the reporter gene lacZ. Distinct expression was observed in the brain, kidneys, stomach, and skeletal muscles. Weak expression was observed in a portion of the parenchymal cells of the salivary glands and pancreas, and expression was hardly detected in the lungs, intestine, or immune and hematopoietic organs such as the thymus, spleen, lymph nodes, and bone marrow. The spectrum of organs positive for expression was narrower than that of the HCMV MIE promoter-lacZ transgenic mice reported previously and showed a greater degree of cell-type specificity. Interestingly, astrocyte-specific expression of the transgene was observed in the brain and primary glial cultures from the transgenic mice by combination of beta-galactosidase (beta-Gal) expression and immunostaining for cell markers. However, the transgene was not expressed in neurons, oligodendroglia, microglia, or endothelial cells. Furthermore, the beta-Gal expression in glial cultures was stimulated significantly by MCMV infection or by addition of calcium ionophore. These observations indicated that expression activity of the MCMV IE promoter is strictly cell-type specific, especially astrocyte-specific in the brain. This specific pattern of activity is similar to that of natural HCMV infection in humans.

Selective neuronal expression of green fluorescent protein with cytomegalovirus promoter reveals entire neuronal arbor in transgenic mice

In simple nervous systems, identified groups of neurons can be studied in depth. To allow the same advantage in the mammalian brain, we have generated green fluorescent protein (GFP) transgenic mice in which only a few types of neurons are strongly labeled with a fluorescent molecule, which the neurons synthesize internally, allowing the cells, their dendrites, filopodia, and axons to be identified in both living and fixed CNS, in slices and culture. The same neurons, with GFP expression controlled by part of the major immediate early promoter of human cytomegalovirus (CMV), show GFP beginning early in development, from one generation to the next, allowing cellular and physiological studies of axonal and dendritic growth, fate mapping, anatomical connections, and synapse formation in identified neurons. The human CMV promoter sequence we used was different from that used in previous work with other reporter genes and gave a dramatically different pattern of expression. Two transgenic lines with the same CMV promoter show similar anatomical patterns of expression in the present study. Strong GFP labeling was found in a subpopulation of mossy fibers that innervated parasagittal bands in the cerebellar cortex and olfactory axons that projected into the olfactory bulb, subsets of motoneurons and dorsal root ganglion cells, granule but not mitral cells of the olfactory bulb, and a group of neurons in the hypothalamic suprachiasmatic nucleus. A novel type of neuron was strongly labeled in the olfactory bulb external plexiform layer. In normal brains, CMV does not constitute a threat, but in the developing brain, CMV can cause debilitating neurodegeneration and death; studies using the CMV promoter aid in understanding the affinity of CMV that has been suggested for specific brain regions.

MDM2 overexpression generates a skin phenotype in both wild type and p53 null mice

The MDM2 proto-oncogene is overexpressed in human tumours and regulates the activities of the tumour suppressors p53 and pRB. We created mice that overexpress MDM2 under the control of the CMV promoter. These mice did not display an increased tumour incidence, but rather a specific skin phenotype, characterized by desquamation and hyperkeratosis. Transgenic MDM2 was found to be overexpressed in the epidermis, a tissue that normally expresses high levels of MDM2. The phenotype appeared during the first week after birth and then lessened with age, closely following the level of expression of the transgene. MDM2 overexpression was associated with an increase in proliferation in the basal layer, thickening of the epidermis, altered expression of the differentiation markers cytokeratin CK14, CK10 and CK1, and a decrease in the size and the number of granules that contain products of differentiation. Transgenic mice on a p53 null background displayed similar although not identical changes, showing that the effects of MDM2 are to a certain degree p53 independent. The skin is a major site of MDM2 expression in mice, raising the possibility that MDM2 overexpression perturbs the normal pattern of MDM2 expression and inhibits differentiation of the epidermis.