Prolonged infection of mouse brain neurons with murine cytomegalovirus after pre- and perinatal infection

Immune surveillance of cytomegalovirus in tissues

Cytomegalovirus (CMV), a representative member of the Betaherpesvirinae subfamily of herpesviruses, is common in the human population, but immunocompetent individuals are generally asymptomatic when infected with this virus. However, in immunocompromised individuals and immunologically immature fetuses and newborns, CMV can cause a wide range of often long-lasting morbidities and even death. CMV is not only widespread throughout the population but it is also widespread in its hosts, infecting and establishing latency in nearly all tissues and organs. Thus, understanding the pathogenesis of and immune responses to this virus is a prerequisite for developing effective prevention and treatment strategies. Multiple arms of the immune system are engaged to contain the infection, and general concepts of immune control of CMV are now reasonably well understood. Nonetheless, in recent years, tissue-specific immune responses have emerged as an essential factor for resolving CMV infection. As tissues differ in biology and function, so do immune responses to CMV and pathological processes during infection. This review discusses state-of-the-art knowledge of the immune response to CMV infection in tissues, with particular emphasis on several well-studied and most commonly affected organs.

Glial estradiol synthesis after brain injury

Glial cells are important contributors to the hormonal milieu of the brain, particularly following damage. In birds and mammals, neural injury induces the expression of aromatase in astroglia at and around the site of damage. This review describes the progression of our understanding about the incidence, regulation, and function of estrogens synthesized in glia. Following a quick discussion of the landmark studies that first demonstrated steroidogenesis in glia, I go on to describe how the inflammatory response following perturbation of the brain results in the transcription of aromatase and the resultant rise in local estradiol. I end with several unanswered questions, the answers to which may reveal the precise manner in which neurosteroids protect the brain from injury, both prior to and immediately following injury.

Murine Models of Central Nervous System Disease following Congenital Human Cytomegalovirus Infections

Human cytomegalovirus infection of the developing fetus is a leading cause of neurodevelopmental disorders in infants and children, leading to long-term neurological sequela in a significant number of infected children. Current understanding of the neuropathogenesis of this intrauterine infection is limited because of the complexity of this infection, which includes maternal immunological responses that are overlaid on virus replication in the CNS during neurodevelopment. Furthermore, available data from human cases are observational, and tissues from autopsy studies have been derived from only the most severe infections. Animal models of this human infection are also limited by the strict species specificity of cytomegaloviruses. However, informative models including non-human primates and small animal models have been developed. These include several different murine models of congenital HCMV infection for the study of CMV neuropathogenesis. Although individual murine models do not completely recapitulate all aspects of the human infection, each model has provided significant information that has extended current understanding of the neuropathogenesis of this human infection. This review will compare and contrast different murine models in the context of available information from human studies of CNS disease following congenital HCMV infections.

Inhibitory Molecules PD-1, CD73 and CD39 Are Expressed by CD8+ T Cells in a Tissue-Dependent Manner and Can Inhibit T Cell Responses to Stimulation

The salivary gland is an important tissue for persistence and transmission of multiple viruses. Previous work showed that salivary gland tissue-resident CD8⁺ T cells elicited by viruses were poorly functional ex vivo. Using a model of persistent murine cytomegalovirus (MCMV) infection, we now show that CD8⁺ T cells in the salivary gland and other non-lymphoid tissues of mice express multiple molecules associated with T cell exhaustion including PD-1, CD73 and CD39. Strikingly however, these molecules were expressed independently of virus or antigen. Rather, PD-1-expressing T cells remained PD-1⁺ after migration into tissues regardless of infection, while CD73 was activated on CD8⁺ T cells by TGF-β signaling. Blockade of PD-L1, but not CD73, improved cytokine production by salivary gland T cells ex vivo and increased the expression of granzyme B after stimulation within the salivary gland. Nevertheless, salivary-gland localized CD8⁺ T cells could kill PD-L1-expressing targets in vivo, albeit with modest efficiency, and this was not improved by PD-L1 blockade. Moreover, the impact of PD-L1 blockade on granzyme B expression waned with time. In contrast, the function of kidney-localized T cells was improved by CD73 blockade, but was unaffected by PD-L1 blockade. These data show that tissue localization per se is associated with expression of inhibitory molecules that can impact T cell function, but that the functional impact of this expression is context- and tissue-dependent.

Prolonged activation of cytomegalovirus early gene e1-promoter exclusively in neurons during infection of the developing cerebrum

The brain is the major target of congenital cytomegalovirus (CMV) infection. It is possible that neuron disorder in the developing brain is a critical factor in the development of neuropsychiatric diseases in later life. Previous studies using mouse model of murine CMV (MCMV) infection demonstrated that the viral early antigen (E1 as a product of e1 gene) persists in the postnatal neurons of the hippocampus (HP) and cerebral cortex (CX) after the disappearance of lytic infection from non-neuronal cells in the periventricular (PV) region. Furthermore, neuron-specific activation of the MCMV-e1-promoter (e1-pro) was found in the cerebrum of transgenic mice carrying the e1-pro-lacZ reporter construct. In this study, in order to elucidate the mechanisms of e1-pro activation in cerebral neurons during actual MCMV infection, we have generated the recombinant MCMV (rMCMV) carrying long e1-pro1373- or short e1-pro448-EGFP reporter constructs. The length of the former, 1373 nucleotides (nt), is similar to that of transgenic mice. rMCMVs and wild type MCMV did not significantly differed in terms of viral replication or E1 expression. rMCMV-infected mouse embryonic fibroblasts showed lytic infection and activation of both promoters, while virus-infected cerebral neurons in primary neuronal cultures demonstrated the non-lytic and persistent infection as well as the activation of e1-pro-1373, but not -448. In the rMCMV-infected postnatal cerebrum, lytic infection and the activation of both promoters were found in non-neuronal cells of the PV region until postnatal 8 days (P8), but these disappeared at P12, while the activation of e1-pro-1373, but not -448 appeared in HP and CX neurons at P8 and were prolonged exclusively in these neurons at P12, with preservation of the neuronal morphology. Therefore, e1-pro-448 is sufficient to activate E1 expression in non-neuronal cells, however, the upstream sequence from nt -449 to -1373 in e1-pro-1373 is supposed to work as an enhancer necessary for the neuron-specific activation of e1-pro, particularly around the second postnatal week. This unique activation of e1-pro in developing cerebral neurons may be an important factor in the neurodevelopmental disorders induced by congenital CMV infection.

A Review of Murine Cytomegalovirus as a Model for Human Cytomegalovirus Disease-Do Mice Lie?

Since murine cytomegalovirus (MCMV) was first described in 1954, it has been used to model human cytomegalovirus (HCMV) diseases. MCMV is a natural pathogen of mice that is present in wild mice populations and has been associated with diseases such as myocarditis. The species-specific nature of HCMV restricts most research to cell culture-based studies or to the investigation of non-invasive clinical samples, which may not be ideal for the study of disseminated disease. Initial MCMV research used a salivary gland-propagated virus administered via different routes of inoculation into a variety of mouse strains. This revealed that the genetic background of the laboratory mice affected the severity of disease and altered the extent of subsequent pathology. The advent of genetically modified mice and viruses has allowed new aspects of disease to be modeled and the opportunistic nature of HCMV infection to be confirmed. This review describes the different ways that MCMV has been used to model HCMV diseases and explores the continuing difficulty faced by researchers attempting to model HCMV congenital cytomegalovirus disease using the mouse model.

NMDA receptor subunit and CaMKII changes in rat hippocampus by congenital HCMV infection: a mechanism for learning and memory impairment

The aim of this study was to investigate the effects of congenital human cytomegalovirus infection on the expression levels of N-methyl-D-aspartate receptors (NRs) and Ca/calmodulin-dependent protein kinase II (CaMKII) in the hippocampal neurons of neonatal Sprague-Dawley (SD) rats. Pregnant SD rats were divided into an experimental group and a control group (n=10 in each group). Spatial learning and memory of the offspring of SD rats were evaluated using the Morris water-maze test. Pathological studies of hippocampus sections were carried out. The concentration of [Ca] was measured using a dual-wavelength spectrophotometer method. The expression levels of NRs were detected by an immunohistochemical study. Western blot was performed to detect the expression level of CaMKII. In the Morris water-maze test, the rats in the experimental group showed significantly increased escape latency and distance traveled than the control group. Damaged and structural disorders of the dentate granule in the hippocampus were found in the experimental rats. Immunohistochemistry results showed that the expression levels of NR subunits in the hippocampus of the experimental group were significantly decreased. The concentration of [Ca] in the experimental group was significantly increased. In contrast, the level of CaMKII in the experimental group was significantly decreased. The expressions of the NR subunit and CaMKII were decreased in rat hippocampus by human cytomegalovirus congenital infection, which may be associated with the mechanism underlying the impairment of learning and memory function.

Glial cells suppress postencephalitic CD8+ T lymphocytes through PD-L1

Engagement of the programmed death (PD)−1 receptor on activated cells by its ligand (PD‐L1) is a mechanism for suppression of activated T‐lymphocytes. Microglia, the resident inflammatory cells of the brain, are important for pathogen detection and initiation of innate immunity, however, a novel role for these cells as immune regulators has also emerged. PD‐L1 on microglia has been shown to negatively regulate T‐cell activation in models of multiple sclerosis and acute viral encephalitis. In this study, we investigated the role of glial cell PD‐L1 in controlling encephalitogenic CD8⁺ T‐lymphocytes, which infiltrate the brain to manage viral infection, but remain to produce chronic neuroinflammation. Using a model of chronic neuroinflammation following murine cytomegalovirus (MCMV)‐induced encephalitis, we found that CD8⁺ T‐cells persisting within the brain expressed PD‐1. Conversely, activated microglia expressed PD‐L1. In vitro, primary murine microglia, which express low basal levels of PD‐L1, upregulated the co‐inhibitory ligand on IFN‐γ‐treatment. Blockade of the PD‐1: PD‐L1 pathway in microglial: CD8⁺ T‐cell co‐cultures increased T‐cell IFN‐γ and interleukin (IL)−2 production. We observed a similar phenomenon following blockade of this co‐inhibitory pathway in astrocyte: CD8⁺ T‐cell co‐cultures. Using ex vivo cultures of brain leukocytes, including microglia and CD8⁺ T‐cells, obtained from mice with MCMV‐induced chronic neuroinflammation, we found that neutralization of either PD‐1 or PD‐L1 increased IFN‐γ production from virus‐specific CD8⁺ T‐cells stimulated with MCMV IE1_168–176 peptide. These data demonstrate that microglia and astrocytes control antiviral T‐cell responses and suggest a therapeutic potential of PD1: PD‐L1 modulation to manage the deleterious consequences of uncontrolled neuroinflammation. GLIA 2014;62:1582–1594

Microglia and astrocytes exert regulatory control over T‐cells during chronic neuroinflammation following viral brain infection.

Post-encephalitic glial cells express PD‐L1 and suppress persistent CD8 T‐cells via the PD‐1: PD‐L1 inhibitory pathway.

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.

Cytomegalovirus contributes to glioblastoma in the context of tumor suppressor mutations

To study the controversial role of cytomegalovirus (CMV) in glioblastoma, we assessed the effects of murine CMV (MCMV) perinatal infection in a GFAP-cre; Nf1(loxP/+); Trp53(-/+) genetic mouse model of glioma (Mut3 mice). Early on after infection, MCMV antigen was predominantly localized in CD45+ lymphocytes in the brain with active viral replication and local areas of inflammation, but, by 7 weeks, there was a generalized loss of MCMV in brain, confirmed by bioluminescent imaging. MCMV-infected Mut3 mice exhibited a shorter survival time from their gliomas than control Mut3 mice perinatally infected with mock or with a different neurotropic virus. Animal survival was also significantly shortened when orthotopic gliomas were implanted in mice perinatally infected with MCMV versus controls. MCMV infection increased phosphorylated STAT3 (p-STAT3) levels in neural stem cells (NSC) harvested from Mut3 mice subventricular zone, and, in vivo, there was increased p-STAT3 in NSCs in MCMV-infected compared with control mice. Of relevance, human CMV (HCMV) also increased p-STAT3 and proliferation of patient-derived glioblastoma neurospheres, whereas a STAT3 inhibitor reversed this effect in vitro and in vivo. These findings thus associate CMV infection to a STAT3-dependent modulatory role in glioma formation/progression in the context of tumor suppressor mutations in mice and possibly in humans.

A comparison of different murine models for cytomegalovirus-induced sensorineural hearing loss

OBJECTIVES/HYPOTHESIS

To compare three different inoculation techniques for the development of cytomegalovirus (CMV)-induced sensorineural hearing loss (SNHL) in a mouse model.

STUDY DESIGN

A prospective experimental animal study.

METHODS

BALB/c mice underwent inoculation using green fluorescent protein-expressing mouse cytomegalovirus (mCMV-GFP) via transtympanic (TT), intraperitoneal (IP), or intracranial (IC) routes. Control mice received an equal volume of saline. Hearing thresholds were measured using both distortion product otoacoustic emissions (DPOAE) and evoked auditory brainstem response studies (ABR). Cochleas were harvested for histological examination and cytocochleogram.

RESULTS

No mice in the TT or IP groups showed significant hearing loss. All infected mice in the IC group showed significantly elevated ABR and DPOAE thresholds at 4 weeks of age. Ten mice (55%) had profound hearing loss (≥80 dB) at 4 weeks of age, while the other eight mice (45%) initially showed moderate hearing loss (≤20 dB), which progressed to profound hearing loss by 6 to 8 weeks. Asymmetric hearing loss was seen in 40% of the mice. Temporal bone histology showed diffuse loss of outer hair cells (OHC). Green fluorescent protein (GFP)-labeled virus was abundant in the spiral ganglion and adjacent to the scala tympani at the basal region of the cochlea at 7 days postinjection, and devoid of GFP labeling by 14 days postinfection.

CONCLUSIONS

Intracerebral injection of mCMV preferentially causes mCMV-mediated hearing loss relative to IP or TT injections. These results are consistent with the hearing loss reported in human congenital infection and may have implications for understanding the pathophysiology of CMV-mediated labyrinthitis.

Persistent humoral immune responses in the CNS limit recovery of reactivated murine cytomegalovirus

Background

Experimental infection of the mouse brain with murine CMV (MCMV) elicits neuroimmune responses that terminate acute infection while simultaneously preventing extensive bystander damage. Previous studies have determined that CD8⁺ T lymphocytes are required to restrict acute, productive MCMV infection within the central nervous system (CNS). In this study, we investigated the contribution of humoral immune responses in control of MCMV brain infection.

Methodology/Principal Findings

Utilizing our MCMV brain infection model, we investigated B-lymphocyte-lineage cells and assessed their role in controlling the recovery of reactivated virus from latently infected brain tissue. Brain infiltrating leukocytes were first phenotyped using markers indicative of B-lymphocytes and plasma cells. Results obtained during these studies showed a steady increase in the recruitment of B-lymphocyte-lineage cells into the brain throughout the time-course of viral infection. Further, MCMV-specific antibody secreting cells (ASC) were detected within the infiltrating leukocyte population using an ELISPOT assay. Immunohistochemical studies of brain sections revealed co-localization of CD138⁺ cells with either IgG or IgM. Additional immunohistochemical staining for MCMV early antigen 1 (E1, m112–113), a reported marker of viral latency in neurons, confirmed its expression in the brain during latent infection. Finally, using B-cell deficient (Jh^−/−) mice we demonstrated that B-lymphocytes control recovery of reactivated virus from latently-infected brain tissue. A significantly higher rate of reactivated virus was recovered from the brains of Jh^−/− mice when compared to Wt animals.

Conclusion

Taken together, these results demonstrate that MCMV infection triggers accumulation and persistence of B-lymphocyte-lineage cells within the brain, which produce antibodies and play a significant role in controlling reactivated virus.

Effects of congenital HCMV infection on synaptic plasticity in dentate gyrus (DG) of rat hippocampus

This study was carried out to investigate whether the congenital HCMV infection affect the induction and maintenance of LTP /DP. Rat models of Sprague-Dawley rats congenitally infected by HCMV were made. Field excitatory postsynaptic potentials (EPSPs) were recorded in the hippocampal slices of offspring rats (50-65days) to study alterations of LTP /DP in area dentate gyrus (DG) of the hippocampus after congenital infection. The Ca(2+) and mRNA level of calmodulin (CaM) in the hippocampus neurons of the experiment group (congenital infected by HCMV) and the control group were measured;The input/output (I/O) curves of the EPSP slope PS amplitude in area DG in experiment group were significantly depressed when compared to control group (P<0.05). LTP of the EPSP slope and PS amplitude in area DG of the hippocampus was 137±4% (EPSP) and 225±11% (PS) in control rats and 115±9% (EPSP) and 163±7% (PS) in experiment rats (EPSP: F=25.29,P<0.05;PS: F=74.33 P<0.05, two-way ANOVA with Tukey test); DP of the EPSP slope and PS amplitude was 86±3% (EPSP) and 85±2% (PS) in control rats and 94±5% (EPSP) and 93±4% (PS) in congenitally infected rats (EPSP: F=5.62, P<0.05;PS: F=4.22, P<0.05, two-way ANOVA with Tukey test) . At the same time, intracellular [Ca(2+)] and mRNA level of CaM in the hippocampus neurons of the experiment group were significantly increased than that of in the controls ([Ca(2+)]: P<0.01;CaM mRNA: P<0.01) . The results demonstrate that congenital HCMV infection could reduce the range of synaptic plasticity in the Sprague-Dawley rats, which may trigger the dysfunction of learning and memory through disrupting the calcium balance.

Transmission of murine cytomegalovirus in breast milk: a model of natural infection in neonates

Vertical transmission of viruses in breast milk can expose neonates to infectious pathogens at a time when the capacity of their immune system to control infections is limited. We developed a mouse model to study the outcomes of acquisition of murine cytomegalovirus (MCMV) when neonates are breastfed by mothers with acute or latent infection. Breast milk leukocytes collected from lactating mice were examined for the presence of MCMV IE-1 mRNA by reverse transcription-PCR (RT-PCR) with Southern analysis. As determined by this criterion, breast milk leukocytes from both acute and latent mothers were positive for MCMV. This mimics the outcome seen in humans with latent cytomegalovirus infection, where reactivation of virus occurs specifically in the lactating mammary gland. Interestingly, intraperitoneal injection of breast milk collected from mothers with latent infection was sufficient to transfer MCMV to neonatal mice, demonstrating that breast milk was a source of virus. Furthermore, we found that MCMV was transmitted from infected mothers to breastfed neonates, with MCMV IE-1 mRNA or infectious virus present in multiple organs, including the brain. In fact, 1 day of nursing was sufficient to transmit MCMV from latent mothers to breastfed neonatal mice. Together, these data validate this mouse model of vertical transmission of MCMV from mothers with acute or latent MCMV infection to breastfed neonates. Its relevance to human disease should prove useful in future studies designed to elucidate the immunological and pathological ramifications of neonatal infection acquired via this natural route.

Effects of cytomegalovirus infection on embryogenesis and brain development

Congenital cytomegalovirus (CMV) infection is a significant cause of brain disorders, such as microcephaly, mental retardation, hearing loss and visual disorders in humans. The type and severity of brain disorder may be dependent on the stage of embryonic development when the congenital infection occurs. Developmental disorders may be associated with the type of embryonic cells to which CMV is susceptible and the effects of the infection on the cellular functions of these cells. Early murine embryos, including embryonic stem (ES) cells, are not susceptible to CMV infection. A part of the embryonic cells acquire susceptibility during early development. Mesenchymal cells are the targets of infection at midgestation, affecting organogenesis of the brain, eyes and oral-facial regions. In contrast to ES cells, neural stem progenitor cells (NSPC) from fetal brains are susceptible to murine CMV (MCMV) infection. The viral infection inhibits proliferation and differentiation of the NSPC to neuronal and glial cells in addition to induction of neuronal cell loss. These cellular events may cause brain malformations, such as microcephaly and polymicrogyria. Furthermore, MCMV persists in neuronal cells in developing brains, presumably resulting in neuronal dysfunction.

Mouse embryonic stem cells are not susceptible to cytomegalovirus but acquire susceptibility during differentiation

BACKGROUND

Cytomegalovirus (CMV) is the most significant infectious cause of congenital anomalies of the central nervous system caused by intrauterine infection in humans. The timing of infection and the susceptibility of cells in early gestational stages are not well understood. In this study we investigated the susceptibility of embryonic stem (ES) cells to CMV infection during differentiation.

METHODS

ES cell lines were established from transgenic mice integrated with the murine CMV (MCMV) immediate-early (IE) promoter connected with a reporter lacZ gene. The susceptibility of the ES cells was analyzed in terms of viral gene expression and viral replication after induction of differentiation.

RESULTS

ES cells were nonpermissive to MCMV infection in the undifferentiated state. Upon differentiation, permissive cells appeared approximately 2 weeks after the leukemia inhibitory factor was removed. Upon neural differentiation by retinoic acid (RA), glial cells showed specific susceptibility in terms of expression of the viral antigen. The MCMV IE promoter was not activated in ES cells from the transgenic mice. Activation of the IE promoter was detected approximately 2 weeks after induction of differentiation and observed predominantly in glial cells. Upon MCMV infection of the ES cells, viral infection was correlated with the activation of the IE promoter.

CONCLUSIONS

ES cells are nonpermissive to MCMV infection and acquire permissiveness about 2 weeks after induction of differentiation, especially in glial cells. Acquisition of permissiveness in differentiated ES cells may be associated with activation of the IE promoter.

Neuropathogenesis of congenital cytomegalovirus infection: disease mechanisms and prospects for intervention

Congenital cytomegalovirus (CMV) infection is the leading infectious cause of mental retardation and hearing loss in the developed world. In recent years, there has been an improved understanding of the epidemiology, pathogenesis, and long-term disabilities associated with CMV infection. In this review, current concepts regarding the pathogenesis of neurological injury caused by CMV infections acquired by the developing fetus are summarized. The pathogenesis of CMV-induced disabilities is considered in the context of the epidemiology of CMV infection in pregnant women and newborn infants, and the clinical manifestations of brain injury are reviewed. The prospects for intervention, including antiviral therapies and vaccines, are summarized. Priorities for future research are suggested to improve the understanding of this common and disabling illness of infancy.

Brain trauma enhances transient cytomegalovirus invasion of the brain only in mice that are immunodeficient

Cytomegalovirus (CMV) is one of the most common viral pathogens leading to neurological dysfunction in individuals with depressed immune systems. How CMV enters the brain remains an open question. The hypothesis that brain injury may enhance the entrance of CMV into the brain was tested. Insertion of a sterile needle into the brain caused a dramatic increase in mouse CMV in the brains of immunodeficient SCID mice inoculated peripherally within an hour of injury and examined 1 week later; peripheral inoculation 48 h after injury and a 1-week survival resulted in only a modest infection at the site of injury. In contrast, uninjured SCID mice, as well as injured immunocompetent control mice, showed little sign of viral infection at the same time intervals. Direct inoculation of the brain resulted in widespread dispersal and enhanced replication of mCMV in SCID brains tested 1 week later but not in parallel control brains. Differential viremia was unlikely to account for the greater viral load in the SCID brain, since increased mCMV in the blood of SCID compared to controls was not detected until a longer interval. These data suggest that brain injury enhances CMV invasion of the brain, but only when the adaptive immune system is compromised, and that the brain's ability to resist viral infection recovers rapidly after injury.

Induction of cytomegalovirus-infected labyrinthitis in newborn mice by lipopolysaccharide: a model for hearing loss in congenital CMV infection

Congenital cytomegalovirus (CMV) infection is the most common infectious cause of sensorineural hearing loss in children. Here, we established an experimental model of hearing loss after systemic infection with murine CMV (MCMV) in newborn mice. Although almost no viral infection was observed in the inner ears and brains by intraperitoneal (i.p.) infection with MCMV in newborn mice, infection in these regions was induced in combination with intracerebral (i.c.) injection of bacterial lipopolysaccharide (LPS). The susceptibility of the inner ears was higher than that of the brains in terms of viral titer per unit weight. In the labyrinths, the viral infection was associated with the mesenchymal vessels and accompanied by inflammatory cells induced by LPS, causing hematogenous targets of infection in the labyrinths. Viral infection also spread in the perilymph regions such as the scala tympani and scala vestibuli, probably from infected brains via meningogenic and cochlear nerve routes. Viral infection was not observed in the scala media in the endolymph, including the Corti organ. However, viral infection was observed in the spiral limbus, including the stria vascularis. These results suggest that hearing loss caused by labyrinthitis after congenital CMV infection may be enhanced by inflammation caused by systemic bacterial infection in the neonatal period.

Roles of neural stem progenitor cells in cytomegalovirus infection of the brain in mouse models

Cytomegalovirus (CMV) is the most significant infectious cause of brain disorders in humans. Although the brain is the principal target organ for CMV infection in infants with congenital infection and in immunocompromised patients, little has been known about cellular events in pathogenesis of the brain disorders. Mouse models have been developed by the authors for studying the cell tropism, infectious dynamics of CMV infection and the effects of CMV infection on proliferation, regeneration and differentiation of neural cells. It has been shown, using brain slice cultures and neurospheres, that neural stem progenitor (NSP) cells are the most susceptible to CMV infection in developing brains. The NSP cells are also susceptible to CMV infection in adult and aged brains. The susceptibility can be enhanced by stimulation of neurogenesis. It was shown that latent murine CMV infection occurs in NSP cells by demonstrating the reactivation in brain slice culture or neurospheres. It is hypothesized that CMV brain disorder such as microcephaly is caused by disturbance of cellular events in the ventricular regions, including proliferation and differentiation of the neural stem cells, whereas neurons are also targets in persistent CMV infection, presumably resulting in functional disorders such as mental retardation.

Two novel NKG2D ligands of the mouse H60 family with differential expression patterns and binding affinities to NKG2D

H60, originally described as a dominant minor histocompatibility Ag, is an MHC class I-like molecule that serves as a ligand for the NKG2D receptor. In the present study, we identified two novel mouse chromosome 10-encoded NKG2D ligands structurally resembling H60. These ligands, which we named H60b and H60c, encode MHC class I-like molecules with two extracellular domains. Whereas H60b has a transmembrane region, H60c is a GPI-anchored protein. Recombinant soluble H60b and H60c proteins bound to NKG2D with affinities typical of cell-cell recognition receptors (K(d) = 310 nM for H60b and K(d) = 8.7 muM for H60c). Furthermore, expression of H60b or H60c rendered Ba/F3 cells susceptible to lysis by NK cells, thereby establishing H60b and H60c as functional ligands for NKG2D. H60b and H60c transcripts were detected only at low levels in tissues of healthy adult mice. Whereas H60b transcripts were detectable in various tissues, H60c transcripts were detected mainly in the skin. Infection of mouse embryonic fibroblasts with murine cytomegalovirus induced expression of H60b, but not H60c or the previously known H60 gene, indicating that transcriptional activation of the three types of H60 genes is differentially regulated. The present study adds two new members to the current list of NKG2D ligands.

Sequential polymicrobial infections lead to CNS inflammatory disease: possible involvement of bystander activation in heterologous immunity

VV(PLP) is a recombinant vaccinia virus (VV) encoding myelin proteolipid protein (PLP) that has been used to investigate molecular mimicry and autoimmunity. Since virus infections can cause bystander activation, mice were first infected with VV(PLP), and later challenged with wild-type VV, lymphocytic choriomeningitis virus (LCMV), or murine cytomegalovirus (MCMV). Among the VV(PLP)-primed mice, only MCMV challenge induced significant Ki-67(+), CD3(+)T cell infiltration into the central nervous system (CNS) with a mild PLP antibody response. While MCMV alone caused no CNS disease, control VV-infected mice followed with MCMV developed mild CNS inflammation. Thus, heterologous virus infections can induce CNS pathology.

Enhancement of susceptibility of adult mouse brain to cytomegalovirus infection by infusion of epidermal growth factor

Neural precursor cells, including neural stem and progenitor cells, in the subventricular zone (SVZ) are the main targets for cytomegalovirus (CMV) infection in developing brains. The neural precursor cells in the SVZ of the adult brain have been reported to respond by proliferating after infusion with epidermal growth factor (EGF). Here we report the susceptibility of the precursor cells in the adult mouse brain to murine CMV (MCMV) infection. Adult mouse brains from 10-, 25-, and 70-week-old (W) mice were infused with either phosphate-buffered saline or EGF into the brain for 3 days, and then intracerebrally infected with MCMV for 5 days. The susceptibility of the adult brains to MCMV was significantly increased by infusion of EGF in terms of viral titers and viral antigen-positive cells. The susceptibility of the young adult brain from 10-week-old mice to MCMV was higher than that of the adult brains from 25-week-old or 70-week-old mice. Both the ependymal and the SVZ cells were susceptible to MCMV infection. The number of virus-infected cells in the SVZ was significantly increased by infusion of EGF, whereas the number of infected ependymal cells was not significantly increased. Among the virus-infected cells in the SVZ, 73% were positive for nestin, 87% were positive for Musashi, 86% were positive for GFAP, and 96% were positive for PCNA. These results indicate that the susceptibility of the adult brain to MCMV is correlated with the proliferative ability of the neural precursor cells in the SVZ of the adult brain.

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.

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/).]

Differential expression of the immediate-early 2 and 3 proteins in developing mouse brains infected with murine cytomegalovirus

Murine cytomegalovirus (MCMV) immediate-early (IE) 2 protein has been reported to be dispensable for growth and latency in mice. Therefore, its role in viral pathogenesis and tissue tropism is not known. Here we prepared specific antibodies to the IE2 and IE3 proteins by using fusion proteins expressed in Escherichia coli as antigens. Immunostaining of MCMV-infected cultured fibroblasts revealed IE2 protein to be expressed diffusely in the nucleoplasm similar to the IE1 protein. In contrast, expression of the IE3 protein, 88 kDa, exhibited a punctate pattern in the nucleus in the early phase of infection then diminished. In the brain of neonatal mice infected with MCMV, both IE2 and IE3 proteins were detected immunohistochemically in the cells of the ventricular walls early in infection. When the infection was prolonged, the IE2 protein was expressed in neurons of the cortex and hippocampus, while the IE3 protein was preferentially expressed in glial cells in the early phase of infection, and its levels declined during the infection. These results suggest that the IE2 protein may play a role in persistent infection in neurons, whereas the IE3 protein, expressed preferentially in glial cells, may play the main role in acute infection.

Cytomegalovirus infection inhibits the expression of N-methyl-D-aspartate receptors in the developing mouse hippocampus and primary neuronal cultures

Cytomegalovirus (CMV) is the most significant infectious cause of developmental brain disorders in humans. The infection occasionally persists and causes neurological disorders. The N-methyl-D-aspartate (NMDA) subtype of glutamate receptors is essential for the development and plasticity of synapses, but also is involved in neuronal excitotoxicity during viral infection. Here we investigated the effects of murine CMV (MCMV) infection on the expression of NMDA receptors in the hippocampal neurons of neonatal mice and primary neuronal cultures. Viral antigen was mostly found in hippocampal pyramidal neurons from the CA1 to CA3. Image analysis of immunohistochemistry demonstrated that the expression of NMDA receptor subunit 1 (NMDA-R1) protein in CA1 neurons of MCMV-infected brain was reduced to 40% of that in uninfected brain. The signal of in situ hybridization for NMDA-R1 mRNA was also decreased in CA1 neurons of MCMV-infected brain. In primary neuronal cultures, reduction of NMDA-R1 expression in MCMV-infected neurons was also detected by immunocytochemistry and Western blotting. These results suggest that reduction of NMDA receptor expression by MCMV infection may cause a decrease in the susceptibility of the neurons to excitotoxic cell death, and may be related to the establishment of viral persistence and functional disturbances in MCMV-infected neurons.

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.

Systemic immune deficiency necessary for cytomegalovirus invasion of the mature brain

Cytomegalovirus (CMV) is a significant opportunistic pathogen associated with AIDS and immunosuppressive therapy. Infection of the mature central nervous system (CNS) can cause significant pathology with associated neurological deficits, mental disorders, and cognitive impairment and may have potentially fatal consequences. Using genetically immunocompromised mice, we studied mechanisms of CMV invasion into, and behavior within, the CNS. Adult immunodeficient (nude and SCID) and control mice were peripherally infected with recombinant mouse CMV expressing a green fluorescent protein reporter gene. Control mice actively eliminated acute peripheral infection and were resistant to invasion of CMV into the brain. In contrast, virus infected brains of immunodeficient mice but only after a minimum of 21 days postinoculation. After inoculation, CMV was found in circulating leukocytes (MAC-3/CD45(+)) and in leukocytes within the brain, suggesting these cells as a possible source of CMV entry into the CNS. CNS infection was observed in many different cell types, including neurons, glial cells, meninges, ependymal cells, and cells of cerebral vessels. Infection foci progressively expanded locally to adjacent cells, resulting in meningitis, choroiditis, encephalitis, vasculitis, and necrosis; clear indication of axonal transport of CMV was not found. Regional distribution of CMV was unique in each brain, consisting of randomly distributed, unilateral foci. Testing whether CMV gained access to brain through nonspecific vascular disruption, vascular injections of a tracer molecule revealed no obvious disruption of the blood brain barrier in mice with CMV in the brain. Results indicate the importance of host adaptive immunity (particularly T cells) in controlling entry and dissemination of CMV into the brain and are consistent with the view that virus may be carried into the brain by circulating mononuclear cells that traffic through the blood brain barrier.

Neuron-specific activation of murine cytomegalovirus early gene e1 promoter in transgenic mice

The brain is the main target in congenital cytomegalovirus (CMV) infection and immunocompromised patients. No definite evidence that a CMV has special affinity for the central nervous system (CNS) has been published. Here, we generated transgenic mice with an e1 promoter/enhancer region connected to the reporter gene lacZ. Surprisingly, expression of the transgene was completely restricted to the CNS in all lines of transgenic mice. The transgene was expressed in subpopulation of neurons in the cerebral cortex, hippocampus, diencephalon, brainstem, cerebellum, and spinal cord in all of the lines. Non-neuronal cells in the CNS were negative for transgene expression. Activation of the transgene was first observed in neurons of mesencephalon in late gestation, and then the number of positive neurons increased in various parts of the brain as development proceeded. Upon infection of the transgenic mouse brains with MCMV, the location of the activated neurons became more extensive, and the number of such neurons increased. These results suggest that there are host factor(s) that directly activate the MCMV early gene promoter in neurons. This neuron-specific activation may be associated with persistent infection in the brain and may be responsible for the neuronal dysfunction and neuronal cell loss caused by CMV infection.

The amount of immature glial cells in organotypic brain slices determines the susceptibility to murine cytomegalovirus infection

Cytomegalovirus (CMV) is the most common infectious cause of congenital anomalies of the brain and also causes brain damage in immunocompromised individuals. We investigated the effects of murine cytomegalovirus (MCMV) infection on the developing mouse brain in terms of susceptible cells and age-related resistance to MCMV in brain slice cultures. Brain slices from BALB/c mice at different developmental stages were infected with recombinant MCMV in which the lacZ gene was inserted into a late gene. The subventricular zone and cortical marginal region were the sites most susceptible to MCMV infection, and the susceptibility declined with the development of the brain. Immunohistochemical staining showed that the virus-susceptible cells were positive for GFAP, nestin, and Musashi-1, and that most of the infected cells were positive for the proliferative cell nuclear antigen and labeled with bromodeoxyuridine. These results suggest that the susceptible cells in the subventricular zone are immature glial cells, including neural progenitor cells. Immature glial cells proliferated when the brain slices were cultured for a prolonged time and furthermore, they showed themselves to be susceptible to virus infection even under serum-free conditions. These results suggest that the amount of immature glial cells, which include neural progenitor cells, in the developing brain or in the damaged brain with neural proliferation may be closely associated with the susceptibility of the brain to CMV infection in humans.

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.

Innate immune responses to cytomegalovirus infection in the developing mouse brain and their evasion by virus-infected neurons

Cytomegalovirus (CMV) is the most frequent infectious cause of developmental brain disorders in humans. Here we show the role of innate immune responses caused by natural killer (NK) cells and nitric oxide (NO) derived from brain macrophages during murine CMV (MCMV) infection of the developing brain. Viral replication in the brain of newborn mice was significantly enhanced by administration of anti-asialo-GM1 antibody, specific for NK cells, or L-N6-(1-imminoethyl)-lysine, a specific inhibitor of NO synthase 2 (NOS2). These results suggest that NK cells and NO contribute to the viral clearance from the brain. At 3 days postinfection (dpi) MCMV early antigen (Ag)-positive cells were immunohistochemically detected in the periventricular area, where most of the positive cells were macrophages. At 7 dpi MCMV-Ag was found not only in cells of the periventricular area but also in neurons of the hippocampus and cortex. At 11 dpi MCMV-Ag disappeared from the periventricular area, but persisted in neurons. In the periventricular area, NK cells and NOS2-positive macrophages were associated with MCMV-Ag-positive cells. In contrast, there were very few NK cells and NOS2-positive macrophages around the MCMV-Ag-positive neurons. In situ hybridization for MCMV DNA demonstrated that positive signals were found mostly in the periventricular cells, and rarely in neurons. These results suggest that the innate immune responses are restricted to the virus-replicating cells, and do not affect MCMV-infected neurons. Therefore, evasion of the innate immune responses by MCMV-infected neurons may be an important factor in supporting the viral persistence in the developing brain.

Reactivation of latent cytomegalovirus infection in mouse brain cells detected after transfer to brain slice cultures

Cytomegalovirus (CMV) is the most significant infectious cause of brain disorders in humans involving the developing brain. It is hypothesized that the brain disorders occur after recurrent reactivation of the latent infection in some kinds of cells in the brains. In order to test this hypothesis, we examined the reactivation of latent murine CMV (MCMV) infection in the mouse brain by transfer to brain slice culture. We infected neonatal and young adult mice intracerebrally with recombinant MCMV in which the lacZ gene was inserted into a late gene. The brains were removed 6 months after infection and used to prepare brain slices that were then cultured for up to 4 weeks. Reactivation of latent infection in the brains was detected by beta-galactosidase (beta-Gal) staining to assess beta-galactosidase expression. Viral replication was also confirmed by the plaque assay. Reactivation was observed in about 75% of the mice infected during the neonatal period 6 months after infection. Unexpectedly, reactivation was also observed in 75% of mice infected as young adults, although the infection ratio in the brain slices was significantly lower than that in neonatally infected mice. Beta-Gal-positive cells were observed in marginal regions of the brains or immature neural cells in the ventricular walls. Immunohistochemical staining showed that the beta-Gal-positive reactivated cells were neural stem or progenitor cells. These results suggest that brain disorders may occur long after infection by reactivation of latent infection in the immature neural cells in the brain.

Activation of murine cytomegalovirus immediate-early promoter in cerebral ventricular zone and glial progenitor cells in transgenic mice

Cytomegalovirus (CMV) is the most common infectious cause of congenital anomalies of the CNS in humans. We recently reported that the murine cytomegalovirus (MCMV) immediate-early (IE) gene promoter directs astrocyte-specific expression in adult transgenic mice. In the present study, we analyzed the activation of the MCMV IE promoter in developing transgenic mouse brains and compared the activation with that of the Musashi 1 (Msi1) gene, which is expressed in neural progenitor cells, including neural stem cells. During the early phase of neurogenesis, the transgene was expressed predominantly in endothelial cells of the vessels, but not in neuroepithelial cells in which Msi1 was expressed. During later stages of gestation, expression of the transgene was largely restricted to the ventricular zone (VZ) in the CNS, similar to the expression of Msi1. In neurosphere cultures from transgenic embryos in the late phase of neurogenesis, the transgene was expressed in some cells of neurospheres expressing Msi1 and nestin. In neural precursor cells induced to differentiate from stem cells, expression of the transgene was detected in glial progenitor cells, expressing GFAP, nestin, and Msi1, but not in cells expressing MAP2 or MAG. In postnatal development, persistent expression of the transgene was observed in astrocyte lineage cells as was Msi1. These spatiotemporal changes of the MCMV IE promoter activity during development of transgenic mice correlated with susceptible sites in congenital HCMV infection. Moreover, this transgenic mouse model may provide useful model for analysis of the regulation of the switching of neuronal and astrocyte differentiation, and the maintenance of the astrocyte lineage.

Growth retardation and microcephaly induced in mice by placental infection with murine cytomegalovirus

BACKGROUND

The placenta is regarded as a site of congenital cytomegalovirus (CMV) infection. The placental infection of fetuses with murine CMV (MCMV) was investigated in a mouse model.

METHODS

The placentas and fetuses were examined using the polymerase chain reaction (PCR) and Southern blotting for viral DNA and immunostaining for viral antigen. Since the transplacental infection rarely occurs, the placentas were directly injected with MCMV at day 12.5 of gestation; the embryos were then allowed to develop until day 18.5 of gestation.

RESULTS

Formation of infected foci at day 18. 5 of gestation was found in more than 60% of the injected placentas. Infection of about 50% of the embryos occurred from the infected placentas. The frequency of infection in the brain was 27%, which was the same as that in the liver and higher than that in the lungs. In the brains, infected cells were often observed in the ventricular zone of the cerebrum and sometimes in the cortical plate and the hippocampus. Developmental retardation with microcephaly was observed in about 25% of offspring exposed to infection in utero.

CONCLUSIONS

These results suggest that formation of infected foci in the placenta is important for embryonic congenital infection, and that the cerebral ventricular zone is one of the most susceptible sites for CMV infection in the embryonic stage.

In vitro activities of methylenecyclopropane analogues of nucleosides and their phosphoralaninate prodrugs against cytomegalovirus and other herpesvirus infections

Human cytomegalovirus (HCMV) infection does not generally cause problems in the immunocompetent adult but can result in severe clinical disease in the fetus, neonate, and immunocompromised host. Ganciclovir (GCV), the agent currently used to treat most HCMV infections, has resulted in much therapeutic success; however, efficacy remains suboptimal. Therefore, there is still a need to develop new compounds for use against HCMV infections. In the present study, several Z- and E-series methylenecyclopropane analogues and their phosphoroalaninate prodrugs were tested initially for activity against HCMV, strain AD169, and murine cytomegalovirus (MCMV) in vitro. Many were found to exhibit efficacy comparable to that of GCV against HCMV in plaque assays and were active against MCMV as well. The compounds were also tested for efficacy against herpes simplex virus types 1 and 2, varicella-zoster virus, and Epstein-Barr virus, and some had levels of activity that were comparable to that of acyclovir. In addition, the compounds synguanol (QYL-438) and 2-amino-6-cyclopropylamino analogue (QYL-769) were chosen for further evaluation and were found to be effective against additional laboratory and clinical isolates of HCMV and GCV-resistant isolates. QYL-438 and QYL-769 were found to be nontoxic in human and mouse fibroblasts and were considerably less toxic than GCV in granulocyte macrophage CFUs and erythroid burst-forming units. These results provide evidence for the high activity of some of these methylenecyclopropane analogues against various herpesviruses, particularly HCMV, in tissue culture and suggest that further evaluation is warranted to determine their potential for use in future clinical studies.

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.

Effective treatment of murine cytomegalovirus infections with methylenecyclopropane analogues of nucleosides

A number of new nucleoside analogues with a Z- or E-methylenecyclopropane structure exhibited significant activity against human and murine cytomegaloviruses (HCMV, MCMV) in tissue culture that was generally comparable to, or greater than, 9-[(1-3-dihydroxy-2-propoxy)methyl]guanine (ganciclovir, GCV). Several of these analogues were chosen for further evaluation of therapeutic efficacy utilizing a MCMV infection. Intraperitoneal (i.p.) inoculation of 3-week-old Balb/c mice with 2.0 x 10(5) plaque forming units (pfu) of MCMV results in an acute, lethal infection with rapid virus replication in visceral and glandular tissue, thus, making it an ideal model for identifying compounds that have potential for use in humans. Synadenol (QYL-284A) and synguanol (QYL-438) were administered i.p. once daily for 5 days initiated 6, 24, or 48 h post-viral infection. Significant protection was demonstrated at 50 and 16.7 mg/kg compared to placebo, with efficacy comparable to GCV. When delivered orally once or twice daily at 100 mg/kg per day, QYL-438 was active, but less effective than GCV. In addition, 2-amino-6-methoxypurine analogue (QYL-941) was active at 60 mg/kg administered orally twice daily, comparable to GCV, while it's prodrug (QYL-972) was as effective as GCV at 40 mg/kg when delivered twice daily for 5 days. Additionally, analogue 2-amino-6-cyclopropylaminopurine (QYL-769) was found to be highly efficacious when given orally twice daily for 5 days. Mortality of 0% and 13% was observed at 60 and 20 mg/kg, respectively, which was similar to GCV. Oral treatment with QYL-769 or GCV reduced virus replication in target organs, but neither resulted in complete clearance of MCMV. These data indicate that these new analogues have activity comparable to GCV when given orally to mice and should be evaluated further to assess their potential for use in humans.

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.

Natural pathogens of laboratory mice, rats, and rabbits and their effects on research

Laboratory mice, rats, and rabbits may harbor a variety of viral, bacterial, parasitic, and fungal agents. Frequently, these organisms cause no overt signs of disease. However, many of the natural pathogens of these laboratory animals may alter host physiology, rendering the host unsuitable for many experimental uses. While the number and prevalence of these pathogens have declined considerably, many still turn up in laboratory animals and represent unwanted variables in research. Investigators using mice, rats, and rabbits in biomedical experimentation should be aware of the profound effects that many of these agents can have on research.