Oral Treponema denticola Infection Induces Aß1-40 and Aß1-42 Accumulation in the Hippocampus of C57BL/6 Mice.

Accumulation of amyloid-ß (Aß) in the brain is a central component of pathology in Alzheimer's disease. A growing volume of evidence demonstrates close associations between periodontal pathogens including Porphyromonas gingivalis (P. gingivalis) and Treponema denticola (T. denticola) and AD. However, the effect and mechanisms of T. denticola on accumulation of Aß remain to be unclear. In this study, we demonstrated that T. denticola was able to enter the brain and act directly on nerve cells resulting in intra- and extracellular Aß1-40 and Aß1-42 accumulation in the hippocampus of C57BL/6 mice by selectively activating both ß-secretase and γ-secretase. Furthermore, both KMI1303, an inhibitor of ß-secretase, as well as DAPT, an inhibitor of γ- secretase, were found to be able to inhibit the effect of T. denticola on Aß accumulation in N2a neuronal cells. Overall, it is concluded that T. denticola increases the expression of Aß1-42 and Aß1-40 by its regulation on beta-site amyloid precursor protein cleaving enzyme-1 and presenilin 1.

Host strain-dependent difference in susceptibility in a rat model of herpes simplex type 1 encephalitis.

Herpes simplex encephalitis (HSE) is characterized by severe focal brain inflammation leading to substantial loss of nervous tissue. The authors established a model of Herpes simplex virus type 1 (HSV)-1-induced acute encephalitis in the rat by injecting into the whiskers' area a virus strain isolated from a fatal human HSE case. The model might resemble natural propagation of HSV-1 in humans; spreading from the mouth and lips via the trigeminal nerve to trigeminal ganglia and subsequently entering the central nervous system (CNS). HSV-1 infected Dark Agouti (DA) rats developed a well-synchronized disease and died 5 days after inoculation. HSV-1 detection by quantitative polymerase chain reaction (qPCR), virus isolation and immunohistochemistry, magnetic resonance imaging, and histopathological examination verified dramatic encephalitis mainly in the brainstem, but also in the olfactory bulb and other segments of the brain of diseased rats. In contrast, Piebald Virol Glaxo (PVG) rats were completely resistant to disease, displaying a more rapid clearance of peripheral infection and no evidence of virus entering into neither the trigeminal ganglia nor the CNS. These results suggest a regulation of susceptibility to HSV-1-induced encephalitis at the level of peripheral infection and subsequent neuronal uptake/transport of the virus. This provides a basis for future positioning of genetic polymorphisms regulating HSE and for dissection of important pathogenetic mechanisms of this severe human disease.

Pneumococcal carriage results in ganglioside-mediated olfactory tissue infection.

Streptococcus pneumoniae cause considerable morbidity and mortality, with persistent neurological sequelae, particularly in young children and the elderly. It is widely assumed that carriage occurs through direct mucosal colonization from the environment whereas meningitis results from invasion from the blood. However, the results of published studies can be interpreted that pneumococci may enter the brain directly from the nasal cavity by axonal transport through olfactory nerves. This hypothesis is based on findings that (i) teichoic acid of the pneumococcal cell wall interact with gangliosides (GLS), (ii) the interaction of GLS with cholera toxin leads to axonal transport through the olfactory nerves into the brain, and (iii) viruses enter the brain through axonal transport into olfactory nerves. After nasal inoculation, we observe high numbers of pneumococci in nasal washes and the olfactory nerves and epithelium. Significant numbers of pneumococci also infected the olfactory bulbs, brain, and the trigeminal ganglia. The absence of bacteremia in this model makes it unlikely that the bacteria entered the brain from the blood stream. Recovery of colony-forming units from the brain, lungs, olfactory nerves, and epithelium and nasal washes was inhibited by incubating pneumococci with GLS before nasal inoculation. These findings, confirmed by PCR and immunohistochemistry, support a GLS-mediated process of infection and are consistent with pneumococci reaching the brain through retrograde axonal transport.

Colony-stimulating factor 1-dependent cells protect against systemic infection with Listeria monocytogenes but facilitate neuroinvasion.

By using mice genomically lacking the mononuclear phagocytic growth factor colony-stimulating factor 1 and thereby deficient in macrophage and dendritic cell populations, we show that these cells play a dual role: they constitute a major defense against systemic infection but also facilitate cerebral bacterial invasion by Listeria monocytogenes.

Molecular and immunological evidence of oral Treponema in the human brain and their association with Alzheimer's disease.

The purpose of this investigation was to use molecular and immunological techniques to determine whether oral Treponema infected the human brain. Pieces of frontal lobe cortex from 34 subjects were analyzed with species-specific PCR and monoclonal antibodies. PCR detected Treponema in 14/16 Alzheimer's disease (AD) and 4/18 non-AD donors (P < 0.001), and AD specimens had more Treponema species than controls (P < 0.001). PCR also detected Treponema in trigeminal ganglia from three AD and two control donors. Cortex from 15/16 AD subjects and 6/18 controls contained Treponema pectinovorum and/or Treponema socranskii species-specific antigens (P < 0.01). T. pectinovorum and/or T. socranskii antigens were also found in trigeminal ganglia and pons from four embalmed cadavers, and 2/4 cadavers also had Treponema in the hippocampus. These findings suggest that oral Treponema may infect the brain via branches of the trigeminal nerve.

Neural route of cerebral Listeria monocytogenes murine infection: role of immune response mechanisms in controlling bacterial neuroinvasion.

The pathologic features of cerebral Listeria monocytogenes infection strongly suggest that besides hematogenous spread, bacteria might also spread via a neural route. We propose that after snout infection of recombination activating gene 1 (RAG-1)-deficient mice, L. monocytogenes spreads to the brain via a neural route. The neural route of invasion is suggested by (i) the immunostaining of L. monocytogenes in the trigeminal ganglia (TG) and brain stem but not in other areas of the brain; (ii) the kinetics of bacterial loads in snout, TG, and brain; and (iii) the increased resistance of mice infected with a plcB bacterial mutant (unable to spread from cell to cell). Gamma interferon (IFN-gamma) plays a protective role in neuroinvasion; inducible nitric oxide synthase (iNOS) accounts only partially for the protection, as shown by a comparison of the susceptibilities of IFN-gamma receptor (IFN-gamma R)-deficient, iNOS-deficient, and wild-type mice to snout infection with L. monocytogenes. The dramatically enhanced susceptibility of RAG-1-deficient, IFN-gamma R gene-deficient mice indicated the overall importance of innate immune cells in the release of protective levels of IFN-gamma. The source of IFN-gamma appeared to be NK cells, as shown by use of RAG-1-deficient, gamma-chain receptor gene-deficient mice; NK cells played a relevant protective role in neuroinvasion through a perforin-independent mechanism. In vitro evidence indicated that IFN-gamma can directly induce bacteriostatic mechanisms in neural tissue.

Expression of protein encoded by varicella-zoster virus open reading frame 63 in latently infected human ganglionic neurons.

The ganglionic cell type in which varicella-zoster virus (VZV) is latent in humans was analyzed by using antibodies raised against in vitro-expressed VZV open reading frame 63 protein. VZV open reading frame 63 protein was detected exclusively in the cytoplasm of neurons of latently infected human trigeminal and thoracic ganglia. This is, to our knowledge, the first identification of a herpesvirus protein expressed during latency in the human nervous system.

The role of herpes simplex thymidine kinase expression in neurovirulence and latency in newborn vs. adult mice.

Infection by standard thymidine kinase-positive (TK+) and TK- mutant herpes simplex virus (HSV) was performed in order to evaluate the role of HSV TK expression in neurovirulence and in HSV latency. In newborn mice, mortality and trigeminal ganglion (TG) HSV titer correlated (both were high) for TK+ and TK- HSV. In adult mice after TK- HSV infection they also correlated (both were low). After TK+ infection of adult mice, correlation was not present; mortality was low while HSV titer was moderately high. During the period of HSV latent infection (> 28 days after HSV infection), the number of neurons expressing HSV latency-associated transcript (LAT) was much greater for TK- HSV newborn-inoculated mice (average of 943/ganglion) than adult-inoculated mice (average of 138/ganglion). In addition, total amount of TG LAT was greater in the former than the latter. Reactivation from latency was restricted, however, for both groups. This result supported the important role of HSV TK expression in HSV reactivation, even when the number of LAT-positive neurons was greatly increased. The following conclusions were drawn from the study of TK- HSV in newborn mice: (i) HSV TK expression was of limited importance for neurovirulence and in vivo HSV TG infection (but was of importance in adult mice); (ii) increased in vivo HSV TG infection correlated with increased number of LAT-positive neurons, so that HSV replication and establishment of latency were not completely separable; and (iii) even with greatly increased numbers of latently infected neurons, HSV TK expression was important for reactivation from latency. Results in newborn mice suggested that the role of HSV TK expression in reactivation from latency and in neurovirulence were separable. To further investigate HSV replication in newborn and adult mice, ganglia were infected with HSV in vitro and either maintained in vitro or transplanted beneath the renal capsule of adult recipients. In both of these studies, HSV titers in ganglia were much higher in newborn than adult ganglia. This suggested that in addition to the well-know role of the immune system in HSV neurovirulence in newborn mice, it is likely that HSV replication per se in neural tissue is greater in newborn than adult mice. This may be related to the high level of HSV neurovirulence in newborn mice.

Quantification of transcripts from the ICP4 and thymidine kinase genes in mouse ganglia latently infected with herpes simplex virus.

Herpes simplex virus establishes latency in nervous tissue in which it is maintained for the life of the mammalian host, with occasional reactivation leading to subsequent spread. Latency-associated transcripts are abundant during latency, but viral proteins and productive cycle RNAs have not been detected. Using sensitive, quantitative PCR assays, we have quantified certain viral RNAs specific to productive-cycle genes in mouse ganglia latently infected with herpes simplex virus type 1. Sense-strand RNA specific to the essential immediate-early gene, ICP4, was present in most ganglia in variable amounts relative to the amount of viral DNA, with one to seven molecules of RNA per viral genome in about 20% of ganglia. In contrast, the amount of latency-associated transcripts was much less variable, at an average of 4 x 10(4) molecules per viral genome. The amounts of ICP4-specific RNA were similar at 30 and 60 days postinfection, and at least some of these transcripts initiated within a region consistent with utilization of the ICP4 promoter. RNA specific to the thymidine kinase gene, whose transcription in productive infection is dependent on ICP4, was present in latently infected ganglia at a maximum level of 3.2 x 10(6) molecules per ganglion (500 molecules per viral genome). ICP4-specific and tk-specific RNAs measured from the same samples showed a positive correlation extending over 2 orders of magnitude. We conclude that ICP4-specific RNA is expressed in the absence of detectable reactivation and discuss possible implications of our findings for latent gene expression.

Correlation of virus replication, cytokine (TNF-alpha and IL-1) producing cells, neuronal necrosis and inflammation after intranasal infection of mice with herpes simplex virus strains of different virulence.

The number of TNF-alpha and IL-1 beta producing cells was investigated during the acute replication phase of herpes simplex virus (HSV) in trigeminal ganglia after intranasal infection with strains of different virulence. The highly virulent strain WAL replicated strongly and induced many cytokine producing cells early in the ganglia. The low virulent strain HFEM replicated less, only few cytokine producing cells were detected late. The thymidine-kinase negative (TK-) virus 1301 did not replicate but produced some lymphocytic inflammation. The higher the virulence of strains of HSV-1 or -2 was, the stronger was the extent of histopathological lesions; moreover, a dissociation in time between replication and cellular reaction (granulocytic and lymphocytic) could be observed after infection with strains HFEM and TK- virus 1301. CD4 and CD8 positive cells could be detected mainly at the rim of necrotic areas, TNF-alpha and IL-1 beta producing cells, however, were scattered throughout the ganglia.

Detection of latent varicella zoster virus DNA and human gene sequences in human trigeminal ganglia by in situ amplification combined with in situ hybridization.

Varicella zoster virus (VZV) establishes latency in sensory ganglia following primary infection (chickenpox) and may reactivate decades later to produce zoster (shingles). The presence of VZV DNA in latently infected ganglia has been demonstrated by Southern blot hybridization as well as by polymerase chain reaction of DNA extracted from latently infected ganglia. Conflicting results have been obtained by in situ hybridization studies to determine the cell type in the ganglia harboring the latent VZV. To address this controversy we have utilized a more sensitive method than the previous studies. We have applied the technique of polymerase chain reaction to sections of ganglia from latently infected individuals and combined this with in situ hybridization to detect the amplified product. Primers specific for VZV were used to amplify VZV DNA in latently infected human trigeminal ganglia and demonstrated the presence of VZV DNA in neurons only. Sections from human kidney and ganglia from neonates as well as monkey ganglia served as controls and did not show amplification of VZV sequences. Amplification using primers for human genes, alpha tubulin and the oncogene Bcl-2, demonstrated the presence of these sequences in nearly all cells in the human tissues while only weak signals were seen in the monkey tissue. This is the first report where in situ amplification has been utilized to detect latent VZV in human ganglia.

The trigeminal ganglion is a location for equine herpesvirus 1 latency and reactivation in the horse.

Four specific pathogen-free ponies were infected intranasally with equine herpesvirus 1 (EHV-1) and two were similarly infected with an EHV-1 thymidine kinase deletion mutant. The primary infections were characterized by a transient fever accompanied by virus shedding into nasal mucus and viraemia. No virus was detected in clinical specimens after 15 days post-infection. Two months later a reactivation stimulus was administered to all six ponies and only the four that had been previously inoculated with wild-type EHV-1 shed virus into nasal mucus (for 10 days), proving the presence of a latent infection. No recurrence of viraemia was observed. The animals were monitored for a further 6 weeks and were consistently shown to be free from infectious virus. Tissues were then obtained postmortem. Co-cultivation of explanted trigeminal ganglia from two out of the four ponies that carried the wild-type virus yielded cultures positive for infectious virus. Apart from nasal epithelium, no infectious virus was recovered from any other tissue. PCR confirmed the presence of virus DNA in the ganglia from all six ponies. Lymphoid tissues also yielded positive signals using this technique. The relevance of virus detection by PCR in lymphoid and neural tissues is discussed in relation to the potential for reactivation of latent virus in the host. However, evidence is presented to show that EHV-1 is neurotropic and, in common with other members of the alpha-herpesvirus subfamily, establishes latency in sensory ganglia from which virus can be reactivated.

N-linked oligosaccharides on herpes simplex virus glycoprotein gD are not essential for establishment of viral latency or reactivation in the mouse eye model.

Glycoprotein D (gD) is an essential component of the herpes simplex virus (HSV) envelope. It is essential for viral penetration and for cell to cell spread of virus in vitro, and is also important for neuroinvasiveness. We investigated the contribution of N-linked oligosaccharides (N-CHO) on gD to viral pathogenesis. We used F-gD(QAA), a mutant virus derived from strain F of HSV-1. This virus contains three mutations in the gD gene which eliminate all signals for addition of N-CHO. These mutations affect the antigenic structure of gD and also lead to a small plaque phenotype. Otherwise the virus appears normal in in vitro assays. We used the mouse eye model of HSV latency to examine whether the mutations alter the phenotype of the virus in vivo. At 4 days postinfection similar amounts of F-gD(QAA) and F-gD(WT), its wild-type parent, were found in either eyes or trigeminal ganglia (TG) of infected mice. Moreover, both mutant and wild-type viruses exhibited the same ability to establish, maintain, and be reactivated from latency. We conclude that N-CHO on gD are not essential for HSV-1 pathogenesis in this model.

The role of nerve growth factor in modulating herpes simplex virus reactivation in vivo.

The role of nerve growth factor (NGF) in the modulation of herpes simplex virus (HSV) latency and reactivation was investigated in a mouse model. To determine whether NGF depletion would reactivate latent virus, concentrated anti-NGF serum antibodies were administered intraperitoneally to latently infected mice for 9 consecutive days. To determine whether NGF given prophylactically could suppress UV-B-induced viral reactivation, mice were irradiated with UV-B while being treated with NGF using diverse regimes over a 4-day period. Following intraperitoneal administration of anti-NGF antibodies, viral shedding was detected in a small number (10%) of mice, but it was not possible to pharmacologically suppress UV-B-induced viral reactivation with NGF. It would appear, therefore, that HSV latency in neurons innervating the cornea can be sustained and disrupted by physiological factors independent of NGF levels.

Herpes simplex virus type 1 gene expression and reactivation of latent infection in the central nervous system.

Restricted gene expression takes place during latent infection of herpes simplex virus type 1 (HSV-1) in the human peripheral nervous system and has been linked with viral reactivation. The state of HSV-1 gene expression in the central nervous system (CNS) during latency is unclear and we, therefore, examined gene expression in the brainstem of experimental mice and normal humans. Only part of the transcription pattern present during latent infection in peripheral sensory ganglia (PSG) was identified in the human brainstem by in situ hybridization and Northern blot analysis for HSV-1-specific transcripts. Instead of three HSV-1 latency-associated transcripts (LATs) present in PSG and demonstrated by Northern blot analysis, only one was identified in mouse brainstem and none was detected in human brainstem. These findings might be attributed to the relatively low amounts of HSV-1-specific latency-associated RNAs in brainstem tissue. Combined with our inability to reactivate HSV-1 from explanted mouse brainstem, these findings suggest that tissue levels of latency-associated gene expression play a role in HSV-1 reactivation and have relevance to the very low incidence of HSV-1-induced CNS disease compared with peripheral mucocutaneous disease.

A net +1 frameshift permits synthesis of thymidine kinase from a drug-resistant herpes simplex virus mutant.

Clinical resistance to antiviral drugs requires that a virus evade drug therapy yet retain pathogenicity. Thymidine kinase (TK)-negative mutants of herpes simplex virus are resistant to the drug, acyclovir, but are attenuated for pathogenicity in animal models. However, numerous cases of clinical resistance to acyclovir have been associated with viruses that were reported to express no TK activity. We studied an acyclovir-resistant clinical mutant that contains a single-base insertion in its tk gene, predicting the synthesis of a truncated TK polypeptide with no TK activity. Nevertheless, the mutant retained some TK activity and the ability to reactivate from latent infections of mouse trigeminal ganglia. The mutant expressed both the predicted truncated polypeptide and a low level of a polypeptide that comigrated with full-length TK on polyacrylamide gels and reacted with anti-TK antiserum, providing evidence for a frameshifting mechanism. In vitro transcription and translation of mutant tk genes, including constructs in which reporter epitopes could be expressed only if frameshifting occurred, also gave rise to truncated and full-length polypeptides. Reverse transcriptase-polymerase chain reaction analysis coupled with open reading frame cloning failed to detect alterations in tk transcripts that could account for the synthesis of full-length polypeptide. Thus, synthesis of full-length TK was due to an unusual net +1 frameshift during translation, a phenomenon hitherto confined in eukaryotic cells to certain RNA viruses and retrotransposons. Utilization of cellular frameshifting mechanisms may permit an otherwise TK-negative virus to exhibit clinical acyclovir resistance.

Restricted herpes simplex virus type 1 gene expression within sensory neurons in the absence of functional B and T lymphocytes.

Herpes simplex virus type I (HSV-1) establishes a latent infection in sensory ganglion neurons. During latency no viral-specific proteins are detected and virus gene expression is restricted to the latency-associated transcripts. We report here that trigeminal ganglia of mice with severe combined immunodeficiency contain individual sensory neurons exhibiting restricted viral gene expression characteristic of latency; this occurred during acute (4-6 days) infection with the wild-type HSV-1 strain 17+ and after prolonged (4 weeks) infection with the replication impaired HSV-1 mutant in 1814. These results indicate that T and B lymphocytes, while important for the recovery from viral infections, are not required for the establishment or maintenance of latency in neurons.

A novel latency-active promoter is contained within the herpes simplex virus type 1 UL flanking repeats.

Herpes simplex virus type 1 (HSV-1) expresses a unique series of RNA molecules, the latency-associated transcripts or LATs, during latent infection of neuronal tissues. Previous studies by others have described a TATA box-containing latency-active promoter, referred to here as LAP1, located approximately 700 bp upstream of the 5' end of the major 2.0-kb LAT. In this report, transient gene expression assays were employed to identify a second, novel latency-active promoter (LAP2) present within a region downstream of LAP1 and 5' proximal to the major 2.0-kb LAT. In contrast to LAP1, this promoter lacks a TATA box but possesses cis-acting regulatory elements and other features frequently observed within eukaryotic housekeeping gene promoters. Unlike most other HSV promoters, LAP2 was down-regulated by the viral transcriptional activators ICP4 and ICP0. The majority of LAP2-positive regulatory elements were located within sequences from -257 to -58 relative to the 5' end of the 2.0-kb LAT, and the basal promoter mapped within sequences from -14 to +28. RNase protection experiments demonstrated that chimeric LAT-chloramphenicol acetyltransferase transcripts produced in the transient assays initiated at or near the 5' end of the major 2-kb LAT. Tn5 insertional mutagenesis of the ICP4 regulatory gene determined that down-regulation of LAP2 required the ICP4 transactivating domain and targeted the minimal promoter region as the site of action by ICP4. Replicating recombinant viruses containing a LAP2-lacZ reporter gene cassette in an ectopic site (glycoprotein C locus) were shown to be active in mouse trigeminal ganglia. Taken together, these experiments suggest that the LAT region of the HSV-1 genome contains at least two latency-active promoters which may play different roles in expressing the various LATs. Alternatively, these promoters may comprise a larger promoter-regulatory complex which may influence transcription during latency.

Herpes simplex virus type 1 DNA replication and gene expression during explant-induced reactivation of latently infected murine sensory ganglia.

Infectious virus assays and PCR amplification of DNA and RNA were used to investigate herpes simplex virus DNA replication and gene expression in two murine in vitro models for virus reactivation. We examined latent infections with wild-type (wt), precisely defined latency-associated transcript-negative (LAT-) mutants, and LAT+ rescuants of these mutants of the 17syn+ strain of virus in both murine trigeminal and lumbosacral ganglia and of the KOS(M) strain in the latter. In explants of ganglia latently infected with the LAT- mutant of strain 17syn+ virus, a reduction in number of cultures exhibiting cytopathic effects due to virus reactivation and measurable delays in virus recovery were observed compared with wt or the LAT+ rescuant. This LAT-specific effect was not seen in explants of lumbosacral ganglia latently infected with mutants derived from the KOS(M) strain of virus. Although there was appreciable variation between individual animals, no significant difference between LAT+ and LAT- virus in time of onset of viral DNA replication in explanted ganglia was seen with use of either virus strain. There was a slight decrease in the relative amount of viral DNA recovered compared with internal cellular controls in latently infected ganglia harboring the LAT- mutant of 17syn+ compared with the wt virus or the LAT+ rescuant. This reduced relative amount ranged from 0 to as much as 50% but averaged 20%. Such differences were not seen in infections with KOS(M)-derived mutants. In contrast, although expression of productive-cycle transcripts could be detected within 4 h following explant cultivation of latently infected ganglia, no differences between LAT+ and LAT- viruses could be seen. As discussed, these data place specific constraints on possible models for the role of LAT expression in in vitro reactivation systems.

Molecular analysis of herpes simplex virus type 1 during epinephrine-induced reactivation of latently infected rabbits in vivo.

Infectious virus assays and PCR amplification of DNA and RNA were used to investigate herpes simplex virus (HSV) DNA replication and gene expression in the rabbit corneal model for virus reactivation in vivo. We used carefully defined latency-associated transcript-negative (LAT-) and LAT+ promoter mutants of the 17syn+ strain of HSV type 1. In agreement with earlier studies using a more extensive LAT- deletion mutant, the 17 delta Pst(LAT-) virus reactivated with extremely low frequency upon epinephrine induction. In contrast to our findings with murine latency models, amounts of viral DNA recovered from rabbit ganglia latently infected with either LAT+ or LAT- virus were equivalent. Also in contrast with the murine models, no net increase in viral DNA was seen in latently infected rabbit trigeminal ganglia induced to reactivate in vivo by iontophoresis of epinephrine. Despite this, transcription of lytic-phase genes could be detected within 4 h following induction of rabbits latently infected with either LAT+ or LAT- virus; this transcription diminished by 16 h following induction. These results are discussed in relation to models for the mechanism of action of HSV LAT.