The bovine papillomavirus type 4 E8 protein binds to ductin and causes loss of gap junctional intercellular communication in primary fibroblasts

Acute Adenoviral Infection Elicits an Arrhythmogenic Substrate Prior to Myocarditis

BACKGROUND

Viral cardiac infection represents a significant clinical challenge encompassing several etiological agents, disease stages, complex presentation, and a resulting lack of mechanistic understanding. Myocarditis is a major cause of sudden cardiac death in young adults, where current knowledge in the field is dominated by later disease phases and pathological immune responses. However, little is known regarding how infection can acutely induce an arrhythmogenic substrate before significant immune responses. Adenovirus is a leading cause of myocarditis, but due to species specificity, models of infection are lacking, and it is not understood how adenoviral infection may underlie sudden cardiac arrest. Mouse adenovirus type-3 was previously reported as cardiotropic, yet it has not been utilized to understand the mechanisms of cardiac infection and pathology.

METHODS

We have developed mouse adenovirus type-3 infection as a model to investigate acute cardiac infection and molecular alterations to the infected heart before an appreciable immune response or gross cardiomyopathy.

RESULTS

Optical mapping of infected hearts exposes decreases in conduction velocity concomitant with increased Cx43Ser368 phosphorylation, a residue known to regulate gap junction function. Hearts from animals harboring a phospho-null mutation at Cx43Ser368 are protected against mouse adenovirus type-3-induced conduction velocity slowing. Additional to gap junction alterations, patch clamping of mouse adenovirus type-3-infected adult mouse ventricular cardiomyocytes reveals prolonged action potential duration as a result of decreased IK1 and IKs current density. Turning to human systems, we find human adenovirus type-5 increases phosphorylation of Cx43Ser368 and disrupts synchrony in human induced pluripotent stem cell-derived cardiomyocytes, indicating common mechanisms with our mouse whole heart and adult cardiomyocyte data.

CONCLUSIONS

Together, these findings demonstrate that adenoviral infection creates an arrhythmogenic substrate through direct targeting of gap junction and ion channel function in the heart. Such alterations are known to precipitate arrhythmias and likely contribute to sudden cardiac death in acutely infected patients.

Mechanisms of Intracellular Communication in Cancer and Pathogen Spreading

Cell-to-cell interactions are essential for proper development, homeostasis, and complex syncytia/organ formation and function. Intercellular communication are mediated by multiple mechanisms including soluble mediators, adhesion molecules and specific mechanisms of cell to cell communication such as Gap junctions (GJ), tunneling nanotubes (TNT), and exosomes. Only recently, has been discovered that TNTs and exosomes enable the exchange of large signaling molecules, RNA, viral products, antigens, and organelles opening new avenues of research and therapeutic approaches. The focus of this review is to summarize these recent findings in physiologic and pathologic conditions.

Pteridium spp. and Bovine Papillomavirus: Partners in Cancer

Bovine papillomavirus (BPV) are a cause for global concern due to their wide distribution and the wide range of benign and malignant diseases they are able to induce. Those lesions include cutaneous and upper digestive papillomas, multiple histological types of urinary bladder cancers—most often associated with BPV1 and BPV2—and squamous cell carcinomas of the upper digestive system, associated with BPV4. Clinical, epidemiological and experimental evidence shows that exposure to bracken fern (Pteridium spp.) and other related ferns plays an important role in allowing viral persistence and promoting the malignant transformation of early viral lesions. This carcinogenic potential has been attributed to bracken illudane glycoside compounds with immune suppressive and mutagenic properties, such as ptaquiloside. This review addresses the role of BPV in tumorigenesis and its interactions with bracken illudane glycosides. Current data indicates that inactivation of cytotoxic T lymphocytes and natural killer cells by bracken fern illudanes plays a significant role in allowing viral persistence and lesion progression, while BPV drives unchecked cell proliferation and allows the accumulation of genetic damage caused by chemical mutagens. Despite limited progress in controlling bracken infestation in pasturelands, bracken toxins remain a threat to animal health. The number of recognized BPV types has steadily increased over the years and now reaches 24 genotypes with different pathogenic properties. It remains essential to widen the available knowledge concerning BPV and its synergistic interactions with bracken chemical carcinogens, in order to achieve satisfactory control of the livestock losses they induce worldwide.

Connexin 43/47 channels are important for astrocyte/ oligodendrocyte cross-talk in myelination and demyelination

The gap junctions (GJs), which form intercellular communicating channels between two apposing cells or form hemichannel with extracellular environment, perform crucial functions to maintain small molecule homeostasis. The central nervous system (CNS) GJs are important for maintenance of myelin sheath and neuronal activity. Connexin (Cx) proteins are building blocks of GJs. Recent cell-biological investigations show that amongst the CNS specific Cxs, the most abundant Cx protein, Cx43 and its oligodendrocytic coupling partner Cx47 primarily important for maintenance of CNS myelin. Recent investigations elucidate that the expression of Cx43 and Cx47 is very important to maintain K+ buffering and nutrient homeostasis in oligodendrocytes, CNS myelin and oligodendrocyte function. The investigations on Multiple Sclerosis (MS) patient samples and EAE hypothesized that the functional loss of Cx43/Cx47 could be associated with spread of chronic MS lesions. Exploring the mechanism of initial GJ alteration and its effect on demyelination in this model of MS might play a primary role to understand the basis of altered CNS homeostasis, observed during MS. In this review, we mainly discuss the role of CNS GJs, specifically the Cx43/Cx47 axis in the perspective of demyelination.

Connexin 43/47 channels are important for astrocyte/ oligodendrocyte cross-talk in myelination and demyelination

The gap junctions (GJs), which form intercellular communicating channels between two apposing cells or form hemichannel with extracellular environment, perform crucial functions to maintain small molecule homeostasis. The central nervous system (CNS) GJs are important for maintenance of myelin sheath and neuronal activity. Connexin (Cx) proteins are building blocks of GJs. Recent cell-biological investigations show that amongst the CNS specific Cxs, the most abundant Cx protein, Cx43 and its oligodendrocytic coupling partner Cx47 primarily important for maintenance of CNS myelin. Recent investigations elucidate that the expression of Cx43 and Cx47 is very important to maintain K? buffering and nutrient homeostasis in oligodendrocytes, CNS myelin and oligodendrocyte function. The investigations on Multiple Sclerosis (MS) patient samples and EAE hypothesized that the functional loss of Cx43/Cx47 could be associated with spread of chronic MS lesions. Exploring the mechanism of initial GJ alteration and its effect on demyelination in this model of MS might play a primary role to understand the basis of altered CNS homeostasis, observed during MS. In this review, we mainly discuss the role of CNS GJs, specifically the Cx43/Cx47 axis in the perspective of demyelination.

Loss of Cx43-Mediated Functional Gap Junction Communication in Meningeal Fibroblasts Following Mouse Hepatitis Virus Infection

Mouse hepatitis virus (MHV) infection causes meningoencephalitis by disrupting the neuro-glial and glial-pial homeostasis. Recent studies suggest that MHV infection alters gap junction protein connexin 43 (Cx43)-mediated intercellular communication in brain and primary cultured astrocytes. In addition to astrocytes, meningeal fibroblasts also express high levels of Cx43. Fibroblasts in the meninges together with the basal lamina and the astrocyte endfeet forms the glial limitans superficialis as part of the blood-brain barrier (BBB). Alteration of glial-pial gap junction intercellular communication (GJIC) in MHV infection has the potential to affect the integrity of BBB. Till date, it is not known if viral infection can modulate Cx43 expression and function in cells of the brain meninges and thus affect BBB permeability. In the present study, we have investigated the effect of MHV infection on Cx43 localization and function in mouse brain meningeal cells and primary meningeal fibroblasts. Our results show that MHV infection reduces total Cx43 levels and causes its intracellular retention in the perinuclear compartments reducing its surface expression. Reduced trafficking of Cx43 to the cell surface in MHV-infected cells is associated with loss functional GJIC. Together, these data suggest that MHV infection can directly affect expression and cellular distribution of Cx43 resulting in loss of Cx43-mediated GJIC in meningeal fibroblasts, which may be associated with altered BBB function observed in acute infection.

Mouse Hepatitis Virus Infection Remodels Connexin43-Mediated Gap Junction Intercellular Communication In Vitro and In Vivo

Gap junctions (GJs) form intercellular channels which directly connect the cytoplasm between neighboring cells to facilitate the transfer of ions and small molecules. GJs play a major role in the pathogenesis of infection-associated inflammation. Mutations of gap junction proteins, connexins (Cxs), cause dysmyelination and leukoencephalopathy. In multiple sclerosis (MS) patients and its animal model experimental autoimmune encephalitis (EAE), Cx43 was shown to be modulated in the central nervous system (CNS). The mechanism behind Cx43 alteration and its role in MS remains unexplored. Mouse hepatitis virus (MHV) infection-induced demyelination is one of the best-studied experimental animal models for MS. Our studies demonstrated that MHV infection downregulated Cx43 expression at protein and mRNA levels in vitro in primary astrocytes obtained from neonatal mouse brains. After infection, a significant amount of Cx43 was retained in endoplasmic reticulum/endoplasmic reticulum Golgi intermediate complex (ER/ERGIC) and GJ plaque formation was impaired at the cell surface, as evidenced by a reduction of the Triton X-100 insoluble fraction of Cx43. Altered trafficking and impairment of GJ plaque formation may cause the loss of functional channel formation in MHV-infected primary astrocytes, as demonstrated by a reduced number of dye-coupled cells after a scrape-loading Lucifer yellow dye transfer assay. Upon MHV infection, a significant downregulation of Cx43 was observed in the virus-infected mouse brain. This study demonstrates that astrocytic Cx43 expression and function can be modulated due to virus stress and can be an appropriate model to understand the basis of cellular mechanisms involved in the alteration of gap junction intercellular communication (GJIC) in CNS neuroinflammation.

IMPORTANCE

We found that MHV infection leads to the downregulation of Cx43 in vivo in the CNS. In addition, results show that MHV infection impairs Cx43 expression in addition to gap junction communication in primary astrocytes. After infection, Cx43 did not traffic normally to the membrane to form gap junction plaques, and that could be the basis of reduced functional gap junction coupling between astrocytes. This is an important first step toward understanding how viruses affect Cx43 expression and trafficking at the cellular level. This may provide a basis for understanding how structural alterations of astrocytic gap junctions can disrupt gap junction communication between other CNS cells in altered CNS environments due to infection and inflammation. More specifically, alteration of Cx43 may be the basis of the destabilization of Cx47 in oligodendrocytes seen in and around inflammatory demyelinating plaques in MS patients.

Effect of human immunodeficiency virus on blood-brain barrier integrity and function: an update

The blood-brain barrier (BBB) is a diffusion barrier that has an important role in maintaining a precisely regulated microenvironment protecting the neural tissue from infectious agents and toxins in the circulating system. Compromised BBB integrity plays a major role in the pathogenesis of retroviral associated neurological diseases. Human Immunodeficiency Virus (HIV) infection in the Central Nervous System (CNS) is an early event even before the serodiagnosis for HIV positivity or the initiation of antiretroviral therapy (ART), resulting in neurological complications in many of the infected patients. Macrophages, microglia and astrocytes (in low levels) are the most productively/latently infected cell types within the CNS. In this brief review, we have discussed about the effect of HIV infection and viral proteins on the integrity and function of BBB, which may contribute to the progression of HIV associated neurocognitive disorders.

Regulation of gap junction channels by infectious agents and inflammation in the CNS

Gap junctions (GJs) are conglomerates of intercellular channels that connect the cytoplasm of two or more cells, and facilitate the transfer of ions and small molecules, including second messengers, resulting in metabolic and electrical coordination. In general, loss of gap junctional communication (GJC) has been associated with cellular damage and inflammation resulting in compromise of physiological functions. Recently, it has become evident that GJ channels also play a critical role in the pathogenesis of infectious diseases and associated inflammation. Several pathogens use the transfer of intracellular signals through GJ channels to spread infection and toxic signals that amplify inflammation to neighboring cells. Thus, identification of the mechanisms by which several infectious agents alter GJC could result in new potential therapeutic approaches to reduce inflammation and their pathogenesis.

Bovine Papillomavirus: New Insights into an Old Disease

Bovine papillomaviruses (BPVs) are small DNA tumoral viruses able to induce benign cutaneous and/or mucosal epithelial lesions. Generally, the benign tumours affecting the skin or mucosa spontaneously regress, but under special circumstances, the defence system may be overwhelmed, thus leading to cancer, especially in the presence of immunosuppressant and mutagen agents from bracken fern. To date, thirteen different BPV genotypes have been associated with skin and mucosal tumours in cattle, and out of these, only four types (BPV-1, -2, -5 and -13) cross-infect other species. Recent investigations in vivo have revealed new insights into the epidemiology and pathogenesis of this viral infection. This review briefly discusses viral epidemiology, will give data on BPV genome structure and viral genes and will describe the cellular events and new aspects of both cutaneous and mucosal tumours in large ruminants. Finally, some aspects of active immunization will be described.

Role of connexin/pannexin containing channels in infectious diseases

In recent years it has become evident that gap junctions and hemichannels, in concert with extracellular ATP and purinergic receptors, play key roles in several physiological processes and pathological conditions. However, only recently has their importance in infectious diseases been explored, likely because early reports indicated that connexin containing channels were completely inactivated under inflammatory conditions, and therefore no further research was performed. However, recent evidence indicates that several infectious agents take advantage of these communication systems to enhance inflammation and apoptosis, as well as to participate in the infectious cycle of several pathogens. In the current review, we will discuss the role of these channels/receptors in the pathogenesis of several infectious diseases and the possibilities of generating novel therapeutic approaches to reduce or prevent these diseases.

The E5 proteins

The E5 proteins are short transmembrane proteins encoded by many animal and human papillomaviruses. These proteins display transforming activity in cultured cells and animals, and they presumably also play a role in the productive virus life cycle. The E5 proteins are thought to act by modulating the activity of cellular proteins. Here, we describe the biological activities of the best-studied E5 proteins and discuss the evidence implicating specific protein targets and pathways in mediating these activities. The primary target of the 44-amino acid BPV1 E5 protein is the PDGF β receptor, whereas the EGF receptor appears to be an important target of the 83-amino acid HPV16 E5 protein. Both E5 proteins also bind to the vacuolar ATPase and affect MHC class I expression and cell-cell communication. Continued studies of the E5 proteins will elucidate important aspects of transmembrane protein-protein interactions, cellular signal transduction, cell biology, virus replication, and tumorigenesis.

The role of gap junction channels during physiologic and pathologic conditions of the human central nervous system

Gap junctions (GJs) are expressed in most cell types of the nervous system, including neuronal stem cells, neurons, astrocytes, oligodendrocytes, cells of the blood brain barrier (endothelial cells and astrocytes) and under inflammatory conditions in microglia/macrophages. GJs connect cells by the docking of two hemichannels, one from each cell with each hemichannel being formed by 6 proteins named connexins (Cx). Unapposed hemichannels (uHC) also can be open on the surface of the cells allowing the release of different intracellular factors to the extracellular space. GJs provide a mechanism of cell-to-cell communication between adjacent cells that enables the direct exchange of intracellular messengers, such as calcium, nucleotides, IP(3), and diverse metabolites, as well as electrical signals that ultimately coordinate tissue homeostasis, proliferation, differentiation, metabolism, cell survival and death. Despite their essential functions in physiological conditions, relatively little is known about the role of GJs and uHC in human diseases, especially within the nervous system. The focus of this review is to summarize recent findings related to the role of GJs and uHC in physiologic and pathologic conditions of the central nervous system.

Expression of connexin 26 and bovine papillomavirus E5 in cutaneous fibropapillomas of cattle

Bovine papillomaviruses (BPVs) can infect epithelial cells and fibroblasts, inducing fibropapillomas in cattle. Gap junctions are communication channels between cells composed of connexins (Cxs). This study evaluated expression of Cx26 and the major BPV oncoprotein E5 in bovine cutaneous fibropapillomas. BPV DNA was amplified from 20/20 fibropapillomas and 3/3 samples of normal skin. All fibropapillomas (20/20) were positive by immunostaining for E5, whereas the three normal skin samples were negative. Cx26 was expressed faintly in the normal skin epithelium. Positive cytoplasmic and juxtanuclear immunoreactivity for Cx26 was evident in 18/20 (90%) fibropapillomas. Western blot analysis demonstrated higher expression of Cx26 in 6/6 fibropapillomas compared to normal skin samples.

Papillomavirus E5: the smallest oncoprotein with many functions

Papillomaviruses (PVs) are established agents of human and animal cancers. They infect cutaneous and mucous epithelia. High Risk (HR) Human PVs (HPVs) are consistently associated with cancer of the uterine cervix, but are also involved in the etiopathogenesis of other cancer types. The early oncoproteins of PVs: E5, E6 and E7 are known to contribute to tumour progression. While the oncogenic activities of E6 and E7 are well characterised, the role of E5 is still rather nebulous. The widespread causal association of PVs with cancer makes their study worthwhile not only in humans but also in animal model systems. The Bovine PV (BPV) system has been the most useful animal model in understanding the oncogenic potential of PVs due to the pivotal role of its E5 oncoprotein in cell transformation. This review will highlight the differences between HPV-16 E5 (16E5) and E5 from other PVs, primarily from BPV. It will discuss the targeting of E5 as a possible therapeutic agent.

Human immunodeficiency virus infection of human astrocytes disrupts blood-brain barrier integrity by a gap junction-dependent mechanism

HIV infection of the CNS is an early event after primary infection, resulting in neurological complications in a significant number of individuals despite antiretroviral therapy (ART). The main cells infected with HIV within the CNS are macrophages/microglia and a small fraction of astrocytes. The role of these few infected astrocytes in the pathogenesis of neuroAIDS has not been examined extensively. Here, we demonstrate that few HIV-infected astrocytes (4.7 ± 2.8% in vitro and 8.2 ± 3.9% in vivo) compromise blood-brain barrier (BBB) integrity. This BBB disruption is due to endothelial apoptosis, misguided astrocyte end feet, and dysregulation of lipoxygenase/cyclooxygenase, BK(Ca) channels, and ATP receptor activation within astrocytes. All of these alterations in BBB integrity induced by a few HIV-infected astrocytes were gap junction dependent, as blocking these channels protected the BBB from HIV-infected astrocyte-mediated compromise. We also demonstrated apoptosis in vivo of BBB cells in contact with infected astrocytes using brain tissue sections from simian immunodeficiency virus-infected macaques as a model of neuroAIDS, suggesting an important role for these few infected astrocytes in the CNS damage seen with HIV infection. Our findings describe a novel mechanism of bystander BBB toxicity mediated by low numbers of HIV-infected astrocytes and amplified by gap junctions. This mechanism of toxicity contributes to understanding how CNS damage is spread even in the current ART era and how minimal or controlled HIV infection still results in cognitive impairment in a large population of infected individuals.

Expression of gap junction protein connexin 43 in bovine urinary bladder tumours

The aetiopathogenesis of urinary bladder tumours in cattle involves prolonged ingestion of bracken fern and infection by bovine papillomavirus types 1 or 2 (BPV-1/2). The oncogenic activity of BPV is largely associated with the major oncoprotein E5. Gap junctions are the only communicating junctions found in animal tissues and are composed of proteins known as connexins. Alterations in connexin expression have been associated with oncogenesis. The present study investigated biochemically and immunohistochemically the expression of connexin 43 in samples of normal (n=2), dysplastic (n=3) and neoplastic (n=23) bovine urothelium. The tumours included 10 carcinomas in situ, five papillary urothelial carcinomas and eight invasive urothelial carcinomas. Normal and dysplastic urothelium had membrane expression of connexin 43, but this was reduced in samples of carcinoma in situ. Papillary urothelial carcinomas showed moderate cytoplasmic and membrane labelling, while invasive carcinoma showed loss of connexin 43 expression.

Classical swine fever virus down-regulates endothelial connexin 43 gap junctions

Classical swine fever is a contagious disease of pigs characterized by fatal hemorrhagic fever. Classical swine fever virus (CSFV) induces the expression of pro-inflammatory and pro-coagulant factors of vascular endothelial cells and establishes a long-term infection. This study aimed to understand the effect of CSFV on endothelial connexin 43 (Cx43) expression and gap junctional intercellular coupling (GJIC). Porcine aortic endothelial cells were infected with CSFV at different multiplicity of infection for 48 h. Semi-quantitative RT-PCR, immunoconfocal microscopy, and Western blotting showed that the transcription and translation of Cx43 were reduced, and this was associated with an attenuation of GJIC. This decrease occurred in a time-dependent manner. An ERK inhibitor (PD98059), a JNK inhibitor (SP600125), and proteasome/lysosome inhibitors all significantly reversed the reduction in Cx43 protein levels without any influence on the titer of progeny virus. In addition, CSFV activated ERK and JNK in a time-dependent manner and down-regulated Cx43 promoter activity, mainly through decreased AP2 binding. This effect was primarily caused by the replication of CSFV rather than a consequence of cytokines being induced by CSFV infection of endothelial cells.

Activated platelet-derived growth factor beta receptor expression, PI3K-AKT pathway molecular analysis, and transforming signals in equine sarcoids

The equine sarcoid is the most common dermatologic neoplasm reported in horses. Bovine papillomavirus (BPV) types 1 and 2 are associated with sarcoids, in which the expression of the major transforming oncoprotein (E5) is often recorded. The transformation activity of the virus is due to the binding of the E5 to the platelet-derived growth factor beta receptor (PDGFbeta-r). In the present study, we show by Western blot in 4 sarcoid samples and 3 normal equine skin samples that the PDGFbeta-r is more phosphorylated in sarcoid tissue than in normal skin (P < .001). Furthermore, the physical interaction between the activated receptor and the 85-kDa regulatory subunit (p85) of phosphatidylinositol-3-kinase (PI3K) is shown by coimmunoprecipitation. The PI3K-AKT-cyclin D3 molecular pathway downstream to the activation of the PDGFbeta-r is shown to be expressed, and the amount of the investigated molecules is higher than normal (P < .001), suggesting an activation of these effectors in sarcoids. Further, we demonstrate that phospho-JNK and phospho-JUN are more expressed in sarcoids than in normal skin. Our results provide new insights into the pathogenesis of equine sarcoids and support the validity of this in-vivo model to further characterize the molecular pathways underlying BPV E5-induced carcinogenesis.

Bovine papillomaviruses: their role in the aetiology of cutaneous tumours of bovids and equids

Bovine papillomavirus (BPV) is perhaps the most extensively studied animal papillomavirus. In cattle BPVs induce benign tumours of cutaneous or mucosal epithelia, called papillomas or warts. Cattle papillomas are benign tumours and generally regress without eliciting any serious clinical problems in the host, but occasionally persist and provide the focus for malignant transformation to squamous cell carcinoma, as in the case of cancer of the urinary bladder and cancer of the upper alimentary canal. BPV is the only papillomavirus that jumps species: the virus also infects equids, and gives rise to fibroblastic tumours called sarcoids. Sarcoids very rarely regress, more often they persist and can be locally aggressive. These tumours are the most common dermatological tumour of equids worldwide. The purpose of this review is to discuss the biology of BPV, the biology of bovine tumours and equine sarcoids, and present the current understanding of BPV in tumour pathogenesis in its natural host, cattle, and in its heterologous host, equids. Finally, the use of anti-BPV vaccines as a therapy for equine sarcoids will be discussed. Only limited information on the clinical or pathological aspects of either bovine or equine tumours will be provided as this subject has been extensively addressed previously.

Bovine papillomaviruses, papillomas and cancer in cattle

Bovine papillomaviruses (BPV) are DNA oncogenic viruses inducing hyperplastic benign lesions of both cutaneous and mucosal epithelia in cattle. Ten (BPV 1-10) different viral genotypes have been characterised so far. BPV 1-10 are all strictly species-specific but BPV 1/2 may also infect equids inducing fibroblastic tumours. These benign lesions generally regress but may also occasionally persist, leading to a high risk of evolving into cancer, particularly in the presence of environmental carcinogenic co-factors. Among these, bracken fern is the most extensively studied. The synergism between immunosuppressants and carcinogenic principles from bracken fern and the virus has been experimentally demonstrated for both urinary bladder and alimentary canal cancer in cows whose diets were based on this plant. BPV associated tumours have veterinary and agricultural relevance in their own right, although they have also been studied as a relevant model of Human papillomavirus (HPV). Recent insights into BPV biology have paved the way to new fields of speculation on the role of these viruses in neoplastic transformation of cells other than epithelial ones. This review will briefly summarise BPV genome organization, will describe in greater detail the functions of viral oncoproteins, the interaction between the virus and co-carcinogens in tumour development; relevant aspects of immunity and vaccines will also be discussed.

Gap junctions mediate human immunodeficiency virus-bystander killing in astrocytes

Human immunodeficiency virus (HIV) entry into the CNS is an early event after infection, resulting in neurological dysfunction in a significant number of individuals. As people with acquired immunodeficiency syndrome (AIDS) live longer, the prevalence of cognitive impairment is increasing, despite antiretroviral therapy. The mechanisms that mediate CNS dysfunction are still not completely understood, and include inflammation, viral presence, and/or replication. In this report, we characterize a novel role of gap junctions in transmitting and thereby amplifying toxic signals originating from HIV-infected astrocytes that trigger cell death in uninfected astrocytes. HIV-infected astrocytes were resistant to apoptosis; however, uninfected astrocytes forming gap junctions with infected astrocytes were apoptotic. Gap junction blockers abolished apoptosis in uninfected astrocytes, supporting the role of these channels in amplifying cell death. Our findings describe a novel mechanism of toxicity within the brain, triggered by low numbers of HIV-infected astrocytes and amplified by gap junctions, contributing to the pathogenesis of NeuroAIDS.

Bovine papillomavirus type-2 DNA and expression of E5 and E7 oncoproteins in vascular tumours of the urinary bladder in cattle

Cattles suffering from chronic enzootic haematuria frequently develop urinary bladder tumours of both epithelial and mesenchymal origin mainly haemangioma and its malignant counterpart. The role of the bovine papillomavirus type-2 (BPV-2) and of its major transforming oncoprotein in naturally occurring urothelial carcinogenesis has been recently clarified. E5 interacts in vivo as in vitro with the beta receptor for the platelet-derived growth factor (PDGF). However, studies regarding tumours of mesenchymal origin such as those arising from blood vessels are lacking. We show that the BPV-2 is present in 100% of the vascular tumours of the urinary bladder examined. Twenty-six out of twenty-seven tumour samples (96%) expressed E5 while 20 out of 27 (74%) tumour samples expressed E7. The two viral oncoproteins were not expressed in normal endothelial cells. Additionally, they co-localize in neoplastic endothelial cells as demonstrated by confocal immunofluorescence. PDGFbeta receptor was also shown to be expressed and co-localizes with E5 in neoplastic blood vessels. Our results demonstrate, for the first time, that the BPV-2 is present in high percentage in tumours of mesenchymal origin arising in its natural host. Furthermore, the expression of the two viral oncoproteins confirm that the virus may have a causative role in the neoplastic process.

Herpes simplex virus-type 2 infectivity and agents that block gap junctional intercellular communication

In rat liver epithelial (WB) cells, the protein kinase C inhibitor H7 blocked gap junctional intercellular communication (GJIC) and reduced virus infectivity. Octanol, 18-beta-glycyrrhetinic acid, and staurosporine, agents that reduce GJIC, had no effect upon virus infectivity. Previous studies demonstrated that herpes simplex virus-type 2 (HSV-2) infection was accompanied by attenuated GJIC. Of agents tested, only H7 reduced plaque forming unit (pfu) ability in a dose-dependent manner with 100% plaque reduction at 40 microM without evidence of cytotoxicity. Dye transfer indicated that H7 decreased GJIC, although Western blotting revealed that it did not alter phosphorylation of the gap junction protein, connexin 43 (Cx43). Using indirect immunofluorescence, Cx43 was found to localize in membrane plaques in uninfected cells and H7 did not alter this distribution. However, Cx43 was lost from the membrane at 24h in both H7-treated and untreated cells infected with HSV-2. Viral infection increased serine phosphorylation, particularly in the nuclear region, and this effect was reduced following H7 treatment. Thus, H7 attenuated both GJIC and infectivity of HSV-2 in WB cells but the anti-viral effects were due to reduced nuclear protein phosphorylation rather than alterations in phosphorylation or localization of Cx43.

E5 protein of human papillomavirus 16 downregulates HLA class I and interacts with the heavy chain via its first hydrophobic domain

Human papillomavirus type 16 E5 protein (HPV-16 E5) is expressed early in papillomavirus infection and is localised primarily in the cell Golgi apparatus (GA) and endoplasmic reticulum. E5 prevents transport of the major histocompatibility class I (MHC I; HLA class I in humans) to the cell surface and retains the complex in the GA. We report that these effects are due, at least in part, to the interaction between E5 and HLA I heavy chain (HC). We also demonstrate that the down-regulation of surface HLA I and interaction with HC are mediated by the first hydrophobic domain of E5. Although E5 downregulates classical HLA selectively as it does not downregulate non-classical HLA, the interaction with the HC of classical HLA I is not specific for a particular haplotype of HLA I. This suggests that E5 can interfere with antigen presentation by most, if not all, classical HLA I haplotypes, with potentially serious consequences as the ability of infected cells to present antigenic peptides to effector T cells would be compromised.

The E5 protein of BPV-4 interacts with the heavy chain of MHC class I and irreversibly retains the MHC complex in the Golgi apparatus

BPV-4 E5 inhibits transcription of the bovine MHC class I heavy chain (HC) gene, increases degradation of HC and downregulates surface expression of MHC class I by retaining the complex in the Golgi apparatus (GA). Here we report that transcription inhibition can be alleviated by interferon treatment and the degradation of HC can be reversed by treatment with inhibitors of proteasomes and lysosomes. However, the inhibition of transport of MHC class I to the cell surface is irreversible. We show that E5 is capable of physically interacting with HC. Together with the inhibition of the vacuolar ATPase (due to the interaction between E5 and 16k subunit c), the interaction between E5 and HC is likely to be responsible for retention of MHC class I in the GA. C-terminus deletion mutants of E5 are incapable of either downregulating surface MHC class I or interacting with HC, establishing that the C-terminus domain of E5 is important in the inhibition of MHC class I.

Bovine papillomavirus E5 oncoprotein binds to the activated form of the platelet-derived growth factor beta receptor in naturally occurring bovine urinary bladder tumours

Studies regarding the functions of the bovine papillomavirus (BPV) E5 oncoprotein in vivo are lacking and no E5-mediated mechanism underlying epithelial carcinogenesis is known. We have shown that BPV-2 DNA is present in the majority of naturally occurring urinary bladder tumours of cattle and that E5 is expressed in the cancer cells. Here we show that the interaction between the platelet-derived growth factor (PDGF) beta receptor and BPV E5, described in vitro in cultured cells, takes place in vivo in bovine urinary bladder cancers. In these cancers, E5 and PDGF beta receptor colocalize, as shown by confocal microscopy, and physically interact, as shown by coimmunoprecipitation. Furthermore, the PDGF beta receptor associated with E5 is highly phosphorylated, suggesting the functional activation of the receptor upon E5 interaction. Our results demonstrate, for the first time, that E5-PDGF beta receptor interaction occurs during the natural history of bovine urinary bladder tumours, suggesting an important role for E5 in carcinogenesis. Finally, the system provides a suitable animal model of papillomavirus-associated cancer to test therapeutic vaccination against E5. Successful bladder tumour regression would provide a valuable model for therapeutic vaccination against papillomavirus-associated tumours.

Similarities and differences between the E5 oncoproteins of bovine papillomaviruses type 1 and type 4: cytoskeleton, motility and invasiveness in E5-transformed bovine and mouse cells

Bovine papillomaviruses (BPVs) are oncogenic viruses. In cattle, BPV-1/2 is associated with urinary bladder cancer and BPV-4 with upper GI tract cancer. BPV E5 is a small hydrophobic protein localised in the endoplasmic reticulum (ER) and Golgi apparatus (GA). E5 is the major transforming protein of BPVs, capable of inducing cell transformation in cultured mouse fibroblasts and, in cooperation with E7, in primary bovine cells. E5-induced cell transformation is accompanied by activation of several cellular protein kinases, including growth factor receptors, and alkalinisation of endosomes and GA. We have reported that BPV E5 causes swelling and fragmentation of the GA and extensive vacuolisation of the cytoplasm. We now show that E5 from both BPV-1 and BPV-4 disturbs the actin cytoskeleton and focal adhesions in transformed bovine cells, where these morphological and behavioural characteristics are accompanied by hyperphosphorylation of the cellular phosphotyrosine kinase c-src. Both BPV-1 and BPV-4 E5 increase the motility of transformed mouse cells, but only BPV-1 E5 causes transformed mouse cells to penetrate a matrigel matrix. BPV-1 transformed mouse cells, but not BPV-4 transformed mouse cells, have hyperhpsphorylated c-src.

Transactivation of the cyclin A promoter by bovine papillomavirus type 4 E5 protein

Bovine papillomavirus type 4 (BPV-4) E5 (formerly E8) is a 42-residue hydrophobic, membrane-localised protein that can transform NIH-3T3 cells by a poorly defined mechanism. In E5-expressing cells, the observed up-regulation of cyclin A is underpinned by transactivation of the cyclin A promoter. Here we show that E5 transactivates the minimal cell cycle-regulated cyclin A promoter in cells both stably and acutely expressing the viral protein. There are no detectable differences between control and E5 cells in protein complexes binding the E2F-like cell cycle-dependent element (CDE)/cell cycle-regulated element (CCRE) of the cyclin A promoter and E5 does not transactivate E2F reporter plasmids in an E2F-dependent manner in vivo. CCAAT box integrity and functional NF-Y complexes are required for E5-mediated transactivation and a Mr approximately 110 K CCAAT-box binding factor (p110 CBF) associates with NF-YA only in E5 cells. This suggests that E5 sets the extent of cyclin A promoter activation by a mechanism similar to other, structurally unrelated, DNA tumour virus oncoproteins but distinct from the action of serum factors and so is inconsistent with E5 acting through constitutive activation of tyrosine kinase growth factor receptors.

Reduced expression of multiple gap junction proteins is a feature of cervical dysplasia

Cervical dysplasia is a premalignant lesion associated with human papillomavirus (HPV) infection which, over time, can turn cancerous. Previous studies have indicated that loss of gap junctions may be a feature of cervical cancer and premalignant dysplasia. Loss of the gap junction protein connexin43 has been demonstrated in dysplastic cervix, but other connexins have not been investigated. In contrast we previously showed that HPV-associated cutaneous warts – and other hyperproliferative skin conditions – display a dramatic upregulation of certain connexins, in particular connexin26. By performing immunofluorescence staining after antigen retrieval of paraffin-embedded cervical tissue samples, this study reports for the first time that connexin26 and connexin30, in addition to connexin43, are expressed in differentiating cells of normal human cervical epithelia. Moreover, in dysplastic ectocervix, all connexins studied display a dramatic loss of expression compared to adjacent normal epithelia. The role of connexins in keratinocyte differentiation and carcinogenesis is discussed.

E5 protein of human papillomavirus type 16 selectively downregulates surface HLA class I

Papillomaviruses have evolved mechanisms that result in escape from host immune surveillance. The E5 protein is expressed early in papillomavirus infection in the deep layers of the infected epithelium. It is localized to the Golgi apparatus (GA) and endoplasmic reticulum. The E5 protein of bovine papillomavirus (BPV) impairs the synthesis and stability of major histocompatibility (MHC) class I complexes and prevents their transport to the cell surface due to retention in the GA. Here we show that human papillomavirus type 16 (HPV-16) E5 also causes the retention of MHC (HLA) class I complexes in the GA and impedes their transport to the cell surface, which is rescued by treatment with interferon. Unlike BPV E5, HPV-16 E5 does not affect the synthesis of HLA class I heavy chains or the expression of the transporter associated with antigen processing TAP. These results show that downregulation of surface MHC class I molecules is common to both BPV and HPV E5 proteins. Moreover, we determined that HPV-16 E5 downregulates surface expression of HLA-A and HLA-B, which present viral peptides to MHC class I-restricted cytotoxic T lymphocytes (CTLs), but not the natural killer (NK) cell inhibitory ligands HLA-C and HLA-E. Selective downregulation of cell surface HLA class I molecules may allow the virus to establish infection by avoiding immune clearance of virus-infected cells by both CTLs and NK cells.

Presence of bovine papillomavirus type 2 DNA and expression of the viral oncoprotein E5 in naturally occurring urinary bladder tumours in cows

Samples of neoplastic and normal urothelium were obtained from cows originating from areas of southern Italy, a region in which chronic enzootic haematuria is endemic and bracken fern infestation is widespread. Specimens were analysed for bovine papillomavirus type 2 (BPV-2) DNA, BPV-2 E5 expression and telomerase activity. A total of 46 of 60 tumours and 17 of 34 normal bladder mucosa samples harboured BPV-2 DNA. Analysis of a subset of samples showed E5 protein expression and telomerase activity in tumour tissue only. No normal samples positive for BPV DNA showed E5 protein expression or telomerase activity, suggesting the presence of DNA in a latent state. Taken together, these data on naturally occurring bovine bladder tumours corroborate the hypothesis of their virus origin.

The relationship between connexins, gap junctions, tissue architecture and tumour invasion, as studied in a novel in vitro model of HPV-16-associated cervical cancer progression

Disruption of gap junctional intercellular communication (GJIC) and/or connexins (gap junction proteins) is frequently reported in malignant cell lines and tumours. Certain human papillomaviruses (HPV) associated with the development of cancers, especially of the cervix, have previously been reported to downregulate GJIC in vitro. There is also evidence for reduced gap junctions in cervical dysplasia. However, many squamous hyperproliferative conditions, including HPV-induced warts, often show extensive upregulation of certain connexins. The association between HPV and GJIC, and the mechanism and consequence of deregulated GJIC in cervical tumour progression, remains unclear. Therefore, using a variety of nonmalignant and malignant cell lines and an organotypic raft-culture system, we investigated the relationship between HPV, gap junctions and tumour progression. Established cervical tumour cell lines carrying HPV were unable to communicate via gap junctions (when assayed by dye-transfer techniques). This correlated with lack of connexin protein expression, while transfection with connexins 26 or 43 led to functional gap junction membrane plaques. On the other hand, immortal but nonmalignant cell lines that contained episomal or integrated HPV-16, but required feeder-layer and growth-factor support, were consistently well coupled, and expressed multiple connexins at membrane junctions. In vitro selection of feeder-layer and growth-factor-independent variants eventually lead to loss of GJIC, which correlated with loss of membrane and increased cytoplasmic connexin 43 localization. However, this was preceded by loss of differentiation and stromal invasion, as assayed on the organotypic raft-culture model. Using this model, a comparison between noncoupled, well-coupled and connexin-transfected cell lines revealed no firm correlation between GJIC and dysplasia, but GJIC appeared to favour increased stratification. These findings demonstrate that loss of GJIC is frequent, but appears to occur more as a consequence of, rather than being the cause of, epithelial dysplasia, and may be influenced by, but is not directly attributable to, HPV.

Association of bovine papillomavirus with the equine sarcoid

The equine sarcoid, a locally aggressive, fibroblastic skin tumour, is the most common dermatological neoplasm reported in horses; there is no consistently effective therapy. It is widely accepted that bovine papillomavirus (BPV) types 1 and 2 are associated with the pathogenesis of sarcoid disease. Most sarcoids appear to contain detectable viral DNA and RNA and are also known to express the BPV types 1 and 2 major transforming protein, E5, but appear not to produce infectious virions. While the mode of transmission of infection has not been elucidated, viral gene expression, in particular of E5, may contribute to virus persistence and disease pathogenesis by downregulating MHC class I expression. Here, the pathology and epidemiology of the sarcoid and its association with BPV is reviewed; the transforming functions of the BPV oncoproteins and their possible role in sarcoid pathogenesis are discussed; and the practical implications of BPV infection for diagnostic and therapeutic purposes are considered.

The bovine papillomavirus oncoprotein E5 retains MHC class I molecules in the Golgi apparatus and prevents their transport to the cell surface

During papillomavirus infection, the E5 protein localizes in the cell Golgi apparatus and other endomembrane compartments. Cells transformed by E5 do not express major histocompatibility class I complex (MHC I) on the cell surface, while cells transformed by the other transforming proteins E6 and E7 do. In addition, the total amount of both MHC I protein and mRNA is reduced in E5-transformed cells. Here we show that expression of bovine papillomavirus E5 causes the retention of MHC I in the Golgi apparatus, thus preventing its transport to the cell surface. We ascribe this effect to a failure of acidification of the Golgi apparatus, as similar effects are observed in control cells treated with the ionophore monensin. Treatment of E5-transformed cells with either beta- or gamma-interferon increases the synthesis of MHC I, showing that inhibition of MHC I expression by E5 is not irreversible. However, even after interferon treatment, MHC I, although increased in quantity, is not transported to the cell surface. E5 therefore affects MHC I at several levels, but prevention of MHC I transport to the cell surface appears to be the dominant effect. Lack of surface MHC I would have profound consequences for presentation of viral peptides to the immune system.

Antiviral agents alter ability of HSV-2 to disrupt gap junctional intercellular communication between mammalian cells in vitro

In cultured mammalian cells (Vero), different antiviral agents change to differing degrees the ability of HSV2 to down-regulate gap junctions, each agent having a specific effect. Measured by intracellular electrodes, control cell populations showed 49-51% coupling, uninfected populations treated with acyclovir or SDS averaged 43-51% coupling while populations infected with HSV2 had coupling reduced to 8%. The antiviral agent acyclovir (1 microg/ml), which suppresses viral replication, failed to prevent this down regulation (final coupling ratio of 11%). A plant extract (250 microg/ml) from Pilostigma thonningii offered slightly more protection (final coupling ratio of 22%), while sodium dodecyl sulfate (SDS) (50 microM) afforded nearly complete protection (final coupling ratio of 40%). With SDS there was an initial down regulation to only 16% coupling by 3 h post infection, followed by a recovery of intercellular communication to near control levels by 24 h. While SDS was originally believed to alter the viral coat and prevent entry into the cell, our data are in agreement with recent studies which indicate that SDS treated viruses can enter into host cells, but in a severely diminished condition. Our data also suggest that the gap junction antagonist is brought into the cells as part of the entering virus.

Lack of canonical E6 and E7 open reading frames in bird papillomaviruses: Fringilla coelebs papillomavirus and Psittacus erithacus timneh papillomavirus

Determination and analyses of the complete sequence of Fringilla coelebs papillomavirus and Psittacus erithacus timneh papillomavirus indicate that they represent a distinct and distant lineage of papillomaviruses. The lack of canonical E6-E7 open reading frames suggests that they serve adaptive functions during papillomavirus evolution.

Evasion of host immunity directed by papillomavirus-encoded proteins

The nature of the interaction between papillomaviruses (PV) and their infected host has led to the identification of ways in which the viral oncoproteins can transform the infected host cells into cancer cells. As viral persistence is required for malignancy, and persistence requires avoidance of immune attack by the host, defining the relationship between PV and the immune system is also paramount in understanding tumorigenesis. It has emerged that PV have evolved several ways in which to prevent clearance by the host immune system. The limitation of the PV replication cycle to the epithelium, together with low level expression of the virus proteins and an absence of inflammation, minimises the exposure of virus to immune cells. In addition, more recently it has been shown that, like many other viruses, PV can directly subvert the immune response, including interference with the interferon pathway, modulation of antigen presentation, inhibition of interleukin-18 activity and down-regulation of major histocompatibility class I on infected cells. Collectively these mechanisms explain how PV lesions are able to persist for long periods of time in immunocompetent hosts.

Down-regulation of MHC class I by bovine papillomavirus E5 oncoproteins

The papillomavirus E5 protein is localized in the endoplasmic reticulum (ER) and Golgi apparatus (GA) of the host cell. Transformed bovine fibroblasts expressing bovine papillomavirus (BPV) E5 are highly vacuolated and have a much enlarged, distorted and fragmented GA. Major histocompatibility complex class I (MHC I) is processed and transported to the cell surface through the GA. Given the cellular localization of E5 in the GA and the morphologically abnormal GA, we investigated the expression of MHC I in cells transformed by E5 from BPV-1 and BPV-4. Two cell lines were used: bovine cells that also express E6, E7 and activated ras, and NIH3T3 cells that express only E5. In addition, PalF cells acutely infected with a recombinant retrovirus expressing E5 were also examined. In contrast to non-transformed normal cells, or transformed cells expressing other papillomavirus proteins, cells expressing E5 do not express MHC I on their surface, but retain it intracellularly, independently of the presence of other viral or cellular oncogenes, or of whether the cells are long-term transformants or acutely infected. We conclude that expression of E5 prevents expression of MHC I to the cell surface and causes its retention within the cell. In addition, lower amounts of total MHC I heavy chain and of heavy chain RNA are detected in E5-transformed cells than in control cells. As surface expression of another glycosylated membrane protein, the transferrin receptor, is not affected, it appears that E5 targets MHC I with at least a degree of specificity. In papillomavirus lesions this effect would have important implications for antigen presentation by, and immunosurveillance of, virally infected cells.

E5 transforming proteins of papillomaviruses do not disturb the activity of the vacuolar H(+)-ATPase

Papillomaviruses contain a gene, E5, that encodes a short hydrophobic polypeptide that has transforming activity. E5 proteins bind to the 16 kDa subunit c (proteolipid) of the eukaryotic vacuolar H(+)-ATPase (V-ATPase) and this binding is thought to disturb the V-ATPase and to be part of transformation. This link has been examined in the yeast Saccharomyces cerevisiae. The E5 proteins from human papillomavirus (HPV) type 16, bovine papillomavirus (BPV) type 1, BPV-4 E5 and various mutants of E5 and the p12' polypeptide from human T-lymphotropic virus (HTLV) type I all bound to the S. cerevisiae subunit c (Vma3p) and could be found in vacuolar membranes. However, none affected the activity of the V-ATPase. In contrast, a dominant-negative mutant of Vma3p (E137G) inactivated the enzyme and gave the characteristic VMA phenotype. A hybrid V-ATPase containing a subunit c from Norway lobster also showed no disruption. Sedimentation showed that HPV-16 E5 was not part of the active V-ATPase. It is concluded that the binding of E5 and E5-related proteins to subunit c does not affect V-ATPase activity or function and it is proposed that the binding may be due to a chaperone function of subunit c.

Cell transformation by the E5/E8 protein of bovine papillomavirus type 4. p27(Kip1), Elevated through increased protein synthesis is sequestered by cyclin D1-CDK4 complexes

The E5/E8 hydrophobic protein of BPV-4 is, at only 42 residues, the smallest transforming protein identified to date. Transformation of NIH-3T3 cells by E5/E8 correlates with up-regulation of both cyclin A-associated kinase activity and, unusually, p27(Kip1) (p27) but does not rely on changes in cyclin E or cyclin E-CDK2 activity. Here we have examined how p27 is prevented from functioning efficiently as a CDK2 inhibitor, and we investigated the mechanisms used to achieve elevated p27 expression in E5/E8 cells. Our results show that normal subcellular targeting of p27 is not subverted in E5/E8 cells, and p27 retains its ability to inhibit both cyclin E-CDK2 and cyclin A-CDK activities upon release from heat-labile complexes. E5/E8 cells also have elevated levels of cyclins D1 and D3, and high levels of nuclear p27 are tolerated because the inhibitor is sequestered within an elevated pool of cyclin D1-CDK4 complexes, a significant portion of which retain kinase activity. In agreement with this, pRB is constitutively hyperphosphorylated in E5/E8 cells in vivo. The increased steady-state level of p27 is achieved largely through an increased rate of protein synthesis and does not rely on changes in p27 mRNA levels or protein half-life. This is the first report of enhanced p27 synthesis as the main mechanism for increasing protein levels in continuously cycling cells. Our results are consistent with a model in which E5/E8 promotes a coordinated elevation of cyclin D1-CDK4 and p27, as well as cyclin A-associated kinase activity, which act in concert to allow continued proliferation in the absence of mitogens.

Polyamines regulate gap junction communication in connexin 43-expressing cells

The control of cell-cell communication through gap junctions is thought to be crucial in normal tissue function and during various stages of tumorigenesis. However, few natural regulators of gap junctions have been found. We show here that increasing the activity of ornithine decarboxylase, or adding polyamines to the outside of cells, increases the level of gap junction communication between various epithelial cells. Conversely, reduction of ornithine decarboxylase activity decreases the level of gap junction communication. This regulation is dependent upon the expression of connexin 43 (Cx43 or Cxalpha1), which is a major connexin expressed in many different cell types, and involves an increase in Cx43 and its cellular re-distribution.

HSV-2 disrupts gap junctional intercellular communication between mammalian cells in vitro

Infection by herpes simplex virus-2 (HSV-2) disrupts both dye and electrical coupling in Vero (African green monkey kidney) cell cultures. Vero cells in vitro were iontophoretically injected with the fluorescent dye Lucifer yellow CH, the spread of which revealed that cells throughout the confluent sheet shared open gap junctions. However, 24 h after infection with the virus (but before cells became rounded), dye always remained only within the target cell. Intracellular electrophysiological measurements of ionic coupling revealed a 0.4 coupling coefficient for adjacent cells in uninfected control cultures. By 3 h following infection significant down-regulation of gap junctions had begun, preceding by many hours any signs of infection visible with the light microscope. Measurements between adjacent cells 3 h post-infection, a period when HSV-2 gene expression is known to be at a maximum, yielded an average coupling coefficient of 0.35. By 6 h post-infection (a period of known viral DNA replication) average coupling coefficient for adjacent cells was 0.25, while by 24 h post-infection the average fill still further to <0.08. A coupling coefficient of <0.08 suggests that infection by HSV-2 completely disabled the gap junctions.

Binding of human papillomavirus 16 E5 to the 16 kDa subunit c (proteolipid) of the vacuolar H+-ATPase can be dissociated from the E5-mediated epidermal growth factor receptor overactivation

Human papillomavirus type 16 E5 protein (HPV16 E5) upregulates ligand-mediated activation of the epidermal growth factor receptor (EGFR) in transfected human keratinocytes. HPV16 E5 binds to the 16 kDa proteolipid (subunit c) of the vacuolar H+-ATPase (16K), responsible for endosomal acidification, and this binding has been suggested to be responsible for increased recycling of the EGFRs. Using mutant deletions we show here that amino acids 54-78, but not 79-83 are necessary for binding to the 16K proteolipid. EGF treatment of cells expressing wild type or mutants of the E5 protein show that deletion of the last carboxy terminal 5 amino acids results in loss of E5-mediated EGFR overactivation. Thus, our results show that the binding capacity of HPV16 E5 to 16K can be dissociated from the effect of the viral protein on EGFR activation.

A mutagenic analysis of the E5 protein of human papillomavirus type 16 reveals that E5 binding to the vacuolar H+-ATPase is not sufficient for biological activity, using mammalian and yeast expression systems

The E5 gene of human papillomavirus type 16 encodes a highly hydrophobic membrane protein previously shown to inhibit endosomal acidification, presumably by binding to the 16-kDa pore-forming subunit of the vacuolar H(+)-ATPase (v-ATPase). The role of this interaction in the disruption of v-ATPase activity was explored through extensive mutagenesis of E5 to identify residues that mediate binding to the 16-kDa subunit. Coimmunoprecipitations revealed that the hydrophobic span between residues 41 and 54 is primarily responsible for this interaction and can be replaced with random hydrophobic amino acids. Studies using mutated 16-kDa proteins indicated that the fourth transmembrane domain of the pore subunit mediates binding to E5. Analysis of the E5 mutants in a yeast expression system revealed that several mutants that retained the capacity to bind to the 16-kDa subunit in COS-1 cells failed to disrupt vacuolar acidification. These data argue that E5 binding to the pore subunit is not sufficient for the associated activity of disruption of v-ATPase function.

Binding of bovine papillomavirus type 4 E8 to ductin (16K proteolipid), down-regulation of gap junction intercellular communication and full cell transformation are independent events

The E8 open reading frame of bovine papillomavirus type 4 encodes a small hydrophobic polypeptide that contributes to primary cell transformation by conferring to cells the ability to form foci and to grow in low serum and in suspension. Wild-type E8 binds in vitro to ductin, a component of gap junctions, and this binding is accompanied by a loss of gap junction intercellular communication in transformed bovine fibroblasts. However, through the analysis of a panel of E8 mutants, we show here that binding of E8 to ductin is not sufficient for down-regulation of gap junction communication and that there is no absolute correlation between down-regulation of gap junction communication and the transformed phenotype.

The human papillomavirus type 16 E5 protein modulates phospholipase C-gamma-1 activity and phosphatidyl inositol turnover in mouse fibroblasts

The human papillomavirus type 16 E5 (HPV16-E5) protein is a membrane protein that has been associated with malignant growth. The protein affects growth factor-mediated signal transduction in a ligand-dependent manner. We show now that E5 expression in A31 fibroblasts results in an increased level of diacylglycerol (DAG) and inositol phosphates. Immunoprecipitation of phospholipase C-gamma-1 (PLC-gamma-1) with specific antibodies and immunoblotting with anti-phosphotyrosine antibodies reveal a large increase in tyrosine phosphorylation of the enzyme in E5-expressing cells compared to control vector-transfected cells. This activation of tyrosine phosphorylation is growth factor independent. In addition, an enhanced formation of phosphatidic acid (PA) was observed in E5 cells. This increase did not result from activation of phospholipase D (PLD), although the enzyme was activatable by treatment with phorbol ester Thus, a phosphohydrolase-mediated DAG synthesis from PLD-produced PA can be excluded. The observed effects were not further enhanced by EGF showing that the presence of the growth factor is not necessary for maintaining permanent activation of PLC-gamma-1 in E5-expressing cells. The DAG- and inositol phosphate-mediated signal cascade within the cells is thus effectively uncoupled from external control via EGF and its receptor in the presence of E5 protein.

A mutational analysis of the transforming functions of the E8 protein of bovine papillomavirus type 4

The E8 protein of BPV-4 contributes to transformation of primary bovine cells (PalFs) by inducing anchorage-independent growth and by down-regulating gap junction intercellular communication, likely due to its binding to 16K ductin. We show here that, in addition, E8 confers on PalF cells the ability to grow in low serum and to escape from contact inhibition (focus formation). E8 also transactivates an exogenous human cyclin A gene promoter, suggesting that overexpression of cyclin A is responsible for the transformed phenotype. Mutant forms of E8 were generated to establish whether the transforming functions of the protein could be segregated. Mutations were introduced both in the hydrophobic domain and in the hydrophilic C-terminal "tail", and chimeras with BPV-1 E5 were constructed. Cells expressing either wild-type E8 or mutant forms were analyzed for their ability to grow in low serum and in suspension and to form foci. Wild-type E8 and its mutants were also analyzed for their ability to transactivate the cyclin A promoter. We show here that the transforming functions of E8 can be segregated and that both the hydrophilic C-terminal tail and the residue at position 17 in the hydrophobic domain are crucial for E8 functions and for the transactivation of the cyclin A promoter. These results support the hypothesis that the different aspects of cellular transformation brought about by E8 might be due to interaction with different cellular targets. They suggest that E8 might function differently from BPV-1 E5 and demonstrate that the separate domains of E5 and E8 are not functionally interchangeable.