Spontaneous electrical activity induced by herpes virus infection in rat sensory neuron cultures

Alpha herpesvirus exocytosis from neuron cell bodies uses constitutive secretory mechanisms, and egress and spread from axons is independent of neuronal firing activity

Alpha herpesviruses naturally infect the peripheral nervous system, and can spread to the central nervous system, causing severe debilitating or deadly disease. Because alpha herpesviruses spread along synaptic circuits, and infected neurons exhibit altered electrophysiology and increased spontaneous activity, we hypothesized that alpha herpesviruses use activity-dependent synaptic vesicle-like regulated secretory mechanisms for egress and spread from neurons. Using live-cell fluorescence microscopy, we show that Pseudorabies Virus (PRV) particles use the constitutive Rab6 post-Golgi secretory pathway to exit from the cell body of primary neurons, independent of local calcium signaling. Some PRV particles colocalize with Rab6 in the proximal axon, but we did not detect colocalization/co-transport in the distal axon. Thus, the specific secretory mechanisms used for viral egress from axons remains unclear. To address the role of neuronal activity more generally, we used a compartmentalized neuron culture system to measure the egress and spread of PRV from axons, and pharmacological and optogenetics approaches to modulate neuronal activity. Using tetrodotoxin to silence neuronal activity, we observed no inhibition, and using potassium chloride or optogenetics to elevate neuronal activity, we also show no increase in virus spread from axons. We conclude that PRV egress from neurons uses constitutive secretory mechanisms: generally, activity-independent mechanisms in axons, and specifically, the constitutive Rab6 post-Golgi secretory pathway in cell bodies.

Differences and Similarities in Spontaneous Activity Between Animal Models of Cancer-Induced Pain and Neuropathic Pain

Background

Clinical data on cancer-induced pain (CIP) demonstrate widespread changes in sensory function. It is characterized in humans not only by stimulus-invoked pain, but also by spontaneous pain. In our previous studies in an animal model of CIP, we observed changes in intrinsic membrane properties and excitability of dorsal root ganglion (DRG) sensory neurons corresponding to mechanical allodynia and hyperalgesia, of which abnormal activities of Aβ-fiber sensory neurons are consistent in a rat model of peripheral neuropathic pain (NEP).

Objective

To investigate whether there are related peripheral neural mechanisms between the CIP and NEP models of spontaneous pain, we compared the electrophysiological properties of DRG sensory neurons at 2–3 weeks after CIP and NEP model induction.

Methods

CIP models were induced with metastasis tumour-1 rat breast cancer cells implanted into the distal epiphysis of the femur. NEP models were induced with a polyethylene cuff implanted around the sciatic nerve. Spontaneous pain in animals is measured by spontaneous foot lifting (SFL). After measurement of SFL, the animals were prepared for electrophysiological recordings of spontaneous activity (SA) in DRG neurons in vivo.

Results

Our data showed that SFL and SA occurred in both models. The proportion of SFL and SA of C-fiber sensory neurons in CIP was more significantly increased than in NEP models. There was no difference in duration of SFL and the rate of SA between the two models. The duration of SFL is related to the rate of SA in C-fiber in both models.

Conclusion

Thus, SFL may result from SA activity in C-fiber neurons in CIP and NEP rats. The differences and similarities in spontaneous pain between CIP and NEP rats is related to the proportion and rate of SA in C-fibers, respectively.

Viral cell-to-cell spread: Conventional and non-conventional ways

A critical step in the life cycle of a virus is spread to a new target cell, which generally involves the release of new viral particles from the infected cell which can then initiate infection in the next target cell. While cell-free viral particles released into the extracellular environment are necessary for long distance spread, there are disadvantages to this mechanism. These include the presence of immune system components, the low success rate of infection by single particles, and the relative fragility of viral particles in the environment. Several mechanisms of direct cell-to-cell spread have been reported for animal viruses which would avoid the issues associated with cell-free particles. A number of viruses can utilize several different mechanisms of direct cell-to-cell spread, but our understanding of the differential usage by these pathogens is modest. Although the mechanisms of cell-to-cell spread differ among viruses, there is a common exploitation of key pathways and components of the cellular cytoskeleton. Remarkably, some of the viral mechanisms of cell-to-cell spread are surprisingly similar to those used by bacteria. Here we summarize the current knowledge of the conventional and non-conventional mechanisms of viral spread, the common methods used to detect viral spread, and the impact that these mechanisms can have on viral pathogenesis.

Involvement of aberrant calcium signalling in herpetic neuralgia

Alpha-herpesviruses, herpes simplex viruses (HSV) and varicella zoster virus (VZV), are pathogens of the peripheral nervous system. After primary infection, these viruses establish latency within sensory ganglia, while retaining the ability to reactivate. Reactivation of VZV results in herpes zoster, a condition characterized by skin lesions that leads to post-herpetic neuralgia. Recurrent reactivations of HSV, which cause mucocutaneous lesions, may also result in neuralgia. During reactivation of alpha-herpesviruses, satellite glial cells (SGCs), which surround neurons in sensory ganglia, become infected with the replicating virus. SGCs are known to contribute to neuropathic pain in a variety of animal pain models. Here we investigated how infection of short-term cultures of mouse trigeminal ganglia with HSV-1 affects communication between SGCs and neurons, and how this altered communication may increase neuronal excitability, thus contributing to herpetic neuralgia. Mechanical stimulation of single neurons or SGCs resulted in intercellular calcium waves, which were larger in cultures infected with HSV-1. Two differences were observed between control and HSV-1 infected cultures that could account for this augmentation. Firstly, HSV-1 infection induced cell fusion among SGCs and neurons, which would facilitate the spread of calcium signals over farther distances. Secondly, using calcium imaging and intracellular electrical recordings, we found that neurons in the HSV-1 infected cultures exhibited augmented influx of calcium upon depolarization. These virally induced changes may not only cause more neurons in the sensory ganglia to fire action potentials, but may also increase neurotransmitter release at the presynaptic terminals in the spinal cord. They are therefore likely to be contributing factors to herpetic neuralgia.

Cell-fusion events induced by α-herpesviruses

ABSTRACT 

Enveloped viruses encode proteins that can induce cell fusion to allow spread of infection without exposure to immune surveillance. In this review, we discuss cell fusion events caused by neurotropic α-herpesviruses. Syncytia (large, multinucleated cells) are clinically indicative of α herpesvirus infections, and peripheral neuropathies are clinical hallmarks. We examine the viral and cellular factors required for cell fusion, as well as mutations which confer a more aggressive ‘hypersyncytial’ phenotype. Finally, we consider the causes of fusion events in infected neurons, and the implications for neuronal dysfunction and pathophysiology.

Animal models and pharmacology of herpetic and postherpetic pain

Varicella-zoster virus (VZV) causes varicella upon primary infection and subsequently becomes latent in the sensory ganglia. Reactivation of latent VZV in the sensory ganglion results in herpes zoster, which usually begins with pain and dysesthesia. Pain that persists long after healing of the rash is termed postherpetic neuralgia. VZV inoculation into rats induces mechanical allodynia and thermal hyperalgesia without causing herpes zoster. As with VZV, herpes simplex virus 1 (HSV1) is an alphaherpesvirus. HSV1 also becomes latent in the sensory ganglia after primary infection, and reactivation of latent HSV1 in the sensory ganglion results in herpes simplex. HSV1 inoculation into mice causes zoster-like skin lesions together with mechanical allodynia and mechanical hyperalgesia. A marked difference between the two rodent models is whether the herpes virus proliferates in the nervous system after inoculation. VZV-inoculated rats are useful for investigating mechanical allodynia induced by latent infection with herpes virus. HSV1-inoculated mice are useful for investigating mechanical allodynia induced by the proliferation of herpes virus in sensory neurons and for assessing the effects of acute herpetic pain on the incidence of postherpetic allodynia.

In vivo imaging of alphaherpesvirus infection reveals synchronized activity dependent on axonal sorting of viral proteins

A clinical hallmark of human alphaherpesvirus infections is peripheral pain or itching. Pseudorabies virus (PRV), a broad host range alphaherpesvirus, causes violent pruritus in many different animals, but the mechanism is unknown. Previous in vitro studies have shown that infected, cultured peripheral nervous system (PNS) neurons exhibited aberrant electrical activity after PRV infection due to the action of viral membrane fusion proteins, yet it is unclear if such activity occurs in infected PNS ganglia in living animals and if it correlates with disease symptoms. Using two-photon microscopy, we imaged autonomic ganglia in living mice infected with PRV strains expressing GCaMP3, a genetically encoded calcium indicator, and used the changes in calcium flux to monitor the activity of many neurons simultaneously with single-cell resolution. Infection with virulent PRV caused these PNS neurons to fire synchronously and cyclically in highly correlated patterns among infected neurons. This activity persisted even when we severed the presynaptic axons, showing that infection-induced firing is independent of input from presynaptic brainstem neurons. This activity was not observed after infections with an attenuated PRV recombinant used for circuit tracing or with PRV mutants lacking either viral glycoprotein B, required for membrane fusion, or viral membrane protein Us9, required for sorting virions and viral glycoproteins into axons. We propose that the viral fusion proteins produced by virulent PRV infection induce electrical coupling in unmyelinated axons in vivo. This action would then give rise to the synchronous and cyclical activity in the ganglia and contribute to the characteristic peripheral neuropathy.

HSV-1 promotes Ca2+ -mediated APP phosphorylation and Aβ accumulation in rat cortical neurons

Epidemiological and experimental findings suggest that chronic infection with Herpes simplex virus type 1 (HSV-1) may be a risk factor for Alzheimer's disease (AD), but the molecular mechanisms underlying this association have not been fully identified. We investigated the effects of HSV-1 on excitability and intracellular calcium signaling in rat cortical neurons and the impact of these effects on amyloid precursor protein (APP) processing and the production of amyloid-β peptide (Aβ). Membrane depolarization triggering firing rate increases was observed shortly after neurons were challenged with HSV-1 and was still evident 12 hours postinfection. These effects depended on persistent sodium current activation and potassium current inhibition. The virally induced hyperexcitability triggered intracellular Ca(2+) signals that significantly increased intraneuronal Ca(2+) levels. It also enhanced activity- and Ca(2+)-dependent APP phosphorylation and intracellular accumulation of Aβ42. These findings indicate that HSV-1 causes functional changes in cortical neurons that promote APP processing and Aβ production, and they are compatible with the co-factorial role for HSV-1 in the pathogenesis of AD suggested by previous findings.

Pseudorabies virus infection alters neuronal activity and connectivity in vitro

Alpha-herpesviruses, including human herpes simplex virus 1 & 2, varicella zoster virus and the swine pseudorabies virus (PRV), infect the peripheral nervous system of their hosts. Symptoms of infection often include itching, numbness, or pain indicative of altered neurological function. To determine if there is an in vitro electrophysiological correlate to these characteristic in vivo symptoms, we infected cultured rat sympathetic neurons with well-characterized strains of PRV known to produce virulent or attenuated symptoms in animals. Whole-cell patch clamp recordings were made at various times after infection. By 8 hours of infection with virulent PRV, action potential (AP) firing rates increased substantially and were accompanied by hyperpolarized resting membrane potentials and spikelet-like events. Coincident with the increase in AP firing rate, adjacent neurons exhibited coupled firing events, first with AP-spikelets and later with near identical resting membrane potentials and AP firing. Small fusion pores between adjacent cell bodies formed early after infection as demonstrated by transfer of the low molecular weight dye, Lucifer Yellow. Later, larger pores formed as demonstrated by transfer of high molecular weight Texas red-dextran conjugates between infected cells. Further evidence for viral-induced fusion pores was obtained by infecting neurons with a viral mutant defective for glycoprotein B, a component of the viral membrane fusion complex. These infected neurons were essentially identical to mock infected neurons: no increased AP firing, no spikelet-like events, and no electrical or dye transfer. Infection with PRV Bartha, an attenuated circuit-tracing strain delayed, but did not eliminate the increased neuronal activity and coupling events. We suggest that formation of fusion pores between infected neurons results in electrical coupling and elevated firing rates, and that these processes may contribute to the altered neural function seen in PRV-infected animals.

Neuronal activity regulates viral replication of herpes simplex virus type 1 in the nervous system

Herpes simplex virus types 1 and 2 (HSV-1, -2) infect and also establish latency in neurons. In the present study, the authors investigated the influence of neuronal activity on the replication of HSV-1. The results showed that the sodium channel blocker tetrodotoxin (TTX) and the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) could significantly increase viral replication in primary neuronal cultures, by two- to fourfold. In contrast, KCl reduced viral production by at least 80% in the same cultures. Inhibitors of GABA(A) receptors completely abolished the effects of GABA. Intravitreously injected TTX in a mouse corneal scarification model enhanced the viral titers > 10-fold in both the trigeminal ganglia and the brain. At 2 h post infection, both TTX and GABA significantly up-regulated the levels of transcription for the viral immediate early (IE) genes ICP0, ICP4, and ICP27, as revealed by real time PCR. These results indicate that the neuronal excitation status may dictate the efficiency of HSV-1 viral replication, probably by regulating the levels of viral IE gene expression. These are the first findings connecting neuronal activity to the molecular mechanisms of HSV replication in the nervous system, which may significantly influence our view of herpesvirus infection and latency.

Synaptic and morphologic properties in vitro of premotor rat nucleus tractus solitarius neurons labeled transneuronally from the stomach

Neurons in the rat nucleus tractus solitarius (NTS) possess morphologic characteristics that have been correlated with the type of synaptic information they receive. These features have been described for viscerosensory neurons but not for premotor NTS neurons. The morphologic and synaptic features of neurons in the rat caudal NTS were assessed using whole-cell patch-clamp recordings and biocytin labeling in brainstem slices. Gastric-related premotor NTS neurons were identified for recording after inoculation of the stomach wall with a transneuronal retrograde viral label that reports enhanced green fluorescent protein. Three morphologic groups of NTS neurons were identified based on quantitative aspects of soma area and proximal dendritic arborization, measures that were consistent across slice recordings. The most common type of cell (group I) had relatively small somata and one to three sparsely branching dendrites, whereas the other groups had larger somata and more than three dendrites, which branched predominantly close to (group II) or distant from (group III) the soma. Voltage-clamp recordings revealed spontaneous excitatory and inhibitory postsynaptic currents in all neurons, regardless of morphology. Gastric-related premotor NTS neurons composed two of the three morphologic types (i.e., groups I and II). Compared with unlabeled neurons, these cells were less likely to receive constant-latency synaptic input from the tractus solitarius. These results refute the hypothesis that general patterns of synaptic input to NTS neurons depend on morphology. Gastric premotor neurons comprise a subset of NTS morphologic types, the organization of the viscerosensory input to which has yet to be defined.

Are latent, immediate-early genes of herpes simplex virus-1 essential in causing trigeminal neuralgia?

The etiology and pathogenesis of major trigeminal neuralgia remain largely unknown, but are believed to result from an irritative lesion near the semilunar ganglion. We suggest that its primary cause is a single, active DNA sequence in the persistent but non-integrated genome of latent herpes simplex virus type 1 commonly observed in a few infected A-delta nerve fibers of the cheek. Facial pain occurs as a result of herpes virus reactivation and when supplies of neurotrophins controlling normal transport functions of axolemmal ion channels become depleted.

Spontaneous activity of cat spinal ganglion neurons in vivo

Recordings were made from L4-S1 spinal ganglion neurons of anesthetized cats while their associated dorsal root and the sciatic nerve were left intact, locally anesthetized, or locally anesthetized and sectioned. In all three experimental conditions spontaneous discharges were recorded. These discharges occurred in the absence of any electrical stimulation of the dorsal root or sciatic nerve, and were not due to peripheral exploration of receptive fields or sustained firing in joint or muscle afferents. The spontaneous discharges were relatively rhythmic, and their firing frequency ranged from 5 to 100 impulses per s. Interactions between spontaneous and electrically evoked discharges were observed that depended on the impulse's frequency of firing. High frequency discharges always abolished low frequency impulses regardless of whether these latter were spontaneous or evoked. Extra spikes and postspike events that followed impulses evoked by stimulation of the dorsal root or sciatic nerve were also recorded from some spinal ganglion neurons. These results suggest that spontaneous discharges may originate within the spinal ganglion itself, and that they can occur under normal circumstances.

Mutations in the UL53 gene of HSV-1 abolish virus neurovirulence to mice by the intracerebral route of infection

The cell fusion protein, the product of the UL53 gene, is responsible for intracerebral (IC) pathogenicity of HSV-1. Recombinant HSV-1 R15 is apathogenic to mice by the IC route of inoculation, while intratypic recombinants, in which the UL53 gene in R15 was replaced by an analogous sequence from the pathogenic strain R19, regained IC pathogenicity. The nucleotide sequence of the UL53 gene of HSV-1 strains R15 (apathogenic) and R19 (pathogenic) was determined and compared to that of other pathogenic strains. Four mutations were found which are thought to be responsible for the apathogenic phenotype of HSV-1 strain R15. Northern blot hybridization of RNA extracted from BSC-1 cells infected with several HSV-1 strains indicated that all of the virus strains tested expressed equal amounts of UL53 mRNA in infected cell cultures. Demonstration of the expression of UL53 mRNA in brains of mice infected with HSV-1 strains was made possible by the combined use of a rapid method for mRNA extraction (Oligo dT-linked magnetic beads) and a highly sensitive technique for detection of the existence of the UL53-specific mRNA (cDNA synthesis followed by PCR). It was shown that both pathogenic (KOS and P42) and apathogenic (R15) HSV-1 strains expressed the UL53 gene in brains of IC infected mice.

Paramyxovirus infections alter certain functional properties in cultured sensory neurons

Functional properties of cultured mouse dorsal root ganglion cells infected with paramyxoviruses have been studied using intracellular recording techniques. Mumps virus, which causes a persistent non-lytic infection, and Sendai virus, which causes an infection that leads to cell lysis after about a week were used. In the early phase of the infection (24-48 h) both viruses caused a reduction in the influx of calcium ions during the action potential, but did not alter resting membrane potential, action potential amplitude or input resistance. At later times functional properties became normal in mumps infected neurons. In contrast, Sendai virus infected neurons showed a reduction of action potential amplitude and input resistance at 48-72 h after infection, and finally there was also a reduction of membrane potential before the cells disintegrated. These results show that different paramyxovirus infections may cause different types of alterations in the functional properties of neurons. The reduced calcium influx resulting from mumps infection suggests that a non-lytic viral infection may have selective effects on important regulators of neuronal functions. Moreover, a lethal viral infection (Sendai) may influence specific membrane properties, such as calcium channel activation, several days prior to general structural and functional degeneration.

Membrane transport and disease

Four situations in which membrane transport is altered by disease are discussed: (a) non-specific leaks induced by poreforming agents; (b) glucose transport and cellular stress; (c) Ca2+-ATPase and hypertension; (d) Na+ channels and HSV infection.

Bradykinin enhances excitability in cultured rat sensory neurones by a GTP-dependent mechanism

The excitatory action of bradykinin (Bk; 0.1-1.0 microM) on cultured rat dorsal root ganglion neurones (DRGs) was studied using the whole cell clamp technique. In a subpopulation of DRGs, a 1 s depolarising voltage pulse from -70 to +20 mV evoked more than one inward current. In these neurones, local application of Bk increased the inward current frequency from 7.0 +/- 0.7 s-1 to 14.9 +/- 1.0 s-1 (mean +/- S.E.M., n = 53). Intracellular application of the GTP analogue, guanosine 5'O-3-thiotriphosphate (GTP gamma S) mimicked this excitatory action of Bk: the frequency of inward currents increased from 5.0 +/- 0.8 s-1, 30 s after the start of recording to 6.9 +/- 1.1 s-1 at 5 min to a maximum of 18.5 +/- 2.2 s-1 at 15 min (n = 16). In control cells, the frequency decreased from 4.6 +/- 0.8 s-1 to 2.5 +/- 0.5 s-1 at 5 min (n = 12). Bk also increased excitability in 4/11 Herpes Simplex Virus I (HSV-I)-infected DRGs. Thus, we demonstrate an excitatory action of Bk in DRGs, which may involve G-protein activation.

Pathogenesis of post-herpetic neuralgia

A model of post-herpetic neuralgia is put forward based on the physiological effects of herpes simplex type-1 virus infection of rat dorsal root ganglion neurons in tissue culture. This virus causes these normally electrically silent neurons to produce spontaneous action potentials. Furthermore, pairs of neurons tend to have synchronised discharges. This coupling is not synaptically mediated, but due to presumed electrical junctions. It is proposed that post-herpetic neuralgia is caused by abnormal impulses arising in dorsal root ganglion neurons as a direct result of viral infection.

Selective block of inward but not outward rectification in rat sensory neurones infected with herpes simplex virus

Neurones from the dorsal root ganglia of neonatal rat pups were grown in tissue culture and voltage clamped using patch electrodes for whole cell recording. The electrodes were filled with a KMeSO4 solution containing 1.1 mM-EGTA. 15-24 h prior to recording cultures were infected with one of three strains of herpes simplex virus type 1 previously characterized on the basis of a cell fusion assay in cultures of the MRC-5 and BHK-21 cell lines. Infection of sensory neurones with either of the two non-syncitial strains of virus caused a nearly complete loss of the hyperpolarization-activated inward rectifying conductance normally present in these neurones, but did not appear to affect depolarization-activated, calcium-dependent outward currents or voltage-dependent delayed outward rectification. The loss of inward rectification appeared to result from inactivation of the underlying conductance mechanism, rather than from a shift in the gating properties of the conductance mechanism to unphysiological values. In contrast to these results inward rectification was present in sensory neurones infected with another, syncitial, strain of herpes simplex virus. However, quantitative analysis suggested a reduced availability compared to that recorded in uninfected neurones.