Exploiting circuit-specific spread of pseudorabies virus in the central nervous system: insights to pathogenesis and circuit tracers

An atlas of brain-bone sympathetic neural circuits in mice

There is clear evidence that the sympathetic nervous system (SNS) mediates bone metabolism. Histological studies show abundant SNS innervation of the periosteum and bone marrow-these nerves consist of noradrenergic fibers that immunostain for tyrosine hydroxylase, dopamine beta-hydroxylase, or neuropeptide Y. Nonetheless, the brain sites that send efferent SNS outflow to the bone have not yet been characterized. Using pseudorabies (PRV) viral transneuronal tracing, we report, for the first time, the identification of central SNS outflow sites that innervate bone. We find that the central SNS outflow to bone originates from 87 brain nuclei, sub-nuclei, and regions of six brain divisions, namely the midbrain and pons, hypothalamus, hindbrain medulla, forebrain, cerebral cortex, and thalamus. We also find that certain sites, such as the raphe magnus (RMg) of the medulla and periaqueductal gray (PAG) of the midbrain, display greater degrees of PRV152 infection, suggesting that there is considerable site-specific variation in the levels of central SNS outflow to the bone. This comprehensive compendium illustrating the central coding and control of SNS efferent signals to bone should allow for a greater understanding of the neural regulation of bone metabolism, and importantly and of clinical relevance, mechanisms for central bone pain.

Unraveling the Neural Circuits: Techniques, Opportunities and Challenges in Epilepsy Research

Epilepsy, a prevalent neurological disorder characterized by high morbidity, frequent recurrence, and potential drug resistance, profoundly affects millions of people globally. Understanding the microscopic mechanisms underlying seizures is crucial for effective epilepsy treatment, and a thorough understanding of the intricate neural circuits underlying epilepsy is vital for the development of targeted therapies and the enhancement of clinical outcomes. This review begins with an exploration of the historical evolution of techniques used in studying neural circuits related to epilepsy. It then provides an extensive overview of diverse techniques employed in this domain, discussing their fundamental principles, strengths, limitations, as well as their application. Additionally, the synthesis of multiple techniques to unveil the complexity of neural circuits is summarized. Finally, this review also presents targeted drug therapies associated with epileptic neural circuits. By providing a critical assessment of methodologies used in the study of epileptic neural circuits, this review seeks to enhance the understanding of these techniques, stimulate innovative approaches for unraveling epilepsy's complexities, and ultimately facilitate improved treatment and clinical translation for epilepsy.

An Atlas of Brain-Bone Sympathetic Neural Circuits

There is clear evidence that the sympathetic nervous system (SNS) mediates bone metabolism. Histological studies show abundant SNS innervation of the periosteum and bone marrow--these nerves consist of noradrenergic fibers that immunostain for tyrosine hydroxylase, dopamine beta hydroxylase, or neuropeptide Y. Nonetheless, the brain sites that send efferent SNS outflow to bone have not yet been characterized. Using pseudorabies (PRV) viral transneuronal tracing, we report, for the first time, the identification of central SNS outflow sites that innervate bone. We find that the central SNS outflow to bone originates from 87 brain nuclei, sub-nuclei and regions of six brain divisions, namely the midbrain and pons, hypothalamus, hindbrain medulla, forebrain, cerebral cortex, and thalamus. We also find that certain sites, such as the raphe magnus (RMg) of the medulla and periaqueductal gray (PAG) of the midbrain, display greater degrees of PRV152 infection, suggesting that there is considerable site-specific variation in the levels of central SNS outflow to bone. This comprehensive compendium illustrating the central coding and control of SNS efferent signals to bone should allow for a greater understanding of the neural regulation of bone metabolism, and importantly and of clinical relevance, mechanisms for central bone pain.

Directed stepwise tracing of polysynaptic neuronal circuits with replication-deficient pseudorabies virus

Brain functions are accomplished by polysynaptic circuits formed by neurons wired together through multiple orders of synaptic connections. Polysynaptic connectivity has been difficult to examine due to a lack of methods of continuously tracing the pathways in a controlled manner. Here, we demonstrate directed, stepwise retrograde polysynaptic tracing by inducible reconstitution of replication-deficient trans-neuronal pseudorabies virus (PRVΔIE) in the brain. Furthermore, PRVΔIE replication can be temporally restricted to minimize its neurotoxicity. With this tool, we delineate a wiring diagram between the hippocampus and striatum-two major brain systems for learning, memory, and navigation-that consists of projections from specific hippocampal domains to specific striatal areas via distinct intermediate brain regions. Therefore, this inducible PRVΔIE system provides a tool for dissecting polysynaptic circuits underlying complex brain functions.

Live Triple Gene-Deleted Pseudorabies Virus-Vectored Subunit PCV2b and CSFV Vaccine Undergoes an Abortive Replication Cycle in the TG Neurons following Latency Reactivation

Like other alpha herpesviruses, pseudorabies virus (PRV) establishes lifelong latency in trigeminal ganglionic (TG) neurons. Upon stress, the latent viruses in the TG neurons reactivate and are transported anterograde from the neuron cell bodies to the nerve endings in the nasal mucosa, where they replicate and are discharged in the nasal and oral secretions. Consequently, the virus is transmitted to other naïve animals. This cycle of latency and reactivation continues until the animal dies or is slaughtered. We have constructed a PRV triple mutant virus (PRVtmv) and used it as a live subunit vaccine vector against porcine circovirus 2b (PCV2b) and classical swine fever virus (CSFV) (PRVtmv+). We compared the latency reactivation properties of PRVtmv+ with its parent wild-type (wt) Becker strain following intranasal infection. The results showed that PRV wt and PRVtmv+ established latency in the TG neurons. Based on nasal virus shedding, immediate early (infected cell protein 0; ICP0) and late genes, MCP (major capsid protein) and gC (glycoprotein C) transcriptions, and viral DNA copy numbers in the TGs of latently infected and dexamethasone (Dex)-treated pigs, both PRV wt and PRVtmv+ reactivated from latency. We noticed that PRV wt virus replicated productively in the terminally differentiated, postmitotic TG neurons, but PRVtmv+ failed to replicate and, therefore, there was no virus production in the TG. In addition, we found that only the PRV wt virus was shed in the nasal secretions following the Dex-induced reactivation. Our results demonstrated that the PRVtmv+ is safe as a live viral subunit vaccine vector without the possibility of productive replication in the TG upon reactivation from latency and without subsequent nasal virus shedding. This property of PRVtmv+ precludes the possibility of vaccine virus circulation in pigs and the risk of reversion to virulence.

Neuropilin-1 Facilitates Pseudorabies Virus Replication and Viral Glycoprotein B Promotes Its Degradation in a Furin-Dependent Manner

Pseudorabies virus (PRV), which is extremely infectious and can infect numerous mammals, has a risk of spillover into humans. Virus-host interactions determine viral entry and spreading. Here, we showed that neuropilin-1 (NRP1) significantly potentiates PRV infection. Mechanistically, NRP1 promoted PRV attachment and entry, and enhanced cell-to-cell fusion mediated by viral glycoprotein B (gB), gD, gH, and gL. Furthermore, through in vitro coimmunoprecipitation (Co-IP) and bimolecular fluorescence complementation (BiFC) assays, NRP1 was found to physically interact with gB, gD, and gH, and these interactions were C-end Rule (CendR) motif independent, in contrast to currently known viruses. Remarkably, we illustrated that the viral protein gB promotes NRP1 degradation via a lysosome-dependent pathway. We further demonstrate that gB promotes NRP1 degradation in a furin-cleavage-dependent manner. Interestingly, in this study, we generated gB furin cleavage site (FCS)-knockout PRV (Δfurin PRV) and evaluated its pathogenesis; in vivo, we found that Δfurin PRV virulence was significantly attenuated in mice. Together, our findings demonstrated that NRP1 is an important host factor for PRV and that NRP1 may be a potential target for antiviral intervention. IMPORTANCE Recent studies have shown accelerated PRV cross-species spillover and that PRV poses a potential threat to humans. PRV infection in humans always manifests as a high fever, tonic-clonic seizures, and encephalitis. Therefore, understanding the interaction between PRV and host factors may contribute to the development of new antiviral strategies against PRV. NRP1 has been demonstrated to be a receptor for several viruses that harbor CendR, including SARS-CoV-2. However, the relationships between NRP1 and PRV are poorly understood. Here, we found that NRP1 significantly potentiated PRV infection by promoting PRV attachment and enhanced cell-to-cell fusion. For the first time, we demonstrated that gB promotes NRP1 degradation via a lysosome-dependent pathway. Last, in vivo, Δfurin PRV virulence was significantly attenuated in mice. Therefore, NRP1 is an important host factor for PRV, and NRP1 may be a potential target for antiviral drug development.

Advancements in the Quest to Map, Monitor, and Manipulate Neural Circuitry

Neural circuits and the cells that comprise them represent the functional units of the brain. Circuits relay and process sensory information, maintain homeostasis, drive behaviors, and facilitate cognitive functions such as learning and memory. Creating a functionally-precise map of the mammalian brain requires anatomically tracing neural circuits, monitoring their activity patterns, and manipulating their activity to infer function. Advancements in cell-type-specific genetic tools allow interrogation of neural circuits with increased precision. This review provides a broad overview of recombination-based and activity-driven genetic targeting approaches, contemporary viral tracing strategies, electrophysiological recording methods, newly developed calcium, and voltage indicators, and neurotransmitter/neuropeptide biosensors currently being used to investigate circuit architecture and function. Finally, it discusses methods for acute or chronic manipulation of neural activity, including genetically-targeted cellular ablation, optogenetics, chemogenetics, and over-expression of ion channels. With this ever-evolving genetic toolbox, scientists are continuing to probe neural circuits with increasing resolution, elucidating the structure and function of the incredibly complex mammalian brain.

Strategies for Targeting Neural Circuits: How to Manipulate Neurons Using Virus Vehicles

Viral strategies are the leading methods for mapping neural circuits. Viral vehicles combined with genetic tools provide the possibility to visualize entire functional neural networks and monitor and manipulate neural circuit functions by high-resolution cell type- and projection-specific targeting. Optogenetics and chemogenetics drive brain research forward by exploring causal relationships among different brain regions. Viral strategies offer a fresh perspective for the analysis of the structure-function relationship of the neural circuitry. In this review, we summarize current and emerging viral strategies for targeting neural circuits and focus on adeno-associated virus (AAV) vectors.

Anterograde transneuronal tracing and genetic control with engineered yellow fever vaccine YFV-17D

Transneuronal viruses are powerful tools for tracing neuronal circuits or delivering genes to specific neurons in the brain. While there are multiple retrograde viruses, few anterograde viruses are available. Further, available anterograde viruses often have limitations such as retrograde transport, high neuronal toxicity or weak signals. We developed an anterograde viral system based on a live attenuated vaccine for yellow fever-YFV-17D. Replication- or packaging-deficient mutants of YFV-17D can be reconstituted in the brain, leading to efficient synapse-specific and anterograde-only transneuronal spreading, which can be controlled to achieve either monosynaptic or polysynaptic tracing. Moreover, inducible transient replication of YFV-17D mutant is sufficient to induce permanent transneuronal genetic modifications without causing neuronal toxicity. The engineered YFV-17D systems can be used to express fluorescent markers, sensors or effectors in downstream neurons, thus providing versatile tools for mapping and functionally controlling neuronal circuits.

The axonal sorting activity of pseudorabies virus Us9 protein depends on the state of neuronal maturation

Alpha-herpesviruses establish a life-long infection in the nervous system of the affected host; while this infection is restricted to peripheral neurons in a healthy host, the reactivated virus can spread within the neuronal circuitry, such as to the brain, in compromised individuals and lead to adverse health outcomes. Pseudorabies virus (PRV), an alpha-herpesvirus, requires the viral protein Us9 to sort virus particles into axons and facilitate neuronal spread. Us9 sorts virus particles by mediating the interaction of virus particles with neuronal transport machinery. Here, we report that Us9-mediated regulation of axonal sorting also depends on the state of neuronal maturation. Specifically, the development of dendrites and axons is accompanied with proteomic changes that influence neuronal processes. Immature superior cervical ganglionic neurons (SCGs) have rudimentary neurites that lack markers of mature axons. Immature SCGs can be infected by PRV, but they show markedly reduced Us9-dependent regulation of sorting, and increased Us9-independent transport of particles into neurites. Mature SCGs have relatively higher abundances of proteins characteristic of vesicle-transport machinery. We also identify Us9-associated neuronal proteins that can contribute to axonal sorting and subsequent anterograde spread of virus particles in axons. We show that SMPD4/nsMase3, a sphingomyelinase abundant in lipid-rafts, associates with Us9 and is a negative regulator of PRV sorting into axons and neuronal spread, a potential antiviral function.

Curcumin protects rat hippocampal neurons against pseudorabies virus by regulating the BDNF/TrkB pathway

Pseudorabies virus (PRV) infection can elicit nervous system disorders. Curcumin has been reported to have neuroprotective effects. However, whether curcumin can protect neurons against PRV infection and the underlying mechanisms remain unclear. In the present study, for the first time, the protective effects of curcumin against PRV-induced oxidative stress, apoptosis, and mitochondrial dysfunction in rat hippocampal neurons and the brain-derived neurotrophic factor (BDNF)/tropomyosin-related kinase B (TrkB) pathway were investigated. Results indicated that PRV with a titer of 3.06 × 10⁶ TCID50 (50% tissue culture infective dose) induced oxidative damage of hippocampal neurons 2 h post-infection and that 10 μM curcumin improved the viability of PRV-infected hippocampal neurons. Blocking the BDNF/TrkB pathway reversed the neuroprotective effects of curcumin, which were imparted by decreasing the PRV-induced upregulation of nitric oxide synthase expression, repressing the PRV-activated mitochondrial apoptotic pathway, and mitochondrial dysfunction. To conclude, curcumin exhibited a neuroprotective role against PRV infection by upregulating the BDNF/TrkB pathway. This study provides insight into the anti-PRV neuroprotective application of curcumin and the underlying mechanism in the prophylaxis and treatment of neurological disorders caused by PRV infection.

Mapping of spinal interneurons involved in regulation of the lower urinary tract in juvenile male rats

Coordination between the urinary bladder (BL) and external urethral sphincter (EUS) is necessary for storage and elimination of urine. In rats interneuronal circuits at two levels of the spinal cord (i.e., L6-S1 and L3-L4) play an important role in this coordination. In the present experiments retrograde trans-synaptic transport of pseudorabies virus (PRV) encoding fluorescent markers (GFP and RFP) was used to trace these circuits. To examine the relative localization of EUS-related and BL-related interneuronal populations we injected PRV-GFP into the EUS and PRV-RFP into the BL wall. The PRV infected populations of spinal interneurons were localized primarily in the dorsal commissure (DCM) of L6/S1 and in a hypothesized lumbar spinal coordinating center (LSCC) in L3/L4 above and lateral to central canal (CC). At both sites colocalization of markers occurred in a substantial number of labeled interneurons indicating concomitant involvement of these double-labelled neurons in the EUS- and BL-circuits and suggesting their role in EUS-BL coordination. Intense GFP or RFP fluorescent was detected in a subpopulation of cells at both sites suggesting that they were infected earlier and therefore likely to represent first order, primary interneurons that directly synapse with output neurons. Larger numbers of weakly fluorescent neurons that likely represent second order interneurons were also identified. Within the population of EUS-related first order interneurons only 3-8 % exhibited positive immunoreaction for an early transcription factor Pax2 specific to GABAergic and glycinergic inhibitory neurons suggesting that the majority of interneurons in DCM and LSCC projecting directly to the EUS motoneurons are excitatory.

Viral Vectors for Neural Circuit Mapping and Recent Advances in Trans-synaptic Anterograde Tracers

Viral tracers are important tools for neuroanatomical mapping and genetic payload delivery. Genetically modified viruses allow for cell-type-specific targeting and overcome many limitations of non-viral tracers. Here, we summarize the viruses that have been developed for neural circuit mapping, and we provide a primer on currently applied anterograde and retrograde viral tracers with practical guidance on experimental uses. We also discuss and highlight key technical and conceptual considerations for developing new safer and more effective anterograde trans-synaptic viral vectors for neural circuit analysis in multiple species.

Neuroendocrine Regulation of Energy Metabolism Involving Different Types of Adipose Tissues

Despite tremendous research efforts to identify regulatory factors that control energy metabolism, the prevalence of obesity has been continuously rising, with nearly 40% of US adults being obese. Interactions between secretory factors from adipose tissues and the nervous system innervating adipose tissues play key roles in maintaining energy metabolism and promoting survival in response to metabolic challenges. It is currently accepted that there are three types of adipose tissues, white (WAT), brown (BAT), and beige (BeAT), all of which play essential roles in maintaining energy homeostasis. WAT mainly stores energy under positive energy balance, while it releases fuels under negative energy balance. Thermogenic BAT and BeAT dissipate energy as heat under cold exposure to maintain body temperature. Adipose tissues require neural and endocrine communication with the brain. A number of WAT adipokines and BAT batokines interact with the neural circuits extending from the brain to cooperatively regulate whole-body lipid metabolism and energy homeostasis. We review neuroanatomical, histological, genetic, and pharmacological studies in neuroendocrine regulation of adipose function, including lipid storage and mobilization of WAT, non-shivering thermogenesis of BAT, and browning of BeAT. Recent whole-tissue imaging and transcriptome analysis of differential gene expression in WAT and BAT yield promising findings to better understand the interaction between secretory factors and neural circuits, which represents a novel opportunity to tackle obesity.

Gene Expression Profiling with Cre-Conditional Pseudorabies Virus Reveals a Subset of Midbrain Neurons That Participate in Reward Circuitry

The mesolimbic dopamine pathway receives inputs from numerous regions of the brain as part of a neural system that detects rewarding stimuli and coordinates a behavioral response. The capacity to simultaneously map and molecularly define the components of this complex multisynaptic circuit would thus advance our understanding of the determinants of motivated behavior. To accomplish this, we have constructed pseudorabies virus (PRV) strains in which viral propagation and fluorophore expression are activated only after exposure to Cre recombinase. Once activated in Cre-expressing neurons, the virus serially labels chains of presynaptic neurons. Dual injection of GFP and mCherry tracing viruses simultaneously illuminates nigrostriatal and mesolimbic circuitry and shows no overlap, demonstrating that PRV transmission is confined to synaptically connected neurons. To molecularly profile mesolimbic dopamine neurons and their presynaptic inputs, we injected Cre-conditional GFP virus into the NAc of (anti-GFP) nanobody-L10 transgenic mice and immunoprecipitated translating ribosomes from neurons infected after retrograde tracing. Analysis of purified RNA revealed an enrichment of transcripts expressed in neurons of the dorsal raphe nuclei and lateral hypothalamus that project to the mesolimbic dopamine circuit. These studies identify important inputs to the mesolimbic dopamine pathway and further show that PRV circuit-directed translating ribosome affinity purification can be broadly applied to identify molecularly defined neurons comprising complex, multisynaptic circuits.SIGNIFICANCE STATEMENT The mesolimbic dopamine circuit integrates signals from key brain regions to detect and respond to rewarding stimuli. To further define this complex multisynaptic circuit, we constructed a panel of Cre recombinase-activated pseudorabies viruses (PRVs) that enabled retrograde tracing of neural inputs that terminate on Cre-expressing neurons. Using these viruses and Retro-TRAP (translating ribosome affinity purification), a previously reported molecular profiling method, we developed a novel technique that provides anatomic as well as molecular information about the neural components of polysynaptic circuits. We refer to this new method as PRV-Circuit-TRAP (PRV circuit-directed TRAP). Using it, we have identified major projections to the mesolimbic dopamine circuit from the lateral hypothalamus and dorsal raphe nucleus and defined a discrete subset of transcripts expressed in these projecting neurons, which will allow further characterization of this important pathway. Moreover, the method we report is general and can be applied to the study of other neural circuits.

Contribution of adaptive thermogenesis to the hypothalamic regulation of energy balance

Obesity and its related disorders are among the most pervasive diseases in contemporary societies, and there is an urgent need for new therapies and preventive approaches. Given (i) our poor social capacity to correct unhealthy habits, and (ii) our evolutionarily genetic predisposition to store excess energy as fat, the current environment of caloric surplus makes the treatment of obesity extremely difficult. During the last few decades, an increasing number of methodological approaches have increased our knowledge of the neuroanatomical basis of the control of energy balance. Compelling evidence underlines the role of the hypothalamus as a homeostatic integrator of metabolic information and its ability to adjust energy balance. A greater understanding of the neural basis of the hypothalamic regulation of energy balance might indeed pave the way for new therapeutic targets. In this regard, it has been shown that several important peripheral signals, such as leptin, thyroid hormones, oestrogens and bone morphogenetic protein 8B, converge on common energy sensors, such as AMP-activated protein kinase to modulate sympathetic tone on brown adipose tissue. This knowledge may open new ways to counteract the chronic imbalance underlying obesity. Here, we review the current state of the art on the role of hypothalamus in the regulation of energy balance with particular focus on thermogenesis.

Hypothalamic-autonomic control of energy homeostasis

Regulation of energy homeostasis is tightly controlled by the central nervous system (CNS). Several key areas such as the hypothalamus and brainstem receive and integrate signals conveying energy status from the periphery, such as leptin, thyroid hormones, and insulin, ultimately leading to modulation of food intake, energy expenditure (EE), and peripheral metabolism. The autonomic nervous system (ANS) plays a key role in the response to such signals, innervating peripheral metabolic tissues, including brown and white adipose tissue (BAT and WAT), liver, pancreas, and skeletal muscle. The ANS consists of two parts, the sympathetic and parasympathetic nervous systems (SNS and PSNS). The SNS regulates BAT thermogenesis and EE, controlled by central areas such as the preoptic area (POA) and the ventromedial, dorsomedial, and arcuate hypothalamic nuclei (VMH, DMH, and ARC). The SNS also regulates lipid metabolism in WAT, controlled by the lateral hypothalamic area (LHA), VMH, and ARC. Control of hepatic glucose production and pancreatic insulin secretion also involves the LHA, VMH, and ARC as well as the dorsal vagal complex (DVC), via splanchnic sympathetic and the vagal parasympathetic nerves. Muscle glucose uptake is also controlled by the SNS via hypothalamic nuclei such as the VMH. There is recent evidence of novel pathways connecting the CNS and ANS. These include the hypothalamic AMP-activated protein kinase-SNS-BAT axis which has been demonstrated to be a key modulator of thermogenesis. In this review, we summarize current knowledge of the role of the ANS in the modulation of energy balance.

Brown adipose tissue has sympathetic-sensory feedback circuits

Brown adipose tissue (BAT) is an important source of thermogenesis which is nearly exclusively dependent on its sympathetic nervous system (SNS) innervation. We previously demonstrated the SNS outflow from brain to BAT using the retrograde SNS-specific transneuronal viral tract tracer, pseudorabies virus (PRV152) and demonstrated the sensory system (SS) inflow from BAT to brain using the anterograde SS-specific transneuronal viral tract tracer, H129 strain of herpes simplex virus-1. Several brain areas were part of both the SNS outflow to, and receive SS inflow from, interscapular BAT (IBAT) in these separate studies suggesting SNS-SS feedback loops. Therefore, we tested whether individual neurons participated in SNS-SS crosstalk by injecting both PRV152 and H129 into IBAT of Siberian hamsters. To define which dorsal root ganglia (DRG) are activated by BAT SNS stimulation, indicated by c-Fos immunoreactivity (IR), we prelabeled IBAT DRG innervating neurons by injecting the retrograde tracer Fast Blue (FB) followed 1 week later by intra-BAT injections of the specific β3-adrenoceptor agonist CL316,243 in one pad and the vehicle in the contralateral pad. There were PRV152+H129 dually infected neurons across the neuroaxis with highest densities in the raphe pallidus nucleus, nucleus of the solitary tract, periaqueductal gray, hypothalamic paraventricular nucleus, and medial preoptic area, sites strongly implicated in the control of BAT thermogenesis. CL316,243 significantly increased IBAT temperature, afferent nerve activity, and c-Fos-IR in C2-C4 DRG neurons ipsilateral to the CL316,243 injections versus the contralateral side. The neuroanatomical reality of the SNS-SS feedback loops suggests coordinated and/or multiple redundant control of BAT thermogenesis.

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.

Neural innervation of white adipose tissue and the control of lipolysis

White adipose tissue (WAT) is innervated by the sympathetic nervous system (SNS) and its activation is necessary for lipolysis. WAT parasympathetic innervation is not supported. Fully-executed SNS-norepinephrine (NE)-mediated WAT lipolysis is dependent on β-adrenoceptor stimulation ultimately hinging on hormone sensitive lipase and perilipin A phosphorylation. WAT sympathetic drive is appropriately measured electrophysiologically and neurochemically (NE turnover) in non-human animals and this drive is fat pad-specific preventing generalizations among WAT depots and non-WAT organs. Leptin-triggered SNS-mediated lipolysis is weakly supported, whereas insulin or adenosine inhibition of SNS/NE-mediated lipolysis is strongly supported. In addition to lipolysis control, increases or decreases in WAT SNS drive/NE inhibit and stimulate white adipocyte proliferation, respectively. WAT sensory nerves are of spinal-origin and sensitive to local leptin and increases in sympathetic drive, the latter implicating lipolysis. Transsynaptic viral tract tracers revealed WAT central sympathetic and sensory circuits including SNS-sensory feedback loops that may control lipolysis.

Connections of the superior colliculus to shoulder muscles of the rat: a dual tracing study

Previous investigations indicate that the superior colliculus (SC) is involved in the initiation and execution of forelimb movements. In the present study we investigated the tectofugal, in particular the tecto-reticulo-spinal projections to the shoulder and arm muscles in the rat. We simultaneously retrogradely labeled the premotor neurons in the brainstem by injection of the pseudorabies virus PrV Bartha 614 into the m. rhomboideus minor and m. acromiodeltoideus, and anterogradely visualized the tectofugal projections by intracollicular injection of the tracer FITC dextrane. Our results demonstrate that the connection of the SC to the skeletal muscles of the forelimb is at least trisynaptic. This was confirmed by long survival times after virus injections into the muscles (98-101 h) after which numerous neurons in the deep layers of the SC were labeled. Transsynaptically retrogradely labeled brainstem neurons connected disynaptically to the injected muscles with adjacent tectal terminals were predominantly located in the gigantocellular nuclear complex of the reticular formation. In addition, putative relay neurons were found in the caudal part of the pontine reticular nucleus. Both tectal projections to the nucleus gigantocellularis and the pontine reticular nucleus were bilateral but ipsilaterally biased. We suggest this projection to be involved in more global functions in motivated behavior like general arousal allowing fast voluntary motor activity.

Genetic strategies to investigate neuronal circuit properties using stem cell-derived neurons

The mammalian brain is anatomically and functionally complex, and prone to diverse forms of injury and neuropathology. Scientists have long strived to develop cell replacement therapies to repair damaged and diseased nervous tissue. However, this goal has remained unrealized for various reasons, including nascent knowledge of neuronal development, the inability to track and manipulate transplanted cells within complex neuronal networks, and host graft rejection. Recent advances in embryonic stem cell (ESC) and induced pluripotent stem cell (iPSC) technology, alongside novel genetic strategies to mark and manipulate stem cell-derived neurons, now provide unprecedented opportunities to investigate complex neuronal circuits in both healthy and diseased brains. Here, we review current technologies aimed at generating and manipulating neurons derived from ESCs and iPSCs toward investigation and manipulation of complex neuronal circuits, ultimately leading to the design and development of novel cell-based therapeutic approaches.

A wide extent of inter-strain diversity in virulent and vaccine strains of alphaherpesviruses

Alphaherpesviruses are widespread in the human population, and include herpes simplex virus 1 (HSV-1) and 2, and varicella zoster virus (VZV). These viral pathogens cause epithelial lesions, and then infect the nervous system to cause lifelong latency, reactivation, and spread. A related veterinary herpesvirus, pseudorabies (PRV), causes similar disease in livestock that result in significant economic losses. Vaccines developed for VZV and PRV serve as useful models for the development of an HSV-1 vaccine. We present full genome sequence comparisons of the PRV vaccine strain Bartha, and two virulent PRV isolates, Kaplan and Becker. These genome sequences were determined by high-throughput sequencing and assembly, and present new insights into the attenuation of a mammalian alphaherpesvirus vaccine strain. We find many previously unknown coding differences between PRV Bartha and the virulent strains, including changes to the fusion proteins gH and gB, and over forty other viral proteins. Inter-strain variation in PRV protein sequences is much closer to levels previously observed for HSV-1 than for the highly stable VZV proteome. Almost 20% of the PRV genome contains tandem short sequence repeats (SSRs), a class of nucleic acids motifs whose length-variation has been associated with changes in DNA binding site efficiency, transcriptional regulation, and protein interactions. We find SSRs throughout the herpesvirus family, and provide the first global characterization of SSRs in viruses, both within and between strains. We find SSR length variation between different isolates of PRV and HSV-1, which may provide a new mechanism for phenotypic variation between strains. Finally, we detected a small number of polymorphic bases within each plaque-purified PRV strain, and we characterize the effect of passage and plaque-purification on these polymorphisms. These data add to growing evidence that even plaque-purified stocks of stable DNA viruses exhibit limited sequence heterogeneity, which likely seeds future strain evolution.

Alphaherpesviruses such as herpes simplex virus (HSV) are ubiquitous in the human population. HSV causes oral and genital lesions, and has co-morbidities in acquisition and spread of human immunodeficiency virus (HIV). The lack of a vaccine for HSV hinders medical progress for both of these infections. A related veterinary alphaherpesvirus, pseudorabies virus (PRV), has long served as a model for HSV vaccine development, because of their similar pathogenesis, neuronal spread, and infectious cycle. We present here the first full genome characterization of a live PRV vaccine strain, Bartha, and reveal a spectrum of unique mutations that are absent from two divergent wild-type PRV strains. These mutations can now be examined individually for their contribution to vaccine strain attenuation and for potential use in HSV vaccine development. These inter-strain comparisons also revealed an abundance of short repetitive elements in the PRV genome, a pattern which is repeated in other herpesvirus genomes and even the unrelated Mimivirus. We provide the first global characterization of repeats in viruses, comparing both their presence and their variation among different viral strains and species. Repetitive elements such as these have been shown to serve as hotspots of variation between individuals or strains of other organisms, generating adaptations or even disease states through changes in length of DNA-binding sites, protein folding motifs, and other structural elements. These data suggest for the first time that similar mechanisms could be widely distributed in viral biology as well.

Anterograde or retrograde transsynaptic labeling of CNS neurons with vesicular stomatitis virus vectors

To understand how the nervous system processes information, a map of the connections among neurons would be of great benefit. Here we describe the use of vesicular stomatitis virus (VSV) for tracing neuronal connections in vivo. We made VSV vectors that used glycoprotein (G) genes from several other viruses. The G protein from lymphocytic choriomeningitis virus endowed VSV with the ability to spread transsynaptically, specifically in an anterograde direction, whereas the rabies virus glycoprotein gave a specifically retrograde transsynaptic pattern. The use of an avian G protein fusion allowed specific targeting of cells expressing an avian receptor, which allowed a demonstration of monosynaptic anterograde tracing from defined cells. Synaptic connectivity of pairs of virally labeled cells was demonstrated by using slice cultures and electrophysiology. In vivo infections of several areas in the mouse brain led to the predicted patterns of spread for anterograde or retrograde tracers.

Genetic approaches to reveal the connectivity of adult-born neurons

Much has been learned about the environmental and molecular factors that influence the division, migration, and programmed cell death of adult-born neurons in the mammalian brain. However, detailed knowledge of the mechanisms that govern the formation and maintenance of functional circuit connectivity via adult neurogenesis remains elusive. Recent advances in genetic technologies now afford the ability to precisely target discrete brain tissues, neuronal subtypes, and even single neurons for vital reporter expression and controlled activity manipulations. Here, I review current viral tracing methods, heterologous receptor expression systems, and optogenetic technologies that hold promise toward elucidating the wiring diagrams and circuit properties of adult-born neurons.

Bilateral bulbospinal projections to pudendal motoneuron circuitry after chronic spinal cord hemisection injury as revealed by transsynaptic tracing with pseudorabies virus

Complications of spinal cord injury in males include losing brainstem control of pudendal nerve-innervated perineal muscles involved in erection and ejaculation. We previously described, in adult male rats, a bulbospinal pathway originating in a discrete area within the medullary gigantocellularis (GiA/Gi), and lateral paragigantocellularis (LPGi) nuclei, which when electrically microstimulated unilaterally, produces a bilateral inhibition of pudendal motoneuron reflex circuitry after crossing to the contralateral spinal cord below T8. Microstimulation following a long-term lateral hemisection, however, revealed reflex inhibition from both sides of the medulla, suggesting the development or unmasking of an injury-induced bulbospinal pathway crossing the midline cranial to the spinal lesion. In the present study, we investigated this pathway anatomically using the transsynaptic neuronal tracer pseudorabies virus (PRV) injected unilaterally into the bulbospongiosus muscle in uninjured controls, and ipsilateral to a chronic (1-2 months) unilateral lesion of the lateral funiculus. At 4.75 days post-injection, PRV-labeled cells were found bilaterally in the GiA/Gi/LPGi with equal side-to-side labeling in uninjured controls, and with significantly greater labeling contralateral to the lesion/injection in lesioned animals. The finding of PRV-labeled neurons on both sides of the medulla after removing the mid-thoracic spinal pathway on one side provides anatomical evidence for the bilaterality in both the brainstem origin and the lumbosacral pudendal circuit termination of the spared lateral funicular bulbospinal pathway. This also suggests that this bilaterality may contribute to the quick functional recovery of bladder and sexual functions observed in animals and humans with lateral hemisection injury.

The use of PRV-Bartha to define premotor inputs to lumbar motoneurons in the neonatal spinal cord of the mouse

Background

The neonatal mouse has become a model system for studying the locomotor function of the lumbar spinal cord. However, information about the synaptic connectivity within the governing neural network remains scarce. A neurotropic pseudorabies virus (PRV) Bartha has been used to map neuronal connectivity in other parts of the nervous system, due to its ability to travel trans-neuronally. Its use in spinal circuits regulating locomotion has been limited and no study has defined the time course of labelling for neurons known to project monosynaptically to motoneurons.

Methodology/Principal Findings

Here we investigated the ability of PRV Bartha, expressing green and/or red fluorescence, to label spinal neurons projecting monosynaptically to motoneurons of two principal hindlimb muscles, the tibialis anterior (TA) and gastrocnemius (GC). As revealed by combined immunocytochemistry and confocal microscopy, 24–32 h after the viral muscle injection the label was restricted to the motoneuron pool while at 32–40 h the fluorescence was seen in interneurons throughout the medial and lateral ventral grey matter. Two classes of ipsilateral interneurons known to project monosynaptically to motoneurons (Renshaw cells and cells of origin of C-terminals) were consistently labeled at 40 h post-injection but also a group in the ventral grey matter contralaterally. Our results suggest that the labeling of last order interneurons occurred 8–12 h after motoneuron labeling and we presume this is the time taken by the virus to cross one synapse, to travel retrogradely and to replicate in the labeled cells.

Conclusions/Significance

The study establishes the time window for virally - labelling monosynaptic projections to lumbar motoneurons following viral injection into hindlimb muscles. Moreover, it provides a good foundation for intracellular targeting of the labeled neurons in future physiological studies and better understanding the functional organization of the lumbar neural networks.

Advanced tracing tools: functional neuronal expression of virally encoded fluorescent calcium indicator proteins

Pseudorabies virus (PrV) strains such as PrV-Bartha and its marker protein-expressing variants have been used in numerous studies as retrograde transneuronal tracing tools, defining the synaptic organization of mammalian neuronal circuits. However, the possibilities for functional examination of virus-infected neurons are limited to electrophysiological approaches or bulk loading strategies using calcium-sensitive dyes. Herein we report the generation and functional characterization of three PrV-Bartha-derived recombinant virus mutants that express different fluorescent calcium indicator proteins (FCIPs). All three generated virus recombinants are able to infect murine trigeminal neurons and express the corresponding FCIP (GCaMP2, camgaroo-2, or inverse pericam). Functionality of these virally expressed constructs was verified by using confocal Ca-imaging technologies. These FCIP-expressing virus recombinants provide a new tool for the functional analysis of whole circuits of synaptically connected neurons in vitro and in vivo.

Pregnancy-related changes in connections from the cervix to forebrain and hypothalamus in mice

The transneuronal tracer pseudorabies virus was used to test the hypothesis that connections from the cervix to the forebrain and hypothalamus are maintained with pregnancy. The virus was injected into the cervix of nonpregnant or pregnant mice, and, after 5 days, virus-labeled cells and fibers were found in specific forebrain regions and, most prominently, in portions of the hypothalamic paraventricular nucleus. With pregnancy, fewer neurons and fibers were evident in most brain regions compared to that in nonpregnant mice. In particular, little or no virus was found in the medial and ventral parvocellular subdivisions, anteroventral periventricular nucleus, or motor cortex in pregnant mice. By contrast, labeling of virus was sustained in the dorsal hypothalamus and suprachiasmatic nucleus in all groups. Based upon image analysis of digitized photomicrographs, the area with label in the rostral and medial parvocellular paraventricular nucleus and magnocellular subdivisions was significantly reduced in mice whose cervix was injected with virus during pregnancy than in nonpregnant mice. The findings indicate that connections from the cervix to brain regions that are involved in sensory input and integrative autonomic functions are reduced during pregnancy. The findings raise the possibility that remaining pathways from the cervix to the forebrain and hypothalamus may be important for control of pituitary neuroendocrine secretion, as well as for effector functions in the cervix as pregnancy nears term.

Electrophysiological characteristics of identified kidney-related neurons in adult rat spinal cord slices

Whole-cell patch-clamp recordings were made from kidney-related neurons in the intermediolateral cell column (IML) in horizontal slices of thoracolumbar spinal cord from adult rats. Kidney-related neurons were identified in vitro subsequent to inoculation of the kidney with a fluorescent, retrograde, transynaptic pseudorabies viral label (i.e., PRV-152). Kidney-related neurons detected in the IML expressed choline acetyltransferase, characteristic of spinal preganglionic motor neurons. Their mean resting potential was -51+/-4 mV and input resistance was 448+/-39 MOmega. Both spontaneous inhibitory and excitatory post-synaptic currents (i.e., sIPSCs and sEPSCs) were observed in all neurons. The mean frequency for sEPSCs (3.1+/-1 Hz) was approximately 2.5 times that for sIPSCs (1.4+/-0.3 Hz). Application of the glycine and GABA(A) receptor-linked Cl(-) channel blocker, picrotoxin (100 microM) blocked sIPSCs, while the ionotropic glutamate receptor antagonist, kynurenic acid (1 mM) blocked all sEPSCs, indicating they were mediated by GABA/glycine and glutamate receptors, respectively. Thus, using PRV-152 labeling allowed whole-cell patch-clamp recording of neurons in the adult spinal cord, which were kidney-related. Excitatory glutamatergic input dominated synaptic responses in these cells, the membrane characteristics of which resembled those of immature IML neurons. Combined PRV-152 pre-labeling and whole-cell patch-clamp recordings may allow more effective analysis of synaptic plasticity seen in adult models of injury or chronic disease.

Sequence variability in clinical and laboratory isolates of herpes simplex virus 1 reveals new mutations

Herpes simplex virus 1 (HSV-1) is a well-adapted human pathogen that can invade the peripheral nervous system and persist there as a lifelong latent infection. Despite their ubiquity, only one natural isolate of HSV-1 (strain 17) has been sequenced. Using Illumina high-throughput sequencing of viral DNA, we obtained the genome sequences of both a laboratory strain (F) and a low-passage clinical isolate (H129). These data demonstrated the extent of interstrain variation across the entire genome of HSV-1 in both coding and noncoding regions. We found many amino acid differences distributed across the proteome of the new strain F sequence and the previously known strain 17, demonstrating the spectrum of variability among wild-type HSV-1 proteins. The clinical isolate, strain H129, displays a unique anterograde spread phenotype for which the causal mutations were completely unknown. We have defined the sequence differences in H129 and propose a number of potentially causal genes, including the neurovirulence protein ICP34.5 (RL1). Further studies will be required to demonstrate which change(s) is sufficient to recapitulate the spread defect of strain H129. Unexpectedly, these data also revealed a frameshift mutation in the UL13 kinase in our strain F isolate, demonstrating how deep genome sequencing can reveal the full complement of background mutations in any given strain, particularly those passaged or plaque purified in a laboratory setting. These data increase our knowledge of sequence variation in large DNA viruses and demonstrate the potential of deep sequencing to yield insight into DNA genome evolution and the variation among different pathogen isolates.

Retrograde tracing of spinal cord connections to the cervix with pregnancy in mice

In contrast to the uterus, the cervix is well innervated during pregnancy and the density of nerve fibers increases before birth. To assess neural connections between the cervix and the spinal cord, the cervix of pregnant mice was injected with the trans-synaptic retrograde neural tract tracer pseudorabies virus (PRV). After 5 days, the virus was present in nerve cells and fibers in specific areas of the sensory, autonomic, and motor subdivisions of the thoracolumbar spinal cord. In nonpregnant controls, the virus was predominantly distributed in laminae I-III in the dorsal gray sensory areas with the heaviest label in the substantia gelatinosa compared with the autonomic or motor areas. Labeled cells and processes were sparse in other regions, except for a prominent cluster in the intermediolateral column (lamina VII). Photomicrographs of spinal cord sections were digitized, and the total area with the virus was estimated. Compared with nonpregnant controls, the area with PRV was significantly decreased in all the spinal cord subdivisions in pregnant mice except in the intermediolateral column. However, areas with the virus were equivalent in mice injected with PRV at 4 days or 1 day before birth. These findings suggest that the predominant innervation of the murine cervix is from the sensory regions of the thoracolumbar spinal cord, and that these connections diminish with pregnancy. The results raise the possibility that the remaining connections from sensory and autonomic subdivisions, particularly the intermediolateral column, of the thoracolumbar spinal cord may be important for increased density of nerve fibers in the cervix as pregnancy nears term.

Directional transneuronal spread of α-herpesvirus infection

Most α-herpesviruses are pantropic, neuroinvasive pathogens that establish a reactivateable, latent infection in the PNS of their natural hosts. Various manifestations of herpes disease rely on extent and direction of the spread of infection between the surface epithelia and the nervous system components that innervate that surface. One aspect of such controlled spread of infection is the capacity for synaptically defined, transneuronal spread, a property that makes α-herpesviruses useful tools for determining the connectivity of neural circuits. The current understanding of intra-axonal transport and transneuronal spread of α-herpesviruses is reviewed, focusing on work with herpes simplex virus and pseudorabies virus, the available in vitro technology used to study viral transport and spread is evaluated and how certain viral mutants can be used to examine neural circuit architecture is described in this article.

Bovine herpesvirus type 1 (BoHV-1) anterograde neuronal transport from trigeminal ganglia to nose and eye requires glycoprotein E

The requirement of bovine herpesvirus type 1 (BoHV-1) envelope protein gE (Us8 homolog) for establishment of latency and reactivation in trigeminal ganglia (TG) was examined. Although BHV-1 gE-rescued and gE-deleted viruses were isolated from nasal or ocular swabs during primary infection, only the gE-rescued virus was isolated following dexamethasone-induced reactivation. Furthermore, gC protein expression, which requires viral DNA replication for its expression, was detected in TG of calves infected with either virus following reactivation. These studies suggest that gE is required for anterograde transport of BoHV-1 from neuronal cell bodies in the TG to their nerve processes.

Transneuronal circuit tracing with neurotropic viruses

Because neurotropic viruses naturally traverse neural pathways, they are extremely valuable for elucidating neural circuits. Naturally occurring herpes and rabies viruses have been used for transneuronal circuit tracing for decades. Depending on the type of virus and strain, virus can travel preferentially in the anterograde or the retrograde direction. More recently, genetic modifications have allowed for many improvements. These include: reduced pathogenicity; addition of marker genes; control of synaptic spread; pseudotyping for infection of selected cells; addition of ancillary genetic elements for combining circuit tracing with manipulation of activity or functional assays. These modifications, along with the likelihood of future developments, suggest that neurotropic viruses will be increasingly important and effective tools for future studies of neural circuits.

Repair of the UL21 locus in pseudorabies virus Bartha enhances the kinetics of retrograde, transneuronal infection in vitro and in vivo

The attenuated pseudorabies virus (PRV) strain Bartha contains several characterized mutations that affect its virulence and ability to spread through neural circuits. This strain contains a small genomic deletion that abrogates anterograde spread and is widely used as a retrograde-restricted neural circuit tracer. Previous studies showed that the retrograde-directed spread of PRV Bartha is slower than that of wild-type PRV. We used compartmented neuronal cultures to characterize the retrograde defect and identify the genetic basis of the phenotype. PRV Bartha is not impaired in retrograde axonal transport, but transneuronal spread among neurons is diminished. Repair of the U(L)21 locus with wild-type sequence restored efficient transneuronal spread both in vitro and in vivo. It is likely that mutations in the Bartha U(L)21 gene confer defects that affect infectious particle production, causing a delay in spread to presynaptic neurons and amplification of infection. These events manifest as slower kinetics of retrograde viral spread in a neural circuit.

Oral Delivery of Pathogens from the Intestine to the Nervous System

Most therapeutic agents are delivered orally. Consequently, the major classes of therapeutically useful chemicals are partially lipophilic, small molecular weight compounds. They have reasonable permeability coefficient values across cell membranes, including those of intestinal epithelia and vascular endothelia. In contrast, large molecular weight biotechnology compounds have limited usefulness by non-injected routes as a consequence of their low membrane permeability and variable solubility. However, a wide range of infectious agents have developed strategies or have hijacked physiological routings in order to enter the host by the oral route. Efforts to address such issues have refreshed interest in mechanisms by which different types of payloads (including particulates and microorganisms) translocate across gut epithelia and then distribute to target tissues. Special attention is given to the potential role of the enteric nervous system and its plasticity.

Genetic dissection of neural circuits

Understanding the principles of information processing in neural circuits requires systematic characterization of the participating cell types and their connections, and the ability to measure and perturb their activity. Genetic approaches promise to bring experimental access to complex neural systems, including genetic stalwarts such as the fly and mouse, but also to nongenetic systems such as primates. Together with anatomical and physiological methods, cell-type-specific expression of protein markers and sensors and transducers will be critical to construct circuit diagrams and to measure the activity of genetically defined neurons. Inactivation and activation of genetically defined cell types will establish causal relationships between activity in specific groups of neurons, circuit function, and animal behavior. Genetic analysis thus promises to reveal the logic of the neural circuits in complex brains that guide behaviors. Here we review progress in the genetic analysis of neural circuits and discuss directions for future research and development.

Anterograde transsynaptic tracing in the murine somatosensory system using Pseudorabies virus (PrV): a "live-cell"-tracing tool for analysis of identified neurons in vitro

The Pseudorabies virus (PrV) strain Bartha is widely used as a tool for retrograde transneuronal tracing in mammals. Traced neurons can be identified in cell culture allowing the analysis of their physiological features ("live-cell"-tracing). Compared to PrV-Bartha, PrV-Kaplan is known for higher virulence and transsynaptic spread in both retrograde and anterograde direction. Herein we assess the authors assess PrV-Kaplan for transsynaptic anterograde "live-cell"-tracing. Following intranasal application in mice, labelled trigeminal and brainstem neurons could be identified in vitro. Detailed electrophysiological analysis indicated that viral infection did not affect neuronal properties, making PrV-Kaplan eligible for functional analysis of identified neurons within somatosensory systems.

Envelope protein Us9 is required for the anterograde transport of bovine herpesvirus type 1 from trigeminal ganglia to nose and eye upon reactivation

In this study, the authors examined the role of bovine herpesvirus type 1 (BHV-1) Us9 in the anterograde transport of the virus from trigeminal ganglia (TG) to nose and eye upon reactivation from latency. During primary infection, both BHV-1 Us9-deleted and BHV-1 Us9-rescued viruses replicated efficiently in the nasal and ocular epithelium. However, upon reactivation from latency, only the BHV-1 Us9-rescued virus could be isolated in the nasal and ocular shedding. By real-time polymerase chain reaction, comparable DNA copy numbers were detected in the TGs during latency and reactivation for both the viruses. Therefore, Us9 is essential for reactivation of the virus in the TG and anterograde axonal transport from TG to nose and eye.

Influence of pseudorabies virus proteins on neuroinvasion and neurovirulence in mice

Neurotropism is a distinctive feature of members of the Alphaherpesvirinae. However, its molecular basis remains enigmatic. In the past, research has been focused mainly on the role of viral envelope proteins in modulating herpesvirus neuroinvasion and neurovirulence (T. C. Mettenleiter, Virus Res. 92:192-206, 2003). To further analyze the molecular requirements for neuroinvasion of the alphaherpesvirus pseudorabies virus (PrV), adult mice were infected intranasally with a set of single- or multiple-deletion mutants lacking the UL3, UL4, UL7, UL11, UL13, UL16, UL17, UL21, UL31, UL34, UL37, UL41, UL43, UL46, UL47, UL48, UL51, US3, US9, glycoprotein E (gE), gM, UL11/US9, UL11/UL16, UL16/UL21, UL11/UL16/UL21, UL11/gE, UL11/gM, UL43/gK, UL43/gM, or UL43/gK/gM genes. Neurovirulence was evaluated by measuring mean survival times compared to that after wild-type virus infection. Furthermore, by immunohistochemical detection of infected neurons, the kinetics of viral spread in the murine central nervous system was investigated.

Relevance of the interaction between alphaherpesvirus UL3.5 and UL48 proteins for virion maturation and neuroinvasion

The UL3.5 and UL48 genes, which are conserved in most alphaherpesvirus genomes, are important for maturation of pseudorabies virus (PrV) particles in the cytoplasm of infected cells (W. Fuchs, B. G. Klupp, H. J. Rziha, and T. C. Mettenleiter, J. Virol. 70:3517-3527, 1996; W. Fuchs, H. Granzow, B. G. Klupp, M. Kopp and T. C. Mettenleiter, J. Virol. 76:6729-6742, 2002). In bovine herpesvirus 1 (BoHV-1), the homologous gene products pUL3.5 and pUL48 have been demonstrated to interact physically (N. Lam and G. Letchworth, J. Virol. 74:2876-2884, 2000). Moreover, BoHV-1 pUL3.5 partially complemented a pUL3.5 defect in PrV (W. Fuchs, H. Granzow, and T. C. Mettenleiter, J. Virol. 71:8886-8892, 1997). By using coimmunoprecipitation and yeast two-hybrid studies, we observed a similar interaction between pUL3.5 and pUL48 of PrV, as well as a heterologous interaction between the PrV and BoHV-1 gene products. The relevant domain could be confined to the first 43 amino acids of PrV pUL3.5. Unlike its BoHV-1 homologue, PrV pUL3.5 is processed by proteolytic cleavage, and only an abundant 14-kDa fragment consisting of amino acids 1 to >or=116 could be detected by peptide mass fingerprint analysis of purified wild-type PrV particles, which also contain the pUL48 tegument component. To determine the biological relevance of the protein-protein interaction, pUL3.5-, pUL48-, and double-negative PrV mutants were analyzed in parallel. All deletion mutants were replication competent but exhibited significantly reduced plaque sizes and virus titers in cultured rabbit kidney cells compared to wild-type and rescued viruses, which correlated with a delayed neuroinvasion in intranasally infected mice. Remarkably, the defects of the double-negative mutant were similar to those of pUL48-negative virus. Electron microscopy of cells infected with either deletion mutant revealed the retention of naked nucleocapsids in the cytoplasm and the absence of mature virus particles. In summary, our studies for the first time demonstrate the relevance of the pUL3.5-pUL48 interaction for secondary envelopment of an alphaherpesvirus, give a molecular basis for the observed trans-complementation between the PrV and BHV-1 pUL3.5 homologs, yield conclusive evidence for the incorporation of a proteolytically processed pUL3.5 into PrV virions, and demonstrate the importance of both proteins for neuroinvasion and neurovirulence of PrV.

In vitro analysis of transneuronal spread of an alphaherpesvirus infection in peripheral nervous system neurons

The neurotropic alphaherpesviruses invade and spread in the nervous system in a directional manner between synaptically connected neurons. Until now, this property has been studied only in living animals and has not been accessible to in vitro analysis. In this study, we describe an in vitro system in which cultured peripheral nervous system neurons are separated from their neuron targets by an isolator chamber ring. Using pseudorabies virus (PRV), an alphaherpesvirus capable of transneuronal spread in neural circuits of many animals, we have recapitulated in vitro all known genetic requirements for retrograde and anterograde transneuronal spread as determined previously in vivo. We show that in vitro transneuronal spread requires intact axons and the presence of the viral proteins gE, gI, and Us9. We also show that transneuronal spread is dependent on the viral glycoprotein gB, which is required for membrane fusion, but not on gD, which is required for extracellular spread. We demonstrate ultrastructural differences between anterograde- and retrograde-traveling virions. Finally, we show live imaging of dynamic fluorescent virion components in axons and postsynaptic target neurons.

The UL4 gene of pseudorabies virus encodes a minor infected-cell protein that is dispensable for virus replication

Although homologues of the open reading frame (ORF) UL4 of herpes simplex virus 1 (Human herpesvirus 1) have been found in the genomes of all hitherto-analysed alphaherpesviruses, little is known about their function. In a project to analyse systematically, in an isogenic and standardized assay system, the gene products of the alphaherpesvirus pseudorabies virus (PrV; Suid herpesvirus 1), the PrV UL4 gene product was identified using a monospecific rabbit antiserum prepared against a bacterial fusion protein. Western blot and immunofluorescence analyses revealed that the 146 codon UL4 ORF of PrV was translated into a nuclear 15 kDa protein which was detectable from 6 h after infection of rabbit kidney cells, but was not found in purified virus particles. For functional analysis, a UL4-negative virus recombinant (PrV-DeltaUL4F) was generated by mutagenesis of an infectious full-length clone of the PrV genome in E. coli. PrV-DeltaUL4F was replication-competent in rabbit kidney cells, and plaque formation was not affected by the mutation. However, maximum virus titres of PrV-DeltaUL4F were decreased about fivefold compared with wild-type PrV, and electron microscopy of infected cells demonstrated an impairment of release of mature virions. This growth defect of PrV-DeltaUL4F could be corrected completely by propagation in UL4-expressing cells. Correlating with the inconspicuous in vitro phenotype, neurovirulence of PrV-DeltaUL4F was also not affected significantly. Thus, UL4 encodes a non-structural protein of PrV that enhances virion formation but is not essential for PrV replication in vitro or in vivo.