The retrograde tracer Fluoro-Gold interferes with the infectivity of herpes simplex virus

Fundus imaging of retinal ganglion cells transduced by retrograde transport of rAAV2-retro

Access of adeno-associated virus (AAV) to ganglion cells following intravitreal injection for gene therapy is impeded by the internal limiting membrane of the retina. As an alternative, one could transduce ganglion cells via retrograde transport after virus injection into a retinal target nucleus. It is unknown if recombinant AAV2-retro (rAAV2-retro), a variant of AAV2 developed specifically for retrograde transport, is capable of transducing retinal ganglion cells. To address this issue, equal volumes of rAAV2-retro-hSyn-EGFP and rAAV2-retro-hSyn-mCherry were mixed in a micropipette and injected into the rat superior colliculus. The time-course of viral transduction was tracked by performing serial in vivo fundus imaging. Cells that were labeled by the fluorophores within the first week remained consistent in distribution and relative signal strength on follow-up imaging. Most transduced cells were double-labeled, but some were labeled by only EGFP or mCherry. Fundus images were later aligned with retinal wholemounts. Ganglion cells in the wholemounts matched precisely the cells imaged by fundus photography. As seen in the fundus images, ganglion cells in wholemounts were sometimes labeled by only EGFP or mCherry. Overall, there was detectable label in 32-41% of ganglion cells. Analysis of the number of cells labeled by 0, 1, or 2 fluorophores, based on Poisson statistics, yielded an average of 0.66 virions transducing each ganglion cell. Although this represents a low number relative to the quantity of virus injected into the superior colliculus, the ganglion cells showed sustained and robust fluorescent labeling. In the primate, injection of rAAV2-retro into the lateral geniculate nucleus might provide a viable approach for the transduction of ganglion cells, bypassing the obstacles that have prevented effective gene delivery via intravitreal injection.

Transneuronal circuit analysis with pseudorabies viruses

Our ability to understand the function of the nervous system is dependent upon defining the connections of its constituent neurons. Development of methods to define connections within neural networks has always been a growth industry in the neurosciences. Transneuronal spread of neurotropic viruses currently represents the best means of defining synaptic connections within neural networks. The method exploits the ability of viruses to invade neurons, replicate, and spread through the intimate synaptic connections that enable communication among neurons. Since the method was first introduced in the 1970s, it has benefited from an increased understanding of the virus life cycle, the function of viral genome, and the ability to manipulate the viral genome in support of directional spread of virus and the expression of transgenes. In this unit, we review these advances in viral tracing technology and the way in which they may be applied for functional dissection of neural networks.

New developments in tracing neural circuits with herpesviruses

Certain neurotropic viruses can invade the nervous system of their hosts and spread in chains of synaptically connected neurons. Consequently, it is possible to identify entire hierarchically connected circuits within an animal. In this review, we discuss the use of neurotropic herpesviruses as neuronal tract tracers. Although a variety of tract tracing viruses are available, each with its own unique infection characteristics, we focus on the widespread use of attenuated strains of pseudorabies virus (PRV), a swine herpesvirus with a broad host range. In particular, we focus on new applications of PRV for tract tracing including use of multiple infections by PRV reporter viruses to test for circuit convergence/divergence within the same animal. We provide examples of these combined application techniques within the context of an animal model to study the naturally occurring reversal of seasonal obesity in Siberian hamsters.

Tracing functionally identified neurones in a multisynaptic pathway in the hamster and rat using herpes simplex virus expressing green fluorescent protein

Using a genetically modified herpes simplex virus encoding green fluorescent protein we sought to establish if this viral modification could be used in transneuronal tracing studies of the sympathetic nervous system. The herpes simplex virus encoding green fluorescent protein was injected into the adrenal medulla of three hamsters and six rats. After a suitable survival period, neurones in the sympathetic intermediolateral cell column of the thoracolumbar spinal cord, rostral ventral medulla and paraventricular nucleus of the hypothalamus were clearly identified by the presence of a green fluorescence in the cytoplasm of the neurones of both species. Thus, herpes simplex virus encoding green fluorescent protein labelled chains of sympathetic neurones in the hamster and rat and therefore has the potential to be used in transneuronal tracing studies of autonomic pathways in these species.

Characterization of the central nervous system innervation of the rat spleen using viral transneuronal tracing

Splenic immune function is modulated by sympathetic innervation, which in turn is controlled by inputs from supraspinal regions. In the present study, the characterization of central circuits involved in the control of splenic function was accomplished by injecting pseudorabies virus (PRV), a retrograde transynaptic tracer, into the spleen and conducting a temporal analysis of the progression of the infection from 60 hours to 110 hours postinoculation. In addition, central noradrenergic cell groups involved in splenic innervation were characterized by dual immunohistochemical detection of dopamine-beta-hydroxylase and PRV. Infection in the CNS first appeared in the spinal cord. Splenic sympathetic preganglionic neurons, identified in rats injected with Fluoro-Gold i.p. prior to PRV inoculation of the spleen, were located in T(3)-T(12) bilaterally; numerous infected interneurons were also found in the thoracic spinal cord (T(1)-T(13)). Infected neurons in the brain were first observed in the A5 region, ventromedial medulla, rostral ventrolateral medulla, paraventricular hypothalamic nucleus, Barrington's nucleus, and caudal raphe. At intermediate survival times, the number of infected cells increased in previously infected areas, and infected neurons also appeared in lateral hypothalamus, A7 region, locus coeruleus, subcoeruleus region, nucleus of the solitary tract, and C3 cell group. At longer postinoculation intervals, infected neurons were found in additional hypothalamic areas, Edinger-Westphal nucleus, periaqueductal gray, pedunculopontine tegmental nucleus, caudal ventrolateral medulla, and area postrema. These results demonstrate that the sympathetic outflow to the spleen is controlled by a complex multisynaptic pathway that involves several brainstem and forebrain nuclei.

Pseudorabies virus membrane proteins gI and gE facilitate anterograde spread of infection in projection-specific neurons in the rat

The membrane proteins gI and gE of Pseudorabies virus (PRV) are required for viral invasion and spread through some neural pathways of the rodent central nervous system. Following infection of the rat retina with wild-type PRV, virus replicates in retinal ganglion neurons and anterogradely spreads to infect all visual centers in the brain. By contrast, gI and gE null mutants do not infect a specific subset of the visual centers, e.g., the superior colliculus and the dorsal lateral geniculate nucleus. In previous experiments, we suggested that the defect was not due to inability to infect projection-specific retinal ganglion cells, because mixed infection of a gE deletion mutant and a gI deletion mutant restored the wild-type phenotype (i.e., genetic complementation occurred). In the present study, we provide direct evidence that gE and gI function to promote the spread of infection after entry into primary neurons. We used stereotaxic central nervous system injection of a fluorescent retrograde tracer into the superior colliculus and subsequent inoculation of a PRV gI-gE double null mutant into the eye of the same animal to demonstrate that viral antigen and fluorescent tracer colocalize in retinal ganglion cells. Furthermore, we demonstrate that direct injection of a PRV gI-gE double null mutant into the superior colliculus resulted in robust infection followed by retrograde transport to the eye and replication in retinal ganglion neuron cell bodies. These experiments provide additional proof that the retinal ganglion cells projecting to the superior colliculus are susceptible and permissive to gE and gI mutant viruses. Our studies confirm that gI and gE specifically facilitate anterograde spread of infection by affecting intracellular processes in the primary infected neuron such as anterograde transport in axons or egress from axon terminals.

The retrograde tracer fluoro-gold interferes with the expression of fos-related antigens

The retrograde tracer, fluoro-gold (FG) has been used in combination with immediate early gene (IEG) immunohistochemistry to identify neural circuits activated by pharmacological, physiological or behavioral manipulations. However, since FG has been shown to be toxic to cell bodies, axons and terminals at the injection site, the question arises as to whether FG alters the detection of IEG products. To examine this question, FG was microiontophoresed unilaterally into the nucleus accumbens (NAc) of rats and Fos-related antigens (FRAs) were examined in both hemispheres 12 days later. Approximately half as many FRA-positive nuclei were observed in the tracer-injected NAc as were found in the contralateral NAc. Similar results were observed in the ventral subiculum of the hippocampus and the basolateral and central amygdaloid nuclei, but not in the lateral septum or lateral habenula. These results suggest that FG microiontophoresed into the NAc interferes with the expression of FRAs at the injection site and also at other ipsilateral limbic sites.

Simultaneous identification of two populations of sympathetic preganglionic neurons using recombinant herpes simplex virus type 1 expressing different reporter genes

We generated neurotropic herpes simplex type 1 viruses expressing human placental alkaline phosphatase and studied the utility of this enzyme as a marker of infected neurons. The neurotropism of these viruses was assessed by their ability to infect sympathetic preganglionic neurons after adrenal injection in hamsters. The transneuronal transfer of these viruses was examined by their ability to cross the peripheral synapse from the kidney to renal preganglionic neurons or to cross the central synapse from the adrenal gland to the medulla oblongata. Finally, we injected an alkaline phosphatase-expressing herpes simplex virus into the adrenal gland and a beta-galactosidase-expressing herpes simplex virus (US5gal) into the muscular wall of the small intestine to label two neural circuits in one animal and to assess the feasibility of a dual-virus labelling system. The alkaline phosphatase gene was inserted into the glycoprotein J locus or the virus-induced host shut-off locus in the herpes simplex genome to create viruses which replicate (gJHAP HSV or vhsHAP HSV) or into the thymidine kinase locus to generate a virus that does not replicate in neurons in vivo (TK- HAP HSV). Each of the three viruses was retrogradely transported from the adrenal gland of hamsters to sympathetic preganglionic neurons, suggesting that the neurotropism of these viruses was maintained. gJHAP HSV travelled transneuronally from the kidney to sympathorenal preganglionic neurons and from the adrenal gland to neurons in the rostral ventrolateral medulla. Neuronal infection with alkaline phosphatase-expressing virus could be identified using histochemistry but detailed morphology of these neurons was not revealed. However, staining by anti-herpes simplex virus immunoperoxidase demonstrated that they had normal morphology. Identification of two distinct neural circuits in one animal was achieved with our dual-virus labelling system. The nonreplicating TK- HAP HSV was used in combination with US5gal to identify intestinal and adrenal sympathetic preganglionic neurons. The beta-galactosidase-expressing intestinal neurons were labelled bilaterally in the nucleus intermediolateralis, pars principalis, and alkaline phosphatase-expressing adrenal neurons were found ipsilaterally. Some clusters of sympathetic preganglionic neurons in the nucleus intermediolateralis, pars principalis contained mostly intestinal sympathetic preganglionic neurons and a few adrenal sympathetic preganglionic neurons. In other areas, the opposite pattern occurred. About 3-7% of the labelled sympathetic preganglionic neurons were double-labelled by both markers. The distinct and crisp morphology and dendritic processes of neurons stained by beta-galactosidase histochemistry contrasted with the partial staining of neurons by alkaline phosphatase, revealing beta-galactosidase as a better marker of infected neurons. In conclusion, alkaline phosphatase-expressing herpes simplex viruses are yet neurotropic after insertion of this marker enzyme into any of three different loci of the herpes simplex genome. One replicating alkaline phosphatase-expressing virus travelled transneuronally. These alkaline phosphatase-expressing herpes simplex virus can be used together with beta-galactosidase-expressing herpes simplex viruses to determine the target specificity of sympathetic preganglionic neurons controlling visceral organs or can be used to express two different recombinant genes in two targeted neuronal populations. This study suggests that sympathetic preganglionic neurons controlling the intestine and adrenal gland are almost completely distinct.

Identification of spinal interneurons antecedent to adrenal sympathetic preganglionic neurons using trans-synaptic transport of herpes simplex virus type 1

Control of sympathetic preganglionic neurons appears to be mediated, in part, through polysynaptic pathways using spinal interneurons. To identify spinal interneurons antecedent to adrenal sympathetic preganglionic neurons, we injected herpes simplex virus type 1 into the adrenal gland of hamsters as this virus is an effective trans-synaptic tracer of neural pathways. After a three day survival period, immunocytochemistry was used to visualize virus-infected spinal cord cells. Infected sympathetic preganglionic neurons with somata that were either kite-shaped, elliptical or fusiform and that had extensive dendrite arbors were identified as well as a group of smaller round cells with finer processes. For comparison, in additional hamsters, labelling with the retrograde tracer Fluoro-Gold and histochemical reactions for the enzyme nicotinamide adenine dinucleotide phosphate-diaphorase were used to identify sympathetic preganglionic neurons. Sympathetic preganglionic neurons identified with Fluoro-Gold or herpes virus were present mostly in the nucleus intermediolateralis, pars intermediolateralis and nucleus intermediolateralis, pars funicularis of the spinal cord. The smaller herpes virus-infected cells were found mostly medial to the preganglionic neurons in lamina VII and also dorsally in lamina V of the spinal cord. Assessing immunoreactivity for glial fibrillary acidic protein demonstrated that the smaller herpes virus-infected cells were not reactive astrocytes. Furthermore, these cells were immunoreactive for two neuronal markers, neuron-specific enolase and for microtubule-associated protein 2. These findings suggest that these smaller round cells with finer processes are distinct from sympathetic preganglionic neurons and astrocytes and may be interneurons antecedent to the sympathetic preganglionic neurons.