Hitchhiking on the neuronal highway: Mechanisms of transsynaptic specificity

Analysis of Monosynaptic Inputs to Thalamic Paraventricular Nucleus Neurons Innervating the Shell of the Nucleus Accumbens and Central Extended Amygdala

The paraventricular nucleus of the thalamus (PVT) sends dense projections to the shell of the nucleus accumbens (NAcSh), dorsolateral region of the bed nucleus of the stria terminalis (BSTDL) and the lateral region of central nucleus of the amygdala (CeL). Projection specific modulation of these pathways has been shown to regulate appetitive and aversive behavioral responses. The present investigation applied an intersectional monosynaptic rabies tracing approach to quantify the brain-wide sources of afferent input to PVT neurons that primarily project to the NAcSh, BSTDL and CeL. The results demonstrate that these projection neurons receive monosynaptic input from similar brain regions. The prefrontal cortex and the ventral subiculum of the hippocampus were major sources of input to the PVT projection neurons. In addition, the lateral septal nucleus, thalamic reticular nucleus and the hypothalamic medial preoptic area, dorsomedial, ventromedial, and arcuate nuclei were sources of input. The subfornical organ, parasubthalamic nucleus, periaqueductal gray matter, lateral parabrachial nucleus, and nucleus of the solitary tract were consistent but lesser sources of input. This input-output relationship is consistent with recent observations that PVT neurons have axons that bifurcate extensively to divergently innervate the NAcSh, BSTDL and CeL.

Synaptic configuration and reconfiguration in the neocortex are spatiotemporally selective

Brain computation relies on the neural networks. Neurons extend the neurites such as dendrites and axons, and the contacts of these neurites that form chemical synapses are the biological basis of signal transmissions in the central nervous system. Individual neuronal outputs can influence the other neurons within the range of the axonal spread, while the activities of single neurons can be affected by the afferents in their somatodendritic fields. The morphological profile, therefore, binds the functional role each neuron can play. In addition, synaptic connectivity among neurons displays preference based on the characteristics of presynaptic and postsynaptic neurons. Here, the author reviews the "spatial" and "temporal" connection selectivity in the neocortex. The histological description of the neocortical circuitry depends primarily on the classification of cell types, and the development of gene engineering techniques allows the cell type-specific visualization of dendrites and axons as well as somata. Using genetic labeling of particular cell populations combined with immunohistochemistry and imaging at a subcellular spatial resolution, we revealed the "spatial selectivity" of cortical wirings in which synapses are non-uniformly distributed on the subcellular somatodendritic domains in a presynaptic cell type-specific manner. In addition, cortical synaptic dynamics in learning exhibit presynaptic cell type-dependent "temporal selectivity": corticocortical synapses appear only transiently during the learning phase, while learning-induced new thalamocortical synapses persist, indicating that distinct circuits may supervise learning-specific ephemeral synapse and memory-specific immortal synapse formation. The selectivity of spatial configuration and temporal reconfiguration in the neural circuitry may govern diverse functions in the neocortex.

Convergence of monosynaptic inputs from neurons in the brainstem and forebrain on parabrachial neurons that project to the paraventricular nucleus of the thalamus

The paraventricular nucleus of the thalamus (PVT) projects to areas of the forebrain involved in regulating behavior. Homeostatic challenges and salient cues activate the PVT and evidence shows that the PVT regulates appetitive and aversive responses. The brainstem is a source of afferents to the PVT and the present study was done to determine if the lateral parabrachial nucleus (LPB) is a relay for inputs to the PVT. Retrograde tracing experiments with cholera toxin B (CTB) demonstrate that the LPB contains more PVT projecting neurons than other regions of the brainstem including the catecholamine cell groups. The hypothesis that the LPB is a relay for signals to the PVT was assessed using an intersectional monosynaptic rabies tracing approach. Sources of inputs to LPB included the reticular formation; periaqueductal gray (PAG); nucleus cuneiformis; and superior and inferior colliculi. Distinctive clusters of input cells to LPB-PVT projecting neurons were also found in the dorsolateral bed nucleus of the stria terminalis (BSTDL) and the lateral central nucleus of the amygdala (CeL). Anterograde viral tracing demonstrates that LPB-PVT neurons densely innervate all regions of the PVT in addition to providing collateral innervation to the preoptic area, lateral hypothalamus, zona incerta and PAG but not the BSTDL and CeL. The paper discusses the anatomical evidence that suggests that the PVT is part of a network of interconnected neurons involved in arousal, homeostasis, and the regulation of behavioral states with forebrain regions potentially providing descending modulation or gating of signals relayed from the LPB to the PVT.

The connectome from the cerebral cortex to skeletal muscle using viral transneuronal tracers: a review

Connectomics has developed from an initial observation under an electron microscope to the present well-known medical imaging research approach. The emergence of the most popular transneuronal tracers has further advanced connectomics research. Researchers use the virus trans-nerve tracing method to trace the whole brain, mark the brain nerve circuit and nerve connection structure, and construct a complete nerve conduction pathway. This review assesses current methods of studying cortical to muscle connections using viral neuronal tracers and demonstrates their application in disease diagnosis and prognosis.

Uncovering the Connectivity Logic of the Ventral Tegmental Area

Decades of research have revealed the remarkable complexity of the midbrain dopamine (DA) system, which comprises cells principally located in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc). Neither homogenous nor serving a singular function, the midbrain DA system is instead composed of distinct cell populations that (1) receive different sets of inputs, (2) project to separate forebrain sites, and (3) are characterized by unique transcriptional and physiological signatures. To appreciate how these differences relate to circuit function, we first need to understand the anatomical connectivity of unique DA pathways and how this connectivity relates to DA-dependent motivated behavior. We and others have provided detailed maps of the input-output relationships of several subpopulations of midbrain DA cells and explored the roles of these different cell populations in directing behavioral output. In this study, we analyze VTA inputs and outputs as a high dimensional dataset (10 outputs, 22 inputs), deploying computational techniques well-suited to finding interpretable patterns in such data. In addition to reinforcing our previous conclusion that the connectivity in the VTA is dependent on spatial organization, our analysis also uncovered a set of inputs elevated onto each projection-defined VTADA cell type. For example, VTADA→NAcLat cells receive preferential innervation from inputs in the basal ganglia, while VTADA→Amygdala cells preferentially receive inputs from populations sending a distributed input across the VTA, which happen to be regions associated with the brain's stress circuitry. In addition, VTADA→NAcMed cells receive ventromedially biased inputs including from the preoptic area, ventral pallidum, and laterodorsal tegmentum, while VTADA→mPFC cells are defined by dominant inputs from the habenula and dorsal raphe. We also go on to show that the biased input logic to the VTADA cells can be recapitulated using projection architecture in the ventral midbrain, reinforcing our finding that most input differences identified using rabies-based (RABV) circuit mapping reflect projection archetypes within the VTA.

Application of AAV1 for Anterograde Transsynaptic Circuit Mapping and Input-Dependent Neuronal Cataloging

Viruses that spread transsynaptically provide a powerful means to study interconnected circuits in the brain. Here we describe the use of adeno-associated virus, serotype 1 (AAV1), as a tool to achieve robust, anterograde transsynaptic spread in a variety of unidirectional pathways. A protocol for performing intracranial AAV1 injections in mice is presented, along with additional guidance for planning experiments, sourcing materials, and optimizing the approach to achieve the most successful outcomes. By following the methods presented here, researchers will be able to reveal postsynaptically connected neurons downstream of a given AAV1 injection site and access these input-defined cells for subsequent mapping, recording, and manipulation to characterize their anatomical and functional properties. © 2022 Wiley Periodicals LLC. Basic Protocol: Stereotaxic injection of AAV1 for anterograde transsynaptic spread.

The Serendipity of Viral Trans-Neuronal Specificity: More Than Meets the Eye

Trans-neuronal viruses are frequently used as neuroanatomical tools for mapping neuronal circuits. Specifically, recombinant one-step rabies viruses (RABV) have been instrumental in the widespread application of viral circuit mapping, as these viruses have enabled labs to map the direct inputs onto defined cell populations. Within the neuroscience community, it is widely believed that RABV spreads directly between neurons via synaptic connections, a hypothesis based principally on two observations. First, the virus labels neurons in a pattern consistent with known anatomical connectivity. Second, few glial cells appear to be infected following RABV injections, despite the fact that glial cells are abundant in the brain. However, there is no direct evidence that RABV can actually be transmitted through synaptic connections. Here we review the immunosubversive mechanisms that are critical to RABV’s success for infiltration of the central nervous system (CNS). These include interfering with and ultimately killing migratory T cells while maintaining levels of interferon (IFN) signaling in the brain parenchyma. Finally, we critically evaluate studies that support or are against synaptically-restricted RABV transmission and the implications of viral-host immune responses for RABV transmission in the brain.

Acute Neurologic Manifestations of Respiratory Viruses

PURPOSE OF REVIEW

Understanding the pathophysiology of COVID-19 and the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus that causes the disease has demonstrated the complexity of acute respiratory viruses that can cause neurologic manifestations. This article describes the most common respiratory viruses that have neurologic manifestations, with a focus on SARS-CoV-2 and COVID-19.

RECENT FINDINGS

In vitro and in vivo studies have better elucidated the neurotropism of various respiratory viruses. Understanding host cell receptors that mediate viral binding and entry not only demonstrates how viruses enter host cells but also provides possible mechanisms for therapeutic interventions. Elucidation of SARS-CoV-2 binding and fusion with host cells expressing the angiotensin-converting enzyme 2 (ACE2) receptor may also provide greater insights into its systemic and neurologic sequelae. Respiratory virus neurotropism and collateral injury due to concurrent inflammatory cascades result in various neurologic pathologies, including Guillain-Barré syndrome, encephalopathy, encephalitis, ischemic stroke, intracerebral hemorrhage, and seizures.

SUMMARY

Numerous respiratory viruses can infect the cells of the peripheral and central nervous systems, elicit inflammatory cascades, and directly and indirectly cause various neurologic manifestations. Patients with neurologic manifestations from respiratory viruses are often critically ill and require mechanical ventilation. Neurologists and neurointensivists should be familiar with the common neurologic manifestations of respiratory viruses and the unique and still-evolving sequelae associated with COVID-19.

Engagement of Neurotropic Viruses in Fast Axonal Transport: Mechanisms, Potential Role of Host Kinases and Implications for Neuronal Dysfunction

Much remains unknown about mechanisms sustaining the various stages in the life cycle of neurotropic viruses. An understanding of those mechanisms operating before their replication and propagation could advance the development of effective anti-viral strategies. Here, we review our current knowledge of strategies used by neurotropic viruses to undergo bidirectional movement along axons. We discuss how the invasion strategies used by specific viruses might influence their mode of interaction with selected components of the host’s fast axonal transport (FAT) machinery, including specialized membrane-bounded organelles and microtubule-based motor proteins. As part of this discussion, we provide a critical evaluation of various reported interactions among viral and motor proteins and highlight limitations of some in vitro approaches that led to their identification. Based on a large body of evidence documenting activation of host kinases by neurotropic viruses, and on recent work revealing regulation of FAT through phosphorylation-based mechanisms, we posit a potential role of host kinases on the engagement of viruses in retrograde FAT. Finally, we briefly describe recent evidence linking aberrant activation of kinase pathways to deficits in FAT and neuronal degeneration in the context of human neurodegenerative diseases. Based on these findings, we speculate that neurotoxicity elicited by viral infection may involve deregulation of host kinases involved in the regulation of FAT and other cellular processes sustaining neuronal function and survival.

Circuit organization of the excitatory sensorimotor loop through hand/forelimb S1 and M1

Sensory-guided limb control relies on communication across sensorimotor loops. For active touch with the hand, the longest loop is the transcortical continuation of ascending pathways, particularly the lemnisco-cortical and corticocortical pathways carrying tactile signals via the cuneate nucleus, ventral posterior lateral (VPL) thalamus, and primary somatosensory (S1) and motor (M1) cortices to reach corticospinal neurons and influence descending activity. We characterized excitatory connectivity along this pathway in the mouse. In the lemnisco-cortical leg, disynaptic cuneate→VPL→S1 connections excited mainly layer (L) 4 neurons. In the corticocortical leg, S1→M1 connections from L2/3 and L5A neurons mainly excited downstream L2/3 neurons, which excite corticospinal neurons. The findings provide a detailed new wiring diagram for the hand/forelimb-related transcortical circuit, delineating a basic but complex set of cell-type-specific feedforward excitatory connections that selectively and extensively engage diverse intratelencephalic projection neurons, thereby polysynaptically linking subcortical somatosensory input to cortical motor output to spinal cord.

Viruses in connectomics: Viral transneuronal tracers and genetically modified recombinants as neuroscience research tools

Connectomic studies have become 'viral', as viral pathogens have been turned into irreplaceable neuroscience research tools. Highly sensitive viral transneuronal tracing technologies are available, based on the use of alpha-herpesviruses and a rhabdovirus (rabies virus), which function as self-amplifying markers by replicating in recipient neurons. These viruses highly differ with regard to host range, cellular receptors, peripheral uptake, replication, transport direction and specificity. Their characteristics, that make them useful for different purposes, will be highlighted and contrasted. Only transneuronal tracing with rabies virus is entirely specific. The neuroscientist toolbox currently include wild-type alpha-herpesviruses and rabies virus strains enabling polysynaptic tracing of neuronal networks across multiple synapses, as well as genetically modified viral tracers for dual transneuronal tracing, and complementary viral tools including defective and chimeric recombinants that function as single step or monosynaptically restricted tracers, or serve for monitoring and manipulating neuronal activity and gene expression. Methodological issues that are crucial for appropriate use of these technologies will be summarized. Among wild-type and genetically engineered viral tools, rabies virus and chimeric recombinants based on rabies virus as virus backbone are the most powerful, because of the ability of rabies virus to propagate exclusively among connected neurons unidirectionally (retrogradely), without affecting neuronal function. Understanding in depth viral properties is essential for neuroscientists who intend to exploit alpha-herpesviruses, rhabdoviruses or derived recombinants as research tools. Key knowledge will be summarized regarding their cellular receptors, intracellular trafficking and strategies to contrast host defense that explain their different pathophysiology and properties as research tools.

Anterograde Neuronal Circuit Tracers Derived from Herpes Simplex Virus 1: Development, Application, and Perspectives

Herpes simplex virus type 1 (HSV-1) has great potential to be applied as a viral tool for gene delivery or oncolysis. The broad infection tropism of HSV-1 makes it a suitable tool for targeting many different cell types, and its 150 kb double-stranded DNA genome provides great capacity for exogenous genes. Moreover, the features of neuron infection and neuron-to-neuron spread also offer special value to neuroscience. HSV-1 strain H129, with its predominant anterograde transneuronal transmission, represents one of the most promising anterograde neuronal circuit tracers to map output neuronal pathways. Decades of development have greatly expanded the H129-derived anterograde tracing toolbox, including polysynaptic and monosynaptic tracers with various fluorescent protein labeling. These tracers have been applied to neuroanatomical studies, and have contributed to revealing multiple important neuronal circuits. However, current H129-derived tracers retain intrinsic drawbacks that limit their broad application, such as yet-to-be improved labeling intensity, potential nonspecific retrograde labeling, and high toxicity. The biological complexity of HSV-1 and its insufficiently characterized virological properties have caused difficulties in its improvement and optimization as a viral tool. In this review, we focus on the current H129-derived viral tracers and highlight strategies in which future technological development can advance its use as a tool.

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.

Modular output circuits of the fastigial nucleus for diverse motor and nonmotor functions of the cerebellar vermis

The cerebellar vermis, long associated with axial motor control, has been implicated in a surprising range of neuropsychiatric disorders and cognitive and affective functions. Remarkably little is known, however, about the specific cell types and neural circuits responsible for these diverse functions. Here, using single-cell gene expression profiling and anatomical circuit analyses of vermis output neurons in the mouse fastigial (medial cerebellar) nucleus, we identify five major classes of glutamatergic projection neurons distinguished by gene expression, morphology, distribution, and input-output connectivity. Each fastigial cell type is connected with a specific set of Purkinje cells and inferior olive neurons and in turn innervates a distinct collection of downstream targets. Transsynaptic tracing indicates extensive disynaptic links with cognitive, affective, and motor forebrain circuits. These results indicate that diverse cerebellar vermis functions could be mediated by modular synaptic connections of distinct fastigial cell types with posturomotor, oromotor, positional-autonomic, orienting, and vigilance circuits.

Synaptic Specificity and Application of Anterograde Transsynaptic AAV for Probing Neural Circuitry

Revealing the organization and function of neural circuits is greatly facilitated by viral tools that spread transsynaptically. Adeno-associated virus (AAV) exhibits anterograde transneuronal transport, however, the synaptic specificity of this spread and its broad application within a diverse set of circuits remains to be explored. Here, using anatomic, functional, and molecular approaches, we provide evidence for the preferential transport of AAV1 to postsynaptically connected neurons and reveal its spread is strongly dependent on synaptic transmitter release. In addition to glutamatergic pathways, AAV1 also spreads through GABAergic synapses to both excitatory and inhibitory cell types. We observed little or no transport, however, through neuromodulatory projections (e.g., serotonergic, cholinergic, and noradrenergic). In addition, we found that AAV1 can be transported through long-distance descending projections from various brain regions to effectively transduce spinal cord neurons. Combined with newly designed intersectional and sparse labeling strategies, AAV1 can be applied within a wide variety of pathways to categorize neurons according to their input sources, morphology, and molecular identities. These properties make AAV1 a promising anterograde transsynaptic tool for establishing a comprehensive cell-atlas of the brain, although its capacity for retrograde transport currently limits its use to unidirectional circuits.SIGNIFICANCE STATEMENT The discovery of anterograde transneuronal spread of AAV1 generates great promise for its application as a unique tool for manipulating input-defined cell populations and mapping their outputs. However, several outstanding questions remain for anterograde transsynaptic approaches in the field: (1) whether AAV1 spreads exclusively or specifically to synaptically connected neurons, and (2) how broad its application could be in various types of neural circuits in the brain. This study provides several lines of evidence in terms of anatomy, functional innervation, and underlying mechanisms, to strongly support that AAV1 anterograde transneuronal spread is highly synapse specific. In addition, several potentially important applications of transsynaptic AAV1 in probing neural circuits are described.

Neuroanatomical tract-tracing techniques that did go viral

Neuroanatomical tracing methods remain fundamental for elucidating the complexity of brain circuits. During the past decades, the technical arsenal at our disposal has been greatly enriched, with a steady supply of fresh arrivals. This paper provides a landscape view of classical and modern tools for tract-tracing purposes. Focus is placed on methods that have gone viral, i.e., became most widespread used and fully reliable. To keep an historical perspective, we start by reviewing one-dimensional, standalone transport-tracing tools; these including today’s two most favorite anterograde neuroanatomical tracers such as Phaseolus vulgaris-leucoagglutinin and biotinylated dextran amine. Next, emphasis is placed on several classical tools widely used for retrograde neuroanatomical tracing purposes, where Fluoro-Gold in our opinion represents the best example. Furthermore, it is worth noting that multi-dimensional paradigms can be designed by combining different tracers or by applying a given tracer together with detecting one or more neurochemical substances, as illustrated here with several examples. Finally, it is without any doubt that we are currently witnessing the unstoppable and spectacular rise of modern molecular-genetic techniques based on the use of modified viruses as delivery vehicles for genetic material, therefore, pushing the tract-tracing field forward into a new era. In summary, here, we aim to provide neuroscientists with the advice and background required when facing a choice on which neuroanatomical tracer—or combination thereof—might be best suited for addressing a given experimental design.

Monosynaptic Tracing Success Depends Critically on Helper Virus Concentrations

Monosynaptically-restricted transsynaptic tracing using deletion-mutant rabies virus (RV) has become a widely used technique in neuroscience, allowing identification, imaging, and manipulation of neurons directly presynaptic to a starting neuronal population. Its most common implementation is to use Cre mouse lines in combination with Cre-dependent "helper" adeno-associated viral vectors (AAVs) to supply the required genes to the targeted population before subsequent injection of a first-generation (ΔG) rabies viral vector. Here we show that the efficiency of transsynaptic spread and the degree of nonspecific labeling in wild-type control animals depend strongly on the concentrations of these helper AAVs. Our results suggest practical guidelines for achieving good results.

Structural Neural Connectivity Analysis in Zebrafish With Restricted Anterograde Transneuronal Viral Labeling and Quantitative Brain Mapping

The unique combination of small size, translucency, and powerful genetic tools makes larval zebrafish a uniquely useful vertebrate system to investigate normal and pathological brain structure and function. While functional connectivity can now be assessed by optical imaging (via fluorescent calcium or voltage reporters) at the whole-brain scale, it remains challenging to systematically determine structural connections and identify connectivity changes during development or disease. To address this, we developed Tracer with Restricted Anterograde Spread (TRAS), a novel vesicular stomatitis virus (VSV)-based neural circuit labeling approach. TRAS makes use of replication-incompetent VSV (VSVΔG) and a helper virus (lentivirus) to enable anterograde transneuronal spread between efferent axons and their direct postsynaptic targets but restricts further spread to downstream areas. We integrated TRAS with the Z-Brain zebrafish 3D atlas for quantitative connectivity analysis and identified targets of the retinal and habenular efferent projections, in patterns consistent with previous reports. We compared retinofugal connectivity patterns between wild-type and down syndrome cell adhesion molecule-like 1 (dscaml1) mutant zebrafish and revealed differences in topographical distribution. These results demonstrate the utility of TRAS for quantitative structural connectivity analysis that would be valuable for detecting novel efferent targets and mapping connectivity changes underlying neurological or behavioral deficits.

The Recognition of the Distribution Features of Corticospinal Neurons by a Retrograde Trans-synaptic Tracing to Elucidate the Clinical Application of Contralateral Middle Trunk Transfer

Corticospinal neurons (CSNs) undertake direct cortical outputs to the spinal cord and innervate the upper limb through the brachial plexus. Our previous study has shown that the contralateral middle trunk transfer to the paralyzed upper extremity due to cerebral injury can reconstruct the functional cerebral cortex and improve the function of the paralyzed upper extremity. To interpret the cortical reconstruction and the motor improvement after the middle trunk transfer, we explored the distribution of CSNs connecting to the middle, upper, and lower trunk of the brachial plexus by retrograde trans-neuronal tracing using pseudorabies virus (PRV-EGFP or PRV-mRFP). We show that, rather than an individual specific area, these CSNs labelled by each trunk of the brachial plexus were widespread and mainly assembled within the primary motor cortex (M1), secondary motor cortex (M2), primary somatosensory cortex (S1), and slightly within the secondary somatosensory cortex (S2). The three trunk-labelled CSNs were intermingled in these cortices, and mostly connected to more than two trunks, especially the middle trunk-labelled CSNs with higher proportion of co-labelled neurons. Our findings revealed the distribution features of CSNs connecting to the adjacent spinal nerves that innervate the upper limb, which can improve our understanding of the corticospinal circuits associated with motor improvement and the functional cortical reconstruction after the middle trunk transfer.