Long-term labeling and imaging of synaptically connected neuronal networks in vivo using double-deletion-mutant rabies viruses

Rabies-virus-based monosynaptic tracing is a widely used technique for mapping neural circuitry, but its cytotoxicity has confined it primarily to anatomical applications. Here we present a second-generation system for labeling direct inputs to targeted neuronal populations with minimal toxicity, using double-deletion-mutant rabies viruses. Viral spread requires expression of both deleted viral genes in trans in postsynaptic source cells. Suppressing this expression with doxycycline following an initial period of viral replication reduces toxicity to postsynaptic cells. Longitudinal two-photon imaging in vivo indicated that over 90% of both presynaptic and source cells survived for the full 12-week course of imaging. Ex vivo whole-cell recordings at 5 weeks postinfection showed that the second-generation system perturbs input and source cells much less than the first-generation system. Finally, two-photon calcium imaging of labeled networks of visual cortex neurons showed that their visual response properties appeared normal for 10 weeks, the longest we followed them.

Third-generation rabies viral vectors allow nontoxic retrograde targeting of projection neurons with greatly increased efficiency

Rabies viral vectors have become important components of the systems neuroscience toolkit, allowing both direct retrograde targeting of projection neurons and monosynaptic tracing of inputs to defined postsynaptic populations, but the rapid cytotoxicity of first-generation (ΔG) vectors limits their use to short-term experiments. We recently introduced second-generation, double-deletion-mutant (ΔGL) rabies viral vectors, showing that they efficiently retrogradely infect projection neurons and express recombinases effectively but with little to no detectable toxicity; more recently, we have shown that ΔGL viruses can be used for monosynaptic tracing with far lower cytotoxicity than the first-generation system. Here, we introduce third-generation (ΔL) rabies viral vectors, which appear to be as nontoxic as second-generation ones but have the major advantage of growing to much higher titers, resulting in significantly increased numbers of retrogradely labeled neurons in vivo.

Compensatory remodeling of a septo-hippocampal GABAergic network in the triple transgenic Alzheimer's mouse model

BACKGROUND

Alzheimer's disease (AD) is characterized by a progressive loss of memory that cannot be efficiently managed by currently available AD therapeutics. So far, most treatments for AD that have the potential to improve memory target neural circuits to protect their integrity. However, the vulnerable neural circuits and their dynamic remodeling during AD progression remain largely undefined.

METHODS

Circuit-based approaches, including anterograde and retrograde tracing, slice electrophysiology, and fiber photometry, were used to investigate the dynamic structural and functional remodeling of a GABAergic circuit projected from the medial septum (MS) to the dentate gyrus (DG) in 3xTg-AD mice during AD progression.

RESULTS

We identified a long-distance GABAergic circuit that couples highly connected MS and DG GABAergic neurons during spatial memory encoding. Furthermore, we found hyperactivity of DG interneurons during early AD, which persisted into late AD stages. Interestingly, MS GABAergic projections developed a series of adaptive strategies to combat DG interneuron hyperactivity. During early-stage AD, MS-DG GABAergic projections exhibit increased inhibitory synaptic strength onto DG interneurons to inhibit their activities. During late-stage AD, MS-DG GABAergic projections form higher anatomical connectivity with DG interneurons and exhibit aberrant outgrowth to increase the inhibition onto DG interneurons.

CONCLUSION

We report the structural and functional remodeling of the MS-DG GABAergic circuit during disease progression in 3xTg-AD mice. Dynamic MS-DG GABAergic circuit remodeling represents a compensatory mechanism to combat DG interneuron hyperactivity induced by reduced GABA transmission.

Targeting thalamic circuits rescues motor and mood deficits in PD mice

Although bradykinesia, tremor and rigidity are the hallmark motor defects in patients with Parkinson's disease (PD), patients also experience motor learning impairments and non-motor symptoms such as depression¹. The neural circuit basis for these different symptoms of PD are not well understood. Although current treatments are effective for locomotion deficits in PD^2,3, therapeutic strategies targeting motor learning deficits and non-motor symptoms are lacking^4-6. Here we found that distinct parafascicular (PF) thalamic subpopulations project to caudate putamen (CPu), subthalamic nucleus (STN) and nucleus accumbens (NAc). Whereas PF→CPu and PF→STN circuits are critical for locomotion and motor learning, respectively, inhibition of the PF→NAc circuit induced a depression-like state. Whereas chemogenetically manipulating CPu-projecting PF neurons led to a long-term restoration of locomotion, optogenetic long-term potentiation (LTP) at PF→STN synapses restored motor learning behaviour in an acute mouse model of PD. Furthermore, activation of NAc-projecting PF neurons rescued depression-like phenotypes. Further, we identified nicotinic acetylcholine receptors capable of modulating PF circuits to rescue different PD phenotypes. Thus, targeting PF thalamic circuits may be an effective strategy for treating motor and non-motor deficits in PD.

Brain-wide mapping of inputs to the mouse lateral posterior (LP/Pulvinar) thalamus-anterior cingulate cortex network

The rodent homolog of the primate pulvinar, the lateral posterior (LP) thalamus, is extensively interconnected with multiple cortical areas. While these cortical interactions can span the entire LP, subdivisions of the LP are characterized by differential connections with specific cortical regions. In particular, the medial LP has reciprocal connections with frontoparietal cortical areas, including the anterior cingulate cortex (ACC). The ACC plays an integral role in top‐down sensory processing and attentional regulation, likely exerting some of these functions via the LP. However, little is known about how ACC and LP interact, and about the information potentially integrated in this reciprocal network. Here, we address this gap by employing a projection‐specific monosynaptic rabies tracing strategy to delineate brain‐wide inputs to bottom‐up LP→ACC and top‐down ACC→LP neurons. We find that LP→ACC neurons receive inputs from widespread cortical regions, including primary and higher order sensory and motor cortical areas. LP→ACC neurons also receive extensive subcortical inputs, particularly from the intermediate and deep layers of the superior colliculus (SC). Sensory inputs to ACC→LP neurons largely arise from visual cortical areas. In addition, ACC→LP neurons integrate cross‐hemispheric prefrontal cortex inputs as well as inputs from higher order medial cortex. Our brain‐wide anatomical mapping of inputs to the reciprocal LP‐ACC pathways provides a roadmap for understanding how LP and ACC communicate different sources of information to mediate attentional control and visuomotor functions.

Shared and distinct transcriptomic cell types across neocortical areas

The neocortex contains a multitude of cell types that are segregated into layers and functionally distinct areas. To investigate the diversity of cell types across the mouse neocortex, here we analysed 23,822 cells from two areas at distant poles of the mouse neocortex: the primary visual cortex and the anterior lateral motor cortex. We define 133 transcriptomic cell types by deep, single-cell RNA sequencing. Nearly all types of GABA (γ-aminobutyric acid)-containing neurons are shared across both areas, whereas most types of glutamatergic neurons were found in one of the two areas. By combining single-cell RNA sequencing and retrograde labelling, we match transcriptomic types of glutamatergic neurons to their long-range projection specificity. Our study establishes a combined transcriptomic and projectional taxonomy of cortical cell types from functionally distinct areas of the adult mouse cortex.

Nontoxic, double-deletion-mutant rabies viral vectors for retrograde targeting of projection neurons

Recombinant rabies viral vectors have proven useful for applications including retrograde targeting of projection neurons and monosynaptic tracing, but their cytotoxicity has limited their use to short-term experiments. Here we introduce a new class of double-deletion-mutant rabies viral vectors that left transduced cells alive and healthy indefinitely. Deletion of the viral polymerase gene abolished cytotoxicity and reduced transgene expression to trace levels but left vectors still able to retrogradely infect projection neurons and express recombinases, allowing downstream expression of other transgene products such as fluorophores and calcium indicators. The morphology of retrogradely targeted cells appeared unperturbed at 1 year postinjection. Whole-cell patch-clamp recordings showed no physiological abnormalities at 8 weeks. Longitudinal two-photon structural and functional imaging in vivo, tracking thousands of individual neurons for up to 4 months, showed that transduced neurons did not die but retained stable visual response properties even at the longest time points imaged.

Lentiviral vectors for retrograde delivery of recombinases and transactivators

Lentiviral vectors pseudotyped with the rabies virus (RV) envelope glycoprotein efficiently infect via axon terminals to stably deliver transgenes to distant neurons projecting to an injection site, but the resulting expression levels are too low and variable for most neuroscientific applications. If used to deliver recombinases or transactivators, however, lentiviral vectors are excellent means of targeting projection neurons when used in reporter mice or in combination with a second virus to express "payload" transgenes at high levels. For retrograde infection of significant numbers of neurons, high virus titers are critical. Here we present reagents and a protocol for generating high-titer supernatants that can be concentrated 1000-fold for final titers in excess of 10(10) infectious units per milliliter. We demonstrate the usefulness of these vectors by selectively targeting corticothalamic and corticotectal neurons for high-level expression of a fluorophore in knock-in reporter mice.

Concentration and purification of rabies viral and lentiviral vectors

Rabies viral and lentiviral vectors are very useful tools for neuroscientists, but high titer and purity are critical for in vivo applications. Here we present a protocol for concentration and purification of viral stocks by ultracentrifugation on a sucrose step gradient to remove impurities of both higher and lower densities than the virus itself, with sucrose removed by a subsequent pelleting step. The final stocks are concentrated in volume by a factor of up to 1000, with higher expected purity than is obtained following previously published protocols for preparing G-deleted rabies viral vectors.

Axonal and subcellular labelling using modified rabies viral vectors

An important aspect of any neural circuit is the placement of its output synapses, at levels ranging from macroscopic to subcellular. The many new molecular tools for locating and manipulating synapses are limited by the viral vectors available for delivering them. Adeno-associated viruses are the best current means of labeling and manipulating axons and synapses, but they have never expressed more than one transgene highly enough to label fine axonal structure while also labeling or perturbing synapses. Their slow expression also makes them incompatible with retrograde and transsynaptic vectors, preventing powerful combinatorial experiments. Here we show that deletion-mutant rabies virus can be specifically targeted to cells local to an injection site, brightly labeling axons even when coexpressing two other transgenes. We demonstrate several novel capabilities: simultaneously labeling axons and presynaptic terminals, labeling both dendrites and postsynaptic densities, and simultaneously labeling a region’s inputs and outputs using coinjected vectors.

Production of glycoprotein-deleted rabies viruses for monosynaptic tracing and high-level gene expression in neurons

Recombinant rabies viruses rendered replication-deficient by the deletion of their envelope glycoprotein gene are useful tools for neuroscientists, permitting (1) extraordinarily high transgene expression levels within neurons, (2) retrograde infection of projection neurons through their axon terminals, (3) targeted infection of genetically specified neurons and (4) monosynaptic tracing of neuronal inputs. Here we present a detailed protocol for the production of high-titer and high-purity viral stocks, from initial generation of infectious virus from cDNA through amplification on complementing cell lines, pseudotyping if desired, purification by ultracentrifugation and titering. The procedure requires 3-4 weeks to complete.

Metropolitan Detroit's network: "...informed by magic numbers"

Wayne State University Library's serials automation project is evolutionary in concept, with its first completed stage designed to give information on titles and holdings. Its master tape includes, in addition to Wayne State University holdings, data for more than twenty biomedical libraries in the area, and print options are available that will produce a full union list or individual lists singly or in any combination.