Transgenic targeting of recombinant rabies virus reveals monosynaptic connectivity of specific neurons

Abstract

Understanding how neural circuits work requires a detailed knowledge of cellular-level connectivity. Our current understanding of neural circuitry is limited by the constraints of existing tools for transsynaptic tracing. Some of the most intractable problems are a lack of cellular specificity of uptake, transport across multiple synaptic steps conflating direct and indirect inputs, and poor labeling of minor inputs. We used a novel combination of transgenic mouse technology and a recently developed tracing system based on rabies virus (Wickersham et al., 2007a,b) to overcome all three constraints. Because the virus requires transgene expression for both initial infection and subsequent retrograde transsynaptic infection, we created several lines of mice that express these genes in defined cell types using the tetracycline-dependent transactivator system (Mansuy and Bujard, 2000). Fluorescent labeling from viral replication is thereby restricted to defined neuronal cell types and their direct monosynaptic inputs. Because viral replication does not depend on transgene expression, it provides robust amplification of signal in presynaptic neurons regardless of input strength. We injected virus into transgenic crosses expressing the viral transgenes in specific cell types of the hippocampus formation to demonstrate cell-specific infection and monosynaptic retrograde transport of virus, which strongly labels even minor inputs. Such neuron-specific transgenic complementation of recombinant rabies virus holds great promise for obtaining cellular-resolution wiring diagrams of the mammalian CNS.

Figure 1.

Transgenic specification of viral infection. A, Location of injection site of EnvA-pseudotyped lentivirus control. B–D, All infected (green) neurons were encircled and compared with TVA ISH (B) to determine transgene expression (C) and a Nissl stain to count nuclei (D). Although only a minority of neurons were mRNA-positive (191/1015), every infected neuron was mRNA-positive (43/43), suggesting restriction of infection to transgenic neurons. CA1, Field CA1 of hippocampus; CA3, field CA3 of hippocampus; DG, dentate gyrus; S, subiculum.

Figure 2.

Monosynaptic restriction of viral transport. A sagittal section from a transgenic cross injected with SADΔG-mCherry(EnvA) in the CA1 pyramidal cell layer. A, Viral mCherry label alone. B, Same section as A hybridized to TVA mRNA. This cross only expresses TVA mRNA in a small minority of neurons in the subiculum and CA1 pyramidal cell layer, not in CA3 pyramidal neurons. As can be seen in A, viral transport is restricted to direct presynaptic inputs of CA1 pyramidal neurons (CA3 neurons), with no label in dentate granule neurons, known to be presynaptic to CA3 pyramidal neurons. Note the anticorrelation between mRNA expression and viral signal around the injection site, presumably caused by viral interference with host cell gene expression. CA1, Field CA1 of hippocampus; CA3, field CA3 of hippocampus; DG, dentate gyrus; hf, hippocampal fissure.

Figure 3.

Strong labeling of minor inputs. A–E, Composite images of sagittal (A, B, E) and coronal (C, D) sections demonstrating retrograde viral (mCherry) transport to relatively minor inputs to hippocampal pyramidal neurons, including limbic thalamus (A, C), ventral tegmental area (B), and basal forebrain cholinergic nuclei (D, E). As well as mCherry label (middle), E shows connectivity from CA1 neurons through the fornix fimbria to the basal forebrain cholinergic nuclei via Nissl (left) and Nissl overlay (right). CA1, Field CA1 of hippocampus; DG, dentate gyrus; 3V, third ventricle; AcbC, accumbens nucleus, core; cc, corpus callosum; Cg2, cingulate cortex, area 2; CM, central medial thalamic nucleus; df, dorsal fornix; f, fornix; fr, fasciculus retroflexus; gcc, genu of the corpus callosum; ic, internal capsule; LSI, lateral septal nucleus, intermediate part; LV, lateral ventricle; MDM, mediodorsal thalamic nucleus, medial part; MS, medial septum; mt, mammillothalamic tract; ns, nigrostriatal bundle; opt, optic tract; Re, reuniens thalamic nucleus; scc, splenium of the corpus callosum; VDB, vertical limb of the diagonal band of Broca; VTA, ventral tegmental area.