The mouse brain contains about 75 million neurons interconnected in a vast array of neural circuits. The identities and functions of individual neuronal components of most circuits are undefined. Here we describe a method, termed “Connect-seq,” which combines retrograde viral tracing and single-cell transcriptomics to uncover the molecular identities of upstream neurons in a specific circuit and the signaling molecules they use to communicate. Connect-seq can generate a molecular map that can be superimposed on a neuroanatomical map to permit molecular and genetic interrogation of how the neuronal components of a circuit control its function. Application of this method to hypothalamic neurons controlling physiological responses to fear and stress reveals subsets of upstream neurons that express diverse constellations of signaling molecules and can be distinguished by their anatomical locations.
The mouse brain contains ~100 million neurons interconnected in a vast array of neural circuits.The identities and functions of individual neuronal components of most circuits are undefined.Here we describe a method, termed 'Connect-seq', which combines retrograde viral tracing and single cell transcriptomics to uncover the molecular identities of upstream neurons in a specific circuit and the signaling molecules they use to communicate. Connect-seq can generate a molecular map that can be superimposed on a neuroanatomical map to permit molecular and genetic interrogation of how the neuronal components of a circuit control its function.
Gene regulation occurs through trans-acting factors (e.g. transcription factors) acting on cis-regulatory elements (e.g. enhancers). Massively parallel reporter assays (MPRAs) functionally survey large numbers of cis-regulatory elements for regulatory potential, but do not identify the trans-acting factors that mediate any observed effects. Here we describe transMPRA -- a reporter assay that efficiently combines multiplex CRISPR-mediated perturbation and MPRAs to identify trans-acting factors that modulate the regulatory activity of specific enhancers.
Animals exhibit instinctive behavioral and physiological responses to a variety of stressors to overcome danger and restore homeostasis. The physiological response to stress is governed by hypothalamic corticotropin-releasing hormone (CRH) neurons which regulate the hypothalamic-pituitary-adrenal axis to control blood levels of stress hormones. At present, the neural circuits and signaling mechanisms through which different stress signals are transmitted to CRH neurons are poorly understood. Here, we devised a new method, termed “Connect-Seq,” which couples single-cell transcriptomics and retrograde viral tracing to define the molecular identities of individual neurons in neural circuits. As a proof of concept, using Connect-Seq, we profiled single-cell transcriptomes of 124 brain neurons upstream of CRH neurons and identified subpopulations that are likely to communicate stress-related signals to CRH neurons. Analyses of single-cell transcriptomes for ‘fast-acting’ neurotransmitters revealed subsets of upstream neurons that expressed markers of inhibitory GABAergic neurons or excitatory glutamatergic neurons. Further analyses showed a number of other neuromodulators/neurotransmitters in upstream neurons, including acetylcholine, dopamine, histamine, and 43 different neuropeptides, each expressed in individual neurons or subsets of neurons. These findings reveal extreme molecular heterogeneity among upstream neurons and suggest the upstream neurons use diverse neurochemical messengers to transmit signals to CRH neurons. Many neurons coexpressed different neurotransmitters/neuromodulators, suggesting the co-release of neurochemical messengers. Dual labeling of brain sections verified expression of specific neuromodulators in virus-infected neurons upstream of CRH neurons in selected brain areas. Our results indicate that Connect-Seq can be applied to genetically dissect neural circuits and uncover molecular identities of neurons upstream of specific neuronal types of known function. Molecular markers identified in those neurons lay a foundation for the application of cell-specific genetic tools to investigate the functions and physiological significance of diverse neuronal subsets within complex neural circuits.
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