Can transsynaptic viral strategies be used to reveal functional aspects of neural circuitry?
Section snippets
Brief history of neural circuit mapping and development of one-step viral methods
The brain consists of a highly complex network of neurons which transmit information largely via synaptic connections. The understanding of how neurons are connected is thus critical to understanding both the basic and emergent properties of neural circuits. Though the advent of the microscope enabled an appreciation of the cellular structure of most tissues, the brain remained recalcitrant to study, in large part because thin sections yielded gray blobs of tissue that showed little about the
What is the evidence that viruses label synaptically-connected populations?
RABV and HSV/PRV are commonly assumed to spread through synaptic connections, and this assumption is rarely questioned. The original assertions about the transsynaptic preference of viral transmission can be traced back to an observational study by Ernest Goodpasture and Oscar Teague in 1923, where HSV was transferred from the lip of a human patient onto a rabbit’s cornea, where it transmitted via the optic nerve to the brain and produced encephalitis (Goodpasture and Teague, 1923). RABV and
How efficient is viral transmission? Do viruses label input cells equally?
An important consideration in any viral transneuronal experiment is the percentage of total input cells to the cell population of interest that get labeled by the virus. This number is difficult to discern when tracing inputs onto populations of starter neurons but is feasible when examining inputs onto single cells. The first study to provide this estimate was from Marshel and colleagues, who performed RABV one-step input tracing experiments from single layer 2/3 cortical neurons (Marshel et
To what extent does viral mapping reflect functional connectivity?
The premise of neuroanatomy is that an understanding of neural connectivity is necessary for understanding brain function. However, it is clear that even a detailed understanding of connectivity is not sufficient. An important limitation of viral tracing is that the extent to which viral labeling reflects functional aspects of neuronal connectivity has not been thoroughly explored. One significant barrier is that for most circuits in the brain, we lack a rigorously validated connectivity map of
Conclusion
In consideration of over a decade of RABV one-step tracing experiments from multiple cell types over many different circuits, there is no clear evidence that the extent of viral labeling can be used on its own to reproduce all aspects of functional connectivity. However, the majority of the data suggest that RABV input mapping can be used to detect synaptically-connected neurons, as where RABV labeling has been reported and tested, synaptic connections have also been observed, at least on a
CRediT authorship contribution statement
Alexandra Rogers: Writing - original draft, Writing - review & editing. Kevin T. Beier: Conceptualization, Visualization, Writing - original draft, Writing - review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest
We have no competing interests to declare.
Acknowledgements
We would like to thank Michael Toledano and other members of the Beier lab for a critical review of this manuscript. This review was supported by the NIH (R00 DA041445, DP2 AG067666), Tobacco Related Disease Research Program (T31KT1437, T31IP1426), American Parkinson Disease Association (APDA-5589562), Alzheimer’s Association (AARG-NTF-20-685694), New Vision Research (CCAD2020-002), and the Brain and Behavior Research Foundation (NARSAD 26845).
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2023, Cell Reports MethodsAll-viral tracing of monosynaptic inputs to single birthdate-defined neurons in the intact brain
2022, Cell Reports MethodsCitation Excerpt :The question of whether transmission of RABVs is strictly synapse specific is also under debate. It is therefore advisable to use RABV tracing for comparative and explorative studies rather than aiming for complete quantification of all monosynaptic inputs to cells (see Rogers and Beier [2021] for a comprehensive review of these limitations). This point becomes particularly pertinent when using Cre-dependent viruses because leaky expression of TVA can result in spurious rabies expression in cells that (combined with the staining issues described below) might be wrongly identified as input cells (Figure 2B; Miyamichi et al., 2013).