Unraveling the developmental logic of cortical long-range projections using in situ sequencing-based neuroanatomy

Project Summary
The connectivity of neurons allows complex functions to emerge from a circuit composed of diverse neuronal
types. In the mammalian nervous system, recent advances in single-cell transcriptomics make it appealing to
define neuronal types by their gene expression patterns (i.e. transcriptomic types). At a high level, classes of
neurons defined by transcriptomics are also distinct in other neuronal properties, including their long-range
projection patterns. Finer-level transcriptomic types, however, do not correspond to projection patterns:
neurons of different transcriptomic types may share similar projections, and neurons of the same
transcriptomic type can project diversely. This lack of correspondence at a fine level raises the question of how
neuronal types are wired into complex circuits and, furthermore, how cell types can be defined by both gene
expression and connectivity. Knowing the developmental relationship between gene expression and
projections may help understand this complex relationship, because the projection pattern of an adult neuron is
the cumulative result of many developmental processes. However, interrogating the developmental relationship
between gene expression and projections is challenging, because conventional single-cell anatomical
approaches can only map the projection patterns of a small number of neurons and are difficult to associate
projections with gene expression measured in the same cells. Here I propose to overcome this challenge by
massively improving the resolution and scale of in situ sequencing-based neuroanatomical approaches. In situ
sequencing-based neuroanatomy achieves high throughput and cellular resolution in mapping projections by
labeling each neuron with a unique RNA barcode. These RNA barcodes and endogenous mRNAs can both be
sequenced in situ to associate projection patterns with gene expression for many neurons in parallel. By
improving both the resolution and the throughput of in situ barcode sequencing, I will generate an
unprecedented view of the relationship between gene expression and the complete brain-wide projection
patterns of neurons in the primary and higher visual cortex over the course of post-natal development. I will
complement this systematic but correlational approach with perturbation experiments to establish causal
relationship between key genes and projections. By unraveling how the complex relationship between gene
expression and projections is established step-by-step in development, this combined approach will provide
insights into the wiring rules of cortical neuronal types. The dataset generated will provide a reference for
future research into long-range connectivity defects in neurodevelopmental disease models. Finally, the
improvement in in situ sequencing-based neuroanatomy will achieve broad impact beyond the developmental
focus of this proposal by enabling similar systematic approaches in understanding long-range projections
during aging, across individual animals, and across species.

Grant Number: 4DP2MH132940-02
NIH Institute/Center: NIH
Principal Investigator: XIAOYIN CHEN