How do neurons recognize self from non-self?

PROJECT SUMMARY
This essay aims to understand a fundamental property of neurons: their ability to self-recognize and self-
avoid. Self-avoidance is an essential aspect of a neuron's function as it ensures that branches from the same
cell minimize their overlap while maximizing their interactions with branches from other cells. In mammals, at
the core of this process is the generation of sufficient Protocadherin (Pcdh) protein isoform diversity such that
essentially every neuron in the brain is differentially barcoded at its surface and therefore appears different to
other neurons. The generation of Pcdh protein isoform diversity requires complex mechanisms of Pcdh
transcriptional and pre-mRNA splicing such that distinct Pcdh mRNA molecules - bearing a different 5' end
(variable exon) but an identical 3' end (constant exons) - are expressed in individual cells. Understanding how
different Pcdh mRNA molecules are produced represents a long-term mystery in the field of neuroscience.
Answering this fundamental mystery is key in illuminating the process of neuronal self-avoidance and
represents the first essential step toward dissecting how dysregulation of this Pcdh mediated self-avoidance
can lead to severe neurological disorders, such as for instance autism spectrum disorder and schizophrenia.
Despite their critical function in the brain, however, limited progress has been made in understanding how
Pcdh mRNAs are transcribed and properly spliced as general models of gene expression regulation have
failed to recapitulate this complex mechanism and as the tools required to study it directly in vivo have lagged
behind. In this proposal, we aim to (i) test a paradigm-shifting hypothesis of Pcdh RNA transcription and
splicing based on alternative trans-splicing of variable and constant exons encoded in tandem on the same
DNA strand - a mechanism that we propose to be regulated by the 3D genome topology of the Pcdh locus -
and (ii) design technological innovations that will allow precise manipulation of the Pcdh gene cluster in vivo
to test our hypothesis directly in neurons. These studies have the potential to illuminate the complex
mechanism of the generation of Pcdh isoform diversity and its role in neuronal self-avoidance and wiring of
healthy and disease brains. The findings from these studies are also poised to open up a new class of
regulatory mechanisms of RNA processing reactions, previously unobserved and uncharacterized in
mammals but that we speculate are utilized by cells to overcome challenging problems of pre-mRNA splicing
associated with complex gene architectures.

Grant Number: 4DP2MH129955-02
NIH Institute/Center: NIH
Principal Investigator: Daniele Canzio