# The rewiring of a terminal selector regulatory cascade generates convergent neuronal laterality

## TL;DR

The study reveals how a nematode species uses miRNA regulation to generate distinct neuronal identities and left-right asymmetry.

## Contribution

The paper identifies a miRNA-mediated regulatory mechanism that controls neuronal fate decisions in Pristionchus pacificus.

## Key findings

- P. pacificus ASE neurons show left-right asymmetry in receptor-type guanylyl-cyclase expression.
- AFD neuron precursors transiently co-express multiple ASE and AFD markers before specializing.
- miR-8345 and pash-1 regulate neuronal fate by toggling between ASE and AFD identities via the cog-1 3’ UTR.

## Abstract

Neuronal identity is established and maintained by “terminal-selector” transcription factors, yet how these networks evolve remains unclear. We examined the specification of the chemosensory ASE and thermosensory AFD neurons in the nematode Pristionchus pacificus, a species that expresses the terminal-selector, Ppa-CHE-1, in both sensory neurons. To determine if the ASE neurons exhibit left-right laterality, we used HCR-FISH and transgenic reporters to discover 8 ASE left-right-specific and 3 AFD-specific receptor-type guanylyl-cyclases. Late embryos exhibit a multipotential state in which AFD precursors transiently co-express all three types of ASEL, ASER and AFD markers. A forward genetic screen for defects in ASER asymmetry identified a Ppa-DIE-1 homolog, whereas targeted mutations revealed the maintenance of AFD neuronal identity requires another terminal-selector, Ppa-TTX-1, and CNG channels, Ppa-TAX-2/TAX-4. Mutations in the microRNA miR-8345 and pash-1 responsible for miRNA-processing convert ASEL to ASER fate while changes to other conserved regions in the 3′ UTR of the cog-1 homolog reveal multiple sites that act as a toggle between left/right ASE versus AFD identities. Together, these results demonstrate that P. pacificus deploys a miRNA-mediated regulatory repertoire to generate three distinct neuronal fates through the Ppa-cog-1 3’ UTR as a key regulatory nexus.

Transcription factors known as terminal selectors help neurons acquire and maintain their specific identities, such as sensing soluble chemicals and temperature. In nematodes, while most sensory neurons develop together in pairs, the chemosensory neurons further exhibit left-right laterality in chemosensory receptor expression. Surprisingly, we discovered that even the thermosensory neurons not destined for chemosensory receptor expression show potential for multiple terminal fates, transiently co-expressing chemosensory markers early in embryogenesis before restricting their expression to only thermosensory receptors. Like the nematode C. elegans, P. pacificus uses microRNA-based regulation to produce distinct left-right neuron sub-types. Unlike C. elegans however, P. pacificus employs multiple post-transcriptional regulators in a conserved negative feedback loop as a toggle switch between 3 distinct terminally differentiated developmental fates. Since little is known about how miRNA and their targets evolve, this study highlights a recurring miRNA regulatory architecture that may improve our understanding of nervous system evolution.