# The homeodomain transcription factor Phox2 in the stellate ganglion of the squid Loligo pealei

## TL;DR

This paper identifies a homeodomain transcription factor, lpPhox2, in the squid Loligo pealei and explores its role in neuronal development and function.

## Contribution

The study identifies the squid ortholog of Phox2a/b and shows its expression pattern and potential regulatory role in neuronal identity.

## Key findings

- lpPhox2 is expressed in the developing and adult stellate ganglion of squid, specifically in the giant fiber lobe.
- lpPhox2 shares homology with mammalian Phox2 only in the homeodomain and cannot transactivate the DBH promoter in mammalian cells.
- The Fmrf gene's upstream region contains potential Phox2a/b binding sites, suggesting regulatory interaction.

## Abstract

Homeodomain transcription factors regulate development of embryos and cellular physiology in adult systems. Paired-type homeodomain genes constitute a subclass that has been particularly implicated in establishment of neuronal identity in the mammalian nervous system. We isolated fragments of eight homeodomain genes of this subclass expressed in the stellate ganglion of the North Atlantic long finned squid Loligo pealei (lp) [Note: Loligo pealei has been officially renamed Doryteuthis pealei. For reasons of uniformity and clarity Loligo pealei (lp) is used here]. Of the most abundant ones, we cloned a full length cDNA which encoded the squid ortholog of the paired-type homeodomain proteins Phox2a/b. The homology of lpPhox2 to invertebrate and mammalian Phox2 was limited to the homeodomain. In contrast to mouse Phox2b, lpPhox2 was unable to transactivate the dopamine beta-hydroxylase (DBH) promoter in a heterologous mammalian transfection system. In vivo, lpPhox2 was expressed in the developing stellate ganglion of stage 27 squid embryos and continued to be expressed in the adult stellate neurons where expression was confined to the giant fiber lobe containing the neurons that form the giant axons. The expression of lpPhox was similarly timed and distributed as the Fmrf gene. Furthermore, the Fmrf upstream region contained putative Phox2a/b binding sites. These results suggest a role of lpPhox2 in the developmental specification of neuronal identity and regulation of neurons of the squid giant axon.