Retinoic acid signaling and neurogenic niche regulation in the developing peripheral nervous system of the cephalochordate amphioxus
The retinoic acid (RA) signaling pathway regulates axial patterning and neurogenesis in the developing central nervous system (CNS) of chordates, but little is known about its roles during peripheral nervous system (PNS) formation and about how these roles might have evolved. This study assesses the requirement of RA signaling for establishing a functional PNS in the cephalochordate amphioxus, the best available stand-in for the ancestral chordate condition. Pharmacological manipulation of RA signaling levels during embryogenesis reduces the ability of amphioxus larvae to respond to sensory stimulation and alters the number and distribution of ectodermal sensory neurons (ESNs) in a stage- and context-dependent manner. Using gene expression assays combined with immunohistochemistry, we show that this is because RA signaling specifically acts on a small population of soxb1c-expressing ESN progenitors, which form a neurogenic niche in the trunk ectoderm, to modulate ESN production during elongation of the larval body. Our findings reveal an important role for RA signaling in regulating neurogenic niche activity in the larval amphioxus PNS. Although only few studies have addressed this issue so far, comparable RA signaling functions have been reported for neurogenic niches in the CNS and in certain neurogenic placode derivatives of vertebrates. Accordingly, the here-described mechanism is likely a conserved feature of chordate embryonic and adult neural development.
KeywordsEvolution of development Lancelet Neural stem cells Retinoid pathway Sensory functions
The authors would like to thank Thurston C. Lacalli, Nicholas D. Holland, and Linda Z. Holland for fruitful discussions. We are also grateful to Ram Reshef for his vital support with administrative issues.
EZ designed and performed experiments, analyzed and interpreted data, and wrote the manuscript. GG supported the collection of gene expression data, NSMR carried out phylogenetic analyses, and JKY contributed important advice concerning the selection of candidate genes. JKY, JCC, and SC provided methodological assistance, supported data analyses, and commented the manuscript. MS designed and supervised the study, analyzed and interpreted data, and wrote the manuscript. All authors have read and approved the manuscript.
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All data used in this study are included in this published article and its supplementary materials.
Conflict of interest
The authors declare that they have no competing interests.
Additional file 1: Movie S1. Movie showing reactions of amphioxus larvae at 48 hpf (hours post fertilization) to mechanical stimulation. Response A = quick muscular swimming movement away from the stimulus. Response B = intense wiggling and bending movements without clear directionality. Response C = short wiggling motion on the spot. Response D = short disconnected twitches or bends on the spot. Response E = no visible reaction (MP4 26973 kb)
Additional file 2: Movie S2. Movie showing responses of amphioxus larvae at 48 hpf (hours post fertilization) to chemical stimulation. As indicated in the movie, the amphioxus embryos were exposed to dimethyl sulfoxide (DMSO) (Control), the retinoic acid receptor (RAR) antagonist BMS493 or all-trans retinoic acid (RA), starting from treatment time points (t) at 6 or 24 hpf. Upon reaching the 48 hpf stage, the larvae were further exposed to agarose blocks, which had either been dissolved in artificial seawater (negative control) or in artificial seawater supplemented with 0.1 M l-glutamate (MP4 88999 kb)
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