A cellular and molecular analysis of SoxB-driven neurogenesis in a cnidarian

Abstract

Neurogenesis is the generation of neurons from stem cells, a process that is regulated by SoxB transcription factors (TFs) in many animals. Although the roles of these TFs are well understood in bilaterians, how their neural function evolved is unclear. Here, we use Hydractinia symbiolongicarpus, a member of the early-branching phylum Cnidaria, to provide insight into this question. Using a combination of mRNA in situ hybridization, transgenesis, gene knockdown, transcriptomics, and in vivo imaging, we provide a comprehensive molecular and cellular analysis of neurogenesis during embryogenesis, homeostasis, and regeneration in this animal. We show that SoxB genes act sequentially at least in some cases. Stem cells expressing Piwi1 and Soxb1, which have broad developmental potential, become neural progenitors that express Soxb2 before differentiating into mature neural cells. Knockdown of SoxB genes resulted in complex defects in embryonic neurogenesis. Hydractinia neural cells differentiate while migrating from the aboral to the oral end of the animal, but it is unclear whether migration per se or exposure to different microenvironments is the main driver of their fate determination. Our data constitute a rich resource for studies aiming at addressing this question, which is at the heart of understanding the origin and development of animal nervous systems.

Pluripotent, germ cell competent adult stem cells underlie cnidarian plant-like life history

“…Hydractinia i-cells derive from early blastomeres, first appearing lodged in the interstitial spaces of the embryo’s internal tissue. They are marked by Piwi1, Piwi2 , and Soxb1 expression (Chrysostomou et al, 2022). Post metamorphosis of the swimming larva, the animal enters a sedentary life, attached to the substratum.…”

“…The cell cycle of i-cells lacks a pronounced G1 phase (Chrysostomou et al ., 2022) but whether these cells continuously cycle is unknown. We found that animals incubated in EdU for 4 days had nearly 100% of their polyp i-cells labeled (Figure S1).…”

“…Hydractinia i-cells are a relatively rare cell population, comprising only 1% of the animal’s cells (Chrysostomou et al ., 2022; DuBuc et al ., 2020). Our data suggest that most if not all i-cells, marked by high Soxb1, Piwi1 , and Piwi2 expression, are pluripotent stem cells.…”

Senescence-induced cellular reprogramming drives cnidarian whole-body regeneration

“…Oligo probes were prepared according to (Kishi et al, 2019) using hairpins 27 and 30. Tissue was fixed and dehydrated as previously described in (Chrysostomou et al ., 2022; DuBuc et al ., 2020): Tissue samples were incubated for 15 minutes in 4% MgCl2 (in 50% distilled water / 50% filtered seawater) and fixed in two steps. Fixative one: ice cold 0.2% glutaraldehyde (stock: 25%, Sigma-Aldrich; G5882), 4% paraformaldehyde (stock: 16%, Alfa Aesar; 43368) in filtered seawater for 90 seconds.…”

“…The generation of stable transgenic reporter animals was carried out as previously described (Künzel et al, 2010). Wnt3::GFP (DuBuc et al, 2020) and RFamide::GFP (Chrysostomou et al, 2022) reporters were previously generated in our lab. We used the GENEius online tool from Eurofins Genomics to codon optimize all the synthetic gene sequences for Hydractinia symbiolongicarpus .…”