Atoh1 is required for the formation of lateral line electroreceptors and hair cells, whereas Foxg1 represses an electrosensory fate

Abstract

In electroreceptive jawed fishes and amphibians, individual lateral line placodes form lines of neuromasts on the head containing mechanosensory hair cells, flanked by fields of ampullary organs containing electroreceptors - modified hair cells that respond to weak electric fields. Extensively shared gene expression suggests that conserved molecular mechanisms are involved in their development, but a few transcription factor genes are restricted either to the developing electrosensory or mechanosensory lateral line. Here, we used CRISPR/Cas9-mediated mutagenesis in F0-injected sterlet embryos (Acipenser ruthenus, a sturgeon) to test the function of three such genes. We found that the 'hair cell' transcription factor gene Atoh1 is required for both hair cell and electroreceptor differentiation in sterlet, and for Pou4f3 and Gfi1 expression in both neuromasts and ampullary organs. These data support the conservation of developmental mechanisms between hair cells and electroreceptors. Targeting ampullary organ-restricted Neurod4 did not yield any phenotype, potentially owing to redundancy with other Neurod genes that we found to be expressed in sterlet ampullary organs. After targeting mechanosensory-restricted Foxg1, ampullary organs formed within neuromast lines, suggesting that Foxg1 normally represses their development. We speculate that electrosensory organs may be the 'default' fate of lateral line primordia in electroreceptive vertebrates.

Two opposing roles for Bmp signalling in the development of electrosensory lateral line organs

“…Different 1-2 cell embryos from some of the same batches were injected with Cas9 protein complexed with different combinations of single-guide (sg) RNAs targeting Bmp5 . Embryos targeted for the melanin-producing enzyme gene tyrosinase ( Tyr ) were used as negative controls: this yields a visible phenotype (i.e., defects in pigmentation), but should not affect other developmental processes (preprint, Minařík et al, 2024b).…”

“…Although these phenotypes are distinct, a common theme emerges, namely the active repression of ampullary organ formation during normal development: within neuromast lines by Foxg1, and within ampullary organ fields by Bmp signalling. Taken together, this suggests that lateral line primordia are ’poised’ to form ampullary organs (indeed potentially that ampullary organs are the ’default’ fate for lateral line primordia in electroreceptive species; preprint, Minařík et al, 2024b), and this must be controlled to ensure that ampullary organs develop in the ’correct’ number and location.…”

“…Four CRISPR single guide RNAs (sgRNAs) were designed using the CRISPR Guide RNA Design Tool from Benchling (https://benchling.com) to target a 450-base region within exon 1 that encodes part of the TGFβ propeptide domain (Table 1; Supplementary Figure S5A). The previously published guides against tyrosinase were designed as described in Minařík et al (2024b, preprint).…”

“…We recently reported that the transcription factor gene Foxg1 is expressed in paddlefish and sterlet in the central region of sensory ridges where neuromasts form (Minařík et al, 2024a), and that targeting Foxg1 for CRISPR/Cas9-mediated led to ampullary organs forming within neuromast lines (preprint, Minařík et al, 2024b). Here, we found that Bmp signalling is required to prevent supernumerary ampullary organ formation within ampullary organ fields, including ectopic ampullary organs within the small dorsalmost fields, although neuromast lines developed normally (apart from the ectopic offshoot of the supraorbital line; see next section).…”

“…Investigation of additional candidate genes from the paddlefish lateral line-enriched dataset in paddlefish and sterlet identified both mechanosensory-restricted and electrosensory-restricted transcription factor gene expression (Modrell et al, 2017a; Minařík et al, 2024a). We recently used CRISPR/Cas9-mediated mutagenesis in G0-injected sterlet embryos to identify a conserved requirement for the ’hair cell’ transcription factor Atoh1 in electroceptor formation and identified a role for mechanosensory-restricted Foxg1 in blocking ampullary organ formation within neuromast lines (preprint, Minařík et al, 2024b).…”