Inter‐rhombomeric interactions reveal roles for fibroblast growth factors signaling in segmental regulation of EphA4 expression

Abstract

Background The basic ground plan of vertebrate hindbrain is established through a process of segmentation, which generates eight transient lineage‐restricted cellular compartments called rhombomeres (r). The segments adopt distinct individual identities in response to axial patterning signals. It is unclear whether signaling between rhombomeres plays a conserved role in regulating segmental patterning during hindbrain development. Results Using tissue manipulations of rhombomeres in chicken embryos, we have uncovered roles for r2 and r4 in regulating the expression of EphA4 in r3 and r5. Perturbations of signaling pathways reveal that these regulatory inputs from r2 and r4 into EphA4 expression are mediated independent of inputs from Krox20 through cues involving fibroblast growth factor (FGF) signaling. These interactions are stage dependent and are set up in embryos with <10 somites. Conclusions We show that r2 and r4 function as temporally dynamic signaling centers in the early patterning of adjacent hindbrain segments and this activity is dependent upon the FGF pathway. These results reveal that inter‐rhombomeric signaling is a conserved feature of the regulatory networks that control the specification of individual rhombomere identities in vertebrate hindbrain segmentation. However, the timing of when restricted domains of FGF signaling are coupled to formation of r4 may vary between the species.

New developments in the biology of fibroblast growth factors

“…In the hindbrain, FGF3, FGF8, and RA signaling from hindbrain primordia are essential for setting up the initial identities of hindbrain segments (rhombomeres) by regulating the expression of transcription factors such as Krox20 that control segment identity (Frank & Sela‐Donenfeld, 2019; Parker et al, 2016). In recent studies, rhombomeres 2 and 4 were found to function as temporally dynamic signaling centers that use FGF to regulate the expression of Ephrin type‐A receptor 4 ( EphA4 ) independent of Krox20 in adjacent rhombomeres 3 and 5 (Cambronero et al, 2020).…”

Segmentation and patterning of the vertebrate hindbrain

“…In combination with vhnf1 (Hnf1b) and FGF signalling, these transcription factors specify individual rhombomere identities in the caudal hindbrain (r4-r7) (Parker and Krumlauf, 2017;Sun and Hopkins, 2001;Wiellette and Sive, 2003). In chick embryos, there is evidence for secondary Fgf signalling centres in r2 and r4 that participate in regulating Krox20-independent EphA4 expression (Cambronero et al, 2020). Collectively, this illustrates the presence of crosstalk and regulatory feedback between modules in the GRN.…”