Cell fate specification in the lingual epithelium is controlled by antagonistic activities of Sonic hedgehog and retinoic acid

Abstract

The interaction between signaling pathways is a central question in the study of organogenesis. Using the developing murine tongue as a model, we uncovered unknown relationships between Sonic hedgehog (SHH) and retinoic acid (RA) signaling. Genetic loss of SHH signaling leads to enhanced RA activity subsequent to loss of SHH-dependent expression of Cyp26a1 and Cyp26c1. This causes a cell identity switch, prompting the epithelium of the tongue to form heterotopic minor salivary glands and to overproduce oversized taste buds. At developmental stages during which Wnt10b expression normally ceases and Shh becomes confined to taste bud cells, loss of SHH inputs causes the lingual epithelium to undergo an ectopic and anachronic expression of Shh and Wnt10b in the basal layer, specifying de novo taste placode induction. Surprisingly, in the absence of SHH signaling, lingual epithelial cells adopted a Merkel cell fate, but this was not caused by enhanced RA signaling. We show that RA promotes, whereas SHH, acting strictly within the lingual epithelium, inhibits taste placode and lingual gland formation by thwarting RA activity. These findings reveal key functions for SHH and RA in cell fate specification in the lingual epithelium and aid in deciphering the molecular mechanisms that assign cell identity.

Distinct and Overlapping Expression Patterns of the Homer Family of Scaffolding Proteins and Their Encoding Genes in Developing Murine Cephalic Tissues

“…At E14.5 and at 1 dpp, Homer1, Homer2 and Homer3 transcripts and/or proteins were expressed in the developing tongue ( Figure 7; Table S1; see also Figure 5). In a previous study, we showed that Homer1 protein is expressed in taste buds of postnatal mice [61]. In the present study, we found that not only postnatal taste buds ( Figure 7G,J) but also embryonic differentiating taste buds ( Figure 7A,D) expressed Homer1 protein and mRNA.…”

“…NBP2-32607) were from Novus Biologicals (Abingdon, UK). Immunohistochemistry was carried out in dewaxed tissue sections as described previously [61,104,105]. Briefly, following antigen unmasking in 10 mM citrate buffer, pH 6, endogenous peroxidase was blocked by incubating the sections in 3% hydrogen peroxide in methanol.…”

“…The sections were not counterstained after visualization of hybridization signals. Hybridization signals detected by oligonucleotide probes and the RNAscope method using chromogenic substrates appear as sparse puncta/precipitates when weak to moderate levels of transcripts are expressed in a given tissue/cell [106][107][108] or as aggregated puncta/precipitates when the hybridization signals are robust [61,[106][107][108]. For detection of Homer mRNAs by in situ hybridization, sections across prenatal and postnatal heads encompassing the brain and other cephalic tissues were processed under the same conditions.…”

Regulating Retinoic Acid Availability during Development and Regeneration: The Role of the CYP26 Enzymes

“…The same mechanism in the tongue epithelium of Shh LOF mutants led to change in cell fate specification causing the formation of ectopic salivary glands and increased size taste-buds. Thus, Shh-mediated activation of Cyp26a1/c1 acts to control RA availability in these tissues [345].…”

Onset of taste bud cell renewal starts at birth and coincides with a shift in SHH function

“…In addition to marking taste placodes in embryonic tongue and post-mitotic precursor cells in mature taste buds, SHH is a key regulator of taste bud development and homeostasis. In embryos, SHH functions to repress taste fate as taste placodes are specified and patterned (El Shahawy et al, 2017;Hall et al, 2003;Iwatsuki et al, 2007;Mistretta et al, 2003); while once taste placodes are established, embryonic taste primordia no longer respond to pharmacological manipulation of Hedgehog signaling (Liu et al, 2004). By contrast, in adults SHH promotes and is required for TRC differentiation.…”