Disruption of sonic hedgehog signaling alters growth and patterning of lingual taste papillae

Abstract

Taste buds on the anterior part of the tongue develop in conjunction with epithelial-mesenchymal specializations in the form of gustatory (taste) papillae. Sonic hedgehog (Shh) and Bone Morphogenetic Protein 4 (BMP4) are expressed in developing taste papillae, but the roles of these signaling molecules in specification of taste bud progenitors and in papillary morphogenesis are unclear. We show here that BMP4 is not expressed in the early tongue, but is precisely coexpressed with Shh in papillary placodes, which serve as a signaling center for both gustatory and papillary development. To elucidate the role of Shh, we used an in vitro model of mouse fungiform papillary development to determine the effects of two functional inhibitors of Shh signaling: anti-Shh (5E1) antibody and cyclopamine. Cultured E11.5 tongue explants express Shh and BMP4(LacZ) in a pattern similar to that of intact embryos, localizing to developing papillary placodes after 2 days in culture. Tongues cultured with 5E1 antibody continue to express these genes in papillary patterns but develop more papillae that are larger and closer together than in controls. Tongues cultured with cyclopamine have a dose-dependent expansion of Shh and BMP4(LacZ) expression domains. Both antibody-treated and cyclopamine-treated tongue explants also are smaller than controls. Taken together, these results suggest that, although Shh is not involved in the initial specification of papillary placodes, Shh does play two key roles during pmcry development: (1) as a morphogen that directs cells toward a nonpapillary fate, and (2) as a mitogen, causing expansion of the interplacodal epithelium and underlying mesenchyme.

Differential expression of a BMP4 reporter allele in anterior fungiform versus posterior circumvallate taste buds of mice

“…This reporter allele comprises a lacZ knockin to the BMP4 coding region and regulation of transcription of this locus should thus be unaltered. Further, ß-galactosidase expression in this mouse line has been shown repeatedly and in a variety of tissues and developmental stages to accurately reflect BMP4 mRNA expression [44,45,48], including faithfully replicating BMP4 mRNA expression in the developing taste bud progenitor cells of mouse embryos [41,60]. Nonetheless, we were unable to confirm this concordance in adult mouse taste epithelium.…”

“…BMP4 regulates embryonic development of taste papillae [41,42], but its expression and role in adult taste buds has not been investigated. We show here that BMP4, via a genetically engineered LacZ reporter mouse line, is expressed in a subset of cells associated with mature taste buds, but that the pattern of expression differs between the circumvallate and fungiform taste papillae.…”

“…BMPs regulate many cellular processes, including differentiation, proliferation, apoptosis, adhesion, and migration (reviewed by Chen, 2004) [40]. In the developing taste organs of embryos, BMP4 is precisely co-expressed with Shh in taste placodes [41]. A recent study using cultured embryonic tongue explants revealed that BMP influences formation of fungiform taste papillae [42].…”

Wnt signaling interacts with Shh to regulate taste papilla development

“…Previous reports with rodent tongue cultures showed that neutralizing Shh with a monoclonal anti- body (5E1) or blocking Shh's activity with cylcopamine enhanced taste papillae formation (both size and number) accompanied by increased expression of Shh (15,16,18). Because we have seen coexpression of Wnt signaling elements and Shh, and because we determined that Wnt/␤-catenin signaling upregulates Shh expression, we set out to more closely examine interactions of these two signaling pathways.…”

“…Expression of Shh, Bmp, Noggin, Fgf, and EGF is associated with taste papilla initiation and patterning, indicating the potential role of these factors in taste papilla development (11)(12)(13)(14). Disruption of Shh signaling during the prenatal period by neutralizing antibody or by cyclopamine caused aberrant development and patterns of fungiform papillae (15,16). Although Wnt signaling pathways are critical in the development of many epithelial tissues, they had not been examined in the developing taste system, nor had their interaction with Shh been previously examined in the developing taste papillae.…”