The Formation of Endoderm-Derived Taste Sensory Organs Requires a <i>Pax9</i>-Dependent Expansion of Embryonic Taste Bud Progenitor Cells

<div><p>In mammals, taste buds develop in different regions of the oral cavity. Small epithelial protrusions form fungiform papillae on the ectoderm-derived dorsum of the tongue and contain one or few taste buds, while taste buds in the soft palate develop without distinct papilla structures. In contrast, the endoderm-derived circumvallate and foliate papillae located at the back of the tongue contain a large number of taste buds. These taste buds cluster in deep epithelial trenches, which are generated by intercalating a period of epithelial growth between initial placode formation and conversion of epithelial cells into sensory cells. How epithelial trench formation is genetically regulated during development is largely unknown. Here we show that <i>Pax9</i> acts upstream of <i>Pax1</i> and <i>Sox9</i> in the expanding taste progenitor field of the mouse circumvallate papilla. While a reduced number of taste buds develop in a growth-retarded circumvallate papilla of <i>Pax1</i> mutant mice, its development arrests completely in <i>Pax9</i>-deficient mice. In addition, the <i>Pax9</i> mutant circumvallate papilla trenches lack expression of K8 and <i>Prox1</i> in the taste bud progenitor cells, and gradually differentiate into an epidermal-like epithelium. We also demonstrate that taste placodes of the soft palate develop through a <i>Pax9</i>-dependent induction. Unexpectedly, <i>Pax9</i> is dispensable for patterning, morphogenesis and maintenance of taste buds that develop in ectoderm-derived fungiform papillae. Collectively, our data reveal an endoderm-specific developmental program for the formation of taste buds and their associated papilla structures. In this pathway, <i>Pax9</i> is essential to generate a pool of taste bud progenitors and to maintain their competence towards prosensory cell fate induction.</p></div

Absence of <i>Pax9</i> causes an endoderm-specific disruption of the Shh pathway in taste papillae.

<p>(<b>A–F</b>) Whole mount in situ hybridization of <i>Shh</i>, its receptor (<i>Ptc1</i>) and the downstream effector transcription factor (<i>Gli1</i>) at E14.5. (<b>A,B</b>) In the wild type CVP (A), <i>Shh</i> is expressed in the central dome as well as in a ring of accessory papillae (arrowheads). <i>Ptc1</i> and <i>Gli</i> are expressed in a similar pattern. In the absence of <i>Pax9</i>, <i>Shh</i>, <i>Ptc1</i> and <i>Gli1</i> are only expressed in the central dome of the CVP (B). (<b>C,D</b>) In wild type embryos (C), patches of <i>Shh</i>, <i>Ptc1</i> and <i>Gli1</i> expression are detectable in the region of the developing FOP, whereas these expression patterns are missing (<i>Shh</i>) or are greatly reduced (<i>Ptc1</i>, <i>Gli1</i>) in <i>Pax9^−/−</i> embryos (D). (<b>E,F</b>) <i>Shh</i> expression in FUP placodes is similar in wild type (E) and <i>Pax9</i>-deficient (F) embryos. (<b>G, H</b>) Histological sections of <i>Pax9</i>-deficient, cultured embryonic tongues. (G) In control medium the <i>Pax9^−/−</i> CVP of cultured tongues is small and is not visible externally (inset). (H) In the presence of purmorphamine (PUR) the number of epithelial cells is increased in the dome of the CVP. Note the absence of trenches. Inset shows enlarged, protruded CVP dome (arrowhead) of the cultured tongue. Scale bars: 100 µm in A,C,G: 200 µm in E.</p

Expression patterns of Pax9 in different taste papillae of the embryonic mouse tongue.

<p>(<b>A</b>) Drawing showing the localization of the circumvallate papilla (CVP), foliate papillae (FOP), and fungiform papillae (FUP) in the mouse tongue. (<b>B</b>) Whole mount X-Gal staining of a <i>Pax9^+/LacZ</i> mouse tongue at embryonic day 13.5 (E13.5). Note that expression is also seen in the mesenchyme adjacent to the developing FOP (arrowheads) and that the color reaction was stopped before epithelial staining began to obscure the mesenchymal expression domain. (<b>C–N</b>) Pax9 immunostaining of taste papillae during development on cross sections (C–F; K–N) and horizontal sections of the tongue (G–J). (<b>C–F</b>) Pax9 is expressed in the epithelium during CVP morphogenesis and is down-regulated in some regions of the trenches at E18.5 (arrowhead in F). (<b>G–J</b>) In addition to the epithelium, Pax9 is also expressed in the mesenchyme during FOP development, while reduced Pax9 levels were observed in the trenches at E18.5 (arrowhead in J). (<b>K–N</b>) In the anterior part of the tongue Pax9 is expressed in the FUP epithelium and in filiform papillae (FIP). Note that the expression is very weak or absent in the taste placodes (arrowheads). Scale bars: 200 µm in B; 50 µm in other panels.</p

Arrest of CVP and FOP development in <i>Pax9</i>-deficient mouse embryos.

<p>(<b>A,C</b>) In wild type (WT) embryos, the invaginating CVP epithelium forms deep trenches. (<b>B,D</b>) Rudimentary CVP trenches form in <i>Pax9^−/−</i> embryos at E16.5 (B) but these trenches fail to invaginate (D). (<b>E,G</b>) A series of invaginations develop in the FOP of wild type embryos. (<b>F,H</b>) FOP trenches are absent in <i>Pax9</i> mutants. (<b>I–L</b>) FUP development on the dorsal tongue. The FUP of wild type embryos (I,K) and <i>Pax9^−/−</i> embryos (J,L) are morphologically indistinguishable. (<b>M,N</b>) FOP development in <i>Pax9^fl/fl</i> embryos. (M) Without <i>Cre</i> expression, FOP development at E14.5 is normal and Pax9 expression is detectable in both epithelium and mesenchyme of the tongue (t), as well as in the adjacent lower jaw mesenchyme (arrow; inset shows a coronal section of the posterior region of the tongue). (N) <i>Wnt1^Cre</i>-mediated inactivation of <i>Pax9^fl/fl</i> did not disrupt the formation of epithelial invaginations. Note that Pax9-positive cells are not detectable in the tongue mesenchyme (asterisk in inset) or in the mesenchyme of the non-elevated secondary palate (p). Scale bars: 50 µm.</p

FUP maintenance and FUP taste bud renewal do not require Pax9 functions.

<p>All analyses were carried out using 3–5 months old mice. (<b>A,B</b>) Pax9 immunostaining of FUPs. In <i>Pax9^fl/fl</i> mice (A), Pax9 expression is detected in the FUP epithelium and in isolated taste bud cells (area of taste bud is indicated by dotted line). (B) No Pax9-positive cells are detectable in the FUP after <i>K14^Cre</i>-mediated recombination of <i>Pax9^fl/fl</i>. (<b>C,D</b>) Histological sections of FUP. <i>Pax9^fl/fl</i> FUP (C) and <i>K14^Cre</i>;<i>Pax9^fl/fl</i> FUP (D) are morphologically indistinguishable. (<b>E,F</b>) Scanning electron microscopy images of FUP. The FUP of both <i>Pax9^fl/fl</i> (E) and <i>K14^Cre</i>;<i>Pax9^fl/fl</i> (F) form taste pores (arrowhead), whereas the non-sensory FIP of the mutants (F) are hypoplastic. (<b>G–L</b>) Indirect immunofluorescent detection of keratins. Nuclei were stained with DAPI (blue). (G) In <i>Pax9^fl/fl</i> mice, K14 is expressed in basal cells of the epithelium and K1 expression was seen in isolated epithelial cells of the FUP epithelium (arrowhead). (H) While K14 expression was not affected in the FUP of <i>K14^Cre</i>;<i>Pax9^fl/fl</i> mice, the number K1 expressing cells was strongly increased. (<b>I,J</b>) K10 expression is mainly restricted to the apical end of the FUP in <i>Pax9^fl/fl</i> mice (I) whereas its expression is more extended in <i>K14^Cre</i>;<i>Pax9^fl/fl</i> mice (J). (<b>K,L</b>) K8 expression marks taste bud cells in both genotypes. (<b>M,N</b>) Immunohistochemical staining showing that Sox2 is expressed in mature taste buds of both <i>Pax9^fl/fl</i> (M) and <i>K14^Cre</i>;<i>Pax9^fl/fl</i> (N) mice. Scale bars: 50 µm in A,C,G,M; 500 µm in E.</p

<i>Pax1</i> and <i>Sox9</i> are <i>Pax9</i> targets in the proliferating compartment of the CVP trenches.

<p>(<b>A–F</b>) Immunohistochemical staining on sections of the CVP at E15.5. (<b>A,B</b>) Pax1 is strongly expressed in the tips of epithelial trenches and in periderm cells covering the central dome of the wild type CVP (A), but not in the <i>Pax9</i>-deficient CVP (B). (<b>C,D</b>) Similarly, Sox9 expression is strongest in the epithelial trenches (C) and is barely detectable in the <i>Pax9</i> mutant CVP (D). (<b>E,F</b>) BrdU-positive cells were counted in defined areas (boxed) of the CVP trenches from three wild type (n = 29 sections) and three <i>Pax9</i> mutant CVPs (n = 28 sections). (<b>G</b>) The number of proliferating cells in the <i>Pax9</i>-deficient CVP is significantly reduced. Error bars illustrate standard deviation. (<b>H</b>) Pax1 immunostaining of one CVP trench in a 3 months old wild type mouse. (<b>I,J</b>) Morphology of the CVP at E18.5. The lengths of the CVP trenches (indicated by bars) were measured and shown to be reduced in the absence of <i>Pax1</i> (for summary of measurements see <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1004709#pgen.1004709.s009" target="_blank">Table S1</a>). (<b>K,L</b>) Morphology of the CVP at postnatal day 16. In <i>Pax1</i> mutants (n = 3) the trenches are growth-retarded and contain fewer taste buds. Scale bars: 50 µm in A,C,E; 100 µm in H,I,K.</p