Transient retinoic acid signaling confers anterior-posterior polarity to the inner ear

Abstract

Vertebrate hearing and balance are based in complex asymmetries of inner ear structure. Here, we identify retinoic acid (RA) as an extrinsic signal that acts directly on the ear rudiment to affect its compartmentalization along the anterior-posterior axis. A rostrocaudal wave of RA activity, generated by tissues surrounding the nascent ear, induces distinct responses from anterior and posterior halves of the inner ear rudiment. Prolonged response to RA by posterior otic tissue correlates with Tbx1 transcription and formation of mostly nonsensory inner ear structures. By contrast, anterior otic tissue displays only a brief response to RA and forms neuronal elements and most sensory structures of the inner ear.

Fig. 1.

(A) Paint-filled inner ears from mouse and chicken embryos. (B) The replacement of a host's right-sided otic cup with a donor's left-sided otic cup, including adjacent ectoderm (blue arrowheads). (C and D) Transplanting an otic cup alone results in normal patterning of anterior inner ear neurosensory markers, Lfng and NeuroD1 (arrows). (E and F) Transplanting an otic cup plus adjacent ectoderm causes an inversion of Lfng and NeuroD1 patterning (red arrows). (G) Percentages of samples from various surgical conditions with a normal or inverted A-P axis, or duplicated anterior domains. aa, anterior ampulla; asc, anterior semicircular canal; bp, basilar papilla; cd, cochlear duct; ed, endolymphatic duct; es, endolymphatic sac; la, lateral ampulla; lsc, lateral semicircular canal; pa, posterior ampulla; psc, posterior semicircular canal; s, saccule; u, utricle.

Fig. 2.

Expression patterns of Raldh2 and Cyp26C1 in chicken embryos. Cyp26C1 is expressed in the ectoderm anterior to the otic placode/cup region (A–C, arrowheads), whereas Raldh2 is expressed in the mesodermal tissues caudal to the otic tissues such as somites and the lateral mesoderm (D–F, arrows). Brackets indicate the location of the otic placode/cup. Weak Cyp26C1 expression in the otic region in B is associated with the mesoderm beneath the otic cup.

Fig. 3.

β-Gal histochemical staining in RARE-lacZ embryos. (A and C–H) β-Gal histochemical staining at E8.25 (A), E8.75 (C and D), E9 (E and F), and E9.5 (G and H). Arrow in A indicates the anterior border of β-gal staining at E8.25, which is comparable to the r4/r5 boundary indicated in B (arrow) as assessed by Hoxb1 expression. Arrows in C, E, and G indicate the anterior-most extent of β-gal staining, which lies at the r5/r6 boundary at E8.75 (C Inset and D) and posterior to r6 by E9.5 (G); arrowheads in C, D, and F indicate the anterior-most extent of otic epithelial β-gal staining. Otocyst: E Inset, dashed line, and G, bracket.

Fig. 4.

RA signaling confers posterior identity to the inner ear. (A–F) Administering RA to pregnant mice at E7.75 affects the size of the otocyst, down-regulates anterior neurosensory markers Lfng (D, asterisk; n = 5) and NeuroD1 (E, asterisk; n = 6), and up-regulates Tbx1 anteriorly (F, bracket; n = 3), compared with controls at E9.5 (A–C). Similar gene expression changes are observed less frequently and with less severe dysmorphology when RA is administered at E8.25 (G–I). (J–O) Implantation of an RA bead in mesoderm anterior to the otic cup in chicken causes down-regulation of Lfng (M, asterisk; n = 16) and NeuroD1 (N, asterisk; n = 15), and up-regulation of Tbx1 anteriorly (O, bracket; n = 6) in comparison with controls wherein beads are soaked with DMSO alone (J–L). Tbx1 expression in the branchial mesoderm is down-regulated in response to exogenous RA (L and O, asterisks).

Fig. 5.

Effects of implanting a citral bead posterior (A–K) or anterior (L and M) to the otic cup. (A) Posterior citral-bead (red) implantation diagram. (B–E) Posterior citral-bead implantation causes down-regulation of Tbx1 (D, bracket; n = 5/10) and SOHo1 (E, bracket; n = 6), compared with controls (B and C). No detectable change is seen in Tbx1 expression in the branchial mesoderm (arrowheads). (F–K) Posterior citral-bead implantation causes ectopic expression of Lfng (I; n = 5/6) and NeuroD1 (J and K; n = 11/18) in the posterior otocyst (red arrows). (L and M) Anterior citral-bead implantation causes down-regulation of Lfng (L, arrow; n = 16/18) and NeuroD1 (M, arrow; n = 14/17).

Fig. 6.

Paint-filled chicken inner ears. Anterior structures (A, red labels) fail to develop in RA-treated ears, and only two posterior canal-like structures are evident (B, n = 5/9; C, n = 4/9). Box in A highlights the normal posterior canal's more ventral point of insertion into the common crus relative to the anterior canal's insertion and is comparable to regions highlighted by arrows in B and C. (D) Whole-mount anti-HCA staining of chicken inner ear at E9 showing locations within the membranous labyrinth of the auditory sensory organ, the basilar papilla (bp) and six vestibular sensory organs: anterior crista (ac), lagena (lg), lateral crista (lc), utriculi (mu), sacculi (ms), and posterior crista (pc). (E and F) RA-treated chicken ears lack all sensory organs except for two cristae, each associated with a posterior-like canal (n = 5).

Fig. 7.

RA induces otic Tbx1 transcription in the presence of cycloheximide. (A–C) Ectopic Tbx1 transcription in the anterior otic cup is observed within 3 h (B, arrowheads; n = 9/9) and 6 h (C, arrowheads; n = 12/12) of RA bead implantation. Tbx1 expression is down-regulated in mesoderm at 6 h after RA bead implantation (C, bracket). (D–F) Ectopic Tbx1 transcripts in the anterior otic cup within 3 h (E, arrowheads; n = 12/15) and 6 h (F, arrowheads; n = 17/19) of RA bead implantation in embryos pretreated with the protein synthesis blocker cycloheximide. Cycloheximide blocks the protein synthesis-dependent down-regulation of mesodermal Tbx1 by RA (n = 17/19; compare brackets in F and C). (G) Summary diagram of the otic epithelium in relation to the source and sink configuration of RA synthesis and degradation enzymes, which is a key component in establishing the A-P axis of the inner ear.