Wnt signaling during tooth replacement in zebrafish (Danio rerio): pitfalls and perspectives

Abstract

The canonical (β-catenin dependent) Wnt signaling pathway has emerged as a likely candidate for regulating tooth replacement in continuously renewing dentitions. So far, the involvement of canonical Wnt signaling has been experimentally demonstrated predominantly in amniotes. These studies tend to show stimulation of tooth formation by activation of the Wnt pathway, and inhibition of tooth formation when blocking the pathway. Here, we report a strong and dynamic expression of the soluble Wnt inhibitor dickkopf1 (dkk1) in developing zebrafish (Danio rerio) tooth germs, suggesting an active repression of Wnt signaling during morphogenesis and cytodifferentiation of a tooth, and derepression of Wnt signaling during start of replacement tooth formation. To further analyse the role of Wnt signaling, we used different gain-of-function approaches. These yielded disjunct results, yet none of them indicating enhanced tooth replacement. Thus, masterblind (mbl) mutants, defective in axin1, mimic overexpression of Wnt, but display a normally patterned dentition in which teeth are replaced at the appropriate times and positions. Activating the pathway with LiCl had variable outcomes, either resulting in the absence, or the delayed formation, of first-generation teeth, or yielding a regular dentition with normal replacement, but no supernumerary teeth or accelerated tooth replacement. The failure so far to influence tooth replacement in the zebrafish by perturbing Wnt signaling is discussed in the light of (i) potential technical pitfalls related to dose- or time-dependency, (ii) the complexity of the canonical Wnt pathway, and (iii) species-specific differences in the nature and activity of pathway components. Finally, we emphasize the importance of in-depth knowledge of the wild-type pattern for reliable interpretations. It is hoped that our analysis can be inspiring to critically assess and elucidate the role of Wnt signaling in tooth development in polyphyodonts.

Beta-Catenin and Plakoglobin Expression during Zebrafish Tooth Development and Replacement

“…Our immunohistochemistry results show absence of nuclear β-catenin at morphogenesis stage but presence of nuclear β-catenin from cytodifferentiation onwards in some of the odontoblasts. These observations largely correspond to the data of Huysseune and colleagues (2014) based on dkk1 expression and therefore suggests that also in zebrafish tooth development Wnt signalling is required for normal tooth development [ 53 ]. We were unable to determine the role of β-catenin during tooth development using β-catenin MO-injected zebrafish since these do not survive until the start of tooth formation.…”

“…An active Wnt pathway is necessary for normal murine tooth development [ 45 – 52 ]. A recent study on zebrafish teeth suggests that Wnt signalling is repressed during morphogenesis and cytodifferentiation stage [ 53 ]. Our immunohistochemistry results show absence of nuclear β-catenin at morphogenesis stage but presence of nuclear β-catenin from cytodifferentiation onwards in some of the odontoblasts.…”

The Wnt Co-Receptor Lrp5 Is Required for Cranial Neural Crest Cell Migration in Zebrafish

“…In zebrafish lrp5 morphants, the most severe defects in viscero-cranial development were observed in ceratobranchials 1–4, while the 5 th ceratobranchial containing pharyngeal teeth and other dermal skeletal elements, such as cleitra and operculae, appeared unaffected at least in classI morphants, further underlining the complex role of Wnt signaling in zebrafish pharyngeal tooth formation [ 50 ]. In contrast to ceratobranchials 1–4, the 5 th ceratobranchials consist mostly of sox10 :GFP-negative cells (data not shown).…”

Supernumerary teeth in the pharyngeal dentition of slow-developing zebrafish (Danio rerio , Hamilton, 1822)

“…For the current experiment we used zebrafish, a cyprinid with an invariant tooth formula (Huysseune, Soenens, & Elderweirdt, ; Van der Heyden et al., ), and artificially induced a retardation of development by deficiency of thyroid hormones. This study revealed the presence of two distinctive types of supernumerary teeth: supernumerary teeth anterior to the first regular tooth of a tooth row (STAs), i.e.…”