Sequential and Opposing Activities of Wnt and BMP Coordinate Zebrafish Bone Regeneration

Abstract

SUMMARY Zebrafish fully regenerate lost bone, including after fin amputation, through a process mediated by dedifferentiated, lineage-restricted osteoblasts. Mechanisms controlling the osteoblast regenerative program from its initiation through reossification are poorly understood. We show that fin amputation induces a Wnt/β-catenin-dependent epithelial to mesenchymal transformation (EMT) of osteoblasts in order to generate proliferative Runx2+ preosteoblasts. Localized Wnt/β-catenin signaling maintains this progenitor population toward the distal tip of the regenerative blastema. As they become proximally displaced, preosteoblasts upregulate sp7 and subsequently mature into re-epithelialized Runx2−/sp7+ osteoblasts that extend preexisting bone. Auto-crine bone morphogenetic protein (BMP) signaling promotes osteoblast differentiation by activating sp7 expression and counters Wnt by inducing Dickkopf-related Wnt antagonists. As such, opposing activities of Wnt and BMP coordinate the simultaneous demand for growth and differentiation during bone regeneration. This hierarchical signaling network model provides a conceptual framework for understanding innate bone repair and regeneration mechanisms and rationally designing regenerative therapeutics.

Osteoblast de- and redifferentiation is controlled by a dynamic response to retinoic acid during zebrafish fin regeneration

“…After several rounds of proliferation, preosteoblasts become slowcycling, differentiating cells in more proximal parts of the regenerate and eventually mature to matrix-producing osteoblasts (Stewart et al, 2014). The expression patterns of aldh1a2 and cyp26b1 at 3 dpa imply that RA concentrations within the regenerate decrease from distally high to proximally low levels ( Fig.…”

“…We next examined possible mechanisms that could explain how RA signaling interferes with preosteoblast differentiation. Wnt/β-catenin and BMP signaling have been shown to coordinate preosteoblast proliferation and differentiation (Stewart et al, 2014). Wnt/β-catenin signaling promotes proliferation towards the distal tip of the regenerate, whereas BMP signaling positively regulates differentiation by activating expression of osx and of the dickkopf WNT signaling pathway inhibitor 1b (dkk1b).…”

“…Several signaling pathways have been implicated in fin regeneration (e.g. FGF, Igf, RA and Wnt/β-catenin signaling) (Blum and Begemann, 2012;Chablais and Jazwinska, 2010;Poss et al, 2000;Stewart et al, 2014;Stoick-Cooper et al, 2007;Wehner et al, 2014;Whitehead et al, 2005). However, our understanding of their tissuespecific functions is very limited.…”

“…However, the molecular mechanisms that control the transition between the distinct differentiation states within the osteoblast lineage have only recently begun to be addressed. Wnt/β-catenin and BMP signaling have been shown to regulate preosteoblast proliferation and differentiation via opposing activities (Stewart et al, 2014). The signals that control osteoblast dedifferentiation in the stump are unknown so far.…”

“…After several rounds of division, preosteoblasts redifferentiate to form new bone tissue. Differentiation progresses in a distal-to-proximal direction, so that fast cycling preosteoblasts at the distal leading edge of aligned osteoblasts become slow-cycling differentiating cells, which subsequently mature into non-cycling matrix-producing cells (Stewart et al, 2014). Despite their dedifferentiation, osteoblasts remain lineage restricted (Knopf et al, 2011;Stewart and Stankunas, 2012).…”