Evidence that the canonical Wnt signalling pathway regulates deer antler regeneration

Abstract

Wnt signalling regulates many developmental processes, including the fate specification, polarity, migration, and proliferation of cranial neural crest. The canonical Wnt pathway has also been shown to play an important role in bone physiology and there is evidence for its recapitulation during organ regeneration in lower vertebrates. This study explores the role of the Wnt signalling pathway in deer antlers, frontal bone appendages that are the only mammalian organs capable of regeneration. Immunocytochemistry was used to map the distribution of the activated form of beta-catenin ((a)betaCAT). A low level of (a)betaCAT staining was detected in chondrocytes and in osteoblasts at sites of endochondral bone formation. However, (a)betaCAT was localised in cellular periosteum and in osteoblasts in intramembranous bone, where it co-localised with osteocalcin. The most intense (a)betaCAT staining was in dividing undifferentiated cells in the mesenchymal growth zone. Antler progenitor cells (APCs) were cultured from this region and when the canonical Wnt pathway was inhibited at the level of Lef/TCF by epigallocatechin gallate (EGCG), the cell number decreased. TUNEL staining revealed that this was as a result of increased apoptosis. Activation of the pathway by lithium chloride (LiCl) had no effect on cell number but inhibited alkaline phosphate activity (ALP), a marker of APC differentiation, whereas EGCG increased ALP activity. This study demonstrates that beta-catenin plays an important role in the regulation of antler progenitor cell survival and cell fate. It also provides evidence that beta-catenin's function in regulating bone formation by osteoblasts may be site-specific.