Xenopus bagpipe-related gene, koza, may play a role in regulation of cell proliferation

The homeobox gene koza is a new member of the vertebrate bagpipe-related gene family. Embryonic expression of koza is observed at highest levels in the muscle layer of the somites and, during later development, is restricted to the lateral somitic cells, which correspond to slow twitch muscle tissue. Expression of koza is also observed in the myocardial layer of the heart and in the cement gland. In each of these tissues, koza transcription commences only after the expression of terminal differentiation markers. By injection of synthetic mRNA, we show that overexpression of koza leads to an apparent decrease in the number of cells in the somites. No reduction in cell number is observed when koza is present in neural tissues, suggesting that koza exhibits some tissue specificity in regulation of cell proliferation. Embryonic manipulations show that restriction of koza expression to the slow twitch muscle layer is independent of axial structures but is, at least partly, regulated by signals arising in ectodermal tissue. Finally, in Drosophila, bagpipe expression is regulated by the hedgehog signaling pathway. By using ectopic expression, we show that koza transcription is positively regulated by banded hedgehog. This result indicates that regulation of bagpipe expression by hedgehog signaling is evolutionarily conserved.