Wnt signaling, Ca2+, and cyclic GMP: visualizing Frizzled functions.

Wnts control the specification of cell fate, cell adhesion, migration, polarity, and proliferation. Their roles in development have been probed in fruit flies, nematodes, zebrafish, frogs, and mice. Some Wnts inhibit the degradation of beta-catenin, which can regulate transcription of specific genes. Other Wnts exert their influences in other ways, such as increasing intracellular concentrations of Ca2+ and decreasing intracellular concentrations of cyclic guanosine monophosphate (cGMP). Heterotrimeric guanine nucleotide-binding proteins (G proteins) and RGS proteins have been implicated in Wnt signaling. Wnt regulation of intracellular Ca2+ and cGMP levels requires the G protein transducin and a cGMP-specific phosphodiesterase, which are major elements in signaling of the visual pathway.