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 β-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.