Glycoprotein hormone GPA2/GPB5: discoveries and insights from invertebrates.

Vertebrate thyrostimulin and the other classical pituitary glycoprotein hormones are considered to have originated from a common ancestral hormone system early in Metazoan evolution. The invertebrate glycoprotein hormone, GPA2/GPB5, is the homolog of vertebrate thyrostimulin, and the only member of these glycoprotein hormones to be found in invertebrates. This review focuses on the GPA2/GPB5 signaling pathway in invertebrates, outlining its evolutionary origin and conservation, discovery, distribution, including receptor localization, and physiological relevance. Although the physiological roles of GPA2/GPB5 in invertebrates have only recently been investigated, the emerging evidence supports functions like that in the vertebrates, namely stress responses, development, and reproduction. In particular, the signaling pathway appears to act as an endocrine "switch" to integrate complex physiological processes.