Comparative genomics of leucine-rich repeats containing G protein-coupled receptors and their ligands

Thyrotropin and homologous glycoprotein hormone receptors: structural and functional aspects of extracellular signaling mechanisms

The TSH receptor (TSHR) together with the homologous lutropin/choriogonadotropin receptor and the follitropin receptor are glycoprotein hormone receptors (GPHRs). They constitute a subfamily of the rhodopsin-like G protein-coupled receptors with seven transmembrane helices. GPHRs and their corresponding hormones are pivotal proteins with respect to a variety of physiological functions. The identification and characterization of intra- and intermolecular signaling determinants as well as signaling mechanisms are prerequisites to gaining molecular insights into functions and (pathogenic) dysfunctions of GPHRs. Knowledge about activation mechanisms is fragmentary, and the specific aspects have still not been understood in their entirety. Therefore, here we critically review the data available for these receptors and bring together structural and functional findings with a focus on the important large extracellular portion of the TSHR. One main focus is the particular function of structural determinants in the initial steps of the activation such as: 1) hormone binding at the extracellular site; 2) hormone interaction at a second binding site in the hinge region; 3) signal regulation via sequence motifs in the hinge region; and 4) synergistic signal amplification by cooperative effects of the extracellular loops toward the transmembrane region. Comparison and consolidation of data from the homologous glycoprotein hormone receptors TSHR, follitropin receptor, and lutropin/choriogonadotropin receptor provide an overview of extracellular mechanisms of signal initiation, conduction, and regulation at the TSHR and homologous receptors. Finally, we address the issue of structural implications and suggest a refined scenario for the initial signaling process on GPHRs.

Bursicon, the tanning hormone of insects: recent advances following the discovery of its molecular identity

Bursicon was identified in 1965 as a peptide neurohormone that initiates the tanning of the insect cuticle immediately after the shedding of the old one during the final stages of the molting process. Its molecular identity as an approximately 30 kDa bioactive heterodimer consisting of two cystine knot proteins resisted elucidation for 43 years. The sequence of the two bursicon subunits is highly conserved among arthropods, and this conservation extends even to echinoderms. We review the efforts leading to bursicon's characterization, the identification of its leucine-rich repeat-containing, G protein-coupled receptor (LGR2), and the progress towards revealing its various functions. It is now clear that bursicon regulates different aspects of wing inflation in Drosophila melanogaster besides being involved at various points in the cuticle tanning process in different insects. We also describe the current knowledge of the expression of bursicon in the central nervous system of different insects in large homologous neurosecretory cells, and the changes in its expression during the development of Manduca sexta and D. melanogaster. Although much remains to be learned, the elucidation of its molecular identity and that of its receptor has provided the breakthrough needed for investigating the diverse actions of this critical insect neurohormone.