Of Juno, neuropeptide release, and light-activated chloride channels

This month's installment of Generally Physiological explores the identification of an oocyte cell surface receptor required for fertilization, the role of calcium stores in large dense core vesicles in neuropeptide secretion, and the conversion of a chan nelrhodopsin to a light-activated chloride channel. The ability of a sperm and an egg to fuse to form the diploid zygote depends on their recognizing each other, which in turn depends on interactions between cell surface receptors. The sperm protein Izumo1, which translocates to the plasma membrane during the process whereby sperm become competent to fertilize eggs, is required for sperm–egg fusion; its binding partner on the egg surface, however, has remained unclear (see Wassarman, 2014). Using an iterative expression cloning approach , Bianchi et al. (2014) probed HEK293 cells transfected with mouse oocyte cDNA with an oligomerized mouse Izumo1 ectodomain and identified a folate receptor paralogue, folate receptor 4 (Folr4), as an Izumo1-binding partner. After determining that Folr4, unlike Folr1 and Folr2, failed to bind folate, the authors renamed it Juno. Noting that an egg receptor essential for fertilization is anchored by glycophosphatidylinosi-tol (GPI), the authors determined that cell surface Juno immunore-activity was lost after treatment with phospho inositide phospholipase C, indicative of anchoring by GPI. Oo-cyte pretreatment with a monoclo-nal antibody directed against Juno blocked Izumo1 binding in an in vitro fertilization; moreover, female mice lacking Juno were infertile, whereas Juno / males were not, and Juno / eggs could not be fertilized in vitro. Cell surface Juno was lost after fertilization , undergoing redistribution to extracellular vesicles, suggesting a possible mechanism for the membrane block to polyspermy that takes place subsequent to fertilization. Izumo1 and Juno orthologues exist in all sequenced mammalian genomes, and binding between the ectodo-mains of Izumo1 and Juno ortho-logues was conserved in humans, pigs, and opossums. The authors thus conclude that Juno is the Izumo1 receptor and that, like Izumo1, it is required for fertilization of mamma-lian eggs. Although calcium influx through voltage-gated channels provides the primary trigger for neuropeptide secretion, calcium released from internal stores may play a modulatory role. Noting that the large dense core vesicles (LDCVs) in which neu-ropeptides are stored contain substantial calcium, in this issue, McNally et al. explored the possibility that calcium released from LDCVs gives rise to the ryanodine-sensitive calcium transients that occur in neuro-hypophysial terminals, providing a local source of calcium to modulate neuropeptide release. Immunofluo-rescence analysis of …