OR1D2 receptor mediates bourgeonal-induced human CatSper activation in a G-protein dependent manner

Abstract

33During fertilization, sperm are guided towards eggs by physiological chemokines, a 34 process named sperm chemotaxis. Human sperm chemotaxis is speculated to be 35 mediated by olfactory receptor OR1D2 in a pathway requiring calcium influx. 36 Bourgeonal, an artificial ligand of OR1D2, can activate CatSper, the primary calcium 37 channel in human sperm. However, whether bourgeonal-induced CatSper activation 38 requires OR1D2 and how CatSper is activated remain unclear. Herein, we show that 39 65 Surprisingly, the expression of functional ORs is not tightly restricted to olfactory 66 sensory neurons. For example, some ORs have been identified in early stage germ 67 cells and mature sperm of rats and dogs (12, 13), suggesting potential roles of ORs in 68 the mammalian reproductive system. In 2003, the mRNA transcript of OR1D2 was 69 identified in human testis. More importantly, bourgeonal, a synthetic ligand of 70 OR1D2, was shown to increase human sperm [Ca 2+ ] i (cytosolic free calcium 71 concentration) and induce sperm chemotatic movement (8). In the follow-up studies, 72 although other OR members (OR7A5/OR4D1) have been identified in human testis, 73 there is no evidence supporting their involvement in the regulation of human sperm 74 3 chemotaxis (14). Therefore, OR1D2 receptor remains the only candidate as a mediator 75 of human sperm chemotaxis, although the consequent signaling after OR1D2 76 activation remain unclear. 77 In sea urchin, regulation of [Ca 2+ ] i fluctuations is the key feature of sperm 78 chemotaxis (15, 16). Bourgeonal-induced human sperm chemotaxis also requires 79 calcium influx (8), suggesting that calcium channels play key roles in human sperm 80 chemotaxis. Although various calcium channels have been proposed to be present in 81 human sperm (17), only CatSper, the sperm specific calcium channel, has been 82 confirmed by patch clamping (18-21). Notably, CatSper gene deletion in both mice 83 and humans produces male infertility, indicating the vital role of CatSper (22-26). It 84 has been proposed that bourgeonal activates human sperm CatSper (27), although 85 whether OR1D2 is necessary for bourgeonal to activate CatSper remains unclear. 86 Since ORs usually transfer extracellular signals through G-protein dependent 87 signaling pathways (11, 28), the involvement of molecules in the G-protein pathways 88 should be expected in bourgeonal-mediated effects, if bourgeonal indirectly activates 89 CatSper through binding with OR1D2. However, 250 μM GDP-β-S showed no 90 inhibitory effect on bourgeonal-induced CatSper activation (27). In addition, no 91 significant increase of cAMP concentration in human sperm had been detected after 92 bourgeonal incubation (27). Those results led to the proposal that bourgeonal may 93 activate CatSper directly (27), raising the question whether OR1D2 participates in the 94 bourgeonal-induced human sperm chemotaxis. 95 Because CatSper is present in species ranging from lower invertebrates to higher 96 mammals and plays crucial role in sperm function regula...

The Chemosensing Role of CatSper in Mammalian Sperm: An Updated Review

“…Sperm chemotaxis guides sperm toward the oocyte and is closely related to sperm capacitation, hyperactivation, the acrosome reaction, and male fertility. In humans, bourgeonal is a typical odorant and chemoattractant that is proposed to activate olfactory receptors (OR1D2) and to open CatSper to increase [Ca 2+ ] i via a G-protein-coupled receptor/olfactory G-protein/cAMP/PKA pathway [ 9 , 56 , 57 ]. Moreover, men with idiopathic infertility and low sensitivity to bourgeonal have decreased OR1D2 protein expression and bourgeonal-activated CatSper current in their sperm [ 58 ].…”