Persistent cAMP Signaling by Internalized LH Receptors in Ovarian Follicles

Abstract

A crucial event in female reproduction occurs at midcycle, when a LH peak induces the final maturation of ovarian follicles. LH signals via a G protein-coupled receptor selectively expressed in the outermost follicular cell layers. However, how LH signals are relayed inside these cells and finally to the oocyte is incompletely understood. Here, we monitored LH signaling in intact ovarian follicles of transgenic mice expressing a fluorescent cAMP sensor. We found that LH stimulation induces 2 phases of cAMP signaling in all cell layers surrounding the oocyte. Interfering with LH receptor internalization abolished the second, persistent cAMP phase and partially inhibited oocyte meiosis resumption. These data suggest that persistent cAMP signals from internalized LH receptors contribute to transmitting LH effects inside follicle cells and ultimately to the oocyte. Thus, this study indicates that the recently proposed paradigm of cAMP signaling by internalized G protein-coupled receptors is implicated in receptor function and is physiologically relevant.

Cannabinoid Receptors in the Central Nervous System: Their Signaling and Roles in Disease

“…Interestingly, these location-specific signaling events appear to be widespread among several GPCRs. For example, the LH receptor, β2 adrenergic receptor and the CB 2 receptor can signal from intracellular compartments either by β-arrestins or G proteins via a “super-complex” ultimately resulting in three different spatio-temporal signaling waves (Brailoiu et al, 2014; Irannejad and von Zastrow, 2014; Lyga et al, 2016; Nogueras-Ortiz and Yudowski, 2016; Thomsen et al, 2016). Constitutive activation also plays a role in their trafficking (Leterrier et al, 2006; McDonald et al, 2007).…”

Regulation of Mammalian Oocyte Meiosis by Intercellular Communication Within the Ovarian Follicle

“…LH signaling begins with the activation of its G-protein–coupled receptor in the mural granulosa cells. The duration of LH signaling is extended by internalization of the receptors after LH binding, such that signaling continues even in the absence of extracellular LH (127, 136). The LH receptor can activate multiple G proteins (137, 138), but most of its effects on meiotic resumption appear to be due to the activation of G s .…”

“…This results in an elevation of cAMP in the follicle, as measured by immunoassays (56, 128, 139, 141, 142), and specifically in the mural granulosa cells, as detected with a FRET sensor (136). cAMP also increases in the cumulus cells, based on immunoassays of isolated cumulus-oocyte complexes, but this was not seen until ~1.5 h after LH application (56).…”

“…cAMP also increases in the cumulus cells, based on immunoassays of isolated cumulus-oocyte complexes, but this was not seen until ~1.5 h after LH application (56). FRET sensor measurements suggest that the cAMP increase in the cumulus cells might occur earlier, but this is uncertain because of the large region of measurement that extended beyond the 2–3 layers of cumulus cells (136). …”

“…Transiently elevating cAMP by treating follicles with forskolin to activate adenylyl cyclase (127, 141), or elevating cAMP in the granulosa cells with an inhibitor of the cAMP phosphodiesterase PDE4 (61, 136), causes meiotic resumption. The forskolin-induced cAMP increase activates PKA (112, 115), which phosphorylates and activates PDE5 to the same extent as LH stimulation (109) (Figure 5).…”