254. Evidences for a novel cAMP-phosphodiesterase expressed in the bovine ovarian follicle

Abstract

3′5’-Cyclic adenosine monophosphate (cAMP) is an important second messenger in the mammalian ovarian follicle implicated in gonadotrophin signalling as well as oocyte meiotic arrest. Cyclic AMP-degrading phosphodiesterases (PDE) modulate cAMP levels in the ovarian follicle, but the specific PDE subtypes responsible for this degradation in the different cellular compartments within the bovine follicle remain unknown. The current dogma, established principally in rodent, presents PDE3A as the ‘oocyte PDE’, while PDE4D is the ‘granulosa/cumulus PDE’. Our PDE activity measurements suggested that a PDE3 (cilostamide-sensitive, 10µM) was representing 79% of the total cAMP-PDE activity in the bovine oocyte, in agreement with the dogma. However, our results suggested that PDE4 (rolipram-sensitive, 10µM) is representing only 19% of the cAMP-PDE activity in the cumulus cells, while 65% of the activity was due to PDE8 (IBMX-insensitive, 500µM), a result in direct opposition with the accepted PDE distribution in the ovarian follicle. Mural granulosa cells were displaying equal amounts of PDE4 (31%) and PDE8 (30%) cAMP-PDE activities. Interestingly, cAMP-PDE activities were not varying during the first 9 h of IVM in the bovine cumulus-oocyte complexes (COC), as seen in rat. COCs treated with an adenylyl cyclase stimulator (forkolin 100µM) in combinaison with the only known inhibitor for the PDE8 family, dipyridamole, are showing a dose-dependant increase of cAMP levels and a significant delay nuclear maturation, whereas a potent PDE4 inhibitor, rolipram (up to 100µM), was ineffective. This study provides the first insight into subtype-specific PDE cAMP degrading activities in the bovine ovarian follicle, especially around oocyte nuclear maturation. It demonstrates dramatic differential PDE subtype compartmentalisation between ovarian somatic cells and the germ cell, including the important contribution of a new PDE family member in the ovarian follicle, PDE8. PDE8 could be a novel pharmacological target to improve bovine oocyte IVM conditions and to increase developmental competence.