In Vitro Activation Early Follicles: From the Basic Science to the Clinical Perspectives

Abstract

Development of early follicles, especially the activation of primordial follicles, is strictly modulated by a network of signaling pathways. Recent advance in ovarian physiology has been allowed the development of several therapies to improve reproductive outcomes by manipulating early folliculogenesis. Among these, in vitro activation (IVA) has been recently developed to extend the possibility of achieving genetically related offspring for patients with premature ovarian insufficiency and ovarian dysfunction. This method was established based on basic science studies of the intraovarian signaling pathways: the phosphoinositide 3-kinase (PI3K)/Akt and the Hippo signaling pathways. These two pathways were found to play crucial roles in folliculogenesis from the primordial follicle to the early antral follicle. Following the results of rodent experiments, IVA was implemented in clinical practice. There have been multiple recorded live births and ongoing pregnancies. Further investigations are essential to confirm the efficacy and safety of IVA before used widely in clinics. This review aimed to summarize the published literature on IVA and provide future perspectives for its improvement.

Keywords: PI3K/Akt/FOXO3 pathway; hippo signaling pathway; in vitro activation.

Figure 1

The schematic folliculogenesis from primordial to ovulatory stages.

Figure 2

The phosphoinositide 3-kinase (PI3K)/Akt/ forkhead box O3 (FOXO3) pathway in oocytes regulates PF activation. Upon growth factors binding to tyrosine kinase receptors (Kit ligand, insulin-like growth factor (IGF), epidermal growth factor (EGF), etc.), the autophosphorylation of intracellular regions of these receptors takes place. Activated receptors then stimulate PI3K activity, leading to increases in phosphatidylinositol-3,4,5-triphosphate (PIP3) levels and phosphatidylinositol-dependent kinase 1 (PDK1) as well as Akt stimulation. Activated Akt then migrates to the cell nucleus and suppresses FOXO3 actions to promote primordial follicle growth [8,10]. Akt also promotes the phosphorylation of tuberous sclerosis 2 (TCS2), which enhances mammalian target of rapamycin complex (mTORC1) activation. S6 kinase (S6K), ribosomal protein S6 (rpS6) and eukaryotic translation initiation factor 4E binding protein 1 (4E-BP1) are the downstream substrates of mTORC1 which promote the translation of related mRNAs following by cell growth and proliferation.

Figure 3

Tissue fragmentation and mechanical manipulation disrupt ovarian Hippo signaling pathway and promote follicle growth. (Left) Without tissue fragmentation and mechanical manipulation, Hippo signaling is activated. Kinase complexes consisting of the mammalian sterile-20 like serine/threonine kinase 1/2 (MST 1/2) and Salvador (SAV) are phosphorylated. Subsequently, large tumor suppressor 1/2 (LATS1/2) phosphorylates Yes-associated protein (YAP)/ transcriptional co-activator PDZ-binding motif (TAZ), leading to the degradation of YAP/TAZ or retention of YAP/TAZ in the cytoplasm. Consequently, transcriptional enhanced associate domains (TEADs) cannot promote the expression of target genes. (Right) Ovarian fragmentation leads to actin polymerization, resulting in nuclear translocation of YAP. Nuclear YAP interacted with TEADs to increase the expression CCN growth factors and baculoviral inhibitors of apoptosis repeat containing (BIRC) apoptosis inhibitors, resulting in follicle growth.

Figure 4

Comparison of the clinical approach between conventional IVA and drug-free IVA. In conventional IVA, one or both ovaries from POI patients were removed under laparoscopic surgery and cut into strips before vitrification. After thawing, strips were fragmented into 1–2 mm cubes, before incubation with Akt stimulators. Two days later, cultured cubes were autografted under second laparoscopic surgery beneath the serosa of Fallopian tubes. In drug-free IVA, the ovarian cortical tissues were removed and fragmented into 1–2 mm cubes followed by grafting back without culture in remaining ovaries and beneath the serosa of Fallopian tubes within the same operation.

Figure 5

The schematic hypothesis for correlation of polycystic ovary syndrome (PCOS) and Hippo signaling pathway. (1) The mechanical damage and/or degrading enzyme approaches or (2) local administration of the Hippo downstream CCN growth factors and/or actin polymerization drugs for disruption of Hippo signaling aim to ameliorate the dysregulation of Hippo signaling pathway in PCOS patients for resumption of follicle growth.