Exosomes: New regulators of reproductive development

Abstract

Exosomes are a subtype of extracellular vesicles (EVs) with a size range between 30 and 150 ​nm, which can be released by the majority of cell types and circulate in body fluid. They function as a long-distance cell-to-cell communication mechanism that modulates the gene expression profile and fate of target cells. Increasing evidence has indicated exosomes' central role in regulating various complex reproductive processes. However, to our knowledge, a review that focally and vividly describes the role of exosomes in reproductive development is still lacking. This review highlights our knowledge about the contribution of exosomes to early mammalian reproduction, such as gametogenesis, fertilization, early embryonic development, implantation, placentation and pregnancy. The discussion is primarily drawn from literature pertaining to the mammalian lineage with emphasis on the roles of exosomes in human reproduction and laboratory and livestock models.

Keywords: Early embryonic development; Exosomes; Fertilization; Gametogenesis; Pregnancy.

Graphical abstract

Fig. 1

Production and secretion of exosomes. The production of exosomes involves the double invaginations of plasma membrane and the formation of Intraluminal vesicles (ILVs) and Multivesicular bodies (MVBs). The first invagination of the plasma membrane forms early-sorting endosome (ESE). ESEs develop into mature late-sorting endosomes (LSEs) and eventually form MVBs. MVBs contains multiple ILVs. MVBs are degraded by fusion with lysosomes or autophagosomes, or fuse with plasma membranes to release the contained ILVs as exosomes. The secretion and transport of exosomes are mainly mediated by the Endosomal sorting complex Required for transport (ESCRT) on the MVBs membrane. ESCRT is mainly composed of ESCRT-0, –I, –II and –III complexes. Escrt-0 (HRS and STAM) regulates content aggregation through ubiquitination dependent pathways, ESCRT-I and ESCRT-II (Vps36, Vps22 and double-copy Vps25) induce bud formation. Escrt-III is composed of four core subunits: Vps20, Snf7 (Vps32), Vps24, and Vps2, as well as accessory proteins Did2, Vps60, and Ist1, which mainly promote membrane separation and vesicle cleavage. In addition, Rab27a and Rab27b, members of the Rab family GTPases, act on the docking of MVBs to the plasma membrane. ARF6 is a regulator of ILVs budding and exosome biosynthesis, which can promote the formation of ILVs.

Fig. 2

The role of exosomes in spermatogenesis and maturation. Increasing evidences have reported that exosomes derived from seminal plasma are involved in the regulation of spermatogenesis, modification and fertilization ability. (A) Exosomes secreted by Sertoli cells in the testis can get into the spermatogenic tubules during sperm differentiation. (B) Sperm produced by testicular spermatogenic tubules are not fertile. Exosomes mediate numerous modifications and gains when sperm leave the testes and get into the epididymis [[26], [27], [28], [29]]. The cargo of epididymosomes derives from epididymal fluid, caudal and distal have differential cargo loading [30]. Epididymal caput, corpus and cauda derived exosomal proteins are related to motility (cSrc and MIF), fertilization (SPAM1, GliPr1L1 and Metalloproteases) and acrosome reaction (Liri α3) [31]. (C) Sperm within the ejaculate are not yet fully functional and must first undergo capacitation, and prostasome can drive the capacitation response [43] and modulate the immune response of female genital tract [46].

Fig. 3

The role of exosomes in oogenesis. Communication between the ovarian follicle and the cumulus cells is crucial for the follicle maturation, and to produce an oocyte capable of fertilization and supporting subsequent embryonic development. (A) Recent studies have shown that follicular fluid (FF)-derived exosomes play a vital and supportive role in various reproductive processes such as cumulus expansion [51] and meiotic resumption of oocytes [52], ovarian physiology, oviduct modulation in preparation for fertilization and embryonic development [53]. (B) Oviduct exosomes (Oc-Exo) can improve the physiological state of cumulus cells, including cell density, viability and proliferation, and reduce the accumulation of reactive oxygen species and apoptosis [59].

Fig. 4

Regulation of fertilization by exosomes. Sperm need to pass through the vagina, uterus and fallopian tube to reach the site of fertilization, exosomes derived from female genital tract cells play important roles in maintaining sperm fertilization ability. (A) When sperm passes through the uterus, the utero-derived exosomes (uterosomes) are essential for sperm fertilization, and enhance their ability to cross the cumulus cells [62]. (B) When sperm passes through the oviduct, Oocyte exosomes (Oc-Exo) carrying specific sugar protein membrane Ca²⁺-ATpase 4 (PMCA4) to the sperm surface, increases resistance to zona pellucida hydrolysis, hardens zona pellucida, reduces multiple sperm fertilization, improves sperm motility and prevents premature sperm capacitation [63]. (C) In the process of sperm-egg fusion, semen-derived exosomes and Oocyte exosomes (Oo-Exo) have been shown to plays an important role in acrosome reaction [65].

Fig. 5

The role of exosomes in early embryonic development. The development of the mammalian preimplantation embryo encompasses the period from fertilization to implantation [68]. During embryo migration from the oviduct to the uterus, the embryo undergoes distinct metabolic stages (cleavage, morula, blastula and gastrula). Exosomes derived from oviduct exert a positive effect on development of embryos [121]. Uterine exosomes can promote embryo implantation (miR-30d) and induce embryonic diapause (Let-7 transportation and targeting C-MYC/mTORC1 and mTORC2). Blastula exosomes can inhibit embryo adhesion and guarantee dissociation (miR-661 transportation) and reduce transcription levels (LINC00478 and ZNF81 transportation). In addition, ICM exosomes can also promote the development of blastocyst (laminin and fibronectin transportation).

Fig. 6

The role of exosomes in embryo implantation. The implantation of the embryo in the uterus depends mainly on the proper communication between the endometrium and the blastula. A blastula is composed of the ICM and the trophoblast cells (Tc). A subsequent study showed that endometrial-derived exosomes regulate the apoptosis and adhesion of blastula (Transporting FBlN1, CYR61, CD55, HSPG2, miR-30d, PRDX2, IDHC et al.), influence uterine physiology (Transporting ICM, VEGF, JAK-STAT, TLR, S100A4, ANXA2 et al.), promote the proliferation of trophoblast cells and increase the phosphorylation of focal adhesion kinase and the production of fibronexin. At the same time, embryo-derived exosomes interact with integrns and stimulate trophoblast (JNK and FAK transportation) and regulate the immune system of uterus (CTCS, IL6, CASP4, IKBKE and miR-98 transportation) [89].

Fig. 7

The role of exosomes in placentation and pregnancy. (A) Trophoblast cells (Tc)-derived exosomes are involved in regulating fetal blood supply (Transporting VEGFA, miR-126–5p et al.) and promoting the invasion of trophoblast cells (MMPs, MAPK and miR-486–5p transportation) in the process of placentation [122]. At the same time, endometrial cell-derived exosomes increase the migration and invasion of trophoblast cells (N-cadherin, SMAD2/3) and promote angiogenesis [103]. (B) During pregnancy, Tc-derived exosomes promote mocrophages into M2 phenotype, increase the migration of monocytes (IL-1β, IL-6, GCSF, GM-CSF and TNF-α transportation) and regulate the activity of NK cells (IL-10, ISGs and IFN-tau transportation) [115].