With a 1% global incidence, premature ovarian insufficiency (POI) is a complicated endocrine condition that is prevalent in women under 40. Primordial follicle pool early exhaustion is typically caused by a number of causes. Autoimmunity, genetic disorders, radiotherapy, chemotherapy, and surgery are all potential causes of this illness. Menstrual disorders, low oestrogen levels, and elevated follicle-stimulating hormone (FSH) are the major symptoms. Early-life onset of POI has deleterious consequences on reproductive health and may result in infertility. Moreover, low bone density, cardiovascular disease, significant psychological stress, and sexual dysfunction are all linked to oestrogen deficiency. Hormone replacement therapy (HRT) is the standard clinical therapeutic option for managing individuals with polycystic ovary syndrome (POI); nevertheless, due to severe side effects and non-fundamental restoration of ovarian function, HRT has a poor efficacy [1, 2].
When compared to other artificial nanoparticles, exosomes—which are naturally occurring nanoscale particles—have a number of advantages. Exosome-related basic studies and clinical studies have permeated numerous medical specialties in the past ten years. How to select the best cells for research is an important subject because exosomes can be released by nearly all cells and various kinds of exosomes that can perform different activities. A significant subset of stem cells, mesenchymal stem cells (MSCs) have drawn interest as potential seed cells for studies on tissue engineering, cell therapy, and regenerative medicine. Therefore, studies using exosomes made from MSCs—also known as MSC-exosomes—have a lot of significance [3].
MSC-exosomes combine the benefits of exosomes with the attributes of MSCs. Numerous studies have demonstrated their therapeutic value in treating a wide range of illnesses, such as wound healing, neurological diseases, tumours, cardiovascular, and cerebrovascular diseases. MSC-exosomes have the potential to be used as nanotherapeutic agents in this regard. Furthermore, a study indicated that MSC-exosomes, naturally occurring nano-drug delivery vehicles, when paired with engineering technologies have shown promise in the treatment of disease [4, 5].
It has been demonstrated that a number of growth factors from the transforming growth factor β (TGF-β) superfamily, along with their receptors and intracellular signalling molecules, the Smads, are essential for vital ovarian processes like oocyte development and formation and ovarian folliculogenesis. The Smads are categorised as inhibitory (I-) Smads (Smad6 and − 7), common-partner (Co-) Smads (Smad4, Xenopus Smad4α and Smad4β), and receptor-activated (R-) Smads (Smad1, -2, -3, -5, and − 8). These transcription factors operate within cells and mediate the signalling of the TGF-β superfamily, which currently has over 40 members, including growth differentiation factors (GDFs), activins, inhibins, and bone morphogenetic proteins in addition to three isoforms of TGF-β [6, 7].
Mice gene knockout studies have demonstrated the requirement for specific BMPs and their downstream Smad effectors for germ line formation. Despite dying in the uterus during development, Smad1 and Smad5 knockout mice exhibit abnormal primordial germ cell (PGC) production, similar to BMP-4 and − 8b deficient mouse embryos that survive gastrulation. Rats express Smad2 and Smad3 at distinct stages of ovarian folliculogenesis, which may allow for distinct effects from TGF-β superfamily ligands that utilise the same signalling pathways [8, 9]
The N-terminal Mad homology 1 (MH1) domain and the C-terminal MH2 domain of the Smads are the two primary domains that are conserved in the protein family. They are joined by a non-conserved proline-rich linker region. While the N-terminal section of the I-Smads only has modest sequence similarity with the MH1 domain, the MH1 domain is largely conserved between the R-Smads and Co-Smads. The MH2 domain, on the other hand, is very similar among all Smads [10, 11].
While I-Smads localise to the cell nucleus, inactive R-Smads are primarily found in the cytoplasm as monomers. Smad4 is located in the cytoplasm of unstimulated cells because of active nuclear export, and it constitutively shuttles between the cytoplasm and the nucleus. Additionally involved in Smad location and signalling are cytoskeletal proteins. SARA is a scaffolding protein with an FYVE domain that interacts with the inactive Smad2 and Smad3 MH2 domain to direct them to early endosomes and help bind Smads to their receptors, hence increasing TGF-β signalling and Smad phosphorylation [13, 14]. The receptor complex is degraded during internalisation via a different pathway, a lipid raft/caveolar dependent process, which in turn controls Smad activation and receptor turnover. After phosphorylation, Smads can form heterotrimers with two R-Smads and one Smad4 (Smad3), or heterodimers with an R-Smad (Smad2) and a Co-Smad. These oligomers can develop at different stochiometries. Activated Smads gather into the nucleus, where they interact with other transcription factors, corepressors, and coactivators to regulate target gene expression in a cell type-specific manner. Different Smad-interacting transcription factors and their interaction with other signalling pathways lead to diverse ligand responses in various cell types [13].
Folliclinogenesis and ovarian organogenesis are mediated by TGF-β superfamily ligands:
Fundamental to animal reproduction is the establishment of the germ line. The extraembryonic ectoderm in mice stimulates the determination of germ cells. Uncommitted epiblast cells in the extra-embryonic mesoderm in mice undergo involution through the primitive streak to the yolk sac endoderm during the early stages of embryonic development, whereupon they commit to become primordial germ cells (PGCs). As the PGCs exit the dorsal mesentery, the process of gametogenesis begins. It continues when the PGCs enter and colonise the genital ridges to establish the potential gonad. Numerous animal studies have examined the function of ligands belonging to the TGF-β superfamily in both folliculogenesis and ovarian organogenesis. Specifically, it has been demonstrated that the BMPs and their antagonists play a significant role throughout organogenesis and embryonic development. BMP4, -8b, and − 2 have been found to be regulators of primordial germ cell (PGC) production from epiblast cells in gene ablation studies in mice. BMP-2 is derived from the embryonic endoderm, whereas BMP-4 and − 8b are derived from the extra-embryonal ectoderm. In the embryos that survive gastrulation, targeted mutations of either BMP-4 or BMP-8b result in significant abnormalities in PGC development. Additionally, it has been found that aberrant germ cell migration occurs when TGF-β signalling via ALK-5 is absent [14, 15].
Follicle growth is thought to be gonadotropin-independent up to the small antral stage, and in these early stages, local autocrine and paracrine cues from the egg and the surrounding somatic cells appear to be the main drivers of folliculogenesis. The course of follicular development through successive stages is driven by a complicated bi-directional communication between the granulosa and thecal cells as well as between the oocyte and granulosa cells. This relationship involves a variety of TGF-β superfamily ligands expressed by distinct ovarian cell types, the expression of which is regulated in a developmental stage-dependent way [12]. The growth of follicles is believed to be influenced by a number of local factors, including activins, inhibins, TGF-β s, BMP-6, GDF-9 and its homologue GDF-9B (also known as BMP-15), and anti-Müllerian hormone (AMH, also known as Müllerian inhibiting substance, or MIS). Research has demonstrated that the developing oocyte expresses TGF-β 2, BMP-6, GDF-9, and GDF-9B, albeit it's possible that the TGF-β protein isn't secreted. At different stages of folliculogenesis, granulosa cells produce activins, inhibins, TGF-β, BMP-2, BMP-3, and BMP-6, as well as AMH. On the other hand, theca cells have been found to generate all isoforms of TGF-β, BMP-3b, BMP-4, and BMP-7. While GDF-9B, BMP-2, -4, -6, and − 7 use the Smad1/5/8 pathway, TGF-β, activins, and GDF-9 signal through the Smad2/3 pathway [13, 15].