Modulation of Female Reproductive Functions through Nerve Growth Factor: A Systematic Review

Neurotrophins (NTs) are a family of small polypeptide growth factors that were initially discovered to play a role in the central and peripheral nervous systems (Ref. 1 ). Subsequent studies have gradually revealed the widespread distribution of NT and its receptors in other tissues, including the digestive tract, lymph nodes, spleen, breast and reproductive system (Refs 2 , 3 , 4 ). Nerve growth factor (NGF), discovered by Levi Montalcini in 1952, was the earliest identified member of the NT family (Ref. 5 ). To date, it is the most thoroughly studied NT and its main functions include regulating the growth, development, differentiation, survival and repairment of neurons after injury. In addition, NGF is also involved in multiple processes such as pain, inflammation, tumour proliferation and metastasis, as well as reproductive physiology. Clinical studies have found that NGF concentration has increased in diseases with chronic pain symptoms such as osteoarthritis, chronic headache, cancer-related pain and diabetes neuropathy. NGF gene mutations (R121W, V232fs and R221W) will lead to painless diseases, suggesting that NGF-mediated signals are continuously active in chronic nociceptive and neuropathic pain (Refs 6 , 7 ). At present, monoclonal antibodies that block the NGF-tropomyosin receptor kinase A (TrkA) signalling pathway, such as Fasinumab (REGN475) and Tanezumab (PF-04383119), have advanced into clinical trials for the treatment of osteoarthritis-related pain (Ref. 6 ). Recently, Zaninelli et al. suggested that anti-NGF treatment reduced endometriosis-associated pain in mice (Ref. 8 ). Human NGF gene is located on chromosome 1, and it first synthesizes two types of long and short NGF precursor after transcription and translation (Ref. 9 ). After processing and hydrolysis, the long NGF precursor is transformed into 7.5S NGF, which is a complex composed of two α subunits (α-NGF), one β subunits (β-NGF) and two γ subunits (γ-NGF) with a relative molecular weight of around 140000 Da. In contrast, the short NGF precursor is converted into 2.5S NGF, which mainly contains β-NGF and can be dissociated from long NGF precursor. α-NGF does not have activity, β-NGF has biological activity, and γ-NGF is a unique serine proteinases, which may be related to the conversion of NGF precursor to mature NGF (Ref. 10 ). β-NGF is a homodimer formed by noncovalent binding and parallel assembly of two identical peptide chains, with disulfide bonds present among six cysteine residues, which are crucial for maintaining the biological activity of NGF. Each peptide chain contains 118 amino acid residues, with a relative molecular weight of approximately 26518 Da and an isoelectric point of 9.3 (Ref. 11 ). The biological actions of NGF are mediated by binding to two different types of membrane surface receptors: TrkA, which belongs to the tyrosine kinase receptor superfamily and is a specific high-affinity receptor for NGF, and p75 neurotrophic receptor (p75NTR), which belongs to the tumour necrosis factor superfamily and is a non-specific low-affinity receptor for NGF (Refs 12 , 13 , 14 , 15 ). Multiple studies have shown that NGF and its receptors are expressed in oocytes, granulosa cells and other cells within the human ovary from embryo to adulthood ( Table 1 ), and play an important role in reproductive physiology processes, such as follicle development and ovulation. As early as the embryonic period at 19–33 weeks of gestation, NGF are expressed in oogenia/oocytes of human embryo ovaries (Ref. 16 ). In ovaries of adolescents/adults, NGF is expressed in granulosa cells of preantral follicles and antral follicles, as well as in all oocytes ( 16 , 17 ). In the embryonic period, TrkA is mainly expressed in oogenia/oocytes in most cases, with a few expressed in granulosa cells (Ref. 16 ). In ovaries of adolescents/adults aged 13–39, TrkA is expressed in granulosa cells of preantral follicles and antral follicles, theca cells of antral follicles and most oocytes (Refs 16 , 17 ). P75NTR is expressed in some stroma cells of human foetal ovaries (Refs 16 , 18 ). Additionally, NGF also exists in human follicular fluid (Refs 19 , 20 , 21 ). Table 1. Distribution of NGF and its receptors in the human ovaries Table 1. long description. Ligands/receptors Localization Expression pattern Detection methods References NGF Oocytes/oogonia Protein IHC (Ref. 16 ) Granulosa cells Protein IHC (Refs 16 , 17 ) Granulosa cells RNA RT-qPCR (Ref. 17 ) Theca cells Protein IHC (Ref. 17 ) stroma cells Protein IHC (Refs 16 , 17 ) Follicular fluid Protein ELISA (Ref. 19 ) Follicular fluid Protein Emax Immunoassay Systems (Ref. 20 ) Follicular fluid Protein ELISA (Ref. 21 ) TrkA Oocytes/oogonia Protein IHC (Refs 16, 17 ) Granulosa cells Protein IHC (Ref. 16 ) Granulosa cells RNA RT-qPCR (Ref. 17 ) Theca cells Protein IHC (Ref. 17 ) Stroma cells Protein IHC (Ref. 16 ) P75NTR Stroma cells Protein IHC (Refs 16 , 18 ) A data table summarizing the localization, expression patterns, and detection methods for NGF, TrkA, and P75NTR in human ovarian tissues. See long description. IHC: immunohistochemistry; NGF: nerve growth factor; p75NTR: p75 neurotrophic receptor; RT-qPCR: quantitative reverse transcription polymerase chain reaction; TrkA: tropomyosin receptor kinase A. Distribution of NGF and its receptors in the human ovaries Table 1. long description. IHC: immunohistochemistry; NGF: nerve growth factor; p75NTR: p75 neurotrophic receptor; RT-qPCR: quantitative reverse transcription polymerase chain reaction; TrkA: tropomyosin receptor kinase A. Both granulosa cells and theca cells of human ovaries can synthesize NGF, with theca cells serving as the primary site of secretion (Ref. 17 ). Experiments have shown that NGF concentration in follicular fluid is related to the follicle size. In sheep, large antral follicles (>4 mm in diameter) produce NGF, whereas small antral follicles and early atresia follicles produce minimal to undetectable levels (Ref. 22 ). The NGF concentration in follicular fluid is regulated by gonadotrophin-releasing hormone (GnRH) and gonadotropins. GnRH causes a dramatic increase of NGF in large follicles, while gonadotrophins stimulate cultured granulosa cells and follicles to produce NGF. In the presence of both luteinizing hormone (LH) and follicle-stimulating hormone (FSH), the production of NGF showed a clear dose-dependent response in vitro based on the culture of sheep antral follicles, while no detectable levels of neurotrophic activity accumulated in medium when gonadotrophins were absent or added separately (Refs 22 , 23 ). In addition, some researchers reported that NGF and its receptors changed with seasons in squirrels. Stronger immunostaining of NGF, TrkA and p75 were observed in granulosa cells, thecal cells, interstitial cells and ovarian surface epithelium in the breeding season compared to the nonbreeding season (Refs 24 , 25 ).

PCOS is the most common reproductive endocrine disorder, with a prevalence of 6–15% (Ref. 83 ). Experimental evidence from rodent models demonstrates a strong association between NGF dysregulation and PCOS pathogenesis. In rats, steroid administration significantly upregulated intraovarian NGF and p75NTR synthesis, resulting in polycystic ovaries marked by multiple follicular cysts (Ref. 84 ). Similarly, ovarian-specific transgenic NGF overexpression in mice recapitulated key PCOS features, including both reproductive and metabolic abnormalities (Refs 43 , 85 ). Histopathological analysis revealed that NGF overproduction disrupted normal folliculogenesis, causing follicular arrest and promoting antral follicle apoptosis (Ref. 43 ). These consistent findings across species strongly implicate ovarian sympathetic hyperactivation in PCOS development and progression (Ref. 85 ). Clinical observations further support this mechanistic link. IVF patient analyses demonstrated significantly elevated NGF levels in patients with PCOS, with approximately twofold higher follicular fluid concentrations and sixfold greater granulosa cell expression compared to controls (Ref. 43 ). However, conflicting data from Buyuk failed to replicate these NGF elevation findings, highlighting potential methodological or population differences in clinical PCOS studies (Ref. 20 ). Recently, clinical evidence from Robeva’s study demonstrated that significantly elevated NGF in women with PCOS showed a positive correlation with plasma-free normetanephrine levels, suggesting that NGF may contribute to PCOS pathogenesis through enhanced sympathetic noradrenergic activity (Ref. 86 ). Furthermore, NGF may promote the occurrence and development of PCOS pathological features by enhancing androgen and testosterone secretion. In in vitro experiments, NGF acted on bovine theca cells transfected with TrkA, resulting in excessive production of androgen and testosterone (Ref. 81 ). Consistent with this, in in vivo experiments, 17NF mice with ovarian local overexpression of NGF produced excessive testosterone under the stimulation of gonadotropins (Ref. 76 ). However, there is currently no experimental evidence to show that excessive NGF can induce a PCOS disease model. Diminished ovarian reserve (DOR) refers to a condition characterized by a reduction in the quantity and quality of oocytes stored in the ovaries, leading to infertility, assisted reproductive technology failure and miscarriage. Based on previous literature, the diagnosis is primarily based on decreased AMH levels and antral follicle counts, and elevated FSH levels (Ref. 87 ). The prevalence of DOR has increased significantly in recent years (Refs 88 , 89 , 90 , 91 ). Women with DOR exhibit significantly higher follicular fluid NGF levels compared to those with normal ovarian function. This finding is supported by clinical data from IVF patients, particularly in advanced maternal age populations (Ref. 92 ). However, this apparent NGF elevation may represent a compensatory mechanism rather than a primary pathogenic factor, as evidenced by reduced expression of NGF receptors TrkA and p75NTR in cumulus cells from DOR patients undergoing assisted reproductive technology. These in vitro findings suggest that impaired NGF receptor signalling, rather than NGF availability itself, may contribute to the low ovarian reserve (Ref. 35 ). Endometriosis, which affects approximately 10% of reproductive-aged women, is a chronic gynaecological disorder characterized by ectopic implantation of endometrial tissue (Refs 93 , 94 ). While extensive research has been conducted, the exact pathogenesis and molecular mechanisms underlying this prevalent condition remain incompletely understood (Ref. 95 ). Notably, immunohistochemical analyses revealed significantly stronger immunostaining for both NGF and its receptor TrkA in endometriotic lesions from patients experiencing deep dyspareunia compared to those without this symptom (Refs 96 , 97 , 98 ). In mouse, anti-NGF treatments reduced endometriosis-associated pain (Ref. 8 ). These findings suggest that NGF signalling may contribute specifically to endometriosis-associated sexual pain. Supporting this notion, clinical investigations have demonstrated elevated follicular fluid NGF levels in endometriosis patients relative to women with male factor infertility (Ref. 20 ).

NGF is a pivotal regulator of female reproductive functions, including follicular assembly, follicle development, oocyte maturation, ovulation and steroidogenic regulation. In particular, the role of NGF in inducing ovulation by acting on the hypothalamus has garnered considerable scholarly attention. Clinically, dysregulated NGF signalling has been implicated in the pathogenesis of several reproductive disorders, such as PCOS, DOR and endometriosis. These investigations may yield novel diagnostic and therapeutic approaches for managing NGF-associated reproductive conditions. Nevertheless, current research has two major limitations that warrant emphasis: (1) the underlying mechanisms by which NGF participates in reproductive physiology remain incompletely understood and (2) the clinical translation of NGF-based applications is still at an early stage. Future investigations should prioritize these two aspects to advance our understanding and optimize the clinical utility of NGF in reproductive health.