Kidney diseases cause serious health problems worldwide, progressing from the early stages of chronic kidney disease (CKD) to end-stage renal disease [1]. Morbidity and mortality rates are very high, and many patients require dialysis or kidney transplantation [2–4]. Renal fibrosis is one of the most common causes of CKD [5]. The concept of CKD is constantly being refined in response to new research findings. CKD is currently defined as a 3-month period of gradual loss of normal kidney function or kidney damage involving structural or functional renal abnormalities with or without a low estimated glomerular filtration rate [5,6]. In 2017, approximately 1.2 million people worldwide were projected to die of CKD, positioning CKD as the 12th most prevalent cause of death globally. Moreover, 35.8 million people were estimated to experience disability-adjusted life years [7,8]. Various abnormalities such as tubulointerstitial fibrosis, loss of renal parenchyma, glomerulosclerosis, and inflammatory cell infiltration are consequences of renal fibrosis [9]. These pathologic outcomes are frequently the result of intricate underlying biological processes such as epithelial–mesenchymal transition (EMT) activation, fibroblast activation, monocyte/macrophage infiltration, and cellular death [9,10]. Following the first insult, resident kidney cells become activated, resulting in the synthesis and secretion of proinflammatory chemotactic cytokines such as transforming growth factor-1 (TGF-1) and tumor necrosis factor [11]. These cytokines cause inflammatory monocytes/macrophages and T cells to infiltrate the damaged areas, and then these cells in the glomerulus or interstitial space become activated and create reactive oxygen species as well as inflammatory and fibrogenic cytokines. Myofibroblasts are generated by the EMT, which stimulates mesangial cells, fibroblasts, and tubular epithelial cells [12,13]. As a result, a large number of extracellular matrix (ECM) components are produced, which are deposited in extracellular compartments and are resistant to destruction. Progressive deposition alters the kidney architecture, leading to collapse of the renal parenchyma and loss of kidney function [14,15]. TGF-1 is a member of the TGF-β superfamily, which includes glial cell-derived neurotrophic factor (GDNF), a strong neurotrophic survival factor [16]. GDNF was first discovered to be a survival factor for noradrenergic, dopaminergic, and spinal motor neurons. Studies have demonstrated its usefulness for treating Parkinson’s disease and other neurological diseases [17–19]. The ligands of this family tend to bind to TGF-β receptors and cause the recruitment and activation of Smad family transcription factors, which in turn influence gene expression. The Smad protein binds to the rearranged during transfection (RET) proto-product, an oncogene that belongs to the receptor tyrosine kinase (RTK) subfamily and is required for the development of the enteric nervous system and kidney [20]. After GDNF knockout in one study, mice demonstrated impairment in both ureteric bud (UB) formation and branching during metanephros development; this was the discovery of non-neuronal GDNF functions [21]. The mice did not survive long after birth because of renal agenesis. Thus, the involvement of GDNF in renal development and illness was revealed.

GDNF was first purified from B49 glial cells. It is an effective neurotrophic factor that can protect nigral dopaminergic neurons [22]. A recent study showed that human amniotic fluid-derived stem cells (AFSCs) have enhanced regeneration ability. In addition, these cells exhibit improved survival, differentiation, and secretion of regeneration factors, particularly in the presence of GDNF [23]. GDNF-modified adipose-derived mesenchymal stem cells (GDNF-AMSCs) inhibit renal inflammation and oxidative stress in mice with unilateral ureteral obstruction (UUO). They also improve peritubular capillary (PTC) injury, promote angiogenesis, and delay the progression of interstitial fibrosis to some extent. Moreover, GDNF-engineered human AFSCs have been shown to play a protective role in kidney injury [24]. GDNF has both dopaminergic and nephrogenic renal morphogenetic actions [25]. However, little is known about the detailed mechanisms underlying these processes.

In this review, we objectively evaluate recent studies on the role of GDNF in renal function. We also extrapolate the mechanism by which GDNF protects the damaged kidney.

Mammalian embryos contain a pair of mesonephric organs called the Wolffian ducts (WDs), which are induced by signals from metanephric mesenchyme (MM) cells to develop into an epithelial growth called the UB [26,27]. This is the first step in the development of the mammalian (metanephric) kidney. The UB grows and branches from the connecting segment to its entrance into the bladder, eventually giving rise to the tree-like collecting system of the kidney [28]. The induction and proper growth of the UB are critical for normal formation of the kidneys because the subsequent elongation and branching of the UB dictate the renal architecture, which is a fundamental determinant of kidney function. This occurs through the induction of the mesenchymal-to-epithelial transition of the MM, which governs the spatial arrangement of nephrons [27,28]. According to Song et al. [29], GDNF plays a significant role in renal morphogenesis. RTK-activated signaling is a critical regulator of kidney development. The two primary inducers of RTK in the developing kidney are RET (induced by GDNF) and fibroblast growth factor (FGF). RET and GDNF family receptor alpha-1 (GFRA1) receptors and coreceptors are responsible for critical tasks [30,31]. GDNF plays several roles during kidney embryogenesis; it is involved in budding of the ureteric duct from the WD, branching of the ureteric epithelium inside the MM, and in the production of new nephrons [32]. Exogenous GDNF increases the number of ureteric branches and developing nephrons in metanephric culture [32,33]. The number of nephrons that develop during embryogenesis is likewise controlled by GDNF. Deletion of one GDNF allele reduces the number of nephrons in adult kidneys, possibly due to impaired morphogenesis of UB branching [33]. Towers et al. [34] discovered that GDNF promotes UB outgrowth and improves UB cell survival in vitro. Similarly, Pepicile et al. [35] found that GDNF promotes ureter tip proliferation in mice. This results in an increase in the number of ureter tips as well as growth and fusion of neighboring tips, with some tips growing toward the GDNF source. In the late 1990s, knockout investigations established that homozygous null mutants for GDNF, RET, and GFRA1 showed bilateral renal agenesis and died shortly after delivery [36–38].

Renal fibrosis is common in patients with advanced CKD. The most common pathogenic characteristics of renal fibrosis are hypoxia, oxidative stress, inflammatory cell infiltration, fibroblast activation, and ECM accumulation. This causes the degradation of kidney parenchyma, which manifests as tubular atrophy, capillary loss, podocyte depletion, and end-stage renal disease [48]. GDNF was originally reported to be a potential therapeutic agent for the treatment of neurodegenerative diseases before many studies revealed its further roles in treating renal fibrosis and other anomalies [49,50]. One study demonstrated that kidney agenesis or dysgenesis in GDNF^−/− mutant mice may result from the promotion of GDNF in ureteric branching, suggesting that GDNF may acts as a morphogen in kidney development [36]. GDNF expression is associated with epithelial differentiation of the nephrogenic mesenchyme in the kidney, and the absence of this gene results in renal aplasia or severe hypodysplasia [21,51]. Therefore, GDNF as a trophic factor has great promise for its use in organogenesis. GDNF is a tissue morphogen that enhances the migration and differentiation of stem cells and reverses kidney injury [52]. Some authors have speculated that pretreatment of AMSCs with GDNF may better alleviate renal fibrosis by strengthening the ability of AMSCs to differentiate into endothelial cells [23,53]. GDNF has therapeutic effects on mesenchymal stem cells, which have emerged as ideal cell candidates for functional and structural restoration of injured kidneys. It can help ameliorate kidney disease, and the hypothesized mechanism involves the transfer of endogenous molecules through paracrine factors to apprehend injured cells. However, the precise mechanism remains unclear. GDNF may activate an angiogenesis pathway in surviving PTCs after injury has occurred, leading to activation of the sirtuin-1 (SIRT1)/endothelial nitric oxide synthase signaling pathway. By inhibiting cell apoptosis and fibrosis, SIRT1 shows cytoprotective effects and inhibits inflammation in the kidney, thus contributing to the maintenance of renal homeostasis. Its downregulation leads to acute and CKDs. Angiogenesis helps regulate the interaction between SIRT1 and endothelial nitric oxide synthase. However, whether angiogenesis and its regulation of SIRT1 are involved in the development of renal fibrosis remains unclear, and the role of GDNF in attenuating this process remains to be defined (Fig. 1).

GDNF requires numerous mediators in distinct signaling pathways to carry out its functions in vivo and in vitro. In this review, we examine the phosphoinositide 3-kinase (PI3K)/Akt and mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) pathways and how they relate to GDNF-mediated kidney protection.

Renal fibrosis is a complex process that includes macrophage/lymphocyte infiltration, podocyte apoptosis, accumulation of activated fibroblasts, EMT, and enhanced ECM deposition, among other processes [88]. Therefore, an ideal therapy should be able to suppress tubular and glomerular interstitial fibrosis by targeting several events along the pathogenic process. Although GDNF plays an important role in kidney development, it has also been shown to help with renal fibrosis [53].

Li et al. [24] examined whether transplanting GDNF-modified AFSCs for the treatment of renal interstitial fibrosis is more effective than transplanting unmodified AFSCs. GDNF-modified AFSCs significantly reduced oxidative stress and inflammation and regulated the protein levels of PTCs, vascular endothelial growth factor (VEGF), hypoxia-inducible factor-1 (HIF-1), and TGF-1. The GDNF gene enhanced the ability of AFSCs to control inflammation and oxidative stress, which helps alleviate renal fibrosis. They also found that transplanting AFSCs into mice with ureteral ligation boosted tubular epithelial cell proliferation and reduced renal interstitial fibrosis. Anti-inflammatory and anti-oxidative pathways in AFSCs were found to help mice recover more rapidly from UUO.

Wang et al. [89] investigated whether GDNF improves the anti-inflammatory effects of human AMSC-based therapy in renal interstitial fibrosis. They focused on how AMSCs work biologically and whether the GDNF gene can help stem cells to become more anti-inflammatory. Interleukin (IL)-10, IL-4, inducible nitric oxide synthase, and tumor necrosis factor mRNA expression levels were measured via quantitative reverse-transcription polymerase chain reaction (qRT-PCR). Two M2 macrophage markers were discovered using flow cytometry (CD163 and CD206). UUO models were established in nude mice, with six mice in each group. IL-6, TGF-1, smooth muscle actin, and cyclooxygenase 2 were subsequently evaluated via Western blotting. In all groups, pathological changes in the kidneys were observed via Masson staining and hematoxylin and eosin staining. All of the preceding observations led to the conclusion that AMSCs can cause M1 macrophages to change into M2 macrophages and that GDNF can enhance this transformation. Furthermore, AMSC therapy improved renal function and reduced interstitial fibrosis in the UUO model.

Wang et al. [90] examined whether GDNF-AFSCs could reduce apoptosis in the mouse renal tubular epithelial cell (mRTECs) and improve renal dysfunction. They used an mRTEC hypoxia/reoxygenation (ischemia/reperfusion) model. In a Transwell system, damaged mRTECs were cocultured with GDNF-AFSCs. The expressions of FGF and hepatocyte growth factor mRNA were measured via qRT-PCR. The levels of superoxide dismutase, malondialdehyde, and reactive oxygen species were also measured. Bax, Bcl-2, Gp91-phlox, and nitrotyrosine expression were examined via Western blotting. The GDNF-AFSC-treated groups showed an increase in growth factor expression and a considerable counteraction of oxidative stress. Hypoxia/reperfusion-induced apoptosis was also reduced in mRTECs.

Some of the factors that contribute to renal disease progression, which can be affected by chronic tissue hypoxia and PTC rarefaction stemming from microvascular injury, can easily accelerate interstitial kidney fibrosis [91,92]. Hypoperfusion of PTCs can induce hypoxia and increase the progression of tubulointerstitial fibrosis [93]. Chen et al. [94] investigated whether GDNF-AMSC-exos could alleviate kidney hypoxia and fibrosis in UUO models. They specifically examined the expression levels of HIF-1α and VEGF. In a group treated with GDNF-AMSC-exos, there was an increase in VEGF expression, while the expression of HIF-1α was significantly reduced. For summary check Table 1 [24,89,90,94–96]. These results suggest that treatment with GDNF-AMSC-exos can significantly improve kidney hypoxia and thus ameliorate kidney fibrosis.

Tubulointerstitial fibrosis, defined as progressive deposition of damaging connective tissue in the kidney parenchyma, appears to be a destructive process that ultimately leads to renal function decline, regardless of the primary renal disease that causes the initial kidney injury. Despite current CKD treatment choices and efforts to eradicate the condition, further research is required. The use of GDNF is a new modality that has the potential to benefit patient outcomes and complement existing therapies. The importance of GDNF in renal development and the repercussions of its absence during embryogenesis were highlighted in this review. Furthermore, GDNF kidney receptors were discussed, and signaling pathways employed to reduce renal apoptosis, inflammation, and fibrosis were examined. Most studies to date have reported positive effects of GDNF although some have suggested that GDNF might increase the growth of cancerous cells; additionally, high levels of GDNF are associated with pancreatic tumors. A limitation of this review is that few studies have focused on the role of GDNF in the kidney and the mechanism underlying its above-described roles, which limits a more comprehensive understanding. Consequently, few clinical trials have been conducted to assess the effects of GDNF on patients; most have been conducted on stem cells and animal models. Therefore, more studies are needed to gain a better understanding of the safety of GDNF for the kidney.