Gremlin and renal diseases: ready to jump the fence to clinical utility?

Gremlin1: a BMP antagonist with therapeutic potential in Oncology

Gremlins, originating from early 20th-century Western folklore, are mythical creatures known for causing mechanical malfunctions and electronic failures, aptly dubbed "little devils". Analogously, GREM1 acts like a horde of these mischievous entities by antagonizing the bone morphogenetic protein (BMP signaling) pathway or through other non-BMP dependent mechanisms (such as binding to Fibroblast Growth Factor Receptor 1and Epidermal Growth Factor Receptor) contributing to the malignant progression of various cancers. The overexpression of GREM1 promotes tumor cell growth and survival, enhances angiogenesis within the tumor microenvironment, and creates favorable conditions for tumor development and dissemination. Consequently, inhibiting the activity of GREM1 or blocking its interaction with BMP presents a promising strategy for suppressing tumor growth and metastasis. However, the role of GREM1 in cancer remains a subject of debate, with evidence suggesting both oncogenic and tumor-suppressive functions. Currently, several pharmaceutical companies are researching the GREM1 target, with some advancing to Phase I/II clinical trials. This article will provide a detailed overview of the GREM1 target and explore its potential role in cancer therapy.

Anti-VEGFR2 F(ab')2 drug conjugate promotes renal accumulation and glomerular repair in diabetic nephropathy

Poor renal distribution of antibody-based drugs is the key factor contributing to low treatment efficiency for renal diseases and side effects. Here, we prepare F(ab')2 fragmented vascular endothelial growth factor receptor 2 antibody (anti-VEGFR2 (F(ab')2) to block VEGFR2 overactivation in diabetic nephropathy (DN). We find that the anti-VEGFR2 F(ab')2 has a higher accumulation in DN male mice kidneys than the intact VEGFR2 antibody, and simultaneously preserves the binding ability to VEGFR2. Furthermore, we develop an antibody fragment drug conjugate, anti-VEGFR2 F(ab')2-SS31, comprising the anti-VEGFR2 F(ab')2 fragment linked to the mitochondria-targeted antioxidant peptide SS31. We find that introduction of SS31 potentiates the efficacy of anti-VEGFR2 F(ab')2. These findings provide proof of concept for the premise that antibody fragment drug conjugate improves renal distribution and merits drug validation in renal disease therapy.

Cellular crosstalk of mesangial cells and tubular epithelial cells in diabetic kidney disease

Diabetic kidney disease (DKD) is a major cause of end-stage renal disease and imposes a heavy global economic burden; however, little is known about its complicated pathophysiology. Investigating the cellular crosstalk involved in DKD is a promising avenue for gaining a better understanding of its pathogenesis. Nonetheless, the cellular crosstalk of podocytes and endothelial cells in DKD is better understood than that of mesangial cells (MCs) and renal tubular epithelial cells (TECs). As the significance of MCs and TECs in DKD pathophysiology has recently become more apparent, we reviewed the existing literature on the cellular crosstalk of MCs and TECs in the context of DKD to acquire a comprehensive understanding of their cellular communication. Insights into the complicated mechanisms underlying the pathophysiology of DKD would improve its early detection, care, and prognosis. Video Abstract.

Current status and prospects of GREM1 research in cancer (Review)

GREM1 is a secreted protein that antagonizes bone morphogenetic proteins (BMPs) and participates in critical biological processes, including embryonic development, organogenesis and tissue differentiation. Gremlin 1 (GREM1) is also an inhibitor of TGF-β and a ligand for vascular endothelial growth factor receptor 2. In addition, GREM1 can induce cells, participate in the process of epithelial-mesenchymal transition, and then participate in tumor development. GREM1 has a variety of biological functions and can participate in the malignant progression of a variety of tumors through the BMP signaling pathway. GREM1 also can inhibit TGF-β in some tumors, thereby inhibiting tumors, and its involvement in tumor development varies in different types of cancer. The present review examines the role and function of GREM1 in tumors. GREM1 is expressed in a variety of tumor types. GREM1 expression can affect the epithelial-mesenchymal transformation of tumor cells. GREM1 has been studied in breast and colon cancer, and its potential role is to promote cancer. However, in pancreatic cancer, which was found to act differently from other cancer types, overexpression of GREM1 inhibits tumor metastasis. The present review suggests that GREM1 can be a diagnostic and prognostic indicator. In future studies, the study of GREM1 based on single-cell sequencing technology will further clarify its role and function in tumors.

Role of gremlin-1 in the pathophysiology of the adipose tissues

Gremlin-1 is a secreted bone morphogenetic protein (BMP) antagonist playing a pivotal role in the regulation of tissue formation and embryonic development. Since its first identification in 1997, gremlin-1 has been shown to be a multifunctional factor involved in wound healing, inflammation, cancer and tissue fibrosis. Among others, the activity of gremlin-1 is mediated by its interaction with BMPs or with membrane receptors such as the vascular endothelial growth factor receptor 2 (VEGFR2) or heparan sulfate proteoglycans (HSPGs). Growing evidence has highlighted a central role of gremlin-1 in the homeostasis of the adipose tissue (AT). Of note, gremlin-1 is involved in AT dysfunction during type 2 diabetes, obesity and non-alcoholic fatty liver disease (NAFLD) metabolic disorders. In this review we discuss recent findings on gremlin-1 involvement in AT biology, with particular attention to its role in metabolic diseases, to highlight its potential as a prognostic marker and therapeutic target.

Insights into the Role of Gremlin-1, a Bone Morphogenic Protein Antagonist, in Cancer Initiation and Progression

The bone morphogenic protein (BMP) antagonist Gremlin-1 is a biologically significant regulator known for its crucial role in tissue differentiation and embryonic development. Nevertheless, it has been reported that Gremlin-1 can exhibit its function through BMP dependent and independent pathways. Gremlin-1 has also been reported to be involved in organ fibrosis, which has been correlated to the development of other diseases, such as renal inflammation and diabetic nephropathy. Based on growing evidence, Gremlin-1 has recently been implicated in the initiation and progression of different types of cancers. Further, it contributes to the stemness state of cancer cells. Herein, we explore the recent findings on the role of Gremlin-1 in various cancer types, including breast, cervical, colorectal, and gastric cancers, as well as glioblastomas. Additionally, we highlighted the impact of Gremlin-1 on cellular processes and signaling pathways involved in carcinogenesis. Therefore, it was suggested that Gremlin-1 might be a promising prognostic biomarker and therapeutic target in cancers.

Epigenetic Modulation of Gremlin-1/NOTCH Pathway in Experimental Crescentic Immune-Mediated Glomerulonephritis

Crescentic glomerulonephritis is a devastating autoimmune disease that without early and properly treatment may rapidly progress to end-stage renal disease and death. Current immunosuppressive treatment provides limited efficacy and an important burden of adverse events. Epigenetic drugs are a source of novel therapeutic tools. Among them, bromodomain and extraterminal domain (BET) inhibitors (iBETs) block the interaction between bromodomains and acetylated proteins, including histones and transcription factors. iBETs have demonstrated protective effects on malignancy, inflammatory disorders and experimental kidney disease. Recently, Gremlin-1 was proposed as a urinary biomarker of disease progression in human anti-neutrophil cytoplasmic antibody (ANCA)-associated crescentic glomerulonephritis. We have now evaluated whether iBETs could regulate Gremlin-1 in experimental anti-glomerular basement membrane nephritis induced by nephrotoxic serum (NTS) in mice, a model resembling human crescentic glomerulonephritis. In NTS-injected mice, the iBET JQ1 inhibited renal Gremlin-1 overexpression and diminished glomerular damage, restoring podocyte numbers. Chromatin immunoprecipitation assay demonstrated BRD4 enrichment of the Grem-1 gene promoter in injured kidneys, consistent with Gremlin-1 epigenetic regulation. Moreover, JQ1 blocked BRD4 binding and inhibited Grem-1 gene transcription. The beneficial effect of iBETs was also mediated by modulation of NOTCH pathway. JQ1 inhibited the gene expression of the NOTCH effectors Hes-1 and Hey-1 in NTS-injured kidneys. Our results further support the role for epigenetic drugs, such as iBETs, in the treatment of rapidly progressive crescentic glomerulonephritis.

Relationship of Plasma Gremlin 1 Levels with Body Adiposity and Glycemic Control in Saudi Female Type 2 Diabetes Patients

OBJECTIVE

Gremlin 1 is a novel adipokine that plays an important role in obesity and type 2 diabetes mellitus (T2DM). In the current study, we aimed to evaluate plasma levels of Gremlin 1 in diabetic and non-diabetic Saudi adult females and its correlation with body composition, glycemic control and lipid profile.

METHODS

A case-control study was conducted among 41 T2DM and 31 non-diabetic adult age matched females (controls). All patients underwent body composition by bioelectrical impedance analysis, with a commercially available body analyzer. Fasting venous samples were analyzed for glycemic markers and lipids, while plasma Gremlin 1 was measured by ELISA. The results were compared between the two groups and correlated with other anthropometric and adiposity parameters.

RESULTS

Gremlin 1 levels were elevated in T2DM patients (345 ± 26 ng/mL) when compared to control subjects (272 ± 16 ng/mL, p < 0.05). Diabetic patients having poor glycemic control had significantly higher Gremlin 1 levels (382 ± 34 ng/mL) compared to patients with good glycemic control (291 ± 37 ng/mL, p < 0.05). Pearson correlation analysis revealed a positive correlation of Gremlin 1 with fat mass (r = 0.246, p = 0.012), HbA1C (r = 0.262, p = 0.008) and HOMA-IR index (r = 0.321, p = 0.001).

CONCLUSION

Our study demonstrates an important role of Gremlin 1 in glycemic control and body adiposity in the pathophysiology of obesity and T2DM. Gremlin 1 may emerge as a promising biomarker and therapeutic target in obesity and T2DM.

Gremlin1 and TGF-β1 protect kidney tubular epithelial cells from ischemia-reperfusion injury through different pathways

BACKGROUND

Gremlin1 belongs to the superfamily members of transforming growth factor (TGF)-β1, playing a profibrotic role in chronic kidney diseases (CKD) and the transition from the late stage of acute kidney injury (AKI) to CKD, but the effect it plays in the early stage of AKI is unclear. This study aimed to investigate the role of Gremlin1on apoptosis in renal tubular epithelial cells under ischemia-reperfusion (I/R) induction.

METHODS

We detected Gremlin1 and TGF-β1 expression in the kidneys of mice undergoing renal ischemia-reperfusion injury bilaterally. We induced apoptosis through depletion and reperfusion of oxygen and serum in human kidney tubular epithelial cells (HKCs), mimicking I/R injury in vivo, and detected the role and pathways of Gremlin1 and TGF-β1on HKCs injury.

RESULTS

Mice undergoing bilateral I/R surgery presented AKI with a significant increase in serum creatinine, obvious renal tubular injuries, and increased macrophage cell and T-cell infiltration in interstitial areas. Gremlin1 expression was significantly increased along with TGF-β1 in the kidneys of AKI mice compared to sham mice. Exogenous Gremlin1 inhibited I/R-induced caspase3 expression in HKCs, which was blocked by a VEGFR2 kinase inhibitor III (SU5416). TGF-β1 also inhibited I/R-induced cell apoptosis in HKCs but had no synergic effect with Gremlin1. The TGF-β1's inhibitory effect could be blocked by the TGF-β1 type I receptor (activin receptor-like kinase 5, and ALK5)-specific inhibitor SB431542.

CONCLUSIONS

Gremlin1 and TGF- β1 protect kidney tubular epithelial cells from ischemia-reperfusion-induced apoptosis through VEGFR2 and Smad2 signaling pathways.

Interplay between extracellular matrix components and cellular and molecular mechanisms in kidney fibrosis

Chronic kidney disease (CKD) is characterized by pathological accumulation of extracellular matrix (ECM) proteins in renal structures. Tubulointerstitial fibrosis is observed in glomerular diseases as well as in the regeneration failure of acute kidney injury (AKI). Therefore, finding antifibrotic therapies comprises an intensive research field in Nephrology. Nowadays, ECM is not only considered as a cellular scaffold, but also exerts important cellular functions. In this review, we describe the cellular and molecular mechanisms involved in kidney fibrosis, paying particular attention to ECM components, profibrotic factors and cell-matrix interactions. In response to kidney damage, activation of glomerular and/or tubular cells may induce aberrant phenotypes characterized by overproduction of proinflammatory and profibrotic factors, and thus contribute to CKD progression. Among ECM components, matricellular proteins can regulate cell-ECM interactions, as well as cellular phenotype changes. Regarding kidney fibrosis, one of the most studied matricellular proteins is cellular communication network-2 (CCN2), also called connective tissue growth factor (CTGF), currently considered as a fibrotic marker and a potential therapeutic target. Integrins connect the ECM proteins to the actin cytoskeleton and several downstream signaling pathways that enable cells to respond to external stimuli in a coordinated manner and maintain optimal tissue stiffness. In kidney fibrosis, there is an increase in ECM deposition, lower ECM degradation and ECM proteins cross-linking, leading to an alteration in the tissue mechanical properties and their responses to injurious stimuli. A better understanding of these complex cellular and molecular events could help us to improve the antifibrotic therapies for CKD.

Gremlin2 Activates Fibroblasts to Promote Pulmonary Fibrosis Through the Bone Morphogenic Protein Pathway

Idiopathic pulmonary fibrosis (IPF) is a progressive lung disease causing unremitting extracellular matrix deposition. Transforming growth factor-β (TGF-β) superfamily involves bone morphogenetic proteins (BMPs) and TGF-β, and the balance between the activation of TGF-β-dependent SMADs (Smad2/3) and BMP-dependent SMADs (Smad1/5/8) is essential for fibrosis process. GREM2, initially identified as a TGF-β-inducible gene, encodes a small secreted glycoprotein belonging to a group of matricellular proteins, its role in lung fibrosis is not clear. Here, we identified Gremlin2 as a key regulator of fibroblast activation. Gremlin2 was highly expressed in the serum and lung tissues in IPF patients. Bleomycin-induced lung fibrosis model exhibited high expression of Gremlin2 in the bronchoalveolar lavage fluid (BALF) and lung tissue. Isolation of primary cells from bleomycin-induced fibrosis lung showed a good correlation of Gremlin2 and Acta2 (α-SMA) expressions. Overexpression of Gremlin2 in human fetal lung fibroblast 1 (HFL-1) cells increased its invasion and migration. Furthermore, Gremlin2 regulates fibrosis functions through mediating TGF-β/BMP signaling, in which Gremlin2 may activate TGF-β signaling and inhibit BMP signaling. Therefore, we provided in vivo and in vitro evidence to demonstrate that Gremlin2 may be a potential therapeutic target for the treatment of IPF.

Targeting Gremlin 1 Prevents Intestinal Fibrosis Progression by Inhibiting the Fatty Acid Oxidation of Fibroblast Cells

Intestinal fibrosis is a consequence of continuous inflammatory responses that negatively affect the quality of life of patients. By screening altered proteomic profiles of mouse fibrotic colon tissues, we identified that GREM1 was dramatically upregulated in comparison to that in normal tissues. Functional experiments revealed that GREM1 promoted the proliferation and activation of intestinal fibroblast cells by enhancing fatty acid oxidation. Blocking GREM1 prevented the progression of intestinal fibrosis in vivo. Mechanistic research revealed that GREM1 acted as a ligand for VEGFR2 and triggered downstream MAPK signaling. This facilitated the expression of FAO-related genes, consequently enhancing fatty acid oxidation. Taken together, our data indicated that targeting GREM1 could represent a promising therapeutic approach for the treatment of intestinal fibrosis.

Potent Therapy and Transcriptional Profile of Combined Erythropoietin-Derived Peptide Cyclic Helix B Surface Peptide and Caspase-3 siRNA against Kidney Ischemia/Reperfusion Injury in Mice

Cause-specific treatment and timely diagnosis are still not available for acute kidney injury (AKI) apart from supportive therapy and serum creatinine measurement. A novel erythropoietin-derived cyclic helix B surface peptide (CHBP) protects kidneys against AKI with different causes, but the underlying mechanism is not fully defined. Herein, we investigated the transcriptional profile of renoprotection induced by CHBP and its potential synergistic effects with siRNA targeting caspase-3, an executing enzyme of apoptosis and inflammation (CASP3siRNA), on ischemia/reperfusion (IR)-induced AKI. Utilizing a mouse model with 30-minute renal bilateral ischemia and 48-hour reperfusion, the renoprotection of CHBP or CASP3siRNA was demonstrated in renal function and structure, active caspase-3 and HMGB1 expression. Combined treatment of CHBP and CASP3siRNA further preserved kidney structure and reduced active caspase-3 and HMGB1. Furthermore, differentially expressed genes (DEGs) were identified with fold change >1.414 and P < 0.05. In IR kidneys, 281 DEGs induced by CHBP were mainly involved in promoting cell division and improving cellular function and metabolism (upregulated signal transducer and activator of transcription 5B and solute carrier family 22 member 7). The additional administration of CASP3siRNA caused 504 and 418 DEGs in IR + CHBP kidneys with or without negative control small-interfering RNA, with 37 genes in common. These DEGs were associated with modulated apoptosis and inflammation (upregulated BCL6, SLPI, and SERPINA3M) as well as immunity, injury, and microvascular homeostasis (upregulated complement factor H and GREM1 and downregulated ANGPTL2). This proof-of-effect study indicated the potent renoprotection of CASP3siRNA upon CHBP at the early stage of IR-induced AKI. Underlying genes, BCL6, SLPI, SERPINA3M, GREM1, and ANGPTL2, might be potential new biomarkers for clinical applications. SIGNIFICANCE STATEMENT: It is imperative to explore new strategies of cause-specific treatment and timely diagnosis for acute kidney injury (AKI). CHBP and CASP3siRNA synergistically protected kidney structure after 48-hour ischemia/reperfusion-induced AKI with reduced injury mediators CASP3 and high mobility group box 1. CHBP upregulated cell division-, function-, and metabolism-related genes, whereas CASP3siRNA further regulated immune response- and tissue homeostasis-associated genes. Combined CHBP and CASP3siRNA might be a potent and specific treatment for AKI, and certain dysregulated genes secretory leukocyte peptidase inhibitor and SERPINA3M could facilitate timely diagnosis.

Pathogenic Pathways and Therapeutic Approaches Targeting Inflammation in Diabetic Nephropathy

Diabetic nephropathy (DN) is associated with an increased morbidity and mortality, resulting in elevated cost for public health systems. DN is the main cause of chronic kidney disease (CKD) and its incidence increases the number of patients that develop the end-stage renal disease (ESRD). There are growing epidemiological and preclinical evidence about the close relationship between inflammatory response and the occurrence and progression of DN. Several anti-inflammatory strategies targeting specific inflammatory mediators (cell adhesion molecules, chemokines and cytokines) and intracellular signaling pathways have shown beneficial effects in experimental models of DN, decreasing proteinuria and renal lesions. A number of inflammatory molecules have been shown useful to identify diabetic patients at high risk of developing renal complications. In this review, we focus on the key role of inflammation in the genesis and progression of DN, with a special interest in effector molecules and activated intracellular pathways leading to renal damage, as well as a comprehensive update of new therapeutic strategies targeting inflammation to prevent and/or retard renal injury.

Targeting the progression of chronic kidney disease

Chronic kidney disease (CKD) is a devastating condition that is reaching epidemic levels owing to the increasing prevalence of diabetes mellitus, hypertension and obesity, as well as ageing of the population. Regardless of the underlying aetiology, CKD is slowly progressive and leads to irreversible nephron loss, end-stage renal disease and/or premature death. Factors that contribute to CKD progression include parenchymal cell loss, chronic inflammation, fibrosis and reduced regenerative capacity of the kidney. Current therapies have limited effectiveness and only delay disease progression, underscoring the need to develop novel therapeutic approaches to either stop or reverse progression. Preclinical studies have identified several approaches that reduce fibrosis in experimental models, including targeting cytokines, transcription factors, developmental and signalling pathways and epigenetic modulators, particularly microRNAs. Some of these nephroprotective strategies are now being tested in clinical trials. Lessons learned from the failure of clinical studies of transforming growth factor β1 (TGFβ1) blockade underscore the need for alternative approaches to CKD therapy, as strategies that target a single pathogenic process may result in unexpected negative effects on simultaneously occurring processes. Additional promising avenues include preventing tubular cell injury and anti-fibrotic therapies that target activated myofibroblasts, the main collagen-producing cells.

VEGFR2 Blockade Improves Renal Damage in an Experimental Model of Type 2 Diabetic Nephropathy

The absence of optimal treatments for Diabetic Nephropathy (DN) highlights the importance of the search for novel therapeutic targets. The vascular endothelial growth factor receptor 2 (VEGFR2) pathway is activated in experimental and human DN, but the effects of its blockade in experimental models of DN is still controversial. Here, we test the effects of a therapeutic anti-VEGFR2 treatment, using a VEGFR2 kinase inhibitor, on the progression of renal damage in the BTBR ob/ob (leptin deficiency mutation) mice. This experimental diabetic model develops histological characteristics mimicking the key features of advanced human DN. A VEGFR2 pathway-activation blockade using the VEGFR2 kinase inhibitor SU5416, starting after kidney disease development, improves renal function, glomerular damage (mesangial matrix expansion and basement membrane thickening), tubulointerstitial inflammation and tubular atrophy, compared to untreated diabetic mice. The downstream mechanisms involved in these beneficial effects of VEGFR2 blockade include gene expression restoration of podocyte markers and downregulation of renal injury biomarkers and pro-inflammatory mediators. Several ligands can activate VEGFR2, including the canonical ligands VEGFs and GREMLIN. Activation of a GREMLIN/VEGFR2 pathway, but not other ligands, is correlated with renal damage progression in BTBR ob/ob diabetic mice. RNA sequencing analysis of GREMLIN-regulated genes confirm the modulation of proinflammatory genes and related-molecular pathways. Overall, these data show that a GREMLIN/VEGFR2 pathway activation is involved in diabetic kidney disease and could potentially be a novel therapeutic target in this clinical condition.

Molecular Regulation of Notch Signaling by Gremlin

Gremlin is a member of the TGF-β superfamily that can act as a BMP antagonist, and recently, has been described as a ligand of the vascular endothelial growth factor receptor 2 (VEGFR2). Gremlin shares properties with the Notch signaling pathway. Both participate in embryonic development and are reactivated in pathological conditions. Gremlin is emerging as a potential therapeutic target and biomarker of renal diseases. Here we review the role of the Gremlin-VEGFR2 axis in renal damage and downstream signaling mechanisms, such as Notch pathway.

Gremlin, A Potential Urinary Biomarker of Anca-Associated Crescentic Glomerulonephritis

Gremlin renal overexpression has been reported in diabetic nephropathy, pauci-immune crescentic glomerulonephritis and chronic allograft nephropathy and has been implicated in the pathophysiology of the progression of renal damage. However, it is unknown whether urinary Gremlin can be associated with renal functional status, renal biopsy findings and outcome. To examine these associations we studied 20 patients with ANCA+ renal vasculitis and very high urinary Gremlin (354 ± 76 ug/gCr), 86 patients with other glomerular diseases and moderately elevated urinary Gremlin (83 ± 14 ug/gCr) and 11 healthy controls (urinary Gremlin 11.3 ± 2.4 ug/gCr). Urinary Gremlin was significantly correlated with renal expression of Gremlin (r = 0.64, p = 0.013) observed in cellular glomerular crescents, tubular epithelial cells and interstitial inflammatory cells. Moreover, urinary Gremlin levels were correlated with the number of glomerular crescents (r = 0.53; p < 0.001), renal CD68 positive cells (r = 0.71; p < 0.005), tubulointerstitial fibrosis (r = 0.50; p < 0.05), and serum creatinine levels (r = 0.60; p < 0.001). Interestingly, Gremlin expression was colocalized with CD68, CD163 (monocyte/macrophage markers) and CCL18 positive cells. ROC curve analysis showed that the cutoff value of urinary Gremlin in glomerular diseases as 43 ug/gCr with 72% of sensitivity and 100% of specificity [AUC: 0.96 (CI 95% 0.92–0.99] (p < 0.001). For ANCA+ renal vasculitis the value of urinary Gremlin of 241 ug/gCr had 55% of sensitivity and 100% of specificity [AUC: 0.81 (CI 95% 0.68–0.94) (p < 0.001]. Based on these results we propose that urinary Gremlin represents a non-invasive biomarker in ANCA+ renal vasculitis, and suggest a role of Gremlin in the formation of crescents.

Gremlin Regulates Tubular Epithelial to Mesenchymal Transition via VEGFR2: Potential Role in Renal Fibrosis

Chronic kidney disease (CKD) is emerging as an important health problem due to the increase number of CKD patients and the absence of an effective curative treatment. Gremlin has been proposed as a novel therapeutic target for renal inflammatory diseases, acting via Vascular Endothelial Growth Factor Receptor-2 (VEGFR2). Although many evidences suggest that Gremlin could regulate renal fibrosis, the receptor involved has not been yet clarified. Gremlin, as other TGF-β superfamily members, regulates tubular epithelial to mesenchymal transition (EMT) and, therefore, could contribute to renal fibrosis. In cultured tubular epithelial cells Gremlin binding to VEGFR2 is linked to proinflammatory responses. Now, we have found out that in these cells VEGFR2 is also involved in the profibrotic actions of Gremlin. VEGFR2 blockade by a pharmacological kinase inhibitor or gene silencing diminished Gremlin-mediated gene upregulation of profibrotic factors and restored changes in EMT-related genes. Moreover, VEGFR2 inhibition blocked EMT phenotypic changes and dampened the rate of wound healing in response to Gremlin. The role of VEGFR2 in experimental fibrosis was evaluated in experimental unilateral ureteral obstruction. VEFGR2 inhibition diminished the upregulation of profibrotic genes and EMT changes, as well as the accumulation of extracellular matrix proteins, such as fibronectin and collagens in the obstructed kidneys. Notch pathway activation participates in renal damage progression by regulating cell growth/proliferation, regeneration and inflammation. In cultured tubular epithelial cells, Notch inhibition markedly downregulated Gremlin-induced EMT changes and wound healing speed. These results show that Gremlin regulates the EMT process via VEGFR2 and Notch pathway activation, suggesting that the Gremlin/VEGFR2 axis could be a potential therapeutic target for CKD.

Gremlin activates the Notch pathway linked to renal inflammation

Preclinical studies suggest that Gremlin participates in renal damage and could be a potential therapeutic target for human chronic kidney diseases. Inflammation is a common characteristic of progressive renal disease, and therefore novel anti-inflammatory therapeutic targets should be investigated. The Notch signaling pathway is involved in kidney development and is activated in human chronic kidney disease, but whether Gremlin regulates the Notch pathway has not been investigated. In cultured tubular cells, Gremlin up-regulated gene expression of several Notch pathway components, increased the production of the canonical ligand Jagged-1, and caused the nuclear translocation of active Notch-1 (N1ICD). In vivo administration of Gremlin into murine kidneys elicited Jagged-1 production, increased N1ICD nuclear levels, and up-regulated the gene expression of the Notch effectors hes-1 and hey-1 All these data clearly demonstrate that Gremlin activates the Notch pathway in the kidney. Notch inhibition using the γ-secretase inhibitor DAPT impaired renal inflammatory cell infiltration and proinflammatory cytokines overexpression in Gremlin-injected mice and in experimental models of renal injury. Moreover, Notch inhibition blocked Gremlin-induced activation of the canonical and noncanonical nuclear factor-κB (NF-κB) pathway, identifying an important mechanism involved in the anti-inflammatory actions of Notch inhibition. In conclusion, Gremlin activates the Notch pathway in the kidney and this is linked to NF-κB-mediated inflammation, supporting the hypothesis that Notch inhibition could be a potential anti-inflammatory strategy for renal diseases.