The binding of the bone morphogenetic protein antagonist gremlin to kidney heparan sulfate: Such binding is not essential for BMP antagonism

GREM1 signaling in cancer: tumor promotor and suppressor?

GREMLIN1 (GREM1) is member of a family of structurally and functionally related secreted cysteine knot proteins, which act to sequester and inhibit the action of multifunctional bone morphogenetic proteins (BMPs). GREM1 binds directly to BMP dimers, thereby preventing BMP-mediated activation of BMP type I and type II receptors. Multiple reports identify the overexpression of GREM1 as a contributing factor in a broad range of cancers. Additionally, the GREM1 gene is amplified in a rare autosomal dominant inherited form of colorectal cancer. The inhibitory effects of GREM1 on BMP signaling have been linked to these tumor-promoting effects, including facilitating cancer cell stemness and the activation of cancer-associated fibroblasts. Moreover, GREM1 has been described to bind and signal to vascular endothelial growth factor receptor (VEGFR) and stimulate angiogenesis, as well as epidermal and fibroblast growth factor receptor (EGFR and FGFR) to elicit tumor-promoting effects in breast and prostate cancer, respectively. In contrast, a 2022 report revealed that GREM1 can promote an epithelial state in pancreatic cancers, thereby inhibiting pancreatic tumor growth and metastasis. In this commentary, we will review these disparate findings and attempt to provide clarity around the role of GREM1 signaling in cancer.

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.

Effect and Related Mechanism of Platelet-Rich Plasma on the Osteogenic Differentiation of Human Adipose-Derived Stem Cells

Objective

Human adipose-derived stem cells (hADSCs) are ideal seed cells for the regeneration of alveolar bone defects. Platelet-rich plasma (PRP), which is rich in growth factors, promotes tissue repair. The purpose of the present study was to investigate whether PRP promotes the osteogenic differentiation of hADSCs and to perform high-throughput sequencing to explore the possible mechanism.

Methods

hADSCs were divided into the three following groups: CON group, OM group, and PRP group. Osteogenesis was detected by Alizarin Red staining on day 14. Total RNA was extracted from the OM and PRP groups for high-throughput sequencing. The target genes of the differentially expressed osteogenic-related miRNAs were predicted, and combined miRNA/mRNA analysis was then performed. The mRNA and protein expression levels of hsa-miR-212-5p, type 1 cannabinoid receptor (CNR1), alkaline phosphatase (ALP), Runx2, osteocalcin (OCN), and collagen 1 A1 (COL1A1) in the OM and PRP groups were detected by qRT–PCR and Western blot analyses. The binding between hsa-miR-212-5p and CNR1 was detected by a dual-luciferase reporter assay.

Results

Both the OM and PRP groups exhibited enhanced proliferation of hADSCs, and the differences at 48 h and 72 h were statistically significant (P < 0.05). The PRP group had significantly more calcium nodules than the CON group (P < 0.05). Through high-throughput sequencing analysis, differential miRNA and mRNA expression profiles were obtained. During hADSC osteogenesis, the expression of hsa-miR-212-5p was downregulated, and the expression of CNR1 was upregulated. hsa-miR-212-5p was found to bind directly to the 3′ UTR of CNR1.

Conclusions

The present findings indicated that downregulation of hsa-miR-212-5p and upregulation of CNR1 may be involved in the process by which PRP promotes the osteogenic differentiation of hADSCs.

Effects of bone morphogenetic proteins on epithelial repair

Epithelial tissue has important functions such as protection, secretion, and sensation. Epithelial damage is involved in various pathological processes. Bone morphogenetic proteins (BMPs) are a class of growth factors with multiple functions. They play important roles in epithelial cells, including in differentiation, proliferation, and migration during the repair of the epithelium. This article reviews the functions and mechanisms of the most profoundly studied BMPs in the process of epithelial damage repair and their clinical significance.

Heparan sulfate proteoglycans regulate BMP signalling during neural crest induction

Bone morphogenetic protein (BMP) signalling is key to many developmental processes, including early regionalisation of the ectoderm. The neural crest is induced here by a combination of BMP and Wnt signals from nearby tissues with many secreted factors contributing to its initial specification at the neural plate border. Gremlin 1 (Grem1) is a secreted BMP antagonist expressed in the neural crest in Xenopus laevis but its function here is unknown. As well as binding BMPs, Grem1 has been shown to interact with heparan sulfate proteoglycans (HSPGs), a family of cell surface macromolecules that regulate a diverse array of signalling molecules by affecting their availability and mode of action. This study describes the impact of HSPGs on the function of Grem1 in neural crest induction. It shows for the first time that Grem1 is required for neural crest development in a two-step process comprising an early HSPG-independent, followed by a late HSPG-dependent phase.

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

The current therapeutic strategy for the treatment of chronic kidney diseases only ameliorates disease progression. During renal injury, developmental genes are re-expressed and could be potential therapeutic targets. Among those genes reactivated in the adult damaged kidney, Gremlin is of particular relevance since recent data suggest that it could be a mediator of diabetic nephropathy and other progressive renal diseases. Earlier studies have shown that Gremlin is upregulated in trans-differentiated renal proximal tubular cells and in several chronic kidney diseases associated with fibrosis. However, not much was known about the mechanisms by which Gremlin acts in renal pathophysiology. The role of Gremlin as a bone morphogenetic protein antagonist has clearly been demonstrated in organogenesis and in fibrotic-related disorders. Gremlin binds to vascular endothelial growth factor receptor 2 (VEGFR2) in endothelial and tubular epithelial cells. Activation of the Gremlin-VEGFR2 axis was found in several human nephropathies. We have recently described that Gremlin activates the VEGFR2 signaling pathway in the kidney, eliciting a downstream mechanism linked to renal inflammatory response. Gremlin deletion improves experimental renal damage, diminishing fibrosis. Overall, the available data identify the Gremlin-VEGFR2 axis as a novel therapeutic target for kidney inflammation and fibrosis and provide a rationale for unveiling new concepts to investigate in several clinical conditions.

Gremlin is a potential target for posterior capsular opacification

Objective: The present study was conducted to determine the role of gremlin during the development of posterior capsular opacification (PCO) via in vitro and in vivo experiments. Methods: The activation, roles and relationships of the BMPs/Smad1/5, MAPK, FAK and AKT signaling pathways in human lens epithelial cells (HLECs) after gremlin induction were detected by western blotting and real-time PCR. Wound-healing, transwell, capsular bag models and rat PCO models assays were used to test the effects of gremlin on HLECs' migration, proliferation, EMT-specific protein α-smooth muscle actin（α-SMA）and development of PCO in rats. Results: Our data showed that knockdown of the gremlin inhibited the development of PCO and reduced expression of α-SMA in rats. While gremlin did not alter the migration of HLECs, it increased the expression of p-ERK and p-AKT. Knockout of Smad2 or Smad3 inhibited the expression of p-ERK and p-AKT proteins induced by gremlin. Gremlin also reduced BMP4-induced expression of the p-Smad1/5 protein. Finally, knockout of Smad1/5 increased gremlin-induced expression of α-SMA, fibronectin and type I collagen (COL-1) in HLECs. Conclusion: These results suggested that gremlin contributed to the development of PCO by promoting LEC proliferation, activation of TGF-β/Smad, ERK and AKT signaling and inhibition of BMPs/Smad1/5 signaling. Furthermore, inhibiting gremlin effectively impaired both PCO development in rats and EMT in the lens capsule. Thus, our data suggest that gremlin might be a potential target for PCO.

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.