Arkadia enhances BMP signalling through ubiquitylation and degradation of Smad6

Deleterious variants in RNF111 impair female fertility and induce premature ovarian insufficiency in humans and mice

Premature ovarian insufficiency (POI) is a heterogeneous female disorder characterized by the loss of ovarian function before the age of 40. It represents a significant detriment to female fertility. However, the known POI-causative genes currently account for only a fraction of cases. To elucidate the genetic factors underlying POI, we conducted whole-exome sequencing on a family with three fertile POI patients and identified a deleterious missense variant in RNF111. In a subsequent replication study involving 1,030 POI patients, this variant was not only confirmed but also accompanied by the discovery of three additional predicted deleterious RNF111 variants. These variants collectively account for eight cases, representing 0.78% of the study cohort. A further study involving 500 patients with diminished ovarian reserve also identified two additional RNF111 variants. Notably, RNF111 encodes an E3 ubiquitin ligase with a regulatory role in the TGF-β/BMP signaling pathway. Our analysis revealed that RNF111/RNF111 is predominantly expressed in the oocytes of mice, monkeys, and humans. To further investigate the functional implications of RNF111 variants, we generated two mouse models: one with a heterozygous missense mutation (Rnf111+/M) and another with a heterozygous null mutation (Rnf111+/-). Both mouse models exhibited impaired female fertility, characterized by reduced litter sizes and small ovarian reserve. Additionally, RNA-seq and quantitative proteomics analysis unveiled that Rnf111 haploinsufficiency led to dysregulation in female gonad development and negative regulation of the BMP signaling pathway within mouse ovaries. In conclusion, our findings strongly suggest that monoallelic deleterious variants in RNF111 can impair female fertility and induce POI in both humans and mice.

The role of E3 ubiquitin ligases in bone homeostasis and related diseases

The ubiquitin-proteasome system (UPS) dedicates to degrade intracellular proteins to modulate demic homeostasis and functions of organisms. These enzymatic cascades mark and modifies target proteins diversly through covalently binding ubiquitin molecules. In the UPS, E3 ubiquitin ligases are the crucial constituents by the advantage of recognizing and presenting proteins to proteasomes for proteolysis. As the major regulators of protein homeostasis, E3 ligases are indispensable to proper cell manners in diverse systems, and they are well described in physiological bone growth and bone metabolism. Pathologically, classic bone-related diseases such as metabolic bone diseases, arthritis, bone neoplasms and bone metastasis of the tumor, etc., were also depicted in a UPS-dependent manner. Therefore, skeletal system is versatilely regulated by UPS and it is worthy to summarize the underlying mechanism. Furthermore, based on the current status of treatment, normal or pathological osteogenesis and tumorigenesis elaborated in this review highlight the clinical significance of UPS research. As a strategy possibly remedies the limitations of UPS treatment, emerging PROTAC was described comprehensively to illustrate its potential in clinical application. Altogether, the purpose of this review aims to provide more evidence for exploiting novel therapeutic strategies based on UPS for bone associated diseases.

N-butyldeoxynojirimycin (miglustat) ameliorates pulmonary fibrosis through inhibition of nuclear translocation of Smad2/3

Idiopathic pulmonary fibrosis (IPF) is a chronic progressive lung disease. The disease involves excessive accumulation of fibroblasts and myofibroblasts, and myofibroblasts differentiated by pro-fibrotic factors promote the deposition of extracellular matrix proteins such as collagen and fibronectin. Transforming growth factor-β1 is a pro-fibrotic factor that promotes fibroblast-to-myofibroblast differentiation (FMD). Therefore, inhibition of FMD may be an effective strategy for IPF treatment. In this study, we screened the anti-FMD effects of various iminosugars and showed that some compounds, including N-butyldeoxynojirimycin (NB-DNJ, miglustat, an inhibitor of glucosylceramide synthase (GCS)), a clinically approved drug for treating Niemann-Pick disease type C and Gaucher disease type 1, inhibited TGF-β1-induced FMD by inhibiting the nuclear translocation of Smad2/3. N-butyldeoxygalactonojirimycin having GCS inhibitory effect did not attenuate the TGF-β1-induced FMD, suggesting that NB-DNJ exerts the anti-FMD effects by GCS inhibitory effect independent manner. N-butyldeoxynojirimycin did not inhibit TGF-β1-induced Smad2/3 phosphorylation. In a mouse model of bleomycin (BLM)-induced pulmonary fibrosis, intratracheal or oral administration of NB-DNJ at an early fibrotic stage markedly ameliorated lung injury and deterioration of respiratory functions, such as specific airway resistance, tidal volume, and peak expiratory flow. Furthermore, the anti-fibrotic effects of NB-DNJ in the BLM-induced lung injury model were similar to those of pirfenidone and nintedanib, which are clinically approved drugs for the treatment of IPF. These results suggest that NB-DNJ may be effective for IPF treatment.

To Ub or not to Ub: a regulatory question in TGF-β signaling

The transforming growth factor β (TGF-β) superfamily controls a wide spectrum of biological processes in metazoans, including cell proliferation, apoptosis, differentiation, cell-fate determination, and embryonic development. Deregulation of TGF-β-Smad signaling contributes to developmental anomalies and a variety of disorders and diseases such as tumorigenesis, fibrotic disorders, and immune diseases. In cancer, TGF-β has dual effects through its antiproliferative and prometastatic actions. At the cellular level, TGF-β functions mainly through the canonical Smad-dependent pathway in a cell type-specific and context-dependent manner. Accumulating evidence has demonstrated that ubiquitination plays a vital role in regulating TGF-β-Smad signaling. We summarize current progress on ubiquitination (Ub) and the ubiquitin ligases that regulate TGF-β-Smad signaling.

E3 Ubiquitin Ligases: Potential Therapeutic Targets for Skeletal Pathology and Degeneration

The ubiquitination-proteasome system (UPS) is crucial in regulating a variety of cellular processes including proliferation, differentiation, and survival. Ubiquitin protein ligase E3 is the most critical molecule in the UPS system. Dysregulation of the UPS system is associated with many conditions. Over the past few decades, there have been an increasing number of studies focusing on the UPS system and how it affects bone metabolism. Multiple E3 ubiquitin ligases have been found to mediate osteogenesis or osteolysis through a variety of pathways. In this review, we describe the mechanisms of UPS, especially E3 ubiquitin ligases on bone metabolism. To date, many E3 ubiquitin ligases have been found to regulate osteogenesis or osteoclast differentiation. We review the classification of these E3 enzymes and the mechanisms that influence upstream and downstream molecules and transduction pathways. Finally, this paper reviews the discovery of the relevant UPS inhibitors, drug molecules, and noncoding RNAs so far and prospects the future research and treatment.

Arkadia-SKI/SnoN signaling differentially regulates TGF-β-induced iTreg and Th17 cell differentiation

TGF-β signaling is fundamental for both Th17 and regulatory T (Treg) cell differentiation. However, these cells differ in requirements for downstream signaling components, such as SMAD effectors. To further characterize mechanisms that distinguish TGF-β signaling requirements for Th17 and Treg cell differentiation, we investigated the role of Arkadia (RNF111), an E3 ubiquitin ligase that mediates TGF-β signaling during development. Inactivation of Arkadia in CD4⁺ T cells resulted in impaired Treg cell differentiation in vitro and loss of RORγt⁺FOXP3⁺ iTreg cells in the intestinal lamina propria, which increased susceptibility to microbiota-induced mucosal inflammation. In contrast, Arkadia was dispensable for Th17 cell responses. Furthermore, genetic ablation of two Arkadia substrates, the transcriptional corepressors SKI and SnoN, rescued Arkadia-deficient iTreg cell differentiation both in vitro and in vivo. These results reveal distinct TGF-β signaling modules governing Th17 and iTreg cell differentiation programs that could be targeted to selectively modulate T cell functions.

E3 Ubiquitin Ligase-Mediated Regulation of Osteoblast Differentiation and Bone Formation

The ubiquitin–proteasome system (UPS) is an essential pathway that regulates the homeostasis and function of intracellular proteins and is a crucial protein-degradation system in osteoblast differentiation and bone formation. Abnormal regulation of ubiquitination leads to osteoblast differentiation disorders, interfering with bone formation and ultimately leading to osteoporosis. E3 ubiquitin ligases (E3) promote addition of a ubiquitin moiety to substrate proteins, specifically recognizing the substrate and modulating tyrosine kinase receptors, signaling proteins, and transcription factors involved in the regulation of osteoblast proliferation, differentiation, survival, and bone formation. In this review, we summarize current progress in the understanding of the function and regulatory effects of E3 ligases on the transcription factors and signaling pathways that regulate osteoblast differentiation and bone formation. A deep understanding of E3 ligase-mediated regulation of osteoblast differentiation provides a scientific rationale for the discovery and development of novel E3-targeting therapeutic strategies for osteoporosis.

SUMO and Transcriptional Regulation: The Lessons of Large-Scale Proteomic, Modifomic and Genomic Studies

One major role of the eukaryotic peptidic post-translational modifier SUMO in the cell is transcriptional control. This occurs via modification of virtually all classes of transcriptional actors, which include transcription factors, transcriptional coregulators, diverse chromatin components, as well as Pol I-, Pol II- and Pol III transcriptional machineries and their regulators. For many years, the role of SUMOylation has essentially been studied on individual proteins, or small groups of proteins, principally dealing with Pol II-mediated transcription. This provided only a fragmentary view of how SUMOylation controls transcription. The recent advent of large-scale proteomic, modifomic and genomic studies has however considerably refined our perception of the part played by SUMO in gene expression control. We review here these developments and the new concepts they are at the origin of, together with the limitations of our knowledge. How they illuminate the SUMO-dependent transcriptional mechanisms that have been characterized thus far and how they impact our view of SUMO-dependent chromatin organization are also considered.

Arkadia protein expression is reduced in the liver during the progression of hepatic fibrosis

Arkadia is able to degrade key signaling molecules in the transforming growth factor (TGF)-β1 signaling pathway; however, the expression of Arkadia in the liver during development and progression of TGF-β1/Smad signaling-regulated hepatic fibrosis remains to be elucidated. The present study aimed to examine Arkadia expression in the livers of two rat models of hepatic fibrosis induced by bile duct ligation and carbon tetrachloride intoxication, and in human liver samples from patients with hepatic fibrosis. Expression was analyzed by quantitative polymerase chain reaction, immunohistochemistry and western blot analysis. The results indicated that Arkadia was predominantly expressed in the cytoplasm of cholangiocytes and hepatocytes. The protein expression levels of Arkadia were significantly decreased in fibrotic livers, whereas the mRNA expression levels of Arkadia were significantly increased in fibrotic livers compared with in nonfibrotic livers. In conclusion, these data indicated that Arkadia may regulate the pathogenesis and progression of hepatic fibrosis.

BMP Signalling at the Crossroad of Liver Fibrosis and Regeneration

Bone Morphogenetic Proteins (BMPs) belong to the Transforming Growth Factor-β (TGF-β) family. Initially identified due to their ability to induce bone formation, they are now known to have multiple functions in a variety of tissues, being critical not only during development for tissue morphogenesis and organogenesis but also during adult tissue homeostasis. This review focus on the liver as a target tissue for BMPs actions, devoting most efforts to summarize our knowledge on their recently recognized and/or emerging roles on regulation of the liver regenerative response to various insults, either acute or chronic and their effects on development and progression of liver fibrosis in different pathological conditions. In an attempt to provide the basis for guiding research efforts in this field both the more solid and more controversial areas of research were highlighted.

Increased PLEKHO1 within osteoblasts suppresses Smad-dependent BMP signaling to inhibit bone formation during aging

Emerging evidence indicates that the dysregulation of protein ubiquitination plays a crucial role in aging‐associated diseases. Smad‐dependent canonical BMP signaling pathway is indispensable for osteoblastic bone formation, which could be disrupted by the ubiquitination and subsequent proteasomal degradation of Smad1/5, the key molecules for BMP signaling transduction. However, whether the dysregulation of Smad1/5 ubiquitination and disrupted BMP signaling pathway is responsible for the age‐related bone formation reduction is still underexplored. Pleckstrin homology domain‐containing family O member 1 (PLEKHO1) is a previously identified ubiquitination‐related molecule that could specifically target the linker region between the WW domains of Smurf1 to promote the ubiquitination of Smad1/5. Here, we found an age‐related increase in the expression of PLEKHO1 in bone specimens from either fractured patients or aging rodents, which was associated with the age‐related reduction in Smad‐dependent BMP signaling and bone formation. By genetic approach, we demonstrated that loss of Plekho1 in osteoblasts could promote the Smad‐dependent BMP signaling and alleviated the age‐related bone formation reduction. In addition, osteoblast‐specific Smad1 overexpression had beneficial effect on bone formation during aging, which could be counteracted after overexpressing Plekho1 within osteoblasts. By pharmacological approach, we showed that osteoblast‐targeted Plekho1 siRNA treatment could enhance Smad‐dependent BMP signaling and promote bone formation in aging rodents. Taken together, it suggests that the increased PLEKHO1 could suppress Smad‐dependent BMP signaling to inhibit bone formation during aging, indicating the translational potential of targeting PLEKHO1 in osteoblast as a novel bone anabolic strategy for reversing established osteoporosis during aging.

Regulation of TGF-β Family Signaling by Inhibitory Smads

Inhibitory Smads (I-Smads) have conserved carboxy-terminal MH2 domains but highly divergent amino-terminal regions when compared with receptor-regulated Smads (R-Smads) and common-partner Smads (co-Smads). Smad6 preferentially inhibits Smad signaling initiated by the bone morphogenetic protein (BMP) type I receptors ALK-3 and ALK-6, whereas Smad7 inhibits both transforming growth factor β (TGF-β)- and BMP-induced Smad signaling. I-Smads also regulate some non-Smad signaling pathways. Here, we discuss the vertebrate I-Smads, their roles as inhibitors of Smad activation and regulators of receptor stability, as scaffolds for non-Smad signaling, and their possible roles in the nucleus. We also discuss the posttranslational modification of I-Smads, including phosphorylation, ubiquitylation, acetylation, and methylation.

Tril targets Smad7 for degradation to allow hematopoietic specification in Xenopus embryos

In Xenopus laevis, bone morphogenetic proteins (Bmps) induce expression of the transcription factor Gata2 during gastrulation, and Gata2 is required in both ectodermal and mesodermal cells to enable mesoderm to commit to a hematopoietic fate. Here, we identify tril as a Gata2 target gene that is required in both ectoderm and mesoderm for primitive hematopoiesis to occur. Tril is a transmembrane protein that functions as a co-receptor for Toll-like receptors to mediate innate immune responses in the adult brain, but developmental roles for this molecule have not been identified. We show that Tril function is required both upstream and downstream of Bmp receptor-mediated Smad1 phosphorylation for induction of Bmp target genes. Mechanistically, Tril triggers degradation of the Bmp inhibitor Smad7. Tril-dependent downregulation of Smad7 relieves repression of endogenous Bmp signaling during gastrulation and this enables mesodermal progenitors to commit to a blood fate. Thus, Tril is a novel component of a Bmp-Gata2 positive-feedback loop that plays an essential role in hematopoietic specification.

Arkadia: Characteristics, function and role in development of human diseases

Ubiquitination of proteins is a post-translational modification that involves targeting and degrading misfolded or unwanted proteins by the proteasome. Arkadia, a RING-type E3 ubiquitin ligase also known as RNF111, confers the substrate specificity for ubiquitination and has a pivotal role in catalyzing the degradation of key signaling molecules. Recent research reveals that Arkadia plays a pivotal role in the transforming growth factor-β1 signaling pathway by catalyzing the degradation of key signaling molecules. In this review, we highlight the recent progress in understanding the characteristics, function and the role of Arkadia in the development of human diseases.

Quantitative Proteomics of the SMAD (Suppressor of Mothers against Decapentaplegic) Transcription Factor Family Identifies Importin 5 as a Bone Morphogenic Protein Receptor SMAD-specific Importin

Gene-specific transcription factors (GSTFs) control gene transcription by DNA binding and specific protein complex recruitment, which regulates promoter accessibility for transcription initiation by RNA polymerase II. Mutations in the GSTFs Suppressor of Mothers Against Decapentaplegic 2 (SMAD2) and SMAD4 are frequently associated with colon and rectal carcinomas. These proteins play an important role in bone morphogenic protein (BMP) and transforming growth factor β (TGF-β) signaling pathways controlling cell fate and proliferation. To study the protein interactome of the SMAD protein family we generated a quantitative proteomics pipeline that allows for inducible expression of GFP-tagged SMAD proteins followed by affinity purification and quantitative mass spectrometry analysis. Data are available via ProteomeXchange with identifier PXD004529. The nuclear importin IPO5 was identified as a novel interacting protein of SMAD1. Overexpression of IPO5 in various cell lines specifically increases nuclear localization of BMP receptor-activated SMADs (R-SMADs) confirming a functional relationship between IPO5 and BMP but not TGF-β R-SMADs. Finally, we provide evidence that variation in length of the lysine stretch of the nuclear localization sequence is a determinant for importin specificity.

TGF-β and BMP signaling in osteoblast, skeletal development, and bone formation, homeostasis and disease

Transforming growth factor-beta (TGF-β) and bone morphogenic protein (BMP) signaling has fundamental roles in both embryonic skeletal development and postnatal bone homeostasis. TGF-βs and BMPs, acting on a tetrameric receptor complex, transduce signals to both the canonical Smad-dependent signaling pathway (that is, TGF-β/BMP ligands, receptors, and Smads) and the non-canonical-Smad-independent signaling pathway (that is, p38 mitogen-activated protein kinase/p38 MAPK) to regulate mesenchymal stem cell differentiation during skeletal development, bone formation and bone homeostasis. Both the Smad and p38 MAPK signaling pathways converge at transcription factors, for example, Runx2 to promote osteoblast differentiation and chondrocyte differentiation from mesenchymal precursor cells. TGF-β and BMP signaling is controlled by multiple factors, including the ubiquitin–proteasome system, epigenetic factors, and microRNA. Dysregulated TGF-β and BMP signaling result in a number of bone disorders in humans. Knockout or mutation of TGF-β and BMP signaling-related genes in mice leads to bone abnormalities of varying severity, which enable a better understanding of TGF-β/BMP signaling in bone and the signaling networks underlying osteoblast differentiation and bone formation. There is also crosstalk between TGF-β/BMP signaling and several critical cytokines’ signaling pathways (for example, Wnt, Hedgehog, Notch, PTHrP, and FGF) to coordinate osteogenesis, skeletal development, and bone homeostasis. This review summarizes the recent advances in our understanding of TGF-β/BMP signaling in osteoblast differentiation, chondrocyte differentiation, skeletal development, cartilage formation, bone formation, bone homeostasis, and related human bone diseases caused by the disruption of TGF-β/BMP signaling.