Osteopontin as a novel substrate for the proprotein convertase 5/6 (PCSK5) in bone

Pro-fibrotic effect of the susceptible gene PCSK5 in vascular fibrosis of Takayasu arteritis via TGF-β and SMAD3 signaling pathway activation

BACKGROUND

Vascular fibrosis directly causes vascular thickening in Takayasu arteritis (TAK), in which sustained transforming growth factor beta (TGF-β) activation is critical. Understanding TGF-β activation regulation and blocking it might yield a therapeutic effect in TAK. Proprotein convertase subtilisin/kexin type 5 (PCSK5) rs6560480 (T/C) is associated with TAK development. In this study, we assessed the association between the PCSK5 rs6560480 genotype and PCSK5 expression in TAK and explored its molecular role in TGF-β activation and vascular fibrosis development.

METHODS

In TAK patients, PCSK5 and TGF-β expression in plasma and aortic tissue was examined by ELISA and immunohistochemical staining, and PCSK5 rs6560480 was genotyped. The correlation between PCSK5 and extracellular matrix (ECM) expression was examined by Western blotting (WB) and immunohistochemistry staining. Detection by co-immunoprecipitation was performed to detect the interaction between PCSK5 and TGF-β in adventitial fibroblasts (AAFs). Downstream signaling pathways were detected by WB and validated with appropriate inhibitors. Potential immunosuppressive agents to inhibit the effects of PCSK5 were explored in cell culture and TAK patients.

RESULTS

Patients with PCSK5 rs6560480 TT patients had significantly higher PCSK5 levels and more thickened vascular lesions than patients with PCSK5 rs6560480 CT. PCSK5 expression was significantly increased in alpha smooth muscle actin (α-SMA)-positive myofibroblasts in TAK vascular lesions. Overexpressing PCSK5 facilitated TGF-β and downstream SMAD2/3 activation and ECM expression in AAFs and aorta in in-vitro culture. The mechanistic study supported that PCSK5 activated precursor TGF-β (pro-TGF-β) to the mature form by binding the pro-TGF-β cleavage site. Leflunomide inhibited PCSK5 and pro-TGF-β binding, decreasing TGF-β activation and ECM expression, which was also partially validated in leflunomide-treated patients.

CONCLUSION

The findings revealed a novel pro-fibrotic mechanism of PCSK5 in TAK vascular fibrosis via TGF-β and downstream SMAD2/3 pathway activation. Leflunomide might be anti-fibrotic by disrupting PCSK5 and pro-TGF-β binding, presenting a new TAK treatment approach.

Apoptotic exosome-like vesicles transfer specific and functional mRNAs to endothelial cells by phosphatidylserine-dependent macropinocytosis

Apoptosis of endothelial cells prompts the release of apoptotic exosome-like vesicles (ApoExos), subtype extracellular vesicles secreted by apoptotic cells after caspase-3 activation. ApoExos are different from both apoptotic bodies and classical exosomes in their protein and nucleic acid contents and functions. In contrast to classical apoptotic bodies, ApoExos induce immunogenic responses that can be maladaptive when not tightly regulated. In the present study, we elucidated the mechanisms by which ApoExos are internalized by endothelial cells, which leads to shared specific and functional mRNAs of importance to endothelial function. Using flow cytometry and confocal microscopy, we revealed that ApoExos were actively internalized by endothelial cells. SiRNA-induced inhibition of classical endocytosis pathways with pharmacological inhibitors showed that ApoExos were internalized via phosphatidylserine-dependent macropinocytosis independently of classical endocytosis pathways. An electron microscopy analysis revealed that ApoExos increased the macropinocytosis rate in endothelial cells, setting in motion a positive feedback loop that increased the amount of internalized ApoExos. Deep sequencing of total RNA revealed that ApoExos possessed a unique protein-coding RNA profile, with PCSK5 being the most abundant mRNA. Internalization of ApoExos by cells led to the transfer of this RNA content from the ApoExos to cells. Specifically, PCSK5 mRNA was transferred to cells that had taken up ApoExos, and these cells subsequently expressed PCSK5. Collectively, our findings suggest that macropinocytosis is an effective entry pathway for the delivery of RNAs carried by ApoExos and that these RNAs are functionally expressed by the endothelial cells that internalize them. As ApoExos express a specific mRNA signature, these results suggest new avenues to understand how ApoExos produced at sites of vascular injury impact vascular function.

Constitutive bone marrow adipocytes suppress local bone formation

BM adipocytes (BMAd) are a unique cell population derived from BM mesenchymal progenitors and marrow adipogenic lineage precursors. Although they have long been considered to be a space filler within bone cavities, recent studies have revealed important physiological roles in hematopoiesis and bone metabolism. To date, the approaches used to study BMAd function have been confounded by contributions by nonmarrow adipocytes or by BM stromal cells. To address this gap in the field, we have developed a BMAd-specific Cre mouse model to deplete BMAds by expression of diphtheria toxin A (DTA) or by deletion of peroxisome proliferator-activated receptor gamma (Pparg). We found that DTA-induced loss of BMAds results in decreased hematopoietic stem and progenitor cell numbers and increased bone mass in BMAd-enriched locations, including the distal tibiae and caudal vertebrae. Elevated bone mass appears to be secondary to enhanced endosteal bone formation, suggesting a local effect caused by depletion of BMAd. Augmented bone formation with BMAd depletion protects mice from bone loss induced by caloric restriction or ovariectomy, and it facilitates the bone-healing process after fracture. Finally, ablation of Pparg also reduces BMAd numbers and largely recapitulates high-bone mass phenotypes observed with DTA-induced BMAd depletion.

β-catenin-controlled tubular cell-derived exosomes play a key role in fibroblast activation via the OPN-CD44 axis

Tubular injury and peripheral fibroblast activation are the hallmarks of chronic kidney disease (CKD), suggesting intimate communication between the two types of cells. However, the underlying mechanisms remain to be determined. Exosomes play a role in shuttling proteins and other materials to recipient cells. In our study, we found that exosomes were aroused by β-catenin in renal tubular cells. Osteopontin (OPN), especially its N-terminal fragment (N-OPN), was encapsulated in β-catenin-controlled tubular cell-derived exosome cargo, and subsequently passed to fibroblasts. Through binding with CD44, exosomal OPN promoted fibroblast proliferation and activation. Gene deletion of β-catenin in tubular cells (Ksp-β-catenin-/- ) or gene ablation of CD44 (CD44-/- ) greatly ameliorated renal fibrosis. Notably, N-OPN was carried by exosome and secreted into the urine of patients with CKD, and negatively correlated with kidney function. The urinary exosomes from patients with CKD greatly accelerated renal fibrosis, which was blocked by CD44 deletion. These results suggest that exosome-mediated activation of the OPN/CD44 axis plays a key role in renal fibrosis, which is controlled by β-catenin.

Assessing SPP1/Osteopontin (OPN) Splice Variants and Their Association to Nonmelanoma Skin Cancer by Absolute Quantification: Identification of OPN-5 Subvariants and Their Protein Coding Potential

The study evaluated whether SPP1/osteopontin (OPN) splice variants are differentially expressed in nonmelanoma skin cancer compared to normal skin. The absolute number of mRNA molecules of OPN-a predominated in normal skin and nonmelanoma skin cancer compared to OPN-b, OPN-c, and OPN-5. However, mRNAs of OPN-a, OPN-b, and OPN-c were expressed in higher levels in cutaneous squamous cell carcinomas (cSCCs) and basal cell carcinomas relative to normal skin. Additionally, OPN-5 expression was higher than OPN-b and OPN-c, and OPN-c, in normal skin and nonmelanoma skin cancer, respectively. Furthermore, we identified four OPN-5 splice variants, which were cloned and analyzed for protein expression.

The consequences of PCSK9 inhibition in selected tissues

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is one of nine members of the proprotein convertase family. These serine proteases play a pivotal role in the post-translational modification of proteins and the activation of hormones, enzymes, transcription factors and growth factors. As a result, they participate in many physiological processes like embryogenesis, activity of central nervous system and lipid metabolism. Scientific studies show that the family of convertases is also involved in the pathogenesis of viral and bacterial infections, osteoporosis, hyperglycaemia, cardiovascular diseases, neurodegenerative disorders and cancer. The inhibition of PCSK9 by two currently approved for use monoclonal antibodies (alirocumab, evolocumab) slows down the degradation of low-density lipoprotein cholesterol receptors (LDLRs). This leads to increased density of LDLRs on the surface of hepatocytes, resulting in decreased level of low-density lipoprotein cholesterol (LDL-C) in the bloodstream, which is connected with the reduction of cardiovascular risk. PCSK9 inhibitors (PCSK9i) were created for the patients who could not achieve appropriate level of LDL-C using current statin and ezetimibe therapy. It seems that high therapeutic efficacy of PCSK9i will make them more common in the clinical use. The pleiotropic effects of previously mentioned lipid-lowering therapies were the reasons for literature review of possible positive and negative effects of PCSK9 inhibition beyond cholesterol metabolism.

Mouse Models of Human Proprotein Convertase Insufficiency

The kexin-like proprotein convertases perform the initial proteolytic cleavages that ultimately generate a variety of different mature peptide and proteins, ranging from brain neuropeptides to endocrine peptide hormones, to structural proteins, among others. In this review, we present a general introduction to proprotein convertase structure and biochemistry, followed by a comprehensive discussion of each member of the kexin-like subfamily of proprotein convertases. We summarize current knowledge of human proprotein convertase insufficiency syndromes, including genome-wide analyses of convertase polymorphisms, and compare these to convertase null and mutant mouse models. These mouse models have illuminated our understanding of the roles specific convertases play in human disease and have led to the identification of convertase-specific substrates; for example, the identification of procorin as a specific PACE4 substrate in the heart. We also discuss the limitations of mouse null models in interpreting human disease, such as differential precursor cleavage due to species-specific sequence differences, and the challenges presented by functional redundancy among convertases in attempting to assign specific cleavages and/or physiological roles. However, in most cases, knockout mouse models have added substantively both to our knowledge of diseases caused by human proprotein convertase insufficiency and to our appreciation of their normal physiological roles, as clearly seen in the case of the furin, proprotein convertase 1/3, and proprotein convertase 5/6 mouse models. The creation of more sophisticated mouse models with tissue- or temporally-restricted expression of specific convertases will improve our understanding of human proprotein convertase insufficiency and potentially provide support for the emerging concept of therapeutic inhibition of convertases.

Application of microRNA in Human Osteoporosis and Fragility Fracture: A Systemic Review of Literatures

MicroRNAs (miRNAs) could serve as ideal entry points to the deregulated pathways in osteoporosis due to their relatively simple upstream and downstream relationships with other molecules in the signaling cascades. Our study aimed to give a comprehensive review of the already identified miRNAs in osteoporosis from human blood samples and provide useful information for their clinical application. A systematic literature search for relevant studies was conducted in the Pubmed database from inception to December 2020. We set two essential inclusion criteria: human blood sampling and design of controlled studies. We sorted the results of analysis on human blood samples according to the study settings and compiled the most promising miRNAs with analyzed diagnostic values. Furthermore, in vitro and in vivo evidence for the mechanisms of the identified miRNAs was also illustrated. Based on both diagnostic value and evidence of mechanism from in vitro and in vivo experiments, miR-23b-3p, miR-140-3p, miR-300, miR-155-5p, miR-208a-3p, and miR-637 were preferred candidates in diagnostic panels and as therapeutic agents. Further studies are needed to build sound foundations for the clinical usage of miRNAs in osteoporosis.

MicroRNA‑338‑3p regulates age‑associated osteoporosis via targeting PCSK5

Bone loss is a disease that is highly associated with aging. This deleterious health condition has become a public concern worldwide, and there is an urgent need to discover more novel therapeutic strategies for the development of age‑associated osteoporosis. The present study aimed to explore the association between proprotein convertase subtilisin/kexin type 5 (PCSK5) and microRNA(miR)‑338‑3p in bone‑formation and bone‑loss processes. Western blotting assay and reverse transcription‑quantitative PCR were employed to analyze PCSK5 and miR‑338‑3p expression levels in bone mesenchymal stem cells (BMSCs). Dual‑luciferase reporter and RNA pull‑down assays were used to determine the target. For osteoblastic differentiation verification, alkaline phosphatase activity, osteocalcin secretion detection, bone formation‑related indicators (osterix, runt‑related gene 2, osteopontin and bone sialoprotein), hematoxylin and eosin staining and Alizarin Red S staining were performed. The findings of the present study indicated that the expression level of PCSK5 was higher in BMSCs from young rat samples, whereas the expression level of miR‑338‑3p was higher in BMSCs from samples of old rats. Experimental results also revealed that unlike miR‑338‑3p, downregulation of PCSK5 inhibited osteoblastic differentiation and osteogenesis by inhibiting alkaline phosphatase, osteocalcin, osterix, runt‑related transcription factor 2, osteopontin, bone sialoprotein and mineralized nodule formation. Overall, the results suggested that miR‑338‑3p could suppress age‑associated osteoporosis by regulating PCSK5.

Hypophosphatemic osteosclerosis, hyperostosis, and enthesopathy associated with novel homozygous mutations of DMP1 encoding dentin matrix protein 1 and SPP1 encoding osteopontin: The first digenic SIBLING protein osteopathy?

The SIBLINGs are a subfamily of the secreted calcium-binding phosphoproteins and comprise five small integrin-binding ligand N-linked glycoproteins [dentin matrix protein-1 (DMP1), secreted phosphoprotein-1 (SPP1) also called osteopontin (OPN), integrin-binding sialoprotein (IBSP) also called bone sialoprotein (BSP), matrix extracellular phosphoglycoprotein (MEPE), and dentin sialophosphoprotein (DSPP)]. Each SIBLING has at least one "acidic, serine- and aspartic acid-rich motif" (ASARM) and multiple Ser-x-Glu/pSer sequences that when phosphorylated promote binding of the protein to hydroxyapatite for regulation of biomineralization. Mendelian disorders from loss-of-function mutation(s) of the genes that encode the SIBLINGs thus far involve DSPP causing various autosomal dominant dysplasias of dentin but without skeletal disease, and DMP1 causing autosomal recessive hypophosphatemic rickets, type 1 (ARHR1). No diseases have been reported from gain-of-function mutation(s) of DSPP or DMP1 or from alterations of SPP1, IBSP, or MEPE. Herein, we describe severe hypophosphatemic osteosclerosis and hyperostosis associated with skeletal deformity, short stature, enthesopathy, tooth loss, and high circulating FGF23 levels in a middle-aged man and young woman from an endogamous family living in southern India. Both shared novel homozygous mutations within two genes that encode a SIBLING protein: stop-gain ("nonsense") DMP1 (c.556G>T,p.Glu186Ter) and missense SPP1 (c.769C>T,p.Leu266Phe). The man alone also carried novel heterozygous missense variants within two additional genes that condition mineral homeostasis and are the basis for autosomal recessive disorders: CYP27B1 underlying vitamin D dependent rickets, type 1, and ABCC6 underlying both generalized arterial calcification of infancy, type 2 and pseudoxanthoma elasticum (PXE). By immunochemistry, his bone contained high amounts of OPN, particularly striking surrounding osteocytes. We review how our patients' disorder may represent the first digenic SIBLING protein osteopathy.

In Vitro Evaluation of a Novel Osteo-Inductive Scaffold for Osteogenic Differentiation of Bone-Marrow Mesenchymal Stem Cells

BACKGROUND

Demineralized bone matrices (DBMs) were demonstrated to be a promising candidate for bone regeneration by previous studies. However, the limited osteoinductivity of DBMs was insufficient for a better repairing of bone defect. Osteoblasts (OBs), the major cellular component of bone tissues, play an important role in the formation of new bone. The extracellular matrix (ECM) of OB is one of the main components of bone formation niche.

OBJECTIVE

To combine the DBMs with the ECM of OBs to construct a novel scaffold that could be used for bone reconstruction.

METHODS

In this study, OBs were cultured on the surface of DBMs for 10 days and removed by Triton X-100 and ammonium hydroxide to prepare the OBs-ECM-DBMs (OEDBMs). A series of material features such as residues of OBs and ECM, cytotoxity, and osteoinductive capability of OEDBMs were evaluated.

RESULTS

Low cell residues and low content of DNA were observed in OEDBMs. Compared with DBMs, OEDBMs possessed more bone tissues organic matrix proteins, such as osteocalcin, osteopontin, and collagen I. Rat bone marrow mesenchymal stem cells (rBMSCs) presented a good viability when cultured on both 2 materials. The significant upregulations of osteogenic genes and proteins of rBMSCs were observed in OEDBMs group compared with DBMs group.

CONCLUSION

Taken together, these findings suggested that the OB-secreted ECM may be qualified as an ideal modification method for enhancing the performance of engineered bone scaffold.

Identification of genetic loci associated with growth traits at weaning in yak through a genome-wide association study

A multilocus GWAS was performed to explore the genetic architecture of four growth traits in yak. In total, 354 female yaks for which measurements of body weight (BW), withers height (WH), body length (BL) and chest girth (CG) at weaning were available underwent genotyping with the Illumina BovineHD BeadChip (770K). After quality control, we retained 98 688 SNPs and 354 animals for GWAS analysis. We identified seven, 18, seven and nine SNPs (corresponding to seven, 17, seven and eight candidate genes) associated with BW, WH, BL and CG at weaning respectively. Interestingly, most of these candidate genes were reported to be involved in growth-related processes such as muscle formation, lipid deposition, feed efficiency, carcass composition and development of the central and peripheral nervous system. Our results offer novel insight into the molecular architecture underpinning yak growth traits. Further functional analyses are thus warranted to explore the molecular mechanisms whereby these genes affect these traits of interest.

Monotropein attenuates ovariectomy and LPS-induced bone loss in mice and decreases inflammatory impairment on osteoblast through blocking activation of NF-κB pathway

Estrogen deficiency and inflammation are known to play important roles in bone metabolism and occurrence of osteoporosis. Monotropein as an iridoid glycoside is reported to decrease estrogen deficiency-induced bone loss and inhibit inflammatory response in LPS-induced RAW 264.7 macrophages. However, the effect of monotropein on bone loss in chronic inflammatory conditions remains unclear. It was found in the present study that monotropein significantly inhibited bone mass reduction and improved bone micro-architectures by enhancing bone formation and blocking increased secretion of inflammatory cytokines in osteoporotic mice induced by combined ovariectomy and LPS. Our in vitro experiment further demonstrated that monotropein was able to increase the proliferation and activity of alkaline phosphatase (ALP), bone matrix mineralization and the expression of bone matrix protein osteopontin (OPN) in osteoblastic MC3T3-E1 cells injured by LPS. In addition, monotropein significantly decreased the production of IL-6 and IL-1β, inhibited the nuclear translocation of p65 and NF-κB P50, and down-regulated the phosphorylation of NF-κB p65 and IKK, indicating that monotropein could attenuate inflammatory impairment to MC3T3-E1 cells by suppressing the activation of NF-κB pathway. All these results suggest that monotropein may prove to be a promising candidate for the prevention and treatment of inflammatory bone loss.