Gla-rich protein is involved in the cross-talk between calcification and inflammation in osteoarthritis.

Osteoarthritis (OA) is a whole-joint disease characterized by articular cartilage loss, tissue inflammation, abnormal bone formation and extracellular matrix (ECM) mineralization. Disease-modifying treatments are not yet available and a better understanding of osteoarthritis pathophysiology should lead to the discovery of more effective treatments. Gla-rich protein (GRP) has been proposed to act as a mineralization inhibitor and was recently shown to be associated with OA in vivo. Here, we further investigated the association of GRP with OA mineralization-inflammation processes. Using a synoviocyte and chondrocyte OA cell system, we showed that GRP expression was up-regulated following cell differentiation throughout ECM calcification, and that inflammatory stimulation with IL-1ß results in an increased expression of COX2 and MMP13 and up-regulation of GRP. Importantly, while treatment of articular cells with γ-carboxylated GRP inhibited ECM calcification, treatment with either GRP or GRP-coated basic calcium phosphate (BCP) crystals resulted in the down-regulation of inflammatory cytokines and mediators of inflammation, independently of its γ-carboxylation status. Our results strengthen the calcification inhibitory function of GRP and strongly suggest GRP as a novel anti-inflammatory agent, with potential beneficial effects on the main processes responsible for osteoarthritis progression. In conclusion, GRP is a strong candidate target to develop new therapeutic approaches.

Gla-rich protein acts as a calcification inhibitor in the human cardiovascular system.

OBJECTIVE: Vascular and valvular calcifications are pathological processes regulated by resident cells, and depending on a complex interplay between calcification promoters and inhibitors, resembling skeletal metabolism. Here, we study the role of the vitamin K-dependent Gla-rich protein (GRP) in vascular and valvular calcification processes. APPROACH AND RESULTS: Immunohistochemistry and quantitative polymerase chain reaction showed that GRP expression and accumulation are upregulated with calcification simultaneously with osteocalcin and matrix Gla protein (MGP). Using conformation-specific antibodies, both γ-carboxylated GRP and undercarboxylated GRP species were found accumulated at the sites of mineral deposits, whereas undercarboxylated GRP was predominant in calcified aortic valve disease valvular interstitial cells. Mineral-bound GRP, MGP, and fetuin-A were identified by mass spectrometry. Using an ex vivo model of vascular calcification, γ-carboxylated GRP but not undercarboxylated GRP was shown to inhibit calcification and osteochondrogenic differentiation through α-smooth muscle actin upregulation and osteopontin downregulation. Immunoprecipitation assays showed that GRP is part of an MGP-fetuin-A complex at the sites of valvular calcification. Moreover, extracellular vesicles released from normal vascular smooth muscle cells are loaded with GRP, MGP, and fetuin-A, whereas under calcifying conditions, released extracellular vesicles show increased calcium loading and GRP and MGP depletion. CONCLUSIONS: GRP is an inhibitor of vascular and valvular calcification involved in calcium homeostasis. Its function might be associated with prevention of calcium-induced signaling pathways and direct mineral binding to inhibit crystal formation/maturation. Our data show that GRP is a new player in mineralization competence of extracellular vesicles possibly associated with the fetuin-A-MGP calcification inhibitory system. GRP activity was found to be dependent on its γ-carboxylation status, with potential clinical relevance.

Four-and-a-half LIM domains protein 2 (FHL2) is associated with the development of craniofacial musculature in the teleost fish Sparus aurata.

Four-and-a-half LIM domains protein 2 (FHL2) is involved in major cellular mechanisms such as regulation of gene transcription and cytoskeleton modulation, participating in physiological control of cardiogenesis and osteogenesis. Knowledge on underlying mechanisms is, however, limited. We present here new data on FHL2 protein and its role during vertebrate development using a marine teleost fish, the gilthead seabream (Sparus aurata L.). In silico comparison of vertebrate protein sequences and prediction of LIM domain three-dimensional structure revealed a high degree of conservation, suggesting a conserved function throughout evolution. Determination of sites and levels of FHL2 gene expression in seabream indicated a central role for FHL2 in the development of heart and craniofacial musculature, and a potential role in tissue calcification. Our data confirmed the key role of FHL2 protein during vertebrate development and gave new insights into its particular involvement in craniofacial muscle development and specificity for slow fibers.

Ucma/GRP inhibits phosphate-induced vascular smooth muscle cell calcification via SMAD-dependent BMP signalling.

Vascular calcification (VC) is the process of deposition of calcium phosphate crystals in the blood vessel wall, with a central role for vascular smooth muscle cells (VSMCs). VC is highly prevalent in chronic kidney disease (CKD) patients and thought, in part, to be induced by phosphate imbalance. The molecular mechanisms that regulate VC are not fully known. Here we propose a novel role for the mineralisation regulator Ucma/GRP (Upper zone of growth plate and Cartilage Matrix Associated protein/Gla Rich Protein) in phosphate-induced VSMC calcification. We show that Ucma/GRP is present in calcified atherosclerotic plaques and highly expressed in calcifying VSMCs in vitro. VSMCs from Ucma/GRP-/- mice showed increased mineralisation and expression of osteo/chondrogenic markers (BMP-2, Runx2, ß-catenin, p-SMAD1/5/8, ALP, OCN), and decreased expression of mineralisation inhibitor MGP, suggesting that Ucma/GRP is an inhibitor of mineralisation. Using BMP signalling inhibitor noggin and SMAD1/5/8 signalling inhibitor dorsomorphin we showed that Ucma/GRP is involved in inhibiting the BMP-2-SMAD1/5/8 osteo/chondrogenic signalling pathway in VSMCs treated with elevated phosphate concentrations. Additionally, we showed for the first time evidence of a direct interaction between Ucma/GRP and BMP-2. These results demonstrate an important role of Ucma/GRP in regulating osteo/chondrogenic differentiation and phosphate-induced mineralisation of VSMCs.

Identification of Sparus aurata bone morphogenetic protein 2: molecular cloning, gene expression and in silico analysis of protein conserved features in vertebrates.

Bone morphogenetic protein 2 (BMP-2) is a secreted signaling molecule that acts as an inducer of bone formation and a regulator of embryonic development. The objectives of this work were as follows: (1) to clone the full-length cDNA of BMP-2 in a marine fish model, (2) analyze its gene expression during development, in adult tissues and in cell lines, and (3) identify protein conserved features of vertebrate BMP-2. Using a combination of RT- and 5'-RACE-PCR, a 1653-bp fragment corresponding to Sparus aurata BMP-2 cDNA (SaBMP-2) was amplified. Levels of SaBMP-2 gene expression were estimated using quantitative real-time PCR and shown to be strongly increased (150-fold induction) at gastrulation, thus suggesting a key role for BMP-2 in fish development. Tissue distribution of SaBMP-2 mRNA revealed highest levels in the calcified tissues bone, caudal fin and scales and in liver. BMP-2 was also found to be highly expressed in S. aurata bone-derived cell lines VSa13 and VSa16 and to be up-regulated (more than 10-fold induction) in mineralized VSa13 chondrocyte-like cells. Using bioinformatic tools and all vertebrate protein sequences available, conserved features of BMP-2 were characterized. The mature protein was shown to be highly conserved across 20 species indicating that BMP-2 function has been conserved throughout evolution, a finding that is in agreement with the widely accepted view of the important role played by BMPs in vertebrate development.

Insights into the association of Gla-rich protein and osteoarthritis, novel splice variants and γ-carboxylation status.

SCOPE: Gla-rich protein (GRP) is a vitamin K dependent protein, characterized by a high density of γ-carboxylated Glu residues, shown to accumulate in mouse and sturgeon cartilage and at sites of skin and vascular calcification in humans. Therefore, we investigated the involvement of GRP in pathological calcification in osteoarthritis (OA). METHODS AND RESULTS: Comparative analysis of GRP patterning at transcriptional and translational levels was performed between controls and OA patients. Using a RT-PCR strategy we unveiled two novel splice variants in human-GRP-F5 and F6-potentially characterized by the loss of full γ-carboxylation and secretion functional motifs. GRP-F1 is shown to be the predominant splice variant expressed in mouse and human adult tissues, particularly in OA cartilage, while an overexpressing human cell model points it as the major γ-carboxylated isoform. Using validated conformational antibodies detecting carboxylated or undercarboxylated GRP (c/uc GRP), we have demonstrated cGRP accumulation in controls, whereas ucGRP was the predominant form in OA-affected tissues, colocalizing at sites of ectopic calcification. CONCLUSION: Overall, our results indicate the predominance of GRP-F1, and a clear association of ucGRP with OA cartilage and synovial membrane. Levels of vitamin K should be further assessed in these patients to determine its potential therapeutic use as a supplement in OA treatment.

Gla-rich protein is a potential new vitamin K target in cancer: evidences for a direct GRP-mineral interaction.

Gla-rich protein (GRP) was described in sturgeon as a new vitamin-K-dependent protein (VKDP) with a high density of Gla residues and associated with ectopic calcifications in humans. Although VKDPs function has been related with γ-carboxylation, the Gla status of GRP in humans is still unknown. Here, we investigated the expression of recently identified GRP spliced transcripts, the γ-carboxylation status, and its association with ectopic calcifications, in skin basal cell and breast carcinomas. GRP-F1 was identified as the predominant splice variant expressed in healthy and cancer tissues. Patterns of γ-carboxylated GRP (cGRP)/undercarboxylated GRP (ucGRP) accumulation in healthy and cancer tissues were determined by immunohistochemistry, using newly developed conformation-specific antibodies. Both GRP protein forms were found colocalized in healthy tissues, while ucGRP was the predominant form associated with tumor cells. Both cGRP and ucGRP found at sites of microcalcifications were shown to have in vitro calcium mineral-binding capacity. The decreased levels of cGRP and predominance of ucGRP in tumor cells suggest that GRP may represent a new target for the anticancer potential of vitamin K. Also, the direct interaction of cGRP and ucGRP with BCP crystals provides a possible mechanism explaining GRP association with pathological mineralization.

Overexpression of four and a half LIM domains protein 2 promotes epithelial-mesenchymal transition-like phenotype in fish pre-osteoblasts.

FHL2 is a multifunctional protein involved in gene transcription regulation and cytoarchitecture modulation that has been recently associated with epithelial-mesenchymal transition (EMT) in colon cancer. Overexpression of FHL2 in a fish pre-osteoblastic cell line promoted cell dedifferentiation and impaired its extracellular matrix mineralization capacity. Cell cultures also acquired a novel three-dimensional structure organization, their proliferation rate was enhanced and gene expression profile was altered in agreement with an EMT-like phenotype upon overexpression of FHL2. Altogether, our results provide additional support to the relevance of FHL2 for cell differentiation and its association with hallmarks of cancer phenotype.

Establishment of primary cell cultures from fish calcified tissues.

Fishes have been recently recognized as a suitable model organism to study vertebrate physiological processes, in particular skeletal development and tissue mineralization. However, there is a lack of well characterized in vitro cell systems derived from fish calcified tissues. We describe here a protocol that was successfully used to develop the first calcified tissue-derived cell cultures of fish origin. Vertebra and branchial arches collected from young gilthead seabreams were fragmented then submitted to the combined action of collagenase and trypsin to efficiently release cells embedded in the collagenous extracellular matrix. Primary cultures were maintained under standard conditions and spontaneously transformed to form continuous cell lines suitable for studying mechanisms of tissue mineralization in seabream. This simple and inexpensive protocol is also applicable to other calcified tissues and species by adjusting parameters to each particular case.