Do Media Extracellular Vesicles and Extracellular Vesicles Bound to the Extracellular Matrix Represent Distinct Types of Vesicles?

Mineralization-competent cells, including hypertrophic chondrocytes, mature osteoblasts, and osteogenic-differentiated smooth muscle cells secrete media extracellular vesicles (media vesicles) and extracellular vesicles bound to the extracellular matrix (matrix vesicles). Media vesicles are purified directly from the extracellular medium. On the other hand, matrix vesicles are purified after discarding the extracellular medium and subjecting the cells embedded in the extracellular matrix or bone or cartilage tissues to an enzymatic treatment. Several pieces of experimental evidence indicated that matrix vesicles and media vesicles isolated from the same types of mineralizing cells have distinct lipid and protein composition as well as functions. These findings support the view that matrix vesicles and media vesicles released by mineralizing cells have different functions in mineralized tissues due to their location, which is anchored to the extracellular matrix versus free-floating.

Additional Insights Into the Role of Osteocalcin in Osteoblast Differentiation and in the Early Steps of Developing Alveolar Process of Rat Molars

This study investigated whether osteocalcin (OCN) is present in osteoblast precursors and its relationship with initial phases of alveolar process formation. Samples of maxillae of 16-, 18-, and 20-day-old rat embryos (E16, E18, and E20, respectively), and 05-, 10-, and 15-day-old postnatal rats (P05, P10, and P15, respectively) were fixed and embedded in paraffin or araldite. Immunohistochemistry for osterix (Osx), alkaline phosphatase (ALP), and OCN detection was performed and the number of immunolabelled cells was computed. Non-decalcified sections were subjected to the von Kossa method combined with immunohistochemistry for Osx or OCN detection. For OCN immunolocalization, samples were fixed in 0.5% glutaraldehyde/2% formaldehyde and embedded in LR White resin. The highest number of ALP- and OCN-immunolabelled cells was observed in dental follicle of E16 specimens, mainly in basal portions of dental alveolus. In corresponding regions, osteoblasts in differentiation adjacent to von Kossa-positive bone matrix exhibited Osx and OCN immunoreactivity. Ultrastructural analysis revealed OCN immunoreactive particles inside osteoblast in differentiation, and in bone matrix associated with collagen fibrils and within matrix vesicles, at early stages of alveolar process formation. Our results indicate that OCN plays a role in osteoblast differentiation and may regulate calcium/phosphate precipitation during early mineralization of the alveolar process.

AnnexinA6: a potential therapeutic target gene for extracellular matrix mineralization

The mineralization of the extracellular matrix (ECM) is an essential and crucial process for physiological bone formation and pathological calcification. The abnormal function of ECM mineralization contributes to the worldwide risk of developing mineralization-related diseases; for instance, vascular calcification is attributed to the hyperfunction of ECM mineralization, while osteoporosis is due to hypofunction. AnnexinA6 (AnxA6), a Ca2+-dependent phospholipid-binding protein, has been extensively reported as an essential target in mineralization-related diseases such as osteoporosis, osteoarthritis, atherosclerosis, osteosarcoma, and calcific aortic valve disease. To date, AnxA6, as the largest member of the Annexin family, has attracted much attention due to its significant contribution to matrix vesicles (MVs) production and release, MVs-ECM interaction, cytoplasmic Ca2+ influx, and maturation of hydroxyapatite, making it an essential target in ECM mineralization. In this review, we outlined the recent advancements in the role of AnxA6 in mineralization-related diseases and the potential mechanisms of AnxA6 under normal and mineralization-related pathological conditions. AnxA6 could promote ECM mineralization for bone regeneration in the manner described previously. Therefore, AnxA6 may be a potential osteogenic target for ECM mineralization.

Osteoblast-Derived Matrix Vesicles Exhibit Exosomal Traits and a Unique Subset of microRNA: Their Caveolae-Dependent Endocytosis Results in Reduced Osteogenic Differentiation

Matrix vesicles (MVs) are nano-sized extracellular vesicles that are anchored in the extracellular matrix (ECM). In addition to playing a role in biomineralization, osteoblast-derived MVs were recently suggested to have regulatory duties. The aims of this study were to establish the characteristics of osteoblast-derived MVs in the context of extracellular vesicles like exosomes, assess their role in modulating osteoblast differentiation, and examine their mechanism of uptake. MVs were isolated from the ECM of MG63 human osteoblast-like cell cultures and characterized via enzyme activity, transmission electron microscopy, nanoparticle tracking analysis, Western blot, and small RNA sequencing. Osteoblasts were treated with MVs from two different culture conditions (growth media [GM]; osteogenic media [OM]) to evaluate their effects on the differentiation and production of inflammatory markers and on macrophage polarization. MV endocytosis was assessed using a lipophilic, fluorescent dye and confocal microscopy with the role of caveolae determined using methyl-β-cyclodextrin. MVs exhibited a four-fold enrichment in alkaline phosphatase specific activity compared to plasma membranes; were 50-150 nm in diameter; possessed exosomal markers CD63, CD81, and CD9 and endosomal markers ALIX, TSG101, and HSP70; and were selectively enriched in microRNA linked to an anti-osteogenic effect and to M2 macrophage polarization. Treatment with GM or OM MVs decreased osteoblast differentiation. Osteoblasts endocytosed MVs using a mechanism that involves caveolae. These results support the hypothesis that osteoblasts produce MVs that participate in the regulation of osteogenesis.

Apigenin Modulates AnxA6- and TNAP-Mediated Osteoblast Mineralization

Mineralization-competent cells like osteoblasts and chondrocytes release matrix vesicles (MVs) which accumulate Ca2+ and Pi, creating an optimal environment for apatite formation. The mineralization process requires the involvement of proteins, such as annexins (Anx) and tissue-nonspecific alkaline phosphatase (TNAP), as well as low molecular-weight compounds. Apigenin, a flavonoid compound, has been reported to affect bone metabolism, but there are doubts about its mechanism of action under physiological and pathological conditions. In this report, apigenin potency to modulate annexin A6 (AnxA6)- and TNAP-mediated osteoblast mineralization was explored using three cell lines: human fetal osteoblastic hFOB 1.19, human osteosarcoma Saos-2, and human coronary artery smooth muscle cells HCASMC. We compared the mineralization competence, the morphology and composition of minerals, and the protein distribution in control and apigenin-treated cells and vesicles. The mineralization ability was monitored by AR-S/CPC analysis, and TNAP activity was determined by ELISA assay. Apigenin affected the mineral structure and modulated TNAP activity depending on the concentration. We also observed increased mineralization in Saos-2 cells. Based on TEM-EDX, we found that apigenin influenced the mineral composition. This flavonoid also disturbed the intracellular distribution of AnxA6 and TNAP, especially blocking AnxA6 aggregation and TNAP attachment to the membrane, as examined by FM analysis of cells and TEM-gold analysis of vesicles. In summary, apigenin modulates the mineralization process by regulating AnxA6 and TNAP, as well as through various effects on normal and cancer bone tissues or atherosclerotic soft tissue.

Correction of Vascular Calcification and Hyperphosphatemia in CKD Rats Treated with ASARM Peptide

Background

Abnormalities in calcium, phosphorus, PTH, vitamin D metabolism, bone, and vascular calcification occur in chronic kidney disease mineral bone disorder (CKD-MBD). Calciphylaxis, involving painful, ulcerative skin lesions, is also a major problem associated with CKD-MBD. There are no quality medical interventions to address these clinical issues. Bone ASARM peptides are strong inhibitors of mineralization and induce hypophosphatemia by inhibiting phosphate uptake from the gut. We hypothesize treatment of CKD-MBD rats with ASARM peptides will reverse hyperphosphatemia, reduce soft-tissue calcification, and prevent calciphylaxis.

Methods

To test our hypothesis, we assessed the effects of synthetic ASARM peptide in rats that had undergone a subtotal 5/6th nephrectomy (56NEPHREX), a rodent model of CKD-MBD. All rats were fed a high phosphate diet (2% Pi) to worsen mineral metabolism defects. Changes in serum potassium, phosphate, BUN, creatinine, PTH, FGF23, and calcium were assessed in response to 28 days of ASARM peptide infusion. Also, changes in bone quality, soft-tissue calcification, and expression of gut Npt2b (Slc34a2) were studied following ASARM peptide treatment.

Results

Rats that had undergone 56NEPHREX treated with ASARM peptide showed major improvements in hyperphosphatemia, blood urea nitrogen (BUN), and bone quality compared with vehicle controls. Also, ASARM-infused 56NEPHREX rats displayed improved renal, brain, and cardiovascular calcification. Notably, ASARM peptide infusion prevented the genesis of subdermal medial blood vessel calcification and calciphylaxis-like lesions in 56NEPHREX rats compared with vehicle controls.

Conclusions

ASARM peptide infusion corrects hyperphosphatemia and improves vascular calcification, renal calcification, brain calcification, bone quality, renal function, and skin mineralization abnormalities in 56NEPHREX rats. These findings confirm our hypothesis and support the utility of ASARM peptide treatment in patients with CKD-MBD.

The Role of Matrix-Bound Extracellular Vesicles in the Regulation of Endochondral Bone Formation

Matrix vesicles are key players in the development of the growth plate during endochondral bone formation. They are involved in the turnover of the extracellular matrix and its mineralization, as well as being a vehicle for chondrocyte communication and regulation. These extracellular organelles are released by the cells and are anchored to the matrix via integrin binding to collagen. The exact function and makeup of the vesicles are dependent on the zone of the growth plate in which they are produced. Early studies defined their role as sites of initial calcium phosphate deposition based on the presence of crystals on the inner leaflet of the membrane and subsequent identification of enzymes, ion transporters, and phospholipid complexes involved in mineral formation. More recent studies have shown that they contain small RNAs, including microRNAs, that are distinct from the parent cell, raising the hypothesis that they are a distinct subset of exosomes. Matrix vesicles are produced under complex regulatory pathways, which include the action of steroid hormones. Once in the matrix, their maturation is mediated by the action of secreted hormones. How they convey information to cells, either through autocrine or paracrine actions, is now being elucidated.

From Matrix Vesicles to Miniature Rocks: Evolution of Calcium Deposits in Calf Costochondral Junctions

OBJECTIVE

Initial stages of cartilage matrix calcification depend on the activity of matrix vesicles. The purpose of the study was to describe how calcified matrix vesicles join into larger structures, to present their up-to-date undescribed 3-dimensional image, and to observe how calcified matrix relates to chondrocyte lacunae.

DESIGN

Calcified cartilage was obtained from the zone of provisional calcification of calf costochondral junctions, then enzymatically isolated and studied by microtomography, scanning electron microscopy, atomic force microscopy and X-ray diffraction, and Fourier transform infrared spectroscopy.

RESULTS

Hyaluronidase digestion released packets of granules surrounded by the cartilage matrix. Further digestion, with collagenase and trypsin, removed matrix and exposed granules with dimensions within 50 to 150 nm range, which we consider as equivalent of calcified matrix vesicles. Granules joined into larger groups with dimensions of 0.5 to 2 μm, which we call globular units. Certain matrix vesicles appeared well connected but contained globular units that had spaces filled with electron lucent material, presumably matrix or chondrocyte remnants. Globular units were organized into massive structures taking the shape of oval plates. Comparison of these plates with lacunae containing isogenous groups of chondrocytes from proliferative zone of costochondral junction suggests that the cells from a single lacuna were responsible for the formation of one plate. The plates were connected with each other and extended over provisional calcification zone.

CONCLUSIONS

The outcome showed how particular calcified matrix vesicles associate into globular units, which organize into massive structures assuming the shape of oval plates and eventually cover large areas of cartilage matrix.

Enzymatic Approach in Calcium Phosphate Biomineralization: A Contribution to Reconcile the Physicochemical with the Physiological View

Biomineralization is the process by which organisms produce hard inorganic matter from soft tissues with outstanding control of mineral deposition in time and space. For this purpose, organisms deploy a sophisticated "toolkit" that has resulted in significant evolutionary innovations, for which calcium phosphate (CaP) is the biomineral selected for the skeleton of vertebrates. While CaP mineral formation in aqueous media can be investigated by studying thermodynamics and kinetics of phase transitions in supersaturated solutions, biogenic mineralization requires coping with the inherent complexity of biological systems. This mainly includes compartmentalization and homeostatic processes used by organisms to regulate key physiological factors, including temperature, pH and ion concentration. A detailed analysis of the literature shows the emergence of two main views describing the mechanism of CaP biomineralization. The first one, more dedicated to the study of in vivo systems and supported by researchers in physiology, often involves matrix vesicles (MVs). The second one, more investigated by the physicochemistry community, involves collagen intrafibrillar mineralization particularly through in vitro acellular models. Herein, we show that there is an obvious need in the biological systems to control both where and when the mineral forms through an in-depth survey of the mechanism of CaP mineralization. This necessity could gather both communities of physiologists and physicochemists under a common interest for an enzymatic approach to better describe CaP biomineralization. Both homogeneous and heterogeneous enzymatic catalyses are conceivable for these systems, and a few preliminary promising results on CaP mineralization for both types of enzymatic catalysis are reported in this work. Through them, we aim to describe the relevance of our point of view and the likely findings that could be obtained when adding an enzymatic approach to the already rich and creative research field dealing with CaP mineralization. This complementary approach could lead to a better understanding of the biomineralization mechanism and inspire the biomimetic design of new materials.

Aesculetin Accelerates Osteoblast Differentiation and Matrix-Vesicle-Mediated Mineralization

The imbalance between bone resorption and bone formation in favor of resorption results in bone loss and deterioration of bone architecture. Osteoblast differentiation is a sequential event accompanying biogenesis of matrix vesicles and mineralization of collagen matrix with hydroxyapatite crystals. Considerable efforts have been made in developing naturally-occurring plant compounds, preventing bone pathologies, or enhancing bone regeneration. Coumarin aesculetin inhibits osteoporosis through hampering the ruffled border formation of mature osteoclasts. However, little is known regarding the effects of aesculetin on the impairment of matrix vesicle biogenesis. MC3T3-E1 cells were cultured in differentiation media with 1–10 μM aesculetin for up to 21 days. Aesculetin boosted the bone morphogenetic protein-2 expression, and alkaline phosphatase activation of differentiating MC3T3-E1 cells. The presence of aesculetin strengthened the expression of collagen type 1 and osteoprotegerin and transcription of Runt-related transcription factor 2 in differentiating osteoblasts for 9 days. When ≥1–5 μM aesculetin was added to differentiating cells for 15–18 days, the induction of non-collagenous proteins of bone sialoprotein II, osteopontin, osteocalcin, and osteonectin was markedly enhanced, facilitating the formation of hydroxyapatite crystals and mineralized collagen matrix. The induction of annexin V and PHOSPHO 1 was further augmented in ≥5 μM aesculetin-treated differentiating osteoblasts for 21 days. In addition, the levels of tissue-nonspecific alkaline phosphatase and collagen type 1 were further enhanced within the extracellular space and on matrix vesicles of mature osteoblasts treated with aesculetin, indicating matrix vesicle-mediated bone mineralization. Finally, aesculetin markedly accelerated the production of thrombospondin-1 and tenascin C in mature osteoblasts, leading to their adhesion to preformed collagen matrix. Therefore, aesculetin enhanced osteoblast differentiation, and matrix vesicle biogenesis and mineralization. These findings suggest that aesculetin may be a potential osteo-inductive agent preventing bone pathologies or enhancing bone regeneration.

Extracellular vesicles and the extracellular matrix: a new paradigm or old news?

Extracellular vesicles (EV) are implicated in a variety of functions affecting the extracellular matrix (ECM), including matrix degradation, cross-linking of matrix proteins and matrix calcification. These processes are important in many physiological contexts such as angiogenesis and wound healing, and dysregulation of ECM homeostasis contributes to a wide range of diseases including fibrosis, cancer and arthritis. Most studies of EV have focussed on their roles in cell:cell communication, but EV can exist as integral components of the ECM. By far the most well-characterised ECM-resident EV are matrix vesicles (MV) in bone, but the broader role of EV in the ECM is not well understood. This review will explore what is known of the roles of EV in the ECM and will also highlight the similarities and differences between MV and other EV.

Overexpression of miR-125b in Osteoblasts Improves Age-Related Changes in Bone Mass and Quality through Suppression of Osteoclast Formation

We recently reported an unexpected role of osteoblast-derived matrix vesicles in the delivery of microRNAs to bone matrix. Of such microRNAs, we found that miR-125b inhibited osteoclast formation by targeting Prdm1 encoding a transcriptional repressor of anti-osteoclastogenesis factors. Transgenic (Tg) mice overexpressing miR-125b in osteoblasts by using human osteocalcin promoter grow normally but exhibit high trabecular bone mass. We have now further investigated the effects of osteoblast-mediated miR-125b overexpression on skeletal morphogenesis and remodeling during development, aging and in a situation of skeletal repair, i.e., fracture healing. There were no significant differences in the growth plate, primary spongiosa or lateral (periosteal) bone formation and mineral apposition rate between Tg and wild-type (WT) mice during early bone development. However, osteoclast number and medial (endosteal) bone resorption were less in Tg compared to WT mice, concomitant with increased trabecular bone mass. Tg mice were less susceptible to age-dependent changes in bone mass, phosphate/amide I ratio and mechanical strength. In a femoral fracture model, callus formation progressed similarly in Tg and WT mice, but callus resorption was delayed, reflecting the decreased osteoclast numbers associated with the Tg callus. These results indicate that the decreased osteoclastogenesis mediated by miR-125b overexpression in osteoblasts leads to increased bone mass and strength, while preserving bone formation and quality. They also suggest that, in spite of the fact that single miRNAs may target multiple genes, the miR-125b axis may be an attractive therapeutic target for bone loss in various age groups.

Annexins A2, A6 and Fetuin-A Affect the Process of Mineralization in Vesicles Derived from Human Osteoblastic hFOB 1.19 and Osteosarcoma Saos-2 Cells

The mineralization process is initiated by osteoblasts and chondrocytes during intramembranous and endochondral ossifications, respectively. Both types of cells release matrix vesicles (MVs), which accumulate P_i and Ca²⁺ and form apatites in their lumen. Tissue non-specific alkaline phosphatase (TNAP), a mineralization marker, is highly enriched in MVs, in which it removes inorganic pyrophosphate (PP_i), an inhibitor of apatite formation. MVs then bud from the microvilli of mature osteoblasts or hypertrophic chondrocytes and, thanks to the action of the acto-myosin cortex, become released to the extracellular matrix (ECM), where they bind to collagen fibers and propagate mineral growth. In this report, we compared the mineralization ability of human fetal osteoblastic cell line (hFOB 1.19 cells) with that of osteosarcoma cell line (Saos-2 cells). Both types of cells were able to mineralize in an osteogenic medium containing ascorbic acid and beta glycerophosphate. The composition of calcium and phosphate compounds in cytoplasmic vesicles was distinct from that in extracellular vesicles (mostly MVs) released after collagenase-digestion. Apatites were identified only in MVs derived from Saos-2 cells, while MVs from hFOB 1.19 cells contained amorphous calcium phosphate complexes. In addition, AnxA6 and AnxA2 (nucleators of mineralization) increased mineralization in the sub-membrane region in strongly mineralizing Saos-2 osteosarcoma, where they co-localized with TNAP, whereas in less mineralizing hFOB 1.19 osteoblasts, AnxA6, and AnxA2 co-localizations with TNAP were less visible in the membrane. We also observed a reduction in the level of fetuin-A (FetuA), an inhibitor of mineralization in ECM, following treatment with TNAP and Ca channels inhibitors, especially in osteosarcoma cells. Moreover, a fraction of FetuA was translocated from the cytoplasm towards the plasma membrane during the stimulation of Saos-2 cells, while this displacement was less pronounced in stimulated hFOB 19 cells. In summary, osteosarcoma Saos-2 cells had a better ability to mineralize than osteoblastic hFOB 1.19 cells. The formation of apatites was observed in Saos-2 cells, while only complexes of calcium and phosphate were identified in hFOB 1.19 cells. This was also evidenced by a more pronounced accumulation of AnxA2, AnxA6, FetuA in the plasma membrane, where they were partly co-localized with TNAP in Saos-2 cells, in comparison to hFOB 1.19 cells. This suggests that both activators (AnxA2, AnxA6) and inhibitors (FetuA) of mineralization were recruited to the membrane and co-localized with TNAP to take part in the process of mineralization.

Matrix Vesicles: Role in Bone Mineralization and Potential Use as Therapeutics

Bone is a complex organ maintained by three main cell types: osteoblasts, osteoclasts, and osteocytes. During bone formation, osteoblasts deposit a mineralized organic matrix. Evidence shows that bone cells release extracellular vesicles (EVs): nano-sized bilayer vesicles, which are involved in intercellular communication by delivering their cargoes through protein–ligand interactions or fusion to the plasma membrane of the recipient cell. Osteoblasts shed a subset of EVs known as matrix vesicles (MtVs), which contain phosphatases, calcium, and inorganic phosphate. These vesicles are believed to have a major role in matrix mineralization, and they feature bone-targeting and osteo-inductive properties. Understanding their contribution in bone formation and mineralization could help to target bone pathologies or bone regeneration using novel approaches such as stimulating MtV secretion in vivo, or the administration of in vitro or biomimetically produced MtVs. This review attempts to discuss the role of MtVs in biomineralization and their potential application for bone pathologies and bone regeneration.

Role of Macrophages in the Progression and Regression of Vascular Calcification

Vascular calcification is an abnormal cell-mediated process in which bone-specific hydroxyapatite crystals are actively deposited on the blood vessel wall and is a significant pathological basis for the increased incidence and mortality of adverse cardiovascular events. Macrophages play an important regulatory role in the occurrence, development, and regression of vascular calcification. After the tissue microenvironment changes, macrophages subsequently change their polarity and phenotype or secrete functional substances as an adaptive response. As research on macrophages continue to move into this field, we gain a new understanding of the mechanism of the formation and regression of vascular calcification, which might offer valuable new intervention targets for the prevention and inhibition of vascular calcification. This review summarizes a wealth of research in this field and explores the roles of macrophages in the development process of vascular calcification.

Localization of Annexin A6 in Matrix Vesicles During Physiological Mineralization

Annexin A6 (AnxA6) is the largest member of the annexin family of proteins present in matrix vesicles (MVs). MVs are a special class of extracellular vesicles that serve as a nucleation site during cartilage, bone, and mantle dentin mineralization. In this study, we assessed the localization of AnxA6 in the MV membrane bilayer using native MVs and MV biomimetics. Biochemical analyses revealed that AnxA6 in MVs can be divided into three distinct groups. The first group corresponds to Ca²⁺-bound AnxA6 interacting with the inner leaflet of the MV membrane. The second group corresponds to AnxA6 localized on the surface of the outer leaflet. The third group corresponds to AnxA6 inserted in the membrane's hydrophobic bilayer and co-localized with cholesterol (Chol). Using monolayers and proteoliposomes composed of either dipalmitoylphosphatidylcholine (DPPC) to mimic the outer leaflet of the MV membrane bilayer or a 9:1 DPPC:dipalmitoylphosphatidylserine (DPPS) mixture to mimic the inner leaflet, with and without Ca²⁺, we confirmed that, in agreement with the biochemical data, AnxA6 interacted differently with the MV membrane. Thermodynamic analyses based on the measurement of surface pressure exclusion (π_exc), enthalpy (ΔH), and phase transition cooperativity (Δt_1/2) showed that AnxA6 interacted with DPPC and 9:1 DPPC:DPPS systems and that this interaction increased in the presence of Chol. The selective recruitment of AnxA6 by Chol was observed in MVs as probed by the addition of methyl-β-cyclodextrin (MβCD). AnxA6-lipid interaction was also Ca²⁺-dependent, as evidenced by the increase in π_exc in negatively charged 9:1 DPPC:DPPS monolayers and the decrease in ΔH in 9:1 DPPC:DPPS proteoliposomes caused by the addition of AnxA6 in the presence of Ca²⁺ compared to DPPC zwitterionic bilayers. The interaction of AnxA6 with DPPC and 9:1 DPPC:DPPS systems was distinct even in the absence of Ca²⁺ as observed by the larger change in Δt_1/2 in 9:1 DPPC:DPPS vesicles as compared to DPPC vesicles. Protrusions on the surface of DPPC proteoliposomes observed by atomic force microscopy suggested that oligomeric AnxA6 interacted with the vesicle membrane. Further work is needed to delineate possible functions of AnxA6 at its different localizations and ways of interaction with lipids.

Role of Sortilin and Matrix Vesicles in Nε-Carboxymethyl-Lysine-Induced Diabetic Atherosclerotic Calcification

BACKGROUND AND AIMS

To investigate the role of Sortilin and matrix vesicles (MVs) in Nε-Carboxymethyl-lysine (CML)-induced diabetic atherosclerotic calcification (AC).

METHODS

At human level, the correlation between Sortilin and CD9 (marker proteins of MVs) in serum MVs and CML in serum was explored by enzyme-linked immunosorbent assay (ELISA) detection and Pearson correlation analysis. After a diabetic apoE^-/- mouse model was constructed, the calcification of aorta and the expressions of related proteins under CML and MVs injection were observed by calcification staining, immunofluorescence staining, and Western blot. MVs levels released by smooth muscle cells (SMCs) under different treatments was detected by nanometer tracking analysis (NTA). After treating SMCs with MVs and Anti-Sortilin, cell calcification was observed by Alizarin red staining.

RESULTS

Serological analysis of patients showed that the concentrations of Sortilin and CD9 in serum MVs were positively correlated with the concentration of CML in serum. Animal experiments showed that CML could promote the progression of diabetic AC and the high expression of Sortilin in plaques. Diabetic apoE^-/- mouse tail vein injection of CML-induced SMCs-derived MVs obviously aggravated AC. Cell experiment results showed that a high concentration of CML significantly promoted the release of MVs from SMCs. MVs from this source could markedly worsen cell calcification, while the administration of GW4869 (a widely used extracellular vesicles biogenesis inhibitor) significantly reduced cell calcification. Finally, treatment of high concentrations of CML could also promote the recruitment of Sortilin to MVs, and administration of Anti-Sortilin could markedly reduce cell calcification caused by MVs.

CONCLUSION

We proved that CML not only affects the release of MVs from SMCs but also affects the recruitment of Sortilin to MVs, thereby promoting diabetic AC. This discovery may provide a new strategy for targeted prevention of vascular calcification in diabetes.

Cholesterol Regulates the Incorporation and Catalytic Activity of Tissue-Nonspecific Alkaline Phosphatase in DPPC Monolayers

Matrix vesicles (MVs) are a special class of extracellular vesicles that drive bone and dentin mineralization by providing the essential enzymes and ions for the nucleation and propagation of mineral crystals. Tissue-nonspecific alkaline phosphatase (TNAP) is an integral protein of MV membrane and participates in biomineralization by hydrolyzing extracellular pyrophosphate (PP_i), a strong mineralization inhibitor, and forming inorganic phosphate (P_i), necessary for the growth of mineral crystals inside MVs and their propagation once released in the extracellular matrix. MV membrane is enriched in cholesterol (CHOL), which influences the incorporation and activity of integral proteins in biologic membranes; however, how CHOL controls the incorporation and activity of TNAP in MV membrane has not yet been elucidated. In the present study, Langmuir monolayers were used as a MV membrane biomimetic model to assess how CHOL affects TNAP incorporation and activity. Surface pressure-area (π-A) isotherms of binary dipalmitoilphosphatidylcholine (DPPC)/CHOL monolayers showed that TNAP incorporation increases with CHOL concentration. Infrared spectroscopy showed that CHOL influences the conformation and orientation of the enzyme. Optical-fluorescence micrographs of the monolayers revealed the tendency of TNAP to incorporate into CHOL-rich microdomains. These data suggest that TNAP penetrates more efficiently and occupies a higher surface area into monolayers with a lower CHOL concentration due to the higher membrane fluidity. However, the quantity of enzyme transferred to solid supports as well as the enzymatic activity were higher using monolayers with a higher CHOL concentration due to increased rigidity that changes the enzyme orientation at the air-solid interface. These data provide new insights regarding the interfacial behavior of TNAP and CHOL in MVs and shed light on the biochemical and biophysical processes occurring in the MV membrane during biomineralization at the molecular level.

Src and ROCK Kinases Differentially Regulate Mineralization of Human Osteosarcoma Saos-2 Cells

Osteoblasts initiate bone mineralization by releasing matrix vesicles (MVs) into the extracellular matrix (ECM). MVs promote the nucleation process of apatite formation from Ca²⁺ and P_i in their lumen and bud from the microvilli of osteoblasts during bone development. Tissue non-specific alkaline phosphatase (TNAP) as well as annexins (among them, AnxA6) are abundant proteins in MVs that are engaged in mineralization. In addition, sarcoma proto-oncogene tyrosine-protein (Src) kinase and Rho-associated coiled-coil (ROCK) kinases, which are involved in vesicular transport, may also regulate the mineralization process. Upon stimulation in osteogenic medium containing 50 μg/mL of ascorbic acid (AA) and 7.5 mM of β-glycerophosphate (β-GP), human osteosarcoma Saos-2 cells initiated mineralization, as evidenced by Alizarin Red-S (AR-S) staining, TNAP activity, and the partial translocation of AnxA6 from cytoplasm to the plasma membrane. The addition of 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo [3,4-d] pyrimidine (PP2), which is an inhibitor of Src kinase, significantly inhibited the mineralization process when evaluated by the above criteria. In contrast, the addition of (R)-(+)-trans-4-(1-aminoethyl)-N-(4-pyridyl) cyclohexane carboxamide hydrochloride (Y-27632), which is an inhibitor of ROCK kinase, did not affect significantly the mineralization induced in stimulated Saos-2 cells as denoted by AR-S and TNAP activity. In conclusion, mineralization by human osteosarcoma Saos-2 cells seems to be differently regulated by Src and ROCK kinases.

Proteomic profiling of extracellular vesicles released from vascular smooth muscle cells during initiation of phosphate-induced mineralization

Purpose/Aim: Elevated serum phosphate is one of the major factors contributing to vascular calcification. Studies suggested that extracellular vesicles released from vascular smooth muscle cells significantly contribute to the initiation and progression of this pathology. Recently, we have demonstrated that elevated phosphate stimulates release of extracellular vesicles from osteogenic cells at the initiation of the mineralization process. Here, we used MOVAS cell line as an in vitro model of vascular calcification to examine whether vascular smooth muscle cells respond to high phosphate levels in a similar way and increase formation of extracellular vesicles.

MATERIALS AND METHODS

Vesicles residing in extracellular matrix as well as vesicles released to culture medium were evaluated by nanoparticle tracking analyses. In addition, using mass spectrometry and protein profiling, protein composition of extracellular vesicles released by MOVAS cells under standard growth conditions and upon exposure to high phosphate was compared.

RESULTS

Significant increase of the number of extracellular vesicles was detected after 72 h of exposure of cells to high phosphate. Elevated phosphate levels also affected protein composition of extracellular vesicles released from MOVAS cells. Finally, the comparative analyses of proteins in extracellular vesicles isolated from extracellular matrix and from conditioned medium identified significant differences in protein composition in these two groups of extracellular vesicles.

CONCLUSIONS

Results of this study demonstrate that exposure of MOVAS cells to high phosphate levels stimulates the release of extracellular vesicles and changes their protein composition.

Extracellular vesicle and mesenchymal stem cells in bone regeneration: recent progress and perspectives

Transplanting mesenchymal stem cells (MSCs) has been widely perceived as an ideal treatment for bone repair and regeneration, owing to their differential potential. However, researchers found that very few intravenous MSCs could stay in the target tissue, whereas the majority of them are trapped in liver, spleen, and lung, largely reducing its therapeutic effects. Recently, extracellular vesicles (EVs) have attracted increased attention due to their function in bone repair and advantages over traditional cell therapy. Also, MSCs-derived EVs are likely to achieve the osteogenic goal via modulating the cells and cytokines involved in bone metabolism. This review aims at summarizing the function of EVs and MSCs in bone metabolism and regeneration. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 243-250, 2019.

Matrix Vesicles-Containing Microreactors as Support for Bonelike Osteoblasts to Enhance Biomineralization

Therapeutic cell mimicry aims to provide a source of cell-like assemblies, which exhibit the core structural or functional properties of their natural counterparts with broad envisioned applications in biomedicine. Bone tissue engineering (BTE) aims at promoting and inciting the natural healing process of, for instance, critically sized bone defects. Microreactors designed to co-assemble with biological bone-forming osteoblasts like SaOS-2 cells to start biomineralization are reported for the first time. The alginate-based microparticles are equipped with active alkaline phosphatase-loaded artificial liposomes or SaOS-2-derived matrix vesicles (MVs). Spheroids assembled from SaOS-2 cells and microreactors not only exhibit higher cell viability, but also show enhanced biomineralization when MVs are present. The active biomineralization stimulation of the microreactors is illustrated by colorimetric calcium quantification and micro-computed tomography. These findings show the promising potential of applying cell mimicry in BTE.

The role of extracellular vesicles in biomineralisation: current perspective and application in regenerative medicine

Extracellular vesicles comprise a heterogenous population of exosomes and microvesicles that have critical roles in intercellular signalling and tissue development. These complex particles have been implicated as mediators of the therapeutic effects of stem cells via the transfer of an assorted cargo of proteins and nucleic acids, which can modulate inflammation and enhance endogenous regeneration in a range of tissues. In addition, extracellular vesicles have the capacity to be loaded with therapeutic molecules for targeted delivery of pharmaceuticals. The versatility, biostability and biocompatibility of extracellular vesicles make them appealing for regenerative medicine and may endow considerable advantages over single molecule approaches. Furthermore, since production can be optimised and assessed ex vivo, extracellular vesicles present a decreased risk of neoplastic transformation when compared with cell-based methods. To date, the contribution of vesicles to tissue development has perhaps been most comprehensively defined within hard tissues, such as endochondral bone, where they were first identified in 1969 and henceforth referred to as matrix vesicles. Within developing bone, vesicles function as vehicles for the delivery of pro-osteogenic factors and initiate early nucleational events necessary for matrix mineralisation. However, advancement in our understanding of the biogenesis and characterisation of matrix vesicles has occurred largely in parallel to associated developments in wider extracellular vesicle biology. As such, there is a requirement to align current understanding of matrix vesicle-mediated mineralisation within the context of an evolving literature surrounding exosomes and microvesicles. In this review, we present an overview of current progress and opinion surrounding the application of vesicles in regenerative medicine with a primary focus on their potential as an acellular approach for enhancing hard tissue regeneration. This is balanced with an assessment of areas where further development is required to maximise their application for regenerative medicine.

High magnesium prevents matrix vesicle-mediated mineralization in human bone marrow-derived mesenchymal stem cells via mitochondrial pathway and autophagy

Magnesium, as a physiological calcium antagonist, plays a vital role in the bone metabolism and the balance between magnesium and calcium is crucial in bone physiology. We recently demonstrated that matrix mineralization in human bone marrow-derived mesenchymal stem cells (hBMSCs) can be suppressed by high Mg²⁺ . However, a complete understanding of the mechanisms involved still remains to be elucidated. As mitochondrial calcium phosphate granules depletion manifests concurrently with the appearance of matrix vesicles (MVs) and autophagy are associated with matrix mineralization, we studied the effect of high extracellular Mg²⁺ on these pathways. Our results first demonstrated that high Mg²⁺ has a significant inhibitory effect on the generalization of extracellular mineral aggregates and the expression of collagen 1 along which the mineral crystals grow. Transmission electron microscope results showed that less amount of MVs were observed inside hBMSCs treated with high Mg²⁺ and high Mg²⁺ inhibited the release of MVs. In addition, high Mg²⁺ significantly suppressed mitochondrial Ca²⁺ accumulation. Autophagy is promoted as a response to osteogenesis of hBMSCs. High Mg²⁺ inhibited the level of autophagy upon osteogenesis and autophagy inhibitor 3-MA significantly suppressed mineralization. Exogenous ATP can reverse the inhibitory effect of high Mg²⁺ by increasing the level of autophagy. Taken together, our results indicate that high Mg²⁺ may modulate MVs-mediated mineralization via suppressing mitochondrial Ca²⁺ intensity and regulates autophagy of hBMSCs upon osteogenesis, resulting in decreased extracellular mineralized matrix deposition. Our results contribute to the understanding of the role of magnesium homeostasis in osteoporosis and the design of magnesium alloys.

Extracellular Vesicles As Mediators of Cardiovascular Calcification

Involvement of cell-derived extracellular particles, coined as matrix vesicles (MVs), in biological bone formation was introduced by Bonucci and Anderson in mid-1960s. In 1983, Anderson et al. observed similar structures in atherosclerotic lesion calcification using electron microscopy. Recent studies employing new technologies and high- resolution microscopy have shown that although they exhibit characteristics similar to MVs, calcifying extracellular vesicles (EVs) in cardiovascular tissues are phenotypically distinct from their bone counterparts. EVs released from cells within cardiovascular tissues may contain either inhibitors of calcification in normal physiological conditions or promoters in pathological environments. Pathological conditions characterized by mineral imbalance (e.g., atherosclerosis, chronic kidney disease, diabetes) can cause smooth muscle cells, valvular interstitial cells, and macrophages to release calcifying EVs, which contain specific mineralization-promoting cargo. These EVs can arise from either direct budding of the cell plasma membrane or through the release of exosomes from multivesicular bodies. In contrast, MVs are germinated from specific sites on osteoblast, chondrocyte, or odontoblast membranes. Much like MVs, calcifying EVs in the fibrillar collagen extracellular matrix of cardiovascular tissues serve as calcification foci, but the mineral that forms appears different between the tissues. This review highlights recent studies on mechanisms of calcifying EV formation, release, and mineralization in cardiovascular calcification. Furthermore, we address the MV–EV relationship, and offer insight into therapeutic implications to consider for potential targets for each type of mineralization.

Ultrastructural, Elemental and Mineralogical Analysis of Vascular Calcification in Atherosclerosis

Over the past few decades, remarkable progress has been achieved in terms of understanding the molecular and cellular mechanisms of atherosclerotic vascular calcification and the important role of matrix vesicles in initiating and propagating pathologic tissue mineralization has been widely recognized. Despite these recent advances, however, no definitive data are currently available regarding the texture and composition of the minerals that grow in the vessel wall during the course of the disease. Using different electron microscopy imaging and analysis, we demonstrate that vascular cells can produce and secrete more than one type of matrix vesicles which act as sites for initial mineral deposition independently of their structural features. Our results reveal that apatite formation in the atherosclerotic lesions of the human aorta occur through the deposition of amorphous calcium phosphate that matures over time, transforms into crystalline hydroxyapatite, and radiates towards the lumen of the vesicles, finally forming the calcified spherules. Elemental and mineralogical analyses also demonstrate that the presence of mature and stable amorphous calcium phosphate deposits in the affected tissues is linked to the incorporation of magnesium, which probably delay the conversion to the crystalline phase. Though more rarely, the presence of calcium oxalate crystals has been also documented.

End stage renal disease-induced hypercalcemia may promote aortic valve calcification via Annexin VI enrichment of valve interstitial cell derived-matrix vesicles

Patients with end‐stage renal disease (ESRD) have elevated circulating calcium (Ca) and phosphate (Pi), and exhibit accelerated progression of calcific aortic valve disease (CAVD). We hypothesized that matrix vesicles (MVs) initiate the calcification process in CAVD. Ca induced rat valve interstitial cells (VICs) calcification at 4.5 mM (16.4‐fold; p < 0.05) whereas Pi treatment alone had no effect. Ca (2.7 mM) and Pi (2.5 mM) synergistically induced calcium deposition (10.8‐fold; p < 0.001) in VICs. Ca treatment increased the mRNA of the osteogenic markers Msx2, Runx2, and Alpl (p < 0.01). MVs were harvested by ultracentrifugation from VICs cultured with control or calcification media (containing 2.7 mM Ca and 2.5 mM Pi) for 16 hr. Proteomics analysis revealed the marked enrichment of exosomal proteins, including CD9, CD63, LAMP‐1, and LAMP‐2 and a concomitant up‐regulation of the Annexin family of calcium‐binding proteins. Of particular note Annexin VI was shown to be enriched in calcifying VIC‐derived MVs (51.9‐fold; p < 0.05). Through bioinformatic analysis using Ingenuity Pathway Analysis (IPA), the up‐regulation of canonical signaling pathways relevant to cardiovascular function were identified in calcifying VIC‐derived MVs, including aldosterone, Rho kinase, and metal binding. Further studies using human calcified valve tissue revealed the co‐localization of Annexin VI with areas of MVs in the extracellular matrix by transmission electron microscopy (TEM). Together these findings highlight a critical role for VIC‐derived MVs in CAVD. Furthermore, we identify calcium as a key driver of aortic valve calcification, which may directly underpin the increased susceptibility of ESRD patients to accelerated development of CAVD.

Cross-talk between Src kinases and Rho small GTPases regulates biomineralization and simplify imaging of the mineralization process

The process of physiological mineralization that occurs during bone ossification is a tightly regulated cascade of molecular events leading to formation of bony skeleton. Its deregulation associated with aging leads to pathological mineralization not only in osseous but also in soft tissues. Recent discoveries in the field of bone biology indicate the participation of the Src family of tyrosine kinases as well as the Rho family of small GTPases in mineral formation. Cross-talk between these two signaling pathways is activated during an early step of the mineralization process, and leads to reorganization of the cytoskeleton of mineralization-competent cells and to matrix vesicles release. The understanding of mechanisms of the cross-talk between Src kinases and Rho small GTPases that regulate the mineralization process is crucial for the development of novel simply imaging techniques and therapeutic strategies in relation to pathological mineralization.

Articular cartilage vesicles and calcium crystal deposition diseases

PURPOSE OF REVIEW

Articular cartilage vesicles (ACVs) are small extracellular vesicles that serve as foci of pathologic calcium crystal deposition in articular cartilage matrix. In this review, I have summarized the role of ACVs in calcium crystal formation and discuss recent findings that impact our understanding of the content, behavior, and origin of ACVs in healthy and diseased joints. The burgeoning interest in extracellular vesicles in other fields renders this a timely and relevant topic.

RECENT FINDINGS

I have highlighted recent studies demonstrating that some ACVs originate in the autophagic pathway in healthy articular chondrocytes. I have reviewed accumulating evidence that nonmineralizing functions of ACVs contribute to osteoarthritis. I have also discussed new work supporting a role for extracellular vesicles in interleukin-1β-induced mineralization and in mediating the catabolic effects of synovial inflammation in osteoarthritis.

SUMMARY

We are making slow and steady progress in understanding the origin and function of ACVs and other relevant extracellular vesicles in arthritis. Further work in this interesting area is warranted.

Membranes and pathophysiological mineralization

Vascular calcification accompanies the pathological process of atherosclerotic plaque formation. Artery calcification results from trans-differentiation of vascular smooth muscle cells (VSMCs) into cells resembling mineralization-competent cells such as osteoblasts and chondrocytes. The activity of tissue-nonspecific alkaline phosphatase (TNAP), a GPI-anchored enzyme necessary for physiological mineralization, is induced in VSMCs in response to inflammation. TNAP achieves its mineralizing function being anchored to plasma membrane of mineralizing cells and to the surface of their derived matrix vesicles (MVs), and numerous important reports indicate that membranes play a crucial role in initiating the crystal formation. In this review, we would like to highlight various functions of lipids and proteins associated to membranes at different stages of both physiological mineralization and vascular calcification, with an emphasis on the pathological process of atherosclerotic plaque formation.

Compounded PHOSPHO1/ALPL deficiencies reduce dentin mineralization

Phosphatases are involved in bone and tooth mineralization, but their mechanisms of action are not completely understood. Tissue-nonspecific alkaline phosphatase (TNAP, ALPL) regulates inhibitory extracellular pyrophosphate through its pyrophosphatase activity to control mineral propagation in the matrix; mice without TNAP lack acellular cementum, and have mineralization defects in dentin, enamel, and bone. PHOSPHO1 is a phosphatase found within membrane-bounded matrix vesicles in mineralized tissues, and double ablation of Alpl and Phospho1 in mice leads to a complete absence of skeletal mineralization. Here, we describe mineralization abnormalities in the teeth of Phospho1(-/-) mice, and in compound knockout mice lacking Phospho1 and one allele of Alpl (Phospho1(-/-);Alpl(+/-) ). In wild-type mice, PHOSPHO1 and TNAP co-localized to odontoblasts at early stages of dentinogenesis, coincident with the early mineralization of mantle dentin. In Phospho1 knockout mice, radiography, micro-computed tomography, histology, and transmission electron microscopy all demonstrated mineralization abnormalities of incisor dentin, with the most remarkable findings being reduced overall mineralization coincident with decreased matrix vesicle mineralization in the Phospho1(-/-) mice, and the almost complete absence of matrix vesicles in the Phospho1(-/-);Alpl(+/-) mice, whose incisors showed a further reduction in mineralization. Results from this study support prominent non-redundant roles for both PHOSPHO1 and TNAP in dentin mineralization.

Tooth root dentin mineralization defects in a mouse model of hypophosphatasia

Tissue-nonspecific alkaline phosphatase (TNAP) is expressed in mineralizing tissues and functions to reduce pyrophosphate (PP(i) ), a potent inhibitor of mineralization. Loss of TNAP function causes hypophosphatasia (HPP), a heritable disorder marked by increased PP(i) , resulting in rickets and osteomalacia. Tooth root cementum defects are well described in both HPP patients and in Alpl(-/-) mice, a model for infantile HPP. In Alpl(-/-) mice, dentin mineralization is specifically delayed in the root; however, reports from human HPP patients are variable and inconsistent regarding dentin defects. In the current study, we aimed to define the molecular basis for changes in dentinogenesis observed in Alpl(-/-) mice. TNAP was found to be highly expressed by mature odontoblasts, and Alpl(-/-) molar and incisor roots featured defective dentin mineralization, ranging from a mild delay to severely disturbed root dentinogenesis. Lack of mantle dentin mineralization was associated with disordered and dysmorphic odontoblasts having disrupted expression of marker genes osteocalcin and dentin sialophosphoprotein. The formation of, initiation of mineralization within, and rupture of matrix vesicles in Alpl(-/-) dentin matrix was not affected. Osteopontin (OPN), an inhibitor of mineralization that contributes to the skeletal pathology in Alpl(-/-) mice, was present in the generally unmineralized Alpl(-/-) mantle dentin at ruptured mineralizing matrix vesicles, as detected by immunohistochemistry and by immunogold labeling. However, ablating the OPN-encoding Spp1 gene in Alpl(-/-) mice was insufficient to rescue the dentin mineralization defect. Administration of bioengineered mineral-targeting human TNAP (ENB-0040) to Alpl(-/-) mice corrected defective dentin mineralization in the molar roots. These studies reveal that TNAP participates in root dentin formation and confirm that reduction of PP(i) during dentinogenesis is necessary for odontoblast differentiation, dentin matrix secretion, and mineralization. Furthermore, these results elucidate developmental mechanisms underlying dentin pathology in HPP patients, and begin to explain the reported variability in the dentin/pulp complex pathology in these patients.

Annexin-mediated matrix vesicle calcification in vascular smooth muscle cells

In bone, osteoblasts and chondrocytes synthesize matrix vesicles (MVs) that interact with collagen to initiate calcification. MVs have been identified in human calcified arteries but are poorly characterized. The objective of this study is to determine the role of annexins and fetuin-A in MV formation and activity during calcification in bovine vascular smooth muscle cells (BVSMCs). BVSMCs were treated with control or calcification (high phosphorus) media, and cellular MVs were isolated by collagenase digestion and secreted MVs were isolated from cultured media by ultracentrifugation. The results showed that alkaline phosphatase (ALP) activity was significantly increased in MVs from calcified BVSMCs compared with noncalcified BVSMCs, as was annexin II and VI content and (45)Ca uptake. We also determined that MVs from calcifying BVSMCs could mineralize type I collagen but not type II collagen in the absence of cells in a dose- and time-dependent manner. Blockade of annexin calcium channel activity by K201 significantly decreased ALP activity and reduced the ability of the MVs to subsequently calcify on collagen, whether the K201 was added during or after MV formation. Furthermore, cellular MVs had significantly increased ability to calcify on collagen compared with secreted MVs, likely because of their increased ALP activity and annexin II content but low fetuin-A content. In conclusion, our results suggest that mineralization in VSMCs requires both active MVs and an interaction of the MVs with type I collagen, and both steps require annexin activity.

1alpha,25(OH)2D3 is an autocrine regulator of extracellular matrix turnover and growth factor release via ERp60 activated matrix vesicle metalloproteinases

Growth plate chondrocytes produce proteoglycan-rich type II collagen extracellular matrix (ECM). During cell maturation and hypertrophy, ECM is reorganized via a process regulated by 1alpha,25(OH)(2)D(3) and involving matrix metalloproteinases (MMPs), including MMP-3 and MMP-2. 1alpha,25(OH)(2)D(3) regulates MMP incorporation into matrix vesicles (MVs), where they are stored until released. Like plasma membranes (PM), MVs contain the 1alpha,25(OH)(2)D(3)-binding protein ERp60, phospholipase A(2) (PLA(2)), and caveolin-1, but appear to lack nuclear Vitamin D receptors (VDRs). Chondrocytes produce 1alpha,25(OH)(2)D(3) (10(-8)M), which binds ERp60, activating PLA(2), and resulting lysophospholipids lead to MV membrane disorganization, releasing active MMPs. MV MMP-3 activates TGF-beta1 stored in the ECM as large latent TGF-beta1 complexes, consisting of latent TGF-beta1 binding protein, latency associated peptide, and latent TGF-beta1. Others have shown that MMP-2 specifically activates TGF-beta2. TGF-beta1 regulates 1alpha,25(OH)(2)D(3)-production, providing a mechanism for local control of growth factor activation. 1alpha,25(OH)(2)D(3) activates PKCalpha in the PM via ERp60-signaling through PLA(2), lysophospholipid production, and PLCbeta. It also regulates distribution of phospholipids and PKC isoforms between MVs and PMs, enriching the MVs in PKCzeta. Direct activation of MMP-3 in MVs requires ERp60. However, when MVs are treated with 1alpha,25(OH)(2)D(3), PKCzeta activity is decreased and PKCalpha is unaffected, suggesting a more complex feedback mechanism, potentially involving MV lipid signaling.