A facilitative role for carbonic anhydrase activity in matrix vesicle mineralization

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.

Rapid bioinspired mineralization using cell membrane nanofragments and alkaline milieu

Bone formationin vivooccurs in alkaline environment, which determines the optimal pK_aof phosphatases, the optimal amount of calcium for mineral precipitation, and the spherical shape of initial minerals. Manipulation of environmental pH forin vitrosynthesis of bone-like tissue, showed a markedly rapid mineralization with nanofragments and alkaline milieu.

DLX3-Dependent Regulation of Ion Transporters and Carbonic Anhydrases is Crucial for Enamel Mineralization

Patients with tricho-dento-osseous (TDO) syndrome, an ectodermal dysplasia caused by mutations in the homeodomain transcription factor DLX3, exhibit enamel hypoplasia and hypomineralization. Here we used a conditional knockout mouse model to investigate the developmental and molecular consequences of Dlx3 deletion in the dental epithelium in vivo. Dlx3 deletion in the dental epithelium resulted in the formation of chalky hypomineralized enamel in all teeth. Interestingly, transcriptomic analysis revealed that major enamel matrix proteins and proteases known to be involved in enamel secretion and maturation were not affected significantly by Dlx3 deletion in the enamel organ. In contrast, expression of several ion transporters and carbonic anhydrases known to play an important role in enamel pH regulation during maturation was significantly affected in enamel organs lacking DLX3. Most of these affected genes showed binding of DLX3 to their proximal promoter as evidenced by chromatin immunoprecipitation sequencing (ChIP-seq) analysis on rat enamel organ. These molecular findings were consistent with altered pH staining evidenced by disruption of characteristic pH oscillations in the enamel. Taken together, these results show that DLX3 is indispensable for the regulation of ion transporters and carbonic anhydrases during the maturation stage of amelogenesis, exerting a crucial regulatory function on pH oscillations during enamel mineralization. © 2016 American Society for Bone and Mineral Research.

Nanoanalytical Electron Microscopy Reveals a Sequential Mineralization Process Involving Carbonate-Containing Amorphous Precursors

A direct observation and an in-depth characterization of the steps by which bone mineral nucleates and grows in the extracellular matrix during the earliest stages of maturation, using relevant biomineralization models as they grow into mature bone mineral, is an important research goal. To better understand the process of bone mineralization in the extracellular matrix, we used nanoanalytical electron microscopy techniques to examine an in vitro model of bone formation. This study demonstrates the presence of three dominant CaP structures in the mineralizing osteoblast cultures: <80 nm dense granules with a low calcium to phosphate ratio (Ca/P) and crystalline domains; calcium phosphate needles emanating from a focus: "needle-like globules" (100-300 nm in diameter) and mature mineral, both with statistically higher Ca/P compared to that of the dense granules. Many of the submicron granules and globules were interspersed around fibrillar structures containing nitrogen, which are most likely the signature of the organic phase. With high spatial resolution electron energy loss spectroscopy (EELS) mapping, spatially resolved maps were acquired showing the distribution of carbonate within each mineral structure. The carbonate was located in the middle of the granules, which suggested the nucleation of the younger mineral starts with a carbonate-containing precursor and that this precursor may act as seed for growth into larger, submicron-sized, needle-like globules of hydroxyapatite with a different stoichiometry. Application of analytical electron microscopy has important implications in deciphering both how normal bone forms and in understanding pathological mineralization.

Expression profile of carbonic anhydrases in articular cartilage

Carbonic anhydrases (CAs), which catalyze the reversible reaction of carbonate hydration, are important for cartilage homeostasis. The full spectrum of CA activity of all 13 isoenzymes in articular cartilage is unknown. This study quantified the mRNA profile of CAs in rat articular cartilage, using quantitative polymerase chain reactions. Among the 13 functional CAs, CAs II, III, Vb, IX, XII and XIII were significantly expressed at mRNA level by the chondrocytes in articular cartilage. To verify these significantly expressed CAs in articular cartilage at protein level, immunohistochemistry was performed. While CAs III, Vb and XII distributed in the full-thickness of cartilage, including the calcified zone of cartilage, CA II was mainly localized in the proliferative zone of cartilage. CA IX was limited in the superficial zone of cartilage and CA XIII expressed in the superficial and partially mid zone. These results provide a framework for understanding individual CAs as well as the integrated CA family in cartilage biology, including matrix mineralization.

Proteomic characterization of biogenesis and functions of matrix vesicles released from mineralizing human osteoblast-like cells

Matrix vesicles (MVs), released by budding from apical microvilli of osteoblasts during bone formation and development, are involved in the initiation of mineralization by promoting the formation of hydroxyapatite in their lumen. To gain additional insights into MV biogenesis and functions, MVs and apical microvilli were co-isolated from mineralizing osteoblast-like Saos-2 cells and their proteomes were characterized using LC-ESI-MS/MS and compared. In total, 282 MV and 451 microvillar proteins were identified. Of those, 262 were common in both preparations, confirming that MVs originate from apical microvilli. The occurrence of vesicular trafficking molecules (e.g. Rab proteins) and of the on-site protein synthetic machinery suggests that cell polarization and apical targeting are required for the incorporation of specific lipids and proteins at the site of MV formation. MV release from microvilli may be driven by actions of actin-severing proteins (gelsolin, cofilin 1) and contractile motor proteins (myosins). In addition to the already known proteins involved in MV-mediated mineralization, new MV residents were detected, such as inorganic pyrophosphatase 1, SLC4A7 sodium bicarbonate cotransporter or sphingomyelin phosphodiesterase 3, providing additional insights into MV functions.

Induction and characterization of metallothionein in chicken epiphyseal growth plate cartilage chondrocytes

Following exposure to cadmium or zinc, chickens were sacrificed and the liver, kidney, and bone epiphyseal growth plates harvested. When cytosolic extracts of the growth plate cartilage were fractionated by gel filtration chromatography, a protein with high metal-binding capacity and low ultraviolet (UV) absorbance eluted in the same position as liver metallothionein (MT) and a MT standard. Cd or Zn treatment resulted in a 25-fold or 5-fold induction in growth plate MT, respectively. In liver the greatest level of MT induction was seen with short-term Cd exposures. In contrast, MT levels in the growth plate increased as the duration of Cd exposure increased. Induction of MT in growth plate chondrocyte cell cultures was observed for media Cd concentrations of > or = 0.1 microM and Zn concentrations of > or = 100 microM. Basal and inducible levels of MT declined through the culture period and were lowest in the terminally differentiated mineralized late stages of the culture. Alkaline phosphatase activity was also lowest in the late-stage cultures, while total cellular protein increased throughout the culture period. Treatment of chondrocytes with Zn prior to Cd exposure resulted in a protective induction of MT. Pre-treatment of chondrocytes with dexamethasone resulted in suppressed synthesis of MT upon Cd exposure and greater Cd toxicity. Both Cd and Zn resulted in significantly increased levels of MT mRNA in chondrocyte cell cultures. Dexamethasone treatment resulted in an approximate 2- to 3-fold increase in MT mRNA. This is contrary to the finding that MT protein levels were decreased by dexamethasone. The findings suggest that an increased rate of MT degradation in dexamethasone-treated and late-stage chondrocyte cultures may be associated with the terminally differentiated phenotype.

Metallothionein induction in calcifying growth plate cartilage chondrocytes

Metallothionein (MT) induction was studied in mineralizing cultures of chicken growth plate chondrocytes and quantitated using a Cd-saturation assay. In serum free media, MT induction was observed for Cd concentrations of 0.1 microM and greater and at Zn concentrations of 100 microM and greater. Supplementation of culture media with cysteine and/or methionine resulted in higher levels of MT induction and reduced toxicity during Cd exposure. Maximum MT induction appeared to coincide with the earliest culture stages during which important enzymes and matrix components are being synthesized. Of non-metal MT inducers tested, sodium butyrate caused a low level induction of MT while interleukin-1 had no effect on basal MT levels. 1,25-dihydroxyvitamin D increased MT induction. The steroid hormone dexamethasone caused a reduction in basal and induced MT levels. These findings suggest that MT regulation in growth plate chondrocytes differs significantly from what is known in other cell types and that this difference may be related to the mineralization of this tissue.

Effects of zinc and divalent cation chelators on ATP hydrolysis and Ca deposition by rachitic rat matrix vesicles

Elsewhere it has been shown that zinc is highly concentrated in the hypertrophic zone of epiphyseal cartilage. It has also been shown that zinc deficiency can result in abnormal bone development, suggesting a direct or indirect role for zinc in calcification. Because matrix vesicles have been implicated in the initiation of calcification, we tested the effect of zinc and its chelators, such as EGTA and phenanthroline, on ATP-dependent Ca uptake by rat matrix vesicles. EGTA pretreatment of matrix vesicles inhibited ATP-dependent Ca uptake by 50%. To see if zinc depletion by EGTA pretreatment is responsible for decreased levels of ATP-dependent Ca uptake, ZnCl2 concentrations, ranging from 5 to 100 mumol/L, were tested for their ability to restore Ca deposition. Zinc exerted a striking enhancing effect on ATP-dependent Ca uptake of both untreated and EGTA-pretreated matrix vesicles in a dose-dependent manner. A 50% activation occurs at about 16 mumol/L Zn2+. At 63 mumol/L Zn2+, there was a fourfold increase in Ca-depositing activity. Addition of an excess amount of phenanthroline relative to Zn2+ concentration to the reaction mixture failed to abolish activation of Ca uptake by Zn2+, indicating that the putative chelator-Zn2+ complex formation did not prevent activation. The observed partial inhibition of ATPase and the activation of ATP-dependent Ca uptake of Zn2+ suggest that, in addition to ATPase, some other Ca and/or Pi uptake activators responsive to Zn2+ treatment are present in mammalian matrix vesicles.