The in vitro phosphorylation of the native rat incisor dentin phosphophoryns

Expression of phosphophoryn is sufficient for the induction of matrix mineralization by mammalian cells

Mineralized tissues such as dentin and bone assemble extracellular matrices uniquely rich in a variety of acidic phosphoproteins. Although these proteins are presumed to play a role in the process of biomineralization, key questions regarding the nature of their contributions remain unanswered. First, it is not known whether highly phosphorylated proteins alone can induce matrix mineralization, or whether this activity requires the involvement of other bone/dentin non-collagenous proteins. Second, it remains to be established whether the protein kinases that phosphorylate these acidic proteins are unique to cells responsible for producing mineralized tissues. To begin to address these questions, we consider the case of phosphophoryn (PP), due to its high content of phosphate, high affinity for Ca(2+), and its potential role in hydroxyapatite nucleation. We have created a model system of biomineralization in a cellular environment by expressing PP in NIH3T3 fibroblasts (which do not produce a mineralized matrix); as a positive control, PP was expressed in MC3T3-E1 osteoblastic cells, which normally mineralize their matrices. We show that expression of PP in NIH3T3 cells is sufficient for the induction of matrix mineralization. In addition, assessment of the phosphorylation status of PP in these cells reveals that the transfected NIH3T3 cells are able to phosphorylate PP. We suggest that the phosphorylation of PP is essential for mineral formation. The principle goal of this study is to enrich the current knowledge of mineralized tissue phosphorylation events by analyzing them in the context of a complete cellular environment.

Synthesis of a potentially bioactive, hydroxyapatite-nucleating molecule

A human phosphophoryn (PP) cDNA was previously cloned from immature root apex total RNA in our laboratory. This cDNA comprises 2,364 bp, encoding 788 amino acids. More than 80% of the sequences are arranged as (DSS)(n) (n = 1-16), DS, and NSS motifs. We hypothesize that the capability of PP to bind Ca(2+) and nucleate hydroxyapatite may depend on these repeated sequences. Two polypeptides were synthesized based on the human PP cDNA sequence to test the hypothesis. One polypeptide has the amino acid sequence DDPNSSDESNGNDD (synthetic polypeptide 1, SP1), which is from the N-terminal end of PP; the other polypeptide, DSKSDSSKSESDSS (synthetic polypeptide 2, SP2), is the PP repeated sequence motif. Phosphorylation of the polypeptides was accomplished by reacting them with adenosine triphosphate and casein kinases I and II. The ability of these molecules to cause mineralization was tested in a steady-state agarose gel system. The results show that phosphorylated SP2 (P-SP2) precipitated approximately 60% of the total Ca + PO(4) precipitated by PP. P-SP1 precipitated about 23% of that precipitated by PP and was similar to the amount precipitated in the control gel, that is, without added peptides. Transmission electron microscopy and X-ray diffraction analysis showed that the precipitate formed in the P-SP2-containing gel was hydroxyapatite. The capability of P-SP2 to nucleate Ca + PO(4) and precipitate hydroxyapatite is a result of the repeated sequence motif, which contains a high percentage of phosphorylated serine. This molecule could be used in the repair and regeneration of dental tissue.

A novel rat 523 amino acid phosphophoryn: nucleotide sequence and genomic organization

Phosphophoryns (PP), the major noncollagenous proteins (NCPs) in dentin, are believed to play a crucial role in mineral nucleation and hydroxyapatite growth during dentin mineralization. Previously we identified two mature rat PP transcripts, one coding for a 240 amino acid protein (designated as PP(240)) (H.H. Ritchie, L.-H. Wang, J. Biol. Chem. 271 (1996) 21695-21698), and another coding for a 171 amino acid protein (PP(171)) (H. Ritchie, L. Wang, Biochim. Biophys. Acta 1493 (2000) 27-32). We now have identified a third novel dentin sialoprotein (DSP)-PP cDNA transcript that encodes a 523 amino acid protein (PP(523)) with typical PP characteristics including DSS and DS motifs suitable as potential casein kinase I and II phosphorylation sites. Based on amino acid composition, the PP(523) protein product is identical to native rat HP2. We also show that the PP(523) sequence is identical to the corresponding genomic DNA sequence. Taken together, the existence of multiple DSP-PP transcripts, each significantly different from the other in net negative charge, suggests that dentin mineralization processes may be under fine-tune control by these PP protein isoforms.

Fluoride alters casein kinase II and alkaline phosphatase activity in vitro with potential implications for dentine mineralization

Dentine phosphoprotein (DPP), a major non-collagenous acidic protein of dentine, undergoes altered phosphorylation in vivo in the presence of high fluoride concentrations. This has major implications for the altered mineralization patterns found during fluorosis. In dentine, casein kinase II is involved in phosphorylating DPP, and alkaline phosphatase (ALP) is ascribed roles in the dephosphorylation of DPP, increasing the inorganic phosphate at the mineralization front and the removal of pyrophosphate. Here the influence of fluoride in vitro on the activity of purified casein kinase II and ALP and its relation to altered patterns of mineralization were examined. Kinetic analysis showed that casein kinase II activity was completely inhibited at 0.04 M NaF. Vmax when compared to the control assay was significantly decreased (P < 0.0001) between concentrations 4 x 10(-4)-4 x 10(-8) M NaF. Significant changes to the Km (P < 0.0001) were also observed. ALP activity was inhibited by NaF (0.09-9 x 10(-8) M), with Vmax significantly decreased (P < 0.0001) at 0.09 M NaF. Alterations in the activity of these enzymes in the presence of fluoride may in part explain the decreased phosphorylation observed in DPP isolated from fluorotic dentine and may aid understanding of the altered matrix mediated mineralization patterns found during fluorosis.

Enamel specific protein kinases and state of phosphorylation of purified amelogenins

Ameloblastic tissue samples from unerupted bone molars were used to prepare subcellular enamel protein kinase preparations, nuclear + plasma membrane, cytosolic and microsomal, and used in in vitro phosphorylation of purified 20 kDa bovine amelogenin in the presence of 32P-ATP. Both cytosolic and microsomal preparations can phosphorylate purified native amelogenins, the addition of Ca2+ slightly increased the microsomal enzyme activity or at least did not inhibit the activity, whereas the presence of Ca2+ substantially decreased the cytosolic kinase activity towards phosphorylation of amelogenins. A comparative analysis using the enamel microsomal kinase against osteopontin, dephosphorylated casein and bone sialoprotein showed no phosphorylation of the first two proteins, and only minor phosphorylation of the bone sialoprotein. Overall, the present work demonstrates for the first time that the protein kinase responsible for the phosphorylation of amelogenins is a novel kinase, which is not inhibited by Ca2+, unlike the microsomal protein kinase (casein kinase type-II) of bone which phosphorylates secretory proteins osteopontin and bone sialoprotein and is strongly CaZ+ inhibited. The direct phosphoserine analysis on the purified bovine 20 kDa amelogenin indicated the presence of 0.8 moles of phosphoserine/mole protein naturally occurring, consistent with the quantitative analysis of 14C-radiolabeling of phosphoserines by conversion to dehydroalanine and in situ reaction with the thiol agent, 14C-mercaptoethanol, 0.64 moles 14C-incorporated/mole 20 kDa amelogenin. The purified low Mramelogenins 5.3 kDa E4 (TRAP) and 7.2 kDa E3 (LRAP), were also derivatized by 14C-mercaptoethanol, providing 0.46 and 0.88 moles 14C-incorporated/mole respectively. Further studies of the 14C-radiolabeled E4 amelogenin by sequence analysis confirmed one site of label to be at position 16 from the N-terminal and hence provided a direct evidence for the naturally occurring phosphoserine residue at this position.

The presence of multiple rat DSP-PP transcripts

Phosphoproteins or phosphophoryns (PPs) are the most abundant (>50%) non-collagenous proteins (NCPs) in dentin. PPs bind to calcium and hydroxyapatite and are believed to play a crucial role in dentin mineralization. Dentin sialoprotein (DSP), a highly glycosylated protein, comprised 5-8% of NCPs in dentin. The coding sequences for these two major NCPs are known to be contiguously located (i.e. DSP-PP) at the cDNA and genomic DNA levels in both rat and mouse. Previous studies have demonstrated the presence of multiple DSP-PP transcripts in the total RNA of adult rat incisors. To further understand the nature of these multiple transcripts, we performed reverse transcription-PCR and obtained a PP cDNA variant which encoded a 171 amino acid peptide (PP(171)) that shares many of the same characteristics as that of the published rat PP(240) sequence [Ritchie, H.H. and Wang, L.-H., J. Biol. Chem. 271 (1996) 21695-21698]. Due to its reduced size, as compared to PP(240), this cDNA encodes a phosphorylated protein with a reduced negative charge that may differentially affect mineralization processes. We provide evidence that there are multiple DSP-PP transcripts with various sizes of PP sequences in rat.

In situ phosphorylation of bone and dentin proteins by the casein kinase II-like enzyme

Our previous studies suggested the possibility of extracellular phosphorylation of matrix phosphoproteins into more phosphorylated forms by mature odontoblasts and osteocytes (Mikuni-Takagi et al., 1995; Satoyoshi et al., 1995). To elucidate such phosphorylation of bone and dentin proteins, we developed a histochemical method using frozen sections to determine the sites of enzymatic processing by the casein kinase II-like enzyme. It was observed that proteins in bone, dentin, and predentin are phosphorylated by the endogenous enzyme when the tissue slices were incubated with [gamma-32P] GTP, suggesting that there are both substrates and the enzyme in these matrices. In vivo, phosphate donors, ATP and GTP, may be supplied through dentinal canals and osteocyte canaliculi. Immunohistochemical analysis of frozen sections showed that the extremely intense staining of phosphoserine residues by anti-phosphoserine antibodies appeared in dentin only after demineralization of the tissue samples. It implies that these phosphoserine residues become bound to mineral as soon as the phosphorylation is completed, thereby being inaccessible to the antibodies without demineralization. The data support our notion that the extracellular phosphorylation of dentin/bone proteins, regulated by the developmental stages of bone and dentin cells, occurs prior to matrix mineralization.

Effects of inositol hexasulphate, a casein kinase inhibitor, on dentine phosphorylated proteins in organ culture of mouse tooth germs

To study the effects of impaired protein phosphorylation on dentine formation and mineralization, inositol hexasulphate, an intracellular type I and type II casein kinase inhibitor, was used in an in vitro organotypic culture system. Mandibular first molar tooth germs were dissected from 18-day-old mouse embryos and cultured for 11 days with and without inositol hexasulphate at different concentrations. At 0.04-0.08 mM inhibitor, cellular alterations were not detected. Dentine displayed the characteristic purple-blue colour when Stains all, a specific stain for extracellular phosphoproteins, was used. At 0.1 mM, dentine failed to stain and mineralization did not occur, as seen from the von Kossa method. The presence of numerous lysosome-like vesicles inside cells indicated that the experiment was at the limits of cytotoxicity; higher concentrations induced severe cellular alterations. Therefore, quantitative radioautography was carried out on germs treated or not with the inhibitor at 0.1 mM. [33P]-phosphate incorporation showed that grain density in inhibited germs compared with that in control germs was about double in odontoblasts and half in the predentine/dentine compartment. In the presence of inositol hexasulphate the incorporation of [3H]serine into odontoblast cell bodies was unchanged between 2 and 24 h while in predentine/dentine, grain density was higher between 1 and 4 h, and reduced at 24 h. Both with [33P]phosphate and [3H]serine, labelling was seen throughout the porous dentine formed in vitro and not as a band located at the predentine/dentine junction, as is the case in vivo. With [3H]proline, in the presence of the inhibitor, a small reduction of grain density occurred in cell bodies, no significant difference was seen between 1 and 4 h in predentine/dentine, and more silver grains were present after 24 h both in cells and in the matrix. The radioautographic data support the view that the inhibitor interacts mostly with post-transductional phosphorylation and does not alter significantly other cell synthetic pathways and functions. Finally, the experiments presented here confirm that phophorylated proteins have a key role in dentine mineralization.

Protein kinase CK2 ("casein kinase-2") and its implication in cell division and proliferation

Protein kinase CK2 (also termed casein kinase-2 or -II) is a ubiquitous Ser/Thr-specific protein kinase required for viability and for cell cycle progression. CK2 is especially elevated in proliferating tissues, either normal or transformed, and the expression of its catalytic subunit in transgenic mice is causative of lymphomas. CK2 is highly pleiotropic: more than 160 proteins phosphorylated by it at sites specified by multiple acidic residues are known. Despite its heterotetrameric structure generally composed by two catalytic (alpha and/or alpha') and two non catalytic beta-subunits, the regulation of CK2 is still enigmatic. A number of functional features of the beta-subunit which could cooperate to the modulation of CK2 targeting/activity will be discussed.

Phosphorylation of the proteins of the extracellular matrix of mineralized tissues by casein kinase-like activity

The extracellular matrix of the connective tissue contains non-collagenous proteins (NCP) which are acidic in character. The NCP of mineralizing systems (bone, dentin) differ from those of the non-mineralizing systems (skin, tendon) in that the mineralized tissue NCP are frequently phosphorylated. The phosphorylated proteins have been implicated in various aspects of the mineralization process. Thus, it is of interest to consider the mechanism and regulation of phosphorylation of the major matrix NCP. The majority of the phosphorylation takes place at Ser or Thr residues embedded within acidic sequences, and therefore are targets for casein kinase I (CK1) or casein kinase II (CK2)-like kinases. CK1 and CK2 are distantly related members of the protein kinase family. They are ubiquitous, constitutively active, second-messenger-independent kinases. CK1 is found in a variety of isoforms, all homologous to the alpha-subunit of the protein kinase family. It acts as a monomer. The active form of CK2 is a tetrameric holoenzyme, with 2 alpha catalytic subunits and 2 beta regulatory subunits. The CK2 alpha has activity alone, but the holoenzyme is four- to five-fold that activity. CK2 can use either ATP or GTP as the phosphate donor, but CK1 can use only ATP. The CK2 activity which phosphorylates the mineralized tissue NCP appears to be localized to membrane-associated cell fractions, and is present in the endoplasmic reticulum and Golgi compartments in osteoblasts, where phosphorylation of the secreted proteins appears to take place as co- and post-translational processes. Data indicate that both alpha and beta subunits of the membrane-associated CK2 are isoforms of the cytosolic CK2 in the same cells. The CK1 has not been specifically localized. Studies of dephosphorylated NCP such as phosphophoryn (PP) have shown that CK1 will not phosphorylate dephosphorylated dPP unless prior phosphorylation with CK2 has been carried out. In turn, CK2 activity may be initiated only after an initial phosphorylation of one of the messenger-dependent kinases. Thus, the phosphorylation reactions in mineralized tissues may be a tightly regulated hierarchical or sequential cascade of intracellular phosphorylation events.

Characterization of human sublingual-gland protein kinase by phosphorylation of a peptide related to secreted proteins

Phosphoproteins in human saliva include proline-rich proteins, statherins, histatin 1 and cystatin SA-III. The presence of phosphate in these proteins is necessary for various functions in the mouth including calcium binding, inhibition of precipitation of calcium phosphate, inhibition of growth of hydroxyapatite crystals and adherence to hydroxyapatite. To elucidate the process of phosphorylation of these proteins, the phosphorylation of a peptide (APRP8) with an amino acid sequence identical to one of the phosphorylated sites in acidic proline-rich proteins by a kinase from the human sublingual gland was investigated. The kinase, which was highly labile, was purified 58-fold by fractionation of sublingual gland homogenate and gel filtration, but the enzyme was inactivated when further purification by chromatographic techniques commonly used for protein kinases was attempted. To compare the enzyme with other kinases, and to obtain information that could be used in its further purification, a characterization was undertaken. The enzyme required 10 mM Mg2+ for optimum activity, it had a KM of 0.09 mM for ATP and the KM for the peptide substrate APRP8 was 0.42 mM. It was not activated by cAMP or calmodulin, characteristics that are shared with casein kinases and mammary gland kinase. The sublingual kinase as well as casein kinase 2 were inhibited by heparin, but in other respects the two kinases had different properties. While casein kinase 2 is activated by polylysine and has optimal activity in 150 mM KCl, sublingual kinase was inhibited by polylysine and the addition of KCl. Moreover, casein kinase 2 can utilize both ATP and GTP as phosphoryl donors, but GTP was not a substrate for sublingual kinase. The sublingual kinase shared a substrate recognition sequence with mammary gland kinase, but, unlike that kinase, it could not utilize Ca2+ instead of Mg2+. While the sublingual kinase thus shared some properties with both casein kinase 2 and mammary gland kinase, distinct differences were also seen and the relationship to these enzymes remains to be determined. The characterization of the sublingual kinase will be useful in its further purification.

Identification of the phosphorylated sites of metabolically 32P-labeled osteopontin from cultured chicken osteoblasts

Osteopontin (OPN) is one of the major secretory phosphoproteins in both calcifying and non-calcifying tissues. Evidence has accumulated for the biological importance of the phosphoproteins and, in particular, the phosphate groups in bone formation, resorption, and calcification. The precise locations of the phosphate groups in the OPN molecule were determined by metabolically labeling OPN with 32P in cultured chicken osteoblasts, followed by purification to homogeneity. N-terminal sequencing showed a single sequence of WPVSKRQHAISA, consistent with that deduced from both cDNA, and previous amino acid sequencing of the protein isolated from chicken bone. Three 32P-labeled peptides were isolated by reverse-phase high performance liquid chromatography of thrombin-digested, 32P-labeled OPN. The N-terminal sequencing of each of these thrombin fragments gave single sequences as follows: WPVSKSRQHAIS, SHHTHRYHQDHVD, and ASKLRKAARKL, with approximate molecular masses of 5, 30, and 20 kDa. These data demonstrate that 32P was incorporated throughout the N- to C-terminal sequence of the protein. Thrombin specifically cleaved chicken OPN at two sites: between Arg-22 and Ser-23, which generated the 5-kDa N-terminal end fragment, and another between Lys-138 and Ala-139, which generated the 30- and 20-kDa fragments. To further define the exact locations of the phosphorylated amino acids and the surrounding amino acid sequences, OPN was digested with trypsin, which generated seven major 32P-labeled peptides whose amino acid sequences were determined. The phosphorylated peptide regions of osteopontin were identified as amino acids 8-18 (QHAIS*AS*S*EEK), 39-54 (LASQQTHYS*S*EENAD), 150-171 (LIEDDAT*AEVGDSQLAGLWLPK), 179-191 (ELAQHQSVENDSR), 194-205 (FDS*PEVGGDSK), 214-219 (ES*LASR), and 239-248 (HSIENNEVTR). The phosphorylated amino acid sites are followed by an asterisk (*). Of the seven identified phosphorylated peptide regions, three were localized on the N-terminal end of the osteopontin molecule (with five phosphorylated serines) and contained the sequence motifs that were phosphorylated by casein kinase II type(s), whereas the remaining four peptides are concentrated toward the C-terminal half of the molecule (with five phosphorylated residues) and contained recognition motifs for other kinases as well as casein kinase II.

Endoplasmic reticulum stress-inducible protein GRP94 is associated with an Mg2+-dependent serine kinase activity modulated by Ca2+ and GRP78/BiP

The 94-kDa glucose-regulated protein (GRP94) is a glycoprotein in the endoplasmic reticulum (ER). It has been characterized as a Ca2+-binding protein and a molecular chaperone. In this report we show that highly purified GRP94 exhibits an active Mg2+-dependent serine kinase activity (termed 94-kinase). The 94-kinase can be recovered from ER membrane fractions and is able to phosphorylate both the constitutive and stress-induced forms of GRP94, correlating with their induction kinetics. The 94-kinase activity is distinct from casein kinase II. In contrast to the heat-stable, Ca2+-dependent autophosphorylation activity recently reported for GRP94, the labile 94-kinase activity is inhibited by Ca2+. We determined that the phosphopeptide map of in vitro phosphorylated GRP94 by the 94-kinase resembles that of the in vivo phosphorylated GRP94. Further, the 94-kinase activity can be specifically stimulated by GRP78, a coregulated protein in the ER known to interact with GRP94.

The mechanism of beta-glycerophosphate action in mineralizing chick limb-bud mesenchymal cell cultures

Differentiating chick limb-bud mesenchymal cells plated in micromass culture form a cartilage matrix that can be mineralized in the presence of 4 mM inorganic phosphate (Pi), and 1 mM calcium. Previous studies showed that when beta-glycerophosphate (beta GP) is used in place of Pi, the mineral crystals formed are larger and differ in distribution. The present study shows that the difference in distribution is not associated with alterations in cell proliferation, protein synthesis, or with collagen, proteoglycan core protein, or alkaline phosphatase gene expression. Cultures with 2.5, 5, and 10 mM beta GP did show different levels of alkaline phosphatase activity, and in the presence of low (0.3 mM) Ca had different Pi contents (4, 6 and 9 mM, respectively), indicating that the increase in CaxP product may in part be responsible for the altered pattern of mineralization. However, cultures with beta GP in which alkaline phosphatase activity was inhibited with levamisole still had an altered mineral distribution as revealed by Fourier transform-infrared (FT-IR) microspectroscopy. The presence of a casein kinase II-like activity in the mineralizing cultures, the ability of specific inhibitors of this enzyme to block mineralization, and the known ability of beta GP to block phosphoprotein phosphatase activity suggests that altered patterns of matrix protein phosphorylation may influence mineral deposition in these cultures.

Protein kinases of cultured osteoblasts: selectivity for the extracellular matrix proteins of bone and their catalytic competence for osteopontin

The enzyme activities of the major kinases found within the cytosolic and microsomal fractions of embryonic avian calvaria osteoblasts were assayed for their specificity for various noncollagenous extracellular matrix (ECM) proteins of bone. At least 6 proteins with M(r)'s of 66, 58, 50, 36, 30, and 22 kD out of more than 30 of the noncollagenous proteins of the bone ECM were phosphorylated by the kinase(s) found in both osteoblast cellular fractions. The purification and N-terminal sequence analysis of three of the above proteins, M(r)'s 66 and 58 kD (+50 kD), identified them as chicken bone sialoprotein (BSP) and osteopontin (OPN), respectively. Heparin, a specific inhibitor of factor-independent protein kinase (FIPK) activity, blocked the phosphorylation of all six ECM proteins by the microsomal kinase(s) but only inhibited the phosphorylation of the 66, 50, and 36 kD by the cytosolic enzyme(s). Casein kinase II (a known FIPK) showed a similar phosphorylation pattern of the same bone ECM proteins as the FIPK(s) found in osteoblast cell extracts, while purified cyclic adenosine monophosphate (cAMP)-dependent protein kinase did not phosphorylate any of the ECM proteins. Use of dephosphorylated casein showed that in comparison with casein kinase II, casein was a poor substrate for the FIPK found in the osteoblast cellular extracts. Further studies, using FIPK(s) of osteoblasts and purified chicken OPN or bacterially produced recombinant murine OPN as a substrate, showed that both species of OPN were excellent substrates for the FIPK(s) found in osteoblasts. The phosphorylation of the purified chicken and recombinant mouse OPNs were evaluated by quantitative analysis using commercially available protein kinases. cAMP-dependent kinase showed no phosphorylation of either protein, and cyclic guanodine monophosphate (cGMP)-dependent kinase and protein kinase C incorporated 1.2 and 0.5 mol phosphate/mol OPN, respectively. However, both chicken and mouse OPNs were significantly phosphorylated by casein kinase II (9.3 and 9.0 mol of phosphate/mol of OPN, respectively). These results demonstrate that the noncollagenous proteins of the bone ECM, and in particular OPN, are predominantly phosphorylated by FIPK(s), and this class of kinase is the major enzyme found within the microsomal fraction of osteoblasts.

Sequence determination of an extremely acidic rat dentin phosphoprotein

The mineralization process associated with the conversion of predentin to dentin is believed to be initiated and controlled by a set of acidic regulatory noncollagenous proteins (NCPs) which include phosphophoryn, the major NCP in dentin. Phosphophoryn binds tightly to collagen and is believed to initiate the formation of apatite crystals which play a central role in the mineralization process. During the process of analyzing the 3' end of an odontoblast-specific cDNA which codes for dentin sialoprotein (Ritchie, H. H., Hou, H., Veis, A., and Butler, W. T. (1994) J. Biol. Chem. 269, 3698-3702), we discovered a 801-base pair open reading frame. This downstream open reading frame encodes a putative leader sequence and a very acidic mature protein sequence having a deduced amino acid composition containing high percentages of both Ser (43%) and Asp (31%) residues which closely coincides with the amino acid composition of phosphophoryns from human, bovine, rat, and rabbit (i. e. Asp (30-40%) and Ser (38-50%)). This newly identified cDNA therefore encodes a protein with characteristics similar to phosphophoryn. Here we present the cDNA sequence, the deduced amino acid sequence, and the prospective Ser residue-specific casein kinase I and II phosphorylation sites for this putative phosphophoryn.

Phosphorylation of purified bovine bone sialoprotein and osteopontin by protein kinases

The large number of covalently bound phosphates on the extracellular phosphoproteins osteopontin (OPN) and bone sialoprotein (BSP) have been implicated in biological functions such as mineral deposition and osteoclast binding. In the present study the state of phosphorylation of BSP and OPN was evaluated by in vitro 32P labeling using a series of protein kinases and quantification. Both the purified bovine BSP and OPN were radiolabeled by [32P]ATP and factor-independent protein kinase. Quantification of 32P radioactivity incorporated on dephosphorylated BSP and OPN provided 6.6 and 8.9 mol of phosphate incorporated/mol, respectively. Native OPN incorporated 1.07 and BSP 2.46 mol of phosphate/mol by factor-independent protein kinase. These data led to calculations that OPN and BSP, respectively, contain 7.83 and 4.14 mol of phosphate/mol in their natural state. Thrombin digests of 32P-labeled BSP showed radioactivity to be associated with fragment of approximately molecular mass values 30 kDa (N-terminal half), with no observable radioactivity associated with the 40-kDa fragment (C-terminal half). Similar experiments with 32P-labeled OPN provided two radiolabeled thrombin fragments, with molecular mass 30 kDa (N-terminal half) and 20 kDa (C-terminal half), both were radioactive. The major phosphorylation was associated with the N-terminal half containing 7.0 mol of phosphate, and 1.9 mol of phosphate were associated with the C-terminal half. Additional experiments of in vitro phosphorylation of OPN and BSP by several other known protein kinases were carried out. cAMP-dependent protein kinase showed no phosphorylation of OPN or BSP, while protein kinase C and cGMP-dependent protein kinase led to minor phosphorylation, each of the latter introduced about 1 mol of phosphate/mol of OPN and BSP molecule.

Enzymes in mineralizing systems: state of the art

The hallmark of biological mineralization is the precise regulation of mineral deposition in space and time. The cells which produce mineralized tissues are themselves controlled by developmental programs and hormonal signals which result in regulation of gene expression and modulation of protein function. These signals are transduced into changes in enzyme levels and/or activity. Upon activation, cellular enzymes then act to synthesize the organic matrix and process it extracellularly, utilize metabolic energy to transport ions from the blood to the matrix, and to initiate the mineralization cascade. The first enzyme activity described in mineralizing tissues was alkaline phosphatase and it is still the best characterized enzyme in the mineralization process. Yet, important questions about the role of this protein remain unanswered, and it continues to occupy a central focus in mineralized tissue investigation. Other phosphatases, including protein tyrosine phosphatases are important in regulating tyrosine kinase mediated signals. Investigators have now begun to look closely at several groups of kinases which are also important for proper mineralization. As peptide hormones are important modulators of mineralized tissues, protein kinase A has always been presumed to play a key role in phosphorylating intracellular proteins. There is also considerable interest in protein kinase C, as well as tyrosine kinases in mineralized tissue signal transduction. Another group of kinases important in mineralized tissues are the enzymes which phosphorylate the matrix phosphoproteins. Of these, casein kinase II appears to be involved in intracellular and extracellular protein phosphorylation. Several enzymes present in the premineralized matrix are thought to be significant in triggering mineralization. Alkaline phosphatase may act at this level, but new data also suggests that metalloproteases and gelatinases, by modifying or digesting matrix components, may be important in the initiation of calcification.

Characterization and partial purification of microsomal casein kinase II from osteoblast-like cells: an enzyme that phosphorylates osteopontin and phosphophoryn

Microsomal casein kinase II (mCKII) is a membrane-bound enzyme present in the microsomal fractions of ROS 17/2.8 osteoblast-like cells. It phosphorylates acidic matrix phosphoproteins such as phosphophoryn and osteopontin. Addition of 1.0% Nonidet P-40 facilitates extraction of the optimum amount of detergent-solubilized and -activated enzyme from microsomal fractions. mCKII was partially purified over 3000-fold by sequential chromatography over DEAE-cellulose and heparin-agarose. SDS-polyacrylamide gels, showed that mCKII contained 43 kDa and 31 kDa polypeptides, corresponding to the alpha- and beta-subunits of the enzyme, respectively. The alpha subunit was identified by anti-CKII antiserum and the beta subunit, by its ability to undergo autophosphorylation. The enzyme was inhibited by 50% with 0.4 micrograms/ml heparin and stimulated by 100% with 1.0 mM spermine when casein was used as a substrate. The phosphorylation of phosphophoryn was reduced to 50% by 0.8 micrograms/ml heparin, but was increased to 2-2.5 fold by 5 to 15 mM spermine, which may be due to substrate-directed effects. Kinetic analysis showed that the apparent Km values for phosphophoryn (0.39 microM) and for osteopontin (2.1 microM) were lower than that for casein (21.3 microM). Vmax values of phosphophoryn and osteopontin were 2.2-fold and 4.6-fold higher than that of casein. Using the ratio Vmax/Km as a measure of kinetic specificity, osteopontin and phosphophoryn appear to be the more specific substrates than casein for mCKII. Thus, both proteins can be considered as physiological substrates for mCKII.

Extracellular processing of bone and dentin proteins in matrix mineralization

There are two steps in the process of matrix-mediated bone and dentin mineralization. First, as in other soft tissues, osteoblasts/odontoblasts synthesize collagenous matrices and second, mineral deposits in these matrices at a location distant from the cells that synthesized the matrices. We suggest a sequence of events that lead the matrix to mineralization: the phosphoproteins of bone and dentin are posttranslationally processed by limited proteolysis, then they are extracellularly processed into a more phosphorylated species that, we believe, facilitates mineralization. Our in situ phosphorylation experiments done with [gamma-32P] GTP suggest the existence of extracellular phosphorylation by a casein kinase II (CKII)-like enzyme, the enzyme known to phosphorylate most of the phosphate residues in dentin phosphophoryn and bone sialoproteins (osteopontin and BSP II).

The membrane associated kinases which phosphorylate bone and dentin extracellular matrix phosphoproteins are isoforms of cytosolic CKII

Bone and dentin contain many phosphoproteins in their extracellular matrix. It has been postulated that the anionic character of these proteins, and the presence of phosphate groups in particular, is important for various functions related to biomineralization. Phosphophoryns (PP), are the most highly phosphorylated dentin matrix components. However, the tissue form of PP can be further phosphorylated in vitro by cytosolic and membrane associated-endogenous messenger-independent kinases from osteoblast-like cells. To examine the kinases, a 2-dimensional zymogram technique has been developed for the detection of casein kinase II (CKII) activity of purified kinases using intact PP as the substrate. After isolation by subcellular fractionation and ion-exchange chromatographic techniques, the enzymes are electrophoresed on an isoelectric focusing gel in the absence of SDS and disulfide bond breaking reagents. This first dimension gel is then layered on the zymogram gel containing PP + SDS. After electrophoresis, the gels are incubated in 250-500 microCi [gamma-32P] ATP. Autoradiography then detects kinase activity. Comparison of the UMR 106 CKII cytosolic and membrane-bound fractions showed that they were different in M(r) and focusing pH, suggesting the presence of CKII isoforms. CKI, which could phosphorylate the native PP in vitro, could not phosphorylate a dephosphorylated preparation (dPP). However, if the dPP was first exposed to CKII and unlabeled ATP then reacted with CKI and [gamma-32P]ATP, the PP was phosphorylated. This prerequisite phosphorylation by CKII indicates that in vivo PP phosphorylation probably occurs in a series of regulated steps as a co- or post-translational process.

Microsomal casein kinase II in endoplasmic reticulum- and Golgi apparatus-rich fractions of ROS 17/2.8 osteoblast-like cells: an enzyme that modifies osteopontin

Osteopontin is an acidic phosphoprotein containing casein kinase II (CKII) phosphorylatable sites and an acidic amino acid cluster. The metabolically 32P-labelings of both serines and threonines in vitro in osteopontin immunoprecipitated from rat osteoblast-like ROS 17/2.8 cells may suggest that casein kinase II catalyzes this modification. The enzyme occurs in microsomal fractions of rat osteoblast-like ROS 17/2.8 cells. Subcellular fractions containing endoplasmic reticulum and Golgi apparatus were isolated by differential centrifugation and were identified according to their ultrastructures and the presence of marker enzymes such as glucose-6-phosphatase and thiamine pyrophosphatase, respectively. both fractions phosphorylated the partially dephosphorylated osteopontin and the specific substrate peptide RRREEETEEE. Endoplasmic reticulum-catalyzed peptide phosphorylation was 2.7 times lower than that of Golgi although both endoplasmic reticulum- and Golgi-catalyzed peptide reactions were 50% inhibited by 20 and 100 ng/ml heparin, respectively. Western blot analysis revealed that both fractions contained osteopontin and microsomal CKII. Furthermore, microsomal CKII was immunogold-labeled in endoplasmic reticulum and Golgi apparatus. Heparin inhibition and utilization of [gamma-32P]GTP as a phosphate donor by both fractions confirmed their capacity to phosphorylate osteopontin. The results suggest that microsomal CKII modifies the acidic matrix proteins during transportation. These matrix phosphoproteins may participate in the mineralization process of hard tissues.

Extracellular processing of dentin matrix protein in the mineralizing odontoblast culture

Odontoblasts that we prepared from bovine incisors produced a dentin-specific protein, phosphophoryn, and accumulated it in mineralized nodules. The time course of mineralization was detected by measuring osteocalcin and mineral in the nodules. The sequence of developmental expression of proteins in this mineralizing dentin cell culture is very similar to that in bone cells, suggesting a common mechanism for matrix mineralization in bone and dentin. Casein kinase II, which phosphorylates bone phosphoproteins and dentin phosphorylates bone phosphoproteins and dentin phosphophoryn, also emerges coinciding with the initiation of mineralization. Furthermore, we have detected extracellular phosphorylation by casein kinase II of a dentin protein of M(r) 60,000, which we recovered from the phosphophoryn fraction in CaCl2 precipitate.

Casein kinase localization in the endoplasmic reticulum of the ROS 17/2.8 cell line

Phosphophoryns (PPs) are phosphoproteins specific to the dentin matrix and are the major noncollagenous matrix proteins in rat incisor dentin. It has been hypothesized that their phosphate groups are important in dentin mineralization. PPs have many sequences which are substrates for membrane-associated endogenous messenger-independent kinases. The objective of this study was to localize the protein kinases involved in phosphorylating the PPs. Osteoblast-like ROS 17/2.8 cells, which secrete extracellular matrix phosphoproteins, were lysed. After removal of the nuclei and mitochondria by low-speed centrifugation, the membrane associated organelles were isolated at higher speed from the cytosol. The Golgi vesicle and rough microsome fractions were collected from 29-43.7% sucrose density gradients. Each fraction was tested for casein kinase II (CKII) activity using an in vitro phosphorylation assay with PPs as substrate. To characterize and confirm the nature of the components of the sucrose gradient fractions, the activities of specific enzymes such as N-acetylglucosamine galactosyltransferase and cytochrome c reductase, which are exclusively associated with the Golgi and rough microsomes, respectively, were determined. Electron microscopy of the isolated fractions confirmed the enzyme assay characterizations. CKII activity capable of phosphorylating the PP was found in the rough microsome fraction but not in the Golgi. Thus, phosphorylation of the secreted phosphoproteins would appear to take place in the endoplasmic reticulum as a cotranslational event.

Matrix mineralization and the differentiation of osteocyte-like cells in culture

Osteocyte-like cells were prepared by sequentially treating calvaria from newborn rats with collagenase and chelating agents. On a reconstituted gel of basement membrane components, cells from the third collagenase digest displayed a round shape and expressed the highest level of alkaline phosphatase with minimal osteocalcin deposition into the matrix. On the other hand, cells derived from the interior after EDTA treatment exhibited well-developed dendritic cell processes and expressed essentially no alkaline phosphatase. The latter population also showed quite distinct characteristics such as higher extracellular activities of casein kinase II and ecto-5'-nucleotidase and the extracellular accumulation of a large amount of osteocalcin associated with mineral. These diverse phenotypic and protein expressions as well as the sites from which each population of cells were recovered strongly suggest that we have isolated osteoblastic and osteocytic cells. Bone sialoprotein II was extracellularly phosphorylated by casein kinase II in osteocytic cells but not in osteoblastic cells. We discuss the possibility that differentiation of young osteocytes from osteoblasts may facilitate the biochemical sequence of mineral deposition in the bone matrix.

Characterization of protein kinases involved in dentinogenesis

Protein phosphorylation and dephosphorylation control many different cell functions as well as responses to internal and external signals. It has also been shown that highly phosphorylated acidic proteins have an important role in matrix mediated biomineralization, perhaps functioning as nucleators for crystal formation. Dentine phosphoprotein (DPP) is one of such proteins which is exclusively synthesized by the odontoblast cells and therefore a likely candidate to play a significant role in normal and abnormal dentine biomineralization. These studies are directed at characterizing the protein kinases involved in dentinogenesis and in particular the enzyme(s) responsible for DPP phosphorylation. In this report we present data which indicate that there are several different types of kinases in the odontoblast-enriched dental papilla mesenchyme (DPM), some of which can phosphorylate DPP, such as casein kinase I and II. However, a different DPP-kinase activity was identified. This enzyme(s) appears to be different from other reported kinases, and it is the only kinase that can phosphorylate both phosphorylated DPP and enzymatically dephosphorylated DPP.

Mineral-matrix interactions in bone and dentin

The bone, dentin, and cementum of the mature individual are comprised from a dense collagenous fiber network into which the carbonate-apatite mineral phase is deposited. It is hypothesized that a set of collagen-interactive acidic phosphoproteins are secreted by the osteoblasts, odontoblasts, and cementoblasts into the preformed collagenous matrix. These proteins then interact specifically with the collagen and nucleate apatite formation on and within the fibrils. These phosphoproteins may also regulate the morphology, rate of growth, and stability of the mineral phase crystals. The acidic matrix phosphoproteins may thus be considered as the crucial regulators of mineralization and tissue stability. In the dentin system, these regulatory proteins are synthesized, posttranslationally modified, and secreted in vesicles different from the collagen secretory vesicles. Mineralization occurs as the regulatory proteins are deposited on the preformed fibrils. This model requires testing in the bone system. In dentin, in the absence of tissue turnover, the resident phosphoproteins are degraded in situ over time, perhaps changing the properties of the tissue. Regulation of synthesis, secretory pathways and retention of integrity within the matrix are thus important areas for further investigation.