Cartilage nucleoside triphosphate (NTP) pyrophosphohydrolase. I. Identification as an ecto-enzyme

Bone formation controlled by biologically relevant inorganic ions: role and controlled delivery from phosphate-based glasses

The role of metal ions in the body and particularly in the formation, regulation and maintenance of bone is only just starting to be unravelled. The role of some ions, such as zinc, is more clearly understood due to its central importance in proteins. However, a whole spectrum of other ions is known to affect bone formation but the exact mechanism is unclear as the effects can be complex, multifactorial and also subtle. Furthermore, a significant number of studies utilise single doses in cell culture medium, whereas the continual, sustained release of an ion may initiate and mediate a completely different response. We have reviewed the role of the most significant ions that are known to play a role in bone formation, namely calcium, zinc, strontium, magnesium, boron, titanium and also phosphate anions as well as copper and its role in angiogenesis, an important process interlinked with osteogenesis. This review will also examine how delivery systems may offer an alternative way of providing sustained release of these ions which may effect and potentiate a more appropriate and rapid tissue response.

Treatment and management of pseudogout: insights for the clinician

Pseudogout and the associated calcium pyrophosphate dihydrate (CPPD)- crystal-related arthropathies are common conditions that present particular management problems in clinical practice as they often affect older patients with multiple medical comorbidities. The epidemiology, metabolic and endocrine disease associations, and routine investigations used in the diagnostic workup are briefly reviewed. Current treatment approaches that are mainly directed at relieving the symptoms of joint inflammation are outlined. Unlike gout, there are no agents available that have been shown to decrease crystal load in CPPD-related joint disease. Recent novel insights into the pathogenesis of crystal-induced joint inflammation and subsequent joint degeneration are also discussed. The potential of colchicine as a prophylactic agent in managing recurrent attacks and the likely mechanisms of its effects on the NACHT, LRR and PYD domains-containing protein 3 (NALP-3) inflammasome of the innate immune system are highlighted. The use of agents that directly target the inflammasome, in particular drugs which inhibit the interleukin 1 pathway, in the treatment of severe, refractory pseudogout is also discussed. Finally, there is particular emphasis on the likely pathogenic role of CPPD crystal deposition in degenerative joint disease and the use of targeted anticrystal therapies as potential disease-modifying drugs.

Parallel regulation of extracellular ATP and inorganic pyrophosphate: roles of growth factors, transduction modulators, and ANK

OBJECTIVE

Extracellular inorganic pyrophosphate (ePPi) is a key regulator of pathologic mineralization in articular cartilage. Articular chondrocytes generate ePPi by the transportation of intracellular PPi (iPPi) through transport mechanisms such as ANK or by the degradation of extracellular adenosine triphosphate (eATP) by ectoenzymes. Although numerous modulators of ePPi have been characterized, little is known about eATP elaboration in cartilage. We sought to determine (1) whether eATP is coordinately regulated with ePPi and (2) whether ANK transports ATP.

METHODS

Primary articular chondrocytes were treated with factors known to modulate ePPi levels including growth factors (TGFβ1 and IGF-1), anion channel inhibitors, and chemicals that alter adenylyl cyclase and protein kinase C activities. Additional chondrocyte monolayers were infected with adenovirus containing functional (Ad-ANK) or mutated (Ad-ANK mutant) ANK sequences. eATP levels were measured with a bioluminescent assay.

RESULTS

TGFβ1 enhanced eATP accumulation by 33%, whereas IGF-1 decreased eATP accumulation by 63% and attenuated TGFβ1-induced eATP release by 72%. Forskolin and probenecid diminished eATP accumulation by 55% and 89%. Phorbol-12-myristate-13-acetate increased eATP by 29%. Transfection of chondrocytes with Ad-ANK caused a 10-fold increase in eATP compared with control values.

CONCLUSION

Modulation of eATP by various factors paralleled their effects on ePPi production, suggesting a shared pathway of ePPi and eATP production and implicating ANK in eATP transport. As eATP directly contributes to pathologic mineralization in articular cartilage, understanding eATP regulation may lead to effective therapies for crystal-associated arthritis.

Spontaneous oscillation and mechanically induced calcium waves in chondrocytes

The characteristics of spontaneous calcium (Ca(2+)) oscillation and mechanically induced Ca(2+) waves in articular chondrocytes were studied. In some, but not all, chondrocytes in sliced cartilage and primary cultures, we observed spontaneous oscillation of intracellular Ca(2+) that never spread to adjacent cells. In contrast, a mechanical stimulus to a single cell by touching with a glass rod induced an increase of intracellular Ca(2+) that spread to neighboring cells in a wave-like manner, even though there was no physical contact between the cells. This indicated the release of some paracrine factor from the mechanically stimulated cells. Application of ultrasonic vibration also induced an oscillation of intracellular Ca(2+). The application of a uridine 5'-triphosphate (UTP), UTP, induced a transient increase in intracellular Ca(2+) and the release of adenosine 5'-triphosphate (ATP) in cultured chondrocytes. A P2 receptor antagonist (suramin) and blockers of Cl(-) channels, niflumic acid and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), reduced the UTP-induced ATP release. The results indicated that Cl(-) channels were involved in the extracellular release of ATP following mechanical or P2Y receptor stimulation. Thus, ATP stimulation of P2Y receptors elicits an increase in intracellular Ca(2+), triggering further release of ATP from adjacent cells, thereby expanding the Ca(2+) wave in chondrocytes.

Calcium crystal deposition diseases: lessons from histochemistry

PURPOSE OF REVIEW

Recent progress in molecular biology and biochemistry has enabled researchers to identify possible key players in physiologic and pathologic calcification. However, important lessons from immunohistochemical studies have contributed greatly to our current understanding of the pathogenesis of calcium crystal deposition disease.

RECENT FINDINGS

Histologic findings led to the hypothesis of the important role of hypertrophic differentiation of articular chondrocytes in calcium crystal deposition. In addition, histologic studies have confirmed the importance of individual proteins that may have direct or indirect roles in calcium crystal formation.

SUMMARY

Future studies will determine whether in vitro data showing key roles for certain factors in mineralization and calcification in cartilage are relevant to crystal deposition disease in humans.

Upregulated ank expression in osteoarthritis can promote both chondrocyte MMP-13 expression and calcification via chondrocyte extracellular PPi excess

OBJECTIVE

In idiopathic chondrocalcinosis and in osteoarthritis (OA), increased extracellular PP(i) (ecPP(i)) promotes calcification. In chromosome 5p-associated familial chondrocalcinotic degenerative arthropathy, certain mutations in the membrane protein ANK may chronically raise ecPP(i) via enhanced PP(i) channeling. Therefore, we assessed if dysregulated wild-type ANK expression could contribute to pathogenesis of idiopathic degenerative arthropathy through elevated ecPP(i).

DESIGN

Using cells with genetic alterations in expression of ANK and the PP(i)-generating nucleotide pyrophosphatase phosphodiestrase (NPP) PC-1, we examined how increased ANK expression elevates ecPPI, testing for codependent effects with PC-1. We also evaluated the effects of ANK expression on chondrocyte growth, matrix synthesis, and MMP-13 expression and we immunohistochemically examined ANK expression in situ in human knee OA cartilages.

RESULTS

Using cells expressing defective ANK, as well as PC-1 knockout cells, we demonstrated that ANK required PC-1 (and vice versa) to raise ecPP(i) and that the major ecPP(i) regulator TGFbeta required both ANK and PC-1 to elevate ecPP(i). Upregulation of wild-type ANK by transfection in normal chondrocytes not only raised ecPP(i) 5-fold to approximately 100nM but also directly stimulated matrix calcification and inhibited collagen and sulfated proteoglycans synthesis. In addition, upregulated ANK induced chondrocyte MMP-13, an effect that also was stimulated within 2h by treatment of chondrocytes with 100nM PP(i) alone. Finally, ANK expression was upregulated in situ in human knee OA cartilages.

CONCLUSION

Elevation of ecPP(i) by ANK critically requires the fraction of cellular PP(i) generated by PC-1. The upregulation of ANK expression in OA cartilage and the capacity of increased ANK expression to induce MMP-13 and to promote matrix loss suggest that increased ANK expression and ecPP(i) exert noxious effects in degenerative arthropathies beyond stimulation of calcification.

Up-regulated expression of cartilage intermediate-layer protein and ANK in articular hyaline cartilage from patients with calcium pyrophosphate dihydrate crystal deposition disease

OBJECTIVE

Excess accumulation of extracellular inorganic pyrophosphate (ePPi) in aged human cartilage is crucial in calcium pyrophosphate dihydrate (CPPD) crystal formation in cartilage matrix. Two sources of ePPi are ePPi-generating ectoenzymes (NTPPPH) and extracellular transport of intracellular PPi by ANK. This study was undertaken to evaluate the role of NTPPPH and ANK in ePPi elaboration, by investigating expression of NTPPPH enzymes (cartilage intermediate-layer protein [CILP] and plasma cell membrane glycoprotein 1 [PC-1]) and ANK in human chondrocytes from osteoarthritic (OA) articular cartilage containing CPPD crystals and without crystals.

METHODS

Chondrocytes were harvested from knee cartilage at the time of arthroplasty (OA with CPPD crystals [CPPD], n = 8; OA without crystals [OA], n = 10). Normal adult human chondrocytes (n = 1) were used as a control. Chondrocytes were cultured with transforming growth factor beta1 (TGFbeta1), which stimulates ePPi elaboration, and/or insulin-like growth factor 1 (IGF-1), which inhibits ePPi elaboration. NTPPPH and ePPi were measured in the media at 48 hours. Media CILP, PC-1, and ANK were determined by dot-immunoblot analysis. Chondrocyte messenger RNA (mRNA) was extracted for reverse transcriptase-polymerase chain reaction to study expression of mRNA for CILP, PC-1, and ANK. NTPPPH and ANK mRNA and protein were also studied in fresh frozen cartilage.

RESULTS

Basal ePPi elaboration and NTPPPH activity in conditioned media from CPPD chondrocytes were elevated compared with normal chondrocytes, and tended to be higher compared with OA chondrocytes. Basal expression of mRNA for CILP (chondrocytes) and ANK (cartilage) was higher in both CPPD chondrocytes and CPPD cartilage extract than in OA or normal samples. PC-1 mRNA was less abundant in CPPD chondrocytes and cartilage extract than in OA chondrocytes and extract, although the difference was not significant. CILP, PC-1, and ANK protein levels were similar in CPPD, OA, and normal chondrocytes or cartilage extracts. Both CILP and ANK mRNA expression and ePPi elaboration were stimulated by TGFbeta1 and inhibited by IGF-1 in chondrocytes from all sources.

CONCLUSION

CILP and ANK mRNA expression correlates with chondrocyte ePPi accumulation around CPPD and OA chondrocytes, and all respond similarly to growth factor stimulation. These findings suggest that up-regulated CILP and ANK expression contributes to higher ePPi accumulation from CPPD crystal-forming cartilage.

Animal models of pathologic calcification

Recent progress in genetics and mouse genomics enables researchers to unveil the molecular basis for mouse phenotypes that express pathologic calcification in soft tissue and/or articular tissues. A newly identified multipass transmembrane protein, ANK, appears to function as an inorganic pyrophosphate (PPi) transporter or regulator of PPi transport. Abnormal extracellular PPi (ePPi) metabolism has been implicated in abnormal calcification, decreased concentrations predisposing to basic calcium phosphate (BCP) deposition, and increased concentrations promoting calcium pyrophosphate dihydrate (CPPD) crystal deposition in articular tissues. The chromosomal location of human ANK overlaps the locus identified in several kindreds affected with familial chondrocalcinosis. Deficient generation of ePPi by the ectoenzyme nucleoside triphosphate pyrophosphohydrolase also results in excessive ossification and ectopic deposition of BCP crystals in tiptoe-walking mice and PC-1 null mice. Recent studies reinforce the important regulatory role of ePPi in pathologic and physiologic calcification.

Rheumatic manifestations of primary hyperparathyroidism and parathyroid hormone therapy

A frequent manifestation of severe primary hyperparathyroidism was the bone disease osteitis fibrosa cystica. Rarely, excess parathyroid hormone (PTH) was associated with gout and calcium pyrophosphate crystal deposition disease. Surgical cure of primary hyperparathyroidism was occasionally associated with pseudogout. Today, primary hyperparathyroidism is generally asymptomatic. Clinically overt rheumatologic and skeletal effects are mainly of historical interest. Skeletal disease is still detectable by more sensitive techniques. In certain circumstances, PTH may be protective and anabolic for the skeleton.

Ophidian envenomation strategies and the role of purines

Snake envenomation employs three well integrated strategies: prey immobilization via hypotension, prey immobilization via paralysis, and prey digestion. Purines (adenosine, guanosine and inosine) evidently play a central role in the envenomation strategies of most advanced snakes. Purines constitute the perfect multifunctional toxins, participating simultaneously in all three envenomation strategies. Because they are endogenous regulatory compounds in all vertebrates, it is impossible for any prey organism to develop resistance to them. Purine generation from endogenous precursors in the prey explains the presence of many hitherto unexplained enzyme activities in snake venoms: 5'-nucleotidase, endonucleases (including ribonuclease), phosphodiesterase, ATPase, ADPase, phosphomonoesterase, and NADase. Phospholipases A(2), cytotoxins, myotoxins, and heparinase also participate in purine liberation, in addition to their better known functions. Adenosine contributes to prey immobilization by activation of neuronal adenosine A(1) receptors, suppressing acetylcholine release from motor neurons and excitatory neurotransmitters from central sites. It also exacerbates venom-induced hypotension by activating A(2) receptors in the vasculature. Adenosine and inosine both activate mast cell A(3) receptors, liberating vasoactive substances and increasing vascular permeability. Guanosine probably contributes to hypotension, by augmenting vascular endothelial cGMP levels via an unknown mechanism. Novel functions are suggested for toxins that act upon blood coagulation factors, including nitric oxide production, using the prey's carboxypeptidases. Leucine aminopeptidase may link venom hemorrhagic metalloproteases and endogenous chymotrypsin-like proteases with venom L-amino acid oxidase (LAO), accelerating the latter. The primary function of LAO is probably to promote prey hypotension by activating soluble guanylate cyclase in the presence of superoxide dismutase. LAO's apoptotic activity, too slow to be relevant to prey capture, is undoubtedly secondary and probably serves principally a digestive function. It is concluded that the principal function of L-type Ca(2+) channel antagonists and muscarinic toxins, in Dendroaspis venoms, and acetylcholinesterase in other elapid venoms, is to promote hypotension. Venom dipeptidyl peptidase IV-like enzymes probably also contribute to hypotension by destroying vasoconstrictive peptides such as Peptide YY, neuropeptide Y and substance P. Purines apparently bind to other toxins which then serve as molecular chaperones to deposit the bound purines at specific subsets of purine receptors. The assignment of pharmacological activities such as transient neurotransmitter suppression, histamine release and antinociception, to a variety of proteinaceous toxins, is probably erroneous. Such effects are probably due instead to purines bound to these toxins, and/or to free venom purines.

Calcium crystal-induced inflammation

Recent studies have added to our knowledge regarding the mechanisms of calcium crystal deposition. Calcium pyrophosphate dihydrate (CPPD) crystal deposition is associated with elevated levels of PPi in joints. Cyclic compression of cartilage transiently elevated ATP levels in culture media. Extracellular ATP may be hydrolyzed by nucleoside triphosphate pyrophosphohydrolase (NTPPPH), yielding an elevated PPi concentration. CPPD crystal deposition increases with age. Nitric oxide may alter cartilage matrix by interfering with chondrocyte mitochondrial function and ATP production. Transglutaminase in adult, but not young, porcine articular chondrocytes was able to activate latent transforming growth factor beta, a potent stimulus to PPi production. Basic calcium phosphate crystals are more likely to form in a milieu of reduced PPi concentration. The ank gene mutation results in higher intracellular PPi concentration and lower extracellular concentration. The ANK protein is thought to be a transmembrane protein necessary for transport of PPi out of cells. A mutation that results in reduced synthesis of NTPPPH PC-1 caused infantile wrist and ankle periarticular calcification and vascular calcification.

The ank gene story

The underlying molecular defect resulting in the abnormal calcification observed in ank/ank mice has been identified. The responsible nonsense mutation affects the protein product of ank, resulting in diminished production of extracellular inorganic pyrophosphate, an important inhibitor of nucleation and of the growth of apatite crystals. The ank gene product is one of several cell membrane proteins, including ectonucleoside triphosphate pyrophosphohydrolase enzymes and alkaline phosphatase, that regulate extracellular inorganic pyrophosphate levels and thereby regulate mineralization.

Expression of cartilage intermediate layer protein/nucleotide pyrophosphohydrolase parallels the production of extracellular inorganic pyrophosphate in response to growth factors and with aging

OBJECTIVE

To evaluate the role of the extracellular inorganic pyrophosphate (ePPi)-generating ectoenzyme cartilage intermediate layer protein/nucleotide pyrophosphohydrolase (CILP/NTPPH) in chondrocyte PPi elaboration, we studied CILP/NTPPH expression in response to growth factors during aging.

METHODS

Porcine chondrocytes from adult (3-4-year-old) and young (2-week-old) animals were stimulated with transforming growth factor beta1 (TGFbeta1), which enhances ePPi elaboration, and/or insulin-like growth factor 1 (IGF-1), which diminishes ePPi elaboration. Measurements of ePPi, NTPPH enzyme activity, Western blot analysis, reverse transcriptase-polymerase chain reaction (RT-PCR), and Northern blot analysis were performed.

RESULTS

Elaboration of ePPi into conditioned media from adult chondrocytes was significantly increased by TGFbeta1 and significantly inhibited by IGF-1, but no significant differences were observed in young chondrocytes. The protein levels of CILP/NTPPH by Western analysis in the media from adult and young porcine chondrocytes were increased by TGFbeta1. RT-PCR and Northern analysis showed that CILP/NTPPH messenger RNA (mRNA) expression in both adult and young chondrocytes was increased by TGFbeta1 and decreased by IGF-1, but these changes were less significant in the young chondrocytes. Basal and TGFbeta1-up-regulated levels of CILP/NTPPH expression were higher in adult chondrocytes than in young chondrocytes.

CONCLUSION

These results provide evidence that CILP/NTPPH expression and ePPi elaboration are concomitantly stimulated by TGFbeta1 and down-regulated by IGF-1, especially in adult chondrocytes, implicating CILP/NTPPH as a functional participant in ePPi elaboration. Increased CILP/NTPPH mRNA expression in chondrocytes derived from aged animals compared with young animals might promote the formation of calcium pyrophosphate dihydrate crystals in aged cartilage.

Exclusion of the gene for human cartilage intermediate layer protein in currently mapped calcium pyrophosphate dihydrate deposition syndromes

OBJECTIVE

To map the gene for human cartilage intermediate layer protein (CILP) in order to assess its involvement in some familial forms of calcium pyrophosphate dihydrate (CPPD) deposition disease.

METHODS

A radiation hybrid panel was analyzed for chromosomal assignment of the CILP gene within a 1-cM limit of resolution. The location of the gene for CILP was confirmed to reside at the observed radiation hybrid locus by fluorescence in situ hybridization.

RESULTS

The human CILP gene resides at chromosome 15q21.

CONCLUSION

This map location definitively excludes mutations in the CILP gene as the cause of certain familial forms of CPPD deposition disease that have been genetically mapped to chromosomes 8q and 5p.

The role of crystals in osteoarthritis

The deposition of calcium-containing crystals in articular tissues is probably an under-recognized event. Clinical observations indicate that an exaggerated and uniquely distributed cartilage degeneration is associated with these deposits. Measurements of putative markers of cartilage breakdown suggest that the presence of these crystals magnifies the degenerative process. In vitro studies indicate two potential mechanisms by which crystals cause degeneration. These involve the stimulation of mitogenesis in synovial fibroblasts and the secretion of proteases by cells that phagocytose these crystals. Approaches that might ameliorate the degenerative process may ensue from new information about how crystals form and how they exert their biologic effects.

Refinement of the chromosome 5p locus for familial calcium pyrophosphate dihydrate deposition disease

Familial calcium pyrophosphate dihydrate deposition disease (CPPDD) is a disease of articular cartilage that is radiographically characterized by chondrocalcinosis due to the deposition of calcium-containing crystals in affected joints. We have documented the disease in an Argentinean kindred of northern Italian ancestry and in a French kindred from the Alsace region. Both families presented with a common phenotype including early age at onset and deposition of crystals of calcium pyrophosphate dihydrate in a similar pattern of affected joints. Affected family members were karyotypically normal. Linkage to the short arm of chromosome 5 was observed, consistent with a previous report of linkage of the CPPDD phenotype in a large British kindred to the 5p15 region. However, recombinants in the Argentinean kindred have enabled us to designate a region<1 cM in length between the markers D5S416 and D5S2114 as the CPPDD locus.

Molecular cloning and expression of a porcine chondrocyte nucleotide pyrophosphohydrolase

The porcine 127-kDa nucleotide pyrophosphohydrolase (NTPPHase) had been previously purified from the conditioned culture media of porcine articular cartilage. Protein sequencing of an internal 61-kDa proteolytic fragment of NTPPHase (61-kDa NTPPHase) determined the 26 N-terminal amino acids. This sequence was used to amplify a DNA fragment, which was used as a probe to clone the gene encoding the 61-kDa NTPPHase from a porcine chondrocyte cDNA library. DNA sequence analysis showed the cDNA insert to be 2509 bp, corresponding to a predicted open reading frame (ORF) encoding 599 amino acids. The 26 N-terminal amino acids of the 61-kDa NTPPHase were located within the ORF immediately downstream of a putative protease recognition region, RRKRR. This is consistent with this cDNA insert representing an internal proteolytic fragment of the full length 127-kDa NTPPHase. BLAST and FASTA analysis confirmed that the deduced amino acid sequence of 61-kDa NTPPHase was unique and did not possess a high degree of homology to sequence in the non-redundant protein and nucleotide databases. Proteins that possess limited homology (< 17%) with the 61-kDa NTTPPHase include several prokaryotic and eukaryotic ATP pyrophosphate-lyases (adenylate cyclase). Northern blot analysis of porcine chondrocyte RNA showed that the DNA encoding the 61-kDa NTPPHase hybridized to a single 4.0-kb RNA transcript. This DNA probe also hybridized to a single species of human chondrocyte RNA. Expression of a 61-kDa protein was detected by coupled in-vitro transcription/translation. Western blot analysis of this in-vitro transcription/translation reaction detected a 61-kDa protein, using an antibody raised against the peptide sequence that was originally used to clone the 61-kDa NTPPHase. These data indicate the successful in-vitro cloning and expression of the porcine chondrocyte 61-kDa NTPPHase. Future studies that utilize the gene encoding the 61-kDa NTPPHase may allow the characterization of the role of NTPPHase in calcium pyrophosphate dihydrate (CPPD) crystal deposition disease.

Inhibition of calcium pyrophosphate dihydrate crystal formation in articular cartilage vesicles and cartilage by phosphocitrate

Articular cartilage vesicles (ACV), isolated by differential centrifugation of adult hyaline articular cartilage collagenase digests, mineralized in the presence of calcium and ATP. Mineral analysis by microscopy, chemical analysis, energy-dispersive analysis, and infrared spectroscopy revealed crystals resembling calcium pyrophosphate dihydrate (CPPD). Adult articular cartilage also underwent ATP-dependent mineralization, supporting the contention that vesicles in situ fostered adult articular cartilage mineralization. Phosphocitrate (PC) is a recognized in vitro inhibitor of hydroxyapatite and calcium oxalate monohydrate crystal formation, but it is not known whether PC can similarly restrict CPPD crystal development. In the present study we examine the effect of PC, citrate, and n-sulfo-2-amino-tricarballylate (SAT, a PC analogue) on the ATP-induced CPPD crystal formation in both ACV and articular cartilage models. Only PC (10-1000 microM) blocked both the ATP-dependent and -independent mineralization in ACV in a dose-dependent fashion. At 1 mM, SAT and citrate blocked the ATP-independent mineralization. Similarly, only PC blocked both the ATP- and non-ATP-dependent mineralization in native articular cartilage slices. PC, SAT, and citrate had no effect on ACV nucleoside triphosphate pyrophosphohydrolase activity, suggesting that none of these agents blocked mineralization through the inhibition of nucleoside triphosphate pyrophosphohydrolase activity, which generates inorganic pyrophosphate from ATP.

Identification of a nucleotide pyrophosphohydrolase from articular tissues in human serum

OBJECTIVE

To characterize the nucleotide pyrophosphohydrolase (NTPPHase) in human serum.

METHODS

NTPPHase activity and kinetic analysis were performed using thymidine monophosphate paranitrophenyl ester (TMPNP) or 32Pgamma-labeled ATP as substrate. Sera were chromatographed (dye column), and peak fractions were analyzed kinetically and by immunoblot using antibodies to 127-kd articular cartilage vesicle (ACV) NTPPHase as well as to PC-1 and to 58 kd, two plasma membrane ecto-NTPPHases. Enzyme activity was measured before and after sample ultracentrifugation.

RESULTS

NTPPHase activity was found in all sera tested (2 normal subjects, 9 arthritis patients). Specific activity was increased 9-32-fold after chromatography; 60-80% of total activity was recovered in a single peak containing an approximately 100-kd soluble peptide related to the 127-kd ACV enzyme. The apparent Km of this peptide (TMPNP) was virtually identical to that of the porcine ACV 127-kd enzyme. No immunoreactivity against PC-1 or 58-kd NTPPHase was found.

CONCLUSION

Human serum NTPPHase is derived from 127-kd ACV-related enzyme.

Calcium pyrophosphate dihydrate crystal deposition disease: a review of the literature and a light and electron microscopic study of a case of the temporomandibular joint with numerous intracellular crystals in the chondrocytes

The pathogenesis of calcium pyrophosphate dihydrate (CPPD) crystal deposition disease of synovial joints is still unclear, although overproduction of extracellular pyrophosphate (PPi) is thought to play a key role. We studied the light and electron microscopic appearances of a case of CPPD crystal deposition disease of the temporomandibular joint (TMJ) in search of new clues for its pathogenesis. Light microscopic examination of CPPD-containing material from the joint space revealed cartilaginous nodules with various degrees of crystallization. Transmission electron microscopic examination revealed numerous extra- as well as intracellular crystals and crystal shaped spaces in the chondrocytes. Other striking ultrastructural features of the chondrocytes included the presence of many mitochondria, frequently containing crystalline material, and the presence of highly dilated rough endoplasmic reticulum and large glycogen islands. The presence of intramitochondrial crystals may hypothetically imply a derangement in mitochondrial adenosine triphosphate or PPi metabolism. The finding of intracellular CPPD crystals in chondrocytes points to the existence of an intracellular pathway of CPPD crystal formation in CPPD crystal deposition disease of the TMJ and possibly in CPPD crystal deposition disease in general.

A unique ectonucleotide pyrophosphohydrolase associated with porcine chondrocyte-derived vesicles

Previous studies have shown increased nucleotide pyrophosphohydrolase (EC 3.6.1.8) (NTPPHase) activity in detergent extracts of degenerated human cartilage containing calcium pyrophosphate dihydrate (CPPD) crystals relative to those from osteoarthritis or normal cartilage. NTPPHase was later shown to be an ectoenzyme and its activity was increased in synovial fluid from patients with CPPD crystal deposits relative to fluids from other types of arthritis. We have purified a soluble 61-kD NTPPHase from conditioned media of organ-cultured porcine articular cartilage to electrophoretic homogeneity. Its NH2-terminal sequence through 26 cycles showed < 30% homology to any previously reported protein sequence. An antibody raised to a synthetic peptide corresponding to this sequence reacted with denatured but not native enzyme. This antibody reacted against a sedimentable vesicle-associated 127-kD protein in conditioned media from cultured articular cartilage or from chondrocytes in primary monolayer culture and against a series of soluble proteins in conditioned media supernatant, including a 61-kD protein representing our original isolate. No reactivity was found in 1% SDS extracts of washed cultured chondrocytes, although these contained greater NTPPHase activity than the conditioned media. Antibody to PC-1, another ectoNTPPHase, reacted with 1% SDS extracts of whole chondrocytes but not against those chromatographic fractions containing the major portion of NTPPHase activity. Release of the vesicle-associated 127-kD enzyme into conditioned medium was stimulated three- to sevenfold by TGF beta 1. The antibody also reacted with a series of soluble proteins and with 127-kD sedimentable protein in human synovial fluid. Kinetic studies supported the existence of a unique vesicle-associated NTPPHase; apparent Km (mM) of chondrocyte membrane NTPPHase was 1.5 and 3.0 at pH 7.3 and 9.88, respectively; apparent Km (mM) of vesicle associated NTPPHase was 0.83 and 1.28 at pH 7.3 and 9.88. The data suggest the existence of a unique ecto-NTPPHase associated with vesicles derived from normal articular cartilage.

Expression of the murine plasma cell nucleotide pyrophosphohydrolase PC-1 is shared by human liver, bone, and cartilage cells. Regulation of PC-1 expression in osteosarcoma cells by transforming growth factor-beta

A bone and cartilage enzyme with both 5'-nucleotide phosphodiesterase I and nucleotide pyrophosphohydrolase (NTPPPH) activity modulates physiologic mineralization and pathologic chondrocalcinosis by generating inorganic pyrophosphate. We hypothesized that, as for alkaline phosphatase, expression of an NTPPPH gene can be shared by cells from bone, cartilage, and liver and by certain leukocytes. Recently, we demonstrated the hepatocyte and murine plasma cell membrane glycoprotein PC-1 to have both 5'-nucleotide phosphodiesterase I and NTPPPH activity. We detected polypeptides cross-reactive with PC-1 in human U20S osteosarcoma cells, articular chondrocytes, homogenized human knee cartilages, human knee synovial fluids, hepatoma cells, and murine plasmacytoma cells. Constitutive low abundance PC-1 mRNA expression was detected in U20S cells and chondrocytes by a nested RNA-PCR assay and by Northern blotting. TGF beta is known to substantially increase NTPPPH activity in primary osteoblast cultures. We demonstrated that TGF beta 1 increased NTPPPH activity and the level of PC-1 mRNA and immunoprecipitable [35S]-methionine-labeled PC-1 polypeptides in U20S cells. The identification of PC-1 as an NTPPPH expressed in cells derived from bone and cartilage may prove useful in furthering the understanding of the role of NTPPPH i n physiologic and pathologic mineralization.

Crystals and arthritis

Monosodium urate, calcium pyrophosphate dihydrate, and basic calcium phosphate (carbonate-substituted hydroxyapatite and octacalcium phosphate) crystal aggregates are associated with gout, pseudogout, and cartilage degeneration (osteoarthritis, Milwaukee Shoulder/Knee Syndrome), respectively. Hyperuricemia is a frequent but nonspecific and inconstant feature of gout just as an elevated synovial fluid inorganic pyrophosphate level is an inconstant feature of pseudogout. Monosodium urate, calcium pyrophosphate dihydrate, or basic calcium phosphate crystals can cause acute inflammation associated with phagocytosis by neutrophilic leukocytes. Each induces neutral protease synthesis and secretion and arachidonic acid metabolism by synoviocytes and macrophages in a dose-dependent fashion, postulated to produce the damage to bone, cartilage, and other joint tissues that is perceived clinically as tophaceous destruction or degenerative joint disease. Crystals containing calcium are potent mitogens. All three types of crystals are more common in older persons and will attract additional attention as the mean age of our population increases. Gout is perhaps the most treatable disease in medicine, although mistakes in diagnosis and in choice of appropriate therapy are very common. Acute pseudogout and acute calcific periarthritis are readily treated medically, but the chronic effects of crystals containing calcium are not. New approaches using drugs derived from scientific study of the biologic effects of these crystals may become useful therapeutically.

Causal link between nucleotide pyrophosphohydrolase overactivity and increased intracellular inorganic pyrophosphate generation demonstrated by transfection of cultured fibroblasts and osteoblasts with plasma cell membrane glycoprotein-1. Relevance to calcium pyrophosphate dihydrate deposition disease

OBJECTIVE

In subjects with idiopathic calcium pyrophosphate dihydrate (CPPD) deposition disease, cartilage chondrocytes elaborate increased amounts of PPi. The mechanism of the intracellular PPi elevation is not known. Plasma membrane 5'-nucleotide phosphodiesterase I/nucleotide pyrophosphohydrolase (NTPPPH) activity also is elevated in chondrocytes and dermal fibroblasts of patients with idiopathic CPPD deposition disease. NTPPPH, as an ecto-enzyme, could act within certain intracellular compartments. Thus, we hypothesized a potential causal link between increased NTPPPH activity and increased intracellular PPi.

METHODS

Transformed simian fibroblasts (COS cells) and human osteoblasts (U2OS cells) were transfected with the 5'-nucleotide phosphodiesterase I ecto-enzyme plasma cell membrane glycoprotein-1 (PC-1), recently shown to be expressed in cartilage, osteoblasts, and fibroblasts.

RESULTS

Transfection with PC-1 markedly up-regulated 5'-nucleotode phosphodiesterase I activity and increased intracellular PPi concentrations by increasing the capacity of cells to generate PPi. Importantly, this did not require supplementation with exogenous nucleotides.

CONCLUSION

Cellular overexpression of PC-1 produces NTPPPH overactivity and increased intracellular PPi generation in vitro. These findings support the potential importance of NTPPPH overactivity in PPi generation, both inside and outside the cell, in some subjects with CPPD deposition disease.

ATP-induced chondrocalcinosis

OBJECTIVE

To determine whether adult articular cartilage mineralizes in the presence of ATP.

METHODS

Intact adult porcine articular cartilage and monolayers of chondrocytes were cultured in physiologic media containing ATP, and mineralization was measured as retention of 45Ca. Cartilage was analyzed by electron microscopy.

RESULTS

Articular cartilage sequestered 45Ca when incubated with 100 microM ATP: Use of the ATP analog alpha, beta-methylene ATP did not promote mineralization and addition of pyrophosphatase inhibited mineralization, indicating that hydrolysis of ATP to AMP and inorganic pyrophosphate is necessary for the process to occur. Mineral was concentrated in articular cartilage vesicles in the perichondral area.

CONCLUSION

Adult articular cartilage mineralizes in the presence of ATP, in a manner similar to that found with isolated matrix or articular cartilage vesicles. This supports the notion that these structures have a role in chondrocalcinosis.

Synovial fluid 5'-nucleotidase activity. Relationship to other purine catabolic enzymes and to arthropathies associated with calcium crystal deposition

We measured 5'-nucleotidase (5NT) activity in synovial fluid from 159 patients with various diagnoses. The activity of 5NT was compared with activities of nucleotide pyrophosphohydrolase, alkaline and neutral phosphatases, and adenosine deaminase, in the same samples. Higher levels of 5NT activity occurred in synovial fluid from osteoarthritic joints than from joints of patients with gout, pseudogout, or rheumatoid arthritis. The highest levels of 5NT activity were found in synovial fluid from patients with Milwaukee shoulder syndrome and from osteoarthritis patients in whom deposition of calcium-containing crystals was also present.

Synovial fluid pyrophosphate and nucleoside triphosphate pyrophosphatase: comparison between normal and diseased and between inflamed and non-inflamed joints

Deposition of intra-articular calcium pyrophosphate is associated with both aging and arthropathy; increased concentrations of free pyrophosphate (PPi) may contribute to such deposition. Free pyrophosphate and nucleoside triphosphate pyrophosphatase (NTPase) were estimated in synovial fluids from 50 subjects with normal knees and from 44 patients with rheumatoid arthritis, 61 with pyrophosphate arthropathy, and 59 with osteoarthritis. For arthropathic knees clinically assessed inflammation was classified as active or inactive using a summated score of six clinical features. The order of PPi (mumol/l) and NTPase (mumol PPi/30 min/mg protein) was pyrophosphate arthropathy greater than osteoarthritis greater than rheumatoid arthritis (median PPi, NTPase respectively: for pyrophosphate arthropathy 15.9, 0.45; for osteoarthritis 9.3, 0.25; for rheumatoid arthritis 4.4, 0.18), with significant differences between all groups. In pyrophosphate arthropathy both PPi (mumol/l) and NTPase (mumol PPi/30 min/mg protein) were higher than normal (15.9, 0.45 v 8.6, 0.2 respectively), but findings in osteoarthritis did not differ from normal. The inflammatory state of the knee had a distinct but variable effect on synovial fluid findings in rheumatoid arthritis and pyrophosphate arthropathy, but not in osteoarthritis. There was no correlation of either PPi or NTPase with age, or between PPi and NTPase in any group. This study provides in vivo data for synovial fluid PPi and NTPase. It suggests that factors other than PPi need to be considered in a study of crystal associated arthropathy. Clinical inflammation, as well as diagnosis, is important in synovial fluid studies.

Cartilage inorganic pyrophosphate elaboration is independent of sulfated glycosaminoglycan synthesis

Inorganic pyrophosphate (PPi), a product of glycosaminoglycan synthesis, may be cosecreted with matrix proteoglycan to reach the extracellular site where calcium pyrophosphate dihydrate crystals form. To test this hypothesis, sulfated glycosaminoglycan synthesis by articular cartilage in culture was stimulated or inhibited while the effect on extracellular PPi was measured. When stimulated by 0.8 mM xyloside to increase 35SO4 incorporation (mean +/- SEM % of control 183 +/- 16, n = 5), PPi accumulation changed little (from 54 +/- 6 pmoles/mg to 63 +/- 8 pmoles/mg of cartilage wet weight). Inhibition of sulfation with monensin or diethylcarbamazine disproportionately lowered 35SO4 incorporation compared with PPi elaboration. Using 60 mM diethylcarbamazine, PPi production was preserved (105 +/- 8% mean +/- SEM) compared with control cultures, while sulfation was markedly inhibited (7 +/- 1%). This dissociation of sulfate incorporation and PPi secretion indicates that it is not likely that glycosaminoglycan sulfation is the source of the PPi that escapes from chondrocytes to participate in the formation of extracellular crystals.

Augmentation of inorganic pyrophosphate elaboration in cartilage by serum factors

The disordered production of inorganic pyrophosphate (PPi) by articular cartilage is thought to have an important role in the pathogenesis of calcium pyrophosphate dihydrate deposition disease and perhaps osteoarthritis. We have previously shown that fetal calf serum added to the culture media of porcine articular cartilage explants increases the elaboration of PPi into the ambient media. We have examined this PPi stimulatory activity by studying the effects of adult human serum (HS), serum derived from adult human plasma (HP), and an acid-alcohol extract of human platelets (PE) on PPi production in cartilage organ culture. Ten percent HS produces a 1.4-fold increase in PPi production after 48 h of culture, while cartilage incubated in media containing 10% HP produces no more PPi than that incubated in media alone. PE stimulates a mean 2-fold increase in PPi production at 48 h in the presence of low concentrations of HP, and has no effect alone. It does not appear to up-regulate the activity of the ectoenzyme nucleoside triphosphate pyrophosphohydrolase (NTPPPH), nor does it promote the release of enzyme substrate into the extracellular space. Cartilage exposed to 0.5% HP and PE has 1.51 +/- 0.36 units of NTPPPH activity whereas cartilage exposed to 0.5% HP alone has 1.52 +/- 0.41 units of enzyme activity. PE does not increase the release of [14C]adenine-labeled compounds into the media. Approximately 13% of soluble 14C counts was found in the media of chondrocytes treated with PE while 18% of counts was released in the presence of HP alone. We have demonstrated a factor or factors present in FCS, HS, and an acid-ethanol extract of human platelets which represent(s) the first known physiologic modulators of PPi production in articular cartilage and may increase PPi production without affecting NTPPPH activity.

Stimulation of inorganic pyrophosphate elaboration by cultured cartilage and chondrocytes

Inorganic pyrophosphate elaboration by articular cartilage may favor calcium pyrophosphate dihydrate crystal deposition. Frequently crystal deposits form in persons affected with metabolic diseases. The cartilage organ culture system was used to model these metabolic conditions while measuring the influence on extracellular pyrophosphate elaboration. Alterations of ambient pH, thyroid stimulating hormone levels, and parathyroid hormone levels did not change pyrophosphate accumulation in the media. However, subphysiologic ambient calcium concentrations (25, 100, 500 microM) increased pyrophosphate accumulation about chondrocytes 3- to 10-fold. Low calcium also induced release of [14C]adenine-labeled nucleotides from chondrocytes, potential substrates for generation of extracellular pyrophosphate by ectoenzymes. Exposing cartilage to 10% fetal bovine serum also enhanced by 50% the egress of inorganic pyrophosphate from the tissue.

Inorganic pyrophosphate release by rabbit articular chondrocytes in vitro

Release of inorganic pyrophosphate (PPi) by rabbit articular chondrocytes in vitro was measured by a newly developed assay which utilizes radioactive orthophosphate (32Pi) labeling and anion exchange high performance liquid chromatography. Chondrocytes in monolayer and high density culture failed to release PPi. Explants (cartilage fragments), however, released newly formed PPi into the culture medium. Trypsin treatment of cartilage fragments almost completely blocked the PPi extrusion. Collagenase treatment had no effect on PPi extrusion. There was no clear correlation between proteoglycan synthesis, measured by 35SO4 incorporation, and PPi release. Suppression of proteoglycan synthesis with tunicamycin did not influence the PPi release of the explants.

Partial characterization of synovial fluid nucleotide pyrophosphohydrolase

Synovial fluid adenosine triphosphate pyrophosphohydrolase, an enzyme which manifests increased activity in chondrocalcinosis and osteoarthritis, was partially characterized in synovial fluids from 41 patients who had a variety of arthropathies. Activity was found to be a soluble, heat labile, and divalent cation-dependent nonspecific nucleotide pyrophosphohydrolase with pH optimum 9.0-9.5.

Adenosine triphosphate pyrophosphohydrolase and neutral inorganic pyrophosphatase in pathologic joint fluids. Elevated pyrophosphohydrolase in calcium pyrophosphate dihydrate crystal deposition disease

Adenosine triphosphate pyrophosphohydrolase (ATPPPH) and neutral inorganic pyrophosphatase activities were assayed in synovial fluids (SF) from 37 patients with a variety of arthropathies. ATPPPH activity was detected in all fluids, but was highest in patients with chronic chondrocalcinosis; its activity in patients with osteoarthritis was higher than that in patients with rheumatoid arthritis, gout, or pseudogout. ATPPPH activity correlated positively with SF pyrophosphate concentration and negatively with SF white blood cell count. Pyrophosphatase activity did not correlate with diagnosis, pyrophosphate level, or white blood cell count.

Identification of ecto-nucleoside triphosphate pyrophosphatase in human articular chondrocytes in monolayer culture

In cultured monolayers of human articular chondrocytes we have observed an enzyme activity which catalyzes the extracellular conversion of ATP to AMP and PPi. The enzyme was active at very low concentrations of ATP (microM) and exhibited optimal activity at concentrations of ATP of approx. 100 microM. The enzyme was active in intact cells as judged by measurement of the release of the cytoplasmic marker enzyme lactate dehydrogenase. No increase in production of PPi from ATP was observed on mechanically disrupting the cells and no activity was shed into the medium by intact cells. Activity was stable between days 4 and 8 after subculturing the cells and was not affected by the timing of the final medium change prior to assay. Activity was also observed with other nucleoside triphosphates (GTP, CTP and UTP). We suggest that this activity is attributable to ecto-nucleoside triphosphate pyrophosphatase. This observation may be important in relation to the pathogenesis of the human disease of chondrocalcinosis in which crystals of calcium pyrophosphate dihydrate deposit in articular cartilage.

Cartilage nucleoside triphosphate pyrophosphohydrolase. II. Role in extracellular pyrophosphate generation and nucleotide metabolism

Extracellular generation of inorganic pyrophosphate (PPi) in cartilage organ culture is markedly augmented by ATP.ATP, not an ATP metabolite (ADP, AMP, adenosine) is necessary for this augmentation. Excess PPi production is effectively blocked by known inhibitors of nucleoside triphosphate (NTP) pyrophosphohydrolase (EDTA, EGTA, dithiothreitol). Excess 32P-PPi is generated directly from gamma 32P-ATP by cartilage, as substrate and product have similar specific activities. These findings strongly favor ecto-NTP pyrophosphohydrolase as the source of extracellular PPi generation in the presence of NTP. Additionally, active nucleotide and nucleoside catabolism is demonstrated in these cartilage organ cultures.