Identification of a nucleotide pyrophosphohydrolase from articular tissues in human serum

Cartilage intermediate layer protein affects the progression of intervertebral disc degeneration by regulating the extracellular microenvironment (Review)

Intervertebral disc degeneration (IDD), which is caused by multiple factors, affects the health of individuals and contributes to low back pain. The pathology of IDD is complicated, and changes in the extracellular microenvironment play an important role in promoting the process of degeneration. Cartilage intermediate layer protein (CILP) is a matrix protein that resides in the middle of human articular cartilage and is involved in numerous diseases that affect cartilage. However, there is no detailed review of the relationship between CILP and degenerative disc disease. Growing evidence has revealed the presence of CILP in the extracellular microenvironment of intervertebral discs (IVDs) and has suggested that there is a gradual increase in CILP in degenerative discs. Specifically, CILP plays an important role in regulating the metabolism of the extracellular matrix (ECM), an important component of the extracellular microenvironment. CILP can combine with transforming growth factor-β or insulin-like growth factor-1 to regulate the ECM synthesis of IVDs and influence the balance of ECM metabolism, which leads to changes in the extracellular microenvironment to promote the process of IDD. It may be possible to show the correlation of CILP with IDD and to target CILP to interfere with IDD. For this purpose, in the present study, the current knowledge on CILP was summarized and a detailed description of CILP in discs was provided.

Enzymes involved in metabolism of extracellular nucleotides and nucleosides: functional implications and measurement of activities

Extracellular nucleotides and nucleosides mediate diverse signaling effects in virtually all organs and tissues. Most models of purinergic signaling depend on functional interactions between distinct processes, including (i) the release of endogenous ATP and other nucleotides, (ii) triggering of signaling events via a series of nucleotide-selective ligand-gated P2X and metabotropic P2Y receptors as well as adenosine receptors and (iii) ectoenzymatic interconversion of purinergic agonists. The duration and magnitude of purinergic signaling is governed by a network of ectoenzymes, including the enzymes of the nucleoside triphosphate diphosphohydrolase (NTPDase) family, the nucleotide pyrophosphatase/phosphodiesterase (NPP) family, ecto-5'-nucleotidase/CD73, tissue-nonspecific alkaline phosphatase (TNAP), prostatic acid phosphatase (PAP) and other alkaline and acid phosphatases, adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP). Along with "classical" inactivating ectoenzymes, recent data provide evidence for the co-existence of a counteracting ATP-regenerating pathway comprising the enzymes of the adenylate kinase (AK) and nucleoside diphosphate kinase (NDPK/NME/NM23) families and ATP synthase. This review describes recent advances in this field, with special emphasis on purine-converting ectoenzymes as a complex and integrated network regulating purinergic signaling in such (patho)physiological states as immunomodulation, inflammation, tumorigenesis, arterial calcification and other diseases. The second part of this review provides a comprehensive overview and basic principles of major approaches employed for studying purinergic activities, including spectrophotometric Pi-liberating assays, high-performance liquid chromatographic (HPLC) and thin-layer chromatographic (TLC) analyses of purine substrates and metabolites, capillary electrophoresis, bioluminescent, fluorometric and electrochemical enzyme-coupled assays, histochemical staining, and further emphasizes their advantages, drawbacks and suitability for assaying a particular catalytic reaction.

A new Enpp1 allele, Enpp1(ttw-Ham), identified in an ICR closed colony

We recently have reported on a novel ankylosis gene that is closely linked to the Enpp1 (ectonucleotide pyrophosphatase/phosphodiesterase 1) gene on chromosome 10. Here, we have discovered novel mutant mice in a Jcl:ICR closed colony with ankylosis in the toes of the forelimbs at about 3 weeks of age. The mutant mice exhibited rigidity in almost all joints, including the vertebral column, which increased with age. These mice also showed hypogrowth with age after 16 weeks due to a loss of visceral fat, which may have been caused by poor nutrition. Histological examination and soft X-ray imaging demonstrated the ectopic ossification of various joints in the mutant mice. In particular, increased calcium deposits were observed in the joints of the toes, the carpal bones and the vertebral column. We sequenced all exons and exon/intron boundaries of Enpp1 in the normal and mutant mice, and identified a G-to-T substitution (c.259+1G>T) in the 5' splice donor site of intron 2 in the Enpp1 gene of the mutant mice. This substitution led to the skipping of exon 2 (73 bp), which generated a stop codon at position 354 bp (amino acid 62) of the cDNA (p.V63Xfs). Nucleotide pyrophosphohydrolase (NPPH) activity of ENPP1 in the mutant mice was also decreased, suggesting that Enpp1 gene function is disrupted in this novel mutant. The mutant mice reported in this study will be a valuable animal model for future studies of human osteochondral diseases and malnutrition.

Genetic and enzymatic analysis for two Japanese patients with idiopathic infantile arterial calcification

Idiopathic infantile arterial calcification (IIAC) is a life-threatening disorder in young infants. Cardiovascular symptoms are usually apparent within the first month of life. The symptoms are caused by calcification of large and medium-sized arteries, including the aorta, coronary arteries, and renal arteries. Most of the patients die by 6 months of age because of heart failure. Recently, homozygous or compound heterozygous mutations for the ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) gene were reported as causative for the disorder. ENPP1 regulates extracellular inorganic pyrophosphate (PPi), a major inhibiter of extracellular matrix calcification. Two Japanese patients with IIAC were studied. One, from first-cousin parents, showed a typical clinical course. The onset in the second patient was late. Both of the patients were clinically compatible for IIAC; arterial calcification was shown, and hypertension was prominent. We sequenced all the exons and exon-intron boundaries of the gene and measured nucleotide pyrophosphohydrolase (NPPH) activity of ENPP1. Homozygous Arg730Stop was detected in the typical IIAC patient. The mutation was a novel nonsense mutation and not detected in 60 healthy controls. His NPPH activity was 4% of normal. On the other hand, the late-onset patient was not shown to have any mutations. NPPH activity in this patient was 70% of normal. We confirmed that ENPP1 was also responsible for the Japanese patient with IIAC. The atypical late-onset phenotype may not be associated with ENPP1 abnormalities. IIAC is considered to be a clinically and genetically heterogeneous disorder.

Characterisation of cartilage intermediate layer protein (CILP)-induced arthropathy in mice

OBJECTIVES

To characterise cartilage intermediate layer protein (CILP)-induced arthropathy in mice.

METHODS

The first and second halves of the nucleotide triphosphate pyrophosphohydrolase (NTPPHase) non-homologous region of human CILP were prepared as recombinant proteins (C1 and C2, respectively), including three overlapping fragments of C2 (C2F1, C2F2, and C2F3). C57BL/6 mice were immunised with these proteins to induce arthritis. In addition, a separate group of mice were immunised repeatedly with the mixture of C1 and C2 to see the effect of chronic immunisation. Arthritis developed in the mice, and cellular and humoral immune responses against CILP were analysed.

RESULTS

Immunisation with C2 and with the mixture C2F1/C2F2/C2F3 caused the severest arthritis to develop in mice. Immunisation with one of C1, C2F1, C2F2, or C2F3 caused milder arthritis, even though each of the fragments carried T cell epitopes. Immunisation either with C1 or C2 alone evoked cellular and humoral immune responses to both the C1 and C2 proteins. Further, the repeated immunisation with the C1/C2 mixture caused tendon calcification and bone irregularity, together with decreased NTPPH activity.

CONCLUSIONS

The results show that multiple T cell epitopes are needed for the development of CILP-induced arthritis, and present the characteristic new model of mild arthropathy accompanied by extra-articular calcifications. An immune response to putative murine CILP/NTPPH may be involved in the ectopic calcifications in the arthritic mice.

Soluble purine-converting enzymes circulate in human blood and regulate extracellular ATP level via counteracting pyrophosphatase and phosphotransfer reactions

Extracellular ATP and other purines play a crucial role in the vasculature, and their turnover is selectively governed by a network of ectoenzymes expressed both on endothelial and hematopoietic cells. By studying the whole pattern of purine metabolism in human serum, we revealed the existence of soluble enzymes capable of both inactivating and transphosphorylating circulating purines. Evidence for this was obtained by using independent assays, including chromatographic analyses with 3H-labeled and unlabeled nucleotides and adenosine, direct transfer of gamma-terminal phosphate from [gamma-32P]ATP to NDP/AMP, and bioluminescent measurement of ATP metabolism. Based on substrate-specificity and competitive studies, we identified three purine-inactivating enzymes in human serum, nucleotide pyrophosphatase (EC 3.6.1.9), 5'-nucleotidase (EC 3.1.3.5), and adenosine deaminase (EC 3.5.4.4), whereas an opposite ATP-generating pathway is represented by adenylate kinase (EC 2.7.4.3) and NDP kinase (EC 2.7.4.6). Comparative kinetic analysis revealed that the Vmax values for soluble nucleotide kinases significantly exceed those of counteracting nucleotidases, whereas the apparent Km values for serum enzymes were fairly comparable and varied within a range of 40-70 micro mol/l. Identification of soluble enzymes contributing, along with membrane-bound ectoenzymes, to the active cycling between circulating ATP and other purines provides a novel insight into the regulatory mechanisms of purine homeostasis in the blood.

Variations in site and levels of expression of chondrocyte nucleotide pyrophosphohydrolase with aging

The aim of this study was to identify changes in cartilage intermediate layer protein/nucleotide pyrophosphohydrolase (CILP/NTPPH) expression in articular cartilage during aging. Adult (3-4 years old) and young (7-10 days old) porcine articular hyaline cartilage and fibrocartilage were studied by Northern blot analysis, in situ hybridization, and immunohistochemistry using a complementary DNA (cDNA) probe encoding porcine CILP/NTPPH and antibody to a synthetic peptide corresponding to a CILP/NTPPH sequence. Northern blot analysis of chondrocytes showed lower expression of CILP/NTPPH messenger RNA (mRNA) in young cartilage than in adult cartilage. In adult cartilage, extracellular matrix from the surface to the middeep zone was immunoreactive for CILP/NTPPH, especially in the pericellular matrix surrounding the middeep zone chondrocytes. In young cartilage, chondrocytes were moderately immunoreactive for CILP/NTPPH throughout all zones except the calcified zone. The matrix of young cartilage was negative except in the superficial zone. In young cartilage, CILP/NTPPH mRNA expression was undetectable. In adult cartilage, chondrocytes showed strong mRNA expression for CILP/NTPPH throughout middeep zones. Protein and mRNA signals were not detectable below the tidemark. CILP/NTPPH secretion into matrix around chondrocytes increases with aging. In this extracellular site it may generate inorganic pyrophosphate and contribute to age-related calcium pyrophosphate dihydrate crystal deposition disease.

Calcifications of the disc of the temporomandibular joint

Calcified lesions of the temporomandibular joint discs obtained from 135 human cadavers were studied. Calcifications were observed in 92 of 250 discs by soft x-ray radiography. Studies by light and electron microscopy and x-ray diffraction revealed that there were two different types of calcification in the discs: apatite crystal deposition with or without ossification, and calcium pyrophosphate dehydrate (CPPD) crystal deposition. Calcifications were recognized more frequently posteriorly than anteriorly, and were related to disc perforation. The results of this investigation suggest that disc degeneration, which may occur as a result of aging or mechanical stress, causes calcifications.

Abnormal distribution of Fc gamma receptor type IIa polymorphisms in Korean patients with systemic lupus erythematosus

OBJECTIVE

Abnormal immune complex clearance is a feature of systemic lupus erythematosus (SLE). Polymorphisms of the Fc gamma receptor type IIa (Fc gammaRIIa) genes (the receptor binds IgG2 and IgG3) are important disease susceptibility factors in some populations. This study sought to determine the effects of these polymorphisms among Korean patients with SLE.

METHODS

Polymerase chain reaction of genomic DNA and allele-specific oligonucleotide hybridization were used to determine Fc gammaRIIa genotypes in Korean patients with SLE and healthy control subjects. Clinical manifestations were analyzed in each patient and correlated with the genotypes.

RESULTS

Among the 73 SLE patients, there was an abnormal distribution of Fc gammaRIIa alleles when compared with 64 controls: 11.0% of the SLE patients were homozygous for Fc gammaRIIa-H131 compared with 34.4% of the controls (odds ratio [OR] 0.20, 95% confidence interval [95% CI] 0.04-0.95, chi2 = 5.7, P = 0.01699). The allelic frequency of Fc gammaRIIa-H131 was significantly lower in the SLE patients than in the controls (49.3% versus 63.3%; P = 0.02019), and it was also significantly lower in lupus patients with nephritis compared with the normal population (OR 0.53, 95% CI 0.29-0.95, chi2 = 5.15, P = 0.02330), but was not significantly lower in lupus patients without nephritis (P = 0.13663 versus controls). Clinically, the level of proteinuria was significantly higher in the lupus nephritis patients who had R/R131 than in those who had H/H131 or R/H131.

CONCLUSION

An abnormal distribution of Fc gammaRIIa polymorphisms was associated with SLE in Korean patients. There was a significant decrease in the Fc gammaRIIa-H/H131 genotype and H131 allelic frequency in SLE patients, particularly in those with nephritis. This suggests that the H131 allele confers some protection from SLE in this population.

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.