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J. Biol. Chem., Vol. 280, Issue 14, 14288-14292, April 8, 2005
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From the Graduate School of Natural Science and Technology, Okayama University, 1-1-1, Tsushima-naka, Okayama 700-8530, Japan
Received for publication, January 19, 2005
| ABSTRACT |
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0.8-cM region of chromosome 4, and natriuretic peptide receptor 2 (Npr2) gene was identified as the most potent candidate for the cn mutant in this region. This gene encodes a receptor for C-type natriuretic peptide (CNP) that positively regulates longitudinal bone growth by producing cGMP in response to CNP binding to the extracellular domain. Sequence analyses of the Npr2 gene in cn/cn mice revealed a T to G transversion leading to the amino acid substitution of highly conserved Leu with Arg in the guanylyl cyclase domain. In cultured chondrocytes of cn/cn mice, stimulus with CNP did not significantly increase intracellular cGMP concentration, whereas it increased in +/+ mice. Transfection of the mutant Npr2 gene into COS-7 cells also showed similar results, indicating that the missense mutation of the Npr2 gene in cn/cn mice resulted in disruption of the guanylyl cyclase activity of the receptor. We therefore concluded that the dwarf phenotype of cn/cn mouse is caused by a loss-of-function mutation of the Npr2 gene, and cn/cn mouse will be a useful model to further study the molecular mechanism regulating endochondral ossification by CNP/natriuretic peptide receptor B signal. | INTRODUCTION |
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The natriuretic peptide (NP)1 family comprises atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) (4), and three receptors for NPs, including natriuretic peptide receptor A (NPRA), natriuretic peptide receptor B (NPRB), and natriuretic peptide receptor C (NPRC), have been identified in mammals (57). NPRA and NPRB consist of extracellular ligand binding, transmembrane, protein kinase homology, and guanylyl cyclase catalytic domains. These receptors mediate ligand signals by producing an intracellular second messenger, cyclic GMP (cGMP) (2). Both ANP and BNP bind to NPRA with high affinity, and CNP prefers binding to NPRB (8). NPRC has a ligand-binding domain and a short cytoplasmic domain that lacks guanylyl cyclase activity and is considered to be involved in NP clearance (7). ANP and BNP mainly reside in the atrium and ventricle, respectively, and act as cardiac hormones that regulate central fluid volume, blood pressure, and the development of cardiovascular tissues (9). On the other hand, CNP gene (Nppc) and NPRB gene (Npr2) expressions have been detected in mice tibial growth plates (10, 11) and in the chondrogenic cell line ATDC5 (12). Since CNP stimulates the longitudinal growth of cultured fetal mouse tibias (13) and CNP-deficient mice develop dwarfism due to impaired endochondral ossification (11), the CNP/NPRB pathway has been considered to be involved in the process of endochondral ossification. However, there is no direct evidence that NPRB effects longitudinal bone growth by regulating endochondral ossification.
The achondroplastic mouse is a mutant strain with an autosomal recessive gene (cn) that arose spontaneously in the AKR/J mouse strain (14). The homozygous (cn/cn) mouse exhibits disproportionate dwarfism with short limbs, short tail, and a domed skull. These phenotypes are distinguishable from normal littermates by 7 days after birth and thereafter gradually become more prominent (15, 16). Histological analyses have revealed that the tibial epiphyseal growth plates of cn/cn mice are thinner than normal mice, and the hypertrophic chondrocyte zone is considerably narrowed (17, 18). These findings suggested that the dwarf phenotype of cn/cn mouse was caused by retarded longitudinal bone growth due to disturbed endochondral ossification. Thus, the cn/cn mouse has been believed to be a useful model for hereditary human skeletal dysplasias. However, the gene responsible for the skeletal abnormalities of the cn/cn mouse has not yet been identified. In this study, we determined the chromosomal localization of the cn locus and identified the causative mutation in the Npr2 gene.
| MATERIALS AND METHODS |
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Linkage AnalysisF1 mice were generated by mating heterozygous (cn/+) mice with JF1 (+/+) mice, and the F2 progeny was subsequently obtained from intercrossing heterozygous (cn/+)F1 mice. Genomic DNA was extracted from mouse livers by phenol/chloroform extraction. PCR reactions for microsatellite markers proceeded as follows: 35 cycles at 94 °C for 30 s, 5560 °C for 30 s, and 72 °C for 45 s. The primer sequences of newly generated microsatellite markers, D4Mok1 (5'-GATTTGAGTTTCTGACACCTCCC-3' and 5'-ACTTGCTTGTTTGCGTGTGGGC-3') and D4Mok2 (5'-TCCTTGGCTGATGCCTAAGC-3' and 5'-GGTCTAGGAAGCTAACTCAGTGG-3'), were obtained from genomic sequences located at 42.2 Mb (GenBankTM accession number NT_039260 [GenBank] ) and 43.0 Mb (GenBankTM accession number NT_039296 [GenBank] ) positions on mouse chromosome 4, respectively. The reaction mixture for PCR (10 µl) contained 1x PCR buffer, 0.2 mM dNTP, a 0.5 µM concentration of each primer, and 0.25 unit of TaqDNA polymerase (Amersham Biosciences). The PCR products were fractionated on 3.0% agarose gels and stained with ethidium bromide. The data were analyzed using Map Manager computer software.
Detection of Mutation in Npr2 GeneTibial RNA was extracted using TRIzol reagent (Invitrogen) according to the manufacturer's instructions. The extracted RNA was then incubated with RNase-free DNase I (Takara, Ohtsu, Japan) for 1 h at 37 °C to remove contaminating genomic DNA. First strand cDNA was synthesized for reverse transcription-PCR using the Superscript preamplification system (Invitrogen) according to the manufacturer's instructions. Npr2 cDNA was amplified with specific primer (5'-TGTATCCGGGCCGACTAAGCTTGC-3 and 5'-GGAAACACAGTGACCATTGCCCATCC-3') under the following conditions: 94 °C for 2 min followed by 35 cycles of 94 °C for 30 s, 64 °C for 20 s, and 72 °C for 4 min. The amplified fragments were sequenced using an ABI PRIZM 310 Genetic Analyzer (Applied Biosystems, Foster City, CA).
To detect the mutation in the genomic DNA of mice by restriction enzyme digestion, DNA fragments, including exon 18 of Npr2 gene, were amplified from genomic DNA using specific primers (5'-TTCACAGCGCTGTCAGCTGAG-3' and 5'-ACTTAGGGAGCGCTGACTGTGG-3'). PCR reaction was carried out as follows: 35 cycles of 94 °C for 30 s, 62 °C for 20 s, and 72 °C for 30 s. The PCR products were digested with AflIII restriction enzyme at 37 °C, and electrophoresed on 3% agarose gel.
Culture of Mouse Chondrocytes and COS-7 CellsChondrocytes were prepared from the costal cartilage of cn/cn or +/+ mice (n = 5) at postnatal day 7 as described (19). COS-7 cells were obtained from the RIKEN cell bank (Tsukuba, Japan). These cells were cultured in Dulbecco's modified Eagle's medium containing 10% fetal bovine medium, 100 units/ml penicillin, and 100 µg/ml streptomycin at 37 °C under 5% CO2 in air. The chondrocytes were seeded in 12-well multiwell plates, and cGMP levels were measured when the cells reached confluence. The COS-7 cells were plated at a density of 1 x 104 cells/well in 96-well multiwell plates and used for transfection after incubation for 24 h.
Transient Expression of Npr2 Gene in Cultured CellsFull-length Npr2 cDNA was amplified by reverse transcription-PCR with tibial RNA extracted from cn/cn or +/+ mice using primers containing EcoRI or the NotI site (5'-CGGAATTCTATCGCCATGGCACTGCCATC-3 and 5'-AGCAGAAGGCGGCCGCCCTGGGCTTTACAG-3'). PCR was performed at 94 °C for 2 min followed by 35 cycles of 94 °C for 30 s, 64 °C for 20 s, and 72 °C for 4 min. Amplified fragments were digested with EcoRI and NotI restriction enzyme and then ligated into pcDNA3.1 mammalian expression vector (Invitrogen). The COS-7 cells were transfected with the DNA constructs using FuGENE 6 (Roche Diagnostics), and the transfected cells were subjected to cGMP level measurement after incubation for 24 h.
Measurement of cGMP Levels in Cultured CellsThe cultured cells were washed twice with phosphate-buffered saline, and incubated in Dulbecco's modified Eagle's medium containing 0.1% fetal bovine serum and 0.5 mM 3-isobutyl-1-methylxanthine at 37 °C for 15 min. Thereafter, various concentrations of CNP were added to the medium, and the incubation proceeded at 37 °C for a further 15 min. The amount of cGMP was measured using a cGMP enzyme immunoassay Biotrak system (Amersham Biosciences).
| RESULTS |
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20-cM intervals using the 115 affected mice. Significant linkage was detected between the cn locus and markers on the proximal region of chromosome 4, indicating the localization of the cn locus on this region. To narrow the critical interval of the cn locus, we further genotyped eight markers located at the proximal region of chromosome 4. As shown in Fig. 1A, no recombination was observed between the cn locus and D4Mok1, and at least one recombination was observed with the other markers. These data indicated that the cn locus was mapped on the 0.8-cM interval between D4Mit182 and D4Mit109, D4Mit213, and D4Mit257 (Fig. 1B).
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| DISCUSSION |
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The cn/cn mice exhibit disproportionate dwarfism with short limbs and tail, and the prominent abnormality during endochondral ossification consists of decreased numbers of proliferating and hypertrophic chondrocytes in a narrowed epiphyseal growth plate (17, 18, 25, 26). These skeletal phenotypes of cn/cn mice are morphologically and histologically very similar to those of CNP-deficient mice (11). In addition, decreased levels of cGMP are detected in both the tailbone of CNP-deficient mice and in cells expressing the mutated Npr2 gene (11). Although CNP not only binds to NPRB with high affinity but also binds NPRA with low affinity (8, 27), NPRA is expressed slightly in mouse tibia (13), and NPRA-deficient mice show no skeletal abnormality (29). Taken together, these findings indicate that the NPRB/cGMP pathway selectively mediates the CNP signal to promote longitudinal bone growth by regulating endochondral ossification.
Although the phenotypes of cn/cn and CNP-deficient mice are similar in terms of gross appearance and in the histological features of tibial growth plate, the survival rates of these mice are quite different. The mortality of CNP-deficient mice is 70% at 100 days of age (11), while that of the cn/cn mice used in this study was less than 10% (data not shown). CNP functions not only in endochondral ossification but also in cardiovascular regulation and vascular tone (30). In addition, CNP has been detected in various tissues such as the brain, spinal cord, heart, kidney, lung, thymus, liver, stomach, uterus, and ovary (31). Thus, the CNP signal, possibly mediated by other receptor(s) including NPRA, might play a role in these tissues other than skeletal tissues. Another possible explanation for this discrepancy in mortality is their distinct genetic backgrounds. Lane and Dickie (14) described that most cn/cn mice used in their study died before 3 months of age. The cn/cn mice used in their study had an AKR/J background, while the cn/cn mice used in this study had a mixed background of C57BL/6J, C3H/HeDiSn, and LG/J (14, 25). Since it is unclear whether cn/cn and the CNP-deficient mice died from the same causes, further studies on the cause of death of cn/cn and the CNP-deficient mice with an identical genetic background would provide a novel function of CNP and/or NPRB in tissues other than in bone growth.
Acromesomelic dysplasia Maroteaux type (AMDM) is an autosomal recessive human disorder characterized by disproportionate short stature and shortening of the extremities (32), but its exact etiology remains unclear because of the lack of a suitable animal model for this disorder. Mutations in the NPR2 gene have recently been identified in affected families, and declined guanylyl cyclase activities of NPRB of mutant alleles have also been observed (20). With respect to the symptoms of AMDM, the disproportionately short limbs become more apparent during childhood than during the postnatal and neonatal periods (33). The skeletal abnormalities of cn/cn mice also became progressively more obvious with age (15, 16). Therefore, cn/cn mouse is a useful model for investigating the pathogenesis and therapeutic approaches of AMDM.
It is apparent that both CNP and cGMP signals play important roles in the regulation of endochondral ossification. However, the precise role of the signal pathway through NPRB during endochondral ossification remains unclear. The cn/cn mouse is the first animal model with a mutated Npr2 gene and can be used to evaluate the physiological function of NPRB. In cn/cn mice, the proliferative activities of proliferating chondrocytes in the epiphyseal growth plate are significantly lower than in control mice (25, 26). The numbers of proliferating and hypertrophic chondrocytes are also severely reduced in cn/cn mice (17, 18). The signal pathway through NPRB therefore appears to positively affect chondrocyte proliferation and differentiation. However, further studies are needed to elucidate how this pathway regulates chondrocyte proliferation and differentiation. Fibroblast growth factor 3 (FGFR3) is a one of the most important factors that negatively regulate the proliferation and differentiation of chondrocytes (34). Mice expressing constitutively activated Fgfr3 histologically exhibit very similar chondrocytic abnormalities in the epiphyseal growth plates to those of cn/cn mice (34). Furthermore, CNP transgenic mice rescue the achondroplastic phenotype of Fgfr3 mutant mice by inhibiting the MAPK pathway through FGFR3, and new therapeutic approaches via the activation of CNP/NPRB have been suggested for the treatment of human achondroplasia (28). Taken together, CNP signals mediated by NPRB might regulate chondrocyte proliferation and differentiation during endochondral ossification through modification of the signal from FGFR3. Further studies using cn/cn mice, as well as CNP-deficient mice, will help to clarify the role of the CNP/NPRB/cGMP pathway in chondrocytes during endochondral ossification and to develop novel therapeutic approaches for human skeletal dysplasias.
| FOOTNOTES |
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To whom correspondence should be addressed. Tel.: 81-86-251-8325; Fax: 81-86-251-8388; E-mail: takehito{at}cc.okayama-u.ac.jp.
1 The abbreviations used are: NP, natriuretic peptide; ANP, atrial NP; BNP, brain NP; NPRA, NP receptor A; NPRB, NP receptor B; NPRC, NP receptor C; CNP, C-type NP; AMDM, acromesomelic dysplasia Maroteaux type. ![]()
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