Takehito Tsuji

A Loss-of-Function Mutation in Natriuretic Peptide Receptor 2 (Npr2) Gene Is Responsible for Disproportionate Dwarfism in cn/cn Mouse

The achondroplastic mouse is a spontaneous mutant characterized by disproportionate dwarfism with short limbs and tail due to disturbed chondrogenesis during endochondral ossification. These abnormal phenotypes are controlled by an autosomal recessive gene (cn). In this study, linkage analysis using 115 affected mice of F2 progeny mapped the cn locus on an ∼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.

Positional cloning of the gene LIMBIN responsible for bovine chondrodysplastic dwarfism

Chondrodysplastic dwarfism in Japanese brown cattle is an autosomal recessive disorder characterized by short limbs. Previously, we mapped the locus responsible for the disease on the distal end of bovine chromosome 6. Here, we narrowed the critical region to ≈2 cM by using linkage analysis, constructed a BAC and YAC contig covering this region, and identified a gene, LIMBIN (LBN), that possessed disease-specific mutations in the affected calves. One mutation was a single nucleotide substitution leading to an activation of a cryptic splicing donor site and the other was a one-base deletion resulting in a frameshift mutation. Strong expression of the Lbn gene was observed in limb buds of developing mouse embryos and in proliferating chondrocytes and bone-forming osteoblasts in long bones. These findings indicate that LBN is responsible for bovine chondrodysplastic dwarfism and has a critical role in a skeletal development.

Role of osteoclast extracellular signal-regulated kinase (ERK) in cell survival and maintenance of cell polarity.

Morphological changes of osteoclasts by a MEK1 inhibitor, PD98059, were investigated to clarify a role of ERK. PD98059 promoted apoptosis of osteoclasts and the loss of ruffled borders. This study supports the importance of ERK in survival and polarity of osteoclasts.

CNP/NPR2 signaling maintains oocyte meiotic arrest in early antral follicles and is suppressed by EGFR-mediated signaling in preovulatory follicles.

Oocyte meiosis is arrested at prophase I by factors secreted from surrounding somatic cells after oocytes acquire meiotic competence at an early antral stage, and meiosis resumes in preovulatory follicles as a result of the luteinizing hormone (LH) surge. Recently, signaling by C‐type natriuretic peptide (CNP) through its receptor, natriuretic peptide receptor 2 (NPR2), was found to be essential for meiotic arrest at the late antral stage. Whether or not CNP/NPR2 signaling maintains oocyte meiotic arrest in earlier follicular stages and how it is associated with meiotic resumption induced by the LH surge is unclear. In this study, we examined the expression of Nppc and Npr2, respectively encoding CNP and NPR2, in the ovaries of immature mice. Nppc and Npr2 mRNA were specifically expressed in the outer and inner granulosa cell layers, respectively, in early antral follicles. Histological analysis of mice with a mutation in Npr2 revealed precocious resumption of oocyte meiosis in early antral follicles. Ovaries of mice treated with excess human chorionic gonadotropin (hCG) exhibited markedly decreased Nppc mRNA levels in granulosa cells of preovulatory follicles. Moreover, we found that amphiregulin, a mediator of LH/hCG activity through epidermal growth factor receptor (EGFR), suppressed Nppc mRNA levels in cultured granulosa cells. These results suggest that CNP/NPR2 signaling is essential for oocyte meiotic arrest in early antral follicles and that activated LH/amphiregulin/EGFR signaling pathway suppresses this signal by downregulating Nppc expression. Mol. Reprod. Dev. 79: 795–802, 2012.

NPPC/NPR2 signaling is essential for oocyte meiotic arrest and cumulus oophorus formation during follicular development in the mouse ovary

Natriuretic peptide type C (NPPC) and its high affinity receptor, natriuretic peptide receptor 2 (NPR2), have been assumed to be involved in female reproduction and have recently been shown to play an essential role in maintaining meiotic arrest of oocytes. However, the overall role of NPPC/NPR2 signaling in female reproduction and ovarian function is still less clear. Here we report the defects observed in oocytes and follicles of mice homozygous for Nppc(lbab) or Npr2(cn), mutant alleles of Nppc or Npr2 respectively to clarify the exact consequences of lack of NPPC/NPR2 signaling in female reproductive systems. We found that: i) Npr2(cn)/Npr2(cn) female mice ovulated a comparable number of oocytes as normal mice but never produced a litter; ii) all ovulated oocytes of Npr2(cn)/Npr2(cn) and Nppc(lbab)/Nppc(lbab) mice exhibited abnormalities, such as fragmented or degenerated ooplasm and never developed to the two-cell stage after fertilization; iii) histological examination of the ovaries of Npr2(cn)/Npr2(cn) and Nppc(lbab)/Nppc(lbab) mice showed that oocytes in antral follicles prematurely resumed meiosis and that immediately before ovulation, oocytes showed disorganized chromosomes or fragmented ooplasm; and iv) ovulated oocytes and oocytes in the periovulatory follicles of the mutant mice were devoid of cumulus cells. These findings demonstrate that NPPC/NPR2 signaling is essential for oocyte meiotic arrest and cumulus oophorus formation, which affects female fertility through the production of oocytes with developmental capacity.

An insertion mutation of the bovine F11 gene is responsible for factor XI deficiency in Japanese black cattle

Factor XI deficiency in Japanese black cattle is an hereditary mild bleeding disorder with an autosomal recessive mode of inheritance. To characterize the molecular lesion causing factor XI deficiency in cattle, we isolated an entire coding region of the bovine F11 gene, which comprises 15 exons and 14 introns, and determined its nucleotide sequences. Comparison of the nucleotide sequences of the F11 gene between affected and unaffected animals revealed an insertion of 15 nucleotides in exon 9 of the affected animals. The insertion results in a substitution of one amino acid with six amino acids in a highly conserved amino acid sequence in the fourth apple domain of factor XI protein. Genotyping of the F11 gene in 109 Japanese black cattle revealed that the insertion clearly corresponded to the factor XI activities of the animals. We therefore concluded that the insertion of 15 nucleotides in the F11 gene is the causative mutation for factor XI deficiency in Japanese black cattle. Genotyping of the F11gene by detecting the insertion will be an effective DNA-based diagnostic system to prevent incidence of the disease.

Hypomorphic mutation in mouse Nppc gene causes retarded bone growth due to impaired endochondral ossification

Long bone abnormality (lbab/lbab) is a spontaneous mutant mouse characterized by dwarfism with shorter long bones. A missense mutation was reported in the Nppc gene, which encodes C-type natriuretic peptide (CNP), but it has not been confirmed whether this mutation is responsible for the dwarf phenotype. To verify that the mutation causes the dwarfism of lbab/lbab mice, we first investigated the effect of CNP in lbab/lbab mice. By transgenic rescue with chondrocyte-specific expression of CNP, the dwarf phenotype in lbab/lbab mice was completely compensated. Next, we revealed that CNP derived from the lbab allele retained only slight activity to induce cGMP production through its receptor. Histological analysis showed that both proliferative and hypertrophic zones of chondrocytes in the growth plate of lbab/lbab mice were markedly reduced. Our results demonstrate that lbab/lbab mice have a hypomorphic mutation in the Nppc gene that is responsible for dwarfism caused by impaired endochondral ossification.

A missense mutation of the Dhh gene is associated with male pseudohermaphroditic rats showing impaired Leydig cell development.

Development of the male gonads is a complex process with interaction of various cells in the gonads including germ, Sertoli, Leydig, and myoid cells. TF is a mutant rat strain showing male pseudohermaphroditism, with agenesis of Leydig cells and androgen deficiency controlled by an autosomal single recessive gene (mp). The mp locus was mapped on the distal region of rat chromosome 7 by linkage analysis, but the gene responsible for the mp mutation has not been identified. In this study, we performed fine linkage mapping and sequence analysis to determine the causative gene of the mp mutation, and performed an immunohistochemical study using a Leydig cell-specific marker to investigate detailed phenotypes of the mutant rats during the testicular development. As a result, we found a missense mutation of the gene encoding Desert hedgehog (Dhh) in the mutant rat, which could result in loss of function of the DHH signaling pathway. Histochemical examination revealed remarkably reduced number of fetal Leydig cells and lack of typical spindle-shaped adult Leydig cell in the mp/mp rats. These phenotypes resembled those of the Dhh-null mice. Additionally, testosterone levels were significantly lower in the mp/mp fetus, indicating androgen deficiency during embryonic development. These results indicate that the mutation of the Dhh gene may be responsible for the pseudohermaphrodite phenotypes of the mutant rat, and that the Dhh gene is probably essential for the development of Leydig cells.

Reduced Expression of the Endothelin Receptor Type B Gene in Piebald Mice Caused by Insertion of a Retroposon-like Element in Intron 1

Mice carrying the piebald mutation exhibit white coat spotting due to the regional absence of neural crest-derived melanocytes. We reported previously that the piebald locus encodes the Ednrb gene and that piebald mice express low levels of structurally intact Ednrb mRNA and EDNRB protein (Hosoda, K., Hammer, R. E., Richardson, J. A., Baynash, A. G., Cheung, J. C., Giaid, A., and Yanagisawa, M. (1994) Cell 79, 1267–1276). Here, we report that both the life span of the Ednrb mRNA and the promoter activity of the Ednrb gene are indistinguishable between wild-type and piebald mice. Introns 2–6 of the Ednrb gene in piebald mice were correctly excised with an efficiency indistinguishable from those in wild-type mice in exon trapping experiments. We found that the piebald allele of the Ednrb gene has a 5.5-kb retroposon-like element in intron 1 possessing canonical sequences of a polyadenylation signal and a splice acceptor site. Abnormal hybrid transcripts carrying exon 1 of the Ednrb gene and a portion of the 5.5-kb element are expressed in piebald mice. The insertion of the 5.5-kb element into a heterologous intron in a mammalian expression vector markedly reduced the expression of the reporter gene. Premature termination and aberrant splicing of the Ednrb transcript caused by the retroposon-like element in intron 1 lead to a reduced level of the normal Ednrb transcript, which is responsible for the partial loss-of-function phenotype of piebald mice.

Localization of Osteoprotegerin (OPG) on Bone Surfaces and Cement Lines in Rat Tibia

Osteoprotegerin (OPG), a soluble member of the tumor necrosis factor (TNF) receptor family, is an osteoclastogenesis inhibitory factor. We investigated the localization of OPG in rat tibia using a specific peptide antibody to clarify the role of OPG in bone remodeling. OPG reactivity was mainly seen on bone surfaces. In bone matrices, OPG was also localized on cartilage/bone interfaces and cement lines. However, labeling was scarcely detected in the region of contact between osteoclasts and stromal cells. Some osteoblasts and osteocytes showed weak labeling. Immunoreactivity was not seen in chondrocytes or osteoclasts. Immunoelectron microscopic observation revealed that OPG is localized on the bone surfaces under osteoclasts. These findings suggest that OPG derived from osteoblast lineage cells and/or serum may be concentrated on resorbed bone surfaces and subsequently on cement lines. OPG may play an important role in the prevention of excess bone resorption by inhibiting differentiation and activity of osteoclasts in bone remodeling.