Changes with age of the rat fetuin concentration in serum and its mRNA expression

Biosynthesis of the vitamin K-dependent matrix Gla protein (MGP) in chondrocytes: a fetuin-MGP protein complex is assembled in vesicles shed from normal but not from osteoarthritic chondrocytes

OBJECTIVE

Mineralization has been observed in osteoarthritic cartilage but the mechanisms are incompletely understood. Vitamin K is an essential cofactor in post-translational modification of proteins where specific Glu residues become modified to Ca(++) binding gamma-carboxyglutamic acid residues (Gla). One such protein, matrix Gla protein (MGP), is a known mineralization inhibitor. This study determined if synthesis of MGP and formation of a fetuin-MGP protein complex was altered in chondrocytes and vesicles from osteoarthritis (OA) cartilage.

METHODS

Chondrocytes and vesicles were isolated from normal and OA human articular cartilage and lysates prepared. Specific antibodies were used in immunoblotting to detect the mature fully gamma-carboxylated form of MGP (cMGP) and non-gamma-carboxylated MGP (ucMGP) as well as fetuin and MGP-fetuin complexes. gamma-carboxylase activity was measured by (14)CO(2) incorporation into the carboxylase peptide substrate FLEEL. Immunocytochemistry was used to examine fetuin in cartilage sections and uptake of biotin-labeled fetuin by isolated chondrocytes.

RESULTS

Chondrocytes and vesicles from osteoarthritic tissue produced significantly less cMGP compared to those from normal cartilage. This correlated with significantly less vitamin K-dependent gamma-carboxylase enzyme activity in OA chondrocytes. Fetuin was found to be present in articular cartilage and cultured chondrocytes were capable of fetuin uptake. A fetuin-MGP complex was identified in normal chondrocytes and in vesicles shed from these cells but not in OA cells or vesicles.

CONCLUSIONS

The absence of cMGP and of the cMGP-fetuin complex in OA cells and OA vesicles may be an important mechanism for increased mineralization of osteoarthritic cartilage.

Overexpression of fetuin-a counteracts ectopic mineralization in a mouse model of pseudoxanthoma elasticum (abcc6(-/-))

The pathologic hallmark of pseudoxanthoma elasticum (PXE) is ectopic mineralization of soft connective tissues. Recent studies have suggested that PXE is a metabolic disease, and perturbations in a number of circulatory factors have been postulated. One of them is fetuin-A, a 60-kDa glycoprotein synthesized in the liver and secreted into blood. Observations in targeted mutant mice (Ahsg(-/-)) and in cell culture model systems have shown that fetuin-A is a powerful anti-mineralization factor in circulation, and the serum levels of fetuin-A in patients with PXE as well as in a mouse model of PXE (Abcc6(-/-)) have been shown to be reduced by up to 30%. In this study, we tested the hypothesis that overexpression of fetuin-A in Abcc6(-/-) mice counteracts the ectopic mineralization. Delivery of an expression construct containing full-length mouse fetuin-A complementary DNA (cDNA), linked to a His-tag, to the liver of these mice resulted in elevated serum levels of this protein. As a consequence, soft tissue mineralization, which is a characteristic of Abcc6(-/-) mice, was reduced by approximately 70% at 12 weeks of age, but the effect was transient when examined 4 weeks later. The results suggest that normalization of serum fetuin-A, either through gene therapy approaches or by direct protein delivery to the circulation, may offer strategies for treating PXE and perhaps other heritable disorders of soft tissue mineralization.

Fetuin A stabilizes m-calpain and facilitates plasma membrane repair

Yeast two-hybrid experiments identified alpha(2)-Heremans-Schmid glycoprotein (human fetuin A) as a binding partner for calpain domain III (DIII). The tandem DIIIs of calpain-10 interacted under the most selective culture conditions, but DIIIs of m-calpain, calpain-3, and calpain-5 also interacted under less stringent selection. DIIIs of mu-calpain, calpain-6, and the tandem DIII-like domains of the Dictyostelium Cpl protein did not interact with alpha(2)-Heremans-Schmid glycoprotein in the yeast two-hybrid system. Bovine fetuin A stabilized proteolytic activity of purified m-calpain incubated in the presence of mm calcium chloride and prevented calcium-dependent m-calpain aggregation. Consistent with the yeast two-hybrid studies, fetuin A neither stabilized mu-calpain nor prevented its aggregation. Confocal immunofluorescence microscopy of scratch-damaged L6 myotubes demonstrated accumulation of m-calpain at the wound site in association with the membrane repair protein, dysferlin. m-Calpain also co-localized with fluorescein-labeled fetuin A at the wound site. The effect of fetuin A on calpain-mediated plasma membrane resealing was investigated using fibroblasts from Capns1(-/-) and Capns1(+/+) mouse embryos. Capns1 encodes the small noncatalytic subunit that is required for the proteolytic function of m- and mu-calpains. Thus, Capns1(-/-) fibroblasts do not express these calpains in active form. Fetuin A increased resealing of scrape-damaged wild-type fibroblasts but not Capns1(-/-) fibroblasts. These studies identify fetuin A as a potential extracellular regulator of m-calpain at nascent sites of plasma membrane wounding.

Biochemical characterization of the serum fetuin-mineral complex

The present study was carried out to characterize the fetuin-mineral complex (FMC), a high molecular mass complex of calcium phosphate mineral and the proteins fetuin and matrix Gla protein (MGP) that was initially discovered in serum of rats treated with etidronate and appears to play a critical role in inhibiting calcification in vivo. Fetuin purified from the FMC contains 3.3 mol of protein-bound phosphate. There is 1.3 mg of FMC/ml of serum 6 h after etidronate injection, and the FMC is 46% fetuin and 53% mineral by mass. Formation of the FMC in the first 6 h after etidronate injection does not increase serum fetuin despite the fact that 50% of serum fetuin is associated with the FMC, and clearance of the FMC in the 9-24-h interval lowers total serum fetuin by 50%. These observations suggest that the fetuin component of the FMC is derived from fetuin initially in serum and that clearance of the FMC removes the associated fetuin from circulation. One additional protein was consistently present in all preparations of the FMC, spp24 (secreted phosphoprotein 24). This 24-kDa protein is similar in domain structure to fetuin and, like fetuin and MGP, contains several residues of phosphoserine and accumulates in bone. Exogenous spp24 associated strongly with the FMC when added to serum containing it. These observations suggest that spp24 may, like fetuin and MGP, play a role in inhibiting calcification.

Tornado extraction: a method to enrich and purify RNA from the nephrogenic zone of the neonatal rat kidney

BACKGROUND

Development of the kidney is a complicated and tightly regulated process. Although several genes responsible for the renal development have been identified to date, the precise mechanisms of spatial and temporal regulation remain to be elucidated. Therefore, expanding our knowledge of molecules that are associated with nephrogenesis will be helpful to understand the whole process.

METHODS

To extract RNA selectively from the nephrogenic zone of the developing kidney, we developed a simple and reliable method.

RESULTS

This method, named "tornado extraction," enriched RNA of the nephrogenic zone by about 30-fold. In combination with the suppression subtractive hybridization, a considerable number of genes that were differentially expressed in the nephrogenic zone were obtained. These genes included a series of endodermal markers such as albumin and alpha-fetoprotein as well as GDNF (glia-derived neurotrophic factor), osteoblast-specific factor-2 (OSF-2)/periostin and fetuin (one of the major serum proteins in the fetus).

CONCLUSION

Tornado extraction has great value in studying genes in the nephrogenic zone of the developing kidney. Since the quality of RNA obtained by this method is excellent, tornado extraction is suitable in combination with other techniques including the subtractive hybridization method and DNA microarray analysis.

Discovery of a high molecular weight complex of calcium, phosphate, fetuin, and matrix gamma-carboxyglutamic acid protein in the serum of etidronate-treated rats

In the present study we report the discovery of a novel protein-mineral complex in the serum of rats treated with doses of the bone-active bisphosphonate etidronate that inhibit normal bone mineralization. The composition of this high molecular mass protein-mineral complex consists of about 18% mineral, 80% fetuin, and 2% matrix Gla protein (MGP) by weight, and the presence of the complex in serum after an injection of 8 mg etidronate/100 g of body weight elevates calcium by 1.8-fold (to 4.3 mm), phosphate by 1.6-fold (to 5.6 mm), and MGP by 25-fold (to 12 microg/ml). The serum mineral complex reaches maximal levels at 6 h after subcutaneous injection of etidronate and is subsequently cleared from serum by 24 h. This highly specific complex of fetuin, MGP, and mineral prevents the growth, aggregation, and precipitation of the mineral component, which indicates that the previously reported calcification inhibitory activities of fetuin and MGP may be related to their ability to form stable complexes with nascent mineral nuclei. Treatment with the vitamin K-antagonist warfarin prevents the increase in serum MGP after etidronate injection, which shows that the increase in serum MGP is due to new synthesis and that the gamma-carboxylation of MGP is necessary for its binding to the serum mineral complex.

Fetuin in the developing neocortex of the rat: distribution and origin

Immunocytochemical distribution of the fetal protein fetuin in the neocortex of developing rat brain and the presence of its mRNA, as detected by using reverse transcriptase-polymerase chain reaction analysis, was studied in fetuses at embryonic day 15 (E15) through E22, in neonates at postnatal day 0 (P0) through P20, and in adults. Quantitative estimates of fetuin in cerebrospinal fluid (CSF) and plasma were obtained over the same period. Exogenous (bovine) fetuin injected intraperitoneally into fetal and postnatal rats was used to study the uptake of fetuin into CSF and brain and its distribution compared with endogenous fetuin; bovine albumin was used as a control. Fetuin was identified immunocytochemically in the cortical plate and subplate cells of the developing neocortex. In the rat fetus, fetuin first was apparent at E17, mainly in cell processes, but a few subplate cells also were positive. By E18, there was strong staining in subplate neurons and in inner cells of the cortical plate. At E21, these inner cells of the cortical plate were beginning to differentiate into layer VI neurons, many of which were positive for fetuin. By P0-P1, more layer VI neurons and some layer V neurons had become positive for fetuin. Fetuin immunoreactivity generally was weaker at P1, and, by P2-P3, it had disappeared from all of the layers of the developing neocortex. Bovine fetuin (but not albumin), probably taken up through CSF over the neocortical dorsal surface, had a cytoplasmic distribution; endogenous rat fetuin was both cytoplasmic and membrane bound. Thus, much of this fetuin can be accounted for by uptake, although the presence of fetuin mRNA indicates that in situ synthesis may also contribute.