Influence of inorganic phosphate on the activity determination of isoenzymes of alkaline phosphatase in various buffer systems

Restriction of Dietary Phosphate Ameliorates Skeletal Abnormalities in a Mouse Model for Craniometaphyseal Dysplasia

Craniometaphyseal dysplasia (CMD), a rare genetic bone disorder, is characterized by lifelong progressive thickening of craniofacial bones and metaphyseal flaring of long bones. The autosomal dominant form of CMD is caused by mutations in the progressive ankylosis gene ANKH (mouse ortholog Ank), encoding a pyrophosphate (PPi) transporter. We previously reported reduced formation and function of osteoblasts and osteoclasts in a knockin (KI) mouse model for CMD (Ank^KI/KI) and in CMD patients. We also showed rapid protein degradation of mutant ANK/ANKH. Mutant ANK protein displays reduced PPi transport, which may alter the inorganic phosphate (Pi) and PPi ratio, an important regulatory mechanism for bone mineralization. Here we investigate whether reducing dietary Pi intake can ameliorate the CMD-like skeletal phenotype by comparing male and female Ank^+/+ and Ank^KI/KI mice exposed to a low (0.3%) and normal (0.7%) Pi diet for 13 weeks from birth. Serum Pi and calcium (Ca) levels were not significantly changed by diet, whereas PTH and 25-hydroxy vitamin D (25-OHD) were decreased by low Pi diet but only in male Ank^+/+ mice. Importantly, the 0.3% Pi diet significantly ameliorated mandibular hyperostosis in both sexes of Ank^KI/KI mice. A tendency of decreased femoral trabeculation was observed in male and female Ank^+/+ mice as well as in male Ank^KI/KI mice fed with the 0.3% Pi diet. In contrast, in female Ank^KI/KI mice the 0.3% Pi diet resulted in increased metaphyseal trabeculation. This was also the only group that showed increased bone formation rate. Low Pi diet led to increased osteoclast numbers and increased bone resorption in all mice. We conclude that lowering but not depleting dietary Pi delays the development of craniofacial hyperostosis in CMD mice without severely compromising serum levels of Pi, Ca, PTH, and 25-OHD. These findings may have implications for better clinical care of patients with CMD. © 2020 American Society for Bone and Mineral Research.

Bioactive characteristics of an implant surface coated with a pH buffering agent: an in vitro study

Purpose

The purpose of this study was to evaluate the effectiveness of conventional sandblasted, large-grit, acid-etched (SLA) surface coated with a pH buffering solution based on surface wettability, blood protein adhesion, osteoblast affinity, and platelet adhesion and activation.

Methods

Titanium discs and implants with conventional SLA surface (SA), SLA surface in an aqueous calcium chloride solution (CA), and SLA surface with a pH buffering agent (SOI) were prepared. The wetting velocity was measured by the number of threads wetted by blood over an interval of time. Serum albumin adsorption was tested using the bicinchoninic acid assay and by measuring fluorescence intensity. Osteoblast activity assays (osteoblast adhesion, proliferation, differentiation, mineralization, and migration) were also performed, and platelet adhesion and activation assays were conducted.

Results

In both the wetting velocity test and the serum albumin adsorption assay, the SOI surface displayed a significantly higher wetting velocity than the SA surface (P=0.000 and P=0.000, respectively). In the osteoblast adhesion, proliferation, differentiation, and mineralization tests, the mean values for SOI were all higher than those for SA and CA. On the osteoblast migration, platelet adhesion, and activation tests, SOI also showed significantly higher values than SA (P=0.040, P=0.000, and P=0.000, respectively).

Conclusions

SOI exhibited higher hydrophilicity and affinity for proteins, cells, and platelets than SA. Within the limits of this study, it may be concluded that coating an implant with a pH buffering agent can induce the attachment of platelets, proteins, and cells to the implant surface. Further studies should be conducted to directly compare SOI with other conventional surfaces with regard to its safety and effectiveness in clinical settings.

In vitro responses to electrosprayed alkaline phosphatase/calcium phosphate composite coatings

Surface modification of titanium implants to improve their fixation in bone tissue is of great interest. We present a novel approach to enhance implant performance by applying important principles of bone mineralization to biomedical coatings. As an attempt to mimic the biphasic biomineralization process, both the enzyme alkaline phosphatase (ALP) and calcium phosphate (CaP) were immobilized onto Ti discs, thereby triggering enzymatically and physicochemically controlled biomineralization pathways. ALP, CaP and ALP-CaP composite coatings with preserved functionality of ALP were successfully deposited using electrospray deposition. In vitro soaking studies in cell culture medium revealed that crystal growth initially proceeded at a faster rate on CaP-coated Ti than on ALP-containing coatings, but mineral deposition onto ALP-coated Ti caught up with the calcification behaviour of CaP coatings upon long-term soaking. Cell culture experiments with osteoblast-like cells, however, demonstrated the opposite effect in mineral deposition on the electrosprayed CaP and ALP coatings. The ALP-CaP composite coatings showed delayed proliferation as well as accelerated mineralization in comparison to cells cultured on the CaP-coated and uncoated Ti. In conclusion, these in vitro results showed that the osteogenic potential of Ti can be stimulated by ALP-containing coatings.

A novel fluorometric assay for ADP-ribose pyrophosphatase activity

ADP-ribose pyrophosphatase (ADPRase) hydrolyzes ADP-ribose to ribose-5-phosphate and AMP. The ADPRase activity have been assessed by coupling the reaction to alkaline phosphatase and colorimetrically measuring the amount of inorganic phosphate released from AMP that is one of the products of ADPRase. Another but less sensitive colorimetric method has been employed: the reaction mixture was treated with charcoal to adsorb the adenine-containing compounds such as AMP and ADPR and subsequently remaining ribose-5-phosphate was measured colorimetrically. However, the measurement of inorganic phosphate cannot be feasible to assay ADPRase in phosphate-containing samples and the determination of ribose-5-phosphate also is less sensitive. Here we develop a fluorescent assay for ADPRase that utilizes 1, N(6)-etheno ADP-ribose, a fluorescent analogue of ADP-ribose. This method measures fluorescent 1, N(6)-etheno adenosine that is produced by coupling the hydrolysis of 1, N(6)-etheno ADP-ribose to dephosphorylation with alkaline phosphatase. The fluorometric assay is comparable in sensitivity and useful for ADPRase assay in phosphate-containing samples.

Mechanism of action of beta-glycerophosphate on bone cell mineralization

Experiments were performed to determine whether beta-glycerophosphate (beta-GP) promoted mineralization in vitro by modulating bone cell metabolic activity and/or serving as a local source of inorganic phosphate ions (Pi). Using MC3T3-E1, ROS 17/2.8, and chick osteoblast-like cells in the presence of beta-GP or Pi, we examined mineral formation, lactate generation, alkaline phosphatase (AP) activity, and protein and phospholipid synthesis. Neither beta-GP nor Pi modulated any of the major biosynthetic activities of the bone cells. Thus, we found no change in the levels of phospholipids, and the total protein concentration remained constant. Measurement of lactate synthesis showed that beta-GP did not effect the rate of anaerobic glycolysis. Evaluation of medium Pi levels clearly indicated that beta-GP was hydrolyzed by bone cells; within 24 hours, almost 80% of 10 mM beta-GP was hydrolyzed. It is likely that this local increase in medium Pi concentration promoted rapid mineral deposition. Chemical, energy dispersive X-ray, and Fourier transform infrared analysis of the mineral formed in the presence of beta-GP showed that it was nonapatitic; moreover, mineral particles were also seen in the culture medium itself. Experiments performed with a cell-free system indicated that mineral particles formed spontaneously in the presence of AP and beta-GP and were deposited into a collagen matrix. We conclude that medium supplementation with beta-GP or Pi should not exceed 2 mM. If this value is exceeded, then there will be nonphysiological mineral deposition in the bone cell culture.