Cloning and analysis of the 5' region of the rat bone/liver/kidney/placenta alkaline phosphatase gene. A dual-function promoter

Tissue-Nonspecific Alkaline Phosphatase, a Possible Mediator of Cell Maturation: Towards a New Paradigm

Alkaline phosphatase (ALP) is a ubiquitous membrane-bound glycoprotein capable of providing inorganic phosphate by catalyzing the hydrolysis of organic phosphate esters, or removing inorganic pyrophosphate that inhibits calcification. In humans, four forms of ALP cDNA have been cloned, among which tissue-nonspecific ALP (TNSALP) (TNSALP) is widely distributed in the liver, bone, and kidney, making it an important marker in clinical and basic research. Interestingly, TNSALP is highly expressed in juvenile cells, such as pluripotent stem cells (i.e., embryonic stem cells and induced pluripotent stem cells (iPSCs)) and somatic stem cells (i.e., neuronal stem cells and bone marrow mesenchymal stem cells). Hypophosphatasia is a genetic disorder causing defects in bone and tooth development as well as neurogenesis. Mutations in the gene coding for TNSALP are thought to be responsible for the abnormalities, suggesting the essential role of TNSALP in these events. Moreover, a reverse-genetics-based study using mice revealed that TNSALP is important in bone and tooth development as well as neurogenesis. However, little is known about the role of TNSALP in the maintenance and differentiation of juvenile cells. Recently, it was reported that cells enriched with TNSALP are more easily reprogrammed into iPSCs than those with less TNSALP. Furthermore, in bone marrow stem cells, ALP could function as a "signal regulator" deciding the fate of these cells. In this review, we summarize the properties of ALP and the background of ALP gene analysis and its manipulation, with a special focus on the potential role of TNSALP in the generation (and possibly maintenance) of juvenile cells.

Liver-type of tissue non-specific alkaline phosphatase is induced during mouse bone and tooth cell differentiation

BACKGROUND AND OBJECTIVE

Tissue non-specific alkaline phosphatase (TNSALP) contains two types-bone- and liver-type-which are produced from the same gene due to differences in splicing. These two differ in their promoter, but the amino acid sequences of the mature proteins are identical. In this study, we examined the relationship between the two types of TNSALP expression and osteoblast differentiation.

DESIGN

Gene expression of the two types of TNSALP was observed by reverse transcription-polymerase chain reaction. MC3T3-NM4 was sub-cloned from an established mouse osteoblastic cell line in which osteoblast characters do not appear without dexamethasone. The C2C12 mouse myoblastic cell line, which can be induced to osteoblasts with bone morphogenic protein 2, and organ-cultured tooth germs were also used in this work.

RESULTS

The gene expression of liver-type TNSALP was observed in only MC3T3-NM4 activated by dexamethasone. For C2C12, the gene expression of bone-type TNSALP was observed even in non-induced conditions where myotubes were formed, whereas the liver-type TNSALP mRNA was only expressed when C2C12 differentiated into osteoblasts by bone morphogenic protein 2. Furthermore, in the organ-cultured tooth germs, the liver-type TNSALP mRNA was expressed according to differentiation of tooth germs.

CONCLUSION

These results suggest that the liver-type TNSALP mRNA is induced according to differentiation of bone and tooth.

Human bone marrow stromal cells are efficiently transduced by vesicular stomatitis virus-pseudotyped retrovectors without affecting subsequent osteoblastic differentiation

This study tested the transduction efficiency of human bone marrow stromal cells (hBMSCs) with vesicular stomatitis virus (VSV)-pseudotyped retrovectors and their subsequent osteogenic differentiation in vitro. Two different retrovectors encoding beta-galactosidase (beta-gal) or enhanced green fluorescent protein (eGFP) as marker genes were examined for transduction of hBMSCs. hBMSCs were obtained from bone marrow filtrates of normal donors (aged 5-35 years), cultured in alpha-minimal essential medium (alpha-MEM) containing 10% fetal calf serum and infected with retrovectors soon after the adherent cells started to form individual colonies. Transduced hBMSCs were observed to express eGFP protein 4-7 days after infection in primary cultures, and the majority of hBMSCs were eGFP-positive. hBMSCs were also stained for beta-gal in the secondary cultures and virtually all hBMSCs expressed beta-gal activity. Transduced hBMSCs were examined for their osteogenic potential. These cells were found to express markers of osteogenic differentiation, including alkaline phosphatase, type I collagen, bone sialoprotein, decorin, and osteocalcin, as strongly as uninfected control cells. Mineralization was also induced by dexamethasone in transduced cells as well as control cells. These results demonstrate that hBMSCs are highly susceptible to infection with VSV-pseudotyped retrovectors with the majority of cultured cells expressing the viral transgenes without antibiotic selection. Transduced cells retain their osteogenic potential in vitro. hBMSCs are a promising cellular vehicle for systemic human gene therapy and VSV-pseudotyped retrovectors should be effective for their in vitro transduction prior to cellular engraftment.

Melatonin promotes osteoblast differentiation and bone formation

Prior studies have demonstrated that the pineal hormone, melatonin, can stimulate chloramphenicol acetyltransferase activity in Drosophila SL-3 cells transfected with a chloramphenicol acetyltransferase reporter construct containing the response element of rat bone sialoprotein (BSP). Based on these findings, studies were performed to determine whether melatonin could similarly modulate the expression of BSP in two cell lines, the MC3T3-E1(MC3T3) pre-osteoblast and rat osteoblast-like osteosarcoma 17/2.8 cell. Initial studies demonstrated that MC3T3 cells grown in the presence of 50 nM melatonin underwent cell differentiation and mineralization by day 12 instead of the 21-day period normally required for cells grown in untreated media. Melatonin increased gene expression of BSP and the other bone marker proteins, including alkaline phosphatase (ALP); osteopontin; secreted protein, acidic and rich in cysteine; and osteocalcin in MC3T3 cells in a concentration-dependent manner. Levels of melatonin as low as 10 nM were capable of stimulating transcription of these genes when cells were grown in the presence of beta-glycerophosphate and ascorbic acid. Under these conditions, melatonin induced gene expression of the bone marker proteins; however, this does not occur until the 5th day after seeding the culture dishes. Thereafter, MC3T3 cells responded to melatonin within 2 h of treatment. The fully differentiated rat osteoblast-like osteosarcoma 17/2.8 cells responded rapidly to melatonin and displayed an increase in the expression of BSP, ALP, and osteocalcin genes within 1 h of exposure to the hormone. To determine whether melatonin-induced osteoblast differentiation and bone formation are mediated via the transmembrane receptor, MC3T3 cells were treated in the presence and absence of melatonin with either luzindole, a competitive inhibitor of the binding of melatonin to the transmembrane receptors, or pertussis toxin, an uncoupler of G(i) from adenylate cyclase. Both luzindole and pertussis toxin were shown to reduce melatonin-induced expression of BSP and ALP. These results demonstrate, for the first time, that the pineal hormone, melatonin, is capable of promoting osteoblast differentiation and mineralization of matrix in culture and suggest that this hormone may play an essential role in regulating bone growth.

Phosphate regulates the stability of skeletal alkaline phosphatase activity in human osteosarcoma (SaOS-2) cells without equivalent effects on the level of skeletal alkaline phosphatase immunoreactive protein

Inorganic phosphate (P(i)) can regulate the level of skeletal alkaline phosphatase (ALP) activity in human osteoblast-like cells, but not by means of changes in transcription or release from the cell surface. The current studies were intended to determine whether (1) P(i) affected the inactivation of ALP activity in human osteosarcoma (SaOS-2) cells; and (2) P(i)-dependent changes in ALP-specific activity were associated with equal, concomitant changes in the level of ALP immunoreactive protein. The results of these studies revealed that P(i) increased the stability of skeletal ALP activity without equivalent effects on the level of ALP immunoreactive protein. An increase in P(i) (from 0 to 1.8 mmol/liter) caused a time-dependent increase in the amount of skeletal ALP activity in the SaOS-2 cells, without a parallel increase in the amount of skeletal ALP immunoreactive protein, and a decrease in P(i) (from 1.8 to 0 mmol/liter) caused a time-dependent decrease in the amount of ALP activity, without a significant decrease in the total cellular content of ALP immunoreactive protein. Together, these observations suggest that P(i) may alter the level of skeletal ALP activity in SaOS-2 cells by inhibiting a process of irreversible inactivation that does not effect equal, concomitant changes in the level of skeletal ALP immunoreactive protein.

Tissue-specific and dexamethasone-inducible expression of alkaline phosphatase from alternative promoters of the rat bone/liver/kidney/placenta gene

The rat Bone/Liver/kidney/Placenta Alkaline Phosphatase (ALP) is transcribed from two alternative promoters spaced over 25 kb apart, resulting in two variant transcripts that are identical in their coding sequence. We investigated the steady-state levels of the two variant transcripts in various rat tissues and cell lines using the polymerase chain reaction (PCR) amplification for RNA phenotyping, RNase protection, and northern blot analysis. Our results demonstrate that ALP transcripts from the upstream promoter are preferentially expressed in calvariae, and are almost exclusively expressed in ROS17/2.8 osteogenic sarcoma cells. In contrast, the downstream promoter is preferentially expressed in kidney. Moreover, the increase in ALP activity and mRNA levels following dexamethasone treatment of ROS17/2.8 cells is correlated with an increase in the level of transcripts from the upstream promoter. Thus, the two alternative promoters of the rat BLKP ALP gene are involved in cell-specific and dexamethasone-inducible regulation of its expression.

Regulation of alkaline phosphatase and alpha 2(I) procollagen synthesis during early intramembranous bone formation in the rat mandible

We have studied intramembranous bone formation in the developing rat mandible. In this system discrete developmental stages can be readily distinguished: mesenchymal condensation, osteoid deposition, and mineralization. In mandibles of 14-day rat embryos avascular condensed mesenchymal cells can be discerned in a region lateral to Meckel's cartilage and anterior to the first molar bud. In 18-day embryos primary bone structures with mineral deposition are evident, and at 2 days postnatally the mandible is extensively mineralized. In the developing mandible we investigated the pattern of bone/liver/kidney/placenta (BLKP) alkaline phosphatase (ALP) and alpha 2(I) procollagen expression in the differentiating osteoblasts. The level of ALP activity in loose mesenchymal tissue is close to background levels. In contrast, the condensed mesenchymal cells in 14-day embryos, which will subsequently form bone, display intense ALP activity prior to discernible osteoid or mineral deposition. ALP activity in the condensed mesenchymal cells can be inhibited by levamisole, indicating activity of the BLKP gene product. We could not detect a corresponding increase in transcript level for either ALP or alpha 2(I) in the condensed mesenchyme in 14-day embryo using in situ hybridization, probably due to low message abundance. At 18 days, cells throughout the developing mandible express ALP activity, and intense in situ hybridization to BLKP ALP probes is evident in cells lining the developing bone trabeculae. Alpha 2(I) procollagen transcripts have accumulated in cells of the developing mandibular bone, but are not specifically localized to osteoblastic cells. Our results demonstrate that ALP activity is a very early marker of differentiation of cells of the osteogenic lineage, since a marked increase in ALP enzyme activity is clearly detectable in condensed mesenchymal cells prior to osteoid or mineral deposition. In contrast, Wright and Leblond, using the same model system and immunohistochemistry, could not localize type I collagen to preosteoblastic cells surrounding the developing bone trabeculae, and demonstrated localization of type I collagen to osteoblasts bordering developing trabeculae, indicating a substantial increase in type I collagen expression (at least at the protein level) during preosteoblast to osteoblast differentiation. These results indicate a discrete pattern of regulation for both the ALP and alpha 2(I) genes during osteogenic differentiation, which may involve both transcriptional and posttranscriptional regulation.