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Osteoblast tissue-nonspecific alkaline phosphatase antagonizes and regulates PC-1
K A Johnson1, L Hessle, S Vaingankar
1Veterans Affairs Medical Center/University of California San Diego, La Jolla 92161, USA.
Abstract:
Tissue-nonspecific alkaline phosphatase (TNAP) is essential for bone matrix mineralization, but the central mechanism for TNAP action remains undefined. We observed that ATP-dependent (45)Ca precipitation was decreased in calvarial osteoblast matrix vesicle (MV) fractions from TNAP-/- mice, a model of infantile hypophosphatasia. Because TNAP hydrolyzes the mineralization inhibitor inorganic pyrophosphate (PP(i)), we assessed phosphodiesterase nucleotide pyrophosphatase (PDNP/NTPPPH) activity, which hydrolyzes ATP to generate PP(i). Plasma cell membrane glycoprotein-1 (PC-1), but not the isozyme B10 (also called PDNP3) colocalized with TNAP in osteoblast MV fractions and pericellular matrix. PC-1 but not B10 increased MV fraction PP(i) and inhibited (45)Ca precipitation by MVs. TNAP directly antagonized inhibition by PC-1 of MV-mediated (45)Ca precipitation. Furthermore, the PP(i) content of MV fractions was greater in cultured TNAP-/- than TNAP+/+ calvarial osteoblasts. Paradoxically, transfection with wild-type TNAP significantly increased osteoblast MV fraction NTPPPH. Specific activity of NTPPPH also was twofold greater in MV fractions of osteoblasts from TNAP+/+ mice relative to TNAP-/- mice. Thus TNAP attenuates PC-1/NTPPPH-induced PP(i) generation that would otherwise inhibit MV-mediated mineralization. TNAP also paradoxically regulates PC-1 expression and NTPPPH activity in osteoblasts.
Insights
Tissue-nonspecific alkaline phosphatase (TNAP) is crucial for bone mineralization. It prevents the inhibitor pyrophosphate (PPi) from blocking mineralization by regulating its generation and activity.
Area of Science:
- Biochemistry
- Cell Biology
- Mineralization Biology
Background:
- Tissue-nonspecific alkaline phosphatase (TNAP) is vital for bone matrix mineralization.
- The precise mechanism of TNAP's action in mineralization is not fully understood.
- TNAP hydrolyzes inorganic pyrophosphate (PPi), a known inhibitor of mineralization.
Purpose of the Study:
- To elucidate the mechanism by which TNAP regulates bone mineralization.
- To investigate the role of phosphodiesterase nucleotide pyrophosphatase (PDNP/NTPPPH) and PC-1 in TNAP-mediated mineralization.
- To understand the interplay between TNAP, PC-1, and PPi in osteoblast matrix vesicles.
Main Methods:
- Analysis of matrix vesicle (MV) fractions from TNAP-/- and TNAP+/+ mice calvarial osteoblasts.
- Assessed ATP-dependent (45)Ca precipitation and PDNP/NTPPPH activity.
- Investigated colocalization of TNAP and PC-1 using immunofluorescence.
- Utilized transfection of wild-type TNAP in osteoblasts.
Main Results:
- TNAP-/- osteoblast MVs showed reduced (45)Ca precipitation.
- Plasma cell membrane glycoprotein-1 (PC-1) colocalized with TNAP and increased MV PPi, inhibiting mineralization.
- TNAP antagonized PC-1's inhibitory effect on MV-mediated (45)Ca precipitation.
- TNAP transfection paradoxically increased osteoblast MV NTPPPH activity and specific activity.
Conclusions:
- TNAP mitigates PC-1/NTPPPH-induced PPi generation, thereby preventing inhibition of MV-mediated mineralization.
- TNAP plays a dual role, directly antagonizing PPi inhibition and paradoxically regulating PC-1 expression and NTPPPH activity.
- These findings clarify TNAP's complex mechanism in regulating bone mineralization.