The Mesenchymal Stem Cell Antigen MSCA-1 is Identical to Tissue Non-specific Alkaline Phosphatase

We have recently identified 2 distinct CD271(bright)MSCA-1(dim)CD56(+) and CD271(bright)MSCA-1(bright)CD56(-) MSC subsets in primary femur-derived bone marrow (BM), which differ in their expression pattern and morphology as well as in their clonogenic and differentiation capacity. Here, we show that MSCA-1 is identical to tissue non-specific alkaline phosphatase (TNAP), an ectoenzyme known to be expressed at high levels in liver, bone, and kidney as well as in embryonic stem (ES) cells. SDS-PAGE of WERI-RB-1 cell lysate and supernatant from phosphatidylinositol-specific phospholipase C (PI-PLC)-treated WERI-RB-1 cells resulted in the appearance of a prominent 68-kDa band. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) sequence analysis revealed TNAP-specific peptides. Screening of the MSCA-1-specific antibody W8B2 on HEK-293 cells transfected with the full-length coding sequence of TNAP showed specific reactivity with transfected but not with parent cell line. In addition, TNAP-specific mRNA expression was selectively detected in the transfectant line. In agreement with these findings, enzymatic activity of TNAP was exclusively detected in sorted MSCA-1(+) BM cells but not in the MSCA-1-negative fraction. Surface marker analysis revealed coexpression of the embryonic marker SSEA-3 but not SSEA-4, TRA-1-60, and TRA-1-81. In endometrium, TNAP is expressed at intermediate levels on CD146(+) cells and at high levels in the luminal space of glandular epithelia. Our results demonstrate that TNAP is a selective marker for the prospective isolation of BM-derived MSC and MSC-like cells in endometrium.