Differential Properties of Human ALP+ Periodontal Ligament Stem Cells vs Their ALP- Counterparts

Characterizing subpopulations of stem cells is important to understand stem cell properties. Tissue-nonspecific alkaline phosphatase (ALP) is associated with mineral tissue forming cells as well as stem cells. Information regarding ALP subpopulation of human periodontal ligament stem cells (hPDLSCs) is limited. In the present study, we examined ALP+ and ALP- hPDLSC subpopulations, their surface markers STRO-1 and CD146, and the expression of stemness genes at various cell passages. We found that ALP+ subpopulation had higher levels of STRO-1 (30.6 ± 5.6%) and CD146 (90.4 ± 3.3%) compared to ALP- (STRO-1: 0.5 ± 0.1%; CD146: 75.3 ± 7.2%). ALP+ cells expressed significantly higher levels of stemness associated genes, NANOG, OCT4 and SOX than ALP- cells at low cell passages of 2-3 (p<0.05). ALP+ and ALP- cells had similar osteogenic, chondrogenic and neurogenic potential while ALP-, not ALP+ cells, lacked adipogenic potential. Upon continuous culturing and passaging, ALP+ continued to express higher stemness genes and STRO-1 and CD146 than ALP- cells at ≥passage 19. Under conditions (over-confluence and vitamin C treatment) when ALP+ subpopulation was increased, the stemness gene levels of ALP+ was no longer significantly higher than those in ALP- cells. In conclusion, ALP+ hPDLSCs possess differential properties from their ALP- counterparts.