FBXL12 regulates T-cell differentiation in a cell-autonomous manner

Aldehyde dehydrogenase (ALDH) activity is a hallmark of stem cells including embryonic, adult tissue and cancer stem cells. The SCF(FBXL) (12) complex is an authentic ubiquitin ligase that targets ALDH3 for degradation. FBXL12 is essential for the differentiation of trophoblast stem cells into specific cell types in the placenta during mouse embryogenesis, but its physiological functions in adult tissues have remained unknown. We have now investigated the role of the FBXL12-ALDH3 axis in the thymus, in which FBXL12 was most abundant among adult mouse tissues examined. During T-cell differentiation, FBXL12 is most abundant in CD4(+) CD8(+) (DP) cells, with its expression declining as these cells differentiate into CD4(+) CD8(-) or CD4(-) CD8(+) (SP) cells. T cells of FBXL12-null mice manifested a differentiation block at the DP-SP transition that was associated with ALDH3 accumulation in DP cells. This differentiation block was also apparent in wild-type mouse recipients of FBXL12-null bone marrow transplants as well as in FBXL12-null fetal thymic organ culture, suggesting that it is a cell-autonomous phenomenon in the thymus rather than an indirect effect of altered systemic conditions. Our results thus indicate that, in addition to its role in placental development, the FBXL12-ALDH3 axis is required for maturation of undifferentiated thymocytes.