Wnt4 and LAP2alpha as pacemakers of thymic epithelial senescence

Age-associated thymic involution has considerable physiological impact by inhibiting de novo T-cell selection. This impaired T-cell production leads to weakened immune responses. Yet the molecular mechanisms of thymic stromal adipose involution are not clear. Age-related alterations also occur in the murine thymus providing an excellent model system. In the present work structural and molecular changes of the murine thymic stroma were investigated during aging. We show that thymic epithelial senescence correlates with significant destruction of epithelial network followed by adipose involution. We also show in purified thymic epithelial cells the age-related down-regulation of Wnt4 (and subsequently FoxN1), and the prominent increase in LAP2alpha expression. These senescence-related changes of gene expression are strikingly similar to those observed during mesenchymal to pre-adipocyte differentiation of fibroblast cells suggesting similar molecular background in epithelial cells. For molecular level proof-of-principle stable LAP2alpha and Wnt4-over-expressing thymic epithelial cell lines were established. LAP2alpha over-expression provoked a surge of PPARgamma expression, a transcription factor expressed in pre-adipocytes. In contrast, additional Wnt4 decreased the mRNA level of ADRP, a target gene of PPARgamma. Murine embryonic thymic lobes have also been transfected with LAP2alpha- or Wnt4-encoding lentiviral vectors. As expected LAP2alpha over-expression increased, while additional Wnt4 secretion suppressed PPARgamma expression. Based on these pioneer experiments we propose that decreased Wnt activity and increased LAP2alpha expression provide the molecular basis during thymic senescence. We suggest that these molecular changes trigger thymic epithelial senescence accompanied by adipose involution. This process may either occur directly where epithelium can trans-differentiate into pre-adipocytes; or indirectly where first epithelial to mesenchymal transition (EMT) occurs followed by subsequent pre-adipocyte differentiation. The latter version fits better with literature data and is supported by the observed histological and molecular level changes.