AGING OF THYMIC EPITHELIAL PROGENITOR POOL IS DETERMINED BY THE P63-FOXN1 REGULATORY AXI

The thymus is a vital organ necessary to fight infections. Unfortunately, it shrinks with age resulting in a compromised function. Our lab uses mice to study the events leading to the shrinkage, hoping to one day develop therapeutic agents to reverse the compromised function. Purpose (a): The postnatal thymic epithelial progenitor (TEP) pool is proposed to be regulated by the p63 and FoxN1 genes through proliferation and differentiation, respectively. However, the combined role of these two genes in the aging TEP is still a mystery. Evidence from murine models has elucidated contrasting roles of the p63 isoforms during the aging process. Methods (b): Wild type and FoxN1 cKO mice were used throughout the experiment to evaluate the amount of p21, p63, and FoxN1 present. Immunofluorescence and senescence staining was performed on frozen sections harvested from euthanized mice. PEI intrathymic injections were performed with either TAp63 or FoxN1 cDNA. Results (c): We found that TAp63+, but not ΔNp63+, thymic epithelial cells (TECs) were increased with age, accompanied with increased senescence associated β-gal clusters and p21+ TECs. Senescent clusters also developed after intrathymic infusion of exogenous TAp63 cDNA into young wild-type mice. Using our conditional FoxN1 gene knockout mouse model to disrupt TEP differentiation accelerated this senescent phenotype to early middle age. However, upon infusion of exogenous FoxN1 cDNA into aged wild-type mice resulted in only an increase in ΔNp63+ TECs, but no change in TAp63+ TECs in the partially rejuvenated aged thymus. Interestingly, using a novel FoxN1 transgenic mouse model to enhance TEP differentiation, ΔNp63+ TECs were decreased in young thymus. Additionally, the TAp63+ population contained a high percentage of phosphorylated-p53 and apoptotic TECs, but showed no changes in BrdU-labeled proliferation. Conclusions (d): FoxN1 controlled TEC differentiation as a bottleneck to determine TEP pool via affecting TAp63 and DNp63 levels. Thus, TEC homeostasis during aging has been determined through the p63-FoxN1 regulatory axis.