Deficient Ghrelin Receptor-mediated Signaling Compromises Thymic Stromal Cell Microenvironment by Accelerating Thymic Adiposity^*

Abstract

With progressive aging, adipocytes are the major cell types that constitute the bulk of thymic microenvironment. Understanding the origin of thymic adipocytes and mechanisms responsible for age-related thymic adiposity is thus germane for the design of long lasting thymic rejuvenation strategies. We have recently identified that ghrelin, an orexigenic anti-inflammatory peptide, can partially reverse age-related thymic involution. Here we demonstrate that Ghrl and ghrelin receptor (growth hormone secretagogue receptor (GHSR)) are expressed in thymic stromal cells and that their expression declines with physiological aging. Genetic ablation of ghrelin and GHSR leads to loss of thymic epithelial cells (TEC) and an increase in adipogenic fibroblasts in the thymus, suggesting potential cellular transitions. Using FoxN1Cre;R26RstopLacZ double transgenic mice, we provide qualitative evidence that thymic epithelial cells can transition to mesenchymal cells that express proadipogenic regulators in the thymus. We found that loss of functional Ghrl-GHSR interactions facilitates EMT and induces thymic adipogenesis with age. In addition, the compromised thymic stromal microenvironment due to lack of Ghrl-GHSR interactions is associated with reduced number of naive T cells. These data suggest that Ghrl may be a novel regulator of EMT and preserves thymic stromal cell microenvironment by controlling age-related adipocyte development within the thymus.