A novel Foxn1^eGFP/+ mouse model identifies Bmp4‐induced maintenance of Foxn1 expression and thymic epithelial progenitor populations

Abstract

Although forkhead-box n1 (Foxn1) is a critical thymic epithelial cell regulator in thymus organogenesis, its association with epithelial differentiation and homeostasis in the postnatal and aged thymic microenvironment remains conflicting. Consequently, we have generated a Foxn1 knock-in mouse model that allows for refined investigation of the aging thymic epithelium. This reporter line differs from those previously published in that concomitant expression of enhanced green fluorescent protein enables live cell sorting of Foxn1 cell populations. Our heterozygotes did not exhibit haploinsufficiency, with Foxn1 expression resembling that of wild-type mice. Comparative analysis between Foxn1 and enhanced green fluorescent protein at both the transcriptional and translational levels revealed co-localization, with progressive down-regulation observed predominantly in the aging cortical epithelium. Supplementation with bone morphogenetic protein (Bmp)-4 enhanced Foxn1 expression and colony forming efficiency in both embryonic and adult progenitor 3D cultures. Strikingly, selective maintenance of immature cortical and medullary epithelial cells was observed which is consistent with the higher Bmp receptor 2 expression levels seen in these progenitor populations. This study demonstrates the significance of our mouse model in unraveling the role of this master regulator in thymus development, homeostasis and aging, providing a faithful reporter system for phenotypic and functional investigations.

Gender Disparity Impacts on Thymus Aging and LHRH Receptor Antagonist-Induced Thymic Reconstitution Following Chemotherapeutic Damage

“…The post-pubertal increase in Bmp4 expression by supporting stromal cells was, however, greater in females, and implicates better progenitor maintenance. Its receptor, Bmpr2, is primarily expressed on cTEC lo and mTEC lo progenitors (42), and was proposed to have a role in maintaining progenitor populations at the expense of differentiation (10). However, Bmp4 induced self-renewal of progenitors may also have a role in thymus regeneration following damage, as demonstrated following irradiation (43).…”

Interplay between Follistatin, Activin A and Bmp4 Signaling Regulates Postnatal Thymic Epithelial Progenitor Cell Differentiation During Aging

“…Furthermore, endogenous regeneration following thymic damage was shown to involve TEC stimulation by endothelial cell-derived BMP4, which subsequently increased FOXN1 expression (Wertheimer et al, 2018). TEC expression of the BMP4 receptor BMPR2 was predominantly in TEPCs, Sca-1 lo cTEC lo precursors, and mTEC lo precursors of 7-week-old mice (Barsanti et al, 2017). Although the majority of BMP4 production in the adult thymus is derived from fibroblasts and endothelial cells (Barsanti et al, 2017), in addition to an increase in fibroblast BMP4 expression with aging, we identified increased BMP4 expression by TECs ( Figure 3F).…”

“…We have previously shown that BMP4 supplementation of in vitro 3D postnatal TEC cultures maintained cTEC lo and mTEC lo precursors at the expense of differentiation into their mature counterparts (Barsanti et al, 2017). Furthermore, endogenous regeneration following thymic damage was shown to involve TEC stimulation by endothelial cell-derived BMP4, which subsequently increased FOXN1 expression (Wertheimer et al, 2018).…”

Epithelial Development Based on a Branching Morphogenesis Program: The Special Condition of Thymic Epithelium

“…Transgenic expression of Noggin under the control of a FoxN1 promoter leads to a hypoplastic spheric thymus always containing big cystic structures [125]. These structural alterations seem to be the result of a branching defect in consonance with the known role of BMP signaling in regulating branching morphogenesis of different organs [26] and to affect Foxn1 expression [147]. Figure 2.…”