Defining The Molecular Networks Necessary For Thymus Fate And Organogenesis

Author

Kaitlin A. Reeh, The University of Texas Graduate School of Biomedical Sciences at HoustonFollow

Date of Graduation

12-2015

Document Type

Dissertation (PhD)

Program Affiliation

Epigenetics and Molecular Carcinogenesis

Degree Name

Doctor of Philosophy (PhD)

Advisor/Committee Chair

Ellen R. Richie, PhD

Committee Member

Sharon Y. R. Dent, PhD

Committee Member

Richard D. Wood

Committee Member

Mark T. Bedford, PhD

Committee Member

Steven A. Vokes, PhD

Abstract

The thymus and parathyroid (PT) glands originate from endodermal progenitors in the bilateral third pharyngeal pouches (3^rd pps). By E11.5 during mouse development, cells committed to the thymus lineage express Foxn1 whereas PT-fated cells express Gcm2. While these transcription factors are required for organ-specific differentiation, the exact molecular mechanisms that specify endodermal progenitors to either the thymus or parathyroid lineage are not well defined. Tbx1 is initially expressed throughout the 3^rd pp endoderm, as it is required for segmentation of the pharyngeal apparatus, but is downregulated in the thymus-fated domain by E10.5. Despite the widely held notion that Tbx1 is required for thymus organogenesis, we have shown that ectopic expression of Tbx1 in thymic epithelial cells (TECs) suppresses FOXN1, inhibits TEC proliferation and arrests TEC differentiation, suggesting Tbx1 must be tightly regulated in the 3^rd pp endoderm for proper thymus organogenesis to occur. Members of the miR-17-92 cluster downregulate Tbx1 in cardiac progenitor cells to permit cardiomyocyte differentiation, and we demonstrated that members of this cluster are expressed in the 3^rd pp endoderm and mesenchyme. We find that global or TEC-specific deletion of miR-17-92 enhances TBX1 expression and reduces FOXN1 in the 3^rd pp. Furthermore, global deletion of miR-17-92 results in an ectopic, hypoplastic thymus lobe, while deletion in TECs results in TBX1⁺ progenitor cells that persist in the fetal thymus. In contrast, overexpression of miR-17-92 in TECs results in downregulation of TBX1 in the dorsal 3^rd pp and surrounding mesenchyme. Therefore, these data suggest that miR-17-92 plays an essential role in thymus development by regulating Tbx1 expression in the 3^rd pp.

Additionally, previous work from our lab has shown that a genetic deficiency in neural crest cells (NCCs) expands thymus fate in the 3^rd pp. We show that NCCs mediate this effect in part by promoting TBX1 expression in the dorsal 3^rd pp. Finally, we present evidence consistent with the dual hypothesis that Fgf8 expression in the ventral 3^rd pp promotes thymus fate by restricting TBX1. Based on these results, we generated a working model describing the signaling networks that contribute to thymus fate and 3^rd pp patterning.

Keywords

Thymus, Tbx1, Foxn1, miR-17-92