The Role and Molecular Mechanisms of Rex1 in Pluripotent Stem Cells

Abstract

Pluripotent stem cells (PSCs) are unique in their capability to self-renew and differentiate into cell types of all three embryonic germ layers. Since their discovery, PSCs have become an indispensable tool for modeling development, disease onset/progression, and drug discovery. The pluripotent state is known to be regulated by a core network of transcription factors including Oct4, Sox2, and Nanog. However, the roles of other contributing transcription factors remain understudied. Our research focused on defining the roles and molecular mechanisms of Rex1, a zinc finger transcription factor whose expression is strongly correlated with the pluripotent state. Attempts by our lab to elucidate the role of Rex1 in embryonic stem cells (ESCs) revealed the presence of two smaller protein products that result from the initiation of translation at downstream start codons within the REX1 open reading frame. We hypothesized that the full-length Rex1 protein and its shorter alternative translation isoforms were acting to regulate the expression of lineage-determining genes in PSCs. To evaluate this hypothesis, we generated mouse embryonic stem cell (mESC) lines expressing FLAG-tagged versions of the full-length Rex1 protein, and its isoforms, from the endogenous locus. Through the use of these lines, we demonstrated the formation of multiple Rex1 isoforms by alternative translation, a novel observation that has yet to be reported. Furthermore, our results indicate that Rex1 is a negative regulator of differentiation-related genes and endogenous retroviral elements, suggesting Rex1 is acting to maintain the tightly regulated transcriptional network of pluripotency, while also maintaining genomic integrity through the repression of repetitive elements.