ESRP1-Induced CD44 v3 Is Important for Controlling Pluripotency in Human Pluripotent Stem Cells.

The importance of alternative splicing (AS) events in pluripotency regulation has been highlighted by the determination of different roles and contributions of different splice isoforms of pluripotency-related genes and by the identification of distinct pluripotency-related splicing factors. In particular, epithelial splicing regulatory protein 1 (ESRP1) has been characterized as an essential splicing factor required for the regulation of human pluripotency and differentiation. Nevertheless, a detailed molecular characterization of ESRP1 (mRNA splice variants 1-6) in human pluripotency is lacking. In this study, we determined that ESRP1 splice variants are differentially expressed in undifferentiated and differentiated human pluripotent stem cells (PSCs). Undifferentiated human PSCs predominantly expressed the ESRP1 v1, v4, and v5, and their expression was downregulated upon differentiation. Ectopic expression of ESRP1 v1, v4, or v5 enhanced the pluripotent reprogramming of human fibroblasts and restored the ESRP1 knockdown-mediated reduction of reprogramming efficiency. Notably, undifferentiated human PSCs expressed the cell surface protein CD44 variant 3 (CD44 v3), and isoform switching from CD44 v3 to CD44 variant 6 (CD44 v6) occurred upon differentiation. Importantly, the human PSC-specific ESRP1 variants influenced CD44 v3 expression. CD44 knockdown or inhibition of binding of CD44 with its major ligand, hyaluronan, significantly induced the loss of human PSC pluripotency and the reduction of reprogramming efficiency. Our results demonstrate that the effect of ESRP1 and CD44 on human PSC pluripotency is isoform-dependent and that ESRP1-induced CD44 v3 is functionally associated with human PSC pluripotency control. Stem Cells 2018;36:1525-1534.