MOUSE EMBRYONIC STEM CELLS EXPRESS FUNCTIONAL TOLL LIKE RECEPTOR 2

Abstract

Embryonic stem cells (ESCs) are unique in that they have potential to give rise to every cell type of the body. Little is known about stimuli that promote mouse (m)ESC differentiation and proliferation. Therefore the purpose of this study was to determine the role of Toll Like Receptor (TLR) ligands in mESCs proliferation, survival, and differentiation in the presence of Leukemia Inhibitory Factor (LIF). We hypothesized that TLRs are expressed and functional, and when activated by their ligand will induce survival, proliferation, and prevent differentiation. In this study, mESC line E14 was used to determine the expression of TLRs at the mRNA level and three mESC lines, R1, CGR8, and E14, were used to determine cell surface protein levels. We found expression of TLRs 1, 2, 3, 5, and 6 at the mRNA level, but no expression of TLRs 4, 7, 8, and 9 in the E14 mESC line. We confirmed the presence of TLR-2 but not of TLR-4, protein on the cell surface using flow cytometric analysis for all three cell lines. We focused our studies mainly on TLR-2 using the E14 cell line. Pam3Cys, is a synthetic triacyl lipoprotein and a TLR-2 ligand, which induced a significant increase in mESC proliferation on Days 3, 4, and 5 and enhanced survival of mESC in a dose dependent manner in the context of delayed addition of serum. All the latter experiments were performed in triplicate and student T-test was performed to establish significant differences. Next, we demonstrated functionality of TLR-2 via the MyD88/IKK pathway, where MyD88 was expressed and IKKα/β phosphorylation was enhanced. This was associated with increased NF-κB nuclear translocation upon activation by Pam3Cys. Finally, we showed that there were no changes in expression of mESCs markers Oct-4, KLF-4, Sox-2, and SSEA-1, thus illustrating that the mESCs may have remained in a pluripotent state after activation with the TLR-2 ligand in the presence of LIF. These results demonstrate that mESCs can respond to microbial products, such as Pam3Cys, and can induce proliferation and survival of the mESCs. This finding expands the role of TLRs and has some implications in understanding embryonic stem cell biology.