Lipid rafts integrity is essential for the prolactin-induced mitogenesis in mouse embryonic stem cells.

Embryonic stem cells, ESCs, retain the capacity to self-renew, yet, the protein machinery essential in maintaining this undifferentiated status remains largely undefined. Signalling interactions are initiated and enhanced at the plasma membrane lipid rafts, within constrains and regulation applied by the actin and tubulin cytoskeleton systems. First, we undertook a comprehensive approach using twodimensional gel electrophoresis and mass spectrometry analysis combined with Western blotting and immunofluorescence analyses at the single cell level to compile the proteome profile of detergentfree preparations of lipid rafts of E14 mouse embryonic stem cells. In comparison with the proteomic profiles of other membrane fractions, recovery of actin and tubulin network proteins, including folding chaperones, was impressively high. At equally high frequency we detected annexins, pleiotropic proteins that may bind membrane lipids and actin filaments to regulate important membrane processes, and we validated their expression in lipid rafts. Next, we tested whether lipid raft integrity is required for completion of mitogenic signalling pathways. Disruption of the rafts with the cholesterol sequestering methyl-β-cyclodextrin (MCD) greatly downregulated the mitotic index of ESCs, in a dose- and time of exposure-dependent manner. Moreover, MCD greatly reduced the mitogenic actions of prolactin, a hormone known to stimulate proliferation in a great variety of stem and progenitor cells. Taken together, our data postulate that lipid rafts in ESCs are in close association with the actin and tubulin cytoskeletons to support signal compartmentalization, especially for signalling pathways pertinent to symmetric divisions for self-renewal.