UC San Diego

c-Mpl, an cell surface receptor in hematopoietic stem and progenitor cells, activates survival and differentiation signaling pathways upon binding its ligand, thrombopoitein. Understanding the regulation of this influential receptor is of great importance to hematology for academic studies and clinical science. In this study, we characterized the clathrin-mediated internalization of activated c-Mpl and the processes leading to c-Mpl degradation. RNAi experiments revealed the importance of adaptor protein 2 (AP2) in mediating the internalization of c-Mpl. We identify, in c-Mpl, two motifs (Y₈RRL and Y7₈RRL) that match a consensus sequence shown to bind to AP2 (YXX[Phi]). Experiments with c-Mpl mutants in these mofits showed great inhibition of c-Mpl internalization. Y₈RRL was also found to influence targeting of c-Mpl to lysosomal degradation. Previous studies have reported presence of endosomal signaling from activated receptors internalized from the cell surface. Through inhibition of receptor internalization experiments with wildype c-Mpl, we also presented data that suggest c-Mpl exhibit endosomal signaling and showed receptor internalization plays a crucial role for c-Mpl to trigger AKT, ERK1/2, and Jak2 activation pathways