MplK, a natural variant of the thrombopoietin receptor with a truncated cytoplasmic domain, binds thrombopoietin but does not interfere with thrombopoietin-mediated cell growth

OBJECTIVE: Interaction of thrombopoietin (TPO) with its receptor c-Mpl is responsible for the formation of megakaryocytes and platelets. In humans, there are two major c-mpl molecules, MplP and MplK, which are generated by alternative splicing. In contrast to MplP, MplK has none of the intracellular sequences required for typical signal transduction but instead has a unique 27 amino acid sequence that is coded by intron 10. We tested to determine if MplK exerts a negative effect on TPO Mpl signal transduction by interfering with the normal homodimerization of MplP. MATERIALS AND METHODS: A cassette coding for MplK cDNA was introduced into parental and MplP-expressing BaF3 cells and TPO-mediated cell growth studied. RESULTS: Cells expressing MplK alone did not respond to TPO compared to cells that expressed MplP. When MplK was coexpressed with MplP on the cell surface of BaF3, no modification in cell growth was observed when compared to those expressing MplP alone. To determine if the normal homodimerization process was negatively influenced, two genetically engineered variants of c-Mpl, one lacking the box1 sequence and the other containing only the first nine amino acids of the intracellular domain, were introduced into MplP-expressing cells. In contrast to MplK, these mutants had a dominant negative effect on TPO-mediated cell growth. CONCLUSIONS: MplK does not influence TPO-mediated growth of Mpl-expressing cells. Our data suggest that the absence of a dominant negative effect of MplK most probably is due to the inability of MplK to dimerize with the MplP receptor.