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Identification and characterization of an isoform of murine Mpl
Diana F Sabath1, Cathy Lofton-Day, Nancy Lin
1Department of Medicine, University of Washington, Harborview Medical Center, 325 Ninth Ave, Box 359756, Seattle, WA 98104, USA.
Abstract:
A new isoform of the full-length murine thrombopoietin (Tpo) receptor was isolated from a murine spleen cDNA library. This isoform, c-mpl-II, differs from full-length c-mpl (c-mpl-I) by virtue of deletion of 180 nucleotides that encode 60 amino acids located in the extracellular domain of Mpl. Normal murine megakaryocytes were found to express both c-mpl-I and c-mpl-II transcripts. BaF3 cells transfected with c-mpl-I expressed a 95 kDa protein that was displayed on the cell surface and bound 125I-Tpo. BaF3 cells transfected with c-mpl-II expressed a 70 kDa protein. However, these cells were not able to bind 125I-Tpo and surface display of Mpl-II could not be detected. In summary, c-mpl-II is an isoform of murine Mpl expressed by megakaryocytes that lacks a 60 amino acid region required for surface expression of the protein.
Insights
A novel thrombopoietin (Tpo) receptor isoform, c-mpl-II, was identified in mice. This isoform lacks a key region for surface expression, impacting Tpo binding in megakaryocytes.
Area of Science:
- Molecular Biology
- Hematology
- Cell Biology
Background:
- The thrombopoietin (Tpo) receptor, c-Mpl, plays a crucial role in megakaryopoiesis and platelet production.
- Alternative splicing can generate receptor isoforms with potentially distinct functions.
Purpose of the Study:
- To identify and characterize novel isoforms of the murine Tpo receptor (c-Mpl).
- To investigate the functional consequences of a newly discovered c-Mpl isoform on Tpo binding and cell surface expression.
Main Methods:
- Screening of a murine spleen cDNA library to isolate Tpo receptor variants.
- Nucleotide sequencing to identify structural differences between isoforms.
- Transfection of BaF3 cells with c-Mpl-I and c-Mpl-II constructs.
- Analysis of protein expression, cell surface localization, and Tpo binding via radioligand assays.
Main Results:
- Isolation of a new murine Tpo receptor isoform, designated c-Mpl-II.
- c-Mpl-II results from a 180-nucleotide deletion, encoding a 60-amino acid loss in the extracellular domain compared to full-length c-Mpl-I.
- Both c-Mpl-I and c-Mpl-II transcripts are expressed in normal murine megakaryocytes.
- BaF3 cells expressing c-Mpl-I produced a cell surface-displayed 95 kDa protein that bound 125I-Tpo.
- BaF3 cells expressing c-Mpl-II produced a 70 kDa protein, but failed to bind 125I-Tpo and showed no detectable surface expression.
Conclusions:
- c-Mpl-II is a functional isoform of the murine Mpl receptor expressed by megakaryocytes.
- The 60-amino acid region absent in c-Mpl-II is essential for the surface expression and Tpo-binding capacity of the Mpl receptor.