UBC Theses and Dissertations
Functional analysis of murine CD43 shedding : a role for the CD43 cytoplasmic tail in nuclear signalling Seo, Wooseok
CD43, a representative of the leukocyte mucin family proteins, is a transmembrane protein highly expressed on most lymphohemopoietic cells and is believed to play a role in the regulation of leukocyte activation and/or migration. CD43 was shown to be proteolytically shed from human cells and high concentrations of soluble CD43 have been found in human plasma. The biological significance of CD43 shedding however remains enigmatic. To study the functional significance of CD43 shedding, we initiated our study by investigating whether CD43 shedding also occurs in the murine system and confirmed using flow cytometry, Western blot and ELISA techniques that murine CD43 is cleaved from the cell surface as is observed in the human system. To examine the biological significance of CD43 shedding, we designed and constructed non-sheddable forms of murine CD43. Ectopic expression of non-sheddable CD43 molecules in primary CD43 deficient bone marrow cells showed that these CD43 mutants have serious negative impacts on cell viability, revealing CD43 shedding as an essential process and implying that the CD43 mutants interfered with intracellular signaling processes. Our data support the hypothesis that CD43 ectodomain shedding is a requirement for release of the cytoplasmic domain and its translocation to the nucleus. In support of our hypothesis, we confirmed that the CD43 cytoplasmic domain is localized in the nucleus and is modified by SUMO (small ubiquitin-like modifier) peptides. In an attempt to determine the functional significance of CD43 nuclear translocation and SUMO modification, we examined nuclei from hemopoietic cells more closely and observed that the CD43 cytoplasmic tail is localized in a subnuclear structure called promyelocytic nuclear bodies, which control many nuclear functions including apoptosis. Consistent with this observation we find that leukocytes from CD43 deficient mice have an increased apoptotic response upon growth factor withdrawal. We conclude that nuclear translocation of the CD43 cytoplasmic tail serves to control the apoptotic response in leukocytes and that CD43 functions as an anti-apoptotic molecule.
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