Research

Molecular analysis of the bacterial toxin entry and application for medicine

The Project Leader's Profile

Eisuke Mekada

Yamagata University, Faculty of Science (1970-1974)
Research Fellow, Research Institute for Microbial Diseases, Osaka University (1974-1979)
Research Associate, Research Institute for Microbial Diseases, Osaka University (1979-1989)  
Associate Professor, Institute of Molecular and Cellular Biology, Osaka University (1989)
Professor, Institute of Life Science, Kurume University (1989-1999)
Professor, Research Institute for Microbial Diseases, Osaka University (2000- )

Research Members
  • Ryo Iwamoto (Associate Professor, Research Institute for Microbial Diseases, Osaka University)
  • Hiroto Mizushima (Assistant Professor, Research Institute for Microbial Diseases, Osaka University)

Summary

Bacterial toxins are frequently responsible for the pathogenesis associated with infection. Many toxins bind to specific proteins (toxin receptors) on the host cell membrane, enter host cells, and exert toxic effects. Binding of diphtheria toxin to its receptor, proHB-EGF, plays a critical role in the generation of toxicity. Diphtheria toxin binds to the EGF domain of proHB-EGF, and this facilitates toxin entry into cells by endocytosis.  The diphtheria toxin receptor forms a complex with membrane proteins such as CD9.   Using cultured cells, we have been conducting comprehensive analyses of the mechanisms of toxin entry through activation of the diphtheria toxin receptor complex.  The diphtheria toxin receptor functions as a growth factor for mammalian cells.  proHB-EGF is a membrane-anchored growth factor belonging to the EGF family, and it promotes the growth and differentiation of various tissues.  It plays a critical role in cardiac formation, myocardial maintenance, and wound healing in the skin.  proHB-EGF exists on the cell membrane as a membrane-anchored form, is cleaved by proteases on the cell surface, and secreted as a soluble form (sHB-EGF).  To function as a growth factor, conversion from the membrane-anchored form (proHB-EGF) to the soluble form (sHB-EGF) is required. Aberrant regulation of this cleavage event gives rise to serious anomalies including fetal tissue hyperplasia. We will conduct analyses of the regulation of the conversion of proHB-EGF to sHB-EGF, the physiological significance of this transformation, and oncogenesis caused by excess cleavage.

14mekata.jpg Figure Legend
(Left)
Diphtheria toxin consists of fragment A (shown in red) and fragment B (shown in green).
Diphtheria toxin binds to the diphtheria toxin receptor in a complex with proteins such as CD9 via fragment B, and it is endocytosed into the endosomal system. Fragment A then passes through the endosomal membrane into the cytosol and inactivates elongation factor 2 (EF2), resulting in the inhibition of protein synthesis in the host cell.
(Right)
proHB-EGF, a diphtheria toxin receptor, is cleaved by a protease on the cell surface to generate a soluble form, sHB-EGF. sHB-EGF binds to EGF receptors in a paracrine or autocrine manner and initiates a signaling cascade via the Ras/ERK pathway.  Both the synthesis and cleavage of proHB-EGF are induced by this signaling cascade. Additionally, multiple GPCR ligands and stress-induced molecules lead to sHB-EGF production.  Consequently, these conditions generate a positive feedback loop and ultimately lead to tissue hyperplasia and malignant transformation.

Some of Recent Papers

  1. Iwamoto, R., et al.  HB-EGF and ErbB signaling is essential for heart function (2003) Proc. Natl. Acad. Sci. U.S.A. 100, 3221-3226.
  2. Yamazaki,  S., et al.   Mice with defects in HB-EGF ectodomain shedding show severe developmental abnormalities. (2003) J. Cell Biol. 163, 469-475.
  3. Miyamoto, S., et al.  Heparin-binding EGF-like growth factor and the LPA-induced ectodomain shedding pathway is a promising target for the therapy of ovarian cancer.  (2004) Cancer Res. 64, 5720-5727.
  4. Mine, N., Iwamoto, R. and Mekada, E. HB-EGF promotes epithelial cell migration in eyelid development.  (2005) Development 132, 4317-4326.
  5. Minami, S., Iwamoto, R. and Mekada, E. HB-EGF decelerates cell proliferation synergistically with TGF-a in perinatal distal lung development. (2008) Dev. Dyn. 237, 247-258.