Research Group / Research Projects / Major publications / Laboratory HomePage /

Research Group

Research Projects

Fig.1 Most membrane-bound organelles inside eukaryotic cells are linked to each other by dynamic membrane trafficking that is regulated by a set of specific proteins. Membrane trafficking is essential not only for the survival of all cells but also for the functions of multicellular networks such as the nervous and immune systems. We aim to elucidate the molecular mechanisms behind membrane trafficking and the roles they play in normal and diseased animal physiology. This knowledge will in turn help improve clinical medical understanding and practice. We are currently focusing on two trafficking routes, namely, autophagy and the endocytic pathway.
Fig.1: ¡ÈRoad map¡É of membrane traffic

Autophagy is a membrane trafficking process that delivers cytoplasmic components to lysosomes for bulk degradation. The process is mediated by the formation of the double membrane-bound autophagosomes. By identifying proteins involved in autophagy in mammalian cells, we have helped to elucidate its molecular basis. In addition, identification of the autophagy-related proteins enabled us to discover a novel physiological role of autophagy, as we found that Group A Streptococcus invading host cells are engulfed by a large autophagosome in the cytosol and then eventually killed. This finding indicates that not only does autophagy play its well known role in metabolism. It also functions¡¡in innate immunity. Furthermore, we found that autophagy participates in removing misfolded proteins that could otherwise gradually accumulate, aggregate and damage the cell. Thus, autophagy may protect against diseases such as Alzheimer's and Creutzfeldt-Jakob diseases, which result from the accumulation of misfolded proteins. We are currently analyzing in more detail the molecular mechanisms behind autophagosome formation and its intracellular dynamics along with examining the cell-protective role of autophagy.
Fig.2
Fig.2 : Membrane dynamics of autophagy.
Upper : Cytosol and organelle are surrounded by flattened membrane sac, isolation membrane, finally to enclose as autophagosome. Upon fusion with lysosomes, the interior is digested.
Lower: We identified a group of autophagy related proteins and revealed their dynamics during autophagosome formation.

Fig.3

Fig.3 : Degradation of pathogenic bacteria by autophagy.
A: Large autophagosome (green) engulfing Group A Streptococcus (GAS) (magenta) in cytoplasm.
B: EM photo of autophagosome engulfing GAS.
C: Destiny of GAS in host cells. Science, 306, 1037-1040 (2004)

Fig.4
Fig.4 : Autophagic degradation of misfolded proteins.
Left panel : In autophagy deficient cells, a lot of aggregates of ¦Á-1 antitrypsin Z mutant (green) accumulate. ER (magenta)
Right panel : In addition to proteasome, autophagy is involved in degradation of misfolded proteins. J. Biol. Chem, 281, 4467-4476 (2006)

Endosomes receive macromolecules taken up by endocytosis from the outside. The cargo is then either sorted to lysosomes or recycled back to the plasma membrane. It is important to maintain a balance between the volume of these two flows because an imbalance can, for example, cause cells to turn cancerous. We are currently investigating the mechanism of this sorting process. We are also analyzing the way many pathogenic bacteria use the endosomal route to invade cells. To date, we have found that Porphyromonas gingivalis, a causative agent of adult periodontitis, enters its host cell by binding to a raft, which is a micro-domain on the plasma membrane. We are currently elucidating this mechanism in more detail.

Fig.5

Fig.5 : Porphyromonas gingivalis invading cells.
Left: SEM image of the bacteria invading from cellular surface.
Right: Video images of entry of beads (red) conjugated with materials of the bacteria into cells. Green shows GPI anchord protein residing in raft. Cell. Struct. Funct, 30, 81-91 (2005)


Major publications

  1. Nakagawa, I., Amano, A., Mizushima, N., Yamamoto, A., Yamaguchi, H., Kamimoto, T., Nara, A., Funao, J., Nakata, M., Tsuda, K., Hamada, S. and Yoshimori, T. (2004). Autophagy defenses cells against invading group A Streptococcus. Science 306, 1037-1040.
  2. Fujita, N., Itoh, T., Fukuda, M., Noda, T. and Yoshimori, T. (2008). The Atg16L Complex Specifies the Site of LC3 Lipidation for Membrane Biogenesis in Autophagy. Mol Biol Cell. 19, 2092-2100.
  3. Fujita, N., Hayashi, M., Fukumoto, H., Omori, H., Yamamoto, A., Noda, T. and Yoshimori, T. (2008). An Atg4B Mutant Hampers the Lipidation of LC3 Paralogues and Causes Defects in Autophagosome Closure. Mol Biol Cell. 19, 4651-4659.
  4. Saitoh, T*., Fujita, N*., Jang, MH., Uematsu, S., Yang, B.G., Satoh, T., Omori, H., Noda, T., Yamamoto, N., Komatsu, M., Tanaka, K., Kawai, T., Tsujimura, T., Takeuchi, O., Yoshimori, T. and Akira, S. (*These authors contributed equally to this work.) (2008). Loss of the autophagy protein Atg16L1 enhances endotoxin-induced IL-1 beta production. Nature 456, 264-268.
  5. Matsunaga, K., Saitoh, T., Tabata, K., Omori, H., Satoh, T., Kurotori, N., Maejima, I., Shirahama-Noda, K., Ichimura, T., Isobe, T., Akira, S., Noda, T. and Yoshimori, T. (2009). Two Beclin-1 binding proteins, Atg14L and Rubicon, reciprocally regulate autophagy at different stages. Nat Cell Biol. 11, 385-396.

Links

RIMD Home | Overview | Research Groups | Recent Publications | Staff | Links | Contact Us | ©1997-2009 Research Institute for Microbial Diseases