title: Imaging of Rab5 activity identifies essential regulators for phagosome maturation.
abstract: Efficient phagocytosis of apoptotic cells is crucial for tissue homeostasis and the immune response. Rab5 is known as a key regulator of the early endocytic pathway and we have recently shown that Rab5 is also implicated in apoptotic cell engulfment; however, the precise spatio-temporal dynamics of Rab5 activity remain unknown. Here, using a newly developed fluorescence resonance energy transfer biosensor, we describe a change in Rab5 activity during the engulfment of apoptotic thymocytes. Rab5 activity on phagosome membranes began to increase on disassembly of the actin coat encapsulating phagosomes. Rab5 activation was either continuous or repetitive for up to 10 min, but it ended before the collapse of engulfed apoptotic cells. Expression of a dominant-negative mutant of Rab5 delayed this collapse of apoptotic thymocytes, showing a role for Rab5 in phagosome maturation. Disruption of microtubules with nocodazole inhibited Rab5 activation on the phagosome membrane without perturbing the engulfment of apoptotic cells. Furthermore, we found that Gapex-5 is the guanine nucleotide exchange factor essential for Rab5 activation during the engulfment of apoptotic cells. Gapex-5 was bound to a microtubule-tip-associating protein, EB1, whose depletion inhibited Rab5 activation during phagocytosis. We therefore propose a mechanistic model in which the recruitment of Gapex-5 to phagosomes through the microtubule network induces the transient Rab5 activation.
authors.affiliation: Laboratory of Bioimaging and Cell Signaling, Graduate School of Biostudies, Kyoto University, Yoshida Konoe-cho, Sakyo-ku, Kyoto 606-8501, Japan.
authors.forename: Masahiro
authors.initials: M
authors.lastname: Kitano
authors.forename: Michio
authors.initials: M
authors.lastname: Nakaya
authors.forename: Takeshi
authors.initials: T
authors.lastname: Nakamura
authors.forename: Shigekazu
authors.initials: S
authors.lastname: Nagata
authors.forename: Michiyuki
authors.initials: M
authors.lastname: Matsuda
mesh_terms: D000199:Actins / Q000378:metabolism; D000818:Animals; D017209:Apoptosis; D002478:Cells, Cultured; D031541:Fluorescence Resonance Energy Transfer; D005799:Genes, Dominant; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D008869:Microtubule-Associated Proteins / Q000378:metabolism; D008870:Microtubules / Q000187:drug effects; D015739:Nocodazole / Q000494:pharmacology; D010587:Phagocytosis / Q000187:drug effects; D010588:Phagosomes / Q000187:drug effects / Q000378:metabolism*; D041701:Swiss 3T3 Cells; D013950:Thymus Gland / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D020696:rab5 GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D000199:Actins; C405804:EB1 microtubule binding proteins; C529585:Gapex-5 protein, mouse; D020662:Guanine Nucleotide Exchange Factors; D008869:Microtubule-Associated Proteins; D020696:rab5 GTP-Binding Proteins; D015739:Nocodazole