title: Cdk5 Regulation of the GRAB-Mediated Rab8-Rab11 Cascade in Axon Outgrowth.
abstract: Neurons communicate with each other through their axons and dendrites. However, a full characterization of the molecular mechanisms involved in axon and dendrite formation is still incomplete. Neurite outgrowth requires the supply of membrane components for surface expansion. Two membrane sources for axon outgrowth are suggested: Golgi secretary vesicles and endocytic recycling endosomes. In non-neuronal cells, trafficking of secretary vesicles from Golgi is regulated by Rab8, a member of Rab small GTPases, and that of recycling endosomes is by Rab11, another member of Rabs. However, whether these vesicles are coordinately or independently transported in growing axons is unknown. Herein, we find that GRAB, a guanine nucleotide exchange factor for Rab8, is a novel regulator of axon outgrowth. Knockdown of GRAB suppressed axon outgrowth of cultured mouse brain cortical neurons. GRAB mediates the interaction between Rab11A and Rab8A, and this activity is regulated by phosphorylation at Ser169 and Ser180 by Cdk5-p35. The nonphosphorylatable GRAB mutant S169/180A promoted axonal outgrowth to a greater extent than did the phosphomimetic GRAB mutant S169/180D. Phosphorylation of GRAB suppressed its guanine nucleotide exchange factor activity and its ability to recruit Rab8A- to Rab11A-positive endosomes. In vivo function of GRAB and its Cdk5-phophorylation were shown in migration and process formation of developing neurons in embryonic mouse brains. These results indicate that GRAB regulates axonal outgrowth via activation and recruitment of Rab8A- to Rab11A-positive endosomes in a Cdk5-dependent manner.
  
  
  
  While axon outgrowth requires membrane supply for surface expansion, the molecular mechanisms regulating the membrane transport in growing axons remain unclear. Here, we demonstrate that GRAB, a guanine nucleotide exchange factor for Rab8, is a novel regulator of axon outgrowth. GRAB promotes the axonal membrane transport by mediating the interaction between Rab11 and Rab8 in neurons. The activity of GRAB is regulated by phosphorylation with Cdk5. We describe an in vivo role for GRAB and its Cdk5 phosphorylation during neuronal migration and process formation in embryonic brains. Thus, the membrane supply for axonal outgrowth is regulated by Cdk5 through the Rab11-GRAB-Rab8 cascade.
authors.affiliation: Department of Biological Sciences, Graduate School of Science, Tokyo Metropolitan University, Minami-Osawa, Hachioji, Tokyo 192-0397, Japan, furusawa-kotaro@ed.tmu.ac.jp hisanaga-shinichi@tmu.ac.jp.
authors.forename: Kotaro
authors.initials: K
authors.lastname: Furusawa
authors.affiliation: Department of Biological Sciences, Graduate School of Science, Tokyo Metropolitan University, Minami-Osawa, Hachioji, Tokyo 192-0397, Japan.
authors.forename: Akiko
authors.initials: A
authors.lastname: Asada
authors.affiliation: Department of Biology, Faculty of Sciences, Universidad de Chile, 7800024 Nunoa, Chile.|FONDAP Geroscience Center for Brain Health and Metabolism, 7500922 Santiago, Chile.
authors.forename: Pamela
authors.initials: P
authors.lastname: Urrutia
authors.affiliation: Department of Biology, Faculty of Sciences, Universidad de Chile, 7800024 Nunoa, Chile.|FONDAP Geroscience Center for Brain Health and Metabolism, 7500922 Santiago, Chile.|Buck Institute for Research on Aging, Novato, California 94945, and.
authors.forename: Christian
authors.initials: C
authors.lastname: Gonzalez-Billault
authors.affiliation: Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Aoba-ku, Sendai, Miyagi 980-8578, Japan.
authors.forename: Mitsunori
authors.identifier: 0000-0002-8620-5853
authors.initials: M
authors.lastname: Fukuda
authors.affiliation: Department of Biological Sciences, Graduate School of Science, Tokyo Metropolitan University, Minami-Osawa, Hachioji, Tokyo 192-0397, Japan, furusawa-kotaro@ed.tmu.ac.jp hisanaga-shinichi@tmu.ac.jp.
authors.forename: Shin-Ichi
authors.identifier: 0000-0002-7572-472X
authors.initials: SI
authors.lastname: Hisanaga
mesh_terms: D000818:Animals; D019556:COS Cells; D002478:Cells, Cultured; D002522:Chlorocebus aethiops; D005260:Female; D020662:Guanine Nucleotide Exchange Factors / Q000502:physiology*; D051379:Mice; D008813:Mice, Inbred ICR; D000071446:Neuronal Outgrowth / Q000502:physiology*; D010766:Phosphorylation; D010770:Phosphotransferases / Q000502:physiology*; D011247:Pregnancy; D051381:Rats; D015398:Signal Transduction / Q000502:physiology; D020691:rab GTP-Binding Proteins / Q000378:metabolism*; D000099193:rab11 GTP-Binding Proteins
chemical_list: D020662:Guanine Nucleotide Exchange Factors; D010770:Phosphotransferases; D020691:rab GTP-Binding Proteins; D000099193:rab11 GTP-Binding Proteins; C492236:Cdk5r1 protein, mouse; C000620981:Rab3il1 protein, mouse; C521976:Rab8 protein, mouse
keywords: Cdk5; GRAB; Rab; axon outgrowth; phosphorylation; vesicle transport