The neuron is crucial for establishing the complex neural networks in the brain, which is comprised of two structurally and functionally distinct compartments, an axon and dendrites. Both axon outgrowth and dendritic arborization is tightly regulated by coordinated actions of cytoskeletal rearrangements and directed membrane delivery. Although the cytoskeletal components involved in have been studied extensively, little is known about the molecular mechanism of membrane supply to both axon and dendrites. Lemur kinase 1 (LMTK1)/Apoptosis-associated tyrosine kinase 1 (AATYK1) is an uncharacterized Ser/Thr kinase, which is highly expressed in mammalian brains (Gaozza et al., 1997; Baker et al., 2001; Tomomura et al., 2007). I have recently shown that LMTK1 localizes to RabllA-positive recycling endosomes and is phosphorylated at Ser34 by cyclin-dependent kinase 5 (Cdk5). However, not only a role of LMTK1 and its phosphorylation in recycling endosomal trafficking but also physiological functions of LMTK1 in neuron remains unclear. In this thesis, I have investigated a role of LMTK1 in recycling endosomal trafficking and its neuronal functions. I first examined a role of LMTK1 and its phosphorylation in recycling endosomal trafficking using CHO-K1 cells. LMTK1 localizes predominantly to RabllA-positive pericentrosomal endocytic recycling compartment (ERC). Phosphorylation at Ser34 of LMTK1 disrupted its accumulation in the pericentrosomal ERC. Consistently, phosphorylation-mimic mutant (LMTK1-S34D) attenuated ERC formation and suppressed transporting of recycling endosomes from perinuclear region to ERC. I reveal a regulatory role for LMTK1 in the formation of pericentrosomal ERC (chapter I). Next, I expanded these finding and explore the physiological functions of LMTK1 using mouse brain cortical neurons. LMTK1 was expressed and was phosphorylated at Ser34, the Cdk5 phosphorylation site, at the time of axonal outgrowth in culture and colocalized with RabllA, the small GTPase that regulates recycling endosome traffic, at the perinuclear region and in the axon. Overexpression of the unphosphorylated mutant LMTK1-S34A dramatically promoted axonal outgrowth in cultured neurons. Enhanced axonal outgrowth was diminished by inactivation of RabllA, placing LMTK1 upstream of RabllA. Unexpectedly, downregulation of LMTK1 by knockdown or gene targeting also significantly enhanced axonal elongation. The enhanced axonal outgrowth was reversed by LMTK1-WT or the LMTK1-S34D mutant, which mimics the phosphorylated state, but not by LMTK1-S34A. These results suggest that LMTK1 suppresses axon outgrowth via RabllA activity in a Cdk5-dependent manner (chapter II). Neurons have another type of processes, dendrites, different in structure and functions from axon. It is important to know whether LMTK1 plays a role also in dendritic formation. Downregulation of LMTK1 by miRNAs or LMTK1-/- neurons significantly enhanced dendritic arborization and spine formation as well as axon outgrowth. Pyramidal neurons in layer V of cerebral cortex in LMTK1-/- mice brain exhibited enhancement of dendritic arborization. It is known that recycling endosomes supply membrane components to neurites and be transported along microtubules in axon and dendrites. It is important to know how trafficking of RabllA-posotive recycling endosomes are affected by polarity of microtubule is different between axon and dendrites and, if so, how LMTK1 is involved in. I analyzed how RabllA-positive recycling endosomes are transported in axon and dendrites using live-cells imaging. LMTK1-/- neurons significantly increased both the amount of anterogradely recycling endosomes and their moving velocity in dendrites as well as in axon (chapter III). Taken together, LMTK1 can negatively control axonal outgrowth and dendritic aroborization by regulating RabllA-positive recycling endosomal trafficking in a Cdk5-dependent manner, and Cdk5-LMTK1-RabllA is a novel signaling pathway involved in axonal outgrowth and dendritic arborization. 首都大学東京, 2013-03-25, 博士(理学)