Axonal outgrowth is a coordinated process of cytoskeletal dynamics and membrane trafficking; however, little is known about proteins responsible for regulating the membrane supply. LMTK1 (lemur kinase 1)/AATYK1 (apoptosis-associated tyrosine kinase 1) is a serine/ threonine kinase thatis highly expressed in neurons. We recently reported that LMTK1 playsa role in recycling endosomal trafficking in CHO-K1 cells. Here we explore the role of LMTK1 in axonal outgrowth and its regulation by Cdk5 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 Rab11A, the small GTPase that regulates recycling endosome traffic, at the perinuclear region and in the axon. Over expression of the un phosphorylated mutant LMTK1-S34A dramatically promoted axonal outgrow thin cultured neurons. Enhanced axonal outgrowth was diminished by the inactivation of Rab11A, placing LMTK1 upstream of Rab11A. Unexpectedly, the down regulation of LMTK1 by knockdown or gene targeting also significantly enhanced axonal elongation. Rab11A-positive vesicles were transported anterogradelymorequicklyintheaxonsofLMTK1-deficientneuronsthaninthoseofwild-typeneurons. The enhanced axonal out growth wasreversedbyLMTK1-WTorthe LMTK1-S34Dmutant,which mimics the phosphorylated state, but not by LMTK1-S34A.Thus,LMTK1 cannegatively control axonal out growth by regulating Rab11AactivityinaCdk5-dependent manner, and Cdk5-LMTK1-Rab11isanovel signaling pathway involved in axonal outgrowth.
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