The Motor KIF5C Links the Requirements of Stable Microtubules and IGF-1 Receptor Membrane Insertion for Neuronal Polarization Mariana Oksdath 1 & Alvaro F. Nieto Guil 1 & Diego Grassi 1 & Lucas J. Sosa 1 & Santiago Quiroga 1 Received: 30 March 2016 /Accepted: 19 September 2016 # Springer Science+Business Media New York 2016 Abstract Three early signals of asymmetry have been de- scribed to occur in a single neurite of neurons at stage 2 of differentiation (before polarization) and shown to be essential for neuronal polarization: (i) accumulation of stable microtu- bules, (ii) enrichment of the plasma membrane with activatable IGF-1r, and (iii) polarized transport of the micro- tubular motor KIF5C. Here, we studied the possible relation- ship between these three phenomena. Our results show that the activatable (membrane-inserted) IGF-1r and stable micro- tubules accumulate in the same neurite of cells at stage 2. The polarized insertion of IGF-1r depends on microtubule dynam- ics as shown using drugs which modify microtubule stability. Silencing of KIF5C expression prevents the polarized inser- tion of IGF-1r into the neuronal plasmalemma and neuronal polarization. Syntaxin 6 and VAMP4, necessary for the polar- ized insertion of the IGF-1r, are associated to vesicles carried by the microtubular motor KIF5C and is transported preferen- tially to the neurite where KIF5C accumulates. We conclude that the enrichment of stable microtubules in the future axon enhances KIF5C-mediated vesicular transport of syntaxin 6 and VAMP4, which in turn mediates the polarized insertion of IGF-1r in the plasmalemma, a key step for neuronal polar- ization. We herewith establish a mechanistic link between three early polarity events necessary for the establishment of neuronal polarity. Keywords Neuronal differentiation . Neuronal polarization . IGF-1 receptor . KIF5C . Stable microtubules . Syntaxin 6 Introduction The formation of a polarized neuron, containing one long axon and several branching dendrites, requires the action of two interrelated processes, specification of the axon and axo- nal elongation. It has been shown that axon specification is defined early, at the time of generation of the first two, oppo- sitely positioned, neurites [1]. After specification, initial axo- nal outgrowth is essential for neuronal polarization and is a particularly early event, occurring in neurons that have not yet exhibited a discernible axon (stage 2 of differentiation). Initial axonal outgrowth is regulated via the segregation of activatable (membrane-inserted) IGF-1 receptors (IGF-1r) in a single neurite [2, 3]. Subsequently, upon activation of IGF- 1r, active phosphatidylinositol-3 kinase (PI3k) and its product, phosphatidylinositol 3,4,5-trisphosphate (PIP3), accumulate at the growth cone of the neurite. These events are critical for the outgrowth of the future axon [4, 5]. In order to become activatable, the IGF-1r needs to be inserted to the neuron plas- malemma and expose the IGF-1 binding motifs to the extra- cellular space. This occurs by incorporation of exocytic vesi- cles containing IGF-1r with the participation of the exocyst complex [6] and the SNAREs proteins VAMP4, Syntaxin 6, and SNAP23 [7]. However, the molecular events responsible for the polarized insertion of IGF-1r to one neurite (out of four or five in hippocampal neurons) have not yet been described. Besides the polarized insertion of IGF-1r, two more early asymmetric events have been described in developing Alvaro F. Nieto Guil and Diego Grassi contributed equally in this article. Electronic supplementary material The online version of this article (doi:10.1007/s12035-016-0144-4) contains supplementary material, which is available to authorized users. * Santiago Quiroga squiroga@fcq.unc.edu.ar 1 Departamento de Química Biológica-CIQUIBIC, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba-CONICET, Haya de la Torre esquina Medina Allende, Ciudad Universitaria, 5000 Córdoba, Argentina Mol Neurobiol DOI 10.1007/s12035-016-0144-4