In order for neurons to perform their function, they must establish a highly polarized morphology characterized, in most of the cases, by a single axon and multiple dendrites. Herein we find that the evolutionarily conserved protein Kidins220 (kinase D-interacting substrate of 220-kDa), also known as ARMS (ankyrin repeat-rich membrane spanning), a downstream effector of protein kinase D and neurotrophin and ephrin receptors, regulates the establishment of neuronal polarity and development of dendrites. Kidins220/ARMS gain and loss of function experiments render severe phenotypic changes in the processes extended by hippocampal neurons in culture. Although Kidins220/ARMS early overexpression hinders neuronal development, its down-regulation by RNA interference results in the appearance of multiple longer axon-like extensions as well as aberrant dendritic arbors. We also find that Kidins220/ARMS interacts with tubulin and microtubule-regulating molecules whose role in neuronal morphogenesis is well established (microtubule-associated proteins 1b, 1a, and 2 and two members of the stathmin family). Importantly, neurons where Kidins220/ARMS has been knocked down register changes in the phosphorylation activity of MAP1b and stathmins. Altogether, our results indicate that Kidins220/ARMS is a key modulator of the activity of microtubuleregulating proteins known to actively regulate neuronal morphogenesis and suggest a mechanism by which it contributes to control neuronal development.Neuronal differentiation comprises several steps, among which the acquirement of a polarized axon-dendrite phenotype, with the corresponding asymmetrical distribution of proteins, is crucial. The morphological changes, followed by a neuron in order to polarize and form a single axon and multiple dendrites, are triggered by signaling cascades evoked by both intracellular and extracellular cues (1-4).Embryonic hippocampal neurons in culture constitute a model to study the mechanisms governing the establishment of polarity (2, 5, 6). These neurons undergo clear morphological changes during in vitro polarization. First, neurons attach to the plate and form lamellipodia and filopodia (stage 1). After several hours, they extend several minor immature neurites of apparent equivalent nature (stage 2) until one of these minor processes extends rapidly and becomes the axon (stage 3). The remaining neurites develop into dendrites (stage 4), after which neurons become morphologically and functionally mature (stage 5) (5, 6).During the early events of the establishment of polarity in this model, differences in local actin polymerization among the immature neurites play a crucial role in axonal determination (5, 7). In a similar manner, microtubule dynamics influence neuronal polarization, because local microtubule stabilization in one neurite specifies an axonal fate (8). Other known regulators of neuronal polarity and axon specification include proteins involved in polarized trafficking (4, 9, 10). However, how these different molecules are linked to t...