Mutations in the X-linked gene DCX result in lissencephaly in males, and abnormal neuronal positioning in females, suggesting a role for this gene product during neuronal migration. In spite of several known protein interactions, the involvement of DCX in a signaling pathway is still elusive. Here we demonstrate that DCX is a substrate of JNK and interacts with both c-Jun N-terminal kinase (JNK) and JNK interacting protein (JIP). The localization of this signaling module in the developing brain suggests its functionality in migrating neurons. The localization of DCX at neurite tips is determined by its interaction with JIP and by the interaction of the latter with kinesin. DCX is phosphorylated by JNK in growth cones. DCX mutated in sites phosphorylated by JNK affected neurite outgrowth, and the velocity and relative pause time of migrating neurons. We hypothesize that during neuronal migration, there is a need to regulate molecular motors that are working in the cell in opposite directions: kinesin (a plus-end directed molecular motor) versus dynein (a minus-end directed molecular motor).

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The mammalian cortex is generally subdivided into six organized layers, which are formed during development in an organized fashion. This organized cortical layering is disrupted in case of mutations in the doublecortin (DCX) gene. DCX is a Microtubule Associated Protein (MAP). However, besides stabilization of microtubules, it may be involved in additional functions. The participation of this molecule in signal transduction is beginning to emerge via discovery of interacting molecules and its regulation by phosphorylation using several different kinases. We raise the hypothesis, that the combinatorial phosphorylation of DCX by different kinases and at different sites may be a molecular regulatory switch in the transition of a migrating neuron through multiple phases of migration. Our recent research has suggested the involvement of DCX in the JNK (Jun-N-terminal Kinase) pathway. The JNK pathway is linked to the reelin pathway, known to regulate cortical layering. Positioning of DCX in this signaling pathway opens up additional possibilities of understanding how migrating neurons are controlled.Neuronal positioning in the cortex is a genetically determined paradigm highly conserved among mammals. Therefore, accumulated molecular information both from human and mouse allowed initially the identification of many key players in this process, followed by the rudimentary positioning of these molecules in genetic pathways (reviewed in ref. 1). One of the major challenges in the field is to uncover physiologically relevant pathways that initiate at receptor activation, affect intracellular signal transduction, and are translated to neuronal movement coupled with target recognition and circuit formation. MODES OF RADIAL MIGRATIONThe advent of time-lapse imaging has changed our view regarding how cortical neurons migrate radially. Previous studies have pointed to the fact that neurons migrating along radial glia have a distinct morphology, with a growth-cone-like structure frequently present at the end of the leading process. [2][3][4][5] Time-lapse studies have indicated that each migrating neuron undergoes a series of cellular morphology changes, 5-7 (reviewed in refs. 8,9). More specifically three different modes of movement have been defined: somal translocation, cellular locomotion, 5 and a multipolar mode. 7 Somal translocation is defined by the translocation of the entire cell body. The leading process is frequently branched, with retractions following pauses. In case of cellular locomotion, the cells have a distinct morphology resembling features of glial-guided migrating neurons. The former is the prevalent form of radial movement of the early-born cortical neurons, while the latter is adopted by those generated later in corticogenesis (reviewed in ref. 10). The third mode of multipolar radial migration is quite different from the other two. These multipolar neurons extend numerous thin processes in various directions independently of the radial glial fibers. They do not exhibit fixed cell polarity, and ...

Mutations in doublecortin (DCX) result in X-linked lissencephaly in males. To explore the role of DCX in differentiation and signal transduction we overexpressed DCX in PC12 cells. Our results indicate that DCX stabilizes microtubules and inhibits neurite outgrowth in nerve growth factor-induced differentiation. However, neurite length is increased when differentiation is induced by epidermal growth factor and forskolin or by dibutyryl-cAMP. Furthermore, CREB-mediated transcription is downregulated, supporting the notion that cytoskeletal regulatory proteins can affect the transcriptional state of a cell. Using different constructs and mutations we reach the conclusion that microtubule stabilization is a key factor, but not the only one, in controlling neurite extension. Overexpression of a mutation found in a lissencephaly patient (S47R), completely blocks neurite outgrowth. We propose that these functions are important during normal and abnormal brain development.