The potential role of doublecortin (Dcx), encoding a microtubule-associated protein, in brain development has remained controversial. Humans with mutations show profound alterations in cortical lamination, whereas in mouse, RNAi-mediated knockdown but not germline knockout shows abnormal positioning of cortical neurons. Here, we report that the doublecortin-like kinase (Dclk) gene functions in a partially redundant pathway with Dcx in the formation of axonal projections across the midline and migration of cortical neurons. Dosage-dependent genetic effects were observed in both interhemispheric connectivity and migration of cortically and subcortically derived neurons. Surprisingly, RNAi-mediated knockdown of either gene results in similar migration defects. These results indicate the Dcx microtubule-associated protein family is required for proper neuronal migration and axonal wiring.

The ability of the mature mammalian nervous system to continually produce neuronal precursors is of considerable importance, as manipulation of this process might one day permit the replacement of cells lost as a result of injury or disease. In mammals, the anterior subventricular zone (SVZa) region is one of the primary sites of adult neurogenesis. Here we show that Doublecortin (DCX), a widely used marker for newly generated neurons, when deleted in mouse results in a severe morphological defect in the rostral migratory stream and delayed neuronal migration that is independent of direction or responsiveness to Slit chemorepulsion. DCX is required for nuclear translocation and maintenance of bipolar morphology during migration of these cells. Our data identifies a critical function for DCX in the movement of newly generated neurons in the adult brain.In the rodent and primate brain, the anterior region of the lateral ventricle and the hippocampus are the primary sites of adult neurogenesis [1][2][3][4][5][6][7] . These neuroblasts, upon completing migration, are capable of maturing into fully differentiated neurons, integrating into local synaptic circuits and may be important in adult cognitive processing [8][9][10][11] .The mechanisms controlling the targeted translocation of large numbers of cells to the olfactory bulb (OB) over a considerable distance (>5 mm in the adult mouse) are an issue of intense investigation. These neurons translocate using a process known as chain migration, whereby the cells use each other as the migratory substrate 12 . These newly generated neurons are derived from SVZa glial fibrillary acidic protein (GFAP)-positive astrocytes 13,14 , which also ensheath them as they move through extensive interconnected networks along the lining of the lateral ventricle in the rostral forebrain and the rostral migratory stream (RMS) 15 . There, they are under the control of extracellular migration guidance cues, including the secreted ligands Slit and Reelin [16][17][18][19] , the receptors ErbB4 and Deleted in Colorectal Carcinoma (DCC) 20,21 , as well as integrins and neural cell adhesion molecule [21][22][23] . Upon reaching the bulb, the neurons turn and migrate in a radial direction 24 , integrating into either the granule cell layer or the periglomerular network 25 . 3The X-linked gene doublecortin (DCX) is mutated in some patients with the severe neuronal migration defect called classical lissencephaly or double cortex syndrome 26,27 . This migration defect is likely to be cell autonomous, because in females with a heterozygous mutation, approximately half of the neurons are misplaced within the cerebral cortex as would be expected from random X-chromosome inactivation. As a result, males display a four-layered agyric cortex and females display a subcortical band of heterotopic neurons. It was surprising, therefore, that the Dcx knockout mice displayed no appreciable defect in cortical neuronal lamination or positioning 28 . One possible explanation for these species differences ...

Disrupted-In-Schizophrenia-1 (DISC1) is a unique susceptibility gene for major mental conditions, because of the segregation of its genetic variant with hereditary psychosis in a Scottish pedigree. Genetic association studies reproducibly suggest involvement of DISC1 in both schizophrenia and bipolar disorder in several ethnic groups. The DISC1 protein is multifunctional, and a pool of DISC1 in the dynein motor complex is required for neurite outgrowth in PC12 cells as well as proper neuronal migration and dendritic arborization in the developing cerebral cortex in vivo. Here, we show that a specific interaction between DISC1 and nuclear distribution element-like (NDEL1/NUDEL) is required for neurite outgrowth in differentiating PC12 cells. Among several components of the dynein motor complex, DISC1 and NDEL1 are selectively upregulated during neurite outgrowth upon differentiation in PC12 cells. The NDEL1 binding site of DISC1 was narrowed down to a small portion of exon 13, corresponding to amino acids 802-835 of DISC1. We demonstrate that genetic variants of DISC1, proximal to the NDEL1 binding site, affect the interaction between DISC1 and NDEL1.

Intracellular type I platelet activating factor-acetylhydrolase is a phospholipase that consists of a dimer of two homologous catalytic subunits ␣1 and ␣2 as well as LIS1, a product of the causative gene for type I lissencephaly. LIS1 plays an important role in neuronal migration during brain development, but the in vivo function of the catalytic subunits remains unclear. In this study, we generated ␣1-and a2-deficient mice by targeted disruption. ␣1 ؊/؊ mice are indistinguishable from wild-type mice, whereas ␣2 ؊/؊ male mice show a significant reduction in testis size. Double-mutant male mice are sterile because of severe impairment of spermatogenesis. Histological examination revealed marked degeneration at the spermatocyte stage and an increase of apoptotic cells in the seminiferous tubules. The catalytic subunits are expressed at high levels in testis as well as brain in mice. In wild-type mice, ␣2 is expressed in all seminiferous tubule cell types, whereas ␣1 is expressed only in the spermatogonia. This expression pattern parallels the finding that deletion of both subunits induces a marked loss of germ cells at an early spermatogenic stage. We also found that the LIS1 protein levels, but not the mRNA levels, were significantly reduced in ␣2 ؊/؊ and double-mutant mice, suggesting that the catalytic subunits, especially ␣2, are a determinant of LIS1 expression level.

During brain development, neurons migrate great distances from proliferative zones to generate the cortical gray matter. A series of studies has identified genes that are critical for migration and targeting of neurons to specific brain regions. These genes encode three basic groups of proteins and produce three distinct phenotypes. The first group encodes cytoskeletal molecules and produces graded and dosage-dependent effects, with a significant amount of functional redundancy. This group also appears to play important roles during the initiation and ongoing progression of neuronal movement. The second group encodes signaling molecules for which homozygous mutations lead to an inverted cortex. In addition, this group is responsible for movement of neurons through anatomic boundaries to specific cortical layers. The third group encodes enzymatic regulators of glycosylation and appears to delineate where neuronal migration will arrest. There is significant cross-talk among these different groups of molecules, suggesting possible points of pathway convergence.