Mutations in Eml1 lead to ectopic progenitors and neuronal heterotopia in mouse and human.

Neuronal migration disorders such as lissencephaly and subcortical band heterotopia are associated with epilepsy and intellectual disability. DCX, PAFAH1B1 and TUBA1A are mutated in these disorders; however, corresponding mouse mutants do not show heterotopic neurons in the neocortex. In contrast, spontaneously arisen HeCo mice display this phenotype, and our study revealed that misplaced apical progenitors contribute to heterotopia formation. While HeCo neurons migrated at the same speed as wild type, abnormally distributed dividing progenitors were found throughout the cortical wall from embryonic day 13. We identified Eml1, encoding a microtubule-associated protein, as the gene mutated in HeCo mice. Full-length transcripts were lacking as a result of a retrotransposon insertion in an intron. Eml1 knockdown mimicked the HeCo progenitor phenotype and reexpression rescued it. We further found EML1 to be mutated in ribbon-like heterotopia in humans. Our data link abnormal spindle orientations, ectopic progenitors and severe heterotopia in mouse and human.

Alternative transcripts of Dclk1 and Dclk2 and their expression in doublecortin knockout mice.

The doublecortin (DCX) gene, mutated in X-linked human lissencephaly, has 2 close paralogs, doublecortin-like kinase 1 and 2 (Dclk1 and 2). In this study we attempted to better understand the dramatic differences between human and mouse DCX/Dcx-deficient phenotypes, focusing on the Dclk genes which are likely to compensate for Dcx function in the mouse. Using sequence database screens, Northern blot analyses and in situ hybridization experiments, we characterized the developmental transcripts of Dclk1 and 2, questioning their conservation between mouse and human, and their similarity to Dcx. Like Dcx, Dcx-like transcripts of the Dclk1 gene are expressed in postmitotic neurons in the developing cortex. No changes of expression were observed at the RNA level for these transcripts in Dcx knockout mice. However, a minor change in expression at the protein level was detected. The Dclk2 gene is less well characterized than Dclk1 and we show here that it is expressed both in proliferating cells and postmitotic neurons, with a notably strong expression in the ventral telencephalon. No major differences in Dclk2 expression at the RNA and protein levels were identified comparing Dcx knockout and wild-type brains. We also analyzed Dclk1 and 2 expression in the hippocampal CA3 region which, unlike the neocortex, is abnormal in Dcx knockout mice. Interestingly, each transcript was expressed in CA3 neurons, including in the heterotopic pyramidal layer of Dcx knockout animals, but is presumably not able to compensate for a lack of Dcx. These results, in addition to characterizing the transcript diversity of an important family of genes, should facilitate further studies of compensation in Dcx-deficient mice.

Large spectrum of lissencephaly and pachygyria phenotypes resulting from de novo missense mutations in tubulin alpha 1A (TUBA1A).

We have recently reported a missense mutation in exon 4 of the tubulin alpha 1A (Tuba1a) gene in a hyperactive N-ethyl-N-nitrosourea (ENU) induced mouse mutant with abnormal lamination of the hippocampus. Neuroanatomical similarities between the Tuba1a mutant mouse and mice deficient for Doublecortin (Dcx) and Lis1 genes, and the well-established functional interaction between DCX and microtubules (MTs), led us to hypothesize that mutations in TUBA1A (TUBA3, previous symbol), the human homolog of Tuba1a, might give rise to cortical malformations. This hypothesis was subsequently confirmed by the identification of TUBA1A mutations in two patients with lissencephaly and pachygyria, respectively. Here we report additional TUBA1A mutations identified in six unrelated patients with a large spectrum of brain dysgeneses. The de novo occurrence was shown for all mutations, including one recurrent mutation (c.790C>T, p.R264C) detected in two patients, and two mutations that affect the same amino acid (c.1205G>A, p.R402H; c.1204C>T, p.R402C) detected in two other patients. Retrospective examination of MR images suggests that patients with TUBA1A mutations share not only cortical dysgenesis, but also cerebellar, hippocampal, corpus callosum, and brainstem abnormalities. Interestingly, the specific high level of Tuba1a expression throughout the period of central nervous system (CNS) development, shown by in situ hybridization using mouse embryos, is in accordance with the brain-restricted developmental phenotype caused by TUBA1A mutations. All together, these results, in combination with previously reported data, strengthen the relevance of the known interaction between MTs and DCX, and highlight the importance of the MTs/DCX complex in the neuronal migration process.

Human disorders of cortical development: from past to present.

Epilepsy and mental retardation, originally of unknown cause, are now known to result from many defects including cortical malformations, neuronal circuitry disorders and perturbations of neuronal communication and synapse function. Genetic approaches in combination with MRI and related imaging techniques continually allow a re-evaluation and better classification of these disorders. Here we review our current understanding of some of the primary defects involved, with insight from recent molecular biology advances, the study of mouse models and the results of neuropathology analyses. Through these studies the molecular determinants involved in the control of neuron number, neuronal migration, generation of cortical laminations and convolutions, integrity of the basement membrane at the pial surface, and the establishment of neuronal circuitry are being elucidated. We have attempted to integrate these results with the available data concerning, in particular, human brain development, and to emphasize the limitations in some cases of extrapolating from rodent models. Taking such species differences into account is clearly critical for understanding the pathophysiological mechanisms associated with these disorders.

Branching and nucleokinesis defects in migrating interneurons derived from doublecortin knockout mice.

Type I lissencephaly results from mutations in the doublecortin (DCX) and LIS1 genes. We generated Dcx knockout mice to further understand the pathophysiological mechanisms associated with this cortical malformation. Dcx is expressed in migrating interneurons in developing human and mouse brains. Video microscopy analyses of such tangentially migrating neuron populations derived from the medial ganglionic eminence show defects in migratory dynamics. Specifically, the formation and division of growth cones, leading to the production of new branches, are more frequent in knockout cells, although branches are less stable. Dcx-deficient cells thus migrate in a disorganized manner, extending and retracting short branches and making less long-distant movements of the nucleus. Despite these differences, migratory speeds and distances remain similar to wild-type cells. These novel data thus highlight a role for Dcx, a microtubule-associated protein enriched at the leading edge in the branching and nucleokinesis of migrating interneurons.