Direct association of LIS1, the lissencephaly gene product, with a mammalian homologue of a fungal nuclear distribution protein, rNUDE

Tyrosine Kinase Inhibitor Imatinib Mesylate Alters DMBA-Induced Early Onco/Suppressor Gene Expression with Tissue-Specificity in Mice

Tyrosine kinases play crucial roles in cellular development and tumorigenesis. Tyrosine kinase inhibitors (TKIs) are effective and widely used drug molecules in targeted cancer therapies. Altered expressions of protooncogenes and tumor suppressor genes after DMBA (7,12-dimethylbenz[a]anthracene) treatment have been described as early markers of tumor induction; however their tissue-specific effects remain still unclear. Our study was aimed at examining the short-term possible antineoplastic and chemopreventive effects of a TKI compound (imatinib mesylate) on a DMBA-induced mouse tumor model. In addition, we also investigated the tissue-specific expressions of Hras, Kras, Myc, and Trp53 genes in the brain, bone marrow, spleen, liver, abdominal lymph nodes, thymus, lungs, and kidneys, respectively. 24 hours after the imatinib mesylate injection, we observed significant Kras downregulation in the bone marrow and lung of the DMBA-treated mice. Moreover, the mRNA expression of Myc was also found to be decreased significantly in the spleen. Interestingly, while Trp53 expression was significantly increased in the lung, it was decreased in the other tissues. However, there was also a tendency in the decreased Myc level in the bone marrow, brain, kidneys, lungs, and lymph nodes and in the decreased Hras level in the bone marrow, kidneys, and lungs, although no significant differences were observed. Our findings indicate rapid tissue-specific impact of imatinib mesylate on DMBA-induced gene expression in vivo, supporting the chemopreventive potential of imatinib mesylate in cancer.

Using mouse transgenic and human stem cell technologies to model genetic mutations associated with schizophrenia and autism

Solid progress has occurred over the last decade in our understanding of the molecular genetic basis of neurodevelopmental disorders, and of schizophrenia and autism in particular. Although the genetic architecture of both disorders is far more complex than previously imagined, many key loci have at last been identified. This has allowed in vivo and in vitro technologies to be refined to model specific high-penetrant genetic loci involved in both disorders. Using the DISC1/NDE1 and CYFIP1/EIF4E loci as exemplars, we explore the opportunities and challenges of using animal models and human-induced pluripotent stem cell technologies to further understand/treat and potentially reverse the worst consequences of these debilitating disorders.

This article is part of a discussion meeting issue ‘Of mice and mental health: facilitating dialogue between basic and clinical neuroscientists’.

Fundamental Elements in Autism: From Neurogenesis and Neurite Growth to Synaptic Plasticity

Autism spectrum disorder (ASD) is a set of neurodevelopmental disorders with a high prevalence and impact on society. ASDs are characterized by deficits in both social behavior and cognitive function. There is a strong genetic basis underlying ASDs that is highly heterogeneous; however, multiple studies have highlighted the involvement of key processes, including neurogenesis, neurite growth, synaptogenesis and synaptic plasticity in the pathophysiology of neurodevelopmental disorders. In this review article, we focus on the major genes and signaling pathways implicated in ASD and discuss the cellular, molecular and functional studies that have shed light on common dysregulated pathways using in vitro, in vivo and human evidence.

Highlights

Autism spectrum disorder (ASD) has a prevalence of 1 in 68 children in the United States.

ASDs are highly heterogeneous in their genetic basis.

ASDs share common features at the cellular and molecular levels in the brain.

Most ASD genes are implicated in neurogenesis, structural maturation, synaptogenesis and function.

NDE1 and NDEL1 from genes to (mal)functions: parallel but distinct roles impacting on neurodevelopmental disorders and psychiatric illness

NDE1 (Nuclear Distribution Element 1, also known as NudE) and NDEL1 (NDE-Like 1, also known as NudEL) are the mammalian homologues of the fungus nudE gene, with important and at least partially overlapping roles for brain development. While a large number of studies describe the various properties and functions of these proteins, many do not directly compare the similarities and differences between NDE1 and NDEL1. Although sharing a high degree structural similarity and multiple common cellular roles, each protein presents several distinct features that justify their parallel but also unique functions. Notably both proteins have key binding partners in dynein, LIS1 and DISC1, which impact on neurodevelopmental and psychiatric illnesses. Both are implicated in schizophrenia through genetic and functional evidence, with NDE1 also strongly implicated in microcephaly, as well as other neurodevelopmental and psychiatric conditions through copy number variation, while NDEL1 possesses an oligopeptidase activity with a unique potential as a biomarker in schizophrenia. In this review, we aim to give a comprehensive overview of the various cellular roles of these proteins in a "bottom-up" manner, from their biochemistry and protein-protein interactions on the molecular level, up to the consequences for neuronal differentiation, and ultimately to their importance for correct cortical development, with direct consequences for the pathophysiology of neurodevelopmental and mental illness.

Regulation of neuronal migration, an emerging topic in autism spectrum disorders

Autism spectrum disorders (ASD) encompass a group of neurodevelopmental diseases that demonstrate strong heritability, however, the inheritance is not simple and many genes have been associated with these disorders. ASD is regarded as a neurodevelopmental disorder, and abnormalities at different developmental stages are part of the disease etiology. This review provides a general background on neuronal migration during brain development and discusses recent advancements in the field connecting ASD and aberrant neuronal migration. We propose that neuronal migration impairment may be an important common pathophysiology in autism spectrum disorders (ASD). This review provides a general background on neuronal migration during brain development and discusses recent advancements in the field connecting ASD and aberrant neuronal migration.

Molecular pathways controlling the sequential steps of cortical projection neuron migration

Coordinated migration of newly-born neurons to their target territories is essential for correct neuronal circuit assembly in the developing brain. Although a cohort of signaling pathways has been implicated in the regulation of cortical projection neuron migration, the precise molecular mechanisms and how a balanced interplay of cell-autonomous and non-autonomous functions of candidate signaling molecules controls the discrete steps in the migration process, are just being revealed. In this chapter, I will focally review recent advances that improved our understanding of the cell-autonomous and possible cell-nonautonomous functions of the evolutionarily conserved LIS1/NDEL1-complex in regulating the sequential steps of cortical projection neuron migration. I will then elaborate on the emerging concept that the Reelin signaling pathway, acts exactly at precise stages in the course of cortical projection neuron migration. Lastly, I will discuss how finely tuned transcriptional programs and downstream effectors govern particular aspects in driving radial migration at discrete stages and how they regulate the precise positioning of cortical projection neurons in the developing cerebral cortex.

NDE1 and NDEL1: twin neurodevelopmental proteins with similar 'nature' but different 'nurture'

Nuclear distribution element 1 (NDE1, also known as NudE) and NDE-like 1 (NDEL1, also known as Nudel) are paralogous proteins essential for mitosis and neurodevelopment that have been implicated in psychiatric and neurodevelopmental disorders. The two proteins possess high sequence similarity and have been shown to physically interact with one another. Numerous lines of experimental evidence in vivo and in cell culture have demonstrated that these proteins share common functions, although instances of differing functions between the two have recently emerged. We review the key aspects of NDE1 and NDEL1 in terms of recent advances in structure elucidation and cellular function, with an emphasis on their differing mechanisms of post-translational modification. Based on a review of the literature and bioinformatics assessment, we advance the concept that the twin proteins NDE1 and NDEL1, while sharing a similar 'nature' in terms of their structure and basic functions, appear to be different in their 'nurture', the manner in which they are regulated both in terms of expression and of post-translational modification within the cell. These differences are likely to be of significant importance in understanding the specific roles of NDE1 and NDEL1 in neurodevelopment and disease.

Novel NDE1 homozygous mutation resulting in microhydranencephaly and not microlyssencephaly

Lissencephaly is characterized by deficient cortical lamination. Recently homozygous NDE1 mutations were reported in three kindred afflicted with extreme microcephaly with lissencephaly or microlissencephaly. Another severe developmental defect that involves the brain is microhydranencephaly which manifests with microcephaly, motor and mental retardation and brain malformations that include gross dilation of the ventricles with complete absence of the cerebral hemispheres or severe delay in their development. In the three related patients with microhydranencephaly that we had reported previously, we identified a homozygous deletion that encompasses NDE1 exon 2 containing the initiation codon. The mutation is predicted to result in a null allele. Herein we compare the clinical phenotypes of our research patients to those reported as microlissencephaly. The clinical findings in our patients having the fourth NDE1 mutation reported so far widen the spectrum of brain malformations resulting from mutations in NDE1.

DISC1-binding proteins in neural development, signalling and schizophrenia

In the decade since Disrupted in Schizophrenia 1 (DISC1) was first identified it has become one of the most convincing risk genes for major mental illness. As a multi-functional scaffold protein, DISC1 has multiple identified protein interaction partners that highlight pathologically relevant molecular pathways with potential for pharmaceutical intervention. Amongst these are proteins involved in neuronal migration (e.g. APP, Dixdc1, LIS1, NDE1, NDEL1), neural progenitor proliferation (GSK3β), neurosignalling (Girdin, GSK3β, PDE4) and synaptic function (Kal7, TNIK). Furthermore, emerging evidence of genetic association (NDEL1, PCM1, PDE4B) and copy number variation (NDE1) implicate several DISC1-binding partners as risk factors for schizophrenia in their own right. Thus, a picture begins to emerge of DISC1 as a key hub for multiple critical developmental pathways within the brain, disruption of which can lead to a variety of psychiatric illness phenotypes.

Lissencephaly: mechanistic insights from animal models and potential therapeutic strategies

Lissencephaly is a severe human neuronal migration defect characterized by a smooth cerebral surface, mental retardation and seizures. The two most common genes mutated in patients with lissencephaly are LIS1 and DCX. LIS1 was the first gene cloned that was important for neuronal migration in any organism, and heterozygous mutations or deletions of LIS1 are found in the majority of patients with lissencephaly, while DCX mutations were found in males with X-linked lissencephaly. In this review, we will discuss how an understanding of the molecular and cellular pathways disrupted in model organisms with Lis1 and Dcx mutations or knock-down not only provide insights into the normal processes of neuronal migration, including neurogenesis, but they also may lead to potential novel therapeutic strategies for these severe cortical malformations.

Disease-associated mutations in TUBA1A result in a spectrum of defects in the tubulin folding and heterodimer assembly pathway

Malformations of cortical development are characteristic of a plethora of diseases that includes polymicrogyria, periventricular and subcortical heterotopia and lissencephaly. Mutations in TUBA1A and TUBB2B, each a member of the multigene families that encode alpha- and beta-tubulins, have recently been implicated in these diseases. Here we examine the defects that result from nine disease-causing mutations (I188L, I238V, P263T, L286F, V303G, L397P, R402C, 402H, S419L) in TUBA1A. We show that the expression of all the mutant proteins in vitro results in the generation of tubulin heterodimers in varying yield and that these can co-polymerize with microtubules in vitro. We identify several kinds of defects that result from these mutations. Among these are various defects in the chaperone-dependent pathway leading to de novo tubulin heterodimer formation. These include a defective interaction with the chaperone prefoldin, a reduced efficiency in the generation of productive folding intermediates as a result of inefficient interaction with the cytosolic chaperonin, CCT, and, in several cases, a failure to stably interact with TBCB, one of five tubulin-specific chaperones that act downstream of CCT in the tubulin heterodimer assembly pathway. Other defects include structural instability in vitro, diminished stability in vivo, a compromised ability to co-assemble with microtubules in vivo and a suppression of microtubule growth rate in the neurites (but not the soma) of cultured neurons. Our data are consistent with the notion that some mutations in TUBA1A result in tubulin deficit, whereas others reflect compromised interactions with one or more MAPs that are essential to proper neuronal migration.

Roles of the Drosophila NudE protein in kinetochore function and centrosome migration

We examined the distribution of the dynein-associated protein NudE in Drosophila larval brain neuroblasts and spermatocytes, and analyzed the phenotypic consequences of a nudE null mutation. NudE can associate with kinetochores, spindles and the nuclear envelope. In nudE mutant brain mitotic cells, centrosomes are often detached from the poles. Moreover, the centrosomes of mutant primary spermatocytes do not migrate from the cell cortex to the nuclear envelope, establishing a new role for NudE. In mutant neuroblasts, chromosomes fail to congress to a tight metaphase plate, and cell division arrests because of spindle assembly checkpoint (SAC) activation. The targeting of NudE to mitotic kinetochores requires the dynein-interacting protein Lis1, and surprisingly Cenp-meta, a Drosophila CENP-E homolog. NudE is non-essential for the targeting of all mitotic kinetochore components tested. However, in the absence of NudE, the 'shedding' of proteins off the kinetochore is abrogated and the SAC cannot be turned off, implying that NudE regulates dynein function at the kinetochore.

Polarity regulation in migrating neurons in the cortex

The formation of the cerebral cortex requires migration of billions of cells from their birth position to their final destination. A motile cell must have internal polarity in order to move in a specified direction. Locomotory polarity requires the coordinated polymerization of cytoskeletal elements such as microtubules and actin combined with regulated activities of the associated molecular motors. This review is focused on migrating neurons in the developing cerebral cortex, which need to attain internal polarity in order to reach their proper target. The position and dynamics of the centrosome plays an important function in this directed motility. We highlight recent interesting findings connecting polarity proteins with neuronal migration events regulated by the microtubule-associated molecular motor, cytoplasmic dynein.

NDE1 and NDEL1: multimerisation, alternate splicing and DISC1 interaction

Nuclear Distribution Factor E Homolog 1 (NDE1) and NDE-Like 1 (NDEL1) are highly homologous mammalian proteins. However, whereas NDEL1 is well studied, there is remarkably little known about NDE1. We demonstrate the presence of multiple isoforms of both NDE1 and NDEL1 in the brain, showing that NDE1 binds directly to multiple isoforms of Disrupted in Schizophrenia 1 (DISC1), and to itself. We also show that NDE1 can complex with NDEL1. Together these results predict a high degree of complexity of DISC1-mediated regulation of neuronal activity.

A pachygyria-causing alpha-tubulin mutation results in inefficient cycling with CCT and a deficient interaction with TBCB

The agyria (lissencephaly)/pachygyria phenotypes are catastrophic developmental diseases characterized by abnormal folds on the surface of the brain and disorganized cortical layering. In addition to mutations in at least four genes--LIS1, DCX, ARX and RELN--mutations in a human alpha-tubulin gene, TUBA1A, have recently been identified that cause these diseases. Here, we show that one such mutation, R264C, leads to a diminished capacity of de novo tubulin heterodimer formation. We identify the mechanisms that contribute to this defect. First, there is a reduced efficiency whereby quasinative alpha-tubulin folding intermediates are generated via ATP-dependent interaction with the cytosolic chaperonin CCT. Second, there is a failure of CCT-generated folding intermediates to stably interact with TBCB, one of the five tubulin chaperones (TBCA-E) that participate in the pathway leading to the de novo assembly of the tubulin heterodimer. We describe the behavior of the R264C mutation in terms of its effect on the structural integrity of alpha-tubulin and its interaction with TBCB. In spite of its compromised folding efficiency, R264C molecules that do productively assemble into heterodimers are capable of copolymerizing into dynamic microtubules in vivo. The diminished production of TUBA1A tubulin in R264C individuals is consistent with haploinsufficiency as a cause of the disease phenotype.

Lissencephaly 1 linking to multiple diseases: mental retardation, neurodegeneration, schizophrenia, male sterility, and more

Lissencephaly 1 (LIS1) was the first gene implicated in the pathogenesis of type-1 lissencephaly. More than a decade of research by multiple laboratories has revealed that LIS1 is a key node protein, which participates in several pathways, including association with the molecular motor cytoplasmic dynein, the reelin signaling pathway, and the platelet-activating factor pathway. Mutations in LIS1-interacting proteins, either in human, or in mouse models has suggested that LIS1 might play a role in the pathogenesis of numerous diseases such as male sterility, schizophrenia, neuronal degeneration, and viral infections.

Type I platelet-activating factor acetylhydrolase catalytic subunits over-expression induces pleiomorphic nuclei and centrosome amplification

LIS1, a causative gene product for type I lissencephaly, binds to and regulates the dynein motor and the centrosome. LIS1 also forms a complex with the catalytic subunits alpha1 and alpha2 of type I platelet-activating factor acetylhydrolase [PAF-AH (I)]. However, the cellular function of the catalytic subunits remains unknown. In this study, we showed that over-expression of the catalytic subunits, especially alpha2, in cultured cells induced dramatic phenotypical changes including nuclear shape change, centrosomal amplification and microtubule disorganization. We examined if these effects were due to the catalytic activity and/or binding of alpha2 to LIS1. Substitution of a single amino acid Glu39 of murine alpha1 and alpha2 by Asp (alpha2-E39D) did not affect catalytic activity but completely abolished LIS1 binding. Over-expression of either alpha2-E39D or the catalytically inactive alpha2-S48C revealed that alpha2-E39D, but not alpha2-S48C, lost its ability to induce above-mentioned phenotypic changes. Biochemical analyses showed that LIS1 present in the precipitate fraction of murine brain homogenates could be translocated to the soluble fraction by alpha2, but not by alpha2-E39D. These results suggest that over-expression of the PAF-AH (I) catalytic subunits induces centrosomal amplification and microtubule disorganization by disturbing intracellular localization of LIS1.

Lissencephaly and LIS1: insights into the molecular mechanisms of neuronal migration and development

Lissencephaly is a severe human neuronal migration defect characterized by a smooth cerebral surface, mental retardation and seizures. LIS1 was first gene cloned in an organism important for neuronal migration, as it was deleted or mutated in patients with lissencephaly in a heterozygous fashion. Studies in model organisms, particularly Aspergillus nidulans, as well as those in the mouse, have uncovered an evolutionarily conserved pathway that involves LIS1 and cytoplasmic dynein. This pathway codes for proteins in a complex with cytoplasmic dynein and positively regulates its conserved function in nuclear migration. This complex appears to be important for proliferation and neuronal survival as well as neuronal migration. One of the components of this complex, NDEL1, is a phosphoprotein that is a substrate for CDK5 (or CDK2 in fibroblasts) and Aurora-A, two mitotic kinases. CDK5-phosphorylated NDEL1 binds to 14-3-3epsilon, which protects it from phosphatase attack. Interestingly, 14-3-3epsilon is located 1 Mb from LIS1 and is heterozygously deleted with LIS1 in patients with a severe form of lissencephaly, Miller-Dieker syndrome. Mouse models confirm that 14-3-3epsilon plays an important role in neuronal migration, and mice that are double heterozygotes for mutations in Lis1 and 14-3-3epsilon, display more severe neuronal migration defects. The identification of LIS1 as the first lissencephaly gene, and the first gene required for neuronal migration has revealed the importance of the regulation of cytoplasmic dynein in the control of neuronal migration by modulating nuclear migration in a pathway conserved in virtually all eukaryotes.

DEHP, bis(2)-ethylhexyl phthalate, alters gene expression in human cells: possible correlation with initiation of fetal developmental abnormalities

Diethylhexylphthalate (DEHP) is a widely distributed phthalate, to which humans are exposed to due to its variety of commercial and manufacturing uses. As a plasticiser, it is found in a wide number of products, and metabolites of DEHP have been detected in urine samples from a high percentage of the people screened for phthalates. We utilised DNA microarray analysis to evaluate DEHP for gene expression disrupting activity using the human cell line MCF-7, and found that DEHP significantly dysregulated approximately 34% of the 2400 genes spotted on the NEN2400 chip we used. The results suggest that DEHP, a known estrogen agonist and probable androgen antagonist, alters the expression of a number of genes, many of which are critical for fetal development. Down-regulation of two genes, FGD1 and PAFAH1B1, related in that both are essential for fetal brain development, was corroborated using quantitative real time PCR. These studies show DEHP to be a highly effective human gene expression-altering chemical, and that, at appropriate concentrations, it has the possibility of altering fetal central nervous system development, resulting in the birth defects lissencephaly and/or faciodigitogenital dysplasia.

Similarities and differences between the Wnt and reelin pathways in the forming brain

One of the key features in development is the reutilization of successful signaling pathways. Here, we emphasize the involvement of the Wnt pathway, one of the five kinds of signal transduction pathway predominating early embryonic development of all animals, in regulating the formation of brain structure. We discuss the interrelationships between the Wnt and reelin pathways in the regulation of cortical layering. We summarize data emphasizing key molecules, which, when mutated, result in abnormal brain development. This integrated view, which is based on conservation of pathways, reveals the relative position of participants in the pathway, points to control mechanisms, and allows raising testable working hypotheses. Nevertheless, although signaling pathways are highly conserved from flies to humans, the overall morphology is not. We propose that future studies directed at understanding of diversification will provide fruitful insights on mammalian brain formation.

LIS1 RNA interference blocks neural stem cell division, morphogenesis, and motility at multiple stages

Mutations in the human LIS1 gene cause the smooth brain disease classical lissencephaly. To understand the underlying mechanisms, we conducted in situ live cell imaging analysis of LIS1 function throughout the entire radial migration pathway. In utero electroporation of LIS1 small interference RNA and short hairpin dominant negative LIS1 and dynactin cDNAs caused a dramatic accumulation of multipolar progenitor cells within the subventricular zone of embryonic rat brains. This effect resulted from a complete failure in progression from the multipolar to the migratory bipolar state, as revealed by time-lapse analysis of brain slices. Surprisingly, interkinetic nuclear oscillations in the radial glial progenitors were also abolished, as were cell divisions at the ventricular surface. Those few bipolar cells that reached the intermediate zone also exhibited a complete block in somal translocation, although, remarkably, process extension persisted. Finally, axonal growth also ceased. These results identify multiple distinct and novel roles for LIS1 in nucleokinesis and process dynamics and suggest that nuclear position controls neural progenitor cell division.

Measuring the stoichiometry and physical interactions between components elucidates the architecture of the vertebrate kinetochore

Vertebrate kinetochores contain over 50 different proteins organized into three distinct regions: the inner plate, outer plate, and fibrous corona. The present study characterizes numerous precursors of kinetochore assembly in a system free of centromeric chromatin, Xenopus extracts. Hydrodynamic analysis suggests there are a minimum of two monomeric proteins and six pre-assembled complexes that accumulate on centromeres to form the kinetochore. The inner and outer kinetochore assemble from at least two distinct kinetochore complexes containing the proteins Mis12, Zwint, and Ndc80, all of which interact by immunoprecipitation. There is also a network of interactions between the fibrous corona proteins that is dissociated by microtubules. We quantify the number of molecules of specific proteins assembled into a single kinetochore. There are between 800 and 1200 molecules of the measured inner and outer kinetochore proteins, demonstrating that the components in these regions are in similar stoichiometry. In contrast, the measured fibrous corona proteins are present at 250-300 molecules per kinetochore. Zwint, but not Mis12, requires the Ndc80 complex for assembly into the kinetochore. Further, Ndc80 requires Zwint for assembly, indicating a co-dependency for these two proteins. Our data provide a model for the structural architecture and assembly pathway of the vertebrate kinetochore.

Inhibition of NUDEL (nuclear distribution element-like)-oligopeptidase activity by disrupted-in-schizophrenia 1

Recently, nuclear distribution element-like (NUDEL) has been implicated to play a role in lissencephaly and schizophrenia through interactions with the lissencephaly gene 1 (Lis1) and disrupted-in-schizophrenia 1 (DISC1) products, respectively. Interestingly, NUDEL is the same protein as endooligopeptidase A (EOPA), a thiol-activated peptidase involved in conversion and inactivation of a number of bioactive peptides. In this study, we have cloned EOPA from the human brain and have confirmed that it is equivalent to NUDEL, leading us to suggest a single name, NUDEL-oligopeptidase. In the brain, the monomeric form of NUDEL-oligopeptidase is responsible for the peptidase activity whose catalytic mechanism is likely to involve a reactive cysteine, because mutation of Cys-273 fully abolished NUDEL-oligopeptidase activity without disrupting the protein's secondary structure. Cys-273 is very close to the DISC1-binding site on NUDEL-oligopeptidase. Intriguingly, DISC1 inhibits NUDEL-oligopeptidase activity in a competitive fashion. We suggest that the activity of NUDEL-oligopeptidase is under tight regulation through protein-protein interactions and that disruption of these interactions, as postulated in a Scottish DISC1 translocation schizophrenia cohort, may lead to aberrant regulation of NUDEL-oligopeptidase, perhaps providing a substrate for the pathology of schizophrenia.

Coupling PAF signaling to dynein regulation: structure of LIS1 in complex with PAF-acetylhydrolase

Mutations in the LIS1 gene cause lissencephaly, a human neuronal migration disorder. LIS1 binds dynein and the dynein-associated proteins Nde1 (formerly known as NudE), Ndel1 (formerly known as NUDEL), and CLIP-170, as well as the catalytic alpha dimers of brain cytosolic platelet activating factor acetylhydrolase (PAF-AH). The mechanism coupling the two diverse regulatory pathways remains unknown. We report the structure of LIS1 in complex with the alpha2/alpha2 PAF-AH homodimer. One LIS1 homodimer binds symmetrically to one alpha2/alpha2 homodimer via the highly conserved top faces of the LIS1 beta propellers. The same surface of LIS1 contains sites of mutations causing lissencephaly and overlaps with a putative dynein binding surface. Ndel1 competes with the alpha2/alpha2 homodimer for LIS1, but the interaction is complex and requires both the N- and C-terminal domains of LIS1. Our data suggest that the LIS1 molecule undergoes major conformational rearrangement when switching from a complex with the acetylhydrolase to the one with Ndel1.

CORTICAL NEURONAL MIGRATION MUTANTS SUGGEST SEPARATE BUT INTERSECTING PATHWAYS

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.

The structure of the N-terminal domain of the product of the lissencephaly gene Lis1 and its functional implications

Mutations in the Lis1 gene result in lissencephaly (smooth brain), a debilitating developmental syndrome caused by the impaired ability of postmitotic neurons to migrate to their correct destination in the cerebral cortex. Sequence similarities suggest that the LIS1 protein contains a C-terminal seven-blade beta-propeller domain, while the structure of the N-terminal fragment includes the LisH (Lis-homology) motif, a pattern found in over 100 eukaryotic proteins with a hitherto unknown function. We present the 1.75 A resolution crystal structure of the N-terminal domain of mouse LIS1, and we show that the LisH motif is a novel, thermodynamically very stable dimerization domain. The structure explains the molecular basis of a low severity form of lissencephaly.

Expression of NUDEL in manchette and its implication in spermatogenesis

Nuclear distribution gene E-like product (NUDEL) is a mammal homologue of fungal NUDE and is involved in neuronal migration during brain development. High levels of NUDEL were expressed in murine testis as well as brain. During spermatogenesis, NUDEL was not detected until postnatal day 12 (P12), rising to significant levels at P27. NUDEL was localized predominantly along microtubules of the manchette in elongated spermatids. In maturing spermatids, NUDEL was observed only in the centrosomes, while mature testicular spermatozoa did not show any NUDEL expression. These results suggest that NUDEL plays an important role in germ cell formation, including nucleoplasmic transport and nucleus shaping by manchette microtubules.

Epileptic-like convulsions associated with LIS-1 in the cytoskeletal control of neurotransmitter signaling in Caenorhabditis elegans

Cortical malformations are a collection of disorders affecting brain development. Mutations in the LIS1 gene lead to a disorganized and smooth cerebral cortex caused by failure in neuronal migration. Among the clinical consequences of lissencephaly are mental retardation and intractable epilepsy. It remains unclear whether the seizures result from aberrant neuronal placement, disruption of intrinsic properties of neurons, or both. The nematode Caenorhabditis elegans offers an opportunity to study such convulsions in a simple animal with a defined nervous system. Here we show that convulsions mimicking epilepsy can be induced by a mutation in a C. elegans lis-1 allele (pnm-1), in combination with a chemical antagonist of gamma-aminobutyric acid (GABA) neurotransmitter signaling. Identical convulsions were obtained using C. elegans mutants defective in GABA transmission, whereas none of these mutants or the antagonist alone caused convulsions, indicating a threshold was exceeded in response to this combination. Crosses between pnm-1 and fluorescent marker strains designed to exclusively illuminate either the processes of GABAergic neurons or synaptic vesicles surprisingly showed no deviations in neuronal architecture. Instead, presynaptic defects in GABAergic vesicle distribution were clearly evident and could be phenocopied by RNAi directed against cytoplasmic dynein, a known LIS1 interactor. Furthermore, mutations in UNC-104, a neuronal-specific kinesin, and SNB-1, a synaptic vesicle-associated protein termed synaptobrevin, exhibit similar convulsion phenotypes following chemical induction. Taken together, these studies establish C. elegans as a system to investigate subtle cytoskeletal mechanisms regulating intrinsic neuronal activity and suggest that it may be possible to dissociate the epileptic consequences of lissencephaly from the more phenotypically overt cortical defects associated with neuronal migration.

Yeast two-hybrid screens implicate DISC1 in brain development and function

DISC1 is a candidate gene for involvement in the aetiology of major psychiatric illnesses including schizophrenia. We report here the results of DISC1 yeast two-hybrid screens using human foetal and adult brain libraries. Twenty-one proteins from a variety of subcellular locations were identified, consistent with observations that DISC1 occupies multiple subcellular compartments. The cellular roles of the proteins identified implicate DISC1 in several aspects of central nervous system development and function, including gene transcription, mitochondrial function, modulation of the actin cytoskeleton, neuronal migration, glutamate transmission, and signal transduction. Intriguingly, mutations in one of the proteins identified, WKL1, have been previously suggested to underlie the aetiology of catatonic schizophrenia.

Disrupted in Schizophrenia 1 and Nudel form a neurodevelopmentally regulated protein complex: implications for schizophrenia and other major neurological disorders

Disrupted In Schizophrenia 1 (DISC1) was identified as a potential susceptibility gene for schizophrenia due to its disruption by a balanced t(1;11) (q42;q14) translocation, which has been shown to cosegregate with major psychiatric disease in a large Scottish family. We have demonstrated that DISC1 exists in a neurodevelopmentally regulated protein complex with Nudel. The complex is abundant at E17 and in early postnatal life but is greatly reduced in the adult. Nudel has previously been shown to bind Lis1, a gene underlying lissencephaly in humans. Critically, we show that the predicted peptide product resulting from the Scottish translocation removes the interaction domain for Nudel. DISC1 interacts with Nudel through a leucine zipper domain and binds to a novel DISC1-interaction domain on Nudel, which is independent from the Lis1 binding site. We show that Nudel is able to act as a bridge between DISC1 and Lis1 to allow formation of a trimolecular complex. Nudel has been implicated to play a role in neuronal migration, together with the developmental variation in the abundance of the DISC1-Nudel complex, may implicate a defective DISC1-Nudel complex as a neurodevelopmental cause of schizophrenia.

Inactivation of a testis-specific Lis1 transcript in mice prevents spermatid differentiation and causes male infertility

Lis1 protein is the non-catalytic component of platelet-activating factor acetylhydrolase 1b (PAF-AH 1B) and associated with microtubular structures. Hemizygous mutations of the LIS1 gene cause type I lissencephaly, a brain abnormality with developmental defects of neuronal migration. Lis1 is also expressed in testis, but its function there has not been determined. We have generated a mouse mutant (LIS1GT/GT) by gene trap integration leading to selective disruption of a Lis1 splicing variant in testis. Homozygous mutant males are infertile with no other apparent phenotype. We demonstrate that Lis1 is predominantly expressed in spermatids, and spermiogenesis is blocked when Lis1 is absent. Mutant spermatids fail to form correct acrosomes and nuclei appear distorted in size and shape. The tissue architecture in mutant testis appears severely disturbed displaying collapsed seminiferous tubules, mislocated germ cells, and increased apoptosis. These results provide evidence for an essential and hitherto uncharacterized role of the Lis1 protein in spermatogenesis, particularly in the differentiation of spermatids into spermatozoa.

Cell migration in the forebrain

The forebrain comprises an intricate set of structures that are required for some of the most complex and evolved functions of the mammalian brain. As a reflection of its complexity, cell migration in the forebrain is extremely elaborated, with widespread dispersion of cells across multiple functionally distinct areas. Two general modes of migration are distinguished in the forebrain: radial migration, which establishes the general cytoarchitectonical framework of the different forebrain subdivisions; and tangential migration, which increases the cellular complexity of forebrain circuits by allowing the dispersion of multiple neuronal types. Here, we review the cellular and molecular mechanisms underlying each of these types of migrations and discuss how emerging concepts in neuronal migration are reshaping our understanding of forebrain development in normal and pathological situations.

Interaction of reelin signaling and Lis1 in brain development

Loss-of-function mutations in RELN (encoding reelin) or PAFAH1B1 (encoding LIS1) cause lissencephaly, a human neuronal migration disorder. In the mouse, homozygous mutations in Reln result in the reeler phenotype, characterized by ataxia and disrupted cortical layers. Pafah1b1(+/-) mice have hippocampal layering defects, whereas homozygous mutants are embryonic lethal. Reln encodes an extracellular protein that regulates layer formation by interacting with VLDLR and ApoER2 (Lrp8) receptors, thereby phosphorylating the Dab1 signaling molecule. Lis1 associates with microtubules and modulates neuronal migration. We investigated interactions between the reelin signaling pathway and Lis1 in brain development. Compound mutant mice with disruptions in the Reln pathway and heterozygous Pafah1b1 mutations had a higher incidence of hydrocephalus and enhanced cortical and hippocampal layering defects. Dab1 and Lis1 bound in a reelin-induced phosphorylation-dependent manner. These data indicate genetic and biochemical interaction between the reelin signaling pathway and Lis1.

Disregulated RhoGTPases and actin cytoskeleton contribute to the migration defect in Lis1-deficient neurons

Lissencephaly is a severe brain malformation caused by impaired neuronal migration. Lis1, a causative gene, functions in an evolutionarily conserved nuclear translocation pathway regulating dynein motor and microtubule dynamics. Whereas microtubule contributions to neuronal motility are incompletely understood, the actin cytoskeleton is essential for crawling cell movement of all cell types investigated. Lis1 haploinsufficiency is shown here to also result in reduced filamentous actin at the leading edge of migrating neurons, associated with upregulation of RhoA and downregulation of Rac1 and Cdc42 activity. Disruption of RhoA function through pharmacological inhibition of its effector kinase, p160ROCK, restores normal Rac1 and Cdc42 activity and rescues the motility defect in Lis1+/- neurons. These data indicate a previously unrecognized role for Lis1 protein in neuronal motility by promoting actin polymerization through the regulation of Rho GTPase activity. This effect of Lis1 on GTPases does not appear to occur through direct Lis1 binding of Rho, but could involve Lis1 effects on Rho modulatory proteins or on microtubule dynamics.

LIS1 missense mutations: variable phenotypes result from unpredictable alterations in biochemical and cellular properties

Mutations in one allele of the human LIS1 gene cause a severe brain malformation, lissencephaly. Although most LIS1 mutations involve deletions, several point mutations with a single amino acid alteration were described. Patients carrying these mutations reveal variable phenotypic manifestations. We have analyzed the functional importance of these point mutations by examining protein stability, folding, intracellular localization, and protein-protein interactions. Our data suggest that the mutated proteins were affected at different levels, and no single assay could be used to predict the lissencephaly phenotype. Most interesting are those mutant proteins that retain partial folding and interactions. In the case of LIS1 mutated in F31S, the cellular phenotype may be modified by overexpression of specific interacting proteins. Overexpression of the PAF-AH alpha1 subunit dissolved aggregates induced by this mutant protein and increased its half-life. Overexpression of NudE or NudEL localized this mutant protein to spindle poles and kinetochores but had no effect on protein stability. Our results implicate that there are probably different biochemical and cellular mechanisms obstructed in each patient yielding the varied lissencephaly phenotypes.

A study of the nature of embryonic lethality in LIS1-/- mice

Homozygous deletion of the Lis1 gene (Lis1(-/-)) in mouse resulted in early embryonic lethality immediately after embryo implantation by an undefined mechanism. We seek to define the nature of this demise. LIS1 (pafah1b1) is a 46 kDa protein with seven tryptophan-aspartate (WD) repeats. It docks with many proteins and has been implicated in microtubular function, cell division, intercellular transport, and nuclear and cellular motility. Combined Western and quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analyses showed that LIS1 expression from the blastocyst stage required new transcription from the embryonic genome. Consequently, the death of post-implantation embryos may not reflect the first time during development that LIS1 was required, rather, it may reflect the first time following depletion of gametic stores that its actions were essential. Following culture of blastocysts in vitro for 96 hr the inner cell mass (ICM) of null embryos were significantly smaller than ICM of wild-type siblings. Normal blastocyst outgrowths after 96-hr culture had high levels of LIS1 expression in the outer cells of developing ICM and extensive expression in trophoblast cells. Lis1(-/-) embryos had significantly smaller trophoblast nuclei than wild-type embryos. The results show that LIS1 expression is required for the continued normal development of the ICM and optimal trophoblast giant cell formation.

Involvement of platelet-activating factor and LIS1 in neuronal migration

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active lipid mediator. We have previously shown the expression of PAF receptor in neurons and microglia. PAF is produced in the brain from its precursor, and degraded by the enzyme PAF acetylhydrolase. LIS1 is a regulatory subunit of PAF acetylhydrolase, and is identical to a gene whose deletion causes the human neuronal migration disorder, type I lissencephaly. Indeed, Lis1 mutant mice display defects in neuronal migration and layering in vivo, and also in cerebellar granule cell migration in vitro. However, the roles of PAF and the PAF receptor in the neuronal migration remain to be determined. Here, we show that PAF receptor-deficient mice exhibited histological abnormalities in the embryonic cerebellum. PAF receptor-deficient cerebellar granule neurons migrated more slowly in vitro than wild-type neurons, consistent with the observation that a PAF receptor antagonist reduced the migration of wild-type neurons in vitro. Synergistic reduction of neuronal migration was observed in a double mutant of PAF receptor and LIS1. Unexpectedly, PAF affected the migration of PAF receptor-deficient neurons, suggesting a receptor-independent pathway for PAF action. The PAF receptor-independent response to PAF was abolished in granule neurons derived from the double mutant mice. Thus, our results suggest that the migration of cerebellar granule cells is regulated by PAF through receptor-dependent and receptor-independent pathways, and that LIS1 is a pivotal molecule that links PAF action and neuronal cell migration both in vivo and in vitro.

Targeted disruption of intracellular type I platelet activating factor-acetylhydrolase catalytic subunits causes severe impairment in spermatogenesis

Intracellular type I platelet activating factor-acetylhydrolase is a phospholipase that consists of a dimer of two homologous catalytic subunits alpha1 and alpha2 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 alpha1- and a2-deficient mice by targeted disruption. alpha1(-/-) mice are indistinguishable from wild-type mice, whereas alpha2(-/-) 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, alpha2 is expressed in all seminiferous tubule cell types, whereas alpha1 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 alpha2(-/-) and double-mutant mice, suggesting that the catalytic subunits, especially alpha2, are a determinant of LIS1 expression level.

Roles of NUDE and NUDF proteins of Aspergillus nidulans: insights from intracellular localization and overexpression effects

The NUDF protein of the filamentous fungus Aspergillus nidulans functions in the cytoplasmic dynein pathway. It binds several proteins, including the NUDE protein. Green fluorescent protein-tagged NUDF and NUDA (dynein heavy chain) localize to linearly moving dashes ("comets") that coincide with microtubule ends. Herein, deletion of the nudE gene did not eliminate the comets of NUDF and NUDA, but affected the behavior of NUDA. Comets were also observed with the green fluorescent protein-tagged NUDE and its nonfunctional C-terminal domain. In addition, overexpressed NUDA and NUDE accumulated in specks that were either immobile or bounced randomly. Neither comets nor specks were observed with the functional N-terminal domain of NUDE, indicating that these structures are not essential for NUDE function. Furthermore, NUDF overproduction totally suppressed deletion of the nudE gene. This implies that the function of NUDE is secondary to that of NUDF. Unexpectedly, NUDF overproduction inhibited one conditional nudA mutant and all tested apsA mutants. An allele-specific interaction between the nudF and nudA genes is consistent with a direct interaction between NUDF and dynein heavy chain. Because APSA and its yeast homolog Num1p are cortical proteins, an interaction between the nudF and apsA genes suggests a role for NUDF at the cell cortex.

Nuclear positioning: the means is at the ends

The nucleus, like other smaller organelles in the cell, is dynamic and can move about in the cytoplasm. In some cells, nuclear movements are concerned with mitosis or meiosis; in others, they are concerned with orienting nuclear divisions; and in still others, they deal with distributing nuclei through the cytoplasm. Recent interest in nuclear positioning has shown that nuclear movements are often mediated by the interactions of dynein and other proteins at the plus ends of astral microtubules with the cell cortex. How the microtubule minus ends interact with the nucleus also affects nuclear movements.

Disrupted-in-Schizophrenia-1 (DISC-1): mutant truncation prevents binding to NudE-like (NUDEL) and inhibits neurite outgrowth

Disrupted-in-Schizophrenia-1 (DISC-1) is a gene whose mutant truncation is associated with major psychiatric illness with a predominance of schizophrenic symptomatology. We have cloned and characterized rodent DISC-1. DISC-1 expression displays pronounced developmental regulation with the highest levels in late embryonic life when the cerebral cortex develops. In yeast two-hybrid analyses, DISC-1 interacts with a variety of cytoskeletal proteins. One of these, NudE-like (NUDEL), is associated with cortical development and is linked to LIS-1, the disease gene for a form of lissencephaly, a disorder of cortical development. The disease mutant form of DISC-1 fails to bind NUDEL. Expression of mutant, but not wild-type, DISC-1 in PC12 cells reduces neurite extension. As schizophrenia is thought to reflect defects in cortical development that are determined by cytoskeletal protein activities, the cellular disturbances we observe with mutant DISC-1 may be relevant to psychopathologic mechanisms.

Brain malformations, epilepsy, and infantile spasms

The models of cortical dysplasia discussed earlier--the Lis1 knockout, the MAM-induced cobblestone LIS, the spontaneous tish mutant, and focal freeze injury-induced PMG--illustrate several important insights into epileptogenesis in malformed brain. First, the appearance of epilepsy varies according to the pathogenesis of the dysplasia and may well depend more on the intrinsic properties of the neurons in these models rather than on the disturbed position of the cells. This is supported by models such as the reeler mouse, in which the dysfunctional extracellular matrix molecule leads to a form of lissencephaly in mouse and human, but there is a far less impressive association with seizures than for LIS1 mutations. However, Lis1 and Dex mutations that appear to affect the cytoskeleton and perhaps intracellular protein trafficking are frequently associated with infantile spasms and epilepsy. Second, the possible mechanisms of epileptogenesis in these models include (a) a loss of subsets of neurons, (b) altered neurotransmitter release, (c) differences in neurotransmitter receptor levels and changes in receptor subunit composition, (d) altered neurite density and/or synaptogenesis, (e) changed membrane properties (e.g., altered voltage-gated channels), (f) altered cell morphology (neuronal differentiation), and (g) effects on cytoskeletal function. Finally, it is important to note that the "generator" of excitability in affected brain may be within the heterotopia or in the normotopic cortex. As additional genetic models come to light and the ability to distinguish their clinical counterparts improves, more individually tailored therapies, including standards for surgical interventions, will surely evolve.

Platelet-activating factor acetylhydrolase

Platelet-activating factor (PAF) is one of the most potent lipid mediators and is involved in a variety of physiological events. The acetyl group at the sn-2 position of its glycerol backbone is required for its biological activity, and deacetylation of PAF induces loss of activity. The deacetylation reaction is catalyzed by PAF-acetylhydrolase (PAF-AH). A series of biochemical and enzymological studies have revealed that there are at least three types of PAF-AH in mammals, namely the intracellular type I and II and plasma type. Type I PAF-AH is a G-protein-like complex of two catalytic subunits (alpha1 and alpha2) and a regulatory beta subunit. The beta subunit is a product of the LIS1 gene, mutations of which cause type I lissencephaly. Recent studies indicate that LIS1/beta is important in cellular functions such as induction of nuclear movement and control of microtubule organization. Although circumstantial evidence is accumulating supporting the idea that the catalytic subunits are also involved in microtubule function, it is still not known what role PAF plays in the process and whether PAF is a native endogenous substrate of this enzyme. Type II PAF-AH is a single polypeptide and shows significant sequence homology with plasma PAF-AH. Type II PAF-AH is myristoylated at the N-terminus and like other N-myristoylated proteins, is distributed in both the cytosol and membranes. Plasma PAF-AH is also a single polypeptide and exists in association with plasma lipoproteins. Type II PAF-AH as well as plasma PAF-AH may play roles as scavengers of oxidized phospholipids which are thought to be involved in diverse pathological processes, including disorganization of membrane structure and PAF-like proinflammatory actions. In this chapter, author focuses on the structures and possible biological functions of intracellular PAF-AHs.

Smooth, rough and upside-down neocortical development

Lissencephaly, which means 'smooth cortex', is caused by defective neuronal migration during development of the cerebral cortex and has devastating clinical consequences. 'Classical' lissencephaly seems to reflect mutations in regulators of the microtubule cytoskeleton, whereas 'cobblestone' lissencephaly is caused by mutations in genes needed for the integrity of the basal lamina of the central nervous system. Reelin, which is mutated in a third type of lissencephaly, may represent a unifying link because it encodes an extracellular protein that regulates neuronal migration and may also regulate the microtubule cytoskeleton.

Life is a journey: a genetic look at neocortical development

Although the basic principles of neocortical development have been known for quite some time, it is only recently that our understanding of the molecular mechanisms that are involved has improved. Such understanding has been facilitated by genetic approaches that have identified key proteins involved in neocortical development, which have been placed into signalling pathways by molecular and cell-biological studies. The challenge of current research is to understand the manner in which these various signalling pathways are interconnected to gain a more comprehensive picture of the molecular intricacies that govern neocortical development.

LIS1-no more no less

LIS1 is one of the genes that has a principle role in brain development since hemizygote mutations in LIS1 result in a severe brain malformation known as lissencephaly ('smooth brain'). LIS1 is a WD repeat protein and is known to be involved in several protein complexes that are likely to play a functional role in brain development. We discuss here the brain developmental phenotype observed in mice heterozygote for an N-terminal truncated LIS1 protein in view of known LIS1 protein interactions.

Role of dynein, dynactin, and CLIP-170 interactions in LIS1 kinetochore function

Mutations in the human LIS1 gene cause type I lissencephaly, a severe brain developmental disease involving gross disorganization of cortical neurons. In lower eukaryotes, LIS1 participates in cytoplasmic dynein-mediated nuclear migration. We previously reported that mammalian LIS1 functions in cell division and coimmunoprecipitates with cytoplasmic dynein and dynactin. We also localized LIS1 to the cell cortex and kinetochores of mitotic cells, known sites of dynein action. We now find that the COOH-terminal WD repeat region of LIS1 is sufficient for kinetochore targeting. Overexpression of this domain or full-length LIS1 displaces CLIP-170 from this site without affecting dynein and other kinetochore markers. The NH2-terminal self-association domain of LIS1 displaces endogenous LIS1 from the kinetochore, with no effect on CLIP-170, dynein, and dynactin. Displacement of the latter proteins by dynamitin overexpression, however, removes LIS1, suggesting that LIS1 binds to the kinetochore through the motor protein complexes and may interact with them directly. We find that of 12 distinct dynein and dynactin subunits, the dynein heavy and intermediate chains, as well as dynamitin, interact with the WD repeat region of LIS1 in coexpression/coimmunoprecipitation and two-hybrid assays. Within the heavy chain, interactions are with the first AAA repeat, a site strongly implicated in motor function, and the NH2-terminal cargo-binding region. Together, our data suggest a novel role for LIS1 in mediating CLIP-170-dynein interactions and in coordinating dynein cargo-binding and motor activities.