Biomedical subjects

T Sapir

Publications and source records attributed to T Sapir.

11 recordsLinked to original sources

PubMed2001

Targeted mutagenesis of Lis1 disrupts cortical development and LIS1 homodimerization.

Lissencephaly is a severe brain malformation in humans. To study the function of the gene mutated in lissencephaly (LIS1), we deleted the first coding exon from the mouse Lis1 gene. The deletion resulted in a shorter protein (sLIS1) that initiates from the second methionine, a unique situation because most LIS1 mutations result in a null allele. This mutation mimics a mutation described in one lissencephaly patient with a milder phenotype. Homozygotes are early lethal, although heterozygotes are viable and fertile. Most strikingly, the morphology of cortical neurons and radial glia is aberrant in the developing cortex, and the neurons migrate more slowly. This is the first demonstration, to our knowledge, of a cellular abnormality in the migrating neurons after Lis1 mutation. Moreover, cortical plate splitting and thalomocortical innervation are also abnormal. Biochemically, the mutant protein is not capable of dimerization, and enzymatic activity is elevated in the embryos, thus a demonstration of the in vivo role of LIS1 as a subunit of PAF-AH. This mutation allows us to determine a hierarchy of functions that are sensitive to LIS1 dosage, thus promoting our understanding of the role of LIS1 in the developing cortex.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗

PubMed2000

Interaction between LIS1 and doublecortin, two lissencephaly gene products.

Mutations in either LIS1 or DCX are the most common cause for type I lissencephaly. Here we report that LIS1 and DCX interact physically both in vitro and in vivo. Epitope-tagged DCX transiently expressed in COS cells can be co-immunoprecipitated with endogenous LIS1. Furthermore, endogenous DCX could be co-immunoprecipitated with endogenous LIS1 in embryonic brain extracts, demonstrating an in vivo association. The two protein products also co-localize in transfected cells and in primary neuronal cells. In addition, we demonstrate homodimerization of DCX in vitro. Using fragments of both LIS1 and DCX, the domains of interaction were mapped. LIS1 and DCX interact with tubulin and microtubules. Our results suggest that addition of DCX and LIS1 to tubulin enhances polymerization in an additive fashion. In in vitro competition assays, when LIS1 is added first, DCX competes with LIS1 in its binding to microtubules, but when DCX is added prior to the addition of LIS1 it enhances the binding of LIS1 to microtubules. We conclude that LIS1 and DCX cross-talk is important to microtubule function in the developing cerebral cortex.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗

PubMed2000

Doublecortin mutations cluster in evolutionarily conserved functional domains.

Mutations in the X-linked gene doublecortin ( DCX ) result in lissencephaly in males or subcortical laminar heterotopia ('double cortex') in females. Various types of mutation were identified and the sequence differences included nonsense, splice site and missense mutations throughout the gene. Recently, we and others have demonstrated that DCX interacts and stabilizes microtubules. Here, we performed a detailed sequence analysis of DCX and DCX-like proteins from various organisms and defined an evolutionarily conserved Doublecortin (DC) domain. The domain typically appears in the N-terminus of proteins and consists of two tandemly repeated 80 amino acid regions. In the large majority of patients, missense mutations in DCX fall within the conserved regions. We hypothesized that these repeats may be important for microtubule binding. We expressed DCX or DCLK (KIAA0369) repeats in vitro and in vivo. Our results suggest that the first repeat binds tubulin but not microtubules and enhances microtubule polymerization. To study the functional consequences of DCX mutations, we overexpressed seven of the reported mutations in COS7 cells and examined their effect on the microtubule cytoskeleton. The results demonstrate that some of the mutations disrupt microtubules. The most severe effect was observed with a tyrosine to histidine mutation at amino acid 125 (Y125H). Produced as a recombinant protein, this mutation disrupts microtubules in vitro at high molar concentration. The positions of the different mutations are discussed according to the evolutionarily defined DC-repeat motif. The results from this study emphasize the importance of DCX-microtubule interaction during normal and abnormal brain development.

Amino Acid Sequence↗

PubMed1999

LIS1 is a microtubule-associated phosphoprotein.

Lissencephaly, a severe brain malformation, may be caused by mutations in the LIS1 gene. LIS1 encodes a microtubule-associated protein (MAP) that is also part of the enzyme complex, platelet-activating factor acetylhydrolase. LIS1 is also found in a complex with two protein kinases; a T-cell Tat-associated kinase, which contains casein-dependent kinase (CDK) activating kinase (CAK), as well as CAK-inducing activity, and with a spleen protein-tyrosine kinase similar to the catalytic domain of p72syk. As phosphorylation is one of the ways to control cellular localization and protein-protein interactions, we investigated whether LIS1 undergoes this post-translational modification. Our results demonstrate that LIS1 is a developmentally regulated phosphoprotein. Phosphorylated LIS1 is mainly found in the MAP fraction. Phosphoamino acid analysis revealed that LIS1 is phosphorylated on serine residues. Alkaline phosphatase treatment reduced the number of visible LIS1 isoforms. In-gel assays demonstrate a 50-kDa LIS1 kinase that is enriched in microtubule-associated fractions. In vitro, LIS1 was phosphorylated by protein kinase CKII (casein kinase II), but not many other kinases that were tested. We suggest that LIS1 activity may be regulated by phosphorylation.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗

PubMed1999

Analysis of lissencephaly-causing LIS1 mutations.

Mutations in the LIS1 gene may result in severe abnormalities of brain cortical layering known as lissencephaly. Most lissencephaly-causing LIS1 mutations are deletions that encompass the entire gene, therefore the mechanism of the disease is regarded as haploinsufficiency. So far, 13 different intragenic mutations have been reported: one point mutation, H149R; deletion of exon 9, which results in deleted acids Delta301-334; deletion of exon 4, which results in deleted amino acids Delta40-64; 10 mutations resulting in truncated proteins and one predicted to result in extra amino acids. We studied the consequences of the point mutation, deletion mutation and one of the reported truncations. In order to study LIS1 structure function, we introduced an additional point mutation and other truncations in different regions of the protein. The consequences of these mutations to protein folding were studied by gel filtration, sucrose density gradient centrifugation and measuring resistance to trypsin cleavage. On the basis of our results, we suggest that all truncation mutations and lissencephaly-causing point mutations or internal deletion result in a reduction in the amount of correctly folded LIS1 protein.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗

PubMed1999

Doublecortin, a stabilizer of microtubules.

X-linked lissencephaly is a severe brain malformation affecting males. Recently it has been demonstrated that the doublecortin gene is implicated in this disorder. In order to study the function of Doublecortin, we analyzed the protein upon transfection of COS cells. Doublecortin was found to bind to the microtubule cytoskeleton. In vitro assays (using biochemical methods, DIC microscopy and electron microscopy) demonstrate that Doublecortin binds microtubules directly, stabilizes them and causes bundling. In vivo assays also show that Doublecortin stabilizes microtubules and causes bundling. Doublecortin is a basic protein with an iso-electric point of 10, typical of microtubule-binding proteins. However, its sequence contains no known microtubule-binding domain(s). The results obtained in this study with Doublecortin and our previous work on another lissencephaly gene ( LIS1 ) emphasize the central role of regulation of microtubule dynamics and stability during neuronal morphogenesis.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗

PubMed1998

Abnormal cortical development; towards elucidation of the LIS1 gene product function (review).

Lissencephaly is a relatively common brain malformation. Lissencephaly type 1 is characterized by the smooth appearance of the cortex and the presence of four abnormally positioned layers instead of the normal six. Lissencephaly is considered to be an abnormality in neuronal migration. The gene mutated in type 1 lissencephaly was cloned by us and designated LIS1. Recently, several genes involved in cortical development have been cloned in the mouse. In human an additional X-linked lissencephaly gene has been identified. We summarize here our current knowledge on the LIS1 gene and its function. It has been identified as a non-catalytic subunit of PAF-acetylhydrolase, a heterotrimeric enzyme which inactivates the platelet-activating factor (PAF). In addition, we have demonstrated that LIS1 interacts with tubulin, and affects the dynamics properties of microtubles. LIS1 contains seven WD repeats and may structurally resemble the beta-subunit of heterotrimeric G proteins. Interestingly, the catalytic subunit of PAF-acetylhydrolase was found to resemble the alpha subunit of heterotrimeric G proteins. We raise the possibility that LIS1 is part of an intracellular signaling pathway involved in neuronal migration.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗

PubMed1997

Reduction of microtubule catastrophe events by LIS1, platelet-activating factor acetylhydrolase subunit.

Forming the structure of the human brain involves extensive neuronal migration, a process dependent on cytoskeletal rearrangement. Neuronal migration is believed to be disrupted in patients exhibiting the developmental brain malformation lissencephaly. Previous studies have shown that LIS1, the defective gene found in patients with lissencephaly, is a subunit of the platelet-activating factor acetylhydrolase. Our results indicated that LIS1 has an additional function. By interacting with tubulin it suppresses microtubule dynamics. We detected LIS1 interaction with microtubules by immunostaining and co-assembly. LIS1-tubulin interactions were assayed by co-immunoprecipitation and by surface plasmon resonance changes. Microtubule dynamic measurements in vitro indicated that physiological concentrations of LIS1 indeed reduced microtubule catastrophe events, thereby resulting in a net increase in the maximum length of the microtubules. Furthermore, the LIS1 protein concentration in the brain, measured by quantitative Western blots, is high and is approximately one-fifth of the concentration of brain tubulin. Our new findings show that LIS1 is a protein exhibiting several cellular interactions, and the interaction with the cytoskeleton may prove to be the mode of transducing a signal generated by platelet-activating factor. We postulate that the LIS1-cytoskeletal interaction is important for neuronal migration, a process that is defective in lissencephaly patients.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗

PubMed1995

LIS2, gene and pseudogene, homologous to LIS1 (lissencephaly 1), located on the short and long arms of chromosome 2.

We report here the isolation of a novel cDNA, designated LIS2, that maps to chromosome 2p11.2 by in situ hybridization and demonstrates extremely high sequence similarity to the recently identified LIS1 gene involved in Miller-Dieker lissencephaly at 17p13.3. Specific probes for LIS2 revealed a pattern of expression resembling that of LIS1, although LIS2 is less abundant. Surprisingly, LIS2 detected an additional, higher molecular weight transcript in adult skeletal muscle. Isolated YAC clones and P1 clones mapped by in situ hybridization to two loci on chromosome 2,2p11.2 and 2q13-q14. This hybridization was due to the existence of LIS2 pseudogene LIS2P on the long arm of chromosome 2.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗

PubMed1995

Lissencephaly gene (LIS1) expression in the CNS suggests a role in neuronal migration.

Miller-Dieker lissencephaly syndrome (MDS) is a human developmental brain malformation caused by neuronal migration defects resulting in abnormal layering of the cerebral cortex. LIS1, the gene defective in MDS, encodes a subunit of brain platelet-activating factor (PAF) acetylhydrolase which inactivates PAF, a neuroregulatory molecule. We have isolated murine cDNAs homologous to human LIS1 and mapped these to three different chromosomal loci (Lis1, Lis3, Lis4). The predicted sequences of murine Lis1 protein and its human homolog LIS1 are virtually identical. In the developing mouse and human, Lis1 and LIS1 genes were strongly expressed in the cortical plate. In the adult mouse Lis1 transcripts were abundant in cortex and hippocampus. The direct correlation between cortical defects in MDS patients and Lis1 expression in the murine cortex suggest that the mouse is a model system suitable to study the mechanistic basis of this intriguing genetic disease.

1-Alkyl-2-acetylglycerophosphocholine Esterase↗

PubMed1994

TNF-alpha associated with fibronectin enhances phorbol myristate acetate- or antigen-mediated integrin-dependent adhesion of CD4+ T cells via protein tyrosine phosphorylation.

The effects of cytokines on immune cells may be influenced by their milieu, such as the extracellular matrix (ECM), in the vicinities of which cytokines and inflammatory cells interact and function. Previously, we demonstrated that TNF-alpha bound to fibronectin (FN) and augments the level of adhesion of activated CD4+ cells. Herein, we examined the mechanisms of this pro-adhesive activity of TNF-alpha and its putative physiologic consequences using human or rat CD4+ cells. A brief exposure of CD4+ cells to low dosages of soluble TNF-alpha or of FN- or laminin-bound TNF-alpha synergized with PMA to enhance the integrin-mediated binding of CD4+ cells to these immobilized ECM moieties. TNF-alpha-enhanced adhesion of CD4+ cells did not delay or inhibit the subsequent detachment of the cells from the substrate, and adhesion was increased provided the cells were treated with TNF-alpha immediately after their exposure to PMA. This indicates that the enhancing effect of TNF-alpha requires a previous activation of the cells. When TNF-alpha was immobilized on FN, less TNF-alpha was required to induce CD4+ cell binding to FN. Soluble, and to a greater extent FN-bound, TNF-alpha synergizes with PMA to intensify protein tyrosine phosphorylation in FN-bound CD4+ cells, and this effect of TNF-alpha was inhibited by inhibitors of tyrosine kinase. That FN-bound or soluble TNF-alpha also amplified the binding of an Ag-specific autoimmune rat T cell line to immobilized FN, emphasizes the physiologic relevance of our findings. Thus, the signal transduction and cell adhesive properties of ECM glycoproteins may be modulated upon their association with TNF-alpha, and matrix-linked TNF-alpha may recruit and direct immune cells to inflammatory sites.

Animals↗