High-resolution genetic map and YAC contig around the mouse neurological locus reeler

Regulation of cortical neuron migration by the Reelin signaling pathway

Reeler is a mutant mouse with defects in layered structures of the central nervous system, such as the cerebral cortex, hippocampus, and cerebellum, and has been extensively examined for more than half a century. The full-length cDNA for the responsible gene for reeler, reelin, was serendipitously identified, revealing that Reelin encodes a large secreted protein. So far, two Reelin receptors, apolipoprotein E receptor 2 and very low-density lipoprotein receptor, and the cytoplasmic adaptor protein Disabled homolog 1 (Dab1) have been shown to be essential for Reelin signaling. Although a number of downstream cascades of Dab1 have also been reported using various experimental systems, the physiological functions of Reelin in vivo remain controversial. Here, we review recent advances in the understanding of the Reelin-Dab1 signaling pathway in the developing cerebral cortex.

Rat neurological disease creeping is caused by a mutation in the reelin gene

Reelin (Reln) is an extracellular matrix protein secreted from distinct neuronal populations and controls neural cell positioning during brain development. Alterations of human RELN have been reported in two pedigrees with an autosomal recessive lissencephaly. Although several alleles of the mouse reeler mutation were identified as disruptions of Reln, there is no other animal model with a confirmed mutation in Reln. We recently established the Komeda Zucker creeping (KZC) rat strain with an autosomal recessive mutation creeping (cre), showing a reeler-like phenotype. We also found that creeping was located in the genomic segment on rat chromosome 4 containing Reln and that the expression level of Reln mRNA was markedly reduced in cre/cre homozygous mutant animals. Here we report positional candidate cloning of creeping, and identify a nucleotide insertion mutation in Reln. This mutation leads to a translational frameshift and results in truncation of the predicted protein in the fourth reelin-specific repeat, removing the C-terminal region required for secretion and function of the protein. We conclude that the mutation detected here is causative and is probably a null allele. The KZC rat is the first rat model with a confirmed Reln mutation and would therefore contribute to the understanding of the Reln function.

Interdigitated deletion complexes on mouse chromosome 5 induced by irradiation of embryonic stem cells

Chromosome deletions have several applications in the genetic analysis of complex organisms. They can be used as reagents in region-directed mutagenesis, for mapping of simple or complex traits, or to identify biological consequences of segmental haploidy, the latter being relevant to human contiguous gene syndromes and imprinting. We have generated three deletion complexes in ES (Embryonic Stem) cells that collectively span approximately 40 cM of proximal mouse chromosome 5. The deletion complexes were produced by irradiation of F(1) hybrid ES cells containing herpes simplex virus thymidine kinase genes (tk) integrated at the Dpp6, Hdh (Huntington disease locus), or Gabrb1 loci, followed by selection for tk-deficient clones. Deletions centered at the adjacent Hdh and Dpp6 loci ranged up to approximately 20 cM or more in length and overlapped in an interdigitated fashion. However, the interval between Hdh and Gabrb1 appeared to contain a locus haploinsufficient for ES cell viability, thereby preventing deletions of either complex from overlapping. In some cases, the deletions resolved the order of markers that were previously genetically inseparable. A subset of the ES cell-bearing deletions was injected into blastocysts to generate germline chimeras and establish lines of mice segregating the deletion chromosomes. At least 11 of the 26 lines injected were capable of producing germline chimeras. In general, those that failed to undergo germline transmission bore deletions larger than the germline-competent clones, suggesting that certain regions of chromosome 5 contain haploinsufficient developmental genes, and/or that overall embryonic viability is cumulatively decreased as more genes are rendered hemizygous. Mice bearing deletions presumably spanning the semidominant hammertoe locus (Hm) had no phenotype, suggesting that the classic allele is a dominant, gain-of-function mutation. Overlapping deletion complexes generated in the fashion described in this report will be useful as multipurpose genetic tools and in systematic functional mapping of the mouse genome.

Neuropathologic findings in a case of OFDS type VI (Váradi syndrome)

Oral-facial-digital syndrome type VI (OFDS VI) or Váradi syndrome is a rare autosomal-recessive disorder distinguished from other oral-facial-digital syndromes by metacarpal abnormalities with central polydactyly and by cerebellar abnormalities. Histopathologic characterization of the cerebellar abnormalities has not been described previously. We describe the neuropathologic findings in a stillborn, 21-week estimated gestational age (EGA) male fetus diagnosed antenatally with signs of OFDS VI. Autopsy findings included: facial abnormalities, postaxial central polydactyly of the right hand, bilateral bifid toes, and absence of cerebellar vermis with hypoplasia of the hemispheric cortex. Microscopic analysis of the cerebellum demonstrated absence of the subpial granular cell layer and disruption or dysgenesis of the glial architecture. These histopathologic findings suggest that a primary neuronal or glial cell defect, rather than an associated Dandy-Walker malformation, may account for the cerebellar abnormalities in this form of oral-facial-digital syndrome.

Reelin is preferentially expressed in neurons synthesizing gamma-aminobutyric acid in cortex and hippocampus of adult rats

During embryonic development of brain laminated structures, the protein Reelin, secreted into the extracellular matrix of the cortex and hippocampus by Cajal-Retzius (CR) cells located in the marginal zone, contributes to the regulation of migration and positioning of cortical and hippocampal neurons that do not synthesize Reelin. Soon after birth, the CR cells decrease, and they virtually disappear during the following 3 weeks. Despite their disappearance, we can quantify Reelin mRNA (approximately 200 amol/ g of total RNA) and visualize it by in situ hybridization, and we detect the translated product of this mRNA by immunocytochemistry preferentially in gamma-aminobutyric acid (GABA)ergic neurons of adult rat cortex and hippocampus. In adult rat cerebellum, Reelin is expressed in glutamatergic neurons (granule cells). The translated product of this mRNA is readily exported from the granule cell somata to the parallel fibers, where it has been detected by electron microscopy in axon terminals located presynaptically to Purkinje cell dendrites.

Reeler: new tales on an old mutant mouse

Neurological mouse mutants provide an opportunity to dissect the complex mechanisms that underlie vertebrate brain development. Advances in genetic technologies have permitted the identification of genes disrupted in many mutants, allowing a molecular interpretation of the phenotypes. For several decades, the spontaneous mutant mouse reeler has been used as a model for the analysis of the development of laminated brain structures. In this ataxic mutant, the migration of many neurons is aberrant, resulting in disrupted cellular organization. Recently, reelin, the gene disrupted in the reeler mouse, has been identified, reelin encodes a novel extracellular molecule that controls neural cell positioning through mechanisms that are not yet completely understood. Analysis of the expression pattern and the properties of the reelin gene product (Reelin) suggests models for its function during brain development. Furthermore, the recent identification of genes that may function in the Reelin signaling pathway advances our knowledge of the molecular basis of neuronal migration.

High-speed positional cloning based on restriction landmark genome scanning

Restriction landmark genome scanning (RLGS) was developed as a method of genome analysis that is based on the concept that restriction enzyme sites can be used as landmarks. In this article, we demonstrate how this method can be used for the systematic, successful positional cloning of mouse mutant reeler gene. The major advantage of the RLGS method is that it allows the scanning of several thousand spots/loci throughout the genome with one RLGS profile. High-speed positional cloning based on the RLGS method includes (1) high-speed construction of a linkage map (RLGS spot mapping), (2) high-speed detection of RLGS spot markers tightly linked to the mutant phenotype (RLGS spot bombing method), and (3) construction of YAC contigs covering the region where tightly linked spot markers are located (RLGS-based YAC contig mapper). We introduced a series of these procedures by using them to positionally clone the reeler gene. High-speed construction of the whole genetic map and spots/loci (less than 1 cM) within the closest flanking markers is demonstrated. The RLGS-based YAC contig mapper also efficiently yielded the YAC physical contig map of the target region. Finally, we cloned the reeler gene, which is the causal gene for the perturbation of the three-dimensional brain architecture due to the abnormal migration of neuroblasts in reeler mouse. Since the RLGS method itself can be used for any organism, we conclude that the total RLGS-based positional cloning system can be used to identify any mutant gene of any organism.

Genomic organization of the mouse reelin gene

Reelin is the protein defective in reeler mice, an extensively studied model of brain development. The reelin gene (symbol Reln) codes for a protein of the extracellular matrix that contains eight successive repeats of 350 to 390 amino acids. In this work, we describe the genomic structure of the mouse reelin gene and the 5'-flanking genomic DNA sequences. The reelin gene is composed of 65 exons spread over approximately 450 kb of genomic DNA. We identified different reelin transcripts, formed by alternative splicing of a microexon as well as by use of two different polyadenylation sites. All splice sites conform to the GT-AG rule, except for the splice donor site of intron 30, which is GC instead of GT. A processed pseudogene is present in intron 42. Its nucleotide sequence is 86% identical to the sequence of the rat RDJ1 cDNA, which codes for a DnaJ-like protein of the Hsp40 family. Comparison of 8 intron positions in mouse and human reelin genes reveals a highly conserved genomic structure, suggesting a similar structure of the whole gene in both species. We identified two transcription start sites embedded within a CpG. The promoter region contains putative recognition sites for the transcription factors Sp1 and AP2 but lacks TATA and CAAT boxes. The presence of tandemly repeated regions in the Reelin protein suggests that gene duplication events occurred during evolution. By comparison of the amino acid sequences of the eight repeats and the positions of introns, we suggest a model for the evolution of the repeat coding portion of the reelin gene from a putative ancestral minigene.

Reln(rl-Alb2), an allele of reeler isolated from a chlorambucil screen, is due to an IAP insertion with exon skipping

The reeler Albany2 mutation (Reln(rl-Alb2) in the mouse is an allele of reeler isolated during a chlorambucil mutagenesis screen. Homozygous animals had drastically reduced concentrations of reelin mRNA, in which an 85-nt exon was absent. At the genomic level, the mutation was shown to be due to an intracisternal A-particle insertion leading to exon skipping. This appears to be the first observation of retrotransposon insertion during chlorambucil mutagenesis.

Reelin mRNA expression during mouse brain development

Using in situ hybridization, expression of the mRNA for reelin, the gene most probably responsible for the reeler trait, was studied during mouse brain development, from embryonic day 13 to maturity. The highest level of expression was found in Cajal-Retzius neurons, while a high signal was also seen in the olfactory bulb, the external granular layer of the cerebellum and, particularly at early developmental stages, in hypothalamic differentiation fields, tectum and spinal cord. A moderate to low level of expression was found in the septal area, striatal fields, habenular nuclei, some thalamic nuclei, particularly the lateral geniculate, the retina and some nuclei of the reticular formation in the central field of the medulla. Paradoxically, no reelin expression was detected in radial glial cells, the cortical plate, Purkinje cells, inferior olivary neurons and many other areas that are characteristically abnormal in reeler mutant mice. Together with other preliminary studies, the present observations suggest that the action of reelin is indirect, possibly mediated by the extracellular matrix. Most of the data can be explained by supposing that reelin is a cell-repulsive molecule which prevents migrating neurons from invading reelin-rich areas, and thus facilitates the deployment of radial glial cell processes and the formation of early architectonic patterns.

Rapid genotyping of mutant mice using dried blood spots for polymerase chain reaction (PCR) analysis

Spontaneous neurologic mutations in the mouse provide powerful tools for the study of mammalian central nervous system development. The study of mouse neurologic mutants has led to a better understanding of the complex mechanisms involved in the development of the nervous system. Because few of these mutations have been identified, molecular probes distinguishing heterozygotes from homozygotes are generally unavailable. Further, most neurologic mouse mutants breed poorly as homozygotes, making it necessary to breed heterozygotes and select homozygous mutant progeny based on phenotype. The requirement for heterozygous breeding and the lack of molecular markers specific for the mutation have hampered developmental studies because the underlying neurologic perturbations occur before the mutant mice can be identified by phenotype. The recent identification and chromosomal assignment of simple sequence repeats (SSRs), repetitive sequences of DNA found at a high density throughout the mouse genome, provide the tools for mapping mutations in the mouse and for subsequent genotyping of potential mutants prior to phenotype onset. The SSRs are useful because these markers are polymorphic (for review see Weber, J.L., Human DNA polymorphisms based on length variations in simple-sequence tandem repeats. In: K.E. Davies and S.M. Tilghman (Eds.), Genetic and Physical Mapping. Genome Analysis, Vol. I, Cold Spring Harbor Laboratory Press, Plainview, NY, 1990, pp. 159-181 [16]), that is, the size of the individual SSRs differs among strains of mice. Following polymerase chain reaction (PCR) amplification of an SSR and separation of PCR products by polyacrylamide gel electrophoresis, one can easily visualize differences in the size of the PCR product between mouse strains. Many mutations in the mouse arose spontaneously on inbred strains and were subsequently backcrossed onto a different strain. After many generations of congenic backcrosses, the only DNA retained from the original mutant strain is composed of the mutant gene and closely linked regions. Thus, it is possible to cross the mutant strain to a different mouse strain and map the mutation by correlating mutant phenotype to SSRs the same size as the original mutant strain. We have mapped the tottering (tg), Purkinje cell degeneration (pcd), and nervous (nr) mutations using SSRs in backcrossed mouse strains. The SSRs distinguishing mutant from normal strains can then be used to genotype potential mutant pups before the onset of the mutant phenotype. The protocol described below can be adapted to almost any mutation congenically inbred for genotyping. Here we describe a method for selecting primers appropriate for genotyping potential mouse mutants and a rapid protocol for genotype screening. Even with SSRs distinguishing mutant from normal mice, genotyping several mice simultaneously can be a daunting task. This is primarily because the protocols available for preparing DNA for PCR amplification are time-consuming, requiring several purification steps including phenol extractions. Although kits are commercially available for DNA preparation without organic extractions, these kits tend to be expensive. The protocol described is a rapid, inexpensive method of determining the genotype of mice using PCR analysis of dried blood spots. The protocol only requires PCR primers distinguishing among alleles and is therefore ideal for the rapid identification of potential mutants for those mouse mutations which have been mapped using microsatellite markers. The DNA preparation protocol may also be used in rapid screening of potential transgenic mice.

The reeler gene encodes a protein with an EGF-like motif expressed by pioneer neurons

We have identified a strong candidate cDNA for the mouse reeler gene. This 5 kb transcript encodes a 99.4 kD protein consisting of 881 amino acids and possessing two EGF-like motifs. We assayed two independent mutant alleles--'Jackson reeler', which has a deletion of the entire gene, and 'Orleans reeler' which exhibits a 220 bp deletion in the open reading frame, including the second EGF-like motif and resulting in a frame shift. In situ hybridization reveals that the transcript is detected exclusively in the pioneer neurons which guide neuronal cell migration along the radial array. Our findings offer an explanation for how the reeler mutant phenotype causes a disturbance of the complex architecture of the neuronal network.

A YAC contig containing the reeler locus with preliminary characterization of candidate gene fragments

The reeler mutation in the mouse maps to proximal chromosome 5 and defines a key gene involved in brain development and evolution. No gene product is known, and the locus is currently being characterized by positional cloning. YAC clones corresponding to the closest markers D5Mit61 and D5Mit72 have been isolated. Cloned extremities of the YAC inserts were used to construct a 1.1-Mb contig, a 700-kb fragment of which was shown to contain the reeler locus. The integrity of the contig was verified by physical mapping on genomic DNA. The classical allele of the reeler mutation was associated with a 150-kb deletion between D5Mit61 and D5Mit72, while no gross chromosomal anomaly was found in the Orleans allele. Candidate coding sequences were isolated to construct a preliminary transcriptional map of the reeler region. Cosmid clones mapping within the rl deletion revealed a large transcript of more than 11 kb, which was present in normal embryonic brain but barely detectable in homozygous rlOrl/rlOrl embryonic brain, suggesting strongly that it corresponds to the reeler transcript.