A novel locus on proximal chromosome 18 associated with agenesis of the corpus callosum in mice

PlexinA1 is crucial for the midline crossing of callosal axons during corpus callosum development in BALB/cAJ mice

The corpus callosum (CC) is the biggest commissure that links cerebral hemispheres. Guidepost structures develop in the cortical midline during CC development and express axon guidance molecules that instruct neurons regarding the proper direction of axonal elongation toward and across the cortical midline. Neuropilin-1 (Npn1), a high affinity receptor for class 3 semaphorins (Sema3s) localized on cingulate pioneering axons, plays a crucial role in axon guidance to the midline through interactions with Sema3s. However, it remains unclear which type of Plexin is a component of Sema3 holoreceptors with Npn1 during the guidance of cingulate pioneering axons. To address the role of PlexinA1 in CC development, we examined with immunohistochemistry the localization of PlexinA1, Npn1, and Sema3s using embryonic brains from wild-type (WT) and PlexinA1-deficient (PlexinA1 knock-out (KO)) mice with a BALB/cAJ background. The immunohistochemistry confirmed the expression of PlexinA1 in callosal axons derived from the cingulate and neocortex of the WT mice on embryonic day 17.5 (E17.5) but not in the PlexinA1 KO mice. To examine the role of PlexinA1 in the navigation of callosal axons, the extension of callosal axons toward and across the midline was traced in brains of WT and PlexinA1 KO mice at E17.5. As a result, callosal axons in the PlexinA1 KO brains had a significantly lower incidence of midline crossing at E17.5 compared with the WT brains. To further examine the role of PlexinA1 in CC development, the CC phenotype was examined in PlexinA1 KO mice at postnatal day 0.5 (P0.5). Most of the PlexinA1 KO mice at P0.5 showed agenesis of the CC. These results indicate the crucial involvement of PlexinA1 in the midline crossing of callosal axons during CC development in BALB/cAJ mice.

Peri-implantation lethality in mice carrying megabase-scale deletion on 5qc3.3 is caused by Exoc1 null mutation

We found a novel spontaneous mouse mutant with depigmentation in the ventral body, which we called White Spotting (WS) mouse. Genetic investigation revealed deletion of a > 1.2-Mb genomic region containing nine genes (Kit, Kdr, Srd5a3, Tmeme165, Clock, Pdcl2, Nmu, Exoc1, and Cep135). We designated this mutant allele Kit^WS. Interestingly, homozygous mutants (Kit^WS/WS) showed a peri-implantation lethal phenotype. Expression analyses of these nine genes in blastocysts suggested that Exoc1 was a prime candidate for this phenotype. We produced Exoc1 knockout mice, and the same peri-implantation lethal phenotype was seen in Exoc1^−/− embryos. In addition, the polygenic effect without Exoc1 was investigated in genome-edited Kit^WE mice carrying the Mb-scale deletion induced by the CRISPR/Cas9 system. As Kit^WE/WE embryos did not exhibit the abnormal phenotype, which was seen in Kit^WS/WS. We concluded that peri-implantation lethality in Kit^WS/WS was caused by a monogenic defect of Exoc1.

Generation and characterization of Ins1-cre-driver C57BL/6N for exclusive pancreatic beta cell-specific Cre-loxP recombination

Cre/loxP system-mediated site-specific recombination is utilized to study gene function in vivo. Successful conditional knockout of genes of interest is dependent on the availability of Cre-driver mice. We produced and characterized pancreatic β cell-specific Cre-driver mice for use in diabetes mellitus research. The gene encoding Cre was inserted into the second exon of mouse Ins1 in a bacterial artificial chromosome (BAC). Five founder mice were produced by microinjection of linearized BAC Ins1-cre. The transgene was integrated between Mafa and the telomere on chromosome 15 in one of the founders, BAC Ins1-cre25. To investigate Cre-loxP recombination, BAC Ins1-cre25 males were crossed with two different Cre-reporters, R26R and R26GRR females. On gross observation, reporter signal after Cre-loxP recombination was detected exclusively in the adult pancreatic islets in both F₁ mice. Immunohistological analysis indicated that Cre-loxP recombination-mediated reporter signal was colocalized with insulin in pancreatic islet cells of both F₁ mice, but not with glucagon. Moreover, Cre-loxP recombination signal was already observed in the pancreatic islets at E13.5 in both F₁ fetuses. Finally, we investigated ectopic Cre-loxP recombination for Ins1, because the ortholog Ins2 is also expressed in the brain, in addition to the pancreas. However, there was no Cre-loxP recombination-mediated reporter signal in the brain of both F₁ mice. Our data suggest that BAC Ins1-cre25 mice are a useful Cre-driver C57BL/6N for pancreatic β cell-specific Cre-loxP recombination, except for crossing with knock-in mice carrying floxed gene on chromosome 15.

Truncated Cables1 causes agenesis of the corpus callosum in mice

Agenesis of the corpus callosum (ACC) is a congenital abnormality of the brain structure. More than 60 genes are known to be involved in corpus callosum development. However, the molecular mechanisms underlying ACC are not fully understood. Previously, we produced a novel transgenic mouse strain, TAS, carrying genes of the tetracycline-inducible expression system that are not involved in brain development, and inherited ACC was observed in the brains of all homozygous TAS mice. Although ACC was probably induced by transgene insertion mutation, the causative gene and the molecular mechanism of its pathogenesis remain unclear. Here, we first performed interphase three-color fluorescence in situ hybridization (FISH) analysis to determine the genomic insertion site. Transgenes were inserted into chromosome 18 ∼12.0 Mb from the centromere. Gene expression analysis and genomic PCR walking showed that the genomic region containing exon 4 of Cables1 was deleted by transgene insertion and the other exons of Cables1 were intact. The mutant allele was designated as Cables1(TAS). Interestingly, Cables1(TAS) mRNA consisted of exons 1-3 of Cables1 and part of the transgene that encoded a novel truncated Cables1 protein. Homozygous TAS mice exhibited mRNA expression of Cables1(TAS) in the fetal cerebrum, but not that of wild-type Cables1. To investigate whether a dominant negative effect of Cables1(TAS) or complete loss of function of Cables1 gives rise to ACC, we produced Cables1-null mutant mice. ACC was not observed in Cables1-null mutant mice, suggesting that a dominant negative effect of Cables1(TAS) impairs callosal formation. Moreover, ACC frequency in Cables1(+/TAS) mice was significantly lower than that in Cables1(-/TAS) mice, indicating that wild-type Cables1 interfered with the dominant negative effect of Cables1(TAS). This study indicated that truncated Cables1 causes ACC and wild-type Cables1 contributes to callosal formation.

Combining M-FISH and Quantum Dot technology for fast chromosomal assignment of transgenic insertions

Background

Physical mapping of transgenic insertions by Fluorescence in situ Hybridization (FISH) is a reliable and cost-effective technique. Chromosomal assignment is commonly achieved either by concurrent G-banding or by a multi-color FISH approach consisting of iteratively co-hybridizing the transgenic sequence of interest with one or more chromosome-specific probes at a time, until the location of the transgenic insertion is identified.

Results

Here we report a technical development for fast chromosomal assignment of transgenic insertions at the single cell level in mouse and rat models. This comprises a simplified 'single denaturation mixed hybridization' procedure that combines multi-color karyotyping by Multiplex FISH (M-FISH), for simultaneous and unambiguous identification of all chromosomes at once, and the use of a Quantum Dot (QD) conjugate for the transgene detection.

Conclusions

Although the exploitation of the unique optical properties of QD nanocrystals, such as photo-stability and brightness, to improve FISH performance generally has been previously investigated, to our knowledge this is the first report of a purpose-designed molecular cytogenetic protocol in which the combined use of QDs and standard organic fluorophores is specifically tailored to assist gene transfer technology.