Mice with the enhanced green fluorescent protein gene knocked in to chromosome 11 exhibit normal transmission ratios

Widespread expression of the Supv3L1 mitochondrial RNA helicase in the mouse

Supv3L1 is an evolutionarily conserved helicase that plays a critical role in the mitochondrial RNA surveillance and degradation machinery. Conditional ablation of Supv3L1 in adult mice leads to premature aging phenotypes including loss of muscle mass and adipose tissue and severe skin abnormalities. To get insights into the spatial and temporal expression of Supv3L1 in the mouse, we generated knock-in and transgenic strains in which an EGFP reporter was placed under control of the Supv3L1 native promoter. During development, expression of Supv3L1 begins at the blastocyst stage, becomes widespread and strong in all fetal tissues and cell types, and continues during postnatal growth. In mature animals reporter expression is only slightly diminished in most tissues and continues to be highly expressed in the brain, peripheral sensory organs, and testis. Together, these data confirm that Supv3L1 is an important developmentally regulated gene, which continues to be expressed in all mature tissues, particularly the rapidly proliferating cells of testes, but also in the brain and sensory organs. The transgenic mice and cell lines derived from them constitute a valuable tool for the examination of the spatial and temporal aspects of Supv3L1 promoter activity, and should facilitate future screens for small molecules that regulate Supv3L1 expression.

Disruption of Supv3L1 damages the skin and causes sarcopenia, loss of fat, and death

Supv3L1 is a conserved and ubiquitously expressed helicase found in numerous tissues and cell types of many species. In human cells, SUPV3L1 was shown to suppress apoptotic death and sister chromatid exchange, and impair mitochondrial RNA metabolism and protein synthesis. In vitro experiments revealed binding of SUPV3L1 to BLM and WRN proteins, suggesting a role in genome maintenance processes. Disruption of the Supv3L1 gene in the mouse has been reported to be embryonic lethal at early developmental stages. We generated a conditional mouse in which the phenotypes associated with the removal of exon 14 can be tested in a variety of tissues. Disruption mediated by a Mx1 promoter-driven Cre displayed a postnatal growth delay, reduced lifespan, loss of adipose tissue and muscle mass, and severe skin abnormalities manifesting as ichthyosis, thickening of the epidermis, and atrophy of the dermis and subcutaneous tissue. Using a tamoxifen-activatable Esr1/Cre driver, Supv3L1 disruption resulted in growth retardation and aging phenotypes, including loss of adipose tissue and muscle mass, kyphosis, cachexia, and premature death. Many of the abnormalities seen in the Mx1-Cre mice, such as hyperkeratosis characterized by profound scaling of feet and tail, could also be detected in tamoxifen-inducible Cre mice. Conditional ablation of Supv3L1 in keratinocytes confirmed atrophic changes in the skin and ichthyosis-like changes. Together, these data indicate that Supv3L1 is important for the maintenance of the skin barrier. In addition, loss of Supv3L1 function leads to accelerated aging-like phenotypes.

Expression and loss of alleles in cultured mouse embryonic fibroblasts and stem cells carrying allelic fluorescent protein genes

Background

Loss of heterozygosity (LOH) contributes to many cancers, but the rate at which these events occur in normal cells of the body is not clear. LOH would be detectable in diverse cell types in the body if this event were to confer an obvious cellular phenotype. Mice that carry two different fluorescent protein genes as alleles of a locus would seem to be a useful tool for addressing this issue because LOH would change a cell's phenotype from dichromatic to monochromatic. In addition, LOH caused by mitotic crossing over might be discernable in tissues because this event produces a pair of neighboring monochromatic cells that are different colors.

Results

As a step in assessing the utility of this approach, we derived primary embryonic fibroblast populations and embryonic stem cell lines from mice that carried two different fluorescent protein genes as alleles at the chromosome 6 locus, ROSA26. Fluorescence activated cell sorting (FACS) showed that the vast majority of cells in each line expressed the two marker proteins at similar levels, and that populations exhibited expression noise similar to that seen in bacteria and yeast. Cells with a monochromatic phenotype were present at frequencies on the order of 10^-4 and appeared to be produced at a rate of approximately 10^-5 variant cells per mitosis. 45 of 45 stably monochromatic ES cell clones exhibited loss of the expected allele at the ROSA26 locus. More than half of these clones retained heterozygosity at a locus between ROSA26 and the centromere. Other clones exhibited LOH near the centromere, but were disomic for chromosome 6.

Conclusion

Allelic fluorescent markers allowed LOH at the ROSA26 locus to be detected by FACS. LOH at this locus was usually not accompanied by LOH near the centromere, suggesting that mitotic recombination was the major cause of ROSA26 LOH. Dichromatic mouse embryonic cells provide a novel system for studying genetic/karyotypic stability and factors influencing expression from allelic genes. Similar approaches will allow these phenomena to be studied in tissues.

Controlled and localized genetic manipulation in the brain

Brain structure and function are determined in part through experience and in part through our inherited genes. A powerful approach for unravelling the balance between activity-dependent neuronal plasticity and genetic programs is to directly manipulate the genome. Such molecular genetic studies have been greatly aided by the remarkable progress of large-scale genome sequencing efforts. Sophisticated mouse genetic manipulations allow targeted point-mutations, deletions and additions to the mouse genome. These can be regulated through inducible promoters expressing in genetically specified neuronal cell types. However, despite significant progress it remains difficult to target specific brain regions through transgenesis alone. Recent work suggests that transduction vectors, like lentiviruses and adeno-associated viruses, may provide suitable additional tools for localized and controlled genetic manipulation. Furthermore, studies with such vectors may aid the development of human genetic therapies for brain diseases.