Centralized mouse repositories

Because the mouse is used so widely for biomedical research and the number of mouse models being generated is increasing rapidly, centralized repositories are essential if the valuable mouse strains and models that have been developed are to be securely preserved and fully exploited. Ensuring the ongoing availability of these mouse strains preserves the investment made in creating and characterizing them and creates a global resource of enormous value. The establishment of centralized mouse repositories around the world for distributing and archiving these resources has provided critical access to and preservation of these strains. This article describes the common and specialized activities provided by major mouse repositories around the world.     

Mutations in the gene encoding the low-density lipoprotein receptor LRP4 cause abnormal limb development in the mouse

Positional cloning of two recessive mutations of the mouse that cause polysyndactyly (dan and mdig-Chr 2) confirmed that the gene encoding MEGF7/LRP4, a member of the low-density lipoprotein receptor family, plays an essential role in the process of digit differentiation. Pathologies observed in the mutant mice provide insight into understanding the function(s) of LRP4 as a negative regulator of the Wnt-beta-catenin signaling pathway and may help identify the genetic basis for common human disorders with similar phenotypes.     

Physiological genomic analysis of the brain renin-angiotensin system

The brain renin-angiotensin system (RAS) has long been considered pivotal in cardiovascular regulation and important in the pathogenesis of hypertension and heart failure. However, despite more than 30 years of study, the brain RAS continues to defy explanation. Our lack of understanding of how the brain RAS is organized at the cellular and regional levels has made it difficult to resolve long-sought questions of how ANG II is produced in the brain and the precise mechanisms by which it exerts its actions. A major reason for this is the difficulty in experimentally dissecting the brain RAS at the regional, cellular, and whole organism levels. Recently, we and others developed a series of molecular tools for selective manipulation of the murine brain RAS, in parallel with technologies for integrative analysis of cardiovascular and volume homeostasis in the conscious mouse. This review, based in part on a lecture given in conjunction with the American Physiological Society Young Investigator Award in Regulatory and Integrative Physiology (Water and Electrolyte Homeostasis Section), outlines the physiological genomics strategy that we have taken in an effort to unravel some of the complexities of this system. It also summarizes the principles, progress, and prospects for a better understanding of the brain RAS in health and disease.     

Toward understanding the mechanism of the complex cyclization reaction catalyzed by imidazole glycerolphosphate synthase: crystal structures of a ternary complex and the free enzyme

Imidazole glycerol phosphate synthase catalyzes formation of the imidazole ring in histidine biosynthesis. The enzyme is also a glutamine amidotransferase, which produces ammonia in a glutaminase active site and channels it through a 30-A internal tunnel to a cyclase active site. Glutaminase activity is impaired in the resting enzyme, and stimulated by substrate binding in the cyclase active site. The signaling mechanism was investigated in the crystal structure of a ternary complex in which the glutaminase active site was inactivated by a glutamine analogue and the unstable cyclase substrate was cryo-trapped in the active site. The orientation of N(1)-(5'-phosphoribulosyl)-formimino-5-aminoimidazole-4-carboxamide ribonucleotide in the cyclase active site implicates one side of the cyclase domain in signaling to the glutaminase domain. This side of the cyclase domain contains the interdomain hinge. Two interdomain hydrogen bonds, which do not exist in more open forms of the enzyme, are proposed as molecular signals. One hydrogen bond connects the cyclase domain to the substrate analogue in the glutaminase active site. The second hydrogen bond connects to a peptide that forms an oxyanion hole for stabilization of transient negative charge during glutamine hydrolysis. Peptide rearrangement induced by a fully closed domain interface is proposed to activate the glutaminase by unblocking the oxyanion hole. This interpretation is consistent with biochemical results [Myers, R. S., et al., (2003) Biochemistry 42, 7013-7022, the accompanying paper in this issue] and with structures of the free enzyme and a binary complex with a second glutamine analogue.     

Recombination Suppression by Heterozygous Robertsonian Chromosomes in the Mouse

Robertsonian chromosomes are metacentric chromosomes formed by the joining of two telocentric chromosomes at their centromere ends. Many Robertsonian chromosomes of the mouse suppress genetic recombination near the centromere when heterozygous. We have analyzed genetic recombination and meiotic pairing in mice heterozygous for Robertsonian chromosomes and genetic markers to determine (1) the reason for this recombination suppression and (2) whether there are any consistent rules to predict which Robertsonian chromosomes will suppress recombination. Meiotic pairing was analyzed using synaptonemal complex preparations. Our data provide evidence that the underlying mechanism of recombination suppression is mechanical interference in meiotic pairing between Robertsonian chromosomes and their telocentric partners. The fact that recombination suppression is specific to individual Robertsonian chromosomes suggests that the pairing delay is caused by minor structural differences between the Robertsonian chromosomes and their telocentric homologs and that these differences arise during Robertsonian formation. Further understanding of this pairing delay is important for mouse mapping studies. In 10 mouse chromosomes (3, 4, 5, 6, 8, 9, 10, 11, 15 and 19) the distances from the centromeres to first markers may still be underestimated because they have been determined using only Robertsonian chromosomes. Our control linkage studies using C-band (heterochromatin) markers for the centromeric region provide improved estimates for the centromere-to-first-locus distance in mouse chromosomes 1, 2 and 16.     

Identification and genetic mapping of the murine gene and 20 related sequences encoding chromosomal protein HMG-17

HMG-17 is an abundant, nonhistone chromosomal protein that binds preferentially to nucleosomal core particles of mammalian chromatin. The human gene for HMG-17 has been localized to Chromosome (Chr) 1p, but the murine gene has not been previously mapped. Here we identify the murine functional gene, Hmg17, from among more than 25 related sequences (probably processed pseudogenes) and show that it is located on mouse Chr 4, in a region known to have conserved linkage relationships with human Chr 1p. We also report the map locations of 20 additional Hmg17-related sequences on mouse Chrs 1, 2, 3, 5, 7, 8, 9, 13, 15, 16, 17, 18, and X. The multiple, dispersed members of the Hmg17 multigene family can be detected efficiently with a single cDNA probe and provide useful markers for genetic mapping studies in mice.     

Efficient liver-specific deletion of a floxed human angiotensinogen transgene by adenoviral delivery of Cre recombinase in vivo.

Tissue-specific ablation of gene function is possible in vivo by the Cre-loxP recombinase system. We generated transgenic mice containing a human angiotensinogen gene flanked by loxP sites (hAGT(flox)). To examine the physiologic consequences of tissue-specific loss of angiotensinogen gene function in vivo, we constructed an adenovirus expressing Cre recombinase. Studies were performed in several independent lines of hAGT(flox) mice before and after intravenous administration of either Adcre or AdbetaGal as a control. Systemic administration of Adcre caused a significant decrease in circulating human angiotensinogen and markedly blunted the pressor response to administration of purified recombinant human renin. Southern blot analysis of genomic DNA from various organs revealed that the Cre-mediated deletion was liver-specific. Further analysis revealed the absence of full-length human angiotensinogen mRNA and protein in the liver but not the kidney of Adcre mice, consistent with the liver being the target for adenoviruses administered intravenously. These studies demonstrate that extra-hepatic sources of angiotensinogen do not contribute significantly to the circulating pool of angiotensinogen and provide proof-of-principle that the Cre-loxP system can be used effectively to examine the contribution of the systemic and tissue renin-angiotensin system to normal and pathological regulation of blood pressure.     

Lens epithelial proliferation cataract in segmental trisomy involving mouse Chromosomes 4 and 17

A dominant induced mutation in the mouse, tightly associated with a reciprocal chromosomal translocation between Chrs 4 and 17, causes abnormal head tossing and circling behavior (the translocation induced circling mutation, Tim). Affected mice develop an unusual anterior subcapsular cataract that appears after birth and is progressive. The most likely explanation for the phenotypic observations is that the translocation breakpoint disrupted a gene or its regulation. Although the Mos protooncogene is located close to the translocation breakpoint and transgenic mice that overexpress Mos demonstrate cataracts and circling behavior, there were no gross changes in the Mos gene or in its level of expression. The morphological changes observed in the lens resemble those seen in some human congenital cataract syndromes. Received: 31 July 1998 / Accepted: 14 October 1998