Strain distribution pattern of 25 simple sequence length polymorphisms in the AXB and BXA recombinant inbred strains

White, brite, and brown adipocytes: the evolution and function of a heater organ in mammals

Brown fat is a specialized heater organ in eutherian mammals. In contrast to the energy storage function of white adipocytes, brown adipocytes dissipate nutrient energy by uncoupling of mitochondrial oxidative phosphorylation, which depends on uncoupling protein 1 (UCP1). UCP1, as well as UCP2 and UCP3, belong to the family of mitochondrial carriers inserted into the inner mitochondrial membrane for metabolite trafficking between the matrix and the intermembrane space. UCP1 transports protons into the mitochondrial matrix when activated by a rise in free fatty acid levels in the cell. This UCP1-dependant proton leak drives high oxygen consumption rates in the absence of ATP synthesis and dissipates proton motive force as heat. The enormous heating capacity of brown fat is supported by dense vascularization, high rates of tissue perfusion, and high mitochondrial density in brown adipocytes.

It has been known for more than 50 years that nonshivering thermogenesis in brown fat serves to maintain body temperature of neonates and small mammals in cold environments, and is used by hibernators for arousal from torpor. It has been speculated that the development of brown fat as a new source for nonshivering thermogenesis provided mammals with a unique advantage for survival in the cold. Indeed brown fat and UCP1 is found in ancient groups of mammals, like the afrotherians and marsupials. In the latter, however, the thermogenic function of UCP1 and brown fat has not been demonstrated as of yet. Notably, orthologs of all three mammalian UCP genes are also present in the genomes of bony fishes and in amphibians. Molecular phylogeny reveals a striking increase in the substitution rate of UCP1 between marsupial and eutherian lineages. At present, it seems that UCP1 only gained thermogenic function in brown adipocytes of eutherian mammals, whereas the function of UCP1 and that of the other UCPs in ectotherms remains to be identified.

Evolution of thermogenic function required expression of UCP1 in a brown-adipocyte-like cell equipped with high mitochondrial density embedded in a well-vascularized tissue. Brown-adipocyte-like cells in white adipose tissue, called “brite” (brown-in-white) or “beige” adipocytes, emerge during adipogenesis and in response to cold exposure in anatomically distinct adipose tissue depots of juvenile and adult rodents. These brite adipocytes may resemble the archetypical brown adipocyte in vertebrate evolution. It is therefore of interest to elucidate the molecular mechanisms of brite adipocyte differentiation, study the bioenergetic properties of these cells, and search for the presence of related brown-adipocyte-like cells in nonmammalian vertebrates.

Ethanol teratogenesis in the C57BL/6J, DBA/2J, and A/J inbred mouse strains

Research has shown variations in susceptibility to alcohol-related birth defects in humans. Genetic differences are one reason for this variability. This study compared three inbred mouse strains to determine whether they differ in their susceptibilities to ethanol teratogenesis because previous studies have generated conflicting data. Pregnant C57BL/6J (B6), DBA/2J (D2), and A/J (A) dams were intubated intragastrically with either an acute dose of ethanol (5.8 g/kg) or an isocaloric amount of maltose-dextrine on day 9 of pregnancy. Litters were removed on day 18 of pregnancy and examined for gross, soft-tissue, and skeletal malformations. Results showed that ethanol-exposed B6 litters had a higher percentage of digit (19%), kidney (24%), and skeletal (32%, mostly vertebral) malformations than their maltose-exposed controls (7% or below). Prenatal exposure to ethanol increased skeletal (68%, both rib and vertebral) malformations for A litters when compared to their maltose-exposed controls (4%), but did not increase digit or kidney malformations. Ethanol-exposed D2 litters did not differ from maltose-exposed controls. Maternal blood ethanol levels did not differ among the B6, D2, and A strains. These results provide additional evidence suggesting a genetic component to ethanol teratogenesis.

New murine polymorphisms detected by random amplified polymorphic DNA (RAPD) PCR and mapped by use of recombinant inbred strains

Oligonucleotide primers of random sequence that were 12 bases in length, 58% in GC content, and lacking internal palindromes were designed. By random amplified polymorphic DNA (RAPD) PCR, these primers were used to survey for DNA variations between the progenitors of the mouse AXB and BXA recombinant inbred sets (A/J and C57BL/6J). We identified 17 DNA variants detected by 10 primers. Map positions for these variants were determined by comparing their strain distribution patterns in the AXB, BXA recombinant inbred sets with strain distribution patterns of previously published loci. When necessary, BXD and NXSM recombinant inbred sets were also used. These 17 new loci mapped to 12 chromosomes. The 10 primers were also used to survey 20 inbred mouse strains including the progenitors of other recombinant inbred sets and four mouse strains recently inbred from the wild (CAST/Ei, MOLF/Ei, PERA/Ei, and SPRET/Ei).

Glycogen synthase: a putative locus for diet-induced hyperglycemia

Inbred mouse strains fed a diabetogenic diet have different propensities to develop features analogous to type 2 diabetes mellitus. To define chromosomal locations that control these characteristics, recombinant inbred strains from diabetes-prone C57BL/6J (B/6J) and diabetes-resistant A/J strains were studied. Insulin levels and hyperglycemia correlated with two different regions of mouse chromosome 7 (two point LOD scores > 3.0). For insulin levels, 15 of 16 recombinant inbred strains were concordant with a region that contains the tubby mutation that results in hyperinsulinemia. For hyperglycemia, 19 of 23 strains were concordant with the D7Mit25 marker and 20 of 23 strains with the Gpi-1 locus on proximal mouse chromosome 7. Using more stringent criteria for hyperglycemia, 10 of 11 strains characterized as A/J or B/6J like were concordant with D7Mit25. This putative susceptibility locus is consistent with that of the glycogen synthase gene (Gys) recently suggested as a candidate locus by analyses of type 2 diabetes patients. Fractional glycogen synthase activity in isolated muscle was significantly lower in normal B/6J diabetic-prone mice compared with normal diabetic-resistant A/J mice, a finding similar to that reported in relatives of human patients with type 2 diabetes. These data, taken together, raise the possibility that defects in the Gys gene may in part be responsible for the propensity to develop type 2 diabetes.

"RFLP subtraction": a method for making libraries of polymorphic markers

We developed a method, "RFLP subtraction," that isolates large numbers of unique sequence restriction fragment length polymorphisms (RFLPs) in a single experiment. The technique purifies small restriction fragments from one genome containing sequences that reside on large fragments in a related genome. We first isolate samples containing the small restriction fragments from two polymorphic strains. Subtractive hybridization then removes the fragments that are present in both samples. The remaining sequences are RFLPs: they occur on small fragments in one strain but not in the other. Here we use RFLP subtraction to make a library of hundreds of unique sequence RFLPs from two inbred mouse strains. We analyze and map a subset of the RFLPs and show that the genetic linkage of these markers can be rapidly determined by an efficient dot blot mapping technique. Several other potential applications of RFLP subtraction, including isolating region specific markers, are discussed.

Typing recombinant inbred mouse strains for microsatellite markers

In total, 41 different microsatellite variants have been typed in one or more of four different sets of recombinant inbred (RI) mouse strains. Microsatellite variants were selected that were located in chromosomal regions previously lacking markers. These markers extend the regions swept in these RI strains.