The central melanocortin system is critical for the long term regulation of energy homeostasis. Null mutations of the melanocortin-4 receptor (MC4-R) are associated with hyperphagia, obesity, and accelerated longitudinal growth in mice and humans. However, little is known about the function of another central melanocortin receptor, the MC3-R. To assess the role of the MC3-R in energy homeostasis, the majority of the mc3r coding sequence was deleted from the mouse genome. In contrast to the MC4-R knockout, which exhibits increased food intake, increased somatic growth, and defects in metabolism, mc3r-/- mice exhibit an exclusively metabolic syndrome. Homozygous null mc3r mice, while not significantly overweight, exhibit an approximately 50% to 60% increase in adipose mass. Mc3r-/- mice also exhibit an unusual increase in respiratory quotient when transferred onto high fat chow, suggesting a reduced ratio of fat/carbohydrate oxidation. Furthermore, male mc3r-/- mice also exhibit an approximately 50% reduction in locomotory behavior on the running wheel, suggesting reduced energy expenditure.
Hypocalcemic vitamin D-resistant rickets is a human genetic disease resulting from target organ resistance to the action of 1,25-dihydroxyvitamin D3. Two families with affected children homozygous for this autosomal recessive disorder were studied for abnormalities in the intracellular vitamin D receptor (VDR) and its gene. Although the receptor displays normal binding of 1,25-dihydroxyvitamin D3 hormone, VDR from affected family members has a decreased affinity for DNA. Genomic DNA isolated from these families was subjected to oligonucleotide-primed DNA amplification, and each of the nine exons encoding the receptor protein was sequenced for a genetic mutation. In each family, a different single nucleotide mutation was found in the DNA binding domain of the protein; one family near the tip of the first zinc finger (Gly----Asp) and one at the tip of the second zinc finger (Arg----Gly). The mutant residues were created in vitro by oligonucleotide directed point mutagenesis of wild-type VDR complementary DNA and this cDNA was transfected into COS-1 cells. The produced protein is biochemically indistinguishable from the receptor isolated from patients.
We developed standards for creatine kinase (CK; EC 2.7.3.2) assays by expressing human CK cDNAs in COS cells. Cells were transiently transfected with full-length cDNAs for CK subunits M and B, individually and in combination; and subsequently, high concentrations of CK activity were present in the cell lysate (1.2 U/mg protein). These proteins exhibited the characteristic isoenzyme-specific electrophoretic mobilities for CK MM and BB isoenzymes. We also produced subforms of CK MM and MB, identical to the modified CK variants produced in plasma after muscle or myocardial injury, by mutating the cDNA for the CK M subunit to delete the carboxy-terminal lysine residue. When this construct was cotransfected with the normal cDNAs for CK M and B, five electrophoretically distinct CK isoenzymes were detected by nondenaturing electrophoresis: MM3, MM2, MM1, MB2, and MB1. These proteins retained 100% of their activity after storage of the cell lysates -20 or 4 degrees C for 3 months.
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