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ABSTRACTderived from the 129/sv strain (Clontech) was screened using a mouse D3 cDNA probe (17). A positive clone encompassing exon 2 of the murine D3 gene was isolated and further characterized. A 7-kb Xho I-Asp718 fragment was engineered for targeted mutagenesis by introducing the GKNeo cassette (16) in antisense orientation at the Sal I site in exon 2 (17). Integration of sequences derived from the pGKNeo cassette generates a novel open reading frame, resulting in the following peptide sequence appended after Arg-148: PASDGIRT-WQNNTENEVYVEQRLLISFFRL Opal (Stop). The sequence of the mutant allele was confirmed by direct sequencing of reverse transcription-PCR (rPCR) products derived from brain mRNAs of -/-and +/-mice (data not shown).Transfection ofES Cells and Embryo Manipulations. J-1 ES cells (a kind gift of R. Jaenisch, Massachussetts Institute of Technology) at passage 13 were grown on mitomycin C-treated embryonic fibroblasts derived from a homozygous neomycin (Neo)-resistant transgenic mouse (16). Cells (2 x 107) were electroporated in a 1-ml cuvette (path length-0.2 cm) at 0.4 kV and 25 ,uF. Cells were plated onto 40 gelatin-coated Petri dishes (6 cm) on embryonic feeder cells. Selection with G418 (0.3 mg/ml; active concentration of 0.66 ,vg/mg of dry powder; GIBCO) was applied 24 hr after plating and was continued for 7-9 days. Individual Neo-resistant colonies were picked using a dissection microscope and expanded as described (16). Genomic DNA was prepared from an aliquot of cells for each clone using previously described techniques and analyzed by Southern blotting (18). Recovery, microinjection, and transfer of 3.5 day postcoitus embryos was performed as described (16).

We have investigated the metabolic actions of recombinant human IGF-1 in mice genetically deficient of insulin receptors ( IR ϪրϪ ). After intraperitoneal administration, IGF-1 caused a prompt and sustained decrease of plasma glucose levels in IR ϪրϪ mice. Plasma free fatty acid concentrations were unaffected. Interestingly, the effects of IGF-1 were identical in normal mice ( IR ϩրϩ ) and in IR ϪրϪ mice. Despite decreased glucose levels, IR ϪրϪ mice treated with IGF-1 died within 2 Ϫ 3 d of birth, like sham-treated IR ϪրϪ controls. In skeletal muscle, IGF-1 treatment caused phosphorylation of IGF-1 receptors and increased the levels of the phosphatidylinositol-3-kinase p85 subunit detected in antiphosphotyrosine immunoprecipitates, consistent with the possibility that IGF-1 stimulates glucose uptake in a phosphatidylinositol-3-kinase-dependent manner. IGF-1 receptor phosphorylation and coimmunoprecipitation of phosphatidylinositol-3-kinase by antiphosphotyrosine antibodies was also observed in liver, and was associated with a decrease in mRNA levels of the key gluconeogenetic enzyme phosphoenolpyruvate carboxykinase. Thus, the effect of IGF-1 on plasma glucose levels may be accounted for by increased peripheral glucose use and by inhibition of hepatic gluconeogenesis. These data indicate that IGF-1 can mimic insulin's effects on glucose metabolism by acting through its own receptor. The failure of IGF-1 to rescue the lethal phenotype due to lack of insulin receptors suggests that IGF-1 receptors cannot effectively mediate all the metabolic actions of insulin receptors. ( J. Clin. Invest. 1997. 99:2538-2544.)

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