We are currently characterizing the abnormalities for regulation of human muscle glycogen synthesis in insulin-resistant subjects. In insulin- resistant subjects, fasting glycogen synthase phosphatase (PP-1) activities are reduced and fail to show the peak insulin stimulation observed for insulin-sensitive subjects at 10-20 minutes. Phosphorylation of the G-subunit bound to the PP-1 catalytic subunit in the GM fraction regulates PP-1 activity. Western blots of muscle extracts indicate that the G-subunit has increased immunoreactivity in insulin resistant subjects. Glycogen synthase kinase-3 (GSK3A) and inhibitor-2 (IPP2) have been studied as candidate genes because the kinase regulates IPP2 which can inactivate the phosphatase responsible for activation of glycogen synthase and insulin mediated glucose storage. GSK3A has been localized to 19p13 and partially characterized for intron/exon boundries and structural identity with rat GSK3A. Two dinucleotide microsatellite markers from this loci have been sequenced and used to type DNA from 900 Pima Indians. Marker data will be used to determine possible linkage or association of this loci with insulin resistance and obesity. IPP2 has been localized to 3q29. A psuedogene lacking intron/exon boundries has been identified on chromosome 5. Intron/exon boundries have been identified for a total of 6 exons. Sequence analysis on 17 insulin resistant and 4 insulin sensitive subjects indicates no abnormalities in exons 2, 3 and 4.
