Dual roles for glucokinase in glucose homeostasis as determined by liver and pancreatic beta cell-specific gene knock-outs using Cre recombinase

Glucokinase (GR) gene mutations cause diabetes mellitus in both humans and mouse models, but the pathophysiological basis is only partially defined. W e have used cre-loxP technology in combination with gene targeting to perfo rm global, beta cell-, and hepatocyte-specific gene knock-outs of this enzy me in mice. Gene targeting was used to create a triple-loxed gk allele, whi ch was converted by partial or total Cre-mediated recombination to a condit ional allele lacking neomycin resistance, or to a null allele, respectively . beta cell- and hepatocyte-specific expression of Cre was achieved using t ransgenes that contain either insulin or albumin promoter/enhancer sequence s. By intercrossing the transgenic mice that express Cre in a cell-specific manner with mice containing a conditional gk allele, we obtained animals w ith either a beta cell or hepatocyte-specific knock-out of GK, Animals eith er globally deficient in GK, or lacking GK just in beta cells, die within a few days of birth from severe diabetes. Mice that are heterozygous null fo r GK, either globally or just in the beta cell, survive but are moderately hyperglycemic, Mice that lack GR only in the liver are only mildly hypergly cemic but display pronounced defects in both glycogen synthesis and glucose turnover rates during a hyperglycemic clamp. Interestingly, hepatic GK kno ck-out mice also have impaired insulin secretion in response to glucose. Th ese studies indicate that deficiencies in both beta cell and hepatic GK con tribute to the hyperglycemia of MODY-2.