Disruption of the allosteric phosphorylase a regulation of the hepatic glycogen-targeted protein phosphatase 1 improves glucose tolerance in vivo.

Abstract Type 2 diabetes is characterised by elevated blood glucose concentrations, which potentially could be normalised by stimulation of hepatic glycogen synthesis. Under glycogenolytic conditions, the interaction of hepatic glycogen-associated protein phosphatase-1 (PP1–G L ) with glycogen phosphorylase a is believed to inhibit the dephosphorylation and activation of glycogen synthase (GS) by the PP1–G L complex, suppressing glycogen synthesis. Consequently, the interaction of G L with phosphorylase a has emerged as an attractive anti-diabetic target, pharmacological disruption of which could provide a novel mechanism to lower blood glucose levels by increasing hepatic glycogen synthesis. Here we report for the first time the in vivo consequences of disrupting the G L –phosphorylase a interaction, using a mouse model containing a Tyr284Phe substitution in the phosphorylase a -binding region of the G L protein. The resulting G L Y284F/Y284F mice display hepatic PP1–G L activity that is no longer sensitive to allosteric inhibition by phosphorylase a , resulting in increased GS activity under glycogenolytic conditions, demonstrating that regulation of G L by phosphorylase a operates in vivo . G L Y284F/Y284F and G L Y284F/+ mice display improved glucose tolerance compared with G L +/+ littermates, without significant accumulation of hepatic glycogen. The data provide the first in vivo evidence in support of targeting the G L –phosphorylase a interaction for treatment of hyperglycaemia. During prolonged fasting the G L Y284F/Y284F mice lose more body weight and display decreased blood glucose levels in comparison with their G L +/+ littermates. These results suggest that, during periods of food deprivation, the phosphorylase a regulation of G L may prevent futile glucose–glycogen cycling, preserving energy and thus providing a selective biological advantage that may explain the observed conservation of the allosteric regulation of PP1–G L by phosphorylase a in mammals.