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

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–GL) with glycogen phosphorylase a is believed to inhibit the dephosphorylation and activation of glycogen synthase (GS) by the PP1–GL complex, suppressing glycogen synthesis. Consequently, the interaction of GL 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 GL–phosphorylase a interaction, using a mouse model containing a Tyr284Phe substitution in the phosphorylase a-binding region of the GL protein. The resulting GLY284F/Y284F mice display hepatic PP1–GL activity that is no longer sensitive to allosteric inhibition by phosphorylase a, resulting in increased GS activity under glycogenolytic conditions, demonstrating that regulation of GL by phosphorylase a operates in vivo. GLY284F/Y284F and GLY284F/+ mice display improved glucose tolerance compared with GL+/+ littermates, without significant accumulation of hepatic glycogen. The data provide the first in vivo evidence in support of targeting the GL–phosphorylase a interaction for treatment of hyperglycaemia. During prolonged fasting the GLY284F/Y284F mice lose more body weight and display decreased blood glucose levels in comparison with their GL+/+ littermates. These results suggest that, during periods of food deprivation, the phosphorylase a regulation of GL 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–GL by phosphorylase a in mammals.