Role of TAPP1 and TAPP2 adaptor binding to PtdIns(3,4)P-2 in regulating insulin sensitivity defined by knock-in analysis

Insulin sensitivity is critically dependent on the activity of PI3K (phosphoinositide 3-kinase) and generation of the PtdIns(3,4,5)P-3 second messenger. PtdIns(3,4,5)P-3 can be broken down to PtdIns(3,4)P-2 through the action of the SHIPs (Src-homology-2-domain-containing inositol phosphatases). As PtdIns(3,4)P-2 levels peak after those of PtdIns(3,4,5)P-3, it has been proposed that PtdIns(3,4)P-2 controls a negative-feedback loop that down-regulates the insulin and PI3K network. Previously, we identified two related adaptor proteins termed TAPP [tandem PH (pleckstrin homology)-domain-containing protein] 1 and TAPP2 that specifically bind to PtdIns(3,4)P-2 through their C-terminal PH domain. To determine whether TAPP1 and TAPP2 play a role in regulating insulin sensitivity, we generated knock-in mice that express normal endogenous levels of mutant TAPP1 and TAPP2 that are incapable of binding PtdIns(3,4)P-2. These homozygous TAPP1(R211L/R211L)TAPP2(R218L)/R-218L double knock-in mice are viable and exhibit significantly enhanced activation of Akt, a key downstream mediator of insulin signalling. Consistent with increased PI3K and Akt activity, the double knock-in mice display enhanced whole body insulin sensitivity and disposal of glucose uptake into muscle tissues. We also generated wild-type and double TAPP1(R211L/R211L)TAPP2(R218L/R218L) knock-in embryonic fibroblasts and found that insulin triggered enhanced production of PtdIns(3,4,5)P-3 and Akt activity in the double knock-in fibroblasts. These observations provide the first genetic evidence to support the notion that binding of TAPP1 and TAPP2 adaptors to PtdIns(3,4)P-2 function as negative regulators of the insulin and PI3K signalling pathways.