Investigation of molecular signaling defects in PTP alpha-deficient mice: Insulin and NMDA receptor signaling.

摘要

The physiological roles of PTPalpha in insulin and NMDA receptor (NMDAR) signaling were investigated using gene-targeted mice deficient in PTPalpha.;PTPalpha-/- animals had normal body weights and circulating levels of glucose and insulin. In glucose and insulin tolerance tests, their efficiency of blood glucose clearance was comparable to wild-type mice. Kinetics and extents of insulin-stimulated insulin receptor and IRS-1 tyrosine phosphorylation were similar in wild-type and PTPalpha-/- liver, muscle, and adipose tissue. However the association of IRS-1 and PI3-K was altered in PTPalpha-/- liver, with increased insulin-independent and reduced insulin-stimulated association compared to wildtype samples. This did not affect activation of the downstream signaling effector Akt. Thus, PTPalpha is not a negative regulator of insulin signaling and does not perform an essential role in mediating the physiological action of insulin.;PTPalpha-/- mice exhibit defects in NMDAR-associated processes. In vivo molecular effectors linking PTPalpha and the NMDAR were investigated in wild-type and PTPalpha-/- mice. Tyrosine phosphorylation of the NMDAR NR2A and NR2B subunits was reduced upon PTPalpha ablation, indicating a positive effect of this phosphatase on NMDAR phosphorylation via intermediate molecules. The NMDAR is a substrate of src family tyrosine kinases (SFKs), and reduced activity of src, fyn, yes, and lck, but not lyn, was detected in the absence of PTPalpha. In addition, autophosphorylation of Pyk2, a tyrosine kinase linked to NMDAR signaling, was also reduced in PTPalpha-/- samples. In an HEK293 cell expression system, PTPalpha actions on NR2A/B phosphorylation mediated by src and fyn were examined. The expression of PTPalpha enhanced fyn- but reduced src-mediated NR2A/B phosphorylation. This is partly due to the fact that PTPalpha complexes with src and fyn with strikingly distinct affinities and via different mechanisms of binding. Interestingly, PTPalpha Tyr789 phosphorylation was found to regulate PTPalpha action on fyn- and src-mediated NR2A/B phosphorylation.;These studies eliminate PTPalpha as a candidate for the development of PTP-directed therapeutics for the treatment of diabetes and obesity. They demonstrate a key upstream regulatory role for PTPalpha in NMDAR signaling and function, and shed light on the involvement of specific SFKs and their potentially distinct targeting by PTPalpha in this process.