UBC Theses and Dissertations
Investigation of molecular signaling defects in PTP alpha-deficient mice : insulin and NMDA receptor signaling Le, Hoa Thi
The physiological roles of PTPα in insulin and N M D A receptor (NMDAR) signaling were investigated using gene-targeted mice deficient in PTPα. PTPα [sup -/-] animals had normal body weights and circulating levels of glucose and insulin. In glucose and insulin tolerance tests, their efficiency o f blood glucose clearance was comparable to wild-type mice. Kinetics and extents o f insulin-stimulated insulin receptor and IRS-1 tyrosine phosphorylation were similar in wild-type and PTPα [sup -/-] liver, muscle, and adipose tissue. However the association of IRS-1 and PI3-K was altered in PTPα [sup -/-] liver, with increased insulin-independent and reduced insulin-stimulated association compared to wild-type samples. This did not affect activation of the downstream signaling effector Akt. Thus, PTPα is not a negative regulator of insulin signaling and does not perform an essential role in mediating the physiological action of insulin. PTPα [sup -/-] mice exhibit defects in NMDAR-associated processes. In vivo molecular effectors linking PTPα and the NMDAR were investigated in wild-type and PTPα [sup -/-] mice. Tyrosine phosphorylation of the NMDAR NR2A and NR2B subunits was reduced upon PTPα 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 o f src, fyn, yes, and lck, but not lyn, was detected in the absence of PTPα. In addition, autophosphorylation of Pyk2, a tyrosine kinase linked to NMDAR signaling, was also reduced in PTPα [sup -/-] samples. In an HEK293 cell expression system, PTPα actions on NR2A/B phosphorylation mediated by sre and fyn were examined. The expression of PTPα enhanced fyn- but reduced sre-mediated NR2A/B phosphorylation. This is partly due to the fact that PTPa complexes with src and fyn with strikingly distinct affinities and via different mechanisms of binding. Interestingly, PTPα Tyr789 phosphorylation was found to regulate PTPα action on fyn- and src-mediated NR2A/B phosphorylation. These studies eliminate PTPα 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 PTPα in NMDAR signaling and function, and shed light on the involvement of specific SFKs and their potentially distinct targeting by PTPα in this process.
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