TI - Discussion . AB - Considerable evidence implicates insulin resistance in the pathogenesis of AD and underlies current efforts to treat AD by improving insulin sensitivity . However , we find that disrupting Irs2 in Tg2576 mice results in improvement of both Abeta plaque burden and behaviour despite an exacerbation of tau PHOSphorylation and the presence of insulin resistance . Previously , over-expression of APP in mice was shown to induce transthyretin expression together with evidence of increased IIS [23] . It was suggested that these are protective mechanisms resulting in the absence of the amyloid cascade in mice . Subsequently , and supportive of this hypothesis , transthyretin protein was shown to prevent Abeta toxicity in vitro[24,25] and in mice over-expressing APP , neutralisation with antibody or deletion of the TTR gene both enhance pathology [26,27] . In addition to these roles , transthyretin has been shown recently to be , like IDE , an Abeta protease [19] and transthyretin protein levels in CSF are decreased in AD [28,29] . Our data demonstrates a substantial increase in transthyretin expression accompanying the similarly substantial amelioration of plaque pathology , Abeta fibrillisation and behavioural deficits ; in line with a protective effect of transthyretin . In marked contrast to the potentially beneficial reduction in Abeta pathology , we found increased tau PHOSphorylation in Tg2276 mice with disruption of Irs2 . However we found no evidence of tangle formation , consistent with previous observations [9] . The increase in tau PHOSphorylation was observed at many but not all epitopes examined . The most pronounced changes were at epitopes positioned at the 396/404 , and 235 and 231 sites , sites known to be PHOSphorylated in AD [30] . However , the AT8/TAU1 epitope covering Ser199/Ser202/Thr205 , a key GSK-3 site which is also highly PHOSphorylated in AD brain , was unaffected in the Tg2576/Irs2-/- animals . When we examined tau kinase activity in Irs2-/- mice we found no increase in the activities of GSK-3 , like others previously [9] , or in the GSK3 and CDK5 SUBstrate CRMP-2 ( data not shown ) . These data are consistent with the pattern of tau phosphorylaTION changes we observed , in particular the absence of an increase in PHOSphorylation at the key GSK-3 sites--Ser199/Ser202/Thr205 . However we did find a highly significant decrease in the tau-phosphatase PP2A in Irs2-/- mice , suggesting that the mechanism of effect might be mediated not by an increase in kinase activity but by a decrease in the activity of this phosphatase . The relative role of amyloid versus tau pathologies in influencing neuronal dysfunction and cognitive impairment has been of considerable interest and indeed controversy . The generation of a mouse model with both decreased Abeta aggregation and deposition but increased tau PHOSphorylation permitted us to directly address this question . Using a standard paradigm of hippocampal dependent learning , contextual fear conditioning , we observed a complete reversal of behavioural deficits in the context of Irs2 deletion . Interestingly it has been reported that the Abeta induced impairment in LTP , known to be present in the Tg2576 animals [22] , is reversed by insulin [31] . The mechanism whereby insulin might restore LTP is not known but one promising candidate is GSK-3 as we and others have recently demonstrated that GSK-3 inhibition is essential for LTP [32,33] . In the Tg2576/Irs2-/- animals the reduction in Abeta and the relative inhibition of GSK-3 might both , together or separately , contribute to the reversal of the behavioural phenotype . Although much of the current literature suggests that insulin resistance is an aetiological factor in AD , we have recently demonstrated that mice lacking Irs1 have increased lifespan and reduced age-related pathology [34] and deletion of Irs2 in the mouse brain increases longevity [35] . In C.elegans , abrogating IIS protects against a range of proteotoxic neuropathologies , including Abeta toxicity [13,36] . Our new findings demonstrate that this is also the case for mammals with specific disruption of Irs2 and suggests that for therapeutic manipulation of this pathway to be beneficial in the treatment of AD an increased understanding of the complex signalling and gene expression mechanisms downstream of IIS will be required .