**5. Conclusions**

Our findings indicate that the phosphorylation of IRS1 at disease-specific Ser residues in the hippocampus may be a potential marker of Aβ-unrelated memory impairments induced by T2DM and aging. Alternatively, in Aβ-related memory decline in AD, the modification of IRS1 via its phosphorylation at multiple Ser sites accompanied by the activation of AMPK, a sensor of energy metabolism, may be a marker of the response to an early detection of elevated Aβ42 level before the onset of memory decline in AD.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2072-6643/11/8/1942/s1: Figure S1: No difference in memory function between wild-type (WT) and DIO mice at a young age.; Figure S2: Levels of blood insulin in the respective mouse models.; Figure S3: Evaluation of the other signaling factors associated with IRS1 signaling in the hippocampus.; Figures S4 and S5: Full images of Western blots; Table S1: Summary of phosphorylated Ser residues on hippocampal IRS1 in all models.

**Author Contributions:** W.W., D.T., Y.F., M.M., and C.K. researched data. W.W. and D.T. analyzed data. W.W., D.T., and Y.F. wrote the manuscript. T.S. and T.C.S. provided the APPKI mice and interpreted the data. A.T. designed experiments, analyzed data, and wrote and edited the manuscript. All authors read and approved the final manuscript.

**Funding:** This work was supported by JSPS KAKENHI (JP26282026, JP17K19951, JP17H02188) (A.T.), the Research Funding for Longevity Science from National Center for Geriatrics and Gerontology, Japan (A.T.), and grants from the Mitsubishi Foundation (A.T.)

**Acknowledgments:** We thank Kohei Tomita (Laboratory of Experimental Animal, National Center for Geriatrics and Gerontology) for animal care and M. Sawaura and K. Tabata for providing aged wild-type mice (Charles river, Japan).

**Conflicts of Interest:** The authors declare no conflict of interest.
