Fasting Reduces Elevated Hippocampal pAkt-S473 and pGSK3 β -S9 but not ptau-S396 in Male Obese Zucker Rats (OZRs) With No Effect in Lean Zucker Rats (LZRs) or Female Zucker Rats

Altered phosphorylation in the PI3K/Akt/GSK3β insulin signaling pathway in the hippocampus contributes to increased phosphorylated tau (ptau) and impaired learning and memory in obese and diabetic rodents, similar to Alzheimer's disease in humans. Some studies report reduced pGSK3β at the inhibitory Ser9 residue (pGSK3β-S9) contributes to elevated ptau-S396 in male obese and diabetic rodents, but other studies report elevated pGSK3β-S9 in male obese and diabetic rodents. We hypothesized that pGSK3β-S9 levels may reflect the feeding state at time of euthanasia rather than the disease stage in diabetic male OZRs. Because obese females are resistant to diabetes and understudied, we also determined whether insulin resistance and overnight fasting alter pGSK3β-S9 in female OZRs. In 18-week-old males, plasma insulin (ng/mL) was higher in fed and fasted OZRs vs. LZRs, and fasting reduced insulin only in OZRs (fed: 12.4 ± 1.1 vs. 0.87 ± 0.09; fasted: 5.4 ± 0.9 vs. 0.76 ± 0.06; fed vs. fasted OZRs: 12.4 ± 1.1 vs. 5.4 ± 0.9, p <0.01 for all, 9-10/group). Blood glucose (mg/dL) was higher in fed and fasted male OZRs vs. LZRs, and fasting reduced glucose only in LZRs (fed: 157.6 ± 5.7 vs. 124.9 ± 2.3; fasted: 149.5 ± 6.6 vs. 96.9 ± 2.2; fed vs. fasted LZRs: 124.9 ± 2.3 vs. 96.9 ± 2.2, p < 0.01 for all, 5/group). Western blotting of hippocampus was performed to measure phosphorylated protein/total protein expression. The expression of pAkt-S473/Akt was higher in fed male OZRs vs. LZRs but comparable in fasted OZRs vs. LZRs, because fasting reduced pAkt-S473/Akt only in the OZRs (fed OZRs vs. LZRs and fed vs. fasted OZRs, p <0.01; 6/group). Likewise, pGSK3β-S9/GSK3β was higher in fed male OZRs vs. LZRs but comparable in fasted OZRs vs. LZRs because fasting reduced pGSK3β-S9/GSK3β in OZRs (fed OZRs vs. LZRs: p < 0.01; fed vs. fasted OZRs: p < 0.05; 6/group). In contrast, expression of ptau-S396/tau was higher in fed and fasted male OZRs vs. LZRs with no changes after fasting (fed OZRs vs. LZRs and fasted OZRs vs. LZRs, p < 0.01; 5-7/group). In 30-week-old females, plasma insulin was elevated in fed and fasted OZRs vs. LZRs, and fasting caused no changes in insulin (fed: 3.4 ± 0.6 vs. 0.71 ± 0.04; fasted: 3.4 ± 1.0 vs. 0.56 ± 0.01, p < 0.01 for both; 6-8/group). Blood glucose was in the normal range in fasted and fed female OZRs but slightly higher than LZRs. Fasting reduced glucose only in LZRs (fed: 110.2 ± 1.6 vs. 100.2 ± 1.6, p < 0.05; fasted: 110.2 ± 2.6 vs. 91.9 ± 1.7, p < 0.01; fed vs. fasted LZRs: 100.2 ± 1.62 vs. 91.9 ± 1.7, p < 0.05; 6/group). In contrast to the males, in females no differences were observed between OZRs vs. LZRs in the fed or fasted state for the expression of pAkt-S473/Akt, pGSK3β-S9/GSK3β, or ptau-S396/tau (5-7/group). Together, these data suggest hyperglycemic 18-week-old male OZRs have elevated pAkt-S473 and pGSK3β-S9, and overnight fasting temporarily reduces phosphorylation in this insulin signaling pathway while leaving ptau-S396 elevated. The 30-week-old insulin-resistant female OZRs with normal glucose show no differences in pAkt-S473, pGSK3β-S9, or pTau-S396 compared to LZRs and no changes with fasting. Mechanisms for transient effects of overnight fasting remain to be determined, but these data highlight the importance of considering the feeding status as well as the disease state when measuring changes in phosphorylation in the PI3K/Akt/GSK3β insulin signaling pathway for increased ptau-S396. Funded by R01HL132568-S1, JES Edwards Foundation, Summerfield G. Roberts Foundation. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.