Feeding Status Transiently Alters the Phosphorylation of Enzymes in the PI3K/Akt/GSK3β Pathway but not Phosphorylated Tau in the Cortex of Male Obese Zucker Rats
Authors: Das-Earl Paromita , Schreihofer Derek , Schreihofer Ann 
Published: May 2024

Metabolic syndrome (MetS) fosters pathological brain markers such as excess phosphorylated tau (ptau) to produce neuronal dysfunction. Activation of the PI3K/Akt/GSK3β pathway by increasing pAkt-Ser473 and pGSK3β-Tyr216 promotes phosphorylation of tau at residues such as Ser396. Insulin also increases pGSK3β-Ser9 to limit phosphorylation of tau. Deficient insulin signaling has been proposed to reduce pGSK3β-Ser9 and promote pathological excess ptau-Ser396. However, studies of these pathways in rodent models of MetS report both elevated and reduced expression of pAkt-Ser473 and pGSK3β-Ser9, raising the question of the predictive value and role of altered pGSK3β-Ser9 expression in producing excess ptau. Studies reporting reduced pGSK3β-Ser9 in insulin resistant and diabetic rodents often used fasted animals for tissue collection, but other studies without reported fasting showed increased pGSK3β-Ser9. However, elevated ptau was consistently observed under both conditions. Because short-term changes in insulin and glucose alter pAkt-Ser473 and pGSK3β-Ser9 expression, we hypothesized that phosphorylation was related to the feeding status at time of tissue collection rather than the chronic disease state, and excess ptau was resistant to these short-term changes. We examined whether fasting altered pAkt-Ser473, pGSK3β-Tyr216, and pGSK3β-Ser9, and ptau-Ser396 levels in 18-week-old male obese (OZR) and lean (LZR) Zucker rats. After measurement of blood glucose by tail sample in their home cage, fasted (16 hours) and non-fasted rats were euthanized for tissue collection. Though baseline glucose was higher in OZRs vs. LZRs in the fed state (202.4±21.9 vs. 115.9±2.7 mg/dl, t-test) and fasted state (169.4±5.8 vs. 95.3±3.8 mg/dl, t-test), fasting significantly reduced glucose in OZRs but not LZRs (2-way ANOVA with Tukey’s post hoc, P=0.023 for OZRs and P=0.143 for LZRs, n=5-8/group). Protein expression in the cerebral cortex was measured by Western blotting and normalized to actin (n=3-5 for all groups). Expression of ptau-Ser396/total tau was significantly elevated in OZRs vs. LZRs in both feeding conditions (p<0.001) and was not altered by fasting in either phenotype. Expression of pAkt-Ser476 was elevated in non-fasted OZRs vs. LZRs (p<0.001). Fasting greatly reduced pAkt-Ser476 in OZRs (p<0.001) but not LZRs (P=0.741), making expression in fasted OZRs and LZRs comparable (P=0.674). Expression of pGSK3β-Ser9 was significantly elevated in fed OZRs vs. LZRs (p<0.001). Fasting reduced expression of pGSK3β-Ser9 only in OZRs (p<0.001 in OZRs, P=0.952 in LZRs), but expression of was still elevated in fasted OZRs vs. LZRs (P=0.009). Expression of pGSK3β-Tyr216 was significantly elevated in fed OZRs vs. LZRs (p<0.001) but significantly lower in fasted OZRs vs. LZRs (p<0.001). These data show that feeding status does not alter the ptau-Ser396 in the cerebral cortex of OZRs vs. LZRs, but phosphorylation of the enzymes of the PI3K/Akt/GSK3β signaling pathway are greatly affected by MetS and feeding status at time of tissue collection. Thus, when OZRs have access to food, elevated pGSK3β-Tyr216 may contribute to the GSK3β-mediated rise in ptau-Ser396 despite elevated pGSK3β-Ser9. Supported by R01Hl132568S1, JES Edwards Foundation, and Summerfield G. Roberts Foundation. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.