Fasting and systemic insulin signaling regulate phosphorylation of brain proteins that modulate cell morphology and link to neurological disorders

Diabetes is strongly associated with cognitive decline, but the molecular reasons are unknown. We found that fasting and peripheral insulin promote phosphorylation and dephosphorylation, respectively, of specific residues on brain proteins that included cytoskeletal regulators such as slit-robo GTPase-activating protein 3 (srGAP3) and microtubule affinity-regulating protein kinases (MARKs), whose deficiency or dysregulation are linked to neurological disorders. Fasting activates protein kinase A (PKA) but not PKB/Akt signaling in the brain, and PKA can phosphorylate the purified srGAP3. The phosphorylation of srGAP3 and MARKs were increased when PKA signaling was activated in primary neurons. Knockdown of PKA decreased phosphorylation of srGAP3. Furthermore, WAVE1, an A-kinase anchoring protein (AKAP), can form a complex with srGAP3 and PKA in the brain of fasted mice to facilitate the phosphorylation of srGAP3 by PKA. Although brain cells have insulin receptors, our findings are inconsistent with the down-regulation of phosphorylation of target proteins being mediated by insulin signaling within the brain. Rather, our findings infer that systemic insulin through a yet unknown mechanism inhibits PKA or protein kinase(s) with similar specificity and/or activates an unknown phosphatase in the brain. Ser858 of srGAP3 was identified as a key regulatory residue, whose phosphorylation by PKA enhanced the GAP activity of srGAP3 towards its substrate Rac1 in cells, thereby inhibiting the action of this GTPase in cytoskeletal regulation. Our findings reveal novel mechanisms linking peripheral insulin sensitivity with cytoskeletal remodelling in neurons, which may help to explain the association of diabetes with neurological disorders such as Alzheimer's disease (AD).