BILIVERDIN REDUCTASE-A MEDIATES THE BENEFICIAL EFFECTS OF INTRANASAL INSULIN ADMINISTRATION ON AD PATHOLOGY IN THE BRAIN OF 3XTG-AD MICE

Abstract

Biliverdin reductase-A (BVR-A) is a novel direct target of the insulin receptor, which phosphorylate BVR-A activating its Ser/Thr/Tyr kinase activity. Through this activity, BVR-A negatively regulates IRS1 activation, thus allowing the correct transduction of the insulin-mediated signaling. Along the progression of AD pathology we identified two phases in which: (1) the early impairment of BVR-A is responsible for the hyper-activation of IRS1, which then (2) causes the stimulation of feedback mechanisms including mTOR, aimed to turn-off IRS1 hyper-activity (Fig.1), thus promoting brain insulin resistance (b.i.r.). Reduced BVR-A activity is therefore an early event triggering the onset b.i.r̤ Intranasal insulin (I-Ins) administration is under evaluation as therapeutic strategy to alleviate b.i.r. in AD. However, the exact molecular mechanisms underlying I-Ins beneficial effects are still unclear. The goal of our project was to clarify whether the I-Ins-associated beneficial effects were mediated by the restoration of BVR-A activity. Changes of (1) the insulin signaling machinery (IR/IRS1/ ERK1/2/AKT/mTOR levels and activation) (2) total OS markers (PC, HNE, 3-NT) and (3) Aβ and tau levels, were evaluated in the hippocampus and cortex of 3xTg-AD and WT mice undergoing an early (4 months) or late (10 moths) I-Ins treatment (1 U/day, 3 times per week, for 2 months) (Fig.2). The morris water maze (MWM) and the novel object recognition (NOR) tasks were used to test cognitive functions. Cell-based experiments to support in vivo data were performed in HEK-APPSwe cells. I-Ins administration rescues the activation of BVR-A both in young and old 3xTg-AD mice. Improved BVR-A activity is associated with (1) a restoration of the insulin signaling cascade, (2) reduced OS markers and (3) a reduction of Tau pathology. All these changes parallel an improved cognition (Fig.3). Cell-based experiments confirmed the central role of BVR-A by showing that the effects of insulin are abolished when BVR-A is knocked-down. Our data highlight that BVR-A plays a pivotal role in the regulation of the insulin signaling in the brain. Restoration of BVR-A activity first, sheds light on the molecular mechanisms underlie I-Ins-mediated beneficial effects, and then suggest the role of BVR-A as potential therapeutic target to prevent b.i.r.in AD. (A) Proposed mechanism leading to b.i.r. in AD. Under physiological condition BVR-A regulates the acitvation of IRS1, thus controlling the correct transduction of insulin signaling cascade. In 3xTg-AD mice, an early impairment of BVR-A (6 months) is responsible for the hyper-activation of IRS1, which sustains the activation of the insulin signaling cascade for a time longer than normal (6 months). This phenomenon is aggravated by the further rise of the oxidative/nitrosative stress levels (PC, HNE and 3-NT), which results in the oxidative stress-induced impairment of BVR-A, which finally contributes to maintain IRS1 hyperactive. Persistence of IRS1 hyper-activation represents a signal for the activation of feedback mechanisms including mTOR, which at the end is responsible for the inhibitory phospshorylation of IRS1 and thus for the onset of brain insulin resistance (b.i.r.) (12 months). (B) Temporal profile of the events promoting BIR in the hippocampus 3×Tg-AD mice. Arrows, promotion; dotted lines, inhibition; Y, phosphor-Tyr residues; S, phospho-Ser residues; Y-NO2, 3-NT modifications. Scheme of the treatment used in this project. Intranasal insulin (I-Ins) administration prevents the impairment of BVR-A and the hyper-activation of IRS1. Normalization of the activity of the BVR-A/IRS1 axis is associated the correct transduction of the insulin signaling cascade, which finally prevents the cognitive decline observed in the 3xTg-AD mice.