mTOR-driven brain vascular dysfunction in Alzheimer's disease
Short-term mTOR attenuation reverses neurovascular coupling (NVC) impairments in symptomatic hAPP(J20) mice modeling Alzheimer's disease.
(A-C) Fold change in cerebral blood flow are means ±SEM of n=4-5/group during whisker pad stimulation (30 sec, bold black line), stimulations conducted sequentially in the presence of (A) aCSF (vehicle), (B) 200 nM L-NPA to inhibit nNOS specifically and (C) 200 nM L-NPA+ 10 μM L-NAME to inhibit all NOS, thus defining the contribution of the non-nNOS-dependent, L-NAME sensitive eNOS.
(D) NVC responses during 30-s whisker stimulations. Area under the curve was calculated as an increase relative to baseline (i.e. only upward peaks). Baseline NVC impairments in hAPP(J20) mice relative to WT (q(33)=12.81, ****p<0.0001) are reversed by 2 months of rapamycin (q(33)=26.01, ****p<0.0001), with enhancement of NVC in rapamycin-treated hAPP(J20) mice as compared to WT (q(33)=13.93, ****p<0.0001). L-NPA superfusion significantly reduces NVC in WT mice [q(33)=5.25, *p=0.02], but not in Tg hAPP(J20) mice (q(33)=0.22, p>0.99), suggesting a preexisting nNOS deficit in this group that is restored by rapamycin treatment in the hAPP(J20)+Rapa group, indicated by a significant inhibition of NVC in the presence of 200 nM L-NPA (q(33)=12.28 vs baseline, ****p<0.0001). The remaining NVC response in the presence of L-NPA+L-NAME (inhibiting all remaining NOS activity, i.e. eNOS) was not significantly different vs L-NPA alone in WT (q(33)=1.05, p=0.99), hAPP(J20) (q(33)=0.09, p>0.99), nor in hAPP(J20)+Rapa (q(33)=3.60, p=0.24).
(E) Contributions of nNOS, eNOS and non-NOS components to total NVC. Deficits in nNOS-dependent NVC in hAPP(J20) mice (q(33)=4.47, **p=0.009 vs WT) are negated by mTOR inhibition in rapamycin-treatedhAPP(J20) mice (q(33)=11.31, ****p<0.0001 vs hAPP) and nNOS-dependent NVC is enhanced over WT (q(33)=7.10, ++++p<0.0001). Additionally, rapamycin enhances L-NAME sensitive activities (eNOS) in hAPP(J20) mice relative to vehicle-treated hAPP(J20) mice (q(33)=3.93, *p=0.02). Deficits in the remaining non-NO mediated NVC response in hAPP(J20) mice (q(33)=6.06, ***p=0.0004 vs WT) are negated in rapamycin-treated hAPP(J20) mice(q(33)=7.87, ****p<0.0001 vs vehicle-treated hAPP).
(F) Hippocampal-dependent contextual memory impairment in 12-month-old hAPP(J20) mice (hAPP(J20) vs WT, q(16)=4.19, *p=0.024) is negated by 2 months of rapamycin treatment (q(16)=3.93, *p=0.034, hAPP(J20) vs hAPP(J20)+Rapa).
(G) Baseline NVC response is correlated with contextual memory performance (r=0.668, p=0.009). Behavioral studies used n=6-7 mice per group. All post hoc analyses are Tukey’s multiple comparison tests. Unless otherwise indicated, asterisks (*) in the figure represent a significant difference relative to hAPP+Veh and pluses (+) represent a significant difference relative to WT+Veh.
mTOR dependent regulation of nitric oxide synthase and cerebrovascular outcomes. mTOR inhibits neurovascular coupling through the regulation of nNOS, eNOS, and non-NO dependent components of neurovascular coupling at the neurovascular unit. Prior research in our lab showed that mTOR drives vascular reactivity deficits and reduced baseline cerebral blood flow through inhibition of eNOS activity (Van Skike and Galvan, 2018) in models of AD (Lin et al., 2013; Lin et al., 2017; Van Skike et al., 2018), in models of vascular cognitive impairment (Jahrling et al., 2018; Van Skike et al., 2018), and in normative aging (Van Skike et al., 2020). Figure created in part with BioRender.com.