A Dual Tracer PET-MRI Protocol for the Quantitative Measure of Regional Brain Energy Substrates Uptake in the Rat

We present a method for comparing the uptake of the brain&#39;s two key energy substrates: glucose and ketones (acetoacetate [AcAc] in this case) in the ...

Université de Sherbrooke

Lower brain glucose metabolism is present before the onset of clinically measurable cognitive decline in two groups of people at risk of Alzheimer's disease--carriers of apolipoprotein E4, and in those with a maternal family history of AD. Supported by emerging evidence from in vitro and animal studies, these reports suggest that brain hypometabolism may precede and therefore contribute to the neuropathologic cascade leading to cognitive decline in AD. The reason brain hypometabolism develops is unclear but may include defects in brain glucose transport, disrupted glycolysis, and/or impaired mitochondrial function. Methodologic issues presently preclude knowing with certainty whether or not aging in the absence of cognitive impairment is necessarily associated with lower brain glucose metabolism. Nevertheless, aging appears to increase the risk of deteriorating systemic control of glucose utilization, which, in turn, may increase the risk of declining brain glucose uptake, at least in some brain regions. A contributing role of deteriorating glucose availability to or metabolism by the brain in AD does not exclude the opposite effect, i.e., that neurodegenerative processes in AD further decrease brain glucose metabolism because of reduced synaptic functionality and hence reduced energy needs, thereby completing a vicious cycle. Strategies to reduce the risk of AD by breaking this cycle should aim to (1) improve insulin sensitivity by improving systemic glucose utilization, or (2) bypass deteriorating brain glucose metabolism using approaches that safely induce mild, sustainable ketonemia.

Brain fuel metabolism, aging, and Alzheimer's disease.

Proteolytic degradation has emerged as a key pathway involved in controlling levels of the Alzheimer's disease (AD)-associated amyloid-β peptides (Aβ) in the brain. The ectopeptidase, neprilysin (NEP), has been reported as the major Aβ-degrading enzyme in mice and human brains. We have previously shown that NEP expression and activity are regulated by AICD, the intracellular domain of the amyloid-β protein precursor (AβPP) generated by γ-secretase. Thus, NEP transcription, expression, and enzymatic activity are dramatically reduced in fibroblasts devoid of AβPP (the precursor of AICD) or lacking both presenilin (PS) 1 and 2 (two parent proteins contributing to AICD formation). We demonstrate here that NEP expression and activity are influenced by a number of cell passages and density, and we confirm a drastic reduction of NEP expression and activity in AβPP and PS null fibroblasts examined at similar passages and cell densities. Furthermore, Imatinib (Gleevec), a known tyrosine kinase inhibitor was recently shown to elevate AICD in H4 human neuroglioma cells, and this was accompanied by concomitant increases of NEP protein, mRNA levels, and activity. However, the demonstration of a causal link between Imatinib and AICD levels was still lacking. We show here an Imatinib-dependent effect on NEP expression and activity in murine fibroblasts and establish that Imatinib-induced modulation of NEP was abolished by the depletion of AβPP or its homologues APLP1 and APLP2, thereby confirming that Imatinib-mediated control of NEP could indeed be accounted for its effect on AICD.

γ-Secretase-mediated regulation of neprilysin: influence of cell density and aging and modulation by imatinib.

Despite decades of study, it is still unclear whether regional brain glucose uptake is lower in the cognitively healthy elderly. Whether regional brain uptake of ketones (β-hydroxybutyrate and acetoacetate [AcAc]), the main alternative brain fuel to glucose, changes with age is unknown. We used a sequential, dual tracer positron emission tomography (PET) protocol to quantify brain (18)F-fluorodeoxyglucose ((18)F-FDG) and (11)C-AcAc uptake in two studies with healthy, male Sprague-Dawley rats: (i) Aged (21 months; 21M) versus young (4 months; 4M) rats, and (ii) The effect of a 14 day high-fat ketogenic diet (KD) on brain (18)F-FDG and (11)C-AcAc uptake in 24 month old rats (24M). Similar whole brain volumes assessed by magnetic resonance imaging, were observed in aged 21M versus 4M rats, but the lateral ventricles were 30% larger in the 21M rats (p=0.001). Whole brain cerebral metabolic rates of AcAc (CMR(AcAc)) and glucose (CMR(glc)) did not differ between 21M and 4M rats, but were 28% and 44% higher, respectively, in 24M-KD compared to 24M rats. The region-to-whole brain ratio of CMR(glc) was 37-41% lower in the cortex and 40-45% lower in the cerebellum compared to CMR(AcAc) in 4M and 21M rats. We conclude that a quantitative measure of uptake of the brain's two principal exogenous fuels was generally similar in healthy aged and young rats, that the % of distribution across brain regions differed between ketones and glucose, and that brain uptake of both fuels was stimulated by mild, experimental ketonemia.

The ketogenic diet increases brain glucose and ketone uptake in aged rats: a dual tracer PET and volumetric MRI study.

In humans consuming a normal diet, we investigated 1) the capacity of a medium-chain triacylglycerol (MCT) supplement to stimulate and sustain ketonemia, 2) ¹³C-β-hydroxybutyrate and ¹³C-trioctanoate metabolism, and 3) the theoretical contribution of the degree of ketonemia achieved to brain energy metabolism.

Stimulation of mild, sustained ketonemia by medium-chain triacylglycerols in healthy humans: estimated potential contribution to brain energy metabolism.

The effect of long-term calorie restriction (CR) on metabolites, fatty acid profiles and energy substrate transporter expression in the brain was assessed in aged rats. Three groups of male Sprague-Dawley rats were studied: (i) a 2 month old ad libitum-fed (2AL group), (ii) a 19 month old ad libitum-fed (19AL group), and (iii) a 19 month old group subjected to 40% CR from the age of 7.5 to 19 months (19CR group). The diet contained high sucrose and low n-3 polyunsaturated fatty acids (PUFA) so as to imitate a Western-style diet. High resolution magic angle spinning-(1)H NMR showed an effect of aging on brain cortex metabolites compared to 2AL rats, the largest differences being for myo-inositol (+251% and +181%), lactate (+203% and +188%), β-hydroxybutyrate (+176% and +618%) and choline (+148% and +120%), in 19AL and 19 CR rats, respectively. However, brain metabolites did not differ between the 19AL and 19CR groups. Cortex fatty acid profiles showed that n-3 PUFA were 35-47% lower but monounsaturated fatty acids were 40-52% higher in 19AL and 19CR rats compared to 2AL rats. Brain microvessel glucose transporter (GLUT1) was 68% higher in 19AL rats than in 2AL rats, while the monocarboxylate transporter, MCT1, was 61% lower in 19CR rats compared to 19AL rats. We conclude that on a high-sucrose, low n-3 PUFA diet, the brain of aged AL rats had higher metabolites and microvessel GLUT1 expression compared to 2AL rats. However, long-term CR in aged rats did not markedly change brain metabolite or fatty acid profile, but did reduce brain microvessel MCT1 expression.

Maggie Roy

Research Center on Aging and Department of Physiology and Biophysics

Université de Sherbrooke

A Dual Tracer PET-MRI Protocol for the Quantitative Measure of Regional Brain Energy Substrates Uptake in the Rat

Maggie Roy

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Research Center on Aging and Department of Physiology and Biophysics

Université de Sherbrooke

A Dual Tracer PET-MRI Protocol for the Quantitative Measure of Regional Brain Energy Substrates Uptake in the Rat

Research Center on Aging and Department of Physiology and Biophysics