BACKGROUND

Alzheimer's disease (AD) is an age-related neurodegenerative disorder, and the most common form of dementia.¹AD is characterized by the presence of amyloid beta plaques and neurofibrillary tangles associated with loss in memory, progressive cognitive impairment and behavioral changes. Epidemiological studies suggest that two-thirds of American AD subjects are women.² However; it is not clear whether the higher prevalence of AD in women is due to longer life expectancy or increased susceptibility. This study evaluates the memory and neurometabolism in female and male AβPP-PS1 mouse model of AD.

METHODS

All animal experiments were performed under protocols approved by the Institutional Animal Ethics Committee of CSIR-CCMB. Male (n=23) and female (n=22) AβPP-PS1 mice of 6 months age were used for the study. Working memory of the mice was assessed using Y-maze³, while the spatial memory was evaluated by the Morris Water Maze (MWM) test^­.⁴ For metabolic analysis, the mice were anesthetized with urethane (1.5 g/kg, intraperitoneal),­­­­­­­ and infused with [1,6-¹³C₂]glucose for 10 min using a bolus rate infusion protocol.⁵ Blood was collected from the retro orbital sinus, and mice were euthanized by a focused beam microwave irradiation (3kW, 1.2s). The metabolites were extracted from the cerebral cortex, hippocampus and cerebellum. Proton observed Carbon-13 edited NMR spectroscopy was done to determine the concentration of ¹³C labeled neurometabolites.⁶The cerebral metabolic rate of glucose oxidation (CMR_glc(ox)) in glutamatergic and GABAergic neurons was estimated by ¹³C labeled trapped into different neurometabolites as described previously.⁵

RESULTS

The escape latency of male AβPP-PS1 (70.8±23.8 s) in MWM test was not significantly different from the male WT mice (61.6±29.8 s). Similarly, the female AβPP-PS1 could reach the escape platform in (59±26.2s) and was not different from the female WT (59.1±27.6s). The spontaneous alternation of male AβPP-PS1 mice(54.5±11.7 %) in Y-maze was similar to the WT(58.5±1.0 %).Similarly, the female AβPP-PS1 mice (65.2±7.6 %)did not show change in the spontaneous alternation WT (63.4±10.5 %). Female AβPP-PS1 mice showed reduction in levels of NAA (5.8±0.3 vs. 6.2±0.3 µmol/g/min; p=0.023)in the cerebral cortex, and glutamine (5.9±0.4 vs. 6.3±0.3 µmol/g/min; p=0.036)in the cerebellum when compared with WT. The neurometabolite homeostasis was unperturbed in male AβPP-PS1 mice.
The labeling of Glu_C4 (1.51±0.13 vs.1.81±0.15 µmol/g; p=0.007), Asp_C3 (0.15±0.02 vs. 0.19±0.01 µmol/g; p=0.004), GABA_C2 (0.17±0.02 vs. 0.19±0.01 µmol/g; p=0.017) and Glu_C3 (0.28±0.05 vs. 0.34±0.04 µmol/g; p=0.02) from [1,6-¹³C₂]glucose was decreased in the cerebral cortex of female AβPP-PS1 mice when compared with female WT mice (Table 1). The male AβPP-PS1 mice exhibited reduced labeling only in Gln_C4 (0.28±0.05 vs. 0.34±0.04 µmol/g; p=0.02) and GABA_C4 (0.28±0.05 vs. 0.34±0.04 µmol/g; p=0.02) in the cerebral cortex. In female AβPP-PS1 hippocampus, ¹³C labeling of Glu_C4 (p=0.023) and GABA_C2 (p=0.05) were decreased. In contrast, male AβPP-PS1 mice showed a reduction in the labeling of GABA_C4 (p=0.001), Gln_C4 (p=0.016), Asp_C3 (p=0.021) and Glu_C3 (p=0.015). In Cerebellum, The female AβPP-PS1 exhibited a reduction in Glu_C4, GABA_C2, Asp_C3 and Glu_C3 (p<0.05) when compared to female WT (Table 1). However, Male AβPP-PS1 showed reduction in Gln_C4 (p=0.021) alone (Table 1).

The cerebral metabolic rate of glucose oxidation in glutamatergic neurons CMR_Glc(Glu)was decreased in the cerebral cortex (AβPP-PS1 0.18±0.02 µmol/g/min, WT 0.22±0.02 µmol/g/min; p=0.009), hippocampus (0.19±0.02 µmol/g/min vs. 0.21±0.02 µmol/g/min; p=0.03), and cerebellum (0.18±0.02 µmol/g/min vs. 0.21±0.02 µmol/g/min; p=0.01),of female AβPP-PS1 (Fig. 3). In contrast, male AβPP-PS1 mice showed reduction in CMR_Glc(Glu) in the hippocampus (0.20±0.03 µmol/g/min vs. 0.23±0.04 µmol/g/min; p=0.04) only, while cerebral cortex and cerebellum were unperturbed. The GABAergic neurons exhibit reduction in CMR_Glc(GABA) in cerebral cortex (0.04±0.004 µmol/g/min vs. 0.05±0.00 µmol/g/min; p=0.009) and cerebellum (0.06±0.00 µmol/g/min vs. 0.07±0.01 µmol/g/min; p=0.045) of female AβPP-PS1 mice, while male exhibit reduction in GABAergic neurometabolism in the hippocampus (0.05±0.01 µmol/g/min vs. 0.06±0.01 µmol/g/min, p=0.02) only (Fig. 4).

DISCUSSION

The working and spatial memory was intact in male and female AβPP-PS1 mice at 6 months of age. The neurometabolite homeostasis was largely unperturbed except a reduction in the cortical NAA suggestive of neurodegeneration in female APP-PS1 mice. The reduced CMR_Glc(Glu) is suggestive of decreased neuronal viability in the cerebellum along with cerebral cortex and hippocampus of female AβPP-PS1 mice. In contrast, only hippocampal neurometabolic activity was reduced in the male mice. These data suggest differential progression of AD in male and female AβPP-PS1 mice.

Acknowledgements

This study was supported by Council of Scientific and Industrial Research (CSIR-CCMB: NCP/MLP0139).

References

1. WHO (2017) Global action plan on the public health response to dementia 2017–2025

2. Alzheimer’s Association (2017) Alzheimer’s disease facts and figures. Alzheimers Dement13:325–373

3. Holcomb et al (1998) Accelerated Alzheimer-type phenotype in transgenic mice carrying both mutant amyloid precursor protein and presenilin 1 transgenes. Nat Med4:97-100.

4. Morris R (1981) Spatial Localization Does Not Require the Presence of Local Cues. Learning and Motivation12:239-260.

5. Tiwari et al (2013) Glutamatergic and GABAergic TCA cycle and neurotransmitter cycling fluxes in different regions of mouse brain. J Cereb Blood Flow Metab33:1523-1531.

6. deGraaf RA, Brown PB, Mason GF, Rothman DL, & Behar KL(2003). Detection of [1,6-¹³C²]-glucose metabolism in rat brain by in vivo ¹H-[¹³C]-NMR spectroscopy. Magnetic resonance in medicine, 49(1), 37–46.