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Background:
Alzheimer disease (AD) is characterized by early and important changes in cerebral glucose metabolism, mitochondrial respiration, and neuroinflammation1-5. These changes and their role in the neurodegeneration observed in AD are not fully understood. Hyperpolarized (HP) 13C MR spectroscopy (MRS) has emerged as a powerful method to detect in vivo metabolic fluxes in real time6. HP 13C MRS provides key information on the fate of pyruvate, the end product of glycolysis, by measuring the conversion of HP [1-13C]pyruvate (Pyr) to [1-13C]lactate (Lac) and [13C]bicarbonate (Bic), thus allowing to investigate changes in glycolytic and oxidative (mitochondria) metabolism, respectively7.
Hypothesis/Objective:
Here, we investigated whether HP 13C MRS of [1-13C]pyruvate can be used to non-invasively assess brain energy metabolism alterations in a novel and clinically relevant model of neurodegeneration, the TE4 mouse model of tauopathy8.
Methods:
Animals: Four groups of female mice were used: the tauopathy mouse model (TE4) expressing tau (P301S) and human-APOE4 at 2M (early asymptomatic stage, n=7) and 11-13M (late pathological stage, n=15) and mice expressing human-APOE4 (E4) at 2M (n=6) and 11-13M (n=17)(Persuasive data). All mice underwent anatomical T2-weighted (T2w), T1-weighted (T1w) MRI, and metabolic HP 13C MRS (Persuasive data).
MR acquisitions: HP 13C MRS acquisitions were performed on a 3T MRI scanner with a 13C surface coil. For 13C MRS, Pyr was polarized in a SPINlab polarizer. Slab dynamic 13C data were acquired using a spectral-spatial pulse sequence9 following an intravenous injection of HP Pyr (80 mM). T2w and T1w MR images were acquired using a 9.4T MRI scanner. To evaluate blood-brain-barrier (BBB) integrity, T1w images were acquired 2.5 minutes post intravenous gadolinium-DTPA injection. MR parameters are summarized in Persuasive data.
MR analyses: 13C spectra were summed over time and HP Pyr, Lac and Bic levels were calculated as the fit integrals. T2w images were manually delineated for volumetric analysis.
Enzyme activity assay: Pyruvate dehydrogenase (PDH, converts Pyr into Bic) and lactate dehydrogenase (LDH, converts Pyr into Lac) enzyme activities were quantified using a colorimetric assay.
Statistical analyses: Statistical significance (p<0.05) was evaluated using t-tests.
Results:
Following injection, HP Pyr was converted into Lac and Bic and informs on glycolytic and oxidative (mitochondria) metabolism respectively (Fig.1A). After intravenous injection of HP Pyr, clear signals from HP Pyr, Lac and Bic were observed in the brain of E4 and TE4 mice (Fig.1B). No difference was observed for Pyr and Lac levels between E4 and TE4 mice at 2M or 11-13M old (Fig.C-D). Bic levels, Bic/Pyr and Bic/Lac ratios were decreased in TE4 compared to E4 mice at 11-13M old, but not at 2M (Fig1.E,G,H), demonstrating impaired oxidative metabolism in the tauopathy model at an advanced pathology stage. The lower Pyr into Bic conversion was supported by enzyme activity assays, which revealed lower PDH enzymatic activity at 11-13M in TE4 mice compared to E4, but no differences in LDH enzymatic activity (Persuasive data).
Next, anatomical T2w MR images confirmed smaller hippocampal volume and larger ventricle volume in TE4 mice at 11-13M but not at 2M (Fig.1I-J). Qualitative analysis of T1w maps revealed hyperintensities indicative of BBB leakage in the hippocampus of TE4 mice at 11-13M but not at 2M (Persuasive data).
Conclusion:
This is the first report showing that HP 13C MRS can detect lower HP Bic production in a clinically relevant tauopathy AD mouse model at an advanced pathology stage, but not prior to pathology onset. These results reveal brain oxidative metabolism impairment, which are explained by lower PDH enzymatic activity and align with changes observed in people with AD1. Hence, HP 13C MRS may provide a new non-invasive way to study mitochondrial anomalies in AD pathogenesis and aid new treatment development in AD.
Image/Figure:
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Image/Figure Caption:
Figure 1: (A) Schematic showing HP [1-13C]pyruvate’s conversion into subsequent metabolites. (B) Schematic 13C acquisition slice and spectra from the dynamic MRS acquisition (black), and the summed spectra (red) from 11-13M E4 and TE4 mice. HP (C) [1-13C]pyruvate, (D) [1-13C]lactate, (E) [13C]bicarbonate, (F) lactate/pyruvate, (G) bicarbonate/pyruvate, and (H) bicarbonate/lactate. (I) T2w images with markers indicating the hippocampus (green dashed) and ventricles (red) from a half hemisphere. (J) 3D rendering of hippocampus (green) and ventricles (red) in E4 and TE4 mice at 11-13M (K) Mean volume of whole bilateral hippocampus and ventricles for E4 and TE4 mice. Error bars=SD. Unpaired t-tests, *p<0.05, ***p<0.001.
Author
Nicholas David Vidas-Guscic
Washington University in St. Louis