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Functioning of the glucose transporter and glymphatic systems in the tauopathy AD mouse brain studied by onVDMP MRI and D-glucose infusion

Lin Chen^1,2, Zhiliang Wei^1,2, Kannie W.Y. Chan^1,2,3, Jianpan Huang⁴, Xiang Xu^1,2, Philip C. Wong^5,6, Hanzhang Lu^1,2, Peter C.M. van Zijl^1,2, Tong Li^5,6, and Jiadi Xu^1,2
¹Department of Radiology and Radiological Science, The Johns Hopkins University School of Medicine, Baltimore, MD, United States, ²F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Research Institute, Baltimore, MD, United States, ³Department of Biomedical Engineering, City University of Hong Kong, Hong Kong, China, ⁴City University of Hong Kong, Hong Kong, China, ⁵Department of Pathology, The Johns Hopkins University School of Medicine, Baltimore, MD, United States, ⁶Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, MD, United States

Synopsis

In this study, we used onVDMP MRI to detect glucose uptake in tauopathy Alzheimer's disease (AD) mouse brain. Compared to wild-type mice, significantly reduced glucose uptake was observed in both cerebrospinal fluid (CSF) and parenchyma of AD mouse brain. Clearance of glucose through CSF was found in wild-type mice, but not in AD mice, which implicates dysfunction of the glymphatic system in AD mouse brain. The results in this study suggest that onVDMP MRI could be a cost-effective and widely available method for evaluating the functions of glucose transporter and glymphatic system, and hence diagnosing AD.

Introduction

Glucose hypometabolism has been hypothesized to be an upstream event of AD that can be used for its early diagnosis (1-4). Currently, glucose uptake can be assessed using 2-[18F]fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) (5) and in vivo magnetic resonance spectroscopy (MRS) (6,7). However, the limited availability of FDG-PET and low sensitivity of MRS hamper their widespread clinical application. Glucose chemical exchange saturation transfer (glucoCEST) MRI is a recently developed technique (8,9) that can detect unlabeled glucose at physiologically relevant concentrations using standard MRI scanners. Among various glucoCEST methods, on-resonance variable delay multiple pulse (onVDMP) MRI is promising due to its excellent labeling efficiency and sensitivity (10-12). In this study, we applied onVDMP MRI in a AD mice with tauopathy and to examine the impact of tau protein on the glucose uptake in parenchyma and CSF (glymphatic function) in brain.

Methods

The glucose transport pathways in mouse brain are illustrated in Fig. 1. The animal study was carried out under the approval of local ACUC. Four Tau4RDK (Tau) mice (13) with an age of 7-8 months and four age-matched C57BL/6J mice were used for the AD study. MRI experiments were performed on a horizontal bore 11.7 T Bruker Biospec system. The onVDMP sequence with phase cycling scheme is shown in Fig. 2. Two independent onVDMP sequences with different saturation-module lengths (i.e. 36 ms and 300 ms) were repeated in an interleaved fashion to detect the glucose uptake in brain parenchyma and CSF, respectively. The dynamic glucose enhanced (DGE) images were acquired continuously for 27.5 min. A bolus of 0.15mL 50% w/w glucose was given at 10th min of the dynamic experiment through the tail vein. Dynamic in vivo MRS spectroscopy was performed to validate the change in glucose concentration in brain tissue after glucose infusion.
Area-under-curve (AUC) was utilized to reflects glucose uptake and retention(14), and the following equations were used to fit the amplitude and lifetime of glucose uptake for MRS and DGE images, respectively:
$$\triangle DGE(t) = A_{DGE}(1-e^{-t/T_{u,DGE}}) [1]$$
$$[\triangle Glc](t) = A_{MRS}(1-e^{-t/T_{u,MRS}}) [2]$$
where$$$\triangle DGE(t)$$$ and $$$[\triangle Glc](t)$$$ are the signal difference between pre and post-glucose infusion determined by onVDMP and MRS, respectively.$$$T_{u,DGE}$$$ and $$$T_{u,MRS}$$$ are the glucose uptake lifetimes, and $$$A_{DGE}$$$ and $$$ A_{MRS}$$$ refer to the amplitudes of glucose uptake. The half-life time of glucose uptake was calculated by:$$$T_{half}=T_{u}\times0.6931$$$.

Results and Discussion

From the AUC maps of parenchyma in Figs. 3b&f, Tau mice exhibited overall lower AUC intensity compared to the WT counterpart. The average AUC value of a cortical region (cx) for Tau mice (1.6±1.3%) was significantly reduced compared to the same region in WT mice (4.3±1.3%, p = 0.016, N = 4). This is consistent with the observation by others that the glucose uptake is impaired in AD brain due to the reductions of both GLUT1 and GLUT3 expression at the BBB (15).
The dynamic AUC maps were utilized to monitor the glucose uptake in brain, as shown in Fig. 4. By fitting the dynamic AUC maps using Eq. 1, the half-life time of WT parenchyma was 4.71±1.03min, which is not significantly different from that of Tau mice (5.14±0.30min, p = 0.511). The DGE signal of WT CSF built up quickly after glucose infusion and plateaued around 10 min. The half-life time for WT CSF was min. The DGE signal of CSF in tau mice kept increasing after glucose infusion and reached a plateau around 25 min with a half-life time of min, which is significantly longer than WT counterpart (p < 0.001). The buildup process of the DGE curve for CSF is again related to the glucose transport at the blood-cerebrospinal fluid barrier (BCSFB). The slower glucose uptake in AD mouse brain could be an impaired glucose transporter at the BCSFB, but that would need to be verified independently. Interestingly, after reaching a plateau, the DGE signal of WT CSF began to decay, as shown in both dynamic AUC maps and DGE curves in Fig. 4. However, this phenomenon was not observed in CSF of the tau mice. From previous studies, glucose can alternatively be redistributed to brain parenchyma through a CSF and interstitial fluid (ISF) exchanging process, i.e. the glymphatic pathway, (16,17) and the glymphatic pathway of AD brain has been reported to be dysfunctional (18). This may explain the decay of WT CSF glucose reaching a maximum around 15 minutes, while the DGE signal of Tau CSF kept increasing.
From Fig. 5, similar glucose uptake patterns and consistent glucose uptake half-life times (MRS:4.16±0.69min; onVDMP: 4.71±1.18min) were observed between the dynamic curves obtained by in vivo MRS and onVDMP, which validates that the signal change obtained by onVDMP is dominated by the glucose concentration.

Conclusion

The results in this study suggest that onVDMP MRI can be used for evaluating the functions of glucose transporter and glymphatic system, which may have potential for diagnosing AD.

Acknowledgements

Funding Support: NIH: R01EB019934, R03NS109664, and DOD W81XWH-18-1-0797.

References

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Figures

Figure 1. Illustration of the D-glucose transport pathways in mouse brain after intravenous infusion of glucose. D-glucose in blood crosses the BBB's luminal and abluminal membranes and reaches parenchyma. Part of the D-glucose rapidly enters the CSF through the BCSFB and recirculates to the parenchyma through the glymphatic system.

Figure 2. Illustration of the onVDMP sequence with phase cycling for detecting glucose uptake. The onVDMP saturation module is composed of a train of bionomical pulses separated by a delay tmix. The flip angle of the binomial pulse is 360 degrees.

Figure 3. (a,e) Representative onVDMP dynamic images with a saturation length of 36 ms on (a) WT and (e) Tau mice, and the corresponding DGE-based AUC maps (b&f). (c,g) Representative onVDMP dynamic images with a saturation length of 300 ms on (c) WT and (g) Tau mice at the same slice, and the corresponding DGE-based AUC maps (d &h). (i) Statistical analysis of AUC values extracted from (b, f, d & h). The cortical (cx) and thalamic (th) regions are prescribed by red and green lines in (a). Error bars indicate standard deviation across mice (N = 4).

Figure 4. Dynamic AUC maps for WT (a,b) and Tau (c,d) mice over the 27.5 min DGE experiment. Each dynamic AUC map was calculated by averaging 10 successive DGE images, which lead to a time window of 150 s. The averaged time-resolved DGE curves for the (e) cortex and (f) CSF of WT and Tau mice. The ROIs used for determining the DGE curves are indicated in the inserted images with red lines. Shaded regions in the graphs represent the standard deviation of the curves (n=4). The averaged DGE curves (solid lines) and the corresponding curves from fitting with Eq. 1 (dotted lines) are shown.

Figure 5. (a) MRS spectra for a typical mouse brain over a voxel in the thalamus before (blue), and after (red, 30 minutes) glucose infusion. Difference spectra at three-time points (green) are also shown. (b) Dynamic MRS signal changes as a function of time relative to the start of glucose infusion (n=2). The glucose to total creatine (tCr) ratios were calculated by integrating the chemical shifts between 3.0 to 4.2 ppm in the difference spectrum and divided by the area of the tCr methyl group centering at 3 ppm. The solid line is the curve fitted with a single exponential function (Eq. 2).

Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)

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