Synopsis

Keywords: Neurofluids, Diffusion Tensor Imaging, Glymphatic system, DTI-ALPS

Excessive alcohol intake seriously damages the brain. Previous animal studies reported that the glymphatic system, which is a brain waste clearance system via the cerebral spinal fluid, is affected by chronic high alcohol consumption. Glymphatic dysfunction is related to cognitive impairment. The changes of diffusivity along the perivascular space (ALPS) indices associated with heavy, moderate, and no‑alcohol intake and executive function with or without aldehyde dehydrogenase 2 (ALDH2) rs671 polymorphism was evaluated. The present study revealed the glymphatic function and executive function decline in heavy drinkers. Furthermore, ALDH2 rs671 variants may increase vulnerability to alcohol-induced glymphatic dysfunction.

Introduction

Long-term excessive alcohol consumption can affect the central nervous system and cause cognitive decline¹. The glymphatic system hypothesis was recently proposed as a brain clearance system² and glymphatic dysfunction is associated with amyloid-β accumulation and cognitive impairment³. Previous animal studies indicated that chronic heavy drinking might cause glymphatic dysfunction and increase the risk of dementia⁴. However, the human involvement in alcohol consumption is still unclear. This study aimed to evaluate the glymphatic function associated with alcohol consumption by diffusion tensor image analysis along the perivascular space (DTI-ALPS)⁵. Furthermore, the effect of aldehyde dehydrogenase 2 (ALDH2) genetic polymorphism on glymphatic function was compared.

Materials and Methods

Study participants
In this study, 137 community-dwelling Japanese older adults were enrolled from the Bunkyo Health study cohort⁶. Current alcohol consumption was assessed by a brief-type self-administered diet history questionnaire⁷. The subjects were divided into heavy-drinker (25 males and 9 females), moderate-drinker (24 males and 24 females), and non-drinker (15 males and 40 females) (Fig. 1). “Heavy drinking” was defined as >14 drinks (196 g) a week for men and >7 drinks (98 g) for women according to the US Federal Dietary guidelines⁸. ALDH2 rs671 genetic polymorphisms were analyzed with GG-type subjects as the ALDH2 wild-type group and GA- and AA-type subjects as the ALDH2 variants group. In this cohort, only the ALDH2 gene rs671 polymorphism was available for genetic data on alcohol metabolism.
MRI acquisition
MAGNETOM Prisma 3T MRI scanner (Siemens, Erlangen, Germany) with a 64-channel head coil was used to acquire all diffusion weighted images (DWI). Whole brain DWI were acquired using multislice echo-planar imaging along 64 diffusion gradient directions in the anterior–posterior direction at a b-value = 1000 s/mm² with one nondiffusion-weighted (b = 0) volume using the following parameters: TR/TE = 3300/70 ms, matrix size = 130 × 130, resolution = 1.8 × 1.8 mm, slice thickness = 1.8 mm, FOV = 229 × 229 mm, and acquisition time = 7 min 29 s.
Analysis ALPS index
DWI data were analyzed using FSL version 6.0. A validated semi-automated pipeline was used to calculate the ALPS index⁹. In brief, a 5-mm diameter spherical regions of interest (ROIs) in the projection and association areas was placed (Fig. 2) using a color-coded FA map at the lateral ventricle body level. The x-, y-, and z-axis diffusivity values in the ROIs were obtained for each individual. In the lateral ventricular plane, medullary vessels run in the right–left (x-axis) direction. Perivascular spaces are oriented along the x-axis and white matter fibers (projection or association fiber) run orthogonally to the x-axis at that level. To estimate the perivascular space diffusivity, the ALPS index was calculated as the average x-axis diffusivity ratio of the projection (D_xxproj) and the association region (D_xxassoc) to the average y-axis diffusivity of the projection region (D_yyproj) and the average z-axis diffusivity of the association region (D_zzassoc) ^5. (Fig. 2) Finally, the ALPS index was calculated by averaging the left and right sides.
Statistical analysis
A general linear model analysis was performed to compare the ALPS indices of the heavy-drinker, moderate-drinker, and non-drinker groups, including age, years of education, handedness, body mass index and the Brinkman index as covariates. Pairwise comparisons of groups were performed using Tukey's tests. In addition, the correlations between the ALPS indices and the time of the trail-making test part B (TMT-B) for drinking subjects after adjusting for age, years of education, handedness, body mass index and the Brinkman index was estimated. Analysis was divided between men and women, with and without ALDH2 variants. A p-value <0.05 was considered significant. Statistical analysis was performed using R version 4.12.

Results

The male and female heavy-drinker groups had significantly lower ALPS indices than the non-drinker groups (Fig. 3). In men with ALDH2 variants, the heavy drinkers' ALPS index was significantly lower than in the other groups and also significantly lower in moderate drinkers than in non-drinkers. While there was no significant difference in ALDH2 wild-type men. In ALDH2 wild-type women, heavy drinkers had significantly lower ALPS indices than the moderate and non-drinker groups. While in women with ALDH2 variants, the ALPS index of moderate drinkers was significantly lower than non-drinkers (Fig. 4). In only drinking men, a significant negative correlation between the ALPS index and the TMT-B time (Fig. 5) was found.

Discussion

This study showed significantly lower ALPS indices in heavy-drinker groups than in non-drinker groups in men and women. This means that in the heavy drinkers’ brains, there is a relative decrease in diffusivity along the perivascular space. The previous animal study reported that chronic alcohol exposure caused fluorescent tracer influx suppression along the cortical vessel⁴. Therefore, the ALPS index reduction could reflect alcohol-induced glymphatic dysfunction. Furthermore, male drinkers with ALDH2 variants were found to have decreased ALPS indices and a significantly negative correlation with executive function. Previous reports indicated chronic alcohol exposure caused amyloid-β accumulation in the brain of mice harboring ALDH2 variants¹⁰. ALDH2 genetic variant may cause executive function impairment in subjects due to alcohol-induced glymphatic dysfunction.

Conclusion

This study suggested that heavy-drinking older adults have a decreased ALPS index and the ALDH2 genetic variant may increase vulnerability to alcohol-induced glymphatic dysfunction.

Acknowledgements

This research was supported by Grants-in-Aid for Scientific Research of the Japan Society for the Promotion of Science (JSPS KAKENHI; Grant Numbers: 19K17244 and 18H02772), Brain/MINDS Beyond program (grant no. JP19dm0307101) of the Japan Agency for Medical Research and Development (AMED), AMED under grant number JP21wm0425006, a Grant-in-Aid for Special Research in Subsidies for ordinary expenses of private schools from The Promotion and Mutual Aid Corporation for Private Schools of Japan, and the Juntendo Research Branding Project.