Locus coeruleus microstructural integrity is linked to plasma GFAP concentration and Aβ42/40 ratio in cognitively unimpaired individuals: a 7T MRI study.

Background: Recent autopsy and in vivo MRI studies supported structural integrity of the brainstem locus coeruleus (LC) as a potential indicator of initial AD‐related pathological processes. Importantly, in the earliest stages of the disease, these changes to the neuronal density of the LC are associated with changes to the somatic morphology and dendritic atrophy. Here, we aimed to examine whether LC microstructural integrity, assessed in vivo with advanced diffusion‐weighted magnetic resonance imaging (dMRI), is related to plasma AD biomarkers in a sample of cognitively unimpaired individuals. Method: Fifty‐seven cognitively unimpaired participants (mean age = 59.3±14.7y; 28 females) (Table 1) underwent a dMRI scanning session using a multi‐shell, high angular resolution acquisition protocol in a 7T scanner. LC microstructure was evaluated by applying the Neurite Orientation Dispersion and Density Imaging (NODDI) biophysical model on the dMRI data, yielding two metrics of microstructural integrity: Neurite Density Index (NDI) and Orientation Dispersion Index (ODI). Additionally, blood samples were analyzed to assess AD plasma biomarkers, including Aβ42/40 ratio, glial fibrillary acidic protein (GFAP), neurofilament light (NfL), and tau phosphorylated at threonine 181 (pTau181) or threonine 231 (pTau231). Result: Higher age was associated with lower LC ODI (t = ‐2.77, p = 0.008) but not LC NDI values (t = 0.44, p = 0.658). Multiple linear regressions adjusted for age, sex, and estimated Total Intracranial Volume (eTIV) showed that lower NDI in the LC was associated with higher plasma GFAP levels (t = ‐2.08, p =.04, Figure 1). In addition, we found a negative association between ODI within the LC and Aβ42/40 (t = ‐2.79, p =.007, Figure 2). No significant relationships were found between LC NDI/ODI values and plasma NfL or pTau biomarkers. Conclusion: Consistent with animal studies, these findings suggest that microstructural changes in the LC are associated with astroglial activation in the earliest stages of the disease. Associations with Aβ42/40 were unexpected but might reflect loss of white matter compartments or changes in lipid density. In future analyses, we will further examine the biological interpretation of the NODDI metrics in subcortical regions, in particular in the context of AD‐related processes. [ABSTRACT FROM AUTHOR]