Search

Objective: Elevated entorhinal cortex (EC) tau in low beta‐amyloid individuals can predict accumulation of pathology and cognitive decline. We compared the accuracy of magnetic resonance imaging (MRI)‐derived locus coeruleus integrity, neocortical beta‐amyloid burden by positron emission tomography (PET), and hippocampal volume in identifying elevated entorhinal tau signal in asymptomatic individuals who are considered beta‐amyloid PET‐negative. Methods: We included 188 asymptomatic individuals (70.78 ± 11.51 years, 58% female) who underwent 3T‐MRI of the locus coeruleus, Pittsburgh compound‐B (PiB), and Flortaucipir (FTP) PET. Associations between elevated EC tau and neocortical PiB, hippocampal volume, or locus coeruleus integrity were evaluated and compared using logistic regression and receiver operating characteristic analyses in the PiB− sample with a clinical dementia rating (CDR) of 0. Associations with clinical progression (CDR‐sum‐of‐boxes) over a time span of 6 years were evaluated with Cox proportional hazard models. Results: We identified 26 (21%) individuals with high EC FTP in the CDR = 0/PiB− sample. Locus coeruleus integrity was a significantly more sensitive and specific predictor of elevated EC FTP (area under the curve [AUC] = 85%) compared with PiB (AUC = 77%) or hippocampal volume (AUC = 76%). Based on the Youden‐index, locus coeruleus integrity obtained a sensitivity of 77% and 85% specificity. Using the resulting locus coeruleus Youden cut‐off, lower locus coeruleus integrity was associated with a two‐fold increase in clinical progression, including mild cognitive impairment. Interpretation: Locus coeruleus integrity has promise as a low‐cost, non‐invasive screening instrument to detect early cortical tau deposition and associated clinical progression in asymptomatic, low beta‐amyloid individuals. ANN NEUROL 2024;96:650–661 [ABSTRACT FROM AUTHOR]

Tau accumulation in and neurodegeneration of locus coeruleus (LC) neurons is observed in Alzheimer's disease (AD). We investigated whether tangle and neuronal density in the rostral and caudal LC is characterized by an asymmetric pattern in 77 autopsy cases of the Rush Memory and Aging Project. We found left-right equivalence for tangle density across individuals with and without AD pathology. However, neuronal density, particularly in the caudal-rostral axis of the LC, is asymmetric among individuals with AD pathology. Asymmetry in LC neuronal density may signal advanced disease progression and should be considered in AD neuroimaging studies of LC neurodegeneration. [ABSTRACT FROM AUTHOR]

Objective: While studies suggested that locus coeruleus (LC) neurodegeneration contributes to sleep–wake dysregulation in Alzheimer's disease (AD), the association between LC integrity and circadian rest‐activity patterns remains unknown. Here, we investigated the relationships between 24‐hour rest‐activity rhythms, cognitive trajectories, and autopsy‐derived LC integrity in older adults with and without cortical AD neuropathology. Methods: This retrospective study leveraged multi‐modal data from participants of the longitudinal clinical‐pathological Rush Memory and Aging Project. Indices of 24‐hour rest‐activity rhythm fragmentation (intradaily variability) and stability (interdaily stability) were extracted from annual actigraphic recordings, and cognitive trajectories were computed from annual cognitive evaluations. At autopsy, LC neurodegeneration was determined by the presence of hypopigmentation, and cortical AD neuropathology was assessed. Contributions of comorbid pathologies (Lewy bodies, cerebrovascular pathology) were evaluated. Results: Among the 388 cases included in the study sample (age at death = 92.1 ± 5.9 years; 273 women), 98 (25.3%) displayed LC hypopigmentation, and 251 (64.7%) exhibited cortical AD neuropathology. Logistic regression models showed that higher rest‐activity rhythm fragmentation, measured up to ~7.1 years before death, was associated with increased risk to display LC neurodegeneration at autopsy (odds ratio [OR] = 1.46, 95% confidence interval [CI95%]: 1.16–1.84, pBONF = 0.004), particularly in individuals with cortical AD neuropathology (OR = 1.56, CI95%: 1.15–2.15, pBONF = 0.03) and independently of comorbid pathologies. In addition, longitudinal increases in rest‐activity rhythm fragmentation partially mediated the association between LC neurodegeneration and cognitive decline (estimate = ‐0.011, CI95%: ‐0.023–‐0.002, pBONF = 0.03). Interpretation: These findings highlight the LC as a neurobiological correlate of sleep–wake dysregulation in AD, and further underscore the clinical relevance of monitoring rest‐activity patterns for improved detection of at‐risk individuals. ANN NEUROL 2024;95:653–664 [ABSTRACT FROM AUTHOR]

Background: The noradrenergic locus coeruleus (LC) nucleus is critical in regulating many brain functions and processes, including arousal and emotion. Autopsy studies suggested that the LC is one of the earliest regions harboring hyperphosphorylated tau. Previous imaging work demonstrated that lower novelty‐related LC activity correlates with tau deposition in the entorhinal cortex. Here, we investigated the variability in activity across the rostro‐caudal LC under different arousal levels and its relationship with plasma markers of phosphorylated tau, including p‐tau231, p‐tau217, and p‐tau181. Method: 61 individuals (age range = [51,85], Male = 30) underwent a task‐based 7T BOLD‐fMRI scan and fasting blood draw. The task consisted of 100 high‐ or low‐arousing context images. The fMRI data were preprocessed using a brainstem‐specific denoising methodology to increase the LC signal‐to‐noise ratio. P‐tau181 and p‐tau231 were measured using in‐house Simoa methods, and p‐tau217 using MSD immunoassay. Voxel‐wise linear model analyses were performed to detect brainstem regions exhibiting significant high>low arousal (h>l) activation. Average h>l contrast values across activated voxels within an ex‐vivo validated LC region of interest (ROI) were extracted, and age‐ and sex‐adjusted associations between LC activity and p‐tau were evaluated using robust linear regression. Similar associations were also investigated for a control ROI defined ventral to the LC. Result: We observed greater h>l activation of voxels in the bilateral LC (p<0.05 uncorrected–Fig.1A). We observed no regional differences in h>l activity along the LC rostral‐caudal axis (Fig.1B). Post‐hoc Mann‐Kendall trend analyses revealed a negative‐to‐positive trend in h>l LC activity across caudal‐to‐rostral LC. Lower h>l LC activity was associated with greater p‐tau217 (p = 0.042–Fig.2A). No significant associations were observed between h>l LC activity and either p‐tau231 or p‐tau181 (p>0.05–Fig.2B‐C), or between h>l activity in the control ROI and any plasma marker (p>0.05–Fig.2D‐F). Conclusion: Our results show that lower h>l LC activation is associated with greater concentrations in p‐tau217. As p‐tau217 has been shown to correlate with LC neuronal loss, cortical tau‐ and amyloid‐β‐PET measures, our results suggest that LC dysfunction in higher states of arousal may signal LC degeneration. Future work includes investigating the mediating role of amyloid‐β on the observed relationship between h>l LC activation and p‐tau217. [ABSTRACT FROM AUTHOR]

Background: In preclinical human Alzheimer's disease (AD), novelty‐related functional connectivity of the locus coeruleus (LC) with the medial temporal lobe (MTL) (ConnLC‐MTL) predicted beta‐amyloid (Aβ)‐related longitudinal cognitive decline. In animals, LC lesions in AD models led to dwindling LC projections to the hippocampus, hippocampal atrophy, and cognitive impairment. Following these observations, we investigated the contribution of cross‐sectional and longitudinal atrophy in early Braak stages as a potential mechanism underlying this relationship. Method: We included 128 participants of the Harvard Aging Brain Study (HABS) with baseline Pittsburgh Compound‐B Positron Emission Tomography – Distribution Volume Ratio (PiB PET, DVR; 36 Aβ+), as well as baseline functional and longitudinal structural MRI (ntot = 372; ∼ every 3 years; follow‐up 5±3.4 years) and yearly cognitive assessments (PACC5 = Preclinical Alzheimer's Cognitive Composite; ntot = 874; follow‐up 6.5±3.3 years) (Table 1). ConnLC‐MTL values were derived voxel‐wise and averaged across significant clusters. FreeSurfer‐estimated cortical thickness and hippocampus/amygdala volume (adjusted for intracranial volume) were z‐scored and averaged in two variables (THV2/THV3), encompassing regions associated with Braak stage II and III respectively (Table 1). Baseline associations were evaluated with linear models, longitudinal relationships between ConnLC‐MTL and THV2/THV3 with mixed‐effects models (random intercept and slope). Mediation of ConnLC‐MTL and PACC5 slopes (year 5‐13) by THV2/THV3 slopes (year 0‐5) was calculated (n = 79; bootstrapped n = 1000; Table 1). All models contained age, sex and education as covariates. Result: At baseline, ConnLC‐MTL positively associated with THV2 (p = 0.005) and with PACC5 (p = 0.045), but not with THV3 (p = 0.26; Figure 1A‐C). Longitudinally, ConnLC‐MTL did not predict changes in THV2 nor THV3. However, we observed faster cortical atrophy with lower ConnLC‐MTL at higher levels of neocortical PiB (THV2: p = 0.02; THV3: p<0.001; Figure 1D‐E). Finally, THV2, but not THV3, mediated the relationship between ConnLC‐MTL and PACC5 decline (p = 0.024) and this effect was stronger in PiB+ individuals compared to PiB‐ (p = 0.026; Figure 2). Conclusion: Worse LC‐MTL connectivity was associated with downstream atrophy in the target Braak‐stage II areas, which impacted cognitive performance in preclinical AD. Given that in similar previous animal findings activation of the norepinephrine LC system reversed memory deficits, maintaining optimal ConnLC‐MTL may be a promising intervention target to delay disease progression. [ABSTRACT FROM AUTHOR]

Background: The locus coeruleus (LC), the primary noradrenergic nucleus in the brainstem, is among the earliest brain structures to accumulate tau pathology, one of the neuropathologic hallmarks of Alzheimer's disease (AD). Tau pathology in the earlier stages and significant cell loss in the disease's later stages can affect the metabolic turnover of the LC. In addition, alterations in the LC's metabolic turnover have been associated with tau formation and propagation. We investigated differences in LC glucose metabolism across AD clinical diagnostic and biomarker groups and its association with longitudinal tau, beta‐amyloid, and cognitive measurements. Method: We investigated LC FDG‐PET signal of 295 participants from the ADNI‐1 and ADNI‐2/GO cohorts, who were equally distributed across the CSF beta‐amyloid (A) and phosphorylated‐tau (T) biomarker groups (Table 1). An in‐house developed joint‐entropy‐penalized image deblurring algorithm was used to quantify Positron Emission Tomography (PET) signal within small brain regions (Figure 2A). ANCOVA was used to investigate diagnostic and biomarker‐based group differences in LC metabolism. Post‐hoc Tukey contrasts corrected for multiple comparisons in subsequent pairwise analyses. Linear mixed‐effects models investigated the predictive value of LC metabolism on the rate of change in CSF‐based p‐tau, beta‐amyloid and cognitive scores. Result: Low LC FDG‐PET signal was observed in the amyloid and tau‐positive group compared to the other groups (p<0.0001). Pair‐wise differences between the diagnostic groups did not survive correction for multiple comparisons. Combining biomarker and diagnostic status, we observed lower LC metabolism in A+T+ MCI and AD groups compared to CN and MCI A+T‐ and A‐T‐ groups (p<0.001; Figure 1B). Lower LC metabolism predicted accumulation of p‐tau (t(127) = ‐2.65, p = 0.009) and PACC decline (t(128) = 2.2, p = 0.029; Figure 2). LC metabolism did not significantly predict changes in beta‐amyloid levels or MMSE scores. Conclusion: This is the first study relating in‐vivo spatially specific measures of LC metabolism to AD‐related biomarker status. Low LC metabolism was most pronounced in those with evidence of underlying AD pathologic change and cognitive impairment. Furthermore, lower LC metabolism predicted greater CSF‐based p‐tau accumulation and declining cognition over time, providing an early marker of AD progression. [ABSTRACT FROM AUTHOR]

Background: The locus coeruleus (LC) is one of the first regions to accumulate tau in Alzheimer's disease (AD). As the disease progresses, tau in the LC has been related to increasing allocortical tau. Recent autopsy work reported that LC neurodegeneration correlated with parietal amyloid, suggesting that the LC may impact both Ab and tau, but with regionally varying contributions. We investigated whether cross‐sectional and longitudinal relationships between in vivo LC integrity and regional tau or Ab are uniquely determined by one pathology or exhibit shared vulnerabilities. Method: 213 individuals from the Harvard Aging Brain Study (mean age:71.6 years, 58% female; 11% cognitively impaired; Figure 1) who underwent 3T LC‐MRI, Ab‐ and tau‐PET imaging were included. Of these, 62 individuals received a second MRI and PET session. For the LC, we extracted the 5 highest normalized intensity voxels. PET‐data was referenced to cerebellar gray and partial volume corrected. Linear regressions associated LC integrity to tau or Ab and variance contributions were quantified. Mixed effects models examined LC changes to changes in tau or Ab. Mediation analyses examined whether local Ab mediated relationships between LC integrity and local tau. Analyses were adjusted for age, sex and multiple comparisons using FDR‐correction. Result: LC integrity was negatively associated with medial‐lateral temporal tau, and widespread Ab. Multivariable analyses demonstrated that LC integrity associated uniquely with tau in medial temporal lobe (MTL) regions and with Ab in frontoparietal regions. LC integrity was associated with both tau and Ab in inferior temporal (IT) and posterior cingulate cortices, and mediation analyses showed that LC integrity – tau associations in these regions were Ab‐mediated (Figure 2). Longitudinal analyses revealed stronger local associations between LC integrity and tau changes, compared to Ab. LC integrity changes were uniquely associated with tau changes in MTL, but longitudinal LC integrity‐IT tau correlations were mediated by local Ab (Figure 3). Conclusion: The LC may have anatomically distinct cortical tau and Ab‐pathways in AD, with MTL correlations being almost uniquely tau‐related, frontoparietal associations uniquely Ab‐related and lateral temporal regions showing Ab‐mediated tau accumulation. Potential underlying mechanisms can include synaptic plasticity alterations, glial activation or neuronal hyperactivation. [ABSTRACT FROM AUTHOR]

Background: 7T MRI allows implementation of high‐resolution quantitative susceptibility mapping (QSM) which can be used to assess the cerebral microvascular/hypoperfusion pathogenic hypothesis in Alzheimer's disease (AD) and in the post‐SARS‐CoV2 setting to explore if there may be similarities in brain changes in these two conditions. The magnetic susceptibility (reflective of tissue iron) of hippocampal subfields was assessed in patients with 1) CSF‐Amyloidß‐status AD; 2) mild COVID‐19 over 6 months previously (Cv); 3) severe COVID‐19 with ICU admission >6 months previously (ICU‐Cv); and 4) age‐matched healthy controls (HC). Methods: 33 participants (10 HC, 14 Cv, 9 ICU‐Cv) for the COVID study and 24 participants (14 AD, 11HC) from AD patients with confirmed CSF‐Amyloidß levels, aged 42‐79, were scanned on a 7T scanner with the following protocol T2* GRE for the AD groups (0.7×0.7×0.7mm3,TE/TR = 20/31ms) and T2* multi‐echo GRE for the Cv groups (0.38×0.38×0.75mm3,TE1/TE2/TR = 8.2/18.4/24ms). Data were processed using QSMbox1 to produce susceptibility maps. PSIR (0.55×0.55×0.55mm3,TE/TR = 3.1/6.9ms) and T2‐weighted FSE (0.38×0.39×1.5mm3, TE/TR = 117/5900ms) images were used to segment the hippocampal subfields (Figure 1)2. One‐way analyses were used in SPSS for comparison between the HC group and each of the Cv, ICU‐Cv, and AD groups. Results: There was no change in susceptibility for the whole hippocampus between AD and HC, but there was a significant difference for the DG subfield (p = 0.045), Figure 2. For COVID patients, there was a significant difference between the susceptibility of the whole hippocampus in the ICU‐Cv group (p = 0.035), but not in the Cv group (p = 0.788), Figure 3, compared to HC. In subfield analyses there was a trend towards decreased susceptibility in the subiculum, an area known to be affected by Alzheimer pathology, in the ICU‐Cv group (p = 0.032) compared to HC. Conclusion: The lower susceptibility within the hippocampal structures observed in AD and ICU‐Cv patients may reflect less iron either from cerebral hypoperfusion or vascular injuries in AD or additionally result from sequelae of hypoxaemia in ICU‐Cv. The study provides preliminary evidence of detectable in vivo microstructural changes in hippocampal subfields in both AD and post‐COVID settings. Funding: Medical Research Council, UK (MR/T005580/1); National Institute of Health, USA (1R56AG074467‐01) References: 1. https://doi.org/10.1016/j.neuroimage.2018.07.065 2. https://doi.org/10.1002/hbm.22627 [ABSTRACT FROM AUTHOR]

Background: Despite considerable advances in beta amyloid (Aβ)‐PET imaging over the last decade, the standard approach of estimating the average neocortical Aβ burden remains largely unchanged. However, as research and clinical trials increasingly shift earlier in the disease process, measures of how far Aβ has spread throughout the cortex (spatial extent) may prove more sensitive than average neocortical magnitude for detecting and quantifying early Aβ deposits and their association with future tau proliferation and cognitive decline. Method: Clinically‐normal individuals (n = 214) were included from the Harvard Aging Brain Study with longitudinal PIB‐PET (2‐4 scans, median = 4.7±2.7 years) and annual cognitive data (median = 5.2±2 years), as well as a subset (n = 181) with flortaucipir (FTP)‐PET. Spatial extent (EXT) was computed as the number of cortical ROIs (n = 62, Desikan atlas) above their ROI‐specific threshold for Aβ positivity. EXT was compared to a traditional mean neocortical DVR using logistic growth modeling. Receiver Operator Characteristic (ROC) curve analyses evaluated EXT's ability to identify baseline PIB‐ individuals (<1.19DVR/24CL) who progressed to PIB+ at 3‐year follow‐up. Linear Mixed Effects (LME) modeling assessed baseline EXT (or DVR) as a predictor of increasing inferior temporal tau (IT FTP SUVR) and cognitive decline on the Preclinical Alzheimer's Cognitive Composite (PACC). Result: EXT begins rising below the neocortical DVR threshold (Figure1A), reaching a maximal growth rate of 14 ROIs per 0.1 DVR increase (∼10CL) and plateauing as full cortical EXT is achieved starting at ∼1.5DVR/68CL. A 3‐ROI EXT threshold predicts progression from PIB‐ to PIB+ in 3 years (AUC =.97, SE =.82,SP =.97), outperforming neocortical DVR (AUC =.92, SE =.65,SP =.94, Figure1B). EXT provides a stronger biomarker of Aβ change than DVR (lower coefficient of variation, Figure2) across the Aβ continuum due to its low variance, even after EXT has plateaued. Baseline EXT is also a stronger predictor of increases in IT FTP SUVR (Figure 3A, η2EXT =.25, η2DVR =.20) and PACC decline (Figure3B, η2EXT =.28, η2DVR =.22). Conclusion: By describing the spread of Aβ throughout the cortex rather than average neocortical Aβ burden, spatial extent provides a more sensitive measure of Aβ at early, preclinical stages of AD that may improve design of AD prevention trials and open new avenues for research into AD pathogenesis. [ABSTRACT FROM AUTHOR]

Background: Topographical staging of tauopathy in preclinical Alzheimer's disease (AD) posits early deposition in the entorhinal region (medial temporal lobe; MTL) followed by spread to adjacent neocortical (NEO) regions. Predictive models support a cascade of tau deposition as baseline MTL tau robustly predicts NEO tau accumulation. In clinically normal (CN) Ab+, elevated tau in MTL and NEO is associated with accelerated progression to dementia relative to those with only elevated MTL tau. The frequency and profile of neocortical tau resistance (defined as MTL+/NEO‐) remain unclear. Method: We defined tau resistance using two longitudinal datasets. To compare directly across MTL (entorhinal/amygdala/parahippocampal) and NEO (inferior‐temporal/fusiform/middle‐temporal/inferior‐parietal) composites, we averaged regional SUVrs within each composite then normed them to independent samples of younger CNAb‐ adults. We selected CN participants from ADNI and HABS with ≥2 time‐points of Flortaucipir‐PET (nHABS = 200; nADNI = 149; cerebellar‐greyreference). We extracted slopes from a linear mixed model covarying for age, APOEe4 and Abstatus. We compared baseline MTLnormed values with slopes in a NEOnormed. We defined tau resistance as those with elevated baseline MTLnormed tau‐PET but no change in NEOnormed tau‐PET over time. We used non‐parametric group comparisons to determine differences in a range of variables (Table1). Result: We found a poor to moderate correlation between baseline MTLnormed values and adjusted NEOnormed slopes (rHABS = 0.12; rADNI = 0.36; Fig.1), with a non‐linear association at higher levels of MTLnormed. In HABS, tau resistance was identified in 13% (n = 10) of those with elevated MTLnormed. There was a trend for Black/African American individuals to be less tau resistant (Table.1). In ADNI, 26% were identified (n = 11), there was a trend toward less APOEe4 carriers in the resistant group. Although the ranges of change were different across the two cohorts, adjusted MTLnormed and NEOnormed slopes were similarly strongly correlated (rHABS = 0.69; rADNI = 0.74; Fig.2). Conclusion: Our preliminary findings suggest that resistance to neocortical tau in CN exists in up to a quarter of those with elevated MTL. Given the complexities with determining cross‐sectional and longitudinal tau‐PET thresholds, our approach to identifying MTL+ or NEO+ may be too conservative. Future work will validate tau resistance via cognitive decline and clinical progression. [ABSTRACT FROM AUTHOR]

Background: The Stress and Adversity Inventory (STRAIN) systematically assesses lifetime stressor exposure, but its validity has yet to be established in The Netherlands. This work aims to validate the Dutch translation of STRAIN and its association with key biological correlates: cortisol measures and the locus coeruleus (LC), a brainstem nucleus involved in stress responses and accumulating phosphorylated tau early in adulthood. Methods: Eighty‐seven cognitively unimpaired Dutch participants (31‐81 years; ∼45% female; Table 1) completed stress‐ and affective‐related questionnaires, cognitive testing, salivary and plasma cortisol measurements, and 7T‐MRI. The STRAIN was administered after translation into Dutch. LC MRI signal intensity was extracted from an MT‐TFL sequence by normalizing LC signal against the pontine tegmentum (reference region) and creating a sample‐specific LC template. Pearson correlation models assessed associations between STRAIN summary scores (total stressor count and severity), and stress‐related (MIA‐Anxiety, PSS, DASS‐Stress, NEO PI‐R‐Neuroticism), affective (DASS‐Anxiety‐Depression, HDRS), cognitive (WLTDR, LDST, STR3), stress‐related biological (salivary‐, plasma cortisol), and LC measures (Table 1). HDRS models employed Spearman correlations. Age and sex were included as covariates, additionally including education for cognitive measures. Analyses were FDR‐corrected and conducted based on complete cases. Results: Overall, the sample had relatively low lifetime stressor counts (M = 14.08; Figure 1A). When assessing concurrent validity, the STRAIN was positively associated with other stress measures (r =.21,.36; pFDR<.05; Figure 1B,C). Models including cortisol revealed that less stressor exposure and severity were associated with higher cortisol levels (r = ‐.22,‐.23; pFDR<.05). Affective and cognitive measures were investigated for discriminant validity. STRAIN was not associated with DASS, but was positively associated with HDRS (r =.17,.28; pFDR<.05). Generally, greater stressor exposure was related to poorer cognitive performance (r =.13,‐.19; pFDR<.05), except for a positive association between WLTDR and stressor severity (r =.18; pFDR<.05). No significant associations were found with LC MRI signal intensity (Figure 1B). Conclusion: This study demonstrates preliminary good concurrent and discriminative validity for the Dutch STRAIN translation, even among individuals with low‐to‐moderate self‐reported stress levels. We found no direct associations between LC integrity and lifetime stressors. Future research will investigate associations with other STRAIN domains and potential moderating effects of Alzheimer's Disease‐related biomarkers on the relationship between cumulative stressor exposure and LC integrity. [ABSTRACT FROM AUTHOR]

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]

Background: Altered 24‐h rest‐activity rhythms have been previously associated with higher burden of Alzheimer's disease (AD) pathological hallmarks, as measured with positron emission tomography or cerebrospinal fluid collection. Recent developments in fluid biomarkers now allow to non‐invasively measure levels of AD‐related pathology in the plasma. Here, we examined whether actigraphy‐derived 24‐h rest‐activity patterns could be linked to plasma markers of neurodegeneration and AD pathology in a sample of cognitively unimpaired participants. Method: Ninety‐three asymptomatic individuals (mean age = 59.73 ± 13.74 y., range = 30 – 85 y., 48 females, Table 1) underwent 10 days of actigraphic recordings, and blood drawing. Standard non‐parametric indices of 24‐h rest‐activity rhythm fragmentation (intradaily variability, IV) and stability (interdaily stability, IS) were extracted using the GGIR package (v2.8‐2) in R. Plasma concentrations of neurofilament light chain (NfL), glial fibrillary acidic protein (GFAP), 40 and 42 amino acid‐long amyloid‐β (Aβ40 and Aβ42, respectively), total tau, and phosphorylated tau at threonine 181 (pTau181) or threonine 231 (pTau231) were measured using Single molecule array technology. Result: Multiple linear regression models adjusted for age, sex, education, and body mass index, showed that worse rest‐activity rhythm fragmentation, as indicated by higher IV values, was strongly associated with elevated levels of plasma NfL (t = 4.53, p <.0001, Figure 1), but also with higher levels of plasma GFAP (t = 2.44, p =.02, Figure 2) and lower plasma Aβ42/40 ratio (t = ‐2.23, p =.03, Figure 2). Similarly, increased day‐to‐day rest‐activity rhythm stability, as indicated by higher IS values, was associated with lower levels of plasma NfL (t = ‐2.37, p =.02, Figure 1). By contrast, no significant relationships were found between IV or IS values and total tau or phosphorylated tau plasma biomarkers (all p >.27). Conclusion: These findings provide further evidence that disrupted rest‐activity patterns, in particular fragmentation of rest‐activity rhythms, may constitute a sensitive marker of incipient AD pathology and neurodegeneration in cognitively unimpaired individuals. Our study therefore highlights the clinical utility of monitoring rest‐activity patterns for improved detection of individuals at‐risk for neurodegenerative diseases, including AD. [ABSTRACT FROM AUTHOR]