Diffusion-weighted magnetic resonance imaging detection of basal forebrain cholinergic degeneration in a mouse model

Loss of basal forebrain cholinergic neurons is an early and key feature of Alzheimer's disease, and magnetic resonance imaging (MRI) volumetric measurement of the basal forebrain has recently gained attention as a potential diagnostic tool for this condition. The aim of this study was to determine whether loss of basal forebrain cholinergic neurons underpins changes which can be detected through diffusion MRI using diffusion tensor imaging (DTI) and probabilistic tractography in a mouse model. To cause selective basal forebrain cholinergic degeneration, the toxin saporin conjugated to a p75 neurotrophin receptor antibody (mu-p75-SAP) was used. This resulted in similar to 25% loss of the basal forebrain cholinergic neurons and significant loss of terminal cholinergic projections in the hippocampus, as determined by histology. To test whether lesion of cholinergic neurons caused basal forebrain, hippocampal, or whole brain atrophy, we performed manual segmentation analysis, which revealed no significant atrophy in lesioned animals compared to controls (Rb-IgG-SAP). However, analysis by DTI of the basal forebrain area revealed a significant increase in fractional anisotropy (FA; + 7.7%), mean diffusivity (MD; + 6.1%), axial diffusivity (AD; + 8.5%) and radial diffusivity (RD; +4.0%) in lesioned mice compared to control animals. These parameters strongly inversely correlated with the number of choline acetyl transferase-positive neurons, with FA showing the greatest association (r(2) = 0.72), followed by MD (r(2) = 0.64), AD (r(2) = 0.64) and RD (r(2) = 0.61). Moreover, probabilistic tractography analysis of the septo-hippocampal tracts originating from the basal forebrain revealed an increase in streamline MD (+5.1%) and RD (+4.3%) in lesioned mice. This study illustrates that moderate loss of basal forebrain cholinergic neurons (representing only a minor proportion of all septo-hippocampal axons) can be detected by measuring either DTI parameters of the basal forebrain nuclei or tractography parameters of the basal forebrain tracts. These findings provide increased support for using DTI and probabilistic tractography as non-invasive tools for diagnosing and/or monitoring the progression of conditions affecting the integrity of the basal forebrain cholinergic system in humans, including Alzheimer's disease. Crown Copyright (C) 2012 Published by Elsevier Inc. All rights reserved

Live imaging of neuronal connections by magnetic resonance: Robust transport in the hippocampal–septal memory circuit in a mouse model of Down syndrome

Connections from hippocampus to septal nuclei have been implicated in memory loss and the cognitive impairment in Down syndrome (DS). We trace these connections in living mice by Mn^(2+) enhanced 3D MRI and compare normal with a trisomic mouse model of DS, Ts65Dn. After injection of 4 nl of 200 mM Mn^(2+) into the right hippocampus, Mn^(2+) enhanced circuitry was imaged at 0.5, 6, and 24 h in each of 13 different mice by high resolution MRI to detect dynamic changes in signal over time. The pattern of Mn^(2+) enhanced signal in vivo correlated with the histologic pattern in fixed brains of co-injected 3kD rhodamine–dextran–amine, a classic tracer. Statistical parametric mapping comparing intensity changes between different time points revealed that the dynamics of Mn2+ transport in this pathway were surprisingly more robust in DS mice than in littermate controls, with statistically significant intensity changes in DS appearing at earlier time points along expected pathways. This supports reciprocal alterations of transport in the hippocampal-forebrain circuit as being implicated in DS and argues against a general failure of transport. This is the first examination of in vivo transport dynamics in this pathway and the first report of elevated transport in DS

Basal forebrain volume reliably predicts the cortical spread of Alzheimer\u27s degeneration

© The Author(s) (2020). Published by Oxford University Press on behalf of the Guarantors of Brain. Alzheimer\u27s disease neurodegeneration is thought to spread across anatomically and functionally connected brain regions. However, the precise sequence of spread remains ambiguous. The prevailing model used to guide in vivo human neuroimaging and non-human animal research assumes that Alzheimer\u27s degeneration starts in the entorhinal cortices, before spreading to the temporoparietal cortex. Challenging this model, we previously provided evidence that in vivo markers of neurodegeneration within the nucleus basalis of Meynert (NbM), a subregion of the basal forebrain heavily populated by cortically projecting cholinergic neurons, precedes and predicts entorhinal degeneration. There have been few systematic attempts at directly comparing staging models using in vivo longitudinal biomarker data, and none to our knowledge testing if comparative evidence generalizes across independent samples. Here we addressed the sequence of pathological staging in Alzheimer\u27s disease using two independent samples of the Alzheimer\u27s Disease Neuroimaging Initiative (n1 = 284; n2 = 553) with harmonized CSF assays of amyloid-b and hyperphosphorylated tau (pTau), and longitudinal structural MRI data over 2 years. We derived measures of grey matter degeneration in a priori NbM and the entorhinal cortical regions of interest. To examine the spreading of degeneration, we used a predictive modelling strategy that tests whether baseline grey matter volume in a seed region accounts for longitudinal change in a target region. We demonstrated that predictive spread favoured the NbM!entorhinal over the entorhinal!NbM model. This evidence generalized across the independent samples. We also showed that CSF concentrations of pTau/amyloid-b moderated the observed predictive relationship, consistent with evidence in rodent models of an underlying trans-synaptic mechanism of pathophysiological spread. The moderating effect of CSF was robust to additional factors, including clinical diagnosis. We then applied our predictive modelling strategy to an exploratory whole-brain voxel-wise analysis to examine the spatial specificity of the NbM!entorhinal model. We found that smaller baseline NbM volumes predicted greater degeneration in localized regions of the entorhinal and perirhinal cortices. By contrast, smaller baseline entorhinal volumes predicted degeneration in the medial temporal cortex, recapitulating a prior influential staging model. Our findings suggest that degeneration of the basal forebrain cholinergic projection system is a robust and reliable upstream event of entorhinal and neocortical degeneration, calling into question a prevailing view of Alzheimer\u27s disease pathogenesis

The effect of cholinergic loss on the structure and function of the neurovascular unit: implications for cerebral amyloid angiopathy

Innervation of cerebral blood vessels is important for the regulation of vascular tone and adequate cerebral perfusion. Alzheimer’s disease (AD) is characterised by a loss of cholinergic innervation of the neurovascular unit (NVU). This loss may contribute not only to inefficient cerebrovascular perfusion, but also to the failure of removal of amyloid-β (Aβ), leading to its accumulation as cerebral amyloid angiopathy (CAA). This hypothesis was tested by mimicking loss of cholinergic innervation of the cortex and hippocampus in adult male C57BL/6 mice using the immunospecific toxin mu-p75-saporin. Using quadruple labelling immunohistochemistry and 3D reconstructions of the NVU, loss of perivascular cholinergic innervation was observed at the smooth muscle cells and basement membrane in the cortex and hippocampus, with additional perivascular loss at the astrocyte endfeet in the cortex. Arterial spin labelling fMRI revealed no differences in resting cerebral blood flow between control and saporin-treated mice in either the cortex or hippocampus. However, denervated vessels in the cortex, but not the hippocampus, failed to respond to pharmacological stimulation of endothelial nitric oxide synthase (eNOS). Further studies revealed a decrease in eNOS expression in the cortex, but an increase in eNOS expression and activity in the hippocampus following loss of cholinergic input. No differences were noted between control and saporin-treated mice in the efficiency of removal of Aβ along perivascular basement membranes in either the cortex or hippocampus. Treatment of TetO-APPSweInd mice with saporin resulted in a trend towards higher CAA load. These data suggest that there are innate differences between NVUs of the cortex and the hippocampus and a difference in their functional response to loss of cholinergic input. Loss of cholinergic input at the NVU may contribute to accumulation of Aβ; however, this likely requires additional pathological factors for CAA to develop

Multimodal connectivity of the human basal forebrain

The cholinergic innervation of the cortex originates from neurons in the basal forebrain (BF) and plays a crucial role in cognitive processing. However, it is unclear how the organization of BF cholinergic neurons in the human brain is related to their functional and structural integration with the cortex. To address this, we have used high-resolution 7 Tesla diffusion and resting-state functional MRI to examine multimodal forebrain cholinergic connectivity with the neocortex in humans. Discrete parcellation analyses revealed that structural and functional parcellation broadly differentiated the anteromedial from posterolateral nuclei of BF. Next, we used gradient estimation to capture more fine-grained connectivity profile of the BF-cortical projectome and found moving from anteromedial to posterolateral BF, structural and functional gradients became progressively detethered, with the most pronounced dissimilarity localized in the nucleus basalis of Meynert (NbM). Additionally, functional but not structural connectivity with the BF grew stronger at shorter geodesic distances, with weakly myelinated transmodal cortical areas most strongly expressing this divergence. Moreover, [18F] FEOBV PET imaging was used to demonstrate that these transmodal cortical areas are also among the most densely innervated regions. This intrinsic BF cholinergic connectivity map of cortex was compared with meta-analytic connectivity map of cholinergic modulation on attention, demonstrating that patterns of brain activity evoked by directed attention are altered by pharmacological activation of acetylcholine (ACh) compared to placebo and these patterns spatially overlap with the intrinsic BF cholinergic connectivity map. Altogether, our findings provide new insights into how cholinergic signaling is organized in the human brain

Cholinergic white matter pathways along the Alzheimer's disease continuum

Nemy et al. investigate cholinergic white matter projections along the Alzheimer's disease continuum. They show that alterations are already present in individuals with subjective cognitive decline, preceding the more widespread alterations seen in mild cognitive impairment and Alzheimer's disease dementia. Previous studies have shown that the cholinergic nucleus basalis of Meynert and its white matter projections are affected in Alzheimer's disease dementia and mild cognitive impairment. However, it is still unknown whether these alterations can be found in individuals with subjective cognitive decline, and whether they are more pronounced than changes found in conventional brain volumetric measurements. To address these questions, we investigated microstructural alterations of two major cholinergic pathways in individuals along the Alzheimer's disease continuum using an in vivo model of the human cholinergic system based on neuroimaging. We included 402 participants (52 Alzheimer's disease, 66 mild cognitive impairment, 172 subjective cognitive decline and 112 healthy controls) from the Deutsches Zentrum für Neurodegenerative Erkrankungen Longitudinal Cognitive Impairment and Dementia Study. We modelled the cholinergic white matter pathways with an enhanced diffusion neuroimaging pipeline that included probabilistic fibre-tracking methods and prior anatomical knowledge. The integrity of the cholinergic white matter pathways was compared between stages of the Alzheimer's disease continuum, in the whole cohort and in a CSF amyloid-beta stratified subsample. The discriminative power of the integrity of the pathways was compared to the conventional volumetric measures of hippocampus and nucleus basalis of Meynert, using a receiver operating characteristics analysis. A multivariate model was used to investigate the role of these pathways in relation to cognitive performance. We found that the integrity of the cholinergic white matter pathways was significantly reduced in all stages of the Alzheimer's disease continuum, including individuals with subjective cognitive decline. The differences involved posterior cholinergic white matter in the subjective cognitive decline stage and extended to anterior frontal white matter in mild cognitive impairment and Alzheimer's disease dementia stages. Both cholinergic pathways and conventional volumetric measures showed higher predictive power in the more advanced stages of the disease, i.e. mild cognitive impairment and Alzheimer's disease dementia. In contrast, the integrity of cholinergic pathways was more informative in distinguishing subjective cognitive decline from healthy controls, as compared with the volumetric measures. The multivariate model revealed a moderate contribution of the cholinergic white matter pathways but not of volumetric measures towards memory tests in the subjective cognitive decline and mild cognitive impairment stages. In conclusion, we demonstrated that cholinergic white matter pathways are altered already in subjective cognitive decline individuals, preceding the more widespread alterations found in mild cognitive impairment and Alzheimer's disease. The integrity of the cholinergic pathways identified the early stages of Alzheimer's disease better than conventional volumetric measures such as hippocampal volume or volume of cholinergic nucleus basalis of Meynert

Longitudinal Alzheimer\u27s Degeneration Reflects the Spatial Topography of Cholinergic Basal Forebrain Projections

© 2018 The Author(s) The cholinergic neurons of the basal forebrain (BF) provide virtually all of the brain\u27s cortical and amygdalar cholinergic input. They are particularly vulnerable to neuropathology in early Alzheimer\u27s disease (AD) and may trigger the emergence of neuropathology in their cortico-amygdalar projection system through cholinergic denervation and trans-synaptic spreading of misfolded proteins. We examined whether longitudinal degeneration within the BF can explain longitudinal cortico-amygdalar degeneration in older human adults with abnormal cerebrospinal fluid biomarkers of AD neuropathology. We focused on two BF subregions, which are known to innervate cortico-amygdalar regions via two distinct macroscopic cholinergic projections. To further assess whether structural degeneration of these regions in AD reflects cholinergic denervation, we used the [ 18 F] FEOBV radiotracer, which binds to cortico-amygdalar cholinergic terminals. We found that the two BF subregions explain spatially distinct patterns of cortico-amygdalar degeneration, which closely reflect their cholinergic projections, and overlap with [ 18 F] FEOBV indices of cholinergic denervation

Investigating the relationship between cholinergic system integrity and Parkinson’s disease symptoms using MRI and EEG

Cholinergic cells of the basal forebrain (cBF) and pedunculopontine nucleus (PPN) are implicated in Parkinson’s disease (PD), but current understanding of their role in PD symptomology is limited. Neuropathological and recent in vivo imaging research implies that cBF and PPN degeneration is associated with PD cognitive and mobility impairments. There remains a need to identify and validate widely accessible markers of cholinergic system degeneration to better understand its contribution to these symptoms. The aim of this thesis was to investigate how structural changes in the cBF and PPN relate to cortical activity and cognitive and mobility performance in people with PD, people with mild cognitive impairment (MCI), and healthy age-matched controls. T1 and diffusion-weighted images were used in combination with stereotactic maps of the cBF and PPN to extract volumetric and diffusivity metrics from these regions as in vivo surrogate markers of structural integrity. These structural measures were assessed for their relationship with resting-state EEG, and cognitive and functional mobility performance. People with PD showed reduced cBF volumes compared to healthy controls, and elevated PPN diffusivity compared to people with MCI. Subregional cBF volumes correlated with EEG changes in the theta-alpha range in people with PD and people with MCI. Volume loss in the cBF was also shown to mediate the relationship between executive function and Timed Up and Go dual-task performance in people with PD. PPN diffusivity metrics demonstrated correlations with cognitive performance and EEG changes in the alpha range in people with PD, and in the beta-gamma range in people with MCI. Cortical activity measured with EEG may hold physiological relevance for structural changes occurring in the cBF and PPN. Volumetric loss in the cBF may impair the attentional-executive control of mobility functions. Elevated PPN diffusivity may impair attentional performance during tasks that require sensorimotor integration

Measuring cortical connectivity in Alzheimer's disease as a brain neural network pathology: Toward clinical applications

Objectives: The objective was to review the literature on diffusion tensor imaging as well as resting-state functional magnetic resonance imaging and electroencephalography (EEG) to unveil neuroanatomical and neurophysiological substrates of Alzheimer’s disease (AD) as a brain neural network pathology affecting structural and functional cortical connectivity underlying human cognition. Methods: We reviewed papers registered in PubMed and other scientific repositories on the use of these techniques in amnesic mild cognitive impairment (MCI) and clinically mild AD dementia patients compared to cognitively intact elderly individuals (Controls). Results: Hundreds of peer-reviewed (cross-sectional and longitudinal) papers have shown in patients with MCI and mild AD compared to Controls (1) impairment of callosal (splenium), thalamic, and anterior–posterior white matter bundles; (2) reduced correlation of resting state blood oxygen level-dependent activity across several intrinsic brain circuits including default mode and attention-related networks; and (3) abnormal power and functional coupling of resting state cortical EEG rhythms. Clinical applications of these measures are still limited. Conclusions: Structural and functional (in vivo) cortical connectivity measures represent a reliable marker of cerebral reserve capacity and should be used to predict and monitor the evolution of AD and its relative impact on cognitive domains in pre-clinical, prodromal, and dementia stages of AD. (JINS, 2016, 22, 138–163