Axonal diameter and density estimated with 7-Tesla hybrid diffusion imaging in transgenic Alzheimer rats

Diffusion-weighted MR imaging (DWI) is a powerful tool to study brain tissue microstructure. DWI is sensitive to subtle changes in the white matter (WM), and can provide insight into abnormal brain changes in diseases such as Alzheimer’s disease (AD). In this study, we used 7-Tesla hybrid diffusion imaging (HYDI) to scan 3 transgenic rats (line TgF344-AD; that model the full clinico-pathological spectrum of the human disease) ex vivo at 10, 15 and 24 months. We acquired 300 DWI volumes across 5 q-sampling shells (b=1000, 3000, 4000, 8000, 12000 s/mm^2). From the top three b-value shells with highest signal-to-noise ratios, we reconstructed markers of WM disease, including indices of axon density and diameter in the corpus callosum (CC) – directly quantifying processes that occur in AD. As expected, apparent anisotropy progressively decreased with age; there were also decreases in the intra- and extra-axonal MR signal along axons. Axonal diameters were larger in segments of the CC (splenium and body, but not genu), possibly indicating neuritic dystrophy – characterized by enlarged axons and dendrites as previously observed at the ultrastructural level (see Cohen et al., J. Neurosci. 2013). This was further supported by increases in MR signals trapped in glial cells, CSF and possibly other small compartments in WM structures. Finally, tractography detected fewer fibers in the CC at 10 versus 24 months of age. These novel findings offer great potential to provide technical and scientific insight into the biology of brain disease

Axonal diameter and density estimated with 7-Tesla hybrid diffusion imaging in transgenic Alzheimer rats

Diffusion-weighted MR imaging (DWI) is a powerful tool to study brain tissue microstructure. DWI is sensitive to subtle changes in the white matter (WM), and can provide insight into abnormal brain changes in diseases such as Alzheimer’s disease (AD). In this study, we used 7-Tesla hybrid diffusion imaging (HYDI) to scan 3 transgenic rats (line TgF344-AD; that model the full clinico-pathological spectrum of the human disease) ex vivo at 10, 15 and 24 months. We acquired 300 DWI volumes across 5 q-sampling shells (b=1000, 3000, 4000, 8000, 12000 s/mm^2). From the top three b-value shells with highest signal-to-noise ratios, we reconstructed markers of WM disease, including indices of axon density and diameter in the corpus callosum (CC) – directly quantifying processes that occur in AD. As expected, apparent anisotropy progressively decreased with age; there were also decreases in the intra- and extra-axonal MR signal along axons. Axonal diameters were larger in segments of the CC (splenium and body, but not genu), possibly indicating neuritic dystrophy – characterized by enlarged axons and dendrites as previously observed at the ultrastructural level (see Cohen et al., J. Neurosci. 2013). This was further supported by increases in MR signals trapped in glial cells, CSF and possibly other small compartments in WM structures. Finally, tractography detected fewer fibers in the CC at 10 versus 24 months of age. These novel findings offer great potential to provide technical and scientific insight into the biology of brain disease

Multi-Shell Hybrid Diffusion Imaging (HYDI) at 7 Tesla in TgF344-AD Transgenic Alzheimer Rats

Diffusion weighted imaging (DWI) is widely used to study microstructural characteristics of the brain. Diffusion tensor imaging (DTI) and high-angular resolution imaging (HARDI) are frequently used in radiology and neuroscience research but can be limited in describing the signal behavior in composite nerve fiber structures. Here, we developed and assessed the benefit of a comprehensive diffusion encoding scheme, known as hybrid diffusion imaging (HYDI), composed of 300 DWI volumes acquired at 7-Tesla with diffusion weightings at b = 1000, 3000, 4000, 8000 and 12000 s/mm^2 and applied it in transgenic Alzheimer rats (line TgF344-AD) that model the full clinico-pathological spectrum of the human disease. We studied and visualized the effects of the multiple concentric “shells” when computing three distinct anisotropy maps–fractional anisotropy (FA), generalized fractional anisotropy (GFA) and normalized quantitative anisotropy (NQA). We tested the added value of the multi-shell q-space sampling scheme, when reconstructing neural pathways using mathematical frameworks from DTI and q-ball imaging (QBI). We show a range of properties of HYDI, including lower apparent anisotropy when using high b-value shells in DTI-based reconstructions, and increases in apparent anisotropy in QBI-based reconstructions. Regardless of the reconstruction scheme, HYDI improves FA-, GFA- and NQA-aided tractography. HYDI may be valuable in human connectome projects and clinical research, as well as magnetic resonance research in experimental animals

Preventing Neurodegenerative Memory Loss in Hopfield Neuronal Networks Using Cerebral Organoids or External Microelectronics

Developing technologies have made significant progress towards linking the brain with brain-machine interfaces (BMIs) which have the potential to aid damaged brains to perform their original motor and cognitive functions. We consider the viability of such devices for mitigating the deleterious effects of memory loss that is induced by neurodegenerative diseases and/or traumatic brain injury (TBI). Our computational study considers the widely used Hopfield network, an autoassociative memory model in which neurons converge to a stable state pattern after receiving an input resembling the given memory. In this study, we connect an auxiliary network of neurons, which models the BMI device, to the original Hopfield network and train it to converge to its own auxiliary memory patterns. Injuries to the original Hopfield memory network, induced through neurodegeneration, for instance, can then be analyzed with the goal of evaluating the ability of the BMI to aid in memory retrieval tasks. Dense connectivity between the auxiliary and Hopfield networks is shown to promote robustness of memory retrieval tasks for both optimal and nonoptimal memory sets. Our computations estimate damage levels and parameter ranges for which full or partial memory recovery is achievable, providing a starting point for novel therapeutic strategies

DEVELOPMENT OF IMAGING MARKERS TO TRACK ALZHEIMER¿S DISEASE PROGRESSION IN HUMANS AND MOUSE MODELS

La Malattia di Alzheimer (AD) \ue8 la forma pi\uf9 comune di demenza nella popolazione anziana e affligge pi\uf9 35 milioni di persone nel mondo. Ad oggi, le uniche terapie approvate per la sua cura sono dirette a ridurre i sintomi. Lo sviluppo di nuovi farmaci \ue8 lungo e costoso. Il processo di scoperta \ue8 arduo in quanto i trial clinici coinvolgono un ampio campione di pazienti e implicano dei follow-up troppo lunghi. Inoltre il valore predittivo dei modelli sperimentali \ue8 limitato a causa della mancanza di marcatori omologhi nell\u2019uomo e nei modelli animali. Questo lavoro si inserisce in Pharmacog, un progetto europeo che vede la collaborazione di universit\ue0 ed industrie allo scopo di identificare biomarcatori affidabili e sensibili alla progressione di malattia in pazienti affetti da decadimento cognitivo lieve (MCI) e modelli animali di AD allo scopo di colmare il vuoto tra risultati clinici e preclinici. Nell\u2019uomo, i marcatori di neuroimmagine sono tra i pi\uf9 promettenti candidati nel tracciare la progressione di malattia. Innovazioni nelle tecniche di risonanza magnetica (MRI) rendono possibile l\u2019identificazione di marcatori omologhi nell\u2019uomo e nel topo. Prima dello studio di neuroimmagine nei pazienti MCI, \ue8 necessario verificare che eventuali cambiamenti individuati siano dovuti all\u2019effettiva progressione di malattia e non causati dalla variabilit\ue0 intra e tra i diversi scanner utilizzati nel progetto. Il primo scopo di questo lavoro \ue8 lo studio dei cambiamenti morfometrici e di diffusione in tre diversi modelli murini di Malattia Alzheimer (TASTPM, TauPS2APP e PDAPP da 3 a 22 mesi) tramite l\u2019utilizzo di tecniche MRI. A nove mesi abbiamo trovato una significativa riduzione rispetto ai controlli del volume del caudato-putamen e della corteccia frontale nei TASTPM e nei TauPS2APP (p< 0.001). L\u2019assottigliamento della corteccia entorinale era significativo alla stessa et\ue0 in tutte e tre i modelli (p< 0.001). Abbiamo inoltre individuato delle anormalit\ue0 dipendenti dall\u2019et\ue0 anche in diverse regione di sostanza bianca. Quelle pi\uf9 precoci erano nella commissura anteriore e nel corpo calloso dei TASTPM di 13 mesi (p< 0.001). I danni dei TASTPM sono associabili al pesante carico di amiloide ed alla marcata attivazione della glia e degli astrociti. Il secondo scopo dello studio \ue8 la valutazione e la comparazione della riproducibilit\ue0 di misure volumetriche e di spessore tra test e retest ottenute utilizzando due diversi metodi di processazione esistenti (Freesurfer sulla singola acquisizione o Freesurfer longitudinale). Inoltre abbiamo saggiato la riproducibilit\ue0 di un\u2019analisi per le immagini di diffusione messa a punto nel nostro laboratorio. A questo scopo ognuno degli otto centri europei coinvolti nel progetto e con diversi scanner a 3T ha arruolato un gruppo di 5 volontari sani e anziani sottoponendoli a 2 acquisizioni di risonanza ad almeno una settimana di distanza l\u2019una dall\u2019altra. Abbiamo trovato che la variabilit\ue0 intra e tra i diversi centri nei volumi estratti da queste acquisizioni era inferiore al 3% per le strutture pi\uf9 grandi (come il talamo) e minore del 6% per quelle pi\uf9 piccole (es. amigdala). La variabilit\ue0 degli spessori era meno del 6% e le variazioni dei parametri di diffusione erano prevalentemente nell\u2019intervallo del 2-3%. In conclusione, abbiamo identificato nei modelli analizzati dei marcatori di immagine sensibili alla progressione dell\u2019AD simili a quelli visti nell\u2019uomo e questo apre la strada al possibile utilizzo di una \u201cdistintiva collezione\u201d di marcatori murini di immagine nei trial clinici. I dati collezionati nella parte umana mostrano un pi\uf9 altra riproducibilit\ue0 dei risultati morfometrici ottenuti con l\u2019analisi longitudinale rispetto a quella sulla singola acquisizione (p< 0.01). Infine, abbiamo dimostrato che l\u2019analisi delle immagini di diffusione messa a punto nel nostro laboratorio d\ue0 risultati ugualmente riproducibili a quelli riportati in letteratura.Alzheimer\u2019s disease (AD) is the most common form of dementia in elderly population, affecting more than 35 million people worldwide. To date, the only approved therapies for AD focus on symptomatic relief. The development of new therapeutic agents is time consuming and costly. Drug discovery process is arduous because clinical trials are currently involving too wide sample of patients and long follow-up. Moreover, the predicting value of experimental models used nowadays is limited due to the lack of homologous markers in humans and animals. This work is a branch of Pharmacog, an industry-academic European project aimed at identifying reliable biomarkers that are sensitive to disease progression in patients with Mild Cognitive Impairment (MCI) and in AD animal models in order to bridge the gap between preclinical and clinical outcomes. Human neuroimaging markers are among the most promising candidates to track disease progression. In addition, advanced magnetic resonance imaging (MRI) allow the identification of homologous biomarkers in humans and mice. Prior to investigate neuroimaging biomarkers on MCI patients, we have to test that there is no significant effect of within and across MRI sites variability on brain AD-related longitudinal changes. The first aim of this work is the study of the morphometric and diffusion changes in three different AD mouse model (TASTPM, TauPS2APP and PDAPP from 3 to 22 months of age) through MRI. We found significant volume reduction starting at 9 months in the caudate-putamen and frontal cortex of TASTPM and TauPS2APP (p< 0.001) compared to non transgenic mice. The decrease in the enthorinal cortex thickness was significantly lower in all the strains (p< 0.001). We also found age-related diffusion abnormalities in different white matter regions of TASTPM. The earlier changes were found in the corpus callosum and anterior commissure of 13 months old mice (p< 0.001). In TASTPM, deficits detected with MRI are related to heavy amyloid pathology, marked gliosis and astrocitosys. The second aim of this study is the evaluation and comparison of test-retest reproducibility of brain volumes and thicknesses by two existing Freesurfer pipelines (longitudinal and cross-sectional). Moreover, we assessed the reliability of a diffusion pipeline developed in our lab. Eight different 3T MRI sites in Europe enrolled a group of 5 healthy elderly subjects scanned twice at least a week apart. We found that the within and across sites variability of volumes was less than 3% for larger brain structures (such as thalamus) and less than 6% for smaller regions (i.e., hippocampus). The thickness variability was less than 6% and diffusion indices variations were mostly within the range 2-3%. In conclusion, the present data identify imaging biomarkers of disease progression in mice similar to that seen in humans and pave the way of a murine \u201cimaging signature\u201d usefulness in clinical trials. Human data show significantly higher reproducibility of brain morphometry using the longitudinal pipeline than using the cross-sectional one (p< 0.01). Finally, we demonstrated that the reliability of the analysis of brain diffusion we implemented in our lab is comparable to data reported in the literature

Monitoring Alzheimer's disease in transgenic mice with ultra high field magnetic resonance imaging

While aging remains one of the most significant risk factors for development of Alzheimer disease (AD), increasing evidence strongly points to the potential roles of cerebrovascular and white matter abnormalities in the disease development. A better understanding of the manner in which these abnormalities contribute to disease progression can be achieved by in vivo characterization of AD related pathologies. To this end, MR based techniques serve as effective non-invasive tools to longitudinally monitor changes in AD brain. In this thesis, a variety of MR based techniques were optimized and employed to longitudinally monitor the AD progression in transgenic mouse models of the disease at 9.4T and 17.6T. In Chapter 2, age-dependent blood flow alterations were examined in a Tg2576 mouse model of Alzheimer's disease using MR angiography at 17.6T. AD is linked to abnormalities in the vascular system. In Chapter 3, in vivo T2 changes were longitudinally monitored in the corpus callosum, of the Tg2576 mice. In Chapter 4, age-dependent regional brain T1 and T2 changes in healty mice were established at 17.6T. In vivo imaging of these mouse models at ultra-high magnetic field strengths can permit a better understanding of the underlying cellular mechanism of AD.The Centre for Medical Systems Biology (CMSB), Internationale Stichting Alzheimer Onderzoek and Alzheimer NederlandSolid state NMR/Biophysical Organic Chemistr

Understanding the brain through its spatial structure

The spatial location of cells in neural tissue can be easily extracted from many imaging modalities, but the information contained in spatial relationships between cells is seldom utilized. This is because of a lack of recognition of the importance of spatial relationships to some aspects of brain function, and the reflection in spatial statistics of other types of information. The mathematical tools necessary to describe spatial relationships are also unknown to many neuroscientists, and biologists in general. We analyze two cases, and show that spatial relationships can be used to understand the role of a particular type of cell, the astrocyte, in Alzheimer's disease, and that the geometry of axons in the brain's white matter sheds light on the process of establishing connectivity between areas of the brain. Astrocytes provide nutrients for neuronal metabolism, and regulate the chemical environment of the brain, activities that require manipulation of spatial distributions (of neurotransmitters, for example). We first show, through the use of a correlation function, that inter-astrocyte forces determine the size of independent regulatory domains in the cortex. By examining the spatial distribution of astrocytes in a mouse model of Alzheimer's Disease, we determine that astrocytes are not actively transported to fight the disease, as was previously thought. The paths axons take through the white matter determine which parts of the brain are connected, and how quickly signals are transmitted. The rules that determine these paths (i.e. shortest distance) are currently unknown. By measurement of axon orientation distributions using three-point correlation functions and the statistics of axon turning and branching, we reveal that axons are restricted to growth in three directions, like a taxicab traversing city blocks, albeit in three-dimensions. We show how geometric restrictions at the small scale are related to large-scale trajectories. Finally we discuss the implications of this finding for experimental and theoretical connectomics

The Development and Validation of a Molecular Imaging Probe Targeted to Cathepsin D for the In-vivo Detection of Alzheimer Disease

Background: Currently there is no widely accepted test to diagnose AD. The involvement of the lysosomal system in Alzheimer’s disease (AD) progression provides an opportunity to develop associated biomarkers. The lysosomal enzyme Cathepsin D (CatD) has been shown to be over-expressed in the AD brain before clinical onset. We have developed a dual modality contrast agent (CA) to detect CatD activity which consists of an HIV-1 Tat Cell Penetrating Peptide (CPP) conjugated to a CatD cleavage sequence and two imaging moieties consisting of a fluorescently- tagged probe and a DOTA cage for chelating Gallium-68. The purpose of this work was to validate CatD as an AD biomarker across multiple AD disease models and to test our novel CA in-vivo by means of optical near infra-red (NIR) fluorescence imaging and positron emission tomography (PET). Methods: Three transgenic (Tg) mouse AD model strains were tested for CatD expression by Western blot and immunohistochemistry analysis. The chosen mouse line (5XFAD) and controls were imaged at 5 and 12 months of age using an eXplore Optix scanner (GE Healthcare, Milwaukee, WI, USA). Next, mice at 2, 6 and 9 months of age were tested using an Inveon microPET system (Siemens Medical Solutions, Knoxville TN, USA) using a 68Ga-labeled CatD targeted CA. Results: All 3 AD mice demonstrated an elevation of CatD expression in parallel with AD pathology. The 5XFAD had the highest levels of CatD, making it the best mouse model to study CatD upregulation. The rate of the NIR CatD Targeted CA washout was significantly slower in the 5XFAD mice (