Hippocampal metabolic subregions and networks: Behavioral, molecular, and pathological aging profiles.

INTRODUCTION: Hippocampal local and network dysfunction is the hallmark of Alzheimer's disease (AD). METHODS: We characterized the spatial patterns of hippocampus differentiation based on brain co-metabolism in healthy elderly participants and demonstrated their relevance to study local metabolic changes and associated dysfunction in pathological aging. RESULTS: The hippocampus can be differentiated into anterior/posterior and dorsal cornu ammonis (CA)/ventral (subiculum) subregions. While anterior/posterior CA show co-metabolism with different regions of the subcortical limbic networks, the anterior/posterior subiculum are parts of cortical networks supporting object-centered memory and higher cognitive demands, respectively. Both networks show relationships with the spatial patterns of gene expression pertaining to cell energy metabolism and AD's process. Finally, while local metabolism is generally lower in posterior regions, the anterior-posterior imbalance is maximal in late mild cognitive impairment with the anterior subiculum being relatively preserved. DISCUSSION: Future studies should consider bidimensional hippocampal differentiation and in particular the posterior subicular region to better understand pathological aging.

Age-related differences in the neural correlates of vivid remembering.

When recollecting events, older adults typically report similar memory vividness levels as young adults, while they actually retrieve fewer episodic details. This suggests that young and older adults use episodic details differently to calibrate their vividness judgements. Capitalizing on the idea that remembering reactivates brain regions that initially processed details at encoding, the current fMRI study sought to examine these age-related changes in the basis of vivid recollection. At encoding, young and older adults saw pictures associated with labels and these labels were then used as retrieval cues for recalling the associated pictures and making memory vividness judgments. Results showed that highly vivid memories were associated with greater activity in the precuneus in young than older adults. Furthermore, the direct comparison between encoding and retrieval patterns of activity using Representational Similarity Analyses revealed stronger item-specific reactivation in the posterior cingulate/retrosplenial cortex in young than older adults. Taken together, these results provide new evidence that aging is associated with reduced reinstatement of activity in brain regions that processed the encoding of complex stimuli, but older individuals judge these impoverished memory representations as subjectively vivid.

In vivo imaging of synaptic loss in Alzheimer's disease with [18F]UCB-H positron emission tomography.

PURPOSE: Loss of brain synapses is an early pathological feature of Alzheimer's disease. The current study assessed synaptic loss in vivo with positron emission tomography and an 18F-labelled radiotracer of the synaptic vesicle protein 2A, [18F]UCB-H. METHODS: Twenty-four patients with mild cognitive impairment or Alzheimer's disease and positive [18F]Flutemetamol amyloid-PET were compared to 19 healthy controls. [18F]UCB-H brain uptake was quantified with Logan graphical analysis using an image-derived blood input function. SPM12 and regions-of-interest (ROI) analyses were used for group comparisons of regional brain distribution volumes and for correlation with cognitive measures. RESULTS: A significant decrease of [18F]UCB-H uptake was observed in several cortical areas (11 to 18% difference) and in the thalamus (16% difference), with the largest effect size in the hippocampus (31% difference). Reduced hippocampal uptake was related to patients' cognitive decline (ROI analysis) and unawareness of memory problems (SPM and ROI analyses). CONCLUSIONS: The findings thus highlight predominant synaptic loss in the hippocampus, confirming previous autopsy-based studies and a recent PET study with an 11C-labelled SV2A radiotracer. [18F]UCB-H PET allows to image in vivo synaptic changes in Alzheimer's disease and to relate them to patients' cognitive impairment.

The Rise of Synaptic Density PET Imaging.

Many neurological disorders are related to synaptic loss or pathologies. Before the boom of positrons emission tomography (PET) imaging of synapses, synaptic quantification could only be achieved in vitro on brain samples after autopsy or surgical resections. Until the mid-2010s, electron microscopy and immunohistochemical labelling of synaptic proteins were the gold-standard methods for such analyses. Over the last decade, several PET radiotracers for the synaptic vesicle 2A protein have been developed to achieve in vivo synapses visualization and quantification. Different strategies were used, namely radiolabelling with either 11C or 18F, preclinical development in rodent and non-human primates, and binding quantification with different kinetic modelling methods. This review provides an overview of these PET tracers and underlines their perspectives and limitations by focusing on radiochemical aspects, as well as preclinical proof-of-concept and the main clinical outcomes described so far.

Evaluating the In Vivo Specificity of [18F]UCB-H for the SV2A Protein, Compared with SV2B and SV2C in Rats Using microPET.

The synaptic vesicle protein 2 (SV2) is involved in synaptic vesicle trafficking. The SV2A isoform is the most studied and its implication in epilepsy therapy led to the development of the first SV2A PET radiotracer [18F]UCB-H. The objective of this study was to evaluate in vivo, using microPET in rats, the specificity of [18F]UCB-H for SV2 isoform A in comparison with the other two isoforms (B and C) through a blocking assay. Twenty Sprague Dawley rats were pre-treated either with the vehicle, or with specific competitors against SV2A (levetiracetam), SV2B (UCB5203) and SV2C (UCB0949). The distribution volume (Vt, Logan plot, t* 15 min) was obtained with a population-based input function. The Vt analysis for the entire brain showed statistically significant differences between the levetiracetam group and the other groups (p < 0.001), but also between the vehicle and the SV2B group (p < 0.05). An in-depth Vt analysis conducted for eight relevant brain structures confirmed the statistically significant differences between the levetiracetam group and the other groups (p < 0.001) and highlighted the superior and the inferior colliculi along with the cortex as regions also displaying statistically significant differences between the vehicle and SV2B groups (p < 0.05). These results emphasize the in vivo specificity of [18F]UCB-H for SV2A against SV2B and SV2C, confirming that [18F]UCB-H is a suitable radiotracer for in vivo imaging of the SV2A proteins with PET.

Comparative assessment of 6-[18 F]fluoro-L-m-tyrosine and 6-[18 F]fluoro-L-dopa to evaluate dopaminergic presynaptic integrity in a Parkinson's disease rat model.

Because of the progressive loss of nigro-striatal dopaminergic terminals in Parkinson's disease (PD), in vivo quantitative imaging of dopamine (DA) containing neurons in animal models of PD is of critical importance in the preclinical evaluation of highly awaited disease-modifying therapies. Among existing methods, the high sensitivity of positron emission tomography (PET) is attractive to achieve that goal. The aim of this study was to perform a quantitative comparison of brain images obtained in 6-hydroxydopamine (6-OHDA) lesioned rats using two dopaminergic PET radiotracers, namely [18 F]fluoro-3,4-dihydroxyphenyl-L-alanine ([18 F]FDOPA) and 6-[18 F]fluoro-L-m-tyrosine ([18 F]FMT). Because the imaging signal is theoretically less contaminated by metabolites, we hypothesized that the latter would show stronger relationship with behavioural and post-mortem measures of striatal dopaminergic deficiency. We used a within-subject design to measure striatal [18 F]FMT and [18 F]FDOPA uptake in eight partially lesioned, eight fully lesioned and ten sham-treated rats. Animals were pretreated with an L-aromatic amino acid decarboxylase inhibitor. A catechol-O-methyl transferase inhibitor was also given before [18 F]FDOPA PET. Quantitative estimates of striatal uptake were computed using conventional graphical Patlak method. Striatal dopaminergic deficiencies were measured with apomorphine-induced rotations and post-mortem striatal DA content. We observed a strong relationship between [18 F]FMT and [18 F]FDOPA estimates of decreased uptake in the denervated striatum using the tissue-derived uptake rate constant Kc . However, only [18 F]FMT Kc succeeded to discriminate between the partial and the full 6-OHDA lesion and correlated well with the post-mortem striatal DA content. This study indicates that the [18 F]FMT could be more sensitive, with respect of [18 F]FDOPA, to investigate DA terminals loss in 6-OHDA rats, and open the way to in vivo L-aromatic amino acid decarboxylase activity targeting in future investigations on progressive PD models.

Nanofitin as a New Molecular-Imaging Agent for the Diagnosis of Epidermal Growth Factor Receptor Over-Expressing Tumors.

Epidermal growth-factor receptor (EGFR) is involved in cell growth and proliferation and is over-expressed in malignant tissues. Although anti-EGFR-based immunotherapy became a standard of care for patients with EGFR-positive tumors, this strategy of addressing cancer tumors by targeting EGFR with monoclonal antibodies is less-developed for patient diagnostic and monitoring. Indeed, antibodies exhibit a slow blood clearance, which is detrimental for positron emission tomography (PET) imaging. New molecular probes are proposed to overcome such limitations for patient monitoring, making use of low-molecular-weight protein scaffolds as alternatives to antibodies, such as Nanofitins with better pharmacokinetic profiles. Anti-EGFR Nanofitin B10 was reformatted by genetic engineering to exhibit a unique cysteine moiety at its C-terminus, which allows the development of a fast and site-specific radiolabeling procedure with 18F-4-fluorobenzamido-N-ethylamino-maleimide (18F-FBEM). The in vivo tumor targeting and imaging profile of the anti-EGFR Cys-B10 Nanofitin was investigated in a double-tumor xenograft model by static small-animal PET at 2 h after tail-vein injection of the radiolabeled Nanofitin 18F-FBEM-Cys-B10. The image showed that the EGFR-positive tumor (A431) is clearly delineated in comparison to the EGFR-negative tumor (H520) with a significant tumor-to-background contrast. 18F-FBEM-Cys-B10 demonstrated a significantly higher retention in A431 tumors than in H520 tumors at 2.5 h post-injection with a A431-to-H520 uptake ratio of 2.53 ± 0.18 and a tumor-to-blood ratio of 4.55 ± 0.63. This study provides the first report of Nanofitin scaffold used as a targeted PET radiotracer for in vivo imaging of EGFR-positive tumor, with the anti-EGFR B10 Nanofitin used as proof-of-concept. The fast generation of specific Nanofitins via a fully in vitro selection process, together with the excellent imaging features of the Nanofitin scaffold, could facilitate the development of valuable PET-based companion diagnostics.

Pharmacokinetic Characterization of [18F]UCB-H PET Radiopharmaceutical in the Rat Brain.

The synaptic vesicle glycoprotein 2A (SV2A), a protein essential to the proper nervous system function, is found in presynaptic vesicles. Thus, SV2A targeting, using dedicated radiotracers combined with positron emission tomography (PET), allows the assessment of synaptic density in the living brain. The first-in-class fluorinated SV2A specific radioligand, [18F]UCB-H, is now available at high activity through an efficient radiosynthesis compliant with current good manufacturing practices (cGMP). We report here a noninvasive method to quantify [18F]UCB-H binding in rat brain with microPET. Validation study in rats confirmed the need of high enantiomeric purity to target SV2A in vivo. We demonstrated the reliability of a population-based input function to quantify SV2A in preclinical microPET setting. Finally, we investigated the in vivo metabolism of [18F]UCB-H and confirmed the negligible amount of radiometabolites in the rat brain. Hence, the in vivo quantification of SV2A using [18F]UCB-H microPET seems a promising tool for the assessment of the synaptic density in the rat brain, and opens the way for longitudinal follow-up in neurodegenerative disease rodent models.

Neural correlates of event clusters in past and future thoughts: How the brain integrates specific episodes with autobiographical knowledge.

When remembering the past or envisioning the future, events often come to mind in organized sequences or stories rather than in isolation from one another. The aim of the present fMRI study was to investigate the neural correlates of such event clusters. Participants were asked to consider pairs of specific past or future events: in one condition, the two events were part of the same event cluster (i.e., they were thematically and/or causally related to each other), whereas in another condition the two events only shared a surface feature (i.e., their location); a third condition was also included, in which the two events were unrelated to each other. The results showed that the processing of past and future events that were part of a same cluster was associated with higher activation in the medial prefrontal cortex (PFC), rostrolateral PFC, and left lateral temporal and parietal regions, compared to the two other conditions. Furthermore, functional connectivity analyses revealed an increased coupling between these cortical regions. These findings suggest that largely similar processes are involved in organizing events in clusters for the past and the future. The medial and rostrolateral PFC might play a pivotal role in mediating the integration of specific events with conceptual autobiographical knowledge 'stored' in more posterior regions. Through this integrative process, this set of brain regions might contribute to the attribution of an overarching meaning to representations of specific past and future events, by contextualizing them with respect to personal goals and general knowledge about one's life story.

Resting-state Network-specific Breakdown of Functional Connectivity during Ketamine Alteration of Consciousness in Volunteers.

BACKGROUND: Consciousness-altering anesthetic agents disturb connectivity between brain regions composing the resting-state consciousness networks (RSNs). The default mode network (DMn), executive control network, salience network (SALn), auditory network, sensorimotor network (SMn), and visual network sustain mentation. Ketamine modifies consciousness differently from other agents, producing psychedelic dreaming and no apparent interaction with the environment. The authors used functional magnetic resonance imaging to explore ketamine-induced changes in RSNs connectivity. METHODS: Fourteen healthy volunteers received stepwise intravenous infusions of ketamine up to loss of responsiveness. Because of agitation, data from six subjects were excluded from analysis. RSNs connectivity was compared between absence of ketamine (wake state [W1]), light ketamine sedation, and ketamine-induced unresponsiveness (deep sedation [S2]). RESULTS: Increasing the depth of ketamine sedation from W1 to S2 altered DMn and SALn connectivity and suppressed the anticorrelated activity between DMn and other brain regions. During S2, DMn connectivity, particularly between the medial prefrontal cortex and the remaining network (effect size ß [95% CI]: W1 = 0.20 [0.18 to 0.22]; S2 = 0.07 [0.04 to 0.09]), and DMn anticorrelated activity (e.g., right sensory cortex: W1 = -0.07 [-0.09 to -0.04]; S2 = 0.04 [0.01 to 0.06]) were broken down. SALn connectivity was nonuniformly suppressed (e.g., left parietal operculum: W1 = 0.08 [0.06 to 0.09]; S2 = 0.05 [0.02 to 0.07]). Executive control networks, auditory network, SMn, and visual network were minimally affected. CONCLUSIONS: Ketamine induces specific changes in connectivity within and between RSNs. Breakdown of frontoparietal DMn connectivity and DMn anticorrelation and sensory and SMn connectivity preservation are common to ketamine and propofol-induced alterations of consciousness.

Multimodality imaging assessment of the deleterious role of the intraluminal thrombus on the growth of abdominal aortic aneurysm in a rat model.

OBJECTIVES: To evaluate imaging changes occurring in a rat model of elastase-induced abdominal aortic aneurysm (AAA), with emphasis on the intraluminal thrombus (ILT) occurrence. METHODS: The post-induction growth of the AAA diameter was characterized using ultrasound in 22 rats. ILT was reported on 13 rats that underwent 14 magnetic resonance imaging (MRI) 2-18 days post-surgery, and on 10 rats that underwent 18 fluoro-deoxyglucose (FDG) positron emission tomography (PET)/microcomputed tomography examinations 2-27 days post-surgery. Logistic regressions were used to establish the evolution with time of AAA length, diameter, ILT thickness, volume, stratification, MRI and FDG PET signalling properties, and histological assessment of inflammatory infiltrates. RESULTS: All of the following significantly increased with time post-induction (p < 0.001): AAA length, AAA diameter, ILT maximal thickness, ILT volume, ILT iron content and related MRI signalling changes, quantitative uptake on FDG PET, and the magnitude of inflammatory infiltrates on histology. However, the aneurysm growth peak followed occurrence of ILT approximately 6 days after elastase infusion. CONCLUSION: Our model emphasizes that occurrence of ILT precedes AAA peak growth. Aneurysm growth is associated with increasing levels of iron, signalling properties changes in both MRI and FDG PET, relating to its biological activities. KEY POINTS: • ILT occurrence in AAA is associated with increasing FDG uptake and growth. • MRI signalling changes in ILT reflect activities such as haemorrhage and RBC trapping. • Monitoring ILT activities using MRI may require no exogenous contrast agent.

Intrinsic functional connectivity differentiates minimally conscious from unresponsive patients.

Despite advances in resting state functional magnetic resonance imaging investigations, clinicians remain with the challenge of how to implement this paradigm on an individualized basis. Here, we assessed the clinical relevance of resting state functional magnetic resonance imaging acquisitions in patients with disorders of consciousness by means of a systems-level approach. Three clinical centres collected data from 73 patients in minimally conscious state, vegetative state/unresponsive wakefulness syndrome and coma. The main analysis was performed on the data set coming from one centre (Liège) including 51 patients (26 minimally conscious state, 19 vegetative state/unresponsive wakefulness syndrome, six coma; 15 females; mean age 49 ± 18 years, range 11-87; 16 traumatic, 32 non-traumatic of which 13 anoxic, three mixed; 35 patients assessed >1 month post-insult) for whom the clinical diagnosis with the Coma Recovery Scale-Revised was congruent with positron emission tomography scanning. Group-level functional connectivity was investigated for the default mode, frontoparietal, salience, auditory, sensorimotor and visual networks using a multiple-seed correlation approach. Between-group inferential statistics and machine learning were used to identify each network's capacity to discriminate between patients in minimally conscious state and vegetative state/unresponsive wakefulness syndrome. Data collected from 22 patients scanned in two other centres (Salzburg: 10 minimally conscious state, five vegetative state/unresponsive wakefulness syndrome; New York: five minimally conscious state, one vegetative state/unresponsive wakefulness syndrome, one emerged from minimally conscious state) were used to validate the classification with the selected features. Coma Recovery Scale-Revised total scores correlated with key regions of each network reflecting their involvement in consciousness-related processes. All networks had a high discriminative capacity (>80%) for separating patients in a minimally conscious state and vegetative state/unresponsive wakefulness syndrome. Among them, the auditory network was ranked the most highly. The regions of the auditory network which were more functionally connected in patients in minimally conscious state compared to vegetative state/unresponsive wakefulness syndrome encompassed bilateral auditory and visual cortices. Connectivity values in these three regions discriminated congruently 20 of 22 independently assessed patients. Our findings point to the significance of preserved abilities for multisensory integration and top-down processing in minimal consciousness seemingly supported by auditory-visual crossmodal connectivity, and promote the clinical utility of the resting paradigm for single-patient diagnostics.

Probing the conformation and 2D-distribution of pyrene-terminated redox-labeled poly(ethylene glycol) chains end-adsorbed on HOPG using cyclic voltammetry and atomic force electrochemical microscopy.

The present paper aims at illustrating how end-attachment of water-soluble flexible chains bearing a terminal functional group onto graphene-like surfaces has to be carefully tuned to ensure the proper positioning of the functional moiety with respect to the anchoring surface. The model experimental system considered here consists of a layer of poly(ethylene glycol) (PEG) chains, bearing an adsorbing pyrene foot and a ferrocene (Fc) redox functional head, self-assembled onto highly oriented pyrolytic graphite (HOPG). Cyclic voltammetry is used to accurately measure the chain coverage and gain insights into the microenvironment experienced by the Fc heads. Molecule-touching atomic force electrochemical microscopy (Mt/AFM-SECM) is used to simultaneously probe the chain conformation and the position of the Fc heads within the layer, and also to map the 2D-distribution of the chains over the surface. This multiscale electrochemical approach allows us to show that whereas Fc-PEG-pyrene readily self-assembles to form extremely homogeneous layers, the strongly hydrophobic nature of graphite planes results in a complex coverage-dependent structure of the PEG layer due to the interaction of the ferrocene label with the HOPG surface. It is shown that, even though pyrene is known to adsorb particularly strongly onto HOPG, the more weakly adsorbing terminal ferrocene can also act as the chain anchoring moiety especially at low coverage. However we show that beyond a critical coverage value the Fc-PEG-pyrene chains adopt an ideal "foot-on" end-attached conformation allowing the Fc head to explore a volume away from the surface solely limited by the PEG chain elasticity.

Study protocol: Cerebral characterization of sensory gating in disconnected dreaming states during propofol anesthesia using fMRI.

Background: Disconnected consciousness describes a state in which subjective experience (i.e., consciousness) becomes isolated from the external world. It appears frequently during sleep or sedation, when subjective experiences remain vivid but are unaffected by external stimuli. Traditional methods of differentiating connected and disconnected consciousness, such as relying on behavioral responsiveness or on post-anesthesia reports, have demonstrated limited accuracy: unresponsiveness has been shown to not necessarily equate to unconsciousness and amnesic effects of anesthesia and sleep can impair explicit recollection of events occurred during sleep/sedation. Due to these methodological challenges, our understanding of the neural mechanisms underlying sensory disconnection remains limited. Methods: To overcome these methodological challenges, we employ a distinctive strategy by combining a serial awakening paradigm with auditory stimulation during mild propofol sedation. While under sedation, participants are systematically exposed to auditory stimuli and questioned about their subjective experience (to assess consciousness) and their awareness of the sounds (to evaluate connectedness/disconnectedness from the environment). The data collected through interviews are used to categorize participants into connected and disconnected consciousness states. This method circumvents the requirement for responsiveness in assessing consciousness and mitigates amnesic effects of anesthesia as participants are questioned while still under sedation. Functional MRI data are concurrently collected to investigate cerebral activity patterns during connected and disconnected states, to elucidate sensory disconnection neural gating mechanisms. We examine whether this gating mechanism resides at the thalamic level or results from disruptions in information propagation to higher cortices. Furthermore, we explore the potential role of slow-wave activity (SWA) in inducing disconnected consciousness by quantifying high-frequency BOLD oscillations, a known correlate of slow-wave activity. Discussion: This study represents a notable advancement in the investigation of sensory disconnection. The serial awakening paradigm effectively mitigates amnesic effects by collecting reports immediately after regaining responsiveness, while still under sedation. Ultimately, this research holds the potential to understand how sensory gating is achieved at the neural level. These biomarkers might be relevant for the development of sensitive anesthesia monitoring to avoid intraoperative connected consciousness and for the assessment of patients suffering from pathologically reduced consciousness. Clinical trial registration: European Union Drug Regulating Authorities Clinical Trials Database (EudraCT), identifier 2020-003524-17.

Prediction of cognitive decline in healthy aging based on neuropsychiatric symptoms and PET-biomarkers of Alzheimer's disease.

Neuropsychiatric symptoms (NPS) have been associated with a risk of accelerated cognitive decline or conversion to dementia of the Alzheimer's Disease (AD) type. Moreover, the NPS were also associated with higher AD biomarkers (brain tau and amyloid burden) even in non-demented patients. But the effect of the relationship between NPS and biomarkers on cognitive decline has not yet been studied. This work aims to assess the relationship between longitudinal cognitive changes and NPS, specifically depression and anxiety, in association with AD biomarkers in healthy middle-aged to older participants. The cohort consisted of 101 healthy participants aged 50-70 years, 66 of whom had neuropsychological assessments of memory, executive functions, and global cognition at a 2-year follow-up. At baseline, NPS were assessed using the Beck Depression and Anxiety Inventories while brain tau and amyloid loads were measured using positron emission topography. For tau burden, THK5351 uptake is used as a proxy of tau and neuroinflammation. Participants, declining or remaining stable at follow-up, were categorized into groups for each cognitive domain. Group classification was investigated using binary logistic regressions based on combined AD biomarkers and the two NPS. The results showed that an association between anxiety and prefrontal amyloid burden significantly classified episodic memory decline, while the classification of global cognitive decline involved temporal and occipital amyloid burden but not NPS. Moreover, depression together with prefrontal and hippocampal tau burden were associated with a decline in memory. The classification of participants based on executive decline was related to depression and mainly prefrontal tau burden. These findings suggest that the combination of NPS and brain biomarkers of AD predicts the occurrence of cognitive decline in aging.

The Osteoblast Transcriptome in Developing Zebrafish Reveals Key Roles for Extracellular Matrix Proteins Col10a1a and Fbln1 in Skeletal Development and Homeostasis.

Zebrafish are now widely used to study skeletal development and bone-related diseases. To that end, understanding osteoblast differentiation and function, the expression of essential transcription factors, signaling molecules, and extracellular matrix proteins is crucial. We isolated Sp7-expressing osteoblasts from 4-day-old larvae using a fluorescent reporter. We identified two distinct subpopulations and characterized their specific transcriptome as well as their structural, regulatory, and signaling profile. Based on their differential expression in these subpopulations, we generated mutants for the extracellular matrix protein genes col10a1a and fbln1 to study their functions. The col10a1a-/- mutant larvae display reduced chondrocranium size and decreased bone mineralization, while in adults a reduced vertebral thickness and tissue mineral density, and fusion of the caudal fin vertebrae were observed. In contrast, fbln1-/- mutants showed an increased mineralization of cranial elements and a reduced ceratohyal angle in larvae, while in adults a significantly increased vertebral centra thickness, length, volume, surface area, and tissue mineral density was observed. In addition, absence of the opercle specifically on the right side was observed. Transcriptomic analysis reveals up-regulation of genes involved in collagen biosynthesis and down-regulation of Fgf8 signaling in fbln1-/- mutants. Taken together, our results highlight the importance of bone extracellular matrix protein genes col10a1a and fbln1 in skeletal development and homeostasis.

Relationships between brain metabolism decrease in normal aging and changes in structural and functional connectivity.

Normal aging is characterized by brain glucose metabolism decline predominantly in the prefrontal cortex. The goal of the present study was to assess whether this change was associated with age-related alteration of white matter (WM) structural integrity and/or functional connectivity. FDG-PET data from 40 young and 57 elderly healthy participants from two research centers (n=49/48 in Center 1/2) were analyzed. WM volume from T1-weighted MRI (Center 1), fractional anisotropy from diffusion-tensor imaging (Center 2), and resting-state fMRI data (Center 1) were also obtained. Group comparisons were performed within each imaging modality. Then, positive correlations were assessed, within the elderly, between metabolism in the most affected region and the other neuroimaging modalities. Metabolism decline in the elderly predominated in the left inferior frontal junction (LIFJ). LIFJ hypometabolism was significantly associated with macrostructural and microstructural WM disturbances in long association fronto-temporo-occipital fibers, while no relationship was found with functional connectivity. The findings offer new perspectives to understand normal aging processes and open avenues for future studies to explore causality between age-related metabolism and connectivity changes.

Metabolic and structural connectivity within the default mode network relates to working memory performance in young healthy adults.

Studies of functional connectivity suggest that the default mode network (DMN) might be relevant for cognitive functions. Here, we examined metabolic and structural connectivity between major DMN nodes, the posterior cingulate (PCC) and medial prefrontal cortex (MPFC), in relation to normal working memory (WM). DMN was captured using independent component analysis of [18F]fluorodeoxyglucose positron emission tomography (FDG-PET) data from 35 young healthy adults (27.1 ± 5.1 years). Metabolic connectivity, a correlation between FDG uptake in PCC and MPFC, was examined in groups of subjects with (relative to median) low (n=18) and high (n=17) performance on digit span backward test as an index of verbal WM. In addition, fiber tractography based on PCC and MPFC nodes as way points was performed in a subset of subjects. FDG uptake in the DMN nodes did not differ between high and low performers. However, significantly (p=0.01) lower metabolic connectivity was found in the group of low performers. Furthermore, as compared to high performers, low performers showed lower density of the left superior cingulate bundle. Verbal WM performance is related to metabolic and structural connectivity within the DMN in young healthy adults. Metabolic connectivity as quantified with FDG-PET might be a sensitive marker of the normal variability in some cognitive functions.

Episodic autobiographical memory in amnestic mild cognitive impairment: what are the neural correlates?

Autobiographical memory in amnestic Mild Cognitive Impairment (aMCI) is characterized by impaired retrieval of episodic memories, but relatively preserved personal semantic knowledge. This study aimed to identify (via FDG-PET) the neural substrates of impaired episodic specificity of autobiographical memories in 35 aMCI patients compared with 24 healthy elderly controls. Significant correlations between regional cerebral activity and the proportion of episodic details in autobiographical memories from two life periods were found in specific regions of an autobiographical brain network. In aMCI patients, more than in controls, specifically episodic memories from early adulthood were associated with metabolic activity in the cuneus and in parietal regions. We hypothesized that variable retrieval of episodic autobiographical memories in our aMCI patients would be related to their variable capacity to reactivate specific sensory-perceptual and contextual details of early adulthood events linked to reduced (occipito-parietal) visual imagery and less efficient (parietal) attentional processes. For recent memories (last year), a correlation emerged between the proportion of episodic details and activity in lateral temporal regions and the temporo-parietal junction. Accordingly, variable episodic memory for recent events may be related to the efficiency of controlled search through general events likely to provide cues for the retrieval of episodic details and to the ability to establish a self perspective favouring recollection.

Multimodal imaging of microstructural cerebral alterations and loss of synaptic density in Alzheimer's disease.

Multiple neuropathological events are involved in Alzheimer's disease (AD). The current study investigated the concurrence of neurodegeneration, increased iron content, atrophy, and demyelination in AD. Quantitative multiparameter magnetic resonance imaging (MRI) maps providing neuroimaging biomarkers for myelination and iron content along with synaptic density measurements using [18F] UCB-H PET were acquired in 24 AD and 19 Healthy controls (19 males). The whole brain voxel-wise group comparison revealed demyelination in the right hippocampus, while no significant iron content difference was detected. Bilateral atrophy and synaptic density loss were observed in the hippocampus and amygdala. The multivariate GLM (mGLM) analysis shows a bilateral difference in the hippocampus and amygdala, right pallidum, left fusiform and temporal lobe suggesting that these regions are the most affected despite the diverse differences in brain tissue properties in AD. Demyelination was identified as the most affecting factor in the observed differences. Here, the mGLM is introduced as an alternative for multiple comparisons between different modalities, reducing the risk of false positives while informing about the co-occurrence of neuropathological processes in AD.