Naguib Mechawar

Introduction: Myelin water imaging provides a non-invasive approach for indirectly investigating myelin content of the brain by magnetic resonance Imaging (MRI). Myelin water fraction (MWF) reflects the fraction of water signal associated with water trapped between myelin layers. Postmortem imaging provides a unique opportunity to validate MWF metrics against gold-standard postmortem neuropathology. However, there is a need for validated sequences that would be applicable in postmortem settings. In this study, we validated and compared the applicability of turbo spin echo (TSE) and gradient-and-spin-echo (GRASE) techniques for myelin water imaging in formaldehyde fixed postmortem human brains. Methods: 40 postmortem human brain hemispheres were scanned with TSE and GRASE sequences. Acquired data were reconstructed using a non-parametric multicomponent T ₂ relaxometry approach using a consistent framework to support comparison between GRASE- and TSE-derived measures. Agreement between derived measures was quantified at both voxel and regional level. Maps derived from the full 32-echo GRASE reconstruction were compared against a truncated 14-echo reconstruction to examine the contribution of the longer echoes to the derived maps. Finally, a random forest regression model was trained based on TSE data to predict the GRASE-derived MWF maps. Results: Both sequences provided robust multicomponent T ₂ characterization across the brain, and the derived measures showed anatomical patterns consistent with expected differences across tissue types. TSE and GRASE derived maps had moderate to strong agreement at voxel (0.41< ρ < 0.82, all p _FDR FDR 0.98, p _FDR < 0.001). The nonlinear random forest regression model trained on TSE maps was able to accurately reproduce the MWF maps derived from GRASE (r = 0.91, RMSE = 8.87). Discussion: TSE and GRASE32 capture strongly related multicomponent T₂ information in fixed postmortem human brain tissue, but with systematic and tissue-dependent quantitative differences across derived measures. These differences suggest that direct interchangeability should not be assumed and that cross-sequence mapping or calibration is required when quantitative equivalence is desired.

Depression is a highly prevalent mood disorder with a complex etiology that has yet to be fully characterized. Alterations in the generation of new granule neurons in the hippocampal dentate gyrus have been hypothesized to contribute to the pathophysiology of depression and recovery from this mood disorder, a theory coined the “neurogenic hypothesis of depression”. This article revisits current evidence of this hypothesis in humans, which reveals varying perspectives from post-mortem studies. Drawing on recent findings in the field of human adult hippocampal neurogenesis, we also offer here potential research directions regarding how immature granule neurons, which have been so far understudied in this context, may be affected in the pathogenesis of depression. Finally, this review explores to what extent immature granule neurons are involved in the neurobiological basis of this mood disorder and how their properties may increase their vulnerability to stress-mediated changes in the hippocampus. Together, this review provides crucial insight into immature granule neurons as a particularly interesting therapeutic target to mitigate stress-related behavioural impairments.

INTRODUCTION: Tau pathology typically occurs in Alzheimer's disease (AD), however is also frequently present in Parkinson's disease dementia (PDD) and Dementia with Lewy Bodies (DLB), yet its disease-specific signature is unclear. METHODS: Five tau, amyloid-β, α-synuclein and neuronal markers were analysed across hippocampal subfields in non-demented controls (CTLs), AD, PDD and DLB using multiplex immunohistochemistry, single-tangle classification and confocal imaging. RESULTS: AT8, pTau217, and GT38 were predominatly detected in AD, while pS422 was enriched in PDD and pS396 showed a region- and disease-specific pattern. DLB resembled AD in subregional tau distribution. Tau marker correlation were different comparing AD, PDD and CTL. Single-tangle analyses revealed disease-specific immunophenotypes but conserved mature intra-tangle epitope organisation. Distinct tau signatures were associated with inhibitory interneuron vulnerability, while regional tau co-occurrence with amyloid-β and α-synuclein remained conserved. DISCUSSION: Disease-specific tau signatures vary across hippocampal subregions and neuronal populations, implicating the contribution of regional and cell-specific factors beyond pathology burden.

Postmortem human brains are important research resources to study the mechanisms underlying cerebrovascular features in various neurodegenerative disorders. Immunohistochemical and histochemical staining have been used to examine human brains fixed in neutral-buffered formalin (NBF) for months, years, or decades. Previously, we found that prolonged NBF fixation resulted in differential effects on immunohistochemistry and histochemistry staining on postmortem brains. Here, we further examined the effects of prolonged fixation (1, 5, 10, 15, and 20 years) on stains of known biomarkers of cerebrovascular diseases in the human prefrontal cortex. We included microvasculature markers of the blood vessel wall (anti-collagen-IV and claudin-5), a type III intermediate filament marker (anti-vimentin), an activated microglia marker (anti-CD68), a biomarker for proteolipid protein (anti-PLP) of oligodendrocytes and a marker for iron accumulation (anti-ferritin). We also included Masson’s trichrome stain (MTS) and Bielschowsky silver stain (BSS). We found that staining intensities of ferritin, vimentin, collagen-IV, and BSS decreased with prolonged fixation. No significant differences were observed in the staining intensity of other markers. We therefore recommend performing IHC and HC staining for human brains with the same fixation times to offset any impact on downstream neuropathological analyses, as well as adding the fixation duration as a covariate in the analysis.

Pregnancy and postpartum experiences represent transformative physiological states that impose lasting demands on the maternal body and brain, resulting in lifelong neural adaptations ^1–6 . However, the precise molecular mechanisms that drive these persistent alterations remain poorly understood. Here we used brain-wide transcriptomic profiling to define the molecular landscape of neuroplasticity induced by reproductive experience, identifying the dorsal hippocampal formation (dHF) as a key site of transcriptional remodelling. Combining single-cell RNA sequencing with a maternal–pup separation paradigm, we additionally found that chronic postpartum stress significantly disrupts dHF adaptations by altering dopamine dynamics, leading to changes in the dopamine-dependent histone post-translational modification, H3 dopaminylation, which causally mediates downstream alterations in gene expression and behaviour. In human dorsal subiculum, a brain structure within the dHF, we uncovered conserved patterns of parity-dependent alterations in H3 dopaminylation and transcription. We further established the sufficiency of dopamine modulation in regulating these adaptations via chemogenetic suppression of dopamine release into the dHF, which recapitulated key epigenomic and behavioural features of reproductive experience in virgin female mice. In sum, our findings establish dopamine as a central regulator of parity-induced neuroadaptations in humans and mice, revealing a fundamental transcriptional mechanism by which female reproductive experience remodels the brain to sustain long-term behavioural adaptations.

Until the discovery of white matter neurons (WMN) in the 19th century, white matter (WM) was considered to be completely devoid of neuronal cell bodies. Despite evidence consistently showing neuronal soma within cortical WM and their purported implication in neuropsychiatric disorders, these neurons are understudied and have not been characterized in human long-range WM tracts. Using postmortem human brain tissue, we investigated the presence, densities and proportions of excitatory/inhibitory neurons in the uncinate fasciculus (UF) and corpus callosum (CC). We also investigated the ventromedial prefrontal cortex (vmPFC) to validate our methods by comparing our results with previously reported densities of neurons in cortical WM. To identify WMN, we employed fluorescence in situ hybridization with excitatory (SLC17A7) and inhibitory (GAD1) neuronal markers and subsequently validated these neurons at the protein level with NeuN immunohistochemistry. We found that the density of WMN in the vmPFC corresponded with previous independent estimates. The UF displayed a similar, though slightly lower density of WMN compared to the vmPFC, while the CC had a far lower density of WMN than both of these regions. Due to the higher-than-expected density of WMN in the UF, we validated the findings at a second location along the UF temporal segment and confirmed the presence of substantial numbers of WMN in this tract. This research constitutes the first ever validated observation of WMN in human long-range WM tracts, laying the foundation for future research on the phenotype and function of these neurons, and how they may be affected in brain disorders.

The uncinate fasciculus (UF) is a long‐range association fiber tract connecting the anterior temporal lobe with the orbitofrontal cortex and has been linked to a multitude of physiological and pathophysiological conditions such as aging, epilepsy, and the vulnerability to psychopathology posed by a history of childhood abuse (CA). Since the myelin sheath is highly enriched in lipids, changes in white matter (WM) microstructure observed via neuroimaging may reflect alterations in the myelin lipid profile. Given that the UF does not exist in rodents, its molecular properties are highly understudied. Therefore, we sought to quantify the phospholipid FA and cholesterol quantities of the human postmortem UF and evaluate any lipid‐related or myelin‐constituent gene/protein changes associated with age and history of CA. UF samples were analyzed from individuals with depression who died by suicide with (DS‐CA) or without (DS) severe CA, and control individuals (CTRL), with an age span of 15 to 85 years. Phospholipids were separated by thin‐layer chromatography; FAs and nonderivatized cholesterol were quantified by gas chromatography‐flame ionization detection. The relative expression of myelin‐constituent genes and proteins was measured by RT‐qPCR and immunoblotting, respectively. We found no robust relationships between CA or depression and lipid measures or myelin‐constituent gene/protein levels. In contrast, phospholipids showed pronounced age effects that differed by fraction, with an overall trend of monounsaturated FAs increasing and long‐chain omega‐6 polyunsaturated FAs decreasing with age. The expression of most myelin‐constituent genes and proteins declined with age; PLP1 and MAG showed significant decreases. Therefore, changes in lipid composition and lipid‐protein interactions likely contribute to age‐related myelin deficits and may in part underlie age‐associated cognitive decline. image

SignificanceCharacterizing the ultrastructure of myelin in the human brain is key to understanding the neurobiology of both health and disease. In postmortem human brain tissue, electron microscopy is often technically unfeasible due to poorer tissue quality.AimThe uncinate fasciculus (UF) is a long-range white matter association tract that connects the anterior temporal lobe with the orbitofrontal cortex. The UF is not present in rodents yet is highly expanded in humans and nonhuman primates. As such, its molecular and ultrastructural properties are virtually unknown.ApproachHere, we develop and validate a spectral-focusing coherent anti-Stokes Raman spectroscopy (sf-CARS) system coupled with a custom AxonDeepSeg segmentation model to characterize UF ultrastructure in the human postmortem brain (n=6).ResultsWe provide a proof of concept of this new methodological pipeline in the UF temporal segment and observe that the mean axon diameter detected is 0.93 μm±0.54 and the mean myelin thickness is 0.48 μm±0.14. We also observe that the UF axons are thicker than those in the anterior cingulate cortex white matter.ConclusionsWe detail and validate the full methodology, including tissue fixation and sectioning, sf-CARS acquisition settings, as well as the AxonDeepSeg deep learning model parameters, such that this pipeline can be utilized by others in the field.

The uncinate fasciculus (UF) is a hook-shaped long-range association white matter tract that serves to bidirectionally transmit information between the anterior temporal lobe and the orbitofrontal cortex. Neuroimaging studies have suggested that changes in UF microstructure are involved in the neurobiological sequalae of childhood abuse (CA). Given that the UF is not present in rodents, it is vastly understudied with no cellular and molecular information available. To this end, we aimed to perform a multimodal characterization of the UF between individuals diagnosed with depression who died by suicide with (DS-CA) and without a history of severe CA (DS) and psychiatrically healthy individuals (CTRL). Fresh frozen UF tissue was obtained from the Douglas Bell-Canada Brain Bank, with phenotypic information collected via psychological autopsy. Immunohistochemistry with PDGFRα and NogoA was used to label oligodendrocyte precursor cell (OPC) and oligodendrocyte (OL), respectively, and stereology was performed to ascertain cell density and soma volume. Single nucleus RNA sequencing (snRNAseq) was used to generate a transcriptomic survey of the distinct cell types found in the UF. Finally, spectral focusing Coherent Anti-Stokes Raman Scattering (sf-CARS) microscopy was employed in tandem with a custom AxonDeepSeg segmentation model to measure axon diameter, myelin thickness, and g-ratio. No group differences were observed in histology or ultrastructure metrics, but nearly 50 differentially expressed genes (DEG) were identified between groups. Interestingly NECTIN3, the top DEG downregulated in OL1 and OL3 of DS-CA, is a computationally predicted target of the microRNA MIR646, the host gene of which was significantly downregulated in multiple cell types in DS-CA. Age-associated changes were pronounced and observed in all modalities, including an age-related increase in OL density, extensive changes in glial gene expression, as well as decreases in axon diameter and g-ratio. This study serves as a foundational resource on the molecular and cellular properties of the human UF. Our results suggest that observable myelin-related traces of depression or CA are limited in the UF and highlights the need for future research on the cellular and molecular properties of white matter tracts during aging.