BACKGROUND:APPE693Q ("Dutch") transgenic mice develop aging-related learning deficits and accumulate endogenously generated non-fibrillar aggregates (NFAs) of amyloid beta (Aβ) and amyloid precursor protein α-carboxy terminal fragments. NFA-Aβ correlates with synaptic loss and memory deficits more closely than does fibrillar Aβ. METHODS:We assessed the physiological, transcriptomic, ultrastructural, histological, and metabolic changes associated with the accumulation of NFA of Dutch Aβ in brains of APPE693Q mice. RESULTS:Aging-related accumulation of NFA-Aβ in APPE693Q mice was revealed by A11 immunohistochemistry and cyclic D,L-α-peptide-fluorescein-5-isothiocyanate microscopy. Presynaptic termini of APPE693Q mice developed physiological abnormalities in post-tetanic potentiation, synaptic fatigue, synaptic vesicle replenishment, and an aging-related reduction in mitochondrial complex I activity. Single-cell RNA sequencing showed that excitatory neurons exhibited an altered transcriptomic profile involving "protein translation" and "oxidative phosphorylation." DISCUSSION:Accumulation of NFA-Aβ alters neuronal metabolism but does not activate inflammation. Depletion of all forms of Aβ may be required to eliminate Aβ toxicity with anti-amyloid antibodies.

OBJECTIVE:In lowlanders, ascent to and prolonged stay at high altitude (HA) can trigger maladaptive changes in neurovascularization, immune function, and hippocampal-dependent cognitive impairment. Mild traumatic brain injury (mTBI) induces similar neuropathological and behavioral alterations. The primary objective of this study was to characterize the impact of mTBI at HA on behavioral and neuropathological outcomes in controlled murine models. METHODS:Baseline magnetic resonance imaging (MRI), diffusion tensor imaging (DTI), and [18F]fluorodeoxyglucose positron emission tomography (PET) were conducted in male C57BL/6J mice prior to exposure to either sea level (SL) or chronic HA (12 weeks, 5000 m simulated in a hypobaric chamber). Mice were then assigned to mTBI (three repetitive closed head injuries) or Sham groups. The impact of HA-mTBI on novel object recognition and contextual fear memory recall, analogs of human subcategories of memory altered at HA, was evaluated 9-12 days postinjury, followed by imaging. RESULTS:Relative to their post-HA imaging assessment, HA-mTBI cerebral blood flow within the cortex, hippocampus, and thalamus was reduced, as was global brain volume. Moreover, statistical parametric mapping analysis detected clear HA-mTBI interactions in glucose uptake with reductions at the injury site and contralateral increases in the cortex and amygdala. Significant clusters were identified by MRI and DTI in the cortex, caudate, and corpus callosum for T2 values, trace, and radial diffusivity in HA-mTBI animals relative to their post-HA assessments. The principal component analysis and Pearson's correlations reveal distinct imaging signatures associated with HA exposure and TBI hippocampal integrity and thalamocortical connectivity, which may contribute to deficits in spatial memory and alterations in exploratory behavior. SUMMARY:The combined burden of chronic HA exposure and mTBI induces a complex neuropathological interaction, evidenced by region-dependent white matter vulnerability, gray matter alterations, and impaired cerebral glucose metabolism. Although HA exposure and mTBI independently produced behavioral deficits, a synergistic behavioral effect was not detected, revealing a critical dissociation between structural/metabolic injury and functional outcomes.

Evidence shows that ultra-high dose-rate FLASH-radiotherapy (FLASH-RT) provides relative protection against normal tissue complications and functional decrements in the irradiated brain. Past work has shown that radiation-induced cognitive impairment, neuroinflammation and reduced structural complexity ofgranule cell neurons were not observed to the same extent after FLASH-RT (> MGy/s) compared to conventional dose-rate (CONV, 0.1 Gy/s) delivery. In this study, we explored the sensitivity of hippocampal CA1 and medial prefrontal cortex (mPFC) pyramidal neurons to cranial irradiation and dose-rate modulation using electron and confocal microscopy. Neuron ultrastructural analyses by electron microscopy after 10 Gy FLASH- or CONV-RT exposures indicated that irradiation had little impact on dendritic complexity and synapse density in the CA1, but did increase the length and head diameter of smaller non-perforated synapses. Similarly, irradiation caused no change in mPFC prelimbic/infralimbic axospinous synapse density, but reductions in non-perforated synapse diameters. While irradiation resulted in thinner myelin sheaths compared to controls, none of these metrics were dose-rate sensitive. Analysis of fluorescently labeled CA1 neurons revealed no radiation-induced or dose-rate-dependent changes in overall dendritic complexity or spine density, in contrast to our past analysis of granule cell neurons. Super-resolution confocal microscopy following a clinical dosing paradigm (3 × 10 Gy) showed significant reductions in excitatory vesicular glutamate transporter 1 and inhibitory vesicular GABA transporter puncta density within the CA1 that were largely dose-rate independent. Collectively, these data reveal that, compared to granule cell neurons, CA1 and mPFC neurons are relatively more radioresistant irrespective of radiation dose-rate.

Clinicopathological studies of Alzheimer’s disease (AD) have demonstrated that synaptic or neuronal loss and clinical cognitive decline do not reliably correlate with fibrillar amyloid burden. We created a transgenic mouse model overexpressing Dutch (E693Q) mutant human amyloid precursor protein ( APP ) driven by the pan-neuronal Thy1 promoter. Accumulation of APP carboxyl-terminal fragments was observed in the brains of these mice, which develop an impaired learning phenotype directly proportional to brain oAβ levels. Male and female TgAPP E693Q mice and wildtype controls were compared using learning behavioral studies, immunocytochemistry, transmission electron microscopy, electrophysiology, protofibril-specific assays, and single cell RNA sequencing. Brain levels of nonfibrillar oAβ in Dutch mice were shown to increase aging-dependently using A11 immunocytochemistry and FITC-cyclic peptide (FITC-CP-2) microscopy. Two assays excluded the presence of protofibrils. Electrophysiological characterization of hippocampal synapses in Dutch and wildtype mice at ∼7 and ∼11 months revealed no change in basal excitatory transmission, consistent with normal density and morphology of mGluR2/3+ synapses in hippocampal CA1 of the same mice. One exception was increased postsynaptic density in non-perforated mGluR-2/3+ synapses in the Dutch mice. Functional characterization of the presynaptic terminal showed abnormalities in post-tetanic potentiation, synaptic fatigue, and synaptic replenishment after depletion in Dutch mice. Single cell RNA-seq to elucidate cell-type specific transcriptional responses to oAβ revealed altered transcriptional profiles in multiple cell types. Unexpectedly, no obvious differences existed between profiles of microglia from Dutch compared to those from wildtype mice. Excitatory neurons showed the most altered profile which was associated with ‘protein translation’ and ‘oxidative phosphorylation’. Ultrastructural analysis of presynaptic mitochondria at excitatory synapses revealed fewer mitochondria in the presynaptic terminals of Dutch mice. The profound learning behavior deficits in Dutch mice are associated with presynaptic functional deficits and mitochondrial abnormalities in excitatory neurons of the hippocampus. Nonfibrillar oAβ deposits were revealed by co-localization of A11 immunoreactivity with FITC-CP-2 microscopy. Mice accumulating only oAβ may be especially useful for further characterization of the oligomer-specific cyclic azaglycine PET tracer Lys ( 64 Cu/NOTA) 1 ]-CP-7 that shows robust PET signal from 44-day-old presymptomatic 5xFAD mice [Habashi, M. et al. Proc. Natl. Acad. Sci. U.S.A . 2022].

APP E693Q transgenic mice develop aging-related learning deficits and accumulate endogenously generated nonfibrillar aggregates of Aβ (NFA-Aβ) and APP α-carboxy terminal fragments. The APP E693Q mutation disrupts amyloid fibril formation, and no plaques develop in these mice. In the current study, the aging-related accumulation of NFA-Aβ in APP E693Q mice was revealed by A11 immunohistochemistry and NFA-Aβ-detecting cyclic D,L-α-peptide-FITC microscopy. The presynaptic termini of APP E693Q mice developed aging-related physiological abnormalities in post-tetanic potentiation, synaptic fatigue, and synaptic vesicle replenishment. Single-cell RNA sequencing showed that excitatory neurons exhibited the most altered transcriptomic profile, especially involving "protein translation" and "oxidative phosphorylation". Direct measurements of electron transport chain catalysis revealed reduction in mitochondrial complex I activity in Dutch mice. Microglial transcript analysis revealed no evidence of inflammation. The depletion or neutralization of both fibrillar and NFA-Aβ may be needed for complete elimination of Aβ toxicity. Teaser:APP E693Q "NFA-Aβ only" mice reveal clinically relevant mechanisms despite the absence of detectable inflammation.

Due to their interactions with the neurovasculature, microglia are implicated in maladaptive responses to hypobaric hypoxia at high altitude (HA). To explore these interactions at HA, pharmacological depletion of microglia with the colony-stimulating factor-1 receptor inhibitor, PLX5622, was employed in male C57BL/6J mice maintained at HA or sea level (SL) for 3-weeks, followed by assessment of ex-vivo hippocampal long-term potentiation (LTP), fear memory recall and microglial dynamics/physiology. Our findings revealed that microglia depletion decreased LTP and reduced glucose levels by 25% at SL but did not affect fear memory recall. At HA, the absence of microglia did not significantly alter HA associated deficits in fear memory or HA mediated decreases in peripheral glucose levels. In regard to microglial dynamics in the cortex, HA enhanced microglial surveillance activity, ablation of microglia resulted in increased chemotactic responses and decreased microglia tip proliferation during ball formation. In contrast, vessel ablation increased cortical microglia tip path tortuosity. In the hippocampus, changes in microglial dynamics were only observed in response to vessel ablation following HA. As the hippocampus is critical for learning and memory, poor hippocampal microglial context-dependent adaptation may be responsible for some of the enduring neurological deficits associated with HA.

Technological advancements have facilitated the implementation of realistic, terrestrial-based complex 33-beam galactic cosmic radiation simulations (GCR Sim) to now probe central nervous system functionality. This work expands considerably on prior, simplified GCR simulations, yielding new insights into responses of male and female mice exposed to 40–50 cGy acute or chronic radiations relevant to deep space travel. Results of the object in updated location task suggested that exposure to acute or chronic GCR Sim induced persistent impairments in hippocampus-dependent memory formation and reconsolidation in female mice that did not manifest robustly in irradiated male mice. Interestingly, irradiated male mice, but not females, were impaired in novel object recognition and chronically irradiated males exhibited increased aggressive behavior on the tube dominance test. Electrophysiology studies used to evaluate synaptic plasticity in the hippocampal CA1 region revealed significant reductions in long-term potentiation after each irradiation paradigm in both sexes. Interestingly, network-level disruptions did not translate to altered intrinsic electrophysiological properties of CA1 pyramidal cells, whereas acute exposures caused modest drops in excitatory synaptic signaling in males. Ultrastructural analyses of CA1 synapses found smaller postsynaptic densities in larger spines of chronically exposed mice compared to controls and acutely exposed mice. Myelination was also affected by GCR Sim with acutely exposed mice exhibiting an increase in the percent of myelinated axons; however, the myelin sheathes on small calibur (< 0.3 mm) and larger (> 0.5 mm) axons were thinner when compared to controls. Present findings might have been predicted based on previous studies using single and mixed beam exposures and provide further evidence that space-relevant radiation exposures disrupt critical cognitive processes and underlying neuronal network-level plasticity, albeit not to the extent that might have been previously predicted.

The interplay between AMPA-type glutamate receptors (AMPARs) and major histocompatibility complex class I (MHC-I) proteins in regulating synaptic signaling is a crucial aspect of central nervous system (CNS) function. In this study, we investigate the significance of the cytoplasmic tail of MHC-I in synaptic signaling within the CNS and its impact on the modulation of synaptic glutamate receptor expression. Specifically, we focus on the Y321 to F substitution (Y321F) within the conserved cytoplasmic tyrosine YXXΦ motif, known for its dual role in endocytosis and cellular signaling of MHC-I. Our findings reveal that the Y321F substitution influences the expression of AMPAR subunits GluA2/3 and leads to alterations in the phosphorylation of key kinases, including Fyn, Lyn, p38, ERK1/2, JNK1/2/3, and p70 S6 kinase. These data illuminate the crucial role of MHC-I in AMPAR function and present a novel mechanism by which MHC-I integrates extracellular cues to modulate synaptic plasticity in neurons, which ultimately underpins learning and memory.

Major histocompatibility complex class I (MHC-I) proteins are expressed in neurons, where they regulate synaptic plasticity. However, the mechanisms by which MHC-I functions in the CNS remains unknown. Here we describe the first structural analysis of a MHC-I protein, to resolve underlying mechanisms that explains its function in the brain. We demonstrate that Y321F mutation of the conserved cytoplasmic tyrosine-based endocytosis motif YXXΦ in MHC-I affects spine density and synaptic structure without affecting neuronal complexity in the hippocampus, a region of the brain intimately involved in learning and memory. Furthermore, the impact of the Y321F substitution phenocopies MHC-I knock-out (null) animals, demonstrating that reverse, outside-in signalling events sensing the external environment is the major mechanism that conveys this information to the neuron and this has a previously undescribed yet essential role in the regulation of synaptic plasticity.

Abstract Major histocompatibility complex class I (MHC-I) proteins are expressed in neurons, where they regulate synaptic plasticity. However, the mechanisms by which MHC-I functions in the CNS remains unknown. Here we describe the first structural analysis of MHC-I, to resolve underlying mechanisms that explains its function. We demonstrate that Y321F mutation of the conserved cytoplasmic tyrosine-based endocytosis motif YXXΦ in MHC-I affects spine density and synaptic structure without affecting neuronal complexity. Furthermore, the impact of the Y321F substitution phenocopies the MHC-I null animals, demonstrating that reverse, outside-in signalling events sensing the external environment is the major mechanism that conveys this information to the neuron and this has an essential role in the regulation of synaptic plasticity.

The proposed deep space exploration to the moon and later to Mars will result in astronauts receiving significant chronic exposures to space radiation (SR). SR exposure results in multiple neurocognitive impairments. Recently, our cross-species (mouse/rat) studies reported impaired associative memory formation in both species following a chronic 6-month low dose exposure to a mixed field of neutrons (1 mGy/day for a total dose pf 18 cGy). In the present study, we report neutron exposure induced synaptic plasticity in the medial prefrontal cortex, accompanied by microglial activation and significant synaptic loss in the hippocampus. In a parallel study, neutron exposure was also found to alter fluorescence assisted single synaptosome LTP (FASS-LTP) in the hippocampus of rats, that may be related to a reduced ability to insert AMPAR into the post-synaptic membrane, which may arise from increased phosphorylation of the serine 845 residue of the GluA1 subunit. Thus, we demonstrate for the first time, that low dose chronic neutron irradiation impacts homeostatic synaptic plasticity in the hippocampal-cortical circuit in two rodent species, and that the ability to successfully encode associative recognition memory is a dynamic, multicircuit process, possibly involving compensatory changes in AMPAR density on the synaptic surface.

Military veterans who experience blast-related traumatic brain injuries often suffer from chronic cognitive and neurobehavioral syndromes. Reports of abnormal tau processing following blast injury have raised concerns that some cases may have a neurodegenerative basis. Rats exposed to repetitive low-level blast exhibit chronic neurobehavioral traits and accumulate tau phosphorylated at threonine 181 (Thr181). Using data previously reported in separate studies we tested the hypothesis that region-specific patterns of Thr181 phosphorylation correlate with behavioral measures also previously determined and reported in the same animals. Elevated p-tau Thr181 in anterior neocortical regions and right hippocampus correlated with anxiety as well as fear learning and novel object localization. There were no correlations with levels in amygdala or posterior neocortical regions. Particularly striking were asymmetrical effects on the right and left hippocampus. No systematic variation in head orientation toward the blast wave seems to explain the laterality. Levels did not correlate with behavioral measures of hyperarousal. Results were specific to Thr181 in that no correlations were observed for three other phospho-acceptor sites (threonine 231, serine 396, and serine 404). No consistent correlations were linked with total tau. These correlations are significant in suggesting that p-tau accumulation in anterior neocortical regions and the hippocampus may lead to disinhibited amygdala function without p-tau elevation in the amygdala itself. They also suggest an association linking blast injury with tauopathy, which has implications for understanding the relationship of chronic blast-related neurobehavioral syndromes in humans to neurodegenerative diseases.

Traumatic brain injury (TBI) is a risk factor for the later development of neurodegenerative diseases that may have various underlying pathologies. Chronic traumatic encephalopathy (CTE) in particular is associated with repetitive mild TBI (mTBI) and is characterized pathologically by aggregation of hyperphosphorylated tau into neurofibrillary tangles (NFTs). CTE may be suspected when behavior, cognition, and/or memory deteriorate following repetitive mTBI. Exposure to blast overpressure from improvised explosive devices (IEDs) has been implicated as a potential antecedent for CTE amongst Iraq and Afghanistan Warfighters. In this study, we identified biomarker signatures in rats exposed to repetitive low-level blast that develop chronic anxiety-related traits and in human veterans exposed to IED blasts in theater with behavioral, cognitive, and/or memory complaints. Rats exposed to repetitive low-level blasts accumulated abnormal hyperphosphorylated tau in neuronal perikarya and perivascular astroglial processes. Using positron emission tomography (PET) and the [18F]AV1451 (flortaucipir) tau ligand, we found that five of 10 veterans exhibited excessive retention of [18F]AV1451 at the white/gray matter junction in frontal, parietal, and temporal brain regions, a typical localization of CTE tauopathy. We also observed elevated levels of neurofilament light (NfL) chain protein in the plasma of veterans displaying excess [18F]AV1451 retention. These findings suggest an association linking blast injury, tauopathy, and neuronal injury. Further study is required to determine whether clinical, neuroimaging, and/or fluid biomarker signatures can improve the diagnosis of long-term neuropsychiatric sequelae of mTBI.

Long-term operations carried out at high altitude (HA) by military personnel, pilots, and astronauts may trigger health complications. In particular, chronic exposure to high altitude (CEHA) has been associated with deficits in cognitive function. In this study, we found that mice exposed to chronic HA (5000 m for 12 weeks) exhibited deficits in learning and memory associated with hippocampal function and were linked with changes in the expression of synaptic proteins across various regions of the brain. Specifically, we found decreased levels of synaptophysin (SYP) (p < 0.05) and spinophilin (SPH) (p < 0.05) in the olfactory cortex, post synaptic density−95 (PSD-95) (p < 0.05), growth associated protein 43 (GAP43) (p < 0.05), glial fibrillary acidic protein (GFAP) (p < 0.05) in the cerebellum, and SYP (p < 0.05) and PSD-95 (p < 0.05) in the brainstem. Ultrastructural analyses of synaptic density and morphology in the hippocampus did not reveal any differences in CEHA mice compared to SL mice. Our data are novel and suggest that CEHA exposure leads to cognitive impairment in conjunction with neuroanatomically-based molecular changes in synaptic protein levels and astroglial cell marker in a region specific manner. We hypothesize that these new findings are part of highly complex molecular and neuroplasticity mechanisms underlying neuroadaptation response that occurs in brains when chronically exposed to HA.

Neuronal circuits within the prefrontal cortex (PFC) mediate higher cognitive functions and emotional regulation that are disrupted in psychiatric disorders. The PFC undergoes significant maturation during adolescence, a period when cannabis use in humans has been linked to subsequent vulnerability to psychiatric disorders such as addiction and schizophrenia. Here, we investigated in a rat model the effects of adolescent exposure to Δ 9 -tetrahydrocannabinol (THC), a psychoactive component of cannabis, on the morphological architecture and transcriptional profile of layer III pyramidal neurons—using cell type- and layer-specific high-resolution microscopy, laser capture microdissection and next-generation RNA-sequencing. The results confirmed known normal expansions in basal dendritic arborization and dendritic spine pruning during the transition from late adolescence to early adulthood that were accompanied by differential expression of gene networks associated with neurodevelopment in control animals. In contrast, THC exposure disrupted the normal developmental process by inducing premature pruning of dendritic spines and allostatic atrophy of dendritic arborization in early adulthood. Surprisingly, there was minimal overlap of the developmental transcriptomes between THC- and vehicle-exposed rats. THC altered functional gene networks related to cell morphogenesis, dendritic development, and cytoskeleton organization. Marked developmental network disturbances were evident for epigenetic regulators with enhanced co-expression of chromatin- and dendrite-related genes in THC-treated animals. Dysregulated PFC co-expression networks common to both the THC-treated animals and patients with schizophrenia were enriched for cytoskeletal and neurite development. Overall, adolescent THC exposure altered the morphological and transcriptional trajectory of PFC pyramidal neurons, which could enhance vulnerability to psychiatric disorders.