Quantifying the accretion of hyperphosphorylated tau in the locus coeruleus and dorsal raphe nucleus: the pathological building blocks of early Alzheimer's disease

Alzheimer's disease and sleep disorders: A bidirectional relationship

Alzheimer's disease (AD) is the most prevalent dementia, pathologically featuring abnormal accumulation of amyloid-β (Aβ) and hyperphosphorylated tau, while sleep, divided into rapid eye movement sleep (REM) and nonrapid eye movement sleep (NREM), plays a key role in consolidating social and spatial memory. Emerging evidence has revealed that sleep disorders such as circadian disturbances and disruption of neuronal rhythm activity are considered as both candidate risks and consequence of AD, suggesting a bidirectional relationship between sleep and AD. This review will firstly grasp basic knowledge of AD pathogenesis, then highlight macrostructural and microstructural alteration of sleep along with AD progression, explain the interaction between accumulation of Aβ and hyperphosphorylated tau, which are two critical neuropathological processes of AD, as well as neuroinflammation and sleep, and finally introduce several methods of sleep enhancement as strategies to reduce AD-associated neuropathology. Although theories about the bidirectional relationship and relevant therapeutic methods in mice have been well developed in recent years, the knowledge in human is still limited. More studies on how to effectively ameliorate AD pathology in patients by sleep enhancement and what specific roles of sleep play in AD are needed.

Ageing-related tau astrogliopathy severely affecting the substantia nigra

AIMS

Astrocytic tau pathology is a major feature of tauopathies and ageing-related tau astrogliopathy (ARTAG). The substantia nigra (SN) is one of the important degenerative areas in tauopathies with parkinsonism. Nigral tau pathology is usually reported as neuronal predominant with less prominent astrocytic involvement. We aimed to identify cases with prominent astrocytic tau pathology in the SN.

METHODS

We use the term nigral tau-astrogliopathy (NITAG) to describe cases showing an unusually high density of ARTAG with less neuronal tau pathology in the SN. We collected clinical information and studied the distribution of tau pathology, morphological features and immunostaining profiles in three cases.

RESULTS

Three cases, all males with parkinsonism, were identified with the following clinicopathological diagnoses: (i) atypical parkinsonism with tau pathology reminiscent to that in postencephalitic parkinsonism (69-year-old); (ii) multiple system atrophy (73-year-old); (iii) traumatic encephalopathy syndrome/chronic traumatic encephalopathy (84-year-old). Double-labelling immunofluorescence confirmed co-localization of GFAP and phosphorylated tau in affected astrocytes. Staining profiles of NITAG revealed immunopositivity for various phosphorylated tau antibodies. Some astrocytic tau lesions were also seen in other brainstem regions and cerebral grey matter.

CONCLUSIONS

We propose NITAG is a rare neuropathological feature, and not a distinct disease entity, in the frame of multiple system ARTAG, represented by abundant tau-positive astrocytes in various brain regions but having the highest density in the SN. The concept of NITAG allows the stratification of cases with various background pathologies to understand its relevance and contribution to neuronal dysfunction.

Greater physical fitness ( VO 2 max ) in healthy older adults associated with increased integrity of the locus coeruleus-noradrenergic system

AIM

Physical activity (PA) is a key component for brain health and Reserve, and it is among the main dementia protective factors. However, the neurobiological mechanisms underpinning Reserve are not fully understood. In this regard, a noradrenergic (NA) theory of cognitive reserve (Robertson, 2013) has proposed that the upregulation of NA system might be a key factor for building reserve and resilience to neurodegeneration because of the neuroprotective role of NA across the brain. PA elicits an enhanced catecholamine response, in particular for NA. By increasing physical commitment, a greater amount of NA is synthetised in response to higher oxygen demand. More physically trained individuals show greater capabilities to carry oxygen resulting in greaterVo 2 max - a measure of oxygen uptake and physical fitness (PF).

METHODS

We hypothesized that greaterVo 2 max would be related to greater Locus Coeruleus (LC) MRI signal intensity. In a sample of 41 healthy subjects, we performed Voxel-Based Morphometry analyses, then repeated for the other neuromodulators as a control procedure (Serotonin, Dopamine and Acetylcholine).

RESULTS

As hypothesized, greaterVo 2 max related to greater LC signal intensity, and weaker associations emerged for the other neuromodulators.

CONCLUSION

This newly established link betweenVo 2 max and LC-NA system offers further understanding of the neurobiology underpinning Reserve in relationship to PA. While this study supports Robertson's theory proposing the upregulation of the NA system as a possible key factor building Reserve, it also provides ground for increasing LC-NA system resilience to neurodegeneration viaVo 2 max enhancement.

Unveiling a novel memory center in human brain: neurochemical identification of the nucleus incertus, a key pontine locus implicated in stress and neuropathology

BACKGROUND

The nucleus incertus (NI) was originally described by Streeter in 1903, as a midline region in the floor of the fourth ventricle of the human brain with an 'unknown' function. More than a century later, the neuroanatomy of the NI has been described in lower vertebrates, but not in humans. Therefore, we examined the neurochemical anatomy of the human NI using markers, including the neuropeptide, relaxin-3 (RLN3), and began to explore the distribution of the NI-related RLN3 innervation of the hippocampus.

METHODS

Histochemical staining of serial, coronal sections of control human postmortem pons was conducted to reveal the presence of the NI by detection of immunoreactivity (IR) for the neuronal markers, microtubule-associated protein-2 (MAP2), glutamic acid dehydrogenase (GAD)-65/67 and corticotrophin-releasing hormone receptor 1 (CRHR1), and RLN3, which is highly expressed in NI neurons in diverse species. RLN3 and vesicular GABA transporter 1 (vGAT1) mRNA were detected by fluorescent in situ hybridization. Pons sections containing the NI from an AD case were immunostained for phosphorylated-tau, to explore potential relevance to neurodegenerative diseases. Lastly, sections of the human hippocampus were stained to detect RLN3-IR and somatostatin (SST)-IR.

RESULTS

In the dorsal, anterior-medial region of the human pons, neurons containing RLN3- and MAP2-IR, and RLN3/vGAT1 mRNA-positive neurons were observed in an anatomical pattern consistent with that of the NI in other species. GAD65/67- and CRHR1-immunopositive neurons were also detected within this area. Furthermore, RLN3- and AT8-IR were co-localized within NI neurons of an AD subject. Lastly, RLN3-IR was detected in neurons within the CA1, CA2, CA3 and DG areas of the hippocampus, in the absence of RLN3 mRNA. In the DG, RLN3- and SST-IR were co-localized in a small population of neurons.

CONCLUSIONS

Aspects of the anatomy of the human NI are shared across species, including a population of stress-responsive, RLN3-expressing neurons and a RLN3 innervation of the hippocampus. Accumulation of phosphorylated-tau in the NI suggests its possible involvement in AD pathology. Further characterization of the neurochemistry of the human NI will increase our understanding of its functional role in health and disease.

Lower Locus Coeruleus Integrity Signals Elevated Entorhinal Tau and Clinical Progression in Asymptomatic Older Individuals

OBJECTIVE

Elevated entorhinal cortex (EC) tau in low beta-amyloid individuals can predict accumulation of pathology and cognitive decline. We compared the accuracy of magnetic resonance imaging (MRI)-derived locus coeruleus integrity, neocortical beta-amyloid burden by positron emission tomography (PET), and hippocampal volume in identifying elevated entorhinal tau signal in asymptomatic individuals who are considered beta-amyloid PET-negative.

METHODS

We included 188 asymptomatic individuals (70.78 ± 11.51 years, 58% female) who underwent 3T-MRI of the locus coeruleus, Pittsburgh compound-B (PiB), and Flortaucipir (FTP) PET. Associations between elevated EC tau and neocortical PiB, hippocampal volume, or locus coeruleus integrity were evaluated and compared using logistic regression and receiver operating characteristic analyses in the PiB- sample with a clinical dementia rating (CDR) of 0. Associations with clinical progression (CDR-sum-of-boxes) over a time span of 6 years were evaluated with Cox proportional hazard models.

RESULTS

We identified 26 (21%) individuals with high EC FTP in the CDR = 0/PiB- sample. Locus coeruleus integrity was a significantly more sensitive and specific predictor of elevated EC FTP (area under the curve [AUC] = 85%) compared with PiB (AUC = 77%) or hippocampal volume (AUC = 76%). Based on the Youden-index, locus coeruleus integrity obtained a sensitivity of 77% and 85% specificity. Using the resulting locus coeruleus Youden cut-off, lower locus coeruleus integrity was associated with a two-fold increase in clinical progression, including mild cognitive impairment.

INTERPRETATION

Locus coeruleus integrity has promise as a low-cost, non-invasive screening instrument to detect early cortical tau deposition and associated clinical progression in asymptomatic, low beta-amyloid individuals. ANN NEUROL 2024.

Electroacupuncture Alleviates Memory Deficits in APP/PS1 Mice by Targeting Serotonergic Neurons in Dorsal Raphe Nucleus

OBJECTIVE

Alzheimer's disease (AD) has become a significant global concern, but effective drugs able to slow down AD progression is still lacked. Electroacupuncture (EA) has been demonstrated to ameliorate cognitive impairment in individuals with AD. However, the underlying mechanisms remains poorly understood. This study aimed at examining the neuroprotective properties of EA and its potential mechanism of action against AD.

METHODS

APP/PS1 transgenic mice were employed to evaluate the protective effects of EA on Shenshu (BL 23) and Baihui (GV 20). Chemogenetic manipulation was used to activate or inhibit serotonergic neurons within the dorsal raphe nucleus (DRN). Learning and memory abilities were assessed by the novel object recognition and Morris water maze tests. Golgi staining, western blot, and immunostaining were utilized to determine EA-induced neuroprotection.

RESULTS

EA at Shenshu (BL 23) and Baihui (GV 20) effectively ameliorated learning and memory impairments in APP/PS1 mice. EA attenuated dendritic spine loss, increased the expression levels of PSD95, synaptophysin, and brain-derived neurotrophic factor in hippocampus. Activation of serotonergic neurons within the DRN can ameliorate cognitive deficits in AD by activating glutamatergic neurons mediated by 5-HT1B. Chemogenetic inhibition of serotonergic neurons in the DRN reversed the effects of EA on synaptic plasticity and memory.

CONCLUSION

EA can alleviate cognitive dysfunction in APP/PS1 mice by activating serotonergic neurons in the DRN. Further study is necessary to better understand how the serotonergic neurons-related neural circuits involves in EA-induced memory improvement in AD.

Exploring advancements in early detection of Alzheimer's disease with molecular assays and animal models

Alzheimer's Disease (AD) is a challenging neurodegenerative condition, with overwhelming implications for affected individuals and healthcare systems worldwide. Animal models have played a crucial role in studying AD pathogenesis and testing therapeutic interventions. Remarkably, studies on the genetic factors affecting AD risk, such as APOE and TREM2, have provided valuable insights into disease mechanisms. Early diagnosis has emerged as a crucial factor in effective AD management, as demonstrated by clinical studies emphasizing the benefits of initiating treatment at early stages. Novel diagnostic technologies, including RNA sequencing of microglia, offer promising avenues for early detection and monitoring of AD progression. Therapeutic strategies remain to evolve, with a focus on targeting amyloid beta (Aβ) and tau pathology. Advances in animal models, such as APP-KI mice, and the advancement of anti-Aβ drugs signify progress towards more effective treatments. Therapeutically, the focus has shifted towards intricate approaches targeting multiple pathological pathways simultaneously. Strategies aimed at reducing Aβ plaque accumulation, inhibiting tau hyperphosphorylation, and modulating neuroinflammation are actively being explored, both in preclinical models and clinical trials. While challenges continue in developing validated animal models and translating preclinical findings to clinical success, the continuing efforts in understanding AD at molecular, cellular, and clinical levels offer hope for improved management and eventual prevention of this devastating disease.

Evidence for a novel neuronal mechanism driving Alzheimer's disease, upstream of amyloid

This perspective offers an alternative to the amyloid hypothesis in the etiology of Alzheimer's disease (AD). We review evidence for a novel signaling mechanism based on a little-known peptide, T14. T14 could drive neurodegeneration as an aberrantly activated process of plasticity selective to interconnecting subcortical nuclei, the isodendritic core, where cell loss starts at the pre-symptomatic stages of the disease. Each of these cell groups has the capacity to form T14, which can stimulate production of p-Tau and β-amyloid, suggestive of an upstream driver of neurodegeneration. Moreover, results in an animal AD model show that antagonism of T14 with a cyclated variant, NBP14, prevents formation of β-amyloid, and restores cognitive function to that of wild-type counterparts. Any diagnostic and/or therapeutic strategy based on T14-NBP14 awaits validation in clinical trials. However, an understanding of this novel signaling system could bring much-needed fresh insights into the progression of cell loss underlying AD. HIGHLIGHTS: The possible primary mechanism of neurodegeneration upstream of amyloid. Primary involvement of selectively vulnerable subcortical nuclei, isodendritic core. Bioactive peptide T14 trophic in development but toxic in context of mature brain. Potential for early-stage biomarker to detect Alzheimer's disease. Effective therapeutic halting neurodegeneration, validated already in 5XFAD mice.

Early and selective localization of tau filaments to glutamatergic subcellular domains within the human anterodorsal thalamus

Widespread cortical accumulation of misfolded pathological tau proteins (ptau) in the form of paired helical filaments is a major hallmark of Alzheimer's disease. Subcellular localization of ptau at various stages of disease progression is likely to be informative of the cellular mechanisms involving its spread. Here, we found that the density of ptau within several distinct rostral thalamic nuclei in post-mortem human tissue (n = 25 cases) increased with the disease stage, with the anterodorsal nucleus (ADn) consistently being the most affected. In the ADn, ptau-positive elements were present already in the pre-cortical (Braak 0) stage. Tau pathology preferentially affected the calretinin-expressing subpopulation of glutamatergic neurons in the ADn. At the subcellular level, we detected ptau immunoreactivity in ADn cell bodies, dendrites, and in a specialized type of presynaptic terminal that expresses vesicular glutamate transporter 2 (vGLUT2) and likely originates from the mammillary body. The ptau-containing terminals displayed signs of degeneration, including endosomal/lysosomal organelles. In contrast, corticothalamic axon terminals lacked ptau. The data demonstrate the involvement of a specific cell population in ADn at the onset of the disease. The presence of ptau in subcortical glutamatergic presynaptic terminals supports hypotheses about the transsynaptic spread of tau selectively affecting specialized axonal pathways.

Microstructural associations between locus coeruleus, cortical, and subcortical regions are modulated by astrocyte reactivity: a 7T MRI adult lifespan study

The locus coeruleus-norepinephrine system plays a key role in supporting brain health along the lifespan, notably through its modulatory effects on neuroinflammation. Using ultra-high field diffusion magnetic resonance imaging, we examined whether microstructural properties (neurite density index and orientation dispersion index) in the locus coeruleus were related to those in cortical and subcortical regions, and whether this was modulated by plasma glial fibrillary acidic protein levels, as a proxy of astrocyte reactivity. In our cohort of 60 healthy individuals (30 to 85 yr, 50% female), higher glial fibrillary acidic protein correlated with lower neurite density index in frontal cortical regions, the hippocampus, and the amygdala. Furthermore, under higher levels of glial fibrillary acidic protein (above ~ 150 pg/mL for cortical and ~ 145 pg/mL for subcortical regions), lower locus coeruleus orientation dispersion index was associated with lower orientation dispersion index in frontotemporal cortical regions and in subcortical regions. Interestingly, individuals with higher locus coeruleus orientation dispersion index exhibited higher orientation dispersion index in these (sub)cortical regions, despite having higher glial fibrillary acidic protein levels. Together, these results suggest that the interaction between locus coeruleus-norepinephrine cells and astrocytes can signal a detrimental or neuroprotective pathway for brain integrity and support the importance of maintaining locus coeruleus neuronal health in aging and in the prevention of age-related neurodegenerative diseases.

Locus coeruleus integrity and left frontoparietal connectivity provide resilience against attentional decline in preclinical alzheimer's disease

BACKGROUND

Autopsy work reported that neuronal density in the locus coeruleus (LC) provides neural reserve against cognitive decline in dementia. Recent neuroimaging and pharmacological studies reported that left frontoparietal network functional connectivity (LFPN-FC) confers resilience against beta-amyloid (Aβ)-related cognitive decline in preclinical sporadic and autosomal dominant Alzheimer's disease (AD), as well as against LC-related cognitive changes. Given that the LFPN and the LC play important roles in attention, and attention deficits have been observed early in the disease process, we examined whether LFPN-FC and LC structural health attenuate attentional decline in the context of AD pathology.

METHODS

142 participants from the Harvard Aging Brain Study who underwent resting-state functional MRI, LC structural imaging, PiB(Aβ)-PET, and up to 5 years of cognitive follow-ups were included (mean age = 74.5 ± 9.9 years, 89 women). Cross-sectional robust linear regression associated LC integrity (measured as the average of five continuous voxels with the highest intensities in the structural LC images) or LFPN-FC with Digit Symbol Substitution Test (DSST) performance at baseline. Longitudinal robust mixed effect analyses examined associations between DSST decline and (i) two-way interactions of baseline LC integrity (or LFPN-FC) and PiB or (ii) the three-way interaction of baseline LC integrity, LFPN-FC, and PiB. Baseline age, sex, and years of education were included as covariates.

RESULTS

At baseline, lower LFPN-FC, but not LC integrity, was related to worse DSST performance. Longitudinally, lower baseline LC integrity was associated with a faster DSST decline, especially at PiB > 10.38 CL. Lower baseline LFPN-FC was associated with a steeper decline on the DSST but independent of PiB. At elevated PiB levels (> 46 CL), higher baseline LFPN-FC was associated with an attenuated decline on the DSST, despite the presence of lower LC integrity.

CONCLUSIONS

Our findings demonstrate that the LC can provide resilience against Aβ-related attention decline. However, when Aβ accumulates and the LC's resources may be depleted, the functioning of cortical target regions of the LC, such as the LFPN-FC, can provide additional resilience to sustain attentional performance in preclinical AD. These results provide critical insights into the neural correlates contributing to individual variability at risk versus resilience against Aβ-related cognitive decline.

Spatiotemporal patterns of locus coeruleus integrity predict cortical tau and cognition

Autopsy studies indicated that the locus coeruleus (LC) accumulates hyperphosphorylated tau before allocortical regions in Alzheimer's disease. By combining in vivo longitudinal magnetic resonance imaging measures of LC integrity, tau positron emission tomography imaging and cognition with autopsy data and transcriptomic information, we examined whether LC changes precede allocortical tau deposition and whether specific genetic features underlie LC's selective vulnerability to tau. We found that LC integrity changes preceded medial temporal lobe tau accumulation, and together these processes were associated with lower cognitive performance. Common gene expression profiles between LC-medial temporal lobe-limbic regions map to biological functions in protein transport regulation. These findings advance our understanding of the spatiotemporal patterns of initial tau spreading from the LC and LC's selective vulnerability to Alzheimer's disease pathology. LC integrity measures can be a promising indicator for identifying the time window when individuals are at risk of disease progression and underscore the importance of interventions mitigating initial tau spread.

Serotonergic dysfunction may mediate the relationship between alcohol consumption and Alzheimer's disease

The impact of Alzheimer's disease (AD) and its related dementias is rapidly expanding, and its mitigation remains an urgent social and technical challenge. To date there are no effective treatments or interventions for AD, but recent studies suggest that alcohol consumption is correlated with the risk of developing dementia. In this review, we synthesize data from preclinical, clinical, and epidemiological models to evaluate the combined role of alcohol consumption and serotonergic dysfunction in AD, underscoring the need for further research on this topic. We first discuss the limitations inherent to current data-collection methods, and how neuropsychiatric symptoms common among AD, alcohol use disorder, and serotonergic dysfunction may mask their co-occurrence. We additionally describe how excess alcohol consumption may accelerate the development of AD via direct effects on serotonergic function, and we explore the roles of neuroinflammation and proteostasis in mediating the relationship between serotonin, alcohol consumption, and AD. Lastly, we argue for a shift in current research to disentangle the pathogenic effects of alcohol on early-affected brainstem structures in AD.

Behavioral and Neurophysiological Implications of Pathological Human Tau Expression in Serotonin Neurons

Alzheimer's disease (AD) is a progressive degenerative disorder that results in a severe loss of brain cells and irreversible cognitive decline. Memory problems are the most recognized symptoms of AD. However, approximately 90% of patients diagnosed with AD suffer from behavioral symptoms, including mood changes and social impairment years before cognitive dysfunction. Recent evidence indicates that the dorsal raphe nucleus (DRN) is among the initial regions that show tau pathology, which is a hallmark feature of AD. The DRN harbors serotonin (5-HT) neurons, which are critically involved in mood, social, and cognitive regulation. Serotonergic impairment early in the disease process may contribute to behavioral symptoms in AD. However, the mechanisms underlying vulnerability and contribution of the 5-HT system to AD progression remain unknown. Here, we performed behavioral and electrophysiological characterizations in mice expressing a phosphorylation-prone form of human tau (hTauP301L) in 5-HT neurons. We found that pathological tau expression in 5-HT neurons induces anxiety-like behavior and alterations in stress-coping strategies in female and male mice. Female mice also exhibited social disinhibition and mild cognitive impairment in response to 5-HT neuron-specific hTauP301L expression. Behavioral alterations were accompanied by disrupted 5-HT neuron physiology in female and male hTauP301L expressing mice with exacerbated excitability disruption in females only. These data provide mechanistic insights into the brain systems and symptoms impaired early in AD progression, which is critical for disease intervention.

Respiratory Dysfunction in Alzheimer's Disease-Consequence or Underlying Cause? Applying Animal Models to the Study of Respiratory Malfunctions

Alzheimer's disease (AD) is a neurodegenerative brain disease that is the most common cause of dementia among the elderly. In addition to dementia, which is the loss of cognitive function, including thinking, remembering, and reasoning, and behavioral abilities, AD patients also experience respiratory disturbances. The most common respiratory problems observed in AD patients are pneumonia, shortness of breath, respiratory muscle weakness, and obstructive sleep apnea (OSA). The latter is considered an outcome of Alzheimer's disease and is suggested to be a causative factor. While this narrative review addresses the bidirectional relationship between obstructive sleep apnea and Alzheimer's disease and reports on existing studies describing the most common respiratory disorders found in patients with Alzheimer's disease, its main purpose is to review all currently available studies using animal models of Alzheimer's disease to study respiratory impairments. These studies on animal models of AD are few in number but are crucial for establishing mechanisms, causation, implementing potential therapies for respiratory disorders, and ultimately applying these findings to clinical practice. This review summarizes what is already known in the context of research on respiratory disorders in animal models, while pointing out directions for future research.

Social activity mediates locus coeruleus tangle-related cognition in older adults

The locus coeruleus-noradrenaline system regulates brain-wide neural activity involved in cognition and behavior. Integrity of this subcortical neuromodulatory system is proposed to be a substrate of cognitive reserve that may be strengthened by lifetime cognitive and social activity. Conversely, accumulation of tau tangles in the brainstem locus coeruleus nuclei is recently studied as a very early marker of Alzheimer's disease (AD) pathogenesis and cognitive vulnerability, even among older adults without cognitive impairment or significant cerebral AD pathologies. This clinical-pathologic study examined whether locus coeruleus tangle density was cross-sectionally associated with lower antemortem cognitive performance and social activity among 142 cognitively unimpaired and impaired older adults and whether social activity, a putative reserve factor, mediated the association of tangle density and cognition. We found that greater locus coeruleus tangle density was associated with lower social activity for the whole sample and in the cognitively unimpaired group alone and these associations were independent of age, sex, education, depressive symptoms, and burden of cerebral amyloid and tau. The association of locus coeruleus tangle density with lower cognitive performance was partially mediated by level of social activity. These findings implicate the locus coeruleus-noradrenaline system in late-life social function and support that locus coeruleus tangle pathology is associated with lower levels of social activity, independent of cerebral AD pathologies, and specifically among older adults who are cognitively unimpaired. Early brainstem pathology may impact social function, and level of social function, in turn, influences cognition, prior to canonical stages of AD.

14-3-3 proteins-a moonlight protein complex with therapeutic potential in neurological disorder: in-depth review with Alzheimer's disease

Alzheimer's disease (AD) affects millions of people worldwide and is a gradually worsening neurodegenerative condition. The accumulation of abnormal proteins, such as tau and beta-amyloid, in the brain is a hallmark of AD pathology. 14-3-3 proteins have been implicated in AD pathology in several ways. One proposed mechanism is that 14-3-3 proteins interact with tau protein and modulate its phosphorylation, aggregation, and toxicity. Tau is a protein associated with microtubules, playing a role in maintaining the structural integrity of neuronal cytoskeleton. However, in the context of Alzheimer's disease (AD), an abnormal increase in its phosphorylation occurs. This leads to the aggregation of tau into neurofibrillary tangles, which is a distinctive feature of this condition. Studies have shown that 14-3-3 proteins can bind to phosphorylated tau and regulate its function and stability. In addition, 14-3-3 proteins have been shown to interact with beta-amyloid (Aβ), the primary component of amyloid plaques in AD. 14-3-3 proteins can regulate the clearance of Aβ through the lysosomal degradation pathway by interacting with the lysosomal membrane protein LAMP2A. Dysfunction of lysosomal degradation pathway is thought to contribute to the accumulation of Aβ in the brain and the progression of AD. Furthermore, 14-3-3 proteins have been found to be downregulated in the brains of AD patients, suggesting that their dysregulation may contribute to AD pathology. For example, decreased levels of 14-3-3 proteins in cerebrospinal fluid have been suggested as a biomarker for AD. Overall, these findings suggest that 14-3-3 proteins may play an important role in AD pathology and may represent a potential therapeutic target for the disease. However, further research is needed to fully understand the mechanisms underlying the involvement of 14-3-3 proteins in AD and to explore their potential as a therapeutic target.

Neuromelanin-sensitive MRI as a promising biomarker of catecholamine function

Disruptions to dopamine and noradrenergic neurotransmission are noted in several neurodegenerative and psychiatric disorders. Neuromelanin-sensitive (NM)-MRI offers a non-invasive approach to visualize and quantify the structural and functional integrity of the substantia nigra and locus coeruleus. This method may aid in the diagnosis and quantification of longitudinal changes of disease and could provide a stratification tool for predicting treatment success of pharmacological interventions targeting the dopaminergic and noradrenergic systems. Given the growing clinical interest in NM-MRI, understanding the contrast mechanisms that generate this signal is crucial for appropriate interpretation of NM-MRI outcomes and for the continued development of quantitative MRI biomarkers that assess disease severity and progression. To date, most studies associate NM-MRI measurements to the content of the neuromelanin pigment and/or density of neuromelanin-containing neurons, while recent studies suggest that the main source of the NM-MRI contrast is not the presence of neuromelanin but the high-water content in the dopaminergic and noradrenergic neurons. In this review, we consider the biological and physical basis for the NM-MRI contrast and discuss a wide range of interpretations of NM-MRI. We describe different acquisition and image processing approaches and discuss how these methods could be improved and standardized to facilitate large-scale multisite studies and translation into clinical use. We review the potential clinical applications in neurological and psychiatric disorders and the promise of NM-MRI as a biomarker of disease, and finally, we discuss the current limitations of NM-MRI that need to be addressed before this technique can be utilized as a biomarker and translated into clinical practice and offer suggestions for future research.

Sparse Asymmetry in Locus Coeruleus Pathology in Alzheimer's Disease

 Tau accumulation in and neurodegeneration of locus coeruleus (LC) neurons is observed in Alzheimer's disease (AD). We investigated whether tangle and neuronal density in the rostral and caudal LC is characterized by an asymmetric pattern in 77 autopsy cases of the Rush Memory and Aging Project. We found left-right equivalence for tangle density across individuals with and without AD pathology. However, neuronal density, particularly in the caudal-rostral axis of the LC, is asymmetric among individuals with AD pathology. Asymmetry in LC neuronal density may signal advanced disease progression and should be considered in AD neuroimaging studies of LC neurodegeneration.

REM Sleep Loss-Induced Elevated Noradrenaline Plays a Significant Role in Neurodegeneration: Synthesis of Findings to Propose a Possible Mechanism of Action from Molecule to Patho-Physiological Changes

Wear and tear are natural processes for all living and non-living bodies. All living cells and organisms are metabolically active to generate energy for their routine needs, including for survival. In the process, the cells are exposed to oxidative load, metabolic waste, and bye-products. In an organ, the living non-neuronal cells divide and replenish the lost or damaged cells; however, as neuronal cells normally do not divide, they need special feature(s) for their protection, survival, and sustenance for normal functioning of the brain. The neurons grow and branch as axons and dendrites, which contribute to the formation of synapses with near and far neurons, the basic scaffold for complex brain functions. It is necessary that one or more basic and instinct physiological process(es) (functions) is likely to contribute to the protection of the neurons and maintenance of the synapses. It is known that rapid eye movement sleep (REMS), an autonomic instinct behavior, maintains brain functioning including learning and memory and its loss causes dysfunctions. In this review we correlate the role of REMS and its loss in synaptogenesis, memory consolidation, and neuronal degeneration. Further, as a mechanism of action, we will show that REMS maintains noradrenaline (NA) at a low level, which protects neurons from oxidative damage and maintains neuronal growth and synaptogenesis. However, upon REMS loss, the level of NA increases, which withdraws protection and causes apoptosis and loss of synapses and neurons. We propose that the latter possibly causes REMS loss associated neurodegenerative diseases and associated symptoms.

Examining the diagnostic value of the mnemonic discrimination task for classification of cognitive status and amyloid-beta burden

Alzheimer's disease (AD) is the most common type of dementia, characterized by early memory impairments and gradual worsening of daily functions. AD-related pathology, such as amyloid-beta (Aβ) plaques, begins to accumulate many years before the onset of clinical symptoms. Predicting risk for AD via related pathology is critical as the preclinical stage could serve as a therapeutic time window, allowing for early management of the disease and reducing health and economic costs. Current methods for detecting AD pathology, however, are often expensive and invasive, limiting wide and easy access to a clinical setting. A non-invasive, cost-efficient platform, such as computerized cognitive tests, could be potentially useful to identify at-risk individuals as early as possible. In this study, we examined the diagnostic value of an episodic memory task, the mnemonic discrimination task (MDT), for predicting risk of cognitive impairment or Aβ burden. We constructed a random forest classification algorithm, utilizing MDT performance metrics and various neuropsychological test scores as input features, and assessed model performance using area under the curve (AUC). Models based on MDT performance metrics achieved classification results with an AUC of 0.83 for cognitive status and an AUC of 0.64 for Aβ status. Our findings suggest that mnemonic discrimination function may be a useful predictor of progression to prodromal AD or increased risk of Aβ load, which could be a cost-efficient, noninvasive cognitive testing solution for potentially wide-scale assessment of AD pathological and cognitive risk.

Exploring the Role of Locus Coeruleus in Alzheimer's Disease: a Comprehensive Update on MRI Studies and Implications

PURPOSE OF REVIEW

Performing a thorough review of magnetic resonance imaging (MRI) studies assessing locus coeruleus (LC) integrity in ageing and Alzheimer's disease (AD), and contextualizing them with current preclinical and neuropathological literature.

RECENT FINDINGS

MRI successfully detected LC alterations in ageing and AD, identifying degenerative phenomena involving this nucleus even in the prodromal stages of the disorder. The degree of LC disruption was also associated with the severity of AD cortical pathology, cognitive and behavioral impairment, and the risk of clinical progression. Locus coeruleus-MRI has proved to be a useful tool to assess the integrity of the central noradrenergic system in vivo in humans. It allowed to test in patients preclinical and experimental hypothesis, thus confirming the specific and marked involvement of the LC in AD and its key pathogenetic role. Locus coeruleus-MRI-related data might represent the theoretical basis on which to start developing noradrenergic drugs to target AD.

Mitochondrial Interaction with Serotonin in Neurobiology and Its Implication in Alzheimer's Disease

Alzheimer's disease (AD) is a lethal neurodegenerative disorder characterized by severe brain pathologies and progressive cognitive decline. While the exact cause of this disease remains unknown, emerging evidence suggests that dysregulation of neurotransmitters contributes to the development of AD pathology and symptoms. Serotonin, a critical neurotransmitter in the brain, plays a pivotal role in regulating various brain processes and is implicated in neurological and psychiatric disorders, including AD. Recent studies have shed light on the interplay between mitochondrial function and serotonin regulation in brain physiology. In AD, there is a deficiency of serotonin, along with impairments in mitochondrial function, particularly in serotoninergic neurons. Additionally, altered activity of mitochondrial enzymes, such as monoamine oxidase, may contribute to serotonin dysregulation in AD. Understanding the intricate relationship between mitochondria and serotonin provides valuable insights into the underlying mechanisms of AD and identifies potential therapeutic targets to restore serotonin homeostasis and alleviate AD symptoms. This review summarizes the recent advancements in unraveling the connection between brain mitochondria and serotonin, emphasizing their significance in AD pathogenesis and underscoring the importance of further research in this area. Elucidating the role of mitochondria in serotonin dysfunction will promote the development of therapeutic strategies for the treatment and prevention of this neurodegenerative disorder.

Locus coeruleus pathology is associated with cerebral microangiopathy at autopsy

INTRODUCTION

We investigated the link between locus coeruleus (LC) pathology and cerebral microangiopathy in two large neuropathology datasets.

METHODS

We included data from the National Alzheimer's Coordinating Center (NACC) database (n = 2197) and Religious Orders Study and Rush Memory and Aging Project (ROSMAP; n = 1637). Generalized estimating equations and logistic regression were used to examine associations between LC hypopigmentation and presence of cerebral amyloid angiopathy (CAA) or arteriolosclerosis, correcting for age at death, sex, cortical Alzheimer's disease (AD) pathology, ante mortem cognitive status, and presence of vascular and genetic risk factors.

RESULTS

LC hypopigmentation was associated with higher odds of overall CAA in the NACC dataset, leptomeningeal CAA in the ROSMAP dataset, and arteriolosclerosis in both datasets.

DISCUSSION

LC pathology is associated with cerebral microangiopathy, independent of cortical AD pathology. LC degeneration could potentially contribute to the pathways relating vascular pathology to AD. Future studies of the LC-norepinephrine system on cerebrovascular health are warranted.

HIGHLIGHTS

We associated locus coeruleus (LC) pathology and cerebral microangiopathy in two large autopsy datasets. LC hypopigmentation was consistently related to arteriolosclerosis in both datasets. LC hypopigmentation was related to cerebral amyloid angiopathy (CAA) presence in the National Alzheimer's Coordinating Center dataset. LC hypopigmentation was related to leptomeningeal CAA in the Religious Orders Study and Rush Memory and Aging Project dataset. LC degeneration may play a role in the pathways relating vascular pathology to Alzheimer's disease.

Bioelectronic Zeitgebers: targeted neuromodulation to re-establish circadian rhythms

Existing neurostimulation systems implanted for the treatment of neurodegenerative disorders generally deliver invariable therapy parameters, regardless of phase of the sleep/wake cycle. However, there is considerable evidence that brain activity in these conditions varies according to this cycle, with discrete patterns of dysfunction linked to loss of circadian rhythmicity, worse clinical outcomes and impaired patient quality of life. We present a targeted concept of circadian neuromodulation using a novel device platform. This system utilises stimulation of circuits important in sleep and wake regulation, delivering bioelectronic cues (Zeitgebers) aimed at entraining rhythms to more physiological patterns in a personalised and fully configurable manner. Preliminary evidence from its first use in a clinical trial setting, with brainstem arousal circuits as a surgical target, further supports its promising impact on sleep/wake pathology. Data included in this paper highlight its versatility and effectiveness on two different patient phenotypes. In addition to exploring acute and long-term electrophysiological and behavioural effects, we also discuss current caveats and future feature improvements of our proposed system, as well as its potential applicability in modifying disease progression in future therapies.

Declining locus coeruleus-dopaminergic and noradrenergic modulation of long-term memory in aging and Alzheimer's disease

Memory is essential in defining our identity by guiding behavior based on past experiences. However, aging leads to declining memory, disrupting older adult's lives. Memories are encoded through experience-dependent modifications of synaptic strength, which are regulated by the catecholamines dopamine and noradrenaline. While cognitive aging research demonstrates how dopaminergic neuromodulation from the substantia nigra-ventral tegmental area regulates hippocampal synaptic plasticity and memory, recent findings indicate that the noradrenergic locus coeruleus sends denser inputs to the hippocampus. The locus coeruleus produces dopamine as biosynthetic precursor of noradrenaline, and releases both to modulate hippocampal plasticity and memory. Crucially, the locus coeruleus is also the first site to accumulate Alzheimer's-related abnormal tau and severely degenerates with disease development. New in-vivo assessments of locus coeruleus integrity reveal associations with Alzheimer's markers and late-life memory impairments, which likely stem from impaired dopaminergic and noradrenergic neurotransmission. Bridging research across species, the reviewed findings suggest that degeneration of the locus coeruleus results in deficient dopaminergic and noradrenergic modulation of hippocampal plasticity and thus memory decline.

Poorer aging trajectories are associated with elevated serotonin synthesis capacity

The dorsal raphe nucleus (DRN) is one of the earliest targets of Alzheimer's disease-related tau pathology and is a major source of brain serotonin. We used [18F]Fluoro-m-tyrosine ([18F]FMT) PET imaging to measure serotonin synthesis capacity in the DRN in 111 healthy adults (18-85 years-old). Similar to reports in catecholamine systems, we found elevated serotonin synthesis capacity in older adults relative to young. To establish the structural and functional context within which serotonin synthesis capacity is elevated in aging, we examined relationships among DRN [18F]FMT net tracer influx (Ki) and longitudinal changes in cortical thickness using magnetic resonance imaging, longitudinal changes in self-reported depression symptoms, and AD-related tau and β-amyloid (Aβ) pathology using cross-sectional [18F]Flortaucipir and [11C]Pittsburgh compound-B PET respectively. Together, our findings point to elevated DRN [18F]FMT Ki as a marker of poorer aging trajectories. Older adults with highest serotonin synthesis capacity showed greatest temporal lobe cortical atrophy. Cortical atrophy was associated with increasing depression symptoms over time, and these effects appeared to be strongest in individuals with highest serotonin synthesis capacity. We did not find direct relationships between serotonin synthesis capacity and AD-related pathology. Exploratory analyses revealed nuanced effects of sex within the older adult group. Older adult females showed the highest DRN synthesis capacity and exhibited the strongest relationships between entorhinal cortex tau pathology and increasing depression symptoms. Together these findings reveal PET measurement of the serotonin system to be a promising marker of aging trajectories relevant to both AD and affective changes in older age.

Greater physical fitness (Vo2Max) in healthy older adults associated with increased integrity of the Locus Coeruleus-Noradrenergic system

Physical activity (PA) is a key component for brain health and Reserve, and it is among the main dementia protective factors. However, the neurobiological mechanisms underpinning Reserve are not fully understood. In this regard, a noradrenergic (NA) theory of cognitive reserve (Robertson, 2013) has proposed that the upregulation of NA system might be a key factor for building reserve and resilience to neurodegeneration because of the neuroprotective role of NA across the brain. PA elicits an enhanced catecholamine response, in particular for NA. By increasing physical commitment, a greater amount of NA is synthetised in response to higher oxygen demand. More physically trained individuals show greater capabilities to carry oxygen resulting in greater Vo2max - a measure of oxygen uptake and physical fitness (PF). In the current study, we hypothesised that greater Vo2 max would be related to greater Locus Coeruleus (LC) MRI signal intensity. As hypothesised, greater Vo2max related to greater LC signal intensity across 41 healthy adults (age range 60-72). As a control procedure, in which these analyses were repeated for the other neuromodulators' seeds (for Serotonin, Dopamine and Acetylcholine), weaker associations emerged. This newly established link between Vo2max and LC-NA system offers further understanding of the neurobiology underpinning Reserve in relationship to PA. While this study supports Robertson's theory proposing the upregulation of the noradrenergic system as a possible key factor building Reserve, it also provide grounds for increasing LC-NA system resilience to neurodegeneration via Vo2max enhancement.

Interactive effects of locus coeruleus structure and catecholamine synthesis capacity on cognitive function

Background

The locus coeruleus (LC) produces catecholamines (norepinephrine and dopamine) and is implicated in a broad range of cognitive functions including attention and executive function. Recent advancements in magnetic resonance imaging (MRI) approaches allow for the visualization and quantification of LC structure. Human research focused on the LC has since exploded given the LC's role in cognition and relevance to current models of psychopathology and neurodegenerative disease. However, it is unclear to what extent LC structure reflects underlying catecholamine function, and how LC structure and neurochemical function are collectively associated with cognitive performance.

Methods

A partial least squares correlation (PLSC) analysis was applied to 19 participants' LC structural MRI measures and catecholamine synthesis capacity measures assessed using [18F]Fluoro-m-tyrosine ([18F]FMT) positron emission tomography (PET).

Results

We found no direct association between LC-MRI and LC-[18F]FMT measures for rostral, middle, or caudal portions of the LC. We found significant associations between LC neuroimaging measures and neuropsychological performance that were driven by rostral and middle portions of the LC, which is in line with LC cortical projection patterns. Specifically, associations with executive function and processing speed arose from contributions of both LC structure and interactions between LC structure and catecholamine synthesis capacity.

Conclusion

These findings leave open the possibility that LC MRI and PET measures contribute unique information and suggest that their conjoint use may increase sensitivity to brain-behavior associations in small samples.

The integrity of dopaminergic and noradrenergic brain regions is associated with different aspects of late-life memory performance

Changes in dopaminergic neuromodulation play a key role in adult memory decline. Recent research has also implicated noradrenaline in shaping late-life memory. However, it is unclear whether these two neuromodulators have distinct roles in age-related cognitive changes. Here, combining longitudinal MRI of the dopaminergic substantia nigra-ventral tegmental area (SN-VTA) and noradrenergic locus coeruleus (LC) in younger (n = 69) and older (n = 251) adults, we found that dopaminergic and noradrenergic integrity are differentially associated with memory performance. While LC integrity was related to better episodic memory across several tasks, SN-VTA integrity was linked to working memory. Longitudinally, we found that older age was associated with more negative change in SN-VTA and LC integrity. Notably, changes in LC integrity reliably predicted future episodic memory. These differential associations of dopaminergic and noradrenergic nuclei with late-life cognitive decline have potential clinical utility, given their degeneration in several age-associated diseases.

Repeated Methamphetamine Administration Results in Axon Loss Prior to Somatic Loss of Substantia Nigra Pars Compacta and Locus Coeruleus Neurons in Male but Not Female Mice

Methamphetamine (meth) is a neurotoxic psychostimulant that increases monoamine oxidase (MAO)-dependent mitochondrial oxidant stress in axonal but not somatic compartments of substantia nigra pars compacta (SNc) and locus coeruleus (LC) neurons. Chronic meth administration results in the degeneration of SNc and LC neurons in male mice, and MAO inhibition is neuroprotective, suggesting that the deleterious effects of chronic meth begin in axons before advancing to the soma of SNc and LC neurons. To test this hypothesis, mice were administered meth (5 mg/kg) for 14, 21, or 28 days, and SNc and LC axonal lengths and numbers of neurons were quantified. In male mice, the SNc and LC axon lengths decreased with 14, 21, and 28 days of meth, whereas somatic loss was only observed after 28 days of meth; MAO inhibition (phenelzine; 20 mg/kg) prevented axonal and somatic loss of SNc and LC neurons. In contrast, chronic (28-day) meth had no effect on the axon length or numbers of SNc or LC neurons in female mice. The results demonstrate that repeated exposure to meth produces SNc and LC axonal deficits prior to somatic loss in male subjects, consistent with a dying-back pattern of degeneration, whereas female mice are resistant to chronic meth-induced degeneration.

Alzheimer's disease phenotypes show different sleep architecture

INTRODUCTION

Sleep-wake disturbances are a prominent feature of Alzheimer's disease (AD). Atypical (non-amnestic) AD syndromes have different patterns of cortical vulnerability to AD. We hypothesized that atypical AD also shows differential vulnerability in subcortical nuclei that will manifest as different patterns of sleep dysfunction.

METHODS

Overnight electroencephalography monitoring was performed on 48 subjects, including 15 amnestic, 19 atypical AD, and 14 controls. AD was defined based on neuropathological or biomarker confirmation. We compared sleep architecture by visual scoring and spectral power analysis in each group.

RESULTS

Overall, AD cases showed increased sleep fragmentation and N1 sleep compared to controls. Compared to atypical AD groups, typical AD showed worse N3 sleep dysfunction and relatively preserved rapid eye movement (REM) sleep.

DISCUSSION

Results suggest differing effects of amnestic and atypical AD variants on slow wave versus REM sleep, respectively, corroborating the hypothesis of differential selective vulnerability patterns of the subcortical nuclei within variants. Optimal symptomatic treatment for sleep dysfunction in clinical phenotypes may differ.

HIGHLIGHTS

Alzheimer's disease (AD) variants show distinct patterns of sleep impairment. Amnestic/typical AD has worse N3 slow wave sleep (SWS) impairment compared to atypical AD. Atypical AD shows more rapid eye movement deficits than typical AD. Selective vulnerability patterns in subcortical areas may underlie sleep differences. Relatively preserved SWS may explain better memory scores in atypical versus typical AD.

New perspectives on the basal forebrain cholinergic system in Alzheimer's disease

The basal forebrain cholinergic system (BFCS) has long been implicated in age-related cognitive changes and the pathophysiology of Alzheimer's disease (AD). Limitations of cholinergic interventions helped to inspire a shift away from BFCS in AD research. A resurgence in interest in the BFCS following methodological and analytical advances has resulted in a call for the BFCS to be examined in novel frameworks. We outline the basic structure and function of the BFCS, its role in supporting cognitive and affective function, and its vulnerability to aging and AD. We consider the BFCS in the context of the amyloid hypothesis and evolving concepts in AD research: resilience and resistance to pathology, selective neuronal vulnerability, trans-synaptic pathology spread and sleep health. We highlight 1) the potential role of the BFCS in cognitive resilience, 2) recent work refining understanding about the selective vulnerability of BFCS to AD, 3) BFCS connectivity that suggests it is related to tau spreading and neurodegeneration and 4) the gap between BFCS involvement in AD and sleep-wake cycles.

Altered structural covariance of locus coeruleus in individuals with significant memory concern and patients with mild cognitive impairment

The locus coeruleus (LC) is the site where tau accumulation is preferentially observed pathologically in Alzheimer's disease (AD) patients, but the changes in gray matter co-alteration patterns between the LC and the whole brain in the predementia phase of AD remain unclear. In this study, we estimated and compared the gray matter volume of the LC and its structural covariance (SC) with the whole brain among 161 normal healthy controls (HCs), 99 individuals with significant memory concern (SMC) and 131 patients with mild cognitive impairment (MCI). We found that SC decreased in MCI groups, which mainly involved the salience network and default mode network. These results imply that seeding from LC, the gray matter network disruption and disconnection appears early in the MCI group. The altered SC network seeding from the LC can serve as an imaging biomarker for discriminating the patients in the potential predementia phase of AD from the normal subjects.

Ultra-high field imaging, plasma markers and autopsy data uncover a specific rostral locus coeruleus vulnerability to hyperphosphorylated tau

Autopsy data indicate that the locus coeruleus (LC) is one of the first sites in the brain to accumulate hyperphosphorylated tau pathology, with the rostral part possibly being more vulnerable in the earlier stages of the disease. Taking advantage of recent developments in ultra-high field (7 T) imaging, we investigated whether imaging measures of the LC also reveal a specific anatomic correlation with tau using novel plasma biomarkers of different species of hyperphosphorylated tau, how early in adulthood these associations can be detected and if are associated with worse cognitive performance. To validate the anatomic correlations, we tested if a rostro-caudal gradient in tau pathology is also detected at autopsy in data from the Rush Memory and Aging Project (MAP). We found that higher plasma measures of phosphorylated tau, in particular ptau231, correlated negatively with dorso-rostral LC integrity, whereas correlations for neurodegenerative plasma markers (neurofilament light, total tau) were scattered throughout the LC including middle to caudal sections. In contrast, the plasma Aβ42/40 ratio, associated with brain amyloidosis, did not correlate with LC integrity. These findings were specific to the rostral LC and not observed when using the entire LC or the hippocampus. Furthermore, in the MAP data, we observed higher rostral than caudal tangle density in the LC, independent of the disease stage. The in vivo LC-phosphorylated tau correlations became significant from midlife, with the earliest effect for ptau231, starting at about age 55. Finally, interactions between lower rostral LC integrity and higher ptau231 concentrations predicted lower cognitive performance. Together, these findings demonstrate a specific rostral vulnerability to early phosphorylated tau species that can be detected with dedicated magnetic resonance imaging measures, highlighting the promise of LC imaging as an early marker of AD-related processes.

Priorities for research on neuromodulatory subcortical systems in Alzheimer's disease: Position paper from the NSS PIA of ISTAART

The neuromodulatory subcortical system (NSS) nuclei are critical hubs for survival, hedonic tone, and homeostasis. Tau-associated NSS degeneration occurs early in Alzheimer's disease (AD) pathogenesis, long before the emergence of pathognomonic memory dysfunction and cortical lesions. Accumulating evidence supports the role of NSS dysfunction and degeneration in the behavioral and neuropsychiatric manifestations featured early in AD. Experimental studies even suggest that AD-associated NSS degeneration drives brain neuroinflammatory status and contributes to disease progression, including the exacerbation of cortical lesions. Given the important pathophysiologic and etiologic roles that involve the NSS in early AD stages, there is an urgent need to expand our understanding of the mechanisms underlying NSS vulnerability and more precisely detail the clinical progression of NSS changes in AD. Here, the NSS Professional Interest Area of the International Society to Advance Alzheimer's Research and Treatment highlights knowledge gaps about NSS within AD and provides recommendations for priorities specific to clinical research, biomarker development, modeling, and intervention. HIGHLIGHTS: Neuromodulatory nuclei degenerate in early Alzheimer's disease pathological stages. Alzheimer's pathophysiology is exacerbated by neuromodulatory nuclei degeneration. Neuromodulatory nuclei degeneration drives neuropsychiatric symptoms in dementia. Biomarkers of neuromodulatory integrity would be value-creating for dementia care. Neuromodulatory nuclei present strategic prospects for disease-modifying therapies.

Sex Differences in Alzheimer's Disease

Reviewing the research presented in this article, it is evident that from an epidemiological perspective, it is important to evaluate the extent to which findings of sex and gender differences in Alzheimer's dementia (AD) are due to differences in longevity, survival bias, and comorbidities. Medical, genetic, psychosocial, and behavioral factors, in addition to hormonal factors, can differentially affect the risk and progression of AD in women versus men. Further, evaluation of sex differences in AD progression and the trajectory of change in cognitive function, neuroimaging, cerebrospinal fluid (CSF), and blood-based biomarkers of AD is needed. Finally, identifying sex differences in AD biomarkers and change across the lifespan is critical for the planning of prevention trials to reduce the risk of developing AD.

Age-dependent dysregulation of locus coeruleus firing in a transgenic rat model of Alzheimer's disease

Hyperphosphorylated tau in the locus coeruleus (LC) is ubiquitous in prodromal Alzheimer's disease (AD), and LC neurons degenerate as AD progresses. Hyperphosphorylated tau alters firing rates in other brain regions, but its effects on LC neurons are unknown. We assessed single unit LC activity in anesthetized wild-type (WT) and TgF344-AD rats at 6 months, which represents a prodromal stage when LC neurons are the only cells containing hyperphosphorylated tau in TgF344-AD animals, and at 15 months when amyloid-β (Aβ) and tau pathology are both abundant in the forebrain. At baseline, LC neurons from TgF344-AD rats were hypoactive at both ages compared to WT littermates but showed elevated spontaneous bursting properties. Differences in footshock-evoked LC firing depended on age, with 6-month TgF344-AD rats demonstrating aspects of hyperactivity, and 15-month transgenic rats showing hypoactivity. Early LC hyperactivity is consistent with appearance of prodromal neuropsychiatric symptoms and is followed by LC hypoactivity which contributes to cognitive impairment. These results support further investigation into disease stage-dependent noradrenergic interventions for AD.

Proteostasis failure exacerbates neuronal circuit dysfunction and sleep impairments in Alzheimer's disease

Failed proteostasis is a well-documented feature of Alzheimer's disease, particularly, reduced protein degradation and clearance. However, the contribution of failed proteostasis to neuronal circuit dysfunction is an emerging concept in neurodegenerative research and will prove critical in understanding cognitive decline. Our objective is to convey Alzheimer's disease progression with the growing evidence for a bidirectional relationship of sleep disruption and proteostasis failure. Proteostasis dysfunction and tauopathy in Alzheimer's disease disrupts neurons that regulate the sleep-wake cycle, which presents behavior as impaired slow wave and rapid eye movement sleep patterns. Subsequent sleep loss further impairs protein clearance. Sleep loss is a defined feature seen early in many neurodegenerative disorders and contributes to memory impairments in Alzheimer's disease. Canonical pathological hallmarks, β-amyloid, and tau, directly disrupt sleep, and neurodegeneration of locus coeruleus, hippocampal and hypothalamic neurons from tau proteinopathy causes disruption of the neuronal circuitry of sleep. Acting in a positive-feedback-loop, sleep loss and circadian rhythm disruption then increase spread of β-amyloid and tau, through impairments of proteasome, autophagy, unfolded protein response and glymphatic clearance. This phenomenon extends beyond β-amyloid and tau, with interactions of sleep impairment with the homeostasis of TDP-43, α-synuclein, FUS, and huntingtin proteins, implicating sleep loss as an important consideration in an array of neurodegenerative diseases and in cases of mixed neuropathology. Critically, the dynamics of this interaction in the neurodegenerative environment are not fully elucidated and are deserving of further discussion and research. Finally, we propose sleep-enhancing therapeutics as potential interventions for promoting healthy proteostasis, including β-amyloid and tau clearance, mechanistically linking these processes. With further clinical and preclinical research, we propose this dynamic interaction as a diagnostic and therapeutic framework, informing precise single- and combinatorial-treatments for Alzheimer's disease and other brain disorders.

Neuropathological correlates of neuropsychiatric symptoms in dementia

INTRODUCTION

Neuropsychiatric symptoms (NPS) are common in Lewy body disease (LBD), but their etiology is poorly understood.

METHODS

In a population-based post mortem study neuropathological data was collected for Lewy body (LB) neuropathology, neurofibrillary tangles (NFT), amyloid beta burden, TDP-43, lacunar infarcts, cerebral amyloid angiopathy (CAA), and hyaline atherosclerosis. Post mortem interviews collected systematic information regarding NPS and cognitive status. A total of 1038 cases were included: no pathology (NP; n = 761), Alzheimer's disease (AD; n = 189), LBD (n = 60), and AD+LBD (n = 28).

RESULTS

Hallucinations were associated with higher LB Braak stages, while higher NFT Braak staging was associated with depression, agitation, and greater number of symptoms in the Neuropsychiatric Inventory. Cases with dual AD+LBD pathology had the highest risk of hallucinations, agitation, apathy, and total symptoms but a multiplicative interaction between these pathologies was not significant.

DISCUSSION

LB and AD pathology contribute differentially to NPS likely with an additive process contributing to the increased burden of NPS.

Revisiting a Telencephalic Extent of the Ascending Reticular Activating System

Is the cerebrum involved in its own activation to states of attention or arousal? "Telencephalon" is a term borrowed from embryology to identify not only the cerebral hemispheres of the forebrain, but also the basal forebrain. We review a generally undercited literature that describes nucleus basalis of Meynert, located within the substantia innominata of the ventrobasal forebrain, as a telencephalic extension of the ascending reticular activating formation. Although that formation's precise anatomical definition and localization have proven elusive over more than 70 years, a careful reading of sources reveals that there are histological features common to certain brainstem neurons and those of the nucleus basalis, and that a largely common dendritic architecture may be a morphological aspect that helps to define non-telencephalic structures of the ascending reticular activating formation (e.g., in brainstem) as well as those parts of the formation that are telencephalic and themselves responsible for cortical activation. We draw attention to a pattern of dendritic arborization described as "isodendritic," a uniform (isos-) branching in which distal dendrite branches are significantly longer than proximal ones. Isodendritic neurons also differ from other morphological types based on their heterogeneous, rather than specific afferentation. References reviewed here are consistent in their descriptions of histology, particularly in studies of locales rich in cholinergic neurons. We discuss the therapeutic implications of a basal forebrain site that may activate cortex. Interventions that specifically target nucleus basalis and, especially, the survival of its constituent neurons may benefit afflictions in which higher cortical function is compromised due to disturbed arousal or attentiveness, including not only coma and related syndromes, but also conditions colloquially described as states of cognitive "fog" or of "long-haul" mental compromise.

A conformal TOF-DOI Prism-PET prototype scanner for high-resolution quantitative neuroimaging

BACKGROUND

Positron emission tomography (PET) has had a transformative impact on oncological and neurological applications. However, still much of PET's potential remains untapped with limitations primarily driven by low spatial resolution, which severely hampers accurate quantitative PET imaging via the partial volume effect (PVE).

PURPOSE

We present experimental results of a practical and cost-effective ultra-high resolution brain-dedicated PET scanner, using our depth-encoding Prism-PET detectors arranged along a compact and conformal gantry, showing substantial reduction in PVE and accurate radiotracer uptake quantification in small regions.

METHODS

The decagon-shaped prototype scanner has a long diameter of 38.5 cm, a short diameter of 29.1 cm, and an axial field-of-view (FOV) of 25.5 mm with a single ring of 40 Prism-PET detector modules. Each module comprises a 16 × 16 array of 1.5 × 1.5 × 20-mm3 lutetium yttrium oxyorthosillicate (LYSO) scintillator crystals coupled 4-to-1 to an 8 × 8 array of silicon photomultiplier (SiPM) pixels on one end and to a prismatoid light guide array on the opposite end. The scanner's performance was evaluated by measuring depth-of-interaction (DOI) resolution, energy resolution, timing resolution, spatial resolution, sensitivity, and image quality of ultra-micro Derenzo and three-dimensional (3D) Hoffman brain phantoms.

RESULTS

The full width at half maximum (FWHM) DOI, energy, and timing resolutions of the scanner are 2.85 mm, 12.6%, and 271 ps, respectively. Not considering artifacts due to mechanical misalignment of detector blocks, the intrinsic spatial resolution is 0.89-mm FWHM. Point source images reconstructed with 3D filtered back-projection (FBP) show an average spatial resolution of 1.53-mm FWHM across the entire FOV. The peak absolute sensitivity is 1.2% for an energy window of 400-650 keV. The ultra-micro Derenzo phantom study demonstrates the highest reported spatial resolution performance for a human brain PET scanner with perfect reconstruction of 1.00-mm diameter hot-rods. Reconstructed images of customized Hoffman brain phantoms prove that Prism-PET enables accurate radiotracer uptake quantification in small brain regions (2-3 mm).

CONCLUSIONS

Prism-PET will substantially strengthen the utility of quantitative PET in neurology for early diagnosis of neurodegenerative diseases, and in neuro-oncology for improved management of both primary and metastatic brain tumors.

Dissociation of Arousal Index Between REM and NREM Sleep in Elderly Adults with Cognitive Impairment, No Dementia: A Pilot Study

BACKGROUND

Sleep disruption in elderly has been associated with an increased risk of cognitive impairment and its transition into Alzheimer's disease (AD). High arousal indices (AIs) during sleep may serve as an early-stage biomarker of cognitive impairment non-dementia (CIND).

OBJECTIVE

Using full-night polysomnography (PSG), we investigated whether CIND is related to different AIs between NREM and REM sleep stages.

METHODS

Fourteen older adults voluntarily participated in this population-based study that included Mini-Mental State Examination, Neuropsi battery, Katz Index of Independence in Activities of Daily Living, and single-night PSG. Subjects were divided into two groups (n = 7 each) according to their results in Neuropsi memory and attention subtests: cognitively unimpaired (CU), with normal results; and CIND, with -2.5 standard deviations in memory and/or attention subtests. AIs per hour of sleep during N1, N2, N3, and REM stages were obtained and correlated with Neuropsi total score (NTS).

RESULTS

AI (REM)  was significantly higher in CU group than in CIND group. For the total sample, a positive correlation between AI (REM)  and NTS was found (r = 0.68, p = 0.006), which remained significant when controlling for the effect of age and education. In CIND group, the AI (N2)  was significantly higher than the AI (REM) .

CONCLUSION

In CIND older adults, this attenuation of normal arousal mechanisms in REM sleep are dissociated from the relative excess of arousals observed in stage N2. We propose as probable etiology an early hypoactivity at the locus coeruleus noradrenergic system, associated to its early pathological damage, present in the AD continuum.

State-of-the-art imaging of neuromodulatory subcortical systems in aging and Alzheimer's disease: Challenges and opportunities

Primary prevention trials have shifted their focus to the earliest stages of Alzheimer's disease (AD). Autopsy data indicates that the neuromodulatory subcortical systems' (NSS) nuclei are specifically vulnerable to initial tau pathology, indicating that these nuclei hold great promise for early detection of AD in the context of the aging brain. The increasing availability of new imaging methods, ultra-high field scanners, new radioligands, and routine deep brain stimulation implants has led to a growing number of NSS neuroimaging studies on aging and neurodegeneration. Here, we review findings of current state-of-the-art imaging studies assessing the structure, function, and molecular changes of these nuclei during aging and AD. Furthermore, we identify the challenges associated with these imaging methods, important pathophysiologic gaps to fill for the AD NSS neuroimaging field, and provide future directions to improve our assessment, understanding, and clinical use of in vivo imaging of the NSS.

Associations of Stages of Objective Memory Impairment with Cerebrospinal Fluid and Neuroimaging Biomarkers of Alzheimer's Disease

OBJECTIVE

To investigate cerebrospinal fluid (CSF) and neuroimaging correlates of Stages of Objective Memory Impairment (SOMI) based on Free and Cued Selective Reminding Test (FCSRT) performance, and to evaluate the effect of APOE ε4 status on this relationship.

METHODS

Data from 586 cognitively unimpaired individuals who had FCSRT, CSF, and volumetric magnetic resonance imaging (MRI) measures available was used. We compared CSF measures of β-amyloid (Aβ42/Aβ40 ratio), phosphorylated tau (p-Tau181), total tau (t-Tau), hippocampal volume, and PIB-PET mean cortical binding potential with partial volume correction (MCBP) among SOMI groups in the whole sample and in subsamples stratified by APOE ε4 status.

RESULTS

Participants had a mean age of 67.4 (SD=9.1) years, had 16.1 (SD=2.6) years of education, 57.0% were female, and 33.8% were APOE ε4 positive. In the entire sample, there was no significant difference between SOMI stages in Aβ42/Aβ40 ratio, p-Tau181, t-Tau, or PIB-PET MCBP when adjusted for age, sex, and education. However, higher SOMI stages had smaller hippocampal volume (F=3.29, p=0.020). In the stratified sample based on APOE ε4 status, in APOE ε4 positive individuals, higher SOMI stages had higher p-Tau181 (F=2.94, p=0.034) higher t-Tau (F=3.41, p=0.019), and smaller hippocampal volume (F=5.78, p<0.001). There were no significant differences in CSF or imaging biomarkers between SOMI groups in the APOE ε4 negative subsample.

CONCLUSION

Cognitively normal older individuals with higher SOMI stages have higher in-vivo tau and neurodegenerative pathology only in APOE ε4 carriers. These original results indicate the potential usefulness of the SOMI staging system in assessing of tau and neurodegenerative pathology.

Alzheimer's Disease Pathology Outside of the Cerebrum Is Related to a Higher Odds of Dementia

BACKGROUND

Assessments of Alzheimer's disease pathology do not routinely include lower brainstem, olfactory bulb, and spinal cord.

OBJECTIVE

Test if amyloid-β (Aβ) and paired helical filament (PHF) tau-tangles outside the cerebrum are associated with the odds of dementia.

METHODS

Autopsies were obtained in decedents with cognitive testing (n = 300). Aβ plaques and PHF tau-tangles were assessed in 24 sites: cerebrum (n = 14), brainstem (n = 5), olfactory bulb, and four spinal cord levels. Since spinal Aβ were absent in the first 165 cases, it was not assessed in the remaining cases.

RESULTS

Age at death was 91 years old. About 90% had Aβ in cerebrum and of these, half had Aβ in the brainstem. Of the latter, 85% showed Aβ in the olfactory bulb. All but one participant had tau-tangles in the cerebrum and 86% had brainstem tau-tangles. Of the latter, 80% had tau-tangles in olfactory bulb and 36% tau-tangles in one or more spinal cord levels. About 90% of adults with tau-tangles also had Aβ in one or more regions. In a logistic model controlling for demographics, Aβ and tau-tangles within the cerebrum, the presence of Aβ in olfactory bulb [OR, 1.74(1.00, 3.05)]; tau-tangles in brainstem [OR, 4.00(1.1.57,10.21)]; and spinal cord [OR, 1.87 (1.21,3.11)] were independently associated with higher odds of dementia.

CONCLUSION

Regional differences in Aβ and tau-tangle accumulation extend beyond cerebrum to spinal cord and their presence outside the cerebrum are associated with a higher odds of dementia. Further studies are needed to clarify the extent, burden, and consequences of AD pathology outside of cerebrum.

Early Chronic Stress Induced Changes within the Locus Coeruleus in Sporadic Alzheimer's Disease

Chronic exposure to stress throughout the lifespan has been the focus of many studies on Alzheimer's disease (AD) because of the similarities between the biological mechanisms involved in chronic stress and the pathophysiology of AD. In fact, the earliest abnormality associated with the disease is the presence of phosphorylated tau protein in locus coeruleus neurons, a brain structure highly responsive to stress and perceived threat. Here, we introduce allostatic load as a useful concept for understanding many of the complex, interacting neuropathological changes involved in the AD degenerative process. In response to chronic stress, aberrant tau proteins that begin to accumulate within the locus coeruleus decades prior to symptom onset appear to represent a primary pathological event in the AD cascade, triggering a wide range of interacting brain changes involving neuronal excitotoxicity, endocrine alterations, inflammation, oxidative stress, and amyloid plaque exacerbation. While it is acknowledged that stress will not necessarily be the major precipitating factor in all cases, early tau-induced changes within the locus coeruleus-norepinephrine pathway suggests that a therapeutic window might exist for preventative measures aimed at managing stress and restoring balance within the HPA axis.

Medial Temporal Lobe Tau Aggregation Relates to Divergent Cognitive and Emotional Empathy Abilities in Alzheimer's Disease

BACKGROUND

In Alzheimer's disease (AD), the gradual accumulation of amyloid-β (Aβ) and tau proteins may underlie alterations in empathy.

OBJECTIVE

To assess whether tau aggregation in the medial temporal lobes related to differences in cognitive empathy (the ability to take others' perspectives) and emotional empathy (the ability to experience others' feelings) in AD.

METHODS

Older adults (n = 105) completed molecular Aβ positron emission tomography (PET) scans. Sixty-eight of the participants (35 women) were Aβ positive and symptomatic with diagnoses of mild cognitive impairment, dementia of the Alzheimer's type, logopenic variant primary progressive aphasia, or posterior cortical atrophy. The remaining 37 (22 women) were asymptomatic Aβ negative healthy older controls. Using the Interpersonal Reactivity Index, we compared current levels of informant-rated cognitive empathy (Perspective-Taking subscale) and emotional empathy (Empathic Concern subscale) in the Aβ positive and negative participants. The Aβ positive participants also underwent molecular tau-PET scans, which were used to investigate whether regional tau burden in the bilateral medial temporal lobes related to empathy.

RESULTS

Aβ positive participants had lower perspective-taking and higher empathic concern than Aβ negative healthy controls. Medial temporal tau aggregation in the Aβ positive participants had divergent associations with cognitive and emotional empathy. Whereas greater tau burden in the amygdala predicted lower perspective-taking, greater tau burden in the entorhinal cortex predicted greater empathic concern. Tau burden in the parahippocampal cortex did not predict either form of empathy.

CONCLUSIONS

Across AD clinical syndromes, medial temporal lobe tau aggregation is associated with lower perspective-taking yet higher empathic concern.

Waning locus coeruleus integrity precedes cortical tau accrual in preclinical autosomal dominant Alzheimer's disease

INTRODUCTION

Autopsy studies recognize the locus coeruleus (LC) as one of the first sites accumulating tau in Alzheimer's disease (AD). Recent AD work related in vivo LC magnetic resonance imaging (MRI) integrity to tau and cognitive decline; however, relationships of LC integrity to age, tau, and cognition in autosomal dominant AD (ADAD) remain unexplored.

METHODS

We associated LC integrity (3T-MRI) with estimated years of onset, cortical amyloid beta, regional tau (positron emission tomography [PET]) and memory (Consortium to Establish a Registry for Alzheimer's Disease (CERAD) Word-List-Learning) among 27 carriers and 27 non-carriers of the presenilin-1 (PSEN1) E280A mutation. Longitudinal changes between LC integrity and tau were evaluated in 10 carriers.

RESULTS

LC integrity started to decline at age 32 in carriers, 12 years before clinical onset, and 20 years earlier than in sporadic AD. LC integrity was negatively associated with cortical tau, independent of amyloid beta, and predicted precuneus tau increases. LC integrity was positively associated with memory.

DISCUSSION

These findings support LC integrity as marker of disease progression in preclinical ADAD.

Sulfanegen stimulates 3-mercaptopyruvate sulfurtransferase activity and ameliorates Alzheimer's disease pathology and oxidative stress in vivo

Increased oxidative stress and inflammation are implicated in the pathogenesis of Alzheimer's disease. Treatment with hydrogen sulfide (H2S) and H2S donors such as sodium hydrosulfide (NaSH) can reduce oxidative stress in preclinical studies, however clinical benefits of such treatments are rather ambiguous. This is partly due to poor stability and bioavailability of the H2S donors, requiring impractically large doses that are associated with dose-limiting toxicity. Herein, we identified a bioavailable 3-mercaptopyruvate prodrug, sulfanegen, which is able to pose as a sacrificial redox substrate for 3-mercaptopyruvate sulfurtransferase (3MST), one of the H2S biosynthetic enzymes in the brain. Sulfanegen is able to mitigate toxicity emanating from oxidative insults and the Aβ1-42 peptide by releasing H2S through the 3MST pathway. When administered to symptomatic transgenic mouse model of AD (APP/PS1; 7 and 12 months) and mice that were intracerebroventricularly administered with the Aβ1-42 peptide, sulfanegen was able to reverse oxidative and neuroinflammatory consequences of AD pathology by restoring 3MST function. Quantitative neuropathological analyses confirmed significant disease modifying effect of the compound on amyloid plaque burden and brain inflammatory markers. More importantly, sulfanegen treatment attenuated progressive neurodegeneration in these mice, as evident from the restoration of TH+ neurons in the locus coeruleus. This study demonstrates a previously unknown concept that supplementation of 3MST function in the brain may be a viable approach for the management of AD. Finally, brought into the spotlight is the potential of sulfanegen as a promising AD therapeutic for future drug development efforts.