Localization of endogenous amyloid-β to the coeruleo-cortical pathway: consequences of noradrenergic depletion

Chronic sleep deprivation altered the expression of circadian clock genes and aggravated Alzheimer's disease neuropathology

Circadian disruption is prevalent in Alzheimer's disease (AD) and may contribute to cognitive impairment, psychological symptoms, and neurodegeneration. This study aimed to evaluate the effects of environmental and genetic factors on the molecular clock and to establish a link between circadian rhythm disturbance and AD. We investigated the pathological effects of chronic sleep deprivation (CSD) in the APP^swe/PS1^ΔE9 transgenic mice and their wild‐type (WT) littermates for 2 months and evaluated the expression levels of clock genes in the circadian rhythm‐related nuclei. Our results showed that CSD impaired learning and memory, and further exaggerated disease progression in the AD mice. Furthermore, CSD caused abnormal expression of Bmal1, Clock, and Cry1 in the circadian rhythm‐related nuclei of experimental mice, and these changes are more significant in AD mice. Abnormal expression of clock genes in AD mice suggested that the expression of clock genes is affected by APP/PS1 mutations. In addition, abnormal tau phosphorylation was found in the retrosplenial cortex, which was co‐located with the alteration of BMAL1 protein level. Moreover, the level of tyrosine hydroxylase in the locus coeruleus of AD and WT mice was significantly increased after CSD. There may be a potential link between the molecular clock, Aβ pathology, tauopathy, and the noradrenergic system. The results of this study provided new insights into the potential link between the disruption of circadian rhythm and the development of AD.

Late-in-life neurodegeneration after chronic sleep loss in young adult mice

Chronic short sleep (CSS) is prevalent in modern societies and has been proposed as a risk factor for Alzheimer's disease (AD). In support, short-term sleep loss acutely increases levels of amyloid β (Aβ) and tau in wild type (WT) mice and humans, and sleep disturbances predict cognitive decline in older adults. We have shown that CSS induces injury to and loss of locus coeruleus neurons (LCn), neurons with heightened susceptibility in AD. Yet whether CSS during young adulthood drives lasting Aβ and/or tau changes and/or neural injury later in life in the absence of genetic risk for AD has not been established. Here, we examined the impact of CSS exposure in young adult WT mice on late-in-life Aβ and tau changes and neural responses in two AD-vulnerable neuronal groups, LCn and hippocampal CA1 neurons. Twelve months following CSS exposure, CSS-exposed mice evidenced reductions in CA1 neuron counts and volume, spatial memory deficits, CA1 glial activation, and loss of LCn. Aβ 42 and hyperphosphorylated tau were increased in the CA1; however, amyloid plaques and tau tangles were not observed. Collectively the findings demonstrate that CSS exposure in the young adult mouse imparts late-in-life neurodegeneration and persistent derangements in amyloid and tau homeostasis. These findings occur in the absence of a genetic predisposition to neurodegeneration and demonstrate for the first time that CSS can induce lasting, significant neural injury consistent with some, but not all, features of late-onset AD.

Identification of intraneuronal amyloid beta oligomers in locus coeruleus neurons of Alzheimer's patients and their potential impact on inhibitory neurotransmitter receptors and neuronal excitability

AIMS

Amyloid β-oligomers (AβO) are potent modulators of Alzheimer's pathology, yet their impact on one of the earliest brain regions to exhibit signs of the condition, the locus coeruleus (LC), remains to be determined. Of particular importance is whether AβO impact the spontaneous excitability of LC neurons. This parameter determines brain-wide noradrenaline (NA) release, and thus NA-mediated brain functions, including cognition, emotion and immune function, which are all compromised in Alzheimer's patients. Therefore, the aim of the study was to determine the expression profile of AβO in the LC of Alzheimer's patients and to probe their potential impact on the molecular and functional correlates of LC excitability, using a mouse model of increased Aβ production (APP-PSEN1).

METHODS AND RESULTS

Immunohistochemistry and confocal microscopy, using AβO-specific antibodies, confirmed LC AβO expression both intraneuronally and extracellularly in both Alzheimer's and APP-PSEN1 samples. Patch clamp electrophysiology recordings revealed that APP-PSEN1 LC neuronal hyperexcitability accompanied this AβO expression profile, arising from a diminished inhibitory effect of GABA due to impaired expression and function of the GABA-A receptor (GABAA R) α3 subunit. This altered LC α3-GABAA R expression profile overlapped with AβO expression in samples from both APP-PSEN1 mice and Alzheimer's patients. Finally, strychnine-sensitive glycine receptors (GlyRs) remained resilient to Aβ-induced changes and their activation reversed LC hyperexcitability.

CONCLUSIONS

The data suggest a close association between AβO and α3-GABAA Rs in the LC of Alzheimer's patients, and their potential to dysregulate LC activity, thereby contributing to the spectrum of pathology of the LC-NA system in this condition.

The Locus Coeruleus- Norepinephrine System in Stress and Arousal: Unraveling Historical, Current, and Future Perspectives

Arousal may be understood on a spectrum, with excessive sleepiness, cognitive dysfunction, and inattention on one side, a wakeful state in the middle, and hypervigilance, panic, and psychosis on the other side. However, historically, the concepts of arousal and stress have been challenging to define as measurable experimental variables. Divergent efforts to study these subjects have given rise to several disciplines, including neurobiology, neuroendocrinology, and cognitive neuroscience. We discuss technological advancements that chronologically led to our current understanding of the arousal system, focusing on the multifaceted nucleus locus coeruleus. We share our contemporary perspective and the hypotheses of others in the context of our current technological capabilities and future developments that will be required to move forward in this area of research.

Adrenergic β receptor activation reduces amyloid β1-42-mediated intracellular Zn2+ toxicity in dentate granule cells followed by rescuing impairment of dentate gyrus LTP

Adrenergic β receptor activation prevents human soluble amyloid β (Aβ)-induced impairment of long-term potentiation (LTP) in slices. On the basis of the evidence that human Aβ_1-42-induced impairment of LTP is due to Aβ_1-42-mediated Zn²⁺ toxicity, we postulated that adrenergic β receptor activation reduces Aβ_1-42-mediated intracellular Zn²⁺ toxicity followed by rescuing Aβ_1-42 toxicity. To test the effect of adrenergic β receptor activation, LTP was recorded at perforant pathway-dentate granule cell synapses of anesthetized rats 60 min after Aβ_1-42 injection into the dentate granule cell layer. Human Aβ_1-42-induced impairment of LTP was rescued by co-injection of isoproterenol, an adrenergic β receptor agonist, but not by co-injection of phenylephrine, an adrenergic α₁ receptor agonist. Isoproterenol did not reduce Aβ_1-42 uptake into dentate granule cells, but reduced increase in intracellular Zn²⁺ in dentate granule cells induced by Aβ_1-42. In contrast, phenylephrine did not reduce both Aβ_1-42 uptake and increase in intracellular Zn²⁺ by Aβ_1-42. In the case of human Aβ_1-40 and rat Aβ_1-42, which do not increase intracellular Zn²⁺, human Aβ_1-40- and rat Aβ_1-42-induced impairments of LTP were not rescued by co-injection of isoproterenol. The present study indicates that adrenergic β receptor activation reduces Aβ_1-42-mediated increase in intracellular Zn²⁺ in dentate granule cells, resulting in rescuing Aβ_1-42-induced impairment of LTP. It is likely that noradrenergic neuron activation by stimulating the locus coeruleus is effective for rescuing Aβ_1-42-induced cognitive decline that is caused by intracellular Zn²⁺ dysregulation in the hippocampus.

Sex differences in the effect of cannabinoid type 1 receptor deletion on locus coeruleus-norepinephrine neurons and corticotropin releasing factor-mediated responses

Cannabinoids are capable of modulating mood, arousal, cognition and behavior, in part via their effects on the noradrenergic nucleus locus coeruleus (LC). Dysregulation of LC signaling and norepinephrine (NE) efflux in the medial prefrontal cortex (mPFC) can lead to the development of psychiatric disorders, and CB1r deletion results in alterations of α2- and β1-adrenoceptors in the mPFC, suggestive of increased LC activity. To determine how CB1r deletion alters LC signaling, whole-cell patch-clamp electrophysiology was conducted in LC-NE neurons of male and female wild type (WT) and CB1r-knock out (KO) mice. CB1r deletion caused a significant increase in LC-NE excitability and input resistance in male but not female mice when compared to WT. CB1r deletion also caused adaptations in several indices of noradrenergic function. CB1r/CB2r-KO male mice had a significant increase in cortical NE levels and tyrosine hydroxylase and CRF levels in the LC compared to WT males. CB1r/CB2r-KO female mice showed a significant increase in LC α2-AR levels compared to WT females. To further probe actions of the endocannabinoid system as an anti-stress neuromediator, the effect of CB1r deletion on CRF-induced responses in the LC was investigated. The increase in LC-NE excitability observed in male and female WT mice following CRF (300 nM) bath application was not observed in CB1r-KO mice. These results indicate that cellular adaptations following CB1r deletion cause a disruption in LC-NE signaling in males but not females, suggesting underlying sex differences in compensatory mechanisms in KO mice as well as basal endocannabinoid regulation of LC-NE activity.

Localization of amyloid beta peptides to locus coeruleus and medial prefrontal cortex in corticotropin releasing factor overexpressing male and female mice

A culmination of evidence from the literature points to the Locus Coeruleus (LC)-Norepinephrine system as an underappreciated and understudied area of research in the context of Alzheimer's Disease (AD). Stress is a risk factor for developing AD, and is supported by multiple clinical and preclinical studies demonstrating that amplification of the stress system disrupts cellular and molecular processes at the synapse, promoting the production and accumulation of the amyloid beta (Aβ42) peptide. Stress-induced activation of the LC is mediated by corticotropin releasing factor (CRF) and CRF receptors exhibit sex-biased stress signaling. Sex differences are evident in the neurochemical, morphological and molecular regulation of LC neurons by CRF, providing a compelling basis for the higher prevalence of stress-related disorders such as AD in females. In the present study, we examined the cellular substrates for interactions between Aβ and tyrosine hydroxylase a marker of noradrenergic somatodendritic processes in the LC, and Dopamine-β-Hydroxylase (DβH) in the infralimbic medial prefrontal cortex (ILmPFC) in mice conditionally overexpressing CRF in the forebrain (CRFOE) under a Doxycycline (DOX) regulated tetO promoter. CRFOE was sufficient to elicit a redistribution of Aβ peptides in the somatodendritic processes of the LC of male and female transgenic mice, without altering total Aβ42 protein expression levels. DOX treated groups exhibited lysosomal compartments with apparent lipofuscin and abnormal morphology, indicating potential dysfunction of these Aβ42-clearing compartments. In female DOX treated groups, swollen microvessels with lipid-laden vacuoles were also observed, a sign of blood-brain-barrier dysfunction. Finally, sex differences were observed in the prefrontal cortex, as females responded to DOX treatment with increased frequency of co-localization of Aβ42 and DβH in noradrenergic axon terminals compared to vehicle treated controls, while male groups showed no significant changes. We hypothesize that the observed sex differences in Aβ42 distribution in this model of CRF hypersignaling is based on increased responsivity of female rodent CRFR1 in the LC. Aβ42 production is enhanced during increased neuronal activation, therefore, the excitation of DOX treated female CRFOE LC neurons projecting to the mPFC may exhibit more frequent co-localization with Aβ due to increased neuronal activity of noradrenergic neurons.

Noradrenergic depletion causes sex specific alterations in the endocannabinoid system in the Murine prefrontal cortex

Brain endocannabinoids (eCB), acting primarily via the cannabinoid type 1 receptor (CB1r), are involved in the regulation of many physiological processes, including behavioral responses to stress. A significant neural target of eCB action is the stress-responsive norepinephrine (NE) system, whose dysregulation is implicated in myriad psychiatric and neurodegenerative disorders. Using Western blot analysis, the protein expression levels of a key enzyme in the biosynthesis of the eCB 2-arachidonoylglycerol (2-AG), diacylglycerol lipase-α (DGL-α), and two eCB degrading enzymes monoacylglycerol lipase (MGL) and fatty acid amide hydrolase (FAAH) were examined in a mouse model that lacks the NE-synthesizing enzyme, dopamine β-hydroxylase (DβH-knockout, KO) and in rats treated with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP-4). In the prefrontal cortex (PFC), DGL-α protein expression was significantly increased in male and female DβH-KO mice (P < 0.05) compared to wild-type (WT) mice. DβH-KO male mice showed significant decreases in FAAH protein expression compared to WT male mice. Consistent with the DβH-KO results, DGL-α protein expression was significantly increased in male DSP-4-treated rats (P < 0.05) when compared to saline-treated controls. MGL and FAAH protein expression levels were significantly increased in male DSP-4 treated rats compared to male saline controls. Finally, we investigated the anatomical distribution of MGL and FAAH in the NE containing axon terminals of the PFC using immunoelectron microscopy. MGL was predominantly within presynaptic terminals while FAAH was localized to postsynaptic sites. These results suggest that the eCB system may be more responsive in males than females under conditions of NE perturbation, thus having potential implications for sex-specific treatment strategies of stress-related psychiatric disorders.

High resolution approaches for the identification of amyloid fragments in brain

BACKGROUND

It is now widely recognized that endogenous, picomolar concentrations of the 42 amino acid long peptide, amyloid-β (Aβ₄₂) is secreted under normal physiological conditions and exerts important functional activity throughout neuronal intracellular compartments. Transgenic animal models that overexpress Aβ₄₂ and its precursor, amyloid precursor protein (APP), have not provided predictive value in testing new treatments for Alzheimer's disease (AD), resulting in failed clinical trials. While these results are discouraging, they underscore the need to understand the physiological roles of Aβ₄₂ and APP under normal conditions as well as at early pre- symptomatic stages of AD. New method: We describe the use of acrolein-perfusion in immunoelectron microscopy in combination with novel antibodies directed against endogenous murine Aβ₄₂ and APP fragments to study abnormalities in the endolysosomal system at early stages of disease. The specific requirements, limitations and advantages of novel antibodies directed against human and murine Aβ₄₂, APP and APP fragments are discussed as well as parameters for ultrastructural analysis of endolysosomal compartments.

RESULTS

Novel antibodies and a detailed protocol for immunoelectron microscopy using acrolein as a fixative are described. Acrolein is shown to preserve intraneuronal Aβ₄₂ species, as opposed to paraformaldehyde fixed tissue, which primarily preserves membrane bound species. Comparison with existing method(s): Technology sensitive enough to detect endogenous Aβ₄₂ under physiological conditions has not been widely available. We describe a number of novel and highly sensitive antibodies have recently been developed that may facilitate the analysis of endogenous Aβ₄₂.

CONCLUSIONS

Using novel and highly specific antibodies in combination with electron microscopy may reveal important information about the timing of aberrant protein accumulation, as well as the progression of abnormalities in the endolysosomal systems that sort and clear these peptides.