Gliovascular alterations in sporadic and familial Alzheimer's disease: APOE3 Christchurch homozygote glioprotection

In response to brain insults, astrocytes become reactive, promoting protection and tissue repair. However, astroglial reactivity is typical of brain pathologies, including Alzheimer's disease (AD). Considering the heterogeneity of the reactive response, the role of astrocytes in the course of different forms of AD has been underestimated. Colombia has the largest human group known to have familial AD (FAD). This group carries the autosomal dominant and fully penetrant mutation E280A in PSEN1, which causes early-onset AD. Recently, our group identified an E280A carrier who did not develop FAD. The individual was homozygous for the Christchurch mutation R136S in APOE3 (APOEch). Remarkably, APOE is the main genetic risk factor for developing sporadic AD (SAD) and most of cerebral ApoE is produced by astroglia. Here, we characterized astrocyte properties related to reactivity, glutamate homeostasis, and structural integrity of the gliovascular unit (GVU), as factors that could underlie the pathogenesis or protection of AD. Specifically, through histological and 3D microscopy analyses of postmortem samples, we briefly describe the histopathology and cytoarchitecture of the frontal cortex of SAD, FAD, and APOEch, and demonstrate that, while astrodegeneration and vascular deterioration are prominent in SAD, FAD is characterized by hyperreactive-like glia, and APOEch displays the mildest astrocytic and vascular alterations despite having the highest burden of Aβ. Notably, astroglial, gliovascular, and vascular disturbances, as well as brain cell death, correlate with the specific astrocytic phenotypes identified in each condition. This study provides new insights into the potential relevance of the gliovasculature in the development and protection of AD. To our knowledge, this is the first study assessing the components of the GVU in human samples of SAD, FAD, and APOEch.

Glutamatergic systems in neuropathic pain and emerging non-opioid therapies

Neuropathic pain, a disease of the somatosensory nervous system, afflicts many individuals and adequate management with current pharmacotherapies remains elusive. The glutamatergic system of neurons, receptors and transporters are intimately involved in pain but, to date, there have been few drugs developed that therapeutically modulate this system. Glutamate transporters, or excitatory amino acid transporters (EAATs), remove excess glutamate around pain transmitting neurons to decrease nociception suggesting that the modulation of glutamate transporters may represent a novel approach to the treatment of pain. This review highlights and summarizes (1) the physiology of the glutamatergic system in neuropathic pain, (2) the preclinical evidence for dysregulation of glutamate transport in animal pain models, and (3) emerging novel therapies that modulate glutamate transporters. Successful drug discovery requires continuous focus on basic and translational methods to fully elucidate the etiologies of this disease to enable the development of targeted therapies. Increasing the efficacy of astrocytic EAATs may serve as a new way to successfully treat those suffering from this devastating disease.

The gut microbiome and Alzheimer's disease: Complex and bidirectional interactions

Structural and functional alterations to the gut microbiome, referred to as gut dysbiosis, have emerged as potential key mediators of neurodegeneration and Alzheimer disease (AD) pathogenesis through the "gut -brain" axis. Emerging data from animal and clinical studies support an important role for gut dysbiosis in mediating neuroinflammation, central and peripheral immune dysregulation, abnormal brain protein aggregation, and impaired intestinal and brain barrier permeability, leading to neuronal loss and cognitive impairment. Gut dysbiosis has also been shown to directly influence various mechanisms involved in neuronal growth and repair, synaptic plasticity, and memory and learning functions. Aging and lifestyle factors including diet, exercise, sleep, and stress influence AD risk through gut dysbiosis. Furthermore, AD is associated with characteristic gut microbial signatures which offer value as potential markers of disease severity and progression. Together, these findings suggest the presence of a complex bidirectional relationship between AD and the gut microbiome and highlight the utility of gut modulation strategies as potential preventative or therapeutic strategies in AD. We here review the current literature regarding the role of the gut-brain axis in AD pathogenesis and its potential role as a future therapeutic target in AD treatment and/or prevention.

Traditional herbal formula Jiao-tai-wan improves chronic restrain stress-induced depression-like behaviors in mice

OBJECTIVES

Jiao-tai-wan (JTW) has been often used to treat insomnia and diabetes mellitus. Recent studies found its antidepressant activity, but the related mechanism is not clear. This study is to evaluate the therapeutic effects of JTW on chronic restraint stress (CRS)-induced depression mice and explore the potential mechanisms.

METHODS

CRS was used to set up a depression model. Mice in different groups were treated with 0.9 % saline, JTW and fluoxetine. After the last day of CRS, the behavioral tests were conducted. The levels of neurotransmitters, inflammatory cytokines and HPA axis index were detected and the protein expressions of NLRP3 inflammasome complex were determined. H&E, NISSL, TUNEL and immunofluorescence staining were used to observe histopathological changes and the activation of microglia and astrocytes. The potential mechanisms were explored via network pharmacology and verified by Western blot.

RESULTS

The assessment of liver and kidney function showed that JTW was non-toxic. Behavioral tests proved that JTW can effectively ameliorate depression-like symptoms in CRS mice, which may be related to the inhibition of NLRP3 inflammasome activation. JTW can also improve the inflammatory state and HPA axis hyperactivity in mice, and has a protective effect on CRS-induced hippocampal neurons damage. The network pharmacology analysis and the results of Western blot suggested that the antidepressant effects of JTW may be related to the MAPK signaling pathway.

CONCLUSION

Our findings indicated that JTW may exert antidepressant effects in CRS-induced mice by inhibiting NLRP3 inflammasome activation and improving inflammatory state, and MAPK signaling pathway may also be involved.

Stress-Induced Depression and Alzheimer's Disease: Focus on Astrocytes

Neurodegenerative diseases and depression are multifactorial disorders with a complex and poorly understood physiopathology. Astrocytes play a key role in the functioning of neurons in norm and pathology. Stress is an important factor for the development of brain disorders. Here, we review data on the effects of stress on astrocyte function and evidence of the involvement of astrocyte dysfunction in depression and Alzheimer's disease (AD). Stressful life events are an important risk factor for depression; meanwhile, depression is an important risk factor for AD. Clinical data indicate atrophic changes in the same areas of the brain, the hippocampus and prefrontal cortex (PFC), in both pathologies. These brain regions play a key role in regulating the stress response and are most vulnerable to the action of glucocorticoids. PFC astrocytes are critically involved in the development of depression. Stress alters astrocyte function and can result in pyroptotic death of not only neurons, but also astrocytes. BDNF-TrkB system not only plays a key role in depression and in normalizing the stress response, but also appears to be an important factor in the functioning of astrocytes. Astrocytes, being a target for stress and glucocorticoids, are a promising target for the treatment of stress-dependent depression and AD.

Pharmacogenomics of Alzheimer's Disease: Novel Strategies for Drug Utilization and Development

Alzheimer's disease (AD) is a priority health problem in developed countries with a high cost to society. Approximately 20% of direct costs are associated with pharmacological treatment. Over 90% of patients require multifactorial treatments, with risk of adverse drug reactions (ADRs) and drug-drug interactions (DDIs) for the treatment of concomitant diseases such as hypertension (>25%), obesity (>70%), diabetes mellitus type 2 (>25%), hypercholesterolemia (40%), hypertriglyceridemia (20%), metabolic syndrome (20%), hepatobiliary disorder (15%), endocrine/metabolic disorders (>20%), cardiovascular disorder (40%), cerebrovascular disorder (60-90%), neuropsychiatric disorders (60-90%), and cancer (10%).For the past decades, pharmacological studies in search of potential treatments for AD focused on the following categories: neurotransmitter enhancers (11.38%), multitarget drugs (2.45%), anti-amyloid agents (13.30%), anti-tau agents (2.03%), natural products and derivatives (25.58%), novel synthetic drugs (8.13%), novel targets (5.66%), repository drugs (11.77%), anti-inflammatory drugs (1.20%), neuroprotective peptides (1.25%), stem cell therapy (1.85%), nanocarriers/nanotherapeutics (1.52%), and other compounds (<1%).Pharmacogenetic studies have shown that the therapeutic response to drugs in AD is genotype-specific in close association with the gene clusters that constitute the pharmacogenetic machinery (pathogenic, mechanistic, metabolic, transporter, pleiotropic genes) under the regulatory control of epigenetic mechanisms (DNA methylation, histone/chromatin remodeling, microRNA regulation). Most AD patients (>60%) are carriers of over ten pathogenic genes. The genes that most frequently (>50%) accumulate pathogenic variants in the same AD case are A2M (54.38%), ACE (78.94%), BIN1 (57.89%), CLU (63.15%), CPZ (63.15%), LHFPL6 (52.63%), MS4A4E (50.87%), MS4A6A (63.15%), PICALM (54.38%), PRNP (80.7059), and PSEN1 (77.19%). There is also an accumulation of 15 to 26 defective pharmagenes in approximately 85% of AD patients. About 50% of AD patients are carriers of at least 20 mutant pharmagenes, and over 80% are deficient metabolizers for the most common drugs, which are metabolized via the CYP2D6, CYP2C9, CYP2C19, and CYP3A4/5 enzymes.The implementation of pharmacogenetics can help optimize drug development and the limited therapeutic resources available to treat AD, and personalize the use of anti-dementia drugs in combination with other medications for the treatment of concomitant disorders.

The Conformationally Sensitive Spatial Distance Between the TM3-4 Loop and Transmembrane Segment 7 in the Glutamate Transporter Revealed by Paired-Cysteine Mutagenesis

Excitatory amino acid transporters can maintain extracellular glutamate concentrations lower than neurotoxic levels by transferring neurotransmitters from the synaptic cleft into surrounding glial cells and neurons. Previous work regarding the structural studies of Glt_Ph, Glt_TK, excitatory amino acid transporter 1 (EAAT1), EAAT3 and alanine serine cysteine transporter 2 described the transport mechanism of the glutamate transporter in depth. However, much remains unknown about the role of the loop between transmembrane segment 3 and 4 during transport. To probe the function of this loop in the transport cycle, we engineered a pair of cysteine residues between the TM3-TM4 loop and TM7 in cysteine-less EAAT2. Here, we show that the oxidative cross-linking reagent CuPh inhibits transport activity of the paired mutant L149C/M414C, whereas DTT inhibits the effect of CuPh on transport activity of L149C/M414C. Additionally, we show that the effect of cross-linking in the mutant is due to the formation of the disulfide bond within the molecules of EAAT2. Further, L-glutamate or KCl protect, and D,L-threo-β-benzyloxy-aspartate (TBOA) increases, CuPh-induced inhibition in the L149C/M414 mutant, suggesting that the L149C and M414C cysteines are closer or farther away in the outward- or inward-facing conformations, respectively. Together, our findings provide evidence that the distance between TM3-TM4 loop and TM7 alter when substrates are transported.

Assessment of astrocytes as a mediator of memory and learning in rodents

The classical view of astrocytes is that they provide supportive functions for neurons, transporting metabolites and maintaining the homeostasis of the extracellular milieu. This view is gradually changing with the advent of molecular genetics and optical methods allowing interrogation of selected cell types in live experimental animals. An emerging view that astrocytes additionally act as a mediator of synaptic plasticity and contribute to learning processes has gained in vitro and in vivo experimental support. Here we focus on the literature published in the past two decades to review the roles of astrocytes in brain plasticity in rodents, whereby the roles of neurotransmitters and neuromodulators are considered to be comparable to those in humans. We outline established inputs and outputs of astrocytes and discuss how manipulations of astrocytes have impacted the behavior in various learning paradigms. Multiple studies suggest that the contribution of astrocytes has a considerably longer time course than neuronal activation, indicating metabolic roles of astrocytes. We advocate that exploring upstream and downstream mechanisms of astrocytic activation will further provide insight into brain plasticity and memory/learning impairment.

Charging Up the Periphery: Glial Ionic Regulation in Sensory Perception

The peripheral nervous system (PNS) receives diverse sensory stimuli from the environment and transmits this information to the central nervous system (CNS) for subsequent processing. Thus, proper functions of cells in peripheral sense organs are a critical gate-keeper to generating appropriate animal sensory behaviors, and indeed their dysfunction tracks sensory deficits, sensorineural disorders, and aging. Like the CNS, the PNS comprises two major cell types, neurons (or sensory cells) and glia (or glia-like supporting neuroepithelial cells). One classic function of PNS glia is to modulate the ionic concentration around associated sensory cells. Here, we review current knowledge of how non-myelinating support cell glia of the PNS regulate the ionic milieu around sensory cell endings across species and systems. Molecular studies reviewed here suggest that, rather than being a passive homeostatic response, glial ionic regulation may in fact actively modulate sensory perception, implying that PNS glia may be active contributors to sensorineural information processing. This is reminiscent of emerging studies suggesting analogous roles for CNS glia in modulating neural circuit processing. We therefore suggest that deeper molecular mechanistic investigations into critical PNS glial functions like ionic regulation are essential to comprehensively understand sensorineural health, disease, and aging.

Conformationally Sensitive Proximity Between the TM3-4 Loop and Hairpin Loop 2 of the Glutamate Transporter EAAT2 Revealed by Paired-Cysteine Mutagenesis

Excitatory amino acid transporters (EAATs) serve to maintain extracellular neurotransmitter concentrations below neurotoxic levels by transporting glutamate from the synaptic cleft into apposed glia and neurons. Although the crystal structures of the archaeal EAAT homologue from Pyrococcus horikoshii, Glt_Ph, and the human glutamate transporter, EAAT1_cryst, have been resolved, the transport mechanism of the transmembrane 3-4 (TM3-4) loop and its structural rearrangement during transport have remained poorly understood. In order to explore the spatial position and function of the TM3-4 loop in the transport cycle, we engineered a pair of cysteine residues between the TM3-4 loop and hairpin loop 2 (HP2) in cysteine-less EAAT2 (CL-EAAT2). We observed that the oxidative cross-linking reagent Cu(II)(1,10-phenanthroline)₃ (CuPh) had a significant inhibitory effect on transport in the disubstituted A167C/G437C mutant, whereas dl-dithiothreitol (DTT) reversed the effect of cross-linking A167C/G437C on transport activity, as assayed by d-[³H]-aspartate uptake. Furthermore, we found that the effect of CuPh in this mutant was due to the formation of disulfide bonds in the transporter molecule. Moreover, dl-threo-β-benzyloxyaspartic acid (TBOA) attenuated, while l-glutamate or KCl enhanced, the CuPh-mediated inhibitory effect in the A167C/G437C mutant, suggesting that the A167C and G437C cysteines were farther apart in the outward-facing configuration and closer in the inward-facing configuration. Taken together, our findings provide evidence that the TM3-4 loop and HP2 change spatial proximity during the transport cycle.

A Novel Concept is Needed for Combating Alzheimer's Disease and NeuroHIV

Both Alzheimer’s disease (AD) and HIV-associated neurocognitive disorders (HAND) could progress to dementia, a severe consequence of neurodegenerative diseases. Cumulating evidence suggests that the β-amyloid (Aβ) theory, currently thought to be the predominant mechanism underlying AD and AD-related dementia (ADRD), needs re-evaluation, considering all treatments and new drug trials based upon this theory have been unsuccessful. Similar intention for treating HAND, including HIV-associated dementia (HAD), has also failed. Thus, novel theory, hypothesis, and therapeutic strategies are desperately needed for future study and effective treatments of AD/ADRD and HAND. There are numerous potential upstream mechanisms that may cause AD and/or HAND; but it is unrealistic to identify all of them. However, it is realistic and feasible to intervene the downstream mechanism of these two devastating neurodegenerative diseases by blocking the final common path to neurotoxicity mediated by overactivation of NMDA receptors (NMDARs) and voltage-gated calcium channels (VGCCs). Such a combined pharmacological intervention will likely ameliorate neuronal Ca²⁺ homeostasis by diminishing overactivated NMDAR and VGCC-mediated Ca²⁺ dysregulation (i.e., by reducing excessive Ca²⁺ influx and intracellular levels, [Ca²⁺]in)-induced hyperactivity, injury, and death of neurons in the critical brain regions that regulate neurocognition in the context of AD/ADRD or HAND, especially during aging. Here we present a novel theoretical concept, hypothesis, and working model for switching the battlefield from searching-and-fighting the original mechanism that may cause AD or HAND, to abolishing AD- and neuroHIV-induced neurotoxicity mediated by NMDAR and VGCC over activation, which may ultimately improve the therapeutic strategies for treating AD and HAND.

The Increased Densities, But Different Distributions, of Both C3 and S100A10 Immunopositive Astrocyte-Like Cells in Alzheimer's Disease Brains Suggest Possible Roles for Both A1 and A2 Astrocytes in the Disease Pathogenesis

There is increasing evidence of astrocyte dysfunction in the pathogenesis of Alzheimer’s disease (AD). Animal studies supported by human post-mortem work have demonstrated two main astrocyte types: the C3 immunopositive neurotoxic A1 astrocytes and the S100A10 immunopositive neuroprotective A2 astrocytes. A1 astrocytes predominate in AD, but the number of cases has been relatively small. We examined post-mortem brains from a larger cohort of AD cases and controls employing C3 and S100 immunohistochemistry to identify the astrocytic subtypes. There were a number of C3 immunopositive astrocyte-like cells (ASLCs) in the control cases, especially in the lower cerebral cortex and white matter. In AD this cell density appeared to be increased in the upper cerebral cortex but was similar to controls in other regions. The S100A10 showed minimal immunopositivity in the control cases in the cortex and white matter, but there was increased ASLC density in upper/lower cortex and white matter in AD compared to controls. In AD and control cases the numbers of C3 immunopositive ASLCs were greater than those for S100A10 ASLCs in all areas studied. It would appear that the relationship between A1 and A2 astrocytes and their possible role in the pathogenesis of AD is complex and requires more research.

Transport rate of EAAT2 is regulated by amino acid located at the interface between the scaffolding and substrate transport domains

Excitatory Amino Acid Transporters (EAATs) are plasma membrane proteins responsible for maintenance of low extracellular concentrations of glutamate in the CNS. Dysfunction in their activity is implicated in various neurological disorders. Glutamate transport by EAATs occurs through the movement of the central transport domain relative to the scaffold domain in the EAAT membrane protein. Previous studies suggested that residues located within the interface of these two domains in EAAT2, the main subtype of glutamate transporter in the brain, are involved in regulating transport rates. We used mutagenesis, structure-function relationship, surface protein expression and electrophysiology studies, in transfected COS-7 cells and oocytes, to examine residue glycine at position 298, which is located within this interface. Mutation G298A results in increased transport rate without changes in surface expression, suggesting a more hydrophobic and larger alanine results in facilitated transport movement. The increased transport rate does not involve changes in sodium affinity. Electrophysiological currents show that G298A increase both transport and anion currents, suggesting faster transitions through the transport cycle. This work identifies a region critically involved in setting the glutamate transport rate.

Amyloid Fibril-Induced Astrocytic Glutamate Transporter Disruption Contributes to Complement C1q-Mediated Microglial Pruning of Glutamatergic Synapses

The complement C1q plays a critical role in microglial phagocytosis of glutamatergic synapses and in the pathogenesis of neuroinflammation in Alzheimer's disease (AD). We recently reported that upregulation of metabotropic glutamate receptor signaling is associated with increased synaptic C1q production and subsequent microglial phagocytosis of synapses in the rodent models of AD. Here, we explored the role of astrocytic glutamate transporter in the synaptic C1q production and microglial phagocytosis of hippocampal glutamatergic synapses in a rat model of AD. Activation of astrocyte and reduction glutamate transporter 1 (GLT1) were noted after bilateral microinjection of amyloid-beta (Aβ_1-40) fibrils into the hippocampal CA1 area of rats. Ceftriaxone is a β-lactam antibiotic that upregulates GLT1 expression. Bilateral microinjection of ceftriaxone recovered GLT1 expression, decreased synaptic C1q production, suppressed microglial phagocytosis of glutamatergic synapses in the hippocampal CA1, and attenuated synaptic and cognitive deficits in rats microinjected with Aβ_1-40. In contrast, artificial suppression of GLT1 activity by DL-threo-beta-benzyloxyaspartate (DL-TBOA) in naïve rats induced synaptic C1q expression and microglial phagocytosis of glutamatergic synapses in the hippocampal CA1 area, resulting in synaptic and cognitive dysfunction. These findings demonstrated that impairment of astrocytic glutamate transporter plays a role in the pathogenesis of AD.

Vitamin D3 repressed astrocyte activation following lipopolysaccharide stimulation in vitro and in neonatal rats

Vitamin D3 has been reported to be an immunity modulator and high levels of vitamin D3 are correlated with a decreased risk for developing diseases in the central nervous system. Astrocytes are important immune cells and contribute toward inflammation during neurological diseases. The vitamin D receptor has been reported to be expressed in astrocytes; however, the effect of vitamin D3 on astrocyte activation has not been studied. Here, we found that lipopolysaccharide stimulation in astrocytes could enhance the expression of vitamin D receptor and Cyp27B1, which encodes the enzyme for converting vitamin D3 into its active form. Vitamin D3 suppressed the expression of proinflammatory cytokines tumour necrosis factor-α, interleukin-1β, vascular endothelial growth factor, and also TLR4 in activated astrocytes. Astrocyte activation was further found to be suppressed after the administration of vitamin D3 in neonatal rats injected with lipopolysaccharide in vivo. We demonstrated the antiactivation effect of vitamin D3 in astrocytes after lipopolysaccharide stimulation. Considering the function of reactive astrocytes in augmenting inflammatory response in neurodegeneration and brain injury, the finding that vitamin D3 administration may inhibit astrocyte activation may be potentially useful for the treatment of central nervous system disorders.

YKL-40 in the brain and cerebrospinal fluid of neurodegenerative dementias

BACKGROUND

YKL-40 (also known as Chitinase 3-like 1) is a glycoprotein produced by inflammatory, cancer and stem cells. Its physiological role is not completely understood but YKL-40 is elevated in the brain and cerebrospinal fluid (CSF) in several neurological and neurodegenerative diseases associated with inflammatory processes. Yet the precise characterization of YKL-40 in dementia cases is missing.

METHODS

In the present study, we comparatively analysed YKL-40 levels in the brain and CSF samples from neurodegenerative dementias of different aetiologies characterized by the presence of cortical pathology and disease-specific neuroinflammatory signatures.

RESULTS

YKL-40 was normally expressed in fibrillar astrocytes in the white matter. Additionally YKL-40 was highly and widely expressed in reactive protoplasmic cortical and perivascular astrocytes, and fibrillar astrocytes in sporadic Creutzfeldt-Jakob disease (sCJD). Elevated YKL-40 levels were also detected in Alzheimer's disease (AD) but not in dementia with Lewy bodies (DLB). In AD, YKL-40-positive astrocytes were commonly found in clusters, often around β-amyloid plaques, and surrounding vessels with β-amyloid angiopathy; they were also distributed randomly in the cerebral cortex and white matter. YKL-40 overexpression appeared as a pre-clinical event as demonstrated in experimental models of prion diseases and AD pathology. CSF YKL-40 levels were measured in a cohort of 288 individuals, including neurological controls (NC) and patients diagnosed with different types of dementia. Compared to NC, increased YKL-40 levels were detected in sCJD (p < 0.001, AUC = 0.92) and AD (p < 0.001, AUC = 0.77) but not in vascular dementia (VaD) (p > 0.05, AUC = 0.71) or in DLB/Parkinson's disease dementia (PDD) (p > 0.05, AUC = 0.70). Further, two independent patient cohorts were used to validate the increased CSF YKL-40 levels in sCJD. Additionally, increased YKL-40 levels were found in genetic prion diseases associated with the PRNP-D178N (Fatal Familial Insomnia) and PRNP-E200K mutations.

CONCLUSIONS

Our results unequivocally demonstrate that in neurodegenerative dementias, YKL-40 is a disease-specific marker of neuroinflammation showing its highest levels in prion diseases. Therefore, YKL-40 quantification might have a potential for application in the evaluation of therapeutic intervention in dementias with a neuroinflammatory component.