Extracellular tau oligomers affect extracellular glutamate handling by astrocytes through downregulation of GLT-1 expression and impairment of NKA1A2 function

Divergent disruptive effects of soluble recombinant tau assemblies on synaptic plasticity in vivo

Neurofibrillary tangles (NFTs), composed of tau protein fibrils, together with brain inflammation and synaptic loss, are neuropathological hallmarks of several neurodegenerative diseases, including Alzheimer's disease. Compared with tau fibrils, more water-soluble assemblies of either recombinant or patient-derived tau have been reported to exert relatively potent rapid synaptotoxic effects, including inhibition of synaptic long-term potentiation (LTP) in the hippocampus. Less is known regarding the action of exogenous tau soluble assemblies on the opposite form of synaptic plasticity, long-term depression (LTD). We compared the synaptic plasticity actions of two relatively standard preparations of soluble recombinant tau assemblies, (i) fibril-derived soluble sonicated tau aggregates (SτAs) and (ii) oligomer-enriched tau (oTau) prepared from monomers. Consistent with previous findings, intracerebroventricular injections of either SτAs or oTau acutely inhibited high-frequency stimulation-induced LTP at CA3-to-CA1 synapses in the anaesthetized rat hippocampus. However, LTP inhibition by oTau, but not SτAs, was prevented by co-injection with the conformational anti-tau monoclonal antibody, TOMA1. Furthermore, in contrast to SτAs, which inhibited LTD, injection of oTau potently facilitated LTD, decreasing the threshold for LTD induction by low-frequency stimulation. To test the role of pro-inflammatory cytokines in mediating the disruptive effects of the two forms of soluble tau on synaptic plasticity we pre-injected etanercept, a decoy receptor for tumor necrosis factor alpha (TNFα). Etanercept reduced the disruption of synaptic plasticity by oTau but not by SτAs. Moreover, injection of exogenous TNFα mimicked the facilitation of LTD by oTau, consistent with a role of this pro-inflammatory cytokine in LTD facilitation.These data provide evidence that preparations of soluble tau containing either monomer- or fibril-derived assemblies disrupt LTP and LTD via different mechanisms. Intriguingly, when oTau and SτAs were applied together, LTD block predominated. Thus, if similar synaptotoxic soluble tau assemblies are present together later during the disease process, as seems likely, inhibition of synaptic weakening processes is predicted to predominate. Equally, reducing TNFα would be expected to be more effective when the monomer-derived soluble tau assemblies are the dominant synaptotoxic species. We conclude that oTau and SτAs provide useful means to explore ways of targeting different synaptotoxic soluble tau species in tauopathies.

Disrupted astrocyte-neuron signaling reshapes brain activity in epilepsy and Alzheimer's disease

Astrocytes establish dynamic interactions with surrounding neurons and synchronize neuronal networks within a specific range. However, these reciprocal astrocyte-neuronal interactions are selectively disrupted in epilepsy and Alzheimer's disease (AD), which contributes to the initiation and progression of network hypersynchrony. Deciphering how disrupted astrocyte-neuronal signaling reshapes brain activity is crucial to prevent subclinical epileptiform activity in epilepsy and AD. In this review, we provide an overview of the diverse astrocyte-neuronal crosstalk in maintaining of network activity via homeostatic control of extracellular ions and transmitters, synapse formation and elimination. More importantly, since AD and epilepsy share the common symptoms of neuronal hyperexcitability and astrogliosis, we then explore the crosstalk between astrocytes and neurons in the context of epilepsy and AD and discuss how these disrupted interactions reshape brain activity in pathological conditions. Collectively, this review sheds light on how disrupted astrocyte-neuronal signaling reshapes brain activity in epilepsy and AD, and highlights that modifying astrocyte-neuronal signaling could be a therapeutic approach to prevent epileptiform activity in AD.

Extracellular PHF-tau modulates astrocyte mitochondrial dynamics and mediates neuronal connectivity

BACKGROUND

Tau is an intracellular protein that plays a crucial role in stabilizing microtubules. However, it can aggregate into various forms under pathological conditions and be secreted into the brain parenchyma. While the consequences of tau aggregation within neurons have been extensively studied, the effects of extracellular paired helical filaments of tau (ePHF-tau) on neurons and astrocytes are still poorly understood.

METHODS

This study examined the effect of human ePHF-tau (2N4R) on primary cultures of rat neuroglia, focusing on changes in neurites or synapses by microscopy and analysis of synaptosome and mitochondria proteomic profiles after treatment. In addition, we monitored the behavior of mitochondria in neurons and astrocytes separately over three days using high-speed imaging and high-throughput acquisition and analysis.

RESULTS

ePHF-tau was efficiently cleared by astrocytes within two days in a 3D neuron-astrocyte co-culture model. Treatment with ePHF-tau led to a rapid increase in synaptic vesicle production and active zones, suggesting a potential excitotoxic response. Proteomic analyses of synaptosomal and mitochondrial fractions revealed distinct mitochondrial stress adaptations: astrocytes exhibited elevated mitochondrial biogenesis and turnover, whereas neuronal mitochondria displayed only minor oxidative modifications. In a mixed culture model, overexpression of tau 1N4R specifically in astrocytes triggered a marked increase in mitochondrial biogenesis, coinciding with enhanced synaptic vesicle formation in dendrites. Similarly, astrocyte-specific overexpression of PGC1alpha produced a comparable pattern of synaptic vesicle production, indicating that astrocytic mitochondrial adaptation to ePHF-tau may significantly influence synaptic function.

CONCLUSIONS

These findings suggest that the accumulation of PHF-tau within astrocytes drives changes in mitochondrial biogenesis, which may influence synaptic regulation. This astrocyte-mediated adaptation to tauopathy highlights the potential role of astrocytes in modulating synaptic dynamics in response to tau stress, opening avenues for therapeutic strategies aimed at astrocytic mechanisms in the context of neurodegenerative diseases.

Internalization of extracellular Tau oligomers in Alzheimer's disease

A key factor in the progression of Alzheimer's disease (AD) is internalization of extracellular Tau oligomers (ecTauOs) by neuroglial cells. Aberrant hyperphosphorylation of Tau results in their dissociation from microtubules and formation of toxic intracellular Tau oligomers (icTauOs). These are subsequently released to the extracellular space following neuronal dysfunction and death. Although receptor mediated internalization of these ecTauOs by other neurons, microglia and astrocytes can facilitate elimination, incomplete degradation thereof promotes inflammation, exacerbates pathologic spread and accelerates neurodegeneration. Targeting Tau oligomer degradation pathways, blocking internalization receptors, and mitigating neuroinflammation are proposed as therapeutic strategies to control Tau propagation and toxicity. This review highlights the urgent need for innovative approaches to prevent the spread of Tau pathology, emphasizing its implications for AD and related neurodegenerative diseases.

Na+-K+-ATPase/GLT-1 interaction participates in EGCG protection against cerebral ischemia-reperfusion injury in rats

BACKGROUND

In China, stroke is the primary cause of adult death and disability. Because of the increased rate of blood vessel reperfusion, it is important to prevent cerebral ischemia-reperfusion injury, in which glutamate (Glu) excitotoxicity plays a critical role. The most important Glu transporter, GLT-1, is essential for the regulation of Glu, which is dependent on Na+-K+-ATPase (NKA)-induced ion concentration gradient differences. EGCG, a substance found in tea polyphenols, can reduce infarct areas in ischemia-reperfusion models, reduce stroke incidence, and prolong life in which NKA is involved.

PURPOSE

In this study, we investigated the potential of EGCG in protecting against cerebral ischemia-reperfusion injury by regulating the interaction between NKA and GLT-1.

STUDY DESIGN

This study was designed to investigate the protective effects of EGCG against cerebral ischemia-reperfusion injury by modulating the interaction between NKA and GLT-1, utilizing both the rat middle cerebral artery occlusion/reperfusion (MCAO/R) model and the oxygen-glucose deprivation/reoxygenation (OGD/R) model in co-cultures of rat hippocampal neurons and astrocytes.

METHODS

The neuronal survival rate was assessed using CCK8, and the cerebral infarction area and neurological function were determined by TTC staining and neurological deficit scores. NKA activity was measured using an inorganic phosphorous detection method, and NKA and GLT-1 expression was detected using western blotting. The interaction between NKAα2 and GLT-1 was identified by co-immunoprecipitation (CoIP) assay, laser confocal microscopy, and Imaris 3D confocal rendering technology. An adenovirus vector with overexpression of NKAα2 was constructed, packaged, and injected into the rat lateral ventricle. Neurological function and the cerebral infarction area were identified, and the interaction between NKAα2 and GLT-1 was identified using CoIP assay.

RESULTS

EGCG reduced the infarction area and neurological deficit scores, restored NKA activity, alleviated the decrease in membrane NKAα2 and GLT-1 expression, and relieved the uncoupling of NKAα2 and GLT-1 in the hippocampal CA1 after rat MCAO/R injury. By promoting the coupling of NKAα2 and GLT-1 in rat MCAO/R models, overexpression of NKAα2 reduced the cerebral infarction area and neurological impairment scores.

CONCLUSION

EGCG improved cerebral ischemia-reperfusion injury by restoring NKA activity and increasing membrane GLT-1 expression due to NKA-GLT-1 interaction. For the first time, our findings demonstrate the critical role that NKA and GLT-1 colocalization plays in cerebral ischemia-reperfusion damage. Our findings provide new strategic directions for the pathogenesis and prevention of thrombolytic injury in the clinical treatment of stroke, while also serving as a basis for further development and utilization of EGCG.

Brain clearance of protein aggregates: a close-up on astrocytes

Protein misfolding and accumulation defines a prevailing feature of many neurodegenerative disorders, finally resulting in the formation of toxic intra- and extracellular aggregates. Intracellular aggregates can enter the extracellular space and be subsequently transferred among different cell types, thus spreading between connected brain districts.Although microglia perform a predominant role in the removal of extracellular aggregated proteins, mounting evidence suggests that astrocytes actively contribute to the clearing process. However, the molecular mechanisms used by astrocytes to remove misfolded proteins are still largely unknown.Here we first provide a brief overview of the progressive transition from soluble monomers to insoluble fibrils that characterizes amyloid proteins, referring to α-Synuclein and Tau as archetypical examples. We then highlight the mechanisms at the basis of astrocyte-mediated clearance with a focus on their potential ability to recognize, collect, internalize and digest extracellular protein aggregates. Finally, we explore the potential of targeting astrocyte-mediated clearance as a future therapeutic approach for the treatment of neurodegenerative disorders characterized by protein misfolding and accumulation.

ApoE4 expression disrupts tau uptake, trafficking, and clearance in astrocytes

Tauopathies are a collection of neurodegenerative diseases characterized by the accumulation of pathogenic aggregates of the microtubule-associated protein tau. Despite the prevalence and diversity of tau astrogliopathy in tauopathies, the interactions between astrocytes and tau in the brain, and the influence of neurodegenerative genetic risk factors like the apolipoprotein E4 (apoE4) isoform, are largely unknown. Here, we leveraged primary and immortalized astrocytes expressing humanized apoE isoforms to characterize the mechanisms by which astrocytes interact with and eliminate extracellular tau, and the influence of apoE genotype on these processes. Our work indicates that astrocytes rapidly internalize, process, and release tau via an exosomal secretory mechanism under physiological conditions. However, we found that apoE4 disrupted these processes in comparison to apoE3, resulting in an astrocytic phenotype prone to intracellular tau accumulation. Furthermore, exposure to repetitive mild traumatic brain injuries exacerbated the apoE4-induced impairments in tau processing and elimination by astrocytes in apoE4 targeted-replacement mice. The diminished ability of apoE4 astrocytes to eliminate extracellular tau can lead to an accumulation of pathogenic tau, which induces mitochondrial dysfunction, as demonstrated by our studies. In total, our findings suggest that the apoE4 isoform lowers the threshold of astrocytic resilience to pathogenic tau, rendering them susceptible to bioenergetic deficits in the early stages of neurodegenerative diseases such as traumatic brain injury, potentially contributing to neurological decline.

Tau and Amyloid β Protein in Patient-Derived Aqueous Brain Extracts Act Concomitantly to Disrupt Long-Term Potentiation in Vivo

Amyloid β protein (Aβ) and tau, the two main proteins implicated in causing Alzheimer's disease (AD), are posited to trigger synaptic dysfunction long before significant synaptic loss occurs in vulnerable circuits. Whereas soluble Aβ aggregates from AD brain are well recognized potent synaptotoxins, less is known about the synaptotoxicity of soluble tau from AD or other tauopathy patient brains. Minimally manipulated patient-derived aqueous brain extracts contain the more diffusible native forms of these proteins. Here, we explore how intracerebral injection of Aβ and tau present in such aqueous extracts of patient brain contribute to disruption of synaptic plasticity in the CA1 area of the male rat hippocampus. Aqueous extracts of certain AD brains acutely inhibited long-term potentiation (LTP) of synaptic transmission in a manner that required both Aβ and tau. Tau-containing aqueous extracts of a brain from a patient with Pick's disease (PiD) also impaired LTP, and diffusible tau from either AD or PiD brain lowered the threshold for AD brain Aβ to inhibit LTP. Remarkably, the disruption of LTP persisted for at least 2 weeks after a single injection. These findings support a critical role for diffusible tau in causing rapid onset, persistent synaptic plasticity deficits, and promoting Aβ-mediated synaptic dysfunction.SIGNIFICANCE STATEMENT The microtubule-associated protein tau forms relatively insoluble fibrillar deposits in the brains of people with neurodegenerative diseases including Alzheimer's and Pick's diseases. More soluble aggregates of disease-associated tau may diffuse between cells and could cause damage to synapses in vulnerable circuits. We prepared aqueous extracts of diseased cerebral cortex and tested their ability to interfere with synaptic function in the brains of live rats. Tau in these extracts rapidly and persistently disrupted synaptic plasticity and facilitated impairments caused by amyloid β protein, the other major pathologic protein in Alzheimer's disease. These findings show that certain diffusible forms of tau can mediate synaptic dysfunction and may be a target for therapy.

Interleukin 1β triggers synaptic and memory deficits in Herpes simplex virus type-1-infected mice by downregulating the expression of synaptic plasticity-related genes via the epigenetic MeCP2/HDAC4 complex

Extensive research provides evidence that neuroinflammation underlies numerous brain disorders. However, the molecular mechanisms by which inflammatory mediators determine synaptic and cognitive dysfunction occurring in neurodegenerative diseases (e.g., Alzheimer's disease) are far from being fully understood. Here we investigated the role of interleukin 1β (IL-1β), and the molecular cascade downstream the activation of its receptor, to the synaptic dysfunction occurring in the mouse model of multiple Herpes simplex virus type-1 (HSV-1) reactivations within the brain. These mice are characterized by neuroinflammation and memory deficits associated with a progressive accumulation of neurodegenerative hallmarks (e.g., amyloid-β protein and tau hyperphosphorylation). Here we show that mice undergone two HSV-1 reactivations in the brain exhibited increased levels of IL-1β along with significant alterations of: (1) cognitive performances; (2) hippocampal long-term potentiation; (3) expression synaptic-related genes and pre- and post-synaptic proteins; (4) dendritic spine density and morphology. These effects correlated with activation of the epigenetic repressor MeCP2 that, in association with HDAC4, affected the expression of synaptic plasticity-related genes. Specifically, in response to HSV-1 infection, HDAC4 accumulated in the nucleus and promoted MeCP2 SUMOylation that is a post-translational modification critically affecting the repressive activity of MeCP2. The blockade of IL-1 receptors by the specific antagonist Anakinra prevented the MeCP2 increase and the consequent downregulation of gene expression along with rescuing structural and functional indices of neurodegeneration. Collectively, our findings provide novel mechanistic evidence on the role played by HSV-1-activated IL-1β signaling pathways in synaptic deficits leading to cognitive impairment.

A Medicinal Chemistry Perspective on Excitatory Amino Acid Transporter 2 Dysfunction in Neurodegenerative Diseases

The excitatory amino acid transporter 2 (EAAT2) plays a key role in the clearance and recycling of glutamate - the major excitatory neurotransmitter in the mammalian brain. EAAT2 loss/dysfunction triggers a cascade of neurodegenerative events, comprising glutamatergic excitotoxicity and neuronal death. Nevertheless, our current knowledge regarding EAAT2 in neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS) and Alzheimer's disease (AD), is restricted to post-mortem analysis of brain tissue and experimental models. Thus, detecting EAAT2 in the living human brain might be crucial to improve diagnosis/therapy for ALS and AD. This perspective article describes the role of EAAT2 in physio/pathological processes and provides a structure-activity relationship of EAAT2-binders, bringing two perspectives: therapy (activators) and diagnosis (molecular imaging tools).