Astrocytic uptake of neuronal corpses promotes cell-to-cell spreading of tau pathology

Alzheimer's disease: insights into pathology, molecular mechanisms, and therapy

Alzheimer's disease (AD), the leading cause of dementia, is characterized by the accumulation of amyloid plaques and neurofibrillary tangles in the brain. This condition casts a significant shadow on global health due to its complex and multifactorial nature. In addition to genetic predispositions, the development of AD is influenced by a myriad of risk factors, including aging, systemic inflammation, chronic health conditions, lifestyle, and environmental exposures. Recent advancements in understanding the complex pathophysiology of AD are paving the way for enhanced diagnostic techniques, improved risk assessment, and potentially effective prevention strategies. These discoveries are crucial in the quest to unravel the complexities of AD, offering a beacon of hope for improved management and treatment options for the millions affected by this debilitating disease.

Amyloid-β oligomers increase the binding and internalization of tau oligomers in human synapses

In Alzheimer's disease (AD), the propagation and spreading of CNS tau pathology closely correlates with cognitive decline, positioning tau as an attractive therapeutic target. Amyloid beta (Aβ) has been strongly implicated in driving tau spread, whereas primary tauopathies such as primary age-related tauopathy (PART)-which lack Aβ pathology-exhibit limited tau spread and minimal-to-no cognitive decline. Emerging evidence converges on a trans-synaptic mechanism of tau spread, facilitated by the transfer of misfolded tau aggregates (e.g. soluble oligomers). However, it is unclear whether Aβ oligomers modulate the binding and internalization of tau oligomers in human synapses. Our translationally focused paradigms utilize post-mortem brain specimens from Control, PART, and AD patients. Synaptosomes isolated from the temporal cortex of all three groups were incubated with preformed recombinant tauO (rtauO), ± preformed recombinant AβO (rAβO), and oligomer binding/internalization was quantified via flow cytometry following proteinase K (PK) digestion of surface-bound oligomers. TauO-synapse interactions were visualized using EM immunogold. Brain-derived tau oligomers (BDTO) from AD and PART PBS-soluble hippocampal fractions were co-immunoprecipitated and analyzed via mass spectrometry to compare synaptic tauO interactomes in primary and secondary tauopathies, thereby inferring the role of Aβ. AD synaptosomes, enriched in endogenous Aβ pathology, exhibited increased rtauO internalization compared to PART synaptosomes. This observation was mirrored in Control synaptosomes, where recombinant rAβO significantly increased rtauO binding and internalization. PK pre-treatment abolished this effect, implicating synaptic membrane proteins in AβO-mediated tauO internalization. While both PART and AD BDTO were broadly enriched in synaptic proteins, AD BDTO exhibited differential enrichment of endocytic proteins across pre- and post-synaptic compartments, whereas PART BDTO showed no significant synaptic enrichment. This study demonstrates that Aβ oligomers enhance tau oligomer binding and drive its internalization through synaptic membrane proteins. These findings offer novel mechanistic insights underlying pathological tau spreading directly within human synapses and emphasize the therapeutic potential of targeting Aβ-tau interactions.

Tau processing and tau-mediated inflammation differ in human APOEε2 and APOEε4 astrocytes

Alzheimer's disease (AD) and progressive supra-nuclear palsy (PSP) are both proteinopathies, characterized by the accumulation of tau aggregates. APOEε4 is the greatest genetic risk factor for developing AD, while APOEε2 is a significant risk factor for developing PSP. In the brain, astrocytes are the predominant producer of ApoE, but they are also important for inflammation and overall brain homeostasis. Although, tau inclusions appear frequently in astrocytes in both AD and PSP brains, their connection to ApoE remains unclear. Here, we show that hiPSC-derived APOE 2/2 astrocytes accumulate, process, and spread pathogenic tau aggregates more efficiently than isogenic APOE 4/4 astrocytes. Moreover, the APOE 2/2 astrocytes display a more robust inflammatory response, which could be of relevance for the disease course. Taken together, our data highlight a central role of ApoE in astrocyte-mediated tau pathology.

Neuroinflammatory Biomarkers in Alzheimer's Disease: From Pathophysiology to Clinical Implications

The identification of neuroinflammation as a critical factor in Alzheimer's disease (AD) has expanded the focus of research beyond amyloid-β and tau pathology. The neuroinflammatory fluid biomarkers GFAP, sTREM2, and YKL-40 have gained attention for their potential in early detection and monitoring of disease progression. Plasma GFAP has demonstrated promise in predicting the conversion from mild cognitive impairment to AD dementia, while sTREM2 highlights microglial activation, although there are conflicting results regarding its dynamics in AD pathogenesis. Advanced imaging techniques, such as PET tracers targeting TSPO and MAO-B, have also been developed to visualize glial activation in vivo, offering spatial and temporal insights into neuroinflammatory processes. However, the clinical implementation of these biomarkers faces challenges due to their lack of specificity, as many of them can be elevated in other conditions. Therapeutic strategies targeting neuroinflammation are emerging, with TREM2-targeting therapies and antidiabetic drugs like GLP-1 receptor agonists showing potential in modulating microglial activity. Nevertheless, the complexity of neuroinflammation, which encompasses both protective and harmful responses, necessitates further research to fully unravel its role and optimize therapeutic approaches for AD.

Cellular and pathological functions of tau

Tau protein is involved in various cellular processes, including having a canonical role in binding and stabilization of microtubules in neurons. Tauopathies are neurodegenerative diseases marked by the abnormal accumulation of tau protein aggregates in neurons, as seen, for example, in conditions such as frontotemporal dementia and Alzheimer disease. Mutations in tau coding regions or that disrupt tau mRNA splicing, tau post-translational modifications and cellular stress factors (such as oxidative stress and inflammation) increase the tendency of tau to aggregate and interfere with its clearance. Pathological tau is strongly implicated in the progression of neurodegenerative diseases, and the propagation of tau aggregates is associated with disease severity. Recent technological advancements, including cryo-electron microscopy and disease models derived from human induced pluripotent stem cells, have increased our understanding of tau-related pathology in neurodegenerative conditions. Substantial progress has been made in deciphering tau aggregate structures and the molecular mechanisms that underlie protein aggregation and toxicity. In this Review, we discuss recent insights into the diverse cellular functions of tau and the pathology of tau inclusions and explore the potential for therapeutic interventions.

The role of the gut microbiome in the regulation of astrocytes in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder and is the most common cause of dementia. AD is characterized pathologically by proteinaceous aggregates composed of amyloid beta (Aβ) and tau as well as progressive neurodegeneration. Concurrently with the buildup of protein aggregates, a strong neuroinflammatory response, in the form of reactive astrocytosis and microgliosis, occurs in the AD brain. It has recently been shown that the gut microbiome (GMB), composed of trillions of bacteria in the human intestine, can regulate both reactive astrocytosis and microgliosis in the context of both amyloidosis and tauopathy. Many studies have implicated microglia in these processes. However, growing evidence suggests that interactions between the GMB and astrocytes have a much larger role than previously thought. In this review, we summarize evidence regarding the gut microbiome in the control of reactive astrocytosis in AD.

Stable ubiquitin conjugation for biological interrogation of ubiquitinated tau repeat domain

Protein semisynthesis approaches are key for gaining insights into the effects of post-translational modifications (PTMs) on the structure and function of modified proteins. Among PTMs, ubiquitination involves the conjugation of a small protein modifier to a substrate amino acid residue and is unique in controlling a variety of cellular processes. Interest has grown in understanding the role of ubiquitination in neurodegenerative conditions, including tauopathies. The latter are characterized by the accumulation of the intrinsically disordered protein tau in the form of neurofibrillary tangles in the brains of patients. The presence of ubiquitinated tau in the pathological aggregates suggests that ubiquitination might play a role in the formation of abnormal protein deposits. In this study, we developed a new strategy, based on dehydroalanine chemistry, to install wild type ubiquitin on a tau repeat domain construct with site-specificity. We optimized a three-step reaction which yielded a good amount of highly pure tau repeat domain ubiquitinated in position 353. The structural features of the conjugate were examined by circular dichroism and NMR spectroscopy. The ubiquitinated tau was challenged in a number of assays: fibrils formation under aggregating conditions in vitro, chemical stability upon exposure to a variety of biological media including cell extracts, and internalization into astrocytes. The results demonstrated the wide applicability of the new semisynthetic strategy for the investigation of ubiquitinated substrates in vitro or in cell, and in particular for studying if ubiquitination has a role in the molecular mechanisms that underlie the aberrant transition of tau into pathological aggregates.

Inflammatory aspects of Alzheimer's disease

Alzheimer´s disease (AD) stands out as the most common chronic neurodegenerative disorder. AD is characterized by progressive cognitive decline and memory loss, with neurodegeneration as its primary pathological feature. The role of neuroinflammation in the disease course has become a focus of intense research. While microglia, the brain's resident macrophages, have been pivotal to study central immune inflammation, recent evidence underscores the contributions of other cellular entities to the neuroinflammatory process. In this article, we review the inflammatory role of microglia and astrocytes, focusing on their interactions with AD's core pathologies, amyloid beta deposition, and tau tangle formation. Additionally, we also discuss how different modes of regulated cell death in AD may impact the chronic neuroinflammatory environment. This review aims to highlight the evolving landscape of neuroinflammatory research in AD and underscores the importance of considering multiple cellular contributors when developing new therapeutic strategies.

Cell autonomous microglia defects in a stem cell model of frontotemporal dementia

Neuronal dysfunction has been extensively studied as a central feature of neurodegenerative tauopathies. However, across neurodegenerative diseases, there is strong evidence for active involvement of immune cells like microglia in driving disease pathophysiology. Here, we demonstrate that tau mRNA and protein are expressed in microglia in human brains and in human induced pluripotent stem cell (iPSC)-derived microglia like cells (iMGLs). Using iMGLs harboring the MAPT IVS10+16 mutation and isogenic controls, we demonstrate that a tau mutation is sufficient to alter microglial transcriptional states. We discovered that MAPT IVS10+16 microglia exhibit cytoskeletal abnormalities, stalled phagocytosis, disrupted TREM2/TYROBP networks, and altered metabolism. Additionally, we found that secretory factors from MAPT IVS10+16 iMGLs impact neuronal health, reducing synaptic density in neurons. Key features observed in vitro were recapitulated in human brain tissue and cerebrospinal fluid from MAPT mutations carriers. Together, our findings that MAPT IVS10+16 drives cell-intrinsic dysfunction in microglia that impacts neuronal health has major implications for development of therapeutic strategies.

Astrocyte-Neuron Interaction via the Glutamate-Glutamine Cycle and Its Dysfunction in Tau-Dependent Neurodegeneration

Astroglia constitute the largest group of glial cells and are involved in numerous actions that are critical to neuronal development and functioning, such as maintaining the blood-brain barrier, forming synapses, supporting neurons with nutrients and trophic factors, and protecting them from injury. These properties are deeply affected in the course of many neurodegenerative diseases, including tauopathies, often before the onset of the disease. In this respect, the transfer of essential amino acids such as glutamate and glutamine between neurons and astrocytes in the glutamate-glutamine cycle (GGC) is one example. In this review, we focus on the GGC and the disruption of this cycle in tau-dependent neurodegeneration. A profound understanding of the complex functions of the GGC and, in the broader context, searching for dysfunctions in communication pathways between astrocytes and neurons via GGC in health and disease, is of critical significance for the development of novel mechanism-based therapies for neurodegenerative disorders.

Amyloid-β deposits in human astrocytes contain truncated and highly resistant proteoforms

Alzheimer's disease (AD) is a neurodegenerative disorder that develops over decades. Glial cells, including astrocytes are tightly connected to the AD pathogenesis, but their impact on disease progression is still unclear. Our previous data show that astrocytes take up large amounts of aggregated amyloid-beta (Aβ) but are unable to successfully degrade the material, which is instead stored intracellularly. The aim of the present study was to analyze the astrocytic Aβ deposits composition in detail in order to understand their role in AD propagation. For this purpose, human induced pluripotent cell (hiPSC)-derived astrocytes were exposed to sonicated Aβ42 fibrils and magnetic beads. Live cell imaging and immunocytochemistry confirmed that the ingested Aβ aggregates and beads were transported to the same lysosomal compartments in the perinuclear region, which allowed us to successfully isolate the Aβ deposits from the astrocytes. Using a battery of experimental techniques, including mass spectrometry, western blot, ELISA and electron microscopy we demonstrate that human astrocytes truncate and pack the Aβ aggregates in a way that makes them highly resistant. Moreover, the astrocytes release specifically truncated forms of Aβ via different routes and thereby expose neighboring cells to pathogenic proteins. Taken together, our study establishes a role for astrocytes in mediating Aβ pathology, which could be of relevance for identifying novel treatment targets for AD.

The Neurovascular Unit as a Locus of Injury in Low-Level Blast-Induced Neurotrauma

Blast-induced neurotrauma has received much attention over the past decade. Vascular injury occurs early following blast exposure. Indeed, in animal models that approximate human mild traumatic brain injury or subclinical blast exposure, vascular pathology can occur in the presence of a normal neuropil, suggesting that the vasculature is particularly vulnerable. Brain endothelial cells and their supporting glial and neuronal elements constitute a neurovascular unit (NVU). Blast injury disrupts gliovascular and neurovascular connections in addition to damaging endothelial cells, basal laminae, smooth muscle cells, and pericytes as well as causing extracellular matrix reorganization. Perivascular pathology becomes associated with phospho-tau accumulation and chronic perivascular inflammation. Disruption of the NVU should impact activity-dependent regulation of cerebral blood flow, blood-brain barrier permeability, and glymphatic flow. Here, we review work in an animal model of low-level blast injury that we have been studying for over a decade. We review work supporting the NVU as a locus of low-level blast injury. We integrate our findings with those from other laboratories studying similar models that collectively suggest that damage to astrocytes and other perivascular cells as well as chronic immune activation play a role in the persistent neurobehavioral changes that follow blast injury.

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.

Aggregation, Transmission, and Toxicity of the Microtubule-Associated Protein Tau: A Complex Comprehension

The microtubule-associated protein tau is an intrinsically disordered protein containing a few short and transient secondary structures. Tau physiologically associates with microtubules (MTs) for its stabilization and detaches from MTs to regulate its dynamics. Under pathological conditions, tau is abnormally modified, detaches from MTs, and forms protein aggregates in neuronal and glial cells. Tau protein aggregates can be found in a number of devastating neurodegenerative diseases known as "tauopathies", such as Alzheimer's disease (AD), frontotemporal dementia (FTD), corticobasal degeneration (CBD), etc. However, it is still unclear how the tau protein is compacted into ordered protein aggregates, and the toxicity of the aggregates is still debated. Fortunately, there has been considerable progress in the study of tau in recent years, particularly in the understanding of the intercellular transmission of pathological tau species, the structure of tau aggregates, and the conformational change events in the tau polymerization process. In this review, we summarize the concepts of tau protein aggregation and discuss the views on tau protein transmission and toxicity.