Alzheimer's disease is characterized by the accumulation of amyloid-beta in plaques, aggregation of hyperphosphorylated tau in neurofibrillary tangles and neuroinflammation, together resulting in ...neurodegeneration and cognitive decline
. The NLRP3 inflammasome assembles inside of microglia on activation, leading to increased cleavage and activity of caspase-1 and downstream interleukin-1β release
. Although the NLRP3 inflammasome has been shown to be essential for the development and progression of amyloid-beta pathology in mice
, the precise effect on tau pathology remains unknown. Here we show that loss of NLRP3 inflammasome function reduced tau hyperphosphorylation and aggregation by regulating tau kinases and phosphatases. Tau activated the NLRP3 inflammasome and intracerebral injection of fibrillar amyloid-beta-containing brain homogenates induced tau pathology in an NLRP3-dependent manner. These data identify an important role of microglia and NLRP3 inflammasome activation in the pathogenesis of tauopathies and support the amyloid-cascade hypothesis in Alzheimer's disease, demonstrating that neurofibrillary tangles develop downstream of amyloid-beta-induced microglial activation.
The highest risk factor for the development of neurodegenerative diseases like tauopathies is aging. Many physiological decrements underlying aging are linked to cellular senescence. Senescent cells ...are characterized by an irreversible growth arrest and formation of a senescence‐associated secretory phenotype (SASP), a proinflammatory secretome that modifies the cellular microenvironment and contributes to tissue deterioration. Microglia, the innate immune cells in the brain, can enter a senescent state during aging. In addition, senescent microglia have been identified in the brains of tau‐transgenic mice and patients suffering from tauopathies. While the contribution of senescent microglia to the development of tauopathies and other neurodegenerative diseases is a growing area of research, the effect of tau on microglial senescence remains elusive. Here, we exposed primary microglia to 5 and 15 nanomolar (nM) of monomeric tau for 18 h, followed by a recovery period of 48 h. Using multiple senescence markers, we found that exposure to 15 nM, but not 5 nM of tau increased levels of cell cycle arrest and a DNA damage marker, induced loss of the nuclear envelope protein lamin B1 and the histone marker H3K9me3, impaired tau clearance and migration, altered the cell morphology and resulted in formation of a SASP. Taken together, we show that exposure to tau can lead to microglial senescence. As senescent cells were shown to negatively impact tau pathologies, this suggests the presence of a vicious circle, which should be further investigated in the future.
Although microglial senescence was shown to contribute to the development of tauopathies, no evidence exists for the effect of tau on microglial senescence. In the present study, we showed that exposure to monomeric tau can lead to senescence in primary microglia, illustrated by increased cell cycle arrest, DNA damage, loss of the nuclear envelope protein lamin B1 and the histone mark H3K9me3, impaired phagocytosis and migration, altered cell morphology, and formation of a SASP. As senescent cells were shown to negatively impact tau pathologies, the mechanism(s) by which monomeric tau induces senescence in microglia should be further investigated in the future (figure created with BioRender.com).
The renal filtration barrier is maintained by the renal podocyte, an epithelial postmitotic cell. Immortalized mouse podocyte cell lines-both in the differentiated and undifferentiated state-are ...widely utilized tools to estimate podocyte injury and cytoskeletal rearrangement processes in vitro. Here, we mapped the cultured podocyte proteome at a depth of more than 8,800 proteins and quantified 7,240 proteins. Copy numbers of proteins mutated in forms of hereditary nephrotic syndrome or focal segmental glomerulosclerosis (FSGS) were assessed. We found that cultured podocytes express abundant copy numbers of endogenous receptors, such as tyrosine kinase membrane receptors, the G protein-coupled receptor (GPCR), NPR3 (ANP receptor), and several poorly characterized GPCRs. The data set was correlated with deep mapping mRNA sequencing ("mRNAseq") data from the native mouse podocyte, the native mouse podocyte proteome and staining intensities from the human protein atlas. The generated data set was similar to these previously published resources, but several native and high-abundant podocyte-specific proteins were not identified in the data set. Notably, this data set detected general perturbations in proteostatic mechanisms as a dominant alteration during podocyte differentiation, with high proteasome activity in the undifferentiated state and markedly increased expression of lysosomal proteins in the differentiated state. Phosphoproteomics analysis of mouse podocytes at a resolution of more than 3,000 sites suggested a preference of phosphorylation of actin filament-associated proteins in the differentiated state. The data set obtained here provides a resource and provides the means for deep mapping of the native podocyte proteome and phosphoproteome in a similar manner.
Tauopathies are characterized by the progressive accumulation of hyperphosphorylated, aggregated forms of tau. Our laboratory has previously demonstrated that passive immunization with an anti-tau ...antibody, HJ8.5, decreased accumulation of pathological tau in a human P301S tau-expressing transgenic (P301S-tg) mouse model of frontotemporal dementia/tauopathy. To investigate whether the F
domain of HJ8.5 is required for the therapeutic effect, we engineered single-chain variable fragments (scFvs) derived from HJ8.5 with variable linker lengths, all specific to human tau. Based on different binding properties, we selected two anti-tau scFvs and tested their efficacy in vivo by adeno-associated virus-mediated gene transfer to the brain of P301S-tg mice. The scFvs significantly reduced levels of hyperphosphorylated, aggregated tau in brain tissue of P301S-tg mice, associated with a decrease in detergent-soluble tau species. Interestingly, these mice showed substantial levels of scFvs in the cerebrospinal fluid without significant effects on total extracellular tau levels. Therefore, our study provides a novel strategy for anti-tau immunotherapeutics that potentially limits a detrimental proinflammatory response.
There is an urgent need to improve the understanding of neuroinflammation in Alzheimer’s disease (AD). We analyzed cerebrospinal fluid inflammatory biomarker correlations to brain structural volume ...and longitudinal cognitive outcomes in the DELCODE study and in a validation cohort of the F.ACE Alzheimer Center Barcelona. We investigated whether respective biomarker changes are evident before onset of cognitive impairment. YKL-40; sTREM2; sAXL; sTyro3; MIF; complement factors C1q, C4, and H; ferritin; and ApoE protein were elevated in pre-dementia subjects with pathological levels of tau or other neurodegeneration markers, demonstrating tight interactions between inflammation and accumulating neurodegeneration even before onset of symptoms. Intriguingly, higher levels of ApoE and soluble TAM receptors sAXL and sTyro3 were related to larger brain structure and stable cognitive outcome at follow-up. Our findings indicate a protective mechanism relevant for intervention strategies aiming to regulate neuroinflammation in subjects with no or subjective symptoms but underlying AD pathology profile.
•Neuroinflammation biomarker study on cerebrospinal fluid of DELCODE and F.ACE cohorts•Synchronous pre-dementia elevation of inflammatory, tau, and neurodegeneration markers•Among these, sTyro3 and sAXL positively relate to MRI structure and cognition•Could TAM signaling be protective during pre-dementia neuroinflammation?
In the DELCODE and F.ACE cohorts, Brosseron, Maass, Kleineidam, et al. find that inflammatory CSF biomarkers rise in pre-dementia stages alongside tau and neurodegeneration markers. Within this pattern, soluble TAM receptors sTyro3 and sAXL positively relate to MRI structure and cognition, potentially as proxies of immune regulatory protective mechanisms.
Maintenance of the glomerular filtration barrier with its fenestrated endothelium, the glomerular basement membrane, and the podocytes as the outer layer, is a major prerequisite for proper renal ...function. Tight regulation of the balance between plasticity and rigidity of the podocytes' architecture is required to prevent the onset of glomerular disease, mainly proteinuria. The underlying cellular signaling pathways that regulate the organization of the podocytes' cytoskeleton are still a matter of controversial debate. In this study, we investigated the role of the NF-kB signaling pathway in podocyte cytoskeletal dynamics. As previously published, genetic inhibition of the NF-kB essential modulator (NEMO) in podocytes does not affect glomerular function under physiological, nonstressed conditions nor does it alter the initial podocyte response in an experimental glomerulonephritis (NTN) model (Brahler S, Ising C, Hagmann H, Rasmus M, Hoehne M, Kurschat C, Kisner T, Goebel H, Shankland SJ, Addicks K, Thaiss F, Schermer B, Pasparakis M, Benzing T, Brinkkoetter PT. Am J Physiol Renal Physiol 303: F1473-F1475, 2012). Quite the contrary, podocyte-specific NEMO null mice recovered significantly faster and did not develop glomerulosclerosis and end-stage renal failure over time. Here, we show that cytoskeletal rearrangements and increased podocyte motility following stimulation with IL-1, TNF-a, or LPS depend on NEMO. NEMO also regulates the phosphorylation of the MAP kinase ERK1/2 and suppresses the activation of RhoA following stimulation with IL-1. The migratory response and altered ERK1/2 phosphorylation is independent of NF-kB signaling as demonstrated by expression of a mutant IkB resistant to phosphorylation and degradation. In conclusion, signaling through NEMO might not only be involved in the production of NF-kB proinflammatory chemokines but also regulates podocyte dynamics independently of NF-kB, most likely through small GTPases and MAP kinases.
Neuroinflammation constitutes a pathological hallmark of Alzheimer's disease (AD). Still, it remains unresolved if peripheral inflammatory markers can be utilized for research purposes similar to ...blood-based beta-amyloid and neurodegeneration measures. We investigated experimental inflammation markers in serum and analyzed interrelations towards AD pathology features in a cohort with a focus on at-risk stages of AD.
Data of 74 healthy controls (HC), 99 subjective cognitive decline (SCD), 75 mild cognitive impairment (MCI), 23 AD relatives, and 38 AD subjects were obtained from the DELCODE cohort. A panel of 20 serum biomarkers was determined using immunoassays. Analyses were adjusted for age, sex, APOE status, and body mass index and included correlations between serum and CSF marker levels and AD biomarker levels. Group-wise comparisons were based on screening diagnosis and routine AD biomarker-based schematics. Structural imaging data were combined into composite scores representing Braak stage regions and related to serum biomarker levels. The Preclinical Alzheimer's Cognitive Composite (PACC5) score was used to test for associations between the biomarkers and cognitive performance.
Each experimental marker displayed an individual profile of interrelations to AD biomarkers, imaging, or cognition features. Serum-soluble AXL (sAXL), IL-6, and YKL-40 showed the most striking associations. Soluble AXL was significantly elevated in AD subjects with pathological CSF beta-amyloid/tau profile and negatively related to structural imaging and cognitive function. Serum IL-6 was negatively correlated to structural measures of Braak regions, without associations to corresponding IL-6 CSF levels or other AD features. Serum YKL-40 correlated most consistently to CSF AD biomarker profiles and showed the strongest negative relations to structure, but none to cognitive outcomes.
Serum sAXL, IL-6, and YKL-40 relate to different AD features, including the degree of neuropathology and cognitive functioning. This may suggest that peripheral blood signatures correspond to specific stages of the disease. As serum markers did not reflect the corresponding CSF protein levels, our data highlight the need to interpret serum inflammatory markers depending on the respective protein's specific biology and cellular origin. These marker-specific differences will have to be considered to further define and interpret blood-based inflammatory profiles for AD research.
Maintenance of the glomerular filtration barrier with its fenestrated endothelium, the glomerular basement membrane, and the podocytes as the outer layer, is a major prerequisite for proper renal ...function. Tight regulation of the balance between plasticity and rigidity of the podocytes' architecture is required to prevent the onset of glomerular disease, mainly proteinuria. The underlying cellular signaling pathways that regulate the organization of the podocytes' cytoskeleton are still a matter of controversial debate. In this study, we investigated the role of the NF-κB signaling pathway in podocyte cytoskeletal dynamics. As previously published, genetic inhibition of the NF-κB essential modulator (NEMO) in podocytes does not affect glomerular function under physiological, nonstressed conditions nor does it alter the initial podocyte response in an experimental glomerulonephritis (NTN) model (Brähler S, Ising C, Hagmann H, Rasmus M, Hoehne M, Kurschat C, Kisner T, Goebel H, Shankland SJ, Addicks K, Thaiss F, Schermer B, Pasparakis M, Benzing T, Brinkkoetter PT. Am J Physiol Renal Physiol 303: F1473-F1475, 2012). Quite the contrary, podocyte-specific NEMO null mice recovered significantly faster and did not develop glomerulosclerosis and end-stage renal failure over time. Here, we show that cytoskeletal rearrangements and increased podocyte motility following stimulation with IL-1, TNF-α, or LPS depend on NEMO. NEMO also regulates the phosphorylation of the MAP kinase ERK1/2 and suppresses the activation of RhoA following stimulation with IL-1. The migratory response and altered ERK1/2 phosphorylation is independent of NF-κB signaling as demonstrated by expression of a mutant IκB resistant to phosphorylation and degradation. In conclusion, signaling through NEMO might not only be involved in the production of NF-κB proinflammatory chemokines but also regulates podocyte dynamics independently of NF-κB, most likely through small GTPases and MAP kinases.
The kidney filtration barrier consists of three well-defined anatomic layers comprising a fenestrated endothelium, the glomerular basement membrane (GBM) and glomerular epithelial cells, the ...podocytes. Podocytes are post-mitotic and terminally differentiated cells with primary and secondary processes. The latter are connected by a unique cell-cell contact, the slit diaphragm. Podocytes maintain the GBM and seal the kidney filtration barrier to prevent the onset of proteinuria. Loss of prohibitin-1/2 (PHB1/2) in podocytes results not only in a disturbed mitochondrial structure but also in an increased insulin/IGF-1 signaling leading to mTOR activation and a detrimental metabolic switch. As a consequence, PHB-knockout podocytes develop proteinuria and glomerulosclerosis and eventually loss of renal function. In addition, experimental evidence suggests that PHB1/2 confer additional, extra-mitochondrial functions in podocytes as they localize to the slit diaphragm and thereby stabilize the unique intercellular contact between podocytes required to maintain an effective filtration barrier.
Background
Alzheimer’s disease is characterized by the extracellular accumulation of amyloid beta (Aß), intraneuronal formation of neurofibrillary tangles made of hyperphosphorylated tau and ...activated microglial cells, the innate immune cells of the brain. Activation of microglia by Aß or other DAMPs results in the assembly of the NLRP3 inflammasome consisting of NLRP3, ASC and Caspase‐1. This leads to subsequent production and secretion of inflammatory mediators including IL‐1β and ASC specks. Here, we investigate a contribution of the NLRP3 inflammasome to the development and progression of tau pathology.
Method
Tau22 transgenic mice, that express human 4‐repeat tau mutated at sites G272V and P301S under a Thy1.2‐promotor, were crossed into either NLRP3 inflammasome knockout or ASC knockout mice and analyzed before development of tau pathology (3 month of age) and when robust tau pathology is present (11 month of age). Animals underwent behavioral phenotyping, analysis of pathology as well as assessement of microglial activation. Additionally, Tau22/NLRP3‐knockout and Tau22/ASC‐knockout mice received an injection of Aß‐containing APP/PS1 mouse brain derived lysates to study Aß‐induced spreading of tau pathology. For in vitro studies, primary mouse microglia were treated with different forms of tau and analyzed for NLRP3 inflammasome activation.
Result
NLRP3 inflammasome activation was increased in fronto‐temporal dementia patients as well as in Tau22 mice. Knockout of ASC or NLRP3 in Tau22 mice decreased levels of active tau kinases while increasing activity of phosphatase PP2A in the hippocampus. This strongly protected from accumulation of hyperphosphorylated, misfolded tau in ASC knockout and more robustly in NLRP3 knockout mice while attenuating behavioral deficits. Furthermore, loss of NLRP3 inflammasome function ameliorated Aß‐induced tau pathology. In vitro, treatment of microglia with tau‐containing mouse brain homogenates or recombinant tau monomers and oligomers resulted in IL‐1β release in a NLRP3‐dependet manner, most likely mediated via TLR4. However, the same concentration of recombinant tau fibrils did not evoke this response.
Conclusion
These findings are in line with the hypothesis that innate immune activation represents an important pathogenic factor for tau pathology. In Alzheimer’s disease, early Aß deposition may cause subsequent tau pathology and neuronal demise through NLRP3‐mediated innate immune pathways.
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![The NF-[kappa]B essential m...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/1931-857X/small "The NF-[kappa]B essential modulator (NEMO) controls podocyte cytoskeletal dynamics independently of NF-[kappa]B")
