Inflammation drives the progression of neurodegenerative brain diseases and plays a major role in the accumulation of tau proteins within neurons.
An international research team led by the German Center for Neurodegenerative Diseases (DZNE) and the University of Bonn comes to this conclusion in the journal Nature.
The findings are based on the analyses of human brain tissue and further lab studies.
In the particular case of Alzheimer’s the results reveal a hitherto unknown connection between Amyloid Beta and tau pathology.
Furthermore, the results indicate that inflammatory processes represent a potential target for future therapies.
Tau proteins usually stabilize a neuron’s skeleton. However, in Alzheimer’s disease, frontotemporal dementia (FTD), and other “tauopathies” these proteins are chemically altered, they detach from the cytoskeleton and stick together.
As a consequence, the cell’s mechanical stability is compromised to such an extent that it dies off. In essence, “tau pathology” gives neurons the deathblow.
he current study led by Prof. Michael Heneka, director of the Department of Neurodegenerative Diseases and Gerontopsychiatry at the University of Bonn and a senior researcher at the DZNE, provides new insights into why tau proteins are transformed. As it turns out, inflammatory processes triggered by the brain’s immune system are a driving force.
A Molecular Switch
A particular protein complex, the “NLRP3 inflammasome”, plays a central role for these processes, the researchers report in Nature.
Heneka and colleagues already studied this macromolecule, which is located inside the brain’s immune cells, in previous studies.
It is a molecular switch that can trigger the release of inflammatory substances. For the current study, the researchers examined tissue samples from the brains of deceased FTD patients, cultured brain cells, and mice that exhibited hallmarks of Alzheimer’s and FTD.
“Our results indicate that the inflammasome and the inflammatory reactions it triggers, play an important role in the emergence of tau pathology”, Heneka said.
In particular, the researchers discovered that the inflammasome influences enzymes that induce a “hyperphosphorylation” of tau proteins.
This chemical change ultimately causes them to separate from the scaffold of neurons and clump together.
“It appears that inflammatory processes mediated by the inflammasome are of central importance for most, if not all, neurodegenerative diseases with tau pathology.”
A Link between Amyloid Beta and Tau
This especially applies to Alzheimer’s disease. Here another molecule comes into play: “amyloid beta” (Amyloid Beta). In Alzheimer’s, this protein also accumulates in the brain. In contrast to tau proteins, this does not happen within the neurons but between them. In addition, deposition of Amyloid Beta starts in early phases of the disease, while aggregation of tau proteins occurs later.
Furthermore, the results indicate that inflammatory processes represent a potential target for future therapies.
In previous studies, Heneka and colleagues were able to show that the inflammasome can promote the aggregation of Amyloid Beta.
Here is where the connection to the recent findings comes in.
“Our results support the amyloid cascade hypothesis for the development of Alzheimer’s. According to this hypothesis, deposits of Amyloid Beta ultimately lead to the development of tau pathology and thus to cell death,” said Heneka. “Our current study shows that the inflammasome is the decisive and hitherto missing link in this chain of events, because it bridges the development from Amyloid Beta pathology to tau pathology. It passes the baton, so to speak.”
Thus, deposits of Amyloid Beta activate the inflammasome. As a result, formation of further deposits of Amyloid Beta is promoted. On the other hand, chemical changes occur to the tau proteins resulting into their aggregation.
A Possible Starting Point for Therapies
“Inflammatory processes promote the development of Amyloid Beta pathology, and as we have now been able to show, of tau pathology as well.
Thus, the inflammasome plays a key role in Alzheimer’s and other brain diseases,” said Heneka, who is involved in the Bonn-based “ImmunoSensation” cluster of excellence and who also teaches at the University of Massachusetts Medical School. With these findings, the neuroscientist sees opportunities for new treatment methods.
“The idea of influencing tau pathology is obvious. Future drugs could tackle exactly this aspect by modulating the immune response. With the development of tau pathology, mental abilities decline more and more. Therefore, if tau pathology could be contained, this would be an important step towards a better therapy.”
Neurodegenerative diseases share the fact that they derive from altered proteins that undergo an unfolding process followed by formation of β-structures and a pathological tendency to self-aggregate in neuronal cells.
This is a characteristic of tau protein in Alzheimer’s disease and several tauopathies associated with tau unfolding, α-synuclein in Parkinson’s disease, and huntingtin in Huntington disease. Usually, the self-aggregation products are toxic to these cells, and toxicity spreads all over different brain areas. We have postulated that these protein unfolding events are the molecular alterations that trigger several neurodegenerative disorders.
Most interestingly, these events occur as a result of neuroinflammatory cascades involving alterations in the cross-talks between glial cells and neurons as a consequence of the activation of microglia and astrocytes. The model we have hypothesized for Alzheimer’s disease involves damage signals that promote glial activation, followed by nuclear factor NF-kβ activation, synthesis, and release of proinflammatory cytokines such as tumor necrosis factor (TNF)-α, interleukin (IL)-1, IL-6, and IL-12 that affect neuronal receptors with an overactivation of protein kinases.
These patterns of pathological events can be applied to several neurodegenerative disorders. In this context, the involvement of innate immunity seems to be a major paradigm in the pathogenesis of these diseases. This is an important element for the search for potential therapeutic approaches for all these brain disorders.
Cross-Talks Between Glial Cells and Neurons and Origins of Alzheimer’s Disease
The German physician Alois Alzheimer discovered, in the beginning of the past century, a neuropsychiatric disorder, with clinical features of a dementia, called Alzheimer’s disease (AD) after him. He analyzed the postmortem brain of an elderly woman with cognitive impairment and found anomalous structures which correspond to the intracellular neurofibrillary tangles (NFTs) formed by aggregates of hyperphosphorylated tau protein. These along with the oligomers of β-amyloid (Aβ) peptide became the major hallmarks of this disease. Along with these hallmarks, during many years of research, several factors have been elucidated, neuroinflammation being a key element in the development of the disease. In dementia, one of the most frequent is AD that affects mainly people over 65 years old. Because of the expansion of life expectancy, AD has become a major health problem, with an estimated 50 million people all over the world having it (Bettens et al., 2010). According to the World Health Organization (WHO), AD progressively affects learning and memory as well as mood and behavior, displaying a constantly increasing prevalence and impact (Maccioni, 2012; Guzman-Martinez et al., 2013).
A major constituent of NFTs is a hyperphosphorylated form of the axonal protein tau, whereas a major constituent of senile plaques (SPs) is Aβ protein. SPs are extracellular deposits and correspond to deposition of Aβ peptides, derived from the amyloid precursor protein (AβPP) (Chapman et al., 2002). Aβ is generated by a sequential processing of the AβPP by two proteases and usually exported from the brain to the cerebrospinal fluid (CSF) and local degradation by microglia, the major constituent of the brain’s innate immune system. In principle, microglia can engulf Aβ by phagocytosis (Heneka et al., 2015).
Hyperphosphorylated tau protein originally forms oligomeric structures called paired helical filaments (PHFs); then it turns into NFTs. The deposition of these structures causes loss of synaptic function and finally neuronal death (Giannakopoulos et al., 2003). Evidence supports the toxicity of tau aggregates when they are exported into the extracellular environment, along with being spread all over the brain (Neumann et al., 2011; Andrade et al., 2017). Studies of cell morphology and organelle distribution under tau overexpression show alterations in transport through the axis by motor axonal microtubule-associated proteins (MAPs) (Cambiazo et al., 1995).
On the other hand, in AD pathophysiology, a key event is neuroinflammation in the central nervous system (CNS). Thus, in this review, we will focus on how neuroinflammatory processes are directly related to cognitive impairment and to the neurodegenerative processes, describing yet the implications of the involvement of both astrocytes and microglia in the inflammatory and neuro-immunomodulatory processes (Fernandez et al., 2008; Morales et al., 2010; Maccioni, 2011; Neumann et al., 2011).
The microglial cells regulate the innate immune functions of astrocytes, under both physiological and pathological conditions; the inflammatory factors released by activated microglia can induce transduction of intracellular signals in astrocytes. On the other hand, the reactive astrocytes release factors that favor changes in the permeability of the blood–brain barrier (BBB), resulting in the recruitment of immune cells in the brain parenchyma.
This leads to an amplification of the initial innate immune response. In turn, these reactive astrocytes secrete a wide range of factors, such as neurotrophic factors, growth factors, and cytokines, promoting neuronal survival, neurite growth, and neurogenesis. Both the microglia and the astrocytes release various signaling molecules, establishing an autocrine feedback. The feedback between both types of glial cells generates a close reciprocal modulation for various lesions in the CNS (Jha et al., 2019).
There are several neuroinflammatory factors that are involved in both the onset and the progression of AD. This process depends on the innate immune system which includes microglia and astrocytes (Maccioni et al., 2009). Residues from bacteria, viruses, fungi, abnormal endogenous proteins, iron overload, complement factors, antibodies, cytokines, and chemokines, including toll-like receptors (TLRs) and receptor for advanced glycation end products (RAGE), comprise a large number of damage signals, which represent a danger for homeostasis of the CNS, and participate in microglial action and its activation (Shastri et al., 2013).
Under these conditions, microglial cells regulate the expression of different surface markers, such as the major histocompatibility complex II (MHC-II) molecular pattern recognition receptors (PPRs), which produce cytokine proinflammatory drugs such as interleukin (IL) 1 beta (IL-1β), IL-6, IL-12, interferon (IFN) gamma (IFN-γ), and tumor necrosis factor (TNF) alpha (TNF-α). They also synthesize and release short-lived cytotoxic factors, such as superoxide radicals (O2−), nitric oxide (NO), and reactive oxygen species (ROS) (Meda et al., 2001; Colton and Wilcock, 2010).
Therefore, and in relation to the above, microglial cells have an important role in innate immunity and are the main source of proinflammatory factors in the human brain. The microglial activation process depends on phenotypic characteristics and is functionally diverse, because the response depends on the type, intensity, and context of the stimulus that generates it. The factors that affect microglia can also generate neuroprotection. Under pathological conditions, neurotoxicity will be expressed, due to the breakdown of the delicate balance between neurotoxic and neuroprotective effects.
Microglial cells exhibit ramified processes having high motility and allowing a dynamic and continual survey of the healthy brain as observed by using in vivo two-photon imaging (Nimmerjahn et al., 2005). They sample, detect, and eliminate debris or apoptotic neurons by phagocytosis, but this ability is considerably decreased in a proinflammatory context (Koenigsknecht-Talboo and Landreth, 2005). Microglia is involved in multiple processes such as neurogenesis, synapse elimination in a complement-dependent manner, or synapse plasticity (Paolicelli et al., 2014). The involvement of microglia in AD pathogenesis was studied in the light of the Aβ (Guillot-Sestier et al., 2015; Heneka et al., 2015) and also in the context of tau oligomerization (Maccioni et al., 2009; Morales et al., 2010; Maccioni, 2012; Morales et al., 2014).
Another key factor is the accumulation of monocytes and microglia around blood vessels, due to the CCL2 chemoattractant protein and its affinity receptor CCR2. Studies showed that removal of the receptor increases the microglia accumulation phenomena, possibly through recruitment of mononuclear phagocytes and bone marrow, which promotes the deposition of perivascular Aβ (Ransohoff, 2016b). Care should be taken, since most experiments of circulating monocytes include conditions in which the BBB is open by irradiation procedures in AD, leading to controversy. Interestingly, the reduction of monocyte infiltration following ccr2 deficiency has been involved in tau hyperphosphorylation in traumatic brain injury (TBI) (Ransohoff, 2016b).
It is known that Aβ oligomers induce the activation of microglia through oligomers-surface receptors such as TLRs, being part of a physiological duty to eliminate them via phagocytosis (Walter et al., 2007). Certain receptors are associated with the reduction of microglial Aβ phagocytic capacity like triggering receptors expressed in myeloid cells 2 (TREM2), whose specific missense mutations increase the risk of AD (Jonsson et al., 2013; Parhizkar et al., 2019). Deficiencies in receptor CX3CR1, a chemokine CX3CL1 microglial receptor which partly mediates the infiltration of monocytes, induce overexpression or activation of microglia and tau hyperphosphorylation (Maphis et al., 2015), increased CD33 with a specific single-nucleotide polymorphism (SNP) in the promoter which inhibits immune response promoting Aβ1–42 accumulation (Griciuc et al., 2013), B3 domain-containing transcription factor ABI3 (ABI3) (Sims et al., 2017), and several other factors.
On the other hand, some variants in phospholipase C gamma 2 (PLCG2) have a positive outcome for AD, reducing the late onset of the disease (Sims et al., 2017). Friedman et al. (2018) determined the gene expression profile associated with neurodegeneration, where 75% of these genes are linked with gene ontology (GO) related to plasmatic membrane. Altogether, this information and the effect of mutations in several receptors and other plasma membrane proteins suggest that changes depend on the interaction with the environment. The paper of Keren-Shaul et al. (2017) describes a new kind of microglia, the disease-associated microglia (DAM) that only gets expressed in AD.
The study determined several gene modules. In DAM, a neurodegeneration gene core is expressed. Other modules include the IFN gene. In AD animal models, there are abundant cells that express the IFN module.
The DAM gene expression changes as follows: there is downregulation of homeostatic genes like CX3CR1 and upregulation of genes associated with the disease like Apoε and phagocytic genes for plaque clearance. Every microglia has promoters and enhancers associated with DAMs, indicating that these stage changes might pass through an epigenomic change.
They also discovered the three stages of microglia: homeostatic, intermediated, and finally DAM stage through an unknown Trem2-independent mechanism. In microglia gene expression modules, lipopolysaccharide (LPS) and neutrophil/monocyte are exacerbated, suggesting that inflammation and infiltration elements are involved in the neurodegenerative disease (ND).
Along with microglia, astrocytes are involved in the neuroinflammation process. Astrocytes have roles in metabolic regulation, neuronal scaffold, and synaptogenesis. In addition, there is a close contact with microglia and blood vessels in BBB (Morales et al., 2014). It also participates in the clearance of Aβ, by enzyme secretion (Mulder et al., 2012), and APOε from the ε2 allele is considered a protective factor. (Koistinaho et al., 2004). Like microglia, astrocytes also surround Aβ plaques (Medeiros and LaFerla, 2013), turning into an activated phase. Calcium deregulation, expression of the APOε4 allele, gives rise to APOε4 activity, which does not affect the synthesis of Aβ but does increase the deposition of the same, meaning a defect in the Aβ clearance (Holtzman et al., 2000).
The astrocytes can be activated through a pathway involving NF-κB, to release a C3 complement which binds to the C3aR receptor, inducing neuronal damage (Lian et al., 2015), along with soluble CD40, which binds to microglia and induces the release of TNF-α and other proinflammatory cytokines (Frankola et al., 2011). In neuroinflammation, astrocytes also contribute to NO toxicity, by expressing inducible NO synthase (iNOS) (Phillips et al., 2014).
Besides, there is overexpression of the glial fibrillary acidic protein (GFAP), a protein essential in the astrocyte cytoskeleton, related to astrocyte activation (Hol and Pekny, 2015). In the tauopathy context, Aβ can bind to the calcium sensing receptor (CaSR) in astrocytes, which triggers signaling pathways involved in the production and release of phosphorylated tau (Chiarini et al., 2017).
Neuroinflammation in AD
Neuroinflammation is a process related with the onset of several neurodegenerative disorders and it is an important contributor to AD pathogenesis and progression. Several damage signals appear to induce neuroinflammation, such as trauma, infection, oxidative agents, redox iron, oligomers of tau, and Aβ. In effect, neuroinflammation is responsible for an abnormal secretion of proinflammatory cytokines that trigger signaling pathways that activate brain tau hyperphosphorylation in residues that are not modified under normal physiological conditions.
Indeed, evidence exists that AD pathogenesis is not restricted to the neuronal compartment but includes strong interactions with immunological cells in the brain such as astrocytes, microglia, and infiltrating immune cells from the periphery, which could contribute to the modification of the process of neuroinflammation and neurodegeneration in AD brains.
In this context, this is where our theory of neuroimmunomodulation plays an important role and focuses on the link between neuronal damage and brain inflammatory process, mediated by the progressive activation of astrocytes and microglial cells with the consequent overproduction of proinflammatory agents (Maccioni et al., 2009). Despite clinical and pathological differences, increasing experimental evidence indicates that neuroinflammatory events lead to tau protein misfolding (Cortes et al., 2018).
The participation of the innate immune system in disease progression has shown a harmful bidirectional connection with regard to tau pathology. It is known that the tau protein belongs to the family of MAPs and is expressed mainly by neurons with preferential axonal localization. It has been observed that tau in vitro promotes the polymerization of tubulin and decreases the transition rate between the phases of growth and contraction, generating a stable but dynamic state in microtubules (Weingarten et al., 1975; Drechsel et al., 1992).
Tau is found mainly in axons, but a small amount is distributed physiologically in dendrites. The postsynaptic function of tau is not yet well defined, but it may be involved in synaptic plasticity. On the other hand, in addition to axons and dendrites, a nuclear function of tau (Citron, 2010) has been discovered, which could be regulating transcriptional activity and maintaining DNA/RNA integrity under physiological and stress conditions (Weingarten et al., 1975; Violet et al., 2014).
The tau structure corresponds to a hierarchical phosphorylation process in which different sites modulate the conformation of the protein, promoting the action of secondary kinases. In AD, different sites are phosphorylated earlier than others, leading to the creation of new epitopes. This sequential process has been studied by the use of antibodies such us AT100, whose epitopes in PHFs only appear after successive phosphorylation of residues Thr212 and Ser214, by glycogen synthase kinase (GSK)-3β and protein kinase A (PKA) along with Ser199, Ser202, Ser208, and Thr205 (Bussiere et al., 1999; Malia et al., 2016).
It was also shown that the expression of tau by microglial cells promotes its activation (Wang et al., 2013). Overall, the exact pathway leading to phosphorylation of tau remains poorly defined, but subsequent structural changes induce its detachment from the microtubules and produce higher levels of soluble free tau. Before the formation of NFTs, the hyperphosphorylation of tau favors a dynamic and progressive self-assembly of tau in oligomeric forms and insoluble materials such as PHFs throughout the disease with different degrees of neurotoxicity (Braak and Braak, 1991).
Marcus Neitzert – DZNE
The image is in the public domain.
Original Research: Closed access
“NLRP3 inflammasome activation drives tau pathology”. Christina Ising et al.