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Neurodegenerative diseases affect millions of people all over the world; for example over 5 million people in the United States alone have been diagnosed with Alzheimer’s disease (AD). Although all underlying causes are not fully understood, neuroinflammation and immune responses play a big part in the onset of neurodegeneration. Evidence shows that microglia, astrocytes, oligodendrocytes, and neurons are all capable of initiating immune responses in the brain that could contribute to AD as they signal via neuroinflammatory components like cytokines, interleukins, chemokines, leukocytes, and endothelial adhesion molecules. Research focuses on decyphering the role played by reactive oxygen species, toll like receptor (TLR) and NFkappaB (NF-kB) mediated signaling pathways.
The role of TLRs in neuroinflammation
Is neuroinflammation a cause of or a consequence of neurodegeneration and is it protective or destructive? Acute neuroinflammation is required for the removal of amyloid beta (Aβ) plaques by microglia. However, on-going neuroinflammation results in glial dysfunction and neuronal compromise. Therefore, the time period and intensity of the inflammatory response is critical with respect to whether the response is protective or destructive.
Injury to the brain leads to the activation of repair pathways such as the recovery of the neuronal network, enhancing plasticity and providing alternative methods of performing tasks affected by the loss of brain tissue. Neuroinflammation is the response by glial cells (e.g. microglia and astrocytes) to infection or injury as part of the innate immune system. Activated glial cells produce pro-inflammatory cytokines, enzymes, and adhesion molecules - a response aided by TLRs.
Ischemic mice negative for tumor necrosis factor alpha (TNFα) and interleukin-6 (IL-6), show a larger amount of necrotic tissue when their inflammatory response is activated, indicating a protective role of the inflammation mediator products TNFα and IL-6. In addition to that, other cytokines like the IL-1 family play a detrimental role in inflammation; e.g. the deletion of IL-1α and IL-1β in mice reduces ischemic brain damage (Boutin et al. 2001). IL-1 is expressed after injury and in an NF-kB dependent manner. It therefore appears that timing, as well as specific cytokine expression, which is often regulated by TLR signaling, is crucial in determining whether inflammation helps or hinders repair.
During aging, secretion of pro-inflammatory cytokines is increased, whereas levels of anti-inflammatory cytokines decrease. Similarly TLR signaling is also altered during aging. This suggests that TLRs and inflammation are important factors in the onset of all kinds of age related diseases.
TLRs play a key role in the development and regulation of inflammation, neurodegeneration, and brain trauma. They are crucial components in activating the innate immune response against invading pathogens in addition to playing a key role in neuronal degeneration, neurogenesis, and neurite outgrowth. All these roles taken together suggest an important role in maintaining neuronal plasticity. In total 11 human and 13 mouse TLRs have been identified, all belonging to the pattern recognition receptor (PRR) family, along with other receptors such as CD206, and NALP (a Nod-like receptor). TLRs are expressed on a large variety of immune-related cells, including microglia, astrocytes, oligodendrocytes, and neurons in the brain. Their expression patterns are dependent on the presence of pathogens, cytokines, and environmental stresses.
TLRs are either expressed on the cell surface (TLR1, 2, 4, 5, and 6) or in endosomal vesicles (TLR3, 7, 8, and 9). Characteristic for all TLRs is a leucine-rich extracellular domain, which is involved in recognizing pathogen associated molecular patterns (PAMPs). Also conserved is a cytoplasmic Toll-IL-1 receptor (TIR) domain, which initiates downstream signaling via mediating interactions between TLRs and TIR-adapter proteins like myeloid differentiation factor 88 (MyD88), TIR-adapter proteins (TIRAP), TIR-domain-containing adapter-inducing interferon-β (TRIF) and TRIF-related adaptor molecule (TRAM). These interactions result in the upregulation and expression of inflammation regulating transcription factors such as NF-kB.
TLR4 is highly conserved from Drosophila melanogaster to humans. The active form of TLR4 is able to activate the NF-kB pathway thereby leading to the expression of NF-kB-dependent inflammatory genes and CD80, which are crucial in a T cell reponse.These are all necessary for the activation of naïve T lymphocytes, key players in an innate immune response (Medzhitov et al. 1997). Repeated stress exposure upregulates TLR4 expression suggesting a key role of TLR4 in stress-induced NF-κB activation, iNOS and COX-2 upregulation, and cellular oxidative/nitrosative damage responses. TLR4 signaling also affects IL-1β, IL-10, and IL-7 levels.
TLRs form either homo- or heterodimers and are categorized into three main groups (Table 1) according to the specific PAMP motif they recognize. These classes are:
Table 1. TLR overview.
|Cell surface||Activated by lipids||TLR1 (CD281)||TLR1|
|TLR2 (CD282)||TLR1||Pam3Cys (bacterial peptide) triacylated lipopeptides|
|TLR2||TLR6||Diacylated lipopeptides (Pam2CSK4), peptidoglycan|
|TLR2||TLR2||Heat shock proteins|
|TLR4 (CD284)||TLR4||Lipopolysaccharide (LPS) (gram negative bacteria/bacteria flagellin), heat shock proteins|
|TLR10 (only in humans)||TLR1/2|
|Activated by protein ligation||TLR5||TLR5||Bacterial flagellin proteins|
|TLR11||TLR11||Unknown ligand of bacteria and Toxoplasma gondii|
|Endosomal vesicles||Activated by bacterial and viral nucleic acid||TLR3 (CD283)||TLR3||Double stranded viral RNA|
|TLR7 (murine homolog of TLR8)||TLR7||Imidazoquinoline and viral ssRNA|
|TLR9 (CD289)||TLR9||Unmethylated CpG dinucleotides|
After PAMP recognition, TLR signaling is mediated in either a MyD88 dependent or MyD88 independent/Trif dependent manner. MyD88 is an adapter protein that mainly interacts with TIRAP/ MyD88-adapter-like (Mal) and TRAM proteins and plays a crucial role in enabling the recruitment of the IL-1R associated kinases (IRAK ) -1, 2, 3, 4. TIRAP /Mal and TRAM are also required for the activation of TRIF dependent signaling. All TLRs signal via the MyD88-dependent pathway, except for TLR3 which signals exclusively via TRIF.
Key steps in both MyD88 and TRIF mediated TLR signaling have been outlined below using TLR4 as an example. The outcome of both pathways is the activation of IRF3, NF-kB, and AB-1, which regulate the expression of pro-inflammatory cytokines and type 1 IFN.
Microglia are macrophage-like cells present in the central nervous system (CNS) that mediate neuronal immune interactions. Activated microglia appear to have mainly a neuroprotective role in acute injury. Stress conditions, such as hypoxia, result in increased TLR signaling in microglia. Apart from TLR10, all TLRs are expressed in microglia (Table 2). TLR4 and TLR2 for example have been implicated in sensitizing microglial apoptosis suggesting a role in preventing excessive inflammation. Microglia produce pro-inflammatory cytokines, enzymes, and adhesion molecules, which initiate leukocyte migration through the blood brain barrier and promote effector functions in these infiltrating cells.
Table 2. TLRs in microglial.
|TLRs in Microglia||Agonists||Outcome||Comments|
|TLR2||LPS, gram positive and negative bacteria||IL-6, IL-10||Initial microglial response to axonal injury|
|IFNβ||Decreases MHC II and CD4 T cell proliferation|
|Group B Streptococci||Causes microglial apoptosis|
|TLR3||Poly I:C||TNFα, IL-6, IL-10, IL-12, CXCL-10, IFNβ||Induces TH1 polarization and IFNy secretion of CD4 T cells|
|TLR4||LPS||TNFα, IL-6, IL-10, IL-1β and IL-7||Decreases MHC II and CD4 T cell proliferation|
|IFNβ||Increases apoptosis in microglia via IFNβ production|
|NF-kB independent/JNK-mediated GDNF gene expression||Neuroprotective role|
Human astrocytes express TLR1-7, 9, and 10 (see table 3). Drugs like statins increase astrocyte TLR4-mediated cytokine production and are linked to neuropathies. Cytokines and TLR agonists in return also increase the expression of chemokine ligands such as CCL2, CCL3, and CCL5, which go on to attract immune cells to the site of inflammation.
Table 3. TLRs in astrocytes.
|TLRs in Astrocytes||Agonists||Outcome||Comments|
|TLR2||CD14||IL-6, IL-12, IL-12p40, CXCL8|
|TLR3||Poly I:C||IL-6, TNFα, IFNα4, IFNβ, iNOS, CXCL10||These products lead to growth inhibition of astrocytes and endothelial cells and survival of neurons suggesting a neuroprotective role|
|TLR4||LPS||IL-6, TNFα, IFNα4, IFNβ, iNOS|
|LPS and dsRNA||
IL-1α, IL-1β, IL-6, TNFα, GM-CSF, LTB, TGF-B3
Oligodendrocytes are a type of neuroglia that support and insulate axons in the CNS by creating a myelin sheath. Injury to these cells can result in demyelination, and subsequently development of diseases such as multiple sclerosis. Little evidence is available on the role of TLRs in oligodendrocytes, apart from the fact that TLR2,3, and 4 are expressed in these cells.
Oligodendrocytes, in which TLR signaling has been activated by treatment with the TLR2 agonist zymosan, have been shown to play a role in the repair of damage to the CNS and inflammation-mediated remyelination (Setzu et al. 2006). This strongly suggests a neuroprotective role of TLR2. However, contradicting results imply that zymosan-mediated TLR2 activation results in complete oligodendrocyte loss and demyelination of intact myelin axons around lesions (Schonberg et al. 2007). Further research is needed to fully understand the role of TLR2.
There is increasing evidence for the importance of aberrant TLR neuronal expression in the development of pathological conditions. A distinct difference is seen in TLR activation in differentiated neurons versus neuronal progenitor cells, although both cell types express TLR2 and TLR4. TLR2 is involved in hippocampal neurogenesis, while TLR4 reduces proliferation and neuronal differentiation. Both TLRs regulate cell fate via MyD88 and NF-kB pathways, however, this is specific to progenitor cells, the mediators of differentiated cells are unknown.
Table 4. TLRs in neurons.
|TLRs in Neurons||Location||Agonists||Comments|
|TLR3||Growth cones of neurons||Poly I:C||Causes collapse of growth cones, inhibits neurite extension of cortical and hippocampal neurons, reduces proliferating cells and neurosphere formation|
|TLR4||Nociceptive neurons||LPS and CD14|
|TLR8||R848||Inhibits neurite outgrowth and induces neuronal apoptosis|
|TLR2 deficiency||Adult neuronal progenitor cells||Impairs hippocampal neurogenesis|
|TLR4 deficiency||Adult neuronal progenitor cells||Increases proliferation and neuronal differentiation|
This mini-review has emphasized the importance that TLRs play in the mediation and regulation of neuroinflammation. Although a lot of progress has been made in researching neuroinflammation, especially in a disease context, many questions remain open. These include why different neuronal cell types show different levels of damage vulnerability and identifying the molecular mechanisms underlying inflammatory neurodegeneration. Additionally it is essential to understand the control of recovery and neuronal plasticity after inflammation induced brain damage.