The immune system plays a crucial role in defending the body against invading pathogens. Toll-like receptors (TLRs) are a class of pattern recognition receptors (PRRs) that recognize conserved molecular patterns shared by a wide range of pathogens, known as pathogen-associated molecular patterns (PAMPs). Activation of TLRs initiates a cascade of intracellular signaling events leading to the production of pro-inflammatory cytokines, which orchestrate the immune response against the invading pathogens. This post will delve into the intricate signaling pathways through which PAMPs activate TLRs, leading to the production of pro-inflammatory cytokines.
Structure and Function of Toll-Like Receptors
TLRs are a family of transmembrane receptors expressed on various immune cells, including macrophages, dendritic cells, and B cells. They play a pivotal role in innate immunity by recognizing structurally conserved molecules derived from pathogens. It means they have an essential role as part of the host’s defence to microbial infection.
The TLRs are germline-encoded meaning they are inherited via sex cells – the sperm and ova. They are also pattern recognition receptors and found in all microorganisms. They function using pattern recognition.
Their function is to activate intracellular signalling pathways when they bind PAMPs. Their activation then induces those genes involved in both immune responses and inflammation.
TLRs are characterized by an extracellular domain containing leucine-rich repeats (LRRs), which are involved in PAMP recognition, and a cytoplasmic Toll/IL-1 receptor (TIR) domain responsible for initiating downstream signaling pathways upon ligand binding.
They are seen as a bridge between both Innate and Adaptive immunity because they mediate the maturation of dendritic cells nd activate pathogen-specific T lymphocytes.
Activation of Toll-Like Receptors by PAMPs
The activation of TLRs begins with the recognition of PAMPs, such as bacterial lipopolysaccharide (LPS), lipoproteins, viral nucleic acids, and fungal cell wall components, by the LRR domain of TLRs. Each TLR recognizes specific PAMPs, leading to receptor dimerization and activation.
Upon ligand binding, TLRs undergo conformational changes that facilitate the recruitment of adaptor proteins to their TIR domains, initiating downstream signaling cascades. The two main adaptor proteins involved in TLR signaling are MyD88 (myeloid differentiation primary response 88) and TRIF (TIR domain-containing adaptor protein inducing interferon-β). These adaptors serve as molecular bridges between activated TLRs and downstream signaling molecules.
MyD88-Dependent Signaling Pathway
The MyD88-dependent pathway is utilized by all TLRs except TLR3. Upon ligand binding, TLRs recruit MyD88 to their TIR domains, leading to the formation of a signaling complex known as the Myddosome. The Myddosome comprises MyD88, interleukin-1 receptor-associated kinases (IRAKs), and tumor necrosis factor receptor-associated factor 6 (TRAF6).
Within the Myddosome, IRAKs are phosphorylated and activated, leading to the subsequent activation of TRAF6. TRAF6 serves as a ubiquitin ligase, catalyzing the polyubiquitination of target proteins. Polyubiquitinated proteins, such as TGF-β-activated kinase 1 (TAK1), are crucial for the activation of downstream signaling pathways.
Activated TAK1 phosphorylates and activates two distinct kinase complexes: the IKK (IκB kinase) complex and the MAPK (mitogen-activated protein kinase) cascade. The IKK complex phosphorylates IκB (inhibitor of κB), leading to its degradation via the proteasome. This allows NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) to translocate into the nucleus and initiate the transcription of pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1β), and interleukin-6 (IL-6).
Simultaneously, the MAPK cascade, comprising three major kinases: ERK (extracellular signal-regulated kinase), JNK (c-Jun N-terminal kinase), and p38 MAPK, is activated. These kinases phosphorylate various transcription factors, such as AP-1 (activator protein 1), leading to the transcriptional activation of genes encoding pro-inflammatory cytokines.
TRIF-Dependent Signaling Pathway
TLR3 and TLR4 utilize the TRIF-dependent pathway in addition to the MyD88-dependent pathway. Upon ligand binding, TLR3 and TLR4 recruit TRIF to their TIR domains. TRIF initiates signaling by interacting with TRAF3, leading to the activation of downstream kinases, including TBK1 (TANK-binding kinase 1) and IKKε (IκB kinase epsilon).
TBK1 and IKKε phosphorylate the transcription factor IRF3 (interferon regulatory factor 3), promoting its dimerization and nuclear translocation. In the nucleus, IRF3 induces the transcription of type I interferons (IFNs), such as interferon-beta (IFN-β), which contribute to antiviral responses and further enhance the production of pro-inflammatory cytokines.
Moreover, TRIF activates NF-κB indirectly through the activation of TBK1, which phosphorylates the NF-κB essential modulator (NEMO). Phosphorylated NEMO then activates the canonical NF-κB pathway, leading to the transcription of pro-inflammatory cytokines.
Negative Regulation of TLR Signaling
To prevent excessive inflammation and maintain immune homeostasis, TLR signaling is tightly regulated by various mechanisms. Negative regulators, such as IRAK-M (IRAK inhibitor), SOCS1 (suppressor of cytokine signaling 1), and A20 (TNF-α-induced protein 3), inhibit TLR signaling at multiple levels. These regulators interfere with the activation of signaling molecules or promote their degradation, thereby attenuating the inflammatory response.
Furthermore, endosomal TLRs, such as TLR3, TLR7, TLR8, and TLR9, are sequestered within intracellular compartments to prevent aberrant activation by self-nucleic acids. Endosomal TLR signaling is also regulated by compartment-specific adaptor proteins, such as TRAM (TRIF-related adaptor molecule) and TIRAP (TIR domain-containing adaptor protein), which ensure proper signaling initiation.
The Toll-like receptors play a pivotal role in innate immunity by sensing PAMPs and initiating pro-inflammatory responses to combat invading pathogens. The signaling pathways triggered by TLR activation involve complex molecular interactions orchestrated by adaptor proteins and downstream kinases, ultimately leading to the production of pro-inflammatory cytokines. Understanding these signaling pathways is essential for elucidating the mechanisms underlying immune responses and developing targeted therapies for infectious and inflammatory diseases.
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