Shedding light on the molecular and regulatory mechanisms of TLR4 signaling in endothelial cells under physiological and inflamed conditions - PubMed (original) (raw)

Review

Shedding light on the molecular and regulatory mechanisms of TLR4 signaling in endothelial cells under physiological and inflamed conditions

Anna Stierschneider et al. Front Immunol. 2023.

Abstract

Toll-like receptor 4 (TLR4) are part of the innate immune system. They are capable of recognizing pathogen-associated molecular patterns (PAMPS) of microbes, and damage-associated molecular patterns (DAMPs) of damaged tissues. Activation of TLR4 initiates downstream signaling pathways that trigger the secretion of cytokines, type I interferons, and other pro-inflammatory mediators that are necessary for an immediate immune response. However, the systemic release of pro-inflammatory proteins is a powerful driver of acute and chronic inflammatory responses. Over the past decades, immense progress has been made in clarifying the molecular and regulatory mechanisms of TLR4 signaling in inflammation. However, the most common strategies used to study TLR4 signaling rely on genetic manipulation of the TLR4 or the treatment with agonists such as lipopolysaccharide (LPS) derived from the outer membrane of Gram-negative bacteria, which are often associated with the generation of irreversible phenotypes in the target cells or unintended cytotoxicity and signaling crosstalk due to off-target or pleiotropic effects. Here, optogenetics offers an alternative strategy to control and monitor cellular signaling in an unprecedented spatiotemporally precise, dose-dependent, and non-invasive manner. This review provides an overview of the structure, function and signaling pathways of the TLR4 and its fundamental role in endothelial cells under physiological and inflammatory conditions, as well as the advances in TLR4 modulation strategies.

Keywords: endothelium; lipopolysaccharide; optogenetic control; pro-inflammatory response; toll-like receptor 4 signaling.

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Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

Figure 1

Conserved structure of TLRs. TLRs are characterized by an extracellular domain with leucine-rich repeats responsible for the recognition of PAMPs and DAMPs, a transmembrane domain, and an intracytoplasmic Toll/IL-1 receptor (TIR) domain necessary for signal transduction. TLR, toll-like receptor; LRR, leucine-rich repeats; PAMP, pathogen-associated molecular pattern; DAMP, damage-associated molecular pattern; TIR domain, Toll/interleukin-1 (IL-1) receptor domain. Created with

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Figure 2

Schematic representation of the TLR signaling pathways. Depending on the type of PAMP and its respective binding affinity to different extracellular domains of the TLRs, TLR signaling is initiated by the dimerization of the extracellular domains of the receptors after recruitment of the adaptor proteins MyD88 and TIRAP (MyD88-dependent pathway) or TRIF and TRAM (TRIF-dependent pathway) through the interactions with the TIR domains of the TLRs. TLR, toll-like receptor; LPS, lipopolysaccharide; dsRNA, double-stranded ribonucleic acid; ssRNA, single-stranded ribonucleic acid; CpG DNA, 5’-cytosine-phosphate-guanine-3’ deoxyribonucleic acid; MyD88, myeloid differentiation factor 88; TRIAP, TIR domain-containing adaptor protein; TRIF, TIR domain-containing adaptor inducing interferon beta; TRAM, TRIF-related adaptor protein; IRAK, interleukin 1 receptor-associated kinase; TRAF, tumor necrosis factor receptor-associated factor; TAK-1, transforming growth factor beta-activated kinase 1; TAB, TAK-1 binding protein; MKK, mitogen-activated protein kinase kinase; JNK, Jun N-terminal kinase; ERK1/2, extracellular signal−regulated protein kinase 1/2; CREB, cyclic adenosine monophosphate-responsive element-binding protein; AP-1, activator protein 1; NF-κB, nuclear factor kappa B; NEMO, NF-κB essential modulator; IKK, IκB kinase; IKKi, inhibitor of NF-κB kinase; RIP1, receptor-interacting protein 1; TBK, TANK-binding kinase; IFN, interferon; IRF, interferon regulatory factor. Created with

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Figure 3

Pathogens and their ligand targets for different human TLRs. Cell surface TLRs primarily recognize components of microbial membranes such as lipoteichoic acid (TLR2), lipopeptides (TLR1, 2, 6) and lipopolysaccharide (TLR4, 10) as well as flagellin (TLR5, 10). Additionally, TLR2 detects zymosan glycan of fungi and, like TLR4, protist GPI anchors. In contrast, cytosolic TLR3, TLR7, TLR8, and TLR9 mainly recognize microbial nucleic acids. TLR, toll-like receptor; DNA, deoxyribonucleic acid; RNA, ribonucleic acid; LTA, lipoteichoic acid. LP, lipopeptides. LPS, lipopolysaccharide. GPI, glycosylphosphatidylinositol. Adapted from “Pathogen Ligand Targets for Different TLRs”, by

BioRender.com

(2023).

Figure 4

Schematic representation of the general architecture of blood vessels. ECs line the innermost wall of blood vessels, separating the circulating blood from the surrounding tissue. Adjacent smooth muscle cells are trapped between the internal and the external elastic membranes. Created with

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Figure 5

Simplified schematic representation of leukocyte extravasation. PRR, particularly TLR4-induced chemokine secretion initiates the expression of E- and P-selectins on endothelial surfaces, which have a high affinity for sialyl Lewis X glycan epitopes expressed on leukocytes. These selectin-glycan interactions facilitate leukocyte tethering, rolling, and final endothelial transmigration to the site of inflammation. Leukocytes can migrate from the luminal to the abluminal side of the vascular barrier either by the vesicle-based transcellular route through the EC body or by the junctional paracellular route between adjacent ECs. Lymphocytes express L-selectins on their surface and interact with sialyl Lewis X glycan epitopes expressed on ECs to facilitate diapedesis into the subendothelial compartment. Adapted from “Leukocyte Extravasation - The Role of Glycans in Inflammation”, by

BioRender.com

(2023).

Figure 6

Schematic representation of the coagulation pathways and fibrinolytic pathways. (A) Tissue factor (initiation of the extrinsic pathway) and contact activation (initiation of the intrinsic pathway) lead to a common pathway that generates thrombin. Created with

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. (B) During fibrinolysis, t-PA and u-PA convert plasminogen to plasmin, which degrades the fibrin network. Adapted from “Process of Blood Clot Formation”, by

BioRender.com

(2023).

Figure 7

Schematic representation of anti-coagulant mediators expressed on ECs. Under physiological conditions, the endothelium provides an anti-coagulant/anti-thrombotic environment to prevent thrombus formation by free circulating platelets and red blood cells within the blood vessel. NO, PGI2, and ADPase prevent platelet adhesion and aggregation. HS cooperates with AT to interfere with thrombin, factors IXa, Xa, XIa, and XIIa. TFPI binds to thrombin, which associates with EPCR, and subsequently released APC interacts with PS to block factors Va and VIIIa. EC, endothelial cell; NO, nitric oxide; PGI2, prostacyclin; ADPase, adenosine diphosphatase; AT, antithrombin; HS, heparan sulfate proteoglycan; TM, thrombomodulin; EPCR, endothelial cell protein C receptor; PC, protein C; APC, activated protein C; PS, protein S. Created with

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Figure 8

The propagation of immunothrombosis by leukocytes and platelets. During inflammation, PRRs, particularly TLR4, initiate the exocytosis of WPB from ECs and subsequently the expression of P-selectins and vWFs. Leukocytes expressing PSGL-1 are recruited upon interaction with endothelial P-selectin and platelets adhere upon the interaction of endothelial vWF and platelet-derived GPIbα. Neutrophils release NETs that trap pathogens, facilitate thrombus formation, and activate platelets. The activated platelets, recruit leukocytes and recognize pathogens. Active TFs on the surface of monocytes and microvesicles further enhance thrombus propagation by inducing fibrin formation and trapping RBCs. The resulting thrombus promotes pathogen capture. PRR, pathogen recognition receptor; WPB, Weibel-Palade bodies; vWF, von Willebrand factor; PSGL-1, P-selectin glycoprotein ligand-1; NET, neutrophil extracellular trap; TF, tissue factor; RBC, red blood cell. Adapted from “Propagation of Immunothrombosis by Leukocytes and Platelets”, by

BioRender.com

(2023).

Figure 9

Endothelial permeability. Endothelial integrity and permeability are determined by intracellular junctions, including gap junctions (connexin 32, 37, 40, and 43), adherens junctions (VE-cadherin, nectin), tight junctions (claudin, occludin, JAM A, B, and C), and PECAM-1 to regulate the extravasation of water, plasma proteins, electrolytes, nutrients, metabolic waste products, and immune cells. Created with

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Figure 10

TLR4 modulation strategies. Common approaches used to study TLR4 signaling rely on genetic manipulation through gain- or loss-of-function mutations of the TLR4 or downstream signaling molecules, treatment with its naturally occurring ligand LPS, or optogenetic manipulation. Created with

BioRender.com

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Shedding light on the molecular and regulatory mechanisms of TLR4 signaling in endothelial cells under physiological and inflamed conditions - PubMed (original) (raw)