Toll-like receptors in ischaemia and its potential role in the pathophysiology of muscle damage in critical limb ischaemia

Abstract

Toll-like receptors (TLRs) are key receptors of the innate immune system which are expressed on immune and nonimmune cells. They are activated by both pathogen-associated molecular patterns and endogenous ligands. Activation of TLRs culminates in the release of proinflammatory cytokines, chemokines, and apoptosis. Ischaemia and ischaemia/reperfusion (I/R) injury are associated with significant inflammation and tissue damage. There is emerging evidence to suggest that TLRs are involved in mediating ischaemia-induced damage in several organs. Critical limb ischaemia (CLI) is the most severe form of peripheral arterial disease (PAD) and is associated with skeletal muscle damage and tissue loss; however its pathophysiology is poorly understood. This paper will underline the evidence implicating TLRs in the pathophysiology of cerebral, renal, hepatic, myocardial, and skeletal muscle ischaemia and I/R injury and discuss preliminary data that alludes to the potential role of TLRs in the pathophysiology of skeletal muscle damage in CLI.

Figure 1

TLR signalling pathway. MyD88-dependent signalling pathway is used by all TLRs except TLR 3. Signalling through the MyD88-dependent pathway leads to the activation of MAPKK and IKK complex resulting in activation and nuclear translocation of AP-1 and NF-κB, respectively. TLR 4 is capable of signalling through the MyD88-independent pathway as well; however this is the sole signalling mechanism for TLR 3. TRIF is the main adaptor protein in the MyD88-independent pathway and can associate with TRAF6 to activate AP-1 and NF-κB. Alternatively it also activates NF-κB by interacting with RIP-1. TRIF can further interact with TRAF3 and the phosphatidylinositol 3-kinase (PI3K)-AKT pathway resulting in the nuclear translocation of IRF3 and IRF2, respectively.

Figure 2

Representative western blots showing (a) increased TLR 2, TLR 4, and TLR 6 protein expression in gastrocnemius muscle biopsies obtained from patients with CLI compared to controls. (b) Densitometric quantification of TLR 2, 4, and 6 levels in CLI muscle. *P < 0.05 compared to control. The human experiments were conducted in accordance with the Declaration of Helsinki (1964), and informed consent was obtained from patients with prior approval from the local ethics committee.

Figure 3

Proposed pathophysiological mechanism of skeletal muscle damage in CLI. Skeletal muscle ischaemia initiates muscle cell apoptosis and necrosis leading to the release of endogenous ligands such as HMGB-1. Subsequently TLRs are activated in other viable muscle cells causing signalling through one or more TLR signalling pathways. This may lead to the activation of transcription factors such as NF-κB, AP-1, IRF 3, and 2. The consequent activation of transcription factors leads to the induction and release of proinflammatory cytokines, chemokines, and interferons that propagate the skeletal muscle damage.