The innate immune response has come under the spotlight in recent years due to its central role in propagating the pathogenesis of several diseases, and specifically in driving neurodegenerative aetiology (1-3). Toll-like receptors (TLRs), the mammalian homologue of the Drosophila melanogaster Toll, are highly conserved innate immune receptors and master regulators of the cellular innate immune response (4-6). Research by several groups in the early 1990s discovered the pivotal role of TLRs in the initiation and propagation of the inflammatory signalling in response to bacterial, viral or microbial nucleic acids, known as Pathogen Associated Molecular Patterns (PAMPs) or Danger Associated Molecular Pattern (DAMPs) (5, 7, 8), primarily focusing on what is now termed TLR4. To date, 13 murine and 11 human TLRs identified (9, 10), with several intracellularly expressed on endosomes (TLR3, TLR7, TLR8 and TLR9), whereas others are characterized as transmembrane receptors (all other TLRs, including TLR2 and TLR4).
Glial Cell Activation
It is widely established that reactive gliosis, a term used to describe the activation of glial cells in response to pathogens (11), is concurrent with neuronal death in the Central Nervous System (CNS) (12-14), with microglial cells eliciting the primary response to pathogenic insult (15, 16) and driving chronic neuroinflammation (17-19). Importantly, microglia highly express TLRs (20, 21). TLR signalling, specifically increased levels of TLR2 and TLR4, is reported in multiple neurodegenerative pathologies (22, 23), with mounting evidence for misfolded protein aggregate induced activation of TLR2 and TLR4 in Alzheimer’s disease (24, 25), Amyotrophic Lateral Sclerosis (2), and Prion disease (26). In the CNS, glial cell crosstalk potentiates a TLR pro-inflammatory feedback loop, with evidence for TLR signaling in initiating the adaptive immune response (27). These studies highlight the importance of TLR-induced inflammation in the chronic neuroinflammatory cascade seen in neurodegenerative aetiology.
All TLRs signal as dimers, with stimulation of TLR2 and TLR4 leading to the recruitment of extracellular co-receptors CD14 (28) and MD2 (TLR4 only) (29), preceding homodimerization (TLR4/TLR4), or heterodimerization (TLR1/TLR2) (30) and subsequent assembly of a cytoplasmic Toll/Interleukin-1 Receptor (TIR) adaptor domain (31-33). The TIR-adaptor domain is comprised of MyD88 (Myeloid Differentiation Primary Response Gene 88), Mal (MyD88 adaptor-like), TRIF (TIR-domain-containing adapter-inducing interferon-β) and TRAM (34-36). MyD88 facilitates downstream signalling in all TLRs, with the exception of TLR3 (37). Mal (also known as TIRAP) bridges MyD88 to TLR2 and TLR4 (38), with TRAM adapting signals from TLR4 to TRIF (39).
TLR2 and TLR4 activation
Activation of both TLR2 and TLR4 leads to MyD88-dependent signal propagation, whereas TLR4 can additionally initiate a MyD88-independent, also termed TRIF-dependent, signalling pathway, ultimately culminating in the activation of transcription factors, such as NF-κB, Mitogen Activating Protein Kinase (MAPK) and Interferon Regulatory Factor 3 (IRF3), promoting inf