Bruton’s tyrosine kinase (Btk)—the critical tyrosine kinase in LPS signalling?

Abstract

The discovery of the Toll-like receptors (TLRs) has revolutionised the field of innate immunity. One unresolved question regarding LPS signalling is whether there is a role for tyrosine kinases downstream of the LPS receptor. Studies in mice deficient in Bruton’s tyrosine kinase have previously shown that they are defective in their responses to LPS. Further investigation into the role of Btk in LPS signalling has directly implicated Btk downstream of TLR4, both with respect to p38 MAPK activation and activation of the transcription factor NFκB. In fact Btk is activated by LPS and has been shown to directly bind TLR4 and the key proximal signalling proteins involved in LPS-induced NFκB activation, MyD88, Mal and IRAK-1. These recent findings point to a direct role for Btk in LPS signal transduction and raise interesting questions regarding the mode of activation of Btk following LPS stimulation and the precise nature of the pathways activated downstream of Btk. A better understanding of how Btk functions in LPS signalling will have important implications for inflammatory and autoimmune disorders and therapies thereof.

Introduction

The innate immune response has evolved to recognise structures that are characteristic of microbial pathogens and does so by means of germline encoded pathogen recognition receptors. Recognition of microbial products by receptors on effector cells such as neutrophils, monocytes and macrophages, results in the induction of pro-inflammatory cytokines and activation of the inflammatory response. Toll-like receptors (TLR) are a family of pathogen recognition receptors that discriminate between diverse microbial signatures. To date, genes encoding 10 TLRs have been found in the human genome, each recognising distinct microbial ligands (Fig. 1a). They are characterised as having leucine-rich repeats extracellularly and signal via a conserved intracellular domain that they share with the interleukin-1 (IL-1) receptor family—the Toll/IL-1 receptor (TIR) domain. Of the 10 human TLR members, ligands for all but TLR10 have been described, with TLR2 recognising peptidoglycan, TLR3 recognising double stranded viral RNA and TLR9 recognising bacterial hypomethylated DNA motifs, for example (reviewed in [1]). The discovery that TLR4 was the long sought after signalling component of the lipopolysaccharide (LPS) receptor complex (composed of CD14 and MD2 on myeloid cells) has resulted in intense research into how signalling pathways are regulated downstream of TLR4.

A key consequence of LPS signalling in cells is the induction of pro-inflammatory cytokines such as TNFα and IL-1, via activation of the transcription factor NFκB. The pathway to NFκB activation in response to LPS has been characterised in molecular detail, resulting in the discovery of a novel family of TIR domain-containing adapter proteins, which serve to regulate and fine tune TLR responses [2]. The first identified member of this adapter family was MyD88 [3]. MyD88 has an N terminal death domain and a C terminal TIR domain. In brief, many of the TLRs recruit MyD88 to their intracellular domains once activated. The death domain of MyD88 subsequently recruits the death-domain-containing serine/threonine kinases, IRAK-1 and IRAK-4. These function to activate the downstream adapter protein TRAF-6, which links TIR-domain-containing receptors to the IκB kinase (IKK) complex regulating NFκB activation (reviewed in [4], [5]). The importance of MyD88 in TLR signalling has been demonstrated by the inability of MyD88-deficient mice to respond appropriately to a variety of TLR ligands, namely LPS, peptidoglycan and bacterial CpG motifs [6], [7].

Interestingly, analysis of MyD88-deficient cells demonstrates that MyD88-independent pathway exists regulating late NFκB activation and the induction of IRF-3-dependent genes in response to LPS. This observation has given rise to the discovery and characterisation of two additional TIR domain-containing adapters, Mal (MyD88-adapter like) and Trif (TIR-containing adaptor-inducing IFN-β). Mal appears to be specific for TLR2 and TLR4 signalling and was initially thought to lie on the MyD88-independent pathway [8], [9]. However, Mal knockout mice have demonstrated that, like MyD88, Mal regulates MyD88-dependent signals such as those leading to NFκB activation and pro-inflammatory cytokine production, possibly via its ability to interact with MyD88 [10], [11]. The third adapter identified, Trif (also known as TICAM-1), interacts with TLR3 and mediates TLR3-dependent induction of IFN-β via NFκB and IRF-3 activation [12], [13]. Trif is also involved in the MyD88-independent pathway activated by TLR4. Meanwhile, a fourth adapter has recently been characterised, TRAM (TRIF-related adaptor molecule), which, unlike any of the other adaptors, is specific for TLR4 signalling and regulates the MyD88-independent pathway to NFκB and IRF-3 activation [14]. It also regulates late NFκB activation in response to TLR4 (summarised in Fig. 1b) [15]. Thus it appears that TLRs use different combinations of adaptor proteins in order to mount an appropriate response to specific microbial pathogens. These recent discoveries have important implications for other downstream events that occur following LPS stimulation, particularly tyrosine kinase activation and it is this aspect of LPS signalling which will form the focus of this review.