Human MyD88-KO Dual Reporter THP-1 Cells

MyD88

The Myeloid Differentiation Primary Response Gene 88 (MyD88) is the canonical adaptor for inflammatory signaling pathways downstream of members of the Toll-like receptor (TLR) and interleukin-1 receptor (IL-1R) families [1]. Based on the adaptor proteins involved, the TLR/IL-1Rs pathways can be classified into MyD88-dependent and MyD88-independent (TRIF-dependent) responses. With the exception of TLR3 and TLR4, all TLRs mediate the downstream signaling pathway via MyD88 [2-3].

The MYD88 protein contains three key domains: a N-terminal death domain (DD), a short intermediate domain (INT), and a C-terminal Toll-IL1R domain (TIR). In the resting state, inactive MyD88 appears to be dispersed in the cytosol [3]. Stimulation of TLRs and IL1R leads to the direct recruitment of cytosolic MyD88. In the case of TLR2 and TLR4 signaling, a bridging adaptor, TIRAP (aka Mal), is required for MyD88 recruitment. Upon receptor-adaptor binding through TIR-TIR interactions, an oligomeric protein complex called 'the Myddosome' is assembled [3].

The Myddosome

Within this complex, activated MyD88 recruits IL-1R associated kinase 4 (IRAK4), a serine-threonine kinase, via their DD domains. Together they phosphorylate IRAK1 or IRAK2. Phosphorylated IRAK1 and IRAK2 interact with tumor necrosis factor receptor-associated factor 6 (TRAF6), resulting in activation of transforming growth factor beta-activated kinase 1 (TAK1). TAK1 subsequently activates the inhibitor of the NF-κB kinase (IKK) complex. After activation, this complex phosphorylates the inhibitor of NF-κB (IκB) proteins that are bound to NF-κB, which prevent the translocation of NF-κB to the nucleus. Phosphorylation of these IκB proteins results in ubiquitylation and proteasomal degradation of IκB and release of the NF-κB subunits. Subsequently, NF-κB migrates to the nucleus where it bind to specific DNA-binding sites and induces gene expression [4].

Pathology

Based on its biological role, MyD88 has been in the spotlight as a therapeutic target for different inflammatory disorders. Gain-of-function mutations or overexpression of MyD88 persistently activate NF-κB, resulting in inflammatory response syndrome, cancer, and autoimmune diseases [3]. Sequencing studies recently identified mutations in the MYD88 gene as an important oncogenic driver and biomarker in B-cell lymphomas [4]. Indeed, MyD88 deficiency and loss-of-function has anti-inflammatory effects in several conditions such as arthritis, artherosclerosis, and autoimmune CNS diseases. Also, it alters MyD88-independent pathways [3]. Therefore, understanding the diverse functions of this unique adaptor protein and finding inhibitors targeting MyD88 will offer therapeutic opportunities for numerous indications [3]. 

References:

1. Deguine J, Barton GM. 2014. MyD88: a central player in innate immune signaling. F1000Prime Rep.;6:97.
2. Chao Zheng et al., 2020. Inflammatory Role of TLR-MyD88 Signaling in Multiple Sclerosis. Front. Mol. Neurosci. 12:314.
3. Chen L, et al., 2020. Myeloid Differentiation Primary Response Protein 88 (MyD88): The Central Hub of TLR/IL-1R Signaling. J Med Chem. ;63(22):13316-13329.
4. de Groen RAL, et al., 2019. MYD88 in the driver's seat of B-cell lymphomagenesis: from molecular mechanisms to clinical implications. Haematologica. 2019 Dec;104(12):2337-2348.