Human TBK1: A Gatekeeper of Neuroinflammation

Abstract

The importance of TANK binding kinase-1 (TBK1), a multimeric kinase that modulates inflammation and autophagy, in human health has been highlighted for the first time by the recent discoveries of mutations in TBK1 that underlie amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), normal tension glaucoma (NTG) or childhood herpes simplex encephalitis (HSE). Gain-of-function of TBK1 are associated with NTG, whereas loss-of-function mutations result in ALS/FTD or in HSE. In light of these new findings, we review the role of TBK1 in these seemingly unrelated, yet allelic diseases, and discuss the role of TBK1 in neuroinflammatory diseases. This discovery has the potential to significantly increase our understanding of the molecular basis of these poorly understood diseases.

Keywords: TBK1; amyotrophic lateral sclerosis; autophagy; dementia; glaucoma; herpes simplex encephalitis; interferon.

Figure 1. Disease-causing Mutations in Human TBK1

TBK1 is an 82 kDa, 729 amino-acid protein composed of a kinase domain, an ubiquitin-like domain (ULD), and coiled-coiled domains 1 and 2 (CCD1; CCD2). The kinase domain is critical for the phosphorylation of various substrates such as IRF3 [15], whereas the ULD domain regulates kinase activation and interactions with other proteins in the TBK1 pathway [112]. The CCD1 domain harbors leucine zipper (LZ) and helix-loop-helix (HLH) domains which specifically control dimerization. The C-terminus CCD2 harbors an adaptor-binding motif facilitating the interaction of TBK1 with its adaptors TANK, NAK–associated protein (NAP1), and Similar to NAP1 TBK1 Adaptor (SINTBAD) [89]. Germline human TBK1 mutations have been reported to be disease-causing in (a) normal tension glaucoma (NTG), (b) herpes simplex encephalitis (HSE), (c) amyotrophic lateral sclerosis-frontotemporal dementia ALS-FTD, (d) FTD and (e) ALS : these mutations are shown with respect to their amino acid positions within the TBK1 protein. The black horizontal box in (a) indicates duplications in kb reported to include TBK1. Open circles represent LoF variants; filled circles represent missense variants. (See Table 1).

Figure 2. TBK1 Molecular Pathways

TBK1 and IKKɛ function as the non-canonical IkB kinases downstream of TLRs, RLRs, DDX3X, and DNA receptors leading to the activation of the transcription factors NF-kB (p65/p50) and IRFs (IRF3), resulting in the production of proinflammatory cytokines and antiviral IFNs. TLR3 recognizes dsRNA initiating the recruitment of adaptors such as TRIF and TRAF3 (TNF receptor-associated factor 3), which then activate TBK1 found in complex with its interacting proteins NAP1 (NF-kB-activating kinase-associated protein-1), SINTBAD (similar to NAP1 TBK1 adaptor) and TANK. LPS recognition by TLR4 can also recruit TRIF and subsequently TRAF3, mediating TBK1 activation. Activated TBK1 can then phosphorylate IRF3, leading to its homodimerization and subsequent translocation into the nucleus where it induces the production of IFNs. Cytosolic RLRs and DDX3X, as well as DNA sensor cGAS signal via TBK1 following recognition of their ligands viral 5′-ppp RNA and DNA, respectively. RLRs typically signal via the adaptor MAVS (mitochondrial antiviral-signaling protein; also known as IPS-1, CARDIF or VISA), which activates TBK1. cGAS detects dsDNA and stimulates STING (stimulator of interferon genes) to bind and activate TBK1 directly. TBK1 is also involved in autophagy where it directly phosphorylates the autophagy receptors optineurin and p62, which target cargo to the autophagosome. Ubiquilin-2 can also target ubiquitinated cargo to autophagosomes [113]. Target cargo may be comprising pathogen or ubiquitinated protein aggregates. Proteins whose genes have been reported to predispose to diseases are indicated in red, HSE; green, ALS or ALS-FTD; blue, NTG. Yellow denotes TBK1, where all pathways converge.

Figure 3. Dynamic Interplay between Cells in CNS in ALS, FTD, HSE and NTG

In ALS/FTD, motor neurons accumulate toxic protein aggregates (e.g.: TDP-43 inclusions) which contribute to neurodegeneration. In addition, other cells are known to mediate neuroinflammation, leading to cell death. Activated microglia, infiltrating monocytes and T cells produce inflammatory cytokines; astrocytes downregulate their supportive function, contributing to neurodegeneration [65]. In HSE, studies using iPSCs-derived neurons from a TLR3-deficient patient demonstrated that TLR3-dependent cell-intrinsic immunity was critical in preserving the integrity of neurons and oligodendrocytes during a primary HSV1 infection [54]. In NTG, progressive degeneration of retinol ganglion cells occurs and is poorly understood [114].