TRIM21-From Intracellular Immunity to Therapy

Abstract

Tripartite motif containing-21 (TRIM21) is a cytosolic ubiquitin ligase and antibody receptor that provides a last line of defense against invading viruses. It does so by acting as a sensor that intercepts antibody-coated viruses that have evaded extracellular neutralization and breached the cell membrane. Upon engagement of the Fc of antibodies bound to viruses, TRIM21 triggers a coordinated effector and signaling response that prevents viral replication while at the same time inducing an anti-viral cellular state. This dual effector function is tightly regulated by auto-ubiquitination and phosphorylation. Therapeutically, TRIM21 has been shown to be detrimental in adenovirus based gene therapy, while it may be favorably utilized to prevent tau aggregation in neurodegenerative disorders. In addition, TRIM21 may synergize with the complement system to block viral replication as well as transgene expression. TRIM21 can also be utilized as a research tool to deplete specific proteins in cells and zebrafish embryos. Here, we review our current biological understanding of TRIM21 in light of its versatile functions.

Keywords: TRIM21; antibody; gene therapy; infection; virus.

Figure 1

TRIM21 in disease and therapy. (A) The dual effector and signaling responses mediated by TRIM21 during Ad5 infection. (1) Ad5 coated with antibody enters the cytosol via CAR and αvβ3/5 integrin, (2) and is intercepted by TRIM21 that mediates proteasomal degradation and induces innate signaling. (B) TRIM21 mediated block to gene therapy. (1) An Ad5 based gene therapy vector enters the cytosol via CAR and αvβ3/5 integrin upon which (2) TRIM21 targets the vector for proteasomal degradation and induces innate signaling. (3) This hinders nuclear delivery and transgene expression. (C) (1) Tau protein bound by antibodies enters the cytosol via endocytosis and is targeted for destruction by TRIM21 (2). (3) This prevents formation of large intracellular tau aggregates. (D) TRIM-Away; (1) Antibodies are microinjected or electroporated into cells where they bind their antigen, (2) TRIM21 is recruited and directs the targeted protein to proteasomal degradation. The figure was made in BioRender™.

Figure 2

The TRIM21-IgG interaction. (A) Illustration showing binding of dimeric full-length TRIM21 to antibody (blue). The RING (pink), B-Box (red), Coiled-coil (orange), and PRYSPRY (yellow) domains of TRIM21 are shown. (B) Structural illustration of the interaction between the globular human PRYSPRY domain (orange) of TRIM21 and human IgG1 Fc (blue) (25). Insertion of Fc loop 429–436 into the PRYSPRY binding pocked is highlighted with a square. (C) Close-up view showing the insertion of IgG1 Fc loop 429–436 into the hydrophobic binding pocket on TRIM21 PRYSPRY. A was made in BioRender™ while B,C were made in PyMOL using PBD ID: 2IWG crystallographic data (25).

Figure 3

Mechanism of TRIM21 mediated anti-viral function. (1) Ad5 engages CAR and αvβ3/5 integrin at the cell surface. This triggers endocytosis of the Ad5:antibody complex and loss of fiber from the Ad5 capsid. (2) Fiber loss exposes protein VI that lyse the endosomal membrane which allows the virus:antibody complex to escape to the cytosol. (3) TRIM21 binds to the Fc part of the antibody in which auto-inhibition is released by B-box phosphorylation and undergoes auto-ubiquitination by the E2 enzymes Ube2W and Ube2N/Ube2V2. (4) This directs the Ad5:antibody complex to VCP and the proteasome for degradation. (5) Liberation of K63-linked ubiquitin chains by Poh1 activates IKKα-IKKβ-NEMO and TAK-TAB1-TAB2 which in turn induces NF-κB, AP-1 and IRFs resulting in the production of pro-inflammatory cytokines and an anti-viral state. (6) Exposed viral genomes trigger a second wave of immune signaling via the cytosolic DNA sensor cGAS. The figure was made using BioRender™.

Figure 4

TRIM21 synergizes with the complement system. (A1) Ad5 with deposited C3 enter cells via CAR and αvβ3/5 mediated endocytosis, upon which (A2) Ad5:C3 complexes escape into the cytosol where C3 is detected. This may occur synergistically with TRIM21 engagement (B1, B2, A2). In both cases, Ad5 is directed to proteasomal degradation (A3) followed by ubiquitin or MAVS mediated induction of NF-κB, AP-1, and IRFs (A4). (C1) Antibody coated Ad5 bound by C1 may result in C4b deposition on Ad5 prior to endocytosis via CAR and αvβ3/5 integrin. (C2) C4b prevents exposure of the Ad5 membrane lytic protein VI and blocks endosomal escape. Instead, Ad5 is routed into lysosomes where it is degraded (C3). The figure was made using BioRender™.