Early studies first demonstrated that TAPE is an activator of the NF-κB pathway (Matsuda et al. 2003; Zhao et al. 2010). Proteomics research indicates that TBK1 and TAPE are present in a signaling complex containing the nuclear hormone receptor co-activator SRC-2 (Jung et al. 2005). Subsequent biochemical analyses further confirmed that TAPE interacts with TBK1 and IKKε via its N-terminal region in mammalian cells (Chang et al. 2011). Ectopic expression of TAPE in mammalian cells leads to the activation of the IFN-β, NF-κB and Erk pathways, and the decrease of viral replication (Chang et al. 2011), implying a key role for TAPE in antiviral immunity. As a TBK1-interacting protein and its endosomal location, TAPE was then demonstrated to be involved in the endosomal TLR3 and TLR4 pathways but not other surface TLR pathways (Chang et al. 2011). TAPE is able to interact with Trif and potently enhance Trif-mediated IFN-β activation. Of interest, silencing of TAPE by siRNA impairs TLR3-mediated IFN-β activation but not Trif-mediated IFN-β activation, suggesting that TAPE functions upstream of Trif in the TLR3 pathway (Fig. 5.2). Likewise, silencing of TAPE blocks the TLR4–Trif pathway to IFN-β activation. Conversely, TAPE knockdown has no obvious blocking effect on MyD88-mediated or surface TLR-mediated NF-κB activation. These findings link the TAPE endosomal location to its regulatory role in the TLR3 and TLR4 pathways. Currently, the mechanistic mechanism by which TAPE links TLR3 and TLR4 to Trif-mediated downstream pathways has yet to be explored. Further research to confirm the in vivo function of TAPE in the TLR3 and TLR4 pathways is warranted.

Since TBK1 is a key protein kinase implicated in multiple PRR signaling mentioned above, it is tempting to speculate that TAPE might participate in other PRR pathways in addition to TLR3 and TLR4. A recent study provides biochemical and genetic evidence to reveal a functional role for TAPE in the RIG-I and MDA5 pathways (Chen et al. 2012). TAPE forms a complex with MAVS and RIG-I or MDA5 in mammalian cells. Silencing of TAPE impairs RIG-I and MDA5-mediated IFN-β activation but not MAVS-mediated IFN-β activation, suggesting that TAPE functions upstream of MAVS during RLR signaling. This kind of regulation was also confirmed by the genetic knockout approach. TAPE-deficient mouse embryonic fibroblasts and macrophages are impaired in type I IFN and inflammatory cytokine production upon RLR ligand stimulation. Furthermore, TAPE deficiency or knockdown diminishes cytokine production and antiviral immunity against RNA virus infection, including influenza A virus and vesicular stomatitis virus. These data reveal a vital role for TAPE in linking the RLR pathways to antiviral immunity. Further investigation using genetic knockout mice is essential to validate the in vivo role of TAPE in this kind of regulation. Studies from independent groups show that disruption of TAPE leads to a neonatal lethality in newborn mice (Chen et al. 2012; Zhao et al. 2011; Al-Tawashi et al. 2012). The conditional knockout approach will be an alternative to study the in vivo role of TAPE. Another key issue is the underlying mechanism of how an endolysosomal adaptor such as TAPE might participate in cytosolic RLR signaling. Two hypothetical models are proposed to explain this role of TAPE in RLR signaling (Fig. 5.3). One possibility is that upon signaling transduction, TAPE translocate onto mitochondria to couple RLRs to MAVS. The other possibility is that activated RLRs first engage with TAPE at endolysosomes and then form a complex with MAVS on mitochondria. Further research on this regulation is key to understanding the spatial and temporal control of RLR signaling.

Identification of novel PRRs and regulators in the innate immune system has advanced our understanding of the activation of innate immunity over the past decade. In addition to plasma membrane, subcellular compartments such as endolysosomes, peroxisomes, and mitochondria have also emerged as signaling platforms for relaying innate immune signals. It is, however, less clear how PRRs and innate immune regulators interact with these subcellular compartments to enact their roles in innate immune sensing and signaling during pathogen infection. Pathogens frequently invade host cells to utilize different subcellular compartments or organelles to facilitate their propagation. Thus, it is also important to understand how pathogens develop strategies to target innate immune regulators or subcellular compartments to intercept innate immune signaling. TAPE is an endolysosomal adaptor recently shown to involve in the endosomal TLR (TLR3 and TLR4) and cytosolic RLR pathways. Several features regarding this TAPE-mediated innate immune regulation are of interest. First, it is noted that TAPE acts upstream of Trif/Ticam-1 or MAVS to regulate TLRs or RLRs, respectively. To the best of our knowledge, TAPE is the first regulator implicated in both the endosomal TLR and cytosolic RLR pathways at such an early step. The second feature is that TAPE interacts with the different innate immune sensors and regulators mentioned above (Chang et al. 2011; Chen et al. 2012) and other host and viral factors (our unpublished results). These proteins share very little similarity in structural features. Thus, it is unlikely that TAPE uses its limited protein domains to directly interact with diverse host and viral proteins; instead, it may interact with them through a common subcellular compartment such as endolysosomes. The third feature is that although TAPE is a relatively weaker IFN-β activator, it potently enhances the activation of IFN-β by other innate immune regulators, including Trif/Ticam-1, MAVS, and TBK1. Taken together, these features suggest that TAPE represents a new type of innate immune regulator, which controls a key cellular mechanism essential for both endosomal TLR signaling and cytosolic RLR signaling. Several outstanding questions need to be addressed to appreciate the importance of TAPE in innate immunity. First of all, in vivo functions of TAPE in the TLR3, TLR4, and RLR pathways have to be further confirmed by the conditional knockout approach. Secondly, it is critical to explore the mechanistic mechanisms by which TAPE regulates these TLR and RLR pathways. Particular attention is needed to reconcile the current idea of RLR signaling, centered on MAVS-located mitochondria, with the emerging RLR signaling model (Fig. 5.3), which is involved in TAPE-containing endolysosomes. Lastly, it will be interesting to explore the potential roles of TAPE in other innate immune pathways, such as endosomal TLR7/8, TLR9, and cytosolic DNA sensors. Better understanding of TAPE-mediated innate immune signaling may reveal novel regulatory mechanisms in innate immunity, and also provide insights into targeting the TLR and RLR pathways when they are deregulated in infectious or inflammatory diseases.