Figure 2. Post-transcriptional regulation contributes to negative feedback loop formation at multiple stages of the TLR4 signaling pathway.

The TLR4 signaling pathway consists of several targets for post-transcriptional regulation, several of which contribute to the formation of negative feedback loops that modulate pro-inflammatory responses. (i) Upon detecting LPS, TLR4 is activated and binds several adaptor molecules that facilitate propagation of signaling, in part through the NFκB pathway. (ii) NFκB signaling leads to the initiation of transcription of many cytokines and chemokines. (iii) In response to activation of TLR signaling, the adaptor protein MD2 undergoes alternative splicing and is produced in a shorter isoform (lacking exon 2 in human macrophages). (iv) The protein product of this short isoform competes with canonical MD2 for binding to TLR4 to inhibit signal transduction. (v) Similarly, after TLR activation, adaptor protein MyD88 undergoes alternative splicing and is produced in a short isoform (MyD88-S) lacking exon 2. MyD88 isoform usage is dictated by the balance of SRSF1 and hnRNP U, where SRSF1 binding to the MyD88 transcript promotes inclusion of exon 2, and hnRNP U binding promotes removal of exon 2. (vi) MyD88-S acts as a dominant-negative inhibitor of IL-1 and LPS-induced activation of NFκB. (vii) Finally, TLR4 itself is also alternatively spliced to include an additional exon. (viii) This results in production of a soluble form of TLR4 that inhibits NFκB activation. Thus, regulation at the level of post-transcriptional processing allows for tight control over multiple stages of the TLR4 signaling cascade, facilitating rapid returns to homeostasis.