Systems Biological Approaches Reveal Non-additive Responses and Multiple Crosstalk Mechanisms between TLR and GPCR Signaling

A variety of ligands differ in their capacity to bind the receptor, elicit gene expression, and modulate physiological responses. Such receptors include Toll-like receptors (TLRs), which recognize various patterns of pathogens and lead to primary innate immune activation against invaders, and G-protein coupled receptors (GPCRs), whose interaction with their cognate ligands activates heterotrimeric G proteins and regulates specific downstream effectors, including immuno-stimulating molecules. Once TLRs are activated, they lead to the expression of hundreds of genes together and bridge the arm of innate and adaptive immune responses. We characterized the gene expression profile of Toll-like receptor 4 (TLR4) in RAW 264.7 cells when it bound with its ligand, 2-keto-3-deoxyoctonate (KDO), the active part of lipopolysaccharide. In addition, to determine the network communications among the TLR, Janus kinase (JAK)/signal transducer and activator of transcription (STAT), and GPCR, we tested RAW 264.7 cells with KDO, interferon-beta, or cAMP analog 8-Br. The ligands were also administered as a pair of double and triple combinations.

Gene expression profiling of light-induced retinal degeneration in phototransduction gene knockout mice

Exposure to light can induce photoreceptor cell death and exacerbate retinal degeneration. In this study, mice with genetic knockout of several genes, including rhodopsin kinase (Rhok-/-), arrestin (Sag-/-), transducin (Gnat1-/-), c-Fos (c-Fos-/-) and arrestin/transducin (Sag-/-/Gnat1-/-), were examined. We measured the expression levels of thousands of genes in order to investigate their roles in phototransduction signaling in light-induced retinal degeneration using DNA microarray technology and then further explored the gene network using pathway analysis tools. Several cascades of gene components were induced or inhibited as a result of corresponding gene knockout under specific light conditions. Transducin deletion blocked the apoptotic signaling induced by exposure to low light conditions, and it did not require c-Fos/AP-1. Deletion of c-Fos blocked the apoptotic signaling induced by exposure to high intensity light. In the present study, we identified many gene transcripts that are essential for the initiation of light-induced rod degeneration and proposed several important networks that are involved in pro- and anti-apoptotic signaling. We also demonstrated the different cascades of gene components that participate in apoptotic signaling under specific light conditions.

Toll-like receptor signal transduction

Toll-like receptors (TLRs) are the archetypal pattern recognition receptors in sensing exogenous pathogens. Activation of TLRs is a first line of defense of the immune system, leading to the activation and recruitment of neutrophils and macrophages to sites of infection and enhances antimicrobial activity. The TLR signaling through different intracellular molecules, such as MAP kinases and IkappaB kinases which are conserved signaling elements for many receptors, leads to a distinct set of proinflammatory gene expressions. However, how these pathways differentially and precisely control the transcription of identical genes remains largely unknown. Our review focuses on the details of up-to- date signaling molecules including negative regulators and their role in controlling innate immune response. We also stress the importance of developing systemic approaches for the global understanding of TLR signaling so that appropriate drug therapeutic targets can be identified for regulating inflammatory diseases.