The immune response of a host to an invading pathogen is dependent on the capacity of its immune cell compartment to recognize highly conserved pathogen components using an ancient class of pattern recognition receptors known as Toll-like receptors (TLRs). Initiation of TLR-mediated signaling results in the induction of proinflammatory cytokines that help govern the scale and duration of any ensuing response. Specificity for TLR signaling is, in part, a result of the differential recruitment of intracellular adaptor molecules. Of these, MyD88 is required for the majority of TLR signaling. Zebrafish have been shown to possess TLRs and adaptor molecules throughout early development, including MyD88, strongly suggesting conservation of this ancient defense mechanism. However, information about which embryonic cells/tissues possess this conserved signaling potential is lacking. To help define which embryonic cells, in particular, those of the innate immune system, have the potential for MyD88-dependent, TLR-mediated signaling, we generated transgenic reporter lines using regulatory elements of the myd88 gene to drive the fluorescent reporters enhanced GFP and Discosoma red fluorescent protein 2 within live zebrafish. These lines possess fluorescently marked cells/tissues consistent with endogenous myd88 expression, including a subset of myeloid leukocytes. These innate immune cells were confirmed to express other TLR adaptors including Mal, trif, and Sarm. Live wound-healing and infection assays validated the potential of these myd88-expressing leukocytes to participate in immune responses. These lines will provide a valuable resource for further resolving the contribution of MyD88 to early vertebrate immunity.
Inflammation is an important and appropriate host response to infection or injury. However, dysregulation of this response, with resulting persistent or inappropriate inflammation, underlies a broad range of pathological processes, from inflammatory dermatoses to type 2 diabetes and cancer. As such, identifying new drugs to suppress inflammation is an area of intense interest. Despite notable successes, there still exists an unmet need for new effective therapeutic approaches to treat inflammation. Traditional drug discovery, including structure-based drug design, have largely fallen short of satisfying this unmet need. With faster development times and reduced safety and pharmacokinetic uncertainty, drug repositioning – the process of finding new uses for existing drugs – is emerging as an alternative strategy to traditional drug design that promises an improved risk-reward trade-off. Using a zebrafish in vivo neutrophil migration assay, we undertook a drug repositioning screen to identify unknown anti-inflammatory activities for known drugs. By interrogating a library of 1280 approved drugs for their ability to suppress the recruitment of neutrophils to tail fin injury, we identified a number of drugs with significant anti-inflammatory activity that have not previously been characterized as general anti-inflammatories. Importantly, we reveal that the ten most potent repositioned drugs from our zebrafish screen displayed conserved anti-inflammatory activity in a mouse model of skin inflammation (atopic dermatitis). This study provides compelling evidence that exploiting the zebrafish as an in vivo drug repositioning platform holds promise as a strategy to reveal new anti-inflammatory activities for existing drugs.
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