The role of MK2/TTP pathway in regulating early development and innate immunity in zebrafish
The origin of novel structures and the genes that regulate their formation is a challenging question that has been difficult to address. One such novel structure is the yolk syncytial layer (YSL) in teleost fishes like zebrafish. I studied the genetic pathways operating in the YSL using the betty boop (bbp) mutant. betty boop encodes mitogen activated protein kinase activated protein kinase 2a (mapkapk2a), also known as mk2a. One of the downstream targets of MK2a is an RNA binding protein Tristetraprolin (TTP). bbp mutants lose the expression of YSL specific genes mxtx1 and mxtx2. Here, we show that these genes are regulated through their 3’untranslated region (UTR) by MK2a/TTP pathway required in the YSL during early zebrafish development to limit their expression to the YSL. I developed an in vivo reporter assay to detect spatial regulation of mRNA stability by the MK2/TTP pathway in early zebrafish embryos which can also be used to test other potential TTP targets. MK2a is the zebrafish homolog of mammalian MK2. In mammals, MK2 is not active during early development but rather is required to regulate genes that are responsible for inflammatory response. I show that MK2/TTP reporters are not regulated in early Xenopus embryo either, implying that the early requirement for MK2 arose after the division of tetrapod and teleost lineages. The role of mk2a and its gene duplicate mk2b in regulating immune response has not been previously studied in zebrafish. I developed zebrafish as a model infection system for infection by multiple species of Candida, an opportunistic pathogen and cause of an increasing number of hospital acquired infections. I developed novel transgenic zebrafish lines that allow inducible ablation of different phagocyte classes and identified different inflammatory responses to different Candida species. Preliminary results using these tools show that mk2b-/- mutants show a higher susceptibility to Candida hyphal infection, demonstrating that mk2b plays a role in innate immunity. Whether that role is the same that MK2 plays in mammals in regulating inflammatory cytokines still need to be determined. Taken together, all these findings suggest a model in which the ancestral MK2/TTP pathway was co-opted in teleosts to restrict expression of the mxtx genes to the yolk cell, but also retains its function in innate immune response.
Earle development, Immunity, Zebrafish, Betty boop
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