During embryogenesis the expression of
During embryogenesis, the expression of zGpr84 was found higher at the cleavage stage and decreased at both blastula and gastrula stages. It indicates that maternal zGpr84 transcripts are present at the cleavage and blastula stages, since zygotic transcripts are not produced before the gastrula stage. Whole mount in situ hybridization on zebrafish larva showed that the zGpr84 mRNA were restricted to the intestine, branchial arches, pectoral fin, and head region after 48hpf. These results indicated a role of zGpr84 during embryogenesis. Further studies, such as loss-of-function or dominant-negative approaches, are needed to clarify the roles of the gene in zebrafish embryo development.
Our study demonstrated that the expression of zGpr84 mRNA was up-regulated after fasting or LPS challenge, indicating the roles of zGpr84 in both nutritional control and immune response. In human or mouse, the mRNA expression levels of Gpr84 are also markedly increased by LPS challenge (Venkataraman and Kuo, 2005), suggesting the conservation of the immune function of Gpr84 between fish and mammals. In order to expand and understand the immune role of this gene, it is necessary to further check the response of zGpr84 to other pathogen-associated molecular patterns (PAMPs) such aslipoteichoic poly ic (LTA), peptidoglycan (PGN) and polyinosinic polycytidylic acid (PolyI:C).
Zebrafish as a model organism is becoming an increasingly powerful tool in lipid research (Hölttä-Vuori et al., 2010). In his paper, we checked whether the overexpression of zGpr84 in cells results in the accumulation of lipid droplets. The cells transfected with zGpr84 had much more lipid droplets than the control groups (cells un-transfected with zGpr84), indicating that the lipid droplets formation in cells is mediated by zGpr84. Further work is being done to check whether zGpr84 can also be induced by other MCFAs such as capric acid and lauric acid.
It has been reported that MCFAs can amplify LPS-induced IL-12 p40 production in the macrophage-like cell line RAW264.7 (Wang et al., 2006), providing a potential link between fatty acid metabolism and immunological regulation. Here we further showed that this effect is mediated through zGpr84 activation since the expression of IL-12 p40 mRNA decreased much more in the un-transfected cells compared to the zGpr84-overexpressed cells. Coincidentally, Suzuki et al. (2013) recently demonstrated that the activation of Gpr84 amplifies LPS-stimulated production of the proinflammatory cytokine IL-8 in human polymorphonuclear leukocytes and TNFα in macrophages. Therefore, medium-chain FFAs can affect pathogen elimination processes by inducing IL-12, IL-8 and TNFα through Gpr84, highlighting the importance of FFAs as signaling molecules in regulating immunity. Further work is needed to investigate whether other fatty acid receptors such as Gpr40, Gpr41, Gpr43 and Gpr120 are also involved in this process, as well as the mechanism by which FFAs exert their effects on immune cells.
In conclusion, zGpr84 was identified and characterized in this paper. zGpr84 is highly expressed in the tissues of intestine, heart and liver, and is markedly induced by fasting or LPS challenge. Furthermore, undecanoic acid can amplify LPS stimulated production of IL-12 p40 through zGpr84, supporting the proposal that Gpr84 plays a role in directly linking fatty acid metabolism to immunological regulation.
Acknowledgements We are very grateful to Jennifer Yoo (Smith College, USA) for her critical reading and language editing of the manuscript. This work was supported by the grants of National Natural Science Foundation of China (31340034), Qingdao Municipal Science and Technology Council of China (No. 12-1-4-1-(6)-jch) and the Fundamental Research Funds for the Central Universities (201362019).
Introduction The amyloid cascade hypothesis states that altered processing of amyloid precursor protein lead to the release of β-amyloid peptides (e.g., Aβ42), which initiate the pathological process underlying AD (Tanzi and Bertram, 2005). These peptides can aggregate into soluble oligomers and then into insoluble fibrils that accumulate to form macroscopic amyloid plaques. Whether all of these forms are toxic and how they cause toxicity are questions that are still debated. A prevailing view is that the oligomers, more toxic than the fibrils, bind to and alter the function of certain membrane proteins, resulting in synaptic dysfunction, dendritic degeneration and neuronal death (Benilova et al., 2012).