A medium-chain fatty acid receptor Gpr84 in zebrafish: Expression pattern and roles in immune regulation

https://doi.org/10.1016/j.dci.2014.03.017Get rights and content

Highlights

  • The expression pattern of zGpr84 was revealed.

  • The expression of zGpr84 was up-regulated after fasting or LPS challenge.

  • zGpr84 is involved in the lipid droplets accumulation in cells.

  • Undecanoic acid can amplify LPS-stimulated production of IL-12 p40 through zGPR84.

Abstract

Gpr84 was recently identified as a receptor for medium-chain fatty acids, but its functions remain to be clarified. We reported the identification of a zebrafish Gpr84 homologue (zGpr84), which has a higher gene expression in the tissues of intestine, heart and liver. During embryogenesis, zGpr84 is maternally expressed and a significant increase is observed at segmentation period, and it is mainly restricted to the head region, pectoral fins, branchial arches, intestine and lateral line neuromast. Fasting or treatment with lipopolysaccharide (LPS) can induce significant up-regulation of zGpr84. We further demonstrated that zGpr84 is involved in the accumulation of lipid droplets in cells. Moreover, undecanoic acid (UA) can amplify LPS induced production of the proinflammatory cytokine IL-12 p40 through zGpr84, supporting the proposal that Gpr84 may play a role in directly linking fatty acid metabolism to immunological regulation. The resulting data in fish lay a foundation for a comprehensive exploration of the functions and evolution of Gpr84.

Introduction

Free fatty acids (FFAs) are not only essential nutrients but they also contribute to many cellular functions. FFAs have been demonstrated to act as ligands of several G-protein-coupled receptors (GPCRs) such as Gpr40, Gpr41, Gpr43 and Gpr120 (Ichimura et al., 2009, Briscoe et al., 2003, Itoh et al., 2003, Kotarsky et al., 2003). Gpr41 and Gpr43 can be specifically activated by short-chain FFAs (Brown et al., 2003, Zhang et al., 2013, Covington et al., 2006). Gpr40 is activated by medium- and long-chain FFAs (Mancini and Poitout, 2013, Godinot et al., 2013) and Gpr120 is activated by long-chain FFAs. These fatty acid receptors are suggested to take part in the regulation of energy homeostasis (Briscoe et al., 2003, Itoh et al., 2003, Blad et al., 2012, Oh et al., 2010, Overton et al., 2006, Hirasawa et al., 2005).

In the last decade, another receptor Gpr84 was identified to be activated by medium-chain FFAs (MCFAs) with carbon chain lengths of 9–14 (Suzuki et al., 2013). Gpr84 is mainly expressed in immune-related tissues such as bone marrow, spleen, lymph nodes and thymus of mice (Ichimura et al., 2009), and is significantly up-regulated upon lipopolysaccharide (LPS) stimulation (Wang et al., 2006). A study using Gpr84-deficient mice further revealed that Gpr84 can regulate early IL-4 gene expression in activated T cells (Venkataraman and Kuo, 2005). Thus, it is proposed that Gpr84 effects predominately the immune system.

Very recently, it was shown that Gpr84 mRNA expression is up-regulated in adipose tissues from C57BL/6J male mice fed with a high-fat diet (Nagasaki et al., 2012). Moreover, MCFAs or surrogate agonists can regulate inflammatory responses through activation of Gpr84 (Wang et al., 2006, Lattin et al., 2008, Suzuki et al., 2013). Compared with the research in mammalian species, little information about Gpr84 is available in fish. In this paper, we explored whether zebrafish Gpr84 (zGpr84) is structurally and functionally similar to that of mammalian species. It was revealed that zGpr84 is involved in the accumulation of lipid droplets and that the up-regulation of zGpr84 can be significantly induced through fasting or treatment of LPS. Furthermore, UA can amplify LPS induced production of the proinflammatory cytokine IL-12 p40 through zGpr84, indicating a possible role of zGpr84 in directly linking fatty acid metabolism to immunological regulation.

Section snippets

Sequence and bioinformatic analysis

The sequence of zGpr84 gene was acquired from Ensemble database (http://www.ensembl.org). The protein domain and the transmembrane regions were analyzed using the SMART program (http://smart.embl-heidelberg.de/). Multiple alignments of the protein sequences were generated using the Clustal W program (Chenna et al., 2003) within the MegAlign of the DNASTAR software package (version 5.0). The information of exon–intron organization was obtained from NCBI database (http://www.ncbi.nlm.nih.gov/)

Bioinformatic analysis of zGpr84

The gene of zGpr84 is acquired from the Ensemble database (number: ENSDARG00000077308), and it encodes a protein of 410 amino acids. The analysis of the exon–intron organization of Gpr84 from different species suggests that Gpr84 genes from a mouse, rat, pig and zebrafish are all composed of 2 exons interspaced by 1 intron, while human Gpr84 has only one exon (data not shown). Alignment of the amino acid sequences of Gpr84 proteins, including zGpr84, revealed that zGpr84 shared 47.6–50.3% amino

Discussions

GPR84 is recently found to be a putative receptor for medium-chain fatty acids (MCFAs). To date, there are only a few published reports with respect to the function of this receptor and little information is available in fish. In this study, we reported the identification and characterization of zGpr84 as well as the investigation of the function of this receptor. zGpr84 mRNA is most abundantly present in liver, heart and intestine among the tested tissues. In human and mouse, GPR84 is

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).

References (24)

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