Expression Changes of MHC and Other Immune Genes in Frog Skin during Ontogeny

Quintin Lau,Yoko Satta,Shohei Komaki,Takeshi Igawa

doi:10.3390/ani10010091

Abstract

Simple SummaryTadpoles undergo many changes in physiology and immunology until metamorphosis into adult frogs. Major histocompatibility complex (MHC) molecules are an important part of vertebrate adaptive immunity, and our study measured the expression of two MHC genes (MHC class I and II) in skin during six tadpole stages of the Montane Brown frog (Rana ornativentris). First, using a qPCR method, we found that both MHC class I and II expression significantly increased between stage 24/25 (‘early’) and stage 28 (‘mid’) tadpole skin. Then, we conducted next-generation sequencing for ‘early’, ‘mid’ and ‘late’ stage tadpole skin mRNA of both R. ornativentris and a model species, Xenopus tropicalis, and confirmed that MHC expression increased from the ‘mid’ stage. We also performed further analyses of transcriptome data and found that several immune-related gene ontology terms were upregulated from the ‘mid’ tadpole stage. Our findings probably support that both MHC class I and II have a functional role during tadpole development.Anuran amphibians undergo major physiological and immunological changes following metamorphosis. Genes of the major histocompatibility complex (MHC) code for receptors important for vertebrate adaptive immunity. We used qPCR to measure skin MHC expression in six different ontological stages of Rana ornativentris (n = 10 per stage); normalized MHC class I and II expression at the mRNA level was significantly higher in stage 28 (mid-larval) compared to stages 24/25 (early-larval) tadpoles. Subsequent transcriptomic analyses of three tadpole (early-, mid-, and late-larval) stages of R. ornativentris and model species Xenopus tropicalis focused on mRNA expression of immune-related genes in the skin. Normalized expression of most MHC class I and II transcripts in both species were significantly higher in mid- and late-larval stages compared to early-larval stage. In addition, gene ontology (GO) analyses of differentially expressed transcripts revealed several immune-related GO terms that were significantly upregulated from the mid-larval stage. Our study provides evidence that both MHC class I and II is expressed during development in both R. ornativentris and X. tropicalis.

Highlights

Anuran amphibians have a biphasic life cycle featuring a pre-metamorphic tadpole stage usually inhabiting fully aqueous environments, and a post-metamorphic adult stage inhabiting aqueous and/or terrestrial environments depending on the species
We checked whether the low major histocompatibility complex (MHC) class I and II expression in the skin of stage 24/25 tadpoles was low in the ‘whole body’ samples of the same individuals
We found that expression was slightly higher in whole body of tadpoles than the skin, but this was statistically significant only for MHC class II (MHC-II) (MHC-I: p = 0.0567, MHC-II: p < 0.001, Figure 2)

Summary

Introduction

Anuran amphibians have a biphasic life cycle featuring a pre-metamorphic tadpole stage usually inhabiting fully aqueous environments, and a post-metamorphic adult stage inhabiting aqueous and/or terrestrial environments depending on the species. Animals 2020, 10, 91 tadpoles are considered to have a poor or immature adaptive immune system [1,2] as well as some components of innate immunity [3,4]. Anurans are known to go through a complete physiological reorganization; this includes components of the immune system like expression of major histocompatibility complex (MHC) molecules (reviewed by Rollins-Smith, 1998) [5]. The MHC is an essential component of vertebrate adaptive immunity because MHC genes code for membrane-bound glycoproteins that recognize, bind and present specific antigens to T lymphocytes. There are two major classes of MHC: MHC class I (MHC-I) molecules predominantly recognize and present endogenous antigenic peptides (e.g., from viruses) to cytotoxic T cells, while MHC class II (MHC-II) molecules present exogenous antigens (e.g., from fungi and bacteria) to helper T cells.

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