Beta3-adrenergic receptors--studies on rainbow trout reveal ancient evolutionary origins and functions distinct from the thermogenic response

Functional Genomics of Fish Erythrocytes

Fish red blood cells contain nucleus and therefore carry on active transcription throughout their life. As the steady state level of mRNA depends both on the production and breakdown of the mRNA, it is important to pay attention to sampling, cell separation, mRNA extraction, and transcript stability to conduct a comprehensive erythrocyte genomics study. In addition, particular attention should be made to tie the transcriptional changes to corresponding protein activities, as only those would impact cellular functions. Here, we thus describe the different steps to conduct comprehensive functional genomics studies with fish erythrocytes.

Beta-adrenergic receptor stimulation influences the function of monocytes/macrophages in ayu (Plecoglossus altivelis)

Adrenergic receptors (ARs) are members of the G-protein-coupled receptor superfamily that can be categorized into αARs and βARs. The specific function of ARs in teleost monocytes/macrophages (MO/MФ) remains unknown. We determined the cDNA sequence of ARs from ayu (Plecoglossus altivelis; PaαAR and PaβAR). Sequence comparisons showed that PaαAR was most closely related to the αAR of the Japanese flounder and Nile tilapia, while PaβAR was most closely related to the βAR of Atlantic salmon. The AR transcripts were mainly expressed in the spleen, and their expression was altered in various tissues upon infection with Vibrio anguillarum. PaαAR and PaβAR proteins were upregulated in MO/MФ after infection, and PaβAR knockdown resulted in a pro-inflammatory status in ayu MO/MФ upon V. anguillarum infection and lowered the phagocytic activity of MO/MФ. Our results indicate that PaβAR plays the role of an anti-inflammatory mediator in the immune response of ayu against bacterial infection.

Regulation of erythrocyte function: Multiple evolutionary solutions for respiratory gas transport and its regulation in fish

Gas transport concepts in vertebrates have naturally been formulated based on human blood. However, the first vertebrates were aquatic, and fish and tetrapods diverged hundreds of millions years ago. Water-breathing vertebrates live in an environment with low and variable O₂ levels, making environmental O₂ an important evolutionary selection pressure in fishes, and various features of their gas transport differ from humans. Erythrocyte function in fish is of current interest, because current environmental changes affect gas transport, and because especially zebrafish is used as a model in biomedical studies, making it important to understand the differences in gas transport between fish and mammals to be able to carry out meaningful studies. Of the close to thirty thousand fish species, teleosts are the most species-numerous group. However, two additional radiations are discussed: agnathans and elasmobranchs. The gas transport by elasmobranchs may be closest to the ancestors of tetrapods. The major difference in their haemoglobin (Hb) function to humans is their high urea tolerance. Agnathans differ from other vertebrates by having Hbs, where cooperativity is achieved by monomer-oligomer equilibria. Their erythrocytes also lack the anion exchange pathway with profound effects on CO₂ transport. Teleosts are characterized by highly pH sensitive Hbs, which can fail to become fully O₂ -saturated at low pH. An adrenergically stimulated Na⁺ /H⁺ exchanger has evolved in their erythrocyte membrane, and plasma-accessible carbonic anhydrase can be differentially distributed among their tissues. Together, and differing from other vertebrates, these features can maximize O₂ unloading in muscle while ensuring O₂ loading in gills.