RED FLAG. Salmonid red blood cells as sensors of stress and infection

Red blood cells serve the essential function of transporting oxygen to the organs of the body, and keep us alive. In contrast to human red blood cells that lack a nucleus and DNA, red blood cells in fish, reptiles and birds have the cell nucleus intact and can respond to changes in blood plasma components that reflect the state of health, and this may again affect their natural role as oxygen carriers. Salmonid aquaculture give rise to healthy and nutritious food that are transported all over the world. Ensuring that farmed salmonids are in good health is very important for sustainability of the industry. One still lack effective tools for monitoring salmonid health, and understanding of salmonid biology is still incomplete. The RED FLAG Project aims to provide and share information on how salmonid red blood cells sense danger - like stress and infection, and on how these danger signals affects gas exchange and fish survival. Based on the integration of signals in red blood cells, a small blood sample will be tested as source of information on the state of health in salmon and rainbow trout. We have optimized an ex vivo model for the study of salmonid red blood cell responses, focusing on the integration between responses to stress and infection. We also study the activity of intracellular signaling pathways that couples external signals to gene regulation, and compare these responses in Atlantic salmon and rainbow trout. We have studied the red blood cell transcriptome and changes induced in contact with piscine orthoreovirus in cell culture, compared with two salmonid cell lines (presented internationally/manuscript in prep). These results are being compared to red blood cell responses in vivo, where we study how the blood cell transcriptome reflects differences between PRV variants that cause heart-and skeletal muscle inflammation (PRV-1) versus no disease, but protection (PRV-3) in Atlantic salmon (conference abstract), and two in vivo stress pilot trials have been performed. An exchange of the PhD employed in the project to Vancouver, Canada for the study of physiological properties of red blood cells, and of a PhD from the lab of partner in Elche, Spain to Norway to study responses in A. salmon vs rainbow trout, was performed in 2022. A master student was recruited (Aquamedicine, UiT, 2022) to help establish stress models in salmon.

RED FLAG Salmonid red blood cells as sensors of internal health The RED FLAG project aims to clarify the ability of nucleated pluripotent red blood cells (RBCs) of salmonids to sense “danger” like stress and infection. The initial focus is on how the signals elicited by acute/chronic stress hormones and viral infections impact red blood cell fate, regulation of gas Exchange, immunological functions, and thereby fish health and survival. The project will establish and optimize red blood cell ex vivo models for mechanistic studies, and determine blood cell fate in vivo through use of cell tracers. The project covers exploration of the RBC intracellular signalling pathways and transcriptional/translational control, cell signalling crosstalk and effects on gene expression, cellular functions and differentiation potential. The new knowledge on RBC responses will be used to introduce a minimal blood sample as the source of biomarkers of previous exposure to infection and stress. RED FLAG will represent a starting point of establishing red blood cells as sensors and information providers, with the expanding potential of using RBC responses to also evaluate effects of environmental conditions, chemical treatments, and different composition of feed– i.e. fish RBCs as the source of integrated information on internal Health.