Unusual Diversity of Myoglobin Genes in the Lungfish

Myoglobin is a respiratory protein that serves as a model system in a variety of biological fields. Its main function is to deliver and store O₂ in the heart and skeletal muscles, but myoglobin is also instrumental in homeostasis of nitric oxide (NO) and detoxification of reactive oxygen species (ROS). Almost every vertebrate harbors a single myoglobin gene; only some cyprinid fishes have two recently duplicated myoglobin genes. Here we show that the West African lungfish Protopterus annectens has at least seven distinct myoglobin genes (PanMb1-7), which diverged early in the evolution of lungfish and showed an enhanced evolutionary rate. These myoglobins are lungfish specific, and no other globin gene was found amplified. The myoglobins are differentially expressed in various lungfish tissues, and the brain is the main site of myoglobin expression. The typical myoglobin-containing tissues, the skeletal muscle and the heart, have much lower myoglobin mRNA levels. Muscle and heart express distinct myoglobins (PanMb1 and PanMb3, respectively). In cell culture, lungfish myoglobins improved cellular survival under hypoxia albeit with different efficiencies and reduced the production of reactive oxygen species. Only Mb2 and Mb6 enhanced the energy status of the cells. The unexpected diversity of myoglobin hints to a functional diversification of this gene: some myoglobins may have adapted to the O₂ requirements of the specific tissue and help the lungfish to survive hypoxic periods; other myoglobins may have taken over the roles of neuroglobin and cytoglobin, which appear to be missing in the West African lungfish.