More than hemoglobin - the unexpected diversity of globins in vertebrate red blood cells

In many multicellular organisms, oxygen is transported by respiratory proteins, which are globins in vertebrates, between respiratory organs and tissues. In jawed vertebrates, eight globins are known which are expressed in a highly tissue-specific manner. Until now, hemoglobin (Hb) had been agreed to be the only globin expressed in vertebrate erythrocytes. Here, we investigate for the first time the mRNA expression of globin genes in nucleated and anucleated erythrocytes of model vertebrate species by quantitative real-time reverse transcription PCR (qRT-PCR). Surprisingly, we found transcripts of the whole gnathostome globin superfamily in RBCs. The mRNA expression levels varied among species, with Hb being by far the dominant globin. Only in stickleback, a globin previously thought to be neuron-specific, neuroglobin, had higher mRNA expression. We furthermore show that in birds transcripts of globin E, which was earlier reported to be transcribed only in the eye, are also present in RBCs. Even in anucleated RBCs of mammals, we found transcripts of myoglobin, neuroglobin, and cytoglobin. Our findings add new aspects to the current knowledge on the expression of globins in vertebrate tissues. However, whether or not the mRNA expression of these globin genes has any functional significance in RBCs has to be investigated in future studies.