Hereditary spherocytosis in zebrafish riesling illustrates evolution of erythroid beta-spectrin structure, and function in red cell morphogenesis andmembrane stability

Spectrins are key cytoskeleton proteins,vith roles in membrane integrity, c ell morphology, organelle transport and cell polarity of varied cell types during development. Defects in erythroid spectrins in humans result in cong enital hemolytic anemias with altered red cell morphology, Although well ch aracterized in mammals and invertebrates, analysis of the structure and fun ction of nonmammalian vertebrate spectrins has been lacking. The zebrafish riesling (ris) suffers from profound anemia, where the developing red cells fail to assume terminally differentiated erythroid morphology, Using compa rative genomics, erythroid beta -spectrin (sptb) was identified as the gene mutated in ris, Zebrafish Sptb shares 62.3% overall identity with the huma n ortholog and phylogenetic comparisons suggest intragenic duplication and divergence during evolution, Unlike the human and murine orthologs, the ple ckstrin homology domain of zebrafish Sptb is not removed in red cells by al ternative splicing. In addition, apoptosis and abnormal microtubule margina l band aggregation contribute to hemolysis of mutant erythrocytes, which ar e features not present in mammalian red cells with sptb defects. This study presents the first genetic characterization of a non-mammalian vertebrate sptb and demonstrates novel features of red cell hemolysis in non-mammalian red cells. Further, we propose that the distinct mammalian erythroid morph ology may have evolved from specific modifications of Sptb structure and fu nction.